Note: Descriptions are shown in the official language in which they were submitted.
~z~z~
This invention relates to a method of bonding heat
activated organic peroxide vulcani~ed silicone rubber to the
surface of substrates during the process of vulcanizing the
rubber.
Certain uses for silicone rubber such as shock mounts and
metal-enclosed shaft seal~ require that the rubber be firmly
bonded to the surface of the substrate.
Two general methods are used for bonding silicone rubber
to surfaces. The silicone rubber can be formed to shape and
vulcanized, as in a mold, then bonded to a substrate surface with
an adhesive. Alternatively, the unvulcanized silicone rubber
stock can be applied to the subs~rate surface and then vulcanized.
In either case, most types of substrate surfaces must be
carefully cleaned and then treated with special primers in order
to obtain satisfactory adhesion of the vulcanized silicone rubber
to the substrate surface. ~he priming of the substrate surface
before the bonding step is a costly and time consuming operation
that is not necessary in the method of this invention.
A majority of the commercial primers presently available
are activated when applied to a substrate surface by the moisture
in the air. The variability of the drying and hydrolyzing
conditions due to day to day differences in the humidity in the
air can lead to variability of results.
A different method of obtaining adhesion to a substrate
surface is throu~h the addition of adhesion additives to the
unvulcanized silicone rubber stock. U.S. Patent 4,033,924 to Mine
et al. discloses a heat curable organopolysiloxane composition
containing an organosilicon compound having at least one A~R~o)2si
group and at least one alkyl, low molecular weight alkenyl group,
or hydrogen atom bound to silicon, A is a monovalent epoxy
. ~ . . . . .. . ... . . . .
i~%~23~
containing hydrocarbon group and R' is a low molecular weight
alkyl group.
An improved method of obtaining bonding of a heat
activated organic peroxide vulcanized silicone rubber to substrate
surfaces has been developed. A silicone rubber composition of the
type commonly known as "high viscosity" that is vulcanized through
the use of organic peroxides is used as the basic material. The
first step of the method is mixing into the silicone rubber
composition an acryloxyalkylsilane. ThiS modified composition is
then formed into the desired shape in contact with the cleaned
substrate surface to which it is to be bonded. The modified
composition is then heated to vulcanize it while it is in contact
with the substrate surface. Bonding of the modified composition
to the substrate surface takes place during the vulcanization
~ step.
This invention relates to a method for improving the
bonding of a vulcanized silicone rubber to a substrate surface
comprising
(a) mixing 100 parts by weight of silicone rubber base
consisting essentially of polydiorganosiloxane ;
containing organic radicals selected from the group
consisting of methyl, vinyl, phenyl and
3,3,3-trifluoropropyl, reinforcing silica filler,
and anticrepe-hardening agent; from 0 to 150 parts
by weight of siliceous extending filler with an
average particle size of less than 25 micrometres
and a surface area of less than 50m2/g; from 0.1 to
5 parts by weight of organic peroxide vulcanizing
agent suitable for vulcanizing the silicone rubber
base; and from greater than 0.1 to 1.5 parts by
~ 2
.Z;3~
weight of an acrylo~yalkylsilane of the formula
R O Ra
C~2=C-C-O-R'~Six(3-a)
in which R is selected from the group consisting of
hydrogen and methyl radicals, R' is an alkylene
radical of from 1 to 4 inclusive carbon atoms, X is
a radical selected from the group consisting of
alkoxy radicals of from 1 to 3 inclusive carbon
atoms and acetoxy radicals, and a is from 0 to 2
}0 inclusive, to yield a curable silicone rubber
composition,
(b) forming a composition wherein the curable silicone
rubber composition contacts a surface of a
substrate, and thereafter
(c) heating the combination to a temperature
sufficiently high to vulcanize the composition,
thereby yielding a vulcanized silicone rubber bonded to the
substrate surface.
The silicone rubber base used in the present invention
can be any mixture of polydiorganosiloxane and reinforcing silica
filler including types commercially available. The polydiorgano-
siloxane of this invention contains organic radicals selected from
the group consisting of methyl, vinyl, phenyl and 3,3,3-trifluoro-
propyl, said radicals being bonded to the silicon atoms of the
polydiorganosiloxane. The polydiorganosiloxanes are commonly of a
viscosity of from 1000 Pa-s up to and including non-flowing gums.
These polydiorganosiloxanes are well known in the art and are
commercially available.
A silicone rubber base contains a reinforcing silica
30~ filler to improve the physical strength of the polymer.
2~ ~3~
Reinforcing silica fillers have surface areas of from 1~0 to
greater than 400 m2/g. These reinforcing silica fillers are well
known in the art and can be obtained commercially. The
reinforciny filler can be untreated, treated, or treated in situ
during the manufacture of the silicone rubber base. The treated
reinforcing silica fillers can be treated by any of the
conventional methods described in the prior art, wherein the
treating agents include organosilanes, organosiloxanes and
silazanes. The amount of reinforcing filler can vary from 10 to
as much as 100 parts by weight and the usual amount varying
between 15 to 75 parts by weight per 100 parts by weight of the
polydiorganosiloxane.
A silicone rubber base can also contain anti-crepe
hardening agents. These anti-crepe hardening agents are used to
reduce the reaction between the polydiorganosiloxane and the
reinforcing silica that causes the base to become harder or
pseudo-vulcanized. Such a reaction can cause the base to become
too "nervy" to be of further use.
Suitable anti-crepe hardening agents are well known in
the art. They can be such additives as hydroxyl endblocked short
chain polydimethylsiloxane fluids. If the reinforcing filler is
treated as discussed above, the silicone rubber base may not need
an additional anti-crepe hardening agentO
The silicone rubber base may also contain minor amounts
of additives to improve, among other things, the heat stability,
handling, compression set and oil resistance. A single silicone
rubber base may be used or a mixture of bases may be used to
obtain the desired range of physical properties for the cured
silicone rubber.
1~L%~.Z3~
In use, a silicone rubber base may be extended with an
extending filler to increase the bulk of the composition. This
helps to lower the cost of the finished part as the extending
fillers are much lower in cost than the silicone rubber base.
When a silicone rubber base i5 extended with an extending filler
such as ground quartz, the tensile strength of the cured
composition is lower than that of the beginning base. The amount
of tensile strength lost is dependent upon the relative amounts of
base and extending filler used as well as ~he exact nature of both
I0 ingredients.
The addition of an extending filler may also lower the
bond strength of a composition intended to be bonded to a
substrate surface. The method of this invention provides a means
whereby compositions containing large amounts of extending filler
can still be successfully bonded to substrate surfaces. As higher
levels of extending filler are added, it becomes more difficult to
achieve a satisfactory bond to a substrate surface. The maximum
amount of extending filler that can be used and still obtain
satisfactory bonding to a substrate surface will depend upon the
nature of the silicone rubber base used and the extending filler
used. The maximum is about 150 parts by weight of extending
filler per 100 parts by weight of silicone rubber base. ;~
The siliceous extending fillers used with silicone rubber
bases are finely ground particles of heat stable inorganic
materials with an average particle size of under 25 micrometres.
The finest extending fillers approach a particle size and
configuration such that they have a surface area of as high as
50 m2/g. Examples of siliceous extending fillers include ground
quartz, diatomaceous earth and glass.
~ 5
~`` 3~ ;23~3
About 25 parts by weight of extending filler per 100
parts by weight of silicone rubber base is necessary to
signficantly lower the cost of the composition. The preferred
siliceous extending fillers for use with the present invention are
ground quartz and diatomaceous earth with the most preferred
filler being ground quartz with an average particle size of about
5 micrometres.
- The composition of this invention contains an organic
peroxide vulcanizing agent suitable for vulcanizing the polydi-
organosiloxane in the silicone rubber base. When the polydiorgano-
siloxane does not contain any vinyl radicals, it must be
vulcanized with organic peroxides that are efficient in causing
reactions in such polydiorganosiloxanes. Such organic peroxides
are labeled "non-vinyl specific" and are represented by such well
known organic peroxides as benzoylperoxide, dicumylperoxide and
2,4-dichlorobenzoylperoxide. When the polydiorganosiloxane
contains vinyl radicals, it can be vulcanized with either
"non-vinyl specific" or "vinyl specific" organic peroxides.
Representative of the vinyl specific organic peroxides are
ditertiary-butyl peroxide and 2,5-bis-~tert-butylperoxy)-2,5-
dimethylhexane. All these organic peroxide vulcanizing agents and
their properties are well known in the art. The properties of the
vulcanized silicone rubber can be altered by the type and amount
of vulcanizing agent used to vulcanize the composition. Typical
changes due to such choices are well recognized in the art. The
vulcanizing agent can be present in amounts of from 0.1 to 5 parts
by weight 2er 100 parts by weight of silicone rubber base,
preferably from 0.5 to 2O0 parts by weight.
The critical component of the composition used in the
3~ method of this invention is an acryloxyalkylsilane of the formula
: ,. .: : : , :
:: ' :'
.
L2~
R O Ra
CH2=C-C-O-R'-Six~3-a)
in which R is selected from the group consisting of hydrogen and
methyl radicals, R' is an alkylene radical of from 1 to 4
inclusive carbon atoms, X is a radical selected from the group
consisting of alkoxy radicals of from 1 to 3 inclusive carbon a~oms
and acetoxy radicals, and a is from 0 to 2 inclusive. The silane
is preferred where R is a methyl radical, a is 0, and X is a
methoxy radical or acetoxy radical. The most preferred silane is
gamma-methacryloxypropyltrimethoxysilane because of its efficiency
in causing the vulcanized silicone rubber to bond to a substrate
surface against which the silicone rubber has been vulcanized.
The acryloxyalkylsilanes used in this invention are known
in the art. They are disclosed in U.S. Patent No. 3,567,497 by
Plueddemann and Clark which describes the silanes and their method
of manufacture. The preferred gamma-methacryloxypropyltrimethoxy-
silane is commercially available.
The compositions of this invention bond to a substrate
surface when the composition is cured while in contact with the
subtrate surface during the vulcanization of the composition. In
order to obtain bonding, it is necessary to use at least about 0.1
part by weight of silane per 100 parts by weight of silicone
rubber base. The exact amount of silane necessary to obtain
bonding along with the optimum property profile of the cured
silicone rubber composition can be easily determined by simple
experimentation. The results will depend upon the silicone rubber
base selected, the kind and amount of extending filler used, the
kind and amount of vulcanizing agent used, and the nature of the
substrate surface to be adhered to. The preferred amount of
silane is from greater than 0.1 to 1.0 parts by weight per 100
~ ,~ ~
.
3~
parts by weight of silicone rubber base. The addition of more
than about 1.5 parts by weight of silane will not improve the
adhesion and will begin to adversely effect the physical
properties of the cured silicone rubber composition.
The mixing step of this invention used to prepare the
composition can be any suitable means that will lead to a
homogeneous mix~ure of the several components. Methods of mixing
that are common in the silicone rubber art and which are suitable
for this invention include mixing with a dough mixer, a rubber
1~ compounding mill or with a Banbury mixer. The order of mixing is
not critical. Ordinarily, the silicone rubber base is placed in
the mixer, the extending filler and silane are added and mixed
until homogeneous, then the vulcanizing agent is added and mixing
continued until homogeneous. Any additional additives such as
heat stability additives, antioxidants, processing aids, pigments,
etc. would ordinarily be added before the vulcanizing agent.
The compositions can be formed to the desired
configuration by any of the well known methods of forming
elastomeric curable compositions such as press molding, injection
molding, calendering and extruding, both supported and
unsupported. Since the compositions bond without primers, special
precautions must be taken during vulcanizing operations to insure
that the vulcanized composition adheres only to surfaces where
adhesion is desirable. The surfaces of press plates or molds for
instance must be well coated with a suitable release agent. ~ `~
Suitable release agents for the method of this invention
are heavy coats of a 2 to 5 percent by weight solution of
detergent in water, or more preferably, a coating of fluorocarbon
mold release agent. For flat surfaces, a sheet of polytetrafluoro-
ethylene is satisfactory.
`.: 8
. . ~ , ' , ~ , . .
~: ,. .: .
^: , , . . .. . .: .:
The formed compositions of this invention can bevulcanized by any suitable means that will cause decomposition of
the organic peroxide vulcanizing agent. Heating is the preferred
method. The time and temperature necessary to cause vulcanization
of the composition is dependent upon the organic peroxide
vulcanizing agent chosen, the method of heating, the method of
shaping the composition to the desired configuration, and the
thickness of the part. The temperature that is appropriate for a
given set of conditions is well known in the silicone rubber art.
Typical temperatures are from 110C. to 175C. for molding
operations, to as high as 300C. for the ovens used in continuous
hot air vulcanization operations.
The method of this invention is useful for making
silicone rubber articles that are bonded to a subtrate surface.
Examples of such articles are metal enclosed shaft seals, shock
mounts, rolls, and various types of fabric reinorced articles
such as tubing, tapes and diaphragms.
The following examples are included for illustrative
purposes only and should not be construed as limiting the
invention which is properIy delineated by the appended claims.
All parts are parts by weight.
Example 1
A series of samples were made to evaluate the bonding
characteristics of stock containing an acryloxyalkylsilane as
compared to the same stock without the silane.
A stock was compounded consisting of:
(a) 100 parts of commercial silicone rubber base
described as a vinyl-containing silicone rubber
designed for compounding general purpose silicone
rubber stock. The base was translucent with a ;~
g
... .
.
,.; . ., . ' . , '':.'' ~ - ' , ' :, ,:
specific gravity after curing of 1.09. The base
consisted of a vinyl containing polydimethyl-
siloxane, a reinforcing fume silica, and a hydroxyl
endblocked polydimethylsiloxane fluid to prevent
crepe-hardening of the base,
(b) 50 parts of ground quartz with an average particle
size of 5 micrometres,
(c) ~ parts of iron oxide paste,
(d) 1 part of organic peroxide vulcanizing agent
consisting of 50 weight percent
2,5-bis(tert-butylperoxy)-2,5-dimethylhexane
dispersed on an inert carrier powder.
To a portion of this stock was added 0.5 parts of
gamma-methacryloxypropyltrimethoxysilane based on 100 parts of the
silicone rubber base.
,
Each stock was then calendered onto a piece of glass
fiber fabric to a total thickness of 0.5 mm. The silicone rubber
surface of the calendered fabric was then press molded against the ;
cleaned surface of metal panels as shown in Table I. The surface
of the metal panels was cleaned by wiping thoroughly with
chlo~othene, then with acetone. Two pieces o~ the calendered
fabric were also molded against each other with the rubber `
sur~aces in contact. The moldings were for 10 minutes at 171C.
After molding, each sample was cut into 2504 mm wide
strips. The calendered fabric strips were then pulled from the
various substrate surfaces using a standard tensile testing
machine with a rate of 50.8 mm per minute. The strips were ~ulled `
from the substrate surface at an angle of 180. The glass fiber
fabric samples were pulled from one another At a total angle of
180 or at 90 each at the point of peeling apart.
, 10
,
~2~ Z3~
The method of failure was noted for each sample. If
there was no adhesion, it was recorded as ~ero percent cohesive
failure. If the sample failed by tensile failure within the
rubber itself, it was recorded as 100 percent cohesive failure.
The results are shown in Table I. The addition of the
silane to the silicone rubber stock greatly improved the adhesion
to all of the substrate surfaces tested.
TABLE I
Amount of Subs~rateAdhesion Failure
lOSilane kN/m TyPe
none Aluminum 0.05 0%
cohesion
none C. Ro Steel 0.13 0
none Stainless0.13 0
Steel
none Glassfiber0.56 5
Fabric
0.5 part Aluminum 2.45 100
0.5 part C. R. Steel 2.62 100
0.5 part Stainless1.75 50
Steel
0.5 part Glassfiber2.80 80
Fabric
Example 2
A different commercial silicone rubber base was used to
prepare samples in the same manner as in Example 1.
A stock was compounded consisting of
(a) lO0 parts of a commercial silicone rubber base
similar to that of Example 1 but with a higher
loading of reinforcing silica. The specific gravity
of the translucent base was l.10,
~ZL1%~23~
(b) 2 parts of iron oxide paste, and
(c) 1 part of the organic peroxide vulcaniæing agent of
Example 1.
To a portion of the stock, 0.5 parts of the silane used
in Example 1 was added based on 100 parts of stock.
Test samples were prepared and tested in the same manner
as described in Example 1. The results are recorded in Table II.
The addition of the silane to a silicone rubber stock not
containing extending filler greatly improved the adhesion to all
the substrate surfaces tested.
A sample of each type of substrate surface was primed
with a commercial primer. The primer is used with organic
peroxide catalyzed, heat cured, silicone rubber stock to bond
without oven post curing. A sample was prepared using the above
stock of this Example without the silane added. The samples were
molded and tested as described in Example 1. The results are
recorded in Table II. The stock bonded with the primer did not
give as high a bond strength as that of the s ock bonded by adding
the silane to the stock. The adhesion shown by the glassfiber
fabric samples is partially due to mechanical trapping of the
rubber into the rough surface of the fabric. The amount of
cohesive failure is also more difficult to judge due to the very
uneven nature of the surface.
3a :
12
2:~23~
TABLE II
-
Amount of Substrate Adhesion Failure
Silane _ _ _ _ _kN/m ___Type_
none Aluminum 0.12 0~
cohesion
C. R. Steel 0.35 0
Stainless 0O53 0
Steel
Glassfiber 0.88 0
Fabric
0.5 Aluminum 3.7 100
C. R. Steel 3.3 100
Stainless 3.2 95
Steel
Glassfiber 3.8 80
~abric
1.O Aluminum 4.3 100
C. R. Steel 4.0 100 . .
Stainless 3.4 100
Steel
Glassfiber 4.2 .20
Fabric
primer Aluminum 0.88 40
C. R. Steel 1.1 20
Stainless O.g6 50
Steel
Glassfiber 4.1 15
Fabric
Example 3
Different types of additives were mixed with a commercial
silicone rubber base to compare their usefulness in improving
bonding to glassfiber fabric.
13
3CD
A stock was compounded consisting essentially of
(a) 100 parts of a commercial silicone rubber base
designed to give a 70 durometer, high tensile
strength product. The specific gravity of the base
was 1.~1,
(b) 50 parts of ground quartz with an average particle
size of 5 micrometres, and
(c) 1 part of the organic peroxide vulcanizing agent of
Example lo
Portions of the stock were mixed with 1 part of the
additives, detailed below, on a 2 roll mill. Each sample was then
molded under low pressure in a press against a coarse weave
untreated glassfiber fabric for 10 minutes at 171C. The samples
were then evaluated by pulling the cured silicone rubber stock and
the glassfiber fabric apart. The results are recorded in
Table III.
Sample 1 had no additive.
Sample 2 was a mixture of a trimethylsiloxy endblocked
polymethylhydrogensiloxane with a silicon-bonded hydrogen atom
content of about 1.6 weight percent and ethylpolysilicate. This
mixture is known to give bonding.
Sample 3 was gamma-glycidoxypropyltrimethoxysilane. This
epoxy functional silane is used to aid bonding with many di~ferent
types of polymer.
Sample 4 was gamma-methacryloxypropyltrimethoxysilane.
The results show that the silane of this invention gave
superior bonds as compared to the other additives tested.
14
.
2~3~
TABLE III
Sample Result
1 Blank weak mechanical bond,
0% cohesive failure
2 Comparative Example some bond, 0% cohesive
failure
3 Comparative Example stronger than 2,
o% cohesive failure
4 This invention strong bond,
I0 100% cohesive failure
Example 4
A series of samples were made to evaluate the effect of
adding acryloxyalkylsilane on the physical properties of the cured
silicone rubber composition. -
A stock was compounded consisting of 100 parts of the
commercial silicone rubber base of Example 1, 100 parts of the
ground quartz extending filler of Example 1, and 1 part of the
organic peroxide vulcanizing agent of Example 1.
Portions of the above stock were then mixed with
gamma-methacryloxypropyltrimethoxysilane in the amounts shown in
Table IV for 100 parts of base.
Each portion was molded into test slabs in a press, using ;
aluminum plates treated with a commercial soap-type release agent
specified for use with heat cured silicone rubber. The molding
was for 10 minutes at 171C. The samples containing 0.5 and 1.0
part of the silane were very difficult to remove from the aluminum
plates, even though the plates were coated with a release agent.
The physical properties of the slab were determined in
accordance with the procedures described by ASTM-412 for tensile
strength and elongation, by ASTM-D625, die B for tear strength,
:
2;3~
and by ASTM-D2240 for durometer, type A. The measured physical
properties were as shown in Table IV in which the tensile strength
is recorded in megapascals tMPa) and the tear streng~h is recorded -
in kilonewtons per meter ~kN/M).
The addition of the silane to the compounded stock
containing a siliceous extending filler did not harm the physical
properties. It caused a significant improvement in the tensile
strength.
TABLE IV
Tensile
Amount of Silane Durometer StrengthElongation
parts/100 p_rts base _ _ _ MPa _ %
none 60 3.1 400
0.1 62 6.8 200
0.5 62 6.9 180
1.0 63 ~ 6.0 150
Example 5
A series of samples were made to evaluate the level of
acryloxyalkylsilane needed in a vulcanized silicone rubber stock
to bond to glassfiber fabric~
A stock was compounded consisting of 100 parts of the
comme,rcial silicone rubber base of Example 3, 25 parts of the
ground guartz of Example 3, 1 part of a commercial heat stability
additive, and 1 part of the organic peroxide vulcanizin~ agent of
Example 1.
Portions of the stock were then mixed with the amounts of
gamma-methacryloxypropyltrimethoxysilane shown in Table V based on
100 parts of the silicone rubber base~
Each portion was then calendered onto style 1528
glassfiber fabric to a total thickness of 0.5 mm. Test samples
16
-
were prepared by placing pieces of each calendered sample together
such that the sample was 4 plies thick. The two center plies were
rubber face to rubber face, while the outer plies were rubber face
to glassfiber fabric face. Each sample was molded for 10 minutes
at 171C. in a press under light pressure to vulcanize the stock
and bond the pieces together.
The samples were then evaluated by pulling the pairs of
plies apart in a standard test machine at a rate of 50.8 mm per
minute causing the plies to separate at the center interface where
two rubber layers were together. The plies were pulled from one
another at a total angle of 180 or at go each at the point of
peeling apart.
The method of failure was noted for each sample. The
results are shown in Tablè V.
The addition of the silane to the stock used in this
method of bonding improved the bond over that obtained with no
silane. The failure at the 0.1 part level appeared to be an
adhesive failure, but the higher peel strength shows that some
adhesion must have been taking place. The lower peel strengths
for the samples with 0.75 part silane and 1.0 part silane are
probably due to the higher modulus of those stocks and its effec~
on the geometry of the failure point as the pieces are pulled
apart.
30`
~ .
17
:. : . . - . -, . .
;. ~ .,. : :.
- - , : ~ - .- .
.
.
23~1
TABLE V
Amount of Adhesion Failure
S ilane _ kN/m _Type
21one 0. 35 0%
cohes ion
Ool . 0~78 10
O~ 25 1~3 100
0~5 1~1 100
O ~ 75 0 ~ 91 100
1~0 0~96 100
,
18
