Polyolefin

Polyethylene and polypropylene

Although many specialized kinds of polyolefins are known, two families dominate: polyethylene (PE) and polypropylene (PP). Within the realm of polyethylene, variations involve the incorporation of comonomers. Introduced at the level of a few percent, these components strongly affect the properties of the polyethylene. Comonomers include 1-octene, 1-hexene, etc. In some cases the resulting polyethylenes are referred to as ethylene-octene, ethylene-hexene, etc. copolymers.

Within the realm of polypropylene, variations involve the tacticity (stereochemistry or connectivity) of the individual propylene subunits. The methyl branch groups on a polypropylene polymer are not long enough to make typical commercial polypropylene more flexible than polyethylene.

Production

The polymerization of ethylene and propylene is highly exothermic but still requires catalysts. For polyethylene, Ziegler–Natta catalysts are often 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 used as a slurry. Chromium-containing Phillips catalysts are used also. When applied to the polymerization of ethylene, these catalysts produce high density polyethylene (HDPE) and linear (vs branched) low density polyethylene (LLDPE). Kaminsky catalysts represent a related family of catalysts but are homogeneous (soluble in reaction medium). Because all catalytic centers are identical ("single site"), Kaminsky catalysts are particularly amenable to systematic changes to modify the tacticity of the polymer, especially applicable to polypropylene.

Branched forms of polyethylene, so-called low density polyethylene (LDPE), are produced by free-radical-catalysis. The reaction is conducted at high temperatures (>200 °C) and high pressures. A suitable initiator is azobisisobutyronitrile (AIBN), which breaks down to give alkyl radicals.

Properties

Properties, which range from liquid-like to rigid solids, are primarily determined by their molecular weight and degree of crystallinity. 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 crystallinity over 50% are associated with rigid and sometimes brittle plastics.

Polyolefins lack polar substituents. This deprives these materials of certain properties, in particular dye-ability and adhesion. Those properties require the incorporation of polar groups.

Joining and welding

They inherently have very low surface energies. As a result, thermal welding is a common bonding technique.

They have excellent chemical resistance and are unaffected by common solvents. Consequently, polyolefins are not amenable to solvent welding. They can be adhesively bonded after surface treatment, and by some superglues (cyanoacrylates) and reactive (meth)acrylate glues.

Uses

Polyethylene:

• HDPE: 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, 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.
• automotive applications, 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.