TAKVEEN – KHWARZIMIC SCIENCE SOCIETY http://www.khwarzimic.org/takveen/index.asp

Environmental Waste Management and Plastics Recycling - An Overview Dr Ghulam S Ashraf, Brunel University, London. U.K. Life Member Khwarzimic Science Society

This study is concerned with looking into a wide range of research literature available around the area of environmental waste management, plastics recycling, and related valorisation processes (esp high density polyethylene) and documenting their main concerns, trends and findings.

1.1

Strategies For Recycling and Disposal Of Thermoplastics

Integrated resource management describes an holistic approach to waste management, which addresses the world-wide solid waste issue and has gained acceptance among policy makers and industry leaders. The advantage of this concept lies in its promise to reduce the use of natural resources in the basic manufacturing process, conserve energy in production and shipping, and minimise the final impact on the environment after the product is scrapped. Plastics have a favourable position relative to each of these areas, and one important element, plastics recycling, continues to progress with a wide range of old and new technologies. Because disposal of post consumer plastics is increasingly being constrained by legislation and escalating costs, there is considerable demand for alternatives to disposal or landfilling. Among the alternatives available are source reduction, reuse, recycling, and recovery of the inherent energy value 7

through waste-to-energy incineration and processed fuel applications. Each of these options potentially reduces waste and conserves natural resources.[15] Conventional (or mechanical) recycling includes flaking or granulating, washing, decontamination, and repelletizing of recovered plastic products so they may be fabricated into new, useful, and marketable products. Mechanical recycling can be economically viable, as with the recycling of HDPE milk jugs and PET soda bottles. The process does have some drawbacks. Among them is the requirement for a relatively clean source of post-consumer plastics, the need for efficient separation technology to obtain generically pure resin types, some current end-use market limitations, and often, a labour-intensive process. Developing And Reclaiming The Plastics Value Chain Feedstock Oil/

Monomers

natural gas

Polymers

Fabricators

(resins)

Marketplace - Consumer product companies - Industrial products - Retailers - Consumers

Advanced

Advanced

recycling

recycling

Conventional

technologies

technologies

recycling

(Depolymerization

(Depolymerization

- flake

to feedstock)

to monomers)

- wash - pelletize - Collection - Sorting - Processing

Figure 1.1: Overview of the Plastics Recycling Process A rapidly evolving group of advanced recycling technologies, depolymerization to monomers, involves collecting plastic products, sorting by resin type, and then depolymerizing the plastics back into their basic building blocks or monomers. The recovered monomers are then used to produce new resins of the same type (see figure 1.1). 8

Feedstock recycling, (or thermal depolymerization) recycles plastic products that cannot easily be broken down into their pure generic resin types, or have some level of contamination. A typical feedstock recycling process operates in an oxygen-free environment to prevent the plastics from burning, and results in the recovery of liquid feedstocks. These can then be used in place of virgin oil for the production of new plastic resins, fibres, and other valuable petroleum derivatives [15]. Pyrolytic gasification processes usually require harsh conditions, such as high temperatures or catalysts, and produce an olefin-rich hydrocarbon gas or a synthesis gas product. Thermal (or steam) cracking of plastics at elevated temperatures will produce ethylene and propylene in good yields but requires a sophisticated distillation to separate and purify the olefins. Synthesis gas, a mixture of carbon monoxide and hydrogen, is produced by the partial oxidation of plastics using high temperatures and a controlled amount of oxygen. The products can be used to synthesise higher value products, such as methyl-t-butyl-ether, methanol, and acetic acid. Several pyrolytic liquefaction processes also produce high yields of olefins when operated at elevated temperatures. High (over 40%) olefin yields have been achieved. Advanced recycling processes to produce either monomers or feedstocks have been demonstrated on a commercial scale. Technical issues seem surmountable, but economic and political hurdles remain [6]. The recovery of monomers or oil from waste plastic by a depolymerisation process is called tertiary recycling. Reprocessing scrap as part of a product production is defined as primary recycling, while melt recycling is considered secondary recycling, and burning with energy recovery is considered quaternary recycling. There are two types of tertiary recycling, chemical and thermal. Depolymerization of the plastic by chemical means is called solvolysis, and the process produces a monomer or oligomers. The decomposition of polymers by heat is called thermolysis. If the process is done in the absence of air, it is called pyrolysis or if done with a controlled amount of oxygen, it is called gasification. Pyrolysis will produce a liquid fraction, which is a synthetic crude oil and should be suitable as a refinery feedstock. The non-condensable fraction created during pyrolysis is normally used to provide process heat and any excess is flared. Gasification of plastic takes place at a higher temperature than pyrolysis and with controlled oxygen addition. The result is a syngas that is composed primarily of carbon monoxide and hydrogen. As a mixture, the syngas is valued only as a fuel. But if the gases are separated, the carbon

9

monoxide and the hydrogen are valued as chemical intermediates, which can have 2 to 3 times the fuel value of the mixture. A third form of thermolysis is hydrogenation, where the plastic is depolymerized by heat and exposed to an excess of hydrogen at a pressure of over 100 atmospheres. The cracking and hydrogenation are complementary, with the cracking reaction being endothermic and the hydrogenation reaction being exothermic. The surplus of heat normally encountered is handled by using cold hydrogen as a quench for this reaction. Hydrotreating can remove many heteroatoms. The resultant product is usually a liquid fuel like gasoline or diesel fuel. Thermolysis is a much more versatile and forgiving technology for tertiary recycling than solvolysis (see figure 1.2 below). It can handle mixed polymer waste streams along with some level of non-plastic contaminants. Solvolysis requires a relatively pure polymer stream and has little tolerance for contaminants; therefore the raw material preparation costs are larger. Thermolytic processes can be used for mixed polymer streams from municipal solid waste, auto shredder residue, medical waste, and mixtures of rubber and plastic. Some pre-treatment for sizing or certain contaminant removal may be needed, but it will be much less than that required for solvolysis.

10

The most commonly discussed processes for the thermolysis of waste plastic include hydrogenation, gasifiers, fluidized beds, kilns/retorts and degradative extrusion [11, 121]. Another procedure for recycling or decomposing waste plastic involves placing the post-consumer plastic in a diluent, such as hot oil, with a free radical precursor, such as PVC or PU, at a low temperature. The thermal decomposition (or pyrolysis) reaction lasts for 1 hour at 375oC and useable products, such as distillate, coke, and oil are recovered [12]. Pure or mixed, clean or contaminated plastic wastes, can be cracked (i.e. broken down) to smaller molecules in low pressure plasmas at low substrate temperatures, and then recycled [8, 63]. Recycling economics depends on finding the recyclates most valuable form: resin or energy. Integrated resource management provides the greatest number of options for effectively achieving minimal use of natural resources, combined with maximum conservation and value recovery [15]. HDPE is primarily used for the production of bottles especially for food products, detergents and cosmetics, containers, toys, houseware, fuel tanks and industrial wrapping and film, sheets, gas and waste pipes. However recycled HDPE is used for producing industrial bags, detergent bottles, pipes, containers and wood substitutes e.g. animal flooring and fencing [112]. Microbial and Polymer Degradation Biodegradation of general nonmedical materials can be divided into microbial and chemical degradation. Chemical degradation includes wind and rain erosion, oxidation, photodegradation, acid/base water, and thermal degradation [92]. Alternative methods for detoxifying potentially harmful plastic contaminants include biodegradation (using micro organisms) or degradation of the polymer itself. One way to deal with the problem of polymer wastes is to make polymers degradable, however this seems to eliminate the greatest asset of these materials, namely their durability. It also wastes the time, effort, and energy put into making materials in the first place. Sometimes, the degradation products can be more of a problem than the polymers themselves. Water pollution, stems from toxic substances leaching out of landfilled materials and is a growing problem for the nation’s 6000 landfills. So on the surface, degradability is a poor 11

second choice to recyclability, given that degradability does not necessarily make materials disappear; it may only make them physically or chemically smaller. However, not all synthetic materials are recoverable or even worth recovering. The materials found in disposable diaper linings, plastic grocery sacks, six-pack beverage rings and agricultural films are a few such examples. So for products such as these, degradable polymers could solve a difficult problem, if the degradation products are found to be ecologically benign. One method which can initiate the degradation process is to make a polymer photolabile so that it begins degrading when exposed to the ultraviolet (UV) component of sunlight e.g. a photolabile polymer is used in degradable six-pack rings, where the product incorporates small numbers of carbon monoxide molecules into a LDPE polymer chain; the UV-sensitive carbonyl joints break when struck by UV light. The polymer has the same physical properties as “pure” LDPE but begins falling apart after six-hours of exposure to sunlight. Six-pack rings made of the material, called E/CO, fall apart within a week. E/CO resin costs about 15% more than conventional polymers. A slightly different product, called Ecolyte, is used in polyethylene grocery sacks. It costs about 10% more than unmodified resin. Photosensitive additives such as organometallic compounds, can also make a polymer fall apart by initiating a chemical chain reaction on initial exposure to sunlight, breaking open any polyolefin chain, continues in the absence of sunlight, so that the polymer will fall apart even if it is later buried in a landfill. Some synthetic polymers can serve as food for micro-organisms that live in soil and water. ICI Americas has developed a thermoplastic resin, called PHBV random copolymer that is stable in air and sunlight but falls apart when it is exposed to bacteria in soil, water, or a sewage-treatment plant. The polymer is actually made by certain soil bacteria when fed a diet supplemented with the monomer 3hydroxyvalerate. The resulting thermoplastic is a highly crystalline, stiff material that can be processed into film or blown into bottles. Incorporating a biodegradable filler, such as starch, into the polymer formulation is also possible. When the polymer is buried, microbes detect the starch and release enzymes that convert the starch into simple sugars that the microbes then absorb as food. This loss of filler causes the polymer product to disintegrate into smaller pieces of polymer. As part of the starch-metabolising process, however, the

12

released enzymes produce harsh substances known as superoxides and peroxides. These chemicals can then attack and degrade the polymer chain [32]. Micro-organisms (such as ATCC #53922, a mixed culture of Pseudomonas picketti and a Bacillus species) can also be used to degrade and remove coatings, such as paint, epoxy resins, and combustible polymeric coatings, from the surface of aluminium beverage cans [33, 34, 2]. Paint and coatings, which are frequently used on plastics from durable goods for both functional and decorative reasons, represent challenges to both the identification and recycling of plastics. These coatings, if not removed, can cause property reductions in recycled plastics from stress concentrations created by the coating particulates and/or degradation of the coating leading to chemical degradation of the plastic during reprocessing. The level of potential property reduction depends on the combination of the type of plastic substrate, coating type and coating thickness. Residual paint and coatings can also affect appearance, properties and surface characteristics. The aerospace industry has developed numerous abrasive paint removal techniques in response to environmental concerns with solvent stripping methods. These techniques, however, are more applicable to very large whole parts and a manual approach. Several more continuous and automated abrasive techniques have been investigated using large flakes of coated plastics in an effort to identify a dry coating removal technique, but so far all have proven unsatisfactory. High temperature steam shows promise and is being investigated further [62]. HDPE milk jugs can be recycled by a cold water wash step to remove bacteria-generated odour and in the separation stage utilises a three-compartment sump to separate HDPE using water as the medium, and two further stages using heavier media for separation of PVC cap liners and aluminium from the PET [35, 26]. Thermal Recycling and Degradation Solid household waste is made up of a mixture of largely polyolefin-based resins, such as HDPE, LDPE, PP, PET, etc., i.e. resins that have melt temperatures ranging between 110oC and 160oC. A procedure is described whereby only those resins melting at a temperature below a pre-determined value, typically 170-180o C, are melted and homogenised in an extrusion process. The remaining resins, in particular PVC, PET and Teflon compounds, are not melted, but are on the contrary

13

inglobated, i.e. filled in the resin mass, so that they act as inert fillers, owing also to the action of the alloying agents and the other additives that are present in the compound fed into the extruder [14]. There are many economical and ecological advantages to recycling waste plastics. Discarded waste plastics are available at comparatively negligible costs since they are essentially garbage. Generally, waste plastics have lower tensile strength and relatively poorer flex and thermal properties when compared to new plastics fresh off the production line. Since waste polyolefins are not completely biodegradable, they have life cycles, which are much longer than conventional wooden building materials. In addition, construction materials made from waste polyolefins have chemical, biological, mechanical, electrical and flame resistance properties superior to counterpart natural lumber products (see U.S. Patent no. 4,003,866 which describes construction material made from waste thermoplastic resins and other non-plastic fillers) [8]. A process called ET-1 has been described for producing substitute construction materials from waste plastics, transforms mass plastics directly into a large range of moulded end products without presorting of any kind, and without the need for inserting any additives to the intermediate resin. The ET-1 process melts resins in a short-screw extruder, and then forces the heated extrudate into a series of linear moulds, which are then mounted onto a turret. The heated moulds cool as the turret rotates them through a water filled tank. The end products are air-ejected from open ends of the moulds. The ET-1 end product is essentially a solid with randomly spaced voids. It has a typical density slightly higher than 1.0 g/cm3, making it heavier than most natural timber products (ordinarily, wood floats on water because it has a density less than that of water). The length of the end products is limited by the size of the mould into which the extruder can inject and fill with resin. Practically, as construction material, these end products are generally difficult to cut, saw, nail or drill holes into [17]. Further improvements to this construction lumber product were made by adding to the waste plastic an alkali metal bicarbonate (e.g. NaHCO3); a molar equivalents of the bicarbonate salt and of a saturated fatty acid, such as a sodium bicarbonate/solid stearic acid combination. The extruded products have a density ranging from 0.4 g/cm3 to 0.9 g/cc, in contrast to conventional recycled waste plastic compositions, which generally have densities in excess of 1.0 g/cm3. The foaming serves to reduce the density in the final product thereby saving the amount of raw materials required for a given volume and increases the strength-to-weight ratios of the end-products.

14

The compositions may also include fibrous reinforcing agents (e.g. strands of glass fibre) for providing strength and improved impact properties to the moulded end products, and filler materials (e.g. CaCO3, asbestos, mica) for providing stiffness, additional strength, and enhanced mechanical and heat resistance. Appropriate coupling agents, such as silanes or organo-titanes, can also be used to enhance reinforcement. The use of the foaming agent enables the production of a recycled plastic, which has wood-like densities evenly and continuously, distributed throughout the end-product, and which can be extruded to any desirable dimension. These composites can be nailed, screwed, sawed and bolted with conventional woodworking tools and skills, and unlike wood, these products will not rot and degrade when exposed to the environment and the strength of the product will remain constant whether wet or dry. Ultraviolet absorbers and antifungal agents may also be added depending on the intended use of the final extruded product. In addition to extrusion, the compositions of this invention may be injection moulded to produce commercially usable products. Consequently, other additives can be used, including impact modifiers, viscosity stabilisers, processing aids, and colouring agents [18]. Many other examples of such composite materials exist, whereby recycled or post-consumer plastic has been combined with rubbers, elastomers, virgin plastic or other appropriate additives [19-21].

1.2

Polyolefins

HDPE is a linear polymer with the chemical composition of polymethylene, (CH2)n, and is defined by ASTM D1248-84 as a product of ethylene polymerisation with a density of 0.94 g/cm3 or higher. One of the chain ends in an HDPE molecule is a methyl group; the other chain end can be either a methyl group or a double bond (usually the vinyl group). The number of branches in HDPE resins is low, at most 5 to 10 branches per 1000 carbon atoms in the chain. Even ethylene homopolymers produced with some transition-metal based catalysts are slightly branched; they contain 0.5-3 branches per 1000 carbon atoms. Most of these branches are short, methyl, ethyl, and n-butyl (6-8), and their presence is often related to traces of α-olefins in ethylene. The branching degree is one of the important structural features of HDPE resins [76]. 15

HDPE crystallises from the melt under typical conditions as densely packed morphological structures known as spherulites. Spherulites are small spherical objects (usually from 1 to 10µm) composed of even smaller structural subunits: rod-like fibrils that spread in all directions from the spherulite centres, filling the spherulite volume. These fibrils, in turn, are made up of the smallest morphological structures distinguishable, small planar crystallites called lamellae. These crystallites contain folded polymer chains that are perpendicular to the lamella plane and tightly bend every 5 to 15nm. Lamellae are interconnected by a few polymer chains, which pass from one lamella, through a small amorphous region, to another. These connecting chains, or tie molecules, are ultimately responsible for mechanical integrity and strength of all semicrystalline polymer materials. Crystalline lamellae offer the spherulites rigidity and account for their high softening temperature, whereas the amorphous regions between lamellae provide flexibility and high impact strength to HDPE articles. [76]. Polyethylene is semi-crystalline, but introducing an alkene comonomer in the side chains reduces crystallinity. This in turn has a dramatic effect on polymer performance, which improves significantly as the side chain branch length increases up to hexene, and becomes less significant with octene and longer chains. It is by manipulating this side branching that companies have made various grades of PE suitable for different applications. Grades of PE There are three main types of commercial water-grade polyethylene (PE) resins, which are generally available. Type (I) corresponds to low density polyethylene (LDPE) with a density range of 910-925 kgm-3 and is produced by the high pressure process. Type (II) is classified as medium density polyethylene (MDPE) with densities in the range of 926-960 kgm-3. Type (III) corresponds to high density polyethylene (HDPE) and has a density range of 941-965 kgm-3. MDPE and HDPE are produced by a low pressure process utilising either metal oxide catalysts (Philips Process) or aluminum alkyl or similar materials (Ziegler Process). In the UK, LDPE is used for low pressure applications whereas MDPE and HDPE are used in high pressure pipe systems. 80% of all mains and 90% of all services are made of HDPE. The usage of PE for pipe related applications accounts for about 3-4% of the total PE consumption worldwide. The total quantity of PE resin consumed globally for pipes has been estimated at 1,080,000 tonnes for 1995 with European consumption accounting for half of global. Effect of Processing on Polyethylene 16

An extrusion process manufactures pipes. In principle the extrusion process consists of metering polymer usually in the granular form into a heated barrel with a rotating screw. The rotation of the screw causes shear mixing of the molten polymer and also moves the polymer up the barrel where it is forced under pressure through the breaker plate and into an annular die. The resulting extruded pipe is calibrated by means of water cooled sizing die or vacuum sizing. Both the short term and long term mechanical properties of polyethylene are dependent on the molecular weight and degree of branching of the polymer. As with other polymers, the short term properties are also dependent on the rate of testing, the temperature of the test, the method of the specimen preparation, the size and shape of the specimen and the conditioning of the samples before testing. In general, increasing the density or testing rate or decreasing temperature causes an increase in modulus and lowers the ductility. Morphology The general morphology of crystalline polymers has been extensively studied. Polymers are considered to be either crystalline or amorphous, although they may not be completely one or the other. Crystalline polymers are more rightly termed semi-crystalline as the measured densities for perfect materials differ from those obtained for such. The dominant and most widespread morphological entity formed when polymers crystallise from the melt under normal conditions is the spherulite. Spherulites consist principally of chain folded lamellae radiating from a central point. In polyethylene, spherulites may vary in size from a fraction of a micrometre to several millimetres in diameter, depending on the cooling rate from the melt. It has been suggested that semi-crystalline polymers are made up of spherulites, which are composed of lamellae. However, if the spherulites are to be considered to be discrete entities, the mechanical properties of such polymers would be extremely poor due to fracture along inter spherulite boundaries. It has therefore been suggested that lamellae are joined together by inter crystalline links or tiemolecules. Tie-molecules have been used to explain both ductile and brittle modes of failure. During ductile deformation, the tie-molecules stretch when a tensile load is applied to the lamellae. At a critical point a limit may be reached, where the lamellae break up into smaller units. In general, the tie-molecules act to 17

produce ductile deformation. In brittle failure, the analysis follows the above where the failure occurs over a much longer period and at lower stress levels. However, the stress needed to obtain fibre pullout is not attained due to lower stress levels. Tie-molecules begin to untangle and relax over long periods. After a critical period of time, most of the tie-molecules are presumed to have untangled, leaving behind only a few tie-molecules, which cannot maintain the applied stress where the material consequently fails in a brittle manner.

Figure 1.3: Schematic illustration of the general molecular structure and arrangement of typical semicrystalline materials. Other structural parameters affecting tie-molecules are as follows:(i) molecular weight - the longer the polymer chain, the greater the number of tie-molecules and therefore the greater the number of tie-molecule entanglements; (ii) co-monomer content - small amounts of monomer tend to inhibit crystallinity due to the formation of short branches, therefore producing improved brittle fracture resistance; 18

(iii) degrees of crystallinity - the higher the crystallinity content in a material, the fewer the amorphous intercrystalline tie-molecules and hence increased brittle behaviour; (iv) lamellae orientation - orientation of lamellae perpendicular to the tensile stress direction results in a greater tendency for the interlamellar failure compared to the situation where the lamellae are orientated parallel to the applied stress direction. HDPE: linear molecule, ca. 4 to 10 short side chains per 1000 C atoms

LDPE: long chain branching

LLDPE: linear molecule, ca. 10 to 35 short side chains per 1000 C atoms

Figure 1.4: Schematic illustration of the molecular structure of different polyethylenes In polymers, the residual stresses (or frozen-in strains) resulting from processing can be sufficient to cause cracking in the presence of many organic liquids (c.f. crazing and the term environmental stress cracking (ESC) tends to be used. This also affects the response of polymers to external loading e.g. creep rupture and fatigue [56]. Degradation and Stabilisation of Polyethylene The oxidation of PE is a well-researched topic where both photo and thermal oxidation can occur. On exposure to sunlight most polymers can be degraded to some extent. The process of degradation by light, natural or artificial is called photo-oxidation. The mechanism for oxidation of simple hydrocarbons was established many years ago and it is now generally accepted that polyolefin oxidation follows a 19

quite similar pattern. The mechanism involves an initiation step in which a hydrogen atom is removed from a polymer molecule creating a free radical Ro. Propagation then follows in a series of reactions. The first of these is a rapid reaction of Ro with oxygen to form a peroxy radical ROOo. This is then followed by the rate controlling reaction, the abstraction of H from the same or another polymer molecule by the ROOo radical. Evidently each propagation step produces another polymer radical Ro which is essentially the same as the initiation reaction. Thermal Degradation and Analysis Many analytical techniques can be used to monitor the thermal degradation of polymers. As a result of irreversible chemical reactions, changes in the molecular composition during degradation provide the means for following this deterioration of the polymer. The analytical techniques for monitoring oxidation can be divided into two main areas, namely spectroscopic and thermal analysis. The measurement of the carbonyl index using infrared spectroscopy is well known and widely used to study thermo-oxidative degradation of polymers. Thermal analysis is used to measure the stability of the polymer under processing conditions as well as to evaluate the long-term degradation behaviour. Differential scanning calorimetry (DSC) is a widely used analytical tool in the study of polymers. The theoretical aspects of the DSC are well chronicled and its quantitative capabilities have been established. The basic technique involves heating an aluminium pan containing the sample and an empty reference pan. The two pans are maintained at the same temperature. By monitoring the difference in energy input required to do this, it is possible to observe both endothermic and exothermic thermal events. Traditional stabiliser systems use a combination of phosphites and sterically hindered phenols. These antioxidants scavenge the oxygen-centred radicals, but have some shortcomings when processing temperature increases or the end-use application becomes more demanding – requiring greater quantities of stabiliser and increasing costs. Ciba, the world’s leading producer of antioxidants, has come with a new range of stabilisers to combat these problems. The Irganox HP series of additives is also based on sterically hindered phenol and phosphite processing stabilisers, but they are boosted by a small amount of lactone stabiliser (3-arylbenzofuran-2-one). Lactones have exceptional stabilising activity, because they behave as powerful hydrogen donors and are effective scavengers of many free

20

radicals. Most importantly, the addition of the lactone stabiliser allows considerably less stabiliser system to be used to achieve the same result [58]. Crystallinity and orientation The tendency of a particular polymer to crystallise depends not only on the cooling rate but also on the flexibility and regularity of its chain structure and the strength of inter-chain bonding. Linear HDPE, for example, has a regular backbone of great flexibility, impeding the disentanglement needed for crystallisation, but its regularity allows close packing and HDPE crystallises readily by upto 90%. Branched LDPE, no less flexible, can only attain around 60% crystallinity because the side-chains interfere with regular packing. Isotactic and, to a lesser extent, syndiotactic polymers are regular in structure and usually crystallise, while polymers with irregularly spaced bulky side-groups such as PMMA are necessarily amorphous, while isotactic and syndiotactic forms are semicrystalline; most commercial grades are based on the isotactic form, with a crystallinity of about 70%. For a given polymer of given structure, the degree of crystallinity is also affected by molecular weight, since longer chains are more difficult to draw from amorphous zones into crystallites. For copolymers, regularity is again the key to successful crystallisation. Thus alternating structures (ABABAB) and copolymers with large block lengths can generally crystallise whilst random, graft and block copolymers lacking longrange regularity cannot [52]. Property

Low Density

High Density

Density (g/cm )

0.91-0.925

0.941-0.965

% Crystallinity

42-53

64-80

Melting Temperature (oC)

110-120

130-136

Tensile Modulus (MPa)

17-26

41-124

Tensile Strength (MPa)

4.1-16

21-38

3

Table 1.0: Influence of Crystallinity on Properties for LDPE and HDPE

21

Figure 1.5: Schematic representation of the crystalline structure of polyethylene

Figure 1.6: Influence of degree of crystallinity and molecular weight on different properties of polyethylene

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Figure 1.7: Thermal diffusivity as a function of temperature for various semi-crystalline thermoplastics Processing Two other characteristics of thermoplastic melts significantly affect their processing. Melt elasticity is most visible as die swell after extrusion: the melt having been deformed by radial compression to force it through the die, springs back on exit to increase the diameter of the extrudate. Melt elasticity varies widely between polymers, reflecting differences in the persistence of entanglements in the melt. Branched polyethylene is much more elastic in the melt state than linear polyethylene, since individual molecules find it much more difficult to reptate and relieve applied stress. Polymers melts are relatively brittle. Their tensile strengths, of the order of 1 MN/m2, are not very different from those of other liquids, but constitute a problem because high viscosity requires large stresses to be applied during forming. A common manifestation of melt fracture is the break-up of extrudate emerging too rapidly from an extruder die, under the tensile stresses which accompany die swell [52]. Consequently the effect of contaminant presence on processing properties must be taken into consideration. Thermal analysis Dollimore and Lerdkanchanaporn [64] have provided an extensive review detailing research carried out involving a whole series of thermal techniques such as DSC, TGA, DTA in combination with other instruments and a variety of materials have been mentioned. Higher engine room temperatures and longer warranty periods for automobiles in recent years make it imperative for automotive designers involved in plastic parts to have a very good understanding of the thermo-oxidative reactions and their effect on the life of plastic parts. Nohara [65] has detailed a method for rapidly predicting the thermal deterioration and the service life using thermal analysis i.e. TGA, whereby the weight of the material (nylon 6,6 in this case) is measured under constant temperature rise 23

over a period of time. The fundamental premise of service life prediction is that the change of characteristic of a material when it is exposed to heat is strongly related to the fraction of the material that has undergone a change in chemical structure due to the thermal deterioration reaction. Thermal deterioration of plastic material during service life is attributed to auto-oxidation due to free radical chain reaction. There is considerable interest in preparing extremely strong and stiff polymeric materials, since the vast majority of commercially produced polymeric materials such as polyolefins have strengths and stiffness much lower than the inherent ones of the carbon-carbon bond in polymeric molecules. The properties of polymers can be increased by forming almost perfectly oriented, extended chain structures in them. This is the so-called self-reinforced technique of polymeric materials. Self-reinforcement of flexible-chain polymers such as polyethylene and polypropylene during extrusion can be achieved by two routes: solid state deformation and melt deformation. In the latter the high property parts are produced by inducing oriented crystallisation in a flowing polymer melt. The melt orientation is induced from the extensional flow, and the induced extended-chain crystallisation is retained by exactly controlled cooling under a higher pressure. Extensional flow is much more effective than shear flow in causing molecule orientation for a given level of stress. Therefore, an extrusion die with a convergent channel must be used. The flow-induced crystallisation takes place in an extremely narrow temperature region, and so the melt temperature within the crystallisation area must be controlled with high precision [65]. Thermal degradation of polyethylene gives rise to a continuous spectrum of saturated and unsaturated hydrocarbons from C2-C90, with lower temperatures favouring larger fragments [81]. 1.3

Influence of chemicals on polyolefins

The effects of multiple processing on the material characteristics of thermoplastics has been studied using both uncontaminated thermoplastics (such as polycarbonate, glass fibre-reinforced poly(butylene terephthalate) (I), an elastomer-modified I-polycarbonate blend, and a polypropylene-EPDM rubber blend) and thermoplastics contaminated with dust, paint, and other polymers (e.g. ABS polymer). The mechanical properties of the recyclates are in many cases better than is often assumed. However, the effect of multiple processing as well as of contaminants on the mechanical properties is material 24

dependent. For the optimum utilisation of the material properties of recyclates, the use-specific detection of the material parameters, e.g. the detection of the recyclability of structural parts, is necessary. Specific solutions for some recycling problems are discussed in this research paper [36]. Experimental investigations have shown that the extrudability (expressed as screw torque, mass intensity of flow, melt flow index, and Barus effect) of recycled polyethylene (I) (9002-88-4) bags contaminated with mineral fertilisers (urea, NH4NO3, nitro-chalk, superphosphates) depend on the amount and type of fertiliser but not on the particle size of the fertiliser or moisture in the recycled sample. The screw torque and mass intensity of flow decreased and the Barus effect increased with increasing amount of fertiliser contaminant, the changes being more pronounced at low contaminant concentration. The melt index of recycled polyethylene decreased in the presence of urea, NH2, NO3, and superphosphates but increased in the presence of nitro-chalk, apparently due to polyethylene degradation [37]. Melted waste plastic materials such as polyethylene, in which quantities of oil have been entrapped can be mixed with a given dose of a neutralising agent, e.g. calcium hydroxide, at temperatures of 220-300 o

C, to neutralise the contaminant, thus allowing the bulk polymer to be recycled and to be re-moulded

into other plastic products, or disposed of in a landfill [40]. Demand for recycled HDPE exceeds supply and much of the recycled resin goes into applications where physical properties are not critical, such as plastic lumber and selected injection moulding applications. Recycled HDPE is also being considered in “higher-end” applications such as blow moulding, where the material is utilised as a layer in a coextruded container. Experimental evaluations of post-consumer recycled HDPE have been carried out using, two homopolymer samples and two copolymer samples provided by one plastics recyclers/converters, with another one homopolymer sample and one copolymer sample provided by a different plastics recyclers/converters. Results indicate that post-consumer recycled HDPE exhibits adequate processability and a balance of physical properties adequate for a number of non-critical applications in blow moulding, summarised as follows:(a) none of the recycled samples exhibited the colour of virgin homopolymer. (b) no evidence of melt fracture was encountered during moulding of bottles from the recycled plastics.

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(c) the melt index shifts and swelling indicated that either shear modification or some other form of degradation occurred before or during recycling. (i)

The recycled copolymers exhibited approx. 25% increase in swell and

0.10 - 0.20 g/10 min increase in melt index from the nominal 0.35 g/10 min

for

the

virgin

copolymer HDPE. (ii) For homopolymer recycle, an approximate 25% decrease in swell accompanied a 0.10- to 0.20- g/10 min decrease in melt index from a nominal 0.70g/10 min common for this resin type. (d) the odour and contamination levels in the recycled polymers still need to be reduced to successfully utilise the recycled ethylene polymers in blow moulding. Further research to rationalise the observed losses in certain physical properties of the recycled resins is still in process [27]. It has been further shown experimentally that the type of recycled plastic used in multilayer containers, whether copolymer or homopolymer, has a significant influence on environmental stress crack resistance (ESCR) performance, i.e. the copolymers usually outperform the homopolymers. This fact can be reversed if the copolymer is more contaminated than the homopolymer, as the copolymer is more difficult to clean. The concentration of recycled plastic in the middle layer has minimum effect on the container performance. The 3-layer bottle is equivalent in ESCR performance to the virgin monolayer bottle, and the bottles show a slight improvement in ESCR with a thicker inner wall [28]. Rotational moulding is a method for manufacturing hollow articles. It differs from most other processes because the molten plastic takes the shape of the mould under very low stresses and cools at a slow rate relative to, say, injection moulding. For polyethylene well-developed spherulitic morphologies are formed under such conditions. Rotomoulded polyethylene commercial parts are often coloured with pigments. Due its versatility and economy, dry blending of the pigment with the polymer powder occurs by tumble mixing in the mould or by high speed mixing in turbo-blenders. The mixing efficiency of the dry blending is poor and as there are no stresses to assist with dispersion of additives in rotational moulding, the pigment tends to concentrate at the polymer particle boundaries. Comparing extrusion compounding of pigments to dry blending lead to improved impact strength of rotomoulded polyethylene [66]. However Crawford et al [67] observed that the improvement imparted by extrusion compounding depends on the type of pigment and on the pigment-concentration level. The 26

nucleating effect of pigments and other additives on the crystalline structure of polymers depends on the polymer/additive system. For example, titanium dioxide acts as a nucleator when added to polypropylene but does not show that effect when added to polyethylene. Potassium stearate, on the other hand, has the opposite effect. The use of nucleating additives causes the spherulite size to decrease and the crystallisation temperature to increase with respect to the base polymer [68]. Environmental stress cracking (ESC) relates to the premature failure of polymeric materials in real life services under both small loads and the presence of active environments such as detergent solutions at about room temperature. In particular, ESC limits the lifetime of polyethylenes used in critical applications such as pipes, containers, linings under landfills, geomembranes, etc. The failure is associated with long-term low-level loading conditions and it is considered to develop more rapidly in the presence of certain chemical environments. This mode of failure is characterised by the presence of ‘macroscopic’ cracks in the material with a microscopic fibrous nature at the fracture surface. These cracks arise from the previous existence of a ‘craze’ ahead of the crack, i.e. point of stress concentration, which develops further with time [71]. ESC characteristics of polymers are affected differently depending upon the environment. For example, water seems not to have a marked effect on ESC polymer properties [72] whilst short chain length alcohols seem to swell the polymer by diffusing inside [73]. Stress cracking is also a major concern in the case of plastic containers that are used to house industrial chemicals. Determining stabilisation methods, or producing mouldings with a low affinity to stress cracking is of importance to the plastics industry. Commonly, heavy alkali solutions (such as industrial strength soaps) are found to promote stress cracking, and typically, temperatures above 50oC accelerate the induction time to failure. It is known that for any given polymer, the stress/strain behaviour is based on the polymer’s thermal history. The thermal history of a specimen controls the crystallinity and crystalline texture, which in turn affects the time to failure. This behaviour can be marked for a crystalline material such as HDPE, which being of syndiotactic structure, and of good stereo regularity, is often highly crystalline. The crystalline structure, and rate of crystal growth, will determine a polymer’s stress crack behaviour. Large crystalline structures, molecular orientation effects, and lack of even crystal growth can all accelerate stress cracking. Thus, small, even, and regular crystal growth is desirable for a resilient moulding. Large crystalline structures are undesirable because, while being strong in themselves, they have a line of weakness along the grain boundaries. These 27

boundaries develop, as crystals grow, until they are physically restrained from further growth by meeting with other crystals. The individual crystals are held together by molecular entanglement and, at the boundaries, Van der Waal forces couple the crystalline structures to one another. This type of structure is susceptible to stress cracking because when a stress is applied, the moment of force (or hinge for movement) will be exerted along the grain boundaries. Thus, contact with an incompatible substance will lead to a reduction in the dipole forces, and subsequent failure. Molecular orientation effects also contribute to ESC. When a heated polymer is stretched in any particular direction, the individual polymer molecules will align in the direction of the deforming stress. Hence, if the polymer is then rapidly cooled, the orientation will be ‘frozen’ in. This will affect stress cracking for two reasons: firstly, the strained molecules will try to regain an unstressed state, hence causing irregularities in the polymer structure. Secondly, there will be increased crystallinity along the direction of the pull and therefore more grain boundaries, and the susceptibility to breakdown of the dipole forces. Additionally, lack of crystal growth also affects stress cracking. A lack of even crystal growth leads to large amorphous regions. There will be a lesser degree of chain entanglement, and more dipole forces holding the polymer structure together. These dipole forces will be susceptible to attack by an incompatible substance, which may permeate the structure in a similar manner to that of diffusion. This will cause a reduction in the dipole forces, which in turn may lead to chain disentanglement, or chain scission. Thermal expansion in polymers will cause a reduction in dipole forces and by causing molecules to move away from one another, there will be larger interstitial gaps. This reduction of dipole force, and the increase in both number and size of interstices, will allow an incompatible agent more chance to find, or utilise, a weak spot [75]. When the branching degree in HDPE increases, its crystallinity and the thickness of its crystalline lamellae decrease. This change brings about significant alterations in the mechanical properties of HDPE, two of the most strongly affected are tensile strength and tensile elongation. HDPE with increased degree of branching are softer and more elastic. An increase in the branching degree from 2 to 10 per 1000 carbon atoms results in a decrease of the resin tensile strength but a large increase in tensile elongation. Highly oriented HDPE is approx 10 times stronger than non-oriented polymer because the mechanical strength of a polymer is determined by the number of intercrystalline links: the tie chains anchored in adjacent anchored in adjacent crystallites and binding them together. Because these links are few, intercrystalline boundaries are the weakest elements of the polymer structure. However, since the process of polymer stretching and the dismantling of its original morphological 28

elements is accompanied by a significant increase in the number of intercrystalline chains, polymer strength thus increases greatly. Similarly, orientation significantly increases polymer rigidity; thus the elastic modulus of highly oriented HDPE filaments is increased about six times [80, 87]. Studies with HDPE have shown that samples with the short chain branching content above 3 branches per 1000 carbon atoms showed a significantly different thermal behaviour from those with less than 3 branches per 1000 carbon atoms [86]. Numerous studies of plastic straining of semicrystalline polymers in extensional flow have now established that important morphological reorganisation occurs as a result of deformation, with the structure changing from spherulitic to a highly oriented one consisting of alternating crystalline and amorphous layers. At the same time, crystallographic axes of the crystalline lamellae and macromolecular chains of the amorphous component rotate and tend to align preferentially with respect to principal axes of macroscopic deformation. Thus it has been established that, when plastically deformed, semicrystalline polymers develop three important types of texture: (i)

a crystallographic texture, due to preferential orientation of crystallographic axes in the crystalline lamellae;

(ii)

a morphological texture in the crystalline lamellae;

(iii)

a macromolecular texture in the amorphous phase, promoted by molecular alignment with the direction of maximum stretch.

The evolution of texture with large plastic deformation strongly affects the macroscopic mechanical behaviour of semicrystalline polymers [84, 85]. It has been found that low molecular weight rolled HDPE possesses high modulus and yield stress in the roll direction and showed brittle fracture in the direction perpendicular to the roll direction. The morphology of the internal surface showed a transitional change from fibrillar structure for low molecular weight samples to a smooth surface for high molecular weight samples. It is suggested that samples with high molecular weight possess more entanglements among the tie chains connecting the lamellar blocks. This results in higher orientation of the three crystallographic axes a, b and c, along the thickness, transverse and roll directions of the sample, respectively [111]. HDPE is a saturated linear hydrocarbon and, for this reason, exhibits very low chemical reactivity. The most reactive parts of HDPE molecules are the double bonds at chain ends and tertiary CH bonds at 29

branching points in polymer chains. Because its reactivity to most chemicals is reduced by high crystallinity and low permeability, HDPE does not react with organic acids or most inorganic acids such as HCl and HF. Concentrated solutions of H2SO4 (>70%) at elevated temperatures slowly react with HDPE with the formation of sulfo-derivatives. HDPE can be nitrated at room temperature with concentrated HNO3 (approx 50%) and its mixtures with H2SO4. Under more severe conditions, at 100150oC, these acids decompose the polymer and produce mixtures of organic acids. HDPE is also stable in alkaline solutions of any concentration as well as in solutions of all salts, including oxidising agents such as KMnO4 and K2Cr2O7. At room temperature, HDPE is not soluble in any known solvent, but at a temperature above 80-100 oC, most HDPE resins dissolve in some aromatic, aliphatic and halogenated hydrocarbons [76, 82, 87, 92]. Oxidising acids such as nitric acid will cause scission at weak polymer links, as seen by attacks on the folds of polyethylene crystals to produce α,ω-dicarboxylic acids. Acids and alkalis will also hydrolyse ester and amide linkages in the polymer chain [92]. At elevated temperatures, oxygen attacks HDPE molecules in a series of radical reactions. These reactions reduce the molecular weight of HDPE and introduce oxygen-containing groups, such as hydroxyl and carboxyl groups, into polymer chains. Other oxidation products are low molecular weight compounds such as water, aldehydes, ketones, and alcohols. Oxidative degradation in HDPE is initiated by impurities, which are mainly catalyst residues containing transition metals, e.g. titanium and chromium. Since thermo-oxidative degradation can occur during pelletisation and processing of HDPE resins, molten resins are protected from oxygen attack by incorporation of antioxidants (radical inhibitors) at 0.1-1.0% by weight concentration such as napthylamines or phenylenediamines, hindered phenols, quinones, and alkyl phosphites [76, 87]. Since typical small molecules and large molecules with molecular weights less than a critical value required for chain entanglement are weak and are readily attacked by appropriate reagents. Thus the following properties are related to molecular weight – melt viscosity, tensile strength, modulus, impact strength or toughness, and resistance to heat and corrosives are dependent on the molecular weight of amorphous polymers and their molecular weight distribution. In contrast, density, specific heat capacity and refractive index are essentially independent of the molecular weight values above the critical molecular weight. Most thermodynamic and colligative properties are related to the number of particles 30

present and are thus dependent on Mn. Mn values are independent of molecular size and are highly sensitive to small molecules present in the mixture [87]. Experiments with LDPE showed that high processing temperature and high residence times strongly enhance the degradation processes and reduce the mechanical properties, in particular the elongation at break. Greater thermomechanical degradation, better homogenisation and better dispersion of the non-polymeric impurities are responsible for this behaviour. It was also found that by introducing additives, like antioxidants (phosphite stabiliser), inert fillers (CaCO3) and impact modifiers (calcium silicate), improves mechanical properties (especially elastic modulus and elongation at break) approaching those of virgin polyethylene. [83]. Plasticisers are incorporated in a material to increase its workability and flexibility, which leads to lower melt viscosity, elastic modulus and glass transition temperature of a plastic. The effect of plasticisers may be explained by the lubricity, gel, and free volume theories. The first states that the plasticiser acts as an internal lubricant and permits the polymer chains to slip by each other. The gel theory, which is applicable to amorphous polymers, assumes that a polymer such as PVC has many intermolecular attractions, which are weakened by the presence of a plasticiser such as dioctyl phtahalate. It is assumed that the addition of a plasticiser increases the free volume of a polymer and that the free volume is identical for polymers at Tg. the presence of bulky groups on the polymer chain increases segmental motion. Thus, the flexibility increases as the size of the pendant group increases. However, linear bulky groups with more than 10 carbon atoms will reduce flexibility because of side-chain crystallisation when the groups are regularly spaced [87]. Grafting of acrylic acid onto HDPE yields products with more hydrophilic, functionalised surfaces with lower crystallinity than the virgin material. The presence of the acrylic acid was confirmed by FTIR [95]. Studies evaluating the effects of polymer fillers such as CaCO3 on the mechanical [96-98] and thermal [99] properties have been carried out. The extent to which additives affect the thermal properties depends upon their concentration, nature and molecular weight of the polymer, and molecular weight distribution and processing temperatures. Metallic impurities can also arise from contaminated fillers. The extent of adhesion between a polymer matrix and discrete moieties dictates the variation between mere filling and reinforcement. A true reinforcement occurs when there is chemical bonding between

31

the solid filler and the polymer: it is often achieved by using coupling agents such as zirconates. Therefore many additives are known to have multiple roles. CaCO3 is widely used as a filler to obtain mechanical strength (i.e. increased modulus and impact energy) due its attractive price factor and availability, with a narrow particle size distribution being favourable to the enhancement of the toughness of polymer composites. CaCO3 depresses the onset degradation temperature by 100oC and actually catalyses the breakdown of HDPE. HDPE in the presence of basic CaCO3 became thermally unstable and degraded much earlier than the virgin sample. The adhesion between HDPE and CaCO3 can be improved greatly by phosphate treatment [98]. It has also been found that there is a decrease of microhardness of HDPE with increasing molecular weight, mainly due to the increase in thickness of the interlamellar layers (i.e. decrease of crystallinity). Chemical treatment with chlorosulfonic acid and with osmium tetroxide the samples show a drastic hardness increase. The hardness increase is explained in terms of the large reduction in molecular mobility of the amorphous, interlamellar layers [100]. Polymers degrade to a certain extent during processing in a corotating intermeshing twin screw extruder due to the high temperatures and shear stresses experienced by them. The chemical changes that occur in a polymer are chain scission and crosslinking. Due to these changes, the quality of the product is considerably affected in terms of its properties. Degradation is related to extruder temperature profile, screw speed and feed rate [101]. Studies involving virgin HDPE/recycled HDPE composites found that the mechanical property most affected was the elongation at break, which decreased with increasing amounts of recycled HDPE. Recycled HDPE was obtained from post-consumer cycle of milk bottles. However, generally it was found that recycled HDPE was found to be a material with useful properties not largely different from those of virgin resin and thus could be used, at an appropriate concentration in virgin HDPE, for different applications [102]. The recycling of homogeneous HDPE from containers for liquids has been found to give rise to materials having mechanical properties that are strongly dependent on the reprocessing apparatus and the processing conditions. The thermomechanical degradation during processing gives rise to different 32

modifications of the structure depending on the temperature, residence time and applied stress. Generally, if the reprocessing operations are carried out in apparatus with low residence time, the mechanical and rheological properties of the raw materials are only slightly influenced by the recycling operations. Significant degradation phenomena and reduction of some mechanical properties are observed on increasing the number of recycling steps in apparatus with larger residence times. By adding antioxidant agents the polymer maintains the initial properties even after several recycling cycles. The competition between formation of chain branching and chain scission is considered to be responsible for this behaviour [103]. Woo et al [104] studied the thermal diffusivity of HDPE over a wide range of temperatures (25 to 100oC) by melting powdered HDPE in a cylindrical mould at several pressures and recording the temperature profiles at several radial positions. The thermal conductivity of a packed bed of HDPE powder was found to increase with pressure because of the decreased porosity. A sparsely cross-linked polyethylene (approx 5 cross-linking sites per 1000 carbon atoms) exhibits improved creep strength, impact strength (in the cold), and resistance to stress cracking while showing slightly diminishing hardness and rigidity. Radiation cross-linking of polymers (esp amorphous PE) in the melt will result in products with different physical properties than cross-linking at lower temperatures [105, 113]. Molecular modelling and thermal analysis (DSC) techniques are also being utilised to study the effects of real-world problem solving on the chemical structure of polymers [106]. The effect of mould temperature [107] and pressure [114] on the mechanical performance and microstructure of self-reinforced HDPE prepared by melt deformation in oscillating stress field have been studied. The mechanical properties e.g. modulus, yield strength, have been greatly improved in oscillating stress field, due to the production of shish-kebab crystals and the orientation of molecular chains. For the development of a thermal energy storage material, HDPE was cross-linked by electron beam irradiation. The characteristics of the cross-linked HDPE was analysed by the thermal and spectroscopic methods. Effects of ethylene glycol (EG) as a heat transfer fluid on the cross-linked HDPE was also investigated. The melting temperature and the heat of fusion of HDPE were not 33

changed by the cross-linking. No degradation was observed in the HDPE kept in vacuum or immersed completely in EG even at 150oC for 1000 hours. But the degradation due to oxidation was observed to occur in the HDPE, which was exposed in air [108]. Sensory studies have shown that bottles containing vitamin E yielded less off flavour than bottles containing other commercial antioxidants. The GC-MS study showed that bottles containing vitamin E yielded less aldehydes and ketones, which were considered to be major contributors of off flavour. Hexadec-1-ene was found to correlate well with off flavour. The result suggested that vitamin E was an effective antioxidant in commercial scale applications for reducing off flavour of HDPE bottles [110]. Failla et al exposed HDPE samples to doses of gamma radiation. It was found that the mechanical behaviour of the polymers changed progressively from ductile to brittle as the crystallinity was increased. The extensibility of originally ductile samples decreases with increasing radiation dose [115].

1.4

Identification of contaminated plastics

Plastics used in bottles can be identified by a variety of techniques such as optical, near and mid infrared, ultraviolet, and x-ray fluorescence and then effect mechanical separation based on this identification. Separation techniques based on spectroscopic examination are not directly applicable to plastics from durable goods because their greater wall thickness preclude transmission of most sources of probing radiation, paints and coatings are impenetrable to most of the existing technologies, the wider variety of shapes make the probing difficult on an automated system, and the quantity of materials make the identification process more time-consuming and technically challenging [2]. Portable devices To illustrate an example of “portable” identification techniques, a recent article publicised an invention by Southampton University, who in conjunction with Ford Motor Co, has produced a hand-held device, which can identify polymers. The operator shines an infrared light at a flat section of each piece of plastic and gets a readout saying what the plastic is, together with a “match value” - a measure of the reliability of the identification. For example, it may identify PVC, with 90% certainty, consequently if the certainty is low, the operator knows it may not be the right material. The machine called Polyana, for polymer analyser, measures the spectrum of the infra-red radiation reflected from the sample. This is 34

determined by the molecular structure of the plastic. Polyana can identify up to 200 plastics in three seconds. The machine is to be initially used by carmakers, under increasing pressure to recycle plastics, by plastic recycling companies and by other product manufacturers anxious to know what materials their rivals are using [3]. The sorting of plastics by type represents an important step in the production of high quality recycled plastics. One approach relies on density to effect the sortation during the recycling operation. This has limitations when mixed streams of plastics have overlapping density ranges, due to the wide range of additives, fillers and pigments used in engineering plastics. Parts made from composites and structural foam can actually have wide ranging densities within the same part [2]. In another process the shredding of the plastic containers into pieces, heats the pieces to a temperature sufficient to vaporise some of the contaminants therein in order to emit volatiles. Accordingly, it is particularly advantageous to analyse the emitted volatiles either during or immediately after the shredding of the containers. It is particularly advantageous to test the shredded plastic materials just after the washing process, again due to the fact that there are high temperatures associated with the washing process that will liberate volatiles of contaminants in the plastic material if any are present. It is also important to maintain the temperature of the washed shredded material below a level that would emit detectable levels of vapours derived from the plastic material itself, which would create background interference with volatiles of any contaminants emitted from the plastic materials. This paper is concerned with chemicals that are volatile and can be analysed, but the tests used for chemical determination are not detailed [16]. A device known as Neotronics Olfactory Sensing Equipment (NOSE) developed by Neotronics Scientific is designed to analyse complex vapours and compare this analysis with a user defined reference. The NOSE utilises neotronics patented conducting polymer sensors which, due to their design, have rapid response times and stable outputs. The reaction of the vapour with the conducting polymer causes a change in conductivity. This change is dependent on a complex interaction between the components of the vapour and the polymers, as each sensor responds to a number of components in a unique manner. The use of an expanding range of polymers makes comparative analysis of complex vapour structure realisable. This equipment has been successfully applied in the food, beverage, tobacco, petrochemicals, packaging, health care and other industries [59]. A company called AromaScan has developed another technique based on a similar principle to the NOSE above, to characterise volatile odours and chemicals and is called Olfractroscopy [60]. 35

Spectroscopic techniques, such as IR, NMR, UV and visible light absorption, Raman, photo-correlation spectroscopy have been cited as good methods for determining the polymer type present and which will give a number of intensity peaks in the spectrum and these data can then be correlated with corresponding acid, alkali, nitro-, hydrocarbons, and other functional group peaks. The structure of crystalline polymers can be studied using standard x-ray and electron diffraction methods. Gel Permeation Chromatography is used as a method for determining a complete molar mass distribution of a polymer, and this value can then be used to compare with original results. Chemical methods, such as Beilstein and Lassaigne tests for the presence of particular elements (e.g. halogens, nitrogen, sulphur) have also been used historically for the identification of unknown polymers, but have now largely been superseded by spectroscopic techniques, such as XPS and XRF [49, 53]. Antioxidants, UV absorbers, lubricants, antistatic agents and optical brighteners can be extracted by solvents (such as chloroform, hexane, diethyl ether, toluene, carbon disulphide, cyclohexane) and gas liquid chromatography, then observed by visible spectroscopy. The literature lists numerous chemical and spectroscopic methods for the identification of trace non-metallic elements (Cl, N, S, Na, Fe, Al, Ti, Cu) such as x-ray fluorescence, and flame photometric procedures. Combustion analysis for quantitative determination of elemental composition can be used to confirm the purity of a homopolymer and to determine the average chemical composition of a copolymer for which the repeat units are known and have significantly different elemental compositions, can be evaluated from their % carbon and/or % nitrogen contents. Numerous methods have been cited in the literature for the identification of compounds such as hydrocarbons, cadmium and selenium pigments, and elucidation of polymer structure. These techniques include x-ray fluorescence spectrometry, gas chromatography, gas-liquid chromatography, density and optical birefringence, differential thermal analysis, IR, and NMR [49, 4, 22]. In the recycling process of polymers, at least two steps might require an automatic identification tool: the sorting of the plastics, and the qualification of the regenerated product. A comparative analysis on the efficiencies of reflectance Fourier Transform Middle Infra Red Spectroscopy (FTMIR) and Fourier Transform Raman Spectroscopy (FT Raman) with respect to these identification applications that has been carried out. Although it was possible with both techniques to recognise most usual polymers, severe limitations of FTMIR were evidenced such as a high sensitivity to the surface state, and a weak 36

(noisy) reflectance signal for -CH bonds. On the other hand FT Raman proved to be a rapid (~1s per object) and highly selective method, giving information even on the mineral fillers present in plastics. The polymers analysed in this paper were PVC, HDPE, LDPE, PS, PP, PC, PMMA, PET, PTFE and ABS [50]. Detector choices depend on separation objectives Detectors fall into four categories: x-ray, single wavelength infrared (IR), full-spectrum IR, and colour. The earliest automatic separation systems used x-rays, which are still the most effective means of determining the presence of PVC. The chlorine atom in the PVC molecule emits a unique signal in the presence of x-rays presented by either x-ray transmission (XRT) or x-ray fluorescence (XRF). The XRT signal passes through the container, ignoring labels and other surface contaminants, and is capable of detecting a second container that may be stuck to the first. XRF bounces off the surface of the container and is useful for finding any PVC, including labels and caps. Systems for separating multiple types of plastics utilise a single wavelength of the near infrared (NIR) spectrum. These systems focus on simple determination of opacity; they separate the stream of mixed containers into clear (PET and PVC), translucent (HDPE and polypropylene), and normally mixed, coloured HDPE streams. The newest and most sophisticated detectors employ full spectrum NIR. Since all materials absorb IR to different degrees, each resin has a unique “fingerprint” which allow these detectors to accurately separate each of the resins. Currently, filters for individual wavelengths are used for rapid identifications and there is promise for even faster, lower-cost systems. Mid-range IR, due to its stronger signal and ability to “see through” black pigments, is being used for separating plastic durable goods. The colour detectors are very small cameras capable of identifying a number of colours. When combined with a resin-specific detector, they permit a variety of sorts. Particulate-sorting units capable of sorting by colour represent the new wave in auto-sort technology. Similar equipment has been used for years by the food processing industry to remove defective goods such as burnt potato chips and green blueberries from product streams. Several systems have already been used to separate green and clear PET and to remove closures from ground milk bottles. A system to remove PVC from a reclaimed PET stream using low level x-rays was commercialised in 1994. 37

Future applications will involve the identification and removal of other resins. This equipment has promise as a final quality-control step for the recycler. Other separation techniques are seen on the horizon. There are six areas of technology that are being applied to the plastics separation challenge which look promising for the future:(i)

Markers: This technique involves marking plastic products with an easily identified sign. It has

been suggested that the UPC code could be used to carry the material identification, but getting manufacturers to agree to utilise such a system may prove difficult. A second proposal is the addition of unique molecular markers to the backbone of each polymer. The participation requirement for this is of a smaller group of companies - the resin manufacturers. At least one resin manufacturer is currently reviewing the patent situation for a series of resin markers it has developed. (ii) Solvent Evaporation (or selective dissolution): This technique involves applying a solvent to a mixture of ground plastics placed in a reactor. The first pass dissolves and removes one of the components; a second pass (of a different solvent or the same at a different temperature) removes another polymer, and so on. This technique involves significant capital expenditures to achieve the required economy of scale. (iii) Froth Flotation: Mixed plastics may be treated with surfactants to take advantage of their different surface wetting potentials. As air percolates through the slurry, bubbles stick to select polymers, causing these materials to rise to the surface where they can be skimmed off. (iv) Density: Density separations are common in plastics recycling. In one case, base cups made of HDPE are easily removed from PET soda bottles, since HDPE floats in water while PET sinks. More sophisticated techniques are being evaluated to separate other heavier-than-water resins using heavymedia solutions and precisely controlled density liquids (supercritical liquids). (v) Electrostatics: Plastics have different electric surface potentials, which attracts some polymers to positive charges and others to negative charges. Cascading a mixture of plastic chips between oppositely charged plates can result in an accurate separation. However, the surface potentials of post-

38

consumer flake frequently change. Until a process to restore the original surface potentials is found, a commercial technique is not viable. (vi) Cryogenics: When they are at temperatures below 0oC., plastics become more brittle. They can be ground to form different sized particles, which can be separated by simple screening into resin type since each forms particles of similar size. The drawbacks involve the smallest particles which form from each resin. These particles resist separation by screening, remain mixed and contaminate the separation process. As the plastics recycling industry learns through experience, separation techniques for mixed plastics will continue to grow in number and sophistication and costs will continue to drop [51]. Model studies were carried out by BP Chemicals [46], where contaminated HDPE was evaluated for detergent, oil, bleach and white spirit contaminants using Thermal Gravimetry, high performance liquid chromatography, size exclusion chromatography, NMR, GC/MS, x-ray fluorescence, Energy Dispersive x-ray analysis, AAS, IR and Mass Spectrometry. The tests found that in general, contaminants absorbed into the walls of the original containers were found to persist throughout the recycling process, although levels were sometimes reduced at successive stages. The main contaminants identified in the reprocessed pellets were: (a)

white spirit (1.2 - 1.7 % by weight)

(b)

oil (0.7 % by weight)

(c)

chlorine (600 ppm, from bleach)

(d)

traces of limonene (from detergent) and oil additives.

The outstanding challenge which remains following this present work is to investigate the potential for contamination by more toxic substances. This could arise for example by pick-up of highly toxic garden chemicals (e.g. pesticides or weedkillers) by containers which are put to non-intended use by consumers before entering the waste stream. The approach being taken by BP Chemicals is to define the scope of the problem using selected contaminants in laboratory-scale experiments, in order to give the experimental data needed to make adequate assessment of any risk. Measurements of volatility, equilibration levels in HDPE, and analysis of residues are being carried out. Calculations of the dilution effect on processing will be backed by confirmatory processing experiments carried out under safe conditions.

39

The differences in the crystallinities of HDPE samples conditioned by three different procedures with different cooling rates were followed by use of FTIR, DSC and X-ray scattering techniques. From FTIR, relative values of crystallinities are obtained and if absolute values from DSC and X-ray scattering are compared, the latter yielded higher values. All three groups of results showed the expected trend of increase of crystallinities with decrease of rate of cooling [109, 120]. Exact knowledge of an additive and its concentration in a plastics material can be determined by pyrolysis GC-MS, and comparing the pyrolysis fragments with an additive spectrum database [123].

1.5

Decontamination of plastics

Often melted plastic becomes contaminated with various foreign objects and material such as scrap metal, floor sweepings, etc. The presence of such foreign objects in the melted plastic can lead to loss of production due to their containment in the moulded parts, and also creates other problems as by lodging of the particles in the mould or cavities, tending to block flow. Filters of known diameter sizes have been employed to filter the plastic melt prior to injection into the mould [69, 70]. The removal of volatile components from a polymer solution (referred to as ‘devolatilisation’) is a necessary step in the commercial manufacture of many polymers. High viscosity polymer solutions can be devolatilised by passing through a heated zone of indirect heat exchange with a residence time of 5120 seconds [119]. A water injection foaming devolatilising method includes the steps of: melting and kneading a polymer in water-injection dispersing zone of an extruder having a screw; injecting water into a polymer melt so as to be dispersed into the polymer melt which is being kneaded; and vaporising volatile components contained in the polymer melt together with water in a devolatilising zone having a vent port and located on a downstream side of the water-injection dispersing zone, so that the volatile components are removed and discharged through the vent port [116-8]. In the face of increased awareness of environmental aspects and the use of plastics in food-contact applications, operators and manufacturers of devolatilisation equipment must meet more stringent requirements regarding residual devolatilisation and recover of monomers and solvents. For 40

manufacturers and operators of devolatilisation extruders, this means further advances in the details of the process. Simply arbitrarily increasing the number of devolatilisation steps is not the answer, since this would increase costs and often impair product quality. To increase the degree of reduction in each devolatilisation stage, greater use must be made of entraining agents. The aim, must be during froth devolatilisation, to control the cell membrane in such a way that residual devolatilisation can be carried out in a single stage [124]. A process for the valorisation of organic substances containing waste and melting of the inorganic substances by incineration has also been described [24]. Another method for the continuous treatment and valorisation of oil & water containing solids describes a process whereby different particulate sizes are initially separated, followed by a series of solvent treatments to separate the oil-water-plastic mixtures from one another [25]. Waste products consisting almost exclusively of PVC contain what are now frequently regarded as environmentally unacceptable auxiliary materials and so must be converted to remove completely these auxiliary materials. One method that can be used for disposal and conversion of even small volumes is degradative extrusion between 250 and 400 oC. At these temperatures the chlorine is more or less rapidly split off and can be recovered as HCl and reused in producing new monomers. PVC or plastic waste products containing PVC are freed of hydrochloric acid by thermal separation under 350oC. The residual plastic is then liquefied in an extruder, using halogen-free thermoplastic admixtures and the auxiliary materials used in the known degradative extrusion process, and these liquids are then burned with air or oxygen in a combustion chamber. Alternatively, the waste products consisting solely of PVC are freed of hydrochloric acid and then the temperature is raised high enough that the PVC is broken down. The resulting product is burned with air or oxygen or the resulting pyrolysis coke is used for metallurgical purposes. For this process variant, a synchronous twin-screw extruder is used. These methods can also be used in other degradative processes for recycling plastics and are especially recommended for facilities that process large volumes [10].

PVC can be recovered from a mixture including PVC and a non-PVC component(s), in which the material is mixed with a sufficient amount of a plasticizer at an effective temperature in the range of 100200oC, to give a PVC: plasticizer ratio such that the PVC and plasticizer form a mixture which is a liquid at that temperature so that the liquid PVC/plasticizer can be separated from the non-PVC component or 41

components [29]. Other tests have been carried out to dehalogenate plastics, which showed that the degree of success depends upon the type of halogen compound used and the level of contamination [122]. A waste high-polymer mixture (of polyolefines, polystyrenes, PVC, thermosetting and natural high polymers) can be fractionated using their dissimilar solubilities in different organic solvents (such as oxylene, p-xylene, or m-xylene, the solvents being used either singly or in combination) at temperatures within a certain range or ranges to dissolve and fractionate the polystyrenic and polyolefinic high polymers. The PVC’s in the remainder are dissolved and fractionated by using a solvent selected from either one of tetrahydrofuran, cyclohexane, dioxane, and methyl ethyl ketone at a suitable temperature. The balance of the mixture consists of thermosetting (epoxy and phenolic) and natural high polymers (such as paper). Many other similar fractionation techniques have also been described in the literature to obtain fractions, which can then be individually analysed [30]. Another plastic from plastic separation technique described in the literature involves the steps of dissolving a mixture consisting principally of paper and plastics, in a mixed medium of a hydrocarbon type solvent e.g. xylene and water, and then classifying the mixture into the following four groups. (i) insoluble plastics (principally PVC) (ii) paper disintegrated into a fibrous matter in the aqueous phase (iii) polyolefin-type plastics precipitated by cooling the solution of plastics dissolved in a hydrocarbon-type solvent; (iv) Polystyrene-type plastics obtained by evaporating the solvent from the solution from which the polyolefin-type plastics have been removed [31] Waste plastics, can be disintegrated into manageable pieces using water and a suitable emulsifying agent, without pre-sorting. After this a separation of the different kinds of plastic into individual fractions occurs by flotation and sedimentation in the washing liquid. Polyolefins, on account of their low density rise to the surface in the tub. The recycled material can be dried and granulated to be used in the manufacture of products, e.g. tubes. During the process the liquid utilised for separation i.e. water containing a suitable emulsifying agent for grease and oil, has a density of 1. Polyolefins, such as HDPE, LDPE, and polypropylene have a density, which is in the range between 0.90 and 0.96 and consequently rises to the surface of the liquid. Other materials, such as PVC and polyesters e.g. have a density above 1, and consequently sink to the bottom [7]. 42

Isocyanate contaminated drums can be decontaminated by using general purpose liquid decontaminants, such as mixtures of water, concentrated ammonia and a liquid detergent, or alternatively, water/Na2CO3/liquid detergent. Additional separation techniques include using either a separation tank or a plurality of liquid containers to separate contaminants (usually polymeric, metallic, etc.) from plastic materials [41-42]. Other methods described involve separating organic and/or inorganic contaminants from one another, with no plastic component involved at all [43-45]. Another US patent describes the separation of chemically contaminated drums from one another (but not bulk polymer and contaminant), followed by shredding, compartmentalising, rinsing, grinding, pelletizing and then moulding into a new plastic drum. Whereby the contaminant is not specifically removed, except that by rinsing and is incorporated within the recycled material [47]. Plastic waste can be upgraded by a washing and sink-float separation process, which can be followed by melt-filtration to produce a regular shaped pellet for subsequent conversion into new products. The two main techniques which can be employed for this purpose provide effective means for the comminution of plastic waste, for washing the film and for removing heavy impurities from PE and PP which unlike PVC both float in water. Both techniques present the washed and separated film to flake in a form, which ensures high and consistent output rates during ultimate compounding. The first of the two techniques include wash separation methods i.e. the Reichling (Cologne, W.Germany). A washing and drying plant was developed around the Buckan-Walther turbulent screen flake driers. The system consists of size reduction equipment, a washing and separation trough, drying plant and in its complete form, a final extrusion stage for the production of pellets for subsequent use. The second technique is a compactor orientated method which is used to increase the density of film, bottles and filament waste to form a free flowing agglomerate for subsequent conversion, of which the most well known is the “Krauss Maffei” from Austria, known as zerglomat machines [48].

1.6

Processing Technologies

Twin screw extruders are generally classified by whether the screws co-rotate or counter-rotate, and by whether the screws are tangential or intermesh. All combinations have been used for devolatilisation [125]. In polymer processing, mixing significantly affects material properties, processability and cost. 43

Consequently mixing in polymer processing is performed in a continuous mode using extrusion. Mixing is the intermingling by action of two or more initially segregated components. Mixing involves two distinct physical phenomena, dispersion and blending [126]. The technology for twin screw mixers is designed to1. Incorporate and homogenise additives without exceeding degradation temperatures 2. Generate high shear stresses for dispersion of non-reinforcing fillers or pigments 3. Homogenise two or more materials of different melt viscosities without creating a stratified or layered final matrix 4. Provide a uniform shear stress and heat history to each particle 5. Allow precise control over the mixing process The evolution of intermeshing, corotating twin-screw extruders for polymer mixing is based on these criteria. Fully intermeshing twin screws provide a narrow residence time distribution, which is critical for control of the mixing process. Corotating screws are effective in alternating the direction of applied stress through the use of mixing elements, thus producing a favourable mixing geometry. This feature makes intermeshing and corotating twin screws unique compared to other systems. In the predominant multiscrew system used for polymer mixing, the channel profiles are designed so that the tip of one screw wipes the flanks and roots of the other, resulting in a self cleaning action. This type of twin-screw mechanism (intermeshing and self-wiping) provides efficient conveying, mixing, and pressure build up capabilities. The direction of rotation of the screws, either corotating or counter rotating, determines the operating principles and applications in processing. Material transport in intermeshing, corotating twin screw extruders is generally dependent on drag flow. The screws pick up the material as they rotate, and where the two screws meet, a complete transfer of the material from one screw to the other takes place. Since the direction of circumferential movement is opposite at the intermesh, there is little opportunity for material to pass through the gap. As the material is transferred from one screw to the other, the direction of material flow is changed; new material surfaces are created with each screw revolution. Most corotating extruders are designed and manufactured in a modular construction; the screws are made up of individual sections that slide onto a keyed or spined shaft. Different screw profiles can be arranged at any location along the shaft to generate controlled shear or mixing effects (see diagram 44

below). Selection of an optimum screw configuration is based upon the process requirements (feeding, conveying, melting, dispersing, venting and others) and a knowledge of the working function of each type of screw element. For example, screw pitch can be used to control residence time and degree of fill in conveying zones. Polymer mixing is usually accompanied by several unit operations, including venting, pumping, and filtering. The modular design of barrel segments or corotating extruders provides process flexibility for locating downstream feed ports or vent openings. Changes in reprocessing requirements are accommodated through rearrangement of the screw configuration and/or barrel segments.

Figure 1.8: Different screw arrangements in the plasticising zone of a twin screw, co-rotating extruder (a) left-hand screw, (b) right-hand kneading block, left-hand screw, (c) right-hand kneading block, (d) large pitch left-hand screw Intermeshing and corotating twin screws provide the processing flexibility to suit most laminar mixing applications. The ability to generate pressure several times during extrusion operation, combined with close tolerances, allows intensive mixing with controlled shear input. Gentle mixing is also possible with a high degree of surface regeneration. Kneading and mixing elements, effective in the reorientation of interfaces, can be arranged for dispersive or distributive mixing. The positive conveying characteristics of intermeshing, corotating geometries are due to the influence of the intermesh region. Thus efficient transport of material occurs even in partially filled screw channels. The self-wiping feature assures reproducible performance with precise control of residence time distribution [127]. 45

1.7

Conclusions

The increasing use of plastics in everyday life and in the national economy of industrially developed countries makes it necessary to find a global solution to the problem of plastics waste management by economically sound and ecologically responsible methods. The limited space available for community and industrial tipping sites and the high costs of their construction, and also the resistance of plastics to biodegradation and the danger presented by emission of toxic gases during uncontrolled burning – these are factors which suggest that the rational use of these wastes requires rapid and effective remedial measures to be taken. It is apparent from the evidence collected that it is not possible to point out a single all purpose and optimum strategy for dealing with plastics under all economic conditions. Among the activities undertaken for the purpose of at least partly solving the ecological problems associated with the use of plastics, the following should be mentioned: limiting the use of plastics in the national economy and everyday life, of plastics, replacing materials hitherto used by new types of biodegradable plastics, direct energy utilisation of plastics waste as solid fuel and raw materials recycling by thermal and chemical methods. However, these activities in most cases come up against important barriers that limit the versatility of their application. The greatest potential so far has been offered by thermal methods of materials recycling, which can be used for basically all types of plastics, irrespective of the kind of polymer used, or of the fillers and other additives employed, or also of the form and character of the waste. In this case there is no need for preliminary separation of the wastes, of washing or removal of elements of other organic substances (paper labels, remnants of oils and food products, rubber seals, etc.). The products obtained after relatively simple treatment can be used either gaseous, liquid or solid fuels or employed as a chemical raw material [61]. A survey of the literature reveals that much of the research to date in the thermoplastics recycling and polymer decontamination refers to the separation of plastic from plastic i.e. purely polymer composite based systems, or involves incorporation of contaminants into the recycled plastic without separation, with some detailed description of techniques and processes that will bring about this. Other work to date includes studying the effects of contaminant addition such as calcium carbonate, antioxidants, with respect to mechanical property changes and suppressing thermo-oxidation. Numerous literature cites

46

the availability of a wide range of spectroscopic and chemical techniques in identifying a whole range of chemical contaminants.

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