NOTES INTRODUCTION EXPERIMENTAL

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1 NOTES Stripping Rates of Vinyl Chloride from PVC Resin INTRODUCTION Ever since the discovery that vinyl chloride is a carcinogen, PVC manufacturers have been concerned about the stripping of residual vinyl chloride monomer (VCM) from PVC resins, slurries, and finished products.' Using gravimetric methods, Berens2s3 obtained the basic solubility and diffusion data of VCM in dry PVC resin. Pate et al.4 have measured the distribution of VCM in PVC-water-vapor phases under equilibrium. Here we supplement these studies, which pertain to VCM removal from PVC, by reporting the kinetics of VCM stripping from PVC. The rate of decay of VCM concentration in PVC as the resin is flushed with water-air at various temperatures is reported. EXPERIMENTAL PVC resin with a particle size range of p was obtained from suspension polymerization. Desired levels of VCM concentrations in the PVC resin were obtained by pressurizing the PVC with VCM in a stainless-steel bomb. Adequate time was allowed for VCM to diffuse into the PVC particle to achieve a desirable concentration of VCM. Figure is a schematic of the experimental setup. It consists of a 350-ml glass Buchner funnel FUNNEL HOLDER- I DRAINING VALVE - TO DRAIN r t 350 rnl GLASS FUNNEL Fig.. Experimental apparatus. THERMOCOUPLE Journal of Polymer Science: Polymer Chemistry Edition, Vol. 7, (979) CQ 979 John Wiley & Sons, Inc /79/ $0.00

2 3776 J. POLYM. SCI.: POLYM. CHEM. ED. VOL. 7 (979)

3 K(Mi").l low low0 c, (Pm) Fig. 4. VCM decay constant in PVC rinsed with water. with a fritted glass filter of p pore size. The funnel is fitted with a stopper provided with sampling, drainage, and water inlet lines, a stirrer, and a thermocouple. The funnel was mounted in an inverted position to allow any trapped air to escape through the glass filter. A sample of the PVC resin with a known initial VCM concentration was flushed with a stream of water through the water inlet line. Inlet water temperature was maintained at a desired level by passing it through a heating bath with a thermostat. Inlet water pressure was maintained at 0 psig by a pressure regulator. To determine the concentration of VCM in PVC at different times during the experiment samples of PVC slurry were collected via the sampling line. An aspirator was used to filter the free water from the slurry, and a part of the wet PVC resin was dried to determine the water content. Another part of the wet PVC resin was dissolved in THF. The concentration of VCM in PVC was determined by measuring the concentration of VCM in THF with a gas chr~matograph.',~ In another set of experiments water was replaced by air. In these experiments a sample of PVC in a Buchner funnel was fluidized with hot air. Desired air temperature was achieved by passing the air through a copper tube packed with lead pellets and heated with a heating tape. A sample of PVC was scooped from the funnel at various intervals. VCM concentration in PVC was determined by dissolving PVC in THF and analyzing THF for VCM concentration. RESULTS Figures 2 and 3 show the decay in VCM concentration when the resin is rinsed with water and air, respectively. The VCM concentration C, as a fraction of initial VCM concentration CSo is plotted as a function of time. It is hypothesized that VCM removal from PVC slurry is the result of a two-mode mass transfer mechanism. According to the first mechanism, VCM will boil away from the PVC particle when the sum of the partial pressures of water and VCM dissolved in PVC is greater than the ambient pressure. This mechanism may lead to a dominant VCM flux under the conditions of high VCM concentration in the resin, low pressure, or high temperature. Mass transfer, which results from a concentration driving force between the resin and the surrounding medium, is the second mode of mass transfer.

4 3778 J. POLYM. SCI.: POLYM. CHEM. ED. VOL. 7 (979) 90% 80% 0..I 50% I ~ ( ~ i ~ j - aw 25% aow amw woo c, (m) Fig. 5. VCM decay constant in PVC rinsed with air. From an analysis of the three-phase equilibrium data of Pate et al4 it can be seen that VCM boiling is not possible in the present experiments and that the removal of VCM is a result of the second mechanism only. Under the assumption that the fluid flow rate is so high that VCM concentration in the fluid is essentially zero the decay in the VCM concentration can be approximated by -= dcs -k(c,- 0) dt or -- d In C, - -k dt

5 NOTES 3779 The decay constant k is a lumped mass transfer coefficient derived by assuming that the film coefficient and the intraparticle diffusional resistance shown by GlueckauF are linearly additive. Figures 4 and 5 show the decay constant k calculated from the slopes of the decay curves in Figures 2 and 3. The decay constant is sensitive to the temperature. Changes in the value of k with C, can be attributed to two factors. In Figures 4 and 5 increases in the value of k with increasing C, may be due to plasticization of PVC with VCM. Another reason for variation in k with C, may be the use of an over-simplistic mass transfer model. The model does not account for the relative ease with which the resin can be stripped when VCM is present near the surface of the particle at the beginning of the experiment. With the progress of time the VCM core shrinks and the stripping becomes relatively more difficult. A decreasing value of k with decreasing C, in Figures 4 and 5 reflects these two possible factors. The authors express their appreciation to Dr. G. J. Mantell for guidance and to Air Products and Chemicals, Inc., for permission to publish this note. References. G. J. Mantell, J. T. Barr, and R. K. S. Chan, Chem. Eng. Prog., 7,54 (975). 2. A. R. Berens, Polym. Prepr., 5,97 (975). 3. A. R. Berens, Polym. Prepr., 5,203 (975). 4. C. B. Patel, R. E. Grandin, E. M. Phillips, C. E. Reynolds, R. K. S. Chan, and R. Gupta, Polym. J.,,43 (979). 5. E. Glueckauf, Trans. Faraday SOC., (955). Air Products and Chemicals, Inc. Process and Products R&D P.O. Box 538 Allentown, Pennsgvania 805 Received October 20,978 C. B. PATEL R. HOUCK R. GUPTA R. K. S. CHAN