Radiation curable organosiloxane gel compositions

Abstract: Organosiloxane compositions that cure in the presence of ultraviolet radiation comprise a polymethylsiloxane containing a specified combination of mono-, di- and trifunctional siloxane units, an amount of a mercaptoalkyl-substituted polyorganosiloxane or an organic mercaptan 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 ultraviolet radiation. The cured compositions are particularly useful as coatings and encapsulants to protect electronic devices against the deleterious effects of materials present in the environment adjacent to said devices.

Description: This invention relates to radiation curable organosiloxane compositions. More particularly, this invention relates to one-part organosiloxane compositions that cure upon exposure to ultraviolet radiation to yield gels that will not harden at low temperatures and provide suitable coatings and encapsulants for electronic components. Organosiloxane gels are characterized by their softness and resiliency. The hardness of cured gels is typically too low to be measured on any of the Shore durometer scales, including the "00" scale. The hardness of these gels is sometimes expressed as the depth to which a probe of specified dimensions and weight will penetrate a cured gel. One of the more popular uses of organosiloxane gels is as protective coatings and encapsulants for electrical and electronic components, particularly solid state devices such as integrated circuits and electrical connectors. The gel prevents penetration of atmospheric moisture and other materials that can corrode or otherwise damage the component. The present inventors discovered that when curable compositions containing an alkenyl-substituted polyorganosiloxane of the type described in Brown et al., U.S. Patent No. 4,374,967, a photoinitiator and a relatively small amount of a mercapto-substituted polyorganosiloxane as the curing agent are exposed to ultraviolet radiation the compositions cure in a minute or less. Moreover, cured gels prepared using the present compositions remain soft and flexible even at low temperatures and are suitable for encapsulation of hybrid circuits and other electronic devices. When applied to metal surfaces preferred compositions cause less corrosion relative to prior art compositions that cure by a similar mechanism. Organosiloxane compositions that cure in the presence of ultraviolet radiation comprise a polymethylsiloxane containing a specified combination of mono-, di- and trifunctional siloxane units, an amount of a mercaptoalkylsubstituted 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 ultraviolet radiation. The present invention provides an organosiloxane gel composition that cures upon exposure to ultraviolet radiation, said composition comprising (A) a polyorganosiloxane being liquid under ambient conditions consisting essentially of at least 80 mole percent of (CH 3 ) 2 SiO units, 2 to 10 mole percent of CH 3 SiO 1,5 units, 1 to 6 mole percent of (CH 3 ) 3 SiO O,5 units and 0,2 to 4 mole percent of units represented by the formula (CH 3 ) a R 1 SiO (3-a)/2 , wherein the sum of (CH 3 ) 2 SiO units, CH 3 SiO 1,5 units, (CH 3 ) 3 SiO 0,5 units and (CH 3 ) a R 1 SiO (3-a)/2 units adds up to 100 mole percent, 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, optionally in combination with other compatible liquid organosiloxanes containing an average of at least 2 silicon-bonded ethylenically unsaturated hydrocarbon radicals per molecule, wherein the concentration of the additional organosiloxane (s) can be up to about 50 weight percent, based on the total concentration of ethylenically unsaturated organosiloxanes present in the curable composition; (B) as the curing agent for said composition, a liquid organic compound containing an average of at least two mercapto groups per molecule in combination with carbon, hydrogen and optionally oxygen or a liquid mercaptoalkyl substituted 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; wherein the compatibility is determined by blending one weight percent of the candidate with the other liquid ingredients of the curable composition and mixing the resultant composition at room temperature or with heating to a temperature of about 80°C, the resultant composition being optically clear without any dispersed or precipitated material; and (C) a photoinitiator in an amount sufficient to promote curing of said composition in the presence of ultraviolet radiation, where the molar ratio of mercapto groups to alkenyl radicals in said composition is 0.5 or less; (D) optionally up to 1 weight percent of a viscosity stabilizer for inhibition of gelation during storage; (E) optionally an organic peroxide having a decomposition temperature of 100 to 250°C. The alkenyl-substituted polyorganosiloxanes, also referred to in this specification as ingredient A, are liquids under ambient conditions and contain at least 80 mol percent of difunctional dimethylsiloxane units, (CH 3 ) 2 SiO, from 2 to 10 mol percent of trifunctional monomethylsiloxy units, CH 3 SiO 3/2 , from 1 to 6 mol percent of trimethylsiloxy units, (CH 3 ) 3 SiO 1/2 and from 0.2 to 4 mol percent of organoalkenylsiloxy units of 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 presence of the trifunctional monomethylsiloxy units is considered responsible for the excellent low temperature properties of gels prepared using the present alkenyl-substituted polymers. In the absence of these units, the polymer would be in a hard crystalline phase at temperatures below about -40°C. One class of polyorganosiloxanes that can be combined with a curing agent containing at least two mercapto groups per molecule and a photoinitiator to form the present curable compositions 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 polyorganosiloxanes contain up to 95 mole 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, wherein the sum of dimethylsiloxane units, monomethylsiloxy units, trimethylsiloxy units and dimethylvinylsiloxy units adds up to 100 mole percent. The alkenyl substituted polyorganosiloxanes described as ingredient A can be used in combinations with other compatible liquid organosiloxanes containing an average of at least two silicon-bonded ethylenically unsaturated hydrocarbon radicals per molecule. The concentration of the additional organosiloxane(s) can be up to about 50 weight percent, based on the total concentration of ethylenically unsaturated organosiloxanes present in the curable composition. Diorganosiloxanes wherein the ethylenically unsaturated radical is vinyl and the silicon atoms are bonded to at least one methyl radical are preferred. Any hydrocarbon radicals other than methyl or vinyl are preferably phenyl or 3,3,3-trifluoropropyl. The alkenyl-substituted polyorganosiloxane (Ingredient A) of the present compositions, is cured by reacting with a liquid polyorganosiloxane curing agent (ingredient B) containing an average of at least two silicon-bonded mercaptoalkyl radicals per molecule or with an organic compound containing an average of at least two mercapto (-SH) groups per molecule. The silicon atoms that constitute the mercaptoalkyl-substituted polydiorganosiloxane can be arranged in a linear or branched configuration. One requirement of this ingredient is that it be compatible with the other ingredients of the present curable compositions. Preferred embodiments of ingredient B are can be represented by the average formula I R 3 2 R 4 SiO(R 2 R 3 SiO) y (R 3 R 5 SiO) z SiR 4 R 3 2 where R 2 represents a mercaptoalkyl radical containing from 2 to 6 carbon atoms, each R 3 represents methyl, phenyl or 3,3,3-trifluoropropyl, R 4 represents R 2 , R 3 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 3 , R 4 and R 5 are methyl and ingredient B is represented by the average formula II (CH 3 ) 3 SiO{(CH 3 ) 2 SiO} z (R 2 CH 3 SiO) y Si(CH 3 ) 3 where R 2 , y and z have the same definitions as in formula I and y is at least 2. R 2 is preferably 3-mercaptopropyl or mercaptoisobutyl, based on the availability of the corresponding intermediates that are used to prepare ingredient B, y is from 40 to 45 inclusive and z is from 3 to about 10. The second class of curing agents are organic compounds containing an average of at least 2 mercapto (-SH) groups per molecule. In addition to these groups the compounds 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, a mercaptan, a mercaptocarboxylic acid or an ester derived from a mercaptocarboxylic acid and a mono- or polyhydric alcohol. Examples of suitable organic compounds include but are not limited to 2,2'-dimercaptodiethylether, dipentaerythritolhexa(3-mercaptopropionate), glycol dimercaptoacetate, trimethylolpropane trithioglycolate and trimethylolpropane trimercaptopropionate. The molar ratio of mercapto groups in ingredient B to alkenyl radicals in ingredient A is critical to the physical properties of cured gels prepared using the compositions of this invention. This ratio is 0.5 or less to avoid any adverse effect on the long-term electrical properties of integrated circuits and other electronic components that are coated or encapsulated using the present compositions. The photoinitiator (ingredient C) can be any compound that will initiate a reaction between the silicon-bonded alkenyl radicals of ingredient A and the mercapto groups or silicon-bonded mercaptoalkyl radicals of ingredient B when the curable compositions of this invention are exposed to ultraviolet light. One requirement of the photoinitiator is that it be compatible with the other ingredients of the present compositions. Compatibility can readily be determined by blending one weight percent of the candidate with the other liquid ingredients of the 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. A second requirement of the photoinitiator is that it does not cause the ingredients of the curable composition to react with one another in the absence of ultraviolet light. Photoinitiators that can be used in the present curable compositions include but are not limited to benzophenone, acetonaphthone, acetophenone, benzoin methyl ether, benzoin isobutyl ether, 2-hydroxy-2-methyl-1-phenyl-propan-1-one, 1-hydroxycyclohexylphenyl ketone, 2,2-diethoxyacetophenone, 3- hydroxypropylphenyl ketone and 3-hydroxypropyl-p-isopropylphenyl ketone. Preferred photoinitiators include 2-hydroxy-2-methyl-1-phenyl-propan-1-one, 1-hydroxycyclohexylphenyl ketone, 2,2-diethoxyacetophenone and 3-hydroxypropyl phenyl ketone. Other classes of useful photoinitiators include compatible polysilanes of the type described by West in U.S. Patent No. 4,260,780, which issued on April 7, 1981; the aminated methylpolysilanes described by Baney et al. in U.S. Patent No. 4,314,956, which issued on February 9, 1982; the methylpolysilanes described by Peterson et al. in U.S. Patent No. 4,276,424, which issued on June 30, 1981; and the polysilastyrene described by West et al, in U.S. Patent No. 4,324,901, which issued on April 13, 1982. In addition to the alkenyl-substituted polyorganosiloxane, curing agent and photoinitiator the present compositions can contain optional ingredients such as up to about one weight percent of conventional viscosity stabilizers to inhibit gelation during storage. Acceptable stabilizers limit the increase in viscosity during a 24 hour period at 100°C. to a factor of less than two, i.e. a doubling of viscosity. 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. To ensure complete curing of the present compositions, it may be desirable to include as one of the ingredients an organic peroxide that decomposes at a temperature within the range of from about 100 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 curable gel 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, which issued to Michael A. Lutz on August 17, 1982. The patent teaches silica fillers suitable for use in the optically clear gel compositions of this invention. The ingredients of the present 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 ultraviolet 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 ingredient A be added first followed by the viscosity stabilizer and, lastly, the mercapto-containing curing agent and photoinitiator. Curable compositions containing a viscosity stabilizer can typically be stored for longer than about 6 months under ambient conditions. The present compositions cure rapidly, typically requiring about one second or less of exposure to the 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 radiation used to cure the present 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. The following examples are intended to describe preferred embodiments of the present invention and should not be interpreted as limiting the scope of the invention as defined in the accompanying claims. Unless otherwise specified all parts and percentages specified in the examples are by weight and viscosities were measured at 25°C. Example 1 A curable composition of this invention was prepared by blending the following ingredients to homogeneity: 97.5 parts of an organosiloxane copolymer containing the following units, expressed in mol percent: 94 percent dimethylsiloxane units, 1 percent dimethylvinylsiloxy units, 2 percent trimethylsiloxy units and 3 percent monomethylsiloxy units. The viscosity of the copolymer was 10 -5 m 2 /s at 25°C.; 1.25 parts of a trimethylsiloxy-terminated diorganosiloxane copolymer containing an average of 43 dimethylsiloxane units and 4 methyl-3-mercaptopropylsiloxane units per molecule; and 1.25 parts of 2-hydroxy-2-methyl-1-phenylpropan-1-one as the photoinitiator. The test sample used to evaluate the composition was an electronic device comprising an integrated circuit having an aluminum lead frame and mounted on a small circuit board enclosed on all sides by walls formed from an organic polymer. The circuit board was covered with a layer of curable composition having a thickness of about 0.6 cm. The test sample was then passed under a mercury vapor arc at a speed of two feet per minute to cure the composition. The amount of radiation received by the sample was 3 joules/cm 2 . The composition cured to a gel during this exposure. The test sample containing the cured gel was then placed in a chamber and subjected to 1000 temperature cycles of from -55 to 125°C. to simulate the conditions in the engine compartment of an automobile. There was no evidence of any electrical failure of the electronic circuit following this test. The test sample was then placed in a chamber and exposed to 85°C. and a relative humidity of 85% for 100 hours. No electrical failures were observed following this portion of the test procedure. Finally the test sample was exposed to a salt water spray for 500 hours, at which time the exposed portion of the aluminum lead frame of the integrated circuit was visually inspected for evidence of corrosion. No evidence of corrosion was observed. Example 2 This example describes curable compositions containing mercapto-substituted curing agents that are within the scope of the present invention. The vinyl-containing polymer (A1) was of the same type described in the preceding Example 1 and contained 0.4 weight percent of vinyl radicals. One of the compositions evaluated contained 7.5 parts by weight of A1 and 2.5 parts by weight of a second polyorganosiloxane (A2). A2 was a liquid dimethylvinylsiloxy-terminated polyorganosiloxane containing 93 mole percent of dimethylsiloxane units, 7 mole percent methyl-3,3,3-trifluoropropylsiloxane units and 0.6 weight percent of vinyl radicals. One of the two curing agents (B1) was a dimethylmercaptoisobutylsiloxy-terminated polydimethylsiloxane containing an average of 5 dimethylsiloxane units per molecule. The second curing agent (B2) was a trimethylsiloxy-terminated random copolymer containing an average of 90 dimethylsiloxane units and 8 methylmercaptoisobutylsiloxane units per molecule. Curing agent B1 was used in combination with polymer A1 at a concentration of 5(C), 4.3(C) and 2.8 (I) weight percent, based on vinyl-containing polymer, which was equivalent to a molar ratio of mercapto to vinyl groups of 1.0 (C), 0.75 (C) and 0.5 (I), respectively wherein (C) refers to a comparison example and (I) refers to an example of the present invention. The curable compositions contained 2,2-diethoxyacetophenone (Ingredient C) as the photoinitiator in an amount specified in the following table. Each of the curable compositions were coated as a 0.008 inch (0.2 mm)-thick film onto a sheet of aluminum and then placed on a conveyer that passed under two medium pressure mercury vapor arc lamps housed in an Ashdee Model UV-24H Lab Reactor. The speed of the conveyor was 90 feet (27.4 meters) per minute, equivalent to an exposure time of one second. The quantity of radiation reaching the coated sheet was 38 mJ/cm 2 , measured using a model 205 UV Powermeter supplied by Optical Associates and equipped with a 365 nanometer probe. The distance between the lamps and the coated sheet was about 10 cm. All of the compositions cured to a soft, flexible, transparent gel under these conditions. The types and amounts of ingredients present in the curable compositions evaluated are summarized in the following table. Analysis by differential scanning calorimetry indicated a glass transition temperature of -123°C. for all of the cured gels, making them ideally suited for use in low temperature environments. The penetration values for the gels prepared using samples 1 and 2 were 45 and 100 micrometers/gram, respectively. These values were measured using a DuPont Instruments Model 943 Thermomechanical Analyzer equipped with a 0.6 cm. (1/4 inch) -diameter probe.

Claims: An organosiloxane gel composition that cures upon exposure to ultraviolet radiation, said composition comprising (A) a polyorganosiloxane being liquid under ambient conditions consisting essentially of at least 80 mole percent of (CH 3 ) 2 SiO units, 2 to 10 mole percent of CH 3 SiO 1,5 units, 1 to 6 mole percent of (CH 3 ) 3 SiO 0,5 units and 0,2 to 4 mole percent of units represented by the formula (CH 3 ) a R 1 SiO (3-a)/2 , wherein the sum of (CH 3 ) 2 SiO units, CH 3 SiO 1,5 units, (CH 3 ) 3 SiO 0,5 units and (CH 3 ) a R 1 SiO (3-a)/2 units adds up to 100 mole percent, 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, optionally in combination with other compatible liquid organosiloxanes containing an average of at least 2 silicon-bonded ethylenically unsaturated hydrocarbon radicals per molecule, wherein the concentration of the additional organosiloxane (s) can be up to about 50 weight percent, based on the total concentration of ethylenically unsaturated organosiloxanes present in the curable composition; (B) as the curing agent for said composition, a liquid organic compound containing an average of at least two mercapto groups per molecule in combination with carbon, hydrogen and optionally oxygen or a liquid mercaptoalkyl substituted 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; wherein the compatibility is determined by blending one weight percent of the candidate with the other liquid ingredients of the curable composition and mixing the resultant composition at room temperature or with heating to a temperature of about 80°C, the resultant composition being optically clear without any dispersed or precipitated material; and (C) a photoinitiator in an amount sufficient to promote curing of said composition in the presence of ultraviolet radiation, where the molar ratio of mercapto groups to alkenyl radicals in said composition is 0.5 or less; (D) optionally up to 1 weight percent of a viscosity stabilizer for inhibition of gelation during storage; (E) optionally an organic peroxide having a decomposition temperature of 100 to 250°C. A composition according to claim 1 where said composition contains a viscosity stabilizer to inhibit curing of said composition during storage. A composition according to claim 1 where said composition contains an organic peroxide. A method for protecting an electronic device against the deleterious effects of materials present in the environment of the device, said method comprising coating or encapsulating said device with a composition according to claim 1 and subsequently exposing said device to ultraviolet radiation for a time period sufficient to cure said composition.

Inventors: LEE CHI-LONG, LUTZ MICHAEL ANDREW, VANWERT BERNARD, LEE, CHI-LONG, LUTZ, MICHAEL ANDREW, VANWERT, BERNARD
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1990
Main classification: C08L83
IPC codes: C08L83, H01L23, H01L21, C09D183, C08K5, C09D5