The Research On Blending Modification Of Soy Protein Adhesives

Soy protein is a nontoxic material with wide variety of sources. However, unmodified soy protein itself has low water resistance and shear adhesive strength which could not satisfy the demand of adhesive used in wood industry. It's urgent to improve its workability and water resistance by using physical, chemical, and biological modification methods. The research selected polyvinyl acetate (PVAc) and polyvinyl alcohol(PVA) as modifiers to blend with soy protein isolate(SPI) in order to enhance the water resistance. And a study on the structure and property of the modified composite adhesive was conducted.In this research, SPI was selected as raw material, then it was treated by urea or sodium dodecyl sulfate(SDS) so that the contents of ?-helix, ?-sheet, ?-turn and random coil structure in the secondary structures of SPI were changed. The content of each structure conformation was estimated by circular dichroism spectra, which was used to evaluate the effect of modification. The results displayed that the contents of a-helix decreased while the random coil structure within the secondary structure of SPI after modification rose. Hexamethylenetetramine and PVAc were added into the urea-modified SPI adhesive for blending, and a kind of PVAc/SPI blending adhesive with good dry and wet shear adhesive strengths was prepared. The chemical microstructure and the thermal properties of the adhesive were obtained by means of Fourier transform infrared spectrometry(FTIR), differential scanning calorimetry(DSC) and themogravimetric analysis(TGA). The results have shown as follows. When the mass ratio of the urea-modified SPI adhesive and PVAc was 10:1, the blending time was 1h, the mass concentration of the crosslinking agent was 1.0%, the crosslinking time was 1.5h, the hot pressing temperature was 120?, the hot pressing pressure was 1.2 MPa, and the hot pressing time was 2min/mm and the amount of gum was 250 g/m2, the dry and wet shear adhesive strengths were respectively 2.01 MPa and 1.04MPa by national plywood testing standard for type I. FT-IR and TGA test results have shown that the blending system formed some associating hydrogen bonds, but did not produce any change of new chemical structure and any chemical reaction between the chemical groups within the system.Hexamethylenetetramine is easy to break down into formaldehyde and ammonia in aqueous solution, which has some constraint as a result of the presence of formaldehyde. To avoid the environment problem caused by formaldehyde, a kind of inert crosslinker was prepared by using methylparaben reaction with toluene-2 ,4-diisocyanate(TDI). The results have shown that a high closing ratio was acquired when the molar ratios of the blocking agent and TDI was 3.0:1, the reaction temperature was 80?, the reaction time was 3h, the mass percent of catalyst was 0.5% based on total mass of the blocking agent and TDI. FTIR results has displayed that the peaks of -NCO groups at 2254 cm'1 disappeared completely.SPI was modified by urea to unfold its secondary structures and then was blended with polyvinyl alcohol (PVA). Therefore the PVA/SPI blending adhesive was prepared using the synthetic blocked TDI as crosslinker, and the structure and properties of the blending adhesives were obtained by means of FT-IR, solid-state nuclear magnetic resonance (NMR), scanning electron microscopy (SEM) analysis. When the mass ratio of the urea-modified SPI and PVA was 12:1, the blending time was 1h, then 1% blocked TDI was added as crosslinking agent and reacted again 2h with them, a blending adhesive with superior performance was obtained, and its viscosity was 12.8Pa.s and the solid content was 25.1%. Using it as the adhesive of the wood, the plywood was prepared under the conditions of the hot-pressing temperature 130?, the hot-pressing time 1min/mm, the hot-pressing pressure 1.2MPa and the amount of adhesive 250 g/m2, then the test pieces were prepared and the adhesive strengths were tested according to the national plywood testing standard for type I. The dry and wet shear adhesive strengths were 1.96 MPa and 0.89MPa respectively. FT-IR, NMR, and the other test results have shown that there was neither chemical reaction between the groups nor the association of hydrogen bond within the PVA/SPI blending adhesive system. The PVA/SPI blending adhesive was completely kept in a physically mixing state, which guarantees a longer saving time and improves the pot life.