The properties of polyethylene vary depending on the grade of the virgin material used to synthesize this thermoplastic and its structural crystallization. Generally, the integrity of end product polyethylene-based material in the plastics industry is based on density and melting point. However, since this substance can be produced by a number of methods to polymerize ethane, each will yield differences in specific polyethylene properties. The variations likely to lead to these differences involve molecular weight, density, and the degree of branching of carbon molecular attachment.
Most people are familiar with the two most common forms of this polymer: Low-density polyethylene (LDPE) and high-density polyethylene (HDPE). Since the polyethylene properties of the former contain a greater degree of long chain branching, the matrix is less dense and doesn’t offer as much tensile strength as the latter. Since this is the case, LDPE is less expensive to produce and process than similar polymers.
HDPE, on the other hand, is a more durable grade of polyethylene due to a higher density and linear crystallization. The polyethylene properties of this material make it suitable for use in producing many types of stress-resistant plastics intended for consumer and commercial use. For example, HDPE is used to make gallon-sized milk containers, as well as plumbing fixtures.
As previously mentioned, polyethylene properties differ depending on the method of polymerization employed. The process may be induced by stimulating free radical polymerization, by the introduction of an alkali amide or secondary monomer (anionic addition polymerization and ion coordination polymerization, respectively), or by the use of a catalyst. In addition, the application of high voltage is used to achieve extended chain crystal polyethylene (ECC), which results in dense crystallization but renders transparency.
Catalysts are often the method of choice to obtain material that exhibits desirable polyethylene properties. For instance, ultra high molecular weight polyethylene (UHMWPE) and high molecular weight polyethylene (HMWPE) are both catalyst induced and have an unusually elevated density and molecular weight. Due to a high degree of durability, UHMWPE is used to manufacture various machine parts, artificial joints and implants, and bulletproof vests. HMWPE is used to make chemical resistant articles, such as fuel and storage tanks.
In addition to density, molecular weight, and stress resistance, other polyethylene properties are considered to rate function and application. These include shock or impact strength, shear rate -- rate of polymer melt and flow -- and sag resistance, which refers to how the material holds up to melting and remolding. All of these factors impact how the material will perform during processing. In addition, since much of the raw material used in the plastics industry today may come from recycled polymer resins, polyethylene properties can vary widely, even between different batches presented as the same material.