SSP (Solid-State Polycondensation)

SSP (Solid-State Polycondensation) is a thermal post-treatment process applied to semi-crystalline polyesters — primarily PET and PBT — to increase polymer chain length (molecular weight) after melt extrusion. During melt extrusion of recycled PET, the high temperature (260–280°C) causes hydrolytic and thermal chain scission, reducing Intrinsic Viscosity (IV) from virgin levels (0.76–0.84 dL/g) to 0.60–0.72 dL/g in recycled pellets. SSP reverses this by heating the solid, crystallised PET pellets to 190–230°C (below the melting point of 250°C) under deep vacuum (1–3 mbar) or a continuous inert gas (nitrogen) sweep, promoting chain extension and driving off the reaction byproducts (ethylene glycol and water vapour). Residence time in the SSP reactor is 8–24 hours depending on target IV; a 12-hour SSP run at 210°C typically raises IV by 0.10–0.18 dL/g.

The SSP process also performs a critical decontamination function for food-contact rPET: at 210°C under vacuum, volatile organic contaminants (VOCs) that have diffused into the PET matrix during the previous bottle's use — residual beverages, cleaning agents, surfactants — migrate out of the solid pellet and are removed by the vacuum or gas sweep. This is the basis of the super-clean recycled PET claim accepted by EFSA (European Food Safety Authority) under its Opinion on PET recycling processes: a correctly designed SSP step, coupled with upstream hot washing, is sufficient decontamination to produce food-contact rPET. FSSAI has not issued equivalent Indian guidance as of mid-2024, but Indian food-grade rPET producers follow EFSA and FDA (21 CFR 177.1630) standards as the accepted technical benchmarks.

Equipment for SSP: a complete SSP system consists of a pre-crystalliser (raises crystallinity to 40–55% to prevent pellet sticking), a vessel dryer (pre-dries to ≤30 ppm moisture before SSP entry), and the SSP reactor (typically a vertical cylinder or tumble dryer with vacuum system). For a continuous horizontal-flow SSP unit processing 1,000–5,000 TPY of rPET, capital cost is Rs 2–6 crore for locally engineered systems; Rs 8–20 crore for imported Buehler or Bühler Fischer SSP lines. Energy consumption: 0.08–0.15 kWh per kg of rPET processed (mainly nitrogen or vacuum pumping). The investment is justified only when a premium food-grade rPET market is accessible — the Rs 20–40/kg pellet price premium over standard rPET pellets covers the SSP capex in 2–4 years at 2,000+ TPY throughput.

For Indian rPET recyclers targeting the food and beverage packaging market, SSP is not optional — it is the minimum technology requirement for IV restoration and decontamination that major beverage brand owners (Coca-Cola, PepsiCo, Bisleri, Kinley) specify in their rPET procurement standards. Recyclers without SSP capability are limited to the fibre and non-food packaging market. A practical entry point is to offer rPET flake to existing SSP-equipped players as an intermediate product (flake buy) rather than investing in SSP independently, until sufficient volume and customer commitment justify the capex.