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The influence of native fatty acids on the formation of glue bonds with heat-treated wood Hancock, William Vollor

Abstract

The underlying cause was sought for "inactivation", a heat-induced change in the surface of veneer that inhibits diffusion of moisture into the wood. It was demonstrated, by the use of Douglas fir wood, that the formation of an inactivated surface could be prevented by extraction with certain organic solvents, but not with water, either before or after the wood was dried. When the extractable materials were removed, or were originally absent, no specimen examined showed degradation, through the effect of heat, of the potential for forming strong glue bonds. The frequently postulated statement related to formation of ether linkages from hydroxyl groups, in heated wood, was found to be not correct. Although oxidation had been suggested as a contributing factor, inactivated surfaces were prepared in the absence of oxygen in anything but minute amounts. Veneer was collected from Coastal and Interior type Douglas fir trees that exhibited severe, moderate or no susceptibility to the development of inactivation. Representative samples were extracted with simple alkanes and a complete separation of the various components carried out, using the techniques of gas/liquid, paper and thin-layer chromatography. Qualitative variability was found, between and within trees. Strong correlation was found between the formation of an inactivated surface and the presence of saturated fatty acids with carbon-chain lengths in excess of eighteen atoms. The role of long-carbon-chain, saturated fatty acids as the causal agent of inactivation was substantiated by the application of commercially-prepared acids, of various chain lengths, to veneer that had proven insusceptible. When heated, this veneer developed an inactivated surface. A theory was proposed, based on the work described in this thesis, and the information available in the literature, to explain the mechanism of surface modification. It states that removal of some or all of the last molecular layer of water from the wood surface, and the application of heat, permits the saturated long-chain fatty acids to hydrogen-bond with the hydroxyl groups contained in the wood cellulose. The surface of the wood is then shielded by a hydrocarbon layer with very low free surface energy that is not- readily wet-table by the water contained in applied glue.

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