Ethylene propylene diene monomer (EPDM) rubber is a terpolymer elastomer derived from ethylene and propylene, along with small amounts of a non-conjugated diene as a third monomer (Figure 1). With a completely saturated polymer backbone, this random elastomeric copolymer exhibits interesting properties, including excellent weathering stability and heat resistance, good insulating properties as well as good resistance to chemicals, moisture and steam.
FIGURE 1. EPDM rubber is a random copolymer of ethylene and propylene, with a diene (norbornene, in this case) added as a third monomer
EPDM has excellent ozone resistance compared to natural rubber and its synthetic substitutes, such as styrene-butadiene rubber, butadiene rubber and isoprene rubber. These features make EPDM a versatile synthetic rubber, with several applications in the automotive industry, in the construction industry and as a plastic modifier.
Commercial-scale production of EPDM rubber is mainly based on solution and suspension polymerization processes, and the polymerization conditions determine the structure of the final product obtained. Different products vary mainly according to their ethylene-to-proplyene ratio, monomer sequence distribution, molar mass and molar mass distribution, as well as by the content and type of the diene used. In this context, EPDM rubbers are produced in different grades by each manufacturer according to the specialty applications targeted, in such a way that it is not standardized in the way that general-purpose rubbers are.
The ethylene percentage of EPDM products can vary between 15 and 85%. It may be produced in different physical forms, from solid to friable bales, pellets and granular forms and oil blends.
The process
EPDM rubber production from polymer-grade (PG) propylene and ethylene comprises three major sections: (1) polymerization; (2) purification; and (3) finishing (Figure 2).
FIGURE 2. The diagram shows the polymerization process for making EPDM rubber
Polymerization. Initially, the monomers are treated and mixed and the mixture is continuously supplied to a stirred, jacketed reactor, as well as a solution containing the catalysts. The reaction occurs substantially in the liquid phase, with an excess of liquid propylene, which is used as a diluent. The rate of polymerization is controlled by the rate of catalyst addition, while temperature control is readily accomplished by controlled evaporation of the propylene. As the polymer is formed, it precipitates out of the reaction medium.
Purification. The slurry is pumped into a vessel, where reaction is terminated and the slurry is mixed with small amounts of toluene and water. The toluene allows the water to extract the catalyst entrained in the polymer. Subsequently, unreacted monomers and solvent are removed from the catalyst-free polymer slurry in steam strippers containing water as dispersing medium, and are recycled to the polymerization reactor.
Finishing. The rubber polymer, present in the form of wet crumbs in water, is fed to a drum with auxiliary agents, such as an anti-agglomerant, so as to ensure that copolymer particles stay suspended and show a reduced tendency to agglomerate. The crumb slurry, with moisture content as high as 90 wt.%, is then pumped to a rotary screen, where most of the free water is removed. The wet polymer is conveyed through a single screw extruder. While being conveyed through the extruder, water is expelled from the polymer. The extruded dried crumb, containing less than 0.5 wt.% water, is cooled down, baled and packed.
Uses and pathways
EPDM is widely used across several market segments, including automotive applications, where it can be found in weather stripping, hoses, tubing, brake components, isolators, mounts and grommets. EPDM is also found in plastics, such as in impact-modified polypropylene and thermoplastic olefins, as well as in construction, such as in roofing membrane, glass sealers, gaskets and tapes. Other industrial applications for EPDM include O-rings, mounts, cable jacketing, insulation, cable filler, connectors, footwear, and carpet underlayment.
Editor’s note: The content for this column is developed by Intratec Solutions LLC (Houston; www.intratec.us) and edited by Chemical Engineering. The analyses and models presented are based on publicly available and non-confidential information. The content represents the opinions of Intratec only.