Mercapto group-containing silicone rubber compositions essentially consisting of a mercapto group-containing organopolysiloxane having a viscosity exceeding 100,000 cSt and represented by the average unit formula EQU R.sub.a SiO.sub.(4-a/2) in which R is a monovalent hydrocarbon group substantially free from aliphatic unsaturation, at least two of the R groups in a molecule being mercaptoalkyl groups and a is between 1.98 and 2.05, a certain filler and sulfur, a metal oxide or a metal peroxide as a curing agent. Products from the composition have excellent chemical and mechanical properties and can be bonded to an organic synthetic rubber or natural rubber as well as metals in completely cured state.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to novel mercapto group-containing silicone rubber 
compositions. 
2. Description of the Prior Art 
For purposes of the crosslinking and curing of silicone rubbers, there have 
been proposed a method in which an organic peroxide is incorporated as a 
curing agent in a diorganopolysiloxane, the main raw material, and a 
method in which a platinum compound as a catalyst for addition reaction 
and an organohydrogenpolysiloxane as a crosslinking agent are incorporated 
in a vinyl group-containing, high-molecular-weight diorganopolysiloxane. 
According to these conventional methods, however, it is difficult to 
satisfactorily cure the silicone composition in which a strong reducing 
agent is included. Further, according to the conventional methods, the 
curing of the silicone rubber is insufficient at the areas where the 
silicone rubber is contacted with a substance having reducing activities 
or a material containing a reducing agent, or with an organic synthetic 
rubber of the sulfur vulcanization type, since the synthetic rubber 
usually contains a sulfur compound, antioxidant, or aging-retarder. 
Further, the carbon black as a most widely used filler in the organic 
rubbers may affect the curing of the silicone rubber very badly when the 
latter is in contact with the former. 
With recent development of silicone rubbers, it has become required to 
prepare laminations of a silicone rubber and an organic synthetic rubber 
by simultaneous vulcanization and further, for the purpose, to develop new 
crosslinking and curing methods in a convenient and effective manner. 
On the other hand, when the silicone rubber composition to be cured 
contains unsaturated groups, for example, vinyl groups, of the 
organopolysiloxane in an amount not lower than 10 mole % based on the 
total organic groups (for example, the amount is in general between 0.1 
and 1 mole %), sulfur-vulcanization is possible. However, the resultant 
sulfur-vulcanized silicone rubbers having a substantial content of vinyl 
groups tend to have very poor physical properties, particularly mechanical 
strengths, and those products can hardly be put to practical use. 
SUMMARY OF THE INVENTION 
This invention is based, in part, on the discovery that when a 
mercaptoalkyl group-containing organopolysiloxane is the silicone rubber 
base, the silicone rubber composition can be cured not only by a 
conventional method for ordinary silicone rubbers but also by a method 
using sulfur, a metal oxide or a metal peroxide as a curing agent, and 
further that the resultant cured silicone rubbers have physical 
properties, particularly mechanical strengths, comparable or rather 
superior to those of conventional cured silicone rubbers. 
This invention provides a mercapto group-containing silicone rubber 
composition consisting essentially of 
(a) 100 parts by weight of a mercapto group-containing organopolysiloxane 
having a viscosity of at least 100,000 cSt measured at 25.degree. C., said 
mercapto group-containing organopolysiloxane being represented by the 
following average unit formula 
EQU R.sub.a SiO.sub.(4-a/2) (I) 
wherein R stands for a substituted or unsubstituted monovalent hydrocarbon 
group free from aliphatic unsaturation, at least two of groups R in a 
molecule being mercaptoalkyl groups bonded directly to the silicon atoms, 
and a is a number in the range from 1.98 to 2.05, 
(b) from 5 to 200 parts by weight of a filler having a specific surface 
area of at least 50 m.sup.2 /g, and 
(c) from 0.1 to 10 parts by weight of a curing agent selected from the 
group consisting of sulfur, metal oxides and metal peroxides. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The organopolysiloxane as component (a) useful in the composition of this 
invention is represented by the above average unit formula (I), and in 
this formula, R are unsubstituted monovalent hydrocarbon groups, such as 
methyl, ethyl, propyl, and phenyl groups or the corresponding substituted 
groups in which some of the hydrogen atoms bonded to the carbon atoms in 
the hydrogen groups are substituted by halogen atoms, cyano groups, or the 
like. Further according to this invention, it is essential that at least 
two of the R groups in a molecule are mercaptoalkyl groups bonded directly 
to the silicon atoms. If the organopolysiloxane molecule is free of 
mercaptoalkyl groups or contains only one mercaptoalkyl group, no good 
curing can be attained, when contacted with a strong reducing agent or a 
material containing it. 
The content of the mercaptoalkyl groups in the organopolysiloxane as 
component (a) is not particularly critical, but it is preferred that it is 
from about 0.1 to about 3 mole %, preferably from about 0.1 to about 0.7 
mole %, based on the total organic groups bonded to the silicon atoms in 
the organopolysiloxane. The position where the mercaptoalkyl groups are 
bonded is not particularly critical, and may be on silicon atoms in the 
midway of polymer chains or on terminal silicon atoms. 
The monovalent hydrocarbon groups represented by the symbol R in the 
mercaptoalkyl-containing organopolysiloxane must be substantially free 
from aliphatic unsaturation. In other words, the aliphatically unsaturated 
groups such as vinyl and allyl groups are undesirable. This is because 
that the presence of the aliphatically unsaturated groups as the pendant 
groups of the mercaptoalkyl-containing organopolysiloxane has an adverse 
effect on some of the mechanical properties such as compression set, 
especially at high temperatures, and the heat stability, in particular, 
with poor ultimate elongation after thermal aging of the cured elastomer 
products formulated with such an organopolysiloxane admixed with a filler 
and a curing agent. 
In accordance with the present invention, it is essential that the 
organopolysiloxane (a) has a degree of polymerization high enough to 
correspond to the viscosity of at least 100,000 cSt as measured at 
25.degree. C. If it is lower, the resultant cured product tends to have 
insufficient mechanical strengths. 
The mercapto group-containing organopolysiloxane (a) can be prepared by 
various convenient methods. For example, the organopolysiloxane can be 
easily produced by a process that involves copolymerizing a hydrolysis 
product of 3-mercaptopropylmethyldimethoxysilane obtained by the reaction 
of 3-chloropropylmethyldimethoxysilane and sodium hydrogen sulfide or a 
hydrolysis product of mercaptopropylmethyldimethoxysilane obtained by the 
addition reaction of allylmethyldimethoxysilane and hydrogen sulfide, with 
a hydrolysis product of a chlorosilane, such as dimethyldichlorosilane or 
methylphenyldichlorosilane in the presence of an acid catalyst, such as 
Fuller's earth or phosphonitrile chloride. No ordinary alkali catalysts 
can be used in this copolymerization, since they are readily reactive with 
mercapto groups. 
It is noteworthy that, although small part of the component (a) in the 
inventive composition may be replaced with an organopolysiloxane 
containing no mercaptoalkyl groups, the amount of such a 
mercaptoalkyl-free organopolysiloxane should be limited as small as 
possible because such a mercaptoalkyl-free organopolysiloxane does not 
contribute to the crosslink formation by the curing agent but it serves 
rather as a plasticizer. For example, some of the mechanical properties, 
especially compression set, are adversely influenced by the use of a blend 
of a mercaptoalkyl-containing organopolysiloxane and a substantial amount 
of a mercaptoalkyl-free organopolysiloxane as the component (a). 
The filler useful in this invention as component (b) serves to impart 
reinforcement or sufficient mechanical strengths to the silicone rubber 
products. For the purpose it is required that its specific surface area 
should be at least 50 m.sup.2 /g, preferably from 100 to 300 m.sup.2 /g, 
as measured according to the BET method. Illustrative of fillers (b) are 
silica aerogel and precipitated silica. These silica fillers may be 
treated in advance with a chlorosilane, such as trimethylchlorosilane or 
dimethyldichlorosilane, or a silazane, such as hexamethyldisilazane, 
whereby the surface silanol groups in the silica filler become blocked 
with organosilyl groups, bringing about improved dispersibility to the 
filler in the organopolysiloxane matrix. 
Illustrative further of fillers (b) of the other type are carbon black and 
any one of furnace black, channel black, acetylene black, and other carbon 
black products that may be incorporated in the silicone rubbers when 
electroconductivity is desired according to use. 
It may be added that the use of a filler having basicity is not recommended 
since the mercaptoalkyl groups are unstable to the action of any alkaline 
substances. 
Further in accordance with this invention, small amounts of aluminum oxide, 
titanium dioxide, calcium silicate, aluminum silicate, or quartz powder 
may be added to the above-named silica fillers. 
The amounts of component (b) may be determined depending on its kind and 
the intended use of the silicone rubber products. In general, the amounts 
vary from 5 to 200 parts by weight per 100 parts by weight of component 
(a). When the amount of component (b) is smaller that 5 parts by weight 
per 100 parts by weight of component (a), no sufficient mechanical 
strengths can be imparted to the silicone rubber products. On the other 
hand, when it is larger than 200 parts by weight, kneading of component 
(b) with component (a) becomes extremely difficult, resulting in the 
formation of unsatisfactory compositions. 
The curing agent as the component (c) in the inventive compositions is 
selected from sulfur, metal oxides and metal peroxides. Organic peroxides, 
which are the most conventional curing agents in silicone rubber 
formulations, are not recommended because of rather unsatisfactory bonding 
of the organic peroxide-formulated silicone rubber to an organic rubber 
containing various kinds of reducing substances and vulcanized with sulfur 
although curing of silicone rubbers per se with an organic peroxide is not 
so unsatisfactory. The metal oxides and metal peroxides suitable for use 
as a curing agent in the inventive composition are exemplified by 
PbO.sub.2, ZnO.sub.2, CaO.sub.2, MgO.sub.2, ZnO, MgO and CaO. These curing 
agents may be used singly or in the form of a mixture of two or more of 
them. The sulfur used as the curing agent in the present invention may be 
of any grade suitable for use in vulcanizing conventional organic rubbers. 
The amount of the component (c) used is from 0.1 to 10 parts by weight per 
100 parts by weight of the component (a), variable depending on its kind, 
desired curing time, and other factors. 
The composition of this invention can be obtained by uniformly mixing the 
foregoing components (a), (b) and (c). It is optional that the composition 
of the invention is admixed with various additives that are conventionally 
used in the preparation of ordinary silicone rubbers, including dispersing 
agents, e.g., hydroxy-terminated low molecular weight organopolysiloxane 
fluid; heat stability improvers, e.g., ceric oxide and iron oxide; flame 
retardants, e.g., halogen-containing organic compounds; coloring agents, 
and the like. 
The composition of the present invention may easily be converted to 
silicone rubber elastomers by heating, if necessary, under pressure. The 
schedule for the heat treatment depends largely on the kind of the curing 
agent (c), but it is approximately the same as applied for the curing of 
any ordinary silicone rubber that is formulated with an organic peroxide 
as the curing agent. 
New to and different from the ordinary unsaturated group-containing 
organopolysiloxane rubbers for which the curing agent is limited to an 
organic peroxide or an organohydrogenpolysiloxane in the presence of a 
platinum catalyst, the mercapto group-containing composition of the 
present invention can be cured also with sulfur as the curing agent to 
form adhesive bonding with an organic synthetic rubber or natural rubber 
formulated with sulfur as the vulcanizing agent by simultaneous 
vulcanization, thus rendering it possible to develop the field of 
application of the silicone rubbers. 
A carbon black-filled, electrically conductive silicone rubber can not be 
cured by an acyl peroxide, such as benzoyl peroxide or 2,4-dichlorobenzoyl 
peroxide, and it has been required to use an alkyl peroxide at a high 
temperature for the purpose. To contrast, the mercapto group-containing 
composition of this invention, even if filled with carbon black can be 
cured without any hindrance at a relatively low temperature in a very 
short time by use of sulfur on a metal oxide or peroxide as the curing 
agent. 
Furthermore, since the mercaptoalkyl groups present in the component (a) 
are highly reactive, the composition of the present invention can cure 
even in contact with natural rubber, an organic synthetic rubber, plastics 
or a metal, to form a sufficiently strong bond. 
Silicone rubber products prepared and shaped from the composition of this 
invention are used in a wide field, as wire-insulating tubes, packings, 
gaskets, boots, and the like. Further, by utilizing the adhesive bonding 
of the silicone rubber of the present invention to the surfaces of an 
organic synthetic rubber or natural rubber, there can be obtained a 
variety of silicone rubber-clad articles, such as rubber rolls having 
improved heat resistance, mold releasability, water repellency and other 
distinguished properties as are inherent in the silicone rubbers in 
general.

This invention will be further illustrated by the following examples. In 
the examples all parts are based on weight. 
EXAMPLE 1 
An organopolysiloxane mixture composed of 99.48 mole % of 
octamethylcyclotetrasiloxane, 0.5 mole % of 
1,3,5,7-tetra(mercaptopropyl)-1,3,5,7-tetramethylcyclotetrasiloxane, and 
0.02 mole % of hexamethyldisiloxane was added with 0.05% by weight of 
phosphonitrile dichloride. The resultant mixture was reacted in a nitrogen 
gas atmosphere at 40.degree. to 50.degree. C. for 10 hours to give a 
polymerizate with gum-like consistency. 
The reaction product was a mercapto group-containing organopolysiloxane 
having a relative viscosity of 2.5 at 25.degree. C. as measured with a 1% 
solution in toluene. Then, to 100 parts of this mercapto group-containing 
organopolysiloxane was added 40 parts of a trimethylsilyl-surface blocked 
silica filler having a specific surface area of 200 m.sup.2 /g (Silanox 
101 manufactured by Cabot Corp.), and uniformly mixed together on a 
two-roll mill. One of the curing agents indicated in Table I was added in 
the amount indicated. The resulting compositions were subjected to molding 
at 170.degree. C. under a pressure of 50 kg/cm.sup.2 for 30 minutes to 
form sheets 2 mm thick. Each sheet thus produced was heated at 200.degree. 
C. for 4 hours to cure completely. The cured sheets were tested for 
hardness, elongation and tensile strength in accordance with Japanese 
Industrial Standard, K 6301, with the results set out in Table I. 
Table I 
______________________________________ 
Curing Elonga- Tensile 
agent Amount Hardness tion strength 
______________________________________ 
PbO.sub.2 
5 parts 45 650% 63 kg/cm.sup.2 
Sulfur 3 parts 50 520% 70 kg/cm.sup.2 
______________________________________ 
EXAMPLE 2 
One hundred parts of the mercapto group-containing organopolysiloxane 
obtained in Example 1 was uniformly mixed on a two-roll mill with 50 parts 
of a silica filler with the surface blocked by dimethylsiloxane units and 
having a specific surface area of 100 m.sup.2 /g (Aerosil 972 manufactured 
by DEGUSSA) and 1.5 parts of .alpha.,.omega.-dihydroxydimethyl 
polysiloxane having a viscosity of 20 cSt as measured at 25.degree. C. The 
resulting composition was called "Compound A." 
Separately, another composition was prepared in the same manner as above 
except that a methylvinylpolysiloxane obtained by polymerizing a mixture 
consisting of 99.48 mole % of octamethylcyclotetrasiloxane, 0.5 mole % of 
1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane and 0.02 mole % 
of hexamethyldisiloxane in the presence of potassium hydroxide at 
150.degree. C. for 6 hours was used instead of the mercapto 
group-containing organopolysiloxane. This composition was called "Compound 
B." 
Then, 3 parts of sulfur was added to 100 parts of Compound A, while 4 parts 
of dicumyl peroxide was added to 100 parts of Compound B. From each of 
these two mixtures, a 1-mm thick sheet was formed in uncured state. 
Each uncured sheet was put on a 2-mm thick uncured silica-filled EPDM 
rubber sheet containing 1.5% by weight of sulfur, followed by heating at 
170.degree. C. under a pressure of 50 kg/cm.sup.2 for 35 minutes, to form 
a lamination. 
The sheet from Compound A was found completely cured even at the areas in 
contact with the EPDM rubber sheet, with a bonding strength of 15 kg/cm as 
determined by the peeling test. On the other hand, the sheet from Compound 
B was found insufficiently cured and remaining in sticky state at the 
areas in contact with the EPDM rubber sheet. 
EXAMPLE 3 
Three mixtures were prepared each by adding 45 parts of acetylene black 
(Denka Black manufactured by Denki Chemical Co., Ltd.) to 100 parts of the 
mercapto group-containing organopolysiloxane obtained in Example 1. To 
each mixture dicumyl peroxide, sulfur, or ZnO.sub.2 was added in amounts 
of 2, 1.5 and 4 parts, respectively. The resultant three compositions were 
called "Compounds C, D, and E," respectively. 
Separately, another composition was prepared by adding to 100 parts of the 
methylvinylpolysiloxane obtained in Example 2 and mixed with 45 parts of 
the same acetylene black, 1.5 parts of methylhydrogenpolysiloxane (KF 99 
manufactured by Shin-Etsu Chemical Co., Ltd.), and 0.1 part of a solution 
of chloroplatinic acid in isopropyl alcohol in a concentration of 2% by 
weight as platinum. The composition was called "Compound F." 
Each of Compounds C, D, E and F was spread over the surface of a 1-mm thick 
sulfur-cured natural rubber sheet, to form a layer 1 mm-thick, followed by 
heating at 165.degree. C. under a pressure of 100 kg/cm.sup.2 for 30 
minutes, to form a lamination. 
The layers from Compounds C, D and E were found cured and bonded to the 
natural rubber sheet with adhesive strengths of 3, 6 and 4 kg/cm, 
respectively, as determined by the peeling test. On the other hand, the 
layer from Compound F wholly was found uncured or semi-cured, the uncured 
or semi-cured state remaining unchanged even with continued heating under 
pressure for another 30 minutes. 
EXAMPLE 4 
A mercapto group-containing organopolysiloxane composed of 5 mole % of 
diphenylsiloxane units, 0.3 mole % of 3-mercaptopropylmethylsiloxane 
units, 94.68 mole % of dimethylsiloxane units and 0.02 mole % of 
trimethylsiloxane units was prepared in the same manner as described in 
Example 1. Then, to 100 parts of the mercapto group-containing 
organopolysiloxane were added 45 parts of a fumed silica (Tullanox 500 
manufactured by Tulco Co.) and 3 parts of sulfur, and uniformly mixed 
together on a 2-roll mill. 
The resulting composition was heated at 170.degree. C. under a pressure of 
100 kg/cm.sup.2 for 30 minutes, to form a sheet 2 mm thick. The sheet was 
then cured by heating at 150.degree. C. for 4 hours. The cured sheet was 
tested for hardness, elongation, tensile strength, tear strength, and 
elastic resilience in accordance with Japanese Industrial Standard, K 
6301. The results are shown below. 
Hardness: 45 
Elongation: 750% 
Tensile strength: 80 kg/cm.sup.2 
Tear Strength: 18 kg/cm 
Elastic resilience: 45% 
The above composition that had not been subjected to curing was spread over 
the surface of a carbon black-filled and uncured ethylene-propylene 
terpolymer rubber containing a binary curing agent of 0.3% by weight of 
sulfur and 3% by weight of dicumyl peroxide (both based on the weight of 
the terpolymer), followed by heating at 170.degree. C. under a pressure of 
50 kg/cm.sup.2 for 45 minutes, to form a lamination. As a result, the 
lamination was found completely cured into an integrated product. 
Separately an organopolysiloxane was prepared in the same manner as 
described above except that the 3-mercaptopropylmethylsiloxane units were 
substituted by methylvinylsiloxane units. Then, 100 parts of the 
organopolysiloxane was uniformly mixed with 45 parts of the same fumed 
silica as used above (Tullanox 500) and 3 parts of dicumyl peroxide (C-3 
manufactured by Shin-Etsu Chemical Co., Ltd.). The resulting mixture was 
spread over the surface of the same ethylene-propylene terpolymer rubber 
sheet, followed by a heat treatment. As a result, the combination of the 
two sheets remained uncured at the contacting areas, and the sheets were 
readily peeled from each other. 
EXAMPLE 5 
Five kinds of organopolysiloxanes with gum-like consistency composed of 
dimethylsiloxane units, mercaptopropylmethyl or methylvinylsiloxane units 
and trimethylsiloxy units were prepared in the same manner as in Example 1 
with the molar ratios of the individual siloxane or siloxy units as 
indicated in Table II (Organopolysiloxanes 5-1 to 5-5). 
Table II 
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Organopolysiloxane 
5-1 5-2 5-3 5-4 5-5 
______________________________________ 
Dimethylsiloxane units, mole % 
99.48 99.48 98.98 
99.38 
98.98 
Mercaptopropylmethylsiloxane 
0.50 -- 0.50 0.10 1.00 
units, mole % 
Methylvinylsiloxane unit, 
-- 0.50 0.50 0.50 -- 
mole % 
Trimethylsiloxy unit, mole % 
0.02 0.02 0.02 0.02 0.02 
______________________________________ 
Silicone rubber compositions were prepared by blending 100 parts each of 
the above organopolysiloxanes, 40 parts of a fumed silica with surfaces 
blocked with trimethylsilyl groups having a specific surface area of 200 
m.sup.2 /g and a curing agent as indicated in Table III below and and each 
of the compositions was fabricated into a sheet 2 mm thick and cured 
similarly to Example 1. The mechanical properties of these cured silicone 
rubber sheets are given in Table III. 
Table III 
__________________________________________________________________________ 
Organopolysiloxane 
5-1 
5-1 
5-1 
5-2.sup.a) 
5-2 
5-2.sup.(a) 
5-3 
5-4 
5-5 
__________________________________________________________________________ 
Curing agent (parts) 
Sulfur 3 -- -- 3 -- -- -- -- -- 
Dicumyl peroxide 
-- 1.5 
-- -- 1.5 
-- -- -- -- 
ZnO.sub.2 -- -- 5 -- -- 5 5 5 5 
Hardness (JIS) 
48 49 46 -- 56 -- 48 21 64 
Ultimate elongation, % 
570 
560 
590 
-- 410 
-- 510 
780 
400 
Tensile strength, kg/cm.sup.2 
70 72 66 -- 80 -- 64 20 70 
Elastic resilience, % 
50 54 47 -- 60 -- 49 20 62 
__________________________________________________________________________ 
.sup.(a) Not cured at all. 
EXAMPLE 6 
A silicone rubber composition was prepared by uniformly blending on a 
two-roll mill 100 parts of a mercapto group-containing organopolysiloxane 
with gum-like consistency composed of 99.65 mole % of dimethylsiloxane 
units and 0.35 mole % of mercaptopropylmethylsiloxane units, 50 parts of 
precipitated silica filler (Nipsil VN.sub.3, product of Nippon Silica Co., 
Japan), 5 parts of zinc oxide and 5 parts of the same 
.alpha.,.omega.-dihydroxydimethylpolysiloxane fluid as used in Example 2 
followed by admixture of 2 parts of sulfur as the curing agent. 
(Composition 6-1). 
For comparative purpose, the amount of the mercapto group-containing 
organopolysiloxane gum was decreased to 50 parts or 70 parts and, instead, 
50 parts or 30 parts of a methylvinylpolysiloxane with gum-like 
consistency composed of 99.65 mole % of dimethylsiloxane units and 0.35 
mole % was admixed with the other ingredients being admixed in the same 
amounts as above (Compositions 6-2 and 6-3). 
Each of the above compositions was fabricated into a sheet 2 mm thick by 
press-curing at 160.degree. C. for 20 minutes under a pressure of 50 
kg/cm.sup.2 followed by heating at 150.degree. C. for 2 hours to complete 
the curing. The mechanical properties of these silicone rubber sheets are 
given in Table IV below. 
Table IV 
______________________________________ 
Composition 6-1 6-2 6-3 
______________________________________ 
Hardness (JIS) 50 30 35 
Ultimate elongation, % 
370 280 310 
Tensile strength, kg/cm.sup.2 
68 37 43 
Compression set, % 
40 90 77 
______________________________________ 
The values of the compression set were determined in acordance with the 
procedure specified in JIS K 6301. 
EXAMPLE 7 
Three kinds of organopolysiloxane with gum-like consistency (7-1), (7-2) 
and (7-3) were prepared which were composed of 99.40 mole % of 
dimethylsiloxane units and 0.60 mole % of mercaptopropylmethylsiloxane 
units; 99.40 mole % of dimethylsiloxane units and 0.40 mole % of 
methylvinylsiloxane units; or 99.40 mole % of dimethylsiloxane units, 0.30 
mole % of mercaptopropylmethylsiloxane units and 0.30 mole % of 
methylvinylsiloxane units, respectively. 
One hundred parts of the organopolysiloxane (7-1), 50 parts each of the 
organopolysiloxanes (7-1) and (7-2), or 100 parts of the 
organopolysiloxane (7-3) was blended with 40 parts of a silica filler 
Tullanox 500 (see Example 4), 10 parts of an acetylene black, 1 part of 
diphenylsilane diol, 3 parts of zinc oxide (ZnO) and 2.3 parts of sulfur 
to give silicone rubber compositions, which were then shaped into rubber 
sheets 2 mm thick by press-curing at 160.degree. C. for 20 minutes under a 
pressure of 50 kg/cm.sup.2. The mechanical properties of these cured 
rubber sheets are given in Table V below. 
Further two aluminum or steel plates of 25 mm width were adhesively bonded 
with each of the above prepared silicone rubber composition with the 
length of overlapping 10 mm with application of Primer A-10 (product of 
Shin-Etsu Chemical Co.) on the plate surfaces. After curing at 160.degree. 
C. for 20 minutes under a pressure of 30 kg/cm.sup.2, the shearing 
adhesive strength of these bonded plates was examined to give the results 
as given in Table V. 
In comparison, three comparative compositions were prepared by replacing 
2.3 parts of sulfur with 3 parts each of dicumyl peroxide and similar 
tests for shearing adhesive strength were undertaken to give the results 
given in the same table. 
Table V 
______________________________________ 
(parts) (7-1) (50) 
Organopolysiloxane (parts) 
(7-1) (100) 
+(7-2) (50) 
(7-3)(100) 
______________________________________ 
Hardness (JIS) 57 35 44 
Ultimate elongation, % 
457 335 410 
Tensile strength, kg/cm.sup.2 
75 35 44 
Compression set, % 
35 76 58 
Shearing adhesion, 
kg/cm.sup.2 : 
Sulfur-cured aluminum 
23 5.7 18 
Sulfur-cured steel 
26 9.4 14 
Peroxide-cured aluminum 
19 0 12 
Peroxide-cured steel 
20 5.3 9 
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