ACCELERATED TESTING | MATERIALS
thermo-oxidative failure of plastics at a highly accelerated rate. There, in addition to (moderately) higher temperatures, increased oxygen partial pressures are used to accelerate ageing. Using an Arrhenius approach modified by the ageing factor oxygen partial pressure, statements can be made about thermo-oxidative failure under service conditions,” he says (Figure 1).
strated for a variety of plastics,” according to Dr Ruben Schlutter, Group Leader of the Component Properties Research Group at German training and research centre SKZ. “In many cases, the acceleration factors can be described via an Arrhenius relationship or the WLF relationship [an empirical equation associated with time–temperature superposition]. In recent years, many time-lapse or rapid test methods have been (further) developed and evaluated at SKZ to describe the long-term behaviour of plastics. These include the Stepped Isothermal Method (SIM) and the Stepped Stress Method (SSM) for the acceler- ated determination of long-term creep behaviour,” he says.
“In the creep test, the specimens pass through
temperature stages (SIM) or stress stages (SSM) and a creep master curve is generated from the resulting strain response. This makes it possible to predict creep behaviour over many years using measurements of one to two days,” Schlutter explains. “To predict the slow crack growth (stress crack
failure) of polyolefin materials, two new test methods have been developed, the Cracked Round Bar (CRB) test and the Strain Hardening Test (SHT), which have recently found their way into the ISO standard,” he continues. “The strain hardening modulus determined in
the SHT correlates with the failure times deter- mined in media creep tests. Instead of ageing tests in heating ovens, ageing tests in high-pressure autoclaves have been carried out to evaluate the
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UV weathering Temperature and humidity cycling is often grouped with the impact of UV exposure under the ‘weather- ing’ banner. At last year’s K Show in Düsseldorf, Atlas Material Testing Solutions — a division of Ametek that has offered weathering testing instrumentation, laboratory, and outdoor testing services for over a century — showed its latest weathering test instruments for plastic materials and products. These included the Xenotest 440, a versatile weathering instrument suitable for use with a variety of materials that features a twin-lamp test chamber using the company’s XenoLogic technology to enable faster testing. Powered by two 2,200W xenon lamps, the 440 can reach 2-sun irradiance levels for significantly shorter test times. Also on display was the high-capacity Suntest
XXL+, which meets all common international weathering standards for polymers and coatings, and the Suntest XLS+, a compact bench top instrument for weathering and colour lightfastness testing of plastics boasting a 1,170cm² test area. Both instruments have been designed for testing 3D specimens. The release of a new sealed lamp for the established Atlas Ci4000 and Ci4400 Weather- Ometers was also highlighted. The new design simplifies installation by incorporating all assembly elements – xenon-arc lamp, inner glass filter, outer
Figure 1: Using an Arrhenius approach modified by aging factor oxygen partial pressure, statements can be deduced for thermo-oxidative failure
Left: Ageing testing in high pressure autoclaves underway at SKZ
Source: SKZ September 2023 | COMPOUNDING WORLD 45
IMAGE: SKZ
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