Development of methodologies to evaluate tanning blocking coatings
C Reynaud, L Podgorski
In Europe an increased interest in using home-grown hardwoods as sustainable and renewable construction materials started a few years ago. Amongst these species oak and chestnut are two interesting candidates as their heartwood contain a significant amount of tannins which contribute to their natural durability. To avoid wood greying and dimensional variations, oak and chestnut must be protected by coatings. The European VOC Directive has been the driving force behind the change from solvent borne to waterborne coatings. Therefore bleeding of water- soluble tannins can be observed during the coating application. This phenomenon may also occur during service life due to humidity and rainfall, leading to some ungraceful aesthetic aspect for the construction. Manufacturers have therefore developed specific coatings to control tannin-staining. Moreover due to the specific anatomy of oak and chestnut (large vessels), coatings need also to be flexible enough to cover the irregular surface of these woods. In Europe a standardized test method to assess the performance of these coatings is not yet available. Each manufacturer has developed anti-bleeding coatings using its own tests. It is then difficult to compare objectively the performance of the different products available on the market and to advise the manufacturers for possible improvement of their coatings. This paper describes the advantages and drawbacks of some methods used to qualify different coatings available on the market. Permeability to liquid water and immersion tests are suitable and easy methods to provide evidence of the performance of anti-tanning staining coatings. These methods give information regarding the barrier properties to tannin bleeding and the capability of the coating not to be discoloured.