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George GAINES, Jr.
Mica Paper
CA644427
MOISTURE RESISTANT MICA PRODUCT
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This invention relates to the preparation of mica paper. More
particularly, this invention relates to sheets of mica paper
which are characterized by improved tensile strength and
moisture resistance and to the process by -which these improved
products .are obtained.
Heretofore, the preparation of mica paper has been known in the
art. For example, mica paper preparation is described in U.
S. patents 2,549,880 -Bardet, 2,614,055 - de Senarclens,
2,709,158 - Bouchet. Mica paper prepared by the methods of
these patents as well as by other known methods is gaining
commercial acceptance because of the excellent electrical
properties of the mica paper and because of its inertness at
elevated temperatures. However, there are two serious drawbacks
to the universal acceptance of mica paper in many applications.
These drawbacks are the relatively low tensile strength of the
paper and the fact that the mica paper disintegrates completely
on contact with water.
It is an object of the present invention to provide an improved
mica paper.
A further object of my invention is to provide a mica paper
which is moisture resistant.
These and other objects of my invention are accomplished by
contacting mica paper with a chloro-silane or a mixture of
chlorosilanes having the formula
(1) (R)nSi(Cl)4_n where R is a hydrocarbon radical, e.g. an
alkyl radical, for example, methyl, ethyl, propyl, butyl, octyl,
etc. radicals, an aryl radical, e.g. phenyl, naphthyl,
di-phenyl, tolyl, ethylphenyl, etc. radical, an aralkyl radical,
e.g. benzyl, phenylethyl, etc; substituted alky 1 radicals, e.g.
chlororaethyl, j8-îluoroethy^., etc. radicals, substituted aryl
radicals, e.g. chlorophenyl, dichlorophenyl etc. radicals; and n
has a value of from 0.8 to 1.9 inclusive.
The term "mica paper" in this application is used in its usual
sense to refer to a sheet-like aggregate of mica particles which
has been prepared by any of the general methods described in the
three U. S, patents mentioned above. In general, mica paper is
prepared by heating mica which may comprise phlogophite,
lepidolite, or preferably muscovite, at a temperature and time
sufficient to partially "dehydrate" the mica. In general, the
heating of the mica is carried out at a temperature of about
000°C., e.g., from about 750-Ô50°C. for a time of about 10
minutes, e.g., from 5-20 minutes. This heating step causes a
loss in weight of the mica equal to about 2 per cent by weight
of the original weight of the mica. The heating has the effect
of softening the mica while at the same time delarainating and
increasing the bulk volume of the mica. This heat-treated mica
is then added to an aqueous medium, generally plain water, and
agitated by any suitable device, such as, a high speed
comminuter or mixer., to convert the mica into small particles
or platelets. Usually, the comminution of the mica takes place
in a suspension containing about 1 per cent by weight of mica.
This results in a pulplike suspension of mica in which the
particle size of the mica flakes have a wide distribution. The
extrafine and extra-coarse particles in this comminuted aqueous
suspension are then removed and the resulting slurry is formed
into sheets of paper on conventional paper-making apparatus. In
alternative methods of making mica sheet, the aqueous medium,
instead of being pure water, is sometimes acidic or basic in
nature.
Thus, the fired mica flakes are sometimes ground up in an
alkaline carbonate solution and the solution is then neutralized
by a suitable acid such as, for example, hydrochloric acid.
Regardless of the method of preparation of the wet sheets of
mica paper, the wet sheets are then dried by evaporation with or
without the application of external heat, and the dried sheets
are then sometimes pressed or calendered at elevated
temperatures to form the final mica paper.
Mica paper prepared by any of these processes or by analogous
processes may be employed in the practice of the present
invention. In addition, mica paper which has been treated
subsequent to formation may also be employed in my process.
Thus, mica paper may be employed which has been treated with an
alkyl orthotitanate as described in the Canadian application of
Myron L. Corrin, Serial No. 736,532 filed September 17, 1957 and
assigned to the same assignee as the present invention. In the
aforementioned Corrin application, which is hereby incorporated
by reference into the present application, mica paper is
impregnated with a solution of an alkyl orthotitanate such as,
for example, tetrapropyl titanate or tetrabutyl titanate, and
the impregnated sheet is then hydrolyzed by
tacting the sheet with water vapor and the resulting hydrolyzed
sheet is tlaen dried at an elevated temperature to remove
volatile hydrolysis products. The mica sheet of this Corrin
application is characterized by improved tensile strength, tear
resistance and crease resistance over prior art materials.
However, despite the excellent physical properties of mica paper
treated by the method of this Corrin application, the resulting
paper is still subject to complete disintegration when brought
into contact with water.
The chlorosilanes within the scope of formula (1) which are
employed in the practice of my invention are well known in the
art. Specific chlorosilanes within the scope of formula (1)
include, for example, methyltrichlorosilane,
ethyltrichlorosilane, /?-chloro-ethyltrichlorosilane,
phenyltrichlorosilane, benzyl-trichlorosilane,
naphthylchlorosilane, etc. Among the various specific
chlorosilanes within the scope of formula (1), I prefer to
employ methyltrichlorosilane. It should be understood, however,
that in addition to using specific chlorosilanes within the
scope of formula (1), mixtures of various chlorosilanes may be
employed so long as the average composition of the mixture falls
within the scope of the formula. Thus, mixtures of two or more
compounds within the following formula
(2) (R)aSiCl^_a where R is as previously defined and a has a
value of from 0 to 3, may be mixed to form a mixture of
materials having an average composition within the scope of
formula (1). Specific compounds within the scope of formula (2)
include, for example, silicon tetrachloride,
methyltrichlorosilane dimethyldichlorosilane,
trimethylchlorosilane, ethyltrichlorosilane,
methyl-ethyldichlorosilane, butyltri chlorosilane, diphenyl-
dichlorosilane, benzyltrichlorosilane,
dlbu-t-yltrl-chlorosilane, chloromethyltrichlorosilane etc. In
the preferred embodiment of my invention, the R radicals of all
of the specific materials within the scope of formula (2) are
methyl radicals.
The process of the present invention may be carried out in any
suitable method. Two of the available methods for contacting
mica paper with the chlorosilanes within the scope of formula
(1) are by dipping the mica paper in the chlorosilane and by
subjecting the mica paper to the vapors of the chlorosilane. In
either base, the relative amounts of mica paper and chlorosilane
are completely immaterial since only a very minor amount of the
chlorosilane reacts with the mica paper. Where the treatment is
effected by dipping the mica paper in the liquid chlorosilane,
the only requirement is that there be present sufficient
chlorosilane, so that the mica paper is completely immersed in
the liquid. When the treating process of the present invention
is carried out with vapors of the chlorosilane or chlorosilane
mixture, I prefer to carry out the treatment at the boiling
point of the chlorosilane. By carrying out the treatment at the
boiling point, there is assurance that sufficient chlorosilane
comes into contact with the surface of the mica sheet so that
some of it condenses on the surface of the sheet and is able to
react with the sheet.
The time required for the treatment of the mica sheet is only a
few seconds whether immersion or vapor treatment is employed.
Thus, the time required for immersing a sheet of mica paper into
the liquid and withdrawing the sheet is sufficient to effect
satisfactory treatment.
After contacting the mica paper with the chlorosilane, any
unreacted chlorosilane is removed.
This is accomplished by allowing the chlorosilane to evaporate
at room temperature or by merely heating the treated sheet to a
temperature above the boiling point of the chlorosilane for
sufficient time to remove all volatile materials. The
temperature suitable for evaporation of unreacted chlorosilanes
from the surface of the mica sheet, of course, varies with the
composition of the chlorosilane. Where the chlorosilane is a
mixture of dimethyldichlorosilane and methyltrichlorosilane, the
boiling point of the mixture is from 65-70°C. With this mixture,
I prefer to employ a temperature of about 100°C. for
evaporation. Where the chlorosilane mixture contains some
silicon-bonded aryl radicals, the boiling point of the mixture
is much higher, e.g., up to 150°C. oi? more. Where such high
boiling chlorosilane mixtures are employed, I prefer to use a
temperature of 200°C, or more to cause evaporation of volatile
products.
Although the exact chemical mechanism involved in the present
process is not fully understood, it is clear that a reaction
takes place between the chlorosilane mixture and the mica paper.
The speed and efficiency of this reaction dépends to some ex-
tent on the reactivity of the silicon-bonded chlorine atoms in
the chlorosilane. Thus, I prefer to employ chlorosilanes in
which the R radicals attached to silicon are methyl since this
type of chlorosilane contains the most reactive chlorine atoms.
Where the silicon-bonded R radical is an aryl radical, for
example, a phenyl radical, the reactivity of the silicon-bonded
chlorine atoms are substantially reduced so that the rate of
reaction of chlorosilane and the mica paper is retarded.
Unexpectedly, I have discovered that all chlorosilanes are not
effective in the process of the present invention. Thus, where a
sheet of mica paper is treated with a single chlorosilane
consisting of trimethylchlorosilane, there is no improvement in
either the tensile strength or moisture resistance of the mica
paper. This result is quite surprising in view of the fact that
trimethylchlorosilane is an effective surface coating agent for
many materials. Similarly, diraethyldichlorosilane and
diphenyldi-chlorosilane are completely ineffective,
individually, in the process of the present invention. And,
finally, a highly functional material such as, for example,
silicon tetrachloride is also ineffective in the process of the
present invention. However, it should be understood that
although these particular chlorosilanes mentioned are
ineffective when used alone in my process, when these materials
are used in combination with other materials giving a mixture
having an average composition within the scope of formula
(1), the resulting mixture imparts improved tensile strength and
moisture resistance to mica paper. Thus, a mixture of silicon
tetrachloride and trimethylchlorosilane, neither of which alone
will improve mica paper, will both strengthen and give moisture
resistance to mica paper if the mixture is selected so that the
average composition is within the scope of formula (l).
The following examples are illustrative of the practice of my
invention and are not intended for purposes of limitation.
In all of the examples the mica paper employed was prepared by
firing flakes of muscovite mica at a temperature of about Ô00°G.
for about 10 minutes.
The fired sheets of mica were then added to water to form a
slurry containing 1 per cent by weight of mica.
This slurry was violently agitated to break up the mica into
fine particles. The ultra-fine particles and the course
particles were then separated from the slurry and the resulting
material was formed into a paperlike sheet of mica on a
conventional paper-forming apparatus. The mica sheets were then
calendered at a temperature of about 150°C. to remove all
moisture from the sheet and to yield a uniform sheet of mica
paper having a metallic sheen.
EXAMPLE 1
A number of samples of mica paper were cut from a large sheet of
mica paper 2 mils thick. Some of these samples of mica paper
were treated with a mixture of dimethyldichlorosilane and
methyltrichlorosilane having an average composition
corresponding to the following formula xxx
(CH3)i.6Si(Cl)2.4 Some of the sheets were prepared by immersing
the mica paper samples in the chlorosilane mixture while others
were prepared by holding the mica paper over a vessel of the
boiling chlorosilane (boiling point 65-70°C.) for about 30
seconds. The treated samples were then heated in an oven at
about 110°G. for about 30 seconds to remove any unreacted
volatile material. At the end of this time measurements were
made of the tensile strength and the electrical characteristics
of both the untreated mica paper and the treated mica paper. The
untreated mica paper had a tensile strength of about 2000 pounds
per square inch while the mica paper treated with the
chlorosilane mixture either by immersion or by vapor treatment
had a tensile strength of about 4000 pounds per square inch. The
electrical / strengths of both the uivfcreated and treated mica
paper remained about the same, with dielectric breakdown
occurring at about 1000 volts per mil and with a D.C. resistance
at 500 volts of about 10^ ohm centimeters. When the untreated
mica paper was placed under a stream of water, it disintegrated
completely in a few seconds. .The treated samples of mica paper
were completely imeffected by a stream of water or by being
immersed in water.
EXAMPLE 2
A sample of 4 mil mica paper was treated by immersing it in
methyltrichlorosilane for about 4 seconds. At the end of this
time the treated sheet was placed in an oven at 110°C. for about
1 minute to remove volatile materials. The resulting treated
mica sheet had a tensile strength about twice as great as the
tensile strength of the untreated sheet and was completely
uneffeeted by immersion in water.
EXAMPLE 3
Following the procedure of Example 2, a sheet of mica paper was
treated by immersion in phenyl-trichlorosilane. After heating
this treated sample at-a temperature of about 200°G. to remove
volatile products, the tensile strength was about 1.5 times as
great as the tensile strength of the untreated material. In
addition, this treated mica sheet was uneffected by water.
EXAMPLE 4
This example illustrates the treatment of mica paper with
individual chlorosilanes outèide of the scope of the present
invention. Four mil samples of mica sheet were treated with
trimethylchlorosilane, diphenyldichlorosilane, and silicon
tetrachloride by the method of example 2. In each case, the
treatment with the chlorosilane with subsequent heating of the
treated sheet failed to improve either the tensile strength or
moisture resistance of the ajica paper, EXAMPLE 5
This example illustrates the treatment of mica paper with a
mixture of two chlorosilanes, neither of which alone will
improve the properties of mica paper. Following the procedure of
Example 2, a sheet of k mil mica paper was treated with a
mixture of silicon tetrachloride and trimethylchlorosilane
having an average composition corresponding to the following
formula;
<aH3>:i.2sl<01>2.8
This treated sheet had a tensile strength over twice as great as
an untreated sample and did not disintegrate when immersed in
water.
EXAMPLE 6
Following the procedure of Example 2 a sheet of 4 mil mica paper
was treated with a mixture of phenyltrichlorosilane, silicon
tetrachloride, di-methyldichlorosilane, methyltrichlorosilane,
and dipehnyldichlorosilane having an average composition
corresponding to the following formula:
<06Vo.5(0%>0.5sl(01'3
This treated sheet had a tensile strength about 70 per cent
greater thafitthe tensile strength of an untreated sample and
did not disintegrate when immersed in water.
EXAMPLE 7
Following the procedure of Example 1, a sheet of 4 mil mica
paper was treated with a mixture o& dimethyldichlorosilane
and methyltrichlorosilane having an average composition
corresponding to the formula
<CH3>i.2Si(01)2.6
This treated sheet exhibited a tensile strength over two times
as great as the tensile strength of the untreated sheet and was
completely uneffected by immersion in water, while the untreated
sheet disintegrated completely upon immersion.
EXAMPLE 8
This example illustrates the process of the present
invention applied to "titanized" mica paper prepared by the
method of the aforementioned Corrin application. A sheet of 2
mil mica paper was cut into a number of samples and these
samples were "titanized" by taking 1 part by weight of the mica
paper and sucking l80 parts by weight of a 22 per cent by weight
tetrabutyl. titan ate., solution in benzene through the paper in
about 15-30 seconds. These sheets were then exposed to saturated
water vapor in a closed container for approximately 24 hours and
were then pressed at '500 pounds per square inch pressure at
200°C. for 1 minute. Several of these titanized sheets were then
treated with the mixture of dimethyldiohlorosilane and
methyltrichlorosilane described in Example 1. The treatment
consisted of immersing the "titanized" mica sheet in the liquid
chlorosilane mixture and then heating the mixture at about
120°C. until all volatile materials had evaporated. The tensile
strength and moisture resistance of the untreated mica paper,
the "titanized" mica paper and the "titanized" and chlorosilane-
treated mica paper were investigated. The untreated mica sheet
had a tensile strength of about l600 pounds per square inch, the
"titanized" sheet had a tensile strength of about 4000 pounds
per square inch, while the mica sheet which was both "titanized"
and treated with the chlorosilane mixture had a tensile strength
of about 4200 pounds per square inch. Neither the untreated nor
the "titanized" mica paper had any water resistance. The mica
paper which was both "titanized" and treated with the
chlorosilane mixture was completely unaffected by immersion in
water.
Although the foregoing examples have illustrated the use of
specific chlorosilanes or chlorosilane mixtures without the use
of solvents, it should be understood that the chlorosilane or
chlorosilane mixtures may be dissolved in any suitable inert
solvent in carrying out the process of the present invention..
However, it should be understood that no particular advantage is
gained by the use of such solvents. In fact, the use of solvents
presents the additional problem of having to remove the solvent
from the mica paper after the chlorosilane treatment. This
removal may be effected by evaporating the solvent during the
evaporation of the volatile unreacted chlorosilanes. The
following example illustrates the use of solvents in the process
of the present invention,
EXAMPLE 9
A solution of chlorosilanes was prepared by adding 1 part of
a mixture of dimethyldiëhlorosilane and methyltrichlorosilane
having an average composition corresponding to the formula xxx
to 4 parts of benzene. A sample of k mil mica paper was immersed
in this solution for about lOsseconds and then heated at a
temperature of about 120°G. for about 2 minutes. The resulting
treated sheet had a tensile strength about 60 per cent greater
than the tensile strength of the untreated mica sheet and was
completely uneffected by immersion in water,
Although the foregoing examples have illustrated the process of
the present invention in connection with mica paper prepared
only by one process, it should be understood that the
chlorosilane treatment of the present invention is independent
of the method by which the mica paper is formed. Thus, the
process of
the present invention is applicable to mica paper prepared by
any of the methods described in the three patents previously
enumerated as well as to mica paper prepared by any other
analogous method.
The improved mica paper prepared by the process of the present
invention can be employed in all of those applications in which
prior art mica paper can be used. In addition, the improved mica
paper of this invention can also be employed in those
applications where a product having increased physical
properties is desirable in combination with a product which is
resistant to moisture. Thus, this mica paper can be employed as
a dielectric medium in capacitors, can be employed as insulation
in dynamoelectric machines, for example, as slot insulation in
electric motors,
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