Chapter 11 Plastics and composites 11.1 Utilization of polymeric various molecular relaxation processes associated with materials the glass transition. In linear viscoelastic behaviour, total strain comprises a linear elastic( Hookean)com- 11.1.1 Introduction ponent and a linear viscous(Newtonian) component. The stress-strain ratios depend upon time alone. In the arrangements of long-chain molecules were described to polymer, strain is a function of time and stress classified as thermoplastics, elastomers and thermosets Certain aspects of their practical utilization will now to chart the way in which the behaviour of a given be examined, with special attention to processing, polymer changes from glassy to rubbery Figure 11.1 his stage plays a decisive role in deciding if a shows the non-linear response of a polymer that is marketable commodity. The final section(11.3)will time t. The relaxation modulus E, at time t is give concern composites, extending from the well-known by the expression glass-reinforced polymers to those based upon ceramic Er=o/ee (11.1) 11.1.2 Mechanical aspects of Tg Thus Er, which is represented by the dotted join lines, decreases with time. This As indicated in Chapter 2, it is customary to quote a is shown more precisely by a plot of log er glass-transition temperature Tg for a polymer because log time t, as in Figure 11.2a. The thermoplastic it separates two very different regimes of mechanical behaviour. (The value of Tg is nominal, being subject to the physical method and procedure used in its deter mination). Below Tg, the mass of entangled molecule is rigid. Above Tg, viscoelastic effects come into play and it is therefore the lower temperature limit for pro- cessing thermoplastics. The structural effect of raising the temperature of a glassy polymer is to provide ar input of thermal energy and to increase the vibrations of constituent atoms and molecules. Molecular mobilt ncreases significantly as Tg is approached: rotation about C-C bonds in the chain molecules begins, the free volume of the structure increases and intermolecu. lar forces weaken. It becomes easier for applied forces to deform the structure and elastic moduli to fall Tensile The mechanical properties of polymers are high dependent upon time and temperature, the response E to stress being partly viscous and partly elastic. For instance,natural time periods ociated with the Figure 11.1 Stress relaxation at constant strain
Chapter 11 Plastics and composites 11.1 Utilization of polymeric materials 11.1.1 Introduction In Chapter 2 the basic chemistry and structural arrangements of long-chain molecules were described and it was shown how polymers can be broadly classified as thermoplastics, elastomers and thermosets. Certain aspects of their practical utilization will now be examined, with special attention to processing; this stage plays a decisive role in deciding if a particular polymeric material can be produced as a marketable commodity. The final section (11.3) will concern composites, extending from the well-known glass-reinforced polymers to those based upon ceramic and metallic matrices. 11.1.2 Mechanical aspects of Tg As indicated in Chapter 2, it is customary to quote a glass-transition temperature Tg for a polymer because it separates two very different r6gimes of mechanical behaviour. (The value of Tg is nominal, being subject to the physical method and procedure used in its determination). Below Tg, the mass of entangled molecules is rigid. Above Tg, viscoelastic effects come into play and it is therefore the lower temperature limit for processing thermoplastics. The structural effect of raising the temperature of a glassy polymer is to provide an input of thermal energy and to increase the vibrations of constituent atoms and molecules. Molecular mobilty increases significantly as Tg is approached: rotation about C-C bonds in the chain molecules begins, the free volume of the structure increases and intermolecular forces weaken. It becomes easier for applied forces to deform the structure and elastic moduli to fall. The mechanical properties of polymers are highly dependent upon time and temperature, the response to stress being partly viscous and partly elastic. For instance, 'natural' time periods are associated with the various molecular relaxation processes associated with the glass transition. In linear viscoelastic behaviour, total strain comprises a linear elastic (Hookean) component and a linear viscous (Newtonian) component. The stress-strain ratios depend upon time alone. In the more complex non-linear case, which usually applies to polymers, strain is a function of time and stress because molecular movements are involved. The phenomenon of stress relaxation can be used to chart the way in which the behaviour of a given polymer changes from glassy to rubbery. Figure 11.1 shows the non-linear response of a polymer that is subjected to constant strain e0. Stress cr relaxes with time t. The relaxation modulus Er at time t is given by the expression: Er = ort / eo (11.1) Thus Er, which is represented by the slope of dotted join lines, decreases with time. This variation is shown more precisely by a plot of log Er versus log time t, as in Figure l l.2a. The thermoplastic t Tensile Stress o (~t fl Tensile Strain s i Eo Figure 11.1 Stress relaxation at constant strain
352 Modern Physical Metallurgy and Materials Engineering (a) Glassy Leat Rubbery 6 5 Liquid Figure 11.2 Time-temperature dependence of elastic modulus in thermoplastic polymeric solid: (a)change in relaxation modulus E,(r)as function of time,(b) change in tensile modulus as function of temperature ( from Hertzberg, 1989: by polymer changes in character from a glassy solid, been added Numerous types of additive are used by where the relaxation modulus is a maximum, to a manufacturers and fabricators; in fact, virgin polymers rubbery solid. In the complementary Figure 11. 2b, are rarely used. An additive has a specific function data from standard tensile tests on the same polymer Typical functions are to provide (1)protection fror at different temperatures are used to provide values the service environment(anti-oxidants, anti-ozonants of elastic moduli(E). The similarity of profiles anti-static agents, flame-retardants, ultraviolet radia Figures 11.2a and 11.2b illustrates the equivalent tion absorbers),(2)identification(dyes, pigments), time and temperature (Theoretically, the modulus for (3)easier processability(plasticizers ),(4)toughness, a short time and a high temperature may be taken to and(5)filler equate to that for a combination of a long time In many instances the required amount of additive low temperature; this concept is used in the prepa ranges from 0. 1% to a few per cent. Although uitra- of relaxation modulus versus time graphs. )The violet (UV) components of sunlight can structurally transition temperature Tg has been superimposed upon alter and degrade polymers, the effect is particularly s 11.a and 11. 2b marked in electric light fittings(e.g. yellowing ). Stabi ough single values of Tg are usually lizing additives are advisable as some artificial light the process of molecular rearrangement is co sources emit considerable amounts of UV radiation and minor transitions are sometimes detectable. Thus, with wavelengths in the range 280-400 nm. For any for Pvc, the main glass transition occurs at ten polymer, there is a critical wavelength which will have peratures above 80C but there is a minor transition the most damaging effect. For instance, a wavelengt at-40C. Consequently, at room temperature, PVc of 318.5 nm will degrade PS, which is a commo exhibits some rigidity yet can elongate slightly before choice of material for diffusers and refractors, by either fracture. Addition of a plasticizer liquid, which has a causing cross-linking or by producing free radicals that very low Tg, lowers the Tg value of a polymer. Sim- react with oxygen arly, Tg for a copolymer lies between the Tg values The action of a plasticizer (3)is to we of the original monomers; its value will depend upon molecular bonding by increasing the n of the ionomer proportions hain molecules. The plasticizer may take the form of increased, the relatively few crosslinks begin to vibrate before processing. Additions of a particulate toughener gorously at Tg and the elastomer becomes increas (4)such as rubber may approach 50% and the material ngly rubbery. As one would anticipate, Tg values is then normally regarded as a composite for rubbers lie well below room temperature. Increas- a wide variety of fillers(5)is used for polymers. In ing the degree of crosslinking in a given polymer the case of thermosets, substances such as mica, glass has the effect of raising the entire level of the lower fibre and fine sawdust are used to improve engineering rubbery' plateau of the modulus versus temperature properties and to reduce the cost of moulded prod- lot upwards as the polymer becomes more gla cts. Ptfe has been used as a filler(15%)to improve d are accordingly hard and brittle conductive by loading them with an appropriate temperature filler(e.g. electromagnetic shieiding, specimen mounts 11.1.3 The role of additives in SEM analysis). Fillers and other additives play an important role in the production Industrially, the term 'plastic Inert fillers facilitate handling of the material before fying agents have vulcanization(e.g. clay, barium sulphate). Reinforcing
(a) (b) ~" 9 E Z8 7 u~ m 6 0 .J 5 i i rg r Glassy ~a ~ - ? 9 , E thery z 8 Z i ~,..~~ery ua o tm 7 _a 6 ' Liquid 5 log t 352 Modem Physical Metallurgy and Materials Engineering Figure 11.2 Time-temperature dependence of elastic modulus in thermoplastic polymeric solid: (a) change in relaxation modulus Er(t) as function of time; (b) change in tensile modulus as function of temperature (from Hertzberg, 1989; by permission of John Wiley and Sons). polymer changes in character from a glassy solid, where the relaxation modulus is a maximum, to a rubbery solid. In the complementary Figure l l.2b, data from standard tensile tests on the same polymer at different temperatures are used to provide values of elastic moduli (E). The similarity of profiles in Figures 11.2a and 11.2b illustrates the equivalence of time and temperature. (Theoretically, the modulus for a short time and a high temperature may be taken to equate to that for a combination of a long time and a low temperature; this concept is used in the preparation of relaxation modulus versus time graphs.) The glass transition temperature Tg has been superimposed upon Figures 11.2a and 11.2b. Although single values of Tg are usually quoted, the process of molecular rearrangement is complex and minor transitions are sometimes detectable. Thus, for PVC, the main glass transition occurs at temperatures above 80~ but there is a minor transition at -40~ Consequently, at room temperature, PVC exhibits some rigidity yet can elongate slightly before fracture. Addition of a plasticizer liquid, which has a very low Tg, lowers the Tg value of a polymer. Similarly, Tg for a copolymer lies between the Tg values of the original monomers; its value will depend upon monomer proportions. In elastomeric structures, as the temperature is increased, the relatively few crosslinks begin to vibrate vigorously at Tg and the elastomer becomes increasingly rubbery. As one would anticipate, Tg values for rubbers lie well below room temperature. Increasing the degree of crosslinking in a given polymer has the effect of raising the entire level of the lower 'rubbery' plateau of the modulus versus temperature plot upwards as the polymer becomes more glassy in nature. The thermosets PMMA (Perspex, Lucite) and PS have Tg values of 105~ and 81~ respectively, and are accordingly hard and brittle at room temperature. 11.1.3 The role of additives Industrially, the term 'plastic' is applied to a polymer to which one or more property-modifying agents have been added. Numerous types of additive are used by manufacturers and fabricators; in fact, virgin polymers are rarely used. An additive has a specific function. Typical functions are to provide (1) protection from the service environment (anti-oxidants, anti-ozonants, anti-static agents, flame-retardants, ultraviolet radiation absorbers), (2) identification (dyes, pigments), (3) easier processability (plasticizers), (4) toughness, and (5) filler. In many instances the required amount of additive ranges from 0.1% to a few per cent. Although ultraviolet (UV) components of sunlight can structurally alter and degrade polymers, the effect is particularly marked in electric light fittings (e.g. yellowing). Stabilizing additives are advisable as some artificial light sources emit considerable amounts of UV radiation with wavelengths in the range 280-400 nm. For any polymer, there is a critical wavelength which will have the most damaging effect. For instance, a wavelength of 318.5 nm will degrade PS, which is a common choice of material for diffusers and refractors, by either causing cross-linking or by producing free radicals that react with oxygen. The action of a plasticizer (3) is to weaken intermolecular bonding by increasing the separation of the chain molecules. The plasticizer may take the form of a liquid phase that is added after polymerization and before processing. Additions of a particulate toughener (4) such as rubber may approach 50% and the material is then normally regarded as a composite. A wide variety of fillers (5) is used for polymers. In the case of thermosets, substances such as mica, glass fibre and fine sawdust are used to improve engineering properties and to reduce the cost of moulded products. PTFE has been used as a filler (15%) to improve the wear resistance of nylon components. Although usually electrically non-conductive, polymers can be made conductive by loading them with an appropriate filler (e.g. electromagnetic shielding, specimen mounts in SEM analysis). Fillers and other additives play an important role in the production of vulcanized rubbers. Inert fillers facilitate handling of the material before vulcanization (e.g. clay, barium sulphate). Reinforcing
Packaging LILDPE angell Total 1990 72 million tonnes furnture etcl Total 1990 13 6 mallon tonnes Figure 11.3(a)World consumption of plastics, (b) plastics consumption by market sector, Western Europe, and (c) destination fillers restrict the movement of segments between the limited. Research is mainly concerned with improv branching points shown in Figure 2.24. For instance, ing and reducing the cost of established material carbon black has long been used as a filler for car tyres, ( e.g. improved polymerization catalysts, composites, giving substantial improvements in shear modulus, tear thermoplastic rubbers, waste strength, hardness and resistance to abrasion by road The low- and high-density forms of polyethylene, dur- LDPE and HDPE, were developed in the 1940s and ing their working life and it has been found useful 1950s, respectively. Extruded LDPE is widely used as to express their stress-strain behaviour in terms of thin films and coatings(e.g. packaging ) HDPE is used a dynamic shear modulus(determined under cyclic for blow-moulded containers, injection-moulded crates stressing conditions at a specified frequency). Time is and extruded pipes. An intermediate form of adjustable required for molecular rearrangements to take place density known as linear low-density polyethylene, in an elastomer; for this reason the dynamic modu LLDPE, became available in the 1980s. Although lus increases with the frequency used in the modulus more difficult to process than LDPE and HDPE, film est. The dynamic modulus at a given frequency is extruded LLdPE is now used widely in agriculture, significantly enhanced by the crosslinking action of horticulture and the construction industry (e.g. heavy vulcanization and by the presence of carbon black. duty sacks, silage sheets, tunnel houses, cloches, damp hese carbon particles are extremely small, typically proof membranes, reservoir linings). Its tear strength 20-50 nm diameter. Small amounts of anti-oxidants and toughness have enabled the gauge of PE film to reduced a further variant of pE is ultra-high molecu agents of degradation under service conditions are ar weight polyethylene, UHMPE, which is virtually extreme temperatures, oxygen and ozone, various liq uids. In the last case a particular liquid may penetrate ization. Because of its high molecular mass, it needs etween the chains and cause swellin to processed by sintering UHMPE provides the wear resistance and toughnes ired in artificial joints of 11. 1. 4 Some applications of important Polyvinyl chloride(pvc) is the dominant plastic in plastics the building and construction industries and has effe tively replaced many traditional materials such as steel, Figures 11. 3a and 11.3b summarize 1990 data on cast iron, copper, lead and ceramics. For example world and West European consumption of plastics. The the unplasticized version(UPvC) is used for win- survey included high-volume, low-price commodity dow frames and external cladding panels because of plastics as well as engineering and advanced plastics. its stiffness, hardness, low thermal conductivity and Thermoplastics dominate the market (i.e. PE, PVC and weather resistance. PVc is the standard material fo PP). Development of an entirely new type of plastic is piping in underground distribution systems for potable extremely expensive and research in this direction is water(blue)and natural gas(yellow), being corrosion resistant and offering small resistance to Auid flow. In the 1930s. concern with oxidation led the Continental Ithough sizes of Pvc pipes tend to be restricted, PVC Rubber Works, Hannover, to experiment linings are used to protect the bore of large-diameter nitrogen-infation of tyres intended for use on Mercedes pipes(e.g. concrete). The relatively low softening tem- Iver Arrows, Grand Prix racing cars capable of 300 perature of Pvc has stimulated interest km/h. This practice was not adopted by Mercedes-Benz for piping materials for underfloor heating systems. Poly track events butylene(PB)has been used for this application; being
Plastics and composites 353 (a) By region By product (b) (c) Western North AmerIca Packaging Euro~ / r ILDPE PVC Landfill ~struchon ~ Japan ABS Incmeratton Recyc recovery Lahn Arnenca HDPE electronics " ~ -- Automohve/ Inc~nerahon- " Total 1990 72 rndhOn tonnes furnIture, etc) transport w~thout heat recovery Total consumption 1990: 24 mdhon tonnes (Source Sema Group/Sotres ) Total 1990 13 6 mdhon tonnes (Source Sema Group/Sofres) Figure 11.3 (a) Worm consumption of plastics, (b) plastics consumption by market sector, Western Europe, and (c) destination of post-consumer plastic waste, Western Europe (courtesy of Shell Briefing Services, London). fillers restrict the movement of segments between the branching points shown in Figure 2.24. For instance, carbon black has long been used as a filler for car tyres, giving substantial improvements in shear modulus, tear strength, hardness and resistance to abrasion by road surfaces. Tyres are subject to fluctuating stresses during their working life and it has been found useful to express their stress-strain behaviour in terms of a dynamic shear modulus (determined under cyclic stressing conditions at a specified frequency). Time is required for molecular rearrangements to take place in an elastomer; for this reason the dynamic modulus increases with the frequency used in the modulus test. The dynamic modulus at a given frequency is significantly enhanced by the crosslinking action of vulcanization and by the presence of carbon black. These carbon particles are extremely small, typically 20-50 nm diameter. Small amounts of anti-oxidants ~ and anti-ozonants are often beneficial. The principal agents of degradation under service conditions are extreme temperatures, oxygen and ozone, various liquids. In the last case a particular liquid may penetrate between the chains and cause swelling. 11.1.4 Some applications of important plastics Figures l l.3a and l l.3b summarize 1990 data on world and West European consumption of plastics. The survey included high-volume, low-price commodity plastics as well as engineering and advanced plastics. Thermoplastics dominate the market (i.e. PE, PVC and PP). Development of an entirely new type of plastic is extremely expensive and research in this direction is l ln the 1930s, concern with oxidation led the Continental Rubber Works, Hannover, to experiment with nitrogen-inflation of tyres intended for use on Mercedes 'Silver Arrows', Grand Prix racing cars capable of 300 km/h. This practice was not adopted by Mercedes-Benz for track events. limited. Research is mainly concerned with improving and reducing the cost of established materials (e.g. improved polymerization catalysts, composites, thermoplastic rubbers, waste recycling). The low- and high-density forms of polyethylene, LDPE and HDPE, were developed in the 1940s and 1950s, respectively. Extruded LDPE is widely used as thin films and coatings (e.g. packaging). HDPE is used for blow-moulded containers, injection-moulded crates and extruded pipes. An intermediate form of adjustable density known as linear low-density polyethylene, LLDPE, became available in the 1980s. Although more difficult to process than LDPE and HDPE, filmextruded LLDPE is now used widely in agriculture, horticulture and the construction industry (e.g. heavyduty sacks, silage sheets, tunnel houses, cloches, dampproof membranes, reservoir linings). Its tear strength and toughness have enabled the gauge of PE film to be reduced. A further variant of PE is ultra-high molecular weight polyethylene, UHMPE, which is virtually devoid of residual traces of catalyst from polymerization. Because of its high molecular mass, it needs to processed by sintering. UHMPE provides the wear resistance and toughness required in artificial joints of surgical prostheses. Polyvinyl chloride (PVC) is the dominant plastic in the building and construction industries and has effectively replaced many traditional materials such as steel, cast iron, copper, lead and ceramics. For example, the unplasticized version (UPVC) is used for window frames and external cladding panels because of its stiffness, hardness, low thermal conductivity and weather resistance. PVC is the standard material for piping in underground distribution systems for potable water (blue) and natural gas (yellow), being corrosionresistant and offering small resistance to fluid flow. Although sizes of PVC pipes tend to be restricted, PVC linings are used to protect the bore of large-diameter pipes (e.g. concrete). The relatively low softening temperature of PVC has stimulated interest in alternative piping materials for underfloor heating systems. Polybutylene (PB) has been used for this application; being
354 Modern Physical Metallurgy and Materials Engineering supported, it can operate continuously at a temperature rubbers has encouraged innovative engineering design of soc and can tolerate occasional excursions (e.g. motorway bridge bearings, 110C. However, hot water with a high chlorine con- and earthquake-proof buildings, tent can cause failure. Other important building plastics systems). are PP, ABS and polycarbonates. Transparent roofing Silicone rubbers be regarded as being inter- sheets of twin-walled polycarbonate or Pvc provide mediate in character to polymers and ceramics. From rials need to be stabilized with additives to prevent Uv sist of alternating silicon and oxygen atoms. Although degradation weaker than organic polymers based upon carbon The thermoplastic polypropylene, PP(Propathene) chains, they retain important engineering properties. ecame available in the early 1960s. Its stiffness, tough- such as resilience, chemical stability and electrical ness at low temperatures and resistance to chemicals, insulation, over the very useful temperature range of heat and creep(Tm =165-170.C)are exceptional. PP -100C to 300 C. These outstanding characteristics, has been of particular interest to car designers in their combined with their cost-effectiveness, have led to the quest for weight-saving and fuel efficiency. In a typi- adoption of silicone rubbers by virtually every industry cal modern saloon car, at least 10%o of the total weight (e.g. medical implants, gaskets, seals, coatings) is plastic (i.e. approximately 100 kg). PP, the lowest density thermoplastic(approximately 900 kg m), is 11.1.5 Management of waste plastics increasingly used for interior and exterior automotive Concern for the world's environment and fut fans, body panels, bumpers(fenders)). It is amenable supplies has focused attention on the fate copolymer(with ethylene or nylon), and can be pro- and electricalelectronics sectors. Although recovery duced as mouldings (injecti blow), films and of values from metallic wastes has long been prac filaments tised. the diversity and often complex chemical nature Polystyrene(PS)is intrinsically brittle. Engineering of plastics raise some difficult problems. Nevertheless, polymers such as PS, PP, nylon and polycarbonates despite the difficulties of re-use and recycling, it must throughout the polymeric matrix; these particles con- and are frequently more cost- and energy-effective than sites of crazing. The toughened high- impact form of glasses. Worldwide, production of plastics accounts for urally transparent but easily coloured. When supplied for about 54%. Enlightened designers now consider as expandable beads charged with a blowing agent, the whole life-cycle and environmental impact of a insulating foam endeavour to economize on mass(e.g. thinner thick Some of the elastomers introduced in Chapter 2 will nesses for PE film and PET containers (lightweight unfortunately, because of reactive C-C links in the in sunlight(photodegradation) or by microbial action chains it does not have a high resistance to chemical(biodegradation)represents a loss of material resource attack and is prone to surface cracking and degradation as they cannot be recycled; accordingly, their use tends (perishing). Styrene-butadiene rubber (SBR), inti to be restricted to specialized markets(e.g. agriculture, duced in 1930, is still one of the principal synthetic medicine rubbers(e.g. car tyres). In this copolymer. repea Figure 113c portrays the general pattern of plas- units of butadiene? are combined randomly with those tics disposal in Western Europe. Landfilling is the of styrene. Polychloroprene(Neoprene). introduced in main method but sites are being rapidly exhausted in 1932, is noted for its resistance to oil and heat and some countries. The principal routes of waste ma is used for automotive components (e.g. seals. water agement are material recycling. energy recycling and circuit pipes). chemical recycling. The first opportunity for material As indicated previously, additives play a vital role in recycling occurs during manufacture, when uncontam- rubber technology, The availability of a large family of inated waste may be re-used. However, as in the case of recycled paper. there is a limit to the number of I Synthetic(methyl) rubber was first produced in Germ times that this is possible. Recycling of post-consumer ring World War I as a result of the materials blockade waste is costly. involving problems of contamination, yhen used for tyres, vehicles had to be jacked-up overnight collection, identification and separation.Co-extruded to prevent flat areas developing where they contacted the German legislation requires that, by 1995, 80%e of all or but-2-ene. is an unsaturated derivative of packaging (including plastics) must be collected separately hat the original from other waste and 64% of total waste recycled as ene mono material
354 Modern Physical Metallurgy and Materials Engineering supported, it can operate continuously at a temperature of 80~ and can tolerate occasional excursions to 110~ However, hot water with a high chlorine content can cause failure. Other important building plastics are PP, ABS and polycarbonates. Transparent roofing sheets of twin-walled polycarbonate or PVC provide thermal insulation and diffuse illumination: both materials need to be stabilized with additives to prevent UV degradation. The thermoplastic polypropylene, PP (Propathene) became available in the early 1960s. Its stiffness, toughness at low temperatures and resistance to chemicals, heat and creep (Tm = 165-170~ are exceptional. PP has been of particular interest to car designers in their quest for weight-saving and fuel efficiency. In a typical modern saloon car, at least 10% of the total weight is plastic (i.e. approximately 100 kg). PP, the lowestdensity thermoplastic (approximately 900 kg m-3), is increasingly used for interior and exterior automotive components (e.g. heating and ventilation ducts, radiator fans, body panels, bumpers (fenders)). It is amenable to filament-reinforcement, electroplating, blending as a copolymer (with ethylene or nylon), and can be produced as mouldings (injection- or blow-), films and filaments. Polystyrene (PS) is intrinsically brittle. Engineering polymers such as PS, PP, nylon and polycarbonates are toughened by dispersing small rubber spheroids throughout the polymeric matrix; these particles concentrate applied stresses and act as energy-absorbing sites of crazing. The toughened high-impact form of polystyrene is referred to as HIPS. PS, like PP, is naturally transparent but easily coloured. When supplied as expandable beads charged with a blowing agent, such as pentane, PS can be produced as a rigid heatinsulating foam. Some of the elastomers introduced in Chapter 2 will now be considered. Polyisoprene is natural rubber; unfortunately, because of reactive C-C links in the chains it does not have a high resistance to chemical attack and is prone to surface cracking and degradation ('perishing'). Styrene-butadiene rubber (SBR), introduced in 1930, is still one of the principal synthetic rubbers (e.g. car tyres), l In this copolymer, repeat units of butadiene 2 are combined randomly with those of styrene. Polychloroprene (Neoprene), introduced in 1932, is noted for its resistance to oil and heat and is used for automotive componev.ts (e.g. seals, water circuit pipes). As indicated previously, additives play a vital role in rubber technology. The availability of a large family of ~Synthetic (methyl) rubber was first produced in Germany during World War 1 as a result of the materials blockade: when used for tyres, vehicles had to be jacked-up overnight to prevent flat areas developing where they contacted the ground. 2Butadiene, or but-2-ene, is an unsaturated derivative of butane C4HI0; the central digit indicates that the original butene monomer C4H8 contains two double bonds. rubbers has encouraged innovative engineering design (e.g. motorway bridge bearings, mounts for oil-rigs and earthquake-proof buildings, vehicle suspension systems). Silicone rubbers may be regarded as being intermediate in character to polymers and ceramics. From Table 2.7 it can be seen that the long chains consist of alternating silicon and oxygen atoms. Although weaker than organic polymers based upon carbon chains, they retain important engineering properties, such as resilience, chemical stability and electrical insulation, over the very useful temperature range of -100~ to 300~ These outstanding characteristics, combined with their cost-effectiveness, have led to the adoption of silicone rubbers by virtually every industry (e.g. medical implants, gaskets, seals, coatings). 11.1.5 Management of waste plastics Concern for the world' s environment and future energy supplies has focused attention on the fate of waste plastics, particularly those from the packaging, car and electrical/electronics sectors. Although recovery of values from metallic wastes has long been practised, the diversity and often complex chemical nature of plastics raise some difficult problems. Nevertheless, despite the difficulties of re-use and recycling, it must be recognized that plastics offer remarkable properties and are frequently more cost- and energy-effective than traditional alternatives such as metals, ceramics and glasses. Worldwide, production of plastics accounts for about 4% of the demand for oil: transport accounts for about 54%. Enlightened designers now consider the whole life-cycle and environmental impact of a polymeric product, from manufacture to disposal, and endeavour to economize on mass (e.g. thinner thicknesses for PE film and PET containers ('lightweighting')). Resort to plastics that ultimately decompose in sunlight (photodegradation) or by microbial action (biodegradation) represents a loss of material resource as they cannot be recycled; accordingly, their use tends to be restricted to specialized markets (e.g. agriculture, medicine). Figure l l.3c portrays the general pattern of plastics disposal in Western Europe. Landfilling is the main method but sites are being rapidly exhausted in some countries. The principal routes of waste management are material recycling, energy recycling and chemical recycling. The first opportunity for material recycling occurs during manufacture, when uncontaminated waste may be re-used. However, as in the case of recycled paper, there is a limit to the number of times that this is possible. Recycling of post-consumer waste is costly, involving problems of contamination, collection, identification and separation. 3. Co-extruded 3German legislation requires that, by 1995, 80% of all packaging (including plastics) must be collected separately from other waste and 64% of total waste recycled as material
Plastics and composites 355 blow-moulded containers are being produced with a These blocks are then drawn into tandem sequences three-layer wall in which recycled material is sand- known as microfibrils( Figure 11. 4). The individual wiched between layers of virgin polymer. In the Ger- blocks retain their chain-folding conformation and are man car industry efforts are being made to recycle linked together by the numerous tie molecules which flexible polyurethane foam, ABS and polyamides. New form as the original lamellae unfold. A bundle of ABs radiator grilles can incorporate 30% from old these highly-oriented microfibrils forms a fibril(small fibre). The microfibrils in a bundle are separated Plastics have a high content of carbon and hydrogen by amorphous material and are joined by surviving and can be regarded as fuels of useful calorific value. interlamellar tie molecules. The pronounced molecular Incinerating furnaces act as energy-recycling devices, orientation of this type of fibrous structure maximizes converting the chemical energy of plastics into ther- the contribution of strong covalent bonds to strength mal/electrical energy and recovering part of the energy and stiffness while minimizing the effect of weak originally expended in manufacture. Noxious fumes intermolecular forces and vapours can be evolved (e.g. halogens); control Industrially, cold-drawing techniques which take and cleaning of flue gases are essential advantage of the anisotropic nature of polymer crys Chemical recycling is of special interest because tallies are widely used in the production of synthetic direct material recycling is not possible with some fibres and filaments (e.g. Terylene ).(Similarly, biax wastes. Furthermore, according to some estimates ial stretching is used to induce exceptional strength in only 20-30% of plastic waste can be re-used after film and sheet and bottles e.g. Melinex). Crystalliza- material recycling. Chemical treatment, which is tion in certain polymers can be very protracted. For indirect material recycling route, recovers monomers instance, because nylon 6, 6 has a Tg value slightly and polymer-based products that can be passed crysti feedstocks to chemical and petrochemical ind lize and densify over a long period of time during nor- Hydrogenation of waste shows promise and is mal service, causing undesirable after-shrinkage. This produce synthetic oil metastability is obviated by annealing nylon briefly fect crystals melt while the more stable crystals gro 11.2 Behaviour of plastics during Stretching nylon 6,6 at room temperature during the ctual freezing process also encourages crystallization and develops a strengthening preferred orientation of 11.2.1 Cold-drawing and crazing crystallites Let us now turn from the bulk effect of cold-drawing A polymeric structure is often envisaged as an enan- to a form of localized inhomogeneous deformation, or lled mass of chain molecules. as the T value for many commercial polymers are fairly low, one yielding. In crazing, thin bands of expanded material form in the polymer at a stress much lower than the assumes that thermal agitation causes molecules to bulk yield stress for the polymer. Crazes are usually wriggle at ambient t tures. Raising the temper associated with glassy polymers(PMMA and ps)but ture increases the violence of molecular agitation and, may occur in semi-crystalline polymers(PP). They are under the action of stress, molecules become more ide past each othe oil as they rotate bout their carbon-carbon bonds, and extend in length We will first concentrate upon mechanistic aspects f two important modes of deformation; namely, the development of highly-preferred molecular orienta- tions in semi-crystalline polymers by cold-drawing and the occurrence of crazing in glassy polymers talline structure containing spherulites is subjecte to a tensile test at room temperature. a neck appears in of the test-piece. As th extends. this neck remains constant in cross-section but increases considerably in length. This process forms a necked length that is stronger and stiffer than material beyond the neck. At first, the effect of applied tensile stress is to produce relative movement ella and the interlamellar regions of disordered molecules. Lamellae that normal to the direction of principal stress rotate in a manner reminiscent of slip-plane rotation in metallic Figure 11.4 Persistence of crystalline block structure in single crystals, and break down into smaller blocks. three microfibrils during defor
Plastics and composites 355 blow-moulded containers are being produced with a three-layer wall in which recycled material is sandwiched between layers of virgin polymer. In the German car industry efforts are being made to recycle flexible polyurethane foam, ABS and polyamides. New ABS radiator grilles can incorporate 30% from old recycled grilles. Plastics have a high content of carbon and hydrogen and can be regarded as fuels of useful calorific value. Incinerating furnaces act as energy-recycling devices, converting the chemical energy of plastics into thermal/electrical energy and recovering part of the energy originally expended in manufacture. Noxious fumes and vapours can be evolved (e.g. halogens); control and cleaning of flue gases are essential. Chemical recycling is of special interest because direct material recycling is not possible with some wastes. Furthermore, according to some estimates, only 20-30% of plastic waste can be re-used after material recycling. Chemical treatment, which is an indirect material recycling route, recovers monomers and polymer-based products that can be passed on as feedstocks to chemical and petrochemical industries. Hydrogenation of waste shows promise and is used to produce synthetic oil. 11.2 Behaviour of plastics during processing 11.2.1 Cold-drawing and crazing A polymeric structure is often envisaged as an entangled mass of chain molecules. As the Tg values for many commercial polymers are fairly low, one assumes that thermal agitation causes molecules to wriggle at ambient temperatures. Raising the temperature increases the violence of molecular agitation and, under the action of stress, molecules become more likely to slide past each other, uncoil as they rotate about their carbon-carbon bonds, and extend in length. We will first concentrate upon mechanistic aspects of two important modes of deformation; namely, the development of highly-preferred molecular orientations in semi-crystalline polymers by cold-drawing and the occurrence of crazing in glassy polymers. Cold-drawing can be observed when a semicrystalline structure containing spherulites is subjected to a tensile test at room temperature. A neck appears in the central portion of the test-piece. As the test-piece extends, this neck remains constant in cross-section but increases considerably in length. This process forms a necked length that is stronger and stiffer than material beyond the neck. At first, the effect of applied tensile stress is to produce relative movement between the crystalline lamellae and the interlamellar regions of disordered molecules. Lamellae that are normal to the direction of principal stress rotate in a manner reminiscent of slip-plane rotation in metallic single crystals, and break down into smaller blocks. These blocks are then drawn into tandem sequences known as microfibrils (Figure 11.4). The individual blocks retain their chain-folding conformation and are linked together by the numerous tie molecules which form as the original lamellae unfold. A bundle of these highly-oriented microfibrils forms a fibril (small fibre). The microfibrils in a bundle are separated by amorphous material and are joined by surviving interlamellar tie molecules. The pronounced molecular orientation of this type of fibrous structure maximizes the contribution of strong covalent bonds to strength and stiffness while minimizing the effect of weak intermolecular forces. Industrially, cold-drawing techniques which take advantage of the anisotropic nature of polymer crystallites are widely used in the production of synthetic fibres and filaments (e.g. Terylene). (Similarly, biaxial stretching is used to induce exceptional strength in film and sheet and bottles e.g. Melinex). Crystallization in certain polymers can be very protracted. For instance, because nylon 6,6 has a Tg value slightly below ambient temperature, it can continue to crystallize and densify over a long period of time during normal service, causing undesirable after-shrinkage. This metastability is obviated by 'annealing' nylon briefly at a temperature of 120~ which is below Tm: less perfect crystals melt while the more stable crystals grow. Stretching nylon 6,6 at room temperature during the actual freezing process also encourages crystallization and develops a strengthening preferred orientation of crystallites. Let us now turn from the bulk effect of cold-drawing to a form of localized inhomogeneous deformation, or yielding. In crazing, thin bands of expanded material form in the polymer at a stress much lower than the bulk yield stress for the polymer. Crazes are usually associated with glassy polymers (PMMA and PS) but may occur in semi-crystalline polymers (PP). They are f L 1 ,l JJ; u Figure 11.4 Persistence of crystalline block structure in three microfibrils during defolTnation