Page 34 - Plastics News June 2019

FEATURES

A Processor's Most Important Job- Part 11

Michael Sepe
It’s the processors job to ensure molded parts contain enough stabilizer to perform to the expectations of the
end use

he final aspect of processing that is designed to ensure to protect the polymer from degradation that can occur
Tgood retention of mechanical properties involves during processing and in use.
taking good care of the additive package. All commercial The exact amounts of these additives will depend upon
polymers contain an additive package, a combination of the polymer being stabilized and the applications that the
compounds designed to provide the stability needed to material is intended for. Polypropylene and polyethylene
allow melt processing to occur without degrading the are good examples of materials that require protection
polymer. Resins may also contain additives provided to from oxidation. Generally, PE is more resistant to
enhance end-use performance. The constituents in the oxidation than PP and therefore requires lower loadings
additive package can vary from compound to compound,
of the additives. But all commercial PE and PP materials
require some minimum level just to get through melt
processing. If after the part is molded it is expected to
withstand elevated temperatures while in application,
a higher level of stabilization will be needed to ensure
that the product does not fail. For PP, these types of
high-temperature applications can be anything from
coffee-maker components to tanks for holding automobile
engine coolant.
The material supplier will incorporate a certain amount of
the stabilization package in the material, making this the
starting point for that material. It is now the processor’s
job to handle the material during processing so that the
molded part contains enough stabilizer to perform to
the expectations of the end user. The molding process
will always consume some amount of the antioxidant
and may include UV stabilizers, nucleating agents, in the material. This is expected. But the amount of
antistatic agents, antimicrobials, colorants and flame stabilizer that is consumed will depend upon the process
retardants. But at minimum they are likely to include conditions, most notably the melt temperature and the
some level of lubrication and some degree of stabilization time the material spends in the molten state. Lower melt
against thermal and oxidative degradation. It is this temperatures and shorter residence times will produce
stabilizer package that we are most interested in for parts that retain a higher level of stabilization, and these
purposes of this discussion. parts will be more capable of handling the application
environment. If melt temperatures become elevated or
Antioxidants, as these compounds are generally referred residence times become extended, the molded part will
to, are incorporated in a material for two purposes: to be less capable of managing the application demands.
allow the material to survive the high temperatures and
shear stresses of the molding process; and to provide In the lab we can measure the stability of a material
stability that may be needed so that the part can fulfill against oxidation. Using an analytical technique known as
its application requirements. Often these functions are differential scanning calorimetry (DSC), we can expose a
delegated to two different sets of chemicals, classified sample of material to a constant elevated temperature in
as primary and secondary antioxidants. The chemistry the presence of oxygen and determine the time it takes
of these materials is fascinating and complex. But when for the material to undergo oxidation. The measurement
all is said and done, the purpose of these substances is is known as the oxidation induction time (OIT) and the

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