Methods for the preparation of a friction material and for the manufacture of a brake pad using such friction material

Abstract

Method for manufacturing an asbestos-free friction material having as component materials inorganic and/or organic and/or metallic fibers, at least one organic binder, at least one friction modifier or lubricant and at least a filler or abrasive. The raw components of the friction material are mixed together to obtain a raw mixture which is then molded to obtain a block or layer of friction material. The mixing step includes a first step of hot blending of at least part of the organic binder with at least part of the other components of the friction material by a rolling mill blender that is open to atmospheric pressure at a temperature lower than the polymerization temperature of the organic binder but greater than or equal to its softening temperature, in order to obtain a semifinished solid product. A second step of grinding the semifinished solid product reduces the product to a powder.

Claims

1. A method for manufacturing an asbestos-free friction material comprising as raw component materials inorganic and/or organic and/or metallic fibers, at least one organic binder, at least one friction modifier or lubricant and at least a filler or abrasive, the method comprising: a mixing step involving the raw component materials of the friction material in order to obtain a raw mixture; and a molding step in which the raw mixture is molded under pressure in order to obtain a block or layer of friction material; wherein the mixing step comprises the steps of: a) a first hot blending step carried out by feeding at least part of the at least one organic binder and at least part of at least one other raw component material of the friction material in a first mixer provided with at least two hot rotating rollers and open to atmospheric pressure in order to have the at least part of the organic binder and said at least part of said other raw component material of the friction material passing through the rollers in a gap delimited therebetween at a temperature which is lower than the polymerization temperature of the organic binder, in order to obtain at atmospheric pressure and after the rollers a semifinished solid product in the shape of chips or ribbon or sheet; and b) a grinding step of the semifinished solid product, in order to completely reduce the semifinished solid product to a powder; and c) a second blending step performed in a second mixer, the second blending step being performed prior to or after the first hot blending step and including at least the remaining raw components of the friction material.

2. The method according to claim 1, wherein the second blending step is performed after the first hot blending step and the grinding step, wherein the other raw component materials of the friction material and the powder obtained by the grinding of the semifinished solid product are introduced within the second mixer.

3. The method according to claim 2, wherein the powder obtained by the grinding of the semifinished solid product is subjected to a sieving step prior to the second blending step in order to obtain the raw mixture.

4. The method according to claim 1, wherein the second blending step is performed before the first blending step and the grinding step, wherein the other raw component materials of the friction material are introduced within a second mixer.

5. The method according to claim 1, wherein the organic binder used in the first hot blending step, consists of a polymer selected from the group consisting of: at least a thermosetting polymer; at least a thermoplastic polymer; or mixtures thereof.

6. The method according to claim 5, wherein the organic binder used in the first hot blending step is in a powder form.

7. The method according to claim 1, wherein during the first hot blending step configured to obtain the semifinished solid product, all the organic binder and part of the raw component materials of the friction material are mixed with each other.

8. The method according to claim 1, wherein the product obtained by the grinding of the semifinished solid product that is used to form the raw mixture has a particle size of between 5 and 500 microns.

9. The method according to claim 1, wherein during the first blending step configured to obtain the semifinished solid product all the organic binder and part or the totality of the raw component materials of the friction material having a fibrous structure are mixed with each other.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will now be described in more detail with reference to the following practical non-limiting embodiment examples and with reference to FIGS. 1 to 3 of the appended drawings, wherein:

(2) FIG. 1 illustrates schematically by means of blocks the method for manufacturing a friction material according to the invention highlighting schematically one of the steps that characterizes the method of the invention;

(3) FIG. 2 illustrates the results of a diffraction analysis and a SEM analysis performed at two different magnifications on one of the raw components of the friction material that can be made using the method of the invention before and after a step of the method of the invention; and

(4) FIG. 3 illustrates the results in graphical form of braking efficiency tests on the same friction material made using traditional methods and the method of the invention.

DETAILED DESCRIPTION

(5) The examples and comparative examples are reported here by way of illustration and are not intended to limit the invention.

(6) With reference to FIG. 1, a non-limiting implementation is illustrated schematically in blocks of a method for obtaining a friction material according to the invention.

(7) The block indicated with 100 represents the set or complex of raw component materials of the required friction material: this block is composed of, indicated with hatching, an organic binder 1, fillers 2, lubricants or friction modifiers 3, abrasives 4, fibers 5 and metallic powders 6.

(8) The block indicated with 50 represents a mixing step, for example performed in a Henschel, Loedige or Eirich type blender, of all or only part of the raw component materials of the block 1.

(9) The block indicated with 60 represents a characteristic step of the method of the invention consisting of a hot blending step of some or all of the raw components of the block 100, possibly pre-mixed or not, in correspondence to the block 50. If only some of the raw components of the block 100 are blended in the blending phase according to the block 60, these must be in accordance with a preferred embodiment of the invention, selected from at least part and, preferably, all of the organic binder 2, which should preferably be solid and in the form of a powder or granules and at least part of the other components. According to the prospective expansion of the block 60 shown schematically in FIG. 1, during this step the raw components, at least consisting of part or all of the organic binder 2 and of part or all of the remaining components of the friction material, possibly but not necessarily pre-mixed in the block 50, are fed into a hopper 7 and made to fall from said hopper, under atmospheric pressure conditions, between two (or more) heated and motorized counter-rotating rollers 8. The rollers 8 are heated to a higher temperature than the softening temperature and, preferably, higher than the complete melting temperature of the organic binder but below the polymerization or curing temperature of the organic binder, which is a resin or mixture of thermosetting resins, in such a way as to bring the organic binder up to a temperature that is greater than or equal to the softening temperature but lower than its polymerization temperature in order to maintain the ability of the organic binder to soften or liquefy again.

(10) The rollers 8 and the hopper 7 form part of a roll blender 9 which is open to the atmosphere, therefore not pressurized. Within this blender 9 the raw component materials of the desired friction material are subjected to a high shear stress in the presence of the organic binder in the fluid state. At the output of the roll blender 9 a semifinished solid product 11 in the form of chips or ribbon or sheet is produced.

(11) The rollers 8 are made to rotate at a speed between 10 and 30 revolutions/minute and are maintained at a temperature between 40 and 150 C. The gap between the rollers 8, which determines the magnitude of the applied shear stress and the thickness of the semifinished product 11 at the output, is between 0.01 and 5 mm.

(12) The block indicated with 12 represents a grinding step, preferably performed in a ball or hammer mill, of the semifinished product 11, which is reduced to the form of a powder.

(13) The block indicated with 13 represents a sieving or screening step of the powder obtained from the grinding of the semifinished product 11, which is made to have a controlled particle size distribution between 5 and 500 microns.

(14) The block indicated with 14 and illustrated with hatching represents an optional further mixing step using a Henschel, Loedige or Eirich blender, of the predefined particle size powder obtained from the grinding of the semifinished product 11 with any other raw component materials of the desired friction material, for example consisting of all or part of the friction modifiers and/or fillers, that did not participate in the blending step in the block 60.

(15) Finally, the block indicated with 16 represents a traditional molding step of a block or layer of friction material on a metallic support in order to obtain a braking element, typically a brake pad, obtained by feeding at block 16 a raw mixture of component materials of the friction material to be obtained at the output of block 14 or, directly, from the block 13. The block indicated with 17 represents an optional known step of thermal treatment of a braking element obtained using the method described.

EXAMPLE1

(16) Five formulations were prepared, marked with the initials AMFN/194, prepared using the traditional method, and 329, 330, 331 and 328, respectively, with a composition that is similar/identical to that of the formulation AMFN/194 and obtained using the method of the invention, changing the raw component materials subjected to hot blending with rollers.

(17) The compositions of the five formulations described above are shown in table 1 below.

(18) TABLE-US-00001 TABLE 1 AMFN/ MFN/ MFN/ MFN/ MFN/ FORMULA 194 328 329 330 331 Aramid fiber 6 6 6 6 6 friction powder 8 8 8 8 8 Graphite 10 10 10 10 10 phenolic resin 23 23 23 23 23 Silicates 9 9 9 9 9 Mild abrasive 6 6 6 6 6 Filler 5 5 5 5 5 Sulfides 3 3 3 3 3 Strong abrasive 13 13 13 13 13 Medium abrasive 17 17 17 17 17 TOTALS 100 100

(19) The components shown in Table 1, which indicates values of % by volume compared to the total volume of the blend/mixture were uniformly blended within a Loedige blender in the case of the AMFN/194 mixture.

(20) In the case of the MFN329 mixture at least 40% of the envisaged phenolic resin and mild abrasive content was treated according to the invention in a hot roll blender at a temperature of 75 C., with the rollers rotating at a speed of 20 g/min. and with a gap between the rollers of 1 mm, obtaining a semifinished product in sheet form with a thickness of 1.3 mm, which was subsequently ground and sieved to obtain a powder having a particle size between 5 and 500 microns and preferably between 5 and 250 microns. The powder was mixed with the remaining components in a Loedige blender.

(21) In the case of the MFN330 mixture the same procedure was followed as with the MFN329 mixture, though mixing together, using the roll blender, a phenolic resin and a medium abrasive; in the case of the MF 328 mixture, a phenolic resin, a medium and strong abrasive were mixed together; in the case of the MFN331 blend a phenolic resin and a strong abrasive were mixed together.

(22) Subsequently, all of the mixtures/compositions were subjected to an identical molding and thermal treatment process, subjecting them to molding within a die under a pressure of 20 tons for 3 minutes at a temperature of 160 C., then cured with 240 minutes of thermal treatment at 210 C., thus producing brake pads with a friction material of a substantially identical composition but obtained using different processes.

(23) The brake pads produced as described were subjected to the following tests:

(24) Efficiency tests comprising: bedding in braking events, braking events at different fluid pressures, cold evaluation braking events (<50 C.), freeway simulation braking events, two series of high energy braking events (first FADE test) interspersed by a series of regenerative braking events. From this test it is also possible to extrapolate, in a manner known to a person skilled in the art, the wear to which the pad is subjected.

(25) Some comparative test results are reported in FIG. 3 of the annexed drawings and in table 2 below.

(26) From an examination of FIG. 3 it is evident that the braking coefficient remains more constant under equal conditions in the case of mixtures MFN329, 328, 330, whilst there are great variations in the case of the MFN331 mixture.

(27) TABLE-US-00002 TABLE 2 AMFN/ MFN MFN MFN MFN FORMULA 194 328 329 330 331 Outboard Pad 26.8 20.1 21.7 23.7 25.4 Wear [gr] Inboard Pad 25.5 19.8 21.7 22.1 25.3 Wear [gr]

(28) Comparing the wear of the brake pads on the outboard side (towards the outside of the vehicle) and the inboard-side (towards the inside of the vehicle) there is an obvious reduction in the wear of the pads for the formulations/mixtures MFN329, 328 and 330 manufactured according to the invention compared to the comparison (reference) formulation AMFN/194 and to the MFN331 formulation wherein only a non-fibrous material as the strong abrasive was treated together with the phenolic resin for blending using the rolling mill blender.

(29) Finally, from FIG. 2, it is clear that the treatment according to the method of the invention provides a much finer and homogeneous microstructure of the fiber material with loss of the overall fibrous structure. The greater homogeneous nature of the friction material obtained by means of the method of the invention is particularly advantageous, both in terms of performance and robustness of the friction material.

(30) The objectives of the invention are therefore fully achieved.