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PLASTICS: Playing For Time?
Caught off guard by the ferocity with which steel defended its turf, the plastics industry is regrouping to tell automakers its side of the materials story.
What do you do when your material is a relatively new kid on the block, it doesn’t have dozens of case histories backing up each upgrade like its more established competitors, the computer-aided tools for it aren’t yet as plentiful, and past pronouncements on how it will sweep all the other available materials from its path have not even come close to being true? If you are the plastics industry, you try to learn from your mistakes and start over. “I don’t think anyone in the plastics industry thinks in terms of an all-plastics automotive nirvana out there anymore,” says Rob Krebs, director, Plastic Communications, American Chemistry Council. “Instead we see ourselves operating in a matrix with traditional materials to help OEMs reach their lightweighting and durability goals.”
Part of this is due to the fact that executives whose livelihood depends on strong sales for those other materials—especially steel—fought back by showing how new formulations and processes can reduce cost and weight without degrading strength, and retained an entrenched knowledge base built on approximately 100 years of practical experience. “It’s only in the past 50 years that plastics have come into their own,” says Krebs, “and only since the 1970s that we began graduating a growing number of polymer engineers who can manipulate the material at the molecular level in order to create new compounds or address new needs.” Nevertheless, he says: “Studies continue to show that, for many engineers in the auto industry, plastics still lies outside their comfort level.”
To combat that—and the perception that “lightweight” means “less safe”—the American Chemistry Council has put together 14 brief outlines of current thinking on how plastics can help OEMs reach their weight, durability, and crash targets. (The list includes: composite fuel tanks, composite driveshafts, composite front end, crumple zones, door interiors, plastic fuel tanks, headlamps, laminated glass, plastic foam, composite structures, roll over safety, self-reinforced plastics, size and weight studies, and steering columns.) These vignettes cover everything from plastic fuel tanks to composite driveshafts and front ends to full composite structures like that found on Porsche’s Carrera GT. It also covers the relationship between weight, vehicle size, and crashworthiness. “We are working with the national labs, USCAR, and the Recycling Partnership to answer end-of-life concerns as well as working with companies to create computer-aided design tools that eventually will cover everything from predictive engineering to recycling,” says Krebs. Still, the plastics industry has yet to put together a cohesive mix of near- and medium-term examples of its material’s unique properties and capabilities in the non-denominational and production-feasible form that worked so well for the steel industry. Without this, it will take time for polymer engineers to work their way into positions powerful enough to see that plastics technology gets a fair hearing. By that time, it might be too late.
Three Plastics Technologies to Watch
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| Lotus Elise |
Self-reinforcing polypropylene
Created by heating and weaving polypropylene fibers to stretch and align their molecular chains, Self-reinforcing Polypropylene (SrPP) allows greater deflection of a panel while withstanding the required force and at a weight much lower than steel or aluminum. A leading candidate for underhood panels designed to protect pedestrians in the event of an accident, it also is being investigated for underbody shields and cosmetic panels. Lotus replaced the front access panel (shown) on its Elise and Exige models with a SrPP part that is 57% lighter than the plastic panel it replaces, passed all mechanical and paint durability testing, and—because it is a homogenous polypropylene composition free of other fibers—can be easily recycled.
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| Mercedes SLR |
Composite Crash Structures
According to material suppliers, carbon fiber-reinforced polymer composites—of which carbon fiber is the leading candidate—weigh about 50% less than steel yet can absorb energy levels on the order of 100 kJ/kg versus steel’s 25 kJ/kg. An automotive front structure tested by the Automotive Composite Consortium in a 35-mph barrier test passed initial testing, and showed that in small- to medium-size vehicles a composite front structure could weigh 30% less than its steel counterpart. Designing for the characteristics of the material, especially its ability to put strength where necessary through strand orientation and weave patterns, may make it possible to reduce the size of the crash structure without reducing its energy-absorbing capability. Adding similar structures to the front of light trucks may also reduce the severity of a side impact with a car while providing a platform onto which entire assemblies could be fitted prior to attachment to the vehicle.
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| Chevy Volt |
Polycarbonate Glazing
The desire to increase the glass area of a vehicle—particularly by fitting large transparent roof panels—can bring a weight penalty if traditional laminated safety glass is used. Substituting a molded polycarbonate panel with an abrasion-resistant coating can reduce the weight of this assembly by as much as 50%. Though not yet legal, windshields made of this material are under development, while integrated window designs that incorporate elements of the vehicle’s body design and the window support structure are close to production. |
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For more on polycarbonate glazing, see: