| Soy-based adhesives have the advantages of wide raw material sources,low cost,renewable,no formaldehyde release,etc.,but they also have the bottleneck problems of poor water resistance and low cementing strength.Crosslinking modification of epoxy resin is one of the strategies to improve its properties.At present,the raw materials used for preparation of modified resin are mostly from non-renewable petroleum resources.In this study,industrial alkali lignin,a by-product of pulp and paper industry,was used as raw material to increase the phenolic hydroxyl content of lignin through demethylation treatment,and then the demethylated lignin-epoxy resin(DLEP)was synthesized by reaction with epichlorohydrin under alkaline condition,which was used to modify soy-based adhesives.The main research contents are as follows:Firstly,sodium sulfite was used as a demethylation reagent to increase the content of phenolic hydroxyl groups in lignin by removing part of methoxyl groups in lignin.The results showed that after methylation,the content of phenolic hydroxyl group increased by nearly 35%,and DLEP was successfully synthesized under alkaline conditions,which was nearly 22%higher than that of epoxy resin(LEP)synthesized directly from lignin.Infrared spectroscopy(FTIR)confirmed the grafting of epoxy group of lignin after epoxidation.The results of bonding strength showed that when the addition amount of DLEP was 4%,the wet bonding strength of the adhesive reached 1.14 MPa,which was 87%higher than that of SM adhesive,basically reaching the modification effect of commercial epoxy resin,indicating that the method was feasible.Secondly,in order to further improve the effect of DLEP modified soy-based adhesive,the overall optimization of the process was carried out.The optimal reaction conditions for demethylation of lignin were as follows:sodium sulfite dosage was 1.0 g,sodium hydroxide dosage was 2.0 g,reaction time was2 h,reaction temperature was 70℃.Under these conditions,the phenolic hydroxyl content of methylated lignin could reach 2.98 mmol/g,which was about 51%higher than that of unmodified lignin.The best reaction conditions for the epoxidation of DLEP were as follows:the hydroxyl mole ratio of sodium hydroxide to lignin was 3:1,the reaction time was 5 h,and the reaction temperature was 60℃.Under these conditions,the epoxidation value of DLEP was up to 0.297 mol/100 g,which was nearly 29%higher than that of LEP.DLEP prepared under the optimized conditions was used to modify soy-based adhesive.When the addition amount was 4%,the wet bonding strength of the adhesive can reach 1.34 MPa,which was nearly 120%higher than that of SM adhesive,and met the requirements of Chinese national standard(GB/T 9846-2015)class II plate(≥0.70 MPa).By means of FTIR and SEM,it was concluded that when DLEP modified soy-based adhesives,the epoxy group reacted with amino,carboxyl and hydroxyl groups in protein molecules to form a continuous and dense cross-linking structure,which effectively prevented the invasion of water and thus improved the bonding strength of adhesives.Finally,in order to face the industrial application,the hot-pressing process and dosage of adhesive were explored and optimized.When the hot-pressing temperature was 150℃,the hot-pressing time was 10 min,the hot-pressing pressure was 1.0 MPa and adhesive dosage was 240 g/m~2,the preparation process of plywood was the most economical and effective,and its dry bonding strength reached 2.39 MPa and wet bonding strength reached 1.34 MPa. |
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