Page 27 - Plastcs News January 2019

FeAtures

There are some important differences in the temperature- run in the hotter mold show a negligible change in
dependent behavior of these two samples that resemble apparent modulus over the tested times of 4-12 months.
what we saw in the PPA material discussed in the earlier The samples run in the cooler mold undergo very rapid
issue. changes that result in nearly an order of magnitude decline
in apparent modulus over this time frame. The vertical
First, even at room temperature there is an obvious
difference in the modulus of the two samples. The part line drawn through the creep curves represents the 2-hr
run in the cooler mold has a modulus of 10 GPa (1450 point. Even at this early stage of the tests, the differences
ksi) while the part run in the hotter mold is nearly 25% in performance as a function of the mold temperature at
stiffer. But the most important difference occurs as the which the samples were produced is evident, particularly
two samples enter the glass-transition region. The modulus for the tests conducted at 176 F.
decline begins in both samples at approximately the same
temperature. But the way these two samples pass through
this transition is very different.
The sample molded in the hotter mold exhibits a modulus
decline of a little over 50% by the time the temperature
reaches 302 F (150 C). It then establishes a new plateau
that extends to 500 F (260 C) before displaying a second
decline associated with the initial stages of crystal
melting. The part produced in the cooler mold exhibits a
much sharper decline in modulus that ultimately results
in more than a 90% reduction in the room-temperature
stiffness before the material reaches 302 F. The upward
trend in the modulus beyond this point represents
the attempt of the polymer to form the crystals that should
have been created during the molding process.
The logic behind the advice that optimal crystallinity does These results show that any suggestion that optimal
not matter if the application temperature never reaches crystallinity does not matter at lower temperatures
the glass transition may appear to make sense, since the is simply misguided thinking that arises largely from
modulus of the two samples changes in a similar fashion the failure to take into account the full spectrum of
up to a temperature of about 185 F (85 C). However, it is performance as a function of temperature, time, and
important to understand that the behavior of a material applied load.
over time when under constant or cyclic loading will
reflect the way the material responds to an increase in We have spent a lot of time discussing crystallinity and
temperature. there is a lot more that we could say. While the cooling
rate controlled by the temperature of the mold is the most
Consequently, the earlier and sharper decline of the important variable, crystallization can be influenced by
modulus in the sample produced in the cooler mold the stresses associated with pack and hold pressures, the
will be reflected in creep tests performed at the low effects of orientation, and a process known as nucleation.
temperatures leading up to the glass transition. This can Crystallization is also a process that releases heat,
be seen in Fig. 2, which shows plots of apparent modulus, lengthening the cycle time in unexpected ways. Next
sometime referred to as creep modulus, as a function of month we will close out the conversation on crystallinity
time for samples produced at both mold temperatures with a brief review of these other factors.
when tested at temperatures of 140 F (60 C) and 176 F
(80 C). The differences are striking and very important to (Mike Sepe is an independent, global materials and
the long-term integrity of the molded part. The samples processing consultant)

27 Januar y 2019 Plastics News