The structural difference
LDPE is produced at very high pressure by free-radical polymerisation, which creates both long- and short-chain branches along the backbone. Those branches act like burrs: the chains cannot pack closely, crystallinity stays around 45–55%, and the resulting material is low-density, soft and easy to stretch.
HDPE is produced at low pressure with Ziegler-Natta or metallocene catalysts, giving nearly linear chains with very few branches. They pack into a crystallinity of 70–80%, which raises density, stiffness, tensile strength, melting point and barrier performance — and reduces transparency, because crystalline regions scatter light.
Property comparison
The numbers below are typical ranges for general-purpose grades; specific grades vary considerably, and copolymers such as LLDPE sit between the two.
Where each one is used
LDPE dominates anywhere flexibility, sealing or clarity is the priority: blown and cast film, bags, shrink and stretch wrap, agricultural film, squeeze bottles, cable insulation, and — importantly for the recycling world — the polymer laminate that makes beverage cartons liquid-tight.
HDPE dominates rigid, load-bearing and chemically demanding parts: milk and detergent bottles, jerrycans and IBCs, pressure and gas pipe, geomembranes, crates, fuel tanks and industrial containers. If a part must hold its shape under load or resist solvents, HDPE is the default.
- Pick LDPE for: film under 100 µm, heat-seal layers, flexible tubing, coatings, squeeze packaging.
- Pick HDPE for: pipe, rigid containers, crates, tanks, structural mouldings.
- Pick LLDPE for: stretch film and heavy-duty sacks needing puncture resistance.
- Pick a filled or recycled polyolefin for: durable non-food-contact parts where stiffness and cost matter more than clarity.
Recyclability compared
HDPE is the easier of the two to recycle at scale, mostly because of format: rigid bottles are heavy, easy for NIR sorters to detect and simple to bale, so kerbside HDPE recovery rates are high and the recyclate has a well-established market. LDPE is thin, light and frequently contaminated, so its recovery depends on dedicated film collection rather than mixed kerbside streams.
Neither should be mixed indiscriminately: blending significant HDPE into an LDPE film compound reduces clarity and seal performance, while LDPE in an HDPE pipe compound lowers stiffness and pressure rating. Controlled blends are a different matter — many recycled polyolefin grades are deliberately formulated mixtures with a stated ratio.
Where recycled polyolefin blends fit
Real post-consumer streams are rarely a single polymer. The recycled PolyAl pellet we supply, for example, is roughly 66% LDPE, 18% HDPE and 6% PP, plus about 10% aluminium recovered from carton barrier layers. That composition is not a defect — it is a stable, characterised blend that behaves like a stiffened LDPE.
The lesson for specifiers is to stop thinking in binary LDPE/HDPE terms when buying recyclate and start reading the actual property sheet: MFI, density, ash content and Vicat point tell you how a material will run on your tool far better than the polymer name on the label.
LDPE vs HDPE: typical values
| Density | LDPE 915–935 kg/m³ · HDPE 940–970 kg/m³ |
|---|---|
| Crystallinity | LDPE 45–55% · HDPE 70–80% |
| Molecular structure | LDPE branched · HDPE linear |
| Tensile strength | LDPE 8–15 MPa · HDPE 20–37 MPa |
| Melting point | LDPE ≈105–115 °C · HDPE ≈125–135 °C |
| Flexibility | LDPE high · HDPE low to moderate |
| Clarity | LDPE translucent to clear · HDPE opaque |
| Chemical resistance | LDPE good · HDPE excellent |
| Resin code | LDPE 4 · HDPE 2 |
| Typical use | LDPE film and liners · HDPE pipe and rigid containers |