Modification of wood wettability by plasma and corona treatments

Abstract

The durability of an outdoor wood-coating system is low, especially when the coating is a stain. The most important degradations come from the dimensional variations of wood which strain the coating and lead to its cracking. Thus the service life of a wood-coating system can be increased through different routes: • Improvement of the coating flexibility: it can be obtained by decreasing the glass transition temperature ( T g) below the temperature of use. But in this case the coating is more sensitive to water • Dimensional stabilization of wood: thermal and chemical treatments (acetylation) are efficient but most of them decrease the wettability of the wood surface and therefore the adhesion of the coating is no more guaranteed. Surface treatments are convenient either to improve wettability and therefore coating adhesion or to decrease wettability in order to provide an extra protection.That is why plasma and corona treatment have been applied to wood in two ways: • The first way consists in improving the wettability of wood surface to increase the coating adhesion especially after a thermal treatment. First experiments consist in selecting treatment parameters (type of gas, treatment duration, power) leading to better surface characteristics of wood, before and after thermal treatments. Plasma and corona treatments are effective to improve the wettability of wood but no improvement in the coating adhesion has been observed. • The second way consists in decreasing the wettability so that the surface becomes waterproof. Some tests have been performed to deposit thin films (based on ethylene or fluorine) by plasma or corona treatments. This kind of deposits can be considered either on bare wood or on coated wood with a low T g stain, that is to say, a stain whose sensitivity against water will be solved because of this kind of films. This is the aim of an on-going research programme.