ISO 182-1 PVC compounds  thermal stability test Congo red method. The determination is carried out on a sample of the PVC compound which is maintained at an agreed temperature in still air until the colour of a Congo red paper held above it changes from red to blue. If universal indicator paper is used, the colour change is that corresponding to pH 3.

This test method is intended to determine the thermal stability of a PVC compound when processed at a high temperature. The method is applicable to all PVC compounds, copolymers and products based on them. A test specimen of the PVC material is heated at a defined temperature, normally 180°C.