HEAT CURABLE SILICONE COMPOSITIONS

COMPOSITIONS DE SILICONE DURCISSABLE A CHAUD

English Abstract

60SI-725,
IMPROVED HEAT CURABLE
SILICONE COMPOSITIONS
ABSTRACT OF THE DISCLOSURE
In a heat curable silicone rubber composition comprising a
polydiorganosiloxane base polymer and a curing catalyst, the
improvement comprising an effective amount of co-curing agent.
Preferably, the co-curing agent has the general formula
<IMG> or
<IMG>
where R is H or a C1-5 alkyl radical, R1 is a C1-5 alkylene
radical, each R2 is independently selected from H and CH3,
R3 is H or CH3, n equals 0 or 1, and m is an integer from 3
to 6, inclusive.

Claims

- 16 - 60SI-725
The embodiments of the invention in which an
exclusive property or privilege is claimed are defined
as follows:
1. A heat curable silicone rubber
composition, comprising:
(a) a polydiorganosiloxane base polymer having
the general formula:
<IMG>
wherein R is selected from monovalent substituted or
unsubstituted organic radicals, a varies from about 1.95
to about 2.02, inclusive, and the viscosity is at least
about l00,000 centipoise at 25°C:
(b) an effective amount of catalyst to promote
curing;
(c) from 0.2 to 0.8 parts by
weight of co-curing agent per 100 parts by weight
polydiorganosiloxane (a), said co-curing agent
comprising a silicone soluble monomer having at least
three olefinic functional groups to improve tear
strength and lower compression set: and
(d) reinforcing filler in an amount from
5 to 40 parts by weight silica per 100 parts by weight
said polydiorganosiloxane (a).
2. A composition as in claim 1 wherein the
co-curing agent is present in an amount of from
0.2 to 0.4 parts by weight per 100 parts by weight
of polydiorganosiloxane (a).
3. A composition as in claim 1 wherein the
co-curing agent has the general formula

- 17 - 60SI-725
<IMG>
where R is hydrogen or a lower alkyl radical having from
1 to 5 carbon atoms, R1 is a lower alkylene radical
having from l to 5 carbon atoms, R2 is hydrogen or a
methyl radical, R3 is hydrogen or a methyl radical, and
n equals 0 or 1.
4. A composition as in claim 3, wherein the
co-curing agent is trimethylol propane trimethacrylate.
5. A composition as in claim 1, wherein the
co-curing agent has the general formula
<IMG>
where R1 is a lower alkylene radical having from 1 to 5
carbon atoms, each R2 is independently selected from
hydrogen or methyl, and m is an integer from 3 to 6,
inclusive.
6. A composition as in claim 5, wherein the
co-curing agent is triallyl trimellitate.
7. A heat curable silicone rubber
composition, comprising:
(a) 100 parts by weight of polydiorgano-
siloxane base polymer having the general formula-
<IMG>

- 18 - 60SI-725
where R is selected from monovalent substituted or
unsubstituted organic radicals, a varies from about 1.95
to about 2.02, inclusive, and the viscosity is at least
about 100,000 centipoise at 25°C:
(b) from 0.1 to 5 parts by weight
of organic peroxide catalyst,
(c) from 5 to 40 parts by weight
of reinforcing filler, and
(d) from 0.2 to 0.8 parts by weight
of co-curing agent per 100 parts by weight of
polydiorganosiloxane (a) to improve tear strength and
lower compression set, said co-curing agent selected
from compounds having the general formula:
<IMG> or
<IMG>
where R is hydrogen or a lower alkyl radical having from
1 to 5 carbon atoms, R1 is a lower alkylene radical
having from 1 to 5 carbon atoms, each R2 is
independently selected from hydrogen or methyl, R3 is
hydrogen or methyl, n equals 0 or 1, and m is an integer
from 3 to 6, inclusive.
8. A composition as in claim 7, wherein poly-
diorganosiloxane (a) is a fluorosilicone polymer and
reinforcing filler (c) is fluorosilicone treated fumed
silica present in an amount ranging from 30 to

- 19 - 60SI-725
50 parts by weight per 100 parts by weight of
polydiorganosiloxane (a).
9. The cured composition of claim 1.
10. A method for making a heat curable
silicone rubber composition, comprising mixing:
(a) a polydiorganosiloxane base polymer having
the general formula:
<IMG>
where R is selected from monovalent substituted or
unsubstituted organic radicals, a varies from about 1.95
to about 2.02, inclusive, and the viscosity is at least
about 100,000 centipoise at 25°C;
(b) an effective amount of catalyst to promote
curing;
(c) from 0.2 to 0.8 parts by weight
of co-curing agent per 100 parts by weight
polydiorganosiloxane (a), said co-curing agent
comprising a silicone soluble monomer having at least
three olefinic functional groups to improve tear
strength and lower compression set; and
(d) reinforcing filler in an amount from
5 to 40 parts by weight silica per 100 parts by weight
said polydiorganosiloxane (a);
11. A method as in claim 10, wherein the
co-curing agent has the general formula
<IMG> or

- 20 - 60SI-725
where R is hydrogen or a lower alkyl radical having from
1 to 5 carbon atoms, R1 is a lower alkylene radical
having from 1 to 5 carbon atoms, R2 is hydrogen or a
methyl radical, R3 is hydrogen or a methyl radical, and
n equals o to 1.
12. A method as in claim 10, wherein the
co-curing agent is trimethylol propane trimethacrylate.
13. A method as in claim 10, wherein the
co-curing agent has the general formula
<IMG>
where R1 is a lower alkylene radical having from 1 to 5
carbon atoms, each R2 is independently selected from
hydrogen or methyl, and m is an integer from 3 to 6,
inclusive.
14. A method as in claim 13, wherein the
co-curing agent is triallyl trimellitate.
15. A method for making a heat curable
silicone rubber composition, comprising mixing:
(a) 100 parts by weight of polydiorgano-
siloxane base polymer having the general formula:
<IMG>
where R is selected from monovalent substituted or
unsubstituted organic radicals, a varies from about 1.95
to about 2.02, inclusive, and the viscosity is at least
about 100,000 centipoise at 25°C;
(b) from 0.1 to 5 parts by weight
of organic peroxide catalyst,

- 21 - 60SI-725
(c) from 5 to 40 parts by weight
of reinforcing filler, and
(d) from 0.2 to 0.8 parts by weight
of co-curing agent per 100 parts by weight of
polydiorqanosiloxane (a) to improve tear strength and
lower compression set, said co-curing agent selected
from compounds having the general formula:
<IMG> or
<IMG>
where R is hydrogen or a lower alkyl radical having from
1 to 5 carbon atoms, R1 is a lower alkylene radical
having from 1 to 5 carbon atoms, each R2 is
independently selected from hydrogen or methyl, R3 is
hydrogen or methyl, n equals 0 or 1, and m is an integer
from 3 to 6, inclusive.
16. A method as in claim 15, wherein
polydiorganosiloxane (a) is a fluorosilicone polymer and
reinforcing filler (c) is fluorosilicone treated fumed
silica present in an amount ranging from about 30 to
about 50 parts by weight per 100 parts by weight of
polydiorganosiloxane (a).

Description

i3~95~
60SI 725
IMPROVED HEAT CURABLE
SILICONE COMPOSITIONS- --
Background of the Invention
The present invention relates to heat curable silicone
rubber compositions. More particularly, the present invention
relates to heat curable silicone rubber compositions containing
an amount of certain co-curing agents effective to allow the
use of less reinforcing filler while obtaining improved tear
strength, suitable durometer and lower compression set.
Heat curable silicone rubber compositions comprising ta) a
polydiorganosiloxane base polymer, (b) an effective amount of
reinforcing filler, and (c) an effective amount of free radical
generating catalyst are well known in the art. It is
appreciated by those skilled in the art that the amount of
rein~orcing filler employed is directly related to many of the
physical properties of the resultant silicone rubber. For
example, if the amount of reinforcing filler in a silicone
rubber composition is increased, the durometer, tear strength
and compression set will all generally increase somewhat. It
is also recognized by those skilled in the art that if the
filler loading is too high, the rubber becomes very difficult
to process.
For most applications the artisan can obtain suitable
properties simply by adjusting the filler level. However, in
some applications, such as calendering solvent resistant

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silicone rubber to fabric, when sufficient filler is added to
obtain the necessary durometer, the green strength or bulk
mDdulus of the rubber is detrimental to calendering to fabric
since it does not form the desired intimate, void-free
compound/fabric interface. Moreover, the filler loading
necessary to obtain a suitable durometer also increases the
compression set beyond acceptable limits.
It has now been discovered that certain co-curing agents
when added to heat curable silicone rubber compositions allow
the use of less reinforcing filler while obtaining improved
tear strength, substantially unchanged durometer, and lower
compression set.
Konkle, U.S. Pat. No. 2,684,957, teaches that low
compression set heat curable silicone rubber compositions can
be made from (a) an organopolysiloxane, (b) a vulcanizing
agent, and (c) from .75 to 8 parts by weight per 100 parts by
weight of the organopolysiloxane of a cadmium compound selected
from cadmium oxide, cadmium peroxide and cadmium carbonate.
Konkle and Talcott, U.S. Pat. No. 2,927,908, relates to the
use of polytetrafluoroethylene to improve the tensile strength
and tear resistance of fluorinated organopolysiloxane rubber.
Modic~ U.S. Pat. No. 2,979,479, discloses that heat curable
silicone rubber compositions having improved tear strength can
be prepared from (a) an organopolysiloxane, (b) a finely
divided silica filler, (c) a curing agent, and (d) a copolymer
composed essentiall~ of trimethylsiloxy groups and SiO2
groups, there being present from about 1.0 to 1.5 methyl groups
per silicon atom.
-

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Grabicki et al., U.S. Pat. No. 3,027,3449 provides heat
curable silicone rubber compositions characterized by improved
tenSjle and tear strength comprising (a) an organopolysiloxane,
(b) silica filler, (c) 1 to 20 parts of organic amine, and (d)
1 to 20 parts of a hydroxy organosilane selected from
diphenylsilanediol and triphenylsilanol.
Wada et al., U.S. Pat. No. 3,652,475, teaches heat curable
silicone rubber compositions comprising (a) 100 parts by weight
of polydiorganosiloxane having z degree of polymerization of at
least 3000 and containing at most 0.3 mole percent vinyl
groups, (b) from 1 to 10 parts by weight of polydiorganosilox-
ane having a degree of polymerization of at least 3000 and
containing from 5 to 20 mole percent vinyl groups, (c) from 0.5
to 5 parts by weight of polydiorganosiloxane having a degree of
polymerization of from 10 to 1000 and containing from 5 to 90
mole percent vinyl units, (d) 20 to 200 parts by weight of
silica filler, and (e) a catalytic amount of organic peroxide;
said compositions having improved tear strength and superior
compression set.
None of the foregoing references teach or suggest that
improved tear strength, acceptable durometer -and lower
compression set can be obtained by using less reinforcing
filler, but including a co-curing agent such as trimethylol
propane trimethacrylate or triallyl trimellitate.

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6051-725/GLL:mz/01~5p
Summary of the Invention
It is an ob~ect of the present invention to provide heat
curable silicone rubber compositions which penmit the use of
less reinforcing filler while obtaining improved tear strength,
substantially unchanged durometer, and lower compression set.
It is another object ~f the present invention to provide
heat curable silicone rubber compositions suitable for
calendering to fabric.
Still another object of the present invention is to provide
a method for making the compositions of the present invention.
In accordance with one aspect of the present invention
there are provided heat curable silicone rubber compositions,
comprising:
(a) a polydiorganosiloxane base polymer;
(b) an effective amount of catalyst; and
(c) an effective amount of co-curing agent.
Description of the Invention
According to the present invention there are provided heat
curable silicone rubber compositions~ comprising:

~o~
60SI -725/GLL :mz/01 95p
(a) a polydiorganosiloxane base polymer;
~b) an effective amount of catalyst;
(c~ an effective amount of co-curing agent; and
(d) optionally, but preferably, an effective amount of
reinforcing filler.
Polydiorganosiloxane (a) can be any polymer or mixture of
polymers known by those of ordinary skill in the art to be
suitable for use in heat curable silicone rubber compositions.
Generally, such polydiorganosiloxanes have the general formula
Ra SiO4 a
where R is selected from monovalent substituted and unsubstitu-
ted organic radicals, a can vary from about 1.95 to about 2.02,
inclusive, and the viscosity is at least about 100,000 centi-
poise at 25~C.
Illustrative of suitable R radicals are aryl radicals and
halogenated aryl radicals such as phenyl, chlorophenyl, xylyl,
and tolyl; aralkyl radicals such as benzyl and phenylethyl;
aliphatic, haloaliphatic and cycloaliphatic radicals such as
allkyl, alkenyl, cycloalkyl and haloalkyl, including methyl,
ethyl, propyl, vinyl, allyl9 cyclhexyl, chloromethyl, chloro-
ethyl and 3,3,3-trifluoropropyl; and cyanoalkyl radicals such
as cyanoethyl.

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- 6 - 60SI-725
Preferably, R is selected from methyl,
vinyl, phenyl and 3,3,3-trifluoropropyl radicals. It
is also preferred that the viscosity of
polydiorganosiloxane (a) be from about 1,000,000
centipoise to 200,000,000 centipoise at 25C.
Also within the scope of
polydiorganosiloxane (a) are copolymers containing two
or more different diorganosiloxane units, such as, for
example, copolymers of dimethylsiloxane units and
methylphenylsiloxane units; copolymers of
dimethylsiloxane units, diphenylsiloxane units, and
methylphenylsiloxane units; and copolymers of
dimethylsiloxane units, methylvinylsiloxane units and
diphenylsiloxane units. There is also included within
the scope of polydiorganosiloxane (a) mixtures or
blends of polymers and/or copolymers.
If a solvent resistant heat curable silicone
rubber is to be prepared, polydiorganosiloxane (a)
should be a fluorosilicone polymer, for example, of
20 the type described in U.S. Patent No. 4,355,121,
issued October 19, 1982 to Evans, U.S. Patent
4,492,786, issued January 8, 1985 to Evans, or U.S.
Patent No. 4,529,774, issued July 16, 1985 to Evans.
Other suitable fluorosilicone polymers are well known
in the art. Such fluorosilicone compositions have
been found to be especially suitable for practicing
the present invention.
Catalyst (b) can be any of the
conventionally used means for promoting curing of the
silicone rubber and includes both chemical vulcanizing
agents and high energy radiation sources such as gamma
radiation from a cobalt-60 source. Preferably,

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60SI-725/GLL:mz/0195p
catalyst (b) is one of the organic peroxides coTmonly used in
the silicone art such as 2,4-dichlorobenzoyl peroxide, benzoyl
peroxide, di-t-butyl pero~ide, 2,5-bis(t-butylperoxy)2~-di-
methylhexane or dicumyl peroxide.
Organic peroxides for use as a catalyst for heat curable
silicone rubber are commercially available, usually mixed with
an inert carrier to form a powder or paste.
The amount of peroxide used in heat curable silicone rubber
compositions is not critical, but generally is from about 0.1
to 5 parts by weight per 100 parts by weight of uncatalyzed
silicone polymer. The peroxide catalyst is initiated by
heating the composition tn a temperature sufficient to activate
(e.g. decompose~ the organic peroxide used.
Co-curing agent (c) can be any silicone soluble (i.e.
somewhat polar) monomer having at least three olefinic
functional groups. These monomers are called co-curing agents
because they do not initiate curing, however, once curing has
begun said co-curing agents rapidly form free radicals, thereby
increasing the efficiency of the catalyst and, because they are
polyfunctional, take part in establishing a tight cure matrix.
In practicing the present invention it is preferable to
include from about 0.1 to about 3 parts by weight of co-curing
agent per 100 parts by weight of polydiorganosiloxane (a).
More preferably, cD-curing agent (c) is utilized in an amount
ranging from about 0.2 to about 0.8 parts by weight or, more
preferably, from about 0.2 to about 0.4 parts by weight per 100
parts by weight of uncatalyzed silicone polymer.

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60SI-725/6LL:mz/0195p
-8-
6enerally, the co-curing agents utili~ed in the practice of
the present invention have the general fonmula
I. C ~ R3 ) or
0 R
II. ~C - O - R - C = I R ~
where R is hydrogen or a lower alkyl radical having from 1 to 5
carbon atoms, Rl is a lower alkylene radical having from 1 to
5 carbon atoms, each R2 is independently selected from
hydrogen and methyl, R3 is hydrogen or a methyl radical, n
equals 0 or 1, and m is an integer from 3 to 6, inclusive.
Preferably, R is ethyl, R is methylene, R2 is hydrogen,
R3 is methyl, n equals 1, and m equals 3. Thus, the
preferred co-curing agents of the present invention are
trimethylol propane trimethacrylate,

~ 3 ~ 0
6051-725/GLL:mz/0195p
CH2 = C - C - O
Il ~
ICH3 l H2
CH2 = C - C - 0 - CH2 - C - CH2 CH3
I
fH3 1 2
C H 2 - C - C - O
and triallyl trimellitate,
O O
CH2 = CH - CH20 - C ~ C - OCH2 - ~H = CH2
~ ll OCH2 CH CH2 .

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60SI-725~GLL:mz/0195p
-10-
It should be noted that the olefinic functional groups of
the co-curing agent need not all be the same and that, in
formula II, it is not critical to which carbon of the benzene
ring the olefinic functional groups are bonded. Moreover, it
should be understood that the foregoing formulas are merely
illustrative of suitable co-curing agents and that other
variations will be obvious to the artisan and which are include
within the scope of the invention as well as the appended
claims.
The use of such co-curing agents in heat curable silicone
rubber compositions allows the artisan to use less reinforcing
filler than heretofore required, yet provides a cured
composition having improved tear strength, substantially
unchanged durometer, and lower compression set. Furthermore,
I5 the use of co-curing agents disclosed hereinabove may allow the
artisan to completely eliminate the need for reinforcing filler
in some applications. ~owever, it is most preferable that
reinforcing filler be included in the heat curable silico~e
rubber composition, only in a smaller amount.
Illustrative of suitable reinforcing filler (d) are
included, for example, fumed silica, treated fumed silica,
precipitated silica and treated precipitated silica. Of
course, mixtures of such reinforcing fillers are also within
the scope of the present inYention.
Preferably, the reinforcing filler is a treated silica
filler and, most preferably, is treated fumed silica.
Treatment of the filler can be effected by any method known in
the art, such as, for example, as taught in U.S. Pat. No.

~ 3~95~Q
60SI-725/GLI:mz/0195p
2,938,009 to Lucas, U.S. Pat. No. 3,635,743 to 5mith,
United States Patent Number 3,837,878 to
Beers or United States Patent Num~er 4,529,774 to
Evans et al, issued July 16, 1~85.
In the case of fluorosilicone heat curable silicone rubbers
suitable for calendering to fabric, it is especially preferable
to employ the fluorosilicone treated fillers disclosed in
Evans, IJnited States Patent Number 4,529,774.
Generally, the amount of reinforcing filler included in the
heat curable silicone rubber composition will range from about
5 to about 100 parts by weight and, preferably, from about 30
to 50 parts by weight.
In addition to the ingredients listed above, the heat
curable silicone compositions of the present invention may
contain additives known in the art, such as antioxidants,
pigments, heat stability additives, and the like.
The compositions of the present invention are prepared by
mixing the various ingredients in any manner known in the art,
for example, in a doughmixer.
In order to better enable the artisan to practice the
present invention the following examples are provided by way of
illustration and not by way of limitation. All parts are by
weight unless otherwise noted.

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60SI-725/GLL:mz/019Sp
-12-
EXAMPLES
. . . = . = =
Example 1
A base fluorosilicone composition was prepared by mixing
100 parts by weight of methylvinyl tenminated polydiorgano-
S siloxane ha~ing a viscosity of 100,000,000 centipoise at 25C;
3 parts by weight of a polydimethylsiloxane diol process aid
having the formula H0 ( Si (Me)20 )5 H; 1 part by
weight of methylvinyl tenminated polydimethylsiloxane process
aid; 0.25 parts by weight bis-dimethylvinylsilazane; 0.65 parts
by weight cerium hydroxide and 0.8 parts by weight 2,5 bis-
(t-butyl peroxy)-2,5-dimethylhexane.
4764 grams of this base composition was placed in a 3 1/2
quart doughmixer along with 429 grams of fumed silica treated
with fluorosilicone in accordance with United States
Patent Number, 4,529,774 issued July 16, 1~5. The
amount of filler was sufficient to provide a filler loading of
40 parts by weight.
After efflecting uniform dispersion of the filler, 47.7
grams red colorant and amounts of trimethylol propane
20 ~ ~ trimethacrylate (available as Sartomer~r~ 50 from Sartomer
Company, West Chester, Pa.~ ranging from 0.25 to 2.0 parts by
weight of polymer was added (see Table I). The ingredients
were thoroughly mixed for 30 minutes and then strained through
a 100 mesh 316 SS screen. The strained compound was then
catalyzed on a mill with 2,4-dichlorobenzoyl peroxide (50
weight percent in SF-96 ~ 1000 centistoke fluiJ containing
12.5 weight percent dibutylphthalate~ at a le~el of 1.6 parts
by weight per 100 parts by weight of compound. See Table I,
compositions B, C, E, F and G.

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Example 2
The same procedure as in Ex~mple 1 was followed except that
the trimethylol propane trimethacrylate was replaced with 0.25
parts triallyl trimellitate per 100 parts by weight of
polymer. See Table I, composition D.
Example 3
For purposes of comparison, FSE-7140, available from
General Electric Company, was evaluated since it is an example
of a composition that is ineffective for calendering to
fabric. Also for purposes of comparison there was prepared a
composition as in Example 1 except the co-curing agent was not
included. See Table I, Composition A. Lastly, a composition
manufactured by Dow Corning Corporation (LS-63) and believed to
be used for calendering to fabric was also tested.
. _ _ _ _ _

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-14-
TABLE I
FSE
7140 A B C D E F G LS-63
Base Compound 100 100 100 100 100 100 100lQO U
Total Filler Level28 46 46 40 40 39 4040 N
Trimethylol propane K
trimethacrylate -- -- 0.25 0.25 --0.30 0.25 2.0 N
Triallyl trimelliate -- -- -- -- 0.25 -- -- -- O
Peroxide/100 pt. cpd. 1.6 1.6 1.61.6 1.6 1.6 1.6 1.6 W
N
Post Cure 15 min/350F, Post Bake 4 hrs/400F
Physical Properties
Shore A 4661 66 64 61 60 62 68 62
Tensile, psi13951485 1285 1440 1150 1460 1455 1210 1163
Elongation, ~400330 320 330 410 430 380 460 240
Tear, Die B, pi140145 190 165 190 175 160 190 115
Specific gravity 1.42 1.45 1.46 1.45 1.46 1.45 1.45 1.43 1.48
Comprsssion Set B
22 hrs/350F27.929.1 28.0 23.0 21.0 12.2 17.7 16.3 14.9
Fuel Immersion - Fuel B (22 hours, room temperature)
% Vol. Change 24.5 29.0 29.0 22.1 22.8 23 23 24.9 24.8
Calendererformance Exc. Some No Good Exc. Exc. Exc. Exc. Good
Voids 600d

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The data in Table I clearly shows the superior perfor~ance of
compositions C through G, relative to A and LS-63, which are
prepared with lower filler loadings and the co-curing agents of
the present invention. The silicone elastomers prepared
S therefrom have a substantially unchanged durometer (relative to
A) and exhibit better resistance to tear, compression set and
fuel B solvent swell while maintaining good processability.