Study On The Depolymerization Of Lignin And The Preparation Of Phenolic Resin

Phenolic resin is a widely used adhesive,which is industrially produced by the polymerization of phenol and formaldehyde.Phenol is a basic petrochemical product,and due to China’s oil-poor and gas-poor resource endowment,the search for phenol replacement raw materials has been a hot spot for domestic research.Lignin is the only naturally renewable substance containing phenolic structure,therefore,the use of lignin and its depolymerization products to replace phenol has received widespread attention,which can effectively use lignin and reduce the production cost of resin,but also improve the resin performance.Lignin comes from a wide range of sources(hardwoods,softwoods and herbs)and has a complex structure.The depolymerization efficiency and the properties of the resulting synthetic resin can be influenced by the variation in location and quantity of phenolic hydroxyl groups among different types of lignin.This thesis will focus on the relationship between lignin structure-depolymerization product composition-phenolic resin properties.Two types of lignin,an herbaceous milled wood lignin and a softwood organosolv lignin,were selected for the paper.In this study,depolymerization of lignin was conducted using an autoclave,and the depolymerization products were analyzed using two-dimensional nuclear magnetic resonance and gas chromatography.The effects of lignin source,reaction temperature,and reaction time on both the phenolic yields and the structural characteristics of the depolymerization products were investigated.The study investigated the impact of substitution rate on resin properties by utilizing depolymerization products and lignin as phenol substitutes,and established a correlation between the content of phenolic hydroxyl groups in depolymerization products and the free aldehyde content of the resin performance parameters.The following main conclusions were drawn:(1)The phenolic hydroxyl content of milled wood lignin was 1.81mmol/g,of which 14.59 % was S-type structure,61.22 % was G-type structure,and the rest was H-type structure.The depolymerization of milled wood lignin was carried out using methanol and formic acid as solvents.When the temperature was lower than 240 ℃,the depolymerization products detectable by GC were mainly guaiacols,and when the temperature was higher than 240 ℃,the depolymerization products detectable by GC were mainly phenols with guaiacol macromolecule products.Under the optimal depolymerization conditions,the S-type structure disappeared,and the G-type structure accounted for 79.70 % of the depolymerized products and the H-type accounted for 20.30 %.(2)The phenolic hydroxyl content of the organosolv lignin was 2.25mmol/g,of which 99.20 % was of G-type structure and 0.80 % was of H-type structure.The depolymerization of this lignin in methanol and formic acid showed that the highest phenolic yield of 6.70 wt.% was obtained at 340 ℃for 2 h.The phenolic hydroxyl content increased to 3.96 mmol/g,of which65.16 % was G-type structure and 34.84 % was H-type structure,and the small molecules of the depolymerization products were phenols containing substituents and a small amount of benzenes,while the large molecules were guaiacols.(3)The curing temperature of the pure phenolic resin was 145 ℃,the bonding strength was 1.57 MPa,and the free aldehyde content was 0.17 %under the selected synthesis conditions in this paper.The bonding strength of the resin was significantly reduced by replacing a small amount of phenol with milled wood lignin,and the bonding strength increased with the increase of the substitution rate,but the free aldehyde content increased significantly,and the curing temperature,bonding strength and free aldehyde content of the resin were 155 ℃,1.34 MPa and 0.46 %,respectively,at 20 % substitution rate.When 20 % phenol was replaced by milled wood lignin depolymerization products,the curing temperature,bonding strength and free aldehyde content of the resin were 150 ℃,0.92 MPa and 0.28 %,which were in accordance with the national standard.The bonding strength of the resin exhibited a trend of initial decline,followed by an increase and then a subsequent decrease with the increase in the replacement rate of phenol with organosolv lignin.Moreover,a significant increase in free aldehyde content was observed upon the utilization of organosolv lignin as a replacement for phenol.20 % of the replacement rate had the highest strength of 1.44 MPa,and the replacement rate increased to 30 %,and the strength decreased to 0.64 MPa,the free aldehyde content decreased from 0.37 % to 0.24 %,and the curing temperature was 152 ℃.(4)The free aldehyde content of the phenolic resin was negatively correlated with the amount of phenolic hydroxyl groups in the raw material.When phenol was partially replaced with lignin,the free aldehyde content of the resin was 0.3 % when the ratio of total phenolic hydroxyl content to formaldehyde addition was 0.44,and the free aldehyde content of the resin decreased slightly by further increasing the ratio of the two.Moreover,the phenolic hydroxyl content increased and the free aldehyde content of the resin was low when lignin was depolymerized and substituted for phenol,compared to the direct substitution of lignin for phenol,in accordance with the national standard.