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In
previous columns I have discussed some of the ways in which plastics
are stabilized in order to minimize the degradation that occurs during
processing and end-use. A key aspect in the development and application
of a stabilized material, is gauging the effectiveness of the
stabilization package. Many plastic articles, such as outdoor siding,
plastic pipes, and telecommunication cables have useful
lifetimes that are measured in decades. Predicting service lifetimes
has
been a major challenge for the plastics industry. Not only do we need
methods
to predict these lifetimes, but we also need ways to compare and
evaluate
new materials or formulations that are in the development stages. It is
clear
that accelerated aging tests must be used to measure the stability of
these
systems.
The type of accelerated test used depends on the nature of the
environment that the end-use part is exposed to. One of the most
important environmental stresses seen by polymers is high temperature
exposure in the presence of oxygen, also known as thermoxidative (TO)
degradation. The general method for accelerating the TO degradation is
to expose the material to elevated temperatures in either an air or
pure oxygen environment. Forced air ovens are often used for this
purpose. The test specimens may consist of either molded tensile bars,
plaques, films, or fibers. The extent of degradation can be assessed
via changes in appearance (color, gloss, gross embrittlement), or a
reduction in a physical property such as strength, elongation, or
impact. For polyolefins containing a standard hindered phenol/phosphite
formulation plus a thioester (added for long term thermal stability),
there is often a
long induction time when no apparent changes are seen in either
appearance or physical properties. At some point rapid catastrophic
degradation occurs when the additive package is depleted, and the
sample literally crumbles into
pieces.
In order to predict performance at lower temperatures, the oven aging
is done at several different elevated temperatures. An Arrhenius plot
is then constructed where the time to embrittlement (on a log scale) is
plotted vs the inverse of the absolute temperature. If a single
chemical process controls the rate of degradation, this plot will often
be linear, and the extrapolated degradation time at the temperature of
interest can then be determined. A common problem with this technique,
however, is that the degradation mechanism at elevated temperatures may
be different from the mechanism at lower temperatures. This is
particularly true if hindered amine light stabilizers (HALS) are being
compared to thioesters such as DSTDP for their effectiveness as long
term
heat stabilizers. Below about 125 oC, HALS are generally superior
relative to thioesters. Most accelerated oven aging of polypropylene is
performed at
temperatures in the range of 140 - 160 oC, where thioesters are
superior. If testing is done at this higher temperature range, and the
results are extrapolated
to an end-use temperature of 70 oC, a thioester based formulation will
appear
to be much better than a HALS formulation, even though testing at 100 -
125
oC would show a clear superiority for the HALS! Unfortunately,
the
exposure times needed to observe degradation are much longer at the
lower test temperatures, so expediency often wins out over technical
merit. The most meaningful results are obtained by testing at
temperatures as close as
possible to that expected for the end-use part. Although we would like
to
run tests involving the least amount of extrapolation, this sometimes
means
that the testing time could be longer than our careers in the job.
An even more extreme example of test acceleration is the use of the
Oxygen Induction Time (OIT) test to measure stability. Here a small
sample (a few mg) is placed in a DSC (Differential Scanning
Calorimeter) at a temperature of about 200 oC with pure oxygen, and the
time to degradation, as measured by the evolution of heat due to sample
oxidation, is measured. Typical OIT values range from a few minutes up
to about 90 minutes. This test is used to qualify polymers for fiber
optic cables (to sheath the fiber optic strand) which are expected to
have service lifetimes in excess of 40 years. The validity of this
test, which is done in the molten state, is very questionable, and it
has been shown to give far different lifetime predictions compared to
the
oven aging of solid specimens as discussed above.
In
this brief discussion we have only considered thermoxidative aging, and
we have not talked about the issues involved in accelerated UV testing
to predict outdoor performance. A future article will deal with these
issues. If you would like more information about accelerated testing,
and the influence of
different additives on long term performance, please feel free to
contact me at pjacoby@mayzo.com
or at 770-449-9066, ext. 14.
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