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Comment on “Physical and chemical changes of organic disc pads in service”
Weor, 51 (1978) 385 - 386 0 Elsevier Sequoia S.A., Lausanne
385 - Printed
in the Netherlands
Letter to the Editor
Comment
on “Physical
and chemical
changes of organic disc pads in service”
The method of measuring the temperatures quoted by Jack0 [I] was defined, but assuming that they were rotor surface temperatures measured by rubbing thermocouples, the work reported in the paper complements earlier work, reported by Bush et al. [Z] , Rowson [3,4] and Lynch [5], in which little or no free asbestos fibre was found in the wear debris from disc brake pads. Rowson obtained brake application temperatures (measured by rubbing thermocouples on the disc) up to 175 “C whilst Lynch used conditions of “normal driving”. However, Jacko [l] shows that the rubbing surface of the disc brake pad is reduced in asbestos content, whilst by neutron activation analysis Bush et al. [2] showed that the asbestos component in the debris was in the same proportion to the other pad constituents as in the unused pad formulation up to an initial brake application tempemture of 150 “C. Other work f3] showed this to be true up to 250 “C. Their pads contained phenol& resin, friction dust, rubber, barytes, chrysotile asbestos, chrome green and copper particles; this is a very similar formulation to that used by Jacko [2]. Degradation of the phenolic resin, rubber and friction dust was also noted in this work and the observed exponential increase in pad wear with application temperature from 70 to 250 “C was explained in terms of the thermal degradation of the phenolic resin matrix which causes a softening of the material, provided that the temperature of the real area of contact was some 300 “C higher than that shown by rubbing thermocouples on the rotor. There was no indication of olivines nor of chrysotile asbestos when the original debris was subjected to an X-ray analysis, which is contrary to the finding of Jacko [l] . In addition thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of debris produced at a brake application ~rnpemtu~ of 175 “C showed none of the changes associated with the dehydroxylation of chrysotile and its subsequent transformation to forsterite (one of the olivine series of minerals) although sintering the debris to 1100 “C in uucuo gave sharp X-ray diffraction patterns of these olivines not
[41’ It has also been shown that the wear of organic disc brake pads is extremely dependent on the immediate pest history of the sample, requiring a finite time at the operating temperature to reach an equilibrium value [3] . This was shown by using previously unheated pads on a brake taking the app~cation temperature to 150 “C by making three repeated brake applications from 880 rev min-l to rest at a deceleration of 1.93 m se2 and holding it at this temperature whilst measuring the subsequent wear. The results are
386
Number Fig.
1. Effect
of stops
of temperature
soak on the wear of organic
friction
pads at 150 “C.
shown in Fig. 1, which indicates that it took nearly 50 brake applications before an equilibrium wear rate was reached. Thus it is likely that the frictional heat affected layers (FHALs) studied by Jacko [l] were not at their equilibrium state in view of the few brake applications used in each fade test and recovery test sequence. D. M. ROWSON Department Birmingham
of Physics, University (Gt. Britain)
of Aston
in Birmingham,
Gosta Green,
M. G. Jacko, Physical and chemical changes of organic disc pads in service, Wear, 46 (1978) 163 - 175. H. D. Bush, D. M. Rowson and S. E. Warren, The application of neutron activation analysis to the measurement of the wear of a friction material, Wear, 20 (1972) 211 225. D. M. Rowson, Ph.D. Thesis, University of Bradford, 1971. D. M. Rowson, The chrysotile content of the wear debris of brake linings, Wear, 47 (1978) 315 - 321. J. R. Lynch, Brake lining decomposition products, J. Air Pollut. Control Assoc., 18 (1968) 824 - 826.
(Received
March
21, 1978)
-
385 - Printed
in the Netherlands
Letter to the Editor
Comment
on “Physical
and chemical
changes of organic disc pads in service”
The method of measuring the temperatures quoted by Jack0 [I] was defined, but assuming that they were rotor surface temperatures measured by rubbing thermocouples, the work reported in the paper complements earlier work, reported by Bush et al. [Z] , Rowson [3,4] and Lynch [5], in which little or no free asbestos fibre was found in the wear debris from disc brake pads. Rowson obtained brake application temperatures (measured by rubbing thermocouples on the disc) up to 175 “C whilst Lynch used conditions of “normal driving”. However, Jacko [l] shows that the rubbing surface of the disc brake pad is reduced in asbestos content, whilst by neutron activation analysis Bush et al. [2] showed that the asbestos component in the debris was in the same proportion to the other pad constituents as in the unused pad formulation up to an initial brake application tempemture of 150 “C. Other work f3] showed this to be true up to 250 “C. Their pads contained phenol& resin, friction dust, rubber, barytes, chrysotile asbestos, chrome green and copper particles; this is a very similar formulation to that used by Jacko [2]. Degradation of the phenolic resin, rubber and friction dust was also noted in this work and the observed exponential increase in pad wear with application temperature from 70 to 250 “C was explained in terms of the thermal degradation of the phenolic resin matrix which causes a softening of the material, provided that the temperature of the real area of contact was some 300 “C higher than that shown by rubbing thermocouples on the rotor. There was no indication of olivines nor of chrysotile asbestos when the original debris was subjected to an X-ray analysis, which is contrary to the finding of Jacko [l] . In addition thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of debris produced at a brake application ~rnpemtu~ of 175 “C showed none of the changes associated with the dehydroxylation of chrysotile and its subsequent transformation to forsterite (one of the olivine series of minerals) although sintering the debris to 1100 “C in uucuo gave sharp X-ray diffraction patterns of these olivines not
[41’ It has also been shown that the wear of organic disc brake pads is extremely dependent on the immediate pest history of the sample, requiring a finite time at the operating temperature to reach an equilibrium value [3] . This was shown by using previously unheated pads on a brake taking the app~cation temperature to 150 “C by making three repeated brake applications from 880 rev min-l to rest at a deceleration of 1.93 m se2 and holding it at this temperature whilst measuring the subsequent wear. The results are
386
Number Fig.
1. Effect
of stops
of temperature
soak on the wear of organic
friction
pads at 150 “C.
shown in Fig. 1, which indicates that it took nearly 50 brake applications before an equilibrium wear rate was reached. Thus it is likely that the frictional heat affected layers (FHALs) studied by Jacko [l] were not at their equilibrium state in view of the few brake applications used in each fade test and recovery test sequence. D. M. ROWSON Department Birmingham
of Physics, University (Gt. Britain)
of Aston
in Birmingham,
Gosta Green,
M. G. Jacko, Physical and chemical changes of organic disc pads in service, Wear, 46 (1978) 163 - 175. H. D. Bush, D. M. Rowson and S. E. Warren, The application of neutron activation analysis to the measurement of the wear of a friction material, Wear, 20 (1972) 211 225. D. M. Rowson, Ph.D. Thesis, University of Bradford, 1971. D. M. Rowson, The chrysotile content of the wear debris of brake linings, Wear, 47 (1978) 315 - 321. J. R. Lynch, Brake lining decomposition products, J. Air Pollut. Control Assoc., 18 (1968) 824 - 826.
(Received
March
21, 1978)
-