A azeotroDe comDosition of SAN
I
I
8
7
I
I
I P
1
6
I
I 1
.
I 0
ppm
of the two NMR methods is of the same magnitude as the difference between any of them and the chemical analysis data. There is a tendency, however, for the results of the chemical analysis to be a little higher, which may mean somewhat less nitrogen content as revealed through the laborious Kjeldahl treatment of the copolymers; this is in accordance with the results of Ritchey and Ball (3). As far as the reproducibility of the two NMR methods is concerned, it should be pointed out that in Method No. 1 repeated recording of spectra only is needed in order to estimate the reproducibility, as the results are independent of concentration. In Method No. 2, however, the preparation of the solutions should be ( 3) W. M.
Ritchey and L. E. Ball, J. Polymer Sci. B, 4,557 (1966).
repeated to that purpose. We find that the average error is 2-4 %. It seems that high-resolution nuclear magnetic resonance can match chemical analysis in the determination of the composition of styrene-acrylonitrile copolymers, having the additional advantage of greater simplicity. We could not apply it to compositions less than about 50% per weight styrene, such copolymers being too scarcely soluble in the used solvents. This, however, is no serious drawback, because SAN copolymers with low styrene content are of rather little commercial and technological interest.
RECEIVEDfor review June 13, 1967. Accepted September 22, 1967.
Pyrolysis-Ilnfrared Spectrophotometric Analysis of Rubber Vulcanizates Duncan A. MacKillop Dunlop Research Centre, Sheridan Park, Ontario, Canada
VULCANIZED ELASTOMERS may be readily characterized by pyrolysis followed by examination of the infrared spectrum of the pyrolyzate. When the conditions of pyrolysis are maintained the same-i.e., temperature and time of heating-the breakdown of the polymer is reproducible and the spectral data are quantitative. Vulcanized rubber has been analyzed by infrared spectrophotometry by Dinsmore and Smith (1) whereby the sample is broken down to soluble form by oxidative degradation in boiling o-dichlorobenzene in the presence of a free-radical generating compound. This method requires a long dissolution time, tedious filtration of the carbon black, and suffers from the selective adsorption of different elastomer types on the filtration medium. Another infrared spectrophotometric method for the analysis of vulcanizates is based on transmission through an extremely thin slice of cured rubber (2). This method requires a microtome and considerable manual skill. In the infrared spectrophotometric studies on pyrolyzates of rubber vulcanizates by Harms (3) and Hummel(4), no quantitative measurements were made. Lerner and Gilbert (5) de-
scribe the determination of bound styrene in styrene-butadiene rubber by a pyrolysis-infrared spectrophotometric method. Infrared spectrophotometry on pyrolyzates has been employed for the quantitative analysis of natural rubber-styrene-butadiene rubber covulcanizates (6), ethylene-propylene copolymers (3, and for the determination of acrylonitrile in nitrile rubber (8). Quantitative assay of the pyrolysis products of elastomers and their vulcanizates has been obtained by gas chromatographic techniques (9). Essentially all of the present day automotive tires are made with the elastomers natural rubber (NR), styrene-butadiene rubber (SBR), and cis-1,4-polybutadiene rubber (BR) in various compositions. The pyrolysis-infrared technique described below is a useful quantitative method for the rapid, routine analysis of these three elastomers in vulcanized rubber compounds.
(1) H. L. Dinsmore and D. C. Smith. Rubber Chem. Technol.. 21, 22 (1949). (2) P. J. Corish. J. ADD].Polvmer Sci.. 4. 86 (1960). (3) D. L. Harms, ANAL.CHEM.,25, 1146 (1953). ' (4) D. Hummel, Rubber Chem. Technol., 32,854 (1959). (5) M. Lerner and R. C. Gilbert, ANAL.CHEM., 36, 1382 (1964).
(6) M. Tryon, E. Horowitz, J. J. Mandel, J. Res. Nat. Bur. Std., 55,219 (1955). (7) J. E. Brown, M. Tryon, J. J. Mandel, ANAL.CHEM., 35, 2172 (1963). (8) F. F. Bentley and G. Rappaport, Zbid., 26, 1980 (1954). (9) G. M. Brauer, J. Polymer Sci., C-8, 3 (1965).
I
__
EXPERIMENTAL
Pyrolysis Apparatus. Pyrolysis was conducted in a borosilicate glass tube (1-inch i.d.) with a nichrome toaster element of 10 ohms resistance wound about the center of the tube,
VOL. 40, NO. 3, MARCH 1960
607
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Z
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0
v)
m
U I
20
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I
40
60
80
I
100
I 20 WT.%
W T . % SBR
Figure 1. Curve (l), calibration curve of the ratio of A 14.3p/ A 6.9 p us. wt Z SBR for covulcanized blends of SBR-NR, and Curve (2), calibration curve of the ratio of A 14.3 p / A 6.9 p us. wt Z SBR for covulcanized blends of SBR-BR
and asbestos string wound between and over the windings as electrical and thermal insulation. One end of the 10inch tube is fitted with a stopcock, and the other with a female 24/40 taper. To the tapered end is joined a 3-inch long, 1-inch i.d. glass tube fitted with a stopcock. For 30V applied potential, the temperature at the center of the tube rises rapidly over a 10-min period to level off at 390" C, repeatable to & l o " C. Pyrolysis Procedure. A 1-gram sample of vulcanized rubber previously extracted with acetone is cut into small pieces and wrapped in aluminum foil to form a 3-inch long cylinder. The foil packet, with the upper end loosely folded, is centered in the bottom of the vertically mounted pyrolysis tube. After the tube is evacuated (a mechanical pump capable of reducing the pressure to 0.1 mm of Hg is used), heat is applied electricelly for 10 min and then switched off. After a cooling period of 10 min, the yellow vapors condense and collect at the bottom of the tube. The liquid condensate is drawn from the tube and sandwiched between two sodium chloride disks with a 0.25-mm Teflon (Du Pont) spacer. The infrared spectra were recorded on the Perkin-Elmer spectrophotometer Models 21 and 521 and a Grubb-Parsons (Cambridge, England) grating instrument. The calibration curves shown are for the PerkinElmer Model 521 instrument. Calibration for SBR-NR Covulcanizates. Curve 1 of Figure 1 shows the calibration curve obtained on the pyrolyzates of NR-SBR covulcanizates and vulcanizates of natural rubber and SBR. The plot is based on the ratio of absorbances at 14.3 p and 6.9 p because the 14.3-p band is due to SBR (hydrogen atoms out-of-plane vibrations of the styryl group), The assumption is made that the bound styrene content (usually 2 3 . 5 z ) of SBR is constant. Also, the aromatic ring undergoes little decomposition at 400" C. Calibration Curves for NR-BR Covulcanizates. The absorption band at 11.3 p due to isopropenyl groups is the most characteristic band in the spectrum of the N R pyrolyzates. N R in NR-BR blends The plot of A 11.3 p / A 6.9 p us. wt (Curve 1 of Figure 2) yields a smooth curve with reasonable sensitivity over its entire range. The approximate linearity of this curve confirms the proportionality of the 11.3-p absorbance to natural rubber content. Because the infrared absorbance band at 11.3 p is characteristic of natural rubber and that at 10.4 p is characteristic of the rruns-vinylene of the polybutadiene pyrolyzate, some calibration plots employing their intensities were obtained.
z
608
ANALYTICAL CHEMISTRY
I
NATURAL 40
I
RUBBER 60
I
80
100
Figure 2. Curve (l), calibration curve of the ratio of A 11.3 p / A 6.9 p us. wt NR for covulcanized blends of NR-BR, and Curve (2), calibration curve of the ratio of A 11.3 p / A 10.4 p plus A 11.3 p us. wt NR for covulcanized blends of NR-BR
x
x
Curve 2 of Figure 2 shows the plot of the ratio A 11.3 p us. wt Z N R in the blend. This slightly A 10.4 p A 11.3 p curved relationship is of good analytical accuracy over all compositions. Calibration Curve for SBR-BR Covulcanizates. Because the 14.3-p absorbance band is due to the styryl group of SBR, the ratio A 14.3 p / A 6.9 p is a sensitive measure of the SBR content in SBR-BR blends, as confirmed by the almost linear plot of Curve 2 of Figure 1 . Other absorbance ratios were plotted against composition for each blend and gave some useful analytical curves. For example, a plot of wt SBR in SBR-BR covulcanizates us. A 14.3 p / A 10.4 p is a linear curve with a useful analytical slope. When the polybutadiene or any hydrocarbon chain of three or more carbon atoms is pyrolyzed, vinylic type unsaturation is generated by chain scission. This type of unsaturation gives rise to an IR absorption at 11.0 p because of out-of-plane vibrations of the geminal hydrogen atoms. The plot of A 14.3 p / A 11.0 p us. wt Z SBR gives a relationship with low slope for high SBR content and increased slope for low SBR content. However, the calibration points are widely scattered which suggests that this ratio A 14.3 p / A 1 1 .O p would give results of low accuracy. Method for Determination of Terblend (BR-NR-SBR) Composition. In actual practice, the three rubbers N R , SBR, and BR are frequently compounded into the same vulcanizate composition. In order to analyze these compounds by the method of pyrolysis-infrared spectrophotometry, a calculation technique based on the method of successive approximation and the two-component calibration curves of this study was employed. The composition of a given blend is calculated, as outlined
+
z
below, with the assumption that the ratio A 2 l4 p as a fraction A 6.9 p of that for pure SBR pyrolyzate, is proportional to the wt SBR in the covulcanizate. This is a reasonable assumption for terpolymer compositions in that the average of the two curves of Figure 1 would be a straight line passing nearly through the origin, and the curves converge at low concentrations of each elastomer. The absorbances at 10.4 p and 11.3, p are corrected for their SBR contributions from the spectral pattern for the pure SBR pyrolyzate, and the NR/BR ratio is deduced as for a simple blend from the two-com-
ponent calibration curves. The A 14.3 p is corrected for BR contributions from the spectrum of the pure BR pyrolyzate Table I. Comparison of Terblend Analyses with Composition and the calculation repeated for a better estimate of comBlended Composition, Wt Analysis, Wt position. BR NR SBR BR NR SBR Example Calculation for Terblend. FIRST APPROXIMATION. A 14.3 p 50 25 25 55 18 27 Estimate the SBR content from the absorbance ratio 25 25 50 32 18 50 A 6.9 p 25 50 25 24 53 23 as follows15 70 15 34 55 11 15 15 70 20 8 72 A 14.3 p A ' 14.3 p 70 15 15 68 17 15 Le., Wt % SBR = (A 6 . 9 p / A 1 6 . 9 ~ ) loo 30 40 30 37 35 28 30 30 40 42 28 30 = (0.465/1.385) X 100 40 30 30 42 28 30 Average: = 34% 35.6 28.8 35.6 37.8 26.4 35.8 A 14.3 p Average difference from blend : + 2 . 2 -2 . 4 $0.2 where is the ratio for the sample [example for A6.9p blend # 9 of Table I] Table 11. Infrared Absorbances in Ratio to A 6.9 I.( for Pure A' 14.3 p is the ratio for the pure SBR pyrolyzate, as Elastomer Pyrolyzates and A ' 6 . 9 b cis- 1,4listed in Table 11. Wavelength, p SBR NR polybutadiene ~
~
~
The absorbances at 10.4 p and 11.3 p are corrected for the SBR contributions by subtracting the absorbance values determined by prorating the spectrum of the pure SBR pyrolyzate to the sample absorbance at 14.3 p . Wavelength 10.4 p 11.3p 14.3 p
Sample absorbance 0.170 0.155 0.200
SBR contribution
A Corrected
0.048 0.021 0.200
+
A 11.3 I.( = 0.523 AlO.4p+ A11.3p From Curve 2 of Figure 2 the BR/NR ratio is 61/39, and the first approximation to the overall composition analysis is BR/NR/SBR = 40/26/34. SECONDAPPROXIMATION. For a pure BR vulcanizate the
Then, for this example, the contribution at 14.3 p from BR is 0.40 X 0.050 = 0.020 The absorbance at 14.3 p due to SBR is corrected by subtracting the BR contribution-i.e., 0.200 - 0.020 = 0.180. The percentage of SBR is recalculated from
x
100 i.e., wt % SBR
=
(: ~
:;;I
0'417 X 100 1.385 ~
=
30%.
The SBR contributions to 10.4 p and 11.3 p are subtracted from the sample absorbances and the BR/NR ratio determined as before. Wavelength Absorbance SBR Contribution A Corrected 10.4 p 0.170 0.043 0.127 1 1 . 3 ~ 0.155 0.019 0.136 1 4 . 3 fi 0.200 0.180 0.136 A 1 1 . 3 ~ -0.263 A 10.411 A 1 1 . 3 p
+
6.9
7.3 10.4 11.0 11.3
14.3
2.285 0.625 1 .o
1.940
... 1.0
...
0.573
0.332 0.535 0.146 1 385
0.122 0.138 0.700
I
...
2.755 0.152 1.0 ... 0.413 0.405 0.075 0.050
0.122 0.134
In order to calculate the BR/NR ratio, the value of A 11.3 p is determined from the corrected abA 10.4 fi A 11.3 p sorbances at 10.4 p and 11.3 p.
A ' 14.3 p ) A ' 6.9p
3.4 6.7
=
0.517
From curve 2 of Figure 2 the BR/NR ratio is 62/38. The overall composition is then BR/NR/SBR = 42/28/30 which is in good agreement with the blended proportions 40/30/30. DISCUSSION
The data of Table I show the comparison of analyses with blended composition for nine terblends of various compositions of BR/NR/SBR. In a series of tests on the precision of the absorbance ratios for individual samples of a two-component covulcanizate pyrolyzed 3 times each, the standard deviation is *4% relative for each component. From these data and examination of the calibration curves, the limit of detection for each elastomer is approximately 5 wt %. This method is limited to compositional analysis in that pyrolysis destroys the stereostructure of the polymers. The estimate of SBR content assumes constant bound styrene content, although commercial products vary from 21 to 26 wt % in styrene content. Extender oils are removed by acetone extraction to prevent interference with the SBR determination and thus the content of oil-extended polymer is estimated from the wt extractable data. The pyrolysis tube is inexpensive and functions as well as one heated by a furnace. Low cost allows several tubes to be operated at the same time to increase analytical productivity. Tungsten filament heaters are not desirable because best quantitative results are obtained at a pyrolysis temperature not greater than 450" C. This pyrolysis-infrared spectrophotometric method has been used, routinely, to give a rapid quantitative assay of elastomer compositions of tire rubber compounds.
RECEIVED for review September 28, 1967. Accepted December 4,1967. VOL. 40, NO. 3, MARCH 1968
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