The present invention relates to an asbestos-free friction material, in particular a friction material of the type known as NAO ("Non Asbestos Organic"), i.e. asbestos- free organic friction.

In particular, the invention relates to an asbestos- free organic friction- material adapted for use as a friction - material for automobile brakes and other similar applications (for example, clutch disks) , generally in the automotive field because of the good resistance to wear, the mildness with reference to the mating surfaces (within the field of brakes for vehicles, the surface of the brake disk or drum), which makes it possible to avoid scoring of the mating surface, and adequate braking capacity (friction stability, in particular at high temperature) .

State of the art

The friction materials used as brake shoes for drum brakes and as brake pads in disk brakes for vehicles and other equipment (for example, clutch disks) , comprises a base made of fibrous material, a binder and a filler. In the past, asbestos was used in relatively large measure as a fibrous material, but presented considerable environmental problems and had well known toxic effects on human health, and . for this reason has been banned by legislation for some time. Therefore, this material was replaced by other materials, both inorganic, like rock wool, wollastonite , fiberglass, and organic, like aramid fiber and carbon fiber, as well as metallic, such as fibers or powder of copper, tin, iron, aluminum and other metals or alloys, such as bronze or brass. The binder is a thermosetting polymer, usually pheno,lic-resin based. Various materials are used as the filler, such as baryte (barium sulfate) , calcium carbonate, magnesium oxide, talc, zirconium silicate, zirconia, mica, metal pulverulents , molybdenum disulfide, rubber ( powder or granules), graphite. In particular, graphite and molybdenum bisulfide improve the resistance to wear.

EP1227262 discloses a friction material of the type described above, including approx. 10% by volume of copper fiber as the fibrous material and 0.1 to 15% by volume of tin and/or tin sulfide for the purpose of eliminating the use of antimony compounds while still preserving good resistance to wear, especially at high temperature.

EP482204 too discloses a friction material of the type described above including from 4 to 9% by volume of bronze fibers, which resolves the problem of avoiding attachment or "gluing" of the brake shoe or brake . pad to the contact surface of the disk or drum during use via the simultaneous presence of a mixture of three tin compounds (including metallic tin) that are different from each other and have different melting points.

Nevertheless, for the purpose of preserving the atmosphere and avoiding possible damage to human health, the various national and international standards impose more and more frequently the use of friction materials that are not only free of asbestos and heavy metals but that also have a reduction or complete elimination of copper (standards already approved by some American states and that will enter into force in the coming years) .

Therefore, in the art the necessity exists of providing a friction material free of asbestos, heavy metals and copper, but at the same time preserving performances at least comparable to those of friction materials that include such substances.

Summary of the invention

The object of the present invention is to provide an asbestos -free organic friction material free of copper and copper compounds, such as bronze and brass, capable of overcoming the disadvantages of the prior art and exhibiting braking performances comparable to those of asbestos-free organic friction material containing copper.

The invention is therefore related to a friction material free of asbestos as defined in claim 1. In particular, the friction material according to the invention is formed by a composition or mixture comprising a fibrous base, a filler and an organic binder, is completely free of copper and copper compounds or alloys, but includes in the fibrous base together with inorganic and/or organic fibers from 1% to 10% by volume, calculated on the total volume of the composition of stainless steel fibers, for example the classic UNI X 10 Cr Ni 1808 (known as "inox 18-8") or the equivalent stainless steel, 10% (by weight) of Ni (known as "inox 18-10") .

The friction material according to the invention also contains from 1% to 25% by volume of a "system of fibrous oxides", according to the invention based on a metallic titanate, preferably of potassium hexatitanate (K ₂ Ti ₆ 0i ₃ ) or other titanates of potassium and/or sodium, added with zirconia (which, per se, is not a fibrous material), in variable proportions.

In particular, in. the composition or mixture making up the friction material according to the invention, the ratio between the percentage content by volume of zirconia and of the titanate (in particular of potassium hexatitanate) is between 1 : 1 and 1:2.

Furthermore, the ratio between the percentage content by volume of stainless steel fiber and the sum of the percentage content by volume of zirconia and titanate must be between 1 : 4 and 1:8.

The composition of the friction material utilized to form the asbestos -free organic friction material of the invention may comprise from 0% to 10% by volume of solid lubricants, such as tin sulfides, like SnS and SnS ₂ .

It is also desirable to include graphite and/or coke in the composition.

The., graphite can be any known graphite commonly utilized in friction materials.

The graphite (and/or coke) is added in an appropriately selected quantity, which preferably comprises between 2% and 15% by volume of the total composition of friction material.

The other components of the composition ^■ of the friction material can be components utilized in friction materials already known in the art.

In particular, the rest of the fibrous base can be any organic fiber or inorganic fiber other than asbestos that is commonly used in friction materials. Exemplary embodiments include inorganic fibers, such as glass, fibers, rock wool, wollastonite, sepiolite and attapulgite, and organic fibers, such as carbon fibers, aramid fibers, polyimide fibers, polyamide fibers, phenolic fibers, cellulose and acrylic fibers or PAN (polyacrylonitrile) . The fibrous base can be utilized in short fiber or powder form.

It is preferable that the fibrous base, and in particular the stainless steel fibers used, has a total fiber length of between 0.5 and 1.5 mm and a fiber diameter from 40 to 150 microns.

Numerous materials known in the art can be used as organic or inorganic fillers. Exemplary embodiments include calcium carbonate precipitate, barium sulfate, magnesium oxide, calcium hydroxide, calcium fluoride, slaked lime, ^" talc, molybdenum trioxide, zirconium silicate, iron oxide, mica, iron sulfide, silicon dioxide, vermiculite, rubber powder (rubber powder and granules) , nitrile rubber powder (vulcanized product) , metal powders (excluding copper and its alloys), acrylic rubber powder (vulcanized product) .

These can be used by themselves or in combinations of two or more of them. The quantity of such fillers is preferably comprised from 2% to 40% by volume based on the total composition of the friction material.

The binder can be any known binder commonly used in friction materials.

Exemplary embodiments of suitable binders include phenolic resins, melamine resins and epoxy resins; various modified phenolic resins, such as epoxy-modified phenolic resins, oil-modified phenolic resins,, alkylbenzene-modified phenolic resins and acrylonitrile butadiene rubber (NBR) .

Any one of these compounds or combination of two or more of them may be used. The binder is included in a quantity preferably from 2% to 30% by volume based on the total^ composition of the friction material.

According to another aspect of the invention, the friction material according to the invention, in addition to stainless steel fibers , may also include zinc fibers having a total length of between 0.2 and 1.5 mm, total diameter of between 50 and 250 μττι in a total quantity of between 1 and 10% by volume.

More generally, the friction material according to the invention exhibits the following percentage composition calculated by volume relative to the total volume:

ORGANIC FIBERS 1 - 10

INORGANIC FIBERS 1 - 20

RESI 5- 30

CARBON MATERIALS 1 - 15

OTHER METALS - 0 -10

INORGANIC SULFURS 0 - 15

INORGANIC SALTS 0 - 30

INORGANIC OXIDES 1 - 30

SILICATES 1 - 20

TITANATE 1 - 25 STAINLESS STEEL 1.- 10

The friction material of the invention is generally produced by uniformly mixing the specific quantities of fibrous base described above together with the binder and filler in a suitable mixer, for example, a Henschel, Loedige or Eirich mixer.

The pressing is carried out at a temperature between 130 and 200°C (266 and 392°F)at a pressure from 150 to 1500 Kg/cm2 (14.7-147.1 MPa) for a time between 3 and 10 minutes or preforming the mixture in a die and thereafter pressing at a temperature from 130 to 180°C (266 to 356°F)at a pressure from 150 to 500 kg/cm ² (14.7-49 MPa) for a period of from 3 to 10 minutes.

The resulting pressed item is typically post-cured via thermal treatment at 150-250°C (302-482°F) for from 2 to 10 hours, then spray-painted or powder-coated, oven-dried and possibly machined where necessary to produce the final product-

The friction material of the invention can be utilized in applications such as disk linings, shoes and pads for brakes for automobiles, trucks, railroad cars and other various types of vehicles and industrial machines or in the clutch disks.

Brief description of the drawings

The present invention will now be described in more detail with reference to the following practical and comparative examples and with reference to figures 1 to 4 of the appended drawings, which illustrate in graphical form the results of the braking performance test.

Detailed description

The examples and comparative examples are recorded for the purpose of illustration and . are not intended to limit the invention.

EXAMPLES

The components shown in table 1 were uniformly mixed in a Loedige mixer and pressed in a die under a pressure of 200 kg/cm ² (19.6 MPa) for 3 minutes at a temperature of 160°C (320°F) . then cured with 5 hours of heat treatment at 200°C (392°F) , producing a friction material according to the invention, indicated as "Formula 2," the comparative material, indicated as "Formula 3," having a composition substantially identical to the material of the invention, but free of stainless steel fiber, replaced by an equal quantity of barium sulfate, the comparative material, indicated as "Formula 4," having a composition identical to the one according to "Formula 2" but in which the potassium titanate was substituted with baryte, and the comparative material according to the known art, indicated as "Formula 1," containing copper. TABLE 1

The material obtained in this way was mounted on a vehicle and tested with a bench test according to the following cycle:

• Test setup: 96 brakings from 80 to 30 km/h (49.7 to 18.6 mph)

• Characteristic value: 8 brakings from 80 to 40 Km/h (49.7 to 24.8 mph)

• High speed fade test: 5 brakings from 168 to 80 km/h (104.4 to 49.7 mph) and 5 brakings from 168 to 5 km/h

(104.4 to 3.1 mph)

• High speed fade test: 9 brakings from 168 to 80 Km/h (104.4 to 49.7 mph)

• Characteristic value: 8 brakings ^' from 80 to 40 Km/h (49.7 to 24.8 mph)

The results obtained are illustrated in the graphics of figures 1-4.

In particular, figure 1 refers to the "Formula 3" comparative material free . of both copper and its compounds/alloys and stainless steel fibers.

Figure 2 refers to the material of the "Formula 1" state of the art containing copper.,

Figure 3 refers to the "Formula 2" material of the invention free of copper but containing stainless steel fiber together with potassium titanate and zirconia.

Figure 4 refers to the "Formula 4" comparative materials free of copper and containing stainless steel fibers, but free of titanate.

As is evident, observing in particular the curves described by the points signed with a cross and representing the consumption -of oil during braking > the friction material according to the invention, free of copper, has a braking performance comparable overall to that of the material known in the art, containing approx. 6.9% in volume of copper. Vice versa, the "Formula 3" comparative material, free of both copper and stainless steel, while still having an overall assimilable composition, for the other components, to the material of the invention, exhibits an oil consumption during braking that grows exponentially, which means that the braking of the vehicle gets progressively. "longer," behavior generally unacceptable on a vehicle.

From a comparative examination of figure 4 with the other figures, it is further evident how the material having the combination of stainless steel fibers with potassium titanate (figure 3) performs decidedly better than the same material, with stainless steel in place of the copper, but free of the potassium titanate. Nevertheless, the latter "formula ^' 4" material, exhibits a behavior decidedly better than that of the "formula 3" control material, inasmuch as, while exhibiting greater oil consumption than that of the "formula 1" state of the art material, it does not exhibit the exponential trend that the "formula 3" control material instead exhibits, indicating that the presence of the specifically selected stainless steel fibers, even by themselves and in the absence of copper and its alloys., renders an innovative result.

The objectives of the invention are therefore fully- achieved.