Composition and production of ethylene-propylene-diene rubber
EPDM rubber belongs to the elastomers group. These are dimensionally stable, but are elastically deformable under load. The elasticity depends on the degree of crosslinking. EPDM has been produced in industry by polymerisation since 1963. Unlike ethylene-propylene rubber (EPM), which is formed by copolymerisation of ethylene and propylene, the production of EPDM involves the use of a diene as an additional terpolymer, and so the resulting products form sulfur-crosslinkable double bonds in the side chains.
Ethylene and propylene are crosslinked by means of suspension or solution polymerisation. Industrial EPM grades contain 40 to 80 percent ethylene. The following dienes can be used for EPDM:
- trans-hexadiene-1,4,
- dicyclopentadiene (DCP), or
- ethylidene norbornene (ENB; IUPAC name: 5-ethylidene-2-norbornene)
A metallocene catalyst or a Ziegler-Natta catalyst is used for EPDM production. Incorporation of the diene is dependent on the catalyst type, with the distribution of the diene along the main polymer chain influencing the crosslinking properties.
The ter components have different crosslinking rates: ENB has the highest reactivity, DCP the lowest. The two double bonds of the dienes must have differing reactivities, so that only one is copolymerised, while the other remains available for subsequent crosslinking.
The nature and the amount of the ter components used determine the vulcanising characteristics, the degree of crosslinking and the mechanical properties. Ethylene contents of around 45 to 60 percent lead to amorphous, non-self-reinforcing polymers. With an ethylene content of 70 to 80 percent, partially crystalline polymers, also known as sequential polymers, are formed.
In terms of their processing characteristics, partially crystalline sequential polymers differ considerably from amorphous polymers: they form thermally reversible physical crosslink points, which give the polymers good strength values, even in the uncured state. Conventional ethylene-propylene-diene rubbers have molecular weights from 200,000 to 300,000. High molecular weight EPDM grades are oil-extended, giving them good processing characteristics.
Fillers for special characteristics
In order to achieve improved mechanical properties or resistance to glycol-based brake fluid, fillers need to be added to amorphous EPM and EPDM grades. Ethylene-propylene-diene rubber can usually be more highly filled than EPM, with the same viscosity. If there are specific ageing resistance requirements, unsaturated EPDM grades can be treated with antioxidants.
Processing aids like zinc soaps and stearic acid ensure better distribution of the fillers and make processing easier. Furthermore, resins sometimes have to be added to EPDM mixes because the building tack is too low. Peroxides or sulfur and accelerators are used as vulcanising chemicals.
The production of EPDM compounds takes place almost exclusively in internal mixers. Onward processing can be carried out by any of the methods commonly used in the rubber industry.
Properties of ethylene-propylene-diene rubber
EPDM vulcanisates have excellent chemical resistance to dilute acids, alkalis, acetone, alcohol, ketones and hydraulic fluids such as brake fluid. Because of their non-polar character, they are attacked by aliphatic, chlorinated and aromatic hydrocarbons, petrol and oils and swell strongly in these substances. Direct contact with concentrated mineral acids can cause the vulcanisates to harden or break down.
The mechanical properties of ethylene-propylene-diene rubber are determined by the type and amount of fillers that are used. The following characteristics are typical:
- Good strength values (especially in the case of sequential vulcanisates)
- Hardness can be varied over a broad range
- Good elastic properties
- Low compression set
- Tear propagation resistance at elevated temperature comparable with that of natural rubber (NR)
- Low-temperature flexibility comparable with NR vulcanisates
- Operating temperature range -70 to +150°C (up to +170°C for optimally adjusted peroxide vulcanisates)
- Heat deformation resistance comparable with vulcanisates of isobutene-isoprene rubber (IIR) (much lower than in silicone or AEM vulcanisates)
- Relatively good electrical properties even after hot air ageing or at elevated temperatures
- Excellent electrical insulating properties (also suitable for high-voltage cable insulation)
- Outstanding resistance to high-voltage corona discharges
- Very good resistance to hot water and steam under high pressure
- Very good sunlight and ozone resistance
- Resistant to non-mineral oil based brake fluid
- Very good ageing resistance
Ethylene-propylene-diene rubber is non-melting and does not deform, even under the influence of heat. High temperatures cause the material to break down or decompose. EPDM is non-soluble and cannot be welded.
Characteristics of commercial EPDM rubber grades
The ethylene content of commercial ethylene-propylene-diene rubber is between 45 and 75 percent. Grades with an ethylene content of 45 to 55 weight percent are amorphous and offer the best low-temperature flexibility. If the ethylene content rises, the crystallinity increases too. EPDM grades with an ethylene content of 55 to 65 weight percent are partially crystalline. Terpolymers containing more than 65 weight percent ethylene have larger crystalline regions and behave like thermoplastic elastomers; even in the uncured state their tear strength is very high.
The diene content of commercial products is between 2 and 12 weight percent. This corresponds to 3 to 16 double bonds per 1000 carbon atoms. A higher diene content leads to a higher crosslinking rate, higher strength values and a lower permanent deformation. By contrast, the weathering, ozone and ageing resistance drops as the diene content increases.
Use of ethylene-propylene-diene rubber
There is almost no limit to the range of uses of EPDM. Applications extend from the automotive industry, through heating and sanitary applications to power plant and air-conditioning technology. Thanks to its superb properties, it has replaced natural rubber in many areas.
The saturated backbone structure in EPDM leads to properties such as high weather and ozone resistance and high thermal resistance. Ethylene-propylene-diene rubber is also resistant to chemicals, so among other things it is used for seals such as O-rings in mechanical seals or flat seals. In addition, it is a common material for hoses used for hot water or steam.
Blended with other polymers, EPDM plays an important part in the production of thermoplastic vulcanisates (TPV) and elastomer-modified plastomers (EMP). The main areas of application include cable sheaths and insulation along with weather- and heat-resistant or sea water-resistant technical products.
EPDM rubber in automotive engineering
In the automotive sector, ethylene-propylene-diene rubber is used in the form of hoses (e.g. radiator hoses), seals and profiles (e.g. door seals) and diaphragms. Special grades of EPDM can also be used for brake fluids based on glycol ethers and polyglycols. It is completely incompatible with mineral oil products (fuels, lubricants), however.
EPDM rubber in construction
Ethylene-propylene-diene rubber is used to manufacture high-quality construction and flat roof membranes. Special designs include fabric- or bitumen-coated EPDM sealing membranes. EPDM is also used for absorber mats for heating swimming pools. These mats are UV, weathering and ozone resistant, resistant to chlorinated water, weight-bearing and also frost-proof when filled. High-quality pond liners, which are UV and ageing resistant and are highly flexible even at cold temperatures, are also made from EPDM. In addition, flexible low-pressure gas storage in biogas plants is achieved with membranes made of ethylene-propylene-diene rubber.
Other applications of EPDM rubber
Other typical products made from EPDM include conveyor belts, dock fenders, domestic appliance parts, roller coverings, industrial and washing machine hoses, speaker diaphragms and flexible pipes. EPDM can also be foamed with blowing agents to make sponge rubber, which has exceptionally good resilience and high elasticity of compression and is used for glass run channels and seals, for example.