High-Density Polyethylene (HDPE)
Definition and meaning of High-Density Polyethylene (HDPE) in chemistry.
High-density polyethylene (HDPE) is a thermoplastic polymer of ethylene monomers with very little chain branching, giving it tight molecular packing, high crystallinity, and a density of at least 0.941 g/cm³. This linear structure makes it stiffer, stronger, and more chemically resistant than its branched counterpart, low-density polyethylene (LDPE).
In more detail
HDPE is manufactured by coordination (addition) polymerization of ethylene gas, typically using Ziegler-Natta or metallocene catalysts under relatively low pressure and temperature. Because the resulting chains are almost unbranched, they pack closely together in an ordered, crystalline arrangement (often 60-80% crystallinity), which raises tensile strength, rigidity, and resistance to solvents and moisture compared to LDPE. These properties make HDPE a workhorse plastic for rigid, durable containers and piping. It is thermoplastic, so it can be melted and remolded, which supports its widespread recycling.
Key facts
| Formula | (C2H4)n, repeat unit -CH2-CH2- |
|---|---|
| Density | 0.941-0.965 g/cm3 |
| Recycling code | 2 (PETE is 1; HDPE is 2) |
| Field | Organic Chemistry |
Plastic milk jugs, detergent bottles, and municipal water pipes are commonly molded from HDPE, identifiable by the recycling code "2" often stamped into the plastic.
Frequently asked questions
What makes HDPE different from LDPE?
HDPE chains have minimal branching, so they pack tightly into a highly crystalline, dense structure. LDPE chains have significant short and long branching, which prevents tight packing and results in a softer, more flexible, less dense material.
Why is HDPE considered chemically resistant?
Its high crystallinity leaves little amorphous, permeable region for solvents, acids, or bases to penetrate, so HDPE resists chemical attack and does not readily degrade in contact with most household chemicals.