Effects of urea and catalysts added to phenol-formaldehyde resin binders for strand board

The effects of adding urea to phenol-formaldehyde (PF) resins as a co-polymer component were investigated in conjunction with cure accelerating catalysts. Urea was added at 10% by total resin weight. Several methods for the addition of urea during the advancement of the PF base resin were used to determine if the urea addition leads to co-polymerization with PF-base resins and changes in resin performance properties. The urea was added at the beginning, at three different middle stages, and at the end of PF resin synthesis. No co-polymerized methylene or methylene ether linkage between urea and PF resin was identified by 13C nuclear magnetic resonance. The curing of urea-modified PF resins, evaluated by dynamic mechanical analysis, showed some differences among the resins. The bonding performances of the urea-modified PF resins, examined by making homogeneous strand board, indicated that the optimum method of adding urea to PF resins is to add in the later part of resin synthesis and also that catalysts can lower the thickness swelling values of boards.