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- Durability (3)
- Geosynthetics (3)
- Oxidation (3)
- Oxidative resistance (3)
- Long-term behavior (2)
- Long-term testing (2)
- Test methods (2)
- Antioxidant (1)
- Antioxidant depletion (1)
- Creep behavior (1)
Hydrolytic degradation and functional stability of a segmented shape memory poly(ester urethane)
(2009)
In order to understand the effects of water and hydrolytic ageing on semi-crystalline poly(ester urethane) and its shape memory functionality, water immersion experiments at elevated temperature have been performed on a model substance and various parameters were monitored: change of the melting/crystallisation temperatures, substantial increase in crystallinity, temperature dependence of the water diffusion coefficient and solubility, hydrogen-bonding index and phase mixing by peak deconvolution of the FT-IR carbonyl region and day-to-day tensile and thermo-mechanical cyclic tensile tests. A rising fraction of freezable water agglomerates in the polymer was found for specimens cooled from the immersion temperature. The degradation process could be divided into three phases: an induction phase, a phase of continuous degradation and a phase of accelerated degradation. Shape recovery remains fairly constant during phase one and decreases slowly during phase two. The increase in crystallinity in phase two is accompanied by an increase in shape fixing ability.
Funktionsdauer von Kunststoff-Dränelementen für Oberflächenabdichtungen von Deponien und Altlasten
(2007)
Geosynthetic clay liners (GCLs) often have a sandwich-like multilayer structure, e.g. bentonite encased between two geotextile layers connected by fibers or yarns, either by needle-punching or stitch-bonding. Therefore, the internal shear strength of the GCL depends on the strength of reinforcing fiber bundles or yarns and their anchoring strength in the cover and carrier geotextiles. When used on long and steep slopes and covered with thick soil layers, the GCL is permanently exposed to a combined action of compressive and shear stress. Such load conditions are characteristic for landfill covers and the slope stability of the overall cover system in the long run strongly depends on the long-term internal shear strength of the GCL. A new test method was developed to study this long-term shear behavior. The focus was not only on creep, as it is normally done, but on aging effects. The shear test devices allow the measurement of creep curves and times-to-failure at elevated temperatures in different media (tap water and de-ionized water). In this publication, the main findings of the experiments on needle-punched GCLs with and without thermal treatment are summarized. Tap water as a test medium was essential to ensure sodium to calcium ion exchange in the bentonite layer. Under this condition extremely long test durations without failure were achieved. Sliding failure occurred when de-ionized water was used. Two failure modes were observed: brittle failure of the GCLs with thermal treatment and slow disentanglement of fiber bundles for untreated GCLs. Short-term shear strength (e.g. peel strength) is unrelated to the actual long-term shear strength, i.e. to the times-to-failure achieved in long-term shear strength test. Hence, short-term shear strength alone will not provide reliable dimensioning data for product design and choice of resins. Therefore, the often suggested approach, namely, restriction to short-term tests only and application of factors of safety, is challenged by these results.
Long-term water flow capacity of geosynthetic drains and structural stability of their drain cores
(2008)
The German Federal Institute for Materials Research and Testing (BAM) has established a testing guideline to assess the suitability of geosynthetic or geocomposite drain (GCD) elements for final landfill cover systems. In an earlier paper the uniform procedure according to the BAM guideline for determining long-term water flow capacity was described, and the structural stability of drain cores under creep was investigated. The extrapolation of creep curves is permissible only when it can be shown, over the extrapolated period (at least 100 years), that oxidative aging does not invoke any relevant changes in the polyolefin material. This can be shown by measuring antioxidant depletion and mechanical degradation of the drain cores and filter/protection geotextiles in oven aging and water immersion tests. In this paper the results of such aging studies on three different GCD products are reported. Degradation of the tensile strength of HALS-stabilized polypropylene needle-punched nonwoven geotextiles (PP NWGTs) starts right from the beginning of the oven aging. This has to be taken into account for lifetime predictions based on residual tensile strength. The rate of oxidative degradation of NWGTs decreases significantly with increasing mass per area. Therefore the expected service life sets a limit on acceptable geotextile grammage. Antioxidant depletion in the drain cores, which are made of relatively thick PP strands or HDPE ribs, is about an order of magnitude slower than in filter and protection NWGTs, which are made of thin fibers. Service life values in excess of 100 years may be obtained for the products when the geotextiles and drain cores are properly stabilized and designed.