Quantification of hydrolytic degradation of cured urea-formaldehyde resin adhesives using confocal laser scanning microscopy

This study reports a method of measuring the extent of hydrolytic degradation of cured urea-formaldehyde (UF) resins trapped in the lumen of softwood tracheids, using confocal laser scanning microscopy (CLSM) which enables quantitative determination of the area of a cured UF resin in the lumen as a function of acid etching time. Two formaldehyde/urea (F/U) mole ratios (1.6 and 1.0) and two hardener levels were employed to compare the extent of hydrolytic degradation of cured UF resins. For comparison, we also measured the mass loss of cured UF resin films after acid etching. Cured UF resins with a 1.6 F/U mole ratio resulted in greater hydrolytic degradation than those with a 1.0 F/U mole ratio. The hardener level also showed a clear influence on the extent of degradation with a limited consistency. These results indicate that cured UF resins with higher F/U mole ratios are more easily hydrolyzed than those with low F/U mole ratios, which could be ascribed to a more branched network structure of cured UF resins with a high F/U mole ratio.