CA1104284A - Low temperature transmission room temperature vulcanized silicone composition - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The water vapor permeability of RTV silicone com-positions is significantly reduced by incorporating therein a particulate mica filler such compositions are of parti-cular utility in the construction of multiple pane in-sulating windows, whereby fogging may be decreased.

Description

---- llV~ 4 The present invention relates to silicone rubber compositions having low vapor moisture transmission rate. More particularly the present invention relates to room temperature vulcanizable silicone rubber compositions of the aforesaid type.
Room temperature vulcanizable silicone rubber compositions are well known. Such compositions are utilized for a variety of pur-poses such as, for instance formed-in-place gaskets but are most widely used in sealant applications and specifically in the sealing of window panes, as well as other ceramic surfaces.
It has been found that conventional silicone sealants are not sufficiently impermeable to permit their use in the construction of multiple pane insulating windows, and problems of fogging have been encountered.
One attempted solution to this problem was to utilize butyl tape which has a low moisture vapor transmission rate as an initial sealant in the edges of such insulated glass panes in which tapes there are located micro sleves or silica gel and thereover apply the conventional sealant or conventional silicone sealant to seal or prepare the insulated glass pane from the two or three individual glass window panes. Unfortunately, such conventional silicone sealants or other conventional sealants even with the butyl tape with the dessicant in the space over which the sealant was applied still resulted in a larger than desirable moisture vapor transmission rate into the air space between the window panes. As a result, in some cases, these types of constructed insulated glass panes would tend to fog up when the unit was tested and cycled at temperatures going from -60 F to 150 F. In addition, the use of silica gel along with the butyl tape and the conventional sealant would unduly increase the cost of the construction of the insulated glass. Accordingly, it was highly desirable to produce a silicone sealant which would have a moisture vapor transmission rate considerably below - llV4Z~4 that of conventional silicone sealants which moisture vapor transmission rate for conventional silicone sealants is in the range of 40 to 50 grams per square meter in 24 hours, through a layer of silicone material varying any-where from 60 to 75 mills thick.
In a most advantageous preferred embodiment in the construction of such insulated glass panes, such low moisture vapor transmission rate silicone sealants could be applied over butyl tape with dessicants and then the insulated glass pane would then be again sealed with such low moisture vapor transmission rate silicone sealants against the window frame to allow for construction of an insulated glass pane which was considerably below the cost of a thermopane and which at the same time would function as well as the thermo-pane without excessive transmission of vapor from the atmosphere into the air space between the glass panes of the insulated glass unit and as such the insulated glass unit would not fog up at excessively low temperature and visi-bility through it would not be impaired as was the case with some prior art insulated glass panes.
Accordingly, it is one object of the present invention to provide room temperature vulcanizable silicone rubber composition which has a low moisture vapor transmission rate.
It is another object of the present invention to provide a room temperature vulcanizable silicone rubber composition which has a low moisture vapor transmission rate and good weather resistance.
It is an additional object of the present invention to provide, for a room temperature vulcanizable silicone rubber composition which has a low moisture vapor trans-mission rate and which can be applied to seal and form insulated glass panes.
It is yet an additional object of the present invention 1~42~4 60SI-71 to provide for a process for producing a room temperature vulcanizable silicone rubber composition which has a low moisture vapor transmission rate.
These and other objects of the present invention are accomplished by means of the disclosure set forth herein-below.
In accordance with the above objects, there is provided an improved, room temperature vulcanizable silicone rubber of low water vapor transmission. The room temperature vulcanizable silicone rubber is itself of a conventional nature, and may comprise for example a "one component"
or "two component" type. Amongst the former are included silanol end stopped diorganpolysiloxanes having a viscosity varying from 100 to 500,000 centipoise at 25C where the organo groups are selected from the class consisting of monovalent hydrocarbon radicals and halogenated hydro-carbon radicals, in admixture in a single package with a cross linking agent therefor and catalyst. Suitable cross linking agents known in the art are silanes or siloxanes having a functionality selected from the class consisting of alkoxy, acyloxy, amine, amide, ketoximino for example. Amongst the two component type of rubbers may be mentioned those consisting of the above diorg-anopolysiloxane and, as crosslinking agent therefore, organosilicates of the type Ra (OR') 4-a and partial hydrolysis products thereof, where R and R' and selected from the class consisting of monovalent hydrocarbon radi-cals and halogenated monovalent hydrocarbon radicals, and a has a valueof 0 or 1. Also suitable as a crosslinking agent for this type of two component rubber are resinous copolymers having R23 SiO.5 groups and SiO2 groups, where R2 is defined as are R and R', A still further type of llV4Z84 room temperature vulcanizable silicone rubber of the two component type comprises a diorganopolysiloxine which is end stopped with vinyl radicals, and, as cross linking agent therefor, a silhydrogen functional silane or siloxane.
Each of the above curing systems includes catalysts known in the art, and may further optionally include fillers, processing aids and other conventional adjuvants. The improvement comprises the incorporation into the curable compositions of a particulate mica filler, in general about 75 to 150 parts of such filler being used for 100 parts of the base diorganopolysiloxane used in the room temperature vulcanizable silicone rubber. The mica filler may optionally be comprise up to about 30% talc.
The invention further provides the process of producing a silicone elastomer of reduce water vapor transmission by applying a composition as described above to a sub-strate. In these improved compositions, it is preferred that the vapor barrier filler is a 100~ mica filler, but as stated previously, up to 30~ may be substituted by talc. The mica filler whether wet or dry ground is the one that imparts to the instant silicone composition the desired properties of being resistant to moisture vapor transmission through the silicone elastomer in its cured state or in some respects, even in its uncured state.
The vapor barrier filler whether it be mica or mica-talc mixture can have a particle size varying anywhere from 50 to 4,000 mesh. In addition to the above filler, there may be utilized in the compositions of the instant case from 5 to 30 parts by weight of a second, additional filler which may be a reinforcing filler such as, fumed silica, precipitated silica or may be an extending filler such as, zinc oxide, diatomaceous earth and etc. In the llV4Z~4 6OSI-71 present composition, the only necessity for using the foregoing additional fillers is to give the sealants some sag control so that when it is applied to the substrate which is in a vertical position, the sealant will stay in place for a sufficient period of time to cure to a silicone elastomer without flowing away from the point of application.
Any of the other well known additives for one-component room temperature vulcanizable silicone rubber compositions can be utilized in the present composition if they do not interact with the mica filler. Specifi-cally, almost all known ingredients that can be utilized for room temperature vulcanizable silicone rubber composi-tions can be utilized in the instant composition. In the case of additional filler additives, they have to be adjusted so as to not overload the composition with filler in view of the large amounts of mica and optionally talc that may be present in the instant composition.
The base polymers, cross-linking agents and the cat-alyst are well known ingredients. In the instant one-component RTV silicone rubber compositions any well known cross-linking agents for such systems along with the cat-alyst most suitable in combination therewith may be utilized.
For instance in the case of the utilization of an alkoxy functional silane as a cross-linking agent then it is preferred that a titanium chelate catalyst be used, either itself or in combination with a metal salt of a carboxylic acid as is well known in the art.
Basically, the instant invention lies in the utiliz-ation of a particulate mica filler as disclosed above, and optionally the talc in any room temperature vulcanizable silicone rubber composition for the purpose of rendering , , , 11~)42~ 60SI-71 the composition resistant to the passage of moisture vapor.
It should be noted that the mica filler may be treated or untreated in the instant case and also the optional talc.
The additional filler such as, fumed silica or precipited silica, if such are used, are preferably treated since such treatment prevents the viscosity of the composition from being undesirably high. The advantage of the treated fumed silica or precipitated silica is that it can be utilized in the composition without unduly increasing its viscosity and at the same time give desirable sag control as well as impart to the compositions good tensile strength in the final cured composition. Normally, such fumed silica or precipitated silica are treated with cycliciloxanes. In any case, for particular application, untreated fumed silica or precipitated silica can be utilized in smaller amounts than would be the case with the treated filler to impart to the composition the desired sag control without necessarily increasing the tensile strength of the cured elastomer. The invention will more distinctly be set forth hereinbelow.
The base silanol-terminated diorganopolysiloxane type polymer having a vicosity varying from 100 to 50,000 centipoise at 25C is a well known ingredient for room temperature vulcanizable silicone rubber compositions.
The organo groups may be selected from monovalent hydro-carbon radicals and halogenated monovalent hydrocarbon radicals such as, for instance, alkyl radicals, methyl, ethyl, etc. radicals having 1 to 8 carbon atoms; alkenyl ~ radicals having 2 to 8 carbon atoms such as, vinyl, allyl;
mononuclear aryl radicals such as, phenyl, methylphenyl, ethylphenyl and etc.; cycloalkyl radicals such as, cyclo-hexyl, cycloheptyl and etc., and fluoroalkyl radicals such ~, li~)428~
as, 3, 3, 3 trifluoropropyl. More preferably, such organic groups are preferably selected from alkyl radicals of 1 to 8 carbon atoms, phenyl radicals, vinyl radicals and fluoroalkyl radicals of 1 to 8 carbon atoms. Generally such linear diorganopolysiloxane polymers may have a small amount of triorganosiloxy termination in the polymer of up to 10~ by weight of the polymer and may have some trifunctional siloxy units also in the polymer with the total amount of monofunctionality and trifunctionality not exceeding 10~ by weight. Most preferably, the silanol-terminated linear diorganopolysiloxane polymer has the formula, HO _ 1 S io - ¦ - H
R
n In the above formula, R is selected from the class consisting of alkyl radicals of 1 to 8 carbon toms, phenyl, radicals, vinyl radicals and fluoroalkyl radicals where the R radicals may be the same or different and n varies from 150 to 1500. In accordance with the above disclosure, the polymer preferably has the formula shown above but, as stated previously, a silanol-terminated diorganopoly-siloxane polymer need not be strictly within the formula shown above and may have a certain amount of monofunctional siloxy units and a certain amount of trifunctional siloxy units which at most can comprise up to 10~ by weight of the polymer. It can also be appreciated that such a polymer need not be a single polymer specie but may be a blend of various linear diorganopolysiloxane polymers which are silanol-terminated and such that the blend of the final polymers has a viscosity varying from 100 to 500,000 centi-.

~1~4284 60SI-71 poise at 25C. Such silanol-terminated diorganopolysiloxane polymers are well known in the art as the necessary in-gredients or base ingredients for one-component room tem-perature vulcanizable silicone rubber compositions and two-component room temperature vulcanizable silicone rubber compositions. Such silanol-terminated diorganopolysiloxane polymers may be prepared by two general procedures. In one procedure, cyclictetrasiloxanes are equilibrated in the presence of a small amount of chain-stoppers such as, hexamethyldisiloxane and in the presence of 50 to 500 parts per million of alkali metal hydroxide and by such equili-bration there are produced high viscosity polymers in a viscosity varying from 500,000 to 200,000,000 centipoise.
The resulting triorganosiloxy terminated diorganopolysilo-xane polymers are then taken and steam is pass through them to disrupt the polymer chain and as a result there is formed silanol-terminated linear diorganopolysiloxane polymers in the viscosity range diselosed above for use in the instant invention. In an alternate procedure and pre-ferably for low viseosity and low molecular weight silanol-terminated diorganopolysiloxane polymers, a hydrolyzate of diorganodiehlorosilanes is taken and mixed with the proper quantities of cyelietetrasiloxanes and the result-ing mixture is equilibrated in the presenee of a mild aeid sueh as, sulfurie aeid treated elay sueh as the elays known under the trade mark (Filtrol Corp. of Los Angeles, California) or equilibrated with a mild aeid sueh as, toluene sulfonie aeid to produee the desired low moleeular weight and low viseosity silanol-terminated diorganopoly-siloxane polymers. As ean be appreeiated, through either of these proeedures and speeifieally by the seeond proeedure diselosed above for the preparation of silanol-terminated ~.
' 42~84 60SI-71 diorganopolysiloxane polymers, a small amount of mono-functional and trifunctional siloxy units may be present in the hydrolyzate of the diorganodichlorosilanes and as a result the final polymer that is obtained after equili-bration a polymer having up to 10% of monofunctional siloxy units and tetrafunctional siloxy units. It should be noted that in both procedures set forth above, after the equilibration has terminated or has reached its maximum point, which is roughly 85%, that is after conversion of 10 85% of the cyclictetrasiloxanes to the polymer, the equilibration is stopped and the residual cyclic are stripped off and the catalyst-is neutralized with well known neutralizing agents. The polymer is then ready to be utilized in a subsequent step with steam in accordance with the first method for producing the silanol-terminated diorganopolysiloxane polymer or is simply utilized as such in the second step for producing the low molecular weight silanol-terminated diorganopolysiloxane polymers.
It is important that per 100 parts by weight of the silanol-terminated diorganopolysiloxane polymer there be incorporated into it from 75 to 150 parts by weight of a vapor barrier filler selected from the class consisting of mica, wherein up to 30% of weight of the mica may be substituted by talc. More preferably, 100 to 150 parts by weight of the vapor barrier filler is utilized. Although ranges of mica can be utilized outside of the above ranges, when there is utilized a vapor barrier filler loading of less than 75 parts by weight of the silanol-terminated diorganopolysiloxane polymer then the resulting cured silicone elastomer does not have a desirably high enough rsistance to vapor moisture transmission. As can be ap-precited, more than 150 parts by weight of the mica may be _ 9 _ ~1~)42~ 4 60SI-71 utilized, however, the viscosity of the composition un-duly increases to undesirable levels for many application techniques and especially in view of the fact that such one-component room temperature vulcanizable silicone rubber compositions have to be dispensed from caulking tubes and accordingly a low viscosity of the final composition is desired. The preferred ranges, as stated previously for the mica, is from lO0 to 150 parts by weight since it has been found that through the above preferred range there is obtained the maximum benefits for resistance to moisture vapor transmission in the one-component room temperature vulcanizable silicone rubber compositions of the instant case.
It should be noted that either wet ground or dry ground mica may be utilized in the instant invention and that such mica preferably has a US mesh size varying any- -where from 50 to 4000 mesh. In accordance with the above, the particle size of the mica in a composition is not that critical, the critical aspect being, in the instant case, to have sufficient mica in the one-component room temperature vulcanizable silicone rubber compositions of the instant case such that the proper resistance to moisture vapor transmission is realized. In addition, the mica may be treated or untreated. The only difference with the utilization of treated mica and treated talc is that more of the mica or talc can be added to the com-positions of the instant case without increasing the viscosity of the composition as much as is the case with the untreated mica or talc.
The ingredients that may be utilized to treat the mica or talc are preferably selected from cyclicsiloxanes or organic fatty acids such as, stearic acid, which impart to ~1~4284 60SI-71 the mica and talc a certain amount of hydrophobicity and results in that when the mica and talc is incorporated into the compositions of the instant case that the com-positions do not unduly increase above the desirable viscosity for the composition. It should also be noted that preferably 100% of the vapor barrier filler is selected from wet ground mica or from dry ground mica. To save in costs, in certain situations, up to 30% by weight of the mica may be substituted by talc. With this vapor barrier filler there, advantageously, may be utilized in the instant composition a concentration of 5 to 30 parts or more pre-ferably at a concentration of 5 to 20 parts by weight of a reinforcing or an extending filler for the purpose of rendering the resulting uncured composition so that it has a certain amount of sag control. This addition of the traditional reinforcing and extending fillers to the instant composition is not necessary. It is only desirably utilized in the instant composition so that the composition does not sag as much as would be the case when such fillers are not inserted into the composition. It should be noted that the foregoing traditional reinforcing and extending fillers for silicone rubber compositions do not impart any resistance to moisture vapor transmission of the composition. As will be shown later on in the examples of the instant case, in silicone compositions in which there is utilized the traditional reinforcing and extending fillers, such silicone compositions have very little, if any, resistance to moisture vapor transmission. The fore-going above weights given for the weight of reinforcing or extending fillers may be utilized in the instant com-position are based, per 100 parts of the silanol-terminated diorganopolysiloxane polymer. The reinforcing fillers are, 11~)42~ 60SI-71 of course, well known fumed silica and precipitated silica.
The extending flllers are also well known for Silicone rubber compositions and are preferably selected from titanium dioxide, iron oxide, aluminum oxide, as well as the inorganic filler materials known as inert fillers which can include, among others, diatomaceous earth, calcium carbonate and quartz, as well as diatomaceous silica, aluminum silicate, zinc oxide, zirconium silicate, barium sulfate, zinc sulfate, and finely divided silica having surface bonded alkoxy groups. It should be noted that such fillers may be treated or untreated and preferably are treated so that they will not unduly increase the viscosity of the uncured compositions of the instant case and also such treated fillers result in better sag control. The treating agents for such fillers and specifically for the reinforcing fillers, as is well known, are liquid siloxanes, silazanes and cyclicsilazanes in combination with hydroxyl amines. Another basic in-gredient in the instant one-component room temperature vulcanizable silicone composition comprises from 1 to 15 20, parts by weight of the 100 parts of the silanol-terminated diorganopolysiloxane of a silane or siloxane having func-tionality selected from the class consisting of alkoxy functionality, acyloxy functionality, amine functionality, amide functionality, tert-alkoxy functional silanes and ketoximino functionality. All the above cross-linking agents with the above functionalities are well known in the art. It is the intent of the instant invention to claim and to cover within its scope all known room tem-perature vulcanizable silicone rubber compositions which have mica inserted into them within the foregoing ranges disclosed above where the cross-linking agent, whether it be a silane or siloxane, has any functionality (the ilQ4284 60SI-71 above functionalities being given as being exemplary) and also with whatever type of catalyst is suitable for such silane functionality cross-linking agent.
Accordingly, in the description the preferred cross-linking agents, as stated above, may be a silane which has the formula, RlSi ~oR2~3 where Rl is selected from the class consisting of alkyl radicals of 1 to 8 carbon atoms, phenyl radicals, a alkenyl radicals of 2 to 8 carbon atoms and fluoro-alkyl radicals of 3 to 8 carbon atoms and where R
is selected from the class consisting of alkyl radicals of 1 to 8 carbon atoms. Such alkoxy functional silanes with the base silanol-terminated diorganopolysiloxane for the purpose of producing a one-component room temperature vulcanizable silicone rubber composition are weLl known in the art as, for instance, as exemplified by Beers, U.S. Patent No. 3,708,467 dated January 2, 1973. The method of preparation of such alkoxy functional silane cross-linking agents is also weIl known in the art as exemplified by the foregoing Beers U.S. patent. It should be noted that with such compositions it is preferable that the catalyst be a titanium chelate catalyst as disclosed in the foregoing Beers patent. It should be noted, however, that the catalyst in such compositions and specifically the titani~um cheIate catalyst in such compositions preferably comprises from .01 to 5 parts by weight based on 100 parts of the base silanol-terminated diorgano-polysiloxane polymer. Other metal salts of carboxylic ~ ~ - 13 -1~42~9L 60SI-71 acids suitable for use as catalysts in these compositions may be optionally employed in place of the titanium chelate catalyst, although such metal salts are, to varying degrees, not as effective in curing the composition as the titanium chelate catalyst is. Caution must be exercised when selecting catalysts other than the titanium chelate catalyst since not all of the metal salts of carboxylic acids are suitable for such use. In place of the alkoxy functional silane cross-linking agent there may be utilized in the instant composition, acyloxy functional silanes having the formula, R3Si (ooCR4)3 wherein in the foregoing formula R3 is selected from the class consisting of alkyl radicals of 1 to 8 carbon atoms, phenyl radicals, alkenyl radicals of 2 to 8 carbon atoms and fluoroalkyl radicals of 3 to 8 carbon atoms and wherein R4 is selected from alkyl radicals of 1 to 8 carbon atoms. The acyloxy functional silane cross-linking agents for one-component room temperature vulcanizable silicone rubber compositions are also weIl known. For instance, see the following patents of Harvey P. Shaw - U.S. Patent 3,701,753 dated October 3I, 1972 and U.S. patent 3,872,054 dated March 18, 1975.
The preparation and utilization of such acyloxy functional silanes in one-component room temperature vulcanizable silicone rubber compositions is also well known in the art as exemplified by the foregoing Shaw U.S. patents. Another silane that may be utilized as a cross-linking agent in the one-component composition of the instant case is, for instance, an amide functional silane having the ~, ~ ~ - 14 -~{~42~4 formula, R5 n-b R b sio wherein R5 is selected from the class consisting of hydrogen, alkyl radicals of 1 to 8 carbon atoms and phenyl, n is at least but it does not exceed 4, b has a positive value equal to at least 3, and R6 is a Si-N bonded carboxylic acid amide radical having alkyl and aryl substituents of 1 to 8 carbon atoms. Examples of such amide functional silanes and siloxanes as cross-linking agents in the instant com-position is that, for instance, to be found in U.S. patent 3,417,047 dated December 24, 1968. It should be noted again that the preparation, manufacture, and use of such amide functional silanes to produce one-component room tem- `
perature vulcanizable silicone rubber compositions is well known in the art is as exemplified by the foregoing '047 patent.
Another cross-linking agent that may be utilized to prepare a one-component room temperature vulcanizable silicone rubber composition within the scope of the present case, which composition has enchanced resistance to moisture vapor transmission, is a ketoximino functional silane which has the formula, (C ~ 5' ~0 C ~ `

wherein Rll is an alkyl radical of 1 to 8 carbon atoms, R7 and R8 may be the same or different and are alkyl radicals of 11()4Z~4 60SI-71 l to 8 carbon atoms, R10 is an alkylene radical of 2 to 8 carbon atoms, R7 is an alkyl radical of 1 to 8 carbon atoms a is a whole number varying from 1 to 3, d is a whole number varying from 1 to 3, f is a w~ole number varying from 0 to

2 and the sum of a, d and f is 4. Utilization of such ketoximino cross-linking agents and their preparation to produce one-component room temperature vulcanizable silicone rubber compositions is well known in the art as exemplified by the Beers et al U.S. patent 3,962,160 dated June 8, 1976. Accordingly, it is not necessary to go into the description in preparation of such ketoximino functional silanes for the preparation of one-component room temperature vulcanizable silicone rubber compositions since they are described in the foregoing Beers et al '160 patent.
There can also be utilized a amine functional silane as a cross-linking agent in the instant composition. Such amine functional silane having the formula, R+18 Si (N Rl9 R20) where R 8 is selected from the class consisting of alkyl radicals, phenyl radicals and fluoroalkyl radicals of l to % carbon atoms and Rl9 and R20 are selected from the class consisting of hydrogen, alkyl radicals of l to 8 carbon atoms and phenyl radicals, + is a whole number equal to l or 2, and z is 4. In addition, such silane cross-linking agents are well known in the art, that is their manufacture and preparation and also their utilization in one-component room temperature vulcanizable silicone rubber compositions as cross-linking agents.
Generally, as stated previously, to per lO0 parts of the base silanol-terminated diorganopolysiloxane polymer -there may be utilized from 1 to 15 parts by weight of the silane or siloxane cross-linking agent and more preferably .

from 1 to 10 parts by weight of the silane or siloxane cross-linking agent. With such silanes as cross-linking agents it has been desirable to have from .01 to 5 parts by weight of a catalyst of the foregoing catalyst disclosed above, with the exception of the alkoxy functional silane in which is is highly desirable to utilize a titanium chelate catalyst or less desirably a suitable metal salt of a carboxylic acid, otherwise the composition will cure too slowly in the presence of atmospheric moisture.
Accordingly, a composition with the foregoing ingredients set forth above, that is, the base silanol-terminated diorganopolysiloxane polymer, a vapor barrier filler, optionally a reinforcing or extending filler, a cross-linking agent and the catalyst simply mixed in an anhydrous state, and packaged as such when the mixed ingredients come into contact with atmospheric moisture they will cure to a silicone eIastomer. It should be noted that when the components are mixed together to form a one-component room temperature vulcanizable silicone rubber composition, the mixing operation does not have to be completely anhydrous although there is a drying cycle after such mixing which takes place to remove most moisture from the composition. If such composition is then stored in the absence of moisture (say in a well sealed caulking tube) the composition has a shelf life of 6 months or more and ....
. "

LZ~34 can at any time be taken and dispensed for a particular application and when the composition comes into contact with atmospheric moisture it will then cross-link and produce a silicone elastomer with the foregoing exceptional resistance to moisture vapor transmission.
To this basic composition there may be added other ingredients. For instance, there may be added from .l to 5 parts by weight based on 100 parts of the base polymer of a silane of the formula, (R )2 Si (OCOR )2 where Rl2 and Rl3 are selected from alkyl and aryl radicals of l to 8 carbon atoms and may be the same or different.
It should be noted that the foregoing alkoxy, acyloxy func-tional silane is an exceptional adhesion promoter for one-component room temperature vulcanizable silicone rubber compositions as stated and disclosed in Kulpa, U.S. patent No. 3,296,161 dated January 3, 1967. Although such alkoxy, acyloxy functional silanes, need not be incorporated into the instant composition as for as increasing the resistance to moisture vapor transmission of the instant composition, nevertheless they can be so included to increase the self-bonding characteristics of the instant composition without the use of a primer.
Another ingredient that may be advantageously added to the instant composition, at a concentration of .l to 20 parts by weight of the base polymer, is of a polysiloxane fluid having therein Rl4 SiO units with units of the formula Rl4Sio.5 and units of the formula (Rl4 )3 SiO 5 where the polysiloxane has .1 to 8% by weight of silanol groups and the ratio of organosiloxy units to diorganosiloxy units varies from about 0.11:1 to 1.4:1 inclusive, and a ratio of triorganosiloxy units to diorganosiloxy units of from 4 2~ ~ 60SI-71 about .02:1 to l:l inclusive radicals, where R14 is selected from alkyl radicals of 1 to 8 carbon atoms, phenyl, vinyl and fluoroalkyl radicals of 3 to 8 carbon atoms. The fore-going polysiloxane fluid composed of a difunctional, tri-functional and monofunctional units is disclosed in Beers, U.S. patent No. 3,382,205 dated May 7, 1968. As such, the preparation and the use of such component in the one-com-ponent room temperature vulcanizable silicone rubber com-positions is set forth in the foregoing Beers '205 patent, and as such it is not necessary to explain the preparation and utilization of this ingredient in a one-component system.
Accordingly, this optional ingredient may advantageously be included in the instant composition to increase its adhesion to substrates without primer. It should be noted, as was the case with the Kulpa adhesion promoter, the Beers adhesion promoter did not affect the basic moisture vapor transmission properties of the uncured silicone or the cured silicone elastomer but, advantageously, may be added to the composition to increase its self-bonding characteri-stics to substrates in the absence of a primer. Another type of cross-linking agent that may be utilized is from 1 to 15 parts by weight of a silane of the formula, (R 3 CO) R 2 SiX
where Rl5 is an alkyl radical of l to 8 carbon atoms and Rl6 is selected from the class eonsisting of alkyl radicals, phenyl radicals, alkenyl radicals and fluoroalkyl radicals of l to 8 carbon atoms and X is a hydrolyzable radical.
This eross-linking agent ean be added to improve the modulus of the eomposition, its self-bonding eharacteristics, and to increase its tensile strength without affecting in any way or manner the resistance to moisture vapor transmission of the compositions of the instant case. This cross-linking l~t)428~ 60SI-71 agent is more fully described in Beers, U.S. patent No.

3,438,930 dated April 15, 1969. The preparation and use of such silanes in one-component room temperature vul-canizable silicone rubber compositions does not have to be explained since the foregoing Beers U.S. patent No.
3,438,930 dated April 15, 1969.
There may also be added any of a variety of well known additives to the instant composition to produce a one-component room temperature vulcanizable silicone rubber composition which has outstanding resistance to moisture vapor transmission and in addition has additional characteristics and properties as the result of the optional ingredients inserted into the composition so that the composition may meet certain requirements that may be necessary for a low moisture vapor transmission rate one-component room temperature vulcanizable silicone rubber composition that may be desired in a particular case.
All of these optional additives are added to the base composition in the same way that the other ingredients are mixed into the composition, that is basically the mixing takes place of the one-component system in a sub-stantially anhydrous state by the manufacture, and the mixture is then packaged and sealed from atmospheric moisture in substantially an anhydroud state. When it is desired to cure the composition, the composition is simply applied to whatever substrate is specified for particular applica-tion and allowed to cure in the presence of atmosphere moisture. In the foregoing examples, the moisture vapor transmission rate was determined for test materials utiliz-ing ASTM test method E-96-66 Condition E. This method comprises taking a sample, placing it on the top of a petri dish so as to blanket the dish in which there is Z8~
maintained a dessicant so that there is 0% relative humidity below the sample in the atmosphere of the petri dish.
This sample over the petri dish is then inserted into another covered dish which is sealed to atmospheric moisture and in which there is maintained a potassium salt and water solution such that inside in the second dish the atmosphere is 90% relative humidity. The sample on the Petri dish within the sealed outer container is then placed in an oven which is maintained at 100OE and a run is made for a two-week period. Every 24 hours the sample is taken and examined for moisture pick-up and the average reading of moisture pick-up of the sample for a 24 hour period in the second week of the two-week testing period is then taken as the moisture vapor transmission rate of the sample. It is not felt necessary to go further into a de-scription of the test method since such is fully set forth in the publication of the ASTM Test Method E-96-66 Con-dition E, as specified above.
In the two component RTV compositions based upon silanol end stopped diorganopolysiloxanes as previously defined, second component comprises generally from 1 to 15 parts by weight based on a hundred parts of the silanol end-stopped linear diorganopolysiloxane of a cross-linking agent selected from the class consisting of (2) R~a Si (OR")4 a and partial hydrolysis products thereof where R' and R' are selected from the class consisting of monov-alent hydrocarbon radicals and halogenated monovalent hydrocarbon radicals and a is 0 or 1. Generally, the organo substituting groups as disclosed above for R' and R"
are same as the organo groups of the silanol end-stopped linear diorganopolysiloxane polymer. In Formula (2) above a can be either 0 or 1. Either type of alkoxylated silane ~:1()4284 60SI-71 would provide the proper cross-linking in the instant composition. It should also be noted as atated above that partial hydrolysis products of such silicates may also utilized as cross-linking agents in the two component room temperature vulcanizable silicone rubber composition of the instant case. Such silicates and partial hydroly-sis products of such silicates are well known in the art, as for instance set forth in the U.S. patent 3,888,815 dated June 10, 1975 to Bessemer and Lampe. Such silicates may be utilized by themselves as cross-linking agents or their may be utilized in place of them as a cross-linking agent a resinous copolymer composed of R' 3 SiOo 5 units and SiO2 units in a weight ratio of 0.5:1 to 1:1 where R2 is selected from monovalent hydrocarbon radicals and halogenated monovalent hydrocarbon radicals and mixtures . "
thereof. The R radical may generally represent the same groups as those given above for the R radicals, as well as the R' groups for the silicates. Most preferably the R,R',R substituting group are selected from alkyl -radiclas, phenyl radicals, alkenyl radicals, fluoroalkyl radicals such as trifluoroproply and mixtures thereof.
Such substituting groups preferably do not have more than 8 carbon atoms. Such resinous copolymers as cross-linking agents for two component room temperature vulcanizable silicone rubber compositions is set forth in the Modic U.S.
Patent No. 3,457,214 dated July 22, 1969. It should be also noted that generally it is not necessary that either the silicates for Formula (2) or the resinous copolymer as discussed above that either one or the other be utilized as a cross linking agent in the composition of the instant case, since it is suitable in some cases to utilize as a cross-linking agent a mixture of the silicate of the partial ,~

.

1~42~4 60SI-71 hydrolysis products that are with the resinous copolymer.
Such cross-linking agents are utilized at a concentration of 1 to 15 parts by weight based on the undred parts of the silanol end-stopped linear diorganopolysiloxane polymer.
More preferably there is utilized 1 to 10 parts by weight of such cross-linking agents either one alone or mixtures thereof. Further, there must be present in the composition from .01 to 5 parts by weight based on a hundred parts of the silanol end-stopped linear diorganopolysiloxane polymer of a catalyst selected from metal salts of a carboxylic acid or dicarboxylic acids where the metal varies from lead to maganese in the Periodic Table. Preferred cat-alysts are dibutyl tin oxide and dibutyl tin dilaurate for utilization in the instant invention. The catalyst is mixed with the cross-linking agent and kept separate from the base polymer which usually contains the filler ingredients in it. When it is desired to cure the com-position the two components are mixed immediately prior to usage and then applied or molded to form the desired sealant bead and the mixed silicone composition is then cured to a silicone elastomer over a period of time vary-ing from 1/2 hour to 12 hours. However, in accordance with high speed of cure of such systems that can be formulated the work life of silicone compositions is within 10 minutes after being mixed and applied to provide the proper sealant function. Other ingredients for such low vapor transmission rate room temperature vulcanizable silicone rubber com-positions can be utilized in the instant compositions for instance the nitrogen functional silanes of the foregoing Lampe/Bessemer U.S. Patent can be utilized in the instant compositions to provide a self-bonding two component room temperature vulcanizable silicone rubber composition which ' ~)4Z~4 6OSI-71 composition will bond in a superior manner to adhere to most substrates as to metal substrates, concrete, glass, and various plastics. In addition to the other two ingredients mentioned above in the present two component room temperature vulcanizable silicone rubber composition there may be utilized from 3 to 35 parts by weight based on a hundred parts of silanol end-stopped linear diorganopolysiloxane polymer, and organopolysiloxane polymer having triorgano-siloxy terminal units and silanol terminal units where the ratio of triorganosiloxy terminal units to silanol terminal units varies from 1 to 1, 1 to 10 and which polymer has a viscosity varying from 50 to 1000 centipoise at 25C
and where such organo groups are preferably selected from the class of alkyl radicals, vinyl radicals, phenyl radicals and fluoroalkyl radicals. This ingredient which is pre-ferably mixed with silanol end-stopped based polymer slows down the cure and increases the modulus of the silicone elastomer or sealant that is formed from the composition.
It should be noted that this polymer species has in it also some trifunctional polymer species in the polymer mixture. This ingredient does not have to be utilized with the instant composition. However, it is advantageously utilized to produce a cured silicone sealant for insulated window panes with lower modulus and the improved tear strength. Such polymers are well known in the silcone art and are simply prepared by hydrolyzing with water a mixture of diorganodichlorosilanes and monoorganotrichloro-silanes as is well known in the art. Another optional and additional ingredient which facilitates the deep section of the two component room temperature vulcanizable silicone rubber composition in the instant case is a low molecular weight silanol terminated diorganopolysiloxane polymer.

1~()42~ 6osI-7l .

Such low molecular weight silanol terminated diorgano-polysiloxane polymer may be utilized at a concentration of 2 to 10 parts by weight of the base polymer and has a viscosity varying anywhere from 100 to 500 centipoise at 25 C where the organo groups of such low molecular weight diorganopolysiloxane polymer are again preferably selected from the class consisting of alkyl radicals, alkenyl radicals, alkenyl radicals, phenyl radicals and fluoroalkyl radicals of from 1 to 8 carbon atoms. Such low molecular weight silanol terminated diorganopolysiloxane polymers are preferably formed from again the hydrolysis of diorgano-dichlorosilanes. The use of such polymers in room tem-perature vulcanizable silicone rubber compositions is well known as can be seen in the U.S. Patent of Dale Beers 3,845,161 dated October 29, 1974. Such low molecular weight silanol terminated diorganopolysiloxane polymer is preferably mixed in the~base polymer. If it was mixed with the cross-linking ingredient it would be polymerized and not functional effectively as a deep section curing agent in the instant composition. Another optional ingredient that may be utilized in the instant composition is for instance from 2 to 10 parts by weight of a triorgano-siloxy terminated linear diorganopolysiloxane polymer having a viscosity varying from 5 to 500 centipoise at 25C wherein the said organo groups are again selected from the groups consisting of alkyl radicals, alkenyl radicals, phenyl radicals and fluoroalkyl radicals of 1 to 8 carbon atoms. Such linear diorganopolysiloxane polymer having a low molecular weight is simply utilized as a plastizing agent in the instant composition. It is pre- ~ -ferably mixed with the catalyst and the cross-linking agent to facilitate the mixing of the catalyst in the base com-~lC)4Z~ 60SI-71 positions so that the composition will have a unifrom cure rate throughout the mass thereof. The RTV compositions into which the mica filler may be incorporated may comprise a vinyl containing polysiloxane polymer of the formula CH2 = CH , 2 r~ 2 1 R2 SiO t SiO ~ Si CH = CH2 +

where R is selected from the class consisting of monovalent hydrocarbon radicals and halogenated monovalent hydrocarbon radicals and + varies such that the polymer has a viscosity varying from 1,000 to 500,000 centipoise viscosity at 25C.
The composition further comprises at least 0.1 parts per million of a platinum catalyst and from 1 to 50 parts by weight per hundred parts of the vinyl terminated base polymer of a hydraulic cross-linking agent selected from the class consisting of hydrogen containing silanes and hydrogen containing siloxanes. The vinyl terminated poly-siloxanes and the blends of such vinyl containing poly-siloxanes as well as the hydride cross-linking agent their method of preparation are fully disclosed in the Jeram/
Striker U.S. Patent No. 3,884,866 dated May 20,1975. It should be noted that there is envisioned in the present case that there need not be a blend of a high viscosity and a low viscosity vinyl terminated polysiloxanes for use within the scope of the present invention. There only need be only one vinyl terminated diorganopolysiloxane polymer in the SiH olefin platinum catalyzed compositions of the instant case. A blend of polymers as disclosed in the foregoing Jeram/Striker Patent may be utilized for certain applications. The normal reinforcing and extending fillers are optionally used in the composition for sag control purposes again. The cross-linking agents ~4284 60SI-71 that may be utilized in the compositions of the instant case whether it be a hydrogen containing silanes, siloxanes or siloxane resins is optional as set forth in the foregoing Jerman/Striker Patent. As far as the platinum catalysts are concerned many types of platinum compounds for this SiH olefin addition reactions are known and such platinum catalysts may be used for the reaction of the present case. The preferred platinum catalysts are those platinum compounds catalysts which are soluble in the present reaction mixture. The platinum compound can be selected from those having the formula (Pt C12Olefin) as described i. U.S. Patent No. 3,159,601 dated December 1, 1964 Ashby.
The olefin shown in the previous two formulas can be almost any type of olefin, but is preferably an alkenylene having from 2 to 8 carbon atoms, a cycloalkenylene having from 5 to 7 carbon atoms or styrene. Specific olefins utilizable in the above formulas are ethylene, propylene, the various isomers of butylene, octylene, cyclopentene, cyclohexene, cycloheptene, etc. A further platinum con-taining material usable in the composition of the present invention is the platinum chloride cyclopropane complex (P+C12C3H6)2 described in U.S. Patent No. 3,159,662 dated December 1, 1964 Ashby. Still, further, the patent con-taining material can be a complex formed from chloro-platinic acid with up to 2 moles per gram of platinum of a member selected from the class consisting of alcohols, ethers, aldehydes and mixtures of the above as described in U.S. Patent No. 3,220,972 dated November 30, 1965 -Lamoreaux.
The following examples are given for the purpose of illustrating the scope of the instant invention and are not given for any purpose of defining the scope or the ~ Z84 60SI- 71 limits of the instant specification and claims.

Example 1 There was prepared a base composition comprised of 250 parts of trimethylsiloxy silanol-terminated dimethyl-polysiloxane polymer having 15,000 centipoise at 25C and to which was mixed 30 parts by weight of octamethyl-cyclictetrasiloxane treated fumed silica. Into this composition there was also mixed 200 parts by weight of dry ground mica (3000 mesh size) and 96 parts of the resulting composition there was mixed into it as a cat-alyst cross-linking ingredient 4 parts by weight of a composition comprising 99.4% by weight methyltriacetoxysi-lane, 6% by weight dibutyl tin dilaurate. After a drying cycle of 1 hour, before the mixing of the catalyst with the other ingredients, sample sheets were prepared from the composition which sample sheets were allowed to cure in the presence of a atmospheric moisture for 4 days at 25C. The resulting cured sheets had the following physical properties:
Tensile (psi) 325 Elongation (%) 50 Shore A 60 Tear (Die B) lbs./in. 30 Samples were also taken for the moisture vapor trans-mission rate test discussed above and these samples shall be known as Samples A.
Example 2 There was prepared a composition comprising 900 parts of a silanol-terminated dimethylpolysiloxane polymer having 600 -centipoise viscosity at 25C. Into this polymer there was 11042~4 60SI- 71 mixed 100 parts by weight of a silanol oil composed of trimethysiloxy monofunctional units, dimethylsiloxy di-functional units and trifunctional units which oil had a viscosity of 45cps and a silanol content of .5%. To these ingredients there was added 100 parts by weight of fumed silica treated with octamethyltetrasiloxane and 1000 parts of a 3000 mesh mica. To 96% by weight of the above composition there was added 4~ by weight of the same catalyst system and a cross-linking agent system as utilized in Example 1. The resulting composition was prepared in a Ross change can mixer with a hour, 105C
drying cycle. The resulting composition, after the mixture was prepared, was then taken and sample sheets, hereafter referred to as Sample B, were prepared and allowed to cure for a period of time of 4 days at 25 C.
These samples of silicone elastomer, known as Sample B, were then measured in ASTM press cured sheets and for their moisture vapor transmission rate properties as will be set forth hereinbelow. The physical properties of the ASTM press cured sheets after 4 days were as follows:
Tensile, psi550 Elongation, ~63 Shore A 60 Tear, die B, lbs./in. 27 Afterwards samples of Sample A and Sample B were then taken and the moisture vapor transmission properties were tested in accordance with the above described ASTM Test Method E-96-66 Condition E and compared with a sample of a control room temperature vulcanizable composition containing no mica or talc in it (just ordinary reinforcing fillers, specifically fume silica) and there was also tested a ` -` 11(~428~ 60SI-71 typical polysulfide two-part sealant. The results of such tests in moisture vapor transmission rate are set forth hereinbelow:
MVTR Thickness gms/M in 24 hrs. mls.
RTV Control44.8 .74 Sample A 18.0 70 Sample B 14.6 74 Polysulfide1 - 14.8 75 2 - 15.0 75 The above results clearly indicate that Samples A and B containing the mica filler were considerably improved in moisture vapor transmission rate to the control RTV having no mica and had comparable moisture vapor transmission rates to polysulfides. It should be noted further that the moisture vapor transmission rate of Samples A and B could be decreased even further by the addition of additional mica filler in the compositions.
Example 3 There was mixed 770 parts of a silanol end-stopped dimethylpolysiloxane having a viscosity of 600 cs. with 70 parts by weight of cyclicsiloxane treated fumed silica, 105 parts by weight of a silanol oil composed of trimeth-ysiloxy monofunctional units, dimethylsiloxy difunctional units and methylsiloxy trifunctional units which oil has a viscosity of 45 cps at 25C and a silanol content of 0.5% by weight and into which there was mixed 700 parts by weight of 160 mesh mica. To 96 parts of this mixture there was added 4 parts by weight of the catalyst mixture of Example 1 having therein methyltriacetoxysilane and dibutyl tin dilaurate. After a drying cycle of 1 hour after the mixing of the ingredients with the exception of

4~8~ 605I-71 the catalyst, the catalyst was then mixed and sample sheets were preared and cured in presence of atmospheric moisture for 4 days at 25C.
The resulting cured sheets had the following physical properties:
Tensile, psi 660 Elongation, % 50 Shore A Hardness 71 In the moisture vapor transmission rate test this sample has a value of 7.42 grams per meter square in a sample 71 mils thick.
Example 4 There was prepared a base composition comprising 800 parts by weight of a silanol terminated dimethyl- :
polysiloxane polymer having 3000 centipoise at 25 C, 200 parts by weight of a trimethylsiloxy silanol end-stopped methylpolysiloxane polymer of 600 centipoise viscosity at 25C 100 parts by weight of octamethylcyclictetrasiloxane treated funed silica and 810 parts by weight of water ground mica 325 mesh to 100 parts of the foregoing base composition which is hereafter referred to as component A
there was mixed 10 parts of component B which was formed by mixing 33 parts by weight of a trimethylsiloxy end-stopped dimethylpolysiloxane polymer of 10 centipoise viscosity at 25C 4.0 parts by weight of gamma amino-propyltriethoxy silane 2.04 parts of partially hydrolyzed ethyl silicate and .68 parts of dibutyl tin oxide. The resulting composition had the following properties:
Tensile strength psi 400 Elongation % 50 Shore A Hardness 60 The resulting composition also in the Lap Sheer Test ~)4Z84 60SI-71 with Tinius Olsen Apparatus had an extension rate of .5 inches per minute. Its adhesion on stainless steel screen on Alclad aluminum of 70 pounds per inch with 5% cohesive failure. Its adhesion on Alclad aluminum plus glass screen with 1/4 inch bond line was 52 lbs. per inch and 35% co-hesive failure.
Further Component A was catalyzed with 10 parts per 100 parts of component A of a catalyst system prepared by mixing 22.5 parts of a gamma aminopropyltriethoxy silane 11:26 parts of partially hydrolyzed ethyl silicate 3.75 parts of tin oxide which was solubilized with a phthylate and 62.48 parts of minerial spirits when such a composition was prepared the adhesion was measured on an aluminum screen on Alclad aluminum which gave a value in one test of 105 pounds per inch with 50% of cohesive failure and a value of 83 pounds per inch with 50% cohesive in the second test. In another test with an Alclad aluminum with a stainless steel screen test the composition gave a result of 80 pounds per inch plus 60% cohesive failure. When the ASTM E-96-66 Test condition E was applied to a sample of this material 69 mils thick the sample had a moisture vapor transmission rate of 8.63 grams per square meter.
Example 5 There was mixed with 480 parts of silanol end-stopped dimethylpolysiloxane of 4200 centipoise viscosity at 25C, 120 parts of trimethylsiloxy end-stopped, silanol end-stopped polysiloxane oil, and 18 parts of a cyclicsiloxane treated fumed silica and 480 parts of 160 mesh mica which was treated with 4% by weight of stearic acid. To 100 parts of the above composition there was added 10 parts of a catalyst composition prepared by mixing 102 parts of a vinyl terminated dimethylpolysiloxane of 3,000 cps at ~ 4~4 60SI~71 25C, 120 parts of Ca CO3, 48 parts of gamma aminopropylt-riethoxysilane, 24 parts of partially hydrolyzed ethyl silicate and 3.6 parts of dibutyl tin dilaurate.
Sample sheets of the composition which had cured for 24 hours at 25C gave the following physicals and moisture vapor transmission rate (MVTR):
Tensile Strength psi 330 Elongation ~ 70 Shore A Hardness 62 MVTR - 8.25 grams/M on sheet 74 mils thick.
Example 6 There was mixed into 800 parts of silanol end-stopped dimethylpolysiloxane having a viscosity 600 centi-poise at 25C, 200 parts of a trimethylsiloxy, end-stopped, silanol end-stopped methylpolysiloxane oil, 30 parts of cyclicsiloxane treated fumed silica and 800 parts of 160 mesh mica. To 100 parts of the above composition there was added 10 parts by weight of the catalyst composition of Example 2. The resulting sample sheets which were cured for 24 hours at 25C had the following properties and moisture vapor transmission rate:
Tensile psi 310 -Elongation % 60 Shore A Hardness 60 MVTR = 9.32 grams/M on a sheet 76 mils thick.

: :

Claims (13)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a room temperature vulcanizable (RTV) silicone rubber composition comprising a diorganopolysiloxane base polymer, a cross linking agent and a catalyst therefor, the improvement which comprises incorporating in said com-position from about 75 parts to about 150 parts by weight on 100 parts of said diorganosiloxane base polymer of particulate mica whereby the water vapor transmission of said composition in the cured state is decreased.

2. The composition of claim 1 wherein said RTV rubber comprises (a) 100 parts by weight of a silanol-terminated diorganopolysiloxane having a viscosity varying from 100 to 500,000 centipoise at 25°C where the organo groups are selected from the class consisting of monovalent hydro-carbon radicals and halogenated monovalent radicals;
(b) and from 1 to 15 parts by weight of cross linking agent selected from a silane or siloxane having functionality selected from the class consisting of alkoxy functionality, acyloxy functionality, amine function-ality, amide functionality ketoximino functionality, and tert-alkoxy functional silanes and (c) from 0.01 to 5 parts by weight of a catalyst.

3. The composition of claim 1 wherein said RTV rubber comprises (a) 100 parts by weight of a diorganopolysiloxane polymer having a viscosity varying from 100 to 500,000 centipoise at 25°C wherein said organo groups are selected from the class consisting of monovalent hydrocarbon radicals and halogenated monovalent hydrocarbon radicals;
(b) from 1 to 15 parts by weight of a cross-linking agent selected from the class consisting of R'aSi (0R")4-a and partial hydrolyzide products thereof where R' and R"
are selected from the class consisting of monovalent hydrocarbon radicals and halogenated monovalent hydrocarbon radicals, a is 0 or 1 and a resinous copolymer of R'''Si00.5 units and Si02 units in a weight ratio of 0.5:1 to 1:1 when R is selected from monovalent hydrocarbon radicals and halogenated monovalent hydrocarbon radicals and mixture thereof; and (c) from 0.01 to 5 parts by weight of a catalyst.

4. The composition of claim 1 where said RTV
rubber comprises (a) 100 parts by weight of a vinyl containing polysiloxane polymer of the formula, where R0 is selected from the class consisting of monovalent hydrocarbon radicals and halogenated monovalent hydrocarbon radicals and + varies such that the polymer has a viscosity varying from 100 to 500,000 centipoise viscosity at 25°C.;
(b) an effective amount of a platinum catalyst;
and (c) from 1 to 50 parts by weight of a hydride cross-linking agent selected from the class consisting of hydrogen containing silane and hydrogen containing siloxane.

5. The composition of claim 1, 2 or 3 wherein said mica has a particle size varying from 50 to 400 mesh.

6. The composition of Claim 1, 2 or 3 wherein there is additionally present from 5 to 30 parts by weight of a reinforcing filler selected from the class consisting of treated fumed silica and precipitated silica.

7. The composition of Claim 1, 2 or 3 wherein said mica comprises up to about 30% by weight of talc.

8. The composition of Claim 2 or 3 wherein said silanol-terminated diorganopolysiloxane has the formula, wherein R is selected from the class consisting of alkyl radicals of 1 to 8 carbon atoms, phenyl radicals, vinyl radicals and fluoroalkyl radicals and n varies from 150 to 1500.

9. The composition of Claim 2 wherein in (b) said cross-linking agent is a silane having the formula, R1 Si (OR2)3 wherein R1 is selected from the class consisting of alkyl radicals of 1 to 8 carbon atoms, phenyl radicals, alkenyl radicals of 2 to 8 carbon atoms and fluoroalkyl radicals 3 to 8 carbon atoms and wherein R2 is selected from the calss consisting of alkyl radicals of 1 to 8 carbons atoms.

10. The composition of Claim 2 wherein in (b) said cross-linking agent is a silane having the formula, R3 Si (OOCR4)3 wherein R3 is selected from the class consisting of alkyl radicals of 1 to 8 carbon atoms, phenyl radicals, alkenyl radiclas of 2 to 8 carbon atoms, and fluoroalkyl radicals of 3 to 8 carbon atoms and wherein R4 is selected from alkyl radicals of 1 to 8 carbon atoms.

11. The composition of claim 2 wherein in (b) said cross-linking agent is a silane having the formula, wherein R5 is selected from the class consisting of hydrogen, alkyl of 1 to 8 carbon atoms and phenyl, n is at least 1 but does not exceed 4, b has a positive value agreed to of at least 3, and R6 is a Si-N bonded carboxylic acid amine radical having alkyl and aryl substituents of 1 to 8 carbon atoms.

12. The composition of claim 2 wherein (b) said cross-linking agent is a silane having the formula, wherein R11 is an alkyl radical of 1 to 8 carbon atoms, R7 and R8 may be the same or different and are alkyl radicals of 1 to 8 carbon atoms, R10 is an alkylene radical of 2 to 8 carbon atoms, R9 is an alkyl radical of 1 to 8 carbon atoms, varying from 1 to 3, f is a number varying from 0 to 2, and the sum of a, d and f is 4.

13. The composition of claim 9, 10 or 11 wherein the catalyst is a titanium chelate.