Page 23 - Plastics News August 2020

Annealing Tips for Semicrystalline Polymers

For these polymers, annealing is done to establish a rate of crystallization is not constant across the entire
level of crystallinity that cannot be practically temperature range between Tg and Tm (melting point.
obtained within the parameters of a normal molding In many polymers, crystals form most quickly at a
cycle. Here's some guidance on setting annealing time temperature approximately midway between these two
and temperature. extremes. Therefore, to achieve the most efficient rate
Mike Sepe of crystallization in PPS, we would use a mold
temperature of 205 C (401 F). This is a more challenging
Annealing of amorphous polymers is typically performed
mold temperature to maintain, and the difference in
to reduce the internal stress in a part below the levels mechanical properties between a part produced at this
achievable during the molding process. However, in higher mold temperature and one produced at the lower
semicrystalline polymers the objective of annealing is to
mold temperature is relatively small. Therefore, the
establish a level of crystallinity that cannot be typical practice is to use the lower mold temperature.
practically obtained within the parameters of a normal
molding cycle. Each semicrystalline polymer has the However, if the molded part will need to operate at 200
ability to crystallize to a certain extent that depends C, exposure to this application temperature will produce
upon the chemical structure of the polymer chain. HDPE additional crystallization while the product is in use. We
know that as materials crystallize, they shrink. So, a part
has a flexible, streamlined chain that allows for efficient
crystallization to a very high percentage, while a that goes into the field molded to the proper dimensions
material like PEEK attains a modest level of crystallinity and is then exposed to very high application
even under the most carefully controlled process temperatures may change size while in use. If this
conditions. dimensional change creates a functional problem for the
product, then it is necessary to stabilize the dimensions
Optimum levels of crystallinity enhance a wide range of of the part before it goes into use. This is done through
properties that include strength, modulus, creep and annealing. The target annealing temperature is often
fatigue resistance, and dimensional stability. This last the midpoint between Tg and Tm. Lower temperatures
property is very important in applications where very
will require a longer annealing time.
tight tolerances must be maintained in parts that will be
used at elevated temperatures. Crystallization is
controlled by cooling rate and occurs at a rapid rate
during the fabrication process. To achieve what is
considered to be an optimal level of crystallization, the
temperature of the mold must be maintained above the
glass-transition temperature of the polymer. This
promotes a level of molecular mobility that allows
crystals to form.
Crystallization can only occur in the temperature
window below the crystalline melting point and above
the glass-transition temperature (Tg). Consider PPS as
an example. The melting point of PPS is 280 C (536 F)
while the Tg is approximately 130 C (266 F) when
determined from a particular dynamic mechanical
property. Therefore, the guideline for setting the mold
temperature to ensure proper crystallization is a
minimum of 135 C (275 F). Processors that pay attention
to this requirement will typically select mold
temperatures of 135-150 C (275-302 F). But even when
this parameter is properly controlled, the relatively
rapid rate of cooling involved in melt processing and the
limited time that the part spends in the mold will limit
the achievement of the crystalline structure to about
90% of what is theoretically obtainable.
In amorphous polymers the annealing temperature
One determining factor in selecting an annealing needs to approach the Tg of the polymer. However, to
temperature is the maximum temperature to which the produce the desired result when annealing a
part will be exposed in application. We know that the
semicrystalline material, the annealing temperature

August 2020 21 Plastics News