Polyolefin elastomers (or POEs) are a relatively new class of polymers that emerged with recent advances in metallocene polymerisation catalysts. Representing one of the fastest growing synthetic polymers, POE’s can be substituted for a number of generic polymers including ethylene propylene rubbers (EPR or EPDM), ethylene vinyl acetate (EVA), styrene-block copolymers (SBCs), and poly vinyl chloride (PVC).
Polyolefin elastomers are compatible with most olefinic materials, are an excellent impact modifier for plastics, and offer unique performance capabilities for compounded products.
Polyolefin elastomers are copolymers of ethylene and another alpha-olefin such as butene or octene. The metallocene catalyst selectively polymerises the ethylene and comonomer sequences and increasing the comonomer content will produce polymers with higher elasticity as the comonomer incorporation disrupts the polyethylene crystallinity. Furthermore, the molecular weight of the copolymer will help determine its processing characteristics and end-use performance properties with higher molecular weights providing enhanced polymer toughness.
The Metallocene Catalyst
Polyolefin elastomers are produced using refined metallocene catalyst often referred to as single-site or constrained geometry catalysts. These catalysts have a constrained transition metal (generally a Group 4B metal such as titanium, zirconium, or hafnium) sandwiched between one or more cyclopentadienyl ring structures to form a sterically hindered polymerisation site. This unique catalyst provides a single polymerisation site instead of the multiple sites of conventional catalysts and provides the capability to tailor the molecular architecture of ethylene copolymers. (Note: Metallocene catalysts and process technologies can also be used to produce ethylene propylene rubbers).
Metallocene Catalyst Usage
The metallocene catalyst can be used in a number of polymerisation processes including slurry, solution, and gas phase operations. The catalyst is usually first mixed with an activator or co-catalyst, which can significantly enhance the polymerisation efficiencies to beyond a million units of polymer per unit of catalyst. Very low levels of the catalyst mixture are continuously metered into a reactor along with a predetermined ratio of ethylene and comonomer of choice. The molecular weight of the polymer continues to build with the polymerisation of ethylene and comonomer at the catalyst site until stopped by catalyst deactivation or chain termination with hydrogen introduction to the reactor.
Polyolefin Elastomer Processing
Polymerisation is very exothermic and requires efficient heat removal from the transport media of gas or solvent. Furthermore, reactor conditions must be carefully maintained to avoid loss of process control.
Post-reactor processes involve additives addition and isolation of the polymer from the transporting media and the high catalyst efficiencies generally do not require removal of the deactivated catalysts. The final product is then packaged per manufacturer capability and end-user need, but can range from bags to railcars.