US2350987A

US2350987A - Method of making highly glazed film coatings

Info

Publication number: US2350987A
Application number: US440573A
Authority: US
Inventors: William R Collings
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1942-04-27
Filing date: 1942-04-27
Publication date: 1944-06-13
Anticipated expiration: 1961-06-13

Description

June 13, 1944. w. R. COLLINGS METHOD OF MAKING HIGHLY GLAZED FILM COATINGS Filed April 27, 1942 IN VEN TOR.

W/fl/bm /Z.. COM/175 .fffOL/VEYS Patented June 13, 1944 UNITED STATES PATENT OFFICE LIETHOD OF MAKING HIGHLY GLAZED FILM COATINGS 'wlunm a. Ceilings, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich, a corporation of Michigan Application April 27, 1942, Serial No. 440,513

7 Claims.

sheet material, such as fabrics made "from natuml or synthetic fibers, sheets made from synthetic resins, cellulose derivatives, etc., but this has not heretofore been commercially feasible exceptin a fewinstances because of the relatively large amount of coating material required when using known processes and because of the expensive nature of the operation required to produce such surfaces. The usual methods of producing a highly glazed finish on such flexible sheets has heretofore involved the so-called "plate finishing" operation. In this operation the coated sheets are placed between polished plates of stainless steel, nickel or copper and subjected to high pressure in a press or between the rollers of a "plating" machine. This is a slow and expensive process which requires much handling of the sheet and the use of special presses, particularly if large sheets are desired. It is frequently necessary to heat the coated sheet to soften the coating material, and this may lead to the tendering of organic fibers. Furthermore, the method can-: not be adapted readily to continuous operation. It is known that film-forming materials will produce a shiny appearance when coated'on paper and other flexible fibrous sheets by conventional coating methods, such as by spraying, rolling, or doctoring a solution of the coating material in a volatile solvent onto the sheet. andsubsequently drying the coated sheet or by the methods of U. S. Patents 2,175,125 and 2,169,288; but to produces. smoothsurface of good appearance and apparent optical depth a relatively thick coating of the film-forming material is usually required. To overcome these diqculties lamination of precast films on the surface of the flexible sheet has in' some cases been practiced but the coating is product cannot readily be adapted to many commercial uses because of its stiffness. w I

It is accordingly an. object, of the present invention to provide an economical and continuous method for applying thin, uniform, glazed sur face coatings to previously coated flexible sheets.

An additional object of the invention is to provide a compositely coated'fiexible sheet, the surface coating of which iss'niooth, highly glazed,

and which has the'app'ealfance ofjgreat optical depth. additional object, is to provide a. method whereby highly g a d coatings may be. i applied to flexible sheet material without-involving excessively long drying of the coated sheet.

According to the present invention the foregoing and related objects are readily accomplished by first coating a flexible sheet in any convenient manner witlr abase coating of a plastic film-forming material, drying the coated sheet and applying to the dry coated sheet a thin finishing coat of a film-forming material in the manner hereinafter described. The finishing coat is applied by depositing a layer of a solution of the coating material in a volatile solvent on a polished surface, preferably on a polished roll or drum or on a traveling polished metal belt if continuous operation isdesired, partially drying the deposited layer, pressing the dry previously coated flexible sheet onto the .partially dried, tacky, cementitious layer of coating material while the latter is still on the polished surface, and subsequently stripping the compositely coated sheet from the polished surface.

For visual illustration of one mode in which the invention may be carried out, reference is had to the accompanying drawingwherein:

Fig. 1 is a diagrammatic elevation of apparatus in which the invention may be carried out; and

Fig. 2 is an enlarged diagrammatic cross-section, exaggerated in certain respects, of a coated sheet made by the method of the invention.

Referring to Fig. 1. a thin, uniform layer of a solution of a cellulose ether or other film-forming substance in a volatile solvent is cast from a hopper 3 onto the polished surface of a heated (heating means not shown) revolving metal drum 4 in such quantity as to form a thin film 5 thereon. A dry previously coatedfiexible sheet'i is drawn directly from a. supply roll I or from the dry end" sioned that it exerts a nun pressure on the coated then often of suchthick'nessthat the coated sheet and the finishing film between it and the drum 6; The roll and the drum are rotated at' the same peripheral speed. The roll may be adjusted by means not shown to permit control of the distance the finishing film cast from the hop pet is carried on the surfaceof the drum before it meets the jboated flexible sheet. This furnishes one method of controlling the consistency of the film on the surface of the drum before it meets the coated'fiexible sheet. The surface tempera ture' and peripheral speed ofthe drum may also f be adjusted to assist in controlling the consistency of the film. As the metal drum rotates,jthe flexible sheet with the formed composite coating thereon is carrledalong with it and is stripped from it at a pointa ad of the hopper Al'- though a, certainlamount of drying of the com-.

v surface of the drum 'one side in terially increase ing on the sheet, a finishing coat between 0.05 and 2.0 mils thick will be found advantageous.

sumcient of the solvent remaining in the relatively thin finishing coat into the relatively thick base coat to enable the sheet bearing the comstripped from the polished without distorting or disrupting the film surface. The coated and highly glazed sheet is then passed through a dryer and the dried sheet wound on a roll it for storing posite coating to be or used continuously in a subsequent operation.

flexible sheet I! Referring now to Fig. 2, a which may, be'of paper or sheets formed from natural or synthetic fibers, or the like coated on a previous operation with a base coating II is shown after application of a highly glazedfinishing coat l4 by the method of the sheet and coating layers are shown greatly ex-' aggerated for the sake of clarity. The finishing coat ll has" a smooth, highly glazed surface and is firmly adherent to the base coating it.

Although any flexible sheet material, such as sheets composed of cotton, wool, silk, or other natural or synthetic fibers may be then glazed by the method of the present in-' ventlon, the invention 'has been found of particular value in the application of a highly glazed surface coating to previously coated paper stock. The finishing coat is preferably composed of the same material as the base coating, although it may be composed of any material which will adhere to the base coating and which is soluble in a solvent in which the material of the base coating is soluble. Thus, paper or other sheet material may be readily coated with ethyl cellulose dissolved in solvents and applied in known manner and, after drying, the coated sheet may be glazed by applying to it a very thin layer of ethyl cellulose dissolved in the same or similar solvents. If desired, the finishing coat may be of a harder and more abrasion resistant ethyl cellulose than that used to form the base coating, thus imparting to the coated article these desirable surface characteristics.

The degree of drying of the finishing coat which is effected on the polished surface before it is applied to the coated flexible sheet will vary considerably with the nature of the material being applied, the thickness of the finishing coat desired and with other factors. Usually the film is dried sumciently on the polished surface so that it has a semisolid consistency, but is still tacky, non-self-supporting, cementitious, and in-'- capable ofbeing stripped from the polished sureface. The thickness of the finishing coat applied to the coated flexible sheet may be varied over wide limits. Usually, unless it is desired to ma the total thickness of the coat It is usually desirable to apply a finishing coat which is relatively thin with respect to the base coating since, as mentioned above. it appears that the solvent remaining in the finishing coat when it meets the base coating almost immediately diffuses into the latter and is distributed coated and.

, present invention. The thickness of the flexible the coated sheet from the polished surface. Thicker finishing coats may, of course, be applied in which case additional drying of the composite before it is stripped from the polished surface may be desirable. -An influencing factor is, of course, the proportion of the solvent remaining in the finishing film when it comes in contact with the base coating on the flexible sheet. The maximum amount of solvent which may be present in the composite coating and still permit the coated sheet to be stripped from the polished surface will depend to some extent upon the nature and material of the'polished surface and A upon the material of which-the finishing coat is composed. It is usually desirable, however, that the total amount of volatile solvent in the composite coating be less than 25 per cent, and preferably between 3 per cent and 20 per cent of the l weight of the coating. a To facilitate removal of the coated sheet from the polished surface, it is often desirable to add to the solution, from which the finishing film is cast, small amounts-of a stripping agent or of a substance which acts as a lubricant to prevent sticking of the finishing coat to the polished surface. Examples of compounds which have been tested and found suitable for this purpose include stearic acid, glycerol monosiearate, triisopropanol amine and amine salts of higher fatty acid. The subsequent drying operation after the compositely coated sheet is stripped from the polished surface maybe carried out in any convenient manner, such as by blowing hot air over substantially uniformly throughout the comthe coated heet. Since the actual amount of solvent pres t is very small, the drying is accomplished oidly. a

The folloi g example illustrates the practice of the inve: m, but it is not to be construed scope thereof:

as limiting 1 Example 1 A coating solution is prepared having the composition:

' Parts by weight Ethyl cellulose (57 cps. type) 1 80 Di-(ortho-xenyl) monophenyl phosphate--- 20 Benzene 210 Methanol 86 Ethyl cellulose viscosity determined on a 5 soluisszsieass asses? by The solution has a viscosity of about seconds, measured by the falling ball method as described in the American Society for TestingMaterials Standards for 1933; D-301-33 (p. 720). Paper is coated by the method of U.'S. Patent 2,175,125 with a layer of this solution in amount sufficient to form a film having a thickness when dry of about 0.4 mil. The coated paper is dried and passed in continuous manner through the coating machine illustrated in Fig. l. The polished steel drum is 4 feet in diameter and is heated to a temperature of l00-ll0 O. A solution consisting of 20 per cent by weight of ethyl cellulose cps. type), together with 0.2 per cent of stearic acid in a mixture of 80 parts by volume of toluene and 20 parts of methanol, is deposited from the hopper on the surface of the rotating drum in such quantities that it forms, when dry. a film about 0.1 mil thick. The dry coated side of the paper is pressed smoothly against the partially dried film on the drum, and the two are kept in contact for, the greater part of one revolution of the drum. The assembly is then .a volatile solvent on a polished metal surface in stripped from the drum and is conducted through a drying chamber to complete the drying operation. The coating, which has a total thickness of about 0.5 mil, has a smooth, highly polished surface, and has the appearance of being much thicker than it actually is.

amount sufficient to form a relatively thin film thereon upon evaporation .of the solvent; par- Whlle the example given above illustrates the invention as applied to formation of a highly polished surface cdating of ethyl cellulose on paper, the invention can .be applied with equal advantage to formation of finishing films of other film-forming materials, such as water-soluble.

methyl cellulose, organo-soluble methyl cellulose,

benzyl cellulose, salts of celluose glycolic acid, celulose acetate, cellulose nitrate, arcylic acid ester polymers, etc., on other flexible, coated sheets,-such as coated cotton, wool or silk cloth, rayon, etc. The process may also be used to apply a surface coating to a precast plastic film to give the latter improved surface characteristics. Flexible coated sheets may be glazed on each side by first glazing one side of the sheet and then,

after turning it over, glazing the other side. In

its broader aspects the invention concerns a continuous process for forming smooth highly glazed surface coatings on flexible coated sheet mate-- rials. In itspr'actical application the invention is limited in its scope only by the requirement that the ingredients of the film forming substances used in the base coating and in the finishing coat be soluble in the same solvent and capable of adhering to each other.

Among the advantages of the invention may be mentioned the following: (1) the coating may be applied rapidly and economically, (2) the coating applied by the process of the invention improves the surface characteristics of the coated sheet and provides a uniform appearance, (3) the composite coating gives the desirableimpres- 'sion of apparent optical depth to-ink imprints on the flexible sheet material beneath the coating which it has been impossible to obtain-heretofore except by lamination methods using thicker films than are necessary in carrying out the present invention, and-i4) it provides a method whereby finishing coats may be applied to flexible sheets without the..use of long drying periods.

I claim:

l. The method which includes: depositing a uniform thin layer of a solution of a film-forming substance in' a volatile solvent on a polished metal surface; partially drying the solution on the polished surface; superposing and pressing thereon a dry coated face of a flexible sheet coated on at least one side with a film soluble in the said voltaile solvent and adapted to adhere to the said film-forming substance; stripping the compositely coated sheet from the polished surface; and drying the coated sheet.

2. The method which includes: depositing a uniform thin layer of a solution of a cellulose ether in a volatile solvent on a polished metal surface: partially drying the solution on the polished surface; superposing and pressing thereon a dry coated face of a flexible sheet coated on at least one side with a cellulose ether soluble in the said volatile solvent; and stripping the compositely coated sheet from the polished surface.

3. The method which includes: depositing a uniform layer ofa solution of a cellulose ether in tially drying the solution on the polished surface; superposing and pressing thereon a dry coated face of a flexible sheet coated on at least one side with a relatively thick coating of a cellulose ether soluble in the said volatile solvent; and stripping the compositely coated sheet from the polished surface.

4. The method which includes: depositing a uniform layer of a solution of an organo-soluble ethyl cellulose in a volatile solvent on a polished metal surface in amount sufilcient to form a relatively thin film thereon upon evaporation of the solvent; partially drying the solution on the polished surface: superposing and pressing thereon a dry coated face of a flexible sheet coated on at least one side with a relatively thick coating of an organo-soluble ethyl cellulose soluble in the said volatile solvent; and stripping the compositely coated sheet from the polished surface.

5. The method which includes: depositing a uniform layervof a solution of an organo-soluble ethyl cellulose in a volatile solvent on the polished face of a heated rotating drum in amount suflicientto form a relatively thin film thereon upon evaporation of the solvent; partially drying the solution on the drum face; superposing and pressing thereon a dry coated face of a flexible paper sheet coated on at least one side with a relatively thick coating of an organo-soluble ethyl cellulose soluble in the said volatile solvent; stripping the compositely coated paper sheet from the drum face and drying the compositely coated sheet.

6. The method which includes: depositing a uniform layer of a solution of an organo-soluble ethyl cellulose in a volatile solvent on the polished face of a heated rotating drum in amount sumcient to form a relatively thin film thereon upon evaporation of the solvent; partially drying the solution on the drum face; superposing and pressing thereon a dry coated face of a flexible paper sheet coated on at least one side with a relatively thick coating of an organo-soluble ethyl cellulose soluble in the said volatile solvent; reducing the proportion of said volatile solvent in the composite coating to from 3 per cent to 20 per cent of the weight thereof; and stripping the compositely coated paper sheet from the drum face.

ethyl cellulose in a volatile solvent on the polished face of a heated rotating drum in amount sufficient to form a film from 0.05 to 2 .0 mils in thickness thereon upon evaporation of the solvent: partially drying the solution on the drum face; superposing and pressing thereon a dry coated face of a flexible paper sheet coated on at least one side with a coating from 3 to 8 mils in thickness of organo-soluble ethyl cellulose soluble in said volatile solvent; reducing the proportion. of said volatile solvent in the composite coating to from 3 per cent to 20 per cent of the weight thereof; stripping the compositely coated paper sheet from the drum face and drying the coated paper sheet.

WILLIAM R. COHJNGS.

7. The method which includes: depositing a uniform layer of a solution of organo-soluble,