Preparation And Properties Of Formaldehyde-Free Wood Adhesives Based On Corn Stalk Lignin

Traditional oil based wood adhesives e.g.urea-formaldehyde resin and phenol-formaldehyde resin are widely used in wood-based panel industry since the 20th century,due to their own high performance-price ratio.Although some of these adhesives have already won the recognition of the market,there were some shortcomings and hidden dangers have appeared gradually to this day,such as the depletion of oil resources and the worry about environment and healthy.All of these problems intensified the interest in environment-friendly biomass-based wood adhesives.Lignin is the most abundant in all above bio-resources on earth except of cellulose.Lignin with a large number of active functional groups can be used to prepare a variety of biomass materials.In this study,the corn stalks enzymatic hydrolyzed lignin was used to liquefy and modify under the catalysis of sodium hydroxide solution and the environmental friendly wood adhesive based on the glyoxalated lignin was prepared.Three dissolution systems,1,4-dioxane,polyethylene glycol-400(PEG-400)and alkali liquor,were used for liquefaction of lignin.Three liquefaction systems with the same amount of lignin were compared and analyzed,the alkali solution system is better than the other two groups in stability and homogeneity,at the same time,the viscosity is more suitable for the preparation of wood adhesives.In this study,the corn stalks lignin was used to react with non-volatile and non-toxic glyoxal under the catalysis of sodium hydroxide solution and the glyoxalated corn stalks lignin was prepared.The effect of reaction conditions such as the ratio of glyoxal to lignin,reaction time,reaction temperature and pH value for the mixed solution on the hydroxylation reaction were studied by Fourier transform infrared spectroscopy(FTIR),ultraviolet spectrophotometer(UV),Gel chromatography(GPC),thermo gravimetric analysis(TGA),differential scanning calorimeter(DSC)and titration test.The optimal glyoxalation reaction conditions were chosen as 4h of reaction time,60 ? of reaction temperature,12 of pH value for mixed solution and 1:3 of the mass ratio of glyoxal to lignin.Compared with unmodified lignin,the glyoxalated corn stalks lignin from the optimal reaction conditions had a significantly improvement in hydroxyl content,thermal stability and reactivity.The GPC analysis shows that the molecular weight of modified lignin increases with the modification time,and decreases at high pH and temperature conditions.With the increase of the amount of epichlorohydrin(ECH),the shear strength of plywood first increases and then decreases.When the added amount is 30 parts per 100 parts of modified lignin,the shear strength is the largest(1.58 MPa for dry bonding strength)and can reach the standard of type ? plywood.The mechanical properties and water resistance of the wood adhesive could be improved by mixing with some aqueous emulsion,such as acrylic ester(AE)emulsion resin or polyurethane(PU)emulsion resin.The dry adhesive strength of the adhesive could be up to 2.73MPa and the wet adhesive strength could be up to 1.17MPa after adding the emulsion.When the temperature was higher than 400 ? the curing system degrades rapidly due to the breakage of the polyurethane or polyacrylate chain.