Weathering of coatings - formulation and evaluation

• Published in: Progress in Organic Coatings 1989-07, Vol.17 (2), p.107-114
• Main Author: Pappas, S.Peter
• Format: Article
• Language: English
• Subjects: Applied sciences; Coatings. Paints, varnishes and inks; Exact sciences and technology; Film formation and curing, properties, testing; Polymer industry, paints, wood
• ISSN: 0300-9440; 1873-331X
• DOI: 10.1016/0033-0655(89)80017-2

The weathering of coatings refers to changes occurring during exterior exposure, which are generally undesirable and include changes in mechanical properties, such as modulus and strength, loss of adhesion, discoloration, embrittlement, chalking, loss of gloss, acid-spotting, etc. The most common processes leading to degradation are photo-initiated oxidation and hydrolysis, resulting from unavoidable exposure to light, air and water. Furthermore, inter-relationships between these processes have been reported for acrylic-melamine coatings in studies which provide evidence for enhanced photo-oxidative degradation in high humidity as well as enhanced hydrolytic degradation during light exposure. Hydrolytic degradation is also expected to be enhanced by exposure to acid (e.g. acid rain). Other atmospheric degradants include ozone, and the oxides of nitrogen and sulfur. Changes in temperature and humidity may result in cracking arising from expansion and contraction of coatings. The rate at which these processes occur varies widely depending on the exposure site(s), time of year, coating composition and substrate. Consequently, weatherability is ideally evaluated in the field of performance. However, it is obvious that such evaluation could not serve as the basis for on-going studies directed toward improving weatherability owing to the unacceptably long periods of time required. Alternatively, one may utilize early detection or accelerated weathering methods. Nevertheless, careful records of field performance have indisputable long-term value.