GB2089819A - Conformal coating curable by combination of radiation and heat - Google Patents

Abstract

A process for conformally coating an irregularly shaped article, e.g. an electronic circuit board with components attached, comprises coating the article, e.g., by dipping or spraying with a composition comprising (1) a solventless, radiation and heat curable, reactive, liquid resin containing at least two olefinic carbon- to-carbon double bonds per molecule; (2) a thermal initiator for (1); and optionally; (3) a photoinitiator for (1); and exposing said coated article to radiation and heating the article to a temperature in the range 50 to 250 DEG C for a time sufficient to obtain a completely cured solid coating. The radiation exposure is preferably effected before the heating. If high energy ionizing radiation is used, no photoinitiator is necessary in the composition. Preferred compositions are UV-curable and contain a photoinitiator. r

Description

SPECIFICATION Conformal coating curable by combination of UV radiation and heat This invention is directed to a process for conformally coating an irregularly shaped article with a curable composition which is both radiation and heat activated. More particularly, this invention relates to a process for conformally coating an article which comprises coating the article with a UV and heat curable composition and, thereafter, subjecting the thus coated article to UV radiation and heat.

Background of the invention UV curable, allylic terminated polyene/ polythiol compositions are well known from U.S. 3,661,744; U. S. 3,697,402; U. S.

3,697,395 and 3,697,396. UV curable, acrylic terminated polyene/polythiol compositions and methods of UV curing are also known from U. S. 3,898,349 and U. S.

4,008,341. Photocuring of acrylates, per se, are also known from U. S. 2,760,863. Chemical free radical heat curing of allylic terminated polyenes with polythiols is also known from U. S. 3,662,023; U. S. 3,697,621; U.S. 3,699,084; U. S. 3,714,290 and U. S.

3,653,959. British 1,490,372 teaches a chemical free radical heat cure of an acrylate/polythiol system.

It is also known from U. S. 4,020,233 to form cured polythioethers by admixing a composition comprising an ethylenically unsaturated compound containing at least 2 unsaturated carbon-to-carbons per molecule, a polythiol containing at least 2 thiol groups per molecule, a photoinitiator and a catalytic amount of a pinacol and, thereafter, subjecting the admixture to UV radiation and heat. It is further known to partially polymerize acrylates in the presence of a pinacol from U. S.

3,378,533. Further, in copending application having Serial No. 177,658, filed August 13, 1980, (equivalent to Australian Application No 73509/81), there are taught radiation and heat activated compositions comprising liquid di- and polyacrylic terminated materials in combination with a photoinitiator and a catalytic amount of a substituted or unsubstituted pinacol and processes of curing same.

U. S. 4,171,252 teaches photopolymerization of a polymerizable mass in the presence of a photoinitiator, an organic peroxygen containing compound and naphthalene or substituted naphthalene.

U. S. 4,222,835 teaches that a composition comprising a liquid vinyl monomer, a photoinitiator, a thermal initiator and an accelerator on exposure to UV light initiates polymerization in the surface layer of the composition thereby generating sufficient heat to activate the thermal initiator and cause polymerization to proceed throughout the bulk of the monomer.

In present day methods, conformal coating materials are applied to circuit board assemblies to protect them from moisture during use. Conventional conformal coatings (which are based on silicones, epoxy resins, urethanes or acrylates) are either solvent-based or 2-component or both. Typical disadvantages of these present day systems involve solvent flammability, short pot life, long curing time and unreliable coating coverage.

Additionally, in certain systems in which UV light cannot penetrate because of geometrical constraints causing shadow areas or in systems rendered partially UV opaque due to fillers such as coatings on electronic circuit boards with components attached, it is desirable to have a system which is dual UV/heat cured. By use of a dual cure, the part of the composition exposed to the UV light can be radiation cured, thereby immobilizing the entire coating, and the part of the composition in the shadow areas can be subsequently heat cured.

The present invention provides a process for applying a solventless conformal coating which can be cured by exposure to UV radiation followed by the application of heat to produce a cured conformal coating.

The present invention is directed to a process for conformally coating an irregularly shaped article, e.g., an electronic circuit board with components attached which comprises coating the article, e.g., by dipping or spraying with a composition comprising (1) a solventless, radiation and heat curable, reactive, liquid resin containing at least two olefinic carbon-to-carbon double bonds per molecule; (2) a thermal initiator for (1); and optionally (3) a photoinitiator for (1); exposing the thus coated article to high energy ionizing radiation or actinic radiation and heating the article to a temperature in the range 50 to 250"C for a time sufficient to obtain a cured solid coating. If high energy ionizing radiation is used, no photoinitiator is necessary in the composition.The following description relates primarily to the preferred compositions which are UV curable and contain a photoinitiator.

The solventless UV and heat curable reactive liquid resins operable 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 of the formula:

wherein R is H or CH3, R2 is an organic moiety and n is at least 2, (b) a polythiol in combination with (a) supra, (c) a polythiol in combinatiin with a liquid ethylenically unsaturated monomer, oligomer or prepolymer of the formula:

wherein R2 is H or CH3, R3 is an organic moiety and n is at least 2.

Although the aforesaid compositions, per se, are operable herein to form useful products, they may also be used in combination with conventional copolymerizable monomeric compounds or reactive diluents. The admixture of the composition of the instant invention with other monomers is employed usually to control viscosity and other application variables such as rate of cure as well as final film or coating properties such as hardness and flexibility. These reactive diluents cocure with the ethylenically unsaturated group member on exposure to UV radiation and heat.Examples of conventional copolymenzable compounds useful as reactive diluents include, but are not limited to, monofunctional acrylic esters, monofunctional methacrylic esters, styrene, vinyl-toluene, acrylonitrile, methacrylonitrile, vinyl acetate, vinyl pyrrolidone, vinyl chloride, vinylidene chloride, butadiene, isoprene, chloroprene, divinyl benzene, di(vinylphenyl) carbonate, diallyl phthalate, diallyl carbonate, di-(allyphenyl) carbonate, diallyl furmarate, triallyl isocyanurate, triallyl cyanurate, diallyl chlorendate, diallyl maleate and unsaturated polyesters and mixtures thereof.

By the term unsaturated polyesters herein is meant the usual polycondensation products which consist of ester-like linked residues of polyvalent, especially divalent, alcohols, as well as possibly also residues of monovalent alcohols and /or of monovalent carboxylic acids, whereby 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, adipic acid, phthalic acid, tetrachlorophthalic acid, hexachloroendomethylenetetrahydrophthalic acid, trimellitic acid, benzoic acid, linseed oil fatty acid and ricinoleic fatty acid and mixtures thereof. Examples of alcohols include ethylene glycol, diethylene glycol, propane, butane and hexane diols, trimethylolpropane, pentaerythritol, butanol and tetrahydrofurfuryl alcohol.

The reactive diluents can be added to the system in amounts ranging up to 90% by weight of the ethylenically unsaturated group member, preferably 20 to 50% by weight on the same basis.

The thermal initiators used herein are selected from substituted or unsubstituted pinacols and organic peroxides. Azo initiators such as azobisisobutyronitrile are also operable as thermal initiators but, since nitrogen is evolved, the resulting covering or coating will contain bubbles or be in the form of a foam.

The organic peroxides operable are of the general formula: R-O-O-(R,-O-O),-R wherein n = O or 1, R is independently selected from hydrogen, aryl, alkyl, aryl carbonyl, alkaryl carbonyl, aralkyl carbonyl and alkyl carbonyl and R1 is alkyl or aryl, said alkyl groups containing 1 to 20 carbon atoms.

Examples of operable organic peroxides include, but are not limited to 2,5-dimethyl-2,5di (t-butylperoxy)hexane, 1 , 3-bis(t-butyl peroxy- isopropyl)benzene, 1,3-bis (cumylperoxyisopropyl) benzene, 2,4-dichlorobenzoyl peroxide, caprylyl peroxide, lauroyl peroxide, t-butyl peroxyisobutyrate, benzoyl peroxide, p-chlorobenzoyl peroxide, hydroxyheptyl peroxide, dit-butyl diperphthalate, t-butyl peracetate, tbutyl perbenzoate, dicumyl peroxide and di-tbutyl peroxide.

The organic peroxide is added to the composition in an amount ranging from 0.01-10%, preferably 0.1-5%, by weight based on the weight of the ethylenically unsaturated group member.

The substituted or unsubstituted pinacols operable herein as a thermal initiator have the general formula:

wherein R, and R3 are the same or different substituted or unsubstituted aromatic radicals, R2 and R4 are substituted or unsubstituted aliphatic or aromatic radicals and X and Y which may be the same or different are hydroxyl, alkoxy or aryloxy.

Preferred pinacols are those .wherein Rt, R2, R3 and R4 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'-diiodobenzopinacol, 4,4', 4",4"'-tetrachlorobenzopinacol, 2,4-2',4'-tetrachlorobenzopinacol, 4,4'-dimethylbenzopinacol, 3,3'-dimethylbenzopinacol 2,2'-dimethylbenzopinacol, 3,4-3',4'-tetramethylbenzopina- col, 4,4'-dimethoxybenzopinacol,4,4',4",4"'- tetramethoxybenzopinacol, 4,4'-diphenylbenzopinacol, 4, 4'-dichloro-4",4"'-dimethylben- zopinacol, 4,4'-dimethyl-4",4t"-diphenylben- zopinacol, xanthonpinacol, fluorenonepinacol, acetophenonepinacol, 4,4'-dimethylacetophe none-pinacol, 4,4'-dichloroacetophenonepina col, 1,1,2-triphenyl-propane-1,2-diol, 1 ,2,3,4- tetraphenyl-butane-2, 3-diol, 1,2-diphenylcy clobutane-1,2-diol, propiophenone-pinacol, 4,4'-dimethylpropiophenone-pinacol, 2,2' ethyl-3, 3'-dimethoxypropiophenone-pinacol, 1 , 1 , 1 ,4,4,4-hexafluoro-2,3-diphenyl-butane- 2, 3-diol.

As further compounds according to the pre sent invention, there may be mentioned: benzopinacol-mono methylether, benzopinacol mono-phenylether, benzopinacol and monoiso propyl ether, benzopinacol monoisobutyl ether, benzopinacol mono (diethoxy methyl) ether and the like.

The pinacol is added to the composition in amounts ranging from 0.01-10%, preferably 0.1 -5 h, by weight based on the weight of the ethylenically unsaturated group member.

The thermal initiator can be added to the system in various ways. That is, the thermal initiator, per se, can be admixed with the ethylenically unsaturated group member. Additionally, it can be admixed with a photoinitiator and added to the ethylenically unsatu rated group member. Furthermore, the ther mal initiator can be dissolved or suspended in well known commercially available solvents such as dibutyl phthalate; ketones, e. g., acetone and methylethyl ketone or chlorinated hydrocarbons such as methylene chloride, and then added to the system.

In practicing the instant invention it is sometimes desirable to add a polythiol to the composition prior to curing. This is especially true when the ethylenic unsaturation is an allylic group. In this case, during the curing step, the polythiol adds across the double bond of the allylic group resulting in solid cured materials in a commercially acceptable time period. In the instance where the ethylenic unsaturation in the polyene is an acrylic or methacrylic group, the addition of a polythiol to the system precludes the occurrence of a tacky surface due to air inhibition of the curing.

As used herein, the term polythiols refers to simple or complex organic compounds having a multiplicity of pendant or terminally positioned -SH functional groups per average molecule.

On the average the polythiols must contain 2 or more -SH groups/molecule and usually have a viscosity range of slightly above 0 to 20 million centipoises (cps) at 70"C, as measured 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 viscosity range set out above at 70"C. Operable polythiols in the instant invention usually have molecular weights in the range 94-20,000, preferably 100-10,000.

The polythiols operable in the instant invention can be exemplified by the general formula: R8-(SH)" where n is at least 2 and R8 is a polyvalent organic moiety free from reactive carbon-to-carbon unsaturation. Thus, R8 may contain cyclic groupings and minor amounts of hetero atoms such as N, S. P or 0 but primarily contains carbon-hydrogen, carbonoxygen or silicon-oxygen containing chain linkages free of any reactive carbon-to-carbon unsaturation.

One class of polythiols operable with polyenes in the instant invention to obtain essentially odorless polythio-ether products are esters of thiol-containing acids of the general formula HS-Rg-COOH, where Rg is an organic moiety containing no "reactive" carbonto-carbon unsatu ration, with polyhydroxy compounds of the general structure R10(OH)n where Rlo is an organic moiety containing no "reactive" carbon-to-carbon unsaturation and n is 2 or greater. These components will react under suitable conditions to give a polythiol having the general structure

wherein R9 and R10 are organic moieties containing no "reactive" carbon-to-carbon unsaturation and n is 2 or greater.

Certain polythiols such as the aliphatic monomeric polythiols (ethane dithiol, hexamethylene dithiol, decamethylene dithiol, tolylene2,4-dithiol, etc.) and some polymeric polythiols such as a thiol-terminated ethylcyclohexyl dimercaptan poiymer, etc., and similar polythiols which are conveniently and ordinarily synthesized on a commercial basis, although having obnoxious odors, are operable in this invention but many of the end products are not widely accepted from a practical, commercial point of view.Examples of the polythiol compounds preferred for this invention because of their relatively low odor level include, but are not limited to, esters of thioglycolic acid (HS-CH2COOH), alpha-mercaptopropionic acid (HS-CH(CH3)-COOH) and betamercaptopropionic acid (HS-CH2 CH2COCH) with polyhydroxy compounds such as glycols, triols, tetraols, pentaols, hexaols, etc. Specific examples of the preferred polythiols include, but are not limited to, ethylene glycol bis (thioglycolate), ethylene glycol bis (beta-mercaptopropionate, trimethylolpropane tris (thioglycolate), trimethylolpropane tris (beta-mercaptopropionate), pentaerythritol tetrakis (thioglycolate) and pentaerythritol tetrakis (beta-mercaptopropionate), all of which 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., Pluracoi P2010, Wyandotte Chemical Corp.) and beta-mercaptopropionic acid by esterification.

Additionally, polythiols operable herein to give cured solid polythioether products with the polyene in the presence of a free radical generator include the mercaptoester derivatives of styrene-allyl alcohol copolymers set out in U. S. Patent No. 3,904,499 and the isocyanurate containing polythiols disclosed in U. S. Patent No. 3,676,440 and liquid thiolterminated polymers made in accord with U.

S. Patent No. 3,258,495, all incorporated hereby by reference. An example of the aforesaid latter type liquid thiol-terminated polymer is CAPCURE 3-800, commercially available from Diamond Shamrock Chemical Company.

The preferred polythiol compounds are characterized by a low ievel of mercaptan-like odor initially and, after reaction, give essentially odorless polythioether end products which are commercially attractive and practically useful resins or elastomers for both indoor and outdoor applications.

In the case of a polythiol in combination with allylic polyenes the mole ratio of thiol/ ene groups for preparing the curable composition is from 0.2/1 to about 2/1 and desirably about 0.75/1 to about 1.5/1 group ratio.

In the case of a polythiol in combination with acrylic components the mole ratio of thiol/ene groups for preparing the curable composition is from 0.01 /1 to about 1/1 and desirably about 0.02/1 to about 0.5/1 group ratio.

Prior to curing, the polyene and polythiol components are admixed in a suitable manner so as to form a homogeneous liquid curable mixture. Thus, the polyene and polythiol reactants can be admixed without the necessity of using a solvent at room temperature or slightly elevated temperatures up to about 40iC when one of the components is a solid or, if desired, the reactants may be dissolved in a suitable solvent and, thereafter, the solvent can be removed by suitable means such as evaporation.

It should be understood that in order to obtain the maximum strength, solvent resistance, creep resistance, heat resistance and freedom from tackiness, the reaction components consisting of the reactive liquid resin containing at least two olefinic carbon-to-carbon double bonds per molecule (i. e., polyene) and polythiol of this invention are formulated in such a manner as to give solid, crosslinked, three dimensional network polythioether polymer systems on curing. In order to achieve such infinite network formation, the individual polyenes and polythiols must have a functionality of at least 2 and the sum of the functionalities of the polyene and polythiol components must always be greater than 4.

Blends and mixtures of the polyenes and the polythiols containing said functionality are also operable herein.

The compositions of the present invention may, if desired, include such additives as antioxidants, inhibitors, fillers, antistatic agents, flame-retardant agents, thickeners, thixotropic agents, surface-active agents, viscosity modifiers, plasticizers, tackifiers and the like within the scope of this invention. Such additives are usually preblended with the ethylenically unsaturated compound prior to or during the compounding step. Operable fillers 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 aforesaid additives may be present in quantities up to 500 parts or more per 100 parts of the ethylenically unsaturated compound by weight and preferably about 0.005 to about 300 parts on the same basis.

Additionally, conventional UV stabilizers and antioxidants such as hydroquinone, tertbutyl hydroquinone, tert-butyl catechol, p-benzoquinone, 2,5-diphenylbenzoquinone, 2, 6-di- tert-butyl-p-cresol, benzotriazoles such as Tinuvin-P (manufactured by Geigy Corp.), hydroxybenzophenones, such as 2,4-hydroxy-benzophenone, 2-hydroxy-4-methoxy-benzophenone, 4-dodecyl-2-hydroxybenzophenone, substituted acrylonitriles such as ethyl-2cyano-2,3-diphenyl acrylate, 2-ethylhexyl-2cyano-3,3-diphenyl acrylate, etc., are added to the system.

In practicing the instant invention the composition is preferably subjected to UV radiation followed by heat activation. Reversing the curing steps is also operational but generally less desirable. It preferably is necessary to add photoinitiators in order to initiate the UV reaction. One class of photoinitiators are the aldehyde and ketone carbonyl compounds having at least one aromatic nucleus attached directly to the

group.Various photoinitiators include, but are not limited to, benzophenone, acetophenone, o-methoxy-benzophenone, acenapthene-quinone, methyl ethyl ketone, valerophenone, hexanophenone, alpha-phenylbutyrophenone, p-morpholinopropionphenone, dibenzosuberone, 4-morpholino-benzophenone, 4'-morpholinodeoxybenzoin, p-diacetyl-benzene, 4-amino- benzophenone,4'-methoxyacetophenone, benzaldehyde, alpha-tetralone, 9-acetylphpan- threne, 2-acetylphenanthrene, 1 0-thioxanthe- none, 3-acetyl-phenanthrene, 3-acetylindone, 9-fluorenone, 1-indanone, 1,3,5-triacetylbenzene, thioxanthen-9-one, xanthrene-9-one,7-H benz[de]anthracen-7-one, 1 -naphthaldehy- de, 4,4'-bis (dimethylamino) benzophinone, fluorene-9-one, 1 '-acetonaphthone, 2'-acetonaphthone, 2, 3-butanedione, triphenylphos phine, tri-o-tolyl phosphine, acetonaphthone, 2,3-butanedione, benz[a]anthracene 7.12 dione, etc. Another class of photoinitiators is the benzoin ethers, such as benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin isobutyl ether and 2,2-dimethoxy-2-phenylacetophenone. A third class of photoinitiators is exemplified by benzil dimethyl ketal. The photoinitiator or mixtures thereof are usually added in an amount ranging from 0.005 to 30% by weight of the ethylenically unsaturated compound.

A type of radiation useful herein for curing is ultraviolet light and other forms of actinic radiation which are normally found in radiation emitted from the sun or from artificial sources such as Type RS Sun lamps, carbon arc lamps, xenon arc lamps, mercury vapor lamps, tungsten halide lamps and the like.

Ultraviolet radiation may be used most efficiently if the photocurable composition contains a suitable photoinitiator. Curing periods may be adjusted to be very short and hence commercially economical by proper choice of ultraviolet source, photoinitiator and concentration thereof, temperature and molecular weight and reactive group functionality of the ethylenically unsaturated group member. Curing periods of about 1-80 seconds duration are possible, especially in film applications such as desired for example, in conformal coatings.

When UV radiation is used, an intensity of 0.0004 to 60.0 watts/cm2 in the 200-400 nanometer region is usually employed.

The heating step to cure requires heating the reactants to at least 50"C, preferably in the range 80"-250"C, in order to form the cured product.

In practicing the instant invention the dual UV/heat curable composition is admixed in any order and applied to the substrate by various conventional means such as spraying or dipping. Following application the composition is exposed to UV radiation for a short period of time, e. g., about 1 second to 1 minute, and, thereafter, exposed to heat by, e. g., residence in or passage through a conventional heating oven for a time sufficient to fully cure the composition to a solid product. In addition to conventional heat sources, one can also use radiation such as infrared, microwave or radio frequency which are capable of generating the required temperature within the coating.

The following examples will aid to explain, but specifically not limit, the instant invention.

Unless otherwise noted, all parts and percentages are by weight.

In the automobile industry, circuits for under the hood use must survive extremes and resist attack by antifreeze mixtures. To predict such performance, the thermal cycling test and an ethylene glycol resistance test are used.

In the thermal cycling test, 2" X 2" X 0.010" Mylar circuit boards with copper circuitry but without attached components are dipped into the same dual UV/heat curable compositions of the instant invention. The coatings are cured by UV radiation for 20 seconds followed by heating at 120"C for 60 minutes. The boards are subjected to thermal cycling by placing them in a - 25"C freezer for one hour followed by immediate transfer to a 120"C oven for 1 hour. After 20 such cycles the deterioration, is any, is visibly noted.

In the ethylene glycol resistance test boards of the same dimension as used in the thermal cycling test are coated and cured as in the thermal cycling test. The boards are placed in a 100"C oven for 1 hour, removed and immediately immersed in a 70/30 ethylene glycol/water solution at ambient temperatures for 1 hour. The boards are removed, rinsed, dried and inspected. Deterioration, if any, is visibly noted after 12 such cycles.

Exam pie 1 To 231 g of triallyl cyanurate were added 30 g of Irgacure-651, a photoinitiator commercially available from Ciba-Geigy, 5 g of pulverized benzopinacol, 0.5 g of pyrogallol, 0.2 g of phosphorous acid, 2 g of triphenylphosphine and 10 g of triphenyl phosphite and the additives were dissolved by mixing at 35"C for 2 hours. To the mixture was added 769 g of CAPCURE-3-800, a polythiol commercially available from Diamond Shamrock, and 2.5 g of Schoenmann #2180 odor mask.

The formulation had a viscosity of about 2,700 centipoises. Into the above formulation was dipped an electronic circuit board, approximately 4 X 4" (10 X 10 cm) which had attached resistors, diodes and other components. The total surface area of the board and components was estimated to be about 48 in.2 (310 cm2). After dipping, the board was immediately rotated by hand under a 60 watt/in. medium pressure mercury lamp in a parabolic reflector at a distance of approximately 30 cm from the lamp for a total of 40 seconds. The coating was cured in place on the board, but areas under components shaded from the UV illumination were still uncured. The board was then heated at 110"C for 60 minutes to complete the cure of these shaded areas. Total coating weight was 41 g.Another board was given an initial 18 g coating by allowing it to drip for a few seconds before UV curing, then given a second 10 g coating followed by UV curing and heating at 110 C for 60 minutes. For the thermal cycling test the same formulation was coated and cured onto three 2 X 2 x 0.10 in.

Mylar circuit boards with copper circuitry but without attached components. Each board had a coating weight of 4 to 6 g. After 20 cycles, no deterioration in the coating was observed.

For the ethylene glycol resistance test, 3 boards were coated and cured as for the thermal cycling test. After 1 2 cycles, no deterioration in the coatings was observed.

Example 2 Preparation of urethane triacrylate oligomer with diacrvlate diluent.

To a mixture of 341.25 g (1.27 moles) of methylene-bis (cyclohexylisocyanate) containing 0.315 g of stannous octoate and 5.625 g of triphenyl phosphite was added 170.40 g (1.31 moles) of hydroxypropyl acrylate. After 2 hours at 50 C, 0.225 g of hydroquinone monomethyl ether, 0.315 g of stannous octoate and 280.05 g of hexanediol diacrylate were added, followed by 324.75 g (0.44 moles) of PLURACOL TP-740 (a triol of MW 730 made by reacting trimethylpropane with propylene oxide). After 1 hour at 80"C the isocyanate content had reached zero, and 0.300 g of hydroquinone monomethyl ether, 0.375 g of pyrogallol, 0.195 g of phosphorous acid and 376.20 g of hexanediol diacrylate were added. The resulting product contains approximately 56% of a urethane triacrylate oligomer and 44% of hexanediol diacrylate.

Example 3 1 60 g of the urethane triacrylate/hexanediol diacrylate product of Example 2 were mixed with 40 g of CAPCURE 3-800, 4.0 g of benzopinacol, 2.0 g of 2,2-dimethoxy-2phenylacetophenone, commercially available from Ciba-Geigy under the tradename IRGA CURE-651 and 2.0 g of triphenyl phosphite.

The resulting formulation had a viscosity of 2.300 cp. Portions of this formulation could be cured by UV (10mW/cm2) in less than 10 seconds or by 30 minutes heating at 120"C.

Circuit boards were dip-coated, exposed to UV (10mW/cm2 for 20 seconds, then heated for 1 hour at 1 20'C and subjected to thermal cycling between - 25 and + 1 20"C. After 20 cycles there was no cracking of the coating. Similarly coated and cured boards were subjected to 70% ethylene glycol and after 1 2 cycles showed no deterioration.

Example 4 To 25 g of the urethane triacrylate/hexanediol diacrylate product of Example 2 was added 0.125 g of benzoyl peroxide, 0.025 g of hydroquinone monomethyl ether and 0.25 g of IRGACURE-651. The resulting formulation had a viscosity of 2,900 cp. Portions of this formulation could be cured by UV (1 OmW/cm2) in less than 10 seconds or by 5 minutes at 110"C.

Example 5 To 50 g of the urethane triacrylate/hexandiol diacrylate product of Example 2 was added 0.25 g of 90% t-butyl hydroperoxide, 0.05 g of hydroquinone monomethyl ether and 0.5 g of IRGACURE-651. The resulting formulation had a viscosity of 2,800 cp. Portions of this formulation could be cured by UV (10mW/cm2) in less than 10 seconds or by 30 minutes heating at 1 80"C.

Claims (8)

1. A process for conformally coating an irregularly shaped article which comprises coating the article with a composition comprising (1) a solventless, radiation and heat curable, reactive, liquid resin containing at least two olefinic carbon-to-carbon double bonds per molecule; (2) a thermal initiator for (1); and optionally (3) a photoinitiator for (1); exposing said coated article to high energy ionizing radiation or actinic radiation and heating the article to a temperature in the range 50 to 250"C for a time sufficient to obtain a completely cured solid coating.

2. The process according to Claim 1 wherein the thermal initiator is an organic peroxide of the general formula: R-O-O-(R, -O-O),-R wherein n = O or 1, R is independently selected from hydrogen, aryl, alkyl, aryl carbonyl, alkaryl carbonyl, aralkyl carbonyl and alkyl carbonyl and R1 is alkyl or aryl, said alkyl groups containing 1 to 20 carbon atoms.

3. The process according to Claim 1 wherein the thermal initiator is a substituted or unsubstituted pinacol having the general formula:

wherein R1 and R3 are the same or different substituted or unsubstituted aromatic radicals, R2 and R4 are substituted or unsubstituted aliphatic or aromatic radicals and X and Y which may be the same or different are hydroxyl, alkoxy or aryloxy.

4. A process according to any one of claims 1 to 3 wherein the composition contains the photoinitiator and is exposed to UV radiation.

5. A process according to any of claims 1 to 4 wherein the said liquid resin comprises (a) a liquid ethylenically unsaturated monomer, oligomer or prepolymer of the formula:

wherein R is H or CH3, R1 is an organic moiety and n is at least 2, (b) a polythiol in combination with (a) supra, or (c) a polythiol in combination with a liquid ethylenically unsaturated monomer, oligomer (r prepolymer of the formula:

wherein R2 is H or CH3, R3 is an organic moiety and n is at least 2.

6. A process according to claim 5 wherein the composition also contains a reactive diluent.

7. A process according to claim 1 substantially as described in any one of Examples 1, 3,4 or 5.

8. Irregularly shaped articles provided with coatings by the process of the preceding claims.