GB2068989A - Low solids content silicone emulsions methods of preparing them and substrates coated therewith - Google Patents

Abstract

A silicone emulsion which provides an elastomeric product upon removal of water under ambient conditions comprises (A) 100 parts by weight of an anionically stabilized, hydroxyl endblocked polydiorganosiloxane, present as an oil-in-water emulsion, (B) from 1 to 150 parts by weight of amorphous silica, (C) from 0 to 200 parts by weight of filler other than amorphous silica, and (D) from 0.1 to 1.5 parts by weight of alkyl tin salt, and has a pH of 9 or greater and a solids content of less than 40 percent by weight. It is obtained by (I) emulsifying the diorganosiloxane using an anionic surfactant in water, (II) adding the amorphous silica, any other filler, the alkyl tin salt, and from 0 to 3 parts by weight of an organic amine composed of carbon, hydrogen, and nitrogen atoms, or carbon, hydrogen, nitrogen, and oxygen atoms, said amine being soluble in the amount of water present in the emulsion, and (III) adjusting the pH of the resulting emulsion to 9 or greater, the amount of water in (I) being such that the solids content of the final silicone emulsion is 40 percent by weight or less. Substrates may be treated with these emulsions.

Description

SPECIFICATION Low solids content silicone emulsions methods of preparing them and substrates coated therewith This invention relates to a low solids content silicone emulsion which provides an elastomeric product and to methods of preparing said emulsions.

Emulsions of organopolysiloxanes, such as described by Hyde et al. in U.S. Patent No.

2,891,920 issued in 1959, have been known for many years. Hyde et al. considered one of the problems in the organosilicon field to be the lack of a good method for preparing stable emulsions of extremely high molecular weight siloxanes having a molecular weight at least high enough to make suitable protective coatings. Although Hyde et al. described a concept for preparing, on a commercial scale, emulsion of siloxane suitable for protective coating, there are few products available today in the field of emulsions of siloxanes for protective coatings. The primary contribution of Hyde et al. appears to have been a method of polymerizing siloxanes in the emulsified state with strong mineral acids or strong alkaline catalysts which are characterized by their ability to rearrange siloxane bonds.Hyde et al. disclose that the emulsion polymerization can be carried out with cationic, nonionic or anionic type dispersing agents, and that it is best to use anionic emulsifying agents for acid catalysts, cationic emulsifying agents for alkaline catalysts and nonionic emulsifying agents for either acid or alkaline catalysts. The emulsions prepared by Hyde et al. are described as extremely stable in that they can stand for years without separation, and can also be centrifuged or diluted without separation. Hyde et al. also teach that the emulsion can be changed from one type of surfactant system to another after or during polymerization, such as from cationic to anionic or nonionic or vice versa, and that anionic systems provide better surface wetting. Hyde et al.

describe their emulsions as useful for release agents and coating compositions. The emulsions are also described as particularly adaptable for the preparation of latex paints. For example, they can be mixed with pigment or other filler and applied to a surface where the water will evaporate, leaving a continuous coating. Although Hyde et al.

teach stability and continuous coating formation, they do not teach that stable emulsions can be obtained when additional materials, such as the pigments, are added and that continuous coatings can be obtained after storage or what the properties of such coatings will be.

Findlay et al. in U.S. Patent No. 3,294,725 describe a method for polymerizing organosiloxanes in emulsion using a surface active sulfonic acid catalyst. Findlay et al. describe this method as a method for preparing stable organopolysiloxane latex emulsions. According to Findlay et al., the resulting emulsion can be neutralized to a pH near 7 with an alkaline material unless it is desired to remove the polysiloxane from the emulsion. Findlay et al.

teach that alkali metal salts of the surface active sulfonic catalyst are useful when additional emulsifying agent is desired, particularly where silica fillers are to be used. The emulsions described by Findlay et al. are taught as having the same stability expressed by Hyde et al. and also as having the same utility. Findlay et al., however, teach that the emulsions, either with or without the added filler, are quite stable, and for maximum stability of the emulsion, it is desirable to neutralize the acid catalyst in the emulsion with a base to a pH of approximately 7. The neutralization of the acid catalyst can be done either prior to or after the addition of filler. Such systems are taught by Findlay et al. as providing an excellent method of obtaining coatings of tough rubbery siloxane films for release coatings.

Findlay et al. describe tough rubbery films obtained from colloidal silica and neutralized emulsions made from polymerized hydroxylated dimethylpolysiloxane reacted with a trialkoxysilane such as methyltrimethoxysilane before emulsification. Findlay et al. do not show the use of fillers with emulsions other than those prepared in this manner with a trialkoxysilane. In one example, Findlay et al. added a colloidal silica sol which had a pH of 8.5.

Although Findlay et al. made stable emulsions of organosiloxanes, they apparently did not provide a silicone latex which would be stable on storage and from which a curable polymer could be deposited and which cured to give a tough elastomer, because Cekada in U.S. Patent No.

3,355,406 stated that there was still a need in the silicone industry for such a silicone latex. Cekada described his latexes as useful for many applications including various coating applications. The silicone latexes described by Cekada are prepared from a colloidal suspension of a curable essentially linear silicone polymer in water using a suitable dispersing agent. To the polymer in the colloidal state, a silsesquioxane in the form of a colloidal suspension, preferably neutral, is added. Cross-linking agents and curing catalysts can be added. Those silicone latexes, according to Cekada, which contain no curing catalyst, can be cured by exposing the deposited coating to a suitable radiation source. Cekada teaches that the curing catalysts can be mineral acids, strong bases, dialkyltin diacylates and organic and inorganic peroxides.The cross-linking agents taught by Cekada include alkoxysilanes and methylhydrogenpolysiloxanes. Although Cekada alleges a silicone latex which is stable on storage and from which a curable polymer can be deposited and cured to a tough elastomer, he offered no storage properties for his composition.

Cekada shows neutral latexes, except for one example in which the latex was acidic. In the Cekada invention, a neutral emulsion is not stable on storage and does not cure to an elastomer after storage.

A caulk prepared from cationic emulsions of siloxane block copolymers is described by Butler et al. in U.S. Patent No. 3,817,894. Butler et al.

teach that six ingredients are required to prepare the caulk and that the emulsion of siloxane block polymer is neutralized to a pH of 7 for use in the caulk and that the caulk contains from 20 to 30 parts by weight of cationic surfactant per 300 to 600 parts by weight siloxane copolymer.

It is known in the silicone art that siloxane bonds rearrange in the presence of alkaline materials, particularly alkali metal hydroxides.

Such siloxane bond rearrangement can be very useful in silicone manufacturing for the polymerization of polydiorganosiloxanes from cyclic polydiorganosiloxanes under carefully controlled conditions. However, it is also known that polydiorganosiloxanes under basic conditions in the presence of water can be converted to very low molecular weight species, including -monomeric species. It was therefore unexpected that a stable emulsion could be prepared at high pH, stored and still produce an elastomeric coating or film after storage. Under conditions of a pH greater than 9, it would be expected that depolymerization would occur and result in water soluble species which would not give an elastomeric product when the water was removed. As taught by the prior art cited above, the curable compositions containing filler contain a trifunctional material to act as a cross-linking site.

A silicone emulsion having a pH of 9 or higher and a solids content of less than 40 percent by weight comprising an anionically stabilized, hydroxyl endblocked polydiorganosiloxane aqueous emulsion, amorphous silica, an alkyl tin salt and, optionally, filler other than amorphous silica and an organic amine, produces an elastomeric product when water is removed under ambient conditions. These silicone emulsions can be prepared by emulsifying a hydroxylated polydiorganosiloxane in water using an anionic surfactant, adding amorphous silica and alkyl tin salt, and adjusting the pH to 9 or greater. These silicone emulsions can be converted to a cured elastomer by removal of the water under ambient conditions to produce useful coatings on substrates.

This invention relates to a silicone emulsion suitable to provide an elastomeric product upon removal of water under ambient conditions comprising (A) 100 parts by weight of an anionically stabilized, hydroxyl endblocked polydiorganosiloxane, present as an oil-in water emulsion, (B) from 1 to 1 50 parts by weight of amorphous silica, (C) from 0 to 200 parts by weight of filler other than amorphous silica, and (D) from 0.1 to 1.5 parts by weight of alkyl tin salt, said silicone emulsion having a pH of 9 or greater and a solids content of less than 40 percent by weight.

The combination of anionically stabilized hydroxyl endblocked polydiorganosiloxane emulsion and amorphous silica in an emulsion with a pH of 9 or greater can produce a cured elastomeric film upon removal of the water under ambient conditions without the presence of a monoorganotrifunctional silane or siloxane as required in the past. The alkyl tin salt also used in the composition of this invention imparts practical manufacturing procedures in and storage stability to the composition.

The silicone emulsions of this invention are fluid to thixotropic paint-like materials with a solids content by weight of 40 percent or less.

These emulsions provide a cured elastomeric film merely by evaporation of the water under ambient conditions. Such hydroxyl endblocked polydiorganosiloxanes should have a weight average molecular weight (Mw) of at least 5,000.

Hydroxyl endblocked polydiorganosiloxanes with the lower Mw range, such as 5,000 to 10,000, do not provide strong elastomeric products, but are useful for certain coating applications. Tensile strengths and elongations at break improve with increasing molecular weight. Reasonable tensile strengths and elongations are obtained above 30,000 Mw and the best tensile strengths and elongations are obtained above 50,000 Mw. The maximum weight average molecular weight for the hydroxyl endblocked polydiorganosiloxane is one at which it can be emulsified and which will give elastomeric properties to the product obtained after the water is removed from the emulsion. Weight average molecular weights up to about 1,000,000 for the hydroxyl endblocked polydiorganosiloxane are expected to be practical for this invention.The preferred Mw values for the hydroxyl endblocked polydiorganosiloxanes are in the range of 200,000 to 700,000.

The organic radicals of the hydroxyl endblocked polydiorganosiloxane can be monovalent hydrocarbon radicals containing less than seven carbon atoms per radical and 2 (perfluoroalkyl)ethyl radicals containing less than seven carbon atoms per radical. Examples of monovalent hydrocarbon radicals include methyl, ethyl, propyl, butyl, isopropyl, pentyl, hexyl, vinyl, cyclohexyl and phenyl and examples of 2 (perfluoroalkyl)ethyl radicals include 3,3,3trifluoropropyl and 2-(perfluorobutyl)ethyl. The hydroxyl endblocked polydiorganosiloxanes preferably contain organic radicals in which at least 50 percent are methyl. The preferred hydroxyl endblocked polydiorganosiloxanes are the hydroxyl endblocked polydimethylsiloxanes.

The most preferred hydroxyl endblocked polydiorganosiloxanes are those prepared by the method of anionic emulsion polymerization described by Findlay et al. in U.S. Patent No.

3,294,725 which shows the method of polymerization and the hydroxyl endblocked polydiorganosiloxane in emulsion. Another method of preparing hydroxyl endblocked polydiorganosiloxane is described by Hyde et al. in U.S. Patent No. 2,891,920 which shows the hydroxyl endblocked polydiorganosiloxanes and their method of preparation. These methods and others are known in the art. The hydroxyl endblocked polydiorganosiloxanes of this invention are those which are anionically stabilized. For the purpose of this invention "anionically stabilized" means the hydroxyl endblocked polydiorganosiloxane is stabilized in emulsion with an anionic surfactant. The emulsion is in the form of an oil-in-water emulsion.

Anionic surfactants are preferably the salt of the surface active sulfonic acids used in the emulsion polymerization to form the hydroxyl endblocked polydiorganosiloxane as shown in U.S.

Patent No. 3,294,725 cited above which shows the surface active sulfonic acids and salts thereof.

The alkali metal salts of the sulfonic acids are preferred, particularly the sodium salts. The sulfonic acid can be illustrated by aliphatically substituted benzenesulfonic acids, aliphatically substituted naphthalene sulfonic acids, aliphatic sulfonic acids, silylalkylsulfonic acids and aliphatically substituted diphenylethersulfonic acids.

One of the advantages of the present invention is the relatively small amount of surfactant or emulsifying agent needed to maintain a stable emulsion. The amount of anionic emulsifying agent may be less than 2 weight percent of the emulsion, wherein this amount may result from the neutralized sulfonic acid wherein the sulfonic acid is used in the emulsion polymerization method for the preparation of the hydroxyl endblocked polydiorganosiloxane.Other anionic emulsifying agents can be used, for example, alkali metal sulforicinoleates, sulfonated glyceryl esters of fatty acids, salts of sulfonated monovalent alcohol esters, amides of aminosulfonic acids such as the sodium salt of oleyl methyl tauride, sulfonated aromatic hydrocarbon alkali salts such as sodium alpha-naphthalene monosulfonate, condensation products of naphthalene sulfonic acids with formaldehyde, and sulfates such as ammonium lauryl sulfate, triethanol amine lauryl sulfate and sodium lauryl ether sulfate.

Although not specifically required in the present invention, one can optionally include nonionic emulsifying agents in addition to the anionic emulsifying agents. Such nonionic emulsifying agents can be illustrated by saponins, condensation products or fatty acids with ethylene oxide such as dodecyl ether of tetraethylene oxide, condensation products of ethylene oxide and sorbitan trioleate, condensation products of phenolic compounds having side chains with ethylene oxide such as condensation products of ethylene oxide with isododecylphenol, and imine derivatives such as polymerized ethylene imine.

Amorphous silica is a required ingredient in this invention. The silicone emulsion does not yield a cured film upon drying if amorphous silicas is not present in the composition. Any of the finely divided amorphous silica that are capable of being dispersed in the silicone emulsion can be used.

The common forms of amorphous silica are colloidal silicas available as colloidal silica dispersions in water and as dry powders of fume silica or precipitated silica and the mined amorphous silicas that are known commercially as diatomaceous earth. It is believed that any amorphous silica in a sufficiently finely divided form is useful in this invention.

One of the embodiments of this invention is a paintlike emulsion that has shown usefulness as a coating for glass fiber fabric. A colloidal silica available as a dispersion in water has been shown to be particularly useful in this embodiment. These commercially available colloidal silica dispersions are normally used in a stabilized form, being stabilized with sodium ion, ammonia, or aluminum ion. Aqueous colloidal silica dispersions that have been stabilized with sodium ion are particularly useful for this embodiment of this invention because the pH requirement of this invention can be aided by using such a sodium ion stabilized colloidal silica to bring the pH above 9. Colloidal silicas as used herein are those silicas which have particle diameters of from 0.0001 to 0.1 micrometre. Preferably, the particle diameters of the colloidal silicas are from 0.001 to 0.05 micrometre.

The silicone emulsion of this invention has a continuous water phase in which there are dispersed phases which comprise an anionically stabilized hydroxyl endblocked polydiorganosiloxane and amorphous silica with an alkyl tin salt also present. For this silicone emulsion to maintain a storage stability and also be curable to an elastomer after the emulsion is stored, the pH of the silicone emulsion must be 9 or greater. The silicone emulsions of this invention which have the best storage stability and still form elastomers at ambient conditions at any point during the storage period are those which have a pH in the range of 10.5 to 11.5.

These silicone emulsions which contain in the dispersed phase the hydroxyl endblocked polydiorganosiloxane, amorphous silica, and alkyl tin salt and have a pH of 9 or greater do not require additional ingredients to obtain an elastomeric product after the water is removed at ambient conditions. However, certain additional ingredients have been found useful in providing certain advantageous characteristics to the silicone emulsion and the elastomeric products obtained therefrom. For example, a thickener can be added to give improved handling characteristics in the silicone emulsion, such as thixotropy and structural viscosity. The thickener is useful for increasing the working viscosity of the silicone emulsion to provide a material which can be used to coat a substrate with a film of elastomeric product.Such silicone emulsions with thickener permit the application of thicker coats which form thicker elastomeric films. The use of a thickener also permits a broader versatility of the silicone emulsion by allowing one to select the proper and most convenient emulsion consistency for the specific application. Suitable thickeners are available commercially and would be selected for their stability and usability, at pH of 9 or greater.

Some of the useful thickeners include the classes of cellulose derivatives, alkali salts of polyacrylates and polymethylacrylates, sodium and ammonium salts of carboxylate copolymers and colloidal clays. These and other thickeners can be used, but it is advised that a particular thickener be tried on a small scale to determine that it does not adversely effect the storage stability of the emulsion, the formation of the elastomeric product or the resulting properties of the elastomeric product. For the silicone emulsions of this invention, the best thickeners are the sodium salts of polyacrylates.

Another useful ingredient for addition to the silicone emulsions of this invention is a filler other than amorphous silica. Such fillers can be added to provide pigmentation which can be used, for example, as a colorant as in a paint or an ultraviolet light screening agent. Other fillers can be used as extending fillers which can be used to reduce the cost per unit of the elastomeric product. Examples of some fillers other than amorphous silica include carbon blacks, titanium dioxide, clays, aluminum oxide, quartz, calcium carbonate, zinc oxide, mica and various colorant pigments. Titanium dioxide has been found to be particularly useful as an ultraviolet light screening agent.These fillers, other than amorphous silica, should be finely divided and it may be advantageous to use aqueous dispersions of such fillers if they are commercially obtainable such as aqueous dispersions of carbon black. However, the silicone emulsions of the present invention do not require that these fillers be added in the form of aqueous dispersions. The silicone emulsion readily accepts the finely divided fillers in a dry form.

The silicone emulsion of this invention requires a solids content by weight of less than 40 percent.

For purposes of this invention, the solids content is defined as the nonvolatile content of an emulsion.

The nonvolatile content is determined by placing 29 of emulsion in an aluminum weighing dish of 50 mm diameter and heating in an air circulating oven for 1 hour at 1 500 C. After cooling, the dish is reweighed and the percentage of the original 2g remaining is determined. This percent remaining is the percent solids present in the original emulsion.

The minimum solids content that is useful will depend upon the viscosity of the polydiorganosiloxane, the viscosity of the silicone emulsion, and the ratios of ingredients used as these will all affect the manner in which the coating reacts during the removal of the water.

The substrate upon which the silicone emulsion is placed will also have an effect. Silicone emulsions of 5 to 10 percent solids have been used for coating paper to produce a water repellent coating. If too low a solids content is used on a particular substrate, the drying film will not form a continuous film, but will rupture due to shrinkage.

The minimum useful solids content can be determined by simple experimentation with a particular substrate and silicone emulsion formulation.

When the emulsion of this invention is dried, shrinkage takes place. In order to maintain a continuous coating on a substrate, it is necessary to maintain the shrinkage below an amount that causes the coating to rupture during the drying step. The rupture of the film is less of a problem with thin films than with thick films. Thicker films need to be dried at a slower rate than thin films.

There are also factors to consider concerning the ratio of hydroxyl endblocked polydiorganosiloxane to amorphous silica to extending filler. The curing mechanism of the instant invention requires that from 1 to 1 50 parts by weight of amorphous silica be present per 100 parts by weight of the hydroxyl endblocked polydiorganosiloxane. The useful upper limit for the amount of amorphous silica will normally be determined by the modulus of eiasticity of the cured elastomer produced when the emulsion is dried. Increasing the amorphous silica content increases the modulus of the resulting elastomer.

The modulus is also effected by the physical form of the amorphous silica. The more surface area or the finer the particles, the less required to give a particular level of modulus.

Filler other than amorphous silica is used to raise the total solids content of the emulsion while affecting the modulus to a much lesser degree than the amorphous silica. The amount of these extending fillers used depends upon the type of extending filler chosen, the degree of fineness of the particles, and the properties desired in the final cured elastomer. The optimum amount can be easily determined by simple experimentation. The extending fillers are generally considered to have particle diameters in the range of about 1 to 30 micrometres or surface areas of less than 50m2/g.

The ratio of hydroxyl endblocked polydiorganosiloxane to reinforcing amorphous silica to extending filler also affects the modulus of elasticity. As the amount of filler is raised in relation to the amount of polymer, the modulus rises. At too high a filler loading the cured product will not be sufficiently elastomeric to function properly.

The dispersed phases require hydroxyl endblocked polydiorganosiloxane and amorphous silica. In view of the required pH range of the silicone emulsion, the hydroxyl endblocked polydiorganosiloxane need not contain exciusively silicon-bonded hydroxyl radicals. Some of the hydrogen atoms of the silicon-bonded hydroxyl radicals may be replaced with an alkali metal ion, such as sodium ion; may be complexed with an amine, or may be associated with an emulsifying agent. Thus, the term "hydroxyl endblocked polydiorganosiloxane" as used herein covers all the species of terminating groups which may be formed by emulsifying a hydroxyl endblocked polydiorganosiloxane at a pH of 9 or greater.

The best method of preparing the silicone emulsions is to emulsify a hydroxyl endblocked polydiorganosiloxane using an anionic surfactant, add the amorphous silica, and then adjust the pH within the range of 10.5 to 11.5 inclusive. One of the best methods for emulsifying a hydroxyl endblocked polydiorganosiloxane is to prepare this siloxane polymer by emulsion polymerization as described in U.S. Patent No. 3,294,725, starting with polydiorganocyclosiloxanes. This emulsion polymerization uses an anionic polymerization catalyst and thus the resulting hydroxyl endblocked polydiorganosiloxane contains an anionic surfactant and thus it is ready to be used to make the silicone emulsions of this invention.

There are other methods of emulsifying a hydroxyl endblocked polydiorganosiloxane using an anionic surfactant, such as described in U.S. Patent No.

2,891,920. Although these other methods can be used to emulsify a hydroxyl endblocked polydiorganosiloxane to provide an anionically stabilized siloxane polymer, they are less convenient inasmuch as additional steps are involved, as well as additional ingredients. The concentration of the hydroxyl endblocked polydiorganosiloxane in the anionically stabilized emulsion is not critical; however, it must be high enough to provide a suitable concentration of dispersed phase in the final silicone emulsion when mixed with the other ingredients.

The amorphous silica can be added to the anionically stabilized hydroxyl endblocked polydiorganosiloxane in the form of a dry powder or as an aqueous dispersion. The best method to add colloidal silica is in the form of a sodium ion stabilized aqueous dispersion of collodial silica.

There are many such sodium ion stabilized aqueous dispersions of colloidal silica which are commercially available. These commercial colloidal silicas are usually available in aqueous dispersion having from 1 5 to 50 weight percent colloidal silica and having a pH in the range of 8.5 to 10.5. The best method to add fume silica is by simply stirring it into the hydroxyl endblocked polydiorganosiloxane emulsion.

After the amorphous silica is added, the pH is adjusted to 9 or greater. Silicone emulsions, as described herein, are not storage stable or do not form an elastomeric product over the entire storage period if the pH is adjusted below 9. The resulting silicone emulsion does not provide a useful elastomeric product when the water is allowed to evaporate at ambient conditions immediately after the emulsion is prepared. It has been found that the addition of an alkyl tin salt, preferably a dialkyltindicarboxylate, can be used to reduce the storage time between the preparation of the silicone emulsion and the time an elastomeric product can be obtained from the silicone emulsion by removal of the water under ambient conditions to an acceptable range of one to three days. Such storage times are well within the time required to package and distribute a commercial product.Dialkyltin salts can be used in amounts of from 0.1 to 1.5 parts by weight for each 100 parts by weight of the hydroxyl endblocked polydiorganosiloxane, preferably about 0.1 to 1.0 parts by weight for each 100 parts by weight of hydroxyl endblocked polydiorganosiloxane. Dialkyltincarboxylates, including dibutyltindiacetate, dibutyltindilaurate and dioctyltindilaurate are preferred. The preferreddialkyltindicarboxylate is dioctyltindilaurate.

Dibutyltindibromide has also been found to be operable.

The pH of the silicone emulsion prepared as described herein can be adjusted within the defined range by any of a number of methods, such as with a basic compound or an ion exchange means, such as an ion exchange resin.

The best methods have-been found to be with a basic compound, such as an organic amine, an alkali metal hydroxide or a combination thereof.

The organic amines can be primary, secondary or tertiary amines which contain carbon, hydrogen and nitrogen, and can also contain oxygen, and which are water soluble in the amounts required.

These organic amines include diethylamine, ethylenediamine, butylamine, hexylamine, morpholine, monoethanolamine, triethylamine and triethanolamine. The preferred organic amine for maximum storage stability is diethylamine. The alkali metal hydroxides include sodium hydroxide, potassium hydroxide, lithium hydroxide, rubidium hydroxide and cesium hydroxide. The preferred alkali metal hydroxide is sodium hydroxide. The organic amine can be added neat or in aqueous solution. The alkali metal hydroxides are preferably added as an aqueous solution. A combination of diethylamine and sodium hydroxide has been found to be particularly suitable to provide long term storage stability for these silicone emulsions, maintaining the useful elastomeric forming ability, and maintaining useful elastomeric properties in the product obtained after removal of the water at ambient conditions.

The useful upper pH level is determined by practical considerations. The higher the pH, the more corrosive the silicone emulsion becomes so there should not be an excess of basic compound added. When the pH is above 12, amorphous silica present tends to be dissolved. The system tends to change in pH with time, adjusting to a range of from 10.5 to 11.5.

With the proper selection of anionically stabilized hydroxyl endblocked polydiorganosiloxane and amorphous silica, the mixing of these two ingredients can automatically adjust the pH within the required range and an additional step of adjusting the pH is not needed.

Thus, the mixing of the siloxane and amorphous silica can encompass the step of adjusting the pH.

The selection of a hydroxyl endblocked polydiorganosiloxane which has a pH of at least 9 and an aqueous dispersion of a colloidal silica which has a pH of at least 9 can provide a silicone emulsion within the scope of this invention without further necessity of adjusting the pH by adding additional ingredients. An aqueous dispersion of a colloidal silica which is sodium ion stabilized is preferably used as the colloidal silica with a pH above 9. It is not necessary that both the siloxane and colloidal silica have a pH above 9, but the resulting combination would be required to have a pH greater than 9; if not, adjusting the pH would be required as stated above. To obtain the preferred pH range of 10.5 to 11.5, it will usually require adjusting the pH after the siloxane polymer and colloidal silica are mixed.For the purpose of this invention, the term "pH" means the electrical potential measured on commercially available glass electrodes designed for this purpose when the glass electrode is immersed in the emulsion. The electrical potential is read from a scale on a commercial instrument in terms of the negative logaO of hydrogen ion activity. The electrode is calibrated with a standard buffer solution which gives a pH of 10.

In the method of preparing the silicone emulsion as described above, frothing can be encountered. It is therefore advantageous to add an antifoaming agent to control such frothing. A preferred class of antifoams are those based on silicones which are available commercially.

The silicone emulsions of this invention are particularly suitable for coatings applied to a substrate such as wood or masonry. The coating is elastomeric, forms at temperatures as low as 40C, does not evolve organic solvents which can be irritating to the user as well as environmentally unsuitable and is tough enough to protect the substrate. The coating can be subjected to weathering for long periods without faiiing. The coating is useful at high temperatures, such as at least 1 500 C, and at low temperatures, such as at least -260C. The coating is sufficiently strong and elastic that if the substrate develops cracks, the film can bridge the crack and still protect the substrate sufficiently to present a waterproof surface.

The silicone emulsions of this invention can be useful for coating fabrics, films and papers. When applied to such a substrate, the substrate is rendered water-repellent.

When heavier coatings are applied to fabrics, the fabric openings can be covered over and the fabric made waterproof. Such coated fabric is useful as the covering of a greenhouse. The coated fabric is waterproof, weatherable, flexible at high and low temperatures, and can be transparent to sunlight, if opaque fillers are not added.

The weatherability and flexibility of the cured elastomer resulting from the drying of the silicone emulsion of this invention makes the silicone emulsion useful for binding soil particles together to reduce water penetration and wind erosion.

After an initial coat to bind soil particles together, additional coats can be applied to form a weatherable, waterproof coating suitable for canal or drainage ditch liners, or as the lining of a water storage pond.

Some additional advantages of these silicone emulsions are that relatively small amounts of emulsifying agents are required to maintain stability and, as a result, the elastomeric product is not loaded with large amounts of unreacted ingredients such as the emulsifying agent which can evolve from the elastomeric product, such as blooming, or reduce the strength of the elastomeric product. The elastomeric product forms without curing catalysts or the application of heat or radiation. It was unexpected that a silicone emulsion could produce under ambient conditions an elastomeric product with strong elastic properties from a medium which had such a high pH and still be storage stable over periods of up to one year or more. Practical silicone emulsions of this invention would be storage stable for at least six months at ambient temperatures.

These silicone emulsions can form elastomeric products by removal of the water under ambient conditions. When the silicone emulsion is spread out to form a coating, the water evaporates to leave a cured silicone elastomer. Silicone emulsion coatings can skin over in about 1 5 minutes becoming tack free in about one hour and obtaining substantial physical properties in one day and maximum properties in a few days. The curing characteristics can take place in shorter time periods depending upon the film thickness and method of application. It is also expected that heating the silicone emulsions can produce the elastomeric products. It should be understood that this invention is not limited to removal by evaporation of water; other methods of coagulation may be useful.

The following examples are presented for purposes of illustrating the invention and should not be construed as limiting the scope of the invention which is properly delineated in the claims.

EXAMPLE 1 An anionically stabilized emulsion polymerized polydimethylsiloxane was first prepared containing about 58 percent by weight of an emulsion polymerized hydroxyl endblocked polydimethylsiloxane having a weight average molecular weight of about 325,000 (hereinafter referred to as Polymer Emulsion A). The aqueous emulsion of the polydimethylsiloxane was anionically stabilized with the sodium salt of dodecylbenzene sulfonic acid which was present in an amount of about one percent based on the weight of the emulsion.

A reinforced silicone emulsion was prepared by mixing 100 parts by weight of an aqueous sodium stabilized colloidal silica dispersion having about 1 5 percent by weight silica with 2 parts by weight diethylamine. Then 1 67 parts by weight Polymer Emulsion A, 0.3 part by weight of an antifoam emulsion, and 1 part by weight of a tin emulsion containing 50 weight percent dioctyltindilaurate, 9 weight percent of a sodium alkylarylpolyether sulfonate and 41 percent water (Tin Emulsion A) were mixed until uniform. Then 10 parts by weight of an acrylic thickening agent was mixed in until a uniform mixture resulted. This reinforced silicone emulsion had a viscosity of about 25 Pa s at 230C, a pH of about 11, and a solids content of 39 percent by weight.

A piece of loose weave glass fabric cloth was placed on a sheet of polyethylene coated paper. A portion of the reinforced silicone emulsion described above was poured over the cloth and uniformly spread with a draw bar. The coating was allowed to air dry. Two more coats were applied in a similar manner. After the final coat had dried, the coated fabric was approximately 0.25 millimeters thick and translucent in appearance.

The sheet of coated fabric was placed in a weatherometer for 1000 hours where it was exposed to a cycle of carbon arc light and periodic water spray. After the 1000 hours of exposure, the fabric was still translucent and elastomeric.

EXAMPLE 2 A polydimethylsiloxane emulsion was prepared by homogenizing 54 parts by weight of hydroxyl endblocked polydimethylsiloxane having a viscosity of about 0.075 Pas at 259C, 44 parts by weight of water, and 2.5 parts by weight of a 30 percent aqueous solution of sodium lauryl sulphate. After homogenization, the emulsion was catalyzed with 0.6 part by weight of dodecylbenzene sulfonic acid. Polymerization was then allowed to proceed to equilibrium at room temperature. The emulsion had a pH of less than 3 with a solids content of about 55 percent by weight. The polydimethylsiloxane had a peak molecular weight of about 350,000.

A reinforced silicone emulsion was prepared by mixing together 1 00g of a sodium stabilized colloidal silica dispersion containing about 1 5 percent by weight colloidal silica with a pH of about 10.7 with 29 of diethylamine. Then 200g of the above described polydimethylsiloxane emulsion were stirred in. The mixture was catalyzed with 1 g of Tin Emulsion A. The final emulsion had 39 percent by weight solids and a pH of greater than 9.

About 2009 of the above reinforced silicone emulsion was sprayed over the surface of a 0.2 by 0.3 metre pan full of ordinary sand. After drying for 2 days, the top 6 to 10 mm of sand were found to be bound together into a firm, but flexible layer.

EXAMPLE 3 A reinforced silicone emulsion was prepared similar to that of Example 1 except that the acrylic thickening agent was not used. This emulsion was then diluted with water to a 10 percent by weight solids emulsion. Three different thicknesses of glass fiber fabric were dipped into the emulsion, allowed to drain, and then air dried for 2 hours.

Half of the samples of each thickness of fabric were then cured for 1 5 minutes at 1 500C.

All of the precoated samples of glass fabric were then dipped into a 70 percent by weight solvent solution of a methoxy-functional polysiloxane resin which had been formulated to give a flexible film possessing heat resistance and weatherability. The samples were cured for 1 hour at 1000C.

Additional samples were prepared as above without the emulsion precoat for use as controls.

Each of the coated samples was then folded over and creased. The samples which had been precoated with the reinforced silicone emulsion did not show the effect of the creasing to as great a degree as did the samples that were not precoated.

EXAMPLE 4 A. A sample of open weave glass fiber fabric was coated with the reinforced emulsion of Example 3 by dipping and air drying.

B. A sample of the fabric was coated with the reinforced emulsion of Example 1 by laying the fabric on a piece of polyethylene coated paper, then spreading the reinforced emulsion over the fabric with a draw bar. The coating was air-dried, then a second coat was applied and air-dried. The coated fabric had a smooth surface on both sides.

C. A reinforced emulsion was prepared as in Example 1 but using a different acrylic thickener.

A sample of fabric was then prepared using this reinforced emulsion in the same manner as in B.

Each of the coated fabric samples was then evaluated for near-normal/hemispherical spectral transmittance in accordance with ASTM E-424-7 1, Method A. The results obtained were: Sample %Solar Transmittance A 78.6 B 75.4 C 72.8 Such coated fabrics would be useful in applications where the passage of sunlight through the coated fabric is desirable.

EXAMPLE 5 A reinforced silicone emulsion was prepared by mixing 100 parts of a 1 5 percent by weight sodium stabilized colloidal silica emulsion with 232 parts of an emulsion containing approximately 43 percent by weight emulsion polymerized polydimethylsiloxane, then adding 2 parts of Tin Emulsion A to give a reinforced silicone emulsion of approximately 35 percent by weight solids, and a pH greater than 9.

A piece of glass fiber fabric was cleaned in water, then in acetone, and dried. The cleaned fabric was dipped into the above reinforced emulsion, drained, and air-dried. The cured coated fabric was flexible with the coating well adhered to the fabric. The weave of the cured coated fabric was sealed, passing a hydrostatic water test.

Claims (13)

1. A silicone emulsion suitable to provide an elastomeric product upon removal of water under ambient conditions comprising A) 100 parts by weight of an anionically stabilized, hydroxyl endblocked polydiorganosiloxane, present as an oil-in-water emulsion, B) from 1 to 1 50 parts by weight of amorphous silica, C) from 0 to 200 parts by weight of filler other than amorphous silica, and D) from 0.1 to 1.5 parts by weight of alkyl tin salt, said silicone emulsion having a pH of 9 or greater and a solids content of less than 40 percent by weight.

2. The silicone emulsion in accordance with claim 1 in which the polydiorganosiloxane is polydimethylorganosiloxane.

3. The silicone emulsion in accordance with claim 2 in which there is also present more than 1 part by weight of organic amine composed of carbon, hydrogen, and nitrogen atoms or carbon, hydrogen, nitrogen, and oxygen atoms, said amine being soluble in the amount of water present in the emulsion.

4. The silicone emulsion in accordance with claim 3 in which (B) is from 1 to 50 parts by weight, (C) is from 0 to 50 parts by weight, (D) is from 0.1 to 1.0 part by weight of a dialkyl tin salt, and the amine is selected from the group consisting of diethylamine, monoethanolamine, and morpholine.

5. The silicone emulsion in accordance with claim 4 in which the polydimethylsiloxane of (A) had a weight average molecular weight in the range of 200,000 to 700,000; (B) is in the form of a sodium stabilized colloidal silica dispersion; (C) is selected from the group consisting of calcium carbonate, titanium dioxide, alumina, ground quartz, and clay; and (D) is from 0.25 to 1.0 part by weight of dialkyl tin dicarboxylate.

6. The silicone emulsion in accordance with claim 5 in which there is also present a thickener suitable for use in an aqueous emulsion at a pH of 9 and greater.

7. A method of preparing a silicone emulsion suitable to provide an elastomeric product upon removal of the water under ambient conditions comprising (I) emulsifying a diorganosiloxane using an anionic surfactant and water to yield a hydroxyl endblocked polydiorganosiloxane containing emulsion, (II) adding from 1 to 1 50 parts by weight of amorphous silica, from 0 to 200 parts by weight of filler other than amorphous silica, from 0.1 to 1.5 parts of alkyl tin salt, from 0 to 3 parts by weight of an organic amine composed of carbon, hydrogen, and nitrogen atoms, or carbon, hydrogen, nitrogen, and oxygen atoms, said amine being soluble in the amount of water present in the emulsion, and (III) adjusting the pH of the resulting emulsion to 9 or greater, the amount of water in (I) being restricted in that the solids content of the final silicone emulsion must be 40 percent by weight orless.

8. The method in accordance with claim 7 in which (I) is obtained by preparing an emulsion of a.

hydroxyl endblocked polydiorganosiloxane by anionic emulsion polymerization of a siloxane selected from the group consisting of polydiorganosiloxane cyclics, hydroxyl endblocked polydiorganosiloxane having a viscosity no greater than 0.2 Pa-s at 250C, and mixtures thereof; the amorphous silica of (II) has an average particle diameter of 0.0001 to 0.1 micrometre; the alkyl tin salt is a dialkyl tin carboxylate; and step (III) is accomplished by the addition of the organic amine defined in step (II).

9. The method in accordance with claim 7 in which step (III) is accomplished by the addition of an alkali metal hydroxide.

10. The method in accordance with claim 7 in which step (III) is accomplished by the addition of sodium hydroxide.

11. The silicon elastomer product obtained by removing the water from the silicone emulsion of claim 1.

12. A method of coating a substrate comprising (t) applying a continuous film of the silicone emulsion of claim 1 to a substrate, and (II) removing the water from the emulsion.

13. A coated substrate produced by the method of claim 12.