Plastics: The Tortoise and the Hare
Thirty years ago, plastics were on the move. Pontiac’s Fiero introduced the idea of a steel spaceframe clad with plastic outer panels, and this was repeated with the droop-snoot Chevrolet Lumina APV and Pontiac Transport minivans. Concurrently, BMW launched its Z1 sports car, a vehicle tipped to show BMW’s future. It featured high-intensity discharge headlamps, BMW’s first multi-link independent rear suspension (the “Z axle”), and doors that slid down into the sills so the car could be driven with them open while the thick sills provided exceptional side-impact protection. However, the feature that caught the most attention from the industry was its corrosion- and dent-resistant plastic body panels. Based on GM’s multiple models and BMW’s growing interest, plastics obviously were the future.
Today, however, there are no large-volume programs with plastic body panels in the pipeline, and the material of the moment is aluminum. “I want to believe that plastic body panels are not being dismissed because of a ‘been there, done that’ mentality,” says Jeff Sternberg, DuPont’s Global Automotive Technology Director. If anything, the 54.5 mpg CAFE requirement is causing increasing angst in the auto industry as 2025 approaches, and automakers are getting more aggressive in adopting lightweight materials, including plastics. However, Sternberg admits that when it comes to plastics deployment, “most of the current effort is being spent expanding current applications while nurturing those that are just emerging.” This includes items like transmission baffles and polymeric mufflers, but one of the most interesting near-term projects that has emerged is a side intrusion beam that began with Peugeot, but since has expanded to include other automakers.
A composite-optimized finite element analysis program was used to create multiple beam designs using a polyamide 6.6 thermoplastic composite with continuous woven glass fibers (DuPont Vizilon TPC). The first design was produced in two steps, which includes thermo-stamping the composite sheet, then over-molding it with a network of ribs. It delivers a claimed 5.4 times more energy absorption than a short-glass-fiber polymer beam, and a 40% weight reduction when compared to ultra-high strength (UHS) steel. Also, the material’s thermal capabilities make it possible to fit it to the body-in-white prior to E-coat. A second design proposal improves weight reduction compared to UHS steel to 60%, and further reduces part cost.
“Unfortunately, I can’t talk about programs in the early developmental stage,” says Sternberg, “but we are looking at a number of applications. Some of them are thermoplastic composites alone, and some with glass or carbon fibers either separately or in combination. However, I think we also have to acknowledge multi-material applications as well.” In multi-material designs, carbon fiber, steel, aluminum and plastics each will be used advantageously to create a whole that’s greater than the sum of its parts, much as tailor-welded blanks placed the best grade of steel in the most optimum position. Only in this scenario, different materials will be used, with plastics taking on multiple roles like parts integration through over-molding as well as structural duties. “I think we will see more multi-material components,” says Sternberg, “and though we see a lot of binary structures today, we may see tertiary- and quaternary-type structures in the future.” This will come through new chemistry as well as new filler technology that will allow the entire unit to act as a single structure. But it won’t be easy. According to Sternberg, “A lot of the low-hanging fruit already has been picked, and we are going after applications for physical and chemical properties that are extremely challenging.”
Another challenge for plastics is recyclability and sustainability. With oil as the traditional feedstock for many modern plastics, environ-mentalists single the material out as a bad choice for the planet. “I’m not sure these perceptions are accurate,” chides Sternberg, “as many of today’s thermoplastics are fully recyclable, and we have infrastructure in place to make this process both practical and economical.” Recycling alone isn’t enough, however. Sustainability is equally important, in both environmental and economic terms. The roller coaster ride for oil prices make budgetary planning difficult, which reduces the desire to use plastics. “There’s a huge opportunity for plastics in renewably sourced polymers,” claims Sternberg. Currently DuPont has a thermoplastic elastomer named Tirel made up of two monomer units, one of which is renewably sourced. Another, Sorona, is a polyester with a renewably sourced monomer, and it can be made into a fiber that is used in carpeting. “This material is much more stain resistant, which means automakers will be able to use lighter colored carpeting in their cars and not have to worry about stains, and it has a very soft feel,” he says. Even more interesting is where DuPont sources the renewable monomer for these products.
“We use corn stover, not corn itself, as the feedstock,” says Sternberg. Corn stover includes the stalk, husk, cob and leaf that remain after the corn has been harvested. DuPont has a cellulosic ethanol facility in Nevada, Iowa, that can generate 30-million gallons of ethanol annually from 375,000 dry tons of corn stover from 500 farms in a 30-mile radius. This knowledge is being used to support the production of Sorona and Tirel as they share a lot of ingredients, and help defray DuPont’s $200-million investment in the cellulosic refinery.
On the downside, stover use may have adverse environmental impacts of its own (reduction in soil carbon and increases in nitrogen application, to name the two most important), as well as a price that is sensitive and linked to corn prices. However, it’s still early in the process, and it may be possible to mitigate or eliminate some or all of these concerns with further work.
Until the time that these programs reach fruition, plastic’s auto industry growth will come by taking ground a gram at a time. “Arguably,” says Sternberg, “this is an incremental approach that we believe is going to have a larger overall impact than showcase vehicles like the F-150 have for aluminum. We are the turtle in the race, but the tortoise beat the hare by being slow and steady.”
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