Note: Descriptions are shown in the official language in which they were submitted.
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Docket No. TSL1551
PATENT APPLICATION
FOR
MOLDABLE SILICONE RUBBER SPONGE COMPOSITION, SILICONE
RUBBER SPONGE, AND METHOD FOR PRODUCING
SILICONE RUBBER SPONGE
BACKGROUND OF INVENTION
This invention relates to a moldable silicone rubber sponge composition, to
silicone rubber sponge, and to a method for producing silicone rubber sponge.
Silicone rubber sponge, by virtue of its excellent resistance to heat and
weathering
and its light weight, is used for automotive components such as packings,
gaskets, and O-
rings; as a surface covering for the rolls used in copiers; and for a variety
of sealing
materials. A large number of moldable silicone rubber sponge compositions have
already
been disclosed. For example, Japanese Published Patent Application Sho 44-461
(461/1969) and Japanese Laid Open Patent Application Hei 7-247436
(247,436/1995)
teach moldable silicone rubber sponge compositions that contain a thermally
decomposable blowing agent as typified by azobisisobutyronitrile. Japanese
Laid Open
Patent Application Number Hei 10-36544 (36,544/1998) teaches a composition
comprising hollow thermoplastic silicone resin particles blended into a liquid
silicone
rubber composition that evolves gas during its cure. In the case of the former
two
compositions, however, the decomposition products produced by the thermally
decomposable blowing agent during foaming are harmful to humans and thus are
problematic from the standpoint of environmental pollution. The silicone
rubber sponge
produced by the composition taught in Japanese Laid Open Patent Application
Number
Hei 10-36544 has a poor mechanical strength and hence suffers from limitations
in its
applications.
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The inventors achieved the present invention as the result of investigations
directed to solving the problems described above. More specifically, the
object of this
invention is to provide a moldable silicone rubber sponge composition that
during
foaming does not generate gas harmful to humans and that cures to give a
silicone rubber
sponge that has uniform and microfine cells and a high mechanical strength.
SUMMARY OF INVENTION
A moldable silicone rubber sponge composition comprising
(A) 100 weight parts silicone rubber base compound with a Williams plasticity
at 25°
C of 50 to 600 comprising
(a) 100 weight parts diorganopolysiloxane gum that contains at least 2 alkenyl
groups in each molecule and
(b) 1 to 120 weight parts wet-process silica,
(B) 0.01 to 50 weight parts hollow gas-containing thermoplastic resin powder,
and
(C) curing agent in an amount sufficient to effect curing of the composition.
The invention is further a silicone rubber sponge afforded by the
thermosetting of the
above-defined moldable silicone rubber sponge composition and a method for
producing
the silicone rubber sponge.
DESCRIPTION OF INVENTION
A moldable silicone rubber sponge composition comprising
i
(A) 100 weight parts silicone rubber base compound with a Williams plasticity
at 2~°
C of SO to 600 comprising
(a) 100 weight parts diorganopolysiloxane gum that contains at least 2 alkenyl
groups in each molecule and
(b) 1 to 120 weight parts wet-process silica,
(B) 0.01 to 50 weight parts hollow gas-containing thermoplastic resin powder,
2
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and
(C) curing agent in an amount sufficient to effect curing of the composition.
The invention is further the silicone rubber sponge afforded by the
thermosetting of the
above-defined moldable silicone rubber sponge composition and the method for
producing the silicone rubber sponge, said method being characterized by
curing the
composition by heating to at least the softening point of the thermoplastic
resin.
To explain the preceding in greater detail, the silicone rubber base compound
(A)
used in this invention is the base component of the present composition. The
diorganopolysiloxane gum (a) encompassed by component (A) must contain at
least 2
silicon-bonded alkenyl groups in each molecule. This alkenyl can be
exemplified by
vinyl, allyl, propenyl, and hexenyl. The non-alkenyl silicon-bonded organic
groups can
be exemplified by alkyl groups such as methyl, ethyl, and propyl; aryl groups
such as
phenyl and tolyl; and halogenated alkyl groups such as 3,3,3-trifluoropropyl
and 3-
chloropropyl. The molecular structure of component (a) can be straight chain
or branch-
containing straight chain. This component in general will have a degree of
polymerization of 3,000 to 20,000 and will be a gum. Component (a) can be a
homopolymer or copolymer or a mixture of these polymers. The units
constituting
component (a) can be specifically exemplified by the dimethylsiloxane unit,
methylvinylsiloxane unit, methylphenylsiloxane unit, and 3,3,3-
trifluoropropylmethylsiloxane unit. The molecular chain terminals of component
(a) are
preferably capped by a triorganosiloxy group or the hydroxyl group. The group
present at
the molecular chain terminal position in this component can be exemplified by
trimethylsiloxy, dimethylvinylsiloxy, methylvinylhydroxysiloxy, and .
dimethylhydroxysiloxy. The diorganopolysiloxane gum can be exemplified by
dimethylvinylsiloxy-endblocked dimethylsiloxane-methylvinylsiloxane copolymer
gums,
dimethylvinylsiloxy-endblocked dimethylpolysiloxane gums, hydroxyl-endblocked
dimethylsiloxane-methylvinylsiloxane copolymer gums, and
methylvinylhydroxysiloxy-
endblocked dimethylsiloxane-methylvinylsiloxane copolymer gums.
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The wet-process silica (b) functions to promote formation of the foam cells
during
foaming and curing of the present composition and also functions to impart
mechanical
strength to the cured silicone rubber sponge. Wet-process silica denotes the
microparticulate silica produced by a wet process, for example, by
neutralization of
sodium silicate with acid or decomposition of an alkaline earth metal silicate
with acid.
Wet-process silicas are already well-known as reinforcing fillers for silicone
rubber
compositions and are readily acquired in commerce. They are marketed, for
example,
under such trade names as Nipsil LP (Nippon Silica Kogyo Kabushiki Kaisha),
Tokusil
USA (Tokuyama Kabushiki Kaisha), and Carplex SOS (Kabushiki Kaisha Shionogi).
Wet-process silicas with a BET specific surface area of at least 50 m2/g are
preferred.
The wet-process silica (b) also includes wet-process silica whose surface has
been treated
with, for example, methylchlorosilane, organoalkoxysilane, a silane coupling
agent, or
hexamethyldisilazane. The wet-process silica (b) preferably has a water
content less than
4 weight%. This component is added at from 1 to 120 weight parts and
preferably at
from 5 to 100 weight parts, in each case per 100 weight parts component (a).
The Williams plasticity at 25°C of component (A) must be in the range
from 50 to
600 and preferably is in the range from 150 to 450. A high tackiness occurs at
a Williams
plasticity below 50, while a Williams plasticity in excess of 604 results in
reduced
extrusion moldability.
Component (A) can be exemplified by the material afforded by intermixing
components (a) and (b) and subjecting the mixture to aging for an extended
period of
time; the material afforded by intermixing components (a) and (b) while
heating; and the
material afforded by intermixing components (a) and (b) while heating and
applying
reduced pressure thereto. During this preparative step, silanol-functional
organosiloxane
oligomer, silanol-functional organosilane, or hexaorganodisilazane may also be
introduced and admixed as a surface-treatment agent for component (b). This
organosiloxane oligomer can be exemplified by hydroxyl-endblocked
dimethylsiloxane
oligomers, hydroxyl-endblocked methylvinylsiloxane oligomers, hydroxyl-
endblocked
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dimethylsiloxane-methylvinylsiloxane co-oligomers, hydroxyl-endblocked
methylphenylsiloxane oligomers, and hydroxyl-endblocked dimethylsiloxane-
methylphenylsiloxane co-oligomers. The silanol-functional organosilane can be
exemplified by dimethylsilanediol and diphenylsilanediol. The
hexaorganodisilazane can
be exemplified by hexamethyldisilazane. This component should generally be
added at
from 0.01 to 5 weight parts for each 100 weight parts component (a).
The curing agent (C) can be an organoperoxide or the combination of a platinum
group catalyst with SiH-functional organopolysiloxane crosslinker. The
organoperoxide
can be exemplified by benzoyl peroxide, tent-butyl perbenzoate, o-
methylbenzoyl
peroxide, p-methylbenzoyl peroxide, m-methylbenzoyl peroxide, dicumyl
peroxide, and
2,5-dimethyl-2,5-di(tert-butylperoxy)hexane. Organoperoxide will generally be
added at
from 0.1 to 10 weight parts for each 100 weight parts component (A).
The platinum group catalyst encompassed by the platinum group catalyst/SiH-
functional organopolysiloxane combination can be exemplified by very finely
divided
platinum, platinum black, chloroplatinic acid, alcohol-modified chloroplatinic
acid, olefin
complexes of chloroplatinic acid, complexes of chloroplatinic acid and
divinyltetramethyldisiloxane, rhodium compounds, and palladium compounds. The
platinum group catalyst will generally be used at from 1 to 200 weight parts
as platinum
metal for each 1,000,000 weight parts component (A). The SiH-functional
organopolysiloxane operates to cure the composition of this invention by
addition-
reaction with component (a) in the presence of the platinum group catalyst.
The SiH-
functional organopolysiloxane can be exemplified by trimethylsiloxy-endblocked
methylhydrogenpolysiloxanes, trimethylsiloxy-endblocked dimethylsiloxane-
methylhydrogensiloxane copolymers, and dimethylhydrogensiloxy-endblocked
dimethylsiloxane-methylhydrogensiloxane copolymers. The SiH-functional
organopolysiloxane will generally be added in an amount that provides a value
of 0.5 : 1
to 20 : 1 for the ratio of moles of silicon-bonded hydrogen in this component
to moles of
alkenyl in component (a). When a platinum group catalyst/SiH-functional
organopolysiloxane combination is used as component (C), a compound as known
in the
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art may be added, insofar as the object of this invention is not impaired, as
an inhibitor of
the catalytic activity of the platinum group catalyst. These inhibitors can be
exemplified
by 1-ethynylcyclohexanol, 3-methyl-1-pentyn-3-ol, and benzotriazole.
The gas present in the hollow thermoplastic resin powder (B) used by this
invention functions to accelerate foaming and at the same time to make the
developing
foam cells uniform. Component (B) has a thermoplastic resin as its outer shell
and
contains an inert gas therein. The thermoplastic resin under consideration can
be
exemplified by silicone resins, acrylic resins, and polycarbonate resins. The
softening
point of this thermoplastic resin is preferably from 40 to 200°C and
more preferably from
60 to 180°C. The inert gas can be exemplified by air, nitrogen, and
helium. As far as the
size of component (B) is concerned, its average particle size is preferably
from 0.1 to 500
pln and more preferably from 5 to 50 pm. Component (B) can be produced, for
example,
by spraying a dispersion of solvent-dissolved thermoplastic resin and water
into a hot gas
current from a nozzle in order to drive off the organic solvent concomitant
with
conversion of the thermoplastic resin into particulate form: Component (B) is
added at
from 0.01 to 50 weight parts and preferably at from 0.1 to 40 weight parts, in
each case
per 100 weight parts component (A).
While the composition of this invention comprises the components (A), (B), and
(C) described above, it may also contain those additives known in the art for
addition to
silicone rubber compositions insofar as the object of the invention is not
impaired. These
additives can be exemplified by reinforcing fillers such as dry-process
silicas; semi-
reinforcing and non-reinforcing fillers such as diatomaceous earth, powdered
quartz,
calcium carbonate, mica, aluminum oxide, titanium oxide, and carbom black;
pigments
such as iron oxide red; heat stabilizers such as cerium silanolate and the
cerium salts of
fatty acids; flame retardants; plasticizers; and adhesion promoters.
The composition of this invention can be prepared simply by mixing components
(A), (B), and (C) and any optional components to homogeneity. Mixers and
compounders such as kneader mixers and continuous compounding extruders can be
used
for this purpose.
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In order to produce silicone rubber sponge from the composition of this
invention,
the composition is cured by heating it to a temperature equal to or greater
than the
softening point of the hollow thermoplastic resin powder (B). During this
process, the
composition of this invention undergoes both foaming and curing and thereby
forms a
silicone rubber sponge containing uniform and microscopic cells. The resulting
silicone
rubber sponge, because it contains uniform and microscopic cells and has an
excellent
mechanical strength, can be used, for example, as a gasket, e.g., gaskets for
maintaining
an airtight condition and fire-resistant gaskets, as a sealing material, for O-
rings, and as a
surface covering for the rolls used in copiers.
This invention is explained in greater detail below through working examples,
in
which parts denotes weight parts. The values for viscosity and Williams
plasticity
reported in the examples were measured at 25°C.
Reference Example 1
Distilled water and a solution (30 weight% solids) of a silicone resin
dissolved in
dichloromethane were intermixed by passage through a dynamic mixer, the
distilled water
at a rate of 25 cc/minute and the silicone resin-in-dichloromethane solution
at 100
cc/minute, to give a water-based dispersion. The silicone resin used to
prepare the
dichloromethane solution had a softening point of 80°C and a specific
gravity of 1.2 and
was composed of the methylsiloxane unit and methylphenylsiloxane unit in a 22
: 78
molar ratio. The water-based dispersion was continuously sprayed using a dual-
fluid
nozzle into a spray dryer that used a hot nitrogen gas current. The
temperature of the hot
nitrogen gas current during this process was 70°C and the pressure was
0.5 kg/cm2 (0.05
MPa). The resulting hollow silicone resin powder was immersed for 24 hours in
an
aqueous solution of 100 parts distilled water and 1 part nonionic surfactant
(ethylene
oxide adduct of trimethylnonanol), and the hollow silicone resin powder that
floated was
fractionated and collected. The resulting hollow silicone resin powder had an
average
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particle size of 40 pm, contained nitrogen in its interior space, and had an
average shell
thickness of 4 Vim.
Reference Example 2
A dichloromethane solution containing 10 weight% solids was prepared by
dissolving an acrylic resin with a softening point of 85°C (Elvacite
2008 from DuPont) in
dichloromethane. Using a dual-fluid nozzle, this dichloromethane solution and
distilled
water were continuously sprayed into a spray dryer, the dichloromethane
solution at 100
cc/minute and the distilled water at 25 cc/minute using a hot nitrogen gas
current as
propellant. The temperature of the hot nitrogen current during this process
was 80°C and
the pressure was 0.25 kg/cm2 (0.025 MPa). The resulting hollow acrylic resin
particulate
was immersed for 24 hours in an aqueous solution of 100 parts distilled water
and 1 part
nonionic surfactant (ethylene oxide adduct of trimethylnonanol), and the
hollow acrylic
resin powder that floated was fractionated and collected. The resulting hollow
acrylic
resin powder had an average particle size of 20 pm, contained nitrogen in its
interior
space, and had an average shell thickness of 4 Vim.
Example 1
The following were introduced into and mixed to homogeneity in a kneader
mixer: 100 parts dimethylvinylsiloxy-endblocked dimethylsiloxane-
methylvinylsiloxane
copolymer gum (weight-average molecular weight = 500,000) composed of 99.85
mole% dimethylsiloxane units and 0.15 mole% methylvinylsiloxane units, 45
parts wet-
process silica with a BET specific surface area of 130 m2/g (Nipsil LP from
Nippon .
Silica Kogyo Kabushiki Kaisha), and 3 parts hydroxyl-endblocked
dimethylsiloxane
oligomer with a viscosity of 50 mPa~s. The resulting mixture was heated for 60
minutes
at 175°C to produce a silicone rubber base compound that had a Williams
plasticity of
260. A moldable silicone rubber sponge composition was then prepared by mixing
the
following to homogeneity on a two-roll mill into 100 parts of the silicone
rubber base
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compound: 1 part trimethylsiloxy-endblocked dimethylsiloxane-
methylhydrogensiloxane
copolymer with a viscosity of 25 mPa~s (molar ratio of silicon-bonded hydrogen
in this
component to vinyl in the gum = 3.3 : 1 ), sufficient chloroplatinic acid/
divinyltetramethyldisiloxane complex to provide 2 ppm platinum metal, 0.03
part 1-
ethynylcyclohexanol as cure inhibitor, and 1 part of the hollow silicone resin
powder
prepared in Reference Example 1.
This composition was formed into a sheet with a thickness of 2 mm, placed in a
230°C oven and cured for 5 minutes to give a sheet of silicone rubber
sponge. This
silicone rubber sponge sheet was evaluated for its expansion ratio and the
diameter of the
contained cells; the results of these measurements are reported in Table 1
below. The
foam cell diameter was measured by sectioning the silicone rubber sponge and
inspecting
the sectioned surface with a microscope. Measurement of the tensile strength
of the
silicone rubber sponge according to JIS K-6251 gave a value of 1.9 mPa.
Example 2
The following were introduced into and mixed to homogeneity in a kneader
mixer: 100 parts dimethylvinylsiloxy-endblocked dimethylsiloxane-
methylvinylsiloxane
copolymer gum (weight-average molecular weight = 500,000) composed of 99.85
mole%
dimethylsiloxane units and 0.15 mole% methylvinylsiloxane units, 45 parts wet-
process
silica with a BET specific surface area of 130 m2/g (Nipsil LP from Nippon
Silica Kogyo
Kabushiki Kaisha), and 3 parts hydroxyl-endblocked dimethylsiloxane oligomer
with a
viscosity of 50 mPa~s. The resulting mixture was heated for 60 minutes at
175°C to
produce a silicone rubber base compound that had a Williams plasticity of 260.
A
moldable silicone rubber sponge composition was then prepared by mixing the
following
to homogeneity on a two-roll mill into 100 parts of the silicone rubber base
compound: 1
part trimethylsiloxy-endblocked dimethylsiloxane-methylhydrogensiloxane
copolymer
with a viscosity of 25 mPa~s (molar ratio of silicon-bonded hydrogen in this
component to
vinyl in the gum = 3.3 : 1), sufficient chloroplatinic
acid/divinyltetramethyldisiloxane
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complex to provide 2 ppm platinum metal, 0.03 part 1-ethynylcyclohexanol as
cure
inhibitor, and 1 part of the hollow acrylic resin powder (average particle
size = 20 Vim)
prepared as described in Reference Example 2. This composition was formed into
a sheet
with a thickness of 2 mm, placed in a 230°C oven, and cured for 5
minutes to give a sheet
of silicone rubber sponge. This silicone rubber sponge sheet was evaluated for
its
expansion ratio and contained foam cell diameter as in Example 1 and the
results of these
measurements are reported in Table 1 below.
Example 3
The following were introduced into and mixed to homogeneity in a kneader
mixer: 100 parts dimethylvinylsiloxy-endblocked dimethylsiloxane-
methylvinylsiloxane
copolymer gum (weight-average molecular weight = 500,000) composed of 99.85
mole%
dimethylsiloxane units and 0.15 mole% methylvinylsiloxane units, 45 parts wet-
process
silica with a BET specific surface area of 130 m2/g (Nipsil LP from Nippon
Silica Kogyo
Kabushiki Kaisha), and 3 parts hydroxyl-endblocked dimethylsiloxane oligomer
with a
viscosity of 50 mPa~s. The resulting mixture was heated for 60 minutes at
175°C to
produce a silicone rubber base compound that had a Williarns plasticity of
260. A
moldable silicone rubber sponge composition was then prepared by mixing the
following
to homogeneity on a two-roll mill into 100 parts of the silicone rubber base
compound:
0.3 part o-methylbenzoyl peroxide, 0.6 part dicumyl peroxide, and 1 part of
the hollow
silicone resin powder prepared as described in Reference Example 1. This
composition
was formed into a sheet with a thickness of 2 mm, placed in a 230°C
oven, and cured for
minutes to give a sheet of silicone rubber sponge. This silicone rubber sponge
sheet
was evaluated for its expansion ratio and contained foam cell diameter as in
Example 1
and the results of these measurements are reported in Table 1 below.
Example 4
The moldable silicone rubber sponge composition prepared in Example 1 was
introduced into an extruder configured for tube extrusion and was extruded to
give a tube
1o
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having an outer diameter of 2 cm and inner diameter of 1 cm. This tube was
then
introduced into a forced hot-air convection oven and heated for 3 minutes at
250°C to
produce a silicone rubber sponge tube. This silicone rubber sponge tube was
cut with a
knife and the diameter of the contained foam cells was measured. The cells
diameters
were in the range from 0.2 to 0.5 mm and were essentially uniform. The
expansion ratio
was 2.55-fold.
Example 5
The following were introduced into and mixed to homogeneity in a kneader
mixer: 100 parts dimethylvinylsiloxy-endblocked dimethylsiloxane-
methylvinylsiloxane
copolymer gum (weight-average molecular weight = 500,000) composed of 99.85
mole%
dimethylsiloxane units and 0.1 S mole% methylvinylsiloxane units, 45 parts wet-
process
silica with a BET specific surface area of 130 m2/g (Nipsil LP from Nippon
Silica Kogyo
Kabushiki Kaisha), and 3 parts hydroxyl-endblocked dimethylsiloxane oligomer
with a
viscosity of 50 mPa~s. The resulting mixture was heated for 60 minutes at
175°C to
produce a silicone rubber base compound. A moldable fire-resistant silicone
rubber
sponge composition was then prepared by mixing the following to homogeneity on
a two-
roll mill into 100 parts of the silicone rubber base compound: 30 parts mica
powder, 1
part trimethylsiloxy-endblocked dimethylsiloxane-methylhydrogensiloxane
copolymer
with a viscosity of 25 mPa~s (molar ratio of silicon-bonded hydrogen in this
component to
vinyl in the gum = 3.3 : 1 ), sufficient chloroplatinic
acid/divinyltetramethyldisiloxane
complex to provide 4 ppm platinum metal, 0.03 part 1-ethynylcyclohexanol as
cure
inhibitor, and 1 part of the hollow silicone resin powder prepared as
described in '
Reference Example 1. This moldable fire-resistant silicone rubber sponge
composition
was introduced into an extruder configured for molding gaskets and a fire-
resistant gasket
of this composition with a diameter of 3 cm was molded. This gasket was placed
in a
forced hot-air convection oven and heated for 3 minutes at 250°C to
produce a fire-
resistant silicone rubber sponge gasket. This fire-resistant silicone rubber
sponge gasket
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was cut with a knife and the diameter of the contained foam cells was
measured. The
cells diameters were in the range from 0.2 to 0.5 mm. The expansion ratio was
2.25-fold.
Example 6
The moldable silicone rubber sponge composition prepared in Example 1 was
coated over the circumference of a roll core and the coated roll core was
placed in a
compression mold designed for roll molding. A silicone rubber sponge-coated
roll was
then molded by curing the moldable silicone rubber sponge composition by
heating for 10
minutes at 170°C. Measurement of the expansion ratio of the silicone
rubber sponge
coating layer gave a value of 2.55-fold. The foam cells present in the
silicone rubber
sponge had diameters ranging from 0.2 to 0.5 mm and were essentially uniform.
Comparative Example 1
The following were introduced into and mixed to homogeneity in a kneader
mixer: 100 parts dimethylvinylsiloxy-endblocked dimethylsiloxane-
methylvinylsiloxane
copolymer gum (weight-average molecular weight = 500,000) composed of 99.85
mole%
dimethylsiloxane units and 0.15 mole% methylvinylsiloxane units, 110 parts wet-
process
silica with a BET specific surface area of 130 m2/g (Nipsil LP from Nippon
Silica Kogyo
Kabushiki Kaisha), and, as a surface-treatment agent for the silica, 10 parts
hydroxyl-
endblocked dimethylsiloxane oligomer with a viscosity of 50 mPa~s. The
resulting
mixture was heated for 60 minutes at 175°C to produce a silicone rubber
base compound
that had a Williams plasticity of 640. A moldable silicone rubber sponge
composition
was then prepared by mixing the following to homogeneity on a two-roll mill
into 100
parts of the silicone rubber base compound: I part trimethylsiloxy-endblocked
dimethylsiloxane-methylhydrogensiloxane copolymer with a viscosity of 25
mPa~s,
sufficient chloroplatinic acid/divinyltetramethyldisiloxane complex to provide
2 ppm
platinum metal, 0.013 part 1-ethynylcyclohexanol as cure inhibitor, and 1 part
of the
hollow silicone resin powder prepared in Reference Example 1. This composition
was
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formed into a sheet with a thickness of 2 mm, placed in a 230°C oven,
and cured for 5
minutes to give a sheet of silicone rubber sponge. Measurement of the
expansion ratio on
this silicone rubber sponge sheet as in Example 1 gave a value of 1.45-fold.
Comparative Example 2
A moldable silicone rubber sponge composition was prepared as in Example 1,
but without addition of the 1 part hollow silicone resin powder that was used
in Example
1. The properties of this composition were measured as described in Example 1
and the
results are reported in Table 1 below.
Comparative Example 3
A moldable silicone rubber sponge composition was prepared as in Example 3,
but without addition of the 1 part hollow silicone resin powder that was used
in Example
3. The properties of this composition were measured as described in Example l
and the
results are reported in Table 1 below.
Comparative Example 4
A silicone rubber sponge was produced in accordance with the method described
in Japanese Laid Open (Kokai or Unexamined) Patent Application Number Hei 10-
67875
(67,875/1998). A liquid silicone rubber base compound was prepared by
introducing the
following into a mixer and mixing to homogeneity: 100 parts
dimethylvinylsiloxy-
endblocked dimethylpolysiloxane (vinyl content = 0.14 weight%) having a
viscosity of
1,000 centipoise, 7 parts hexamethyldisilazane-treated fumed silica having a
specific
surface of 200 m2/g, and S parts hydroxyl-endblocked dimethylsiloxane oligomer
having
a viscosity of 50 mPa~s. A liquid A was prepared by mixing 100 parts of this
liquid
silicone rubber base compound to uniformity with 0.4 part chloroplatinic
acid/divinyltetramethyldisiloxane complex (platinum content = 0.4 weight%) and
5 parts
of the hollow silicone resin powder (average particle size = 40 pm, average
shell
13
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thickness = 4 p,m) whose preparation is described in Reference Example 1. This
liquid A
was filled into a cartridge. A liquid B was separately prepared by mixing 100
parts of the
liquid silicone rubber base compound to homogeneity with 10 parts
trimethylsiloxy-
endblocked methylhydrogenpolysiloxane having a viscosity of 20 centipoise
(silicon-
bonded hydrogen content = 1.5 weight%), 2 parts 3,5-dimethyl-1-hexyn-3-of
(cure
inhibitor), and 5 parts benzyl alcohol (solvent). This liquid B was also
filled into a
cartridge. Liquids A and B were then mixed to uniformity at a 1 : 1 ratio by
passage
through a 12-element static mixer and the product was extruded into a metal
frame to
produce a 2 mm-thick sheet. This sheet was cured for 1 hour at room
temperature to
produce silicone rubber sponge. The tensile strength of this silicone rubber
sponge
measured in accordance with JIS K-6251 was 0.25 MPa.
Table 1.
Example Example Example ComparativeComparative
1 2 3
Example Example
2 3
expansion 2.60 2.65 2.25 2.26 1.45
ratio
cell diameter0.3-0.5 0.3-0.6 0.2-0.5 3.0-8.0 2.0-6.0
evaluationuniform uniform uniform nonunifonm,nonunifonn,
of cell abnormal abnonmal
uniformi foamin foamin
14
