US2321732A - Apparatus for evaporative castings - Google Patents

Description

Jun 15, 1943. G. w. BRANT 2,321,732

APPARATUS FOR EVAPORATIVE CASTING Filed July 5, 1941 G'ilbert (Brant INVENTOR Patented June 15, 1943 APPARATUS Fo'n avaroan'rrvn CASTINGS Gilbert W. Brant, Buifalo, N. Y.,' assignor to E. ii. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware Application July 5, 1941, Serial No. 401,289

9 Olaima- (Cl. 18-47) This invention relates to evaporative (dry) casting apparatus, especially to the surface upon which the casting solution is spread, and compreehends the discovery that the common .metal casting surfaces, when coated with sodium silicate,

are rendered satisfactorily resistant to the chemical and physical actions normally encountered in the casting of thin sheeting of organic solvent soluble compounds.

The conventional details of evaporative casting involve the laying of a film of dope (solution of the material to be sheetedi on to a casting wheel or belt mounted for rotation and arranged for removal of the solvent from the dope during its rotation. The means for the solvent removal may include heating the casting surface or surfaces adjacent thereto, together with devices such as fans for carrying the solvent or solvent-laden atmosphere away from the film. Ordinarily the speed and degree of heating area'djusted so that the desired amount of solvent is removed in somewhat less thanfonefr'erolution of the .casting wheel or belt.

In dry casting.'a film is formed which adheres relatively tenaciously to the casting surface. This cohesion, which is particularly tenacious in the case of some of the more common and potentially interesting sheet forming materials, has

- invention. How the foregoing objects and related ends are accomplished will be apparent from the following exposition, in which are disclosed the principle and divers embodiments of the nvention, including the best mode contemplated for carrying out the same. The written description is amplified by the accompanying drawing,

in which: I

Figure l is a side elevation view partly in section, w th some parts exaggerated in the interest of clarity, of a wheel casting device embodying the present invention; and

- Figure 2 is a similar view of a portion of the casting wheel with the relative dimensions of the parts somewhat out of proportion in order to better illustrate the details of the casting wheel. It has now been found that such objectives can 'be realized by applying the solution of sodium silicate to the metal casting surface and there- 7 after drying and baking the coating to render it hard, smooth and resistant to the action of organic solvents.

The most outstanding results have been obtained when the casting surface was made of stainless steel, copper or nickel. Improvements I have also been obtained with other metallic surfaces such as iron, steel, nickel alloys and the like.

- The metal surface should be uniformly smooth and preferably polished to a mirror-like finish and be'carefully cleaned, especially if only a thin coating of sodium silicate is to be applied. before the coating operation.

Manyvarieties of sodium silicate are commercially available. They differ chiefly with respect to the ratio of-the silica to the alkali, which may be anywhere from 1:1 to 4.2:1 or even higher. This ratio affects the characteristics and properties of the dried coating, and it is therefore important in determining the sodium silicate to be used. The higher the ratio of SiO: to NazO:

(l) The less easily the dried coating is dissolved or removed;

(2) The less the dried coating is affected by high humidity;

(3) The more brittle is the dried coating;

(4) The faster the coating dries with less reta ned water: and

(5) The less is the tendency of the coating to bloom.

If it is desired that the coat ng be easily removed. and if the casting surface is to be constantly maintained'under atmospheric conditions of low humidity. a sodium s l cate solution wherein the ratio of SOa to NazO is as low as 2:1.

may be used with fairly good results. For most purposes the ratio of slot to Naro should not be factory casting surface over a long period of time.

The concentration of the aqueous sodium silicate (calculated as SiOa+NazO solution is important. In general, concentrations in the range 20% to 50% are satisfactory. The lower the alkalinity of the solution, that is, the higher the ratio of 810; to NazO, the more desirable it is to reduce the concentration. A concentration of 50% or even more may be used when the ratio of SiOz to Nazo is2zl or less. For ratios of 4:1 or more, the concentration should be in the neighborhood of 20% or even less.

The choice of a concentration is influenced somewhat by the method by which itis to be applied to the casting surface. As an example, a solution of sodium silicate wherein the ratio of SiOz to NazO is 3.3:1 should have a concentration of about 25% to 30% for 'spray application,

30% to 35% for flow-on application, and 35% to 40% for spreading by means of a doctor knife or the like.

As previously indicated, the coating solution may be applied to the metal surface in any convenient manner. The more common schemes are spraying, flowing-on, dipping and spreading. When a plurality of thin coats are applied, as by spraying, each coat should be air dried before the subsequent coat is applied. Sodium silicate coatings, when subjected only to air drying,

check or crack. Coatings which are force dried do not exhibit these undesirable characteristics, so it is essential that the air drying not proceed too far and that the coating be subjected to an elevated temperature while it contains a substantial amount of water. It is therefore advisable to carry out the several coatings in as short a time as possible. It has been found empirically that the time of air drying between coatings should not exceed about ten minutes at ordinary room temperature and ordinary normal or low room humidities. If the humidity is unusually high the time of drying (between coatings) may be prolonged.

If the sodium silicate coating is to be applied by flowing-on or dipping, time should be allowed for draining and to initially set the coating by air drying before a second coat is applied. A somewhat higher concentration of solution is used for this method of application, and as a result, the individual coatings are somewhat heavier. It follows that the number of coats may be considerably reduced except in those cases where a very heavy total coating is desired.

In those instances where a still more concen-.

' still hydrated to a considerable extent. These deleterious actions do not occur at 100 C. ,or 110 C. even when the coating contains a'fairly high percentage of water. It is therefore generally desirable, when .subjecting the coated-surface to forced drying, to use an initial tempera.- ture of about 100 C. and gradually raise it to the range 140-l50 C. or even more. Excessive drying should be prevented while the temperature is rising to C., it having been found important that the coating still contain at least 25% water when it reaches this point. The coating should be head soon enough and/or high enough to fuse it.

It is preferred that the final coating be a few thousandths (preferably five thousandths) of an inch thick. Any irregularities in the surface may be ground ofi and polished.

For convenience the invention has been described in terms of sodium silicate. It is to be understood that other alkaline (alkali metal) silicates, such as potassium silicate, may be used if desired.

The new coatings resist the action of the commonly employed neutral organic solvents such as acetone, toluene, ethyl alcohol, and those mentioned in the prior art patent cited earlier in the specification.

The alkaline silicate surfaces of this invention are useful in casting any of the types of organic useful in the casting of organic solvent soluble ethyl cellulose.

The new sodium silicate coatings are extremely hard and smooth and show excellent adhesion to clean stainless steel, copper, and nickel surfaces.

They form excellent bases for dry casting materials dissolved in organic liquids because inter alia they have high dielectric strength, .low thermal conductivity, and permit ready stripping of water insoluble films, sheets and pellicles therefrom. Such pellicles are of uniform thickness and have smooth surfaces which are free from imperfections, blemishes, scratches, etc. The properties of the sodium silicate coatings are such that they can be easily and quickly removed from the casting surface and econom-' ically renewed when desired. It is unnecessary to incorporate softeners or plasticizers in these coatings;

Asmany apparently widely different embodiments of this invention may be made without departing from thespirit and scope thereof, it-

is to be understood that this invention is not limited to the specific embodiments thereof except as defined in the appended claims.

I claim: 1. A metallic casting surface from the group consisting of. stainless steel, copperand nickel, having a thin coating of dried and baked alkali metal silicate. l

2. A metallic casting surface from the groupconsisting of stainless steel, copper and nickel, having a thin coating of dried and baked sodium silicate.

3. A nickel casting wheel surface having a. thin coating of dried and baked alkali metal silicate.

4. A nickel casting belt surface having a thin coating of dried and baked sodium silicate.

including the cellulose 5. A metallic casting surface having a thin coating of dried and baked alkali metal silicate.

6. A metallic casting surface having a thin coating of dried and baked sodium silicate.

7. A metallic nickel casting surface having a thin coating of dried and baked sodium silicate, the ratio of 8102 to NazO in the silicate being in the range 1:1 to 42:1.

8. A metallic nickel casting surface having a the ratio of SiO: to NazO in the silicate being in the range 3:1 to 3.6:1.

9. A metallic nickel casting surface having a thin coating of dried and baked sodium silicate,-

. 4 cxmn'icAm (IF commence. rtsnt in. 2,321,732. June 1 ,1915.

' GILBERT w. BRANT.

It is hereby certified that error appears 1n the printed. Specification I of theebote numbered patent requiring correction asfollowe: Page 2 second. column, line 5; for "head' read ----heatedand that the said Letters Potent should be read with this correction therein that the same may'conform to the record of the case in the Patent Office.

Signed and sealed this 27th day .of Jui A. n. 1915.

Henry Van A'rsdalle, (Seal) 4 Acting Commiesioner of Patents.

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