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) adhesives on the performance of medium density fiberboard (MDF). To obtain different <(M)over bar (w)> :PF resins, a series of PF resoles were prepared by blending low <(M)over bar (w)> (LMW) and high <(M)over bar (w)> (HMW) resins in different proportions. Six blending ratios of LMW:HMW were chosen: 100:0, 80:20, 60:40, 40:60, 20:80, and 0:100. The prepared resins were characterized with size exclusion chromatography (SEC) for their <(M)over bar (w)> determination and differential scanning calorimetery (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 energy (Ea) of cure decreased with increasing resin <(M)over bar (w)> as determined by SEC. Test result for a series of fiberboards prepared with the blended resins showed that the LMW:HMW blending ratio of 40:60 gave the highest internal bond (IB) strength. The optimum viscosity of PF 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 profile 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.