PACKAGING TECHNOLOGY AND SCIENCE VOL. 1 177-183 (1988)

Study of the Polyvinyl Chloride Polymer Structure in Relation to Migration of Residual Vinyl Chloride Monomer by Inverse Gas Chromatography

Dimitrios Apostolopoulos* and Seymour G. Gilbert' Department of Food Science, Cook College, New Jersey Agricultural Experiment Station, Rutgers University, New Brunswick, NJ 08903, USA

The polyvinyl chloride (PVC) polymer structure was studied using inverse gas chromatogra- phy (IGC). The specific retention volume (Vg") of a vinyl chloride monomer (VCM) probe, as well as the thermodynamic parameters of the interaction between PVC and VCM were calculated.

Changes in Vg", free energy(A@J, enthalpy (AWJ and entropy (APJ, observed as the temperature and the amount of VCM decreased, clearly indicate that the PVGVCM interaction was both concentration and temperature dependent. The Vg" and thermodyna- mic parameters also varied with changes in polymer structure. Data were interpreted in terms of the active site hypothesis. Active sites in the PVC matrix strongly bind VCM at low enough concentrations and temperatures. Inferably, migration of VCM from PVC packaging materials containing very low concentrations of residual monomer should be for all practical purposes, essentially zero, particularly at low temperatures.

Keywords: Polymer structure, Inverse gas chrornatography, Vinyl chloride monomer (VCM), Migration

INTRODUCTION

Polyvinyl chloride (PVC) was used for years as a food packaging material on the assumption that no harmful species migrated from the package to the food. This assumption was shown to be erroneous when it was found that alcoholic beverages packaged in PVC bottles were con- taminated with vinyl chloride monomer (VCM) extracted from the plastic. Further studies have shown that VCM migrates by diffusion from domains of high concentration to those of lower

5 concentration, whether these be the surroundin air or food in contact with the PVC plastic.4* Sigce VCM is a potential carcinogen, consider- able attention has been focussed on the desorp- tion or migration of residual monomer resent in

The migration of residual VCM from a glassy PVC polymer matrix under fixed conditions has been considered as directly proportional to its concentration in the packaging material. This is

PVC plastics used for food packaging. B

'Present address: Hercon Laboratories Corporation, 200B Corporate Court, So. Plainfield, NJ 07080, USA. tAuthor to whom correspondence should be addressed.

0894-32 14/88/0401 7747$0S.(K) 0 1988 by John Wiley & Sons, Ltd.

Received I June I988

178 D. APOSTOLOPOULOS AND S. G. GILBERT

true for concentrations higher than lppm. At concentrations below 1 ppm, however, the VCM migration mode becomes non-linear in favour of the polymer.' This has been attributed to the sorption of initial residual VCM molecules on submicroscopic structural irregularities of active sites of unusually high binding energy in the PVC matrix. As the amount of VCM immobilized by active sites increases at the expense of the unbound VCM, the partition coefficient rises exponentially, the interaction between PVC and VCM also increases, and as a result the migration of VCM becomes minimal.^" At higher VCM concentrations, the active sites become saturated and additional VCM molecules tend to migrate in accordance with classical diffusion theory. The importance of the number of active sites in the PVC matrix and their binding energy is apparent. It was theorized that both of those characteristics are an inherent part of the polymer structure, which in turn is affected by the polymerization process used for manufacturing the polymer. The PVC polymers made by emulsion and suspension polymerization were assumed to exhibit different surface configurations, which consequently affect differently the number and the binding energy of active sites and eventually the mass transport of residual VCM.

The employment of classical partition equilib- ria methods to confirm this hypothesis is not feasible due to the inadequate sensitivity of such methods. However, since the monomer migrating is directly related to the propensity of VCM-PVC interaction, a measurement of the latter could offer a means by which the effect of active sites on the migration of VCM at low concentrations might be studied. Inverse gas chromatography, (IGC) has been used widely in studying mono- mer-polymer interactions,l2-'' and requires the packing of a chromatographic column with the polymeric resin under study and the injection of small amounts of monomer/migrant in the mobile phase. The chromatographic system is assumed to attain true equilibrium instantaneously, since small amounts of monomer are injected and relatively low flow rates used. During equilibra- tion of the system, the monomer undergoes partitioning between the stationary and the mobile phase. The volume of carrier gas neces- sary to elute the sorbed monomer from the polymer under specific conditions of temperature and pressure is the net retention volume, V,, given by Equation 1

where t, is the retention time of the monomer; fa is the retention time of a un-sorbed species (air); F, is the measured flow rate; P, is the column outlet pressure; Tcol is the column temperature; Pw is the water vapour pressure at room temperature; T, is the room temperature; and J is the James and Martin" compressibility factor accounting for pressure drop along the column, which is calculated from Equation 2

J = - ( 3 (P,lPo)2 - 1 ) 2 (Pi/Po)3 - 1

where Pi is the column inlet presure. The net retention volume per unit weight of

polymer, corrected to O'C, is defined as the specific retention volume, V;, and is given by Equation 3

(3) 273 1

Vg" = JF,(t, - fa) [(Po - Pw/Po)] - - Tr ws where W , is the polymer weight.

Since the chromatographic system is assumed to attain instantaneous local equilibrium, Vi is considered to be strictly dependent on the thermodynamic equilibrium of the polymer- monomer interaction, unaffected by any oper- ational parameters. Thus, the V; values at or near infinite dilution can be used to calculate the therymodynamic parameters of the interaction between the polymer and the monomer. The enthalpy of sorption (AH:) is obtained from equation 4.

(4) A H : - alnVg -

1 a - Tcol

where R is the universal gas constant.

from Equation 5

R

The free energy (AG:) of sorption is calculated

AG: = -RTcoIlnK, ( 5 ) where K , is the partition coefficient of monomer in the polymer/mobile phase derived from Equa- tion 6

va Tcol K , = - 273

where p is the density of the polymer

STUDY OF PVC POLYMER STRUCTURE 179

The entropy of sorption (AS:) is then calcu- lated using Equation 7

(7) AH: - AG:

m AS: = ' col

The present work was undertaken with the objective to study the effect of PVC polymer structure, monomer concentration and tempera- ture on VCM migration as measured by the PVC-VCM interaction using IGC.

MATERIALS

The PVC polymers studied were unplasticized resins E and S made by emulsion and suspension polymerization, respectively. Both resins were manufactured and supplied by Unilever and had a density of 1.41 g/ml. The VCM was of chromato- graphic grade and high purity (1066ppm), sup- plied by Matheson Gas Products.

METHODS

Preparation of chromatographic columns

Both PVC resins were sieved and the 100/150 mesh fractions were collected. Sieved resins were further stripped of their residual monomer by heating at 60-65 "C in a vacuum oven for ca. 24 h. The removal of residual VCM from the resins was confirmed by using gas chromatography in combination with the hot jar technique sensitive to ca. 2ng VCM/g PVC, as modified by Gilbert (ASTM F-151-72).18 An amount of monomer- free resin was packed in a 6ft x 1/4-in aluminium chromatographic-grade column. The packing was done with the aid of a vacuum pump and a mechanical vibrator to assure adequate settling of the PVC resin. The amount of polymer packed was determined by weighing the column before and after packing, and was 14.4g and 20.8g for resins E and S, respectively. Both columns were conditioned before use by purging overnight with nitrogen.

Preparation of VCM standards

Known volumes of VCM were introduced directly into glass serum vials which had been tightly sealed and pre-flushed with pure nitrogen.

Aliquots of 0.51111 of VCM standards within the concentration range 2-1066 ppm (v/v) were in- jected into the gas chromatograph.

Apparatus

A Hewlett-Packard gas chromatograph (Model 5990A) equipped with a dual-flame ionization detector was used for measuring the retention time of VCM. Constant oven temperature was maintained to within f l "C by means of circulat- ing air. A uniform temperature was assured throughout the chromatographic column by plac- ing insulated spacers on the column ends to prevent overheating from contact with the injec- tion port and detector. Nitrogen was used as the carrier gas. Its flow rate was measured at room temperature (25 "C) by a soap-bubble flow meter attached to the column outlet in combination with a stopwatch. The inlet pressure was measured on a mercury manometer with a range up to 100cmHg. The operation conditions for the gas chromatography were as follows:

(a) For column packed with resin E: column temperature 35 "C, 45 "C, 55 "C; injection port temperature 40 "C, 50 "C, 60 "C; detector temper- ature 210°C; flow rate of carrier gas 60ml/min; inlet pressure 77.0cmHg, 76.5 cmHg, 77.7 cmHg.

(b) For column packed with resin S: column temperature 35 "C, 45 "C, 55 "C; injection port temperature 40 "C, 50 "C, 60 "C; detector temper- ature 210°C; flow rate of carrier gas 60ml/ min; inlet pressure 77.7 cmHg, 77.7 cmHg, 78.2 cmHg.

Microscopic examination of PVC resins

Resins E and S were examined by a JEOL 50A scanning electron microscope at 1500X magnifica- tion to observe any characteristic surface differ- ences.

RESULTS AND DISCUSSION

The first point to be considered in this investiga- tion was whether or not there was any evidence indicating the presence of active VCM binding sites in PVC. This would be confirmed by observing a concentration-dependent rise in the

180 D. APOSTOLOPOULOS AND S. G. GILBERT

Table 1. Relative magnitude of specific retention volumes, thermodynamic parameters and peak shapes of resins E and S Resin V: AGs AH, AS. Shape of Peaks

E 1 1 2 2 Gaussian with diffused tailing S 2 2 1 1 Sharp leading edge and diffused tailing

1 = Highest values (more positive or negative values, respectively). 2 = Lowest values (least positive or negative values, respectively).

specific retention volume ( V i ) for the VCM probe. The thermodynamic parameters (AH:, AGS and AS:) should also show VCM concentra- tion dependence if active sites of varying binding energy were present or differences between the two resins exist. The diffused, tailed, elution band of the VCM probe molecules could also serve as an indication of active site binding of VCM to PVC at low concentration.

40 I

Evaluation of V t values and thermodynamic parameters

Comparison of VgO, AG:, AH: and AS: values, as well as a description of the gas chromatographic peak shapes for resins E and S are given in Table 1.

55°C I

- - - - - 10

Amount of injected VCI.1 per qromof PVC restn(g/g XU’)

I , I . I . .

0 10 20 30 4 0 50

Figure 1. Specific retention volume (V:) for resin E as a function of VCM concentration and temperature

0.3 I values. The specific retention volume (V i ) for

both resins E and S as a function of the amount of injected VCM, at three different temperatures, is given in Figures 1 and 2. These figures show that the VCM specific retention volume for both resins are concentration and temperature depen- dent. This concentration dependence of the VgO, mainly observed at low VCM concentration, was attributed to the presence of active sites in the PVC polymer matrix. These active sites sorbed the VCM molecules very strongly at low VCM concentrations. As a result, the VgO of the system increased. As the VCM concentration increased and the active sites were saturated, the Vg became concentration independent. An increase in temperature resulted in a decrease in Vg due to the higher kinetic energy of VCM molecules causing less active site binding. Resin E exhibited greater specific retention rolumes as compared to resin S. (Table 1). This would imply that resin E had a greater number of available active sites than

I3 35°C 45°C

55°C

0.1 ‘ I 1 I

0 10 20 3 0 40 Amount of injected VCM per gromof PVC resin (g/g xlO-’)

Figure 2. Specific retention volume (V:) for resin S as a function of VCM concentration and temperature

Gibb’s free energy of sorption (AGZ). The free energy of VCM sorption as a function of the amount of injected VCM for both resins at three temperatures is shown in Figures 3 and 4. Table 1

resin S. and-Figures 3 and 4 showthat the AG: values

STUDY OF PVC POLYMER STRUCTURE

25.5

- - 0 E 25.4 1 I s o * 2 25.3

181

Both resins also showed that AG: becomes more rapidly concentration independent at high temperatures than at low temperatures. This provided evidence that there was less site binding or greater matrix penetration at high temperat- rures. Therefore it is easier for residual VCM to migrate from a PVC matrix at higher rather than at lower temperatures.

45°C >& ' 55"c ~

-

- * . rn

2.5 I 1

2.4 iL = 35°C

55°C 2.0

i a I I -

I , 1 . I . I . m I .Y

0 10 20 30 40 50

Amount of injected VCM per gram of PVC resin (g/g x lo-')

Figure 3. Gibbs free energy of sorption (AG,O) for resin E as a function of VCM concentration and temperature

0.5 I 1 I I 0 10 20 30 40

Amount of injected VCM per gram of PVC resin (g/g x lo-') Figure 4. Gibbs free energy of sorption (AG.") for resin S as a function of VCM concentration and temperature

follow the trend of the V i values. This supported the presence of active sites in the PVC matrix and signified their importance on the free energy of the PVC-VCM system.

Resin E showed the more negative AG: values as compared with resin S (Table 1 and Figure 3 and 4). In other words, resin E was more reactive than resin S. This may be due to the greater number of available active sites in resin E.

The more negative AGp values of resin E, or the less positive AG: values of resin S obtained at the lower temperature showed that the sorption of VCM was favoured at lower temperatures. Thus, the desorption and migration of VCM from a PVC package into the food with which it is in contact is less likely to occur at low temperatures.

Entropy of sorption (AS:). The difference in AS': values between resins E and S, and the different pattern of concentration dependence of these entropic values indicated the existence of structu- ral differences between the two resins. Resin S, which according to Vz values was proposed to have the least number of active sites, showed more negative AS: values than resin E (Table 1). This led to the postulation that the PVC-resin SNCM system exhibited a more ordered struc- ture than the PVC-resin E/VCM system. Such a more ordered PVC-resin S/VCM system can indeed occur if the active sites of resin S have a greater binding energy than those of resin E. Temperature within the range studied had no effect on the entropy changes of the PVCNCM system (Figures 5 and 6).

Enthalpy of sorption (AH:). The enthalpic values (AH:) of VCM sorption, calculated for both resins E and S at different amounts of injected VCM, are shown in Figure 7.

The AH: values correspond to the AS: values (Table 1). This indicated that the enthalpy of the

25.6 El 35°C

t = 0 10 20 3 0 4 0 50

Amount of injected VCM per gram of PVC resin (g/g x lo-') Figure 5. Entropy of sorption (AS.") for resin E as a function of VCM concentration and temperature

182 D. APOSTOLOPOULOS AND S. G. GILBERT

60 Evaluation of peak shape

Resin E gave Gaussian peaks with a little diffused probe tailing. Resin S gave peaks which showed a very sharp leading edge and diffused probe tailing which was more pronounced than resin E.

In every instance, at high probe concentrations and/or high temperatures the leading edges became sharper than those obtained at low probe concentrations and/or low temperatures. Also, the diffused tailing was concentration and

20 temperature dependent. All of this is further 40 evidence for the existence of active sites in the

Amount of injected VCM per gromof PVC resln (919 x lo-’) resins.

- - 50 e x 9

1 0 40 0 -

30

30 0 10 20

Figure 6. Entropy of sorption (AS,”) for resin S as a function of VCM concentration and temperature

Evaluation of PVC matrix microscopic examination

18

16

0 - * 10

1 . 1 . 1

50

Amount of injected VCM per gram of PVCresin (g/g x lo-’) 8

0 10 20 30 40

Figure 7. Enthalpies of sorption (AH,”) for resins E and S as a function of VCM concentration

VCM-PVC system was related to the structure of the polymer.

Resin S showed more negative AH: values than resin E (Table 1 and Figure 7), which clearly suggested that resin S must sorb VCM more strongly than resin E despite the fact that resin S had a smaller number of active sites, as indicated from the V i values. A smaller number of active sites in resin S with very high binding energies could account for that. Thus, for a given concentration of residual VCM and temperature, less VCM will migrate from a packaging material made of resin S than of resin E.

When resins E and S were examined by a scanning electron microscope it was observed that resin E particles exhibited a smooth, open and non-convoluted surface, while the particles of resin S showed a significant surface convolution. In addition, the powder particles of resin E appeared to be smaller than those of resin S. Thus, the surface area to volume ratio in resin E was larger than in the other resin. Both character- istics of resin E, the open structure and the high ratio of surface area to volume, seem to be related to the greater number of available active sites.

Acknowledgements

A paper of the Journal Series, New Jersey Agricultural Experiment Station, Department of Food Science, Cook College, Rutgers University, New Brunswick, NJ 08903, USA. This work was performed as part of NJAES Project of the New Jersey Agricultural Experiment Station.

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