Stretchable polymer thick film carbon black composition for portable heaters and articles

Abstract

Polymer thick-film carbon black composition, comprising: (a) 6 to 13 wt.% of a conductive carbon black powder; and (b) 87 to 94 wt.% organic medium comprising 10 to 30 wt.% thermoplastic polyurethane resin dissolved in an organic solvent, wherein the thermoplastic polyurethane has a percentage elongation of at least 200%, the wt.% of the thermoplastic polyurethane resin being based on the total wt.% of the organic medium and the wt.% of the conductive carbon black powder and the organic medium being based on the total wt.% of the composition; wherein the ratio of the wt.% of polyurethane resin to the wt.% of conductive carbon black powder is in the range of 1.50 to 1.75.

Description

FIELD OF THE INVENTION

[0001] This invention is directed to a polymer thick-film carbon black composition. More specifically, the polymer thick-film carbon black composition can be used to form heaters in applications where significant stretching is required, particularly on substrates that can be highly elongated, which can be used particularly in wearable clothing applications. Another approach utilizes direct printing onto the clothing fabric, either nonwoven or knitted, to produce a stretchable heater. BACKGROUND OF THE INVENTION

[0002] Polymer thick film (PTF) circuits have long been used as electrical elements. Although they have been used as electrical elements, the use of PTF silver or carbon conductors in highly stretchable applications such as wearable clothing has not been common. The ability to be stretched and subjected to multiple washing and drying cycles while still maintaining conductivity is essential. One of the purposes of this invention is to address the above requirements and produce a stretchable PTF ink that can be used in the construction of a functional battery that can be used on a substrate that can be used as wearable clothing or that can be applied to a fabric that can be used as wearable clothing.

[0003] Carbon polymer thick-film (PTF) pastes can be printed to produce dried films that can be significantly more resistant than those made from silver pastes. This makes them ideal candidates for resistive heating elements in printed electric resistance heaters. Silver printed circuits can provide the low-resistance busbar with little or no parasitic heating, which can deliver power to the active carbon elements. In many cases, a somewhat positive temperature coefficient (PTC) of resistance inherent in the carbon formulation is desirable to limit the maximum operating temperature. However, in a heater attempting to extract maximum power from small power sources, a strong PTC effect can cause problems.If the design is designed to deliver power at operating temperatures, the cold resistance may be too low, and the current demand will trigger the battery current, limiting the circuitry and resulting in shutdown. If the design is designed to reliably turn on when cold, the heater may not deliver sufficient power when warm. A low-temperature PTC carbon that is stable over the desired operating temperature range is required.

[0004] US 2018 / 0346757 A1 discloses a stretchable polymer thick film composition suitable for wearable garments. More specifically, the polymer thick film can be used in applications requiring significant elongation, particularly on substrates that can be stretched to a high degree. SUMMARY OF THE INVENTION

[0005] The invention relates to a polymer thick film carbon black composition as claimed in claim 1. The invention further relates to an article as claimed in claim 4 containing a stretchable heater with a resistance element.

[0006] The polymer thick-film carbon black composition comprises: (a) 6 to 13 wt. % of a conductive carbon black powder; and (b) 87 to 94 wt. % organic medium comprising 10 to 30 wt. % thermoplastic polyurethane resin dissolved in an organic solvent, wherein the thermoplastic polyurethane has a percent elongation of at least 200%, wherein the weight percent of the thermoplastic polyurethane resin is based on the total weight of the organic medium, and the weight percent of the conductive carbon black powder and the organic medium is based on the total weight of the composition. The ratio of the weight of polyurethane resin to the weight of conductive carbon black powder is in the range of 1.50 to 1.75.

[0007] The invention is further directed to the use of the composition for forming the resistive portion of heaters for articles requiring stretchable heaters, e.g., wearable clothing. Therefore, the invention provides an article containing a stretchable heater formed from a polymer thick film carbon black composition comprising: (a) 6 to 13 wt. % of a conductive carbon black powder; and (b) 87 to 94 wt. % organic medium comprising 10 to 30 wt. % thermoplastic polyurethane resin dissolved in an organic solvent, wherein the thermoplastic polyurethane has a percent elongation of at least 200%, wherein the weight percent of the thermoplastic polyurethane resin is based on the total weight of the organic medium, and the weight percent of the conductive carbon black powder and the organic medium is based on the total weight of the composition.The ratio of the weight of polyurethane resin to the weight of conductive carbon black powder is in the range of 1.50 to 1.75.

[0008] In one embodiment, the article is wearable clothing. DETAILED DESCRIPTION OF THE INVENTION

[0009] The invention relates to a polymer thick-film carbon black composition for use in forming a heater, particularly for use in highly stretchable circuits such as those applications where a heater is formed on fabric for clothing. This is often referred to as wearable electronics. Additionally, the composition is useful for forming heaters in applications such as heated seats. A layer of the conductor is printed and dried on a substrate to produce a heater, after which the entire circuit is subjected to the typical bending / crimping of a fabric. In addition, as is typical for fabrics, they must be washed and dried regularly, and the conductivity and integrity of the conductor must be maintained.

[0010] Here, weight percent is written as wt%. ORGANIC MEDIUM

[0011] The organic medium consists of a thermoplastic polyurethane resin dissolved in an organic solvent. The polyurethane resin must achieve good adhesion to the underlying substrate. The polyurethane resin must be compatible with the performance of the heater and must not compromise its performance after deformation and washing and drying cycles.

[0012] The thermoplastic polyurethane resin comprises 10 to 30 wt.% of the total weight of the organic medium. In one embodiment, the thermoplastic polyurethane resin is a polyurethane homopolymer. In another embodiment, the polyurethane resin is a polyester-based copolymer. In one embodiment, the thermoplastic polyurethane resin is a predominantly linear hydroxyl polyurethane.

[0013] The thermoplastic polyurethane resin has a % extension of at least 200%.

[0014] The percentage extension is defined in the usual way: Percentage extension = Final length − Initial length × 100 Initial length

[0015] The polymer resin is typically added to the organic solvent by mechanical mixing to form the medium. Solvents suitable for use in the polymer thick film composition will be recognized by one of skill in the art and include acetates and terpenes such as carbitol acetate and alpha- or beta-terpineol, or mixtures thereof with other solvents such as kerosene, dibutyl phthalate, butyl carbitol, butyl carbitol acetate, hexylene glycol, and high-boiling alcohols and alcohol esters. Additionally, volatile liquids may be included to promote rapid curing after application to the substrate. In many embodiments of the present invention, solvents such as glycol ethers, ketones, esters, and other solvents with similar boiling points (in the range of 180°C to 250°C), and mixtures thereof, may be used.Various combinations of these and other solvents are formulated to achieve the desired viscosity and volatility requirements. The solvents used must dissolve the resin. The solvent can be added to the composition to adjust the viscosity and is considered part of the organic medium.

[0016] The amount of organic medium is in the range of 87 to 94 wt% based on the total weight of the composition. CONDUCTIVE CARBON BLACK COMPOSITION

[0017] Many carbon composite films incorporate graphite. It is easy to achieve high conductivity with moderate graphite loadings. The contact between the graphite sheets is easily disrupted by thermal expansion of the polymer matrix, and the resistance increases rapidly with temperature. Therefore, highly structured carbon black (CB) powder is used in the instant composition, such as Vulcan XC-72 and Monarch 700 (both available from Cabot Corp., Boston, Mass., USA). When properly dispersed, carbon black can provide a conductive network that is more difficult to disrupt as the matrix expands. Too low a level of CB leads to too high a resistance and too high a positive temperature coefficient (PTC) of resistance. The electrical resistance generally improves as the carbon black loading increases, until the volume fraction of carbon particles is well above the percolation threshold for the particular CB.PTC also decreases as the CB concentration increases. Improvements become smaller as the percolation threshold is passed. However, too high a CB load in the dry film can lead to poor mechanical properties and cracking during the drying process. High shear mixing can be used to provide better carbon dispersion, lower the percolation threshold, and provide better electrical performance at lower CB loading. This results in more crack-resistant and low-PTC formulations. The instant conductive carbon black composition forms crack-free films with acceptably low resistance and low PTC when processed using standard PTF mixing and roll mills and processes. The amount of conductive carbon black powder ranges from 6 to 13 wt.% based on the total weight of the composition.The ratio of the weight of the polyurethane resin to the weight of the conductive carbon black powder is in the range of 1.50 to 1.75.

[0018] Some additional processing when premixing surfactants with the solvent and carbon black can further improve dispersion, but the surfactants are not burned out as would happen with a paste processed at high temperature, which would raise toxicological concerns as it is a volatile compound in a film that could come into contact with the skin. ADDITIONAL POWDER

[0019] Various powders or additives can be added to the PTF composition to improve adhesion, modify rheology, and increase low shear viscosity to enhance printability, as long as they do not have a damaging effect on the skin. APPLICATION OF PTF COMPOSITIONS

[0020] The PTF carbon black composition, also referred to as a "paste," is deposited onto a substrate that can be used in a wearable garment or that can be applied to a fabric that can be used as a wearable garment. One substrate is a thermoplastic polyurethane substrate, such as Bemis ST-604, available from Bemis Associates, Inc., Shirley, MA. Another possible substrate is a thermoplastic polyester, such as Hytrel®, available from DuPont Co., Wilmington, DE. The substrate can also be a film of a composite material comprising a combination of plastic film with a permeable coating deposited thereon.

[0021] The deposition of the PTF carbon black composition onto the substrate is typically performed by screen printing; other deposition techniques such as stamping, spraying, or coating techniques can be used. In the case of screen printing, the mesh size controls the thickness of the deposited thick film.

[0022] Generally, a thick-film composition comprises a functional phase that imparts appropriate functional properties to the composition. The functional phase may comprise electrically functional powders dispersed in an organic medium that serves as a carrier for the functional phase. Typically, the composition is fired to burn out both the polymer and the solvent of the organic medium and impart the electrically functional properties. In the case of a polymer thick-film composition, the polymer portion of the organic medium remains as an integral part of the composition after drying.

[0023] The PTF carbon black composition is processed for the time and at the temperature necessary to remove all solvent. For example, the deposited film is dried by exposure to heat at 130 °C for typically 10 to 15 minutes. HEATER CONSTRUCTION

[0024] The PTF carbon black composition is printed onto a substrate and dried under the above conditions. One or more layers of the resistive PTF carbon black composition can be printed and dried onto the substrate to form the battery's resistive element. The bus and conductors to the busbars can be printed before or after the resistive PTF carbon black composition.

[0025] In one embodiment, the substrate can be applied to a fabric that can be used to form a wearable garment. Either side of the substrate can be applied to the fabric, i.e., the side of the substrate with the carbon black film can be adjacent to the fabric, or the other side of the substrate can be adjacent to the fabric. A thermoplastic polyurethane substrate, such as DuPont™ Intexar™ TE11C or Bemis ST-604, adheres to polyester, nylon, and polyurethane- or polyvinyl chloride-coated fabrics.

[0026] In another embodiment, the carbon black composition can be applied directly to a stretchable permeable fabric. One such nonwoven fabric is one constructed of Evolon®, available from Fruedenberg Evolon, Colmar, France. Another permeable substrate that can be used for this type of application is a woven polyamide-coated polyester, e.g., Cetus® OS5000U, available from Dynic Corp., Kyoto, Japan. EXAMPLES AND COMPARISON EXPERIMENTS EXAMPLE 1

[0027] The PTF carbon black composition was prepared as follows. 69 parts by weight of an organic starting medium was used and prepared by mixing 28.50 wt% Desmocoll® 406 polyurethane (Covestro LLC, Pittsburgh, PA) with 71.50 wt% Dowanol™ DPM dipropylene glycol methyl ether (Dow Co., Midland, MI) organic solvent. This mixture was stirred at 90°C for 1 to 2 hours to dissolve all of the resin. 12.31 parts by weight of Vulcan XC-72 conductive carbon black powder (Cabot Corp, Boston, Mass.) was added. Finally, 18.69 parts by weight of Dowanol™ DPM dipropylene glycol methyl ether was added for thinning purposes to bring the composition to a desired viscosity of 70 Pas. The amount of organic medium, including solvent, added to adjust viscosity is 87.70 wt%, consisting of 68.03 wt% solvent and 19.67 wt% polyurethane resin, where wt.-% based on the total weight of the composition. The polyurethane resin constituted 22.4 wt.% of the total weight of the organic medium. The ratio of the weight of polyurethane resin to the weight of carbon black was 1.60.

[0028] This composition was mixed for 30 minutes in a planetary mixer and then subjected to several passes in a three-roll mill to provide good dispersion of the carbon black powder.

[0029] The composition was screen-printed onto a polyester (PET) substrate and dried at 130 °C for 10 minutes in a forced-air oven. No cracking occurred during the drying process. The volume percent of carbon black in the dried film was 36.6.

[0030] The resistance of the dried film was 162 ohms / sq. An indication of the PTC was obtained by measuring the film's resistance at 25°C, 40°C, and 65°C. The PTC resistance factor at 40°C (the ratio of the resistance at 40°C to that at 25°C) was 1.06, and the PTC resistance factor at 65°C (the ratio of the resistance at 65°C to that at 25°C) was 1.18, indicating a relatively low PTC. EXAMPLE 2

[0031] The PTF carbon black composition was prepared as follows. 84.2 parts by weight of an organic starting medium was used and prepared by mixing 20.50 wt.% Desmocoll® 530 / 1 polyurethane (Covestro LLC, Pittsburgh, PA) with 79.50% organic solvent of diethylene glycol monoethyl acetate (Eastman Chemical Co., Kingsport, Tenn.). This mixture was stirred at 90°C for 1 to 2 hours to dissolve all of the resin. 10.80 parts by weight of Vulcan XC-72 conductive carbon black powder (Cabot Corp, Boston, Mass.) was added. Finally, 5.00 parts by weight of diethylene glycol monoethyl acetate was added for thinning purposes to bring the composition to a desired viscosity of 70 Pas. The amount of organic medium, including solvent, added to adjust viscosity is 89.2 wt%, consisting of 71.94 wt% solvent and 17.26 wt% polyurethane resin, where wt.-% based on the total weight of the composition. The polyurethane resin constituted 19.4 wt.% of the total weight of the organic medium. The ratio of the weight of polyurethane resin to the weight of carbon black was 1.60.

[0032] This composition was mixed for 30 minutes in a planetary mixer and then subjected to several passes in a three-roll mill to provide good dispersion of the carbon black powder.

[0033] The composition was screen-printed onto a polyester (PET) substrate and dried at 130 °C for 10 minutes in a forced-air oven. No cracking occurred during the drying process. The volume percent of carbon black in the dried film was 36.6.

[0034] The resistance of the dried film was 187 ohms / sq. A reading of the PTC was obtained by measuring the film's resistance at 25°C, 40°C, and 65°C. The PTC resistance factor at 40°C (the ratio of the resistance at 40°C to that at 25°C) was 1.14, and the PTC resistance factor at 65°C (the ratio of the resistance at 65°C to that at 25°C) was 1.28, indicating a relatively low PTC. EXAMPLE 3

[0035] The PTF carbon black composition was prepared as follows. 58.50 parts by weight of an organic starting medium was used and prepared by mixing 20.50 wt.% Desmocoll® 530 / 1 polyurethane (Covestro LLC, Pittsburgh, PA) with 79.50 parts by weight of diethylene glycol monoethyl acetate (Eastman Chemical Co., Kingsport, Tenn.). This mixture was stirred at 90°C for 1 to 2 hours to dissolve all of the resin. 7.00 parts by weight of Vulcan XC-72 conductive carbon black powder (Cabot Corp, Boston, Mass.) was added. Finally, 34.50 parts by weight of diethylene glycol monoethyl acetate were added for thinning purposes to bring the composition to a desired viscosity of 70 Pas. The amount of organic medium, including solvent, added to adjust viscosity is 93.00 wt%, consisting of 81.01 wt% solvent and 11.99 wt% polyurethane resin, where wt% is based on the total weight of the composition.The polyurethane resin constituted 12.9 wt% of the total weight of the organic medium. The ratio of the weight of polyurethane resin to the weight of carbon black was 1.71.

[0036] This composition was mixed for 30 minutes in a planetary mixer and then subjected to several passes in a three-roll mill to provide good dispersion of the carbon black powder.

[0037] The composition was screen-printed onto a polyester (PET) substrate and dried at 130 °C for 10 minutes in a forced-air oven. No cracking occurred during the drying process. The volume percent of carbon black in the dried film was 35.

[0038] The resistance of the dried film was 179 ohms / sq. A reading of the PTC was obtained by measuring the film's resistance at 25°C, 40°C, and 65°C. The PTC resistance factor at 40°C (the ratio of the resistance at 40°C to that at 25°C) was 1.11, and the PTC resistance factor at 65°C (the ratio of the resistance at 65°C to that at 25°C) was 1.24, indicating a relatively low PTC. EXAMPLE 4

[0039] The PTF carbon black composition was prepared as follows. 66.07 parts by weight of an organic starting medium was used and prepared by mixing 27.50 wt% Desmocoll® 406 polyurethane (Covestro LLC, Pittsburgh, PA) with 72.50 wt% Dowanol™ DPM dipropylene glycol methyl ether (Dow Co., Midland, MI) organic solvent. This mixture was stirred at 90°C for 1 to 2 hours to dissolve all of the resin. 12.00 wt. parts of Vulcan XC-72 conductive carbon black powder (Cabot Corp, Boston, Mass.) were added. 0.06 wt. parts of surfactant were added. Finally, 18.17 wt. parts of Dowanol™ DPM diethylene glycol monoethyl acetate were added for thinning purposes to bring the composition to a desired viscosity of 70 Pas. The amount of organic medium, including solvent, added to adjust viscosity is 87.40 wt%, consisting of 69.23 wt% solvent and 18.17 wt%.-% polyurethane resin, where wt% is based on the total weight of the composition. The polyurethane resin constituted 20.7 wt% of the total weight of the organic medium. The ratio of the weight of polyurethane resin to the weight of carbon black was 1.51.

[0040] This composition was mixed for 30 minutes in a planetary mixer and then subjected to several passes in a three-roll mill to provide good dispersion of the carbon black powder.

[0041] The composition was screen-printed onto a polyester (PET) substrate and dried at 130 °C for 10 minutes in a forced-air oven. No cracking occurred during the drying process. The volume percent of carbon black in the dried film was 37.

[0042] The resistance of the dried film was 122 ohms / sq. A reading of the PTC was obtained by measuring the film's resistance at 25°C and 40°C. The PTC resistance factor at 40°C (the ratio of the resistance at 40°C to that at 25°C) was 1.2, indicating a relatively low PTC. COMPARISON EXPERIMENT A

[0043] A PTF carbon black composition was prepared as follows. 61.07 parts by weight of an organic starting medium was used and prepared by mixing 27.50 wt% Desmocoll® 406 polyurethane (Covestro LLC, Pittsburgh, PA) with 72.50 wt% Dowanol™ DPM dipropylene glycol methyl ether (Dow Co., Midland, MI) organic solvent. This mixture was stirred at 90°C for 1 to 2 hours to dissolve all of the resin. 13.00 wt. parts of Vulcan XC-72 conductive carbon black powder (Cabot Corp, Boston, Mass.) were added. 0.06 wt. parts of surfactant were added. Finally, 25.33 wt. parts of Dowanol™ DPM diethylene glycol monoethyl acetate were added for thinning purposes to bring the composition to a desired viscosity of 70 Pas. The amount of organic medium, including solvent, added to adjust viscosity is 86.40 wt%, consisting of 69.61 wt% solvent and 16.79 wt%.-% polyurethane resin, where wt% is based on the total weight of the composition. The polyurethane resin constituted 19.4 wt% of the total weight of the organic medium. The ratio of the weight of polyurethane resin to the weight of carbon black was 1.29.

[0044] This composition was mixed for 30 minutes in a planetary mixer and then subjected to several passes in a three-roll mill to provide good dispersion of the carbon black powder.

[0045] The composition was screen-printed onto a polyester (PET) substrate and dried at 130 °C for 10 minutes in a forced-air oven. Cracking occurred during the drying process. The volume percent of carbon black in the dried film was 40.8. The higher relative amount of carbon black led to cracking. COMPARISON EXPERIMENT B

[0046] A PTF carbon black composition was prepared as follows. 69 parts by weight of an organic starting medium was used and prepared by mixing 28.50 wt% Desmocoll® 406 polyurethane (Covestro LLC, Pittsburgh, PA) with 71.50 wt% Dowanol™ DPM dipropylene glycol methyl ether (Dow Co., Midland, MI) organic solvent. This mixture was stirred at 90°C for 1 to 2 hours to dissolve all of the resin. 4.45 parts by weight of Vulcan XC-72 conductive carbon black powder (Cabot Corp, Boston, Mass.) and 11.1 parts by weight of graphite were added. Finally, 35.93 parts by weight of Dowanol™ DPM dipropylene glycol methyl ether were added for thinning purposes to bring the composition to a desired viscosity of 70 Pas. The amount of organic medium, including solvent, added to adjust viscosity is 84.45 wt%, consisting of 67.77 wt% solvent and 16.68 wt% polyurethane resin, where wt.-% based on the total weight of the composition. The polyurethane resin constituted 19.7 wt.% of the total weight of the organic medium. The ratio of the weight of polyurethane resin to the weight of carbon black and graphite was 1.07.

[0047] This composition was mixed for 30 minutes in a planetary mixer and then subjected to several passes in a three-roll mill to provide good dispersion of the carbon black powder.

[0048] The composition was screen-printed onto a polyester (PET) substrate and dried at 130 °C for 10 minutes in a forced-air oven. No cracking occurred during the drying process. The volume percent of carbon in the dried film was 37.5.

[0049] The resistance of the dried film was 150 ohms / sq. A reading of the PTC was obtained by measuring the film's resistance at 25°C, 40°C, and 65°C. The PTC resistance factor at 40°C (the ratio of the resistance at 40°C to that at 25°C) was 2.2, and the PTC resistance factor at 65°C (the ratio of the resistance at 65°C to that at 25°C) was 2.9, indicating a relatively high PTC due to the presence of graphite.

Claims

[1] A polymer thick film carbon black composition comprising: (a) 6 to 13 wt.% of a conductive carbon black powder; and (b) 87 to 94 wt.% organic medium comprising 10 to 30 wt.% thermoplastic polyurethane resin dissolved in an organic solvent, wherein the thermoplastic polyurethane has a percent elongation of at least 200%, wherein the weight percent of the thermoplastic polyurethane resin is based on the total weight of the organic medium and the weight percent of the conductive carbon black powder and the organic medium is based on the total weight of the composition; wherein the ratio of the weight of polyurethane resin to the weight of conductive carbon black powder is in the range of 1.50 to 1.

75. [2] The polymer thick film carbon black composition of claim 1, wherein the thermoplastic polyurethane resin is selected from the group consisting of a polyester-based polymer, a urethane homopolymer, and a predominantly linear hydroxyl polyurethane. [3] The polymer thick film carbon black composition of claim 2, wherein the thermoplastic polyurethane resin is a predominantly linear hydroxyl polyurethane. [4] An article comprising a stretchable heater having a resistive element formed from a polymer thick film carbon black composition comprising: (a) 6 to 13 wt.% of a conductive carbon black powder; and (b) 87 to 94 wt.% organic medium comprising 10 to 30 wt.% thermoplastic polyurethane resin dissolved in an organic solvent, wherein the thermoplastic polyurethane has a percent elongation of at least 200%, wherein the weight percent of the thermoplastic polyurethane resin is based on the total weight of the organic medium and the weight percent of the conductive carbon black powder and the organic medium is based on the total weight of the composition; wherein the ratio of the weight of polyurethane resin to the weight of conductive carbon black powder is in the range of 1.50 to 1.

75. [5] The article of claim 4, wherein the thermoplastic polyurethane resin is selected from the group consisting of a polyester-based polymer, a urethane homopolymer, and a predominantly linear hydroxyl polyurethane. [6] The article of claim 5, wherein the thermoplastic polyurethane resin is a predominantly linear hydroxyl polyurethane. [7] An article according to any one of claims 4 to 6, wherein the article is a wearable garment. [8] An article comprising a stretchable heater having a resistive polymer thick film element, wherein the ratio of the weight of polyurethane resin to the weight of carbon black is in the range of 1.50 to 1.

75. [9] The article of claim 8, wherein the thermoplastic polyurethane resin is a predominantly linear hydroxyl polyurethane. [10] An article according to any one of claims 8 or 9, wherein the article is a wearable garment.

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