Effect Of Acid And Alkali Treatment On The Adhesion Properties For The Soybean Protein Adhesive

The changes for the physical mechanical properties and micro-structure of soybean protein modified with sodium hydroxide(NaOH),monoethanolamine(MEA) and acetic anhydride respectively through the factor such as the pH,reaction time and reaction temperature were studied in this paper.And the effect of the change for the micro-structure of soybean protein on the bonding properties of the soybean protein adhesive was also studied.The experimental results showed that at the reaction condition of the pH value of 11.00, the reaction temperature of 60℃and the reaction time of 1h,the viscosity of the soybean protein adhesive modified with NaOH was 2800m Pa·s,the bonding shear strength reached the highest of 7.3Mpa,while the storage period was 4 days.And in the structures of the soybean protein modified with NaOH,the content for the random coil structure was 18.09%,the content for the a-helical structure andβ-sheet structure was 11.25%and 23.14%separately.While using monoethanolamine as the modifier,at the reaction condition of the pH value of 10.00,the reaction temperature of 60℃and the reaction time of 1h,the viscosity of the soybean protein adhesive was 9800mPa·s,the bonding shear strength reached the highest of 8.6Mpa,and the storage period was 8 days.And in the structures of the soybean protein modified with monoethanolamine,the content for the random coil structure was 16.07%,the content for the a-helical structure andβ-sheet structure was 10.33%and 28.47%separately.The experimental results also indicated that the soybean protein adhesive modified solely with acetic anhydride has lower viscosity,poor storage period as well as easier for isoelectric point precipitation. When acetic anhydride and NaOH were used simultaneously as modifier,at the condition of pH value control with NaOH,the resulted adhesive has higher viscosity,higher bonding properties as well as storage period for 4 days.The microstructure analysis indicated that when using NaOH or MEA as modifier to prepare the soybean adhesive,the content of a-helix structure andβ-sheet structure was decreased as far as possible in order to obtain higher bonding strength and better fluidness and storage period,namely the random coil structure has greater contribution to the performance as well as the bonding strength of the adhesive.When using acetic anhydride as modifier to prepare the soybean protein adhesive,the content of a-helical structure andβ-sheet structure decreased,and the content of random coil structure does not changed so much,but the bonding property for the adhesive modified with acetic anhydride was improved than that for the adhesive modified with alkali,it can be seen that the bonding properties for the soybean protein adhesive was affected not only by the structure of soybean protein,but also by the structure of modifier.