Study On Amino Acid Enhanced Soy Protein Adhesive And Its Mechanism

The improvement of public awareness of environmental protection and the increasing reduction of non-renewable resources have put forward higher requirements for the environmental protection of the wood-based panel industry.Bio-based environmentally friendly adhesives have become the focus of research.As one of the bio-based adhesives,soy protein adhesives can effectively replace the use of aldehyde adhesives because of the wide materials sources,no formaldehyde pollution,renewable and easy degradation.However,the unmodified soy protein adhesive has poor performance in mechanical properties and water resistance,thence the unmodified soy protein adhesive needs to be treated or modified to meet the industrial requirements for wood-based panels.Two main directions are carried out to improve the bonding performance of the adhesive to the board,including improving the cohesion of the adhesive itself and strengthening the interface bonding force between the adhesive and the board.Cross-linking modification is currently the most effective method to modify soy protein adhesives.For this reason,glycerin epoxy resin as modifier,amino acid was used as enhancer for the interface bonding between soybean protein adhesive and sheet in this study.The effects of amino acid and glycerin epoxy resin on mechanical properties,water resistance and interface microscopic morphology of soy protein adhesives were studied.Through the infrared spectrometer,universal mechanical testing machine,thermogravimetric analyzer and electron microscopy characterization methods.The main conclusions are as follows:(1)Soy protein-based films were prepared by modifying soy protein isolate with glycerol epoxy resin as cross-linking agent.Partial replacement of glycerol with glycerol epoxy resin can not only reduce the amount of glycerol,but also improve the tensile strength and water resistance of soy protein-based films.The results of mechanical experiments show that when the additions of glycerol epoxy resin and glycerin are 25%of soy protein isolate,the toughness is the best,and the tensile modulus and tensile strength of soy protein-based films are 42.5 MPa and 9.2 MPa,respectively.Compared with the unmodified soy protein-based film(28.1 MPa,4.1 MPa),the tensile modulus and tensile strength increased by 50%and 130%,respectively,and the elongation at break decreased from 320.3%to 265.2%.When the addition amounts of glycerol epoxy resin and glycerin were 45%and 5%of the soy protein isolate,respectively,the soy protein-based film reaches the maximum tensile strength of 11.2 MPa,compared with the unmodified soy protein-based film,the tensile strength increased by 180%.(2)Lysine,glutamic acid,serine and leucine were selected as the representative of soy protein,and the conditions required for the reaction of the four amino acids with glycerol epoxy resin and the difficulty of making films were studied.The research shows that:lysine has the best performance when the temperature is 70℃,and the heating time is 1 h,the molar ratio of lysine and glycerol epoxy resin is 1:0.75,and the tensile modulus,tensile strength and elongation at break are the best of 7.83 MPa,1.78 MPa and 75.32%,respectively.The difference between the performance of GER-Lys films under boiling condition and 70℃is not obvious.Glutamic acid and glycerol epoxy resin can only form films under boiling conditions.When the heating time is 1h,the molar ratio of glutamic acid and glycerol epoxy resin is 1:1,the performance is the best.The tensile modulus,the tensile strength and elongation at break are 60.58 MPa,2.85 MPa and 17.33%,respectively.The results showed that the film formed by glutamic acid and glycerol epoxy resin has poor water resistance,and the GER-Glu films will partially dissolve into a viscous liquid when placed in the air for about 10 min.When the boiling condition and the heating time are 2 h,films of GER-Leu and GER-Ser cannot be formed under the selected ratio.(3)The soy protein adhesives were prepared by mixing lysine,soy protein isolate and glycerol epoxy resin to glue poplar veneers to prepare plywood.The process conditions for preparing plywood were as follows:the temperature was 120°C,and the sizing amount was 200 g/m~2,the pressure is 1.0 MPa,and the hot press time is 300 s.The experimental results showed that the wet shear strength was 0.64 MPa when the unmodified soy protein adhesive was used to prepare plywood.When the sum of lysine and soy protein isolate was constant,and the weight of glycerol epoxy resin and lysine were both 9%of the total,the wet shear strength of the plywood prepared by soy protein adhesive reached the maximum of 1.22 MPa,which was compared with the modified soy protein adhesive by glycerol epoxy resin(0.94 MPa),the wet shear strength of plywood was improved obviously.