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Stability and extraction features in the determination of Irganox-1330 in a polyalkene copolymer
Analytica Chimica Acta, 166 (1984) 243-251 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
STABILITY AND EXTRACTION FEATURES IN THE DETERMINATION OF IRGANOX-1330 IN A POLYALKENE COPOLYMER
ULLA GASSLANDER Astra Liikemedel (Sweden)
and HANS JAEGFELDT**
AB, Research
and Development
Laboratories,
S-l 51 85 Siidertiilje
(Received 21st May 1984)
SUMMARY The determination of Irganox-1330 in a polypropene/polyethene copolymer is described. The unstable antioxidant can be extracted from the copolymer without degradation at refluxing temperature by using decalin, hexane, chloroform or tetrahydrofuran if adequate protection is provided. The time needed for quantitative extraction is 30 min if the particle size of the polymer is
Polyolefins such as polypropene and polyethene are subject to thermal and oxidative degradation particularly during processing at elevated temperatures. To protect the polymer during this phase of handling and to give longterm stability, antioxidants are added. The antioxidants act as radical scavengers and interrupt the chain propagation steps of the auto-oxidation of the polymer [l] . The procedures used to determine antioxidants in polymers, and the problems associated with these methods, have been reviewed by Wheeler [2], Crompton [3] and Walter and Johnson [4]. The main problems arise from the more or less insoluble polymer matrix, the high reactivity and low stability of the antioxidant and its low concentration (0.1-l%). A successful analysis depends primarily on extracting the antioxidant without decomposition at the low concentrations encountered. Several different solvents have been suggested as well as different extraction equipment and milling procedures [ 2-41. Milling the polymer samples to smaller particles increases the surface/weight ratio and thus speeds up the rate of solid/liquid extraction. It is probable that a large amount of work has been done in this direction but not published because of the proprietory nature of many in-house methods. However, a few papers have appeared. Spell and Eddy [5] showed that the extraction rate was a function of the particle size and the permeability of the solvent. This was confirmed by aPresent address: Research and Development Lund, Sweden. 0003-2670/84/$03.00
Laboratories,
AB Draco, Box 34, 22100
0 1984 Elsevier Science Publishers B.V.
244
Wims and Swarm [6], who concluded that tetrahydrofuran (THF) was the most efficient extraction solvent. In a recent paper by Vadakkott et al. [‘7], carbon tetrachloride and THF were shown to be suitable solvents for lowdensity polyethene and propene, respectively; higher-boiling solvents led to the decomposition of the antioxidant during the extraction. However, Schabron and co-workers [ 8, 91 used hot (110” C) decalin (decahydronaphthalene) to dissolve polyethene granules completely for extraction of the antioxidants. Using this procedure, they avoided problems connected with solvent penetration into the polymer; they did not refer to antioxidant degradation. Apart from a paper by Lichtentaler and Ranfelt [lo], no systematic investigation of the problems of quantitative solid/liquid extraction has been reported. Also, no attempts to study and to minimize the decomposition and loss of antioxidant during extraction seem to have been made. In particular, knowledge about decomposition seems vital because most methods of analysis involve heating to increase the speed of extraction. This paper reports on the determination of Irganox-1330 in a commercial polypropene/ polyethene (98/2 % w/w) copolymer. Different extraction solvents are compared. The importance of protecting the antioxidant from oxidation during extraction is discussed. A simple standard-addition procedure is used to prevent “matrix effects” from the polymer from affecting the accuracy of the determination. A nitro-group modified direct-phase column was chosen for the highperformance liquid chromatographic (l.c.) separations. This column yielded reproducible retention times, fast separations and better selectivity [ 11-131 than some reverse-phase non-aqueous systems recently reported [ 7,141. The compatibility between the chromatographic system and the extractants used here is discussed. In view of the fact that monitoring of polymer additives has become increasingly important because of stricter laws and regulations, the need for fast and reliable analytical methods has increased. This is especially true for the pharmaceutical industry, as particular interest is focused on additives in medical plastics and food packaging. EXPERIMENTAL
Chemicals and chromatographic equipment Commercial-grade Irganox-1330 {2,2’,2”,6,6’,6’‘-hexa-t-butyl-4,4’ ,4”-[(2,46-trimethyl-1,3,5-benzenetriyl)trismethylene] triphenol) was obtained from Ciba-Geigy. Reagent-grade 2,6-di-t-butyl-4-methylphenol (BHT) was obtained from Janssen Chimica (Belgium). A commercial polypropene containing only Irganox-1330 as antioxidant was used. The solvents were reagent-grade (Merck) for the extractions and spectroscopic grade (Fluka) for the chromatographic work. For chromatography, a Waters 6000-A pump was used with a Rheodyne injector (20~~1 loop). The steel column was 150 X 4 mm i.d. and the support
245
was Nucleosil-5 NO, (5 pm; Macherey-Nagel, Diiren). The mobile phase was 99.0-99.5% hexane and 0.5-1.0% THF. The detector was a Pye-Unicam 4020 variable-wavelength model and the measurements were made at 230 nm. The integrator was a Pye-Unicam PU-4810 model. Sample preparation and extraction procedure The polymer granules were frozen either with liquid nitrogen or in a COz(s)/ethanol bath to temperatures well below the glass-point. The brittle granules were then pulverized in an ultracentrifuge mill (Retsch, Type ZMl). Pulverized samples (0.5-5.0 g) were immediately soaked in 25-100 ml of solvent and extracted with reflux for 40 min. The extraction solvents used were decalin, hexane, chloroform and THF. When the antioxidant was extracted with decalin, no pulverization was necessary as the granules dissolved at the reflux temperature (ca. 180” C). After reflux and upon cooling in any of the solvents used, greater or smaller amounts of dissolved polymer always precipitated, leading to the necessity of filtration. The sample solutions were filtered through glass microfibre filters (Whatman GF/A) in order to avoid clogging of the inlet of the chromatographic column. After filtration, the polymer particles on the filter were carefully rinsed and the sample solution was made up to volume. When chloroform or THF was used as the extracting solvent, the sample solutions were diluted with hexane 5- and lo-fold, respectively. This was done in order to achieve acceptable chromatographic behaviour (see below). For quantitative work (Table l), a lOO-fold molar excess of BHT (0.5-1.5 mg ml-‘) was added to the extracting solvent in order to protect the Irganox-1330 from oxidation during extraction. Soxhlet extraction was not used, because it would not have allowed the protection of the antioxidant in the polymer with another synergic antioxidant in solution. This kind of additional protection, besides nitrogen purging, was found to be particularly important for the extraction with decalin. In the determination of the mass/particle size distribution of the pulverized polymer granules, four brass sieves were used (Retsch; 0.43-, LO-, 1.6- and 2.0-mm mesh, sieve diameter 10 cm). Pulverized samples (ca. 15 g) were TABLE 1 Determination of Irganox-1330 in polypropylene by refluxing with solvents of widely different polarity and boiling temperatures. Samples were milled to < 1.0 mm particle size. Sample
Irganox-1330
found (pg g-l)
Decalin
Hexane
CHCl,
THF
1 2 3 4
800 815 -
764 771 771 822
810 801 -
793 813 -
Mean * SD
808 f 10
782 i 27
806 * 7
803 f 14
246
sifted for 20 min (Fritsch Analysette sieve shaker) after which the different fractions were collected and weighed. Standard-addition procedure For this method, four samples of polymer were weighed. To each sample was added the appropriate solvent containing known amounts of Irganox1330. This procedure was used in order to reach equilibrium in the solid/ liquid extraction for each sample. The peak areas resulting from the chromatographic recordings for each sample were then plotted against the known added concentration and the concentration in the polymer was calculated in the usual way. This procedure was used so that the data could be quantified even when the sample was steeped directly in the extractant solution. The requirement for success of the method is that distribution equilibrium of the antioxidant between polymer and solution is achieved (see below). Full mathematical justification of this standard-addition procedure is available from the authors on request. RESULTS
AND DISCUSSION
Stability As antioxidants are designed to be sensitive to oxidation, they obviously have to be protected from this during extraction. Figure 1 summarizes the results of different treatments. As can be seen, when oxygen-saturated decalin solutions containing 10 pg ml-’ Irganox-1330 were refluxed at 180” C, the antioxidant concentration decreased dramatically within 30 min (curve A). Even when the solution was continuously purged with nitrogen, there was slow degradation of the antioxidant (curve B). However, when the nitrogen purging was combined with a loo-fold molar excess of BHT, the decomposition rate was virtually negligible (curve C). Thus, it is possible to protect a sensitive antioxidant even at high temperature if suitable precautions are taken. Another way to retard or stop oxidation is to lower the temperature of the extraction [7]. Figure 2 shows the results when oxygen-saturated solutions of Irganox-1330 were refluxed at 60-70°C with hexane or THF with and without a lOO-fold molar excess of BHT. No appreciable degradation was observed for the hexane solutions with or without BHT, but for THF protection against oxidation was necessary for prolonged extractions. This is not surprising, given the tendency for THF itself to react with oxygen to form peroxides which will, of course, oxidize the antioxidant. When chloroform was used, no detectable degradation with or without BHT was observed. It can be concluded, therefore, that low temperature during an extraction will lead to good stability if no other effects can cause degradation. The rate of extraction from milled polymer was then studied at room temperature. The aim was not to elucidate stability problems, but to study the influence of particle size on extraction rate. As shown in Fig. 3, when the
247
80 70
\
8
I
.n
Time (h)
Fig. 1. Degradation of Irganox-1330 (10 pg ml-‘) during reflux in decalin at ca. 180°C: (A) oxygen-saturated solution; (B) continuous nitrogen purging; (C) continuous nitrogen purging and a loo-fold molar excess of BHT. Fig. 2. Reflux of Irganox-1330 (50 fig ml-‘) in hexane or THF at 60-7O’C: with or without BHT; (B) THF without BHT.
(A) hexane
particle size was O-1.0 mm, a plateau was reached after about 4 h (curve A) but, as expected, extraction was slower when particles smaller than 0.4 mm were removed (curve B). However, when the larger sifted polymer particles were extracted without excess of BHT, severe degradation of the antioxidant was observed (curve C). This effect was not observed for unsieved polymer. It is concluded that very small amounts of metal (from the brass sieve used) can severely increase the rate of degradation of the antioxidant. This is of course not new, but it emphasizes the importance of protection against oxidation even at low temperatures. In this context, it should be kept in mind that even trace catalyst residues in the polymer might exert a catalytic effect on degradation during extraction in the presence of oxygen, particularly at high temperatures. The degradative effect of minute amounts of trace metal from the brass sieve was observed also for chloroform and hexane extractions. It was smallest for hexane and greatest for THF. Extraction As discussed above, degradation of the antioxidant can be retarded by lowering the temperature during extraction. However, lowering of the
248
60
b I
2
3
4
5
b I
2
3
4
Tlme (h)
Fig. 3. Extraction of Irganox-1330 of different particle sizes with THF at ambient temperature. Particle size: (A) O-1.0 mm, excess of BHT added; (B) 0.4-1.0 mm, excess of BHT added; (C) 0.4-1.0 mm, no BHT added. Fig. 4. Extraction of the antioxidant from two 3-g samples by refluxing with THF. Particle size: (A) O-l.6 mm;(B) 3-6 mm.
temperature by refluxing with, for example, THF, chloroform or hexane changes the system from a liquid/liquid (refluxing with decalin) to a solid/ liquid extraction, because of the poor solubility of polypropene in lowboiling solvents. However, the slow rate of solid/liquid extractions is not necessarily a problem in quantifying an antioxidant in a polymer matrix. The surface/volume ratio of the solid sample (and so the speed of extraction) may be increased considerably by milling. Still, precise knowledge of the degree of extraction versus time is essential in order to define when the extraction process is complete. When two samples were extracted by refluxing with THF, the sample with the smaller particle size (<1,6 mm) gave a complete yield much more quickly than the other (3-6 mm), as shown in Fig. 4. Both extraction curves levelled off at the same concentration, indicating that the solvent completely penetrated both samples. The steady state also suggests that distribution equilibrium between the solid polymer and the liquid extractant is reached. When the extraction is done by placing the sample directly into the extractant (as in this work), distribution equilibrium defines when the
249
extraction process is complete. Obviously, 100% extraction is never quite achieved because of the finite distribution between the two phases, but 100% extraction is not essential if a properly designed standard-addition procedure is applied. Yet, it could be argued that a steady-state extraction level does not necessarily mean that distribution equilibrium prevails throughout the sample, because crystallites, for example, may not be penetrated completely. Total penetration and extraction by a low-boiling solvent was indicated by the following experiments. It was shown (Fig. 1) that the antioxidant can be stabilized even in refluxing decalin (ca. 180°C) which completely dissolves the polypropene. When Irganox-1330 was determined in a commercial polypropene/polyethene (98/2 %I w/w) granulate by the standard-addition procedure described under Experimental with different solvents, including some in which the polymer did not dissolve, the results obtained were very similar. Table 1 summarizes the data acquired with decalin, hexane, chloroform and THF as extractants. All these solvents yielded the same concentration of the antioxidant in the sample, within the precision of the method (*3%). It can therefore be concluded that any of these solvents can be used successfully to extract the antioxidant concentration in the polymer for quantitative work, provided that the sample is milled to particles < 1.0 mm. Figure 5 shows the mass/particle-size distribution given by the milling procedure (see Experimental) for the determinations in Table 1. Figure 6 shows that the difference in the rate of extraction at ambient temperature with hexane, chloroform or THF is surprisingly small, considering the wide range of polarity of these solvents, When the polymer particles are small enough, differences in polarity obviously play a minor role in the extraction rate. When extractions were done with hexane at refluxing temperature, the extraction “plateau” was reached after 30 min. Choice of extraction method The choice of extraction method depends on its simplicity, chemical efficiency and compatibility with the chromatographic system. Dissolution of the polypropene sample in decalin requires a minimum of work-up. The procedure is only slightly complicated by the necessity of nitrogen purging and addition of another antioxidant. However, if many samples are to be analyzed, practical considering of cleaning glassware tends to favour one of the other solvents, which do not dissolve the polymer. The risk of degradation is negligible for any of the “protected” solvents and the recovery time for a sample is about the same (
250
4
loo-
0.4
1.0
1.6
I
5
2.0 Time (h)
Particle sue (mm)
Fig. 5. Mass/particle-size distribution determinations in Table 1.
obtained
by the milling procedure
used for the
Fig. 6. Rate of extraction of Irganox-1330 from polypropene (
If THF is chosen, the solution must be diluted 10 times in order to obtain acceptable peak shapes. Figure 8 shows the relative peak heights for sample solutions containing increasing volumes of chloroform or THF added to hexane. Clearly, these rather polar solvents block the adsorption sites to a large extent and cause the performance of the column to deteriorate. The combined effects of dilution and peak-broadening lower the sensitivity by a factor of 10 or 20 for chloroform or THF, respectively. The final choice of hexane as the best extractant for routine work based on the system described is obvious from the point of view of the chromatography. Hexane has the further advantages of low toxicity, ease of disposal, and low absorption at the wavelength used. Sample filtration is easy, because of the low solubility of the polymer, and hexane is less dense than the polymer so that submerging the samples is no problem.
251
i
P
2
\
A
0 0
1.
C
b IO
T Ime AdmIxture
to hrxone
20 (% v/v)
Fig. 7. Chromatograms from 20-~1 samples containing 20 pg ml-’ Irganox-1330 with the same eluent, hexane/THF (99/l): (A) antioxidant dissolved in pure hexane; (B) antioxidant dissolved in 20/80 chloroform/hexane; (C) antioxidant dissolved in 20/80 THF/ hexane. Fig. 8. Effect on peak height of adding chloroform (A) or THF (B) to hexane solutions of Irganox-1330 (20 rg ml-‘). Other conditions as for Fig. 7. REFERENCES 1 R. Giichter and H. Miiller, Taschenbuch der Kunststoff-Additive, Carl Hanser Verlag, Miinchen, 1979, pp. 4-68. 2 D.A. Wheeler, Talanta, 15 (1968) 1315. 3 T. R. Crompton, Chemical analysis of additives in plastics, 2nd edn., Pergamon Press, 1977. 4 R. B. Walter and J. F. Johnson, J. Polym. Sci., Macromol. Rev., 15 (1980) 29. 5 H. L. Spell and R. D. Eddy, Anal. Chem., 32 (1960) 1811. 6 A. M. Wims and S. J. Swarm, J. Appl. Polym. Sci., 19 (1975) 1243. 7 C. F. Vadakkott, N. S. Yoginder and S. B. Ishwar, Angew. Makromol. Chem., 113 (1983) 219. 8 J. F. Schabron and L. E. Fenska;Anal. Chem., 52 (1980) 1411. 9 J. F. Schabron, V. J. Smith and J. L. Ware, J. Liquid Chromatogr., 5 (1982) 613. 10 R. G. Lichtentaler and F. Ranfelt, J. Chromatogr., 149 (1978) 553. 11 P. Roumeliotis, K. K. Unger, G. Tesarek and E. Miihlberg, Z. Anal. Chem., 298 (1979) 241. 12 J. Berg, C. Mohl, R. Deelder and J. Thijssen, Clin. Chim. Acta, 78 (1977) 165. 13 E. D. Lankmayer and K. Milller, J. Chromatogr., 170 (1979) 139. 14 M. A. Haney and W. A. Dark, J. Chromatogr. Sci., 18 (1980) 655.
STABILITY AND EXTRACTION FEATURES IN THE DETERMINATION OF IRGANOX-1330 IN A POLYALKENE COPOLYMER
ULLA GASSLANDER Astra Liikemedel (Sweden)
and HANS JAEGFELDT**
AB, Research
and Development
Laboratories,
S-l 51 85 Siidertiilje
(Received 21st May 1984)
SUMMARY The determination of Irganox-1330 in a polypropene/polyethene copolymer is described. The unstable antioxidant can be extracted from the copolymer without degradation at refluxing temperature by using decalin, hexane, chloroform or tetrahydrofuran if adequate protection is provided. The time needed for quantitative extraction is 30 min if the particle size of the polymer is
Polyolefins such as polypropene and polyethene are subject to thermal and oxidative degradation particularly during processing at elevated temperatures. To protect the polymer during this phase of handling and to give longterm stability, antioxidants are added. The antioxidants act as radical scavengers and interrupt the chain propagation steps of the auto-oxidation of the polymer [l] . The procedures used to determine antioxidants in polymers, and the problems associated with these methods, have been reviewed by Wheeler [2], Crompton [3] and Walter and Johnson [4]. The main problems arise from the more or less insoluble polymer matrix, the high reactivity and low stability of the antioxidant and its low concentration (0.1-l%). A successful analysis depends primarily on extracting the antioxidant without decomposition at the low concentrations encountered. Several different solvents have been suggested as well as different extraction equipment and milling procedures [ 2-41. Milling the polymer samples to smaller particles increases the surface/weight ratio and thus speeds up the rate of solid/liquid extraction. It is probable that a large amount of work has been done in this direction but not published because of the proprietory nature of many in-house methods. However, a few papers have appeared. Spell and Eddy [5] showed that the extraction rate was a function of the particle size and the permeability of the solvent. This was confirmed by aPresent address: Research and Development Lund, Sweden. 0003-2670/84/$03.00
Laboratories,
AB Draco, Box 34, 22100
0 1984 Elsevier Science Publishers B.V.
244
Wims and Swarm [6], who concluded that tetrahydrofuran (THF) was the most efficient extraction solvent. In a recent paper by Vadakkott et al. [‘7], carbon tetrachloride and THF were shown to be suitable solvents for lowdensity polyethene and propene, respectively; higher-boiling solvents led to the decomposition of the antioxidant during the extraction. However, Schabron and co-workers [ 8, 91 used hot (110” C) decalin (decahydronaphthalene) to dissolve polyethene granules completely for extraction of the antioxidants. Using this procedure, they avoided problems connected with solvent penetration into the polymer; they did not refer to antioxidant degradation. Apart from a paper by Lichtentaler and Ranfelt [lo], no systematic investigation of the problems of quantitative solid/liquid extraction has been reported. Also, no attempts to study and to minimize the decomposition and loss of antioxidant during extraction seem to have been made. In particular, knowledge about decomposition seems vital because most methods of analysis involve heating to increase the speed of extraction. This paper reports on the determination of Irganox-1330 in a commercial polypropene/ polyethene (98/2 % w/w) copolymer. Different extraction solvents are compared. The importance of protecting the antioxidant from oxidation during extraction is discussed. A simple standard-addition procedure is used to prevent “matrix effects” from the polymer from affecting the accuracy of the determination. A nitro-group modified direct-phase column was chosen for the highperformance liquid chromatographic (l.c.) separations. This column yielded reproducible retention times, fast separations and better selectivity [ 11-131 than some reverse-phase non-aqueous systems recently reported [ 7,141. The compatibility between the chromatographic system and the extractants used here is discussed. In view of the fact that monitoring of polymer additives has become increasingly important because of stricter laws and regulations, the need for fast and reliable analytical methods has increased. This is especially true for the pharmaceutical industry, as particular interest is focused on additives in medical plastics and food packaging. EXPERIMENTAL
Chemicals and chromatographic equipment Commercial-grade Irganox-1330 {2,2’,2”,6,6’,6’‘-hexa-t-butyl-4,4’ ,4”-[(2,46-trimethyl-1,3,5-benzenetriyl)trismethylene] triphenol) was obtained from Ciba-Geigy. Reagent-grade 2,6-di-t-butyl-4-methylphenol (BHT) was obtained from Janssen Chimica (Belgium). A commercial polypropene containing only Irganox-1330 as antioxidant was used. The solvents were reagent-grade (Merck) for the extractions and spectroscopic grade (Fluka) for the chromatographic work. For chromatography, a Waters 6000-A pump was used with a Rheodyne injector (20~~1 loop). The steel column was 150 X 4 mm i.d. and the support
245
was Nucleosil-5 NO, (5 pm; Macherey-Nagel, Diiren). The mobile phase was 99.0-99.5% hexane and 0.5-1.0% THF. The detector was a Pye-Unicam 4020 variable-wavelength model and the measurements were made at 230 nm. The integrator was a Pye-Unicam PU-4810 model. Sample preparation and extraction procedure The polymer granules were frozen either with liquid nitrogen or in a COz(s)/ethanol bath to temperatures well below the glass-point. The brittle granules were then pulverized in an ultracentrifuge mill (Retsch, Type ZMl). Pulverized samples (0.5-5.0 g) were immediately soaked in 25-100 ml of solvent and extracted with reflux for 40 min. The extraction solvents used were decalin, hexane, chloroform and THF. When the antioxidant was extracted with decalin, no pulverization was necessary as the granules dissolved at the reflux temperature (ca. 180” C). After reflux and upon cooling in any of the solvents used, greater or smaller amounts of dissolved polymer always precipitated, leading to the necessity of filtration. The sample solutions were filtered through glass microfibre filters (Whatman GF/A) in order to avoid clogging of the inlet of the chromatographic column. After filtration, the polymer particles on the filter were carefully rinsed and the sample solution was made up to volume. When chloroform or THF was used as the extracting solvent, the sample solutions were diluted with hexane 5- and lo-fold, respectively. This was done in order to achieve acceptable chromatographic behaviour (see below). For quantitative work (Table l), a lOO-fold molar excess of BHT (0.5-1.5 mg ml-‘) was added to the extracting solvent in order to protect the Irganox-1330 from oxidation during extraction. Soxhlet extraction was not used, because it would not have allowed the protection of the antioxidant in the polymer with another synergic antioxidant in solution. This kind of additional protection, besides nitrogen purging, was found to be particularly important for the extraction with decalin. In the determination of the mass/particle size distribution of the pulverized polymer granules, four brass sieves were used (Retsch; 0.43-, LO-, 1.6- and 2.0-mm mesh, sieve diameter 10 cm). Pulverized samples (ca. 15 g) were TABLE 1 Determination of Irganox-1330 in polypropylene by refluxing with solvents of widely different polarity and boiling temperatures. Samples were milled to < 1.0 mm particle size. Sample
Irganox-1330
found (pg g-l)
Decalin
Hexane
CHCl,
THF
1 2 3 4
800 815 -
764 771 771 822
810 801 -
793 813 -
Mean * SD
808 f 10
782 i 27
806 * 7
803 f 14
246
sifted for 20 min (Fritsch Analysette sieve shaker) after which the different fractions were collected and weighed. Standard-addition procedure For this method, four samples of polymer were weighed. To each sample was added the appropriate solvent containing known amounts of Irganox1330. This procedure was used in order to reach equilibrium in the solid/ liquid extraction for each sample. The peak areas resulting from the chromatographic recordings for each sample were then plotted against the known added concentration and the concentration in the polymer was calculated in the usual way. This procedure was used so that the data could be quantified even when the sample was steeped directly in the extractant solution. The requirement for success of the method is that distribution equilibrium of the antioxidant between polymer and solution is achieved (see below). Full mathematical justification of this standard-addition procedure is available from the authors on request. RESULTS
AND DISCUSSION
Stability As antioxidants are designed to be sensitive to oxidation, they obviously have to be protected from this during extraction. Figure 1 summarizes the results of different treatments. As can be seen, when oxygen-saturated decalin solutions containing 10 pg ml-’ Irganox-1330 were refluxed at 180” C, the antioxidant concentration decreased dramatically within 30 min (curve A). Even when the solution was continuously purged with nitrogen, there was slow degradation of the antioxidant (curve B). However, when the nitrogen purging was combined with a loo-fold molar excess of BHT, the decomposition rate was virtually negligible (curve C). Thus, it is possible to protect a sensitive antioxidant even at high temperature if suitable precautions are taken. Another way to retard or stop oxidation is to lower the temperature of the extraction [7]. Figure 2 shows the results when oxygen-saturated solutions of Irganox-1330 were refluxed at 60-70°C with hexane or THF with and without a lOO-fold molar excess of BHT. No appreciable degradation was observed for the hexane solutions with or without BHT, but for THF protection against oxidation was necessary for prolonged extractions. This is not surprising, given the tendency for THF itself to react with oxygen to form peroxides which will, of course, oxidize the antioxidant. When chloroform was used, no detectable degradation with or without BHT was observed. It can be concluded, therefore, that low temperature during an extraction will lead to good stability if no other effects can cause degradation. The rate of extraction from milled polymer was then studied at room temperature. The aim was not to elucidate stability problems, but to study the influence of particle size on extraction rate. As shown in Fig. 3, when the
247
80 70
\
8
I
.n
Time (h)
Fig. 1. Degradation of Irganox-1330 (10 pg ml-‘) during reflux in decalin at ca. 180°C: (A) oxygen-saturated solution; (B) continuous nitrogen purging; (C) continuous nitrogen purging and a loo-fold molar excess of BHT. Fig. 2. Reflux of Irganox-1330 (50 fig ml-‘) in hexane or THF at 60-7O’C: with or without BHT; (B) THF without BHT.
(A) hexane
particle size was O-1.0 mm, a plateau was reached after about 4 h (curve A) but, as expected, extraction was slower when particles smaller than 0.4 mm were removed (curve B). However, when the larger sifted polymer particles were extracted without excess of BHT, severe degradation of the antioxidant was observed (curve C). This effect was not observed for unsieved polymer. It is concluded that very small amounts of metal (from the brass sieve used) can severely increase the rate of degradation of the antioxidant. This is of course not new, but it emphasizes the importance of protection against oxidation even at low temperatures. In this context, it should be kept in mind that even trace catalyst residues in the polymer might exert a catalytic effect on degradation during extraction in the presence of oxygen, particularly at high temperatures. The degradative effect of minute amounts of trace metal from the brass sieve was observed also for chloroform and hexane extractions. It was smallest for hexane and greatest for THF. Extraction As discussed above, degradation of the antioxidant can be retarded by lowering the temperature during extraction. However, lowering of the
248
60
b I
2
3
4
5
b I
2
3
4
Tlme (h)
Fig. 3. Extraction of Irganox-1330 of different particle sizes with THF at ambient temperature. Particle size: (A) O-1.0 mm, excess of BHT added; (B) 0.4-1.0 mm, excess of BHT added; (C) 0.4-1.0 mm, no BHT added. Fig. 4. Extraction of the antioxidant from two 3-g samples by refluxing with THF. Particle size: (A) O-l.6 mm;(B) 3-6 mm.
temperature by refluxing with, for example, THF, chloroform or hexane changes the system from a liquid/liquid (refluxing with decalin) to a solid/ liquid extraction, because of the poor solubility of polypropene in lowboiling solvents. However, the slow rate of solid/liquid extractions is not necessarily a problem in quantifying an antioxidant in a polymer matrix. The surface/volume ratio of the solid sample (and so the speed of extraction) may be increased considerably by milling. Still, precise knowledge of the degree of extraction versus time is essential in order to define when the extraction process is complete. When two samples were extracted by refluxing with THF, the sample with the smaller particle size (<1,6 mm) gave a complete yield much more quickly than the other (3-6 mm), as shown in Fig. 4. Both extraction curves levelled off at the same concentration, indicating that the solvent completely penetrated both samples. The steady state also suggests that distribution equilibrium between the solid polymer and the liquid extractant is reached. When the extraction is done by placing the sample directly into the extractant (as in this work), distribution equilibrium defines when the
249
extraction process is complete. Obviously, 100% extraction is never quite achieved because of the finite distribution between the two phases, but 100% extraction is not essential if a properly designed standard-addition procedure is applied. Yet, it could be argued that a steady-state extraction level does not necessarily mean that distribution equilibrium prevails throughout the sample, because crystallites, for example, may not be penetrated completely. Total penetration and extraction by a low-boiling solvent was indicated by the following experiments. It was shown (Fig. 1) that the antioxidant can be stabilized even in refluxing decalin (ca. 180°C) which completely dissolves the polypropene. When Irganox-1330 was determined in a commercial polypropene/polyethene (98/2 %I w/w) granulate by the standard-addition procedure described under Experimental with different solvents, including some in which the polymer did not dissolve, the results obtained were very similar. Table 1 summarizes the data acquired with decalin, hexane, chloroform and THF as extractants. All these solvents yielded the same concentration of the antioxidant in the sample, within the precision of the method (*3%). It can therefore be concluded that any of these solvents can be used successfully to extract the antioxidant concentration in the polymer for quantitative work, provided that the sample is milled to particles < 1.0 mm. Figure 5 shows the mass/particle-size distribution given by the milling procedure (see Experimental) for the determinations in Table 1. Figure 6 shows that the difference in the rate of extraction at ambient temperature with hexane, chloroform or THF is surprisingly small, considering the wide range of polarity of these solvents, When the polymer particles are small enough, differences in polarity obviously play a minor role in the extraction rate. When extractions were done with hexane at refluxing temperature, the extraction “plateau” was reached after 30 min. Choice of extraction method The choice of extraction method depends on its simplicity, chemical efficiency and compatibility with the chromatographic system. Dissolution of the polypropene sample in decalin requires a minimum of work-up. The procedure is only slightly complicated by the necessity of nitrogen purging and addition of another antioxidant. However, if many samples are to be analyzed, practical considering of cleaning glassware tends to favour one of the other solvents, which do not dissolve the polymer. The risk of degradation is negligible for any of the “protected” solvents and the recovery time for a sample is about the same (
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4
loo-
0.4
1.0
1.6
I
5
2.0 Time (h)
Particle sue (mm)
Fig. 5. Mass/particle-size distribution determinations in Table 1.
obtained
by the milling procedure
used for the
Fig. 6. Rate of extraction of Irganox-1330 from polypropene (
If THF is chosen, the solution must be diluted 10 times in order to obtain acceptable peak shapes. Figure 8 shows the relative peak heights for sample solutions containing increasing volumes of chloroform or THF added to hexane. Clearly, these rather polar solvents block the adsorption sites to a large extent and cause the performance of the column to deteriorate. The combined effects of dilution and peak-broadening lower the sensitivity by a factor of 10 or 20 for chloroform or THF, respectively. The final choice of hexane as the best extractant for routine work based on the system described is obvious from the point of view of the chromatography. Hexane has the further advantages of low toxicity, ease of disposal, and low absorption at the wavelength used. Sample filtration is easy, because of the low solubility of the polymer, and hexane is less dense than the polymer so that submerging the samples is no problem.
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i
P
2
\
A
0 0
1.
C
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T Ime AdmIxture
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Fig. 7. Chromatograms from 20-~1 samples containing 20 pg ml-’ Irganox-1330 with the same eluent, hexane/THF (99/l): (A) antioxidant dissolved in pure hexane; (B) antioxidant dissolved in 20/80 chloroform/hexane; (C) antioxidant dissolved in 20/80 THF/ hexane. Fig. 8. Effect on peak height of adding chloroform (A) or THF (B) to hexane solutions of Irganox-1330 (20 rg ml-‘). Other conditions as for Fig. 7. REFERENCES 1 R. Giichter and H. Miiller, Taschenbuch der Kunststoff-Additive, Carl Hanser Verlag, Miinchen, 1979, pp. 4-68. 2 D.A. Wheeler, Talanta, 15 (1968) 1315. 3 T. R. Crompton, Chemical analysis of additives in plastics, 2nd edn., Pergamon Press, 1977. 4 R. B. Walter and J. F. Johnson, J. Polym. Sci., Macromol. Rev., 15 (1980) 29. 5 H. L. Spell and R. D. Eddy, Anal. Chem., 32 (1960) 1811. 6 A. M. Wims and S. J. Swarm, J. Appl. Polym. Sci., 19 (1975) 1243. 7 C. F. Vadakkott, N. S. Yoginder and S. B. Ishwar, Angew. Makromol. Chem., 113 (1983) 219. 8 J. F. Schabron and L. E. Fenska;Anal. Chem., 52 (1980) 1411. 9 J. F. Schabron, V. J. Smith and J. L. Ware, J. Liquid Chromatogr., 5 (1982) 613. 10 R. G. Lichtentaler and F. Ranfelt, J. Chromatogr., 149 (1978) 553. 11 P. Roumeliotis, K. K. Unger, G. Tesarek and E. Miihlberg, Z. Anal. Chem., 298 (1979) 241. 12 J. Berg, C. Mohl, R. Deelder and J. Thijssen, Clin. Chim. Acta, 78 (1977) 165. 13 E. D. Lankmayer and K. Milller, J. Chromatogr., 170 (1979) 139. 14 M. A. Haney and W. A. Dark, J. Chromatogr. Sci., 18 (1980) 655.