High and low temperatures greatly influence the behavior of elastomers
High-Temperature effects on O-Rings
At high temperatures, the compounds are subject to deterioration causing different effects:
- the hardness decreases, but the effect is reversed if the temperature drops again.
- the percentage elongation increases and, consequently, the load required to break it decreases.
If the high temperatures persist for a long time, they can trigger premature ageing of the compound causing changes in hardness, breaking load, elongation and volume that can reduce the characteristics of the product.
In the presence of high pressure, the decrease in hardness can cause the extrusion effect in one seat. Basically, the O-Ring, made softer by the temperature, is pressed against the wall of the seat where it is housed and, as a result of the pressure thrust, tends to slip into the empty space between the two components on which the seal is made. This effect commonly referred to as “extrusion”, is destructive to the O-Ring and can cause seal malfunctions (see figure in point 1.1).
Figure1.1 “The O-ring as an active element”.
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Each elastomer has a different coefficient of thermal expansion and is divided into:
- linear: represents the ratio between the change in length at 1°C (or °F) and the original length at 0°C (or °F).
- volumetric: represents the ratio between the change in volume and the product of the original volume and the change in hardness. It is usually equal to 3 times the coefficient of linear thermal expansion.
Elastomers have a much higher coefficient of thermal expansion than metals (about 8/10 times).
This considerable variation should be taken into account when designing an O-Ring seat because the excessive expansion of the compound could cause the seat to fill and cause misalignment of the parts and/or seal problems.
Low-Temperature effects on O-Rings
Unlike the effects caused by high temperature, low temperature usually does not cause permanent changes in elastomers. The most evident effect of the low temperature is the increase in hardness until the product is completely stiffened, but if the temperature increases, the elastomer reacts by returning to its initial state. Defining a limit for use at low temperatures is very difficult. In this regard, there are conflicting theories because many manufacturers of rubber-moulded articles tend to “broaden” the limits imposed by the results obtained in the laboratory with standard tests.
Therefore, there are technical data sheets with limits of use much lower than the tested values, perhaps differentiating limits for static and dynamic seals. Usually for static seals -5°C / – 10°C are “added” to the TR10 reference test value, but these are subjective suggestions based on the experience of use or the fact that “the rubber part does not reach the low temperatures indicated in the specification because it is inserted in a larger system“, etc.
An interesting study on the effect of low temperatures on O-Rings can be found in this article
The crystallisation temperature of O-Rings
If an O-Ring inserted in a seat to perform a static seal reaches the crystallisation temperature, the rigidity of the material no longer allows an elastic reaction and, since the article is completely rigid, the sealing effect is lost. Differences in pressure inside the seat can cause the movement of the ring which, being crystallised and rigid, cannot react causing a leakage.
- Material selection – FKM
FKM, also known generically as FPM, are a family of elastomers that can be defined as “fluorinated compounds”. Thanks to the presence of fluorine in the formulation, they are considered high-quality compounds, with generally excellent resistance to high temperatures and the attack of chemical substances.
Characteristics and fields of use of fluorinated materials
FKM can be used in standard formulations with temperatures between -20°C and +210°C, for short periods up to +230°C. Thanks to specific compounds and advanced formulations, the limit of use at low temperatures can be extended to -45°C/-50°C (always considered on the basis of TR10).
They are resistant to ageing, ozone, UV rays, oxygen, fuels, many organic solvents and chemical substances. It also has excellent compatibility with mineral and synthetic hydraulic fluids and vegetable oils.
In the nuclear field, they can be defined as compatible with high-energy radiation.
The low gas permeability also makes FKM the main choice in high (or absolute) vacuum applications, guaranteeing excellent results in Outgassing tests.
Weak points and limitations of FKM use
FKM have poor resistance to hot water and steam (although there are specific formulations for these uses that improve their performance in this environment) as well as electrical insulation.
This type of compound is not compatible with polar solvents (MEK), some organic acids, some hydraulic fluids based on esters and methanol, ammonia and some amines.
As for all elastomers, the vulcanization process is activated by substances called “accelerators” (or simply vulcanizing agents) and in the case of the FKM family, you can have bisphenol compounds (the vulcanized agent is bisphenol), which guarantee better performance at high temperatures, or peroxidic compounds (with peroxide accelerator, more commonly called “perox”), which guarantee better resistance to vapours and hydrocarbons.
The basic polymers used to package the compounds can be:
- Fluorine content 65% / 66%
- Usually defined with the letter “A”
- They are among the most widely used polymers for general applications
- Fluorine content 66.5% / 67%
- Usually defined with the letter “B”
- They offer better resistance to fluids and oils than copolymers
- Fluorine content: 67% / 69%
- Usually defined with the letter “G”
- Even better resistance to fluids and oils. Good resistance even to vapour (Viton® GF type), for low-temperature applications (Viton® GLT type) or as a combination of the two (Viton® GFLT type)
The major manufacturers of fluorinated polymers are:
Solvay Solexis: Tecnocflon®
The ORINGONE offer for FKMs
The different application needs and some specific applications have led ORINGONE to develop a complete and varied range of fluorinated compounds. In addition to materials that can be defined as standard (copolymers) in various colours and hardnesses (from 60ShA to 90 ShA), the choice includes solutions for food and pharmaceutical applications with compounds certified according to FDA and 3-A Sanitary Standard, materials developed for the valve and Oil&Gas industry in general with NORSOK M-710 certified materials for Anti Explosive Decompression, as well as the entire family of tetrapolymers for low-temperature applications down to -50°C. Find out more about the techniques and materials for producing O-Rings, read this article.
For further information on all these materials available, you can consult the complete list, including datasheets, available here