(649f) Predicting Volatile Emissions from Automotive Sealants and Empirical Correlations for the Key Mass Transfer Parameters for Phthalate Emissions from PVC Materials

The lightweighting strategy in the automotive industry of using mixed materials in the vehicle reduces emissions of greenhouse gases, however, necessitates greater use of sealants due to the increased amount of material joints and seams. Phthalates are widely used compounds in sealant formulations based on polyvinylchloride (PVC), which are emitted from the material during use. Common phthalates are classified as Semi-Volatile Organic Compounds (SVOCs), which typically have a higher molecular weight and a lower gas phase concentration. Phthalates present in the air environment have shown to enter the human body through inhalation, ingestion, and dermal contact, and have potential to bring about harmful effects.

The rate of emissions of volatile compounds from a solid material is governed by diffusion, convection, and thermodynamics. Values for the diffusion coefficient, solid-air partition coefficient (K), and the mass transfer coefficient are needed to predict emissions in specified scenarios. Empirical relationships for combinations of classes of volatile compounds with classes of solid materials have been published for common volatile compounds, but not phthalates in PVC. New relationships are presented in this study for estimation of the diffusion and solid/air partition coefficients for phthalates in PVC materials using data compiled from studies of other products, such as vinyl flooring. The relationships are functions of vapor pressure of the phthalates, which is determined from a Clausius-Clapeyron equation for four phthalates.

A test chamber was constructed to test the concentration of phthalate in an air sample. Solid Phase MicroExtraction (SPME) fibers were positioned into the test chamber with the axis perpendicular to the flow direction. A mass transfer model was applied to calculate the air concentration based on the mass of phthalate adsorbed over time with the flow set as a continuous loop. The predicted K value from the empirical relationship was tested using di-octyl terephthalate (DOTP) in a PVC-based automotive sealant, in which the results fell within reasonable error of the prediction. The test chamber was used to measure the air concentration of an environmental chamber that contained sealant samples. Equilibrium is reached quickly in the chamber as predicted by the unsteady state mass transfer mass transfer model combined with a mass balance, due to low convective resistance from a high rate of air circulation.

The model was used to evaluate omitting diffusion from the model. Under the base conditions, the difference in the final equilibrium concentrations is negligible due to a high K value, which brings about a negligible loss in mass when diffusion is included. The final concentrations start to significantly diverge at a K value three orders of magnitude lower, resulting in a significant mass loss when diffusion is included, bringing about a lower air concentration.

The model with and without diffusion are applied to outdoor and manufacturing scenarios with four different phthalates and different temperatures. Each of the four phthalates were evaluated at temperatures of 25 oC and 41 oC, which is a typical high outdoor temperature in Phoenix in July. The emission rate is significantly dependent on the type of phthalate and temperature due to the relationship between vapor pressure and K. Under conditions of lower temperature and mass transfer coefficient, the system can be modeled adequately with convection only for years. In an extreme situation of sustained conditions of 41 oC and hm = 5 m/h, the loss of phthalate at 1.5 years is 10% higher for the convection only model than the model with diffusion. Thus, using a convection only model is sufficient for most realistic scenarios, such as in an assembly plant where the sealant is applied in the open air. The model is sensitive to temperature and mass transfer coefficient and it is important to determine an accurate value.

A simplified equation is presented for estimating the mass emission rate for phthalates in PVC sealants in open spaces based on the mass transfer coefficient, exposed area, initial concentration of phthalate in the PVC, and vapor pressure of the phthalate. Changes in temperature and mass transfer coefficient over time can be expressed as functions and inserted into the equation to predict the emissions based on a dynamic scenario. In order to investigate minimizing emissions, strategies could involve reducing the inputs of exposed area and concentration in the sealant, as well as using phthalates with lower vapor pressures.