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Paper des Monats
- ja (4)
Epoxy carbon-fibre prepreg, Hexcel Type 6376 HTS, was investigated using Dynamic Mechanical Analysis (DMA). The DMA characteristic parameters are storage modulus E', loss modulus E' and loss factor tanδ. These parameters are ideally suited to observe the vitrification, referred to as glass transition, resulting from the cross-linking reaction. Detection of the cure state may also be achieved by determining the momentary glass transition temperature of partially cured samples. The consequent use of a multi-frequency measuring regime was used to derive the apparent activation energy for the glass transition process. Different temperature programs were also applied to monitor the curing process directly, as well as to investigate the different states of incomplete cure reached in preceding curing steps. The intention was to provide better understanding of the consequences of an interrupted autoclave curing process and to use DMA to detect the cure state achieved. With DMA, the continuation of an incomplete curing process also can be monitored. DMA measurements up to 300 °C showed, furthermore, that the final glass transition temperature was reduced by thermal degradation at high temperatures.
Both chain scission and crosslinking reactions occur during ageing of EPDM rubber. However, with many conventional polymer analysis methods such as hardness and DMA, it is hardly possible to obtain information about the contribution of each reaction type to the measured data. For example, hardness and Tg both increase during ageing of EPDM, indicating crosslinking during ageing, but it is not clear whether this is partly counterbalanced by chain scission reactions which would lower hardness and Tg. An indication that chain scission reactions probably counteract the hardness or Tg increase by crosslinking is given by Compression Set (CS) measurements. CS exhibits a higher change than hardness or Tg, as CS increases additively through both chain scission and crosslinking reactions. In order to elucidate the share of chain scission reactions in the total degradation, a method testing relaxation and recovery behaviour using DMA equipment was applied. The method revealed the strong influence of chain scissions, leading to more pronounced relaxation and higher residual strain after compression.
The long time-frame of the necessary safe enclosure of radioactive waste in containers gains interna-tionally an increasing attention. As well extended storage periods as a usually necessary subsequent transportation after storage have to be evaluated.
In Germany, the concept of dry interim storage in dual purpose metal casks before disposal is being pursued for spent nuclear fuel (SNF) and high level waste (HLW) management. The initially planned and established dry interim storage license duration of up to 40 years will be too short and its exten-sion will become necessary since there is no repository available in due time.
The main safety relevant components are the thick-walled dual purpose metal casks. These casks consist e.g. of a monolithic cask body with integrated neutron shielding components (polymers, e.g. polyethylene) and a monitored double lid barrier system equipped with metal and elastomer seals. The metal seals of this bolted closure system guarantee the required leak-tightness whereas the elastomer seals allow for leakage rate measurement of the metal seals.
At BAM we focus on the long-term performance of the sealing system and neutron shielding materials as their application conditions have to be concerned as rather demanding.
Therefore, BAM began to investigate the overall function and long-term behavior of elastomer and metal seals and also the irradiation influence on neutron shielding materials.
Recently a new project was initiated to expand our investigations to get more and in some context better applicable data.
This paper describes the approach of our new research project with the aim of further closure of existing knowledge gaps and presents an overview of investigations concerning the low temperature behavior of elastomer seals, of running long-term tests on elastomer seals at different temperatures under static conditions and the influence of gamma irradiation on neutron shielding materials.
Polymer Testing - Editorial
(2016)
Polymer Testing - Editorial
(2016)
The investigation of the long-term performance of sealing systems employed in containers for radioactive waste and spent nuclear fuel is one research focus area for division 3.4 “Safety of Storage Containers” at the Bundesanstalt für Materialforschung und -prüfung (BAM). Our investigations comprise investigations on metallic and elastomeric seals and covers experimental investigations to get a database on the component/material behaviour, work on analytical descriptions and numerical analysis. Our aim is to understand the long-term behaviour of the sealing systems for evaluation of their performance during possible extended interim storage and subsequent transportation.
Due to their simple and reliable application, elastomers are used for many tasks. Many fields of application like transport, aviation and process technology, entail low temperatures where proper functionality of the employed seals has to be ensured. Therefore, understanding the low temperature behavior of elastomeric seals is important for safe seal operation to restrict the temperature application range.
For all areas of application, the question of seal lifetime is also important for replacement or maintenance intervals. The lifetime is especially important in applications that allow no seal replacement. Since the seal can determine the lifetime of storage in such cases, reliable lifetime estimation is necessary for these applications, and requires an extensive knowledge of the specific aging behavior.
THz and Mid IR Fourier transform spectroscopy on polyethylene irradiated with Gamma- Co60 radiation
(2014)
THz and mid IR spectroscopy of high molecular PE (HMW) and ultra high molecular PE (UHMW) reveals modifications of the molecular structure induced by γ-Co60 radiation. UHMW-PE and HMW-PE can be distinguished. The integral absorption in the B1u THz-region can quantitatively describe the amorphization of crystallinity. The formation of trans vinylene unsaturations and the decay of vinyl can be followed by mid IR spectroscopy
Characterisation of degradation and determination of end-of-lifetime criterion for O-ring seals
(2017)
The ageing of polymers, including elastomers, is an undesirable but inevitable process leading to a limited lifetime of rubber products such as seals. Independent of the application, as e.g. automotive, piping or container applications, a long lifetime or at least a timely exchange of a seal before occurence of critical failure is desired.
In order to investigate the degradation of material properties and to determine the lifetime of elastomers used as seals, an ageing programme was started with hydrogenated acrylonitrile butadiene rubber (HNBR), ethylene propylene diene rubber (EPDM) and fluorocarbon rubber (FKM) [1-3]. Both O-ring seals (uncompressed and compressed by 25 %) with a cord diameter of 10 mm as well as sheets with a thickness of 2 mm were aged at 75 °C, 100 °C, 125 °C and 150 °C for up to two years. The changes of material properties during ageing were characterised for each material using the sheets by means of e.g. dynamic-mechanical analysis and tensile tests, while sealing properties such as compression stress relaxation (CSR), compression set (CS) and leakage rate were determined on O-rings. The experimental results indicate that while material properties, CSR and CS show considerable degradation effects, the static leakage rate stays constant or even decreases before failure occurs quite suddenly at advanced degradation levels. This reveals that static leakage rate has only limited sensitivity for the degradation of the seal material.
Our approach to determine a suitable end of lifetime criterion, which involves a partial decompression of the seal during the leakage test, is presented and discussed.
Component and material investigations on metal seals for high level radioactive waste containers
(2018)
Long-term investigations performed at BAM look to extend the state of knowledge on safety-related components of interim storage containers. Metal seals act as the primary sealing barrier in the bolted double lid closure system of the containers. The behaviour of metal seals has been investigated for ageing times up to 8.5 years and for various temperatures. The main cause for reduction in useable resilience overtime was due to creep deformation of the outer jacket of the seal.