Effect of Extended Mat Open Assembly Time on Properties of OSB Bonded With PMDI

In this study, the effect of increasing mat open assembly time on the strength properties of oriented strandboards bonded with polymeric methylene diphenol di-isocyanate (pMDI) resin was examined. Isocyanates are more sensitive to open assembly times than other resin systems such as phenol formaldehyde (PF), and the storage capacity of pMDI-blended wood furnish is believed to be no more than about 5 h. On the other hand, previous research works on bonding of wood elements with pMDI allude to the growth of an interpenetrating network (IPN) of cross-linked polyurea as being responsible for the high bonding capacity with pMDI and that this is enhanced with long pre-cure times (greater than 10-15 h). There was no evidence that this effect enhances the strength of OSB made from shortleaf pine, as boards pressed after a delay of 10 or more hours after blending were significantly lower in internal bond (IB) strength than those pressed within half an hour of blending. However, after a reduction of approximately 28% between 0 and 10 h, IB remained relatively stable with open assembly times up to 18 h, suggesting the potential for recycling such strands by partial substitution of them with freshly blended strands. The discrepancy between our findings and the pMDI bonding theory based on previous small-scale laboratory experiments was thought to have arisen from differences in resin distribution on the strands due to their different methods of application, and the cure temperature and moisture conditions in the core of boards being sub-optimal for the formation of an extensive network of fully cross-linked polyurea