UV curable transparent conductive compositions

[0001] This application is a continuation of U.S. application Ser. No. 10/193,389 filed Jul. 11, 2002, now U.S. Pat. No. 6,784,223, which, in turn, is a continuation-in-part of International Application Ser. No. PCT/US01/00976, filed Jan. 11, 2001 which, in turn, claims the benefit of U.S. Provisional Patent Application Ser. No. 60/175,971, filed Jan. 13, 2000.

[0002] The present invention relates to ultraviolet light (UV) curable compositions capable of producing a transparent conductive coating.

[0003] UV radiation curable transparent conductive compositions are applied to a substrate through spraying, screen printing, dipping or brushing, thus forming a transparent conducting film or coating. Transparent conductive coatings transmit visible light while possessing electrical conductivity. Accordingly such coatings find application in automobiles, airplanes, etc. as electrodes for liquid crystal devices, exothermic resistors, and photosemiconductors.

[0004] UV curable conductive films offer advantages over typical heat curable films typically produced by chemical vapor deposition, sputtering, and sol-gelling. Heat curable compositions used for example in the sol-gel process require the use of organic solvents that contain a significant amount of volatile organic compounds (VOCs). These VOCs escape into the atmosphere while the heat curable composition dries. Such solvent based systems are undesirable because of the hazards and expenses associated with VOCs. The hazards include water and air pollution and the expenses include the cost of complying with strict government regulation on solvent emission levels. In contrast, UV curable compositions contain reactive monomers instead of solvents; thus eliminating the detrimental effects of the VOCs.

[0005] The use of heat curable compositions not only raises environmental concerns but other disadvantages exist with their use as well. Heat curable compositions suffer from slow cure times which lead to decreased productivity. These compositions require high energy for curing due to energy loss as well as the energy required to heat the substrate. Additionally, many heat curable compositions yield poor film properties that result in decreased value of the end product.

[0006] Although UV curable compositions exhibit superior properties and performance over their heat curable counterparts, UV curable compositions themselves suffer from certain disadvantages. Generally, UV compositions have high molecular weights and a substantial degree of cross linkage due to the highly reactive nature of the composition. As a result, many of these compositions suffer from low durability and resin shrinkage. With the use of many such compositions, an inordinately high amount of UV light is required to cure. With some compositions, suspended solids fall out of solution after a period of one to two days. This dispersion adversely affects the gloss and clarity of the finished product.

[0007] Accordingly, there exists a need to provide environmentally safe UV curable transparent conductive compositions which exhibit improved appearance and workability. Additionally, there is a need to provide a method of applying an improved composition which furthers the goal of improved performance.

[0008] It is an object of the present invention to provide an improved composition that upon curing by ultraviolet light produces a transparent conductive coating.

[0009] It is another object of the present invention to provide an improved composition suitable for use in touch screens, membrane switches, TV screens, and VCRs.

[0010] It is another object of the present invention to provide an improved composition suitable for coating a suitable substrate that can be applied by spraying, screen printing, dipping, and brushing.

[0011] It is still another object of the present invention to provide an improved composition that does not contain any significant amount of volatile organic solvents that do not become incorporated in the active layer after curing.

[0012] The present invention discloses an ultraviolet light curable transparent conductive composition and method for making such a composition that may be used to produce a transparent conductive coating on a suitable substrate. The disclosed composition does not contain any significant amount of volatile organic solvents that do not become incorporated in the active layer after curing. Specifically, the transparent conductive composition contains 5% or less volatile organic solvents by weight.

[0013] In accordance with one aspect of the invention, an ultraviolet light curable transparent conductive composition is provided. The transparent conductive composition comprises at least one aliphatic acrylated oligomer, an electrically conductive powder, and a photoinitiator. The aliphatic acrylated oligomer is present in an amount of about 10% to 40% of the total weight of the transparent conductive composition, the electrically conductive powder is present in an amount of about 20% to 50% of the transparent conductive composition, and the photinitiator is present in an amount of 2% to 10% of the total weight of the transparent conductive composition. All percentages of the transparent conductive composition as expressed in this document refer to the mass percentage of the stated component to the total mass of the transparent conductive composition in its fluid state at standard temperature and pressure.

[0014] The transparent conductive composition preferably further comprises an acrylated epoxy oligomer in an amount of about 3% to 11%, an isobornyl acrylate monomer in an amount of about 10% to 40% of the transparent conductive composition, and a flow promoting agent in an amount of about 0.1% to 8% of the transparent conductive composition.

[0015] In accordance with yet another aspect of the invention, a method is provided for depositing a transparent conductive coating on a substrate. The method comprises a first step of applying to the substrate a transparent conductive fluid-phase composition (“transparent conductive composition”). The transparent conductive composition comprises a mixture of one or more aliphatic acrylated oligomers, an electrically conductive powder, and a photoinitiator. Preferably, the aliphatic acrylated oligomer is present in an amount of about 10% to 40% of the total weight of the transparent conductive composition, the electrically conductive powder is present in an amount of about 20% to 50% of the total weight of the transparent conductive composition, and the photoinitiator is present in an amount of about 2% to 10% of the total weight of the transparent conductive composition. The transparent conductive composition preferably further comprises an acrylated epoxy oligomer in an amount of about 3% to 11% of the total weight of the transparent conductive composition, an isobornyl acrylate monomer in an amount of about 10% to 40% of the total weight of the transparent conductive composition, and a flow promoting agent in an amount of about 0.1% to 8% of the total weight of the transparent conductive composition.

[0016] The method also includes a second step of illuminating the transparent conductive composition on the substrate with an ultraviolet light to cause the transparent conductive composition to cure into the transparent conductive coating.

[0017] In accordance with this method, the transparent conductive composition can be selectively deposited on the substrate at specific locations where transparent conductive plating is desired. It need not be applied to the entire substrate.

Transparent Conductive Compositions

[0018] Reference will now be made in detail to presently preferred compositions or embodiments and methods of the invention, which constitute the best modes of practicing the invention presently known to the inventor.

[0019] In accordance with one aspect of the invention, a presently preferred ultraviolet light curable transparent conductive composition (“transparent conductive composition”) is provided. In this preferred embodiment, the transparent conductive composition includes a mixture of one or more aliphatic acrylated oligomers. The aliphatic acrylated oligomer mixture is present in an amount of about 10% to 40% of the total weight of the transparent conductive composition. The aliphatic acrylated oligomer mixture is more preferably present in an amount of about 20% to 30% of the total weight of the transparent conductive composition, and most preferably about 27% of the total weight of the transparent conductive composition. The aliphatic acrylated oligomer preferably comprises one or more urethane oligomers. Suitable aliphatic acrylated oligomers include Radcure Ebecryl 244 (aliphatic urethane diacrylate diluted 10% with 1,6-hexanediol diacrylate), Ebecryl 264 (aliphatic urethane triacrylate diluted 15% with 1,6-hexanediol diacrylate), Ebecryl 284 (aliphatic urethane diacrylate diluted 12% by weight with 1,6-hexanediol diacrylate) urethanes, commercially available from Radcure UCB Corp. of Smyrna, Ga.; Sartomer CN-961E75 (aliphatic urethane diacrylate blended with 25% ethoxylated trimethylol propane triacylate), CN-961H81 (aliphatic urethane diacrylate blended with 19% 2(2-ethoxyethoxy)ethyl acrylate), CN-963A80 (aliphatic urethane diacrylate blended with 20% tripropylene glycol diacrylate), CN-964 (aliphatic urethane diacrylate), CN-966A80 (aliphatic urethane diacrylate blended with 20% tripropylene glycol diacrylate), CN-982A75 (aliphatic urethane diacrylate blended with 25% tripropylene glycol diacrylate) and CN-983 (aliphatic urethane diacrylate), commercially available from Sartomer Corp. of Exton, Pa.; TAB FAIRAD 8010, 8179, 8205, 8210, 8216, 8264, M-E-15, UVU-316, commercially available from TAB Chemicals of Chicago, Ill.; and Echo Resin ALU-303, commercially available from Echo Resins of Versaille, Mo.; and Genomer 4652, commercially available from Rahn Radiation Curing of Aurora, Ill. The preferred aliphatic acrylated oligomers include Ebecryl 264 and Ebecryl 284. Ebecryl 264 is an aliphatic urethane triacrylate of 1200 molecular weight supplied as an 85% solution in hexanediol diacrylate. Ebecryl 284 is aliphatic urethane diacrylate of 1200 molecular weight diluted 10% with 1,6-hexanediol diacrylate. Combinations of these materials may also be employed herein.

[0020] The preferred transparent conductive composition still further includes a conductive powder preferably in an amount of about 20% to 50% of the total weight of total weight of the transparent conductive composition. Specifically, the conductive powder is an electrically conductive powder. The conductive powder is more preferably present in an amount of about 30% to 40% of the total weight of total weight of the transparent conductive composition, and most preferably about 33% of the total weight of total weight of the transparent conductive composition. Preferred conductive powders include metal powders, metal oxide powders, metal nitride powders, or mixtures thereof. Suitable conductive powders include silver powder, tin oxide powder, antimony tin oxide, and indium tin oxide powder. The preferred conductive powders are the antimony tin oxide powders, Minatec 30 and Minatec 40, commercially available from EM Industries located in Hawthorne, N.Y.

[0021] This preferred transparent conductive composition also includes a photoinitiator in an amount of about 2% to 10% of the total weight of total weight of the transparent conductive composition. The photoinitiator is more preferably present in an amount of about 4% to 6% of the total weight of total weight of the transparent conductive composition, and most preferably about 5% of the total weight of total weight of the transparent conductive composition. Suitable photoinitiators include Irgacure 184 (1-hydroxycyclohexyl phenyl ketone), Irgacure 907 (2-methyl-1-[4-(methylthio)phenyl]-2-morpholino propan-1-one), Irgacure 369 (2-benzyl-2-N , N-dimethylamino-1-(4-morpholinophenyl)-1-butanone), Irgacure 500 (the combination of 50% 1-hydroxy cyclohexyl phenyl ketone and 50% benzophenone), Irgacure 651 (2,2-dimethoxy-1,2-diphenylethan-1-one), Irgacure 1700 (the combination of 25% bis(2,6-dimethoxybenzoyl-2,4-,4-trimethyl pentyl) phosphine oxide, and 75% 2-hydroxy-2-methyl-1-phenyl-propan-1-one) DAROCUR 1173 (2-hydroxy-2-methyl-1-phenyl-propan-1-one) and DAROCUR 4265 (the combination of 50% 2,4,6-trimethylbenzoyldiphenyl-phosphine oxide and 50% 2-hydroxy 2-methyl-1-phenyl-propan-1-one), available commercially from Ciba-Geigy Corp., Tarrytown, N.Y.; CYRACURE UVI-6974 (mixed triaryl sulfonium hexafluoroantimonate salts) and CYRACURE UVI-6990 (mixed triaryl sulfonium hexafluorophosphate salts) available commercially from Union Carbide Chemicals and Plastics Co. Inc., Danbury, Conn.; and Genocure CQ, Genocure BOK, and Genocure M.F., commercially available from Rahn Radiation Curing. The preferred photoinitiator is Irgacure 1700 commercially available from Ciba-Geigy of Tarrytown, N.Y. Combinations of these materials may also be employed herein.

[0022] This preferred transparent conductive composition further includes an acrylated epoxy oligomer. The acrylated epoxy oligomer is present in an amount of about 3% to 11% of the total weight of total weight of the transparent conductive composition. The acrylated epoxy oligomer is more preferably present in an amount of about 5% to 9% of the total weight of total weight of the transparent conductive composition, and most preferably about 7% of the total weight of total weight of the transparent conductive composition. Suitable acrylated epoxy oligomers include Radcure Ebecryl 3603 (novolac epoxy acrylate diluted 20% by weight with tripropylene glycol diacrylate), commercially available from Radcure UCB Corp.; Sartomer CN-120 (difunctional bisphenol based epoxy acrylate) and CN-124 (difunctional bisphenol based epoxy acrylate), commercially available from Sartomer Corp.; and Echo Resin TME 9310 and 9345, commercially available from Echo Resins. The preferred acrylated epoxy oligomer is Ebecryl 3603, which is a tri-functional acrylated epoxy novolac. Combinations of these materials may also be employed herein.

[0023] The photocurable mixture of the lubricating composition preferably includes an ethylenically unsaturated monomer having Formula I:

[0024]
wherein R₁ is hydrogen or substituted or unsubstituted alkyl; and R₂ is substituted or unsubstituted alkyl having more than 4 carbon atoms, cycloalkyl, cycloalkenyl, or substituted or unsubstituted aryl. Preferably R₁ is hydrogen or methyl; and R₂ is isobornyl, phenyl, benzyl, dicylcopentenyl, diclypentenyl oxyethyl, cyclohexyl, and naphthyl. The most preferred ethyleneically unsaturated monomers are isobornyl acrylate monomers. The isoborynl acrylate monomers are preferably present in an amount of about 10% to 40% of the total weight of total weight of the transparent conductive composition. The isobornyl acrylate monomer is more preferably present in an amount of about 20% to 35% of the total weight of total weight of the transparent conductive composition, and most preferably about 28% of the total weight of total weight of the transparent conductive composition. Suitable isobornyl acrylate monomers include Sartomer SR-423 (isobornyl methacrylate):

[0025]
and SR-506 (isobornyl acrylate):

[0026]
available from Sartomer Corp.; Radcure IBOA (isobornyl acrylate), commercially available from Radcure Corp.; IBOA and IBOMA, commercially available from CPS Chemical of Bradford, England; and Genomer 1121, commercially available from Rahn Radiation Curing. The preferred isobornyl acrylate monomer is Radcure IBOA, commercially available from Radcure Corp. Radcure IBOA is a high purity, low color monomer. Combinations of these materials may also be employed herein.

[0027] The preferred transparent conductive composition optionally includes a flow promoting agent in an amount of about 0.1% to 8%. The flow promoting agent is more preferably present in an amount of about 3% to 5% of the total weight of total weight of the transparent conductive composition, and most preferably about 3.5% of the total weight of total weight of the transparent conductive composition. Suitable flow promoting agents include Genorad 17, commercially available from Rahn Radiation Curing; and Modaflow, commercially available from Monsanto Chemical Co., St. Louis, Mo. The preferred flow promoting agent is Modaflow which is an ethyl acrylate and 2-ethylhexyl acrylate copolymer that improves the flow of the composition. Combinations of these materials may also be employed herein.

[0028] To illustrate, the following example sets forth a presently preferred transparent conductive composition according to this aspect of the invention.

[0029] This example provides a preferred transparent conductive composition according to the invention. The transparent conductive composition was made from the following components:

[0030]

		Approximate
	Component	Mass %
	Ebecryl 264	26.7
	IBOA	28.3
	Irgacure 1700	5.0
	Ebecryl 3603	6.6
	Modaflow	3.5
	Minatec 30	33.4
	Total	100.00

[0031] In this example the IBOA and Irgacure 1700 are mixed in a pan with a propeller blade mixer for 30 seconds at a speed of 500 to 1000 rpm. In the next step, the Ebecryl 264, the Ebecryl 3603, and Modaflow are introduced into the pan and mixed for 1 to 2 minutes at a speed of 2000 rpm. In the final step, the Minatec 30 is added and mixed at 2000 rpm for 1 to 2 minutes. The mixing is temporarily suspended if the temperature exceed 100° F.

[0032] This example provides a preferred transparent conductive composition according to the invention. The transparent conductive composition was made from the following components:

[0033]

		Approximate
	Component	Mass %
	Ebecryl 264	26.7
	IBOA	28.3
	Irgacure 1700	5.0
	Ebecryl 3603	6.6
	Modaflow	3.5
	Minatec 40	33.4
	Total	100.00

[0034] In this example the IBOA and Irgacure 1700 are mixed in a pan with a propeller blade mixer for 30 seconds at a speed of 500 to 1000 rpm. In the next step, the Ebecryl 264, the Ebecryl 3603, and Modaflow are introduced into the pan and mixed for 1 to 2 minutes at a speed of 2000 rpm. In the final step, the Minatec 40 is added and mixed at 2000 rpm for 1 to 2 minutes. The mixing is temporarily suspended if the temperature exceed 100° F.

Method for Depositing a Transparent Conductive Coating

[0035] In accordance with still another aspect of the invention, a method is provided for depositing an transparent conductive coating on a suitable substrate. The method comprises a first step of applying a transparent conductive fluid-phase composition (“transparent conductive composition”) to the substrate.

[0036] The transparent conductive composition comprises a mixture of one or more aliphatic acrylated oligomers, an electrically conductive powder, and a photoinitiator. Preferably, the aliphatic acrylated oligomer is present in an amount of about 10% to 40% of the total weight of the transparent conductive composition, the electrically conductive powder is present in an amount of about 20% to 50% of the total weight of the transparent conductive composition, and the photoinitiator is present in an amount of about 2% to 10% of the total weight of the transparent conductive composition. The transparent conductive composition preferably further comprises an acrylated epoxy oligomer in an amount of about 3% to 11% of the total weight of the transparent conductive composition, an isobornyl acrylate monomer in an amount of about 10% to 40% of the total weight of the transparent conductive composition, and a flow promoting agent in an amount of about 0.1% to 8% of the total weight of the transparent conductive composition. The preferred transparent conductive compositions according to this method are those described herein, for example, including the compositions described in examples 1 and 2.

[0037] The transparent conductive composition may be applied to the substrate using a number of different techniques. The transparent conductive composition may be applied, for example, by direct brush application, or it may be sprayed onto the substrate surface. It also may be applied using a screen printing technique. In such screen printing technique, a “screen” as the term is used in the screen printing industry is used to regulate the flow of liquid composition onto the substrate surface. The transparent conductive composition typically would be applied to the screen as the latter contacts the substrate. The transparent conductive composition flows through the silk screen to the substrate, whereupon it adheres to the substrate at the desired film thickness. Screen printing techniques suitable for this purpose include known techniques, but wherein the process is adjusted in ways known to persons of ordinary skill in the art to accommodate the viscosity, flowability, and other properties of the liquid-phase composition, the substrate and its surface properties, etc. Flexographic techniques, for example, using pinch rollers to contact the transparent conductive composition with a rolling substrate, also may be used.

[0038] The method includes a second step of illuminating the transparent conductive fluid-phase composition on the substrate with an ultraviolet light to cause the transparent conductive fluid-phase composition to cure into the transparent conductive coating. This illumination may be carried out in any number of ways, provided the ultraviolet light or radiation impinges upon the transparent conductive composition so that the transparent conductive composition is caused to polymerize to form the coating, layer, film, etc., and thereby cures.

[0039] Curing preferably takes place by free radical polymerization, which is initiated by an ultraviolet radiation source. The photoinitiator preferably comprises a photoinitiator, as described above.

[0040] Various ultraviolet light sources may be used, depending on the application. Preferred ultraviolet radiation sources for a number of applications include known ultraviolet lighting equipment with energy intensity settings of, for example, 125 watts, 200 watts, and 300 watts per square inch.

[0041] While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.