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A report is presented on the development of a test for determining the long-term oxidation resistance of polyolefin materials, which is accelerated not only by elevated temperatures but also by increased oxygen pressure and a stirred aqueous medium. The method permits durability tests to be carried out at markedly lower temperatures with reasonable testing durations and significantly reduced diffusion limited oxidation effects and allows for physical and chemical impacts of aqueous media. Oxidative durability can be assessed by autoclave immersion exposures at three different temperatures (60, 70 and 80 degrees C) and 50 bar oxygen pressure and at 80 degrees C and two different oxygen pressures (usually 10 and 20 bar). Accelerated test data are evaluated by modified Arrhenius equations with the application of a three-dimensional regression analysis. The application of the test method to various materials, including PP non-woven materials and geosynthetics is demonstrated. 10 refs.
The time-dependent sorption of biodiesel in a typical polyethylene for container applications is investigated in comparison to conventional diesel fuel at three different temperatures. In this context, the desorption behavior is also addressed. Subsequently, the effects of both penetrants on mechanical properties are characterized in terms of impact strength and dynamic-mechanical analysis. The discussion of property changes is firstly based on the sorption kinetics of biodiesel and diesel, which is determined by immersion experiments allowing for the calculation of respective diffusion coefficients. Changes in impact strength as determined by the Charpy method are further characterised in more detail by analyzing the fracture surfaces, and correlated with results of dynamic-mechanical analysis.
Atomistic simulation of gas sorption isotherms in a polymer of intrinsic microporosity (PIM-1)
(2009)
Service life of geosynthetics made of polyolefin materials is mainly limited by oxidative degradation. Durability of polyolefin products depend on formulation, morphology resulting from manufacturing, design and particularly on the antioxidants used. Although the fundamental oxidative reactions are known, the complex effect of geosynthetic characteristics and external influences mean that durability assessments require practical tests.
The assessment of oxidative durability involves exposure to accelerating conditions as well as the material characterisation with respect to the state of stabilisation and/or degradation (e.g. by tensile testing and/or oxidation induction time).
The autoclave test developed by BAM for durability assessments of polyolefin geosynthetics is based on the simultaneous application of moderately elevated temperatures (up to 80 degrees Celsius) and elevated oxygen pressures (up to 5.0 MPa) in combination with a surrounding aqueous medium. Thus this test method provides several advantages in comparison to conventional oven testing, especially with respect to test duration, impact of surrounding environment and the potential to differentiate and rank oxidative resistance between products of very different dimensions (surface/volume) and stabilisation.
In view of the increasing demand for durability assessments of geosynthetic products with lifetimes significantly exceeding 25 years, the contribution is focused on practical aspects concerning implementation and optimisation of test procedures, conditions and evaluation of results. Furthermore, advanced approaches based on multiple exposure conditions will also be included and corresponding calculations of expected service life in typical applications will be evaluated with respect to significance and repeatability. Thereby involved standards and recommendations - such as EN ISO 13438, Part C or EAG-EDT - are considered.
Whenever geosynthetics are used to reinforce or stabilise infrastructural elements such as road- and railways, bridge abutments, run- and taxiways on airports or to protect coastlines of canals and other steep slopes they may be exposed to cyclic loads. Therefore, a reduction factor for dynamic effects on the geosynthetic tensile strength has been included into the calculation of the Long Term Design Strength (LTDS). An approach to assess the behaviour of geosynthetics is to adopt Woehler- or Smith graphs and Haigh-diagrams for polymers. This is because an endurance limit for polymers, which defines an infinite life under a defined cyclic load, has not been seen so far. A huge amount of mechanical testing is needed to predict the behaviour of geosynthetics against this particular background.
To reduce these efforts while getting an indication of the cyclic effect on geosynthetics, a combined method of mechanical and chemical analysis has been developed. The pure tensile strength tests to determine the residual strength after cyclic stress has been amended by IR-spectroscopy and Differential Scanning Calorimetry (DSC). In this context the chain change model of PET has been proven as valid for cyclic loads as well, which helps to define the endurance limit of materials made out of PET. For polyolefins such as PP and PE, a limit for the maximum alternating load has been identified at the point of the stress strain graph where the actual and the numerical stress of a material diverge from each other. Cyclic tensile strength tests have been carried out in a hydraulic test frame. The test parameters have been set to simulate the service life of a railway bed trafficked by high speed trains (ICE).
