US2360919A - Method of coating paper - Google Patents

Description

Patented Oct. 24,

r 2,360,919 uNrrEo STATES PATENT OFFICE- 2,360,919 METHOD or COATING PAPER Edgar Warner, Dayton, Ohio, assig'nor to The Champion Paper and Fibre Company, Hamilton, Ohio, a corporation of Ohio I .No Drawing. Application July 5, 1,940, Serial No. 343,950

cast" surface-coated paper such as produced by the process described in prior Patent 1,719,166. In accordance with the process there described,

when aqueous coatings are used, the paper with" the wet coating material on its surface is pressed against, a solid casting surface and dried with the coating in intimate contact therewith until the coating material has set or hardened sumciently to no longer adhere to the casting surface. When the coated paper is removed the finish on the coated surface of the paper is an accurate replica of the casting surface. Because of the impervious nature of the casting surface, it is a characteristic of this process that at, least enough drying to bring about the necessary setting 'of the coating must be accomplishedby transmission of'moisture from the coating through the fibrous web to the point where evaporation can take place.

The primary object ofthe present invention process, considerably decreases the length of time which is necessary to dry the paper in contact with the casting surface. In other words; it has been found that the time required for evaporatlon'can be materially decreased if a large increase is made in the amount of water which must be evaporated, or, a substantial increase in' operating speed is made possible by a large increase in the amount of evaporation which must take place.- It has further been observed that this same wetting of the back of the web serves to largely neutralize the usual tendency of finished paper to curl towards the side which has been wetted by the coating composition.

This wetting. is preferably done while the web is in contact with the casting surface, but may sometimes be accomplished with advantage before the freshly-applied coating is pressed into contact with the finished casting surface and either before or after the coating operation. It is most advantageously accomplished as soon as possible after the coated'web has been pressed into contact with the castin surface and before the heated metal has had time to materially heat the coating material. The longer the interval is to increase the rate. of drying whereby the between the pressing of the web onto the casting surface and the wetting of the back, the less effective is the wetting in reducing the drying time, until, if the wetting is not accomplished until the coated web is substantially dried, it serves only to correct the tendency to curl and does not make possible any increase in the speed of operation. v

The exact amount of water used does not appear to be critical. with types of paper such as are generallyused as bases for coating, I find that the purposes of the present invention are accomplished by wetting with the amount of water which the surface of the paper will freely take up without prolonged soaking. Any convenient method of wetting may be used. For example, the back of the web may be flooded with water and the excess then removed from the surface. If the wetting is done while the paper is on the casting surface, the excess water may.

be removed by a yielding wiper of felt, rubber, or the like, or it may advantageously be removed by a flat, uniform jet of air which is directed backward against the oncoming web of freshly wetted paper. If the wetting is done when the paper is out of contact with the casting surface, the excess may be-removed by one of the aforementioned methods or by wiping over the rounded edge of afixed blade. Alternatively a properly regulated and controlled amount of water may be applied according to known methods by sprays, rolls, etc. so that there is no excess to be removed, but it is usually more satisfactory to apply an abundance of water and remove the excess by simple means such as above described. The procedure now practiced is to press the paper with the wet coating into contact with the casting surface and as soon as possible thereafter to flood the back of the web with water by means of a shower or the like, and then almost immediately to remove the unabsorbed surplus by a high velocity air jet blowing backwards against the oncoming freshly wetted paper surface.

11134218111161 in which the wetting of the back serves to increase operating speed and improve the quality of the finished product is not definitely understood. Among the explanations proposed, the following may be noted. First, it is thought that a wetted web transmits moisture from the coating, through the web to the point where evaporation takes place,at a more rapid rate than does a web with a dry back. This is inferred from theobserved fact that the'wetting of the back materially increases thedrying rate even though the weight and character of the coating and the temperature of the casting surface and of the drying air are unchanged. Second, it is thought that there is a tendency, increasing with speed, for the fibres to spring back water pervious' coating.

after pressing, and pullthe wet coating loose from the casting surface in spots which form irregularities or crevices 'in the finished cast coat,

andthat wetting the back to soften the fibers greatly reduces this tendency. Third, wetting the back is found to make possible a considerable increase in the operating temperature of the casting surface without. causing the defects which usually accompany such increases. This may be due'tolocal cooling of the casting surface which prevents excessive temperaturesdrying on the locally cooled area appears to materially shorten the total drying time required without deleteriously affecting the quality of the final product.

It has also been discovered that the wetting of the back of the web as described, in addition to making possible considerably higher operating speeds, also serves to more or less compltely correct the tendency of'the finished paper to curl towards the freshly coated side. Due to the wetting of one side of the web inthe coating operation and the drying of the coating while in contact with the casting surface, a very severe curl is found to exist in the dried coated paper and this tendencyto curl is largely neutralized by the wetting of the other surface of the paper inaccordance with the present invention. If this is insufiicient to completely correct the curling tendencies, a small additional wetting, as by steaming, of the back, preferably by steaming after the freshly coated web has been dried, and advantageously afterit has been removed from the casting surface, will ordinarily completely removethe curl.

This invention, when applied in the preferred form as described, has been found to make possible increases of from twenty-five to fifty per cent or more, in the speed of operation of the process of the aforementioned patent as heretofore practiced. It therefore makes possible increases up to fifty per cent or more in the plant capacity and in the production obtainable from the same equipment without any sacrifice in quality of the finished product.

The invention may also advantageously be applied to the manufacture of cast surfaced coated paper according to the modified process as disclosed in Patent No. 2,029,273, preferably by wetting the back of the web just after it is pressed into contact with the coating on the drum. Any of the above described procedures may, however, be used'if desired. The paper to which the fresh coating is ap plied may be of any desired weight, and if desired, may previously have been coated with a I claim: l. In the manufacture of cast-surfaced-coated paper in which an aqueous coating is freshly apweb, before evaporation of moisture from the freshly-applied coating has been completed, and causing at least a part of the evaporation of the moisture of the freshly-applied coating to take place through the web while it is wet from said application of water.

2. The process of manufacturing cast-surfaced-coated paper as defined in claim 1 in which the water is applied to the web prior to the application thereto of the coating material to form the freshly-applied coating.

3. The process of manufacturing cast-surfaced-coated paper as defined in claim 1 in which the'water is applied to the web after the coating material to form the freshly-applied coating is applied thereto and prior to bringing the freshly-applied coating in contact with-the finished casting surface, and in which the water is applied to the surface of the web which does not receive the freshly-applied coating. 1

4. In the manufacture of cast-surfaced-coated paper by application of aqueous coating composition to one side of a paper web and subsequent drying of the coating while in contact with a finished casting surface, the step which comprises applying .water to the exposed back side of the paper web while said coated side is in contact with said casting surface.

5. The process of making cast-surfaced-coated paper which comprises: the application of aqueous coating to one side of a pa or web; the pressing of the paper and coating on a finished casting surface; followed, before any substantial evaporation of water from the coating material, by the application of water to the other side of the web; and the evaporation of said applied water and of aqueous vehicle from the coating composition, from the exposed side of the web, while the coating is in intimate contact with the castin surface.

. 6. The-process of making cast-surfaced-coated pape which comprises: the application of aqueous coating to one side of a paper web; the pressing of the paper and coating onto a finished casting surface followed substantially immediately by a flooding of the back of the web with water andthe substantially immediate blowing on of the unabsorbed surplus water therefrom;

and the evaporation of the remaining applied water, and of aqueous vehicle in the coating, from the exposed side of the web, while the coating is in intimate contact with the surface of said cylinder.

7. In the manufacture of cast-surfaced-coated paper by application of aqueous coating composition to one side of a paper web and subsequent drying of the coating while in contact with a finished casting surface, the step which comprises plied to a web of paper and, the coating dried in contact with a finished casting surface, .with evaporation of moisture from the coating through the paper web and from the exposed side thereof, the improvement which comprises applying water-to the webother than by absorption from the coating, and in amount sufficient to wet the applying water to the back of the paper web substantially immediately after the coated side has been brought into contact with the casting surface.

-8. In. the manufacture of coated paper by application of aqueous coating composition to one side of a paper web and subsequent drying of the coating while in contact with a finishing surface,

the step which comprises promptly applying water to the exposed uncoated side of the paper web after the coated side has been brought into contact with the finishing surface.

9. The process of manufacturing cast-surfacedcoated paper as definedin claim 1 in which said exposed side of the web is further moistened after the evaporation of moisture from the freshlyapplied coating is substantially complete.

EDGAR WARNER.