The Research Of E₁ Grade Urea-Formaldehyde Resin For Man-Made Board

The advantages of urea-formaldehyde adhesives are their excellent adhesion to ligno-cellulose and low cost. It also has two principal disadvantages, 1) lack of resistance to weather and water, 2) their emission of formaldehyde. In this paper, the low formaldehyde/urea molar ratio was adopted and technological parameters of acid condensation stage were improved for the preparation of UF resin. The manufacturing conditions of urea-formaldehyde resin modified with melamine(MUF resin)were studied. The chemical structure of urea-formaldehyde resin in every reaction stage was quantitatively characterized by 13C—nuclear magnetic resonance. It has been proved that the F/U molar ratio is the most important factor in decreasing formaldehyde emission of plywood. Improving the condensation reaction conditions of UF resin could increase the amount of methylene and decrease the amount of methylene ether, both low formaldehyde emission and satisfied bonding strength of plywood were reached. Adding 1 % accelerator agent to UF resin, manufactured plywood satisfy the bonding strength requirement of type Ⅱplywood. It was possible to decrease the formaldehyde emission by adding 10 % formaldehyde scavenger without weaken the bonding strength. The quantitative characterization of UF resin by 13C—NMR demonstrated that methylene was mainly produced in acid stage through the condensation of methylolureas. A part of methylene ether bridges broke and the amount of methylolureas was increased in last basic stage. The 13C—NMR spectrum also shown that the condensation reaction of melamine with formaldehyde formed many methylene ether bridges which liberated less formaldehyde, and formaldehyde emission of plywood were decreased. The bonding strength of plywood produced with MUF resin was increased as amount of melamine increased, and decreased while the content of melamine was beyond 3 % (based on urea). It made the later stage of condensation reaction mild by controlling the pH value and reaction time. There would be more methylene bridges and less ether bridges if formaldehyde/urea molar ratio was lower than 1.6 in the acid stage.