Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
PROCESS FOR THE PREPARATION OF SILICONE POLYMER/SOLID
PREMIXES HAVING A HIGH FILLER CONTENT
BACKGROUND OF THE INVENTION
Field of the Invention:
The invention relates to a process for the preparation of silicone polymer/solid
premixes having a high filler content which are employed for the preparation of silicone
rubbers.
Description of the Background:
Processes for distributing solids in liquids are known. The solid is usually
incorporated into the liquid with the aid of mixing organs which rotate at more or less rapid
speeds, and as a result the solid is wetted completely. The resulting mixture of wetted solid
and liquid is then subjected to mechanical stress in a mixing chamber for a sufficiently long
time to achieve as homogeneous as possible a distribution of the solid in the liquid.
To intensify this mechanical stress, a procedure is often followed (Masterbatch
process, c~ Degussa"Schriftenreihe Pigmente" [Pigments Publication Series] Number 63,
1995) which incorporates the total amount of a solid initially in only a portion of the liquid.
As a result of the higher consistency of the solid/liquid mixture, which can be achieved via a
higher solids content, kneading and/or shear energy can be introduced considerably more
effectively by a mixing organ, since the introduction of mixing energy has a greater effect on
regions of the goods to be mixed which are close to the mixing organ. After mechanical
stress on the mixture of solid and the portion of liquid, the mixture is diluted with further
liquid.
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Incorporation of a large amount of solids into liquids by the masterbatch process
presents problems. It is irrelevant here whether the process is carried out in an open mixing
unit, such as a twin-shaft kneader or dissolver, or in a closed mixer, such as an internal mixer
or compression mixer, or in a continuous mixer, for example in a twin-screw extruder. If too
much solid is added, the compact mixture disintegrates and therefore cannot be processed
further, and in the case of mixtures with semi-active or active solids in particular, crumbling
occurs.
The distribution of highly disperse silicic acids as rheology ~llxili~ries into organic
polymers is described in a brochure from Wacker-Chemie GmbH ("HDK Das
Verdickungsmittel" [HDK The Thickener], 1985), but the silicic acid content sought here in
the finished formulation is relatively low. During preparation of silicone rubber mixtures, to
improve the mechanical strength of the crosslinked elastomer, it is necessary to incorporate
large amounts of chiefly active solids, such as highly disperse silicic acids, precipitated or
naturally occurring chalks or highly disperse metal oxides, such as aluminum oxide or
titanium dioxide, into silicone polymers.
It is known to accelerate the incorporation of solids by addition of incorporation
auxiliaries. For the preparation of silicone rubber mixtures having a filler content of silicic
acid, for example, monomeric or oligomeric silanes which are reactive in respect of the silicic
acid surface, such as hexamethyl(lisil~7~ne or short-chain polydimethylsiloxanes having
hydroxyl end groups, are incorporated as ~llxili~ries, the silicic acid surface is simultaneously
hydrophobized, i.e. subjected to a chemical treatment (NOLL "Chemie und Technologie der
Silicone" [Chemistry and Technology of the Silicones] 1968, p. 346). The process of in situ
hydrophobization of silicic acid with hexamethyl~ 7~ne has been used for a long time for
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the preparation of two-component silicone rubbers which crosslink at room temperature and
have a filler content of silicic acid (cf., for example, German Patent application DE 24 33
697). In spite of the use of these known auxiliaries and the use of the so-called masterbatch
process, the processes known to date impose limits on increasing the silicic acid proportion in
the mixture and therefore the intensity of the stress on the goods to be mixed. If other solids
are employed, it is possible to employ dispersing auxiliaries. Only relatively small amounts
are used here, and these as a rule remain in the product. However, limits are also imposed
here on increasing the proportion of solids in the mixture.
DE 25 35 334 describes a process for the homogeneous distribution of highly disperse
fillers into polysiloxanes. In this process, the solid is hydrophobized in situ while being
mixed into the polymer, and the mixture is then subjected to a mechanical treatment. A
silicic acid proportion of about 25% by weight is achieved.
An extrudable material comprising polymer and filler is described in DE 44 42 871.
In this case, up to 33 to about 65% by weight of solid is combined with a polymer under
conditions of a relatively high shear action, and the components are then mixed, while
retaining the high shear forces. In this process, it is already necessary to employ a high shear
action during mixing of the filler into the polymer, and therefore an a~plopl;ate mixing unit.
This can be realized technologically only with difficulty, since high shear forces must already
be introduced during the incorporation into the mixture, which is still a relatively thin liquid
at this point. A more effective introduction of energy is possible only in mixtures having a
higher filler content.
No processes are known which describe the preparation of silicone polymer/solid
premixes having a very high filler content in simple mixing units.
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SUMMARY OF THE INVENTION
An object of the invention is to increase the proportion of solids in silicone
polymer/solid premixes. Using the premixes having a high filler content, it is possible to
prepare silicone rubber mixtures which are distinguished by improved flow properties of the
non-crosslinked mixtures and very good mechanical properties, such as, a higher extensibility
and lower Shore A Hardness of the cured elastomers.
The invention relates to a process for increasing the proportion of solids in silicone
polymer/solid premixes and the associated better distribution of the solid in a preferably
linear polyorganosiloxane with the aim of improving both the rheological properties of the
non-crosslinked mixtures and the elastomer characteristic values.
The proportion of solids is increased according to the invention by a procedure in
which a readily volatile siloxane which contains no hydrolyzable groups is first added for
incorporation of the solid into a polyorganosiloxane, and is then removed again by means of a
vacuum and/or by elevated temperature. As a result, it is possible to increase the proportion
of solids in silicone polymer premixes far beyond the maximum proportion in filler/polymer
mixtures which can be achieved by processes known to date, without the mixture
disintegrating .
These objects are achieved by a process comprising incorporating into a mixture (A)
of (a) 50 to 100 parts by weight of at least one polyorganosiloxane cont~ining up to 5 mol %
cro~slink~ble groups, and (b) 5 to 200 parts by weight, preferably 20 to 100 parts by weight,
of at least one readily volatile organosilicon compound cont~ining no hydrolyzable groups,
100 to 500 parts by weight, preferably 150 to 300 parts by weight, of a solid (B). The
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compound (b) is removed and the mixture having a high filler content is then subjected to
mechanical stress.
DETAILED DESCRIPTION OF THE INVENTION
Polyorganosiloxanes (a) which are employed are linear and/or branched
polyorganosiloxanes in which up to 5 mol % of the organo radicals are crosslink~ble groups
and which preferably have more than 50 Si atoms and viscosities of between 0.05 and
100 Pas. Examples of crosslink~ble groups are hydroxyl and/or alkoxy groups in
con~len~tion-crosslinking systems, dimethylvinylsiloxy and/or hydrogen end groups and/or
hydrogen and/or vinyl side groups in addition-crosslinking systems, or vinyl groups in
peroxidically crosslinking systems. Methyl, phenyl and/or trifluolol)lo~yl radicals, inter alia,
can be bonded as further groups which do not have a crosslinking action. Branched
polyorganosiloxanes which are employed are liquid siloxanes which, in addition to di- and
monofunctional siloxy units, also contain tri- and tetrafunctional units. The ratio of tri- and
tetrafunctional to monofunctional groups is usually chosen here such that the polymers are
liquids at room temperature (25~C). They are used, for example, with the aim of achieving
specific flow properties in the elastomers. Mixtures of different polyorganosiloxanes may
also be employed. Preferably, the polyorganosiloxanes (a) have a molecular weight of at
least 1000.
Readily volatile organosilicon compounds (b) which contain no hydrolyzable groups
are those which contain no SiX groupings, in which X is a hydrolyzable radical, for example,
hydroxyl, alkoxy, chlorine, oxime, nitrogen or hydrogen radicals. A hexaalkyldisiloxane,
further low molecular weight dialkylsiloxanes having up to 5 Si atoms and/or one or more
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cyclic dialkyl siloxanes having 3 to 6 silicon atoms are preferably employed. The alkyl
groups may contain 1-10 carbon atoms. Advantageously, and for reasons of industrial
availability, the compounds employed contain methyl groups as the alkyl groups. The readily
volatile organosilicon compound (b) has a low molecular weight, preferably at most 1000.
Readily volatile means that at least 99 weight % of the organosilicon compourid (b) can be
removed from the mixture at a temperature of at most 200~C at a pressure of at least 1 torr in
24 hours or less. No hydrolyzable groups means that less than 1 mol% of the groups bounded
to silicon will hydrolyze when the compound is mixed with water at room temperature, in 24
hours.
Solids (B) which can be employed are all the solids known for use in silicone rubber
mixtures, in particular active solids, such as highly disperse silicic acids, precipitated or
naturally occurring chalks, kieselguhrs or highly disperse metal oxides, such as aluminum
oxide or titanium dioxide. Pyrogenic and precipitated silicic acids, for example having a BET
surface area of greater than 50 m2/g, are particularly preferred. The surfaces of the solids used
may be pretreated.
If highly disperse hydrophilic silicic acid is used, it is advantageous to add, before or
during incorporation thereof, 0.1 to 50 parts by weight, preferably 1 to 10 parts by weight, of
at least one reactive compound (aa) which is suitable for surface treatment of the silicic acid,
the sum of all the parts by weight of mixture (A) being 100. Organosilicon compounds having
1 to 2 Si atoms, for example ~ 7~nes, such as hexamethyl~ 7~ne, or trialkylsilanols, for
example trimethylsilanol, are preferably used. If silazanes are used, it is customary to work in
the presence of water if the moisture adhering to the silicic acid is not sufficient. If silanols
are employed, it is favorable to add ~ nes or an NH3/water mixture. Short-chain siloxanes
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having up to 10 Si atoms and alkoxy, acetoxy, chlorine and/or hydroxyl groups can
furthermore be employed as reactive compounds. Any readily volatile compounds formed
are removed from the premix together with compound (b), after the silicic acid has been
incorporated.
The incorporation of solids (B) into mixture (A) can be carried out, for example,
discontinuously in a twin-shaft kneader, compression mixer, dissolver or in other known
mixing units. The process according to the invention may of course also be carried out in
continuously operating mixing units, such as a twin-screw extruder. The incorporation may
be carried out at room temperature or elevated temperature and under pressures above
atmospheric pressure.
After the incorporation, compound (B) and, if app~op~iate, readily volatile compounds
contained in the premix or formed, for example, during the hydrophobization, are removed
from the premix, usually by means of a vacuum and/or an increase in temperature.
The resulting mixture having a high filler content is then subjected to mechanical
stress by a rotating mixing organ, preferably in the same mixing unit in which the solid has
also been mixed in, for example a twin-shaft kneader. The time that the mixture is subjected
to stress can last from a few minutes to several hours, and like the intensity of the stress,
depends on the desired degree of mixing and the particle size sought for the solid. The
increased proportion of solids here allows for considerably more intensive introduction of
energy into the premix than is possible with previously known processes. As a result, a
considerably better distribution of the solid in the polyorganosiloxane can be achieved.
The mechanical stress can be controlled via the speed of rotation of the mixing organ,
the highest possible speed of rotation being sought. As a result of the high proportion of solid
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in the premix, however, it is also possible to incorporate the desired kneading and/or shear
energy in mixing units with a slowly rotating mixing organ. If a completely filled mixing
chamber (free from dead space) is used, it is possible to achieve average particle sizes of
silicic acid of less than 100 nm, for example less than 50 nm, as a result of the intensive
mechanical stress exerted here on the premix having a high filler content. Mechanical stress
under the action of pressure is also possible. The use of a mixing unit which can be heated is
also favorable, in order to remove any readily volatile compounds still contained in the
mixture from the mixture during or after the mixing operation.
Depending on the later intended use, the premixes having a high filler content
obtained by the procedure according to the invention can either be diluted with
polyorganosiloxane (a) and/or stored as a silicone rubber premix, or be processed to the
ready-cured elastomer, if ap~rupl;ate after addition of further customary mixture constituents,
such as crosslinking agents, plasticizers, catalysts, adhesion promoters, pigments, compounds
having a reinforcing action, such as MQ silicone resins, and further active or inactive fillers.
To achieve a homogeneous and storage-stable mixture, the dilution is preferably carried out at
temperatures above 1 00~C.
Silicone rubber mixtures which result from the premixes prepared according to the
invention have the great advantage over conventional mixtures in that they are exceptionally
storage-stable, i.e. retain their viscosity even over several months, and can also be prepared as
mixtures having a high filler content and good flow properties. Furthermore, they are of
considerably lower viscosity than conventionally prepared mixtures, while retaining good
mechanical characteristic values of the elastomers.
The silicone polymer/solid premixes prepared according to the invention can be
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employed, for example, in silicone rubber mixtures with good flow properties, as impression
and embedding compositions and, if chalk is used, as a solid for the ~l~pa~lion of sealing
compositions.
The chemistry of silicones, and the compounds used for their preparation are well
known to those of ordinary skill in the art, and are described in "The Encyclopedia of
Chemical Technology" Kirk-Othmer, 4th ed., Vol. 22, pages 1-154 (1997, John Wiley & Sons,
Inc.), hereby incorporated by reference.
Having generally described this invention, a further understanding can be obtained by
reference to certain specific examples which are provided herein for purposes of illustration
only and are not intended to be limiting unless otherwise specified.
Embodiment Examples
All viscosity data are based on 25~C.
Example 1
90g of a pyrogenic silicic acid which has not been pretreated and has a surface area of
200 m2/g were incorporated into a mixture of 120 g of a polydimethylsiloxane having
hydroxyl end groups and a viscosity of 5 Pas, 15 g of hexamethyldisiloxane, 24 g of
hexamethyl~ 7~ne and 6 g of water in a twin-shaft kneader of 0.5 ~ capacity. This mixture
was kneaded at 130~C for 1 hour. It was then heated up to 160~C in vacuo, the
hexamethyldisiloxane and readily volatile compounds formed being removed. The silicone
rubber premix having a high filler content thus obtained was kneaded at temperatures
between 130 and 160~C for a further hour. It was then rediluted with 120 g of the
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polydimethylsiloxane having hydroxyl end groups and 60 g of a polydimethylsiloxane having
trimethylsiloxy end groups and a viscosity of 0.1 Pas. The viscosity of the resulting mixture
was 34 Pas.
100 g of the mixture prepared was cured at room temperature for 7 days with 5 g of a
crosslinking agent liquid which comprised 60 parts by weight of tetraethoxysilane, 35 parts
by weight of methyltriethoxysilane and 5 parts by weight of dibutyltin dilaurate. The cured
elastomer had a hardness of 31 Shore A and a tear propagation resistance of 25 N/mm.
Example 2
Example 1 was repeated, with the modification that the silicic acid was incorporated
into a mixture of 90 g of a polydimethylsiloxane having hydroxyl end groups and a viscosity
of 5 Pas, 45 g of hexamethyldisiloxane, 24 g of hexamethyldisilazane and 6 g of water. The
viscosity of the diluted mixture was 27 Pas. The material cured with the crosslinking agent
liquid had a hardness of 28 Shore A and a tear propagation resistance of 24 N/mm.
Example 3 (comparative example)
Example 1 was repeated, with the modification that the silicic acid was incorporated
into a mixture of 135 g of a polydimethylsiloxane having hydroxyl end groups and a viscosity
of 5 Pas, 24 g of hexamethyl~lisil~7~ne and 6 g of water. No hexamethyldisiloxane was
added. The viscosity of the diluted mixture was 59 Pas. The material cured with the
crosslinking agent liquid had a hardness of 29 Shore A and a tear propagation resistance of 25
N/mm.
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The comparative example shows that the mixture, which has the same composition as
that of Examples 1 and 2, has a considerably higher viscosity which is undesirable for later
use, with the same high tear propagation resistance.
Example 4
Example 1 was repeated, with the modification that the silicic acid had a surface area
of 300 m2/g and was incorporated into a mixture of 65 g of a polydimethylsiloxane having
dimethylvinylsiloxy end groups and a viscosity of 6 Pas, 45 g of hexamethyldisiloxane, 28.5
g of hexamethyl(1i~ 7~ne, 1.5 g of divinyltetramethyl(1isil~7~ne and 7.5 g of water, and the
mixture was kneaded at 130~C for 1 hour. The resulting mixture was diluted with 145 g of
polydimethylsiloxane having dimethylvinylsiloxy end groups and a viscosity of 6 Pas. The
viscosity of the resulting mixture was 172 Pas.
90 g of the mixture were cured at 1 50~C for 30 minutes with a cro~linking agent
component comprising 6.9 g of polydimethylsiloxane having dimethylvinylsiloxy end groups
and a viscosity of 6 Pas, 0.3 g of a 1% strength Pt catalyst and 2.8 g of polydimethylsiloxane
having dimethylhydrogensiloxy end groups and an H-Si content of 4.0 mmol/g. The cured
material had a hardness of 28 Shore A and a tear propagation resistance of 30 N/mm.
Example 5 (comparative example)
Example 4 was repeated, with the modification that no hexamethyldisiloxane was
incorporated. The viscosity of the diluted mixture was 272 Pas. The material cured with the
cros~linking agent component according to Example 4 had a hardness of 29 Shore A and a
tear propagation resistance of 30 N/mm.
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The comparative example without addition of hexamethyl(li~ 7~ne shows that the
finished mixture which has the same composition as that from Example 4 has a considerably
higher viscosity which is undesirable for later use, with the same high hardness and tear
propagation resistance.
Example 6
Example 4 was repeated, with the modification that the silicic acid was incorporated
into a mixture of 65 g of a polydimethylsiloxane having dimethylvinylsiloxy end groups and
a viscosity of 60 Pas, 45 g of octamethylcyclotetrasiloxane, 28.5 g of hexamethyl~ 7~ne,
1.5 g of divinyltetramethyl~ 7~ne and 7.5 g of water. Redilution was carried out with 295
g of the polydimethylsiloxane having dimethylvinylsiloxy end groups and a viscosity of 60
Pas. The viscosity of the resulting mixture was 239 Pas.
90 g of this mixture was cured at 1 50~C for 30 minutes with a crosslinking agent
component comprising 8.3 g of polydimethylsiloxane having dimethylvinylsiloxy end groups
and a viscosity of 6 Pas, 0.3 g of a 1% strength Pt catalyst and 1 g of polydimethylsiloxane
having dimethylhydrogensiloxy end groups and an H-Si content of 7.0 mmol/g. The cured
material had a hardness of 30 Shore A and a tear propagation resistance of 20 N/mm.
Example 7
Example 1 was repeated, with the modification that the resulting silicone rubber
premix having a high filler content was subjected to intensive mechanical stress in a
laboratory compression mixer with no dead space at a speed of rotation of the mixing organ
of 10 ms~~ under a pressure of 9 bar and at a temperature of 120~C for 120 minutes. Before
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further dilution, an average particle size of the filler of 40 nm was determined by means of
tr~n~mi~ion electron microscopy. The viscosity of the diluted mixture was 28 Pas.
Example 8
Example 1 was repeated, with the modification that after the exposure to mechanical
stress, the mixture having a high filler content was diluted at a temperature of 1 20~C. The
viscosity of the resulting mixture was 32 Pas.
Example 9
270 g of a precipitated chalk treated with stearic acid and having a surface area of 4
m2/g were incorporated into a mixture of 90 g of a polydimethylsiloxane having hydroxyl end
groups and a viscosity of 30 Pas, 6 g of a polydimethylsiloxane having hydroxyl end groups
and a viscosity of 0.05 Pas, 30 g of a polydimethylsiloxane having trimethylsiloxy end groups
and a viscosity of 0.1 Pas and 60 g of hexamethyldisiloxane in a twinshaft kneader of 0.5 Q
capacity. This mixture was heated up to 1 1 0~C in vacuo, with constant kneading, the non-
reactive hexamethyldisiloxane being removed from the mixture. The silicone rubber premix
having a high filler content thus obtained was then kneaded at 80~C for one hour. After the
mechanical stress, the mixture was rediluted with a further 180 g of the polydimethylsiloxane
having hydroxyl end groups and a viscosity of 30 Pas. The viscosity of the diluted mixture
was 129 Pas.
100 g of the resulting mixture were cured at room temperature for 7 days with 10 g of
a crosslinking agent liquid comprising 20 parts by weight of a partial hydrolyzate of
methyltrimethoxysilane, 10 parts by weight of propyltriethoxysilane, 55 parts by weight of an
13
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adhesion promoter combination of aminopropyltriethoxysilane and
glycidoxytrimethoxysilane, 10 parts by weight of carbon black and 5 parts by weight of
dibutyltin dilaurate. The cured material had a hardness of 52 Shore A. Glass test specimens
produced from the material had an elongation at break of 120% and a tensile strength of 1.0
MPa.
Example 10
Example 9 was repeated, with the modification that 270 g of a precipitated chalk
treated with stearic acid and having a surface area of 4 m2/g were incorporated into a mixture
of 60 g of a polydimethylsiloxane having hydroxyl end groups and a viscosity of 30 Pas, 6 g
of a polydimethylsiloxane having hydroxyl end groups and a viscosity of 0.05 Pas, 30 g of a
polydimethylsiloxane having trimethylsiloxy end groups and a viscosity of 0.1 Pas and 90 g
of hexamethyldisiloxane. After the mechanical stress, the mixture was rediluted with a
further 210 g of the polydimethylsiloxane having hydroxyl end groups and a viscosity of 30
Pas. The viscosity of the resulting mixture was 124 Pas.
100 g of the finished mixture were cured at room temperature for 7 days with 10 g of
the crosslinking agent liquid. The cured material had a hardness of 50 Shore A. An
elongation at break of 150% and a tensile strength of 1.1 MPa were measured on glass test
specimens.
Example 11 (colllpal~Li~e example)
Example 9 was repeated, with the modification that 270 g of a precipitated chalk
treated with stearic acid and having a surface area of 4 m2/g were incorporated into a mixture
14
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of 150 g of a polydimethylsiloxane having hydroxyl end groups and a viscosity of 30 Pas, 6 g
of a polydimethylsiloxane having hydroxyl end groups and a viscosity of 0.05 Pas and 30 g of
a polydimethylsiloxane having trimethylsiloxy end groups and a viscosity of 0.1 Pas. After
kneading at 80~C for 1 hour, the resulting mixture was rediluted with a further 210 g of the
polydimethylsiloxane having hydroxyl end groups and a viscosity of 30 Pas. The viscosity of
the resulting mixture was 182 Pas.
100 g of the finished mixture were cured at room temperature for 7 days with 10 g of
the cro~linking agent liquid. The cured material had a hardness of 56 Shore A. An
elongation at break of 110% and a tensile strength of 0.9 MPa were measured on glass test
specimens.
Comparative example 11 shows that the mixture, which has the same composition as
that of Examples 4 and 5, has a high viscosity which is undesirable for later use. The glass
test specimens produced with this mixture do not achieve sufficiently high tensile strengths
and elongations at break.
Obviously, additional modifications and variations of the present invention are
possible in light of the above teachings. It is therefore to be understood that within the scope
of the appended claims, the invention may be practiced otherwise than as specifically
described herein.
The priority document of the present application, German Patent Application No. DE
196 53 993, filed December 21, 1996, is hereby incorporated by reference.
