© 2006 by Taylor & Francis Group, LLC

Weathering and Aging of Paint 103
Ideally, selection of binders that absorb little or no UV radiation should minimize
the potential damage from this source. In reality, however, even paints based on
these binders can prove vulnerable because other components — both those intentionally
added and those that were not — often compromise the coating as a whole.
Components that can be said to have been added intentionally are, of course,
pigments and various types of additives: antiskinning, antibacterial, emulsifying,
colloid-stabilizing, flash-rust preventing, flow-controlling, thickening, viscositycontrolling,
additives, ad infinitum. Examples of unintentional components are catalysts
or monomer residues left over from the polymer processing; these may include
groups that are highly reactive in the presence of UV radiation, such as ketones and
peroxides. Interestingly, impurities can sometimes show a beneficial effect. When
studying waterborne acrylics, Allen and colleagues [12] have found that low levels
of certain comonomers reduced the rate of hydroperoxidation. The researchers speculate
that the styrene comonomer reduced the unzipping reaction that the UV
otherwise would cause.
6.2 MOISTURE
Moisture (water or water vapor) can come from several sources, including water
vapor in the surrounding air, rain, and condensation as temperatures drop at night.
Paint films constantly absorb and desorb water to maintain equilibrium with the
amount of moisture in the environment. Water is practically always present in the
coating. In a study of epoxy, chlorinated rubber, alkyd and linseed oil paints,
Lindqvist [13] found that even in stagnant air at 25˚C and 20% relative humidity
(RH), the smallest equilibrium amount of water measured was 0.04 wt %.
Water or water vapor is taken up by the coating as a whole through pores and
microcracks; the binder itself also absorbs moisture. Water uptake is not at all
homogeneous; it enters the film in several different ways and can accumulate in
various places [13, 14]. Within the polymer phase, water molecules can be randomly
distributed or aggregate into clusters, can create a watery interstice between binder
and pigment particle, can exist in pores and voids in the paint film, and can accumulate
at the metal-coating interface. Once corrosion has begun, water can exist in
blisters or in corrosion products at the coating-metal interface.
Water molecules can exist within the polymer phase because polymers generally
contain polar groups that chemisorb water molecules. The chemisorbed molecules
can be viewed as bound to the polymer because the energy for chemisorption (10 to
100 kcal/mole) is similar to that required for chemical bonding. The locked, chemisorbed
molecule can be the center for a water cluster to form within the polymer
phase [13].
When water clusters form in voids or defects in the film, they can behave as
fillers, stiffening the film and causing a higher modulus than when the film is dry.
Funke and colleagues [14] concluded that moisture in the film can have seemingly
contradictory effects on the coating’s mechanical properties because several different
— and sometimes opposite — phenomena are simultaneously occurring.
Two of the most important parameters of water permeation are solubility and
diffusion. Solubility is the maximum amount of water that can be present in the
© 2006 by Taylor & Francis Group, LLC