Plasma treatment of heat treated beech wood – investigation on surface free energy

Abstract

With an increased interest in finding alternatives for tropical timbers and preservative-treated wood, several new wood treatments have recently been commercialised, e.g., acetylation, furfurylation and heat treatment. For the latter, diverse techniques protected by trademarks have been developed, such as the Plato process, retification process, oil-heat treatment, thermo wood and Stellac treatment, whereby the chemical and physical properties of the treated wood are altered (Boonstra et al. 2006a,b). Beneficial changes are the improved dimensional stability and durability which, among other reasons, are caused by the loss of hydroxyl groups which results in reduced hygroscopicity and increased hydrophobicity of surfaces (Tjeerdsma and Militz 2005; Petric et al. 2007). On the other hand, wettability is a prerequisite for good adhesion, but it is reduced by less hydrophilic surfaces after heat treatment. Accordingly, the deposition of waterborne finishes is generally more difficult on heat treated wood (Petrissans et al. 2003; Geradin et al. 2007). Polymer processing industries are using plasma treatment techniques to improve wettability and adhesion on hydrophobic polymer surfaces (Kogelschatz 2003). By plasma treatment, the O/C ratio is increased by the formation of hydroxyl and carboxyl groups which brings about an increase in surface polarity (Jie-Rong et al. 1999; Drnovska et al. 2003) and may be able to compensate partly the loss of functional groups by heat treatment. Several publications deal with plasma treatment of wood or wooden materials for hydrophilicity increment: elevated surface free energy and an ameliorated wetting behaviour with water were found (Rehn and Viol 2003; Rehn et al. 2003; Evans et al. 2007; Toth et al. 2007; Wolkenhauer et al. 2007a,b,c). In general, air, nitrogen or oxygen are the gaseous media to increase hydrophilicity; however, plasma techniques can also be applied for this purpose, i.e., to decrease hydrophilicity. For example, in the presence of special chemicals or reactive gases (e.g., HMDSO, silane) thin layers can be deposited on wood surfaces for creating water repellent characteristics (Denes et al. 1999; Bente et al. 2004; Odraskova et al. 2007). Podgorski et al. (2000) investigated the wetting behaviour of water on heat treated wood after application of oxygen plasma treatment at low pressure (0.08 mbar) and found a significant decrease of water contact angles. To explore the surface modification by plasma treatment in more detail, the surface free energy and the work of adhesion based on the Lifshitz-van der Waals/acidbase approach were determined (Van Oss et al. 1988). In this approach, the total surface energy is divided into (1) a polar or acid-base component, and (2) a dispersive or Lifshitz-van der Waals component. The polar part is further subdivided into an acid part (electron acceptor) and a basic part (electron donor) with regard to the Lewis acid-base model. The latter includes hydrogen bonding, whereas the disperse part or the Lifshitz-van der Waals part comprises London, Keesom and Debye interactions. In the present study, a dielectric barrier discharge (DBD) was used for plasma treatment of heat treated wood. DBD was performed at atmospheric pressure and ambient air to avoid the impact of low pressure.