Some factors affecting the flow and penetration of powdered phenolic resins into wood
Creators: name: Ellis, Simon Colin
Publisher: name: University of British Columbia
Year: 2010

description: Powdered adhesive resins based on polymers of phenol and formaldehyde are prevalent in the Canadian waferboard industry. In the development of faster curing resins, problems associated with low levels of resin flow and subsequent penetration into wood have been encountered. Little is known about how resin characteristics affect flow. This study involved the analysis of eleven samples of commercial resins and six produced in the laboratory. Molecular weight distributions and functional group characteristics were determined and related to flow characteristics. Two-ply parallel laminate boards were produced from Picea glauca (Moench.) Voss and Populus tremuloides Michx. using four resins of each type. Samples from these boards were tested in tension shear to produce strength and wood failure values. Resin penetration was evaluated using electron and light microscopy. The resins produced in the laboratory incorporated a raeta-bromophenol label. This allowed the distribution of the resin to be determined at the glueline using SEM/WDX. All the resins were of the resol type and contained an average of 0.38 to 1.32 methylol groups per phenol ring. Resins containing greater proportions of low molecular weight species generally exhibited more flow which tended to occur at lower temperatures than in higher molecular weight resins. A resin sample which had been stored for over seven years exhibited greatly reduced flow compared to fresh samples of the same resin. For the laboratory resins, the extent of penetration appeared to be the limiting factor in the quality of the bond formed only for the highest molecular weight resin. An optimum number average molecular weight was identified. Above this value, insufficient low molecular weight species were present to bring about good bond formation. Below this value excess low molecular weight species lengthened the time required for cure. The quality of the bonds as determined from percentage wood failure values was most dependent on pressing time and wood species, less dependent on resin type, with the moisture content of the wood stock having the least effect. The wood failure values obtained with the highest molecular weight laboratory resin increased with higher wood moisture contents. This effect was believed to be due, in part, to an increase in resin flow properties in addition to a more rapid heat transfer to the glueline. Species anatomy was observed to influence the extent to which the resins flowed down the lumens of the wood cells away from the glueline. In aspen gluelines, resin was observed to have flowed in the longitudinal direction down the lumens of vessels elements to a distance of 300 μm from the glueline on the cross-section. Flow down the lumens of longitudinal tracheids in spruce confined the resin to the three or four rows of cells immediately adjacent to the glueline (within approximately 100 of the glueline on the cross-section). The greater retention of resin at the glueline of the spruce bonds manifested itself as higher wood failure values at all pressing conditions. This study indicated possible directions that designs of future resin systems might follow.