Preparation And Characterization Of Water-resistant Soybean Meal-based Adhesives

Soybean meal(SM)is the by-product of soybean oil extraction,which is divided into low-temperature defatted soybean meal(low-temperature meal,LM),high-temperature defatted soybean meal(high-temperature meal,HM)and physical defatted soybean meal(physical meal,PM)according to the different oil extraction process.Nowadays,LM is the main raw material used to prepare commercial SM-based adhesives that have higher cost than urea-formaldehyde resin because of the small yield and high price of LM.In addition,the performance of SM-based adhesives is not desired so that limits its wide application in wood industry.Aiming at the shortcomings of SM-based adhesives such as high cost and undesired bond performance,a new SM-based adhesive with good bond property and anti-aging ability was developed by thermochemical modification and cross-linking modification of low-cost HM.The structure and properties of the optimized HM-based adhesives were characterized by a variety of modern analytical techniques combined with conventional analytical methods,and the related modification mechanism was explored.In view of the high cost of LM-based adhesive,the effects of three typical SM species,namely LM,HM and PM,and their particle sizes on the structure and properties of SM-based adhesives were compared.The content order of active groups(such as amino group and carboxyl group)of three SMs was:LM>HM>PM.LM has relatively more active groups and can be more sufficiently crosslinked by crosslinker,which results in higher crosslinking density and better bond properties of LM-based adhesives.The adhesive prepared with 160-200 mesh SM flour had moderate viscosity,suitable wettability and fine spreading property,resulting in the good process usability,high bond strength and desired water resistance.The type II wet strengths of LM-based adhesive and HM-based adhesive reached 1.46 MPa and 1.01 MPa,respectively,which met the performance requirement of the national standard for Type II plywood(≥1.0 MPa).Compared with the cost of LM-based adhesive(3154 yuan/ton),the cost of HM-based adhesives(2738 yuan/ton)decreased by 13.19%.Therefore,HM can replace LM to prepare SM-based adhesives with lower cost and desirable bond properties.In view of the undesired bond properties of HM-based adhesive,thermochemical modification was applied to activate HM.The effects of modifier species,dosage of modifier,modification temperature and modification time on the properties of HM-based adhesive were investigated.From the evaluation of bond properties,the optimum thermochemical modification formulation was optimized as follows:NaOH as modifier with a dosage of 2.0%and heat treatment temperature at 100℃ for 70 min.Under these conditions,dry strength,type II wet strength and type I wet strength of modified HM-based adhesive increased by 32%,47%and 81%,respectively.The thermochemical modification process could effectively unfold globular structure of soybean protein,release the active groups in HM,and promote the crosslinking reaction of modified HM with crosslinker EMPA,thus improving the thermal stability,bond strength and water resistance of the modified HM-based adhesive.The type I and type II wet strengths of the modified HM-based adhesive were increased to 0.96 MPa and 1.48 MPa,respectively,indicating its bond strength and water resistance were better than those of LM-based adhesive(0.92 MPa and 1.46 MPa).The ambient aging phenomenon of SM in long time storage was discovered,and the characteristics and mechanism of ambient aging in unmodified LM and thermochemical modified LM were evaluated.The unmodified SM was obviously aged after storage for 3 to 4 months.Owing to the oxidative degradation,reaggregation and structural rearrangement of molecular chains during the ambient aging process,the active amino and carboxyl groups in SM were continuously consumed,and the crosslinking density and thermal stability of the adhesive decreased accordingly,which adversely affected the bond strength and water resistance of the adhesive.Ultimately(after aging for 6 months),the dry strength,type II and type I wet strengths of aged SM-based adhesive decreased by 52.6%,39.0%and 53.8%,respectively.However,as for thermochemical modified SM,it was insensitive to ambient aging because the SM had undergone degradation,re-aggregation and rearrangement during thermochemical modification process that was regarded as an accelerated aging process in the presence of oxygen and NaOH.Even after 6 months of ambient aging,the type I wet strength of thermochemical modified LM-based adhesive could still reach 1.00 MPa,which met the required value for Type I plywood(≥1.0 MPa).In general,it is feasible to prepare soybean meal adhesives by substituting HM for LM.Thermochemical modification of HM can significantly improve the bond strength and water resistance of SM-based adhesives,and effectively promote the ambient-aging resistance and the storage stability of SM and reduce the raw material cost of SM-based adhesives,which is of positive significance for expanding the application field of soybean protein-based adhesives and promoting the application of soybean meal-based adhesives in wood industry.