© 2006 by Taylor & Francis Group, LLC

Composition of the Anticorrosion Coating 25
2.2.5.5 Darkness Degradation
Byrnes notes an interesting phenomenon in some alkyds: if left in the dark for a
long time, they become soft and sticky. This reaction is most commonly seen in
alkyds with high linseed oil content [18]. The reason why light is necessary for
maintaining the cured film is not clear.
2.2.6 CHLORINATED RUBBER
Chlorinated rubber is commonly used for its barrier properties. It has very low
moisture vapor transmission rates and also performs well under immersion conditions.
General characteristics of these coatings are:
• Very good water and vapor barrier properties
• Good chemical resistance but poor solvent resistance
• Poor heat resistance
• Comparatively high levels of VOCs [1,19]
• Excellent adhesion to steel [19]
Chlorinated rubber coatings have been more popular in Europe than in North
America. In both markets, however, they are disappearing due to increasing pressure
to eliminate VOCs.
2.2.6.1 Chemistry
The chemistry of chlorinated rubber resin is simple: polyisoprene rubber is chlorinated
to a very high content, approximately 65% [19]. It is then dissolved in solvents,
typically a mixture of aromatics and aliphatics, such as xylene or VM&P naphtha
[19]. Because of the high molecular weight of the polymers used, large amounts of
solvent are needed. Chlorinated rubber coatings have low solids contents, in the 15%
to 25% (vol/vol) range.
Chlorinated rubber coatings are not crosslinked; the resin undergoes no chemical
reaction during cure [1]; they are cured by solvent evaporation; in effect, the film
is formed by precipitation. However, the chlorine on the rubber molecule undergoes
hydrogen bonding. The tight bonding of these secondary forces gives the coating
very low moisture and oxygen transmission properties.
Because the film is formed by precipitation, chlorinated rubber coatings are very
vulnerable to attack by the solvents used in their formulation and have poor resistance
to nearly all other solvents. They are also vulnerable to attach by organic carboxylic
acids, such as acetic and formic acids [19].
2.2.6.2 Dehydrochlorination
Chlorinated rubber resins tend to undergo dehydrochlorination; that is, a hydrogen
atom on one segment of the polymer molecule joins with a chlorine atom on an
adjacent segment to form hydrogen chloride. When they split off from the polymer
molecule, a double bond forms in their place. In the presence of heat and light, this
© 2006 by Taylor & Francis Group, LLC