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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper II covers organic chemistry and spectroscopy.

UPSC Civil Services (Mains), Chemistry optional · 2018 · Paper II

Q5(c). HDPE and LDPE: production, properties, uses

  • 10 marks
  • Answered
Question

Distinguish between HDPE and LDPE with reference to their mode of production and physical properties. Mention any two applications for each.

Answer in brief

HDPE is made by low-pressure coordination (Ziegler–Natta) polymerisation and is linear, dense, highly crystalline and rigid (pipes, bottles). LDPE is made by high-pressure radical polymerisation and is branched, less dense, less crystalline and flexible (films, bags, cable insulation).

Key points

  • Introduction. Both are polyethylene, -(CH₂-CH₂)ₙ⁻. They differ in chain branching, which is fixed by the polymerisation method: branching decides how well the chains pack, and so the density, crystallinity, melting point and…
  • Figure: Few branches allow crystalline packing (HDPE); many short and long branches keep the chains apart (LDPE).

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

Introduction. Both are polyethylene, -(CH₂-CH₂)ₙ⁻. They differ in chain branching, which is fixed by the polymerisation method: branching decides how well the chains pack, and so the density, crystallinity, melting point and strength.

Figure: Few branches allow crystalline packing (HDPE); many short and long branches keep the chains apart (LDPE).

HDPE (high-density)LDPE (low-density)
Polymerisationcoordination (Ziegler–Natta, TiCl₄/Al(C₂H₅)₃; or Phillips CrO₃/silica)radical (traces of O₂ or a peroxide initiator)
Conditionslow pressure (a few atm), about 60–100°C, in a hydrocarbonvery high pressure (about 1000–3000 atm), about 150–250°C
Chain structurelinear, very few branchesbranched: short branches by back-biting (intramolecular H transfer), long branches by chain transfer to polymer
Densityabout 0.94–0.97 g cm⁻³about 0.91–0.93 g cm⁻³
Crystallinityhigh (about 80–90%)lower (about 50–60%)
Melting pointabout 130–137°Cabout 105–115°C
Mechanicalrigid, higher tensile strength and hardness; opaquesoft, flexible, tough; translucent in films
Usespipes and fittings, bottles and jerry cans for detergents and milk, crates, bucketscarry bags and packaging film, squeeze bottles, cable insulation, coatings

Conclusion. HDPE is made by low-pressure coordination (Ziegler–Natta) polymerisation and is linear, dense, highly crystalline and rigid (pipes, bottles). LDPE is made by high-pressure radical polymerisation and is branched, less dense, less crystalline and flexible (films, bags, cable insulation).

Application. Linear low-density polyethylene (LLDPE), made by Ziegler–Natta copolymerisation of ethene with a little but-1-ene or hex-1-ene, combines short branches with a linear backbone and now replaces LDPE in stretch films.

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Model answer to Q5(c), 2018 Paper II: solution page 17
Solution page 17 of 24
Read the whole 2018 solution2018 question paper

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