© 2006 by Taylor & Francis Group, LLC
© 2006 by Taylor & Francis Group, LLC
© 2006 by Taylor & Francis Group, LLC
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6<br />
Weathering and Aging<br />
of Paint<br />
This chapter presents a brief overview of the major mechanisms that cause aging,<br />
and subsequent failure, of organic coatings. Even the best organic coatings, properly<br />
applied to compatible substrates, eventually age when exposed to weather, losing<br />
their ability to protect the metal.<br />
In real-life environments, the aging process that leads to coating failure can<br />
generally be described as follows:<br />
1. Weakening of the coating <strong>by</strong> significant amounts of bond breakage within<br />
the polymer matrix. Such bond breakage may be caused chemically (e.g.,<br />
through hydrolysis reactions, oxidation, or free-radical reactions) or<br />
mechanically (e.g., through freeze–thaw cycling, which leads to alternating<br />
tensile and compressive stresses in the coating).<br />
2. Overall barrier properties may be decreased as bonds are broken in the<br />
polymeric backbone — in other words, as transportation of water, oxygen,<br />
and ions through the coating increases. The polymeric network may be<br />
plasticized <strong>by</strong> absorbed water, which softens it and makes it more vulnerable<br />
to mechanical damages. The coating may begin to lose small,<br />
water-soluble components, causing further damage. Flaws such as microcracks<br />
develop or, if preexisting, are enlarged in the coating.<br />
3. Even more transportation of water, oxygen, and ions through the coating.<br />
4. Deterioration of coating-metal adhesion at this interface.<br />
5. Development of an aqueous phase at the coating/metal interface.<br />
6. Activation of the metal surface for the anodic and cathodic reactions.<br />
7. Corrosion and delamination of the coating.<br />
Many factors can contribute in various degrees to coating degradation, such as:<br />
• Ultraviolet (UV) radiation<br />
• Water and moisture uptake<br />
• Elevated temperatures<br />
• Chemical damage (e.g., from pollutants)<br />
• Thermal changes<br />
• Molecular and singlet oxygen<br />
• Ozone<br />
• Abrasion or other mechanical stresses<br />
The major weathering stresses that cause degradation of organic coatings are the<br />
first four in the list above: UV radiation, moisture, heat, and chemical damage. And,<br />
<strong>©</strong> <strong>2006</strong> <strong>by</strong> <strong>Taylor</strong> & <strong>Francis</strong> <strong>Group</strong>, <strong>LLC</strong><br />
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