Plastics Material Selection PDF

 /. ,I. / j...

 /. ,I. / j POLYOLEFINES INTRODUCfION Polyolefines are hydro-carbon polymers which is to say, they contain only carbon and hydrogen. The carbon atoms form the backbone of the polymer. The principal members of the the family are the polyethylenes (PE) and the polypropylenes (PP). Polyethylene has been commercially available in the Low Density form since the early 1930s. In the mid 1950s, the Ziegler-Natta type of catalysts for polymerising olefines were discovered and as a result, a new type of polyethylene, High Density Polyethylene and Polypropylene became available from around 1960. Various developments based on the Ziegler-Natta catalysts led to polypropylene copolymers (block and random), Medium Density polyethylenes and Linear Low Density polyethylenes as well as ethylene-propylene cross linkable elastomers and thermoplastic elastomers based upon polypropylene and EPR. In the last few years, a development as revolutionary as the Ziegler-Natta catalysts were in the 1950s has occurred This is the use of metallocene catalysts to produce polyolefines. TIle use of metallocenes allows much greater control of polymer and co-polymer structures giving rise a vast range of new polyolefine materials with almost tailored properties., POLYETHYLENES It is convenient to start with the two basic forms of polyethylene, LDPE and HDPE. Low Density (LDPE) with a density of about 0.91 gjcc and High Density (HDPE) with a density of about 0.96 gjec. Both are made from ethylene and the repeat unit of both is TIley are, however, made by different polymerisation processes which leads to structurally different polymers which have different physical properties. Both types are normally partly crytstalline. LDPE is made by the so-called high pressure process which produces a version of polyethylene which consists of long chains of ethylene units joined together. These long chains are in fact branched with long sequences of ethylene units. Along the branches are very short branches, eg - -CH2 - CH3 or -CH2 - CH2-CH2 - CH3, which occur on average about every 30 carbon units on a main / chain. These short chains interfere with the ability of the polymer chains to crystallise efficiently and the result is that LDPE has low crystallinity (typically about 55%) compared with HDPE, a lower melting point (110°C) and a lower density. HDPE is made by the so-called low or medium pressure processes which produce essentially linear polyethylene chains. Since there are no short branches of the type found in LDPE, HDPE polymer chains can crystallise efficiently with the result that HDPE has high crystallinity (about 80%), a higher melting point (BO°C) and a higher density. It is the above structural factors that produce the differences in physical properties of the two classical types of polyethylene. The essential difference in this respect is that HDPE is significantly more crystalline and therefore has less amorphous material. Amorphous polyethylene softens at about -100°C and therefore at ambient temperatures, it is a fairly fluid material which can flow under stress. Since LDPE has about 45% amorphous material, it is a low strength, low modulus and ductile material. It will creep under load. It also has a low softening point (HDT) of about 50°C maximum which severely li nuts its applications as far as high temperatures are concerned. On the other hand the semi-liquid amorphous material imparts exceptional toughness and impact resistance to LDPE. Since the amorphous material does not begin to stiffen until temperatures approaching -80°C, LDPE retains flexibility and impact resistance down to at least -60°C. 1 '~.. C LDPE (Low Density Polyethylene) Key Points ~ 1 - Short (eg, 30 -CH2- unit length) Branches - Interfere with crystalisation. 2 - Max crystallinity is.approx, 55%. 3 - Typical Density 0.91/0.92 glcc 4 - Melt temperature range 110 to 150°C HDPE (High Density Polyethylene) Key Points 1 - Linear Branches Enable Higher % Crystalinity 2 - Max crystallinity is approx. 80%. 3 - Typical Density 0.96 up to 0.98 glcc 4 - Melt temperature range 130 to 180°C  By contrast, HDPE typically has only 20% amorphous material. It is therefore significantly stronger, stiffer and more creep resistant than LDPE although it is by no means as good in any respect as engineering polymers, nor even polypropylene. Its low temperature performance is not as good as LDPE but nevertheless will have useful properties down to below -20°e. The extra crystallinity allows it to be used at higher temperatures than LDPE as it does not soften until 60-80°C depending upon grade. Some grades currently in production can even withstand boiling water (l00°C) for a short time. Although impact resistance is very good, it can be broken in standard impact tests (eg Izod) whereas LDPE does not break. Both types of polyethylene have excellent chemical resistance and there are no known solvents at room temperatures. However, some low molecular weight grades of LDPE are swollen by hydrocarbons such as petrol and some grades of LDPE undergo environmental stress cracking in contact with certain liquids. This is not a major problem as most grades today do not suffer swelling or stress cracking. UV resistance is not good, especially with LDPE, and leads to embrittlement. This is remedied where necessary by incorporating UV stabilisers. There are no toxicological problems with polyethylenes and the polymer has acceptance for a wide range of food contact and medical applications. This may not be the case if additives are present however. HDPE is much more stable to UV and more resistant to swelling in liquids than LDPE./ Electrical insulation properties of all polyethylenes are excellent and LDPE is used as the primary insulation in for example, power cables and other electrical and electronic systems. The choice of LDPE is largely because of its greater flexibility compared with HDPE. The excellent electrical insulation properties are in part due to the properties inherent in the polyethylene and in part to the fact that it absorbs virtually no water at all.. LDPE is used mainly (80%) for film (eg plastic bags) and sheet. The remainder is used for extruded tube (eg, dip tube in aerosols, ink tubes in ball point pens) and blow moulded containers. Relatively little is injection moulded HDPE is used in injection moulding a wide range of components such as closures for blow moulded containers, bottle crates, industrial trays, tote boxes, pallets, toys and a variety of components for machines. Considerable amounts of HDPE are converted into film which may be by the blown film process but mainly by the cast film process. This film can be uni- or biaxially orientated and either single or multi-layer. The major process for HDPE, however, is blow moulding - something like 50% of HDPE is used in blow moulded containers of one sort or another. Blow moulded components are also used in under-the-bonnet applications in the automotive industry, for example; radiator header tanks, petrol tanks. The production of pipe (eg, for mains water, gas ) is also a major use of HDPE It may be noted here that shrinkage on cooling is significantly higher with HDPE when compared with LDPE LINEAR LOW DENSITY AND MEDIUM DENSITY POLYETHYLENE TYPES Among the other types of polyethylene, the most widely known of these is the so-called Linear Low Density Polyethylene. In fact, linear low density is but a member of a larger family of polyethylenes which range in density from about 0.866 to 0.96. The original types were made by a process similar to that used for HDPE but newer methods of polymerisation, principally the "metallocene" catalysts, have enabled the range to be extended. All of these polyethylenes are co-polymers of ethylene with controlled amounts of a comonomer such as butene, octene and methyl pentene among others. The co-polymer chain is itself linear, ie; no long chain branching as in LDPE. However, the introduction of the co-monomer produces very short side chains 2 eg butene octene methyl pentene -CH2 - CH- -CHo- CH- -CHo- CHo- I. I I - CH CH3 CH3 These polymers therefore resemble LDPE (although here is no long chain branching) but with the important differences that the type of side chain group is controlled the amount of side chain units is controlled These two factors, together with the control of molecular weight (chain length) means that a very wide range of polyethylenes with densities ranging from 0.866 to 0.955 can be made and more importantly, with a wide range of properties. Those polyethylenes normally referred to as LLDPE have densities in the range 0.91-0.92 and are used almost exclusively for the production of blown film with the advantage over LDPE that the films are stronger and can therefore be produced in thinner gauge compared with conventional LDPE film of the same strength. The higher density polyethylenes of this type (0.93-0.95) are generally referred to as Medium Density Polyethylenes (MDPE) and which are used in competition with HDPE in blow moulded containers, injection moulded components and extrusions. It will be apparent that these types of polyethylenes are really a whole family of different poyethylenes and it is difficult to generalise on properties other than to state that MDPE wil tend to be similar to HDPE and LLDPE will tend to be similar to LDPE, but in general terms only. OTHER POLYETHYLENES Ultra High Molecular Weight Polyethylene (UHMWPE) is, as the name suggests, an extremely high molecular weight version of polyethylene. The high molecular weight is so high that it is not melt processable but, the high molecular weight gives much better stiffness and other mechanical properties compared with the melt processable types. Shapes are produced by machining sheet or bar stock which has been made by compressing powder at high pressures with heating. It is a special purpose polyethylene Ethylene is copolymerised with numeous vinyl monomers to produce co-polymers which are produced in relatively small amounts but whichhave important uses. Examples are Ethylene Vinyl Acetate (EVA) which is made by copolymerising ethylene 'with vinyl acetate using the high pressure LDPE process. Control of the vinyl acetate gives a range of properties. Increasing the VAc content progressively diminishes the amount of crystallinity until a totally amorphous rubbery polymer is obtained. Four types may be conveniently distinguished around 3% vinyl acetate - a tougher and more flexible form of LDPE is produced which is used mainly in flexible film production at around 15% vinyl acetate, the copolymer resembles plasticised PVC with which it competes in applications where plasticiser loss might cause problems (eg food and medical applications). Pipes for conveying fluids (eg in the food industry and in medical applications) may be made from this type of EVA at around 30% vinyl acetate, the EVA becomes soluble in several common solvents and use of this is made in adhesive formulations at around 45% vinyl acetate, the polymer is a rubber which can be moulded but needs to be cross- linked like natural rubber When copolymerised with small amounts (1%) of an acrylic acid (eg, methacrylic acid), a polymer is produced which when treated with sodium methoxide, for example, gives an ionic polymer. These are called ionomers. They are elastomeric thermoplastic materials which are 3 extremely tough and also transparent. They are used, JOr. example, in the manufacture of puncture resistant film and golf balls. POLYPROPYLENES Polypropylene has the repeat unit -CH~ - CH- - I CH3 and is therefore, in chemical terms, a close relation of the polyethylenes. It has been available in its well known form for nearly 40 years now. Prior to 1956, polypropylene could only be made by the high pressure process for LDPE and the result was a form (atactic) of polypropylene that could not crystallise. Since the glass transition temperature is -18°C, this form of PP is a rubber at room temperatures and therefore of little use as a plastic. If the low pressure processes are applied to propylene, then a different form of polypropylene (isotactic) is produced which will crystallise, typically to about 75%. Since the melting point of the crystalline polypropylene is nominally around 160-170°C, isotactic polypropylene is a useful plastics material not dissimilar to HDPE in many ways. Because of the higher Tg, the higher melting point and the high degree of crystallinity, it differs in the following ways from HDPE it is stiffer and stronger at room temperatures it is less tough and less impact resistant it has a higher softening point - HDT (1.81 MPa) is around 60°C it has a higher brittle temperature (around O°C) Although the HDT is only 60°C, many polypropylene grades will stand temperatures of around 100°C for a short time provided that they are not under mechanical stress In order to lower the brittle temperature to extend the service temperature range, mainly to improve low temperature impact resistance, a small amount (5-10%) of ethylene is copolymerised into the polymer. This type is known as polypropylene copolymer (that without the ethylene is referred to as polypropylene homopolymer). Whilst the presence of ethylene units in the polymer chain has the desired effect of lowering the brittle temperature, it also lowers the softening point, makes the polypropylene more ductile, less stiff, less strong and less creep resistant. It is now common practice to produce glass fibre reinforced polypropylenes to increase strength, stiffness, heat resistance and creep resistance which it does at the cost of reducing impact resistance, especialyat low temperatures. Another common additive is talc (typically at 40% loading). This is added principally to improve dimensional stability in injection moulded components. It may improve strength, stiffness etc but is not as efficient as using glass fibre for this purpose. Talc filled polypropylenes are more brittle than unfilled PP and will often show lower impact resistance. The chemical resistance of polypropylenes is excellent and similar to polyethylene. It is inherently more resistant to UV light than LDPE but less so than HDPE. There are no toxicological effects and many grades are approved for food and medical use. Electrical properties are similar to polyethylenes. Water absorption is approximately zero. Today, there are many grades of polypropylene and they range in MFR values from about 0.5 to 55. The increase in range is in part because of the increase in demand for PP in many applications. This demand has in tum found an increasing number of uses for PP. The high molecular weight grades, especially when reinforced with glass fibre, meet many of the requirements of low performance engineering applications. It is easy to understand the increased usage of PP in engineering applications when one considers that the price is about 25% of the cheapest of the recognised engineering polymers. 4  Polypropylene shows its versatility in the range of processes used to make products. These may be sunnarised as injection moulding which accounts for over 35% of polypropylene used. Products include a wide range of automotive components, domestic and business appliances, toys, medical components, bottle crates and other storage containers and packaging items. It is also used for garden furniture and seating in sports stadia. film production, mainly as chill-cast (flat film) and especially as biaxially orientated film. It has high strength and clarity. It is widely used in the packaging of foodstuffs, cosmetics, cigarettes etc. Where improved reduction of gas permeation is required a multi-layer film containing a gas barrier is used. Significant amounts of film are converted into tape for use as magnetic tape and adhesive tape. Some tape is converted into fibrillated 'string' for use as twine or weaving into sacking. blow moulding, - a wide range of packaging containers for food, cosmetic, medical and chemical contents are now produced. Containers up to 5 000 litres have been produced. Some food containers are made by the injection-stretch-blow moulding process to give greater strength and clarity. extrusion process are used to produce pressure pipe, tubing, hoses, drinking straws, etc vacuum forming of food (eg yoghurt pots) and other containers is increasingly being used / Whether homo- or co-polymer grades are used in any particular application will depend primarily upon whether impact resistance and use at low temperatures are important. POLYPROPYLENE ELASTOMER SYSTEMS An alternative way of toughening polypropylene is to incorporate a rubber as a blend. TIle rubber normally used is ethylene propylene rubber (EPR or EPDM). Increasing the amount of EPR in the blend increases toughness and inpact resistance but decreases strength, rigidity and heat resistance. TIle amount of crystallinity diminishes with increasing EPR content and disappears when sufficient has been added. This has led to the widespread use of thermoplastic olefine elastomers (TPO) which normally contain about 70% of rubber and this class of thermoplastic elastomers is now one of the major TPEs in use today. The rubber toughened PP grades (as opposed to TPOs) are used where impact resistance and low temperature performance are required at low cost. Blending two polymers is a relatively easy and cheap process as it is a simple matter to change the ratio of the components during blending. This is not the case with manufacturing copolymers where the demand for sales of a particular product will dictate whether or not that grade is produced by copolymerisation. The rubber modified polypropylenes are mainly injection moulded, a major use being the manufacture of car bumpers ·OTHER POLYOLEFINES Other polyolefines are used but in relatively small quanities. Two are of some general importance although not widely used Polymethyl pentene (more usually known as TPX) is a moderately crystalline polymer (40-60%) which is transparent. It is stiffer and stronger than PP and at the same time, more brittle. It is more expensive than PP and is used in applications where the general properties of polyolefines are required together with transparency. Examples might be laboratory equipment (eg beakers), medical and pharmaceutical equipment, etc Polybutene is generally very similar polypropylene but it has better creep resistance due to its high crystallinity and better toughness due to its low glass transition temperature. It is less resistant to hydrocarbons than polyethylenes or polypropylenes. The biggest problem is that on moulding, it 5  crystallises in one form and then recrystallises to a second fonu over a period of several days. This leads to dimensional instability (the density increases from 0.89 to 0.95 g/cc), It is used mainly in the manufacture of water pipes and pipe fittings MET ALLOCENE POL YOLEFINES As indicated earlier, metallocenes are commercially new catalysts which allow much greater control of the polymer structures. TIle catalysts themselves can be relatively easily modified and designed to do specific things which means that the polymers can be designed to a much greater extent than previously to achieve required properties. There are two broad aspects to "metallocene" polyolefines better control of molecular weights better control of co-polymer structure The principal advantage of molecular weight control is that the molecular weight distribution is much narrower. Compared with the conventional materials, this leads to./ smaller melting ranges and can also produce lower melting points. Low molecular weight materials will have a lower Tm compared with a high molecular weight polymer. different rheological properties because there are no very long or very short chains. A metallocene LDPE will have very different processing characteristics from a conventional LDPE of the same MFR value. The melts are less shear thinning (psuedoplastic). higher heat resistance (if higher molecular weight polymers are used). For example, it is possible to obtain grades of PP which do not soften (HDT) until temperatures above 100°C The advantage of controlling copolymer structure is that the co-monomers can be placed in the polymer with greater precision and with less variation in composition from chain to chain. This leads to control of crystallinity and therefore properties of products. For example, it is now possible to obtain polypropylenes and polyethylenes which have high transparency or higher strength properties depending upon the co-monomer, the amount of comonomer and how it is placed in the polymer chain. Polyethylenes are now available with virtually no crystallinity and polypropylenes are available with up to around 85% crystallinity control of processing characteristics. To summarise, the use of mettalcene catalysts has greatly extended the range of polyolefine materials in terms of processing characteristics and properties of products. 6 Design and Processing Data Polyolefines Design Data: Shrinkage Critical wall (mm) Flow Ratio LDPE 1.5 to 4.0 % 2.1mm 160 - 250 HOPE 1.5 to 4.0 % 2.3mm 150 - 20t) PP 1.0 to 3.0 % 2.6mm 150 -350 Comments: High shrinkage materials with low Tg values, will tend to creep under load at room temperature. Very good chemical resistance at room temperature. All Olefines are susceptible to UV attack if used outside without UV protection (eg. Carbon Black loading). Avoid thick wall sections, sharp corners and thick to thin section changes - warping, distortion and related processing defects can be problems as a consequence. Waxy and milky natural finish is poor from aesthetic point of view - good chemical resistance a problem with regard to finishing. Good toughness and resilience under cyclic loading (eg. hinges). Material becomes tougher if orientated (eg. LDPE based films, PP rope & fibres). Huge range oflow cost polymers commercially available. PP now available in many grades with different properties, transparency, elastomeric, harder mineral loaded (ego talc), stiffer (glass fibre loaded), etc. Typical Uses: LDPEILLDPE - Blown Films HOPE - Blow Moulded Components (chemical bottles, packaging, etc) PP - Injection Moulded Components, fibres, etc Processing Nature: Low melt temperatures, good thermal stability and large processing windows mean these polymers are easy to process. Olefins suffer from high specific heat contents (they absorb more energy to melt and cool when compared to other polymers - ego Styrinics). Energy use footprints can be high as a result and should be taken into consideration during selection. Typical injection force requirements: LDPE - 2 tsi or 30 MPa HOPE - 2.5 tsi or 37.5 MPa PP 2.5 - 3 tsi or 37.5 to 45 MPa  Typical Properties of Commodity Plastics © PSC Associates Ltd - , Polymer Tensile Flexual Izod Un- HDT 1.82 Density Strength Strength Notched MPaload glcc MPa MPa" D4812JIM QC Olefins - LDPE 12 16 No break 25 0.92 LLDPE 13 1"1 NQ break 37 0.93 MDPE 15 19 , - No break 39 0.94 HDPE 20 ,. 21 No break 46 0.95 PP 32 41 No break 54 &.91 PP+20% 29 45 1390 63 1.04 Talc ,. (viscous faU) / PP+30% 55 83 240 127 1.12 Glass Fibre PP +30 0/0 101 165 214 149 1.11 LGF...- Styrinics PS S3 - 83 ,l(iO 82 1.04 HIPS 24 45 1068 79 1.03 (viscous fail) SAN 72 124 214 100 1.07 ADS 46 74 1762 90 1.06 (viscous fail) ADS+30% 91 134 268 93 1.27 Glass Fibre ASA 50.. 67 2:;0' ss 1.0' DDS 28 Elastomeric No break 10 0.89 Elastomer Acrylic PMMA 65 90 232 - 92 - 1.18 Vinyls , PVC 48 75 200 70 1.43 Unplastieised PVC 22 8 No break 20 1.28 Plasticised (Flexible /elastomerie).I STYRENE BASED POLYMERS INTRODUCTION This family of polymers consists of a range of homopolymers, copolymers and blends, all of which contain styrene as the major constituent. The parent polymer, polystyrene (PS), is one of the oldest synthetic plastics materials having been commercially available since about 1930. From about 1950, toughened polystyrene (TPS or lllPS), styrene acrylonitrile (SAN) and acrylonitrile- butadiene-styrene (ABS) have been available. Today, TPS is as widely used as polystyrene itself. In the last twenty years or so, a number of other styrene based copolymers have been marketed most of which have remained in small scale production. - POLYSTYRENE Known either simply as polystyrene or sometimes as general purpose polystyrene (GPPS), the polymer has the repeat unit of It is made by a free radical polymerisation process which produces atactic polymer. It is therefore amorphous and its properties are governed by one transition temperature, Tg. Being amorphous, it is also transparent which gives rise to another common name for the polymer, crystal polystyrene. This name can be confusing to the unwary. The name does not imply crystallinity but probably derives from the optical properties in the same way as crystal glass. The polymer is slightly branched with long chain branching. Typical average molecular weights are in the range 60000 (600 units) to 100000 (1000 units). The higher average molecular weight marks the upper limit ofprocessability at acceptable processing temperatures. The lower end marks the lower limit for acceptable properties and these grades are often known as easy flow because melt viscosity is relatively low. Some high molecular weight grades (up to 120000) are available but are the most difficult to process. The glass transition temperature is about 100°C and because there is no other transition below Tg, there is no significant molecular motion at room temperatures. Polystyrene is therefore a stiff creep-resistant polymer at room temperatures and shows brittle behaviour. Break strains are of the order of 2% and the resistance to impact is very poor. Heat resistance is limited by Tg and softening points rarely exceedXr'C except with high molecular weight grades which is one of the principle reasons for their existence. In general, polystyrene products are not recommended for temperatures above about 60°C for any length of time. Chemical resistance is poor, especially to a wide range of common solvents at room temperatures in which it readily dissolves or swells. It is also slowly attacked by alcohols and oils which can ultimately result in mouldings crazing or cracking, especially if they contain moulded stress. Polystyrene is generally resistant to aqueous solutions but not concentrated acids and the like. Resistance to UV is poor and the polymer quickly yellows on exposure and eventually shows embrittlement From the above, it will be apparent that the three major limitations of polystyrene with regard to applications are poor impact resistance, poor heat resistance and poor chemical resistance. There are advantages of the latter however, principally that polystyrene components can be readily glued together (hence the use of polystyrene in model kits) and that the polymer readily takes print, adhesive labels and other forms of surface decoration such as metallisation. Electrical properties are good and are enhanced by the fact that water absorption is low. Unive sitv of rfr..London  i TOUGHENED POLYSTYRENE (TPS, HIPS) Toughened polystyrene is used in approximately the same quantities as polystyrene itself. It is also known as High Impact Polystyrene (HIPS) which is something of a misnomer as the impact resistance, though better than polystyrene is significantly less than most other plastics. The toughening is achieved by incorporating rubber into the polystyrene. To obtain the most effective toughening, the rubber needs to be dispersed uniformly throughout the polystyrene and the rubber particle size is also critical, 10-20 microns being the most efficient size. The manufacture of 1PS is therefore the free radical polymerisation of a solution of the rubber in styrene. Control of the polymerisation system produces the desired effect and there is also some beneficial linking of the rubber to the polystyrene by chain transfer mechanisms. Polybutadiene is normally the rubber used these days but others such as styrene-butadiene rubber (SBR) have been used. ' The amount of rubber used is important. Increasing the rubber content increases toughness, ductility and impact resistance but reduces stiffness, creep resistance, softening point. Since the aim is to have a tougher form of polystyrene whilst maintaining rigidity, rubber contents are normally around 10%. Although tougher than polystyrene, 1PS is still essentially a brittle material. Tensile test results are very dependent on strain rate. At high strain rates, many grades exhibit polystyrene- like brittleness but at low speeds, the polymer may yield and draw to 15-25% before failure, Impact resistance is increased by up to 5 times that of polystyrene but softening points are about lOoC lower than equivalent polystyrene grades, Chemical resistance is similar to polystyrene but the presence of the nibber gives an advantage in one respect. The rubber can be chemically etched from the surface of mouldings thereby giving keying for surface treatments such as painting, metallisation etc. and therefore TPS is preferred to polystyrene when such surface decoration is required. The presence of the rubber produces mouldings that are opaque. They are off-white unless coloured with pigment. Special grades of TPS however are available which will produce transparent mouldings with about 90% light transmission. It is believed that these are produced by dispersing the rubber in such a way that the particle size is smaller than the wavelength of light (0.4 microns). STYRENE ACRYLONITRILE (SAN) This is a random copolymer of styrene and acrylonitrile, the repeat units being -CHz---CH- and -CHz---CH- I ~Ol V"" CN· It is made by the free radical polymerisation of a mixture of the two monomers. The copolymer is amorphous and transparent. ' The purpose of the acrylonitrile is to impart chemical resistance to to a styrene based polymer. Polyacrylonitrile is resistant to. a wide range of common solvents due to the high polarity of the nitrile group. Unfortunately, polyacrylonitrile is not sufficiently thermally stable to be melt processed. However, when acrylonitrile units are incorporated into a polystyrene chain chemical resistance is significantly greater than polystyrene whilst melt processability is maintained. 2 (J uversr= of Nc h' =don  Normally, SAN grades contain about 30% acrylonitrile, the greatest amount of acrylonitrile that can be incorporated without serious detriment to processability. Some special grades have higher acrylonitrile content and in the past, very high acrylonitrile polymers (about 60%) have been available. SAN may be regarded as a more chemically resistant form of polystyrene. It is resistant to fats, oils and alcohols but, although more resistant than polystyrene, it is not resistant to chlorinated solvents, esters and ketones. There are further advantages to incorporating the acrylonitrile in the polymer chain. Compared with polystyrene the polymer is more rigid, more creep resistant and has a higher softening point (about 20°C higher) giving a continuous service temperature of about 80-95°C depending on acrylonitrile content. Higher temperatures can be tolerated short term and it can withstand boiling water for a short time. It has better impact resistance than polystyrene. The impact resistance is comparable with TPS but SAN is naturally transparent. Furthermore, this improved impact resistance is maintained down to -40°c. The presence of the-acrylonitrile produces a yellow tint in the polymer which is normally masked by incorporating a blue dye during manufacture, hence the tell-tale tint of SAN products. A further advantage of incorporating the nitrile group is that a high gloss finish can be obtained when. the polymer' is correctly moulded. Electrical properties are similar to polystyrene. Glass reinforced grades are available to increase strength and stiffness at the expense of optical clarity ACRYLONITRILE-BUTADIENE-STYRENE (ABS) ABS is not a polymer but a blend of polymers and copolymers. It was originally made by blending SAN with a rubber to..produce a form of polystyrene with the advantages of TPS (toughness) and SAN (chemical and heat resistance). Today it is made by more sophisticated techniques allowing greater control of properties. In essence, styrene and acrylonitrile monomers are blended with polybutadiene latex. The mixture is then emulsion polymerised to form polybutadiene dispersed in an SAN with the rubber being grafted onto the SAN. By controlling the amount of rubber, styrene and acrylonitrile in the initial blend and the molecular weight of the SAN produced, a wide range of ABS compounds can be produced. Additional SAN can be blended in by using a twin screw compounding extruder. It is therefore possible to tailor an ABS for a particular application by enhancing properties such as chemical resistance, heat resistance, impact resistance and low temperature performance. Two principle types of ABS may be distinguished; rigid materials for moulding and high rubber content materials for calendering. The former are by far the most important. The rigid moulding grades of ABS contain 10-20% rubber and the mechanical properties in general lie between polystyrene and SAN. Creep resistance is good and impact resistance is superior TPS. Furthermore, it can be maintained to temperatures down to -40°C in some cases. Heat resistance is good and many grades are able to withstand boiling water. Chemical resistance is similar to SAN but the inclusion of a diene rubber means that weatherability suffers. ABS needs to be stabilised if prolonged exposure to sunlight is anticipated in service. The ability to tailor properties has enabled some grades of ABS, like polypropylene, to be regarded as an engineering polymer. Dimensional stability and the ability to mould to close tolerances are advantages in such applications. Properties for engineering applications can be adjusted by the use of glass and mineral powder fillers. ABS is capable of being moulded with a high gloss finish or with a matt finish depending on composition and moulding conditions. Higher rubber content grades are, like TPS, particularly suitable for surface decoration by painting and metallisation by surface etching prior to the coating being applied. ABS materials are normally opaque but transparent materials (over 90% light transmission) are available but are more expensive. The cost of 'normal grades of ABS is between 2,5 and 3 times that of polystyrene. The high rubber content ABS materials contain upto 60% rubber and can therefore be quite flexible whilst retaining strength and toughness. The amount of rubber determines the flexibility and hardness of the sheet. 3 ----=;:--_  ACRYLATE-STYRENE-ACRYLONITRILE (ASA) ASA is a similar material to ABS but instead of using polybutadiene as the rubber phase, an acrylic ester polymer is used. The acrylic elastomer is dispersed and grafted to the SAN matrix. The acrylic elastomer has no unsaturation and so ASA is more stable to UV light and oxygen than ABS. ASA is also much more resistant (100 times) to yellowing than even UV stabilised ABS. The properties of ASA are otherwise much the same as ABS and in general terms, ASA can be regarded as a UV/oxygen resistant form of ABS BUTADIENE-STYRENE BLOCK COPOLYMERS (BDS) Block copolymers of styrene and butadiene are made by an anionic 'living polymer' technique whereby the monomers are introduced to the reactor in sequence. By controlling the sequence of monomer addition to the reactor and the amount of each monomer, a range of different block copolymers can be obtained Styrene-Butadiene-Styrene (SBS) tri-block copolymer, in which the butadiene content is around 80%, is the major member of the most Widely used family of thermoplastic elastomers. However, if the styrene is the major constituent, the result is rubber phase dispersed in polystyrene, ie a rubber toughened plastic (polystyrene). Unlike TPS however, the rubber phase particles are so small that the product is transparent, even when the rubber content is as high as 25%. Such polymers have achieved commercial recognition in recent years. The commercial polymers are not linear but star-block. """'---- styrene ~ r styrene-- ". butadiene ---- styrene~ ~ styrene _ The mechanical properties of BDS depend very much upon the rubber content which, in.the grades in common use, is from about 15% to 25%. Increasing the rubber content reduces strength and stiffness properties and softening point but increases toughness, softness and flexibility. The major attraction of these polymers is the clarity coupled with the very high impact resistance that is possible. The impact behaviour, though less good, resembles polycarbonate, It requires about half the energy of polycarbonate to break it under impact if the specimen is uni-notched. However, BDS is extremely notch sensitive and surface scratches can reduce the impact resistance to that of polystyrene. Careful moulding to reduce internal stresses will help to minimise this notch sensitivity. With regard to chemical and UV resistance, BDS is similar to polystyrene. Unless stabilised, it tends to yellow on exposure to sunlight due largely to the rubber present. The polymer is not suitable for contact with oils as these are absorbed and act as plasticisers to soften the polymer. Fats and oils can also produce stress cracking. The cost is about twice that of polystyrene. " PROCESSING The major processing method of converting styrene based materials to finished products is by injection moulding. Typical barrel and mould temperatures are given below Material Barrel Temperature Mould Temperatures °C °C PSITPS 200-250 20-60 SAN 210-280 40-80 ABS 200-280 40-90 ASA 240-280 40-80 BDS 170-240 20-50 4 tv rth I "n Drying is not normally necessary as the materials absorb only very small amounts of water. However, it is advisable to dry ABS and ASA if high quality surface finishes are required. With SAN, ABS and ASA, high mould temperatures are used if a high gloss surface finish is required. Extrusion processes are also used Polystyrene and BDS are both blow moulded to make small bottles. Some polystyrene is converted into sheet but considerably more TPS sheet is produced and this is widely used for vacuum forming. Likewise, ABS sheet is also produced and much of it is also vacuum formed to the final product. ABS is also extruded as pipe and profile whereas polystyrene is extruded mainly as profile (eg light diffusers). Both PS and BDS are converted to film in small amounts by the blown film process. Polystyrene film is produced for electrical applications such as miniature capacitors and, especially in the USA, for farm produce packaging. BDS film is produced for medical applications because of the material's suitability for particular sterilisation processes The high rubber content ABS materials (contain upto 60% rubber) are usually calendered to form sheet which, in the case of the more rigid sheet can be vacuum formed. APPLICA nONS The uses of polystyrene and toughened polystyrene are restricted because of the deficiencies listed above. Their low cost and easy moulding characteristics are their main reasons for use. Polystyrene is used where transparency is required but where this is not essential, TPS is usually preferred because of its superior impact performance. Examples of products made from PS include kitchen-ware (eg butter dishes, jugs), picnic ware, disposable fast food tableware, toys, ball point pen outer tubes, combs, tape spools, cassette cases, transparent panels for appliances and meters, tube light covers (when suitably stabilised) and a variety of advertising/display equipment. TPS products include a wide range of machine components and housings (eg radio and cassette housings, fan blades), toys (including model kits and models such as electric trains) and domestic components such as wall clocks and kitchen-ware. Extruded sheet is vacuum formed to produce a wide range of packaging (including food) containers, disposable drinking cups and the like. SAN competes with polystyrene in many applications but at about twice the material cost. Therefore it is used where its superior properties dictate or where a generally superior quality transparent product is required. Its total use is less than 10 % of polystyrene (GPPS + TPS). Typical applications include kitchen-wares (jugs, blenders, etc) picnic-ware (cups, tumblers, plates, cutlery), toothbrush handles, lenses, transparent covers on audio-cassettes, radio and meter dials and scales, cosmetic containers and components, transparent machine covers (eg portable cassette players.) ABS is widely used in the automotive industry for dashboards, knobs, mirror housings, hub caps, etc. It is used for a wide range of domestic and office machine and instrument housings eg power tools, telephones, vacuum cleaners, computers, calculators, radios, televisions and kitchen equipment. Pipes and pipe fittings in ABS are widely used in domestic appliances such as washing machines and refrigerators. Impact resistant grades are used for safety helmets in industry and civil engineering. Extruded sheet is used to vacuum form a wide range of packaging containers, including food containers, and also refrigerator liners and for example, large mouldings for medical equipment. The high rubber content calendered sheet materials are used for leather-look covers for luggage and similar applications. ' Products made from ASA are principally used in outdoor applications. Typical examples are automotive radiator grilles, signs, cats-eyes along roads, garden furniture, radio aerial components and sports equipment, all of which, of course are also made in ABS. 5 U iversity f North London  Typical applications of BDS include toys, bottles and other containers for solid pharmaceuticals, medical components such as mouth tubes, storage containers and packaging containers such as egg boxes. Drawing instruments, commonly made from polystyrene Or acrylic materials have also been made from BDS. EXPANDED POLYSTYRENE (XPS) Expanded polystyrene is the second most important expanded plastics material after polyurethanes accounting for about 25% of the market. It is made by using a blowing agent to produce products of densities down to about 20 kg/m" by a number of processes. These processes are outlined briefly below. The principle applications of the low density products involve thermal insulation and protective packaging. Moulded products of low density are made by the bead process in the UK. In this process, suspension polymerised beads (about Imm diameter) are made containing pentane (bp 60°C) as the blowing agent. Since the pentane migrates slowly out of the beads, they should not be stored for too long before use. Conversion of the beads to a moulded product takes place in three stages. Stage one is the free expansion of the beads using steam at around 100°C to increase their size to between 3 and 5 mm diameter. Stage two is the equilibration stage where the beads are left for several hours to allow the internal pressure in the beads to reach atmospheric pressure by air diffusing in. In stage three, the expanded beads are packed into a closed mould through which superheated steam (eg 120°C) is passed to cause the beads to soften, expand and coalesce to form a unified moulding. In such mouldings, the fused beads can be clearly seen. Mouldings made by this process usually have densities in the range of 20-100 if:g/m3. Mouldings include insulated cups for use with hot drink dispensers, moulded draught excluders, insulated cavity linings for refrigerators, picnic boxes etc and protective cushioning for the transportation of delicate instruments and machines. Low density expanded polystyrene sheet is produced by extruding polystyrene containing a blowing agent such as an azo-carbamide which decomposes at melt temperatures to produce a gas. In the case of azo-carbamides, the gas is nitrogen which is produced at about 150°C On emerging from a specially designed die, the gas causes the melt to expand, the degree of expansion being controlled principally by the amount of blowing agent. The gas escapes through the sheet surface leaving a- porous surface. If a non porous surface is required, the melt can be extruded onto a non-expanded polystyrene sheet. Expanded sheet made in this way is vacuum formed into plates, dishes, food packaging trays etc. Chips in a variety of shapes are also produced for protective packaging purposes. Polystyrene containing a blowing agent is also injection moulded. The densities here are generally much higher than in the above processes (300 to 1000 kg/nr'. The density is controlledby the amount of blowing agent and the amount of shot fed to the mould There are several reasons for moulding with expandable polystyrene, One reason is to counteract sinking where thick sections are necessary. Many of the cheaper mass produced speaker cabinets, for example, are made by this process, A disadvantage is the fact that gas escape through the surface during moulding mars the surface with streaks. This has to be disguised, hidden or taken advantage of. The streaks produce a wood grain type of finish and, if the polystyrene is correctly coloured, a good imitation wood effect is produced, Cigar boxes and umbrella handles are two examples of this type of moulding. An advantage, as far as injection moulding is concerned; is that since the mould filling pressure is produced by the gas and not the injection unit, much lower mould clamping forces can be used 6  Representative Properties-of Styrene Based Polymers PS HIPS SAN ADS ASA BDS Tensile :MFa 35-60 15-50 65-85 25-60 30-35 26-30 \ Strength Yield :MFa 14-50 20-30 - 36-42 Strength Break % 1-3 5-20 2-3 5-75 20-40 20-260. Elong'n Tensile GPa 2-4 1-3 3-4 1-3 Modulus Flexural MPa 70-110 25-60 120-140 40-90 40-55 34-44 Strength Flexural GPa 2.5-3.5 1.0-2.5 3.5-4.0 1-3 1.5-2.0 1.4-1.5 Modulus Notched J/12.7 0.1-0.3 0.4-3.0 0.2-0.4 2-8 5-7 0.2-0.5 !zod nun HDT °C 70-105 65-95 100-104 90-105 85-90 70-77 1.81 :MFa "", Vol ncm >1016 >1016 >1016 >1016 Resist ReI 2.4-3.1 2.4-3.8 2.6-3.1 2.7-4.8 Permit'ty Tan 8 1-4 10-30 70-100 70-300 x 104 7  POLYMETHYL METHACRYLATE (PMMA) INTRODUCTION Polymethyl methacrylate is made by the free radical polymerisation of methyl methacrylate and has the repeat unit '" CH3 I -CH2-CH- I C=O I CH3 It is atactic and therefore amorphous. Its principal assets are its glass-like transparency and its stability to UV light. It has the best weathering characteristics of all transparent polymers. There are two types of polymethyl methacrylate which are distinguished by their methods of manufacture and which give rise to significantly different properties. These are cast PMMA and moulding materials. Both are often referred to as acrylic materials. Moulding Materials As a moulding material, PMMA is generally similar to polystyrene. Molecular weights are in the range 60000 to 100000 and with a Tg of about 100°C, it gives rigid glass like products. It is however less brittle than polystyrene. There is a transition just below room temperature which is... ascribed to the onset of side chain mobility and, while not as significant as the main chain segmental rotation which occurs around Tg, it nevertheless introduces a useful element of ductility and shock absorbance. Consequently, the impact resistance of PMMA is slightly better than polystyrene. It is also harder (Rockwell M85-105) than polystyrene (M65-M90) and as a consequence, is also more scratch resistant. However, its scratch resistance cannot compare with glass. Since the Tg is similar to polystyrene, the softening point is also similar. Chemical resistance is slightly better than PS but it is still dissolved or attacked by a number of common solvents which limits its applications. An advantage of this is that components can be joined by solvents or the use of adhesives which if correctly applied can produce a seamless joint. These adhesives, include solvents, solutions of PMMA which harden by evaporation of the solvent or polymer-monomer solutions which are polymerised in situ and which are sold under proprietary trade names such as Tensol. The polymer is more polar in character than polystyrene and therefore is more resistant to oils and fats. This polarity also affects the electrical properties which, whilst being good at low frequencies, they are less so at higher frequencies and the polymer is unsuitable for many electrical applications.  With regard to melt flow, PMMA is generally more viscous than PS and is therefore not so easy to mould without moulded stress. Flexibilising the chain produces easier flowing polymer. This can be done by adding small amounts (eg 5%) of plasticiser (eg di-butyl phthalate) or by copolymerising methylmethacrylate with another acrylic monomer such as methyl acrylate or methylethacrylate. A disadvantage of plasticisers is that they can migrate out and copolymer grades are generally preferred. Either way, flexibilising the polymer reduces strength, stiffness and softening point but does increase.' toughness and impact resistance. When impact resistance is of importance in moulding, then copolymer grades are usually preferred. The selection of PMMA for a product is usually because of its optical properties. Refractive index is high (around 1.5) and light transmission is about 92%. There is no spectral absorption in the visible region and it is stable to UV and. therefore does not yellow with age. Unfortunately, the difficulties of flow during melt processing lead to anisotropic optical properties. Surface defects and '../ flow lines can also mar the optical clarity. The moulding grades are injection moulded or extruded Injection machine temperatures are in the range 170-275°C (barrel) and 60-90°C (mould) depending on the grade. Extrusion temperatures are normally in the range 200-240°C. It is not usually necessary to dry the material before processing unless the material has been exposed to the atmosphere. Injection moulded products include lenses, sight glasses (eg for drink dispensers), telephone dials, automotive rear lights, light fittings, advertising displays, shelving, road signs and a variety of transparent covers/panels for meters, clocks, hi-fi equipment, business machines etc. Extruded sheet is also produced but the optical quality in general is lower than cast sheet (see below). Nevertheless, this sheet is used for panelling in building construction (indoor and out) and transport as it is cheaper than cast sheet and it can be vacuum formed into a variety of components. Extruded profile (eg strip light diffusers) and tube (eg burettes for school use) are also manufactured. An important use of extruded rod of small diameter is as light guides in fibre optic applications. This is due to the high refractive index which allows internal reflection of the light as it passes along quite severe bends in the fibre. Uses include medical investigations inside the body and similar industrial investigations of machinery and equipment. The use in telecommunications is limited to short distances because the of the absorption of light as it passes along the fibre optic. Glass is far superior in this respect. A more mundane use of fibre optics is in table top ornaments. Cast Products The most important cast product is sheet of various thickness but rod and other stock shapes are also produced. In the production of sheet, the method used involves a series of steps. First, the monomer is polymerised at about 80°C to about 15% conversion to form a viscous syrup. It is important to stop at this stage because autoacceleration occurs soon after and the polymerisation will go very rapidly to completion. Alternatively, polymethyl methacrylate is dissolved in monomer together with the peroxide to form the syrup. The second stage is to pour the cooled syrup into a mould, for example large sheets of heat resistant plate glass separated by a flexible gasket to provide the correct final thickness. The polymerisation of the syrup is then continued by heating the mould at 40°C for several hours (typically 12-16 hours). At 40°C, autoacceleration does not present a problem. However, contraction on polymerisation is nearly 20%. This is reduced somewhat by using the polymer syrup but even so, contraction in the mould is large, hence the flexible gasket to accommodate this shrinkage. The mould is loaded with clamps to compensate for the shrinkage to maintain contact of the polymerising material with the mould surfaces. When done correctly, a bubble free sheet of PMMA is produced. However, at this stage, polymerisation is only about 95%. Diffusion of the monomer is slow and so further polymerisation takes a long time. To speed the process, the mould is gradually heated to about 95°C to take the polymerisation as near to completion as possible. The mould is cooled and the sheet removed. The sheet is then usually annealed by heating to about 140°C to remove residual stresses. The sheet is then covered on both sides by adhesive paper to protect the surfaces. The production of rod stock follows similar procedures but since thicker sections are involved, greater problems of heat dissipation during polymerisation are encountered The monomer boils at just over 100°C and if this temperature is reached, bubbles are formed in the moulding. Various me hods are used to counteract the problem, but most of them involve an external pressure to raise the boiling point of the monomer. The lengthy process described above is necessary if the best optical properties are to be obtained Another facet of this production method is that the molecular weight of the PMMA is very high (about 1 million, ie about 10000 units per chain) which gives the polymer better mechanical properties, heat resistance and chemical resistance than the moulding materials. For example, the long term service temperature of cast sheet is about 80°C which is several degrees higher than most moulding grades. Otherwise, properties are very similar to the moulding materials. Cast sheet is available in standard sizes and can be cut to size and shape as required for a variety of purposes, usually as a substitute for glass. It is found in aircraft, buses (roof lights),  commercial and agricultural vehicles and buildings. Not only is it available as transparent sheet but also available in a wide range of colours, both transparent and opaque. It is widely used in advertising and shop display shelving. Three dimensional shapes can be made by thermo-forming but only thin sheet can be vacuum formed because of the very high molecular weight. The thermo- forming process involves heating the sheet to about 160°C when it softens to a rubbery state which can be handled. It is then moulded over a former and clamped while it cools to shape. A wide variety of mouldings, some quite large, are produced this way. Public walltelephone bubbles and helicopter cockpits are made in this way. Rod stock is also provided in standard sizes for machining (the material has good machining characteristics for shaping by all standard workshop techniques provided guidelines are followed) to quite complex shapes and even simple shapes where the superior properties of the cast material are required. An example is components for medical equipment where it may be desirable to , be able to visually monitor fluid flow whilst having a robust and durable component. ometimes, the polymerisation process is done in a mould to produce the finished article directly. Examples are dentures (where a highly filled material is used) and decorative paper weights in which items are encapsulated. Acrylic sheet also finds use in the fashion world Dress items (eg belt buckles) can be cut to shape (or moulded) and then dyed on the surface using masks where necessary to produce a variety of patterns. It is worth noting that scrap material cannot be recycled as can the moulding grades. Re-ground cast material has a melt viscosity far too high to be melt processed Represen tti a ve pronernes 0f A crylIic M aten ial s Cast Sheet Moulding Materials Tensile Strength MPa 82-86 36-72 Elongation at break % 4 4 Tensile modulus GPa 2.8 2.0-2.5 Flexural Strength MPa 140 105-130 Flexural modulus GPa 3.0 1.4-3.1 Notched Izod J/12.7mm 0.3 0.2"().4 Hardness RockwellM 100 < 100 HDT (1.81 MPa) °C 100-105 60-100 Volume resistivity Qcm >1015 >1015 Permittivity (1kHz) 2.7 3.0-3.2 Tan8 0.04 0.02..().04 Specific gravity 1.16-1.18 1.9-2.0 Water abosorption % 0.2..().4 0.3..().8  i Design and Processing Data Styrinics and Acrylic Design Data: Shrinkage Critical wall (mm) Flow Ratio ABS 0.4 to 0.7 % 4mm 80 -150 TPS 0.2 to 0.8 % 3.5mm 100-130- Acrylic. 0.2 to 1.0 % 5mm 100-150 Comments: Low shrinkage materials with higher Tg values (eg. 80 C for PS) than the olefmes, will not tend to creep under load at room temperature. Very poor chemical resistance at room temperature. Acrylic has excellent UV resistance, whereas ABS, PS and TPS all exhibit poor UV resistance. These materials enable the use of thicker more accurate wall sections due to their low shrinkage rates. Good aesthetic characteristics (PS and Acrylic are clear) and all exhibit a high gloss finish. Poor chemical resistance enables the use of all finishing techniques (painting, glue fixing, plating, foiling, etc) The materials are brittle in nature and have to be impact modified (rubber added, ego ABS & TPS) to resist moderate impact loads. Typical Uses: Acrylic (PMMA) Injection moulded light lenses, extrusions, often employed for light transmission applications, etc. ABS - Injection moulded housings, carcases, electrical cases, etc. TPS Vacuum formed packaging, injection moulded disposable products (eg. knives, forks, spoons, etc), foamed drinking cups, insulation, etc. Processing Nature: Low to mid range melt temperatures (eg. 170 - 240 C), good thermal stability and large processing windows mean these polymers are easy to process. Styrinics exhibit low specific heat contents (they absorb less energy to melt and cool when compared to other polymers - ego olefines). Energy use footprints can be low and the materials have good second hand values (especially if clear). Typical injection force requirements: PMMA - 4 tsi or 60 MPa ABS 3 tsi or 45 MPa TPS 2.5 tsi or 37.5 MPa  I POLYVINYL CHLORIDE INTRODUCTION Polyvinyl chloride (PVC) became commercially available about 1930 and it is therefore, with polystyrene, the oldest of the thermoplastics in use today. In 1995, the UK consumption of PVC amounted to about 600 000 tonnes per year, in Europe the figure is over 6 million tones and worldwide, 20.6 million tonnes were used. The 1995 growth rate of 4.8% means that if this continues, the use in the year 2005 will exceed 32 million tonnes. There are two basic types of PVC, unplasticised PVC (PVC-u or upvq which is a rigid material and plasticised PVC (PVC-p or PPVC) which is a flexible material. The uses of PVC are many and varied as the following table shows " Areas of Use of PVC Western Europe Building and Construction 50 Packaging 18 Wire and Cable 10 Transport 4 Leisure 3 Furniture 3 Clothing and Footwear 2 Domestic Appliances 1 Others 9 There are many reasons for the versatility and popularity of PVC and the following are the more important: Low cost of raw polymer - in recent years, the price of common grades of PVC have been around £500 per tonne which is similar to polyethylenes and polypropylenes. However, since the specific gravity of PVC is about 1.4 compared with the polyolefines (0.90-0.96), PVC is relatively more expensive per unit product. Additional costs are incurred through the need to incorporate additives and the cost of additives and compounding need to be considered. It may be noted that in the PVC industry, the raw polymer is referred to as PVC Resin and the polymer with additives is known as PVC Compound Useful property profile - PVC polymer has reasonably good physical properties at temperatures around room temperature. Its main drawback is its low softening point and it cannot be used above about 60°C. It can have high optical clarity and has good chemical resistance. It is also fire resistant. Extension of the range of properties - the properties of PVC can be improved or modified by the incorporation of additives, However, the properties can be and are extensively modified by the use of plasticisers. In principal, other polymers can be modified by the use of plasticisers but many are not and those that are use plasticisers to a limited extent. The main purpose of the plasticiser is to make the PVC more flexible and if sufficient plasticer is added, the PVC compound can be not only very soft and flexible at room temperatures but can maintain these properties down to around -50°C. Versatility of processing - the polymer as compound can be shaped by all the melt processing techniques associated with thermoplastics, eg extrusion, injection moulding, blow moulding, vacuum forming and film forming techniques. without difficulty. Additionally. the process of calendering is extensively used to produce flexible sheet. The PVC resin can also be made into liquid form without melting using plasticisers to. form a paste. Paste processing methods include rotational moulding, casting, roller coating, spreading and dipping. In contrast with other thermoplastics, injection moulding is relatively unimportant and the bulk of PVC is processed by extrusion..  The popularity of PVC, indeed the very existence of PVC, bas come under increasing threat in recent years because of supposed hazards and the environmental impact on the environment as a packaging material. Despite the environmentalists'claims, which are largely based on un-informed opinion, PVC remains as important as ever as the usage growth figures indicate. STRUCTURE AND PROPERTIES PVC polymer has the repeat unit -CH2--CH- I Cl The word vinyl indicates that the polymer is made from a monomer of the type -CH2=CHX-. This means that polystyrene, polypropylene, polymethylmethacrylate etc are all vinyl polymers but tbe term 'vinyl' is in practice used exclusively in association with PVC The polymer is produced by a free radical polymerisation process and is atactic in structure and therefore essentially amorphous. There is a degree of short range molecular ordering, often referred to as crystallinity, which amounts to less than 10% of the material and therefore, to all intents and purposes, PVC is regarded as amporphous. It is therefore intrinsically transparent and its softening behaviour is determined by its glass transition temperature of about SO°C The Heat Distortion Temperature (I.S2 MPa) of PVC polymer is about 70°C. The short range ordering oftbe polymer chains does not disappear until temperatures of around 250°C resulting in the poor flow and fusion properties of PVC polymer unless it is plasticised. The presence of the chlorine atom in the repeat unit makes the polymer chain somewhat polar due to the desire of the cblorine atom to acquire electrons. The carbon-chlorine is polarised thus -CO+-CIII- The polymer is fherefore soluble in polar solvents such as tetrahydrofuran and cyclohexanone and is swollen by many others but is resistant to non-polar liquids such as fuels, oils and greases. The polarity limits tbe use of PVC as an electrical insulating material to low frequency applications such as domestic wiring. The high chlorine content of the polymer imparts good fire resistance without the use of fire retardants although these may be used to further improve the fire resistance. However, when PVC polymer does burn, large amounts of smoke are produced Since plasticisers are flammable, their incorporation reduces the fire resistance and plasticised PVC compounds may bum readily with smoke. PVC has poor thermal stability. Exposure to temperatures even as low as 100°C will produce discoloration (yellow to black depending on the exposure time). PVC polymers need temperatures of between 160-190°C for melt processing and therefore, PVC compounds contain heat stabilisers to prevent the discoloration and change of properties. Even with heat stabilisers, temperatures higher than about 190°C are not advisable and these processing temperatures are considerably less than the 250°C needed to achieve complete fusion of the polymer giving rise to processing problems which are alleviated by the use of processing aids. Even then, the lack of complete fusion- means that it is doubtful if the full potential of the polymer's strength properties are always developed in UPVC TYPES OF PVC POLYMER There are.four ways of manufucturing PVC from vinyl chloride monomer which give rise to distinct forms of the polymer for use in different applications. All four types are made by one form or other of the free radical type of addition polymerisation. Theses types are: 2  Suspension Polymer - the most widely used type of the polymer. In suspension polymerisation, the monomer is dispersed as small droplets in water with the aid of suspension agents and agititation to prevent the droplets from coalescing. The resulting polymer is small particles (about 0.1 mm diameter) which may be porous or non-porous depending upon the reactor conditions. The particles are coated with a thin coating of residual suspension agents. The porous polymer is used for plasticised PVC sine it readily absorbs plasticiser. Bulk (Mass) Polymer - this is produced by polymerising the vinyl chloride monomer as the sole liquid in the reactor. It is more difficult than the suspension polymer because of the problems of dissipating the heat generated during the exothermic polymerisation. The advantage of this polymer over suspension polymer is that polymer is pure (no suspension agents or other contaminants) and therefore has somewhat better clarity and electrical properties Emulsion Polymer - this is made by polymerising the monomer as an aqueous emulsion. The resulting polymer is similar to suspension polymer except that the particles are much smaller- about 0.001 mm diameter although these often fuse together to form larger particles. The polymer tends to fuse more readily on heating due the more favourable heat transfer conditions of having a much larger surface area and smaller particles. They are sometimes prefered for calendering for this reason and are widely used in making PVC pastes because the fine particles remain in suspension when mixed with a plasticiser. Micro-suspension Polymer - this is made by a suspension polymerisation method but in such a way that the resultant polymer has a particle size that is similar to that of the emulsion polymerisation COPOLYMERS A copolymer is a polymer that is made from two monomers. In the case of PVC, a number of co-monomers are used to reduce the regions of short range order (in effect, to reduce the "crystallinity") which allows improved fusion and flow characteristics in melt processing. The mechanical properties of the copolymers are slightly different from the PVC polymer itselfbut the difference depends upon the co-monomer and the amount incorporated. The most common copolymer is that of vinyl chloride and vinyl acetate (usually 5-20%). It was formerly widely used for the "vinyl" LP and EP records but its main use is now in the production of sheet for vacuum forming. Copolymers with vinylidene chloride, acrylonitrile and other vinyl esters have also been produced. CHAIN LENGTH The mechanical properties and processing characteristics of PVC polymer is, like all polymers, affected by the chain length, normally expressed as the molecular weight. High molecular weight polymer has better physical properties but is more difficult to melt process because of higher melt viscosity at a particular temperature. Low molecular weight polymer is easier to melt process but but the resultant products are lower mechanical strength properties. Whilst it is possible to measure the molecular weight (strictly the average molecular weight since it is impossible to manufacture polymer where all the chains are the same length), it is normal practice with all polymers to grade them by flow characteristics as this is quicker and cheaper than determining the molecular weight. Most thermoplastics are graded according to their melt flow characteristics using Melt Flow Rate (Index) otherwise known as MFR or MFI. This is not the case with PVC because of its thermal instability. Instead, the viscosity of a dilute solution of PVC polymer in a suitable solvent is used. Using standard viscometers, the flow times of the polymer solution of known concentration and the solvent itself are measured. The information is used in the equation given below to obtain what is known as the Fikentscher K value or more usually as the K-value for the polymer../ 3  75 K2 c X 10-6 + Kc X 10-3 1 + l.5Kc X 10-3 where 1s and to are the flow times of the solution and solvent respectively and c is the polymer concentration in gllOO ml. Unfortunately, the value of K for a polymer depends upon the solvent used and the concentration used to measure K which means that K values from different suppliers do not necessarily indicate the same grade of polymer resin The higher the K value, the higher the molecular weight. In practice, K values are normally in the range 45 - 100 although values outside this range may be encountered. When using unplasticised polymer, high values are used for extrusion (eg K84) because the process is capable of handling higher viscosity polymer when produceing simple profiles and better properties result. Injection moulding polymer requires a lower molecular weight (eg K57) and and calendering lower still (eg K45) because of the greater difficulties in processing using these methods of manufacture. An alternative method is the ISO Viscosity Number which is given by ISO Vicosity Number = ~ c to and the solvent is specified as cyclohexane at 25°C. The concentration (c) is normally 0.005 glee. The ISO viscosity number has two advantages over the K-value since the solvent and temperature are specified, the value is independent of the test method. the value is easier to calculate than K The ISO Viscosity Number is now the prefered way of specifying the gade of PVC and most manufacturers use it rather than K. However, old habits die hard and many processors refer to K values. The normal range of ISO Viscosity Numbers is 65 - 140 and as with K values, high numbers indicate higher molecular weight and therefore graeter difficulty in processing ADDITIVES PVC polymer is very difficult to process because of its instability to heat and because of the flow problems. Therefore if PVC is to be processed using heat and flow, a number of additives need to be present. Additionally, a plasticiser may be present to produce flexible products. Other additives may also be present for a variety of reasons. Some product manufacturers do their own compounding and conversion of the compound to the finished product. Two areas where this is likely to be seen is in calendering and in pipe extrusion. The majority of PVC converters buy in compound from specialist PVC compounders. Compounding is done either in specially designed (for PVC) twin screw compounding exttuders or, where plasticisers are used (eg calendering industries), the compounding may be done in an internal mixer followed by a two roll mill in a manner similar to the rubber industry. Various other mixing devices are used and the whole process of compounding may involve several stages to achieve the correct degree of dispersion and the physical form (powder, granules) for the end use of the compound. The essential difference between a rigid compound (UPVC) and a flexible (PPVC) is the presence of a plasticiser in the latter. Given below is a table of the additives that might be found in a PVC compound. Heat Stabiliser prevents discoloration during processing Lubricant reduces friction during processing. Plasticiser iIncreases flexibility of product Filler increases bulk of the product and cheaply Processing aid promotes fusion 4  J Pigment colours the product Impact modifier improves toughness and therefore impact resistance UVabsorber improves weatherability Blowing agent produces cellular product Anti-oxidant prevents oxidation of the plasticiser Antistatic agent prevents the build-up of electrostatic charges Fire retardant improves fire resistance PROCESSING AND PRODUCTS Processing involves the initial fusion of the compound (powder or granule) followed by flow and shaping to the final product. The problems of flow and degradation are more severe with UPVC than PPVC. Melt temperatures are in the range 160-210°C for UPVC and 140-170°C for PPVC. The material must not become overheated in the processing machinery which is therefore designed to produce smooth flow with no dead spots. Additionally, metals should be either corrosion resistant (expensive) or coated with corrosion resistant protective metal coatings to prevent attack by hudrochloric acid which is produced when PVC degrades.~ Extrusion The majority ~fPVC is extruded using single screw machines although twin screw machines have the advantage of producing less shear and therefore shear heating. Among the wide range of product types are hose - including garden hose, medical tubing, sheathing (eg pram handles), wire and cable insulation (using a cross head die) - all made from PPVC pipe - water drainage pipes and waste disposal pipes in buildings, gutters, chemical plant, electrical and other conduit - all from UPVC sheet - for the building industry as roofing panels, internal wall dividers etc - UPVC window frames and doors - UPVC film - by the blown film process (often horizontal rather than vertical) - much of it for medical applications - PPVC Extrusion Blow Moulding. (\ I A wide range of transparent and non-transparent bottles are made from UPVC for use with mineral water, cooking oils, fruit juices and toiletry products. Extrusion stretch blow moulding is increasingly being used to make UPVC products with greater clarity to compete with PET injection stretch blow moulded bottles.- Injection Moulding Not so widely used as with most other polymers but pipe fittings, shoes and shoe soles are moulded from PVC - UPVC and PPVC as appropriate. Some machine housings are alsomade from. UPVC. ' Calendering A great deal of UPVC and PPVC sheet and film is still made by calendering. It is mainly used in packaging applications, often the sheet or film is vacuum forming to the final shape. (see below). PPVC is widely /' used for cling film. PPVC sheet is also used for floor coverings (eg in roll form or as tiles) and as vinyl wall.coverings. In the latter case, it is often cellular PPVC. 5  I Vacuum Forming A high proportion of PVC sheet is converted by vacuum forming to suitable shapes for food' and other packaging: (the other major materials in this respect are toughened polystyrene and polypropylene). Examples are chocolate andbiscuit trays, blister packaging of food stuffs and other items, especially ironmongery. Liquid State Processing A wide range of products are made from PVC pastes which are dispersions of PVC powder in a plasticiser using a variety of processes. These include spreading techniques - for leathercloth, belting and wallpapers by rotational moulding - for balls and dolls dipping - for gloves and boots In all cases, the paste is heated to gel the paste to a solid form. Temperatures are usually around 170°C ENVIRONMENTAL ISSUES Among all plastics materials, PVC has been singled out as being of special concern by environmentalists as part of a wider campaign against organo-chlorine compounds. Many concerns are about waste plastics, principally packaging materials of which PVC is a principal polymer. There are also concerns about using the earth's resources, in this case oil, in a wasteful way. PVC itself does not present a toxic or other environmental hazard. However, various materials associated with PVC manufacture and use do pose environmental problems. The principal ones are The monomer, vinyl chloride (VCM) is a carcinogen and so the levels of residual VCM in PVC must be kept low (usually less than 1 ppm) The heat stabilisers are based upon heavy metals such as lead. These could pose a hazard in dumped waste. More expensive stabilising system avoid the problem There is some concern about migration of plasticisers into food stuffs having potential toxic effects. As yet there is little evidence that this is a problem. Apart from land-fill, incineration is a method of disposing of waste. In this process, hydrochloric acid is produced. Since all incineration plants are required by law to have 'acid scrubbers', the former argument that incineration is not viable with PVC is no longer valid. However, the incineration of PVC may release dioxins although the evidence is that it does not. In principal, PVC can be recycled like any other thermoplastic polymer. The main problem is that PVC compounds used to make a wide variety of products have a wide variety of formulations. This means that the compounds need to be sorted into types which is made more difficult by the fact that precise formulations are not known. At present, much work is being carried out to find viable re- cycling methods rather than dispose of the waste by landfill or incineration. Many of the claims of environmentalists have little foundation and are based on wrong information or the mis-interpretation of information. It is a measure of the usefulness of PVC that despite its 'bad press', it remains one of the major polymers and continues to grow in usage. ,/ 6  J TYPICAL PROPERTIES OF UPVC Property Units UPVC PPVC Tensile strength MPa 40-50 10-25 elongation at break. % 40-80 200-450 compressive strength Mpa 55-110 6-12. flexural strength MPa 70-110 flexural modulus MPa 2.0 - 3.5 Izod impact J/12.5 mm 0.3 - 15 Hardness Shore D65-85 A50-100 HDT (1.82 ~a) DC 60-77 water absorption % in 24 hours 0.04 - 0.4 0.15-0.75 mould shrinkage % 0.1-0.5 1-5 thermal expansion mmlmmx 106 50-100 70-250 Some Trade Names and Suppliers Corvic European Vinyls Corporation (EVC) Hostalit Hoechst Lacovyl Atochem Norvinyl Hydro-Polymers Pevikon Hydro-Polymers Ravinil European Vinyls Corporation (EVC) ".:» Solvic Vestolit Solvay Huls Vinnol Wacker Vinoflex BASF Vipla European Vinyls Corporation (EVC) 7 Design and Processing Data PVC&UPVC Design Data: Shrinkage Critical wall (mm) Flow Ratio PVC 1.5 to 5.0 % 2.3mm up to 180 UPVC - 0.2 to 0.4 % 4.5mm about 60- Comments: UPVC is a cheap, rigid, low shrinkage polymer which exhibits a Tg of approx 80 C. UPVC is never used in its base polymer state, it is always compounded with additives to enhance specific properties. A knowledge of the basic additives is required to effectively use this polymer. PVC (plasticised UPVC) can vary in its properties from almost rigid to a soft rubbery fabric material depending upon the % plasticiser present. Blowing agents are often added to UPVC and PVC to create cellular structures, like rubbery foams. PVC in particular has suffered from bad press and is often mis-understood outside the polymer industry. The cheaper grades of PVC can be compounded from toxic additives (eg.lead stabilizers, carceogenic plasticisers, etc), these compounds should be avoided by designers and organic substitute compounds selected instead. Typical Uses: PVC- Leather type cloth, fabrics, floor tiles, floor coverings, toys, film, vacuum forms, etc. UPVC - Injection moulded building products, carcases, covers, etc. Extruded pipes, gutter sections, fixings, etc. Processing Nature: Low melt temperatures (eg. 140 - 165 C typical), poor thermal stability and narrow processing windows mean these polymers can be tricky to process. PVCIUPVC exhibit low specific heat contents (they absorb less energy to melt and cool when compared to other polymers - ego olefines). Energy use footprints can be very low. Low energy useage is possible when processing UPVC and small carbon footprints are possible for re-pro applications in particular. Typical injection force requirements: PVC - 1.5 to 3.5 tsi (22.5 MPa to 52.5 MPa) depending upon % plasticiser present. UPVC - 4 tsi (60 MPa)  I. Introduction This isa group of plasticsmaterials \Nith a superior I'a1ge of properties cxmpared to the oommodity plastics. Among the oommodity plastics, polyethylene istough but lacks stiffness whereas polystyrene is stiff but brittle. PVC and PPare both intennediate in their properties. In contrast an engineering plastic is one which is both stiff and tough. The engineering performance of a oommodity plastic may be enhanced by various modifications suffident for it to be 'promoted' to the engineering plastic dass. In partirular, polypropylene may be reinforced to increase its stiffness and toughness whilst ABS is well known as a superior toughened form of polystyrene. Nevertheless, people do not consider such modified commodity plastics as true engineering plastics. Apartfrom reinforced PPand ABS. there are five main polymer types that make up the most generally used engineering plastics. These are: Nylons or polyamides (PA) - Ace1as or ~xymethylene (POM) Polycarbonate (PC) Polyphenylene oxide (PPO)or Polyphenylene ether (PPE)- blended with PS Thermoplastic Polyester, inc, ~ene terephthalate (PEl) and PoMlutyiene terephmalate (PBl). Total oonsumption of these materials is about 25 million tonnes per year in Westem Europe. When oompared to a I total plasticsronsumption of about 25 mUIiootonnes, they represent only about 10010 of plastics usage. Of course, on / a value, as opposed to a tonnage basis, they are relatively more important Of these mai