Optimal synthesis of urea-glyoxal resin and its crosslinking modification on soybean-based wood adhesive

KAN Yufei;KAN Haofeng;JIANG Ye;GE Jing;ZHU Mengqi;WU Dong;GAO Zhenhua;Materials Science and Engineering College,Northeast Forestry University;

urea-glyoxal resin;;crosslinking modification;;soybean protein-based adhesive;;adhesive property;;synthetic molar ratio

Formaldehyde-based synthetic adhesive is widely used in the wood-composite industry because of its low cost,excellent bonding strength and good water resistance.However,the formaldehyde-based synthetic adhesive is derived from nor-renewable fossil resources and releases toxic formaldehyde,which results in high intention by both academia and industries to the development and application of bio-based adhesives from renewable,non-toxic and sustainable bio-resources.As a popular bio-based adhesive,commercial formaldehyde-free soybean adhesives in the current wood-composite industry are mainly prepared from the defatted soybean meal flour and polyamidoamine-epichlorohydrin(PAE) resin that is synthesized from diethylenetriamine,adipic acid and epichlorohydrin.The soybean adhesives have higher cost than other commercial adhesives such as the urea-formaldehyde resin due to the expensive raw materials for PAE.In order to reduce the cost of the soybean-based adhesive and promote its wide application in wood composites,a formaldehyde-free urea-glyoxal(UG) resin as the crosslinker for the defatted soybean meal flour-based adhesive was optimally synthesized using the low-cost urea and glyoxal.Based on the examination results from the Fourier transform infrared spectroscopy(FT-IR) analysis,thermogravimetric analysis(TGA),sol-gel test and bondproperty determination,the effects of G/U molar ratio on the structures and properties of defatted soybean meal flourbased adhesives modified by UG resin were evaluated.The UG resin could effectively crosslink defatted soybean meal flour and improve the water resistance of the defatted soybean meal flour-based adhesive,attributing to the crosslinking reaction between amino groups of soybean protein and residue aldehyde groups of UG resin.The G/U molar ratio had important effects on the structures of UG resin and the crosslinking density,thermal stability and bond property of the defatted soybean meal flour-based adhesives modified by UG resin.With the increase of G/U molar ratio,the viscosity and the water resistance in terms of soaking wet strength at 63 water and boiling-dry-boiling cycled wet strength of the defatted soybean meal flour-based adhesive were increased at first and then decreased due to the various crosslinking efficiencies and thermal stabilities of UG resins with different G/U molar ratios.The defatted soybean meal flour-based adhesive modified by UG resin with the G/U molar ratio of 2.0 had the best bond strength and water resistance,but that modified by UG resin with the G/U molar ratio of 1.6 had the desired water resistance with lower cost.The defatted soybean meal flour-based adhesive modified by the optimal UG resin can be satisfactorily used to manufacture Type Ⅱ plywood according to Chinese standard GB/T 9846—2015 "General Plywood".The cost of UG resin was 43.8% lower than the PAE resin,which made the defatted soybean meal flour-based adhesive being possible to commercially apply to the wood-based composite industry.