Equilibrium Relationship between SVOCs in PVC Products and the Air in Contact with the Product

Phthalates and phthalate alternatives are semi volatile organic compounds (SVOCs) present in many PVC products as plasticizers to enhance product performance. Knowledge of the mass-transfer parameters, including the equilibrium concentration in the air in contact with the product surface (ye), will greatly improve the ability to estimate the emission rate of SVOCs from these products and to assess human exposure. The objective of this study was to measure y(0) for different PVC products and to evaluate its relationship with the material-phase concentrations (C-0). Also, C-0 and y(0) data from other sources were included, resulting in a substantially larger data set (N-total = 34, T = 25 degrees C) than found in previous studies. The results show that the material/gas equilibrium relationship does not follow Raoult's law and that therefore the assumption of an ideal solution is invalid. Instead, Henry's law applies, and the Henry's law constant for all target SVOCs consists of the respective pure liquid vapor pressure and an activity coefficient gamma, which accounts for the nonideal nature of the solution. For individual SVOCs, a simple partitioning relationship exists, but Henry's law is more generally applicable and will be of greater value in rapid exposure assessment procedures.