Phthalates (US: /ˈθæleɪts/,[1] UK: /ˈθɑːleɪtsˌ ˈfθælɪts/[2][3]), or phthalate esters, are esters of phthalic acid. They are mainly used as plasticizers, i.e., substances added to plastics to increase their flexibility, transparency, durability, and longevity. They are used primarily to soften polyvinyl chloride (PVC).