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- Gamma irradiation (1)
- HMW-PE (1)
- Neutron radiation shielding material (1)
- UHMW-PE (1)
Radiation induced structural changes of HMW-PE and UHMW-PE as detected by thermo-analytical methods
(2009)
The special properties of high molecular weight polyethylene (HMW-PE) and ultra high molecular
weight polyethylene (UHMW-PE) result basically from their extreme chain lengths and their high
degree of crystallinity. As high-performance polymers, they are used for a variety of applications.
UHMW-PE in particular is often utilized for endoprothesis (due to its excellent slip and wear
properties) and due to its high hydrogen content as a neutron moderator in casks for storage and
transport of radioactive materials.
To prepare the material for instance for its use as a total joint replacement, it is exposed to radiation
for several reasons, such as sterilization and crosslinking, leading to partial improvement of the
mechanical properties (e.g. fracture toughness, crack propagation resistance, wear resistance) and
better chemical stability.
To be applicable for long term radiation shielding purposes for instance over a period of 40 years,
PE has to withstand any type of degradation affecting safety relevant aspects.
The scope of our investigation comprises an estimation of the radiation impact on the molecular and
supra molecular structure of two types of PE and to what extent these changes are detectable by
thermo-analytical (TA) methods, such as Differential Scanning Calorimetry (DSC), Thermo
Mechanical Analysis (TMA), Dynamic Mechanical Analysis (DMA) and Thermo Gravimetric
Analysis (TGA). Additionally FT-IR spectroscopy as well as density and gas sorption
measurements were carried out.
Due to the poor solubility of HMW-PE and UHMW-PE, some classical analytical techniques are
not applicable. But TA-methods represent a feasible approach to detect structural and
morphological features of these materials as well as changes caused by external influences, such as
thermal treatment and/or irradiation. With the combination of the applied TA-techniques it is
possible to distinguish between crosslinking and degradation.
Due to their extreme high hydrogen contents, high molecular weight (HMW-) and ultra-high molecular weight (UHMW-) polyethylene (PE) are a comprehensible choice as neutron radiation shielding material in casks for storage and transport of radioactive materials. But as a direct consequence of inserting radioactive material in such casks, gamma radiation occurs. Hence, the impact of gamma radiation on the molecular structure of polyethylene has to be taken into consideration. Consequently, PE has to withstand any type of gamma radiation induced degradation affecting safety relevant aspects in order to be applicable for long term neutron radiation shielding purposes during the whole storage period (in Germany, for instance, up to 40 years). The scope of our investigation comprises an estimation of the impact of gamma radiation and temperature on the molecular and supra molecular structure of the two types of PE used as neutron radiation shielding cask components. A further point which is worth exploring is to what extent these changes are detectable by conventional analysis methods. Therefore, thermoanalytical measurements were performed such as differential scanning calorimetry (DSC), thermo mechanical analysis (TMA), dynamic mechanical analysis (DMA), and thermo gravimetric analysis (TGA). Additionally optical and weighing methods were applied. With those methods it is possible to detect structural changes in polyethylene induced by exposure to gamma radiation. The observed amounts of changes of the irradiated material are not safety relevant for the application of polyethylene as neutron radiation shielding material; moreover, some properties actually improve via irradiation.