DE2326936A1 - Liq. resistance coating for electric space heaters - addn. of fibres improves rheological properties and strength after hardening - Google Patents

Abstract

A conductive, liq. coating material made from binders, pigments, fillers, additives, solvents and/or water, the novelty being that the material contains conductive fibres and the hardened layer has a specific resistance of 10-3 10+10 ohm. cm. A very wide range of binders can be used. The coating can contain (a) 0.1-100% conductive fibres w.r.t. the wt. of binder, the pref. fibres being carbon or graphite with length 0.01-50 mm and dia. 0.1-100 mu; (b) non-conductive fibres; (c) organic fibres (e.g. nylon) or inorganic fibres such as glass, asbestos, silicates. It was found that the fibre addn. improves the rheological behaviour of the liq. and also the mechanical props. of the hardened coating. The liq. can be used to coat penels for the electric resistance heating of rooms, greenhouses, airport runways etc.

Description

Conductive coating agent The application relates to a conductive one Coating agent in the form of a layer-forming, rheologically advantageous liquid Mixture and its use for electrical resistance elements.

It is known that the electrical resistance in the passage of current is influenced by the nature of the material composition. So diminishes for example, the ability to work in the presence of certain substances. Through this Appearance can be the conversion of electrical energy into other forms of energy such as Thermal energy, radiation energy or light energy can be controlled.

It is known to use electrically conductive layers from coating agents, the metal powder such as aluminum, copper or iron or other conductive substances, w e contain carbon black or graphite. This is how the German patent application describes DOS 2 018 823 a method for producing an electrically conductive paint, according to the graphite means Wet milling is crushed and under Adding a binder, a coating agent is produced, which, on a base Applied as a thin layer and cured, a film with low electrical Resistance results, which delivers thermal energy when current passes through. This well-known paint was specially developed for the purpose of producing sufficiently low, to achieve electrical resistance values, because on the one hand due to the rheological and chemical properties of the known coating agents in only thin layers can be applied in one operation and on the other hand a coating with thick Layers that are obtained by multiple application and intermediate hardening for Crack formation tends to occur, so that in the event of long-term stress as a result of unavoidable temperature differences When the heat is released, the conductivity of the film changes, and finally the film becomes unusable by cracking.

The coating compositions described in DOS 2 018 823 are in their electrical properties compared to other known materials of this type only improved by the fact that a certain and complex selection of raw materials with narrow Tolerances is met and an equally complex manufacturing process is carried out will.

In the rheological properties required for processing the coating compounds are crucial, and in the mechanical properties that are necessary for continuous use of the coatings produced therefrom are essential are those in DOS 2 018 823 described paints suffer from the disadvantages of a conventional type.

The object of the present invention is to remedy these disadvantages with certainty switch off, and a conductive, layer-forming, rheologically advantageous, to find liquid coating agent, when using it, permanent electrical Resistance elements with constant electrical resistance values at one Continuous load can be produced.

Surprisingly, this problem was solved. through a conductive, layer-forming, liquid coating agent made of binders, pigments, fillers, Additives, solvents and / or water that is characterized by dadu.rch that it additionally contains conductive fiber materials and as a hardened layer a has a specific resistance value between 10 3 and 10 + 10 Ohm x cm.

The best results are obtained with fibers that contain a Have a length between 0.01 and 50 mm and a diameter between 0.1 and 100 microns.

A preferred embodiment of the invention provides that the liquid Coating agent optionally additionally contains non-conductive fiber materials. This will make the Strength values of the conductive layers improved.

The liquid coating agent preferably contains it as a conductive one Fiber materials carbon fibers and / or graphite fibers.

The layer-forming binders are all inorganic or organic Substances suitable after the application and drying or hardening of the coating Form coherent films.

The hardening can be achieved by simply drying at normal or increased Temperature can be brought about. It is also possible to use the type of synthetic resin, to carry out hardening by chemical reaction with the aid of catalysts or hardeners.

Typical binders of an inorganic type are alkali metal silicates, e.g. potassium, sodium or lithium silicate in aqueous solution, furthermore hydrolyzable Alkyl silicates such as ethyl silicate. Organic binders are in organic Synthetic resins soluble in solvents, synthetic resins in aqueous dispersion or solvent-free Synthetic resins. Polycondensate resins such as alkyd resins, Urea resins, phenolic resins, melamine resins, polycarboxylic acid resins, polyester resins, Polyaddition resins, such as epoxy resins, polyurethane resins, maleinate resins, and more Polymer resins, such as acrylic or methacrylic polymers, polysulfides, polydienes, synthetic rubbers, polyfluoroethylene, polyolefins, polyvinyl copolymer resins and others, Werner silicone resins and cellulose compounds. The binders can can be used individually or in admixture with one another. The niche can be matched with the customary pigments, fillers, they can also be used as additives Contain leveling agents, anti-foaming agents, etc. It can also be transparent or contain soluble dyes, as well as conductive carbon black or conductive graphite.

It is essential to the invention that the liquid coating agent is conductive Fiber materials contains. These are to be understood as 1. carbon fibers and graphite fibers; 2. Inorganic fibers, such as glass fibers, asbestos fibers, which by applying before metal oxides like indium oxide, tin oxide, cadmium oxide or other metal salts made conductive are.

Treating the inorganic fibers with the conductive ones Mixtures are not claimed here. Organic can also be used in the same way fibrous materials can be made conductive. Even those organic conductive ones Fiber materials are useful in this invention. The manufacture of Carbon fibers or graphite fibers is known and is used, for example, in the U.S. Patent 3,547,584. The conductive fibers should preferably a length between 0.01 and 50 mm and a diameter between 0.1 and 100 microns own. However, lengths of less than 0.1 mm and more than 50 mm can also be used. The liquid coating agent contains the conductive fibers in an amount of 0.1 up to 100%, based on the binder present in the mixture. It should be in the As a rule, so much conductive fiber must be present that the viscosity is in the order of magnitude from 103 to 106 milliPascals x seconds.

In addition, the conductive fibers can be used in the liquid coating agent Graphite and carbon black powder may be included.

In order to further improve the performance properties of the finished coating, The liquid coating agent can optionally also contain non-conductive fiber materials contain. These can be inorganic fibers such as asbestos and glass fibers or organic fibers, such as natural or synthetic fibers. Examples of this are space wool or wool fibers, fibers made from polyamides, polycarylates, polyesters, polyurethanes or rayon such as acetate fibers. Can be used all organic or inorganic materials. which are in fiber form or can be manufactured.

By adding the non-conductive fibers, however, the prescribed Do not change viscosity limits.

It was surprising and unpredictable that the use of Fibers in the liquid superfluous agent of this particularly pronounced positive rheological Conveyed properties that manifest themselves in the following properties: 1. Adjustable high structural viscosity, 2. low tendency of the liquid layer to run off from vertical Surfaces, 3. low tendency of the liquid mixture to separate, 4. low tendency to settle the liquid mixture, 5. easy adaptability to different application methods.

To explain the special rheological behavior of the liquid tbersugsmxttels is used in Figure 1. It shows the rheological behavior at 200 C for two mixtures obtained according to Example 1 and Example 2. On the ordinate is the viscosity and the shear gradient is plotted on the abscissa. Out of the curves it can be seen that the mixture 1 has a high internal structure. This is shown in the high viscosity value at low shear rate versus a low viscosity value at high shear rates.

In comparison, the comparison mixture according to Example 2, in which does not contain any fiber materials, has a much smaller structure, so that their low shear viscosity is below the viscosity of the mixture according to Example 1 and vice versa with a high shear gradient above the viscosity of the Mixture 1. The figure clearly shows that due to the high structural content of the Viscosity the processability of the mixture containing fiber materials is essential is better there with the high shear rates, the application techniques usually require mixture 1 to be extremely fluid.

The formation of the inner and outer film structure takes place on the other hand in an optimal way with a low shear gradient, i.e. in a highly viscous state. That means, among other things, that even on vertical surfaces thick layers in one Operation can be applied without sagging or sagging the layer occur. Because of the low viscosity at low shear rate this is not the case with the fiber-free comparison mixture according to Example 2 Technical progress of the fiber-containing material is thus illustrated in the figure proven.

The conductive layer is produced using the usual methods spraying, painting, pouring, dipping or rolling. Usually you get Conductive layers adhering to insulating layers after hardening. Man However, the conductive, liquid coating agent can also be applied to substrates which make it possible to peel the cured conductive layer off the base or disconnect.

The conductive layer can then be used as a single film and can be embedded between any two insulating layers.

To produce conductive layers from conductive, liquid mixtures, which contain graphite or conductive types of soot, which are usually flaky is known. As can be seen from the DOS 2 018 823 second cited at the beginning, such films can deliver thermal energy when a current passes through them. Such known coating agents when applied in the form of thin coatings, e.g.

with a thickness of 25 to 130 microns, resistance values from 1 to 10 Ohm / cmS or more. In order to lower the resistance values, thicker coatings are used produced, so show the films of the well-known Coating agent im Cracking over time, rendering the layer unusable. Also can such layers are therefore not used as self-supporting layers in film form will.

It was surprising and unpredictable that when a conductive Layer of a layer-forming, liquid conductive coating agent, the conductive Contains fiber materials, these disadvantages do not occur. In addition, you can Layers of any thickness can be produced with continuous use and continuous heating do not crack and their specific resistance, which is independent of the layer thickness of the electrical resistance layer is, always remains constant, which is a considerable technical progress means.

The coating agent according to the invention is therefore particularly suitable for the production of such conductive layers, which are used in electrical resistance elements used to convert electrical energy into thermal energy when current passes through convert and thus serve as a heating layer. For this it is necessary that the conductive layer in the hardened state has a specific resistance value between 10 3 and 10 + 10 ohms x cm.

The required resistivity values are determined by the choice of the binder. The big interval in the resistivity values allows an optimal adaptation of the resistance elements to the required Energy turnover per unit area. The electrical resistance element can be Make use of the coating agent according to the invention so that with any electrical voltages, especially in the area of low voltages, that are operating voltages of less than 42 volts, the specified energy consumption per Area unit is achieved. This ensures that the electrical resistance elements also in open Can be operated without breaking the circuit To violate provisions of the VDI regulations. This simplified operation is wise while respecting the safety regulations, represents a significant asset to the technology.

Those produced using the coating compositions according to the invention electrical resistance elements can be used indefinitely, even under constant stress. There is no cracking of the warming layers, and constant values are obtained Values for the specific electrical resistance. which is independent of the layer thickness of the electrical resistance layer. This makes it possible to use the. conductive Produce layer for heating purposes in any layer thickness. Such electrical Resistance elements can be used as heat-generating elements for heating parts of buildings, how greenhouses, rooms, basements, factory rooms are used. A - Another field of application is the heating of pipes, roads, bridges, Runways at airfields They are used for many industrial and household purposes suitable.

The following examples serve to illustrate the invention, without restricting them. All. Percentages relate to percentages by weight; all parts on parts by weight.

Example 1 A resistance element is produced in the following way: In 35 parts of water, which contains 1.5 parts of alkyl naphthalene sulfonate, there are 32 parts Fine powder graphite (flake graphite) 2.3 parts of conductive carbon black (particle size 15 to 35 microns) 1.48 parts graphite fibers (diameter 8 to 10 microns, length 800 to 1500 microns) 4> 20 parts nylon fibers (diameter 7 to 10 microns, Length 800 to 1,300 microns) and stirred in a high-speed mixer for 5 minutes. Then 25 parts of an aqueous dispersion with a solids content are added of 50% of a styrene-butadiene-carboxylic acid copolymer obtained by copolymerization from 50 to 60% styrene, 37 to 48% butadiene and 2 to 3% unsaturated carboxylic acid was obtained, and stir well until a homogeneous mixture has formed. The viscosity ratios at low and high shear rates are in the table shown.

The liquid coating agent produced in this way is applied with a roller an asbestos cement plate with the dimensions of 50 x 50 cm2 is applied so that after air drying obtained a layer thickness of 100 to 120 microns will. A brass wire mesh strip is then attached to each end, which is sprayed over with the same liquid product and dried. By means of a Terminal takes place the power supply, so that you have a resistance element in the form of a conductive surface with the dimensions of 50 x 50 cm2. The specific resistance is 5.2 x 10-2 ohms x cm.

When passing a direct current of 4 amps, starting from a 12 volt power source, the conductive layer heats up to a temperature of 50 to 600 C. Here, the specific resistance is only insignificantly reduced.

After switching off the power supply, the heated film is replaced by cold air cooled quickly from 00 C, and this process repeated five hundred times.

Microscopic examination of the surface of the resistance element after 500 coolings gave excellent cohesion of the film without the slightest cracking or detachment from the substrate was observed.

Example 2 (comparative example) It is carried out in the same way as in Example 1, a resistance element produced, with the difference that instead of of the fibrous material, 4 parts graphite and 0.68 parts conductive carbon black are used will.

The specific resistance is 1.6 x 10-1 ohm x cm. During the exam the thermal shock, as in Example 1, was already evident 100 cooling hairline cracks in the film surface that increase the electrical resistance value deteriorated by approx. 40%. After 350 coolings, broad cracks appeared Detachments from the base. The element was no longer usable. In the figure is the rheological behavior of the liquid coating agent of Example 1 with compared to that of Example 2. The graphical representation proves the technical Progress of the coating agent of Example 1.

Table rheological behavior of the liquid coating agents and mechanical properties of the cured film. Mechanical Rheological 0 Viscosity at 20 C Example elongation at break low shear high shear L / Lo slope slope 1 19% 2.6 Fa sec - 250 m Fa sec high 1 19% 2.8 Pa sec 250 m Pa sec none structure 2 14% 1.0 Pa sec 450 m Pa sec 3 17% 2.4 Pa sec 150 m-Pa sec high structure 4 8% 2.3 Pa sec 600 m Pa sec 5 *. 20% 2.7 Pa sec 100 m Pa sec high structure 6 8% 1.6 Pa sec 50 m Pa sec *) Self-supporting layer with high flexural strength possible.

Example 3 As in Example 1, 32 parts of fine powder graphite (flake graphite) 2.3 parts conductive carbon black (particle size 15 to 35 microns) 0.48 parts graphite fibers (Diameter 8 microns, length 30 to 100 microns) 4.2 parts cellulose acetate fiber (diameter 25 microns, length 80 to 800 microns) with 20 parts of water, which is 1.5 parts of alkyl naphthalene sulfonate contains, stirred well.

40 parts of 30% potassium silicate solution are then slowly added and the mixture is homogenized.

The liquid mixture is poured onto an asbestos cement board of 50, x 50 cm2 and dried at 120 ° C, so that a film with a thickness of 180 Micron is created.

The specific resistance is 5.6 x 10 2 Ohm x cm The test of the behavior in the event of temperature changes by heating the coating by means of a current flow to 1200 C and rapid cooling by cold air from 0 G resulted, as in example 1 no complaints whatsoever. The viscosity behavior is shown in the table.

Example 4 (comparative example) instead of that used in example 3 Fiber material is 4 parts of fine powder graphite and 0.6 parts of conductive carbon black.

The specific resistance is 6.0 x -2 ohms x cm.

When testing the thermal shock, as in the example 3, the electrical resistance value was found to be around 50% deteriorated.

After 220 cooling, the coating showed numerous cracks after all Directions and began to peel off the pad.

Example 5 In 16 parts of 96%, ethanol, the 0.3 parts of one not ionogenic wetting agent (ethylene oxide adduct of a fatty alcohol) 30 parts block graphite 3 parts conductive carbon black (particle size 15 to 35 microns) 0.5 part carbon fiber (diameter 10 microns, length 20 to 150 microns) -4 Parts of polyamide fibers (diameter 8 to 12 microns, length 600 to 1,200 microns) are stirred in and 45 parts of silicic acid ester solution (Qrganooxysilan) 33% in ethanol added. The viscosity behavior is shown in the table.

The homogeneous mass is applied to a glass plate with the spray gun so that a dry layer thickness of 250 microns is created and dries with it. Warm air. After complete drying, the resulting film can be lifted off the glass plate and forms a self-supporting uniform layer with high flexural strength, the delivers a specific resistance value of 3.4 x 10-2 ohms x cm when current passes through.

Example 6 (Comparative Example) In the composition of matter mentioned in Example 5 the proportion of carbon fibers and polyamide fibers is replaced by 6 parts of graphite and 1 part of conductive soot replaced. The specific resistance is 6.3 x 10-2 ohms x cm.

The one obtained after drying in the same manner as in Example 5 dried coating cannot be lifted off the glass plate without damage. The layer is not self-supporting and cracks and the mass appear when it is lifted off disintegrates.

Example 7 100 g of epoxy resin (glycidyl ether from 2,2-bis- (4-hydrozyphenyl) propane) are mixed with 27 g of carbon fibers at a temperature of 1200 C and then this mixture in a layer thickness of 90 microns on an asbestos base upset.

At the top and bottom, strips of brass fabric 1 cm wide are placed Applied so that the ends protrude about 1 cm.

Hardening takes place through a 10-hour heat treatment at 120 ° C in the drying oven.

The electrical resistance of this resistance element is 1.9 x lo + 2 ohms.

The measured value for this resistance element was unchanged after over 700 thermal shock loads according to example 1.

Claims (9)

P a t e n t a n s @ p r ü c h e 1. Conductive, liquid coating agent made of binders, pigments, Fillers, additives, solvents and / or water, characterized in that that the coating agent additionally contains conductive fiber materials and as hardened Layer has a specific resistance value between 10 3 and .. 10 + 10 Ohm x cm. 2. Coating agent according to claim 1, characterized in that the Coating agent 0.1 to 100 percent by weight of the conductive base materials, based on on the binder present in the coating agent. 3. Coating agent according to Claims 1 and 2, characterized in that that the conductive fiber materials preferably have a length between 0.01 to 50 mm and a diameter between 0.1 to 100 microns. 4. Coating agent according to Claims 1 to 3, characterized in that that the liquid coating agent additionally optionally non-conductive fiber materials contains. 5. Coating agent according to claims 1 to 4, characterized in that that the liquid coating agent as fiber materials are inorganic fibers such as glass fibers, Contains asbestos fibers and / or fibers made from silicate materials. 6. coating agent according to claims 1 to 5, characterized in that that the liquid coating agent contains organic fibers as fiber materials. 7. Coating agent according to Claims 1 to 6, characterized in that that the liquid coating agent as conductive fiber materials carbon fibers and / or contains graphite fibers. 8. The method according to claims 1 to 7, characterized in that that the liquid coating agent additionally, if necessary, conductive carbon blacks and / or Contains graphite. 9. Use of the characterized according to claims 1 to 8, conductive, liquid coating agent for the heating layer which generates heat when the current passes through an electrical resistance element.