Microwave-assisted Catalytic Degradation Of Lignin And Its Preparation Of Adhesives

Phenolic resin occupies an important position in industrial applications.It is widely used as refractory and insulating materials in the fields of electronics,automobiles,aerospace,and construction.However,since phenol,one of the synthetic raw materials,is a non-renewable resource,the decline in fossil fuel reserves and the negative impact on the environment have become a major global problem.Therefore,people are concerned about the development of green chemical raw materials and the use of renewable chemicals to replace fossil energy.Among them,lignin is an easily available natural phenolic cross-linked polymer,and it is the second abundant biomass resource in the world.Because lignin has low reactivity and is usually discharged as waste during production,it is necessary to improve the activity of lignin through modification.Modified lignin can be used as a renewable raw material to replace petroleum resources.In this thesis,the industrial alkali lignin was used as raw material to obtain phenol-rich alkali lignin through microwave alkali catalytic depolymerization,which is copolymerized with phenol and formaldehyde to form a biomass phenolic phenolic resin.The effect of microstructure on macromechanical properties was systematically studied.The following research results have been obtained:1.The solid content of lignin and the conditions of microwave digestion have important influences on the content of phenolic hydroxyl groups in lignin.The experimental results showed that when the solid content of lignin was 40%,the concentration of NaOH was 0.3 mol/L,the temperature was 170℃,the power was400 W and the time was 20 min,it was the best condition for depolymerization.The relative molecular weight（M_n）of lignin decreased to 2300,and the content of active phenolic hydroxyl group increased from 8.20%to 21.68%,an increase of 164%.The reactivity of lignin was improved.2.Using the depolymerized lignin in the microwave alkali catalysis process to replace 40 wt%of phenol,the bonding strength of the synthetic biomass phenol-based phenolic resin adhesive reached 1.934 MPa,which was 2.76 times higher than the national level I plywood standard（0.7 MPa）.The 80 wt%and 100 wt%bonding strengths can reach 1.17 MPa and 0.96 MPa,respectively,which were 1.67 times and1.37 times of the national level I plywood standard（0.7 MPa）.3.In the alkali-catalyzed microwave digestion lignin reaction system,the addition of sodium thiosulfate had an important effect on the depolymerization of lignin.When the concentration of sodium hydroxide was 0.2 mol/L and sodium thiosulfate was 0.4 mol/L,the relative molecular weight（M_n）of lignin decreased to600.The content of active phenolic hydroxyl group increased from 8.20%to 29.82%,an increase of 263%.4.When the concentration of NaOH was 0.3 mol/L and the concentration of Na₂S₂O₃ was 0.01 mol/L,the depolymerized lignin was used to replace 40 wt%of phenol to synthesize the biomass phenol-based phenolic resin adhesive with the bonding strength of 2.94 MPa,which was 4.2 times higher than the national level I plywood standard（0.7 MPa）.The 80 wt%and 100 wt%bonding strengths can reach2.22 MPa and 1.57 MPa,respectively,which were 3.17 times and 2.24 times of the national level I plywood standard（0.7 MPa）.5.The relative curing temperature（ΔT）of the microwave-catalyzed degradation of lignin synthetic resins were reduced by 10-12℃.In this paper,the waste lignin in the process of biomass utilization was converts into biomass phenol through degradation and phenolization,replaces phenol and synthesizes phenol-based phenolic resin adhesive,which greatly improved the mechanical properties of the resin.This provided a green way for phenolic resin production.