Construction Of Water-resistant Structure And Enhancement Mechanism Of Soy Protein Adhesive Based On Tannin And Its Chemical Modification

In recent years,soy protein-based adhesive(SPA)has been researched in wood industry because of its environmental protection and renewable properties.However,SPA has some defects such as high viscosity,poor water resistance,easy to mildew,and poor initial viscosity.It needs to be modified.The synthetic crosslinking agent can effectively improve the water-resistant bonding performance of SPA,but there are some defects such as non-renewable raw materials and large amount of synthetic crosslinking agent additions.SPAs cannot completely get rid of the dependence on fossil resources.In view of the above defects,SPAs were modified based on the theory of crosslinking enhancement in this study and condensed tannin was used as the main raw material.The chemical modification of tannins was carried out by using simple,efficient,and mild modification methods.An effective crosslinking water-resistant structure in the SPA system was constructed,so as to improve the water-resistant bonding performance.By means of FTIR,TGA,SEM and other analytical methods,the effects of chemically modified tannins with different reactive groups on their chemical structure and physical properties were studied.And the chemical modification of tannins and influencing mechanism were analyzed.The modified tannins were used as the crosslinking agent to replace that dependent on fossil resources.The effects of chemically modified tannins on the structure and properties of the tannins and the bonding mechanism of modified SPAs were studied.Meanwhile,the biological activity of tannins against mildew was used to explore the mildew improvement effect on SPAs,so as to provide theoretical guidance and basis for the industrial application of high-performance formaldehyde-free SPAs with high biomass or even whole biomass.The main conclusions are as follows:(1)Only weak interaction forces such as hydrogen bond and hydrophobic bond were formed between tannin and protein molecules.The introducing epoxy compound(PAE)promoted the formation of stable strong interaction between tannin and protein,reducing the amount of modifier addition compared to direct modification of epoxy compound(200%reduction),and improving the water-resistant bonding performance.When the additional amount of tannin was 4 g(based on 100g of SPA),the wet shear strength was increased by 42.3%to 1.11 MPa.Tannin has the dual characteristics of cross-linking agent and mildew inhibitor,which can not only improve the water-resistant bonding strength of SPA as a cross-linking agent,but also improve the mildew resistance of SPA as a mildew inhibitor.The anti-mildew effect of tannin is equal to that of sodium tetraborate,better than that of potassium sorbate,and slightly lower than that of BIT.(2)Phenol-hydroxyl-methylated tannin oligomer(PHTO)and protein molecules were easy to react and form a water-resistant network structure when SPA was hot cured.Besides,PHTO formed another network structure by itself.Both network structures crossed to form interpenetrating network(IPN)structure,effectively improving the water resistance bonding performance of SPAs.When the additional amount of PHTO was 10%(mass ratio of SPA),the crosslinking density of cured adhesive increased by 11.6%to 89.2%.The moisture uptake rate decreased by 22.8%to 2.7%.The wet shear strength of the prepared plywood met the requirement of the domestic class I plywood standard.Formaldehyde emission concentration was only 0.08 mg/L,owning the same level to that of solid wood and lower than the standard of indoor E0 grade(?0.5mg/L)plywood.(3)Hydroxyl-ethylated tannin(TR)was prepared by modification of tannin with glyoxal instead of formaldehyde.TR was dispersed in SPA systems to improve the fluidity of the adhesives.Chemical bond linkage was formed,providing a water-resistant crosslinking network structure,between TR and protein molecules during the thermal curing process.Meanwhile,the self-condensation of TR formed a self-crosslinking network structure,increasing the crosslinking density and improving the water resistance of the SPAs.When the amount of the TR addition was 5 wt%,the dry bonding strength was increased by 83.1%to 2.27 MPa,and the wood failure rate was 70%;the wet shear strength was increased by 105.4%to 1.13 MPa,and the wood failure rate was 50%,which meeting the domestic class II plywood standard.(4)Amino-branched polyamide(BPA)was grafted onto the tannin framework structure to synthesize an amino-branched tannin-based resin(BPAT).The BPAT dispersed in the SPA system,providing abundant physical crosslinking points,forming a ductile crosslinking system,and improving initial stickiness and prepressing strength.The addition of the epoxy compound increased the strong interaction force of the crosslinking system and improved the water-resistance of the adhesives.When the additional amount of BPAT was 15 g(with 2g of TGA and 100g of SPA),the crosslinking density was increased by 5.0%to 82.1%;the prepressing strength was increased by 146.2%to 0.32 MPa.The wet shear strength of the prepared plywood was increased by 44.8%to 0.97 MPa(equivalent to that prepared by commercial UF resin),meeting the standard of the domestic class II plywood.(5)Epoxidated tannin(EPT)was synthesized by grafting epoxy groups onto tannin framework through chemical grafting reaction.EPT was dispersed in SPA system and reacted directly with the active groups on soy protein molecules to reduce the number of hydrophilic groups on the protein molecules,increase the crosslinking density and improve the water resistance of the adhesive during thermal curing process.Adding 12g of additional EPT(100g soybean protein adhesive)modified adhesive,the viscosity was decreased by 20.0%and the wet shear strength of the prepared plywood was increased by 129.8%to 0.85MPa.The formation of crosslinking structure between EPT and soy protein effectively prevented water from invading the curing system and improved the water resistance of the adhesives.(6)Tannin modifiers were introduced into the SPA system to investigate the influence of adhesives and bonding process on the preloading and water resistance of the plywood.The results showed that within the 19%to 23%of veneer moisture content,220 to 230 g/m~2 of glue spread,four hours of opening time,1.1 MPa of prepressing pressure,and one hour of prepressing time,the prepressing property of EPT modified SPA(28g of soybean powder,12g of EPT,30g of PAE and 30g of water)was better than that of the commercial PAE modified SPA.The results showed that the prepressing performance of the EPT-modified SPA met the requirements of industrial production.The boiling time increased by 14.3%and the wet shear strength met the requirement of the domestic class I plywood standard.By adding tannic crosslinkers from biomass instead of synthetic crosslinkers,the addition amount of PAE crosslinker(accounting for the percentage of PAE dosage)could be reduced by12%?19%.Thus,the dependence of adhesive on fossil resources was reduced.