Chemical (feedstock) recycling breaks polyester back into monomer instead of just re-melting shredded fiber. In glycolysis, excess ethylene glycol (EG) attacks the ester bonds of PET chains, catalyzed by a metal salt, converting long chains into bis(2-hydroxyethyl) terephthalate (BHET) — a virgin-quality monomer that can be re-polymerized into new fiber with none of the strength loss that mechanical (shred-and-respin) recycling causes each cycle.
–[OC-C6H4-CO-O-CH2CH2-O]n– + n HOCH2CH2OH
(PET chain) (ethylene glycol)
→ n HOCH2CH2-OOC-C6H4-COO-CH2CH2OH
(BHET monomer)
dp/dt = k·(1 − p) first-order in remaining ester bonds
k = A·exp(−Ea/RT)·f(cat)·g(EG:PET) Arrhenius + reagent terms
Xn = 1 / (1 − p) Carothers degree-of-polymerization
Mn = Xn · M0, M0 ≈ 192 g/mol PET repeat-unit molar mass
- Temperature — sets the Arrhenius term exp(−Ea/RT); every +10°C roughly doubles the rate constant k.
- Catalyst loading — zinc-acetate (or similar) coordinates the carbonyl and lowers the effective activation barrier; more catalyst speeds bond cleavage.
- EG : PET ratio — excess glycol drives the equilibrium toward full monomerization (Le Chatelier); low ratios stall conversion before completion.
- The reactor view shows each PET chain as 10 linked units; as ester bonds cleave they detach into free, faster-drifting BHET monomer (gold) suspended in the glycol.
Real-world relevance: BHET-route glycolysis is the basis of several commercial textile-to-textile polyester recycling processes — it recovers monomer purity independent of how many times the fiber has already been mechanically recycled, at the cost of running a hot, catalyzed chemical reactor instead of a shredder.