Note: Descriptions are shown in the official language in which they were submitted.
~0~9~9a
PATENTS
60SI-1483
HE~T CURABLE SILICONE RUBBER COMPOSITIONS HA~ING
SIMPLIFIED FORMULATIONS
BACKGROUND OF THE INVENTION
The present invention relate~ to heat curable
silicone rubber compositions. More particularly, the
i~vention relates to heat curabl~ silicone rubber
compositions prepared from relativsly simplP
formulation~ having improved tear and ten~ile ~trength,
oil re~istanc~ and co~pres~ion 8et propertie~c
~ eat curable ~ilicone rubber compo~itions are
known in the art. Such cDmposition~ are known for
their ability to re~i~t change at elevated temperature
and expo3ure to advexs~ weather conditions over
extend~d periods of time. Effort~ have been made in
the pa~t to produ~e heat ~urable silicone elastomer
.
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60SI-14B3
composition3 having increa3ed tear and tensile
strength, good oil resi~ance and reduced compre~sion
set properties. However~ a problem ha~ been the
compl~xity of the compositions re~ulting from such
5 effortsO In particular, numerous ~omponents including
expensive resin composition~ have been required to
produce compo~ition~ which have satisfactory
propertie~. Formulations with mzny component~ increase
inventory C08t~ and complic~te ~cheduling and increase
the probability of pro~e~ upsets.
U.S. Patent No. 4,539,357 (Bobaar '357) disclose~
an exemplary heat-curable cilicone composition
comprising a vinyl-termirlated linear
diorganopolysiloxane gum having a viscosity in the
lS range of lx106 to 20x107 eQntipoi~ 25C, a vinyl-
containing diorganopoly~iloxane gum having a viscosity
Yarying from lxlO~ to 20x107 centipoi~ at 25C, a
fill~rr a hydrid~-containing polysiloxane, and a~
organic peroxide or organic hydropero~ide curing agent.
2~ ~he compo~ition may further compri~e a vinyl-stopped
,. , . ~
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60SI-1483
organopolysiloxane resin copolymer. It is stated in
Bobear '357 at column 6, lines 22-29 that there should
be no vinyl-containing fluid in the composition since
it has been found that compositions containing vinyl-
containing fluids of a viscosity of 500,000 centipoiseor less result in elastomers with good tear strength
initially, but after the composition has been subjected
to post-cure, its tear strength properties degrade
dramatically. After post-cure, the Bobear '357
composition ha~ a tear strength of above 200 pi.
U.S. Patent No. 3,652,475 to Wada et al. (Wada
'475) discloses heat curable elastomeric silicona
compositions reportedly having high tear strength,
excellent compression set and re~iliency, comprisiny
two vinyl-unsaturated polydiorganopolysiloxanes each
having a degree of polymerization of at least 3000; a
vinyl-unsaturated polydiorganopolysiloxane having a
degree of polymerization of from 10 to 1000; a silica
filler; and an organic peroxide catalyst. The samples
prepared in the Wada '475 examples had tear strength
values of about 40 kg/cm, i.e., about 225 pi.
U.S. Patent No. 3 t 671,480 to Wada et al. (Wada
'480) discloses a heat curable elastomeric silicone
composition having improved tear strength and superior
compression set, comprising a mixture of two vinyl-
unsaturated polydiorganopolysiloxanes, one of which
appears to have a high molecular weight and the other
of which appear~ to have a low molecular weight; a
polyorganohydrogensiloxane; silica filler; and a
platinum compound. The examples in Wada '480 report
tear strength values of about 50 kg/cm, i.e., about 280
pi. Compression set values are not given.
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60SI-1483
U.S. Patent No. 4,061,609 to Bobear (Bobear '609)
discusses a platinum catalyzed silicone rubber
composition comprising a vinyl-stopped polysiloxane or
a blend of such polysiloxanes; platinum; and a
hydrogen-containing polysiloxane. Preferably, the
vinyl-stopped polysiloxane has a viscosity in the range
of 1000 to 300,000,000. A low viscosity vinyl-stopped
polysiloxane may be added to the basic composition as
a reinforcing agent to give the final composition good
physical strength. Claim 4 in Bobear '609 recites a
first polysiloxane having a viscosity of 1,000,000 to
200,000,000 centipoise at 25C and a second
polysiloxane having a viscosity of 50,000 to 500,000
centipoise at 2SC. The Bobear patent is directed to
improving the work life of silicone rubber
compositions, rather than tear strength. In the
examples, tear strength values of greater than 200 pi
were obtained, but the compositions having these values
contained two high viscoæity vinylpolysiloxanes rather
than one high viscosity vinylpolysiloxane and one low
vi~cosity vinylpolysiloxane.
U.S. Patent No. 3,660,345 to Bobear ~Bobear '345)
discloses oganopolysiloxane compositions which are
convertible to elastomers having high tear strength and
resiliency, comprising a two component blend of vinyl-
stopped organopolysiloxanes silica filler, and a
process aid which can be, for example, a methoxy-
terminated polysiloxane, a silanol-terminated
polydimethylsilox~ne, or hexamethyldisilazane. A
peroxide is used as the catalyst. Th~ viscosi~y of one
of the two vinyl-stopped organopolysiloxanes can be in
the range of 1 to 1 billion centipoise at 25C, while
the viscosity of the other can be between 100,000 to
.
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60SI-1483
2,000,000,000 centipoise at 25C. The examples in
Bobear '345 report tear strength values of greater than
200 pi.
U.S. Patent No. 3,696,068 to Creamer discloses
S heat-curable silicone gumstock compositions having high
"B" tear strength as well as low compression set
values, comprisingavinyl-stopped diorganopolysiloxane
gum and a linear vinyl-stopped fluid having a viscosity
of from 10 to 150,000 centiskokes at 25C. The cross-
linkable vinyl siloxane gum has a viscos~ty in excessof 200,000 centistokes at 25C. The Creamer
composition is cured by a peroxide catalyst. Some of
the samples prepared in Creamer had tear strength
values in excess of 150 pi. Tear strength after post
cure was not measured in any of the examples, which is
significant since, as pointed out previously herein, it
has been found that, in compositions containing low
viscosity vinyl stopped fluids, tear strength drops
drama~ically after post cure.
U.S. Patent No. 3,884,866 to Jeram et al.
discloses a high strength organopolysiloxane
composition suited for low pressure injection molding,
comprising (A) a first component containing (i) a
vinyl-stopped high viscosity organopolysiloxane having
a viscosity of 5000 to 1,000,000 centipoise at 25C;
and (ii) a low vi8c05ity of 50 to 5000 centipoise at
25C; (iii) a filler; and (iv) a platinum catalyst; and
(B) a hydrogen ~ilicon composition. The highest tear
strength reported in the examples was 250 pi.
Although heat curable silicone rubber compositions
having good tear strength, reasonably good tensile
strength and low compression set propertie~ are known
in the ark, it is desirable to provide heat curable
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60SI-1483
silicone rubber compositions having good properties in
a simplified formulation, and it therefore is an object
of the invention to provide one or more simplified
formulations while retaining such desirable properties.
SUNMARY_OF THE INV~NTION
The present invention i3 based upon the discovery
that a simplified combination of a high viscosity
vinyl-stopped diorganopolysiloxane gum and a silica
fill~r combined in a presence of a silanol or alkoxy
stopped process aid may be used to produce a heat
curable silicone rubber. The process aid is preferably
silanol stopped.
In one embodiment of the invention, the
composition consists essentially of a vinyl-stopped
diorganopolysiloxane gum having a viscosity of about
3,000,000 to about 100,000,000 centipoise at 25C, a
silica filler and a process aid. In another embodiment
of the invention a heat curable silicone rubber
composition con ists essentially of the vinyl-stopped
diorganopolysiloxane gum described above, the silica
filler, the process aid, and a silazane coupling agent.
In another embodiment of the invention, the composition
consists essentially of the vinyl-s~opped
diorganopolysiloxane described above, the silica
filler, the process aid, and a cross-linking agent. In
yet another embodiment of the invention a heat curable
~ilicone rubber composition consists essentially of the
vinyl-~topped diorganopolysiloxane gum described above,
the silica filler, the process aid, the cross-linking
agent and the silazane coupling agent.
In a particular embodiment, a heat curable
~ilicone rubber composition having increased tear and
tensile strength, good oil resistance, reduced
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60SI-1483
compression set consists essentially of by weight, tA)
100 parts of a vinyl-stopped organopolysiloxane or
blend wherein the vinyl-stopped organopolysiloxane has
a viscosity of about 3,000,000 to about 100,000,000
centipoise at 25C; (B) up to about 200 parts by
weight, based upon Component (A), of a filler; and (C)
up to about 10 parts by weight, based upon Component
(A), of a silanol- or alkoxy-process aid.
In another embodiment of the invention, a heat
curable silicone rubber composition consists
essentially of Components (A), (B) and (C) as defined
above and (D) up to about .5 parts by weight, based
upon Component (A)~ of a sila~ane coupling agent.
In another embodiment of the invention, a heat
curable silicone rubber composition consists
essentially of Components (A), (B) and (C) as defined
of above and ~E) up to about 10 parts by weight, based
upon Component (A), of an organohydrogenpolysiloxane
cross-linking agent.
In yet another embodiment of the invention a heat
curable silicone rubber composition consists
essentially of Components (A), (B), (C), (D) and (E) as
herein before defined.
DESC~IPTION OF THE INYENTION-
The present inven~ion is directed to a heat
curable silicone composition having improved tear and
tensile strength properties, good oil resistance and
improved compression set having a simplified
formulation. In one embodiment the invention consists
essentially of Componen~ (A) a vinyl-stopped
diorganopolysiloxane gum or blends thereof, ha~ing a
viscosity of about 3,000,000 to about 100,000,000 CPS
at 25C; (B) up to about 200 parts by weigh~, based
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6~SI-14~3
upon Component (A) of a silica filler; and (C) up to
about 10 parts by weight, based upon Component (A), of
a silanol process aid. In another embodiment the
invention consists essentially of only Components (A),
(B) and (C).
In another embodiment, the heat curable silicone
rubber composition consists es~entially of Components
(A), (B) and (C) as defined above and in the
proportions set forth; and (D) up to about .5 weight
percent, based upon Component (A), of 2 silazane
coupling agent. In another embodiment the heat curable
silicone rubber composition of the present invention
consists essentially of only (A), (B), (C) and (D) as
defined and in the proportions set forth.
In another embodiment of the invention the heat
curable silicone rubber composition consists
essentially of Components ~A), (B) and (C) as defined
above and the proportions set forth and (E) up to about
10 parts by weight, based upon Component (A), of an
organohydrogenpolysiloxane cross-linking agent. In
another embodiment of the invention the heat curable
silicone rubber composition consists essentially of
only Components (A), (B), (C), and (E) as defined aboYe
and in the proportions noted.
In another embodiment o the invention, the hea~
curable silicone rubber composition consists
essentially of Components (A), (B), (C), (D) and (E) as
defined above and in the proportions noted. In another
embodiment of the invention a heat curable silicone
rubber composition consists essentially of only
Components (A~, (B), (C), (D) and (E) as defined above
and in the proportions noted.
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60SI-1483
Component (A) is a vinyl-stopped
diorganopolysiloxane or vinyl-stopped diorganosiloxane
blend wherein the vinyl-stopped diorganosiloxane or
blend has a viscosity of about 3,000,000 to about
100,000,000, preferably from ~bout 20,000,000 to about
3~,000,000 and more preferably from about 25,000,000 to
about 30,000,000 centipoise at 25~C. Component (A) has
a vinyl concentration in the range of about 5x10-5 to
about 2x10-4, preferably from about 8xlO-s to about
1.5x10-4 and more preferably from about 8x10-5 to about
1.2x10-4, weight percent. The organo groups in the
vinyl polymer or polymers of Component ~A) are
monovalen~ hydrocarbon radicals.
The vinyl polymer or polymers of Component tA)
contain no significant vinyl substituent units in the
chain but rather contain only vinyl end-groups.
Preferably the vinyl-stopped polymer has the formula:
ViSiORl2(SiORl2)X ~SiOR22)ySiRl2Vi
wherein Vi is vinyl each Rl is independently chosen
from a monovalent hydrocarbon, free of aliphatic
unsaturation, containing 1 to about 8 carbon atoms;
each R2 is independently chosen from monovalent
hydrocarbon radicals, also free of aliphatic
unsaturation, containing 1 to about 8 carbon atoms, and
and y are integers chosen such that Component (A) has
a vi~c06ity which ranges from about 3,000,000 to about
100,000,000 centipoise at 25C and a weight percent of
vinyl concentration as noted above. The vinyl chain-
stopped polymer is preferred because it appears to
rzsult in higher Tear B, Tensile Strength and
Elongation.
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60SI-1483
The vinyl-stopped polymers of Component (A) can be
made by a process well known in the art, for example by
reacting cyclotetrasiloxanes in the presence of low
molecular weight linear vinyl chain-stoppers at high
temperatures in the presence of basic catalysts so as
to yield the polymer of the desired molecular weight.
When the reaction is over, the catalyst i5 neutralized
and the excess cyclics are vented off to result in the
desired polymer. By controlling the amount of chain-
stopper and the temperature of reaction, the molecularweight of the desired vinyl-stopped polymer end product
can be controlled. For more details of the process ~y
which such vinyl-stopped polymers are produced,
reference is made, for example, to U.S. Patent ~o.
3,660,345, which is incorporated herein by reference.
In accordanse with the invention, the amount of
Component (A) present in the final composition may
vary. However, for purposes of explanation herein, it
i8 assumed that 100 parts by weight of the gum i~
combined with varying amounts of the other components.
In some examples, the formulation has 80 parts of
Component (A). In each case, the amount of the other
components in the final product may be calculated or
otherwise readily inferred. In the examples below, (A)
may be (A1) a ViMD~MVi gum with a~out 9000 D units or
(A2) a similar gum having approximately the same number
of D units and up to about 25 ppm RSiO3t2 or T units.
Component (B) comprises from about 5 to about 200,
preferably from 10 to about 100, and more preferably
from about 30 to about 75 parts by weight, based upon
Component ~A), of a reinforcing filler. Such a
reinforcing filler is needed in the compo~ition if it
is to have high tensile and tear strength. Examples of
a
PATENTS
60SI-1~83
reinforcing fillers that can be used include, for
instance, fumed silica and precipitated silica, with
fumed silica being preferred. The filler may be
treated with various agents so as to prevent the cured
composition from structuring. Exemplary treatments
include cyclopolysiloxanes as disclosed in U.S. Patent
No. 2,938,009 to Lucas or silazanes such as disclosed
in U.S. Patent No. 3,635,743 to Smith or both, the
teachings of which are incorporated herein by
reference. The cyclopolysiloxane present may be, for
example, a cyclotetramethyl~iloxane present in an
amount of about 15 to 20 weight percent of the filler.
The preferred fumed silica filler may have a
surface area of about 100 m2tgm to about 300 m2t~m and
preferably about 160 m2~gm to about 240 m2/gm. The
higher surface area fillers tend to re~ult in more
clear, less hazy composition~. However, they are more
expensive than the lower surface area fillers and
require surface treatments or more processing aid to
incorporate them into the gum. In the examples below,
Component (B) is a fumed silica filler treated as in
Lucas above having a surface area in the untreated
state of 240 m2/gm or 160 mZ/gm. A further surface
treatment with a linear vinyl containing silazane may
be provided in-situ.
Extending filler~ can be used in the combination
with the reinforcing fillers, treated or untreated, to
obtain proper balance in the final phy~ical properties.
These extending filler~ include, for example, titanium
dioxide, lithopone, zinc oxide, zirconium silicate,
silica aerogel, iron oxide, diatomaceous earth, calcium
carbonate, glass fibers, magnesium oxide, chromic
oxide, zirconium oxide, aluminum oxide, alpha quartz,
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60SI-1483
calcined clay, carbon, graphite, cotton, synthetic
fibers, aluminum silicate, and calcium silicate. It
should be understood, however, then when clear, heat
curable compositions are desired, fillers which tend to
produce opaque end products are not desirable and hence
are not incorporated into the composition.
Component ~C) may be employed in order to allow
for easier incorporation of the filler into the gum.
Component (C) is a processing aid or plasticizer and in
a preferred embodiment Component (C) is a low viscosity
silanol- or alkoxy-stopped siloxane fluid having a
viscosity of about 3 to 500 cps and preferably 3 to 50
cps at 25C. The processing aid is preferably silanol-
stopped, if alkyl stopped, methyl is preferred. The
siloxane fluid is an equilibrium mix of low molecular
weight oligomers of from about 4 to about 10 repeating
units, and preferably between 4 and 6 repeating units
in length with a minimum amount of cyclics in
equilibrium with the oligomers. Component (C) may have
the form:
R30(R2Sio)XR3
wherein each R is CH3, and x is between 4 and 10 and
preferably between 4 and 6, with resulting cyclics in
the same number of units in equilibrium; and R3 is
selected from the group consisting of alkyl radicals
having 1 to about 8 carbon atoms and hydrogen, with
hydrogen being preferred.
In the present invention the processing aid is
typically present in amounts up to 20 parts by weight,
based upon Component (A~l preferably between 2.5 and 5
parts by weight and more preferably 3 parts by weight,
based upon Component (A). It should be understood that
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60SI-1483
as more filler is used, a greater amount of processin~
aid is employed. In the examples below, Component (C)
has a silanol content of about 7.5 percent by weight.
Componen (D) is a vinyl terminated silazane
surface treatment for the filler which may take the
form:
ViSiR2NHSiR2Vi
wherein R is an organic such as CH3. The surfac~
treatment promotes bonding between Components (A) and
(B3, and may be applied to the filler, Component (B),
prior to mixing with the other components or while
mixing with other components. Component (D) may be
present in an amount up to about 2 parts by weight,
based upon the weight of Component (A~, preferably
between 0.1 and 1 parts by weight and more preferably
.3 parts by weight based upon Component (A).
Component (E) is in the form of hydride cross-
linkin~ agent. In a preferrPd embodiment, Component(E) may be a random copolymer formed of a
polydimethylsiloxane (P DMS~ and a
polymethylhydrogensiloxane (PMHS) having the form:
~3SiO (SiORHSiORz)XSiR3
wherein each R is indepsndently chosen from a hydrogen
or monovalent hydrocarbon radical, free of aliphatic
unsaturation, containing 1 to about 8 carbon atoms, and
wherein x ranges from about 2 to about 100.
In the present invention, the hydride is present
in an amount ranging from about 0.1 to about 10 parts
by weight, based upon Component (A), preferably .5 to
%0~93
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60SI-1483
8.0 parts by weight and more preferably 0.8 to 5.0
parts by weight. When a hydride agent is employed, a
platinum catalyst may be substituted for a peroxide
catalyst to cure the composition. In Component (E), x
may vary so that it has a viscosity ranging from about
5 to about 500 cps, preferably from about 10 to 100 cps
and more preferably from about 10 to 50 cps at 25C.
In the examples below, Component (E) has a viscosity of
about 30 centistokes, a hydrogen content ranging from
about 0.05 to about 2 percent by weight, and a ~hain
length of about 100 units.
The linear hydride described above may be made by
many procedures which are known to those skilled in the
art particularly by hydrolysis of appropriate
chlorosilanes. See for example, U.S. Patent No.
4,041,101 which is incorporated herein by reference.
In order to form a heat curable rubber, an organic
peroxide free radical initiator or a curing catalyst is
provided. The preferred peroxide curing agents are
thermal decomposition organic peroxides convenien~ly
used to cure silicone elastomers. Examples of suitable
organic peroxide free radical initiators for use in the
present invention are disclosed, for example, in U.S.
Patent No. 4,539,357 to Bobear which is incorporated
herein by reference. Suitable peroxide catalysts
include dialkyl peroxide such as di-~ertiary-butyl
peroxide, tertiary-butyl-triathylmethyl peroxide,
tertiary-butyl-tertiary-butyl-tertiary-triphenyl
peroxide, t-butyl perbenzoate and a di-tertiary alXyl
peroxide such as dicumyl peroxide. Under certain
conditions such as when a hydride is used, a platinum
catalyst may be employed instead of an initiator. In
the examples below, the preferred catalyst is a vinyl
.
.
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60SI-1483
specific component such as 2,5 dimethyl-2~5-di(t-butyl
peroxy) hexane e.g., (Lupersol ~101).
A heat age additive e.g., (F) a rare earth
octoate, comprising primarily mixtures of cerium and
S lanthanum octoates may be optionally employed to
improve heat age properties. In the examples below,
(F) is (C7H1sCO2)X La or Ce 6% by weight in mineral
spirits.
Copending patent application Serial No. 07/587,876
filed 9/25/90, attorney docket no. 60SI-1336,
incorporated herein by reference, discloses heat
curable silicone compositions containing blends of
vinyl-stopped organopolysiloxane gums and oils, MQ and
MDQ resins fillers process aids and peroxide
initiators. The compositions are formulated for
increased tear strength and reduced compression set
properties.
The following components were mixed in various
proportions to produce heat curable silicone rubber
compositions:
TABLE_I
Component (Al) - ViMD~9000 MVi - gum
Component (A2) - ViND~gooo MVi (or 25 ppm RSiO3~z or T)-gum
Component (Bl) - Treated Fumed Silica 240 m2/gm
Component (B23 - Treated Fumed Silica 160 m2/gm
Component (C3 - HOD~6OH Process ~id
Component (D) - Divinylsilazane - Coupling Agent
Component (E) - MDsoD~soM - Cross-Linking Agent
Component (F) - (C7Hl5cOz)xLa and (C7HlsCO2)xCe (Rare
Earth Octates) Heat Age Additive, 6%
by weight in mineral spirits
2 ~
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60SI-1483
EXAMPLES I - XI
The Compound~ (A) - ~E) were mixed in various
proportions to produce the compounds li~ted below in
Tables IIA -IIB. The resulting measured properties are
listed immediately below the corresponding composition
in each example.
TA5~B II-A
1 0 COmDOnent I II II} IV V VI
(Al) 80 ao 80 80 80 80
15 ~A2)
(i31) 30 40 50 60
(82)
(c) 4 4 4 4 4 4
(D) 0.15 0.15 0.15 0.15 0.15 0.15
(E) 1.8 1.8 l.B 1.8 1.8 1.8
(F)
PQOPEQTY
Shore A 44 54 66 75 52 55
~enDll~ 1812 1752 1714 1551 1702 1734
8trongth (psl)
Elong~tlon ('s) 958 1026 902 841 925 927
Ts~r 8trength 224 254 265 251 216 227
Dle 8 (pl)
Compre0~ion 39.5 69.2 83.5 89.2 55.2 65.6
8et ~ (22/350)
Compresslon - 26.6 32.8 65.3 18.9 23.7
Set 9~ (70/100)
8A 8hore - 37 29 30 40 36
TA31,B r l-D
co~Donent VII VIII IX X XI
(Al~ 80 100 100 50 80
(A2)
(31)
(82~ 60 25 75 30 30
(C~ 4 2 4 2 4
(D~ 0.15 .4 .4 .2
(B) 1.8 - - 2 1.3
(F) _ .2 .2
-17- PATENTS
60SI-1483
PROP~TY
9hore A 70 33 72 ~ 3S
Ten~lle 1464 1243 1724 lûOO 14~3
3trength (p~l)
~long~tlon (~) 914 779 576 ~1000 1152
1 0 To~r 8trength 250 259 174 ~1~5 179
Dlo ~ ~pl)
Co~praaDlon 79.2 25.9 64.9 32.8
~et ~ (22/350)
Compre~slon 44.9 10.6 34.2 1~.3
~et ~ (70/100)
3A 8hor~ 29 62 31 ~54
Teet d~t~ were ~bt~lned from the ~ollowlng
A~TM methods ~nd procedur~: 8bor~ A - D-2240:
Ton~lle, Elong~tion ~nd ~odulu~ - D-412 (D1~ C)
Ta~r - D-624 (Dle 13); ~nd Comprenulon 80t - D-395 (method E)).
Example I represents a formulation for a clear,
heat curable silicone rubber without a resin component.
This formulation represents a control in an experiment
employing index matching resin. This formulation,
while not as clear as might be desirable in a clear
product, exhibited unexpectedly high physical
properties as illustrated in Table II-~. Examples I-IV
show the effect of increased filler loading. As
expected, the hardness (Shore A) increases with
increasing filler loading. Examples V - VII substitute
a treated lower surface area iller (160 m2/gm) for the
treated high surface area filler (240 m2/gm) from
Examples I-IV. The results in Examples V-VII show
similar high strength properties.
Examples VIII and IX demonstrate a composition
having high strength properties without the cross-
linking agent Component (E).
Example X employs a blend of the gum materials.
Example XI is similar to Example I except that a lower
surface area filler (160 m2/gm) and no coupling agent
is used.
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While there have been described what at present
are considered to be the preferred embodiments of the
present invention, it will be readily apparent to those
skilled in the art that various changes may be made
therein without departing from the invention and it is
intended in the claims to cover such changes and
modifications as fall within the true spirit and scope
of the invention.
