Polyethylene is a polyolefin which is an high molecular weight hydrocarbon. These are the only plastics that have a lower specific gravity than water. The main properties of polyethylene are to be weatherproof, easy to process, low cost and to warranty a good chemical resistance.
When ethylene is polymerized the result is relatively straight polymer chains. From the main chain they can branch out. We get different kinds of Polyethylenes from the varying degree of branching in their molecular structure.
– LDPE (Low Density Polyethylene) has the most excessive branching. This causes the low density to have a less compact molecular structure which is what makes it less dense. It has a density of 0.910-0.925 g/cm3. LLDPE (Linear Low Density Polyethylene) has a significant numbers of short branches. Because it has shorter and more branches its’ chains are able slide against each other upon elongation without becoming entangled like LPDE which has long branching chains that would get caught on each other. This gives LLDPE higher tensile strength and higher impact and puncture resistance than the LDPE. It has a density of 0.91-0.94 g/cm3.
Low Film for packaging is one of the largest markets for low density polyethylene (LDPE/LLDPE). Others applications are : squeeze bottles, toys, carrier bags, high frequency insulation, chemical tank linings, heavy duty sacks, general packaging, gas and water pipes.
– HDPE (High Density Polyethylene) has minimal branching of its’ polymer chains. Because it is denser it is more rigid and less permeable then the LDPE. It has a density of 0.941-0.965 g/cm3. Applications of HDPE are : chemical drums, jerricans, carboys, toys, picnic ware, household and kitchenware, cable insulation, carrier bags, food wrapping material. Low cost, high rigidity and ease of blow moulding has made the material a natural choice in gardening furnishings.
– Some others polyethylene could be find and are commonly used : MDPE (Medium Density Polyethylene), UHMWPE (Ultra High Molecular Weight Polyethylene) and XLPE (Crosslinked Polyethylene).