A New Method for Testing and Evaluating the Long-Time Resistance to Oxidation of Polyolefinic Products

Abstract

Oxidative long time resistance is essential for many polyolefinic products, especially if repair or exchange is not possible as encountered in many building applications. For the time being, CE-standards for geosynthetics are covering the needs of a minimum lifetime of 25 years only, though many applications require longer lifetimes (up to 100 years and more). Testing long time oxidation resistance is complicated by the complex interplay of physico-chemical processes and reactions in combination with the need for relatively short testing durations, usually not exceeding 12 months. Thus, for normal atmospheric oven testing the use of temperatures clearly higher than 80 °C is inevitable. It should be noted that Arrhenius extrapolation exceeding the temperature range of experimental data by more than 20 K requires the consideration of additional safety factors in order to allow for possible changes of the apparent activation energy EA. In general, at elevated testing temperatures care has to be taken that oxygen diffusion does not become the rate limiting process. Furthermore, in many application environments the loss of antioxidants (and other components) is - besides oxidative deactivation - mainly due to extraction by aqueous media and not due to evaporation. For oven testing, however, frequently the latter is the case. With these limiting conditions in mind a test has been designed which in addition to elevated temperatures is accelerated by the presence of an increased oxygen pressure and a stirred aqueous medium. This enables us to perform durability tests at markedly lower temperatures with reasonable testing durations and significantly reduced diffusion limited oxidation (DLO) effects. Also it allows for physical and chemical impacts of aqueous media. Assessments of oxidative durability can be performed by autoclave immersion exposures at three different temperatures (usually 60, 70 and 80 °C) and 50 bar oxygen pressure and additionally at 80 °C and two different oxygen pressures (usually 10 and 20 bar). Results of such accelerated tests are evaluated by means of modified Arrhenius equations with the application of a 3D-regression analysis. Validation of the method is still at an early stage, like for most other lifetime test methods. For purpose of comparison, materials of approved durability were used, such as pipe materials of quality PE100 and PE 80 with well-known minimum lifetime determined according to ISO/TR 9010, i.e. by long-term failure tests under internal hydrostatic pressure at different temperatures and by evaluation using the Arrhenius law. Based on results of such tests it is supposed that a rather quick and relevant method is now at hand for testing the oxidation life of olefinic polymeric materials (like pressed plaques) or specimens of finished products like textiles, sheets, liners, pipes or composites. This test method should be very useful for the effective design of polymer formulations.

Life prediction of the oxidation of geogrids by three different methods

“…The values of A, B and C are similar to those quoted by Schröder et al (2008) for a polypropylene geosynthetic product analysed using the above formula: ln A ¼ 30.4, B ¼ À12045, C ¼ 8.36, allowing for their A being the natural logarithm of our A. BAM, Berlin, Germany performed tests on the same material using the same procedure, from which they derived the parameters ln A ¼ 28.8, B ¼ À11435, C ¼ 7.92, and the durations corresponding to the three conditions used here were 11.6, 20.0 and 73.6 days respectively (BAM, personal communication). These results are comparable.…”

“…If the activation energy was to decrease further at lower temperatures, as suggested by Schröder et al (2008), then all three graphs in Fig. 8 would acquire a lower gradient but remain as far apart.…”

“…where t 50 is the time to 50% retained strength in days, P is pressure in bar and T is temperature in K (Schröder et al, 2008). We refer to this later as the 'linear' formula.…”

“…This method also promotes leaching of antioxidants. The dry method has been described by Hsuan and Li (2005) and the wet method by Schröder et al (2000Schröder et al ( , 2001Schröder et al ( , 2002Schröder et al ( , 2008.…”

“…Since, however, in many applications geosynthetics are intended to function for 100 years or more, there is great interest in extending the validity of these tests to longer lifetimes. Schröder et al (2008) have performed the wet autoclave test over a range of pressures and temperatures and used a modified Arrhenius equation to extrapolate the data to service pressures and temperatures. Current practice is testing the specimens without the application of mechanical stress.…”