Note: Descriptions are shown in the official language in which they were submitted.
~ ~ 67 ~ ~
The avaiiability of ligh~weight plastics has
led to the replacement of glass by such plastics for
numerous uses. Over the past several years, new plastics
have been developed which find use in window glazing, lenses,
clear face shields, aircraft canopies and the like.
Although such plastics have many outstanding properties,
they are deficient in their resistance to scratching. An
outstanding e~ample is the deterioration of plastic
sunglass lenses by common everyday use because such glasses
are frequently removed from the face and laid on hard
substrates, lens down.
Thus, in order to better utili~e the
advantageous properties of today's plastics, there
is a need to render such plastics scratch and abrasion
resistant.
In order to obtain abrasion resistant sur~aces,
such as, for example, polycarbonate surfaces, investigators
have tended to coat very thin coats of organic or silicone
; resins on the surface of the plastics. The intent was to
obtain abrasion resistance without losing the optical
properties of the plastic substrate,
Such an organic coating is disclosed in U.S~
Patent No. 4,018,941. Such an organic coating is prepared
from polyols and urethanes and is cured via melamine
crosslin~ers. Although some degree of abrasion resistance
is afforded by the melamine coating, it has a tendency to
be affected by outdoor exposure and eventually the
coating deteriorates.
In view of the above9 silicone coatings which
exhibit good weather resistance were developed. Such resins
are shown in U.S~ Patents 3,389,114, 3,389,121, 3,634,3~1,
3,642,698 and 3,935,346, all assigned to Owens-Illinois.
The latter patent teaches a method of making an abrasion
resistan~ coating from an alkylated melamine-formaldenyde
resin and a hydroly7ate of CH3Si~OR)3. These resins all
have good wea-ther resistance but only moderate abrasion
resistance.
A siloxane resin having a low degree of organic
substitution has been developed. The coating had hardness
and therefore good abrasion resistance. Such resins are
disclosed in U.S. Patent 3,986,997 issued October 19, 1976
to Harold A. Clark. The ClarX resins are very versatile
materials and find utility as abrasion resistant coatings
on a number oE substrates which requi.re good abrasion resistance.
The only disadvantage of the Clar~ resins is the
fact that they tend to be inflexiblel~ that is, under certain
circumstances the coatings tend to cra~e.
It is well Xnown in the silicone art that
flexibility can be built into a siloxane resin coating
by incorporating a dimethyl containing hydrolyzable
silane in the formulation when the resin is first prepared
(see Canadian Patent No. 1,015,888). Unfortunately, the
presence of dimethyl siloxane in any siloxane resin tends
to also soften the coating so that the abrasion resistance
alls off. Thus, for purposes of obtaining an abrasion
resistant coatin$ with flexibility, one would not suggest
using the above approach in preparing the resins.
What is needed is a weather resistant, abrasion
resistant, flexible, c ear coating.
There has now been discovered a means of
improving the flexibility of siloxane resins having a low
degree of organic substitution without undue sacrifice of
the abrasion resistance of the coating.
What is disclosed herein is an improvemen~ in
the flexibility of the Clark resins shown in the U.S.
Patent No. 3,986,997 set out above.
Such improved flexibility can be obtain~d
by incorporating in the original formulation for the
Clark resin a certain amount of monophenylsilsesquioxane
structure.
It has been found that the incorporation of
C6H5Si~OH)3 in the Clark resin gives increased flexibility
to the resin without significant loss of abrasion
resistance of the cured coating.
In accordance with the present teachings, a pigment-free
aqueous coating composition is provided which comprises
1) a dispersion of colloidal silica in
2) a solution which has as cosolvent ether esters of
ethylene or propylene glycol and water and has as solute
- 3) a partial condensate of a silanol of the formula RSi(OH)3
in which R is an alkyl radical of 1 to 3 carbon atoms and phenyl
wherein at least 70 weight percent of the silanol is CH3Si(OH)3,
wherein the composition contains 10 to 50 weight percent solids
consisting essentially of 10 to 70 weight percent colloidal
silica and 30 to 50 percent of the partial condensater and wherein
the composition contains sufficient acid to provide a pH in the
range of 2.8 to 6Ø
1 ~ 6~ ~ ~
Not only can one obtain flexibility in siloxane
resins having a low degree o~ substitution, one can obtain
control over the degree of flexibility given to such resins
by merely controlling the amount of monophenyl silanol put into
the formulation, i.e., the degree of flexibility built into
the cured coating of the resin is linearly dependent on the
amount of monophenyl silanol actually incorporated in the
~ormulation. The control is such that flexibility + 5% can
be estimated from the amount of monophenyl silanol incorporated.
At least 1 weight percent of C6H5Si(OH)3, based on
the weight of total RSi(OH)3 present in the composition,
is required to get the flexibility effect. Up to 30
weight percent of C6H5SitOH)3 can be utilized. Generally,
greater than 30 weight percent of C6H5Si~OH)3, even though
giving increased fle~ibility, does not retain the required
abrasion resistance.
The resins are prepared by the methods found
in the above Clark patent and the only di~ference is that
C6H5Si~OCH3)3, in the proper proportions, is mixed with the
CH3Si~OH)3 befoTe the hydrolysis and contact with ~he
colloidal silica. The C6H~Si~OCH3)3 can be pre-hydrolyzed
before mixing with the CH3Si~OH)3 but no significant
advantage is obtained thereby.
The silica component of the composition is
present as colloidal silica. Aqueous colloidal silica
dispersions generally have a particle size in the range
of 5 to 150 millimicrons in diameter. These silica
dispersions are prepared by methods well-known in the
` art and are commercially a~aila~le, It is preferred to
use colloidal silica of 10-3Q millimicron par~icle si~e
in order to obtain dispersions having a greater stability
and to provide coatings having superior optical properties.
Colloidal silicas of this type are relatively free of Na2O
and other alkali metal oxides, generally containing less
than 2 weight percent, preferably less than 1 weight
percent Na2O. They are available as both acidic and basic
hydrosols. Colloidal silica is distinguished from other
water dispersable forms of SiO2, such as nonparticulate
polysilicic acid or alkali metal silicate solutions, which
are not operative in the practice of the present invention.
The silica is dispersed in a solu~ion of the
siloxanol carried in a lower aliphatic alcohol-water, or
ether ester-water, cosolvent. Suitable lower aliphatic
alcohols include methanol, ethanol, isopropanol, and t-butyl
alcohol. Mixtures of such alcohols can be used. Isopropanol
is the preferred alcohol and when mixtures of alcohol are
utilized it is preerred ta utilize ~it least 50 weight
percent of isopropanol in the mixture to obtain optimum
adhesion of the coating. Suitable etheT esters are
ether esters of ethylene or propylene glycol such as
CH3cOOtcH2cH2o)2c2H5~ CH3COO~CH2CH2)2 4 9'
; CH3COOCH~CH2OC2H5~ CHjCOOCH~CH2OCH3 and CH3COOC~2CH2OC4Hg
and analogs o~ such materials prepared from propylene glycol.
The solvent system should contain from about 20 to 75
weight percent of alcohol or ether ester to ensure
solubility o the siloxanol. Optionally one can utilize
an additional water-miscible polar solvent, such as acetone,
butyl cellosolve and the like in a minor amount, for
example, no more than 20 weight percent of the cosolvent
system.
~ 6~7~
To obtain optimum properties in the coating
and to prevent immediate gellation of the coating composition,
sufficient acid to provide a pH of from 2.8 to 6.0 must
.
be present. Suitable acids include both organic and inorganic
acids such as hydrochloric, acetic, chloroacetic~ citric,
benzoic, dimethylmalonic, formic, glutaric, glycolic, maleic,
malonic, toluene-sulfonic, oxalic and the like. The
specific acid utilized has a direct effect on the rate of
silanol condensation which in turn determines shelf life
of ~he composition. The stronger acids, such as hydrochloric
and toluenesulfonic acid, give appreciably shortened shelf
or bath life and require less ageing to obtain the described
soluble partial condensate. It is preferred to add sufficient
water-miscible carboxylic acid selected from the group
consisting of acetic, formic, propionic and maleic acids
to provide pH in the range of 4 to 5.5 in ~he coating
composition. In addition to providing good bath life, the
alkali metal salts of these acîds are soluble, thus allowing
the use of these acids with silicas containing a substantial
Z0 ~greater than 0.2% Na20) amount of alkali metal or metal
oxide.
The composition is easily prepared by adding the
trialkoxysilanes, such as R'Si(OCH3)3, to colloidal silica
hydrosols and adjusting the pH to the desired level by
addition of the organic acid. The acid can be added to
either the silanes or the hydrosol prior to mixing the
two components pro~ided that the mixing is done rapidly
The amount of acid necessary to obtain the desired pH
will depend on the alkali metal content of the silica but
is usually less than one weight percent of the composition.
Alcohol is generated by hydrolysis of the alkoxy substituents
of the silane, for example, hydrolysis of one mole of
-Si(OC2H5)3 generates 3 moles of ethanol. Dependins upon
the percent solids desired in- the final composition~
.. , , .~
additional alcohol e~her ester, water or a water-miscible
solvent can be added. The composition should be ~-ell mi~ed
and allowed to age for a short period of time to ensure
formation of the partial condensate. The coating composition
thus obtained is a clear or slightly hazy low viscosity
fluid which is stable for several days.
Buffered latent condensation catalysts can be
added to the composition so that milder curing conditions
can be utilized to obtain the optimum abrasion resis~ance
in the final coating. Alkali metal salts of carboxylic
acids, such as potassium formate, are one class of such
latent catalysts. The amine carboxylates and quaternary
ammonium carboxylates are another such class of la~ent
catalysts. O~ course the catalysts imust be soluble or at
least miscible in the cosolvent system. The catalysts
are latent to the extent that at room temperature they do
not appreciably shorten the bath life of the composition,
but upon heating the catalyst dissociates and generates
a catalytic species ac~ive to promote condensation.
Buffered catalysts are used to avoid effects on the pH
of the composi~ion. Certain of the commercially a~ailable
colloidal silica dispersions contain free alkali metal base
which reacts with the organic acid during the adjustment of
pH to generate the carboxylate catalysts in situ. This is
particularly true when starting with a hydrosol having a
pH of 8 or 9. The compositions can be cataly~ed by addition
of carboxylates such as dimethylamine acetate~ ethanolamine
--7-
acetate, dimethylaniline formate, te~raethylammonium benzoate,
sodium acetate, sodium propionate, sodium formate or
benzyltrimethylammonium acetate. The amount of catalyst
can be varied depending upon the desired curing condition,
but at about 1.5 weight percent catalyst in the composition,
the bath life is shortened and optical properties of the
coating may be impaired. It is preferred to utilize from
about 0.05 to 1 weight percent of the catalyst.
To provide the grea~est stability in the dispersion
form while obtaining optimum properties in the cured coatin~,
it is preferred to utilize a coating composition having a
pH in the range o 4-5 which contains 10-35 weight percent
solids; the silica portion having a pàrticle size in the
range of 5-30 millimicrons, the partial condensate
CH3Si(OH)3 and C6H5Si(OH)3 being present in an amount in the
range of 35 to 55 weight percent of the total solids in
a cosolvent oE methanol, isopropanol and water or
CH3COOCH2CH20CH3 and water or ether esters, the alcohols
representing from 30 to 60 weight percent of the cosolvent
and a catalyst selected from the group consisting of
sodium acetate and benzyltrimethylammonium acetate being
present in an amount in the range of 0.05 to 0.5 weight
percent of the composition. Such a composition is relatively
s~able and, when coated onto a substrate, can be cured in a
relatively short time at temperatures in the range of
75-1~5C. to provide a transparent abrasion resistant
surface coating.
The coating compositions of the invention can be
applied to solid substrates by conventional methods, such as
flowing, spraying or dippin~T to ~orm a continuous surface
~1~ 6'7~
film. Although substrates of soft plastic sheet material
show the greatest improvement upon application of the coating,
the composition can be applied to other substrates, such
.
as wood, metal, printed surfaces, leatker, glass, ceramics
and textiles. The compositions are especially useful as
coatings for dimensionally stable synthetic organic polymeric
substrates in sheet or film form, such as acrylic polymers,
for example, poly~methylmethacrylate), polyesters, for example
poly(ethyleneterephthalate) and polycarbonates, such as
poly(diphenylolpropane)carbonate, polyamides, polyimides,
copolymers o acryloni~rile-styrene, styrene-acrylonitrile-
butadiene copolymers, polyvinyl chloride, butyrates,
polyethylene and the li~e. Transparent polymeric materials
coated with these compositions are us,e~ul as flat or curved
enclosures, such as windows, skylight:s and l~indshields,
especially for transportation equipme:nt. Plastic lenses,
such as acrylic or polycarbonate opht:halmic lenses, can be
coated ~-ith the compositions of the invention In certain
applications requiring high optical resolution, it may
be desirable to filter the coating composition prior to
applying it to the substrate. In other applications,
such as cor~rosion-resistant coatings on metals, the slight
haziness ~less than 5~) obtained by the use of certain
formulations 3 such as those containing citric acid and
sodium citrate, is not detrimental and filtration is
not necessary.
By choice of proper formulation, including solvent,
application conditions and pretreatment of the substrate,
the coatings can be adhered to substantially all solid
surfaces. A hard solvent-resistan~ surface coating is
~ ~ 6~
obtained by removal of the solvent and volatile materials.
The composition will air dry to a tack-free condition, but
heating in the range of 50 to 150~ is necessary to obtain
condensation of residual silanols in ~he partial condensate.
This final cure results in the formation of silsesquioxanes
of the formula 0SiO3/2 and RSiO3/2 and greatly enhances the
abrasion resistance of the coating. The coating thickness
can be varied by means of the particular application
technique, but coatings of about 0.5 to 20 micron
preferably 2-10 micron thickness are generally utilized.
BspecialIy thin coatings can be obtained by spin coating.
Now so that those skilled in the art can better
understand and appreciate the invention, the following
examples are presented,
In the e~amples and elsewhere in this disclosure,
~he use of the symbols ~ and Me mean "phenyl" and "methyl"
respectively.
Example 1 - Preparation of phenyl containing
resins of this invention.
Six resins were prepared for evaluation.
Sample 1 was prepared according to the
procedure of Example 1 of U.S. 3,986,997 and was used
for comparison purposes.
Samples 2-6 were prepared according to the
following procedure. Samples 2, 3 and 4 fall within the
scope of this invention and Samples 5 and 6 fall outside
the scope of the claims.
5% ~Si(OH)3/45 CH3Si~OH)3
To a three-nec.~ed, round-bottomed flask was
added 154.5 grams of a colloidal sllica having an initial
-10-
pH of 3.1 containing 34% SiO2 of approximately 22 millimicron
particle size and having an Na2O content of less than 0.01
weight percent. It was cooled to 8C and 5.3 grams o~
., . . ~ . . ........... , . . - . . . . - - - - ..
glacial acetic acid was added. 96.0 grams of CH3Si(OMe)3
and 8.1 grams of 0Si(OMe)3 were premixed and slowly added
to the colloidal silica with vigorous st rring and
external cooling. The methoxy silanes were allowed to
hydrolyze with the formation of methanol. After the
hydrolysis was complete, 2.7 grams of a 10% solution of
sodium acetate and 132.4 grams of isopropanol were added.
After seven days standing, 66.2 grams additional alcohol was
added and the solution filtered.
Samples 3 through 6 were prepared in the same
manner as Sample 2 but with appropriate adjustment of the
relative amounts o~ ~CH3)SiO~CH3)3 and C6H5Si~OCH3)3 employed
to give:
Sample 3:
10~ C6H5Si(OH)3/40% CH3sitoH)3;
Sample 4:
15~ C6H5Si~OH)3/35% CH~Si~OH)3;
Sample 5:
20% C6H5Si~OH)3/30% CH3Si(OH)3; and
Sample 6:
25% C6HSSi(OH)3/25% CH3si~O )3
Exam~le 2
Plexiglas~ panels 4" x 4" x 1/8" (10.16 cm x 10.16 cm
x 0.32 cm) after being cleaned with isopropanol and air dried,
were flow coated with a 22.5% solids resin, allowed to air
dry and then cured for 18 hours at 75C.
Similar l" x 4" x 1/8" ~2.54 cm x 10.16 cm x 0.32 cm)
strips were prepared to test the flexibility. The test for
flexibility is relatiue and was ca~ried out in ~he
following manner.
The 1" (2.5~ cm) wide strips were placed in a
vise-like device so that the longest i.e. ~" ~10,16 cm) axis
was horizontal. A strong light was placed on the opposite
side of the strip from ~here the observation was being made,
so that the craze marks when they formed, were more easily
seen. The vise-like device is manipulated by hand to draw
the vise jaws together slowly so that the plastic strip
irst humps in the center and then begins to form a
semi circle with the coating on the outside of the
semi circle. While observing the coating, the jaws are
moved slowly together (decreasing the radius of curvature)
until craze marks are propogated in the coating. When
the craze marks show across the entire width of the
plastic strip, the end point has been reached.
The degree of flexibility is then calculated in
the following manner. The initial length of ~he st~ip
before compression is designated AB. The distance between
the vise jaws at the end of compression is designated AB.
Prior measurements of angle of ~ plotted versus AB
give a graph i.e. ~ = ~B from which ~ can be easily determined.
AB
The radius of curvature ~r) can then be calculated.
r = AB 1~0
__
'IT ~ o
-12-
~ 6~ ~ ~
One has to assume that ~ is an arc of a circle.
The actual shape of the semi-circle in this test is a
parabola which suggests that this is a more severe test
than a test which had a true semi-circular shape.
The abrasion resistance was determined according
to ASTM ~ethod Dl0~4-~6. The instrument ~as the Tabor
A braser. A 500 gram ~est load was used with CS-lOF
abrasive wheels and the test panels subjected to 500
revolutions on the abraser turntable~ The percent change
in haze which is the criterion for determining the abrasion
resistance of the coatlng is determined by measuring the
di~-ference in haze of the unabrased and abrased coatings.
Haze is defined as that percentage of transmitted light
which in passing through the specimen deviates from the
incident beam by forward scattering. In this method, only
light flux that deviates more than 2,5 degrees on the
arerage is considered to be haze. The ~ Haze on the
coatings was determined by ASTl~ Method D1003-61. A
Hunter Haze Meter: Gardner Laboratory, Inc~ was used.
The ~ Haze was calculated by measuring the amount of
diffused light dividing by the amount of transmit~ed
light and multiplying by one hundred.
The adhesion test was the 1/8" crossha*ch
tape pull test in which the cured coating is crosshatched
in 1/8" squares using a sharp object, over a square inch
area. Adhesive tape ~#600 Adhesive - 3M Company) is firmly
pressed onto the crosshatched area and sharply pulled away.
If all of the coating remains, the adhesion is 100~.
-13-
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