What is PRI?
The Plasticity Retention Index measures how well raw natural rubber resists thermal oxidation (ageing).
PRI is the ratio of a sample's plasticity after ageing (30 minutes at 140 °C) to its plasticity before ageing, expressed as a percentage. A higher PRI means better resistance to heat and oxidation.
For SMR 20 the specification typically requires PRI ≥ 40. Low PRI signals rubber that will degrade faster during processing and in service, so it is a key quality gate for tire and industrial compounders.