Influence of natural and artificial weathering on aesthetic and protective properties of organic coatings

Abstract

One of the most common strategy to protect metallic substrates from corrosion is the use of organic coatings. The accurate prediction of long-term performances of this protective system is a very important issue for the coating industry. Outdoor exposure of organic coatings can produce the loss of their aesthetic and anticorrosion properties because of weathering conditions: sunlight, temperature, water, oxygen and pollutants. Coating service life is generally evaluated by using laboratory tests simulating in accelerated way the natural conditions (temperature, UV radiation, moisture, condensation, pollution). It is well known that it is difficult to find a clear correlation between natural exposure and artificial weathering. The time of exposure in a natural site is not sufficient to define the weathering conditions; it is important to consider different environmental parameters affecting the organic coating properties (energy from UV radiation, wet time, temperature). In this work the change of aesthetic and protective properties of organic coatings was investigated. Different systems were studied: epoxy-polyester and polyester coatings crosslinked with different hardeners. The coatings have distinct surface finishes and colours with particular aesthetic effects. The samples were exposed in Trento area (alpine region, north of Italy), monitoring all meteorological parameters. The same samples were artificially weathered by a cyclic test consisting of 72 h of UV exposure (ASTM G154 UVA, 340 nm) and 72 h of salt spray chamber exposure (ASTM B117, 5% NaCl, 35°C, 100% relative humi­dity). The aesthetic degradation was evaluated by gloss and colour variation measurements; the surface chemical changes were studied by FTIR analysis. Environmental scanning electron microscopy was used to obtain some information about morphological characteristics of the exposed surfaces. The degradation of protective properties was studied by electrochemical impedance spectroscopy. Different results have been obtained for aesthetic and protective properties. Concerning gloss and colour variations, a good relationship was found between artificial and natural tests. When considering the protective properties, laboratory test results were more aggressive due to the salt spray environment in which the samples have been exposed.