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 formaldehyde to urea (2.5, 3.0, 3.5, and 4.0). The resins were prepared by placing all formaldehyde and water in reaction kettle and pH was adjusted with sulfuric acid and sodium hydroxide, respectively. Urea was added in 15 equal parts at 1-minute intervals. The proportion of high molecular weight products in the resin increased substantially as the reaction pH decreased. Furthermore, the F/U molar interacted with reaction pH to effect resin molecular weight. At acidic pH, the high molecular weight products increased as F/U ratio decreased; while, at alkaline pH, little difference was evident between the high or low molecular weight products at various F/U ratios. The formation of a high percentage of uron derivatives under strong acidic conditions also indicated these resins differed considerable from conventional UF resins formulated in the past. Panels bonded with resins catalyzed at strong acidic conditions resulted in lowest formaldehyde emission but slightly lower bonding strength. Of the three pH conditions evaluated in the study, both weak and strong acid catalysts systems are not commonly used in conventional UF resin formulation. Based on the bond strength and formaldehyde emission data, however, the weak acid catalysts seems to provide the best compromise between the strong acid and the conventional alkaline-acid catalyst system currently used for formulating UF resin wood adhesives.