PVC, polyester and nylon manufacture

Why it matters

PVC is the main plastic for pipes and cables in Indian construction and agriculture, while polyester (PET) and nylon dominate synthetic fibres and packaging. The three show the two polymerisation families in practice: PVC by free-radical chain growth with a demanding monomer plant, and PET and nylon by step-growth condensation, where stoichiometry and by-product removal decide molecular weight.

Key ideas

Vinyl chloride monomer (VCM) by the balanced process.

1. Direct chlorination: C₂H₄ + Cl₂ → C₂H₄Cl₂ (EDC, 1,2-dichloroethane), liquid phase with FeCl₃ catalyst, about 50–120 °C, exothermic.
2. EDC cracking (pyrolysis): C₂H₄Cl₂ → C₂H₃Cl + HCl in a fired furnace at about 480–530 °C and 10–30 bar, with 50–60 % conversion per pass; the gas is quenched and VCM purified by distillation.
3. Oxychlorination: C₂H₄ + 2HCl + ½O₂ → C₂H₄Cl₂ + H₂O over CuCl₂ on alumina at about 220–300 °C in fluidised or fixed beds. This consumes the HCl from cracking, so the plant makes no net HCl. Overall: 2C₂H₄ + Cl₂ + ½O₂ → 2C₂H₃Cl + H₂O. Chlorine comes from the chlor-alkali plant.

PVC polymerisation. Free-radical addition. About 80–90 % is made by suspension polymerisation: VCM droplets dispersed in water with a protective colloid (PVA, cellulose ethers) and an oil-soluble peroxide initiator, in large jacketed batch reactors at about 50–70 °C and 8–12 bar (the vapour pressure of VCM). Temperature controls molecular weight (expressed as K-value). PVC is insoluble in its monomer, so particles precipitate inside the droplets, giving porous grains that absorb plasticiser. The batch is stopped at about 85–90 % conversion, unreacted VCM is stripped from the slurry with steam (VCM is a carcinogen and residual levels must be in ppm), and the resin is centrifuged and dried. Emulsion PVC gives fine paste resins; bulk (mass) PVC gives very clear resin. Rigid PVC (uPVC) is compounded with heat stabilisers; flexible PVC with plasticisers such as phthalates.

Polyester (PET). Polyethylene terephthalate is made by step-growth from purified terephthalic acid (PTA) and monoethylene glycol (MEG).

1. Esterification: PTA + MEG (molar ratio about 1 : 1.1–1.2) at about 250–270 °C and slight pressure; water is removed by a column. Product: bis(hydroxyethyl) terephthalate (BHET) and short oligomers. The older DMT route uses transesterification with dimethyl terephthalate, releasing methanol.
2. Polycondensation: at about 270–290 °C under high vacuum (down to about 1 mbar) with an antimony catalyst; excess glycol is removed, driving the equilibrium. Melt viscosity rises sharply, so finishers are special disc-ring or cage reactors with large free surface.
3. Solid-state polymerisation (SSP): chips are heated below their melting point in nitrogen to raise intrinsic viscosity for bottle grade. Products: fibre-grade melt spun directly; film and bottle chips.

Nylon.

• Nylon 6,6: hexamethylenediamine (HMDA) + adipic acid. They are first combined into nylon salt (hexamethylenediammonium adipate, AH salt) in water, which guarantees an exact 1 : 1 ratio. The salt solution is concentrated and heated to about 270–280 °C, first under pressure (to keep the volatile diamine in) and then with pressure release to remove water.
• Nylon 6: ring-opening polymerisation of caprolactam initiated by water at about 250–270 °C in a VK (continuous tubular) reactor. Equilibrium leaves about 8–10 % monomer and oligomers, removed by hot-water extraction of the chips before drying.

Step-growth kinetics (Carothers). Molecular weight is high only when conversion of functional groups is above about 99 % and stoichiometry is exact. Any monofunctional impurity or excess of one monomer caps chains.

Formulas

C₂H₄ + Cl₂ → C₂H₄Cl₂; C₂H₄Cl₂ → C₂H₃Cl + HCl; C₂H₄ + 2HCl + ½O₂ → C₂H₄Cl₂ + H₂O

n HOOC–C₆H₄–COOH + n HOCH₂CH₂OH → [–OC–C₆H₄–COO–CH₂CH₂–O–]ₙ + 2n H₂O (end groups neglected)

Xₙ = 1 / (1 − p) (equal stoichiometry) Xₙ = (1 + r) / (1 + r − 2rp) (stoichiometric imbalance)

• Xₙ: number-average degree of polymerisation counted in structural units (each monomer residue is one unit), p: fractional conversion of the limiting functional group, r: ratio of functional groups (≤ 1).

Mₙ = Xₙ × M̄₀

• M̄₀: mean mass of a structural unit (g/mol). For nylon 6,6, repeat unit 226.32 g/mol contains two structural units, so M̄₀ = 113.16 g/mol. For PET, repeat unit 192.17 g/mol, M̄₀ = 96.09 g/mol.

Worked examples

Example 1 (standard): balanced VCM plant. Find the ethylene, chlorine and oxygen consumed per tonne of VCM (62.50 g/mol) in a balanced plant with 100 % yields.

1. VCM = 1000 / 62.50 = 16.0 kmol. EDC cracked = 16.0 kmol, giving 16.0 kmol HCl.
2. Oxychlorination uses 16.0 kmol HCl to make 8.0 kmol EDC, with 8.0 kmol C₂H₄ and 4.0 kmol O₂.
3. Direct chlorination makes the other 8.0 kmol EDC from 8.0 kmol C₂H₄ and 8.0 kmol Cl₂.
4. Ethylene = 16.0 × 28.05 = 448.8 kg; Cl₂ = 8.0 × 70.90 = 567.2 kg; O₂ = 4.0 × 32.00 = 128 kg.

Answer: 449 kg ethylene, 567 kg chlorine, 128 kg oxygen per tonne of VCM.

Example 2 (GATE level): effect of stoichiometric imbalance on nylon 6,6. (a) With exact stoichiometry, find Xₙ at p = 0.995. (b) If the acid groups are in 1 % deficit (r = 0.99), find Xₙ and Mₙ at p = 0.995.

1. (a) Xₙ = 1 / (1 − 0.995) = 200.
2. (b) Xₙ = (1 + 0.99) / (1 + 0.99 − 2 × 0.99 × 0.995) = 1.99 / (1.99 − 1.9701) = 1.99 / 0.0199 = 100.
3. Mₙ = 100 × 113.16 = 11 316 g/mol.

Answer: (a) Xₙ = 200; (b) Xₙ = 100, Mₙ ≈ 11 300 g/mol — a 1 % imbalance halves the chain length, which is why nylon salt is used.

Common mistakes

• Forgetting that each ester or amide link releases one water: about 2n mol water per n mol of diacid for a long chain.
• Counting repeat units instead of structural units in Carothers' equation.
• Saying PVC is made by condensation; it is free-radical addition (suspension, emulsion or bulk are process techniques, not mechanisms).
• Ignoring the HCl recycle in the VCM balance and over-counting chlorine.
• Assuming high pressure is used in PET polycondensation; it runs under vacuum to remove glycol.

For GATE CH

Expect questions on the VCM balanced process and catalysts, why suspension PVC is stripped of VCM, PTA vs DMT routes, why vacuum and SSP are used for PET, nylon salt and caprolactam ring opening, and Carothers' equation. Numericals: raw-material balances, water or methanol released, Xₙ and Mₙ from p and r.

Quick check

1. What catalyst is used in oxychlorination?
2. Why is nylon salt prepared before polymerising nylon 6,6?
3. What is the by-product of the PTA route to PET, and of the DMT route?
4. Find Xₙ at p = 0.98 with equal stoichiometry.

Answers: 1. CuCl₂ on alumina; 2. to fix an exact 1 : 1 diamine : diacid ratio; 3. water, and methanol; 4. 50.