A paper box wrapped with polyolefin shrink film

A polyolefin is a polymer produced from a simple olefin (also called an alkene with the general formula C_nH_2n) as a monomer. For example, polyethylene is the polyolefin produced by polymerizing the olefin ethylene. An equivalent term is polyalkene; this is a more modern term, although polyolefin is still used in the petrochemical industry. Polypropylene is another common polyolefin which is made from the olefin propylene.

[edit] Polyolefin elastomer

Polyolefin elastomer (or POE) is a relatively new class of polymers that emerged with recent advances in metallocene polymerisation catalyst. It represents one of the fastest growing synthetic polymer. 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.

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).

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.

Source: The Institute of Synthetic Rubber Producers.

For more information on this source please visit The Institute of Synthetic Rubber Producers.

[edit] Properties

Polyolefins are impossible to join by solvent cementing because they have excellent chemical resistance and can only be adhesively bonded after surface treatment because they have very low surface energies. They are also extremely inert chemically and exhibit decreased strength at lower temperatures.

A more specific type of olefin is a poly-alpha-olefin (or poly-α-olefin, sometimes abbreviated as PAO), a polymer made by polymerizing an alpha-olefin. An alpha-olefin (or α-olefin) is an alkene where the carbon-carbon double bond starts at the α-carbon atom, i.e. the double bond is between the #1 and #2 carbons in the molecule. Common alpha-olefins used as co-monomers to give a polymer alkyl branching groups are similar to 1-hexene or may be longer (see chemical structure below).

1-hexene, an example of an alpha-olefin

Many poly-alpha-olefins have flexible alkyl branching groups on every other carbon of their polymer backbone chain. These alkyl groups, which can shape themselves in numerous conformations, make it very difficult for the polymer molecules to line themselves up side-by-side in an orderly way. Therefore, many poly-alpha-olefins do not crystallize or solidify easily and are able to remain oily, viscous liquids even at lower temperatures. Low molecular weight poly-alpha-olefins are useful as synthetic lubricants such as synthetic motor oils for vehicles used in a wide temperature range.

Even polyethylenes copolymerized with a small amount of alpha-olefins (such as 1-hexene, 1-octene, or longer) are more flexible than simple straight chain high density polyethylene, which has no branching. The methyl branch groups on a polypropylene polymer are not long enough to make typical commercial polypropylene more flexible than polyethylene.

[edit] Uses

Polyolefin is used for blown film as well as rash guards or under garments for wetsuits. Polyolefin eloastomer POE is used as a main ingredient in the molded flexible foam technology such as in the fabrication of self skinned footwear (think Crocs shoes), seat cushion, arm rest and spa pillow etc.

[edit] External links

Wikimedia Commons has media related to: Polyolefins

• Polymer Structure