关键词: 脲醛树脂, 固化, 水解稳定性, 结晶, pH

Abstract: [Objective] The hydrolysis of cured urea-formaldehyde resin(UF resin) plays an important role on long-term formaldehyde emission from UF resin-bonded panels. The crystallinity of cured UF resin also have an effect on the hydrolysis stability. The influence of pH condition during UF resin curing on the hydrolysis stability and crystallinity of cured UF resin was studied.[Method] Four kinds of UF resin with different F/U mole ratios (0.95,1.20,1.35 and 1.50) were synthesized in the lab.The UF resin was cured on different pH (3.5, 4.0, 4.5, 5.0 and 7.5) conditions. The crystallinity of the cured UF resin was measured by XRD. The hydrolysis stability of cured UF resin was characterized by the mass loss and liberated formaldehyde content of cured UF resin in HCl solution(0.1 mol·L^-1).[Result] The hydrolysis stability of cured UF resin with F/U mole ratio of 0.95 increased with the decreasing of pH (from 7.5 to 4.0),then decreased with the decreasing pH (from 4.0 to 3.5),and there was the same trend of crystallinity of cured UF resin in the same pH range. The hydrolysis stability and crystallinity were exhibited the maximum at pH 4.0.The hydrolysis stability of cured UF resin with F/U mole ratio of 1.50 increased with the decreasing pH (from 7.5 to 3.5),and the crystallinity of cured UF resin with F/U mole ratio of 1.50 was much less than 0.95 and decreased with the decreasing pH value(from 5.0 to 3.5).The hydrolysis stability of cured UF resin with different F/U mole ratio under the same pH condition decreased with the decreasing of F/U mole ratio(from 1.50 to 1.20),and then showed an increasing trend (F/U mole ratio changed from 1.20 to 0.95).The crystallinity of cured UF resin increased with the decreasing F/U mole ratio(from 1.50 to 0.95). The hydrolysis stability of cured UF resin with F/U mole ratio of 1.20 and 0.95 was the worst and is the best,respectively.[Conclusion] The hydrolysis stability of cured UF resin with F/U mole ratio of more than 1.20 was mainly affected by condensation degree of cured UF resin. In other words, decreasing pH value was good for improving the hydrolysis stability, but not in favor of crystalline of cured UF resin. The hydrolysis stability of cured UF resin with F/U mole ratio of less than 1.20 was affected by not only condensation degree but also crystallinity of cured UF resin,and the condensation degree affects the hydrolysis stability by means of the crystallinity. The hydrolysis stability and crystallinity of cured UF resin increased with the decreasing pH value, but decreased when pH value was less than 4.0. The hydrolysis stability of cured UF resin with F/U mole ratio of 1.20 was the worst.

Key words: urea-formaldehyde resin, cure, hydrolysis stability, crystallinity, pH