Patent Application
20070287768
Exemplary embodiments of the invention will be described.
In the exemplary embodiments of the invention, a part of conventional graphite added to a friction material is replaced with a partially graphitized coke having elasticity, so that compression deformation is increased even when the other materials and forming conditions are not changed.
The “partially graphitized coke having elasticity” is preferably one where graphitization is stopped on the way of graphitizing starting cokes for graphitization to obtain an elastic graphite and a graphitization ratio thereof is from 80 to 95%. As the above starting cokes, there may be mentioned those which are used as production starting materials for elastic graphite, such as carbonaceous mesophases or raw cokes formed during thermal treatment of coal tar pitch, coal-based pitch, petroleum-based pitch, or the like at about 350 to 500° C., or those obtained by treating them with nitric acid or a mixed acid of nitric acid and sulfuric acid.
The graphite usually used in the production of the friction material has properties that a compressibility ratio at a load of 900 MPa is about 70% in volume change on the basis of the time when a load of 0.01 MPa is applied and a recovery ratio at the time when the load is removed is 30% or less. Therefore, it is not sufficient for the purpose of compensating insufficiency of parallelism of a friction surface by elastic deformation of the friction material.
Moreover, JP-A-03-282028 describes an elastic graphite excellent in a compressing property and a recovering property wherein a compressibility ratio at a load of 900 MPa is 80% or more in volume change on the basis of the time when a load of 0.01 MPa is applied and a recovery ratio at the time when the load is removed is 50% or more. However, even in the case of the graphite having such physical properties, the compressibility ratio is still small and hence the object of the invention cannot be achieved.
With regard-to the partially graphitized coke (elastic coke) to be used in the friction material of the exemplary embodiment of the invention, the recovery ratio at the time when a load is removed is extremely high, i.e., from 90% or more to almost 100%. Moreover, the compressibility ratio at a load of 900 MPa is 80% or less, preferably 60% or less. Thus, the coke is anelastic graphite having physical properties different from those of the above conventional graphite. More preferably, an elastic graphite having a recovery ratio of 95% or more and a compressibility ratio within the range of 60% to 40% is used.
A graphitization ratio of the partially graphitized coke (elastic graphite) to be used in the invention measured by X-ray diffraction falls within the range of 80% to 95%. With regard to the amount to be added, a sufficient effect is observed by adding a small amount thereof and it is suitable to blend it in a ratio of 0.5 to 2.5 vol %, preferably 2.0 to 2.5 vol %. Furthermore, in the friction material, the partially graphitized coke is used in combination with conventional graphite (natural graphite, artificial graphite).
The compressibility ratio and recovery ratio of the partially graphitized coke to be used in the exemplary embodiments of the invention are measured by the following method. Namely, about 10 g of a graphite sample is placed in a stainless steel cylindrical mold having an inner diameter of 10 mm. After the mold is patted to attain closest packing, a pushrod is inserted from the upper portion and then a load of 10 Pa is applied. Height of the sample at that time is measured and the value is represented by h0. Then, a predetermined load is applied and height of the sample is measured, the value being represented by h1. Thereafter, the load is removed and height of the sample is measured, the value being represented by h2. From these values, a compressibility ratio and a recovery ratio can be determined according to the following equations.
Compressibility ratio (%)=(h1/h0)×100 (1)
Recovery ratio (%)=((h2−h1)/h0)×100 (2)
In the blending of the friction material, those usually used are employed. As the fibrous base material for reinforcement, heat-resistant organic fibers, inorganic fibers, and metal fibers are used. There maybe, for example, mentioned aromatic polyamide fibers and flame-resistant acrylic fibers as the heat-resistant organic fibers; ceramic fibers such as potassium titanate fibers or alumina fibers, glass fibers, and rock wools as the inorganic fibers; and copper fibers and steel fibers as the metal fibers.
As the binder, there may be, for example, mentioned phenol resins (including straight phenol resins and various modified phenol resins with rubber or the like), melamine resins, epoxy resins, polyimide resins, and the like. Moreover, as the friction modifier, there may be, for example, mentioned organic friction modifiers such as rubber dust and cashew dust, abrasives of metal oxides and the like such as alumina, silica, magnesia, zirconia, chromium oxide, and quartz, particles of metals such as copper, aluminum, and zinc, solid lubricants such as graphite and molybdenum disulfide, scale-like inorganic substances such as mica and vermiculite, and inorganic fillers such as barium sulfate and calcium carbonate.
The production of the friction material can be conducted by well-known production processes. For example, the friction material can be produced via steps of pre-forming, thermal forming, heating, grinding, and the like. In the case of a production process of a friction pad for disc brake, there are conducted a step of preparing a pressure plate by forming a predetermined shape by sheet-metal pressing, subjecting it to a degreasing treatment and a primer treatment, and then applying an adhesive; a step of producing a pre-formed product by blending a fibrous base material of a heat-resistant organic fiber, an inorganic fiber, or a metal fiber and powder raw materials such as an inorganic/organic friction modifier, a filler, and a thermosetting resin binder and forming (pre-forming) the raw materials, which have been sufficiently homogenized by stirring, at ordinary temperature under a predetermined pressure; a step of thermal forming of the both members to integrally fix them at a predetermined temperature and pressure; a step of conducting after-cure; and finally a step of conducting a finish treatment. Thus, the friction material can be produced by such a process.
The following will describe the invention in more detail with reference to Examples but the scope of the invention is not limited only to these Examples.
The production process of Examples is as follows.
In any cases, the blend materials shown in Table 1 were charged into a stirrer all at once, followed by stirring.
Each of the above five kinds of stirred products was subjected to steps of pre-forming, thermal forming, heating, grinding, or the like to prepare a friction material as a finished product.
The above stirred product was charged into a mold for a pre-forming press and pressurized at ordinary temperature under a pressure of 40 MPa for 1 minute and thereby pre-formed into a brake pad shape.
The resulting pre-formed product was transferred into a mold for hot pressing in which a pressure plate had been set. After gas-venting was conducted five times at intervals of 10 seconds during heating and pressurization at 150° C. and 50 MPa, thermal forming was conducted at 150° C. and 50 MPa for 4 minutes.
After the thermal forming, the product was further heated in a heating furnace at 250° C. for 3 hours to carry out after-cure.
After the after-cure, the product was grinded so as to be a predetermined thickness by means of a plane grinder to obtain a friction material as a finished product (brake pad).
A partially graphitized coke having elasticity (RGC 14A from Superior Graphite Co.) was added in a ratio of 0.5 to 2.5 vol %. With the formulation, as mentioned above, after steps of dry stirring, pre-forming, thermal forming (see Table 1 for temperature and pressure), and heating were conducted, grinding was carried out to obtain a finished product.
The partially graphitized coke having elasticity was added in a ratio of 3.0 vol %. Production conditions are in accordance with Examples 1 to 3.
The partially graphitized coke having elasticity was not added. Production conditions are in accordance with Examples 1 to 3.
Then, compression deformation was measured on the friction materials of Examples 1 to 3 and Comparative Examples 1 and 2 (thickness of mother material: 2.5 mm, thickness of pressure plate: 5.5 mm, sliding area: 50 cm2). Results of the measurement are shown in
As shown in
As above, the friction materials obtained by utilization of the invention are friction materials having an appropriate compression deformation. Thus, it was possible to obtain a good pedal feeling property.
Since the invention can provide a friction material excellent in pedal feeling property, the material can be used in various kinds of vehicles and hence is industrially very valuable.
It will be apparent to those skilled in the art that various modifications and variations can be made to the described exemplary embodiments of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover all modifications and variations of this invention consistent with the scope of the appended claims and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| P.2006-160963 | Jun 2006 | JP | national |
