Document 70NkGObp3nqzn36ynNymQbvL8

BRAKE LINING DECOMPOSITION PRODUCTS BY Jeremiah R. Lynch Occupational Health Program 1014 Broadway Cincinnati, Ohio 45202 'i ` 7 U. S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE Public Health Service National Center for Urban and Industrial Health Occupational Health Program Presented at the Engineers Round Table Thirtieth Annual Meeting American Conference of. Governmental Industrial Hygienists St. Louis, Missouri, May 12, 1968 HWBUI0009465 ) ) BRAKE LINING DECOMPOSITION PRODUCTS ABSTRACT A number of investigates have found asbestos bodies in the lungs of urban residents who were not occupationally exposed to asbestos. A relationship between the carcinogenic properties of asbestos and the urban excess of lung cancer has been suggested. The decomposition products of brake linings have been described as a possible source of these fibers. Brake testing laboratory methods were used to test fiber emission from a variety of friction products. Only a very small fraction of the asbestos escaped as free fiber while.the remainder was transformed Into some other, non-fibrous, mineral. A significant release of free fiber occurred only under conditions extreme enough to produce brake failure. >. ' ..... HWBUI0009466 BRAKE LINING DECOMPOSITION PRODUCTS Jeremiah R. Lynch* Introduction The occurrence of coated fibers referred to as "asbestos bodies" in the lungs of urban populations not occupationally exposed to asbestos has been noted by several Investigators. The results of several autopsy series, as summarized by Cooper,^ are shown In Table 1. Thomson,^ in suggesting possible modes of exposure that could account for these bodies, stated "The average private motor vehicle wears out three or four sets of brake linings and one or two clutch linings_in its lifetime, and commercial and public transport vehicles wear out many more. These linings consist largely of asbestos which is ground to dust as the lings wear, and most wear occurs in built-up areas. This alone in most cities would involve the discharge of many tons of asbestos dust and fibers in the streets each year." Since asbestos has been implicated as a carcin ogen^^ and lung cancer has a higher incidenceamong urban populations, the fate of the asbestos worn from brake linings becomes significant. *U. S. Department of Health, Education, and Welfare Public Health Service Bureau of Disease Prevention and Environmental Control National Center for Urban and Industrial Health Occupational Health Program 1014 Broadway, Cincinnati, Ohio 45202 i; HWBUI0009467 Location Cape Town (2) Miami (2) Pittsburgh (3) Johannesburg (4) Finland (5) Montreal (6) San Francisco (1) 2 Table 1 Asbestos Bodies in Human Lungs Year 1963 1965 1965 1965 1966 1966 1966 % Positive 26 27 41 39 58 48 42 \ ) HWBUI0009468 3 Brake Lining Composition The average composition of typical brake linings of the type tested is shown in Table 2. Individual mixes may vary considerably from these averages. Table 2 Brake Lining Composition Ingredient Automobile Truck Asbestos Resins and Polymers Oxides and pigments Metals Carbon, graphite, etc. 55 28 9 3 5 100% 33 " ~48 16 2 1 100% Each of these ingredients performs a particular function. The asbestos provides strength, heat resistance and selective decomposition under stress. The resins and polymers hold the other ingredients together. Various oxides are added as pigments and to improve the grip of the lining on the drum. Soft metals such as lead and brass improve wear while carbon and graphite serve as friction modifiers. Brake lining quality depends to a large extent on the type of binder used, "Elnseed oil, which begins to decompose at about 450 F, is used for light service brakes. More demanding service requires brakes made from cashew type resins or oil modified phenolic resins. These ingredients are mixed, either dry or with a solvent added, formed into shape and cured HWBUI0009469 4 in an oven. The hardened lining is then cut, ground and sanded to the precise dimensions of the finished linings. The dust produced by the abrading operations in asbestos friction product factories (Figure 1) contains free asbestos fibers that are similar to those in industries where cancer is known to be in excess (10) The question to be resolved, therefore, is whether the dust produced by the normal wear of brake linings contains this same type of free asbestos fiber. Test Methods In an initial exploratory survey, the dust obtained from inside automobile brake drums removed for brake relining was examined by electron micrograph. Figure 2, which is typical of the series, does not reveal any free fibers. However, this finding did not eliminate the possibility that free asbestos fibers were released and that they had escaped, into the atmosphere. To examine this hypothesis a series of experiments was devised to permit sampling decomposition products of the lining under simulated operating conditions. The tests (Table 3) were performed with the brake testing machines in the laboratory of a major brake lining manufacturer. Most of the tests were performed on a friction-testing machine which used one-inch-square samples of the test lining. Heating and cooling apparatus permitted control of drum temperature from those encountered with intermittent brake use to the maximum temperature which occurred only in extremely rapid or "panic" stops from high speed. Other tests were performed on a brake-testing dynamometer which subjected >, a complete brake assembly to a series oC stops and starts from different speeds and at different deceleration rates to simulate actual driving conditions. In these tests the drum temperatures varied according to the driving condition simulated. During each run at a test condition, a sample was collected on an 0.8 u pore size membrane filter. Electron micrographs of these filters were examined to determine the presence of free asbestos fibers and to quantify these fibers when more than a few occurred. ) ) HWBUI0009471 6 Test Results by Electron Micrograph Product Brand Test Method No. of Samples Conditions Presence of of Test-F Free Fibers 1. Automobile A drum brakes Friction 6 300-800 Few 2. It B Friction 6 250-800 None 3. 99 C Friction 5 300-700 Few 4. 99 c Friction 1 700-900 Numerous 5. 99 D Friction French 5 300-800 Few 6. 7. > ' 9t E Friction U F Friction German 99 G Friction 5 300-700 5 300-800 2 100-500 Few Few Few 9. 99 G Friction 1 600-700 Numerous 10. 99 H Friction 2 100-600 Few 11. Automobile clutch J Dynamometer 1 normal driving None 12. Automobile disk brake K Dynamometer 1 normal driving Few 13. Bus drum brake ,-f L Dynamometer 1 city driving None 14. M Friction 2 450-550 None 15. Truck drum F brake (light) Friction 10 300-800 Few % Free Fiber* <1 0 <1 --10 <1 <1 <1 <1 ^15 <1 0 <5 0 0 <1 Weight estimated from fiber volume. HWBUI0009472 7 Results The object of the tests was to determine what proportion of the asbestos known to be present in the lining appeared as free fiber In the decomposition product. No attempt was made at a mass balance between the material worn from the lining and the decomposition product since it was not possible to collect all of the decomposition product. However, the percentages obtained in the fraction of decomposition product examined are applicable to the total material worn from the brake and may be compared to the percentage of asbestos originally present. In all but a few tests the automobile drum brake linings showed less than IX free fiber in the decomposition product as compared to about SOX in the lining. An electron micrograph of the decomposition product obtained from a typical test In this group is shown in Figure 3. In those tests where a significant mass of free fiber was released (Figure 4), the temperature was in an extremely high range for the lining in question as evidenced by rapid drop in the coefficient of friction. Had these linings been subjected to like conditions in a vehicle, the brakes would have failed. Similar results were obtained in the bus and truck drum brake tests and in the clutch test. The experimental model disk brake tested did release some (>5X) free fibers, but no conclusion can be drawn from a single sample. HWBUI0009473 8 Discussion Except in all but the most extreme driving conditions, only a very small fraction of the 30 to 50% asbestos present in a brake lining escapes into the atmosphere as free fiber. The question remains: What happened to the asbestos? A prevalent theory of brake operation holds that wear occurs not by abrasion of the ling by the drum but by the production of minute areas of Intense heat at the points of contact between the drum and the lining. Decomposition, not only of the binder but of the asbestos as well, occurs at these locations since the break down temperature of asbestos-, about 900 F. is exceeded. -The-decomposition products will thus include not free asbestos fibers, but a different mineral resulting from the thermal metamorphosis of the asbestos. Some independent experiments done at a brake lining research laboratory^have shown that all of the magnesium present in the chrysotile asbestos erginally in the lining can be accounted for in the decomposition products, However, the characteristic X-ray diffraction pattern of chrysotile asbestos had vanished. Thus, it is apparent that under conditions in which asbestos is worn from the lining of a brake, the asbestos is destroyed. Conclusion- -'`---5 Only a very small proportion of the asbestos worn from brake linings is released as free fiber; the remainder is converted into some other mineral as a result of the extreme temperatures generated at small spots on the lining surface. Thus, although urban air contains a few ) s ) HWBUI0009474 9 free fibers as a result of brake lining wear, they represent a very small proportion of the total asbestos used in the manufacture of brakes. Many sources of respirable fibers not associated with asbestos products have been identified, ^2) ancj the free fibers from brake lining wear appear to be an inconsequential health factor in urban air pollution. HWBUI0009475 10 References 1. Cooper, W. C. : Asbestos as a Hazard to Health. Arch, of Env. Health. 15:285 (1967). 2. Thomson, J. G., R. 0. C. (Casehula and R. R. McDonald: Asbestosls as a Modern Urban Hazard. S. Afr. Med. J. 37:77 (1963). 3. Cauna, D. R. S. Totten and P. Gross: Asbestos Bodies In Human Lungs at Autopsy. JAMA 192:37 (1965). 4. Webster, I., in discussion of Thompson, S. F.: Physiological Effects of D20 In Mammals, Ann. N. Y. Acad. Sci. 84:736 (1960). 5. Meurman, L.: Asbestos Bodies and Pleural Plaques in a Finnish series of Autopsy Cases, Acta Path Microbiol Scand Suppl 181:107 (1966). 6. Aryilrel, L. and W. Thurlbeck: The Incidence of Asbestos Bodies in the Lungs of Random Autopsies in Montreal, Canada. Med. Assoc. J. 95:1179 (1966). ,7. Thomson, J, G.: Asbestos and the Urban Dweller. Ann. N. Y. Acad. Sci. 132:196 (1965). 8. Selikoff, 1. J., J. Churg and E. C. Hammond: Asbestos Exposure and Neoplasia. JAMA 188:22.(1964), 9. Buell, P. and J. E. Dunn: Relative Impact of Smoking and Air Pollution on Lung Cancer. Arch, of Env. Health 15:291 (1967). 10. Enterline, P. E. and M. A. Kendrick1: Asbestos Dust Exposures at Various Levels and Mortality. Arch. Env. Health 15:181 (1967). 11. Sinelair, D.: Study of Wear Dust from Brake lining. Johns-Manvilie Research and Engineering Center, personal communication (1967). 12. Cralley, L-;-*J., R. G. Keenan, J. R. Lynch and W. S. Lainhart, M.D.: Sources and Identification of Respirable Fibers. Am. Indust. Hyg. Assoc. J. 29:2 (1968). Figure I. Brake lining factory dust . HWBUI0009477 Figure 2. Dust removed from broke drum. HWBUI0009478 Figure 3. Decomposition product from normal wear of brake lining . HWBUI0009479 Figure 4. Decomposition product from brake lining at failure . HWBUI0009480