The Analytical Approach Edited by Jeanette G. Grasselli Jerry B. Pausch BFGoodrich Research and Development Center Brecksville, Ohio 44141
Analysis of Rubber and Plastic Chemicals by LC/Spectroscopy Thiazoles and jlfenamides
Ultra Accelerators
Processing Aids
Dispersing Agents
Cross-linking Agents
Coating Materials
Bonding Agents
yrophyllite
Figure 1. Uses of polymer chemicals in the rubber and plastic industries (adapted with permission of R. T. Vanderbilt Company)
Vulcanizing Agents
0003-2700/81/0351-089A$01.00/0 © 1981 American Chemical Society
Antioxidants
Anticorrosion Agent
Liquid Accelerators
Clay
Dispersions and Emulsions
Plasticizers
Rubber Extender
ν HeatResistant Agents
Thermal Blacks
Talc
Antiozonants
Retarder
Activator
Aromatic Odors
Vancide Products
A N A L Y T I C A L CHEMISTRY, VOL. 5 4 , NO. 1, JANUARY
Coagents
1982 · 89 A
Natural rubber was first introduced to the European continent by Christo pher Columbus after he observed American native boys playing with strange bouncing balls. However, the unique elastic property of rubber re mained virtually untapped until rub ber vulcanization was discovered by Charles Goodyear in 1839. From 1850 to 1950 numerous chemicals were de veloped and added to rubber to over come two serious problems—its fast deterioration rate and its slow vulcan ization rate. Aniline was developed as the first organic accelerator in 1906; antioxidants based on aromatic amines were discovered in 1921; alkylphenylenediamines became commer cial antiozonants in the 1950s. Today there are many thousands of these chemicals that play a major role in protecting and developing desired physical and chemical properties, not only in rubber (1 ) but in plastics or other synthetic polymers. Figure 1 il lustrates a number of uses for these chemicals in the rubber and plastic in dustries. The wide variety and complexity of these chemicals make analysis very difficult. They usually have high boil ing points and molecular weights ranging from 200 to >2000 amu. 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 because the starting materials are not purified, and the manufacturing conditions can lead to the production of various by-products and/or oligomers. From a chemical analysis standpoint this task is ob viously 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 chromato graphic methods is mandatory in this type of analysis. Majors (3) in 1970 was perhaps the first to show the utili ty 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 identifi cation, we decided to use mass spec troscopy (MS), infrared spectroscopy
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Figure 2. Reversed-phase liquid chromatogram for aniline-acetone reaction prod ucts (β). An asterisk indicates the major components
(IR), and nuclear magnetic resonance (NMR) to give us information on the LC peaks. The complexity of polymer chemi cal analysis with reversed-phase, microparticulate LC columns is illus trated 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 10.0% of the total. For a 10-mm 3 injection of a 10% solution, a 1% peak would repre sent about 10 μg on-column. Assuming 100% recovery, this amount is obvious ly much too small to obtain IR and NMR spectra using conventional methods. Instead, we use either MS or specialized microtechniques for IR and NMR. Mass Spectroscopy Mass spectroscopy has the inherent capability to obtain spectra on micro gram 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 ap proach offers limited applicability in analysis of most polymer chemicals at this time, since polymer chemicals do not give good molecular ion spectra
90 A · ANALYTICAL CHEMISTRY, VOL. 54, NO. 1, JANUARY 1982
from either the electron impact (EI) or chemical ionization (CI) modes. Re quirements in the polymer field dic tate that ion formation must result from a low-energy process. Therefore, we decided to try the field desorption (FD) mode. In most cases the FD spectrum gives only one peak, which is usually the molecular ion [compounds having labile protons often give the (M + 1) ion] (4). After the molecular ion is known, accurate mass analysis from EI spectra can be much more de finitive in pinpointing the exact iden tification of the compound. There are two additional advan tages with FDMS: Direct qualitative analysis of mixtures is often possible on a sample without prior isolation of the components, and the FDMS re sults 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 iden tifying the overall structure. The major limitations of FDMS have been that a small percentage of compounds are poor desorbers. Sodium cation at tachment and fast atom bombardment have been useful in some instances to circumvent this problem. With some polymer chemicals there may also be a mass range limitation unless a high
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Figure 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 3a shows the LC curve for a commercial poly styrene standard. Isolated fractions analyzed by FDMS gave essentially only molecular ions, so each polysty rene oligomer was easily verified. The utility of this approach is demon strated in Figure 3b by comparing the EI and FD spectra on the total sam ple. It readily shows that EI ionization could not be applied for identifying the exact nature of the individual frac tions. A side benefit of these FDMS data on oligomers is that a molecular weight distribution (6) can also be de rived from the same experiment. Poly styrenes 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 extend ed mass range is obviously necessary. A second example was a polyure thane sample that was extracted for
characterization of low molecular weight oligomers. FDMS again showed its unique ability to directly identify molecular species in a com plex 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 re garded 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 av eraged four times for a lO-fig sample and further enhanced by subtracting
out background and interfering com ponents. These data are used routine ly to complement the other spectro scopic results and are most valuable in the total analytical picture for making structural determinations. AgeRite White is an aromatic amine antioxidant produced from p-phenylene diamine and β-naphthol. Under conditions of the manufacturing pro cess, numerous tars may be formed as shown in Figure 5. Proposed struc tures 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 band at 1565 c m - 1 is typi cal of the novel C = N vibration, which is conjugated with a C = C vibration in a ring structure. This band was first observed in previous analysis of ozo nation products (10) of another aro matic amine. Further agreement was gained from the absence of the secon dary Ν—Η stretch at 3420 cm" 1 . Fi nally, the 800 c m - 1 region shows the presence of 2,3-disubstitution of naphthalene.
MW = 200n
MW = 340 + 200n
MW - 680 + 200 n Figure 4. Cyclic polyurethane oligomers identified from extract of high molecular weight polyurethane 92 A · ANALYTICAL CHEMISTRY, VOL. 54, NO. 1, JANUARY 1982
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