New Soy Protein Adhesive: Water Resistant, Strong And Non-Toxic

Sun will present the experimental results for the first time at 9:30 a.m. Sunday, Aug. 23, at the American Chemical Society meeting in Boston. Her paper, with research assistant K. Bian, "Adhesive performance of modified soy protein polymers," is part of a three-day symposium on polymers from renewable resources.

Sun developed the adhesive by chemically modifying the soy protein molecular structure. She found that one group of chemicals will unfold the soy protein molecule in such a way that hydrophobic amino acids come to the surface. That promotes water resistance, and the unfolded molecules increased the contact area resulting in adhesive strength. The KSU Research Foundation has applied for a patent covering the chemical modifications Sun has invented.

"We are using only non-toxic chemicals to modify soy protein structure, and no formaldehyde," Sun said. Phenyl formaldehyde is a toxic chemical used in petroleum-based adhesives and in other protein-based adhesives as a cross-linking agent to increase water resistance.

The search for substitutes to petroleum-based, formaldehyde-laced adhesive has been stepped up in recent years, Sun noted, as formaldehyde poses a health hazard to workers in the adhesives manufacturing and furniture industries, in particular, and to consumers.

The modified soy protein adhesive performed well in two ASTM standard tests for adhesives.

It remained strong after eight weeks in a chamber at 90 percent relative humidity, indicating its suitability for interior uses such as for the furniture industry, and for package labeling uses. "It's excellent for interior uses," Sun said.

For the exterior tests, Sun tested plywood made with the modified adhesive. They held up well after three cycles of being soaked in water for 48 hours, then dried. Cherry, pine and walnut plywood showed zero percent delamination, and adhesive strength was reduced by only 10 percent. Maple and poplar plywood, woods with a much greater expansion property, showed 20-40 percent delamination. By comparison, plywood bonded with non-modified soy protein adhesives came apart.

The next steps in the adhesive research will be to characterize viscosity, stability and the underlying mechanisms. Preliminary tests show it has lower viscosity and is more thermally stable than unmodified soy adhesives. That's desirable in plywood manufacturing where adhesives are sprayed on and need to flow easily and spread uniformly.

For basic research on soy proteins, Sun received a National Science Foundation EPSCoR First Award for the study of new plastics and composite materials from biopolymers. Her research has characterized the molecular structure and the functional properties of soy protein. The modified soy protein adhesive and a related project on making composites from natural fibers are two applied outcomes. She also has research support from Kansas Soybean Commission.