Why does PVC compound for pipe fitting need to add modifier? PVC compound for pipe fitting is polymerized from polyvinyl chloride monomer (VCM). PVC material has the characteristics of non-toxic, anti-aging, acid and alkali resistance, and is very suitable for chemical pipelines. The mixture composed of a certain amount of solid additives (without plasticizers) added to the raw materials of polyvinyl chloride is called rigid polyvinyl chloride (UPVC).