Preparation Of A Wood Adhesive From Defatted Soybean Flour Modified By Soybean-polysaccharide/Isocyanate Microcapsule

With the decrease of non-renewable petroleum reserve and the increasing of environmental concern in China,traditional petroleum-based adhesives will face greater challenges.There is huge consumption of adhesives in wood industry,which is accounts for about 70% of the total depletion of adhesives.Formaldehyde-based adhesives such as ureaformaldehyde adhesive,melamine formaldehyde adhesive,melamine modified urea formaldehyde adhesive,phenolic adhesive occupy more than 80% of the wood industrial adhesives market share.In this case,the soy based adhesives with raw material of renewable and environmentally safe has attracted many attentions in replacement of petroleum based adhesives.Although some commercial applications of soy based adhesives had been reported,such adhesives still have the problems of low solid content,high viscosity,and high curing temperature,which limits the largescale industrial application of soy based adhesives.In this work,soybean polysaccharides were used as stabilizer and shell material to prepare microcapsule by emulsion coating with isocyanate crosslinking agent.The synthesis mechanism and process of microcapsules were analyzed.Next,defatted soy flour was modified by the microcapsule to obtain a good water resistance,high solid content,low viscosity,and low curing temperature soy based wood adhesive.The results and conclusions are as follows:(1)Soybean polysaccharide/isocyanate microcapsules were prepared by the emulsion method.The particle size and distribution,coating rate and wall thickness of the microcapsule gradually decreased as the stirring speed increased during the preparation process of microcapsule.When stirring speed was 800 rpm,the average particle size of the microcapsule was 321μm,with a narrow particle size distribution and 73.76% coating rate and1.37±0.02 μm of average wall thickness.The microcapsule was stored at25 ℃ and RH 60% for 56 days,and its coating rate was decreased to55.73%,showing excellent storage stability.The microcapsule prepared by emulsion method has good sphericity and structure.The density emulsion will protect its core material.(2)Soybean polysaccharide/isocyanate microcapsule mixed with defatted soy flour can effectively reduce the viscosity of the prepared adhesives.When the solid content is about 40%,the viscosity of adhesives is only 8000 m Pa.s.When adhesives were optimized with 20% of microcapsule prepared at 800 rpm,and further hot-pressing with temperature 130 ℃,pressure 2 MPa,and time 1.96 min/mm.Maximum bond strength of plywood samples was 1.21 MPa.When adhesive was cured at the hot-pressing temperature of 90 ℃,the bonding strength still met the requirement of national standard GB/T 9846-2015 for indoor use plywood Ⅱ class.The microcapsule as other biomass base(peanut protein,the cottonseed protein,oxidized starch,sodium alginate and chitosan)adhesive modification agent,also has the obvious positive modification effect.The obtained plywood can reach the national standard GB/T 9846-2015 in indoor use plywood Ⅱ class requirements,except the plywood bonded with oxidized starch.(3)FTIR analysis of the synthesis mechanism of soybean polysaccharide / isocyanate microcapsules and the modifying soybeanbased adhesives showed that the oil phase isocyanate was added dropwise to the aqueous phase containing soybean polysaccharides,and an emulsion was formed during the stirring process.The isocyanate groups in the oil phase droplets then react with the water in the outer layer and the hydroxyl groups on the soybean polysaccharide to form urethane and urea crosslinks,thereby encapsulating the unreacted isocyanate component inside to form microcapsules.The curing and cross-linking process of the microcapsule modified soybean-based adhesive is similar to the formation mechanism of its shell layer.The main reason is that the microcapsules rupture under pressure,the isocyanate groups of the core material component reacted with the carboxyl group,hydroxyl group,amino group and other active functional groups of the defatted soybean flour and eucalyptus veneer to form a three-dimensional network structure,thereby improving the water resistance of the adhesive after curing.TG analysis found that microcapsules can effectively improve the thermal stability of the cross-linking agent and the adhesives after curing.SEM and fluorescence staining analysis found that the microcapsules have good stability in the glue solution and will not agglomerate.At the same time,they can be broken under pressure and release the cross-linking agent.It was found that the cured soybean-based adhesive has no obvious toxic effect on the cells after the cytotoxicity test of the microcapsule modified soybean-based adhesive,which stored for 30 days after curing.The microcapsule modified soybean-based adhesive showed the good safety and environmental performance.