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BACKGROUND OF THE INVENTION
The present invention relates to a flame insulative heat vulcanizable
silicone rubber composition and flame insulative two-part room temperature
vulcanizable silicone rubber composition and more particularly the present
invention relates to such compositions having in them as the flame
insulative additive from 25 to 150 parts of ground glass and more
preferably, glass frits.
Heat vulcanizable silicone rubber compositions are well known. Such
compositions usually consist of a linear diorganopolysiloxane polymer of
1,000,000 to 200,000,000 cps. viscosity at 25.degree. C., an inert filler
which is usually selected from the reinforcing fillers such as, fumed
silica and precipitated silica and also may contain to some extent
extending fillers such as, quartz or zinc oxide. To such a mixture there
is normally added a peroxide curing catalyst which cures the composition
at elevated temperatures to form a silicone elastomer. In the past such
heat vulcanizable silicone rubber compositions were utilized to insulate
wires and cables of various kinds so as to protect or maintain the
electrical integrity of the conducting wires or cables during a fire, that
is, such heat vulcanizable silicone rubber compositions were found to be
highly desirable as insulators for electrical wires and cables. Such
silicone materials tended to resist burning better than organic materials
and organic polymers. Such heat vulcanizable silicone rubber compositions
even when burning leave a white ash which if kept in place would protect
to a certain extent the underlying wires or cables from further burning
and would tend to prolong the maintaining of the electrical integrity of
the conducting wire or cable. In addition, there have been various
modifications in which such heat vulcanizable silicone rubber compositions
were made even more resistant to burning by the addition of small amounts
of platinum alone or with carbon black as, for instance, disclosed in the
Noble/Brower, U.S. Pat. No. 3,514,424. One difficulty with such flame
insulative heat vulcanizable silicone rubber compositions was that the
silicone elastomer that was formed would still, upon burning at highly
elevated temperatures, form a white ash that was easily displaced from its
place by gaseous by-products that were formed during the combustion
process, and by physical vibration of the burned cable. Accordingly, it
was felt highly desirable to develop a flame insulative heat vulcanizable
silicone rubber composition which, upon burning, would form a white ash
that maintained its physical integrity around the cable so as to form a
barrier between the flame and the wire that was being protected so as to
provide further flame protection capabilities to the wire that was
enclosed by such heat vulcanizable silicone rubber composition. Other
attempts have been made to keep the white ash that is formed when the
silicone elastomer was burned in place such as, use of glass braids around
the silicone elastomer. However, with the use of such glass braids, the
resultant cable was bulky and hard to work with in the construction and
installation of electrical systems.
The approach for the protection of such electrical conducting wires or
cables when there was utilized a two-part room temperature vulcanizable
silicone rubber composition as a sealant that was cured in place upon
mixing the two parts and forcing the material in the cavities in the cable
and allowing the composition to cure to a silicone elastomer, was such
that the silicone material was kept in place during the burning process by
the configuration and shapes that it took as it filled the voids of the
electrical cables. The use of such a room temperature vulcanizable
silicone rubber composition and in such a use is known to the industry as
a valley sealant. It was common when such room temperature vulcanizable
silicone rubber compositions were utilized as valley sealants that jackets
were inserted thereover, as an additional means for keeping the white ash
that was formed from the burning silicone elastomer in place. However, as
stated previously, the jackets burned even more rapidly than the silicone
elastomer and, thus, were not very effective, such that the valley
sealant, upon burning, formed a white ash and such white ash still tended
to be moved in its position by the gases that escaped and that were formed
by the burning of the silicone elastomer. Accordingly, it was highly
desirable for such two-part room temperature vulcanizable silicone rubber
compositions that were used as valley sealants to have added to them
additives that would maintain their integrity and result in the white ash
that was formed from the burning of the silicone elastomer being cohesive
so that the white ash of the burned silicone elastomer would remain in
place and act as a barrier between the flame and the electrical conducting
wires that were being protected by such room temperature vulcanizable
silicone rubber composition. It should also be noted that the use of
platinum as well as carbon black and various other additives to two-part
room temperature vulcanizable silicone rubber compositions, while
increasing to some extent the flame insulativeness did not perform any
function as far as allowing the cured silicone elastomer that was burned
into a white ash to maintain its integrity.
Accordingly, it is one object of the present invention to provide for an
improved heat vulcanizable silicone rubber composition which has improved
flame insulative properties by utilizing in the composition the necessary
amount of ground glass and more specifically glass frits so that the
silicone composition after it has burned to a white ash will still
maintain its integrity.
It is another object of the present invention to provide for a process for
producing the heat vulcanizable silicone rubber composition with improved
flame insulative properties by the utilizing in such process ground glass
or glass frits such that the composition after burning to a white ash
still maintains its integrity.
It is an additional object of the present invention to provide for a
two-part room temperature vulcanizable silicone rubber composition with
improved flame insulative properties which by the use of ground glass or
glass frits in the composition results in the composition maintaining its
integrity upon burning.
It is yet an additional object of this invention to provide for an improved
process for producing a two-part room temperature vulcanizable silicone
rubber composition with improved flame insulative properties which by the
addition of ground glass or glass frits in the composition results in the
composition maintaining its integrity.
These and other objects of the present invention are accomplished by means
of the disclosure set forth herein below.
SUMMARY OF THE INVENTION
In accordance with the above objects there is provided by the present
invention a flame insulative heat vulcanizable silicone rubber composition
comprising (a) 100 parts by weight of a linear diorganopolysiloxane
polymer of a viscosity ranging from 1,000,000 to 200,000,000 centipoise at
25.degree. C. with organic groups selected from monovalent hydrocarbon
radicals and halogenated monovalent hydrocarbon radicals, (b) from 50 to
300 parts by weight of an inert filler which preferably is selected from
fumed silica and precipitated silica, and (c) from 25 to 150 parts by
weight of ground glass. Preferably this composition which may be used in
both the uncured and the cured state contains from 0.1 to 10 parts by
weight of a peroxide curing catalyst and the composition is formed into a
silicone elastomer by heating the above ingredients, alone or with
additives, at elevated temperatures of about above 100.degree. C. for a
period of 1 hour to 8 hours to form a flame insulative silicone elastomer.
More specifically, there is desired for utilization in the instant
invention 25 to 150 parts by weight of ground glass known as glass frits,
which is a ground glass having a size of anywhere from 50 to 400 microns
and having a melting point between 1000.degree. to 1400.degree. F.
Preferably it is only this type of ground glass or glass frits that can be
utilized as the basic flame insulative additive in the compositions of the
instant case.
In a more preferred embodiment there is utilized as an additional flame
insulative additive so as to allow the composition, upon burning to a
white ash, to maintain its integrity, glass fibers at a concentrated of
from 5 to 30 parts. The most preferred glass fibers that are utilized as
an additional flame insulative additive in the composition of the instant
case are milled glass fibers having the average size ranging from 0.01 to
1 inch in length and, more preferably, having an average size of anywhere
from 0.01 to 0.5 inches in length.
There is also provided by the instant invention a two-part room temperature
vulcanizable silicone rubber composition comprising (a) 100 parts by
weight of a silanol end-stopped diorganosiloxane having a viscosity
varying from 1000 to 200,000 centipoise at 25.degree. C. (b) from 50 to
300 parts by weight of an inert filler, (c) from 25 to 150 parts by weight
of ground glass or specifically ground glass that is known as glass frits,
(d) from 1 to 15 parts by weight of a silicate having the formula
R.sub.a.sup.1 Si(OR.sup.2).sub.4-a and partial hydrolysis products
thereof, where R.sup.1 and R.sup.2 are selected from monovalent
hydrocarbon radicals and halogenated hydrocarbon radicals, and a is a
whole number that is 0 or 1, and (e) from 0.1 to 0.5 parts by weight as a
curing catalyst for the composition of a metal salt of a carboxylic acid
varying from lead to manganese in the periodic table. In this composition,
as well as in the case of the heat vulcanizable silicone rubber
composition, there may be utilized as an additional flame insulative
additive the glass fibers in the foregoing size range and, more
preferably, milled glass fibers. Such compositions or room temperature
vulcanizable silicone rubber compositions are cured by mixing at room
temperature the one part which usually comprises the silanol end-stopped
diorganopolysiloxane and the inert filler with the second part which
usually comprises the alkyl silicate and the metal salt of a carboxylic
acid. Both the heat vulcanizable silicone rubber composition and the room
temperature vulcanizable silicone rubber composition can have the usual
ingredients or additional flame insulative ingredients in the composition
to enhance their effectiveness. One additive, that is, a flame insulative
additive, which was found to degrade the flame insulative properties of
the instant composition was platinum. It was found that the presence of
platinum with glass frits in the composition did not improve the flame
insulative properties of either the heat vulcanizable silicone rubber
composition or the room temperature vulcanizable silicone rubber
composition from such compositions having no special flame insulative
additives or formulations.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The linear diorganopolysiloxane polymer of a viscosity varying from
1,000,000 to 200,000,000 centipoise at 25.degree. C., as pointed out
above, may have organo groups selected from monovalent hydrocarbon
radicals and halogenated monovalent hydrocarbon radicals. Accordingly,
such organo groups may be selected from alkyl radicals of from 1 to 8
carbon atoms, alkenyl radicals, cycloalkyl radicals and mononuclear aryl
radicals such radicals being, for instance, methyl, ethyl, propyl, vinyl,
allyl, cyclohexyl, phenyl, methylphenyl, ethylphenyl and etc. As examples
of halogenated monovalent hydrocarbon radicals, such halogenated
monovalent hydrocarbon radicals may be selected from various chlorinated
and fluorinated alkyl radicals and more preferably may be such fluorinated
alkyl radicals such as, 3,3,3-trifluoropropyl. Accordingly, such a
diorganopolysiloxane polymer has the formula,
R.sub.n SiO.sub.(4-n)/2
where R is selected from the class consisting of alkyl radicals, aryl
radicals, alkenyl radicals and fluoroalkyl radicals of up to 10 carbon
atoms, n varies from 1.97 to 2.01. Most preferably, the substituent groups
both in the above formula and as defined of a linear diorganopolysiloxane
polymer are preferably selected from methyl, phenyl, vinyl and
3,3,3-trifluoropropyl radicals. Preferably, the linear
diorganopolysiloxane polymer has some unsaturation in it and specifically
from 0.1 to 1 mole percent vinyl substituent groups in the
diorganopolysiloxane polymer. Also, such linear diorganopolysiloxane
polymers may be either a single polymer specie or a blend of various
linear diorganopolysiloxane polymers, however, substituted within the
above definitions and with varying viscosities as long as the viscosity of
the mixture is within 1,000,000 to 2000,000,000 centipoise at 25.degree.
C. Accordingly, it can also be envisioned in such blends that there may be
utilized linear diorganopolysiloxanes of a viscosity below 1,000,000
centipoise at 25.degree. C., so long as that blend or mixture of such
linear diorganopolysiloxane polymer has a viscosity between 1,000,000 to
200,000,000 centipoise at 25.degree. C.
It is also envisioned within the scope of the present invention to utilize
a linear diorganopolysiloxane polymer of a viscosity or blends of linear
diorganopolysiloxane polymers of a viscosity below 1,000,000 centipoise,
such as, a viscosity between 500,000 centipoise and 1,000,000 centipoise
at 25.degree. C. However, such linear diorganopolysiloxane polymers or
blends of polymers having a viscosity below 1,000,000 centipoise at
25.degree. C. are not preferred since the cured polymer does not have the
requisite physical properties. The procedure for producing such linear
diorganopolysiloxane polymers is well known. Briefly, it comprises
equilibrating cyclic polysiloxanes, whether trisiloxanes or
tetrasiloxanes, in the presence of a small amount of an alkali metal
hydroxide as a catalyst at elevated temperatures, that is, at temperatures
between 100.degree. to 250.degree. C. until there is as much of the cyclic
polysiloxanes being formed into a linear diorganopolysiloxane polymer as
there is of the linear diorganopolysiloxane polymer being reconverted to
cyclic polysiloxanes, such linear diorganopolysiloxane polymers being of
the desired viscosity. To control the end viscosity of the mixture there
is preferably added to such equilibration mixture the appropriate amount
of chain stoppers such as, hexamethyldisiloxane, octamethyltrisiloxane,
and etc. The amount of chain stoppers regulates the molecular weight of
the linear diorganopolysiloxane polymers that are formed and as such
determines the final viscosity of the linear diorganopolysiloxane polymers
that are formed.
In the case of fluorinated polymers such fluorinated polymers can be
prepared by the process set forth in the patent of John S. Razzano, U.S.
Pat. No. 3,937,648. For more details as to the method of preparing such
linear diorganopolysiloxanes, such methods are more fully explained in
Glaister et al, U.S. Pat. No. 3,814,722, which is hereby incorporated into
the present specification by reference.
In the present flame insulative composition set forth above, there must be
50 to 300 parts by weight of an inert filler. Preferably, such an inert
filler is selected from fumed silica and precipitated silica (known in the
silicone art as reinforcing fillers). However, up to 100% of the total
inert filler may be comprised of extending fillers. There is preferably
employed finely divided silica based fillers of the highly reinforcing
type which are characterized by particle diameter of less than 50
millicrons and by surface areas of greater than 50 square meters per gram.
The extending fillers of others than those preferred above may be selected
from titanium dioxide, iron oxide, aluminum oxide, as well as the other
inorganic materials known as inert fillers which can be included among
others, diatomaceous earth, calcium carbonate and quartz and can
preferably be employed in combination with highly reinforcing silica
fillers to improve the tensile strength or the hardness of the elastomeric
product. Other examples of suitable fillers are, diatomaceous silica,
aluminum silicate, zinc oxide, zirconium silicate, barium sulfate, zinc
sulfide, aluminum silicate and finely divided silica having surface bonded
alkoxy groups.
As mentioned previously, up to 100% of such inert fillers should be of the
extending type, preferably, the total filler for maximum structuring to
impart flame insulative properties to the heat vulcanizable silicone
rubber compositions of the instant case should be selected from fumed
silica or precipitated silica. However, in certain instances part of such
fillers, as stated previously, up to a maximum of 100% of the total filler
may be any of the foregoing extending fillers.
The third necessary ingredient in the flame insulative heat vulcanizable
silicone rubber compositions of the present case is from 25 to 150 parts
by weight of ground glass and more preferably 50 to 100 parts, but a
specific type of ground glass which is known as glass frits. Briefly, such
glass frits and most preferably such glass frits or ground glass should
have an average particle size varying from 50 microns to 400 microns.
Although glass frits of a smaller size or a larger size can be utilized in
the instant invention the most preferred glass frits utilized within the
scope of the instant invention to produce the maximum structure flame
insulative heat vulcanizable silicone rubber composition of the instant
case is a glass frits having an average particle size within the above
preferred range. By glass frits it is meant ground glass within the above
particle size having a glass transition temperature between 1,000.degree.
and 1,400.degree. F. It is within the scope of the instant invention to
claim, as necessary, in the compositions of the instant case both heat
vulcanizable silicone rubber compositions and room temperature
vulcanizable silicone rubber compositions wherein the ground glass within
the above particle size has the glass transition temperature as specified
above, that is, a glass transition temperature between 1,000.degree. to
1,400.degree. F. As pointed out above, such a type of ground glass is
usually known in the industry as glass frits. However, it may have other
names and it is intended to specify as a critical ingredient in the flame
insulative composition of the instant case ground glass within the above
particle size having a glass transition temperature within the above
ranges.
In one aspect of the instant case, the composition comprising the linear
diorganopolysiloxane polymer, the inner filler and ground glass within the
above particle size and glass transition temperature may be utilized in an
uncured state as a flame insulative composition around cables, although in
such situations it will be necessary to have some type of jacket to hold
the composition in place. Although such uncured heat vulcanizable silicone
rubber compositions are not the preferred insulative compositions of the
instant case because of the need for a jacket to keep the uncured
composition in place, nevertheless, for low flame insulative purposes or
application where low flame insulative properties are desired such uncured
heat vulcanizable silicone rubber compositions can be utilized with the
necessary jacket.
In a more preferred aspect there is utilized to cure the above heat
vulcanizable silicone rubber composition a peroxide curing catalyst and
more generally from 0.1 to 8 parts by weight of a peroxide curing
catalyst. Accordingly, these are all the necessary ingredients for the
instant composition to provide a flame insulative heat vulcanizable
silicone rubber composition which upon burning leaves a structured ash
which is not blown away or easily removed and, thus, obviating the need
for a jacket to hold the ash in place. As a result of this, the structured
ash provides maximum flame insulative properties as against the flame and
allows the electrical system to maintain its integrity for a maximum
period of time. There may be added other well known ingredients to the
instant heat vulcanizable composition either in preparing it so as to
facilitate the process in mixing by the use of well known process aids, or
also as additives to further improve the properties of the heat
vulcanizable silicone rubber composition for certain applications. For
instance, well known pigments may be added to the composition so that the
cured heat vulcanizable silicone rubber composition will be within the
proper color code.
There may also be employed in the present composition 1 to 25 percent and
preferably 5 to 15 percent by weight based on the polydiorganosiloxane gum
of a process aid for preventing the gum and the filler mixture from
structuring prior to curing and after compounding. One example of such a
process aid is a compound of the formula,
##STR1##
where R is a member selected from the class consisting of methyl and
phenyl, X is a member selected from the class consisting of --OH,
--NH.sub.2 or --OR', where R' is methyl or ethyl, n has a value of from 2
to 4, inclusive, and b is a whole number equal to from 0 to 10, inclusive.
Further details as to the properties, as well as the method of preparation
of the compound of Formula (3), are to be be found in the disclosure of
Martellock, U.S. Pat. No. 3,464,945 which is herein incorporated by
reference.
The process aid may also be a dihydrocarbon-substituted polysiloxane oil
having a hydrocarbon substituent to silicon atom ratio of from 1.6 to 2.0
and whose hydrocarbon substituents comprise at least one member selected
from the class consisting of methyl, ethyl, vinyl, allyl, cyclohexenyl and
phenyl groups, said polysiloxane oil comprising polysiloxane molecules
containing an average of from one to two lower alkoxy groups bonded to
each of the terminal silicon atoms where the alkoxy groups are selected
from the class consisting of methoxy, ethoxy, propoxy and butoxy.
Preparation of the alkoxy-containing hydrocarbon-substituted polysiloxane
oils that can be employed as a process aid in the present invention can be
carried out by producing one or more types of cyclic
dihydrocarbon-substituted polysiloxanes from one or more types of
dihydrocarbon-substituted dichlorosilanes and dialkoxysilanes in
accordance with the hydrolysis, depolymerization and fractional
distillation procedures described in detail above with reference to the
preparation of the gum of Formula (1). Then one or more types of cyclic
siloxanes so produced are mixed with predetermined amounts of a
dihydrocarbon-substituted dialkoxysilane and the mixture is subjected to
an equilibration treatment under controlled conditions to produce the
desired alkoxy end-blocked hydrocarbon-substituted linear polysiloxane
oil.
The alkoxy-containing hydrocarbon-substituted polysiloxane oils suitable
for use in the present invention are relatively low molecular weight
polysiloxane oils whose polymer chains have at least four and as much as
thirty-five and more dihydrocarbon siloxy units per molecule. The
polysiloxane oils preferably have an average of at least one and not more
than two alkoxy groups bonded to each of the terminal silicon atoms of the
molecule. A more detailed disclosure of the alkoxy end-blocked
polysiloxane process aids, as well as their method of preparation, is to
be found in the disclosure of Fekete, U.S. Pat. No. 2,954,357, which is
hereby incorporated into this specification by reference.
There may also be used as a process aid hydroxylated organosilanes which
contain from one silicon-bonded hydroxyl per 70 silicon atoms to two
silicon-bonded hydroxyls per silicon atom and contains from 1.9 to 2.1
hydrocarbon radicals per silicon atom. The remaining valences of the
silicon atom are satisfied by oxygen atoms. The hydroxylated materials
include both monomers such as diphenylsilanediol and polymeric materials
which contain two silicon-bonded OH groups in the molecule. In addition,
the hydroxylated organosilane may be a mixture of hydroxyl-containing
siloxanes and completely condensed siloxanes. Irrespective of the
particular composition of the hydroxylated organosiloxane, it is necessary
that there be present in said organosiloxane from one OH to 70 silicon
atoms to two OH per silicon atom.
The hydroxylated siloxanes may be prepared by any suitable method, such as
heating said siloxanes with steam under pressure at temperatures of about
120.degree. C. or hydrolyzing silanes of the formula R.sub.n SiX.sub.4-n
where X is any hydrolyxable group such as Cl, OR, H, --OOR and R is a
monovalent hydrocarbon radical. The former method is preferred for the
preparation of those hydroxylated materials in which the hydrocarbon
radicals are alkyl, while the latter method is best for the siloxanes in
which hydrocarbon radicals are monocyclicaryl hydrocarbon radicals.
Further, detailed information as to the hydroxylated organosiloxanes which
may be used as process aids is to be found in Konkle et al, U.S. Pat. No.
2,890,188, the disclosure of which is being incorporated into this
application by reference.
Any of the above process aids may be used alone or mixtures thereof may be
used in the above-defined concentrations. Further, other suitable process
aids may also be used in the silicone rubber compositions of the present
invention.
Other well known ingredients for heat vulcanizable silicone rubber
compositions as set forth in the foregoing Glaister et al, U.S. Pat. No.
3,814,722, patent may be utilized in the instant composition as desired to
lend to the cured composition the desired properties in accordance with a
specific application.
For a maximum flame insulative effect it is desirable to also incorporate
into the composition from 5 to 50, and preferably 5 to 30, parts by weight
of glass fibers in addition to the ground glass (more commonly known as
glass frits), more preferably at a concentration of 5 to 30 parts by
weight of said glass fibers. Although more glass fibers than 50 parts can
be added, the uncured composition becomes difficult to handle and the
added glass fibers and such additional amount of fibers does not impart to
the cured composition or uncured composition any additional flame
insulative benefits, while if less than 5 parts of glass fibers are added
the desired flame insulative effect is not obtained.
In the most preferred aspect of the instant invention for heat vulcanizable
compositions or room temperature vulcanizable silicone rubber
compositions, when the glass fibers are added to the composition for
maximum flame insulative properties it is preferred that such glass fibers
be milled glass fibers with a size varying from 0.01 to 0.5 inches. If the
glass fibers are larger than 0.5 inches the desired structured ash of
maximum flame insulative benefits is not obtained, and the materials
become hard to process. If glass fibers of less than 0.01 inches are used,
they do not markly add to the structureness of the burnt silicone ash that
is formed. Accordingly, it is generally preferred that milled glass fibers
be added, although any glass fibers may be added for some flame insulative
effect in addition to the glass frits. It is generally preferred that such
glass fibers have an average size, as stated previously, of between 0.01
to 0.5 inches and more preferably from 0.1 to 0.3 inches.
It should be pointed out that also within the general scope of the
invention that the general range of fillers that may be added are 50 to
300 parts by weight per 100 parts of linear diorganopolysiloxane polymer,
more preferably there may be utilized from 100 to 250 parts of an inert
filler. In the preferred case, it is also desirable to add between 50 to
150 parts of the glass frits. However, it should be mentioned that both
for room temperature vulcanizable silicone rubber compositions or heat
vulcanizable silicone rubber compositions that the concept of the instant
invention is the addition of glass frits in sizable quantities in addition
to the inert filler to obtain maximum flame insulativeness or specific
flame insulativeness for a specific cable or electrical system
application. Accordingly, it is not applicant's intention to be bound by
the above general and preferred ranges. In the same way, the use of glass
fibers or the preferred milled glass fibers, for that matter, both in the
use as well as the size of such milled glass fibers is optional, and such
glass fibers do increase the flame insulativeness of the final
composition. It can be categorically stated that the use of such preferred
milled glass fibers in the instant composition will increase the flame
insulativeness of the composition. However, the use of the milled glass
fibers is optional unless maximum flame insulativeness in the heat
vulcanizable silicone rubber composition or room temperature vulcanizable
silicone rubber composition is desired. Again, as with the glass frits and
the use of an inert filler, although ranges have been set above both for a
general and preferred conditions for the amount and size of the milled
glass fibers it is not applicant's intention to be bound by such ranges
except to indicate them as preferred embodiments.
In addition to the foregoing ingredients to further increase the flame
insulativeness of the heat vulcanizable silicone rubber composition or
room temperature vulcanizable silicone rubber composition for maximum
flame insulativeness there may be added from 5 to 30 parts of carbon
black. However, as stated previously, such addition of carbon black is
optional and is only to be added for specific applications.
It should be noted at this point that the addition of platinum as set forth
in the foregoing Noble/Brower patent when added to the instant
composition, in combination with the glass frits, retards rather than
improves the flame insulativeness of the instant composition both in the
case of heat vulcanizable silicone rubber compositions and room
temperature vulcanizable silicone rubber compositions.
The ingredients that were applied above with respect to obtaining an
improved heat vulcanizable silicone rubber composition also applies within
the scope of the instant invention to improving flame insulativeness of
two-part room temperature vulcanizable silicone rubber compositions. Such
room temperature vulcanizable silicone rubber compositions comprise a
silanol end-stopped diorganopolysiloxane polymer having a viscosity
anywhere from 1,000 to 200,000 centipoise at 25.degree. C. Such silanol
stopped linear diorganopolysiloxane polymers are well known in the art as
set forth in the Lampe and Bessmer, U.S. Pat. No. 3,888,815, which is
hereby incorporated by reference. As pointed out in that patent, such
linear silanol end-stopped diorganopolysiloxane polymers may be produced
by various methods such as the equilibration of cyclicsiloxanes in the
presence of a mild acid catalyst such as, sulfuric acid treated clay or
toluene sulfonic acid with a proper amount of water in the composition or
the equilibration of certain silicone hyrolyzates that are obtained by
hydrolysis of diorganodichlorosilanes. Preferably, such silanol
end-stopped diorganopolysiloxane polymer has the formula,
##STR2##
where R.sup.4 is selected from the class consisting of alkyl radicals,
aryl radicals, alkenyl radicals and fluorinated alkyl radicals of up to 10
carbon atoms, and b varies from 380 to 1000. The preparation of such
silanol end-stopped diorganopolysiloxane polymers is more fully explained
in the foregoing Bessmer and Lampe patent set forth above.
It must be specified that more broadly the organo groups of the linear
silanol end-stopped diorganopolysiloxane polymer may be any of the organo
groups set forth in the linear diorganopolysiloxane polymer mentioned
above for preparing the heat vulcanizable silicone rubber compositions.
However, more preferably, the organic groups are those as set forth above.
In addition to the silanol end-stopped diorganopolysiloxane polymer, such
compositions would have the same amount of filler per 100 parts of such
silanol end-stopped polymer, that is, from 50 to 300 parts by weight of an
inert and more preferably from 100 to 250 parts by weight of inert filler.
Such inert filler may be the same in concentration and definitions set
forth for the heat vulcanizable silicone rubber composition. Again, such
inert filler is preferably totally fumed silica or precipitated silica.
There is generally present in the composition from 25 to 150 parts of
ground glass and more preferably 50 to 100 parts of ground glass, such
ground glass being the glass frits mentioned previously, having the glass
transition temperature between 1,000.degree. to 1,400.degree. F. As
explained previously, such ground glass, that is, ground glass known as
glass frits, or known by some other name has a glass transition
temperature between 1,000.degree. and 1,400.degree. F., and preferably an
average particle size varying between 50 to 400 microns. The same
limitations in quantity and type of the ground glass specified above for
the heat vulcanizable silicone rubber composition applies also to room
temperature vulcanizable silicone rubber compositions. Such two-part room
temperature vulcanizable silicone rubber compositions are preferably
stored with the one-part containing the linear silanol end-stopped
diorganopolysiloxane and an inert filler, the second part comprising 1 to
15 parts per 100 parts of the linear silanol polymer of silicate having
the formula,
R.sub.a.sup.1 Si(OR.sup.2).sub.4-a
and partial hydrolysis products thereof where R.sup.1 and R.sup.2 are
selected from monovalent hydrocarbon radicals and halogenated monovalent
hydrocarbon radicals and a is 0 or 1, again the R.sup.1 and R.sup.2
radicals may be any of the radicals set forth previously for the organo
substituting groups for the linear diorganopolysiloxane polymer or silanol
end-stopped polymer. Preferably the radicals are selected from the class
consisting of alkyl radicals, alkenyl radicals, aryl radicals and
fluoroalkyl radicals of up to 10 carbon atoms. More preferably, the
silicate is a partial hydrolysis product of the compound set forth in the
formula above. For more information as to the preparation and use of such
silicate in two-part room temperature vulcanizable silicone rubber
compositions one can refer to the Lampe and Bessmer patent which is hereby
incorporated by reference. Accordingly, in the second part of such
two-part room temperature vulcanizable silicone rubber compositions along
with the silicate there is preferably present 0.01 to 5 parts by weight of
a metal salt of carboxylic acid varying from lead to manganese in the
periodic table. Although the metal salt if preferably of a monocarboxylic
acid, both metal salts of monocarboxylic acid and dicarboxylic acids can
be utilized in the room temperature vulcanizable silicone rubber
composition of the instant case. The most preferred metal salts that may
be utilized as catalysts with the composition of the instant case within
the room temperature vulcanizable silicone rubber compositions in the
instant case are tin salts and specifically dibutyl tin dilaurate. Other
ingredients that may be added to the compositions, as is necessary, are
pigments and the other usual ingredients as disclosed in the foregoing
Lampe and Bessmer patent for the purpose of meeting the requirements of a
particular electrical system. To cure the two-part room temperature
vulcanizable silicone rubber composition of the instant case, the first
part is mixed with the second part and the material is molded or formed
into the desired shape or injected into desired cavities to be insulated
and allowed to cure at room temperature--final curing taking place in 24
hours. Again to obtain maximum flame insulative properties in the two-part
room temperature vulcanizable silicone rubber composition there may be
present from 5 to 50 parts by weight of glass fibers and more preferably 5
to 30 parts by weight of glass fibers, and more specifically, milled glass
fibers having an average length ranging from 0.01 to 0.5 inches in length;
the preferred range of the milled glass fibers having the size of 0.1 to
0.3 inches in length. Glass fibers outside the above ranges and size may
be utilized. Generally, it has been found that no additional benefits have
been gained by exceeding the 50 parts by weight and if there is less than
5 parts by weight of glass fibers, the glass fibers do not add any flame
insulative properties to the composition. As specified previously, within
the broad range for the addition of glass frits to the two-part room
temperature vulcanizable silicone rubber composition, as in the case of
the heat vulcanizable silicone rubber composition, there is preferably
added 50 to 100 parts of glass frits, preferably such glass frits having a
size average particle size ranging from 50 to 400 microns in size.
Finally, as with the heat vulcanizable silicone rubber composition, for
certain additional flame insulative properties there may be added to the
instant composition 5 to 30 parts of carbon black. Preferably, it must be
pointed out, as stated previously, although the above particle size has
been given for the glass frits or ground glass both in the heat
vulcanizable silicone rubber composition and the room temperature
vulcanizable silicone rubber composition, such ranges are general guides.
Preferably the only requirement in the use of the ground glass being the
use of glass frits, that is, ground glass having glass transition
temperatures between 1,000.degree. to 1,400.degree. F. As far as the broad
and preferred ranges for the inert filler and the glass frits in the heat
vulcanizable silicone rubber composition and the room temperature
vulcanizable rubber composition, the ranges are general ranges to be used
as guides. It can be appreciated that the amount of the flame insulative
additives that will be utilized in a specific composition will be the
values that give a particular flame insulativeness or a structured
silicone ash for a particular application. In addition, the uses of milled
glass fibers are optional and would only be necessary to obtain maximum
flame insulativeness for the insulation and protection of certain
electrical systems. This is the case with the addition of the carbon black
which has been found necessary in certain applications. However, again
with respect to the concentrations of such carbon black and glass fibers
the concentrations given in the instant application are given as guides
and the particular concentrations for a specific application to protect a
particular system would be determined by the flame insulative properties
desired for that particular electrical system. The critical aspect of the
present invention lies in the utilization of glass frits, that is, ground
glass, at the concentration set forth above to produce a heat or room
temperature vulcanizable silicone rubber system which will protect and
impart the desirable flame insulativeness to an electrical system so that
as a result when the silicone system burns it will form a structured white
ash which provides maximum protection to the electrical system. To improve
such properties it is also disclosed in the instant case that glass fibers
in certain generally preferred quantities may be added as well as the
additional use of carbon black.
The foregoing examples are given for the purpose of illustrating the
conception and reduction to practice of the instant invention. They are
not given for any purpose in limiting or interpreting the scope of the
instant patent application and claims thereto. All parts are by weight.
EXAMPLE 1
There was prepared an uncured heat vulcanizable silicone composition
comprising the foregoing parts set forth in the table below comprising a
vinyl-terminated diorganopolysiloxane polymer of 10,000,000 viscosity
having an 0.2 mole percent vinyl content, containing the quantity set
forth below of 5 micron size of ground silica, the quantity set forth
below of fumed silica and the quantity set forth in Table I below of glass
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