CA1217895A - Peroxide curing polysiloxane compositions having a high tear strength - Google Patents

Abstract

PEROXIDE CURING POLYSILOXANE COMPOSITIONS
HAVING A HIGH TEAR STRENGTH
ABSTRACT OF THE DISCLOSURE, The present invention relates to a silicone composition that cures to a silicone elastomer having a tear strength of above 100 pounds per inch and comprising a blend of vinyl-containing gums, a silica reinforcing filler, a hydride cross-linking agent, and a peroxide curing catalyst.

Description

~L2~

PEROXI~E 'CURING'PO~YSILOX~NE COM:P'OSITIONS
HAVING A HI'G~ TEAR;'STRENGTH
Backg'rou d of the- Invention The present invention relates to a silicone rubber 5 composition, and more particularly the present invention relates to a silicone elastomeric composition which has a tear strength of over 100'pounds per inch.
There has been much innovative work done on heat curable, silicone rubber compositions. Basically, such 10 Gompositions comprise a diorganopolysiloxane gum having a viscosity of at ~east 1,000,~00 centipoise at 25C., silica filler~ process aids, and a peroxide free radical initiator. See~ for instance, DeZuba et al., U.S. Patent No. 3~730,932, issued May 1, 1973. In such compositions, 15 it became necessary to vary the diorganopolysiloxane polymer. Particularly, it was desirable that the diorganopolysiloxane polymer contain vinyl radicals so that it could polymerize more readily into a elastomeric mass with desirable properties. Accordingly, one of the 20 developments in the area was to vary the vinyl unsaturation in the polymer blend, and particularly have a polymer blend made from various kinds of vinyl-containing polymers;
see for instance, U.S. Patent No. 3~660,345, issued May 2, 1972 to Bobear.
Ano-ther deveIopment was to make a high viscosity, heat-curable silicone rubber composition comprised of vinyl-containing polysiloxanes, a hydride-containing polysiloxane cross-linking agent, and a platinum catalyst with filler and other ingredients. The novel ingredient in ~2~7B9~ 60SI-417 such compositions was the use of a hydroperoxide inhibi~or which will allow the composition to have a long work life in one instance. In another instance, it allows -the compositions to be packaged in a single unit and upon heating the composition at elevated temperatures~ i.e., temperatures above 100C., the composition cured to a silicone elastomeric mass. Such a development is disclosed in Bobear, U.S. Patent No. 4,061~609, issued December 6~ 1977.
One variation on the above compositions is, for instance, to be found in Wada et al, U.S. Patent No.
3,671,480, issued December 18, 1969, which discloses a vinyl blend as the base polymers with a silicone hydride and a platinum catalyst. In this patent, the vinyl blend comprises one polymer containing from .02 to .2 mole percent of vinyl-containing siloxy units which appears to have a high molecular weight, and another vinyl-containing polymer containing at least about 2 mole percent of vinyl-containing siloxy units of somewhat lower molecular weight. From the recitation in the patent, it appears the molecular weight of the second polymer, at the minimum, could be below 100,000 centipoise at 25 C.
The purpose of this patent was to produce a high-tear co~position. It should be noted that the tear obtained by the samples set forth in the examples varied around 50 pi.
Another patent of interest is Polmanteer et al., U.S. Patent No. 3,697,473~ issued October 10, 1972, which describes a heat-curabIe composition comprising a vinyl siloxane, a hydride polysiloxane composed of various polymeric hydrides, and a platinum catalyst. The invention in this case was in the hydride blend. In this case the tear of the sample was as high in some cases as 200 pi.
However, the method of obtaining such high tear strength appears to have been the use of the novel hydride mixture of Polmanteer et al.

3L2~789S

Further developments are disclosed in Wada et al , U.S. Patent No. 3l652,~75, issued March 28, 197Z, ~hich discloses a heat-curable silicone composition comprisin~
a vinyl-containing polysiloxane of hiyh viscosityl and containing at most .3 mole percent of vinyl-containing siloxy units, which was blended with a high viscosity vinyl-containing siloxane of at least the same viscosity as the other polymer, and containing from 5 to 20 mole percent of vinyl-containing siloxy units. Thirdly, there was present another polydiorganosiloxane of a smaller degree of polymerization containing -from 5 to 90 mole percent of vinyl-containing siloxy units. Specifically, the vinyl-containing polysiloxane could be of very low molecular weight such that it could easily have a viscosity in the range of 1000 to 100,000 centipoise at 25C. within most of the degree of polymerization specified in the patent. This composition included silica filler and was cured by an organic peroxide. The tear strength of such compositions as set forth in the Examples was less than 50 p.i., especially after the post cure.
~ Another pertinent patent in this area is U.S.
Patent No. 3,669,068, issued October 3, 1972 to Creamer.
This patent discloses a heat-cura~le silicone stock comprising a vinyl-containing diorganopolysiloxane gum, a linear vinyl-containing fluid having a viscosity from 10 to 150,000 centipoise at 25C., and further including a vinyl-containing resin. This composition as disclosed was cured with peroxides. The tear strength shown ln the examples, and particularlyl Table 1, Table 2 and Table 3, exceeded 100 p.i. in some cases, but in most cases, did not exceed 100 p.i. Tables 2 and 3 disclose samples which had tears in excess of 150 p.i. Again, this composition contained a low viscosity vinyl-containing fluid. However, it does not appear that the inventor in U.S. Patent 3,669,068 carried out any measure of the tear of the samples after post-cure. The reason this is mentioned is ~2~9~
- ~ - 60SI 417 that with a low vinyl-containing fluid in the composition, it has been found that after post-cure~ -the tear strenyth dramatically drops.
Another reference oE interesk is that of Bobear, Canadian Application Serial No. 366,580, filed December 11, 1980, which disclosed a flame retardant, heat-curable composition comprising an organopolysiloxane gum, a filler, a platinum compound and various amounts of a hydrated aluminum, titanium dioxide and maynesium oxide.
There was some experimenta-tion with the variation of the vinyl-blend to increase the tear-strength properties of the composition in accordance with Bobear, U.S. Patent No. 3,660,3~5, wi~h some success. However, it has now been unexpectedly found that by having a particular type of vinyl blend of two high viscosity polymers, a filler, a peroxide curing agent, and a hydride polysiloxane present, that the composition can cure to a silicone elastomer with .~

~ 60SI-~17 _5_ exceedingly high tear strengths (in mos-t cases above 100 p.i. and in some cases exceeding 200 p.i.). This was achieved withouk haviny a complicakea hydride blend which is difficult to make due to the natural instability of the hydride polysiloxanes or by utilizincJ a low viscosity vinyl ~luid which it was later found would result in a composition whose post-cure tear strength would degrade dramatically.
Accordingly r it is one object of the present invention to provide for a high-tear-strength silicone rubber composition comprising a vinyl-containing poly-siloxane gum blend.
It is another object of the present invention to provide for a polysiloxane composition that cures to a silicone elastomer having a tear strength after post-cure of above 100 p.i~ which is formed from a high viscosity vinyl-containing polysiloxane blend, a hydride poly-siloxane, and a peroxide curing agent.
It is an additional object of the present invention to provide a process for forming a heat curing silicone elastomeric composition having a tear strength of above 100 p.i. which is formed from a vinyl-containing polysiloxane blend, a hydride polysiloxane, and a peroxide curing agent.
It is yet still a further object of the present invention to provide medical tubing and spark plug boots which have a tear strength of above 100 p.i. which are formed from a high viscosity polysiloxane blend, a hydride polysiloxane cross-linking agent, and a peroxide curing agent.
These and other obiects of the present invention are accomplished by means of the disclosure set forth herein below.
Summary of the Invention In accordance with the above objects, there is provided by the present invention, a polysiloxane ;

~2~895 60SI-417 composi-tion tha-t cures to a silicone elastomer haviny a tear strenyth of above 100 p.i~ comprising:
(A) 60-95 parts o a first vinyl-terminated linear dior~anopolysiloxane gum having a viscosity in the range of 1 x 106 to 20 x 107 centipoise at 25 C.
and having a vinyl concentration that can vary from 0.005 to 0.1 mole percent of siloxy units with at least one vinyl radical attached to silicon, and where the organo group is a monovalent hydrocarbon radical.
(B) from 5.0 to 40.0 parts by weight of a second vinyl~containing diorganopolysiloxane gum having a viscosity varying from 1 x 106 to 20 x 107 centipoise at 25C wherein the vinyl concentration varies from 0.5 to 15~0 mole percent, wherein the mole percent vinyl concentration (of siloxy units with at least one vinyl radical attached to silicon) increases from 0.5 to 15.0 mole percent, as the second vinyl-containing gum concentration in the composition decreases from 40.0 to 5.0 parts by weight wherein the organo group is a monovalent hydrocarbbn group.
(C) from 10 to 300 parts by weight of a filler, at least part of which is a reinforcing silica filler;
(D) from 0.1 to 25 parts by weight of a hydride-containing polysiloxane having a hydride content varying from 0.05 to 5.0 percent by weight and a viscosity varying from 10 to 1000 centipoise at 25 C; and (E) a curing agent being present at a concentration of at least an effective amount of free radical initiator selected from the class consisting of 3Q organic peroxides and organic hydroperoxides.
Basically, the process for forming this composition comprises mixing the ingredients and heating the composition at elevated temperatures; i~e., temperatures above 100 C. for a period of time varying anywhere from 60 seconds to 30 minutes. Preferably~ the method of vulcanization comprises hot air vulcanization, and just such a method is particularly suited to -- ~2~ S

producing medical tubing and spark plug boots of high tear strength. It is, of course/ obvious to produce the high-tear-strength-composition, there must be present some silica filler in -the composition, and particularly a reinEorcing silica filler in -the composition such as fumed silica and precipitated silica.
A platinum catalyst may be incorporated in the composition to speed up the cure, but it is not necessary;
it may or may not be used.
Description of the Prefer~red Embodiment Before going into the description of the invention, it is necessary also to discuss the fact that the curing of vinyl-containing compounds with hydxides in the presence of peroxide is known as exemplified by the disclosurein the publication Organic Synthesis Via Metal Carbonyls, Vol. 2, p 673 (1977) edited by Irving Wender and Piero Pino. What was not known/ however, was to produce a high-tear compound utilizing a particular type of vinyl blend of diorganopolysiloxane gums which were reacted with a hydride polysiloxane and a peroxide curing agent.
Now, proceeding to the invention, the invention in the first aspect comprises 60-95 parts by weight of a first vinyl-terminated linear diorganopolysiloxane gum having a viscosity in the range of 1 x 106 to 20 x 107 centipoise at 25C. and having a vinyl concentration that can vary anywhere from .005 to .l mole percent vinyl, and where the organo group is a monovalent hydrocarbon radical.
In the first aspect of the invention, preferably the polymer is just vinyl-terminated. Howeverl the polymer can also have vinyl on-chain groups in accordance with the vinyl content of the gum~ It should be noted that this is a gum that has a viscosity of anywhere from l,OOO,OOQ to 200,000,000 centipoise at 25C., and more preferably has a viscosity in the range of 10,000,000 - ~L2~ 9~
60SI~417 to 100,000,000 centipoise at 25C. Alsol preerably, the mole percent vinyl can be anywhere rom .01 to ~B
mole percent vinyl. Preerably, such a irst vinyl~
containing gum has the formula t~ R R

wherein in the abov~ formula~ Vi is vinyl, R is selected from Cl 8 monovalent hydrocarbon radicals free of aliphatic unsaturation, and Rl is a Cl 8 monovalent hydrocarbon radical, and x and t vary such that the viscosity of the gum varies from 1 x 10~ to 20 x 107.
More preferably) it varies from 1 x 107 to 15 x 107 centipoise at 25C. The radical R can be selected from any monovalent hydrocarbon radical such as alkyl radicals from 1 ~o 8 carbon atoms such as methyl, ethyl, propyl, phenyl radicals; mononucleararyl radicals such as phenyl, methylphenyl, etc.; cycloalkyl radicals such as cyclohexyl, cycloheptyl, etc.; fluoroalkyl radicals such as 3,3,3-trifluoropropyl~ etc. Accordingly, R can be any substituted or unsubstituted monovalent hydrocarbon radical which is substantially inert to the addition reactions. The radical Rl can be any of the same foregoing radicals given for R and in addition an alkenyl radical such as vinyll allyl, etc. Most preferably r R is not an alkenyl radical.
A second component of the diorganopolysiloxane gum is another gum which comprises from 5.0 to ~0.0 parts by weight of a second vinyl-containing diorganopolysiloxane gum having a viscosity varying from 1 x 106 to 20 x 107 centipoise at 25C., wherein the vinyl concentration ~2~78gS 60SI-417 _g_ varies from .5 to 15.0 mole percent and wherein as the mole percent vinyl concentration increases from .5 to 15 mole percent, the second vinyl contalning gum concentration in the composition decreases from 40.0 to 5.0 parts by weigl~t where the organo yroup is a monovalent hydrocarbon radical. It is important in the invention that as the second vinyl-containing diorganopolysiloxane gum decreases in concentration in the vinyl bIend~ then its mole percent vinyl preferably increases. By preferred, it is meant that it can either increase or stay the same. However r it is important that if the second vinyl-containing gum contain from 10 to 15 mole percent of vinyl siloxy units, it should not be present in the composition at a concentration of, say, 20 to 40 parts by weight per 80 to 60 parts by weight of the first vinyl-containing gum. I-f there is a high concentration of a high vinyl-containing gum in the composition close to the high viscosity limits given above in the compositionr then the composition will not have as high tear strength as is desirable. It is preferred as the viscosity of the gum increases within the above prescribed units that its vinyl concentration decrease.
Accordingly, as the concentration of the second vinyl-containing gum increases in the blend mixturer then i-t is necessary that its mole percent vinyl concentration in the grum decrease within the range indicated above.
Preferably, the vinyl concentration of the second vinyl-containing gum varies from .5 to 10 mole percent or less.
In addition, it is preferred that the viscosity of the second vinyl-containing gum vary anywhere from 1 to 200 x 10`6 centipoise at 25C. It is desirable that the viscosity of the two gums be as high as possible within the foregoing viscosity limits given abover and experimentations seem to indicate that the higher viscosity blends resul~ in the elastomers with a higher tear.

Further, in the concentration of the -two gums, the more preferred range of utilization of the two gums is ~rom 70 to 95 parts of the first vinyl-terminated linear diorganopolysiloxane gum wi-th rom 5 ~o 30 parts by weight of the second vinyl-containing diorganopolysiloxane gum. This vinyl-containing gum preferably has vinyl on-chain units only. However, in a broader embodiment of the present invention, the second vinyl-containing gum can be vinyl-terminated as well as containing vinyl on-chain within the prescribed limits of vinyl concentration indicated above. It is important that both the vinyl-containing gums do not contain any more vinyl than indicated in the above language since if too much vinyl is present in the polymer blend~ then the cured elastomer will not have optimum tear-strength properties. Further, it should be noted that either gum can be either a singular polymer species or polymer blend with the fore-going viscosities and vinyl concentrations. It is also important to note that there should be no vinyl-containing fluid in the composition. It has been foundthat the compositions with vinyl-containing fluids of a viscosity, say in the 500,000 centipoise range or less, result in elastomers with good tear strength initially;
but after the composition has been subjected to post-cure, its tear-strength properties degrade dramatically.
Preferably the second vinyl-containing gum has the formula R SiO l Sio ~ I O ~ R2 (2) R3 ¦ 14 l l R3 ¦ R
where R , R are selected from Cl 8 monovalent hydrocarbon radicals, R3 is a Cl 8 monovalent hydrocarbon radical free of aliphatic unsaturation, and w and z vary such that the ~ 95 60SI-417 viscosity of the gum varies from l x 106 to 20 x 107 centlpoise at 25C. and, more preferably, varies rom 20 x 106 to ]OO x 106 centipoise at 25C. and wherein as the vinyl concentration of the gum ~aries from 0.5 to 15.0 mole percent provided the viscosity oE
the gum increases as the vinyl concentration approaches .5 mole percent. This is another aspect of the present invention; i~e., it is not desirable to have a high vinyl-containing, high-viscosity gum within the range indicated above since this would result in a composition with poor tear-strength properties when it was cured.
The radical R3 can be any of the radicals given for the radical for the Formula (1) above. Further, the R and R radicals can be any of the Cl_8 monovalent hydrocarbon radicals previously given for the radical R
in formula (l) and in addition can be alkenyl radicals such as vinyl, allyl~ etc.
The vinyl-containing gums can be made by a process well known in the art such as, for instance, reacting vinyl-containing cyclotetrasiloxanes in the presence of low molecular weight linear vinyl chain stoppers at high temperatures in the presence of basic catalysts so as to yieId the polymer of the desired molecular weight. When the reaction is over, then the catalyst is neutralized, the excess cyclics vented off to result in the desired polymer. By controlling the amount of chain stopper and the temperature of reaction, there can be controlled the molecular weight of the desired vinyl-containing polymer end product. For more informa-tion as to the process with which such vinyl-containing polymers are produced, one is referred to the disclosure of Bobear, U.S. Patent No. 3,660,345, disclosed previously.
To the lO0 parts of the vinyl blend of gums, there 35 is added in the composition from 10 to 300 parts by weight of a filler, at least part of which is a reinforcing ~- silica filler. A reinforcing silica filler is need in the ~2~7~9S 60SI~417 composition if the composition is to have high-tear-strength-properties, and particularly tear-strength-properties which are above lO~ p.i. after posk~cure aging. Examples of reinforcing silica fillers that can be utilized are, for instance, fumed silica and precipitated silica, and especially Eumed silic~. The fumed silica or precipi-tated silica can be treated with various agents so as to prevent the uncured composition from structuring such cyclopolysiloxanes as disclosed in Lucas, U.S. Patent No. 2,938~009, issued May 24~ 1960, and silazanes as disclosed in Smith, U.S. Patent No.
3,635,743, issued January 18, 1972. The reinforcing fillers are generally used at a concentration of 10 to 200 parts by weight and/or 10 to 100 parts by weight, and there can be utilized in addition other extending fillers in the composition.
Extending fillers that may be utilized are, for instance, seIected from the class consisting of titanium dioxide, lithopone, zinc oxide, zirconium silicate, silica aerogeI, iron oxide, diatomaceous earth, calcium carbonate, glass fibers, magnesium oxide, chromic oxide, zirconium oxide, aluminum oxide, alpha quartz, calcined clay, carbon, graphite, cotton, and synthetic fibers. These are some of the extending fillers that can be utilized to give the composition additional desired properties for one reason or another. It should be noted that to produce the silicone elastomer, it is not necessary to have a filler, particularly a fumed silica, but if it is desired to have a high-tear-strength-composition, it is necessary to have substantial amountsof fumed silica in the compositionO
Per 100 parts of vinyl diorganopolysiloxane blend, there must be from .1 to 25 parts by weight of a hydride-containing polysiloxane having a hydride content varying fro~ .05 to 5 percent by weight and preferably having a hydride content varying from .05 to 2 percent by weight and a viscosity varying from 12~L7~395 10 to 1000 centipolse at 25C. and, more pre~erably/ having a viscosity varying from 10 to ].00 centipoise at 25 C.
The hydride crosslinking agent may be eithér a linear hyclride or a hydride-containing resin. Preferabl~, -the hydride polysiloxane is a linear diorganopolysilo~ane polymer having the formula H S O - - S - t ii t si H (3) P q where R is selected from Cl 8 monovalent hydrocarbon radicals free of aliphatic unsaturation, and p and q vary such that the viscosity of the hydride polysiloxane varies from 10 to 1000 centipoise at 25C. and, more preferably, the viscosity of the polymer varies from 10 to 100 centipoise at 25~. and the hydride content varies from .05 to 2.0 percent by weight. It is desirable that the hydride polysiloxane not contain too little hydride, otherwise the composition will not cure completely. On the other hand, if it has too much hydride, there will be excess hydride polysiloxane in the composition, possibly acting as a plasticizer, and the composition will not have as high tear-strength as possible. Such hydride polysiloxanes in Formula (3) can be made by many procedures which are known in the art, and particularly by the hydrolysis of the appropriate chlorosilanes. For more information as to the production of such hydrides/ one is referred to the disclosure of U.S. Patent No. 4,041,OIO, issued August 9, 1977 to Jeram.
Another type of hydride crosslinking agent that may be utilized is a hydride resin having ~21~9~ 60SI-417 H SiOo 5 units (4) and SiO2 units whexe R6 is a Cl 8 monovalent hydrocarbon radical free of aliphatic unsaturation and the R ~ H
to SiO ratio varies from 1.0 to 2.7 wherein the hydride content varies from .05 to 2 percent by weight. It should be noted that both in Formula (3) as well as in the hydride resin above r` that R6 radicals can be any of Formula (I). These radicals can be any hydrocarbon radical whether substituted or unsubstituted which do not enter into the hydrosilation reaction. Again such a resin is made by hydrolysis of the appropriate chloro-silanes and purification thereof.
Another type of hydride region is one having difunctional units. Accordingly, they can be utilized in the instant invention, the hydride-containing polysiloxane which is a hydride regin having H - SiOo 5 units mono~unctional units and SiO2 tetrafunctional units, and R SiO difunctional units, where the R6 + H to Si ratio varies from 1.2 to 2.7 and R~ is selected from Cl 8 monovalent hydrocarbon radicals free of aliphatic unsaturation which are selected from the same monovalent h~drocarbon radicals given for the R radical of Formula (I). It should be noted that basically most of the radicals in these polymers are either selected from methyl, phenyl, and vinyl, in accordance with the ~2~78~

disclosure since these are the simplest polysiloxanes to be produced. The phenyl polysiloxanes or some phenyl substitution in the polysiloxane may be desirable so as to give the composition certain properties. Other than that~ it is preferable the polysiloxane contain only methyl for the hydrides and methyl, vinyl substitution for vinyl containing polysiloxanes. These are the simplest and cheapest polysiloxanes to produce. Further the hydride-containing difunctional siloxy units preferably have avinyl concentration that varies anywhere from .05 to 2.0 percent by weight. ~gain, such a hydride resin containing difunctional siloxy units is well known in the art as shown in U.S. Patent No. 4~041~010~ issued August 9r 1977 to Jeram, whether containing fluoro substituent units or not, and can be utilized as a crosslinking agent in the present composition. Further, such hydride crosslinking agent can be produced by methods well known in the art as noted in the foregoing U.S. Patent 4,041,010.
2Q Finally, in the composition there must be a curing agent which is a free radical initiator and which is desirably present at a minimum, as an effective amount and is generally selected from organic peroxides and organic hydroperoxides.
Examples of suitable hydroperoxides are, for instance, disclosed in U.S. Patent No. 4,061~609, issued December 6~ 1977 to Bobear, such as, for instancer tertiary butylhydroperoxide, methylethylketone hydroperoxide, cumene hydroperoxide, 1,1,3,3-tetramethyl-butylhydroperoxide and dimethyl 2,5-dihydroperoxy hexane.
The more preferable peroxides are the organic peroxides, and particularly the vinyl specific peroxides. The preferred curing agents are organic peroxides conventionally used to cure silicon elastomers. Some peroxides that may be used are the dialkyl peroxides which may have the structural formulas, ,~

~2~789S

R R R
R - C - R R- C - ( 2 a C R
O O

R - C _ R R - - C - R R - C - -R
R R

wherein R represents the same alkyl group throughout or alkyl groups of two or more different types and a is zero or a larger integer.
Among the dialkyl peroxide curing catalysts that can be used are di-tertiary-butyl peroxide, tertiary-butyl-triethylmethyl peroxide, tertiary-butyl-tertiary-butyl-tertiary-triphenyl peroxide, t-butyl perbenzoate and a di-tertiary alkyl peroxide such as dicumyl peroxide. Other peroxide catalysts which effect curing through saturated as well as unsaturated hydro-carbon groups on the silicon chain are aryl peroxides which include chloroalkyl peroxides such as 2,4-dichlorobenzoyl peroxide, parachlorobenzoyl peroxide;
~ 15 orthchlorobenzoyl peroxide, benzoyl peroxide, etc. The : best dialkyl peroxide that has been found is

2,5-dimethyl-2,5-bis(t-butyl-peroxy)hexane.
Accordingly, the most preferred peroxides are the vinyl specific peroxides and the ones that have been found to give the best curing tear strength composition are for instance:
2,5 dimethyl-2,5-di(t-butylperoxy)hexane 2,5 dimethyl-2,5-di(t-butylperoxy)hexane-3 di-t-butylperoxide 5-butylcumylperoxide , a~ bis(-t-butylperoxy~di-isopropylbenzene.

~2~9S 60SI-417 Preferably, there is present at least .1 parts by weight of the curing agent per 100 parts of the rest of the composition. More preferably, there is utilized from .1 to 2 parts by weight o~ the curing agent -specifically the peroxide - per 100 parts of the rest of the composition. It should be noted that there is nothing critical about the concentrations o~ the peroxide, that being used in the specific composition tha-t results in the best tear-strength properties.
Finally, there may be utilized as a cure accelerator from 1 to 500 ppm of platinum in the composition as a platinum compound and particularly one of the platinum catalyst compounds well known in the art. The platinum catalyst which may be utilized in the present composition may be a platinum deposited on a solid carrier such as platinum on charcoal or platinum on gamma alumina or may be a solubilized platinum complex. The solubilized platinum complex are preferred in the present composition since they are more reactive.
Preferred platinum catalysts are those platinum compound catalysts which are soluble in the present reaction mixture. The platinum compound can be selected from those having the formula (PtC12 Olefin)2 and H(PtCl Olefin) as described in U.S. Patent No.

3l159,601, issued December 1l 1964 to Ashby. The olefin shown in the previous two formu]as 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, cyclohexane~ cycloheptaneJ etc.
A further platinum containing material usable in the composition of the present invention is the platinum chIoride cyclopropane complex (PtC12 C3H6) described in U.S. Patent No. 3,159,662, issued December lr 1964 to ~shby.

~789~ 60SI-~17 Still, further, the platinum containing material can be a complex formed from chloroplatinic acid wi~h up to 2 moles per gram of platinum of a member selected ~rom the class consisting of alcohols, ethers, aldehydes ~nd mixtures of the above as described in U.S. Patent ~o.

3,220,972, issued November 30, 1965 to Lamoreaux.
The pre~erred platinum compound to be used not only as a platinum catalyst but also as a flame retardant additive is that dlsclosed in U.S. Patent No. 3,775,452, lQ issued November 27, 1973 to Karstedt. Generally speaking, this type of platinum complex is formed by reacting chIoroplatinic acid containing 4 moles of water of hydration with tetravinylcyclotetrasiloxane in the presence of sodium bicarbonate in an ethanol solution.
The Karstedt or Lamoreaux catalysts are preferred in the present composition since they are the most reactive and result in the cure rate of the composition being most easily con-trolled. The Karstedt catalyst which is the most preferred is generally an alkenyl polysiloxane complexed with platinum and which is substantially free of chlorine radicals. The platinum catalyst can or cannot be utilized. It is not necessary for the higher tear-strength properties or the cure of the composition to have the platinum. However, the platinum, if utilized, does act as a cure acceIerator.
Other ingredients which are desirable, but not necessary, in the instant composition are, for instance, process aids. Thus, generally, it is desirable to have per 100 parts of the base vinyl diorganopolysiloxane gum, 3Q from 1 to 25 parts by weight of a process aid. Process aids are necessary in high viscosity compositions so as to process them on mills and fabrication equipment.
The process aid may also be a dihydrocarbon-substituted polysiloxane oil having hydrocarbon substituent to silicon atoms ratio of from 1.6 to 2.0 and whose hydro-carbon substituents comprise at least one member selected from the class consisting of methyl, ethyl~ vinyl, allyl, ~;~17~39S

cyclohexenyl and phenyl groups, said polysiloxane oil comprising polysiloxane molecules containi~g an average of from one to two lower alkoxy groups bonded to each of the termina]. silicon atoms where the alkoxy groups are selected from ~he class consisting of methoxy, ethoxy, propoxy and butoxy.
Preparation of the alkoxy-containing hydro-carbon-substituted polysiloxane oils can be employed as a process aid in the present invention can be carried out by producing one or more types of cyclic dihydrocarbon-substituted polysiloxanes from one or more types of dihydrocarbon-substituted dichIorosilanes and dialkoxysilanes in accordance with the hydrolysis, depolymerization and fractional distillation procedures described in detail above with reference to the pre-paration of the~:gum of Formula (l). Then one or more types of cyclic siloxanes so produced are mixed with predetermined amounts of a dihydrocarbon-substituted dialkoxysilane and the mixture is subjected to an equilibrium treatment under controlled conditions to produce the desired alkoxy end-blocked hydrocarbon-substituted linear polysiloxane oil~
The alkoxy-~ontaining hydrocarbon-substituted polysiloxane oils suitable for use in the present invention are relativeIy low molecular weight polysiloxane oils whose polymer chains have at least four and as much as thirty-five and more dihydrocarbon siloxy units per molecule. The polysiloxane oils preferably have an average of at least one and not more than two alkoxy groups bonded to each of the terminal silicon atoms of the molecule~ A more detailed disclosure of the alkoxy end-blocked polysiloxane process aids, as well as their method of preparation, is to be fcund in the disclosure of Fekete, U.S. Patent No~ 2,954,357, issued September 27, 1960.
There may also be used as a process aid .

78~S
60SI~417 hydroxylated organosilanes which contain ~rom one silicon-bonded hydroxyl per 70 silicon atoms to two silicon-bonded hydroxyls per silicon atom and conkains ~rom 1.9 to 2.1 hydrocarbon radicals per silicon ato~ls.
The remaining valences of the silicon atom are satisfied by oxygen atoms. The hydroxylated materials include both monomers such as diphenylsilanediol and polymeric materials which contain two silicon-bonded O~I groups in the molecule. In addition, the hydroxylated lQ organosilane may be a mixture of hydroxyl-containing siloxanes and completeIy condensed siloxanes~ Irrespective of the particular composition of the hydroxylated organosiloxane, it is necessary that there be present in said organosilane from one OH to 70 silicon atoms to lS two OH per silicon atoms.
The hydroxylated siloxanes may be prepared by any suitable method/ such as heating said siloxanes with steam under pressure at temperatures of about 120 C.
or hydrolyzing silanes of the formula R SiX4 n where X is any hydrolyzable group such as Cl, OR, H OOR
and R is a monovalent hydrocarbon radical. The former method is preferred for the preparation of those hydroxylated materials in which the hydrocarbon radicals are alkyl~ while the latter me-thod is best for the siloxanes in which hydrocarbon radicals are monocyclic aryl hydrocarbon radicals. Further, detailed information as to the hydroxylated or~anosiloxanes which may be used as process aids is to be found in Konkle et al., U.S.
Patent No. 2,890,188, issued June 9, 1959.
Any of the above process aids may be used alone or mixtures thereof may be used in the above-defined concentrations. Further, other suitable process aids may also be used in the silicone rubber composition of the present invention. As stated, the process aids are desirable in the composition so as to facilitate its processing on milling equipment and other fabrication 12~a~5 60SI-417 equipment because of the high viscosity of the yum.
There may also be incorporated in the final composition, a third vinyl-containiny siloxane, and specifically a vinyl-containing resin. The use of such vinyl-containing resins has been found to give the composition good reinforcing tear-strength-properties~
Accordingly, there may be present in the composition per 100 parts of the base vinyl-containing diorganopolysiloxane gum blend from 1 to 50 parts by weight of an organopolysiloxane resinous co-polymer having therein R3SiOo 5 monofunctional units and SiO2 tetrafunctional units, where R8 is a member selected from a class of vinyl radicals and monovalent hydrocarbon radicals free of aliphatic unsaturation where the ratio of monofunctional units to tetrafunctional units being from .5 to 1 to 1 to 1, and from where about 2.5 to 10 mole percent of the silicon atoms contain silicon bonded vinyl groups. The radical R8 can be any of the radicals given for the radical of R in Formula (1). It 2Q should be noted further that this resin should not contain any hydride groups in it.
There can also he utilized a similar resin having difunctional siloxy units. Accordingly, per 100 parts of the base vinyl-containing diorganopolysiloxane gums, there may be incorporated into the composition from 20 to 50 parts by weight of an organopolysiloxane resinous co-polymer comprising R3SiOo 5 monofunctional units and SiO2 tetrafunctional units and R2SiO difunc-tional units, where R8 is a member selected from a class consisting of vinyl radicals and monovalent hydrocarbon xadicals free of aliphatic unsaturation where the ratio of monofunctional units to the tetrafunctional units is .5 to 1 to 1 to 1 and the difunctional units that are preferably prese~t in amount equal to about 1 to 10 mole percent based on the total number of moles of siloxy units in the co-polymer and wherein the resinous copolymer -; contains fro~ about 2.5 to 10 mole percent of vinyl groups.

~2~7a~s Such a resinous co-polymer can be present in the composition in the foregoing quantities indicated above, and more preferably, at the concentration of 5 to 25 parts by weight per 100 parts of the base vinyl-containiny diorganopolysiloxane gum blend. It has been found such vinyl-containing resins produce good cured elastomeric compositions with good tear strength even after post-cure aging. Unlike the low viscosity vinyl-containing fluids~ the compositions with the vinyl-containing resins have good tear-strength-properties even after post-cure aging, i.e., above 100 p.i. Suchresins are known and are produced by methods well known in the art and can be utilized within the prescribed above quantities with good results. Further, such resins are usually produced by hydrolysis of the appropriate chIorosilanes which method is well known in the art. For further information as to such resins/ one is referred to the . .
disclosure of Modic, U.S. Patent No. 3,436,366, issued April lt 1969.
2a There may be incorporated other ingredients in the composition such as compression set additives such as rare earths octoate, and various other ingredients.
Also~ there may be incorporated other process aids and various types of other ingredients, such as self-bonding additives as disclosed in ~eZuba et al., U.S. Patent No.
3,730,932, issued May 1, 1973. However, the above is the basic ingredients that are necessary in the composition with the additional optional desirable additives as disclosed above: i.e.~ the specific process aids disclosed above or the platinum catalyst, or the vinyl-containing resins. These ingredients may be added with desirable efects and, as statedr other ingredients may be added as desired to produce specific results such as sel~-bonding, etc. If platinum is in the composition, there may be present an inhibitor. Specifically, the hydroperoxide compound may also be present as inhibitor in the composition especially if platinum is present.

~2~7~9~ 60SI-417 Further, the composition may be prepared in many ways. First of all, the composition ing~edients may be mixed with or without platinum, and if platinum is present with an inhibitor,~without the peroxide; and ; 5 stored as such. When it is desired to fabricate the composition, the composition can be milled, the peroxide added, and then the composition heated at a temperature above 100C. for periods o-f time varying fxom 60 seconds to 60 minutes to result in a cured elastomer. Then, lQ subsequently, it may be post-cure aged for a period of time varying anywhere from 1 to 4 hours at temperatures above 100C. The means of vulcanization may be hot air vulcanized or other means.
In another methcd with respect to the present composition, the composition - especially if platinum is present - may be packaged in separate packages without the hydride, the vinyl-containing siloxane and platinum being in the same package. Then when it is desired to cure the composition, the two compositions are mixed, there is added a peroxide or hydroperoxide curing agent to the composition, and then the composition is heated at temperatures above 100C. for the foregoing time period.
Irrespectlve of which method oE packaging and preparing the composition is undertaken, desirably the peroxide catalyst is not added to the composition until just before it is molded and fabricated to the desired part just before it is subjected to vulcanization. In some cases the peroxide catalyst may be added beforehand, but the problem with this is that the peroxide tends to degrade 3Q upon storage. It is not usually the practice to incorporate such peroxides in the composition prior to the time they are milled and fabricated and vulcanized into the desired part. By this method, there can be produced various parts such as tubing~ and specifically medical tubing, spark plu~ boots and other parts which have high-tear-strength; i.e., tear strengths of above 100 p~i., and in some cases above 200 p.i., and which ~2~7~9S 60SI-417 even after post-curing aging~ may have tear strengths of above 100 p~i.
The examples given below are given for the purpose of illustrating the present in~ention. The~
are not given for any purpose of sekting limits and boundaries to the instant invention. ~11 parts are by weight.
_ample 1 There was prepared a composition comprising 80 parts by weight of a vinyl-terminated dimethylpolysiloxane gum of 5.33 x 107 centipoise at 25C. and 20 parts of a .6 mole percent vinyl on-chain trimethylsiloxy end-stopped dimethylpolysiloxane gum having a viscosity of 4.54 x 107 centipoise at 25C. where the viscosity of the blend 15 was approximately 5 x 107 centipoise at 25C. To this mixture there was added 1.5 parts of a hydroxy-stop dimethylpolysiloxane oil, process aid. In addition, there was added to this mixture 42 parts by weight of an octamethylcyclotetrasiloxane treated fumed silica. Then 2Q there was added 2 parts by weight of a hydride linear polysiloxane which had hydride terminal groups and hydride ~; on-chain groups and a viscosity in the range of 35 to 75 centipoise at 25C. and a hydride content of .72 to 1.0 percent by weight~ This mixture was cured with .5 parts by weight of 2,5-dimethyl 2,5-ditertiarybutylperoxyhexane per 100 parts of the rest of the mixture. This mixture was press-cured for 10 minutes at 350F. and a sample was taken to evaluate the properties. In addition the mixture was further post-cured by oven baking it for 2 hours at 350F. and the physical properties were evaluated. The results are as follows:

-` 12~78~5 PRESS CURE OVEN BAKI~G
PHYSICALS 10 min./350F. 2 hrs./35~ F
.. ... _ . _ _ ~ardness Durometer ~5 ~9 Tensile Strenyth, psi1169 1222 % Elc~ngatiQn 720 650 Die B, Tear Strenyth, Pi 202 215 Exam~e 2 There was prepared a second composition comprising 80 parts by weight of a vinyl-terminated dimethylpolysiloxane gum having a viscosity of 2.6 x 107 centipoise at 25~C. and 20 parts by weight of a .6 mole percent vinyl on-chain gum having a viscosity of 4.65 x 107 centipoise at 25 C. To this blend there was added 17 parts by weight of a 60 percent solids in xylene solution of a vinyl-containing resin containing monofunctional vinylmethylsiloxy units and tetrafunctional SiO2 units wherein the vinyl content of the resin was in the range of 1.8 to 2.4 weight percent and wherein the monofunctional to tetrafunctional ratio was equal to .6. To this composition ~here was added 3 parts of the same process aid as in Example 1, and 60 parts by weight of the octamethylcyclotetrasiloxane treated fumed silica of Example 1. This mixture was cooked at 150C. for 1 - 1~ hours and cooled. To 172 parts by weight of the cooled mixture, there was added 6 parts by weight of the hydride of Example 1 and 3 parts by weight of a curing catalyst composition which was 33 percent by weight of a dimethylpolysiloxane oil, trimethylsiloxy end-stopped, ha~ing a viscosity less than 100,000 centipoise at 25C. and 67 percent by weight of Varox powder (Varox is the trade name of R.T.
Vanderbilt Co.), Varox being a mixture of 50 percent by weight of 2,5-di~ethyl-2,5-bis(t~butylperoxy)hexane and 50 percent by weight of inert diluent.

~2~7~95 60SI-417 The physical properties of the composition were evaluated by first press-curing the composition at 350~. for 10 minutes and evaluating the physical properties and then pos-t baking the composition for 4 hours at 400 F. and then evaluating the properties. rrhe properties are as follows:

PRESS-CURE PRESS-CURE
PHYSICALS ~10 min.~350 F. 4 hrs./400 F
_ .
Hardness, Durometer 59 69 Tensile Strength, psi lJ01 1096 % Elonyation 680 400 Die B, Tear, pi 297 248 Example 3 There was prepared a composition comprising -the same blend of the same vinyl-terminated gum and vinyl on-chain containing gums of Example 2. To this vinyl gum blend was added instead of a vinyl-containing resin 12.75 parts of a vinyl-containing oil which had a viscosity of 1410 centipoise at 25 C. This vinyl containing oil contained 2 mole percent of methylvinylsiloxy units and was trimethylsiloxy end-stopped. To this composition, there was added 3 parts of the same hydride as in Example 1, and 3 parts of the same catalyst system as Example 2.
In addition, this composition contained the same type and quantity of process aid and octamethylcyclotetrasiloxane treated silica. Thedifference in the composition of Example 2 and this Example was the presence of the vinyl oil in place of the vinyl resin and the concentration of the hydride resin. The composition was press-cured at 350 F. for 10 minutes and its properties evaluated. Then the composition was press-cured for 4 hours at 400F.
and the properties were evaluated. The results are set forth below:

~7~9S 60SI-417 PRESS-CURE PRESS-CURE
P~IYSICALS10 min./350 ~'.4 h;rs /400F.
Hardness, Durometer 68 72 Tensile Strength, psi 1064 976 5 ~ Elongation 400 L70 Die B, Tear, pi303 80 As noted previously, the composition prepared with the vinyl oil had low strength properties a~ter post-cure aging. This is undesirable in the present composition.
Example 4 A blend was prepared of 80 parts by weight of a vinyl-terminated dimethylpolysiloxane gum of 6.25 x 107 centipoise viscosity at 35F. and 20 parts by weight of 0.6 mole percent vinyl gum, which had a vinyl on-chain and was trimethylsiloxy end-stopped and a viscosity of 4.38 x 107 centipoise at 25 C. which were blended to a blend viscosity of 5.7 x 107 centipoise at 25 C. To this composition, there was added 2 parts by weight of the process aid of Example 1 and 40 parts by weight of an octamethylcyclotetrasiloxane treated fumed silica~
There was also added one part by weight of t.he same hydride compound of Example 1. There was present 0.015 parts by weight of a methylethylketone peroxide solution as an inhibitor. The composition was cured with .5 parts by weight of 2,5-dimethyl-2,5-bis(t-butylperoxy) hexane per 100 parts by weight of the rest of the composition. The physicals were evaluated by press-curing the composition for lO minutes at 350 F. The results are as follows:

~7~9S 60SI-417 PRESS-CURE PRESS-CURE
_YSICALS10 min./350 F~4 hrs~/400 F.
~Iardness, Vurometer 68 72 Tensile Strength, psi 106~ 976 % Elongation 400 170 Die B, Tear, pi303 30 As noted previously, the composition prepared with the vinyl oil had low strength properties after post-cure aging. This is undesirable in the present composition.
Example 4 A blend was prepared of 80 parts by weight of a vinyl-terminated dimethylpolysiloxane gum of 6.25 x 107 centipoise viscosity at 25F. and 20 parts by weight of 0.6 mole percent vinyl gum, which had a vinyl on-chain and was trimethylsiloxy end-stopped and a viscosity of

4.38 x 107 centipoise at 25C. which were blended to a blend viscosity of 5.7 x 107 centipoise at 25 C. To this composition, there was added 2 parts by weight of the process aid of Example 1 and 40 parts by weight of an octamethylcyclotetrasiloxane treated fumed silica. There was also added one part by weight of the same hydride compound of Example 1. There was present 0.015 parts by weight of a methylethylketone peroxide solution as an inhibitor. The composition was cured with .5 parts by weight of 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane per 100 parts by weight of the rest of the composition.
The physicals were evaluated by press-curin~ the composition for 10 minutes at 350 F. The results are as follows:

PHYSICALS ~ ~ `10 min./350 F.
Hardness, Durometer 50 Tensile Strength, psi 1297 ~ Elongation 660 35 Die B Tear, pi 149 Example 5 , _ I'here was prepared a composition which ~as the same as Example 4, but a different batch. The composition was press-cured for 10 minutes at 350C. and the physicals evaluated. Then the composition was post-cured by oven baking it ~or 4 hours at 400F. The results are as follows:
PRESS-CURE PRESS-CUR~
PHYSICALS10 min./350 F. 4 hrs./400 F.
Hardness, Durometer51 53 Tensile Strength, psi 1308 1353 % Elongation 660 560 Die B Tear t pi 231 144 Example 6 _ There was prepared a vinyl gum blend comprising 80 parts by weight of a vinyl-terminated dimethylpolysiloxane gum having a viscosity of 2.19 x 107 centipoise at 25C. and 20 parts by weight of a .6 mole percent vinyl on-chain gum which was trimethylsiloxy end-stopped having a viscosity of 4.38 x 107 centipoise at 25 C. To this there was added four parts by weight of the same silanol stopped methyl process aid of Example 1~ 64 parts by weight of octamethylcyclotetrasiloxane treated fumed silica, one part by weight of the same hydride crosslinking agent of Example 1 and .015 parts by weight of methylethylketone peroxide as an inhibitor. This mixture was catalyzed with .5 parts by weight of 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane per 100 parts by weight of the rest of the composition~
The resulting composition was cured by press-curing it for 10 minutes at 350F. and then post-cured by oven-baking it for 4 hours at 400F. The results are asfollows~

~789~ 60SI-417 PRESS-CURE PRESS-C~RE
PHYSICALS10 min~/350 F.~ hrs /400 F.
Hardness, Durometer 70 75 Tensile Strength, psi 1176 ]184

5 % Elongation 410 330 Die B Tear, pi 177 177 Example 7 -There was prepared a composition which was the same as in Example 1 except there was utilized .5 parts by weight o~ the same hydride oil as Examples 1 instead of 2 parts and the composition was catalyzed with .28 parts by weight of dicumyl peroxide per 100 parts of the rest of the composition in place of the catalyst shown in Example 1.
The physicals were cbtained after the composition had been press-cured for 10 minutes at 350F~ The results are as follows:
PRESS-CURE
PHYSICALS 10 min./350 F.
Hardness, Durometer 43 20 Tensile Strength, psi 1103 % Elongation 700 Die B, Tear, pi 78 Example 8 There was prepared a composition that was exactly the same as Example 7 except there was utilized 2 parts by weight of the same hydride compound o~ Example 7 instead of the .5 parts by weight concentration. The composition was press cured for 10 minutes at 350F. and the physicals were evaluated which are as follows:
PRESS-CURE
PHYSICALS ~ 10 min./350 F.
_, Hardness, Durometer 39 Tensile Strengthf psi 1101 % Elongation 820 35 Die B, Tear~ pi 120 .LZ~L7895 Examp~le 9 There was prepared a composition which was -the same as that of Example 7 except -there was used ~ parts by weight of the same hydride compound as in Example 7. ~he physicals were evaluated after the composition was p~ess-cured for 10 minutes at 350F. The results are as ollows:
PRESS-CURE
PHYSICALS - ~_ 10 min.j350 F.
Hardness, Durometer 39 10 Tensile Strength, psi 1155 ~ Elongation 870 Die B Tear, pi 199 As the examples show and as discussed in the specification, in each case there must be balanced the particular vinyl gum blend, the hydride level and the peroxide level to get a system with optimum tear physical properties. However, it should be noted that even in that casel the tear will vary depending on the type of peroxide catalyst utilized. The type of peroxide catalyst used is very important in obtaining a cured composition with a high percent tear strength.
As the experimental results indicate, to obtain a composition with a tear strength above 100 pi, it is necessary -to use the proper peroxide catalyst along with the proper concentrations o~ the vinyl gum blend, the hydride and peroxide. Also, particular vinyl gum blends and hydrides as well as peroxides will result in better tear strengths than are obtainable by the use of other compounds in the composition.

Claims (66)

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

1. A polysiloxane composition curable to a silicone elastomer comprising:
(A) 60-95 parts by weight of a vinyl-terminated linear diorganopolysiloxane gum having a viscosity in the range of 1 X 106 to 20 X 107 centipoise at 25°C and having a vinyl concentration in the range of 0.005 to 0.1 mole percent vinyl and where the organo groups are monovalent hydrocarbon radicals;
(B) 5.0 to 40.0 parts by weight of a vinyl-containing diorganopolysiloxane gum having a viscosity in the range of 20 X 106 to 100 X 106 centipoise at 25°C and having a vinyl concentration in the range of 0.5 to 15.0 mole percent, wherein the organo groups are monovalent hydrocarbon radicals and the sum of (A) and (B) equals 100 parts by weight;
(C) 10 to 300 parts by weight of filler, at least part of which is a reinforcing silica filler;
(D) 0.1 to 25 parts by weight of hydride-containing polysiloxane having a hydride content ranging from 0.05 to 5.0 percent by weight and having a viscosity ranging from 10 to 1000 centipoise at 25°C; and (E) an effective amount of an organic peroxide free radical initiator curing agent.

2. The composition of claim 1 wherein the vinyl-terminated gum has the formula, where Vi is vinyl, R is a C1-8 monovalent hydrocarbon radical free of aliphatic unsaturation, R1 is a C1-8 monovalent hydrocarbon radical, x and t vary such that the viscosity of the gum ranges from 1 X 106 to 20 X 107 centipoise at 25°C and the vinyl concentration of the gum ranges from 0.005 to 0.1 mole percent vinyl.

3. The composition of claim 2 wherein the radical R1 is free of aliphatic unsaturation.

4. The composition of claim 2 wherein the vinyl-terminated gum has a vinyl content varying from 0.01 to 0.08 mole percent.

5. The composition of claim 1 wherein the vinyl-containing gum has the formula, where R2 and R4 are C1-8 monovalent hydrocarbon radicals, R3 is a C1-8 monovalent hydrocarbon radical free of aliphatic unsaturation, w and z vary such that the viscosity of the sum ranges from 20 X 106 to 100 X 106 centipoise at 25°C and wherein the vinyl concentration of the gum varies from 0.5 to 15.0 mole percent.

6. The composition of claim 5 wherein R2 is a C1-8 monovalent hydrocarbon radical free of aliphatic unsaturation.

7. The composition of claim 1 wherein there is present from 10 to 200 parts by weight of a reinforcing filler selected from the group consisting of fumed silica and precipitated silica.

8. The composition of claim 7 wherein an extending filler is utilized in addition to the reinforcing filler.

9. The composition of claim 1 wherein the reinforcing filler is treated with treating agents selected from the class consisting of cyclopolysiloxanes and silazanes.

10. The composition of claim 1 wherein the curing agent is selected from organic peroxides.

11. The composition of claim 1 wherein the curing agent is selected from the group consisting of 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, di-t-butylperoxide and 5-butylcumylperoxide.

12. The composition of claim 1 further comprising from 1 to 500 parts per million of a platinum catalyst cure accelerator.

13. The composition of claim 1 wherein the hydride-containing polysiloxane is a linear polysiloxane of the formula where R5 is a C1-8 monovalent hydrocarbon radical free of aliphatic unsaturation, p and q vary such that the viscosity of the hydride polysiloxane ranges from 10 to 1000 centipoise at 25°C and the hydride content varies from 0.05 to 2.0 percent by weight.

14. The composition of claim 1 where the hydride-containing polysiloxane is a hydride resin having and SiO2 units where R6 is a C1-8 monovalent hydrocarbon radical free of aliphatic unsaturation and the R6 + H to Si ratio varies from 1.0 to 2.7 wherein the hydride content of the resin varies from 0.05 to 2.0 weight percent.

15. The composition of claim 1 wherein the hydride-containing polysiloxane is a hydride resin having and SiO2 units and (R6)2SiO units, where the R6 + H to Si ratio varies from 1.2 to 2.7, and R is a C1-8 monovalent hydrocarbon radical free of aliphatic unsaturation.

16. The composition of claim 1 wherein the curing agent is present at a concentration of at least 0.1 parts by weight for 100 parts by weight of the rest of the composition.

17. The composition of claim 1 further comprising from 1 to 50 parts by weight of an organopolysiloxane resin copolymer comprising R38 - SiO0.5 units and SiO2 units where R8 is selected from the group consisting of vinyl radicals and monovalent hydrocarbon radicals free of aliphatic unsaturation with a ratio of R38 - SiO0.5 units to SiO2 units ranging from about 0.5 to 1 to 1 to 1, and where said copolymer contains about 2.5 to 10 mole percent vinyl groups.

18. The composition of claim 1 further comprising from 1 to 50 parts by weight of an organopoly-siloxane resin copolymer comprising R38SiO0.5 units and SiO2 units and R2 SiO units where R8 is selected from the group consisting of vinyl radicals and monovalent hydro-carbon radicals free of aliphatic unsaturation, where the ratio of R3 -SiO0.5 units to SiO2 units is from about 0.5 to 1 to 1 to 1, and the R2 -SiO units are present in an amount of from about 1 to 10 mole percent based on the total number of moles of siloxy units in the copolymer, and where the resinous copolymer contains from about 2.5 to 10.0 mole percent vinyl groups.

19. The composition of claim 1 further comprising from 1 to 25 parts by weight of a process aid.

20. The composition of claim 19 wherein the process aid is a dihydrocarbon substituted polysiloxane oil having a hydrocarbon substituent silicon atom ratio from 1.0 to 2.0 and where such hydrocarbon substituents comprise at least one member selected from the class consisting of methyl, vinyl, ethyl, allyl, cyclohexenyl and phenyl groups, said polysiloxane oil comprising polysiloxane molecules containing an average of from 1 to 2 lower alkoxy groups bonded to each of the terminal silicon atoms.

21. The composition of claim 10 wherein the curing agent is 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3.

22. The composition of claim 19 wherein the process aid is a hydroxylated organosiloxane, a hydroxylated silane, or mixture thereof, said process aid containing on average from 1.9 to 2.1 hydrocarbon radicals per silicon atom and from 1 silicon-bonded OH per 70 silicon atoms to 2 silicon-bonded OH per silicon atom.

23. The composition of claim 16 wherein the curing agent is present in a concentration of from 0.1 to 2 parts by weight per 100 parts of the rest of the composition.

24. A process for making a polysiloxane composition that is curable to a silicone elastomer com-prising mixing:
(A) 60-95 parts by weight of a vinyl-terminated linear diorganopolysiloxane gum having a viscosity in the range of 1 X 106 to 20 X 107 centipoise at 25°C and having a vinyl concentration in the range of 0.005 to 0.1 mole percent vinyl and where the organo groups are monovalent hydrocarbon radicals;
(B) 5.0 to 40.0 parts by weight of a vinyl-containing diorganopolysiloxane gum having a viscosity in the range of 20 X 106 to 100 X 106 centipoise at 25°C and having a vinyl concentration in the range of 0.5 to 15.0 mole percent, wherein the organo groups are monovalent hydrocarbon radicals and the sum of (A) and (B) equals 100 parts by weight;
(C) 10 to 300 parts by weight of filler, at least part of which is a reinforcing silica filler;
(D) 0.1 to 25 parts by weight of a hydride containing polysiloxane having a hydride content ranging from 0.05 to 5.0 percent by weight and having a viscosity ranging from 10 to 1000 centipoise at 25°C; and (E) an effective amount of an organic peroxide free radical initiator curing agent.

25. A cured silicone elastomeric article formed by curing a mixture comprising:
(A) 60 to 95 parts by weight of a vinyl-terminated linear diorganopolysiloxane gum having a viscosity in the range of 1 X 106 to 20 X 107 centipoise at 25°C
and having a vinyl concentration in the range of 0.005 to 0.1 mole percent vinyl and where the organo groups are monovalent hydrocarbon radicals;
(B) 5.0 to 40.0 parts by weight of a vinyl-containing diorganopolysiloxane gum having a viscosity in the range of 20 X 106 to 100 X 106 centipoise at 25°C and having a vinyl concentration in the range of 0.5 to 15.0 mole percent, wherein the organo groups are monovalent hydrocarbon radicals and the sum of (A) and (B) equals 100 parts by weight;
(C) 10 to 300 parts by weight of filler, at least part of which is a reinforcing silica filler;
(D) 0.1 to 25 parts by weight of hydride containing polysiloxane having a hydride content ranging from 0.05 to 5.0 percent by weight and having a viscosity ranging from 10 to 1000 centipoise at 25°C; and (E) an effective amount of an organic peroxide free radical initiator curing agent.

26. The article of claim 25 wherein the vinyl-terminated gum has the formula, where Vi is vinyl, R is a C1-8 monovalent hydrocarbon radical free of aliphatic unsaturation, R is a C1-8 monovalent hydrocarbon radical, x and t vary such that the viscosity of the gum ranges from 1 X 106 to 20 X 107 centipoise at 25°C and the vinyl concentration of the gum ranges from 0.005 to 0.1 mole percent vinyl.

27. The article of claim 26 wherein the radical R is free of aliphatic unsaturation.

28. The article of claim 26 wherein the vinyl-terminated gum has a vinyl content varying from 0.01 to 0.08 mole percent.

29. The article of claim 25 wherein the vinyl-containing gum has the formula, where R2 and R4 are C1-8 monovalent hydrocarbon radicals, R3 is a C1-8 monovalent hydrocarbon radical free of aliphatic unsaturation, w and z vary such that the viscosity of the gum ranges from 20 X 106 to 100 X 106 centipoise at 25°C and wherein the vinyl concentration of the gum varies from 0.5 to 15.0 mole percent.

30. The article of claim 29 wherein R2 is a C1-8 monovalent hydrocarbon radical free of aliphatic unsaturation.

31. The article of claim 25 wherein there is present from 10 to 200 parts by weight of a reinforcing filler selected from the group consisting of fumed silica and precipitated silica.

32. The article of claim 31 wherein an extending filler is utilized in addition to the reinforcing filler.

33. The article of claim 25 wherein the reinforcing filler is treated with treating agents selected from the class consisting of cyclopolysiloxanes and silazanes.

34. The article of claim 25 wherein the curing agent is selected from organic peroxides.

35. The article of claim 34 wherein the curing agent is selected from the group consisting of 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, di-t-butylperoxide and 5-butylcumylperoxide.

36. The article of claim 25 further comprising from 1 to 500 parts per million of a platinum catalyst cure accelerator.

37. The article of claim 25 wherein the hydride-containing polysiloxane is a linear polysiloxane of the formula, where R5 is a C1-8 monovalent hydrocarbon radical free of aliphatic unsaturation, p and q vary such that the viscosity of the hydride polysiloxane ranges from 10 to 1000 centipoise at 25°C and the hydride content varies from 0.05 to 2.0 percent by weight.

38. The article of claim 25 where the hydride containing polysiloxane is a hydride resin having and SiO2 units where R6 is a C1-8 monovalent hydrocarbon radical free of aliphatic unsaturation and the R6 + H
to Si ratio varies from 1.0 to 2.7 wherein the hydride content of the resin varies from 0.05 to 2.0 weight percent.

39. The article of claim 25 wherein the hydride-containing polysiloxane is a hydride resin having and SiO2 units and (R6)2SiO units where the R6 + H to Si ratio varies from 1.2 to 2.7 and R6 is a C1-8 monovalent hydrocarbon radical free of aliphatic unsaturation.

40. The article of claim 25 wherein the curing agent is present at a concentration of at least 0.1 parts by weight per 100 parts by weight of the rest of the composition.

41. The article of claim 25 further comprising from 1 to 50 parts by weight of an organopolysiloxane resin copolymer comprising R38 - SiO0.5 units and SiO2 units where R8 is selected from the group consisting of vinyl radicals and monovalent hydrocarbon radicals free of aliphatic unsaturation with a ratio of R38SiO0.5 units to SiO2 units ranging from about 0.5 to 1 to 1 to 1, and where said copolymer contains about 2.5 to 10 mole percent vinyl groups.

42. The article of claim 25 further comprising from 1 to 50 parts by weight of an organopolysiloxane resin copolymer comprising R38 - SiO0.5 units and SiO2 units and R28 - SiO units where R8 is selected from the group consisting of vinyl radicals and monovalent hydrocarbon radicals free of aliphatic unsaturation, where the ratio of R38- SiO0.5 units to SiO2 units is from about 0.5 to 1 to 1 to 1, and the R28 - SiO units are present in an amount of from about 1 to 10 mole percent based on the total number of moles of siloxy units in the copolymer, and where the resinous copolymer contains from about 2.5 to 10.0 mole percent vinyl groups.

43. The article of claim 25 further comprising from 1 to 25 parts by weight of a process aid.

44. The article of claim 43 wherein the process aid is a dihydrocarbon substituted polysiloxane oil having a hydrocarbon substituent silicon atom ratio from 1.0 to 2.0 and where such hydrocarbon substituents comprise at least one member selected from the class consisting of methyl, vinyl, ethyl, allyl, cyclohexenyl and phenyl groups, said polysiloxane oil comprising polysiloxane molecules containing an average from 1 to 2 lower alkoxy groups bonded to each of the terminal silicon atoms.

45. The article of claim 34 wherein the curing agent is 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3.

46. The article of claim 43 wherein the process aid is a hydroxylated organosiloxane, a hydroxylated silane or mixtures thereof, said process aid containing an average from 1.9 to 2.1 hydrocarbon radicals per silicon atom and from 1 silicon-bonded OH per 70 silicon atoms to 2 silicon-bonded OH per silicon atom.

47. The article of claim 25 wherein the curing agent is present in a concentration of from 0.1 to 2 parts by weight per 100 parts of the rest of the composition.

48. The process of claim 24 wherein the curing agent is selected from organic peroxides.

49. The process of claim 48 wherein the curing agent is selected from the group consisting of 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, di-t-butylperoxide and 5-butylcumylperoxide.

50. The process of claim 24 further comprising mixing from 1 to 500 parts per million of a platinum catalyst curing accelerator into the mixture.

51. The process of claim 24 wherein the curing agent is present at a concentration of at least 0.1 parts by weight per 100 parts by weight composition.

52. The process of claim 24 further comprising mixing from 1 to 50 parts by weight of an organopoly-siloxane resin copolymer comprising R3 SiO0.5 units and SiO2 units, where R8 is selected from the group consisting of vinyl radicals and monovalent hydrocarbon radicals free of aliphatic unsaturation and having a ratio of R38SiO0.5 units to SiO2 units ranging from about 0.5 to 1 to 1 to 1, and where said copolymer contains about 2.5 to 10 mole percent vinyl groups.

53. The process of claim 24 further comprising mixing from 1 to 50 parts by weight of an organopoly-siloxane resin copolymer comprising R38SiO0.5 units, SiO2 units and R28SiO units, where R8 is selected from the group consisting of vinyl radicals and monovalent hydrocarbon radicals free of aliphatic unsaturation, where the ratio of R38SiO0.5 units to SiO2 units is from about 0.5 to 1 to 1 to 1 and the R28SiO units are present in an amount of from about 1 to 10 mole percent based on the total number of moles of siloxy units in the copolymer, and where the resinous copolymer contains from about 2.5 to 10.0 mole percent vinyl groups.

54. The process of claim 24 further comprising mixing from 1 to 25 parts of a process aid into the mixture.

55. The process of claim 24 wherein the curing agent is present in a concentration of from about 0.1 to 2 parts by weight per 100 parts by weight composition.

56. A method of making an elastomeric silicone article comprising:
(i) mixing:
(A) 60-95 parts by weight of a vinyl-terminated linear diorganopolysiloxane gum having a viscosity in the range of 1 X 106 to 20 X 107 centipoise at 25°C and having a vinyl concentration in the range of 0.005 to 0.1 mole percent, and where the organo groups are monovalent hydro-carbon radicals;
(B) 5.0 to 40.0 parts by weight of a vinyl-containing diorganopolysiloxane gum having a viscosity in the range of 20 X 106 to 100 X 106 centipoise at 25°C and having a vinyl content in the range of 0.5 to 15.0 mole percent, wherein the organo groups are monovalent hydrocarbon radicals and the sum of (A) and (B) equals 100 parts by weight;
(C) 10 to 300 parts by weight of a filler, at least part of which is a reinforcing silica filler;
(D) 0.1 to 25 parts by weight of a hydride-containing polysiloxane having a hydride content ranging from 0.05 to 5.0 percent by weight and having an organic peroxide viscosity ranging from 10 to 1000 centipoise at 25°C; and (E) an effective amount of a free radical initiator curing agent;
(ii) forming the mixture of step (i) into the desired shape; and (iii) heating the shaped composition at an elevated temperature in order to effect curing of the composition.

57. The method of claim 56 wherein the curing agent is selected from the organic peroxides.

58. The method of claim 57 wherein the curing agent is selected from the group consisting of 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, di-t-butylperoxide and 5-butylcumylperoxide.

59. The method of claim 56 further comprisiny mixing 1 to 500 parts per million of a platinum catalyst curing accelerator into the mixture of step (i).

60. The method of claim 56 wherein the curing agent is present in a concentration of at least 0.1 parts by weight per 100 parts by weight composition.

61. The method of claim 56 further comprising mixing in step (i) from 1 to 50 parts by weight of an organopolysiloxane resin copolymer comprising R38SiO0.5 units and SiO2 units, where R8 is selected from the group consisting of vinyl radicals and monovalent hydrocarbon radicals free of aliphatic unsaturation and having a ratio of R38SiO0.5 units to SiO2 units ranging from about 0.5 to 1 to 1 to 1, and where said copolymer contains about 2.5 to 10 mole percent vinyl groups,

62. The method of claim 56 further comprising mixing in step (i) from 1 to 50 parts by weight of an organopolysiloxane resin copolymer comprising R38SiO0.5 units, SiO2 units and R2 SiO units, where R8 is selected from the group consisting of vinyl radicals and monovalent hydrocarbon radicals free of aliphatic unsaturation, where the ratio of R38SiO0.5 units to SiO2 units is from about 0.5 to 1 to 1 to 1 and the R2 SiO units are present in an amount of from about 1 to 10 mole percent based on the total number of siloxy units in the copolymer, and where the reinous copolymer contains from about 2.5 to 10.0 mole percent vinyl groups.

63. The method of claim 56 further comprising mixing in step (i) from 1 to 25 parts of a process aid.

64. The method of claim 56 wherein the curing agent is present in a concentration of from 0.1 to 2 parts by weight per 100 parts by weight composition.

65. The method of claim 56 wherein curing is effected at a temperature greater than 100°C.

66. The method of claim 56 further comprising the step of post curing the article.