Heat strength curable silicone rubber compositions

A high strength heat curable silicone rubber composition comprising (A) 100 parts of a vinyl-terminated diorganopolysiloxane base polymer having up to 0.7 weight percent of aliphatic type unsaturation such as 3,3-difluor-2-propenyl from 5 to 80 parts by weight of a vinyl-terminated diorganopolysiloxane additive polymer having from 0.05 to 10 mole percent of organo vinylsiloxy units and from 0.01 to 10 parts by weight of a curing catalyst.

BACKGROUND OF THE INVENTION 
The present invention relates to heat curable silicone rubber compositions 
and more particularly the present invention relates to flourosilicone heat 
curable silicone rubber compositions. 
Heat curable silicone rubber compositions are well known. Generally such 
compositions comprise a diorganopolysiloxane gum having a viscosity 
varying from 1,000,000 to 300,000,000 centipoise at 25.degree. C., 
optionally a filler which is preferably a silica filler and a curing 
catalyst. The curing catalyst preferably is a peroxide catalyst. In the 
composition, there should be preferably utilized a polysiloxane process 
aid which facilitate the mixing of the filler in the diorganopolysiloxane 
gum and prevent it from structuring. There may be utilized other additives 
in the compositions such as heat flame retardant additives, heat aging 
additives, and self-bonding additives such as the self-bonding additives 
disclosed in De Zuba et al. U.S. Pat. No. 3,730,932 which is hereby 
incorporated by reference. An example of flame retardant additive is a 
platinum additive of Nobel et al. U.S. Pat. No. 3,514,424 which is hereby 
incorporated by reference. 
It should be noted that a diorganopolysiloxane gum preferably contains some 
alkenyl groups which are preferably vinyl groups so as to ease the 
cross-linking and curing of the polymer. Fluorosilicone substituted heat 
curable silicone rubber compositions are also well known as disclosed in 
Brown U.S. Pat. No. 3,179,619. Other such compositions are produced by 
taking fluorosilicone substituted cyclopolysiloxane and heating them at 
elevated temperatures in the presence of an alkali metal hydroxide 
catalyst which is preferably potassium hydroxide in the presence of small 
amounts of chain-stoppers which may be water or low molecular silanol 
terminated diorganopolysiloxane polymers or may be aliphatic alcohol 
chain-stoppers, see for instance the disclosure of Bluestein Ser. No. 
170,272. Such polymers are also disclosed to be produced with low 
molecular weight vinyl terminated chain-stoppers as for instance disclosed 
in the Patent Application of E. Robert Evans Ser. No. 071,152, filed Aug. 
30, 1979, abandoned which is hereby incorporated by reference. 
It should be noted that all the foregoing Patent Applications and Patents 
disclosed in this document are incorporated by reference in the present 
case. Thus utilizing a low molecular weight vinyl terminated 
fluorosilicone substituted copolymer as a chainstopper which is reacted in 
the presence of a sodium hydroxide catalyst with a fluoro substituted 
cyclotrisiloxane, it was possible to obtain diorganopolysiloxane gums of 
high molecular weight in a period of time varing anywhere from 3 minutes 
to 4 hours. 
As disclosed in the copending case of Evans Ser. No. 253,282 filed Apr. 9, 
1981 it was possible to obtain diorganopolysiloxane gums of a high 
viscosity, a viscosity varying from anywhere from 1,000,000,000 to 
300,000,000 centipoise at 25.degree. C. where the 3,3-difluoro-2-propenyl 
or aliphatic unsaturation does not exceed 0.70 weight percent. It was 
found in such gums where the aliphatic unsaturation does exceed 0.7 weight 
percent that the resulting silicone elastomer formed from such gums had 
good strength properties and especially high tear. It was also possible to 
utilize the process of the foregoing patents to produce flurosilicone 
copolymers in which the aliphatic type unsaturation did not exceed 0.70 
weight percent. This was done by controlling the amount of unsaturation 
that was present in the cyclotrisiloxane that was utilized in the process 
by first controlling the amount of unsaturation that was formed in the 
cyclotrisiloxane by the process by which it was made, and also but not 
utilizing batches of cyclotrisiloxanes in which the vinyl unsaturation 
exceeds 0.70 weight percent. 
It was possible to produce fluorosilicone substituted copolymers which were 
vinyl terminated by equilibrating fluoro substituted cyclotrisiloxanes 
with octaorganotetrasiloxanes in the presence of an alkali metal hydroxide 
catalyst at elevated temperatures, that is temperatures above 140.degree. 
C. There was produced a copolymer having from 5 to 40 mole percent of 
trifluoropropyl substituted groups. The method of producing these 
copolymers is disclosed in the copending Patent Application of Evans et 
al. Ser. No. 253,282 filed Apr. 9, 1981. These copolymers have had many 
desirable properties and produce addition cured compositions with good 
strength properties. 
The base homopolymer as well as the copolymer were tested for utilization 
on gas masks. In the past, gas masks had been produced from a 
timethylsilicone polymer in which the lens was formed from transparent 
plastic and in which the frame supporting the lens as well as the straps 
was formed from a dimethylpolysiloxane gum. However, it was necessary to 
coat the dimethylpolysiloxane gum frame with various types of 
fluorosilicone compositions and coatings so as to make the frame 
impervious to nerve gas. 
Dimethylpolysiloxanes are not impervious to nerve gas. However the 
complicated coating procedure that was utilized tended to increase the 
cost of the mask. 
Accordingly it was highly desirable to produce the frame and the straps of 
the gas masks from a fluorisilicone composition of the proper impervious 
ability to nerve gas at a lower cost. The foregoing vinyl terminated 
fluorosilicone homopolymers and copolymers were tested for this 
application. However, it was found that they do not have as high strength 
properties as would be desired although they do indeed have good strength 
properties. 
Accordingly it was highly unexpected by including two vinyl terminated 
polymers with a dramatically different vinyl-on-chain concentration that 
there could be obtained exceptionally high strength fluorosilicone heat 
curable silicone rubber compositions. It should be noted that high 
strength high vinyl blends of diorganopolysiloxane gums to produce 
nonfluoronated heat curable silicone rubber compositions are disclosed in 
Bobear U.S. Pat. No. 3,660,345. However the present fluorosilicone 
compositions were not known prior to the present time. 
It is one object of the present invention to provide for a high strength 
fluoro substituted heat curable silicone rubber composition. 
It is another object of the present invention to provide for a high 
strength heat curable silicone rubber composition which is produced by 
blending diorganopolysiloxane gums of different vinyl-on-chain 
concentrations. 
It is yet an additional object of the present invention to provide a 
process for producing a vinyl terminated high organo vinylsiloxy fluoro 
substituted copolymer of a viscosity varying from 1,000,000 to 300,000,000 
centipoise at 25.degree. C. 
It it still an additional object of the present invention to provide a 
process for producing a high strength fluoro substituted heat curable 
silicone rubber composition. 
It is a further object of the present invention to provide a gas mask in 
which the frame of the mask and the straps are formed from a high strength 
fluoro substituted heat curable silicone rubber composition which is 
impervious to gases. These and other objects 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 high strength heat curable silicone rubber composition 
comprising (A) 100 parts by weight of a vinyl terminated 
diorganopolysiloxane base polymer having a viscosity varying from 
1,000,000 to 300,000,000 centipoise at 25.degree. C. where the organo 
groups are selected from a mixture fluoroalkyl groups and monovalent 
hydrocarbon radicals wherein the aliphatic type unsaturation is 0.7 weight 
percent or less; (B) from 5 to 80 parts by weight of a vinyl terminated 
diorganopolysiloxane additive polymer having a viscosity varying from 
1,000,000 to 300,000,000 centipoise at 25.degree. C. where the organo 
groups are a mixture of fluoroalkyl radicals and monovalent hydrocarbon 
radicals wherein the vinyl-on-chain content of the additive polymer varies 
from 0.05 l to 10 mole percent of organo vinylsiloxy units which were 
derived through copolymerization with methylvinylsiloxy cyclic trimer; and 
(C) from 0.1 to 10 parts by weight of a curing catalyst. Preferably in the 
present composition, there is also incorporated from 5 to 200 parts by 
weight of a filler which is most preferably a silica filler selected from 
fumed silica or precipitated silica. 
There may also be present in the composition from 2 to 30 parts by weight 
of a process aid which is preferably a non-fluorinated process aid. In 
addition to these ingredients, there may self-bonding additives and other 
additives commonly associated with heat curable silicone rubber 
compositions. The basis of the present invention lies in the blending of 
two vinyl terminated fluorosilicone substituted polymers and copolymers in 
which in the base polymer the aliphatic type unsaturation does not exceed 
0.7 weight percent and in which the additive polymer, the vinyl-on-chain 
unsaturation varies from 0.05 to 10 mole percent of organo vinylsiloxy 
units in the molecule. 
It should be noted that the polymer can be a homopolymer in which there is 
50 mole percent of 3,3,3 trifluoropropyl substituted groups in the polymer 
or it can be a copolymer so that the trifluoropropyl substituted group 
concentration varies from 5 to 40 mole percent. It should be noted that 
there may be some silanol groups in either polymer or in both polymers, 
although such silanol groups should be preferably be kept below 0.1 weight 
percent. Such silanol groups may be present as a result of the incomplete 
line of the cyclopolysiloxane that was utilized in and the reaction to 
produce the gum. However, preferably, there are no silanol groups in the 
cyclopolysiloxanes which are utilized. 
In addition to the above, there is within the scope of the present 
invention, a process for producing a fluoropropyl substituted vinyl 
terminated polysiloxane gum which is a copolymer which has a concentration 
of 0.05 to 10 mole percent of organo vinylsiloxy units in the polymer. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
The basic polymer is a heat curable silicone rubber composition and 
comprises a vinyl terminated diorganopolysiloxane base polymer having a 
viscosity varying from 1,000,000 to 300,000,000 centipoise at 25.degree. 
C. and more preferably having a viscosity varying from 15,000,000 to 
300,000,000 centipoise at 25.degree. C. where the organo groups are 
selected from a mixture of fluoroalkyl groups and monovalent hydrocarbon 
radicals where the aliphatic unsaturation is 3,3-difluor-2-propenyl 
unsaturation on the polymer and generally does not exceed 0.70 weight 
percent and more preferably does not exceed 0.40 weight percent. The 
organo groups other than the fluoroalkyl groups can be any monovalent 
hydrocarbon radicals such as alkyl radicals of 1 to 8 carbon atoms, 
cycloalkyl radicals, such as cyclohexyl, cycloheptyl, mononuclear aryl 
radicals, such as phenyl, methylphenyl etc. and alkenyl radicals such as 
vinyl. However as noted before, the amount of unsaturation in the polymer 
chain cannot exceed 0.7 weight percent and preferably does not exceed 0.4 
weight percent. Preferably there is no vinylmethylsiloxy units present. 
However due to the method for preparation of the polymer, there is present 
some aliphatic unsaturation such as 3,3-difluor-2-propenyl unsaturation 
which is preferably kept at the levels indicated above. Preferably, the 
organo groups, other than fluoroalkyl, are selected from methyl and phenyl 
and more preferably are methyl. In addition preferably the fluoroalkyl 
group is 3,3,3 trifluoropropyl. 
The base polymer can either be a homopolymer that is composed of 100 mole 
percent or close to 100 mole percent of orgao or alkyl trifluoropropyl 
siloxy units with the exception of the terminal groups or it can be a 
copolymer where there is present from 10 to 80 mole percent and more 
preferably from 20 to 40 mole percent of alkyl such as methyl, 
trifluoropropyl siloxy units in the polymer. 
Preferably the base polymer has the formula, 
##STR1## 
wherein Vi is vinyl, R.sup.2 is fluoroalkyl radical of 3 to 8 carbon atoms 
and more preferably 3,3,3 trifluoropropyl, R.sup.1 and R.sup.3 are 
monovalent hydrocarbon radicals such as alkyl radicals, phenyl radicals 
and cycloalkyl radicals of 1 to 8 carbon atoms and are most preferably 
methyl. The radical R.sup.4 is is selected from monovalent hydrocarbon 
radicals and fluoralkyl radicals and s and t vary such that the base 
polymer has a viscosity varying from 15,000,000 to 300,000,000 centipoise 
at 25.degree. C. and wherein the unsaturated dihalogenated hydrocarbon 
group-on-chain does not exceed 0.7 weight percent and preferably does not 
exceed 0.4 weight percent. Most preferably R.sup.2 is trifluoropropyl and 
R.sup.1, R.sup.3 are alkyl radicals of 1 to 8 carbon atoms or phenyl. In 
case the base compound of formula 1 is a copolymer then the alkyl 
trifluoropropyl siloxy units varies from 10 to 80 mole percent in the 
polymer. Most preferably it varies from 20 to 40 mole percent. Also within 
the scope of the present invention is when R.sup.2 is 3,3,3 
trifluoropropyl whereupon the polymer of Formula (1) is a homopolymer. It 
should be noted that the polymer of Formula (1) whether a copolymer or 
homopolymer may contain less than 0.01 weight percent of silanol groups. 
These silanol groups result in the polymer as a result of water or trace 
amounts of silanol type chain-stopping impurities being present in the 
cyclotrisiloxane that is utilized to produce the polymer. 
It should be noted that preferably there is no water at all in the 
reactants such that the polymer of Formula (1) does not have any silanol. 
However this may be difficult to carry out. 
A small amount of water or silanol type chain-stopping impurities will not 
affect the properties of a final composition. The polymer of Formula (1) 
is obtained by reacting cyclotrisiloxane containing alkyl and 
trifluoropropyl substituent groups in the presence of small amounts of an 
alkali metal hydroxide catalyst and preferably a sodium hydroxide catalyst 
in the presence of certain vinyl terminated chain-stoppers. 
Preferably the cyclotrisiloxane does not contain a aliphatic type 
unsaturation concentration that exceeds 0.7 weight percent in the total 
cyclotrisiloxane reactant. The chainstopper can be any of two types and is 
more preferably, a chainstopper as defined in Evans Patent Application, 
Ser. No. 071,152 abandoned which is hereby incorporated by reference. The 
chainstopper is formed in accordance with the process of the foregoing 
Patent Application and it comprises forming a low molecular weight vinyl 
terminated polymer which in one case has 30 to 40 repeating dimethylsiloxy 
units and 20 to 30 repeating methyl 3,3,3 trifluoropropyl siloxy repeating 
units. In the other situation, the chainstopper can be a chainstopper 
containing vinyl terminal units with about 20 repeating dimethylsiloxy 
units. It should be noted that in place of methyl, there can be any other 
alkyl radical of 1 to 8 carbon atoms and in place of 3,3,3 trifluoropropyl 
there can be any fluoroalkyl radical of 3 to 8 carbon atoms. 
With these chainstoppers in the appropriate amount, the cyclotrisiloxane 
and the sodium hydroxide catalyst is heated at a temperature of at least 
140.degree. and more preferably at a temperature of 140.degree. to 
180.degree. C. for 30 minutes to 1 hour to produce the hompolymer. If it 
is desired to produce a copolymer, then the alkyl trifluoropropyl 
cyclotrisiloxane is reacted in the appropriate amounts with a 
1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetraiclooxane or more 
preferably 1,3-trimethyl-1,3,5-trivinylclotrisiloxane in the presence of 
the same amounts of alkali metal hydroxide catalyst such as sodium 
hydroxide at the same temperature range for 30 minutes to 6 hours. In the 
reaction with only the cyclotrisiloxane, the cyclotrisiloxane reacts 
quickly to form the diorganopolysiloxane gum in a period of time varying 
from 30 minutes to 60 minutes with the maximum amount of the desired 
polymer being formed in about 30 minutes or slightly over 30 minutes. With 
respect to the case where a copolymer is formed having more than 10 weight 
percent of another cyclo tri or tetrasiloxane, it is necessary to 
equilibrate them into the reaction mixture with the cyclotrisiloxane so as 
to produce a copolymer and this results in the reaction being a 
equilibration reaction taking place in 4 to 6 hours where about 20 percent 
of the cyclotrisiloxane and the cyclotetrasiloxane are being converted to 
the linear polymer at maximum conversion. The rest of the cyclics, that is 
8-20 percent or so of cyclics, remain a cyclic and are stripped off to be 
reused or disposed of as is desired. After the equilibration reaction is 
over in the case of the copolymer or the polymerization reaction in case 
of the cyclotrisiloxane by itself, the polymer mixture is neutralized with 
a silyl phosphate so as the neutralize the alkyl metal hydroxide catalyst, 
then the cyclics are stripped off and the polymer filtered to remove salts 
so as to yield the desired polymer which as stated previously may be 
homopolymer or a copolymer. 
It should be noted that as a result of the process by which it is made, 
that is by utilizing cyclotrisiloxane that does not exceed 0.7 weight 
percent of aliphatic unsaturation such as alkenyl such as vinyl or 
3,3-difluor-2-propenyl type aliphatic unsaturation, there results a 
fluorosilicone base polymer which can be either homopolymer or copolymer 
in which the aliphatic type unsaturation does not exceed 0.7 weight 
percent and more preferably does not exceed 0.4 weight percent. 
It should be noted that the aliphatic unsaturation is controlled in the 
cyclotrisiloxane by using a cyclotrisiloxane that had been produced that 
contains an aliphatic unsaturation that does not exceed the above level 
and also by controlling the preparation of the cyclotrisiloxane such that 
it does not contain aliphatic unsaturation or vinyl unsaturation beyond 
the level indicated above. The manner in which this is done is explained 
in the copending docket of Evans Ser. No. 253,282 filed Apr. 9, 1981 which 
had been referred to previously and which is hereby incorporated by 
reference. In addition more details as to the preparation of such vinyl 
terminated base polymers which have low aliphatic unsaturation content in 
accordance with the above limits are disclosed in the foregoing Evans 
Docket which is hereby incorporated by reference. 
Now proceeding to the second vinyl terminated polymer, per 100 parts by 
weight of the base vinyl terminated diorganopolysiloxane polymer there is 
utilized from 5 to 80 parts by weight of a vinyl terminated 
diorganopolysiloxane additive polymer having a viscosity varying from 
1,000,000 to 300,000,000 at 25.degree. C. where the organo groups are a 
mixture of fluoroalkyl radicals and monovalent hydrocarbon radicals where 
the vinyl-on-chain content of the additive polymer varies from 0.05 to 10 
mole percent of organo vinylsiloxy units. The polymer, that is the 
additive polymer; preferably has a viscosity varying from 15,000,000 to 
300,000,000 centipoise at 25.degree. C. The fluoroalkyl groups which is 
preferably 3,3,3 trifluoropropyl, can be any fluoroalkyl group of 3 to 8 
carbon atoms. 
The other organo groups can be any organo groups such as alkyl radicals of 
1 to 8 carbon atoms, cycloalkyl radicals of up to 8 carbon atoms, 
mononuclear aryl radicals, such as phenyl, methylphenyl etc. and alkenyl 
radicals such as vinyl. As a matter of fact, there has to be vinyl present 
not only in the vinyl terminating positions of the polymer chain but also 
vinyl-on-chain. Specifically, 0.05 to 10 mole percent of diorganosiloxy 
units have to be vinyl organo units where the organo group can be any of 
the groups mentioned previously. The organo groups are preferably selected 
from alkyl radicals of 1 to 8 carbon atoms and phenyl and are most 
preferably, methyl. In the additive polymers or copolymers the organo 
vinyl groups in the vinyl-on-chain content of the additive polymer 
preferably varies from 0.3 to 10 mole percent. Preferably the additive 
polymer has the formula, 
##STR2## 
where in Formula (2), Vi is vinyl, R.sup.5 is a monovalent hydrocarbon 
radical and is preferably an alkyl radical or methyl, R.sup.6 is a 
fluoroalkyl radical of 3 to 8 carbon atoms and preferably 3,3,3 
trifluoropropyl, R.sup.7 is selected from the class consisting of alkyl 
radicals and phenyl and is preferably methyl and p and q vary such that 
the viscosity of the polymer varies from 15,000,000 to 300,000,000 
centipoise at 25.degree. C. and the mole percent of R.sup.7, Vi SiO units 
varies from 0.05 to 10 mole percent in the polymer. More preferably the 
concentration of p and q vary such that the units taking q times varies 
from 0.3 to 7 mole percent in the polymer. 
It is necessary to have this vinyl copolymer in the compositions of the 
instant case to get the high strength properties as disclosed previously. 
Such a polymer is produced by reacting a cylcotrisiloxane of the formula, 
EQU (R.sup.10 R.sup.f SiO).sub.3 
wherein R.sup.f is a fluoroalkyl radical of 3 to 8 carbon atoms and most 
preferably 3,3,3 trifluoropropyl and R.sup.10 is a radical selected from 
the class consisting of alkyl radicals, vinyl and phenyl and is preferably 
alkyl radicals of 1 to 8 carbon atoms, with a cyclotrisiloxane of the 
formula, 
EQU (ViR.sup.11 SiO).sub.3 
where Vi is vinyl, R.sup.11 is a radical selected from the class of alkyl 
and phenyl radicals and is preferably an alkyl radical of 1 to 8 carbon 
atoms such as methyl, with a vinyl chainstopper in the presence of alkali 
metal hydroxide catalyst. 
Preferably, the vinyl chainstopper is a vinyl terminated low molecular 
weight polymer which is prepared and is disclosed in the foregoing Evans, 
Ser. No. 071,152 which is hereby incorporated by reference. Preferably the 
vinyl terminated chainstopper is a copolymer of dialkylsiloxy units and 
alkyl trifluoropropylsiloxy units and it is most preferably a vinyl 
terminated low molecular weight polymer having about 30 to 40 
dimethylsiloxy units and 20 to 30 methyltrifluoropropylsiloxy units. 
Preferably sufficient amounts of this chainstopper are utilized to produce 
the desired polymer with the desired viscosity. The more chainstopper that 
there is utilized, the lower the viscosity of the final polymer or gum. 
The less amount of chainstopper that is utilized, the higher the molecular 
weight and viscosity of the resulting polymer and gum. Preferably there is 
utilized from 4,000 to 50,000 parts per million of chainstopper. 
##STR3## 
Other vinyl terminated low molecular weight fluorosilicone copolymers may 
be utilized as chainstoppers. However, the preferred one is the one 
disclosed above. The preferred alkali metal hydroxide catalyst is sodium 
hydroxide and is desirably present at a concentration of anywhere from 5 
to 200 parts per million and more preferably at a concentration of 
anywhere from 5 to 50 parts per million. Preferably there is no water or 
silanol present in the reaction mixture. However, if the cyclotrisiloxanes 
are not dried completely, there may be tolerated less than 0.1 weight 
percent of water or silanol groups in the reaction mixture. However, 
silanol content in the reaction mixture should not exceed this level, 
otherwise the properties of the final polymer and the cured elastomer are 
affected. More preferably there is no silanol groups or water in the 
reaction mixture. However, if there is less than 0.1 weight percent of 
silanol groups or water in the reaction mixture, then this will not 
markedly affect the final properties of the heat curable silicone rubber 
composition. The time of the reaction which is not an equilibration 
reaction but a non-equilibration polymerization reaction, may be anywhere 
from 30 minutes to 2 hours and is most preferably from 30 minutes to 60 
minutes wherein the reaction temperature varies from 140.degree. to 
180.degree. C. 
Accordingly after 30 minutes or so when there is about 97 percent 
conversion of the cyclics to the polymer, the catalyst is neutralized 
preferably with a silyl phosphate and the remaining cyclics are stripped 
off so that the final polymer has less than 1.0 percent by weight of 
voltiles in it. The resulting polymer is then ready for utilization or 
blending with the base vinyl terminated polymer to produce a high strength 
heat curable silicone rubber composition. 
Per 100 parts of the base polymer, there may be present from 0.1 to 10 
parts by weight of a curing catalyst. Preferably, the curing catalyst is 
an organic peroxide. 
The curing of the silicone rubber composition of the present invention can 
be effected by chemical vulcanizing agents or by high energy electron 
radiation. More often, chemical vulcanizing agents are employed for the 
curing operation and any of the conventional curing agents can be 
employed. The preferred curing agents are organic peroxides conventionally 
used to cure silicone elastomers. Especially suitable are the dimethyl 
peroxides which may have the structural formulas, 
##STR4## 
wherein R represents the same alkyl group throughout or alkyl groups of 
two or more different types and n is zero or a larger integer. 
Amount the specific peroxide curing catalysts that are preferred 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 suitable peroxide catalysts which effect curing through saturated as 
well as unsaturated hydrocarbon groups on the silicone chain are aryl 
peroxides which include benzoyl peroxides, mixed alkyl-aryl peroxides 
which include tertiary-butyl perbenzoate, chloralkyl peroxides such as 
1,4-dichlorobenzoyl peroxide; 2,4-dichlorobenzoyl peroxide, 
monochlorobenzoyl peroxide, benzoyl peroxide, etc. Generally 0.1-10 parts 
by weight of said peroxide per 100 parts of polydiorganosiloxane gum is 
used to cure the silicone rubber composition and referably 0.5-3.0 parts 
by weight. Of the above curing catalyst, t-butyl perbenzoate, is 
preferred. Any of the foregoing peroxides may be utilized in the present 
invention but the following are preferred: 
2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 
2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3, dicumpyperoxide, 
bis(2,5-diaklorobezoyl)peroxide, and alpha, alpha 
bis(t-butylperoxy)-diisopropylbenzene. 
In addition to the curing peroxide, there may be per 100 parts of the base 
polymer, from 5 to 200 parts by weight of a filler. The filler is 
preferably selected from a class consisting of fumed silica and 
percipitated silica and mixtures thereof and is more preferably fumed 
silica. Most preferably the concentration of silica filler is from 5 to 
100 parts by weight. Extending fillers can be employed alone or in 
combination with the preferred fillers with good results. Such filler 
material are extending fillers, such as titanium oxide, iron oxide, 
aluminum oxide as well as the inorganic filler materials known as inert 
fillers which can include along others diatomaceous earth, calcium 
carbonate and quartz. It can be preferably be employed in combination with 
a highly reinforcing silica fillers to improve the tensile strength or the 
hardness of the elastomer product. Other examples of suitable fillers are 
diatomaceous silica, alunimum silicate, zinc oxide, zirconium silicate, 
barium sulfate, zinc sulfide, aluminum silicane and finely divided silica 
having surface-bonded alkoxy groups. 
In the silica filler, there is preferably treated with cyclopolysiloxanes 
as disclosed in Lucas U.S. Pat. No. 2,938,009 or with silizanes as 
disclosed in Smith U.S. Pat. No. 3,635,743 hereby incorporated by 
reference. Most preferably there is utilized a fluorosiloxane to treat the 
filler as disclosed in Matsomato Ser. No. 195,579 which is hereby 
incorporated by reference. In addition to the filler treatment, there may 
be utilized process aids in the composition. Such process aids reducing 
the tendency of the composition to structure. Accordingly, there may be 
utilized from 2 to 30 parts by weight of a process aid. Preferably the 
process aid is a non-halogenated compound and preferably non-fluorondated 
compound which is a diorganopolysiloxane having a viscosity varing from 
100,000 to 10,000,000 at 25.degree. C. where the organo groups are 
selected from alkyl radicals, vinyl radicals, phenyl radicals and mixtures 
thereof and in which the vinyl concentration varies from 5 to 25 weight 
percent. This is the most preferred process aid. 
Another process aid that can be utilized at the same concentration is a 
non-halogenated polysiloxane compound of R.sub.2 SiO and RSiO.sub.1.5 
units and having a viscosity in the range of 5 to 10,000 centipoise at 
25.degree. C. and a silanol content varying from 0.1 to 5 percent by 
weight where R is selected from the class consisting of alkyl radicals of 
1 to 3 carbon atoms, phenyl, vinyl and mixtures thereof and more 
preferably being selected from methyl phenyl and vinyl radicals. Examples 
of this process aid and more details as to the production and use is to 
found in the Patent of Simpson U.S. Pat. No. 4,089,833 which is hereby 
incorporated by reference. 
Other additives that may be incorporated into the composition are 0.25 to 
10 parts by weight of a self-bonding additive. Accordingly the 
self-bonding additive may be selected from the class consisting of silyl 
maleates, silyl fumerates, silyl phthalates, silyl isocyanurates, silyl 
cyanurates and mixtures thereof and other suitable self-bonding additives. 
Disclosure of such self-bonding additives is to be found in De Zuba et al. 
U.S. Pat. No. 3,730,932 which is incorporated by reference. The 
isocyanurate self-bonding additives are disclosed in U.S. Pat. No. 
3,773,819 which is hereby incorporated by reference. There may be added 
other additives to the composition such as flame retardant additives, heat 
compression additives, heat aging additives etc. 
The heat curable composition is prepared by mixing the two vinyl terminated 
polymers and then adding preferably a filler treated or not treated as 
desired with the necessary amount of process aids. The resulting mixture 
can be mixed either in a dough mixer or in a Banbury, stripping off the 
voltiles that are given off as the mixture is formed to homogenous 
mixture. The mixing time can vary anywhere from 5 to 24 or more hours 
especially in a dough mixer. When the composition is formed into a 
homogenous mass, then it is strained to remove dirt and cut up into the 
necessary sections. 
It may then be taken and there may be incorporated into it the necessary 
amount of curing catalyst or it may be sold without the curing catalyst. 
The fabricator may take the composition and mill it so as to plasticize it 
and then add to it the necessary amount of the curing catalyst. At any 
rate after the curing catalyst has been added to the composition and the 
composition formed to the desired slope, then desirably the composition is 
heated at temperatures above 100.degree. C. to cure it after a period of 
time varying from 30 minutes to 2 hours or more to a heat curable silicone 
rubber elastomer. It should be noted that there may be utilized a post 
bake cycle if necessary. There may be utilized varies types such as hot 
air vulcanization. 
It is not meant to delete or not include within the present use 
compositions which do not include curing catalyst therein but the 
composition is claimed with a curing catalyst since the composition can 
only be formed to silicone elastomer with a curing catalyst in it and upon 
being heated at temperatures preferably above 100.degree. C. However, the 
composition can be sold without a curing catalyst in it and a fabricator 
can incorporate the curing catalyst and the composition. 
In addition the filler does not necessarily have to be incorporated into 
the composition to form a silicone elastomer. However filler and 
preferably silica filler is incorporated in the composition to give it the 
high strength properties that are desired. In addition even beyond the 
addition of the filler there results in the present composition the 
additional enhanced physical properties as a result of the blend of vinyl 
polymers. 
It should be noted that also too much filler cannot be incorporated into 
the composition otherwise too much structuring takes place as one result 
and the composition starts to crumble when too much filler is incorporated 
into it is another result.

The examples below are given for the purpose of illustrating the present 
invention. They are not given for any purpose of setting limits and 
boundaries to the present invention. All parts in the examples are by 
weight. 
EXAMPLE 1 
A vinyl terminated polymer which contains less than 0.73 mole percent 
3,3-difluoro-2-propenyl unsaturation and composed of 
methyltrifluoropropylsiloxy units is considered to be base or polymer A. 
Said homopolymer has a viscosity of 180 million centipoise at 25.degree. 
C. 
The additive vinyl polymer which is also vinyl terminated and had 0.3 mole 
percent methyl-vinylsiloxy units derived from the copolymerization 
involving 1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane, had a viscosity 
of 250 million centipoise at 25.degree. C. This polymer will be referred 
to as B. 
The base polymer A and the additive polymer B were blended to provide a 
series of blends wherein the overall vinyl levels varied from 0.06 to 0.24 
mole percent vinyl on chain. The blends were then compounded; there was 
added 23 parts of fumed silica filler which was treated with 
1,1,3,3,5,5,7,7-octamethylcyclotetrasiloxane, process aids, and 1.5 parts 
of a curing catalyst which was 2,5-dimethyl-2,5-di(t-butylperoxy)hexane. 
The compounds were cut into sheets and heated to a temperature of 
177.degree. C. for 15 minutes then post baked for 4 hours at 205.degree. 
C. to provide cured silicone elastomers. The following process aids were 
utilized: 
5-7 parts of a silanol terminated dimethylpolysiloxane of a viscosity 
varying from 50-60 centipoise at 25.degree. C. and a silanol content of at 
least 6 percent by weight; 
3-5 parts of vinyl-on-chain dimethyl polysiloxane gum of a viscosity of 
5,000,000 centipoise at 25.degree. C. and 13.5 mole percent of methyl, 
vinyl siloxy units. 
There was also present 0.25-0.5 parts of rare earth octoate as a heat 
stabilizer. 
The physical properties of these cured compositions are as follows: 
__________________________________________________________________________ 
COMPOUNDS C D E F G CONTROL.sup.1 
__________________________________________________________________________ 
Parts Polymer A 
100 80 50 20 0 -- 
Parts Polymer B 
0 20 50 80 100 -- 
Mole percent voc* 
0.0 0.06 0.15 0.24 0.30 0.50 
Hardnes Shore A 
38 40 41 41 44 44 
Tensile, psi 
1440 1340 1160 1190 1070 880 
Elongation, % 
750 650 550 480 400 430 
Tear,Die B,ppi.sup.2 
147 110 105 70 60 60 
Comp. Set, 
23 19 18 17 15 26 
22 hr/749.degree. C..sup.3 
Vol. Swell in Ethyl 
Acetate, % 
288 273 252 237 229 235 
__________________________________________________________________________ 
*VOC refers to vinylon chain 
.sup.1 Control polymer, silanol terminated polymer which contains 0.5 mol 
percent vinylon-chain and has a viscosity of 260 million centipoise at 
25.degree. C. 
.sup.2 Tear resistance (lbs/in), Die "B", ASTM method D624. 
.sup.3 Compression set (%), method B, ASTM method D395. 
The material made from the compositions of the present invention had a 
marked increase in resistence to compression set, they were better by 
30-50 percent from the known composition. The tensile strength improved by 
as much as 20-50 percent. The resistence to tear improved by as much as 75 
percent, and the elongation improved by as much as 10-50 percent. Other 
examples which illustrate this improvement are as follows: 
EXAMPLE 2 
A vinyl terminated polymer similar to Polymer A in Example 1 but with a 
viscosity of 260 million centipoise at 25.degree. C. called Polymer H; and 
a polymer similar to B in said Example 1 but with a viscosity of 260 
million centipoise at 25.degree. C. called Polymer I were blended 
together. The blend and the control referred to as Composition J were 
compounded for maximum tear resistance and the properties measured. The 
compounds were catalyzed with 1.5 parts of 2,5-dimethyl-2,5-di 
(t-butylperoxy) hexane, cut into sheets and heated to a temperature of 
177.degree. C. for 10 minutes then post baked for 4 hours at 205.degree. 
C. to provide cured silicone elastomers. The physical properties of the 
cured compositions are as follows: 
______________________________________ 
FORMULATION 
CONTROL 
______________________________________ 
Polymer.sup.1 -- 100 
Polymer H 80 -- 
Polymer I 20 -- 
Solid Process Aid.sup.2 
4 5 
Liquid Process Aid.sup.3 
3 4 
Fumed Silica 9 12 
Fumed Silica.sup.4 
21 14 
______________________________________ 
.sup.1 Same polymer is in Control of Example 1. 
.sup.2 High vinylon-chain gum process aid of Example 1. 
.sup.3 Silanol terminated dimethylpolysiloxane process aid defined in 
Example 1. 
.sup.4 Fumed silica filler treated with 
1,1,3,3,5,5,7,7Octamethylcyclotetrasiloxane 
______________________________________ 
Compound J CONTROL 
______________________________________ 
Mole percent VOC 0.3 0.5 
Hardness Shore A 55 59 
Tensile, psi 1360 1180 
Elongation, % 550 390 
Tear, Die B, ppi 215 128 
______________________________________