Silicone usage expanding in European automotive applications.

Silicone usage expanding in European automotive applications

Over the past two decades, a powerful combination of environmental, economic and aesthetic pressures has forced the performance requirements of elastomers in American cars steadily upwards. Part of this pressure has come as the result of environmental control devices, which have significantly impacted the operating temperatures of the engine, exhaust system and drive-train, and have therefore steadily eliminated lower temperature, lower-performance products. And the warranty wars being waged between automakers competing in the U.S. market have also put greater demands on the physical and mechanical properties of elastomeric parts.

One consequence of this drive for higher-performance materials, at least in the elastomers area, is that automakers competing in the U.S. market have significantly increased their usage of silicone materials. For instance, today virtually all engine gaskets on U.S.-made automobiles are a silicone of one form or another.

Silicones have been used in the automotive industry for almost three decades in applications as diverse as ignition cables and boots, shaft seals, self-bleed connector grommets and adhesive sealants. This family of inorganic polymers has a reputation for offering exceptional performance in some of industry's most demanding applications. However, the price premium for such levels of durability and reliability has often precluded their use in cost-sensitive applications. But, as the automotive industry changes on a global scale, and performance requirements on all automotive materials grow tougher, silicone polymers are increasingly becoming the only materials that can handle the job.

Whereas trends in the specification and use of elastomers in the European automotive community have generally followed the same pattern as in the U.S., they have done so at a much slower and less radical rate. Hence, elastomer use in that market is still largely dominated by lower-performing, organic materials. But that situation is expected to change rapidly over the next three to five years.

Keeping up with a changing market

New environmental regulations may prove the strongest motivator yet for European automakers to turn to silicone elastomers for seals, gaskets, hose, shielding and numerous other related applications. Current environmental laws in Europe are as varied as the continent's languages and geography. But that situation will change by 1992 when most Western European nations are scheduled to become united in the European Community (EC), an integrated economic entity with no trade barriers or tariffs between member nations. While much public attention concerning this event has focused on such matters as trading opportunities and a common European currency, the effect of unification on product and environmental standards may prove to be just as significant.

On the environmental front, EC nations are expected to finally and firmly establish uniform regulations on auto emissions to help protect the environment and its fragile infrastructure. These regulations are expected to resemble their counterparts in the U.S., mandating the use of catalytic converters and other environmental controls which heretofore have been more or less optional. They may even mandate a more rapid changeover to high-alcohol-content fuels, widely used in South America, and now selectively being tested in the U.S. Further, there is a good deal of work underway to develop chlorofluorocarbon (CFC)-free heat-exchange products for automotive air-conditioning systems.

Each of these changes - tighter emissions controls, alternative fuels and alternative heat-exchange materials - will push engine temperatures higher, introduce new chemical environments and make more demands on gaskets, seals, hose, diaphragms, fuel lines, injectors and exhaust system components. In turn, these new requirements will push automakers competing in Europe to make use of higher-performing materials.

But environmental controls are not the only factors at work here. Design changes (generally led by the Europeans), a new global emphasis on quality, a potential European warranty war and other new technologies will also be fueling these transitions to silicone polymers.

Environmental controls and the problem of heat

In the U.S., increased automotive operating temperatures frequently blamed on tougher environmental controls have accounted for significant market penetration by silicone elastomers over the past few decades. Engine down-sizing and tight EPA emissions and fuel economy restrictions of the '70s started temperatures climbing. Transverse mounting of engines and turbocharging have also contributed to this temperature rise on select models.

Environmental controls have made exhaust system components run hotter, causing increasing amounts of heat to be radiated upward into the passenger compartment (particularly in the area over the catalytic convertor). This heat is thought to overtax air conditioning systems and cause premature failure of interior carpeting, as backing and foam begin to break down, not from wear and tear, but from thermal degradation. And now new environmental controls are coming into play which will drive operating temperatures even higher, affecting materials specification globally.

The proposed change to gasohol and the interim development of flexible-fuel vehicles, which can adjust from tank to tank to a variety of fuels, ranging from gasoline to methanol to ethanol to mixtures of the three, poses a number of engineering challenges to the materials and design community.

Because gasohol's energy density is about half that of gasoline, a car using this type of fuel would require a significantly larger gas tank, typically almost twice the current size, in order to achieve the same range. And since gasohol has a higher octane rating than gasoline, engines must operate at higher compression ratios to burn this fuel efficiently. Unfortunately, gasoline will not operate at such high compression ratios. Therefore, interim flexible-fuel vehicles will have to offer a compromise in efficiency between both fuels just to be able to handle both fuels and mixtures in between. And the lower volatility of alternative fuels means harder starts in colder weather, so special fuel injectors and high-energy ignition systems will need to be developed.

In the safety area, gasohol, unlike gasoline, can form a flammable mixture in the fuel tank, necessitating the use of special fire-resisting systems around the tank. Further, an alcohol flame is close to invisible and its vapor has minimal odor. Hence, additives will need to be incorporated, as in natural gas, to warn of leaks or fires.

Not only does gasohol make engines run hotter, necessitating the use of higher-temperature materials, but it also represents a new chemical environment, one which is hostile to many traditional automotive materials. Because gasohol softens, dissolves or corrodes some of the plastics, rubbers and metals currently in use as fuel tanks, fuel lines and fuel injection seals, tubing, hose and diaphragms, alternative materials may need to be considered in a number of application areas.

For instance, because of the new chemical environment posed by gasohol mixtures, it may be necessary to switch to stainless steel gas tanks. Further, while fluorohydrocarbon elastomers offer better fuel-swell performance than fluorosilicone elastomers in straight gasoline, the performance of many fluorohydrocarbon elastomers drops significantly as the percentage of alcohol in the fuel mixture increases. Since fluorosilicone elastomers provide adequate swell resistance in any combination of gasoline-alcohol mixture, and since fluorosilicones offer superior low-temperature performance capabilities, it is expected that these silicone polymers will replace organic elastomers as alternative fuels become more widely available.

Alternative refrigerants present a similar situation. As global concerns over the greenhouse effect and depletion of the ozone layer become stronger, there is increasing pressure to develop effective, non-chlorofluorocarbon (CFC)-based coolant materials, all at a time when air conditioning systems are becoming an increasingly popular automobile option, especially in Europe. The first commercially available product offering is reported to be less efficient than CFC refrigerants now in use, and may well impact under-hood materials use.

Condensers using this new refrigerant will require much higher air flow, and working pressures for the alternative system are estimated to be 10-12 percent higher than current technology. Higher pressures and hotter temperatures will more than likely require the use of higher-performance materials to handle the new coolant. And again, a new chemical environment may present challenges to materials currently handling CFC coolants.

Design changes are also accounting for higher operating temperatures, both in the U.S. and abroad. The sleek, windswept lines of today's more aerodynamic automobiles have forced front ends to become lower and more cramped, reducing airflow to the engine block at a time when underhood temperatures are climbing.

Meanwhile, an increase in the use of window glass, particularly the popular high-raked styling of windshields, has further contributed to solar gain, making instrument panels approach the heat-distortion temperatures of some plastics. Rising interior temperatures are not only putting more stress on air conditioning systems, which are in turn causing temperatures to rise further, but this temperature rise is necessitating the use of higher-performance polymers, not only in instrument-panel skins and support structures, but also in duct work. In fact, instrument panel ducting is a promising application area for silicone foam and ducting.

Dealing with the problem of heat: A few solutions

As the under-hood, drive-train and exhaust-system environments grow increasingly hostile, the role of silicone elastomers is becoming more significant, especially in engine hot spots. In figure 1, the progression of under-hood temperatures is plotted against elastic life, the time (in hours) for elongation in generic categories of materials to be reduced to 50 percent. As temperatures rise, the elastic-life capabilities of organic elastomers drop off rapidly, leaving few materials with adequate performance.

Since the heat problem shows no sign of diminishing, new approaches are being sought in order to deal with this situation more effectively. One such approach is to use shielding to isolate heat around hot spots and to protect more thermally sensitive components. In applications where simple shielding is not enough, manufacturers are trading up to higher-performance, higher-temperature materials.

Materials which can be foamed offer better insulating properties and lighter weight, and elastomeric foams not only cushion components, but are better able to handle vibration and abuse. However, as temperatures get higher, fewer flexible foams are appropriate candidates, especially since many of these materials do not offer adequate heat-aging characteristics. Rather, they stiffen and shrink after repeated heating/cooling cycles, pulling away from components they were designed to shield, and often causing more noise and problems than they were initially designed to stop.

The area of thermal shielding is an extremely promising one for silicone foams. These light weight, resilient elastomers offer excellent thermal-aging characteristics, very broad temperature capabilities and non-halogenated flame retardance. Medium- and low-density versions of these products are expected to see significant penetration into numerous automotive and public transportation applications.

For instance, on many vehicles, plastic gas tanks are now necessary given the tighter, more oddly shaped space requirements currently available to designers, and the nearly universal desire to reduce vehicle weight on passenger cars. However, placing a gas tank molded of a material with a heat-deflection temperature of less than 200[degrees]F in close proximity to an exhaust system which may be well over 800[degrees]F causes many to worry.

Early attempts to shield the plastic gas tanks focused on corrugated aluminum shrouds. However, aluminum functions more as a heat sink than a heat shield, and is cumbersome to install around oddly shaped tanks. Hence, a special composite fabric of a medium-density, elastomeric silicone foam and aluminum foil is being evaluated as a replacement. The lightweight foam/foil composite radiates heat back at its source, keeping tanks cooler. It can be easily draped around uneven tank contours, and it trims off several pounds of weight per vehicle.

This same flexible silicone foam/foil composite is also being considered for use as padding under interior carpeting, especially over the catalytic convertor, and as padding on fire walls, and in and around the engine. The material's heat- and sound-shielding properties, light weight and flame retardance are driving such changes.

Other areas where the high-temperature capabilities of silicone elastomers are put to use are in exhaust-pipe hangers, and a variety of under-hood connector and electronic packaging applications. Long flex-life at both high and low temperatures is a silicone hallmark and is the reason silicone rubber has replaced organic rubber in applications such as C.V. boots.

And as previously mentioned, silicone elastomers, whether heat-cured rubber, room-temperature vulcanizing elastomers or liquid-injection-molded silicones, are used extensively as engine gaskets for U.S.-built cars. Typical applications include gaskets for the oil pan and rocker cover, and water-pump and crankshaft seals. Seals using organic elastomers have been displaced by silicone seals in these applications not only because of silicone's superior temperature performance and longevity, but also because of its superior sealing-force retention while exposed to a wide variety of conditions, and the ease of manufacturing and assembly offered by silicone materials. Even more applications are expected to proliferate in the 1990s to better control the increasing heat problem under the hood and in and around the exhaust system.

As European manufacturers are confronted with higher operating temperatures, they too will be forced to upgrade their elastomers to meet tougher performance requirements. Further, global trends toward more efficient, modular engines are also opening up opportunities for silicone gaskets on plastic carriers and/or metal covers and carriers. These one-piece systems also facilitate automated manufacturing and assembly.

How design changes are affecting material selection

While many design changes are affecting material use by virtue of their contribution to vehicle heat, other design changes are also affecting material selection, but for different reasons.

For instance, the increased use of plastics in body panels and bumper systems has spurred developments in alternative fastening systems. Often, the most economical, efficient and durable method of attaching plastic parts to themselves or to dissimilar materials is to use adhesive bonding. Because plastics are generally not as strong as metals, engineers must typically design closer to their yield strength, or stress concentrations induced by mechanical fastening systems can lead to premature part failure. However, adhesive fasteners which are chemically compatible with their substrates not only eliminate the stress concentrations common with mechanical fasteners, but they also provide a more uniform distribution of stress across a wider surface area of the part. This, in turn, gives an engineer more freedom to optimize part designs since stress levels no longer have to be kept as low.

Further, by virtue of their application method, adhesives also provide the advantage of improved aesthetics. The exterior surface of an adhesively bonded assembly can be smooth, uninterrupted and aesthetically attractive. The deformations caused by point fastening or vibration welding are eliminated, reducing or even eliminating the cost and time of additional finishing operations.

While a number of organic adhesives compete with silicones in this arena, the elastomeric properties of silicone adhesive/sealants and their superior temperature capabilities over a much broader range, are accounting for further share-shift by silicone adhesives.

The newest and most promising application for silicone adhesives is in the bumpers area. Several room-temperature-vulcanizing (RTV) elastomeric silicones, a one-part adhesive and a two-part adhesive, have been used successfully to bond together fascias and beams of both injection-and blow-molded thermoplastic bumper systems. Replacing secondary operations such as heat staking and ultrasonic welding, these elastomeric adhesives offer greater energy absorption during impact than either mechanical fasteners or organic adhesives, especially at low temperatures (figure 2). And they also help to alleviate mechanical and thermal-stress differentials between thermoplastic components. The silicones have the further advantage of not being stress concentrators on the plastic they bond. They also offer primerless adhesion to bumpers molded of thermoplastic alloy resin, and they cure more rapidly than primed, multi-part urethane systems.

Other silicone RTV adhesive/sealants are widely used on forward and rear lighting units to keep dirt and moisture out of glass and polycarbonate lenses, as well as to attach emblems and hood ornaments, rear-view mirrors and side mirrors.

Global quality wars

Led by the Japanese, the global automotive industry is engaged in a quality and warranty war which started in the U.S. and has moved to Europe. This battle promises to improve production quality and benefit the consumer, but it may prove costly to automakers who do not build in quality ahead of time. As warranties become longer and more comprehensive, OEMs must decide whether to absorb the cost of replacing lower-performing products several times during the warranty period, or to invest in higher performance materials initially in order to build in quality that lasts.

There are several problems inherent to the use of less expensive, but lower performing materials. First, the cost of the service to change the part is frequently much greater than the cost of the part itself. And the aggravation of too many trips back to the dealer on the part of the consumer, even for defective parts that are completely covered, leaves the consumer questioning the automaker's commitment to quality. This consumer may not buy from that automaker again. Even worse, bad experiences have a way of being communicated faster than good experiences, giving a manufacturer a bad reputation with the international public. Hence, many manufacturers are learning that it is often less expensive in the long run to opt for more expensive but more effective materials from the beginning.

Air ducting and coolant hose provide a good example of this tradeoff. Trends toward hotter cars, longer turn-over times between new-car buying and the decrease of full-service gas stations (and maintenance checks) have been cited for a sharp increase in belt and hose replacement on light trucks and passenger cars by manufacturers of organic rubber belts and hose. But as temperatures rise, organic products will need more frequent changing, or may no longer be appropriate at all. This replacement problem, coupled with longer and more comprehensive OEM warranty periods, are predicted to enable silicone hose to significantly erode the marketshare of organic hose in the air ducting and water coolant systems of most passenger cars and light trucks.

Silicone rubber has long been used for formed and straight air- and coolant-system hose on heavy-duty trucks and fleet vehicles. In fact, these materials have as much as a 17-year performance record on trucks, ambulances, police cars and other vehicles subject to heavy use. Last year, GE Silicones offered to warrant hose made of its silicone rubber products and installed on OEM vehicles for 10 years or 100,000 miles of use. While the initial cost of silicone hose is three times higher than that of organic hose, the gain in performance and reliability is expected to substantially offset the price differential.

Automakers are waging the quality war on the interior of vehicles as well. On a new line of luxury automobiles, one automaker advertises the use of a single piece of leather for both steering wheel and gear-shift knob, so fading will occur at the same rate. The same automaker is offering stainless steel exhaust systems for longer wear. Another automaker is using silicone foam as a sound-deadener in body pillars to keep road noise from intruding into the passenger compartment. Silicone foam is also being considered for additional acoustic-shielding applications on vehicle interiors and under the hood to reduce road noise and enhance the perception of quality.

Productivity gains in the new factory

Although superior performance is probably the primary reason for the sharp increase in specification and use of silicone elastomers in U.S.-made automobiles, these materials offer automakers a number of production benefits as well. For instance, silicones are chemically inert, biocompatible, and have no known, long-term toxicological effects when handled properly. Because they do not require any special handling aside from the general guidelines for handling any industrial material, production workers need not wear special equipment to work with them safely. And because the majority of silicone materials do not contain organic solvents, they pose less risk to the environment and to workers. Solventless products not only reduce inventory, since no solvent needs to be ordered, tracked and stored, but they can also reduce insurance costs since the risk of fire and explosion can likewise be reduced.

Another benefit of silicone elastomers is that there are frequently a number of processes available to produce parts. In the gasketing and sealing area, for instance, heat-cured silicone rubber is molded, frequently off-site, and formed into gaskets, or liquid-injection-molding machines can be set up to inject silicone into a part, forming a molded-in-place gasket. Silicone RTV adhesive/sealant products can be similarly dispensed, either by hand or robotically, on-line for formed-in-place gaskets. While all three products produce slightly different types of seals and gaskets (varying in mechanical strength), manufacturers can select the process which best suits not only their performance requirements, but also their production capabilities. Many of these processes are highly amenable to automation and robotization. And because most production equipment required is simple and readily available, set-up costs can be quite competitive.

Conclusion

The automotive industry is changing globally, and European manufacturers are beginning to find themselves facing challenges already met by their American and Japanese competitors over the past two decades. As Japanese and American automakers work in concert with European manufacturers to keep from being closed out of the EC in 1992, much of their materials and production technologies will be shared. The probable result will be that silicone use will rise dramatically on the continent over the next decade.

The versatility of silicone chemistry, coupled with the reliability, safety and superior performance of these inorganic materials, have already enabled them to penetrate approximately 40 applications on the average U.S. automobile. Figure 3 highlights a number of present and potential automotive applications for silicone products. With new polymer and processing technologies, that number could easily double by 1995. Silicones will continue to play an increasingly significant role in cars of the future.

PHOTO : Figure 1 - "elastic life" time required for elongation to be reduced to 50%

PHOTO : Figure 2 - energy absorption comparison of silicone versus urethane at - 10(degrees)F

PHOTO : Figure 3 - silicone automotive applications