Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
RD-15,60,
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SODIUM FREE SILICONE RESIN COATING COMPOSITIONS
BACKGROUND OF THE INVENTION
This invention relates to silicone resin coating
compositions and methods for producing silicone resin
coatings on substrates. More particularly, it relates to
silicone resin coating compositions having a basic p~
provided by a volatile base, such as ammonia, and methods
for producing coatings therefrom. These coatings
demonstrate improved resistance to ultra violet light,
stress, heat and humidity.
The substitution of glass with transparent
materials which do not shatter has become widespread. For
example, transparent glazing made from synthetic organic
polymers is now utilized in public transportation vehicles,
such as trains, buses an~ airplanes. Lenses for 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
comparison to glass is a further advantage, especially in
the transportation industry where the weight of the vehicle
is a major factQr in its fuel economy.
~ hile transparent plastics provide the major
advantage of being more resistant to shattering and lighter
than glass, a serious drawback lies in the ease with which
these plastics mar and scratch due to everyday contact with
abrasives, such as dust, cleaning e~uipment and/or ordinary
weathering. Continuous scratching and marring results in
impaired visibility and poor esthetics, oftentimes re~uirin~
replacement of the glazing or lens.
1~ 7SS~ 2D-15,607
Attempts have been made to improve the abrasion
resistance of these transparent plastics. For example,
coatings formed from mixtures of silica, such as colloidal
silica or silica gel, and hydrolyzable silanes in a
hydrolysis medium have been developed to impart scratch
resistance. U.S. Patent Nos. 3,708,225, 3,986,997,
3,976,497, 4,36~,235 and 4,324,712, describe such
compositions.
While these aforementioned coating formulations
have been found acceptable, there still remains room for
improvement. For example, this invention provides coa~ings
having an added degree of resistance to moisture, heat,
humidity and ultraviolet light, which does not exist in
similar coatings provided in the patents re~erred to above.
Coating compositions known to the art, having a
basic pH, usually contain the base sodium hydroxide since
the colloidal silicas utllized in these compositions are
stabilized with sodium hydroxidè to prevent agglomeration or
gelation of the colloidal dispersion. The colloidal silicas
provide excess quantities of sodium hydroxide and the pH of
the coating composition must be buffered with acetic acid or
other suitable acids.
Sodium hydroxide is the predominant stabilizing
species for basic colloidal silica sols since most
commercial processes that produce colloidal silica utilize
sodium silicate as a starting material, which generates
sodium hydroxide.
Those colloidal silicas which contain sodium
hydroxide, such as Ludox2 LS, are preferred in the coating
compositions of the prior art. Sodium hydroxide is a
nonvolatile base and will not vaporize from the coating
composition during processing or upon application of said
composition to a substrate.
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It has been discovered that silicone resin coating
compositions ~ith an alkaline p~ provided by a volatile
base, such as ammonia, provide coatings with greater
resistance to the elements than alkaline coating
compositions which contain sodium hydroxide.
Not being bound by theory, it is believed that the
superior propertiès are attributed to the reduced
concentration of base and salts of the base in the cured
coatings since the volatile base vaporizes from the
composition during curinq. Sodium hydroxide and the salts
produced therefrom, such as sodium acetate, do not
volatilize and remain in the cured coating. The alkaline
species within the coating compositions and salts therefrom
function as catalysts which promote the condensation
lS reaction that cures the composition. It is further believed
that when these alkaline species and their salts remain
within the cured coating, they also catalyze hydrolysis
reactions, which result in the formation of cracks in the
coating under conditions of heat, humidity and ultraviolet
light exposure.
All alkaline species, such as potassium hydroxide,
calcium hydroxide, ammonium hydroxide and the like,
including their salts, will provide catalysis for the
hydrolysis reaction within the coating composition.
However, where a volatile base is utilized, such as ammonia,
the coating compositions can be cured under conditions so
that a substantial portion of these catalysts are removed
and cannot aid the formation of cracks. The coatings
obtained therefrom exhibit greater resistance to humidity,
heat, stress and ultraviolet light.
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~L275i5X6
SUMMARY OF THE INVE~TION
This invention provides improved silicon resin
coating compositions and methods for preparing coatings
therefrom. The coating compositions comprise a
water/aliphatic alcohol dispersion having an alkaline pH
provided by a base which is volatile at the temperature
selected for curing said composition. The preferred coating
compositions of the invention comprise about 10-50 weight
percent solids dispersed in a water/alcohol solution, these
solids comprise colloidal silica, preferably about 10-70
weight percent, and a partial condensate, preferably about
30-90 weight percent, derived from organotrialkoxy silanes,
preferably of the formula ~'Si(OR)3, wherein R' is selected
from the group consisting of alkyl radicals having from 1 to
3 carbon atoms and aryl radicals of from 6-13 carbon atoms
and R is selected from alkyl radicals of from 1 to 8 carbon
atoms and aryl radicals of from 6-20 carbon atoms. The
compositions provided have an alkaline pH, preferably from
7.1 to about 7.8, provided by a base which is volatile at
the temperature selected for curing said composition.
Included in this invention are methods for preparing silicon
resin coatings which comprise
(a) applying to a substrate a silicon resin coating
composition of this invention, said silicon resin coating
composition having an alkaline pH of from 7.1 to about 7. a
provided by a base which is volatile at the selected cure
temperature,
(b) heating the resulting composite to a temperature
being sufficiently high to volatilize said base until the
applied silicon resin coating composition is cured, said
temperature falli~g within the range of from about 75C to
about 150C.
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OBJECTS OF THE INVENTION
It is an object of the present invention to
provide an improved silicon resin coating composition which,
when applied to a substrate, is substantially free of
species which catalyze the hydrolysis reaction.
It is another object of the present invention to
provide an improved silicon resin coating composition which,
when applied to a substrate, shows improved resistance to
ultraviolet light, stress, heat and humidity.
It is another object of the present invention to
provide a mèthod for preparing silicon silicone resin
coatings with improved weatherability.
It is a further object of the present invention to
provide a method for preparing a coating ~hich is
substantially free of species which catalyze the hydrolysis
reaction.
DETAILED DESCR~PTION OF THE INVENTION
The silicone resin coating compositions of this
invention contain the hydrolysis product of an aqueous
dispersion of colloidal silica and an organo-
trialkoxysilane. This hydrolysis product is maintained in
an aqueous/alcohol solution having a basic pH to provide a
stabilizing medium for the reactive species until the
composition is cured. For compositions of this invention,
the base utilized is volatile at the selected cure
temperature.
The term "volatile base" as used herein are those
bases having a boiling point within the range of
temperatures utilized to cure the coating compositions.
Cure temperatures for the coating compositions of this
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1275S~6
invention range from 75C to about 200C, and pre~erably
from about 100 to 150C. Any base which is volatile within
these ranges is suitable for use in this invention. Such
bases are well known to those skilled in the art, examples
of which include, alkyl amines of from 1 to 6 carbon atoms
such as methylamine, ethylamine, t-butylamine, diethylamine,
triethylamine, ethylenediamine; aromatic amines of from 5-7
carbon atoms, such as pyridine, aniline and methylaniline;
ammonia; and the like.
Although the principle function of the base is to
stabilize the coating composition by providing the desired
pH, the volatile base also provides catalysis for the curing
reaction and for the reaction which provides the partial
condensate. A significant portion of the base utilized is
often neutralized when adjusting the pH to the desired
value. The salts obtained also provide catalysis for the
curing reaction and the formation of the partial condensate.
Volatilizing the base will also reduce the concentration of
the salts because of the shift in equilibrium. The
preferred bases are those which produce unstable salts upon
neutralization which will dissociate at the cure
temperatures described above. Where the salts of the base
are unstable at the cure temperatures, a larger portion of
the salt catalysts will be rendered ineffective upon
volatilization of the base. An example of such a base is
ammonia, which forms unstable salts with acetic and formic
acids.
The preferred organotrialkoxy silanes utilized in
this invention are of the formula, R'Si(OR)3, wherein R is a
monovalent radical selected from the group consisting of
alkyl radicals of from 1 to 8 carbon atoms and aryl radicals
of from 6-20 carbon atoms and R' is a monovalent radical
selected from the group consisting of alkyl radicals of from
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~27~5~6
1-3 carbon atoms and aryl radicals of from 6-13 carbon
atoms.
The aqueous dispersions of colloidal silica which
can be utilized in the present invention have a particle
5 size of from 5-150 millimicrons and preferably from 10-30
millimicrons average diameter. Such dispersions are known
in the art and commercially available ones include, for
example, those under the trademarks of Ludox (DuPont) and
Nalcoag (Nalco Chemical Company). Such dispersions are
available in the form of acidic or basic hydrosols. The
commercially available basic colloidal silicasols typically
provide a sufficient quantity of base to maintain the pH
within the range of 7.1 to 7.~. Therefore, when utilizing
the colloidal silicas, it is preferable that the alkaline
species within the silica be volatile at the selected cure
temperature. If the alkaline species is not volatile, and
the desired pH is provided by said non-volatile base, such a
colloidal silica is not desirable for use in this invention
unless the non-volatile base is removed. Where the desired
pH is not completely provided by the non-volatile base
within the colloidal silica, such colloidal silicas can be
utilized to achieve some of the desi~ed objects of this
invention.
Colloidal silicas which are initially acidic can
also be used but the pH of the hydrolysis medium must be
adjusted to be basic with a volatile base. Colloidal
silicas having a low alkali content provide a more stable
coating composition and these are preferred. A particularly
preferred colloidal silica for purposes herein is known as
Ludox AS, an ammonium stabilized colloidal silica sold by
DuPont Company. Other commercially available ammonium
stabilized colloidal silicas include Nalcoag 2326 and
Nalcoag 2327, sold by Nalco Chemical Company.
RD-15,~07
1~7~5~6
In preparing the silicone resin coating
compositions of this invention, the aqueous dispersion of
basic colloidal silica is added to a solution of a small
amount of glacial acetic acid (or alkyltriacetoxy silane)
and an organotrialkoxy silane of the formula indicated
above. Where an acidic colloidal silica is utilized, a
volatile base is added to buffer the p~ instead of acetic
acid. The temperature of the reaction mixture is kept in
the range between 20C to 40DC, preferably below 25C. A
reaction time of about 6-8 hours is usually sufficient to
react enough of the organotrialkoxy silane such that the
initial two-phase liquid mixture has been converted to a
single liquid phase in which the colloidal silica is
dispersed. Hydrolysis is usually permitted to continue for
a period of about 8-48 hours. As a rule, the lon~er the
time permitted for hydrolysis, the higher the final
viscosity.
Silanols, R'Si(OH)3, are formed in situ as a
result of admixing the corresponding organotrialkoxysilanes
with the aqueous dispersion of colloidal silica. Alkoxy
functional groups, such as methoxy, ethoxy, isopropoxy,
n-butoxy, and the like generate the hydroxy functional group
upon hydrolysis and liberate the corresponding alcohol, such
as methanol, ethanol, isopropanol, n-butanol, and the like.
During the preparation of the coating formulations
from basic colloidal silica, alkyltriacetoxysilanes may be
employed to buffer the viscosity of the initial two-phase
liquid reaction mixture and to regulate the hydrolysis rate
by producing acetic acid. Preferred are those
alkyltriacetoxysilanes in which the alkyl groups contain
from 1 to 5 carbon atoms and especially 1 to 3 carbon atoms.
Methyltriacetoxysilane is the most preferred. These
alkyltriacetoxysilanes will generate silanols in the same
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~-15,607
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manner as organotrialkoxysilanes. ~s indicated ab~ve, an
alternative to utilizing these alkyltriacetoxysilanes is to
regulate the hydrolysis rate and buffer the pH through the
use of glacial acetic acid or similar acid such as
propionic, butyric, citric, benzoic, formic, oxylic, and the
like.
Upon generating the hydroxyl substituents of these
silanols, a condensation reaction begins to form
silicon~oxygen-silicon bonds. This condensation reaction is
not exhaustive. The siloxanes produced retain a quantity of
silicon-bonded hydroxy groups, typically about at least one
for every true Si-O units, which is why the polymer is
soluble in the water-alcohol solvent mixture. This soluble
partial condensate can be characterized as a siloxanol
polymer having silicon-bonded hydroxyl groups and -SiO-
repeating units.
A major portion of the partial condensate is
obtained from CH3Si(OR)3, depending on the input of ingredi-
ent to the hydrolysis reaction. Minor amounts of the
partial condensate are derived from C2H5Si(OR)3,
C3H7Si(OR)3, C6~5Si(OR)3, and the like where the
corresponding organotrialkoxy silanes are used. It is most
preferable to use only methyltrimethoxysilane for most
silicone resin coating compositions, thus generating only a
methyl-substituted partial condensate.
After hydrolysis has been completed, the solids
content of the coating compositions is typically adjusted by
adding alcohol to the reaction mixture. Suitable alcohols
include lower aliphatics, e.g., having l to 6 carbon atoms,
such as methanol, ethanol, propanol, isopropanol, butyl
alcohol, t-butyl alcohol, and the like, or mixtures thereof.
Isobutanol is preferred. A solvent system, i.e., mixture of
water and alcohol, preerably contains from about 20-75% by
~-,5,607
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weight of the alcohol to ensure that the partial condensate
is soluble.
Optionally, additional water-miscible polar
solvents, such as diacetone alcohol, butyl cellosolve, and
the like can be included in minor amounts, usually no more
than 20% by weight of the solvent system.
After adjustment with solvent, the coating compo-
sitions of this invention preferably contains from about
10-50% by weight solids, most preferably, about 20% by
weight of the total composition. The nonvolatile solids
portion of the coating formulation is a mixture of colloidal
silica and the partial condensate of a silanol. In the
preferred coating compositions herein, the partial
condensate is present in an amount of from about 55-75% by
weight of total solids, with the colloidal silica being
present in the amount of from about 25-45% by weight based
on the total weight of solids within the alcohol/water
cosolvent.
The coating compositions of this invention
preferably have a pH in the range of about 7.1 to 7.8 and
most preferably from about 7.2 to 7.8. After the hydrolysis
reaction, it may be necessary to adjust the pH of the
composition to fall within these values. To raise the pH,
volatile bases are preferred; such as ammonium hydroxide and
to lower the pH, volatile acids are preferred, such as
acetic acid and formic acid. These volatile acids have a
boiling point which falls within the range of temperatures
utilized to cure said compositions.
The coating compositions of this invention will
cure on a substrate in approximately 2 hours at temperatures
of about 120C. Such a cure rate is obtained without the
aid of additional cure catalysts. I milder curing
conditions or an accelerated cure time are desired, it is
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~.2755~6
preferable to add an additional amount of buffered or latent
condensation catalyst to the coating composition. It is
well known that alkaline coating compositions containing
commercially available colloidal silica will generate cure
catalysts in situ when the pH is adjusted below a value of
about 8. The pH is typically adjusted with acetic acid,
generating carboxylate catalysts from the alkaline species.
In addition, the alkaline species within such compositions
will provide some catalysis also.
Where additional catalysts are desired, it i5
preferable to utilize those catalysts which will volatilize
on curing or will dissociate to a volatile species upon
curing. Suitable examples are those catalysts based on
ammonia or amines. These catalysts will dissociate to a
volatile base upon curing. Examples include amine
carboxylates, such as dimethylamine-acetate,
ethylamine-acetate, dimethylaniline-formate, ammonium
acetate, and the like; quaternary ammonium carboxylates,
such as tetramethylammonium-acetate,
benzyltrimethylammonium-acetate; amines, such as
triethylamine, trimethylamine, pyridine, and the like,
including ammonia. Where additional cure catalysts are
utilized, cure times can be reduced to about 30 minutes.
Although other species which do not volatilize or
dissociate to volatile species will provide catalysis, they
are not desired since these catalysts will remain in the
composition after curing. These catalytic species include
alkali metal salts of carboxylic acids, such as sodium
acetate, potassium formate, and the like, and the
nonvolatile bases, such as sodium hydroxide and potassium
hydroxide.
The amount of curing catalyst can vary widely,
depending on particular re~uirements. In general, the
~D-15,60'
~27~5~
catalyst is present in the amount of from about 0.05 to
about 0.5 weight percent and preferably about 0.1% by weight
of the total coating composition. Such compositions are
curable on the substrate within a brief period of time by
the process of this invention, e.g., from 30 to 60 minutes,
using temperatures in the range of from about 75-100C to
provide a transparent abrasion resistant surface coating.
Maintaining the pH in the range of 7.1-7.8 limits the
quantity of base catalysts utilized.
Other ingredients may be present within the
silicone resin coating compositions of this invention
without a significant effect on achieving the desired
objects. These additives are typically introduced to
improve certain characteristics of the coatings obtained.
For example, ultraviolet light-absorbing agents may be
introduced, such as those disclosed by Anthony, U.S. Pat.
4,495,36~-& Ashby et al., in U.S. Patent 4,278,804 and by
Fry in U.S. Patent 4,299,746 of the basic formula
O Z
z~ C~Q
where Q varies in each reference but is of the general
formula
CH2(CH2)nSi(Rl)x(OR2,)y,
and Z', Z, Y, X, y, Rl, R and n are more particularly
defined in the respective references. The radicals Z and Y
are more commonly H and OH, Rl and R2 are more commonly
methyl, y is typically 3 with x and n being zero.
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3L~7~;iSX6
These ultraviolet lig~t-absorbing agents can be
used in amounts of from about 1.0 to about 40.0, and
preferably from about 5~0 to 20.0 parts by weight per 1QO
parts of the resulting hardcoat formulation on the basis
of solids.
Other ingredients which may be added include the
flow modifiers which control 1Ow and prevent flow marks,
dirt marks and the like on the coating surface and increase
the stress cracking resistance of the coating. These are
typically polysiloxane/polyether copolymers having the
formula
R3Si[ o(R42Sio)bR 2SiCcH2c-C-O(CdH2dO)x,R ]3 ,
4 3 5
wherein R and R are monovalent hydrocarbons, R is a lower
alkyl, preferable alkyl having 1 to 7 carbon atoms, b is at
least 2, preferably 2 to about 40, c is from 2 to 3, d is
from 2 to 4 and x' is at least 5, preferably 5 to lOO. By
way of illustration, R4 and R3, independently are alkyls
such as methyl, ethyl, propyl, butyl, octyl, and the like;
cycloalkyls such as cyclohexyl, cycloheptyl and the like;
aryl, such as phenylto'yl, naphthyl, zylyl, and the like;
arylalkyl, such as benzyl, phenylethyl, and the like;
alkenyl or cycloalkenyl, such as vinylallyl, cyclohexenyl,
and the like and halogenated derivatives of any of the
` foregoing, such as chloromethyl, chlorophenyl, dibromo-
phenyl, and the like. Illustratively, R5 is methyl, ethyl,
propobutyl, isobutyl, amyl and the like. Such
polysiloxane-polyether copolymers are described with greater
particularity by Fry in United States Patent Numbers
4,308,315; 4,324,839 and 4,277,287.- The pFeparation
of the above Polysiloxane polyether copolyn~ers --
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~2755~6
is further described in U.S. Patent No. 3,629,165.
availa~le materials are SF-1066 and SF-1141, from General
Electric Company, L-540 from Union Carbide and DC-190 from
Dow Corning.
Other additives, such as thickening agents,
pigments, dyes, antioxidants and the like can also be
included for their conventionally employed purposes. These
are typically added to the compositions after hydrolysis has
been initiated. Thickening agents are more particularly
described by Vaugh,Jr., U.S. Patent 4,30~,319; suitable
adhesion promoting compounds (~-hydroxy ketones) are
disclosed by Conroy, U.S. Patent 4,311,763 and suitable
antioxidants are described by Anthony in United
States Patent Number 4,495,3~0 which patent issued
January 22, 1985.
These compounds may be "ultrafiltered" as
described by Anthony et al. in United States
Patent Number 4,499,224 which patent issued on
February 12, 1985. - ~-
The coating compositions of this invention can be
applied to the surface of an article with or without priming
of the said surface. Priming of the surface with a
thermosetting acrylic prior to application of the silicone
coating composition is often preferred and can be accom-
plished using conventional methods. The cured compositions
are useful as protective coatings on a large variety of
surfaces, either transparent or opaque, including plastic
surfaces and metal surfaces. Examples of such plastics
include synthetic organic polymeric substrates, such as
acrylic polymers, example, poly(methylmethacrylate), and the
like; polyesters, example, poly~ethylene terephthalate~,
poly(butylene terephthalate), and the like; polyamides,
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polyimides, acrylonitrile-styrene copolymers;
styrene~acrylonitrile-butadiene terpoly~ers; polyvinyl
chloride; butyrates, polyethylene, and the like.
Special mention is made of the polycarbonates,
such as those polycarbonates known as Lexan~ polycarbonate
resin, available from General Electric Company, including
transparent panels made of such materials. The compositions
of this invention are especially useful as protective
coatings on the surfaces of such articles.
lG Also included in this invention are methods for
curing silicone resin coating compositions. The silicon
resin coating compositions which are cured by these
processes have a basic pH provided by a volatile base. The
volatile bases have a boiling point at or below a
temperature in the range of 75C to 200C, which is the
range a suitable cure temperature for this process. The use
of silicon resin coating compositions with a pH in the range
of 7.1 to 7.8 is preferred, but those of a higher pH are
also suitable. The use of silicon resin coating
compositions of this invention is most preferred.
The coating compositions are first applied to a
solid substrate. This can be accomplished by conventional
methods such as flowing, spraying, or dip coating to form a
continuous film or layer thereon. The coating thickness can
be varied, but, in general, the coating will have a
thickness in the range between about 0.5 to 20 microns, more
usually from 2-10 microns.
The cured silicone resin coatings can be adhered
to substantially all solid substrates. Therefore, the
substantially all solid substrates are suitable for use in
this process. Examples of suitable substrates include
plastics, both opaque and transparent; metal surfaces, both
bright and dull; wood; leatheri glassi ceramics; textiles;
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~.2755;~
an~ the like. The preferred plastic substrates are those
described above as suitable for the silicon resin coating
compositions of this invention. Most preferred are the
polycarbonate substrates, particularly those known as Lexan~
polycarbonate available from General Electric Company.
After the silicon resin coating composition is
applied to a substrate, it is heated to a temperature
suIficiently high to volatilize the base within. The actual
cure temperature utilized is determined by the volatilizing
temperature of the volatile base, the cure temperature being
at or above the temperature at which the base is volatile.
The suitable cure temperatures fall within the range of 75C
to 200C. Heating to a temperature in this range will also
volatilize the solvent and condense residual silanols in the
lS partial condensate to provide a hard coating. Although the
coating can air dry to a substantially tack free condition
without heating and the temperature utilized may be
significantly higher than is necessary to cure the coating
composition, heating at the selected temperature (boiling
point of base) is necessary in this process to provide
coatings with improved weatherability. Where ammonia is the
volatile base, a cure temperature of about 110 to 120C is
preferred.
The final cure results in the formation of
silsesquioxane (RSiO3/2) moieties. In the final coating,
the ratio of RSiO3/2 units to SiO2 will range from about
0.43 to about 9.0, preferably from l to 3 where R is methyl,
the ratio is most preferably egual to 2.
In order that those skilled in the art will be
better able to practice the invention, the following exam-
ples are given by way of illustration and are not provided
to limit the invention to the embodiments described. All
parts are by weight unless otherwise indicated.
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~7~5~6 ~-15,~C7
EXAMPLE I
A coating composition within the scope of this
invention was prepared by admixing 406 grams
methyltrimethoxysilane and 2.5 grams acetic acid with 251
grams Ludox AS in 83 grams distilled water. The two phase
solution was stirred at 20-30~C for about 16 hours. Then,
740 grams isobutanol, 12 grams of surface reagent SF-1066,
described with greater particularity above, and 67 grams of
"trimethoxy" SHBP- W screener (52% in methanol), of the
formula described above with Y and Z' being hydrogen, Z
being hydroxy, n is 2, x is 0 and y is 3, were added within
the solution and stirred at room temperature for one month.
This coating composition was flow coated on Lexan~
polycarbonate panels 1/4" x 4" x 4". The coating
composition was air dried for 30 minutes and then cured in a
forced air convection oven for 1.5 hours at a temperature of
125C. The resulting hard coating was smooth and clear and
had no evidence of flow marks or stress cracking. The
coated sample was tested for Taber abraser hardness in
accordance with the procedures described below and provided
a value of 5.1. The coated samples were also tested under
Q W in accordance with the procedure described below and
resulted in loss of adhesion (delamination) after 2000
hours. No microcracks were observed. In contrast, when a
similar coating composition containing the nonvolatile base,
sodium hydroxide, was prepared and cured in the same manner
(251 gms Ludox~ LS, 406 gms methyltrimethoxysilane, 2.5 gms
acetic acid, 83 gms water), a Taber hardness value of 5.0
was obtained but microcracks were observed after 1000 hours
of exposure to QW . Loss of adhesion resulted after 2000
hours of exposure to QUV.
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EXAMPLE II
Three silicone resin coating compositions within
the scope of this invention were prepared in a manner
similar to that described in Example 1. A cure catalyst,
benzyltrimethylammonium acetate, was added to two of the
compositions at various levels (0.15 weight % and 0.25
weight %) prior to coating. Three coated samples were
obtained by flow coating each of the compositions on Lexan~
polycarbonate panels 1/4" x 4" x 4", air drying for 30
minutes and curing the coatings in a forced air convection
oven for 30~minutes at 125C. A fourth coated sample was
obtained by flow coating a 1/4" x 4" x 4" Lexan~
polycarbonate panel with the composition containing 0%
catalyst. Thls coating was also allowed to air dry for 30
lS minutes and then cured in a forced air convection oven for
two hours at 135C. Each of the samples were tested for
Taber abrasion hardness, microcracks and loss of adhesion
under Q W . The results are provided in Table I.
TABLE I
QUV Agent of Sodium Free Silicone Hardcoats
Sample Catalyst Taber QW Time to QUV Time to
No. Level Hardness Microcrack Loss of Adhesion
0% * * *
2 0.15% 4.5 1500
3 0.25% 3.0 - 1500
40%** 5.1 - 1500
* - Coating failed initial adhesion under
curing conditions
30** - Coating was cured for ~ hours at 135C
in a ,orced air convection oven
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EXPERIM~NTAL
The adhesion tests referred to above comprised the
cross-hatched method (ASTM-3359). In accordance with this
procedure, coated Lexan~ polycarbonate was cut with a
lattice cutting device and tested with adhesion tape (Scotch
710). Only 100% retention of adhesion after the third fresh
tape pull was considered as passing.
The ~W tests referred to above were performed in
a GM harsh cycle apparatus. The samples were exposed for 8
hours at 70C with UV light followed by 4 hours at 50C of
darkness with condensation. Samples were observed for
microcracks after each cycle. The surface was tested for
adhesion during the wet cycle after one hour of exposure to
water vapor.
The Taber Abraser Hardness Test referred to above
was performed by placing coated polycarbonate samples in
Abraser Model ~174. The samples were exposed to 300 cycles
in a 500 gram load using freshly sanded CS-lOF wheels. The
haze was measured with a Gardner haze meter, Model UX10.
The values given in the Examples above represent the change
in % haze.
Obviously, other modifications and variations of
the present invention are possible in light of the above
teachings. It is, therefore, to be understood that changes
may be made in the particular embodiments described above
which run within the full intended scope of the invention as
defined in the appended claims.
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