Document DMLKYz8GYx5J65kdXwaGyd8K4
FILE NAME Brakes BRK
DATE 1978
DOC BRK103
DOCUMENT DESCRIPTION Journal Article - The Chrysotile Content of the Wear Debris of Brake Linings
Weer 17 1978 315 321 'Elsevier Sequoia S A. Lausanne
Printed in the Netherlands
813
THE CHRYSOTILE CONTENT OF THE WEAR DEBRIS OF BRAKE LININGS
Notice This material may be pro-
tected by copyright law Title 17 U.S.
M. ROWSON
Code
The Higher Petroleum Institute Pubruk lahyar
Received May 31 1977
Summary
Electron microscopy of the wear debris from brake linings has indicated that the particle size is about 10 nm the characteristic chrysotile asbestos fibre being almost absent No evidence of the chrysotile structure was found by either ray diffraction or thermal analysis and it is suggested that the structure has been reduced to an amorphous state or to forsterite
1. Introduction
Present day drum linings and disc brake pads for road vehicles are moulded friction materials containing chrysotile asbestos in fibrous form with other mineral and metallic fillers the whole being bonded with thermo-
setting resins Some 20-50 wt of the friction material is asbestos so that
the debris produced in braking by the wear of the friction material is a potential source of asbestos fibre in the environment as suggested by Sullivan and Athanassiadis 1
The design of disc brakes is such that the wear dust is discharged
directly into the atmosphere whilst with drum brakes only a fraction is discharged the remainder being retained in the drum Lynch 2 has demon-
strated the very low asbestos fibre content of the dust usually less than 17 for both types of brake whilst Hickish and Knight 3 have shown that personal exposure to asbestos fibres by mechanics engaged in brake maintenance is well below the maximum exposure recommended by the Hygiene
Society 4 The above workers have suggested that the low chrysotile fibre
content arises because of the transformation of chrysotile to a different mineral form during braking leaving only trace quantities of the original fibres in the dust Lynch 2 did report however that it was not possible to collect all the wear dust produced in his tests
In the present work wear tests have been run under controlled condi-
tions to give a high collection efficiency for the wear debris Several techniques have been used to examine the asbestos component of the debris
Present address Department of Physics University of Aston in Birmingham Gosta
Green Birmingham Git Britain
Py Pos
316
2. Collection of wear debris
disc brake assembly was mounted on mertia dynamometer 5 so
that automobile braking could be simulated in the
surrounded the disc brake
laboratory A box
assembly and was made airtight by appropriate
seals The heat generated during working was dissipated by a coolant
stream The air was drawn by an extraction fan
air
fibre filter into the box and
through a commercial glass-
over the brake assembly In addition this
stream carried the wear debris onto a filter
air
gauze The filter
paper supported on a metal
paper was Whatman no 90 of put of air being 1700 min .
cm diameter the through-
The debris was produced under conditions of
deceleration usually from an initial
constant angular
brake
speed of 880 rev min 1 to rest The
brake application temperature measured by thermocouples rubbing the
dise was 175 C. These conditions
on
normal driving of a saloon
are typical of those which obtain in
tion
car Debris was also produced for a brake applica-
temperature of 40 C The examination of the debris
showed no significant differences for the two
reported later
consisted of material worn from the brake
temperatures The debris
from the dise
pads together with iron worn
against which the pads rubbed
3. Debris analysis
The use of the
material
apparatus to measure the wear rates of frictional
and the elemental identification of the
in the wear debris by neutron activation
components of the material
analysis- NAA has been reported
elsewhere 6 The following points are relevant to this work
analya sedMore than 80 of the wear debris produced was collected and
oVemnpanatilonel transformation b The quantity of asbestos in both the friction
debris was determined by The element chosen
material and the wear
NAA`through NAA`through the trace element concentration
the
was scandium which is present at a level of 4.2
in
raw chrysotile fibre Previous work had shown that the
ppm
did not alter on heating the fibre to
scandium content
possible to
a temperature of 900 C so that it is
measure the proportion of the original asbestos fibre
the debris independently of any chemical
present in
the wear process
or physical
during
c All other components of the friction material
wear debris and the
were present in the
tion material
proportions were the same as those for the original fric-
d Some 20 wt of the wear debris was iron
against which the pad rubbed
worn from
e Contamination of the wear debris was insignificant
the disc
oe.
. A box
agit by appropriate
d by a coolant air
commercial glass-
addition this air
ported on a metal
ameter the through-
onstant angular nin,,to rest The ples rubbing on the
those which obtain in
d for a brake applica-
is reported later The debris
her with iron wom
3.1 Electron microscopy
Samples of unused and used pad surfaces and of wear debris were
prepared as dilute suspensions in distilled water Ultrasonie vibration was used disperse the particles There was no significant difference between the micrographs of the used and unused pad surfaces In all such micrographs there was a profusion of asbestos fibres and fibrils Fig 1 and this was further illustrated by characteristic electron diffraction patterns The micrographs of the wear debris Fig 2 showed particles of size 10 - 20 nm together with larger agglomerates There were no fibres present 7
es of frictional ents of the material
has been reported
work was collected and
aterial and the wear
tent concentration level of 4.2 ppm in the scandium content
0 C so that it is tos fibre present in insformation during
vere present in the
for the original fric-
orn from the disc
Fig 2. Electron micrograph of wear debris Magnification 100 000
3.2 ray diffraction
the in
Crystalline chrysotile asbestos was identified in the pad material but not
in the wear debris Since the absence of the characteristic chrysotile diffrac-
tion pattern may be due to particles which are very small and highly strained
~
318
rather than to the absence of chrysotile material wear debris was heated in
vacuo at 1100 in an attempt to increase the particle size of any crystalline components of the debris ray diffraction of this material showed welldefined lines and the mineral forsterite was positively identified When chrysotile asbestos fibre is heated it loses its water of crystallization at about 650 C and at 780 C is transformed into forsterite according to the reaction
2 3MgO chrysotile
3 2MgO + SiO2 + 410
forsterite
3.54
UNITS
3.0
UNITS
UNITS
UNITS
2.5
ARBITRARY
ARBITRARY ARBITRARY
ARBITRARY 2.0
ARBITRARY
ARBITRARY
ARBITRARY
1.5- 1.5-
DIFFERENCE DIFFERENCE DIFFERENCE
DIFERENCE 1.0
DIFFERENCE DIFFERENCE
DIF ERENCE
DIFFERENCE
TEMPERATURE 0.5
TEMPERATURE
TEMPERATURE
TEMPERATURE
0.0
TEMPERATURE
TEMPERATURE
TEMPERATURE
TEMPERATURE
. 0.5
Exothermic Endothermic
a .
QL
ee
kame
ae ot
caamet
5
?
LZ
nme Q
a
200
400 TEMPERATURE
600
800
1000
Y
deat
a
Bow 6.5 4
Exothermic
a2
iestam ar)
.
=
5 0.0 J
ag
cm
>
4 anet e? 5 -0.5 J
a8
~
Endothermic
5
fi
T
T
T
Ly
T
0
200
.
400
600
800
1000
b
TEMPERATURE
c
Sieneamnel
Fig 3. a DTA curve of chrysotile asbestos run in nitrogen b DTA curve of wear
cont
.
debris run in nitrogen )
comes
[ewrmces.
A curve of wear
3.3 Thermal analysis
The dehydroxylation of chrysotile asbestos which occurs at about
650 is an endothermic reaction accompanied by a mass loss of about 137
of that of the original sample The transformation to forsterite which occurs
at 780 C is exothermic These energy and mass changes are observed directly by differential thermal analysis DTA and thermogravimetric analysis TGA respectively Figures a and a show the behaviour of 80 mg of
chrysotile asbestos when slowly heated from room temperature to 950 (
the changes indicated above being clearly displayed When debris containing
the equivalent of 50 mg of chrysotile asbestos was heated the results were as
shown in Figs b and b Neither the endothermic reaction at 650 nor
he
the exothermic reaction at 780 C is observed on the DTA trace Further
Booed
tii
80 4
An
)
MGMS
ade
MGMS MGMS
MGMS
(
dicbMetns
Ste
- _;
MASS 75
MASS
indet
SAMPLE SAMPLE SAMPLE
SAMPLE
SAMPLE 70 abd
ah
0
,
200
400
600
806
1005
TEMPERATURE
4 170
)
MGMS MGMS MGMS MGMS
(
150 MASS
MASS
MASS
SAMPLE SAMPLE SAMPLE SAMPLE SAMPLE
rT
0
200
7
400
T
600
T
800
a
1000
b
TEMPERATURE
Fig 4. a TGA curve of chrysotile asbestos run in nitrogen TGA curve of wear debris run in nitrogen
ais. ae 2
7 rue)
Eee
si Pen sg eeeate oeeros ior
wee
ka ght Stn: Ret
eae ae nets Ee
ce
ie
.
Bees Sick ara 5 SES; ee ~IPE,
PORE
Ae
xs ee
320
there is no indication on the TGA trace of mass loss 650 C although there is a mass loss at about 780 ( which is as yet unexplained There is thus no evidence of the chrysotile forsterite transformation when wear debris debris is heated to 950
All the thermal analyses were carried out in an atmosphere of nitrogen
to prevent the effects of oxidative reactions of other components components of the frie-
tion material masking observation of the chrysotile forsterite forsterite transforma-
tion
~-
4. Discussion
ae
poe
ane The electron micrographs clearly showed that the wear debris consisted
-_ , of finely divided particles in the 10 nm range The larger particles appeared
to be agglomerates ranging above 50 nm in size Witen examined with an
wer optical microscope the debris appeared relatively coarse grained but
e individual particles could be readily subdivided until the limiting resolution
of the microscope was reached at about 100 nm This tendency to agglomerate may explain the high collection efficiency with a Whatman no 90 filter
It was evident from the micrographs of the pad surface material that the
method of sample preparation was adequate and that chrysotile fibrils
te
vee should have been detected if they had been present the wear debris
Several samples were prepared and many fields of view were examined A
normal fibri was found in only one field of view suggesting a fibril concenot
tration in the wear debris of less than 0.5 wt This is even less than the low
ae
e concentration reported by Lynch 2 This author does not report whether
his samples were analysed to confirm the elemental composition The tests
me reported here indicate that the debris has the same elemental composition as
e e the original material so that the chrysotile component worn from the pad is
present in some chemical form in the debris but certainly not its original
fibril form
owe
ems Owing to the severe transient conditions of stress or temperature during
the wear process the crystalline structure of asbestos fibrils will be disrupted The most frequently occurring chrysotile fibril is a tube of inner diameter
nm and outer diameter 26 nm 8 these magnitudes are similar to those
we
of the particle size of the debris Further the particle size is so small that it
approaches the limits at which coherent scattering e"o nccur
The ray analysis does not show the presence of chrysotile asbestos
and this may be due to the particle size A further possibility is that frictional heating has decomposed the chrysotile If the temperature at which the wear products are produced is in the range 780 T then the
chrysotile will be reduced to an amorphous form with no recognizable
crystallinity and if it above this temperature forsterite may be formed The
presence of forsterite in debris heated to 1100 is not definite evidence of the formation of forsterite during wear but one can assert that the original
mies, chrysotile structure is not sufficiently degraded for the magnesium and silica
to have reacted appreciably with other components of the debris
loslosss at 650 C although
ret unexplained There is thus mation when wear debris is
an atmosphere of nitrogen
other components of the fric-
forsterite transforma-
hat the wear debris consisted
he larger particles appeared
When examined with an ely coarse grained but
"
the limiting resolution endency to agglomer
th a Whatman no 90 filter urface material that the d that chrysotile fibrils
sent in the wear debris f view were examined A
suggesting a fibril concenThis is even less than the low
rdoes not report whether tal composition The tests e elemental composition as
nent worn from the pad is certainly not in its original
stress or temperature during
tos fibrils will be disrupted
a tube of inner diameter itudes are similar to those ticle size is so small that it
gran occur
ce of chrysotile asbestos her possibility is that . If the temperature at Inge 650 - 780 C then the
1 with no recognizable sterite may be formed The
definite evidence of
. assert that the original or the magnesium and silica ts of the debris
321
The thermal analyses do not show any behaviour consistent with crystalline chrysotile and this could be because forsterite is present in the wear debris However there is insufficient information concerning the behav iour of small particles of either chrysotile or forsterite to be certain of the
interpretation of these data
The elements of the original mineral chrysotile however are present in the wear debris If the health hazard of chrysotile is found to be associated with its chemical behaviour in body tissue rather than with its physical form the
implications of these present findings should be considered
References
1 R. Sullivan and Y. Athanasstadis Nat Air Pollut Control Adm U.S. Publ APTD Ser 69 1969 27
J. R. Lynch Air Pollut Control Assoc 18 1968 12
3 E. Hickish and K. Knight Ann Occup Hyg 13 1970 1 4. R. Lane J. M. Barnes D. E. Hickish G. Jones A. Ronch and E
Occup Hyg 11 1968 17
5 R. Parker Engineering London 167 1919 219 6 H. D. Bush D. M. Rowson and S. Warren Wear 20 2 1972 211 211
M. Rowson Ph.D. Thesis Univ of Bradford 1971
8 W. A. Deer A. Howie and J. Zussman Rock Forming Minerals Vol
Green Harlow 1962
King Ann
II Longmans