© 2006 by Taylor & Francis Group, LLC

100 Corrosion Control Through Organic Coatings
of course, interactions between these stresses are to be expected; for example, as
the polymeric backbone of a coating is slowly being broken down by UV light, the
coating’s barrier properties can be expected to worsen. Ranby and Rabek [1] have
shown that under UV stress, polyurethanes react with oxygen to form hydroperoxides
and that this reaction is accelerated by water. Another example is the temperaturecondensation
interaction. Elevated temperatures by themselves can damage a polymer;
however, they can also create condensation problems, for example, if high daytime
temperatures are followed by cool nights. These day/night (diurnal) variations in
temperature determine how much condensation occurs, as the morning air warms
up faster than the steel.
Various polymers, and, therefore, coating types, react differently to changes in
one or more of these weathering stresses. In order to predict the service life of a
coating in a particular application, therefore, one must know not only the environment
— average time of wetness, amounts of airborne contaminants, UV exposure,
and so on — but also how these weathering stresses affect the particular polymer [2].
6.1 UV BREAKDOWN
Sunlight is the worst enemy of paint. It is usually associated with aesthetic changes,
such as yellowing, color change or loss, chalking, gloss reduction, and lowered
distinctness of image. More important than the aesthetic changes, however, is the
chemical breakdown and worsened mechanical properties caused by sunlight. The
range of potential damage is enormous [3-7] and includes:
• Embrittlement
• Increased hardness
• Increased internal stress
• Generation of polar groups at the surface, leading to increased surface
wettability and hydrophilicity
• Changed solubility and crosslink density
In terms of coating performance, this translates into alligatoring, checking,
crazing, and cracking; decreased permeation barrier properties; loss of film thickness;
and delamination from the substrate or underlying coating layer.
All the damage described above is created by the UV component of sunlight. UV
light is a form of energy. When this extra energy is absorbed by a chemical compound,
it makes bonds and break bonds. Visible light does not contain the energy required to
break the carbon–carbon and carbon–hydrogen bonds most commonly found on the
surface of a cured coating. However, just outside of the visible range light in the
wavelength range of 285 to 390 nm contains considerably more energy, commonly
enough to break bonds and damage a coating. The 285 to 390 nm range causes almost
all weathering-induced paint failure down at ground level [4]. At the short end of the
UV range, we find the most destructive radiation. The damage caused by short-wave
radiation is limited, though, to the topmost surface layers of the coating. Longer wave
UV radiation penetrates the film more deeply, but causes less damage [8-10]. This
leads to an inhomogeneity in the coating, where the top surface can be more highly
© 2006 by Taylor & Francis Group, LLC