Plastics From the Prairie
Whether red, green or blue, tractors are as indispensable to a farmer as seed or land. Someday, parts of those tractors may be made from last year's harvest as researchers from North Dakota State University seek ways to make reinforced composite plastics from the oils, proteins and fibers grown in American fields.
The cover is made from an epoxy resin that uses 30 percent vegetable oil to partially replace the petroleum-based ingredients. Flax fiber has been added to the plastic to give it more strength and stiffness. The combination is called a reinforced composite, and it could be used in a number of applications including parts for automobiles and agricultural equipment such as tractors, says Dennis Wiesenborn, one of a multidisciplinary team of researchers at NDSU's Bio Energy and Products Innovation Center, or BioEPIC. "When you talk about composites, fibers are used to provide structural strength but you need some kind of binder or matrix to hold them," Wiesenborn says. "There are a number of possibilities, including using a polymer-type material. The polymer we used was a chemically modified vegetable oil."
Wiesenborn's lab has developed a method for converting unsaturated fats into epoxy compounds. Epoxies are highly reactive compounds characterized by a ring made of two carbon atoms and an oxygen atom. The chemical bonds in this three-member ring are highly strained, which makes it easy for them to react with other compounds to form polymers. Composite materials made with epoxy resins are versatile and are used as adhesives and as structural resins in applications ranging from electronics to automotive to industrial uses. Fibers, typically fiberglass, are added to epoxy resins to give them strength and stiffness.
Currently, Wiesenborn is working with a blend of 30 percent epoxidized vegetable oil and a petroleum-based epoxy that is mixed with fiberglass and a chemical catalyst called a hardener. To convert the oil into an epoxy, Wiesenborn reacts vegetable oil with hydrogen peroxide and acetic acid in the presence of a catalyst. The double bond in unsaturated fatty acids is converted to an epoxide group. "The epoxy groups then become the active group in cross-linking the fatty acids into the hardened polymeric material," he says.
Saturated fats, such as those found in animal fat or palm oil, are essentially inert in this process. Wiesenborn found that canola oil, being a highly unsaturated fat, was extremely
well-suited for producing epoxys. "It's high in monounsaturated fatty acids and quite low in saturated fatty acids," he says.
Wiesenborn's work is still in the early stages. The batches of resin he is creating measure only about 100 grams. His next challenge is to scale up the process to make kilogram-sized batches. He is also looking for ways to lower the cost of the process such as recycling the catalyst. "We would like to show that we can make some finished types of products-paneling for agricultural equipment, for example-simply to raise awareness that these kinds of things are possible," he says.
Another bioplastic project is also taking shape at NDSU. Rather than converting oil into epoxy, Scott Pryor, an assistant professor in agricultural and biosystems engineering at NDSU, is using the canola meal left over after oil extraction and converting that into a biodegradable bioplastic. "It's been done quite a bit with soy proteins," Pryor says. "We have been using soy for many decades to make adhesives, polymers and composites. We're interested in seeing how proteins from other sources might function in these applications. Hopefully, different proteins might lend themselves to improved properties for certain applications."
He became interested in using canola meal because canola oil is used as a feedstock for biodiesel production in North Dakota. He was approached by a biodiesel producer about other value-added opportunities for canola meal, which is currently sold as animal feed. A surprising number of polymers can be made out of proteins. Pryor listed panels for automobiles or farm equipment as just one example of a potential product.
A mixture of different proteins, called an isolate, can be extracted from oilseed meal. Pryor started by taking canola meal isolate and separating it into its component proteins and examining their properties. One property that is important is water absorbtion. "One problem with protein-based polymers is that they can absorb too much water," he says. "We see that if we can extract the portions of the protein that have higher water solubility, we can decrease the solubility of the final mix of proteins." Proteins can also be modified with heat, enzymes or chemicals to modify their properties.
Pryor's work is also in its beginning stages so he isn't sure for which applications canola-based bioplastics will be best suited. "We didn't go into this research with a specific application in mind," he says. "We saw that there was a hole in the research in that we didn't know what functionality these canola proteins would give for industrial products. We want to explore the possibilities of canola proteins." Pryor will be looking at using the proteins to make composites, but will be keeping an eye open for other applications such as adhesives.
Pryor's current work will be taking the proteins the lab has extracted from canola meal and working to convert it into plastic. He would like to find a way to relate the properties of the protein isolates to the finished biobased composite. "We do want to do a little work on forming the composites," he says. "You find a lot of information in the literature about the functional properties of the proteins and a lot on forming them into composites. But there hasn't really been a clear link between the two. We don't have a good understanding of how the properties of the proteins translate after all the processing." Other work will include optimizing the processes for extracting the proteins from the canola meal and how the extraction process can affect the finished product.
Fibers are key to making commercially viable composites from biomass material. "If you take epoxidized canola oil and blend it with petroleum-based epoxy, you can get it to harden into a good hard plastic," Wiesenborn says. "But when you subject it to mechanical tests, you find it doesn't have a lot of inherent strength."
The typical material for a composite is fiberglass, although carbon fibers are becoming more common. However, Chad Ulven, an assistant professor in NDSU's Department of Engineering and Applied Mechanics, thinks fibers from agricultural products can do just as well, if not better, for some applications. He is looking at products varying from long fibers from crops like flax to short corn fibers extracted from distillers dried grains (DDG).
Long fibers include not only flax but crops such as jute, hemp and kenaf. Ulven is concentrating on flax because it is a major crop in North Dakota and the fibers are underutilized. "So what happens is the farmers have to burn it or try to plow it down," he says. "Those who bale it might send it off for specialty papers, but that is a very low-value use. What I am trying to do is create a much higher value use."
Small-scale experiments in Ulven's lab show flax fiber can compete with the properties of fiberglass in applications such as boat hulls and shrouds on tractors. "We were able to prove that on a weight basis, our composites were just as strong and just as stiff as fiberglass," he says. Composites made with biofibers also exhibit less shrinkage and warping after injection molding, he adds.
Ulven says he started looking at short fiber reinforced composites to take advantage of another waste stream from local industry, DDG from ethanol plants. He was able to fractionate the fibers from DDG and chemically modify the fibers so they adhere to commercial thermoplastics such as polyethylene. "We've been incorporating these waste fibers and have seen improvements in both stiffness and strength," he says.
The composites are similar in concept to wood-fiber-filled plastics used for patio decking. But Ulven says those composites sacrifice strength but gain stiffness. Due to the chemical treatment, the DDG fiber composites improve stiffness while maintaining the polymer's strength. "That's what's more attractive about using this material instead of wood filler in plastics," he explains.
Another attractive aspect of using ag waste as a filler for composites is its low price, Ulven says. Virgin polypropylene can cost $2 a pound. He believes his modified DDG fiber can sell for 60 to 70 cents a pound while paying ethanol producers a premium for their distillers grains. "I've talked to plastics compounders to see if they would be interested in a product at that price that would increase the strength and stiffness of their products and they said Ďabsolutely,'" he says. "So there is a 50- to 60-cent-per-pound incentive for someone to take the research we are doing here in the lab, scale it up and create a higher value use." Ulven hasn't done the economic analysis with flax straw yet, but believes flax fiber could eventually have a 30 percent or more price advantage over fiberglass.
The big challenge for biofiber composites is putting out a consistent product year after year. "How do you maintain consistency in a natural product?" Ulven asks. "Is the fiber I get from this growing season going to be just as strong and stiff as the next growing season? Those are the issues." Ulven is working with the Composites Innovation Center in Winnipeg, Manitoba, and the USDA Cotton Quality Research Center in North Carolina to develop a grading system for flax fibers so producers and buyers can bargain with confidence in the product.
Eventually, the work at the different parts of NDSU could combine to form the "ultimate biocomposite plastic." "That was our initial vision, Dennis, Scott and I, that each of us would grow an area of research that we could combine down the road," Ulven says. "The idea would be that the matrices or plastics that are derived from renewable resources would eventually be combined with the natural fibers so you have a composite that is getting close to 100 percent renewable resources."
Weisenborn thinks biocomposites could be a higher value application for canola oil than biodiesel production. The cost of feedstock is the largest cost in biodiesel production, and canola oil is typically higher priced than other vegetable oils. "One of our reasons for pursuing composites is the cost of the finished product will be higher than the feedstock costs," he says. "The cost of the finished plastics compared with the cost of vegetable oil is much more value added."
BioEPIC is a multidisciplinary center and includes members from all over the campus. For example, Pryor and Wiesenborn are members of the Agribusiness and Biosystems Management Department while Ulven is part of the Mechanical Engineering Department. The NDSU Oilseed Development Center, headed by Bill Wilson of the Agribusiness and Applied Economics Department, is also an important part of the center's work on biocomposites.
Because of that multidisciplinary approach, the team was able to include economists to look at how the researchers' products could compete in the marketplace. "Ultimately the economics question is going to be key for these composites to become commercially feasible," Wiesenborn says. "We are at the point now where we can make samples of this epoxidized canola oil and do the quality characterization of the samples. We can produce hardened resin and even composite samples. We have all of this work going on, but at some point we will have to have an accurate idea what the cost of all of this will be."
Jerry W. Kram is a Biomass Magazine staff writer. Reach him at firstname.lastname@example.org or (701) 738-4920.