CA1233289A - Peroxide-curable fluorosilicone copolymer compositions - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Peroxide-curable fluorosilicone copolymers are provided which, when compounded and cured, form elastomers of superior physical properties, especially in terms of solvent swell and thermal aging. Curable compositions prepared according to the present invention make possible the efficient production of high quality electrical connectors for aerospace and other applications.

Description

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-L-FIELD OF THE INVENTION

The present invention relates to fluorosilicone elastomers. More particularly, it relates to curable fluorosilicone copolymer compositions winch provide superior physical properties and are suitable for the manufacture of nigh quality electrical connectors.

BACKGROUND OF TUBE INVENTION

Platinum catalyzed fluorosilicone/
organosilicone blends and low molecular weight silicone copolymer compositions such as those described in US.
4,122,247 Evans), US. 4,157,337 (Evans), and US.
4,317,899 (Blue stein et at) have found use as low energy molding compounds in the aerospace industry for making high quality electrical connectors and other molded silicone rubber inserts. Such compounds, which cure via nydrosilation addition reaction in the presence of platinum, display the advantages of rapid processing, fast deep section curing, low rejection rate, and good mechanical properties. However, in spite of these advantages, the addition cure systems are susceptible lo-.,. . ~`:~.
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_ 2 koalas. poisoning, e.g., from small amounts of compounds such as carborl monoxide or sulfur or amine, which tie up platinum; the ratio of silicon hydra tee to olefin functionality for optimal properties must fall within a narrow range; a critical range of inhibitor to catalyst is necessary to provide rapid cure along with adequate working shelf life; and the systems are also susceptible Jo organic contaminants which can adversely affect the thermal stability of the cured parts.
In order to secure better thermal aging properties, many electrical connector manufacturers have tested peroxide-cured fluorosilicone systems made by blending heat-curable fluorosilicone compounds with polydimethylsiloxane gums. Because of the nature of tune curing reaction, addition cure compositions do not fully cure initially, and further curing in response to hush temperatures encountered by the compositions in use ma lead to shrinkage of the molded component, making it 23 unsuitable for continued used. Greater dimensional stability is provided by peroxide-curable systems which fully cure and so do not shrink in use. It has been observed that the peroxide-curable blends, however, require a higher overall average fluorine content to provide sufficient solvent resistance. Consequently, their has been a need for a peroxide-curable fluorosilicone copolymer gum which exhibits the desirable process ability of the addition cure systems while providing better thermal aging and good resistance to solvent swell.

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_ 3 SUMMARY OF TOE INVENTION

Accordingly, it is an object of the present invention to provide a fluorosilicone copolymer gum that is curable in the presence of provide catalysts.

It is a further object of the present invention to provide a peroxide-curable fl~orosilicone copolymer composition useful for the manufacture of high quality electrical connectors.

It is a further object of the present invention to provide a curable fluorosilicone composition which, wren compounded and cured exhibits superior physical properties, including low solvent swell and goon thermal aging.

These and other objects are accomplished herein by a peroxide-curaole sullenly- or alkenyl-endstopped diorganopolysiloxane coplanar oil or gum comprising units of the formulae RR'SiO and R" So, wherein R is fluoroalkyl of from 3 to 8 carbon atoms, R' is alkyd of from l to 8 carbon atoms or phenol, and each R" is independently, alkyd of from l to & carbon atoms, alkenyl of from 2 to 8 cordon atoms, or phenol, and wherein sufficient RR'SiO units are present to provide at least about q5 mole percent fluorosilicone content and sufficient R" So units containing alkenyl substituents are present to provide up to about 0.5 30 weight percent vinyl content (as I I along the polysiloxane chain.

Also contemplated is a curable fluorosilicon~
composition comDrisins PA) a silanol-endstopped or 35 alkenyl-endstopped diorganpolysiloxane copolvmer oil Ox -3~89 SUE

gum comprising units of the foretell Rio and R" So, wherein R is fluoroalkyl of from 3 to 8 carbon atoms, R' is alkyd of from 1 to 8 carbon atoms or phenol, and each R" is, independently, alkyd of from 1 to 8 carbon atoms, alkenyl of from 2 to 8 carbon atoms, or phenol, and wherein sufficient RR'SiO units are present to provide at least about 45 mole percent fluorosilicone content and sufficient R" So units containing alkenyl substituents are present to provide up to about 0.5 weight percent vinyl content (as OH I along the polysiloxane chain; and By a peroxide catalyst.

Processes for preparing the copolymers and compositions of the present invention are Allah contemplated.

DETAILED DESCRIPTION OF THE INVENTION

The fluorosilicone copolymers of the present invention may be compounded and cured in conventional ways in the presence of a peroxide catalyst to provide dimensionally stable cured parts having exceptional resistance to solvent swell, superior thermal aging properties and mechanical properties which exceed strict industrial aerospace specifications.

The R, I' and R" substi~uents defined above are representative of monovalent hydrocarbon radicals and halogenated monovalent hydrocarbon radicals that are well known as attachments to silicon atoms. Preferably, the fluoroalkyl substituent, R, contains 3 or more ,. . .

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_ 5 carbon atoms and 1 or more fluorine atoms, the radicals including, for example, 3-fluoropropyl, 3,3-difluorcpro-Pyle 3,3,3-trifluoropropyl, and the like up to 8 carbon atoms. More preferably, the R radical is a substituted alkyd group such as, -OH OH R , wherein R is perfluoro-alkyd containing from 1 to 6 carbon atoms, such as perfluorome,hyl, perfluoroethyl, perfluorohexyl, and the like. Most preferably, the R radical is trifler-propel. R' it preferably methyl; R" is preferably methyl or vinyl, with at least 1 vinyl group being present along the copolymer chain but no more vinyl groups being present than would provide about 0.5 weight percent vinyl, as vinyl, the rest of the R" groups being methyl.
The copolymers of the present invention may De synthesized from diorganodihalogensilanes of the formulae RR'SiX and R"2SiX2, wherein OR and R" are as previously defined, and X is halogen, such a chlorine or bromide (preferably chlorine), as described in the aforementioned Evans patents, United States Patent Number 4,122,247 and United States Patent No. 4,157,337.
The diorganodihalogensilanes, at a purity of at least 99~ by weight, are added to water at room temperature, e.g., 20-25C to provide from about 2 to 10 moles of water per mole of dior~anodihalogensilane. After the diorganodihaIogensilanes have been added to the water it will contain about 20% by weight of Hal.

Optionally, hydrolysis may be carried out in the presence of a water-immiscible solvent such as, for example, Tulane, zillion, Bunsen, and the like. The use of a solvent facilitates the separation of the hydrolyzate from the aqueous acid solution. Preferably, a water-immiscible organic solvent is added to tune waxer prior to the addition of the diorganodihalogensilanes.
The diorganodihalogensilanes, preferably at greater than 99% purity, are added to the water (or water/water-immiscible solvent mixture) over a one-half to two hour period, with agitation. Where a solvent is used, the hydrolyzate is seen to dissolve in the solvent phase, and this is then separated from the water phase. The hydrolyzate is finally neutralized with a mild base, such as sodium bicarbonate, to a pi of about 7 to 8.
Alternatively, the hydrolyzate may be washed repeatedly with water until a neutral pi is reached.
The hydrolyzate product is a mixture containing mostly cyclic polysiloxanes of from 3 to 10 silicon atoms and low molecular weight linear sullenly-end stopped diorganopolysiloxanes. This raw hydrolyzateis useful as a treating agent for fumed silica fillers as described in US. Patent No. 4,529,774, issued July 16, 1985 to Evans et at.
Heating the hydrolyzate at elevated temperatures removes any solvent by overhead distillation. The hydrolyzate is then cracked by a procedure comprising adding from 0.1 to 5% by weight (preferably 0.1 to 2%
by weight) of a cracking catalyst, such as potassium hydroxide or sesame hydroxide, then heating.
The cracking procedure is typically carried out at temperatures between 150 and 200C, and preferably heating is carried out under a vacuum of 1 to 100 millimeters of mercury for a period of from about 1 to 5 hours. A mixture of cyclic polysiloxanes, specifically cyclic trimmers, tetramers and pen tamers viol be continually distilled overhead. The cracking procedure is utilized to maximize the formation of the ~Z~33Z8~

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_ 7 cyclic triter from the broad range in the headrest.
This permits conversion of about us% by weight of the hyarolyzate to cyclic trisiloxanes, cyclic tetrasiloxanes, and cyclic pentasiloxanes, predominantly cyclic trisiloxanes. The cyclic polysiloxanes may be separated by known distillation procedures from the other cyclic.

It is preferred that there be less than 10 parts per million of water in the composition of cyclic selections to be utilized with the catalyst to make polymers. Removal of all but traces of water may be accomplished by heating to 100C or above, with a nitrogen purge. This effectively reduces the water content of the cyclic selection composition to less than 10 parts per million. It has been found that if there is substantially more than this amount of water present, the desired copolymer gum will not be formed in commercially attractive yields.

To prepare the copolymers of the invention, cyclic polysiloxanes of the formulae (RR'SiO) and I So) , where OR and R" are as previously define an x an y are integers from 3 to 6, are placed in a reaction vessel. An alkali metal hydroxide polymerization catalyst (preferably KIWI) in amounts to provide about 5 to 500 parts per million of the catalyst are added to the cyclic mixture. Polymerization is then carried out by heating at a temperature of from 20 to 160C for a period of from hour to 20 hours, during which time equilibrium is reached. At this point, 70 to 88% by weight or more of the cyclic selections will have been converted to the desired diorganopolysiloxane copolymer oil or gum. The reaction mixture can be cooled, e.g., to under 25C, and the catalyst neutralized.

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o A number of conventional neutralizing agents may be used, however neutralizing agents preferrer for the purposes herein include phosphoric acid, tris(2-chloroethyl)phosphite, and sill phosphate (See, e.g., United states Patent Number 4,177,200 (Rosen et at), and also orgànohalosilanes or halosilanes of the formulae R Six , wherein R is alkyd, cycloalkyl, vinyl or phenol, the alkyd and cycloalkyl groups having from 1 to 8 carbon atoms; b is an integer from 1 to 3; and wherein X is bromide or chlorine. These Solon compounds include, e.g., trimethylchlorosilane.

After neutralization, the reaction mixture is heated at elated temperatures, e.g., 150 to 200C
under a vacuum of 1 to 100 millimeters of mercury to strip the unrequited cyclic polysiloxanes to viola the diorganopolysiloxane copolymer oil or gull.

In accordance to known procedures, the viscosity of the copolymer may be controlled by adding a chain stopper to the composition of co-monomers and polymerization catalyst. Such chain stoppers can be, for example, disiloxanes or low molecular weight diorganopolysiloxanes having, preferably, either sullenly ( Sigh) or vinyl terminal units. the organ substituents in such chain stoppers are typically alkyd of from l to 8 carbon atoms, vinyl, phenol, cycloalkyl of from 4 to 8 carbon atoms, or haloalkyl, especially fluoroalkyl, such 30 as trifluoropropyl. Preferred5c6hainstoppers for the purposes herein include, HERR R Sue, where R is alkyd and R it 3,3,3-trifluoropropyl, and VitMe)2-~MeR So) Messiah) Si-(Me)2Vi,6wherein x=23, yo-yo, Vi is vinyl, Me is methyl, and R is 3,3,3-35 trifluoropropyl.

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9 _ Generally, the amount of chain stopper introduced to the equilibration mixture will be selected to produce the desired final molecular weight or viscosity of the diorganopolysiloxane copolymer oil or gum. sigher amounts of chain stopper will produce lower molecular weights, and smaller amounts will produce higher molecule weights. For the purposes of the present invention, it is generally preferred to use sufficient chain stopper to provide a copolymer having a Williams Plasticity I minute reading) of about 160 to 220, or to provide a copolymer having a molecular weight of f rum about 400,000 to 600,000.

As mentioned previously the fluorosilicone copolymers should have a fluorosilicone content of at least 45 mole percent in order to exhibit a desired high resistance to solvent swell. Preferably the fluorosilicone content of the copolymer will fall in Tao range of 45-65 mole percent, more preferably 50-60 mole 20 percent, and most preferably 52-57 mole percent.
Obviously, where solvent swell is not a concern of the practitioner, the fluorosilicone content of tube copoiymer may be varied within wider limit.

In addition, sufficient, alkenyl-functional starting materials should be adder to the polymerization mixture to provide up to about 0.5 weight percent vinyl content (as I SHEA-), preferably 0.02-0.2 weight percent, and most preferably 0.03-0.1 weight percent 30 vinyl along the copolymer chain. This on-chain alkenyl functionality allows for a Tighter" cure and thereby contributes to the higher dimensional stability of the cured compositions according to this invention.
however, higher levels of.on-chain-unsaturation than 35 about 0.5 weight percent (as I act-) leads to too tight :

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_ 10 _ a cure, adversely affecting other mechanical properties of the cured compositions, e.g., compression set.

The aforementioned copolymers are curable in the presence of peroxide catalysts which initiate a cross linking hydrogen abstraction reaction between the pendant alkenyl groups and pendant saturated hydrocarbon groups or possibly a halogen abstraction reaction, in the case of pendant fluorocarbon groups) of the copolymer chains. This type of free-radical initiated abstraction is promoted by small amounts, such as about 0.01~ by weight of the more active organic peroxide inflators.
Greater amounts, such as up to about 5.0% by weight or more of the initiator may be used, but amounts in excess of about 1.5~ by weight may promote coupling reactions which undesirably increase the viscosity of the reaction mixture.

The most suitable peroxide initiators are compounds of the formula, POOH or AYE, in which A is an organic radical, especially those compounds in which at least one peroxide oxygen is attached to a tertiary carbon atom. Preferred such initiators include t-butyl hydroperoxide, cumin hydroperoxiae, decline hydroperoxide, di-t-butyl peroxide, dicumyl peroxide,

2,5-dimethyl-2,5-di-~t-butyl peroxy)hexane; also cyclic peroxides such as ascaridole and l,5-dimethylhexane-1,5-peroxide, per esters such as t-butyl-perbenzoate, t-butylperoxyisopropylcarbonate and t-butyl peroctoate, and kitten peroxides such as acetone peroxide and cyclohexanone peroxide. The peroxides containing tertiary alkoxy radicals have been found to be more efficient in abstracting hydrogen or halogen) atoms from the pendant organic groups linked to the silicon atoms, and the are therefore preferred. 2,5-dimethyl-2,5-di-(t-butyl : .

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peroxy)hexane is most preferred and is available commercially, e.g., at 45~ by weight on an inert filler under the trade names, VAROX- (RUT. Vanderbilt Co., Inc.) and LUPERCO- 101XL (Lucidly Division, Penlight Corp.).
The fluorosilicone copolymer oils or gums of the present invention, when combined with the aforementioned peroxide curing catalysts form curable - compositions having superior physical properties, compared to prior addition cure silicone compositions. Obviously, the copolymers can be formulated, e.g., by mixing with reinforcing fillers, such as fumed silica or precipitated silica; extending fillers, such as zinc oxide, titanium oxide, diatomaceous earth, and the like; heat aging additives, such as iron oxide; pigments or dyes; flame retardants, such as platinum (as platinum or in combination with other materials such as triallylisocyanurate); adhesion promoters, such as organic soullessness, which promote bonding between fillers JO and the gum; and other additives, including anti-oxidants, processing alas (e.g., sullenly fluids), compression set resistance promoters (e.g., curium hydroxide), supplementary curing agents (e.g., materials that provide additional vinyl curing sites) such as trimethylol propane trimethacrylate (Sartomer- 350; Sartomer Co.), triallyl trimellitate tSipomer- TAT; Alcoholic, Inc.), 1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane, 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiiloxane, and the like.
A particularly useful filler for the compositions described herein is a silica filler, preferably fumed silica, that has been treated with the raw hydrolyzate containing mixed cyclic polysiloxanes and low molecular weight linear polysiloxanes described , , . .

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above. Such fillers are disclosed in the aforementioned US. Patent Jo. 4,529,773.
The copolymer gums, mixed into a uniform mass to which is added a peroxide curing agent, may be cured at elevated temperatures, for example from about 100 to 300C, or by exposure to radiation, to produce fluorosilicone elastomers having tune aforementioned superior physical properties, including exceptional resistance to solvent swell and excellent thermal aging characteristics.
In order that persons skilled in this art may more readily practice the present invention, the following examples are provided by way of illustration, and not by way of limitation.
All measurements are by weight.

Sample 1 1716 parts by weight of methyl-3,3,3-tri-fluoropropylsiloxane cyclic triter, 821 parts by weight of dim ethyl selection cyclic tetramer, and about 850 parts per million (Pam) HO-(MeCH2CH2CF3SiO)3-H chain stopper were added to a clean, dry polymerization vessel equipped with stirrer, nitrogen inlet, and thermometer. The 25 mixture was heated to 150-160C and with a nitrogen spurge, 115 parts by weight of dim ethyl-selection cyclic tetramer were removed to azeotropically dry (to fewer than 10 Pam of moisture) the reaction mixture. While maintaining a positive atmosphere of nitrogen, the vessel was allowed to cool to 135-142C, at which point 3.1 parts by weight of methyl vinyl selection cyclic triter ~Z33'~
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_ 13-(99~ purity) and sufficient potassium hydroxide* to provide 25 Pam (as potassium hydroxide) were added. The resulting exotherm cause a 16~C temperature rise within about 30-35 seconds, which subsided over a 50-minute period to the 135-145C range.

A gum began to form after 30 minutes, and the polymerization was continued for 4-6 hours before the catalyst was neutralized with silylphosphate. The unrequited cyclic and other volatile were removed by heating to 160C while agitating and maintaining a strong Newton purge. When the level of volatile reached about 0.8 (+0.5) weight percent in the copolymer solution, the copolymer was discharged from the neutralization vessel to give 1897 parts by weight ~81.6% yield) of a clear gum with a Williams Plasticity (3 minute value) of 178, specific gravity of 1.1708 at 77F, and a refractive index of 1.3810 D . Nuclear magnetic resonance (NOR) indicated a fluorosilicone content of 56.9 mole percent and a dimethylsiloxy content of 43.1 mole percent; Fourier Transform infrared assay (FIR) indicated a vinyl content of 0.04 weight percent.

-. . . _ *OWE is collided in 1,3,5,7-octamethylcyclotetrasiloxane and dried over a AYE molecular solve.

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_14 _ Sample 2 A fluorosilicone copolymer was prepared according to the procedure described above for Sample 1.
A copolymer having a Williams Plasticity (3 minute value) of 208 was obtained; the fluorosilicone content it methyltrifluoropropyl-siloxy content) was 50.4 weight percent and the dimethylsiloxy content was 49.6 weight percent. The vinyl content was 0.03 weight percent.
Sample 3 A vinyl-terminated fluorosilicone copolymer was prepared according to the procedure described for Sample 1. The product (82~ yield) was a clear gum having a Williams Plasticity (3 minute value) of 193.
The copolymer vinyl content was 0.042 weight percent.

The three copolymers were compounded with a number of conventional ingredients and an organic peroxide curing agent as follows:
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_ 15 _ FORMULATION

Ingredient Amounts Parts weight) Copolymer 100 5 Fluorosilicone hydrolyzate treated fumed silica 2 40 Silica filler 6 Sullenly fluid processing aid 3 5 10 Bis-dimethylvinylsilazane 0 1 Iron octet 0 04 Curium hydroxide 0 40 -Curing co-agent 1 0 Red pigment 5 1 7 15 Curing catalyst (1 5 parts/100 of total compound) ... . . _ _ _ . _ _ Treated filler according to cop ending U S Application Serial No 410,004, filed August 20, 1982 Cab-O-Sil- ~S-5(Cabot Crop ) Silanol-terminated polydimethylsiloxane ~PDMS) fluid having the formulae, ~O-(Me2SiO) -owe 4 Trimethylol propane trimethacrylate (Sartomer- 350;
Sartomer Co.) Varox- (R T Vanderbilt, In 45% 2,5-dimethyl-2,5-35 di(t-butvlperoxy)hexane on inert fillers ~233'~39 SUE

_ 16 _ A control composition was also prepared according the same formulation using 100 parts of a blend of fluorosilicone h4mopolymer with a polydimethyl-selection gum. The three sample formulations and the control were press cured 15 minutes at 350F and post baked 4 yours at 400DF. The following properties were observed in the cured elastomers:

Control Physical Properties Sample 1 Sample 2 Sample 3 (blend) Fluorosilicone Content, m% 56.9 50.4 50 66 Shore A 56 56 57 51 Tensile, psi 1070 1035 1060 1050 15 Elongated, 420 410 430 450 Die B Tear, pi 102 100 117 107 Specs Go. 1.3595 1.3300 1.3350 1.345 Ashore . 35 - - -Coup. Set, method B.
22 hrs./350F, % 17.5 13.6 12.7 25 Fuel Immersion, Fuel B 22 hrs./RT
Volume Swell 87.6 108 121.5 91.
teat Age, 7Q hrs.~43? F
Store Change 0 +1 +2 % Tensile Change -9.6% -7% -5%
% Elongation Change -14~2% -15~ -16%
Dimensional Change 1.1% 1.3~ 1.4%

It is seen that in identical formulations the copolymers of the present invention show comparable or superior physical properties, with a significantly US lower fluorosilicone content, when compared to a 2~g SUE

conventional fluorosilicone/PDMS blend. Especially promising results are obtained in the important areas of compression set, solvent swell and dimensional stability after heat aging. The samples prepared in accordance with the invention, especially Sample 1, are believed to meet the strictest current aerospace industry specifications.

Modifications and variations in the present invention are obviously possible in light of the foregoing disclosure. It is understood, however, that incidental changes made in the particular embodiments of the invention as ascribed herein are within the full intended scope of the appended claims.

Claims (19)

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

1. A peroxide-curable silanol-endstopped or alkenyl-endstopped diorganopolysiloxane copolymer having a molecular weight of from about 400,000 to about 600,000 comprising units of the formulae RR'SiO and R"2SiO, wherein R is fluoroalkyl of from 3 to 8 carbon atoms, R' is alkyl of from 1 to 8 carbon atoms, alkenyl of from 2 to 8 carbon atoms, or phenyl, and wherein sufficient RR'SiO units are present to provide about 45-65 mole percent fluorosilicone content and sufficient R"2SiO
units containing alkenyl substituents are present to provide a vinyl content along the polysiloxane chain of about 0.02 weight percent to about 0.5 weight percent as CH2=CH-.

2. A copolymer as defined in claim 1, wherein R is 3,3,3-trifluoropropyl, R' is methyl, and R" is methyl or vinyl.

3. A copolymer as defined in claim 2, wherein sufficient R" groups are vinyl to provide 0.02-0.2 weight percent vinyl content along the polysiloxane chain.

4. A curable fluorosilicone composition comprising (A) a silanol-endstopped or alkenyl-endstopped diorganopolysiloxane copolymer having a molecular weight of from about 400,000 to about 600,000 comprising units of the formulae RR'SiO and R"2SiO, wherein R is fluoro-alkyl of from 3 to 8 carbon atoms, R' is alkyl of from 1 to 8 carbon atoms or phenyl, and each R" is, independently, alkyl of from 1 to 8 carbon atoms, alkenyl of from 2 to 8 carbon atoms, or phenyl, and wherein sufficient RR'SiO units are present to provide about 45-65 mole percent fluorosilicone content and sufficient R"2SiO units containing alkenyl substituents are present to provide a vinyl content along the polysiloxane chain of from about 0.02 weight percent to about 0.5 weight percent as CH2=CH-; and (B) a peroxide curing catalyst.

5. A curable composition as defined in claim 4, wherein said peroxide curing catalyst is an organic peroxide compound in which at least 1 peroxide oxygen is attached to a tertiary carbon atom.

6. A curable composition as defined in claim 5, wherein R is 3,3,3-trifluoropropyl, R' is methyl, and R"
is methyl or vinyl.

7. A curable composition as defined in claim 6, wherein said organic peroxide compound is present in an amount from about 0.01% to about 5.0% by weight.

8. A curable composition as defined in claim 7, wherein sufficient R" groups are vinyl so as to provide 0.02-0.2 weight percent vinyl content along the poly-siloxane chain.

9. A curable composition as defined in claim 8, wherein said fluorosilicone content is in the range of 50-60 mole percent.

10. A curable composition as defined in claim 9, wherein said fluorosilicone content is in the range of 52-57 mole percent.

11. A curable composition as defined in claim 7, wherein said organic peroxide compound is 2,5-dimethyl-2,5-di-(t-butyl peroxy)hexane.

12. A curable composition as defined in claim 7, which is further compounded with a filler or combination of fillers.

13. A curable composition as defined in claim 12, wherein said filler is a silica filler treated with a diorganodihalogensilane hydrolyzate comprising mixed cyclic diorganopolysiloxanes and low molecular weight linear silanol-endstopped diorgano-polysiloxanes.

14. A curable fluorosilicone composition comprising (A) a silanol-endstopped or vinyl-endstopped diorganopolysiloxane copolymer having a molecular weight of from 400,000 to about 600,000 comprising units of the formulae RR'SiO and R"2SiO, wherein R is fluoroalkyl of from 3 to 8 carbon atoms, R' is alkyl of from 1 to 8 carbon atoms, and each R" is, independently, alkyl of from 1 to 8 carbon atoms or vinyl, and wherein sufficient R-containing units are present to provide about 50-60 mole percent fluorosilicone content and sufficient vinyl-functional units are present to provide a vinyl content along the polysiloxane chain of about 0.02-0.5 weight percent as CH2=CH-; (B) a peroxide curing catalyst in which at least one peroxide oxygen is attached to a tertiary carbon atom; and (C) cerium hydroxide.

15. A curable composition as defined in claim 14, wherein said vinyl content is from 0.02 to 0.2 weight percent.

16. A curable composition as defined in claim 15, wherein said curing catalyst is 2,5-dimethyl-2,5-di(t-butyl peroxy)hexane.

17. A curable composition as defined in claim 15, which is further compounded with a silica filler treated with a diorganodihalogensilane hydrolyzate comprising mixed cyclic diorganopolysiloxanes and low molecular weight linear silanol-endstopped diorgano-polysiloxanes.

18. A curable composition as defined in claim 17, which further comprises an organic silazane and a curing co-agent selected from trimethylol propane trimethacrylate, triallyl trimellitate, 1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane, and 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane.

19. A curable composition as defined in claim 18, wherein said organic silazane is bis-dimethyl-vinyl silazane, and said curing co-agent is trimethylol propane trimethacrylate.