Page 35 - Plastics News June 2019

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technique was first used to help polymer scientists retained an OIT of 100 min, and parts that had been
understand the premature failure of wire insulation in exposed to a prolonged elevated temperature routine
the 1960s. The accompanying graph shows a result from were at 60-70 min. As we move in the direction of using
this test. The sudden change in the slope of the plot is more post-consumer recycled material, it will become
associated with the onset of oxidation. This raw material increasingly important that we consider the life cycle of
can withstand 77 min of exposure at these conditions the initial product as well as the types of products that
before the antioxidant package is consumed and the we plan to make from the recycled material. It may be
material rapidly fails. necessary to reconstitute the stabilizers when producing
the regenerated raw material.
If this test is run on parts molded from this material,
the result can be used to determine how the process It has taken us some time to review all the aspects of
influenced the long-term stability of the material. In processing that relate to maintaining the integrity of the
practice, we have observed molded parts that retain as polymer. All of these are factors that are not part of most
much as 75% of the OIT of the raw material. and we have quality plans associated with molding parts. Processors
seen parts that preserve as little as 10% of this baseline are typically focused on making parts to print at a cycle
value. The difference between these two outcomes is time that allows them to make a profit. The molecular
related to the thermal history of the process. And the weight, polymer structure, level of internal stress, and
lifetime of the part in the field will be governed to a integrity of the reinforcements and additives are seldom
significant extent by the level of oxidative stability that considered to be part of the job.
the part has when it goes into use.
Many processors may not even be aware of their role
This has implications for the use of regrind. Regrind can in achieving optimal results in these areas. There are
be expected to exhibit some level of depletion in the some interesting common strategies in doing so. For
antioxidant package. The degree to which this occurs will example, molecular weight and additive retention are
depend upon the process conditions. Often, I am asked both optimized by running materials at the lower end
how much regrind should be used or how many times of the melt-temperature range. Maximizing crystallinity
a material can undergo melt processing. The answer is and minimizing molded-in stress are both achieved by
always that it depends upon the thermal stresses the running higher mold temperatures. Once the processor
material was exposed to when it passed through the becomes aware of their role in these matters, they can use
molding process previously. This depletion can also occur this understanding to inform their process-development
while the part is in use. strategies. The world of plastic products will be a better
place for having gone through the learning process.
Recently, I reviewed an analysis of a material that had
an OIT of 140 min in pellet form. The as-molded parts (With this the 11 part series concludes.)

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35 June 2019 Plastics News