Experimental and theoretical behavior of exterior wood coatings subjected to artificial weathering Jan Van den Bulcke, Joris Van Acker, Marc Stevens Ó FSCT and OCCA 2007 Abstract Several solvent- and water-borne exterior wood coatings were artificially weathered to study their performance behavior. Penetration and layer thickness were measured on unweathered samples and compared to theoretical estimates by means of measured basic parameters of coating and substrate. Color, surface roughness, gloss, and adhesion were monitored during aging, and the latter two were also calculated and compared to their experimental values. Theoretical values of gloss, and especially those of adhesion, were less successful on an absolute scale, but were in accordance with practical values on a relative scale. The influence of solid content, drying speed, and viscosity on penetration depth manifested itself clearly both in theory and in practice. In general, solvent- borne coatings performed well, but some water-borne coatings also showed good performance. By measuring the characteristics of a coating as described in this article, it is possible to rank the coatings and follow their weatherability. Finally, calculation of theoretical values proved a promising method for initial screening purpose. Keywords Exterior wood coating, Artificial aging, Gloss, Penetration, Adhesion Introduction Finishes for wooden surfaces are subjected to severe in-service environmental conditions during natural weathering—both physical and biological. Because of this, lab-based test methods that can accurately and rapidly predict the durability of coatings have been sought since the beginning of the modern coatings era. 1 With this in view, researchers in wood-coating science mostly make use of devices that combine a set of controllable parameters such as temperature, UV irradiance, and humidity (spraying or condensa- tion)—aiming at an acceleration of natural weathering phenomena. A lot of research has focused on the relationship between artificial and natural weather- ing 1–3 to optimize accelerated weathering cycles or devices and to increase a correlation with natural weathering. 4,5 It is a given fact that the stress levels in an artificial weathering device have to be chosen carefully so that the acceleration does not lead to an aging process that is different from the one under service conditions. 4 Obviously, different artificial tests differing in UV light dose, sequence order of stress, and exposure duration can lead to different degrada- tion phenomena. 6 Nowadays, artificial aging is gener- ally accepted as a tool for evaluating the performance of a coating and hence will be used in this article with a specific weathering cycle to imitate natural weathering closely. Yet, assessment of performance is also a much- discussed topic in coating research. Many parameters are available for evaluating the degradation of the coating. In this work, both aesthetic as well as physicochemical parameters were looked at for use in weatherability ranking. In addition to practical mea- surements, theoretical values were also calculated, using some measured basic parameters of substrate and coating. This approach can provide additional value by ranking coatings according to a predicted value as an initial screening exercise. Aesthetic aspects of the coating can be described by processing and interpretation of color and gloss data, and their change during weathering. These parameters can be related to physical and chemical changes of the coating on its surface as well as changes in the interior J. Van den Bulcke (&), J. Van Acker, M. Stevens Laboratory of Wood Technology, Department of Forest and Water Management, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, 9000 Ghent, Belgium e-mail: Jan.VandenBulcke@UGent.be J. Coat. Technol. Res., 5 (2) 221–231, 2008 DOI 10.1007/s11998-007-9074-4 221