| With the exception of curing agents, a certain amount of filler is added to urea-formaldehyde resin adhesives (UF) used to make plywood, which could reduce the comsuption of adhesive and production cost, but improve some properties of adhesive. In practice, flour is most commonly used for the filler in UF. It does not only consume a large number of foods, but also increase the production cost of the plywood. The domestic and foreign scholars have carried out many studies about the fillers which include some residuals of agriculture and forestry, minerals powder, even nano-materials and so on. The purposes are to enlarge the range of UF filler resources, reduce the comsuption of food in the non-food industry, decrease the formaldehyde emission and improve the properties of UF. Moreover, these fillers have not been widely used, and the flour is still being widely used widely in the actual production. Sometimes, talcum powder might be instead of some flour.To study the feasibility of the bamboo powder instead of the flour, the basic characteristics of the bamboo powder, the effects on adhesive properties, the heat treatment technology of bamboo powder, the optimization of filling technology and the filling mechanism were considered. To begin with, the chemical composition and particle shape of the bamboo power were analyzed, and the effects of bamboo powder filled in UF on the curing time and viscosity were discussed. Then, the heat treatment method was used to modify the bamboo powder to improve adhesive performance. Effects of the treatment temperature, treatment time, the moisture content of bamboo powder, and interactions on formaldehyde release were studied by response surface analysis method. Besides that, the bamboo powder was treated with optimized heat treatment technology, and then mixed into the UF adhesive was used to made plywood samples. The filling technological parameters including addition level and particle sizes were optimized with the bond strength and the formaldehyde emission of the plywood samples. After that, the effects of the mixture of the bamboo powder and flour on the bond strength and the formaldehyde emission of plywood were studied by some contrast experiments. Finally, the chemical and physical characteristics of the UF filled with bamboo powder were studied by fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscope (FESEM), and x-ray photoelectron spectroscopy (XPS) in this paper. The results were shown as follows:(1) It was observed that a portion of cell wall was broken, and contents were exposed when bamboo powder grinding was over 200 meshes. As the particle size of bamboo powder decreasing, the characteristic absorption peaks of active groups such as C-H, C-O and the basic structural unit of benzene enhanced.(2) The glue liquid’s viscosity increased with the increase of bamboo powder content with 15% additive quantity and 220-240 meshes.The glue liquid’s viscosity of adhesive reached the maximum, which was better than that filled with 15% flour. The glue liquid’s curing time was longest when the particle size of bamboo powder was 220-240 meshes. However, the change of the particle size from 160-180 meshes to 200-220 meshes did not play an important role in glue liquid’s curing time of UF. Filling bamboo powder or flour can significantly reduce the formaldehyde release of urea-formaldehyde resin adhesive after curing.The formaldehyde release of UF adhesive after curing reached the minimum with 15% additive quantity and 200-220 meshes of bamboo powder.(3) Three factors included heat treatment temperature (140 ℃), heat treatment time (2h) and moisture content of bamboo powder (25%) were chosen as the center point of response surface design. The heat treatment technology of bamboo powder was optimized by response surface software. The results showed as follows:heat treatment temperature was 160 ℃, heat treatment time was 2 h, and the moisture content of bamboo powder was 26.42%.(4) The glue liquid’s viscosity increased with the increase of bamboo powder content after heat treatment. The glue liquid’s viscosity of adhesive reached the maximum when the particle size of bamboo powder was 220-240 meshes. The change of glue liquid’s curing time was not obvious with the additive quantity of bamboo powder increased from 5% to 20%. The glue liquid’s curing time reached the maximum when additive quantity of bamboo powder was 25%. Heat treatment of bamboo powder had little effect on bond strength of plywood, but it was helpful to reduce the formaldehyde release amount of plywood. The addition technology of bamboo powder was optimized by response surface software. The results showed as follows:the additive quantity of bamboo powder was 18%, the particle size of bamboo powder was 220-240 meshes.(5) The bond strength of plywood filled with bamboo powder was slightly higher than flour. Mixed 6% of bamboo powder with 12% of flour or 12% bamboo powder with 6% of flour, the plywood had better bond strength. The amount of formaldehyde emission was declined obviously by adding mixed filler. The formaldehyde release of plywood reached the minimum with 12% of flour and 6% of bamboo powder.(6) Mixed with bamboo powder above 160 meshes, UF had better homogeneity. It was easy to mix with smaller size.(7) The structure unit of lignin was involved in the reactions analyzed by XPS and FTIR. Meanwhile, the bamboo powder reacted with free formaldehyde in the Urea-formaldehyde resin adhesive, which decreased the content of formaldehyde, as well as improved the bonding performance. |
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