Binder-Free Friction Linings, Process for Their Production and Their Use

Abstract

Expanded graphite and/or vermiculite are introduced into the friction lining mixture to eliminate or substantially reduce the amount of binder present therein. The friction lining at most contains from 2.5% by weight of binder. By omitting/reducing binder, the production is very stable and the friction linings can be easily reproduced. The process steps of hardening (including the hardening furnace) and scorching (including the necessary equipment) can be omitted when making such friction linings.

Claims

1. A method for producing a friction lining, comprising: preparing a friction lining mixture that contains from 10% to 65% by weight of steel fibers, stainless steel fibers and/or iron fibers, from 5% to 26% by weight of expanded graphite and/or expanded vermiculite, and optionally a binder in a maximum amount of 2.5% by weight relative to the friction lining mixture, wherein the amount of copper is 0% by weight; compressing the friction lining mixture in a heated mold at a temperature in the range of from 135° C. to 240° C.

2. The method according to claim 1, wherein the friction lining is compressed in the heated mold at a pressure of from 45 Mpa to 200 Mpa for a pressing time duration of from 50 seconds to 300 seconds.

3. The method according to claim 1, wherein the expanded graphite has a particle size in a range of from 0.25 mm to 1.5 mm.

4. The method according to claim 1, wherein the friction lining mixture is binder-free.

5. The method according to claim 1, wherein the friction lining removed from the mold is not hardened.

6. The method according to claim 5, wherein the friction lining removed from the mold is not scorched.

7. A friction lining produced by the method of claim 1.

8. A friction lining produced by the method of claim 4.

9. A friction lining produced by the method of claim 5.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0020] The problem is solved according to the invention by means of a metal-containing friction lining containing no binder or only a very small share of binders. The percentage of binders is preferably a maximum of 2.5% by weight (here and below, all weight % information is based upon the finished friction material mixture from which the actual friction lining is produced), further preferably up to a maximum of 1% by weight, in particular up to a maximum of 0.9% by weight, and particularly preferably 0% by weight. Particularly suitable are organic or inorganic binders that harden during hot pressing in the range between 100° C. and 200° C. The binders that are typical for friction linings, such as phenol resins, novolaks (or novolacs) as condensation products from formaldehyde and excess phenol, polyimides, cyanate ester, phthalonitrile resins, and silicone resin can be used as binders, wherein the phenol novolak resins are preferred according to the invention. Binder mixtures can also be used as binders.

[0021] The friction linings according to the invention are preferably semi-metallic friction linings with metal shares of 30 to 65% by weight, wherein friction lining mixtures/friction linings from the so-called low-metallic range (metal share 10 to 30% by weight) are covered by the invention.

[0022] For the purposes of the present invention, the term “metal” or “metallic” covers all metals or metal alloys in all of their forms occurring in semi- or low-metallic friction materials, such as fibers, wool, etc.

[0023] Fibers are the preferred form, in particular for the metals or metal alloys. These fibers should not be too fine or too short, because otherwise the mechanical stability of the respective friction lining could suffer. Too long fibers or wool can cause problems in the mixing of the starting components and their distribution in the resulting friction material mixture. The fibers preferably have an essentially rectangular cross section and are therefore preferably not drawn out in design. It is therefore preferable to choose the scraped fibers commonly used in the field of brake production. The dimensions of such fibers, which are commercially available, are known to the prior art and to a person skilled in the art.

[0024] With respect to metals, the friction linings according to the invention preferably contain iron or steel, in particular stainless steel, aluminum, or brass. Copper would also be suitable, but is not often used due to existing environmental regulations. Particularly suitable are iron, steel, and stainless steel, which are preferably provided as fibers having the parameters/dimensions described above. Various metals can also be used simultaneously here.

[0025] The preferred share of metals in the finished friction lining mixture from which the friction lining is produced is preferably at least 30% by weight, in particular at least 35% by weight, and particularly preferably at least 40% by weight. The maximum share of metal is 65% by weight.

[0026] The metal fibers present in the friction material, in particular steel or stainless steel fibers, are usually sufficient to provide the friction lining resulting from the pressing of the friction material mixture with the necessary mechanical stability and the necessary braking properties.

[0027] In order to further improve the stability and the property profile of the friction linings according to the invention, it has proven to be advantageous to use, in addition to the components that ensure a good property profile mechanically, at least one additional component that additionally strengthens the cohesion of the friction lining components and thus of the friction lining itself in a not purely mechanical manner. Expanded graphite and expanded vermiculite have proven to be particularly suitable here. Electrode graphite is not suitable for the purposes of the invention. Expanded graphite or exfoliated graphite are particularly suitable. Graphite is constructed from graphite layers, between which only very weak binding forces prevail, whereby its good sliding and lubrication properties can be explained. Expanded graphite is formed by the storage of molecules between these graphite layers. Through the storage of acids, usually sulfuric acid, graphite is converted into exfoliated graphite. Graphite is preferably used with a particle size of approximately 0.25 mm to approximately 1.5 mm, and further preferably from approximately 0.5 mm to approximately 1 mm.

[0028] Vermiculite is a layer silicate belonging to the clay minerals and forming flaky crystals. Exfoliated mica or expanded vermiculite are formed by the processing of mica schist. Here, raw vermiculite is thermally expanded, whereby the chemically bound crystal water between the layers is expelled and the volume is increased by up to 35 times the starting value. Expanded vermiculite or exfoliated vermiculite is suitable according to the invention.

[0029] All materials described are commercially available.

[0030] In the event of a complete or extensive omission of binders (such as phenol resins, novolaks (novolacs)) during the friction lining production, the use of the metal fibers preferred according to the invention (in particular steel or stainless steel fibers) and without expanded graphite or expanded vermiculite can be sufficient to obtain friction linings with a sufficient or good property profile. By comparison, the omission of metal fibers does not lead to the desired result. The use of the specified combination of metal fibers (also in the form of different types simultaneously) and expanded graphite and/or expanded vermiculite (also in the form of different types simultaneously) is preferred.

[0031] In general, the metal fibers are used at 30-65% by weight (in accordance with the semi-metallic friction lining category described above), and the second material category (which is not effective due to mechanical solidification, expanded graphite, and/or expanded vermiculite alone) is used at 5-26% by weight. The % by weight information is based upon the finished friction lining mixture to be pressed.

[0032] The production of the friction linings according to the invention takes place according to the methods well-known to the prior art and to a person skilled in the art by mixing the starting components and pressing the friction lining mixture at increased pressure and increased temperature. Preferred parameters in this respect are, for example, temperatures of 135-240° C. at a pressure of 45-200 MPa for a duration (pressing time) of 50-300 seconds.

[0033] In principle, the process for producing friction linings according to the invention can also be described as follows:

[0034] Assuming a typical semi-metallic friction lining composition such as:

TABLE-US-00001 Component Share in % by weight Graphite 26 Steel fiber 53.9 Phenol resin 0.9 Petroleum coke 8.8 Calcium fluoride 2.6 Silicon carbide 2.6 Magnesium oxide 5.2
the components steel fibers and graphite, and possibly also the binder (here, phenol resin), are mathematically removed from the calculation, and the remaining composition (here, 19.2% by weight remaining) is included in a friction lining mixture according to the invention at, for example, 60% by weight. The missing 40% by weight then consists of metal fibers according to the invention, expanded graphite and/or expanded vermiculite, and possible binder, if no absolutely binder-free friction lining composition is desired. According to the invention, the binder share is a maximum of 2.5% by weight. The remaining composition can be included at more or less than 60% by weight in the friction lining mixture according to the invention. It contains the typical components of the friction lining categories described above.

[0035] The binders that, according to the invention, are preferably not present or only in a very small share in the friction linings/friction lining mixtures are the binders commonly used in the friction lining production of the prior art. Examples of this are given above. These are usually binders that are or become liquid under the process conditions of the friction lining production and then act as chemical adhesives. For the purposes of the present invention, expanded graphite and expanded vermiculite are not considered to be binders, because they are not self-adhesive but realize a mechanical binding under pressure in the friction lining composition according to the invention.

[0036] Due to the complete or at least extensive omission of binders (classic or liquid binders) according to the invention, the production of high-performance friction linings is significantly simplified and made less expensive. Due to the omission of the binder, the production is very stable and the friction linings can be easily reproduced. In particular, it should be pointed out that the process steps of hardening (including the hardening furnace) and scorching (including the necessary equipment) can be omitted for such friction linings.

[0037] Additional objectives, advantages, features and application possibilities of the present invention can be gleaned from the description herein of embodiments. In this context, all of the described features, either on their own or in any meaningful combination, constitute the subject matter of the present invention, also irrespective of their compilation in the claims or in the claims to which they refer back.