New Curing System of Urea-Formaldehyde Resins Under Neutral Condition

The conventional curing system of UF resins is accomplished in the strong acidic state.The difficulties of the resins are considered to be caused from the residual strong acid，which will accelerate the hydrolysis of the final products.From this point of view，the development of the new curing system using neutral condition was investigated in the present report。The new curing system is based on the simple method to mix UF resins with polyfunctional hydrazide compounds under around neutral conditions.Results were concluded as follows：1.The adhesive strengths of the plywood made using four kinds of dihydrazide compounds as hardner of commercial UF resins were not al all inferior to the conventional curing by ammonium chloride both in the normal test and the soaking at 60°C and passed the JAS value.2。The curing system using dihydrazide polymers as hardner not only showed comparable adhesive shear strengths with the conventional curing of UF resins，but also reduced the formaldehyde emission from the final product of plywood.3。It was concluded that the reaction of hydrazide compounds with UF resins involves two stages.The first stage is the reaction of hydrazide group with free formaldehyde。The second stage at the rather higher temperature is due to the cocondensation of hydrazide groups with methylol group of UF resins to give the crosslinking networks between UF resins and hydrazide compound.From the practical point of view，the new curing system has the great advantage of reducing the formaldehyde emission from the final products using urea-formaldehyde resins and to cure UF resins the neutral condition.

The conventional curing system of UF resins is accomplished in the strong acidic state. The difficulties of the resins are considered to be caused from the residual strong acid, which will accelerate the hydrolysis of the final products. From this point of view, the development of the new curing system using neutral condition was investigated in the present report. The new curing system is based on the simple method to mix UF resins with polyfunctional hydrazide compounds under around neutral conditions. Results were concluded as follows:1. The adhesive strengths of the plywood made using four kinds of dihydrazide compounds as hardner of commercial UF resins were not al all inferior to the conventional curing by ammonium chloride both in the normal test and the soaking at 60 ゜C and passed the JAS value.2. The curing system using dihydrazide polymers as hardner not only showed comparable adhesive shear strengths with the conventional curing of UF resins, but also reduced the formaldehyde emission from the final product of plywood.3. It was concluded that the reaction of hydrazide compounds with UF resins involves two stages. The first stage is the reaction of hydrazide group with free formaldehyde. The second stage at the rather higher temperature is due to the cocondensation of hydrazide groups with methylol group of UF resins to give the crosslinking networks between UF resins and hydrazide compound.From the practical point of view, the new curing system has the great advantage of reducing the formaldehyde emission from the final products using urea-formaldehyde resins and to cure UF resins under the neutral condition.