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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.
Oxidative long time resistance is essential for many polyolefin based products, especially if repair or exchange is not possible as encountered in many building and geotechnical applications. Current regulations and recommendations for geosynthetics cover mainly the needs of a maximum lifetime of 25 years, though many applications require longer lifetimes up to 100 years and more. Reliable testing of the long time oxidation resistance is complicated by the complex interplay of physicochemical processes and reactions in combination with the need of relatively short testing durations, usually not exceeding 12 months. Thus for conventional atmospheric oven testing, the use of temperatures higher than 100 °C is inevitable, making conclusive Arrhenius extrapolations often questionable.
Practical assessments of oxidative durability consist of exposures under accelerated ageing conditions with subsequent characterization of exposed material samples with respect to their residual stabilization and/or their mechanical properties.
The autoclave test combines exposures at elevated temperatures with higher oxygen pressures and is performed with material samples immersed in a well defined aqueous medium representing critical environments in geosynthetic applications and including extraction of additives. Under these conditions meaningful results can be obtained within reasonable testing durations at only moderately elevated temperatures up to 80 °C.
Application of different physicochemical methods, such as tensile testing, OIT /HP-OIT and analysis of residual antioxidant content (ICOT), in order to determine the time dependent degradation behaviour, reveal valuable information about mechanisms and effectiveness of the used antioxidants. Corresponding results for exposures at different temperatures and oxygen pressures are the basis for a 3D-extrapolation in order to estimate the expected service life. Different empirical models are presented and discussed with respect to their practical impact.
The impact fracture behavior of two common high-density polyethylene grades for container applications were intensively studied by the instrumented Charpy impact test after well-defined exposure to UV-irradiation. Individual stages of the impact event, such as crack initiation and crack propagation energy as well as maximum impact load, were investigated from the recorded load–deflection curves. UV-induced material property changes were further investigated by infrared spectroscopy, differential scanning calorimetry, and dynamic-mechanical analysis as well as density measurements. Based on the results of the Charpy impact test, three indicators were identified to describe the extend of photooxidation on high-density polyethylene: (a) a reduced Charpy impact strength—at least to half of its initial value for a distinctly brittle impact fracture, (b) a marked decrease in the crack propagation contribution to the impact strength, and (c) an increase of the brittle features of the fracture surface.
The physical and chemical effects of diesel and biodiesel fuels on two high-density polyethylene (PE-HD) types were investigated. Both semi-crystalline PE-HD are common thermoplastic materials for container and storage tank applications. Biodiesel, a composition of unsaturated fatty acid esters from renewable resources, was chosen as it is regarded a possible green alternative to fossil fuels. The study aims at identifying significant differences between biodiesel and conventional diesel fuels based on the differences in the chemical nature of the two. The physical effects of the fuels on the polymer at first comprises the sorption behavior, i.e. kinetics and final equilibrium concentration. Not only are both fuels absorbed by the amorphous phase of the semi-crystalline PE-HD, they also induce a plasticization effect that modifies the molecular mobility and therefore also the characteristic yielding properties, manifest in the obtained stress-strain curves. The chemical effects related to degradation phenomena is investigated by a long-term storage scenario using partially immersed tensile test specimens in diesel and biodiesel. We were able to confirm the proposed co-oxidation mechanism by Richaud et al. for polyethylene-unsaturated penetrant systems on a larger scale based on practical tensile tests. One of the investigated polyethylene grades subjected to tensile drawing showed a significant loss of plastic deformation and the onset of premature failure after 150 days of storage in biodiesel. Further biodiesel storage showed a systematically reduced elongation at break before necking. None of these effects were observed in diesel. Oxidation of fuels and polymer after progressing storage times were analyzed by the evolution of carbonyl species in FT-IR/ATR spectroscopy.