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Aufgrund ihrer einfachen und zuverlässigen Anwendung werden Elastomere für viele Dichtungsaufgaben eingesetzt. In vielen Anwendungsbereichen, wie z.B. Verkehr, Luftfahrt und Verfahrenstechnik, können während des Einsatzes tiefe Temperaturen auftreten, bei denen die Funktionsfähigkeit der Dichtung erhalten bleiben muss. Daher ist die Untersuchung des Verhaltens von Elastomerdichtungen bei tiefen Temperaturen wichtig, um ihren Temperatureinsatzbereich nach unten abzugrenzen und dadurch einen verlässlichen Betrieb zu sichern. Ziel dieser Arbeit ist es, den sicheren Einschluss von gefährlichen Gütern unter verschiedenen Umgebungsbedingungen und über lange Zeiten zu ermöglichen bzw. die Materialauswahl entsprechend zu steuern.
Non-destructive evaluation (NDE) of composites: using ultrasound to monitor the curing of composites
(2013)
Typical composite materials for lightweight construction consist of a fibre (glass or carbon) reinforced thermosetting matrix. The impregnation of the fabric is made with liquid resin – usually epoxy resin or unsaturated polyester resin are used. The main processing methods are resin transfer moulding (RTM) and prepreg technology often used in combination with autoclave curing. After shaping, the resin must be cured. The final properties of the produced parts depend on the curing degree and the final glass transition temperature. Analytical methods and the use of ultrasound for online cure monitoring in the manufacturing process are introduced. Examples for the application of ultrasonic cure monitoring for the production of composite parts for traffic systems are also covered.
To achieve the required tightness levels of Containers for low and intermediate level radioactive wastes rubbers are widely applied as main sealing materials. The save encapsulation of the radioactive Container contents has to be guaranteed according to legislation and appropriate guidelines for long storage periods as well as down to temperatures of-40 °C during transportation.
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 influenced by temperature. 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. Therefore, rubbers are normally used above their glass transition but the minimum working temperature limit is not defrned precisely, what can cause problems during application. The temperature ränge where full functionality is possible is strongly dependent on the application conditions and the material.
For this investigation mainly ethylene propylene diene (EPDM) and fluorocarbon rubbers (FKM) were selected as they are often used for radioactive waste Containers.
Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) are typically used for the determination of the temperature ränge of the glass transition process. The standardized compression set measurement according to ISO 815 is common for investigation of rubber sealing materials as the test simulates the seal behaviour after release. To reduce the test time of the Standard tests a faster technique giving the same information 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 model was developed that allows calculating the minimum working temperature limit of a seal by combining the results of the applied methods.
Rubbers are widely used as sealing material in various applications. In many fieldsthefunction of seal materials at lowtemperatures is required. Therefore the understanding of faiiure mechanisms that lead to leakage at low temperatures is of high importance. 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 results of standardized measurements as the compression set according to ISO 815. To reduce the test time of the Standard tests a faster and more efficient technique was developed and applied. In addition the breakdown temperature of the sealing function of O-ring seals at lowtemperatures is measured in a component test Setup in dependence of the material and the degree of compression.
Carbon fibre prepregs have found widespread application in lightweight constructions. They are based on a carbon-fibre fabric impregnated with reactive epoxy resin. DMA measurements under temperature conditions similar to an autoclave programme were carried out using commercially available prepreg material with a high glass transition temperature. The characteristic of the temperature programme was a dynamic heating segment at 1.5 K/min followed by a longer isothermal segment at 180 °C. The courses of the storage modulus E', loss modulus E'' and tanδ were recorded. The measuring frequency was varied between 1 Hz and 33.3 Hz. Gelation and vitrification are assigned. The influence of the measuring frequency on the time to vitrification and the correlation with DSC are discussed. The reaction does not end even after 10 h curing at 180 °C, which is interpreted as the slow cessation of the reaction caused by vitrification.
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.