GB2305433A - Friction materials for brakes - Google Patents

Abstract

A friction material for brakes which comprises a metallic fiber, a nonmetallic fiber, a binder, carbon, an organic filler, and an inorganic filler, the inorganic filler comprising hydrotalcite in an amount of from 0.5 to 65 vol% based on the total amount of the friction material. The filler may comprise metal silicates, hydroxides, oxides or sulphides or metal powders. The surface of the hydrotalcite may be treated with rubber, surfactant or resin.

Description

FRICTION MATERIALS FOR BRAKES FIELD OF THE INVENTION The present invention relates to friction materials for brakes in such applications as automobiles, railroad cars and industrial machines.

BACKGROUND OF THE INVENTION Friction materials for brakes adopt a variety of formulation depending upon environmental, performance and other requirements as exemplified by asbestos-based materials, steel fiber-based materials and those which contain neither steel fibers nor asbestos. In particular, asbestos are being displaced from the world market on account of the potential risk they cause to the environment.

Currently used alternatives to asbestos include metal fibers such as steel fibers, organic fibers such as aramid fibers and ceramic fibers, with significant dependency of the friction performance upon material characteristics.

However, the friction materials using these substitute fibers have been found to present a common problem, i.e., low frequency noises. Various proposals have been made to deal with this problem and an example is the use of a material having a platy mesh crystal structure.

JP-A-3-181628 teaches specifically the use of silicates such as mica and metal hydroxides such as Mg(OH)2. (The term "JP-A" used herein means an "unexamined published Japanese patent application.) The stated technique is effective to some extent in controlling the noise problem at low frequencies but, on the other hand, its performance is inferior in other aspects as compared with friction materials that do not contain a material having a platy mesh crystal structure. Friction materials using metal hydroxides are incapable of reducing the amount of wear, whereas those using silicates have a tendency to deteriorate in friction performance upon fade.

These deteriorations in characteristics are undesirable to friction materials for brakes which are required to maintain certain minimal performance levels under all conditions.

SUMMARY OF THE INVENTION An object of the present invention is to provide a friction material that exhibits improved friction performance, and in particular, the coefficient of friction z will not assume any abnormal value even under high load, or upon fade; in addition, the development of brake noise at low frequencies is sufficiently damped to provide good feeling for the friction material.

Other objects and effects of the present invention will be apparent from the following description.

The present invention relates to a friction material for brakes which comprises a metallic fiber, a nonmetallic fiber, a binder, carbon, an organic filler, and an inorganic filler, the inorganic filler comprising hydrotalcite in an amount of from 0.5 to 65 vol% based on the total amount of the friction material.

In preferred embodiments of the present invention, the inorganic filler further comprises at least one silicate having a platy mesh crystal structure or at least one of hydroxides of Mg, Al, and Fe; water of crystailization of the hydrotalcite has been partly or entirely released by a heat treatment; the hydrotalcite has been surface treated with a surfactant, a resin, or a rubber; or the hydrotalcite has been granulated with at least one of other ingredients. In these preferred embodiments of the present invention, the effects of the present invention are further enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows the fluid pressure curves that were obtained with friction material sample Al of the invention when it was subjected to a fade test with a dynamometer; Fig. 2 shows the fluid pressure curves that were obtained with friction material sample A2 of the invention when it was subjected to a fade test with a dynamometer; Fig. 3 shows the fluid pressure curves that were obtained with friction material sample A3 of the invention when it was subjected to a fade test with a dynamometer; Fig. 4 shows the fluid pressure curves that were obtained with comparative sample B1 when it was subjected to a fade test with a dynamometer; Fig. 5 shows the fluid pressure curves that were obtained with comparative sample B2 when it was subjected to a fade test with a dynamometer; and Fig. 6 shows the fluid pressure curves that were obtained with comparative sample B3 when it was subjected to a fade test with a dynamometer.

DETAILED DESCRIPTION OF THE INVENTION Hydrotalcite differs from silicates having a platy mesh crystal structure in that it does not have elemental Si in its structure and this difference causes markedly dissimilar results with respect to fade. Figs. 5, 4 and 6 show three examples of the results of a bench test conducted using a dynamometer in accordance with JASO 6914-82 Test Code. Fig. 5 shows curves for the output torque and fluid pressure that developed upon the application of successive brakes in the first fade test on a silicate-free friction material. Since the test was a constant torque test, there was no variation in torque.However, the fluid pressure changes considerably during single brakes and this phenomenon may be explained as follows: the friction material is deteriorated by the heat generated upon friction with the mating member and the resulting drop in friction force is compensated by the increasing fluid pressure.

Compared with Fig. 5, Figs. 4 and 6 show the results of a test conducted on friction materials containing talc and mica (each containing a silicate). Each of the curves for fluid pressure given in Figs. 4 and 6 differs from the one shown in Fig. 5 and drivers will feel uneasy if the fluid pressure undergoes such great changes during single brakes.

Such abnormal fade is principally caused by the following mechanism; Fe in the mating member catalyzes the drop in the melting point of the Si oxide in the silicate, which then lowers the coefficient of friction of the rubbing surface of the friction material. This phenomenon is pronounced if the friction material contains Fe as in steel fibers.

Hydrotalcite is an inorganic substance that has a layered structure similar to the platy mesh structure of mica or talc and which holds the water of crystallization between layers. It occurs not only naturally but also as a synthesized product and is known to have the following structural formulae: Ng6Al2(OH)16CQ.4H20 (native form) Mg4.l2(OH)13CQ.3.5H2O (synthetic form) If the Mg in either formula is replaced by Si, the structure is identical to that of zeolite but not similar to those of mica and talc. Since hydrotalcite has the lamellar structure, it is capable of damping low frequency noises like micas and talc, and at the same time, hydrotalcite is more wear resistant than metal hydroxides.As a further advantage, hydrotalcite has no Si in its structure and, hence, friction materials containing hydrotalcite do not produce an abnormal fluid pressure curves of the profiles shown in Figs. 4 and 6.

In order to exhibit these effects, hydrotalcite must be present in amounts of 0.5 to 65 vol% based on the total amount of the friction material of the invention. If the content of hydrotalcite is less than 0.5 vol%, it is not capable of exhibiting the intended effect over the entire sliding interface; if its content exceeds 65 vol%, hydrotalcite will interfere with the functions of other ingredients (e.g., fibrous reinforcements, binders and heat dispersants). Preferably, hydrotalcite is present in amount of 2 to 40 vol%.

The inherent characteristics of hydrotalcite are not lost even if it is used in combination with micas or talc and, hence, it may effectively be combined with silicates having a platy mesh crystal structure. This may be because the hydrotalcite effectively prevents the drop in the melting point of Si oxides which would otherwise occur in the presence of Fe to produce abnormal fluid pressure curves upon fade. As a result, a friction material can be obtained that not only is capable of preventing the brake noise at low frequencies but also ensures that its friction characteristics will not drop below certain levels even if it is faded.

As mentioned hereinabove, hydrotalcite holds the water of crystallization between layers of its lamellar crystal structure. The effectiveness of hydrotalcite becomes more pronounced by removing the water of crystallization partly or entirely by preliminary heat treatment so as to prevent the evolution of gases even if the friction material containing hydrotalcite becomes very hot upon fade. The temperature for the heat treatment will generally suffice at 8000C or below.

When the hydrotalcite is used in a powder form, its particles are preferably surface treated with surfactants, resins, rubbers, etc. because they improve not only the moldability of the mixture but also permit the hydrotalcite to be dispersed within the friction material in a sufficiently uniform manner that it need be incorporated in only a small amount in order to achieve a desired improvement in the fade characteristics. Examples of the surfactants are soaps and coupling agents, and the resins and rubbers are advantageously used after dilution with organic solvents.

If the mixture of hydrotalcite with at least one of other ingredients of the friction material is granulated preliminarily, not only uniformity in formulation is assured but also a sufficient number of pores are provided in the molded friction material that the gases evolving upon fade can be rejected from the sliding interface rapidly enough to provide better fade properties. Other ingredients that can be granulated together with the hydrotalcite include all that are to be incorporated in the friction material but it is preferred to select the major ingredient (e.g. fibers) and part of powdery materials.

Since hydrotalcite is structurally similar to zeolite and thus has ion adsorptivity, there is another advantage for friction materials as specifically described below. Most of the mating members (e.g. disk rotors and drums) for the friction materials are made of iron or its alloys and are prone to rust formation. Particularly, de-icing chemicals used in winter seasons cause the rusting of the mating members, which may occasionally result in a significant drop in friction performance. Cacti2 and NaCl are commonly used as de-icing chemicals, and the C1 ion will induce the rusting of the mating members. Hydrotalcite adsorbs the C1 ion, thereby working to retard the rusting of the mating members.

Hydrotalcite is also effective in preventing the rust formation in friction materials per se that involve Fecontaining ingredients.

The friction material according to the present invention comprises a metallic fiber, a nonmetallic fiber, a binder, carbon, an organic filler, and an inorganic filler.

These ingredients other than hydrotalcite and their proportions in the friction material are not particularly limited and they may be the same as those used in the field of art of friction material.

Examples of the metallic fiber include copper fibers, brass fibers, steel fibers and stainless steel fibers, and copper fibers and steel fibers are preferably used in the present invention.

Examples of the nonmetallic fiber include alamide fibers, acrylic fibers, carbon fibers, phenolic fibers, glass fibers, alumina fibers, silicate fibers and rock wool fibers, and alamide fibers and glass fibers are preferably used in the present invention.

Examples of the binder include phenolic resins, melamine resins, polyimide resins and epoxy resins, and phenolic resins are preferably used in the present invention.

Examples of the carbon include graphite, coak and carbon black, and graphite is preferably used in the present invention.

Examples of the organic filler include cashew dust, melamine dust, NBR powder and SBR powder, and cashew dust is preferably used in the present invention.

Examples of the inorganic filler include molybdenum disulfide, antimony trisulfide, antimony trioxide, muscovite, phlogopite, alumina, zirconium oxide, zirconium silicate, silica, rutile, magnesia, wollastnite, barium sulfate, calcium carbonate, calcium hydroxide, aluminum hydroxide, magnesium hydroxide, ferrous hydroxide, ferrous oxide, talc, brass chips, copper powder and zinc powder, and muscovite, phlogopite, antimony trisulfide, zirconium oxide, barium sulfate, magnesia, calcium hydroxide, magnesium hydroxide, aluminum hydroxide and ferrous hydroxide are preferably used in the present invention.

The proportions of the components constituting the friction material of the present invention are generally 2 to 20 vol% of metallic fiber, 5 to 20 vol% of nonmetallic fiber, 18 to 25 vol% of binder, 1 to 10 vol% of carbon, 6 to 25 vol% of organic filler, and 10 to 50 vol% of inorganic filler.

The present invention will be described in more detail by referring to the following Examples and Comparative Examples, but the present invention is not construed as being limited thereto.

EXAMPLE 1 The raw materials listed in Table 1 were dry mixed with an Eilich mixer at the proportions also listed in Table 1, thereby providing mixtures for friction materials Al, A2 and A3. Each mixture was metered in a specified amount, charged into a die cavity and formed on a press at a mold temperature of 1500C for 10 min to make a shaped part having a thickness of 12.5 mm. A backing plate to which an adhesive had been applied and dried was set in the mold, and the mixture adhered to the backing plate simultaneously with the shaping. The molding pressure was set at such a value that the formulation would have a porosity of 10%. After postcure at 2300C for 3 hours, the shaped part was ground to a thickness of 12 mm, thereby providing a test sample of a brake pad.

COMPARATIVE EXAMPLE 1 Formulations shown in Table 1 that did not contain hydrotalcite B1, B2 and B3, were processed by the same method as in Example 1, thereby preparing comparative test samples of friction material.

TABLE 1 Comparative ExamPle (vol% Example (vol%) Inaredient Al A2 A3 B1 B2 B3 Cu fiber 5 - - 5 - Steel fiber - 5 10 - 5 10 Aramide fiber 12 10 15 12 10 15 Muscovite - 12 - 12 - 10 Talc - - - 5 - 5 Hydrotalcite 27 2 15 - - Brass chips - 3 - - 3 Ca(OH)2 3 3 - 3 3 Sb203 3 3 3 3 3 3 Graphite 3 8 6 3 8 6 Phenolic resin 23 22 23 23 22 23 Cashew dust 16 18 16 16 18 16 ZrO2 1 0.5 1.5 1 0.5 1.5 BaSO4 7 9 10.5 17 27.5 10.5 Wollastnite - 4.5 - - - Total 100 100 100 100 100 100 EXAMPLE 2 Hydrotalcite was heated at 4000C for 2 hours in a ceramic firing furnace and then cooled to room temperature in a dry atmosphere. The dried hydrotalcite was incorporated to give the same formulation as Al in Table 1, which is hereunder designated as C1.

Hydrotalcite particles were spray coated with a titanium coupling agent as they were agitated in a ball mixer. After the coating, the hydrotalcite powder was dried with a dryer at 1000C; upon measurement, the coupling agent was found to have deposited in an amount of 3 wt%. The thus treated hydrotalcite was incorporated to give the same formulation as Al, which is hereunder designated as C2.

Separately, Cu fiber, Sb2S3, ZrO2 and hydrotalcite were mixed in respective volume fractions of 5, 3, 1 and 27.

The resulting powder mixture was charged into a ball mixer, and the mixture was sprayed with an ethanolic solution of 10% phenolic resin under agitation. At the time when the hydrotalcite particles deposited on the fiber, the mixture was recovered and dried, followed by curing of the phenolic resin in a constant-temperature bath (1500C). The thus granulated product was analyzed for its composition and the phenolic resin was found to have a volume fraction equivalent to 10. The granulation was incorporated to give the same formulation as Al, except that those ingredients already present in the granulation were not added and that the phenolic resin was added in an amount of 13 vol% since 10 wt% was already used to prepare the granulation. The thus prepared formulation is hereunder designated as C3.

Disk pads were molded from C1, C2 and C3 by the same process as in Example 1.

EXPERIMENT EXAMPLE 1 Dynamometer testing was conducted per the specifications assumed for a 3,000 cc FF (front-engine, front-drive) passenger car. The test results are shown in Table 2. The testing conditions were as described in JASO 6914-82. The characteristics evaluated by the test of fade which is of particular relevance to the present invention are extracted for reproduction in Table 2. The individual items of the data shown in Table 2 are explained below.

Abnormal fade: A fluid-pressure curve having a peak in its pattern is found to be "positive" for abnormal fade.

Min. min. p: Among the ten brakes applied in the fade test, the maximum liquid pressure on the curve for the brake with the smallest coefficient of friction (min. > ) was read off and min. min. p was calculated from the relationship with the output torque. If the value of min. min. R is small, the driver will have no sense of "effectiveness" during the application of brake.

Pattern: Fade is tested under constant torque conditions, so a variation in fluid pressure curve means that the moving car cannot be stopped at a constant deceleration rate unless the brake force being applied is altered; hence, the fewer the variations in the pattern of fluid pressure curve, the better.

On the basis of these criteria for evaluation, it is concluded that the patterns of the fluid pressure curves obtained with samples Al (Fig. 1), A2 (Fig. 2) and A3 (Fig.

3) in Example 1 of the present invention reflect preferred results. The patterns for B1 (Fig. 4) and B3 (Fig. 6) are abnormal whereas that for B3 (Fig. 5) is in an acceptable range. It is therefore clear that the friction materials using hydrotalcite exhibited satisfactory friction performance. Although not shown in Table 2, the testing on Cl, C2 and C3 gave substantially the same data as in the testing on Al; it should be particularly noted that C1 produced a min. min. z value of 0.24, thus demonstrating the effectiveness of the heat treatment of hydrotalcite.

TABLE 2 Fade Test Example Comparative Example Check item A1 A2 A3 B1 B2 B3 Abnormal fade negative negative negative positive negative positive Min. min. 0.21 0.18 0.20 0.12 0.16 0.17 Pattern Fig. 1 Fig. 2 Fig. 3 Fig. 4 Fig. 5 Fig. 6 EXPERIMENT EXAMPLE 2 By means of the same dynamometer as used in Experiment Example 1, friction material samples Al to A3 (according to the invention) and comparative samples B1 to B3 were measured for two kinds of friction coefficient, static ( S) and dynamic ( D). During the measurements, the temperature of the mating member (disk rotor) was set at 500C as each test sample was held against it at 10 kg/cm2, thereby developing an increased rotational torque.The results are shown in Table 3 together with the values of R (pS/D). With increased values of S, the chance of "stick slip" will increase, producing an undesired tendency toward greater low frequency noise. The tendency is increased if the ratio R becomes larger and this interferes with the control of low frequency noise. Table 3 shows that the friction materials using hydrotalcite could effectively control the low frequency noise because they had small enough values of S and R.

TABLE 3 Test for zS and zD Comparative Example Example Check Item Al A2 A3 B1 B2 B3 cos 0.32 0.34 0.39 0.39 0.43 0.41 R (=RS/AD) 1.01 1.09 1.11 1.07 1.31 1.14 EXPERIMENT EXAMPLE 3 Friction material samples Al to A3 (of the invention) and comparative samples B1 to B3 were installed on an actual 3,000 cc vehicle for testing the brake noise they would produce. The testing schedule was as set forth in Table 4. The brake noise produced during brake application was measured with a vibration pickup and the sensed vibrations were determined by passage through a bandpass filter for separation into components between 20 Hz and 1 kHz.The generation of vibrations greater than a specified amplitude was counted as the occurrence of brake noise and the incidence of brake noise that occurred at low frequencies during the test brake application as distinguished from higher frequency noise was calculated. The results are shown in Table 5. By comparing the data on gS and R that are shown in Table 3 with the brake noise data shown in Table 5, it can be seen the validity of the test results obtained in Experiment Example 2.

TABLE 4 2. Brake Braking conditions 1. Break in aPDlication test Initial speed 40 km/h 40 km/h Final speed 0 km/h 0 km/h Deceleration rate 0.3 G 0.2 to 0.6 G Application times 30 140 Temperature before 1000C 40 to 2000C the start of brake application TABLE 5 Brake noise test Comparative ExamPle ExamPle Check Item Al A2 A3 B1 B2 B3 Percent occurrence of noise 3 0 5 6 29 9 When hydrotalcite is used as an ingredient of a friction material formulation, improved friction performance is achieved and, in particular, the coefficient of friction, , will not assume any abnormal value even under high load, or upon fade; in addition, the development of brake noise at low frequencies is sufficiently damped to provide good feeling for the friction material.

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.

Claims (6)

1. A friction material for brakes which comprises a metallic fiber, a nonmetallic fiber, a binder, carbon, an organic filler, and an inorganic filler, said inorganic filler comprising hydrotalcite in an amount of from 0.5 to 65 vol% based on the total amount of said friction material.

2. A friction material as claimed in Claim 1, wherein said inorganic filler further comprises at least one silicate having a platy mesh crystal structure or at least one of hydroxides of Mg, Al, or Fe.

3. A friction material as claimed in Claim 1 or Claim 2, wherein water of crystallization of said hydrotalcite has been partly or entirely released by a heat treatment.

4. A friction material as claimed in Claim 1 or Claim 2, wherein said hydrotalcite has been surface treated with a surfactant, a resin, or a rubber.

5. A friction material as claimed in Claim 1 or Claim 2, wherein said hydrotalcite has been granulated with at least one of other ingredients.

6. A friction material substantially as hereinbefore described with reference to Example 1 or Example 2.