The present invention relates to a production process for carbon brushes and to carbon brushes obtainable thereby.

Carbon brushes are usually produced by molding a pulverulent material, substantially based on carbon, with a finely divided binder and, for certain applications, with addition of metal powders, and then subjecting the molded material to a heat treatment for coking, curing or distribution of the binder or sintering of the metal powder. The carbon brush thus obtained is then optionally subjected to final mechanical processing.

For example, pitches and tars, synthetic thermosetting resins, such as, for example, phenolic resins, or thermoplastics have been used to date as binders.

Thus, US-A-5,441 ,683 describes the production of a brush for an electric motor using so-called "distillation binders" (i.e. residues from mineral oil or coal distillation, such as, for example, pitches and tars) or synthetic binders, preferably thermosetting resins, particularly preferably phenolic resins.

DE-A-103 24 855, too, discloses the preferred use of a thermosetting resin, for example of a phenolic resin, or of pitch as a binder in the production of carbon brushes.

In DE-A-101 56 320, a synthetic powder resin, in particular a thermosetting plastic, for example an epoxy or phenolic resin, is used as a binder for the production of a resin-bound graphite material. The binder is preferably a powder resin dissolved in a solvent or a solvent-free liquid resin.

GB-A-641 ,937 relates to the production of electrical contact brushes and describes the use of a binder in a solution of a volatile solvent in order to distribute the binder thoroughly.

DE-A-24 44 957 discloses the production of carbon contact bodies by pressing of powders of graphite and a metal onto one another in a die and subsequent heat treatment. The powder contains from 0.5 to 50 parts by weight of a binder from the group consisting of the monovalent aromatic polymers, e.g. polyarylene sulphides, preferably polyphenylene sulphide in finely milled form.

A thermoplastic binder is also used in the process for the production of a carbon brush, which process is the subject of DE-A-199 00 23. Polyphenylene sulphides, polysulphones, polyphenylene sulphones, polyether ketones, polyethylene terephthalates, polyamides, polyimides and copolymers derived therefrom are mentioned as examples.

DE-A-199 00 24 describes a process for the production of a carbon brush from a pulverulent material, which is characterized by the use of an undissolved pulverulent binder comprising fine-grained thermoplastic powder having an average particle size of 5 μm to 50 μm and a very narrow particle size distribution.

Pitch-bound materials for carbon brushes, which also contain copper, silver or other metal powders and which are produced with pressed-in copper stranded wires, have the disadvantage of sulphide formation on metal powders and stranded wires owing to the ever present sulphur content of the pitches. Similar problems arise with the use of polymers which contain sulphur (e.g. polyphenylene sulphide), which is at least partially liberated under the conditions of production or use. Many pitch-based binders are also carcinogenic.

An inevitable direct disadvantage of all production processes for carbon brushes which operate with dissolved thermoplastic or thermosetting polymers as binders is the handling of a volatile solvent which entails very complicated apparatuses for avoiding health problems and environmental pollution and should as far as possible be avoided. In addition, soluble thermoplastics as binders lead to end products which have comparatively low resistance to solvents and high temperatures of use.

Particularly in association with heat treatments which lead to their partial or complete coking, the use of thermosetting polymers as binders leads to end products which, in applications with stresses due to high temperatures and atmospheric humidity, have insufficient dimensional stability (swelling) and considerable resistance ageing (increasing resistance).

A disadvantage in the case of the abovementioned processes which use a pulverulent thermoplastic as a binder is the poor moldability of the powder mixtures. In addition, difficulties occur in the reproducibility of the material properties of the carbon brushes thus produced.

It is therefore an object of the present invention to provide a process for the production of carbon brushes which avoids the above disadvantages. In particular, in combination with good moldability of the carbon-containing powder, carbon brushes having good reproducibility of the material properties, high dimensional stability and constant resistance together with a long life time when used at high temperatures and quiet running are produced.

The object is achieved by a process for the production of carbon brushes, comprising the steps of

(a) preparing of a pulverulent composition which comprises a carbon powder and a pulverulent thermoplastic binder,

(b) processing the pulverulent composition from step (a) at a temperature above the melting point of the thermoplastic binder in order to obtain a mixture which comprises the carbon powder and the molten thermoplastic binder,

(c) optionally milling and sieving the mixture from step (b),

(d) compression molding the mixture from step (b) or step (c), if carried out, to give a desired shape, and

(e) thermally treating the shaped product form step (d) at a temperature above the melting point of the thermoplastic binder but below its decomposition temperature.

The present invention furthermore relates to a carbon brush which is obtainable by this process.

In step (a) of the process according to the invention a composition comprising a carbon powder and a pulverulent thermoplastic binder is prepared. Optionally, a suitable metal powder, which is described in detail further below, can be added to the composition as early as in step (a).

For example, graphites (natural and synthetic graphites), cokes and anthracites, and intermediates and mixtures prepared from these raw materials, are suitable as carbon powder. In the case of carbon brushes for automotive applications, natural graphite is preferably used. In preferred embodiments, the carbon powder should have an average particle size (D ₅₀ median of the volume distribution (in percent by volume) by means of laser granulometry) of from 30 μm to 50 μm, preferably about 40 μm. The maximum particle size preferably should not exceed 150 μm. The required purity of the carbon powder (ash) depends substantially on application-related requirements and can easily be determined by the person skilled in the art.

Examples of suitable thermoplastic binders, in each case in powder form, are: polyamides; polyimides; polyether ketones; polyether ether ketones (PEEK); polysulphones, in particular polyphenylene sulphones (PPSU); polyphenylene sulphides (PPS); polyethylene terephthalates and copolymers and blends of these polymers. The use of special copolymers which were developed with the objective of improved solubility is neither required nor preferred since this would reduce the stability of the carbon brushes to solvents and high temperatures of use. A preferred thermoplastic binder is polyamide, e.g. polyamide-6, polyamide- 11 and polyamide-12, and copolymers and blends of these polyamides.

Although the process according to the invention can also be carried out with sulphur-containing thermoplastics, such as, for example, PPS, as binders, thermoplastic binders which are not PPS are preferably used. Sulphur-free thermoplastic binders are particularly preferably used. It is presumed that, in the case of metal-containing carbon brushes, the sulphur from the binder can react

with the metal particles or pressed-in copper stranded wires and thus adversely affects the material properties of the carbon brushes.

In a preferred embodiment of the present invention, the pulverulent thermoplastic binder has an average particle size (D ₅₀ median of the volume distribution (in per cent by volume) by means of laser granulometry) of from 5 μm to 70 μm, more preferably from 10 μm to 50 μm and most preferably from 20 μm to 30 μm.

The proportions of the pulverulent thermoplastic binder in the composition from step (a) are preferably from 2 to 20% by weight, more preferably from 3 to 18% by weight, based in each case on the total weight of carbon powder and pulverulent thermoplastic binder. In the case of carbon brushes for use in automobiles (onboard supply from 12 V to 42 V) or battery-operated electrical equipment and tools, from 3 to 8% by weight of pulverulent thermoplastic binder are particularly preferred, and, depending on the specific application, from 3 to 6% by weight (objective of quiet running of the corresponding electric motors, e.g. air- conditioning and heating fans or actuator motors) or from 6 to 8% by weight (objective of stability at high temperatures of use and high atmospheric humidity) of pulverulent thermoplastic binder being preferred, based in each case on the total weight of carbon powder and pulverulent thermoplastic binder. In the case of carbon brushes for use in electrical tools and household appliances (mains voltage 110 V / 230 V), from 8 to 15% by weight of pulverulent thermoplastic binder are particularly preferred, and most preferably from 10 to 12% by weight of pulverulent thermoplastic binder are preferred for this purpose, based in each case on the total weight of carbon powder and pulverulent thermoplastic binder.

In step (a) of the process according to the invention, the individual components, i.e. the carbon powder, the pulverulent thermoplastic binder and optionally metal powder, are simply dry-blended with one another in order to prepare a pulverulent, preferably homogeneous composition. The mixing can be effected in a suitable mixing apparatus, such as, for example, a ploughshare mixer or a Simplex mixer.

The pulverulent composition from step (a) is then processed in step (b) at a temperature above the melting point of the thermoplastic binder. A mixture which comprises the carbon powder, optionally metal powder and the molten thermoplastic binder is obtained as a result. Because of adhesive bonding of the powder particles to one another by the thermoplastic binder, the composition has, as a rule, a coarse to fine crumbly consistency. The diameter of the adhesively bonded particles is frequently in the range of from 40 μm to 2 mm. Without, however, wishing to be tied to this theory, it is assumed that the thermoplastic binder wets and coats the particles of the carbon powder and optionally of the metal powder.

The processing temperature in step (b) is above the melting point of the thermoplastic binder and depends on a number of factors, such as, for example, the type, the amount and the particle size distribution of the carbon powder, of the thermoplastic binder and optionally of the metal powder. It can be determined readily by the person skilled in the art and is preferably from 10 K to 40 K, more preferably from 10 K to 25 K and most preferably from 15 K to 20 K above the melting point of the thermoplastic binder. The maximum processing temperature in step (b) is of course below the decomposition temperature of the binder and, for economic reasons, is chosen to be as low as technically possible.

The processing of the pulverulent composition from step (a) in step (b) is effected in a suitable apparatus having a mixing and kneading effect. Processing in an extruder is preferred, more preferably in a twin-screw extruder. A twin-screw extruder operates with two metal screws which run in parallel and are divided into various transport and kneading regions. The arrangement of these transport and kneading regions, the rotational speeds of the screws and the exact temperature distribution in the heating zones of the extruder are not important for the invention and can be readily optimized by the person skilled in the art by means of preliminary experiments.

In a preferred embodiment, the mixture obtained from step (b) is either directly milled and sieved (step (c)) or optionally, for reasons of better processability, first premoulded and then milled and sieved. The milling process can be effected in

any suitable mill; for example, hammer mills, hammer bar mills, impact disc mills, pinned disc mills, toothed disc mills, roll mills and air jet mills can be used. The milled mixture is preferably sieved at from 0.4 mm to 0.8 mm, more preferably at 0.5 mm, so that granules are obtained. The milling and sieving in step (c) serves for producing an optimum particle size but is not essential for carrying out the present invention. In that embodiment of the process according to the invention which comprises a premoulding step before step (c), this premoulding step is preferably effected on a platen press, preferably at pressures of from 5 x 10 ⁷ to 15 x 10 ⁷ Pa (500 to 1500 bar).

If the mixture from step (b) does not as yet contain any metal powder and if it is intended to produce a metal-containing carbon brush, the metal powder is added before step (d), the subsequent compression molding. Metal powders are usually added for reducing the material resistance and contact resistance between carbon brush and commutator in the case of formulations for carbon brushes which operate in ranges of low voltages, such as, for example, in on-board supplies of automobiles or industrial trucks and battery-operated electrical equipment and tools. Metal powders which may be used are, for example, powders comprising copper, copper alloys, silver or iron, copper powder being preferred. It is also possible to use different metal powders in the form of a mixture. The proportion of the metal powder in the mixture depends on the subsequent use of the carbon brush produced therefrom. In the following applications of the finished carbon brushes, preferred quantity ranges of metal powder in the mixture are: on-board automotive supply 12 V: from 30 to 60 % by weight, on-board automotive supply

24 V: from 15 to 35 % by weight, on-board automotive supply 42 V: from 5 to

25 % by weight, fixed supplies 110 V/230 V: not more than 15 % by weight.

One or more auxiliaries can be added to the mixture from step (b) or (c), if carried out, before the final compression molding in step (d), optionally in addition to metal powder as already explained above. Alternatively, the auxiliaries can also be added at an earlier time in the process, for example during the preparation of the pulverulent composition in step (a). If a plurality of auxiliaries is added, the separate addition of the individual auxiliaries at different times in the process is also possible.

Examples of auxiliaries which can be used in the process according to the invention for the production of carbon brushes are solid lubricants, such as, for example MoS ₂ and WS ₂ , and cleaning agents, such as, for example, silicates, oxides and carbides, in particular silicon carbide.

If at least one further component (i.e. metal powder and/or auxiliary or auxiliaries) is added to the pulverulent composition from step (b) or (c), if carried out, before the final compression molding in step (d), all constituents are preferably homogenized in a suitable mixing apparatus prior to compression molding. Mixing apparatuses of the same type as in step (a) can be used here.

The molding in step (d) is effected in a suitable compression mould consisting of dies and punches, so that the resulting body has the shape of the desired carbon brush. The transmission of the pressure to the punch and/or die is effected by means of mechanical (e.g. eccentric) or hydraulic presses. The compression moulds used may have one or more cavities (single or multiple dies). Metal-free carbon brushes are preferably molded at pressures of from 1 x 10 ⁸ to 2 x 10 ⁸ Pa (1000 to 2000 bar). Metal-containing carbon brushes are preferably molded at pressures of from 2.5 x 10 ⁸ to 4 x 10 ⁸ Pa (2500 to 4000 bar), a metal stranded wire (e.g. copper stranded wire) frequently simultaneously being pressed in in the case of metal-containing carbon brushes.

The molded and compressed carbon brushes thus obtained are thermally treated in step (e) at a temperature above the melting point of the thermoplastic binder but below the decomposition temperature of the thermoplastic binder. This treatment leads to an even more uniform distribution of the binder in the form of thin polymer films between the carbon particles and optionally metal particles, with the result that in particular the strength properties of the carbon brushes are improved. In the case of metal-containing carbon brushes, sintering of the metal particles also takes place, which leads to a further increase in the strength, but in particular to a reduction of the electrical resisitivity. The thermal treatment is preferably effected in continuous or batchwise industrial furnaces, for example belt or retort furnaces. In preferred embodiments, the thermal treatment is

effected in a reducing atmosphere, for example in a mixture of nitrogen and hydrogen.

If required, the thermally treated carbon brushes can be subsequently processed mechanically. Subsequent processing serves for maintaining the required dimensional tolerances or for enhancing the run-in behaviour and is preferably effected by grinding, in particular in the direction of pressing or on the running surface.

In addition to the advantages of the process according to the invention or of the carbon brushes according to the invention, such as the omission of pitches and solvents in the production, good reproducibility of the material properties and high dimensional stability and ageing stability of the resistance even under conditions of high temperatures and high atmospheric humidity, the carbon brushes according to the invention are distinguished in particular by comparatively low hardness but high bending strength and hence good damping and sliding properties, which result in longer service lives, higher speeds, better speed stability and quiet running during use.

Owing to said advantages, the carbon brushes according to the invention are widely used, for example in the automotive industry in on-board supplies of from 12 V to 42 V (e.g. 12 V, 24 V and 42 V), for battery-operated electrical equipment or at mains voltages (110 V/230 V), for household appliances and industrial motors.

The invention will now be illustrated in more detail with reference to examples.

Raw materials used:

Graphite:

Natural graphite FP 99.5 from Graphit Kropfmϋhl AG, Hauzenberg, Germany Sieve analysis (% by weight): 97 % < 90 μm, 89 % < 63 μm, 74 % < 40 μm; Ash content < 0.5%

Polyamide:

Polyamide powder Rilsan D30 from Atofina

Polyamide 11

Sieve analysis (% by weight): 95 % < 35 μm, 50 % < 25 μm, 5 % < 15 μm;

Melting point 186 ⁰ C

Cu Powder:

Dendritic copper powder (D ₅₀ (volume distribution)) = 30 μm; Bulk density < 1 g/cm ³

Molybdenum disulphide (particle size < 20 μm)

Silicon carbide (particle size < 15 μm)

Step 1 : Preparation of the basic graphite-binder mixtures (granules)

Example A (for the production of carbon brushes for use at temperatures up to above 100 ⁰ C and high atmospheric humidity):

94 % by weight of graphite and 6 % by weight of polyamide

Example B (for the production of carbon brushes for use where quiet running is required):

97 % by weight of graphite and 3 % by weight of polyamide

In accordance with Example A or B, graphite and polyamide are homogenized in the dry state in a Lδdige mixer (ploughshare mixer). This homogenized powder is fed continuously to a heatable twin-screw extruder and processed at 170 revolutions per minute. Setting of the temperature zones in the transport direction: from 170 ⁰ C to 22O ⁰ C. The mixing and kneading process results in the formation of a mixture which consists of graphite particles coated with polyamide and has acquired a coarse to fine crumbly consistency (particle size: from 40 μm to 2 mm) by adhesive bonding to one another.

This mixture is compression molded at 15 x 10 ⁷ Pa (1500 bar) to give sheets, milled on a hammer mill and sieved at 0.5 mm so that granules having a typical

particle distribution as follows form: 10 % > 400 μm, 60 % > 125 μm, 90 % > 63 μm

Step 2: Preparation of a metal-containing powder for the compression molding of carbon brushes

48.2 % by weight of granules according to Example A or B prepared in step 1.

50.0 % by weight of copper powder

1.5 % by weight of molybdenum sulphide

0.3 % by weight of silicon carbide as cleaning agent

The above constituents are introduced into a Lδdige mixer (ploughshare mixer) and homogenized for 10-15 min.

Step 3: Production of the carbon brushes

The powders prepared in step 2 are compression molded on a table press in multiple dies at pressures of about 3.5 x 10 ⁸ Pa (3500 bar) with simultaneous pressing in of a copper stranded wire. The molded carbon brushes are subjected to a thermal treatment in a belt furnace at temperatures of 330°C in a reducing atmosphere under a nitrogen-hydrogen mixture.

TEST RESULTS

Example A: Carbon brushes for use at temperatures up to above 100 ⁰ C and high atmospheric humidity

4 carbon brushes produced according to the invention, according to steps 1 to 3, example A (KB A1 - KB A4), and 4 carbon brushes of the prior art (carbon brushes A 553 from Schunk Kohlenstoff-Technik GmbH, phenolic resin as binder, coked) (KB V1 - KB V4) were tested in continuous operation on a motor fan according to VW standard (400 h at 95°C and 50 % relative humidity and 600 h at 50 ⁰ C and 95 % relative humidity). The results are shown in Table 1.

Table 1

It is evident from the results in Table 1 that the carbon brushes according to the invention exhibit less wear, better dimensional stability and a more constant resistance than those of the prior art.

Example B: Carbon brushes for use where quiet running is required

In each case two carbon brushes produced according to the invention, according to steps 1 to 3, Example B, and carbon brushes of the prior art (carbon brushes A 473 from Schunk Kohlenstofftechnik GmbH, phenolic resin as binder, coked) were installed in an air-conditioning fan motor. The airborne sound measurements were effected using a "Pulse Analyser" from Brϋel & Kjaer in a noise measuring cabin, where the microphone was arranged at a distance of 10 cm from the brush holder opening of the motor housing. The results are shown in Table 2.

Table 2

It is evident from the results in Table 2 that the carbon brush according to the invention runs more quietly than that of the prior art.