WO1994029381A1 - Preparation of novolak moulding compounds for the manufacturing of commutators - Google Patents

Abstract

Described are asbestos-free moulding compounds based on phenol-formaldehyde resins, also known as phenolic resins, which are prepared in a turbo mixer with the addition of water. The moulding compounds include up to 30 % by wt. of a novolak resin plus short glass fibres and long glass fibres, the total glass-fibre content being about 30 % by wt. The glass fibres are preferably mixed in the ratio, by weight, of short fibres to long fibres of 2:1. The mineral fillers preferred for the moulding compounds are chalk and clays, together making up about 30 % by wt. Other components are cross-linking agents, mould-release agents, accelerators and, optionally, colorants, in most cases in quantities less than 3 % by wt. The moulding compounds are obtained as granular products with a low dust content. They can be plasticized and further processed using automatic production machinery. The moulding compounds thus obtained have properties which make them suitable for use in the manufacture of commutators for electric motors.

Description

Preparation of a Novolak molding compound for collectors

State of the art

The invention relates to glass fiber reinforced phenoplasts, in particular for collectors and commutators of electric motors in accordance with the preamble of the main claim. In the book A. Knop and L. Pilato, Penolic Resins, Springer Verlag, Berlin, 1985, molding compounds with novolaks are described. Conventional molding compounds, including novolak molding compounds, are usually mechanically stabilized or reinforced by fibers and fabrics. The proportion of phenolic resin, especially novolak, is usually 30-50 percent by weight of the total mass of the molding composition. Phenolic resin molding compositions, in particular novolak molding compositions, are therefore usually prepared on heated roller mills or by use with extruders. The molding compound is obtained in flatbread, which are ground. Grains of dust with an amount of up to 15 percent by weight of the molding compound inevitably form as undesirable by-products, which means that filters that are frequently changed have to be used.

A disadvantage of the preparation in the fluid mixer is that, with conventional resin contents, sufficient fiber impregnation becomes inadequate and reproducible novolak molding compound preparation cannot be guaranteed. Further disadvantages come to light in the case of high proportions of filler in the molding compound, because no granules but woolly products with low filler contents, because undefined conglomerates are created as products.

The following property profile of phenolic resin molding compounds based on solid novolak resins is required for use as a molding compound for collectors:

- Asbestos-free molding compounds

- Temperature-resistant molding compound up to 400 degrees Celsius operating temperature for a short time

- good adhesion to metals, especially copper

- no harmful influences caused by metal corrosion

- Good plasticizability of the molding compound for further processing

- fine-grained, dust-free granules for processing.

In addition, the following requirements must be met for the production of the molding compounds:

- Production in a fluid mixer

- Reproducible fiber and filler impregnation, i.e. homogeneous product

- Coordination of mechanical stability through fiber or fabric mass.

The object of the invention is to produce asbestos-free, plasticizable and / or compressible molding compositions based on phenolic resins, especially solid novolak resins, in a fluid mixer, which meet the above-mentioned property profile for the manufacture of collectors for electric motors.

Advantages of the invention

The molding compositions according to the preamble of the main claim avoid the disadvantages of the prior art. The prejudice of the professional world that phenoplasts from solid Phenol-formaldehyde resins that cannot be produced using turbo mixing processes in fluid mixers become unsustainable for molding compositions whose mass fraction of novolak resin falls below 30 percent by weight of the molding composition and a special ratio of

Glass fibers: Has fillers. The production of fine-grained molding compound granules, which has no dust particles, reduces the manufacturing effort and processing effort by saving filters and breathing protection. Since there is no oversize grain, the grinding process is eliminated. The replacement of asbestos takes into account the legal asbestos ban and the avoidance of carcinogenic substances.

The measures and procedural details listed in the subclaims bring about numerous improvements over the general features given in the main claim. It is possible to achieve a homogeneous mixing of the individual components and reliable impregnation of fibers of the molding compound in the molding compound production and to avoid melting of the phenolic resin by a clever choice of the processing temperature. The mechanical stability can be set by the ratio of the long-fiber to the short-fiber glass fractions as well as the mineral fillers and the mixtures of mineral fillers and their respective proportions of the total mass of the molding composition. The mixing times are extremely short and correspond to the requirements of modern, automated production and enable the production of granulated, fine-grained molding compound granulate that can be plasticized or pressed and thus provides preliminary products, for example tablets, as the basis for the manufacture of final electrical engineering products. Embodiment

Suitable solid novolak resins and modified novolak resins are readily available commercially.

The proportion of the novolak resin component or a corresponding mixture of the phenolic resin component in the total mass of the molding composition was a minimum of 20 to a maximum of 30 percent by weight.

Hexamethylenetetramine (HMTA) is used as the crosslinker in a proportion of 0.1 to 4 percent by weight based on the total mass of the molding composition. Occasionally, epoxy-modified resins can be used for cross-linking.

Advantageously used release agents are stearates, in particular zinc and calcium stearate, their mixtures or chemically related stearates in a proportion of 0.3 to 3.5 percent by weight based on the total mass of the molding composition. Other fatty acid salts, fatty acid levels and fatty acid esters were also beneficial.

Accelerators were oxides of divalent cations, in particular the alkaline earth oxides MgO, CaO, SrO, BaO with a proportion of 0.1 to 0.5 percent by weight based on the total mass of the molding composition.

Glass fibers were used with fiber lengths according to the tables below. The glass fibers are commercially available from conventional silicate glass, in particular E-glass in various fiber lengths of, for example, 3, 4.5, 6, 12 and 30 millimeters. Short glass fibers are obtained by suitable comminution, for example by grinding the glass fibers under defined, reproducible grinding conditions and sieves. Glass fibers with a fiber length of 4.5 millimeters were particularly suitable as long fibers and short fibers with a fiber length of 0.2 millimeters for commutators of the starter motors of starters for motor vehicles. This selection of the fiber lengths proved to be particularly advantageous for the desired property profile given a mass ratio of long fiber: short fibers = 2: 1. It was also possible to obtain molding compositions with the desired property profile by means of short glass fibers, for example 0.5 millimeter in fiber length. In isolated cases, it has proven to be advantageous to replace the glass fibers with polyacrylonitrile fibers or mixtures of glass fibers and polyacrylonitrile fibers. The proportion of glass fibers in the total mass of the molding composition was 20 to 35 percent by weight and a mass ratio of short fibers: long fibers of 4: 1 to 1: 0.

Chalk, leaf silicates, mica, wollastonite and aluminum hydroxide were used as fillers. It is also conceivable to use aluminum oxide, rare earth oxides and their mixtures with 20 to 55 percent by weight of the total mass of the molding composition. A mixture of 13 weight percent chalk and 20 weight percent leaf silicate fulfilled the property profile very well. A mixture of 10 percent by weight of leaf silicate and 20 percent by weight of chalk still fulfilled the property profile well. Wood flour, cellulose, textile fibers, textile fabrics and veneer chips are also conceivable as fillers for special molding compounds.

If desired, the molding composition can also be colored by incorporating a coloring inorganic or organic substance as a pigment, color paste or solution. The coloring substance is optionally added in amounts which are usually between 0.1 to 2.5 percent by weight, based on the sum of the components of the molding composition. The plasticizing of the molding compounds takes place at 60 to 80 degrees Celsius. Pressing the pre-product granules into tablets or strands before further processing in automatic production plants is no problem. Final processing is possible by further processing the molding compound at 180 degrees Celsius.

It would be conceivable to replace or partially replace phenol by modified phenols such as cresols, thermoplastic modified phenol compounds or by aromatic compounds derived from phenols and to use them in the form of mixtures in connection with novolaks. It is conceivable to partially replace water with other compounds having low molecular weight hydroxyl groups. It is also possible to mix glass fibers with more than two different fiber lengths.

The turbo mixing process is carried out in a cylindrical container of a fluid mixer, in particular a heating / cooling mixer from Henschel, for example of the type FM 10 in an open design, speed 560-3800 revolutions / minute stirring power, 3 kW, 2.5 kg batch. The solid phenolic resin, the crosslinking agent, the release agent, the accelerator, the glass fibers and the fillers as well as possibly the dye are weighed out and placed in the mixer. The mixing material introduced is mixed by the rotating, ring-shaped or wing-like tools of the mixer, thrown against the wall as a result of the centrifugal forces and pushed up. It leaves the influence of the rotors and returns to the bottom of the container under the influence of gravity in the middle of the container. Due to the high relative speeds of the individual mixed material particles, intense heating occurs as a result of the friction. The temperature of the mix is kept at 100 degrees Celsius, which is achieved by cooling the container. The power consumption of the stirrer is recorded to record the change in the viscous properties of the mix. The resin is sintered within the mixing time, which is usually between 2 and 15 minutes. If the suitable first threshold value of the stirrer performance is reached due to changed viscous properties of the mix, up to 5% by weight of water is added in order to impregnate the fillers and fibers, to achieve homogeneous mixing of all components and to adjust the granule size. After stirring for a further 1 to 5 minutes, a homogeneous molding composition is obtained as a homogeneous, fine-grained granulate. In contrast to ground molding compounds according to the prior art, this has only a few dust particles. The molding compound is discharged into the cooling mixer and cooled after reaching a second threshold value of the stirring power, coarse particles present are broken up by rotating knives. In an alternative of the method, the molding compound is cooled on a cooling belt. These molding compounds are very free-flowing and almost dust-free. The absence of dust is determined by sieve analysis of the molding compound. The average grain diameter of the granules was 1 to 5 millimeters, depending on the mixture. A grain size of 0.7 millimeters or smaller is defined as dust, which may be screened out. 0 to 5 percent by weight, usually less than 3 percent by weight, of dust, based on the molding composition, is obtained. The granulate can be stored or pre-plastified for automatic production and further processed, for example in the form of tablets, into the end product.

Six examples for the selection of materials for Novolak molding compounds with the

Advantages of the invention are listed in Tables 1 and 2 and give an impression of a selection of possible mixing ratios of the molding compositions. The figures refer to

100 weight percent molding compound. The proportions of glass fibers, fillers and

Dyes. Table 1

Examples

Component 1 2 3

Novolak Novolak Novolak

Phenolic resin 25 20 26

Crosslinker HMTA HMTA HMTA

Release agent calcium stearate calcium stearate zinc stearate 2.5 1 3

CaO accelerator MgO CaO

0.5 0.1 0.5 short glass fibers E-glass E-glass E-glass 20 20 24 long glass fibers E-glass E-glass E-glass 10 10 6

Filler chalk chalk chalk 34 24 19.5 leaf silicate leaf silicate

19.4 12

Dye carbon black carbon black

2 2.5 2

Water 3 1 3

Total 100 100 100

HMTA = hexamethylenetetramine Table 2

Examples

Component 4 5 6

Novolak Novolak Novolak

Phenolic resin 23 23 23

Crosslinker HMTA HMTA HMTA

Release agent zinc stearate zinc stearate zinc stearate 1.8 3 4

Accelerator MgO MgO MgO

0.2 0.5 0.1 short glass fibers E-glass E-glass E-glass 30 20 20 long glass fibers E-glass E-glass E-glass 10 10

Filler chalk chalk chalk 11 17.5 20.9

Leaf silicate Al-hydroxide leaf silicate 25 20 15

Dye 1 - -

Water 5 3 4

Total 100 100 100

HMTA = hexamethylenetetramine

Claims

Expectations 1. Molding compound with (a) Phenolic resin or mixtures of phenolic resins (b) glass fibers and (c) Fillers characterized in that 10 to 30 percent by weight of phenolic resin are mixed in based on 100 percent by weight of molding compound. 2. Molding composition according to claim 1, characterized in that the phenolic resin is novolak resin or a mixture of novolak resin with modified phenolic resins. 3. Molding composition according to claim 1, characterized in that the mixed mass ratio of short-fiber glass fibers: long-fiber glass fibers is from 1: 0 to 4: 1. 4. Molding composition according to claim 1, characterized in that the fiber length of the glass fibers is 0.01 to 30 millimeters. 5. Molding composition according to claim 1, characterized in that the glass fiber length of the long fibers is 1 to 10 millimeters, preferably 4.5 millimeters, the glass fiber length of the short fibers is 0.1 to 1 millimeters, preferably 0.2 millimeters, and the mixing ratio of long fibers: short fibers is 1: 2. 6. Molding composition according to claim 1, characterized in that the fillers are asbestos-free mineral fillers, in particular silicates, rock powder and chalk or mixtures of these substances. 7. Molding composition according to claim 1, characterized in that the fillers additionally contain wood flour, cellulose, textile fibers, textile fabrics, veneer chips. 8. Molding composition according to claim 1, characterized in that the mineral fillers are 1 to 70 percent by weight of the molding composition. 9. Molding composition according to one of claims 1 to 8, characterized in that it is used for the production of collectors of electric motors and generators. 10. A process for the preparation of a molding composition according to one of claims 1 to 9, characterized in that the metered components of the molding composition are mixed intimately in a heating mixer without grinding. 11. A process for the preparation of a molding composition according to one of claims 1 to 10, characterized in that 0.1 to 10 percent by weight of water based on 100 percent by weight of molding composition are added to the mixture of the molding composition when the resin is sintered on. 12. A process for the preparation of a molding composition according to one of claims 1 to 11, characterized in that the mixture of the molding composition is cooled during mixing or is transferred into a cooling mixer or onto a cooling belt after the mixing process. 13. A process for the preparation of a molding composition according to one of claims 1 to 12, characterized in that the The processing temperature of the molding compound is below the melting temperature of the phenolic resin and complete fiber and filler impregnation is achieved without melting the resin.