Optimization of Experimental Parameters for the Determination of ...

Journal of Chromatographic Science, Vol. 2 9 , December 1991

Optimization of Experimental Parameters for the Determination of Polymer Additives using On-Line SFE-SFC Mehdi A s h r a f - K h o r a s s a n i * , David S . Boyer, and J o s e p h M. L e v y Suprex Corporation, 125 William Pitt W a y , Pittsburgh, Pennsylvania 15238

Abstract A n o n - l i n e s u p e r c r i t i c a l fluid e x t r a c t i o n ( S F E ) a n d s u p e r c r i t i c a l fluid c h r o m a t o g r a p h i c ( S F C ) s y s t e m w i t h a c r y o f o c u s i n g t r a p is u s e d t o a n a l y z e q u a n t i t a t i v e l y different a d d i t i v e s in p o l y m e r m a t r i c e s . T h e effects of t e m p e r a t u r e , m a t r i x c o n f i g u r a t i o n , a n d e x t r a c t i o n t i m e a r e s t u d i e d . T h e u s e of h i g h e r t e m p e r a t u r e s h o w s better e x t r a c t i o n e f f i c i e n c y , w h i l e m a t r i x c o n f i g u r a t i o n e x p e r i m e n t s d e m o n s t r a t e that better a n d f a s t e r e x t r a c t i o n c a n b e o b t a i n e d a s t h e s u r f a c e a r e a of t h e m a t r i x i n c r e a s e s .

†

techniques for the extraction and analysis of high molecular weight and thermally labile additives (8-12). This manuscript ex­ pands on this approach by using on-line SFE-SFC with a cry­ ofocusing trap to investigate the extraction efficiencies of dif­ ferent additives from polymer matrices by varying parameters such as extraction temperature, physical characteristic of the ma­ trix, and extraction duration. Also, quantitative and qualitative re­ sults that were obtained from the extraction and separation of dif­ ferent additives in polymer matrices will be described.

Experimental Introduction Supercritical fluids have been used as solvents for both ex­ traction and chromatography (1-3). The solvent strength of a supercritical fluid is directly related to density while the solubility of a specific analyte can be varied by employing different ex­ traction pressures or temperatures. Supercritical fluid extraction (SFE) and supercritical fluid chromatography (SFC) have been used as analytical tools for the extraction and separation of dif­ ferent analytes in different matrices. However, the on-line coup­ ling of SFE to an SFC presents a powerful analytical technical technique for the characterization of different samples. The on­ line mode of SFE-SFC has several advantages that make the use of supercritical fluids more desirable. Also, the addition of a cryofocusing trap allows the user to obtain enhanced versatility (4–7). More specifically, the addition of a cryofocusing trap per­ mits the focusing of low-level analytes into discrete narrow bands, allowing quantitative and reproducible results with better detection limits. In this technique, supercritical fluids solubilize certain samples and are depressurized onto a cryofocusing trap. After extraction and collection in the cryofocusing trap, all of the extracted components are directly injected onto the chromato­ graphic column. The analysis of chemical additives in various polymers has al­ ways been important to the polymer industry. Usually, these ad­ ditives are present at low concentration levels (ppm) in the sample matrix, which makes it more difficult for analysis with current an­ alytical tools. Supercritical fluids have shown much promise in * P r e s e n t a d d r e s s : N a t i o n a l I r a n i a n O i l C o m p a n y , P . O . B o x 1863, T e h r a n , I r a n , Author to w h o m correspondence should be addressed. †

A Suprex MPS/225 on-line supercritical fluid extraction and chromatography system with a cryofocusing trap and packed columns was utilized for this study. The complete description of the system used in this study has been previously described (4). Polymer additives standards and polymer matrices employed in this study were provided by different manufacturers. The polymer matrices containing various additives used were in the form of films or pellets. The polymer additives standards were all dis­ solved in pesticide-grade methylene chloride (J.T. Baker). The columns used in this study included nucleosil octadecyl bondedphase silica columns (Keystone Scientific) with dimensions of 20 or 40 cm × 1.0-mm i.d., 5-μm particle size. Supercritical C O (Scott Specialty Gases) with flame ionization detection (FID) was used for all of the studies. 2

Results and Discussion In the first part of this study, optimum conditions for the ex­ traction of different polymer matrices were pursued. To maximize the extraction efficiency and at the same time to obtain qualita­ tive and quantitative results, the effect of temperature, matrix configuration, and extraction time were investigated. Figure 1 shows the SFE-SFC characterization of polystyrene polymer (melting point of 240°C) which contained 1250 ppm N,N-ethyl bis stearamide (EBS) at two different extraction and chromatog­ raphy temperatures. In the first analysis (Figure 1A), both ex-

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traction and chromatography were achieved at 60°C using 4.8 mg of polystyrene. However, extraction and chromatography of the same amount of polymer under the same flow rate of liquid CO (2 mL/min) through the extraction vessel, the same extraction pressure, and the same volume extraction vessel (150 μL) at a higher temperature (150°C) showed higher extraction efficiency. This enhancement arises from the higher volatility of EBS at the operating densities or the opening of the polymer at glass tran­ sition temperatures, at which the polymer becomes porous and increases its surface area, thus accelerating diffusion. Similar ex­ periments were also performed at 100° and 120°C on the same polymer. Table I shows the EBS extraction efficiency from polystyrene at different temperatures. As the temperature in­ creased, the extraction efficiency of the additive from the polymer also increased. 2

Experiments were also conducted to study the extraction effi­ ciency of different additives from polyethylene films at different temperatures. The polyethylene film had a melting point of 140°C. The additives in the polymer were a mixture of Irganox 1076, Tris-nonylphenylphosphate (TNPP), and Weston 618 with concentrations ranging from 90 to 110 ppm. Extraction and chro­ matography results from the on-line SFE-SFC of 4.2 mg of polyethylene film at 450 arm, 60°C, and 2 mL/min of liquid CO through the extraction vessel showed that only 40 to 60 per­ cent of the additives were extractable (Figure 2A). However, by increasing the extraction temperature to 150°C (higher than the polymer matrix melting point temperature), more than 95 percent of the additives were extracted (Figure 2B). Each additive was identified by retention time comparison using pure components. It was also observed that some of the low molecular weight polyethylene was also extracted at higher temperatures, as shown in Figure 2B. Besides the effect of temperature on extraction efficiency, ex­ periments were also conducted to explore the effects of the par2

Table I. Effect of Temperature on the Extraction Efficiency of N,N-EXhyl Bis Stearamide from Polystyrene

Figure 1. On-line SFE-SFC extraction of N,N-ethyl bis stearmide from polystyrene. Extraction conditions: pressure, 450 atm for 30 min; tem­ perature, (A) 60°C, (B) 150°C; cryofocusing trap temperature -30°C during collection and 80°C during injection. Chromatography conditions: packed column, 20 cm × 1.0-mm i.d., 5-μm particle size, C nucleosil; oven temperature (A) 60°C (B) 150°C; pressure program, flame ionization detection. 1 8

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Extraction temperature (C°)

Sample weight (mg)

Extraction efficiency (%)

60 100 120 150

2.0 3.0 2.4 2.2

77 81 88 94

Figure 2. On-line SFE-SFC extraction of (1) Irganox 1076, (2) Tris-nonyl­ phenylphosphate (TNPP), and (3) Weston 618 from polyethylene. Ex­ traction and chromatography conditions as in Figure 1.

Journal of Chromatographic Science, Vol. 29, December 1991

ticle size of polymer beads (i.e., surface area). For this purpose, a 30-min extraction was conducted on 12 mg of low density polyethylene (LDPE) pellets at 450 atm, 100°C, and 2 mL/min of liquid CO through the extraction vessel. Approximately 50 to 80 percent of the additives were extracted from the initial ex­ traction of the LDPE pellet. Additional experiments were per­ formed using 12-mg pellet sizes that were sliced into small pieces (30-50 mesh size). After extraction, collection, and backflushing of the extracted materials into the chromatographic column, more than 95 percent of the additives were extracted (Table II). The enhanced efficiency of extraction was due to an increase in the overall rate of mass transfer, which was a function of the increased surface area that eventually resulted in higher ex­ traction rates. Experiments were also conducted to optimize the extraction duration for additives in LDPE. For this purpose, 12-mg sample sizes of sliced LDPE pellets with additives were extracted at 450 2

Table II. Percent Recoveries of Various Additives from Polyethylene Pellets

Additives

Whole pellet 12 mg 30 min* Extraction 81.0 72.0

20.0

Extraction 1 2 98.0 96.1 3.8

Extraction 1 2 96.0 96.0 3.5

*Extraction duration.

Extraction CM

CM

BHT/BHEB Irganox 1076

Sliced pellet Sliced pellet Sliced pellet 12 mg 12 mg 12 mg 30 min* 20 min* 10 min*

85.0 84.5

12.0 10.0

atm, 100°C, and 2 mL/min of liquid CO through the extraction vessel. In this experiment, however, the duration of the extraction was decreased from 30 to 20 min. Two complete SFE-SFC runs were performed on the sample. More than 95 percent of the ad­ ditives were extracted in the first extraction (Table II). Similar ex­ periments were repeated with extraction durations of 15 and 10 min. The results after 15 min of extraction were the same as those after 20 and 30 min. However, the recoveries obtained for the 10min extraction were less than 90 percent (Table II). Therefore, by increasing the surface area, not only could better recoveries be achieved, but extraction times could also be reduced. For all of the above studies, the percent recoveries were measured based on known amounts of additives in the matrices. After optimization of extraction conditions, quantitative results for a series of additives in different polymers were obtained. External standard calibration curves for different additives were prepared by spiking the additives into the pure sample. Figure 3 shows calibration curves for BHT and Irganox 1076. Previously, similar results were reported by our group using the same methods to prepare the calibration curves (8). After obtaining the calibration curves for each additive, 12 mg of polymer matrix with the additives was placed in the extraction vessel. All ex­ tractions were performed at 450 atm and 150°C. Supercritical CO flow through the extraction vessel was approximately 2 mL/min. Figure 4 shows the extraction of stearyl stearamide and Irganox 1010 from an ethafoam (polyethylene). Each addi­ tive in the polymer was identified by retention time comparison of known compounds. This particular polymer matrix was placed in the extraction vessel without any slicing because of the large pore size on the surface that was demonstrated by complete ex2

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Figure 4. On-line SFE-SFC extraction of (1) stearyl stearamide and (2) Irganox 1010 from ethafoam. Extraction conditions (30 min): pressure, 450 atm; oven temperature, 150°C; cryofocusing trap temperature, -30°C during collection; and injection temperature, 80°C. Chromatography con­ ditions: packed column, 40 cm × 1.0-mm i.d., 5-μm particle size C nucleosil; flame ionization detection. 1 8

Figure 3. Calibration curves for BHT and Irganox 1076.

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traction of the additives from the polymer as shown in Table III. The additional extraction and chromatography experiments were conducted on a styrofoam (Figure 5) polymer containing ap­ proximately 110 to 120 ppm hexabromocyclododecane (HBCD). The complete extraction of HBCD from the maxtrix was con­ firmed by calculating the additive concentrations from calibration curves. The styrofoam polymer contained 0.5 percent of copper phthalocyanine blue, which was also extracted by supercritical CO . This was previously shown by the extraction and chro­ matography of metal containing compounds (13-15). Figure 6 shows the on-line SFE-SFC of a thin polymer film which con­ tained BHT, stearamide, Irganox 1076, and erucylamide. The

chemical structure of the polymer cannot be identified for con­ fidential reasons. All of the additives were extracted and quan­ tified without any change in the sample configuration. Table III shows the concentration of additives in these polymer films. Additional on-line SFE-SFC characterizations were obtained on two different low density polyethylene (LDPE) matrices.

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Table III. Calculated SFE-SFC Concentration of Different Additives from Different Polymers

Additive BHT, BHEB (in LDPE) lsonox 129 (in LDPE) Irganox 1076 (in LDPE) Irganox 1010 (in LDPE) Irgafos 168 Cyasorb 3346 Cyanox 1790 Stearyl stearamide Irganox 1010 (in Ethafoam) HBCD BHT (in a polymer film) Irganox 1076 (in a polymer film) Stearamide (in a polymer film) Erucamide (in a polymer film)

SFE-SFC calculated concentration (ppm)

Expected concentration (ppm)

510 220 250 250 200 1000 110 95 310 120 240 200 520 95

540 210 265 240 205 1050 120 100 300 140 250 185 510 100

Figure 5. On-line SFE-SFC extraction of hexabromocyclododecane (HBCD) from styrofoam. Extraction and chromatography conditions as in Figure 4.

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Figure 6. On-line SFE-SFC extraction (1) β-hydroxy toluene, (2) Irganox 1076, (3) stearamide, and (4) erucamide from a polymer film. Extraction and chromatography conditions as in Figure 4.

Figure 7. On-line SFE-SFC extraction of (1) Irgafos 168, (2) Cyasorb 3348, and (3) Cyanox 1790 from polyethylene pellet. Extraction and chro­ matography conditions as in Figure 4.

Journal of Chromatographic Science, Vol. 29, December 1991

of Kathryn Cross (Soltex) for the polystyrene SFE temperature variations and Robert M. Ravey for all of his assistance.

References

Figure 8. On-line SFE-SFC extraction of (1) β-hydroxy toluene, (2) hydroxyethyl benzene, (3) Isonox 129, (4) Irganox 1076, and (5) Irganox 1010 from a low density polyethylene pellet. Extraction and chromatog­ raphy conditions as in Figure 4.

Each sample was in bulk form, which necessitated slicing them into smaller pieces for extraction. Figure 7 shows the SFE-SFC results on 12 mg of the polyethylene, which contained Cyasorb 3346, Cyanox 1790, and Irgafos 168 additives. All of the addi­ tives were completely extracted and quantified (Table III). Figure 8 shows the SFE-SFC characterization of an LDPE sample which contained BHT, BHEB, Isonox 129, Irganox 1076, and Irganox 1010. Again all of the additives were extracted from the sliced LDPE matrix and quantified (Table III). Complete ex­ tractions of additives from each matrix were confirmed by cal­ culating the additive concentrations using calibration curves.

Conclusion In summary, this paper shows that the use of on-line SFE-SFC with a cryofocusing trap can achieve the characterization and quantification of chemical additives in a variety of polymer sam­ ples. The results demonstrated that the extraction efficiency and recovery not only increase at higher temperatures for certain polymer matrices, but also that better recoveries can be achieved if a larger surface area of sample matrix is exposed to the su­ percritical extracting fluid.

Acknowledgments The authors would like to acknowledge the experimental work

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Manuscript received J u l y 8, 1991; revision received October 3, 1991.

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