GB2140815A

GB2140815A - Silicone conformal coatings

Info

Publication number: GB2140815A
Application number: GB08413204A
Authority: GB
Inventors: Arnold Torkelson
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1983-06-02
Filing date: 1984-05-23
Publication date: 1984-12-05
Also published as: GB8413204D0; BE899800A; JPS6047024A; CA1335617C; DE3418623A1; FR2546897A1

Abstract

A composition of matter comprises the co-condensation reaction product of (1) a hydroxy-containing resinous copolymer comprised of R3SiO0.5 units and SiO2 units and (2) a linear hydroxy-endstopped diorganopolysiloxane, and may be used to provide a solvent- removable conformal protective coating on electrical and electronic components. e

Description

SPECIFICATION Silicone conformal coatings The present invention relates to protective silicone coating compositions and articles coated therewith. More specifically, the pre sent invention relates to silicone compositions comprising the reaction product of (1) a hy droxy-containing resinous copolymer of RsSiOo.5 units and SiO2 units and (2) a linear organopolysiloxane containing terminal silicon bonded hydroxy groups. Additionally, the in vention relates to articles coated with such compositions and methods for protecting articles such as electrical and electronic devices by coating such devices with the composition of the present invention.

Recent advances in the arts of electrical and electroics engineering have resulted in very complex semiconductor circuits contained in much smaller areas than heretofore possible.

Such circuits place increased demands upon electrical insulation to minimize mechanical damage due to jolting or jostling and to maximize the ability of the system to withstand extreme environmental conditions such as heat, cold and moisture. Furthermore, it is important that the insulating material have very low levels of ionic impurities so as to minimize electrical contamination.

Among the more useful insulative materials presently utilized are silicone compositions such as greases, resins and room temperature vulcanizable polysiloxanes. Although such silicone compositions give the desired protection to sensitive electrical and electronic components, they have the shortcoming that once they are in place it is virtually impossible to remove all or a portion of the coating so as to provide access for repair or servicing of the components.

While silicone compositions formed from a linear hydroxy-chainstopped diorganopolysiloxane and an MQ resin are described in the arts of pressure sensitive adhesives and room temperature vulcanizable (RTV) compositions, so far as is known, none have been provided which are tough durable coating, yet capable of being removed by a solvent.

It, is therefore, an object of the present invention to provide a composition for protecting electrical and electronic equipment from adverse environmental conditions, but which composition is removable by a solvent so as to provide access for repair and servicing of such electronic components and circuits.

Other objects and advantages of the present invention will be obvious from the following detailed description.

The present invention provides a novel silicone conformal coating composition, a method for protecting electronic and electrical components utilizing such composition, and articles having such composition cured thereon. Briefly stated, and in its broadest sense, the silicone conformal coating composition of the present invention comprises the co-condensation reaction product of (1) from approximately 55 to 70 parts by weight of an organopolysiioxane cohydrolysis product of a trialkyl hydrolyzable silane and an alkyl silicate or sodium silicate, in which the hydrolyzable silane and the silicate are reacted in the ratio of from about 0.33 to about 0.55 mole of hydrolyzable silane per mole of silicate, said alkyl radicals having at most four carbon atoms, and (2) 30 to 45 parts by weight of a linear diorganopolysiloxane fluid having terminal silicon-bonded hydroxy groups, said organo groups being selected from the group consisting of alkyl radicals, aryl radicals, alkaryl radicals, aralkyl radicals, haloaryl radicals and alkenyl radicals and mixtures thereof, and said diorganopolysiloxane having a viscosity of from about 200,000 centipoise to about 2,000,000 centipoise at 25"C.

In accordance with the present- invention there are provided novel silicone compositions which are solvent and removable and exhibit primerless adhesion to a wide variety of substrates. Generally, such compositions are the co-condensation reation product of a mixture containing (1) approximately 55 to 70 parts and preferably 60 to 65 parts by weight of a hydroxy containing resinous copolymer of RsSi005 units (M units) and SiO2 (Q units), wherein the ratio of R3Si005 units per SiO2 unit varies from approximately 0.33 to 0.55:1 and preferably 0.35 to 0.45::1, and wherein R is an alkyl radical preferably having not more than four carbon atoms, and (2) approximately 30 to 45 parts, and preferably 35 to 40 parts, by weight of a hydroxy endstopped diorganopolysiloxane having an average of about two organic radicals per silicon atom, said organic radicals being selected from the group consisting of alkyl radicals, aryl radicals, alkaryl radicals, aralkyl radicals, haloaryl radicals and alkenyl radicals and mixtures thereof, said diorganopolysiloxane having a viscosity of from about 200,000 centipoise to about 2,000,000 centipoise at 25 C, and the sum of (1) and (2) equalling 100 parts.

The resinous copolymer of R3SiOo 5 units and SiO2 units employed in the practice of the present invention is well known in the prior art and is described in Dexter, U.S. Pat. No.

2,736,721, Goodwin, Jr., U.S. Pat. No.

2,857,356, Daudt et al., U.S. Pat. No.

2,676,182 and Modic, U.S. Pat. No.

3,017,384, all of which are incorporated by reference into the instant disclosure. Although a variety of methods are available for producing such MQ resins, the present invention does not depend upon the particular method by which the resinous copolymer is made for patentability. However, it has been found that MQ resins made according to the process of Goodwin, Jr., U.S. Pat. No. 2,857,356, are particularly preferable as they directly provide a hydroxy containing resin. The production of such resins involes the cohydrolysis of a trialkyl hydrolyzable silane and an alkyl silicate or sodium silicate; that is, the trialkyl hydrolyzable silane and alkyl silicate or sodium silicate - are added to a suitable solvent and thereafter a sufficient amount of water is added to effect the desired cohydrolysis and co-condensation.

Most preferably, there is utilized trimethylchlorosilane and ethylorthosilicate to obtain such hydroxy-containing MQ resin. Of course, the proportions of hydrolyzable silane and silicate must be such that there results a resinous copolymer containing from approximately 0.33 to 0.55 R3SiOo5 unit per SiO2 unit.

Briefly, the MO resin is obtained by dissolving the two ingredients in a suitable solvent, for example, toluene, xylene or an aliphatic hydrocarbon, and then adding the mixture with stirring, to water maintained at a temperature on the order of 60 to 85 C. Thereafter, the resulting two-phase system is processed to remove the resulting water-alcohol layer and the resinous material is neutralized with an amount of sodium bicarbonate or other alkaline material. The resinous solution is then filtered and the resinous solids content adjused to the desired level, using where necessary, additional amounts of solvent. Those interested in greater detail or additional information are urged to consult the aforementioned patents.

Regardless of the method by which the resinous copolymer is formed, it is preferred that the alkyl groups be lower alkyl groups such as methyl, ethyl, propyl and butyl since higher alkyl radicals undesirably slow down the hydrolysis of the silane and cause a type of co-condensation with the silicate which leads to less desirable products. However, such alkyl groups may be same or different alkyl groups. It is also important that in the final product there should be approximately 0.33 to 0.55 M units per Q unit so that optimal properties, such as toughness, tensile strength and freedom from tackiness, are obtained.

The linear, high viscosity organopolysiloxane used for co-reacting with the hydroxycontaining MQ resin must of necessity have terminal silicon-bonded hydroxy groups to permit ready copolymerization with the resin.

Such organopolysiloxanes may also be prepared by any of the known methods, for example, acid or alkali catalyzed polymerization of the corrsponding cyclic siloxanes.

Again, the processes for preparing hydroxyendstoped polysiloxanes are well known in the art, for example, as described in Goodwin, Jr., U.S. Pat. No. 2,857,356. The Goodwin, Jr. process involves heating cyclic organopolysiloxanes at a temperature of 125no to 150"C in the presence of a small amount of rearrangement catalyst such as potassium hydroxide. In general, the polymerization is carried out for a time sufficient to obtain a high molecular weight product, preferably having a viscosity within the range of about 75,000 to 125,000 centipoise. After such polymerized product is obtained, it is treated to provide terminal silicon-bonded hydroxy groups on the molecules of the organopolysiloxane for coreaction with the hydroxyl groups of the resin.

This can be accomplished by blowing steam through or across the surface of the polymer.

However, this decreases the viscosity of the polymer while at the same time increasing the silanol content of the organopolysiloxane. Although the organopolysiloxane could be used in this form, it is preferable to reheat the organosiloxane, which still contains the rearrangement catalyzed, to a temperature of about 125"C to 150 C to obtain a material having a viscosity of from 200,000 to 2,000,000 centipoise. The preferred range of viscosities is from 400,00 to 800,000 centipoise. Once the desired viscosity has been reached, the organopolysiloxane should be treated in order to inactivate the siloxane rearrangement catalyst.

The organo groups attached to the silicon atoms of the diorganopolysiloxane are selected from the class consisting of alkyl radicals such as methyl, ethyl, propyl, butyl and isobutyl; aryl radicals such as phenyl and naphthyl; alkaryl radicals such as tolyl, xylyl and ethylphenyl; aralkyl radicals such as benzyl and phenylethyl; haloaryl radicals such as chlorophenyl, tetrachlorophenyl, and difluorophenyl; and alkenyl radicals such as vinyl and allyl. It is also intended that mixtures of the foregoing radicals are also within the scope of the present invention. Preferably the organo groups are an alkyl group selected from methyl, ethyl, propyl and butyl and most preferably is methyl.

Preparation of the silicone conformal coating composition from the MO resin and diorganopolysiloxane is relatively simple in that it merely requires mixing the two together and heating the mixture to effect co-condenstaion.

In practicing the present invention, the critical consideration is that the ratio of resin to organopolysiloxane be within the range of 1.22 to 2.33:1. Utilization of higher levels of organopolysiloxane will result in a final product which is too tacky, whereas utilization of higher levels of MO resin will result in a final product which is too brittle.

To accomplish the co-condensation of resin and organopolysiloxane, the resin is heated, for instance, at a temperature of about 100 to 150"C for from one half to six hours or until a product of desired characteristics is obtained.

After a suitable material is obtained the product is dissolved in a solvent such as toluene, xylene, an aliphatic hydrocarbon, or a halogenated aliphatic hydrocarbon, to a convenient solids content, for example from 10 to 50 percent. The amount of solvent in the mixture can vary widely as its only function is to facilitate handling and application of the conformal coating composition to substrates such as electronic circuits and devices.

The present invention further provides a method for protecting electrical and electronic components from adverse environmental conditions comprising applying a 0.00254 to 0.254 millimeter thick coating of the composition of the present invention to such electronic equipment. Throughout the present disclosure the terms electrical and electronic devices, components, equipment and the like are used interchangeably and are meant to include, but not limited to, devices such as circuits, transistors, diodes, resistors, capacitors and the like. Following application of the formulation to the equipment or devices by.

conventional means such as spraying, brushing and dipping, the formulation is air dried for 10 to 60 minutes or until "tack free" and subsequently oven dried at a temperature in the range of 75 to 150"C for 10 to 60 minutes.

Optionally, a catalyst or curing agent is included in the composition. Preferably such curing agent is a peroxide catalyst such as benzoyl peroxide or dichlorobenzoyl peroxide at a level of 1 to 2% based. on the silicone content of the coating composition. However, any organic peroxide present in amounts rangeing from about 0.01 to 3.0% by weight is effective. Of course, any suitable catalyst can be employed. In application, the catalyst is mixed into the 10 to 50 percent solids solution of the conformal coating composition.

The solution is then coated on the desired electronic components, and thereafter the coated component is air dried and then heated at a temperature of from about 70 to 1 50'C for 10 to 60 minutes to evaporate the solvent and effect curing of the conformal coating composition. The solvent must be removed before the temperature is raised to activate the peroxide.

The article thus formed is effective for protecting electronic and electrical components from adverse environmental conditions such .as heat, cold and moisture as well as protecting against mechanical damage caused by jostling or jolting of the equipment. An important advantage of utilizing the composition of the present invention on electronic components, circuitboards and the like is that the composition is easily removable by exposing it to a solvent such as toluene, xylene, or an aliphatic hydrocarbon. Thus, in the event that the coated article malfunctions, it is now possible to remove all or part of the silicone conformal coating composition so as to provide access for repair of the electronic device or circuit. Moreover, it is now possible to gain access to such coated electronic devices for routine servicing, if necessary, rather than only for making repairs. Although it is envisioned that the composition of the present invention will primarily be employed to protect and insulate electronic and electrical equipment; other components, devices, substrates and the like which can be protected by the instant silicone conformal coating composition are also within the scope of the invention.

Claims

1. A composition of matter comprising the co-condensation reaction product of a mixture containing: (a) approximately 55 to 70 parts by weight of a hydroxy-containing resinous copolymer comprised of R3SiOo 5 units and SiO2 units, wherein the ratio of R3Si005 units per SiO2 unit ranges from approximately 0.33 to 0.55::1, and wherein R is an alkyl radical or a mixture of alkyl radicals, and (b) approximately 30 to 45 parts by weight of a hydroxy-endstopped diorganopolysiloxane, wherein the organo radicals of said diorganopolysiloxane are selected from the group consisting of alkyl radicals, aryl radicals, alkaryl radicals, aralkyl radicals, haloaryl radicals and alkenyl radicals and mixtures thereof, wherein said diorganopolysiloxane has a viscosity ranging from about 200,000 centipoise to about 2,000,000 centipoise at 25"C, and wherein the sum of (a) and (b) equals 100 parts.

2. The composition of Claim 1 wherein the amount of hydroxy-containing resinous copolymer varies from about 60 to about 65 parts by weight and the amount of hydroxyendstopped diorganopolysiloxane varies from about 35 to 40 parts by weight.

3. The composition of Claim 1 wherein the ratio of R3SiOo 5 units per SiO2 unit ranges from about 0.35 to about 0.45:1.

4. The composition of Claim 1 wherein the hydroxy-containing resinous copolymer is the reaction product of a trialkyl hydrolyzable silane and an alkyl silicate or sodium silicate.

5. The composition of Claim 4 wherein the trialkyl hydrolyzable silane is trimethylchlorosilane.

6. The composition of Claim 4 wherein the alkyl silicate is tetraethylorthosilicate.

7. The composition of Claim 1 wherein the alkyl radicals of the R3SiOo5 units are selected from the group consisting of methyl, ethyl, propyl and butyl and mixtures thereof.

8. The composition of Claim 1 wherein the alkyl radicals of the R3SiOo 5 units are methyl radicals.

9. The composition of Claim 1 wherein the organo radicals of the diorganopolysiloxane are selected from the group consisting of methyl, ethyl, propyl and butyl and mixtures thereof.

10. The composition of Claim 1 wherein the organo radicals of the diorganopolysilox ane are methyl radicals.

11. The composition of Claim 1 wherein the viscosity of the diorganopolysiloxane ranges from about 400,000 centipoise to about 800,000 centipoise at 25"C.

12. A composition of matter comprising the co-condensation reaction product of a mixture containing: (a) approximately 60 to 65 parts by weight of a hydroxy-containing resinous copolymer comprised of (CH3)3SiOo 5 units and SiO2 units and which is the reaction product of trimethylchlorosilane and tetraethylorthosilicate or sodium silicate, wherein the ratio of (CH3)3sioes units per SiO2 unit ranges from approximately 0.35 to 0.45:1, and (b) approximately 35 to 40 parts by weight of a linear, hydroxy-endstopped dimethylpolysiloxane, wherein the viscosity of said dimethylpolysiloxane ranges from about 400,0()0 to about 800,000 centipoise at 25"C and wherein the sum of (a) and (b) equals 100 parts.

1 3. A method for providing an article having a conformal coating thereon comprising the steps of: (a) applying to a substrate a coating of a composition comprising the co-condensation reaction product of a mixture containing: (1) approximately 55 to 70 parts by weight of a hydroxy-containing resinous copolymer comprised of R3SiOoS units and E;iO2 units, wherein the ratio of R3SiOo 5 units per SiO2 unit ranges from approximately 0.33 to 0.55::1, and wherein R is an alkyl radical or a mixture of alkyl radicals, and (2) approximately 30 to 45 parts by weight of a hydroxy-endstopped diorganopolysiloxane, wherein the organo radicals of said diorganopolysiloxane are selected from the group consisting of alkyl radicals, aryl radicals, alkaryl radicals, aralkyl radicals, haloaryl radicals and alkenyl radicals and mixtures thereof, wherein said diorganopolysiloxane has a viscosity ranging from about 200,000 centipoise to 2,000,000 centipoise at 25"C, and wherein the sum of (1) and (2) equals 100 parts; (b) curing said composition to said substrate.

14. The method of Claim 1 3 wherein the amount of hydroxy-containing resinous copolymer varies from about 60 to 65 parts by weight and the amount of hydroxy-endstopped diorganopolysiloxane varies from about 35 to 40 parts by weight.

1 5. The method of Claim 1 3 wherein the ratio of R3SiOo5 units per SiO2 unit ranges from about 0.35 to about 0.45:1.

1 6. The method of Claim 1 3 wherein the hydroxy-containing resinous copolymer is the reaction product of a tri-alkyl hydrolyzable silane and an alkyl silicate or sodium silicate.

1 7. The method of Claim 16 wherein the trialkyl hydrolyzable silane is trimethylchlorosilane.

18. The method of Claim 16 wherein the alkyl silicate is tetraethylorthosilicate.

1 9. The method of Claim 1 3 wherein the alkyl radicals of the R3Si005 units are selected from the group consisting of methyl, ethyl, propyl and butyl and mixtures thereof.

20. The method of Claim 13 wherein the alkyl radicals of the R3SiOo 5 units are methyl radicals.

21. The method of Claim 1 3 wherein the organo radicals of the diorganopolysiloxane are selected from the group consisting of methyl, ethyl, propyl and butyl and mixtures thereof.

22. The method of Claim 1 3 wherein the organo radicals of the diorganopolysiloxane are methyl radicals.

23. The method of Claim 1 3 wherein the viscosity of the diorganopolysiloxane ranges from about 400,000 centipoise to about 800,000 centipoise at 25"C.

24. The method of Claim 13 wherein the substrate having a conformal coating thereon is an electrical of electronic component.

25. The method of Claim 1 3 wherein the coating is applied to the substrate in a layer having a thickness ranging from about 0.00254 millimeters to about 0.254 millimeters.

26. The method of Claim 1 3 wherein curing of the composition is effected by (a) air drying for about 10 to 60 minutes and (2) thereafter oven drying at a temperature ranging from 75"C to 1 50 C for about 10 to 60 minutes.

27. The method of Claim 1 3 wherein curing is effected in the presence of a catalyst.

28. The method of Claim 27 wherein said catalyst is an organic peroxide.

29. The method of Claim 28 wherein said organic peroxide catalyst is present in an amount ranging from 0.1 to 3 perecent by weight.

30. i:he method of Claim 29 wherein said catalyst is selected from the group consisting of benzoyl peroxide and dichlorobenzoyl peroxide.

31. The method of Claim 30 wherein said catalyst is present in an amount ranging from about 1.0 to 2.0 percent by weight.

32. The method of Claim 27 wherein curing is effected by air drying for about 10 to 60 minutes and thereafter heating the coated article at from about 75"C to about 1 50 C for from 10 to 60 minutes.

33. A method for providing an article having a solvent removable coating thereon comprising the steps of: (a) applying to a substrate a coating ranging from about 0.00254 millimeters to about 0.254 millimeters of a composition consisting essentially of the co-condensation reaction product of a mixture containing: (1) approximately 60 to 65 parts by weight of a hydroxy-containing resinous copolymer comprised of (CH3)3SiOo.s units and SiO2 units, and which is the reaction product of trimethylchlorosilane and tetraethylorothosilicate or sodium silicate, wherein the ratio of (CH3)SiOo.5 units per SiO2 units ranges from approximately 0.35 to 0.45::1, and (2) approximately 35 to 40 parts by weight of a linear, hydroxy-endstopped dimethyl-polysiloxane, wherein the viscosity of said dimethylpolysiloxane ranges from about 400,000 centipoise to about 800,000 centipoise at 25"C, and wherein the sum of (1) and (2) equals 100 parts, and (b) curing said composition to said substrate in the presence of 1.0 to 2.0 percent by weight of a catalyst selected from the group consisting of benzoyl peroxide and dichlorobenzoyl peroxide by air drying foi about 10.two 60 minutes and thereafter heating at a temperature ranging from about 75"C to 150"C for from about 10 minutes to about 60 minutes.

34. An article comprising: (a) a sustrate and (b) a coating composition cured to at least one surface of said substrate, said composition comprising the co-condensation reaction product of a mixture-containing: (1) approximately 55 to 70 parts by weight of a hydroxy-containing resinous copolymer comprised of R3SiOo5 units and SiO2 units, wherein the ratio of R3SiOo5 units per SiO2 unit ranges from approximately 0.33 to 0.55::1 and wherein R is an alkyl radical or a mixture of alkyl radicals and (2) approximately 30 to 45 parts by weight of hydroxy-endstopped diorganopolysiloxane, wherein the organo radicals of said diorganopolysiloxane are selected from the group consisting of alkyl radicals, aryl radicals, alkaryl radicals, aralkyl radicals, haloaryl radicals and alkenyl radicals and mixtures thereof, wherein said diorganopolysiloxane has a viscosity ranging from about 200,000 centipoise to about 2,000,000 centipoise at 25"C, and wherein the sum of (1) and (2) equals 100 parts.

35. The article of Claim 34 wherein the amount of hydroxy-contaning resinous copolymer varies from about 60 to about 65 parts by weight and the amount of hydroxyendstopped diorganopolysiloxane varies from about 35 to about 40 parts by weight.

36. The article of Claim 34 wherein the ratio of R3SiOo5 units per SiO2 unit ranges from about 0.35 to about 0.45:1.

37. The article of Claim 34 wherein the hydroxy-containing resinous copolymer is the reaction product of a trialkyl hydrolyzable si- lane and an alkyl silicate or sodium silicate.

38. The article of Claim 37 wherein the trialkyl hydrolyzable silane is trimethylchlorosilane.

39. The article of Claim 37 wherein the alkyl silicate is tetraethylorthosilicate.

40. The article of Claim 34 wherein the alkyl radicals of the R3Si005 units are selected from the group consisting of methyl, ethyl, propyl and butyl and mixtures thereof.

41. The article of Claim 34 wherein the alkyl radicals of the R3SiO05 units are methyl radicals.

42. The article of Claim 34 wherein the organo radicals of the diorganopolysiloxane are selected from the group consisting of methyl, ethyl, propyl and butyl and mixtures thereof.

43. The article of Claim 34 wherein the organo radicals of the diorganopolysiloxane are methyl radicals.

44. The article of Claim 34 wherein the viscosity of the diorganopolysiloxane ranges from about 400,000 centipoise to about 800,000 centipoise at 25go.

45. The article of Claim 34 wherein the substrate having a conformal coating thereon is an electrical or electronic component.

46. The article of Claim 34 wherein the coating is applied to the substrate in a layer having a thickness ranging from about 0.00254 millimeters to about 0.254 millimeters.

47. The article of Claim 34 wherein the cured coating composition is removable by a solvent.

48. The article of Claim 47 wherein the solvent is toluene, xylene, an aliphatic hydrocarbon or a halogen-substituted aliphatic hydrocarbon.

49. An article comprising: (a) an electrical or electronic component and (b) a coating composition cured to at least one surface of said component, said coating composition consisting essentially of the cocondensation reaction product of a mixture containing: (1) approximately 60 to 65 parts by weight of a hydroxy-containing resinous copolymer comprised of (CH3)3SiOo.5 units and SiO2 units and which is the reaction product of trimethylchlorosilane and tetraethylorthosilicate or sodium silicate, wherein the ratio of (CH3)3SiOo5 units per SiO2 unit ranges from approximately 0.35 to 0.45: :1, and (2) approximately 35 to 40 parts by weight of a linear, hydroxy-endstopped dimethylpolysiloxane, wherein the viscosity of said dimethylpolysiloxane ranges from about 400,000 centipoise to about 800,000 centipoise at 25"C, and wherein the sum of (1) and (2) equals 100 parts.

50. A composition as claimed in claim 1 substantially as herein before described.

51. A method as claimed in claim 13 substantially as herein before described.

52. An article as claimed in claim 34 substantially as herein before described.