Types of polyethylenes
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Polyethylene is classified into several different categories based mostly on its density and branching. The mechanical properties of PE depend significantly on variables such as the extent and type of branching, the crystal structure, and the molecular weight.
UHMWPE is polyethylene with a molecular weight numbering in the millions, usually between 3.1 and 5.67 million. The high molecular weight results in less efficient packing of the chains into the crystal structure as evidenced by densities less than high density polyethylene (e.g. 0.935 - 0.930). The high molecular weight results in a very tough material. UHMWPE can be made through any catalyst technology, although Ziegler catalysts are most common. UHMWPE is used in high modulus fibers and in bulletproof vests.
HDPE is defined by a density of greater or equal to 0.941 g/cc. HDPE has a low degree of branching and thus stronger intermolecular forces and tensile strength. HDPE can be produced by chromium/silica catalysts, Ziegler-Natta catalysts or metallocene catalysts. The lack of branching is ensured by an appropriate choice of catalyst (e.g. Chromium catalysts or Ziegler-Natta catalysts and reaction conditions.
PEX is a medium- to high-density polyethylene containing cross-link bonds introduced into the polymer structure, changing the thermoplast into an elastomer. The high-temperature properties of the polymer are improved, its flow is reduced and its chemical resistance is enhanced. PEX is used in some potable water plumbing systems, as tubes made of the material can be expanded to fit over a metal nipple, and it will slowly return to its original shape, forming a permanent, water-tight connection.
MDPE is defined by a density range of 0.926 - 0.940 g/cc. MDPE can be produced by chromium/silica catalysts, Ziegler-Natta catalysts or metallocene catalysts.
LLDPE is defined by a density range of 0.915 - 0.925 g/cc. is a substantially linear polymer, with significant numbers of short branches, commonly made by copolymerization of ethylene with short-chain alpha-olefins (e.g. 1-butene, 1-hexene, and 1-octene.
LDPE is defined by a density range of 0.910 - 0.940 g/cc. LDPE has a high degree of short and long chain branching, which means that the chains do not pack into the crystal structure as well. It has therefore less strong intermolecular forces as the instantaneous-dipole induced-dipole attraction is less. This results in a lower tensile strength and increased ductility. LDPE is created by free radical polymerization. The high degree of branches with long chains gives molten LDPE unique and desirable flow properties.
VLDPE is defined by a density range of 0.880 - 0.915 g/cc. is a substantially linear polymer, with high levels of short chain branches, commonly made by copolymerization of ethylene with short-chain alpha-olefins (e.g. 1-butene, 1-hexene, and 1-octene. VLDPE is most commonly produced using metallocene catalysts due to the greater co-monomer incorporation exhibited by these catalysts.
The most common household use of HDPE is in containers for milk, liquid laundry detergent, etc; the most common household use of LDPE is in plastic bags. LLDPE is used in flexible tubing and in bags either neat or blended with LDPE.
HDPE is also widely used in the fireworks community. In tubes of varying length (depending on the size of the ordinance), HDPE is used as a replacement for the supplied cardboard mortar tubes for two primary reasons. One, it is much safer than the supplied cardboard tubes because if a shell were to malfunction and explode inside (flower pot) an HDPE tube, the tube will not shatter. The second reason is that they are reusable allowing designers to create multiple shot mortar racks. All pyrotechnicians discourage the use of PVC tubing in mortar tubes because it will shatter, sending shards of plastic at possible spectators, and will not show up in x-rays.
Recently, much research activity has focused on the nature and distribution of Long Chain Branches in polyethylene. These branches are present in all polyethylenes to some degree, and are very common in LDPE. In HDPE however, a relatively small number of these branches (perhaps 1 in 100 or 1000 branches per backbone carbon) can significantly affect the rheological properties of the polymer.
