engleski

Structural effects of weathering on unprotected and painted wood

This paper shows the microscopic evidence (obtained by scanning electron microscopy - SEM) of the structural degradation of wood caused by weathering. Typical damage on radial and tangential surfaces of softwoods due to natural or artificial exposure is presented. The order of appearance of particular structural changes enables the distinguishing between various levels of degradation according to the occurrance and intensity of the changes on certain anatomical features. Subsequently the conditions of the coating, of the substrate and of their link were assessed after exposure of coated samples. Two types of European softwood species were used as a substrate, namely pine and spruce, and they were used either as coated panels or as microtomed wood sections ('thin strips'), which were exposed behind a detached film of a coating. Solvent borne (sb) coatings were compared with modern water-borne (wb) coatings. Semitransparent stains were compared with opaque white paints. The paints did not penetrate the cell wall but firmly adhered to the S3 layer of the lumina. The solvent-borne coatings (both paint and stain) penetrated deeper into the wood surface than the water-borne coatings.The opaque paints fully protect the wood from the effect of light through a 14-month period of natural exposure and maintain the coherent protective coating. The water-borne paint exhibits more brittle characteristics on the fractured transverse surface than the solvent-borne paint. The stains vary in their protective effectiveness. The thick water-borne stain shows a tough, sound failure mode, good adhesion but very poor penetration into wood surface. The thin coating of the solvent-borne stain is degraded by light, and the interface with the wood is affected, resulting in the development of brittleness of both wood and coating and leading to adhesion failures preceding the detachment of the film.

SEM; photodegradation; durability;wood finishing; water-borne coatings; solvent-borne coatings

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