Study On Tannin Adhesive Modified With Hyperbranched Polyamides And Curing Mechanism

With the continuous improvement of people's living standard and growing popularity of the concept green environmental protection, people pay more attention to the impact of formaldehyde emission on health from the interior decoration and the furniture with wood based panels and propose higher requirements of aldehyde adhesive for wood so that the research and development of biomass adhesive once again received extensive attention. This paper emphasized the manufacturing of environment friendly wood adhesive with tannin extracted from the bark. It was based on the structures and characteristics of tannin and used HyperBranched Polyamides?HBPA-NH₂? as modifier which has high density of the functional groups on the surface, excellent solubility and rich amine at the end of chains to improve the degree of cross linking and condensation polymerization and overcome the defects of low water resistance and bonding strength of tannin adhesive. Firstly, this subject focused on the preparation of HBPA-NH₂ through melt polycondensation reaction and the characterization of the chemical structures and stability of HBPA-NH₂ by modern chemical and instrumental analysis methods. Secondly, it explored the effects of polymerization conditions on the appearance, pH,viscosity and other properties of modified tannin adhesive, and investigated the influence of bonding strength and water resistance of the glued products of tannin adhesive modified by HBPA-NH₂ through the preparation of three layers of poplar plywood. It investigated the change of functional groups in the chemical reaction process of tannin adhesive resin modified by HBPA-NH₂ through FTIR and solubility of glue cured, and suggested the preliminary mechanism of crosslinking reaction of tannin adhesive modified by HBPA-NH₂, which laid scientific theoretical foundation for the development and application of the high performance and environment friendly tannin adhesive. The main conclusion of this paper is as follows.1.HBPA-NH₂ was prepared by melt polycondensation under different polycondensation processes with diethylenetriamine?DETA? and succinic anhydride or o-phthalic anhydride as raw materials. Solubility experiment showed that HBPA-NH₂ was soluble in water and methanol when polycondensation temperature was from 130? to 160?, time from 3h to 4h and the molar ratio of DETA and succinic anhydride less than 1. The results of FTIR and 1H NMR indicated that when polycondensation temperature was 140? or time was 3h, primary amines and carboxyl groups reacted to form amide bonds promoting the growth of molecular chains and amide bonds further reacted with carboxyl groups enabling molecular structure to branch.When polycondensation temperature was 150? or time was 4h, the molecular structure branched completely. The results of DSC and TG demonstrated that HBPA-NH₂ had higher glass transition temperature and better thermal stability when polycondensation temperature was 140?.2.The study showed that tannin adhesives modified with HBPA-NH₂ had a certain pot life.The viscosity of modified tannin adhesive first increased in the short term and then graduallydecreased when pH was raised to different values.The stable viscosity decreased with the increasing pH values.The pH value of modified tannin adhesive increased and the viscosity increased to 600 mPa·s with the increasing addition of HBPA-NH₂.3.Effects of amount, polycondensation conditions and types of HBPA-NH₂ on the bonding strength and water resistance of plywood was explored. The results showed that it was feasible to use HBPA-NH₂ as crosslinking agent to improve the degree of polymerization of tannin adhesive and increase cross-linking rigidity in the preparation of plywood. Bonding strength test showed that the bonding strength and water resistance were significantly improved when polycondensation temperature was 140?, time 4h and the molar ratio of DETA and succinic anhydride 0.8:1 with the amount of 3% HBPA-NH₂ in tannin adhesive.4.Water resistance and crosslinking property was evaluated by measuring the solubility of the adhesive layer and Infrared spectrum technology to explore the possible reaction between HBPA-NH₂ and tannin adhesive. The results of solubility showed that HBPA-NH₂ in tannin adhesive caused the decrease in solubility which indicated the improvement of water resistance and crosslinking reaction. It agreed with water resistance and bonding strength of plywood.Especially when polycondensation temperature was 140?, time 4h and the molar ratio of DETA and succinic anhydride 0.8:1 with the amount of 3% HBPA-NH₂ in tannin adhesive, FTIR showed that there was meta-substitution and ortho-substitution on the benzene ring of tannin molecular. It implied that Schiff base and Michael addition reaction may occur between amino and phenolic hydroxyl in Bring.