Prevent Failing of Performance Polymers Applied at High Pressures

Introduction

High-performance polymers are known to retain desirable mechanical, thermal, and chemical properties when subjected to harsh environments such as high temperature, high pressure, and corrosive chemicals. High-performance polymers have the potential to replace traditional materials because of their strength-to-weight ratios and other economic advantages over aluminum or other metals. Performance polymers are used in aviation, automobile, and oil and gas industries as well as in renewable energy, electronics, and military applications. They are designed to resist a wide range of thermal, chemical, and physical stress [1-3]. Polymer degradation is an unwanted change in the properties of the polymer under the influence of one or more environmental factors, such as heat, chemicals, applied force or light radiation.

Degradation is often due to a change in the chemical and/or physical structure of the polymer, which in turn leads to a decrease in the desired properties.

Many analytical methods are used to test polymer products and their raw materials. Thermal, rheological, physical, and mechanical testing ensure that the materials comply with industry specifications and meet quality requirements. The same analytical methods are applied to assess the stability and lifetime of high-performance polymers under the same or even harsher environmental conditions as the material is exposed to in its application.

Thermogravimetric Analysis (TGA) is a proven method for accelerated lifetime testing of polymers. Short term experiments can predict in-use lifetime by assessing mainly thermal and oxidative stability of the materials. The combination of TGA with spectroscopic or chromatographic analysis of the decomposition products provides a more detailed understanding of the degradation mechanisms.

Standard TGA is usually performed at ambient pressure using inert atmospheres or air. However, high-performance polymer materials are particularly suitable for applications under harsh conditions. These materials must be tested under their service conditions to evaluate their stability and durability. These conditions include potentially corrosive, flammable, and explosive atmospheres as well as steam and high pressures.

Results and Discussions

FKM is a family of fluorocarbon-based fluoroelastomer materials described in the ASTM International standard D1418 [1]. All FKMs contain vinylidene fluoride as a monomer [2]. Originally developed by DuPont, FKMs are today produced by many companies. Fluoroelastomers provide more heat and chemical resistance than neoprene or nitrile rubber elastomers. FKMs can be divided into different classes based on either their chemical composition, fluorine content, or cross-linking mechanism. Polymers of this type are considered relatively chemically inert and temperature resistant. Therefore, they are used in the chemical, automotive, aerospace, and energy industries. They are used for hoses, diaphragms, accumulator bladders, gaskets, O-rings and seals, all operating in especially harsh environments [3]. The typical service temperature of FKMs is between -20 °C and 230 °C. They can, however, withstand temperatures up to 300 °C for a short period of time. However, at high service temperatures, FKMs are weak, so the design must prevent any high loads.

Since FKMs have already been in use for decades, the material’s thermal and chemical stability has been tested extensively. However, the influence of pressure on the material’s degradation is not well studied yet. In this application note, the influence of pressure on the thermal stability and the oxidative stability of a commercial FKM was studied by high-pressure TGA.

In table 1 the temperatures of the decomposition steps are summarized for each pressure. Decomposition of FKM takes place at lower temperatures when the pressure is increased from ambient to 40 bar. The differences between the temperatures range from 7 % to 11 % for the different decomposition steps. If the pressure is further increased to 80 bar, the decomposition temperatures do not change significantly compared to 40 bar.

Table 1. Onset temperatures of FKM decomposition in argon atmosphere at different pressures

Pressure	1 Bar	40 Bar	80 Bar
Step 1	515 ºC	457 ºC	453 ºC
Step 2	654 ºC	556 ºC	544 ºC
Step 3	822 ºC	773 ºC	743 ºC

 

A typical finding in high-pressure thermal stability testing is that the decomposition temperatures at the different steps change differently. While the temperature of the first step is 7 % lower at high pressure compared to ambient, the change in the second step is 10 % and only 4 % in the third step. Another typical finding in high-pressure TGA is that the residual weight of the material after decomposition is higher at higher pressure. This effect is considered the result of the higher resistance of volatile components from the sample to evaporate into the higher-pressure gas atmosphere.

Key insight from the high-pressure TGA testing of the thermal decomposition behavior of FKM is that the material degrades at lower temperatures if the pressure is increased. This must be considered if FKM is applied as a sealing material in applications at high pressures.