Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to an improved process for
treating substrates to aid in the release of adhesive materials
therefrom. .~ore specifically, this invention relates to a new
substrate-coating process which uses a silicone release
composition which cures rapidly, has improved resistance to
abrasion and rub-off in the cured form and yet has a viscosity at
room temperature that permits its application to the substrate by
standard coating methods, without the necessity of using a
volatile thinning medium, such as a solvent or a dispersant.
The term "adhesive materials" as employed herein means
materials having a lasting sticky nature, such as pressure
sensitive adhesives, foodstuffs, asphalt, pitch and raw rubber, in
contrast to temporary sticky materials which become non-sticky,
such as cements and moldin~ compositions.
The application of curable silicone release compositions
to substrates to aid in the release of adhesive materials
therefrom is old and well known in the coatings art. However,
most, if not all, curable silicone compositions that have been
used in the release coatings art suffer from a property trade-off
which affects the viscosity of the curable composition, the
release behavior of the cured coating and the abrasion resistance
of the cured coating.
Specifically, with respect to the silicone compositions
which cure by way of a catalyzed reaction of silicon-bonded vinyl
radicals with silicon-bonded hydrogen radicals, this proper~y
trade-off can be summarized in the following manner. Solventless,
low-viscosity coating compositions, such as those disclosed by
Chandra et al., U.S. Patent No. 3,928,629 and Grenoble, U.S.
Patent No. 4,071,644 are based on low-molecular weight,
vinyl-containing polymers and, therefore, frequently lac~ the
- . -
.
~ J~
necessary strength, in the cured state, to sur~ive vigorous
abrasion and flexing forces. Using a higher molecular weight
polymer, instead of the low molecular weight polymer, such as is
disclosed in U.K. Patent ~o. 1,240,523, is ~nown to provide
increased strength to the cured coatins; however, the viscosity of
the uncured composition is correspondingly increased, resulting in
difficulty in applying the curable composition to a substrate.
Low viscosity is thereby traded for increased strength~ Cured
silicone release compositions, whether based on high or low
molecular weight polymer, have excellent release characteristics
and are desired as abhesive, i.e., non-stick, coatings. ~owever,
in many instances, such as in the preparation of a release backing
for pressure-sensitive adhesive labels, easy release is not
desired. To control the adhesive release behavior of a silicone
composition and to provide additional strength, it has been taught
by Sandford, U.S. Patent No. 4,123,604 to mix a vinyl-containing
resin with the curable silicone composition. ~hether this resin
is mixed with a low molecular weight polymer or a high molecular
weight polymer, a considerable increase in the viscosity of the
composition results. To obtain a solventless, low-vlscosity
curable release composition from such a resin-containing mixture,
a very low molecular weight, vinyl-containing polymer, including
vinyl-containing cyclopolysiloxanes, must be used, thereby trading
abrasion resistance for controlled release.
Of course, an obvious way to decrease the viscosity of a
curable silicone release composition is to dissolve or disperse
the composition in a volatile thinning medium; however, this
expedient is undesirable from at least two aspects. First,
environmental considerations suggest that it may be undesirable to
vent a solvent to the atmosphere during the preparation and/or use
7C~
of the release composition. Second, recovery of a solvent from a
release composition may be a prohibitively expensive process for
the manufacturer and/or user o~ the release composition.
It is an object of this invention to provide an improved
process for coating substrates which provides abrasion-resistant,
adhesive-releasing coatings from solventless coating compositions.
This object and others are realized by applying to a
surface of a substrate, ~rom which adhesive release is desired,
certain fast-curing, tough silicone compositions having a high
ratio of silicon-bonded hydrogen radicals to silicon-bonded vinyl
radicals. Certain of the curable silicone compositions which are
useful in the process of this invention have been disclosed as a
marking ink composition and as a repair composition for surface
imperfections on translucent silicone rubber articles. Others
useful herein appear to be new.
This invention relates to an improved process for coating
a substrate to provide release of adhesive materials therefrom,
said process comprising applying a curable silicone release
composition to a surface of the substrate and thereafter curing
the applied composition, the improvement comprising applying, as
the curable silicone release c~mposition, a composition obtained
by mixing components consisting essentially of (I) a polydiorgano-
siloxane having the general formula ViR2Sio(R2SiO)x~RViSiO)ySiR2Vi
wherein x and y are integers whose sum has an average value
sufficient to provide the polydiorganosiloxane with a viscosity at
25C. of at least 1.0 pascal-second, Vi denotes a vinyl radical
and each R denotes, independently, a monovalent radical selected
from the group consisting of methyl, phenyl and saturated
hydrocarbon radicals having from 2 to 6 carbon atoms, the total
number of organic radicals in the polydiorganosiloxane consisting
2~'0
of at least 95 percent rneth~l radicals and no more than 1 percent
vinyl radicals, (II) a xylene-soluble copolymer of (CH3)3SiO1/2,
(CH3)2(CH2=C~)SiOl/2 and SiO4/2 siloxane units, said copolymer
having from 1 to 5 percent by weight of vinyl radisals, based on
the weight of the copolymer, and a total of from 0.6 to 1.1 of
said (CH3)3siOl/2 plus (CH3)2(CH2=CH)SiOl/2 siloxane units for
every said SiO4/2 siloxane unit, (III) a methylhydrogenpoly-
siloxane, soluble in the mixture of (I) plus ~II), and having an
average of at least three silicon-bonded hydrogen radicals per
molecule, said hydrogen radicals being bonded to separate silicon
atoms, and (IV) a catalytic amount of a hydrosilylation catalyst,
said components being mixed in sufficient amounts to provide, in
the curable silicone release composition, from 10 to 70 parts by
weight of (II) for every 100 parts by weight of (I) plus (II) and
from 2 to 10 silicon-bonded hydrogen radicals for every
silicon-bonded vinyl radical.
The four essential components, i.e., polydiorganosiloxane
(I), xylene-soluble copolymer (II), methylhydrogenpolysiloxane
(III) and hydrosilylation catalyst (IV), which are mixed to form
2~ the curable silicone composition which is used as a curable
release agent in the improved process of this invention are all
known, broadly, in the organopolysiloxane art.
Polydiorganosiloxane (I) is a vinyl-endblocked linear
polymer having the general formula ViR2SiO(R2SiO)X(RViSiO)ySiR2Vi.
Each R denotes, independently, a methyl radical, a phenyl
radical or a saturated hydrocarbon radical having 2 to 6,
inclusive; carbon atoms such as alkyl radicals, such as ethyl,
propyl, isopropyl, ~utyl and hexyl and cycloaliphatic radica's,
such as cyclohexyl. At least 9S persent of all organic radicals
3(; in (I) are the methyi radical. Preferably, each termir.al silicon
~ 3 Z~t~
atom o~ poiydiorganosiloxane (I) bears at least one methyl
radical. To avoid overcuring of the release composition, the
total number of vinyl radicals in polydiorganosiloxane (I) should
not exceed 1 percent of all of the silicon-bonded organic radicals
therein.
Examples of preferred siloxane units which form
polydiorganosiloxane (I) include ViMe2SiOl/2, Phl~eViSiOl/2,
Me2SiO2/2 and MeViSiO2/2. Examples of other siloxane units
suitable for use in polydiorganosiloxane (I) include PhViSiO2/2,
Ph2SiO2/2, PhMeSiO2/2, ViEtSiO2/2 and ~eEtSiO2/2 siloxane units.
Herein the symbols ~e, Et, Ph and Vi denote,
respectively, the methyl, ethyl, phenyl and vinyl radical.
Examples of preferred polydiorganosiloxanes (I) to be
used in the process of this invention include
ViMe2SiO(~e2SiO)xSiMe2Vi, ViPhMeSiO(Me2SiO)xSiMePhVi,
ViMe2SiO(Me2SiO)x(ViMeSiO)ySiMe2Vi and
ViPh~eSiO(Me2SiO)x(ViMeSiO)ySiMePhVi.
The average value of the sum of x plus y in the above
formulae is such that the viscosity of the resulting polydiorgano-
siloxane is at least 1.0 pascal-seconds (1000 centipoise) at 25C.
Preferred results, such as rapid cure rate of the curable
composition and high abrasion resistance of the cured release
coating, are obtained when said viscosity of the vinyl-endblocked
polydiorganosiloxane is at least 25 pascal-seconds. There is no
known upper limit to the viscosity of polydiorganosiloxane (I)
unless a solventless coating composition is desired, in which case
an upper viscosity limit is approximately 100 pascal-seconds.
The exact average value for the sum of x plus ~ which
will produce a desired viscosity at 25C. for polydiorganosilox~ne
(I) depends upon the amounts and types of R radicals present
Ji~
therein and is difficult to predict. However, for the
above-delineat~d preferred vinyl-endblocked polydiorganosiloxanes,
an average value for x plus y of approximately 225 will provide a
viscosity of 1.0 pascal-seconds and an average value of
approximately 695 will provide a viscosity of 25 pascal-seconds,
both measured at 25C.
Polydiorganosiloxanes (I) are well known in the
organosilicon polymer art and may be prepared by any suitable
method. While the preparation of polydiorganosiloxane (I) needs
no further elaboration here, it should be noted that, depending
upon the particular polydiorganosiloxane (I) that is prepared and
the particular method for its preparation that is used, there may
be coproduced therewith approximately 0 to 15 percent by weight of
cyclopolydiorganosiloxanes. A large portion of said cyclopolydi-
organosiloxanes may be volatile at temperatures up to 150C. It
is to be noted that the suitability of vinyl-endblocked
polydiorganosiloxane (I) for use in the process of this invention
is determined by its viscosity at 25C., as delineated above, and
does not depend upon the presence or absence therein of the
above-described amounts of coproduced cyclopolydiorganosiloxanes.
That is to say, vinyl-endblocked polydiorganosiloxane (I) may be
optionally freed of any volatile cyclopolydiorganosiloxanes, if
desired, without having a detrimental effect on the process of
this invention.
For the purposes of this invention, however, the
viscosity and the amount of vinyl-endblocked polydiorganosiloxane
(I) that is used, and which is further delineated belo~, are based
on polydiorganosiloxane which has been devolatilized at 1~0C. for
1 hour.
~3
i~hile ~olydior~anosiloxane (I) is stated to be linear and
to ~ear only hydrocar~on radicals on silicon, it is within the
scope anà spirit of this invention to permit the presence therein
of small amounts of non-linear siloxane units, i.e. SiO4/2,
ViSiO3/2 and RSiO3/2 siloxane units wherein ~ is as denoted above,
and trace amounts of other sillcon-bonded radicals, such as
hydroxyl and alkoxyl, which are normally incidentally present in
commercial polydiorganosiloxanes. Preferably, polydiorgano-
siloxane ~I) is free of said non-linear siloxane units and said
other radicals.
The xylene-soluble copolymer (II) of (CH3)3SiOl/2
siloxane units, (CH3)2(CH2=CH)SiOl/2 siloxane units and SiO4/2
siloxane units is ~ell known in the organosilicon polymer art~
Said copolymer is a solid, resinous material which is prepared as,
and usually, but not necessarily, used as, a solution in an
organic solvent. Typical solvents that are used with copolymer
(II) include benzene, toluene, xylene, methylene chloride,
perchloroethylene and naphtha mineral spirits.
Copolymer (II) contains from 1 to 5, preferably 1.5 to
3.5, percent by weight of vinyl radicals, based on the weight of
the copolymer, ancl from 0.6 to 1.1 of the siloxane units bearing
organic radicals for every SiO4~2 siloxane units. Thus, in
copolymer (II), for every SiO4/2 unit there is a total of from 0.6
to 1-1 (CH3)3SiOl/2 units plus (cH3)2(cH2=cH)siol/2 units.
Copolymer (II) is preferably prepared by an adaptation
of the procedure described by Daudt et al. in U.S. Patent Mo.
2,676,182 whereby a silica hydrosol is treated ~t lo~ pH with a
source of trimethylsiloxane units, such as hexamethyldisiloxane or
trimethylchlorosilane and a source of dimethylvinylsiioxane units,
such as divinyltetramethyldisiloxane or dimethylvinylchlorosilane.
.~lternativeiv, a suitable mixture of hydrolyzable
trimethyl-substituted, dimethyl-~inyl-substituted and organic
radical free silanes t such as chlorosilanes and/or alkoxysilanes,
may be cohydrolyzed. In this alternate procedure, the resulting
cohydrolyzate is preferably subsequently treated with a suitable
silylating agent, such as hexamethyldisilazane or divinyltetra-
methyldisilazane, to reduce the hydroxyl content of the resulting
resinous copolymer to less than 1 percent by weightA Minor
amounts of diorganosiloxane units may be present in the copolymer
(II).
Methylhydrogenpolysiloxane (III) operates as a curing
agent for the mixture of polydiorganosiloxane (I) and
xylene-soluble copolymer (II) and therefore must be soluble
therein and must contain an average of at least three, preferably
more than three, silicon-bonded hydrogen radicals per molecule.
The term "methylhydrogenpolysiloxane" means that at least three,
but preferably all, silicon atoms therein which bear the hydrogen
radicals also bear at least one methyl radical. For efficient
curing of the mixture of (I) plus (II), it is preferred that no
silicon atom in IIII) bear more than one silicon-bonded hydrogen
radical. Methylhydrogenpolysiloxane (III) may also contain
silicon-bonded phenyl radicals and silicon-bonded alkyl radicals
having from 2 to 6 carbon atoms, provided that it is soluble in
the mixture of (I) plus (II).
Methylhydrogenpolysiloxane (III) is preferably a fluid
having a low viscosity, such as less than 0.1 pascal-seconds at
25C., thereby considerably and desirably decreasing the initial,
i.e., uncatalyzed, viscosity of the mixture of polydiorgano-
siloxane (I) and copolymer (II) when mixed therewith. It is
desirable to decrease the viscosity of the mixture of (I) plus
1~2~3~, ~
(II) because application of the resulting curable composition
onto, and adhesion of the suhsequently cured composition to, a
substrate is aided thereby. The viscosity of the mixture of (I)
2Ius (II) can be considerably decreased by the use of a low
viscosity methylhydrogenpolysiloxane as component (III) because
the curable silicone compositions which are uni~uely operative in
the process of this invention have an unusually high ratio of
silicon-bonded hydrogen radicals to silicon-bonded vinyl radicals,
further delineated below, thereby permitting the use of relatively
large amounts of any particular methylhydrogenpolysiloxane (III).
Preferred siloxane units which form the methylhydrogen-
polysiloxane (III) include Me3Sil/2~ Me2HSiOl/2~ Me2Si2~2~
MeHSiO2/2, MeSiO3/2, and SiO4/2. Methylhydrogenpolysiloxane (III)
may also further comprise other siloxane units, such as HSiO3/2,
PhHSiO2/2, PhMeHSiOl/2, PhMeSiO2/2 and PhSiO3/2, provided that the
resulting methylhydrogenpolysiloxane is soluble in the mixture of
(I) plus (II).
Examples of methylhydrogenpolysiloxane (III) whicA are
operative in the process of this invention include, but are not
limited to, siloxanes consisting of Me3SiOl/2 units and MeHSiO2/2
units, siloxanes consisting of Me3siol/2 units, Me2SiO2/2 units
and MeHSiO2/2 UllitS, siloxanes consisting of HMe2SiOl/2 units,
Me2SiO2/2 units and MeHSiO2/2 units, siloxanes consisting of
SiO4/2 units, Me3SiOl~2 units and HMe2SiOl/2 units, siloxanes
consisting of SiO4/2 units and HMe2SiOl/2 units, siloxanes
consisting of HMeSiO2/2 units and siloxanes consisting of
HMeSiO2/2 units and Me2SiO2/2 units.
Specific examples of sultable methylhydrogenpolysiloxanes
that may be used as component (III) in the process of this
33 invention include (HMe2sio~4si~ (.YeHSiO)4, MeSi(OSiMe2H~,
2'~
PhSi(OSiMe2H)3 and, preferably, higher molecular weight fluid
siloxanes having t~e average formulae ~e3SiO(MeHSiO)35SiMe3,
Me3SiO(~e2SiO)3(1~eHSiO)5Sii~e3 and HMe2SiO(Me2SiO)3(.~eHSiO)5Si~e2H.
The higher molecular weight methylhydrogenpolysiloxanes are
preferred as a curing component for a silicone release composition
because said higher molecular weight methylhydrogenpolysiloxanes
have a low volatility and will remain with and more effectively
cure the silicon release composition at elevated temperatures.
I~ethylhydrogenpolysiloxanes are well known in the
organosilicon polymer art; their preparation therefore needs no
further elaboration here. As in the case of the preparation of
vinyl-endblocked polydiorganosiloxanes, it should be noted that
the preparation of methylhydrogenpolysiloxanes comprising
diorganosiloxane units may produce small amounts of cyclopolydi-
organosiloxanes. The presence or absence of these cyclopolydi-
organosiloxane species in the methylhydrogenpolysiloxane is of no
importance to this invention as long as the
methylhydrogenpolysiloxane has an average of at least 3
silicon-bonded hydrogens per molecule.
Component (IV) is any hydrosilylation catalyst that is
effective to catalyze the addition reaction of silicon-bonded
hydrogen radicals with silicon-bonded vinyl radicals in the well
known manner. Typically, component (IV) is an active metal
containing composition such as a platinum-containing compound or a
rhodium-containing compound. Examples of these active metal
compositions include chloroplatinic acid, platinum deposited on a
substrate, platinum complexed with organic liquids, such as
ketones, vinylsiloxanes and ethylene, and complexes of rhodium
halides. Preferably, the hydrosilylation catalyst is soluble in
the curable silicone release composition.
3~t~l~3
The platinum-containing catalysts may also contain an
inhibitor to ~oderate its catalytic activity at room temperature,
in the well ~nown manner, if desired. ~hodium-containing
catalysts do not need room temperature inhibiting.
Hydrosilylation catalysts and their inhibitors are well
known in the organosilicon art and need no further delineation
herein. For further details, if needed, the reader is directed to
the teachings of Speier et al., U.S. Patent No. 2,823,218;
Willing, U.S. Patent No. 3,419,593; Xookootsedes et al., U.S.
Patent No. 3,445,~20; Chandra, U.S. Patent No. 3,890,359;
Polmanteer et al., UOS. Patent No. 3,697,473; Nitzsche et al.,
U.S. Patent ~7O. 3,814,731; and Sandford, U.S. Patent No.
4,123,604.
The vinyl-containing components of the curable silicone
release composition are mixed in amounts sufficient to provide
from 10 to 70 parts by weight of the xylene-soluble copolymer (II)
for every 100 parts by weight of the mixture of vinyl-endblocked
polydiorganosiloxane (I) plus copolymer (II). A preferred
combination of properties, for the cured release agent, such as
premium release of aggressive acrylic adhesives and high film
strength, is obtained when the curable composition contains a
mixture of (I) and (II) which is from 20 to 60 percent by weight
ccmponent (II).
The desired amounts of (I) and (II) are used on a
non-~olatile basis. This is easily achieved by subjecting samples
of the polydiorganosiloxane, which frequently contains volatile
cyclopolydiorganosiloxanes, and xylene-soluble copolymer, which is
usually prepared and handled in xylene, to a devolatilization
procedure at 150C. for 1 hour to determine the non-volatile
~0 content of each and using a sufficient quantity of each material
v
to ob_ain the desired amount of the vinyl-elldblocked polydiorgano-
siloxane (I) and :~ylene-soluble copolymer (II) in the curable
composition.
The amount of methylhydrogenpolysiloxane (III) to be
mi~ed when preparing the curable silicone release composition is
merely the amount that will provide from 2 to 10 silicon-bonded
hydrogen radicals for every siiicon-bonded vinyl radical in the
composition and the desired viscosity for the silicone release
composition. For any given resin concentration in the resin plus
polymer mixture, the amount of methylhydrogenpolysiloxane (III)
that is needed to obtain a suitable coating viscosity for the
curable composition is related to the viscosity of the polymer
(I). Thus, a curable composition comprising a polydiorgano-
siloxane (I) having a viscosity of 5 pascal-seconds may have a
suitable coating viscosity when the SiH/SiVi ratio is 2.0; whereas
a curable composition containing the same amount of resin but
comprising a polydiorganosiloxane (I) having a viscosity of 100
pascal-seconds may require a Si~/SiVi ratio of 10.0 to provide a
suitable coating viscosity for the curable composition. A
preferred value for this ratio of hydrogen radicals to vinyl
radicals is from 2 to 5 wherein the hydrogen gassing, sometimes
observed at higher ratios, is usually avoided.
The number of said silicon-bonded hydrogen radicals and
said silicon-bonded vinyl radicals should be measured by suitable
analytical techniques.
The amount of hydrosilylation catalyst to be used in the
curable silicone release composition is merely that amount that
will catalyze the addition of silicon-bonded hydrogen to
silicon-bonded vinyl and provide the desired cure time at a
~articular curing temperature for the curable silicone release
1~L2~Z~7~
compositions. ~ suitable catalytic amount of hydrosilylation
catalyst can be determined by simple experimentation. A
composition soluble, platinum-containing catalyst is typically
used in sufficient amount to provide from 0.5 to 20 parts by
weight of platinum for every one million parts by weight of
components (I) plus (II) plus (III). A composition soluble,
rhodium-containing catalyst is typically used in sufficient amount
to provide from 5 to 40 parts per million of rhodium, on the same
weight basis.
The curable silicone release composition may further
contain up to 95 percent by weight, based on the weight of the
curable composition, of a volatile thinning medium having a normal
boiling point of less than 150C., such as a dispersant or a
solvent, to aid in mixing and using said composition, if desired.
A volatile thinning medium is advantageously employed when further
reduc~ion in the viscosity of the curable silicone composition,
beyond that conferred by the methylhydrogenpolysiloxane, is desired.
Conveniently, said thinning medium is the solvent in which the
xylene-soluble copolymer (II) is normally prepared and handled.
Organic thinning media should be free of aliphatic ~nsaturation.
The curable silicone release composition may further
contain additional components which do not adversely interfere
with the curing of the composition or its use as a release agent,
such as pigments, rheology-control additives, substrate-adhesion
promoters and adjuvants to control substrate penetration by the
release composition.
The curable silicone release composition is prepared by
mixing the desired amounts of the four essential components and
any additional components in any suitable manner such as by
stirringl blending and/or tumbling and in any suitable order.
t~
Prefera~ly, the methylhydrogenpolysiloxane (III) and the
hydrosilylatior. c~talyst ~IV) are brought together in a final
mixing step.
For example, the curable silicone release composition can
be conveniently prepared by preparing two non-curing compositions
which, when blended in proper proportions, will give rise to the
curable silicone release composition. Typically, one of said
non-curing compositions comprises a portion of the polydiorgano-
siloxane (I), the xylene-soluble copolymer (II), optionally
containing its processing solvent such as xylene, and the methyl-
hydrogenpolysiloxane (III) and another of said non-curing
compositions comprises the balance of the polydiorganosiloxane (I)
and the hydrosilylation catalyst (IV) and any inhibitor.
Alternately, one of said non-curing compositions may comprise all
of the ccmponents except the methylhydrogenpolysiloxane, which
constitutes another non-curing composition to be mixed with the
first non-curing composition at the proper time.
Any solid substrate may be treated by the process of this
invention to provide release of adhesive materials therefrom.
Examples of suitable substrates include cellulosic materials, such
as paper, cardboard, and wood; metals, such as aluminum, iron and
steel; siliceous materials, such as ceramics, glass and concrete;
and synthetics, such as polyester and polyepoxide. To assure
proper curing and adhesion of the curable silicone release
composition, the substrate to which it is applied should be clean
and free of materials which undesirably inhibit the cure of the
release composition, such as materials containing amines,
mercaptans and phosphines.
The process of this invention is particularly useful for
coating flexible substrates, such as paper, aluminum foil, and
14
tapes to provide controlled release of pressure-sensitive adhesive
materials, such as aggressive acrylic adhesives. The curable
silicone release composition may be applied in a thin layer to the
surface of the flexible substrate to provide a coating with a mass
of approximately one gram per square metre of coated surface. In
the cured state, ~any of these coatings will release aggressive
acrylic adhesives with a force of no more than 38.61 newtons/metre
as measured by the method described in the examples. It is to be
understood that thinner or thicker coatings of release
composition may be applied to the substrate, if desired. In the
paper coating art, the amoun' of release coating will generally be
applied in an amount between 0.1 and 2.0 grams per square metre of
surface.
In the process of the invention, the curable silicone
release compositions may be applied to a substrate by any suitable
means, such as by brushing, dipping, spraying, rolling and
spreading. Flexible substrates, such as paper, may also be coated
by any of the well known rolling methods, such as by a trailing
blade coater, kiss rolls, gravure rolls and offset printing rolls
as desired.
For general applications, a practical upper limit for the
viscosity of the curable silicone release composition is
ap2roximately lO0 pascal-seconds; however, for coating flexible
substrates by rolling methods, a practical upper limit for said
viscosity is approximately 50 pascal-seconds, preferably 10
pascal-seconds at 25C. In many instances, the release
compositions used in the process of this invention have a
viscosity which is useful in said roller methods of coating
without comprising a significant amount; for example, less than 5
percent by weight, of a volatile thinning medium.
9;2~
After application to a substrate r the curable silicone
release composition is allowed to cure and any volatile thinning
medium is allowed to evaporate. Preferably, said curing and
evaporating are accelerated by the application of heat to the
applied composition. Heating, usually limited to temperatures
less than 300C., preferably less than 200C., may be accomplished
by any suitable means; however, the curable silicone release
composition should not be heated much above room temperature
before it is applied because it will rapidly gel and become
unusable.
The invention having been fully described, it will now be
exemplified, but not limited, by the following examples, which
also include the best mode for practicing the invention.
For this disclosure, all viscosities were measured in
centipoise at 25C. and were converted to pascal-seconds by
multiplying by 0.0001 and rounding off. Adhesive release was
measured in grams/inch and was converted to newtons/metre by
multiplying by 0.3860885 and rounding off. Tensile strength was
measured in pounds/square inch and was converted to mega-pascals
by multiplying by 6.894757 x 10-3 and rounding off. All parts,
percentages and ratios are by weight unless otherwise indicated.
These examples illustrate the viscosity-lowering that is
obtained by using a high SiH/SiVi ratio in a curable, vinyl
resin-containing silicone release composition.
Examples
Release agents, designated by letters A through L in the
Table, were prepared by mixing sufficient poiydiorganosiloxane and
resin copolymer solution to provide the indicated ratio of
non-volatile polydiorganosiloxane to non-volatile resin copoiymer.
The polydiorganosiloxane consis~ed of 91 percent of a non-volatile
16
~ethylpn~nylvinylsilo~rle-endbloc1~ed ~lydimethylsilox~ne having a
viscosity of approximately 65 pascal seconds at 25C. and 9
percent o~ volatile cyclic polydimethylsiloxanes.
The r~sin copolymer solution consisted of 34.5 percent
xylene and 65.5 percent o~ a non-volatile resin copolymer having
(CH3)3SiOl~2, (CH3)2(CH2=CH)SiOl~2 and SiO4/2 siloxane units
wherein the mol ratio of the sum of the methyl- and vinyl-bearing
siloxane units to the SiO4/2 siloxane units had a value of 0.7 and
the vinyl content was 1.7 percent. The resulting mixtures were
devolatilized for 3 hours under reduced pressure and at steam
temperatures and, after being cooled, were mixed with sufficient
methylhydrogenpolysiloxane to provide the indicated ratio of
silicon-bonded hydrogen radicals to silicon-bonded vinyl radicals
and 0.004 percent methylbutynol as a platinum-catalyzed cure
control additive. The methylhydrogenpolysiloxane had the average
formula l~e3SiO(Me2SiO)3(i~eHSiO)5SiMe3.
The viscosities of these mixtures were then measured with
a rotating spindle viscometer at 25C. and are recorded in the
Table. The percent viscosity decrease is based on the viscosity
of the corresponding comparative composition (A, E or I).
Samples were prepared for tensile strengtA and elongation
measurements by catalyzing the devolatilized mixture of
vinyl-containing polymer, vinyl-containing resin copolymer,
methylhydrogenpolysiloxane and cure control additive with 0.012
percent of the platinum-containing catalyst containing 0.6 percent
platinum. The platinum-containing catalyst was a complex of
PtCl6-6H2O and divinyltetramethyldisiloxane, prepared according
to U.S. Patent No. 3,419,593. The catalyzed compositions were
poured into 7.6 cm. x 12.7 cm. x ~0 mil chases and were heated at
70C. for 15 minutes and at 177C. for 5 minutes. The cured
samples, after being cooled, were cut into "dog-bone" samples.
Tensile strength and elongation were measured according to ASTM
D-~12, using these "dog-~one" samples, and are recorded in the
Table.
Adhesi~Je release was measured on compositions A, C, E and
G, except that the mixtures of polydiorganosiloxane and resin
copolymer solution were not devolatilized before being mixed with
methylhydrogenpolysiloxane and catalyst and applied to a
substrate.
Catalyzed formulations A, C, E and G were coated onto 40
pound supercalendared kraft paper uslng a blade coater at 35
p.s.i. pressure to give approximately 0.8 grams of coating per
square metre of surface. The applied coatings were heated at
163C. for 60 seconds to cause curlng. A coating was considered
to be cured if a piece of adhesive tape would stic~ to itself
after having first been adhered to the coating and then removed
and its adhesive-bearing surface doubled back on itself. A11
coatings passed this cure test. The abrasion resistance of each
cured formulation was determined by rubbing the cured coating with
the index finger to see if rub-off occurred. No rub-off occurred.
~11 coatings therefore passed this test for abrasion resistance.
Each cured release coating was prepared for release
testing according to the following procedure. The cured release
coating was coated with adhesive using a solution of a commercial
acrylic adhesive. The acrylic adhesive solution was applied to
the cured coating at a wet thickness of 3 mils (76.2 um) using a
Bird Bar. The applied adhesive was air dried at room temperature
for one minute, heated at 70C. for 30 seconds and then cooled to
room temperature again for 1 minute. A sheet of 60 pound matte
litho was applied to the dried adhesive and the resulting laminate
3~
18
lP~9.~t3~
was rolled with 3 4 . 5 Dound rubber-coated roller and aged for 20
hours at 70C.
Release testing of the laminates was accomplished by
cooling the aged laminates to room temperature, cutting the cooled
laminates into 1 inch (25.4 mm) wide strips and pulling the
matte/adhesive lamina from the kraft paper/coating lamina at an
angle of 180 (~ radians) at 400 inches/minute (0.17 m/s). The
force, in grams per inch, that was required to separate the
laminae was noted. A composition that results in a release value
of no more than 38.61 N/m using this test is considered to display
premium release.
The Table summarizes the release values (converted from
grams/inch to newtons/metre) that were obtained.
19
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