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Service lives of coatings and their prediction.

The service life of a coating is the period of time over which it has acceptable appearance and supplies adequate protection while in its service environment. Service life affects sustainability and is one factor used in a life cycle assessment calculating the environmental and economic impact of a coating over its lifetime (see "Coatings Clinic" in CoatingsTech, 12 (1) 52 (January 2015). One of the reasons for the current interest in service life is the realization that the price of a can of paint may not tell us very much about the cost of using it. A cheap coating with a short service life may well be far more expensive, environmentally and economically speaking, than a coating that lasts a long time. I remember when whitewash was applied every year to exteriors of farm buildings and many other structures. I doubt whether that currently is an economical practice, just considering labor costs. Perhaps it never was.

I think that our industry always has been concerned about service life. However, for many years, formulators could depend on individual or corporate experience to provide guidance on which coatings technologies would provide adequate service lives in specific applications under normal service conditions. The last 40 years or so have seen great changes in the situation. The need for coatings with considerably reduced levels of volatile organic compounds and little or no hazardous chemicals has led to rapid development of products based on new chemistries and alternative technologies such as high solids solventborne, waterborne, powder, and radiation cure. Unfortunately, we rarely have the experience or data to tell how these new coatings will hold up and what sorts of failure modes will occur. In addition, we do not have time to do the 5- to 10-year outdoor exposures that might allow us to sell a new formulation with confidence or make us decide to cancel that product. Accelerated testing is faster, but does not replicate outdoor weathering and often provides erroneous information on lifetimes and failure mechanisms. Natural weathering does not even replicate itself. We cannot depend on weather testing alone to enable us to identify coatings that will be sufficiently long-lived.

A number of investigators have been working on identifying key failure modes in specific coatings and developing ways of making meaningful service life predictions. Many of the techniques that are being used involve analysis of coating surfaces (often on a nanoscale) to follow chemical and physical degradation with weathering (natural, accelerated, or just UV). Other methods follow changes in electrical properties with exposure to corrosive environments. The tools used are considerably more sophisticated than fences and weathering cabinets. They include light microscopy; scanning electron microscopy (SEM); atomic force microscopy (AFM); electrochemical impedance spectroscopy (EIS); laser scanning confocal microscopy (LSCM); Fourier transform IR microscopy (FTIR); nanoindentation; and measurement of gloss/surface roughness, contact angles, and fracture toughness. Computerized databases make it possible to compare coatings, identify failure modes, determine the most meaningful tests, and ultimately, to decide on the best formulations.

In my reading in preparation for this article, I found two schools of thought that I want to mention. Service life research begun by Martin and Nguyen has been going on for over 20 years at the National Institute of Standards and Technology (NIST). Researchers there have long advocated reliability-based methodologies instead of traditional weathering. Croll and Hinderliter at North Dakota State University have been interested in this area for some time as well. They studied failure modes and analysis techniques, carried out Monte Carlo simulations of property changes, validated them with measured data, and showed how statistical analysis can lead to service life predictions. Here are a few selected publications:

(1.) Martin, J.W., Saunders, S.C., Floyd, F.L., and Wineburg, J.P., "Methodologies for Predicting the Service Lives of Coatings Systems," FSCT, Blue Bell, PA, 1996. See b96005.pdf.

(2.) Martin, J.W. "A Systems Approach to the Service Life Prediction Problem for Coatings Systems." See fire.

(3.) Gu, X., et al., "Advance Techniques for Nanocharacterization of Polymeric Coating Surfaces," J. Coat. Technol. Res., 1 (3), 191 (2004).

(4.) Nguyen, T, et al., "Degradation Modes of Crosslinked Coatings Exposed to Photolytic Environment,"! Coat. Technol. Res., 10 (1), 1-14 (2013).

(5.) Hinderliter, B.R. and Croll, S.G., "Simulations of Nanoscale and Macroscopic Property Changes on Coatings with Weathering," J. Coat. Technol. Res., 3 (3), 203-2012 (2003).

(6.) Croll, S.G., Hinderliter, B.R., and Liu, S., "Statistical Approaches for Predicting Weathering Degradation and Service Life," Prog. Org. Coat., 55, 75-87 (2006).

(7.) Croll, S.G. and Hinderliter, B.R., "Estimating Service Lifetimes in Weathering: An Optimistic View," J. Coat. Technol. Res., 4 (3), 217-230 (2007).

(8.) Croll, S.G., "Applications and Limitations of Current Understanding to Model Failure Modes in Coatings," J. Coat. Technol. Res., 10 (1), 15-27 (2013).

By Clifford K. Schoff, Schoff Associates
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Title Annotation:Coatings Clinic
Author:Schoff, Clifford K.
Publication:JCT CoatingsTech
Date:Feb 1, 2015
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