Thermo-oxidative degradation of gasket materials

Abstract

Rubber materials, preferably referred to as elastomers, are used in a wide variety of engineering applications, thanks to their unique properties and characteristics. Fluid channels between the plates of Alfa Laval’s heat exchangers are sealed with different types of elastomers, called gaskets. Gaskets are exposed to both chemical, physical and mechanical stresses during their lifetime, affecting the sealing ability. In order to avoid failure of these vital components in the future, the ambition is to develop a model which describes their diminishing sealing ability. If the service life of gaskets can be predicted, they can be replaced before leakage occurs. This will contribute to better maintenance of operating heat exchangers along with better use of resources in the future.

A gasket’s sealing ability decreases over time due to the inevitable stress relaxation behaviour of rubber materials, which is highly affected by thermo-oxidative degradation. Investigation and quantification of the degradation process is therefore crucial. This thesis work focuses on the thermo-oxidative degradation of two different gasket materials, NBR and EPDM. Their structural, mechanical and thermal characteristics have been investigated with various analytical techniques to find an experimental strategy which can be used to address the aging behaviour. Initially, stress relaxation measurements and oven aging was performed at elevated temperatures and for different time periods. Thereafter, the aged specimens were investigated with swelling measurements, Optical Microscopy (OM), ATR-FTIR Spectroscopy and Dynamic Mechanical Analysis (DMA).

NBR and EPDM degraded differently at the chosen aging temperatures. An additional chemical mechanism could be observed when aging NBR at temperatures above 120 ℃, which contributed to crosslinking of the polymer network. EPDM proved to be more resistant towards oxidation, even when aged at higher temperatures. Furthermore, a shielding effect caused by diffusion-limited oxidation was observed after aging, which is believed to have a significant impact on the degradation behaviour. Combining stress relaxation measurements with techniques addressing ongoing structural mechanisms proved to be a convenient approach to gather crucial insights about thermo-oxidative degradation. Among the examined techniques stress relaxation in compression, swelling, optical microscopy and ATR-FTIR are suggested for future investigations.

Popular Abstract

Rubber materials are very often considered as trivial, but as a matter of fact our everyday life would not function without them. Rubbers are essential in our modern industries and one of their main tasks is to ensure tightness in different applications where sealing is required. The aim of this thesis work is to investigate how rubber materials degrade when exposed to high temperatures and oxygen. This will help companies to avoid expensive downtimes and it will also contribute to better use of resources in the future.

Rubber materials have many striking characteristics. One of them is their elasticity which makes them a perfect choice in sealing applications. This property can be experienced when pulling a regular rubber band. Their chemical structure, which looks very much like long entangled noodles, allows them to stretch and retract rapidly. Rubbers can be processed into vital components, called gaskets, used in for example heat exchangers. A heat exchanger is a system used to transfer heat between two fluids. Usually, the fluids are separated by metal plates, and gaskets are placed between the plates to seal and prevent leakage. Unfortunately, the elasticity of gaskets is affected by external factors such as temperature, air and fluids. When rubber gaskets are used their elastic properties decrease slowly over time, and after a certain period they are not able to ensure proper sealing. This behaviour is called stress relaxation. To optimize the usage of gaskets it is crucial to understand how external factors such as oxygen and temperature affect their ability to seal properly.

In this project, two common rubber materials, called NBR and EPDM, were analysed with various techniques. Initially samples of these materials were aged in ovens. Real working conditions were also simulated by stress relaxation measurements. With this technique it is possible to measure the counterforce from a gasket compressed between plates and exposed to high temperatures and oxygen. This property is closely related to the gasket’s sealing ability. After aging at different temperatures and time-periods, the properties of each sample were analysed. The stiffness of rubbers depends on the number of crosslinks between the long ‘’noodle’’ chains. This property was studied by swelling in solvents, since a highly crosslinked and stiff material swells less than the elastic material. Visible changes and the chemistry of each material was also studied after aging. Their mechanical properties were investigated by deforming them slightly and measure their response.
It was found that the counterforce of EPDM was higher than for NBR when aged at the same temperatures. Both materials crosslinked and became stiffer when aged. However, many differences could be seen. NBR was more affected by oxygen than EPDM. At high temperatures NBR oxidized a lot, but oxygen did not penetrate deep into the material, which resulted in a highly oxidized surface layer. The detected counterforce was also higher at these temperatures. This layer was not formed when aging EPDM at the chosen temperatures. The ability of a gasket to maintain a tight seal depends on both its structure and the environment it operates in. This thesis showed that many important aspects of how gaskets degrade, can be understood by investigating the counterforce and using different methods to examine how their structure is affected. It also showed that a high counterforce does not necessarily lead to a longer service life. Hence, it is important to understand how each specific gasket material behaves in order to determine its lifetime.

• author: Kujundzic, Emina
• supervisor: Baozhong Zhang; Henrik Kockum
• organization: Centre for Analysis and Synthesis
• alternative title: Investigating the degradation behaviour of vital components used in our modern industries
• course: KASM15 20191
• year: 2019
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: rubber, elastomer, polymer technology, gasket, lifetime, aging, degradation, oxidation, stress relaxation, polymerteknologi
• language: English
• id: 8996004
• date added to LUP: 2019-10-17 13:22:19
• date last changed: 2019-10-17 13:22:19