9 Plasticizer Analysis Using Chromatographic Methods DIETRICH B R A U N
Deutsches Kunststoff-Institut, Darmstadt, Germany
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P l a s t i c i z e r a n a l y s i s is i n d i s p e n s a b l e in e s t a b l i s h i n g t h e p u r i t y o f t e c h n i c a l plasticizers and d e t e r m i n i n g t h e chemical n a t u r e o f p l a s t i c i z e r s in finished a n d semi-finished articles. W h e n plastics are used f o r food p a c k a g i n g a n d s i m i l a r p u r p o s e s , o n l y c e r t a i n n o n t o x i c substances a r e a c c e p t a b l e a s p l a s t i c i z e r s , and this is why h e a l t h a u t h o r i t i e s t a k e a n a c t i v e i n t e r e s t in p l a s t i c i z e r a n a l y s i s . R e q u i r e m e n t s such a s t h e s e h a v e p r o m o t e d the d e v e l o p m e n t o f a n a l y t i c a l m e t h o d s t h a t are a s simple a n d a s t i m e - s a v i n g a s p o s s i b l e . This a r t i c l e d e a l s w i t h t h e p r e s e n t art o f plasticizer a n a l y s i s — i t s methods a n d p r o b l e m s — b a s e d o n the l i t e r a t u r e a s well a s o n t h e results o f the a u t h o r ' s o w n i n v e s t i g a tions.
^mong the numerous auxiliaries used in converting high polymers, plasticizers, on a percentage basis, rate high, because many polymers become technically high grade substances only when plasticizers are added to them. For this rea son, chemists have searched for suitable agents that efficiently combine plasticizing action with other requirements. In the past few years, thousands of substances, including many types of chemicals, have been recommended as plasticizers, and plasticizer analysis has become a complicated problem. Fortunately for the analyst, only about 300 of these plasticizing substances are produced on an industrial scale, and of these 300, only 100 are important in converting high polymers; the others are specialized products. Plasticizer production figures indicate the relative value of different groups of these materials and point up some substances that may rate special attention. Table I gives plasticizer production in the U . S. in 1960. Although all production figures since then show considerable growth, the relationships between groups remain fairly constant. The figures show that nearly all plasticizers chemically are esters: esters of phthalic acid account for 57% of total plasticizer production in 1960; esters of phosphoric acid account for 11 %; and esters of five aliphatic mono95 In Plasticization and Plasticizer Processes; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
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and dicarboxylic acids for 10%; the remaining 22% represents a wide range of monomeric and polymeric compositions with plasticizing capacity. Most of these also belong to the ester group. Thus, plasticizer analysis consists primarily of describing the characteristics of esters and determining their acid and alcohol components.
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T a b l e I. U . S . P r o d u c t i o n o f P l a s t i c i z e r s — 1 9 6 0
Cyclic Plasticizers Phosphoric acid esters Tricresyl phosphate Phthalic acid esters Dibutyl phthalate Diethyl phthalate Diisodecyl phthalate Dioctyl phthalates, total All other cyclic plasticizers Total
Tons Produced 14,748 8,601 7,612 16,134 77,280
Acyclic Plasticizers Adipic acid esters Azelaic acid esters Oleic acid esters Phosphoric acid esters Sebacic acid esters Stearic acid esters All other Total
Total 24,891 156,512
20,511 201,914
10,050 3,312 2,823 4,308 6,257 5,579 39,126 71,455
Plasticizer analysis requires use of both physical and chemical methods. Prior to analysis, mixtures must be separated. But plasticizers often have such low vapor pressures and high boiling points that standard separations—distillation and crystallization—are not adequate, and the analyst must use chromatographic separations. Currently, the most popular of these methods are : 1. 2. 3. 4.
Column chromatography Paper chromatography Thin layer chromatography Gas chromatography
Some plasticizer mixes require pretreatment, such as saponification, but in most instances chromatographic separations can be accomplished with the mix. In addition to the usual identification of substances by organochemical analysis, other methods now being used include color tests, physical tests (determinations of boiling point and refractive index), and infrared and ultraviolet spectroscopy. Chromatographic Methods for Separating Plasticizers All chromatographic methods are based on a two-phase partition of the substances or mixtures to be separated. Hence these separating methods utilize differences in physical properties of the components. Liquid adsorption chromatography (now referred to as column chromatography) uses silicious gel or aluminum oxide as the solid phase. Paper chromatography is essentially a liquid-liquid partition; one phase, in most cases, is represented by water adhering to paper, while the eluent represents the second phase. Gas chromatography, as gas adsorption chromatography, adsorbs gaseous substances on solids, mostly inorganic materials
In Plasticization and Plasticizer Processes; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
Downloaded by GEORGE MASON UNIV on June 8, 2014 | http://pubs.acs.org Publication Date: January 1, 1965 | doi: 10.1021/ba-1965-0048.ch009
9.
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Plasticizer
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such as diatomaceous earth. However, as gas partition chromatography, it fixes the liquid phase to a solid substrate by impregnation ; organic substances with high boiling points are best suited for the liquid phase. Thin layer chromatography is not easily classified. However, it works more like partition chomatography than like pure adsorption chromatography. Column Chromatography. Among chromatographic methods, adsorption to solid adsorbents (almost exclusively carried out in columns) has been popular because it can be used on a laboratory scale in connection with syntheses. It is, therefore, also used extensively for separating and purifying mixtures. But the technique is not effective for separating the diverse chemical components that are found in plasticizers. Nevertheless, some results of adsorption chromatography studies are included here because the method is related to thin layer chromatography, which has become an important development in plasticizer analysis. Sease (22) has adsorbed dimethyl and dibutyl phthalate in a silica gel column on a milligram scale with a zinc sulfide—zinc silicate fluorescence indicator. The different zones of the column material were eluted with diethyl ether. Cachia and his team (9) describe the column chromatographic separation of polyvinyl chloride plasticizers. The plasticization agent in the eluate was identified by infrared spectroscopy. These authors expressly state that chromatography is useful for separations only; it is not intended to identify substances. However, they mention that spectroscopy, in the future, may be replaced by combining refractive index data with color tests. Their investigation showed that carbon tetrachloride and isopropyl ether can be used as eluents in separating various types of plasticizing agents. The column used was 30 cm. long and 1 cm. in diameter. It was filled with 5 grams of silica gel 200/300 and Celite 545 in equal portions. Two drops of the plasticizer mixture, dissolved in 1 ml. of carbon tetrachloride, were then added to the top of the column for the separation. Some further tests (4), aimed at adapting these methods to the analysis of larger quantities—for example 100 mg. Under similar conditions, with methylene chloride as eluent, dimethyl and dinonyl phthalates have been adsorbed in a silica gel column (Merck). However, the technique is not really suitable for separating unknown substances when there are a large number of possible components. These tests do prove that, in principle, inorganic adsorbents can be used to separate components, provided that sharpness of separation is improved and that simpler identification methods are developed for the separated substances. Thin layer chromatography, as a kind of chromatography in open columns, has been upgraded to a simple technique in the last few years. But column chromatography obviously does not deserve this much attention in connection with plasticizer analysis. Paper Chromatography. For some time, paper chromatography has provided an analytical method for identifying organic substances, and the technique is still significant. But the literature includes only two reports that deal with the separation of polyvinyl chloride (PVC) plasticizers. The reason for this lack of interest may be that it is often difficult to recognize plasticizer spots on the chromatographic paper after migration. Burns (8) has succeeded in chromatographically fixing a large number of plasticizers on a paper impregnated with 14% PVC and in identifying them by their
In Plasticization and Plasticizer Processes; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
Downloaded by GEORGE MASON UNIV on June 8, 2014 | http://pubs.acs.org Publication Date: January 1, 1965 | doi: 10.1021/ba-1965-0048.ch009
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Rf values. Chromatography without pretreated paper, however, has not given satisfactory results. Chromatography has been carried out ascendingly with a mixture of pyridine, ammonia, isopropyl alcohol, and water as eluent. At the starting point, 0.001 ml. of the plasticizer mixture was applied, and the chromatography was completed within eight hours. The paper subsequently was dried in hot air at 85° C. and then, at room temperature, was gradually treated with Universal indicator solution (British Drug House). An exact analysis of plasticizers according to this method involves fluctuations in the Rp values, characterized by the authors as "daily deviations." To eliminate thesefluctuationsthe authors related their Rp values to tricresyl phosphate (of high meta content) with a mean Rp value equated to 0.66. Figure 1 gives the corrected Rp values for a number of plasticizers and a chart showing location of the spots. The data prove that in certain cases separations have
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