Intrinsic Viscosity (IV)

Also called: IV, inherent viscosity

Intrinsic viscosity (IV) indicates the molecular weight of PET in dl/g and decides whether recycled PET suits bottles, sheet, strap or fibre.

Intrinsic viscosity (IV) is a measure of a polymer molecular size in solution, reported in decilitres per gram (dl/g). It is the main quality parameter for PET, playing the role that MFI plays for polyolefins. Higher IV means longer polymer chains and generally better strength and melt strength.

Why it matters when buying recycled plastic

• Different end uses need different IV. Bottle-grade PET is commonly around 0.7-0.85 dl/g, while fibre grades run lower.
• PET loses IV when it is processed wet or overheated, because moisture breaks the chains. Proper drying is essential; check moisture content.
• Recycled PET IV can be raised again by solid-state polymerisation, a separate process step.
• Ask for an IV result per lot, and the test method used, since results from different labs and methods can differ slightly.

How it is measured

IV is usually measured by dissolving a sample in a solvent and timing its flow through a capillary viscometer, or by an equivalent melt-based method. Read our recycled PET flakes buying guide for how IV fits with other flake specifications such as colour and PVC content.