rubber modulus) in the rubbery plateau. The theoretical
relation between molecular weight between two cross
linking points (Mc) and the tensile storage modulus (E′)
can be expressed as follows:[2]
[ 4] E′ =
3ρRT
Mc
Where E′ is the storage modulus, ρ is the materials
specific gravity, R is the ideal gas constant, T is temperature and Mc is the molecular crosslink density. The density of the material increases as the molecular crosslink
density value, Mc, decreases (shorter molecular chains
between crosslink points in the polymer network).
The glass transition temperature (Tg) can be found
from the storage modulus curve, the loss modulus curve
or the tan δ curve. This is shown below in Figure 2.
Figure 2
This figure shows how Tg can be found from any of the curves
storage modulus (~97°C), loss modulus (~98°C) or tan δ(~108°C).
As seen from Figure 2 the Tg can vary quite a bit (up to
11°C in this case) depending on which function is used to
decide Tg. One should therefore always keep in mind the
degree of uncertainty when it comes to deciding Tg and
always decide Tg from the same function within a set of
experiments. Also, one should take into account that the
glass transition for a polymer blend is never a set temperature but rather a temperature distribution, where Tg
is the maximum.
EXPERIMENTAL PART
Materials: The tank coating investigated here is based
on a novolac epoxy binder with an amine adduct as the
main curing agent. The formulation also contains different additives, extenders and pigments picked specifically
to improve the barrier effects of this coating.
Preparation of the Coating: The two-component coating
was applied to smooth plastic polyester films using a 250 μm
applicator. The panels were cured at different temperatures
in climatic air chambers or in hot water/hot oil baths.
Dynamic Mechanical Analysis: A DMA 2980 analyzer
from TA Instruments was used to determinate storage modulus, loss modulus and tan δ. The Tg was determined from
the peak of the tan δ curve. The samples were heated from
- 50°C to 200°C with a heating rate of 4°C/min. The preload
force was set to 0.020 N and the amplitude to 5 μm.
RESULTS AND DISCUSSION
Calculated results from the primary experiments are given
below in Table 1. All coated plates were cured at a given
temperature for 14 days and then post-cured at an elevated temperature for a variable number of days (1-5 days).
Table 1
Tg is the glass transition temperature maximum found from the
tan δcurve. E'm is the minimum storage modulus while TE'm is
the temperature at minimum storage modulus. Mc,min is the
crosslink density at E'm. The “curing conditions” column gives
the curing temperatures with number of days given in brackets.
Effect of Curing Temperature on Glass Transition
Temperature
As expected, the glass transition temperature (Tg)
increases with increasing post cure temperature. Figure 3
on page 30 shows how varying the post cure temperature
affects the tan δcurve and the Tg of the paint film.
The obvious trend seen from Figure 3 is the increase in
