Research On Synthesis And Performance Of Polyurethane Adhesive Modified By Acetic Acid Lignin

Polyurethane?PU?adhesives are usually prepared by the polymerization of isocyanate and polyols.They have good chemical adhesion to a variety of materials and are widely used.However,due to non-renewablility and the price escalation of fossil-based raw materials as well as environmental considerations,the search for alternatives of fossil-based raw materials based on renewable resources that environmentally friendly and widely sourced is indispensable.Lignin comes from biomass.As a by-product of pulping and biorefining,lignin has a wide range of sources and large yields.It has the potential value to replace polyols to prepare polyurethane adhesive because of its functional groups such as phenol/aliphatic hydroxyl groups.Generally,the structure of lignin is complex,which varies depending on its source and separation process.Acetic acid lignin is an organic solvent lignin,which has better solubility in some organic solvents,which helps to improve interface compatibility and reaction modification.In this paper,bagasse was used as the raw material,and the lignin acetate was prepared by the acetic acid refining method after extraction with the benzene alcohol mixture.Then the carbohydrate content,element composition,molecular weight,and ash content of the extracted acetic acid lignin?AAL?were measured and analyzed.Its functional groups were characterized by ¹H-NMR,¹³C-NMR and ³¹P-NMR,and its structures and properties were discussed.The results showed that the AAL contained only 2.32%moisture,the ash content was only 1.56%,and the total hydroxyl content was 4.21 mmol/g.Further,the potential of bio aromatics was fully unlocked by proper demethylation and hydroxymethylation,thereby promoting the reactivity of lignin by the elimination,substitution or addition of functional groups.Which improves the reactivity of lignin as a polyol,and is used for preparing polyurethane adhesives.The demethylation was carried out in HBr aqueous in the presence of the phase-transfer agent,TBHDPB,in a DMF solvent.The carbon-oxygen bond of methoxy on the phenyl ring was cleavaged and phenolic hydroxyl and methyl halide was produced.The hydroxyl contents of the demethylated product whose reaction temperature range from 105 to 125°C were discussed,and the optimal reaction temperature was determined to be 120°C and the corresponding product was labeled DL-4.Its corresponding hydroxyl content increased by36.5%to 5.25 mmol/g compared to AAL.The PU adhesives were prepared by the condensation reaction of demethylated acetic acid lignin and TDI,and their tensile strength and thermal stability were measured.The product DLPU-4-3 with 25 wt%lignin content had the best performance,its tensile strength and Young's modulus reach 91.2 MPa and 1.02×10³MPa,respectively.In addition,DLPU exhibits enhanced thermal stability due to the improved reactivity and covalent bond density of desmethyl lignin.Hydroxymethylation was carried out with an alkaline formaldehyde solution,and the hydroxymethyl group was introduced to the C5 position of the guaiacyl unit.It was found that 85°C was the optimal temperature for hydroxymethylation.At this temperature,the free formaldehyde in HL-6 was 0.32%,and the aliphatic hydroxyl group increased by 189%to 2.92 mmol/g.The modified hydroxy-rich lignin was used in place of PEG to prepare PU adhesive.For tensile strength,the best performing HLPU-6-3 reaches 74.1 MPa.The research results proved the successful modification of lignin and its potential value in the production of PU adhesives.