Comparison of organic coating accelerated tests and natural weathering considering meteorological data

The protective properties of organic coatings for outdoor applications are generally evaluated by means of accelerated laboratory tests, including electrochemical techniques. The coatings are stressed by different mechanical, chemical, thermal loads and the effects on the protective properties can be measured by using well established electrochemical techniques, like electrochemical impedance spectroscopy, electrochemical noise, etc. An open question is how these accelerated tests can be correlated with natural exposure in different environments. It is well known that many papers did not find any significant correlation between the natural exposure and different laboratory tests. This fact is due to many reasons, but one aspect very important is that the natural exposure is generally not sufficiently defined. The time of exposure (and the season) in a natural site is not sufficient to define the weathering, being the meteorological conditions variables considering different years. In this paper we proposed a new different approach. The starting point is a very simple consideration. We can try to define the natural weathering by considering a few different environmental parameters affecting the organic coatings properties: total amount of energy coming from UV radiation, time with relative humidity higher than a defined threshold (causing surface water condensation), environmental temperature higher that the polymer T, temperature range, etc.). In this way, by carefully monitoring these parameters, it is interesting to verify if a correlation exists with some artificial accelerated tests (UV exposure, thermal cycling, salt spray chamber, etc. This paper is a first attempt to apply this approach to a polyester coil coating for outdoor application. We considered different times of natural exposure in Trento (alpine region, north of Italy), monitoring at the same time all the meteorological parameters. The same coatings were tested in the laboratory using electrochemical impedance spectroscopy, in order to quantify the damage, after some different accelerated tests: thermal cycling, salt spray chamber and UV exposures. The results were compared with the impedance data measured after natural exposure and a correlation was investigated between the sum of the "natural stresses" and the sum of the "artificial stresses". The first results are very interesting and encouraging future work using the same approach on different materials and comparing several natural sites of exposure. (C) 2007 Elsevier B.V. All rights reserved.