IDENTIFICATION OF B R A K E LINING CONSTITUENTS BY P Y R O L Y S I S G A S CHROMATOGRAPHY G E R A L D E . F I S H E R A N D J A M E S C. N E E R M A N
Scientific Research Staf, Ford Motor Co., Dearborn, Mich.
Compositional differences between commercial automotive brake lining materials are distinguishable by means of pyrolysis of the sample followed by chromatographic separation and infrared identification of the degradation products. A standard gas chromatograph equipped with a pyrolysis introduction system provides chromatograms characteristic of the binder resins. The method easily distinguishes major differences in binder materials and, with close control of operating parameters, provides fair repeatability for most peaks. A relatively small sample i s required and valuable information can be obtained in less than 2 hours per sample. HE degradation of complex organic molecules is used in Torganic chemistry for structure identification. The pyrolysis of organic material under controlled time and temperature conditions is commonly used for this purpose. This technique has proved particularly useful in the structural characterization and identification of polymeric materials such as rubber (3, 4 ) , plastics (2, 7), paints (8, 9))and resins (5, 6) by measurement of the quantity and composition of their breakdown products. A very recent development in organic identification methods is the combination of pyrolysis and gas chromatography. In this technique a pyrolysis chamber is directly attached to a gas chromatograph; the sample is pyrolyzed in an inert atmosphere and the products are purged instantaneously into the chromatographic system by the carrier gas for separation and analysis. This investigation relates to the effect of composition and process variables on the properties of brake linings in order to develop optimum compositions and processing techniques. The general aims are threefold: (1) to devise a system of comparing the resin content of brake linings in order to detect deviations in compositions for potential quality control tests; (2) to establish identification procedures whereby a wide variety of brake linings can be identified in reference to linings of known compositions as potential material standards; and (3) to determine whether the technique can be applied to the quantitative analysis of brake lining materials for resin content. Typically, brake lining material consists of 40 to 60y0 inert asbestos-type material ; the remainder is mainly organic resins.
Experimental
Apparatus. The pyrolysis apparatus is attached to an F & M Model 500 programmed temperature gas chromatograph using a hot-wire thermal-conductivity detector and a Micro-Tek high temperature universal inlet system (GET-121). The pyrolyzer and lock valve are inserted in place of the standard sample inlet. The sampling lock helps reduce pressure fluctuations and prevents large amounts of air from entering the system when the pyrolyzer is open. The pyrolyzer consists of a long stainless steel probe to the end of which is attached a platinum foil basket large enough to hold 10 to 20 mg. of sample material. Electrical conductors lead from the basket to the secondary of a stepdown transformer, and pyrolysis temperatures are controlled by a Variac in the transformer primary. The time of pyrolysis is regulated by an onoff switch on the Variac. Because of the type of inlet system used, it is difficult to determine the temperature of the platinum basket accurately during pyrolysis. Instead, a study was made 288
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
a t various transformer settings and a t various heating times to determine the best conditions for pyrolysis. From this investigation it was found that a Variac setting of 90 volts for a duration of 15 to 20 seconds is needed for complete brake lining pyrolysis. Current flow through the basket is approximately 5 amperes, and temperature is estimated to be about 1100' C. A commercial pyrolyzer such as the F & M Model 80 could probably be used by modifying the filament configuration to accommodate a large solid sample. The column used for this work is a 2-foot length of '/(-inch 0.d. copper tubing packed with 25% by weight of silicone rubber on 80- to 100-mesh hexamethyldisilizane-treated Chromosorb W. The hexamethyldisilizane-treated material is used to prevent tailing of water, which might otherwise be detrimental to the analysis. The chromatographic conditions are: bridge current, 150 ma.; block temperature, 250' C.; column temperature, 75' C.; port temperature, 190' C.; helium flow rate, 60 cc. per minute. Conditions for programmed temperature analysis are : starting column temperatures, 75' C.; column heating, 11' C. per minute; finishing column temperature, 200' C. Procedure. Samples of approximately 12 mg. of brake lining are cut and weighed accurately, then placed in the platinum foil basket at the end of the probe. The probe is inserted partially into the sampling lock. After pressure fluctuations have diminished, the sampling lock is opened and the probe is inserted completely into the sampling lock and into the port of the chromatograph. At this point, 5 minutes are allowed for the instrument to reach equilibrium. After equilibrium is reached, the sample is pyrolyzed at 90 volts (5 amperes) for 15 seconds, The column elutions are recorded isothermally for 60 minutes, using the operating conditions described above. After this time, programmed temperature analysis is started a t the 75' C. isothermal temperature and continued up to 200' C. in order to elute any high boiling materials. The amount of sample actually pyrolyzed is determined by carefully transferring and reweighing the residue after each analysis. Results and Discussion
Comparison of Brake Linings. A series of 10 brake linings has been analyzed and compared to detect deviations in composition. Although large quantities of Hz, CH4, CzH4, and other light gases are eluted within the first minute under these conditions, they were not separated and used on a comparative basis. Comparisons were based on peak presence or absence, as well as peak ratios, of intermediate and high molecular weight products. Results of these analyses, including elution times, peak identifications, and approximate peak area-weight pyrolyzed (sq. cm. per mg.) are shown in Table I. From these analyses 10
Table 1.
Comparison of Brake lining Pyrolysis Products
(Peak areas in square centimeters per milligram of sample) Elution Time, Min. 1.7 3.2 6.8 7.8 15.1 20.5 24.0 28.0 32.5 46.1
Sample Component Identifed Benzene Toluene m-Xylene Styrene o-xylene Phenol Indene o-Cresol #-Cresol 2,6-Dimethylphenol 2,4-Dimethylphenol
+
Elution Temp., “C. 120 a 130 a 150 165 e 180 190 a 205 21 5 a a Product not identifed.
1
2
3
4
5
6
2.12 1.02 0.21 0.30 0.085
2.15
2.57 0.77 0.057 0.029 0.26
1.77 0.85 0.05 0.025 0.10
2.04 1.09 0.021 0.021 0.43
1.90
...
...
... ... 0.21 ... ...
0.88
0.25 0.05 0.25 0.78
0.13
...
0.02 0.13
(I
... ... ... ...
...
... 0.28 ...
...
... ...
0.88
1.08
0.05 0.075 0.38
0.05 0.10 1.70
1.60
1.21
1.53
.,.
0.91
0.78
0.85
0.85
... ...
0.06 0.06 0.06
... ... ... ... ... ... ...
0.20 0.18
... ... ...
0.15
... ... 0.14 ... ...
...
.
.
...
...
0.05
0.06
0.20
...
0.15
0.24
...
...
... ...
0.84
..,
0.28
... ...
...
... ... 0.15 ... ... ... ...
... ... ... ...
...
...
...
...
0.10
...
... ... ... ...
I
...
...
0.09
1.50
0.026 0.18 0.79
.., ...
0.73 0.20
10
2.05 0.73 0.025 0.075 0.15
... ...
...
...
9 2.18
8
2.40 1.10 0.05
...
... ...
1.91
...
7 1.28
0.18
...
0.03 0.15
0.82 0.29 0.03 0.03
... ... ... ...
...
0.18 0.03 0.05
...
A, Brake Lining 4.0 M g. Pyrolyzed
al c
-= 0
e
t
?! n
2. c
+
-c
-
t a l
.o)
5 5 s;
Time, Minutes
B.
Brake Lining
3.7 Mg. Pyrolyzed
I
0
5
IO
I
15
I
20 Figure 1.
I
25
30 35 Time, Minutes
I
I
40
45
I
50
I
,
I
1
1
55
Pyrograms of similar brake lining materials VOL. 5
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289
peaks with elution times between 1.5 and 50 minutes were observed for various linings in isothermal operation. All 10 peaks have been identified. The 10 major peaks were benzene, toluene, m-xylene, and styrene together with o-xylene, phenol, indene, 0- andp-cresols, 2,Gdimethylphenol, and 2,4-dimethylphenol. The low molecular weight compounds, benzene through the xylenes, were identified by comparing their retention times with those of known compounds under the same column conditions. These identifications were later verified by infrared spectra. The higher molecular weight compounds, styrene through 2,4-dimethylphenol, were collected by capillary trapping (7) and identified by their infrared spectra. A total of seven peaks appeared on various brake linings during temperature programming following the 60 minutes of isothermal operation. However, on a few linings no evidence of peaks was observed during temperature programming. As yet, none of these temperature-programmed products has been identified. Some brands of linings, as shown in Figure 1, were very similar in pyrolysis products, whereas others were distinctly different (Figure 2). I n each case, however, a majority of the constituents was found in each lining. Identification Procedures. A group of eight known resin
A.
standards was first analyzed by pyrolysis gas chromatography. These materials consisted of Cardolites (polyresin cashew derivatives), Bakelites (oil-modified phenolics), and regular phenolic resins. Their characteristic pyrolysis products are shown in Table 11. Within this coIlection, there are few cases of exact identity, although there are definite family resemblances. Most phenolics give similar patterns and most Cardolites behave in a unique manner. Typically, individual members of a group present pyrolysis patterns sufficiently different to allow identification with a high degree of confidence. Identifications are based primarily upon the moderately volatile products. Where the resin standards are similar in composition, such as the phenolics in Figure 3, pyrolysis gas chromatography is still capable of making a positive identification from the ratios of various components. I n Figure 4, the patterns for different types of resin standards are shown. I n the standard Cardolite, the presence of styrene plus o-xylene and indene is clearly shown, whereas in the standard phenolic neither styrene plus o-xylene nor indene is present, but 2,6-dimethylphenol and 2,4-dimethylphenol are definitely present. Quantitative Analysis. Because of relatively successful qualitative comparison of linings, efforts are being made to compare pyrolysis products quantitatively. Because resin
Brake Lining Sample
m
5.3 Mg. Pyrolyzed
n
Time, Minutes
B.
s g 2
Lc:
Brake Lining Sample 4.0 Mg,. Pyrolyzed
0
t
'ime, Minutes Figure 2. 290
Pyrograms of different types of brake lining materials
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
P ' g ?
Y
Table II.
Pyrolysis Characteristics of Known Standard Resins
(Peak areas in square centimeters per milligram of sample) Samble Identifications Elution Time, Min. 1.7 3.2 6.8 7.8 15.1 20.5 24.0 28.0 32.5 46.1 Elutio; Temp., C. 130 165 190 195 205 215 0
Powder Phenolics
Cardolites Component Identified Benzene Toluene m-Xylene Styrene o-xylene Phenol Indene o-Cresol p-Cresol 2,6-Dimethylphenol 2,4-Dimethylphenol
+
1 1.97 0.98 0.19 0.45 0.16 0.16
2
3
7
2
3
1.19 1.13 0.39 0.68 0.29 0.23
0.49 0.80 0.33 0.47 0.02
0.87
1.27 0.87
Liquid phenolic 3.26 1.32 0.84
0.09
0.23
... ... ...
...
...
...
...
0
0.19 0.29
a
0.10
0
...
(1
0.09
0
...
.z
...
...
...
0.16
...
0.20
0.20
1.96 1.32 0.50
2.76
2.34
9.51
6.55
0.73 0.43
1.20 0.47
4.19 1.23
...
0.07
1 .oo
1.32
3.63 3.63 1.71 1.60
0.11 0.15 0.20
.. .. .. ... ... ...
... ... ... ... ... ...
... ... ... ... ... ...
...
...
0.04
...
0.57
...
...
...
...
...
...
...
...
...
...
...
0.42 0.13
... ... 0.05 ...
Oil-
modified phenolic 1.25 0.60 0.32 0.23 0.30
...
0.28
...
0.05 0.13
...
0.32 0.45 0.13 0.03 0.18
Product not identified.
A.
Standard Resin
I
-
III
2.2 Mg . Pyrolyzed
0 0 C
c
a
Figure 3.
Pyrograms of similar phenolic resin standards VOL. 5
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SEPTEMBER 1966
291
I
B. Standard Phenolic
-
Resin
C 0
3.4Mg. Pyrolyzed
.c 0, Q
[q /I
Start
Pyrolysis
0
5
I
A
\
uu IO
15
20
Figure 4.
t
I
25
f
j
K g g 4
Time, Minutes
Pyrograms of different types of resin standards
decomposition is very sensitive to small changes in pyrolysis conditions, quantitative comparison requires much finer control of the procedure than is necessary for qualitative results. Such factors as total sample size, size and density of individual pellets or powder particles, and the manner of packing the sample into the basket can affect the yield of the decomposition product. Also, to obtain the highest possible sensitivity it is necessary to use the maximum amount of sample the platinum basket will allow. Repeatability of individual peak areas from one chromatogram to another shows deviations as great as *30Yo, but usually within +loo/, of the mean. The ratio of two peak areas within a chromatogram has better repeatability from one chromatogram to the next, usually within *S% of the mean. One useful method of quantitative comparison of materials is the use of a fixed sample quantity and determination of the ratio of an individual peak area to the total area of all peaks on the chromatogram. The coniparison of peak areas has quantitative significance in sorting and evaluating friction materials, although it does not provide a weight yo value for each constituent.
292
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Acknowledgment
The authors thank J. L. Parsons for his skillful analysis and interpretation of the infrared spectra used in the identification of these compounds. Literature Cited
(1) Blake, B. H., Erley, D. S., Berman, F. S., Appl. Spectry. 18, 115 (1964). (2) Ettre, K., Varadi, P. F., Anal.. Chem. 35, 69 (1963). (3'1 Grasselli. J. G.. Ross. B. L.. Huber. H. F., Augi, - J. M., Chem. ' Znd. 4,1~2'(1963j. (4) Higgins, C. E., Baldwin, W. H., Oak Ridge Natl. Lab., Chemistry Division Annual Progress Report for Period Ending June 20, 1963, ORNL-3488, 62-4. ( 5 ) Karr, C., Jr., Comberiati, J. R., Estep, P. A., Fuel 42, 211 (1963). (6) Karr, C., Jr., Comberiati, J. R., Warner, W. C., Anal. Chem. 35, 1441 (1963). (.7 ,) Nelson. D. F.. Yee, J. L., Kirk, P. I., Microchem. J. 6, 225 (1962). (8) Scholl, F., Angew. Chem. 73,778 (1961). ( 9 ) Scholl, F., Deut. Furben. 2.16, 146 (1962). RECEIVED for review October 15, 1965 ACCEPTEDJune 6, 1966
