EFFECTS OF REACTION PH ON PROPERTIES AND PERFORMANCE OF UREA-FORMALDEHYDE RESINS

Urea formaldehyde resins were formulated with combination variables of three reaction pH (1.0, 4.8, and 8.0) and four molar ratios of formal dehyde to urea (2.5, 3.0, 3.5, and 4.0). The resins were prepared by p lacing all formaldehyde and water in reaction kettle and pH was adjust ed with sulfuric acid and sodium hydroxide, respectively. Urea was add ed in 15 equal parts at 1-minute intervals. The proportion of high mol ecular weight products in the resin increased substantially as the rea ction pH decreased. Furthermore, the F/U molar interacted with reactio n pH to effect resin molecular weight. At acidic pH, the high molecula r weight products increased as F/U ratio decreased; while, at alkaline pH, little difference was evident between the high or low molecular w eight products at various F/U ratios. The formation of a high percenta ge of uron derivatives under strong acidic conditions also indicated t hese resins differed considerable from conventional UF resins formulat ed in the past. Panels bonded with resins catalyzed at strong acidic c onditions resulted in lowest formaldehyde emission but slightly lower bonding strength. Of the three pH conditions evaluated in the study, b oth weak and strong acid catalysts systems are not commonly used in co nventional UF resin formulation. Based on the bond strength and formal dehyde emission data, however, the weak acid catalysts seems to provid e the best compromise between the strong acid and the conventional alk aline-acid catalyst system currently used for formulating UF resin woo d adhesives.