T/2 processes in the material and on the correlation with the decrease in static sealing force. To begin with, however, theoretical model concepts will be discussed relating to thermodynamic and kinetic processes during cooling of a polymer melt, i.e. the process of freezing in molecular mobility and volume shrinkage.
The purpose of this study is to provide a guide to the various methods for compression stress relaxation testing (CSR testing), to assessing the influences of various parameters on the measurement results and to understanding different evaluation variants. It also presents a measuring method developed by ElringKlinger AG together with its various evaluation options. As the experiments in this study show, CSR compression methods developed by Shawbury-Wallace, Wykeham-Farrance and Jamak, which often produce very different static long-term sealing force curves, can be replicated sufficiently accurately by the EK method if the right evaluation variant is chosen. In addition, identical compression stress conditions must be established in the specimen by means of its geometrical design. The CSR methods are surprisingly simple to use and versatile in terms of the choice of test parameters and conditions. The various CSR measuring methods are used to plot the static long-term sealing force curves for AEM, ACM and HNBR compounds in hot air and in SF105G engine test oil. The influences of materials and sealing profiles in CSR testing are also discussed.
Selected from International Polymer Science and Technology, 42, No. 10, 2015, reference GK 15/08/548; transl. serial no. 17441
Translated by M. GrangeThis publication first addresses the influence of an increasing chemical network density on the static sealing force behaviour and internal structural mobility of an FKM material in the cold. Cylindrical FKM test specimens were exposed to a high static surface pressure to make it easier to see thermodynamic and immobilising influences on sealing force behaviour as a function of chemical network density. Additional volume swelling trials and PVT measurements focused particularly on the compressible behaviour of the elastomer material as a function of its chemical network density. Furthermore -based on the static sealing force behaviour and mobility of the material in the cold -the loading situation of free compression with distortional strain energy was compared with the extreme case of a completely confined elastomer seal. The completely confined elastomer material was exposed to pure compression work, as in the PVT measurements.
A relevant technical feature of an elastomeric sealing material is its sealing function in the cold down to the glass transition range. Considering homologous elastomer structures as a function of their molar mass, low-viscosity elastomeric materials offer the advantage of shorter relaxation times, giving reason to expect a more marked dynamic structural mobility and flexibility at low temperatures. Identical elastomer structures with a high molar mass and a more complex entanglement due to longer polymer chains will show more significant growth of the potential energy of the molecules by their dislocation to positions of higher energy, as well as the change in entropy, during an increase in static compression. This will be reflected by the linear pressure dependence of the specific volume, which is not significantly influenced by the molar mass, and by the decrease in the specific volume when the molar mass increases, which can be demonstrated by PVT diagrams. A chemical network structure will significantly support the elastic recovery characteristics, interfere with the physical effects but constrain the mobility of the polymer chains which can have various effects depending on the molar mass and compression state. From the mere thermodynamic viewpoint, the elastic recovery behaviour or sealing force in the cold should be supported by an increase in the molar mass of the polymer. However, if the effects are considered in conjunction, opposing behaviour can be detected, i.e. the mobility of the structure decreases and the elastic recovery behaviour improves if the molar mass of the elastomeric material is increased. This gives reason to expect that the sealing force in the cold will pass through a maximum functionality as the molar mass grows. The objective of this study was to investigate the molar mass dependency of the sealing force function at very low temperatures using the example of a range of relatively homologous fluoroelastomer grades. A description of the static measurement method used is also given.
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