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A natural weathering test for coating systems

A natural weathering test for coating systems is being developed. The natural weathering test is accelerated by exposing coated wooden panels at an inclination of 45 degrees and facing south, and by the infliction of an artificial defect, a water trap through the coating and into the wood. The concept of the water trap has caused much controversy and lengthy debate among the participants.

The results so far show that:
the test is applicable to different wood species, but coating degradation on pine sapwood, as specified in prEN 927-3, is faster than on other species;
the correlation of the test method to practical conditions has not been confirmed, contradictory to the findings from other EU-projects (the reason may be artefacts created by the test items chosen as substitutes for practical window joinery details such as L-joints);
the water trap does not affect the permeability of a coating under test;
for most systems tested the moisture balance properties improve considerably during exposure (this may be due to initial leaching of water-soluble substances from the film);
the tape test method for indicating coating adhesion does give meaningful results, but the precision is low;
the water trap is the best of 3 tested alternative challenges to the coating (it accelerates the formation of cracks, but not other parameters monitored);
transfer of test results by means of a Climatic Index does not seem feasible;
test results however may be transferred by means of the internal comparison product (ICP), but the procedure described in prEN 927-2 (specifications) ought to be adjusted, and the composition of the ICP maybe changed so as to increase the response on the climate of the ICP.

Reported by

Maleteknisk Rådgivning
Rugvaenget 23
2630 Taastrup
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