Rheological behavior and bonding performance of an alkaline soy protein suspension

The goal of this work is to study the rheological properties of based-modified soy protein concentrate (SPC) adhesives and the relationship between viscoelastic properties and bonding performance. Chemical modification of SPC with sodium hydroxide was made to evaluate the effect of alkali on the viscoelastic proper ties. Viscosity and solubility depends directly on the 3D structure and the isoelectric point (pI) of the protein. Results show that viscosity is strongly pH dependent due to the protein unfolding. Solubility profiles exhibit the typical U-shaped curve, being higher on either side of the isoelectric point. Fourier transformed infrared analysis was used to analyze Amide I (1720-1600 cm−1) and Amide III (1400-1200 cm−1) band patterns which reflect the different secondary structures in proteins. The intensity of the band at 1250 cm−1 increases with respect to that at 1235 cm−1 for higher pH values. This could be associated with the destruction, at least partially, of the β-sheet structure. Bonding performance was measured in dry conditions and the wetting properties were analyzed by scanning electron microscopy. The bonding performance improves when the SPC is stabilized at pH 12 due to the protein unfolding, revealing a strong interaction between the secondary structure and the wood surface. As part of an ongoing project it was concluded that alkali modification is a suitable procedure to modify a protein suspension, improving application conditions and mechanical properties of bioadhesives of a semistructural type.