Note: Descriptions are shown in the official language in which they were submitted.
60SI-386
FOR: SILICONE RESIN COATING COMPOSITION WITH IMPROVED
SHELF LIFE
BACKGROUND OF THE INVENTION
_ . . _ . ~ . . . _
This invention relates to a protective coating
composition. More particularly, it relates to a
silicone resin coating composition with improved storage
capacity which, when applied to a substrate, forms a
protective, abrasion-resistant coating thereon.
Recently, the substitution of glass glazing with
transparent materials which do not shatter, or are more
resistant to shattering than glass, has become wide-
spread. For example, transparent glazing made from
synthetic organic polymers is now utilized in public
transportation vehicles such as trains, buses, taxis
and airplanes. Lenses in eye glasses and other
optical instruments, as well as glazing for large
buildings, also employ shatter-resistant, transparent
plastics. The lighter weight of these plastics in
comparision to glass is a further advantage, especially
in the transportation industry where the weight of the
vehicle is a factor in its fuel economy.
One of the most promising and widely used trans-
parent plastics for glazing is polycarbonate, such asthat known as Lexan~, sold by General Electric Company.
It is a tough material, having high impact strength~
high heat deElection temperature and good dimentional
stability. It is also self-extinguishing And is easily
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60SI-386
-- 2
fabricated. ~crylics, such as polymethylmethacrylate,
are also widely used transparent plastics for glazing.
Although these plastics provide the advantages of
being more shatter-resistant and lighter than glass, they
are easily marred and scratched by contact with abrasives
such as dust, cleaning equipment and ordinary weathering.
This has led to the d~velopment of coatings which serve to
improve the abrasion resistance of transparent plastics.
For example, scratch-resistant coatings formed from
mixtures of silica, such as colloidal silica or silica gel,
and hydrolyzable silanes in a hydrolysis medium, such as
alcohol and water, are known. U.S. Patents 3,708,225
(Misch et al.), dated January 2, 1973, 3,986,997 to Clark
dated October 19, 1976, 3,976,497 to Clark dated August
24, 1976, and 4~177,315 (Ubersax~ dated December 14, 1979
for example, describe such compositions.
In Canadian application Serial No. 339,539 filed
November 9/ 1979, coating compositions having improved
resistance to moisture and humidity and ultraviolet
light are disclosed. It was discovered therein that,
in direct contrast to the teachings of U.S. Patent
3,986,997 to Clark dated October 19, 1976, compositions
having a basic pH, i.e., 7.1 - 7.8, do not immediately
gel but in fact provide excellent abrasion-resistant
coatings on solid substrates.
Silicone resin coatings such as those disclosed
in U.S. Patent No. 4,309,313 to Vaughn dated January
5, 1982, have shown great promise in protecting
transparent plastics. However, a serious drawback
diminishing their usefulness is that silicone resin hard
coatlngs are short-lived when stored at room temperature.
Refrigerated storage and shipping are required to obtain
extended shelf beyond the usual one-month life at room
temperature.
SUMMARY OF ~HE INV~N~ION
Accordingly, it is the object of this invention to
60SI-386
-- 3 --
provide a silicone resin coating composition with improved
storage stability at room temperature.
This is accomplished herein by preparing a coating
compositi~n comprisiny a dispersion of colloidal silica
in an aliphatic alcohol-water solution of the partial
condensate o~ a silanol of the formula RSi(OH)3, ~herein
R is selected from the group consisting of alkyl having
from 1 to 3 carbon atoms and aryl, at least 70 weight
percent of the silanol being CH3Si(OH)3, said composition
having a pH of from 7.1 to 7.8 and containing 10 to 5~
weight percent solids, said solids consisting essentially
of 10 to 70 weight percent colloidal silica and 30 to 90
weight percent of the partial condensate; diluting said
composition to 10-15% solids; and adding a thickening
agent.
DETAILED DESCRIPTION OF THE INVENT _
The improved coating compositions of this invention
are prepared by hydrolyzing a trialkoxysilane or a mixture
of trialkoxysilanes of the formula R'Si(OR)3, wherein R'
is alkyl of from 1 to 3 carbons or aryl, such as phenyl,
and R is alkyl, in an aqueous dispersion of colloidal
silica.
In the practice of the present invention, suitable
aqueous colloidal silica dispersions generally having a
particle size of from 5 to 150 millimicrons in diameter.
These silica dispersions are well known in the ar-t and
commercially available ones includes, for example,those
sold under the trademarks of Ludox ~ (duPont~ and
Nalcoag~ (NALCO Chemical Co.). Such colloidal silicas
are available as both acidic and basic hydrosols. For
the purpose of this invention, wherein the pH of the
coating compositions is on the basic side, basic colloidal
silica sols are preferred. However, acidic colloidal
silicas, wherein the pH is adjusted to a basic level,
are also contemplated. In addition, it has been found
that colloidal silicas having a low alkali content
~L~&~
60SI-386
-- 4
(e.g., Na2O) yield a more stable coating composition.
Thus, colloidal silicas having an alkali content of less
than 0.35% (calculated as Na2O) have been found to be
preferable. Moreover, colloidal silicas having average
particle size of from lQ to 30 millimicrons are also
preferred. A particularly preferred aqueous colloidal
silica dispersion of the purposes herein is known as
Ludo~ LS~ , sold by duPont Company.
In accordance with this invention, the aqueous
colloidal silica dispersion is added to a solution of a
small amount of alkyltriacetoxylsilane in alkyltrial-
koxysilane or aryltrialkoxysilane. For the purposes
herein, from about 0.07 parts by weight, to about 0.1
parts by weight; based on lO0 parts by weight of the
total composition of the alkyltriacetoxysilane is used.
The temperature of the reaction mixture is maintained
at about 20C to about 40C, preferably 20C to about
30 C, and most preferably below 25C. It has been
found that in about six to eight hours sufficient
trialkoxysilane has hydrolyzed so as to reduce the
initial two phase liquid mi~ture to one liquid phase in
which the now treated silica (i.e., treated by its
admixture with the alkyltrialkoxysilane or aryltrial-
koxysilane~ is dispersed. In general, the hydrolysis
reaction is allowed to continue for a total of about
12 to 48 hours, depending upon the desired viscosity
of the final product. The more time the hydrolysis
reaction is permitted to continue, the higher will be
the viscosity of the product.
After the hydrolysis has been completed to -the
desired extent, the solids content is reduced by the
addition of alcohol or an alcohol mixture, preferably
a mixture of isopropanol and isobutanol, to the reaction
mixture. With resi~ compositions of this invention which
contain thickener, the presence of isopropanol in the
alcohol mixture has been found to provide better optical
~1~6~9~
60SI-386
-- 5
clarity. So long as the thickener is completely soluble
in the alcohol or alcohol mixture, optical clarity in
the finished coating should be obtainable. Other alcohols
which are suitable dilution solvents include aliphatic
alcohols, such as methanol, ethanol, propanol, n-butyl
alcohol, isobutanol and t-butyl alcohol. Mixtures o~
such alcohols are suitable as well. The specific
alcohol or alcohol mixture is not critical unless the
thickener is not completely soluble therein. The
solvent system should contain from about 20 to 75 weight
percent alcohol to also ensure solubility of the partial
condensate (siloxanol). Optionally, additional water-
miscible polar solvents, such as acetone, butyl cellosolve
and the like, in minor amounts (i.e., no more than 20%
of the cosolvent system) can also be used. The solids
content of the coating compositions of this invention is
diluted to a range of from about 10~ to 15~ by weight of
the total composition, which effectively prolongs -the
~helf life of the composition.
~fter dilution of the silicone resin composition,
a small amount of a thickening agent, usually dispersed
in a solvent, preferably aqueous isopropanol, is added
to sufficiently increase the viscosity to yield a cured
coating thick enough to have good solvent and abrasion
resistance. Suprisingly, increasing the viscosity in
this way does not shorten the shelf life of the
composition as was found to occur when viscosi-ty was
increased by raising the solids content of the composition.
There axe many thickening agents suitable for the
purposes herein, including hydroxypropyl guar gum~
hydroxypropyl cellulose, and the like. Amounts contemplated
are about 50 to about 1,000 ppm by weight, preferably about
75 to about 750 ppm by weight, of the total composition.
The pH of the resultant coating compositions of the
invention is in the range of from about 7.1 to about
7.~, preferably higher than 7.2, such as from 7O3 to about
6~
60SI-386
_ 6
7.5. If necessary, dilute base, such as ammonium hydroxide,
or weak acid, such as acetic acid, can be added to the
composition to adjust the final pH to this desired range.
At these basic pH's and low solids contents, the com-
positions are translucent liquids which are stable atroom temperature for at least f:ive months. When stored
at temperatures below about 5C (40 F), the period of
stability is increased even further.
The alkyltriacetoxysilane is used to bufEer the
basicity of the inltial two liquid phase reaction mixtures
and thereby also temper the hydrolysis rate. While the
use of glacial acetic acid is preferred herein, alkylt-
riacetoxysilane may be used in its place, as well as
other acids such as organic like propionic, butyric,
citric, benzoic, formic, oxalic and the like. Alkyltria-
cetoxysilanes wherein the alkyl group contains from 1 to
6 carbon atoms can be used, alkyl groups having from 1
to 3 carbon atoms being preferred. Methyltriacetoxysilane
is the most preferred alkyltriacetoxysilane.
The silanetriols, RSi(OH)3, hereinbefore mentioned,
are forme~ in situ as a result of the admixture of the
corresponding trialkoxysilanes with the aqueous medium,
i.e., the aqueous dispersion of colloidal silica. Exempl-
ary trialkoxysilanes are those containing methoxy, ethoxy,
isopropoxy and n-butoxy substituents which, upon hydrolysis,
generate the silanetriol and further liberate the cor-
responding alcohol. In this way, at least, a portion
of the alcohol content present in the final coating
composition is provided. Of course, if a mixture of
trialkoxysilanes is employed, as proyided for herein-
above, a mixture of different silanetriols, as well as
different alcohols, is generated. Upon the generation
of the silanetriol or mixtures of silanetriols in the
basic aqueous medium, condensation of the hydroxyl sub-
stituents to form -Si-O-Si- bonding occurs. This
condensation takes place'over a period of -time and is
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60SI-386
-- 7
not an exhaustive condensation but rather the siloxane
retains an appreciable quantity of silicon-bonded
hydroxyl groups which render the polymer soluble in the
alcohol-water co-solvent. This soluble partial condensate
can be characterized as a siloxanol polymer having at
least one silicon-bonded hydroxyl group per every three
-SiO- units.
The non-volatile solids portion of the coating
composition herein is a mixture of colloidal silica and the
partial condensate (or siloxanol) of a silanol. The
major portion or all of the partial condensate or siloxanol
is obtained from the condensation of CH3Si(OH)3 and,
depending upon the input of ingredients of the hydrolysis
reaction, minor portions of partial condensate can be
cbtained, for example, from the condensation of CH3Si(OH)3
with C2H5Si(OH)3Or C3H7Si(OH)3; CH3Si(OH)3 with C6H5Si(OH)3,
or even mixtures of the foregoing. For optimum results
in the cured coating, it is preferred to use all methyl~
trimethoxysilane (thus generating all monomethylsilanet-
riol) in preparing the coating compositions herein. Inthe preferred coating compositions herein, the partial
condensate is present in an amount of from about 55 to
75 weight percent of the total solids in a co-solvent of
alcohol and water, the alcohol comprising from about 50
to 95 weight percent O~ ~he co-solvent. Compositions
prepared according to this invention will show no reduction
in abrasion resistance, and no increase in viscosity,
after 5 months of storage at room temperature~
At the low solids contents contemplated herein, it
has been found that the polysiloxane polyether copolymers
disclosed in U.S. Patent 4~277,2~7 to Frye dated July 7,
1981, may be employed in the thickened compositions of
this invention as flow control additives which assist in
the prevention of flowmarks, dirtmarks and the like on
the surface of the substrate which is coated. Generally,
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60SI-386
-- 8
these polysiloxane polyether copolymers may be employed
in amounts of from about 2.5 to about 15 weight percent
of the total solids of the composition.
The coating compositions of this invention will cure
on a substrate at temperatures of, for example, 120 C
without the aid of an added curing catalyst. However, in
order to employ more desirable milder curing conditions,
buffered latent condensation catalysts can be added.
Included in this class of catalysts are alkali metal salts
1~ Of carboxylic acids, such as sodium acetate, potassium
format and the like. Amine carboxylates, such as dimethyl-
amine acetate,ethanolamine acetate~ dimethylaniline formate
and the like, quaternary ammonium carboxylates such as
tetramethylammonium acetate, benzyltrimethylammonium
acetate, metal carboxylates, like tin octoate and amines
such as triethylamine, triethanolamine, pyridine and the
like are also contemplated curing catalysts herein. Alkali
hydroxides, like sodium hydroxide and ammonium hydroxide
can also be used as curing catalysts herein. Moreover,
typical commercially available colloidal silica, especially
those having a basic pH, contain free alkali metal base,
and alkali metal carboxylate catalysts will be generated
in situ during the hydrolysis reaction herein.
The amount of curing catalyst can be varied within
a wide range, depending upon the desired curing conditions.
~owever, in general, catalyst in the amounts of from about
0.05 to about 0.5 weight percent, preferably about 0.1
weight percent, of the composition can be used. Com-
positions containing catalysts in these amounts can be
cured on a solid substrate in a relatively short time at
temperatures in the range of from about 75 - 150 C to
provide a transparent abrasion-resistant surface coating.
The coating compositions of the present invention
can be applied to a variety of solid substrates by
conventional methods, such as flowing, spraying or dipping,
to form a continuous surface film. Substrates which are
60SI-386
g
especially contemplated herein are transparent, as well
an non-transparent, plastics and me-tals. More particularly,
these plastics are synthetic organic polymeric substrates
such as acrylic polymers like poly(methylmethacrylate),
polyesters, such as poly(ethylene terephthalate), poly
(butylene terephthalate) r etc., polyamides, polyimides,
acrylonitrilestyrene copolymers, styrene-acrylonitrile-
butadiene copolymers, polyvinyl chloride, butyrates,
polyethylene and the like. The coating compositions of
this invention are especially useful as coatings for
polycarbonates, such as those polycarbonates known as
Lexan ~ sold by General Electric Company and as coatings
for injection molded or extruded acrylates, such as poly-
methylmethacrylates. ~etal substrates on which the present
protective coatings are also effective include bright
and dull metals like aluminum and bright metallized
surfaces like sputtered chromium alloy. Other solid
substrates contemplated herein include wood, painted
surfaces, leather, glass, ceramics and textilies.
By choice of the proper formulation, application
conditions and pretreatment, including the use of primers,
of the substrate, the coatings can be adhered to sub-
stan-tially all solid substrates. A hard coating having
all of the aforementioned characteristics and advantages
is obtained by the removal of the solvent and volatile
materials. I'he coating composition will air-dry to a
tack-free condition, but heating in the range of 75C
to 200C is necessary to obtain condensation of residual
silanols in the partial condensate. ~his final cure
results in the formation of silsequioxane (RSiO3/2). In
the finished cured coating, the ratio of RSiO3/2 units to
SiO2 will range from about 0.~3 to about 9.0, preferably
1 to 3. A cured coating having a ratio RSiO3~2 to SiO2,
where R is methyl, of 2 is most preferred. Coating thick-
ness may vary but for the improved abrasion resistancedesixed herein, coating thickness of 0.5-20 microns,
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60SI-386
-- 10 --
preferably 2-10 microns, are utilized.
In order that these skilled in the art may better
understand how to practice the present invention, the
following examples are given by way of illustration and
not by way of limitation.
Examples 1-5
80.1 lbs. of Ludox LS (aqueous colloidal silica
dispersion, average particle size of le millimicrons, pH
of 8.2 sold by duPont) is added, over a period of one-
half hour, to a solution of 88g. of glacial acetic acid in97.6 lbs. of methyltrimethoxysilane. The temperature of
the reaction mixture is maintained between 20 C and 30 C.
The hydrolysis i5 allowed to proceed over a period of
sixteen hours. 2.9 lbs. of a silicone polyether flow
control agent (General Electric Company SF-1066) was
added to the reaction mixture. Sufficient isobutanol
was then added to reduce the solids content of the resin
solution to 20% by weight.
Five test samples were then prepared as follows:
SAMPLE
_ 2 3 ~ 5_
resin solution (lbs.) 650 495 495 485 300
- isobutanol (lbs.) -- 165 160 148 --
isopropanol (lbs.~ 255
25 thickening agent (lbs.) -- -- 5 15 45
(1~ hydroxypropyl
cellulose in aqueous
ispropanol)
solids content 20~ 15% 15~ 15% 10~
30 Important coating properties of the samples were then
tested after intervals of approximately one and ~ive
months storage, yielding the following data:
60SI-386
VISCOSITY (centistokes)
_ ___
Sample First Test 1 Month 5 Months
1 5.2 6.3 10.4
2 4.6 5.0 5.6
3 4.8 5.2 5.7
4 5.7 6.2 6.8
7.4 7.5 7.6
For testing, sample compositions were coated and
cured on primed Lexan ~ plaques. Coating appearance
was observed; and abrasion resistance was measured on a
Taber Abraser, which involves the measurement of the
increase in the amount of haze ( ~ %H 500) after abrading
500 cycles using CS-lOF abrasive wheels under a 500 gram
load. The following results were observed:
Coating Appearance
_ _ .
Sample First Test 1 Month 5 Months
1 good good cracked
2 good good cracked
3 good good cracked
20 4 good good cracked
good good no cracks
~ % H 500
SampleFirst Test 1 Month 5 ~onths
_
1 0~5 3.7 3.1
~5 2 1.9 9.7 3.2
3 1~1 16.7 2.5
4 2.8 15.8 3.9
5.3 18.6 1.8
~8~
60SI-386
- 12 -
From the above data it can be seen the the coating
properties of samples prepared according to the present
invention (Samples 3, 4, 5) chanaged less after pro-
longed storage than the control samples (Samples 1, 2).
Although Samples 5 exhibited a slight initial haze, it
gave the overall best performance. The initial haze may be
eliminated by increasing the isopropanol content of the
alcohol mixture.
Obviously, other modifications and variations of
the present invention are possible in the light of the
above teachings. For example, additives and other
modifying agents, such as pigments,dyes and the like, may
be added to the compositions of this invention. It is
to be understood, however, that the changes may be made in
the particular embodiments described above which are within
the full intended scope of the invention as defined in the
appended claims.
