Research On The Bonding Mechanism And Processof Dialdehyde Starch Based Resin

With the rapid development of the domestic real estate industry and decoration industry, decoration and furniture materials grow fast and wood-based panel industry has achieved unprecedented development. Among the types of adhesive used in wood-based panel, synthetic resin is still dominant. With the introduction of national environmental protection laws and regulations, the improvement of people’s living standard and health awareness growing, study on the formaldehyde free adhesive is very necessary.Therefore the development of natural polymer adhesive which has been paid more and more attention is the focus of research in the future. The biodegradable and environmental-friendly starch adhesive with abundant and cheap raw materials, good toughness and ease for use has widespread application prospect. Due to dialdehyde starch containing so much high activity of aldehyde, it own easy gelatinization and cross-linking reaction properties, while not easy to mildew. Dialdehyde starch can replace the substances containing aldehyde to prepare wood adhesives. The production of adhesive based on d property ialdehyde starch is of great importance not only for the expansion of starch adhesive used in wood-based panel industry but also the production of formaldehyde free environment-friendly indoor decoration materials.This paper worked on the development of biomass wood adhesive based on dialdehyde starch, and the preparation of the environmental-friendly plywood. Orthogonal experiment was used to investigate the water/dialdehyde starch ratio, additives/dialdehyde starch ratio, additives feeding ratio, reaction time, pH value, content of fillers and the amount of crosslinking agent and other factors through the performance and characteristics evaluation of Dialdehyde Starch Adhesive. Single-factor and response-surface experiment were used to investigate the effects of hot-pressing pressure, hot-pressing temperature, hot-pressing time, the quantity of adhesive and veneer moisture content on bonding strength of the plywood to obtain a series of optimal parameters about hot-pressing process, and the stability test was included finally. In addition, a variety of characterization methods were used to analyze the structure and curing characteristics of the adhesive. FT-IR, TG-DSC, XRD, AFM, rheometer, contact angle measuring instrument and other technical analysis methods were used to evaluate properties of the adhesive and reveal the reaction process and the bonding mechanism.The main conclusions of this paper are as follows:(1) In the aspect of Urea-Dialdehyde Starch Adhesive reaction and feeding ratio, main factor affecting the bonding strength is water/dialdehyde starch ratio,urea/dialdehyde starch ratio takes second place. A set of optimal parameters was obtained:water/starch ratio is 100:35, urea/starch ratio is 1:4, urea feed ratio is 3:7. While in the aspect of Urea-Dialdehyde Starch Adhesive preparation conditions, effects of soybean protein content on bonding strength was the most significant. The optimal parameters as follows:pH=5, reaction time is 70min and the soybean protein content is 16%. (2) In the preparation process of Phenol-Dialdehyde Starch Adhesive, effect of phenol/dialdehyde Starch ratio on the bonding strength and viscosity are significant, while effects of wate/dialdehyde starch ratio and reaction time on both bonding strength and viscosity have no significance. Comprehensive results of the optimal parameters are as follows:wate/starch ratio is 100:30, phenol/starch ratio is 1:2,and reaction time is 110min. (3) In the preparation of Dialdehyde Starch Graft Copolymer Adhesive, the effects of vinyl acetate/dialdehyde starch ratio and N-hydroxymethyl acrylamide content on water-resistant bonding strength of the plywood are all significant, but the effect of dialdehyde starch/water ratio is Completely not significant; while effects of this three factors on the dry bonding strength are not significant. The optimal parameters are as follows:water/starchratio of is 100:35, vinyl acetate/starch ratio is 1.8 and the amount of N-hydroxymethyl acrylamide is 8%. (4)Urea-Dialdehyde Starch Adhesive and Phenol-Dialdehyde Starch Adhesive have still not provided with good water-resistance characteristic. But compared with the ordinary starch adhesive, they own the properties of high solid content, well-distributed adhesive solution, good storage stability, long active period, corrosion resistant and good softness, especially their dry bonding strength improve significantly. Dialdehyde Starch Graft Copolymer Adhesive can reduce the interface energy, improve the compatibility and overcome the bad mechanical properties of starch itself by grafting modification method, which has a certain strength and water-resistance. (5) The analysis of rheological properties indicated that, three kinds of dialdehyde starch based adhesives are shear-thinning and belong to pseudoplastic fluids. The Phenol-Dialdehyde Starch Adhesive and Urea-Dialdehyde Starch Adhesive are manifested as semi solid semi fluid substance in low frequency region, while in the high frequency region as viscous fluid properties; The Dialdehyde Starch Graft Copolymer Adhesive in the low-frequency region shows a solid state properties and shows viscous fluid properties in high frequency region; The contact angle measurement analysis show that, the order of wetting speed are as follows: Urea-Dialdehyde Starch Adhesive>Dialdehyde Starch Graft Copolymer Adhesive> Phenol-Dialdehyde Starch Adhesive. The wettability of Phenol-Dialdehyde Starch Adhesive is the best, Urea-Dialdehyde Starch adhesive takes second place, dialdehyde starch graft copolymer adhesive is the worst. (6) FTIR analysis showed that the aldehyde in dialdehyde starch exist in the form of acetal, hemiacetal or hydrated mostly. Urea, phenol and dialdehyde starch form high moleculars by methenyl-key or ether bond in the reaction process. The adhesive solution containing hydroxymethyl, aldehyde, amide and other active groups could actually promote the bonding strength as wood adhesive. Absorption peaks in1244cm-1and 1024cm-1 show the presence of vinyl ester side chain in the starch copolymer. Dialdehyde starch based adhesive still exist obvious intramolecular hydrogen bond association.(7) TG-DSC analysis showed that Urea-Dialdehyde Starch Adhesive and Phenol-Dialdehyde Starch Adhesive at the temperature of 130～150 ℃, while Dialdehyde Starch Graft Copolymer Adhesive at about 100℃, moisture evaporation and functional group reaction basically complete. If continuing to increase the temperature, it will cause the thermal degradation of the adhesive layer and reduce the bonding strength. (8) The results of XRD showed that, compared with native starch, the major absorption peaks of dialdehyde starch do not change significantly, but the crystallinity decrease and the amorphous area increase. Thus it has so loose structure and good permeability that it is easy to react chemically. After prepared into starch adhesive, the adhesives basically do not have strong absorption peaks and the crystal structure are completely destroyed, which translate the crystalline structure into amorphous structure. The reaction between the dialdehyde starch and the added substances can be inferred from XRD analysis. (9) Starch particle size analysis showed that, with the improvement of the degree of substitute of aldehyde, molecular weight of starch decrease. Oxidation is accompanied by starch degradation. As can be seen from the results of AFM, three kinds of adhesive film are compact.3D net structures are formed between the molecules, which is conducive to the formation of good tensile strength. Urea and phenol react with dialdehyde starch, which also lead to degradation of the starch molecules and the film is difficult to form a network structure. And the small moleculars degrade due to the hydroxyl groups exposed, which more easily combine with water molecules to produce swelling and damage the adhesive layer, So the adhesive system do not have the water-resistant bonding strength. The starch particles and vinyl acetate polymers distribute uniformly in the Dialdehyde Starch Graft Copolymer Adhesive system, and form network structure. (10) The response-surface analysis showed that, various factors have strong interactive effects by using Urea-Dialdehyde Starch Adhesive to prepare plywood. Eeffect of hot-pressing temperature on bonding strength is the most significant, while hot-pressing time takes second place, and effect of the quantity of adhesive is not significant. The optimal parameters are as follows:the hot pressing temperature is 136 ℃, hot pressing time is 1.99min.mm-1, and the quantity of adhesive is 416g.m-2. Prediction of the bond strength of plywood is 2.12MPa when usinghe optimal parameters; Through the single-factor analysis of the dialdehyde starch graft copolymer adhesive, a group of optimal parameters is obtained as follow:hot-pressing temperature is 80～100 ℃, hot-pressing pressure is 1.4MPa, hot-pressing time is 3.2min.mm-1, the quantity of adhesive is 300g.m-2, and the veneer moisture content is 12%. (11) The results of stability test showed that, Urea-Dialdehyde Starch Adhesive compares favorably with UF resin adhesive on dry bonding strength. The bonding strength result error is small compared with theoretical value and the results show high reliability and stability. The fitting equation match well with the actual situation, the established model is of great significance, and the optimal process parameters have a certain reference value; Dry bonding strength and water-resistant bonding strength of the Dialdehyde Starch Graft Copolymer Adhesive have good performance, which can meet class Ⅲ plywood requirements and achieve the general bonding strength level of the PVAc.