Cross-linking Architecture Regulation And Reinforcement Mechanism Of Soy Protein-based Adhesive

Recently,soy protein adhesives have been extensively researched and used to replace formaldehyde-based adhesives commonly used in the wood-based panel industry to eliminate the hazards of formaldehyde.However,the poor water-resistance,high viscosity and other disadvantages of soy protein adhesives limit their industrial application.Furthermore,the mechanism of curing,bonding,and modification of soy protein adhesives is still not very specific.To solve the above problems,different polyepoxides were used to enhance soy protein adhesives based on the cross-linking enhancement theory in this study,and the structure of cross-linkers was used to control the cross-linking structure of the adhesives.With the characterization of the solid contents,viscosity,residual rate,functional groups,thermal stability,morphology and crystallinity of the soybean meal-based adhesives,the mechanism of curing and bonding of soybean meal-based adhesive is clarified,the interaction between polyepoxides and soy protein is revealed,the relationship between the cross-linked structure of the soybean meal-based adhesive and the adhesive performance is established.The main conclusions were summarized as followed.1)The curing process of soy protein adhesives was the process in which the molecules of the soy protein are entangled with each other to form intermolecular forces and molecular recombination to form crystalline regions after the water evaporation of the resultant adhesive.The denaturing agent made the soy protein molecules swelled and unfolded,increased the order region of the adhesive after curing(the degree of crystallinity increased by 17.6%),imoroved the water resistance of the adhesive,and the wet shear strength of the denatured soybean meal-based adhesive increased by 21.6%.Physical interlocking,intermolecular forces,and crystalline regions are prone to erosion by moisture,so soy protein adhesives have poor water resistance.2)The active groups of soy protein can be reacted with the polyepoxides to form a cross-linking network,therefore,the water resistance of the adhesive was effecitvely improved.(a)The polyepoxide M85 reacted with active groups on protein molecules to form a cross-linking network,this decreased the crystallinity region,made the degree of crystallinity increased by 21.8%,meanwhile,the cohesion in the cured adhesive was converted into chemical bonds by intermolecular forces,therefore,the water resistance of the adhesive improved.Incorporating 12 wt%M85 improved the water resistance of the adhesive by 236.7%and the wet shear strength of the resulting plywood to 1.12 MPa,which met interior use requirement.(b)To introduce rigid triazine ring structure into the system of the soybean meal-based adhesive,the melamine/epichlorohydrin prepolymer(MEP)is used as the cross-linker for the adhesive.The mult-epoxy groups in MEP reacted with active groups on the protein molecule and formed a cross-linking and denser network,which improved the water resistance of the resultant adhesive.In addition,adding MEP increased the thermal ability of the cured adhesive and created a smooth surface with less holes and cracks to prevent moisture intrusion,which further improved the water resistance of the resultant adhesive.Incorporating 6 wt%MEP effectively improved the water resistance of the adhesive by 10.5%.The wet shear strength of the plywood bonded by the resultant adhesive increased by 281.1%to a maximum value 1.41 MPa,which was higher than the soybean meal/PAE adhesive and commercail MUF and UF resin.(c)To introduce long chain structure into the system of the soybean meal-based adhesive,the neopentyl glycol diglycidyl ether(NGDE)is used as the cross-linker for the adhesive.With the intrinsic toughening effect of NGDE,the brittleness of the cured soybean meal-based adhesive was reduced,a smooth and homogeneous fracture surface was created and the water resistance was improved.By adding 6 g NGDE improved the water resistance of the soybean meal-based adhesive by 12.5%,the wet shear strength of the plywood bonded with the soybean meal-based adhesive was 286.2%higher than that without NGDE and attained 1.12MPa.The tensile shear strength of the plywood bonded with 6 g NGDE was 19.1%higher than that of with 6 g PAE and was similar to the MUF resin,which validated that the novel adhesive was suitable for use as an industrial plywood adhesive.(d)Acrylic emulsion(AE)can be used to externally toughen the cross-linked soybean meal-based adhesive to improve adhesive properties.With the introduction of 8%triglycidylamine(TGA)produced an improvement of 15.1%in the water resistance of soybean meal-based adhesive and 86.8%in the wet shear strength of plywood as a result of the chemical cross-linking between epoxy groups and protein molecules.However,this reaction increased the brittleness of adhesive and caused insufficient dry bond strength.Incorporating 8%AE into the SP/TGA adhesive resulted in a water resistance that was improved by 24.6%,a dry shear strength that was increased by 44.0%to 1.80 MPa and a wet shear strength that was increased by 47.9%to 1.05 MPa.The improvements were attributed to the formation of interpenetrating network which increased the solid content,improved the toughness of the adhesive,and promoted a uniform and compact cured structure.3)Using the lignin-based(LR)resin as the cross-linker is an effective way to improve the water resistance of the resultant adhesive and the wet shear strength of the plywood prepared by the resultant adhesive.The LR could react with active groups on the protein molecule and itself to form a cross-linking and interpenetrating network,which improved the water resistance of the resultant adhesive.In addition,adding LR increased the thermal ability of the cured adhesive,forming a cross-linked protein molecule based structure,created a smooth surface with less holes and cracks to prevent moisture intrusion,which further improved the water resistance of the resultant adhesive.Using LR of low molecular weight benefited the adhesive distribution during the hot press process and formed more interlock with wood,thus created good wet shear strength of the resultant plywood.Incorporating 10 wt%LR effectively improved the wet shear strength of the resultant plywood by 200%to 1.05 MPa,which met interior use plywood requirement.4)Epoxidized bark extractives(EBE)can react with active groups(mainly amino groups)on the protein molecule to form a cross-linking network,which improved the water resistance of the resultant adhesive.With addition of 10 wt%EBE,the wet shear strength of the resultant plywood increased by 187.0%and the moisture uptake decreased by 17.7%.The cross-linked network formed by the EBE and soy protein improved the water resistance of the cured adhesive,created a smooth surface with less holes and cracks to prevent moisture intrusion,and improved thermal ability of the cured adhesive.