CONFORMAL COATING AND POTTING SYSTEM

Abstract: Ultraviolet curable polymerizable coating and potting system having a built-in secondary curing mechanism is disclosed. The polymerizable coating and potting system is a one component system comprising, in one embodiment, at least one isocyanate-capped polyether diol and triol, an acrylate or methacrylate reactive diluent, and a polymeriza­ tion initiator of the photo-initiator type. The polymerizable coating and potting system comprises, in a second embodi­ ment, at least one urethane-acrylate or methacrylate, an acrylate or methacrylate compound containing an allylic group; a non-allylic acrylate or methacrylate diluent, a polymerization initiator of the photo-initiator type, and a metal-drier. The coating systems are uv curable, and also possess an additional built-in cure mechanism which assures complete cure even of unexposed (shadow) areas.

Description: Conformal Coating and Potting System This invention concerns polymerizable compositions which have their primary utility in the area of coatings and sealants. The invention has particular utility in the area of conformal coatings for printed circuit boards and pottings for electrical and electronic components and will be described in connection with such utility, although other uses are contemplated. Printed circuit boards and electrical and electronic components may be coated or potted with a protective film to avoid or at least 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 conductors, accelerate high-volta9e breakdown and dendritic growth, and corrode conductors. Other contaminants which may damage printed circuit boards and components include various chemicals which may be residues of the manufacturing process such as fluxes, organic solvents, 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 conditions may also contribute a variety of contaminants such as salt spray, dirt and dust, oil, fuel, acid, corrosive vapor and fungus. Although the list of possible contaminants is almost endless, in all but the most severe cases, their destructive action effectively can be eliminated by provision of a good conformal coating. In addition to providing protection from contÅaminants, conformal coatings and pottings also provide a certain degree of protection to mechanical shock, vibration and tampering. Various conformal coating and potting systems are known in the art and are available commercially. Each has its advantages and disadvantages. One such prior art conformal coating and potting system is based on acrylics. Acrylics make excellent coating and potting systems from a production standpoint because they readily may be applied by spraying, dipping or brushing. Acrylic coatings and pottings have desirable electrical and physical properties and are fungus resistant. Additional advantages of acrylic coatings and pottings include long life, low or no exotherm during cure, and little or no shrinkage during cure. However, acrylic coatings and pottings typically are formed by solvent evaporation and reach optimum physical characteristics during cure typically in a matter of minutes. Conventional-acrylic coatings and pottings are soluble in chlorinated solvents such as tricholorethane or methylene chloride. Another prior art conformal coating and potting system is based on polyurethanes. Polyurethanes are available as either single or two-component systems. polyurethane coatings and pottings offer excellent humidity and chemical resistance and good dielectric properties. Single-component urethanes are relatively easy to apply and exhibit relatively long working pot life. However, single-component polyurethanes typically require a curing time of three to ten days at room temperature to reach optimum physical characteristics. Two-component polyurethanes typically achieve optimum cure at elevated temperatures within one to three hours, but exhibit relatively short working pot life. Also, surface preparation of substrate boards and components prior to application of polyurethane based coatings and pottings is important, since even minute quantities of moisture on a substrate board or component could produce blistering under humid conditions. Blisters, in turn, may lead to electrical failures and mandate costly rework. Polyurethane coatings and pottings are insoluble in most common solvents, which is a drawback to rework. Thus, repair of a polyurethane coated board or potted components requires a corrosive stripper to remove effectively all traces of the polyurethane film. However, extreme caution must be exercised when such a stripper is used, because the stripper also may corrode metallic surfaces on the board or component. Epoxy resin systems also have been employed by the prior art for conformal coatings and pottings. Epoxy resins are available-as two component systems only. Epoxy resin coatings and pottings provide good humidity resistance and high abrasive and chemical resistance. However, epoxy resins are virtually impossible to remove chemically for rework because any stripper that will attack the coating also will attack materials commonly used in making the electronic components and the epoxy-glass of printed circuit boards as well. Thus, the only effective way to repair an epoxy resin coated board or potted component 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, epoxy resins shrink somewhat-during cure. Accordingly, a buffer 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 to seven days at room temperature. Epoxy resins exhibit a relativly short working pot life which is an additional disadvantage. Silicone resins also have been employed by the prior art for conformal coating and pottings. Silicone resin coatings and pottings provide high humidity and corrosion resistance along with high temperature resistance which makes silicone resins preferred for coating and potting assemblies that contain high heat-dissipating components such as power resistors. However, silicone resins are relatively thick making them somewhat difficult to apply. Moreover, silicone resins require a relatively long cure, and repairability is difficult 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 or potted component is to mechanically remove the coating. The prior art also has employed polyimides for conformal coatings and pottings. Polyimide coatings and pottings provide high-temperature, moisture and chemical resistance over extended periods of time. However, polyimide coatings require high temperature cure (one to three hours at 200 to 2500C) which could damage heat sensitive electronic components, and this requirement severely limits the use of polyimide coating systems. Also, since polyimide are high-temperature, moisture and chemical resistant, the only effective way to repair a polyimide coated board or potted component is to mechanically remove the coating. Diallyl phthalate varnishes also have been proposed by the prior art for conformal coatings and pottings, and provide excellent high temperature and chemical resistance. However, diallyl phthalate-varnishes require high temperature cure (approximately 150 e C which severely limits their use. And, diallyl phthalate coatings and pottings also typically must be mechanically removed to permit repair. The preceeding discussion of background in the art of conformal coating circuit boards is taken largely from Printed Circuits Handbook, Clyde F. Coombs, Jr., Editor, McGraw Hill Book Company, Second Edition (1979). As thus is apparent from the above discussion, none of the currently available conformal coatings and pottings are completely satisfactory since all prior art conformal coatings and pottings at best are a tradeoff of preferred 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 resistance), and repairability. It is thus a primary object of the present invention to provide an improved conformal coating and potting system, i.e. method and materials, which overcomes the aforesaid and other disadvantages of the prior art. Other objects of the present invention are to provide an improved composition for conformal coating printed circuit board assemblies and potting electrical and electronic components which compositions demonstrate extended pot life and ease of application, rapid dry-to-touch cure, good electrical properties, superior adhesion and abrasion resistance, excellent high temperature, chemical and moisture resistance, and ease of repairability. Generally, the foregoing and other objects of the invention are achieved by the provision of an ultraviolet (hereinafter "uv") curable polymerizable coating and potting system having a secondary cross-linking curing mechanism built in. More specifically, in accordance with a first embodiment of the present invention the polymerizable system is a one component coating system comprising (1) an isocyanate capped prepolymer formed by reacting a blend of one or more polyether diols and one or more polyether triols with an excess of a diisocyanate, (2) a mono or polyfunctional acrylate or methacrylate reactive diluent, and (3) a polymerization initiator of the photo-initiator type. A particular feature and advantage of the first embodiment is that rhe uv radiation cure produces extremely rapid dry-to-the-touch cure of all exposed areas of the coating or potting thereby permitting substantially immediate handling of the coated products and maintainance of the shape of the coating which might otherwise sag and creep, while secondary cu#ring which occurs by moisture reaction of the isocyanate capped prepolymer provides substantially complete cure of unexposed (shadow) areas of the coating or potting under conditions of ambient temperature and humidity. In accordance with a second embodiment of the present invention the polymerizable system is a one component coating system comprising at least one urethane-acrylate or methacrylate, an acrylate or methacrylate compound containing an allylic group, a non-allylic acrylate or methacrylate diluent, a polymerization initiator of the photo-initiator type, and a metal-drier. A particular feature and advantage of the second embodiment is that the uv radiation cure produces extremely rapid dry-to-the-touch cure of all exposed areas of the coating thereby permitting substantially immediate handling of the coated products and maintenance 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 mono or polyfunctional reactive acrylate or methacrylate diluent 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 "dry-to-the-touch" as 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 wherein a material is readily flowable 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 of surface tackiness. THE FIRST EMBODIMENT The isocyanate-capped prepolymers employed in the first embodiment of the coating and potting system of the instant invention are formed by reacting a blend of one or more polyether diols and one or more polyether triols with an excess of at least one diisocyanate. The preferred polyether diols are selected from those having a molecular weight in the range of about 300 to about 5000, while the preferred polyether triols are selected from those having a molecular weight in the range of about 200 to about 3000. Particularly preferred is a blend of polyether diols having a molecular in the range of about 400 to about 1000, and polyester triols having a molecular weight of about 700. The polyether diol/polyether triol blend typically comprises the polyether diol in substantial major proportion, e.g. seventy or more by weight of the polyether diol, while the polyether triol may be present in quite minor amount, e.g. as little as one to three percent of the polyether triol. The preferred diisocyanate is toluene diisocyanate (TDI) although other relatively low molecular weight diisocyanates of the general formula: (I) ( = C = N)2R wherein R is a C2 20 alkylene, alkenylene or cycloalkylene radical or a C6 40 arylene, alkarylene or aralkylene are useful. The polyether diol/polyether triol blend is reacted with excess diisoeyanate. Generally the equivalent weight reaction ratio of the polyether diol/polyether triol blend to diisocyanate will be in the range of about 1.0 equivalent weights of polyether diol/polyether triol to about 1.6 to about 2.2 equivalent weight of diisocyanate. The preferred reaction ratio is 1.0 equivalents of the polyether diol/polyether triol blend for every 2.0 equivalents of diisocyanate. This preferred reaction ratio is particularly important to the properties of the final composition. The 2:1 ratio yields a final product with a chemical structure having a high degree of chemical, solvent and hydrolytic resistance. At present the preferred prepolymer is based on a mixture of 0.916 equivalents of polyether diol (m.w. 1000 average) and 0.084 equivalents of polyether triol (m.w. 700 average) reacted with 2.0 equivalents of diphenylmethane diisocyanate (MDI). The resin resulting is then blended with an acrylate or methacrylate reactive diluent such as are exemplified by, but not limited to trimethylol propane ethoxylate triacrylate, dicyclopentadiene methacrylate and dicyclopentadiene acrylate. Other reactive methacrylate compounds such as isobutyl methacrylate, isobornyl methacrylate, lauryl methacrylate and cyclohexylomethacrylate also can be beneficially utilized. The reaction preferably is accomplished in the absence of other i.e. non-reactive dilutents. The isocyanate-capped prepolymers typically comprise about 50 to about 75% by weight of the reaction mixture, and preferably will comprise about 65% by weight, while the acrylate or methacrylate reactant diluent typically will comprise about 25 to about 50% by weight of the reaction mixture, and preferably will comprise about 35% by weight. THE SECOND EMBODIMENT The urethane-acrylates or methacrylates employed in the second embodiment of the coating and potting system of the instant invention preferably comprise polyester-urethane-acrylates formed by reacting a polyester diol with a diisocyanate, and reacting the resulting product with a polymerizable acrylic or methacrylate acid ester. The preferred polyester diol is manufactured by the Inolex Company under the trade name Inolex 1400-120. This polyester diol is formed by reaction of neopentyl glycol and 1,6-hexanediol with adipic acid. Other polyester diols useful are formed by the reaction of neopentyl glycol and a diol of more than 3 carbon atoms, e.g., 1-4-butanediol with adipic acid. The preferred diisocyanate is toluene diisocyanate (TDI) although other relatively low molecular weight diisocyanates of the general formula: (I) (O = C = N)2R wherein R is as defined supra, are useful. The reaction ratio of the equivalent weights of the polyester diol to the diisocyanate should generally be in the range of about 1.0 of polyester diol to about 1.7 to about 2.2 of diisocyanate. The preferred reaction ratio is 1.0 equivalents of the polyester diol for every 1.9 equivalents of diisocyanate. This preferred reaction ratio is particularly important to the properties of the final composition. The 1:1.9 ratio 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 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 of 1.6 to 2.0, and the most preferred being 1.8 equivalents. The polymerizable acrylate and methacrylate ester monomers used to cap the polyester diisocyanate reaction product may be mono- or difunctional. Mono-functional monomers are preferred. Those monofunctional monomers found most effective are selected from the group consisting of hydroxyalkyl acrylates and methacrylates, amino alkyl acrylates and methacrylates. The most preferred polymerizable ester monomers are hydroxyethyl methacrylate and hydroxypropyl methacrylate. Additional monofunctional polymerizable ester monomers deemed useful are represented by the following formula: EMI10.1 wherein X is -O- or EMI10.2 R3 is hydrogen or lower alkyl of 1 to 7 cazbon atoms; R1 is selected from the group consisting of hydrogen, chlorine and methyl and ethyl radicals; and R2 is a divalent organic radical selected from the group consisting of lower alkylene of 1-8 carbon atoms, phenylene and naphthylene. Suitable hydroxy- or amine-containing #aterials are exemplified by, but not limited to, such materials as hydroxyethyl acrylate, hydroxyethyl methacrylate, aminoethyl methacrylate, 3-hydroxypropyl methacrylate, aminoethyl methacrylate, hydroxyhexyl acrylate, t-butylaminoethy# methacrylate, hydroxyoctyl methacrylate, and the monoacrylate or monomethacrylate esters of bisphenol-A, the fully hydrogenated derivative of bisphenol-A, cyclohexyl diol, polyethyleneglycol methacrylate, and the like. Preferably but not necessarily the end-capping reaction will be accomplished in the absence of diluents. In the event diluents are employed they preferably will comprise mono or polyfunctipnal reactive acrylate or methacrylate compounds. The mono or polyfunctional reactive acrylate or methacrylate reactive diluents employed in the coating and potting system of the present invention comprise allylic-bond containing acrylates or methacrylates of the formula: EMI11.1 4 in which R is a radical selected from the group consisting of hydrogen and lower alkyl of 1 to 4 carbon 5. atoms, and R 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 consist of an allylic-bond containing aliphatic or substituted aliphatic carbon ring structure of which dicyclopenyloxyethyl is presently preferred. Other allylic-bond containing carbon ring structures such as dicyclopentenyl which is given as exemplary may also be advantageously employed. Other non-allylic reactive diluents also may be included in the coating composition. Such diluents preferably are acrylate or methacrylate compounds of which hydroxyethyl methacrylate (HEMA) is the most 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 acrylate or methacrylate and the non-allylac reactive diluent typically comprise, respectively, about 10 to about 50% 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. Incorporated into the coating and potting system of the second embodiment of-the present invention are appropriate metal-driers, the purpose of which will be discussed in detail hereinafter. Useful metal-driers include salts of metals with a valence of two or greater and unsaturated organic acids. Amongst suitable compounds are mentioned the linoleates, naphthenates and resinates of cobalt, manganese, serium, lead, chromium, iron, nickel, uranium and zinc. The metal driers are usually employed in concentrations of less than between about Q.01 and five percent by weight of the coating system. INGREDIENTS COMMON TO BOTH EMBODIMENTS The polymerization initiators useful in accordance with the present invention preferably comprise ultra-violet (hereafter "uv") activated free-radical generators and typically may be employed in amounts of about 1% to about 10t by weight of the coating and potting composition, although greater or lesser amounts of polymerization initiators may be employed. Many uv activated polymerization initiators are known in the art and may be advantageously employed in the invention. For example, the uv activated initiators may be selected from metal carbonyls of the formula Mx (CO) y wherein M is a metal atom, x is 1 or 2, and y is an integer determined by the total valence of the metal atoms, generally 4 to 10. The preferred uv activated initiators are selected from (a) C1 16 straight or branched chain alkyl diones; and (b) 5 6 carbonyl compounds of the general formula R (CO)R 5 wherein R5 is a C1 10 alkyl, aryi, aralkyl or alkaryl group, and R6 is R5 or 5 6 hydrogen. In addition, R or R can contain any substituents which do not adversely affect the compound in serving its function. For example, R5 or R6 can be alpha-substituted with an alkyl, aryl, alkaryl, alkoxy or aryloxy radical, or with an amino or a monoor dialkylamino derivative thereof, each of the above substituents containing up to about six carbon atoms. In addition, R5 and R6 taken together with carbonyl group form an aromatic or heterocyclic ketone containing up to about 16 carbon atoms. It is useful, but not required to incorporate an adhesion promoter into the coating and potting system. The adhesion promoters 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 mono-and dicarboxylic acids which are capable of copolymerizing with the acrylate or methacrylate reactive diluent. The preferred species of these. classes are methacrylic and acrylic acid. Such acids are used in proportions of 1-208 by weight of coating and potting system, preferably in proportion of 1-10 to 1-15%. The second preferred type of adhesion promoter is of the well-known silane type, present in the composition in proportions of 1-10t by weight of the coating and potting system. Also optional, but recommended, are the inclusion of one or more chelators, cross-linking agents and/or polymerization inhibitors. Chelators and inhibitors are effective in amounts of about 0.1 to about 1% by weight of the coating and potting system. Ethylenediamine tetra-acetic acid and its sodium salt (Na4EDTA), l,l-ethylenebis-nitril methylidyne dipyridine and the class of beta-diketones are generally the most effective and are preferred. Cross-linking agents, optionally present in amounts from about zero to about 10% by weight of the coating and potting system, include such compounds as copolymerizable dimethacrylates. The inhibitor concentration left over in the monomers from manufacture is often high enough for good stability. However, to insure maximum shelf life, the proportions mentioned above (about 0.1 to about 1% by weight of the coating and potting system) are recommended. Of those inhibitors which have been found of adequate utility is the group consisting of hydroquinones, benzoquinones, naphthoquinones, phenanthraquinones, anthraquinones, and substituted compounds of any of the foregoing. Additionally, various #phenols can be employed as inhibitors, the preferred one being l,6-di-tert-butyl-4-methyl phenol. The amount of thickness, viscosity, or thixotropy desired can be varied in accordance with the particular application required. Thickeners, plasticizers, diluents, and various other reagents common to the art can be employed in any reasonable manner to produce the desired characteristics. It also is optional, but recommended, that surfactants be present in the coating and potting system for optimum performance. Selection of suitable surfactants is a matter of simple experimentation. Obviously the surfactant must be soluble in the polymer composition and should be non-reactive with the isocyanate-capped prepolymer. The surfactants may be anionic materials such as, the petroleum sulfonates having the formula: EMI15.1 wherein n is more than 20 and Me is a metal of valence x. Such materials are sold by, among others, Witco Chemical Corp., New York, New York, under the trademark "Alconate" 80, and by the Penreco Division of Penzoil, Butler, Pennsylvania, under the trademark "Petrobase." Another suitable anionic type comprises the sodium alkyl or alkylaryl sulfonates having the formula: EMI15.2 Such materials are sold by E.I. duPont de Amours & BR< Co., Wilmington, Delaware, under the trademarks "Alkanól" 189-S and "DW" 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 ethoxylated types sold as the "Alipal" series by GAF Corp., New York, New York, and as "Neodol" 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 compositions. Surfactant concentration will depend in any given case upon the particular surfactant and reactive diluent 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 0.25 percent surfactant by weight of the coating and potting system 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 and potting system by adversely affecting, for 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 total coating and potting system. It is frequently desirable to add low levels, such as up to about 500 parts per million by weight, of a free-radical or uv 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. The coating and potting systems of the present invention are designed primarily as a uv curing product; however, due to the configuration of many circuit boards and electrical and electronic components there are shadow areas that cannot be cured by uv light. To overcome this deficiency, a second curing mechanism has been built into the coating and potting system. More specifically, in accordance with the first aspect of the invention, after exposure t uv a built-in moisture cure mechanism is available to complete curing of uncured areas of the coating and potting. The overall curing mechanisms of the first embodiment of the invention are as follows: (1) uv light polymerizes the acrylate or methacrylate species to provide fixture of the coating or potting; and (2) the isocyanate-capped prepolymer then reacts with water, specifically atmospheric moistUre i.e. according to the following formula: EMI17.1 This latter reaction provides ultimate physical properties and complete cure to the substrate of unexposed areas of the coating or potting in twenty-four hours or less. The overall curing mechanisms of the second aspect are as follows: (1) uv light polymerizes the acrylate or methacrylate species to provide fixture to the coating or potting; and (2) the allylic bonds then undergo cross-linking in the presence of the metal-dryers. Cross-linking of the allylic bonds typically provides ultimate physical. properties and complete cure to the substrate of unexposed areas of the coating or potting in twenty-four hours or less. EXAMPLE I The invention will now be illustrated by the following description of a specific preferred embodiment thereof, given by way of example only. To a nitrogen-swept, four-necked uv shielded resin kettle equipped with a stainless steel stirrer, nitrogen inlet tube, thermometer, condensor, and entrance port, add 286 grams of Upjohn LF-168 diphenylmethane diisocyanate (MDI). Then slowly add at 600 with mixing 466 grams of polyether diol (1000 m.w. average) and 20 grams of polyether triol (700 m.w. average). Heat to 600C with mixing for one hour and then at 900C for one and one half hours. Cool to room temperature, add 260 grams of trimethylol propane ethoxylate triacrylate and 30 grams of Ciba-Geigy Irgacure 651 dimethoxy-2-phenyl acetophenone, and blend. The resulting blend was applied by dipping the back side surface of an audio speaker cone. Cure of the coating was effected by exposing the speaker to uv radiation (3650 angstroms, 60000 microwatts per square centimeter) for 20 seconds. Those areas of the coating exposed to uv radiation were solid and dry to the touch within 25 to 45 seconds. Shadow areas of the speaker core, i.e. not receiving direct uv radiation, were wet to the touch initially, but were found to be fully cured after standing at ambient temperature and humidity conditions for twenty-four hours. The speaker core was sliced through shadow areas and the coating was found to be fully cured to the board surface after twenty-four hours. The coating was humidity tested as follows: The coated speaker cone was 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 a temperature of 115 C. After heating for 40 hours, the tank was allowed to cool. The fank was then opened, and the cone removed and examined. No visible blistering or other evidence of coating failure was observed. EXAMPLE II The procedure of Example I was repeated except 260 grams of dicyclopentenyl methacrylate was employed in place of the trimethylol propane ethoxylate triacrylate. Similar results to Example I were obtained. EXAMPLE III The procedure of Example II was repeated except 260 grams of dicyclopentadiene acrylate was employed in place of the trimethylol propane ethoxylate triacrylate. Similar results to Example I were obtained. EXAMPLE IV. 'A nitrogen-swept, four-necked uv shielded resin kettle equipped as in Example I was-heated to approximately 40 C., 165 grams of MOBAY TDX toluene 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 (40 to 450C temperature) for one tour, followed by two hours heating with stirring at 100 C. Upon completion of the three hour reaction period! the bath temperature was lowered to 50 C, and 230 grams of hydroxyethylmethacrylate was added. The reaction mixture was then heated, maintained at 500C and stirred for two hours at that temperature. The reaction mixture was permitted to return to room temperature. To 170 grams of the product resin were added the following ingredients, with stirring: 275 grams dicyclopentenyloxyethyl methacrylate; 29 grams acrylic acid; 5.5 grams glycidoxypropyl trimethoxysilanei 20 grams Ciba-Geigy Igacure 651 dimethoxy-2-phenyl-acetophenone; 0.5 grams Union Carbide L-5320 silicone surfactant, and 1.5 grams 3M Company Fluorad FC 430 leveling agent. The mixture was stirred until all ingredients were dissolved. the resulting solution contained about 34 percent concentration of polyester-urethane-methacrylate resin (PEUMA). A mixture coptaing 66.4 percent of the above resin solution, 33.2 percent of hydroxyethylmethacrylate and 0.3 percent of a 6 percent solution of Cobalt Napthanate was used in the following working example. WORKING EXAMPLE The resulting solution was sprayed onto the top side surface (i.e. component side surface) of a printed circuit board. Cure of the coating was effected by exposing the boards to uv radiation (3650 angstroms, 60,000 microwatts per square centimeter). Those areas of the coating exposed to uv radiation were solid and dry. to the touch within 10-45 seconds. Areas in the shadow of components, i.e. not receiving direct uv 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 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 a temperature of 115 C. After heating for 40 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 V The procedure of Example IV was repeated except an equal amount of Alcolac SIPOMER DCPM dicyclopentenyl methacrylate was employed in place of the dicylopentenyloxyethyl methacrylate. Similar results to Example IV were obtained. EXAMPLE VI The procedure of Example IV was repeated except an equal amount of dicyclobutadiene methacrylate was 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 conformal coating and potting printing circuit boards and electrical and electronic components and the like. The coating and potting systems may be applied to the circuit board substrates or components by any convenient manner, for instance by brushing, dripping, rolling, dipping, etc. or by spraying (when suitably diluted). Moreover, the coating and potting systems cure through dual mechanisms including uv cure which permits fast fixture cure thereby achieving almost immediate dry-to-the-touch curing. However, unlike conventional uv cured products, the coating and potting systems of the present invention do not require full area irradiation to achieve full cure due to their built-in secondary cure mechanism. Moreover, the cured coatings and pottings have excellent adherence to plastics, metal, glass and wood, good abrasion resistance, and are hydrolytically stable and resistant to thermal cycling. The coating and pottings also are repairable, i.e., can be removed by selective solvents such as methyl ethyl ketone, and then replaced by brush or spray, and uv cured. The invention has been described partioularly with application to conformal coating circuit board assemblies and to potting audio speaker cores. However, one skilled in the art would appreciate that the coating and potting systems may be applied to other electrical electronic components such as electrical transformers or the like. Moreover, the coating and potting system is not limited to the use in the electronics field but may be employed in any industrial area where conformal protective coating andlor potting is desired.

Claims: CLAIMS 1. A one-part coating composition, especiarly - v useful for conformal coating and potting, characterized by comprising, in combination, (A) a polymerizable system selected from: (1) (a) at least one isocyanate-capped polyether diol and triol; and (b) an acrylate or methacrylate reactive diluent; or (2) (a) at least one urethane-acrylate or urethane-methacrylate compound; (b) an acrylate or methacrylate compound containing an allylic group; (c) a non-allyic acrylate or methacrylate diluent; and (d) a metal-drier: and, (B) a polymerization initiator of the photo-initiator type. 2. The composition of Claim 1, characterized in that said isocyanate-capped polyether diol and triol comprises the reaction product of a blend of at least one polyether diol and at least one polyether triol with excess diisocyanate. 3. The composition of Claim 2, characterized by one or more of the following features: (a) the polyether diol portion comprises a polyether diol of molecular weight in the range of about 300 to about 5000 average; (b) the polyether triol portion comprises a polyether triol of molecular weight in the range of about 200 to about 3000 average; (c) said polyether diol has a molecular weight of about 400 to about 1000 average, and said polyether triol has a molecular weight of about 700 average; (d) said polyether diol and polyether triol blend comprises said polyether diol in major proportion; ; (e) said isocyanate-capped polyether diol and triol comprises the reaction product about 1.0 equivalents of said polyether diol and polyether triol blend with about 1.6 to about 2.2 equivalents of diisocyanate, more preferably about 1.0 equivalents of said polyether diol and polyether triol blend with about 2.0 equivalents of diisocyanate; most preferably about 0.916 equivalents of polyether diol of molecular weight of about 400 to about 1000 average, 0.084 equivalents of a polyether triol of molecular weight of about 700 average, and 2.0 equivalents of diphenyl methane diisocyanate; (f) said acrylate or methacrylate reactive diluent is selected from trimethylol propane ethoxylate triacrylate, dicyclopentadiene methacrylate, dicyclopentadiene acrylate, isobutyl methacrylate, isobornyl methacrylate, lauryl methacrylate and cyclohexylmethacrylate; and (g) said acrylate or methacrylate reactive diluent comprises about 25 to about 50% by weight of said composition; more peferably, about 35% by weight of said composition. 4. The composition of Claim lt characterized in that said urethane-acrylate or urethane-methacrylate compound comprises a polyester-urethane-acrylate or polyester-urethane-methacrylate. 5. The composition of Claim 4, characterized in that the polyester portion of said polyester-urethanemethacrylate comprises a polyester diol, said polyester diol preferably comprising the reaction product of neopentyl glycol and 1,6-hexanediol with adipic acid. 6. The composition of Claim 1, characterized by one or more of the following features: (a) said non-allylic diluent comprises hydroxyethylmethacrylate; (b) said metal-drier comprises a salt of a metal having a valence of at least two and an unsaturated organic acid, said metal-drier preferably being selected from the linoleates, naphthenates and resinates of cobalt, manganese, cerium, lead, chromium, iron, nickel, uranium and zinc, more preferably a cobalt naphthenate; (c) said urethane-acrylate or urethane-methacrylate comprises about 20 to about 50% by weight of said composition; (d) said non-allylic acrylate or methacrylate diluent comprises about 20 to 60% by weight of said coating composition; (e) said allylic-group containing acrylate or methacrylate comprises about 10 to 50% by weight of said composition; and (f) said allylic-group containing acrylate or methacrylate is selected from dicyclopentenyloxyethyl methacrylate and dicyclopentenyl methacrylate. 7. The composition of any of Claims 1 to 6, characterized by further comprising a surfactant. 8. A method of forming a uv curing polymerizable conformal coating or potting on a substrate wherein said substrate has one or more shadow areas, characterized by the steps in sequence of: (1) providing a polymerizable composition selected from (A)(i) at least one isocyanate-capped polyether diol and triol, (ii) an acrylate or methacrylate reactive diluent, and (iii) a polymerization initiator of the photo-initiator part; or (B) (i) at least one urethane-acrylate or urethane-methacrylate compound, (ii) an allylic-group containing acrylate or methacrylate, (iii) * non-allylic acrylate or methacrylate diluent, (iv) a metal-drier, and (v) a polymerization initiator of the photo-initiator part; (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 such as to promote curing of unexposed (shadow) areas of said coating through a built-in secondary cure mechanism. 9. A method according to Claim 8, characterized in that said substrate comprises an electrical or electronic component, if desired mounted on a circuit board and overshadowing areas of said circuit board. 10. An electrical or electronic component, if desired mounted on a circuit board, and covered at least in part with the cured composition of Claim 1.

Inventors: KADZIELA, KRIS, NATIVI, LARRY A.
Applicants: LOCTITE CORPORATION
Application year: 1983
Main classification: C08F283
IPC codes: C08G18, C09D175, C08F299, C08F2, H05K3