Polyethylene is categorized by density and molecular structure into distinct types with different properties. Low-Density Polyethylene (LDPE) has a highly branched molecular structure with density of 0.910–0.940 g/cm³, providing flexibility, toughness, and excellent film properties. Linear Low-Density Polyethylene (LLDPE) features a linear backbone with short, uniform side branches and density of 0.915–0.940 g/cm³, delivering higher tensile strength and puncture resistance than LDPE while maintaining flexibility. High-Density Polyethylene (HDPE) has a linear structure with minimal branching and density of 0.941–0.970 g/cm³, offering maximum stiffness, strength, and chemical resistance. Ultra-High-Molecular-Weight Polyethylene (UHMWPE) provides exceptional wear resistance and impact strength for specialized applications including medical implants and ballistic protection.

Polyethylene is semi-crystalline, with crystallinity ranging from 40–50% for LDPE to 70–90% for HDPE. The degree of crystallinity directly affects mechanical properties, processing behavior, and application performance. Higher crystallinity increases tensile strength, stiffness, hardness, and heat resistance while reducing flexibility and impact resistance. Crystalline regions act as physical crosslinks, improving dimensional stability and chemical resistance. The semi-crystalline structure causes significant processing shrinkage (1.5–4%) as the material crystallizes during cooling. LDPE's lower crystallinity provides transparency and flexibility for film applications. HDPE's higher crystallinity delivers opacity and rigidity for bottles and containers. Understanding crystallinity helps predict part performance and select appropriate processing parameters.

Polyethylene and polypropylene are both polyolefins with similar chemical resistance and low cost, but differ in key properties. PE offers better low-temperature impact resistance, with LDPE remaining flexible below 0 °C while PP becomes brittle. PE provides superior moisture barrier properties with lower water absorption. PP delivers higher heat resistance, with continuous use temperatures 20–30 °C higher than PE. PP offers greater stiffness at similar densities compared to LDPE and LLDPE. PE demonstrates better stress crack resistance in many chemical environments. PP enables living hinge designs through exceptional fatigue resistance. PE costs slightly less than PP for commodity grades. HDPE competes directly with PP for rigid containers, with PE offering better impact resistance and PP providing better heat resistance for hot-fill and dishwasher applications.

Food-contact polyethylene grades meet FDA 21 CFR 177.1520 requirements and European food contact regulations EC 1935/2004 and EU 10/2011. LDPE is widely used for flexible food packaging, wraps, and squeeze bottles. HDPE serves in milk jugs, juice bottles, food storage containers, and cutting boards. These grades withstand refrigeration, freezing, and microwave heating without leaching harmful substances. Medical-grade PE meets USP Class VI and ISO 10993 biocompatibility requirements. LDPE is used in pharmaceutical packaging, medical tubing, and catheter applications. HDPE serves in rigid medical containers and diagnostic equipment housings. UHMWPE provides biocompatible bearing surfaces for hip and knee joint replacements. Medical grades support sterilization through gamma radiation, electron beam, and ethylene oxide without significant property degradation.

Polyethylene is recyclable through established collection and processing infrastructure. HDPE carries recycling code 2 and LDPE carries code 4. Post-consumer HDPE from bottles and containers recycles into new bottles, plastic lumber, drainage pipes, and non-food containers. LDPE film recycling produces trash bags, shipping envelopes, and composite lumber. Recycling requires sorting by type, cleaning, and reprocessing through extrusion. HDPE demonstrates better recyclability than LDPE due to higher strength and easier processing. Mechanical recycling may cause minor property degradation, but recycled PE maintains acceptable performance for many applications. Cross-contamination between LDPE and HDPE reduces recycled material quality. Advanced sorting technologies improve recycling stream purity. Some suppliers offer recycled-content PE grades and bio-based PE produced from renewable ethanol feedstocks.