CONDUCTIVE COATING COMPOSITION AND HEATING SYSTEM

Abstract

The present invention relates to a conductive coating composition (known as heating paint) having high efficiency, which composition can be applied to substrate surfaces for efficient generation of heat from electrical energy, and to a method for the production thereof. The conductive coating composition comprises a silicate salt and particles of expanded graphite which are dispersed in the silicate salt, wherein the graphite is present in a hexagonal and/or rhombohedral crystal structure having crystal lattice planes extending in parallel and silicon is embedded between the crystal lattice planes of the graphite.

Claims

1. Method for producing a conductive composition for generating heat from electrical energy (heating paint), comprising: forming a mixture of graphite and a silicate salt in an aqueous solvent, electrolyzing the mixture, by applying an electrical voltage, to activate a surface of the graphite and form an intercalation compound, and separating the reaction product of the electrolysis from the reaction solution.

2. Method according to claim 1, wherein a total amount of power of at least 200 mA.Math.h/g, relative to the weight content of graphite, is supplied to the mixture.

3. Method according to claim 1, wherein the applied electric potential of the power supply lies in the range between 1 V and 3 V.

4. Method according to one of claim 1, wherein the power supply is accomplished in a potentiostatic operating mode in stages, wherein the applied electric potential of a first stage exceeds the applied potential of a second stage by at least 1.1 times.

5. Method according to claim 4, wherein the applied electric potential of a first stage is 2.1 V to 2.5 V and in a second stage is 1.5 to 1.8 V.

6. Method according to claim 5, wherein the supplied amount of power of a first stage is 300 to 500 mA.Math.h/g relative to the weight content of graphite at a potential of up to 2.5 V, and of a second stage is 100 to 160 mA.Math.h/g V, at a potential of up to 2.3 V relative to the total weight of graphite.

7. Method according to claim 1, further comprising drying and expanding the reaction product separated from the reaction solution by supply of thermal energy.

8. Method according to claim 7, wherein temperatures of 130° C. or higher are applied for the expansion.

9. Method according to claim 1, wherein a total amount of power in the range of 300 to 600 mA.Math.h/g, relative to the weight content of graphite is supplied to the mixture.

10. Method according to claim 7, wherein temperatures of 150° C. to 260° C. are applied for the expansion.

11. Method according to claim 7, wherein temperatures of 150° C. to 180° C. are applied for the expansion.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0024] FIG. 1 shows a first electron micrograph through a first cross-section of a first coating composition according to the invention.

[0025] FIG. 2 shows a second electron micrograph through a second cross-section of the first coating composition according to the invention.

[0026] FIG. 3 shows an electron micrograph through a third cross-section of the first coating composition according to the invention.

[0027] FIG. 4 shows the retention of the electrical capacity of the first coating composition according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0028] In one embodiment of the invention the composition according to the invention is present in the form of expanded particles. In one embodiment of the invention the particles have a bulk density between 0.01 kg/L and 0.1 kg/L, preferably between 0.02 kg/L and 0.05 kg/L. In a further embodiment of the invention the particles have pore sizes of 10 μm-100 μm.

[0029] In order to produce the expanded particles, the intercalation compound formed by the power supply is eluted with water, the eluted particles of the intercalation compound are dried and subjected to a thermal treatment in which the graphite expands. Preferably temperatures of 130° C. or higher, particularly preferably temperatures of 150° C. to 260° C., in particular 150° C. to 180° C. are used for the expansion.

[0030] Preferably thermoplastics or resins are used as matrix polymers which are also designated as binders. Suitable polymers comprise but are not restricted to: polyamides such as polyamide 6 or polyamide 12; acrylic polymers such as polybutyl acrylate (PBA) or polyethylacrylate (PEA); and epoxy resins. The purpose of the matrix polymers is primarily to ensure a permanent contact between substrate surface and coating. Biologically degradable or natural polymers which have a certain adhesiveness can also be used.

[0031] According to a further embodiment, the present invention relates to a substrate which is coated with particles of the conductive composition according to the invention on a surface thereof. According to one embodiment the particles are distributed in a polymer matrix.

[0032] The coating can take place in any manner, for example, by application by means of a brush or a spatula, by spray coating or dip coating and subsequent drying of the coating. Before complete drying, at least two wires, preferably copper wires, are inserted in the coating, which act as anode or cathode to conduct electrical energy. The thickness of the coating on the substrate is preferably 10 μm to 40 μm. In this case a sheet resistance of 10 Ohm to 20 Ohm is preferably achieved.

[0033] The composition according to the invention is per se current-conducting and exhibits excellent properties in the conversion of power into heat output. It is preferably used in low-voltage applications but can also be used in alternating voltage applications. Preferably the composition according to the invention or the heating system according to the invention which comprises a composition coated on a substrate is used in 24V/48 V DC applications. The substrates thus coated can thus be used very efficiently as heating for surfaces and rooms.

EXAMPLES

Example 1

[0034] In an electrolyzer which comprises an anode working chamber disposed between the current drain of the anode and a movable piston with a diaphragm, and a cathode disposed in the electrolyte above the piston, 25 ml of 58% HNO.sub.3 and 10 g of natural graphite having the following particle size composition: 80% 200-290 μm and 20% less than 200 μm were added. For this purpose 10 g of potassium water glass solution was added. An anodic treatment of the graphite was carried out after the potentiostatic procedure. At stage (b) (activation of the surface) a potential E.sub.b=2.1 V was predefined in the course of 15 s. Thereafter, stage (c) of the electrochemical treatment to produce GEV at the potential E.sub.c=1.75 V was carried out for 5 hours at Q=400 mA.Math.h/g graphite. The pressure on the piston was 0.2 kg/cm.sup.2. The product obtained was eluted with water, dried and thermally treated at 200° C. An expanded graphite embedded in potassium water glass (dispersion graphite) having a bulk density of 1.7 g/l was obtained.

Example 2

[0035] A mixture of natural graphite and water glass was produced as described in Example 1, wherein however graphite powder having an average particle size of 200 μm was used, The anodic treatment was carried out in acid having a concentration of 35 to 40% in two stages, with Q of 300 to 420 mA.Math.h/g graphite and foam formation temperatures of 200 to 250° C.

Example 3

[0036] In the working chamber of an electrolyzer disposed between the anode and the cathode having a closely adjacent separator of polypropylene fabric, 2 kg of a mixture of graphite with 80% sulphuric acid was added, which was taken in a ratio of 1:1.6. To this was added 1 kg of potassium water glass solution.

[0037] The graphite suspension was treated anodically after the galvanostatic procedure. Step (b) (activation of the surface) was carried out under action of a current of 160 mA/g until the anode potential E.sub.b=2.3 V was reached (in the course of approximately 2 hours), then stage (c) (formation of GEV) was carried out whilst reducing the current to about 80 mA/g. The voltage at the electrolyzer varied during the synthesis process within the limits of 3.5-4.5 V. The total time of the treatment was 9 hours, Q=450 m.Math.Ah/g graphite. The pressure on the piston was 0.2 kg/cm.sup.2.

[0038] The product obtained was then eluted with water, dried and treated at 250° C. A dispersion graphite having a bulk density of about 1.6 g/l was obtained.

Example 4

[0039] The anode treatment of the graphite suspension in 70% sulphuric acid which was taken in a ratio of 1:1.6 was carried out in agreement with Example 3. When the potential of 2.1 V was reached, the amount of current was reduced, stage (c) was initiated at E=1.8 V. The total time of the treatment is increased to 10 hours, Q=520 mA.Math.h/g graphite.

[0040] The product obtained was eluted with water, dried and thermally treated at 200° C. A dispersion graphite having a bulk density of 2.0 to 2.20 g/l was obtained.

Example 5

[0041] The anode treatment of the graphite suspension in 60% sulphuric acid which was taken in a ratio of 1:1.5 was carried out in agreement with Example 3 but the initial polarization current was 80 mA/g graphite at the activation stage and upon reaching the potential of 1.9-2.0 V the amount of current was reduced. The treatment was then carried out at E=1.7 V with a current of 40 mA/g graphite.

[0042] The total time of the treatment was increased to 12 hours, Q=480-500 mA.Math.h/g graphite. The product obtained was eluted with water, dried and thermally treated at 200° C. A dispersion graphite having a bulk density of 2.2 g/1 was obtained.

Example 6

[0043] The electrical capacity of a first coating composition according to the invention was determined in several cycles. The relative decrease in the electrical capacity over the cycles is shown in FIG. 4.