EPDM Rubber: Properties, Manufacturing and Benefits
EPDM rubber (ethylene-propylene-diene-monomer) is a synthetic elastomer consisting of a copolymer of ethene (ethylene gas), propene (propylene gas), and a small proportion of diene monomer. The diene component (often ethylidene norbornene, dicyclopentadiene or vinylnorbornene) is integrated into the polymer chain and introduces unsaturated side groups, which enable vulcanization with sulfur.
The chemical structure (consisting of ethene and propene) makes EPDM highly resistant to ozone, UV radiation, and heat. EPDM belongs to the polymethylene rubber materials that combine a saturated polymer backbone with the ability to crosslink through sulfur.
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Manufacturing process and additives
EPDM is produced industrially by copolymerization of ethylene, propylene and a non-conjugated diene in solution or suspension using Ziegler-Natta or metallocene catalysts. The process was introduced commercially in 1963. The choice of catalyst affects how the diene component is distributed along the polymer chain, which in turn affects the vulcanization properties. The distribution of ethylene vs. propylene can be varied (typically 45–70% ethylene content) – higher ethylene ratios produce semi-crystalline polymers with better strength in the unvulcanized state, while lower ethylene ratios produce amorphous polymers with more rubber elasticity.
After polymerization, raw EPDM is always mixed with various additives (rubber compound). Typical fillers include carbon black (technical carbon) and calcium carbonate, which improve mechanical strength, and soft oils (e.g. paraffinic process oils) are often added to increase elasticity and facilitate processing. In addition, antioxidants and waxes are usually included to protect against aging, as well as processing aids (such as zinc soaps and stearic acid) to improve mixing and molding properties.
EPDM itself is a thermoplastic in its raw form and only becomes functional after crosslinking (vulcanization). Vulcanization is traditionally done with sulfur and accelerators, which utilizes the diene units to create sulfur bridges between the polymer chains. However, for applications that require higher heat resistance or better compression properties, peroxides are often used as crosslinkers instead of sulfur. Phenolic resins or high-energy radiation (electron beam) can also be used to crosslink or foam EPDM in special applications (e.g. foam rubber, cable insulation).
The mixing of EPDM compounds is essentially done exclusively in internal mixers within the rubber industry, after which the material can be shaped by common methods such as extrusion, injection molding or calendering. The fully mixed rubber is then vulcanized under heat and pressure into its final shape.
Advantages and limitations
Benefits
EPDM is characterised by its outstanding resistance to weathering and ageing. Thanks to its saturated polymer structure, it is hardly affected by ozone or UV light, and the rubber can be used outdoors for many years without cracking or breaking down.
The material also has very good heat resistance – special formulations can withstand up to about 150 °C continuously – and retains its elasticity down to around -40 °C or lower without becoming brittle. Furthermore, EPDM is very resistant to water, steam and polar chemicals such as many acids, bases, alcohols, ketones and glycols.
It does not swell in water and is not affected by water vapor, making it well suited for applications with hot/cold water and water-based liquids. EPDM also has good insulation properties (electrical insulation and gas-tight) as well as high elasticity and extensibility (often over 300% elongation). Overall, EPDM is a durable and weather-resistant rubber suitable for demanding outdoor environments and a long service life.
Restrictions
An important limitation is that EPDM is not compatible with oils, fuels and hydrocarbon-based solvents. In contact with petroleum products (e.g. mineral oils, petrol, diesel) EPDM swells and softens considerably, and the material breaks down relatively quickly. Therefore, it is not used for gaskets or hoses that must withstand oily environments - oil-resistant rubber types are required instead (see comparison below).
EPDM also does not have the same high mechanical strength as some other rubber materials; the pure polymer is relatively soft and needs to be reinforced with fillers for increased strength. For example, EPDM has slightly lower abrasion and tear resistance than natural rubber and chloroprene rubber when compared at similar hardness, although the differences are reduced by reinforcement. Furthermore, EPDM can have adhesion problems – the raw rubber has low self-adhesion (“building tack”), which may require the addition of resins during processing, and vulcanized EPDM can be difficult to bond without special primers.
Finally, EPDM is not resistant to certain strong chemicals; for example, concentrated strong acids can cause hardening/degradation of the rubber. Direct contact between EPDM and bitumen (asphalt) should also be avoided: black EPDM profiles on bituminous surfaces have shown a tendency to chemical degradation. This limits the combination of EPDM rubber with traditional asphalt-based roofing materials.
Technical data and characteristics
- Below are summarized some important properties of vulcanized EPDM rubber (note that values vary depending on specific compounding):
- Hardness: Typically from around 30 up to 90 Shore A depending on filler and formulation. Common seals are often in the 50–80 Shore A range, but soft foam grades can be even lower (down to 25 Shore A for some sheet products).
- Density: The EPDM polymer itself has a low density (~0.86 g/cm³), but practical mixtures are usually around 1.1–1.3 g/cm³. By heavy filling, the density can be increased considerably (theoretically >2.0 g/cm³), while cellular rubber EPDM can be very light (~0.5 g/cm³).
- Tensile strength: Typically 5–12 MPa in tensile strength (ultimate elongation), depending on the degree of reinforcement. Well-reinforced EPDM blends can achieve about 17 MPa in tensile strength, which is high for a saturated rubber but somewhat below the levels of some crystallizing rubbers (e.g. natural rubber).
- Elongation at break: Typically ≥300% and in many cases 400–500%. EPDM is a very elastic material with high elongation before breaking, especially in amorphous grades without crystallinity.
- Temperature resistance: The service temperature for EPDM ranges from approximately -50 °C up to +150 °C. Standard grades are often specified for approximately -40 to +120 °C continuously, but special formulations (e.g. peroxide cured) can withstand short-term temperatures of 150 °C or slightly higher. At low temperatures, the glass transition temperature is around -54 °C, which means that EPDM retains its rubber elasticity down to approximately -50 °C before it begins to harden significantly. However, for applications below -40 °C, other elastomers (e.g. silicone) may be more suitable.
- Chemical resistance: EPDM is excellently resistant to many polar chemicals. It is hardly affected by diluted acids and alkalis, glycols (e.g. brake fluid), alcohols, ketones (acetone, MEK), phosphate esters (fire-resistant hydraulic oils) as well as water and steam. However, EPDM is unsuitable in contact with all kinds of hydrocarbon-based liquids: mineral oils, petrol, diesel, kerosene, aromatic solvents, etc. causing severe swelling and degradation. EPDM has only moderate resistance to e.g. freons and ammonia – chloroprene rubber is often preferred there (see neoprene below) – but overall EPDM is considered to have a very broad chemical resistance on the polar side, and a very limited resistance on the non-polar (oil/fuel) side.
- (Note: The properties above refer to typical vulcanized EPDM mixtures. By composition and choice of curing method, the properties can be optimized within certain limits, e.g. for better heat resistance or lower compression set.)
We manufacture in EPDM!
EPDM is a versatile material with excellent properties for use in environments where weather resistance and flexibility are important. Helsingborgs Gummifabrik manufactures a range of products in EPDM, including rubber sheets, rubber mats, and special profiles. They use compression molding and extrusion to create products that meet specific customer requirements. Their EPDM products are known for their durability and resistance to weathering.
Contact us for more information on how we can help you with EPDM products that meet your specific requirements.
