glass transition temperature with increasing post cure
temperature. As mentioned earlier on, tan δ is the rela-
tionship between storage modulus and loss modulus (see
equation [ 3] on page 28). As post cure temperature is
increased the film becomes less flexible and the value of
storage modulus increases relative to the value of loss
modulus. Consequently, the absolute value of tan
δ
decreases with increasing post cure temperature.
Figure 3
This graph shows tan δof paint films cured at 5°C for 14 days
and post-cured in hot air at different temperatures for five days.
Below Figure 4 shows how varying initial cure temperature (using a fixed post-cure temperature) affects the
tan δ curves and the Tg of the paint film.
Figure 4
This figure shows how tan δand Tg are affected by varying the
initial cure temperatures ( 5°C, 10°C, 23° and 40°C) using a fixed
post cure temperature (60°C).
From Figure 4 one can see that as long as the sample
is post cured at an elevated temperature the initial cure
temperature is close to insignificant when it comes to
influencing Tg. Even crosslink density changes little with
the different initial cure temperatures (see Table 1 on
page 29). This tells us that we can cure this coating over
the interval 5°C - 40°C as long as we post cure at a fixed
elevated temperature. This is valuable information
indeed, as seasonal changes in air temperature at application sites are substantial, and the coating can also be
used at several different locations worldwide. One should
note that these results are based on films that are post-cured only a few weeks after application. There is a time
lag here that has to be taken into account. The desirable
reaction is, of course, the curing of epoxy with the amine
hardener. The competing reaction, between the amine
hardener, carbon dioxide and water will cause problems
over time, at least in poorly controlled humid environments. The product of this reaction is amine carbonates
(amine blush). Amine blushing is described in detail by
Rinker et al. [ 3] Due to this one has to make sure the coating is post-cured within a reasonable time limit (and at
least within three months of application).
Effect of Curing Temperature on Storage Modulus
The general observed trend is an increase in storage
modulus as the post-cure temperature is increased, as
shown below in Figure 5. This is as expected since a higher cure temperature gives a higher crosslink density and,
consequently, a less flexible film.
Figure 5
This graph shows storage modulus (E') of paint films pre-cured at 5°C
for 14 days and post-cured at different temperatures for five days.
From Figure 5, looking at the interval between 0°C and
50°C, we clearly see that the film gets less flexible and
more brittle with increasing post-cure temperature (the
storage modulus curves are shifted upwards). There is little difference in flexibility, however, between the coating
post cured at 60°C and the coating post-cured at 80°C. This
tells us that we can safely increase the post-cure temperature without having to worry too much about brittleness
and cracking of the film. Even though the coating films
post-cured at 60°C and 80°C are less flexible than the film
post-cured at 23°C, neither are what one would call a brittle coating. Especially at working temperatures above 50°C
all the films can be considered quite flexible.
