Polyolefins

A polyolefin is a type of polymer with the general formula (CH₂CHR)_n where R is an alkyl group. They are usually derived from a small set of simple olefins ( alkenes). Dominant in a commercial sense are polyethylene and polypropylene. More specialized polyolefins include polyisobutylene and polymethylpentene. They are all colorless or white oils or solids. Many copolymers are known, such as polybutene, which derives from a mixture of different butene isomers. The name of each polyolefin indicates the olefin from which it is prepared; for example, polyethylene is derived from ethylene, and polymethylpentene is derived from 4-methyl-1-pentene. Polyolefins are not olefins themselves because the double bond of each olefin monomer is opened in order to form the polymer. Monomers having more than one double bond such as butadiene and isoprene yield polymers that contain double bonds ( polybutadiene and polyisoprene) and are usually not considered polyolefins. Polyolefins are the foundations of many chemical industries.

Industrial polyolefins

Most polyolefin are made by treating the monomer with metal-containing catalysts. The reaction is highly exothermic.

Traditionally, Ziegler–Natta catalysts are used. Named after the Nobel laureates Karl Ziegler and Giulio Natta, these catalysts are prepared by treating titanium chlorides with organoaluminium compounds, such as triethylaluminium. In some cases, the catalyst is insoluble and is used as a slurry. In the case of polyethylene, chromium-containing Phillips catalysts are used often. Kaminsky catalysts are yet another family of catalysts that are amenable to systematic changes to modify the tacticity of the polymer, especially applicable to polypropylene.

; Thermoplastic polyolefins
: low-density polyethylene (LDPE),
: linear low-density polyethylene (LLDPE),
: very-low-density polyethylene (VLDPE),
: ultra-low-density polyethylene (ULDPE),
: medium-density polyethylene (MDPE),
: polypropylene (PP),
: polymethylpentene (PMP),
: polybutene-1 (PB-1);
: ethylene-octene copolymers,
: stereo-block PP,
: olefin block copolymers,
: propylene–butane copolymers;
; Polyolefin elastomers (POE)
: polyisobutylene (PIB),
: poly(a-olefin)s,
: ethylene propylene rubber (EPR),
: ethylene propylene diene monomer (M-class) rubber (EPDM rubber).

Properties

Polyolefin properties range from liquidlike to rigid solids, and are primarily determined by their molecular weight and degree of crystallinity. Polyolefin degrees of crystallinity range from 0% (liquidlike) to 60% or higher (rigid plastics). Crystallinity is primarily governed by the lengths of polymer's crystallizable sequences established during polymerization. Examples include adding a small percentage of comonomer like 1-hexene or 1-octene during the polymerization of ethylene, or occasional irregular insertions ("stereo" or "regio" defects) during the polymerization of isotactic propylene. The polymer's ability to crystallize to high degrees decreases with increasing content of defects.

Low degrees of crystallinity (0–20%) are associated with liquidlike-to-elastomeric properties. Intermediate degrees of crystallinity (20–50%) are associated with ductile thermoplastics, and degrees of crystallity over 50% are associated with rigid and sometimes brittle plastics.

Polyolefin surfaces are not effectively joined together by solvent welding because they have excellent chemical resistance and are unaffected by common solvents. They inherently have very low surface energies and don't wet-out well (the process of being covered and filled with resin). They can be adhesively bonded after surface treatment, and by some superglues ( cyanoacrylates) and reactive (meth) acrylate glues. They are extremely inert chemically but exhibit decreased strength at lower and higher temperatures. As a result of this, thermal welding is a common bonding technique.

Practically all polyolefins that are of any practical or commercial importance are poly-''alpha''-olefin (or poly-α-olefin or polyalphaolefin, 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. Alpha-olefins such as 1-hexene may be used as co-monomers to give an alkyl branched polymer (see chemical structure below), although 1-decene is most commonly used for lubricant base stocks.

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 align themselves up side-by-side in an orderly way. This results in lower contact surface area between the molecules and decreases the intermolecular interactions between molecules. 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 and can be used over 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.

Uses

* Polyethylene:
** HDPE: used for film (wrapping of goods), blow molding (e.g. bottles), injection molding (e.g., toys, screw caps), extrusion coating (e.g., coating on milk cartons), piping for distributing water and gas, insulation for telephone cables. Wire and cable insulation.
** LDPE: mainly (70%) used for film.

* Polypropylene: injection molding, fibers, and film. Compared to polyethylene, polypropylene is stiffer but less prone to breaking. It is less dense but shows more chemical resistance.
* Synthetic base oil (by far the most used one): industrial and automotive lubricants.

Polyolefins are used for blow moulded or rotationally moulded components, e.g. toys, for heat-shrink tubing used to mechanically and electrically protect connections in electronics, and for rash guards or undergarments for wetsuits.

Polyolefin sheets or foams are used in a wide variety of packaging applications, sometimes in direct contact with food.

Polyolefin elastomer POE is used as a main ingredient in the molded flexible foam technology such as in the fabrication of self skinned footwear (for example, Crocs shoes), seat cushions, arm rests, spa pillows, etc. Hydrogenated polyalphaolefin (PAO) is used as a radar coolant. Head makes polyolefin tennis racket strings. Polyolefin is also used in pharmaceutical and medical industry for HEPA filter certification—a PAO aerosol is passed through the filters and the air that exits is measured with an aerosol detector.

Elastolefin is a fiber used in fabrics. IKEA's Better Shelter uses structural panels made out of polyolefin foam, stating, "They are tough and durable.". Piping systems for the conveyance of water, chemicals or gases are commonly produced in Polypropylene, and to a much greater extent Polyethylene. Piping systems in high-density Polyethylene (HDPE, PE100, PE80) are fast becoming the most commonly used drinking water, waste water and natural gas distribution piping systems in the world.

Polyalphaolefin, commonly referred to as a synthetic hydrocarbon, is used in various types of air compressors and turbines including reciprocating, centrifugal, and rotary screw compressors where high pressures and temperatures can be an issue. These base fluids are the most widely used variety of synthetic oil blends mainly for their ability to maintain performance in spite of temperature extremes and their similarity to—but improved performance over—mineral oil base fluids.

Polypropylene is commonly used in car bumpers, interior trims, and other components where TiO₂ is added to improve the UV stability of the plastic, ensuring that parts do not degrade or lose color when exposed to sunlight over time. Polyethylene films are widely used in agriculture for greenhouses, mulching, and silage wraps.

Recycling

Despite hype rosier than practice, real recycling of polyolefins has been insufficient in the decades since they became ubiquitous, often not due to technical limitations but because of economic realities. Polyolefin waste can potentially be converted into many different products, including pure polymers, naphtha, clean fuels, or monomers, but only to the extent that money-losing processes are not required, in the reality of the business world. In the 2020s, improved catalysts have been developed that may bring commercial recycling of polyolefins closer to a circular economy of recovery of the monomers, more comparable to the existing situation with PET polyester bottles.

References

External links

MSDS (Material Safety Data Sheet)

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Plastics