Study On The Curing Characteristics And Properties Of Low Molar Ratio Urea-Formaldehyde Resin

Urea-formaldehyde (UF) resin has been widely used in wood based panel, especially in plywood industry, which has advantages of low cost raw material, convenient synthesis process, excellent bonded performance and so on. However, UF resin possesses some disadvantages, such as a short pot time, poor boiling-water resistance and ageing resistance, especially high free formaldehyde content and high formaldehyde emission of its bonded products, which can lead to the environmental pollution and affect people's health. Therefore, to study and prepare modified low molar ratio UF resin with good properties and low formaldehyde emission of its bonded products is becoming an important issue, which has significant social and economic value.In this thesis, the curing characteristics of low molar ratio and melamine modified low molar ratio UF (MUF) resins had been studied, and the property of the plywood bonded with the resins had also been covered. In order to get the optimum molar ratio, different low molar ratio UF resins had been studied. The curing time, pot time, pH value in curing and thermal property had been researched to characterize the curing behavior of low molar ratio UF resins. The curing behaviors and structure of MUF resins had been studied by TGA, FT-IR and XRD to explore the optimum addition amount of melamine. The brief conclusions showed as follows:(1) UF resins with the molar ratio of formaldehyde to urea as 1.1 could get reasonable curing time. The plywood manufactured by the 1.1 molar ratio UF resins mixed with 1% ammonium chloride added as curing agent showed excellent bonded properties.(2) The optimum addition amount of single curing agents, such as ammonium chloride and ammonium sulfate, was weight ratio as 1%. The low molar ratio UF resins using the compound curing agents could also get the superior curing properties, such as ammonium biphosphate/ammonium dibasic phosphate (ABP/ADP).(3) MUF resin with the melamine addition amount of 9% had the excellent curing properties and mechanical properties. The curing time increased with the addition amounts of melamine, meanwhile, pot life and storage life decreased with that. The temperature of exothermic peak lagged behind and the reactivity of curing reaction decreased with the addition amounts of melamine.(4) The hydroxyl group changed into methylene group with the addition amounts of melamine and the crosslink of MUF resins increased with the increasing of melamine. The diffraction peak of XRD became smooth with the addition amounts of melamine, otherwise, the crystallinity of MUF resin decreased after curing.