Particulate-Filled Polymer Composites Second Edition

Editor

R.N. Rothon

Rapra Technology Limited

8

Filled Thermoplastics Chris DeArmitt and Michael Hancock

8.1 Introduction 8.1.1 Thermoplastics and Typical Applications Thermoplastics have become an essential part of our everyday lives. Our cars and appliances contain more and more plastics every year. Even our clothes are often made from synthetic thermoplastics. They are a very important class of material for many reasons. They combine good mechanical and electrical properties with low density and high formability. Clearly, the driving force for their success has been that they can often provide an overall solution that is less expensive than that achievable with other materials such as glass, wood, metal, thermosetting polymers or ceramics. Thermoplastics, as implied by their name, are materials that flow upon heating, and harden when cooled. They can be formed using a wide variety of techniques, such as injection moulding, thermoforming, blow moulding and rotational moulding. Injection moulding in particular allows complex shapes, so it is possible to integrate several smaller parts into one larger part, thus saving on assembly costs. As well as being easily processed when molten, they also have the potential to be recycled by remelting them to form new articles, or burnt and used to generate electrical energy. New legislation is being introduced to encourage recycling of used products; this is expected to favour thermoplastics over other materials, which are not as easy to reprocess. Thermoplastic demand in Western Europe is 37 x 106 tonnes compared to 10 x 106 tonnes for thermosetting polymers. A breakdown of thermoplastics by application area is given in Figure 8.1. Some of the main properties and applications are given next for the five main thermoplastics, which together account for 75% of the total thermoplastics market.

• Polyethylene (low density) LDPE, (linear low density) LLDPE: 7.6 x 106 tonnes Properties: Flexible, translucent, very tough, weatherproof, good chemical resistance, low water absorption, easily processed by most methods, low cost. 357

Particulate-Filled Polymer Composites

Figure 8.1 Use of thermoplastics in Western Europe

Applications: Squeeze bottles, toys, carrier bags, high frequency insulation, chemical tank linings, heavy-duty sacks, general packaging, gas and water pipes.

• Polyethylene (high density) HDPE: 5.0 x 106 tonnes Properties: Semi-rigid, translucent, weatherproof, good low temperature toughness (to –60 °C), easy to process by most methods, low cost, good chemical resistance. Applications: Chemical drums, jerricans, carboys, toys, picnic ware, household and kitchenware, cable insulation, carrier bags, food wrapping material.

• Polypropylene PP: 7.0 x 106 tonnes Properties: Semi-rigid, translucent, good chemical resistance, tough, good fatigue resistance, integral hinge property, steam sterilisable, good heat resistance. Applications: Sterilisable hospital ware, ropes, car battery cases, chair shells, integral moulded hinges, packaging films, electrical kettles, car bumpers and interior trim components, video cassette cases. 358

Filled Thermoplastics

• Polyvinyl chloride PVC: 5.8 x 106 tonnes Properties: Rigid or flexible, clear or opaque, durable, weatherproof, flame resistant, good impact strength, good electrical insulation properties, limited low temperature performance. Applications: Window frames, drain pipes, sewage and soil pipes, roofing sheets, cable and wire insulation, floor tiles, hose and pipes, stationary covers, fashion footwear, leathercloth.

• Polystyrene (general purpose) GPPS: 3.1 x 106 tonnes combined with high impact polystyrene (HIPS) Properties: Brittle, rigid, transparent, low shrinkage, low cost, excellent X-ray resistance, free from odour and taste, easy to process. Applications: Toys and novelties, rigid packaging, refrigerator trays and boxes, cosmetic packs and costume jewellery, lighting diffusers, audio cassette and CD cases.

• Polystyrene (high impact) HIPS Properties: Hard, rigid, translucent, impact strength up to seven times that of GPPS, other properties similar. Applications: Yoghurt pots, refrigerator linings, vending cups, bathroom cabinets, toilet seats and tanks, closures, instrument control knobs.

• Polyesters (thermoplastic) PET: 3.1 x 106 tonnes Properties: Rigid, clear, extremely tough, good creep and fatigue resistance, wide range of temperature resistance (–40 °C to 200 °C). Applications: Carbonated drink bottles, synthetic fibres, video and audio tape, microwave utensils.

8.1.2 Thermoplastic Composites One disadvantage of thermoplastics is that they soften appreciably as they are heated. As this happens, their modulus decreases and they begin to creep (slowly deform over time) 359

Particulate-Filled Polymer Composites and at higher temperatures they progressively lose their shape, and then melt. There has been a great deal of effort spent in trying to overcome this limitation. Filling PP with talc is an early example of this. In fact, addition of mineral fillers increases the modulus of all thermoplastics, and increases their heat distortion temperature (HDT) [1]. It is commonly thought that the primary goal of adding fillers is to lower the overall materials cost of the composite compared to the unfilled polymer. This is, however, rarely the case. Polyethylene (PE) and PP are the world’s number one and two highest volume polymers, respectively, and they are also the least expensive per unit volume. Addition of any common filler, with the exception of calcium carbonate, increases the material cost of these polymers. Even in cases where the main goal is to decrease cost, the addition of filler changes nearly every property of the polymer. It is therefore now common to use the term ‘functional fillers’ to emphasise that the fillers change the polymer, giving many advantages, and naturally, some disadvantages. Making good composites is all about knowing how to find a good balance of properties at the lowest cost. In order to make the right decision, one needs to know about polymers, engineering, fillers and surface science. That is what makes the study and application of composites so challenging, fascinating and rewarding. In this chapter, we will discuss the main polymer properties and how they are influenced by the addition of various common fillers. The global fillers market is estimated as between 5-10 million tons per year, with over 90% of the filler going into rubbers, PVC and polyolefins, e.g., PE and PP. PP is the one of the most commercially important filled polymers [2] and it will therefore be used to illustrate some of the main points. Similar trends are seen when fillers are compounded into other semi-crystalline thermoplastics such as PE¸ PVC [3] and the polyamides. The amorphous polymers such as polystyrene and polycarbonate also respond similarly to filler addition. Incorporation of fillers into thermoplastics alters all the properties of the material. Some of the changes will be beneficial and some will be detrimental. It should be noted that these are not absolutes and the determination of pros and cons is only meaningful to the proposed end-use of the material. Burditt listed 21 reasons why filler may be added to a polymer [4]. In addition to those intentional changes, there are a multitude of unintentional effects that must also be considered. In this chapter the main properties of composites and how they vary with filler type and level of addition will be discussed. An essential point to note, is that the properties of the composite depend upon the volume percentage of filler added [5, 6]. Often in the literature, one sees properties plotted versus the weight percentage of filler, which is not particularly useful and may even be misleading. It is more meaningful to plot properties versus the volume percentage of filler [7]. In many cases, this latter approach gives straight lines, allowing simple, accurate extrapolation and prediction of properties [8, 9]. The properties of a composite are usually in between those of the component materials. Several of the properties such as

360

Filled Thermoplastics density, modulus and yield strength can be predicted by the rule of mixtures, according to the volume fraction of each component. In other cases, there are more complex mathematical models that can be used to describe and predict properties as the volume fraction of the ingredients is varied. Sometimes, the inclusion of filler can chemically or physically modify the polymer phase to such as extent that it becomes more difficult to apply simple models. It is important to realise that these complications may occur, and to look for them when characterising composites. This will help in the understanding and design of new composite materials. One example of this behaviour is changes in crystallisation, such as nucleation of crystal growth [10-14] and changing of the crystal phase of the polymer [15, 16]. Chemical degradation of the polymer may be catalysed by the filler, or impurities in the filler, especially transition metals [17, 18]. Another common example is where the filler surface adsorbs stabilisers and antioxidants, which are then unable to protect the polymer during processing and during its service life [19-21]. Alternatively, mechanical degradation may occur when high levels of filler cause unduly high viscosity, thereby inducing chain scission due to the excessive shear needed to process the material. In this chapter, an attempt has been made to mention each of the factors that influence composite design and performance. For a given application, one must identify the key properties that are important and concentrate on those. It will be seen that there are many different parameters to consider and that in some cases, optimisation of one precipitates an inevitable worsening of some other property. There is no one optimal composite; rather the goal is to seek the best balance of properties through compromise and an awareness of the entire picture in terms of economics and performance. In cases where the filler is less expensive than the polymer, then the goal is to increase the filler loading as much as possible, while still retaining sufficient processability and properties. Conversely, when the filler is more expensive than the polymer on a volume basis, then one seeks to identify the minimum filler loading that gives sufficient properties.

8.2 Bulk and Process Related Properties 8.2.1 Specific Gravity or Relative Density The common fillers used in plastics are minerals (densities from 2.4-2.8 g/cm3), which give a composite of higher density than that of the unfilled polymer (densities of 0.8-1.9 g/cm3). The density of a composite of known composition can be calculated according to the linear rule of mixtures (Equation 8.1), where ρc, ρf and ρp are the densities of the composite, filler and matrix, respectively, and mf is the mass fraction of filler.

361

Particulate-Filled Polymer Composites

ρc =

ρf ρ p ρ pm f + ρf (1 − m f )

Equation 8.1 Composite density Such calculations usually agree well with measured values. A lower than expected density may be due to inclusions of air resulting from poor mixing or poor wetting of the filler surface by the polymer. Higher than expected density occurs when the filler nucleates crystal growth in the polymer. The increased crystallinity increases the density of the composite because crystals are of higher density than amorphous regions in the polymer. The expected density of the composite can be calculated using Equation 8.1, assuming no air inclusions and that the filler does not influence density of the polymer phase by nucleation of crystal growth for example. The filler loading mf is usually determined by ashing, namely burning away the polymer from a known mass of composite and weighing the amount of residual filler. This method is simplest if the filler has enough thermal stability so that it does not lose mass at the high temperatures needed to burn off common polymers (≥ 300 °C). For fillers that are somewhat thermally unstable, such as aluminium trihydrate, it is necessary to correct the ashing result using the mass loss for the filler alone under the same ashing conditions. Recently ‘ovens’ based on microwave ashing have been introduced. These operate at low temperatures, removing the polymer while leaving the filler unaffected. Density can also be determined by measuring the volume of liquid displaced by a known mass of composite or using a density gradient column. Increased density is usually undesirable because products must inevitably be transported to be sold, or installed. This may result in increased transportation costs. Weight increases are undesirable when the material is to be used to make cars, trucks, trains, aeroplanes or spacecraft. Recent European legislation on packaging will also penalise by weight. Decreased density is possible through use of fillers such as wood flour (or fibre), hollow glass microspheres, hollow polymer microspheres [22] (e.g., Expancel®) or hollow spheres from fly ash. Low-density thermoplastic composites are useful for products that must float.

8.2.2 Acoustic Properties It is appropriate to mention acoustic properties here, as they are affected by density. Adding filler usually increases the density compared to the host polymer, and this is usually an unwanted side effect. However, it is common to make sound deadening composites by using high-density fillers such as barium sulfate (BaSO4, 4.5 g/cm3) or magnetite (Fe3O4, 5.1 g/cm3) [23].

362

Filled Thermoplastics In order to dampen sound, a material must be able to absorb vibrational energy (sound) and transform the energy into thermal energy (heat). The loss factor, tan δ, is the parameter that best describes the sound damping ability of materials [5]. It is the inelastic component of the material’s response to deformation. The loss factor can be measured by dynamic mechanical thermal analysis (DMTA), which shows its behaviour with changes in frequency and temperature. It is common to select a matrix with a high tan δ in the frequency range to be damped. Then, high-density filler can be added in order to further improve performance. Mica is also used, as the platy particles cause multiple reflection of the sound waves so they may be absorbed in the composite instead of passing straight through [5]. Adding too much filler should be avoided however, because this leads to particle-particle interactions, and eventually percolation, where a continuous path exists from particle to particle. Under those conditions, the acoustic energy can pass through the interparticle contacts, largely avoiding interaction with the matrix.

8.2.3 Melt Viscosity (MFI) The melt viscosity of polymers is usually measured as the melt flow index (MFI), also known as melt viscosity index (MVI) and melt flow rate (MFR). A pressure is applied to force the molten polymer through a hole at a controlled temperature [24]. All three parameters are set out in standards appropriate to the polymer being measured [24]. The measured value is in grams of polymer extruded through the hole in a set period of time, e.g., ten minutes. This of course, is actually the reciprocal of the viscosity, so a high MFI means low viscosity and vice versa. There are some caveats when using MFI as a measure of viscosity. Firstly, MFI measurements are performed in the medium shear rate range [24], whereas polymer processing is often performed at higher shear rates. Therefore, the MFI may not correspond well to the flowability of the polymer melt during compounding and processing. A further point is that raw MFI data should not be used to compare viscosities of polymer melts containing different filler loadings. This is because adding filler increases the density of the melt, and therefore the MFI will increase because more mass of polymer melt flows in a given time, even if the volume of material flowing remains constant. Therefore, the MFI data must be adjusted by dividing by the density. This gives the volume of material flowing in unit time and allows fair comparison of samples with differing filler amounts [8]. A detailed description of filled polymer melt rheology can be found in a book by Shenoy [24]. Of particular interest is Shenoy’s method for extrapolating MFI data to give an idea of the expected rheology of the polymer melt at the higher shear rates encountered during polymer processing. Another good review has been made by Hornsby [25]. This latter work includes an investigation of the degree of filler dispersion at various points as it passes through a twin-screw extruder.

363

Particulate-Filled Polymer Composites An inevitable consequence of adding filler is that the viscosity of the polymer melt increases [8, 24-26]. This is exacerbated at high volume fractions of filler, and when the filler particles are smaller, especially for nano-sized fillers such as carbon black [27]. In fact, the increase in viscosity often limits the amount of filler in the compound. At low filler addition levels, this effect is very low and is masked by the high inherent viscosity of the polymer melt. As the filler addition increases, the viscosity begins to rise more sharply, until eventually it approaches infinity at some critical filler level, which depends upon particle size, shape and amount of agglomeration. High melt viscosity is to be avoided, as it lessens the throughput during extrusion, increasing production costs. It may also prevent complete filling of the mould, leading to high reject levels. The viscosity of a very dilute dispersion of rigid spherical particles in a Newtonian fluid is described by the Einstein equation (Equation 8.2) [26]. Where η is the viscosity of the dispersion, ηl is the viscosity of the fluid alone, φ is the volume fraction of particles and kE is the Einstein coefficient, which is 2.5 for spherical particles. kE depends upon both particle shape and orientation. η = ηl (1 + kE φ)

Equation 8.2 Einstein equation of dispersion viscosity Although it is a starting point for understanding the effect of filler on viscosity, the Einstein equation is not applicable to filled polymer melts. Polymer melts are nonNewtonian, and the filler concentrations are often too high to ignore particle-particle interactions. A plethora of equations exist for modelling dispersions of particles. However, the best approach is to make the desired formulation and test it under real conditions, such as measuring the torque and volume throughput during extrusion, plus mould filling when injection moulding.

8.2.4 Compounding and Extrusion 8.2.4.1 Introduction Extruders are used to mix ingredients into thermoplastics [28]. The polymer is fed into a hopper and is then forced into the barrel, either by gravity, or by mechanical means, such as a feeder screw. The barrel is heated to melt the polymer and a rotating single or twinscrew arrangement transports the polymer melt down the barrel and out of the die (hole) at the end. There are usually ports at various points along the barrel to allow for the introduction of additives such as lubricants, antioxidants, pigments and fillers. These additives may be added individually, but more commonly they are fed in together as a

364

Filled Thermoplastics concentrate known as a masterbatch. After it emerges from the die, the molten polymer string is usually cooled rapidly by running it through a water bath. The polymer is then dried and lastly, the cooled polymer strand is chopped into small granules using a pelletising machine. If the material is to be stored before use, then it is common to seal the pellets to protect them from contamination and water pick-up. The extrusion step is not particularly costly in comparison with the price of the raw materials, but the cost is still significant and impacts on the overall economics of the final material. It is therefore worthwhile to devote effort to optimisation of the extrusion process in terms of increased throughput (productivity) and decreasing energy consumption and machine wear. In the author’s opinion, the subject of throughput does not receive the attention it deserves. There are countless reports of the mechanical properties of thermoplastic composites but no mention of the extrusion characteristics of the materials. For a meaningful comparison of different composites, one must consider not only their mechanical and aesthetic properties, but also the relative economics of extrusion.

8.2.4.2 Volume Throughput As with all of the other properties of a composite, when considering extruder output one must think in volume terms, not in terms of weight. If one is making a certain amount of composite, that material must be of sufficient volume to create a certain number of parts, each of which has a fixed volume determined by the size of the mould to be filled. Therefore, the mass of material produced is, in itself, of no interest. There have been various reports that filler increases extruder throughput. These claims are often erroneous, or at least greatly exaggerated, because the throughput is given in units of mass per unit time. That is not a valid way to compare throughput, for the same reasons mentioned previously for MFI. Namely, that the addition of mineral fillers increases the density of the filled polymer compared to its unfilled counterpart. Naturally, this elevates the mass throughput of the extruder, giving a false impression of improved productivity. One possible cause for the confusion over throughput, may be the way in which extruders are rated by the manufacturer. Usually, the capacity of the extruder is given in terms of kilograms of polymer extruded per hour. This value conjures up an image that the maximum throughput of the extruder is limited to a given mass of polymer, as shown on the side of the machine. In reality, that value is valid only in the case of a standardised grade of unfilled polymer, and is merely a convenient means for the extruder manufacturer to show the relative capacities of different machines. Perhaps in the future, the extruder manufacturers will consider expressing the maximum throughput in volume terms to avoid confusion. There are two classes of extruder, single-screw and twin-screw. The primary drawback of single-screw extruders is that they give poor dispersion of fillers compared to the

365

Particulate-Filled Polymer Composites more expensive twin-screw variants. Thus, twin-screw extruders are used almost exclusively when good filler or pigment dispersion is required. Recently, it has been suggested that the lack of dispersion associated with single-screw extruders is not an intrinsic limitation [29]. It is argued that single-screw extruders had not been optimised, as all of the attention and R&D has been on the more lucrative twin-screw machines. Furthermore, it is claimed that the newly designed single-screw extruders can now give sufficient dispersion for filled systems along with advantages such as being simpler, less expensive, easier to maintain and giving higher volume throughput compared to twinscrew machines. It will be interesting to see whether single-screw extruders do indeed gain acceptance for the manufacture of thermoplastic composites. The volume throughput and the quality of filler dispersion are the main parameters to consider. One might assume that it would be a simple matter to measure the volume MFI, as described previously, and then correlate that to extruder throughput. However, it has been shown that the transport of the polymer melt in an extruder is more complicated than the simple flow used in measuring MFI. In fact, the mechanism is different for melt transportation through a single-screw compared to a twin-screw machine [28]. The best approach is to extrude the proposed formulations in the production extruder that will be used. The volume throughput can be measured as well as the torque on the screw and the energy input to the motor. However, production extruders can have outputs of well over one thousand kilos (litres) per hour, which means a lot of filler and polymer is needed to run a test. Even more problematic is the loss of productivity when a production extruder is used for testing purposes. Usually, the test material made will not conform to existing specifications and may have to be scrapped/discarded. More commonly an instrumented laboratory extruder is used for initial testing. The feeding and screws of the laboratory machine should be set up to mimic the configuration of the production extruder. When this is done, one can obtain good correlations between the properties of compounds made in the laboratory and production material. A more detailed treatment of compounding is given in Chapter 5.

8.2.4.3 Dispersion Good dispersion is nearly always beneficial for the properties of a composite so one tries to optimise the dispersion of filler. The level of dispersion can be measured directly or indirectly. The most common direct measurement is to perform scanning electron microscopy (SEM) on a cross-section. It is advisable to use two different magnifications to examine the filler distribution on a macroscopic and dispersion on a microscopic scale. An indirect measurement is to measure the unnotched impact strength of the composites, as that property is sensitive to agglomerates. In a rather insightful study, Hornsby showed the degree of dispersion of filler as it passed through a twin-screw

366

Filled Thermoplastics extruder [25]. This was achieved by using a clamshell extruder that could be stopped and opened so that samples of compound could be removed for analysis. It was found that the greater part of the dispersion was imparted in the melt zone where the pellets of polymer are just melting. The high viscosity in that region requires a high energy input and encourages deagglomeration. Feeding the filler into the unmelted polymer may give good dispersion, but it also results in higher wear of the extruder so that this approach is only advisable for soft, surface treated fillers. It is more usual to add the filler when the polymer is already molten although a multitude of feeding possibilities exist [25].

8.2.4.4 Machine Wear Machine wear is a concern, partly because it costs money to replace a worn barrel or screw. The main problem though, is the loss of productivity when the machines must be shut down for maintenance. Potentially, wear may lead to significant metal contamination levels with accompanying polymer stability problems. Although machine wear is an issue, it is not a subject that has received much attention. There are a few studies that have investigated the effect of filler properties on wear [17, 30]. One simple method is to extrude through a plate of soft metal and measure weight loss from the perforated plate at regular intervals. It was concluded that hard, large irregular particles cause most wear. Surface treatment with a lubricating additive such as stearic acid helps alleviate wear because the additive forms a protective layer around the particles.

8.2.5 Thermal Conductivity and Specific Heat Capacity It is not only the final properties of the composite that matter, processability and economy of manufacture are also important. If the part can be cooled more quickly, then money can be saved through improved productivity. Mineral fillers typically have thermal conductivities in the range 0.02–3 WK-1m-1 [5, 22, 31] that are an order of magnitude higher than those of polymers [2, 32-34]. The volume specific heat capacity of mineral fillers (~1900-2000 J litre-1K-1) [5, 22, 31] are very similar to those of polymer melts (~1500-3000 J litre-1K-1) [2, 33-35]. An equation has been proposed for calculating the heat capacity of composites based on the composition and the heat capacities of the components [36]. The result is that the filler speeds heating and cooling of the filled polymer melt through improved conduction. Therefore, filling a polymer often allows for reduced cycle times in injection moulding [30] and thermoforming [34] because the part cools and hardens more quickly, allowing it to be removed from the mould earlier. In the literature, there have been some contradictory statements about •

•

367

Particulate-Filled Polymer Composites the relative specific heat capacities of mineral fillers and polymers. This confusion may have arisen because the specific heat capacity is usually given in terms of mass with units of J·kg-1K-1, whereas to compare fairly, these values must be converted into volume terms, i.e., J·L-1K-1. There is a niche market for thermoplastics with very high thermal conductivity. These are marketed for CPU cooling in laptop computers and other high performance applications. An interesting point to note is that thermal and electrical conductivity actually benefit from poor filler dispersion. Agglomeration and network formation (percolation) allows better heat conduction due to the network of particle – particle contacts.

8.2.6 Thermal Expansion The thermal expansion coefficients (CTE) for polymers (~10 x 10-5 mm mm-1 °C-1) [2, 36, 34] are approximately an order of magnitude higher than those for mineral fillers (~10 x 10-6 mm mm-1 °C-1) [31] or for metals (~20 x 10-5 mm mm-1 °C-1) [31]. This may lead to problems in applications where plastics and metals are in contact, as differential expansion and contraction can cause such parts to warp. It is possible to estimate the CTE of a composite based on the composition and knowledge of the CTE for each component [37]. The polymer chains can become oriented during flow of the polymer melt and this can give rise to a difference in the amount of shrinkage parallel and perpendicular to the flow direction. Addition of particulate fillers such as calcium carbonate, silica and talc tend to lessen the amount of polymer chain orientation, and thereby reduce not only shrinkage, but also shrinkage differentials, with a corresponding decrease in warpage [22, 34]. In contrast, fibrous fillers and other highly anisotropic fillers, tend to partially align in the flow direction, leading to an increased shrinkage differential and a tendency for the part to warp during cooling. Warpage is not easy to predict, and so it is common to use low aspect ratio fillers for parts where warpage must be avoided. Another approach is to use a mixture of low aspect ratio filler and fibres [38], which can ameliorate the high warpage observed when fibres alone are used, whilst maintaining sufficiently good mechanical properties. •

•

•

It is often not possible to use the same mould for filled and unfilled polymer because the change in shrinkage gives parts that are out of specification. On the other hand, adding filler to a polymer allows its shrinkage to be systematically tuned. This tuning method is useful for example, if one is attempting to use an existing mould with a different polymer. Filler may be added to the newly chosen polymer to achieve similar shrinkage to that of the previously used polymer. Injection moulding tools (moulds) are very expensive and it is preferable to keep using the same mould rather than purchasing a new one specifically made to accommodate the new polymer.

368

Filled Thermoplastics

8.2.7 Electrical Properties The high tonnage, commercially important polymers are, in general, excellent insulators with resistivities in the range 1012 – 1018 Ω cm [22]. The top three polymers in volume terms (in descending order) PE, PP and PVC are all used extensively as cable insulation. There are some intrinsically conductive polymers [39-41] such as polyaniline [42, 43] polythiophene and polypyrrole [43], but these are relatively expensive, intractable, niche materials, that must be modified to impart processability [44-46]. •

Most fillers, while having good dielectric properties and resistivities, are in general worse than a plastic. Also, incorporation of a filler will introduce flaws and interfaces, charged ions and traces of water (all fine particles adsorb some water from their environment) and hence reduce electrical properties. Usually, however, because of the good electrical properties of the plastic, fillers may be used simply as an extender, the composite still giving good properties. Calcined clay (produced at just above 1000 °C) and other calcined silicates, do not degrade electrical performance as severely as other fillers, because of low water pickup, and immobilisation of matrix ions. Conversely, metakaolin, because of its highly reactive surface absorbs ions and thus improves the electrical performance of polymers such as plasticised PVC and ethylene vinyl acetate (EVA) copolymers [47]. Due to their good insulation characteristics thermoplastics can suffer from tracking, i.e., the build-up of surface charge which then discharges across the surface, because that path has less resistance than passage through the plastic. Filler particles can reduce this problem by acting as a physical barrier and by distributing the charge before critical build-up occurs [48, 49]. In many cases, it is beneficial to introduce some level of electrical conductivity into a polymeric material [5, 22]. The highly insulating polymers are susceptible to static build up. This may be a nuisance, attracting dust to the surface, or it may lead to damage of sensitive electronic parts, when manufacturing integrated circuits, for example. Even very low levels of surface conductivity will resolve this problem [6]. Organic based antistatic additives are available or alternatively conductive fillers such as carbon black, graphite, or metals may be added [6]. Sometimes, a material of much higher conductivity is required. One example is for electromagnetic interference (EMI) shielding [22]. This is achieved by adding a sufficient level of conductive filler. Initially, as one adds more conductive filler, the conductivity does not rise appreciably, because the conductive particles remain isolated from one another. As the filler loading is gradually increased, the conductivity suddenly rises sharply, by many orders of magnitude, approaching the conductivity of the conductive filler itself, and then levels off [50]. This discontinuity is known as the percolation threshold and it signifies the volume percentage of filler required to attain a continuous network of interconnected particles throughout the matrix [22]. Usually, 10-30 volume percent of filler is needed to

369

Particulate-Filled Polymer Composites achieve percolation, depending upon particle size, shape and the level of dispersion. Smaller, more anisotropic particles show lower percolation thresholds [39]. As with thermal conductivity, good dispersion is detrimental to electrical conductivity [22, 51]. On the contrary, the aim is to achieve percolation by inducing an agglomerated network of particles.

8.2.8 Barrier Properties Thermoplastics are widely used as packaging materials due to their low cost, excellent chemical resistance, good barrier properties and the potential for recycling. In fact, over 35% of all thermoplastic is used in packaging. They are also used in a variety of other applications where barrier properties are required. These include water and gas pipes, as well as car petrol tanks. Permeability of a material to small molecule penetrants, such as oxygen and water, increases with the solubility of the small molecule in the matrix [52] and with the diffusion coefficient in that material [53]. Polymers are very sensitive to plasticisation by small molecules. Thus, the presence of small molecules may greatly increase the diffusion coefficient. Based on these observations, one can envision ways to decrease permeability by reducing the solubility and/or the diffusion coefficient. Molecules can neither dissolve in, nor diffuse through, mineral fillers to any appreciable extent. Therefore the presence of filler reduces the solubility of the diffusant in the composite material, and thereby the permeability, in proportion to the volume fraction of filler. In addition, the presence of impermeable filler in a polymer forces the diffusant molecule to travel further around the filler particles. This physical blocking effect is known as tortuosity, because the filler forces the diffusant to take a more indirect, or tortuous, path through the material. The degree of tortuosity imposed is dependent upon the anisotropy and orientation of the filler particles with respect to the direction of diffusion. For example, platy particles oriented perpendicularly to the diffusion vector will be particularly effective in retarding diffusion. The permeability of a composite can be calculated using an equation that allows for the reduction in permeant solubility and for the tortuosity (Equation 8.3). Where Pc and Pp are the permeability of the composite and the unfilled polymer, respectively. The terms w and t refer to the width and thickness of the filler and φp and φf represent the volume fraction of polymer and filler. φp Pc = Pp 1 + (w / 2t )φ f

Equation 8.3 Composite permeability

370

Filled Thermoplastics As mentioned previously, the addition of filler may also change the amount of crystallinity in the polymer. As polymer crystals are impermeable even to low molecular weight species, an increase in crystallinity also results in improved barrier properties, through increased tortuosity [54]. This effect is expected to be especially prevalent for fillers that induce a high degree of transcrystallinity. Dispersion and wetting of the filler can also affect the permeability of the composite. It has been shown that PE filled with 25 volume percent calcium carbonate was actually four times more permeable to oxygen compared to the unfilled reference PE. This was attributed to poor wetting of the filler, so that the diffusant was able to travel unimpeded along the polymer/filler interface. In contrast, stearic acid coated calcium carbonate at the same loading resulted in three times lower oxygen permeability than the unfilled PE [55]. Similarly, Tiburcio and Manson showed that the water vapour permeability of glass-bead filled phenoxy films decreased sharply as the degree of adhesion between the filler and the matrix was increased [56]. In some cases, it is desirable to increase the permeability of a polymeric material. One example is breathable films. For example, calcium carbonate filled PP films are first made by solvent casting, or extrusion casting or as blown film and subsequently stretched to delaminate the filler – polymer interface [57]. High filler loadings are used to ensure interconnecting voids, giving unimpeded diffusion [58].

8.3 Mechanical Properties 8.3.1 Introduction For any given application, certain mechanical properties will be of more importance than others. It is therefore, essential to identify and rank the most relevant properties and formulate or purchase the least expensive composite material that satisfies the requirements. The key mechanical properties for most applications are modulus (tensile or flexural), yield strength, impact strength and possibly HDT. A distinction is often made between reinforcing and nonreinforcing fillers, but unfortunately, the term reinforcement is rarely defined explicitly. Fibres are usually considered to reinforce and isotropic fillers are not, with platy fillers somewhere in between. As shown later, it is not appropriate to define reinforcement in terms of particle shape, because that definition breaks down with variations in anisotropy and particle size. In agreement with Ram [59], the definition of reinforcement as the simultaneous improvement of both modulus and yield strength will be used in this chapter. Polymer mechanics is a broad subject and the interested reader is directed to specialised texts [60]. Each of the main properties is described here, along with a consideration of

371

Particulate-Filled Polymer Composites how it changes with filler type and level of addition. The effect of surface treatments such as dispersants and coupling agents are also mentioned, where applicable. A more detailed description of surface treatments is given in Chapter 4.

8.3.2 Modulus – Tensile and Flexural One of the main reasons for adding mineral fillers to thermoplastics is to increase the modulus (stiffness). Tensile (under tension) modulus is the ratio of stress to strain, at some low amount of strain, below the elastic limit. Flexural (bending) modulus is also often measured. The most relevant modulus to measure depends upon the expected deformation mode of the part that will be made from the material. Often, the flexural modulus and tensile modulus are rather similar. The exception is the case of anisotropic fillers, which can become aligned in the flow direction when the test specimens are moulded. There is a good understanding of how addition of filler affects the modulus of a polymer. Chow has done an extensive review of the area, and the interested reader can consult that work for a more detailed description [61]. In fact, the simple rule of mixtures is fairly accurate at low strain levels (Equation 8.4). Ec, Ep and Ef are the moduli of the composite, polymer and filler, respectively, and φf is the volume fraction of filler. The moduli of thermoplastics are in the range 1-3 GPa [1, 32, 35, 34] whereas common fillers have much higher moduli [5, 22, 31] (calcium carbonate and dolomite ~ 35 GPa, mica ~ 17.2 GPa and wood flour ~10 GPa). Ec = (1 − φ f )E p + φE f

Equation 8.4 The dependence of composite modulus on volume fraction of filler The modulus increases with increasing volume percentage of filler [62]. In most cases, this relationship is linear for filler concentrations up to approximately 20 volume percent. Filler orientation strongly affects modulus [63]. This is important because injection moulded ASTM standard ‘dog-bone’ shaped test specimens give high filler orientation. The moduli measured on such specimens may be far higher than those attained in a real injection moulded part where the orientation is usually not optimal. Increasing crystallinity in the polymer phase can also lead to a higher modulus because the crystal phase is stiffer than amorphous regions [64, 65]. Various attempts have been made to account for other factors such as polymer-filler interaction and interparticle interactions. One such semi-empirical model is known as the Nielsen equation, also known as the LewisNielsen or modified Kerner equation [66, 67]. In these theories, modulus is independent of particle size. However, Heikens [68] has found that when the polymer is strongly bonded to the plastic composite, modulus is

372

Filled Thermoplastics affected by the filler particle size. Particle size effects are allowed for to some extent in the modified Kerner equation by the introduction of an effective filler volume fraction. The most important filler parameter affecting modulus is its shape. Unfortunately, when the filler is non-spherical theories become much more complicated and the reader is advised to refer to Chow’s review [61]. Shape factors can be incorporated in the models mentioned previously but are only useful when applied to very high aspect ratio materials, e.g., fibres. There is also an almost insurmountable problem with particulate fillers: the difficulty and effort to measure aspect ratios of micrometre sized particles. Pukansky examined the effects of 11 different fillers in polypropylene [69] and concluded that Young’s modulus is affected by the amount of bonded polymer, which is in turn related to surface area, and therefore to both particle size and shape. That observation helps to explain the strong effect that nano-fillers have on the modulus of a composite. Schreiber and Germain showed that modulus depends on the strength of interaction between the polymer and the filler surface [62]. To exemplify the effect of fillers on a thermoplastic, PP homopolymer filled with differing filler types and consequently very different shapes is shown in Figure 8.2. It can be seen that a linear fit can be used successfully for most of the fillers. The exception is mica, which deviates from linearity at high filler levels where interparticle interactions become important. The modulus values also reflect the expected shapes of each of the fillers. Similar trends are reported for other common polymers [22].

Figure 8.2 The effect of common fillers on the tensile modulus of PP homopolymer 373

Particulate-Filled Polymer Composites In conclusion, it can be stated that the effect of fillers on modulus is relatively well understood and may be predicted. The other properties are less easy to predict as they are measured under conditions where the composite is deformed to a greater extent. This means that other factors such as particle debonding, particle re-orientation and polymer orientation, must be considered.

8.3.3 Heat Deflection Temperature (HDT) The HDT is the temperature at which a beam of polymer deflects by a given amount under a specified load. The HDT is a complex function of the composite’s modulus and polymer properties such as glass transition temperature (Tg), melting temperature (Tm), degree of crystallinity and amount of bonded polymer in the filler-polymer interphase. Examining the effect of fillers on the HDT of PP homopolymer (Figure 8.3) shows that the trends are similar to those for modulus. There are two common standard conditions for testing the HDT of PP; one uses a force of 0.46 MPa, whereas the other uses 1.8 MPa. Care must be taken when comparing results for different composites to ensure that the same test conditions were used. The greatest enhancement of HDT is seen with semi-crystalline thermoplastics such as PE, PP, polyamides and PET, with only minor enhancements achieved for filled amorphous

Figure 8.3 The effect of common fillers on the HDT of PP homopolymer

374

Filled Thermoplastics polymers like polystyrene, acrylonitrile-butadiene-styrene (ABS), polycarbonate and polysulfone [1]. Addition of glass fibre to semi-crystalline polymers gives an HDT approaching the melting point of the matrix. In the case of amorphous polymers, incorporation of glass fibre gives an HDT (at 1.8 MPa stress), which is close to the Tg of the matrix. For this reason, filler is most commonly used in semi-crystalline polymers where the HDT is improved most because the crystalline regions help transfer stress to the filler under load [70].

8.3.4 Yield Strength Yield strength is a measure of the force that a material can withstand before it suffers macroscopic plastic deformation. For most materials, e.g., metal, it is taken as the point on the stress-strain curve when the line becomes non-linear (the elastic limit). However, for plastics, it is taken as the peak of the stress-strain curve, as that is simpler to measure. In practice, most parts are designed so that they never experience a force approaching the yield stress because yielding represents failure of the material. Yield strength is a key property when designing parts. Fillers are often added because they increase the yield strength of the polymer, this effect is known as reinforcement if the modulus is also improved [59]. The explanation for reinforcement lies in the fact that adding filler actually changes the polymer phase. It has been shown that polymers interact with the filler surface, forming an interphase of adsorbed polymer [71-74]. The thickness of the interphase can vary widely from system to system. That is to be expected; for example polar polymers such as polyamides are capable of strong, specific interactions with groups on the filler surface. In contrast, non-polar polymers such as PE and PP have weaker interactions with fillers. The apparent thickness of the interphase also depends strongly upon the measurement method. Lower values of around 0.004 µm are reported from extraction experiments, whereby all non-adsorbed polymer is solvent extracted [75, 76]. Values deduced from mechanical data such as by dynamic mechanical analysis or modulus tend to be much larger, in the range 0.012 to 1.4 µm [77]. This interphase has mechanical properties intermediate between those of the polymer and the filler [78, 79], thereby allowing an increased yield strength for the composite. Several factors determine the level of reinforcement attained by adding filler. These include, the volume fraction of filler added, the surface area of the filler (related to particle size), particle shape, the level of adhesion between the filler and polymer [80], as well as the thickness and nature of the interphase between the two phases. A linear correlation between yield strength and heat of crystallisation has also been reported in the case of PP filled with calcium carbonate [81]. It is well known that spherical fillers give least

375

Particulate-Filled Polymer Composites

Figure 8.4 The effect of common fillers on the yield strength of PP homopolymer

reinforcement, platy fillers are better and fibrous fillers are best of all [5, 30]. Usually, it is considered that spherical fillers such as calcium carbonate and dolomite do not reinforce at all, and in fact they usually reduce the yield strength of the material (Figure 8.4). However, that is not necessarily true, as it has been shown that it is possible to increase the yield strength of PP by using very fine spherical filler with a mean diameter of 0.01 µm [78, 79]. This improvement must be due to the high surface area of the filler as the filler is isotropic. The high surface area increases overall polymer-filler adhesion and thereby improves yield strength. It is observed that spherical fillers do not reinforce whereas platy fillers like mica may do, and glass fibres are most effective (Figure 8.4). In this particular example, talc does not reinforce, probably because the talc grade used did not have sufficient anisotropy. As with yield strength, the data shown is for PP homopolymer, but similar trends are seen for a wide range of other thermoplastics [3, 22]. The filler creates an additional complication especially for injection moulded parts. Namely, during mould filling, the filler distribution becomes non-homogeneous due to the flow. One consequence is flow lines and weld lines. These are created when two fronts of molten polymer meet. For an unfilled polymer the melt can easily mix when two melt fronts meet and so the mechanical properties are normally unchanged (except for the special case of liquid crystalline polymers). The uneven distribution of filler at the weld lines creates a weak point, so for example, the measured yield strength and elongation to break are reduced. This effect is not as great for isotropic fillers but for more anisotropic fillers the yield strength may be reduced by more than fifty percent. It is 376

Filled Thermoplastics therefore essential to design with this in mind. This can be done by designing parts to avoid weld lines and by judicious placement of injection points. Additionally, it should be remembered that the reported mechanical properties for composites are for ideal specimens with no weld lines, whereas the actual yield strength in the part may be far lower.

8.3.5 Impact Strength (Toughness) The impact strength of polymers and composites is another key property. In contrast to the other mechanical properties, it is not possible to predict the impact strength of a thermoplastic composite. The reason is that there are too many factors to be considered. One of the major complications is that adding hard filler can change the mode of failure from ductile to brittle [2, 7], or vice versa for rubbery fillers [7]. The filler may act as a flaw, if there are large particles or agglomerates [82, 83]. Alternatively, well dispersed, small particles can improve the impact strength by a crack-pinning mechanism. Another problem in predicting impact strength of composites is that mechanical properties of polymers are very dependent upon the rate at which the testing is performed. Most mechanical data are acquired at low speeds; for example, tensile testing is often performed at a strain rate of 0.1–10 mm·mm-1min-1. In contrast, impact testing is a very rapid event (>10 000 mm·mm-1min-1) and the polymer often responds very differently. For example, a polymer that fails in a ductile way during tensile testing may become brittle under impact test conditions because when the deformation is fast, the polymer chains have insufficient time to move and accommodate the deformation. It is well known that the response of polymers is dependent upon testing rate [60, 84-86]. The WLF (Williams, Landel and Ferry) equation [87] can be used to account for this effect at temperatures near to the Tg. The WLF equation predicts that the effective Tg of a polymer changes by about 6.9 °C for every decade of change in the rate of testing [84]. That means that a polymer which has a Tg below room temperature under tensile testing conditions, may have a Tg well above room temperature under impact test conditions. Therefore, the polymer may behave in a ductile manner, with high strength, under tensile testing but it may be brittle under impact, giving low impact strength. In cases where the mode of fracture does not change with testing speed, then it is expected that the energy to break determined by tensile testing (the area under the stress-strain curve) will correlate with the impact strength. Similar complications exist when it comes to measuring impact strength. Many methods are available, the most common of which involve either a pendulum striking the sample (Izod and Charpy), tensile impact testing, or a falling dart. In general, impact tests do not correlate well with each other, although it has been shown that there is a correlation between Izod and Charpy values [88]. Impact tests may be performed on unnotched (as moulded) samples or pre-notched samples, whereby a well-defined flaw is introduced to ensure that the sample fails at the desired point. Adding a notch improves the reproducibly noticeably, 377

Particulate-Filled Polymer Composites but it may not be realistic as parts in actual use are not notched, at least not intentionally. Unnotched testing is best for detecting agglomerates and flaws because the crack initiates at such imperfections. Notched impact testing is comparatively insensitive to agglomerates and large particles because the sample fails preferentially at the large, introduced flaw. Reverse notched testing such as Izod E is especially suitable for composites. This sample is struck on the opposite side to the notch. In this way the stress is concentrated opposite the notch but the introduced notch is not the site where crack growth occurs. The notch encourages reproducibility by localising the stress on the opposite side of the sample, but the crack initiation occurs at some imperfection in the composite and so the method is still sensitive to agglomerates. In the final analysis though, the lab scale impact tests are of limited value. They may be used to compare materials qualitatively, but the ultimate test is to make the part or prototype and perform impact tests on that, in a manner that closely simulates the way in which the part is to be handled during manufacture, or used. There are many reports describing the effect of fillers on the impact strength of polymers. These are however, hard to compare as they often use different polymers, test methods and filler particle size distributions. It is therefore difficult to make any general comments. One study showed that increased filler anisotropy (aspect ratio) resulted in reduced falling weight impact strength [89]. That is particularly interesting because increased filler anisotropy is known to improve modulus. This is therefore an excellent illustration that one must be prepared to sacrifice some property for the sake of improving another, more important one. There has been some success in creating high modulus composites with good impact strength. This was done using ternary composites where a dispersed, stiff filler is encapsulated in a rubbery layer dispersed in the matrix polymer [90]. It can be stated that fillers can either decrease or increase the impact strength compared to the unfilled polymer. A common technique for increasing impact strength is to disperse a soft, or rubbery, filler in a harder polymer to increase its impact resistance. This method is used to make HIPS and to improve the impact resistance of PP, especially at low temperatures [91]. Hard fillers on the other hand may either increase or decrease impact strength [92]. For example, fine calcium carbonate or talc can increase the impact strength of PP homopolymer by more than a factor of two [93, 94]. This only holds if there is good dispersion to avoid agglomerates, if there are no large particles, above about 10-20 µm in diameter, the calcium carbonate is stearate coated and the particle size distribution has been optimised [94]. There seems to be a definite interaction or synergy between particle size and coating level, and the highest impact strengths are found with calcium carbonates with 1% stearate coating and a mean particle size 1-2 µm, depending on the type of polymer and the nature of the impact test (se Figure 8.5).

378

Filled Thermoplastics

(a)

(b) Figure 8.5 (a) Effect of stearate coating level on a ultrafine calcium carbonate (d50: 0.8 µm) on the properties of polypropylene (b) Effect of particle size of stearate coated calcium carbonate on the properties of polypropylene

379

Particulate-Filled Polymer Composites In the case of PP copolymer and impact modified PP, the effect of filler is very different; impact strength is lowered significantly by hard particulate fillers. This is because the filler interacts with the soft or rubbery component and nullifies its ability to adsorb energy during impact. In conclusion, it seems that soft rubbery fillers improve impact strength by helping to dissipate the energy of impact. Hard fillers decrease the impact strength of ductile materials, which already have high impact strength. On the other hand, well dispersed, hard particles of the correct particle size may improve the impact strength of brittle materials like PP homopolymer or polystyrene [22] by promoting crazing.

8.4 Effects of Filler on the Polymer Phase 8.4.1 Introduction To understand and predict the properties of composites, it is necessary to realise that adding filler may affect the polymer phase, both chemically and physically. Chemical changes may occur if the filler, or impurities on the filler surface, catalyse degradation of the polymer. Alternatively, various physical changes may result from the incorporation of filler. Some fillers nucleate crystal growth in certain polymers, which in turn influences manufacturing and mechanical properties. In the literature, the possibility that the filler may have altered the polymer phase is rarely considered. It is important to realise that the polymer may be altered by the filler, especially in the case of semi-crystalline polymers [95] like PE, PP, PVC, PET and the polyamides. Therefore, these effects are mentioned here to allow a better understanding of the factors affecting the performance of a composite.

8.4.2 Nucleation It is widely recognised that fillers may affect the crystallisation of polymers [96]. The filler may increase the rate of cooling, as mentioned previously, and may thereby affect crystallisation. Some polymers have more that one type of crystal phase, which occur preferentially when cooling within a certain temperature range [97, 98]. These crystal types have different mechanical properties [99, 100] and therefore, the relative amount of each phase will influence to the properties of the thermoplastic composite. In other instances, the filler may nucleate crystal growth. This is often beneficial, as it causes the material to harden more rapidly on cooling, giving the possibility of faster production. In fact, it is common practice to add a small amount of fine talc to nucleate

380

Filled Thermoplastics

Figure 8.6 The effect of some common fillers on the impact strength of PP homopolymer

crystallisation of PP, especially in thin-walled parts. Interestingly, dolomite is also rather effective, and calcium carbonate and mica have a slight effect on crystallisation onset temperature (Figure 8.6). In some cases surface treatment of the filler affected nucleation and in other cases no effect was seen. As well as the crystallisation onset temperature, the peak of the crystallisation endotherm may also be used as a measure of nucleating effectiveness. The two methods show the same qualitative trends. Another important parameter is the proportion of crystallinity in the composite. The crystalline phase has a higher modulus than the amorphous phase, and it has been reported that the yield strength is linearly proportional to the heat of crystallisation [81]. Clearly, the mechanical properties of the composite are influenced by the degree of crystallinity. For a given PP copolymer grade, the degree of crystallinity was 43 weight%, whereas this was 45-48 weight% when 60 weight% of magnesium hydroxide, dolomite or talc was added. When measuring the degree of crystallinity one must correct for the amount of filler added. For example adding 50 volume % filler will reduce the apparent crystallinity [as measured by differential scanning calorimetry (DSC)] by 50%. The explanation is quite simply that 50% of the polymer has been removed. It is quite common that this dilution effect is not accounted for, or it is not stated whether it has been corrected for. This leads to some confusion in the literature. 381

Particulate-Filled Polymer Composites Unfortunately, it is not possible to predict whether a particular filler type will nucleate crystal growth. For some time it was thought that high energy surfaces nucleated, but this has since been proven incorrect [95]. Instead, Hobbs has shown that surface microtopology influences nucleating ability [101]. In that work, Hobbs made a replica of a nucleating surface using a non-nucleating material, to produce a surface that was strongly nucleating. It can be concluded that any thorough study of filled polymers must also consider the possibility of nucleation and changes in the degree of crystallinity.

8.4.3 Transcrystallinity Transcrystallinity may occur when a polymer is cooled in contact with a highly nucleating filler or surface [95, 102]. Usually, polymer crystals are spherulitic, growing out radially from the nucleation site [2, 85, 86]. In contrast, when crystals are nucleated very close together, they impinge on each other almost immediately, and are forced to grow in one direction, away from the nucleating surface [95]. For a polymer where only one crystal form occurs, it has been shown that the microstructure of the transcrystalline phase is the same as that for the spherulites [103]. The transcrystalline layer is typically 10-30 µm thick, which means that for higher filler loadings, the whole matrix may be composed of transcrystalline material [95]. Transcrystallinity has been reported in several systems including glass [104], PET [105, 106] and some types of carbon fibre in PP [107], as well as Kevlar fibres in Nylon 6,6 and even in Nylon-PP blends. Sheets of transcrystalline PP have been prepared by cooling PP from the melt in between PET (Melinex®) sheets and the properties studied [108]. This rather elegant work by Fowkes and Hardwick showed that the transcrystalline sheet had a much higher Young’s modulus (1.09 GPa) compared to the control PP sample that was quenched to give a fine spherulitic sheet (0.67 GPa). The tensile yield strength of the transcrystalline material was also higher at 25.0 MPa, compared to 18.6 MPa for the spherulitic sheet. Some of the properties were much worse compared to normal, spherulitic PP. For example, the transcrystalline sheet showed just 4% elongation to break and an energy to failure of just 28.0 kJm-2, compared to >300% and 48.5 kJm-2, respectively, for the fine spherulitic analogue. Clearly then, the filler can greatly influence the type and level of crystallinity, leading to profound changes in the properties of the resultant composite material.

8.4.4 Interphase Aside from changes in crystallinity, there is another way in which the presence of filler may alter the host polymer. It has been shown that polymer adsorbs onto the filler and that this 382

Filled Thermoplastics adsorbed material has different properties compared to those of the bulk matrix [68, 71, 109, 110]. Pukansky and Fekete have written a review of the importance of the interphase in thermoplastics [9]. The relevance of the interphase to adhesion at planar interfaces, and in composites, has been discussed by Berg [109]. As expected, the thickness of the interphase varies depending upon the extent of interaction between the polymer and filler. It has been shown that the thickness of the interphase is proportional to the reversible work of adhesion [8]. Reported thicknesses are usually in the range 0.004–0.15 µm, depending upon the polymer filler combination and the method used to estimate the thickness [9, 22]. It is anticipated that the interphase thickness should be influenced by the ubiquitous van der Waals forces plus any specific chemical interactions such as Lewis acid-base or hydrogen bonding or covalent bonding [22]. Therefore it should be affected by surface treatment of the filler. The degree to which the interphase affects the properties of the composite should also therefore depend on the total surface area of the filler [22] and is therefore especially important for nano-composites.

8.5 Surface Science Aspects 8.5.1 Introduction The importance of surface interactions in composites is widely recognised, and yet, it is probably the one that leads to most confusion. This is partly because it is difficult to characterise the interphase, but also because the composites field is interdisciplinary, requiring not only surface science and adhesion science but also an understanding of polymer science. This section will briefly mention some of the relevant points, but surface and colloid science is a wide area and specialist texts should be consulted if more detail is required [111-114]. Pugh has reviewed the wetting and dispersion of ceramic powders in liquids, which is highly informative and relevant to the case of mineral particles dispersed in a polymer [115]. The interfacial interactions in particulate filled composites have been reviewed by Pukánsky and Fekete [9]. Most recently, Berg has written an excellent review on the subject of wetting and adhesion [109].

8.5.2 Surface Energy and Surface Tension Surface energy must be introduced in order to be able to understand the forces that drive wetting and adhesion. If one imagines an atom or molecule located in the bulk of a material, the forces acting upon that entity are symmetrical. Each unit is attracted to its neighbours equally. If one then cleaves the material, then the forces acting on the units (atoms or molecules) at the newly formed surface are no longer symmetrical. Those surface units are still attracted towards their neighbours, but they lack attractive 383

Particulate-Filled Polymer Composites interaction in the direction of the new interface. Because these units at the interface are in a new environment, energetically speaking, there must be a change in the total free energy. This change in energy relative to the bulk is termed the surface free energy, with units of mJm-2. For liquids, the net force towards the bulk manifests itself as a tension at the interface. This gives rise to the concept of surface tension in liquids, with units of mNm-1. There are various old and new units of surface energy and surface tension, but fortuitously, it turns out that they are numerically equivalent, i.e., 1 mNm-1 = 1 mJm-2 = 1 dyne cm-1 = 1 erg cm-2. So, in summary, the surface energy is the energy required per unit area to form a new surface of that material in a vacuum. The symbol for surface tension and surface energy is γ, where γ S, γ L and γ SL refer to the surface energies of the solid, the liquid and the interfacial surface energy between the solid and the liquid, respectively. •

•

The surface tension of liquids is easily measured by a wide variety of methods, whereas it is much more difficult to measure the surface energy of solids. A comprehensive overview covering methods applicable for liquids and solids is presented by Adamson [111].

8.5.3 Wetting and Spreading When the filler is mixed with the polymer melt, it is important that the polymer wets the filler. It is desirable for the polymer to penetrate in between the particles, displacing the air and giving intimate contact between the polymer and filler surface. This will prevent trapped air bubbles and aid adhesion between the filler and polymer. Wetting, and more specifically, immersional wetting, is the term used to describe this process [112, 115]. The driving force for wetting to occur is related to the surface energies of the wetting fluid and the solid [8, 109, 111-115]. The surface energy of solids is usually determined using static contact angle measurements. This is a useful method, although the results are affected by surface roughness, drop size and contaminants [116]. Furthermore, contact angle measurements are only valid when the liquid does not penetrate into the solid under analysis [116], which is of particular importance for polymers, as they are often swollen by liquids. Theoretically, under equilibrium conditions, a low surface energy liquid, such as a polymer melt, should completely wet the high energy mineral filler surface [112, 115]. However, this view is overly simplistic, and does not apply in reality for several reasons. One reason is that high energy surfaces attract contaminants and are usually covered in a layer of hydrocarbon material spontaneously adsorbed from the atmosphere [112]. This means that the ‘high energy’ surface no longer behaves as such. Another, more fundamental problem is that polymer processing is a dynamic process and so kinetic aspects of wetting are more important than thermodynamics. Dynamic wetting has been studied as it is of great industrial importance. It can be shown that the low surface energy and the high

384

Filled Thermoplastics viscosity of the polymer melt both disfavour wetting of the filler [111, 112]. Dynamic wetting in composites is an area that deserves further study in order to understand and optimise wetting under real processing conditions. Some studies have been performed, in particular for fibre reinforced composites [117]. The spreading coefficient represents the thermodynamic driving force for the liquid to spread onto and wet the solid (Equation 8.5). For spontaneous spreading to occur, the spreading coefficient must be positive, implying a contact angle of zero. S L / S = γ S − (γ L + γ SL )

Equation 8.5 The spreading coefficient

8.5.4 Adhesion The degree of adhesion between the filler and polymer is expected to influence the mechanical properties of the composite. This can be predicted theoretically, although it is much more difficult to prove experimentally. It is a major challenge to change the adhesion between the polymer and the filler without invalidating the experiment by unintentionally altering other parameters [109] such as filler dispersion level or polymer crystallinity. Even so, many workers have tried to correlate the calculated reversible work of adhesion with the mechanical properties of composites. Studies on planar surfaces have shown that measured adhesive bond strengths are, at best, only one tenth of the calculated value based on van der Waals interactions alone. It might therefore be assumed that the reversible work of adhesion would have rather limited utility as an indicator of adhesion at the interface. Despite that, it has been shown numerous times that the reversible work of adhesion often does correlate rather well with the mechanical properties of composites, in particular yield strength [8, 109]. Berg addressed the question of whether it was worthwhile to use concepts such as reversible work of adhesion to predict and tune adhesion [109]. The conclusion was that the approach is reasonably effective already and will improve in the future. The first criterion for good adhesion is intimate contact, that is good wetting of the surface. Wetting is a necessary, but not sufficient, condition for good adhesion [109]. In addition, one should seek to maximise the work of adhesion. The simplest case is to consider the work of adhesion to be attributable solely to non polar (London) forces between the materials. This is indeed the case when at least the adhesive (polymer melt) or adherand (filler surface) is non-polar. So, for thermoplastics such as PE, PP and polytetrafluoroethylene (PTFE) this simple case should apply.

385

Particulate-Filled Polymer Composites The work of adhesion is given by Equation 8.6. This implies that the work of adhesion can be maximised for a given adherand by increasing the surface energy of the adhesive. However, we also know that wetting is required and so the surface tension of the adhesive must not exceed that of the adherand. So, the ideal case is when the surface tension of the polymer melt is as high as possible, but without exceeding that of the filler surface, so that spontaneous spreading still occurs. This has been verified experimentally for a range of polymeric adhesives on a set of surfaces with controlled surface energy [109]. WA = 2 γ Sγ L

Equation 8.6 Work of adhesion (WA) for the non-polar case Later, it was realised that polar interactions could also play a role in wetting and adhesion. The surface energy was divided into non-polar and polar components and new equations were then proposed to allow for the possibility of polar interactions across the interface (Equation 8.7). From that equation it can be expected that maximal adhesion will occur when the ratio of polar to non-polar surface energy is the same for the adhesive and the adherand. WA = WAd + WAp = 2 γ dS γ Ld + 2 γ Spγ Lp

Equation 8.7 Work of adhesion for the polar case Wu proposed another equation using an harmonic mean approach to estimate the nonpolar and polar interactions across the interface [8, 109, 116]. This approach gives the same criterion for maximising adhesion. Folkes went a step further by arguing the importance of Lewis acid and base interactions. This leads to a new elaboration of the equation for calculating the work of adhesion (Equation 8.8). WA = WAd + WAp + WAAB

Equation 8.8 Work of adhesion in Lewis acid-base terms The premise is that each component can only interact with its corresponding component in the other material. Intuitively, it seems reasonable that this should be the case, although again, it is not clear whether the geometric or harmonic mean, or some other method, is best for estimating the magnitude of the Lewis acid-base interactions across the interface. Fowkes’ proposal has been widely accepted and verified experimentally. For example

386

Filled Thermoplastics Sinicki and Berg varied the Lewis acid-base interactions systematically and found a correlation between adhesion measured by peel testing and the calculated thermodynamic work of adhesion [118]. Schreiber and Germain used plasma treatment to alter the Lewis acid-base balance of three fillers [62]. These were then compounded into two different matrix polymers. Firstly, polyethylene (LLDPE), which is completely apolar and cannot form Lewis acid-base interactions. The other polymer was poly(ethylene-vinyl acetate) [EVA], which is based on polyethylene, but contains some (28 mole%) acetate groups, which they found to be Lewis acidic (although acetate groups are also reported to be Lewis basic [109]). They then measured tensile modulus and elongation at break to assess the affect of adhesion on mechanical performance of the composites. In LLDPE, a fluoropolymer (PTFE-like) coating on the filler gave lowest modulus and highest elongation to break, attributed to poor adhesion. An ammonia plasma was used to give a polar, Lewis basic surface. For this combination, the modulus was lower and the elongation to break higher than for the untreated filler. This was consistent with reduced adhesion compared to the untreated filler, but better than for the fluoropolymer coated filler. The modulus and adhesion were maximised when the filler was plasma treated with methane to make the surface non-polar like the polymer. When the matrix polymer was changed to EVA, the results were quite different. In that case, the highest modulus and least elongation to break were recorded for the ammonia plasma treated filler. This was interpreted as being due to enhanced adhesion from the bonding between the Lewis acidic groups in the polymer and the Lewis basic sites introduced on the filler surface through ammonia treatment.

8.5.5 Dispersion and Agglomeration Good dispersion of fillers and pigments is a prerequisite for good mechanical and aesthetic properties. For example, multiple studies have shown that filler agglomerates act as stress concentrators, reducing tensile and impact strength [82]. Similarly, good dispersion results in a smoother surface, with a concomitant increase in specular gloss [119]. Shenoy [24] and Hornsby [25] have each given detailed descriptions of the factors influencing dispersion of particulates in polymers. In the case of pigments, good dispersion results in a higher tinting strength so that less pigment is needed to attain a given level of pigmentation. The principal exceptions are thermal and electrical conductivity, which are improved by particle agglomeration to form a percolated (continuous) network [22]. The ubiquitous van der Waals forces ensure that particles attract each other, thus favouring agglomeration. The magnitude of this attractive force can be calculated if the Hamaker

387

Particulate-Filled Polymer Composites constants for the disperse phase (filler particles in this instance) and the dispersion medium, i.e., the polymer melt are known. The Hamaker constants for several inorganic materials, including fillers such as calcium carbonate, muscovite mica, silica and titania have been calculated by Bergström and co-workers using Lifshitz theory [120, 121]. In a subsequent study, it was possible to directly measure the van der Waals forces between materials using atomic force microscopy (AFM). The results were found to be in good agreement with the magnitude of the attractive force predicted from the calculated Hamaker constants [122]. Once the particles have agglomerated, the surfaces come into intimate contact and other, short-range forces can become important. Thus, the force required to separate the particles will depend on the van der Waals force plus any specific surface interactions that have occurred such as water bridging, hydrogen bonding or Lewis acid-base interactions. The polymer melt competes to interact with the surface so that the overall energetics depend on whether it is more favourable for the filler surface to interact with another particle, or with the polymer phase. In order to separate the particles, mechanical energy must be added [25]. Then, once separated, the particles must be kept from reagglomerating. This can be achieved by reducing the attractive forces between the particles (by adding a dispersant), or by increasing the affinity of the polymer phase for the filler surface (by adding some polymer modified to interact with the filler). These two approaches are well understood and are described in texts on surface and colloid science [111-115]. A brief description of dispersants and coupling agents is given below, but is dealt with more thoroughly in Chapter 4. A theoretical model has been developed to help in optimising dispersion in twin-screw extruders [123].

8.5.6 Surface Treatments – Dispersants and Coupling Agents Surface treatments for fillers have been extensively reviewed [124]. Ernstsson and Larsson studied a range of mineral fillers in terms of the Lewis acid-base character [125, 126]. It was found that each one had a different surface chemistry and they were all amphoteric (possessing both Lewis acid and basic sites). Another point was that trace impurities of iron on the filler surface, presumably picked up during processing, significantly changed the surface chemistry of silica. In unpublished work, DeArmitt and Breese used a novel rheological method to characterise a wide variety of mineral fillers. The method was especially attractive in that it allowed investigation of the surface chemistry of the fillers, while at the same time showing which types of organic probe molecules were most effective as dispersants. This work confirmed the findings of Ernstsson and Larsson [125, 126]. Each mineral displayed a unique affinity for the probe molecules and each was appreciably amphoteric, adsorbing Lewis acid and 388

Filled Thermoplastics Lewis basic probes. This shows that the dispersant should be chosen to optimise its affinity for the filler or pigment. It is important to ensure that the chosen surface treatment agent does in fact bind to the filler. If the additive does not bond to the surface then it cannot fulfil its function. Excess additive is, at best, a waste of money, but in some cases it may have worse consequences such as destabilising the polymer. For example, it has been shown that calcium stearate, a common dispersant, can destabilise polyolefins and cause yellowing, by interacting with the antioxidant [127].

8.5.6.1 Dispersants As mentioned, dispersant design and mode of action are well understood by surface and colloid scientists [43, 112-115]. In that field, the term dispersant refers to any additive that reduces the interparticle interactions, thereby encouraging dispersion of the particles. This is achievable via a number of mechanisms using low molecular weight, oligomeric, or polymeric additives [128]. Steric stabilisation is most relevant to mineral fillers in polymers because it is the main way to achieve colloidal stability in low polarity solvents. This stabilisation mechanism operates by strong adsorption of a layer of organic additive that physically prevents close interparticle approach. Stearic acid and metal stearates are widely used as dispersants, especially in cases where high filler loadings are required. Examples are polyolefins filled with aluminium hydroxide or magnesium hydroxide where 60 weight percent of filler or more may be needed to achieve sufficient flame retardancy [129, 130]. Of course the correct level of addition depends upon the amount of filler surface to be covered, and therefore upon the amount of filler, and its specific surface area. Excess additive is to be avoided as it can seriously destabilise some polymers and give yellowing problems [127].

8.5.6.2 Coupling Agents Dispersants need only adhere to the filler, and help reduce particle – particle interactions. Whereas for coupling agents, as implied by the name, the additive must be bi-functional, adhering both to the filler and to the polymer. It is therefore found that the best choice of coupling agent varies depending on the filler and polymer. In some instances, the socalled ‘coupling agent’ may not couple to either phase, or may only adhere to the filler. In the latter case it may act solely as a dispersant, rather than as a coupling agent. It is therefore important to note that an additive marketed as a coupling agent will only provide coupling for certain combinations of filler and polymer.

389

Particulate-Filled Polymer Composites

Filled Thermoplastics

It is often assumed that coupling (adhesion) is good, but that is not always the case. For example, good coupling is undesirable during extrusion, because it would dramatically increase the torque (and energy) needed for extrusion. This would reduce extruder throughput and that would result in a significantly more expensive material. Coupling is advantageous when high yield strength is required, but it is often detrimental to the elongation at break. Impact strength is more difficult to predict, it may increase or decrease depending on the filler – polymer combination. The effect of coupling agents on modulus is less clear. The equation for describing modulus has no term for adhesion between filler and polymer, so coupling agents should not affect modulus (Equation 8.4). However, there are reports that they do affect modulus. This may be due to increased orientation of the filler or due to the way that the modulus was determined. The modulus should be determined at low stress, in the linear part of the stress-strain curve, where filler debonding has not yet occurred, and should therefore be insensitive to adhesion.

8.6 Aesthetics 8.6.1 Introduction Often thermoplastic composites are used in applications where the consumer will see the part [131]. In those instances, it is vital to consider the aesthetic aspects of the material as well as the mechanical, electrical and other properties. Fillers affect the surface finish, colour and scratch-resistance of the composite and these factors should be optimised to give a marketable product.

8.6.2 Colour/Pigmentation Pigments may be either organic or inorganic particles that are added to give colour to a plastic, as opposed to dyes, which dissolve in the plastic. Pigments can be considered as a special class of filler, and they influence the polymer in much the same way as any other filler would. So, for example, they can lower the thermal stability of the polymer matrix [18], adsorb antioxidant and nucleate crystallisation of the polymer. They also affect the mechanical properties in the same way as any other filler, but usually pigments are used at concentrations that are too low to significantly affect modulus, yield strength or HDT. It is essential to disperse pigments thoroughly in order to achieve the maximum tinting strength, and to avoid agglomerates, which would lower the impact strength, as discussed previously. As dry powders, several common fillers appear white, because they scatter light strongly, and it might therefore be supposed that they would make good pigments for polymers. This is usually not the case however, because the amount of scattering is determined by the difference in refractive index between the particulate and continuous phases. This

390

Filled Thermoplastics refractive index difference is small for common fillers and thermoplastics and so scattering is limited. In fact, in some cases the filler is invisible in the polymer because the refractive indices match very closely. An example is glass beads in PVC. Calcium carbonate is available in high whiteness and gives a mild pigmentary effect, but if high whiteness is required, then a pigment of higher refractive index, such as titanium dioxide, must be used. Titanium dioxide and carbon black are used to protect against UV radiation as they scatter and adsorb it, respectively. It should be noted that commercial titanium dioxide is always has an inorganic coating, e.g., aluminosilicate, plus an organic additive such as a polyol or silicone [132]. This is necessary because naked titania can oxidatively degrade polymers when exposed to UV light.

8.6.3 Surface Finish and Gloss Surface finish is important to the end-user. It can be altered to be glossy or matte, as fashion dictates, and similarly, textures can be used to convey the right feel when handling the product. The surface may also be tuned for more functional reasons. For example, smoother surfaces are generally more hygienic and easier to clean, whereas a rough surface can prevent blocking (self adhesion) of films or increase the coefficient of friction. It is well known that fillers can affect the surface finish of the host polymer [133]. Coarse particles tend to give a rough surface finish, whereas fine particles decrease roughness with a concomitant increase in gloss [119]. Surface treated filler can give better gloss by aiding dispersion. For a given filler type the gloss is steadily reduced as more filler is added. However factors such as the mould surface and processing parameters can have a very large effect on the final gloss. For low shear situations like gas-assisted injection moulding even 10 weight% of filler may decrease the gloss to unacceptable levels. For a normal injection moulded part the gloss may still be high at filler levels as high as 40 weight%. At low filler loadings it is possible to retain much of the gloss of the unfilled polymer, but above a certain filler level (depending on filler type, size and treatment), there is a sharp reduction in gloss. Special surface treatment of the filler has been shown to decrease scratch visibility.

8.6.4 Scratch and Abrasion Resistance Scratch and abrasion resistance is very important, especially in the home appliance and automotive industries [134]. The manufacturer may be forced to use a more expensive polymer or an unfilled polymer in order to maximise scratch resistance. There are two different aspects of scratch resistance. One is the actual size of the physical scratch, but the more important aspect is usually the visibility of the scratch, because that is what the user sees. Often filler increases the visibility of scratches. A 391

Particulate-Filled Polymer Composites commercially important example is talc-filled polypropylene where the talc particles become exposed by scratching, giving an undesirable white mark. Other fillers suffer the same problem and a great deal of work has been devoted to lessening the scratch depth and visibility of thermoplastic composites. Attempts to correlate the mechanical properties of polymers to their scratch-resistance have been largely unsuccessful. This is partly because the response of polymers to deformation is very dependent on the speed of testing. The mechanical properties of polymers and composites are usually determined at much lower rates than those encountered during scratching. Evans and Fogel used the WLF equation to correct tensile testing results and were then able to correlate the energy to break to the scratch resistance [135]. Another complication is that there is a wide variety of scratch-testing methods. These vary from scratching the surface using pencils of varying hardness (B, HB, H, 2H, etc.), through scratching with hard styli whilst incrementally increasing the load, (e.g., Erichsen pen), all the way to fully instrumented methods that examine the scratch profile and its dependence on load [136, 137]. Many polymeric materials display some recovery over time and this too must be allowed for if the true scratch performance is to be established [138]. Krupicka has reviewed methods and factors affecting scratch performance of organic coatings [138]. One can say that scratch resistance may be improved by one, or some combination of three methods. Lowering the coefficient of friction may make it more difficult to scratch the surface. This can be achieved for example by adding a lubricant that migrates to the surface [136, 137]. Another method is to increase the hardness of the surface, although this is only effective if the scratching medium is not too hard. For example, this method might protect against scratching by another polymer, but not against sand, grit or metal objects. By far the best method is to make the surface elastic using a soft, rubbery coating for example. One prime example is in the plastic flooring industry, where the flooring is usually coated in a polyurethane layer optimised to give scratch-resistance, the desired gloss level and the right level of friction.

8.7 Stabilisation and Recycleability 8.7.1 Introduction Contrary to popular public perception, polymers are not very stable and are readily attacked by atmospheric oxygen, heat and UV light [132, 139, 140]. Most of the high volume polymers require additives that stabilise them during processing at high temperatures, during their service life, and during any subsequent recycling operations. In fact, polypropylene is so unstable that all commercial grades must be stabilised [18] and the same is true of PVC. Once properly stabilised, these polymers can have a useful service life of tens or even hundreds of years.

392

Filled Thermoplastics Fillers often affect the stability of polymers via a variety of mechanisms. Although this is recognised, at least to some extent, it has not been studied as thoroughly as the stability of unfilled polymers. As the stability and recycleability can be critical issues, hopefully this subject will receive more attention in the future. Thermoplastics are usually processed in the molten state, at temperatures in the range 150-350 °C depending on the melting point and viscosity of the polymer. There are many standard stabilisation packages on the market, often containing a process stabiliser and a long-term stabiliser. Most of the stabilisers are synthetic, although recently, a natural hindered phenol, α-tocopherol (vitamin E) was found to be effective in polyolefins [141143] and has been commercialised. For further information, the reader can consult books explicitly dedicated to stabilisation of polymers [132, 144]. During service, most polymers experience mean temperatures in the range 20-40 °C, whereas peak temperatures may be much higher for short periods. The mechanical properties of polymers depend upon molecular weight, and it takes relatively little degradation to seriously impair mechanical performance. The degradation may result in crosslinking or chain scission depending on the chemistry of the polymer and the conditions the polymer is exposed to. By far the most important stabilisers are the hindered phenols, which are used in a wide range of polymers including the polyolefins, (e.g., PE and PP), polyamides, polycarbonate and PET. These stabilisers are effective both during processing at high temperature and for long-term use under ambient conditions. For increased effectiveness, they are usually combined with other stabilisers to attain an optimised combination of stabilisation and other properties such as discoloration. Often, the antioxidant is physically lost, primarily by extraction or volatilisation, rather than by chemical consumption [145]. The trend is therefore to use higher molecular mass antioxidants [132, 139, 146]. Fillers may affect the stability of polymers via a number of mechanisms. The two most important ones are discussed here. Those are the catalysis of degradation by the filler’s surface and the indirect lowering of stability that occurs when the filler surface adsorbs, and thereby deactivates, the antioxidants.

8.7.2 The Effect of Filler Chemistry and Impurities on Stability It is well documented that transition metals such as chromium, copper, iron and vanadium can catalyse the degradation of polymers [132, 139, 144]. These metals promote the decomposition of hydroperoxides, which are important in the degradation mechanism of most polymers.

393

Particulate-Filled Polymer Composites Most common fillers are not based on these elements, but they may be present as impurities in the filler, or they may be picked up by the filler during processing operations such as milling. Talc and mica commonly contain traces of iron. In fact, talc grades with low iron content command a price premium over less pure grades because it is assumed that iron content correlates to polymer stability. In fact, that assumption is completely erroneous. For example, iron oxide (Fe2O3) is used as a pigment, but it does not cause polymer degradation because it is not the total iron concentration that is important, but more the chemical form of the iron, or other metal [5]. It would be more accurate to measure the actual destabilisation caused by each talc grade and then adjust the price of the talc accordingly. However, this has not been done, probably because it would be rather labour intensive and therefore expensive. There is a fast, inexpensive alternative and that to measure the effect of filler on the stability (oxidation induction time) of a low molecular weight model liquid. For example, instead of compounding the filler into PE or PP, one can mix in a small amount of filler into squalane and measure the effect on stability [147]. It has been shown that squalane degrades by the same mechanism as PP [148]. This approach requires only very small samples and the procedure can be automated so it is very rapid to perform. Other model liquids may be used to simulate other polymers.

8.7.3 The Effect of Antioxidant Adsorption on Stability Antioxidants often contain functional groups that are capable of interaction with the filler surface. This can result in antioxidant adsorption depending upon the surface chemistry of the filler and the type of antioxidant. Once adsorbed, the antioxidant becomes ineffective because it is unable to diffuse to, and react with, the radicals that cause polymer degradation. The amount of deactivated antioxidant can be significant, and the usual response in industry is to add more antioxidant to attain the required level of stability. However, that approach raises the cost of the compound significantly. Another commercial approach is to use an epoxy additive that preferentially adsorbs onto the filler surface, physically blocking antioxidant adsorption. That helps to reduce cost, but the epoxy additive is itself still a relatively expensive chemical. DeArmitt, Breese and Lamèthe [149] studied the propensity of calcium carbonate to adsorb Irganox 1010 using squalane as a model liquid to simulate PP (Figure 8.7). Oxidation induction time (OIT) is a well-accepted method for measuring antioxidant concentration [145]. First a calibration curve of Irganox 1010 in squalane was made. Then increasing amounts of Irganox 1010 were added to a 20 weight percent dispersion of calcium carbonate in squalane. This was mixed and left for some time to allow the antioxidant to adsorb. The dispersion was then centrifuged to give a clear supernatant solution, which was analysed by OIT to determine the residual

394

Filled Thermoplastics

Figure 8.7 Effect of fillers on the nucleation of PP

Figure 8.8 The effect of calcium carbonate on the stability of squalane

antioxidant concentration in the squalane. The results showed that the Irganox 1010 was completely ineffective until enough had been added to saturate the filler surface. The 20 weight percent dispersion of calcium carbonate (specific surface area 5 m2g-1)

395

Particulate-Filled Polymer Composites adsorbed and deactivated 300 ppm of Irganox 1010. The same results were found when the calcium carbonate was not removed by centrifugation. The OIT measurements were performed at 190 °C, so the antioxidant must have been strongly bound to the filler, otherwise it would have desorbed and raised the OIT of the squalane. Interestingly, calcium carbonate surface treated with stearic acid did not adsorb any antioxidant. Stearic acid treated calcium carbonate is more expensive than untreated grades, but the cost differential is largely compensated for by the reduction in antioxidant required. If this were more widely recognised, it might promote the use of surface treated calcium carbonate.

8.7.4 Recycleability Thermoplastics may be recycled in a variety of ways such as mechanical recycling (collection, sorting, and reprocessing), burning to give energy, or biological recycling [150, 151]. There is a public perception that synthetic polymers are less friendly to the environment than natural polymers such as cellulose, poly(lactic acid) and poly(hydroxyalkanoates). That view is not supported by the facts. Life cycle analysis reveals a very different picture, favouring the synthetic polymers, especially polyolefins [150]. Although thermoplastics and thermoplastic composites are potentially easy and economical to recycle, in practice there are some impediments to the implementation of widespread recycling. The main one is that the used materials must be collected, separated and cleaned economically. This is feasible in some instances but often it is not. In general, polymers are immiscible with one another, and, if melt processed as a mixture, the result is phase separation to give domains of one polymer in the other. This morphology leads to rather poor mechanical properties. Therefore, there are efforts to find better separation techniques in order to avoid the problem or to use compatibilisers [152] that lower the interfacial tension, improve the adhesion of the two phases, and encourage smaller domains of the disperse phase.

8.8 Uses of Filled Thermoplastics 8.8.1 Uses of Fillers As shown in Table 8.1 more than 80% of the filler used in thermoplastic is based on calcium carbonate minerals. Most is used in PVC, with major sectors being cables, flooring, hose, plastisols, pipe, profiles and fittings. The main reason for this is firstly due to the fact that PVC has to be compounded in order for it to be used. The incorporation of stabilisers is an essential prerequisite for its successful use and therefore fillers can be 396

Fillers European consumption Filled Thermoplastics

Table 8.1 Estimated European consumption (in kilotonnes) of fillers in plastics, 1998 Calcium carbonate

PVC

PP

PE

Thermoset

ETP

Total

850

75

50

100

1

1076

10

1

131

6

15

Talc Calcined kaolin

120 7

Mica Wollastonite Kaolin

5

Total

862

small

2

small

Small

4

4 1

199

52

111

8 6

12

1236

ETP: engineering thermoplastics

included without increasing processing costs substantially. Secondly, PVC is compatible with both organic and inorganic substances so that changes in properties due to the filler can be more readily controlled than the other thermoplastics. Not withstanding the previous statement, the use of fillers in polypropylene, polyethylene and polyamide is of considerable commercial importance and seeing greatest growth and most research and development. The main fillers used are talc, ground calcium carbonate and calcined kaolin. The bulk of the filler comprises low-cost products for which price is the main specification requirement but in all applications, certain criteria are important. The filler should not greatly worsen the colour either directly by increasing light absorption (K) or indirectly by reducing heat stability or by degrading other additives in the plastic. However, in several applications the functionalities – shape, size, size distribution and coating, are very important. In unplasticised PVC (uPVC), extrusions and mouldings, PP mouldings, PP film and PE film, the particle size, size distribution and surface coating play an important role in determining processing, mechanical and aesthetic properties. Talc is used principally in PP, and to a lesser extent polyamide and PE, to give rigidity, a consequence of its (usually) high aspect ratio. In PP it is also used as a nucleating agent. Calcined kaolin is used in film as an anti-blocking agent, in thermal barrier agricultural film, and to allow for carbon dioxide laser printing. Aminosilane treated calcined clay is used principally in polyamide to give rigidity, toughness with low anisotropy. Wollastonite is also used in polyamide to give rigidity.

397

Particulate-Filled Polymer Composites

8.8.2 Fillers in PVC 8.8.2.1 Introduction PVC is produced by the free-radical polymerisation of vinyl chloride in suspension, emulsion, solution and mass, with the first two being the most important. Its fundamental unit is:

CH2

CH Cl

and commercial polymers have molecular weights between 50,000 and 120,000. They have approximately 5% crystallinity. All PVC polymers are unstable to heat and light with hydrogen chloride being evolved in an ‘unzipping’ mechanism. The resulting polymer chains are highly coloured, rigid and infusible. Stabilisers must be added to the PVC for it to be processed and used satisfactorily. These are added in a compounding operation at which time other additives can be added with little cost penalty. Density, refractive index, Tg, melt viscosity and other properties are dependent on the additives used. PVC is produced as a powder containing irregular grains with diameters between 65 and 170 µm (for suspension grades, other types are different). These grains are quite complex structures with a ‘strawberry’ looking surface, because they are composites with very small domains of 10-30 nm diameter which have agglomerated to form ‘spherical’ primary particles of 0.2–1.5 µm in diameter. The surface of the PVC grain (except for the mass polymer) is a skin of surfactants, polymerisation aids and other additives. During processing lubricants, processing aids and plasticisers penetrate this structure aiding melting and homogenisation. This process is known as gelation or fusion and fillers affect it significantly. The level of fusion determines many of the properties of the final PVC article. As a consequence of its complex chemistry and formulations, PVC is used in a very wide range of applications, making it the plastic with the third largest tonnage. However, the last few years have seen a very strong movement against PVC as a material because of its chlorine content and the possibility that, during its production, converting and disposal, chlorinated organic compounds known as dioxins may be formed. Possible formation of dioxins is also of concern during burning and disposal. As a consequence of this converters of PVC have made determined efforts to replace it with non-halogenated polymers. This has led to some changes in the uses of fillers. There are also moves occurring to replace the most commonly used lead-based stabilisers with organic and heavy metal free stabilisers because of concerns over its toxicity. These changes impose more stringent requirements on the purity of the filler. One of the most obvious changes is in colour because lead stabilisers give much greater opacity than the alternatives.

398

Filled Thermoplastics

Table 8.2 Estimated use of calcium carbonate in thermoplastics in Europe Application

Grade of calcium carbonate

kilo tonnes

PVC Cables

Medium and fine coated and uncoated

250

uPVC Extrusions

Ultrafine and fine coated

110

uPVC Windows and Profiles

Ultrafine coated

80

PVC Plastisols

Precipitated, ultrafine coated, fine and coarse

25

PVC Flooring

Ultrafine coated, fine and very fine

177

PVC Flexibles (general)

Fine, coated and uncoated

133

PE Compound and Masterbatch

100

PP Compounds

70

8.8.1.1 Fillers in Plasticised PVC Calcium carbonate is used in virtually all plasticised (flexible) PVC applications as an extender as seen in Table 8.2. The effects that the filler has on mechanical properties, colour and stability, are similar to those reported in Section 8.8.1.2 for cables, and will not be discussed in detail, although the formulations are far more variable.

8.8.1.2 Cable Coverings Most of the calcium carbonate used in plasticised PVC cable (insulation, sheathing and filling) in Europe is fine, good-quality chalk whiting often with a stearate coating, although where non-lead stabilisers are being used white marble based products are being used because of higher colour needs. In North America, Italy, Spain and other countries where relatively pure ground limestone or marble is abundant, then these are already being used. The use of stearate is not necessary as it can be added as extra lubricant in the formulation. In this case allowance must be made for the reaction that will occur between stearic acid and calcium carbonate. Even so, many users prefer a stearate coated filler as the coating improves the powder flow and general handling of the filler. The main type has a high purity usually 94-98% CaCO3, good whiteness 80-95 ISO, a mean particle size of 2-3 µm and low levels of coarse particles 1-10% above 10 µm. The 399

Particulate-Filled Polymer Composites

Figure 8.9 The effects of calcium carbonate fillers in plasticised PVC

Table 8.3 PVC Cable Sheathing Formulation Compound

phr

PVC (suspension grade K67)

100

Di-2-ethylhexylphthalate

50

Tribasic lead sulfate

5

Coated calcium carbonate

100

properties of the calcium carbonate have little effect on the mechanical and electrical properties of a PVC compound. This is shown in Figure 8.9 for the cable-sheathing compound given in Table 8.3, in which the calcium carbonate filler is based on chalk whiting that has been ground to different particle size distributions, and the compounds were extruded as a flat strip. Gloss values show a significant particle size effect increasing markedly with finer fillers. Many high-gloss cable covers are produced using ultrafine fillers. Particle size also effects stress whitening and scratch marking. As a generalisation, it may be said that insulation compounds will be filled with 40-70 parts per hundred resin (phr) and sheathing with 20-100 phr; filler loading, plasticiser level and lubricants are used to control properties of the cable covering. In some applications, such as high temperature resistant or high voltage compounds the electrical properties obtained using calcium carbonate as filler are not good enough. In these cases 400

Filled Thermoplastics metakaolinite (calcined clay produced at around 700 °C) is used at between 5 and 15 phr, although loadings as high as 20 phr are sometimes encountered.

8.8.1.3 Floor Tiles and Homogeneous Flooring Coarse calcium carbonate with a broad particle size distribution, based on chalk, limestone or marble, or dolomite with an average particle size of approximately 15 µm, is the main filler used in PVC floor tiles. Price is the main specification but control of colour and of levels of coarse particles is needed. It is used as an extender at 200-450 phr. To give extra dimensional stability, reduced water pick-up, and green or hot strength during calendering and extrusion of the carpet high aspect ratio platy particles are used with the calcium carbonate. Stabilisation systems have to be modified to allow for the extra reactivity of the silicate surface. In homogeneous flooring all types of calcium carbonate are used but finer products are more common at loadings between 25 and 250 phr. Sometimes stearate coated grades are used. The biggest developments in PVC flooring are, unfortunately, its replacement with other types of floor coverings such as wood and carpet due to fashion, or the replacement of PVC with other polymers such as polypropylene and polyethylene. These can be heavily filled but because the developments are new and ongoing little can be said concerning the filler requirements.

8.8.1.4 Wall Coverings/Leather Cloth/Spread Coatings/Calendered Sheet Fine calcium carbonates (average particle size 2-5 µm) is used as an extender at loadings between 25 and 50 phr; precipitated calcium carbonate is sometimes used as a rheological control, although whatever filler is used there will be a significant effect on rheology, which is of great importance in these applications. Filler dispersion is also important.

8.8.1.5 Hose and Profiles Most grades of calcium carbonate are used, except very coarse, (i.e., above 10 µm average particle size), as extenders at levels of 20-50 phr.

8.8.1.6 Footwear Precipitated and 1-3 µm grades are used for rheological control in rotational moulded products and as extenders in injection moulded products. 401

Particulate-Filled Polymer Composites

8.8.1.7 Plastisol/Sealants Mostly precipitated grades are used as rheological control additives in combination with medium (2-5 µm) or fine (1 µm) grades as extenders, the latter at levels of 40-200 phr.

8.8.2 Uses of Fillers in Unplasticised PVC 8.8.2.1 Introduction As with plasticised PVC applications, most uPVC uses natural calcium carbonate as a filler. Loadings, however, are usually lower, in the range 3-30 phr. Low levels of impact modifier and processing aids are frequently used with the higher loadings to achieve desired mechanical and processing properties. Often non-specification products will use higher loadings with levels up to 100 phr being encountered, especially in pipe in China, Indonesia and the Indian sub-continent. Another quite significant difference with plasticised products is that in uPVC the filler affects processing and end-properties significantly. Usually fine and ultrafine products with top cuts of 10 µm and average particle sizes of 1-2 µm are used, but in lower specification products 3 µm grades are being used. The filler affects processing by affecting fusion of the PVC particles. This is a consequence of the interaction and reaction of the lubricants used in the compound with the surface of the calcium carbonate changing the balance of internal and external lubrication. This, of course, depends on the surface coating used on the filler and the filler’s surface area [153]. The speed and level of fusion or gelation play significant roles on efficiency of processing, and in determining mechanical properties, especially impact strength [154]. The particle size of the filler, especially levels of coarse particles, also affects tensile and impact strengths either positively by the fine particles acting as stress diffusers or crack stoppers or adversely with the coarse particles acting as ‘flaws’ or stress concentrators. Filler loading has a significant effect on processing and on properties, especially on impact strength, which can reach a maximum often higher than the unfilled. The optimum loading is determined by particle size, coating levels and by other additives in the formulation. For coated ground calcium carbonates with a top cut of 10 µm and a d50 of about 0.8 µm, maxima have been found at between 15 and 20 phr for lead stabilised compositions without any impact modifier [153], with acrylic modifier [153] chlorinated PE [155], and with tin stabiliser and styrenics [156].

402

Filled Thermoplastics

8.8.2.2 Pipes, Conduit and Fittings The fillers used are mostly 1-3 µm grades based on chalk in Europe, limestone and marble in the rest of the world principally to reduce costs. The finer grades also act as impact modifiers and processing aids and are preferred to coarser grades in more demanding applications, and in decorative areas where gloss becomes important. In pressure pipes and fittings levels are usually between 1 and 5 phr; in rainwater goods, sewage, soil and agricultural drainage levels are 8-20 phr; and in conduit and ducting levels are 3-40 phr. In non-specification pipes, loadings can be as high as 100 phr but most national and international specifications are now including limit values for fillers, either directly or indirectly by specifying the maximum specific gravity permissible. This differs from application to application but typically will be around 1.46 g/cm3. This equates to a stabiliser plus filler content of around 20 phr.

8.8.2.3 Window and Other Profiles Technical properties supplied by the filler are as described previously for pipes, although in most 4-5 phr TiO2 is added to give whiteness, light and some heat stability. Stearate-coated ultrafine (0.7-0.8 µm) produced from chalk, white limestone and white marbles are most widely used. Growth in the whiter fillers is being spurred by changes in aesthetic requirements; by the move from lead stabilisers to calcium-zinc or tin-based which do not give the same levels of opacity as lead composites; and by developments in coloured profiles. Filler levels for window and other building profiles have been creeping higher in the last few years from 3-5 phr to 5-15 phr with the limiting factors being cold impact strength, extrudate gloss and corner weld-strength. In various non-critical applications such as blinds and roller blinds coarser grades (1-3 µm) are used with levels up to 75 phr being encountered. In these cases, small amounts of plasticisers (7.5 phr) are added to aid processing and properties.

8.8.2.4 Film The market for uPVC film is diminishing under environmental pressure and consequently the use of particulate fillers is also diminishing. Most of the film is transparent for food packaging, display, blister packs and so on, and kaolins and other silicate minerals are used as antiblocking additives, without detriment to colour and transparency of the film.

403

Particulate-Filled Polymer Composites

8.8.3 Uses of Fillers in Polypropylene 8.8.3.1 Introduction Polypropylene has the general formula:

CH2

CH

n

CH3 with n being about 2000. It is a linear polymer, essentially a hydrocarbon with many similarities to PE, being chemically inert, flexible, tough and having fairly low softening and melting points. The presence of the methyl groups, however, introduces several significant differences. Tacticity is introduced due to the various spatial arrangements of the methyl groups that are possible. Commercial polymers are 90-95% isotactic; that is the methyl groups occur on one side of the polymer chains. This introduces some crystallinity (approximately 50%), and higher softening points. However, the methyl groups also induce greater susceptibility to oxidation and chemical attack (usually at the hydrogen atom β to the methyl group). It is the lightest common plastic with a specific gravity of about 0.9. Ethylene can be polymerised at levels of 4-15% with propylene, either randomly or as blocks to give copolymers that are more flexible and tougher than the homopolymer. Alternatively the polypropylene may be compounded with ethylenepropylene rubber to give copolymers as a physical mixture or rubber-modified-grades (depending on the level of the rubber). All suffer from oxidative instability and are always stabilised in service. Particulate fillers and coupled glass fibres are used in all these polymers for many applications to increase rigidity and heat distortion.

8.8.3.2 The Uses of Filled Polypropylene As mentioned previously, the principle reason why fillers are used in PP is to increase rigidity, especially at higher than ambient operating temperatures. The rigidity or modulus of a composite is affected by the modulus of the inorganic component, its loading and by its aspect ratio, that is the ratio of its length (or largest dimension of a particle) to its thickness (or smallest dimension). The modulus of any inorganic filler is very much higher than that of a plastic, and thus differences between types of fillers are not so important in determining modulus as the other two factors. As volume loading of filler increases, the modulus of a composite increases almost linearly at low to moderate filler levels. The higher the aspect ratio the higher the modulus. Thus, where rigidity of the final product is the most important single parameter, high-aspect-ratio fillers such as talc

404

Filled Thermoplastics or glass fibre are preferred. Mica and wollastonite are available with high aspect ratios but their main use is in North America where their cost-performance ratio is advantageous; in Europe, talc is most widely used. Original developments in filled PP were compounds with much higher stiffness than the unfilled, especially at higher than ambient temperatures, but which would be low cost to match the styrenics, especially ABS. Talc, being widely available, low cost and usually having a high aspect ratio is the preferred filler type, and all polypropylene compounders have several talc-filled grades available. Typical uses are in: automotive components such as air-filter covers, timing chain covers, heater boxes, and battery box tops, domestic appliances, such as washing machine soap dispensers, and in some disposable food packaging such as skeletal fruit packages. New initiatives on recycling, particularly in the automotive industry, are tending to limit polymer types used. Polypropylene is strongly favoured and this is helping drive the market for filled grades. Crudely, the talcs that are used can be divided into five types: three based on ‘pure’ talc (less than 10% impurities) with top cuts of 300 BS mesh, 20 and 10 µm; and two based on less pure minerals with top cuts of 300 BS mesh and 20 µm. Particle size has no effect per se on rigidity but, depending on the method of processing, finer types may have higher aspect ratios and therefore will give higher rigidities. Particle size does affect composite impact and tensile strength with smaller particle size products giving higher strengths [157], although the results are not unambiguous because methods of producing fine talcs also produce higher aspect ratio particles [158]. Virtually all applications for talc-filled PP are those that do not require toughness or high strains because the rigidity imparted by talc is accompanied by brittleness. The toughness can be improved by changing the base polymer to a copolymer with ethylene as comonomer, by incorporating ethylene-propylene rubber, or by changing the mineral to stearate coated calcium carbonate. All methods, however, reduce the rigidity of the composite compared with the talc filled equivalent. As discussed in [94], particle size and coating of the calcium carbonate affect impact strength and toughness, while other properties are not affected greatly. Loading of the coated calcium carbonate also affects properties. Rigidity increases, and tensile strength decreases virtually linearly with loading but impact strength can go through a maximum at between 20 and 40 wt%. This high level of toughness coupled with a rigidity which is higher than the unfilled, good flow in mouldings, good light and temperature stability means that calcium carbonate filled PP is regarded as a separate material, with major uses in injection moulded garden furniture, automotive components, food packaging, fibres, tapes and blown oriented PP (BOPP) packaging film. A very rapidly growing market for fine coated calcium carbonate is in breathable films, in which micropores form around the calcium carbonate particles during film orientation (mostly oriented PP). In fibres and tapes, the stearate-coated

405

Particulate-Filled Polymer Composites calcium carbonate is used at around 8 wt% and acts as a delustering agent and also reduces fibrillation. Another large and growing market is in BOPP film used in packaging. Loadings up to 70 wt% are used in the central layer of a three-layer film, produced by co-extrusion, with the outer layers unfilled. Mineral filled polypropylene often replaces ABS, polyamide and other engineering plastics, although their mechanical, processing and optical properties are different. However, by experimenting with the large number of permutations and combinations of type of filler, type of polypropylene and rubber toughening agent, satisfactory matching of cost-property performance is achievable. Reductions in cycle time, shrinkage, sink marking and improved noise reduction are among the benefits resulting from mineral filling, but detrimental effects have usually been observed in the aesthetic properties. Gloss is normally reduced but the surface can be tailored by choice of filler. For example, a high gloss moulding can be obtained by using a blend of talc and calcium carbonate [159]. Colour is also changed by the mineral filler: pure talc gives translucent, almost colourless compounds while high quality calcium carbonate gives a white colour with some opacity. Increasing levels of impurities in both minerals causes colour degradation. Colour will also be caused by any instability introduced into the polypropylene by the mineral. One problem from which all particulate filled plastics, and in particular filled PP, suffer is that of scratch marking and marring; as the plastic is scratched, light scattering occurs at exposed filler particles. This has been overcome to some extent by choosing the correct filler and by modifying the filler plastic interface. Calcined talc is being used in automotive compounds for its improved resistance to scratching. Filled PP sheet is also being used extensively as thermoformed packaging materials because the filler confers good sheet extrudability and thermoforming characteristics [160].

8.8.4 Uses of Fillers in Polyethylene 8.8.4.1 Introduction Polyethylene –(–CH2–CH2–)–n is essentially a high-molecular-weight paraffin, which as a consequence, is inert to most chemicals, flexible with low softening and melting points. Three types are now commercialised: LDPE, produced by polymerisation over a freeradical source at high pressures; HDPE, produced by polymerisation over Ziegler catalysts (complex catalysts based on metallic co-ordination compounds); and LLDPE, produced by a variety of techniques designed to give chains with limited short-chain branching and incorporating low levels of other olefins – butene, hexene and octene. All grades are semi-crystalline with levels of about 60% for low density, and up to 90% for high density,

406

Filled Thermoplastics but these can be lowered by branching. There is much argument about the Tg with values from –20 °C to –130 °C being reported. They have softening points from about 77 °C to 124 °C. The main applications for filled polyethylene, film and bags, blow moulding and electrical insulation dictate the required properties and limit the potential for fillers.

8.8.4.2 The Uses of Filled Polyethylene A considerable amount of calcium carbonate is used in LDPE, LLDPE and HDPE. Medium particle-size grades (with d50 of between 2 and 3 µm) are used in masterbatch either alone or to extend pigments, principally white TiO2 and carbon black. These in fact dominate the masterbatch market, with film and bags being the final destination for most. In natural masterbatch, the filler level will usually be 70-75 wt%; in pigmented products it is used to dilute the prime pigment by amounts dictated by the requirements (opacity, colour and gloss) of the end application. Mostly dilution at 10-20 wt% is used but some masterbatches can be formulated with 10% prime pigment and 60 wt% calcium carbonate. Levels of course are also dictated by any other additives (stabilisers, lubricants, etc.), in the masterbatch. Cost is naturally very important, but the filler does play an important role in several film properties [161]. For example, in LLDPE extruded film, ground calcium carbonate improves efficiency by both increasing the cooling rate of the bubble and the level of fusion; it improves printability; primary pigment dispersion can be improved; it reduces the coefficient of friction by increasing the surface hardness of the film; and it acts as an anti-blocking agent. Other minerals are used to give specific properties. Medium and coarse china clays (average particle size 2 and 5 µm) are used at 5-10 wt% to reduce stretch, to reduce slip, act as anti-blocking additives and to give thermal barrier properties (see below). A variety of minerals are used to anti-block all types of film (although HDPE film is not so prone to blocking because it is much stiffer, so anti-blocking needs are less). Talc, silica (natural and synthetic), aluminosilicates, zeolites and calcined kaolin are all used at levels of 0.11.0 wt%, the loading depending on the ‘stickiness’ of the film and the temperatures at which it may be used. The surface chemistry, shape, purity and refractive index of the mineral determine the blocking, friction, clarity, haze and colour of the film. A major use for calcined clay is in agricultural film. Because of its very strong absorption bands in the far infrared (IR), it renders the plastic opaque to heat (and consequently more like a flexible glass) [162]. Other minerals absorb IR radiation in lesser amounts but are still used by some film producers. All of course are added via masterbatch, which will also include stabilisers and lubricants the levels of which being dictated by the intended service length of the agricultural film.

407

Particulate-Filled Polymer Composites Linear low-density polyethylene is principally used in film and bags, with similar criteria applying as in LDPE. There is some interest in using it (because of its toughness) in engineering and automotive applications, but its stiffness has to be increased by using fillers. In recent years there has been a rapidly growing and very large market for ground calcium carbonates in LLDPE in the production of microporous films, where holes have been incorporated into a plastic sheet or film through de-bonding of the polymer from filler particles dispersed in the matrix. The most common plastic used is LLDPE (often in blends) and the most common filler is coated ground calcium carbonate with average particle size 1-3 µm. The de-bonding is achieved by stretching and orienting the film. Loading levels of the calcium carbonate are around 55-60 wt%, and the particle size and efficiency of coating are key to producing film with controlled pore size. The principal uses are in the production of controlled atmosphere or breathable films and in white opaque films. The former has various uses – hygiene, medical and industrial garments, building membranes, house wrap but the biggest single use is in the production of babies’ nappies. The latter includes food packaging, labels and paper replacement. High-density polyethylene finds its major outlets in blow-moulded bottles and containers, film and carrier bags and pipes (the last often used with LDPE and the blends are known as medium density polyethylene (MDPE)). Although fillers have been and are still being looked at periodically in blow mouldings, processing imposes severe restrictions before costs and product properties are considered. Film and bags will have some ground calcium carbonate included via masterbatch as an extender for the prime pigment. The main pipe sectors are for gas and water transportation, and end-users have imposed very strict regulations on additives and properties; potential use of fillers is very low. There has been some interest in using HDPE in ducting, competing against uPVC, and a filler, at high loadings, will be essential to achieve the required stiffness and cost balance, if this replacement is to be successful.

8.8.5 The Use of Fillers in Polyamides 8.8.5.1 Introduction Nylon is the generic name for the family of polyamides (PA) with PA6 and PA6, 6 being the most common. They are named after the chemical group:

O

H

C

N

formed during the condensation polymerisation which occurs when an organic acid is heated with an organic amine. The numbers which always occur with the name Nylon or 408

Filled Thermoplastics polyamide refer to the types of acid or amine used in the production. The regular spacing of the amide groups means that the polymers crystallise with a high intermolecular attraction leading to high-strength polymers with high melting points. Levels of crystallinity depend on thermal history and can vary from 15% to 50%. Nylon is fairly hygroscopic and its Tg (temperature below which the polymer become brittle) and mechanical properties depend on the amount of absorbed water.

8.8.5.2 Properties of Filled Nylon All polyamides are tough, rigid plastics with high heat distortion temperatures but they are thermoplastic, that is, they exhibit plastic flow (high creep) and soften at elevated temperatures. Mostly they are used unfilled but there is a significant application sector in which higher rigidities and higher heat distortion temperatures are required than can be achieved from unfilled polyamide. Glass fibre (with a size or a silane coating) fills most requirements giving very high rigidity, etc. Other mechanical properties of a properly coupled glass-fibre filled Nylon are also good. Some 200,000 tonnes per year of glass-filled Nylon is now used, and it is regarded as a separate engineering material. However these fibre filled composites suffer from anisotropy due to fibre orientation during processing; that is, the properties of the composite vary depending on the direction in which they are measured. Fibre orientation also causes uneven shrinkage that can lead to unpredictable warpage (bending and distorting), which is not acceptable in products that require good dimensional stability. Anisotropy and warpage are proportional to the aspect ratio of the filler used and can be reduced to zero by using glass spheres [163]. Calcined kaolins give good, low, anisotropy and when treated with a bifunctional aminosilane, give very good mechanical properties and tend to dominate the European and Pacific filled Nylon market. Both particle size and coating level affect impact strengths. Pre-treatment gives much better properties than when calcined clay and silane are added separately to the compounding operation [164]. Some talc and wollastonite is also used giving high rigidity (although this depends on the grade and its aspect ratio), although impact properties are worse. In the case of talc, this has been related to its inability to couple through silanes with the polyamide. Calcined talc is being developed to improve the interaction [154].

8.8.5.3 Uses of Filled Nylon Mineral-filled Nylon is widely used: in automotive applications, such as wheel discs, headlamps, water pumps, air inlets and grills; in electrical engineering; in electronics; appliances and consumer goods [165]. Significant recent developments have seen its use in the production of automotive engine covers. The mineral, as mentioned previously, is 409

Particulate-Filled Polymer Composites frequently used in combination with glass fibre. Loading levels have, in the automotive industry, dropped to around 20 wt% under pressures to reduce vehicle weight but in some cases are still as high as 40 wt%.

8.8.6 Polybutylene Terephthalate Polybutylene terephthalate (PBT) is a fairly tough, rigid thermoplastic which has a very good gloss in mouldings. It is less susceptible than Nylon to moisture when moulded but otherwise most of its mechanical properties are not so good. Many glass-fibrefilled compounds are available from specialty compounders and polymer producers and some mineral filled grades are also widely used. Talc, aminosilane-treated calcined clay, ultrafine calcium carbonate, ultrafine china clays, glass beads and glass flakes have all been encountered. Benefits from the particulate fillers have been reported to be rigidity with good dimensional stability, high impact strength and exceptional surface finish [166].

8.8.7 Polyethylene Terephthalate One of the most important uses of PET is in producing many types of film and tapes. In these a variety of speciality fine grades of china clays, calcined clays, calcium carbonates (natural and precipitated), synthetic silicas and silicates are used at levels of about 0.1 wt% as anti-blocking agents; they may also nucleate crystallite formation. Fillers seem to be precluded from the largest single market for PET – bottles, not only because of the difficulties in successfully blow moulding filled products in general, but also their application properties such as optical clarity and burst strength will be adversely affected by fillers. Many bottles are now being recycled into a variety of plastic and fibre applications; fibres and particulate fillers are being looked at to extend the range of applications. The high HDT and softening temperature of PET have recently led to developments in its use in food ‘cook-in’ applications. However, it’s HDT is not quite high enough for conventional oven use and fillers are being looked at to improve this.

8.8.7.1 Polystyrene, High-Impact Polystyrene and ABS The styrenics are a family of low to medium price, rigid, easily processed plastics with good gloss and optical properties. ABS and HIPS are more tough, due to the acrylonitrile rubber content, which has either been incorporated into the plastic or copolymerised with the styrene monomer, respectively. Fillers have been looked at by several producers 410

Filled Thermoplastics and compounders but do not give good enough properties to allow them to be used in any significant amounts, although products containing kaolin, calcium carbonate and talc have been reported. These may have the filler incorporated as a diluent in colour masterbatches used in the plastic. Some very old patents cover the incorporation of china clays and ultrafine calcium carbonates into the rubber before compounding this into the polystyrene but again no noticeable commercial success has been achieved. Talc is used in expanded polystyrene as a nucleating agent.

8.8.7.2 Polyphenylene Oxide (or Ether) Polyphenylene oxide is an engineering plastic that is commonly blended with polystyrene, with the blend having significantly better properties than the separate plastics. Modified polyphenylene oxide has been available now for approximately 30 years and it’s uses are widespread including automotive components, business machines, domestic appliances, wire covers and water treatment. Other blends with PBT and PA are in the market place offering different property balances. These composites (and other polymer blends) are being trialed for automotive body parts, and particulate fillers are needed to give the correct coefficient of thermal expansion. This type of filler is also needed to give rigidity without affecting impact properties. Very fine kaolins (with mean equivalent spherical diameter (esd) of about 0.3 µm) with and without silane coupling agent give good impact strength [167].

8.8.7.3 Polyphenylene Sulfide (PPS) Polyphenylene sulfide is a high temperature, stiff, fire-resistant, chemically resistant plastic, which has a very low melt viscosity and accepts fillers and reinforcing agents very well. Talc, china clay, dolomite, quartz and glass fibre filled grades are all available but volumes are very small and the situation changes very rapidly.

8.8.7.4 Polyformaldehyde or Polyoxymethylene Polymers These plastics are characterised by high stiffness, good impact resistance, high HDT, good chemical resistance and have the best fatigue resistance of any plastic. Glass fibreand mineral-filled grades are used to increase stiffness, hardness and resistance to distortion. The type of filler that can be used is important as the polymers are badly depolymerised by acids, the unzipping of the polymer chain giving gaseous formaldehyde. Minerals with acid surfaces, such as china and calcined clays, can cause this depolymerisation. Calcium carbonate reduces some mechanical properties but one calcium carbonate filled grade is claimed to have better abrasion resistance. 411

Particulate-Filled Polymer Composites

8.8.7.5 Others The properties of particulate fillers in a number of other thermoplastics are being investigated by plastics producers and academic institutions but usually with a low priority rating. This activity has been growing less and less in recent years as companies, in particular, have been reducing the staffing levels in the research and development departments.

8.9 Conclusions Thermoplastic composites are all around us and their use is increasing every year. The reason for this is that thermoplastics have an excellent combination of cost and performance. The performance can often be further enhanced by addition of fillers while maintaining a favourable cost. The recycleability of thermoplastic composites is an advantage compared to rubbers and thermosetting polymers because the latter two types cannot be melted and reshaped. This favours the continued growth of the thermoplastics and their composites at the expense of other polymeric materials. To understand and optimise composites, one must have an overview of all the different economic, chemical, surface and physical aspects. Furthermore, one must have a clear goal, and be able to correctly prioritise the properties of most import for the intended application. The best composite is the one that makes the best compromise between the multitude of properties, at the lowest cost. The use of fillers has been increasing incrementally for many years and that trend is expected to continue as the use of traditional fillers is optimised and as new nano-fillers eventually become economically attractive. What can we expect in the future? In the near future, composites must be designed with re-use in mind. That means proper stabilisation so that the polymer can be recycled with sufficient retention of mechanical and aesthetic properties. There will be an increased tendency to use fewer, standard materials to reduce cost and to reduce the need for extensive separation of materials for recycling. It can also be anticipated that products will be designed for easy disassembly. It seems probable that surface treated filler will become more popular. Although the treatment adds cost it gives many advantages and when these are summed, the overall cost and performance of the material may be better for the surface treated type. For example, stearic acid coated calcium carbonate in PP homopolymer gives higher extruder throughput, better gloss, better impact strength and improved stability (because it prevents antioxidant from adsorbing onto the filler and becoming inactive). Any one of these

412

Filled Thermoplastics benefits may not justify the extra cost of pre-treated filler, but taken together they give a very attractive combination of price and performance. Another trend for the future may be the increased use of single-screw extruders to make composites. At the moment twin-screw extruders are used almost exclusively, because they are able to achieve better dispersion. However, recent developments have improved mixing in single-screw extruders. Therefore, it may become common to use single-screw extruders because they are cheaper, easier to maintain and give higher throughput. Again, surface treatment of the filler also helps here to give good filler dispersion even for a single-screw extruder. Progress in polymer composites has been held back because it is expensive and timeconsuming to prepare multiple formulations and then perform thorough mechanical testing. It is possible to save time and money by screening new fillers, antioxidants, dispersants and coupling agents in a model liquid instead of the polymer. The screening can then be followed up by full testing, using the polymeric matrix. Hopefully, this method will be used to help develop new filler grades and surface treatments, more quickly and cheaply.

Acknowledgements We would like to thank several people for their assistance in writing this chapter. Firstly, the editor Professor Rothon who has been a great help in making suggestions, proofreading and for general discussions. Professor John Berg (who deserves a special thanks for being such a help with the section on adhesion), Professor Ulf Gedde, Professor Aubrey Jenkins, Kevin Breese, Massimo Sanità, Carlo Tomaselli, Roy Goodman, Chris Paynter, Richard Day, Anna Kron and Werner Posch are all warmly thanked for making significant contributions by reading draft versions and for making valuable comments that improved the quality of the chapter.

References 1.

J.A. Brydson, Plastics Materials, 7th Edition, Butterworth-Heinemann, Oxford, UK, 1999, 189.

2.

J. Jancar in Mineral Fillers in Thermoplastics I: Raw Materials and Processing, Ed., J. Jancar, Advances in Polymer Science Series, Volume 139, Springer-Verlag, Berlin, Germany, 1999, 4.

3.

H.E. Wiebking, Proceedings of Antec 95, Boston, MA, USA, 1995, Volume 3, 4112.

413

Particulate-Filled Polymer Composites 4.

N. Burditt, Proceedings of Mineral Fillers in Polymers, Ed., J. Griffiths, Metal Bulletin plc, London, UK, 1991, 4.

5.

C. DeArmitt and R. Rothon, Plastics Additives & Compounding, 2002, 4, 5, 12.

6.

W. Hohenberger in Plastics Additives Handbook, 5th Edition, Ed., H. Zweifel, Carl Hanser Verlag, Munich, Germany, 2001, 901.

7.

B. Pukánsky in Polypropylene: Structure, Blends and Composites, Volume 3: Composites, Ed., J. Karger-Kocsis, Chapman and Hall, London, UK, 1995, 1.

8.

C. DeArmitt and K. D. Breese, Plastics Additives & Compounding, 2001, 3, 9, 28.

9.

B. Pukánsky and E. Fekete in Mineral Fillers in Thermoplastics I: Raw Materials and Processing, Ed., J. Jancar, Advances in Polymer Science, Volume 139, Springer-Verlag, Berlin, Germany, 1999, 109.

10. J. Menczel and J. Varga, Journal of Thermal Analysis, 1983, 28, 1, 161. 11. M. Fujiyama and T. Wakino, Journal of Applied Polymer Science, 1991, 42, 10, 2739. 12. A.T. Kowalewski and A. Galeski, Journal of Applied Polymer Science, 1986, 32, 1, 2919. 13. F. Rybnikar, Journal of Applied Polymer Science, 1991, 42, 10, 2727. 14. A. Garton, S. W. Kim and D. M. Wiles, Journal of Polymer Science: Polymer Letters Edition, 1982, 20, 5, 273. 15. J. Varga and F. Schulek-Tóth, Angewandte Makromolekulare Chemie, 1991, 188, 11. 16. J. Varga, Journal of Thermal Analysis, 1989, 35, 1891. 17. H-P. Schlumpf, Kunstoffe, 1983, 73, 9, 511. 18. R.F. Becker, L.P.J. Burton and S.E. Amos in Polypropylene Handbook, Ed., E. P. Moore Jr., Carl Hanser Verlag, Munich, Germany, 1996, 177. 19. B. Klingert, Proceedings of Polymer Additives, Product and Market Developments Conference, Chicago, IL, USA, 1995. 20. N.S. Allen, M. Edge, T. Corrales, A. Childs, C. Liauw, F. Catalina, C. Peinado and A. Minihan, Polymer Degradation and Stability, 1997, 56, 2, 125.

414

Filled Thermoplastics 21. A. Childs, N.S. Allen, C.M. Liauw and K.R. Franklin, Proceedings of Eurofillers 97, Manchester, 1997. 22. G. Wypych, Handbook of Fillers, 2nd Edition, ChemTec Publishing, Toronto, Canada, 2000. 23. P.L. Duifhuis and J.M.H. Janssen, Plastics Additives & Compounding, 2001, 3, 11, 14. 24. A.V. Shenoy, Rheology of Filled Polymer Systems, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1999. 25. P.R. Hornsby in Mineral Fillers in Thermoplastics I: Raw Materials and Processing, Ed., J. Jancar, Advances in Polymer Science Series, Volume 139, Springer-Verlag, Berlin, Germany, 1999, 155. 26. A. Einstein, Annales de Physik, 1911, 34, 591. 27. J.L. White and J.W. Crowder, Journal of Applied Polymer Science, 1974, 18, 4, 1013. 28. C.J. Rauwendaal, Polymer Extrusion, 4th Edition, Carl Hanser Verlag, Munich, Germany, 2001. 29. C. Rauwendaal, Plastics Additives & Compounding, 2002, 4, 6, 24. 30. Fillers and Reinforcing Agents in Plastics – Physical Chemical Aspects for the Processor, Omya Technical Note No. 172, Omya, Oftringen, Switzerland, 31. CRC Handbook of Chemistry and Physics, 68th Edition, Eds., R.C. Weast, M.J. Astle and W.H. Beyer, CRC Press Inc., Boca Ratan, FL, USA, 1987. 32. Polymer Handbook, 4th Edition, Eds., J. Brandrup, E.H. Immergut, E.A. Grulke, A. Abe and D.R. Bloch, John Wiley and Sons Inc., New York, NY, USA, 1999. 33. M. Pyda and B. Wunderlich in Polymer Handbook, 4th Edition, Eds., J. Brandrup, E.H. Immergut, E.A. Grulke, A. Abe and D.R. Bloch, John Wiley and Sons Inc., New York, USA, 1999, Volume 1, 483. 34. Polypropylene The Definitive User’s Guide and Databook, Eds., C. Maier and T. Calafut, Plastics Design Library, Norwich, NY, USA, 1998, p.152. 35. Physical Properties of Polymers Handbook, Ed., J.E. Mark, American Institute of Physics Press, New York, NY, USA, 1996.

415

Particulate-Filled Polymer Composites 36. P.T. DeLassus and N.F. Whiteman in Polymer Handbook, 4th Edition, Eds., J. Brandrup, E.H. Immergut, E.A. Grulke, A. Abe and D.R. Bloch, John Wiley and Sons Inc., New York, USA, 1999, V 159. 37. B.T. Åström, Manufacturing of Polymer Composites, Chapman and Hall, London, UK, 1997. 38. J.N. Gaitskell, Fillercon ’84, Kingston Polytechnic/PRI Conference, 1984. 39. Handbook of Conducting Polymers, 2nd Edition, Eds., T.A. Skotheim, R.L. Elsenbaymer and J.R. Reynolds, Marcel Dekker, New York, NY, USA, 1998. 40. Conductive Polymers and Plastics in Industrial Applications, Ed., L. Rupprecht, Plastics Design Library, Norwich, NY, USA, 1999. 41. R.S. Kohlman, J. Joo and A.J. Epstein in Physical Properties of Polymers Handbook, Ed., J. E. Mark, American Institute of Physics Press, New York, NY, USA, 1996, 453. 42. C.L. DeArmitt, Novel Polyaniline Colloids, University of Sussex, Brighton, UK, 1990. [M.Phil. Thesis] 43. C.L. DeArmitt, Novel Colloidal and Soluble Forms of Polyaniline and Polypyrrole, University of Sussex, Brighton, UK, 1995. [D.Phil. Thesis] 44. C. DeArmitt, S.P. Armes, J. Winter, F.A. Uribe, S. Gottesfeld and C. Mombourquette, Polymer, 1993, 34, 1, 158. 45. C. DeArmitt and S.P. Armes, Journal of Colloid and Interface Science, 1992, 150, 134. 46. C. DeArmitt and S.P. Armes, Langmuir, 1993, 9, 3, 652. 47. J.A. Brydsen, Plastic Materials, 7th Edition, Butterworth-Heinemann, Oxford, UK, 1999. 48. F. Breitnerfellner and J. Hrach, inventors; Ciba-Geigy, assignee; DE2,616,754, 1986. 49. D.G. Needham, inventor; Belgian Patent 862,067, 1978. 50. L. Rejón, A. Rosas-Zavala, J. Porcayo-Calderon and V.M. Castaño, Polymer Engineering and Science, 2000, 40, 9, 2101. 51. D.M. Kaylon, E. Birinci, R. Yazici, B. Karuv and S. Walsh, Polymer Engineering and Science, 2002, 42, 7, 1609. 416

Filled Thermoplastics 52. A.S. Michaels and R.B. Parker, Jr., Journal of Polymer Science, 1959, 41, 138, 53. 53. S. Pauly in Polymer Handbook, 4th Edition, Eds., J. Brandrup, E. H. Immergut, E.A. Grulke, A. Abe and D.R. Bloch, John Wiley and Sons Inc., New York, NY, USA, 1999, 543. 54. H. Alter, Journal of Polymer Science, 1962, 57, 165, 925. 55. S. Steingiser, S.P. Nemphos and M. Salame in Encyclopedia of Chemical Technology, 3rd Edition, Ed., H.F. Mark, Wiley Interscience, New York, NY, USA, 1978, 482. 56. A.C. Tiburcio and J.A. Manson in Acid-Base Interactions:Relevance to Adhesion Science and Technology, Eds., K.L. Mittal and H.R. Anderson, Jr., VSP, Utrecht, The Netherlands, 1991, 313. 57. D. Ansari, A. Calhoun and P. Merriman, The Role of Calcium Carbonate in Microporous Film Applications, Imerys Technical Data Sheet PMA 124PL – 2nd Edition, Imerys Performance Minerals, Cornwall, UK, 2001. 58. S. Nakamura, S. Kaneko and Y. Mizutani, Journal of Applied Polymer Science, 1993, 49, 1, 143. 59. A. Ram, Fundamentals of Polymer Engineering, Plenum Press, New York, NY, USA, 1997. 60. I.M. Ward, Mechanical Properties of Solid Polymers, 2nd Edition, John Wiley and Sons, Ltd., Chichester, UK, 1985. 61. T.S. Chow, Journal of Materials Science, 1980, 15, 8, 1873. 62. H.P. Schreiber and F. St. Germain in Acid-Base Interactions: Relevance to Adhesion Science and Technology, Eds., K.L. Mittal and H.R. Anderson, Jr., VSP, Utrecht, The Netherlands, 1991, 273. 63. G. Gibson in Polypropylene Structure Blends and Composites, Volume 3: Composites, Ed., J. Karger-Kocsis, Chapman and Hall, London, UK, 1995, 71. 64. C. Halpin and J.L. Kardos, Journal of Applied Physics, 1972, 43, 5, 2235. 65. J.A. Manson, S.A. Iobst and R. Acosta, Journal of Polymer Science: Part A1, Polymer Chemistry, 1972, 10, 1, 179. 66. L.E. Nielsen, Mechanical Properties of Polymers and Composites, Marcel Dekker, New York, NY, USA, 1974. 417

Particulate-Filled Polymer Composites 67. T.B. Lewis and L.E. Nielsen, Journal of Applied Polymer Science, 1970, 14, 6, 1449. 68. P.H.T. Vollenberg and D. Heikens, Polymer, 1989, 30, 9, 1656. 69. B. Pukánsky, J. Kolarik and F. Lednicky, Polymer Composites: Proceedings of the 28th Microsymposium on Macromolecules, Prague, Czechoslovakia, 1985, 67, 553. 70. J. Humphries, Proceedings of the Filled and Reinforced Thermoplastics Conference, London, UK, 1981. 71. C.Y. Yue and W.L. Cheung, Journal of Materials Science, 1991, 26, 4, 870. 72. E. Morales and J.R. White, Journal of Materials Science, 1988, 23, 10, 3612. 73. A. Galeski and R. Kalinski in Polymer Blends: Processing, Morphology and Properties, Volume 1, Eds., E. Martuscelli, R. Palumbo and M. Kryszewski, Plenum Press, New York, USA, 1980, 431. 74. F.H. Maurer, R. Kosfeld, T. Uhlenbroich and L.G. Bosveliev, Proceedings of the 27th International Symposium on Macromolecules, Strasbourg, France, 1981, Volume 2, p.1251. 75. F.H. Maurer, H.M. Schoffeleers, R. Kosfeld and T. Uhlenbroich in Progress in Science and Engineering of Composites, ICCM-IV, Eds., T. Hayashi, K. Kawata and S. Umekawa, Tokyo, Japan, 1982, p.803-809. 76. G. Akay, Polymer Engineering Science, 1990, 30, 21, 1361. 77. K. Iisaka and K. Shibayama, Journal of Applied Polymer Science, 1978, 22, 3135. 78. B. Pukánsky, B. Turcsányi and F. Tüdös in Interfaces in Polymer, Ceramic and Metal Matrix Composites, Ed., H. Ishida, Elsevier, New York, NY, USA, 1988, 467. 79. B. Pukánsky, Composites, 1990, 21, 3, 255. 80. S.J. Porter, C.L. DeArmitt, R. Robinson, J.P. Kirby and D.C. Bott, High Performance Polymers, 1989, 1, 1, 85. 81. S.N. Maiti and P.K. Mahapatro, International Journal of Polymeric Materials, 1990, 14, 3-4, 205. 82. A.M. Riley, C.D. Paynter, P.M. McGenity and J.M. Adams, Plastics and Rubber Processing and Applications, 1990, 14, 2, 85.

418

Filled Thermoplastics 83. V. Svehlova and E. Poloucek, Angewandte Makromolekulare Chemie, 1987, 153, 197. 84. L.H. Sperling, Introduction to Physical Polymer Science, John Wiley and Sons Inc., New York, NY, USA, 1986. 85. U.W. Gedde, Polymer Physics, Chapman and Hall, London, UK, 1996. 86. J.M.G. Cowie, Polymers: Chemistry and Physics of Modern Materials, 2nd Edition, Chapman and Hall, New York, NY, USA, 1991. 87. L. Williams, R.F. Landel and J.D. Ferry, Journal of the American Chemical Society, 1955, 77, 3701. 88. I. Vincent, Impact Tests and Service Performance of Plastics, Plastics Institute, London, UK, 1971. 89. C. Paynter, Anisotropic Mineral Fillers Compounded with Polypropylene, A Study of the Differing Stiffening Effects of Talc and Kaolin, University of Surrey, UK, 1999. [MSc Thesis] 90. Y-C. Ou, J. Zhu and Y-P. Feng, Journal of Applied Polymer Science, 1996, 59, 2, 287. 91. S.M. Dwyer, O.M. Boutni and C. Shu in Polypropylene Handbook, Ed., E. P. Moore, Jr, Carl Hanser Verlag, Munich, Germany, 1996, 211. 92. H. Liang, W. Jiang, J. Zhang and B. Jiang, Journal of Applied Polymer Science, 1996, 59, 3, 505. 93. P. McGenity, C.D. Paynter and J.M. Adams, Proceedings of Filplas ’89, BPF/PRI Conference, Manchester, 1989, Paper No.14. 94. M. Hancock, P. Tremayne and J. Rosevear, Journal of Polymer Science: Polymer Chemistry Edition, 1980, 18, 11, 3211. 95. M.J. Folkes in Polypropylene Structure Blends and Composites, Volume 3: Composites, Ed., J. Karger-Kocsis, Chapman and Hall, London, UK, 1995, 340. 96. A. Shanks and B.E. Tiganis in Plastics Additives: An A-Z Reference, Ed., G. Pritchard, Chapman and Hall, London, UK, 1998, 464. 97. A. Turner Jones, J.M. Aizelwood and D.R. Beckett, Die Makromolekulare Chemie, 1964, 75, 134.

419

Particulate-Filled Polymer Composites 98. S. Laihonen, Structure and Morphology of Poly(propylene-stat-ethylene) Fractions, Royal Institute of Technology, Stockholm, Sweden, 1995. [Licentiate Thesis] 99. S.C. Tjong, J.S. Shen and R.K.Y. Li, Polymer Engineering and Science, 1996, 36, 1, 100. 100. S.C. Tjong, R.K.Y. Li and T. Cheung, Polymer Engineering and Science, 1997, 37, 1, 166. 101. S.Y. Hobbs, Nature, Physical Sciences Edition, 1972, 239, 28. 102. S. Wu, Polymer Interface and Adhesion, Marcel Dekker Inc., New York, NY, USA, 1982. 103. A. Keller, Journal of Polymer Science, 1955, 15, 31. 104. M.J. Folkes and S.T. Hardwick, Journal of Materials Science, 1990, 25, 5, 2598. 105. M.J. Folkes and S.T. Hardwick, Journal of Materials Science Letters, 1984, 3, 12, 1071. 106. M.G. Huson and W.J. McGill, Journal of Polymer Science: Polymer Chemistry Edition, 1984, 22, 11, 3571. 107. S.Y. Hobbs, Nature, Physical Sciences Edition, 1971, 234, 12. 108. M.J. Folkes and S.T. Hardwick, Journal of Materials Science Letters, 1987, 6, 6, 656. 109. J.C. Berg in Adhesion Science and Engineering, Volume 2: Surface Chemistry and Applications, Ed., A.V. Pocius, Elsevier, Amsterdam, The Netherlands, 2002, 1. 110. M. Sumita, H. Tsukihi, K. Miyasaka and K. Ishikawa, Journal of Applied Polymer Science, 1984, 29, 5, 1523. 111. A.W. Adamson, Physical Chemistry of Surfaces, 5th Edition, John Wiley and Sons Inc., New York, NY, USA, 1990. 112. D. Meyers, Surfaces Interfaces and Colloids, 2nd Edition, John Wiley and Sons, New York, NY, USA, 1999. 113. R.J. Hunter, Introduction to Modern Colloid Science, Oxford University Press, Oxford, UK, 1993. 420

Filled Thermoplastics 114. D.J. Shaw, Introduction to Colloid and Surface Chemistry, 4th Edition, Butterworth-Heinemann Ltd., Oxford, UK, 1992. 115. R.J. Pugh in Surfactant and Colloid Chemistry in Ceramic Processing, Eds., R.J. Pugh and L. Bergström, Surfactant Science Series, Volume 51, Marcel Dekker Inc., New York, NY, USA, 1994, 127. 116. C-M. Chan, Polymer Surface Modification and Characterization, Hanser/Gardner, Cincinatti, OH, USA, 1994. 117. S. Lundström, Permeability and Void Formation in RTM, Luleå University of Technology, Lund, Sweden, 1993. [Licentiate Thesis] 118. R.A. Sinicki and J.C. Berg, Journal of Adhesion Science and Technology, 1998, 12, 10, 1091. 119. Y-J. Lee, I. Manas-Zloczower and D.L. Feke, Polymer Engineering and Science, 1995, 35, 12, 1037. 120. L. Bergström, A. Meurk, H. Arwin and D.J. Rowcliffe, Journal of the American Ceramic Society, 1996, 79, 2, 339. 121. L. Bergström, Advances in Colloid and Interface Science, 1997, 70, 125. 122. A. Meurk, P.F. Luckham and L. Bergström, Langmuir, 1997, 13, 14, 3896. 123. F. Berzin, B. Vergnes, P.G. Lafleur and M. Grmela, Polymer Engineering and Science, 2002, 42, 3, 473. 124. M. Gilbert in Plastics Additives: An A-Z Reference, Ed., G. Pritchard, Chapman and Hall, London, UK, 1998, 590. 125. M. Ernstsson and A. Larsson, Proceedings of EuroFillers 95, MOFFIS/FILPLAS, Mulhouse, France, 1995. 126. M. Ernstsson, Surface Characterization of Minerals used in Asphalt Systems and Filled Plastics: A Multianalytical approach, Royal Institute of Technology, Stockholm, Sweden 1999. [Licentiate Thesis] 127. A. Holzner in Plastics Additives Handbook, 5th Edition, Ed., H. Zweifel, Carl Hanser Verlag, Munich, Germany, 2001, 485. 128. I. Piirma, Polymeric Surfactants, Surfactant Science Series, Volume 42, Marcel Dekker Inc., New York, NY, USA, 1992. 421

Particulate-Filled Polymer Composites 129. B. Haworth, C.L. Raymond and I. Sutherland, Polymer Engineering and Science, 2000, 40, 9, 1953. 130. B. Haworth, C.L. Raymond and I. Sutherland, Polymer Engineering and Science, 2001, 41, 8, 1345 131. C.G. Oertel in Polypropylene Handbook, Ed., E.P. Moore Jr., Carl Hanser Verlag, Munich, Germany, 1996, 349. 132. H. Zweifel, Stabilization of Polymeric Materials, Springer-Verlag, Berlin, Germany, 1998. 133. J. Travis, F. McDowell and C. Baird, Proceedings of the 37th Annual SPI/RPCI Conference, Washington, DC, USA, 1982, Session 25-A, 1. 134. R.P. Higgs, D.A. Taylor, C.D. Paynter and P.N. Sambells, Proceedings of Polypropylene in Automotive Applications, Birmingham, UK, 1992, Paper No.12. 135. R.M. Evans and J. Fogel, Journal of Coatings Technology, 1979, 49, 634, 50. 136. J. Chu, L. Rumao and B. Coleman, Polymer Engineering and Science, 1998, 38, 11, 1906. 137. J. Chu, C. Xiang, H-J. Sue and R.D. Hollis, Polymer Engineering and Science, 2000, 40, 4, 944. 138. A. Krupicka, Use and Interpretation of Scratch Tests on Organic Coatings, Royal Institute of Technology, Stockholm, Sweden, 2002. [PhD Thesis] 139. F. Gugumus in Plastics Additives Handbook, 5th Edition, Ed., H. Zweifel, Carl Hanser Verlag, Munich, Germany, 2001, 141. 140. K. Schwarzenbach, B. Gilg, D. Müller, G. Knobloch, J.-R. Pauquet, P. RotaGraziosi, A. Schmitter and J. Zingg in Plastics Additives Handbook, 5th Edition, Ed., H. Zweifel, Carl Hanser Verlag, Munich, Germany, 2001, 1. 141. S.F. Laermer and P.F. Zambetti, Journal of Plastic Film & Sheeting, 1992, 8, 3, 228. 142. S. Al-Malaika, H. Ashley and S. Issenhuth, Journal of Polymer Science: A, Polymer Chemistry Edition, 1994, 32, 16, 3099. 143. K.D. Breese, J-F. Laméthe and C. DeArmitt, Polymer Degradation and Stability, 2000, 70, 1, 89.

422

Filled Thermoplastics 144. Polymer Durability: Degradation, Stabilization and Lifetime Prediction, Eds., R.L. Clough, N.C. Billingham and K.T. Gillen, American Chemical Society, Washington, DC, USA, 1995. 145. J. Viebke, Theoretical Aspects and Experimental Data on the Deterioration of Polyolefin Hot-Water Pipes, Royal Institute of Technology, Stockholm, Sweden, 1996. [PhD Thesis] 146. H. Bergenudd, P. Eriksson, C. DeArmitt, B. Stenberg and E.M. Jonsson, Polymer Degradation and Stability, 2002, 76, 3, 503. 147. K. Breese, Proceedings of the Polymer Degradation and Stabilisation Conference, Stockholm, Sweden, 1999. 148. P. Gjisman, Proceedings of the Polymer Degradation and Stabilisation Conference, Stockholm, Sweden, 1999. 149. Previously unpublished work 150. G. Scott, Polymer Degradation and Stability, 2000, 68, 1, 1. 151. M. Chanda and S.K. Roy, Plastics Technology Handbook, 3rd Edition, Marcel Dekker Inc., New York, NY, USA, 1998. 152. F.P. La Mantia in Plastics Additives: An A-Z Reference, Ed., G. Pritchard, Chapman and Hall, London, UK 1998. 153. M. Gilbert, A. Plaskett, M. Hancock and R.P. Higgs in Proceedings of PRI Conference on Plastic Pipes VII, Bath, UK, 1988, Paper No.32. 154. B. Terselius and J-F. Jansson, Proceedings of IUPAC Symposium: Interrelations between Processing, Structure and Properties of Polymeric Materials, Athens, Greece, 1982, p.451. 155. H-J. Pfister and A.E. Gruninger, Kunststoffe, 1980, 70, 9, 18. 156. R.A. Baker, L.L. Koller and P.E. Kummer in Handbook of Fillers for Plastics, 2nd Edition, Ed., H.S. Katz and J.V. Milewski, Van Nostrand Reinhold, New York, NY, USA, 1987. 157. N. Burditt, Proceedings of Moffis’91, Conference on Mineral and Organic Functional Fillers for Plastics, Le Mans, France, 1991, Paper No.1. 158. G. Fourty, Proceedings of Moffis ’91, Conference on Mineral and Organic Functional Fillers for Plastics, Le Mans, France, 1991, Paper No.59. 423

Particulate-Filled Polymer Composites 159. P.R. Plant, Proceedings of Fillers ’86 British Plastic Federation/Plastics and Rubber Institute Conference on Filled Plastics, London, UK, 1986, Paper No.7. 160. V.E. Malpass, J.T. Kempthorn and A.F. Dean, Plastics Engineering, 1989, 45, 1, 27. 161. F.A. Ruiz and C.F. Allen, Proceedings of the TAPPI Conference on Polymers Lamination and Coatings, Book 2, Westin St Francis, CA, USA, 1987, p.365. 162. M. Hancock, Plasticulture, 1988, 79, 3, 4. 163. H.M. Caesar, Plastverarbeiter, 1981, 32, 10, 1382. 164. F.J. Washabaugh, Modern Plastics International, 1988, 18, 3, 52. 165. L. Stigter in, Proceedings of the Corporate Development Consultants Conference on Thermoplastic Compounders in Western Europe, 1988, London, UK. 166. Anonymous, European Plastics News, 1987, 14, 9, 23. 167. D.C. Myers and P.S. Wilson, inventors; GE Plastics assignee; US 4,243,575, 1981.

424