US2180145A - Flatting agent and uses therefor - Google Patents

Description

Patented Nov. 14, 1939 PATENT OFFICE mums. AGENT AND uses 'rrmnnron Charles Gordon Harford, Wollaston, Mass., as-

- signor to Monsanto Chemical Company, St.

Louis, Mo., a corporation of Delaware No Drawing. Application January 3, 1938, Serial No. 183,135

r 12 This invention relates to fiat liquid coating compositions and more particularly it has reference to an adjuvant for varnishes, lacquers. and the like which will dull the appearance of the coating prepared therefrom, and methods of those interested early began seeking for someway of dulling the objectionably bright and harshly reflective surface that is the natural result of drying varnish.

The first major advance was development of the so-called fiat varnish containing a component which imparted a dull sheen to the final coating without rubbing. In general the adjuvants that impart flatness to the varnish are substances probably having an index of refraction approximating that of the varnish film. The transparency of the varnish is not appreciably affected by satisfactory fiatting agents, but the dry surface has a soft glowing luster that is similar to the appearance of varnish which has been rubbed with pumice stone and oil by a skilled artisan. Known flattening agents include abestine, clay magnesia, magnesium car- -bonate, wax, and heavy metal soaps such as aluminum stearate and zinc tungate. ,Such substances are generally quite useful in drying oil varnishes, but considerable care must be used in making up the varnish. The fiatting agent and varnish are usually ground together for rather long periods on a paint mill, often requiring eight hours or more for the grinding operation..

With the introduction of nitrocellulose lacquers, the fiatting problem was again raised, but upon finding that the substances which cause varnishes to be fiat have a like effect on lacquers, it was believed that fiatting of lacquers was relatively simple. To the dismay of lacquer producers, known fiatting agents were found to be highly unsatisfactorily for nitrocellulose coating compositions.

The inorganic transparent pigment type of fiatting agent, necessarily present in small amount, showed a marked tendency to cake and settle in the lacquer vehicle. The resulting agglomerates blurred the film, and inorganic fiatting agents were practically eliminated from consideration, leaving wax and insoluble soaps as the only practical fiatting agents for lacquer. 5

It was then found that lacquers fiatted with wax were not of practical commercial value. The wax flat is a very clear, hard lacquer, but the wax tends to break up the continuity of the film, making the composition too short. Over 10 shellac the wax fiatted lacquer is actually brittle. This objection, together with the lack of uniformity of raw material, difliculty of grinding, and other manufacturing difliculties has resulted inabandonment of wax as a fiatting agent 15 for lacquer. Manufacturers have been forced to use the insoluble metal soaps as the only commercially feasible fiatting agent known for nitrocellulose lacquers.

These metallic soaps produce stable suspen- 20v sions of fiatting agent in the lacquer vehicle, and the films of soap fiatted lacquer are clear and of a dull luster. For a time, it seemed that the problem of making fiat lacquers was solved. The effect of sunlight on such lacquers was not 25 anticipated, and since that effect requires a period of two or three years to develop, the market was well stocked with lacquers containing metallic soaps whenserious difficulties in connection with such compositions were pre- 30 sented to the manufacturers.

Soaps tend to break down under the action of ultra-violet light, and the decomposition products are markedly hygroscopic. The soap, which acts as a water-proofing component in the ori,,- inal lacquer, actually absorbs water from the air after decomposition, and the lacquer film becomes fogged. In extreme casesthe surface film turns to anopaque white, and the manufacturer is faced with the alternative of sufiering a loss of reputation or adjusting a claim of considerable size as compared to his price for the original sale of lacquer.

Immediate steps were taken in the attempt to produce a waterproof soap. The soaps of wood oil acids were tested extensively with poor re-' suits. Generally, the films obtained with metallic tungates and the like were obscured by a blue haziness that could be overcome only with great ditficulty.

Diatomaceous earth is used as a fiattingagent for lacquers and varnishes, but I have found that larger quantities are required, very substantially longer grinding is necessary and, whereas with aerogeis one obtains a clear film,

diatomaceous earth tends to produce a cloudy or milky film. It has been suggested that a metallic tungate of great purity is superior to previously known substances as a flatting agent for nitrocellulose lacquers. As is well known, tung oil is a material of complex composition, and it follows that purification to any considerable degree is a diflicult and expensive process. After formation of the metallic tungate, it has been found necessary to carry the flatting compound through intricate purification steps in order to positively remove metal salts and other impurities as they may cause serious trouble if not eliminated.

It is seen that prior to this invention, the art did not know of a fiatting agent capable of general application which could be prepared and incorporated into the finished coating composition with no more than moderate cost.

It has now been discovered that finely divided gels which retain substantially the original structure in the coating composition are excellent fiatting agents for various types of coating compositions. Such. materials may be incorporated in the liquid coating material by replacement of the swelling agent or original continuous phase, or they may be converted to aerogels and ground into the liquid on a paint mill in the well known manner.

A primary object of this invention is the provision of a fiat coating composition.

Another object of the invention is to provide a. stable fiatting agent of improved characteristics.

The invention further provides the definition of a class of substances which are fiatting agents of general application and from which may be chosen a particular adjuvant particularly suitable as a fiatting agent for any given liquid coating composition.

In general it may be stated that the original gel structure can be maintained unimpaired if the original continuous phase is displaced without the formation of interfaces in the pores of the gel. The surface tension at such interfaces is now believed to exert sufiicient force to collapse the gel to a greater or less extent depending on the nature of the interface and the density of the gel.

One manner in which the flatting agents of the present invention may be incorporated in the coating composition is by replacement of the original continuous phase of a gel with the vehicle of the liquid to be flatted. A liquid existing as the continuous phase of a gel may be practically completely replaced by another liquid if the two are miscible. The gelis immersed in successive baths of the replacing liquid until the replacement has proceeded to a satisfactory extent.

continuous phase, no matter what the original which is the vehicle of the liquid coating composition. The gel may then be incorporated into the final product by grinding the finely divided Samuel S. Kistler.

gel with the liquid coating composition or by adding to such composition a previously prepared slurry of the gel in a portion of the vehicle or of the unflatted coating material. When it is desired to prepare the final composition by addition of a preformed slurry to a portion of untreated liquid (either vehicle or mixture of vehicle and film forming constituent), the gel is added to the liquid with mechanical agitation. A convenient apparatus for making up the slurry is a motor driven stirrer in a jar. More concentrated preparations are easily formed by ball milling a mixture of gel and liquid.

Proportions of gel to coating composition may vary widely according to the nature of the gel, the composition of the unflatted coating material and the properties desired in the finished product. A silica gel may be used as a flatting agent for clear varnish in amounts preferably ranging from 0.2% to 7.0% of gel based on the weight of the solid phase thereof, and it has been determined that a clear varnish containing about 4% or 5% silica gel is a relatively stable suspen'sionforming hard clear films of the desirably dull luster. The preferred proportions recited above are not limiting, but the advantages of this invention may be realized 'to some extent over an extremely wide range of compositions. Ten per cent or more of gel may be incorporated into a coating composition to form so-oalled crackle finishes which are fiat. The prepared liquid coating composition is very short, and the film formed therefrom cracks on drying to form a surface which is very attractive for certain uses.

From the great number of gels known may be chosen a material which will give the desired results and yet be compatible with the components of the liquid coating composition under consideration. Gels of metal oxides and hydroxides as silica, magnesia, thoria, ferric hydroxides, and alumina; and organic gels such as albumin, cellulose, and synthetic resins swollen in suitable solvents are suitable for the purposes of this invention.

An alternative, and in many cases more convenient, method of incorporating my novel flatting agent into the liquid coating composition is to grind into the liquid an aerogel of the type described in United States Patent 2,093,454 to When a gel is dried by normal evaporation, the solid structure thereof is collapsed to a large extent. The favored explanation of this phenomenon is that the fine fibers constituting the solid phase of the gel do not possess sufiicient mechanical strength to resist the forces exerted by the menisci of the continuous phase as these recede on evaporation. Dried gels which have shrunk on drying are denominated xerogels to distinguish them from the aerogels which are dried without shrinkage of the gel structure.

Aerogels are formed by removal of the continuous phase of a gel under such conditions that the surface tension of the liquid continuous phase is zero or very small. When the continuous phase is a liquid having a very small surface tension at atmospheric temperature, it may usually be removed by drying at ordinary pressure without susbtantial damage to the gel structure. Some petroleum fractions meet this condition, but most liquids which may occur as the original swellin liquid or become the continuous phase by replacement of the original liquid do have appreciable surface tension. The latter type of liquid medium is conveniently removed by heating to above 76 the critical temperature of the liquid in an autoclave while maintaining the pressure suihciently high to insure that the continuous phase will remain in the liquid state at all temperatures below 5 the critical. At or slightly above the critical temperature, the liquid in the gel will be converted to. gas without attendant formation of menisci and may be allowed to escape without damage to the gel structure. In handling gels in which the continuous phase is a liquid of inconveniently high critical temperature and pressure, the difliculties of. autoclaving atsuch conditions may be avoided by replacing the continuous phase with another liquid of more suitable properties. Silica aerogel has been prepared from the hydro-gel by replacing water with alcohol in the manner described above and heating the resultant alco-gel to a temperature above the critical temperature of alcohol under a pressure slightly in excess of the critical in an autoclave. The product is a light solid having an apparent specific gravity less than 15% of the actual value and possessing a large proportion of voids in sub-microscopic pores. By variation of the concentration of the original gel and of the conditions of treatment, gels having a percentage of voids ranging from the to 50% normally found in xerogels (silica gels of commerce) to 99% or over may be 30 prepared.

The aerogels have been found to be excellent flatting agents for fluid coating compositions of various kinds. They are ground into the liquid composition with great ease and require a much shorter grinding time than commercial silica gels and similar materials now known to the trade. It is important that the aerogel containing liquid be not ground too long, as prolonged grinding destroys the flatting eflect achieved by proper 40 grinding. Further, it has been determined that, as compared to xerogels, the aerogels are effective to the same extent when present in much smaller proportions on the weight of the finished coating composition. Aerogel containing varnishes, lacquers, and a other liquid coating compositions are prepared by making a slurry of aerogel and untreated coating material and grinding the mixture on a paint mill until the aerogel is evenly dispersed throughout the liquid. Alternatively the aerogel may be mixed with a small proportion of the liquid to form a paste or very viscous liquid; and this concentrate is then added to the liquid to be treated. 0n the strength of tests conducted, it has been estimated that the aerogels are approximaely 2 times as eflective as xerogels for flatting varnishes and the like. Enormous savings in time and equipment are realized by using aerogels, since it has been discovered that grindo ing time is reduced tremendously. A clear varnish containing silica xerogel requires about 16 hours grinding, more or less, in order to form a suitable flatted varnish. A proportion of silica aerogel (having an apparent specific gravity of 5 about .10-.15 which will give a varnish comparable to. that of the sixteen hour ground xerogel varnish gives a satisfactory varnish after two hours grinding with varnish on the same paint mill. It is believed that the grinding of 7 aerogel must be suflicient to break up agglomerates of gel and disperse them through the mixture-but must stop short of the point at which small particles are broken up, since prolonged grinding gives a varnish capable of forming a film which cannot be difi'erentiated from that of untreated varnish on examination with the unaided human eye.

The proportions of gel in the final composition when prepared by grinding in dry aerogel are preferably of the same order as those resulting 5 from the first method described herein. Similarly,-dry flatting agents may be the aerogels of any desired substance capable of forming a gel.

It is seen that the present invention includes the preparation of a fiat coating composition by 10 incorporating therein a minor proportion of a gel, the solid structure of which is maintained in substantially the form in which it existed at the .time the original gel was prepared.

The flatting agents of this invention may serve 15 purposes additional to that of flatting the liquid coating composition. A gel having slight tinctorial value as a chromium oxide gel may be used to give pleasing eiiects. Also as pointed out above, crackle finishes are obtained from com- 20 positions containing an excess of the new flatting agent. The invention also contemplates the use of aerogels of siccatives, suchas magnesia aerogel for flatting.

One convenient way of using aerogels accord- 2 ing to my invention involves the preparation of a master batch of the vehicle containing the gel, which batch is ground so that little, if any, ad-

7 ditional grinding is necessary when the material is added to the varnish or lacquer composition. 30 This enables me to add a flatting agent from the master batch to any finished varnishes or lacquers when, and only when, a fiat effect is desired. Furthermore, it is possible to market or distribute a master batch of the flatting agent 35 to be used by the paint and varnish consumer in a conjunction with prepared varnishes and lacquers whenever and to the extent the user deems the flat eifect desirable. For the purpose of the master batch I grind the aerogel in a varnish or 40 lacquer solvent in sufiicient quantities to produce a gel or paste which, however, will disperse readily upon mixing with the lacquer or varnish. For this purpose 8%-l2% of aerogel will usually be found suiiicient. The mixture may be ground 45 in the usual way until a maximum flatting power is developed, after which it can be stored indefinitely.

I claim:

1. A flat liquid coating composition compiis- 5o ing a film forming liquid and a ground aerogel having in the mixture substantially the structure of the gel as originally formed.

2. A flat liquid coating composition comprising a clear varnish and a ground aerogel having in 55 the mixture substantially the structure of the gel as originally formed.

3. A flat liquid coating composition comprising nitrocellulose, a solvent therefor, and a ground preformed aerogel having in the mixture substantially the structure of the gel as originally formed.

4. A fiat liquid coating composition comprising a film forming liquid and a ground silica aerogel having in the mixture substantially the structure of the gel as originally formed.

5. A flat liquid coating composition comprising a clear varnish and a ground silica aerogel having in the mixture substantially the structure of the gel as originally formed. 7

6. A flat liquid coating composition comprising nitrocellulose, a solvent therefor, and a ground silica aerogel having in the mixture substantially the structure of the gel as originally formed.

'4'. A fiat liquid coating composition comprising I a film forming liquid and a ground alumina aerogl having in the mixture substantially the structure of the gel as originally formed.

8. A fiat liquid coating composition comprisinga film forming liquid and a ground thoria aerogel having in the mixture substantially the structure of the gel as originally formed.

9. A fiatting composition to be added in small amounts to varnishes and lacquers 'and capable of imparting a fiat finish to a film obtained thereby comprising a mixture of an aerogel and a liquid vehicle therefor which is compatible with a varnish or lacquer solvent said gel being ground to'impart substantially maximum fiatting effect.

10. A flatting composition to be added in small amounts to varnishes and lacquers and capable of imparting a fiat finish to a film obtained thereby comprising a mixture of a silica aerogel and a liquid vehicle therefor which is compatible with a varnish or a lacquer solvent, said gel being ground to impart substantially maximum fiatting effect.

11. A fiatting composition to be added in small amounts to varnishes and lacquers and capable of imparting a fiat finish to a film obtained thereby comprising a mixture of an alumina aerogel and a liquid vehicle therefor which is compatible with a varnish or a lacquer solvent, said gel being ground to impart substantially maximum fiatting eflect.

12.vA fiatting composition to be added in small amounts to varnishes and lacquers and capable of imparting a flat finish to a film obtained thereby comprising a mixture of a thoria aerogel and a liquid vehicle therefor which is compatible with a varnish or a lacquer solvent, said gel being ground to impart substantially maximum fiatting effect.

' CHARLES GORDON HARFORD.