Optimization of Curing Conditions for a
Chemical Resistant Tank Coating with the
Help of Dynamic Mechanical Analysis
BY GARD REIAN
R&D CHEMIST
JOTUN AS - MARINE & PROTECTIVE COATINGS
Currently, when the necessary curing conditions
for a coating are to be found, the coating is
applied to test panels, which are then cured at
different temperatures and for different periods of time.
The different panels are then immersed in water and
other chemicals of interest, at different temperatures.
These experiments are time consuming to say the least,
as one might have to wait up to a year to get adequate
results. By using DMA to investigate the glass transition state ( Tg) of the coating one can, with reasonable certainty, decide the needed curing conditions for a coating to
be used at a given working temperature.
In addition to the pure temperature factor, it is widely
known that postcuring of epoxy paints at elevated temperatures most often will improve a films chemical resistance. This can be explained theoretically, as elevated
temperatures increase the reaction rate of curing. The
molecules “trapped” at low temperatures can move more
freely and therefore react more easily. This will result in
a “tighter” polymer matrix and a tighter polymer matrix
means less diffusion through the film. Diffusion of molecules through the film is often a problem, especially with
small molecules like methanol and water. A tighter net
will for epoxy films often result in a harder, and less flexible, film. This “tightening” of the polymer network can
be measured by calculating the crosslink density using
results from DMA.
It is also important to know how low we can go on initial curing temperature. Is curing the film for two weeks
at 5°C tantamount to curing for two weeks at 23°C as
long as one post cures the coating at a given elevated
temperature? To know this would save one a lot of time
required for testing.
DYNAMIC MECHANICAL ANALYSIS THEORY
Dynamic mechanical analysis (DMA) is a method involving application of an oscillating force to a sample and
analysis of the material’s response to that force.[1] At
right Figure 1 shows how by applying stress to a film
results in a material response (strain) and a phase lag
between the applied stress and resulting strain.
Definitions of dynamic properties are given below in terms
of maximum oscillatory strain (ε0), maximum resulting
stress (σ0) and phase lag (δ) between strain and stress.
σ0 cos δ
[1] Storage modulus = = E′
ε0
σ0 sin δ
[2] Loss modulus = = E′′
ε0
E′
[ 3] Loss tangent = = tan δ
E′′
Where E′ is the storage modulus, E′′ is the loss modulus and tan δ is the loss tangent. Increase in this ratio
relates to a harder and (often) more brittle polymer.
The crosslink density (Mc) is usually calculated from
the minimum value of storage modulus E′ (also called the
Figure 1
The oscillating force applied to the sample is most commonly
sinusoidal as shown here. By measuring the amplitude of the
deformation (material response) curve and the phase difference
between applied stress and material response (strain), quantities like modulus, damping and Tg can be measured.
