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Ferroelektrete sind zelluläre polarisierte Polymere, die ferroelektrische und dadurch auch piezoelektrische Eigenschaften aufweisen. Unter anderem werden sie für den Bau von Mikrofonen und Ultraschallwandlern für Materialprüfung eingesetzt. Dabei erweist sich ihre besonders niedrige akustische Impedanz als Vorteil. Ursache dafür ist vor allem ihr außergewöhnlich niedriger Elastizitätsmodul in Dickenrichtung. In diesem Beitrag werden zwei Methoden für die Ermittlung des Elastizitätsmoduls des geladenen zellulären Polypropylens (zPP) dargestellt. Die erste ist eine neuartige modellgestützte Methode mittels Laservibrometrie an dem eigenentwickelten Polypropylen-Wandler, wobei die Resonanzschwingung des Wandlers bei 250 kHz gemessen wird.
Die zweite Methode ist dynamisch-mechanische Analyse (DMA) durchgeführt mit Frequenzen von 0,3 bis 50 Hz, erstmalig für das zPP angewendet.
Das Fließ-Härtungs-Verhalten von Duroplastformmassen wird gegenwärtig nur
unvollständig durch empirische Größen beschrieben. Diese ungenauen und
unsicheren Angaben können zu Schwierigkeiten bei der optimalen Einstellung
der Fertigungsmaschinen des Verarbeiters führen. Insbesondere Veränderungen
der Formmasseeigenschaften während des Transportes und der Lagerung stören
die Produktion. Das vorgestellte Messverfahren mit Ultraschall-sensoren ermöglicht
die schnelle und vollständige Erfassung des Fließ-Härtungs-Verhaltens, auch
während des technologischen Prozesses. Experimentelle Ergebnisse zu
Alterungs-erscheinungen dieser Formmassen illustrieren den Nutzen des
Verfahrens für Formmasse-produzenten und -verarbeiter.
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.
Rubbers are widely used as main sealing materials for containers for low and intermediate level radioactive waste and as additional component to metal seals in spent fuel and high active waste containers. The save encapsulation of the radioactive container inventory has to be guaranteed according to legislation and appropriate guidelines for long term storage periods as well as down to temperatures of -40 °C during transport.
Therefore the understanding of failure mechanisms that lead to leakage at low temperatures is of high importance.
It is known that the material properties of rubbers are strongly temperature dependent. At low temperatures this is caused by the rubber-glass transition (abbr. glass transition). During continuous cooling the material changes from rubber-like entropy-elastic to stiff energy-elastic behaviour, that allows nearly no strain or retraction, due to the glass transition. Hence rubbers are normally used above their glass transition but the minimum working temperature limit is not defined precisely, what can cause problems during application. Therefore the lower operation temperature limit of rubber seals should be determined in dependence of the material properties.
The results of Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) are combined with the results of standardized measurements as the compression set according to ISO 815. To reduce the test time of the standard tests a faster technique was developed.
Additionally, the breakdown temperature of the sealing function of complete O-ring seals is measured in a component test setup to compare it with the results of the other tests. The experimental setup is capable of measuring the leakage rate at low temperatures by the pressure rise method.
A fluorocarbon rubber (FKM) was selected for this investigation as it is often used for radioactive waste containers. Some materials (seals and test sheets) were purchased from a commercial seal producer and some materials were compounded and cured at BAM in form of rubber sheets.
Rubbers are widely used as main sealing materials for containers for low and intermediate level radioactive waste and as additional component to metal seals in spent fuel and high active waste containers. According to appropriate guidelines and regulations safe enclosure of the radioactive container contents has to be guaranteed for long storage periods as well as down to temperatures of -40 °C for transportation. Therefore the understanding of seal behavior is of high importance.
In this paper we focus on the behavior of elastomer seals at low temperatures with regard to poten-tial decrease of leak-tightness. In addition, changes in material properties due to aging effects over long periods of time and their influence on the seal performance is investigated.
It is known that material properties of rubbers are strongly temperature dependent. At low tempera-tures this is caused by the rubber-glass transition (abbr. glass transition). During continuous cool-ing, the material changes from rubber-like entropy-elastic to stiff energy-elastic behavior, that al-lows nearly no strain or retraction due to the glass transition. Hence rubbers are normally used above their glass transition but the minimum working temperature limit is not defined precisely.
Aging of elastomer seals is important, as possible dynamic loads may have to be considered during the whole interim storage period (so far approved in Germany for up to 40 years) and for transporta-tion after storage.
For the investigations, fluorocarbon (FKM) and ethylene-propylene-diene (EPDM) rubbers were selected as they are often used in radioactive waste containers. Some materials were purchased from a commercial seal producer and some materials were compounded and cured at BAM. The elastom-ers where studied by several thermo-analytical methods and compression set to characterize the material behavior at low temperatures. Additionally component tests were performed to determine the breakdown temperature of the sealing function of complete elastomer O-rings.