EFFECTS OF WEIGHT AVERAGE MOLECULAR-MASS OF PHENOL-FORMALDEHYDE ADHESIVES ON MEDIUM DENSITY FIBERBOARD PERFORMANCE

This study was conducted to investigate the effects of weight average molecular mass (<(M)over bar (w)>) of phenol-formaldehyde (PF) adhesiv es on the performance of medium density fiberboard (MDF). To obtain di fferent <(M)over bar (w)> :PF resins, a series of PF resoles were prep ared by blending low <(M)over bar (w)> (LMW) and high <(M)over bar (w) > (HMW) resins in different proportions. Six blending ratios of LMW:HM W were chosen: 100:0, 80:20, 60:40, 40:60, 20:80, and 0:100. The prepa red resins were characterized with size exclusion chromatography (SEC) for their <(M)over bar (w)> determination and differential scanning c alorimetery (DSC) for thermal cure kinetics. As the proportion of HMW was increased, <(M)over bar (w)> and hence the viscosity of adhesives increased. The thermal curing kinetics of the blended resins obtained by DSC showed that total thermal energy (Delta H) and activation energ y (Ea) of cure decreased with increasing resin <(M)over bar (w)> as de termined by SEC. Test result for a series of fiberboards prepared with the blended resins showed that the LMW:HMW blending ratio of 40:60 ga ve the highest internal bond (IB) strength. The optimum viscosity of P F resin was approximately 300 mPa.s. The maximum values of MOR and MOE were found at a blending ratio of 80:20 (LMW:HMW). The density profil e indicated that MOR and MOE were influenced by the maximum density of the board surfaces while the IB correlated to the minimum density in the core regions of the board.