The surface inactivation of two wood species, yellow poplar (Liriodendron tulipifera) and southern pine (Pinus taeda), was studied following high temperature drying. Surface analysis involved X-ray photoelectron spectroscopy, sessile drop wettability and fracture mechanics of the adhesively-jointed surfaces. The results showed that wood drying at high temperature (i.e., >160 to 180 °C) caused modifications in surface composition. The oxygen to carbon ratio (O/C) decreased and the ratio of carbon atoms bonded to other carbon or to hydrogen atoms vis-à-vis carbons bonded to oxygen atoms (i.e., the C1/C2 ratio) increased with drying temperature. In addition, the contact angle increased with the temperature of exposure, but decreased with time. A dependence on wood species was evident: southern pine surfaces always exhibited higher contact angles than yellow poplar. Also, the rate of contact angle decline with time, dθ/dt, was found to vary with surface composition: this rate corresponded to O/C ratio-changes, especially in the case of southern pine. Southern pine was most susceptible to inactivation particularly when bonded with PF adhesive. Yellow poplar surfaces did not show significant inactivation when exposed to drying temperatures below ca. 180 °C. The results are explained by a relative enrichment of wood surfaces with non-polar substances, hydrophobic extractives and volatile organic compounds that 'become visually evident during the drying process at temperatures above ca. 160 °C. Little change was observed if drying temperatures remained below 150 °C.
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