Melamine-modified urea formaldehyde resin for bonding particleboards

For the development of a cost-effective melamine-modified urea formaldehyde resin (MUF), the study evaluated the effects of reaction pH and melamine content on resin properties and bond performance of the MUF resin adhesive systems. Eight resins, each with three replicates, were prepared in a factorial experiment that included two formulation variables: two reaction pHs (i.e., 4.5 and 8.0) and four molar ratios of formaldehyde to urea to melamine (i.e., 3/2.2/0.3, 3/2.2/0.2, 3/2.2/0.1, and 3/2.2/0.05). Variables in particleboard preparation were two hot-press cured times (2 and 4 min). Thus, with two panel replications, a total of 96 panels were fabricated. Melamine content significantly affected resin properties and glue bond quality: gel time decreased, solid content increased, internal bond strength increased, thickness swell and water absorption decreased, and formaldehyde emission decreased as melamine content increased. In general, the resins catalyzed under acidic conditions (pH 4.5) resulted in faster gel times, higher internal bond strengths, lower formaldehyde emissions, and lower thickness swell and water adsorption than those catalyzed under alkaline conditions (pH 8.0). Significant correlation between gel time with both internal bond strength and formaldehyde emission suggest that resins with fast curing speeds provided favorable conditions for attaining a higher degree of resin cure, and in turn led to better bond strength and lower formaldehyde emission. Low cost and proven performance have made urea formaldehyde resins (UF) the most important of wood adhesives for interior applications. However, concern about formaldehyde emission from particleboard and weakening glue bond caused by hydrolytic degradation of UF polymers have stimulated efforts to develop improved and/or new adhesives based on UF resins. For instance, in the late 1940s and early 1950s, various fortified UF resins were developed by simply incorporating various portions of melamine resin or resorcinol to improve the durability of hot-press urea resin glue bonds in plywood (Lecher 1946, Blomquist and Olson 1955). It was shown that optimum water resistance was obtained only when the urea and melamine were coreacted in the resin system (Lecher 1946). Paper chromatography, infrared spectroscopy, and elemental analysis were later used to confirm the cocondensation of urea and melamine with formaldehyde (Yanagawa and Matsumura 1962). It should be noted, however, the formaldehyde emission of MUF resins was not evaluated in these studies, because these resins were developed mainly for plywood gluing and the formaldehyde emission was not an issue at the time. The market domination of UF resins has so far made the application of fortified UF or MUF resins to particleboard gluing economically not feasible. This has led to very little technical data on gluing particleboard with MUF resins in the literature. With the development of structural wood composites in recent years, phenol-modified melamine urea formaldehyde resins have been used for the manufacture of exterior grade particleboard in Europe (Clad and Schmidt-Hellerau 1977). In Japan, phenol melamine formaldehyde in combination with urea formaldehyde resins has been used for the manufacture of concrete-form plywood (Tamura et al. 1981). These applications have expanded the interest in melamine-modified UF resins for structural applications other than conventional hotsetting MUF resins in edge gluing and laminating. Recent development of a cost-effective melamine-modified UF resin The authors are, respectively, Principal Wood Scientist, USDA Forest Serv., Southern Research Sta., Pineville, Louisiana ([email protected]); Professor, Inst. of Wood Industry, Chinese Academy of Forestry, Beijing, China ([email protected]); and Assistant Professor, Calhoun Research Sta., Louisiana State Univ. AgCenter, Calhoun, Louisiana ([email protected]). This paper was received for publication in January 2007. Article No. 10301. ✳Forest Products Society Member. ©Forest Products Society 2008. Forest Prod. J. 58(4):56-61.