High-Density Polyethylene (HDPE) and Low-Density Polyethylene (LDPE) are two important types of polyethylene, but they differ in how their molecules are arranged, which changes their properties and uses.
1. Structure
- HDPE (High-Density Polyethylene):
- Chains are linear with very little branching.
- Molecules pack tightly together.
- High crystallinity → more ordered.
- LDPE (Low-Density Polyethylene):
- Chains have lots of branching (short and long side chains).
- Molecules cannot pack closely.
- Low crystallinity → more amorphous (disordered).
2. Density
- HDPE: 0.94 – 0.97 g/cm³ (higher density).
- LDPE: 0.91 – 0.93 g/cm³ (lower density).
3. Strength and Flexibility
- HDPE:
- Stronger, harder, more rigid.
- High tensile strength.
- LDPE:
- Softer, weaker, but more flexible and ductile.
4. Melting Point
- HDPE: ~120–135 °C (higher, due to crystallinity).
- LDPE: ~105–115 °C (lower).
5. Transparency
- HDPE: Opaque (because of high crystallinity).
- LDPE: More transparent or translucent.
6. Chemical Resistance
- HDPE: More resistant to chemicals, acids, and solvents.
- LDPE: Also resistant, but less so compared to HDPE.
7. Common Uses
- HDPE:
- Rigid containers (milk jugs, shampoo bottles).
- Pipes and fittings.
- Hard hats, toys.
- Grocery bags (in some cases).
- LDPE:
- Plastic bags, cling film, squeeze bottles.
- Flexible tubing.
- Coatings (wire insulation, packaging films).
8. Cost
- HDPE: Slightly more expensive.
- LDPE: Cheaper to produce.
In short:
- HDPE → Linear chains, tightly packed, high density, strong, rigid, used for pipes, containers, hard products.
- LDPE → Branched chains, loosely packed, low density, soft, flexible, used for films, bags, coatings.