Chapter 5

Which are the best plastics for use in the auto sector?

Delving under the bonnet to discover prime plastics for automotive applications

Plastics are moving into ever more under-the-bonnet applications in motor vehicles. The materials used have to keep their good mechanical properties at operating temperatures well above 100º C with minimal change.

The properties of plastics, as of all materials, depend on temperature. This takes on special importance for mechanically stressed parts, which are exposed in use to high temperatures. Under-the-bonnet parts near the motor have to withstand operating temperatures of 100-130º C, and sometimes up to 150º C for short periods.

Such peak loads may often last several hours, but in total may not account for more than 5% of the vehicle’s operating lifetime. For example: vehicle lifetime 10 years or 86,600 hours, total distance covered 200,000km, average speed 50km per hour; hence operating lifetime 4,000 hours or 4.5% of total lifetime.

Thermal resistance

Two procedures are generally used to determine short-term thermal resistance rapidly. In each case, a test piece is heated at a fixed rate in a heat transfer medium.

The Vicat softening point measured according to DIN EN ISO 306 indicates at what temperature a weighted test pin of 1mm cross-section penetrates to a depth of 1mm into the surface of the plastic.

The test piece is round (diameter 10mm) or of square cross-section (10mm x 10mm) and 3mm to 6.5mm high. In the most widely used variation, B50, the rate of temperature increase is 50º C per hour and the pin presses on the surface with a force of 50N.

Heat deflection temperature (HDT) indicates at what temperature a test piece under flexural load deforms to a certain extent (or more precisely, reaches an outer fibre strain of 0.2%).

DIN EN ISO 75 lays down that an 80mm long test piece of 10mm x 4mm cross-section rests horizontally on two supports 64mm apart and a load is placed at the centre.

In Process A, the flexural stress is 1.8MPa (0.45MPa in Process B), and the warming up rate at 120º C per hour is about twice the rate used in the usual Vicat test method.

The results of the two tests are not directly comparable and the classification of where various plastics figure in a classification list can also differ, due to the different types of stress compressive strength in the Vicat test and flexural strength for HDT.

Resin and reinforcements

In general, one can say that a plastic’s heat resistance increases as its melt temperature goes up, but the characteristic properties of a family of plastics also play a role.

The hardness and stiffness of polyacetals (POM), for example, brings with it high levels of thermal resistance. In the case of Delrin®, in spite of its relatively low melt temperature of only 178º C, the Vicat softening point is around 160º C and the HDT 100º C.

The adjunction of glass-fibre as a reinforcing additive can substantially improve the heat resistance of plastics. This is particularly marked in the case of HDT, because the addition of reinforcing fibres raises a material’s flexural modulus of elasticity and the higher value is valid almost up to the melt temperature, if the fibres are well anchored in the matrix resin.

Thus, the HDT of unreinforced Rynite® PET thermoplastic polyester is about 60º C, but it reaches 225º C (1.8MPa, 120º C per minute) for Rynite® 555, which contains 55% glass fibre reinforcement.

Fillers such as chalk or talc can also raise the Vicat softening point and HDT, but the effect is less marked. In the case of impact modified or flame retarded plastics, the heat resistance can actually be lower than the base material.

If a part is exposed continuously to high temperature, its thermooxidative ageing, which is accompanied by a gradual loss of mechanical properties, can be slowed down with stabilisers, such as antioxidants.

Ageing is measured in long-term tests lasting between 5,000 hours and 20,000 hours, carried out in hot air circulating ovens, using stressed or unstressed test pieces.

Vicat softening point and heat deflection temperature (HDT) rise with a plastic’s melt temperature, but depend on its material properties. Glass fibre reinforcement considerably improves both of these properties in all plastics.

Under the bonnet

The heat resistance requirements on parts near the motor have risen due to the tight packing of ancillary equipment under the bonnet and to sound damping encapsulation.

In the new three-litre six-cylinder DaimlerChrysler diesel, only a special heat shield of Zytel® HTN51G35 high-performance polyamide can withstand the high temperatures. (See Figure 1 below.) As a result of its high glass-transition temperature and 35% glass fibre reinforcement, this plastic’s heat deflection temperature (HDT) is 288º C and the heat shield retains the necessary stiffness in spite of its thinness.

Figure 2: Glass fibre reinforcement makes plastics resistant to heat deformation. This is the case for housing parts of an auxiliary car heater made of Crastin® SK605BK PBT, as well as for the connecting stubs made of Zytel® 70G30 HSLR nylon 66.

Figure 1: A heat shield of Zytel® HTN high-performance polyamide above the turbo-charger withstands high temperatures and protects the beauty cover made of mineral filled nylon which is in front of it.

Parts of the housing of an auxiliary heater for cars are also exposed to high temperatures. (See Figure 2 above.) In continuous use they reach 125º C and may reach 150º C for short periods. The mechanical behaviour requires a balance of elasticity, strength, stiffness and vibration-damping.

Tests to choose the right material showed that Crastin® SK605BK polybutylene terephthalate meets these optimally.

The connecting stubs for the cooling water circuit are made of a nylon 66 reinforced with 30% glass fibre.

Zytel® 70G30 HSLR, a heat stabilised hydrolysis resistant nylon, is specially designed to withstand high temperatures and chemical attack from radiator fluids.

Glass fibre reinforcement

Operating parts made of plastic situated near the motor must offer top-class performance in terms of strength, stiffness and toughness at temperatures up to 130º C.

Unreinforced engineering plastics with high melt temperatures can meet these requirements. With reinforcement of glass-fibre and/or other reinforcing materials, their improved mechanical properties can be maintained up to just under their melting point.

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The examples in this series of articles are intended to illustrate underlying principles and to explain the main influencing factors.

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