Upgrading silicone resin coating compositions

A method of upgrading silicon resin coating compositions is provided wherein the coating compositions are filtered prior to application on a substrate. The filters utilized are 5 microns or less.

BACKGROUND OF THE INVENTION 
This invention relates to a method of improving silicone resin coating 
compositions. More particularly, it relates to a method of upgrading 
silicone resin coating compositions by ultrafiltration. 
Silicone resin coating compositions are presently being utilized to protect 
the surface of wood, metals, and synthetic polymers. These transparent 
coatings provide excellent mar and scratch resistance to acrylic and 
polycarbonate sheets used as glazing. An example of a widely used 
transparent polycarbonate glazing on which these coating are useful is 
Lexan.RTM. polycarbonate resin sold by General Electric Company. 
The silicone resin coating formulations typically comprise an aqueous 
dispersion of colloidal silica or silica gel and hydrolizable silanes, 
such as the trifunctional silanes of the formula, R'Si(OR).sub.3 and a 
solvent medium such as alcohol and water. 
Degradation of these silicone coating compositions from prolonged exposure 
to moisture, humidity and ultra violet light during outdoor use, has 
presented certain problems. Yellowing and hazing along with delamination 
of these protective coatings is often observed. Attempts to enhance the 
longevity of such silicone coatings have included modification of the 
solvent carrier, as disclosed by Anthony in Ser. No. 373,361, filed Dec. 
1, 1982 and modifications of the composition. These modifications include 
the introduction of additives to the coating as disclosed by Frye in U.S. 
Pat. No. 4,277,287 and altering the pH of the composition as disclosed by 
Vaughn, Jr. in U.S. Pat. No. 4,368,235. The above references are assigned 
to the same assignee as the present invention. 
While the procedures mentioned above have provided acceptable coating 
formulations, there still remains room for improvement. For example, the 
method comprising this invention upgrades the coatings contained in the 
processes identified above in addition to providing coatings having 
improved resistance to ultra violet light, heat, moisture, stress and 
strain. 
SUMMARY OF THE INVENTION 
A method of improving silicone coating compositions is provided which 
comprises filtering a silicone resin coating composition through a filter 
of a size 5 microns or less. 
OBJECTS OF THE INVENTION 
An object of the present invention is to improve those protective silicone 
resin coating compositions presently in use. 
Another object of the present invention is to provide a method of improving 
the resistance to cracking and the resistance to deterioration from heat, 
moisture, humidity and ultra violet light in coatings obtained from 
silicon resin compositions. 
A further object of the present invention is to provide a method of 
upgrading silicon resin coating compositions without introducing additives 
or affecting the concentration of ingredients.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The desired objects of this invention and other objects are achieved by 
filtering a silicone resin coating composition through a filter having a 
size in the range of 5 microns or less prior to application on a 
substrate. 
The term "ultrafiltration", as used herein, refers to a filtration 
procedure wherein a filter having a pore size of 5 microns or less is 
utilized. This range of filter sizes includes those procedures often 
referred to as "microfiltration". 
Filtering of the silicone resin coating compositions can be accomplished 
with filters that are commercially available. Any filter comprised of an 
inert material, such as polypropylene, polyethylene, teflon, silicone, 
etc., is suitable provided it is of a size 5 microns or less. A suitable 
filter is Gelman filter Model No. 4280, which is a size of 0.5 microns. 
The extent to which the silicone resin compositions are upgraded is 
proportional to the size of the filter utilized. The smaller the filter 
size the more the resistance to strain, stress, heat, moisture and ultra 
violet radiation is increased. Compositions passed through a 0.5 micron 
filter exhibit physical properties which are significantly improved over 
those filtered with 5 micron filters. Filtering silicon resin coating 
compositions through filters smaller than 0.5 microns is expected to 
improve the compositions property profiles even further. However, 
conventional methods for testing the effectiveness of the silicone resin 
coatings do not permit a quantitative evaluation of further improvements 
in properties. 
When it is desirable to filter a silicone resin coating composition through 
filters smaller than 1 micron, it is preferable to utilize a series of 
filters of decreasing size. Often attempting to filter the silicone resin 
compositions by a single pass through a small filter cannot be achieved. 
It may be necessary to pass the composition through a series of filters to 
prevent clogging. A typical step down filtration procedure is to utilize a 
5 micron filter, a one micron filter and then a filter of the desired 
size. Filters smaller than 0.02 microns cannot be utilized with the 
coating compositions that are presently available since this is 
essentially the particle size of a majority of the components in the 
silicone resin compositions. Therefore, a substantial portion of material 
will not pass through the filter. 
Ultrafiltration of the silicone resin coating compositions provides 
improvements in properties unlike those obtained from common purification 
procedures utilizing filtration. Filtration procedures remove impurities 
to provide a pure product. In the case of silicone resin coating 
compositions, the purified coatings are typically clearer upon application 
to a substrate; however, ultrafiltration enhances properties unrelated to 
purification or removal of impurities, such as improved resistance to 
stress and strain and improved ultra violet light stability. 
Ultrafiltration will typically separate only 0.1% of the components within 
the silicon resin coating composition. The separated components are 
typically agglomerates of the colloidal silica. 
This process will upgrade essentially any silicone resin coating 
composition which contains colloidal silica. The extent to which the 
particular silicone resin coating composition is upgraded varies with the 
contents of said composition. Examples of such silicone resin coating 
compositions are described in U.S. Pat. Nos. 4,277,287 and 4,278,804; and 
pending application Ser. No. 373,361. The effectiveness of ultrafiltration 
is dependent upon the pH of the coating composition. The coating 
compositions typically have a pH of from about 3.5 to about 8, preferably 
from about 7.1 to about 7.8. Those silicone resin coating compositions 
which have a pH above 7.0 have shown the greatest improvement upon 
ultrafiltration. 
The silicone resin coating composition are comprised of the hydrolysis 
product of an aqueous dispersion of colloidal silica and a trifunctional 
silane of the formula, R'Si(OR).sub.3 wherein --OR are the functional 
groups, R being an alkyl radical of from 1-8 carbon atoms and R' is a 
monovalent radical selected from the group consisting of alkyl radicals of 
from 1-3 carbon atoms and aryl radicals are from 6-10 carbon atoms. The 
quantity of colloidal silica which may be present in the silicone resin 
coating compositions known to the art typically range from 0.01% by weight 
to 35% by weight. The quantity of trifunctional silane typically ranges 
from 3% to 45% by weight. 
In general, the aqueous dispersion of colloidal silica utilized in the 
silicone resin coating compositions is characterized by a particle size of 
from 5-150 millimicrons, and preferably exhibit an average diameter of 
10-30 millimicrons. Such dispersions are known in the art. Commercially 
available materials include Ludox (Dupont) and Nalcoag (Nalco Chemical 
Co.). The effectiveness of ultrafiltration will vary depending on the type 
of colloidal silica present in the silicone resin coating compositions. 
Where the particles are of a large diameter, ultra filtration will improve 
the silicone resin coating composition; however, the improvement in 
compositions containing colloidal silica of smaller average diameters is 
more pronounced. The commercially available colloidal dispersions are in 
the form of acidic or basic hydrosols. The more common colloidal 
silicasols utilized have about 0.35% by weight Na.sub.2 O, which provides 
a slightly basic pH. 
The trifunctional silanes which are utilized as starting materials in 
obtaining the hydrolysis product include aryltrialkoxysilanes, 
alkyltriacetoxysilanes and alkyltrialkyloxysilanes, the functional groups 
being the alkoxy and acetoxy radicals. The silane triols, R'Si(OH).sub.3, 
are formed in situ as a result of admixing the corresponding trifunctional 
silanes with the aqueous dispersion of colloidal silica. Examples of the 
alkoxy functional groups on the trifunctional silanes include methoxy, 
ethoxy, isopropoxy, and n-butoxy which, upon hydrolysis, generate the 
silane triols and liberate the corresponding alcohol, e.g. methanol, 
ethanol, isopropanol and n-butanol, and the like. 
The alkyltriacetoxysilanes also generate silane triols but are principally 
employed to regulate the hydrolysis rate by producing acetic acid. An 
alternative to utilizing these trifunctional silanes to regulate the 
hydrolysis rate is to add glacial acetic acid or other acid to the 
composition. 
Upon generating the hydroxyl substituents of the silane triols, 
R'Si(OH).sub.3, a condensation reaction begins to form 
silicon-oxygen-silicon bonds. This condensation reaction, which takes 
place over a period of time, is not exhaustive. The siloxane produced 
retains a quantity of silicone-bonded hydroxyl groups which render the 
polymer soluble in the alcohol-water solvent mixture. This soluble partial 
condensate can be characterized as a siloxanol polymer having at least one 
silicone-bonded hydroxyl group for every three --SiO-- units. 
A major portion of the partial condensate is typically obtained from the 
condensation of CH.sub.3 Si(OH).sub.3 depending on the input of 
ingredients to the hydrolysis reaction. Minor amounts of the partial 
condensate can also be obtained from the condensation of C.sub.2 H.sub.5 
Si(OH).sub.3, C.sub.3 H.sub.7 Si(OH).sub.3 or C.sub.6 H.sub.5 
Si(OH).sub.3. For most silicone resin coating compositions, it is 
preferable to use only methyltrimethoxysilane, thus generating only 
mono-methylsilanetriol and ultimately the methyl-substituted partial 
condensate. 
The temperature of the hydrolysis reaction mixture is typically kept within 
the range between 20.degree. C. to 40.degree. C. A reaction time of about 
8 hours is sufficient to react enough of the trifunctional silane such 
that the initial two-phase liquid mixture is converted to a single liquid 
phase in which the silica is dispersed. Hydrolysis is usually permitted to 
continue for a period of 8-24 hours. As a rule, the longer the time 
permitted for hydrolysis, the higher the final viscosity. 
After hydrolysis has been completed, the solid content of the coating 
composition is typically adjusted by adding alcohol to the reaction 
mixture. Suitable alcohols include lower aliphatics having 1-6 carbon 
atoms, such as methanol, ethanol, propanol, isopropanol, and butylalcohol, 
tributylalcohol, and the like, or mixtures thereof. Isopropanol is 
typically preferred. The solvent system is a mixture of water and alcohol 
and typically contains from about 20-75% by weight of the alcohol to 
ensure that the partial condensate is soluble. 
After adjustment with solvent, the coating composition usually contains 
from 10-50% by weight solids and preferrably has about 20% by weight of 
total solids. Such compositions are ready for application to a substrate 
and may be ultrafiltered prior to application. 
Other ingredients may be present within the silicone resin coating 
composition without hindering the effectiveness of this process. One such 
ingredient is a buffered latent condensation catalyst. Examples include 
alkali metal salts of carboxylic acids, such as sodium acetate, potassium 
formate, etc.; amine carboxylates, such as dimethylamine acetate, 
ethanolamine acetate, dimethylanaline formate, etc; quaternary ammonium 
carboxylates, such as tetramethylammonium acetate, benzyltrimethylammonium 
acetate, etc.; metal carboxylates, such as tin octoate; amines such as 
triethylamine, triethanolamine, pyridine, etc; and alkali hydroxides, such 
as sodium hydroxide, ammonium hydroxide, etc. Other ingredients which may 
be present include ultraviolet light-absorbing agents such as those 
disclosed by Ashby et al., U.S. Pat. No. 4,278,804, Frye, U.S. Pat. No. 
4,299,746 and Anthony, Ser. No. 373,361. 
Suitable additives also include polysiloxane-polyether copolymers described 
by Frye in U.S. Pat. Nos. 4,308,315; 4,324,839 and 4,277,287. These 
copolymers control flow, flow marks, dirt marks, and the like on the 
coating surface. The preparation of this polysiloxane-polyether copolymer 
is described in U.S. Pat. No. 3,629,165, and incorporated herein by 
reference. Suitable commercially available materials are SF-1066 and 
SF-1141, from General Electric Company and Dow Corning's DC-190. 
Other ingredients, such as thickening agents, pigments, dyes, antioxidants, 
adhesion promoting compounds, and the like, can also be included for their 
conventionally employed purposes and ultrafiltration will still improve 
the resistance of the silicone resin coating compositions to degradation 
from stress, strain, moisture, heat and ultraviolet light. Suitable 
thickening agents are more particularly described by Vaughn Jr., U.S. Pat. 
No. 4,309,319; suitable adhesion promoting compounds are disclosed by 
Conroy, U.S. Pat. No. 4,311,763 and suitable antioxidants are described by 
Anthony in pending application Ser. No. 373,361. 
Upon ultrafiltration of the silicone coating compositions they can be 
applied to the surface of an article. Priming of the surface with a 
thermosetting acrylic prior to application of the filtered compositions is 
often preferred. Conventional methods can be used for coating the 
substrate with the filtered composites, such as spraying or dip coating to 
form a continuous film or layer. The cured filtered compositions are 
useful as protective coatings on a wide variety of surfaces, including 
plastic surfaces and metal surfaces. Examples of such plastics include 
synthetic organic polymer substrates, such as acrylic polymers, 
polyesters, polyamides, polyimides, acrylonitrile-styrene copolymers, 
styrene-acrylonitrile-butadieneterpolymers, polyvinyl chloride, butyrates, 
polyethylene, etc. 
Special mention is made of the polycarbonates, such as those polycarbonates 
known as Lexan.RTM. polycarbonate resin, available from General Electric 
Company. Ultrafiltration of coatings is especially useful when they are 
utilized on such articles. Other types of substrates include wood, 
leather, glass, ceramic, textiles, etc. 
The silicone resin coating is obtained by removing the solvent and other 
volatile materials from the filtered composition. The coating air-dries to 
a substantially tack free condition, but heating in the range of 
75.degree. C. to 200.degree. C. is necessary to obtain condensation of 
risidual silanols in the partial condensate. Final cure typically results 
in the formation of a silsesquioxane (R'SiO.sub.3/2). In the cured 
coating, the ratio of R'SiO.sub.3/2 to SiO.sub.2 is preferably equal to 2. 
The coating thickness can be varied, but in general, the coating will have 
a thickness in the range between 0.5 to 20 microns. 
In order that those skilled in the art will be better able to practice the 
invention, the following examples are given by way of illustration and not 
by way of limitation. 
EXPERIMENTAL 
A microcrackometer was used to evaluate the stress and strain resistance of 
silicone resin coatings. Samples of coated substrates were placed into the 
microcrackometer and were stretched. The stretched samples were observed 
for the formation of cracks. The strain applied to the samples at the 
initial formation of cracks was noted and compared to values obtained for 
other coated samples. The strain applied to the samples in the 
microcrackometer was equal to 
EQU .DELTA.L/L, 
where L is the length of the sample and .DELTA.L is the length the sample 
was stretched at the initial formation of cracks. 
The drawing illustrates a microcrackometer (1). The sample (2) is anchored 
to a moving gripping means (10) attached to sliding base (5) and a fixed 
gripping means (20) attached to base (25). The sliding base (5) slides on 
rods (15) by rotating an adjusting screw (30). Turning the adjusting screw 
(30) stretches the coated sample (2) by sliding the moving gripping means 
(10) away from base (40). The value .DELTA.L is obtained from the dial 
micrometer (50), which measures the distance the moving gripping means 
(10) is moved from base (40). 
Samples of coatings were prepared for testing in the following manner. A 4 
inch piece of 10 mil Lexan.RTM. polycarbonate resin film 12 inches long 
was flow coated at room temperature and allowed to air dry until tacky to 
the touch (approximately 4.5 minutes). The coated film was then cut into 
strips 1/2 inch wide and 12 inches long using an Ingenito paper cutter. 
This procedure was used in order to minimize the influence of boundary 
effects produced by the cutting. The strips were allowed to air dry for 
another 25 minutes and then cured in an air oven for 90 minutes at 
135.degree. C. The strips were mounted in the microcrackometer with the 
initial length preset at 10 inches. 
A QUV Q panel was utilized to evaluate the ultraviolet light humidity and 
thermal resistance of silicone resin coated Lexan.RTM. samples. The 
samples were exposed to 8 hours of ultra violet light at about 70.degree. 
C. and then allowed to cool for four hours in darkness to permit 
condensation. This cycle was repeated until cracks or flaws were observed. 
Samples were prepared for QUV testing by flow coating 4 by 4 inch 
Lexan.RTM. polycarbonate resin specimens which were 1/4 inch thick. After 
air drying for 30 minutes they were cured in an air oven for 90 minutes at 
130.degree. C. After cooling to room temperature, scribed and unscribed 
adhesion were checked. They were also visually inspected for flaws prior 
to placement in the QUV apparatus and then checked periodically for crack 
formation. 
EXAMPLE 1 
Methyltrimethoxysilane (20.3 gms), 0.06 grams of acetic acid, 16.7 grams of 
Ludox LS colloidal silica (30% colloid) were added to a reaction vessel. 
The two phase solution was stirred at a temperature within the range of 
about 20.degree.-30.degree. C. for 16 hours. Isobutanol (38 grams) was 
then introduced, followed by 0.6 grams of flow modifier SF-1066, described 
in the references cited above and sold by General Electric Company. In 
addition, 3.2 grams of a silylated hydroxybenzophenone UV stabilizer of 
the formula below was introduced. 
##STR1## 
The mixture was stirred at room temperature for 10 days before use. The 
composition was them applied to a Lexan.RTM. polycarbonate substrate and 
tested in the microcrackometer and QUV apparatus as described above. The 
results of these tests appear in Table I along with the measurement of 
other physical properties. This silicone resin composition was then 
pressure filtered utilizing a 0.5 micron Gelman filter, Model No. 4280. 
This coating solution was then placed on Lexan.RTM. polycarbonate resin 
substrates and analyized in the microcrackometer and QUV apparatus as 
described above. The results from the microcrackometer and QUV apparatus 
appear in Table I, along with measurements of other physical properties. 
EXAMPLE 2 
To a mixture of 20.3 grams methyltrimethoxysilane and 0.06 grams of acedic 
acid, 16.7 grams of Ludox LS colloidal silica (30% colloid) was added, the 
two phase solution was stirred at 20.degree. C. for 16 hours. Isobutanol 
(38 grams) was then introduced, followed by 0.6 grams of flow modifier SF 
1066 as described in Example I, and 3.2 grams of silylated 
hydroxybenzophenone UV stabilizer of the formula below. 
##STR2## 
The mixture was stirred in at room temperature for 10 days before use. The 
coating solution was applied to Lexan.RTM. polycarbonate resin substrates 
and tested in the microcrackometer and QUV apparatus as described above. 
The results of these tests and measurements of other physical properties 
appear in Table I. A portion of the coating solution was pressure filtered 
utilizing a 0.5 micron Gelman filter Model No. 4280. The filtered solution 
was applied to Lexan.RTM. polycarbonate resin substrate and tested in the 
micro crackometer and QUV apparatus as described above. These results and 
the measurements of other physical data appear in Table I. 
TABLE 1 
______________________________________ 
Material 
Particle Size (Microns) 
##STR3## QUV (hours) 
______________________________________ 
Example I 
1.3-1.5 1.0 950 
Example I 
0.30-0.35 1.9 1500 
(filtered) 
Example II 
4.5-5.0 0.5 700 
Example II 
0.56-0.65 1.4 1100 
(filtered) 
______________________________________ 
The above examples are directed to only a few of the silicon resin coating 
compositions which may be upgraded by ultrafiltration, the present 
invention further includes the upgrading of other silicone resin coating 
compositions. In addition, further modifications are possible by one 
skilled in the art without departing from the scope and spirit of this 
invention.