RIGID POLYVINYL CHLORIDE : UPVC PHYSICAL PROPERTIES. PVC is a substantially amorphous, thermoplastic material which does not exhibit a sharp melting point. With a density of approximately 1.41 g/cm3 due to the high chlorine content the non-filled, non-plasticized material will sink in water. UPVC can be cut easily with a knife and the cuts have smooth edges. The addition of plasticizers will lower the density as they range in density from 0.908 g/cm3 (DOP) to 1.16 g/cm3 ( TTP). The addition of inorganic fillers will raise the density as they have densities greater than 2.5 g/cm3. The density of commercial compounds may range from 1.15 to 1.49 g/cm3 . The natural color of the material is clear when it is unfilled and stabilized with appropriate stabilizers for example, tin stabilizers. If the material is stabilized with lead stabilizers, and /or filled, then the natural color is usually an off-white. In either case this means that a wide color range is possible. PVC in its “natural state” is very unstable to heat and is rapidly degraded at temperatures within its softening range. Without additives such as stabilisers and lubricants it cannot be successfully processed. Examples of such additives include: * Heat stabilizers which are often based on lead or tin compounds. * Lubricants such as butyl stearate and stearic acid. * Processing aids such as acrylic polymers. In addition to these there are many others additives used to modify the properties of PVC rather than stabilise it. These include * Impact modifiers based on thermoplastics such as ABS or MBS or elastomers. * Fillers such as china clay. * Plasticisers. PVC containing plasticizes is referred to as PPVC while that which does not is referred to as UPVC (unplasticized PVC) or sometimes as rigid PVC. The two materials are distinguished in this way because the plasticizers added to confer flexibility and softness also make PPVC much easier to process since the plasticizer acts as a lubricant and reduces frictional heating of the polymer. As a result PPVC can usually be extruded on single screw equipment while rigid PVC develops too much frictional heat in a single screw extruder and a twin screw machines must be used for most jobs. The level of additives used in PVC can be very high and may exceed 50% of the weight of the final compound. Dispersing such large amounts requires considerable shear force but too much shear will degrade the polymer. Depending on the formulation and the equipment available PVC may be purchased as a compound where the polymer has been melt mixed with the additives using a twin screw extruder. Many “standard” compounds designed for specific applications can be obtained from resin suppliers (usually under a different trade name from the raw polymer).

In other cases the raw PVC is blended with the additives in a high speed mixer and then fed directly to the extruder. While this saves cost the physical and processing properties of compounded materials are usually superior because the additives are better dispersed by a compounding extruder. PVC can also be blended, compounded and extruded in-house. This has the advantage of versatility since non-standard compounds are easily made. Also regrind polymer and colours are better dispersed since they can be incorporated at the compounding stage rather than at the extrusion stage. PVC may be made softer by “internal plasticization” when a second monomer is incorporated during manufacture. These copolymers are softer and more readily fabricated than homopolymers. They are however, usually more expensive. To realize the potential of this material it is necessary to put in just sufficient work and heat so that the blend is fully gelled ( fused ) but not so much that it is degraded by overheating or excessive shear. Once decomposition starts it can spread very rapidly because one of the products of decomposition (hydrochloric acid or HCl ) catalyses further degradation. This acid readily attacks steel causing pitting and corrosion and promotes rust by stripping protective coatings. The effects of the HCl on human eyes and lungs are rapid and destructive. Despite these disadvantages PVC is widely used as it is relatively cheap and particularly versatile. Products ranging from very hard to very flexible are easily produced and their properties can be maintained over a wide temperature range. The material is inherently flame retardant because of the high chlorine content and this can be advanced if fillers are used. Although PVC can be easily glued with solvent cements its chemical resistance is generally good. UPVC has high impact strength but can be very notch sensitive. Impact modifiers are used to reduce this problem. Some compounds will provide Shore D hardness above 80. UPVC has a Vicat softening point (50N) of 75 - 90 C. It is not recommended for use above 60 C. When correctly formulated, transparent products are possible: this means using appropriate stabilizers based on tin compounds and avoiding the use of fillers. If impact rnodifiers are used the refractive index of the impact modifier and the PVC must match. This match may only be possible over a limited temperature range. The major application of UPVC is in pipes for waste for drainage etc. Use of UPVC profiles for window frames have increased considerably because of the materials good weatherability, good color range, stiffness, toughness and relatively low cost. Other profile shapes include runners, venetian and other blinds, cladding, framing, fencing, easily assemblable room partitioning and electrical conduit. Widely used in chemical plant because of its good chemical resistance, rigidity and non-inflammability. Rainwater down pipes and guttering are also extruded from UPVC compounds because of their rigidity, UV resistance and excellent chemical resistance.

In film and foil, UPVC is used in the thickness range from 80 to 160 gauge as a twist wrapping film while thicker gauges are used as material to be thermoformed into nestings for cookies, cakes etc. CHEMICAL PROPERTIES. PVC alone has good chemical resistance. Water, salt solutions, concentrated non-oxidising acids, alkalis and dilute oxidising agents have little effect at room temperature although at higher temperatures some hydrolysis may occur. PVC is attacked by concentrated oxidizing acids such as sulfuric, nitric and chromic acids which cause decomposition; the rate of attack may be accelerated in the presence of metals such as zinc and iron. It is attacked by bromine and fluorine even at room temperature. It is resistant to aliphatic hydrocarbons but is unsuitable for use in contact with aromatic and chlorinated hydrocarbons, ketones, nitro compounds, esters and cyclic ethers which penetrate the polymer and cause marked swelling. Some solvents such as cyclohexanone, THF and MEK will dissolve the polymer easily and these are the basis of PVC adhesives. Treatment with methylene chloride is used to detect inadequately gelled polymer. Because of the large amounts used additives can have major effects on chemical and solvent resistance of PVC formulations. The effect of stabilizers, lubricants and pigments on chemical is less marked than the effect of plasticizes and copolymers but the type and amount of these additives will have some effect on the chemical resistance. Use of copolymers or addition of impact modifiers will generally reduce chemical and weathering resistance. Weathering resistance is improved by the use of UV stabilizers, titanium dioxide or carbon black. When PVC is heated in a flame it softens, burns and chars. It is not normally self extinguishing. A dirty yellow flame is seen together with a lot of smoke and a sweet, chemical smell from the plasticizer. If a piece of a PVC compound is heated on a copper wire then the flame will be colored green. This test distinguishes PVC from other common polymers. If the plasticizer is removed by solvent extraction then it may be identified by its boiling point and/or infrared spectrum. COLORING. As the natural color of the material can be clear then a wide color range is possible; this includes both transparent and opaque colors. Sold in both compounded colors and as natural material for coloring on the extrusion machine by techniques such as solid masterbatches and dry color. MATERIALS HANDLING. The monomer from which PVC is made is vinyl chloride (VCM). Inhalation of this material over prolonged periods is linked with carcinogenic effects and the level of VCM in the atmosphere should not exceed 1 ppm. While it is true that PVC compounds can release traces of VCM into the atmosphere these levels in normal plant conditions are much below those considered dangerous to health.

The only situation where special precautions are needed concerns silos and bulk tankers holding PVC powder which has a higher surface area to volume ratio than granules. The contents of tankers and containers should be sampled without personnel entering the container i.e. by use of long handled scoops. If it is necessary to enter a silo or tanker, it should be purged with air. VCM levels should be monitored and breathing apparatus used if these levels are too high. PVC powder blends can produce fine dust and dust masks should be used where there is likelihood of dust being present in the air. As far as possible, powder blends should be contained and not free to blow into the plant atmosphere. In any event PVC dust levels should remain below 10 mg/m3 and respirable dust below 5 mg/m3. Some of the additives (e.g. lead stabilisers) used in PVC compounds can also be toxic. Smoking, drinking and eating should not occur where blends containing toxic ingredients are present. Many users prepare their own dry blend compounds. This has the advantage of reduced cost and the control over compound design which can he kept in house. For major tonnage producers, the weighing and mixing is done under automatic control. Sometimes the main mixing is done on this way and manual, mixing of dry color additive done in ribbon blenders as a separate operation. If extrusion is at low speed then any moisture and volatiles will outgas backwards during extrusion so there is no need to predry. However at rates which are commercially viable it is recommended that material be dried at a temperature of 105 C for 1.5 hours. Application of a vacuum during drying will help. The drying can take place in a heated hopper or by using using a vented extruder. If granules are used then gravity feed hoppers are satisfactory. However for powder blends either a crammer or a hopper vibrator may be necessary. The base of the feed hopper must be kept cool to prevent blockages. FLOW PROPERTIES PVC is too unstable to allow a normal MFI measurement so it is dissolved in a solvent and the viscosity of the solution is measured. The viscosity is the “reverse” of MFI. A higher number means longer polymer chains and therefore a lower MFI and stiffer flow. The effect of additives on the final compound viscosity can be dramatic. Plasticisers lower viscosity and fillers increase it while other additives will have smaller effects. The actual numbers quoted for viscosity vary. The commonest is the K value but the viscosity number (DIN 53726 or ISO - R174) is also used along with the inherent viscosity (ASTM). As an example normal pipe and profile resins would have K values of 66 - 68 which is the same as DIN viscosity numbers of 110 - 116 and inherent viscosities of 1.1 - 1.15. For clear sheet and profiles K values around 62 are used.

As with most polymers the longer chains provide better physical properties so if the article is to be subjected to a wide range of temperatures or requires particularly good mechanical properties are required a resins with K values up to 91 (viscosity number 234) are used.

The viscosity values for a fairly stiff compound designed for pipe are given by ; Viscosity (Ns/m2) = antilog ( 6.597 - 0.0086 x temperature (C) - 0.748 log (shear rate (s-1))) SCREW AND BARREL DESIGN. The largest tonnage of rigid PVC is extruder on twin screw extruders. These are usually of the counter-rotating type although co-rotating designs are also used. Both cylindrical and conical twin screws are used successfully. The advantage of twin screw extrusion is the ability to get high outputs without excessive shear in the melt and hence at lower melt temperatures than is possible with high speed single screw extrusion. Details of the twin screw design are outside the scope of this handbook. The tables below show typical screw designs used for single screw extrusion, firstly at low speeds and low melt temperatures and secondly at higher speeds and temperatures. The high speed screws are designed as two stage screws. The first stage compresses and melts the compound which is then decompressed before the second stage. A vent may be positioned at the decompressed stage to remove volatiles. Care must be taken with two stage designs not to allow material to fill the decompression section because of too great a die resistance. LOW EXTRUSION SPEED SINGLE SCREWS Length of Feed Zone 4D, Compression Zone 16D and Metering Zone 4D. SCREW DIAMETER (mm) Feed zone depth (mm) Metering zone depth (mm)

63 11 2.3

91 16.5 6.9

115 17.5 7.1

HIGH EXTRUSION SPEED SINGLE SCREWS. Length of Feed Zone 3D, Compression Zone 6-7D, 1st Metering Zone 4-5D, Decompression Zone 2D, 2nd Compression Zone 4-5D and Final Metering Zone 3-4D. SCREW DIAMETER (mm) 1 st Feed Zone depth 1 st Metering Zone depth Decompression Zone depth Final Metering Zone depth

63 9.6 3.3 9.9 4.7

91 13.5 4.3 17.3 7.9

Single stage screws are also used for UPVC at high speeds. They should have a compression ratio of about 2. 5: 1. Screws are often fitted with 2 - 4 sets of pin mixing studs. These are a ring of studs standing up from the screw root to a height of about half the channel depth. The screw flight can be broken to accommodate the ring of pins.

Barrier screws have been used successfully for rigid PVC compounds. Other designs include screws incorporating an additional flight inside the normal channel. These NRM designs put the extra flight in the first stage for powder feeds but in the second stage for pellet feed. All high speed screws should be temperature controlled by having a hollow section through which a fluid can be circulated. Heated oil at a temperature between 90 and 160 C is the best cooling medium but air is possible. BARREL AND DIE TEMPERATURES. The table below shows typical zone temperatures in centigrade. SCREW TYPE

Zone 1 Temperature Zone 2 Temperature Zone 3 Temperature Zone 4 Temperature Zone 5 Temperature Adapter Temperature Die Temperature Melt Temperature

LOW SPEED SINGLE 150 160 170

185 185 180 - 190

HIGH SPEED SINGLE (vented) 180 180 190 190 180 200 200 190 - 205

TWIN

165 170 177 184 182 190 193 195

DIE DESIGN AND CONSTRUCTION. The main considerations in designing dies for rigid compounds are the high viscosity of the melt and the poor thermal stability of PVC. In some ways the two considerations lead to opposite solutions. The high viscosity means flow channels should be fairly generous in size to prevent large pressures while to prevent degradation residence time and hence die volume should be small. PVC dies are streamlined and usually chrome plated. In sheet dies a choker bar is not used as this provides a space where material can hold up and degrade. In pipe dies there has to be compression after the spider to encourage the melt to weld together. Typically the torpedo diameter ratio at the spider compared to the land is about 2: 1. The overall volume compression ratio is in the range 7: 1 to 10: 1 and so the channel height also decreases. The length of the die land has to be about 20 times the die exit gap width. For thin walled pipes the ratio can be somewhat less but should be more for thick walled pipes. Pipe dies come with a range of interchangeable rings and pins. This means that one die body can produce a range of pipes though not such a wide range as with a die for polyolefins. The table below shows a typical range of UPVC pipe dies. DIE A B C D E F G

Pipe Diameter (mm) 5 - 15 12 - 50 32 - 75 50 - 150 75 - 220 110 - 380 160 - 450

Wall Thickness (mm) 1-2 1-4 2-7 2 - 10 2 - 14 2 - 23 3 - 28

Maximum Output Rate (Kg/hr) 50 100 250 450 600 800 1,000

H

380 - 680

4 - 30

1,000

Profile dies are designed with similar care to achieve streamlined flow. In designing components for manufacture wall thicknesses should be all about the same and sharp internal or external corners avoided if possible. Large sections are replaced by hollow cored sections with internal struts if necessary. The design where struts meets flat surfaces can cause problems. Dies are made from hardened chrome nickel steels and need to be highly polished. DOWNSTREAM OPERATIONS. Calibration in sizing dies is needed to ensure satisfactory dimensional tolerances. Vacuum calibration is used increasingly. For example, when producing window frame profile, with a production rate around 90 Kg/h. And a line speed of around 2 m. per minute, a set of four vacuum sizing units is used each unit being about 600 mm long. At double the production rate, extra sizing and cooling units have to be added eg another 1.5 - 2 m of unit length. In both profile and pipe lines, lengths are sawn or cut automatically using computer controlled saws. The saw blades are tungsten carbide tipped and the tooth form is specially designed for PVC. The shaft speed is such as to give the saw blade a cutting speed of about 50 m/second. Automatic monitoring of pipe thickness is becoming more common. VDU displays are linked to the monitoring equipment and display machine settings as well as operating temperatures, pressures and pipe thickness profile. The measurements can also be used to give automatic line and die adjustment. EXTRUSION CAPACITY. The figures below refer to typical maximum outputs and screw speeds on single screw units when using high viscosity resins. Extruder Low speed screw High speed screw Screw (mm) Kg/h. r.p.m. Kg/h. r.p.m. 63 30-40 15-25 110-160 40-70 90 40-70 10-20 190-250 40-60 115 50-80 5-15 270-340 20-30 The following table gives outputs for twin screw extruders in pipe production. Extruder Pipe Diameter Maximum Output Screw (mm) (mm) (Kg/hr) 73 16 - 160 180 91 30 - 250 400 125 100 - 650 700 132 100 - 650 700

STARTING UP. Before starting up, ensure that the extruder is not contaminated with any reactive polymer such as acetal or one which contains a halogen flame retardant. If there is any doubt, purge the

extruder. When changing to a food grade also purge the extruder thoroughly of any non-food grade material.

SHUTTING DOWN. Reduce the temperature of the extruder whilst purging with a compatible material such as PMMA or ABS; if these are not available use PP or HDPE. REPROCESSING. Maximum reclaim levels of 15 % are suggested provided no evidence of degradation is present. FINISHING. Inks are available for printing onto PVC which because of its polar nature readily accepts the ink. When drilling or sawing UPVC lubricants should not be used Hot plate welding can be used to join PVC components and has become widely used in the window frame industry. Surfaces to be joined are brought into contact with a PTFE fabric covered hot plate which is at a temperature of 210 to 230 C. Surfaces are held against the hot plate for 30 - 35 seconds after which they are pulled away from the plate which is removed so that the surfaces can be immediately pushed together. After being held together for up to a minute while the weld cools, the welded component is trimmed to remove weld flash. About 2 to 3 mm is lost off the extrudate during hot plate welding. Solvent welding is another widely used method. The initial set up is quick but joints need 24 hours to develop their full strength. joints require finishing after welding to give a polished surface. The process of grinding and polishing can be very time consuming.

.