Mixing method for friction material with a pre-mix in a single mixer

The present invention relates to friction materials and, more particularly, to a mixing method for friction materials using a pre-mix in a single mixer.

Pre-mixes have been used in the manufacturing of brake pad friction materials for some time. Friction materials produced with a pre-mix tend to process quickly, and exhibit improved performance characteristics in the completed brake pad as compared with pads produced without a pre-mix. For example, brake pads formed of pre-mixed materials tend to exhibit improved noise/vibration/harshness characteristics.

Typically, friction materials include rubber, fiber, and filler components. Usually an intensive rubber mixer is needed to masticate the binder material(s) and thoroughly mix it in with the other raw materials. To begin, a first material is added to the mixer. After mixing, the first material may be granulated to a desired size by a chopper to form a pre-mix. This pre-mix is then transferred to a second mixer, where other raw materials are added to the pre-mix. The resulting mixture is then mixed to produce the final friction material. The friction material can subsequently be formed into brake pads or other friction material products through any of a variety of methods, such as compression molding, for example.

The pre-mixing process may be relatively inefficient. First, granulating the premix in between the first mixer and the second mixer can be laborious and costly. Second, the premix has a limited shelf-life due to the tendency of any chemical components of the pre-mix to cure within a short period of time.

The present invention discloses a method of producing a friction material. Initially, a first non-asbestos material is added into a single mixer. The first non-asbestos material is comprised of at least one of a binder material, an abrasive material, and a filler material. At least one of the binder, abrasive, and filler materials doubles as a wetting agent. The first non-asbestos material is then mixed in the single mixer to form a pre-mix.

A second non-asbestos material is next added into the same single mixer. This second non-asbestos material is also comprised of at least one of the binder, abrasive, or filler materials, such that the addition of the second non-asbestos material results in at least one binder material, at least one abrasive material, and at least one filler material being present in the single mixer. The second non-asbestos material is then mixed with the pre-mix to create a friction material.

Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the various embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

FIG. 1 is a cross-sectional view of a disc brake system; and

FIG. 2 is an exemplary flowchart of a friction material pre-mixing process in accordance with the present invention.

The following description of the various embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

In FIG. 1 , the present invention is shown with reference to a simplified and exemplary vehicle disc brake system 10 . The disc brake system 10 includes a rotor 12 , a caliper 14 , and a hub 16 . The disc brake system 10 also includes a pair of outboard and inboard brake elements 18 a and 18 b , respectively, referred to hereinafter as brake elements 18 or brake pads 18 . The brake elements 18 mount to the caliper 14 in a manner known in the art. Each of the brake elements 18 includes a structural backing 20 and a friction material 22 . The friction material 22 mounts to the structural backing 20 using mechanical fasteners and/or chemical bonding (not shown). The brake elements 18 squeeze against rotor 12 to slow the rotation of the rotor 12 to thereby slow the vehicle (not shown) to a desired speed. As noted above, friction is produced when the brake elements 18 come into contact with the rotor 12 ; this in turn, causes the brake elements 18 to heat and ultimately wear. It will be appreciated that the above description is also applicable to a drum brake configuration, a clutch lining configuration and other non-vehicle configurations, for example, slowing machine parts. As such, the scope of the present invention is not to be construed as being limited solely to brake pad applications.

Referring to FIG. 2 , a process for producing a friction material for use in brake pads, brake shoes, clutch linings and other suitable applications is illustrated in accordance with the present invention. Initially a first non-asbestos material is added to an intensive mixer, as shown at block 100 . The mixer must be robust enough to granulate the materials properly. One such suitable mixer is a Littleford Ploughshare mixer commercially available from Littleford Day Inc. in Florence, Ky. Preferably, the mixer will allow for variable mixing speeds, and drying operations such as heat drying or vacuum drying, as may be necessary. The first non-asbestos material includes at least one of a binder material, a fiber material, and a filler material. The first non-asbestos material also includes a wetting agent, which may be a rubber latex (natural, styrene butadiene, acrylic, or silicone rubber), depolymerized rubber, cashew nut shell liquid based resin, or liquid phenolic resole resin. It is contemplated that the wetting agent may comprise about 0.5% to about 10% by weight of the first and second non-asbestos material. Chemical curative agents such as sulfur or peroxides may be added to the wetting agents as required to cure the first non-asbestos material during mixing, if desired. Other solvents may also be added to enhance the effect of the primary wetting agents. These solvents may include water, or alcohols such as isopropynol or propylene glycol. Water may be added directly, or in the form of wet aramid or cellulose pulp.

In one specific example, the first non-asbestos material is comprised of the materials listed below in Table 1, with percentages denoting their percent composition by volume of the pre-mix:

The materials listed in Table 1 are then added to the mixer as discussed above.

Once the first non-asbestos material is added to the single mixer, it is mixed as shown at block 110 to create a pre-mix. This step is carried out at a first mixing speed. The mixing step continues until the first non-asbestos material is granulated to a desired size. The mixing speed may be varied as deemed appropriate for the given material composition. In the specific example referred to above, the first non-asbestos material is mixed at a mixer speed of approximately 1500 R.P.M. for approximately five minutes, or until granulated to a desired size, to create a pre-mix.

The vigorous mixing process may raise the temperature of the first non-asbestos material, which will promote the homogeneity of the resulting pre-mix, and the operation of any curing agents. Additionally, as shown at optional block 115 , heat may be otherwise artificially added to the first non-asbestos material by any other means effective in promoting the curing action, if so desired.

A second non-asbestos material is next added to the pre-mix in the same mixer in which the pre-mix was created, as shown at block 120 . This second non-asbestos material, like the first non-asbestos material, includes at least one of a binder, a fiber, and a filler material. The composition of the second non-asbestos material can be determined according to the desired composition of the final friction material to be produced, taking into account those materials already present in the pre-mix. In the specific example referred to above, the second non-asbestos material is comprised of the materials listed below in Table 2, with percentages denoting their percent composition of the second material by volume:

For the specific example outlined in Tables 1 and 2, the second non-asbestos material should be added in a quantity such that it comprises about 72% of the total material present in the mixer by volume, with the pre-mix comprising about 28% of the total material present in the mixer by volume. The overall material composition of the material present in the mixer should therefore be as listed below in Table 3, with the material and percent composition by volume of the final friction material:

For this specific example, it should be noted that the relative material composition of the pre-mix and second non-asbestos material may be adjusted as required, so long as the end result still achieves the material composition in the final mix shown in Table 3.

After the second raw material mix is added to the same mixer, mixing continues, as shown at block 130 . This second mixing step is performed at a second mixing speed. The second mixing speed may be less than, greater than, or equal to the first mixing speed. In the specific example referred to above, this final mixing step is carried out at about the same speed as for the first mix, approximately 1500 R.P.M., and continues for approximately five minutes, or until granulated to the desired size, to create the final friction material.

When the mixing is completed, and the mixture has achieved a homogeneous consistency, it may be necessary to dry the material with a vacuum drying or heat drying step, as shown at optional step 140 . Whether this will be necessary will depend on the composition of the pre-mix and the second non-asbestos material, and the degree of moisture provided by the pre-mix. Pre-mixes having a higher moisture content will generally lead to a higher moisture content in the final friction material, and may require a longer or more robust drying step.

After the final friction material is completed, it may be processed in a variety of ways to form the final product, such as compression molding.

The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.