depolymerisation (Depolymerization)

Depolymerisation is the reverse of polymerisation — chemical reactions that break polymer chains into monomers or oligomers of defined structure, enabling the monomers to be re-polymerised into new, virgin-quality plastic. Unlike mechanical recycling (which retains the polymer but degrades it) or pyrolysis (which produces a mixture of hydrocarbons of varying composition), depolymerisation produces chemically defined molecules that can be incorporated into food-grade or high-specification plastic manufacturing. The most commercially developed depolymerisation pathway in India is PET depolymerisation.

PET depolymerisation routes: (1) Glycolysis — PET reacts with ethylene glycol (EG) at 180–240°C with catalyst (zinc acetate or similar) to produce BHET (bis-2-hydroxyethyl terephthalate), a PET precursor that can be directly repolymerised with additional TPA; reaction time 3–5 hours; glycolysis is the most commercially mature route in India, used by players like Polygenta (Maharashtra) and Fibre2Fashion feedstock recyclers; (2) Methanolysis — PET reacts with methanol at 180–280°C under pressure to produce DMT (dimethyl terephthalate) + EG; higher temperature, higher cost, but cleaner product; (3) Hydrolysis — PET reacts with water (acid or alkaline catalysis) to produce TPA (terephthalic acid) + EG; TPA purity is the highest of all routes; Carbios (France) is commercialising an enzymatic hydrolysis route using engineered cutinase enzymes at ≤72°C; (4) Aminolysis — PET + amine → terephthalamide derivatives (niche, pharmaceutical-adjacent applications).

For other polymers: PS depolymerisation (thermal cracking of polystyrene to styrene monomer, 350–400°C) is commercialised by Styrenics Circular Solutions and GreenMantra; no Indian commercial operators as of 2024. Nylon depolymerisation (hydrolysis of Nylon 6 to caprolactam) is commercially run by Aquafil and others from fishing net and carpet waste. PC (polycarbonate) depolymerisation to BPA and BPF is a developing commercial route. For polyolefins (PE, PP), depolymerisation to monomers (ethylene, propylene) requires catalytic cracking at 400–600°C — not commercially distinct from pyrolysis in practice, and the monomer recovery is limited.

Investment thresholds in India: a 5,000 TPA PET glycolysis plant costs Rs 8–20 crore for equipment; a 10,000 TPA plant Rs 15–30 crore. Enzymatic hydrolysis (Carbios licensee model) is expected to require Rs 40–100 crore for a first-generation plant. The economics depend on the spread between PET waste cost (Rs 15–35/kg at current market rates), BHET or TPA selling price (Rs 90–130/kg vs virgin TPA at Rs 75–95/kg, with recycled TPA commanding a premium), and processing cost (Rs 18–30/kg in well-run glycolysis plants). Current commercial advantage is driven primarily by EPR credit eligibility (depolymerisation BHET qualifies for EPR Category I credits if CPCB-approved) and the food-contact rPET premium market.