Edited by Jeanette G. Grasselli
Jerry B. Pausch BFGoodrich Research and DBvelOpment Center Brecksville, Ohio 44141
Analysisof
RubberandPlasticChemieals
Flgure 1. Uses of polymer chemicals in the rubber and plastic industries
Anticorrosion
L
0003-2700/81/035 1-089AS01.OO/O t2 1981 American Chemical S o c W
Vancide Products
ANALYTICAL CHEMISTRY. VOL. 54. NO. 1. JANUARY 1982
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89 A
Natural rubber was first introduced to the European continent by Christopher Columbus after he observed American native boys playing with strange bouncing halls. However, the unique elastic property of rubber remained virtually untapped until ruhher vulcanization was discovered by Charles Goodyear in 1839. From 1850 to 1950 numerous chemicals were developed and added to rubber to overcome two serious problems-its fast deterioration rate and its S ~ R Wvulcanization rate. Aniline was developed as the fmt organic accelerator in 1906, antioxidants based on aromatic amines were discovered in 1921; alkylphenylenediiines became commercial antiozonants in the 1950s. Today there are many thousands o these chemicals that play a major role in protecting and developing desired physical and chemical properties, not only in rubber ( I ) hut in plastics or other synthetic polymers. Figure 1illustrates a number of uses for these chemicals in the rubber and plastic in dustries. The wide variety and complexity ol these chemicals make analysis very difficult. They usually have high boiling points and molecular weights ranging from 200 to > 2 W m u . Many are quite polar and contain several heteroatoms. Also, many contain more species than originally anticipated-as many as 20 to 50 components. These mixtures occur hecause the starting materials are not purified, and the manufacturing conditions can lead to the production of various by-produds and/or oligomers. From a chemical analysis standpoint this task is ohviously in the “supersleuth” category (2).
In this article we will describe the joint chromatographic-spectroscopic approach as practiced in the BFGoodrich Analytical Laboratories both for isolating microgram samples and for identifying the individual components from these complex mixtures.
The Analytical Problem Measurement is the basis of all knowledge. Lord Kelvin A separation step using chromatographic methods is mandatory in this type of analysis. Majors (3)in 1970 was perhaps the first to show the utilit y of liquid chromatography (LC) for separating polymer chemicals like antoxidants. Since then, LC has found widespread acceptance for analyzing these types of chemicals in many fields such as agriculture, petroleum, pharmaceuticals, and foods. But since LC is not specific enough for identification, we decided to use mass spectroscopy (MS), infrared spectroscopy 9OA
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1 2
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1 6
1 8
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346’
1 10
1 12
1 14
1 16
1 18
1
20
1 22
l 24
l 26
1 28
1
30
Time (mln) 519’ 652 716 AA
479’
Figure 2. Reversed-phase liquid chromatogram for anillne-acetone reaction products (8).An asterisk indicates the major components (IR), and nuclear magnetic resonance (NMR) to give us information on the LC peaks. The complexity of polymer chemical analysis with reversed-phase, microparticulate LC columns is illustrated in Figure 2 with the resolution of about 40 components from a typical rubber antioxidant, a reaction product of aniline and acetone. Each peak probably represents from 0.1 to 1O.m of the toM. For a 1O-mmsinjection of a 1W solution, a 1%peak would represent about 10 pg on-column. Agsuming loosb recovery, this amount is obviously much too small to obtain IR and NMR spectra using conventional methods. Instead. we use either MS or specialized micro&hniques for IR and NMR.
Mass Spectroscopy Mass spectroscopy has the inherent capability to obtain spectra on microgram or smaller amounts of material. Even for probe analysis, the rule of thumb has generally been, “if you can see it, there is too much.” Scientists have been working for several years to develop interfaces joining the liquid chromatograph to the conventional mass spectrometer. This type of approach offers limited applicability in analysis of most polymer chemicals at this time, since polymer chemicals do not give good molecular ion spectra
ANALYTICAL CHEMISTRY. VOL. 54, NO. 1. JANUARY 1982
from either the electron impact (EI) or chemical ionization (CI) modes. Requirements in the polymer field dictate that ion formation must result from a low-energy process. Therefore, we decided to try the field desorption (FD) mode. In most casea the FD spectrum gives only one peak, which is usually the molecular ion [compounds having M i l e protons often give the (M 1) ion] (4). After the molecular ion is known, accurate mass analysis from E1 spectra can he much more definitive in pinpointing the exact identifcation of the compound. There are two additional advantages with FDMS Direct qualitative analysis of mixtures is often possible on a eample without prior isolation of the components, and the FDMS results give the number of components in the sample to guide development of the subsequent LC procedure. FDMS has been close to a panacea for our needs in supporting liquid chromatography. Just the molecular weight can be a very big factor in identifying the overall structure. The major limitations of FDMS have been that a small percentage of compounds are poor desorhers. Sodium cation attachment and fast atom bombardment have been useful in some instances to circumvent this problem. With some polymer chemicals there may also he a mass range limitation unless a high
+
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 1. JANUARY 1982
SIA
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Flgure 3. (a) Reversed-phase liquid chromatogram of low molecular weight polystyrene: (b) electron impact and field desorption mass spectra of polystyrene field instrument is available. A typical application where FDMS molecular weight values yielded straightforward interpretation of the data concerned a low molecular weight polymer analysis (5).Figure 38 shows the LC curve for a commercial polystyrene standard. Isolated fractions analyzed by FDMS gave essentially only molecular ions, so each polystyrene oligomer was easily verified. The utility of this approach is demonstrated in Figure 3b by comparing the E1 and FD spectra on the total sample. It readily shows that E1 ionization could not be applied for identifying the exact nature of the individual fractions. A side benefit of these FDMS data on oligomers is that a molecular weight distribution ( 6 ) can also be derived from the same experiment. Polystyrenes up to -5300 amu have been characterized successfully. For these materials and other typical products like tackifying resins and polyglycols (7),a mass spectrometer with extended mass range is obviously necessary. A second example was a polyurethane sample that was extracted for
I
characterization of low molecular weight oligomers. FDMS again showed ita unique ability to directly identify molecular species in a complex mixture. The three series of cyclic polyesters/polyurethanes shown in Figure 4 were readily identified (8). Infrared Spectroscopy One of the chief drawbacks to applying IR spectroscopy to problem solving in liquid chromatography is that traditional IR has never been regarded as a trace analytical technique. We have sometimes used FTIR for this purpose, but have relied more on the micro-attenuated total reflectance (ATR) approach (9) using a grating spectrometer. The ATR crystal is a parallel-piped, 0.5-mm-thick KRS-5 crystal with 45' end angles. It is very convenient to add the trapped effluent to the crystal dropwise and let the LC solvent evaporate. This permits some enrichment of the sample compared to examining the sample in solution. The spectra obtained are usually signal averaged four times for a 10-pg sample and further enhanced by subtracting
MW
=
out background and interfering components. These data are used routinely to complement the other spedroscopic results and are most valuable in the total analyticalpicture for making structural determinations. AgeRite White is an aromatic amine antioxidant produced from p-phenylene diamine and &naphthol. Under conditions of the manufacturing process, numerous tara may he formed as shown in Figure 5. Proposed structures were determined as usual from the FDMS and accurate mass data. For complementary data, IR spectra were obtained, for example, on the peak labeled mass 279 in Figure 6. The strong hand a t 1565cn-I is typical of the novel C=N vihration, which is conjugated with a c=C vibration in a ring structure. This hand was first obewed in previous analysis of ozonation products (10) of another aromatic amine. Further agreement was gained from the absence of the secondary N-H stretch a t 3420 em-'. Finally, the 800 cm-1 region shows the presence of 2,3-disuhstitution of naphthalene.
680 + 200"
Figure 4. Cyclic polyurethane oligomers Identified from extract of high molecular weight polyurethane %?A
ANALYTICAL CHEMISTRY. VOL. 54, NO. 1, JANUARY 1982
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Small is better in LC, too. Tiny sample loops I short columns work better. ~
Shortefficient HPLC columns packed with 3-pm particles are beginning to appear on the scene. As you might guess, these small columns require small samples. In fact, if you want to get good resolution, you must often inject samples smaller than 5 microliters.And that's where Rheodyne lends a helping hand. Our new 7410 injector has precision sample loops of 0.5 pl. 1 pl, 2 pI and 5 pl.The four loop sizes help you cope with the inevitable trade-off between resolution and sensitivity. For higher resolution, a smaller loop. For higher sensitivity, a larger loop. The 10 to 1 range in loop size lets you approximate the optimum. The sample loops are so small they fit inside the injector, but -we hasten to add-they're still easy to interchange. In operation, loops are completely filled, ANALYTICAL CHEMISTRY, VOL. 54, NO. 1. JANUARY 1982
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Wavenumber (cm-’)
Time(m1n) Figure 5. Reversed-phase liquid chromatogram of tar by. products from antioxidant production process
Nuclear Magnetic Resonance As with conventional IR, iron-magnet NMR instrumentation does not posseas the necessary sensitivity to examine samples routinely from analytical LC units. However, superconducting magnet NMR instruments can obtain useful proton spectra after averaging a number of pulsed scans. In this work a standard probe with 5-mm sample tubes has proven satisfactory to obtain the desired sensitivity, although we do concentrate the sample to the smallest pmible volume. The biggest problem has been the presence of trace amounts of water. Most reversed-phase LC experiments use water as part of the solvent system, and it is difficult to remove completely. The customary approach is to evaporate the sample to dryness, add Dz0 and reevaporate, then dilute in an appropriate organic solvent. “Solvent suppression” has also been valu-
Flgure 6. Micro-ATR infrared spectrum of -2O-Sg tar byproduct with mlecuiar weight 279 (shown in Figure 5), KRS5 parallel-piped crystal with 45’ end angles, and -25 internal reflections using Perkin-Elmer 180 infrared spectrorneter able in some instances. gram shown in Figure 2. This resinous The usefulness of NMR is shown in material is an acetone-aniline condenan example involving trimethylolprosation product whose principal strucpanetriacrylate (TMPTA), which is a ture is shown in Table I as the oligomcommonly used cross-linking agent for eric series with a monomer molecular polymers and adhesives. Proton NMR weight of 173. The detailed composispectra on LC cuts confirmed the tion of this product was generally unmajor component in the LC curve as known before liquid chromatography the expected prcdud from the various became available. But a total of eight assignments shown in Figure ?a. An oligomeric series containing 42 compoIR spectrum also verified this strucnents has now been identified. These ture. NMR analysis of the primary im- structures were deduced initiallv iust from FDMS molecular weights a i d purity indicated two components. The identifications (even the ratio of logical chemical reactions of the reacamounts) were determined from Figtants. Atomic composition data and w e 7b as the incomplete (diacrylate) IR spectra were obtained on several fractions to confirm assignments of and overreacted prcducta from the various structures. starting materials of trimethylolpropane and acrylic acid. Summary Rubber Antioxidant The combined application of chroThe multicomponent nature of a matography and spectroscopy is mantypical rubber antioxidant was already datory for analyzing polymer and rubillustrated by the liquid chromatober chemicals-chromatography for .
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Flgure 7.20oMHz ’H NMR spectra of (a) trimethylolpropanetriacrylateand (b) two impurities. Spectra obtained on -50 pg in 5mm tube on Bruker WH-200 NMR with quadrature detection: 16 scans averaged ANALYTICAL CMMISTRY, VOL. 54, NO. 1, JANUARY 1982
95A
vulnponents Identifiedin Aniline-Acetone Resin
Table 1.
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280
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226 399 572 745
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290 463 636 809
0 1 2 3 4
306 479 652 625 998
1 2 3 4 5
330 503 676 849 1022
Y,Z AD,AE AG
1 2 3 4 5 6 7
173 346 519 692 665 1038 1211
K' T' AA' AF' AI' AM' AQ'
0 1 2
370 543 716 889
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386 559 732 905 1078 1251
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Acknowledgment The work of several BFGoodrich scientists forms the hasis of this article. It is my pleasure to recognize their analytical team achievements in analyzing polymer chemicals during the past few years: Robert P. Lattimer (mass spectroscopy); E. Ray Hwser (liquid chromatography); Dale J. Harmon (size exclusion and liquid chromatography); Jerome C. Westfahl (proton nuclear magnetic resonance spectroscopy); Hugh E. Diem (infrared spectroscopy); Paul M. Zakriski (liquid chromatography-mass spectroscopy); and C. K. Rhee, E. T. McDonel, J. C. Andries, P. P. Nicholas, and R. W. Layer (ozonation studies). Finally, the support of the B F G d r i c h Company management for funding these laboratories and for permission to publish this article is appreciated. References (11 Rubber World 1979 Hlue Hook,"Matprials, Compounding Ingredients and Machinerv fur Rubber." Hanman Cummu-
niesticks,Inc.: New York.
MW = 370 + 173
3
separation and spectroscopy for identification. Although the molecular weight is unquestionably the single most important piece of information for identifying components, atomic composition determinations from accurate mass data and functional groups information from NMR and IR provide vital spectroscopic support. Very high resolution accurate mass data are not required on compounds containing heteroatoms. Specific isomer configurations or conformations are not usually necessary for problem solving in polymer chemicals. This analytical approach of utilizing spectroscopy as an off-line technique for identifying microgram amounts of isolated LC fractions has proven quite valuable in the BFGoodrich laboratories for determining material balances on production processes and for understanding the performance of these chemicals. It is also invaluable for FDA and TSCA approvals.
(2) Grssselli, J. G. Anal. Chem. 1980,52, 30 A. (3) Majors, R. E. J. Chrornatogr. Sci. 1970, 8,338. (4) Beckey, H. D. "Principles of Field Ionization and Field Desorption Mass Spectrometry"; Pergamon Press:New York, 1977.
(5) Lattimer,R. P.; Hooser, E. R.; Zakriski, P. M. Rubber Chem. Technol. 1980. 53,346.
Lattimer,R. P.; Harmon, D. J.; Hansen, G . E. Anal. Chem. 1980,52,1808. (7) Lattimer,R. P.; Hansen, G. E. Macro(6)
molecules 1981,14,776. (8)Lattimer,R. P.; Welch, K. R. Rubber Chem. Teehnol. 1980;53,151.
(9) Perkin-Elmer Product Bulletin, Accessory Data Sheet L-525.
(10)Lattimer, R. P.; Hooser, E. R.; Diem, H. E.; Layer, R. W.; Rhee,C. K. Rubber Chern. Teehnol. 1980,53,1170. 96 A * ANALYTICAL CHEMISTRY, VOL. 54, NO. 1, JANUARY 1982
