Polythene is the world's most common plastic. It finds innumerable applications in everything from bottles and jugs to shopping bags and children's toys. It owes its surprising versatility to its properties and molecular structure. There are different kinds of polythene available; two of the most common are high-density polythene or HDPE and low-density polythene or LDPE. While they have many similarities, they are distinguished by several important differences.
Ethylene is a simple hydrocarbon with a pair of double-bonded carbon atoms and four hydrogen atoms. All polyethylenes are long polymers of ethylene subunits. They have a central chain of carbon atoms, each of which is bonded to the carbon ahead of it and the carbon behind it and two hydrogen atoms. Some polyetheylenes have secondary chains that branch out from the first chain; HDPE, however, is unbranched. This structural difference between HDPE and other polyethylenes is important for determining its function.
Since HDPE is unbranched, the polymers or chains can be packed together much more tightly, making HDPE more dense than LDPE or other similar polyethylenes. Some polyethylenes formed from exceptionally long chains are even more dense than HDPE; these are called ultra high molecular weight polyethylenes (UHMWPEs).
Differences in structure and density make HDPE tougher, stronger and more opaque than LDPE, although not so strong as UHMWPE. Its ability to withstand heat and its chemical resistance are also somewhat superior to LDPE although not quite so good as that of UHMWPE.
Since it's stronger and more opaque than most other polyethylenes, HDPE is often used for bottles, jugs, trays and containers. LDPE, medium density polythene (MDPE) and very low density polythene (VLDPE) are more often used for shrink wrap, plastic bags, films, hoses and tubing and other applications. UHMWPE is so strong it's sometimes used for machine parts, edge protection on ice rinks and even artificial joints.