6
Plastics in Automotive Engineering
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 plastics
Plastic
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
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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: Polyamide
PBT: Polybutylene terephthalate
PEEK: Polyether ether ketone
PEI: Polyether-imide
PES: Polyether sulfone
PET: Polyethylene terephthalate
PMMA: Polymethyl methacrylate
PP: Propylene
PPO: Polyphenylene oxide
PPS: Polyphenylene sulfide
PSU: Polysulfone
Source: Authors’ own files
Some applications of plastics in the automotive industry are reviewed in the next
sections.
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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 talcfilled 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 interply 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 photooxidation 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, acrylonitrilebutadiene-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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