Crosslinking And Reinforcement Of Tannin Adhesive By Cellulose Nanofibrils Grafted With Hyperbranched Polyamides

With the continuous improvement of wood-based panel production,the amount and variety of adhesives are increasing.Tannin is a natural polyphenols,which has the basic conditions and great potential for the production of wood adhesives.It is an important breakthrough in the development and utilization of biomass adhesives for green sustainable development.In this paper,nanofibrils grafting hyperbranched polyamide as a modifier to enhance the degree of cross-linking of tannin adhesives and improve the mechanical and water resistance of tannin adhesive were investigated,because the condensed tannin adhesive has low degree of cross-linking characteristics which cause poor water resistance,physical and mechanical properties.Firstly,CNF was oxidized by sodium periodate and the oxidation process was discussed.Then,CNF was modified based on the reaction of Schiff base with hyperbranched polyamides which has functional groups of high density at the end,dissolve easily;and contain a large number of amino groups,and which can improve the degree of cross-linking tannin and the water resistance and mechanical properties of tannin adhesive.The structure and performance of CNF modified with HBPA was characterized by instrumental methods.Secondly,the effects on the pH,viscosity,gel time and other properties of modified tannin adhesive were explored.The effect of CNF on the bonding strength was studied by poplar plywood.Finally,the crosslinking reaction between HBPA and tannin was elucidated by the solubility of the adhesive layer,FTIR and solid 13 C NMR.This work lay the theoretical foundation for the utilization of tannin adhesive.The main content of this paper is as follows.(1)Cellulose nanofibrils was oxidized selectively by sodium periodate,the effects of oxidation temperature,oxidation time,pH,sodium periodate and CNF mass ratio on the oxidation were studied,and the optimum oxidation process was determined titration by aldehyde group.The oxidized CNF was grafted with aliphatic hyperbranched polyamides that were derived from butanedioic anhydride and diethylenetriamine and aromatic hyperbranched polyamides that were derived from phthalic anhydride and diethylenetriamine,respectively.The functional groups,crystallinity and thermal properties of cellulose nanofibrils filament oxidation and grafting products were characterized by ATR-FTIR,XPS,XRD,TG,and DSC.The results indicated that the optimum oxidation process was sodium periodate and CNF mass ratio of 1: 1,pH=5,3h in the water 50℃.The results demonstrated that the characteristic peak of ATR-FTIR appeared in the oxidation phase,and the C2 area in XPS C1 s decreased,indicating that the number of C-O was reduced,which proved that the hydroxyl group was oxidized to aldehyde group.Based on the Schiff base reaction,the hyperbranched polyamides was grafted on the surface of cellulose nanofibrils,which has functional groups of high density,dissolve easily and contain a large number of amino groups.The characteristic peaks of aldehyde group in ATR-FTIR disappeared,and the characteristic peaks of imine appeared.And XPS appeared N1 s characteristic peaks,N1 s peak appeared in the C = N peak,indicating that HBPA successfully grafted to CNF.TGA and XRD showed that the thermal stability and crystallinity of the cellulose nanofibrils before and after grafting decreased in different degrees,because the surface of the cellulose nanofibrils was affected by oxidation.(2)The effect of CNF-g-HBPA on the physical properties such as pH,viscosity and gel time of tannin adhesive was studied by CNF-g-HBPA modified tannin and pressing plywood.And the effect of CNF-g-HBPA on the bonding strength and water resistance of plywood was studied.The results showed that the viscosity of CNF-g-HBPA was increased slowly compared with the ungrafted CNF,but it increased linearly with the increase of the amount of tannin,and increased to about 900mPa·s.The pH increased slowly from 4.98 to 5.45,but the addition of modified CNF had little effect on gel time.CNF-g-HBPA can significantly improve the bonding strength and water resistance of tannin plywood.The addition of 2% CNF-g-ArHBPA bond strength was improved,which can reach the national standard class Ⅱlevel,and the grafted aromatic hyperbranched polyamides CNF was superior to the aliphatic hyperbranched polyamides.(3)The curing properties of the cured product were evaluated by simulating the curing conditions of the tannin adhesive.The water resistance and crosslinking properties of the adhesive were evaluated;the functional groups were investigated by FTIR and solid NMR properties during the reaction of HBPA and tannin.The solubility results showed that the water resistance of the adhesive layer was significantly improved after CNF-g-HBPA was added,indicating that hyperbranched polyamide could improve the crosslinking degree of tannin adhesive.The addition of 1% CNF-g-AlHBPA is the best water resistance.The addition of 2% CNF-g-ArHBPA has the best water resistance,and the ArHBPA polycondensation temperature is 150 ℃ and the material ratio is 1: 1,which is consistent with the bonding strength.The reaction between tannin and hyperbranched polyamides by FTIR may cause heterocyclic open and meta-substitution.NMR proved that the B ring part of the phenolic hydroxyl group was converted to NH2,phenolic ring polyamines may occurred,the heterocyclic ring opening and the cross-linking reaction of the-N = bridge may be formed between the flavonoid units.