The compression set (ASTM D395) of a material is the permanent deformation remaining once a force applied to it has been removed. Rubber and elastomeric compounds permanently deform under strain. The rate and degree of permanent deformation is determined by the type of elastomer, the force applied, how long it is applied for and a number of other factors like temperature and chemical exposure and other environmental conditions.
This is of particular concern when elastomers are used to create seals. Deformation and decreasing seal force have the potential to result in leaking seals over time.
To measure compression set, the difference between the height under force and the height 30 minutes post force removal (in order to allow recovery and to measure permanent deformation) is expressed as a percentage. A compression set of 0% means the material completely returned to its original height. 100% compression set means that the material remained in the deformed position and did not recover at all when removed from the fixture.
Nitrile Rubber, also known as Buna-N and NBR, is a copolymer of Butadiene and Acrylonitrile and is the most widely used elastomer at temperatures between -40 C and +125 C. The popularity of Nitrile is due to low cost, wide availability and good performance with oils and chemicals.
Nitrile is widely used in the automotive and aeronautical industries, offering excellent resilience and chemical resistance, making it ideal for standard products and critical components. But despite the wide suitability of Nitrile in many applications, there are many environments that can reach temperatures below -40C.
Using an elastomer below its operating temperature will likely result in excessive shrinkage of the rubber. A rubber elastomer can shrink up to 10 times more than a mating metal or plastic surface. Should the O-Ring or rubber seal shrink excessively, there is a high probability that a leak path could develop. Elastomers used in dynamic rather than static applications are even more sensitive to the effect of low temperature.
However. Nitrile formulations with smaller amounts of Acrylonitrile *can* perform at low temperatures – with a drawback of slightly inferior performance in fuels and oils. Temperatures down to -65F can be accommodated with Acrylonitrile modification. So it simply a case of balancing low temperature performance with the chemical and oil resistance also required.
Silicone materials are generally non-reactive, stable and resistant to extreme environments and temperatures. They can be easily formulated to meet a wide range of requirements, including food, water and medical regulations.
In some instances it is necessary to detect trace silicone. By combining the silicone with fillers that are magnetised, the silicone itself then becomes magnetic. Meadex has been designing magnetic silicone seals since 2010, to be used in applications where the silicone has to be detected throughout the process – for example in food and medical applications.
Compound development, design and tooling can be complicated and they are all application specific. But magnetised silicone can be an excellent solution where traceability is important and in healthcare applications etc.
Like other silicone compounds, fluorosilicone rubber (FSR, FVMQ) resists high temperatures, low temperatures, ozone, aging, ultraviolet light, and weathering. Fluorosilicone offers higher resistance to fuel and oil than standard silicone, and there are varying grades of fluorosilicones that offer higher or lower levels of such resistance.
Applications for these extreme-environment elastomers include automotive, aerospace, defense, electronics, and semiconductor manufacturing but sourcing the right compound and configuring the tooling correctly are important.
In addition to fuel line seals, engine gaskets, and exhaust gas recirculating diaphragms, fluorosilicone rubber is also used in O-rings, tank linings, protective boots for electrical equipment, sliding shaft seals and rotating seals… There are many different applications and uses for fluorosilicone – some fluorosilicone gasket materials also resist abrasion and can provide EMI shielding – as metal-filled fluorosilicones provide electrical conductivity too (e.g. fluorosilicones filled with silver-nickel or silver-aluminum particles, or containing Monel or aluminum wires).
Two of the most commonly approved elastomers are Nitrile (popularly referred to as NBR or Buna-N) and Viton® (Fluorocarbon rubber). Both materials are a great choice for general industrial use, offering excellent compression set resistance and a wide range of achievable properties and applications.
Nitrile is far more cost effective than Viton, but mainly effective within a temperature range of -30°C to 100°C (without formulation / additives ). So whilst it can be used for applications requiring resistance to aliphatic hydrocarbons, petroleum oils, water and hydraulic fluids, it’s temperature performance and chemical resistance is limited.
Viton® or FKM is therefore better suited for diverse sealing jobs that require high temperature and chemical resistance. So, for example, applications involving contact with petroleum oils, acids, silicone fluids and gases, halogenated hydrocarbons (e.g. trichloroethylene and carbon tetrachloride).
Viton® O-rings and seals have a wide range of chemical resistance (e.g. mineral acids, fuels, oils and petroleum based lubricants) and they are also highly resistant to oxidation, UV exposure, fungus, weather, ozone, and mould.
However, whilst Viton® seals can withstand temperatures between -20°C to 210°C, they are prone to failure below 15°C in dynamic applications (as they become inflexible and hard). And so for these applications, Nitrile is a better candidate material.