CA1238443A

CA1238443A - Conformal coating systems

Info

Publication number: CA1238443A
Application number: CA000439413A
Authority: CA
Inventors: Larry A. Nativi; Kris Kadziela
Original assignee: Henkel Loctite Corp
Current assignee: Henkel Loctite Corp
Priority date: 1982-11-12
Filing date: 1983-10-20
Publication date: 1988-06-21
Also published as: US4451523A

Abstract

ABSTRACT OF THE DISCLOSURE

Ultraviolet curable polymerizable coating system having a built-in secondary curing mechanism is disclosed. The polymerizable coating system is a one component system comprising at least one urethane-acrylate or methacrylate, an allyl-group containing acrylate or methacrylate monomer, a non-allylic acrylate or methacrylate, a polymerization initiator of the photo-initiator type, and a metal-drier. The coating system is uv curable, and also possesses an additional cure mechanism.

Description

of This invention concerns polymeri~able compositions which have their primary utility in the area of coating and sealants. The invention has particular utility in -the area of nonformal coatings for printed circuit boards and electronic components and will be described in connection with such utility, although other uses are contemplated.
Printed circuit boards and electronic components may be coated with a protective film -to avoid or at least I minimize degradation in electrical performance due to contamination. The most damaging, and usually the most prevalent contaminant generally is considered to be moisture or humidity. Excessive moisture or humidity will drastically lower insulation resistance between 15 conductors, accelerate high voltage breakdown and dendritic growth, and corrode conductors. Other contaminants which may damage printed circuit boards include various chemicals which may be residues of the manufacturing process such as fluxes, organic solvents, 20 release agents, metal particles and marking inks, and contaminants which inadvertently may be deposited by human handling such as body greases, fingerprints, cosmetics and food stains. Ambient operating condition may also contribute a variety of contaminants such as 25 salt spray, dirt and dust oil, fuel, acid corrosive vapor an fungus. Although the list of possible contaminants is almost endless, in all but the most severe cases, their destrLIctive action effectively can be eliminated by provision of a good nonformal coating.
In adulation to providing protection prom contaminants, nonformal coatings also provide a certain degree of protection to mechanical shock, vibration and tampering.
Various nonformal coating systems are known in top 35 art and are available commercially. Each has its advantages and disadvan~agesO One such prior art nonformal coating system is based on acrylics. Acrylics are excellent coating systems from a production ,-standpoint buckles they readily may be applied by spraying or brushing. Acrylic coatings have desirable electrical and physical properties and are fungus resistant. Additional advantages of acrylic coatings include long life, low or no ex~therm during cure, and little or no shrinkage during cure. However, acrylic 10 coatings typically are formed by solvent evaporation and reach optimum physical characteristics during cure typically in a matter of minutes. Conventional acrylic coatings are soluble in chlorinated solvent such as tricholorethane or methyl en chloride.
Another prior art nonformal coating system is based on polyurethane. Polyurethane are available as either single or two-component systems. Polyurethane coatings offer excellent humidity and chemical resistance and good dielectric properties. Single-component urethanes 20 are relatively easy to apply and exhibit relatively long working pot live. However, single component polyurethane typically require a curing time of three to ten days at room temperature to reach optimum physical characteristics. Two component polyurethane 25 typically achieve optimum cure at elevated temperatures within one to three hour, but exhibit relatively short working pot life. Also, surface preparation of substrate boards prior to application of polyurethane based coatings is important, since even minute 30 quantities of moisture on a substrate board could produce blistering under humid conditions Blisters, in turn, may lead to electrical failures and mandate costly rework. Polyurethane coatings are insoluble in most common solvents, which is a drawback to rework. Thus, 35 replacement of a component on a polyurethane coated board requires a corrosive stripper to remove effectively all traces ox the polyurethane film.
Ilowever, extreme Cockney must be exercised when such a stripper is used, because the stripper also may corrode 5 metallic surfaces on the board.
Epoxy resin systems also have been employed by the prior art for nonformal coating printed circuit boards.
Epoxy resins are available as two component systems only. Epoxy resin coatings provide Cody humidity 10 resistance and high abrasive and chemical resistance.
However, epoxy resins are virtually impossible to remove chemically for rework because any stripper that will attaclc the coating also will attack the epoxy coating of potted electronic components and the epo~y-glass of the 15 printed circuit board as well. Thus, the only effective way to repair an epoxy resin coated board is to burn through the epoxy coating with a hot knife or soldering iron. However, burning introduces a cosmetic defect which is unacceptable to many consumers Moreover, 20 epoxy resins shrink somewhat during cure Accordingly, a buffet material must be placed around fragile electronic components to prevent fracturing from shrinkage. Curing of epoxy systems can be accomplished in one to three hours at elevated temperature, or four 25 to seven days at room temperature. Epoxy resins exhibit a relatively short workinc3 pot life which it an additional disadvantage.
Silicone resins also have been employer by the prior art for nonformal coating circuit board. Silicone I resin coatings provide high humidity and corrosion resistance along with high temperature resistance which makes silicone resins preferred for coating printed circuit assemblies that contain high heat-dissipating components such as power resistors. However, silicone 35 resins ore relatively thiclc making them somewhat _~3~3 difficult to apply Moreover, silicone resins require a relatively Len cure, and repairability is defoliate since silicone resins once cured are essentially insoluble, and cannot be vaporized with the heat of a soldering iron. Accordingly, the only effective way to repair a silicone resin coated circuit board is to mechanically remove the coating The prior art also has employed polyamides for nonformal coating circuit boards. Polyamide coatings 10 provide high-temperature, moisture and chemical resistance over extended periods of time. However, polyamide coatings require high temperature cure (one to three hours at 200 to 250C) which could damage heat sensitive electronic components, and this requirement 15 severely limits the use of polyamide coating systems on most printed circuit board assemblies Also, since polyamides are high temperature, moisture and chemical resistant, the only effective way to repair a polyamide coated board is to mechanically remove the coating.
Delilah phthalate varnishes also have been proposed by the prior art for nonformal coating circuit boards and provide excellent high temperature and chemical resistance. Elowever, Delilah phthalate varnishes require high temperature cure (approximately 150C) 25 which severely limits their use. And, Delilah phthalate coatings also typically must be mechanically removed to permit repair.
The proceeding discussion of background in the art of nonformal coating circuit boards is taken largely 30 from Printed Circuits Handbook, Clyde F. Combs, Jar . .
Editor, McGraw Hill Book Company, Second Edition (1979).
As thus is apparent from the above discussion, none of the currently available nonformal coating systems are completely satisfactory since all prior art nonformal 35 coating systems at best are a tradeoff of preferred . ,, I

application characteristics (i.e., processing speed, ease of application, pot life and cure conditions), preferred physical characteristics, (i.e., electrical properties, temperature, chemical and moisture 5 resistance), and repairability.
It is thus a primary object of the present invention to provide an improved nonformal coating system, i.e.
method and materials which overcomes the aforesaid and other diqadvantag~ of the prior art.
Other objects of the present invention are to provide an improved composition for con~ormal coating printed circuit board assemblies and the like which compositions demonstrate extended pot fife and ease of application, rapid dry-to-touch cure, good electrical 15 properties, superior adhesion and abrasion resistance, excellent high temperature, chemical and moisture resistance, and ease of repairability. Yet other objects will in part appear obvious and will in part appear hereafter.
the invention accordingly comprises the processes involving the several steps and relative order of one or more such steps with respect to each other, and the devices, material and compositions possessing the features, properties and relations of elements which are 25 exemplified in the following disclosure and scope ox i application of which will be indicated in the claims.
SUMMARY OF THE INVENTION
. _ Generally, the Foregoing and other objects of the invention are achieved by the provision of an 30 ultraviolet hereinafter "us") curable polymerizable coating system having a secondary cross-linking curing mechanism built in. More specifically, in accordance with one aspect of the invention there is provided a solvent-free one-part coaxing composition comprising at 35 least one urethane-acrylate or methacrylate, an acrylate or methacrylate compound containing an allylic group, a non-allylic acrylate or methac~ylate Dylan, a polymerization initiator of the photo-initiator type, and a metal drier.
In another aspect of the invention there it provided a method of forming a us curing polymerizable nonformal coating on a substrate wherein said substrate has one or more shadow areas, comprising the steps in sequence of:
(1) providing a polymerizable composition which 10 comprises at least one urethane-acrylate or urethane-methacrylate compound, an allylic-group containing acrylate or methacrylate; a non-allylic acrylate or methacrylate delineate, a polymerization initiator of the photo-initiator type, and a metal-drier;

(2) coating the substrate at least in part with said polymerizable composition;
I exposing the coated substrate to us light of wave length and intensity to effect us cure of exposed areas of the coating; and I storing the coated substrate under conditions of temperature and humitidy which promote curing of unexposed (shadow) areas of the coating through a built-in secondary cure mechanism.
; In yet another aspect of the invention, there are 25 provided circuit boards and components coated with the polymerizable composition as above described.
I A particular feature and advantage of the instant invention it that the us radiation cure produces extremely rapid dry-to-the-touch cure of all exposed 30 areas of the coating thereby permitting substantially immediate handling of the coated products and Montanans of the shape of the coating which might otherwise sag and creep, while secondary curing which occurs by cross linking through the allylic bond of the ~L~3~3~3 moo or polyfunctional reactive acrylate or methacrylate delineate provides substantially complete cure of unexposed (shadow) areas of the coating under conditions of ambient temperature and humidity.
For a fuller understanding of the nature and objects of the present invention, reference should be made to the following detailed description of the invention taken in connection with the accompanying examples.
The terms "liquid", and "solid" or I "dry-to-the-touch" are used herein with reference to physical properties of the materials, are to be understood as to be referring to such properties as they exist under conditions as may be specified. For example, the term "liquid" refers to a physical state lo wherein a material is readily flyable while the terms 'solid" and "dry-to-the-touch" are to be understood as referring respectively to physical states wherein a material is resistant to change in shape and is without free surface moisture or surface tackiness.
DETAILED DESCRIPTION OF THE INVENTION AND
_ ITS PREFERRED EMBODIMENTS
The urethane-acrylates or methacrylates employed in the coating system of the instant invention preferably comprise polyester-urethane-acrylates formed by reacting 25 a polyester dill with a diisocyanate, and reacting the resulting product with a polymerizable acrylic or methacrylate acid ester. The preferred polyester dill is manufactured by the Inolex Company under the trade mark Inolex 1~00~120. This polyester dill is formed by 30 reaction of neopentyl glycol and 1,6-hexanediol with adipic acid. Other polyester dills useful are formed by the reaction of neopentyl glycol and a dill of more than

3 carbon atoms, e.g., buttonwood with adipic acid.
The preferred diisocyanate is Tulane diisocyanate (TDI) although other relatively low molecular weight diisocyanates of the general frill:
(I) (o = C = NOR
wherein R it a C2_20alkylene, alkenylene or cycloalkylene radical or a C6 Arlene, alkarylene or aralkylene are useful.
The reaction ratio of the equivalent weights of the polyester dill to the diisocyanate should generally be in the range of about 1~0 of polyester dill to about 1.7 10 to about 2,2 of diisocyanate. The preferred reaction ratio is 1.0 equivalents of the polyester dill for every 1.9 equivalents ox diisocyanate. This preferred reaction ratio is particularly important to the properties of the final composition. The 1:1.9 ratio 15 yields a composition with a chemical structure having a high degree of flexibility and stability. The copolymer formed is generally of a random copolymer structure.
This copolymer product is then reacted with an hydroxyl-containing acrylate or methacrylate ester I monomer, producing acrylate end-capping. The useful range of equivalents of the ester monomers is about 0.9 to about 3.0, the preferred being in the range ox 1.6 to I and the most preferred being 1.8 equivalents.
The polymerizable acrylate and methacrylate ester 25 monomers used to cap the polyester diisocyanate reaction product may be moo- or difunctional. Mono-functional monomers are preferred Those monofunctional monomers found most effective are selected from the group ;
consisting of hydroxyalkyl acrylates and methacrylates, 30 amino alkyd acrylates and methacrylates. The most preferred polymerizable ester monomers are hydroxyethyl methacrylate and hydroxypropyl methacrylate~
Additional monofunctional polymerizable ester monomers, -'` .` 7 ~2~1~;3~3 deemed useful are represented by the following formula:

S Al o II. H2C = - - O R -X - H

wherein X is -O- or R3 lo -N-R3 is hydrogen or lower alkyd of l to 7 carbon atoms;
Al is selected from the group consisting of hydrogen, chlorine and methyl and ethyl radicals; and R is a diva lent organic radical selected from the group consisting of lower alkaline of 1-8 carbon atom, phenylene and naphthylene.
Suitable hydroxy- or amine-eontaining materials are exemplified by, but not limited to, ugh materials as hydroxyethyl acrylate, hydroxyethyl methacrylate,-I anthill methacryl~te, 3-hydroxypropyl methaerylate, amino ethyl methacrylate, hydroxyhexyl acrylate, t-butylaminoethyl methacrylate~ hydroxyoctyl methacrylate, and the monoacrylate or monomethacrylate esters of bisphenol-A, the fully hydrogenated derivative 250f bisphenol-A, cyclohexyl dill, polyethyleneglycol methacrylate, and the like.
; Preferably but not necessarily the end-capping reaction will be accomplished in the absence of delineates. In the event delineates are employed they preferably will comprise moo or polyfunctional reactive `
acrylate or methacrylate compounds The moo or polyfunctional reactive acrylate or methacrylat~ reactive delineates employed in the coating system of the present invention comprise allylic-bond . . .

. _ . . .. .. " , . , . , 7 containing acrylates or methacrylates of the -formula (III) SHEA = C - C - 0- US
14 11 '' in which R is a radical selected from the group S consisting of hydrogen and lower alkyd of l to 4 carbon atoms, and R5 is an organic radical containing at least one allylic bond and does not contain any group which may adversely affect the coating system for purposes of disclosed herein. Preferably R will lo consist of an'allylic-bc)nd containing aliphatic or substituted aliphatic carbon ring structure of which dicyclopenyloxyethyl is presently preferred. Other allylic-bond containing carbon ring structures such as clicyclopentenyl which is given as exemplary may also be - lo advantageously employed.
Other non-allylic reactive delineates also may be included in the coating composition. Such delineates preferably are aerylate or methacrylate compounds of which hydroxyethyl methacrylate (HEM) is the most 20 preferred compound.
The urethane acrylate or methacrylate typically comprises about 20 to about 50% by weight of the reactive mixture, preferably will comprise about 20 to about 30~ by weight, while the allylic group containing , I acrylate or methacryla-te and the non-allylic reactive delineate typically comprise, respectively about lo to about So and about 60 to about 20% by weight of the reactive mixture; most preferably each will comprise, about 35~ by weight of the reactive mixture.
Ultraviolet ("us") activated polymerization initiators are incorporated into the coating system.
Many us activated polymerization initiators are known in the art and may be advantageously employed in the invention. For example, the us activated initiators may ....... .. .

be selected from metal carbonless of the formula McCoy wherein M is a metal atom, x is 1 or I, and y is an integer determined by the total valence of the metal atoms, generally 4 to 10. The preferred us 5 activated initiators are selected from (a) Of 16 straight or branched chain alkyd drones; and (b) carbonyl compounds of the general formula R (COREY
wherein R is a Of 18 allele, aureole, aralkyl or alkaryl group, and R is R or hydrogen. In 10 addition, R5 or R can contain any substituents which do not adversely affect the compound in serving its function. For example R or R can be alpha-substituted with an alkyd, aureole, alkaryl alkoxy or airlocks radical, or with an amino or a moo- or 15 dialkylamino derivative thereof, each of the above substituents containing up to about six carbon atoms.
In addition, R and R6 -taken together with carbonyl group form an aromatic or heterocyclic kitten containing up to about 16 carbon atoms. --The polymerization initiators are usually~employedin amounts of about I to about 10~ by weight of the urethane acrylate or methacrylate compound of the coating system.
It is useful, but not recolored to incorporate an I adhesion promoter into the coating system. I've adhesion -promoter may be chosen from any of those commonly known to the person reasonably skilled in the art. Two types of promoters are preferred. The first type consists of moo- and dicarhoxylic acids which are capable of I copolymerizirlg with the urethane acrylate or methacryla~e compound. The preferred species of these classes are methacrylic and acrylic acid. Such acids are used in proportions of 1~20~ by weight of the coaxing system, preferably in proportion of 1-10%. oh second preferred type of adhesion promoter is of the well-known Solon type, present in the composition in proportions of lo by weight of the coating system.
It is optional, but recommended, that chelators and inhibitors also be added to coating system for optimum performance. Chelators and inhibitors are effective in amounts of about Owl to about 1% by weight of the coating system. E~hylenediamine tetra-acetic acid and its sodium salt (Nudity), l,l-ethylenebis-nitril 10 methylidyne dipyridine and the class of beta-dike-tones are generally the most effective and are preferred.
The inhibitor concentration left over in the monomers from manufacture is often high enough for good stability. However, to insure maximum shelf life, the 15 proportions mentioned above (about 0.1 to about 1% by weight of the coating system) are recommended. Of those inhibitors which have been wound of adequate utility is the group consisting of hydroquinones, benzoquinones, naphtho~uinones, phenanthraquinones, anthraquinones, and 20 substituted compounds of any of the foregoing.
Additionally, various phenols can be employed as inhibitors, the preferred one being 1,6-di-tert-butyl-4-methyl phenol.
The amount of thickness, viscosity, or thi~otropy 25 desired can be varied in accordance with the particular application required Thickeners, plasticizers, and various other agents common to the art can be employed in any reasonable manner to produce the desired characteristics.
I It also is optional, but recommended, that surfactants be present in the adhesive composition for optimum performance Selection of suitable surfactants is a matter of simple experimentation. Obviously -the surfactan-t MllSt be soluble in the polymer composition I . . ' .
.. -, .. .... ?

~3~(~f~3 and should be non-reactive with the prepolymer composition. The surf~ctants Jay be anionic materials such as the petroleum sulfonates having the formula:
(III) (Cn~i2n-10 S3)xMe wherein n is more than 20 and Me is a metal of valence x. Such materials are sold by, among others, Witch Chemical Corp., New York, New York, under the trademark "Alconate" 80, and by the Penreco Division ox Pencil, 10 Butler, Pennsylvania, under the trademark "Petrobase."
Another suitable anionic type comprises the sodium alkyd or alXylaryl sulfonates having the formula:

(IV) C9-l5Hl9-3l 3 Such materials are sold by ELI. Dupont de Numerous & Co., 20 Wilmington, Delaware, under the trademarks "Alkanol"
189-~ and "DO" and the trademark "Duponol," and by Union Carbide Corp., New York, New York, under the trademark "Tergitol" (numerical series).
Still another useful anionic class is the sulfonated 25 ethoxylated types sold as the Alipal~ series by GAY
Corp., New York, New York, and as Nudely 25-35 by Shell Chemical Co., Houston Texas.
Non-ionic surfactants such as Union Carbide L-5320 also may be usefully employed in the inventive 30 compositions.
Surfactant concentration will depend in any given case upon the particular surfactant and monomer composition being used. Optimization will be a matter of routine experimentation within the skill of the art.

Ordinarily, for anionic surfactants, however, minimum concentration of about 0.10 percent surEactant by weight of the coating composition will be needed to achieve an acceptable degree of emulsifiability, and a concentration of at least about 0.5 percent usually will be preferred. The maximum surfactant concentration will usually be about ten percent since above this level the surfactant may begin to interfere with the properties of the coating composition by adversely affecting, for 10 example, its cure rate, stability or the cured products. As a practical matter, an upper concentration limit of about five percent, is usually satisfactory.
For non-ionic surfactants the optimum concentration may be lower, typically around 0.1% or less by weight of the 15 coating composition.
The coating system of the present invention is designed primarily as a us curing product; however, due to the configuration of many circuit boards, there are areas of the board that are in the shadow of other 20 components such that they cannot be cured by us light.
To overcome this deficiency, a second curing mechanism has been built into the coating system. As noted swooper, the reactive delineates employed in accordance with the present invention comprise acrylate or methacrylate 25 compounds containing one or more allylic bonds. These allylic bonds are available to undergo significant cross linking in the presence of appropriate metal driers. Useful metal-driers include salts of metals with a valence of two or greater and unsaturated organic 30 acids. Amongst suitable compounds are mentioned the linoleates, naphthenates and resonates of cobalt, manganese, serum, lead, chromium, iron, nickel, uranium and zinc. The metal driers are usually employed in concentrations of less than between about 0.01 and five percent by weight of the coating system. Cross-linking of the allylic bonds typically will provide cure of unexposed areas of the coating in tinter hours.
It is frequently desirable to add low levels, such 5 as up to about 500 parts per million by weight, of a free-radical or us stabilizer, many of which are known in the art, to prevent spurious polymerization of the composition prior to the time of its intended use.
Suitable free-radical stabilizers are hydroquinone, p-benzoquinone, butylate of hydroxytoluene, and butylate of hydroxyanisole.
EXAMPLE I.
The invention will now be illustrated by the following description of a specific preferred embodiment thereof, given by way of example only.
A nitrogen-swept, four -necked us shielded resin kettle equipped with a stainless steel stirrer, nitrogen inlet tube, thermometer, condenser, and entrance ports, was heated to approximately 40C., 165 grams of MORAY
OUTED Tulane diisocyanate added, and 460 grams of 1,6-hexanediol/neopentyl glycol adipate (Inolex 1400-120 polyester, Inolex Company, Philadelphia, PA) was slowly added over 20 minutes. At the completion of the polyester addition, heating was continued with stirring 25(40 to 45C temperature) for one hour, followed by two hours heating with stirring at 100C. Upon completion of the three hour reaction period, the bath temperature was lowered to 50C, and 230 grams of hydroxyethylmethacrylate was added.
The reaction mixture was then heated, maintained at 50C and stirred for two hours at that temperature. The reaction mixture was permitted to return to room temperature. To 170 grams ox the product resin were added the following ingredients, with stirring: 275 `:

grams dicyclopentenyloxyethyl methacrylate; 29 grams acrylic acid; 5.5 grams glycidoxypropyltrimethoxysilane;
20 grams of Ciba-Geigy Irgacùre~651 dimethoxy-2-phenyl acetophenone: 0.5 grams Union Carbide L-5320 silicone surfactan~; and 1.5 grams of 3-M Company Flurried ~30 leveling agent. The resulting solution contained approximately I concentration of polyester-urethane-methacrylate resin (PUMA).
A mixture containing 66.4% of the above resin 10 solution, 33.2~ of hydroxyethyl methacrylate, and 0.3~
of a I solution of cobalt naphthenate was used in the following working example.
WORKING EXAMPLE
Spray the resulting blend onto the top side surface 15 (i.e. component side surface) of a printed circuit board. Cure of the coating was effected by exposing the boards to us radiation (3650 angstroms, 60,000 micro watts per square centimeter. Those areas of the coating exposed to us radiation were solid and dry Jo 20 the touch within 10-45 seconds. Areas in the shadow of components, i.e. not receiving direct us radiation, however, were observed to be wet to the touch, but found to be fully cured after standing at ambient temperature and humidity conditions for 24 hours. Circuit boards 25 were sliced through shadow areas after 24 hours and the coating was found to be fully cured to the board.
Coated boards were placed on a rack in a stainless steel tank, and 500 grams of water added to the bottom of the tank. The tank was closed, sealed, and heated to 30 a temperature of 115C. After heating for JO hours, the tank was allowed to cool. The tank was then opened, and the boards removed and examined. No visible blistering or other evidence of coating failure was observed.

EXAMPLE II
The procedure of example I was repeated except an equal amount of Alcoholic SIPOMER PM dicyclopentenyl methacrylate was employed in place of the dicyclopentenyloxy~ethyl methacrylate. Similar results to Example I were obtained.
EXAMPLE III
The procedure of Example I was repeated except an equal amount of dicyclobutadiene methacrylate was I employed in place of the dicyclopentenyloxyethyl methacrylate. Similar results to Example I were obtained.
As will be appreciated, the foregoing invention provides novel and improved coating systems for lo nonformal coating printing circuit boards and the like.
The coating systems may be applied to circuit boards assemblies and the like by any convenient manner, for instance a spraying, brushing, dripping, rolling, dipping, etc. Moreover, the coating systems cure 20 through dual mechanisms including us cure which permits fast fixture cure thereby achieving almost immediate dry-to-~he touch curing. however, unlike conventional us cured products, the coating systems of the present invention do not require full area irradiation to 25 achieve full cure due to their built-in secondary cure mechanism. Moreover, the cured coatings have excellent adherence to plastics, metal, glass and wood, good abrasion resistance, and are hydrolytic ally stable and resistant to thermal cycling. The coatings also are repairable, i.e., can be removed by selective solvents such as methyl ethyl kitten, and then replaced by brush or spray, and us cured.
The invention has been described particularly with applications to nonformal coating circuit board assemblies. ivory, one skilled in the art would appreciate that the coating systems may be applied to other electrical electronic components such as transformers or the like. Moreover, the coating composition it not limited to the use in the electronics field but may be employed in any industrial area where nonformal protective coating is desired.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are described as follows:

1. An essentially solvent-free one-part coating composition, especially useful for conformal coating, comprising:
(1) at least one urethane-acrylate or methane-methacrylate compound;
(2) an acrylate or methacrylate compound containing an allylic group;
(3) a non-allylic acrylate or methacrylate diluent;
(4) a polymerization initiator of the photo-initiator type; and (5) a metal-drier.

2. The composition of Claim 1, wherein said urethane-acrylate or urethane-methacrylate compound comprises a polyester-urethane-acrylate or polyester urethane methacrylate.

3. The coating composition of Claim 2, wherein the polyester portion of said polyester-urethane-methacrylate comprises a polyester diol.

4. The coating composition of Claim 3, wherein said polyester diol comprises the reaction product of neopentyl glycol and 1,6-hexanediol with adipic acid.

5. The coating composition of Claim 1, and further comprising a surfactant.

6. The coating composition of Claim 1, wherein said non-allylic diluent is hydroxyethyl methacrylate.

7. A coating composition of Claim 1, wherein said metal-drier is a salt of a metal having a valence of at least two and an unsaturated organic acid.

8. A coating composition of Claim 7, wherein said metal-drier is selected from the group consisting of the linoleates, naphthenates and resinates of cobalt, manganese, cerium, lead, chromium, iron, nickel, uranium and zinc.

9. A coating composition of Claim 8, wherein said metal drier comprises a cobalt naphthenate.

10. A coating composition of Claim 1, wherein said urethane-acrylate or urethane-methacrylate comprises about 20 to about 50% by weight of said coating composition.

11. A coating composition of Claim 1, wherein said non-allylic acrylate or methacrylate comprises about 20 to 60% by weight of said coating composition.

12. A coating composition of Claim 1, wherein said allylic group containing acrylate or methacrylate comprises about 10 to 50% by weight of said coating composition.

13. The coating composition of claim 1, wherein said allylic-group containing acrylate or methacrylate is selected from dicyclopentenyloxyethyl methacrylate and dicyclopentenyl methacrylate.

14. A method of forming a uv curing polymerizable conformal coating on a substrate wherein said substrate has one or more shadow areas, comprising the steps in sequence of:
(1) providing a polymerizable composition which comprises at least one urethane-acrylate or urethane-methacrylate compound, an allylic-group containing acrylate or methacrylate; a non-allylic acrylate or methacrylate diluent, a polymerization initiator of the photo-initiator type, and a metal-drier;
(2) coating said substrate at least in part with said polymerizable composition;
(3) exposing said coated substrate to uv light of wave length and intensity to effect uv cure of exposed areas of said coating; and (4) storing said coated substrate under conditions of temperature and humidity which promote curing of unexposed (shadow) areas of said coating through a built-in secondary cure mechanism.

15. A method according to Claim 14, wherein said substrate comprises a circuit board, and wherein said circuit board has one or more components mounted thereon overshadowing areas of said circuit board.

16. A printed circuit board having one or more components mounted thereon, and covered at least in part with the cured composition of claim 1.

17. An electrical or electronic component covered at least in part with the cured composition of claim 1.