Paint Thickener

The invention relates to organophilic clay complexes capable of suspending materials and imparting thixotropic properties to aqueous coating systems such as latex paints and caulks.

Rheological agents are added to thicken aqueous systems and to produce thixotropic flow characteristics for proper brush, roller or spray applications. Prior art thickening agents for aqueous systems possessed various defects which are overcome by compositions and processes within the scope of the present invention.

These thickening agents included many organic polymers such as hydroxyethyl cellulose, carboxymethyl cellulose, quar gum or acid containing polyacrylates, partially hydrolyzed polyvinyl acetate, polyvinyl pyrolidone etc. In the last decade, numerous water soluble synthetic polymers known as associative thickeners have been introduced to coating composition for controlling application properties. These thickeners are polymers with water soluble backbone containing long chain hydrophobic pendants. U.S. Pat. Nos. 4,077,028 and 4,426,485 describe thickeners of such composition.

However, there are various problems associated with the use of these thickeners. In one form the prior art teaches a high molecular weight hydroxyethyl cellulose used as the thickening agent for latex paints. Hydroxyethyl cellulose is a solid metal material which must be dissolved before addition to the coating system which adds additional cost. Cellulose derivatives are known to have high "low shear viscosity" and low "high shear viscosity"; the fast viscosity recovery causes poor flow and leveling in paint application. In addition the elastic property of cellulose derivatives gives severe roller spatter. Moreover, cellulose derivatives are subject to microbial degradation and thus requiring the addition of preservatives. Preservatives are not only expensive, they are also a cause of enviromental concern.

A second prior process teaches the use of acid containing polyacrylates as aqueous thickener. This type of thickener is pH dependent, so before the agent will become sufficiently thickened to suspend the mixture, the pH must be carefully adjusted to the basic range.

The third prior process teaches hydrophobicly modified polyurethane or acrylic polymers used as thickening agents. Both the polyacrylates and hydrophobicly modified polymers give improved flow, leveling and roller spatter; however they have poor sag control and poor brush pickup. They are also very expensive.

Finlayson, et al, U.S. Pat. No. 4,412,018 teaches an organophilic clay gellant reacted with a quaternary ammonium, phosphonium or sulfonium compound consisting of alkyl chains and aralkyl chains or mixtures thereof; with each carbon chain consisting of 1 to 22 carbon atoms attached to the specific central element chosen for the cationic compound. However, the use of the aforementioned process is limited to non-aqueous systems, contrary to the present invention.

Mardis et al U.S. Pat. No. 4,391,637 teaches an organophilic clay reacted with a quaternary ammonium compound containing a first chain consisting of a beta, gamma, unsaturated alkyl group or a hydroxalkyl group having 2 to 6 carbon atoms; a second member containing a long chain alkyl consisting of 12 to 60 carbon atoms; and a third and fourth member, each consisting of aralkyl and alkyl or combination thereof containing 1 to 22 carbon atoms for use in non-aqueous systems.

Dohman, et. al. U.S. Pat. No. 3,298,849 describes the use of alkanolamine salt modified clay in the formulation of aqueous base paint to increase hydration rate and control rheological properties.

No prior art method or composition is known utilizing organoclay made by a polyether substituted quaternary ammonium compound as a rheological additive in aqueous paint systems.

The processes and compositions within the scope of the present invention have been unexpectedly found to produce agents which can be used to thicken aqueous coating systems, for example, latex paints and caulks.

Further, the new processes and products have many advantages over other prior art methods and compositions in that they are not subject to microbial degradation that has been found to occur in the previously described cellulose systems.

Further, components within the scope of the present invention give products in slurry form which can be easily dispersed at low shear, for example, during the "let down" stage of latex paint preparation. Processes within the scope of the present invention are not pH sensitive as are many of the other prior art thickeners, thus, eliminating the pH adjustment needed in the prior art procedures.

Briefly the present invention provides a process whereby a smectite clay having a cationic exchange capacity of at least 50 meq./100 gm of clay is reacted with a quaternary ammonium compound to yield a cationic structure suitable as a thickener for aqueous suspensions particularly latex paints and caulks. The general structure of the quaternary ammonium compound is typically a nitrogen atom bonded to four separated carbon chains, one chain being a methyl or alkyl group containing 10 to 22 carbons and the second chain an alkyl group containing from 10 to 22 carbons or a polyoxyethylene chain where the third and fourth chains are polyoxyethylene chains were the total number of ethylene oxide units is from 5 to 200 moles.

In general, in accordance with the present invention, it has been found that certain organoclays can be utilized to modify the rheological characteristics of aqueous coating systems where the organoclays are formed by reaction of a smectite clay with a quaternary ammonium compound represented as follows: ##STR1## Where R.sub.1 is methyl or a C.sub.10 to C.sub.22 carbon chain, R.sub.2 is C.sub.10 -C.sub.22 carbon chain or polyoxyethylene chain, (CH.sub.2 -CH.sub.2 O).sub.Z H, with z reapeating unit where x+y+z=5 to 200. X.sup.- is a suitable anion, for example bromine, sulfate, acetate, chloride etc. The quaternary ammonium compound has been found to be effective when added to the clay at 30 to 130 meq. wt./100 gm clay. The organophilic clay can be prepared by admixing the clay, quaternary ammonium compound and water together, preferably at a temperature within the range of from 40.degree. C.-95.degree. C. and more preferably from 60.degree. C. to 90.degree. C. 140.degree. F. (60.degree. C.) to 170.degree. F. (77.degree. C.) for a period of time sufficient for the organic quaternary ammonium compound to react with the clay particles.

Preferably, the clay is dispersed in water at a concentration from about 5% to 12% by weight and, the slurry is centrifuged to remove non-clay impurities. The slurry is then agitated and heated to a temperature in the range of from 60.degree. C. to 90.degree. C.; and the quaternary ammonium salt added in the desired milliequivalent ratio, preferably as a liquid in isopropanol or dispersed in water. The agitation is continued to effect the reaction.

The amount of the quaternary ammonium compound added to the clay for purposes of this invention must be sufficient to impart to the clay the enhanced dispersion characteristics desired. The milliequivalent ratio is defined as the number of milliequivalents of the quaternary ammonium compound, per 100 grams of clay, 100% active basis. The organophilic clays of this invention have a milliequivalent ratios of from 10 to 150. At lower milliequivalent ratios the organophilic clays will cause pigment flocculation. At higher milliequivalent ratios, the organophilic clays are poor thickeners. However, the preferred milliequivalent ratio within the range of from 30 to 100 will vary depending on the characteristics of the quaternary ammonium compound and the aqueous system to be used. The organophilic clay thickener is employed in amounts sufficient to obtain the desired rheological properties for application as to control sagging of fluid films and prevent settling and hard packing of pigments present in the fluid compositions. Amounts of the organophilic clay thickener employed in a typical latex paint are from 5 lb. to 15 lb./100 gal. depending on the particular formulation.

The resulting organophilic clay is then added to the paint system to provide thickening as described hereinafter.

The following examples are given to illustrate the invention, but are not deemed to be limiting thereof.

One thousand grams of a 6.3% slurry of Wyoming bentonite in water which had been previously treated by centrifugation to remove all non-clay impurities was heated to about 75.degree. C. To the clay slurry, 56.6 gm of methyl, coco-di(polyoxyethylene) quaternary ammonium chloride with an activity of 1.001 meq. wt./gm. was added under mild agitation. The mixture was stirred for 60 min while maintaining the Temperature at 80.degree. C. After cooling the viscosity, solids content and pH are determined. The organophilic clay product contains 90 meq. wt. of quaternary ammonium compound/100 gm clay.

Examples 2 to 5 were prepared according to the procedures of Example 1, but with different quaternary ammonium compounds at different milliequivalents added as shown in table 1. The organophilic clays prepared above were then tested as thickeners in latex paint formulations.

To illustrate the effectiveness of one type of composition within the scope of this invention, various paint formulations (Formulation I to IV) were made and the thickening efficiency and application properties compared with commercially available thickeners. The results are given in Tables 2 to 5.

In the preparation of latex paints, a master batch was made according to the formulation excluding the thickener solution, then divided into small portions. To each portion, the calculated amount of thickener solution was added at low shear. The resulted paints were equilibrated and the properties determined.

A. Sag and leveling were done on leneta antisag bar and leneta leveling bar on leneta form 7B.

b Spatter resistance was measured by roller application of 70 g paint on 4 ft./4 ft. pressed board, 20 strokes over an area of 10 in./12 in. The spatter pattern is collected on a black cardboard paper. The result are rated numerically, the higher the number the better the spatter resistance.

c. Contrast ratio is determined by ACS computer system on a film of 3.0 wet mil. thickness. The higher the contrast ratio, the better the hiding power.

Paint Formulation I was used as an example, where Table II gives the application properties. This example demonstrates that organoclays of this invention can be made to equal the thickening efficiency of associative thickener.

In paint Formulation II, the thickening efficiency of organoclay from Experiment I is equivalent to Acrysol RM-5 whereas organoclay from Experiment II is equivalent to QR-708 but somewhat less effective than HEC (ER-4400). It is generally accepted that associative thickeners offer excellent flow out properties but are relatively poor in sag control. It is shown here that the combined use of associative thickener QR-708 and organoclay can improve paint flow out and in the meantime, maintain proper sag control.

Formulation III is a vinyl acrylic latex flat paint. In this formulation, organoclay from Experiment IV is much more efficient than Acrysol RM-5 but somewhat less efficient than other associative thickeners and HEC as shown in Table IV. When used alone, organoclay gives excellent sag control, leveling property is better than HEC, but not as good as associative thickeners. Compared to HEC, organoclay improves spatter resistance and gives good hiding. However, it should be mentioned that the hiding power of organoclay is superior to that of associative thickeners. The advantages of the combined use of organoclay and associative thickeners are further illustrated in this formulationexcellent sag control, improved spatter resistance and leveling while maintaining higher hiding power.

Formulation IV and Table V give additional data to demonstrate the usefulness of organoclay as a rheological additive. The formulation is different from formulation II, yet similar application properties were obtained and the synergistic effects of organoclay with associative thickeners were again observed.

The present invention has been described in some detail by way of examples, it is understood that certain changes and modifications may be practiced within the scope of the invention, and such variations are not to be regarded as departure from the scope of the invention and all such modifications are intended to be included within the scope of the following claims.