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Chemistry and Manufacturing Processes
Ethylene-propylene rubbers use the same chemical building blocks or monomers as polyethylene (PE) and polypropylene (PP) thermoplastic polymers. These ethylene (C2) and propylene (C3) monomers are combined in a random manner to produce rubbery and stable polymers. A wide family of ethylene-propylene elastomers can be produced ranging from amorphous, non-crystalline to semi-crystalline structures depending on polymer composition and how the monomers are combined. These polymers are also produced in an exceptionally wide range of Mooney viscosities (or molecular weights).
The ethylene and propylene monomers combine to form a chemically saturated, stable polymer backbone providing excellent heat, oxidation, ozone and weather aging. A third, non-conjugated diene monomer can be terpolymerized in a controlled manner to maintain a saturated backbone and place the reactive unsaturation in a side chain available for vulcanization or polymer modification chemistry. The terpolymers are referred to as EPDM (or ethylene-propylene-diene with “M” referring to the saturated backbone structure). An EPDM polymer structure is illustrated in Figure 2. The ethylene-propylene copolymers are called EPM.
The solution polymerization process is the most widely used and is highly versatile in making a wide range of polymers. Ethylene, propylene, and catalyst systems are polymerized in an excess of hydrocarbon solvent. Stabilizers and oils, if used, are added directly after polymerization. The solvent and unreacted monomers are then flashed off with hot water or steam, or with mechanical devolatilization. The polymer, which is in crumb form, is dried with dewatering in screens, mechanical presses or drying ovens. The crumb is formed into wrapped bales or extruded into pellets. The high viscosity, crystalline polymers are sold in loosely compacted, friable bales or as pellets. The amorphous polymers grades are typically in solid bales.
The slurry (or suspension) process is a modification of bulk polymerization. The monomers and catalyst system are injected into the reactor filled with propylene. The polymerization takes place immediately, forming crumbs of polymer that are not soluble in the propylene. Slurry polymerization reduces the need for solvent and solvent handling equipment, and the low viscosity of the slurry helps to control temperature and handle the product. The process is not limited by solution viscosity, so high molecular weight polymer can be produced without a production penalty. Flashing off the propylene and termonomer completes the process before forming and packaging.
Gas-phase polymerization technology was recently developed for the manufacture of ethylene-propylene rubbers. The reactor consists of a vertical fluidized bed. Monomers and nitrogen in gas form along with catalyst are fed to the reactor and solid product is removed periodically. Heat of reaction is removed through the use of the circulating gas that also serves to fluidize the polymer bed. Solvents are not used eliminating the need for solvent stripping, washing and drying. The process is also not limited by solution viscosity, so high molecular weight polymer can be produced without a productivity penalty. Continuous injection of a substantial amount of carbon black used as a partitoning aid is necessary to prevent the polymer
granules sticking to each other and to reactor walls. Products are made in a granular form to enable rapid mixing.
Processing and Vulcanization
The processing, vulcanization and physical properties of ethylene-propylene elastomers are largely controlled by the characteristics of ethylene content, diene content, molecular weight (or Mooney viscosity) and molecular weight distribution. For example, decreasing ethylene content decreases crystallinity and associated properties such as hardness and modulus. General polymer features in rubber compounding are summarized in Table III.
Table III. – General Features of Ethylene-Propylene Elastomers
Characteristics High Low
Ethylene Content Good Green Strength Fast Mixing
Flow at High Extrusion Temperatures Low Temperature Flexibility
High Tensile Strength, Modulus Low Hardness and Modulus
High Loading (Reduced Cost) Calendering and Milling
Diene Content Cure Degree and Fast Rate Scorch Resistance
Acceleration Versatility High Heat Stability
Good Compression Set Low Hardness and Modulus
High Modulus, Low Set
Molecular Weight Good Tensile, Tear, Modulus, Set Fast Mixing
High Loading and Oil Extension High Extrusion Rates
Good Green Strength Good Calendering
Collapse Resistance Low Viscosity, Scorch Resistance
MWD Overall Good Processing Low Die Swell
Extrusion Feed and Smoothness Fast Extrusion Rate
Collapse Resistance High Cure
Good Calendering and Milling Good Physicals
Conclusion
Ethylene-propylene elastomers are one of the most versatile, fastest growing and interesting synthetic rubber polymers. Excellent resistance to heat, oxidation, ozone and weather aging are expected to provide continued value in demanding automotive, construction, and mechanical goods applications. Current and emerging advanced polymerization and catalyst technologies also provide the ability to design polymers to meet application and processing needs that are important to meeting the ever-increasing demands for product quality, uniformity and performance.
References
Richard Karpeles and Anthony V. Grossi, “EPDM Rubber Technology”, Handbook of Elastomers, 2nd Ed., Anil K. Bhowmick and Howard L. Stephens [Editors], pp. 845-876, Marcel Decker, Inc., New York (2001).
John A. Riedel and Robert Vander Laan, “Ethylene Propylene Rubbers”, The Vanderbilt Rubber Handbook, 13th Ed., pp. 123-148, R.T. Vanderbilt Co., Inc., Norwalk, CT (1990).
Gary Ver Strate, “Ethylene Propylene Elastomers”, Encyclopedia of Polymer Science & Engineering, vol. 6, pp. 522-564 (1986).
