Page 24 - Plastics News August 2020

must exceed the Tg of the polymer. The time required that form as the material cools from the melt.
will depend upon the part wall thickness, as is the case Consequently, they do not have the same properties and
for amorphous polymers. But the other factor that they do not impart the same benefits to the overall
influences the required time will be the annealing structure of the material. Specifically, crystals that are
temperature. formed at a particular annealing temperature will melt
As mentioned above, the target annealing temperature at a temperature just a few degrees above the
is often the midpoint between Tg and Tm. Lower temperature at which they were produced. Therefore,
temperatures will require a longer annealing time. crystals that are produced at a temperature below the
maximum use temperature of the part will not survive
Another determining factor in selecting an annealing
temperature is the maximum temperature to which the that exposure and are not useful.
part will be exposed in application. If a part is annealed Because additional shrinkage during annealing of a
at 200 C but is then used at 225 C, new crystals will form semicrystalline material is inevitable, the dimensions of
at the higher use temperature that were not formed the as-molded part must be larger than the final target
during the annealing process. This will produce dimensions. This may require that parts be molded out of
additional dimensional changes that may be print so that they can meet the print once they have
problematic. Therefore, the annealing temperature gone through the annealing process. It is important,
should be equal to or slightly greater than the maximum therefore, that a relationship be established between
temperature at which the part will be used. Just as the as-molded dimensions and the annealed dimensions.
amorphous polymers cannot withstand annealing Annealing temperatures for many semicrystalline
temperatures above their Tg, semicrystalline polymers polymers are high enough to produce other effects on
cannot be annealed at temperatures that exceed their the polymer that are potentially damaging. For
crystalline melting point. example, the midpoint between the Tg and the Tm of
Annealing time is best established experimentally for a nylon 66 is 160 C (320 F). At this temperature nylon can
particular part geometry. In amorphous polymers, the rapidly oxidize. This can cause a change in the color of
test used to establish that the objective of annealing has the material, but more importantly it can result in a
been met is the solvent test that measures residual permanent loss in mechanical properties, particularly
stress in the part. In semicrystalline resins the those associated with ductility. Consequently, for
benchmark is dimensional stability. A properly annealed materials like nylons annealing is best performed either
part molded in a semicrystalline material should be able in an inert atmosphere, under vacuum, or in a fluid that
to withstand exposure to a time-temperature routine will act as an oxygen barrier and will not alter the
representative of a worst-case application environment properties of the material. For example, nylon parts can
without exhibiting an additional change in dimensions. be annealed in hot mineral oil to prevent oxidation and
A good example of this principle can be illustrated for improve heat transfer. Because mineral oil is nonpolar,
parts designed for exposure to a temperature of 85 C the nylon will not absorb the oil and no plasticizing
(185 F) for periods of up to 8 hr. An assembly produced effects will be observed.
from two component parts that had each been annealed Annealing in semicrystalline materials is ideally done in
at 70 C (158 F) for 1 hr exhibited dimensional changes order to perfect the structure of a part that has already
upon exposure to the application conditions. These been molded according to optimal procedures. However,
changes caused the parts to bind when the assembly was some processors use the annealing strategy to avoid the
operated, making it non-functional. Annealing at 110 C demands of the high mold temperatures needed to
for the same 1-hr period resulted in assemblies that properly crystallize high-performance materials such as
displayed no change in function after exposure to the PPS, PEEK, and PPA. This can bring about serious
application environment. deficiencies in part performance and significant
There is another reason for selecting an annealing difficulties with process control. In our next article we
temperature that exceeds the highest anticipated use will look at these problems more closely.
temperature. Crystals that are formed while a material
is in the solid state are not as large or as perfect as those

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August 2020 22 Plastics News