A reliable artificial weathering test for wood coatings: Fid-Bau Portal

A reliable artificial weathering test for wood coatings

Podgorski, Laurence / Arnold, Martin / Hora, Guido / Quill, Jeffrey

Accelerated weathering plays an important part in the development of new finishing systems. But there is no standardised method in existence that is applicable to the wood surface. This study has been conducted at the request of CEN/TC139/WG2 (European Committee for Standardization, Technical Committee 139, Working Group 2, Coating Systems for Exterior Wood) in order to develop an accelerated weathering method for wood finishes to assess the durability of exterior wood coatings in a short time period. The development of the test itself had to comply with an economic criterion - to wit, the use of a device that is accessible to small and medium-sized companies. This is why the studies have been directed at cycles using fluorescent lights, based on modern and relatively low-cost fluorescent lamp apparatus originally developed for coated steel and plastics substrates, adapting equipment and operating cycles to meet the needs of wood substrates. Based on the experience of the different partners, several artificial weathering cycles have been tested on a limited number of 6 finishing systems, in tandem with natural weathering tests carried out at several sites in Europe, including different types of UV radiations, condensation, spray, high and low temperature, freezing, soaking. On completion of these tests, one optimised cycle was singled out, made of 24h of condensation followed by a sub-cycle of 2.5 hours of UVA 340 nm and 0.5 hours of water spray. It was re-tested by several partners in order to assess its reproducibility, which was considered good, even though different sources of variations were identified. The definition and respect of a strict testing procedure should allow for the reduction of these variations. The performance for the alkyd systems followed the general trend which was observed at outdoor exposures on pine panels but also the experience obtained from exposures at joineries, facades etc. For the acrylic systems the performance compared to outdoor exposures is very depending on the pigmentation. The optimised cycle has been tested on different wooden substrate to determine whether it can be used to evaluate the performance of coating systems on different wood substrates. The six main coating systems were tested on oak, beech, spruce, meranti, and exterior plywood. The studies have been pursued to refine the method by broadening, in particular, the number of finishes and wood surfaces used. At the end of this research project, a standard proposal has been drawn up and presented to the CEN/TC 139/WG2 with the reference prEN927-6 (coating materials and coating systems for exterior wood), part 6 (method, assessment and evaluation for an artificial weathering test for wood coatings based on fluorescent lamp apparatus).