Modification Of Alkali Lignin And Its Application In The Synthesis Of Lignin-phenolic Resin

Lignin is a renewable resource of abundant aromatic compounds in higher plants.With the development of modern pulp and paper industry,a large amount of lignin produced by the pulp and paper industry has not been fully utilized.The alkaline pulping process can extract industrial alkali lignin,which provides an important source for obtaining lignin.Industrial alkali lignin contains certain cellulose and hemicellulose,which makes the alkali lignin complex in structure and low in reactivity.Industrial alkali lignin needs to be purified and increased in activity in order to improve its application value and expand its application range.The deep eutectic solvent has good dissolving ability to lignin,and has the effect of retaining the lignin structure,and can be used as an excellent modification solvent for lignin.The traditional phenolic resin is synthesized from phenol,formaldehyde and so on.It is a adhesive with excellent properties and widely used.Lignin has a similar structure to phenol,so lignin can be used to partially replace phenol to synthesize lignin phenolic resin.The modified lignin phenolic resin synthesized from modified lignin,phenol and formaldehyde has higher thermal stability and lower concentration of free formaldehyde.It also saves the use of phenol and replaces petrochemical products with non-toxic renewable resources,which is of great significance to low-carbon economy and green production.In this study,industrial alkali lignin(AL)was selected as raw material,the content of AL was analyzed by absorption spectrum,and the standard curve of lignin was drawn.By studying choline chloride(ChCl)and Lactic acid(Lac),Urea(U),Formic acid(FA),Oxalic acid(OA)and other hydrogen bond donors with different compositions of deep eutectic solvents,the mass ratio of deep eutectic solvent to AL,the molar ratio of choline chloride to hydrogen bond donors in deep eutectic solvent,the effects of temperature and time on the extraction rate of lignin,a single-factor experiment was carried out for these five factors.The results showed that the mass ratio of alkali lignin to ChCl/Lac deep eutectic solvent(ChCl:Lac was 1:12)was 1:20,the temperature was 120?,the treatment time was 12h,and the extraction rate of lignin was 95.37%.The effect of catalyst on the extraction rate of DES-modified lignin was studied.DES modification requires higher temperature,in order to reduce the reaction temperature,Na OH,Na₂CO₃,HCl,H₂SO₄,H₂O₂(p H=11.5)were selected as catalysts.The addition amount was10%,and the single factor experiment was carried out to determine that the maximum modification temperature under normal pressure was 100°C.And the process conditions of the highest extraction rate of lignin obtained by orthogonal test were:catalyst was 8%Na₂CO₃,DES(ChCl:Lac was 1:12),reaction temperature was 100?,reaction time was 12h,and the extraction rate of lignin was 74.87%.The effect of phenolic ratio on the properties of phenolic resin was studied,phenolic resin and alkali lignin phenolic resin adhesives with different phenolic ratios were synthesized,and the performance test and structure characterization of the adhesive were carried out.The experimental results showed that when the ratio of phenol to formaldehyde was 1:1.4,the performance of the adhesive was the best when the substitution rate of alkali wood to phenol was 27%.The dry bonding strengths and wet bonding strengths of the alkali lignin phenolic resin obtained in the experiment were 2.06MPa and 1.91MPa,respectively,the formaldehyde emission was 0.30mg/L,the free formaldehyde was 0.29%,and the solid content of the resin was 48.66%,all meeting the national standard.Catalytic-deep solvent-modified lignin-phenolic resin(DLPF)was synthesized using 8%Na₂CO₃as a catalyst to replace part of phenol with catalyzed ChCl/Lac eutectic-modified lignin,DLPF was modified with urea(U).The results showed that the phenol substitution rate of 8%Na₂CO₃-ChCl/Lac deep eutectic solvent modified lignin was 32%at 1:1.4 phenolic ratio.The urea-deep eutectic solvent-modified lignin-phenolic resin(U-DLPF)obtained by adding 8%urea modified DLPF has a free formaldehyde of 0.12%,a formaldehyde emission of 0.18mg/L,and the bonding strength and solid content all meet the national standards.The curing temperature of DLPF and U-DLPF adhesives were 107.78?and 121.78?,which were higher than ALPF curing temperature and shorter curing time,the thermal stability was improved.