Density values and unit conversions
Polyethylene grades are classified largely by density, and the boundaries are conventions rather than physical constants. LDPE occupies 915–935 kg/m³. Below it sits VLDPE (very low density, under 915). Above it, LLDPE overlaps at 915–925 with a different chain architecture, MDPE runs 926–940, and HDPE takes 940–970.
Converting between units: divide kg/m³ by 1,000 for g/cm³; multiply g/cm³ by 62.428 for lb/ft³. Specific gravity is numerically identical to density in g/cm³ because water is 1.000 g/cm³ at 4 °C. Since every polyethylene grade has a specific gravity below 1, all of them float in water — which is exactly what sink-float separation exploits in recycling plants.
Why LDPE is less dense
The high-pressure free-radical process that makes LDPE creates long- and short-chain branching along the backbone. Those branches physically obstruct crystallisation, so only 45–55% of the polymer forms ordered crystalline regions; the rest stays amorphous, with more free volume between chains. Less efficient packing means lower mass per unit volume.
The same branching explains the rest of the property profile. Lower crystallinity means lower melting point, lower stiffness, higher elongation, better transparency and better heat-seal performance — which is why LDPE dominates film even though HDPE is mechanically stronger.
What changes density in practice
Three things move a real material away from the textbook value.
- Comonomer content — more comonomer means more branching, lower crystallinity, lower density.
- Fillers — calcium carbonate, talc, glass or metal all raise density substantially, roughly in proportion to their volume fraction.
- Polymer contamination in recyclate — HDPE or PP fractions shift measured density up or down depending on the mix.
- Voids and moisture in pellets — affect bulk density (which is different from solid density and typically 500–600 kg/m³ for pellets).
Recycled LDPE-based materials
Post-consumer recycled LDPE that is genuinely a single polymer measures close to virgin values, perhaps slightly higher from residual additives and pigments. Multi-polymer recyclates diverge more: a stream carrying HDPE and PP will read higher than pure LDPE, and one carrying mineral filler higher still.
The recycled PolyAl pellet from beverage cartons is a clear example: ≈66% LDPE, 18% HDPE and 6% PP plus about 10% aluminium by mass, measuring 950–1050 kg/m³. The aluminium is dispersed as fine flakes and adds mass without a proportional volume increase, which is also why the material is stiffer and more dimensionally stable than unfilled LDPE. Its Vicat softening point sits at 90–100 °C and MFI at 2–4 g/10 min.
Using density in material selection
Density is a proxy for crystallinity, and crystallinity drives most of the mechanical property set. In practice, a higher-density polyethylene will be stiffer, stronger, more chemically resistant, less permeable and less transparent; a lower-density one will be more flexible, tougher at low temperature, easier to seal and clearer.
For part design, density also matters commercially: resin is sold by weight but parts are designed by volume. A 10% density increase means 10% more material cost for the same part geometry — worth checking when comparing a filled recyclate against an unfilled virgin resin on price per kilo alone.
LDPE physical properties
| Density | 915–935 kg/m³ (0.915–0.935 g/cm³) |
|---|---|
| Density (imperial) | 57.1–58.4 lb/ft³ |
| Specific gravity | 0.915–0.935 |
| Bulk density (pellets) | ≈500–600 kg/m³ |
| Crystallinity | 45–55% |
| Melting point | 105–115 °C |
| Vicat softening point | 85–95 °C |
| Melt flow index | 0.3–20 g/10 min (grade dependent) |
| Tensile strength | 8–15 MPa |
| Elongation at break | 100–650% |
| Water absorption | <0.01% |
| Processing temperature | 160–230 °C |