6 Plastics in Automotive Engineering - Smithers Rapra · 161 Plastics in Automotive Engineering...

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6.1 Applications

A wide range of plastics are now being used in interior, exterior and under the bonnet applications in the manufacture of automobiles. Some of the major applications are shown in Table 6.1.

Table 6.1 Automobile applications of plasticsPlastic Parts/components

PP Under bonnet applications

PP, 20% talc filled Under bonnet applications

PP, 20% glass fibre filled Under bonnet applications

Ethylene-propylene copolymer Car bumper applications

Ethylene-propylene elastomer Car fascias

PPO Automotive grills, fascia automotive panels

PPO, 10% glass fibre reinforced Automotive instrument panels

PEEK, 20% glass fibre reinforced Automotive applications

PEEK Automotive engine parts

PMMA Automotive components

PBT Under-body parts

PBT, 10−20% glass fibre reinforced Distributor caps

PET, 36% glass fibre reinforced Exterior body parts, casings and housings of wiper arms

PET, 45−55% glass fibre reinforced Automotive bumpers

PET (unreinforced) Automotive mouldings

PET, 45% mineral and glass fibre reinforced (fire retardant)

Automotive ignition

PA 6 Fuel tanks, automotive door closures

PA 6, 30% glass fibre reinforced Automotive parts

PA 6,6 Automotive applications

6 Plastics in Automotive Engineering

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Engineering Plastics

PA 6,1, 40% mineral reinforced Under bonnet mechanical parts

PA 6,6 (high impact) Automotive parts

PA 6,6 carbon fibre reinforced Connecting rods

PA 11 Hoses for automotive use

PA 11, 30% glass fibre reinforced Fan blades

PA 12 Injection moulded automotive parts, fuel lines and air brakes

PA – PPO alloy Automotive applications

Polyimide, 40% glass fibre reinforced Gear boxes

PEI Under bonnet applications

PEI, 10−30% glass fibre reinforced Cooling fans, gears, under body components and fuel systems

Polyamide-imide Parts for internal combustion engines: valves, gears and bearings

Chlorotrifluoro ethylene Chemical resistant O-rings

Chlorinated ethylene-propylene Non-stick valves

PPS, glass fibre and bead reinforced Headlamps, under bonnet applications and parts exposed to oil, petrol and hydraulic fluid

PPS, 40% glass fibre reinforced Headlamps, petrol, oil, and hydraulic fluid resistant parts, exhaust emission and control valves

PPS, 20% PTFE lubricated Anti-friction gears

PES Under bonnet applications, gear box, air ducting and car heating fans

PSU Under bonnet applications

PSU, 10% glass fibre reinforced Under bonnet applications, electronics and ignition components

PSU, 30% carbon fibre reinforced Under bonnet applications

PA: PolyamidePBT: Polybutylene terephthalatePEEK: Polyether ether ketonePEI: Polyether-imidePES: Polyether sulfonePET: Polyethylene terephthalatePMMA: Polymethyl methacrylatePP: PropylenePPO: Polyphenylene oxidePPS: Polyphenylene sulfidePSU: PolysulfoneSource: Authors’ own files

Some applications of plastics in the automotive industry are reviewed in the next sections.

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Plastics in Automotive Engineering

6.1.1 Polypropylene and Polyethylene

Polypropylene is used for large automotive exterior applications requiring durability, ductile impact properties at low temperature, excellent paintability and easy processability. Applications include bumpers and other large exterior parts.

Botkin and co-workers [1] state that improvements in the performance of mechanical properties of the PP used in automotive engineering applications can be achieved through the use of nucleating agents. These additives function by promoting the crystallisation of PP during moulding, providing a wide range of benefits including improved moulding productivity, increased modulus without sacrificing impact strength, enhanced thermal properties and improved clarity for special visual effects. An overview is presented of the various types of nucleating agents and the effects they provide are compared. Glass fibre reinforcement of PP has been discussed by several workers [1-3]. PP formulations with exceptional properties have been developed [2]. This is because of the good overall properties of magnetite such as its thermal and electrical conductivity, its ferrimagnetism and its high density and hardness that have been developed. Apart from these specific properties, it is particularly important in many applications, for example, in the automotive industry, for a filled plastic to have very good mechanical characteristics and to comply with the recycling requirements. Trials have shown that it is relatively easy to compound magnetite into a PP matrix and to use injection moulding with the resultant compound. Mechanical property data are presented.

Pahl and Miklos [3] state that one of the driving forces behind PP acceptance by car designers is its ‘monomaterial construction’. For example, instrument panel parts and fascias can include a long glass fibre PP (LGF-PP) for the structure and a talc-filled a PP for ductwork and a thermoplastic olefin (TPO) for the outer skin. One recent advancement in LGF-PP is the Verton MTX concentrate. The heat-stabilised masterbatch is 75% LGF by weight and is available in 13 and 25 mm long pellets. It is specifically formulated for machine-side blending using neat PP. Verton MTX is based on a commingled fibre technology called Twintex from Saint-Gobain Vetrotex America. Pahl and Miklos compared the performance properties of a fully compounded heat-stabilised 30% LGF-PP to a 75% LGF-PP masterbatch diluted to a comparable 30% glass fibre fill. The tests helped to verify that the key performance properties were not compromised by the masterbatch approach.

Engineering polymers, such as PP can be reinforced at the macroscopic level with a variety of higher modulus materials such as fibres, beads, and cement, to form heterogeneous composites [4, 5]. Thus, reinforcing PP with glass fibres as a co-woven mat has many advantages. Alternatively, reinforcement can also take place at the molecular level. These researchers have reported on the benefits of simultaneous

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molecular and macroscopic reinforcement. Low fractions of polymer liquid crystal interlayers increase the damping energy and also increase the glassy plateau modulus. Long-term dimensional stability is mainly influenced by the presence of the woven glass fibre fraction in the co-woven mat. However, higher loadings of polymer liquid crystal interlayers have detrimental effects on the dimensional stability, causing inter-ply slip at lower temperatures. The benefits of using a polymer liquid crystal interlayer, inter-ply between the co-woven mats are mainly associated with the weight savings to be gained by using a material with a lower specific gravity.

The crystallisation kinetics of the composites are affected by the nucleating effects of the polymer liquid crystal interlayer and the glass fibre. When both components are present the glass fibres dominate the nucleation kinetics. This is also reflected in the degradation kinetics, where complete degradation of the matrix occurs at lower temperatures for the glass reinforced materials.

Basell (USA) [6] have reported on the development of a family of high-density polyethylene (HDPE) resins which have a high resistance to bio-diesel fuels and it is currently being developed for use in automotive fuel tanks. Exposure of fuel tanks to biofuel for 11 years at 40 °C has, to date, produced no significant deterioration of the polymer properties and it is significantly better than the deterioration that occurs with current low-density polyethylene (LDPE) grades used in fuel tanks.

Polyethylene (PE), PP and ultra-high molecular weight polyethylene have been used in gears at lower temperatures in aggressive chemical and high wear environments [7].

6.1.2 Ethylene-propylene-diene

Ethylene-propylene-diene terpolymer (EPDM) is a rubbery polymer which has been applied to polymeric composites for automotive window seals which are generally exposed to complex service environment conditions [8]. Different EPDM rubber compounds (control, bulk modified and surface modified), coated with polyurethane were exposed to controlled ultraviolet (UV) irradiation. The higher the adhesion, the lower the extent of UV penetration through the interface was. The photo-oxidation mainly takes place from the surface, as the energy gets consumed and the intensity of the UV decreased on its passage through the strongly adhered layers to the rubber compound. The coating layer, being on the surface, is more affected and photodegraded by the UV irradiation than the rubber compound. The sample being multi-phasic in nature and with a strong interface, can scatter the UV strongly and thus shows the best weatherability among all the systems.

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Delaval and co-workers [9] examined the effects of two pollutants (engine oil and ethylene glycol) and recycling on the thermal and rheological properties of two talc containing PP-based materials used in the automotive industry. The PP-based materials were a copolymer of PP and ethylene-propylene rubber (EPR) used for front bumper face bars and PP-EPR-talc used for back bumper face bars. The results showed that the pollution and recycling did not decrease the intrinsic properties of these PP-based materials. Indeed the presence of engine oil improved the thermal stability, indicating that engine oil may act as a flame retardant in this particular case.

6.1.3 Polyether Ether Ketone

Hufenbach and co-workers [10] and researchers at LNP Engineering Plastics [11] have discussed the use of polyketones in engineering plastics. This included a study of the damage to high speed impellers in fabricated carbon fibre reinforced PEEK.

Researchers at LNP Engineering Plastics [11] have developed a series of formulations of aliphatic ketone based, thermoplastic compounds containing glass and carbon fibre reinforcement and polytetrafluoroethylene as a lubricant. These polyketones are targeted at the automotive market and are characterised by good impact performance over a broad temperature range and also high chemical resistance, good hydrolytic stability and superior resilience.

6.1.4 Polyesters

Krigbaum [12] has reported on the use of glass fibre reinforced composites, both from Cytec Industries, based on Cyglas 685 unsaturated polyester [bulk moulding compound (BMC)] and Cyglas 695 vinyl ester resin (BMC) in automotive valve covers and other engine cover applications. The recycling of these valve covers is also discussed.

6.1.5 Polyethylene terephthalate

Polyethylene terephthalate (PET) usually incorporating glass fibre reinforcement has been used extensively in automotive body mouldings, bumpers and housings [13-15].

Sepé and co-workers [13] found that automotive components consisting of a metal insert over-moulded with impact modified glass fibre reinforced PET cracked adjacent to the insert following thermal shock testing (100 cycles at – 40 to 180 °C). The melt flow index of the material increased by 140% on drying prior to moulding,

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suggesting a molecular weight reduction because of polymer degradation. However, the intrinsic viscosities before and after drying, and after moulding were similar, indicating minimal degradation. This anomaly was attributed to the melt viscosity and intrinsic viscosity techniques being more sensitive to low molecular weight fractions and high molecular weight fractions, respectively. Gel permeation chromatography measurements confirmed a bimodal molecular weight distribution, the bimodal nature of which increased with increasing exposure to elevated temperatures.

Gobernado and co-workers [14] have discussed the use of PET to produce thermally bonded vehicle roof liners by thermoforming.

Renaud [15] has reviewed developments by the DuPont European Technical Centre in the design and production of automotive components using advanced blow moulding techniques. Examples are presented of automotive hoses manufactured in DuPont’s engineering plastics and Hytrel polyesters elastomers.

PBT gears have extremely smooth surfaces and have a maximum operating temperature of 150 for unfilled and 170 °C for glass reinforced grades. PBT works well against polyacetals and other plastics as well as against metal and is utilised in engine housings [7].

6.1.6 Polyamides

Various PA, particularly PA 6 and PA 6,6 have been used in the manufacture of parts for automotive applications including radiator tanks [15-20], engine covers in Audi cars [20-22], a pedal box [23] and various other components [24-26]. Usually glass fibre reinforcement is included in the formulations [27, 28]. The requirements for radiator tanks are good heat stability, vibration resistance and resistance to coolant additives [18]. Audi use 25% glass fibre, 15% mineral reinforced Nylon 6,6 for the fabrication of rubber covers for their Audi A4 and VW Passat engines. These covers have good dimensional stability even at elevated temperatures, which ensures a low creep tendency that contributes to a long in-service life of the part [27].

Di Pasquale and co-workers [29] have discussed the recycling of glass reinforced PA 6,6 used in the automotive industry.

DSM Engineering Plastics [30] is developing a high performance PA 4,6, called Stanyl for the production of tumble valve bearings and chip carriers for the latest generation of Mercedes Benz V6 and V8 engines. Resistance to high loading, good wear and friction properties play an important role in the lifetime of this component.

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Tumble valve bearings are an integral part of the manifold optimising air flow in the combustion chamber. It is the first time that such tumble valves have been produced by assembly moulding and, according to DCM, Stanyl played an important role in making this possible. With the in-mould assembly technique, components are assembled inside the mould in a two shot (2K) process. This results in significant cost savings compared to traditional processes in terms of assembly, tooling and machinery [31].

For in-mould assembly to work a combination of materials is required which can meet both the in-mould process demands as well as the performance under the operating conditions throughout the life of the engine. In the moulding process, says DCM Engineering, Stanyl has outstanding stiffness at high temperatures – even close to its melting point – which enables 2K over-moulding without distortion of the initial moulding. As the assembly has a bearing function, free movement of the two components is required after production. This requires that no adhesion occurs between the two materials during the moulding process and the shrinkage coefficients are such that free movement is maintained after moulding and at engine operating conditions and temperatures up to 150 °C. In addition, wear and friction behaviour of the component materials is very important. The tumble valve bearings have to resist high loads. Good wear and friction properties play an important role in the lifetime of the component itself and subsequently of the engine. A grade, selected from the Stanyl portfolio of materials that are optimised for stiffness and improved tribological performance, gave the correct performance against the PPS mating material in the assembly [31].

6.1.7 Polyacetal

A polyacetal, Delrin 300CP, from DuPont Engineering Polymers (Wilmington, DE, USA) [6] combines high impact and mechanical performance with high flow, which improves weld-line strength. Its cost is equivalent to standard grades. Delrin 300CP is designed for use in automotive fasteners, as seat-belt components, levers, brackets, and hardware for window and doors.

Delrin 300CP offers consistent impact performance over a broad temperature range. For example, notched Charpy impact of 10.5 kJ/m2 at 23 °C only falls to 10 kJ/m2 at 30 °C. High elongation at yield (22%), high stiffness (440 MPa) and high creep resistance help extend the use of the products in continuously loaded, spring-elastic components.

Use of polyacetal gears is increasing because of their high performance, dimensional stability, high fatigue, chemical resistance at temperatures up to 90 °C, and excellent lubricity against metals and plastic materials in automotives and precision gears [7].

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6.1.8 Polyphenylene Sulfide

A 40% glass fibre reinforced PPS is used in for automotive fuel rails [28]. This engine component conveys fuel from the injection pump to the cylinders and is required to withstand rapid changes of pressure, service temperatures of up to 120 °C, engine vibrations, and direct contact with aggressive fuels. Details are given of the chemical resistance tests carried out on the Fortron grade and also on a branched PPS grade and two PA 6,6 grades. Test fuels were ASTM reference fuel C, two aggressive fuels with differing methanol proportions, and an auto-oxidised fuel containing peroxide and copper additives. Fortron 1140L4, a linear partially crystalline PPS, exhibited the highest impact resistance and dimensional stability. PPS offers high stiffness, dimensional stability, and fatigue and chemical resistance at temperatures as high as 200 °C. It is used in demanding automotive applications.

Liquid crystalline polymers such as thermoplastic elastomer copolyesters are being used in lower power, higher speed gears because it allows them to tolerate inaccuracies and reduce noise while providing sufficient dimensional stability and stiffness [7].

6.2 Acoustic Properties of Polymers

Treny and Duperray [32] have pointed out that issues of human comfort relating to noise and vibration are one of the major priorities for materials structural research and development in the field of transportation. Dynamic mechanical analysis (DMA) testing provides a way to characterise in an accurate manner the viscoelastic properties of all the material used in vehicle interiors. Using a unique database software allowed easy material selection according to their viscoelastic properties. An approach for the optimisation of materials through the combination of selective database software and specific numerical calculation methods, to predict the final acoustic behaviour during the materials selection and systems development period are presented.

A further consideration in sensory studies is the evaluation of tactile properties of automotive components reported by Giboreu and Bardot [33].

Plastic covers for gasoline and diesel engines have become generally established [34]. The trend is towards the use of increasingly high-quality, multi-functional designs. Modern engine covers typically feature combinations of different materials and surface finishes and integrate widely diverse functions such as sound deadening.

Blinkhorn and co-workers [35] developed a new glass fibre reinforced PP substrate for use in car interiors as a semi-structural and acoustic material. It is available in a number of weights per unit area and with various proportions of glass. The composite

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is claimed to have exception expansion properties, which allow components of varying thickness to be moulded.

Palmer [36] discusses Thinsulate™ sound absorbing materials made by 3M for the automotive sector. These materials have enabled design engineers to totally rethink their approach to acoustic absorption in many areas of motor vehicles.

Janisch and co-workers [37] and Browell and Jyawook [38] studied the acoustic properties of automotive weather sealing using a thermoplastic vulcanisate for body sealing applications. EPDM sponge was compared with JyFlex (thermoplastic vulcanisation). The study was performed using multiple acoustic tests (road noise and component noise), supported by finite element analysis as a diagnostic tool. It was shown that primary seal acoustic performance is stiffness controlled at low frequencies and mass controlled at higher frequencies. A series of studies are reported which demonstrate the extent of the acoustic advantage by using higher density material for automotive weather strip sealing applications.

6.3 End of Life of Vehicles

The automotive industry faces worldwide pressure to help find environmentally friendly ways of disposing of end-of-life vehicles [39]. Most national governments have some form of proposed legislation to cover vehicle disposal and the European Union has designated vehicle scrap as a priority waste stream. There are questions of economic viability, environmental balance and lowest energy use, which must be considered, alongside the necessity to satisfy other legal and customer requirements. Cost estimates for the recovery and recycling of plastic material tend to ignore the time and cost associated with identifying the material. Most dismantling studies start with a knowledge of the materials that make up the parts to be removed. In practice the cost of material identification by non-automated systems, such as referring to dismantling manuals or finding and reading material identification marks, may well tip material recycling into the non-economical category. Mistakes in material identification can also have serious consequences on the later stages of the recycling process, where contamination by non-compatible polymers can ruin whole batches of recylate. The development of two different types of automated material identification equipment is described together with some of the other techniques evaluated during the development process.

Viera [40] has discussed the reuse of the waste produced from edge trimming in the extrusion of TPO sheets and final inspection of TPO skin/crosslinked PE (XLPE) foam laminates. Different analysis techniques were used to evaluate the effects of the recyclate on the final product performance. The DMA results show no significant

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changes of the viscoelastic properties for different percentages of recylate. Adding TPO/XLPE foam scrap into the virgin TPO causes a significant increase in viscosity of the compound. Results from oxidation induction time measurements show that over 50% of recycled material may cause some failure in the thermal stability of the compound. Therefore, recycled TPO skin can be reutilised for the sheet extrusion process, without affecting the functionality and quality of the final product performance. The incorporation of TPO/PE foam laminates into some injection moulding applications has proved to be a cost effective way of improving impact resistance plastic applications of the car, such as the wheel arch cover.

6.4 Miscellaneous Applications

6.4.1 Interiors

Traugott and co-workers [41] have discussed the emergence of thermoplastics as primary material choices in automotive interiors. This brings numerous challenges. Increasingly, cost and performance balances are closely scrutinised to maximise the value for the consumer. Careful comparisons and rapid material development aid applications calling for structural rigidity and impact resistance. Great importance is placed on the ability of automotive thermoplastics to be fabricated with durable, low gloss first surfaces. This requirement motivates the development of advanced fabrication processes, mould designs (including grain) and materials.

Traugott and co-workers [41] place the emphasis on aspects of PP, acrylonitrile-butadiene-styrene (ABS) and polycarbonate (PC) blends with ABS or impact modifiers. Material development benefits from a host of recent technical advancements such as catalysis, reaction engineering, impact modification and compatibilisation. Some examples of these are fourth generation Ziegler-Natta and metallocene polyolefin catalysts, highly efficient solution processes (PP and ABS) and copolymer modifiers.

6.4.2 Seals

Leone and Oliver [42] have discussed the applications of fluoroelastomers, nitrile rubber (NBR) and hydrogenated NBR in seals for automotive air-intake manifolds. They review the properties required for such seals including mechanical and low temperature properties, heat and fuel resistance, and design requirements for the seals used in conjunction with the engineering plastics components in this application.

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6.4.3 Tyres

‘Smart’ or ‘intelligent’ tyres, equipped with under-tread sensors are capable of measuring tyre forces directly [43]. Supplied with this information rather than having to presume or infer it, a new generation of suspension, brake and stability control systems promise finer control over vehicle dynamics than has been possible before. However, just as engine performance is not determined solely by engine management electronics, so the fastest microprocessors and cleverest software can only do so much to influence ride, braking and handling. They may exploit the tyre better, but the tyre itself still determines the ultimate performance envelope, and a great deal more besides. It is not possible to make a tyre run-flat using electronics, or lower its rolling resistance, or increase its load rating, or alter its packaging ramifications. For progress in these areas the tyre itself, and ultimately both tyre and rim, must be examined.

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