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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to a protective coating composition. In one aspect
the invention relates to a transparent abrasion resistant coating. In
another aspect, the invention relates to a stable dispersion of colloidal
silica and a silicone resin.
There is a need for transparent glazing materials which exhibit a greater
resistance to shattering than glass. Synthetic organic polymers can be
formed into transparent enclosures and these materials, such as
polycarbonates and acrylics, are finding use in glazing for autos, buses
and aircraft and as windows in public buildings. While these polymers are
easily fabricated into the desired shape, and are less dense and have more
resistance to breakage than glass, their abrasion resistance is relatively
low. This lack of surface hardness and abrasion resistance has severely
restricted the use of these transparent polymeric materials. Other uses of
the polymeric materials, such as glazing decorative architectural panels
and mirrors, are also limited because of this lack of abrasion resistance.
Scratch resistant coatings, such as silica-containing solutions and
polysilicic acid fluorinated copolymer compositions, are available in the
prior art. These materials have found only limited commercial use because
they are difficult to apply, poor in humidity resistance, or expensive.
The coating composition of the present invention is based on relatively
inexpensive commercially available materials which are easily applied to
substrates to provide an abrasion-resistant surface having good weathering
characteristics.
Thus, it is an object of the present invention to provide a composition
suitable as a protective coating for solid substrates. It is another
object of the invention to provide an improved abrasion resistant coating
for solid substrates, especially transparent substrates. It is a further
object of the invention to provide dispersions from which the coatings of
the invention can be applied. These and other objects of the invention
will be apparent to one skilled in the art upon consideration of the
following description and appended claims.
DESCRIPTION OF THE INVENTION
In accordance with the present invention, there is provided an unpigmented
coating composition comprising a dispersion of colloidal silica in lower
aliphatic alcohol-water solution of the partial condensate of a silanol of
the formula RSi (OH).sub.3 in which R is selected from the group
consisting of alkyl radicals of 1 to 3 inclusive carbon atoms the vinyl
radical, the 3,3,3-trifluoropropyl radical, the gamma-glycidoxypropyl
radical and the gamma-methacryloxypropyl radical, at least 70 weight
percent of the silanol being CH.sub.3 Si(OH).sub.3, said composition
containing 10 to 50 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, said composition containing
sufficient acid to provide a pH in the range of 3.0 to 6.0.
As described above the nonvolatile solids portion of the coating
composition is a mixture of colloidal silica and the partial condensate of
a silanol. The major portion of the partial condensate or siloxanol is
obtained from the condensation of CH.sub.3 Si(OH).sub.3 ; a minor portion,
if desired, being obtained from cocondensation with C.sub.2 H.sub.5
Si(OH).sub.3, C.sub.3 H.sub.7 Si(OH).sub.3, CH.sub.2 = CHSi(OH).sub.3,
##STR1##
or mixtures thereof. From both the standpoint of economy and optimum
properties in the cured coating it is preferred to utilize all
monomethyltrisilanol in formulating the composition.
As will be described in detail in the examples, the trisilanols are
generated in situ by adding the corresponding trialkoxysilanes to acidic
aqueous dispersions of colloidal silica. Suitable trialkoxysilanes are
those containing methoxy, ethoxy, isopropoxy and t-butoxy substituents,
which upon hydrolysis liberate the corresponding alcohol; thus, generating
at least a portion of the alcohol present in the coating composition. Upon
generation of the silanol in the acidic aqueous medium, there is
condensation of the hydroxyl substituents to form --Si--O--Si bonding. The
condensation is not complete, but rather the siloxane retains an
appreciable quantity of silicon-bonded hydroxyl groups, thus rendering the
polymer soluble in the water-alcohol 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. During curing
of the coating on a substrate, these residual hydroxyls condense to give a
silsesquioxane, RSiO.sub.3 /.sub.2.
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 available
under such registered trademarks as "Ludox" and "Nalcoag". It is preferred
to use colloidal silica of 10-30 millimicron particle size 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 Na.sub.2 O and other alkali metal oxides, generally
containing less than 2 weight percent, preferably less than 1 weight
percent Na.sub.2 O. They are available as both acidic and basic hydrosols.
Colloidal silica is distinguished from other water dispersable forms of
SiO.sub.2, 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 solution of the siloxanol carried in a lower
aliphatic alcohol-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 preferred to utilize at least 50
weight percent of isopropanol in the mixture to obtain optimum adhesion of
the coating. The solvent system should contain from about 20 to 75 weight
percent alcohol to ensure solubility of 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.
To obtain optimum properties in the coating and to prevent immediate
gellation of the coating composition, sufficient acid to provide a pH of
from 3.0 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 the 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
meleic acids to provide pH in the range of 4 to 5.5 in the coating
composition. In addition to providing good bath life, the alkali metal
salts of these acids are soluble, thus allowing the use of these acids
with silicas containing a substantial (greater than 0.2% Na.sub.2 O)
amount of alkali metal or metal oxide.
The coating compositions are easily prepared by adding trialkoxysilanes,
such as RSi(OCH.sub.3).sub.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 silane or the hydrosol prior to mixing the two
components provided 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(OC.sub.2
H.sub.5).sub.3 generates 3 moles of ethanol. Depending upon the percent
solids desired in the final composition, additional alcohol, water or a
water-miscible solvent can be added. The composition should be well mixed
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. The
condensation of SiOH continues at a very slow rate and the composition
will eventually form gel structures. The bath life of the composition can
be extended by maintaining the dispersion at below room temperature, for
example at 40.degree. F.
Buffered latent condensation catalysts can be added to the composition so
that milder curing conditions can be utilized to obtain the optimum
abrasion resistance 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 latent catalysts. Of course the catalysts must 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 catalysts dissociates and
generates a catalytic species active to promote condensation, for example
an amine. Buffered catalysts are used to avoid effects on the pH of the
composition. Certain of the commercially available 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 catalyzed by addition of carboxylates
such as dimethylamine acetate, ethanolamine acetate, dimethylaniline
formate, tetraethylammonium 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 greatest stability in the dispersion form while obtaining
optimum properties in the cured coating, it is preferred to utilize a
coating composition having a pH in the range of 4-5 which contains 10-25
weight percent solids; the silica portion having a particle size in the
range of 5-30 millimicrons; the partial condensate of CH.sub.3
Si(OH).sub.3 being present in an amount in the range of 35 to 55 weight
percent of the total solids in a cosolvent of methanol, isopropanol and
water, 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 stable, having a bath life of approximately one
month, and, when coated onto a substrate, can be cured in a relatively
short time at temperatures in the range of 75.degree.-125.degree. C. 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 dipping
to form a continuous surface 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, leather, glass, ceramics and textiles. As noted
above, 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 and poly(diethylene glycol bis
allyl)carbonate, polyamides, polyimides, copolymers of
acrylonitrile-styrene, styrene-acrylonitrile-butadiene copolymers,
polyvinyl chloride, butyrates, polyethylene and the like. Transparent
polymeric materials coated with these compositions are useful as flat or
curved enclosures, such as windows, skylights and windshields, especially
for transportation equipment. Plastic lenses, such as acrylic or
polycarbonate ophthalmic lenses, can be coated with 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 corrosion-resistant
coatings on metals, the slight haziness (less than 5%) obtained by the use
of certain formulations, 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 (including the use of primers) of the substrate, the
coatings can be adhered to substantially all solid surfaces. A hard
solvent-resistant surface coating is 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.degree. C is necessary to
obtain condensation of residual silanols in the partial condensate. This
final cure results in the formation of silsesquioxane of the formula
RSiO.sub.3 /.sub.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. Especially thin coatings can
be obtained by spin coating.
The following examples are illustrative and notto be construed as limiting
of the invention delineated in the claims.
EXAMPLE 1
Glacial acetic acid (0.2 grams) was added to 200 grams of a commercially
available aqueous dispersion of colloidal silica having an initial pH of
3.1 containing 34% SiO.sub.2 of approximately 15 millimicron particle size
and having a Na.sub.2 O content of less than 0.01 weight percent.
Methyltrimethoxysilane (138 grams) was added to the stirred acidified
dispersion generating methanol and methyltrisilanol. After standing for
about one hour, the pH of the composition stabilized at 4.5. Portions of
the composition were mixed with ammonium hydroxide or glacial acidic acid
to adjust the pH of individual samples to provide compositions ranging in
pH from 3.7 to 5.6. These compositions were aged for 4 days to ensure
formation of the partial condensate of CH.sub.3 Si(OH).sub.3 in the silica
methanol-water dispersion. The composition contained 40% solids, half of
which was SiO.sub.2 and the other half silicone calculated on the basis
CH.sub.3 SiO.sub.3 /.sub.2 weight available in the cured composition.
Six grams of each composition were flow-coated onto biaxially oriented,
stretched panels of poly (methylmethacrylate). The acrylic panels panels
were 4" .times. 6" .times. 0.187" and had been previously cleaned with
isopropanol. The coated panels were allowed to air dry for 1 1/2 hours at
room temperature and then cured at 185.degree. F for 4 hours in a forced
air oven.
Portions of the aged compositions were diluted to 25 weight % solids by
addition of isopropanol, coated onto acrylic panels ad cured in the same
manner. Other portions of the compositions were aged for a total of eight
days, then coated onto acrylic panels and subjected to the same curing
cycles.
The adhesion and abrasion resistance of all the coatings was determined.
Adhesion, as measured by pulling adhesive tape from a 1/8 inch
crosshatched grid of the coating, was excellent to good except for a
partial failure noted in some of the 8-day coatings. Abrasion resistance
was determined by subjecting the coatings to circular rubbing with No.
0000 steel wool for five revolutions at 25 p.s.i. loading. The increase of
optical haze of the abraded area was then measured by means of a Gardner
large area hazemeter. The abrasion resistance data for the coatings at
various pH levels and days of aging are tabulated below:
__________________________________________________________________________
Abrasion Resistance --% Delta Haze
pH of
Composition
3.7 4.5 5.0 5.6
% Solids in
Composition
40 25 40 25 40 25 40 25
% .DELTA. Haze
4 day Composition
6 4 0.4 0.8 0.4 0.5 6 6
% .DELTA. Haze
8 day Composition
6 4 6 2 2 2 5* 6
__________________________________________________________________________
*small gel particles in sample
Uncoated panels of this same stretched acrylic sheet show an increase in %
haze of 32-35 percent when tested by this method. These data demonstrate
the effect of pH, percent solids and aging. The compositions having a pH
of 4.5 and 5.0 were stable for over 21 days and gave the best scratch
resistance.
A portion of the composition having a pH of 4.5 and diluted to 25% solids
with isopropanol was catalyzed by the addition of 0.28 weight percent
benzyltrimethylammonium acetate after five days ageing. The catalyzed
composition was flow-coated onto transparent, glass-reinforced polyester
panels which are commercially available as a glazing material for solor
energy collectors. The coating was cured at 70.degree. C. for six hours.
Adhesion of the coating was excellent and there was a slight improvement
in light transmission. Abrasion resistance of the coating was good. Based
on testing of other substrates, it is anticipated that long term
weathering tests will show substantial improvement in the weatherability
of the panels.
EXAMPLE 2
A coating composition containing 37 weight percent solids, 50% of which
were SiO.sub.2, was formulated by adding a basic colloidal dispersion of
13-14 millimicron silica (pH of 9.8, Na.sub.2 O content of 0.32%) to
methyltrimethoxysilane which had been acidified by the addition of 2.5
weight percent glacial acetic acid. After four hours mixing, the
composition was divided into portions which were then adjusted to a pH of
3.9, 4.5 or 5.0 by addition of more glacial acetic acid. The compositions
were then diluted to 25% solids by addition of isopropanol, aged for four
days, coated onto acrylic panels, cured and tested in the manner described
in Example 1. All panels showed no change in haze upon being abraded with
the steel wool. This increase in hardness as compared to the coating of
Example 1, especially that obtained from the composition at a pH of 3.7,
is attributed to the catalytic action of sodium acetate which was formed
upon addition of the colloidal silica to the acidified silane. The
undiluted compositions (37% solids) were less stable and gelled within the
four day aging period because of the presence of the catalyst.
A coating composition the same as described above having a pH of 4.5 and
25% solids was aged for 3 days and used to dip coat six spodumene ceramic
heat exchanger core samples. The remaining coating composition was then
diluted to 20% with a 50/50 isopropanol-water cosolvent and used to dip
coat six cordierite heat exchanger core samples having relatively small
air passages. All coated specimens were cured at 100.degree. C for 6
hours. Three of each type of core were cured at 350.degree. C for an
additional 20 hours. All of the coated cores exhibited improved strength
and were more resistant to hot corrosive gases.
EXAMPLE 3
A number of coating compositions utilizing various trimethoxysilanes were
prepared. The compositions were prepared by adding the appropriate amount
of silane to the aqueous colloidal silica dispersion described in Example
1 which had been acidified by the addition of 1 weight percent glacial
acetic acid to a pH of about 4.5. The solids consisted of 50 weight
percent SiO.sub.2 and 50 weight percent of the partial condensate of
RSi(OH).sub.3, calculated as RSiO.sub.3 /.sub.2. After three days, the
compositions were diluted to 20% solids with isopropanol. In the case of
where R was CF.sub.3 CH.sub.2 CH.sub.2 -,
##STR2##
gellation occured within this three day period. Fresh compositions were
formulated and diluted to 20% solids with isopropanol after 4 hours and
allowed to stand for two days.
Th compositions were coated onto clean stretched acrylic panels, as
previously described, allowed to air-dry for 15 minutes and then cured for
4 hours at 85.degree. C. The coated panels were tested for abrasion
resistance by means of the described steel wool abrasion test. Results
obtained by the use of the different silanes are tabulated below:
__________________________________________________________________________
Silane Used Abrasion Resistance of Coating
In Coating Composition
(% Change in Haze)
__________________________________________________________________________
CH.sub.3 Si(OCH.sub.3).sub.3
3.2
C.sub.2 H.sub.5 Si(OCH.sub.3).sub.3
Composition gelled
CH.sub.2CHSi(OCH.sub.3).sub.3
9.0
C.sub.3 H.sub.7 Si(OCH.sub.3).sub.3
45.5
CF.sub.3 CH.sub.2 CH.sub.2 Si(OCH.sub.3).sub.3
33.5
##STR3## 50.7
##STR4## 39.0
__________________________________________________________________________
A mixture of silanes was utilized in formulating the above type of
composition. A mixture of 90 weight percent CH.sub.3 Si(OCH.sub.3).sub.3
and 10 weight percent CF.sub.3 CH.sub.2 CH.sub.2 Si(OCH.sub.3).sub.3 used
in place of the monomethyltrimethoxysilane in the composition gave a
coating exhibiting a delta haze of 2.0%. A similar coating formed from a
mixture of 80% CH.sub.3 Si(OCH.sub.3).sub.3 and 20%
##STR5##
equivalent abrasion resistance.
These data demonstrate the necessity of using compositions containing a
major amount of the partial condensate of CH.sub.3 Si(OH).sub.3. Those
cured coatings based on other silsesquioxanes, such as C.sub.3 H.sub.7
SiO.sub.3 /.sub.2 and
##STR6##
were softer than the acrylic surface itself.
EXAMPLE 4
Various amounts of methyltrimethoxysilane were added to an acidified
aqueous colloidal dispersion as described in Example 1 and the pH of the
compositions was adjusted to 4.5. After four days the compositions were
diluted to 20% solids with isopropanol and flow-coated onto acrylic
panels, air-dried and cured for 2 hours at 85.degree. C. Abrasion
resistance (%.DELTA. haze from the steel wool test) of the different
coatings is tabulated below:
______________________________________
Composition of
% Change
Cured Coating
in Haze
______________________________________
10% CH.sub.3 SiO.sub.3 /.sub.2
90% SiO.sub.2
*
20% CH.sub.3 SiO.sub.3 /.sub.2
80% SiO.sub.2
*
30% CH.sub.3 SiO.sub.3 /.sub.2
70% SiO.sub.2
1.0
40% CH.sub.3 SiO.sub.3 /.sub.2
60% SiO.sub.2
2.0
50% CH.sub.3 SiO.sub.3 /.sub.2
50% SiO.sub.2
0.4
______________________________________
*coating flaked off when cured.
The data demonstrate that a miniumum amount (at least 25-30 weight percent)
of CH.sub.3 SiO.sub.3 /.sub.2 must be present in the coating.
EXAMPLE 5
Different amounts of sodium acetate were added to an acidified colloidal
dispersion which was initially substantially free of alkali metal salts.
Sufficient methyltrimethoxysilane was added to the dispersion to form a
50/50 --SiO.sub.2 /CH.sub.3 SiO.sub.3 /.sub.2 coating and the pH of each
composition was adjusted to 4.5 by addition of glacial acetic acid. After
1 to 4 days the compositions were diluted to 20 percent solids with
isopropanol, coated onto acrylic panels ad cured for 4 hours at 85.degree.
C. Abrasion resistance (%.DELTA. haze by steel wool abrasion) is tabulated
below:
______________________________________
Wt. % Sodium Acetate
% Change in
Present in Original
Haze Upon
Silica Dispersion Abrasion
______________________________________
0 1.0*
0.0625 0.3*
0.125 0.3
0.25 0.5
0.50 0.8
1.0 0.3
2.0 14.8
______________________________________
*coated after four days, all others coated after one day.
Equivalent results (.DELTA. haze of less than 1%) were obtained when
trimethylbenzyl ammonium acetate was used in amounts in the range of 0.05
to 0.25 weight percent of the composition. Following the above procedure,
the optimum amount of any of the latent catalysts described in the
specification can be readily determined.
EXAMPLE 6
For purposes of comparison, ethylorthosilicate was utilized as the
SiO.sub.2 source as disclosed in U.S. Pat. No. 3,642,681. One hundred
grams of ethylorthosilicate was hydrolyzed by addition to 47 grams of
ethanol in 45 grams of 0.1N aqueous hydrochloric acid. After 1 hour
sufficient amounts of various silanes were added to samples of the
hydrolyzed (C.sub.2 H.sub.5 O).sub.4 Si to form a composition in which 50%
of the solids were SiO.sub.2. These solutions were diluted with
isopropanol and acetic acid to give 10% solutions which would wet acrylic
panels. After coating, the panels were heated at 85.degree. C. for 16
hours to ensure complete cure. Abrasion resistance was determined by the
previously described steel wool test. Results are tabulated below:
______________________________________
Silane Added to the
Ethylorthosilicate Solution
% .DELTA. Haze
______________________________________
CH.sub.3 Si(OCH.sub.3).sub.3
18.7
CH.sub.2CHSi(OCH.sub.3).sub.3
25.7
##STR7## 21.5
______________________________________
These data demonstrate the inferior performance of silica solutions of
hydrolyzed ethylorthosilicate when used in the practice of the present
invention. The silica must be in the form of a colloidal dispersion in
order to obtain superior abrasion resistance.
EXAMPLE 6
Methyltrimethoxysilane (50.0 grams) was acidified with 1.0 gram of acetic
acid. A colloidal silica dispersion (66.7 grams) as described in Example 2
was added to the acidified silane to provide a methanol-water dispersion
of silica and soluble partial condensate. The solids were 40% SiO.sub.2
and 60% the partial condensate of CH.sub.3 Si(OH).sub.3 when calculated as
CH.sub.3 SiO.sub.3 /.sub.2. After diluting to 22.5% solids, with
isopropanol, the pH of the composition was adjusted to 5.35. After 5 days,
the composition was filtered and coated onto panels of
poly(diethyleneglycolbis alkyl) carbonate which had been treated by
soaking overnight in a 10% solution of potassium hydroxide. The coating
was cured at 100.degree. C. for 2 hours.
A second portion of the described coating composition was catalyzed by
addition of 0.1 weight percent trimethylbenzyl ammonium acetate, coated
onto silvered acrylic panels (mirrors) and onto panels of polycarbonate,
pretreated as described above. The catalyzed coatings were cured at
85.degree. C. for 2 hours.
The coated panels, both mirrored acrylic and polycarbonate, were tested by
the circular steel wool rubbing method and showed a delta haze of less
than 1%.
The coated polycarbonate panels were tested by the Taber Abrasion test
method. The coatings were abraded until 10% haze was developed, the number
of revolutions being reported as the multiple of the number of revolutions
necessary to give this same amount of haze on uncoated acrylic panels.
______________________________________
Taber Abrasion (X Acrylic)
Minimum Maximum
Panel Abraded Area Abraded Area
______________________________________
Uncoated polycarbonate
24 X 24 X
40 SiO.sub.2 /60 CH.sub.3 SiO.sub.3 /.sub.2
uncatalyzed 580 X 375 X
40 SiO.sub.2 /60 CH.sub.3 SiO.sub.3 /.sub.2
catalyzed 670 X 350 X
______________________________________
As with poly(methylmethacrylate) the poly(diethyleneglycol bis allyl)
carbonate is especially useful in molding ophthalmic lenses. After
molding, such lenses can be coated with the compositions of the invention
to provide an extended service life.
Panels of commercially available transparent polycarbonate (bisphenol-A
polycarbonate sold under the trademark "Lexan") were primed with a 5%
solution of a silane-modified epoxy in Dowanol-EM and allowed to air dry.
The epoxy primer was a mixture of about 20%
beta-aminoethylgamma-aminopropyltrimethoxysilane in a commercially
available liquid epoxy sold under the trademark "DER-331" by The Dow
Chemical Company. The primed panels were coated on a composition the same
as that described in Example 1 (pH = 3.9) which was diluted to 25% solids
with isopropanol and catalyzed by addition of 0.2 weight percent
trimethylbenzyl ammonium acetate. The coating passed the cross-hatching
tape adhesion test and the abrasion resistance was also excellent.
Other polymer substrates which benefit from use of the coating of the
present invention include polyvinyl chloride, polystyrene, silicone resin
and and rubber, cellulosic thermoplastics, polyesters and the like.
EXAMPLE 7
Methyltrimethoxysilane (75.7 parts) which had been acidified with 18.9
parts acetic acid was mixed with 126.1 parts of the 50% solids basic
colloidal silica dispersion described in Example 1. There was a slight
exotherm and the mixture was cooled. After 5 hours another 8 parts acetic
acid was added to provide a pH of 4.5. After eleven hours the composition
was diluted by addition of 100 parts isopropanol. The composition was then
aged for 3 1/2 days and dip coated onto stretched acrylic panels. The
coating was air dried and then cured for 4 hours at 185.degree. F.
Coated panels were placed in a humidity test chamber maintained at
165.degree. F and 100% humidity. Other coated panels were exposed in a
weatherometer under the conditions described in ASTM-G-25-70. For purposes
of comparison, a commercially available acrylic sheet coated with a
polysilicic acid/fluoroolefin-hydroxyalkyl vinyl ether copolymer was also
tested. Results of the steel wool abrasion test (25 p.s.i.) after the
indicated number of revolutions and exposure times are tabulated below:
__________________________________________________________________________
Exposure
Coating Abrasion Resistance (% .DELTA. Haze) After
Appearance
Conditions
Material 5 Rev.
10 Rev.
15 Rev.
before Abrasion
__________________________________________________________________________
Initial 50% SiO.sub.2
(no exposure)
50% SiO.sub.3 /.sub.2
0.7 0.8 1.0 good
Polysilicic acid/
fluoro copolymer
1.6 2.2 3.5 Excellent
120 Hrs. -
50% SiO.sub.2
Humidity
50% CH.sub.3 SiO.sub.3 /.sub.2
0.2 0.2 1.9 good
Chamber Polysilicic acid/
fluoro copolymer
40.2 -- -- permanent
water spots
240 Hrs. -
50% SiO.sub.2
Humidity
50% CH.sub.3 SIO.sub.3 /.sub.2
0.6 1.4 5.6 good
Chamber Polysilicic acid/
fluoro copolymer
44.0 -- -- permanent
water spots
7 Days -
50% SiO.sub.2
Weatherometer
50% CH.sub.3 SIO.sub.3 /.sub.2
0.5 0.7 0.5 good
Polysilicic acid/
fluoro copolymer
1.8 5.4 15.7 good
__________________________________________________________________________
Another acrylic panel coated with the above-described composition of the
invention was exposed for over 21 days in the weatherometer and was not
visibly scratched by the steel wool at 25 p.s.i. loading after 5
revolutions.
Other acrylic panels coated and cured as described above were submitted to
other testing to determine utility as enclosures (windshields) for
transportation equipment. After a gauze pad soaked in the particular
solvent was placed on the cured coating and covered with a watch glass for
24 hours at room temperature, there was no apparent effect from such
contact with benzene, toluene, xylene, trichloroethane, acetone, ethyl
acetate, butylamine, methanol, isopropanol, permanent antifreeze, gasoline
or motor oil.
Another such coated acrylic panel was abraded with a windshield wiper blade
loaded at 0.33 lbs./in. of length moving in an arc at 80 cycles per
minute. A 15% sodium chloride solution was sprayed on the test panel
surface at 5-minute intervals. The test was terminated after 12,420 cycles
and there was no visible effect on the arc surface area.
Thermal shock characteristics of the coating were determined by temperature
cycling of a coated panel from -65.degree. F. to +160.degree. F in about
20 minutes. After six cycles, the coating remained intact with no apparent
effect on optical properties.
These data demonstrate that in addition to abrasion resistance the coatings
of the invention possess excellent weathering characteristics, solvent
resistance and thermal stability.
EXAMPLE 8
A coating composition similar to that described in Example 7 except that
isopropanol was not present was sprayed onto clean aluminum panels. After
air-drying for 24 hours, the coated panels were tested in a Dew Cycle
Weatherometer. After 100 hours of testing, the panels showed very low
corrosion (2%) and there was no blistering in the coating. These data
demonstrate the utility of the compositions as corrosion resistant
coatings for metal.
EXAMPLE 9
The utility of various organic acids in formulating the compositions of the
invention was demonstrated by diluting formic and maleic acid to 25%
solutions with a 50/50 isopropanol-water cosolvent. Oxalic acid was
diluted to a 12.5% solution with the same cosolvent. Each of the diluted
acids was added to 10.0 gram portions of a 30% solids aqueous colloidal
silica having 13-14 millimicron particles, a pH of 9.8 and Na.sub.2 O
(titratable alkali) of 0.32%. Sufficient acid was added to bring the ph
down to the range 3.5 to 4.1. Methyltrimethoxysilane (6.0 grams) was added
to each of the acidified silicas. After 30 minutes mixing, the
compositions were reduced to 25% solids by the addition of isopropanol and
aged for about 18 hours. The composition based on oxalic acid contained a
small amount of precipitate which had settled out during ageing. The aged
compositions were flow coated onto glass microscope slides, air-dried and
then cured at 100.degree. C for 2 1/2 hours.
The clear cured coatings were tested for abrasion resistance by attempting
to mar the surface with a pencil eraser. The abrasion resistance of the
composition acidified with formic acid was excellent, while the coating
based on maleic and oxalic acid exhibited very good abrasion resistance.
EXAMPLE 10
The procedure of Example 9 was repeated utilizing a 25% solution of
glycolic acid and a colloidal silica dispersion containing about 0.05%
Na.sub.2 O and having an initial pH of 3.1. After dilution to 25% solids
with isopropanol the pH was 3.6. After ageing for about 4 hours, about 25
grams of the composition was catalyzed with 0.15 grams of a 10% solution
of benzyltrimethylammonium acetate raising the pH to 4.7. When coated onto
a glass slide and cured, the clear coating had good to very good abrasion
resistance when tested with an eraser.
The coating composition was then further catalyzed by the addition of 0.5
grams of 10% isopropanol solution of triethylamine which elevated the pH
to 5.2. The coating, curred on to glass slides, exhibited very good
abrasion resistance but gave a slight (about 3% haze) reduction in light
transmission upon curing. This small amount of haze is not objectionable
when the coating is used in applications other than the optical area.
EXAMPLE 11
Three different aqueous colloidal silicas were blended to provide a 32%
solids dispersion containing one-third of 50-70 millimicron-sized
particles, one-third of 15-17 millimicron sized particles and one-third
6-7 millimicron sized particles having an Na.sub.2 O content of about 0.2
weight percent. Acetic acid (2.25 grams in 10 ml. of water) was added to
87.5 grams of colloidal silica dispersion. After acidification, 45 grams
of methyltrimethoxysilane was added rapidly and the mixture was shaken.
After 45 minutes, hydrolysis was considered complete and 57.75 grams of
isopropanol was added to provide a coating composition containing 25%
solids (calculated on the weight of SiO.sub.2 plus CH.sub.3 SiO.sub.3
/.sub.2) and having a pH of 5.4.
After two days ageing, the solution/dispersion of colloidal silica and
partial condensate was filtered and a portion was coated onto one-eighth
inch thick stretched acrylic sheet. After air-drying for 30 minutes the
coating was cured for 4 hours at 80.degree. C.
When tested for abrasion resistance with steel wool as described in Example
1 there was no apparent change in haze. The number of revolutions was
increased to 25, but there was still no measurable abrasion. The loading
was then increased from 25 p.s.i. to 35 p.s.i. and after 10 revolutions
there was no measurable increase in haze. The exceptional hardness of this
coating is believed to result from the more dense packing of particles
obtained by use of the blend of different particle sizes.
A second portion of the aged coating composition was flow-coated onto a 100
mil-thick molding of a commercially available styrene-acrylonitrile
copolymer (Tyril from the Dow Chemical Company, Midland, Michigan) which
had been primed with the silane-modified epoxy described in Example 6.
After air-drying for 30 minutes, the coating was cured for 6 hours at
75.degree. C.
The cured coating exhibited excellent abrasion resistance when subjected to
the steel wool rubbing test. A portion of the coating was cross-hatched
into 1/16-inch squares using a razor blade to cut through to the
styrene-acrylonitrile substrate. The cross-hatched coating was not lifted
by rapidly removing pressure-sensitive tape previously pressed onto the
cut surface. These data demonstrate the abrasion resistance and adhesion
obtained by the practice of the invention.
Reasonable modification and variation are within the scope of this
invention which is directed to novel pigment-free coating compositions and
solid surfaces coated with such materials.
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