Polyethylene

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Polyethylene — Note

Polyethylene

1. Introduction

Polyethylene (PE) is the most widely used thermoplastic polymer. It is obtained by polymerisation of ethylene (ethene) monomer, C2H4. A member of the polyolefin family, its properties can be varied by polymerisation method, density, and branching.

General formula: (C2H4)n

2. Types of Polyethylene

Type Structure & Density Properties Uses
LDPE (Low-Density PE) Highly branched, 0.91–0.94 g/cm³ Soft, flexible, transparent Plastic bags, film wraps, squeeze bottles
HDPE (High-Density PE) Linear, minimal branching, 0.94–0.97 g/cm³ Strong, stiff, opaque Pipes, containers, bottles, crates
LLDPE (Linear Low-Density PE) Linear with short branches Flexible, impact resistant Stretch films, packaging, liners
UHMWPE (Ultra-High-Molecular-Weight PE) Very long chains Extremely tough, abrasion-resistant Bulletproof vests, medical implants, industrial gears

3. Methods of Polymerisation

  • Free Radical Polymerisation: Used for LDPE; high pressure (~1000–3000 atm) and temperature (~200–300°C)
  • Ziegler–Natta Polymerisation: Used for HDPE and LLDPE; low pressure and temperature with TiCl3/AlEt3 catalyst; produces linear chains with controlled branching
  • Metallocene Catalysis: Produces uniform polymers with precise branching

4. Structure

  • LDPE: Branched chains --> less dense, flexible
  • HDPE: Linear chains --> tightly packed --> dense, strong
  • LLDPE: Linear with short branches --> combines strength and flexibility
  • UHMWPE: Very long linear chains --> exceptional toughness

5. Properties

  • Mechanical: Strength increases with density; HDPE > LDPE
  • Thermal: Softening point: LDPE 105°C, HDPE 130°C
  • Chemical: Resistant to acids, bases, and organic solvents
  • Electrical: Excellent insulator
  • Processability: Can be molded, extruded, or blown into films

6. Advantages

  • Lightweight and strong
  • Chemically inert and resistant
  • Easily processable by molding and extrusion
  • Can be recycled (codes 1–4 for LDPE/HDPE/LLDPE)

7. Limitations

  • Poor resistance to UV light unless stabilized
  • Flammable
  • Not biodegradable
  • Lower temperature resistance than some engineering plastics

8. Applications

  • Packaging films, grocery bags, shrink wraps
  • Bottles and containers
  • Pipes and tubing
  • Insulation for wires and cables
  • Bulletproof vests (UHMWPE)
  • Medical implants and prosthetics

9. Summary

Polyethylene is a versatile polymer. Its properties range from soft, flexible LDPE to strong HDPE and ultra-tough UHMWPE. Its cost-effectiveness, chemical resistance, and ease of processing make it one of the most widely used plastics in the world.

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