Glass Transition Temperature as Function of Post Cure
Temperature
A plot of Tg versus post-cure temperature gives some
very interesting curves as seen below in Figure 6.
Figure 6
This graph shows the effect of post curing temperature on the
glass transition temperature, Tg. The paint film was first cured for
two weeks at 5°C, 10°C, 23°C and 40°C, respectively. This was followed by a five day post-cure at 23°C, 50°C, 60°C and 80°C,
respectively. Each curve represents an initial cure temperature.
As seen from Figure 6 the range between 50°C and
60°C is critical with a leap in the nearly linear Tg curve.
Although hard to prove, a critical chemical change is
probably the reason for this break in linearity. One
hypothesis is that at temperatures above 60°C we start
to see the effects of some homopolymerization between
epoxy groups, causing a jump in Tg values. But then
again, this is a hypothesis that needs to be studied further before any conclusion is drawn. We can, however, use
what is seen here to argue that increasing post-cure temperature from 50°C to 60°C or above will make the coating more chemical resistant.
Figure 7
This figure shows how Tg and tan δare affected by varying post
cure times and temperatures.
Effect of Curing Time on Glass Transition Temperature
Figure 7 shows how variation in post curing time affects Tg.
It is of great value to know how long one has to post cure for
the film to reach an optimal condition. For this specific coating a Tg of around 100°C is a satisfactory result.
As the desired Tg for this coating is around 100°C we see
from Figure 7 that post-curing for as long as five days is
hardly necessary, neither at 60°C nor at 80°C. For a 60°C
post-cure two days is suffcient and for an 80°C post-cure no
more than one day is necessary to reach the desired Tg. The
results are the same independent of the initial cure temperatures tested ( 5°C, 10°C, 23°C and 40°C).
Crosslink Density
The molecular crosslink density (Mc) can be calculated
using equation [ 4] on page 28. This method can be used to
compare results within the same set relative to each other.
Looking at the results (see Table 1 on page 29), one can
see that the molecular crosslink density of the polymer
0
film, M , decreases when the post-curing temperature is
c
increased from 23°C to 50°C. Increasing the curing temper-
0
ature above this, however, does not seem to affect M rad-
c
ically.
Glass Transition Temperature and Chemical/Corrosive
Resistance
As mentioned in the introduction tank coatings needs to be
thoroughly tested in different chemicals and corrosive environments before a real-life product can be launched. Some
of these tests are summarized in Table 2 on page 32.
In addition to chemical testing the paints are also subjected to accelerated corrosion tests including hot water
testing, salt spray (ASTM B 117), prohesion (ASTM G
85), continuous condensation (ISO 6270), cathodic disbondment (ASTM G8, ASTM G42) and seawater immersion. The plates tested in salt spray, prohesion and seawater are scribed to see how well the paint system can
handle damage in the film.
As seen from Table 2 the 60°C post-cured coating does
not differ much from the coating cured at 23°C. A few
exceptions are seen with acetic acid, ethanol diamine,
tetrahydrofuran, cathodic disbondment and condensation, in which the 23°C cured coating shows some blistering while the post-cured coating is performing better.
It should be noted that the panels in these tests were
post cured using hot air and that one might get completely different results with a hot water or hot oil cure.
Hot Air Post Curing Versus Hot Liquid Post Curing
In practice a tank coating is often post cured using a hot cargo
instead of or in addition to hot air. It was therefore of interest
to use DMA to study some paint cured using hot liquid instead
of hot air. The results are given in Table 3 on page 32.
The results given in Table 3 are very interesting, but their
interpretation is not straight-forward. The Tg is about a factor of 7°C- 14°C higher for hot water post cure than for hot
air post cure. And the crosslink density roughly double.
