Weathering

A coating defect in which ultraviolet degradation of the binder at the surface leaves a loosely bound, powdery layer of pigment that can be rubbed off. It is a normal long term weathering mode of many exterior coatings and is more pronounced in binders with poor ultraviolet resistance.

An additive that protects polymers, coatings and sealants against degradation by ultraviolet light, either by absorbing the radiation (ultraviolet absorbers) or by scavenging the free radicals it generates (hindered amine light stabilisers). It is used to delay chalking, colour fade and embrittlement in exterior applications.

Laboratory exposure of materials to intensified ultraviolet light, moisture and heat cycles, using fluorescent UV or xenon arc apparatus, to simulate outdoor weathering in a compressed timescale. Methods are standardised in the ISO 4892 and ASTM G154 and G155 series, and results are used to compare colour retention, gloss loss, chalking and embrittlement of coatings, membranes and composites.

An accelerated laboratory corrosion test in which coated or plated specimens are exposed to a fine fog of salt solution in a closed chamber under standardised conditions, as described in ISO 9227 and ASTM B117. Results are used for comparative and quality control purposes, as correlation with service life in real atmospheres is limited.