EP1713148B1 - Method of fabricating carbon brushes and carbon brushes made according to said method - Google Patents

Abstract

The method involves processing a pulverulent composition having carbon powder and thermoplastic binder at a temperature above a melting point of the binder to obtain a mixture having the powder and the binder. The mixture is optically milled and sieved and is compression molded to provide a desired shape. The shaped mixture is thermally treated at the temperature above the point and below a decomposition temperature of the binder. An independent claim is also included for a carbon brush.

Description

The present invention relates to a manufacturing method for carbon brushes and carbon brushes available thereafter.

Carbon brushes are usually produced by compressing a powdery material, substantially carbon-based with a finely divided binder and for certain applications with the addition of metal powders in a mold and then the pressed material of a heat treatment for coking, curing or distribution of the binder or Sintering of the metal powder is subjected. The carbon brush thus obtained is then optionally subjected to mechanical finishing.

As binders, hitherto, e.g. Pitches and tars, synthetic thermosetting resins, such as phenolic resins, or thermoplastics.

So describes US-A-5,441,683 the manufacture of a brush for an electric motor using so-called "distillation binders" (ie residues from petroleum or coal distillation, such as pitches and tars) or synthetic binders, preferably thermosetting resins, more preferably phenolic resins.

Also in DE-A-103 24 855 discloses the preferred use of a thermosetting resin, eg, a phenolic resin, or pitch as a binder in the manufacture of carbon brushes.

In DE-A-101 56 320 For example, a synthetic powder resin, in particular a duroplastic, for example an epoxy or phenolic resin, is used as a binder for producing a resin-bonded graphite material. Preferably If the binder is a dissolved in solvent powder resin or a solvent-free liquid resin.

GB-A-641.937 relates to the manufacture of electrical contact brushes and describes the use of a binder in a solution of a volatile solvent to disperse the binder well.

In DE-A-24 44 957 discloses the production of carbon contact bodies by pressing together powder of graphite and a metal in a die and subsequent heat treatment. The powder contains 0.5 to 50 parts by weight of a binder from the group of monovalent aromatic polymers, for example polyarylene sulfides, preferably polyphenylene sulfide in finely ground form.

Also in the process for producing a carbon brush, the subject of DE-A-199 00 23 is, a thermoplastic binder is used. Examples include polyphenylene sulfides, polysulfones, polyphenyl sulfones, polyether ketones, polyethylene terephthalates, polyamides, polyimides, and copolymers derived therefrom.

DE-A-199 00 24 describes a method for producing a carbon brush from a powdery material, which is characterized by the use of an undissolved powdery binder of fine-grained thermoplastic powder with an average grain size of 5 .mu.m to 50 .mu.m and a very narrow particle size distribution.

Pitch-bonded materials for carbon brushes, which also contain copper, silver or other metal powders and which are produced with pressed-in copper strand, have the disadvantage of sulfide formation on metal powders and stranded wire because of the sulfur content of the pitches. Similar problems arise with the use of polymers containing sulfur (eg polyphenylene sulfide) under the conditions of manufacture or application is at least partially set free. Many pitch-based binders are also carcinogenic.

As a direct disadvantage of all manufacturing processes for carbon brushes, which work with dissolved thermoplastic or thermosetting polymers as a binder, it necessarily results in the handling of a volatile solvent, which is connected to avoid health problems and environmental pollution with high equipment costs and should be avoided. In addition, soluble thermoplastics as binders lead to end products whose resistance to solvents and high application temperatures is comparatively low.

The use of thermosetting polymers as binders, especially in combination with heat treatments leading to its partial or complete coking, leads to end products that fail to provide adequate dimensional stability (swelling) in high temperature and humidity applications, and significant resistance aging (resistance increase). exhibit.

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

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, with good compressibility of the carbonaceous powder carbon brushes with good reproducibility of the material properties, high dimensional stability and resistance constancy associated with long life in use at high temperatures and low noise run.

1. (a) Herstellung einer pulverförmigen Zusammensetzung, die ein Kohlenstoffpulver und ein pulverförmiges thermoplastisches Bindemittel enthält,
2. (b) Verarbeitung der pulverförmigen Zusammensetzung aus (a) bei einer Temperatur oberhalb der Schmelztemperatur des thermoplastischen Bindemittels, um eine Mischung zu erhalten, die das Kohlenstoffpulver und das aufgeschmolzene thermoplastische Bindemittel enthält,
3. (c) gegebenenfalls Aufmahlen und Absieben der Mischung aus Schritt (b),
4. (d) Pressen der Mischung aus Schritt (b) oder Schritt (c), falls durchgeführt, in eine gewünschte Form und
5. (e) Thermische Behandlung bei einer Temperatur oberhalb des Schmelzpunkts des thermoplastischen Bindemittels, aber unterhalb dessen Zersetzungstemperatur.

The object is achieved by a method for producing carbon brushes, comprising the steps

1. (a) preparing a powdery composition containing a carbon powder and a powdery thermoplastic binder,
2. (b) processing the powdery composition of (a) at a temperature above the melt temperature of the thermoplastic binder to obtain a mixture containing the carbon powder and the molten thermoplastic binder,
3. (c) optionally grinding and sieving the mixture from step (b),
4. (d) pressing the mixture of step (b) or step (c), if carried out, into a desired shape and
5. (e) thermal treatment at a temperature above the melting point of the thermoplastic binder, but below its decomposition temperature.

The subject of the present invention is also a carbon brush obtainable by this process.

In step (a) of the process according to the invention, a composition is prepared which comprises a carbon powder and a powdery thermoplastic binder. Optionally, a suitable metal powder may be added to the composition already in step (a), which is further described below in detail.

As carbon powder are, for example graphite (natural and artificial graphite), cokes and anthracites, and produced from these raw materials Intermediates and mixtures suitable. Carbon brushes are preferably used for automotive applications natural graphite. The carbon powder should have in preferred embodiments, an average grain size (D ₅₀ -Median the volume distribution (in volume percent) by means of laser granulometry) of 30 μ m to 50 μ m, more preferably about 40 μ m. The maximum grain size should preferably not exceed 150 μm . The required degree of purity of the carbon powder (ash) depends essentially on application requirements and can be easily determined by the person skilled in the art.

Examples of suitable thermoplastic binders, each in powder form, are: polyamides; polyimides; polyether ketones; Polyetheretherketones (PEEK); Polysulfones, in particular polyphenylene sulfones (PPSU); Polyphenylene sulfides (PPS); Polyethylene terephthalates and copolymers and mixtures of these polymers. The use of special copolymers developed with the objective of improved solubility is neither necessary nor preferred as this would reduce the resistance of the carbon brushes to solvents and high application temperatures. A preferred thermoplastic binder is polyamide, e.g. Polyamide-6, polyamide-11 and polyamide-12, as well as copolymers and blends of these polyamides.

Although the process of the present invention may be practiced with sulfur-containing thermoplastics such as PPS as a binder, thermoplastic binders other than PPS are preferably used. Sulfur-free thermoplastic binders are particularly preferably used. It is assumed that in the case of metal-containing carbon brushes, the sulfur from the binder can react with the metal particles or pressed-in copper strands and thus worsens 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 the volume distribution (in volume percent) by means of laser granulometry) of 5 μ m to 70 μ m, more preferably from 10 μ m to 50 μ m, and most preferably from 20 μm to 30 μm .

The proportions of the powdery thermoplastic binder in the composition of the step (a) are preferably 2 to 20% by weight, more preferably 3 to 18% by weight, based on the total weight of the carbon powder and the powdered thermoplastic binder. In carbon brushes for use in automobiles (electrical system 12 V to 42 V) or electrical appliances and tools on battery 3 to 8 wt .-% powdered thermoplastic binder are particularly preferred, depending on the particular application 3 to 6 wt .-% (target position low-noise operation of the corresponding electric motors, eg air conditioning and fan heaters or modulating motors) or 6 to 8 wt .-% (objective stability at high application temperatures and high humidity) powdered thermoplastic binder are preferred, each based on the total weight of carbon powder and powdered thermoplastic binder. In carbon brushes for use in power tools and household appliances (mains voltage 110 V / 230 V) 8 to 15 wt .-% powdered thermoplastic binder are particularly preferred, most preferred for this purpose 10 to 12 wt .-% powdered thermoplastic binder, each based on the total weight of carbon powder and powdered thermoplastic binder.

In step (a) of the process according to the invention, the individual components, ie the carbon powder, the powdered thermoplastic binder and optionally metal powder, simply dry mixed together to produce a powdered, preferably homogeneous composition. The mixing can be done in a suitable mixing device carried out, such as a plowshare mixer or a simplex mixer.

The powdered composition of step (a) is then processed in step (b) at a temperature above the melting point of the thermoplastic binder. As a result, a mixture is obtained which contains the carbon powder, optionally metal powder, and the molten thermoplastic binder. By bonding the powder grains together by the thermoplastic binder, the composition usually has a coarse to feinkrümelige consistency. The diameter of the bonded grains is often in the range of 40 μ m to 2 mm. However, without wishing to be bound by theory, it is believed that the thermoplastic binder wets and coats the particles of the carbon powder and optionally 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 the type, amount and grain size distribution of the carbon powder, the thermoplastic binder and optionally the metal powder. It can be readily determined by one skilled in the art and is preferably 10K to 40K, more preferably 10K to 25K, and most preferably 15K to 20K above the melting point of the thermoplastic binder. Of course, the maximum processing temperature in step (b) is below the decomposition temperature of the binder and is chosen to be as low as technically possible for economic reasons.

The processing of the powdery composition from step (a) in step (b) takes place in a suitable device with mixing and kneading action. Preference is given to processing in an extruder, more preferably in a twin-screw extruder. A twin-screw extruder works with two parallel metal screws, which are divided into various transport and kneading areas. The arrangement of these transport and Knetbereiche, the rotational speeds of the screw, as well as the exact temperature distribution in the heating zones of the extruder are not essential to the invention and can be easily optimized by a person skilled in the art by preliminary experiments.

In a preferred embodiment, the mixture obtained from step (b) is either directly ground and screened (step (c)) or optionally first pre-pressed for reasons of better processability, then ground and screened. The grinding process can be done in any suitable mill; can be used, for example Hammer mills, cross beater mills, disc mills, pin mills, toothed disk mills, roll mills and air jet mills. The milled mixture is preferably sieved at 0.4 mm to 0.8 mm, more preferably at 0.5 mm, so that a granule is obtained. The milling and sieving in step (c) serves to produce an optimum grain size, but is not essential to the practice of the present invention. In the embodiment of the method according to the invention with a prepressing step before step (c), this prepressing step is preferably carried out on a plate press, preferably at pressures of 500 to 1500 bar.

If the mixture of step (b) does not contain any metal powder and if a metal-containing carbon brush is to be produced, the metal powder is added before step (d), the subsequent pressing. Metal powders are commonly added to reduce material resistance and contact resistance between carbon brush and commutator in formulations for carbon brushes operating in low voltage areas, such as on-board automotive or material handling equipment, and electrical appliances and tools on battery power. As metal powder, for example, powders of copper, copper alloys, silver or iron can be used, copper powder is preferred. It can also be different Metal powder can be used in mixture. The proportion of the metal powder in the mixture depends on the later application of the carbon brush produced therefrom. Preferred quantitative ranges of metal powders in the mixture in the following applications of the finished carbon brushes are: Automotive electrical system 12 V: 30 to 60 wt.%, Automotive on-board 24 V: 15 to 35 wt.%, Automotive vehicle electrical system 42 V: 5 up to 25% by weight, fixed networks 110 V / 230 V: max. 15% by weight.

If appropriate, one or more auxiliaries may be added to the mixture of step (b) or (c), if appropriate, before the final pressing in step (d), optionally in addition to metal powder as already explained above. Alternatively, the auxiliaries may also be added at an earlier stage in the process, for example during the preparation of the powdered composition in step (a). If several adjuvants are added, the separate addition of the individual excipients at different times in the process is possible.

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

If at least one further component (ie, metal powder and / or adjuvant (s)) is added to the powdered composition of step (b) or (c), if done, prior to final pressing in step (d), all of the constituents are preferably in one homogenized suitable mixing device before pressing. In this case, similar mixing devices can be used as in step (a).

The pressing in step (d) takes place in a suitable mold consisting of die and stamping, so that the resulting body has the shape of the desired carbon brush. The transmission of the pressing force on punch and / or die occurs via mechanical (eg eccentric) or hydraulic presses. The pressing tools used can have one or more cavities (single or multiple dies). Metal-free carbon brushes are preferably pressed at pressures of 1000 bar to 2000 bar. Metal-containing carbon brushes are preferably pressed at pressures of 2500 bar to 4000 bar, with metal-containing carbon brushes often simultaneously a metal strand (eg copper wire) is pressed with.

The pressed carbon brushes thus obtained are thermally treated in step (e) at a temperature above the melting point of the thermoplastic binder but below its decomposition temperature. This treatment leads to an even more uniform distribution of the binder in the form of thin polymer films between the carbon and optionally metal grains, which in particular improves the strength properties of the carbon brushes. In the case of metal-containing carbon brushes, there is also a sintering of the metal grains, which leads to a further increase in the strength, but in particular to a reduction in the specific electrical resistance. The thermal treatment is preferably carried out in continuous or discontinuous industrial furnaces, e.g. Strip or retort ovens. In preferred embodiments, the thermal treatment is carried out in a reducing atmosphere, such as in a mixture of nitrogen and hydrogen.

If necessary, the thermally treated carbon brushes can be mechanically reworked. The post-processing serves to maintain the required dimensional tolerances or to promote the running-in behavior and is preferably carried out by grinding, in particular in the pressing direction or on the tread.

In addition to the advantages of the method and the carbon brushes according to the invention, such as the elimination of pitches and solvents in the production, good reproducibility of the material properties, and high dimensional and aging resistance of the resistor even under conditions of high temperatures and high humidity, the invention Carbon brushes in particular by comparatively low hardness, but high bending strength and thus good damping and sliding properties, which require increased service life, higher speeds, better speed stability and low-noise operation in use.

The carbon brushes according to the invention are widely used, for example in the automotive industry in vehicle electrical systems of 12 V to 42 V (for example 12 V, 24 V and 42 V) for power tools in battery mode or at mains voltages (110 V / 230 V), for home appliances and industrial engines.

The invention will now be illustrated in more detail by way of examples.

Used raw materials:

Graphit:

Naturgraphit FP 99,5 von Fa. Graphit Kropfmühl AG, Hauzenberg, Deutschland
Siebanalyse (Gew.-%): 97% < 90 µm, 89% < 63 µm, 74% < 40 µm; Aschegehalt < 0,5%

Polyamid:

Polyamidpulver Rilsan D30 von Atofina
Polyamid 11
Siebanalyse (Gew.-%): 95% < 35 µm, 50% < 25 µm, 5% < 15 µm; Schmelzpunkt 186°C

Cu-Pulver:

dendritisches Kupferpulver (d₅₀ (Volumenverteilung) = 30 µm; Schüttgewicht < 1 g/cm³

Molybdändisulfid (Körnung < 20 µm)
Siliciumcarbid (Körnung < 15 µm)

Graphite:

Natural graphite FP 99.5 from Fa. Graphit Kropfmühl 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 ° C

Cu powder:

dendritic copper powder (d ₅₀ (volume distribution) = 30 μm , bulk density <1 g / cm ³

Molybdenum disulphide (grain size <20 μ m)
Silicon carbide (grain size <15 μ m)

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

• Example A (for the production of carbon brushes for use at temperatures above 100 ° C and high humidity):
  94% by weight graphite and 6% by weight polyamide
• Example B (for making carbon brushes for use in low-noise demands):
  97 wt .-% graphite and 3 wt .-% polyamide

Graphite and polyamide are dry-homogenized according to Example A or B in a Lödige mixer (ploughshare mixer). This homogenized powder is fed continuously to a heatable twin-screw extruder and processed at 170 rpm. Setting the temperature zones in the conveying direction: 170 ° C to 220 ° C. As a result of the mixing and kneading process, a mixture consisting of graphite grains coated with polyamide is obtained and, by bonding to each other, has a coarse to fine crumbly consistency (particle size: 40 μm to 2 mm).

This mixture is pressed at 1500 bar into plates, ground on a hammer mill and sieved at 0.5 mm, so that a granule with a typical grain distribution is produced as follows: 10%> 400 μ m, 60%> 125 μ m, 90% > 63 μ m

Step 2: Preparation of a metal-containing powder for pressing carbon brushes

46.0% in step 1 produced granules according to Example A or B.
50.0% copper powder
1.5% molybdenum disulfide
0.3% silicon carbide as a cleaning agent

The above ingredients are placed in a Lödige mixer (ploughshare mixer) and homogenized for 10-15 minutes.

Step 3: Production of carbon brushes

The powders produced in step 2 are pressed on a bench press in multiple dies with pressures of about 3500 bar while simultaneously pressing in a copper strand. The pressed carbon brushes are subjected in a belt furnace to a thermal treatment at temperatures of 330 ° C under a reducing atmosphere under a nitrogen-hydrogen mixture.

TEST RESULTS Example A: Carbon brushes for use at temperatures above 100 ° C and high humidity

Four carbon brushes (KB A1-KB A4) produced according to the invention according to steps 1 to 3, and 4 carbon brushes of the prior art (coal skins A 553 from Schunk Kohlenstoff-Technik GmbH, phenolic resin binder, coked) were used (KB V1 - KB V4) 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 ° C and 95% relative humidity tested). The results are shown in Table 1. <u> Table 1 </ u> Example A Comparative example KB A1 KB A2 KB A3 KB A4 KB V1 KB V2 KB V3 KB V4 Carbon brush wear ( μ m / h) 2.11 2.13 2.41 1.94 5.84 6.52 8.29 9.23 Dimensional change in pressing direction (%) 0.59 0.58 0.62 0.52 2.65 2.55 2.55 2.34 Resistance aging (%) 26 13 7 41 52 113 74 66 Commutator wear ( μ m / h) 0.35 0.9 Speed range (1 / min) 2,900 - 3,200 2,800 - 3,000 Maximum current density (A / cm ² ) 30 15

It can be seen from the results in Table 1 that the carbon brushes of the present invention exhibit less wear, better dimensional stability, and a better resistance constant than those of the prior art.

Example B: Carbon brushes for use in low-noise demand

In each case, two carbon brushes and carbon brushes of the prior art produced according to step 1 to 3, example B (carbon brush A 473 from Schunk Kohlenstoff-Technik GmbH, phenolic resin binder, coked) were installed in a fan motor. The airborne sound measurements were carried out with a "Pulse Analyzer" from Brüel & Kjaer in a noise measurement booth, the microphone being located at a distance of 10 cm from the brush holder opening of the motor housing. The results are shown in Table 2 <u> Table 2 </ u> Example B Comparative example Airborne sound (0 to 20kHz) (dB (A)) 45.2 49.6

It can be seen from the results in Table 2 that the carbon brush of the present invention has lower running noise than that of the prior art.

Claims (11)

1. A process for the production of carbon brushes, comprising the steps of

   (a) preparing 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.
2. The process according to Claim 1, characterized in that the carbon powder is graphite.
3. The process according to Claim 1 or 2, characterized in that the pulverulent thermoplastic binder has an average particle size of from 5 µm to 70 µm (D₅₀ median of the volume distribution).
4. The process according to any of Claims 1 to 3, characterized in that the composition in step (a) comprises from 80 to 98 % by weight of graphite and from 2 to 20 % by weight of pulverulent thermoplastic binder.
5. The process according to any of Claims 1 to 4, characterized in that the mixture from step (b) is premolded before step (c).
6. The process according to any of Claims 1 to 5, characterized in that a metal powder comprising one or more metals is added to the mixture in step (a) or the mixture from step (c) before molding in step (d).
7. The process according to any of Claims 1 to 6, characterized in that the processing of the composition in step (b) is effected at a temperature which is at least 10 K above the melting point of the thermoplastic binder.
8. The process according to any of Claims 1 to 7, characterized in that the processing of the composition in step (b) is effected in an extruder.
9. The process according to any of Claims 1 to 8, characterized in that the thermoplastic binder is selected from the group consisting of polyamides, polyimides, polyether ketones, polyether ether ketones, polysulphones, in particular polyphenylene sulphones, polyethylene terephthalates and copolymers and blends of these polymers.
10. The process according to Claim 9, characterized in that the thermoplastic binder is a polyamide.
11. A carbon brush obtainable by the process according to any of Claims 1 to 10.