Types of epichlorohydrin rubber
There are three types of epichlorohydrin rubber:
- Epichlorohydrin homopolymer (CO) produced in solution
- Copolymers of epichlorohydrin and ethylene oxide (ECO), and
- Terpolymers of epichlorohydrin, ethylene oxide and a diene with a lateral double bond (ETER).
All three variants are saturated rubbers. That is true even in the case of ETER, since the diene double bond is located in the side group rather than the main group. The three types can be blended together in order to achieve beneficial combinations of properties for specific applications, e.g. for hoses and diaphragms. CO-ECO blends are the commonest form.
CO is the most heat-resistant and has the highest resistance to ozone and weathering influences. Ethylene oxide-epichlorohydrin rubber has the best low-temperature resistance, remaining elastic until just before embrittlement. ETER can be vulcanised with sulfur and accelerators, so it can be optimised to the processing method during vulcanisation. This reduces the tendency towards mould contamination. However, in comparison to ECO, the heat resistance is lower and the tendency to creep is significantly higher.
Properties of epichlorohydrin rubbers
The properties of epichlorohydrin rubbers are comparable to those of polyacrylic rubbers. They are resistant to fuels and hot oils and adequately resistant to methanol-based motor fuels. CO and ECO elastomers have a similar swelling resistance to acrylic rubber (ACM) or nitrile rubber (NBR), but better low-temperature flexibility. The gas permeability is similar to that of isobutylene-isoprene rubber (IIR), the flame resistance equivalent to that of chloroprene rubber (CR). The weathering and ozone resistance and damping properties are excellent.
Specific properties of ECO:
- Very low gas permeability and excellent ozone resistance
- Good weathering resistance
- Acid and base stability
- Excellent low-temperature flexibility
- Good resistance to oils, fats, fuels and water
- Electrically conductive
- Good ageing resistance
- Operating range from -40 to +135°C
Curing of ethylene oxide-epichlorohydrin rubber
Unlike thermoplastic materials, elastomers are usually produced by crosslinking crude rubbers. Crosslinking the molecule chains gives the material its final elastic properties.
ECO differs from other rubber compounds in the way that it is cured. This material cannot be cured with sulfur or peroxides, so metal salts or metal oxides in combination with ethylene thiourea (ETU) or diamines are used as crosslinking agents. In addition, like all chlorine-containing polymers, it has to be stabilised by means of acid acceptors in the crosslinking system. The vulcanisates require relatively long cure times and have to be annealed in some cases.
Non-hazardous ECO compounds for the automotive sector
For reasons of environmental and human health protection, epichlorohydrin rubber compounds with ETU or lead-based curing systems can no longer be used in car manufacturing, and their use in other industrial applications is restricted. Therefore, lead-free and ETU-free curing systems based on special materials are used in such cases. In terms of mechanical properties, heat resistance and resistance to fuels, the performance of these compounds is the same as or similar to that of compounds with lead or ETU curing systems. In some cases the vulcanisates actually outperform standard epichlorohydrin rubber.
Through the use of special retarding agents, the storage stability of the environmentally friendly products has been extended to over six weeks. With regard to material and production costs, the new compounds are similar to their lead- and ETU-containing counterparts.
Possible applications for epichlorohydrin rubber
Epichlorohydrin elastomers are used in sectors which require a combination of good fuel and mineral oil resistance together with high heat resistance and low-temperature flexibility. The main area of application for these materials is the automotive industry.
Unlike thermoplastics, which are used as interior components such as instrument panels, seats, buttons and levers in full view of the driver and passengers, ECO components tend to be hidden away. The vulcanisates are used as diaphragms, inner tubes for fuel systems and heat-resistant damping elements, for example. Despite being more expensive, they have managed to replace NBR in some areas.
ECO vulcanisates are also used for roller elements, rubber coatings for textiles, and moulded items.