With strong demand for formaldehyde-free and bio-based wood adhesives, soy flour with crosslinking agent now dominates the North American interior plywood market. It has not expanded to other, larger wood bonding markets because attaining the required strength is cost prohibitive. Here we discuss how different denaturation treatments applied to soy proteins result in wood bonds with widely varying wet strength.
There is a common statement in soy adhesive literature that denatured proteins have higher wet strength because denaturation exposes groups for reaction with the crosslinking agent. Our data on a range of denatured soy proteins shows no correlation between amine reactive groups on the surface (fluorescamine reaction) and wet strength, even when an amine reactive crosslinking agent (polyamidoamine epichlorohydrin, or PAE) was used in the adhesive. The reasons for questioning this often-quoted belief are discussed in further detail, with an emphasis on protein structure and how the structure is modified through (mostly heat-based) processes.

Figure 1: Shear strength of wet bonds made with soy flour and isolate after various treatments. Protein<b><i> conformation </i></b>is much more important to bond strength than protein content. Jet cooing of isolate resulted in much stronger bonds than autoclaved or native state proteins.

Figure 1: Shear strength of wet bonds made with soy flour and isolate after various treatments. Protein conformation is much more important to bond strength than protein content. Jet cooing of isolate resulted in much stronger bonds than autoclaved or native state proteins.

Fig 2 –It is commonly claimed that denatured proteins make better adhesives because the denaturation process exposes reactive groups on the protein. Here we show that the wet strength of soy protein has NO CORRELATION to available amine groups, even when an amine reactive crosslinking agent is used.

Chris Hunt

US Forest Service, Forest Products Laboratory

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