Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND 0~ THE INV~NTION
The invent;on herein relates to friction materials for
brakes suitable for railroad service.
5Over the years a number of different types of composition
railroad brake shoes have been described. These have normally been
composed of a ~ubber and/or resin matrix heavily reinforced with
asbes~os fiber and containing lead in the form of elemental lead
and/or lead oxides and friction modifiers such flS iron chromite,
10silicon carbide and the like as critical components. Typical
compositions have been describecl in U.S. Pat. Nos. 2,686,140;
2,861,964; 3,152,099; 3,390,1139 l,832,325 3,959,194 and 4,]78,278.
Many of the materials described in the afore-mentioned patents have
enjoyed widespread com~ercial success as railroad brake shoe
15compositions under the trademark COBRA. However, environmental and
health concerns have led purchasers and therefore manufacturers to
seek friction material compositions which contain neither asbestos
fiber nor lead metal or its compounds. Recently, a composition
containing little or no lead was disclosed. This was found to be
20quite advantageous in reducing wheel wear, particularly where softer
steel wheels are used. See the aforesaid U.S. Pat. No. 3,959,194.
Efforts to eliminate asbestos fiber from the brake shoe compositions
have been beset with major difficulties, primarily because the
asbestos fiber contributed a unigue combination of reinforcement and
25thermal properties to the compositions. During service application
brake shoes under~o significant physical stresses and also
experience substantial temperature increases, often reaching peak
t0mperatures in excess of 800~. Previously, fibrous materials
other than asbestos such as certain or~anic fibers, steel fibers and
30combinations of steel fiber and metal grit have been found suitable
to adeguately withstand the thermal and physical forces involved in
railraod service.
Conventional AAR (Association of American Railroads)
service for standard freight cars constitutes the large majority of
35the uses of friction materials and therefore many of the previous
compositions were directed to such end useO Conseguently, a basic
friction material composition for molcled brake shoes which is
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nsbestos-free and suitable for ~Ise in AAR service has been sollght,
which woulcl meet the service re(luirements and be economical to
manufacture.
BRI~F SUMMARY OF TH~ INVENTION
The invention herein i8 a molded fiber reinforcement-free
friction material sui~able for railroad use which comprises~ in
percent by volume:
Phenolic Resin Binder: 2-15%
Carbonaceous Component: 30-60%
Qrganic Component: 15-30~
Inorganic Component: 5-35%
Friction ~odifiera 1-15~
Additives: 1- 7%
In the pre~erred embodiments the composition may also
contain from 5~ to 10~ by volume of friction moclifiers and from 5
up 10% by volume of phenolic resin binder; the carbonaceous
component 30-50~ by volume and the inorganic component from 15% to
25% by volume and the organic component from 20~ to 25~ by volume.
D~TAIL~D DESCRIPTION AND PREFERRED EMBODIMENTS
The product of this invention is used in various types of
railroad services which are categorized on the ba~is of the contact
pressure which is applied across the face of the brake shoe during
typical brake application. In service the actual pressures will
depend on the type of railroad eguipment on which the brake is used,
whether li~ht to emergency brake application is considered, and
similar factors. The distinctions, however, are evident and will be
clearly understood by those skilled in the art. In addition, AAR
service and heavy duty service commonly employ tread brakes as
contrasted to disc brakes.
It is readily apparent from the foregoing that any
particular composition selected for AAR use must have good thermal
properties as ~ell as undereoing an evaluation of the effect the
material will have in use on the metal wheel. These factors and
others combine to ma~e the selection of components more of an art
than a precise science. Small changes in tha proportion of one
component with respect to another can produce considerable
differences in the f;nal performance of the brake shoe. For example
some combinations of materials ~hile e~hibitin~ ~ood friction
chflracteristics in use may prove to be too hard on the metal
railroad wheel to be satisfactory for long term use.
With thes~ many considerations and others in mind the
composition of the present invention has surprisin~ly been found to
exhibit the desired characteristics ~or a molded brake shoe friction
material to be a satisfactory substitute for asbestos or other fiber
reinforced materials in railroad service.
The first and largest component i~ the present composition
0 i9 an or~anic component which consists of a rubber component bonded
together with a phenolic resin. The binder component and rubber
matrix are the vehicles in which the other components are embedded.
The or~,anic component may contain mixtures of natural or synthetic
rubber, to~ether with e].astomeric materials or a phenolic resin
which may be vulcanized or otherwise cured to form a hard matrix for
the remaining components. Of the rubbers preferred are the butyl
rubbers, styrene-butadiene rubbers or nitrile rubbers. A number of
varieties of the various natural or synthetic rubbers and phenolic
resins are readily available and are widely described in the
litsrature. Numerous rubbers are described in the Vanderbilt Rubber
Handbook published by the R. T. Vanderbilt Company in 1968.
Phenolic resins ~enerally applicable for use in the present
invention are described in the Modern Plastics Encyclopedia, Vol. 4,
No. 10 (1970~1971). Cashew nut shell resins are preferred,
primarily due to their excellent bondin~ and heat resistant
properties. Such resins and their method or preparation are
discussed in U.S. Patent Nos. 2,686,140 and 2,861,964. The or~anic
component may in some embodiments be composed entirely of rubber,
although most preferably the or~anic component will comprise a
mixture of rubbers and phenolic resin~
Tha components listed herein are variously measured as
percent by volume rather than the more common percent by weight,
except where noted. This i9 done because the volume percenta~e
reflects the physical properties of a given shape and size of brake
shoe despite variations in the actual chemical composition of the
shoe, since in brakin~ service an important factor is the amount of
each material which is exposed at the braking surface which is in
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contact with the wheel tread. It also simpl;fies the ~escription of
the material of the present invention in that it avoids the
necessity Eor recalculation of components to account for varying
densities of materials selected, such as in the organlc component.
The second component of the composition and one of the key
components in any friction composition for brake shoes are the
friction modifying a~ents describecl more fully hereinafteI.
A further component of the present composition will be
inorganic fillers. These may be fl wide variety o~ granulated
inorganic materials which clo not substantially affect the friction
properties of the composition, but are rather used to fill out the
composi~ion and rectuce the necessity for usin~ more of the expensive
organic component. In addition, they also provide thermal
properties to some degree, in that they serve as heat sinks.
Typical of the materials which may be used include alumina, barytes,
silica, iron oxides, whiting, talc, diatomite9 clays and the like as
well as mixtures of these various materials. Particle sizes will
normally be in the ran~e of from 4 to 75 microns. The inor~anic
fillers will normallg be present as from 5 to 35 percent by volume,
preferably 15 to 25 percent by volume of the composition. In the
present invention it is important that the inorganic filler
component contains neither asbestos fiber nor lead metal or lead
compounds.
The third component of the present composition is a
carbonaceous component which is composed of carbon particles in any
of a variety of forms, such as carbon black, carbon flour, graphite
or gound anthracite. A sin~le material such as carbon black or
mixtures of the various carbon-containin~ materials may be used.
This component will normally be present in from 30 to 60 volume
percent, preferably 30 to 50 volume percent. Normall~ the particle
size of the carbonaceous particles will be in the range from .02 to
150 microns. These materials have been and are incorporated in
friction materials for several reasons includln~ color, to provide
resistance to U.V. degradation of the polymerized materials,
reinforcement of the cured rubber component, and the like.
The final materials required in the present invention are
"additives" used to volcanize, cure or otherwise modify the rubbers
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and resln~, in the or~anic component. These will naturally vary
depend-lng upon the nature of the particular or~anic component used,
and the fina] physical properties sought but in general will include
mAterials such as sulsphur, zinc oxide, thiazoles, sulfenamides,
dithiocarbamates, pero~ides, anti-oxidants, retarders and other
processin~ aids. A wide variety of such materials are described in
the patent literature ancl the above-mentioned Vanderbilt Rubber
Handbook. l'he aclditives ~ill be present as from about 1 to 7 volume
percent, preferably 2 to 4 volume percent.
The composition therefore may contain in the cateeories
described: cashew nut shell resin and friction modi~iers such as
those inorganic materiflls which substantially affect the friction
properties of the material. For the purposes of this invention, it
will be considered that such friction modifiers are commonly
minerals or ceramics havin~ a ~ohs hardness or ~reater than 5.
Typical of such materials are silicon carbide, zircon, garnet, iron
chromite and similar materials. The friction modifiers will be in
the form of granules having particle sizes in the ran~e from about
0.05 to 200 microns. The friction modifiers will be present as up
to 15% volume percent, preferably 5~ to 10% volume percent, of the
composition. While the friction modifiers employed herein have been
found to have utility in varying amounts in other bra~e shoe
compositions includin~ asbestos-reinforced friction compositions and
compositions containing steel fibars alone or in combination with
metal particles or "grit", the individual use in the amounts
disclosed in a brake shoe composition havin~ no reinforcing fiber is
somewhat surprising. It will be evident to those skilled in the art
from the descriptions of the examples below how best to mix and
otherwise formlllate the composikions of the present invention.
Additional descriptions of various types of processin~ equipment
including mixers and molds may be found in Stern, ubber: Natural
and Synthetic (2d en. 1~67) and Rubber World, ~achinery and
~g~ _nt for Rubber and Plastics (2d edn, 1963) and the patent
literature.
The followin~ mo].ded brake compositions is ~ preferred
embodiment of this invention, with al:L components listed as percent
by weight and by volume:
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Ex~ e I
~ BY WEIGHT ~ BY VOLUNE
Crumb Rubber with 10~ tfllc 12.74 23.36
~ubbermakers' Sulfur0.51 0.45
TMTD Rubber Accelerator0,25 0.33
MBT Rubber Accelerator0.25 0.31
Antio~idant 0.].3 0.22
Ste~ric Acid 0.13 0.2~
Zinc Oxide 0.64 0.21
Carbon Black Powder 9.17 9.33
Carbon Flour lO.]9 9.33
Iron Chromite 14.0l 5.81
Red Iron Oxide 9.94 3.83
Silicon Cflrbide Nicrogrit 0.64 0.36
Carbon Base Filler 7.39 9.21
Rubbermakers' Hard Clayl8.85 13~39
Hard Rubber Dust 6.37 8.58
~ineral Rubb0r 3.18 5.61
Liquid Cashew Nut Resin5.lO 9.48
Hexamethylenetetramine0.51 0.91
Total lOO.OO lOO.OO
Certain modifications in the foregoirlg composition can be
made without losing the overall advantages of the molded friction
material of the present invention. For example the antio~idant can
be eliminated altogether as well as the stearic acid where
unnecessflry in the molding procesfi. Variations such as these are
commonly made in such formulations and can be made in practice
without deleteriously affecting the advantages provided by the
overall composition.
The important contribution described herein is that a
friction co~position as described herein can be satisfactory for AAR
service without the presence of reinforcing fiber of any kind in a
brake shoe tread material which pflsses the ob~ective tests and other
requirements for AA~ service. The ~mounts of friction modifying
agents, additives and processing and other aids can be Adjusted by
one skilled ln the art to achieve the desired results.
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The invantion ;~ therQîor~ only limit~d by the scope o~ the
appended c 1 a im~ .
