Document Eqa5x2QaokRwx4QDZ2nQGb3LL

Jeremiah R. Lynch Notional Center for jrban and Industrial Health N* Brake Lining CL 7]!$ fjkS* X \D9 ^ Decomposition Products A number of investigators have found asbestos bodies in the lungs of urban residents who were not oc cupationally exposed to asbestos. A relationship between the carinogenic 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, nonfibrous, mineral. A significant release of free fiber occurred only under conditions extreme enough to produce brake failure. Table I. Asbestos bodies in human lungs. Location Cape Town (2) Miami (2) Pittsburgh (3) Johannesburg (4) Finland (5) Montreal (6) San Francisco (1) Year 1963 1965 1965 1965 1966 1966 1966 Q% Positive 26 27 41 39 58 48 42 Table II. 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% Mr. Lynch is Chief uf ihr Lab oratory of Engineering I h'Hpalional Health J'n >nt am. Public llcallh Service, I >r|wi tmeni of Health. Education. ami Welfare. HUt Htoathrav, ('inrinnaii. i ihm IVJtlJ. 8Z4 The occurrence of coated fibers re ferred to as "asbestos bodies'' in the lungs of urban populations not oc cupationally exposed to asbestos has been noted by several investigators. The results of several autopsy series, as summarized by Cooper,1 are shown in Table I.1 6 Thompson,' in suggesting possible modes of exposure that could account for these bodies, stated: The average private motor vehicle wears out three or lour 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 achestos which is ground to dust as the linings wear, and most wear occurs in built-up areas. This alone in mn-i cities would involve the discharge of many tons of asbestos dust and fibers in the srreets each year. Since asbestos has been implicated as a carcinogen5 and lung earner has a higher incidence9 among urban popula tions, the fate of the asbestos worn from brake linings becomes significant. Brake Lining Composition The average composition of typical brake linings of the type tested is shown in Table II. Individual mixes mat" vary considerably from these averages. Each of these ingredients performs a particular function. The asbestos pro vides strength, heat resistance, and selec tive decomposition under stress. The resins and polymers hold the other in gredients together. Various oxides are added as pigments and to improve the grip of the lining on th t drum. .Soft metals such as lead and brass improve wear while carbon and graphite -me as friction modifiers. Brake lining quality depends to a largeT extent on the type of binder used.* Linseed oil, which begins to decompose at about 450F, is used for light service brakes. More demanding service re quires 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 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 can-_ cer is known to be in excess.10 The" question to lie 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 obtailifjLirom inside automobile /brake drums removed for brake refining 1 was examined by electron micrograph. I Figure 2, which is typical of the series, \doos nor reveal any free fibers. How ever. this finding did not eliminate the po.--ibility that free ashesto- fibers were released ami that they had esca|ied into the atmosphere. To examine this hy pothesis a series of exiHuiment- was devised to permit sampling decom|>ositiou products of the lining under simu lated operating condition-. The tests (Table III) were performed with the brake te-ting machines in the laboratory of a major brake lining manufacturer. Mn-t of the tests were performed on a Journal ot the Air Pollution Control Association HWBUI0006346 :f ocd the been fiber free free lids to a laiy binder u-x?il 10 deeoinpo" i light servi'-' ..g service i>cashew typlenolic resined. either dr' formed inn oven. Tic ground, an ..elisions of tliproduced 1>. - in asbestu .es (Figure 1 fibers that at ries where ca: excess.10 Th i. therefore, iuced by th inings contain sbestos fiber. >ry survey, th ie automobi! i brake relinin. n micrograph d of the serii fibers. Hou. t eliminate th. -to- fibers wen id escajied inn amine this byiieriments tvamg deconip"~:ig under simn ii-. The te't ined with tin the latioraioi manufacture: erfurmed on trol Assaciatior Figure 1. Brake lining factory dust. Figure Z. Dust removed from brake drum. Figure 3."* Decomposition product from nor mal wear ot brake lining. ii friction-testing machine which used one-inch-s:|iiare samples of the test lining. Heating and cooling apparatus permitted control of drum temiierature from those encountered with inter mittent 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 of stops and starts from different speeds and at different deceleration rates to simulate actual driving con ditions. In these tests the drum tem peratures varied according to the driv ing condition simulated. During each run at a test condition, a sample was collected on an 0.8 p 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. :i Results | The object of the tests was to deterI mine what proportion of the asbestos ? known to be present in the lining apS peared as free fiber in the decomposi| tion product. No attempt was made at I a mass balance between the material * worn from the lining and the deeom- position product since it was not posf sible to collect all of the decomposition * product. However, the |>ercentages obtained in the fraction of decoiiiposi* tiou product examined are applicable > to the total material 'vorn from the ! brake and may be compared to the c percentage of asbestos originally present. 1 In all but a few tests the automobile drum brake linings sho'ygd~ifiSS._tha.n_ 1% free fiber in the decomposition jiiroSuctascomi^^^aBmiFpO1^. inthe limnr~A.n electron micrograph of the* decomposition product obtained from a typical test in this group is shown in Figure 3. In those tests where a sig nificant ma4s of free fiber was released (Figure 4i. the tem|xrature was in an extremely high range for the lining in question a- 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 dutch test. The experimental model disk brake tested did release some (>5%) free fibers, but no conclusion can be drawn from a single sample. Discussion Except in all but the most extreme driving conditions, only a very small fraction of the 30 to 50% asbestos pres ent in a brake lining escapes into the atmosphere as free fiber. The question remains: What hap[iened to the as bestos? A. prevalent theory of brake operation holds that wear occurs not by abrasion of the lining by the drum but by the production of minute areas of intense heat at the points of contact between the drum and the lining. De composition, not only of the binder hut.,of the asbestos as well, occurs at these WhnriwP~smre~ the hreakdnwn tejwL [imfff^rr^BesforliSo'ut -9QQF. is exceeded. Decomposition products will include different mineral resulting from. thermal metamorphosi- of asbestos. * Figure 4. Decomposition product from brak. lining at failure. ioa Vnliim ta Nn 1? HWBUI0006347 Table III. Test results by electron micrograph. Product Brand Test Method Presence No. of Conditions of Free % Free Samples of Test-F Fibers Fiber* 1. Automobile A Friction drum brakes 6 300 - 800 Few <1 2. Automobile B Friction drum brakes 6 250 - 800 None 0 3. Automobile C Friction drum brakes 4. Automobile c Friction drum brakes 5 300 - 700 Few <1 1 700 - 900 Numerous -10 5. Automobile D Friction drum French brakes 5 300 - 800 Few <1 6. Automobile E Friction drum 5 300 - 700 Few <1 brakes 7. Automobile F Friction drum German brakes 5 300-800 Few <1 8. Automobile 6 Friction drum brakes 2 100 - 500 Few <1 9. Automobile G Friction drum brakes 1 600 - 700 Numerous -15 10. Automobile H Friction drum brakes 2 100-600 Few <1 11. Automobile J dutch Dynamometer 1 Normal None driving 0 12. Automobile K Dynamometer 1 Normal Few disk driving brake <5 13. Bus drum L Dynamometer 1 City None brake driving 0 14. Bus drum M Friction brake 2 450 - 550 None 0 15. Truck drum F Friction brake (light) 10 300 - 800 Few <1 a Weight estimated from fiber volume. a -t,i . t Some independent experiment.' done at a brake lining research laboratory11 have shown that all of the magnesium present in the ehrvsotile asbestos orig inally in the lining can he accounted for in the decomposition products. How ever. the characteristic X-ray diffrac tion pattern of ehrvsotile asbestos had vanished. Thus. it_ is .ammceia-lhaJ-- under conditions in which a.-hestos is worn TronTXKe'Tfiim^ the a-l test us is destroyed. Conclusion Only a very small proportion of the asbestos worn from brake linings js released as free fiber; the remainder is converted into some other mineral as a result of the extreme tem|ieratures gen erated at small .spots on the lining sqri:ii e. Thus, although urban air con tains a few free libers as a result of brake lining wear, they represent a very -mall proportion of the total asbestos u>ed in manufacture of brakes. Many sources of respirable fibers not associated with.asbestos products have been identi fied,15 and free fibers from brake lining wear seem to be an inconsequential health factor in urban air pollution. References 1. Cooper. W. C., "Asbestos as a hazard 'to health," Arch, of Env. Health. IS: 285.19071. 2. Thomson, J. Cl., Ka-ehula, It. O. C., and McDonald, It. It., "Asbestosis as a modern urban hazard," S. A/r. Sled. J. 37 : 77 (1963). 3. Caitna, P., Totten, R. S., and Gross, P., "Asbestos bodies in human longs at autopsy,'' JASIA, 102: 37 (19(53). 4. Webster, I., in discussion of Thomp son, S. F., "Physiological effect of D.O in mammals," Ann. X. 1. Acud. Sa., 34: 736 119601. 5. Mauritian, I.., "Asbestos bodies anti pleural plaques in a Finish series of autopsy rases," Acta Path Micrnbicl. Srand Suppi. 181: IU7 1 PPti: 6. Aryilrel. L and Thurlbeck. W.. "The incidence of asbestos bodies in the lungs of randon autopsies in Montreal, Canada. Sled. Assoc. J., 95: 1179 (1966). 7. Thomson, J. G., "Asbestos and t he ur ban dweller," Ann. N. }'. Acad. Sri., 132: 196 11955). S. ftelikoff,s.X,al-s,,Chufg,,.Jf., and Ham mond, E. C7, "Asbestos exposure and neoplasia," JASIA, 188: 22 (19641- 9. Buell, P. and Dunn, J. E., "Relative impart of smoking and air pollution on lung cancer," Arch, of Env. Health. 15 : 291(1967;. 10. Enterline, P. E. and Kendrirk, M. A., "Asbestos dust exposures at various levels and mortalitv," Arch. Ear. Health, 15: 181 (1967). 11. Sinclair, L>., "Study of wear dust from brake lining," Johns-Mattville lies and Eng Ctr., personal communication (1967). 12. Cralley, L. J., Keenan, R. G.. lynch, J. R., and Lainhart, \V. S., "Source and identification of respirable libers." Amer. lml. llvg. Assoc. J., 29: 129 (1968 1. I j 1 | J , I 1 ` f I \ I I 821 Journal of the Air Pollution Control Association i HWBUI0006348