What is Po (initial plasticity)?
Po is the Wallace plasticity of raw rubber before ageing — an indicator of molecular weight and stiffness.
Po (initial plasticity) is measured with a Wallace rapid plastimeter. It reflects the rubber's molecular weight: higher Po generally means a stiffer, higher-molecular-weight rubber.
SMR 20 typically specifies Po ≥ 30. Po is used together with PRI: Po describes the starting state of the rubber, while PRI describes how well that state survives heat ageing.