CA1046180A - Paint composition based on colloidal silica - Google Patents

Abstract

Abstract of Disclosure Pigments are incorporated in an acidic dispersion of colloidal silica and hydroxylated silsesquioxane in an alcohol-water medium to provide paint compositions.

Description

This lnvention relates to a novel paint composltlon. In one aspect, the lnventlon relates to a water-based palnt vehicle which can be plgmented with a varlety of plgments to obtain flame-resistant coatlngs.
In another aspect, the invention relates to high gloss enamel coatings.
Various paint vehicle systems are available.
Certain paint binders utilize aqueous alkali metal silicate solutions or solvent-soluble organosilicon polymers.
Because of their inorganic nature, the silicate-based paints are generally more flame resistant than coatings based on organic polymers but the silicate coatings are poor in their finish characteristics, often cracking and crazing upon weathering.
The paint compositions of the present invention contain a relatively high proportion of inorganic material in the vehicle resulting in ~lame resistance yet they exhibit good adhesion and weatherability because of the presence of a siloxane polymer. These paint compositions cure to provide an extremely hard, high-gloss finish which is stable at high temperatures.
Thus, it is an object of the present invention to provide a novel paint composition. It is another ob~ect of the present invention to provide coated substrates which exhibit improved flame resistance. It is a further ob~ect of the invention to provide a heat-resistant enameling composition. These and other ob~ects of the present invention will be apparent to one skilled in the art upon consideration of the following description and appended claims.

-1- i~

1046~80 The paint composition of the present invention ls characterlzed ln that it comprlses a pigment or blend of plgments carried in a vehicle consisting essentially of a dispersion of colloidal silica in a lower aliphatlc alcohol-water solution of the partial condensate of a silanol of the formula RSi(OH)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-tri- :
fluoropropyl radical, the gamma-glycidoxypropyl radical and the gamma-methacryloxypropyl radical, at least 70 weight percent of the silanol being CH3Si(OH)3, said vehicle containing 10 to 60 weight percent solids consisting essentially of 10 to 70 weight percent colloidal sllica and 30 to 90 weight percent of the partlal condensate, said vehicle containing sufficient acid to provide a pH in the range of 3.0 to 6.o. :-~
The paint vehicle is a solution dispersion containing from about 10 to 60 weight percent solids based on the weight of colloidal silica and partial condensate present in the vehicle. The major portion of the partial condensate is that of CH3Si(OH) 3 ~ a minor amount (30% or less) of the siloxanol being obtained from : :~
cocondensation with C2H5Si(OH)3, C3H7Si(OH)3, CH2=CHSi(OH)3, CH2=CHCCOO(CH2)3Si(OH) CH2-CHCH20(CH2)3Si(OH)3 and mixtures thereof. From both o the standpoint of economy and optimum properties in the cured paint, it is preferred to utilize all monomethyl-trisilanol in formulating the vehicle. As described in : . . ' ~ . . ' ' :

1046180 ~

the copending patent application entitled ''Pigment-Free Coating Compositions " , filed as Canadian Patent Application Serlal No. 208,621 flled September 6, 1974, the presence of at least 70 weight percent CH3SiO3/2 in the siloxane component of the vehicle provides improved hardness and abrasion resistance.
The trisilanols, RSi(OH)3 are generated in situ by adding the corresponding trialkoxysilanes to an acidic aqueous medium, preferably an acidic aqueous dispersion of colloidal -silica. Exemplary 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 paint vehicle. Upon generation of the silanol in the acidic aqueous medium, there ls condensation of the hydroxyl substltuents to form -Si-O-Si- bonding. The condensation takes place over a period of time and is not complete, but rather, the siloxane retains an appreciable quantity of silicon-bonded hydroxyl groups which render the polymer soluble in the alcohol-water cosolvent. This soluble partial condensate can be characterized as a siloxanol polymer having at least one silicon-bonded hydroxyl group per every three -SiOSl- units. During curing of the paint, these residual hydroxyls condense to provide a silsesquioxane, RSiO3/2.
The silica component of the composition is present as colloidal silica. Aqueous dispersions generally contain colloidal silica having a particle size in the range of 5 to 150 millimicrons in diameter. These silica hydrosols are prepared by methods well known in the art and are commercially available. It is preferred to use colloidal silica of 10 to 60 millimicron particle size in order to provide `:

greater shelf life in the paint vehicle. Colloidal silicas of this type are relatively free of Na20 and other metal preferably less than 1 weight percent Na2O. These sillcas are available as both acidic and basic hydrosols. Colloidal slllca ls to be dlstlngulshed from other water dispersible forms of sio2 ~ such as polysilicic acld or alkali metal slllcate solutlons, whlch are not operatlve ln the practice of the present lnventlon.
The silica ls dlspersed in a solutlon of the slloxanol ln a lower allphatlc alcohol-water cosolvent.
Suitable lower aliphatic alcohols are water miscible and lnclude methanol, ethanol, lsopropanol and t-butanol. Of course, mlxtures of such alcohols can be utllized.
Isopropanol is the preferred alcohol and when mlxtures of alcohol are used, lt ls preferred that at least 50 welght percent of isopropanol be present ln such mlxture.
The solvent system should contain from about 20 to 75 welght percent alcohol to ensure solublllty of the siloxanol.
Optionally mlnor amounts (no more than 20 weight percent) of a water-miscible polar solvent, such as acetone, 2-butoxy ethanol and the like, can be present in the cosolvent system To provide shel~ life and minimize gellation in the paint vehicle, sufficient water-miscible organic acid to provide a pH of from 3.0 to 6.o must be present.
Suitable acids include both organic and inorganic acids such as hydrochloric, acetic, chloroacetic, citric, benzoic, dimethylmalonic, formlc, glutaric, glycolic, malelc, malonic, toluene-sulfonlc, oxallc 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 to:Luenesulfonic acid, give appreciably shortened shelf or bath life and re~uire less ageing to obtain the described soluble partial condensate. It is preferred to add sufficient water-misclble 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 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% Na20) amount of alkali metal or metal oxide.
The vehicle is easily prepared by adding trialkoxysilanes, such as RSi(OCHJ) 3~ to colloidal silica hydrosols and ad~ustlng 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 obtaln the desired pH will vary with 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 silicon-bonded alkoxy substituents, for example, hydrolysis of one mole of -Si(OC2H~)3 generates three moles of ethanol. Depending upon the percent solids desired in the final vehicle composition, additional alcohol, water or a water-miscible solvent can be added. The paint vehicle should be well mixed and allowed to age for a short period of time to ensure formation of the partial condensate. The composition thus obtained is a clear or slightly hazy low viscosity dispersion 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 composltion can be extended by maintaining the dlsperslon at below room temperature, for example, at 5C.
Buffered latent condensation catalysts can be added to the vehicle so that milder curing conditions can be utilized to obtain the optimum properties 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 other classes 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 shelf life of the vehicle composition, but upon heating the catalyst 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 which reacts with the organic acid during the ad~ustment of pH to generate the carboxylate catalyst in situ. This is particularly true when starting with a hydrosol having a pH of 8 or 9. The paint vehicle 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, .

104~i180 but at about 1.5 weight percent catalyst in the vehicle, the shelf life is shortened and physical propertles of the coating may be lmpaired. It is preferred to utilize ~rom about 0.05 to 1 welght percent of the catalyst.
To provide the greatest stabillty ln the dispersion whlle obtalnlng optimum properties ln the cured palnt, lt ls preferred to utillze a vehlcle composltion havlng a pH in the range of ~-5 which contains 25-40 weight percent solids; the silica portion having a particle size in the range of 5-60 millimicrons; the partial condensate of CH3Si(OH)3 belng present in an amount ln 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 conslstlng of sodlum acetate and benzyltrimethylammonlum acetate belng present ln an amount ln the range of 0.05 to 0.5 welght percent of the composltion.
Such a vehicle ls relatlvely stable, having a shelf llfe of approximately one month. When pigmented and coated onto a substrate, the paint can be cured in a relatlvely short tlme at temperatures in the range of 75-125C.
Various pigments can be incorporated in the above described paint vehicle. The term pigment as used -herein means all types of organic and particulate inorganic coloring materlals and lncludes fillers or extenders for pigments.
Suitable pigments include alkali-earth carbonates such as chalk, magnesium carbonate, dolomite, precipitated calcium carbonate and barium carbonate, talcum, alumina hydrates, zinc oxide, magnesium oxide, fluorite, basic lead carbonate, white and colored pigments based on titanium dioxide, iron 10~18~ , ' oxide red, iron oxide black, manganese black, carbons, barium yello~, strontium chromate, calcium chromate, zinc yellow, zinc green, cadmium yellow, cadmium red, cadmium vermilion, cadmopone, vermillon, ultramarine, lead chromate, chromlum yellow, molybdate red, molybdate orange, chromoxide green, chromoxidehydrate green, manganese violet, manganese blue, cobalt blue, cobalt green, cobalt violet, naples yellow, and organic pigments of the azo series.
Ceramic and vitreous frits can also be used in the pigment portion of the paint composition to obtain porcelain~like coatings. Powdered metals, such as zinc dust, aluminum flakes, bronze powder and the like, form another class of pigments suitable for use in the present invention. Glass beads can be incorporated in the pigments to provide a reflective coating such as use on highway signs and pavement markings. The pigments available in the art.
A detailed listing of both organic and inorganic pigments can be found in The Encyclopedia of Chemistry, Clark and Hawley, Reinhold Publishing Corp., New York (1966) beginning at page 833.
The paint compositions of the invention can contain standard additives, such as thickening agents, for example, sodium salts of acrylic and polymethacrylic acid, carboxy-methylcellulose and methylcellulose; mildew-cides, antifoam agents, corrosion inhibitors and the like.
The amount of pigment in the paint composition can vary widely such as from l to 200 parts by weight pigment per lO0 parts by weight of the described vehicle. Most applications for metal require from 20 to 150 parts pigment per l-00 parts vehicle. The pigments can be mixed on a ball mill or other paint mixing devices.

~0461~0 Once the pigment has been incorporated in the paint vehicle, the composition has a relatively short shelf llSe.
For opt;lmum coatlng propertles, the palnt composltlon should be u~ecl wlthln 24 hour~ of incorporatlon of the pigment.
The paints can be applied uslng conventional technlques~
such as flow coatlng, dip coating, roller coating, spraying or brushing. The coatings can be applied to a variety of solld substrates, for example, metal, glass, ceramlc materials, wood, plastics, asphalt, concrete, and the like. Application of the paint composition to flammable substrates such as wood and foamed plastics, for example, foamed polystyrene, is of special interest. If desired, the substrate can be pretreated or primed with conventional paint primers to improve adhesion. The thickness of the paint film i8 a function of the solids content, vlscoslty of the compositlon and the method of appllcation. Generally a paint film of -1 to 25 or 30 mils thickness is obtained by appllcatlon of the paint composltlons of the invention. The pigmented coatings will air dry to a tack-free condition in one to two hours and to a usable hardness in about 24 hours.
Further curing and complete condensation of the siloxanol component of the paint vehicle is obtained by heating the coatings at a temperature of from 75 to 125C. The cured coating comprises pigment uniformly incorporated in a binder matrix of colloidal silica (SiOz) and RSiO3/2.
The following examples are illustrative and not intended as limiting the invention delineated in the claims.
Example 1 A paint vehicle, containing 36 weight percent solids, half of which was SiO2, the other half of which ' ' ' , 1046~80 was siloxanol calculated on the basis of CH3SiO3/2 weight, was formulated by adding 68 grams of CH,Si(OCH3)3 to 100 grams of an acidic aqueous disperslon of 13~14 mllllmicron colloldal silica containing 30% sio2. The colloidal dlspersion was a commercially available material (pH of 9.8, Na2O content of 0.32%) which had been acidified by sufficient. glacial acetic acid to provide a pH of 4.8.
After mixing for about one hour, TiO2 (36 grams) was incorporated in the dispersion of colloidal silica in the solution of CH3Si(OH) 3 partial condensate in the methanol-water cosolvent.
The pigment was incorporated by grinding in a Waring Blender until a homogeneous paint composition was obtained.
After one hour, the paint composition was spray coated onto 7.62 cm. x 22.86 cm. panels of phosphatized steel. Flnal film thlckness was ln the range of 1 to 2 mlls. The paint coating air dried to a tack-free condition ~;
in two hours glving a high gloss white enamel. After aging for 24 hours, the paint composition did not give a satisfactory coating on steel.
The painted panels were tested to determine checking, hardness, adhesion, color and gloss. After 24 hours at room temperature, the coating exhibited no cracking or checking, had a pencil hardness of greater than 9H, gave 100% crosscut adhesion, had a color rating of 10 (no dis-coloration) with a Gardner 60C gloss (% reflectance) of 87. Abrasion of the coating with steel wool had no visible effect. The coating retained these properties, except for a slight reduction in gloss, after being heat-aged at 315C.
for 100 hours.

Since a quantitative determination could not be obtained by the pencil hardness, the air-dried paint descrlbed above was submitted to the Sward Hardness test.
This test ls described ln Gardner and Sward, Paint Testlng Manual, 13th Ed., 1972. The coating exhibited a Sward hardness of 88 whlch ls to be compared to a Sward hardness of 100 for plate glass. Most commercially avallable epoxy or sllicone paints have a Sward hardness ln the range of 30 to 50.
These data demonstrate that the paint composltlons of the lnventlon can be used to provlde heat-reslstant enamel coatings having hardness approaching that of porcelain.
Such a paint would be especially useful as a coating on appliances and the exterior of cookware.
Example 2 The paint vehicle of Example 1 was plgmented by addition of 42.3 grams of mica (1000 mesh) and 21.9 grams of TiO2. After thorough mixing, the paint composition was sprayed on panels of foamed polystyrene (1.9 cm. skin board measuring 10.2 cm. x 60.96 cm.) and allowed to air dry for 24 hours. A fllm thlckness of approximately 2-3 mils was obtained.
A second paint vehicle was formulated by addlng sufficient acetlc acld to an aqueous dispersion of 50-70 millicron colloidal silica (50% solids, pH of 8.5, Na20 content of about 0.2%) to provide a pH of 4.8. Methyltri-methoxysilane (10.8 grams) was added to 24.5 grams of the acidified colloidal dispersion which was further diluted with 16 grams of water. The components of the vehicle were mixed for about one hour to provide a methanol-water solution-dispersion containlng 50% solids, 20 welght percent / - :

of which was colloidal SiO2, the remainder being siloxanol as weight percent CH3SiO3/2. This vehicle was mixed with 43.7 ~rams of mica (1000 mesh) and 21.9 grams TiO2 to provide a second paint composition containing 49 parts pigment per lOO parts vehicle. This paint composition was also applied to panels of foamed polystyrene and cured in the manner described above.
The flammability of the coatings was determined ~
by subjecting the painted panels to a " Two-~oot Tunnel -Test " as described in The Journal of Paint Technology, Vol. 44, No. 575, December 1972 at p. 64. Both paints were completely nonburning and remained reasonably intact even after the coatings became red hot at the point of flame contact. During the test, the polystyrene substrate melted and shrunk away from the paint. No burning occured until a hole or crack formed in the coating and gases could escape. The gases did ignite but were insufficient to provide any reproducable flame spread data.
The two paint compositions were applied to aluminum (Alodized) panels to provide a 4 mil thick coating. After curing at room temperature for 48 hours, the cured paints exhibited 90-95% adhesion to aluminum as -determined by the crosscut tape test.
Example 3 Black iron oxide pigment was mixed with a paint vehicle prepared in accordance with Example l.
Sixty grams of the pigment were ball milled with lll grams of the vehicle for about four hours. The composition ~as then thinned with a sufficient amount of a 50-50 mixture of isopropanol-water to provide a paint containing 40 weight percent of the pigment. The paint was coated onto the outer surface of o.64 cm. diameter stainless steel tubing. After air drying, the coatlng was cured at 100C. for 2 hours.
The palnted surfaces were exposed to sunlight and water passed through the tubing was rapidly heated.
This demonstrates use of the paints of the invention as high heat transfer coatings as applied to surfaces for the collection of solar energy.
Example 4 The paint composition prepared in Example 2 containing the 13-14 millimicron colloidal silica was applled to rigid polyurethane which had been cast in the form of insulators. The paint was air dried. There was no visible effect on the coating after the insulators were used for 60 days in an environment that lncluded elevated temperatures and chlorine vapors.
Example 5 ~ -Acetic acid (5 lbs.) was diluted with 15 lbs.
of water and was added with stirring over a 30 minute period to 180 lbs. of the 30% colloidal silica dispersion described in Example 1. This addition was carried out in an open-top 100 gallon mixing tank equipped with an explosion-proof stirrer. Methyltrimethoxysilane (150 lbs.) was added during a 30 minute period to the acidified colloidal silica. Maximum temperature was about 40C during this addition. Hydrolysis of the methoxysilane was considered complete after an additional 30 minutes stirring. A
blend of mica (-325 mesh) and TiO2 were then mixed with the paint vehicle. A total of 100 lbs. of mica and 1046~80 50 lbs. of ''low-chalk'' TlOz were added. A small amount (3 oz.) of hydroxymethylcellulose, a thickening agent, was subsequently added to the plgmented composltlon.
At thls stage of the formulation process, a five gallon quantity of the paint was removed from the tank and 4.5 lbs.
of zinc dust was rapidly dispersed in the remainder.
Within eight hours of formulation, the 5 gallons of zinc-free paint were sprayed onto approximately 400 sq. ft. of cement roof tile. Another 2000 sq. ft. of ln-place cement roof tile were sprayed wlth 20 gallons of the zinc-containing paint. The hiding power and coverage of both paints was very good. After 24 hours drying, the coatings were resistant to attack by dilute aqueous hydrochloric acid. Roof tiles with a similar coating have shown no change after six months exposure in southern Florida.
Example 6 A pigment mixture, 3.18 grams of TiO 2 and 9.53 grams of china clay, was blended with 42 grams of the paint vehicle of Example 1 by ball milling. This pigmented composition was used to coat roofing granules to provide both color and resistance to asphalt staining.
The paint was poured over 100 grams of uncoated roofing granules. After a sufficient amount to completely wet the particles had been added, the wetted particles were stirred with a spatula while blowing hot air over the surfaces to prevent agglomeration. The dried coated particles were then further cured at 100C for 20 minutes to provide white reflective roofing granules.
3o 104~i180 An asphalt substrate was prepared by placing 15 grams of roofing asp~lalt on a circular metal panel of about 15 sq. in. area. After about 15 minutes in a 205C. oven, the asphalt was melted evenly over the metal surface. The asphalt was allowed to cool and 18 grams of coated granules were sprinkled over the surface. For purposes of comparison, commercially available roofing granules (ceramic-coated mineral aggregate) were adhered to an asphalt substrate in the same manner. To provide an even coating of granules ~he panels were inverted and placed in an 80C oven for one hour. After cooling, the panels were returned to the 80C oven for 4 days to determine the degree of asphalt staining. There was no visible staining of the roofing granules coated with the palnt compositlon of the invention while there was sllght staining of the commercially available granules.
Reasonable modification and variation are within the scope of the present invention which is directed to novel paint compositions and articles coated `
with such compositions.

Claims (10)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A paint composition comprising a pigment carried in a vehicle consisting essentially of a dispersion of colloidal silica in a lower aliphatic alcohol-water solution of the partial condensate of a silanol of the formula Rsi(OH)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 radoca; and the gamma-methacryloxy-propyl radical, at least 70 weight percent of the silanol being CH3Si(OH)3, said vehicle containing 10 to 60 weight percent solids consisting essentially of 10 to 70 weight percent colloidal silica and 30 to 90 weight percent of the partial condensate, said vehicle containing sufficient acid to provide a pH in the range of 3.0 to 6Ø

2. A paint composition in accordance with claim 1 wherein the acid is a water-miscible organic acid selected from the group consisting of acetic acid, formic acid, propanoic acid and maleic acid.

3. A paint composition in accordance with claim 2 in which the alcohol in the alcohol-water cosolvent s present in an amount in the range of 20 to 75 weight percent based on the total weight of the cosolvent.

4. A paint composition in accordance with claim 3 in which at least 50 weight percent of the alcohol is isopropanol.

5. A paint composition in accordance with claim 2 including from 0.05 to 1.5 weight percent, based on the weight of the vehicle, of a buffered latent silanol condensation catalyst.

6. A paint composition in accordance with claim 5 wherein the catalyst is an alkali metal salt of a carboxylic acid or a quaternary ammonium salt of a carboxylic acid.

7. A paint composition in accordance with claim 6 wherein the catalyst is sodium acetate.

8. A paint composition in accordance with claim 1 wherein the vehicle contains 25 to 40 weight percent solids, the colloidal silica is of a 50 to 60 millimicron-diameter particle size, the partial condensate of CH3Si(OH)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, said alcohols being present in amount in the range of 30 to 60 weight percent of the cosolvent.

9. A point composition in accordance with claim l wherein the pigment is present in an amount in the range of 20 to 150 parts by weight per 100 parts of said vehicle.

10. A paint composition in accordance with claim 9 wherein the pigment comprises titanium dioxide or a mixture of titanium dioxide and mica.