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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.
In consideration of the influence of the curing reaction on the material properties of elastomers, it is very important to control the vulcanization process properly.
As shown previously [1], the effects of the vulcanization reaction of an elastomer can be monitored by use of ultrasonic sound waves. This technique has to a certain degree a high similarity to the standard curemeter test according to ISO 6502 but can be applied also inside a production tool.
So far, the method was only applied to the vulcanization of one compound at a given temperature and sample thickness. This is now complemented with measurements on another compound system at different temperatures and sample thicknesses.
The expected effects of temperature and thickness on the curing behaviour were found by use of the ultrasound online control.
Über die in der Technik weit verbreitete typisierte Harnstoffformmasse UF 131.5, ein Aminoplast, wird berichtet, dass sie im Zuge der thermisch aktivierten Vernetzungsreaktion wegen "Überhärtung" mangelhafte Eigenschaften annimmt. Zur Verifizierung dieses Effektes werden mithilfe der Ultraschall-Prozesskontrolle gezielt Proben unterschiedlicher Aushärtegrade hergestellt. Sie werden mit verschiedenen Methoden, wie der "Kochprobe" und der Thermoanalyse, charakterisiert und auf ihre Materialeigenschaften untersucht. Dabei zeigt sich ein deutlicher Einfluss der Härtezeit auf die Glasübergangstemperatur der Formmasse. In den mechanischen Eigenschaften ist eine Veränderung allerdings nur in der Schlagzähigkeit festzustellen, die sich mit steigender Härtezeit sogar wesentlich verbessert.
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.