Method of indicating a cure point for ultraviolet radiation curing compositions by color change

Abstract: Adding a dye with a visible color to an ultraviolet radiation curable composition which contains a photoinitiator which generates free radicals upon exposure to ultraviolet radiation produces a composition which changes visible color upon exposure to ultraviolet radiation. This visible color change indicates that the composition has cured. This cure indication is useful for compositions curable by ultraviolet radiation in the electronics and electrical industry.

Description: This invention relates to a method of curing compositions by ultraviolet radiation. The curing of an ultraviolet radiation curable composition upon exposure to ultraviolet radiation varies with the kind of ingredients making up the composition, the kind of equipment used for generating the ultraviolet radiation, the geometry of the device having a film, coating or encapsulant to be cured and the curing conditions to which the device is exposed. These variables make it difficult to determine at what point the composition is sufficiently cured. Many times to avoid the chance that a composition may be under cured, the dosage of ultraviolet (UV) radiation which is given to the items to be cured is excessive. Excessive dosages may not harm the items, but they could and, furthermore, using excessive dosages is expensive and a waste of resource. Therefore, the discovery of the present method permits establishing curing conditions which can result in sufficient cure without either undercuring or overcuring. Materials for the electrical and electronics industry need to meet stricter requirements because the electronic devices are becoming smaller and more complex. The field of printed circuit boards is no exception. The various coatings require that the materials protect the electronic devices and components from the environments which they may encounter either during processing or in use. Coating printed circuit boards with compositions which will cure to a film and which will provide the required protection without changing the designed electronic properties or otherwise damage the boards components is difficult to achieve because the components are very small and often of shapes and design which demands compositions which have special uncured properties, special curing properties and special application properties. Printed circuit boards often need to be protected from contamination of the electrical and electronic components. This protection can be provided by coating or encapsulating the device bearing board with a protective film to avoid or minimize the reduction in the electrical performance due to contamination. Moisture and humidity are considered to be the worst contaminant because it must be dealt with in most environments. Moisture and humidity can drastically lower insulation resistance between conductors, accelerate high voltage breakdown, accelerate dendritic growth and corrode the conductors. Other than moisture, chemical contaminants from the environment such as dust, dirt, solvents, acids, fungus, oils and fuels or those which are used in the manufacturing process such as organic solvents, fluxes, vapors, acids, release agents and metal dust. Handling a printed circuit board can also cause contamination, for example, from fingerprints. Conformal coatings are also used to protect the electronic components from the harmful aspects of extremes in temperature, shock and abrasion. Many conformal coatings and potting compositions are known in the art and are available commercially. Each has its advantages and disadvantages. One prior art conformal coating material is based on acrylics. Acrylic coatings make excellent coating and potting systems because they have desirable electrical and physical properties, are resistant to fungus growth, have a long life, low or no exotherm during cure and have little or no shrinkage during cure. From some viewpoints, the acrylic coating systems have a production advantage because they can be readily applied by spraying, dipping or brushing. However, this is also a disadvantage because the films are formed from solvent containing acrylic systems. The evaporation of the solvent is a slow and expensive step and the solvent vapors need to be controlled for environmental reasons. The solvents typically used are the chlorinated solvents, such as trichloroethane and methylene chloride. RD-247016 discloses the use of organic nitriles to form photosensitive complexes in the presence of alcohol, acids, esters or amines. This property is then used to provide a visible indication that a radiation sensitive compound such as an adhesive or a potting compound is cured. JP-1-204902 discloses a photocuring resin composition which discolors by irradiation of light. The composition is characterised in that a compound is formed comprising a photopolymerising resin constituent selected from the group of unsaturated polyester resin, (meta)acryloyl group-containing polymerising resin, thiol-ene base addition polymerisation resin and cationic polymerisation resin, a photopolymerisation initiator, and a photodiscoloring compound. Furthermore, JP-1-297483 relates to an ultraviolet irradiation type dicing tape which supports semiconductor wafer dicing of the semiconductor wafer. The dicing tape comprises an adhesive layer made of the adhesive which bonds semiconductor wafer at the time of dicing, ultraviolet transmitting tape base on which the adhesive layer is formed and a discoloring material layer which is formed either selectively or all over the tape base on the opposite side to the adhesive layer. The discoloring material layer discolors by the irradiation of ultraviolet light. Any discoloring material can be used which does not impair the characteristics of the adhesive layer. US-A- 4 698 386 discloses polyorganosiloxane compositions comprising: (A) a linear and/or branched polyorganosiloxane, with a viscosity of 50 to 100,000 cP at 25° C., wherein 0.7 to 1.8 vinyl radicals bonded to silicon atoms, on average, are present in each molecule, and the remaining organic radicals bonded to silicon atoms are substituted or unsubstituted monovalent hydrocarbon radicals which does not include unsaturated aliphatic radicals, and 30% or more portion of the molecules has only one silicon-bonded vinyl radical in the molecule; (B) a polyorganohydrogensiloxane, wherein more than two hydrogen atoms bonded to silicon atoms, on average, are present in each molecule, in an amount such that the number of silicon-bonded hydrogen atoms may be 0.5 to 5 per vinyl radical in the molecule (A); (C) a catalytic amount of platinum compounds; and (D) anthraquinone dyes and/or azo dyes, in an amount from 1 to 2,000 ppm to (A). The combined impact of high energy costs and more stringent control regulations which restrict emissions of volatile solvents into the atmosphere has created a need in the coatings industry for high solids or solventless systems which do not require a large amount of energy for conversion of the system into a high performance coating. Coatings which are 100% solids are known and have a rapid cure at a relatively low conversion energy demand. Such coatings are acrylated coatings which cure by ultraviolet radiation or by electron beam exposure. These are all reasons why it is important to have the ability to know when a UV curable composition is cured so that extra energy usage is avoided and the cost is kept to a minimum. This invention relates to a method of indicating a cure point of an ultraviolet radiation curing composition comprising: adding a non-cure inhibiting amount of a dye with a visible color to an ultraviolet radiation curable composition comprising a photoinitiator which generates free radicals upon exposure to ultraviolet radiation and an organosiloxane gel composition which cures upon exposure to ultraviolet radiation which comprises an alkenyl-containing polymethylsiloxane consisting essentially of 80 to 97 mole percent of (CH 3 ) 2 SiO units, 2 to 10 mol percent of CH 3 SiO 1.5 units, 1 to 6 mol percent of (CH 3 ) 3 SiO 0.5 units and 2 to 4 mol percent of units represented by the formula (CH 3 ) a R 1 SiO (3-a)/2 , where R 1 represents 3-cyclohexenyl, cyclohexenylethyl or CH 2 =CH(CH 2 ) x , a is 1 or 2 and x represents 0 or an integer from 1 to 6, inclusive; and a mercaptoalkyl-containing compound selected from the group consisting of a liquid organic compound having an average of at least two mercapto groups per molecule and a liquid mercaptoalkyl-containing polydiorganosiloxane containing an average of at least two repeating units per molecule corresponding to the formula where R 2 represents a mercaptoalkyl radical, R 3 represents a monovalent hydrocarbon or halohydrocarbon radical and said curing agent is compatible with the other ingredients of said composition; and where the molar ratio of mercapto groups to alkenyl radicals in material is 1 or less, and thereafter exposing the resulting composition to a dosage of ultraviolet radiation which causes the visible color to either disappear or change to a different color indicating that cure has occurred coincidentally. The method of the present invention permits a user of ultraviolet radiation curable compositions to be assured that the resulting material is fully cured. This is accomplished without overcuring by observation of visible color change. This color change occurs when a non-inhibiting amount of a dye with a visible color is added to a composition which cures by exposure to ultraviolet radiation. Than change can be from blue to yellow, from red to yellow, from blue to clear and from yellow to clear. This color change usually lasts over long time periods and does not return upon standing. If the cure is not sufficiently complete, the original color will return upon standing over a period of time, such as from a few minutes, to an hour or more, to a couple weeks. For purposes of this invention, the use of the term "visible dye" means a dye which will impart a color under ordinary visible light. Also, the color changes are those which are observable in ordinary visible light. The visible dyes useful in the present invention are those which, in the presence of free radical generating photoinitiators, change color upon exposure to ultraviolet radiation. This color change occurs and is essentially permanent at the point that a UV curable composition is cured upon exposure to UV radiation. Some visible dyes useful in the present invention include, but are not limited to, anthraquinone dyes and disazo dyes. Examples of anthraquinone dyes include 1-amino-2-bromo-4-hydroxy-anthraquinone of which Sandoplast Red 2B powder, sold by Sandoz Chemicals, Charlotte, North Carolina is an example, a proprietary anthraquinone dye sold by Sandoz Chemical as Nitro Fast Blue 2B. An example of a disazo dye is Nitro Fast Yellow B Powder, sold by Sandoz Chemical which has a formula C 37 H 36 N 4 O 2 . The amount of visible dye useful in a particular UV curable composition should be an amount less than that amount which will inhibit the cure of the composition upon exposure to UV radiation. For many compositions, the amount will be less than about 40 ppm based on the weight of the UV curable composition. From experience, the amount is preferably from about 10 ppm to about 40 ppm of the visible dye based on the weight of the UV curable composition. It was found that amounts of the visible dye which became too high could inhibit the cure of the UV curable composition. The optimum amounts of visible dye in a given composition can readily be determined by adding a specific amount of visible dye to the composition and observing the color change, or lack thereof, upon exposure to UV radiation. If too much dye is added, no color change takes place, indicating the composition did not cure. At this point, one can increase the dosage of UV to determine whether the reason it did not cure was that the dosage was too low. Further, evaluation can be used to determine whether the amount of visible dye concentration is too high. The UV curable compositions in which this color change can occur are those which contain a photoinitiator which produces free radicals upon exposure to UV radiation and in which there is at least one material which cures upon exposure to UV radiation. Example of photoinitiators which generate free radicals include benzophenone, acetonaphthone, acetophenone, benzoin methyl ether, benzoin isobutyl ether, 2-hydroxy-2-methyl-1-phenylpropan-1-one, 1-hydroxycyclohexylphenyl ketone, 2,2-diethoxyacetophenone, 3-hydroxypropylphenyl ketone, 3-hydroxypropyl-p-isopropylphenyl ketone and mixtures thereof such as a mixture of 50 weight percent of 1-hydroxycyclohexylphenyl ketone and 50 weight percent benzophenone. The amount of photoinitiator is that amount which is conventionally required by the particular composition for curing it. The UV curable compositions which can be used in the present invention are those which are curable by photoinitiators which produce free radicals. These compositions can be either silicone compositions, organic compositions or compositions which contain both silicone compounds (includes polymers) which will cure by exposure to UV radiation in the presence of a free radical photoinitiator and organic compounds (includes polymers) which cure by exposure to UV radiation in the presence of a free radical photoinitiator. The compositions containing the visible dye and photoinitiator are exposed to UV radiation from conventional equipment and under conditions normally used for such compositions wherein adjustments are made to reach the cure point by observation of the color change. The point at which cure is achieved results in a color change. Therefore, the length of time or the dosage of UV radiation can be varied to produce the required cure. Compositions curable to gels are organosiloxane compositions which retain their gel properties at low temperature. These organosiloxane compositions cure in the presence of UV radiation and comprise an alkenyl-containing polymethylsiloxane with a specified combination of mono-, di- and trifunctional siloxane units, an amount of a mercaptoalkyl-containing polyorganosiloxane in an amount sufficient to cure the composition to a soft, repairable gel and an amount of a photoinitiator sufficient to initiate curing of the composition in the presence of UV radiation. The alkenyl-containing polymethylsiloxane consists essentially of 80 to 97 mol percent of (CH 3 ) 2 SiO units, 2 to 10 mol percent of CH 3 SiO 1.5 units, 1 to 6 mol percent of (CH 3 ) 3 SiO 0.5 units and 0.2 to 4 mol percent of units represented by the formula (CH 3 ) a R 1 SiO (3-a)/2 , where R 1 represents 3-cyclohexenyl, cyclohexenylethyl or CH 2 =CH(CH 2 ) x -, a is 1 or 2 and x represents 0 or an integer from 1 to 6, inclusive. The alkenyl-containing polymethylsiloxanes are preferably liquids under ambient conditions. These alkenyl-containing polymethylsiloxanes in which the alkenyl group is vinyl is described in United States Patent No. 4,374,967, which issued to Paul Brown et al. on February 22, 1983. A preferred class of alkenyl-substituted polymethylsiloxanes contain from 87 to 95 mol percent of dimethylsiloxane units, from 3 to 6 mol percent of monomethylsiloxy units, from 1.5 to 5 mol percent of trimethylsiloxy units and from 0.5 to 1.5 mol percent of dimethylvinylsiloxy units. Mercaptoalkyl-containing compounds are combined with the alkenyl-containing polymethylsiloxane. The mercaptoalkyl-containing compounds is a liquid organic compound containing an average of at least two mercapto groups per molecule or a liquid mercaptoalkyl-containing polydiorganosiloxane having an average of at least two repeating units per molecule corresponding to the formula where R 2 represents a mercaptoalkyl radical, R 3 represents a monovalent hydrocarbon or halohydrocarbon radical. The compositions are prepared by combining the alkenyl-containing polymethylsiloxane with the mercapto-containing compound such that the molar ratio of mercapto groups to alkenyl radicals in said composition is 1 or less. Preferred embodiments of the mercapto-containing polyorganosiloxane can be represented by the general formula R 6 2 R 4 SiO(R 7 R 6 SiO) y (R 6 R 5 SiO) z SiR 4 R 6 2 where R 7 represents a mercaptoalkyl radical containing from 2 to 6 carbon atoms, each R 6 represents methyl, phenyl or 3,3,3-trifluoropropyl, R 4 represents R 6 , R 7 or a hydroxyl group, each R 5 is methyl or phenyl, y and z are each 0 or greater, with the proviso that R 4 represents a mercaptoalkyl radical when both y and z are 0. Most preferably, the radicals represented by R 6 , R 4 and R 5 are methyl and ingredient B is represented by the average formula (CH 3 ) 3 SiO{(CH 3 ) 2 SiO} z (R 7 CH 3 SiO) y Si(CH 3 ) 3 where R 7 , y and z have the same definitions as above and y is at least 2. R 7 is preferably 3-mercaptopropyl or mercaptoisobutyl. y is preferably from 40 to 45 inclusive and z is preferably from 3 to about 10. The organic compounds containing an average of at least 2 mercapto (-SH) groups per molecule also contain carbon, hydrogen and, optionally, oxygen. These organic compounds preferably contain from 2 to 6 mercapto groups per molecule. The organic compound can be, for example, but are not limited to, 2,2'-dimercaptodiethylether, dipentaerythritolhexa(3-mercaptopropionate), glycol dimercaptoacetate, trimethylolpropane trithioglycolate and trimethylolpropane trimercaptopropionate. The UV curable compositions which cure to gels and which are useful for the present invention are those which contain photoinitiators which generate free radicals. The photoinitiator is present in an amount sufficient to promote curing of the composition upon exposure to UV radiation. The photoinitiator should be compatible with the other ingredients of the present compositions. Compatibility can readily be determined by blending one weight percent of the candidate photoinitiator with the other liquid ingredients of the UV curable composition and mixing the resultant composition at room temperature or with heating to a temperature of about 80°C. The resultant composition should be optically clear without any dispersed or precipitated material. The photoinitiators are exemplified above. In addition to the alkenyl-containing polymethylsiloxane, mercapto-containing compound and photoinitiator, the present compositions can contain up to about one weight percent of conventional viscosity stabilizers to inhibit gelation during storage. Examples of suitable viscosity stabilizers include amines such as 2-(diisopropylamino)ethanol and trioctylamine and free radical scavengers such as p-methoxyphenol, catechol, hydroquinone and 2,6-di-t-butyl-p-methylphenol. It may be desirable to include an organic peroxide that decomposes at a temperature within the range of from about 100°C. to about 250°C. This is particularly true if the substrate to which the composition will be applied is contoured such that a portion of the curable composition is shielded from the ultraviolet radiation to which the coated substrate is subsequently exposed for the purpose of curing the gel composition. The UV curable compositions can contain a reinforcing filler or other type of reinforcing agent to improve the physical properties of the cured material. In those applications where transparency of the cured gel is a requirement, the reinforcing agent is preferably a finely divided hydrophobic silica of the type described in United States Patent No. 4,344,800, issued to Michael A. Lutz on August 17, 1982. The disclosure of this patent is a teaching of silica fillers suitable for use in the optically clear gel compositions of this invention. The ingredients of the compositions are blended to form a homogeneous mixture using any of the known techniques that will not adversely affect the storage stability of the composition in the absence of ultraviolet light. Unless the composition exhibits borderline storage stability in the absence of UV radiation, the order in which the ingredients are added is not critical. If the ingredients have a tendency to react, it is preferable that the alkenyl-containing polymethylsiloxane be combined first with the viscosity stabilizer and, lastly, the mercapto-containing compound and photoinitiator. Curable compositions containing a viscosity stabilizer can typically be stored for longer than about 6 months under ambient conditions. The compositions cure rapidly, typically requiring about one second or less of exposure to UV radiation from a medium pressure mercury vapor arc lamp. The amount of energy required to cure preferred compositions is from 0.1 to about 3 joules per square centimeter, depending upon the ingredients and the thickness of the layer of curable composition applied to a substrate. The intensity of the UV radiation used to cure the compositions can be measured using any of the available techniques. A preferred method employs an integrating photometer which is exposed to a mercury vapor arc lamp or other source of UV radiation under the same conditions used to cure the coated substrate. A commercially available integrating photometer is the "Light Bug" available from International Light. With the method of the present invention, the visible dye is added to the composition and upon exposure to UV radiation, the color changes when cure is achieved. This will eliminate the necessity to constantly check with a photometer during manufacturing operations. In those instances, where a wet film of the composition is exposed to the UV radiation and the color does not change or the color returns after a short period of time, it is apparent that the dosage is insufficient. These organosiloxane gels can include uses as protective coatings and encapsulants for electrical and electronic components, particularly solid state devices such as integrated circuits and electrical connectors. The gel can prevent penetration of atmospheric moisture and other materials that can corrode or otherwise damage the component. An example of such UV curable compositions which cure to gels is described by Lee et al in U.S. Patent No. 5,063,102 to show UV radiation curable compositions useful in the present invention. The method of the present invention can be used for UV curable compositions which make films, coatings, encapsulants in the electrical and electronics industries such as for printed circuit boards, electrical connectors and electrical splices. The following examples are presented for illustrative purposes. In the following examples, "part" or "parts" represents "part by weight" or "parts by weight", "%" are percent by weight unless otherwise stated. Example 1 A curable composition of this invention was prepared by blending 0.002 part of anthraquinone dye purchased from Sandoz Chemical, Charlotte, North Carolina as Nitro-Fast Blue 2B, (which is also known as C.I. Solvent Blue 104) with the following ingredients to homogeneity: 97.5 parts of an organosiloxane copolymer containing the following units, expressed in mol percent: 94% dimethylsiloxane units, 1% dimethylvinylsiloxy units, 2% trimethylsiloxy units and 3% monomethylsiloxy units. The viscosity of the copolymer was 10 -5 m 2 /s at 25°C.; 1.25 parts of a trimethylsiloxy-terminated polydiorganosiloxane copolymer containing an average of 43 dimethylsiloxane units and 4 methyl(3-mercaptopropyl)siloxane units per molecule; and 1.25 parts of 2-hydroxy-2-methyl-1-phenylpropan-1-one as the photoinitiator. The blended composition was blue. A 10g sample of the blue composition was passed through a UV-6 Single Lamp Conveyorized UV Curing Unit manufactured by Colight, Minneapolis, Minnesota. The conveyor was set at about 0.015m/s (3ft/min) and the lamp was at 200 watts. The composition changed to light yellow and was fully cured. The cured sample remained yellow for at least six months. Example 2 UV curable compositions were prepared as described in Example 1, except the 2-hydroxy-2-methyl-1-phenylpropan-1-one were left out and another photoinitiator was used in its place as indicated below. The mixtures were prepared by mixing 0.79 part of the photoinitiators listed below with 49.5 parts of the UV curable composition as described in Example 1 but without the 2-hydroxy-2-methyl-1-phenylpropan-1-one. The Colight system used to expose the samples to UV radiation was the same as described in Example 1. The results observed were as described below along with the photoinitiator used in each case. Sample 1: Benzophenone : passed through the Colight system once did not cure or change color, a second pass through the Colight system changed the color a little to a paler blue and the film was slightly cured, after four passes through the Colight system the blue was almost gone and the film was almost cured. Sample 2: passed through the Colight system once produced a good cure and the color changed to yellow. Sample 3: Diethoxyacetophenone : passed through the Colight system once produced a good cure and the color changed to dark yellow. Sample 4: A mixture of 50 weight percent and 50 weight percent benzophenone : passed through the Colight system once produced a good cure and the color changed to dark yellow. This yellow was the same as observed for Sample 3. These samples showed that the color change occurred with different kinds of free radical producing photoinitiators. Example 3 An ultraviolet radiation curable composition was prepared as described in Example 1 except 1-amino-2-bromo-4-hydroxy-anthraquinone (Sandoplast Red 2B powder, made by Sandoz Chemicals, Charlotte, North Carolina). The composition was red in color before being exposed to ultraviolet radiation by passing it through the Colight system as described in Example 1. After the exposure to the UV radiation, the composition was cured and had changed color from red to yellow. The color did not return upon standing. Example 4 An ultraviolet radiation curable composition was prepared as described in Example 1 except a disazo dye sold by Sandoz Chemicals, Charlotte, North Carolina, as Nitro Fast Yellow B powder (also known as C.I. Solvent Yellow 30, C.I. 21240, has a formula C 37 H 36 N 4 O 2 and a CAS number 3321-10-6). The composition was dark yellow in color before being exposed to ultraviolet radiation by passing it through the Colight system as described in Example 1. After the exposure to the UV radiation, the composition was cured and had changed color from dark yellow to very pale yellow. The color did not return upon standing.

Claims: A method of indicating a cure point of an ultraviolet radiation curing composition comprising: adding a non-cure inhibiting amount of a dye with a visible color to an ultraviolet radiation curable composition comprising a photoinitiator which generates free radicals upon exposure to ultraviolet radiation and an organosiloxane gel composition which cures upon exposure to ultraviolet radiation which comprises an alkenyl-containing polymethylsiloxane consisting essentially of 80 to 97 mole percent of (CH 3 ) 2 SiO units, 2 to 10 mol percent of CH 3 SiO 1.5 units, 1 to 6 mol percent of (CH 3 ) 3 SiO 0.5 units and 2 to 4 mol percent of units represented by the formula (CH 3 ) a R 1 SiO (3-a)/2 , where R 1 represents 3-cyclohexenyl, cyclohexenylethyl or CH 2 =CH(CH 2 ) x , a is 1 or 2 and x represents 0 or an integer from 1 to 6, inclusive; and a mercaptoalkyl-containing compound selected from the group consisting of a liquid organic compound having an average of at least two mercapto groups per molecule and a liquid mercaptoalkyl-containing polydiorganosiloxane containing an average of at least two repeating units per molecule corresponding to the formula where R 2 represents a mercaptoalkyl radical, R 3 represents a monovalent hydrocarbon or halohydrocarbon radical and said curing agent is compatible with the other ingredients of said composition; and where the molar ratio of mercapto groups to alkenyl radicals in material is 1 or less, and thereafter exposing the resulting composition to a dosage of ultraviolet radiation which causes the visible color to either disappear or change to a different color indicating that cure has occurred coincidentally. A method according to claim 1, wherein the mercapto-containing polyorganosiloxane is a compound of the formula: R 6 2 R 4 SiO(R 7 R 6 SiO) y (R 6 R 5 SiO) z SiR 4 R 6 2 wherein R 7 represents a mercaptoalkyl radical containing from 2 to 6 carbon atoms, each R 6 represents methyl, phenyl or 3,3,3-trifluoropropyl, R 4 represents R 6 , R 7 or a hydroxyl group, each R 5 is methyl or phenyl, y and z are each 0 or greater, with the proviso that R 4 represents a mercaptoalkyl radical when both y and z are 0. A method according to claim 1 or claim 2 wherein the organosiloxane gel composition further comprises an organic peroxide which decomposes at a temperature within the range of from 100°C to 250°C.

Inventors: FIELD ARTHUR JAMES, STRONG MICHAEL RAYMOND, FIELD, ARTHUR JAMES, STRONG, MICHAEL RAYMOND
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1991
Main classification: C08K5
IPC codes: C08L75, C08J7, C09D183, C08J3, C08L83, H05K1, H05K3, C08F2, C08F299, C08K5, C09D4, C08F290