polyethylene

The term polyethylene is used to decribe a category of thermoplastics materials that are polymers of the base monomer ethylene. Polyethylene polymers are usually classified by the type (and length) of the polymer chains. These types include:

• low density polyethylene (LDPE)
• linear low density polyethylene (LLDPE)
• very low density polyethylene (VLDPE)
• medium density polyethylene (MDPE)
• cross-linked polyethylene (XLPE)
• high density polyethylene (HDPE)
• high density cross-linked polyethylene (HDXLPE)
• high molecular weight polyethylene (HMWPE)
• ultra high molecular weight polyethylene (UHMWPE)
• ultra low molecular weight polyethylene (ULMWPE)

Polyethylenes are members of the polyolefin family. Polyolefins are polymers of olefin monomers, which are more commonly known alkenes. (Alkenes – ethylene, propylene, butylene, butadiene, isoprene, etc – contain at least one carbon-to-carbon double bond, and are are staples of the petrochemical industry. They are chemically similar to alkanes – methane, ethane, propane, butane, etc – which have a slightly different chemistry, and contain only single carbon bonds.)

As a class, polyethylenes are the most widely used thermoplastics in the world – and for good reason. They have good mechanical properties, great chemical resistance (they are semi-crystalline after all), are very low in cost, and are easy to process. Polyethylenes can be extruded, injection molded, blow molded, rotationally molded, melt-cast, thermo-formed, heat-staked, melt-swaged, thermally fused, ultra-sonically welded, and more. They are used in a wide variety of industries, including consumer goods, industrial and medical applications, and in packaging – in both rigid and film form (did you just say shrink wrap?)

From a design engineer’s perspecitve, one of the more interesting traits of polyethylenes are their surface characteristics. While they are relatively soft materials, they have excellent lubricity and fantastic wear resistance. This makes them a great material choice for gears, cams, and mechanisms (as long as the forces are relatively low). They also have good flexural fatigue properties (which is why they are often used in packaging applications with “living hinges”.)

As I mentioned above, polyethylenes are chemically similar to alkanes, which are also known as paraffins. For most of us, the term paraffin means wax. And if you were to rub a finger on a part made from polyeythylene, you could easily sense that is has a waxy feel to it.

Wax on, wax off – go polytheyene!

P.S. Of all of the various types of polyethylene my favorite has to be UHMWPE (or as I like to call it, oomm-whoopie. I mean come on, how can you not like oomm-whoopie?)