Temperature dependence of wood surface energy.

A thorough understanding of the wood surface is required to engineer adhesive bonding in composite applications. A surface analysis technique, dynamic contact angle (DCA) analysis, was used to examine the effects of temperature on the wood surface as measured by the contact angle and surface energy. A hydrophobic surface transition was found on the wood surface at 60 C, which coincides with the glass transition of lignin as measured by differential scanning calorimetry. The change in the surface at the glass transition can be attributed to the diffusion of nonpolar molecular groups to the surface. This could be the result of the migration and deposition of extractives, reorientation of macromolecules, or a combination of the two. Similar behavior has been observed in synthetic amorphous polymers. Although the surface of wood is complex, the results indicate that it can be investigated and understood like synthetic polymer materials.