Polyolefin

Polyolefin

Family of related polymers

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Polyolefins are a family of polymers produced from olefins, majorlyethylene and propylene. They are used in various applications and represent around two-thirds of the global consumption of synthetic polymers. Polyolefins are colorless or white oils or solids, and different types include polyethylene (PE), polypropylene (PP), and more specialized forms. Polyolefins are the backbone of numerous chemical industries.

Industrial Polyolefins

Most polyolefins are synthesized by treating the monomer with metal-containing catalysts in a highly exothermic reaction. The traditional catalysts for this purpose are Ziegler-Natta catalysts, named after Nobel Prize winners Karl Ziegler and Giulio Natta. These catalysts are made by treating titanium chlorides with organoaluminium compounds, such as triethylaluminium. In some cases, the catalysts are insoluble and used as a slurry. For polyethylene, chromium-containing Phillips catalysts are frequently utilized. Kaminsky catalysts also allow systematic alterations to adjust the tacticity of polymers, particularly polypropylene.

Thermoplastic Polyolefins

The diverse types of thermoplastic polyolefins include:

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

The properties of polyolefins range from liquid-like to rigid solids, depending on molecular weight and degree of crystallinity. Crystallinity, determined during polymerization, can range from 0% (liquid-like) to over 60% (rigid plastics). High degrees of crystallinity yield rigid, sometimes brittle materials, while lower degrees result in more elastic properties.

For enhanced flexibility, copolymers can be created by introducing small amounts of comonomers during polymerization. Examples include polyethylene copolymerized with alpha-olefins like 1-hexene or 1-octene, providing more flexibility than standard high-density polyethylene (HDPE).

While polyolefin surfaces resist solvent welding due to excellent chemical resistance, they can be adhesively bonded after specific surface treatments and using certain glues like cyanoacrylates and (meth)acrylate glues. Thermal welding is a common joining technique given their thermal properties.

Most commercially significant polyolefins are poly-alpha-olefins (PAOs), made by polymerizing alpha-olefins. These polymers, with flexible branched chains, are used in synthetic lubricants, including synthetic motor oils, due to their ability to remain viscous in low temperatures.

Uses

• Polyethylene:
  • High-density polyethylene (HDPE): Used for films, blow molding (e.g., bottles), injection molding, insulation for cables, and water and gas distribution piping.
  • Low-density polyethylene (LDPE): Mainly used for films (about 70%).
• Polypropylene: Used in injection molding, fibers, and films. It is stiffer than polyethylene but has better chemical resistance and lower density.
• Synthetic base oil: Widely used in industrial and automotive lubricants.

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