Soy flour good start for biobased glue

Chris Hunt has been working on soy - based adhesives in collaboration with the industry at the USDA Forest Products Laboratory in Madison, Wisconsin, US. His focus is on understanding the structures and mechanisms that contribute to protein adhesive performance.

“Most of our work has been done with soy, but I think that there's a lot of transferability. A lot of that information is relevant to canola and other proteins”, says Dr Hunt.

At a BioGlue center webinar he pointed out that all plywood originally was bonded with soy and other proteins, and that soy protein is back for interior plywood since almost 20 years, when formaldehyde regulation increased in North America. The European plywood standards are more challenging for soy and protein-based adhesives than the North American standards.

Reasons for using soy flour in adhesives are that it is renewable bio-based content. As a coproduct of oil production, it is cheap, is high in protein and is available in abundant volumes. Another good thing is that soy has commercial use, and companies are funding the research.

The physical form of plant proteins is compact and globular with a wide variety of chemical functionalities, with pros and cons. The polar portion of proteins absorb water: current adhesive films swell on the order of 100%, reducing their wet strength.Polar surfaces are associated with better wet strength, to a point. Getting those globular proteins to form a strong cohesive film is a challenge. 

Currently all commercial systems rely on covalent bond formation provided by a crosslinking additive. Better protein-protein interactions, as well as more, and more cost-effective covalent bond formation, could also improve the wet strength.

Heat (the hotter the better) almost always improves wet strength, and more water during bond formation also helps.  Both heat and moisture dependence suggest that better protein mobility and protein-protein interactions are helpful.

 
“There’s a lot of potential to improve performance of protein adhesives, either through priming the system to develop covalent bonds, or through improving the coalescence and film formation during protein processing or bond formation” says Dr. Hunt.