The reliable prediction of the service lifetime of tunnel constructions is of essential technical and economic interest. In this context polymers used for tunnel liners have to fulfill the requirement of 100 years lifetime. However, today no reliably proven test method is available which confirms such long lifetimes of polymers. The current paper introduces a recently started research project which focus on lifetime prediction of polyethylene (PE) for tunnel geomembranes. Different PE grades in combination with different antioxidant packages are conditioned at elevated temperatures and different media. Initial results confirmed significant material aging already after exposure up to 16 weeks, at least at the highest exposure temperature of 80°C. To consider application relevant material properties the material characterization was done by means of tensile tests and dynamic oxidation induction temperature tests. High pressure liquid chromatography was used for further investigation of antioxidant consumption kinetics.
For a reliable service of modern tunnel constructions a sustainable proofing against water intake is essential for the lifetime of the structure. Geomembranes made of polymeric materials have been used successfully for tunnel lining since several decades. Whereas previous geomembranes usually were made of plasticized polyvinylchloride (PVC-P), due to several technical advantages today polyethylene (PE) has become a more and more popular material for such applications. For modern geomembranes, current guidelines expect minimum lifetimes of at least 100 years (ÖBV-Richtlinie 2012). In contrast to this requirement it must be considered that today no test methodology is available which reliably confirms such long lifetimes of polymeric materials. Beside geomembranes also different other types of polymers like injection resins are used in tunnel constructions. Basically these materials have to meet the same requirements in terms of guaranteed lifetime.
In 2014 a three-year research project was initiated at the Polymer Competence Center Leoben (PCCL) GmbH. It focuses on the subject of long-term lifetime prediction of polymers for tunnel constructions under the consideration of tunnel relevant exposure conditions. The characteristics and kinetics of aging mechanisms in relevant polymers are investigated. The generated data should increase the understanding of responsible degradation mechanisms in the investigated materials and should support the development of material compounds with optimized aging resistance. A critical challenge in long-term lifetime predictions is the extrapolation of long lifetimes (e.g. to guarantee 100 years) based on relatively short testing times (< 1 year). Hence, one main intention of the project is the development of alternative but accelerated testing methods to allow an application oriented material characterization within shorter testing times.
The current paper gives an introduction about the main targets and the strategy of the above described research project. The main focus will be put on the thermo-oxidative material aging of PE grades for tunnel geomembranes. Based on first intermediate experimental results the observed aging mechanisms will be presented and discussed.
