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- Elastomer (6)
- Compression set (5)
- DSC (5)
- Dynamic mechanical analysis (4)
- Glass transition (4)
- Low temperature behaviour (4)
- Sealing material (4)
- Curemeter (3)
- DMA (3)
- EVA (3)
- Prepreg (3)
- Ultrasound (3)
- Cross-linking (2)
- Cure process (2)
- Druckverformungsrest (2)
- Vulcanization (2)
- Acoustic propagation (1)
- Autoclave (1)
- Bauteilversuche (1)
- CFC (1)
- Carbon fibre (1)
- Component test (1)
- Composites (1)
- Cross linked polyethylene insulation (1)
- Crosslinking (1)
- Crystal melting (1)
- Curing (1)
- Degree of cure (1)
- Degree of curing (1)
- Dichtung (1)
- Dichtungen (1)
- Elastomere (1)
- Epoxy (1)
- Ethylene vinyl acetate (1)
- Gelation (1)
- Injection moulding (1)
- Klebfolie (1)
- Low temperature (1)
- Mechanical variables measurement (1)
- Natural rubber (1)
- O-Ring (1)
- O-Ringe (1)
- O-ring (1)
- On-line monitoring (1)
- Online control (1)
- Photovoltaik (1)
- Rheology (1)
- Seal (1)
- Silicone rubber (1)
- Sound velocity (1)
- TMDSC (1)
- Thermal analysis (1)
- Tiefe Temperatur (1)
- Ultrasonic (1)
- Vitrification (1)
- Vulcanisation (1)
Partial discharges may cause damage to electrical insulation of high voltage equipment. They initiate elastic waves in the insulating material, e.g. in the stress cone of an outdoor termination. Localisation of the origin of such elastic waves can help to predict serious damaging processes in the electrical insulation. In order to measure and evaluate the wave propagation effects in typical multilayered elastomeric structures, knowledge of the material properties is required. The propagating velocity and the attenuation of longitudinal waves are important parameters. Values for these quantities found in the literature were not appropriate. Therefore, for cross-linked polyethylene (XLPE) and cured liquid silicone rubber (LSR), the longitudinal wave velocity and the attenuation were evaluated in the temperature interval from -20°C to 50°C and in the frequency range from 200 kHz to 600 kHz using a two-sample ultrasound technique. The loss factor was determined from these measured quantities. Additionally, low frequency Dynamic Mechanical Thermal Analysis (DMTA) was applied to investigate LSR and XLPE in a temperature interval between -100 and 50°C and to check qualitatively the ultrasound data.
EVA is a widely used material for the encapsulation of photovoltaic modules. It melts at elevated temperatures, and seals the module before it is crosslinked at temperatures above 130 °C by a peroxide-initiated crosslinking reaction. EVA has good optical properties necessary for application in solar modules. For process optimization and quality management, a method for the quick and reliable characterization of EVA crosslinking behaviour is of great value. Here, the practicability of ultrasound for online crosslinking monitoring is demonstrated. A sound velocity increase of about 8 m/s during the crosslinking reaction is found. The ultrasound results are compared with rheometer measurements performed with a curemeter typically used for the investigation of rubber crosslinking.
Carbon-fibre prepregs have found widespread use in lightweight applications. They are based on a carbon-fibre fabric impregnated with reactive epoxy resin. Prepreg materials are generally pre-cured so that they have a higher molecular weight than typical resins in order to reduce resin flow, which facilitates storage and later processing properties.
The measurements were carried out using commercially available materials and follow the published DMA investigations of the same material. TMDSC was used to find the correlation between curing conditions, the degree of cure and glass transition temperature. TMDSC has the advantage over standard DSC that it enables better determination of the glass transition temperature, which is often accompanied by an exothermic curing reaction, and thus overshadowed. The influence of the amplitude of temperature modulation was tested in preliminary experiments. For non-cured material a glass transition temperature of approximately 0 °C was determined; whereas for the totally cured material it was approximately 230 °C. The changes in degree of cure, temperature of actual glass transition and post-reaction are given as a function of curing time at 180 °C. The correlation between actual glass transition temperature and degree of cure is derived.
Two types of commercially applied Ethylene/Vinyl Acetate Copolymers (EVA) for encapsulation of photovoltaic modules were investigated by the thermal analysis methods of Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) in the temperature range from -150 °C to 200 °C. Glass transition, crystal melting and cross-linking were analyzed. The aims of the investigations were to gain more information for incoming goods control and to get information about the whole temperature dependent material properties in the investigated temperature range, starting at very low temperatures up to the crosslinking temperature region.
The method for the determination of compression set values with a Dynamic Mechanical Analysis (DMA) setup at low temperatures, which was presented previously, allows a much faster and readily automated procedure than the standardized compression set test according to ISO 815-2. This method is applied to a series of different elastomeric materials that are commonly used for sealing applications. The results of the compression set test are compared with results from thermal analysis to allow an in depth comparison of the material behaviour at low temperatures.
Furthermore, a comparison between two EPDM materials is presented. These materials show very similar properties determined by thermo analytical methods such as Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) but differ clearly in their compression set behaviour. This comparison shows the importance and value of information of the compression set test in addition to thermal analysis to judge the behaviour of sealing materials and confirms the relevance of the new compression set test method for the investigation of low temperature properties of elastomers.
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.
Der Druckverformungsrest (DVR) stellt insbesondere für Dichtungsanwendungen eine wichtige Kenngröße zur Beurteilung der Eignung eines Elastomers dar. Er ermöglicht die Einschätzung des zeitabhängigen Rückstellverhaltens eines Werkstoffs nach Deformation. Neben der Prüfung bei erhöhten Temperaturen, zur Beurteilung des Alterungsverhaltens, lässt sich bei der Durchführung bei tiefen Temperaturen die limitierende Einsatztemperatur bestimmen. Die Versuchsdurchführung unterscheidet sich je nach angewandter Norm und kann einen deutlichen Einfluss auf den DVR-Wert haben. Um bei tiefen Temperaturen eine schnelle Aussage zu gestatten, ist eine im Vergleich zu genormten Methoden deutlich schneller durchzuführende, automatisierte Methode entwickelt und mit Standardmethoden verglichen worden. Durch kontinuierliche Messung des Druckverformungsrests kann die Kinetik der Materialrückstellung betrachtet und dadurch ein effizienter Materialvergleich und eine effiziente Materialauswahl ermöglicht werden. Compression set (CS) is especially for sealing applications an important parameter for material selection. It allows an assessment of the time dependent recovery after deformation.
In addition to testing at elevated temperatures which aims at the evaluation of the ageing behaviour, testing at low temperatures permits the determination of the low working temperature limit. The test procedure varies depending on the applied standard and can show a significant effect on the CS value. In order to allow a quick statement regarding low temperatures an automated procedure being significantly faster realisable in relation to the standardised methods has been developed and compared with standard procedures. Continuous measurement of the compression set allows evaluating the recovery kinetics and therefore an efficient comparison and selection of materials.
Today rubber is due to its unique property profile one of the most important materials for the production of bearings, dampers or also tires and tubes. In the production of rubber goods a chemical cross-linking reaction (vulcanisation) generates the final material properties. This process takes place during the shape giving of the rubber compound in the mould and can proceed for several minutes. The cross-linking process is influenced by the mixture of the basic polymer, the cross-linking agent, the influence of catalysts and inhibitors and especially the processing conditions. The quality of every ingredient, the reproducibility of the mixing ratio, the homogeneity of the mixture and the accurateness in temperature control can have a direct influence on the cross linking reaction. To control the quality of a rubber compound and verify the processing conditions in industry moving die rheometers, named curemeter, are involved in the production process. Starting from our experience in cure monitoring of thermosetting materials by measuring ultrasound velocity in the mould we extended the measuring principle to rubber vulcanisation monitoring. After first investigations using a compression mould we dealt here with on-line monitoring of vulcanisation in an injection moulding process which is one of the most used techniques in rubber industry.
Monitoring the vulcanization of elastomers: Comparison of curemeter and ultrasonic online control
(2009)
The vulcanization of elastomeric materials has a high impact on the properties of the final product. Therefore, it is important to monitor and control this crosslinking process. A common technique to attain the necessary curing time is the use of a curemeter in accordance with ISO 6502 in order to determine the time for full cure of a sample with a standardized geometry. Based on this result and a lot of practical experience, the required curing time for a given product geometry is estimated. Within the scope of this work, a new analysis technique will be compared with the standard procedure. The ultrasonic online control employs ultrasound waves to measure the changes in material properties caused by vulcanization. For this study, a natural rubber compound with a conventional curing system was investigated by both techniques. It was found that the results of the ultrasonic technique show good agreement with the results of the curemeter.