Adhesion strength and pendulum hardness of some coatings in wood heat-treated by different methods

Abstract

Beech (Fagus sylvatica L.) and pine (Pinus sylvestris L.) wood specimens were heat-treated separately at three different temperatures (170 °C, 190 °C, and 210 °C) with steam treatment method (STM), oil treatment method (OTM), and hot-air treatment method (HTM). Then, the specimen surfaces were coated with water-based, polyurethane-based, and oil-based varnishes according to industrial applications. The study results show that both hardness and adhesion strength values of STM and HTM treated specimens were similar for both wood species. In contrast, these values were generally lower in OTM-treated specimens. For all applied methods, heat treatment temperature had no significant effect on hardness values. Varnish adhesion strength decreased in all heat-treated wood specimens compared to untreated specimens. Adhesion strength also decreased in STM- and HTM-treated specimens with increasing heat treatment temperature. Hardness values increased in all specimens coated with polyurethane- and water-based varnish compared to the specimens without varnish. However, hardness decreased in the specimens coated with oil-based varnish. In contrast, the highest adhesion resistance was determined in the specimens coated with oil-based varnish under all heat treatment conditions. In addition, the heat treatment method is more effective relative to the hardness values and the treatment temperature is more effective relative to the adhesion strength of varnished pine and beech specimens.