| In recent years,the research on the composition,viscosity,bonding strength and water resistance of soybean adhesives have made great progress,and realize industrial application.Commercial soybean adhesive modified by polyamide-epoxychloropropane(PAE)was prepared by stiring defatted soybean flour(DSF)and PAE solution at room temperature.Solubility and dispersity of DSF in PAE solution have an important effect on the structure,physicochemical and adhesive properties of soybean adhesive during modifiction and application.Therefore,this paper mainly focused on the dissolution and dispersion characteristics of DSF under different concentrations and pH values of PAE solution,and its effects on the structure and properties of soybean adhesives,providing theoretical guidance for scientific and effective application of soybean adhesives.1)PAE could promote the dissolution of DSF and reach a dissolution equilibrium within 2 h,which was attributed to the secondary bond and chemical bond between PAE resin and reduced the hydrophobicity of the protein molecule.In particular,the electrostatic repulsion between cationic azobutyl of PAE resin and soybean protein increased the stability of dissolved soybean protein.2)Increasing PAE concentration could promote the solublity of DSF.Overly low concentration of PAE,the protein molecules in DSF could not effectively dissolve and release active groups wrapped inside,reducing the cross-linking density and leading to the decrease of bonding performance.Whereas the high concentration resulted in soybean protein excessive dissolved and released numerous active groups,led to the protein aggregation,increased viscosity.Furthermore,a negative impacted in coating ability,water resistace and technology applicability of soybean adhesive.In general,in order to obtain ideal coating properties,bonding strength,water resistance and lower cost,the appropriate concentration of PAE was 8 wt%～10 wt%.3)The pH vaule of adhesive liquid effected not only crosslinking reaction of PAE resin but also the dispersion and solubility of soybean protein.The pH value of PAE solution had important effects on secondary structure of soybean protein,crystallinity,crosslinking characteristic,viscosity,pot life and water resistance of soybean adhesive.Increase the pH value of PAE solution when it was greater than 4.6 led to unfolding of globular structure of soybean protein,and the unfolded protein chain immediately rearranged to form more order structures that decreased the crosslinkable sites on protein.In addition,increaseing the pH value of PAE solution promoted the self-crosslinking reaction of PAE resin.These facts resulted in larger viscosity,shorter pot life,lower crosslinking density and worse water resistance of soybean adhesive prepared by PAE solution with higher pH value.In consideration of bonding property,spreading,pot life of soybean adhesive,the pH value of PAE solution should be less than 5.9 and preferably be less than 4.5.4)DSF could not immediately dissolved in PAE solution at room temperature.But,DSF fully expanded,dissolved,rearranged and configured in the PAE solution need about 2 hours.The soybean protein pre-crosslink with PAE resin inhibit the crosslinking during hot pressing process.Therefore,the potting time or working time of PAE-modified soybean adhesive has a significant effect on the structure and properties of the adhesive,and its water resistance increases first and then decreases with increasing potting time,and reach the maximum value at 2 h.In order to ensure the stable water resistance,the placement and use time of the fresh prepared soybean adhesive should be controlled within 0.5～4 h. |
