What Is Polyethylene?

Polyethylene (PE) is a thermoplastic polymer produced by the polymerization of ethylene monomers (CH₂=CH₂) into long chain macromolecules with the repeating unit (CH₂-CH₂)n. It is the most widely produced synthetic polymer in the world by volume, with annual global output exceeding 100 million metric tons. Polyethylene's properties span an unusually wide range depending on chain length, branching architecture, and crystallinity, making the term "polyethylene" a family of related materials rather than a single substance. The polymer draws its broad utility from its chemical inertness, low density, electrical insulating properties, and ease of processing at relatively modest temperatures.

Polyethylene was first synthesized in 1933 at ICI laboratories in the United Kingdom using high-pressure radical polymerization, producing what is now classified as low-density polyethylene. The development of Ziegler-Natta catalysts in the 1950s enabled controlled polymerization at low pressure, yielding high-density grades with significantly different physical characteristics.

Molecular Structure and Grades

The classification of polyethylene grades is based primarily on chain branching and the resulting density and crystallinity. Low-density polyethylene (LDPE) is produced by high-pressure free-radical polymerization (1,000 to 3,000 bar) and contains extensive short and long side-chain branches that prevent close chain packing, yielding densities of 0.910 to 0.940 g/cm³ and crystallinity of 50 to 60 percent. High-density polyethylene (HDPE) is produced by coordination catalysis at low pressure and has a predominantly linear chain architecture, with density above 0.940 g/cm³ and crystallinity exceeding 90 percent in some grades, producing a much stiffer material. Linear low-density polyethylene (LLDPE) is copolymerized with short-chain alpha-olefins such as butene or hexene to introduce uniform short branches that improve toughness and puncture resistance without sacrificing processability. Ultra-high-molecular-weight polyethylene (UHMWPE), with molecular weights from three to six million g/mol, cannot be melt-processed but offers exceptional abrasion resistance and impact strength. Detailed comparative analysis of LDPE and HDPE properties documents how these structural differences translate directly into different processing requirements and end-use performance.

Processing and Mechanical Properties

Polyethylene is processed by extrusion, blow molding, injection molding, and film blowing. LDPE's low melt viscosity and softening point make it readily processable into thin films, where its transparency and flexibility are assets. HDPE's higher crystallinity and stiffness make it preferred for pressure pipe, rigid containers, and structural geomembranes. Tensile yield strength ranges from approximately 8 MPa for LDPE to 30 MPa or more for HDPE, while elongation at break can exceed 600 percent for LDPE, reflecting its highly ductile character. Electrical properties are consistent across grades: dielectric strength above 20 MV/m, volume resistivity around 10¹⁵ ohm-cm, and a dielectric constant near 2.3 at one megahertz. These values, combined with low dissipation factor, make polyethylene a standard dielectric material in power cables and high-frequency signal cables.

Chemical Resistance and Degradation

Polyethylene resists attack by dilute acids, alkalis, and most aqueous salt solutions at ambient temperatures, which accounts for its dominance in chemical storage tanks, laboratory ware, and food packaging. Above 60 degrees Celsius, resistance to aromatic hydrocarbons and chlorinated solvents diminishes, limiting use in elevated-temperature chemical service. Photodegradation and oxidative degradation in outdoor exposure cause embrittlement over years unless UV stabilizers are incorporated into the compound. Environmental persistence of polyethylene in marine and terrestrial ecosystems has prompted research into oxo-degradable and bio-based alternatives, as well as pyrolytic chemical recycling routes. Research on polyethylene recycling addresses the challenge of separating mixed-grade streams and restoring properties to near-virgin specifications.

Applications

Polyethylene has applications across a wide range of fields, including:

• Flexible and rigid packaging including films, bottles, and containers
• Pressure pipe systems for water, gas, and industrial fluid transport
• Electrical cable insulation and jacketing for power and communication cables
• Geomembranes for landfill liners and environmental containment
• Orthopedic implants using ultra-high-molecular-weight grades
• Agricultural film for crop protection and greenhouse coverings