This part of ISO 1628 defines particular conditions for determining the reduced viscosity (also known as viscosity number) and intrinsic viscosity of polyethylenes and polypropylenes at 135 °C in dilute solution. It is applicable to low, medium and high-density polyethylenes and to a wide range of polypropylenes, including pure isotactic, stereoblock and atactic polymers. The viscosity of polymer solutions may be affected by additives present in the sample. The value of a reduced viscosity determined by this method may therefore be unreliable if the sample contains fillers or other additives. NOTE Reduced viscosity is also known as the Staudinger function (Jv) and intrinsic viscosity as the Staudinger index (Jg).