Polyethylene is a member of the olefin family. Polyethylene can be produced with both branched and linear molecular chains. High density polyethylene is made up of linear molecular chains, as is linear low-density, high molecular weight-high density, and ultrahigh-molecular weight. Low-density polyethylene has branched molecular chains and unlike the other polyethylenes, is produced under very high pressure. This method produces molecular chains with various side chains lengths or branches. The difference between the two is in the degree of crystallinity. Branched polethylenes are limited.
The density of the polyethylene determines many of the properties of the polyethylene. The higher the density, the more stiff the material. It has a higher softening point, higher tensile and creep resistance. With higher density, some other properties are lost. Increased density means lower impact strength, elongation and flexibility. Properties can also be changed by means of changing the molecular weight. By changing the density or molecular weight when making polyethylene, a wide variety of properties can be had.
HDPE is used for small tubing up to large pipe used for natural gas, sewer pipe, or drainage lines. It can be made into profiles and can be used to coat wire and cable. A large amount of HDPE is used in blow molding to produce things like fuel tanks and containers. LLDPE is used for films or bags. They are puncture and tear resistant. LDPE is also used for films and because of the low crystallinity, they typically have good optic qualities. They are also good for coatings to help with heat sealability or moisture barriers. UHMWPE is quite different from the most of the other polyethylenes and therefore has a dedicated material description.
Polyethylene (PE) was first synthesized in the 1930s and today it accounts for the largest volume of all plastics. The features that make PE attractive as an engineering material are low cost, chemical inertness, and easy processing. Polyethylene is available in several grades, the most common of which are low-density polyethylene (LDPE) and high-density polyethylene (HDPE). The low-density grade is a highly branched polymer with lower crystallinity and density. Applications include sheets, film, and wire insulation. HDPE has a more linear structure, with higher crystallinity and density. These differences make HDPE stiffer and stronger and give it a higher melting temperature. HDPE is used to produce bottles, pipes, and housewares. Both grades can be processed by most polymer shaping methods.
Polythylenes are characterized by toughness, near-zero moisture absorption, excellent chemical resistance, excellent electrical insulating properties, low coefficient of friction, and ease of processing.
In general, they are not outstanding load-bearing materials, but high-density polyethylene can be used for some short-term light loads.
Few thermoplastics have the excellent chemical resistance and dielectric properties of polyethylenes. Soluble in some organic solvents above 140°F, polyethylenes resist bases and acids at room temperature. Their resistivity (both volume and surface) and dielectric strength are high.
Polyethylenes can be crosslinked to form infusible thermosetting materials with high heat resistance and crack resistance. Applications are in wire and cable coating, foams, and rotationally molded products.
Polyethylenes are classified by density as follows:
0.880 to 0.915 g/cu cm
ultra- or very low density
0.910 to 0.925 g/cu cm
low density
0.925 to 0.940 g/cu cm
medium density
0.941 to 0.965 g/cu cm
high density
For Polyethylene molecular weight classification the following guidelines can be used:
HDPE (High Density Polyethylene) 100,000 - 400,000
HMWHDPE (High Molecular Weight High Density Polyethylene) 500,000 - 1,999,999
VHMWHDPE (Very High Molecular Weight High Density Polyethylene) 2,000,000 - 3,199,999
UHMWPE (Ultra High Molecular Weight Polyethylene) 3,200,000 +
The primary differences among the types are in rigidity, heat resistance, chemical resistance, and ability to sustain loads. In general, as density increases, hardness, heat resistance, stiffness, and resistance to permeability increase.
Polyethylene, pronounced pahl ee EHTH uh leen, is a major synthetic polymer, an essential ingredient of plastics. Polyethylene plastic products include milk jugs, oil bottles, and plastic bags. About a third of all synthetic polymers produced in the United States are polyethylenes.
There are three main types of polyethylene: high density polyethylene (HDPE), low density polyethylene (LDPE), and linear low density polyethylene (LLDPE). HDPE is the toughest and most rigid type. Manufacturers form it into such products as bottles and jugs. LDPE and LLDPE are relatively soft and flexible. Manufacturers produce them as thin films. One use of LDPE is as bread bags. Garbage bags are made of LLDPE, which is stronger than LDPE.
A polymer is a long, chainlike molecule. The "links" are repeating patterns of simple groups of atoms called monomers. Polyethylene is made from ethylene monomers, each consisting of two carbon atoms and four hydrogen atoms.
Manufacturers make polyethylene by mixing a solution of ethylene gas with a catalyst, a substance that speeds a chemical reaction without being used up by the reaction. Many polyethylene plastics contain special ingredients, such as colorings and a substance that prevents film from sticking to itself.
Polyethylene melts at temperatures of 230 to 300 degrees F (110 to 150 degrees C). It can be melted and re-formed again and again, so it is easy to recycle.
Imperial Chemical Industries (ICI) of Great Britain first produced polyethylene in 1939. It was expensive to make, so little was produced until the 1950's, when less expensive manufacturing techniques were developed.