CA1335617C

CA1335617C - Silicone conformal coatings

Info

Publication number: CA1335617C
Application number: CA 455696
Authority: CA
Inventors: Arnold Torkelson
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1983-06-02
Filing date: 1984-06-01
Publication date: 1995-05-16
Anticipated expiration: 2012-05-16
Also published as: JPS6047024A; DE3418623A1; FR2546897A1; GB2140815A; BE899800A; GB8413204D0

Abstract

A composition of matter comprising the co-condensation reaction product of (1) a hydroxy-containing resinous copolymer comprised of a R3SiO0.5 units and SiO2 units and (2) a linear hydroxy-endstopped diorganopolysiloxane.
There is also provided a method for providing an article having a solvent-removable conformal coating thereon and the article having a solvent-removable conformal coating.

Description

SILICONE CONFOR~AL COATINGS~
BACKGROUND OF THE INVENTION
The present invention relates to protective silicone coating compositions and articles coated therewith. More specifically, the present invention relates to silicone compositions comprising the reaction product of ~1) a hydroxy-containing resinous copolymer of R3SiOo 5 units and SiO2 units and ~2~ a linear organopolysiloxane containing terminal s;licon-bonded hydroxy groups.
Additionally, the invention relates to articles coated with such compositions and methods for protecting articles such as electrical and electronic devices ~y coating such devices with the composition of the present lnventlon.
Recent advances in the arts of electrical and electronics 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 2Q damage due to joltlng or jostling and to minimize the ability of the system to withstand extreme environment conditions such as heat, cold and moisture. Furthermore, it is important that the insulating material have very low leyels of ionic impurities so as to minimize electrical cont~mjn~tion.
Among the more useful insulative materials presently utilized are silicone compositions such as greases, resins and room temperature yulcanizable polysiloxanes. Although such silicone compositions give ,_ ~

the desired protection to sensitive electrical and electronic components, they h~ve the shortcoming that once they are in place it is yirtually impossible to remo~e all or a portion of the coating so as to provide access for rep~ir or seryicin~ of the components.
~ hile silicone compositions formed from a Iinear hydxoxy-chainstopped diorgano~olysiloxane ~nd an MQ
resin are descri~ed in the arts of pressure sensitiye adhesives and room temperature vulcanizable ~RTV~
compositions, so f~r as is known, none have been provided which are tough dur~ble 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 proYide access for repair and servicing of such electronic components and circuits.
Other objects and advantages of the present invention 2Q will be obvious from the following detailed description.
SUMMARY OF THE INVENTION
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 Xeaction product of (1~ from approximately 55 to 7Q parts by weight of an or~anopolysiloxane cohydxolysis product of a trialkyl hydrolyzable silane and an alkyl silicate or sodium silic~te, in which the hydrolyzable silane and the silicate are reacted in the ratio Qf from about Q.33 to about 0.55 mole of hydxolyzable silane per mole of silicate~ said alkyl ~adicals ha~ing at most four carbon atoms, and C2~
3~ to 45 parts by weight of a linear diorganopolysiloxane t fluid having terminal silicone-honded hydroxy groups~
said organo ~roups being selected fxom the ~roup consisti.ng of alkyl radicals, aryl radicals, alk~xyl radicals, aralk~l xadicals, haloaryl radic~ls and alkenyl radicals and mixtures thereof, ~nd said diorganopQlysiloxane having a ~iscosity of from about 200,0Q0 centipoise to about 2,QQ0,QQQ centipoise at 25C.
DES~K~ O~ OF ~HE IN~ENTION
In accordance with the present invention there are provided noYel silicone compositions which are solvent removable and exhibit primerless adhesion to a wide yariety of substrates. Generally, such compositions are the co-condensation reaction product of a mixture containing ~1~ approximately 55 to 70 parts and preferably 6Q to 65 parts by weight of a hydroxy containing resinous copolymer of R3SiOQ 5 units (M units~ and SiO2 (:Q units~, wherein the ratio of R3SiOo 5 units per SiO2 units 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 end-stopped diorgano-polysiloxane having an average weight of about two organic radicals per silicone atom, said organic radicals being selected from the group consisting of alkyl radicals, aryl radicals, alkaryl radicals, aralkyl radicals, haloaryl radicals and alkenyl xadicals and mixtures thereof, said diorganopolysiloxane haying a viscosity of from about 200,QQQ centipoise to about 2,Q0Q,QQ0 centipoise at 25~C, and the sum of ~1~ and C2~ equalling 100 parts.
The resinous copQlymer of R3SiOQ 5 units and SiQ2 units employed in the practice of the p~esent inYention is ~ell known in the prior art and is descxibed in U.S. Patent NQ. 2,736,72I, issued Fehruary 28, lq56 to Dexter, U.$. Patent NO. 2,857,356, issued Octo~er 21, 1958 to Goodwin, Jr., U.5. Patent No.

2,676,182, issued April 2Q, 1954 to Daudt et al., and U.S. Patent No. 3,017,384, issued January 16, 1962 to Modic. Although a Yariety of methods are ayailable for producing such MQ resins, the present invention does not depend upon the p~axticular method by which the resinous copolymer is made ~or patentability. Howeyer, it has been found that MQ resins made according to the process of Goodwin, Jr., U.S. Patent No. 2,857,356, are lQ particularly prefera~le as they directly provide a hydroxy cont~inlng resin. The production of such resins invol~es 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 suita~le solvent and thereafter a sufficient amount of water is added to effect the desired cohydrolysis and co-condensation and ethylorthosilicate to obtain such hydroxy-containing MQ resin. Of course, the propo~tions of hydrolyza~le silane and silicate must be such that there results a resinous copolymer containing from approximately 0.33 to Q.55 R3SiOo 5 unit per SiO2 unit.
Briefly, the MQ 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 to 60 to 85C. Thereafter, the resulting two-phase system is processed to remoYe the resultin~ water-alcohol layex and the resinous material is neutralized with an amount of sodium bicarbonate or other alkaline material. The resinous solution is then filtexed and the resinous solids content adjusted to the desired leYel~ usin~ where necessaXy~ ~dditional amounts of solyent. Those interested in ~reater detail or additional information are ur~ed to consult the aforementioned patents:.

1335617 6QSI-72l Regardless of the method ~y which the resinous copolymer is formed, it is preferred that the alkyl groups be lower alkyl groups such as methyl, ethyl, propyl and ~utyl since highex alkyl radicals undesira~ly slow down the hydrolysis of the silane ~nd cause a type of co-condensation with the silicate w~hich leads to less desirable product$. HoweYer, such alkyl groups may ~e the same or different alkyl groups. It is also important that in the fin~l product there should be lQ approximately Q.33 to a . 55 M units per Q unit so that optimal properties, such as toughness, tensile strength and freedom from tack;ness, are obtained.
The linear, high viscosity organopolysilane used for co-reacting with the hydroxy-containing MQ resin mu$t of necessity have texminal silicon-bonded hydroxy groups to permit ready copolymerization with the resin.
Such organopolysilxanes may also be prepared by any of the known methods, for example, acid or alkali catalyzed polymerization of the corresponding cyclic siloxanes.
2Q Ag~in, the processes for preparing hydroxy-endstopped polysilxanes are well known in the art, for example as described in Goodwin, Jr., U.S. Patent No. 2,857,356.
The Goodwin, Jr. process involyes heating cyclic organopolysilxanes at a temperature of 125~C to 150C
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, prefexably having a viscosity within the xange of about 75,QOQ to 125,QQ0 3~ centipoise. After such polymerized product is obtained, it is treated to proYïde termin~l silicon-bonded hydroxy groups on the molecules of the organopolysiloxane for co-reaction with the hydroxyl groups of the xesin. This can be acco~plished ~y blowing steam through or across the surface of the polymer. Howeyer, this decreases the viscosity of the polymex while ~t the same tLme increasing 13356~7 60SI-721 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 tempexature of about 125~C to 150C to obtain a material haying a viscosity of from 200,000 tQ 2,000,000 centipoise. The preferred range of ~iscosities is from 400,000 to 800,000 centipoise. Once the desired viscosity has been reached, the organopolysiloxane should be treated in order to inactiV~te 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 MQ resin and diorganopolysiloxane is relatively simple in that it merely requires mixing the two together and heating the mixture to effect co-condensation. 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 leyels of organopolysiloxane will result in a final product which is too tacky, ~hereas utilization of higher leyels of MQ resin will result in a final product which is too brittle.
To accompli$h the co-condensation ofresin and organopoly$iloxane, the xesîn is heated, for instance, at a temperature of about 100 to 150C for from one half to six _7 1335617 60SI-721 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 soIvent in the mixture can varY ~idely as its only function is to facilitate handling and application of the conformal coating composition to substrates such as electronic circuits and device$.
The present invention further provides a method for pxotecting electrical and electronic components from adyerse environmental conditions comprising applying a 0.00254 to 0.254 millimeter thick coating of the composition of the present invention to such electronic equipment. Throughout the present disclosure the terms electrical and electronic devices, components, equipment and the like are used interchangeably and are meant to include, but not limited to, devices such as circuits, transistors, diodes, resistors, capacitors and the like. Following application of the formulation to the equipment or devices by conventional means such as spraying, brushing and dipping, the formulation is air dried for 10 to 60 minutes or until "tack free" and subsequently oven dried at a temperature in the range of 75 to 150C for 10 to 60 minutes.
Optionally, a catalyst or curing agent is included in the composition. Pxeferably such curing agent is a peroxide catalyst such as benzoyl perioxide or dichlorobenzoyl perioxide at a level of 1 to 2% based on the silicone content of the coating compositions.
Howe~er, any organic pexioxide present in amounts ranging from about 0.01 to 3.0% by weight is effective.
Of course, any suitable catalyst can be employed. In application, the catalyst is mixed into the 10 to 50 percent solid$ solution of the conformal coating composition. The solution is then coated on the desired electronic components, and thereafter the coating component i5 air dried and then heated at a temperature of from about 75 to 15Q9C for 10 to 60 minutes to evaporate the solvent and effect curing of the confprmal coating composition. The solvent must be removed before the te~perature is r~ised to actiVate the peroxide.
The article thus ,f,o~med 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 ~dvantage 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 a 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.
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Claims

1. 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 R3SiO0.5 units and SiO2 units, wherein the ratio of R3SiO0.5 units per SiO2 unit ranges from approximately 0.35 to 0.45:1, and wherein R is an alkyl radical or a mixture of alkyl radicals, and (b) approximately 35 to 40 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 hydroxy-containing resinous copolymer is the reaction product of a trialkyl hydrolyzable silane and an alkyl silicate or sodium silicate.

3. The composition of claim 2 wherein the trialkyl hydrolyzable silane is trimethylchlorosilane.

4. The composition of claim 2 wherein the alkyl silicate is tetraethylorthosilicate.

5. The composition of claim 1 wherein the alkyl radicals of the R3SiO0.5 units are selected from the group consisting of methyl, ethyl, propyl and butyl and mixtures thereof.

6. The composition of claim 1 wherein the alkyl radicals of the R3SiO0.5 units are methyl radicals.

7. 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.

8. The composition of claim 1 wherein the organo radicals of the diorganopolysiloxane are methyl radicals.

9. 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.

10. 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)3SiO0.5 units and SiO2 units and which is the reaction product of trimethylchlorosilane and tetra-ethylorthosilicate or sodium silicate, wherein the ratio of (CH3)3SiO0.5 units per SiO2 unit ranges from approxi-mately 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 400,000 to about 800,000 centipoise at 25°C and wherein the sum of (a) and (b) equals 100 parts.

11. 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 60 to 65 parts by weight of a hydroxy-containing resinous copolymer comprised of R3SiO0.5 units and SiO2 units, wherein the ratio of R3SiO0.5 units per SiO2 unit ranges from approximately 0.35 to 0.45:1, and wherein R is an alkyl radical or a mixture of alkyl radicals, and (2) approximately 35 to 40 parts by weight of a hydroxy-endstopped diorganopoly-siloxane, 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; and (b) curing said composition to said substrate.

12. The method of claim 11 wherein the hydroxy-containing resinous copolymer is the reaction product of a trialkyl hydrolyzable silane and an alkyl silicate or sodium silicate.

13. The method of claim 12 wherein the trialkyl hydrolyzable silane is trimethylchlorosilane.

14. The method of claim 12 wherein the alkyl silicate is tetraethylorthosilicate.

15. The method of claim 11 wherein the alkyl radicals of the R3SiO0.5 units are selected from the group consisting of methyl, ethyl, propyl and butyl and mixtures thereof.

16. The method of claim 11 wherein the alkyl radicals of the R3SiO0.5 units are methyl radicals.

17. The method of claim 11 wherein the organo radicals of the diorganopolysiloxane are selected from the group consisting of methyl, ethyl, propyl and butyl and mixtures thereof.

18. The method of claim 11 wherein the organo radicals of the diorganopolysiloxane are methyl radicals.

19. The method of claim 11 wherein the viscosity of the diorganopolysiloxane ranges from about 400,000 centipoise to about 800,000 centipoise at 25°C.

20. The method of claim 11 wherein the article having a conformal coating thereon is an electrical or electronic component.

21. The method of claim 11 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.

22. The method of claim 11 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 150°C for about 10 to 60 minutes.

23. The method of claim 11 wherein curing is effected in the presence of a catalyst.

24. The method of claim 23 wherein said catalyst is an organic peroxide.

25. The method of claim 24 wherein said organic peroxide catalyst is present in an amount ranging from 0.1 to 3 percent by weight.

26. The method of claim 25 wherein said catalyst is selected from the group consisting of benzoyl peroxide and dichlorobenzoyl peroxide.

27. The method of claim 26 wherein said catalyst is present in an amount ranging from about 1.0 to 2.0 percent by weight.

28. The method of claim 23 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 150°C for from 10 to 60 minutes.

29. A method for providing an article having an organic 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)3SiO0.5 units and SiO2 units, and which is the reaction product of trimethyl-chlorosilane and tetraethylorthosilicate or sodium silicate, wherein the ratio of (CH3)SiO0.5 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 for about 10 to 60 minutes and thereafter heating at a temperature ranging from about 75°C to about 150°C for from about 10 minutes to about 60 minutes.

30. An article comprising:
(a) a substrate 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 60 to 65 parts by weight of a hydroxy-containing resinous copolymer comprised of R3SiO0.5 units and SiO2 units, wherein the ratio of R3SiO0.5 units per SiO2 unit ranges from approximately 0.35 to 0.45:1 and wherein R is an alkyl radical or a mixture of alkyl radicals and (2) approximately 35 to 40 parts by weight of a hydroxy-endstopped diorganopolysiloxane, wherein the organo radicals of said diorgano-polysiloxane 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 centi-poise to about 2,000,000 centipoise at 25°C, and wherein the sum of (1) and (2) equals 100 parts.

31. The article of claim 30 wherein the hydroxy-containing resinous copolymer is the reaction product of a trialkyl hydrolyzable silane and an alkyl silicate or sodium silicate.

32. The article of claim 31 wherein the trialkyl hydrolyzable silane is trimethylchlorosilane.

33. The article of claim 31 wherein the alkyl silicate is tetraethylorthosilicate.

34. The article of claim 30 wherein the alkyl radicals of the R3SiO0.5 units are selected from the group consisting of methyl, ethyl, propyl and butyl and mixtures thereof.

35. The article of claim 30 wherein the alkyl radicals of the R3SiO0.5 units are methyl radicals.

36. The article of claim 30 wherein the organo radicals of the diorganopolysiloxane are selected from the group consisting of methyl, ethyl, propyl and butyl and mixtures thereof.

37. The article of claim 30 wherein the organo radicals of the diorganopolysiloxane are methyl radicals.

38. The article of claim 30 wherein the viscosity of the diorganopolysiloxane ranges from about 400,000 centipoise to about 800,000 centipoise at 25°C.

39. The article of claim 30 wherein the substrate is an electrical or electronic component.

40. The article of claim 30 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.

41. The article of claim 30 wherein the cured coating composition is removable by an organic solvent.

42. The article of claim 41 wherein the organic solvent is toluene, xylene, an aliphatic hydrocarbon or a halogen-substituted aliphatic hydrocarbon.

43. 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 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)3SiO0.5 units and SiO2 units and which is the reaction product of trimethylchlorosilane and tetraethylorthosilicate or sodium silicate, wherein the ratio of (CH3)3SiO0.5 units per SiO2 unit ranges from approximately 0.35 to 0.45:1, and Claim 43 continued:
(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.