U.S. patent application number 13/391997 was filed with the patent office on 2012-07-05 for process for the preparation of a silicone pressure-sensitive adhesive.
Invention is credited to Stephen Edward Cray, Delphine Davio, Robert Alan Ekeland, Andreas Stammer.
Application Number | 20120172543 13/391997 |
Document ID | / |
Family ID | 43127667 |
Filed Date | 2012-07-05 |
United States Patent
Application |
20120172543 |
Kind Code |
A1 |
Cray; Stephen Edward ; et
al. |
July 5, 2012 |
Process for the Preparation of a Silicone Pressure-Sensitive
Adhesive
Abstract
Silicone pressure sensitive adhesive (PSA) compositions and
methods for their preparation are provided. In one embodiment, PSAs
are formed from a silicone polymer mixture by condensation
polymerization of low viscosity polyorganosiloxanes in inert
solvents and/or silicone fluids, and optionally adding a silicone
resin (MQ) during polymerization. The silicone polymer mixture
formed may also be mixed with a silicone resin (MQ) and bodying
catalyst, and bodying is allowed to continue until the desired
reaction product is formed.
Inventors: |
Cray; Stephen Edward; (
Sully, GB) ; Davio; Delphine; (Le Roeulx, BE)
; Ekeland; Robert Alan; (Greer, SC) ; Stammer;
Andreas; (Pont-A-Celles, BE) |
Family ID: |
43127667 |
Appl. No.: |
13/391997 |
Filed: |
August 24, 2010 |
PCT Filed: |
August 24, 2010 |
PCT NO: |
PCT/US10/46487 |
371 Date: |
February 23, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61236576 |
Aug 25, 2009 |
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|
Current U.S.
Class: |
525/477 ;
525/474; 528/10; 528/21 |
Current CPC
Class: |
C08G 77/44 20130101;
C09J 183/04 20130101 |
Class at
Publication: |
525/477 ;
525/474; 528/10; 528/21 |
International
Class: |
C08L 83/04 20060101
C08L083/04; C08G 77/04 20060101 C08G077/04 |
Claims
1. A silicone pressure-sensitive adhesive composition obtained by a
method comprising the sequential steps of: (I) forming a polymer
mixture comprising: (A) a silicone polymer formed by condensation
polymerization of at least one polyorganosiloxane in the presence
of at least one hydrocarbon solvent or silicone fluid; wherein the
polyorganosiloxane has an average solution viscosity of from about
1 mm.sup.2/s to about 200 mm.sup.2/s at 25.degree. C. and at least
one hydroxyl group capable of undergoing condensation
polymerization; wherein polymerization is facilitated by addition
of at least one condensation catalyst and occurs at a temperature
of from about 30.degree. C. to about 110.degree. C.; wherein the
silicone polymer formed has a solution viscosity of from about
10,000 mm.sup.2/s to about 5,000,000 mm.sup.2/s at 25.degree. C.;
(B) optionally, a silicone resin that has at least one hydroxyl
group capable of undergoing condensation polymerization and that is
soluble in at least one hydrocarbon solvent or silicone fluid;
wherein the resin has a hydroxyl group content of from about 0.5%
to about 2.5% (by weight based on resin solids content); wherein
the resin comprises R.sub.3SiO.sub.1/2 units and SiO.sub.4/2 units
in a molar ratio of from about 0.6 to about 1.5 (R.sub.3SiO.sub.1/2
units:SiO.sub.4/2 units); wherein R is independently selected from
a monovalent hydrocarbon or a halohydrocarbon radical having from 1
to 20 carbon atoms, an alkenyl radical, or a hydroxyl radical; and
(C) optionally, a neutralizing agent; wherein the neutralizing
agent is added in an amount sufficient to neutralize the
condensation catalyst; (II) mixing with the polymer mixture of (I),
at least one bodying catalyst and a silicone resin that has at
least one hydroxyl group capable of undergoing bodying; and then
allowing a bodying reaction to occur at a temperature of from about
40.degree. C. to about 145.degree. C. to form the silicone
pressure-sensitive adhesive composition; wherein the resin is
soluble in at least one hydrocarbon solvent or silicone fluid;
wherein the resin has a hydroxyl group content of from about 0.5%
to about 2.5% (by weight based on resin solids content); wherein
the resin comprises R.sub.3SiO.sub.1/2 units and SiO.sub.4/2 units
in a molar ratio of from about 0.6 to about 1.5 (R.sub.3SiO.sub.1/2
units:SiO.sub.4/2 units); wherein R is independently selected from
a monovalent hydrocarbon or a halohydrocarbon radical having from 1
to 20 carbon atoms, an alkenyl radical, or a hydroxyl radical; and
(III) optionally, adding to the silicone pressure-sensitive
adhesive composition of (II) from about 0.5% to about 3.5% (by
weight) of an organic peroxide.
2. A composition according to claim 1, wherein the
polyorganosiloxane is generally characterized by the formula
R.sup.1O[R.sup.2R.sup.3SiO].sub.xH wherein R.sup.1, R.sup.2, and
R.sup.3 are independently selected from methyl, ethyl, propyl,
isopropyl, butyl, sec-butyl, iso-butyl, tert-butyl,
perfluorobutylethyl, phenyl ethyl, chlorpropyl, fluoropropyl,
vinyl, and phenyl; and wherein x is an integer with a value of at
least 2.
3. A composition according to claim 2, wherein the
polyorganosiloxane is polydimethylsiloxane.
4. A composition according to claim 1, wherein the
polyorganosiloxane has a solution viscosity of from about 10
mm.sup.2/s to about 100 mm.sup.2/s at 25.degree. C.
5. A composition according to claim 1, wherein the at least one
hydrocarbon solvent or silicone fluid is selected from xylene,
heptane, benzene, toluene, naptha, mineral spirits,
polydimethylsiloxane, isododecane, hexane, and decane.
6. A composition according to claim 1, wherein the condensation
catalyst is selected from
[PCl.sub.3.dbd.N--PCl.sub.2.dbd.N--PCl.sub.3].sup.+PCl.sub.6.sup.-.
7. A composition according to claim 1, wherein the silicone polymer
has an average viscosity of from about 80,000 mm.sup.2/s to about
750,000 mm.sup.2/s at 25.degree. C.
8. A composition according to claim 7, wherein the
pressure-sensitive adhesive has from about 20% to about 60% (by
weight) of a silicone polymer.
9. A composition according to claim 1, wherein the silicone resin
of step (I)(B) undergoes condensation polymerization with the
silicone polymer prior to addition of the neutralizing agent.
10. A composition according to claim 9, wherein the silicone resin
of step (I)(B) is the same as the silicone resin of step (II).
11. A composition according to claim 9, wherein the silicone resin
of step (I)(B) is different from the silicone resin of step
(II).
12. A composition according to claim 1, wherein the
pressure-sensitive adhesive has from about 40% to about 80% (by
weight) of silicone resins.
13. A composition according to claim 1, wherein the optional
organic peroxide is selected from benzoyl peroxide and
dichlorobenzoyl peroxide.
14. A composition according to claim 1, wherein the
pressure-sensitive adhesive has less than 0.1 wt %
octamethylcyclotetrasiloxanes or decamethylcyclopentasiloxanes.
15. A composition according to claim 1, wherein the silicone
polymer of step (I)(A) comprises branched polyorganosiloxanes.
16. A composition according to claim 15, wherein the silicone
polymer is formed by condensation polymerization in the presence of
trialkoxyphenylsilanes, tetraalkoxysilanes, or combinations
thereof.
17. A composition according to claim 16, wherein the silicone
polymer is formed by condensation polymerization in the presence of
trimethoxyphenylsilane, tetraethoxysilane, or combinations thereof.
Description
[0001] In various embodiments, the application relates to silicone
pressure sensitive adhesive (PSA) compositions and methods for
their preparation.
[0002] Silicone pressure-sensitive adhesives were introduced into
the market in the mid-1950s and have since been the subject of
ongoing development focused on improving performance capabilities
(especially at extreme temperatures) in order to meet increasing
demands in various industries and applications. Despite such
development, the primary components of conventional PSAs remain (i)
a high molecular weight linear silicone polymer with silanol
functionality at the polymer chain ends, (ii) a highly condensed
low molecular weight silicate resin (MQ) with silanol functionality
on its surface, and (iii) a solvent.
[0003] Known methods of manufacturing PSAs involve mixing a
silicone polymer (usually polydimethyl siloxane-based or
polydimethyl-diphenyl siloxane-based), a MQ resin, and a catalyst
in a hydrocarbon solvent, followed by heating to promote bodying
(i.e. condensation) between the respective silanol functionalities
of the resin and polymer. After bodying, most silicone PSAs known
in the art undergo further cross-linking to enhance cohesive
strength. Known cross-linking methods utilize either a
peroxide-catalyzed free-radical cure system or a platinum-catalyzed
addition cure system.
[0004] PSA performance properties are controlled by, among other
things, a critical balance of molecular weights/viscosities of the
silicone polymer, the structure of the silicone polymer, molecular
weights/viscosities of the MQ resin, the ratio of polymer to resin,
type and level of functionality on the polymer and resin, and the
process conditions (for example, reaction time and temperature) of
manufacture. Slight variations can have dramatic effects on the PSA
properties.
[0005] While PSAs and their methods of manufacture are well known,
there is a need in the art for new PSAs to meet the needs of new
applications, as well as to meet new performance demands for known
applications. Such needs arise from, among other things, increased
use of chemical fasteners instead of mechanical fasteners,
increased use of PSAs in high temperature applications (for
example, electronics), increased use of PSAs on
temperature-sensitive substrates, and increased use of PSAs in
applications where volatile cyclosiloxanes (for example,
octamethylcyclotetrasiloxanes and decamethylcyclopentasiloxanes)
and other byproducts/impurities arising from methods of preparing
the silicone polymer component must be minimized.
[0006] These and other needs in the art are met by embodiments of
the present invention. Provided in various embodiments are silicone
PSAs and methods for their preparation. According to certain
aspects, the inventive method of preparing PSAs involves forming
silicone polymers by condensation polymerization of low viscosity
polyorganosiloxanes in inert solvents (for example, toluene or
xylene) and/or silicone fluids (for example, polydimethylsiloxane).
According to other aspects, a silicone resin is optionally mixed
with the silicone polymer during polymerization and condensation is
allowed to continue before the addition of (where required) a
neutralizing agent. In further aspects, a silicone resin, bodying
catalyst, and the silicone polymer mixture are mixed and bodying is
allowed to continue until the desired reaction product is formed.
In additional aspects, the reaction product is cured by the
addition of an organic peroxide.
[0007] In some aspects, in addition to silicone resins, other solid
particles, coupling agents (for example, alkoxysilanes such as
tetraethoxysilane and tetraorthotitanates), and cross-linking
agents can be added to and further bodied with the silicone
polymer. In other aspects, the silicone polymer can be prepared
using mixed intermediates (for example, methylphenyl siloxanes or
methyl trifluoropropyl siloxanes) in conjunction with the low
viscosity polyorganosiloxanes, yielding co-polymers that can lead
to improved PSA properties. In additional aspects, condensation
polymerization for formation of the silicone polymer can be carried
out in the presence of trialkoxyphenylsilanes, tetraalkoxysilanes,
and other silanes selected to introduce branching into the silicone
polymer, which allows for tailoring of adhesive properties and
viscosities of the PSAs. For example, condensation polymerization
to form the silicone polymer can occur in the presence of
trimethoxyphenylsilane, tetraethoxysilane, or combinations thereof
in order to introduce branching into the resulting silicone
polymer.
[0008] These and additional features and advantages of the
invention will become apparent in the course of the following
detailed description.
[0009] A more complete appreciation of the invention and the many
embodiments thereof will be readily obtained as the same becomes
better understood by reference to the following detailed
description, when considered in connection with the accompanying
drawings, wherein:
[0010] FIG. 1 illustrates that one of the attributes of adding
branching to silicone polymers, particularly high molecular weight
linear gum polymers, is the ability to maintain molecular weight
while reducing solution viscosities. The chart shows solution
viscosity plotted versus molecular weight in linear and branched
dimethyl silicone polymers. The branched polymers are able to reach
a considerably higher weight average molecular weight (M.sub.w;
measured for example, by gel permeation chromotography) at a lower
solution viscosity. This also enables lower solution viscosities
for PSA compositions derived from these polymers.
[0011] FIG. 2 illustrates hot peel testing of PSA compositions in
accordance with embodiments of the invention. An adhesive tape is
applied to a stainless steel plate and then subjected to a
temperature of 250-270.degree. C. for a period of 10 minutes. The
plate is then removed from the oven and the adhesive tape is
quickly removed. The appearance of a residue left by the adhesive
on the plate indicates failure in this particular test. The test
has been given a number scale to indicate the amount of residue
left on the plate, with 0 being total adhesive failure, and 5
meaning that no adhesive residue can be detected on the plate. The
chart illustrates that hot peel performance can be maximized by the
use of higher molecular weight polymers. Also, what is shown is
that the molecular weight required for excellent hot peel
performance is higher in linear polymers than that which is
required for excellent performance from branched
polydimethylsiloxane polymers. The last two data points represent
conventional, commercially-available samples of silicone PSAs.
[0012] Features and advantages of the invention will now be
described with occasional reference to specific embodiments.
However, the invention may be embodied in different forms and
should not be construed as limited to the embodiments set forth
herein. Rather, these embodiments are provided so that this
disclosure will be thorough and complete and will fully convey the
scope of the invention to those skilled in the art.
[0013] Unless otherwise defined, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which the invention belongs. The
terminology used in the description herein is for describing
particular embodiments only and is not intended to be limiting.
[0014] As used in the specification and appended claims, the
singular forms "a," "an," and "the" are intended to include the
plural forms as well, unless the context clearly indicates
otherwise.
[0015] The term "independently selected from," as used in the
specification and appended claims, is intended to mean that the
referenced groups can be the same, different, or a mixture thereof,
unless the context clearly indicates otherwise. Thus, under this
definition, the phrase "X.sup.1, X.sup.2, and X.sup.3 are
independently selected from noble gases" would include the scenario
where X.sup.1, X.sup.2, and X.sup.3 are all the same, where
X.sup.1, X.sup.2, and X.sup.3 are all different, and where X.sup.1
and X.sup.2 are the same but X.sup.3 is different.
[0016] The term "silicone fluid," as used in the specification and
appended claims, is intended to mean a substantially non-volatile
and non-reactive silicone-based fluid that generally does not
chemically participate in a polymerization reaction or otherwise
chemically interact with additives introduced in any steps of the
described process. The inert fluid may or may not be removed during
the process.
[0017] As used in the specification and appended claims, the term
"silicone polymer" is intended to mean a polymer comprising
multiple organosiloxane or polyorganosiloxane groups per molecule.
The term includes, but is not limited to, polymers substantially
containing only organosiloxane or only polyorganosiloxane groups in
the polymer chain, and polymers where the backbone contains both
organosiloxane and polyorganosiloxane groups in the polymeric
chain.
[0018] The term "substituted," as used in the specification and
appended claims in relation to hydrocarbon groups, means one or
more hydrogen atoms in the hydrocarbon group has been replaced with
another substituent. Examples of such substituents include, but are
not limited to, halogen atoms such as chlorine, fluorine, bromine,
and iodine; halogenated organic groups such as chloromethyl,
perfluorobutyl, trifluoroethyl, and nonafluorohexyl; oxygen atoms;
oxygen-containing groups such as (meth)acrylic and carboxyl;
nitrogen atoms; nitrogen atom containing groups such as amines,
amino-functional groups, amido-functional groups, and
cyano-functional groups; sulfur atoms; and sulfur atom containing
groups such as mercapto groups.
[0019] As used in the specification and appended claims, the term
"extender" means a compound typically used to dilute a
silicone-based product to make the product more economically
competitive without substantially affecting the properties of the
product.
[0020] The term "plasticizer," as used in the specification and
appended claims, means a compound that is added to silicone-based
compositions to increase the flexibility and toughness of the
polymer product. This is generally achieved by reduction of the
glass transition temperature (T.sub.g) of the cured polymer
composition, thereby enhancing the elasticity of the elastomer (for
example, a sealant).
[0021] As used in the specification and appended claims, the terms
"bodied" and "bodying" mean a condensation reaction between the
functional hydroxyl groups of a silicone polymer and the functional
hydroxyl groups of a silicone resin in order to increase molecular
weight or crosslinking, or both.
[0022] The terms "viscosity" and "solution viscosity," as used in
the specification and appended claims, mean the viscosity of a
compound wherein about 30-70% of the compound is dissolved in a
solvent. In some aspects, 45-55% of the compound is dissolved in a
solvent. Solution viscosities were measured using standard
procedures with a Brookfield Rotational Viscometer Model DVII+
using Spindle RV7 and rotational speeds between 0.3 RPM to 100 RPM
dictated by the fluid being tested. Measurements were made at
standard conditions.
[0023] Unless otherwise indicated, all numbers expressing
quantities of ingredients, properties such as molecular weight,
reaction conditions, and so forth as used in the specification and
claims are to be understood as being modified in all instances by
the term "about." Accordingly, unless otherwise indicated, the
numerical properties set forth in the specification and claims are
approximations that may vary depending on the desired properties
sought to be obtained in embodiments of the present invention.
Notwithstanding that the numerical ranges and parameters setting
forth the broad scope of the invention are approximations, the
numerical values set forth in the specific examples are reported as
precisely as possible. Any numerical values, however, inherently
contain certain errors necessarily resulting from error found in
their respective measurements.
[0024] The invention provides, in certain aspects, novel silicone
pressure-sensitive adhesive (PSA) compositions and methods of
making such compositions. In embodiments of the present invention,
the PSA compositions are made by a method comprising the sequential
steps of: (i) forming a silicone polymer by condensation
polymerization of at least one polyorganosiloxane in the presence
of at least one hydrocarbon solvent or silicone fluid; optionally,
(ii) mixing with the silicone polymer, a silicone resin that has at
least one hydroxyl group capable of undergoing condensation
polymerization and that is soluble in at least one hydrocarbon
solvent or silicone fluid; (iii) allowing condensation
polymerization to continue before addition of, where required, a
neutralizing agent; (iv) adding at least one bodying catalyst and a
silicone resin having at least one hydroxyl group capable of
undergoing bodying and that is soluble in at least one hydrocarbon
solvent or silicone fluid; (v) allowing bodying between the
silicone polymer and the silicone resin to continue until the
desired reaction product is formed; and (vi) optionally, adding an
organic peroxide and allowing the pressure-sensitive adhesive to
cure. In some embodiments, condensation polymerization to form the
silicone polymer of (i) is carried out in the presence of one or
more silanes selected to introduce branching into the polymer.
[0025] In certain aspects, the PSA compositions made by the present
invention have silicone polymer components with a diverse range of
polymer structures, molecular weights and viscosities (including,
but not limited to, molecular weights greater than 1,000,000 g/mol
and corresponding 50% solids solution viscosities), as well as
diverse organic groups. In other aspects, the PSA compositions made
by the present invention have low cyclosiloxane content (defined
herein as <0.1 weight % of a particular cyclosiloxane) and/or
other by-products and impurities. For example, the PSA compositions
made by the present invention may have <0.1 weight %
octamethylcyclotetrasiloxanes, <0.1 weight %
decamethylcyclopentasiloxanes, and/or <0.1 weight % larger
cyclosiloxanes.
[0026] Embodiments of the inventive method allow for PSA
compositions characterized as having improved performance
characteristics (including, but not limited to, adhesion and tack),
as compared to conventional PSA compositions. For example, such PSA
compositions have superior performance in the hot peel test with
which the skilled artisan will be familiar. Essentially, the hot
peel test involves applying a PSA composition to a stainless steel
plate and then subjecting it to a temperature of 250-270.degree. C.
for a period of time (for example, 10 minutes) and then quickly
removing it. The degree of residue left on the plate is evaluated.
In some aspects, PSA compositions prepared by the inventive method
embody the inventors' discovery that hot peel performance can be
maximized by the use of high molecular weight silicone polymers in
the PSA compositions, and that the molecular weight required for
excellent performance is higher in linear polymers than in branched
polymers.
[0027] According to embodiments of the invention, PSA compositions
are made by a method wherein the silicone polymer component of the
PSA is formed by condensation polymerization of at least one
polyorganosiloxane in the presence of at least one hydrocarbon
solvent or silicone fluid. The polyorganosiloxane may be linear,
substantially linear, or branched. In some aspects, linear or
substantially linear low molecular weight/low viscosity
polyorganosiloxanes having reactive hydroxyl groups are used as
starting materials for condensation polymerization. For example,
such polyorganosiloxanes can be generally characterized by formula
(I):
R.sup.1O[R.sup.2R.sup.3SiO].sub.xH (1)
wherein each R is independently selected from a hydrogen atom, an
alkyl or substituted alkyl group containing 1 to 8 carbon atoms, an
aryl or substituted aryl group containing 1 to 8 carbon atoms, and
wherein x is an integer with a value of at least 2. In certain
aspects, x is an integer with a value range of 2-80. In other
aspects, x is an integer with a value range of 3-49. In additional
aspects, x is an integer with a value range of 50-80. Examples of R
include, but are not limited to, methyl, ethyl, propyl, isopropyl,
butyl, sec-butyl, iso-butyl, tert-butyl, perfluorobutylethyl,
phenyl ethyl, chloropropyl, fluoropropyl, vinyl, and phenyl.
[0028] With the presence of various groups in the polymeric chain,
a polymer may comprise a modest degree of branching and still be
considered "linear" or "substantially linear," as the terms are
used herein. In some aspects, branching is less than 10%. In other
aspects, branching is less than 2%. For reference, a significantly
branched polymer (i.e. where a polymer is neither linear nor
substantially linear) would have structural units selected
from:
##STR00001##
wherein the molecular structure would have at least 1 T unit per
100 D units, or at least 1 Q unit per every 200 D units.
[0029] The linear or substantially linear polyorganosiloxanes of
the inventive method may have, in certain aspects, a solution
viscosity of from about 1 mm.sup.2/s to about 200 mm.sup.2/s at
25.degree. C. Good results have been obtained with
polyorganosiloxanes having a solution viscosity of from about 10
mm.sup.2/s to about 100 mm.sup.2/s at 25.degree. C. Good results
have also been obtained with silanol-ended polydimethylsiloxanes as
the polyorganosiloxane starting material.
[0030] In some aspects, the linear or substantially linear
polyorganosiloxanes undergo condensation polymerization with mixed
intermediates such as methylphenyl siloxanes or methyl
trifluoropropyl siloxanes, yielding co-polymers that can lead to
improved PSA properties. In some aspects, the condensation
polymerization reaction occurs in the presence of silanes selected
to introduce branching into the silicone polymer, thereby allowing
for tailoring of adhesive properties and viscosities of the
resultant PSA compositions. For example, branching may be
introduced into the silicone polymer by carrying out condensation
polymerization in the presence of trialkoxyphenylsilanes, such as
trimethoxyphenylsilane; tetraalkoxysilanes, such as
tetraethoxysilane; or combinations thereof. In some aspects, PSA
compositions prepared by the inventive method embody the inventors'
discovery that adding branching to the silicone polymers used in
PSA compositions (particularly high molecular weight gums) allows
for maintenance of molecular weight while reducing solution
viscosity and thereby increasing tack. Thus, in certain aspects,
the PSA compositions prepared by the inventive method exhibit
decreased solution viscosity and increased tack, as compared to
conventionally prepared PSAs with comparable molecular weights.
[0031] In principle, any suitable condensation polymerization
reaction pathway may be utilized for formation of the silicone
polymer. Similarly, any suitable condensation catalyst known in the
art may be mixed with the siloxane starting materials to facilitate
polymerization. In certain aspects, protic acids, Lewis acids and
bases, organic acids and bases, and inorganic acids and bases are
used. For example, BF.sub.3, FeCl.sub.3, AlCl.sub.3, ZnCl.sub.2,
and ZnBr.sub.2 can be used. Alternatively, organic acids such as
those having the general formula RSO.sub.3H, wherein R represents
an alkyl group having from 6 to 18 carbon atoms (for example, a
hexyl or dodecyl group), an aryl group (for example, a phenyl
group), or an alkaryl group (for example, dodecylbenzyl) can be
used. Other condensation-specific catalysts include, but are not
limited to, n-hexylamine, tetramethylguanidine, carboxylates of
rubidium or cesium, hydroxides of potassium, sodium, magnesium,
calcium or strontium, and phosphonitrile halide ion-based catalysts
having the general formula [X(PX.sub.2.dbd.N).sub.zPX.sub.3].sup.+,
wherein X denotes a halogen atom and wherein z is an integer from 1
to 6. In certain aspects,
[PCl.sub.3.dbd.N--PCl.sub.2.dbd.N--PCl.sub.3].sup.+PCl.sub.6.sup.-
is the catalyst used.
[0032] Typically the amount of catalyst present is from about 2 ppm
to about 208 ppm (by weight, based on weight of the
polyorganosiloxane), including but not limited to, about 2-12 ppm,
about 12-24 ppm, about 24-36 ppm, about 36-48 ppm, about 48-60 ppm,
about 60-72 ppm, about 72-84 ppm, about 84-96 ppm, about 96-108
ppm, about 108-120 ppm, about 120-136 ppm, about 136-148 ppm, about
148-160 ppm, about 160-172 ppm, about 172-184 ppm, about 184-196
ppm, and about 196-208 ppm. In some aspects, the catalyst is
present in amount of from about 3 ppm to about 53 ppm, including
but not limited to about 3-13 ppm, about 13-23 ppm, about 23-33
ppm, about 33-43 ppm, and about 43-53 ppm. In additional aspects,
the catalyst can be present in a solvent in an amount from about
1-50% (w/w).
[0033] One of skill in the art will appreciate that condensation
polymerization involves the production of water as a by-product. In
certain aspects of the invention, it may or may not be necessary to
remove the water formed during condensation. In some aspects,
removal of water is required and is done during or after
condensation polymerization but before neutralization. Methods of
removing water are known in the art.
[0034] In certain aspects, the catalyst(s) chosen, desired reaction
products and their properties, as well as the presence of optional
end-blocking agent and/or other optional additives, may affect how
reaction temperature is chosen. In some aspects, condensation
polymerization is carried out at a temperature of from about
30.degree. C. to about 110.degree. C., including but not limited to
from about 30.degree. C.-40.degree. C., about 40.degree.
C.-50.degree. C., about 50.degree. C.-60.degree. C., about
60.degree. C.-70.degree. C., about 70.degree. C.-80.degree. C.,
about 80.degree. C.-90.degree. C., about 90.degree. C.-100.degree.
C., and about 100.degree. C.-110.degree. C. In other aspects,
condensation polymerization is carried out at a temperature of from
about 70.degree. C.-90.degree. C., including but not limited to,
70.degree. C.-75.degree. C., about 75.degree. C.-80.degree. C.,
about 80.degree. C.-85.degree. C., and about 85.degree.
C.-90.degree. C.
[0035] Where appropriate, any suitable end-blocking agent known in
the art (for example, water, polymethyl siloxanes, or silanes
having one group capable of reacting with the terminal groups of
the polymer) can also be added to introduce the appropriate
end-groups in the polymer and halt the polymerization reaction,
thereby limiting the average molecular weight of the resulting
silicone polymer. The end-blocking agent is present in an amount
calculated to result in the desired molecular weight range of
silicone polymer.
[0036] Where appropriate, any conventional additive known for use
in production of silicone polymers can also be added. The additive
is present in an amount calculated to result in the desired
properties of the silicone polymer. Examples of additives include
other solid particles or reinforcers, extenders, plasticizers,
coupling agents (for example, alkoxysilanes such as
tetraethoxysilane and tetraorthotitanates), cross-linking agents,
and silanes (for example, trimethoxyphenylsilane,
tetraethoxysilane) selected to introduce branching into the
silicone polymer.
[0037] In certain aspects, condensation polymerization occurs in
the presence of at least one hydrocarbon solvent. The hydrocarbon
solvent can be selected from, among others, linear or branched
saturated hydrocarbons, linear or branched unsaturated hydrocarbons
(for example, alkenes), benzenes and substituted benzenes (for
example, alkylbenzenes), cycloaliphatics (for example, cyclohexane)
and substituted cycloaliphatics (for example, alkylcyclohexanes).
Suitable hydrocarbon solvents include, but are not limited to,
xylene, heptane, benzene, toluene, dodecane, isododecane, hexane,
decane, naptha, mineral spirits, paraffins, isoparaffins,
polyisobutenes, ethanol, isopropanol, butanol, ethyl acetate, amyl
acetate, butyl acetate, acetone, dimethyl isosorbide, and propylene
carbonate.
[0038] In other aspects, condensation polymerization occurs in the
presence of at least one silicone fluid. The silicone fluid can be
selected from, among others, trialkylsilyl terminated
polydialkylsiloxanes and derivatives thereof which may comprise a
degree of substitution, provided that any substituted groups do not
participate in the polymerization reaction. Suitable silicone
fluids include, but are not limited to, low molecular weight
polydimethylsiloxanes and cyclosiloxanes. For example, 0.65, 1, 2
and 3 cs trimethylsilylterminated polysiloxane fluids may be
useful.
[0039] The amount of hydrocarbon solvent and/or silicone fluid
which may be included in the PSA composition will depend upon
multiple factors, such as the intended use of the PSA and the
molecular weight of the solvent and/or fluid. In general, PSA
compositions can contain up to 70% w/w of solvent and/or fluid.
[0040] According to embodiments of the inventive method, the
condensation polymerization reaction is allowed to continue until a
silicone polymer with the desired characteristics is formed. The
characteristics of the formed silicone polymer can be affected by
the nature of the polyorganosiloxane starting materials, the
catalyst chosen, the hydrocarbon solvent or silicone fluid chosen,
the temperature of the condensation reaction, the optional addition
of one or more endblockers, and/or the optional addition of one or
more other additives (for example, co-monomers, extenders and
plasticizers).
[0041] In certain aspects, the condensation polymerization reaction
is allowed to continue until a silicone polymer is produced having
lesser amounts of cyclic siloxanes present in the composition than
typically obtained by other polymerization techniques, such as
equilibration. In certain aspects, the silicone polymer has a
concentration of less than 0.1 wt % octamethylcyclotetrasiloxanes
or decamethylcyclopentasiloxanes.
[0042] In certain aspects, condensation polymerization is allowed
to continue until a desired viscosity of the silicone polymer is
reached, followed by addition of a neutralizing agent (where
required). In some aspects, the silicone polymer formed is
characterized as having a solution viscosity of from about 10,000
mm.sup.2/s to about 5,000,000 mm.sup.2/s at 25.degree. C. Good
results have been obtained with the formation of polymers having a
solution viscosity of from about 80,000 mm.sup.2/s to about 750,000
mm.sup.2/s at 25.degree. C. In some aspects, a PSA composition may
comprise from about 20 weight % to about 60 weight % of a silicone
polymer described herein. Typically, such a PSA comprises from
about 30 weight % to about 50 weight % of silicone polymer.
[0043] According to other aspects, condensation polymerization of
the polyorganosiloxanes is allowed to continue until a desired
viscosity of the silicone polymer is reached, followed by addition
of a silicone resin having at least one hydroxyl group capable of
undergoing condensation polymerization, wherein polymerization is
allowed to continue for a period of time prior to addition of a
neutralizing agent (where required). An example of a suitable
silicone resin is one that is soluble in at least one hydrocarbon
solvent or silicone fluid, has a hydroxyl group content of from
about 0.5% to about 2.5% (by weight based on resin solids content),
and comprises R.sub.3SiO.sub.1/2 units and SiO.sub.4/2 units in a
molar ratio of from about 0.6 to about 1.5 (R.sub.3SiO.sub.1/2
units:SiO.sub.4/2 units), wherein R is independently selected from
a monovalent hydrocarbon or a halohydrocarbon radical having from 1
to 20 carbon atoms, an alkenyl radical, or a hydroxyl radical.
[0044] According to some embodiments of the inventive method, once
the polymer mixture (i.e. silicone polymer, solvent/silicone fluid,
catalyst, optional endblocker and/or other additives, optional
silicone resin, and optional neutralizing agent) is formed, at
least one bodying catalyst and a silicone resin having at least one
hydroxyl group capable of undergoing bodying are mixed with the
polymer mixture, and bodying is allowed to occur until the desired
reaction product is formed. An example of a suitable silicone resin
for bodying is one that is soluble in at least one hydrocarbon
solvent or silicone fluid, has a hydroxyl group content of from
about 0.5% to about 2.5% (by weight based on resin solids content),
and comprises R.sub.3SiO.sub.1/2 units and SiO.sub.4/2 units in a
molar ratio of from about 0.6 to about 1.5 (R.sub.3SiO.sub.1/2
units:SiO.sub.4/2 units), wherein R is independently selected from
a monovalent hydrocarbon or a halohydrocarbon radical having from 1
to 20 carbon atoms, an alkenyl radical, or a hydroxyl radical. In
some aspects, a silicone resin is added only during the bodying
steps of the inventive method. In other aspects, the silicone resin
added during the bodying steps of the method is the same as that
added during the steps resulting in the formation of the silicone
polymer mixture. In additional aspects, the silicone resin added
during the bodying steps of the method is different from that added
during the steps resulting in the formation of the silicone polymer
mixture. In any case, bodying is generally allowed to continue
until the desired reaction product is formed. In some aspects, a
suitable PSA composition may comprise from about 40 weight % to
about 80 weight % of the silicone resins described herein (i.e.
inclusive of all silicone resins present without regard to steps of
addition). Typically, such a PSA comprises from about 50 weight %
to about 70 weight % of silicone resins.
[0045] Examples of suitable bodying temperatures include, but are
not limited to, from about 40.degree. C.-50.degree. C., about
50.degree. C.-60.degree. C., about 60.degree. C.-70.degree. C.,
about 70.degree. C.-80.degree. C., about 80.degree. C.-90.degree.
C., about 90.degree. C.-100.degree. C., about 100.degree.
C.-110.degree. C., about 110.degree. C.-120.degree. C., about
120.degree. C. -130.degree. C., about 130.degree. C.-140.degree.
C., and about 140.degree. C.-150.degree. C. Examples of periods of
time suitable for bodying include, but are not limited to, from
about 1-2 hours, about 2-3 hours, and about 3-4 hours.
[0046] In principle, one or more suitable bodying catalysts known
in the art may be used for the bodying steps of the inventive
method. Typically, the amount of bodying catalyst present is from
about 1000 ppm to about 3000 ppm (by weight). In certain aspects,
the bodying catalyst(s) can be selected from liquid silanol
condensation catalysts having a boiling point of less than
200.degree. C. or catalysts which are solid at room temperature.
For example, such catalyst(s) can be selected from alkali metal
hydroxides, alkali metal alkoxides, alkali metal carbonates, alkali
metal silanolates, amines, metal salts of amines, carboxylic acids
or metal salts of carboxylic acids, amines, carboxylic acid salts
of organic amines, and quaternary ammonium salts. Suitable amines
include, but are not limited to, primary amines exemplified by
methylamine, ethylamine, propyl amine, hexylamine, butanolamine and
butylamine; secondary amines exemplified by dimethylamine,
diethylamine, diethanolamine, dipropylamine, dibutylamine,
dihexylamine, ethylamylamine, imidazole and propylhexylamine;
tertiary amines exemplified by trimethylamine, triethylamine,
tripropylamine, tributylamine, trihexylamine, methyldipropylamine,
tripropanolamine, pyridine, N-methylimidazole and
methylpropylhexylamine. Suitable carboxylic acid salts of organic
amines include, but are not limited to, diethylammonium acetate,
butylammonium octoate and trimethylammonium laurate. Suitable
quaternary ammonium salts include, but are not limited to,
tetramethylammonium acetate, methylethyldibutylammonium chloride or
dioctadecyldimethylammonium chloride. Suitable carboxylic acids
include, but are not limited to, acetic acid, propanoic acid,
butanoic acid, formic acid, stearic acid, tetradecanoic acid,
hexadecanoic acid, dodecanoic acid, decanoic acid, benzoic acid,
3,6-dioxaheptanoic acid and 3,6,9-trioxadecanoic acid. Metal salts
of carboxylic acids wherein the metal is selected from the group
consisting of Li, Na, K, Ce and Ca are also suitable for use as a
bodying catalyst. Suitable metal salts of carboxylic acids are
exemplified by potassium formate and potassium acetate.
[0047] In addition to bodying catalyst(s), polymer stabilizers
and/or neutralizers for bodying catalysts may additionally be added
during or after the bodying steps. In some aspects, any high
temperature polymer stabilizer/neutralizer known in the art may be
added. Examples of such additives include, but are not limited to,
alkylsilylphosphates such as trimethylsilylphosphate.
[0048] According to embodiments of the inventive method, once the
desired reaction product is formed, it optionally undergoes curing
by addition of an organic peroxide. In some aspects, the organic
peroxide is selected from benzoyl peroxide and dichlorobenzoyl
peroxide. In other aspects, the organic peroxide is added in an
amount of from about 0.5% to about 3.5% (by weight of silicone
polymer and silicone resin). In another embodiment, curing is
achieved using hydrosilation reactions. Methods of curing are known
in the art.
[0049] The PSA compositions formed by the inventive method are
characterized as having improved performance characteristics,
including but not limited to adhesion, tack, and solution
viscosity, as compared to conventionally-formed PSA compositions.
In certain aspects, the PSA compositions formed by the inventive
method have superior performance in the hot peel test. In some
aspects, the superior PSA compositions comprise linear silicone
polymers, and in other aspects, the compositions comprise branched
silicone polymers. With regard to PSA compositions having branched
silicone polymers, the compositions exhibit lower solution
viscosity and increased tack as compared to conventional PSA
compositions of similar molecular weight. In yet other aspects, the
PSA compositions formed by the inventive method contain less than
0.1 wt % octamethylcyclotetrasiloxanes or
decamethylcyclopentasiloxanes.
[0050] Embodiments of the present invention will be better
understood by reference to the following examples which are offered
by way of illustration not limitation.
EXAMPLES
[0051] The present invention will be better understood by reference
to the following examples which are offered by way of illustration
and which one of skill in the art will recognize are not meant to
be limiting.
Example 1
Formation of Silicone Polymer
[0052] A solution of 1000 grams silanol ended polydimethylsiloxane
fluid (Mw 2500 g/mol) and 1000 grams xylene as a non-reactive
diluent were blended in a sigma blade mixer equipped with N.sub.2
purge and vacuum. The mixture was heated to 80.degree. C. and was
then catalyzed with 0.6 grams of a 5% phosphonitrosyl chloride
catalyst in methylene chloride solution. The system was mixed and
evacuated to remove water of condensation via azeotropic
distillation. Separated xylene solvent was returned to the reactor
from the condenser. The reaction proceeded until a solution
viscosity of 278,000 mm.sup.2/s at 25.degree. C. was reached. This
yielded a polymer (Compound 1; CAS Registry Number 70131-67-8) in
xylene having a Mw of 1.17.times.10.sup.6 g/mol as determined by
Gel Permeation Chromatograpy or GPC (all GPC's herein used toluene
solvent and calibrated using polystyrene standards). The resulting
polymer contained less than 0.1 wt %
octamethylcyclotetrasiloxanes.
Example 2
Formation of Silicone Polymer
[0053] A solution of 900 grams of silanol ended
polydimethylsiloxane fluid (Mw 2500 g/mol) and 100 grams of
diphenylsilane diol (Mw 216 g/mol) and 1000 grams xylene as a
non-reactive diluent were blended in a sigma blade mixer equipped
with N.sub.2 purge and vacuum. The mixture was heated to 80.degree.
C. and was then catalyzed with 0.6 grams of a 5% phosphonitrosyl
chloride catalyst in methylene chloride solution. The system was
mixed and evacuated to remove water of condensation via azeotropic
distillation. Separated xylene solvent was returned to the reactor
from the condenser. The reaction proceeded until a solution
viscosity of 82,000 mm.sup.2/s at 25.degree. C. was reached. This
yielded a polymer (Compound 2; CAS Registry Number 68931-93-9) in
xylene having a Mw of 9.02.times.10.sup.5 g/mol as determined by
GPC and having less than 0.1 wt %
octamethylcyclotetrasiloxanes.
Example 3
Formation of Silicone Polymer
[0054] A solution of 999.7 grams of silanol ended
polydimethylsiloxane fluid (Mw 2500 g/mol) and 0.3 grams of
tetraethoxysilane (Mw 208 g/mol) and 1000 grams xylene as a
non-reactive diluent were blended in a sigma blade mixer equipped
with N.sub.2 purge and vacuum. The mixture was heated to 80.degree.
C. and was then catalyzed with 0.6 grams of a 5% phosphonitrosyl
chloride catalyst in methylene chloride solution. The system was
mixed and evacuated to remove water of condensation as well as
ethanol via azeotropic distillation. Separated xylene solvent was
returned to the reactor from the condenser. The reaction proceeded
until a solution viscosity of 749,000 mm.sup.2/s at 25.degree. C.
was reached. This yielded a polymer (Compound 3) in xylene having a
Mw of 1.89.times.10.sup.6 g/mol as determined by GPC and having
less than 0.1 wt % octamethylcyclotetrasiloxanes.
Example 4
Formation of Silicone Polymer
[0055] A solution of 900 grams of silanol ended
polydimethylsiloxane fluid (Mw 2500 g/mol) and 100 grams of silanol
ended polyphenylmethylsiloxane fluid (Mw 543 g/mol) and 1000 grams
xylene as a non-reactive diluent were blended in a sigma blade
mixer equipped with N.sub.2 purge and vacuum. The mixture was
heated to 80.degree. C. and was then catalyzed with 0.6 grams of a
5% phosphonitrosyl chloride catalyst in methylene chloride
solution. The system was mixed and evacuated to remove water of
condensation via azeotropic distillation. Separated xylene solvent
was returned to the reactor from the condenser. The reaction
proceeded until a solution viscosity of 114,000 mm.sup.2/s at
25.degree. C. was reached. This yielded a polymer (Compound 4) in
xylene having a Mw of 1.06.times.10.sup.6 g/mol as determined by
GPC and having less than 0.1 wt %
octamethylcyclotetrasiloxanes.
Example 5
Formation of Silicone Polymer
[0056] A solution of 900 grams of silanol ended
polydimethylsiloxane fluid (Mw 2500 g/mol) and 100 grams of silanol
ended polytrifluoropropylmethylsiloxane fluid (Mw 577 g/mol) and
1000 grams xylene as a non-reactive diluent were blended in a sigma
blade mixer equipped with N.sub.2 purge and vacuum. The mixture was
heated to 80.degree. C. and was then catalyzed with 0.6 grams of a
5% phosphonitrosyl chloride catalyst in methylene chloride
solution. The system was mixed and evacuated to remove water of
condensation via azeotropic distillation. Separated xylene solvent
was returned to the reactor from the condenser. The reaction
proceeded until a solution viscosity of 326,000 mm.sup.2/s at
25.degree. C. was reached. This yielded a polymer (Compound 5) in
xylene having a Mw of 1.35.times.10.sup.6 g/mol as determined by
GPC and having less than 0.1 wt %
octamethylcyclotetrasiloxanes.
Example 6
PSA
Formation of PSA Composition
[0057] A solution of 203 grams of Compound 1 was combined with 177
grams of a 70% solids trimethylsiloxylated silicic acid (MQ resin)
in xylene (Mw 20000 g/mol), 19 grams of additional xylene, 0.8
grams benzoic acid, 0.1 grams trimethylsilylphosphate in a 0.5
liter 3 necked round bottom flask equipped with N.sub.2 purge,
mechanical stirring and a Dean Stark water trap filled with xylene.
The mixture was refluxed at 143.degree. C. for 3 hours to remove
condensed water. The resulting PSA (Compound 6; CAS Registry Number
68440-70-0) was 83000 mm.sup.2/s at 25.degree. C. viscosity at
56.5% solids, and had a GPC with multimodal resin and polymer
peaks.
Example 7
PSA
Formation of PSA Composition
[0058] A solution of 215 grams of Compound 2 was combined with 177
grams of a 70% solids trimethylsiloxylated silicic acid (MQ resin)
in xylene (Mw 20000 g/mol), 6 grams of additional xylene, 0.8 grams
benzoic acid, 0.1 grams trimethylsilylphosphate in a 0.5 liter 3
necked round bottom flask equipped with N.sub.2 purge, mechanical
stirring and a Dean Stark water trap filled with xylene. The
mixture was refluxed at 143.degree. C. for 3 hours to remove
condensed water. The resulting PSA (Compound 7; CAS Registry Number
68440-62-2) was 12000 mm.sup.2/s at 25.degree. C. viscosity at
56.5% solids, and had a GPC with multimodal resin and polymer
peaks.
Example 8
PSA
Formation of PSA Composition
[0059] A solution of 187 grams of Compound 3 was combined with 189
grams of a 70% solids trimethylsiloxylated silicic acid (MQ resin)
in xylene (Mw 20000 g/mol), 27 grams of additional xylene, 0.8
grams benzoic acid, 0.1 grams trimethylsilylphosphate in a 0.5
liter 3 necked round bottom flask equipped with N.sub.2 purge,
mechanical stirring and a Dean Stark water trap filled with xylene.
The mixture was refluxed at 143.degree. C. for 3 hours to remove
condensed water. The resulting PSA (Compound 8) was 25000
mm.sup.2/s at 25.degree. C. viscosity at 56.5% solids, and had a
GPC with multimodal resin and polymer peaks.
Example 9
PSA
Formation of PSA Composition
[0060] A solution of 187 grams of Compound 4 was combined with 187
grams of a 70% solids trimethylsiloxylated silicic acid (MQ resin)
in xylene (Mw 20000 g/mol), 25 grams of additional xylene, 0.8
grams benzoic acid, 0.1 grams trimethylsilylphosphate in a 0.5
liter 3 necked round bottom flask equipped with N.sub.2 purge,
mechanical stirring and a Dean Stark water trap filled with xylene.
The mixture was refluxed at 143.degree. C. for 3 hours to remove
condensed water. The resulting PSA (Compound 9) was 11000
mm.sup.2/s at 25.degree. C. viscosity at 56.5% solids, and had a
GPC with multimodal resin and polymer peaks.
Example 10
PSA
Formation of PSA Composition
[0061] A solution of 195 grams of Compound 4 was combined with 188
grams of a 70% solids trimethylsiloxylated silicic acid (MQ resin)
in xylene (Mw 20000 g/mol), 14 grams of additional xylene, 0.8
grams benzoic acid, 0.1 grams trimethylsilylphosphate in a 0.5
liter 3 necked round bottom flask equipped with N.sub.2 purge,
mechanical stirring and a Dean Stark water trap filled with xylene.
The mixture was refluxed at 143.degree. C. for 3 hours to remove
condensed water. The resulting PSA (Compound 10) was 33,300
mm.sup.2/s at 25.degree. C. viscosity at 56.5% solids, and had a
GPC with multimodal resin and polymer peaks.
Example 11
Testing of the PSA Samples
[0062] PSA's were formulated to contain 2% by weight benzoyl
peroxide by the addition of a 10 wt % toluene solution of benzoyl
peroxide and coated to a 28-35 micrometers dry film thickness on 50
micrometers polyester film backing for tack and adhesion
measurements. The adhesive formulations were coated from xylene
solution, dried for 2 minutes at 90.degree. C., and then cured for
2 minutes at 178.degree. C. in forced air ovens. Tack testing was
done per ASTM 2979 for measuring probe tack with units of grams
force at 1.0 seconds dwell with a 20 gram weight and a 5 mm
stainless steel probe tip moving at 5 mm/second. Adhesion testing
was done per ASTM 3359-08 measuring adhesion to mirrored stainless
steel plates at 180 degrees peel angle and reported as grams/25.4
mm width at a peel velocity of 0.3 m/minute. For comparison
purposes, two conventional, commercially-available silicone PSAs
were also coated, cured and tested for adhesive properties and
performance. One was a general purpose silicone adhesive, and one
was stabilized for high temperature applications. The results are
as shown in Table 1.
TABLE-US-00001 TABLE 1 Adhesion and Tack Values for PSA
Compositions PSA film Probe Compound thickness (.mu.m) Tack (g)
Adhesion (g) Compound 6 33.2 873.3 1237 Compound 7 31.5 1205.4 1340
Compound 8 30.3 1092.3 1287 Compound 9 29.7 1284.6 1134 Compound 10
30.5 1019.4 1282 DC7406 Adhesive 31 850 1044 (commercial general
purpose adhesive) DC7566 Adhesive 29.8 865 868 (commercial high
Temperature adhesive)
Example 12
Formation of Silicone Polymer
[0063] A solution of 998 grams silanol ended polydimethylsiloxane
fluid (Mw 2500 g/mol), 2 grams silanol ended
polymethylvinyldimethylsiloxane fluid (Mw 432 g/mol) and 1000 grams
xylene as a non-reactive diluent were blended in a sigma blade
mixer equipped with N2 purge and vacuum. The mixture was heated to
80.degree. C. and was then catalyzed with 0.6 grams of a 5%
phosphonitrosyl chloride catalyst in methylene chloride solution.
The system was mixed and evacuated to remove water of condensation
via azeotropic distillation. Separated xylene solvent was returned
to the reactor from the condenser. The reaction proceeded until a
solution viscosity of 223,000 mm.sup.2/s at 25.degree. C. was
reached. This yielded a polymer (Compound 12; CAS Registry Number
67923-19-7) in xylene having a Mw of 0.95.times.10.sup.6 g/mol and
having less than 0.1 wt % octamethylcyclotetrasiloxanes.
[0064] The present invention should not be considered limited to
the specific examples described herein, but rather should be
understood to cover all aspects of the invention. Various
modifications and equivalent processes, as well as numerous
structures and devices, to which the present invention may be
applicable, will be readily apparent to those of skill in the art.
Those skilled in the art will understand that various changes may
be made without departing from the scope of the invention, which is
not to be considered limited to what is described in the
specification.
* * * * *