CA2089290A1 - Heat curable silicone rubber compositions having simplified formulations - Google Patents

Abstract

PATENTS

ABSTRACT OF THE DISCLOSURE
In one embodiment, a heat curable silicone rubber composition consists essentially of a vinyl-stopped diorganopolysiloxane gum having a viscosity of about 3,000,000 to about 100,000,000 centipoise at 25°C, a silica filler and a process aid. In another embodiment, the composition includes a silazane coupling agent. In another embodiment, the composition includes a cross-linking agent.

Description

~0~9~9a PATENTS

HE~T CURABLE SILICONE RUBBER COMPOSITIONS HA~ING
SIMPLIFIED FORMULATIONS

BACKGROUND OF THE INVENTION
The present invention relate~ to heat curable silicone rubber compositions. More particularly, the i~vention relates to heat curabl~ silicone rubber compositions prepared from relativsly simplP
formulation~ having improved tear and ten~ile ~trength, oil re~istanc~ and co~pres~ion 8et propertie~c ~ eat curable ~ilicone rubber compo~itions are known in the art. Such cDmposition~ are known for their ability to re~i~t change at elevated temperature and expo3ure to advexs~ weather conditions over extend~d periods of time. Effort~ have been made in the pa~t to produ~e heat ~urable silicone elastomer .

composition3 having increa3ed tear and tensile strength, good oil resi~ance and reduced compre~sion set properties. However~ a problem ha~ been the compl~xity of the compositions re~ulting from such 5 effortsO In particular, numerous ~omponents including expensive resin composition~ have been required to produce compo~ition~ which have satisfactory propertie~. Formulations with mzny component~ increase inventory C08t~ and complic~te ~cheduling and increase the probability of pro~e~ upsets.
U.S. Patent No. 4,539,357 (Bobaar '357) disclose~
an exemplary heat-curable cilicone composition comprising a vinyl-termirlated linear diorganopolysiloxane gum having a viscosity in the lS range of lx106 to 20x107 eQntipoi~ 25C, a vinyl-containing diorganopoly~iloxane gum having a viscosity Yarying from lxlO~ to 20x107 centipoi~ at 25C, a fill~rr a hydrid~-containing polysiloxane, and a~
organic peroxide or organic hydropero~ide curing agent.
2~ ~he compo~ition may further compri~e a vinyl-stopped ,. , . ~

-organopolysiloxane resin copolymer. It is stated in Bobear '357 at column 6, lines 22-29 that there should be no vinyl-containing fluid in the composition since it has been found that compositions containing vinyl-containing fluids of a viscosity of 500,000 centipoiseor 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. After post-cure, the Bobear '357 composition ha~ a tear strength of above 200 pi.
U.S. Patent No. 3,652,475 to Wada et al. (Wada '475) discloses heat curable elastomeric silicona compositions reportedly having high tear strength, excellent compression set and re~iliency, comprisiny two vinyl-unsaturated polydiorganopolysiloxanes each having a degree of polymerization of at least 3000; a vinyl-unsaturated polydiorganopolysiloxane having a degree of polymerization of from 10 to 1000; a silica filler; and an organic peroxide catalyst. The samples prepared in the Wada '475 examples had tear strength values of about 40 kg/cm, i.e., about 225 pi.
U.S. Patent No. 3 t 671,480 to Wada et al. (Wada '480) discloses a heat curable elastomeric silicone composition having improved tear strength and superior compression set, comprising a mixture of two vinyl-unsaturated polydiorganopolysiloxanes, one of which appears to have a high molecular weight and the other of which appear~ to have a low molecular weight; a polyorganohydrogensiloxane; silica filler; and a platinum compound. The examples in Wada '480 report tear strength values of about 50 kg/cm, i.e., about 280 pi. Compression set values are not given.

-_4_ PATENTS

U.S. Patent No. 4,061,609 to Bobear (Bobear '609) discusses a platinum catalyzed silicone rubber composition comprising a vinyl-stopped polysiloxane or a blend of such polysiloxanes; platinum; and a hydrogen-containing polysiloxane. Preferably, the vinyl-stopped polysiloxane has a viscosity in the range of 1000 to 300,000,000. A low viscosity vinyl-stopped polysiloxane may be added to the basic composition as a reinforcing agent to give the final composition good physical strength. Claim 4 in Bobear '609 recites a first polysiloxane having a viscosity of 1,000,000 to 200,000,000 centipoise at 25C and a second polysiloxane having a viscosity of 50,000 to 500,000 centipoise at 2SC. The Bobear patent is directed to improving the work life of silicone rubber compositions, rather than tear strength. In the examples, tear strength values of greater than 200 pi were obtained, but the compositions having these values contained two high viscoæity vinylpolysiloxanes rather than one high viscosity vinylpolysiloxane and one low vi~cosity vinylpolysiloxane.
U.S. Patent No. 3,660,345 to Bobear ~Bobear '345) discloses oganopolysiloxane compositions which are convertible to elastomers having high tear strength and resiliency, comprising a two component blend of vinyl-stopped organopolysiloxanes silica filler, and a process aid which can be, for example, a methoxy-terminated polysiloxane, a silanol-terminated polydimethylsilox~ne, or hexamethyldisilazane. A
peroxide is used as the catalyst. Th~ viscosi~y of one of the two vinyl-stopped organopolysiloxanes can be in the range of 1 to 1 billion centipoise at 25C, while the viscosity of the other can be between 100,000 to .

-5- PAT~NTS

2,000,000,000 centipoise at 25C. The examples in Bobear '345 report tear strength values of greater than 200 pi.
U.S. Patent No. 3,696,068 to Creamer discloses S heat-curable silicone gumstock compositions having high "B" tear strength as well as low compression set values, comprisingavinyl-stopped diorganopolysiloxane gum and a linear vinyl-stopped fluid having a viscosity of from 10 to 150,000 centiskokes at 25C. The cross-linkable vinyl siloxane gum has a viscos~ty in excessof 200,000 centistokes at 25C. The Creamer composition is cured by a peroxide catalyst. Some of the samples prepared in Creamer had tear strength values in excess of 150 pi. Tear strength after post cure was not measured in any of the examples, which is significant since, as pointed out previously herein, it has been found that, in compositions containing low viscosity vinyl stopped fluids, tear strength drops drama~ically after post cure.
U.S. Patent No. 3,884,866 to Jeram et al.
discloses a high strength organopolysiloxane composition suited for low pressure injection molding, comprising (A) a first component containing (i) a vinyl-stopped high viscosity organopolysiloxane having a viscosity of 5000 to 1,000,000 centipoise at 25C;
and (ii) a low vi8c05ity of 50 to 5000 centipoise at 25C; (iii) a filler; and (iv) a platinum catalyst; and (B) a hydrogen ~ilicon composition. The highest tear strength reported in the examples was 250 pi.
Although heat curable silicone rubber compositions having good tear strength, reasonably good tensile strength and low compression set propertie~ are known in the ark, it is desirable to provide heat curable silicone rubber compositions having good properties in a simplified formulation, and it therefore is an object of the invention to provide one or more simplified formulations while retaining such desirable properties.
SUNMARY_OF THE INV~NTION
The present invention i3 based upon the discovery that a simplified combination of a high viscosity vinyl-stopped diorganopolysiloxane gum and a silica fill~r combined in a presence of a silanol or alkoxy stopped process aid may be used to produce a heat curable silicone rubber. The process aid is preferably silanol stopped.
In one embodiment of the invention, the composition consists essentially of a vinyl-stopped diorganopolysiloxane gum having a viscosity of about 3,000,000 to about 100,000,000 centipoise at 25C, a silica filler and a process aid. In another embodiment of the invention a heat curable silicone rubber composition con ists essentially of the vinyl-stopped diorganopolysiloxane gum described above, the silica filler, the process aid, and a silazane coupling agent.
In another embodiment of the invention, the composition consists essentially of the vinyl-s~opped diorganopolysiloxane described above, the silica filler, the process aid, and a cross-linking agent. In yet another embodiment of the invention a heat curable ~ilicone rubber composition consists essentially of the vinyl-~topped diorganopolysiloxane gum described above, the silica filler, the process aid, the cross-linking agent and the silazane coupling agent.
In a particular embodiment, a heat curable ~ilicone rubber composition having increased tear and tensile strength, good oil resistance, reduced -7_ PATENTS

compression set consists essentially of by weight, tA) 100 parts of a vinyl-stopped organopolysiloxane or blend wherein the vinyl-stopped organopolysiloxane has a viscosity of about 3,000,000 to about 100,000,000 centipoise at 25C; (B) up to about 200 parts by weight, based upon Component (A), of a filler; and (C) up to about 10 parts by weight, based upon Component (A), of a silanol- or alkoxy-process aid.
In another embodiment of the invention, a heat curable silicone rubber composition consists essentially of Components (A), (B) and (C) as defined above and (D) up to about .5 parts by weight, based upon Component (A)~ of a sila~ane coupling agent.
In another embodiment of the invention, a heat curable silicone rubber composition consists essentially of Components (A), (B) and (C) as defined of above and ~E) up to about 10 parts by weight, based upon Component (A), of an organohydrogenpolysiloxane cross-linking agent.
In yet another embodiment of the invention a heat curable silicone rubber composition consists essentially of Components (A), (B), (C), (D) and (E) as herein before defined.
DESC~IPTION OF THE INYENTION-The present inven~ion is directed to a heat curable silicone composition having improved tear and tensile strength properties, good oil resistance and improved compression set having a simplified formulation. In one embodiment the invention consists essentially of Componen~ (A) a vinyl-stopped diorganopolysiloxane gum or blends thereof, ha~ing a viscosity of about 3,000,000 to about 100,000,000 CPS
at 25C; (B) up to about 200 parts by weigh~, based -8- PA~ENTS
6~SI-14~3 upon Component (A) of a silica filler; and (C) up to about 10 parts by weight, based upon Component (A), of a silanol process aid. In another embodiment the invention consists essentially of only Components (A), (B) and (C).
In another embodiment, the heat curable silicone rubber composition consists es~entially of Components (A), (B) and (C) as defined above and in the proportions set forth; and (D) up to about .5 weight percent, based upon Component (A), of 2 silazane coupling agent. In another embodiment the heat curable silicone rubber composition of the present invention consists essentially of only (A), (B), (C) and (D) as defined and in the proportions set forth.
In another embodiment of the invention the heat curable silicone rubber composition consists essentially of Components ~A), (B) and (C) as defined above and the proportions set forth and (E) up to about 10 parts by weight, based upon Component (A), of an organohydrogenpolysiloxane cross-linking agent. In another embodiment of the invention the heat curable silicone rubber composition consists essentially of only Components (A), (B), (C), and (E) as defined aboYe and in the proportions noted.
In another embodiment o the invention, the hea~
curable silicone rubber composition consists essentially of Components (A), (B), (C), (D) and (E) as defined above and in the proportions noted. In another embodiment of the invention a heat curable silicone rubber composition consists essentially of only Components (A~, (B), (C), (D) and (E) as defined above and in the proportions noted.

2 ~

-Component (A) is a vinyl-stopped diorganopolysiloxane or vinyl-stopped diorganosiloxane blend wherein the vinyl-stopped diorganosiloxane or blend has a viscosity of about 3,000,000 to about 100,000,000, preferably from ~bout 20,000,000 to about 3~,000,000 and more preferably from about 25,000,000 to about 30,000,000 centipoise at 25~C. Component (A) has a vinyl concentration in the range of about 5x10-5 to about 2x10-4, preferably from about 8xlO-s to about 1.5x10-4 and more preferably from about 8x10-5 to about 1.2x10-4, weight percent. The organo groups in the vinyl polymer or polymers of Component ~A) are monovalen~ hydrocarbon radicals.
The vinyl polymer or polymers of Component tA) contain no significant vinyl substituent units in the chain but rather contain only vinyl end-groups.
Preferably the vinyl-stopped polymer has the formula:

ViSiORl2(SiORl2)X ~SiOR22)ySiRl2Vi wherein Vi is vinyl each Rl is independently chosen from a monovalent hydrocarbon, free of aliphatic unsaturation, containing 1 to about 8 carbon atoms;
each R2 is independently chosen from monovalent hydrocarbon radicals, also free of aliphatic unsaturation, containing 1 to about 8 carbon atoms, and and y are integers chosen such that Component (A) has a vi~c06ity which ranges from about 3,000,000 to about 100,000,000 centipoise at 25C and a weight percent of vinyl concentration as noted above. The vinyl chain-stopped polymer is preferred because it appears to rzsult in higher Tear B, Tensile Strength and Elongation.

2 ~ 9 ~

The vinyl-stopped polymers of Component (A) can be made by a process well known in the art, for example by reacting cyclotetrasiloxanes in the presence of low molecular weight linear vinyl chain-stoppers at high temperatures in the presence of basic catalysts so as to yield the polymer of the desired molecular weight.
When the reaction is over, the catalyst i5 neutralized and the excess cyclics are vented off to result in the desired polymer. By controlling the amount of chain-stopper and the temperature of reaction, the molecularweight of the desired vinyl-stopped polymer end product can be controlled. For more details of the process ~y which such vinyl-stopped polymers are produced, reference is made, for example, to U.S. Patent ~o.
3,660,345, which is incorporated herein by reference.
In accordanse with the invention, the amount of Component (A) present in the final composition may vary. However, for purposes of explanation herein, it i8 assumed that 100 parts by weight of the gum i~
combined with varying amounts of the other components.
In some examples, the formulation has 80 parts of Component (A). In each case, the amount of the other components in the final product may be calculated or otherwise readily inferred. In the examples below, (A) may be (A1) a ViMD~MVi gum with a~out 9000 D units or (A2) a similar gum having approximately the same number of D units and up to about 25 ppm RSiO3t2 or T units.
Component (B) comprises from about 5 to about 200, preferably from 10 to about 100, and more preferably from about 30 to about 75 parts by weight, based upon Component ~A), of a reinforcing filler. Such a reinforcing filler is needed in the compo~ition if it is to have high tensile and tear strength. Examples of a PATENTS
60SI-1~83 reinforcing fillers that can be used include, for instance, fumed silica and precipitated silica, with fumed silica being preferred. The filler may be treated with various agents so as to prevent the cured composition from structuring. Exemplary treatments include cyclopolysiloxanes as disclosed in U.S. Patent No. 2,938,009 to Lucas or silazanes such as disclosed in U.S. Patent No. 3,635,743 to Smith or both, the teachings of which are incorporated herein by reference. The cyclopolysiloxane present may be, for example, a cyclotetramethyl~iloxane present in an amount of about 15 to 20 weight percent of the filler.
The preferred fumed silica filler may have a surface area of about 100 m2tgm to about 300 m2t~m and preferably about 160 m2~gm to about 240 m2/gm. The higher surface area fillers tend to re~ult in more clear, less hazy composition~. However, they are more expensive than the lower surface area fillers and require surface treatments or more processing aid to incorporate them into the gum. In the examples below, Component (B) is a fumed silica filler treated as in Lucas above having a surface area in the untreated state of 240 m2/gm or 160 mZ/gm. A further surface treatment with a linear vinyl containing silazane may be provided in-situ.
Extending filler~ can be used in the combination with the reinforcing fillers, treated or untreated, to obtain proper balance in the final phy~ical properties.
These extending filler~ include, for example, titanium dioxide, lithopone, zinc oxide, zirconium silicate, silica aerogel, iron oxide, diatomaceous earth, calcium carbonate, glass fibers, magnesium oxide, chromic oxide, zirconium oxide, aluminum oxide, alpha quartz, 2 ~ ~

calcined clay, carbon, graphite, cotton, synthetic fibers, aluminum silicate, and calcium silicate. It should be understood, however, then when clear, heat curable compositions are desired, fillers which tend to produce opaque end products are not desirable and hence are not incorporated into the composition.
Component ~C) may be employed in order to allow for easier incorporation of the filler into the gum.
Component (C) is a processing aid or plasticizer and in a preferred embodiment Component (C) is a low viscosity silanol- or alkoxy-stopped siloxane fluid having a viscosity of about 3 to 500 cps and preferably 3 to 50 cps at 25C. The processing aid is preferably silanol-stopped, if alkyl stopped, methyl is preferred. The siloxane fluid is an equilibrium mix of low molecular weight oligomers of from about 4 to about 10 repeating units, and preferably between 4 and 6 repeating units in length with a minimum amount of cyclics in equilibrium with the oligomers. Component (C) may have the form:
R30(R2Sio)XR3 wherein each R is CH3, and x is between 4 and 10 and preferably between 4 and 6, with resulting cyclics in the same number of units in equilibrium; and R3 is selected from the group consisting of alkyl radicals having 1 to about 8 carbon atoms and hydrogen, with hydrogen being preferred.
In the present invention the processing aid is typically present in amounts up to 20 parts by weight, based upon Component (A~l preferably between 2.5 and 5 parts by weight and more preferably 3 parts by weight, based upon Component (A). It should be understood that as more filler is used, a greater amount of processin~
aid is employed. In the examples below, Component (C) has a silanol content of about 7.5 percent by weight.
Componen (D) is a vinyl terminated silazane surface treatment for the filler which may take the form:

ViSiR2NHSiR2Vi wherein R is an organic such as CH3. The surfac~
treatment promotes bonding between Components (A) and (B3, and may be applied to the filler, Component (B), prior to mixing with the other components or while mixing with other components. Component (D) may be present in an amount up to about 2 parts by weight, based upon the weight of Component (A~, preferably between 0.1 and 1 parts by weight and more preferably .3 parts by weight based upon Component (A).
Component (E) is in the form of hydride cross-linkin~ agent. In a preferrPd embodiment, Component(E) may be a random copolymer formed of a polydimethylsiloxane (P DMS~ and a polymethylhydrogensiloxane (PMHS) having the form:

~3SiO (SiORHSiORz)XSiR3 wherein each R is indepsndently chosen from a hydrogen or monovalent hydrocarbon radical, free of aliphatic unsaturation, containing 1 to about 8 carbon atoms, and wherein x ranges from about 2 to about 100.
In the present invention, the hydride is present in an amount ranging from about 0.1 to about 10 parts by weight, based upon Component (A), preferably .5 to %0~93 8.0 parts by weight and more preferably 0.8 to 5.0 parts by weight. When a hydride agent is employed, a platinum catalyst may be substituted for a peroxide catalyst to cure the composition. In Component (E), x may vary so that it has a viscosity ranging from about 5 to about 500 cps, preferably from about 10 to 100 cps and more preferably from about 10 to 50 cps at 25C.
In the examples below, Component (E) has a viscosity of about 30 centistokes, a hydrogen content ranging from about 0.05 to about 2 percent by weight, and a ~hain length of about 100 units.
The linear hydride described above may be made by many procedures which are known to those skilled in the art particularly by hydrolysis of appropriate chlorosilanes. See for example, U.S. Patent No.

4,041,101 which is incorporated herein by reference.
In order to form a heat curable rubber, an organic peroxide free radical initiator or a curing catalyst is provided. The preferred peroxide curing agents are thermal decomposition organic peroxides convenien~ly used to cure silicone elastomers. Examples of suitable organic peroxide free radical initiators for use in the present invention are disclosed, for example, in U.S.
Patent No. 4,539,357 to Bobear which is incorporated herein by reference. Suitable peroxide catalysts include dialkyl peroxide such as di-~ertiary-butyl peroxide, tertiary-butyl-triathylmethyl peroxide, tertiary-butyl-tertiary-butyl-tertiary-triphenyl peroxide, t-butyl perbenzoate and a di-tertiary alXyl peroxide such as dicumyl peroxide. Under certain conditions such as when a hydride is used, a platinum catalyst may be employed instead of an initiator. In the examples below, the preferred catalyst is a vinyl .

.

specific component such as 2,5 dimethyl-2~5-di(t-butyl peroxy) hexane e.g., (Lupersol ~101).
A heat age additive e.g., (F) a rare earth octoate, comprising primarily mixtures of cerium and S lanthanum octoates may be optionally employed to improve heat age properties. In the examples below, (F) is (C7H1sCO2)X La or Ce 6% by weight in mineral spirits.
Copending patent application Serial No. 07/587,876 filed 9/25/90, attorney docket no. 60SI-1336, incorporated herein by reference, discloses heat curable silicone compositions containing blends of vinyl-stopped organopolysiloxane gums and oils, MQ and MDQ resins fillers process aids and peroxide initiators. The compositions are formulated for increased tear strength and reduced compression set properties.
The following components were mixed in various proportions to produce heat curable silicone rubber compositions:
TABLE_I

Component (Al) - ViMD~9000 MVi - gum Component (A2) - ViND~gooo MVi (or 25 ppm RSiO3~z or T)-gum Component (Bl) - Treated Fumed Silica 240 m2/gm Component (B23 - Treated Fumed Silica 160 m2/gm Component (C3 - HOD~6OH Process ~id Component (D) - Divinylsilazane - Coupling Agent Component (E) - MDsoD~soM - Cross-Linking Agent Component (F) - (C7Hl5cOz)xLa and (C7HlsCO2)xCe (Rare Earth Octates) Heat Age Additive, 6%
by weight in mineral spirits 2 ~

EXAMPLES I - XI
The Compound~ (A) - ~E) were mixed in various proportions to produce the compounds li~ted below in Tables IIA -IIB. The resulting measured properties are listed immediately below the corresponding composition in each example.

TA5~B II-A

1 0 COmDOnent I II II} IV V VI

(Al) 80 ao 80 80 80 80 15 ~A2) (i31) 30 40 50 60 (82) (c) 4 4 4 4 4 4 (D) 0.15 0.15 0.15 0.15 0.15 0.15 (E) 1.8 1.8 l.B 1.8 1.8 1.8 (F) PQOPEQTY

Shore A 44 54 66 75 52 55 ~enDll~ 1812 1752 1714 1551 1702 1734 8trongth (psl) Elong~tlon ('s) 958 1026 902 841 925 927 Ts~r 8trength 224 254 265 251 216 227 Dle 8 (pl) Compre0~ion 39.5 69.2 83.5 89.2 55.2 65.6 8et ~ (22/350) Compresslon - 26.6 32.8 65.3 18.9 23.7 Set 9~ (70/100) 8A 8hore - 37 29 30 40 36 TA31,B r l-D
co~Donent VII VIII IX X XI
(Al~ 80 100 100 50 80 (A2) (31) (82~ 60 25 75 30 30 (C~ 4 2 4 2 4 (D~ 0.15 .4 .4 .2 (B) 1.8 - - 2 1.3 (F) _ .2 .2 PROP~TY
9hore A 70 33 72 ~ 3S
Ten~lle 1464 1243 1724 lûOO 14~3 3trength (p~l) ~long~tlon (~) 914 779 576 ~1000 1152 1 0 To~r 8trength 250 259 174 ~1~5 179 Dlo ~ ~pl) Co~praaDlon 79.2 25.9 64.9 32.8 ~et ~ (22/350) Compre~slon 44.9 10.6 34.2 1~.3 ~et ~ (70/100) 3A 8hor~ 29 62 31 ~54 Teet d~t~ were ~bt~lned from the ~ollowlng A~TM methods ~nd procedur~: 8bor~ A - D-2240:
Ton~lle, Elong~tion ~nd ~odulu~ - D-412 (D1~ C) Ta~r - D-624 (Dle 13); ~nd Comprenulon 80t - D-395 (method E)).

Example I represents a formulation for a clear, heat curable silicone rubber without a resin component.
This formulation represents a control in an experiment employing index matching resin. This formulation, while not as clear as might be desirable in a clear product, exhibited unexpectedly high physical properties as illustrated in Table II-~. Examples I-IV
show the effect of increased filler loading. As expected, the hardness (Shore A) increases with increasing filler loading. Examples V - VII substitute a treated lower surface area iller (160 m2/gm) for the treated high surface area filler (240 m2/gm) from Examples I-IV. The results in Examples V-VII show similar high strength properties.
Examples VIII and IX demonstrate a composition having high strength properties without the cross-linking agent Component (E).
Example X employs a blend of the gum materials.
Example XI is similar to Example I except that a lower surface area filler (160 m2/gm) and no coupling agent is used.

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While there have been described what at present are considered to be the preferred embodiments of the present invention, it will be readily apparent to those skilled in the art that various changes may be made therein without departing from the invention and it is intended in the claims to cover such changes and modifications as fall within the true spirit and scope of the invention.

Claims (39)

1. An improved heat curable silicone composition, having increased tensile and tear strength, good elongation and reduced compression set consisting essentially of:
(A) 100 parts of a vinyl chain-stopped organopolysiloxane or blend having a viscosity of about 3,000,000 to about 100,000,000 centipoise at 25°C;
(B) Up to about 200 parts by weight, based upon Component (A), of a filler; and (C) Up to about 10 parts by weight, based upon Component (A), of a silanol- or alkoxy-stopped siloxane fluid having a viscosity between about 3 and 500 cps at 25°C.

2. The composition of claim 1, wherein Component (A) has a viscosity ranging from about 7,000,000 to about 84,000,000 Cps at 25°C.

3. The composition of claim 1, wherein Component (A) has a viscosity of about 13,000,000 cps at 25°C.

4. The composition of claim 1, wherein Component (A) has the formula:

ViSiOR12(SiOR12)x (SiOR22)ySiR12Vi wherein Vi is vinyl; R1 is a monovalent hydrocarbon radical, free of aliphatic unsaturation, containing from 1 to about 8 carbon atoms, R2 is independently selected from monovalent hydrocarbon radicals containing 1 to about 8 carbon atoms; x and y vary such that Component (A) has a viscosity which ranges from about 3,000,000 to about 100,000,000 cps at 25°C, and has a vinyl concentration which ranges from about 5x10-5 to about 2x10-4 weight percent.

5. The composition of claim 4, wherein the vinyl concentration ranges from about 8x10-5 to about 1.5x10-4.

6. The composition of claim 5, wherein the vinyl concentration ranges from about 8x10-5 to about 1.2x10-4.

7. The composition of claim 4, wherein Component (A) contains about 9000 SiOR2 or D units.

8. The composition of claim 7, wherein Component (A) contains about 25 ppm SiO3/2R or T units.

9. The composition of claim 1, wherein Component (B) is present in an amount ranging from about 30 to about 75 parts by weight, based upon Component (A).

10. The composition of claim 1, wherein Component (B) is a silica filler having a surface area ranging from about 100 to about 300 m2/gm.

11. The composition of claim 10, wherein the filler is fumed silica.

12. The composition of claim 10, wherein the silica filler has a surface area ranging from about 160 to about 240 m2/gm.

13. The composition of claim 1, wherein Component (C) is a low viscosity silanol-stopped siloxane fluid having a viscosity ranging from about 3 to about 500 cps at 25°C.

14. The composition of claim 13, wherein Component (C) has a silanol content of about 7.5 percent by weight.

15. The composition of claim 13, wherein Component (C) has a viscosity ranging from about 3 to 50 cps at 25°C.

16. The composition of claim 13, wherein Component (C) has the formula HO(R2SiO)xH where R is an organic group.

17. The composition of claim 16, wherein R is CH3.

18. The composition of claim 16, wherein x is between 4 and 10.

19. The composition of claim 16, wherein x is between 4 and 6.

20. The composition of claim 1, wherein Component (C) has the formula R3O(R2SiO)xR3 where R is an organic group and R3 is selected from the group consisting of alkyl radicals having from 1 to about 8 carbon atoms and hydrogen.

21. The composition of claim 1, wherein Component (C) is present in an amount ranging from about 2 to about 5 parts by weight, based upon Component (A).

22. The composition of claim 1, further including Component (D), an organohydrogensilazane having the form ViSiR2NHSiR2Vi wherein R is an organic group.

23. The composition of claim 22, wherein R is CH3.

24. The composition of claim 22, wherein Component (D) is a divinyltetramethylsilazane.

25. The composition of claim 22, wherein Component (D) is present in an amount between about 0.1 and about 1 parts by weight, based upon Component (A).

26. The composition of claim 24, wherein Component (D) is present in an amount of about .3 parts by weight based upon Component (A).

27. The composition of claim 1, further comprising (E) up to about 10 parts by weight, based upon Component (A), of an organohydrogensiloxane having a viscosity ranging from about 5 to about 500 centipoise at 25°C.

28. The composition of claim 27, wherein Component (E) is a hydride cross-linking agent.

29. The composition of claim 27, wherein Component (E) comprises a polydimethylsiloxane (PDMS) and polymethylhydrogensiloxane (PMHS) having the form:
R3SiOR(SiORHSiOR2)xSiR3 wherein each R is independently chosen from a hydrogen or monovalent hydrocarbon radical, free of aliphatic unsaturation, containing 1 to about 8 carbon atoms and where x is about 100.

30. The composition of claim 27, wherein Component (E) is present in an amount ranging from about .1 to about 10 parts by weight, based upon Component (A).

31. The composition of claim 27, wherein Component (E) is present in an amount ranging from about .5 to about 8 parts by weight, based upon Component (A).

32. The composition of claim 27, wherein Component (E) is present in an amount ranging from about 0.8 to about 5.0 parts by weight, based upon Component (A).

33. The composition of claim 27, wherein Component (E) has a viscosity ranging from about 10 to about 50 cps at 25°C.

34. The composition of claim 27, wherein Component (E) has a viscosity ranging from about 10 to about 100 cps at 25°C.

35. An improved heat curable silicone composition, having increased tensile and tear strength, good elongation and reduced compression set consisting essentially of:
(A) 100 parts of a vinyl stopped organopolysiloxane or blend having a viscosity of about 3,000,000 to about 100,000,000 centipoise at 25°C;
(B) Up to about 200 parts by weight, based upon Component (A), of a filler; and (C) Up to about 10 parts by weight, based upon Component (A), of a silanol- or alkoxy-stopped siloxane fluid having a viscosity between about 3 and 500 centipoise at 25°C; and (D) Up to about 2 parts based upon Component (A) of an organohydrogen silazane having a hydrogen content ranging from about 0.05 to about 2% by weight and having a viscosity ranging from about 5 to about 500 centipoise at 25°C.

36. An improved heat curable silicone composition, having increased tensile and tear strength, good elongation and reduced compression set consisting essentially of:
(A) 100 parts of a vinyl stopped organopolysiloxane or blend having a viscosity of about 3,000,000 to about 100,000,000 centipoise at 25°C;
(B) Up to about 200 parts by weight, based upon Component (A), of a filler;
(C) Up to about 10 parts by weight, based upon Component (A), of a silanol- or alkoxy-stopped siloxane fluid having a viscosity between about 3 and 500 centipoise at 25°C;

(E) Up to about 10 parts by weight, based upon Component (A), of a hydride cross-linking agent.

37. An improved heat curable silicone composition, having increased tensile and tear strength, good elongation and reduced compression set consisting essentially of:
(A) 100 parts of a vinyl stopped organopolysiloxane or blend having a viscosity of about 3,000,000 to about 100,000,000 centipoise at 25°C;
(B) Up to about 200 parts by weight, based upon Component (A), of a filler; and (C) Up to about 10 parts by weight based upon Component (A), of a silanol- or alkoxy-stopped siloxane fluid having a viscosity between about 3 and 500 centipoise at 25°C;

(D) Up to about 2 parts by weight, based upon Component (A), of an organohydrogen silazane having a hydrogen content ranging from about 0.05 to about 2 parts by weight and having a viscosity ranging from about 5 to about 500 centipoise at 25°C; and (E) Up to about 10 parts by weight, based upon Component (A), of a hydride cross-linking agent.

38. An improved heat curable silicone composition, having increased tensile and tear strength, good elongation and reduced compression set consisting essentially of:
(A) 100 parts of a vinyl chain-stopped organopolysiloxane or blend having a viscosity of about 3,000,000 to about 100,000,000 centipoise at 25°C;
(B) Up to about 200 parts by weight, based upon Component (A), of a filler; and (C) Up to about 10 parts by weight, based upon Component (A), of a silanol stopped siloxane fluid having a viscosity between about 3 and 500 cps at 25°C.

39. The invention as defined in any of the preceding claims including any further features of novelty disclosed.