of Plastics


                                     Long Term Performance of Plastics and Accelerated Aging Tests
                                                                                                   Phil Jacoby   PhD             

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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