The durability of paint films containing titanium dioxide — Contraction,erosion and clear layer theories

For gloss paint films containing titanium dioxide, evidence fundamental to the development of weathering theories, including weight loss, thickness loss and electron microscope examination of weathered surfaces, especially for the stages before chalking, is critically reviewed and discussed. It is concluded that the published data, considered as a whole, cannot be explained by erosion of the surface, but can easily be interpreted in terms of a model based on appreciable degradation beneath the surface, decreasing with depth depending on the penetration of u.v. radiation. This results in differential contraction of the surface layers and an increasing surface pigment concentration.Various ‘clear layer’ theories are also discussed and distinguished by the thickness of the clear layer proposed. It is concluded that there is no convincing evidence that distinct uniform pigment-free layers exist in normal air-drying gloss paints. Even if clear layers ≈ 0.2 – 0.5 μm thick were present, durability results could still not be explained by erosion. Effects of pigment volume concentration on the specular gloss of alkyd paints at various wavelengths, which have been interpreted in terms of ‘clear layers’ and optical interference, can be much more satisfactorily explained by alternative theories (based on gloss reduction at particular wavelengths due to surface micro-defects, and gloss enhancement at higher PVC's due to refractive index effects).Some implications of contraction are considered. Because erosion has been assumed in all visualisations of the weathering process previously published, sketches illustrating the effect of a number of variables on the contraction model are included.