Conductive composition

Abstract

The present invention relates to an electrically conductive composition comprising a) a nitrocellulose resin; b) conductive particles comprising graphite and carbon black, wherein the ratio of the graphite to the carbon black is from 1: 1 to 5: 1; c) a solvent; and d) a bifunctional or polyfunctional isocyanate, wherein the ratio of the conductive particles to the resin is from 0.20: 1 to 4: 1. The composition according to the invention can be used as a structural adhesive or to create an electrically conductive surface on a non-electrically conductive substrate.

Description

TECHNICAL FIELD

[0001] The present invention relates to an electrically conductive composition which can be used to produce an electrically conductive surface on a non-conductive substrate. BACKGROUND

[0002] Ordinary electrically conductive inks and compositions have been applied to substrates to produce electrically conductive surfaces. Typically, these compositions are silver-based and require relatively high curing temperatures (> 120°C) and are therefore not suitable for more delicate materials.

[0003] Therefore, there is a need for an electrically conductive composition which can be used as a structural adhesive and / or to produce an electrically conductive surface on a non-conductive substrate without damaging the substrate. SUMMARY

[0004] The present invention relates to an electrically conductive composition comprising a) a nitrocellulose resin; b) electrically conductive particles comprising graphite and carbon black, wherein the ratio of the graphite and the carbon black is 1 : 1 to 5: 1 ; c) a solvent; and d) a di- or polyfunctional isocyanate, wherein the ratio of the electrically conductive particles and the resin is 0.20: 1 to 4: 1.

[0005] The present invention comprises an electrically conductive film comprising the electrically conductive composition according to the present invention.

[0006] The present invention comprises the use of the electrically conductive composition or the electrically conductive film according to the present invention as a structural adhesive or to produce an electrically conductive surface on a non-conductive substrate. DETAILED DESCRIPTION

[0007] In the following passages, the present invention is described in more detail. Each aspect so described can be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous can be combined with any other feature or features indicated as being preferred or advantageous.

[0008] In the context of the present invention, the terms used are to be construed in accordance with the following definitions, unless a context dictates otherwise.

[0009] As used herein, the singular forms "a", "an" and "the" include singular and plural referents unless the context clearly dictates otherwise.

[0010] As used herein, the term "comprising" is synonymous with "including," "containing," or "comprehending," and is inclusive or open-ended and does not exclude additional, unrecited elements, elements, or method steps.

[0011] As used herein, the term "consisting of" excludes any element, ingredient, component or method step not specified.

[0012] The words "preferred" and "preferably" as used herein refer to embodiments of the disclosure that can provide certain benefits under certain circumstances. However, the recitation of one or more preferred, preferable, desired, or specific embodiments does not imply that other embodiments are not useful, and does not serve to exclude those other embodiments from the scope of the disclosure.

[0013] As used throughout this application, the word "may" is used in a permissive sense — meaning that there is a possibility of — but not a requirement of.

[0014] The recitation of numerical ranges by endpoints includes all numbers and fractions subsumed within the respective range, as well as the recited endpoints.

[0015] All percentages, parts, ratios, etc. as used herein are based upon weight, unless otherwise specified.

[0016] When a quantity, concentration or other value or parameter is given as a range, preferred range or a range of preferred ranges, it is intended to include every narrower range that falls within the broader range, preferred range or range of preferred ranges. It is also intended to include any and all sub-ranges of the named range, preferred range or range of preferred ranges.

[0017] All references cited in this specification are hereby incorporated by reference in their entirety.

[0018] Unless defined otherwise, all terms used herein, including technical and scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. By further guidance, definitions for terms are included herein to better define the teachings of the present application.

[0019] The present invention relates to an electrically conductive composition comprising a) a nitrocellulose resin; b) electrically conductive particles comprising graphite and carbon black, wherein the ratio of the graphite and the carbon black is from 1 : 1 to 5: 1; c) a solvent; and d) a di- or polyfunctional isocyanate, wherein the ratio of the electrically conductive particles and the resin is from 0.20: 1 to 4: 1.

[0020] Applicants have found that the composition according to the present invention provides an electrically conductive coating with high electrical conductivity, low curing temperature, which enables the coating of finer substrates. The composition according to the present invention can also be used as a structural adhesive. In addition to the high cohesive properties resulting from the reactive curing components, the composition according to the present invention maintains its high adhesion to plastics, which is generally considered challenging for structural adhesives.

[0021] The electrically conductive composition according to the present invention comprises a resin. Suitable for use in the present invention is a nitrocellulose resin.

[0022] Nitrocellulose resin is the preferred resin as it easily releases solvent and disperses the conductive particles well. Furthermore, nitrocellulose resin provides low resistance and high conductivity.

[0023] Suitable commercially available nitrocellulose resins for use in the present invention include, but are not limited to, NC-E560 IPA 30% from Dow Chemical.

[0024] The nitrocellulose resin can be present in the conductive composition according to the present invention in an amount of 2 to 25 wt.%, preferably 3 to 23 wt.%, more preferably 4 to 21 wt.% of the total weight of the composition.

[0025] When the amount of nitrocellulose resin exceeds 25%, the viscosity of the composition can increase to high, while too low an amount, less than 2% can result in poor coating, low viscosity, and settling of the resin and conductive particles.

[0026] The conductive composition according to the present invention comprises conductive particles. The conductive particles include a mixture of graphite and carbon black. The ratio of graphite and carbon black is 1 : 1 to 5: 1.

[0027] Applicants have found that good results can be achieved by using a combination of graphite and carbon black as conductive particles. The combination of conductive particles provides good conductivity in thin layers.

[0028] Suitable graphite for use in the present invention preferably has a particle size D90 of 1 pm to 75 pm, more preferably 2 pm to 45 pm, more preferably 3 pm to 25 pm, even more preferably 3 pm to 10 pm. The particle size is measured by laser diffraction therein. In this method, a laser beam illuminates a cell containing a sample of graphite suspended in water, the resulting diffraction pattern is collected by the system and analyzed using light scattering produced by Mie. The particle size distribution is calculated and reported for 90% of the amount.

[0029] In addition to the particle size D90, or as an alternative feature for suitable graphite for use in the present invention is the specific surface area preferably of 0.25 m 2 / g to 25 m 2 / g, more preferably 4 m 2 / g to 22 m 2 / g, and more preferably 7 m 2 / g to 21 m 2 / g, wherein the specific surface area is measured by B.E.T nitrogen adsorption. The measurement is made by measuring the adsorption-desorption isotherms of nitrogen on the surface of the material using the Brunauer-Emmet-Teller algorithm, a determined amount (precision 0.01 mg) of powder is weighed in a sample tube. Subsequently, the sample undergoes a series of heating and cooling runs from a pressure in the sample tube, and the amount of nitrogen adsorbed during the different steps is calculated; from this data, the specific surface area is determined in m2 / g and reporting the specific surface area.

[0030] In a preferred embodiment, the graphite has a particle size D90 of 1 pm to 75 pm, preferably 2 pm to 45 pm, more preferably 3 pm to 25 pm, even more preferably 3 pm to 10 pm, and / or a specific surface area of 0.25 m 2 / g to 25 m 2 / g, preferably 4 m 2 / g to 22 m 2 / g, more preferably 7 m 2 / g to 21 m 2 / g, wherein the particle size is measured by laser diffraction and wherein the specific surface area is measured by B.E.T nitrogen adsorption.

[0031] The preferred specific surface area and particle size of the graphite, either as individual features or as a combination, provide low resistance and good conductivity in the thin layer.

[0032] Generally, too large particle size of the graphite can result in lower specific surface area and difficulty in releasing the particles from the pool during printing. However, too small particle size of the graphite can be too expensive to produce and has a larger specific surface area which can result in higher viscosity.

[0033] A specific surface area of the graphite below 0.25 m 2 / g can result in reduced conductivity, while a specific surface area of the graphite above 25 m 2 / g can result in viscosity problems of the composition and additionally would not be a cost effective component.

[0034] Commercially available graphites suitable for the present application include, but are not limited to, Timrex SFG6 from TIMCAL Graphite & Carbon and Graphite pure 200-09 from Asbury.

[0035] The carbon black suitable for the present application has an oil absorption number of preferably 70 ml / 100g to 500 ml / 100g, more preferably 100 ml / 100g to 300 ml / 100g, more preferably 150 ml / 100g to 200 ml / 100g, wherein the oil absorption number is measured according to ASTM D2414.

[0036] In addition to the oil absorption number, or as an alternative feature, suitable carbon blacks for the present application have a specific surface area of preferably 30 m 2 / g to 1400 m 2 / g, more preferably 100 m 2 / g to 700 m 2 / g, more preferably 150 m 2 / g to 350 m 2The specific surface area is measured according to BET. This method is based on the registration of nitrogen absorption at 77 K. Following the model proposed by Brunauer, Emmet and Teller (BET), the monolayer capacity can be determined. Based on the cross-sectional area of a nitrogen molecule, the monolayer capacity and the weight of the sample, the specific surface area can then be calculated.

[0037] In a preferred embodiment, the carbon black has an oil absorption number of 70 ml / 100 g to 500 ml / 100 g, preferably 100 ml / 100 g to 300 ml / 100 g, more preferably 150 ml / 100 g to 200 ml / 100 g, and / or a specific surface area of 30 m 2 / g to 1400 m 2 / g, preferably 100 m 2 / g to 700 m 2 / g, more preferably 150 m 2 / g to 350 m 2 / g, wherein the oil absorption number is measured according to ASTM D2414 and wherein the specific surface area is measured according to BET.

[0038] The preferred specific surface area and oil absorption number alone or in combination provide a low electrical resistance in the thin layer.

[0039] When the oil absorption number of the carbon black is too low, this means less branched carbon black and thus less contact points in the thin layer are possible and this will negatively affect the electrical conductivity, while an oil absorption number of the carbon black that is too high, mainly greater than 500, can lead to processing difficulties.

[0040] When the specific surface area of the carbon black is too small, mainly less than 30, this can lead to poor electrical conductivity, while a too high specific surface area of the carbon black, mainly greater than 1400, can lead to processing difficulties.

[0041] Suitable commercially available carbon blacks for use in the present application include, but are not limited to, Vulcan XC72 from Cabot and Ensaco 250 G from Imerys.

[0042] According to the present application, the ratio of graphite and carbon black is 1 : 1 to 5: 1, preferably 2: 1 to 4: 1, more preferably the ratio of graphite and carbon black is 3: 1.

[0043] The applicants have found that these ratios of graphite to carbon black provide the composition according to the present application with ideal viscosity and conductive properties. It has been found that a too high concentration of carbon black relative to graphite can lead to thicker printed layers. Furthermore, if the amount of carbon black is too high, the viscosity of the composition can increase too high.

[0044] The electrically conductive particles can be present in the electrically conductive composition according to the present application in an amount of 3 to 45 weight percent, preferably 4 to 43 weight percent, more preferably 4.75 to 41 weight percent, of the total weight of the composition.

[0045] When the amount of electrically conductive particles is too low, primarily less than 3%, the amount can not provide the desired electrical conductivity, and too high an amount, primarily greater than 45%, can result in difficulty in processing the composition.

[0046] Applicants have found that particularly good properties are obtained when the ratio of electrically conductive particles to resin is 0.20:1 to 4:1, preferably 0.25:1 to 3:1.

[0047] Applicants have found that this ratio of electrically conductive particles to resin provides good viscosity and good electrical conductivity properties.

[0048] The electrically conductive composition according to the present application comprises a solvent. The composition can comprise one solvent or a mixture or two or more solvents.

[0049] In preferred embodiments, the solvent is selected from the group consisting of toluene, ethanol, isopropanol, n-propanol, ethyl acetate, n-propyl acetate, n-butyl acetate, isobutyl acetate, isopropyl acetate, methyl ethyl ketone, methyl isobutyl ketone, 2-butoxyethyl acetate, water, 2-(2-butoxyethoxy)ethanol, [2-(2-butoxyethoxy)-ethyl] acetate, 2-butoxyethanol, isophorone, 3,3,5-trimethyl-2-cyclohexen-1-one, dimethyl succinate, dimethyl glutarate, dimethyl adipate, acetic acid, 1-methoxypropan-2-ol, 1-methoxy-2-propyl acetate, dipropylene glycol (mono)methyl ether, glycol ethers, ethoxypropanol, water, and mixtures thereof, and mixtures thereof, preferably the solvent is 2-butoxyethyl acetate.

[0050] These solvents are preferred because they dissolve the resin well and result in a uniform film upon evaporation.

[0051] Suitable commercially available solvents for use in the present application include, but are not limited to, 2-butoxyethyl acetate from Eastman.

[0052] The solvent can be present in the electrically conductive composition of the present application in an amount of 40 to 92 weight percent, preferably 45 to 91 weight percent, more preferably 46 to 90 weight percent, of the total weight of the composition.

[0053] If the amount of solvent is less than 40%, the viscosity of the composition can increase too high, and a higher amount, greater than 90%, can adversely affect the electrical conductivity of the composition.

[0054] The composition according to the present application comprises a di- or polyfunctional isocyanate. The isocyanate can be an aliphatic isocyanate or an aromatic isocyanate.

[0055] Preferably, the di- or polyfunctional isocyanate is selected from the group consisting of methylene diphenyl diisocyanate, hexamethylene diisocyanate, toluene diisocyanate, 1,5-naphthalene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, p-phenylene diisocyanate, 1,3,6-hexamethylene triisocyanate and bicycloheptane triisocyanate and mixtures thereof, preferably the di- or polyfunctional isocyanate is methylene diphenyl diisocyanate.

[0056] The use of di- or polyfunctional isocyanate results in crosslinking of the resin and an increase in cohesive strength.

[0057] Suitable commercially available di- or polyfunctional isocyanates for use in the present application include, but are not limited to, Desmodur VKS 20 from Covestro.

[0058] The di- or polyfunctional isocyanate can be present in the conductive composition according to the present application in an amount of 0.5 to 9 wt.%, preferably 0.5 to 7 wt.%, more preferably 0.75 to 6 wt.% of the total weight of the composition.

[0059] These amounts are preferred as they provide an ideal level of crosslinking. When the amount of di- or polyfunctional isocyanate is greater than 9%, it can have an adverse effect on moisture resistance and shelf life.

[0060] The conductive composition according to the present application can be prepared by mixing all components together. The conductive composition according to the present application is preferably prepared by a bead mill or three-roll mill process. In the bead mill process, the conductive particles, resin, solvent and di- or polyfunctional isocyanate are ground by sand in a large mill for about 15 hours. In the three-roll mill process, the conductive particles, resin, solvent and di- or polyfunctional isocyanate are crushed between three rolls that are counter-rotated to each other to a defined particle size.

[0061] In one aspect of the present application, a conductive film comprising the conductive composition according to the present application can be formed.

[0062] The conductive film according to the present application has one or more layers, wherein each layer has a thickness of 5 pm to 100 pm, preferably 7 pm to 80 pm, and more preferably 8 to 60 pm.

[0063] Thin layers are desirable due to cost effective aspects, but also from a process point of view. However, thicker layers increase the cohesive force that the adhesive can exhibit.

[0064] The conductive composition according to the present application is preferably applied to form a film by rod coating, stencil printing, screen printing, rotogravure printing, rotoscreen printing, flexographic printing and using spray application.

[0065] The electrically conductive composition according to the present application preferably has a viscosity of 0.3 to 30 Pa s, wherein the viscosity is measured according to ISO 3219 using a rheometer at a constant shear rate of 15 s with a 20 mm plate-plate configuration (0.2 mm gap, 60 s at 25 °C).

[0066] A particularly suitable viscosity for screen printing and rotogravure printing is preferably in the range of 2 to 30 Pa s.

[0067] A particularly suitable viscosity for rotogravure printing or flexographic printing, a suitable viscosity is in the range of 0.5 to 4 Pa s.

[0068] The applicant has found that this viscosity range is ideal for the production process for producing the electrically conductive composition according to the present application, in addition, this viscosity range prevents the composition from settling during storage.

[0069] The sheet resistance of the electrically conductive composition according to the present application is preferably 5-100 Ohm / sq / mil, preferably 7-50 Ohm / sq / mil, more preferably 10-30 Ohm / sq / mil, wherein the sheet resistance is measured according to ASTM D257.

[0070] These sheet resistance values are preferred as they can ensure values provided with an aluminum surface.

[0071] The electrically conductive composition according to the present application preferably has a lap shear strength of 4 to 15 MPa, preferably 6 to 12 MPa, wherein the lap shear strength is measured according to ISO 4587-03.

[0072] The electrically conductive composition according to the present application can be cured at a temperature of 23-120 °C, preferably 40-80 °C.

[0073] The low curing temperature enables the use of more delicate non-conductive substrates.

[0074] The present application comprises the use of the electrically conductive composition or the electrically conductive film according to the present application for producing an electrically conductive surface on a non-conductive substrate.

[0075] Suitable non-conductive substrates for the present application are for example Kalix, polybutylene terephthalate (PBT), electrocoated steel, polyethylene terephthalate (PET) and PET-wrapped metal substrates.

[0076] The electrically conductive composition according to the present application can also be used as a structural adhesive.

[0077] Examples The composition details are listed in Table 1 below. The composition was prepared by mixing the components together by mixing at 2000 rpm for 60 seconds.

[0078] Table 1

[0079] The lap shear strength of the example compositions was measured according to ISO 4587-03.

[0080] The compositions from the examples of Table 1 were applied by bar coating to a non-conductive surface. Alternatively, a pre-cured adhesive with a wet thickness of 25-200 μιη was applied to the non-conductive surface. A second surface was applied on top of the conductive coating. The conductive coating was subsequently cured at 60-120 °C for 15-120 minutes. The test results are listed in Table 2 below.

[0081] Table 2

[0082] The compositions according to the application illustrate good adhesive strength compared to comparative compositions.

[0083] The sheet resistance of the example compositions was measured according to ASTM D257.

[0084] The compositions from Table 1 were applied by screen printing (wet thickness 13-17 μιη) in thin rectangles of 2 x 100 mm onto a PET film. After curing the compositions at 120 °C for 15 minutes, the resistance of the rectangles was measured. The test results are listed in Table 3 below.

[0085] Table 3

[0086] The compositions according to the application illustrate that the resistance is only very slightly affected compared to the large effect on the cohesion gain, and that the resistance remains very high.

Claims

1. A conductive composition comprising: a) Nitrocellulose resin; b) Conductive particles comprising graphite and carbon black, wherein the ratio of graphite to carbon black is from 1:1 to 5:

1. c) Solvent; and d) Bifunctional or polyfunctional isocyanates, The ratio of the conductive particles to the resin is 0.20:1 to 4:

1.

2. The conductive composition according to claim 1, wherein the resin is present at 2 to 25% of the total weight of the composition, preferably 3 to 23%, more preferably 4 to 21%.

3. The conductive composition according to claim 1 or 2, wherein the graphite has a particle size D90 of 1 μm to 75 μm, preferably 2 μm to 45 μm, more preferably 3 μm to 25 μm, and even more preferably 3 μm to 10 μm. and / or Specific surface area is 0.25 m² 2 / g to 25m 2 / g, preferably 4m 2 / g to 22m 2 / g, more preferably 7m 2 / g to 21m 2 / g, The particle size is measured by laser diffraction, and the specific surface area is measured by BET nitrogen absorption.

4. The conductive composition according to any one of claims 1 to 3, wherein the carbon black has an oil absorption value of 70 ml / 100g to 500 ml / 100g, preferably 100 ml / 100g to 300 ml / 100g, more preferably 150 ml / 100g to 200 ml / 100g. and / or Specific surface area ranges from 30 to 1400 m² 2 / g, preferably 100 to 700m 2 / g, more preferably 150 to 350m 2 / g, The oil absorption value is measured according to ASTM D2414, and the specific surface area is measured according to BET.

5. The conductive composition according to any one of claims 1 to 4, wherein the ratio of graphite to carbon black is 2:1 to 4:1, preferably 3:

1.

6. The conductive composition according to any one of claims 1 to 5, wherein the conductive particles are present at 3 to 45% by weight, preferably 4 to 43% by weight, and more preferably 4.75 to 41% by weight of the total weight of the composition.

7. The conductive composition according to any one of claims 1 to 6, wherein the ratio of the conductive particles to the resin is 0.25:1 to 3:

1.

8. The conductive composition according to any one of claims 1 to 7, wherein the solvent is selected from toluene, ethanol, isopropanol, n-propanol, ethyl acetate, n-propyl acetate, n-butyl acetate, isobutyl acetate, isopropyl acetate, methyl ethyl ketone, methyl isobutyl ketone, 2-butoxyethyl acetate, water, 2-(2-butoxyethoxy)ethanol, [2-(2-butoxyethoxy)-ethyl]acetate, 2-butoxyethanol, isophorone, 3,3,5-trimethyl-2-cyclohexen-1-one, dimethyl succinate, dimethyl glutarate, dimethyl adipate, acetic acid, 1-methoxyprop-2-ol, 1-methoxy-2-propyl acetate, dipropylene glycol (mono)methyl ether, glycol ether, ethoxypropanol, water, and mixtures thereof, preferably 2-butoxyethyl acetate.

9. The conductive composition according to any one of claims 1 to 8, wherein the solvent is present at 40 to 92% by weight, preferably 45 to 91% by weight, and more preferably 46 to 90% by weight of the total weight of the composition.

10. The conductive composition according to any one of claims 1 to 9, wherein the difunctional or polyfunctional isocyanate is an aliphatic or aromatic isocyanate, preferably selected from methylene diphenyl diisocyanate, hexamethylene diisocyanate, toluene diisocyanate, 1,5-naphthalene diisocyanate, isophorone diisocyanate, xylene diisocyanate, terephthalic diisocyanate, 1,3,6-hexamethylene triisocyanate and bicycloheptane triisocyanate and mixtures thereof, more preferably the difunctional or polyfunctional isocyanate is methylene diphenyl diisocyanate.

11. The conductive composition according to any one of claims 1 to 10, wherein the bifunctional or polyfunctional isocyanate is present in 0.5 to 9% by weight, preferably 0.5 to 7% by weight, more preferably 0.75 to 6% by weight of the total weight of the composition.

12. A conductive film comprising the conductive composition according to any one of claims 1 to 11.

13. The conductive film according to claim 12, wherein the film has one or more layers, and wherein the layers have a thickness of 5 μm to 100 μm, preferably 7 μm to 80 μm, and more preferably 8 to 60 μm.

14. Use of the conductive composition according to any one of claims 1 to 11 or the conductive film according to claim 12 or 13 as a structural adhesive or for creating a conductive surface on a non-conductive substrate.

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