WO2021150172A1

WO2021150172A1 - Electrically conductive coating

Info

Publication number: WO2021150172A1
Application number: PCT/SI2020/050027
Authority: WO
Inventors: Dusan CAF; Matija VRSIC
Original assignee: Dr. Nemo, D.O.O.
Priority date: 2020-01-22
Filing date: 2020-12-11
Publication date: 2021-07-29
Also published as: SI25955A

Abstract

The electrically conductive coating allows the electrical conductivity of various surfaces such as wood, plastics, metals, ceramics, plasters, glass, etc. The coating is quick-drying and waterproof. It adheres well to the surface and is partially flexible, which reduces the issues of cracking. It is intended primarily for use in heating elements, capacitive sensors, protection against electromagnetic radiation, electrolytic metallization of non-conductive materials, manufacture of electronic circuits and devices, enabling thermal conductivity of surfaces, etc. It provides good surface conductivity, which can be further adjusted by the method of application of the coating and the final mechanical treatment.

Description

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ELECTRICALLY CONDUCTIVE COATING

The subject of the invention is a carbon-based electrically conductive coating that ensures the electrical conductivity of the surface to which it is applied and the process of application and subsequent treatment of the coating that ensures the adequate quality of application for each treated surface. In the context of this application, the term "coating" refers to a composition that is applied to a surface and then treated to ensure the appropriate quality of the final application to each surface. The coating is waterproof and flexible. It is designed for application to various materials such as wood, plastics, paper, ceramics, plasters, metal, glass, textiles, etc. The coating is formulated in liquid form with solid conductive particles dispersed in a suitable solvent. In its liquid form, the coating is applied to the surface in established ways for applying liquid coatings to surfaces, such as spray application, brush application, roller application or in any other way known to professionals in the field of applying liquid coatings. The coating can be formulated in powder form and which is then mixed with a solvent before use. In this case, the coating is applied in the same way as the coating in liquid form. Regardless of the coating formulation used and the coating technology used, the final coating can be further processed by compression, which increases the conductivity or reduces the sheet resistance and makes it easier to define the area of conductivity while ensuring better adhesion, lower application thickness and ensure look with a metallic luster.

STATE OF THE ART:

Nowadays, both consumers and industry value products and devices that are compact, inexpensive and easy to use. The progress towards this goal is enabled by new materials and new ways of using them. The conductive coating presented in this invention enables this in a number of different devices as either their main component or as an addition that enables miniaturization and the streamlining of manufacturing, especially when it comes to batch production. The main goal of the coating, according to the invention, is to enable the conductivity of surfaces for the transmission of various signals, protection against interference or the conversion of the electric current into heat, for example when used in a resistance heater. Additionally, many new applications in sensors and other devices are provided by the properties of the invented coating. In particular, it offers excellent adhesion to various surfaces, flexibility, water resistance, various application options, the possibility of after treatment of the coating in terms of improving the conductivity of precisely defined areas and affordable price.

Comparable graphite-based coatings currently available on the market have low conductivity. While the possibilities for improving the conductivity of coatings using conductive fillers and special binders have already been shown, they greatly increase the number of components and increases the cost of production due to expensive raw materials and complex manufacturing processes (e.g.: DE202014009744U1 , DE202016106096U1 , CN104231749A, CN103146259B, US20150240099A1 , JP3400236B2, GB2526591A, US 2012/0020033 A1 ).

Currently known coatings enabling the conductivity of surfaces are mostly water- based and have long drying times, which limits applications to some surfaces. Additionally, their adhesion to different surfaces is poorer and their conductivity is lower. All this can make the process of application very complex.

Industrial conductive coatings, paints and inks are often applied by the screen printing process, which is suitable for creating specific patterns on the substrate but presents greater difficulties when applied to larger surfaces, especially when applied by spraying.

According to the invention, the conductive coating uses standard materials to enable the conductivity of the final product. The production process is simple and safe and raw materials are widely available, which simplifies the entire production process compared to the competition. The coating can be used on several different substrates such as paper, cardboard, wood, textiles, artificial and natural fibers, ceramics, concrete, stone, glass, plastics. By using different solvents, it is possible to apply the coating to different surfaces. An additional advantage is that the coating is quick drying, a particularly desirable property in batch production. The flexibility of the final coating is another very desirable property, as it makes it possible to ensure the conductivity of materials exposed to bending and various deformations, such as various foils, textiles, fibers, paper, etc.

The technical problem solved by the electrically conductive coating of the invention is the formulation of a coating that provides water resistance, flexibility, high electrical conductivity and good adhesion to various surfaces with the possibility of further processing to improve conductivity, which are properties not displayed by existing similar coatings. The coating is quick-drying and enables various application techniques suitable for industrial and domestic uses. Due to its composition, it is not limited to use at low voltage, but can be used at all voltages when adequately protected by electrical insulation. The presented conductive coating also has a completely new formula, which has not been used for similar coatings so far.

Additionally, the coating in the invention can be formulated in powder form, i.e., as a powder, to which a solvent is added according to the required application. The coating conductivity is regulated by changing the ratio of the components, by the thickness of the coating and by mechanical treatment, for example by compressing the final coating, which is not possible with similar coatings.

According to the invention, the electrically conductive coating comprises at least a conductive filler and a binder. The conductive filler comprises graphite particles and carbon black. Graphite can be natural or synthetic, the graphite particles being of various shapes, namely in the form of powder and/or rods and/or flakes, and the size of the graphite particles measuring from 0.5 to 200 micrometers, preferably from 5 to 50 micrometers. The stated graphite particle size dimension refers to the longest dimension of an individual particle shape. Carbon black can be of a variety of types, although a highly conductive carbon black such as, but not limited to, SuperP is preferred. The carbon black particles can be of various shapes, in the form of powders and/or rods and/or flakes, etc. with the size of the carbon black particles measuring between 0.5 to 200 micrometers, preferably from 5 to 100 micrometers. The stated carbon black particle size dimension refers to the longest dimension of an individual particle shape. For some applications, particularly when manufacturing paint for IR heaters, the specific surface area of carbon black is also important. The specific surface area of 50 m2/g or more is preferred. The content of carbon black in relation to the graphite in the conductive filler is from 5 wt. % to 15 wt. %, preferably between 8 wt. % and 13 wt. %.

Additionally, the conductive filler may include additives to increase conductivity in the form of conductive metal particles, such as silver or copper, and/or other various forms of graphite, for example graphene, carbon nanotubes, and the like. The size of these particles is between 0.5 and 200 micrometers, preferably 5 to 50 micrometers. The mass ratio of additives to increase conductivity to graphite is smaller than the ratio of carbon black to graphite, depending on the desired properties of the coating and the material used; it is preferably between 1 :10 and 1 : 100.

The binder ensures the adhesion of the coating to the surface of the substrate and the flexibility of the final coating while providing a solid connection between the particles of the conductive filler in a way that allows electrical conductivity. The binder is polyvinyl butyral (hereinafter PVB) in the form of a powder with particle sizes ranging between 0.5 and 200 micrometers, preferably between 5 and 100 micrometers. The binder content with respect to the conductive filler in the conductive coating is from 10 wt. % to 30 wt. %, preferably from 13 wt. % to 18 wt. %.

The invented electrically conductive coating additionally includes a solvent for formulating the coating in liquid form. Suitable solvents are organic solvents in which the binder is soluble. A suitable solvent is selected from, but not limited to, ethanol, isopropyl alcohol or butanol, or a mixture thereof. To ensure compliance with the legally prescribed volatile organic compound limits and for other purposes, such as regulating the drying time, etc. the solvent can be diluted with various substances, preferably water, so that instead of 96% ethanol, for example, either 80% or 60% ethanol can be used. It should be noted that such dilution can change the quality of the final application in terms of resistance, adhesion to different materials and the appearance of the application. It also extends the drying time. The content of the binder, i.e. , polyvinyl butyral, in relation to the solvent is between 2 wt. % and 20 wt. %, preferably from 5 wt. % to 8 wt. %.

Various additives can be added to the conductive coating to customize it to specific applications. In addition to the aforementioned optional additives to increase conductivity that can be added to the conductive filler, the conductive coating may additionally include additives to strengthen the coating structure (e.g.: milled carbon fibers, glass fibers), additives to prevent sedimentation (e.g.: fumed silica), additives for changing the appearance of the coating and the structure of the coating (e.g.: additives to increase the specific surface area of the application), for changing the physical (e.g.: additives to increase or decrease viscosity) and chemical properties of the coating (e.g.: additives for better adhesion on specific surfaces, reduced reactivity, fungicidal additives, etc.).

The viscosity of the final coating formulation in liquid form is generally between 19 mPas and 500 mPas.

The type of PVB used (i.e., molar mass, number of butyral groups) depends on the desired properties of the final coating such as water resistance, the viscosity of the coating in its liquid state or the solubility in organic solvents. PVB with an average molar mass of 50,000 or more is preferred to allow the control over the viscosity of the coating and with a number of butyral groups of 75 mol% or more to determine the water resistance of the final application. PVB with good solubility in organic solvents such as ethanol, isopropyl alcohol, butanol, etc., and a suitable viscosity in the solution for the desired application technique is used.

The sheet resistance of the dry coating is between 2 W and 1.5 kQ, depending on the substrate, the thickness of the coating and the final treatment of the coat. The process of making the coating consists of the appropriate mixing of the basic components.

To make the coating formulation in powder form, all components, i.e. , conductive filler, binder and optionally various additives mentioned above, are mixed with a suitable powder mixer, preferably a high-shear mixer. Mixing is performed until the mixture is completely homogeneous. Finally, if necessary, the homogeneous mixture is ground again in a mill, preferably a roller mill or a ball mill, or in a similar powder grinding mill, to obtain a homogeneous mixture of powder particles. The coating in powder form can be added to other coatings as an additive or mixed with a solvent and applied in liquid form. A high-shear mixer or another mixer for mixing powders and liquids is the most suitable mixer for adding the coating in powder form to a solvent. The main advantages of the coating in powder form are easy storage, long shelf life and easy transport.

To make the coating formulation in liquid form, each of the powder components separately, i.e., the conductive filler, the binder and the optional various additives mentioned above, is first ground to the required size, if necessary. Individual ingredients are ground in a mill for grinding powder particles or solids, i.e., in a roller mill or a ball mill or any other mill that can grind the particles to the desired size. PVB is then mixed into the solvent at room temperature so that PVB is completely dissolved. This can be done using a mixer designed to mix dust particles into a liquid, especially a high-shear mixer or another suitable mixer. Carbon black is then added to the mixture and mixed until a homogeneous mixture is obtained. Graphite and other various fillers mentioned above are then mixed into the homogeneous mixture so that the mixture is completely homogeneous. A high-shear mixer or another mixer for mixing powders and liquids is the most suitable mixer for adding conductive fillers and other various additives.

The prepared coating is transferred into containers suitable for storage or transport. Caution is advised to prevent the sedimentation between production stages, because in this case the coating no longer has the desired properties, especially in terms of the conductivity of the final coating.

The coating formulated in liquid form is applied to the substrate using various techniques known to those skilled in the art. Suitable application techniques include, but are not limited to brush application, roller application, spray application (ultrasonic nozzles, air) and spin coating. After applying the coating to the substrate, drying follows until the formation of the final coating. Drying can be accelerated by heating the coating with hot air, in an oven, using IR radiation, electric current, electromagnetic waves (eddy currents) and other methods known to those skilled in the art.

The composition of the coating can be adapted to different applications, especially in terms of particle size and raw materials used. This is especially important when used in batch production, as it permits the use of existing devices and tools and eliminates the need for major changes to production lines.

A special feature of the presented coating is also the possibility of processing the final application, i.e. , the coating after application. Exposing the dry final coating to pressure improves the conductivity, a property not previously seen in the segment of conductive coatings. Compression can increase the conductivity of the entire surface or just a part of the surface. Larger surfaces can be compressed using rollers or a press, which can also be part of the process of applying the coating to the substrate, such as, but not limited to, roller application. It is possible to use the compression process to further define the part of the coated surface with higher conductivity. Namely, if a die is present on the compression roller or the press, the impression of the die becomes more conductive than the rest of the coated area. The conductivity can be increased from 10% to 200% compared to the uncompressed dry final coating; the increase in conductivity depends on the substrate, the composition of the coating and the pressure applied. By continuously changing the pressure during the processing of different parts of the coating, it is also possible to continuously change the resistance of the coating, which is interesting for use in sensor, heating and some other applications. Through compression and polishing, the coating also gets a shiny metallic appearance, which is an added advantage when using the coating as part of a finished room architecture or product design, as it eliminates the need for additional application of paint or covering the coating. The pressure required to improve the coating depends on the substrate. Each compression of the coating will increase its conductivity, and it is preferable to use compression with a pressure of 1.5 MPa and more. The pressure itself depends on the substrate and the application of the coating.

The coating or its components can be used as an additive in the production of other coatings, adhesives, plastics, foils, plasters, paints and similar products and semi finished products as an additive to improve electrical and thermal conductivity and transmit other properties inherent in the presented coating.

Examples of use EXAMPLE 1 :

5.5 g of PVB (KT-30H, manufactured by Kunshan Chemtech Co. Ltd.) is slowly dissolved in 130 ml of 99 % isopropyl alcohol. The mixture is stirred with a mixer at room temperature until PVB is completely dissolved. In all steps, a high shear mixer with a rotation speed of 1000 rpm or more is used. The mixing is performed at room temperature. Then 5g of carbon black (SuperP Li, manufactured by Imerys Graphite and Carbon) is slowly stirred into the mixture. Mix the mixture with a mixer until completely homogeneous (15 minutes) Finally, add another 37 g of graphite (particle size <50 micrometers (> 99.5 %), manufacturer Merck KGaA) and mix the mixture with a mixer until completely homogeneous (15 minutes). Immediately after mixing, the coating is applied with a roller to a glass substrate measuring 50 x 80 x 1 mm. The sample is dried for 15 minutes. The measured sheet resistance is 136 W and its thickness is 36 pm. The sample is then compressed in a hydraulic press at a pressure of 5 MPa. The sheet resistance after compression is 63.4 W. EXAMPLE 2:

Slowly dissolve 45 g of PVB (KT-30H, manufactured by Kunshan Chemtech Co. Ltd.) in 630 g of 96 % denatured ethanol. The mixture is stirred with a mixer until PVB is completely dissolved. In all steps, a high-shear mixer with a rotation speed of 1000 rpm or more is used. The mixing is performed at room temperature. 25 g of carbon black (SuperP Li, manufactured by Imerys Graphite and Carbon) is added and mixed with a mixer (15 minutes). Finally, 300 g of graphite (particle size <50 micrometers (> 99.5 %), manufactured by Merck KGaA) is added. The mixture is mixed using a high-shear mixer for at least 15 minutes. Immediately after mixing, a layer of coating is applied with a roller to a glass substrate measuring 50 x 80 x 1 mm. The coating is dried for 15 minutes.

The measured sheet resistance is 159 W and its thickness is 25 pm. The sample is then compressed in a hydraulic press at a pressure of 2,5 MPa. The sheet resistance after compression is 127 W.

In both cases, sheet resistance is measured with a four-point probe and calculated according to the formula R = 4.532 x (V/l), where V is the voltage measured between two internal contacts, and I is the current measured between two external contacts (Electrical Measurement, Signal Processing, and Displays. Ed. John G. Webster. CRC Press, 2003, Chapter 7-1. Heaney, Michael B. "Electrical Conductivity and Resistivity”. Thickness was measured using a contact profilometer.

EXAMPLE 3:

Using a high-shear mixer, 5 g of carbon black (SuperP Li, manufactured by Imerys Graphite and Carbon) was mixed with 37 g of graphite (particle size <50 micrometers (> 99.5 %), manufacturer Merck KGaA). When the mixture is completely homogeneous (mixing time 5 minutes), another 5.5 g of PVB (KT-30H, manufactured by Kunshan Chemtech Co. Ltd.) is added. Mix again until the mixture is completely homogeneous (mixing time 5 minutes). The mixture of carbon black, graphite and PVB is added to 130 ml of 99 % isopropyl alcohol and the resulting mixture is mixed with a high-shear mixer. The mixture is mixed for 15 minutes or until the mixture is completely homogeneous.

Claims

PATENT CLAIMS

1. Electrically conductive coating, wherein the coating comprises at least a conductive filler and a binder, and the binder content relative to the conductive filler in the conductive coating ranges between 10 wt. % and 30 wt. %, and wherein the conductive filler comprises graphite and carbon black particles, and the carbon black content relative to the graphite in the conductive filler ranges between 5 wt% and 15 wt%, and the binder is polyvinyl butyral.

2. Electrically conductive coating according to claim 1, wherein the graphite is natural or synthetic and graphite particles are of various shapes in a form of a powder and/or rods and/or flakes, and the size of the graphite particles measures from 0.5 to 200 micrometers, preferably from 5 to 50 micrometers.

3. Electrically conductive coating according to claims 1 and 2, wherein the carbon black is a highly conductive carbon black and carbon black particles are in a form of a powder and/or rods and/or flakes with the size of the carbon black particles measuring between 0.5 to 200 micrometers, preferably from 5 to 100 micrometers.

4. Electrically conductive coating according to the preceding claims, wherein the conductive filler additionally includes additives to increase conductivity in a form of conductive metal particles and/or other various forms of graphite and the mass ratio of said additives to graphite is between 1:10 and 1: 100 and the size of conductive particles is between 0.5 and 200 micrometers, preferably 5 to 50 micrometers.

5. Electrically conductive coating according to the preceding claims, wherein polyvinyl butyral has the average molar mass of at least 50,000 and the number of butyral groups is at least 75 mol% and wherein the polyvinyl butyral is in a form of a powder with particle sizes ranging between 0.5 and 200 micrometers, preferably between 5 and 100 micrometers.

6. Electrically conductive coating according to the preceding claims, wherein the coating additionally contains an organic solvent in which the polyvinyl butyral is soluble, wherein the solvent is selected from ethanol, isopropyl alcohol or butanol, or a mixture thereof and the content of polyvinyl butyral in relation to the solvent is between 2 wt. % and 20 wt. % and the viscosity of the final coating formulation in liquid form is between 19 mPas and 500 mPas.

7. Electrically conductive coating according to the preceding claims, wherein a sheet resistance of the dry coating is between 2 W and 1.5 kQ.

8. Electrically conductive coating according to the preceding claims, wherein a dry final coat of the electrically conductive coating is exposed to compression with a pressure of at least 1.5 MPa to increase conductivity from 10% to 200% compared to the uncompressed dry final coating.

9. Use of the electrically conductive coating for increasing the electrical conductivity of a substrate to which it is applied, wherein the electrically conductive coating in liquid form is applied to the substrate, followed by drying until the formation of the final coat.

10. Use according to claim 9, wherein an entire substrate surface or a section of the substrate surface with dry final coat of the electrically conductive coating is exposed to compression with a pressure of at least 1.5 MPa to increase conductivity of the entire surface or part thereof from 10% to 200%.