Analysis of the Sealing Force/Elastic Recovery Behaviour of Polymers in the Cold as a Function of Molar Mass, using the Example of Fluoroelastomers

Abstract

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.