fL
Table I.Trends in Kinetic Parameters for Polarograms Simulating Marcus' Behavior Simulated polarograms Marcus' behavior k s , effective.
a
k,, cm/sec
+ 0.26 ( E 0.50+ 0.23 ( E - EO') 0.50+ 0.21 ( E - EO') 0.50 + 0.19 ( E - EO')
0.50
EO')
cm/sec
aefrectlve
5 X
0.40
6.9 X
5X
0.33
5 X IO-' 5 X lo-'
0.27 0.20
1.5 X 5.2 X 4.3 X IO-'
it is desirable to use Equation 18, which is perfectly linear for constant a. At small rate constants either method is suitable. The results of this study suggest a direction of further experimental work on this subject. Systems with heterogeneous rate constants less than 2 x 10-5 cm/sec should be investigated polarographically. Deviation between the inflection point of the polarogram and curvature in the shape analysis plots should be sought; but most importantly the possible existence of the trend given in Table I should be thoroughly investigated.
APPENDIX I a = charge transfer coefficient. Co* = bulk concentration of the oxidized species, mmol/ liter. Do = diffusion coefficient of the oxidized species, cm2/ sec. DK = diffusion coefficient of the reduced species, cm2/sec. E = potential applied to electrode, corrected for double layer effects, volts. EO' = thermodynamic formal potential of the couple
E"'
=
E"
+
= activity coefficient of ith species.
F = faraday. 3G" = molar free energy of activation of charge transfer
RT nF
__ l n ( f o / f K ) , v o l t s
reaction. AGRZ = molar free energy of activation of charge transfer reaction at the thermodynamic formal potential. I = instantaneous kinetically controlled polarographic current, bA. i d = instantaneous diffusion limited current, PA. 2 , er = instantaneous theoretical reversible polarographic current, PA. K = kinetic transition probability. k , -- reverse heterogeneous rate constant, cm/sec. k f =- forward heterogeneous rate constant, cm/sec. k, = standard heterogeneous rate constant, cm/sec. m = inass flow rate of mercury, mg/sec. m , = atomic weight of a single reactant species 0. n = total electrons involved in charge transfer reaction. n, = total electrons from oxidized state to and inclusive of the potential determining step of the reduction. R = gas constant, joule/deg/mole. t = instantaneous time, sec. T = temperature, "K. Zhet = heterogeneous collision frequency, cm/sec. [ = ratio of t h e mean square deviation of the distance between ion a n d electrode to the mean square deviation of the perpendicular distance from the reaction hypersurface. Received for review April 27, 1973. Accepted July 25, 1973. Presented in part at the 165th National Meeting, American Chemical Society, Dallas, Texas, April 1973. Receipt of an NDEA fellowship by D. R. Lewis is gratefully acknowledged. Taken in part from the M.S. thesis of D. R. Lewis submitted to the Graduate School University of Georgia.
Quantitative Determination of the Polymeric Constituents in Compounded Cured Stocks by Curie-Point Pyrolysis-Gas Chromatography Anoop Krishen and Ralph G. Tucker The Goodyear Tire & Rubber Company, Research Division, Akron, Ohio 44316
Quantitative estimation of the polymeric constituents of a compounded and cured stock, without extensive chemical treatment, has been accomplished earlier by pyrolysis-gas chromatography. The increasing complexity of the mixtures now used in various applications, has necessitated the use of more precise analytical techniques. The Curie-point pyrolysis technique, where the sample is heated indirectly by a high frequency current, was used to achieve precise control over a number of the experimental parameters. The higher precision resulting from this technique was combined with the advantages of using a dual gas chromatographic column. Under these conditions, hydrocarbon products ranging from methane to diprene/limonene were quantitatively analyzed. Conditions were established for determining the percentages of
polyisoprene, styrene-butadiene rubber, ethylene-propylene terpolymer rubber, polybutadiene, and chlorobutyl rubber. I t was possible to identify the components of unknown compounded cured stocks by this method. One sample can be analyzed in about an hour and the standard deviation was found to be 1.5%.
Pyrolysis-gas chromatography has been shown to be an excellent technique for quantitative estimation of polymers ( 1 ) . Its acceptance is apparent from the large number of publications reported in the past year (2). The in(1) A . Krishen, Anal. Chem., 44, 494 (1972). (2) C. W . Wadelin and M. C. Morris, Anal. Chem.. 45, 333R (1973).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 1, JANUARY 1974
*> 29
LIGHT ENDS ANALYSIS
10.5 MM
SAMPLE
0.5 X 1.0X 0.01 MM HEAVY
ENDS
ANALYSIS
Figure 2. Sample positioning on Curie-point wire
Figure 1. Connections for light and heavy ends analysis LEC = Light Ends Column, 40 ft X 3/j6 in. 0.d. 10% tricresylphosphate on 60-80 mesh Chromosorb P. HEC = Heavy Ends Column, 10 ft X 3116 in. 0 . d . 20% Carbowax 20 M on 60-80 mesh Chromosorb P. S = Stripper Column, 10 ft X 3/16 in. o.d 10% tricresylphosphate on ~ C J - 8 0mesh Chromosorb P. R = Restrictor Column, 10 f t X 3/16 in. 0.d. 10% tricresylphosphate on 60-80 mesh Chromosorb P. Helium A = Helium stream from Curie-point pyrolyzer. Helium B = Auxiliary h e i u m stream. A = Detector for LEC. B = Detector for HEC
creasing complexity of the constituents used in various applications has necessitated the use of moPe precise and sophisticated control over the experimental variables. The Curie-point pyrolysis technique has been a..ble to provide a degree of control which has not been pos:;ible with any of the other pyrolysis techniques used. In this technique, the sample is heated in a helium stream to a precisely duplicable temperature which is dependent on the composition of the alloy used for the pyrolysis vvire-the maximum temperature being the Curie-point when the ferromagnetic wire becomes paramagnetic and is thus incapable of absorbing any energy from the high frequency induction current source. The fast temperature rise results in more specific products on pyrolysis and any contamination from previous samples can be avoided by using a new wire each time. Application of this technique to complex polymeric mixtures containing a large number of monomeric species is of considerable importance in establishing the composition of these mixtures. The ability to handle cured samples without any elaborate pretreatment makes this technique much more useful.
EXPERIMENTAL Apparatus. Microbalance. A Cahn Electrobalance Model G-2 (Cahn Division, Ventron Instrument Corporation, Paramount, Calif.) was used for weighing the samples. The weighings were accurate to f 0 . 2 pg at the 1-mg range used. PyrolTsis Unit. The Curie-point precolumn pyrolyzer was obtained from Phillips Electronic Instruments, Mount Vernon, K.Y. This unit, Pye-Unicam Pyrolyzer Catalog Number 12558, uses a 550 kHz 30-watts high frequency source to heat the ferromagnetic wire holding the sample. When the source is energized, the rapid cycling of the magnetic flux produces heat by hysteresis. The wire reaches its Curie-point temperature within a few milliseconds where it is no longer ferromagnetic and thus cannot absorb any more energy from the high frequency source. As the source is energized only for an automatically preset interval, the temperature and the time for pyrolysis are precisely controlled. The pyrolysis products are directly swept into the gas chromatograph by the helium stream. S w i t c h i n g Value. A Carle Micro-Volume Eight Port Gas Chromatography Valve No. 2013 (Carle Instruments, Fullerton, Calif.) was used t o facilitate the analysis of the.pyrolysis products on two different gas chromatographic columns. The switching valve was connected to the columns as shown in Figure 1. Gas Chromatography. The pyrolysis products were analyzed using a Hewlett-Packard 5750 gas chromatograph equipped with dual f a m e ionization detectors. The chromatographic peaks were recorded with a 1-mV Moseley 7127A recorder (Hewlett-Packard 30
Company, Moseley Division, San Diego, Calif.). The peaks were recorded at an electrometer range of 100 and a chart speed of %-inch per minute. The area measurements were made either by the triangulation method (peak height X width at half peak height) or obtained directly by use of the on-line computer, Varian Aerograph Chromatography Data System 240 (Varian Aerograph, 2700 Mitchell Drive, Walnut Creek, Calif.). The Light Ends main column was aluminum tubing 40 ft X 3h6-in. o.d., 0.032-in. wall thickness. It was packed with 10% tricresylphosphate on 60-80 mesh Chromosorb P. The Heavy Ends main column was a 10-ft length of the same aluminum tubing packed with 20% Carbowax 20 M on 60-80 mesh Chromosorb P. The stripping and restrictor columns were 10-ft lengths of the aluminum tubing packed with 10% tricresylphosphate on 60-80 mesh Chromosorb P. The columns were operated a t 35 "C for 36 minutes and then programmed to 100 "C at 60 "C per minute. The helium flow for the Light Ends column was 37 ml per minute and for the Heavy Ends column was 77 ml per minute. The hydrogen and air pressures were 20 psi (Flowrator a t 5.0) and 40 psi (Flowrator at 5.0), respectively. The injection port and the detector were maintained at 200 "C while the Carle valve was kept at 150 "C by an auxiliary heating tape wrapped around it. Reagents. Tricresylphosphate, used as the stationary phase in the gas chromatographic column, was obtained from Eastman Organic Chemicals (Catalog No. 1483) while Carbowax 20 M , a polyethylene glycol, the other stationary phase, was obtained from Union Carbide Corporation, Chemicals Division, New York, N.Y. Chromosorb P, 60-80 mesh, the column substrate was supplied by Johns-Manville Products Corporation, Celite Division. Natural rubber (NR), styrene-butadiene rubber (SBR) (23.5% styrene, 76.5% butadiene, Goodyear's Plioflex 1006), ethylenepropylene-terpolymer rubber (EPDM) (Royalene 301T from Uniroyal), polybutadiene (BR) (Goodyear's Budene 1207) and chlorobutyl rubber (CB) (HT1066 from Enjay Chemicals) were used to prepare various standard mixtures. The other styrene-butadiene copolymers used were: 50% styrene, 50% butadiene; 20% styrene, 80% butadiene; 16% styrene, 84% butadiene; and 10% styrene, 90% butadiene (Copolymer Corporation's SBR 1505). These mixtures were then compounded and cured using standard rubber compounding recipes and operational techniques. The following blends were utilized to obtain the standard curves: 1. Natural rubber and styrene-butadiene rubber. 2. Polybutadiene and styrene-butadiene rubber. 3. Natural rubber, styrenebutadiene rubber and ethylene-propylene-terpolymer rubber. The percentage of styrene-butadiene rubber was held at 30% while the percentages of the other two were varied. 4. Natural rubber, styrene-butadiene rubber, ethylene-propylene-terpolymer rubber, and chlorobutyl rubber. The percentages of natural rubber and styrene-butadiene rubber were held at 20% each while the percentages of the other two polymers were varied. Procedure. The cured rubber samples were subdivided into small pieces with a pair of scissors. A 0.1- to 1-gram sample of these pieces was extracted with methanol in an Underwriters' extraction apparatus for 4-6 hours. The solvent was completely removed from the rubber by placing the sample in a vacuum oven at 60 "C for 30 minutes. A 575- to 650-microgram sample of the dried rubber was weighed on the microbalance. One centimeter of one end of the Curie-point wire was filed to half its thickness. The weighed sample was placed on this flattened portion of the wire and the end of the wire was folded onto the sample as shown in Figure 2. The other end of the wire was inserted through the septum on the pyrolyzer cap, positioned to the appropriate length (5.2 cm) from the pyrolyzer cap and the cap was then put on the pyrolyzer. After an interval of two minutes to re-establish equilibrium conditions on the gas chromatograph, the "Pyrolyze" button on the pyrolyzer control unit was pushed to start the high frequency current. The unit was set to turn off automatically after 15 seconds. At the end of 15 minutes, the Carle valve was turned
ANALYTICAL CHEMISTRY, VOL. 46, NO. 1, JANUARY 1974
4
8
W VI
z
SBR-NR rn NR-BR
X 0
10
20
40
30 TIME,
50
Figure area
60
MIN
4.
NATURAL
RUBBER
Determination of natural rubber from isoprene peak
rn -.
a 10000-
Figure 3. Chromatogram of pyrolysis products from SBR/EPDM /NR
\
a
1 , Methane; 2 , Ethane/Ethene; 3, Propane; 4 , Propene; 5, Propadiene/ Butane; 6, 2-Methylpropenejl-Butene; 7 , trans-2-Butene;8, 1,3-Butadiene; 9 , 1 -Pentene; 10, 2-Methyl-2-butene;11, Isoprene; 12, Diprene(6,Ern-Menthadiene)/Limonene( 1 ,E-p-Menthadiene);13, Styrene
to counterclockwise position (Figure 1) to stop the introduction of higher boiling components onto the Light Ends column and to keep them on the stripper column. After 36 minutes, when the chromatographic analysis of the lighter components was complete, the column oven temperature was programmed up to 100 "C at 60 "C per minute to allow the elution of the higher boiling components. The electrometer switch was turned to monitor the Heavy Ends column. When using a single electrometer, it was necessary to physically disconnect the cable from the detector A (Light Ends column) when monitoring detector B (Heavy Ends column), If this procedure was not followed, considerable interference was encountered from electrical cross talk even when a special switch was incorporated in an attempt to isolate the two inputs. An instrument with dual electrometers will not present this problem. The chromatograph was allowed to run for a total of 65 minutes, then the column oven was cooled to 35 " C , the electrometer switch turned to monitor detector A, the Carle valve turned to clockwise position, the old sample and the wire $ere replaced with a new sample on a new wire, and the instrument was ready to run the next sample.
RESULTS AND DISCUSSION Peak Identification. The arrangement of the columns and the operating conditions were specifically designed to give a suitable separation of most of the peaks of interest. The relative retention data obtained on the tricresylphosphate column have been published before ( I ) . While the pyrolysis products from the polymers studied did correspond to these components in relative retention (Figure 3), an exhaustive study to eliminate or identify other possible products with relative retentions identical to those of the compounds listed, was not undertaken. Pyrolysis Temperature. The following alloy wires were used to study the effect of temperature: Material Ni/Fe 48/52 NijFe 51/49 NijFe 70130 Fe
Curie-Point 480 "C 510 "C 610 "C 770 "C
Pyrolysis at 770 and 610 "C produced essentially similar products. The amount of monomeric products was larger when the sample was pyrolyzed a t 770 "C. Lowering the temperature to 510 or 480 "C severely limited the production of volatile hydrocarbons resulting in carbonaceous deposits on the walls of the pyrolysis tube. Pyrolysis Interval. The time allowed for pyrolysis was varied from 0.2 to 15.0 seconds. Smaller samples when pyrolyzed for shorter intervals gave products identical to those produced by pyrolyzing larger samples for longer in-
a
6000-
W
NR-SBR NR- BR
40
20 %
NATURAL
60
100
80
RUBBER
Figure 5. Determination of natural rubber from diprene/limonene peak area 0
NR- BR NR-SBR NR- BR NR-SBR
a a
0
20 %
40 60 BO N A T U R A L RUBBER
loo
Figure 6. Determination of natural rubber from area per cent of isoprene and diprene/limonene Upper curve isoprene Lower curve diprene/limonene
tervals. For 500- to 700-microgram samples, a pyrolysis time of 15 seconds was most suitable. Identification and Quantitation. Polyisoprene. Two series of samples containing natural rubber were examined by the Curie-point pyrolysis-gas chromatographic technique. These samples contained SBR-or polybutadiene as the second component and were compounded and cured. As shown in Figures 4 and 5, the absolute peak area of either isoprene or diprene (6,8-rn-menthadiene)/limonene (l$-p-menthadiene) can be used for quantitation. For the purpose of establishing automatic computational procedures, the equations for isoprene and diprenellimonene areas were calculated to be x = y/9600 and x = y/9450, respectively--x being the polyisoprene per cent and y , the area per microgram. The mode of calculation normally used in most of the published literature, namely area per cent of isoprene or diprenellimonene peaks, was tried for the experimental data obtained (Figure 6). The calculation of percentages is much more difficult with the nonlinear curve obtained. Traditionally, this curve has been converted to a straight line after plotting on a logarithmic scale. This, of course, decreases the accuracy of calculations. The straight line obtained by the use of absolute area measurements is simpler to use for calculations. It is interesting to note that the combined area per cents for the isoprene and diprenellimonene peaks exceeded 80% when 100% natural rubber was pyrolyzed. The specificity of products obtained in the Curie-point pyrolyzer is apparent.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 1, JANUARY 1974
31
600r
cn 24003.
\
7
a w
5
1600-
Y
a w a 800-
BR-NR
mI m.
rn SBR-NR
0
a joO/ E
-
I
20
0
OF
X
40 60 80 POLYMERIC BUTADIENE
Kx)
Determination of polymeric butadiene from 1,3-butadiene peak area Figure 7.
'?
0
/ I
1
I
I
I
,
,
.
,
I
100
40 60 BO % EPDM IN-EPDM/SBR/NR
20
Figure 12.
Determination of EPDM from propene peak area
Figure 13.
Determination of EPDM from 1-pentene peak area
BR-NR eSBR-NR
8 0
60
40
20 %
OF
POLYMERIC
80
100
BUTADIENE
Determination of polymeric butadiene from area per cent of 1,3-butadiene Figure 8.
I
/
ae 3.2
s/ Y
/
2.4
/
Figure 9.
;::p
Determination of SBR from styrene peak area
0
20 %
Figure 14.
60 80 40 EPDM IN EPDM/SBR/NR
100
Determination of EPDM from area per cent of l-pen-
tene
W
:IO 2.
L Figure 10.
0
20
40 % SBR
60
80
100
IN SBR/NR
Determination of SBR from area per cent of styrene 0
% EPOM
IN EPDM/SBR/NR
Figure 11. Determination of EPDM from ethanelethene peak area Butadiene Polymers. Two series of samples, SBR/natural rubber and BR/natural rubber, were examined. Area per microgram of 1,3-butadiene us. the polymeric butadiene content gave a linear relationship (Figure 7 ) . The experimental data plotted as the area per cent us. the polymeric butadiene content showed the nonlinear curve (Figure 8). The linear relationship is expressible as the equation x = y/2100.
32
Under the experimental conditions used, the nature of the matrix-polybutadiene or SBR-does not seem to have a significant effect on the amount of 1,3-butadiene produced from the total polymeric butadiene. Styrene Polymers. One series of styrene-containing polymers was examined. These were mixtures of varying proportions of SBR and natural rubber. The data, plotted on an absolute area basis, are shown in Figure 9. The equation for the straight line was calculated to be y - 3 1 1 . 4 ~- 1.55 = 0. The plot of area per cent of styrene us. SBR content (Figure 10) is again a nonlinear curve. EPDM Rubbers. Royalene 301T, a copolymer of 80120 ethylene/propylene, was compounded with natural rubber and SBR. These samples were then analyzed. Correlations between the EPDM per cent and ethene, propene, and 1-pentene peaks were sought. The gas chromatographic conditions used do not separate ethene from ethane; thus the absolute area of this combined peak was plotted against EPDM percentage (Figure 11). Correlations suitable for quantitation could not be obtained. Similarly the propene peak was considered for quantitation (Figure 12). This peak seems suitable for this analysis for lower concentrations of EPDM. However, the 1-pentene peak, which shows the least interference from other components was judged to be most suitable for quantitation (Figures 13 and 14). The equa-
ANALYTICAL CHEMISTRY, VOL. 46, NO. l , JANUARY 1974
Table I. Determination of Composition of Mixtures Natural Rubber
Chlorobutyl Rubber
SBR
EPDM
Added Found
20 21
20
10 9
50
22
2
Added Found
20 21
20 22
20 18
40 39
3
Added Found
20 21
20 22
30 28
30 29
4
Added Found
20 21
20 22
40 37
20 20
1
q 17000r
48
CB
%
IN N R / S B R / E P D M / C B
Figure 15. Determination of chlorobutyl rubber from 2-methylpropene peak area
tion for the linear graph when absolute peak areas are used (Figure 13) was calculated to b e y - 210x - 21 = 0. The slopes of the ethene and propene peaks in Figure 11 and 12 are too far apart to be used for calculating the ratio of ethene to propene in the polymer used. Since the same EPDM was used in all the mixtures, the slopes should have been identical. The divergence results from interference from the other components of the mixture. In the absence of interfering components, it should be possible to calculate the monomer ratio from the ratio of ethene and propene peaks. Chlorobutyl R u b b e r Mixtures of chlorobutyl rubber with natural rubber, SBR, and EPDM were pyrolyzed. The main pyrolysis product from chlorobutyl rubber was 2-methylpropene. The plots of absolute area and area per cent of 2-methylpropene us. chlorobutyl rubber per cent are shown in Figures 15 and 16. The specificity of the pyrolysis is noticeable from Figure 16 where 50% of chlorobutyl in the mixtures contributes almost 60% 2-methylpropene in the products. The relatively large amounts of the monomer produced by butyl rubbers facilitate their identification and determination. However, the determination of other polymers in the presence of butyl rubbers would not be possible if area per cent values obtained from other mixtures were used. When the absolute area values are used, this difficulty does not arise. The polymeric constituents of mixtures containing chlorobutyl rubber were calculated using the earlier data from SBR/NR and SBR/NR/EPDM mixtures (Table I). Natural rubber values were calculated from diprene/ limonene peak area per microgram, SBR from styrene, EPDM from I-pentene and chlorobutyl from 2-methylpropene. The results are based on the normalized values from duplicate runs. Styrene-Butadiene c'opol> m e r Composition. Five different types of styrene-butadiene copolymers were examined. The ratio of the 1,3-butadiene to styrene peak areas was plotted against the ratio of butadiene to styrene present in the copolymer (Figure 17). In SBR, both the raw polymer and cured stocks gave the same ratio of peak areas. Cured stocks were not examined for comparison in all the other copolymers. In the absence of polybutadiene, the copolymer ratio can be determined by obtaining the ratio of 1,3-butadiene and styrene peak areas even when other polymers are present. If the copolymer composition is known, then it should be possible to quantitate polybutadiene in presence of a copolymer of styrene and butadiene. This can be done by calculating the expected contribution of the copolymer to 1,3-butadiene peak area based on the styrene
A
~0 "
" 20 " %
"
"
40 60 00 NR/SBR/EPDM/CB
100
CB IN
Figure 16. Determination of chlorobutyl rubber from cent of 2-methylpropene
g
area
per
2.0-
K
2
BD/STYRENE
4 RATIO
6
IN
8 COPOLYMER
10
Figure 17. Determination of butadiene styrene ratio in copolymers
peak area and the styrene butadiene ratio in the copolymer. After subtracting this contribution, the rest of the 1,3-butadiene peak area is used to calculate the percentage of BR in the mixture. Compounding Ingredients. The technique of using peak area per microgram helped in giving the absolute amounts of individual polymers in cured stocks containing carbon black or other additives ( 1 ) . This method was equally useful when uncured polymers were used if calibration curves were prepared from identical mixtures. Precision. In order to establish the standard deviation, a mixture of NR/SBR/EPDM was pyrolyzed nine times. The standard deviation was 1.5% absolute on the percentage of the polymer present in the mixture. Interference. No serious mutual interferences were encountered in the analysis of the polymers examined. However, some other polymers may interfere with the quantitative estimation. Limitations. Although structural variations in the polymers used did not affect the results, a styrene-butadiene copolymer or an ethylene-propylene-terpolymer with different monomer ratios will require standard curves obtained from mixtures containing identical rubbers. Received for review May 14, 1973. Accepted July 17, 1973. Presented a t the 103rd meeting of the Division of Rubber Chemistry, American Chemical Society, Detroit, Mich., May 3, 1973. Permission by The Goodyear Tire & Rubber Company to publish is gratefully acknowledged.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 1, JANUARY 1974
33
