Friction material for dampers and process for producing the same

A molded friction material for a damper is disclosed, which contains, as a part of a friction regulating material, porous fibrous particulates having an average particle size of 0.5 to 2 mm and a bulk density of 0.1 to 0.2 g/cm.sup.3 and comprising finely divided ceramic fibers or porous fibrous particulates having an average particle size of 0.1 to 5 mm and a bulk density of 0.2 to 2.0 g/cm.sup.3 and comprising finely divided ceramic fibers, a filler, and a binder. The friction material exhibits excellent performance basically required of a friction material, such as mechanical strength and damping characteristics, and hardly squeaks during damping.

FIELD OF THE INVENTION 
This invention relates to a friction material used in a damper of various 
vehicles and industrial machines and a process for producing the same. 
BACKGROUND OF THE INVENTION 
Various friction materials have hitherto been proposed for use in dampers 
used in automobiles and other vehicles and industrial machines. Generally 
used friction materials are produced by hot-press molding a molding 
material comprising one or more than one fibrous materials selected from 
inorganic fibers, such as asbestos and ceramic fiber, heat-resistant 
organic fibers, and metal fibers as a base material, a friction regulating 
material and other additives, and a thermosetting synthetic resin binder, 
such as a phenolic resin. 
In developing a friction material, not only a coefficient of friction which 
is a factor directly relating to damping capacity and its temperature 
dependence but other various characteristics, such as wear resistance, 
mechanical strength, attack on a metallic material, and squeaks on 
damping, should be considered. These characteristics are controlled by 
properly combining the above-mentioned molding materials. 
It is known that squeaks on damping can be prevented by introducing fine 
pores into a friction material. Effective means proposed therefor include 
reduction of the amount of a binder, use of long fibers as the fibrous 
base material, incorporation of a porous material, such as vermiculite, 
into a molding material, and reduction of the filling rate of a molding 
material. 
However, these conventional methods for preventing squeaks involve 
significant reduction in strength and wear resistance of the friction 
material. Further, since porous materials like vermiculite have low heat 
resistance, incorporation of such materials incurs deterioration of 
high-temperature characteristics, particularly wear resistance, of the 
friction material. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an improved friction 
material which exhibits excellent performance basically required of a 
friction material, such as mechanical strength and damping 
characteristics, and hardly squeaks during damping. 
Another object of the present invention is to provide a process for 
producing the improved friction material. 
The first embodiment of the present invention provides a molded friction 
material for a damper comprising a fibrous base material, a friction 
regulating material, and a thermosetting synthetic resin binder, wherein 
particulate ceramic fiber agglomerates, which are composed of finely 
divided ceramic fibers having a fiber length of 5 to 700 .mu.m and an 
average fiber length of 50 to 150 .mu.m and have an average particle size 
of 0.5 to 2 mm and a bulk density of 0.1 to 0.2 g/cm.sup.3, are dispersed 
in the friction material as a part of the friction regulating material 
while retaining their porosity. 
The friction material of the first embodiment is produced by a process 
comprising hot-press molding a molding material containing, as a part of a 
friction regulating material, particulate ceramic fiber agglomerates 
having an average particle size of 0.5 to 2 mm, a bulk density of 0.1 to 
0.2 g/cm.sup.3 and a degree of tightness of not less than 40% and composed 
of finely divided ceramic fibers having a fiber length of 5 to 700 .mu.m 
and an average fiber length of 50 to 150 .mu.m. 
The second embodiment of the present invention provides a molded friction 
material for a damper comprising a fibrous base material, a friction 
regulating material, and a thermosetting synthetic resin binder, in which 
molded porous particulates, which are composed of finely divided ceramic 
fibers having a fiber length of 5 to 700 .mu.m and an average fiber length 
of 50 to 150 .mu.m, a filler and a binder and have an average particle 
size of 0.1 to 5 mm and a bulk density of 0.2 to 2.0 g/cm.sup.3, are 
dispersed in the friction material while retaining their porosity. 
The friction material of the second embodiment is produced by a process 
comprising hot-press molding a molding material containing, as a part of a 
friction regulating material, molded porous particulates having an average 
particle size of 0.1 to 5 mm and a bulk density of 0.2 to 2.0 g/cm.sup.3 
and composed of finely divided ceramic fibers having a fiber length of 5 
to 700 .mu.m and an average fiber length of 50 to 150 .mu.m, a filler and 
a binder. 
According to the present invention, every particulate ceramic fiber 
agglomerate or every molded porous particulate (hereinafter inclusively 
referred to as a fibrous particulate(s)) which is dispersed in the 
friction material while retaining its porosity provides a dense cluster of 
fine pores localized within a diameter of 0.5 to 2 mm or 0.1 to 5 mm. A 
number of the clusters of fine pores distributed through the friction 
material prevent squeaks of the friction material. Because the clusters of 
fine pores are supported by fine ceramic fibers, they have no adverse 
influences on the strength and damping characteristics of the friction 
material.

DETAILED DESCRIPTION OF THE INVENTION 
The friction material according to the first embodiment of the present 
invention is produced by using particulate agglomerates of finely divided 
ceramic fibers as a part of a friction regulating material. The 
agglomerates are formed by gathering ceramic fibers having a fiber length 
of 5 to 700 .mu.m and an average fiber length of 50 to 150 .mu.m into 
particulates having an average particle size of 0.5 to 2 mm and a bulk 
density of 0.1 to 0.2 g/cm.sup.3. The agglomerates preferably have a 
degree of tightness of not less than 40%. 
The terminologies "bulk density" and "degree of tightness" as used herein 
mean values measured as follows. 
Measurement of Bulk Density 
A sample weighing 100 g is put in an upright metallic cylinder having an 
inner diameter of 150 mm, and a weight is placed on the sample to impose a 
load of 50 g per cm.sup.2 of the opening of the cylinder. Five minutes 
later, the height of the sample is measured to obtain the volume V 
(cm.sup.3). 
Bulk density (g/cm.sup.3)=100/V 
Measurement of Degree of Tightness 
In a 500 ml measuring cylinder are put 500 ml of water and 10 g of a 
sample, lightly stirred, and allowed to stand for 30 minutes, and the 
ceramic fiber sediment volume V.sub.1 was measured. Separately, 10 g of 
the same sample is put in a 500 ml beaker, and 500 ml of water is added 
thereto, followed by stirring by means of a propeller stirrer at 700 rpm 
for 3 minutes. The whole amount of the ceramic fiber suspension is 
transferred to a 500 ml measuring cylinder and allowed to stand for 30 
minutes. The ceramic fiber sediment volume V.sub.2 is measured. 
EQU Degree of Tightness (%)=1-(V.sub.2 -V.sub.1)/V.sub.1 !.times.100 
Although the particulate agglomerates comprise finely divided ceramic 
fibers shorter than the particle diameter and no binding assistant such as 
a binder is used, the state of gathering of ceramic fibers is very stable. 
In particular, those having a degree of tightness of not less than 40% are 
hardly unbound into single fibers on usual handling or during use as a 
molding material. This seems to be because highly rigid ceramic fibers 
cross and support each other to form a stable three-dimensional network 
structure like a bird's net made of twigs (see FIG. 1). 
The particulate ceramic fiber agglomerates can be prepared by means of a 
well-known stirring type granulator used for granulation of powder. That 
is, ceramic fibers finely divided to a fiber length of 5 to 700 .mu.m and 
an average fiber length of 50 to 150 .mu.m are stirred in a stirring type 
granulator. No binder is added. 
Stock fiber compressed in a package usually forms irregular-sized lumps. On 
being stirred in a granulator, the lumps disintegrate and single fibers 
re-gather to form regular-sized fiber agglomerates. The rate of stirring 
influences the particle size of the fiber agglomerates. The gathering 
ceramic fibers are loosely entangled among themselves in the initial stage 
of stirring and, as they undergo a compressional stress of stirring, they 
gradually form agglomerates having a high packing density. On further 
continuing the stirring, while the packing density as measured in terms of 
bulk density is not so changed, the agglomerates become more stable to 
have the above-mentioned tightness probably because the fibers get into a 
stable state. 
Particulate agglomerates having a degree of tightness of less than 40% 
easily disentangle, tending to fail to maintain the gathered state, when 
vigorously stirred with other raw materials in the course of production of 
a friction material. Therefore, in order to facilitate production of 
friction materials, it is preferable to continue stirring for a while 
after particulate agglomerates are seemingly completed so as to achieve a 
degree of tightness of 40% or more. 
The ceramic fiber raw material is not particularly limited. Aluminosilicate 
fiber is the most preferred for its inexpensiveness, commercial 
availability, physical properties, and ease of granulation. Rock wool, 
alumina fiber and carbon fiber are also suitable as ceramic fiber. 
Generally available ceramic fibers, which have a fiber length of several 
millimeters or longer, can be finely divided to lengths suitable for the 
production of the above-mentioned particulate agglomerates by means of a 
dry process or wet process grinder. 
The friction material according to the second embodiment of the present 
invention is produced by using molded porous particulates comprising 
finely divided ceramic fibers, a filler, and a binder. 
The ceramic fiber raw material to be used here is the same as used in the 
first embodiment. 
The filler which can be used in the present invention includes barium 
sulfate, wollastonite, calcium carbonate, cashew dust, and carbon powder. 
The kind and amount of the filler are appropriately selected to control 
frictional characteristics of the friction material. 
Examples of suitable binders are sol binders, such as colloidal silica and 
colloidal alumina, ethyl silicate, and organic binders, such as phenol 
resin, polyvinyl alcohol and carboxymethyl cellulose (CMC). These binders 
may be used either individually or as a combination of two or more 
thereof. 
Where a phenol resin is used as a binder for production of the friction 
regulating material according to the present invention, the resulting 
particulates are improved in the degree of tightness and the bulk density 
thereof is increased while they are cured on drying, and exhibit an effect 
of decreasing the scatter of fibers, i.e., dust generation which is caused 
by impact during the production of the friction material. 
The filler is preferably used in an amount of 1 to 300 parts by weight, 
still preferably 50 to 200 parts by weight per 100 parts by weight of the 
ceramic fiber. 
The inorganic binder is preferably used in an amount of 1 to 40 parts by 
weight (on a solid basis), and the organic binder is preferably used in an 
amount of not more than 25 parts by weight, each per 100 parts by weight 
of the ceramic fiber. Also in the case where a mixture of the inorganic 
binder and organic binder is used as a binder, it is preferred that the 
amount of the inorganic binder and the amount of the organic binder fall 
within the above ranges, respectively. 
The above-described ingredients are stirred in a well-known stirring type 
granulator used for granulation of powder. 
Stock fiber compressed in a package usually forms irregular-sized lumps. On 
being stirred in a granulator, the lumps disintegrate and single fibers 
re-gather to form regular-sized fiber agglomerates with the filler 
adhering onto the surface of the fibers with the aid of the binder. The 
gathering ceramic fibers are loosely entangled among themselves in the 
initial stage of stirring and, as they undergo a compressional stress of 
stirring, the packing density gradually increases. On further continuing 
the stirring, while the packing density as measured in terms of bulk 
density is not so changed, the state of fiber gathering becomes more 
stable. At this point, granulation by stirring is stopped, and the 
resulting particulates are dried by heating to set the binder. With 
properly selected formulation of raw materials and under properly selected 
stirring conditions, molded porous particulates having an average particle 
size of 0.1 to 5 mm and a bulk density of 0.2 to 2.0 g/cm.sup.3 which are 
suitable as a friction regulating material can be obtained. Too dense 
particulates having a bulk density exceeding 2.0 g/cm.sup.3 fail to 
introduce sufficient pores into a friction material. 
The molded porous particulates are very stable and are hardly unbound into 
single fibers on usual handling or during use as a molding material. 
In the first and second embodiments of the present invention, the fibrous 
particulates are preferably used in an amount of 5 to 20% by weight based 
on the total molding material. If the proportion of the fibrous 
particulates is less than 5% by weight, there is a possibility that the 
effect produced is insufficient. If it exceeds 20% by weight, there is a 
possibility that reduction in strength of the resulting friction material 
becomes significant. 
In the present invention, the fibrous particulates serve as a kind of a 
friction regulating material. It means that the fibrous particulates are 
used in appropriate combination with other raw materials generally 
employed in the production of friction materials. That is, one or more of 
fibrous base materials, such as asbestos, ceramic fibers, e.g., 
aluminosilicate fiber and alumina fiber, glass fiber, heat-resistant 
organic fibers, e.g., polyimide fiber and aromatic polyamide fiber, 
metallic fibers comprising copper, brass, steel, etc.; silica, graphite, 
molybdenum sulfide, silicon nitride, boron nitride, metallic powder, 
barium sulfate, wollastonite, cashew dust, and grinds of a thermosetting 
synthetic resin hardened product can be used in combination with the 
fibrous particulates of the present invention for the purpose of 
adjustment of friction characteristics and the like. For example, aramid 
fibers are preferably used in an amount of 5 to 10 parts by weight per 100 
parts by weight of the fibrous particulates. 
The thermosetting synthetic resin which can be used as a binder for 
preparing a friction material is not particularly limited and includes 
those commonly employed, such as novolak or resol phenolic resins and 
modified phenolic resins. For example, phenol resins are preferably used 
in an amount of 10 to 25 parts by weight per 100 parts by weight of the 
fibrous particulates. 
All the raw materials are mixed as uniformly as possible. It is preferable 
to add the fibrous particulates after the other raw materials are 
uniformly mixed so that the porous particulate structure of the fibrous 
particulates may not be destroyed. 
The resulting molding materials is hot-press molded into a prescribed shape 
in a usual manner to give a friction material of the present invention. 
While the mixture is hot-press molded under ordinary conditions, the 
fibrous particulates maintain their original shape without being flattened 
nor disintegrated into single fibers, and their porous structure is not 
destroyed due to, for example, penetration of the binder into the pores. 
Therefore, the fibrous particulates are uniformly dispersed in the final 
product while maintaining the porosity. 
The present invention will now be illustrated in greater detail with 
reference to Examples, but it should be understood that the present 
invention is not deemed to be limited thereto. 
EXAMPLE 1 
Commercially available aluminosilicate fiber (Al.sub.2 O.sub.3 50% by 
weight; SiO.sub.2 : 50% by weight) was treated in a dry process grinder to 
obtain finely divided fibers having an average fiber length of 100 .mu.m 
with not less than 95% by weight of which having a fiber length of 5 to 
500 .mu.m. The fine fibers were stirred in a stirring type granulator. The 
fibers initially had an irregular-sized lumpy form, but the lumps crumbled 
while single fibers gradually gathered into small particulates with 
stirring, and all the fibers turned to particulate agglomerates having a 
diameter of about 0.1 to 5 mm, mostly of 0.5 to 2 mm. 
Six minutes' stirring gave particulate agglomerates having a bulk density 
of 0.147 g/cm.sup.3, a degree of tightness of 95%, and an average particle 
size of 1.2 mm. The scanning electron micrograph of the resulting fibrous 
particulates is shown in FIG. 1. 
A molding material having the composition shown in Table 1 below (unit: 
part by weight) was hot-press molded to obtain a friction material 
(designated 1a to 5a). 
TABLE 1 
______________________________________ 
1a 2a 3a 4a 5a 
______________________________________ 
Fibrous particulates 
-- -- -- 5 10 
Non-granulated 
-- 5 10 -- -- 
ceramic fiber* 
Barium sulfate 
67 62 57 62 57 
Aramid fiber 8 8 8 8 8 
Phenolic resin 
20 20 20 20 20 
Cashew dust 5 5 5 5 5 
______________________________________ 
Note: 
*The finely divided fibers used in the preparation of the fibrous 
particulates. 
The porosity and bending strength of each friction material were measured. 
Porosity (%) was obtained according to the following equation. 
EQU Porosity (%)=(1-bulk density/apparent density).times.100 
Bending strength was measured according to JIS D 4311. 
The results obtained are tabulated in Tables 2 and 3 below taking the 
ceramic fiber content as a parameter. 
TABLE 2 
______________________________________ 
Change in Porosity (%) with Ceramic Fiber Content 
Ceramic Fiber Content 
0% 5% 10% 
______________________________________ 
Fibrous particulates- 
3.8 5.2 8.2 
containing sample 
Non-granulated 
3.8 4.2 6.3 
fiber-containing 
sample 
______________________________________ 
TABLE 3 
______________________________________ 
Change in Bending Strength (kgf/mm.sup.2) 
with Ceramic Fiber Content 
Ceramic Fiber Content 
0% 5% 10% 
______________________________________ 
Fibrous particulates- 
6.4 6.3 5.6 
containing sample 
Non-granulated 
6.4 6.5 6.8 
fiber-containing 
sample 
______________________________________ 
It is seen that friction materials 4a and 5a containing fibrous 
particulates (particulate ceramic fiber agglomerates) attain an increased 
porosity without suffering great reduction in bending strength. As is 
shown in FIG. 2, fibrous particulates 1 are uniformly distributed 
throughout friction material 2. Further, each fibrous particulate retained 
its porosity as is observed from the electron micrographs taken of the 
polished surface of the friction material (FIGS. 3 and 4). 
Friction material 5a according to the present invention and comparative 
friction material 3a were subjected to an abrasion test using an abrasion 
tester in accordance with JIS D4411 "Automobile Brake Lining". Occurrence 
of squeaks was also examined. The results obtained are shown in Table 4 
below. 
TABLE 4 
______________________________________ 
No. 5a No. 3a 
______________________________________ 
Specific wear 
(.times.10.sup.-7 cm.sup.3 /kg .multidot. m): 
100.degree. C. 1.6 1.5 
150.degree. C. 1.1 1.0 
200.degree. C. 1.3 1.2 
250.degree. C. 1.5 1.5 
Squeak not occurred 
occurred 
______________________________________ 
EXAMPLE 2 
Commercially available aluminosilicate fiber (Al.sub.2 O.sub.3 50% by 
weight; SiO.sub.2 : 50% by weight) was treated in a dry process grinder to 
obtain finely divided fibers having an average fiber length of 50 .mu.m 
with not less than 95% by weight of which having a fiber length of 5 to 
500 .mu.m. The fine fibers and an equivalent amount of barium sulfate were 
stirred in a stirring type granulator for 1 minute. Colloidal silica and 
polyvinyl alcohol were added thereto in an amount of 30% by weight (on a 
solid basis) and 2% by weight, respectively, based on the aluminosilicate 
fiber, and the stirring was continued for an additional period of 4 
minutes. The fibers initially had a lumpy form of irregular size, but the 
lumps crumbled while single fibers and barium sulfate are gradually formed 
into small particulates with stirring, and all the fibers turned to 
particulates having a diameter of about 0.1 to 5 mm. 
The resulting fibrous particulates had a bulk density of 0.80 g/cm.sup.3, a 
porosity of 75.0%, and an average particle size of 0.4 mm, with not less 
than 95% by weight thereof having a particle size of 1 mm or less. The 
scanning electron micrograph of the resulting fibrous particulates is 
shown in FIG. 5. 
A molding material having the composition shown in Table 5 (unit: part by 
weight) was hot-press molded to obtain a friction material (designated 1b 
to 7b). 
TABLE 5 
______________________________________ 
Friction material No. 
1b 2b 3b 4b 5b 6b 7b 
______________________________________ 
Fibrous particulates 
-- -- -- -- 5 10 20 
Non-granulated 
-- 5 10 20 -- -- -- 
ceramic fiber* 
Barium sulfate 
67 62 57 47 62 57 47 
Aramid fiber 
8 8 8 8 8 8 8 
Phenolic resin 
20 20 20 20 20 20 20 
Cashew dust 5 5 5 5 5 5 5 
______________________________________ 
Note: 
*The finely divided fibers used in the preparation of the fibrous 
particulates. 
The porosity and bending strength of each friction material were measured. 
The results obtained are tabulated in Tables 6 and 7 below taking the 
ceramic fiber content as a parameter. 
TABLE 6 
______________________________________ 
Change in Porosity (%) with Ceramic Fiber Content 
Ceramic Fiber Content 
0% 5% 10% 20% 
______________________________________ 
Fibrous particulates- 
3.8 4.4 4.5 5.1 
containing sample 
Non-granulated 3.8 2.7 2.7 3.9 
fiber-containing 
sample 
______________________________________ 
TABLE 7 
______________________________________ 
Change in Bending Strength (kgf/cm.sup.2) 
with Ceramic Fiber Content 
Ceramic Fiber Content 
0% 5% 10% 20% 
______________________________________ 
Fibrous particulates- 
6.4 6.3 6.2 5.7 
containing sample 
Non-granulated 6.4 7.2 6.9 6.9 
fiber-containing 
sample 
______________________________________ 
It is seen that friction materials 5b to 7b containing fibrous particulates 
(molded porous particulates) attain an increased porosity without 
suffering great reduction in bending strength. The fibrous particulates 
were uniformly distributed throughout friction material in the same manner 
as the fibrous particulates according to the first embodiment as shown in 
FIG. 2. As is observed from the electron micrographs taken of the polished 
surface of the friction material (FIGS. 6 and 7), each fibrous particulate 
retained its porosity. 
Friction material 6b according to the present invention and comparative 
friction material 3b were subjected to an abrasion test with an abrasion 
tester in accordance with JIS D4411 "Automobile Brake Lining". Occurrence 
of squeaks was also examined. The results obtained are shown in Table 8 
below. 
TABLE 8 
______________________________________ 
No. 6b No. 3b 
______________________________________ 
Specific wear 
(.times.10.sup.-7 cm.sup.3 /kg .multidot. m): 
100.degree. C. 1.6 1.5 
150.degree. C. 0.9 1.0 
200.degree. C. 1.0 1.2 
250.degree. C. 1.3 1.5 
Squeak not occurred 
occurred 
______________________________________ 
As has been fully described, the technique of the present invention 
characterized by incorporation of porous fibrous particulates of ceramic 
fiber makes it possible to introduce fine pores into a friction material 
without causing reductions in strength and wear resistance thereby 
preventing squeaks during damping. 
EXAMPLE 3 
Preparation of Molded Particulates 
To 100 parts of aluminosilicate fiber prepared in the same manner as in 
Example 2, 130 parts by weight of barium sulfate and 20 parts by weight of 
phenol resin powder were added, and the mixture was stirred for 4 minutes. 
After then, 9 parts by weight of colloidal silica (on a solid basis) and 3 
parts by weight of polyvinyl alcohol (on a solid basis) were added 
thereto, and the mixture was stirred for 2 minutes and finally dried at 
70.degree. C. Thus, molded particulates having a bulk density of 0.71 
g/cm.sup.3 and an average particle size of 0.3 mm were obtained. 
Dusting Characteristics 
The molded particulates were measured for dusting characteristics according 
to the following method. 
Five grams of a sample (particulates) was dropped from a height of 1.2 m 
through a pipe having a diameter of 30 mm, and dust flying up by impact at 
dropping was counted for 1 minute with a digital dust meter P-5H2 
(manufactured by Shibata Kagaku Kiki Kogyo Kabushiki Kaisha) at a height 
of 150 mm from the bottom. 
The measurement of dusting characteristics proved that the molded 
particulates exhibited a value as extremely small as 206 CPM (count per 
minute), while the molded particulates of Example 2 exhibited 847 CPM. 
Preparation of Friction Material 
Next, a friction material was prepared in the same manner as Friction 
Material No. 7b of Example 2 except for using the above-prepared 
particulates in place of the fibrous particulates used in Example 2. The 
performance of the friction material is shown in Table 9 below. 
TABLE 9 
______________________________________ 
Content of 20 
Particulates 
(% by weight) 
Porosity 4.7 
(%) 
Bending 5.9 
Strength 
(kgf/mm.sup.2) 
Specific Wear 
(.times.10.sup.-7 cm.sup.3 /kg .multidot. m) 
100.degree. C. 1.6 
150.degree. C. 1.1 
200.degree. C. 1.2 
250.degree. C. 1.4 
Squeak not occurred 
______________________________________ 
While the invention has been described in detail and with reference to 
specific examples thereof, it will be apparent to one skilled in the art 
that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.