Study On Synthetic Process Of Low Toxic Urea-formaldehyde Resins

Urea-formaldehyde (UF) resins are most widely used adhesives. However UF resins and their products will release formaldehyde in the process of production and their application. The adhesive industry has focused on the research to produce UF resins with low free formaldehyde content (FFC) and with high agglutinative strength.The effect of several factors on UF resin properties were firstly studied on the basis of"mild base-mild acid-mild base"synthetic processs, which included adding formaldehyde and urea in two portions respectively. The factors are the molar ratio of formaldehyde to urea, the pH value, the temperature and time of reaction in every stage, and there is no modifying agent used during the synthetic processs.According to the above experiments, the synthetic conditions are determined as follows. The molar ratios of formaldehyde to urea are r₁=1.0, r₂=2.2 and r₃=1.4; the pH values are: pH₁=7.5, pH₂=4.4 and pH₃=9.0, the temperature are T1=60℃, T2=80℃and T3=70℃; the reaction time are t₁=20min, t₂=50min, and t₃=40min. The UF resins produced by the process meet the requests of national standard for low free formaldehyde (<0.5%). The viscosity is 65.2mPa.s, the curing time is 42.0s, the solid content is 49.93%, the free formaldehyde content is 0.44%, and the methylol content is 15.84%, for the UF resin. Especially the agglutinative strength of cold pressing board is as high as 1.52 MPa, exceeding that of the national standard for hot pressing board (>0.8MPa). The cold pressing method is superior to the hot pressing method, not only for easy operation but also for more energy savings.Then, the UF resins are modified by adding melamine and/or polyvinyl alcohol in the process of"mild base-mild acid-mild base"using the experimental method of uniform design, under the condition of that r₁=1.0, r₂=2.6, pH₁=7.5, pH₂=5.8, pH₃=9.0, T1=90℃, T2=80℃, T3=70℃, t₁=40min, t₂=50min and t₃=40min. The quadratic regressive equations between the free formaldehyde content FFC (or the agglutinative strength) and three effecting factors (the melamine, the polyvinyl alcohol and the final molar ratio of formaldehyde to urea) were established.The results show that (1) the FFC and the agglutinative strength of cold pressing...