| With the continuous growth of China’s economy,the artificial wood-based board industry has experienced rapid development,resulting in an increased demand for board adhesives.Due to their low cost and excellent processability,urea-formaldehyde(UF)resin adhesives have become the most prevalent wood adhesives in China.As the second most abundant natural polymer,lignin is a rich renewable resource.Its inherent polyphenolic structure allows it to react with formaldehyde or substitute phenol in adhesive production to a certain extent,leading to the emergence of lignin-based adhesives as a research hotspot.However,the complex structure,low purity,and low reactivity of industrial lignin have significantly impeded its application in UF resins.Based on this,this article first studies the optimal preparation process of lignin modified UF;Secondly,study the effect of purified lignin modification on the performance of UF;Finally,based on the optimized preparation process of purified lignin modified UF adhesive,lignin was hydroxymethylated and modified.The structure of lignin,the physical and chemical properties of adhesive,the application in particle board and the exploration of its physical and mechanical properties were characterized respectively to prepare UF adhesive with excellent performance.The main research contents and results are listed as follows:(1)With lignin as modifier,the preparation process of modified UF resin was optimized to enhance its physical and mechanical properties on particle board,especially the reduction of formaldehyde emission from the board.Therefore,the experimental design explored different addition nodes and different amounts of lignin to modify UF resin and prepare particle board.It was found that the best physical and mechanical properties were obtained when 2%lignin was added during the addition reaction stage;the internal bond strength reached 0.51 MPa and the formaldehyde emission was reduced to 1.3 mg/100g.Fourier Transform Infrared(FT-IR)spectroscopy analysis indicated that the overall structure of the UF resin was not significantly affected by the presence of lignin.X-ray diffraction(XRD)analysis showed that the crystalline region proportion of the UF resin decreased due to the addition of lignin.Thermogravimetric(TG)analysis demonstrated that lignin effectively enhanced the thermal stability of the UF resin.The results indicate that lignin can participate in the reaction of UF resin and extract its physicochemical properties to a certain extent.(2)Priority should be given to graded purification of lignin to enhance its reactivity in UF resin and reduce the impact of impurities on UF resin.Purification of lignin through methanol-acetone organic solvent fractionation was investigated.Based on the optimal preparation process obtained in the first chapter,two types of purified lignin were used to prepare UF resins,and their effects on the physicochemical properties of particleboards were explored.First,the fractionated purified lignin was characterized:the lignin content was measured using ultraviolet spectrophotometry,and it was found that organic solvents effectively purified lignin,increasing the lignin content to 84%.The phenolic hydroxyl content of lignin was determined to be 0.5676 mmol/g using the ultraviolet method,indicating that an increase in lignin content also enhanced its reactivity.Infrared spectroscopy analysis revealed no significant change in the lignin structure.Gel permeation chromatography was employed to analyze the molecular weight of fractionated lignin components,and it was discovered that organic solvents effectively separated high and low molecular weight lignin,improving the homogeneity of lignin molecular weight.Subsequently,the physicochemical properties of the UF adhesive prepared using fractionated lignin were characterized.The results showed that high molecular weight lignin in the fractionated purified lignin effectively reduced the free formaldehyde content in the UF resin to 0.12%.Particleboards prepared with this resin were tested for their physicochemical properties,and it was found that high molecular weight lignin in the fractionated purified lignin was beneficial for the preparation of UF resin,with the internal bond strength increasing to 0.53 MPa and the formaldehyde emission further reduced to 1.2 mg/100g.(3)Based on the above research results,in order to increase the hydroxymethyl content of lignin and enhance its reactivity,the purified lignin was hydroxymethylated,and then used to modify UF resin to prepare particle board with better comprehensive physical and mechanical properties and lower formaldehyde emission.The results showed that hydroxymethylation modification can effectively purify lignin,increase the lignin content to 92%,and significantly increase the average molecular weight(Mn)of lignin to 5527 g/mol.Then the hydroxymethylated lignin modified UF resin and its prepared particle board were characterized and analyzed,Characterization of the hydroxymethylated lignin-modified UF resin and its fabricated particleboards was subsequently conducted.The results revealed no significant differences in the physicochemical properties of the hydroxymethylated lignin-modified UF resin;however,the internal bond strength of particleboards was effectively improved to 0.59 MPa,and formaldehyde emissions were reduced to 1.0 mg/100g.By utilizing fractionated and hydroxymethylated lignin,the overall performance of UF resin can be enhanced,thereby expanding the potential applications of lignin in the modification of UF resins. |
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