Abstrict Heat-treated palygorskite clay, such as attapulgite having controlled
low moisture and carbonate content, is used with steel wool or other
non-asbestos fiber in asbestos-free friction material compositions
such as clutch linings, brake pads, truck blocks and the like. These
compositions include conventional fillers, lubricants, wear modifiers
and the like in order to achieve friction and wear properties comparable
to asbestos-based materials.
Claims I claim:
1. A friction material composition suitable for molded friction
pads comprising a thermosetting binder, a non-asbestos fibrous material,
particles of heat-treated palygorskite clay, said clay having a
volatile matter content below about 5% on a free-moisture-free basis
and a carbonate content of not more than about 5% CO.sub.2 by weight
on a free-moisture-free basis, and additional conventional fillers
and additives, said composition being substantially free from asbestos
and said palygorskite clay being present in amount sufficient to
provide acceptable wear and high temperature friction properties.
2. A friction material composition suitable for molded friction
pads comprising a thermosetting binder, a non-asbestos fibrous material,
particles of heat-treated attapulgite clay having a volatile matter
content below about 5% on a free-moisture-free basis and a carbonate
content of not more than about 5% CO.sub.2 by weight on a free-moisture-free
basis, and conventional fillers and additives, said composition
being substantially free from asbestos and containing from about
10% to 35% by weight of said attapulgite clay.
3. The composition of claim 1 or 2 wherein said binder is present
in amount in the range of 10 to 40% by weight and said non-asbestos
fibrous material and said clay are present in combined amount in
the range of 20 to 60% by weight.
4. The composition of claim 1 or 2 where said additives comprise
friction and wear modifiers.
5. The composition of claim 4 where said wear modifiers comprise
organic polymeric fibers and carbon lubricant in amounts sufficient
to achieve wear properties comparable to asbestos-formulated friction
materials.
6. The composition of claim 1 or 2 wherein said non-asbestos fibrous
material is steel wool and comprises from about 10-50% by weight
of said composition.
7. The composition of claim 1 or 2 which contains barytes as an
additional filler in amount in the range of about 10-40% by weight
of said composition.
8. A friction material composition suitable for molded friction
pads comprising a thermosetting binder, a non-asbestos fibrous material,
particles of heat-treated attapulgite clay, said clay having a volatile
matter content below about 5% on a free-moisture-free basis and
a carbonate content of not more than about 5% CO.sub.2 by weight
on a free-moisture-free basis, and additional conventional fillers
and additives, said composition being substantially free from asbestos
and said attapulgite clay being present in amount sufficient to
provide acceptable wear and high temperature friction properties.
9. An asbestos-free friction composition suitable for molded friction
pads consisting essentially of 10-40% by weight of a thermosetting
binder, 10-50% by weight of a non-asbestos fibrous material, 10
to 35% by weight of particles of heat treated attapulgite clay having
a volatile matter content below about 5% by weight on a free-moisture-free
basis and a carbonate content of not more than about 5% CO.sub.2
by weight on a free-moisture-free basis and conventional fillers
such as barytes and conventional friction and wear modifiers in
amount sufficient to achieve wear properties equivalent to asbestos-formulated
friction materials.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to friction materials such as clutch linings,
brake pads, linings, blocks and the like. More particularly this
invention relates to asbestos-free formulations of friction materials
which incorporate a palygorskite clay exemplified by attapulgite.
2. Prior Art
Friction material compositions currently used in clutch and brake
linings of vehicles, motors and other machinery must be capable
of withstanding severe operating temperatures and high pressures
experienced during repeated applications. In addition the materials
must have surface coefficient of friction suitable to the requirements
of the application and must have wear properties to balance the
cost of their use and avoid undesirable side effects such as glazing,
squeak, fade and grooving in the mating surfaces. Compositions in
general use comprise a thermoset binder exemplified by phenolic
molding compositions, or novolak resins, a fibrous reinforcement
to prevent physical degradation and deterioration in performance,
various fillers and other additives including wear modifiers, lubricants,
friction modifiers and the like. The fibrous reinforcement in widest
use is asbestos. The asbestos provides the phenolic molding composition
with superior mechanical and thermal properties but constitutes
a health hazard to those exposed to it.
Compositions containing other fibrous reinforcement as an asbestos
replacement have been investigated, but such materials have had
limited commercial acceptance. When formulating asbestos-free friction
materials, end-use properties such as friction coefficient and wear,
heat resistance and mechanical strength should be equivalent to
asbestos-containing compositions. In addition, processing properties
such as moldability and shrinkage should also be equivalent.
In U.S. Pat. No. 3835118 to Rhee et al. the use of steel fibers
in combination with sponge iron filler to replace asbestos is disclosed.
In U.S. Pat. No. 3922241 to Barker et al the use of a novolak
resin with aluminum silicate (not further defined), talc and cellulose
fiber in place of asbestos is disclosed. Other asbestos-free formulations
include those disclosed in U.S. Pat. No. 4119591 to Aldrich, wherein
steel and cellulose fiber are used and carbon fiber, mineral fiber
and glass fiber are disclosed; U.S. Pat. No. 4197223 to Bartram,
in which glass fiber, mineral fiber and cellulose are disclosed
as an asbestos replacement, and U.S. Pat. No. 4226759 to Chester,
in which steel fiber alone is disclosed. In order to meet performance
and processing criteria, such as those mentioned above, substituting
another material for asbestos is generally not sufficient in itself;
rather the overall formulation often requires change or adjustment
to meet the demands the product must satisfy.
Thus an object of this invention is to provide a friction material
composition formulated without asbestos but having performance and
processing characteristics equivalent to existing products made
with asbestos.
THE INVENTION
I have discovered that asbestos-free friction material having acceptable
performance may be made by using a palygorskite clay, for example
one selected from the group attapulgite, sepiolite and mixtures
thereof, a non-asbestos fibrous reinforcement and conventional additives
such as wear modifiers and lubricants. The non-asbestos fibrous
reinforcing material may be steel wool, glass fiber, spun mineral
fiber, aramid fiber, or the like and is necessary for cohesiveness
and strength.
It is particularly important that a heat-treated grade of a palygorskite
clay, e.g. attapulgite, with low volatile matter, especially low
carbonate content, be used. When a palygorskite clay not meeting
these restrictions is used, the heat generated during operation
is sufficient to cause release of volatiles from the clay and subsequent
loss of friction properties. Typically, a low volatile matter is
understood to mean volatile matter below about 5% by weight on free-moisture-free
basis, and low carbonate content is understood to mean a carbonate
content below about 5% by weight as carbon dioxide on a free-moisture-free
basis.
Although certain aluminosilicates have been used in friction materials
as fillers the use of a palygorskite clay in conjunction with an
inorganic fibrous material in asbestos-free formulations has heretofore
not been practiced or suggested. When used according to the present
invention a palygorskite clay of low volatile matter allows less
expensive asbestos-free friction materials to be made without loss
of performance. Palygorskite clay has good heat resistance and high
temperature friction properties, so that much of the non-asbestos
fibrous reinforcement, such as steel wool, in asbestos-free friction
formulations can be replaced with less expensive palygorskite clay.
Palygorskite clay also offers advantages in wet process formability
due to a naturally high oil absorption.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A palygorskite clay suitable for practice of the present invention
is attapulgite also known as Attapulgus clay, or Georgia-Florida
fuller's earth, which is composed principally of the mineral attapulgite,
a crystalline hydrated magnesium aluminum silicate, but which may
also contain significant amounts of mineral impurities such as montmorillonite,
calcium carbonate, quartz (silica) and feldspar, and in some cases
sepiolite. This clay and processing thereof by heat-treatment or
calcination to obtain special properties are well known in the art.
Further detail on processing and resultant properties is disclosed
for example in U.S. Pat. No. 3953292 to Burns and U.S. Pat. No.
3041238 to Allegrini.
Attapulgite clay may be calcined by means conventional in the art,
for example a rotary calciner. Calcination temperatures range from
about 400.degree. to 1000.degree. F. Calcination at temperatures
in the lower end of the aforementioned range give rise to an attapulgite
clay having a total volatile matter of about 16%. This clay is known
as an RVM-grade (regular volatile matter) attapulgite clay. The
higher temperature calcination gives rise to an attapulgite known
as an LVM-grade (low volatile matter) clay of volatile matter of
about 5% or below as produced.
The heat-treated mineral readily absorbs moisture from the atmosphere
so that free-moisture content may vary widely depending upon conditions
of exposure subsequent to processing. As produced an LVM-grade clay
has less than about 1% free moisture; as used the free moisture
can be 2-3%, with the increase resulting largely from exposure to
the ambient atmosphere. In general volatile material is classified
according to three levels of thermal treatment: loosely-held water
of hydration known as free-moisture which is measured by heating
to constant weight at 220.degree. F., structural water that is held
chemically in the molecular structure of the clay and is measured
by heating from 220.degree. F. to constant weight at 1200.degree.
F., and other volatile matter such as inorganic carbonates, principally
calcium carbonate, which release CO.sub.2 at 1800.degree. F.
Various bases related to water content or thermal treatment may
be used to express percentages of components in the clay. A free-moisture-free
basis weight, or dry weight, is the weight of the clay after heating
to constant weight at 220.degree. F. A volatile-free basis weight
is the weight of the clay after heating to constant weight at 1200.degree.
F., and a loss-on-ignition or L.O.I. basis weight is determined
by heating to constant weight at 1800.degree. F. Also, volatiles
content may be expressed on an as received basis.
The difference between loss on ignition weight and volatile-free
weight is a fairly accurate representation of carbonate content,
since carbonates generally account for virtually all of the non-aqueous
volatile matter in the clay. An alternative method for measuring
the carbonate content (as % CO.sub.2) is to finely pulverize the
clay and heat in the presence of a strong mineral acid such as hydrochloric
acid to release carbon dioxide. The gas can then be collected and
its volume measured, from which the weight percent of CO.sub.2 can
be calculated. This later method was employed to determine the carbonate
content of clays used in the Examples described hereinafter.
Friction materials routinely experience extreme elevated temperatures
and volatile matter in the clay must be kept to an acceptable maximum
level, otherwise release of volatiles due to high temperature will
cause the friction molding to lose desirable friction properties
and result in loss of performance or even disintegration. Attapulgite
suitable for use in the present invention must be an LVM-grade clay
or the equivalent. In addition non-aqueous volatile matter, e.g.
carbonates, must be below about 5% by weight as CO.sub.2 on a free-moisture-free
basis. It is preferred to have % CO.sub.2 below about 4% by free-moisture-free
weight to minimize the possibility of significant release of volatiles
during high temperature use. Low volatile matter heat-treated attapulgite
clay is very hygroscopic, and care must be exercised to keep the
free moisture content from exceeding about 4% by free-moisture-free
weight.
Ultimate particles of attapulgite are needle-shaped in contrast
to platelet shapes of other clays such as kaolin. As mined and used
the needles are generally agglomerated in irregularly shaped clumps.
Particle sizes of the clumps can be altered by conventional pulverization
techniques for specific end uses. The particle size may be measured
by Sedigraph to give a distribution in terms of equivalent spherical
diameter. The particle size does not appear to be a critical parameter
in the performance of friction materials of the present invention.
Particle size may vary widely, but sizes in the range of about 2
microns average diameter, e.s.d. to about 100 mesh Tyler (149 microns)
are preferred.
Attapulgite clay used in the asbestos-free friction materials of
the present invention may be formulated with other ingredients by
either the wet process or the dry process as practiced in the art.
For example, a typical formulation may include, as percentages by
weight, the following:
Ingredients may be dry mixed (some phenolic resins are available
as powders), and molded under pressure and heat using conventional
techniques and conditions to form the brake pad or friction material.
Phenolic resin may be present in amounts ranging from about 10-40%
by weight. In general the percentage of non-asbestos fibrous reinforcement,
such as steel wool, may vary from 10-50% by weight and the percentage
of attapulgite or other palygorskite clay may vary from about 10%
to 35% by weight. The attapulgite and steel wool may range from
20-60% by weight as a combination. The proportion of attapulgite
to steel wool or other non-asbestos fiber in the combination may
vary widely but generally may be in the range of about 1:3 to 3:1.
Especially good results achieved with 1:1 ratio. The percentage
of graphite or other wear modifiers may range up to 20%, but the
5-10% range is typically adequate. The filler material, barytes,
is a common inexpensive filler material of high density and desirable
inertness with good thermal and friction properties. Other filler
material such as iron oxide, sponge iron particles, or scavengers
such as brass or tin fillings may be included in the formulation.
Practioners of the art may add proprietary materials or other materials
to achieve desired end results and formulations can vary widely
with respect to ingredients and amounts. For example, conventional
lubricants or other special wear modifiers may be added to the asbestos-free
formulations of the present invention in order to achieve the desired
wear properties. Other materials such as polymeric fibers, for example
aramid fiber, having a combination of wear reducing and reinforcing
properties may be particularly desirable.
The following examples are presented in order to more fully explain
and illustrate the invention comprising these novel compositions
for friction materials. The examples are not to be construed as
limiting the invention.
EXAMPLES 1-12
In order to demonstrate the performance of attapulgite asbestos-free
formulations of the present invention friction pads containing various
ingredients were molded from dry-mix into shapes suitable for evaluation
on a Chase Machine friction tester according to the procedure outlined
in SAE J661a as published in the SAE Handbook 1980 pages 31.59
to 31.61 published by the Society of Automotive Engineers, Warrendale,
PA. The Chase Machine is equipped to vary the speed and control
the temperature and braking force on a rotating drum. Specimen samples
are 1 inch square with one flat side and a radius of the working
surface conforming to the radius of the drum. Wear is measured by
specimen thickness in inches at the center and by weight loss in
grams. Coefficient of friction (cold) and coefficient of friction
(hot) are classified according to a letter scale described in SAE
J866a, page 31.58 in the SAE Handbook (supra).
The following samples containing different ingredients in varying
amounts were prepared and evaluated in accordance with the above-described
SAE standard tests. Ingredients were dry-blended, placed into a
circular mold about four inches in diameter and about 1/4-inch thick,
and molded on a platen press at 325.degree. F. for 0.5 hours at
a pressure of 2000 psi. Test samples of requisite size were then
cut from the resulting molded discs. Ingredients in each sample
are shown below in Table I.
With reference to the ingredients listed in the table, the phenolic
resin was SP-8855 a dry powder resin containing hexamethylenetetramine
initiator and excess phenol component, available from Schenectady
Chemicals Corporation, Schenectady, N.Y. The barytes (No. 22) was
a standard grade of barium sulfate of 12 micron average particle
size; the No. 1 grade barytes had an average particle size of 8.3
microns. The steel wool was a No. 1 grade, unannealed, having chopped
fibers of average length 4 mm, and obtained from Durawool, Incorporated,
Queens Village, N.Y. The Kevlar aramid fiber was available from
E. I. DuPont de Nemours, Wilmington, Del. The graphite used was
#9985 amorphous carbon of 100 mesh (U.S. Sieve Series) average particle
size and 80% carbon content available from The Asbury Graphite Mills,
Inc., Asbury, N.J. The Cardolite NC 104 resin is a cashew nut oil
modified phenolic sold by 3M Company, St. Paul, MN. This fully cured,
particulate resin is part of a standard formulation and was about
100 mesh (U.S. Sieve Series) average particle size. The asbestos
5R-04 used was a standard asbestos grade with "well-open"
fiber texture according to Quebec Standard Test rating.
Attapulgite "A" was an LVM-grade attapulgite clay in
the range of about 2-3 microns average particle size and had a free
moisture content as used of about 3.8% by weight. The volatile matter
on a free-moisture-free basis (V.M..sub.0) was about 5.4%. The loss
on ignition (L.O.I.) was 10.7% by weight and % CO.sub.2 by weight
was 2.0. This and other samples of attapulgite clay used in the
examples were obtained from a deposit near Attapulgus, Georgia.
Attapulgite "B" was another LVM grade attapulgite clay
of the same average particle size as Attapulgite "A",
but having a free moisture as used of about 2.0% by weight, a volatile
matter content on a free-moisture-free basis of 4.9%, an L.0.I.
of 8.4% by weight and a % CO.sub.2 of about 0.9.
Attapulgite "C" was an LVM grade attapulgite clay 100%
finer than 200 mesh (U.S. Sieve Series) having a free moisture content
as used of 2.8% by weight, a volatile matter on a free-moisture-free
basis of about 5.5% by weight, an L.O.I. of about 10.9% by weight
and a % CO.sub.2 of about 4.5.
Attapulgite "D" was slightly coarser than "C"
having particles 100% finer than 100 mesh (U.S. Sieve Series). The
free-moisture content as used was 3.2% by weight with a volatile
matter content on a free-moisture-free basis of 5.9%, an L.O.I.
of about 15.3% by weight and a % CO.sub.2 of about 4.1.
Samples of the required size were prepared for Chase Machine testing.
Wear and friction characteristics of the twelve samples were measured
according to the aforementioned standard tests of the Society of
Automotive Engineers. The results appear in Table II below:
From the data in Table II above it can be seen that attapulgite-substituted
asbestos-free friction materials using steel wool can be equivalent
in performance to conventional asbestos formulations when aramid
fiber or graphite lubricant or combinations thereof are used in
the formulation. Particularly good results were realized with Example
5 which contained 14% attapulgite and 10% graphite. The aramid fiber
proved to be a more effective lubricant than graphite on a weight
basis as shown by Examples 2 and 3 and it has additional reinforcement
properties so that some combination of aramid fiber and graphite
may be particularly desirable.
The friction classification is related to the coefficient of friction
(cold) for the first letter and (hot) for the second. According
to SAE J866a (supra), the letter "E" represents a material
with coefficient of friction between 0.25 and 0.35; the letter "F"
represents a material with coefficient of friction 0.35 to 0.45
and the letter "G" represents the range 0.45 to 0.55.
Attapulgite friction materials without wear additives showed higher
wear and higher coefficient of friction than the conventional asbestos-filled
control samples.
EXAMPLES 13-15
Three other samples of attapulgite clay were used to formulate
friction materials, which were subsequently tested on the Chase
machine according to SAE J661a. The coefficient of friction during
the second fade test from 450.degree. F. to 650.degree. F. was compared
to obtain a measure of the fade characteristics of the formulations.
The three attapulgite samples, one of which was a regular volatile
matter grade, were compared along with Example 1. The formulations
of Examples 13-15 were identical to that of Example 1 except in
the attapulgite used. The method of preparing samples was identical
to that used for Examples 1-12 as mentioned previously.
Example 13 contained an LVM-grade attapulgite clay of the same
average particle size as Example 1 and had a free moisture content
as used of about 1.6% by weight, a volatile matter content on a
free-moisture-free basis of 4.6%, an L.O.I. of 12.1% by weight and
a % CO.sub.2 of about 5.1.
Example 14 contained an LVM-grade attapulgite clay of the same
average particle size as Example 1 and had a free moisture content
as used of about 0.7% by weight, a volatile matter content on a
free-moisture-free basis of 4.5% an L.O.I. of 8.9 by weight and
a % CO.sub.2 of about 3.3.
Example 15 contained an RVM-grade attapulgite clay of the same
average particle size as Example 1 and had a free moisture content
as used of about 5.3% by weight, a volatile matter content on a
free-moisture-free basis of 9.8%, an L.O.I. of 17.4% by weight and
a % CO.sub.2 of about 2.3.
As is evidenced by the above data some slight fade is experienced
by Example 1. Very little fade is exhibited by Examples 14 and 15
although Example 15 showed an unacceptably low coefficient of friction,
about half that of the other formulations. Example 15 contained
the RVM-grade clay and is unsuitable for use in the present invention.
Example 13 differed from Example 14 mainly in the % CO.sub.2 level
of the clay. Example 13 had about 5.1% CO.sub.2 and Example 14 had
about 3.3% CO.sub.2. The initial average coefficients of friction
between Examples 13 and 14 are seen to be almost identical, however,
Example 13 showed significant and undesirable fade by virtue of
a 40% decrease in average coefficient of friction in this test.
This fade is attributed to the higher carbonate level of the clay.
Both the water and CO.sub.2 volatile compounds would tend to be
released at the higher temperatures encountered in brake usage,
resulting in unacceptable performance. Thus the water, CO.sub.2
and any other volatile compounds in the clay, must be controlled
to an acceptable level for materials of the present invention to
have utility in friction applications. |