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- 2010 (11) (entfernen)
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- Dichtungen (3)
- Elastomere (3)
- Tieftemperaturverhalten (3)
- Dynamic mechanical analysis (2)
- Compression set (1)
- Dielectric relaxation (1)
- Elastomer (1)
- Ferroelectric polymer (1)
- Low temperature behaviour (1)
- P(VDF-HFP) (1)
Eingeladener Vortrag
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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.
Die Messung des dynamischen Speichermoduls mit der Dynamisch-Mechanischen-Analyse ermöglicht einen schnellen Überblick über das temperaturabhängige Verhalten von elastomeren Dichtungsmaterialien. Ein direkter Rückschluss von der so ermittelten Glasübergangstemperatur Tg auf die Versagenstemperatur einer Dichtung ist aber ohne zusätzliche Messungen bisher nicht möglich.
Mit den Ergebnissen des Druckverformungsrestes DVR und des DVRDMA ist eine ge-nauere Beschreibung der Tieftemperatureigenschaften und dadurch der Funktion von elastomeren Dichtungsmaterialien möglich. Messungen ergeben beispielsweise, dass sich das EPDM bei einer Temperatur von -40 °C laut der Dynamisch Mechanischen Analyse noch im gummielastischen Bereich befindet. Die DVRDMA-Werte zeigen eine Rückstellung um mehr als 40 % der Ausgangsdeformation in wenigen Minuten.
Mit derart detailliert bestimmten Eigenschaften lassen sich Rückschlüsse auf einsatzspezifische Anforderungen wie z. B. die Dichtwirkung in Abhängigkeit von Betriebstemperatur und mechanischer Belastungsgeschwindigkeit ziehen, so dass je nach Einsatzbedingungen für die Dichtungen eine geeignete Materialauswahl getrof-fen werden kann.
A new method for the evaluation of the low temperature properties of rubber materials is presented. The method emulates the standardized compression set measurement, which is frequently used for sealing materials, but can be performed within a considerably shorter time. The results are compared with the standard test and found to be qualitatively the same. Slight differences are discussed on the basis of the differences in the measurement procedures. Further data evaluation is done by fitting functions to describe the material behaviour.
The temperature dependences of dielectric permittivity and elastic modulus of poly(vinylidene fluoride - hexafluoropropylene) (P(VDF-HFP)) are compared and explained by use of structural data obtained by differential scanning calorimetry and X-ray analysis. Special emphasis is put on the effect of uniaxial stretching which renders the polymer ferroelectric. It is shown that dielectric and mechanical relaxations at the glass transition (aa relaxation) are closely related and not significantly affected by stretching. It is further confirmed that stretching destroys the non-polar spherulitic crystalline structure and produces relatively small-sized polar crystallites. This manifests in the disappearence of the so-called ac relaxation which is associated to molecular motions within the spherulitic crystalline lamellae. However, there are discrepancies between dielectric and mechanical losses above the glass transition which point towards a still not described structural transition.