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Polyethylene density is a characteristic used to distinguish between grades of polyethylene plastics that is expressed as g/cm3. This measurement varies among petroleum-derived plastics due to differences in crystallization structure formed during the polymerization of ethene. In addition, density is affected by molecular weight of the final product. Branching, or the bonding of ethene monomers to hydrogen molecules to form polymers, also determines density. In fact, this last factor is the primary difference between high-density polyethylene (HDPE) and low-density polyethylene (LDPE).
Most people are familiar with HDPE and LDPE plastics. However, there are numerous other grades of polyethylene density. That’s because different types of polyethylene are obtained according to the methods employed to induce polymerization. For instance, material produced by the addition of a free-radical initiator (radical polymerization) often produces a different polyethylene density than one made by the use of a catalyst, such as chromium and/or silica. In addition, each type offers specific advantages.
HDPE possesses a density of at least 0.941 g/cm3 and is used to produce various packaging materials, such as garbage bags, milk containers, and laundry detergent bottles. This type of plastic is also used in the construction of tanks to hold fuel and chemicals, as well as in plumbing pipes. Its counterpart, LDPE, has a density of only 0.910 g/cm3 and is primarily used to make plastic film, bags, and cling wrap.
PEX, on the other hand, is a polyethylene of medium-to high density. While it may not be as dense as HDPE, its cross-linked crystallization structure lends it a very unique and valuable property—elasticity. For this reason, PEX is used in the plumbing industry where it can be expanded over fixtures and then permitted shrink to back to size to form a watertight seal.
Very low-density polyethylene (VLDPE) has a density range between 0.880–0.915 g/cm3 and is used to make frozen food storage bags and plastic wrap. Ultra high molecular weight polyethylene (UHMWPE), on the other hand, has a minimum density of 0.930 g/cm3. However, its high molecular weight results in very durable material that is resistant to chemical and environmental erosion. For these reasons, this type of polyethylene is used to manufacture bulletproof vests and artificial replacement parts for hip and knee joints.
As might be expected, polyethylene density affects the environmental impact of the product. Generally, plastics do not biodegrade, but linger in the environment intact for hundreds of years. However, most polyethylene plastics can be recycled to some degree. The best candidate for successful recycling is HDPE, which can be identified by the number “2” in the recycling symbol. LDPE, designated by the number “4,” can be recycled but, unfortunately, is not included in most residential collection programs and must be either returned to the store or the manufacturer for recycling.