《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_brittleness37

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 deter￾mination). 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 pro￾cessing 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 intermolecu￾lar 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) com￾ponent 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 tem￾peratures 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. Sim￾ilarly, 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 increas￾ingly rubbery. As one would anticipate, Tg values for rubbers lie well below room temperature. Increas￾ing 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~ respec￾tively, 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 radia￾tion 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 ultra￾violet (UV) components of sunlight can structurally alter and degrade polymers, the effect is particularly marked in electric light fittings (e.g. yellowing). Stabi￾lizing 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 inter￾molecular 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 prod￾ucts. 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 dur￾ing 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 modu￾lus 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 liq￾uids. 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 improv￾ing 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, film￾extruded LLDPE is now used widely in agriculture, horticulture and the construction industry (e.g. heavy￾duty sacks, silage sheets, tunnel houses, cloches, damp￾proof 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 molecu￾lar weight polyethylene, UHMPE, which is virtually devoid of residual traces of catalyst from polymer￾ization. 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 effec￾tively replaced many traditional materials such as steel, cast iron, copper, lead and ceramics. For example, the unplasticized version (UPVC) is used for win￾dow 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 corrosion￾resistant 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 tem￾perature of PVC has stimulated interest in alternative piping materials for underfloor heating systems. Poly￾butylene (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 con￾tent 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 mate￾rials need to be stabilized with additives to prevent UV degradation. The thermoplastic polypropylene, PP (Propathene) became available in the early 1960s. Its stiffness, tough￾ness 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 typi￾cal modern saloon car, at least 10% of the total weight is plastic (i.e. approximately 100 kg). PP, the lowest￾density 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 pro￾duced 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 con￾centrate 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 nat￾urally transparent but easily coloured. When supplied as expandable beads charged with a blowing agent, such as pentane, PS can be produced as a rigid heat￾insulating 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), intro￾duced 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 inter￾mediate in character to polymers and ceramics. From Table 2.7 it can be seen that the long chains con￾sist 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 prac￾tised, 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 thick￾nesses for PE film and PET containers ('lightweight￾ing')). 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 plas￾tics disposal in Western Europe. Landfilling is the main method but sites are being rapidly exhausted in some countries. The principal routes of waste man￾agement are material recycling, energy recycling and chemical recycling. The first opportunity for material recycling occurs during manufacture, when uncontam￾inated 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 sand￾wiched between layers of virgin polymer. In the Ger￾man 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 ther￾mal/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 entan￾gled 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 tempera￾ture 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 orienta￾tions in semi-crystalline polymers by cold-drawing and the occurrence of crazing in glassy polymers. Cold-drawing can be observed when a semi￾crystalline 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 crys￾tallites are widely used in the production of synthetic fibres and filaments (e.g. Terylene). (Similarly, biax￾ial stretching is used to induce exceptional strength in film and sheet and bottles e.g. Melinex). Crystalliza￾tion 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 crystal￾lize and densify over a long period of time during nor￾mal service, causing undesirable after-shrinkage. This metastability is obviated by 'annealing' nylon briefly at a temperature of 120~ which is below Tm: less per￾fect 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

356 Modern Physical Metallurgy and Materials Engineering several microns wide and fairly constant in width: they chemistry decides the character of individual molecules can scatter incident light and are visible to the unaided but it is the processing stage which enables them to be eye(e. g. transparent glassy polymers). As in the stress- arranged to maximum advantage. Despite the variety orrosion of metals, crazing of regions in tension of methods available for converting feedstock may be induced by a chemical agent(e. g. ethanol on ders and granules of thermoplastics into useful shapes. PMMA). The plane of a craze is always at right angles these methods usually share up to four common stages the principal tensile stress. Structurally, each craze of production; that is, (1)mixing, melting and homog consists of interconnected microvoids, 10-20 nm in enization,(2)transport and shaping of a melt, (3) size, and is bridged by large numbers of molecule- drawing or blowing, and(4)finishing orientated fibrils, 10-40 nm in diameter, The voidage Processing brings about physical, and often chemi- is about 40-50%, As a craze widens, bridging fibrils cal, changes In comparison with energy requirements xtend by drawing in molecules from the side walls. for processing other materials, those for polymers are Unlike the type of craze found in glazes on it is relatively low. Temperature control is vital because not a true crack, being capable of sustaining some load. it decides melt fluidity. There is also a risk of ther- Nevertheless, it is a zone of weakness and can initiate mal degradation because, in addition to having limited brittle fracture. Each craze has a stress-intensifying thermal stability, polymers have a low thermal cor tip which can propagate through the bulk polymer. ductivity and readily overheat. Processing is usuall Crazing can take a variety of forms and may even rapid, involving high rates of shear. The main methods beneficial. For instance, when impact causes crazes that will be used to illustrate technological aspects of to form around rubber globules in ABS polymers, processing thermoplastics are depicted in Figure 11.6 he myriad newly-created surfaces absorb energy and Injection-moulding of thermoplastics, such as PE toughen the material. Various theoretical models of and PS, is broadly similar in principle to the pressure craze formation have been proposed. One suggestion die-casting of light metals, being capable of produc- is that triaxial stresses effectively lower toane en ing mouldings of engineering components rapidly wit tensile strain has exceeded a critical value, induce a repeatable precision(Figure 11 6a). In each cycle, a glass-to-rubber transition in the vicinity of a flaw, or metered amount(shot)of polymer melt is forcibly similar heterogeneity. Hydrostatic stresses then cause injected by the reciprocating screw into a'cold'cavity microcavities to nucleate within this rubbery zone (cooled by oil or water channels). When solidifica As Figure 11.5 shows, it is possible to portray the tion is complete the two-part mould opens and the strength/temperature relations for a polymeric material moulded shape is ejected. Cooling rates are faster on a deformation map. This diagram refers to PMMa than with parison moulds in blow-moulding becaus and shows the fields for cold-drawing, crazing, viscous heat is removed from two surfaces. The capital out contours of strain rate over a range of ub composed lhy for injection-moulding tends to be high because of 11.2.2 Processing methods for thermoplastics multi-impression moulding dies. In die design, special Processing technology has a special place in the different flows coalesce, and of feeding gates. Com- remarkable history of the polymer industry: polymer puter modelling can be used to simulate the melt flow distributions of temperature and pressure Temperatute(C) the mould cavity. This prior simulation helps to lessen nal mo which are ostly. Microprocessors are used to monitor and d feed rates continuously during the io moulding process; for example the flow rates into a complex cavity can be rapid initially and then reduced to ensure that flow-dividing obstructions do not pro- duce weakening weld lines is wide n drawing nto continuous lengths of sheet, tube bar, filament, etc. with a constant and exact ci (Figure 11.6b). A long Archimedean screw (auger PAIVA rotates and conveys feedstock through carefully pro- Contours of portioned feed, compression and metering sections The polymer is electrically heated in each of the three barrel sections and frictionally heated as it is shear erature T/Tl tinned by the inally, it is forced through a .5 Deformation map for PMMA sh die orifice. Microprocessor control systems are avail tble to measure pressure at the die inlet and to keep us normalized temperature (from Ashby and Jones, stational speed of 1986: permission of Elsevier Science Ltd, UK) screw. Dimensional control of the product benefits

356 Modern Physical Metallurgy and Materials Engineering several microns wide and fairly constant in width: they can scatter incident light and are visible to the unaided eye (e.g. transparent glassy polymers). As in the stress￾corrosion of metals, crazing of regions in tension may be induced by a chemical agent (e.g. ethanol on PMMA). The plane of a craze is always at right angles to the principal tensile stress. Structurally, each craze consists of interconnected microvoids, 10-20 nm in size, and is bridged by large numbers of molecule￾orientated fibrils, 10-40 nm in diameter. The voidage is about 40-50%. As a craze widens, bridging fibrils extend by drawing in molecules from the side walls. Unlike the type of craze found in glazes on pottery, it is not a true crack, being capable of sustaining some load. Nevertheless, it is a zone of weakness and can initiate brittle fracture. Each craze has a stress-intensifying tip which can propagate through the bulk polymer. Crazing can take a variety of forms and may even be beneficial. For instance, when impact causes crazes to form around rubber globules in ABS polymers, the myriad newly-created surfaces absorb energy and toughen the material. Various theoretical models of craze formation have been proposed. One suggestion is that triaxial stresses effectively lower Tg and, when tensile strain has exceeded a critical value, induce a glass-to-rubber transition in the vicinity of a flaw, or similar heterogeneity. Hydrostatic stresses then cause microcavities to nucleate within this rubbery zone. As Figure 11.5 shows, it is possible to portray the strength/temperature relations for a polymeric material on a deformation map. This diagram refers to PMMA and shows the fields for cold-drawing, crazing, viscous flow and brittle fracture, together with superimposed contours of strain rate over a range of 10 -6 to 1 s -1. 11.2.2 Processing methods for thermoplastics Processing technology has a special place in the remarkable history of the polymer industry" polymer 10 0 10.~ 10.2 g E 10 "3 ~ 10 .4 Brittle fracture 11"I . Crozing and " shear yielding ~" PMMA E~; l S, OP. = 3?8 K ConlotJre of 10-$ :.., weln rite I Temperature (*C) -200 -100 0 100 200 300 1 I I 1 1 ! I ~ _~.vi,cou, I/~\\'!~ '~ 0 103 102 ,r 4, 10 ~ In 10 0 0.4 0.8 1.2 Normalised temperature (TITs ) 10 -I 1.( Figure 11.5 Deformation map for PMMA showing deformation regions as a fimction of normalized stress versus normalized temperature (from Ashby and Jones, 1986; permission of Elsevier Science Ltd, UK). chemistry decides the character of individual molecules but it is the processing stage which enables them to be arranged to maximum advantage. Despite the variety of methods available for converting feedstock pow￾ders and granules of thermoplastics into useful shapes, these methods usually share up to four common stages of production; that is, (1) mixing, melting and homog￾enization, (2) transport and shaping of a melt, (3) drawing or blowing, and (4) finishing. Processing brings about physical, and often chemi￾cal, changes. In comparison with energy requirements for processing other materials, those for polymers are relatively low. Temperature control is vital because it decides melt fluidity. There is also a risk of ther￾mal degradation because, in addition to having limited thermal stability, polymers have a low thermal con￾ductivity and readily overheat. Processing is usually rapid, involving high rates of shear. The main methods that will be used to illustrate technological aspects of processing thermoplastics are depicted in Figure 11.6. Injection-moulding of thermoplastics, such as PE and PS, is broadly similar in principle to the pressure die-casting of light metals, being capable of produc￾ing mouldings of engineering components rapidly with repeatable precision (Figure l l.6a). In each cycle, a metered amount (shot) of polymer melt is forcibly injected by the reciprocating screw into a 'cold' cavity (cooled by oil or water channels). When solidifica￾tion is complete, the two-part mould opens and the moulded shape is ejected. Cooling rates are faster than with parison moulds in blow-moulding because heat is removed from two surfaces. The capital out￾lay for injection-moulding tends to be high because of the high pressures involved and machining costs for multi-impression moulding dies. In die design, special attention is given to the location of weld lines, where different flows coalesce, and of feeding gates. Com￾puter modelling can be used to simulate the melt flow and distributions of temperature and pressure within the mould cavity. This prior simulation helps to lessen dependence upon traditional moulding trials, which are costly. Microprocessors are used to monitor and con￾trol pressure and feed rates continuously during the moulding process; for example, the flow rates into a complex cavity can be rapid initially and then reduced to ensure that flow-dividing obstructions do not pro￾duce weakening weld lines. Extrusion is widely used to shape thermoplastics into continuous lengths of sheet, tube, bar, filament, etc. with a constant and exact cross-sectional profile (Figure l l.6b). A long Archimedean screw (auger) rotates and conveys feedstock through carefully pro￾portioned feed, compression and metering sections. The polymer is electrically heated in each of the three barrel sections and frictionally heated as it is 'shear￾thinned' by the screw. Finally, it is forced through a die orifice. Microprocessor control systems are avail￾able to measure pressure at the die inlet and to keep it constant by 'trimming' the rotational speed of the screw. Dimensional control of the product benefits

Plastics and composites 357 Nonie from an annular die is drawn upwards and with air to form thin film: stretching and ease when crystallization is complete at abou m 「州画是霄 Similarly, in the blow-moulding of bottles and air- ducting, etc, tubular extrudate(parison) moves ver- ically downwards into an open split-mould. As the ould closes, the parison is inflated with air at a pres movement ure of about 5 atmospheres and assumes the shape of the cooled mould surfaces. Relatively inexpensive alu minium moulds can be used because stresses are low Thermoforming( Figure 116c)is another secondary method for processing extruded thermoplastic sheet, being particularly suitable for large thin-walled hol- low shapes such as baths, boat hulls and car bodies (e.g. ABS, PS, PVC, PMMA). In the basic version of the thermoforming, a frame- held sheet is located above D·Hdra the mould, heated by infrared radiation until rubbery and then drawn by vacuum into close contact with the mould surface. The hot sheet is deformed and thinned Kev:口 pastic Es cooling by biaxial stresses. In a high-pressure version of ther- moforming, air at a pressure of several atmospheres heater acts on the opposite side of the sheet to the vacuum and improves the ability of the sheet to register fine mould detail. the draw ratio, which is the ratio of plastic mould depth to mould width, is a useful control parar eter. For a given polymer, it is possible to construct a plot of draw ratio versus temperature which can be used as a'map to show various regions where there mould table risks of incomplete corner filling, bursts and holes. Unfortunately, thinning is most pronounced at vulnerable corners. Thermoforming offers an econom- ical altermative to moulding but cycle times are rather long and the final shape needs trimming. 11.2.3 Production of thermosets Development of methods for shaping thermosetting able raised materials is restricted by the need to accommodate a uring reaction and the absence of a stable viscoelas- state Until fairly recently, these restrictions tended limit the size of thermoset products. Compres ion moulding of a thermosetting P-F resin(Bakelite) vithin a simple cylindrical steel mould is a well-known boratory method for mounting metallurgical samples. vacuum Resin granules, sometimes mixed with hardening or lectrically-conducting additives, are loaded into the table locked up ould, then heated and compressed until crosslinking actions are complete. In transfer moulding, which can produce more intricate shapes, resin is melted in a primary chamber and then transferred to a vented sheet(from MilLs, 1986; by permission o/ Tawara or auction moulding chamber for final curing In the car indus- plastic pipe by extrusion and (c) thermoforming ic try, body panels with good bending stiffness are pro duced from thermosetting sheet-moulding compounds (SMC). A composite sheet is prepared by laying do layers of randomly-oriented, chopped glass fibres, ca from this device. On leaving the die, the continuously. cium carbonate powder and polyester resin. The sheet formed extrudate enters cooling and haul-off sections. is placed in a moulding press and subjected to heat an Frequently, the extrudate provides the preform for a pressure. Energy requirements are attractively low second operation. For example, in a continuous melt Greater exploitation of thermosets for large car parts inflation technique, tubular sheet of LDPE or HDPE has been made possible by reaction injection-moulding

Plastics and composites 357 (a) Moving Nozzle mould ! Heater bands half / ! , /Screw rotation I ~_ i] I ~ / .Screw Non-return Injection unit valve movement Key:- II hydraulic :.: polymer system (b) m, ~ ~ Cooling water OUt -- ' 9 " I C~176 I / DIe spicier legs water i;r L mixing Fl/~ting tube plug Key: r'n plastic F~ cooling water (c) heater /l I I I I i2" .,..,,c l IIIII I I table ir table raised -:'?:us-- table locked up Figure 11.6 (a) Injection-moulding machine, (b) production of plastic pipe by extrusion and (c) thermoforming of plastic sheet (from Mills, I986; by permission of Edward Arnold). from this device. On leaving the die, the continuously￾formed extrudate enters cooling and haul-off sections. Frequently, the extrudate provides the preform for a second operation. For example, in a continuous melt￾inflation technique, tubular sheet of LDPE or HDPE from an annular die is drawn upwards and inflated with air to form thin film: stretching and thinning cease when crystallization is complete at about 120~ Similarly, in the blow-moulding of bottles and air￾ducting, etc., tubular extrudate (parison) moves ver￾tically downwards into an open split-mould. As the mould closes, the parison is inflated with air at a pres￾sure of about 5 atmospheres and assumes the shape of the cooled mould surfaces. Relatively inexpensive alu￾minium moulds can be used because stresses are low. Thermoforming (Figure 11.6c) is another secondary method for processing extruded thermoplastic sheet, being particularly suitable for large thin-walled hol￾low shapes such as baths, boat hulls and car bodies (e.g. ABS, PS, PVC, PMMA). In the basic version of the thermoforming, a frame-held sheet is located above the mould, heated by infrared radiation until rubbery and then drawn by vacuum into close contact with the mould surface. The hot sheet is deformed and thinned by biaxial stresses. In a high-pressure version of ther￾moforming, air at a pressure of several atmospheres acts on the opposite side of the sheet to the vacuum and improves the ability of the sheet to register fine mould detail. The draw ratio, which is the ratio of mould depth to mould width, is a useful control param￾eter. For a given polymer, it is possible to construct a plot of draw ratio versus temperature which can be used as a 'map' to show various regions where there are risks of incomplete corner filling, bursts and pin￾holes. Unfortunately, thinning is most pronounced at vulnerable comers. Thermoforming offers an econom￾ical alternative to moulding but cycle times are rather long and the final shape needs trimming. 11.2.3 Production of thermosets Development of methods for shaping thermosetting materials is restricted by the need to accommodate a curing reaction and the absence of a stable viscoelas￾tic state. Until fairly recently, these restrictions tended to limit the size of thermoset products. Compres￾sion moulding of a thermosetting P-F resin (Bakelite) within a simple cylindrical steel mould is a well-known laboratory method for mounting metallurgical samples. Resin granules, sometimes mixed with hardening or electrically-conducting additives, are loaded into the mould, then heated and compressed until crosslinking reactions are complete. In transfer moulding, which can produce more intricate shapes, resin is melted in a primary chamber and then transferred to a vented moulding chamber for final curing. In the car indus￾try, body panels with good bending stiffness are pro￾duced from thermosetting sheet-moulding compounds (SMC). A composite sheet is prepared by laying down layers of randomly-oriented, chopped glass fibres, cal￾cium carbonate powder and polyester resin. The sheet is placed in a moulding press and subjected to heat and pressure. Energy requirements are attractively low. Greater exploitation of thermosets for large car parts has been made possible by reaction injection-moulding

358 (RIM). In this process, polymerization takes place dur- Figure 11.7 shows the typical fall in apparent shear cal reactants are pumped at high velocity into a mixing If Newtonian flow prevailed, the plotted line would chamber. The mixture bottom-feeds a closed chamber be horizontal. This type of diagram is plotted for rhere polymerization is completed and a solid forms. fixed values of temperature and hydrostatic pressure. A Mouldings intended for high-temperature service are change in either of these two conditions will displace stabilized, or post-cured, by heating at a temperature the flow curve significantly. Thus, either raising the of 100C for about 30 min. The reactive system in RIM temperature or decreasing the hydrostatic(bulk)pres- can be polyurethane, nylon- or polyurea-forming. sure will lower the apparent shear viscosity. The latter The basic chemical criterion is that polymerization in increases with average molecular mass. For instance the mould should be virtually complete after about fluidity at a low stress, as determined by the standard 30 s. Foaming agents can be used to form compo- melt flow index(MFI) test, is inversely proportional nents with a dense skin and a cellular core. when glass to molecular mass. At low stress and for a giver fibres are added to one of the reactants, the process is molecular mass, a polymer with a broad distribution of RIM now competes with the injection-moulding of than one with a narrow distribution. However, at high thermoplastics. Capital costs, energy requirements and stress, a reverse tendency is possible and the version moulding pressures are lower and, unlike injection- with a broader distribution may be less pseudo-plastic are not su Figure 11.8 provides a comparison of the flo problems (sinks'and voids ). Cycle times for RIM behaviour of five different ther thermosets are becoming comparable with those for for comparing the suitability of different processes. It indicates that acrylics are relatively difficult to extrude ingent control is necessary during the rim changing in the fluid stream and there is a cha sition and viscosity a need to develop appropriate dynamic models nass transport and reaction kinetics 11.2. 4 viscous aspects of melt behaviour Melts of thermoplastic polymers behave in a highly iscous manner when subjected to stress during pro- essing. Flow through die orifices and mould chan nels is streamline (laminar), rather than turbulent with shear conditions usually predominating. Let us now adopt a fuid mechanics approach and conside e effects of shear stress, temperature and hydro static pressure on melt behaviour. Typical rates of strain(shear rates)range from 10-10s(extrusion) to 10-10-s(injection-moulding). When a melt is being forced through a die the shear rate at the die wall is calculable as a function of the volumetric flow ate and the geometry of the orifice. At the necessarily high levels of stress required, the classic Newtonian is not obeyed: an increase in shear stress produces Shear stress(N/m?) other words, the shear stress/shear rate ratio, which is now referred to as the 'apparent shear viscosity, falls. Figure 11.7 Typical plot of apparent shea Terms such as'pseudo-plastic'and'shear-thinning'are shear stress for LDPE ar 210C and atmo applied to this non-Newtonian fow behaviour. I The effects of increasing temperature T and hy general working range of apparent shear viscosity for Imperial Chemical Industries Plc!oA m extrusion, injection-moulding, etc is 10-10 Ns m-2. (Shear viscosities at low and high stress levels are IThis important test, which originated in ICI laboratories measured by cone-and-plate and capillary extrusion techniques, respectively during the development of PE, is used for most hermoplastics by polymer manufacturers and processors The MFI is the mass of melt extruded through a standard IIn thixotropic behaviour, viscosity decreases with increase cylindrical die in a prescribed period under conditions of in the duration of shear (rather than the shear rate) constant temperature and compression load

358 Modern Physical Metallurgy and Materials Engineering (RIM). In this process, polymerization takes place dur￾ing forming. Two or more streams of very fluid chemi￾cal reactants are pumped at high velocity into a mixing chamber. The mixture bottom-feeds a closed chamber where polymerization is completed and a solid forms. Mouldings intended for high-temperature service are stabilized, or post-cured, by heating at a temperature of 100~ for about 30 min. The reactive system in RIM can be polyurethyane-, nylon- or polyurea-forming. The basic chemical criterion is that polymerization in the mould should be virtually complete after about 30 s. Foaming agents can be used to form compo￾nents with a dense skin and a cellular core. When glass fibres are added to one of the reactants, the process is called reinforced reaction injection-moulding (RRIM). RIM now competes with the injection-moulding of thermoplastics. Capital costs, energy requirements and moulding pressures are lower and, unlike injection￾mouldings, thick sections are not subject to shrinkage problems ('sinks' and voids). Cycle times for RIM￾thermosets are becoming comparable with those for injection-moulded thermoplastics and mouldings of SMC. Stringent control is necessary during the RIM process. Temperature, composition and viscosity are rapidly changing in the fluid stream and there is a chal￾lenging need to develop appropriate dynamic models of mass transport and reaction kinetics. 11.2.4 Viscous aspects of melt behaviour Melts of thermoplastic polymers behave in a highly viscous manner when subjected to stress during pro￾cessing. Flow through die orifices and mould chan￾nels is streamline (laminar), rather than turbulent, with shear conditions usually predominating. Let us now adopt a fluid mechanics approach and consider the effects of shear stress, temperature and hydro￾static pressure on melt behaviour. Typical rates of strain (shear rates) range from 10-10 3 s -! (extrusion) to 10 3-10 5 s-l (injection-moulding). When a melt is being forced through a die, the shear rate at the die wall is calculable as a function of the volumetric flow rate and the geometry of the orifice. At the necessarily high levels of stress required, the classic Newtonian relation between shear stress and shear (strain) rate is not obeyed: an increase in shear stress produces a disproportionately large increase in shear rate. In other words, the shear stress/shear rate ratio, which is now referred to as the 'apparent shear viscosity', falls. Terms such as 'pseudo-plastic' and 'shear-thinning' are applied to this non-Newtonian flow behaviour. ~ The general working range of apparent shear viscosity for extrusion, injection-moulding, etc. is 10-10 4 Ns m -2. (Shear viscosities at low and high stress levels are measured by cone-and-plate and capillary extrusion techniques, respectively.) ]In thixotropic behaviour, viscosity decreases with increase in the duration of shear (rather than the shear rate). Figure 11.7 shows the typical fall in apparent shear viscosity which occurs as the shear stress is increased. If Newtonian flow prevailed, the plotted line would be horizontal. This type of diagram is plotted for fixed values of temperature and hydrostatic pressure. A change in either of these two conditions will displace the flow curve significantly. Thus, either raising the temperature or decreasing the hydrostatic (bulk) pres￾sure will lower the apparent shear viscosity. The latter increases with average molecular mass. For instance, fluidity at a low stress, as determined by the standard melt flow index (MFI) test, ~ is inversely proportional to molecular mass. At low stress and for a given molecular mass, a polymer with a broad distribution of molecular mass tends to become more pseudo-plastic than one with a narrow distribution. However, at high stress, a reverse tendency is possible and the version with a broader distribution may be less pseudo-plastic. Figure 11.8 provides a comparison of the flow behaviour of five different thermoplastics and is useful for comparing the suitability of different processes. It indicates that acrylics are relatively difficult to extrude 10 5 . ~ 10 ~ i, '7 m > 10 3 I,. r0 t- o'} .,.., t- 0 2 < 10 -- ,, r / \. X" i / \ ! 1 IAIll I I II!1| 1 _.1.._ I I till 10 3 10 4 10 s 10 6 Shear stress (N/m 2) Figure 11.7 Typical plot of apparent shear viscosi~ versus shear stress for LDPE at 210~ and atmospheric pressure: effects of increasing temperature T and hydrostatic pressure P shown (after Powell, 1974; courtesy of Plastics Division, bnperial Chemical Industries Plc). 1This important test, which originated in ICI laboratories during the development of PE, is used for most thermoplastics by polymer manufacturers and processors. The MFI is the mass of melt extruded through a standard cylindrical die in a prescribed period under conditions of constant temperature and compression load

Plastics and composites 359 to tensile strain rate. At low stresses, tensile viscosity is independent of ter As the level of tensile stress rises tensile either remains constant (nylon 6,6), rises(LDPE)or falls(PP, HDPE). This characteristic is relevant to the stability of dimensions and form. For example, during blow-moulding, thin ning walls should have a tolerance for local weak spots E26>8 or stress concentrations. PP and hdpe lack this tol erance and there is a risk that tension-thinning,will lead to rupture. On the other hand the tensile viscos- ity of LDPE rises with tensile stress and failure during wall-thinning is less likely 11.2.5 Elastic aspects of melt behaviour While being deformed and forced through an extru- sion die, the melt stores elastic strain energy. As xtrudate emerges from the die, stresses are released, some elastic recovery takes ce and the extrudate swells. Dimensionally, the degree of swell is typically expressed by the ratio of extrudate diameter to die 10 diameter; the elastic implications of the shear process are expressed by the following modulus: A=t/yR (11.3) where u is the elastic shear modulus, t is the shear DPE at I70°C;B stress at die wall, and yr is the recoverable shear strain. The magnitude of modulus u depends upon the poly 230C; D moulding-grade acet er, molecular mass distribution and the level of shear moulding-grade nylon at 285C(after 1974; stress. (Unlike viscosity, dependency of elasticity upon courtesy of Plastics Division, Imperial Chemical industries mperature, hydrostatic pressure and average molec ular mass is slight. )If the molecular mass distribution is wide, the elastic shear modulus is low and elastic and that pP is suited to the much faster deformation ecovery is appreciable but slow. For a narrow distribu rocess of injection-moulding. In all cases, Newtonian on, with its greater similarities in molecular lengths How is evident at relatively low levels of shear stress ecovery is less but faster. With regard to stress level the modulus remains constant at low shear stresses but The following type of power law equation has been usually increases at the high stresses used commer- found to provide a reasonable fit with practical data and cially, giving ap le recovery. has enabled pseudo-plastic behaviour to be quantified The balance n elastic to viscous behaviour (11.2) deformation time with the relaxation time or 'natural of the polymer where C and n are constants. Now t= ny, hence the viscosity to elastic shear modulus(n/u), and derives viscosity n=Cym-l. The characteristic term (n-1) from the Maxwell model of deformation. The term vis can be derived from the line gradient of a graphical coelasticity originated from the development of such plot of log viscosity versus log shear rate. In practice, models(e.g. Maxwell, Voigt, standard linear solid he power law index n ranges from unity(Newtonian (SLS)). The Maxwell model is a mechanical analogue that provides a useful, albeit imperfect, simulation of ecreases in magnitude as the shear rate increases viscoelasticity and stress relaxation in linear polymers and the thermoplastic melt behaves in an increasingly above Tg(Figure 11.9). It is based upon conditions pseudo-plastic manner of constant strain. A viscously damped *Newtonian So far attention has been concentrated on the vis- dashpot, representing the viscous component of defor- cous aspects of melt behaviour during extrusion and mation, and a spring, representing the elastic compo- jection-moulding, with emphasis on shear process nent, are combined in series. at time t the stress g is In forming operations such as blow-moulding and exponentially related to the initial stress oo, as follows: filament-drawing, extensional fow predominates and tensile stresses become crucial: for these conditions O= Oo exp(-1/A) it is appropriate to define tensile viscosity, the coun- A is the relaxation time terpart of shear viscosity, as the ratio of tensile stress is sufficient time for viscous movement of chain

Plastics and composites 359 10 5 I- X .~y- "~ i 04 ~,~ p, "~ i0 ~ > Z, there is sufficient time for viscous movement of chain

360 Modern Physical Metallurgy and Materials Engineering 36 kN s m and 4.6 kN m, respectively, the relax ation time is roughly 8 s. Hence sagging of the parison under its own weight will be predominantly elastic 11.2.6 Flow defects The complex nature of possible flow defects under nes the need for careful product design(sections hapes, tools) and close control of raw materials and ing arrangements). The quality of processing makes a vital contribution to the engineering performance of a Ideally, melt flow should be streamlined throughout Figure 11.9 Representation of stress relaxation under the shaping process. If the entry angle of an extru constant strain conditions(Maxwell model sion die causes an abrupt change in flow direction, the melt assumes a natural angle as it converges upon the die entry and a relatively stagnant dead zone is cre- ated at the back of the die. In this region, the melt molecules to take place and stress will fall rapidl vill have a different thermal history. In addition to its When t<i, elastic behaviour predominates. The magnitude of A ranges from infinity, at the start of tains an extensional component that increases rapidly rubbery behaviour, to zero at the start of viscous during convergence. If the extensional stress reaches a behaviour. A real polymer contains different lengths of critical value, localized'melt fracture will occur at a molecules and therefore features a spectrum of relax- frequency depending upon conditions. The fragments ation times. Nevertheless, although best suited to poly- produced recover some of the extensional strain. The mers of low molecular mass. the Maxwell model offers effect upon the emerging extrudate can range from a a reasonable first approximation for melts matt finish to gross helical distortions. The associated Let us now apply the relaxation time concept to flow condition is often an injection-moulding process in which a thermo- the fact that the calculat lue of the dimensionless plastic acrylic at a temperature of 230 C is sheared Reynolds number is ver The choice of entry rapidly at a rate of 105s"-1. Assume that the injection angle for the die is crucial and depends partly upon (deformation)time is 2 s For the shear rate given, the polymer As a melt passes through the die, velocity gradients Figure 11.8 indicates that the apparent shear viscosity develop, with the melt near the die surface moving is 9 Ns m-2 and the corresponding maximum shear stress is 0.9 mN m. At this shear stress, the elastic slower than the central melt. Upon leaving the die, shear modulus for acrylic is 0.21 MN m-2. The value the outer layers of extrudate accelerate, eliminating the of i(=n/p)is 43 us, which is very small compared velocity gradient. Above a critical velocity, the resul tant stresses rupture the surface to give sharkskin to the injection time of 2 s, hence viscous behaviour effect which can range in severity from a matt finish will predominate. A similar procedure can be applied to regular ridging perpendicular to the extrusion direc- to deformation by extrusion, For instance pP with a tion. 'Sharkskin is most likely when the polymer has sion die in 20 s, The time difference is smaller than a high average molecular mass (i highly viscous) the previous example of injection-moulding, indicating elastic) these factors cause surface stress to build up that although deformation is mainly viscous, elastic- rapidly and to relax slowly. Fast extrusion at a low ity will play a greater part than in injection-moulding mperature favours this defect. Heating of the tip of The previously-mentioned phenomenon of die swell the die lowers viscosity and reduces its likelihood then becomes understandable (Swelling is equivalent to the spring action in the Maxwell model. Although recovery of elastic strain at the cooling surface and the degree of elastic behaviour may be relatively nfluence its final texture. Thorough mixing before small during injection-moulding and extrusion, it can, shaping is essential. However, inhomogeneity may nevertheless, sometimes cause serious fow defects. exist on a molecular scale. For instance. in both Relaxation times for extensional flow, as employed injection-moulding and extrusion, a broad distribution in blow-moulding, can be derived from the ratio of of molecular mass gives a more matt finish than a nar apparent tensile viscosity to elastic tensile modulus row distribution. Thus, extrusion of a polymer with =O/ER). Suppose that a PP at a temper- a narrow mass distribution at a rate slow enough to ature of 230"C hangs for 5 s inflation with prevent the development of 'sharkskin'will favour a air. If the tensile viscosity and tensile modulus are high-gloss finish

360 Modern Physical Metallurgy and Materials Engineering t _ _ a E ,, , i illlll E I ~t a : ol Figure 11.9 Representation of stress relaxation under constant strain conditions (Maxwell model). molecules to take place and stress will fall rapidly. When t << L, elastic behaviour predominates. The magnitude of ~. ranges from infinity, at the start of rubbery behaviour, to zero at the start of viscous behaviour. A real polymer contains different lengths of molecules and therefore features a spectrum of relax￾ation times. Nevertheless, although best suited to poly￾mers of low molecular mass, the Maxwell model offers a reasonable first approximation for melts. Let us now apply the relaxation time concept to an injection-moulding process in which a thermo￾plastic acrylic at a temperature of 230~ is sheared rapidly at a rate of 105 s -~ . Assume that the injection (deformation) time is 2 s. For the shear rate given, Figure 11.8 indicates that the apparent shear viscosity is 9 Ns m -2 and the corresponding maximum shear stress is 0.9 MN m -2. At this shear stress, the elastic shear modulus for acrylic is 0.21 MN m -2. The value of ~.(= O//z) is 43 Its, which is very small compared to the injection time of 2 s, hence viscous behaviour will predominate. A similar procedure can be applied to deformation by extrusion. For instance, PP with a relaxation time of 0.5 s might pass though the extru￾sion die in 20 s. The time difference is smaller than the previous example of injection-moulding, indicating that although deformation is mainly viscous, elastic￾ity will play a greater part than in injection-moulding. The previously-mentioned phenomenon of die swell then becomes understandable. (Swelling is equivalent to the spring action in the Maxwell model.) Although the degree of elastic behaviour may be relatively small during injection-moulding and extrusion, it can, nevertheless, sometimes cause serious flow defects. Relaxation times for extensional flow, as employed in blow-moulding, can be derived from the ratio of apparent tensile viscosity to elastic tensile modulus (E = cr/eR). Suppose that a PP parison at a temper￾ature of 230~ hangs for 5 s before inflation with air. If the tensile viscosity and tensile modulus are 36 kN s m -2 and 4.6 kN m -2, respectively, the relax￾ation time is roughly 8 s. Hence sagging of the parison under its own weight will be predominantly elastic. 11.2.6 Flow defects The complex nature of possible flow defects under￾lines the need for careful product design (sections, shapes, tools) and close control of raw materials and operational variables (temperatures, shear rates, cool￾ing arrangements). The quality of processing makes a vital contribution to the engineering performance of a polymer. Ideally, melt flow should be streamlined throughout the shaping process. If the entry angle of an extru￾sion die causes an abrupt change in flow direction, the melt assumes a natural angle as it converges upon the die entry and a relatively stagnant 'dead zone' is cre￾ated at the back of the die. In this region, the melt will have a different thermal history. In addition to its dominant shear component, the convergent flow con￾tains an extensional component that increases rapidly during convergence. If the extensional stress reaches a critical value, localized 'melt fracture' will occur at a frequency depending upon conditions. The fragments produced recover some of the extensional strain. The effect upon the emerging extrudate can range from a matt finish to gross helical distortions. The associated flow condition is often termed 'non-laminar' despite the fact that the calculated value of the dimensionless Reynolds number is very low. The choice of entry angle for the die is crucial and depends partly upon the polymer. As a melt passes through the die, velocity gradients develop, with the melt near the die surface moving slower than the central melt. Upon leaving the die, the outer layers of extrudate accelerate, eliminating the velocity gradient. Above a critical velocity, the resul￾tant stresses rupture the surface to give a 'sharkskin' effect which can range in severity from a matt finish to regular ridging perpendicular to the extrusion direc￾tion. 'Sharkskin' is most likely when the polymer has a high average molecular mass (i.e. highly viscous) and a narrow molecular mass distribution (i.e. highly elastic); these factors cause surface stress to build up rapidly and to relax slowly. Fast extrusion at a low temperature favours this defect. Heating of the tip of the die lowers viscosity and reduces its likelihood. An inhomogeneous melt will produce a non-uniform recovery of elastic strain at the cooling surface and influence its final texture. Thorough mixing before shaping is essential. However, inhomogeneity may exist on a molecular scale. For instance, in both injection-moulding and extrusion, a broad distribution of molecular mass gives a more matt finish than a nar￾row distribution. Thus, extrusion of a polymer with a narrow mass distribution at a rate slow enough to prevent the development of 'sharkskin' will favour a high-gloss finish