Robertson is also researching bioresins and composite recyclability. Current recycling processes, based primarily on thermally treating the materials, work only with thermoplastics. Recent advances in the chemical recycling of polymers, which breaks the plastic down into monomers so the materials can be repolymerized, hold some promise for thermosets.
Since the esters in bio-based resins can undergo chemical degradation reactions like hydrolysis, “our hypothesis was that if we can distribute those ester linkages throughout the epoxy resin network, we can make a network that can be degraded under the right conditions,” says Robertson.
Researchers tested two epoxy resins – one a soybean oil-based and the other made with BPA – in a solution with a very high pH, which accelerates hydrolytic degradation. “We found that if we put a traditional epoxy resin through this process, even after three months we found no noticeable change in the mass,” explains Robertson.
The test with the bio-based resin was more promising. “The soybean-oil based epoxy resin degraded within two weeks; the solid mass completely disappeared,” she adds. Other bio-based epoxies, which also contain high concentrations of esters, have reacted to the solution the same way. The challenge will be to make a thermoset that could maintain the desired properties in applications, but that could be broken down after it reaches the end of its useful life.
The Case for Bio
Deborah Mielewski, Ford’s senior technical leader for materials sustainability, has worked for decades trying to get more bio and recycled materials into the company’s vehicles. One of her first victories came in 2008, when Ford introduced seats for its Mustang that contained foam made from soybean oil. A few years later the company added composite storage bins made with wheat stalk fibers into its Flex SUV, reducing its annual petroleum usage by 20,000 pounds and CO2 emissions by 30,000 pounds.
The wheat straw bin, developed by the Ontario BioCar project, helped focus Mielewski’s team on the benefits of incorporating agricultural waste fibers into composites. “They are lighter in weight than the glass fiber and the talc that we currently put in. So, if we’re worried about fuel economy, here’s a way to lightweight and use something that you would otherwise burn or landfill,” says Mielewski.
A good example of this approach is the incorporation of pulp from Weyerhaeuser’s lumber operations into the Lincoln MKX’s composite armrest. “The pulp industry is struggling in this country because China is producing paper at a very low cost. So, putting pulp in a high-end composite was very attractive to Weyerhaeuser,” says Mielewski.