Die Angewandte Makromolekulare Chemie 192 (1991) 81 -91 (Nr. 3294)

Department of Chemistry, University of Ioannina, Ioannina 451 10, Greece

Gas chromatographic studies on the compatibility of PVC with a migration resistant plasticizer

P. G. Demertzis*, K. A. Riganakos, K. Akrida-Demertzi, M. G. Kontominas

(Received 12 June 1990)

SUMMARY The technique of inverse gas chromatography (IGC) was used to investigate the

thermodynamic miscibility (compatibility) of low molecular weight PVC-poly(adipate ester) plasticizer mixtures. The polymer-plasticizer thermodynamic interaction para- meters were determined from chromatographic retention data for various low molecu- lar weight volatile solutes (probes) on stationary phase mixtures which covered the entire composition range for each component. Experiments were carried out in the tem- perature range of 90 to 110 "C. The Flory-Huggins interaction parameter (1s) values were found to be dependent on the chemical nature of the probes, the temperature, and the composition of the mixed phase. The results predict a relatively high polymer- plasticizer compatibility for a wide range of polymer-plasticizer mixtures (up to approx. 70 wt.-Vo of plasticizer). Results were also compared to our own previous data using high molecular weight PVC.

ZUSAMMENFASSUNG Die Technik der inversen Gaschromatographie (IGC) wurde zur Untersuchung der

thermodynamischen Mischbarkeit (Vertraglichkeit) von niedermolekularem Poly(vi- nylchlorid) (PVC) mit einem Poly(adipinsaureester) als Weichmacher angewendet. Die thermodynamischen Polymer-Weichmacher-Wechselwirkungsparameter wurden aus den chromatographischen Retentionsdaten verschiedener, niedermolekularer fliichti- ger Substanzen a l s mobile Phase auf einer stationaren Phase aus Polymer-Weichma- cher-Mischungen, die sich iiber den gesamten mdglichen Zusammensetzungsbereich erstreckten, ermittelt. Die Untersuchungen wurden im Temperaturbereich von 90 bis 1 10 "C durchgefiihrt. Der Flory-Huggins-Wechselwirkungsparameter (x&) hangt dabei im wesentlichen von der chemischen Natur der als "Sonde" verwendeten fliichti- gen Substanz (mobile Phase), von der lkmperatur und von der Zusammensetzung der Polymer-Weichmacher-Mischung (station&e Phase) ab. Die Ergebnisse deuten auf eine relativ gute Polymer-Weichmacher-Vertraglichkeit iiber einen weiten Zusammenset- zungsbereich (bis ca. 70 Gew.-Yo Weichmacher) hin; die Vertraglichkeit ist damit besser a l s bei friiheren Untersuchungen mit hochmolekularem PVC.

* Correspondence author.

0 1991 Hiithig & Wepf Verlag, Base1 CCC 0003-3146/91/$04.00 81

P. G. Demertzis, K. A. Riganakos, K. Akrida-Demertzi, M. G. Kontominas

Introduction

Polyvinylchloride (PVC) and its copolymers are thermoplastic materials which find a variety of applications including food packaging'. The basic polymer as produced after polymerization is hard and brittle and cannot be used in most applications unless it is first plasticized. Addition of plasticizer enhances processibility and flexibility of the polymer. On the molecular level, plasticization is the weakening or rupture of selective bonds between mole- cules, while leaving the rest intact, to enable the shaping, flexing or molding of the polymeric material2. Thus, plasticization requires the existence of molecular interactions between polymer and plasticizer, defined as compati- bility, in order to produce a homogeneous, stable polymer-plasticizer mixture with reduced potential for plasticizer extractability*.

Commercially available plasticizers for PVC include phthalates, adipates, phosphates, etc. These plasticizers are known as "monomeric" and while improving the workability and flexibility of the polymer resin, they usually exhibit a high potential for migration, extractability, volatility, e t ~ . ~ . These disadvantages become more pronounced when polymeric materials are intended to be used in applications such as food and drug pa~kaging~9~.

A possible approach to reduce the potential for migration of plasticizers from PVC into foods is to replace monomeric with polymeric or permanent plasticizers, compounds of higher molecular weight and, therefore, of reduced tendency for migration due to limited mobility in the polymer m a t r i ~ ~ ? ~ .

Among the methods to study thermodynamic type interactions in polymers, inverse gas chromatography has proven to be very successful in a variety of applications such as interactions in polymer blends and binary mixtures of a non-volatile compound with a polymer6, interactions between polymers and plasticizers as well as polymer blends7-15.

In the present work, the thermodynamic interaction between low molecular weight PVC and a poly(adipate ester) type plasticizer has been studied in the temperature range of 90 to 110 "C. A number of vapor phase solutes (probes) of varying chemical nature were used to determine the Flory-Huggins thermodynamic parameter (&) of the PVC-plasticizer interaction in mixtures of these two components covering the entire composition range. The PVC-plasticizer mixtures comprised the stationary phase of the GC column while the vapor-phase probes comprised the mobile phase of GC system. A second objective of this work was to compare present thermodynamic data using low molecular weight PVC to those previously reported by our group using high molecular weight PVC17.

82

PVC-plasticizer compatibility by inverse GC

Experimental

Materials

The PVC resin was of commercial grade with a molecular weight of 40000 and a density of 1.40 g/ml supplied by Aldrich (Milwaukee/Wisconsin). In all cases the PVC resin was used in combination with 1 wt.-To di-n-butyltin dilaurate (organotin thermal stabilizer). The plasticizer used was a poly(adipate ester) (Santicizer 409 A) of molecular weight of 1 800 supplied by Montsanto (St. Louis/Missouri).

Volatile solutes used are given in Thb. 1 and were supplied by Ferak (Berlin/ Germany).

Tab. 1. Specific retention volume (V3 values for the system: low molecular weight PVC-poly(adipate ester) plasticizer at 90 "C*.

Volatile Column A B C D E F G H compound (probe) PVC (wt.-Yo) 100 92 85 75 60 45 25 0

THF 24.01 25.44 26.60 29.05 31.21 33.50 36.95 41.34 CHCI, 16.48 18.26 21.40 26.39 31.89 34.06 48.67 60.39 CCI, 13.09 13.88 15.42 18.58 22.46 27.84 32.70 38.34 C6H6 27.60 28.54 29.81 32.77 36.51 41.56 47.95 54.96 C6H5CH3 52.50 58.97 62.04 67.93 74.28 86.70 95.76 109.07 CyC10-C6Hlz 6.99 8.26 9.73 12.42 15.11 17.27 19.95 23.42 n-C6H14 3.11 4.17 4.88 7.20 7.40 8.00 8.78 11.24 n-C7H16 7.20 8.16 9.26 11.04 13.37 15.36 18.86 21.51 "-CsH,s 13.40 14.88 17.13 20.75 24.31 29.31 33.60 39.33 n-C9HZ0 26.00 28.34 33.28 39.00 45.85 54.53 61.02 75.91 "-C,oHzz 45.80 53.22 62.44 72.62 84.96 102.03 116.38 144.38

* Respective V: values were calculated for temperatures 100 and 110 "C.

Column preparation

To prepare columns, the polymer-plasticizer mixtures were dissolved in a mutual solvent (2-butanone) at 40 "C and loaded onto the inert support (Chromosorb WAW DMCS, 60 to 80 mesh) by continuous stirring and slow evaporation of the solvent at 50 "C under partial vacuum in a rotavapor. The coated support was dried to constant weight in a vacuum oven at 60 "C, then it was sieved and packed in a 6.4 mm outside diameter, aluminum tubing. The composition of the mixed stationary phase and the loading percentage were adjusted prior to the coating process. The final column

83

P. G. Demertzis, K. A. Riganakos, K. Akrida-Demertzi, M. G. Kontominas

loadings were estimated by ashing an appropriate amount of the coated supports at 550 "C in a furnace, using suitable blank corrections.

Chromatography

Data were generated on a Varian 3700 gas chromatograph equipped with a dual flame ionization detector. Dried helium was used as the carrier gas, at a flow rate of 15 mllmin measured by a soap bubble flow meter. The inlet and outlet column pressures were monitored by a mercury manometer. Methane was used as a marker for retention times. Columns were thermostatted in the range of 90 to llO"C, temperatures corresponding to actual processing conditions for PVC. Retention times of eleven volatile probes were recorded in quadruplicate. The probes were injected into the column using a Hamilton microsyringe. In all cases symmetrical peaks were obtained, indicative of equilibrium sorption conditions16. Peak maxima were used to calculate V: (specific retention volume) values using Eq. (I)":

F 273.16 3 (Pi/Po)2 - 1

W T, 2 (Pi/P$ - 1 v; = (t, - tf)*- a- .- -

where t, and t, are the retention times of the probe and the marker, respectively, F is the carrier gas flow rate measured at room temperature (Tr), W is the mass of stationary phase, Pi and Po are the column inlet and outlet pressure, respectively. m i c a 1 V: values for low molecular weight PVC-poly(adipate ester) plasticizer at 90 O C are given in Thb. 1.

Data reduction

The Flory-Huggins thermodynamic interaction parameters ( x ) between the volatile probe (1) and the binary (polymer, plasticizer) stationary phase (2, 3) were determined from the measured specific retention volume using Eq. (2) to (4)6,18:

(3) 273.16 R v3 PO

1 - - - - (Bll - V,) ) - ( R T x l 3 = In

(4) PO

273.16 R (w2 v2 + w3 v3)

vg, blend vl xl(23) = In

[ (1 - &) m2 + (1 - &) m,] - &BIl - VI)

84

PVC-plasticizer compatibility by inverse GC

where R is the gas constant, wir pi, vi are, respectively, weight fraction, volume fraction and specific volume of polymer (2) and plasticizer (3), Po, V,, B,, are partial molar vapor pressure, molar volume and second virial coefficient of the pure vapor probe, respectively, computed by using appropriate literature dataf9sm. Mi is the polymer (2) and plasticizer (3) molecular weight. The parameter x , (23) is affected by the interaction between polymer-plasticizer (~23):

xI(23) = (P2 XI2 + (P3 x13 - (P2 (P3 x23 (v1/v2) ( 5 )

The polymer-plasticizer interaction coefficient (xZ3) is a rather inconvenient quantity to work with, as it embeds the large molar volume of the polymer within it. Instead, the probe normalized parameter x& defined as6*’*

is employed to characterize the interaction between the two components. ~ 2 3 may be calculated from Eq. (6) if parameters xr2, x13 and x , (23) are known (e. g., via Eq. (2) to (4)). Manipulation of Eq. (2) to (5 ) in order to cancel the common terms leads to a mathematically identical yet simplified form of Eq. (6):

Use of Eq. (7) instead of Eq. ( 5 ) is believed to yield more reliable values of the polymer-plasticizer interaction parameter, in the sense that it saves several intermediate steps in computation and circumvents the necessity of having accurate probe thermodynamic data thus involving less round-off errors in the computation than does Eq. (5). Eq. (7) was used in the present study to calculate the x& interaction para- meter.

Results and discussion

Tab. 2 and 3 present the thermodynamic interaction parameters x12 (probe- PVC) and x i 3 (probe-plasticizer), respectively, as a function of temperature and probe nature. The temperature and the n-alkane chain length dependence of the xI3 values is neglible, but significantly lower values of x i 3 are obtained for chlorinated hydrocarbon and for aromatic probe molecules. This can be attributed to the higher affinity of these molecules for the plasticizer. Analogous results have been reported by other investigators** 13. The x12 values also indicate strong interaction for chlorinated hydrocarbons and weak interactions for alkane probes. The n-alkanes are highly incompatible with PVC, with the incompatibility slightly increasing with increasing chain length.

85

P. G. Demertzis, K. A. Riganakos, K. Akrida-Demertzi, M. G. Kontominas

Tab. 2. Probe - low molecular weight PVC interaction parameters (x12) as a function of temperature and probe nature.

Volatile compound (probe) x12

90 "C 100 "C 110°C

THF CHCI, CCl, C6H6

C6H5CH3 CYCIO-C~HI~ n-C7H16

n-CsH1,

"-C10H22 n-C9H20

0.54 0.39 0.5 1 0.50 0.61 1.69 1.75 1.76 1.77 1.79

0.53 0.39 0.52 0.49 0.63 1.70 1.74 1.75 1.78 1.80

0.55 0.40 0.52 0.50 0.62 1.70 1.76 1.76 1.78 1.79

Tab. 3. Probe - poly(adipate ester) interaction parameters (xis) as a function of temperature and probe nature.

Volatile compound (probe) x13

90 "C 100 "C 110°C

THF 0.21 0.22 0.22 CHCI, 0.08 0.09 0.09 CCI, 0.22 0.22 0.21 C6H6 0.12 0.13 0.12 C6H5CH3 0.20 0.19 0.29 CYCIO-C,HI~ 0.72 0.73 0.73 n-C7H16 0.70 0.71 0.71 "-CsH,* 0.72 0.70 0.72 n-C9H20 0.73 0.74 0.73 n-C10H22 0.75 0.74 0.75

On the other hand, THF, chlorinated hydrocarbons and aromatic probes exhibit low x12 values and could be considered as "plasticizing molecules".

Values for the interaction parameter x& between PVC and plasticizer as a function of the PVC-plasticizer mixture composition for various probes at 90 "C are given in Tab. 4 and selectively in Fig. 1. Increasing positive values for

86

PVC-plasticizer compatibility by inverse GC

Tab. 4. Change in the interaction parameter (&) as a function of PVC - poly(adi- pate ester) plasticizer mixture composition for various probes at 90 "C*.

Volatile Column B C D E F G compound (probe) PVC (wt.-%) 92 85 75 60 45 25

THF CHCI, CCl, C6H6

C6H5CH3 CyClO-C&,, n-C7H16

"-CsH,s

-0.164 +0.039 -0.285 +0.120 -0.510 -0.130 -0.371 -0.192 -0.250 +0.104 +0.321 +0.458 +0.071 +0.257 -0.122 +0.235

+ 0.065 +0.252 -0.183 -0.125 +0.149

+0.417 + 0.498 +0.380

+0.103 +0.133 +0.170 +0.288 +0.300 +0.345 +0.235 +0.380 +0.410 -0.020 +0.060 +0.121 + 0.170 + 0.240 + 0.263 +0.521 +0.615 +0.695 + 0.453 + 0.467 + 0.603 +0.423 +0.500 +0.523

* Respective x& values were calculated for temperatures 100 and 110 "C.

-r

- -0.2 - - -0.1 - - -0.6 4 I 1 - I I I

0.2 0.1 0.6 0.8 1.0 PVC weight fraction

Fig. 1. Dependence of interaction parameter xZ; values on composition of low mole- cular weight PVC-poly(adipate ester) mixtures at 90°C: (0) THF, (+) CHCI,, ( 0 ) CCl,, (A) C6H,, (x) n-C7H,,, and (0) n-C,H,,.

parameter x2+3 indicate increasing incompatibility between the two compo- nents of the mixture. In contrast, values of x2+3 lower than 0.5 and especially negative values are indicative of an increasing degree of compatibility between

87

P. G. Demertzis, K. A. Riganakos, K. Akrida-Demertzi, M. G. Kontominas

the mixture components. As observed in Thb. 4, there is strong dependence of x2+3 values on both: nature of the volatile probes as well as composition of the PVC-plasticizer mixture. Significantly lower x2+3 values are obtained for chlorinated hydrocarbons and aromatic probe molecules than for n-alkanes. This is indicative of a stronger interaction between PVC-plasticizer in the presence of chlorinated hydrocarbons and aromatics than in the presence of n-alkane probes. This trend is observed for all mixture compositions except for the first (92 wt.-Yo PVC-8 wt.-Yo plasticizer). In our opinion these data are of considerable significance in industrial applications (selecting the proper solvent for processing polymer-plasticizer blends). These results are in accordance with those on polymer-polymer or polymer-plasticizer blends presented by other investigators8* 1 3 9 14.

Several reasons for the dependence of x2+3 on probe nature may be mentioned:

(i) Non-random mixing of the components in the stationary phase as assumed in the theory22,

(ii) selective sorption of different probes on either component of the mixed stationary phase8,

(iii) certain sites on the polymer or plasticizer may not be available for probe interaction, and

(iv) dissimilar solubility characteristics of PVC and polymeric plasticizer. Of particular interest to the present study is the dependence of x2+3 on the

composition of the polymer-plasticizer mixture. This dependence is concisely presented in Tab. 4 and Fig. 1. It is observed that for a given probe and temperature, decrease in the amount of PVC in the PVC-plasticizer mixture increases the x2+3 value, indicative of a decreasing compatibility between the PVC-plasticizer mixture. Values of x2+3 in Tab. 4 and Fig. 1 indicate that the PVC-poly(adipate ester) compatibility is relatively high up to a plasticizer fraction of about 45 to 50 wt.-Yo (negative or slightly positive values). From 45 to 50 wt.-Yo to about 70 wt.-Yo of plasticizer the polymer-plasticizer compatibility is generally satisfactory. As the plasticizer content rises above 70 wt.-Yo the values of x2+3 exceed 0.5 which is considered as the compatibility limit value2. This can be explained by the fact that at large poly(adipate ester) concentrations, much of the plasticizer is relatively “mobile”, i. e., in interaction with other plasticizer molecules rather than “bound”, due to inter- action with PVC molecules. For this postulation the x2+3 values of cyclohexane were not considered, because Vs values of this compound are very similar to respective values of the marker used and therefore the resulting x2+3 values are inaccurate.

88

PVC-plasticizer compatibility by inverse GC

The effect of temperature on x& for a given mixture composition (i. e., 92 wt.-Yo PVC-8 wt.-Yo plasticizer) is shown in Thb. 5. It is observed that increase in temperature increases x2f3 values, a trend corresponding to a decrease in PVC-plasticizer compatibility.

lhb. 5. Interaction parameter (xZ;) values of system low molecular weight PVC - poly(adipate ester) plasticizer of composition 92 wt.-Vo PVC and 8 wt.-Vo plasticizer as a function of temperature*.

Volatile compound (probe) XA

90°C loo "C 110°C

THF -0.164 + 0.050 +OM0 CHCI, -0.285 -0.104 + 0.040

C6H5CH3 - 0.250 -0.130 - 0.022

n-C7H16 + 0.07 1 , +0.089 +0.102

cc1, -0.510 -0,180 -0.107 C6H6 -0.371 - 0.095 - 0.029

CYCIO-C~H,, +0.321 + 0.449 + 0.479

n-C8H18 -0.122 +0.017 + 0.06 1 "-C!P20 -0.118 +0.122 +0.162 "-C,oH22 +0.199 +0.230 +0.318

* Respective xZ; values were calculated for all mixture compositions.

Concerning the second objective of the present paper, that is to consider the effect of molecular weight of PVC on the degree of compatibility between PVC-poly(adipate ester) plasticizer, present results were compared to those previously reported" involving PVC of molecular weight about 1OOOOO and the same poly(adipate ester) plasticizer (Santicizer 409A). Comparative results of these two PVC-plasticizer systems using benzene as the probe at 90 "C are shown in Fig. 2. It is observed that while the high molecular weight PVC- plasticizer system showed a relatively high degree of compatibility up to a plasticizer fraction of about 25 wt.-Vo (negative x2f3 values), the low molecular weight PVC-plasticizer system showed an equivalent degree of compatibility up to plasticizer fraction of about 50 wt.-Yo. While the compatibility limit for the high molecular weight PVC-plasticizer system was approximately 50 wt.-Vo, the respective limit was extended to approximately 70 wt.-Yo for the low molecular weight PVC-plasticizer system.

89

P. G. Demertzis, K. A. Riganakos, K. Akrida-Demertzi, M. G. Kontominas

I I I I 7 0.2 0.1 0.6 0.8 1.0

PVC weight fraction

Fig. 2. Dependence of interaction parameter xZ; values on molecular weight of PVC polymer at 90 "C (probe: benzene): (0) high molecular weight PVC, (A) low molecular weight PVC.

Above results show that a decrease in the molecular weight of the PVC used favours the compatibility between PVC and poly(adipate ester). This trend can be possibly correlated with the fact that by decreasing the molecular weight one increases the probability of interaction between PVC-plasticizer at the

molecular level (formation of: hydrogen bonding between Cl-C-H groups I I 0

I1 of PVC and -C-0- groups of the plasticizer, dipole-dipole type interac- tions, etc.).

The reasons for such a behaviour need of course further investigation. A more detailed study on the effect of molecular weight on the compatibility between PVC-plasticizer would help toward this direction.

Conclusions

(i) The systems studied predict. a good compatibility between PVC-poly- (adipate ester) plasticizer up to a plasticizer fraction of 70 wt.-Vo for low

90

PVC-plasticizer compatibility by inverse GC

molecular weight PVC in comparison to 50 wt.4'0 for high molecular weight PVC reported by our group previ~usly'~.

(ii) The present results show considerable variation in compatibility of the investigated polymer-plasticizer system depending on the chemical nature of the probe. This may possibly find industrial application for selecting the appropriate solvent for processing polymer-plasticizer blends.

(iii) The present work, once again, demonstrates the potential of the IGC method in evaluating interaction parameters for a polymer-plasticizer system. The main advantages of this method can be summarized as follows: (a) IGC is presently the only experimental method that yields thermodynamic infor- mation on polymer blends, with a relative ease, under extreme conditions of temperature and composition and (b) a large number of solvents of different chemical nature and solubility characteristics can be used as probes even when their molecular properties are unknown.

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