Note: Descriptions are shown in the official language in which they were submitted.
~95880
The invention relates to pasty compositions of mobile to solidJ
kneadable consistency, comprising essentially silicic acid esters,
polysilicic acid esters or silane crosslinking agents, curing catalysts
and hydrophilic active silicon dioxide as crosslinking components for
silicone rubbers which are vulcanizable at room temperature on the basis
of the silanol condensation.
It has been known for a considerable time to vulcanize silicone
rubber mixtures, which comprise a hydroxyl-containing polyorganosiloxane
as the polymer component, at room temperature by adding a silicic acid ester
and a curing catalyst in the presence of water which is either added to the
mixture or finds access to the mixture in the form of atmospheric moisture.
These products are of great commercial importance, for example in mold
construction, for potting, coatings and similar applications. They belong
to the 2-component silicone rubbers which are vulcanizable at room
temperature, since the component containing crosslinking agents and/or
condensation catalyst is mixed with the second component, namely the polymer
component, only shortly before use.
In the case of these 2-component silicone rubbers it has, for a
considerable time, been desired that the component containing the
crosslinking agent and curing catalyst should also be employed in a pasty
consistency since this makes it possible, for example, by choosing the
appropriate tube orifices, to obtain the quantitative ratios required for
the 2-component composition by means of equal "lengths of ribbon" of
polymer component and of crosslinking component, which offers more precise
and simpler metering than the addition of a liquid second component and
also permits simpler and better mixing of the two components.
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lO9S8~0
There has hitherto been no lack of attempts to solve the problem
of the manufacture and use of a pasty crosslinking agent technically and
in an economically acceptable manner.
DT-AS (German Published Specification) 1,153,169 describes pasty
2-component silicone rubber mixtures. These comprise a polydimethylsiloxane
with both ends terminated by hydroxyl groups, a crosslinking agent - which
can be a hydrogenosilane-containing silicone or a silicic acid ester - and
a filler. The second component contains a condensation catalyst, a diorga-
nopolysiloxane with the ends terminated by triorganosiloxy groups, and
likewise a filler. This composition has the disadvantage that the mixture
which contains the vulcanizable polydiorganosiloxane with ends terminated
by hydroxyl groups, and the crosslinking agent, suffers a noticeable loss,
on storage, in its effectiveness as a crosslinking agent.
DT-OS (German Published Specification) 1,669,940 describes a process
for the manufacture of elastomeric moldings, based on organopolysiloxanes,
having a high tear-starting resistance and peel strength, which process
comprises, inter alia, the use of pasty crosslinking agents, consisting of
a dimethylsiloxane oil containing triorganosilyl groups, hydrogenated
castor oil, a tin-containing condensation catalyst and solid calcium
carbonate. However, this mixture has the disadvantage that calcium carbonate
sediments on storage.
The use of hydrophobic, very finely divided silicon dioxide as a
thickener for pasty crosslinking agents which in addition contain, as
essential constituents, the crosslinking agent and the condensation catalyst,
is described in DT-OS (German Published Specification) 2,313,218. Of course,
this process makes it necessary to render the very finely divided silicon
dioxide used hydrophobic, for example by treatment with organoalkoxysilanes
or hexaorganodisilazanes, before use in the paste. In addition, the amount
of filler, which has been rendered hydrophobic, that must be used is
relatively large since the ability to assume the pasty consistency is of
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- 1~95880
course diminished by the hydrophobic treatment.
The present invention relates to a pasty composition, containing
crosslinking agents and curing catalysts, for organopolysiloxanes which are
vulcanizable at room temperature, comprising essentially crosslinking agents,
a curing catalyst and thickenersJ characte.ized in that the thickener comprises
about 3-40% by weight, relative to the total weight of the pasty composition,
of hydrophilic active silicondioxide.
Thus, this invention provides for a pasty composition for the room
temperature crosslinking and curing of a vulcanizable organopolysiloxane,
comprising a crosslinking agent for an organopolysiloxane, a curing catalyst
therefor, and about 3-40% by weight of the total amount of the pasty com-
position of hydrophilic active silicon dioxide as a thickener.
In a further embodiment, in the production of a crosslinked and
cured silicone rubber by mixing a vulcanizable organopolysiloxane with a
crosslinking agent and a curing catalyst therefor in the presence of water,
this invention provides for the improvement which comprises employing the
crosslinking agent and the curing catalyst in the form of a pasty composition
according to the present invention.
Surprisingly, it has been found that the described dlsadvantages of
the state of the art, such as inadequate storage stability, sedimentation of
the thickener or an additional hydrophobic treatment step do not arise, or
are not essential, if hydrophilic active silicon dioxide, in the stated range
of proportions, is added to a mixture of, essentially, silicic acid esters,
polysilicic acid esters or silane crosslinking agents and curing catalyst.
Furthermore, up to 40% by weight ~of the total mixture) of inactive
filler can be added to the crosslinking paste containing hydrophilic active
silicon dioxide. Examples of inactive fillers are quartz powder, calcium
carbonate, calcium silicate, calcined gypsum, titanium dioxide, diatomaceous
earth, asbestos and pigments.
r
1~95880
Crosslinking agents and curing agents can be activated before
use by heating them together.
Further additives which can be used are inert substances
conventionally employed for silicone rubber pastes, such as, for example,
plasticizing polydimethylsiloxanes with terminal triorganosilyl groups,
paraffin oils, paraffin greases - amongst these, especially "Vaseline
castor oil and dyestuffs or pigments.
If the amount of hydrophilic active silicon dioxide added is
kept below about 3%, the paste mixture is too soft unless further solid
thickeners are added. If inactive fillers are also added, a sufficiently
good pasty consistency can admittedly be obtained, but the excessive content
of inactive filler, as compared to the proportion of active filler, causes
a sediment to form on storage. If, when preparing the pastes, a proportion
of about 40% of hydrophilic active silicon dioxide is exceeded, a paste
consistency which permits good handling is no longer achievable.
The organopolysiloxanes employed are in the main polydiorgano-
siloxanes containing hydroxyl groups. These polydiorganosiloxanes can be
substituted by any of the known monovalent hydrocarbon or halogenated
hydrocarbon radicals, for example by alkyl radicals with 1-8 C atoms, such
as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl and octyl, by alkenyl
radicals with 1-8 C atoms, such as vinyl, allyl and butenyl, by halogenated
alkyl radicals, such as chloropropyl and 3,3,3-trifluoropropyl, by cycloalkyl
radicals, such as cyclobutyl, cyclopentyl and cyclohexyl, and also by
aromatic radicals, such as phenyl, tolyl, xylyl and naphthyl, or by
1-~9S8~
halogeno-aromatic radicalsJ such as chlorophenyl and chloronaphthyl,
as well as by alkylaryl radicals, such as benzyl and phenylethyl.
In addition to the diorganosiloxy units (R2SiO), the dior-
ganopolysiloxanes can also contain triorganosiloxy units (R3SiOo 5)'
monoorganosiloxy units (RSiOl 5) and unsubstituted silicon dioxide
units (SiO2).
Crosslinking substances which can be used are 0-silicic acid
esters, polysilicic acid esters or substituted silane crosslinking
agents. Examples of 0-silicic acid esters are 0-silicic acid methyl,
ethyl, n-propyl, isopropyl, chloroethyl, octyl, allyl, hexenyl,
cyclohexyl, phenyl, benzyl and chlorophenyl ester, and mixed alkylated
0-silicic acid esters, such as 0-silicic acid dimethyl diethyl ester.
Examples of the polysilicic acid esters formed by partial hydrolysis
and condensation and/or by heating the abovementioned 0-silicic
acid esters are methyl, ethyl and n-propyl polysilicate.
Examples of silane crosslinking agents are methyltri-
methoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, ethyl-
triethoxysilane, n-propyltrimethoxysilane, n-propyltriethoxysilane,
phenyltrimethoxysilane, phenyltriethoxysilane and methyltriacetoxy-
silane.
The amount of the crosslinking compound to be added is about
0.1 to 10 parts by weight per 100 parts by weight of polyorganosiloxane.
Curing catalysts which can be used in the process according
to the invention are all catalysts which promote the silanol con-
densation, the reaction of silanol with alkoxy or other groups which
are hydrolyzable by water, and the hydrolysis of silicon-bonded
alkoxy or other groups.
lt~9S880
Examples thereof are the metal salts of organic monocarboxylic
acids which may contain lead, tin, zirconium, antimony, iron, cadmium,
calcium, barium, manganese, bismuth or titanium as metals, such as dibutyl-
tin dilaurate, dibutyl-tin diacetate, tin-II octoate, iron octoate, lead
laurate, cobalt naphthenate, tetrabutyl titanate, tetraoctyl titanate
and tetraisopropyl titanate. Further examples are amines such as
n-hexylamine and cyclohexylamine, and amine salts such as hexylamine
hydrochloride, butylamine acetate and guanidine di-ethylhexoate. Carboxylic
acid salts of tin are preferred as catalysts. The amount of curing catalyst
added is between 0.1 and 10%, based on the organopolysiloxane employed.
The hydrophilic active silicon dioxide employed as the solid
thickener has a surface area, by the BET method, of about 35-600 m2/g,
preferably about 40 to 300 m2/g. The silicon dioxide can be manufactured
as an "aerogel" by combustion of silanes, for example of silicon tetra-
chloride or trichlorosilane, in the gas phase. These aerogels are
commercially available; they are offered, for example, under the name
"Aerosil ~ " or "Cabosil 6~ ". In addition to pyrogenically manufactured
active hydrophilic silicon dioxide, it is also possible to employ, according
to the invention, fillers which are manufactured by dehydrating silicic
acid hydrogels or by precipitating aqueous silicic acid solution.
The crosslinking agent paste which, according to the invention,
consists of silicic acid, polysilicic acid or silane crosslinking agents,
curing catalysts, hydrophilic active silicon dioxide and, optionally, other
constituents, such as, for example, oily or waxy paraffins such as, for
example, "Vaseline ~9", can be produced by mixing the components, for
_ . .
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5~80
example in a kneader. A simple process for handling this crosslinking agent
paste when carrying out the vulcanization is provided by filling both the
crosslinking agent paste and the polymer paste into separate tubes and
choosing the diameters of the tube orifices so that in each case equal
"ribbon lengths" of the pastes pressed out of the tubes can be measured out.
Per lO0 parts of polymer paste, between about 3 and 40 parts by weight of
crosslinking agent paste are used.
The pasty crosslinking agents according to the invention can be
employed, together with polymer mixtures, comprising hydroxyl-containing
polyorganosiloxanes and other constituents, in all cases where two-component
silicone rubbers are used, for example in mold-making. Dental impressions
is the preferred field of use.
The present invention will be explained in further detail with
the aid of the examples which follow:
Example 1
200 g of Vaseline* (paraffin grease) 40 g of Aerosil 130*
(pyrogenic silicic acid from Messrs. Deutsche Gold- und Silber-
scheideanstalt, vorm. Roessler, FrankfurttM., West Germany, having a
specific surface area, by the BET method, of 130 + 25 m2/g), 88 g of
hexamethoxydisiloxane and 72 g of dibutyl-tin dilaurate are introduced,
in the stated sequence, into a laboratory kneader of 0.75 1 capacity,
equipped with Sigma blades. The composition is mixed for 3 hours at
room temperature, with exclusion of moisture. Advantageously, the paste
obtained is immediately filled into tubes.
Example 2
300 g of tetraethoxysilane and 300 g of dibutyl-tin dilaurate
are stirred at 120C in a l 1 flask equipped with a
*Trade Mark
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10958~0
stirrer and a nitrogen connection. 140 g of Vaseline*, 40 g of Aerosil 130*,
40 g of Sikron 6000* (quartz powder from Quarzwerke GmbHJ 5020 Frechen,
West Germany) and 180 g of the above crosslinking agent/curing agent mixture
are introduced, in the stated sequence, into the laboratory kneader of
Example 1. Thereafter the further procedure is as described under Example 1.
Exanple 3
A paste is produced, as described under Example 1, from 140 g of
Vaseline*, 84 g of Aerosil OX 50* (pyrogenic silica from Messrs. Deutsche
Gold- und Silberscheideanstalt, vorm. Roessler, having a specific surface
area, by the BET method, of 50 + 15 m2/g), 8 g of titanium dioxide and 168 g
of the crosslinking agent/curing agent mixture from Example 2.
Example 4 (comparative example)
100 g of Vaseline*, 164 g of Aerosil OX 50*, 8 g of titanium
dioxide and 128 g of the crosslinking agent/curing agent mixture from
Example 2 are introduced into the laboratory kneader of Example 1 and mixed
for 3 hours at room temperature. The resulting composition is very
vi s cous .
Example 5
180 g of a polydimethylsiloxane, with terminal hydroxyl groups,
having a viscosity of 800 cP, and 170 g of a polydimethylsiloxane, with
terminal hydroxyl groups, having a viscosity of 22,o~ cP are heated to 100C
in a 3 1 thick-walled glass cylinder with a metal stirrer.
60 g of Sikron 6000* quartz powder are next addedJ while stirring,
the mixture is stirred for 10 minutes and 590 g of calcined gypsum are then
added gradually. The mixture is then stirred for a further hour at 100C
and for a further two
*Trade Mark
1~9S8BO
hours without supplying heat.
Example 6
Polydimethylsiloxanes with terminal hydroxyl groups, namely
700 g of a material of viscosity 1,000 cP and 300 g of a material of
viscosity 2,000 cP, 150 g of calcium silicate and 50 g of calcined
gypsum are brought together in a 2 l thickwalled glass cylinder with
a metal stirrer, and are stirred for two hours.
Examples 7-11 (vulcanizations~
10 g portions of the polymer mixture according to Example
5 or 6 are intimately mixed with up to 2 g of crosslinking agent
paste according to Examples 1-4. One part is transferred, after one
minute, into a small metal cap; the top is wiped over to give a smooth
surface and the metal cap, containing the vulcanization mixture, is
introduced into a thermostat containing water at 37C. The hardness
of the vulcanized samples, in Shore A, is in each case measured after
4'30", 6', 8' and 10', both when using the crosslinking paste im-
m0diately after its preparation, and when using it after it has been
stored.
The processing behavior is tested, using another portion
of the mixture of polymer mixture and crosslinking agent paste, by
carrying out the so-called "digital test". For this purpose, a part
of the mixture is taken between the thumb and the index finger and
these digits are moved relative to one another until the vulcanization
"tears". The time from starting the mixing to "tearing" represents
the digital test.
The table which follows contains vulcanization results of
mixtures which were determined immediately after preparation
1~)95S~
of the crosslinking agent paste, and after 12 months' storage of the
paste.
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~9S~380
It will be appreciated that the instant specification and
examples are set forth by way of illustration and not limitation,
and that various modifications and changes may be made without
departing from the spirit and scope of the present invention.
