Diffusion and solubility of hazardous compounds in polyvinyl chloride

Accepted Manuscript Title: Diffusion and solubility of hazardous compounds in polyvinyl chloride Author: Ida M. Balashova Edi Meco Ronald P. Danner PII: DOI: Reference:

S0378-3812(14)00027-2 http://dx.doi.org/doi:10.1016/j.fluid.2014.01.012 FLUID 9959

To appear in:

Fluid Phase Equilibria

Received date: Revised date: Accepted date:

28-10-2013 19-12-2013 7-1-2014

Please cite this article as: I.M. Balashova, E. Meco, R.P. Danner, Diffusion and solubility of hazardous compounds in polyvinyl chloride, Fluid Phase Equilibria (2014), http://dx.doi.org/10.1016/j.fluid.2014.01.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

*Revised Manuscript

Diffusion and solubility of hazardous compounds in polyvinyl chloride Ida M. Balashova, Edi Meco, Ronald P. Danner

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The Pennsylvania State University

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University Park, PA, 16802

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Key words: diffusion, solubility, polyvinyl chloride, phthalates, bisphenol-A Abstract

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Polyvinyl chloride (PVC) is a widely used polymer for many types of products, some of which come into close contact with people. To obtain flexibility a variety of plasticizers are added to PVC. Over the years

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these additives have been investigated extensively in terms of their potential health and safety hazards.

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The diffusion and solubility of these materials at higher concentrations (10 – 50 wt%) in PVC have been

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reported by a number of researchers. Similar data at very low concentrations, however, have not been found. At ambient temperatures the diffusivity is so low that direct measurements are very difficult if

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not impossible. In this study data at higher temperatures have been obtained, compared with predictions or correlated with a model, and then extrapolated to ambient conditions. This provides some reasonable estimates of the solubility and diffusivity that can be expected at the lower limits of concentration, low temperature, and atmospheric pressure. 1. Background

Polyvinyl chloride is ubiquitous in people’s daily lives. Some of the common uses are stretch films for food packaging, water bottles, most all water and sewer pipes, electric cables, vinyl siding, vinyl sheets and film, clothing of all kinds, and medical applications such as intravenous tubing and catheters. The predominance of PVC is due to its low cost, biological and chemical resistance, ease of sterilization, transparency, light weight, and flexibility (when plasticizers are used). 1 Page 1 of 20

Clearly for many of the applications flexibility is important. To promote this characteristic plasticizers are added to the PVC. For example various types of phthalates are used: butyl benzyl (BBP), dibutyl (DBP), di-2-ehtylhexyl (DEHP), di-isobutyl (DIBP), dimethyl (DMP), and diethyl (DEP). These

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materials do not covalently bind to the polymer matrix and thus tend to leach out. Another frequently used plasticizer is bisphenol-A (BPA). Typically this is (or was) found in liners of canned goods, baby

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bottles, and drinking water containers.

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There has been extensive research into the health and safety of these additives.[2 , 3-5] The reported potential hazards extend to the parts per billion region. There is, however, significant

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disagreement regarding this issue. In some countries and states moves have been made to ban BPA completely. DBP, BBP, and DEHP are to be phased out in the European Union by early 2015. Some car

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manufactures have banned use of PVC in car interiors. Other reports, however, state that low concentrations are of no concern even to infants.

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This paper is not intended to further the discussion of the health hazards of these compounds.

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Rather it is to examine the rate of migration of these materials at low concentration from the PVC into

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the surroundings. There are essentially no data available for the diffusion of the types of plasticizers being considered herein in the infinite dilution concentration and range of temperatures of interest, i.e., ambient temperatures. The primary reason for this is their very low diffusivities at ambient temperatures. In this study an effort has been made to provide reasonable estimates of the solubility and diffusivity of these compounds based on extrapolation of higher temperature data using a theoretical model (diffusivity) or confirmation of a predictive model at the higher temperature and then using it to predict at ambient temperatures (solubility). For practical purposes the diffusivity is the important parameter. The surroundings of the PVC in most cases have essentially zero concentration of the plasticizer so in theory the equilibrium concentration in the polymer is zero. What matters is the rate at which the remaining material in the polymer will diffuse out.

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The issue is further clouded by the tendency for PVC and some of the plasticizers to degrade at higher temperatures. Since data can only be obtained at the higher temperatures efforts must be made to carry out the experiments quickly without allowing too much time for degradation. The final results,

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however, must be interpreted with this caveat in mind. PVC degrades with the loss of hydrogen

chloride. This is an autocatalytic reaction. Frequently, however, heat stabilizers are used to slow the

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degradation. It has been reported that loss of HCl may occur at temperatures as low as 70°C. The

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results from a set of experiments in our laboratory indicated that significant degradation of the PVC did not occur below 175°C.

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2. Materials

The PVC was purchased from Scientific Polymer Products, Inc. Two capillary columns were made

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with different molecular weights of PVC: 275,000 and 120,000. No differences were detected between

temperature of 85°C.

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the responses of the two columns. PVC had a chlorine content of 56 wt% and a glass transition

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The bisphenol-A (97 % pure) and the diethylphthalate (98% pure) were from Spectrum Chemical and

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Laboratory Products. The dimethylphthalate was from Alfa Alesor (99% pure). 3. Experimental method

An important region of interest is low concentration at ambient temperatures. Unfortunately, measurements cannot be made near ambient temperature because of the very low diffusivities encountered there. The low concentration requirement can be most easily met using the inverse gas chromatography method. The use of this method and the method of data analysis has been thoroughly presented in previous papers. [6-8] The essence of the method is a thorough analysis of the elution profile of a compound from a capillary column which has its walls coated with the polymer. The operative equation is 1   2   1 CL 1 s 2 s   exp   exp   2   tanh  s     C0 ν 4    2      





(1)

3

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Where

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r Kτ

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α

τ2 ν β  Dp L

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Dg νL

(4)

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γ

(3)

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2

(2)

Here C is the eluent concentration, C0 is the initial concentration, L is the length of the

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capillary column, τ is the thickness of polymer coating, r is the void radius of the capillary

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(radius of capillary column minus thickness of polymer coating),and ν is the velocity of the

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carrier gas. Dg is the diffusion coefficient of solvent in the carrier gas and was estimated using the method of Fuller, et al.[9] In the current study the  and  values were obtained by minimizing the deviations of the model and experimental profiles in the time domain. K is then obtained from  and Dp from  . This has proven to be the most reliable method.

4. Treatment of the data Since data can only be obtained at higher temperatures methods are needed to extrapolate these data to ambient temperatures if possible. There are a number of predictive models available for the solubility of solvents in polymers. One that has been reported to be successful in a number of cases is the UNIFAC-vdw-FV model of Kannan et al. [10]. This model is an extension of the UNIFAC model of 4 Page 4 of 20

Fredenslund et al. [11] often used for lower molecular weight systems. For polymer type systems a free volume term has been added based on the van der Waals model. This model is strictly predictive so if it adequately fits the high temperature data it can be extrapolated to estimate the lower temperature

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behavior.

For the diffusivity there is no purely predictive model that is judged to be reliable. One can,

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however, fit two or three parameters to the free-volume model of Duda and Vrentas[12, 13] and

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extrapolate this fit to lower temperatures. The equation for the mutual diffusion coefficient in an

2

  

1

1



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(5)

(6)

VˆFH



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D01  D0 exp(  E / RT )

  1  2  1    

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

  1Vˆ1*  2Vˆ2*  D p  D01 exp    VˆFH /    

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amorphous polymer is

 1

K  K21  Tg1   T   2 12  K22  Tg 2   T       

K11

(7)

Here Dp is the mutual binary diffusion coefficient in the amorphous polymer, D0 is the pre-exponential factor, E is the activation energy for diffusion, i , is the weight fraction of the solvent (1) or the polymer (2), Vˆi * is the specific occupied volume of component i, VˆFH is the hole free volume,  is the overlap factor accounting for shared free volume,  is the ratio of the solvent and polymer jumping units, 1 is the volume fraction of the solvent, and  is the Flory-Huggins interaction parameter. In Eq. (5) the first bracketed term is the estimation of the self-diffusion coefficient and the second term is the 5 Page 5 of 20

thermodynamic term using the Flory-Huggins thermodynamic model[14] to relate the mutual binary diffusion coefficient to the self-diffusion coefficient. 5. Results and Discussion

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Data were measured for three plasticizers in the PVC – dimethyl phthalate (DMP), diethyl phthalate (DEP), and bisphenol-A (BPA) at infinite dilution using the inverse gas chromatography method. Fig. 1

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shows the data that were measured for the partition coefficient of DEP in PVC over the temperature

concentration in the vapor phase, both in the same units.

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range of 210 to 310°C. The partition coefficient is the concentration in the polymer phase divided by the

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In order to compare the UNIFAC-vdw-FV predictions to the experimental partition coefficient, K, a transformation between the predicted weight fraction activity coefficient, 1 , and K is needed. From

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the predicted 1 the “predicted” specific retention volume, Vg, can be calculated from[15]

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 P1s V1  B11    RT  Vg   exp    s RT  1P1 M 1   

(8)

s Here P1 is the saturated vapor pressure, M1 is the molecular weight, V1 is the liquid molar volume, B11 is

the second virial coefficient all for the solvent, R is the universal gas constant, and T is the temperature. The “predicted” K then is calculated from the retention volume, Vg, by the relation[16]

K  Vg  2

(9)

Here 2 is the density of the polymer. The UNIFAC-vdw-FV predicted curve in Fig. 1 is in good agreement with the data. The solubility is estimated to increase by four orders of magnitude when the temperature is decreased from 350 to about 50°C. When the surrounding concentration is zero, the partition coefficient is not of real practical 6 Page 6 of 20

value. When the surrounding concentration is finite, however, the partition coefficient can be used as a Henry’s Law constant over a limited concentration range. The important characteristic is the diffusivity. The data for the diffusivity in DEP-PVC are shown in

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Fig. 2 for the same temperature range. Table 1 lists all the parameters for the free-volume model for all the pure materials and for the binary parameters. The DEP parameters were obtained from the viscosity

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data reported by Kishimoto[17]. The relation of Doolittle[18] was used to relate the amount of free-

 Vˆ1* / K11

K

21

 Tg1   T

(10)

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ln1  ln A1 

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volume in the DEP to the viscosity. The equation of interest as given by Hong [19] is:

From a nonlinear regression of the natural log of the plasticizer viscosity, 1 , as a function of

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temperature the free-volume parameters, K11 /  and (K21 – Tg1), were determined. The PVC free-

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volume parameters are estimates based on a few viscosity data that were found. The best that can be

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said for these values is that they fit the available diffusivity data of all three plasticizers well. The specified occupied volumes of the plasticizers and polymer were calculated using the group contribution

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method of Sugden.[20]

The three binary free-volume parameters remain to be defined. The activation energy for diffusion, E, was set equal to zero. This is often satisfactory if the data can adequately be fitted with it. Thus, two parameters remain to be determined, D01 and  . These were regressed from the free-volume model, Eqns. 5-7, using the experimental data.

The resulting data correlation and predicted extrapolation of the data is shown in Fig. 2. The data obtained by the IGC method are at infinite dilution. That means that 1 in Eqns. (5-7) is set to zero and many of the free-volume parameters are not invoked. They are useful, however, if one wishes to extrapolate in terms of concentration. The free-volume model fits the infinitely dilute experimental data very well. The extrapolation to 100°C clearly shows why one cannot obtain data at ambient 7 Page 7 of 20

conditions. The diffusion coefficient at 100°C is estimated to be 2x10-16 cm2/s. No claim is made that this long extrapolation is precise, but it is a reasonable estimation of how low the diffusivity can go. No other estimates have been found in the literature.

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Figs. 3 and 4 show the same data for the dimethyl phthalate (DMP)-PVC system. The solubility prediction is in reasonable agreement with the data although the slope of the data appears to be

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greater than the predicted slope. The pure component parameters for DMP were taken from Hong[19].

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The diffusivity correlation predicts a value at 100°C of 1x10-14 cm2/s. This is higher than the values for DEP as would be expected since it is a smaller molecule. Again, however, no claim is made as to the

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precision of this value, only its practical interest.

Whereas DMP and DEP are liquids at room temperature and have reasonable vapor pressures, BPA

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is a solid and has very low vapor pressure. It was introduced into the chromatograph by dissolving it in ethanol. The BPA and ethanol separate quickly in the column and thus one can evaluate the elution

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peak for the BPA in the usual manner. Again measurements can only be made at higher temperatures

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and this can lead to the degradation of both the PVC and BPA. Nevertheless data were collected for

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four temperatures as shown in Fig. 5. The UNIFAC-vdw-FV predictions do not agree with the experimental data. The cause of this discrepancy could be the predictions, experimental error, degradation of the materials, or something not identified. The experimental partition coefficients fall, however, in the same range as those for DEP and DMP. Fig. 6 shows the extrapolation of the diffusivity to ambient temperatures using the free-volume model of Vrentas and Duda. The free-volume parameters for BPA were obtained by regressing viscosity data from Daubert and Danner.[21] Again extremely low values of the diffusivity are estimated at ambient temperatures. It is difficult to interpret what these values mean from a practical point of view. A simulation was run for a PVC film 0.1 cm thick with an initial concentration of plasticizer of 50 mg/kg PVC (5x10-4 wt fraction). This was a typical value that has been reported for BPA in some food packaging films. [22] Fig.

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7 shows the predicted concentration of solvent as a function of position in the film after a period of one year. The residual concentration is a strong function of the diffusivity. For a diffusivity of 1x10-9 cm2/s essentially all the solvent has migrated out of the film. For a diffusivity of 1x10-15 cm2/s almost none of

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the solvent has left the film. 6. Comparison with literature data

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There have been a number of studies reported in the literature that address the diffusion of similar

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plasticizers in PVC. Essentially all of these deal with high concentrations of the plasticizer that would be needed to render them adequately flexible for their intended purpose. Without the plasticizer PVC

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becomes quite brittle. Storey et al. [23] studied numerous phthalates migrating from their pure liquids into PVC at temperatures between 60 and 100°C. The initial concentration of plasticizer was 2 to 16

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wt%. The diffusivities increased with higher temperature and decreased as the molecular weight increased. They found diffusivities in the range of 10-7 to 10-10 cm2/s. As expected these diffusivities are

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significantly higher than those found herein because of the increase in free-volume provided by the

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was one.

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finite concentration of plasticizer and the pure liquid interface which meant the activity at the surface

Brouillet and Fugit [24] studied the migration of phthalates out of thin sheets of PVC into liquid nheptane. The PVC had 35 wt% phthalate. The rate of migration was determined by the change in concentration of the phthalate in the n-heptane with time. At 30°C their reported diffusivities were in the range of 10-7 to 10-9 cm2/s. The n-heptane probably diffused into the PVC increasing the free volume, thus raising the diffusion rate. Demir and Ulutan [25] made films of PVC with various phthalates at 35.7 wt% and heated them in air to temperatures between 50 and 160°C. They then measured the weight loss of the samples over time. The diffusivities were in the range of 10-10 to 10-14 cm2/s. In this case the higher diffusivities were expected because of the high concentration of the plasticizer and thus the added free volume.

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While these comparisons with the literature cannot confirm or contradict the data reported herein, they do emphasize that the interpretation of the diffusivities must be made in conjunction with the concentration of plasticizer in the PVC. Just as temperature has such a large effect on the diffusivity so

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does concentration.

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7. Conclusions

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Experimental data have been collected for three plasticizers, dimethyl phthalate, diethyl phthalate, and bisphenol-A in polyvinyl chloride in the infinitely dilute region. At temperatures in the range of 250-

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350°C these compounds exhibit high solubility and relatively high diffusivities. The temperature range of practical interest, however, is ambient conditions. In order to estimate solubility values in this range

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the UNIFAC-vdw-FV model was shown to be in good agreement for the phthalates at high temperature so it was used to extrapolate to ambient conditions. For the diffusivities the Vrentas-Duda free-volume

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model was fit to the available data and then used to extrapolate to lower temperatures. Both the

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solubility and diffusivity are strong functions of temperature. While the extrapolated values are not

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claimed to be precise they are reasonable indications of what can be expected at ambient conditions and very low concentrations. For truly plasticized PVC the large concentrations of plasticizer will significantly increase the rate of diffusion as has been documented in the literature.

Acknowledgement. This work was supported by a grant from the Camille and Henry Dreyfus Senior Scientist Mentor Program.

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References

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[1] Toxicological and Health Aspects of Bisphenol-A, in: Report of Joint FAO/WHO Expert Meeting, World Heath Organization, Ottawa, Canada, 2010. [2] NTP-CERHR Monograph on Potential Human Reproductive and Developmental Effects of Bisphenol A, in, NIH Publication No. 08-5994, National Institute of Health, 2008. [3] M.A. Kamrin, Journal of Toxicology and Environmental Health, Part B, 12 (2009) 157-174. [4] Phthalates and Cumulative Risk Assessment: The Task Ahead, in, National Academic Press, Washington, D.C., 2008. [5] S. Chandra, V. Kumar, A. Prakash, S. Kumari, American-Eurasian Journal of Scientific Research, 7 (2012) 199-202. [6] C.A. Pawlisch, J.R. Bric, R.L. Laurence, Macromolecules, 21 (1988) 1685-1698. [7] C.A. Pawlisch, A. Macris, R.L. Laurence, Macromolecules, 20 (1987) 1564-1578. [8] R.K. Surana, R.P. Danner, F. Tihminlioglu, J.L. Duda, J. Polym. Sci.: Part B Polym. Phys., 35 (1997) 1233-1240. [9] E.N. Fuller, Schettle.Pd, J.C. Giddings, Ind. Eng. Chem., 58 (1966) 19-&. [10] D.C. Kannan, J.L. Duda, R.P. Danner, Fluid Phase Equilib., 228 (2005) 321-328. [11] A. Fredenslund, R.L. Jones, J.M. Prausnitz, AlChE J., 21 (1975) 1086-1099. [12] J.S. Vrentas, J.L. Duda, J. Polym. Sci.: Part B Polym. Phys., 15 (1977) 403-416. [13] J.S. Vrentas, J.L. Duda, J. Polym. Sci.: Part B Polym. Phys., 15 (1977) 417-439. [14] P.J. Flory, Principles of polymer chemistry, Cornell University Press, Ithaca,, 1953. [15] I.H. Romdhane, R.P. Danner, J. Chem. Eng. Data, 36 (1991) 15-20. [16] R.J. Laub, R.L. Pecsok, Physicochemical Applications of Gas Chromatography, John Wiley & Sons, New York, NY, 1978. [17] A.E. Kishimoto, Journal of Polymer Science Part A: General Papers, 2 (1964) 1421-1439. [18] A.K. Doolittle, J. Appl. Phys., 22 (1951) 1471-1475. [19] S.U. Hong, Ind. Eng. Chem. Res., 34 (1995) 2536-2544. [20] S. Sugden, J. Chem. Soc., (1927) 1786-1798. [21] T.E. Daubert, T.E. Daubert, R.P. Danner, Physical and thermodynamic properties of pure chemicals : data compilation, Taylor & Francis, Washington, DC, 1989. [22] J. Lopez-Cervantes, P. Paseiro-Losada, Food Addit. Contam., 20 (2003) 596-606. [23] R.F. Storey, K.A. Mauritz, B.D. Cox, Macromolecules, 22 (1989) 289-294. [24] S. Brouillet, J.L. Fugit, Polym. Bull., 62 (2009) 843-854. [25] A.P.T. Demir, S. Ulutan, J. Appl. Polym. Sci., 128 (2013) 1948-1961.

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Figure Captions Figure 1. Infinite dilution partition coefficient of diethylphthalate in polyvinyl chloride Figure 2. Infinite dilution diffusion coefficient of diethylphthalate in polyvinyl chloride

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Figure 3. Infinite dilution partition coefficient of dimethylphthalate in polyvinyl chloride

Figure 4. Infinite dilution diffusion coefficient of dimethylphthalate in polyvinyl chloride

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Figure 5. Infinite dilution partition coefficient of bisphenol-A in polyvinyl chloride

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Figure 6. Infinite dilution diffusion coefficient of bisphenol-A in polyvinyl chloride

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Figure 7. Concentration profiles through a film of PVC 0.1 cm thick as a function of the diffusion coefficient after a period of 1 year.

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Table 1

DMP(i=1)

PVC(i=2)


∗ (cm3/g)

0.589

0.698

0.609

0.666

K2i – Tgi (K)

59

-190

-198

-262

K1i / γ

1.61x10-4

1.50x10-3

1.29x10-3

4.80x10-4

ρ i (g/cm3)

0.910

0.848

0.935

1.1

Do (cm2/s)

4.40x10-3

1.94x10-6

1.06x10-6

ξ

3.32

1.826

E (J/mol)

0

0

χ

---

0.17

Regression parameters with PVC F-H param.

1.482 0

0.3

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(cm /g•K)

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3

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DEP(i=1)

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Pure comp. parameters

BPA(i=1)

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Table 1. Free-volume parameters

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Figure - 1

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Figure -2

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Figure - 3

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Figure - 4

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Figure - 5

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Figure - 6

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Figure 7

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