Note: Descriptions are shown in the official language in which they were submitted.
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SILICONE PAINTS FOR PROTECTION
OF SILICONE ELASTOMERIC COATING MATERIALS
; The Government has rights in this invention
pursuant to Contract No. F33615-83-C-5084 awarded by the
Department of the Air Force.
The present invention relates to a method for
coating a cured silicone substrate, to a silicone-based paint
composition therefor, and to the coated substrate obtained
therefrom. More specifically, the present invention relates
to a method comprising overcoating, e.g. painting, the
surface of a cured silicone substrate with a silicone-based
coating composition to provide a durably adhered coating
composition on the silicone substrate which provides various
benefits, such as decoration and/or protection.
Silicone substrates comprising a cured silicone
polymer frequently need to be painted and/or protected from
abrasion; however, they typically have low surface energies
and organic paints do not wet the surface of the silicone
substrate well. Several attempts to solve this problem have
been disclosed in the art, which involve either the mixing of
; an additional component, typically called an adhesion
promoter or a paintability component, into the curable
silicone composition prior to its being applied and cured.
The existing methods for painting a cured silicone
substrate have several drawbacks. For example, the steps
needed to modify the silicone substrate are time-consuming,
costly and can alter the physical properties of the cured
silicone substrate. And after the silicone substrate has
been prepared to receive and coated with, an organic paint,
the paint has use limitations, such as temperature exposure
limitations and ultraviolet li.ght exposure limitations.
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There is a need for a method for painting a
silicone substrate with a durably adhering, environmentally
capable overcoating, such as a protective and/or decorative
paint, which does not require the priming or modifying of the
cured silicone substrate prior to applying the overcoating
thereto.
It is an object of the present invention to provide
a method for overcoating a silicone substrate with a durably
adhering decorative and/or protective material. It is a
particular object of this invention to provide a method for
painting a cured silicone substrate which does not involve
the use of a primer. In a preferred embodiment of this
invention it is an object to provide a method for painting an
iron-filled silicone substrate with a protective paint. It
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is also an object of this invention to provide a silicone
composition for overcoating, i.e. painting, the surface of a
cured silicone substrate.
These objects, and others which will occur to one
of ordinary skill in the silicone coatings art, are obtained
by the composition and method of this invention which,
briefly described, comprises applying a continuous layer of a
silicone-based paint composition to the cured silicone
substrate and thereafter curing the applied composition. The
paint can be any of the well-known silicone-based paints or
the new silicone-based paint disclosed herein. It is only
necessary that the paint composition wet, and cure to, the
cured silicone substrate.
In a first aspect, the present invention relates to
a method comprising overcoating a cured silicone elastomeric
substrate with a curable silicone-based paint composition and
thereafter curing the applied paint composition.
In the method of this invention, the curable
silicone-based paint composition can be any of the well-known
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silicone resin-modified, organic-based paints or silicone
resin-based paints that are recommended for painting metallic
substrates such as those in buildings, towers and bridges.
Silicone resins that are used in paint compositions
are three dimensional matrices of mono and diorgano-
substituted siloxane units wherein the organic radicals are
monovalent hydrocarbon radicals such as methyl, ethyl,
propyl, amyl, phenyl and phenethyl. As the average number of
organic groups per silicon atom in the resin increases the
resin becomes softer and more flexible.
By the term silicone resin-modified, organic-based
paint compositions, it is meant the organic-based paint
compositions, such as polyester-, phenolic-, epoxy-, alkyd-
and other organic binder-based paint compositions containing
hydroxyl groups in the binder portion that have been further
mixed and/or reacted with low molecular weight silicone
resins that contain silicon-bonded alkoxy and/or hydroxy
radicals. Examples of the silicone resins include, for
example, Dow Corning(R) Z-6018 Intermediate which contains
silicon-bonded hydroxy radicals, Dow Corning 3037
Intermediate which contains silicon-bonded methoxy radicals
which are particularly useful for reacting with polyesters or
oil-free alkyds to form silicone alkyds, Dow Corning 804
Resin and Dow Corning 840 Resin.
By the term silicone resin-based paint
compositions, it is meant paint compositions which have a
binder portion that is entirely silicone resin-based.
Silicone resins that are useful in silicone resin-based paint
compositions typically have a higher molecular weight and a
lower reactivity than the organic binder-directed silicone
resins noted above. Examples of these higher molecular
weight silicone resins include Dow Corning 805 Resin, Dow
Corning 806A Resin ~n~ Dow Corning 808 ~esin.
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Dow Corning Form Nos. 23-216B-80 and 23-253B-82,
each titled "Information about Silicone Resins" and Dow
Corning Form No. 23-201D-83, titled "Selection guide to
silicone paint resins" show how to make silicone organic-
based paint compositions and silicone resin-based paint
compositions that are useful in the method of this invention.
In the method of this invention, the curable
silicone-based paint composition can also be any of the
well-known silicone elastomer-based coating compositions.
By the term silicone elastomer-based coating
compositions, it is meant any curable compositions that
comprise a polydiorganosiloxane binder having at least two
curing radicals per molecule and about two organic radicals
per silicon atom, a curing agent for the binder, a pigment
and a catalyst to accelerate a curing reaction between the
binder and the curing agent. Silicon-bonded organic radicals
here are monovalent hydrocarbon radicals such as methyl,
ethyl, propyl, amyl, phenyl and phenethyl. Silicon-bonded
curing radicals include alkenyl radicals, hydroxyl radicals,
alkoxyl radicals, acyloxy radicals and the like. Examples of
these compositions include the well-known caulks and sealants
which further comprise a pigment.
In the method of this invention, the curable
silicone-based paint composition can also be a paint
composition of this invention, detailed below, comprising (i)
a binder component consisting essentially of (A) an alkenyl-
containing polydiorganosiloxane; (B) an alkenyl-containing
resin; (C) an organohydrogenpolysiloxane resin; (D) a
platinum-containing compound; and (E) an inhibitor for (D);
and (ii) one or more pigments.
In addition to comprising a silicone-based binder
and one or more pigments the paint composition that is used
in the method of this invention can further comprise any of
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the well-known paint additives such as pigment suspension
agents, leveling agents, solvents, thixotropic agents,
viscosity control agents and driers.
In the method of this invention, the paint
composition, delineated above, including preferred
embodiments thereof, is applied to a cured silicone
substrate, herein also referred to as a substrate. The
substrate that is treated by the method of this invention can
be any solid substrate that is obtained by converting a
liquid composition containing a siloxane polymer to the
elastomeric state. Examples of cured silicone substrates
include sealants, caulks, glazings, encapsulants, adhesives,
moldings, extrusions, foams and coatings.
For example, the cured silicone substrate that is
treated by the method of this invention can be any substrate
that is obtained by exposing a moisture-curable silicone
composition to moisture until its surface becomes tack-free.
Typically the moisture-cured substrate is sufficiently cured
to have a solid, non-tacky form.
MGisture-curable silicone compositions are well
known in the silicones art and need no extensive delineation
herein. Generally, moisture-curable silicone compositions
are of the one-package type that are ready to use; they give
off either an acid, typically acetic acid, or a non-acid,
such as an alcohol or a ketoxime, while curing. The
disclosures of U.S. Patent Nos. 3,035,016; 3,184,427;
3,122,522; 3,766,127; 3,766,128; 3,061,565; 3,161,614;
3,077,465; 3,294,739; 3,175,993; 3,499,859; 3,109,013; and
4,595,610 show the types of moisture-curable silicone
compositions that provide substrates which can be treated by
the method of this invention.
Alternatively, the cured silicone substrate can be
an addition-cured silicone substrate obtained by mixing a
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siloxane polymer containing olefinic unsaturation, a silicon
compound containing silicon-bonded hydrogen atoms and a
catalyst, such as a platinum compound and/or a peroxide
compound; and allowing or causing, ~he mixture to cure.
Addition-curable silicone compositions are well
known in the silicones art and need no extensive delineation
herein. Generally, addition-curable silicone compositions
are of the one- or two-package type and they give off little
or no reaction by-products while curing. The disclosures of
U.S. Patent Nos. 2,823,218; 2,915,497; 3,020,260; 3,284,406;
3,433,760; 3,445,420; 3,595,934; 3,627,851; 4,322,518;
4,537,82g and 4,731,191 show the types of addition-curable
silicone compositions that provide substrates which can be
treated by the method of this invention.
In a preferred embodiment of the present invention
the cured silicone substrate comprises a metallic filler
consisting essentially of iron powder. The disclosures of
U.S. Patent No. 4,731,191; copending and coassigned
Application Serial Nos. 815,436 and 815,437, filed on
December 31, 1985; and copending U.S. application Serial
No. 466,704, filed on December 21, 1989, all teach how to
prepare the preferred silicone substrates.
Any means or combination of means, for applying the
paint composition to the silicone substrate may be used as
long as the paint composition layer is continuous, i.e. free
of discontinuities, such as streaks or holes, commonly called
"fish-eyes". Application means include, but are not limited
to, brushing, dipping, spraying and electrostatic means.
Preferably, the paint composition is applied to a
deglossed substrate. Deglossing is preferably equivalent to
that obtained by rubbing the surface of the substrate with
240 grit sandpaper. For example, a glossy substrate can be
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rubbed with 240 grit sandpaper to remove the gloss, freed of
any loose material, such as by cleansing with alcohol and
then coated with the paint composition. The thus-abraded
surface typically can be described as having been freed of
the shiny surface that most cured silicone coatings typically
display. The loss of gloss can be determined visually;
however, any method may be used, such as by observation with
a reflectometer.
Although not required, the method of this invention
can further comprise applying one or more applications of a
primer composition to the substrate, if desired. The primer
composition that is applied to the substrate can be, for
example, any of the well-known primer compositions that are
used to improve the bonding of an organic coating composition
to a cured silicone substrate.
The paint composition can be applied to the
substrate at any time after the substrate has cured. The
surface of the substrate should be protected from
contamination by, for example, oils and dirt, until it is
overcoated. If it has become so contaminated, it should be
freed of contamination.
After the paint composition has been applied to the
substrate in a continuous, preferably thin, layer it is
allowed or caused to, cure to the solid, non-tacky state.
The composition and method of this invention
provides an article comprising a cured silicone substrate
bearing a silicone-based paint composition durably adhered
thereto. The cured silicone substrate that has been painted
by the method of this invention can be unsupported, such as a
molded or extruded silicone article, or disposed on a
support. The support can be made of any useful material,
such as cellulosics, such as wood or cotton; metals, such as
aluminum, copper, steel, titanium, brass or iron; plastics,
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such as vinyl, polyester or nylon; siliceous, such as glass,
cement, brick, mortar, concrete ancl combinations thereof; and
can take any form, such as a building component, such as a
window assembly or a bead of caulk; a fabricated metal
surface; and a work of art or handicraft.
In a second aspect, the present invention relates
to a curable silicone-based paint composition comprising (i)
a binder component consisting essentially of (A) 100 parts by
weight of a vinyl-terminated polydimethylsiloxane having a
viscosity of from 500 to 50,000 centipoise at 25C.; (B) from
20 to 55 parts by weight of a benzene-soluble silicone resin
copolymer consisting of triorganosiloxy units and SiO4/2
siloxane units in a mol ratio of 0.7/1.0, the triorganosiloxy
units being trimethylsiloxy units and dimethylvinylsiloxy
units taken in sufficient amount to provide from 1.4 to 2.2
weight percent silicon-bonded vinyl radicals in the
copolymer; (C) 10 to 20 parts by weight of a resin copolymer
consisting of monofunc~ional siloxy units and SiO4/2 units in
a mol ratio of 1.6/1.0, the monofunctional siloxy units being
trimethylsiloxy units and dimethylhydrogensiloxy units taken
in sufficient amount to provide 1.0 percent silicon-bonded
hydrogen atoms in the copolymer; (D) a platinum-containing
hydrosilylation catalyst in an amount sufficient to
accelerate a room temperature cure of the composition; and
(E) an effective amount of an inhibitor component for the
hydrosilylation catalyst; and (ii) a pigment component
comprising one or more pigments.
Component (A) can have any viscosity within the
limits stated above; however, it preferably has a viscosity
of from 1,000 to 5,000 centipoise, and most preferably 2,000
centipoise, at 25C. in order to provide a more easily
applied paint composition.
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Component (B) is a well-known resin component in
the silicone arts and needs no extensive delineation herein.
It is prepared, as a solution in a hydrocarbon solvent,
following the teachings of U.S. Patent No. 2,676,182 to Daudt
and Tyler.
Component (C) can be prepared by following the
teachings of U.S. Patent No. 4,707,531 to Shirahata.
Component (D) of the composition of this invention
is a catalyst component which facilitates the curing of the
composition at room temperature and can be any platinum-
containing catalyst component, such as platinum metal,
optionally deposited onto a suitable carrier, such as silica
gel or powdered charcoal; or a compound or complex of a
platinum group metal. Platinum hydrosilylation catalysts are
well known in the organosilicon art, and particularly in the
curable silicone composition art.
A preferred platinum-containing catalyst component
in the compositions of this invention is a form of
chloroplatinic acid, either as the commonly available
hexahydrate form or as the anhydrous form, because of its
easy dispersibility in organosilicon systems. Speier, U.S.
Patent No. 2,823,218, discloses the use of chloroplatinic
acid (H2PtC16) as a hydrosilylation catalyst. A particularly
useful form of chloroplatinic acid is that composition
obtained when it is reacted with an aliphatically unsaturated
organosilicon compound such as divinyltetramethyldisiloxane,
as disclosed by U.S. Patent No. 3,419,593 to Willing.
The amount of platinum group metal-containing
catalyst component that is used in the compositions of this
invention is not narrowly limited as long as there is a
sufficient amount to accelerate a room temperature reaction
between the silicon-bonded hydrogen atoms of component (C)
with the silicon-bonded vinyl radicals of components (A) and
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(B). The exact necessary amount of said catalyst component
will depend upon the particular catalyst and is not easily
predictable. Howe~er, for chloroplatinic acid said amount
can be as low as one part by weight of platinum for every one
million parts by weight of organosilicon components (A) I (B)
~(C). Preferably, said amount is at least 10 parts by
weight, and most preferably 50 to 500 parts by weight, on the
same basis.
Component (E) can be any of the inhibitors for
platinum-containing catalysts that are well known in the
organosilicon art. Examples of various classes of such metal
catalyst inhibitors include ethylenically or aromatically
unsaturated amides, U.S. Patent No. 4,337,332; acetylenic
compounds, U.S. Patent Nos. 3,445,420 and 4,347,346;
ethylenically unsaturated isocyanates, U.S. Patent
No. 3,882,083; olefinic siloxanes, U.S. Patent No. 3,989,667;
unsaturated hydrocarbon diesters, U.S. Patent Nos. 4,256,870;
4,476,166 and 4,562,096, and conjugated ene-ynes, U.S. Patent
Nos. 4,465,818 and 4,472,563.
The pigments that are used in the paint
compositions of this invention can be any of those typically
used in silicone compositions. Examples thereof include, but
are not limited to, titanium dioxide, lampblack, red iron
oxide, cadmium yellow and combinations thereof.
The following examples are disclosed to further
teach how to practice, but not to limit, the present
invention which is properly delineated by the claims appended
hereto. All parts, percentages and ratios are by weight,
unless stated otherwise.
Adhesion of a paint to a silicone elastomeric
substrate was measured by the following tests.
The Cross Hatch test consisted of cutting, through
the coating and into the substrate, a lOxlO grid measuring 1
0~`3
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inch on a side and containing 100-0.1 inch squares. A piece
of masking tape was firmly applied to the squares, in a
parallel direction to one set of cuts and peeled off at an
angle of 1~0 to the surface of the paint. The number of 0.1
inch squares remaining on the substrate was counted and was
expressed as a percentage of the total number of squares
covered b~ the masking tape.
The Tape Anchorage test consisted of cutting,
through the coating and onto the substrate, a single straight
line cut. A piece of duct tape was firmly applied to the cut
at an angle of 90 to the direction of the cut and peeled off
at an angle of 180 to the surface of the paint. The length
of paint that was removed was measured in mm.
The paint compositions that are referenced in the
following examples have the compositions indicated below. In
these compositions the following materials are used.
Asbestine 625 - A fibrous magnesium silicate commonly used as
a pigment and pigment extended in paints and available from
International Pulp Co.
Mica C-3000 - Mica commonly used as a filler in paints and
available from The English Mica Company.
Nytal(NR) 400 - A non-registered trade mark of R.T.
Vanderbilt Co. for a finely divided Thermolitic talc commonly
used as a filler in paints.
Thixcin(R) R - A registered trade mark of Baker Castor Oil
Co. for a finely divided, solidified vegetable-oil derivative
which is used as a thixotropic agent in non-aqueous systems
such as paints, inks, greases and lubricants. Dow Corning(R~
- A registered trade mark of the Dow Corning Corporation,
Midland~ MI for silicone based products available in sample
and/or commercial quantities.
TiO2 R960 - Titanium dioxide available from E. I. duPont de
Nemours.
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NMS 66-3 - Neutral mineral spirits meeting the requirements
of Los Angele~ County's Rule 66.
Paint A - TiO2 R960, 56.07; lampblack, 0.93;
Asbestine 625, 34.35; Mica C-3000, 12.54; Thixcin R, 0.93;
NMS 66-3, 29.26; Dow Corning El-3044 Silicone Alkyd, 156.27;
Dow Corning 3 Paint Additive, 0.62. The foregoing was milled
at 3600 rpm for 30 minutes to a Hegman value of 4 and, when
used, further mixed with 14.63 NMS 66-3; 0.31 Dow Corning 3
Paint Additive and 5.81 of Dow Corning E-3059 Drier.
Paint B - TiO2 R960, 56.07; lampblack, 0.93; Nytal
400, 34.35, Mica C-3000, 12.54; Thixcin R, 0.93; Dow Corning
805 Resin, 112Ø The foregoing was milled at 3600 rpm for
30 minutes and mixed with an additional 75.52 parts of resin;
0.93 Dow Corning 3 Paint Additive and 50 toluene to provide a
Zahn #4 viscosity of 36 seconds. When used this mixture was
mixed with Dow Corning 21 Paint Additive at 3% solids based
on resin solids.
Paint C - TiO2 R960, 56.07; lampblack, 0.93;
Asbestine 625, 34.35; Mica C-3000, 12.54; Thixcin R, 0.93;
Dow Corning 806A Resin, 112Ø The foregoing was milled at
3600 rpm for 30 minutes and mixed with an additional 75.52
parts of resin; 0.93 Dow Corning 3 Paint Additive and 50
toluene to provide a Zahn #4 viscosity of 30 seconds. When
used this mixture was mixed with Dow Corning 21 Paint
Additive at 3% solids based on resin solids.
Paint D - TiO2 R960, 56.07; lampblack, 0.93; Nytal
400 34.35; Mica C-3000, 12.54; Thixcin R, 0.93; Dow Corning
808 Resin, 112Ø The foregoing was milled at 3600 rpm for
30 minutes and mixed with an additional 75.52 parts of resin;
0.93 Dow Corning 3 Paint Additive and 35 toluene. When used
this mixture was mixed with Dow Corning 21 Paint Additive at
3% solids based on resin solids.
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Paint E ~ TiO2 R960, 56.07; lampblack, 0.93; Nytal
400 625, 34.35; Mica C-3000, 12.54; Thixcin R, 0.93; a vinyl-
terminated polydimethylsiloxane having a viscosity of about
2.1 Pa-s (2,100 centipoise) at 25C., 55.77; a benzene-
soluble silicone resin copolymer consisting of triorgano-
siloxy units and SiO4/2 siloxane units in a mol ratio of
0.7/1.0, 30.03 (the triorganosiloxy units are trimethylsiloxy
units and dimethylvinylsiloxy units taken in sufficient
amount to provide from 1.4 to 2.2 weight percent silicon-
bonded vinyl radicals in the copolymer); toluene, 10.00. The
. foregoing mixed with a Jiffy Blade and then mixed with Dow
Corning 3 Paint Additive, 0.93; 0.14 methylbutynol; 60.00
toluene and 7.73 resin copolymer consisting of monofunctional
siloxy units and SiO4/2 units in a mol ratio of 1.6/1.0 (the
monofunctional siloxy units are trimethylsiloxy units and
dimethylhydrogensiloxy units taken in sufficient amount to
provide 1.0 percent silicon-bonded hydrogen atoms in the
copolymer) to a Zahn #4 viscosity of 40 seconds. When used
this mixture was mixed with 1.13 parts of a vinylsiloxane-
complexed chloroplatinic acid hydrosilylation catalyst to
give 100 ppm platinum, based on silicone solids.
Paint F - Dow Corning 3145 Grey RTV, 250; toluene,
150. When used this material was further diluted with 1 part
of toluene for every 3 parts of mixture.
Examples 1-6
The six silicone-based paints (A-F) prepared as
disclosed above were spray-applied to an elastomeric
substrate which had been cured to each of six aluminum
panels. The elastomeric substrate was prepared as disclosed
in Example 3 of copending and coassigned Application Serial
No. 815,437 to Blizzard, filed on December 31, 1985. The
surfaces of the elastomeric substrates were prepared by
roughening with 240 grit paper, rinsed with isopropyl alcohol
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and 1/2 of the roughened surface was primed with a 5%
solution of (CH30)3SiCH2CH2CH2NHCH2CH2NH2 in n-butanol. The
primed surfaces were allowed to dry for 1 hour at 72F.
before the paints were applied to the primed and the unprimed
portions of the substrate and cured thereto. Paint E was
cured by heating at 180C. for 5 minutes. The other paints
were cured at 72F. for 7 days.
The cured paints were evaluated for adhesion by the
tests noted above. The results, TABLE 1 (unprimed portion of
the elastomeric substrate) and TABLE 2 (primed portion of the
elastomeric substrate), show that paints having a silicone
resin base show superior adhesion compared to a paint based
on a silicone alkyd, with or without the use of a primer
composition typically used for organic paints.
TABLE 1
Dry Film Unprimed Adhesion
Example No. Paint No. Thickness X-Hatch 90 TaPe
1 A 2.3 mils10% <1/3 mm
2 B 0.8 " 100% none
3 C 3.0 " 90% <1/2 mm
4 D 3.S " 100% ~1/5 mm
E 3.9 " 100% none*
6 F 3.5 " 100% none*
* Tape did not adhere to the paint.
TABLE 2
Dry Film Primed Adhesion
Example No. Paint No. Thickness X-Hatch 90 Tape
1 A 2.7 mils60% ~1/3 mm
2 B 0.9 " 100% none
3 C 2.8 " 75% ~1/2 mm
4 D 3.8 " 100% none
E 3.8 " 100% none*
- 6 F 3.5 " 100% none*
* Tape did not adhere to the paint.
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Examples 7 and 8
Examples 3 and 6 were repeated except using the
platinum-catalyzed, iron-filled, addition-cured composition
disclosed in cofiled and coassigned U.S. application Serial
No. 466,704 filed December 21, 1989. Results substantially
equivalent to the results no~ed in Examples 3 and 6 were
obtained except Paint F could be peeled from the substrate
whereas it could not be in Example 6.
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