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a) microtomed surface prepared on a sledge microtome (5 samples/species) b) roughly sanded surface (P100) (5 samples/species) Holzforschung / Vol. 55 / 2001 / No. 4 SummaryUsing four different wood species, the wetting behaviour of sanded wood surfaces was compared with that of microtomed surfaces. Sanding caused lower contact angles and lower total surface free energies. Electron donor components were considered to be responsible for the observed changes according to the applied acid-base approach. The time behaviour of sanded surfaces differed from the microtomed reference, insofar as the volume of a deposited drop of water decreased very rapidly as soon as it was applied, indicating fast penetration into the wood. A qualitative analysis by SEM revealed a heavily torn uppermost cell layer on sanded surfaces, which was assumed to be responsible for the increased surface free energy and enhanced penetration.

The surface free energy of several selected wood species was calculated from contact angle measurements according to the Lifschitz-van der Waals/acid-base theory and compared to pH and water-insoluble acid content (i.e., classical acidity). Distinct relationships with increasing statistical significance were found between pH and the Lewis base component and, most clearly, the Lewis acid component. Similar relationships, although less distinct, were observed with the water-insoluble acidity of the examined wood species. It is concluded that the Lewis acid component as determined using contact angle data is a good relative measure for the variability of classical acidity and pH between different wood species.

Spruce and beech wood specimens were sanded with four different grain sizes. Surface morphological and chemical changes were determined by roughness measurement, surface free energy determination according to acid-base theory and X-ray photoelectron spectroscopy. An analysis model was proposed to describe chemical changes due to sanding and the data were analysed according to it. It could be shown that spruce and beech behave similarly over a wide range and that chemical changes are non-linear functions of grain size and thus surface roughness. A maximum of surface free energy and a maximum of carbon C1s sub-peak ratio (ratio of carbon bonded to a single non-carbonyl oxygen to carbon bonded only to hydrogen or other carbon atoms) was found for middle grain size.