IT8124196A1 - PROCESS FOR PREPARING CONDUCTIVE COATINGS - Google Patents

Description

Description of the invention having title?

"H PROCESS FOR PREPARING CONDUCTIVE COATINGS ,,n

SUMMARY

A process for forming a conductive coating on a substrate which comprises (1) applying an electrically conductive material to the surface of a non-crosslinked or partially crosslinked organic resin having a tacky surface, said resin being deposited in a circuit pattern on a substrate and then, in that order or in reverse order, (2)

crosslinking / said resin to fix in place the electrically conductive material on said surface by the action of heat or ultraviolet radiation, preferably in an inert atmosphere, e.g. a nitrogen atmosphere, and (3) removing any still-dissolved, i.e. unfixed, electrically conductive material.

GENERAL INFORMATION OF THE INVENTION

1. Field of invention

The present invention relates to a process for forming a conductive coating on a substrate.

The present invention also relates to a process for forming conductive coatings on substrates for the purpose of forming circuit boards.

2. Description of past technology Electrically conductive coatings are known to the art.

U.S. Patent No. 3,412,043 discloses an electrically conductive resin composition consisting essentially of flaked silver, a resin binder, and a finely divided inert filler, combined in specific weight ratios. In this patent, the resin binder is an epoxy resin system that is crosslinked by the addition of an amine curing agent at temperatures slightly higher than normal.

U.S. Patent No. 3,746,622 discloses electrically conductive coatings

_ _ comprising certain epoxy resins, particles of a tough polymer having carboxylic, hydroxyl, amine or isocyanate substituents which core-link with the epoxy resin, finely divided metal particles and a curing or cross-linking agent for the epoxy resin, the cross-linking being achieved by heating the composition to temperatures of 125°C or higher.

U.S. Patent No. 3,968,056 discloses a radiation-curable ink comprising a material containing an electrically conductive metal in particulate form in combination with an organic resin binder that converts to a conductive coating on the surface of a substrate upon exposure to actinic or ionizing radiation.

Reissued U.S. Pat. No. 30,274 discloses a circuit board for inactivating high voltage flash lamps, said board including a nonconductive thermoplastic substrate having on one of its surfaces an electrically conductive coating applied in a pattern and defining an electrical circuit diagram for the flash lamps, said coating comprising an ultraviolet radiation crosslinkable organic resin matrix and a particulate electrically conductive material selected from the group consisting of a particulate electrically conductive metal and a particulate electrically conductive metal-containing material, including mixtures thereof with not more than about 15% by weight of said particulate electrically conductive material having a diameter/thickness aspect ratio exceeding 20.

Have you noticed that in all of the above prior art compositions used to form conductive coatings, an electrically conductive material is mixed into the composition with an organic resin, which mixture is then exposed to heat, ultraviolet radiation, or ionizing radiation? One of the drawbacks of such a system is that, because the organic resin is not conductive but is required to act as a matrix for the electrically conductive material and adhere to the substrate in a desired pattern, it is necessary to use significant amounts of electrically conductive material, such as silver flakes or beads, to achieve a commercially acceptable degree of conductivity. That is, because the silver flakes or beads are dispersed throughout the organic resin and not in a linear conductive pattern, it is necessary to use a significant amount of the organic resin. excessive amount of silver flakes or beads which constitute the most expensive part of the electrically conductive composition.

The use of large amounts of electrically conductive material mixed with the crosslinkable resin results in the mixture being so thick and of such high viscosity that it is extremely difficult, if not impossible, to prepare the desired circuit pattern using a screen printing system. Conversely, when the amount of electrically conductive material mixed with the crosslinkable resin is small enough to permit screen printing, the conductance obtained in the crosslinked mixture is often not commercially acceptable.

PURPOSES OF THE PRESENT INVENTION

One object of the present invention is to provide a process for forming a conductive coating on a substrate that requires a smaller amount of electrically conductive material to achieve the same conductivity. Another object of the present invention is to provide a process by which a conductive coating can be formed on a substrate using screen printing. Yet another object of the present invention is to provide a process for forming a conductive coating on a substrate by photoreproducing an image.

Other purposes will become apparent from a reading of what follows.

DESCRIPTION OF THE INVENTION

The present invention provides a process for forming a conductive coating on a substrate, which process comprises (1) applying an electrically conductive material to the surface of an uncrosslinked or partially crosslinked organic resin composition having a tacky surface, said composition being deposited on a substrate in a circuit pattern, and then, in that or reverse order, (2) crosslinking said composition to secure the electrically conductive material in place on said surface by ultraviolet radiation, preferably, but not necessarily, in an inert atmosphere or by heat, and (3) removing any still-loose or unsecured electrically conductive material.

The electrically conductive material considered here may be in the form of particles, spheres, beads, powder, fibers, flakes, or mixtures thereof. Regardless of the noble metals and noble metal-coated substrates that may be used in the case considered here as electrically conductive material, are the use of other metals such as copper, aluminum, iron, nickel, and zinc also contemplated? Silver-coated glass spheres, sometimes referred to as "pearls," may also be used and have an average diameter of approximately 6 125 microns.

These materials are produced from glass spheres commonly used as reflective fillers and are commercially available. Glass fibers coated with silver, copper, or nickel may also be used, as indicated in French Patent No. 1,531,272. Electrically conductive materials used for the purposes considered here also include carbon black and graphite.

In the process of the present invention, the amount of electrically conductive material B? requires for conductance is independent of the thickness of the organic resin composition employed. That is, since the electrically conductive material is applied only to the top surface of the organic resin, it does not matter whether the resin is 0.5 mils (12.7 microns) thick or 20 mils (508 microns) thick. Resin thicknesses usable for the purposes herein may range from about 0.05 to 20 mils (1.27 to 508 microns) or more, the preferred range being 0.5 to 2 mils (12.7 to 50.8 microns).

The electrically conductive material employed herein may be used in various sizes depending on its shape. For best results, the largest dimension of the electrically conductive material should not exceed about 75 microns. However, since the electrically conductive material in the system of the present invention is not subject to screen printing, which limits its largest dimension, larger particles, flakes, or beads having a largest dimension on the order of 100 microns or more may also be used. Preferably, the electrically conductive material has a largest dimension in the range of 10 to 60 microns.

The organic resin composition used here to act as a surface matrix for the electrically conductive material consists of a solvent-free, reactive, heat- and/or UV-curable liquid organic resin containing at least two carbon-carbon olefinic double bonds per molecule. The resin is added in combination with its own photoinitiator and, in the case of thermal crosslinking, with a thermoinitiator. If high-energy ionizing radiation is used instead of ultraviolet radiation, a photoinitiator is not required in the composition.

v

The solvent-free, reactive, crosslinkable liquid organic resins which may be used herein include, but are not limited to, at least one ethylenically unsaturated member of the group consisting of

(a) a liquid, ethylenically unsaturated monomer, oligomer, or prepolymer having the formula

(CH =C-C-0-)-H,

2 f n 1

- R

where R is H or CHy R^ is an organic residue and n is 2-4,

(b) a polythiol in combination with the above (a), and

(c) a polythiol in combination with a liquid, ethylenically unsaturated monomer*, oligomer, or prepolymer having the formula:

(CH2=O-CH2)5R3

- ? ?

where Rg is H or CH^, R^ is an organic residue and n is at least 2, as well as mixtures thereof#

While the above organic resins are, as such, usable for the purposes herein considered to form useful products, they may also be used in combination with conventional copolymerizable monomer compounds or reactive diluents. Blending the composition of the present invention with other monomers is usually employed to control viscosity and other application variables such as crosslinking rate as well as final properties of the film or coating, such as hardness and flexibility. These reactive diluents coreticle with the ethylenically unsaturated member of the group upon exposure to ultraviolet radiation and heat.

Examples of common copolymerizable compounds useful as reactive diluents include, but are not limited to, monofunctional acrylic esters, monofunctional methacrylic esters, styrene, vinyltoluene, acrylonitrile, methacrylonitrile, vinyl acetate, vinyl pyrrolidone, vinyl chloride, vinylidene chloride, butadiene, isoprene, chloroprene, divinylbenzene, di(vinylphenyl) carbonate, diallyl phthalate, diallyl carbonate, dl-(allylphenyl) carbonate, diallyl fumarate, triallyl isocyanurate, triallyl cyanideate, diallyl chlorendate, diallyl maleate, and unsaturated polyesters as well as mixtures thereof. The term "unsaturated polyesters" refers here to the usual polycondensation products consisting of linked ester-type residues of polyhydric alcohols, especially dihydric alcohols, as well as possibly also residues of monohydric alcohols and/or monohydric carboxylic acids, the residues must contain at least partially unsaturated groups. Examples of acids include maleic acid, fumaric acid, itaconic acid, mesaconic acid, citraconic acid, succinic acid, glutaric acid, oedipic acid, phthalic acid, tetrachlorophthalic acid, hexachloroendomethylenetetrahydrophthalic acid, trimellitic acid, benzoic acid, linseed oil fatty acid, and castor oil fatty acid, and mixtures thereof. Examples of alcohols: include ethylene glycol, diethylene glycol, propandol, butanediol and hexanediol, trimethylolpropane, pentaerythritol, butanol, and tetrahydrofuryl alcohol.

Reactive diluents may be added to the system in quantities up to 90% by weight of the organic resin, preferably 20 - 50% by weight on the same basis.

The thermoinitiators used here are chosen from substituted or unsubstituted pinacles.

X substituted or unsubstituted pins usable for the purposes considered here as thermo-initiators have the following formula;

V** E?J3

H_-C- C-R.

2 I i 4

- X Y

where R^ and R^ are equal or different substituted or unsubstituted aromatic radicals, R^ and R^ are substituted or unsubstituted aliphatic or aromatic radicals and X and Y which can be equal or different are hydroxyl, alkoxy or aryloxy?

Preferred pinacles are those in which R^, R^, R^ and R^ are aromatic radicals, especially phenyl radical, and X and Y are hydroxyl.

Examples of this class of compounds include, but are not limited to, benzopinacol, 4*4*-dichlorobenzopinacol, 4,4*-dibromobenzopinacol, 4,4*?diiodobenzopinazole,

4,4*,4w*4,,*-tetrachlorobenzopinacol, 2,4,2*,4*-tetrachlorobenzopinacol, 4?4 '-dimethylbenzopinacol, 3,3*?dimethylbenzopinacol, 2,2*?dimethylbenzopinacol, 3,4j3,*4*-tetramethylbenzopinacol, 4,4?-dirnethoxybenzopinacol, 4*4**4***4"*-tetramethoxybenzopinacol, 4*4'?diphenylbenzopinacol, 4*4*-dichloro-4", 4"*?dimethylbenzopinacol, 4*4*-dimethyl-4"*4"*-diphenylbenzopinacol, xanthopinacol, fluorenonpinacol, acetophenonpinacol, 4*4*?dimethylacetophenon-pinacol, 4*4*-dichloroacetophenonpinacol, 1,1,2-triphenyl-propane-1.2-diol, 1*2,3*4-tetraphenylbutan-2, 3-diol,

1.2-diphenylcyclobutan-1,2-diol, propiophenon-pinacol, 4,4'-dimethylpropiophenonpinacol, 2,2*-ethyl-3,3*-dimethoxypropiophenon? pinacol, 1*1* 1*4,4,4?hexafluoro? 2. 3-diphenyl-butane-2,3-diol.

Other compounds according to the present invention may be mentioned as benzopinacol monomethyl ether, benzopinacol monophenyl ether, benzopinacol monoisopropyl ether, benzopinacol monoisobutyl ether, benzopinacol mono(diethoxymethyl) ether and the like.

Pinacol is added to the composition in quantities ranging from 0.01 to 10$ by weight, preferably from 0.1 to 5$ by weight, based on the

weight of organic resin.

The thermoinitiator can be added to the system in various ways. That is, the thermoinitiator, as such, can be mixed with the organic resin. It can also be mixed with a photoinitiator and added to the organic resin.

Additionally, the thermo-initiator can be dissolved or suspended in well-known, commercially available solvents such as dibutyl phthalate; ketones, examples of which are acetone and methyl ethyl ketone; or chlorinated hydrocarbons such as methylene chloride, and then added to the system.

In the practice of this invention, it is sometimes necessary to add a polythiol to the composition prior to crosslinking. This is especially true when the ethylene unsaturation in the organic resin (sometimes hereinafter referred to as polyene) is an allyl group.

In this case, during the crosslinking stage, the polythiol is added across the double bond of the allyl group resulting in crosslinked solids within a commercially acceptable time frame. In the case where the ethylene unsaturation in the polyene consists of an acrylic or methacrylic group, the addition of a polythiol to the system results in a fully crosslinked resin and precludes the possibility of a tacky surface resulting from crosslinking inhibition by air.

As used here, the term "polythiols" refers to simple or complex organic compounds having multiple pendant or terminal -SH functional groups per average molecule?

On average, polythiols must contain 2 or more -SH groups/molecule and typically have a viscosity range from slightly above 0 to 20 million centipoises (cp?) at 70°C, as determined by a Brookfield viscometer. Included in the term "polythiols" as used herein are those materials which, in the presence of an inert solvent, aqueous dispersion, or plasticizer, fall within the aforementioned viscosity range at 70°C. Polythiols capable of operation in the present invention typically have molecular weights in the range of 94 - 20,000, preferably 100 - 10,000.

The polythiols that can operate in the present invention can be exemplified by the general formula: Re-(SH) where n is at least 2

or n

and S. is a polyvalent organic residue free from o

reactive unsaturation between carbon and carbon.

Thus Rg may contain cyclic groupings and minor amounts of heteroatoms such as N, S, P or O, but it contains primarily carbon-hydrogen, carbon-oxygen or silicon-oxygen chain bonds free from any reactive carbon-carbon unsaturation.

One class of polythiols which may be used with polyenes in the present invention to yield essentially odorless polythioether products are esters of thiol-containing acids having the general formula ES-H^-COOH, where E^ is an organic moiety containing no "reactive" carbon-carbon unsaturation, with polyhydroxyl compounds having the general structure R^Q?(OH)^, where R^ is an organic moiety containing no "reactive" carbon-carbon unsaturation and n is 2 or more. These components are capable of reacting under suitable conditions to give a polythiol having the general structure:

0

Bio?(?C-H9-SH)n

in which and are organic residues not containing__ "reactive" unsaturation between carbon and carbon and n and* 2 or more*?

Certain polythiols such as monomeric aliphatic polythiols (ethane dithiol, hexamethylene dithiol, decamethylene dithiol, tolylene-2,4-dithiol, and so on) and certain polymeric polythiols such as thiol-terminated ethylene dimercaptan polymer, and so on, and similar polythiols which are conveniently and routinely synthesized on a commercial scale, although having unpleasant odors, may be used in the present invention, but many of the end products fail to gain widespread practical and commercial acceptance. Examples of the preferred polythiol compounds for the present invention due to their relatively low odor level include, but are not limited to, esters of thioglycolic acid (HS-CHgCOOH), alpha-mercaptopropionic acid (HS-CH(CH^)-COOH) / beta-mercaptopropionic acid (HS-CHgCH^COOH) with polyhydroxy compounds such as glycols, triols, tetrols, pentols, hexols, and so on. Specific examples of the preferred polythiol compounds include, but are not limited to, ethylene glycol bis(thioglycolate), ethylene glycol bis(beta-mercaptopropionate), trimethylolpropan tris(thioglycolate), trimethylolpropan tris(beta-mercaptopropionate), pentaerythritol tetrakis(thioglycolate), and pentaerythritol tetrakis(beta-mercaptopropionate). of which all are commercially available. A specific example of a preferred polymeric polythiol is polypropylene ether glycol bis(beta-mercaptopropionate) which is prepared from polypropylene ether glycol (e.g., Pluracol P2010, Wyandotte Chemical Corp.) and beta-mercapropionic acid by esterification.

Further, polythiols usable in the case considered here with polyene to obtain solid cross-linked polythioether products in the presence of a free radical generator include the mercaptoester derivatives of styrene-allyl alcohol copolymers disclosed in U.S. Pat. No. 3,904,499 and the isocyanurate-containing polythiols disclosed in U.S. Pat. No. 3,676,440, and thiol-terminated liquid polymers prepared in accordance with U.S. Pat. No. 3,258,495, all of which are incorporated herein by reference. An example of a thiol-terminated liquid polymer of the type last mentioned is CAPCUBE 3-800, marketed by Diamond Shamrock Chemical Company.

The preferred polythiol compounds are initially characterized by a low level of mercaptan-like odor and, upon reaction, give essentially odorless polythioether end products that are useful for holding electrically conductive material in place.

In the case of a polythiol in combination with allylic polyenes the molar ratio of thiol groups to ene groups to prepare the crosslinkable composition is between 0.2/1 and about 2/1, and suitably between about 0.75/1 and about 1.5/1?

In the case of a polythiol in combination with acrylic components, the molar ratio of thiol groups to ene groups for preparing the crosslinkable composition is between 0.01/1 and about 1/1, and this ratio of groups is desirably between about 0.02/1 and about 0.5/1. Prior to crosslinking, the polyene and polythiol components are mixed in a suitable manner to form a homogeneous liquid crosslinkable mixture. Thus, the polyene and polythiol reagents can be mixed without the need for a solvent at room temperature or at temperatures slightly above room temperature up to about 40°C when one of the components is a solid, or, if desired, the reagents can be dissolved in a suitable solvent and then the solvent removed by suitable means such as evaporation.

The compositions of the present invention may, if desired, include certain additives such as antioxidants, inhibitors, fillers, antistatic agents, flame retardants, thickeners, thixotropic agents, surface-active agents, viscosity modifiers, plasticizers, tackifiers, and the like, which is within the scope of the present invention. Such additives are usually premixed with the ethylenically unsaturated compound prior to, or during, the stage of combining the various ingredients. Usable fillers that may be added to the system to reduce costs include natural and synthetic resins, glass fibers, wood flour, clay, silica, alumina, carbonates, oxides, hydroxides, silicates, glass flakes, borates, phosphates, diatomaceous earth, talc, kaolin, barium sulfate, calcium sulfate, calcium carbonate, and the like. The foregoing additives may be present in quantities of 0.5 to 100 g. of up to 500 parts or more per 100 parts of the organic resin, parts by weight, and preferably present in quantities of between about 0.005 and about 300 parts on the same weight basis?

In addition, conventional ultraviolet stabilizers and antioxidants such as hydroquinone, tert.-butylhydroquinone, tert.-butylpyrocatechin, p-benzoquinone, 2,5-diphenylbenzoquinone, 2,6-di-tert.-butyl-p-cresol, and so on, are added to the system in normal amounts ranging from 0.001 to 2.0% by weight of the organic resin.

It is preferable to add photoinitiators in order to initiate the reaction under ultraviolet light. One class of photoinitiators consists of aldehyde and ketone carbonyl compounds having at least one aromatic nucleus directly attached to the group.

n

-c-Various photoinitiators of this type include, but are not limited to, benzophenone, acetophenone, o-methoxy-benzophenone, acenaphthenquinone, methyl ethyl ketone, valerophenone, hexanophenone,

alpha-phenylbutyrophenone, p-morpholinopropionphenone, dibenzo-suberone, 4-morpholino-benzophenone, 4'-morpholino-deoxybenzoin, p-diacetyl-bemzene, 4-aminobenzophenone, 4*-methoxyacetophenone, benzaldehyde, alpha-tetrahydrofuran, 9-acetylphenanthrene, 2-acetylphenanthrene, 10-thioxanthenone, 3-acetylphenanthrene,

3-acetylindone, 9-fluorenone, 1-indanone, 1,3,5-triacetylbenzene, thioxanthen?9?one, xantren?9?one, 7-H-benz[d]anthracen-7-one, 1?naphthaldehyde, 4,4'? bis(dimethylamino)benzophenone, fluoren-9-one,

1'-acetonaftone, 2'?acetonaftone, 2,3-butanedione, acetonaftone, 2,3-butanedione, benz[a]anthracene-7,12-dione, and so on. Another class of photoinitiators are benzoin ethers, such as benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin isobutyl ester, and 2,2?dimethoxy-2?phenylacetophenone. A third class of photoinitiators is exemplified by benzyl dimethyl benzoate. Other photoinitiators that may be used in the case considered here include triphenylphosphine and tri-o-tolylphosphine.

Photoinitiators or mixtures thereof are usually added in quantities between 0.0005 and 30/2 by weight of the organic resin.

One type of radiation useful here for crosslinking is ultraviolet light and other forms of actinic radiation commonly found in radiation emitted by the sun or artificial sources, such as ES-type solar lamps, cardboard arc lamps, xenon arc lamps, mercury vapor lamps, tungsten halide lamps, and the like. Ultraviolet radiation can be used most efficiently if the photocrosslinkable composition contains a suitable photoinitiator. Crosslinking periods can be adjusted to be very short and therefore commercially economical by appropriate choice of ultraviolet light source, photoinitiator and its concentration, as well as temperature and the molecular weight and functionality of the reactive group of the organic resin. Depending on these variables, crosslinking periods ranging in duration from about 1 second to 10 minutes can be used.

When using ultraviolet radiation, a specific intensity is usually used.

2

equal to 0.0004 - 60.0 watt/cm in the region of

200-400 nanometers.

As stated above, the non-crosslinked or partially crosslinked organic resin to which electrically conductive material is added can be formed by screen printing or by photoreproduction of images. In the screen printing method, the organic resin is mixed with the photoinitiator (and possibly

The organic resin (prepared with a terminator if heat is required for final crosslinking) is squeezed through a mesh to create a desired circuit pattern. If the ethylene unsaturation in the organic resin consists of an acrylic or methacrylic group, subsequent direct exposure to ultraviolet radiation results in the formation of an uncrosslinked, sticky surface on the resin due to the inhibition of crosslinking by air. The electrically conductive material is applied by sprinkling, dusting, or by spraying. or spray application to this sticky surface and the surface is subjected to final crosslinking by irradiation with ultraviolet rays from above, from the bottom (if the substrate is transparent to ultraviolet rays) or from both sides in an inert atmosphere or by heating, for example in a normal heating oven (air) at a temperature of 50-250°C if a thermo-initiator has been added to the system? In other cases where air does not affect the crosslinking of the organic resin due to the presence of a polythiol mixed with the resin,

The organic resin is applied by screen printing according to a specific circuit design, then sprinkled, powdered, or sprayed with the electrically conductive material, and then subjected to ultraviolet radiation in the presence of air by irradiating from above or below through an ultraviolet-transparent substrate, or from both sides. Furthermore, instead of using ultraviolet radiation for final crosslinking, a thermoinitiator can be mixed with the original formulation, and after adding the electrically conductive material to the surface of the uncrosslinked or partially crosslinked resin, the entire mixture can be subjected to heat to crosslink the organic resin and secure the electrically conductive material in place on its surface.

In the case of photoimaging, the nonconductive substrate is completely covered with a thin coating (up to 5 mils thick) of the organic resin mixed with the photoinitiator and possibly the thermoinitiator. The coating is then exposed to ultraviolet radiation through a slide whose image is positive with respect to the desired circuit. If the organic resin used is one that is not inhibited by air, the formation of a sticky surface over the entire exposed area is avoided. The slide is removed and the electrically conductive material is sprayed, glazed, or spread over the unexposed and therefore uncrosslinked portion of the organic resin, i.e., the desired circuit as outlined by the slide. The thus-coated substrate is then directly exposed to ultraviolet radiation in the presence of air from above, below, or both sides to cross-link the circuit pattern and secure the electrically conductive material. This last step can also be performed if a thermoinitiator is added to the system by exposing the coated substrate to heat at a temperature between 50 and 250°C for a time sufficient to cross-link the organic resin into the circuit pattern, thereby securing the electrically conductive material in place. When using the photoimaging method, where the ethylene unsaturation in the organic resin consists of an acrylic or methacrylic group, it is necessary to carry out the photoexposure step using ultraviolet radiation through a slide in an inert atmosphere, such as nitrogen. In this way, the portions exposed to ultraviolet radiation will cross-link, remaining transparent throughout their entire thickness, and will not result in a sticky surface. The unexposed portion beneath the circuit pattern on the slide, however, will remain uncross-linked. The electrically conductive material is then applied to the uncross-linked circuit pattern on the coated substrate, and the circuit pattern is subsequently re-exposed to ultraviolet radiation by irradiating it from above, below, or both sides, operating in an inert atmosphere. Furthermore, this second cross-linking stage can be performed when a thermoinitiator has been added to the system, exposing the uncross-linked organic resin, which has electrically conductive materials on it, to heat at a temperature of 50 - 250°C for a time sufficient to cross-link the uncross-linked organic resin.

In all of the above processes, loose, i.e., unfixed, particles of the electrically conductive material which, upon application, fall or settle on the substrate or the cross-linked coating on the substrate can be easily removed before the electrically conductive material is fixed in the desired circuit pattern by final cross-linking or after the final cross-linking stage has been performed. Such removal, before final cross-linking, is easily accomplished using traditional methods, for example, by brushing off excess electrically conductive material, by gently tapping the substrate at one end, or by using an air jet. After final cross-linking, in addition to the methods that can be used before final cross-linking, it is also possible to use a spray or bath of water or an aqueous detergent solution. The electrically conductive material thus removed can then be reused in the system after reprocessing to remove oxides.

The heating step using a thermoinitiator to crosslink the organic resin and secure the electrically conductive material in place is typically performed for 1–10 minutes at a temperature of 50–250°C, preferably 80–175°C, which is sufficient to fully crosslink the composition into a solid product. In addition to conventional heat sources, such as a heating stove, which can be used for the heating step to crosslink the organic resin, radiation such as infrared rays, microwaves, or radio frequencies can also be used to generate the required temperature within the organic resin to induce crosslinking.

The following examples will serve to facilitate the explanation of the present invention, but are not intended to specifically limit it. Unless otherwise indicated, all parts and percentages are by weight.

Example 1

Preparation of triacrylic urethane oligomer with diacrylic thinner_ ___

To a mixture of 30.035 parts methylenebis(cyclohexylsocyanate) containing 0.0275 parts stannous octoate and 0.506 parts triphenyl phosphite was added 15.231 parts hydroxypropyl acrylate. After 2 hours at 50°C, 0.020 parts hydroquinone monomethyl ether, 0.0275 parts stannous octoate and 24.5 parts hexanediol diacrylate were added, followed by 29.562 parts PLURACOL IP-740 (a 730 molecular weight triol obtained by reacting trimethylolpropane with propylene oxide). After 1 hour at 80°C the isocyanate content had reached zero. The resulting triacrylic urethane oligomer and diacrylic diluent will hereafter be referred to as Prepolymer A.

Example 2

An organic resin composition applicable by screen printing and photo-crosslinkable was prepared by mixing the following components:

T?BELLA I

Components Prepolymer Parts A de11*Example 1 40.4 Hexanediol diacrylate 25.9 Trimethylolpropane triacrylate 26.5 Diethylacetophenone 1.0 Benzophenone 2.0 Cab-o-Sil 4.0 Phosphorous acid 0.2

the above formulation will hereinafter be referred to as Photocomposition A?

Example 3

1164 g of commercially available redistilled diallylamine were added to a 5-liter round-bottom flask equipped with a stirrer, thermometer, Graham condenser (coil), and a dropping funnel. A stream of nitrogen was passed through the flask, and a protective nitrogen atmosphere was maintained throughout the reaction. The flask was heated to 80°C with stirring, and 140 g of an epoxy resin of the bisphenol A and diglycidyl ether type, marketed by Shell Chemical Co. under the trade name "EPON 828," was slowly added to the flask via the dropping funnel over a period of 2 hours.

while the flask was kept at a temperature of 80-90?C. Once it was complete_

ta the addition, stirring and heating are

were continued at 80 - 90?C for 4 hours. the diallyl?

Excess amine (194 g) was eliminated by

vacuum stripping at 90°C and 1-10 mm Hg.

The epoxy tetraene product, that is?*

OH OH OH

(CH2=CHCH2)2NCH2CHCH2O-// \ / \ 0CH?CHCH_I (OHgCH = CH2)2

2 2

CH

obtained and amounted to 2910 g and will come? from here on

hereinafter referred to as Prepolymer B*

Example 4

An organic resin composition applicable by screen printing and photocurable was prepared by mixing the following components:

TABLE II

Components Parts by weight

Prepolymer B of Example 3 39.4

Triallyl cyanurate 19.5

2,2-dimethoxy-2-phenylacetoph enone 2,0

Pentaerythritol tetrakis (beta-mercapto?

pr?pionate) 39.4

Benzopinacol 1.0

PC-430 (coating additive - 3M Corp.) 0.24 Triphenylphosphine- 0.20

Stabilizers - pyrogallol 0.10

phosphorous acid 0.02

The above formulation will hereinafter be referred to as Photocomposition B.

Example 5

To 10 parts of Photocomposition A in Example 2, 0.1 parts of benzo-pinacol were added. This mixture will be referred to as Photocomposition C from here on.

Example 6

Photocomposition A of Example 2 is*

was applied by screen printing in a way

to create a line circuit diagram (0.64 cm x 10.16 cm) on Itylar substrate through a nylon mesh

from 140 mesh? 11 coated substrate has been exposed

to ultraviolet radiation from a

Addalux mercury vapor lamp of the type for me?

pressure (56 mW/cm ) for 2 minutes in air, the composition thus exposed adhered to the Mylar substrate, but had a superior surface

1

partially cross-linked and sticky. Silver_

to in the form of silver flakes (-325 mesh)

It was spread on the sticky surface of the composition. The excess silver was removed.

from the substrate using a brush and the sticky composition coated with silver was

new exposed to the Addalux lamp for 2 minutes in

a nitrogen atmosphere. A fully cross-linked solid composition having a thickness of inch was obtained.

of 4.2 _0.2 mill/(l06 5 micron) on whose surface it was fixed, in an incorporated position

in composition, silver flakes. The resulting conductive silver coating weighed

had _ 0.099 S and/or a specific resistivity of 1.43x10 ohm-cm.

This example was repeated. The resulting conductive silver coating weighed 0.103 g and had a specific resistivity of

-2

1.88 x 10 ohm-cm.

Example 7

Photocomposition A of Example 2 was applied by screen printing in a line circuit pattern (0.64 cm x 10.16 cm) onto a Mylar substrate through a network of

140 mesh nylon. The coated substrate was exposed to ultraviolet radiation from a medium pressure Addalux mercury vapor lamp (56 mW/cm ) for 2 minutes in the presence of air, the exposed composition adhered

to the Mylar substrate, but had a partially cross-linked and sticky top surface.

Silver in the form of silver-coated glass beads (8fi by weight of silver^) was sprinkled onto the sticky surface of the composition. Excess silver beads were removed.

from the substrate using a brush and the sticky composition covered with silver beads was again exposed to the Addalux lamp for

2 minutes in a nitrogen atmosphere. A fully cross-linked solid composition (4.(1? 0.2 thousandths of an inch) (124+5 microns) thick) was obtained with silver fixed in place embedded in the composition on its surface. The resulting weight of the silver-coated beads was 0.078 g and the conductive coating

showed a specific resistivity of 5.6x10^ohm-cm.

Example 8

The photocomposition C of Example 5 was applied by screen printing

according to a circuit diagram (0.64 cm x 10.16 cm) on a flfylar substrate through a 140 mesh nylon mesh. The coated substrate was exposed to ultraviolet radiation from a medium-pressure Addalux mercury vapor lamp (56 mW/cm ) for 2 minutes in the presence of air. The exposed composition adhered to the MylaTyma substrate and exhibited a partially cross-linked and tacky upper surface. Silver in the form of silver-coated glass beads (8$ in

weight of silver) was sprinkled on the sticky surface of the composition. Excess silver was removed from the substrate using a brush and

the sticky composition covered with silver and*

was heated to 160?C in an air oven for 5 minutes. A solid, fully cross-linked composition was obtained (thickness 4.8 , 0.2 thousandths of

inch (122.5 microns) with silver fixed on its surface in a position embedded in the composition. The resulting weight of silver-coated beads was 0.0508 g and the conductive coating had a specific resistivity of 3

2.3 x 10 ohm-cm pussy.

This example has been repeated. The silver-coated glass beads deposited according to

10 line circuit diagrams weighed 0.035 g and the conductive coating had a specific resistivity of 1.48 x 10-^ ohm-cm.

Example 9

Photocomposition B of Example 4 was deposited on the surface of a Mylar substrate.

up to a thickness of 1 thousandth of an inch (25.4 microns).

The coating was then exposed to ultraviolet radiation from a mer vapor lamp.

Medium pressure Addalux curium (56 mW/cm ) was exposed through a slide whose image was positive with respect to a 9.0 cm x 0.4 cm line circuit pattern. The exposure was carried out for 2 minutes, resulting in a solid reticulated composition in all exposed areas. Silver in the form of silver-coated glass beads (8% weight of silver) was distributed on the unexposed portion and then

?- \

The composition was uncrosslinked and still liquid, while the excess silver beads were brushed off the crosslinked coating on the substrate. The substrate was placed in an air oven and heated to 150°C for 10 minutes. The result was a fully crosslinked solid composition with silver beads fixed in place embedded within the circuit board. The resulting weight of silver-coated beads was 0.062 g, and the conductive coating had a 2.2

resistivity? specification of 2.03 x 10 ohm-cm.

It must be well understood that to obtain the maximum characteristics of mechanical resistance, resistance to solvents, resistance to creep, heat resistance and absence of

Claims (17)

1. A process for forming a conductive coating on a substrate, comprising (1) applying an electrically conductive material to the surface of a non-crosslinked or partially crosslinked organic resin composition having a tacky surface, said composition being deposited in a circuit pattern on a substrate, and then, in that order or in reverse order, (2) crosslinking said composition to secure the electrically conductive material in place on said surface by the action of heat or ultraviolet radiation, and (3) removing any loose, i.e., unsecured, electrically conductive material. 2. Process according to claim 1, characterized in that the crosslinking by ultraviolet radiation is carried out in an inert atmosphere. 3- Process according to claim 1, characterized in that the composition contains a reactive diluent. 4. Process according to claim 1, characterized in that the electrically conductive material is in the form of a particle, sphere, powder, fibre, flake or mixture thereof. 5. A process for preparing a circuit board, comprising (l) printing onto a substrate a desired circuit pattern made of an ultraviolet-curable composition which comprises (a) an ethylenically unsaturated compound having the formula: (C?=C-C-?Bi And 0 where H is* H or CH^ and is* an organic residue with valence n and n is* 2-4? and (b) a photoinitiator; (2) 1*exposure of said composition to ultraviolet radiation for a time sufficient to obtain a solid and adherent printed circuit board with a tacky surface; (3) coating at least the sticky surface of said composition with an electrically conductive material, and then, in that or reverse order, (4) re-exposure of at least the sticky surface covered with said composition to ultraviolet radiation in a substantially oxygen-free atmosphere so as to solidify said sticky surface and form an electrically conductive circuit thereon; and (5) the removal of any electrically conductive material still loose or unfixed. 6. Process according to claim 5, characterized in that the composition contains a reactive diluent. 7 Process according to claim 5, characterized by the fact that the oxygen-free atmosphere consists of an inert gas. 8. A process according to claim 5, characterized in that the electrically conductive material is in the form of a particle, sphere, powder, fiber, flake or a mixture thereof. 9. A process for preparing a circuit board, characterised by comprising (1) printing onto a substrate a desired circuit pattern made of a composition crosslinkable to ultraviolet radiation and heat which comprises (a) an ethylenically unsaturated compound having the formula: (CH ?=C?C-0)-R4 2 , ? 'n 1 HO where E is H or CH^ and is an organic residue of valence n and n is 2-4, and (b) a photoinitiator, and (c) a thermo-initiator; (2) exposing said composition to ultraviolet radiation for a time sufficient to obtain a solid, adherent printed circuit board having a tacky surface; (3) coating at least the sticky surface of said composition with an electrically conductive material, and then, in this order or in reverse order, (4) 1* exposure of the coated composition to a temperature in the range of 50 - 250?C to cause said tacky surface to solidify and form an electrically conductive circuit thereon; and (5) the _removal- of any electrically conductive material that is still loose or not fixed. 10. Process according to claim 9* characterized in that the composition contains a reactive diluent. 11. Process according to claim 9, characterized in that the electrically conductive material is in the form of a particle, sphere, powder, fiber, flake or a mixture thereof. 12. Process according to claim 9, characterized in that the initiator is a substituted or unsubstituted pinnacle. 13. Process for producing a circuit board, characterized by comprising (1) coating a substrate with an ultraviolet-curable composition which comprises (a) a solvent-free, crosslinkable, reactive, liquid organic resin which contains at least two carbon-to-carbon olefinic double bonds per molecule and (b) a photoinitiator; (2) the exposure of said coating ultraviolet radiation through a slide bearing an image, the image of which is positive with respect to the desired circuit so as to form a cross-linked adherent coating on the substrate in the exposed areas not corresponding to the circuit; (3) coating the unexposed and therefore uncrosslinked circuit areas of the composition with an electrically conductive material, and then, in this or reverse order, (4) re-exposure of at least the uncrosslinked area to ultraviolet radiation so as to crosslink that area and fix the electrically conductive material to its surface; and (5) the removal of any electrical material? N conductive tricamefte still loose ov? \ probably not fixed. 14. A process according to claim 13, characterized in that the organic resin consists essentially of an ethylenically unsaturated compound having the formula: (CH ?C-C-O )-R 2 ? H n 1 - R 0 where R is H or CH^ and R^ is an organic residue of valence n and n is 2-4* and by the fact that exposures to ultraviolet radiation are carried out in an atmosphere substantially devoid of oxygen. 15- Process for producing a circuit board, characterised by comprising (1) coating a substrate with a composition crosslinkable to ultraviolet rays and heat which comprises (a) a solvent-free, crosslinkable, reactive, liquid organic resin which contains at least two carbon-to-carbon olefinic double bonds per molecule, (b) a photoinitiator, and (c) a thermo-initiator; (2=) 1*exposure of said coating to ultraviolet radiation through a slide bearing an image whose image is positive with respect to the desired circuit so as to form a cross-linked and adherent coating on the substrate in the exposed area not corresponding to the circuit; (3) coating the unexposed and therefore uncrosslinked circuit area of the composition with an electrically conductive material and then, in this or reverse order, 1*exposure of at least the uncrosslinked circuit area to a temperature in the range of 50-250°C in order to crosslink said area and fix the electrically conductive material on its surface; and (5) the removal of any electrically conductive material that is still loose or not fixed. 16. A process according to claim 15, characterized in that the organic resin consists essentially of an ethylenically unsaturated compound having the formula: (CH =C?C?0) -R 2 , n n 1 ? R 0 where R is H or CH^ and R^ is an organic residue of valence n and n is 2-4? and by the fact that exposure to ultraviolet radiation is carried out in an atmosphere substantially devoid of oxygen. 17. Process according to claim 1, characterized in that the organic resin present in the organic resin composition is a member of the ethylenically unsaturated group consisting of (a) an ethylenically unsaturated liquid monomer, oligomer, or prepolymer having the formula: (CH ?C?C?0-)?R 2 t n 1 - R where R,e1 H or CH^, R^ is* an organic residue and n is? 2-4, (b) a polythiol in combination with the above mentioned (a), And (c) a polythiol in combination with a liquid, ethylenically unsaturated monomer, oligomer, or prepolymer having the formula! (OH2=C?CH2)5E3 ? ? ? where Rg is H or CHy R^ is an organic residue and n is at least 2, as well as mixtures thereof \ I ? I i i ? l I