Document DGz1yzER1JMkKEXbpb22bXEkQ

* SUMMARY AND EVALUATION Subject: Discussion of the possible use of materials other than asbestos for auto brakes. Of interest and importance. Evaluation: The conclusion of the author is that asbestos is practically indispensable for auto brakes at the present time. If asbestos were not available, ve could "eventually find a substitute, but it could veil be at the expense of a considerable loss in performance, and also at a con siderable increase in cost." The author also emphasizes that there is no valid health reason Why asbestos should be abandoned at the present time. He cites the tests demonstrating the very small percentage of asbestos fiber in brake lining dust and concludes that "the wear products from friction materials are probably no more dangerous to health than many other dusts and can be treated as 'merely dirty.'" He also mentions that technical developments have reduced the quantities of asbestos needed in the production^of-nefficient brakes and clutches. Author: Ferodo Company). Journal: Annals of Occupational Hygiene (British), January 1970. Title: '-PossibleAl-temative-s to Asbestos as aFrterCxon"Material.'" Summary: As given above. SCF-FA-6330 industry red, in the neures for mat over a level of 12 erhaiil) on ticular in* %ures even described ne univerrake dust, industrial We have iifTraetion although ere seems ah occurs tween the <) Hyfitnt it. Omr. Hn. Vol. IS. pc.23-29. hnmtfnii 1*19. Prime* ia Gnu Imm January 1S7D POSSIBLE ALTERNATIVES TO ASBESTOS AS.A FRICTION . MATERIAL* ' D. Hatch . FercxJo, Chipcl-en-te-Friih. Derbyshire ' INTRODUCTION I have been asked to talk on the possible alternatives to asbestos as a friction material. The facts are that if by "alternative" we mean a new or better fibrr which might shortly be available as a replacement for asbestos in conventional friction materials, then I think the chances are so remote as to be not worth considering. This is not to say that if the world supplies of asbestos dried up we would not eventually find a substitute, but it could well be at the expense of a considerable loss in performance, and also at a considerable increase in cost. THE ROLE OF ASBESTOS IN FRICTION LININGS ` Having started on what may appear to be a rather depressing note, let us examine the reasons why asbestos occupies Die role of an almost universal constituent in friction materials at the present time. The first automotive friction linings were made from cotton textile material im pregnated with drying oils which were subsequently cured to form a strip of material which, while being flexible and conformable, still exhibited great mechanical strength. The'main function of the drying oil was to protect the cotton from attack by atmo spheric oxygen, which, even at the temperatures reached by early brakes, would have resulted in the cotton burning had its surface been exposed. A second function of the drying oil was to prevent the cotton from fraying under the influence of mechanical abrasion. . As brake operating temperatures increased,'it was found that cotton started to degrade and lose its strength even though still protected from attack by oxygen, fn ofhcrwords-rtistrfTercd thermal degradation instead Of oxidative degradation. Around 1910, a technological break-through was achieved when it was discovered that textiles could be woven front asbestos and used to replace cotton, because asbestos neither burns nor loses its strength below about 500"C. However, as brake operating conditions, became steadily more severe, a point was reached in about the middle 1940`s when, despite the continual improvement of synthetic resins with which the asbestos textile was impregnated, further heat resistance was required, this time because the organic part of the composite was proving inadequate. The solution proved to be to mould brake lining front powdered resins, using short fibre unwoven Thii paper was oripnally presented at a Conference on Exposure to Asbestos during Brake and Clutch Maintenance, held at Brentwood, Essex, March, 1969. ' .' M 26 D. Hatch asbestos to maintain the. necessary strength and flexibility. This made possible the inclusion of heavy-loadings or mineral and metal fillers which imparted greater heat resistance than could be obtained from'resin and asbestos alone, and did 'so at the expense of a reduction in asbestos content. The last point is important in the context of asbestos substitutes. The question is often asked, "why can't asbestos be replaced by something like glass fibre which, after all, roust surely impart sufficient strength?" The answer is possibly "it can", if merely the performance or a textile lining is required, but at the temperatures commonly met in modern brakes, the glass fibre melts even in depth below the operating surface. Furthermore, as a result of the embrittling effect of mineral and other fillers needed in today's brake lining, glass fibre is inadequate in imparting the necessary strength and flexibility. The same car. be said to a greater extent, of the various types of rock wools and mineral fibres which have appeared on the market over recent yean. It is interesting to note that not only is the ultimate strength of asbestos almost three times that of glass fibre, i.e. 550,000 Ib/in.1 but the modulus of rigidity is up to ten times as high, at a value of 30,000,000 Ib/in.1. Typical figures for other materials are say 100,000 Ib/in.1, for steel wool and down-at 60,000 Ib/in.1 for mineral wool. The current situation therefore, is that in drum brake linings of normal configura tion used for passenger can and commercial vehicles, there exists no substitute for asbestos which would not result in a deterioration of performance and strength. DISC BRAKE PADS Rates of energy dissipation continue to rise and we have now passed the point where moulded drum brake linings are adequate for a large percentage of passenger cars. In this country, nearly 50 per cent of the passenger car axles are equipped with disc brakes, and for the second time in nearly half a century, there seem to be technical possibilities of reducing the asbestos content of friction materials. It is purely for tuitous that the friction materials used in disc brakes are designed to a stronger shape, that 5, more or less square or circular pads of considerable thickness, supported, furthermore, b/ a metal plate of adequate stiffness. The friction material, therefore, does not have to stand up to handling during assembly, does not have to withstand riveting, and could, from the point of view of bulk mechanical strength alone, be made without a high loading of fibrous reinforcement of any kind. There remains, however, thermal shrinkage and thermal shock, and in order to prevent the formation .xif.SensiJcjccacks normal to the operating surface, a percentage of asbestos fibre is still retained. Nevertheless, it cannot be said that the use of asbestos in disc brake pads remains a technical necessity, and it is in this field of friction materials that some departure from resin-asbestos based composites could occur in the next few years on technical and performance grounds. FRICTION MATERIALS FOR CLUTCHES As yet, 1 have confuted my comments to brakes, but the development or friction materials for dutches has followed broadly similar lines. In recent years, however, we have seen the start of a growing replacement of manual dutches by automatic tr;; to sin Jea th; set frit of fre is.; res ret in aid sui per bee bcc imj wh but poi of SlO' pro the asty occ* Owe oliv but due by: occ has by the sun . jle the eater heat i to at the he context c replaced strength?" s required, melts even her fillers necessary ious types ver recent os almost y is up to materials t wool, ronfigure stitute for l! oint passenger with itcnnical jrcJy forwr shape, ipported, herefore, vithstand slone, be remains, ormation -vrc*is-slili . ke pads sat some years on * friction >owcvcr, Jtomatic 'Possible alternatives to asbestos as a friction material 27 transmissions where the clutch packs are immersed in oil and therefore do not give rise to airborne eontaminauonT' Tiic'Jrictidn^ materials' vrsetd'kre,"in any case, made from sintered metals or paper based facings in which the1'fibre is largely cellulose. This feature is made possible by the low operating temperatures, due in turn, to the fact that the materials are immersed in oil. - Quiches for heavy duty manual shift gear boxes are, in the U.S.A.., being super seded at a significant rate by the spider clutch. This is a type of dutch in which the friction material, instead of being in tite form of a thin annulus, consists of a number of buttons rather like disc brake pads, and these again could be made from asbestos- free material such as sintered metals'or cermets (ceramic-mctallic compositions). Summarizing the situation at the present time therefore, it can be seen that there is a drift towards the use of smaller quantities of asbestos in friction materials asre result of a number of purely performance considerations and design changes, that is, reduced amounts of asbestos in some disc brakes, and replacement by other materials in automatic transmissions and some dry dutches. `. OTHER FACTORS AFFECTING EXPOSURE TO ASBESTOS There aie, however, several other factors which enter the equation when con sidering the exposure to asbestos during brake and clutch maintenance, which is the subject of this meeting. The first is that the durability of friction materials is increasing, as is the tem perature at which they will operate satisfactorily. 1 mention these facts not only because they tend to offset the adverse effect of increase in vehicle miles per year, but because they affect the whole chemistry of asbestos in friction materials. The second important point is that the type of asbestos used in friction materials is chrysolite or white asbestos, and is different from blue asbestos or crncidolite, not only in colour, but in chemical composition and also in chemical decomposition. The third important foint is that under the influence of heat alone, chrysolite asbestos decomposes by loss of water of Crystallization at a temperature of around 6Q0C- The process is rather slow, taking well over an hour to convert SO per cent of the asbestos to its degradation products. These form a series of iron magnesium silicate materials known as olivine, the end member of which is forstcritc. The olivine series of minerals are quite unlike asbestos;* both physically. and chemically, being non-ftbrous, and in fact, widely occurring as the major part of many common rocks (basic and ultramaflc igneous). One would not expect that a significant amount of asbestos would he convened to olivine during braking except under conditions of very severe duly such as a fade test, but in fact it is exceedingly difficult to detect asbestos in friction maierial wear pro ducts. It has only on rare occasions been possible to detect asbestos in wear products by X-ray diffraction methods, and one has to resort to electron diffraction, selecting occasional individual small fibres to show its existence. This rather surprising fact has been known for several years and current theory suggests that it is accounted for by very high temperatures which arc said to exist at the points of real contact between the lining and drum or disc, even at bulk temperatures as low as I00C. Whether this explanation is correct or not, it remains a fact that very little asbestos survives in the wear products after brakes or clutches have been used under normal l 4 28 . D. Hatch . operating conditions. As the durability or brakes improves, farther reductions will - occur in the percentage of asbestos which .survives the wear process, because each particle of asbestos is then subjected to more heat treatment. Of the approximately 1 per cent of asbestos remaining in wear produets, a large proportion b ground down during the wear process to much less than the S u length at which particles are judged to be fibres. This is essy to understand when one considers that undcr'normal condi tions a lining wQl lose about one-millionth of an inch of thickness for every 20 ft of sliding. Over recent weeks I have had regular tests performed in which employees' private care, and trucks from the Company fleet have been brought into the garage for brake cleaning. The drums have been removed and in a garage with no forced ventilation, all loose dust has been removed from the drums and back plates by the use of a compressed air jet. As each brake was cleaned, instantaneous atmospheric samples were rapidly taken by a hand-pump from the centre of the dust doud. Several of the dust douds were visibly filthy, and would, in my view, be avoided by the operator; the dust counts token at the point of maximum dust density were as given in Table 1. Tails i. Durr counts duuno maxs clbamno ar ' C0MHU3SID AIMU Test Non-fibitnu Fibre count Fibre count No. above 2 p 2-5 p Above 5 p Simples 1 2 3 4 S 7 JO min vcjige . 2000+ >09 402 eo? 475 >313 621 IB! 306 11 n. 21 . 37 49 . 3S 5-0 5-4 2*1 2-5 3-7 5-3 >2 3-2 0-8 The interesting points from this exercise are, firstly, the ratio of the number of paetidcs .cf.asbcssos 40-tfae.mtal-Am panicles-of.a.8iroiiar..sizc,.*howiag.a .very.small percentage of asbestos, and, secondly, the distribution of size of the fibrous element, suggesting that those particles over 5 a must have been only just over. 1 am not suggesting for a minute that the practice of blowing out brake drums with a compressed air line is to be recommended, merely that an intense dust doud of this composition is a lot less harmful than one might think. . Unfortunately, at the present time, no corresponding data is available from clutches but one would expect the situation to be better than in brakes rather than worse, because on the one hand, surface rubbing speeds arc higher, giving rise to higher point contact temperatures, and on the other hand, wear rates are very much lower resulting in an even finer subdivision in the wear products. . I i in. in . Pfl otl to ne Cl Su M a Si ' 'IS will jc each oximateiy and down re judged nl condiy 20 It of s* private for brake niilaiion, use of a samples u! of the operator; Table 1. 9 Possible alternatives to asbestos as a friction material 29 POSSIBLE ALTERNATIVES TO ASBESTOS I have confined my comments to an account of changes which are taking place in friction materials at the present time in the normal course of technical development, in the belief that, although the use of raw asbestos must be tightly controlled, the wear products from friction materials are probably no more dangerous tohealth than many other dusts and can be treated as "merely dirty". If further evidence comes to hand to suggest that a substitute for asbestos must be found, then under the pressure of necessity, a list of possible replacements could be drawn up as follows:-- Glass fibre Low strength--low melting point--severe frictional instability. Steel wool Low availability--high cost--low strength--damage to drums. Mineral wool Very low strength--brittle to the extent of limiting mixing pro cesses. .Carbonfibres 100/lb currently forecast to fall to l/lb in several years' time. Slit! ten limes too expensive. ' Sintered metals Probably the best possibility eventually, but cost will always be and cermets relatively high. ber of small ment, s with *f this tbhes orse, ifher