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New Technique Yields Recyclable Thermoplastics With 85 Percent Ag Fibers, No Formaldehyde

This year”s rise in oil prices reminded many that it makes good sense to substitute renewable resources for petroleum-based compounds.

Now researchers at the University of Wisconsin-Madison and the USDA Forest Products Laboratory have developed an innovative technique that allows them to increase the natural fiber content of thermoplastics from 50 percent to more than 85 percent.

Anand Sanadi and Daniel Caulfield will present their findings today at the Third International Symposium on Natural Polymers and Composites, which is meeting in Sa? Pedro, Brazil. The two are materials scientists. Sanadi works with the Department of Biological Systems Engineering at the College of Agricultural and Life Sciences; Caulfield with the Forest Products Laboratory.

Over the years, thermoplastics manufacturers have added glass and minerals such as calcium carbonate and talc to control the properties of the plastic, according to Sanadi. “Now there”s a growing interest in using agricultural and wood fibers as fillers and for reinforcing thermoplastics,” he says.

Sanadi and Caulfield developed the method, which allows them to increase the fiber content of the composite while reducing its non-renewable components. The resulting product is stronger than conventional fiberboard, yet made without the formaldehyde usually used to produce it, says Sanadi. After the new material is used, it can be recycled into new products.

The researchers produced the new compound from a mixture of polypropylene and kenaf, a hibiscus relative grown as a fiber crop in the United States. Sanadi says that other agricultural crops with long fibers, such as industrial hemp, may work as well as kenaf. The kenaf-polypropylene mix can be molded to form a wide range of products such as table tops, bench seats and cabinet components.

According to Sanadi, natural fibers are actually composites, with cellulose as the reinforcing element and lignin and hemicellulose as the primary matrix. This is analogous to glass fibers reinforcing the polyester matrix in fiberglass. However, the lignin and hemicellulose in natural fibers are themselves thermoplastic in nature. Sanadi and Caulfield used a novel non-toxic method to improve the flow of these natural thermoplastics. The method enhanced the ease with which the kenaf-polypropylene composites can be molded and allowed the materials scientists to increase the percentage of natural fibers in the composite.

Their tests showed that boards made from plastic composite that was 85 percent by weight kenaf were stiffer and stronger than medium-density and high-density fiberboard.

Caulfield and Sanadi continue to explore the structure of these novel materials. To improve the new composites, the researchers will collaborate with others at the Forest Products Laboratory. Also, they will be working with a Madison-based company to develop commercial products that use the composites.

Sanadi hopes the technology can be transferred to other countries after further research and development. Composites produced by the process may be especially useful in developing countries where petroleum-based plastics are relatively expensive, and in countries where deforestation is a key issue. In some countries, people can”t legally cut down trees unless they are grown on plantations, he says.

The study was supported by: state funding to the UW-Madison College of Agricultural and Life Sciences, federal funding to the Forest Products Laboratory, and a grant from the Non-Food Value-Added Research Program of the USDA National Research Initiative.