Shear adhesion strength
Both SPI and UA-modified adhesives had great performance on dry shear adhesion strength, as shown inTable 2. UA had little influence on dry shear strength, as all samples exhibited 100% cohesive wood failure. The wet strengths of UA-modified SPI adhe-sives were improved from 2.04MPa to 3.03MPa at UA amounts up to 10%. The fiber of the wood specimen was pulled from the glued wood surface for the 10% UA-modified sample. The UA graft onto SPI through a reaction with NH2 may have changed the protein conformation, which could increase contact surface areas between protein and wood, thus benefitting adhesion strength. It is also possible that protein molecules formed stronger crosslink-ing network after UA modification and thus resulting in better performance on adhesion strength. At the same time, hydrophilic amino groups were replaced by hydrophobic aliphatic hydrocar-bon chains, which enhanced the hydrophobic interactions among protein molecules. In addition, UA is non-water-soluble, so the hydrophobic aliphatic chains prevented water from penetrating into the interfacial surface of the adhesive and wood. This is an improvement over protein- based adhesives,where incavities form between the protein and wood surface when hydrophilic groups of protein dissolve into water. The decrease in hollow cavities could be the main reason for increased water resistance of UA-modified SPI adhesives