MATERIALS | ACCELERATED TESTING
estimating the corresponding failure times requires a high level of experience,” says Hahn. Similar to other ageing tests, there is a trade-off in test speed and correlation to real-world conditions. “The lower the temperature and pressure, the longer the test duration (and higher the costs), but the closer the test is to real life,” she adds. Hahn says that while this test is not yet widely used in industry, SKZ is continuing to evaluate it and to run long-term tests to correlate it to other ageing tests. For one product, for example, the lab has HPAT data and data for several years of heat ageing in an oven at different temperatures that correlate. “Due to the increased oxygen pressure, and
Figure 1: Results of a High Pressure Autoclave Test (HPAT) carried out on PE at SKZ showing extrapolation to failure at ambient temperature Source: SKZ
assessing long-term use of polyolefins, where oxidation is a primary degradation mechanism, according to Wilma Hahn, Project Manager at the plastics institute, in a presentation given at AMI’s Compounding World Expo in Essen in September. Although in real-world conditions other mecha- nisms besides oxidation may be at work, the accelerated test can be used to estimate service life, she suggests. SKZ’s laboratory conducts various tests on plastic
parts, with durability tests including methods to measure resistance to UV, chemicals/leaching, and oxidation. The HPAT tests, conducted according to EN 1348:2005, are performed at elevated tempera- ture and oxygen pressure and in aqueous medium. SKZ performs the tests at different oxygen pressures and temperatures and prolongs the ageing until the material degrades. Product standards that require HPAT tests include pipes and geosynthetics. Hahn says that the lab has tested other products as well, such as roofing sheets. To date, the SKZ laboratory has primarily tested
PE products using HPAT. Typical test conditions cover temperatures of 60 to 95°C and 0.6 to 51 bar oxygen, with sodium bicarbonate (pH 10) as the media. Degradation is accelerated by the higher temperature and the availability of oxygen. As shown in Figure 1, tests were run at 80°C and several pressures, with the “service life” measured at the point where the sample had 75% residual stress at break (meaning the material had lost 25% of its strength). The service life could then be extrapolated to the real-world condition of atmos- pheric pressure. Additional tests allowed extrapola- tion to real-world temperatures. “Choosing appropriate ageing conditions and
62 COMPOUNDING WORLD | December 2021
therefore oxygen availability, ageing is accelerated and temperatures can be reduced,” says Hahn. “A life-time estimation for the application temperature can mostly be completed within less than one year and often leads to service lives of >100 years at 40°C. A minimum life-time of 100 years is very often required for polymeric products within construction sectors. Using conventional thermo- oxidative ageing methods, the life-time estimation for these well-advanced products leads to much longer test durations compared to HPAT.”
Circular thinking Assessing the degradation behaviour of polymers and its impact on the service life of materials is crucial in the circular economy, says Arjen Boersma, Senior Scientist at TNO, a contract research organisation based in The Netherlands, in a presentation at AMI’s Compounding World Expo in Germany. “In the circular approach, a polymer should last a long time, but it should remain suitable for recycling and not emit microplastics when it degrades during use,” he said. Boersma described how accelerated ageing tests can be monitored with analytical techniques, including infrared spectroscopy to assess chemical changes, indentation hardness to assess surface stiffness, and tensile or flex modulus to assess bulk stiffness. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) can also be used to evaluate changes in polymer structure, crystallinity and stability. As an example, he described a method for
predicting the service life of the polymer encapsu- lation in buried power cables. The cables were exposed to high pressures and a range of high temperatures in salt water, and the material properties were evaluated at different times. Exposure to high temperatures up to 140°C can be used to predict behaviour at ambient tempera- tures; above 140°C, other phenomena (for exam-
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