the mixture to maintain the suspension. PVC may be manufactured from either naphtha or ethylene feedstock. Microstructure The polymers are linear and are strong. The monomers are mainly arranged head-to-tail, meaning that there are chlorides on alternating carbon centres. PVC has mainly an atactic stereochemistry, which means that the relative stereochemistry of the chloride centres are random. Some degree of syndiotacticity of the chain gives a few percent crystallinity that is influential on the properties of the material. About 57% of the mass of PVC is chlorine. The presence of chloride groups gives the polymer very different properties from the structurally related material polyethylene. The density is also higher than for these structurally related plastics. Producers About half of the world's PVC production capacity is in China, despite the closure of many Chinese PVC plants due to issues complying with environmental regulations and poor capacities of scale. The largest single producer of PVC as of 2018 is Shin-Etsu Chemical of Japan, with a global share of around 30%. Additives The product of the polymerization process is unmodified PVC. Before PVC can be made into finished products, it always requires conversion into a compound by the incorporation of additives (but not necessarily all of the following) such as heat stabilizers, UV stabilizers, plasticizers, processing aids, impact modifiers, thermal modifiers, fillers, flame retardants, biocides, blowing agents and smoke suppressors, and, optionally, pigments. The choice of additives used for the PVC finished product is controlled by the cost performance requirements of the end use specification (underground pipe, window frames, intravenous tubing and flooring all have very different ingredients to suit their performance requirements). Previously, polychlorinated biphenyls (PCBs) were added to certain PVC products as flame retardants and stabilizers. Plasticizers Among the common plastics, PVC is unique in its acceptance of large amounts of plasticizer with gradual changes in physical properties from a rigid solid to a soft gel, and almost 90% of all plasticizer production is used in making flexible PVC. The majority is used in films and cable sheathing. Flexible PVC can consist of over 85% plasticizer by mass, however unplasticized PVC (UPVC) should not contain any. A wide variety of substances can be used as plasticizers including phthalates, organophosphates, adipates, trimellitates, polymeric plasticizers and epoxidized vegetable oils. Phthalates The most common class of plasticizers used in PVC is phthalates, which are diesters of phthalic acid. Phthalates can be categorized as high