Influence of weathering on the mechanical properties and performance of exterior wood coatings
L Podgorski, J-D Lanvin
Three commercial coatings were exposed to artificial (EN 927-6) and natural weathering (EN 927-3) on wood samples. Cracking was visually assessed as well as Persoz hardness. Free films of the same coatings were also exposed to the same weathering tests before their tensile properties were measured. Results show that artificial and natural weathering modified the overall mechanical properties of wood coatings. The elastic modulus and the strength increased whereas the strain at break dramatically decreased from the first hours of exposure.
For selecting good performing coatings, our results show that it is useful to consider the variations of the elastic modulus and to calculate the retention of the initial strain at break after weathering instead of considering the strain at break result.
A significant increase in the elastic modulus lead to cracking. Coatings performed better when their modulus remained below 400 MPa and their retention in strain at break was higher than 20%.
The study shows that short artificial ageing tests (< 500 h) on free films are relevant to highlight changes in strain at break observed in natural weathering. They are therefore a valuable tool in the formulation of high-performance products.
The mechanical properties measured using the Persoz hardness test are also interesting to take into consideration in order to anticipate the risk of cracking. For the three coatings, the Persoz hardness increased more or less due to weathering. The results show that coatings with an initial Persoz hardness higher than 80 seconds should not be chosen for wood exposed outdoors because their risk of cracking is higher.