Preparation Of Protein Derived Plywood Adhesives And Their Dry And Wet Bonding Properties

Phenolic resins,urea-formaldehyde resins and melamine-formaldehyde resins are widely used in wood adhesives with excellent wet and dry bonding properties,but they release toxic gases such as formaldehyde in the application process.Therefore,they have a significant impact on the environment and human health.In this regard,natural plant protein-based adhesives which are low-priced and eco-friendly,have entered the vision of scholars.However,their disadvantages such as poor adhesive strength and weak water resistance limit their development in the field of wood bonding.In study,protein-based composite adhesives using protein extracted from waste cottonseed were prepared.Oxidized cellulose extracted from waste sawdust was used as reinforcing phase,modified starch and isophorone diisocyanate as cross-linking agents to improve the adhesive properties of cottonseed protein-based adhesives.The effect of additive contents on the adhesive strength of cottonseed protein-based adhesives was investigated.The content and results are shown as below:（1）The composite adhesive was prepared by mixing urea-modified cottonseed protein,modified starch and supplemented with purified cellulose and oxidized cellulose.The adhesives were applied to the surfaces of two wood veneers by single-sided gluing with a density of 240 g/m~2.According to the national standard requirements for dry and wet bonding performance measurement,three-layer manufactured plywood boards specimen were prepared by hot pressing.The results showed that the overall bonding strength of the cottonseed protein-based adhesive with oxidized cellulose as the reinforcing phase was better than that of the adhesive with purified cellulose.At 15%content of oxidized cellulose,due to the reaction between-CHO on oxidized cellulose and-NH₂ in protein,the cross-linked network of amine bond（-C=N-）was formed which enhanced the bond between cellulose and protein matrix.The maximum dry bonding strength of 2.40 MPa was achieved,increased by 13.2%compared with pure cottonseed protein adhesive.At 10%oxidized cellulose addition,the average wet bonding strength of the adhesive reached 0.93 MPa,which meets the China national standard for TypeⅡplywood（≥0.7 MPa）.（2）In order to improve the wet bonding strength of the adhesive,cottonseed protein was modified by sodium dodecyl sulfonate to expose its embedded reactive groups,and then isophorone diisocyanate with high reactivity was added as a cross-linking agent.The cottonseed protein-based adhesive was prepared by stirring and reaction at room temperature.The viscosity of the adhesive was in the range of 6140-26660 m Pa·s.The above adhesives were applied to wood veneer and hot pressed into three-layer plywood specimen for performance measurement under the China national standard.The water contact angle test shows that the surface contact angle of the adhesive with 20%cross-linking agent can reach58.5°,which is 110.4%higher than the pure cottonseed protein-based adhesive.The bonding strength test shows that the addition of cross-linking can effectively improve the wet bonding strength of cottonseed protein-based adhesives.The highest wet bonding strength of1.43 MPa was achieved with the addition of 20%cross-linking agent,which was 43.57%higher than the pure cottonseed protein adhesives.