Fast Curing of Composite Wood Products

Abstract

The overall objective of this program is to develop low temperature curing technologies for UF and PF resins. This will be accomplished by:• Identifying the rate limiting UF and PF curing reactions for current market resins;• Developing new catalysts to accelerate curing reactions at reduced press temperatures and times.In summary, these new curing technologies will improve the strength properties of the composite wood products and minimize the detrimental effects of wood extractives on the final product while significantly reducing energy costs for wood composites. Background:The low cost and proven performance of urea-formaldehyde (UF) and phenolformaldehyde (PF) resins have made them the most important adhesive systems for composite wood products such as, oriented strandboard (OSB), medium-density fiberboard (MDF), and particleboard (PB). Both UF and PF are most commonly applied to wood fibers, particles, and veneer as aqueous solutions. PF resins are manufactured from phenol and formaldehyde and have an affinity for wood surfaces. In the presence of sodium hydroxide and heat (215-235 o C), the PF resin polymerizes into a threedimensionally crosslinked network thereby providing bonding. The curing chemistry can be accelerated using organic esters such as, propylene carbonate, methyl formate or triacetin (i.e., α-and β-sets acceleration). These catalysts decrease press-time by a factor of 2 -7. The inclusion of the ester accelerator also improves the I.B. strength for dry and boiling tests.UF resins are prepared from urea and formaldehyde and are applied to wood fibers, particles, and flakes as an aqueous solution. These resins are the dominant adhesives for MDF and PB products. This resin has a natural affinity towards wood surfaces and in the presence of heat o C) and an acidic catalyst (or hardener), UF condenses into a 3-dimensionally crosslinked network thereby providing bonding.The formulation of UF resins was initially accomplished using a two-stage alkaline/acid set of reactions. The use of acid catalysts has significantly improved the curing process providing improved curing rates and products. Neither UF nor PF resins are chemically attached to the wood fibers; instead, the adhesion is attributed to secondary bonding and/or physical bonding. The lack of chemical bonding to wood makes the physical performance of the cured resins susceptible to the presence of extractives in wood. One of the main differences between PF and UF products lies in the water resistance of PF as opposed to UF.