Analytical Applications of Supercritical Fluid Extraction

Chapter 20

Analytical Applications of Supercritical Fluid Extraction—Chromatography in the Coatings Industry Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 3, 2017 | http://pubs.acs.org Publication Date: May 8, 1992 | doi: 10.1021/bk-1992-0488.ch020

William J . Simonsick, Jr., and Lance L. Litty Marshall Research and Development Laboratory, E . I. du Pont de Nemours and Company, 3500 Grays Ferry Avenue, Philadelphia, PA 19146 S u p e r c r i t i c a l f l u i d extraction/chromatography can be used to characterize many of the components of today's high performance automobile coatings. For example, a l i p h a t i c isocyanates are highly reactive with protic solvents and are not chromophoric, therefore, t h e i r analysis poses a formidable task to the a n a l y t i c a l chemist. Using carbon dioxide as the mobile phase i n conjunction with flame i o n i z a t i o n detection allows t h e i r quantification in coatings. Supercritical fluid chromatography i s used to obtain molecular weight d i s t r i b u t i o n information on nonchromophoric oligomeric materials which are not amenable to routine gel permeation chromatography. Furthermore, we use the accurate quantitative data afforded by SFC to corroborate and complement the q u a l i t a t i v e data provided by spectroscopic techniques. We have used s u p e r c r i t i c a l f l u i d extration to p r e f e r e n t i a l l y remove U V - s t a b i l i z e r s and/or polymer additives from cured films. Moreover, s u p e r c r i t i c a l f l u i d extraction i s i d e a l for removing undesirable matrices to f a c i l i t a t e other spectroscopic methods for i d e n t i f i c a t i o n . Today's high performance automotive coatings contain a variety of chemical compounds. Coatings are t y p i c a l l y composed of a binder system dissolved i n an appropriate solvent formulation. In addition, s t a b i l i z a t i o n packages (antioxidants, photostabilizers) are added to lengthen the usable l i f e t i m e of the coating. Pigment surfaces are coated with organic dispersing agents to minimize aggregation. The f i n a l products are the glamorous long-lasting finishes seen on today's automobiles. Each of the constituents l i s t e d above contain a v a r i e t y of chemical functionalities encompassing a wide range of molecular weights. Hence, the complete characterization of such coatings represents quite a challenge to the a n a l y t i c a l chemist.

0097-6156/92AM88-O288$06.00A) © 1992 American Chemical Society

Bright and McNally; Supercritical Fluid Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 3, 2017 | http://pubs.acs.org Publication Date: May 8, 1992 | doi: 10.1021/bk-1992-0488.ch020

SIMONSICK & LITTY

Solvatochromic Dyes in SFC

289

Solvents are usually mixtures of lov-molecular-weight materials of high v o l a t i l i t y which are removed from the paint formulation during the curing or drying process. Commercial s t a b i l i z e r s have molecular weights i n the range of 300-2000 Daltons (Da) and are therefore, not v o l a t i l i z e d during the curing process. Binders are traditionally high-molecular-weight polymers (>50,000 Da). These high-molecular-weight polymers can be synthesized from a host of monomers. Unfortunately, high-molecular-weight polymers require large amounts of organic solvents for d i s s o l u t i o n which, during the curing process, are emitted into the atmosphere. Therefore, the use of low-molecular-weight oligomers (

Figure 1. Chromatogram of 3-(Diethoxymethylsilyl) propyl methacrylate (I). Conditions - 10M SB-biphenyl 30 column, 0.25um f i l m thickness. Program - 200 atm for 10 min ramped to 415 atm at a rate of 10 atm/min. Column temperature 100°C; Detector temperature - 325°C.

M/z (DAL TONS)

Figure 2. K+IDS mass spectrum of butanediol d i a c r y l a t e crosslinker (IV). Scan rate 100 - 1000 Da/sec, average of five scans. Consult reference 12 for s p e c i f i c d e t a i l s .


293

294

SUPERCRITICAL FLUID


crosslinkers. Undesirable monofunctional i d e n t i f i e d and quantified.

TECHNOLOGY

materials are

easily

A c r y l i c Macroners. Thus far we have shown applications of SFC to the characterizations of monomers and c r o s s l i n k e r s . The next couple applications w i l l focus upon the analysis of oligomeric methacrylates, s p e c i f i c a l l y methacrylate macromers. Methacrylate macromers are frequently used as building blocks for larger a r c h i t e c t u r a l l y designed polymers. Unfortunately, macromers far exceed the c a p a b i l i t y of GC and do not possess a chromophore for HPLC analysis. Hatada et. a l . has used packed column SFC to analyzed the stereoisomers of oligomeric me thy line thacry late (MMA) prepared by anionic polymerization (13). MMA macromers were prepared by using a cobalt chain-transfer catalyst (CoCTC) and azobisisobutyronitrile (AIBN) as the initiator (14). The macromers were suspected to be low i n molecular weight (

F i g u r e 7. (Top) Chromatogram of polybutylene oxide oligomers (IX) t a r g e t e d a t a m o l e c u l a r weight o f 600. (Bottom) Chromotogram o f p o l y b u t y l e n e o x i d e o l i g o m e r s (IX) t a r g e t e d a t a higher molecular weight. C o n d i t i o n s - 10M SB-biphenyl 30 column, 0.25um f i l m t h i c k n e s s . Program - 200 atm f o r 10 m i n t h e n ramped t o 415 atm a t 10 atm/min and h e l d at 415 atm f o r 10 min. Column temperature - 100°C; D e t e c t o r temperature - 325°C.

F I D Response

(x) (XI) (XII)

T i m e / P r e s s u r e

>

F i g u r e 8. Chromatogram o f hexamethylene d i i s o c y a n a t e ( X ) . Conditions 10M SB-biphenyl-30 column, 0.25um film thickness. Program - 100 atm f o r 5 m i n ramped t o 400 atm a t 10 atm/min, and h e l d a t 400 atm f o r 15 m i n . Column temperature 100°C; D e t e c t o r temperature - 325°C.


300

SUPERCRITICAL

FLUID T E C H N O L O G Y

Soxhlet methods are traditionally employed to remove stabilizers from cured f i n i s h e s . Havthorne has demonstrated s u p e r c r i t i c a l f l u i d extraction to be i d e a l for removing materials from complex matrices (4). Because cured organic coatings are crosslinked networks of i n f i n i t e molecular weight, SFE using C0 will not s o l u b i l i z e any of the polymer matrix. However, stabilizers can be quantitatively removed. The extracted compounds can be e a s i l y loaded onto a column, separated using C 0 , and detected by FID. I d e n t i f i c a t i o n i s based upon retention time matching with authentic standards. Less than 1 mg of cured paint i s necessary to perform the analysis. Figure 9 i s a portion of the chromatogram of the extract from a cured paint f i l m . The coating was extracted at 400 atmospheres (atm) at 125°C for twenty-minutes. The chromatographic conditions are described i n Figure 9. The peak eluting at about twenty-five minutes corresponds to Tinuvin 440 (XIII) which was i d e n t i f i e d based upon retention time comparison to an authentic standard. Tinuvin 440 i s a HALS and does not have a strong chromophore, but i s e a s i l y detected using FID. The l a s t peak appearing at t h i r t y - f o u r minutes i s due to Tinuvin 900 (XIV), also i d e n t i f i e d by retention time comparison. Ve did not i d e n t i f y the peak at twenty-one minutes. This analysis shows the power of SFE for i s o l a t i o n and concentration of low-molecular-weight additives from complex matrices. The benefits of SFE over conventional Soxhlet extraction methods are fast extraction rate, one step extraction and c o l l e c t i o n , low temperature operation, less c o s t l y , nontoxic solvent, and minimal waste disposal costs. Ve have found t h i s approach applicable to many coatings, however, high-molecularweight s t a b i l i z e r s can be d i f f i c u l t to q u a n t i t a t i v e l y extract from highly crosslinked f i l m s . Fortunately, using s t a t i c SFE with a co-solvent to swell the f i l l or r a i s i n g the extraction temperature above the glass t r a n s i t i o n temperature (lj>) of the f i l m f a c i l i t a t e s the extraction process. This work i s currently underway i n our laboratory. 2


2

I s o l a t i o n of Pigment A d d i t i v e s . Many pigments are coated with a long chain a l i p h a t i c organic compound to deactivate the surface and prevent aggregation. The i s o l a t i o n and spectroscopic analysis of such compounds can be challenging because of t h e i r r e l a t i v e l y low concentration and i n t e r f e r i n g compounds. SFE can be used to s e l e c t i v e l y remove i n t e r f e r i n g materials from the analyte of i n t e r e s t . The analyte can be successfully i s o l a t e d , c o l l e c t e d , and i d e n t i f i e d by other spectroscopic methods. Using SFE/SFC we were able to extract from a pigment, a material which gave a single chromatographic peak as seen i n Figure 10. Ve surmise that t h i s peak i s due to the compound used to treat the pigment surface. The large e a r l i e r eluting peak corresponds to a high b o i l i n g solvent, 2 , 2 , 4 - t r i m e t h y l - l , 3 - p e n tanediol-monoisobutyrate (Texanol). Owing to the large abundance of Texanol we were not able to i d e n t i f y the additives by routine gas chromatography/mass spectrometery.


SIMONSICK & LITTY

301



F ID Response

T I NUV | N - 9 0 0

T I NUV I N-4 40

A . 20

A 25

30

Time

P r e s s u r e

35

>

Figure 9. Chromatogram of an extracted paint film. Conditions - 10M SB-biphenyl 30 column, 0.25um film thickness. Program - 80 atm for 5 min ramped to 400 atm at 7 atm/min and held at 400 atm for 5 min. Column temperature 125°C; Detector temperature - 325°C. F I D Response

2 15,

3 3 7 Do

TEXANOL

T i m e / P r e s s u r e Figure 10. Chromatogram 2.5 M SB-biphenyl Program - 100 atm for 5 atm and held for 5 min. temperature - 325°C.

>

of the extracted pigment. Conditions 30 column, 0.25um f i l m thickness. min programmed at 10 atm/min to 400 Column temperature - 125°C; Detector


302

SUPERCRITICAL

FLUID T E C H N O L O G Y

In order to remove the undesirable Texanol, we extracted the pigment with s u p e r c r i t i c a l carbon dioxide which was held at a low pressure (100 atm) for five minutes. The extract was found to contain Texanol. We increased the carbon dioxide pressure to 400 atm for five minutes to extract the pigment additive. The pressure control i n SFE allows us to control the s o l u b i l i z i n g power of the carbon dioxide, therefore, perform selective extractions. Isolation of the pigment additive by SFE followed by K IDS analysis showed the additive had components of molecular weights 186 Da and 298 Da, respectively. We performed a l i t e r a t u r e search, using the Formula Weight Index of the Registry F i l e of Chemical Abstracts Service (18,19) on these molecular weights and found two references each of which i s a mixture of fatty acids and t h e i r corresponding methyl esters. These mixtures are used to s t a b i l i z e pigments. This analysis i l l u s t r a t e s the u t i l i t y of s u p e r c r i t i c a l f l u i d s for rapid selective extraction with no expensive solvent disposal problems, K IDS for molecular weight information, and the usage of on-line databases to a s s i s t i n data i n t e r p r e t a t i o n .


+

Conclusions We have demonstrated the u t i l i t y of SFC and SFE using carbon dioxide with FID detection i n the coatings area. SFC extends the molecular weight regime which i s currently available using GC. Nonchromophoric compounds can be characterized with the universal FID detector. Accurate molecular weight data i s provided on nonchromophoric oligomers. MMA macromers qualitatively characterized by K IDS are quantified by SFC. Preliminary data on copolymers i s also promising. SFC provides quantitative data complementary to the qualitative information afforded by spectroscopic techniques. SFE i s i d e a l for removing low-molecular-weight additives from complex polymer matrices. Owing to the control of density, and therefore, s o l u b i l i z i n g power one can selectively isolate targeted chemicals or remove complex matrices for a n c i l l a r y methods of characterization. Any method which can be used to separate or simplify the constituents of today's coatings i s welcomed by the coatings chemists. S u p e r c r i t i c a l carbon dioxide has proven to be a valuable t o o l to the coatings chemist. +

Acknowledgements The authors thank the Automotive Products sector of Du Pont for allowing publication of t h i s work. Dr. M. J . Darmon synthesized the MMA macromers. We thank Dr. M. J . Mahon for his s c i e n t i f i c and e d i t o r i a l assistance. Maryann S i l v a performed a l l of the mass spectrometric experiments. Audrey Lockton, Christie Connolly, Mary Clavin and Eileen Brennan are acknowledged for t h e i r c l e r i c a l assistance i n the preparation of t h i s manuscript.


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SIMONSICK & LITTY


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L i t e r a t u r e Cited

1.

2.


3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

Armour, A. G . ; Wu, D. T . ; A n t o n e l l i , J. A.; Lowell, J. H. Presented at the 186th American Chemical Society Meeting, Symposium of the History of Coatings, Sept 14, 1989, Paper 38. Lee, M. L . and Markides, K. E. " A n a l y t i c a l S u p e r c r i t i c a l Fluid Chromatography and Extraction", Chromatography Conferences, I n c . , Provo, Utah, 1990. McNally, M.E. and Wheeler, J . R . J. Chromatogr. S c i . , 27, 534 (1989) Hawthorne, S.B. Anal. Chem., 62, 633A (1990). Campbell, R . M . , In: "SFC A p p l i c a t i o n s , " Compiled by K. Markides and M.L. Lee, Brigham Young U n i v e r s i t y Press, Provo, Utah, (1988). Campbell, R . M . , In: "SFC A p p l i c a t i o n s " , Compiled by K. Markides and M.L. Lee, Brigham Young U n i v e r s i t y Press, Provo, Utah, (1989). Pinkston, J . D . ; Owens, G . D . ; Burkes, L . J.; Delaney, T . E . ; M i l l i n g t o n , D . S . ; Maltby, D. A. Anal. Chem., 60, 962 (1988). Ashraf-Khorassani, M. and Taylor, L . T. Anal. Chem., 61, 145 (1989). Fuoco, R . ; Pentoney, S. L.; G r i f f i t h s , P. R. Anal. Chem., 61, 2212 (1989). Bourne, T. A.; Bufkin, B. G . ; Wildman, G. C . ; Grawe, J. R. J. Coat. Technol., 54, 69 (1982). Bombick, D . ; Pinkston, J . D . ; Allison, J . A n a l . Chem., 56, 396 (1984) Simonsick, W. J. Jr.; Appl. Poly. Sci.: Appl. Polym. Symp., 43, 257 (1989). Hatada, K . ; Ute, K . ; Nishimura, T . ; Kashiyama, M . ; Saito, T . ; Takeuchi, M. Poly. Bull., 23, 157 (1990) Janowicz, A. H., US Patent 4,694,054. Solomon, D. H. The Chemistry of Organic Film Formers, John Wiley & Sons, I n c . , New York, NY, 1967. Beck, K. R . ; Korsmeyer R . ; Kurz, R. J. J. Chem. E d . , 61, 668 (1984). Raynor, M. W.; B a r t l e , K. D . ; Davies, I . L.; Williams, A.; Clifford, A. A.; Chalmers, J . M . ; Cook, B. W. Anal. Chem, 60, 427 (1988). L e i t e r , D . P . ; Morgan, H. L . ; Stobaugh, R. E. J. Chem. Document, 5, 238 (1965). Dittmar, P. G . ; Stobaugh, R. E.; Watson, C. E. J. of Chem. Inform. and Comp. Sci., 16, 111 (1976).

R E C E I V E D January 21, 1992


Analytical Applications of Supercritical Fluid Extraction

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