IV (intrinsic viscosity)

Intrinsic viscosity, measured in decilitres per gram (dl/g), is the PET industry's proxy for molecular weight: longer polymer chains give higher IV and stronger, tougher material. The commercial map: fibre grades around 0.58–0.64, sheet and thermoforming 0.70–0.78, bottle preforms 0.76–0.84, and high-strength strapping above that.

For recycled PET, IV is the headline number on every flake and pellet specification, because chains shorten with each heat history and with hydrolysis from moisture — the reason rPET is dried scrupulously before melting, and the reason solid-state polycondensation (SSP) exists: it rebuilds IV, lifting recycled material back to bottle capability. Buying rule of thumb: specify the IV your process needs, and read the lot's measured value on the consignment spec sheet, not the brochure.