Hydraulic and pneumatic systems are used in a wide variety of applications with different operating and environmental conditions. Seal components will determine functionality, reliability, efficiency and safety operations of the systems. Seal materials have significant influence on the seal performance from different aspect. As there is a large choice of seal materials available, the proper selection is very important, which will help to achieve the expected performance and service life.
Historically the leather, felt and rubber gave the natural choice of sealing materials for use in most of the industries. Thanks to the revolutionary changes in the chemical and material engineering/manufacturing technologies, mechanical engineers can select from many different synthetic polymers today, which are available and can be used in broad range of industrial applications.
We can distinguish single-constituent (i.e. homopolymers or copolymers) and multiple-constituent polymers (blend or alloys). In many instances, plastics, rubbers, elastomers and polyurethanes contain additives such as:
• Plasticizers: A substance used to make a plastic more flexible or pliable. In some cases they are also added to improve the processing of the component.
• Fillers: Additives that lower the consumption of more expensive base material or enhance the properties of the manufactured component in some way (examples: strength, performance).
• Reinforcing materials: Additives that that are used to improve the strength of the manufactured component.
• Others: vulcanizing agent, activators and processing aids and antidegradants
There are applicable international standards related to polymers as follows:
• Base material: plastics – ISO 1043 Part 1
• Base material: rubbers – ISO 1629
• Base material: thermoplastic elastomers – ISO 18064
• Fillers and reinforcing materials – ISO 1043 Part 2
Generally, we can divide the modern synthetic seal materials in four large subgroups:
Polyurethanes: polyester urethanes (AU) and polyether urethanes (EU)
Polyurethane seal materials have high mechanical strength, good wear resistance, good flexibility and wide hardness range with good elasticity. That is why in many of the fluid power applications a polyurethane seal is often the best design choice. Urethanes are tough and able to take shock loads better than most elastomers.
Polyurethane seals are generally used in high pressure hydraulic systems where highly stressed parts are subject to wear. Typical application temperature range is –30 °C to +80 °C, while some of the low temp polyurethane materials can be used down to -50°C and high-load resistant materials can reach 120 °C continues operating temperature limit as well in mineral oil based fluids.
Polyurethanes create the bridge between soft-elastomers and hard plastics, thanks to the wide hardness range.
Elastomers (rubbers): chemical or temperature resistance of the seals can require special elastomer in particular fluid power applications, where polyurethanes cannot provide reliable solution.
The most popular elastomers in fluid power applications
• Nitrile rubber (NBR): Nitrile rubber is the general term for acrylonitrile butadiene elastomer. The acrylonitrile content of nitrile sealing compounds varies considerably (18% to 50%) and influences the physical properties of the finished material (just like compression set, gas permeability, elasticity and cold flexibility)
The higher the acrylonitrile content, the better the resistance to oil and fuel. Meanwhile, elasticity and resistance to compression set is adversely affected. In view of these opposing realities, a compromise is often drawn, and a medium acrylonitrile content selected.
Thanks to the good mechanical properties NBR is applicable in a large range of applications.
• Hydrogenated nitrile rubber (H-NBR): HNBR is made via partial or full hydrogenation of the NBR elastomer. The peroxide cross-linking improves the temperature and ozone resistance considerably. HNBR is widely known for its physical strength and retention of properties after long-term exposure to heat, oil, and chemicals. HNBR has better heat resistance; oxidation resistance; tensile strength and abrasion resistance than standard nitrile (NBR). Temperature range is –30 °C to +150 °C.
• Ethylene propylene diene monomer rubber (EPDM): PDM is a copolymer of ethylene, propylene and a small portion of diene. EPDM has good swelling resistance in water, hot water and steam. This seal material can be used in flame retardant hydraulic fluids (HFC and some type of HFD). EPDM has no resistance against mineral oil based fluids and greases. Operating temperature range is –50 °C to +150 °C.
• Fluorocarbon rubbers (FPM): fluorocarbon rubber has excellent resistance to high temperatures, ozone, oxygen, mineral oil, synthetic hydraulic fluids, fuels, aromatics and many organic solvents and chemicals. It has higher resistance to swell in high octane and oxygenated fuel blends. The gas permeability of FPM is low.
Low temperature resistance is normally not favourable, however with special material version it can be -35 °C. Operating temperature range is –20 °C to +200 °C (short term up to 220 °C )
• Vinyl Methyl Silicon Rubber (MVQ): the general term silicone covers a large group of materials in which vinyl-methyl-silicone is most often the base ingredient. Silicone elastomers have excellent high temperature resistance and excellent cold flexibility, that is why silicone generally used in low temperature applications. However the silicone elastomer materials have relatively low tensile strength, poor tear and wear resistance. They also have good ozone and weather resistance as well as good insulating and physiologically neutral properties. Operating temperature range: –60 °C to +200 °C (short term up to +230 °C).
• Tetrafluoroethylene (TFE) TFE/P: is a copolymer of tetrafluoroethylene and propylene with lower fluorine content. This material is unique due to its resistance to petroleum products, steam, and phosphate-esters t the same time. In some respects it exhibits media compatibility properties similar to ethylene propylene and fluorocarbon. The compression set resistance at high temperatures is inferior to standard fluorocarbons.
Fluoroplastics: PTFE and its different compounds (such as bronze-filled, glass, Ekonol®*, carbon/graphite)
Polytetrafluoroethylene (PTFE) is a thermoplastic polymer made of tetrafluoroethylene. PTFE is a non-elastic material having outstanding properties what allow to use those compounds in numerous applications. Some of the most properties:
• Outstanding temperature resistance (Operating temperature range – 200 °C to +260 °C.)
• Very low coefficient of friction
• Static and dynamic friction values are virtually same
• Superior chemical resistance
As the PTFE materials and non-elastic and permanently deformed from a certain load by creep or cold flow, PTFE seals require combination with energizer elements. The additional contact pressure on the contact surface can be created by O-Ring or other profiled ring (made of elastomer) or by springs.
Engineered Hard Plastics: rigid thermoplastics and thermosets and their different composites
In certain fluid power applications mechanically highly loadable, rigid plastics are required. Typical application areas: anti-extrusion rings (back-up rings), wear rings (guide rings) and other components.
Polyacetal (POM): semi crystalline thermoplastic material used in parts that require precision and high mechanical strength.
Some of the important properties:
• High stiffness
• Low coefficient of friction
• High abrasion resistance
• High heat resistance
• Low water absorption
• Excellent dimensional stability
• More creep resistance than nylon
Operating temperature range is –40 °C to +140 °C.
Polyamide (PA): semi crystalline thermoplastics (commonly called nylon)
are widely used hard plastic material in fluid power applications. There are numbers associated with the nylon type such as 6, 66 or 12. The numbers reflect on the molecular structure of the polymer and each type has different properties. Some of the important properties:
• High wear resistance
• Good thermal stability
• High strength and hardness
• High mechanical damping characteristics
• Good sliding properties
Depending on the type of the polyamide, materials absorb different amounts of moisture what will affect the physical properties and dimensional stability of the finished part.
Operating temperature range: –40 °C to +110 °C
As the seal materials play important role in the functionality and service life of the seal components, the careful selection of the seal material is required. Some of the important material properties that should be considered during the selection process are the following:
• Proper durometer (hardness) and elasticity for tight sealing (sealability) and to avoid leaks
• Temperature resistance through a broad temperature range
• Chemical resistance against utilized medias in order to maintain physical properties of the seal seal components
• Gap extrusion resistance to withstand elevated system pressure and shear stress caused by fluid pressure
• Maintain the elasticity over a broad operating temperature range
• Maintain the elasticity over the expected service life, having resistance against compression set and good stress relaxation behaviour
• Wear resistance material in order to avoid early wear out
• Improved tribological properties by low frictional values
• Proper durometer (hardness) and flexibility for easy installation