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This study was conducted to investigate the effects of weight average molecular mass (Mw) of phenol-formaldehyde (PF) adhesives on the performance of medium density fiberboard (MDF). To obtain different Mw PF resins, a series of PF resoles were prepared by blending low Mw (LMW) and high Mw (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 Mw determination and differential scanning calorimetery (DSC) for thermal cure kinetics. As the proportion of HMW was increased, Mw and hence the viscosity of adhesives increased. The thermal curing kinetics of the blended resins obtained by DSC showed that total thermal energy (zXH) and activation energy (Ea) of cure decreased with increasing resin Mw 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. Einflu8 des Gewichtsmittels der Molmosse yon PhenoI-Formoldehydhorzen ouf die Quolit~t yon MDF-PlottenIn dieser Arbiet wird der Einflufl des Gewichtsmittets der Molmasse yon PF-Harzen auf die Qualit~it von MDF-Platten untersucht. Eine Reihe yon PF-Resolen wurde als Mischung von Harzen mit niedriger (LWM) and hoher Molmasse (HMW) hergestellt. Sechs Mischungsverh~ilt-nisse

ABSTRACT:One of the variables in phenol formaldehyde adhesive resin formulation is the addition of urea, which allows the resin manufacturer to manipulate both product functionality and cost. Nitrogen content can be used as a measure of the level of urea addition because most of the nitrogen present is derived from urea added at the end of the preparation process. Nitrogen analyses of a set of commercial oriented strandboard resins were first determined by combustion analysis. IR spectroscopic data were then collected and used along with the values for nitrogen content to generate predictive models. First, principal component analysis demonstrated the ability to separate resin mixtures from low to high nitrogen content. Partial least squares regression was then performed and gave excellent correlations between the measured and predicted nitrogen contents for a set of unrelated (test) resins and a set of mixtures prepared through combinations of resins with known nitrogen contents. Similar results were obtained on both a wet and dry mass basis. Given the flexibility of this instrumentation, such analyses could be placed in-line for real time monitoring of resin applied during panel manufacturing.