Chemical resistance of elastomer gaskets in chemically aggressive environments

Abstract

The chemical resistance is an important property for elastomers, especially for gaskets used in industrial applications related to food processing and packaging. Three common elastomers, namely silicone, FKM (fluorocarbon) and EPDM (hydrocarbon with carbon-carbon double bonds), were aged in harsh chemicals used in specific applications in Tetra Pak filling machines. Gaskets of these materials were immersed in nitric acid (10 %, 60 °C) or sodium hydroxide (10 %, 80 °C) for 50, 100 and 200 h, or subjected to hydrogen peroxide fumes (50 %, 80 °C) for 500 h. The weight and hardness changes were analysed, as were the surfaces with SEM (electron microscope) and IR (infrared spectroscopy). Stress relaxation rigs were designed to analyse how the sealability changed due to relaxation phenomena when being subjected to these chemicals.
Silicone showed degradation in acid, although not in SEM or by IR. Its hardness increased by 10 % and the stress relaxation was reduced by over 10 %. FKM and EPDM were oxidised by the acid and their surfaces roughened, but they showed no decreased sealability. In base, the silicone surface was degraded according to SEM but otherwise it remained intact. Both FKM and EPDM were heavily degraded in base; both showed considerable surface alterations (filler leakage, cracking and chain scission), but their sealability was maintained. The peroxide was less aggressive; EPDM experienced some filler leakage, both FKM and EPDM increased their hardness but silicone was completely intact.
Lifetime estimations based on stress relaxation showed that FKM is the mechanically most resistant, regardless of chemical environment. EPDM relaxed the most in all chemicals. Silicone was comparable to FKM except for considerable relaxation when aged in nitric acid. FKM was estimated to last about 200 days in all environments, EPDM about 50. The lifetime of silicone was about 150 days, but only about 50 days when exposed to nitric acid.

Popular Abstract

Lifetime of rubbers

Rubber components are cheap and sometimes considered as disposables but in fact rubbers play a central role in modern day industry. Virtually no machine or factory can function without rubbers, and neither can your car or shower. Whenever two pipes or tubes are joined, rubbers are necessary to ensure tightness and to avoid leakage. Rubbers are processed into o-rings or gaskets and placed at the joint where it helps containing the fluid inside the pipes or tubes.
If you have ever used a rubber band, you would know that the most prominent property of rubbers is their elasticity. In fact, the elasticity depends on many features of the specific material, but also on external factors such as temperature, surroundings and the time the rubber is used. Even when used in air at room temperature, the ability to maintain the sealing decreases with time. If the rubber is used to seal reactive chemicals at higher temperature, the time to failure will be even shorter, due to physical and chemical changes in the rubber structures. To optimise the use of rubber, it is crucial to find the lifetime of rubber gaskets.
In this project, three common types of rubber – silicone, fluorocarbon and hydrocarbon – were subjected to three common types of chemicals – nitric acid, sodium hydroxide and hydrogen peroxide – and how some key material properties changed was analysed. The silicone rubber is composed of a silicon and oxygen backbone, whereas the fluorocarbon is composed of fluorine and carbon and the hydrocarbon of hydrogen and carbon, with carbon-carbon double bonds. The aim was to compare the materials and estimate their lifetimes. To simulate the real working conditions, the materials were immersed (aged) in the chemicals at high temperatures and then analysed in different ways. The weight change during ageing was followed to detect either swelling due to absorption of the chemical or leakage of filler particles added to the rubber compound. Hardness, a property related to the stiffness and density of crosslinks between the long molecule chains in the rubber structure, was also measured. Logically, the surface of the rubber will be affected first and worst by the chemicals, so the shape and chemistry of the surfaces were analysed too. Lastly, a test rig was built to simulate compression in a joint with the possibility of measuring the counterforce of the rubber gasket, which is closely related to the ability to keep a tight seal.
It was found that nitric acid changed the surface chemistry of the fluorocarbon and hydrocarbon a lot but the sealability seemed intact. Silicone behaved completely different; no changes were detected on the surface but the sealability decreased with more than 10 %. All materials, especially the fluorocarbon, were chemically degraded in sodium hydroxide, but the sealability was unchanged for all materials. Hydrogen peroxide was the least hostile chemical since very little degradation was detected; only some minor chemical changes of the hydrocarbon.
Both the ability to maintain a tight seal and to avoid large chemical changes are important aspects for gaskets. With this in mind it was difficult to say which material was the best for each chemical, but fluorocarbon maintained the sealability the best and silicone was the least affected chemically. Silicone was the top choice for sodium hydroxide and hydrogen peroxide but fluorocarbon behaved best in nitric acid.

• author: Hansson, Oskar
• supervisor: Baozhong Zhang; Manuela Franchi
• organization: Centre for Analysis and Synthesis
• course: KTE720 20162
• year: 2017
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: FTIR, Stress relaxation, Lifetime, polymer technology, polymerteknologi, SEM, Elastomer, Gasket, Degradation, Chemical compatibility
• language: English
• id: 8905414
• date added to LUP: 2017-04-19 11:36:46
• date last changed: 2017-04-19 11:36:46