Elastomer Seals: Creep Behavior and Glass Transition by TMA

This article describes how thermomechanical analysis (TMA) can be used to characterize the creep and viscous flow behavior of two different types of elastomers. This was done by means of different isothermal creep and recovery measurements and thermally stimulated creep (TSC) experiments. These methods allow the glass transition and other relaxation processes (e.g. reversible flow relaxation) to be measured with high sensitivity. Elastic deformation and the viscous flow of elastomers can also be determined. The elastomers studied were SBR (styrene-butadiene rubber) with different degrees of vulcanization and EPDM (ethylene-propylene-diene rubber) containing different amounts of carbon black.

Introduction

Hardness, glass transition, creep and the viscous flow component are some of the more important properties that have to be taken into account when elastomers are used for sealing applications.

The hardness of a material is determined by its elasticity and modulus and predicts the deformation capacity under pressure or load. The determination of the glass transition and the temperature retraction method (ASTM D1329) are often used to characterize the low temperature sealing performance of such elastomers.

The term “creep” refers to the time- and temperature-dependent elastic and plastic deformation of a material when it is subjected to a load or stress. Creep deformation consists of two components: reversible creep relaxation and irreversible viscous flow. The time-independent elastic deformation that also occurs under load is not considered as being part of creep deformation. The creep deformation caused by reversible creep relaxation recovers over time when the stress is reduced or removed. This is a positive factor in sealing applications. Viscous flow however causes permanent deformation and geometry change and often leads to product failure.

These properties can be readily investigated using thermal mechanical analysis (TMA) by performing isothermal creep and recovery experiments and thermally stimulated creep (TSC) measurements. The results obtained from SBR (styrenebutadiene rubber) with different degrees of vulcanization and from EPDM (ethylene-propylene-diene rubber) containing different amounts of carbon black are discussed in detail.

Isothermal Creep and Recovery

In an isothermal creep and recovery experiment, the sample is held isothermally at a specified temperature. A mechanical stress (in this case, the TMA force) is applied, held constant for a certain period, and then quickly removed. The strain (i.e. the relative change of sample thickness) is recorded as a function of time.

Thermally Stimulated Creep

The measurement principle of thermally stimulated creep (TSC) is shown schematically in Figure 1. Step 1: The sample is subjected to a mechanical stress (the TMA force, F₀) at a given temperature T_p for a specified time. This results in a certain amount of orientation of relaxation units (such as polymer chain segments).

Step 2: The sample is then cooled under control to T₀, causing molecular mobility and rearrangements to freeze.

Step 3: The mechanical stress is removed at T₀.

Step 4: The sample is heated at a constant rate, which allows the relaxation units to lose their orientation and recover. Retardation, or time-delayed reorientation, occurs. This appears as a step in the sample thickness and as peak in the first derivative curve. The step height is a measure of the intensity of this retardation.

If the temperatures and force are suitably chosen, the TSC technique even allows the time distribution of retardation processes to be studied.

Experimental Details

The measurements were performed using a TMA/SDTA841e equipped with a 3-mm ball-point quartz probe and liquid nitrogen cooling

Conclusions

The most important physical properties of elastomer used for sealing applications are the elastic modulus, glass transition and the creep and viscous flow behavior. These properties should be measured and compared to ensure adequate and consistent sealing performance. TMA is a powerful technique that can be used for this purpose.

The elastic modulus, creep and flow behavior of materials can be determined in isothermal creep and recovery experiments. Increasing the degree of crosslinking of elastomers through vulcanization not only increases the elastic modulus but also reduces creep relaxation and the undesired viscous flow. Although the addition of carbon black to the elastomer formulation increases the elastic modulus, it does not lead to a reduction of the viscous flow.

The latter is often responsible for the failure of sealing rings. The problem cannot therefore be solved through the addition of carbon black. Due to its sensitivity, the thermally stimulated creep method is ideal for measuring the glass transition of highly filled elastomers and other weak relaxation processes such as the flow relaxation of unvulcanized or lightly vulcanized elastomers.

 _{Elastomer Seals: Creep Behavior and Glass Transition by TMA | Thermal Analysis Application No. UC 284 | Application published in METTLER TOLEDO Thermal Analysis UserCom 28}