Friction material containing steel wool as reinforcing agent

A friction material useful in automobile brakes and the like, comprises as a reinforcing agent steel wool coated with a thermosetting resin such as a phenolic resole resin.

BACKGROUND OF THE INVENTION 
The present invention relates to a high performance friction material for 
use in automobile brakes which consists of steel wool as a major portion 
of the reinforcing material. 
As used herein "friction particle" is intended to mean a particulate 
material having the properties of no substantial softening at elevated 
temperatures and a material which will not flow together or cohere with 
other particles, as would be the case with a "friction binder". A 
"friction particle" will not fuse with like friction particles, and is 
insoluble. A friction particle is held in place with a friction binder. 
As used herein, a "friction binder" has the properties of flowability and 
adhesive and cohesive binding action, for the purpose of binding together 
a reinforcing material and other additives (including a friction particle) 
necessary for building a brake lining or other similar article of 
manufacture. 
As used herein, a "friction material" is a composition useful as linings or 
facings in brakes, main clutches, and banded clutch facings of power 
transmission speed control structures of powerdriven devices such as 
automotive vehicles. Friction materials as now made are composed in 
general of a filamentous reinforcing material, bonded with a friction 
binder and containing other organic or mineral friction controlling agents 
such as friction particles. 
Conventionally, components of friction materials consist of asbestos as a 
major portion of the reinforcing component and thermosetting resins, 
cashew-nut-shell-oil friction particles, barium sulfate, graphite etc. are 
incorporated therein. However, a problem has recently occurred because 
asbestos contains a carcinogenic substance. On the other hand, automotive 
makers have become more severe in requirements for this kind of friction 
material for reduction of friction wear. As a result, some suppliers have 
already begun to substitute other materials for asbestos to meet the 
requirements of better performance of friction material or to circumvent 
the regulations imposed on asbestos. 
Among the reinforcements for non-asbestos friction materials, attention is 
called at the present time to steel wool because of its relatively low 
cost. However, when using steel wool for a reinforcement, the "wetting 
effect" of thermosetting resins, which perform as binders in the molding 
process, is inferior compared to asbestos. This results in a fatal 
drawback of lowering the strength of friction parts which is a vital 
physical characteristic. 
SUMMARY OF THE INVENTION 
To overcome the above described drawback, after much investigation, the 
present inventors have found that such a drawback can easily be overcome 
by using steel wool as a major portion of reinforcement with its surface 
being pretreated with a liquid thermosetting resin. The liquid 
thermosetting resins used in the invention are various kinds of phenolic 
resins, epoxy resins and melamine resins. As the friction articles are 
exposed to an elevated temperature and other several severe conditions, 
heat resistive strength (heat resistance) of the material is important. 
DETAILED DESCRIPTION OF INVENTION 
Phenolic resins are preferably used in this invention among the liquid 
thermosetting resins because of their excellent heat resistance. The 
phenolic resins preferred in this invention are, for instance, those made 
of a condensation reaction between one or several of such phenols as 
phenol, cresol, etc., and a formaldehyde or compounds emitting same, 
further those resins modified by cashew nut shell oil, polyvinylbutyral, 
vegetable oils, melamine, and epoxides. Among those resins, unmodified 
phenolic resins are more preferable to use because of their excellent heat 
resistance. 
Examples of phenols which can be used in preparing a phenol aldehyde resole 
for use in practicing the invention include ortho-, para-directing hydroxy 
aromatic compounds having 6 to 24 carbon atoms such as phenol itself 
(C.sub.6 H.sub.5 OH), naphthol, anthranol and substituted derivatives 
thereof where the substituents on the aromatic compound are independently 
selected from hydrogen, halogen such as Cl, Br, and F, and hydrocarbon 
radicals such as: 
a. alkyl groups or radicals of 1 to 60 carbon atoms, preferably of 1 to 30 
carbon atoms, and their various isomeric forms and substituted on the 
aromatic nucleus in the ortho or para position; 
b. cycloalkyl groups of 5 to 12 carbon atoms such as cyclohexyl, 
cyclopentyl, methylcyclohexyl, butylcyclohexyl, and so forth; 
c. alkyl, aryl and cycloalkyl ketonic groups wherein the hydrocarbon 
portion is as defined above in (a) and (b); 
d. alkyl, aryl and cycloalkyl carboxylic groups wherein the hydrocarbon 
part is defined as above in (a) and (b); 
e. aryl groups of 6 to 24 carbon atoms such as phenyl, naphthyl, anthryl, 
and the like; 
f. aryl substituted alkyl wherein the aryl is phenyl which may contain 
lower alkyl and/or hydroxy substituents so that the resulting hydroxy 
aromatic is, for example, a bisphenol; 
g. the corresponding oxyhydrocarbon radicals; and 
h. mixtures of the aforesaid hydroxy aromatics. 
Suitable substituted phenols include meta-methyl phenol, m-propyl phenol, 
m-isobutyl phenol, m-sec-butyl phenol, m-tert-butyl phenol, m-bromo 
phenol, m-chloro phenol, m-phenyl phenol, m-benzyl phenol, m-cetyl phenol, 
m-cumyl phenol, m-hydroxyacetophenone, m-hydroxybenzophenone, m-d-limonene 
phenol. The corresponding phenols substituted in the para-position can be 
used. 
Among the aldehydes which may be used within the scope of this invention to 
produce either the resole are formaldehyde or any of its variations, such 
as 37 percent or higher concentrations of formalin, or paraldehyde, 
acetaldehyde, propionaldehyde, isobutyraldehyde, isopentaldehyde, and the 
like. The aldehyde should have not more than 8 carbon atoms and should not 
detrimentally affect the resinification of the resin. Preferred aldehydes 
are those having from 1 to 4 carbon atoms, such as formaldehyde, which may 
be in aqueous solution (37 percent or higher), or in any of its low 
polymeric forms such as paraform or trioxane. Other aldehydes include 
para-aldehydes, furfural, 2-ethyl-hexanal, ethylbutyraldehyde, 
heptaldehyde and glyoxal, benzaldehyde and crotonaldehyde. 
The alkaline catalyst used in preparing the resoles to be used in this 
invention may be any of those known in the art; for instance, sodium 
hydroxide and calcium hydroxide. In general, the alkali metal hydroxides 
and the alkaline earth metal hydroxides and ammonium hydroxide and the 
amines such as triethanol amines may be used. 
Liquid phenolic resins are prepared in any of such forms as organic or 
aqueous solution and emulsion. 
When carrying out this invention, the organic solution is preferred 
otherwise there is a possibility of rusting the surface of the steel wool 
reinforcement. When an aqueous solution is applied in this invention, it 
is necessary to have a rust-preventive agent incorporated in the same. 
In this invention, phenolic resole resins are preferred because of their 
excellent ease of processing and "wetting effect" on steel wool. A 
preferable viscosity range of liquid resoles is 1 to 55 centipoises. The 
most suitable liquid resole is prepared by the process of adding 0.8 to 3 
mole, preferably 1 to 2 mole, of formaldehyde to one mole of phenols, e.g. 
phenol or cresol, reacting under such basic conditions as a pH 8 to 10. 
The resulting resin is either neutralized or left as it is thereafter. 
By the incorporation of 0.2 to 5 weight percent of such silane coupling 
agents as epoxy-silanes, amino-silanes, mercapto-silanes and vinyl-silanes 
into the liquid phenolic resins, the "wetting effect" of said resins can 
be greatly improved. 
Following are detailed embodiments of this invention. In the friction 
material of this invention, steel wool with filaments being 0.03 to 0.1 mm 
in diameter and 3 to 20 mm in length is used after it is pretreated by 
liquid thermosetting resins. In the pretreatment process, solid resin is 
coated on the steel wool in a proportion of about 0.3 to 5 weight percent 
of solid resin based on the weight of steel wool. It is no longer 
effective if said weight percentage is below about 0.3, and it is 
unfavorable for ease of processing if it exceeds about 5. Said 
pretreatment in this invention may be selected from any of such methods as 
mixing, impregnation, spraying, etc. of the liquid thermosetting resins so 
long as the liquid thermosetting resins are present as a "predetermined 
quantity" on the surface of steel wool, and the resins are treated by heat 
on said surface. On account of this heat treatment a reaction occurs and 
proceeds in said liquid thermosetting resins, from a half-hardened state 
to a complete hardened state with excellent adherence to the surface of 
the steel wool. 
The pretreatment process of the invention can be carried out in a variety 
of methods such as the following: 
(1) Dipping of steel wool into the liquid thermosetting resins and 
thereafter drying same under heat. 
(2) Spraying the liquid thermosetting resins onto steel wool during mixing 
of it in a mixer and drying same under heat thereafter. 
(3) Addition of the liquid thermosetting resins to mixer during mixing of 
preheated steel wool therein. 
The mixer employed in the aforementioned process is, e.g. Banbury mixer, 
Henschell mixer, kneader, double-cone type blender, and high-speed muller. 
It is also inclusive within the scope of this invention that fillers of 
relatively high density such as barium sulfate or graphite are 
incorporated with steel wool in equal or less content to same during said 
pretreatment of steel wool. To steel wool being mixed in a mixer, such 
binders as phenolic resins, drying oils, thermosetting resins exemplified 
by cashew nut shell oil modified phenolic resins are added at any 
arbitrary occasion during mixing, and also such conventional fillers as at 
least one selected from barium sulfate, dusty cashew powder or graphite 
are added. 
Pretreated steel wool should be present in a proportion of about 25 to 75 
weight percent of the entire friction material involved in this invention. 
This is because the friction material should have to endure several severe 
conditions, elevated temperatures and dynamic pressure when it is molded 
and put to use. Variations of its application require the incorporated 
ratio of reinforcement to be changed, according to the application, and 
further is because an even distribution of the desired quantity of 
incorporated reinforcement in the resin matrix is necessary to maintain 
the strength of the molded articles furnishing performance or preventing 
deterioration in physical properties when in use. 
If required, organic fibers or other inorganic fibers, pretreated or not by 
the same liquid thermosetting resins as are used in the pretreatment of 
steel wool, can be incorporated in the friction material. 
A typical friction material of the resin coated steel wool of the 
invention, contains about 25 to 75 weight percent up to 40 weight percent 
other inorganic filler and abrasives, about 5 to 15 weight percent organic 
filler, including the friction particle, and about 15 to 30 weight percent 
binder; all percents are by weight of total composition. 
The friction binder may be a phenolic resole or novolak resin prepared from 
the phenols and aldehydes disclosed herein. 
The abrasives, that is, the friction imparting agents and fillers, which 
may be used, include, but are not limited to, brass chips, metal shavings 
and filings, silica, talc, wood flour, chalk, clay, mica, fiber glass, 
felt, carbon black, graphite, metal nitrides and oxides, and ground cashew 
nut shell oil polymerizate. 
The mixture of components is molded under an elevated temperature and 
pressure. Further hardening reaction of the resin is accomplished through 
subsequent baking to produce friction articles.

This invention is clarified by the following examples: 
EXAMPLE 1 
A resole type phenolic resin with a 10 percent solid content was prepared 
by reacting phenol and formaldehyde (molar ratio 1:1.20) with ammonia as 
catalyst and diluted with methanol. 70 grams of this resin was sprayed 
onto the surface of steel wool (700 g.) rotated in a high speed muller, 
and heated immediately afterwards. After heated 30 minutes at 120.degree. 
C. with continuing mixing, a pretreated steel wool was obtained, having a 
coating of 1 weight percent of solid resin based on the weight of steel 
wool. 
Then, 700 g. of said pretreated steel wool, 100 g. of a novolac type 
phenolic resin, 100 g. of dusty cashew powder and 100 g. of barium sulfate 
are charged into a double-cone type blender, mixed for 15 minutes, and 
discharged as a uniform mixture. The mixture was molded at a temperature 
of 160.degree. C. and pressure of 200 kg/cm.sup.2 for 10 minutes. After 
baking at 180.degree. C. for 3 hours, test specimens were obtained. 
EXAMPLE 2 
The procedure of Example 1 was repeated, except that 5 percent of an 
amino-silane coupling agent, with respect to the solid content of the 
phenolic resin, was added to the resole type phenolic resin mentioned in 
Example 1. Specimens were obtained in the same manner as described in 
Example 1. 
EXAMPLE 3 
The procedure of Example 1 was repeated, except that the resole type 
phenolic resin mentioned in Example 1 was modified by adding 15 percent 
polyvinyl butyral with respect to the solid content of phenolic resin. 
Specimens were obtained in the same manner as described in Example 1. 
COMATIVE EXAMPLE 1 
The procedure of Example 1 was repeated, except that the steel wool was not 
pretreated by a liquid thermosetting resin. Specimens were obtained in the 
same manner as described in Example 1. 
The measured properties of specimens thus obtained in Examples 1, 2, 3 and 
Comparative Example 1 are listed in Table 1. 
For the test specimens of Example 1 and Comparative Example 1, electron 
microscope photographs of the broken cross-section were taken. 
Comparison of the photomicrographs indicates the resin successfully adheres 
to the surface of steel wool in Example 1. 
TABLE 1 
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Test Example Comparative 
Item Conditioning 
1 2 3 Example 1 
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Bending 
at ambient 700 750 650 440 
strength 
temp. 
(kg/cm.sup.2) 
at 200.degree. C. 
410 420 340 230 
Rockwell 
at ambient 87 89 87 61 
hardness 
temp. 
(Scale L) 
Density 
at ambient 2.95 2.95 2.93 2.92 
(g/cm.sup.3) 
temp. 
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