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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a fluororubber composition, and, more
specifically, relates to a fluororubber composition which is highly
suitable for blending or processing on a roll mill and which is readily
released from molds during a molding operation. With the optional addition
of a specified silane the composition develops excellent adhesion to a
variety of substrates during curing.
2. Description of the Prior Art
Organic fluorine containing rubbers, also referred to as fluororubbers, are
used in a broad range of industrial applications due to their excellent
mechanical properties. A shortcoming of these rubbers is the difficulty of
fabricating them on roll mills and achieving a good release of the cured
fluororubber from molds. In addition, the molds have a definite tendency
to become soiled during molding operations, which together with the poor
release properties results in a high reject rate.
The use of finely divided fluororubber particles as modifiers for a
polyorganosiloxane elastomer is disclosed in Japanese unexamined published
application no. 60/105,557, which issued on June 11, 1985. Japanese
unexamined published application No. 60/112,431, which was published on
June 18, 1985, teaches using these modified fluororubber compositions as
the material for preparing thermal fixing rolls for electrostatic copying
machines.
U.S. Pat. No. 4,488,771, which issued on Dec. 18, 1984, describes molding
compositions comprising a matrix of a fluorosilicone elastomer and a
crosslinking agent containing dispersed particles of a solid fluorocarbon
telomer.
U.S. Pat. No. 4,010,136, which issued on Mar. 1, 1977 discloses
polyorganosiloxane elastomer compositions containing a silica filler an
organic peroxide and powdered polytetrafluoroethylene as a dispersed
phase.
In all of the aforementioned prior art the continuous phase is a
polyorganosiloxane and the dispersed phase is a fluorocarbon polymer.
The present invention was the result of extensive investigations directed
at solving the aforementioned problems with fluororubbers.
The object of the present invention is to provide a curable fluororubber
composition which is excellent with respect to (1) its ability to be
fabricated on roll mills and the low reject rate of articles molded from
these compositions, and (2) the excellent adhesion developed during curing
between the rubber and a variety of substrates.
The present invention also provides a method for improving the
processability of curable fluororubber compositions, specifically the
ability of these compositions to be molded and fabricated using roll mills
typically used for other types of rubber compositions. This invention also
provides additives for improving the adhesion of the cured fluororubber
compositions to metal and plastic substrates.
SUMMARY OF THE INVENTION
The objective of the present invention is achieved by incorporating into a
fluororubber from 0.1 to 30 weight parts per 100 weight parts of
fluororubber of a finely divided particulate silicone material.
DETAILED DESCRIPTION OF THE INVENTION
This invention provides an improved fluororubber composition comprising 100
parts by weight of a fluororubber and an amount of curing agent sufficient
to cure said fluororubber, where the improvement comprises the presence in
said composition of from 0.1 to 30 parts by weight of a finely divided
cured silicone material in the form of a rubber, gel or resin, where said
silicone material exhibits an average particle diameter not exceeding 100
micrometers.
If it is desired to achieve adhesion between the cured fluororubber and a
metal or plastic substrate the curable composition also contains from 0.1
to 30 parts by weight of an alkoxysilane containing an epoxy group as a
substituent.
The present invention also provides a method for improving the
processability of a curable composition comprising 100 parts by weight of
a fluororubber and an amount of curing agent sufficient to cure said
fluororubber, said method comprising combining said composition with from
0.1 to 30 weight parts of a finely divided cured silicone material in the
form of a rubber, gel or resin, where said silicone material exhibits an
average particle diameter not exceeding 100 micrometers.
Cohesive bonding of the cured composition to various substrates, including
metals and plastics, can be achieved by including in the curable
composition from 0.1 to 30 parts of an alkoxysilane containing an epoxy
group as a substituent or a partial hydrolysis product of such a silane.
The Fluororubber Ingredient
To explain the present invention in greater detail, the fluororubber
compositions of this invention contain a curable fluorine-containing
organic or fluorine-containing organosiloxane polymer as its main
ingredient. The cured polymer is a rubbery elastomer. Such fluororubbers
are described, for example, in the "Compendium of Synthetic Rubber
Processing Technology. Fluororubber/Silicone Rubber." [in Japanese]
(Yuzuru Komeya, et al., authors' published by Kabushiki Kaisha
Taisei-sha).
Examples of suitable fluororubbers include but are not limited to (1)
vinylidene fluoride-based polymers such as
chlorotrifluoroethylene/vinylidene fluoride copolymers,
pentafluoropropane/vinylidene fluoride copolymers, and
hexafluoropropene/vinylidene fluoride copolymers: and (2) fluorosilicone
rubbers containing a fluoroalkyl group-containing organopolysiloxane as
the main ingredient.
The Curing Agent
The compositions of this invention typically include at least one of the
conventional curing or vulcanizing agents for the fluororubber ingredient.
Typical curing agents for the vinylidene fluoride-type of fluororubbers
include but are not limited to polyfunctional amines such as
hexamethylenediamine carbamate and organoperoxides such as benzoyl
peroxide and dicumyl peroxide. Curing agents for fluorosilicone rubbers
include but are not limited to organoperoxides such as 2,4-dichlorobenzoyl
peroxide, dicumyl peroxide and 2,5-dimethyl-2,5-di(t-butylperoxy)hexane.
The Cured Silicone Material
The cured particulate silicone material of the present invention consists
essentially of particles or granules having average diameter not in excess
of 100 micrometers. The mechanical strength of the fluororubber
composition of the present invention substantially declines when this
average diameter of the particles exceeds 100 micrometers.
The concentration of particulate cured silicone material in the present
compositions is from 0.1 to 30 weight parts, preferably from 0.1 to 15
weight parts, and most preferably from 0.1 to 5 weight parts per 100
weight parts of the fluororubber ingredient. The ability of the
particulate silicone material to function as a mold release agent is not
apparent at concentrations below about 0.1 weight part, while the
mechanical strength of the cured fluororubber can be adversely affected
when the concentration of particulate silicone material exceeds about 30
weight parts.
The cured particulate silicone material can be a rubber, gel, or resin. The
silicone material, which typically includes at least one
organopolysiloxane, has been cured by an addition, condensation or free
radical reaction.
Addition reaction-cured silicone materials are typically obtained by curing
a composition comprising an organopolysiloxane having at least two
silicon-bonded alkenyl groups per molecule, an organohydrogenpolysiloxane
having two or more silicon-bonded hydrogen atoms per molecule, and a
platinum-containing catalyst.
Organoperoxide-cured silicone materials are typically obtained heating an
organopolysiloxane in the presence of an organoperoxide.
Condensation reaction-cured silicone materials are typically obtained by
the curing a composition comprising a curing catalyst, an
organopolysiloxane containing at least two silanol groups per molecule
and, as the curing agent, an organohydrogenpolysiloxane or a silane
containing at least three silicon-bonded hydrolyzable groups, such as
alkoxy, per molecule. Curing agents containing hydrolyzable groups
typically require the presence of atmospheric moisture to activate them.
It will be understood that the curable silicone rubber composition can
include additives, such as reinforcing fillers, to modify the physical
properties of the cured material.
A preferred type of particulate silicone material is obtained by curing a
silicone rubber composition in which at least a portion of the organic
groups bonded to the silicon atoms of the organopolysiloxane are
fluorinated propyl radicals. The cured silicone material preferably
contains an additive to improve compatibility or miscibility, and thereby
improve the adhesion between the particulate silicone material and the
fluororubber. Examples of suitable additives of this type include but are
not limited to fluoropropyl group-containing organoalkoxysilanes such as
3,3,3-trifluoropropyltrimethoxysilane and
3,3,3-trifluoropropylmethyldimethoxysilane; epoxy groupcontaining
organoalkoxysilanes such as gamma-glycidoxypropyltrimethoxysilane, and
gamma-glycidoxypropylmethyldimethoxysilane, partial hydrolysis condensates
of any of these silanes, and epoxy group-containing organic compounds such
as glycidyl methacrylate and vinylcyclohexene monoxide.
One method for preparing the particulate cured silicone material is
described in U.S. Pat. No. 4,742,142, which issued on May 3, 1988. In
accordance with the teaching in this patent a curable silicone rubber is
blended with water and a surfactant in a colloid mill or homogenizer to
prepare a dispersion of the silicone rubber composition. The silicone
rubber composition is then cured in particulate form by dispersing the
aqueous dispersion in water heated to a temperature of at least 50 degrees
Centigrade.
The Optional Epoxy-Substituted Alkoxysilane
An epoxy-substituted alkoxysilane or a partial hydrolysis product of such a
silane is present in the curable compositions of this invention when it is
desired to adhere the cured fluororubber to a plastic or metallic
substrate. These silanes include but are not limited to
gamma-glycidoxypropyltrimethoxysilane,
gamma-glycidoxypropyltriethoxysilane,
gamma-glycidoxypropylmethyldimethoxysilane, and
beta-(3,4-epoxycyclohexyl)ethyltrimethoxysilane
The concentration of the epoxy-substituted silane is typically from 0.01 to
30 parts by weight per 100 parts of fluororubber. This range is preferably
from 0.01 to 15 parts, most preferably from 0.01 to 2 parts.
Preparation of the Present Curable Compositions
One method for preparing the fluororubber composition of this invention
comprises mixing the fluororubber, particulate cured silicone material,
optional additive and curing agent for the fluororubber to homogeneity
using a two-roll mill.
The uncured fluororubber compositions of this invention excel with respect
to their ability to be processed on roll mills typically used for rubber
compositions. Cured articles that have been molded using these
compositions are readily released from molds without staining the mold
surfaces, resulting a low reject rate for the finished articles.
Compositions containing the optional epoxy-substituted alkoxysilane develop
excellent bonding to both metal and plastic substrates during curing.
The following examples are intended to describe preferred embodiments of
the present compositions and method and should therefore not be
interpreted as limiting the scope of the invention as defined in the
accompanying claims. Unless otherwise specified all parts and percentages
specified in the examples are by weight and viscosities were measured at
25 degrees C. The molding properties of the fluororubber compositions were
tested by the following method.
Determination of Molding Properties and Molding Reject Rate
The fluororubber composition to be evaluated was placed in a
chromium-plated mold used to form key pads for electronic calculators. The
dimensions of the mold were 10.times.10.times.0.5 cm. The composition was
cured at 170 degrees Centigrade for 10 minutes. This molding process was
repeated 100 times, and the mold-releasability was evaluated by visually
examining the surfaces of the resultant key pads. In addition the
percentage of moldings with rough and/or damaged surfaces was determined
and is reported as the molding reject rate in the examples. It was
observed that rough and/or damaged surfaces on the key pad were caused
mainly by poor mold release and/or soiling of the mold surfaces.
EXAMPLE 1
An addition reaction-curing silicone rubber composition was prepared from
100 parts of a dimethylvinylsiloxy-terminated dimethylpolysiloxane having
a viscosity of 2,000 centistokes (2.times.10.sup.-3 m.sup.2 /second) and a
vinyl content of 0.25%, 1.5 parts of a trimethylsiloxy-terminated
methylhydrogenpolysiloxane having a viscosity of 10 centistokes
(1.times.10.sup.-5 m.sup.2 /second) and a siliconbonded hydrogen atom
content of 0.9% and 0.15 parts of 3% isopropanolic chloroplatinic acid
solution. 5 Parts of a nonionic surfactant available as Tergitol TMN-6
from Union Carbide Corporation and 200 parts water were then added and
mixed into this silicone rubber composition.
The resultant aqueous dispersion of a curable silicone rubber composition
was cured by introducing the dispersion into 5000 parts of water
maintained at a temperature of 70.degree. C. The cured silicone rubber was
then isolated and dried by heating to yield a particulate silicone rubber
with an average particle diameter of 10 micrometers.
A curable fluororubber composition of this invention was obtained by
kneading the following ingredients to homogeneity on a two roll mill: 4
parts of the particulate silicone rubber prepared as described in the
preceding paragraph 100 parts of a vinylidene fluoride type fluororubber
(Viton.RTM. E430 from E.I. DuPont Company), 3 parts calcium hydroxide, 6
parts magnesium oxide, 25 parts carbon black, and 2 parts dicumyl
peroxide. This fluororubber composition did not substantially adhere to
the surface of the mill rolls, and was judged to have a good
rollworkability.
The reject rate for the molded articles was 11%, and the mold-releasability
in this test was excellent.
For comparison, a fluororubber composition was prepared as described in the
preceding portion of this example, with the exception that the cured
particulate silicone rubber was not added. This fluororubber composition
readily adhered to the surface of the mill rolls, and was therefore
considered to have poor roll-workability. The reject rate for molded
articles prepared from this composition was 21%.
EXAMPLE 2
A condensation reaction-curing silicone rubber composition was prepared
from 100 parts of a dimethylhydroxysiloxyterminated dimethylpolysiloxane
having a viscosity of 100 centistokes (1.times.10.sup.-4 m.sup.2 /second),
10 parts of the methylhydrogenpolysiloxane described in Example 1, 10
parts gamma-glycidoxypropyltrimethoxysilane to improve the compatibility
of the silicone rubber with the fluororubber, and 1 part of stannous
octoate as the curing catalyst. The composition was cured as described in
Example 1 to yield particles of cured silicone rubber exhibiting an
average diameter of 12 micrometers.
The following ingredients were kneaded on a two-roll mill to produce a
curable fluororubber composition of this invention: 5 parts silicone
rubber powder obtained as described in the first section of this example
100 parts of a vinylidene fluoride-type fluororubber (Dai-el G901 from
Daikin Kogyo Co. Limited), 2 parts dicumyl peroxide, 4 parts triallyl
isocyanurate, and 20 parts carbon black. This fluororubber composition had
an excellent roll-workability during blending. When the composition was
molded, the cured articles released easily from the mold and the reject
rate was 8%.
EXAMPLE 3
A fluororubber composition of this invention was prepared by blending the
following ingredients to homogeneity on a two roll mill: 5 parts of a
cured particulate silicone rubber prepared as described in Example 2, 100
parts of a fluorosilicone rubber (LS63u from Toray Silicone Company Ltd.),
and 1 part of 2,5-dimethyl-2,5-di(t-butylperoxy)hexane. The curable
composition could be processed on a roll mill without difficulty. Cured
articles molded using this composition did not stick to or stain the mold
surfaces.
EXAMPLE 4
A finely divided addition reaction-curing fluorosilicone rubber of this
invention was prepared from 100 parts dimethylvinylsiloxy-terminated
trifluoropropylmethylsiloxanedimethylsiloxane copolymer having a vinyl
group content of 0.15% and containing 60 mole% of
trifluoropropylmethylsiloxane units and 40 mole% dimethylsiloxane units: 1
part of the methylhydrogenpolysiloxane described in Example 1. and 0.15
part of a 3% isopropanolic chloroplatinic acid solution. The resultant
fluorosilicone rubber composition was cured as described for the finely
divided material of Example 1 to yield a powder consisting essentially of
spherical particles with average diameters of 10 micrometers.
A curable fluororubber composition was prepared using the procedure of
Example 2, with the modification that the fluorosilicone rubber powder
prepared as described in the preceding section of the present example was
used in place of the silicone rubber powder described in Example 2. The
resultant composition exhibited a good ability to be blended on the roll
mill. The reject rate for the articles molded from this composition was
5%.
EXAMPLE 5
A silicone rubber powder was prepared using the procedure described in
Example 2, with the modification that trifluoropropyltrimethoxysilane was
used in place of the gamma-glycidoxypropyltrimethoxysilane used in Example
2.
A curable fluororubber composition was then prepared using the procedure of
Example 2, with the modification that the silicone rubber powder obtained
as described in the preceding paragraph of the present example above was
used in place of the silicone rubber powder described in Example 2.
Evaluation of this composition demonstrated its excellent
roll-workability, molding properties, and a molding reject rate of 3%.
Example 6
A silicone rubber powder was prepared according to the procedure of Example
1, but without the surfactant described in Example 1. The cured silicone
rubber consisted of particles with an average diameter of 20 micrometers.
A fluororubber composition was prepared according to the procedure in
Example 1 with the modification that the silicone rubber powder described
in the first paragraph of the present example was used in place of the
silicone rubber powder of Example 1. The composition exhibited excellent
workability on the roll mill and releasability from the mold.
EXAMPLE 7
This example and the following Example 8 demonstrate the use of an optional
epoxy-substituted alkoxysilane to improve the adhesion of the present
fluororubber compositions to metal and polyester substrates.
An addition-curing silicone rubber composition was prepared from 100 parts
dimethylvinysiloxy-terminated dimethylpolysiloxane exhibiting a viscosity
of 2 Pa.s and a vinyl content of 0.25 percent, 1.5 parts
trimethylsiloxy-terminated methylhydrogenpolysiloxane (viscosity=10
centistokes, siliconbonded hydrogen content=0.9%). and 0.15 parts 3%
isopropanolic chloroplatinic acid solution. 5 Parts surfactant (Tergitol
TTMA6. nonionic surfactant from Union Carbide Corp. and 200 parts water
were mixed into this silicone rubber composition, and this mixture was
then poured into 5,000 parts hot water (70.degree. C.); in order to cure
said silicone rubber composition. The resultant cured material was removed
and dried in a hot sir flow to yield a silicone rubber particulate with an
average diameter of 10 micrometers.
A fluororubber composition was then obtained by mixing the following on a
two-roll mill: 4 parts of the silicone rubber particulate obtained as
above, 100 parts vinylidene fluoride-based fluororubber available as Viton
E 430 from DuPont 3 parts calcium hydroxide, 6 parts magnesium oxide, and
2 parts gamaglycidoxypropyltrimethoxysilane. The fluororubber composition
did not stick to the surface of the two-roll mill during this mixing
process, and its roll workability was thus excellent. It could be mixed to
homogeneity by kneading for 12 minutes.
This fluororubber composition was applied on a stainless steel (Type SUS
304) test plate and a polyester resin test plate, and the fluororubber
composition was cured by heating for 10 minutes at 170.degree. C. under a
pressure of 25 kg/cm.sup.2. The cured fluororubber was bonded to both the
stainless steel and polyester substrates to from a single unit. The
fluororubber on each test specimen was then peeled by pulling an end at an
angle of 90.degree. relative to the plane of the test specimen. Failure
occurred entirely in the rubber layer in both test specimens, which was
rated as 100% cohesive failure.
For purposes of comparison, a fluororubber composition was prepared as
described in the first section of this example, with the exception that
the particulate silicone rubber was omitted. This composition stuck to the
surface of the mill roll and its workability wa therefore rated as poor. A
kneading time of 30 minutes was required to obtain a homogeneously mixed
fluororubber composition. A second comparative fluororubber composition
was prepared using the particulate silicone rubber described in the first
part of this example, but omitting the
gammaglycidoxypropyltrimethoxysilane. When this composition was cured in
contact with a stainless steel or a polyester substrate as described in
the first section of this example, separation occurred at the interface
between the rubber later and the substrate, and was rated as 100 percent
interfacial separation.
EXAMPLE 8
A condensation curable silicone rubber composition was prepared using the
following ingredients: 100 parts of a dimethylhydroxysiloxy-terminated
dimethylpolysiloxane exhibiting a viscosity of 0.1 Pa.s, 10 parts of the
methylhydrogenpolysiloxane described in Example 1 and 1 part of stannous
octoate. This composition was converted to a particulate silicone rubber
(A) as described in Example 7.
The following ingredients were blended to homogeneity using a two-roll
mill: 5 parts of the particulate silicone rubber A, 100 parts of a
vinylidene fluoride-based fluororubber available as Dai-el G901 from
Daikin Kogyo Company, Limited, 5 parts of triallyl isocyanurate, and 20
parts of carbon black. The workability of the resultant fluororubber
composition was excellent. When the adhesion of this composition was
evaluated as described in Example 7, 100 cohesive failure was achieved
using both the stainless steel and polyester substrates.
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