Study On The Pretreatment Of Lignin And The Preparation And Performance Evaluation Of Lignin-based Phenolic Resin Adhesive

Phenolic resin adhesive is widely used in wood processing industry because of its outstanding bonding properties.But phenol,a kind of byproduct of fossil industry and a kind of synthetic raw material of phenolic resin adhesive,which has the disadvantage of non-renewable,and phenol has certain toxicity,which causes environmental pollution.Therefore,lignin,a natural renewable resource,has become an important raw material to replace part of phenol in the preparation of adhesives due to its chemical structure characteristics.However,due to the complex structure of lignin,it has the disadvantages of less reaction sites,and high polydispersity,which limits its application in the preparation of lignin-based phenolic resin adhesive.In view of the above problems,based on the process optimization,lignin was pretreated with DES under mild conditions,and the effect of modified lignin on the performance of the synthetic lignin-based phenolic resin adhesive was studied.Then,lignin was classified by an organic solvent fractionation method,the graded lignins were used to synthetize resin,and the effect of the graded lignin-based phenol resin adhesive was studied.The main work and conclusions of this paper are as follows:(1)The physical and chemical properties of industrial alkali lignin were analyzed,and then it was applied to the preparation of lignin phenolic resin(LPF)adhesive.In order to improve the substitution degree of lignin to phenol,the preparation process of lignin-based phenolic resin adhesive was optimized.When the substitution degree of lignin to phenol was 70%,the free formaldehyde content of adhesive was 0.29%,p H was 12.9,solid content was 50.33%,viscosity was 3390 m Pa·s,and the bonding strength of plywood was 0.70 MPa which meets the requirements of national standard GB/T 9846-2015.(2)The modified lignin-based phenolic resin(MLPF)adhesive was prepared by activating industrial alkali lignin under low temperature and atmospheric pressure by deep eutectic solvent(DES).Compared with the unmodified lignin,under the optimized conditions,the content of phenolic hydroxyl groups in lignin increased from 3.12 wt.%to 3.93 wt.%,and the methoxy content decreased from 11.83 wt.%to 6.46 wt.%.The adhesive was prepared by activated lignin,and its bonding strength was higher than that of the control samples.(3)The condition of pressing process of plywood was optimized by Single-Factor Experiment and Response Surface Methodology.When the substitution degree of lignin to phenol was 60%,the optimal pressing process temperature was 160±10℃,platen pressure was1.6±0.2 MPa and resin content was 300 g/m~2,at this time,the maximum bonding strength of plywood was 1.67 MPa,far more than 0.70 MPa stipulated by national standards.(4)The lignin was graded by using organic solvent and then the graded lignin was used to synthetize adhesive of which the properties was studied.The results showed that the comprehensive properties of the adhesive prepared by low polydispersity lignin(F2)were better,and the bonding strength of the plywood prepared by F2 was higher.Compared with LPF-KL and LPF-F1,the viscosity of LPF-F2 was 153 m Pa·s,and the bonding strength of plywood was1.33 MPa.