Refers to a diverse range of substances that are added in the production of PVC, including both the polyvinyl chloride polymer as well as finished PVC compounds. A few additive substances are required for the polymerisation process. Various additives stabilize the structure of PVC against heat, impact or long-term exposure to light, granting PVC a long-term life and then helping its easy reprocessing through mechanical recycling. Other additive substances provide a range of functional, aesthetic and other desired performance properties. Plasticisers, for example, make PVC flexible. Additives may often be introduced into the compounding process as ‘one pack systems’, comprising a range of additives providing a particular set of properties in the finished PVC compound.