High-density polyethylene (HDPE) is a thermoplastic polymer made from the monomer ethylene. It is characterized by a linear molecular structure with minimal branching, which allows its molecules to be packed closely together, thus giving it a high strength-to-density ratio and excellent tensile strength.
HDPE also features high durability, including resistance to many chemicals, to impact, to moisture and to ultra-violet radiation. It is non-leaching and thus said to be safe for use for food and beverage containers and for water pipes.
Moreover, its production requires significantly less energy than for materials for which it is a good substitute in some applications, such as steel, thus resulting in lower greenhouse gas emissions.
Disadvantages include poor weathering resistance, vulnerability to stress cracking, difficulty of bonding and gluing, flammability and difficulty of recycling.
Although HDPE is technically recyclable, a major challenge is the difficulty of removing contaminants such as adhesive labels, food residues, dirt, and incompatible resins, especially polypropylene, using conventional mechanical recycling processes, thus limiting the applications of the recycled materials. Recycling also requires significant energy and water, especially during cleaning, which produces contaminated wastewater that can damage aquatic ecosystems.
Major applications include plastic bottles, household containers, cutting boards, chemical containers, water and sewage pipes, plastic lumber, toys, and some medical products.
Low-density polyethylene (LDPE) differs from HDPE in that it has a highly branched polymer chain, which prevents the tight packing of its molecules and thus results in a lower density, less rigidity and greater flexibility, increased transparency and poor UV resistance. These properties make it suitable for very different applications than HDPE, including plastic bags, shrink wrap, packaging and medical bags.
Recycling HDPE (RID 2) is generally much easier than for LDPE (RID 4) because the latter is both more prone to contamination and can more easily get caught in and jam recycling machinery because of its softness and flexibility. Some sources estimate worldwide recycling rates of roughly ten percent for HDPE versus only about one or two percent for LDPE.