| At present,synthetic polymer adhesive is widely used in wood-based panel industry,which is not only harmful to the environment but also expensive.Starch adhesive has many advantages such as wide source,low price,green environmental protection,etc.Applying starch adhesive to artificial board can produce formaldehyde-free plywood with excellent adhesive performance.However,the application of starch adhesive in wood industry is limited because of its poor water resistance.Although many scholars at home and abroad have modified starch,toxic substances are used in the process of modification,and the disadvantages of starch sizing are not overcome.Therefore,in this paper,starch was used as the main raw material,chitosan and polyvinyl alcohol were used to modify it to prepare a water-resistant adhesive film for starch-based plywood.In this paper,the hot-pressing technology of plywood made from adhesive film is optimized,and the modification mechanism and bonding mechanism are analyzed and summarized.(1)Starch Adhesive(St Adhesive)was prepared from corn starch.The preparation process of starch adhesive is optimized and its micro-morphology and thermal stability are studied.The results showed that the best preparation conditions of starch adhesive were sodium hypochlorite(calculated as available chlorine)4%,reaction temperature 50?,reaction time 3 h,system p H value 10.0.Starch adhesive particles are mostly spherical,uniform in size,and have small gaps,but the particle surface is rough and has many cracks and wrinkles.The initial temperature of thermal decomposition of starch adhesive is 215?,and the maximum thermal decomposition temperature is 302.7?.When the temperature is increased to 700?,the total weight loss of the adhesive is 95.66%,indicating that the thermal stability of starch adhesive is poor.(2)The response surface method was used to optimize the hot-pressing process of starch plywood.The research found that the optimal hot-pressing process parameters were hot-pressing temperature 156.13?,hot-pressing pressure 1 MPa,hot-pressing time 185.70 s,adhesive consumpution 240.72 g/m2(double-sided,solid content 30%).The experimental value and the theoretical value conform to the quadratic regression model:Y=+2.06+0.09X1+0.029X2-0.13X3-0.014X1X2+0.023X1X3-0.013X2X3-0.21X12-0.071X22-0.66X23.The model predicted that the dry shear strength of plywood produced under this hot-pressing condition was 2.07 MPa.The repeatability test proved that the difference between the predicted value and the actual value was small,which proved that the fitting accuracy of the quadratic polynomial model was high.(3)In order to improve the water resistance of starch adhesive,chitosan modified starch adhesive(CS adhesive)was prepared by using chitosan(CA)to modify starch,and its bonding mechanism,modification mechanism,micro morphology and thermal stability were studied.The results showed that when the amount of chitosan was 30%of the dry weight of starch,the dry shear strength was 2.46 MPa,30.05%higher than that of St adhesive,and the wet shear strength was 0.90 MPa,which reached the national standard of class II plywood(?0.80 MPa),while the wet shear strength of unmodified St adhesive was 0 MPa.The condensation reaction between-NH2 in the chitosan molecular chain,-COOH in the starch molecular chain,and-COOH on the wood surface generates a water-resistant amide bond,supplemented by a large number of hydrogen bonds formed between the chitosan,starch,and wood surfaces.The water resistance of starch adhesive is improved.With the addition of chitosan,the ordered structure of starch was destroyed,and the modified adhesive surface was smoother and more delicate,without obvious starch particles.The addition of chitosan improved the thermal stability of starch adhesive,the initial temperature of thermal decomposition of CS adhesive(227?)was 12?higher than St adhesive,and the total weight loss rate(94.55%)was 1.16%lower than St adhesive.(4)In order to improve the film-forming property of starch adhesive,polyvinyl alcohol(PVA)was used to modify it to prepare polyvinyl alcohol modified chitosan starch adhesive film(PCS adhesive film),and its hot-pressing process,bonding mechanism,modification mechanism,micro morphology and thermal stability,film cost were studied.As a result,it was found that PCS adhesive with 5%PVA added had good adhesive properties.Sizing in the form of adhesive film solves the shortcoming of starch adhesive sizing difficulty.When the spraying water volume is 30%of the dry weight of the adhesive film,the dry and wet shear strength of the plywood reaches the national class II plywood standard(?0.80 MPa).There is no chemical reaction between PVA,chitosan and starch,but hydrogen bond is formed.A large number of hydrogen bond formed on the surface of PVA,starch,chitosan and wood is also one of the main sources of shear strength.The surface of the PCS adhesive film is smooth and delicate,and no obvious starch particles are seen,indicating that the adhesive film is mainly disordered.The compatibility of polyvinyl alcohol with chitosan and starch is very good,and there is no phase separation.The addition of polyvinyl alcohol further improved the thermal stability of the adhesive film.The initial temperature of thermal decomposition of the adhesive film(231?)was 16?higher than that of St adhesive and 4?higher than that of CS adhesive.The total weight loss rate(94.19%)is 0.38%lower than that of CS adhesive and 1.54%lower than that of St adhesive.The cost of producing 1m3 plywood using PCS adhesive film is 226.66RMB,which is 24.45%lower than the cost of urea-formaldehyde adhesive,and the economic benefit is significant. |
PDF Full Text Request