DRYING AND CURING OF PAINT FILMS
During the drying/curing process a paint changes from a liquid into a solid. It does this by
various mechanisms and combinations of mechanisms. The time it takes to undergo this
physical change is governed by several factors including temperature. Generally three terms arc
used to refer to drying/curing temperatures.
a) Air Drying
This refers to normal ambient temperatures.
b) Forced Drying
When heat is needed to effect a cure or accelerate the reaction it is called forced drying, but the
temperature range for forced drying is ambient to 65°c.
c) Stoving
When temperatures above 65°c are used, using ovens or infra red, the term used is stoving.
Industrial paints, with a few exceptions e.g. intumescents, are generally in the air Drying
category, and the liquid to solid transition is dependant on one of the four drying mechanisms as
follows.
Solvent Evaporation
Paints employing this drying mechanism are linear polymer materials, sometimes referred to as
solution polymers. Solution polymers dissolve in the solvent, when the paint is applied the
solvent evaporates away allowing the fully formed linear polymers, saturated, with no activity
points, to come out of solution and form a film on the substrate. The polymers lie in a random
interlocking pattern, similar to cooked spaghetti or noodles and loosely bond together by "
secondary Hydrogen bonds". The solvents used by these materials are strong solvents and,
when reapplied onto the paints, easily penetrate between the polymers and split the secondary
bond, allowing the polymer to go back into solution. Materials, which can do this are, called
reversible or non-convertible. Chlorinated rubber, vinyl's, acrylics, cellulosic materials and
laquers fall into this category.
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2 Oxidation
Paints using this mechanism fOffi1 a film by "oxidative cross linking" (polymerisation) using
atmospheric oxygen, and in some cases, the oxygen contained in the driers. First of all if a
solvent is present, the solvent evaporates away, allowing the oxidation to begin. Oxygen then
combines with the unsaturated bonds on the fatty acid esters, progressively linking them
together, to form the film. Once the oxygen has reacted with the binder, it has changed the
chemical structure of the binder and cannot be removed. These materials are therefore
convertible or non-reversible. Because oxygen is in abundance in the atmosphere the reactions
continue, ad infinitum, until the materials crack and peel, having fOffi1ed a very complex cross-
linked matrix. Alkyds, Phenolics, natural oils and resins are materials from this category.
3 Chemical Curing
Chemical curing paints need addition of a second material, (in some cases as in moisture curing,
water from the atmosphere) but generally the second material, the activator, is supplied in a can,
hence the teffi1 2 pack or Multi Component Liquid. In order to obtain the desired film the whole
of the contents of both cans should be thoroughly mixed together and instructions on the
materials data sheet should be strictly observed. Some materials will require an induction period
and most data sheets will state the 'pot life'.
An induction period is "The length of time after mixing which the paint should stand before
use". Induction time is also called stand time or lead time, and is recommended to allow
thorough wetting of the solids. During the induction period the chemical reaction will
commence and will be either: -
a) An exotheffi1ic reaction. Giving offheat, the container will waffi1 up
b) An endothermic reaction. Taking in heat, the container will cool forming condensation.
A typical induction period is 20 - 30 minutes.
Pot life is the period of time after mixing in which the paint must be used, and with industrial
paints, dependant on temperature is usually 6 - 8 hours. After the recommended pot life the
material becomes very user unfriendly and if in bulk, is quite often subject to spontaneous
combustion.
2 pack materials curing agents
Amides - Epoxy curing agents, usually quote seven days to full cross linking at 20°C.
Amines - Epoxy curing agents, three days to full cross linking at 20°c.
Isocyanates - Mainly used for urethanes but also for some epoxies where low temperature
application is unavoidable, _10°c being typical. Ambient temperature urethanes, especially for
pipeline use quote 16 hours to full cure.
NB. Isocyanates are very toxic and need great care during use.
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Chemically curing materials are convertible or non-reversible.
4 Coalescence
Coalescence means to physically join together. In an emulsion the resin droplets are dispersed
in the continuous phase, water. Upon application the water evaporates away allowing the resin
droplets to come close together until they are touching. At this stage small amounts of high
boiling point solvents are concentrated in the voids between the spheres, from where they
migrate into the spheres, plasticise them and allow them to fuse together. In so doing they also
reduce the Tg of the material (Tg = Gloss Transition and is the temperature at which the material
changes from a rubbery to a glossy solid and vice versa). If the Tg wasn't changed, the resulting
mm would stay as a liquid and be easily wiped away.
These materials e.g. acrylics and vinyl's are reversible. It is important to remember in this case
that water is not a solvent, but if the true hydrocarbon solvent was used the material would form
a solution.
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