Polyethylene plastic grades are distinguished by their density, which varies due to differences in crystallization structure, molecular weight, and branching. HDPE and LDPE are the most common types, used for packaging and films respectively. Other types include PEX, VLDPE, and UHMWPE, each with specific benefits. The density of polyethylene affects its environmental impact, but most types can be recycled, with HDPE being the best candidate.
Polyethylene density is a characteristic used to distinguish polyethylene plastic grades expressed as g/cm3. This measurement varies among petroleum-derived plastics due to differences in the crystallization structure formed during the polymerization of ethene. Also, the density is influenced by the molecular weight of the final product. Branching, or the bonding of monomers of ethene to hydrogen molecules to form polymers, also determines density. Indeed, the latter factor is the main difference between high-density polyethylene (HDPE) and low-density polyethylene (LDPE).
Most people are familiar with HDPE and LDPE plastics. However, there are a number of other density grades of polyethylene. This is because different types of polyethylene are obtained according to the methods used to induce polymerization. For example, material produced by the addition of a radical initiator (radical polymerization) often produces a different density of polyethylene than that obtained by the use of a catalyst, such as chromium and/or silica. Additionally, each type offers specific benefits.
HDPE has 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 mainly used to make plastic films, bags and cling film.
PEX, on the other hand, is a medium-high density polyethylene. While it may not be as dense as HDPE, its cross-linked crystallization structure gives it a truly unique and valuable property: elasticity. For this reason, PEX is used in the plumbing industry, where it can be expanded over fixtures and then allowed to shrink down to size to form a watertight seal.
Very low density polyethylene (VLDPE) has a density range of 0.880 to 0.915 g/cm3 and is used to make frozen food storage bags and plastic wraps. 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 a very durable material that is resistant to chemical and environmental erosion. For these reasons, this type of polyethylene is used to make bulletproof vests and artificial replacement parts for hip and knee joints.
As you might expect, the density of polyethylene affects the environmental impact of the product. Generally, plastic does not biodegrade, but remains intact in the environment for hundreds of years. However, most polyethylene plastics can be recycled to some extent. 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 returned to the store or manufacturer for recycling.
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