DE1165699B - Granular product for the manufacture of electrically conductive plastic objects and process for its manufacture - Google Patents

Description

Granular product used to make electrically conductive plastic objects and method for its preparation. The invention relates to a granular Product for the manufacture of electrically conductive plastic objects and a process for making such a granular product. Electrically conductive plastic objects, which can be made from the granular product according to the invention For example, contact elements of multiple switches, as they are, for. B. in information scanning devices and similar instruments may be used, and are referred to below for illustration of the invention may be referred to, although such electrically conductive plastics are also suitable for other purposes, such as. B. for the production of molded circuits, molded commutator rings, high frequency attenuators, Resistors or the like.

Multiple contact switches usually have a number of contact elements, which can be made of metal or conductive plastic, and at least a switching member which is movably arranged to be with a selected one Contact element can be brought into contact to form an electrical circuit close. In some cases the switching element and the contact elements are stationary are moved with respect to the switching element. The relative movement between the switching element and the contact elements cause the switching element to wear the surface causes the contact elements to form a groove, and accordingly it is desirable to produce the contact elements from a relatively hard material so they withstand this wear and tear. It is also desirable that the contact elements have the lowest possible static and dynamic contact resistance. Of the Contact resistance should also be as uniform as possible; h., he should not depending on the particular location of the surface of the contact element, over which the switching element moves, change.

The previously proposed electrically conductive plastics are in generally unable to meet these various requirements.

According to a previously proposed method of manufacturing electrically Conductive plastic objects will be a solution of a resin or plastic prepared in a solvent, a finely divided conductive material such as finely divided Coal, uniformly dispersed in this solution, and then the solvent is evaporated. In the manufacture of such a plastic object, a proportionate large amount of conductive material used to provide sufficient electrical To maintain conductivity. The use of such a large amount of conductive However, material affects the physical properties of the plastic composition, and it also gives a resistivity that is much greater than is that of a metal. If a product of this nature, even if it's an acceptable one Has wear resistance, for the contact elements of a multiple contact switch is used, the contact resistance is undesirably high, especially in those Cases where a low voltage and a small current are possible.

In another prior process for the preparation of electrically conductive polymers (USA. Patent 2,761,854) are coated the surfaces of plastic grains with finely divided metal powder and the metal-coated plastic granules are then pressed in a mold to obtain a product which is a net-like structure made of conductive metal. Such plastics have extremely low resistance to the amount of metal powder used in their manufacture and good molding properties. When conductive plastics of this type are used for the contact elements of a multiple switch, they result in a low contact resistance. However, since they are not completely homogeneous, there is the disadvantage that the contact resistance varies greatly at the various points on the surface of the contact element. If, for example, the switching element passes over a point on the surface of the contact element which is made of metal, then the contact resistance has essentially the value zero. On the other hand, if the switching element passes over a point on the surface of the contact element, which is made of plastic, then the contact resistance is very high. Therefore, conductive plastics of this type are not satisfactory for such contact elements.

The purpose of the invention is to produce a granular product electrically to create conductive plastic objects, which the aforementioned difficulties allowed to fix.

According to the invention, the granular product consists of grains which are in in a manner known per se from a substantially uniform dispersion of finely divided electrically conductive particles in a synthetic organic Plastic made and on its surface with at least one thin layer are coated with an electrically conductive substance.

The process for making such a granular product is characterized in that finely divided electrically in a known manner conductive particles uniformly dispersed in a synthetic organic plastic the resulting dispersion is hardened and converted into grains and then a thin coating of one on the surfaces of these grains electrically conductive substance is applied.

It should be noted at this point that it occurs in the manufacture of mass cores is known from magnetizable metal powder, the metal powder with a liquid, water-soluble, curable, alkaline condensed phenol-formaldehyde condensation product to mix and dry the mixture and press it into molds. It is about hence in this case the manufacture of articles in which the metal powder particles are surrounded by an insulating material, but not to produce an electrical conductive granular product.

It is also known to have electrical resistors with an insulating Protective cover to provide increased heat permeability by being used as the material for this Protective cover a mixture of electrically insulating, but poorly heat conducting Substances and electrically insulating, but heat-conductive particles are used will. Again, it is not a question of producing an electrically conductive one ; granular product.

Plastics that can be used in the invention are for example thermosetting plastics such as phenol and urea-formaldehyde resins, and thermoplastics such as polystyrene, polyethylene, polymethyl methacrylate, Vinyl polymers and copolymers, cellulose acetate, silicone, polymonochlorotrifluoroethylene, Polytetrafluoroethylene or the like .. The one used in the invention as a starting material Plastic can also contain compounding components such as lubricants, plasticizers, Contain dyes, pigments, fillers or the like. To make the grainy Product becomes a dispersion of finely divided conductive particles in one of the Prepares plastics of the aforementioned type. The disperse phase can, for example an electrically conductive carbon powder, a powder of a conductive compound or be a metal powder. The dispersion of the conductive powder in the plastic can be effected in any known manner. For example, can the plastic dissolved in a suitable solvent and the conductive powder be intimately mixed with the solvent solution to form a homogeneous dispersion to build. The solvent is then evaporated to create a solid product, in which the conductive powder is uniformly dispersed. If necessary, can in the case of thermoplastic materials, the plastic is spread out on a grinder and the conductive powder is introduced into the plastic by a grinding or rolling process will. The amount of the conductive powder dispersed in the plastic can be such be that it constitutes 20 to 50 percent by weight of the conductive plastic product. Preferably the conductive powder consists of finely divided carbon such as acetylene black or furnace black, and preferably it is in the conductive plastic product in one Amount present from 30 to 40 percent by weight.

The solid dispersion is then broken up into granules. Although the extent to which the material is crushed is not particularly critical appears to be, it is generally desirable that the grains have a smaller size than that which corresponds to a sieve with a clear mesh size of 0.42 mm, and that the particles that pass through a sieve with a mesh size of 0.044 mm, removed. The preferred particle size is in the range of the sieve mesh sizes between 0.177 and 0.105 mm.

In the next stage, the surfaces of the obtained conductive Plastic grains coated with a layer of a conductive powder, the one has a smaller particle size than the plastic granules. The mean diameter of the Particles of the coating powder should be smaller than one third of the mean diameter of the conductive plastic grains. The coating of the surfaces of the plastic grains Using a thin layer of the conductive powder is made easier when the powder is composed of scale-like particles. Particularly good results can be can be achieved if silver powder is used for the coating, although also other conductive powder, such as nickel powder, an electrically conductive carbon powder or. Like., Can be used. Oxidation-resistant powder made from a precious metal are particularly useful. The amount of coating powder used can be 15 to 40% of the weight of the plastic granules; the preferred amount is 25 to 35% of the weight of the grains.

The desired conductive coating on the plastic grains can thereby obtained that the plastic grains and the finely powdered substance such as it is described in more detail in the specific examples below will. If necessary, the coating consisting of a metal can be applied to the plastic grains by chemical deposition of the metal on the grains or by evaporation of the Metal under reduced pressure and condensing the metal the surfaces of the plastic grains can be obtained.

In cases where the plastic grains covered with a thin layer a conductive metal is to be coated, it has proven to be advantageous first of all to apply an underlayer made of a substance that promotes adhesion. Suitable substances that promote adhesion are molybdenum sulfide and graphite. It is found been that by applying a previous coating of such an adhesion-promoting Fabric on the plastic granules a more effective wrapping of the plastic granules with the conductive metal.

The invention is illustrated below by means of some specific examples explained. Example 1 There were 140 g of an alcoholic phenolic resin in 500 g Ethyl alcohol dissolved using a mechanical stirrer. This solution was 60 g acetylene black with a particle size of about 1 #t added with stirring, and then a further 300 g of ethyl alcohol were added to the suspension with stirring. The suspension was then placed in an approximately 3.81 capacity porcelain grinder with porcelain balls introduced and ground for 2 hours.

The milled suspension was placed in a bowl and in a Oven dried at 65 ° C for 3 hours to evaporate the solvent from it, so that a homogeneous dispersion of the carbon powder remained in the plastic mass. The solid plastic mass was taken out of the bowl and placed in a hammer mill (a micro-pulverizer) crushed to a grain shape. The granular product was sifted, around the grains that have a sieve size between 0.177 and 0.105 mm mesh size corresponded to separate. 25.2 g of these sieved grains were together with 4.2 g of finely divided molybdenum sulfide placed in a drum-shaped 1-1 vessel, which is provided with circulating blades. The jar was turned on for 2 hours at one rate rotated around its axis by about 120 rpm. At the end of this time, 12.6 grams were finely divided Silver in scale form with a mean particle size corresponding to a sieve of about 0.048 mm clear mesh size together with 40 g of stainless steel balls about 3 mm in diameter are introduced into the 1-1 vessel, and those introduced into the vessel Fabrics were milled for an additional 8 hours.

The granular obtained by the procedure described above Product has been molded into contact elements, which are then based on their electrical properties and their wear resistance were checked. These contact elements showed one static contact resistance of only 0.1 ohms compared to around 25 ohms for off known conductive plastics made contact elements.

To the. Testing the wear resistance was made across the surface Hook-shaped tool made from a palladium alloy Wire with a diameter of 1 mm, left painted at a contact pressure of 55 g, with the tool being rotated at 600 rpm. The direction of rotation of the tool was reversed every 15 minutes and the test was run for 24 hours. At the end of this test, the depth of the groove made by the tool was measured; it was 5 - 10-g mm for a contact element made of a known conductive plastic, which contained a comparable amount of conductive material.

Example 2 The procedure of Example 1 was followed with the exception repeats that instead of the phenolic resin, an alcohol-soluble urea-formaldehyde resin was used, and the step of coating with molybdenum sulfide was omitted was and the amount of used. Acetylene black was increased from 60 g to 70 g. Contact elements that have been compression-molded from grains produced in this way, exhibited improved electrical properties and increased wear resistance.

Example 3 The procedure of Example 1 was followed with the exception repeats that instead of the phenolic resin, an alcohol-soluble melamine-formaldehyde resin, instead of 60 g of acetylene black, 80 g of conductive oil furnace black; and instead of Powder -from flaky silver 15 g of finely powdered nickel were used. In this case, too, equally good results were obtained.

Example 4 To a water emulsion made from 140 g of polytrifluoromonochlorethylene 60 g of acetylene black were added with stirring, and the suspension was poured into a about 3.8 l porcelain grinder with porcelain balls introduced and 2 hours ground. The resulting suspension was dried in an oven at 65 ° C for 3 days, in order to evaporate the water from it, so that there is a homogeneous dispersion of the acetylene black revealed in the plastic mass. After that, this solid plastic mass became grains crushed, which then according to the procedure of Example 1 with an adhesion-promoting Underlayer of molybdenum sulfide and a thin layer of silver were coated.

Example 5 The procedure of Example 4 was followed with the exception repeats that instead of the emulsion of polytrifluoromonochlorethylene an emulsion of polytetrafluoroethylene was used. Example 6 The working of the example l was repeated with the exception that the phenolic resin solution was replaced by a solution of 140 g of a polyamide in 500 g of dimethylformamide and instead of silver powder Gold powder was used.

Example? The procedure of Example 1 was with the exception repeats that instead of the phenolic resin solution, a solution of 140 g of allyl phthalate in acetone was used.

Example 8 The procedure of Example 1 was followed with the exception repeats that instead of the phenolic resin solution, 280 g of a melamine-formaldehyde composition with a solids content of 5011 / o was used. One from the The resulting conductive plastic made contact element had a contact resistance of 1 ohm measured against a conductive part of a switch.

From the above it can be seen that by the invention a conductive plastic is created from which articles are made by compression molding which correspond to the purposes mentioned in the introduction. By using a homogeneous dispersion of conductive particles in a plastic, which the Fill the gaps of a conductive network-like skeleton, the shaped, electrically conductive plastic object superior electrical and physical Properties compared to objects that were previously available conductive plastics are made.

Claims (11)

Claims: 1. Granular product for the production of electrically conductive Plastic objects, d a d u r c h g e - indicates that it is made of grains that in a manner known per se from a substantially uniform dispersion of finely divided electrically conductive "particles in a synthetic organic Plastic made and on its surface with at least one thin layer are coated with an electrically conductive substance. 2. Product according to claim 1, characterized in that the layer of the electrically conductive substance on the surfaces of the grains is bound by an adhesion promoting layer. 3. Product according to claim 2, characterized in that for the adhesion-promoting Layer of molybdenum sulfide or graphite is used. 4. Product according to one of the claims 1 to 3, characterized in that for the layer of the electrically conductive substance a conductive metal in finely powdered form is used. 5. Product according to claim 4, characterized in that silver is used as the conductive metal. 6. Product according to one of claims 1 to 5, characterized in that as electrically conductive Particles for the dispersion of acetylene black is used. 7. Method of manufacture a granular product according to any one of claims 1 to 6, characterized in that that in a known manner finely divided electrically conductive particles uniformly dispersed in a synthetic organic plastic, the resulting Dispersion is cured and converted into grains and then applied to surfaces A thin coating of an electrically conductive substance is applied to these grains will. B. Method according to claim 7, characterized in that in per se known Way the dispersion of the electrically conductive particles in the plastic thereby is brought about that a solution of the plastic in an organic solvent is prepared, the finely divided electrically conductive particles into uniform the solvent solution are dispersed and the solvent out of the solution is evaporated to produce a solid composition. 9. The method according to claim 7 or 8, characterized in that the coating consists of the conductive substance on the surfaces of the grains is applied in that the grains first with an adhesion promoting agent, which is made of molybdenum sulfide or graphite in finely divided form, to be circulated to be on the surfaces of the grains to form a coating of the adhesion promoting agent, and then a thin one Coating of an electrically conductive metal applied to the surfaces of the body will. 10. The method according to any one of claims 7 to 9, characterized in that for the electrically conductive particles of the dispersion acetylene black, as electrical conductive substance silver and a phenolic resin is used as plastic. 11. Use of the granular product obtained according to one of claims 7 to 10 for the production of a pressed, electrically conductive plastic object. Documents considered: German Patent No. 744 149; German utility model No. 1513164; USA. Pat. No. 2,761,854.