Engineering polymers in the underhood The underhood of a car is a particularly challenging area because of high-temperature stresses (often above 100 C) and for the presence of aggressive liquids as gasoline, oils and other liquid matters (coolants and braking system liquids). Research into light materials that withstand chemical agents and may replace traditional metals has favoured the extensive use of many families of engineering polymers. These thermoplastic materials are often developed specifically for particular applications. Because of smaller spaces and of more and more performing engines and fuels, research now aims at providing these materials with a higher thermal and chemical resistance. Out of the whole weight of an average European car, plastic materials account for about 105 kilograms, 26 of which are located under the hood (see table below). Function Polymer matrix Kilograms Fuel system PE, POM, PA, PP 8 Electric components PP, PE,, PA 7 Tanks PP, PE, PA 1 Various elements PA, PP, 10 Total under the hood 26 Total plastic in the car 105 Table 1: quantity of plastic in an average European car The following table lists some examples of underhood applications that use plastic. The list is just an indication, since every specific engineering polymer is chosen on the basis of the exact location of the component, of the actual working environment (temperature, contact substances, etc.), of mechanical stresses, and economic considerations. Environment Air Fuel Water Examples of engineering Examples of components polymers Ventilation system ABS, Components of the controls of the air-conditioning system PP, Air intake manifold, 6, PA46 Fans Air sensors Filters Fuel tank PE Fuel pump Gasoline manifold, 6 Gasoline gauge support POM Injector nipple 6 Carburetor components PEI, PPS, PEEK Water tank PE Water pump components, PPA Thermostat support Cooling system PPO Table 2: examples of component-dependant applications (cont'd)
Tappet cover PPA Valve cover Oil Bushings for automatic transmission PPA, PEEK Components of the oil distributing system Components of the oil filter 6 Elements of the gearbox Lobe pump Coil structure PC,, Collectors Distributor cap Electric parts Injection distributor Connectors Cable support, PPE, PP Engine Experimental engines Thermosetting resins Belt tightening pads 6 Sundries Gears POM, PEEK Table 2: examples of component-dependant applications In order to better understand the advantages of thermoplastic engineering polymers, some applications are illustrated below. 1. Engine cover Usually the engine cover receives no mechanical stress; therefore the main requirement is for it to withstand the high temperatures in the car underhood. World car manufacturers have chosen two polymeric materials: A) epoxy resin or vinyl ester (US car manufacturers) B) glass fibre-reinforced polyamide (European car manufacturers) The following table lists the major thermal properties of these two materials: Thermosetting PA 33%FV PA 43%FV Melting point ( C) Does not melt 255 255 Glass transition temperature ( C) >170 80-90 80-90 HDT ( C) >260 249 252 CTLE (mm/mm* C) 0,8*10E-5 1,8*10E-5 1,7*10E-5 Impact strength IZOD (J/m) 500 150 160 Table 3: comparison between thermosetting resin and reinforced polyamide
As you can see, the thermosetting resin shows a far better thermal behaviour (for obvious intrinsic reasons); however its simple and quick processing, together with its being fully and directly recyclable (an aspect on which people's awareness is growing) favours the use of reinforced polyamide. This is why also the biggest US manufacturers are moving to this thermoplastic material. One example is the Chrysler group which is adopting a mineral-reinforced nylon (Minlon by DuPont) for the covers of its new Chrysler Town&Country 2004, of Dodge Caravan and of Grand Caravan with 3.3- and 3.8-litre V6 engines. This solution makes Chrysler cars weigh 30% less than in the past, when aluminium was used. figure 1: valve covers of the new Chrysler Town & City (material: Minlon - Du Pont Engineering Polymers) 2. Air intake manifold One of the motor sector components allowing greater design freedom is the air manifold. The design of effective and efficient air ways had always been limited by difficulties in metal working. The use of die-cast aluminium helped design more complex shapes but it is with engineering polymers that the best benefits are achieved. Many car manufacturers use glass-reinforced polyamide to make air intake manifolds; this offers a number of direct advantages such as: lower weight while preserving same performances lower costs thanks to easy manufacturing design freedom leading to better air flows possibility to integrate the various parts workability corrosion resistance unlike metals better aesthetic performance To these some indirect benefits can be added, such as: lower gasoline consumption without any lower performance smaller dimensions and optimization of the available space in the engine compartment increased performances thanks to a better air flow
The manifolds made with thermoplastic engineering polymers are used not only in production-model cars but on sports cars too, as with some Porsche models. However in the case of sports cars (as with Chevrolet Camaro and Pontiac Firebird) polyphthalamide is preferred to polyamide because of its higher properties such as: high-temperature resistance (up to 120-130 C) lower humidity absorption better creep strength that lowers the risks of losses But the use of PPA presents some disadvantages as well: the cost of the raw material is higher than polyamide. Nevertheless car manufacturers often accept this compromise, because by using PPA they can achieve higher advantages in terms of performances such as: lower weight (5 kg with PPA, 12 kg with aluminium) a 25% higher air flow higher power by 20HP 3. Radiator fan The decision to use glass-reinforced polyamide to make the first fans in the automotive sector, instead of widely used steel, came out of weight reduction reasons. Polyamide low density, together with its good mechanical performances (which are not at all on the same level as steel, but they are satisfactory enough to meet component's needs), made it possible to reduce weight by 60%. Today the decision to make fans with engineering polymers is not only justified by their lightweight advantages but also by their higher flexibility and the reduction of the radial stress on the water pump bearing (and the consequent possibility to use engineering polymer bearings). Easy working and the design freedom that are typical of thermoplastic polymers enable designers to draw complex blade profiles that make fans more efficient. To this the following indirect advantages ought to be added: longer life for the pump bearing, thanks to the lower radial load lower gasoline consumption without any lower performance better performances (also thanks to new blade geometries) higher safety for workers and customers, because of the material s higher flexibility and rust resistance. 4. Fuel cells As hydrogen-powered cars are just appearing on our roads (albeit as prototypes for now), the manufacturers of materials and components are studying some solutions that may lead to an economically-sustainable production of fuel cells in the near future. For the past few years some manufacturers have been busy researching into and developing polymeric materials that may replace metals in the bipolar plates that turn hydrogen into electric power. The reasons for this are economic considerations (plastic costs less than gold, graphite and aluminium) and the lower weight deriving from the use of synthetic materials. For these applications some types of LCP and PPS were developed that can resist corrosion and the chemical substances in cells, while preserving a good dimensional stability with temperatures up to 240 C.
5. Examples of high-tech components made with very high-performance engineering polymers In many types of vehicles specific breakdown can fully prevent them from working. In these cases very high-performance engineering polymers (like PEEK, PEI, PI) prove effective and perform even better than metals, particularly from the tribology standpoint. As for PPA and, even more so, these engineering polymers the high cost of the polymer matrix is well offset by the performances offered by such matrix. In fact the possibility to replace the metal (thus reducing weight) and to eliminate bushings and bearings (curbing the costs of components and assembly), together with their exceptional thermo-mechanical and tribologic performances, makes very high-performance engineering polymers economically profitable in specific market sectors and applications. For example components made with polyimide (DuPont Engineering Polymers) meets 3 out of the most urging performance demands of the automotive industry: high wear resistance even when lubrication is lacking excellent high-temperature behaviour, up to 350 C good seal for fluids But on the other hand: processing is unusual and expensive the cost of the raw material is high materials are little known However the applications that require these materials are the following: bearings, gaskets and valves of the gasoline pump transmission bushings, valves and gas rings gaskets and bushings for the gasoline injector turbine bearings carburetor gaskets piston rings valves, gaskets and bushings for the hydraulic system 6. Conclusions Plastic applications in the underhood are a technical and economic reality. Some parts are exclusively made of thermoplastic polymeric material; as for other components, competition is still underway between thermoplastic and thermosetting materials; finally for other parts still, designers are faithful to the reassuring tradition of metal (steel and aluminium), particularly because of their poor knowledge of the extraordinary intrinsic properties of engineering polymers. References: WEB sites and technical bulletins by: AES, APME, Bayer, Borealis, Citroen, Dow, Du Pont, Fiat, General Electric, General Motors, Honda, SAE.