Moisture transport in coated wood. Part 2: Influence of coating type, film thickness, wood species, temperature and moisture gradient on kinetics of sorption and dimensional change

Abstract

The sorption of moisture by spruce and meranti coated with both waterborne and solventborne coatings was studied during controlled conditions. Experimental variables included: coating ®lm thickness, temperature and gradients in relative humidity (33±98, 65±98 or 33±75%), or exposure to liquid water. Changes in moisture content of the wood and the tangential or the radial dimensions were recorded as a function of time. Apparent moisture diffusion coef®cients were calculated from the initial slope of the fractional weight increase following Fick's second law for unsteady state conditions. The apparent moisture diffusion coef®cients were clearly in¯uenced by both coating and wood species. The fastest moisture adsorption was observed for uncoated spruce which could be explained by the rapid capillary uptake of water. Moisture diffusion in coated samples of spruce or with coated or uncoated meranti was much lower. With the exception of liquid water, moisture diffusion during desorption was faster than during adsorption. The measured moisture diffusion coef®cients should be considered as apparent because they were dependent on the initial and ®nal moisture content. Moisture diffusion was found to be strongly dependent on temperature with activation energies between 55±76 kJ mol )1 . The rate of dimensional change was described by a two-parameter asymptotic regression model that included one constant for the rate of dimensional change and an asymptotic constant given by the dimensions at equilibrium wood moisture content as an asymptote. The differences in rate constant were inuenced by the same factors as those for moisture diffusion. The correlation between rate constant and moisture diffusion coef®cient was good except for very high moisture diffusion coef®cients that were controlled by capillary water uptake.

The Color Changing Effect of the Moisture Content of Wood Materials on Water Borne Varnishes

“…This condition might have resulted from the reddening due to the formation of chromophore groups in the Eastern beech and the high tendency of the resin within the Scots pine to redden as a result of varnish application. Previously conducted studies report that water borne varnishes has a color changing effect, especially on tanniferous tree species (De Meijer and Militz 2001;Budakçı 2006;Pelit 2007).…”

“…The water that might already be present in the wood body or that might enter the body at a later time plays a significant role in the success of finishing (varnish, coloring, dying) applied on wooden furniture and decorative elements (Wheeler 1983;De Meijer and Militz 2001;De Meijer 2002;Sönmez 2005;Sönmez et al 2009;Sönmez et al 2011). The moisture content of the wooden material, as well as its many other characteristics, is an important aspect that needs to be mastered by the people using wood as the raw material in order to prevent faulty applications.…”

“…They raise the grain of the wood; they are weather-sensitive during application; they have only moderate resistance to heat, solvent, acid, alkali, water, and water vapor resistance (about the same as nitrocellulose laquer); and they produce a bland, washed-out appearance on dark and dark-stained woods Flexner 2005). It has been reported in various studies that water-borne varnishes have a color changing effect on especially tanniferous trees (De Meijer and Militz 2001;Budakçı 2006;Pelit 2007).…”

Assessment of moisture protective properties of wood coatings by a portable NMR sensor

“…[3][4][5][6][7][8] However, it has also been demonstrated that the spatial distribution of water close to the surface is inhomogeneous. [9][10][11][12] Typically, the MC does not reach a steady state but is continuously responding to changes in environmental conditions. 11 Hence, monitoring of the MC with spatial and time resolution is required to properly evaluate the effect of coating on moisture migration.…”

Moisture conditions in coated wood panels during 24 months natural weathering at five sites in Europe

“…Coating systems are able to protect wood against moisture ingress and they influence moisture release depending on their permeability for liquid water and water vapour (Wassipaul and Janotta 1972, Janotta 1973, Hora and Böttcher 1995, De Meijer and Militz 2001, Ekstedt 2002, Svane 2004. Moisture conditions in coated panels exposed 45° facing south were investigated by Derbyshire and Miller (1996) by recording wood moisture content with conductivity measurements on a site in the UK.…”