|Publication number||US3330688 A|
|Publication date||Jul 11, 1967|
|Filing date||Oct 29, 1963|
|Priority date||Oct 29, 1963|
|Publication number||US 3330688 A, US 3330688A, US-A-3330688, US3330688 A, US3330688A|
|Inventors||Brenton S Halsey, Robert C Williams|
|Original Assignee||Albemarle Paper Co|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (6), Referenced by (5), Classifications (9)|
|External Links: USPTO, USPTO Assignment, Espacenet|
July 11, 1967 B. s. HALSEY ETAL CAST COATING PROCESS Filed Oct. 29, 1963 \EEE maomom m. mm.
/5mm 9:55 352 W835 United States Patent 3,330,688 CAST COATING PROCESS Brenton S. Halsey and Robert C. Williams, Richmond,
Va., assignors to 'Albemarle Paper Company, a corporation of Virginia Filed Oct. 29, 1963, Ser. No. 319,768 9 Claims. (Cl. 117-64) This invention relates to an improved method and apparatus for cast coating a continuous web. More specifically, this invention relates to an improved method and apparatus for cast coating a paper web when employing a liquid coating composition which is cast at temperatures exceeding the boiling temperature of the liquid contained in the coating.
The method of cast coating which comprises heating a web to a temperature of around 212 F. or more is generally referred to as pressure-cast coating since sufficient pressure must be applied to the web during its heating to prevent the web from being separated or blown away from the dryer drum before its coating is dried or cast, otherwise an imperfect coating will result. In conducting that process, it is desirable to maintain an excess or puddle of the liquid coating material at the entrance nip which is that area defined by the paper web as it comes into contact with the casting surface. While not absolutely essential to achieve high quality cast coatings, this technique of operating serves as an obvious indicator to the machine operator that he is maintaining a suitable coating consistency in relation to other operating parameters.
To increase production rates, the above described type of casting operation can be readily speeded up by increasing the temperature and speed of rotation of the dryer drum upon which casting is effected, viz. increasing the temperature above 212 F. In this manner, the whole operation is speeded up and the web being coated naturally moves through the casting unit at a greater linear speed. It has been found when cast coating a web by the above described procedure that as the temperature of the dryer drum is increased considerably above 212 F. (e.g. 280 F.), maintenance of the proper puddle of coating material in the entrance nip by the machine operator becomes more and more diflicult inasmuch as the liquid in the coating material begins to boil away. This not only results in the coating becoming more viscous which at times will cause sticking of the coating to the drum, but additionally causes spattering of the coating. Spattering is undesirable since the distinct and discrete particles of coating material upon contacting the casting surface of the heated drum coalesce and become dried. These discrete particles of dried coating material upon subsequent contact with the puddle and the wet coated web are not absorbed thereby but rather remain separate and distinct and at times will cause additional coating material to be pulled away from the web and cohere to the casting surface. This results in an imperfect cast coated web having a mottled and distorted surface.
While the above problem can be overcome by having the machine operator adjust the coating viscosity in proportion to any changes in the temperature of the dryer drum, there are intervening factors that must also be correlated simultaneously by the machine operator. The net result is that this extent of operating complexity introduces a wide margin of human error which has proven difficult to control. It can be appreciated that failure to maintain this required degree of control results in high waste and loss of operating time in order to cut back the production rate and establish exactly the proper or right operating conditions.
An object of this invention is to eliminate this close Patented July 11, 1967 degree of operational control. Specifically, an object of this invention is to provide an improved pressure-cast coating method and apparatus whereby a puddle of coating material can be maintained at the entrance nip in a simplified manner without necessitating an extremely close degree of operational control, especially when pressure-cast coating at high pressures and temperatures. These and further objects will come to light as the discussion proceeds.
The drawing depicts a schematic elevational view of the apparatus of this invention wherein the process of this invention can be practiced.
The above objects are accomplished pursuant to the practice of this invention which provides a method of coating a web with a mineral coating which method comprises: passing the web through an entrance nip between a pair of turning rollers, one of said rollers being main: tained at a temperature at least equivalent to the boiling temperature of the liquid in a wet thermosettable mineral coating; simultaneously passing a cooled endless metal belt through said nip; and maintaining a puddle of said wet thermosettable mineral coating within the nip defined by said belt and said web. In conducting the above process, it is preferred to employ a metal belt having an extremely smooth surface, especially a chromium plated extremely smooth surfaced flexible metal belt. Moreover, the improved method of cast coating made possible by way of this invention is especially applicable to pressurecast coating with vapor receiving means in combination with pressure means for urging the paper web against the dryer drum which can be heated to temperatures well above 212 F., e.g. on the magnitude of 350 F. When pressure-cast coating in this manner, it is preferred to heat the dryer drum 18 to a temperature within the range of from about 220 F. to about 350 F., especially at about 240 F. to about 280 F. Moreover, it is preferred to exert a pressure upon the web by pressure means 20 within the range of from about 5 p.s.i. to about 50 p.s.i., particularly from about 10 p.s.i. to about 35 p.s.i. By this embodiment and operating under these conditions, cast coated paper having optimum properties is produced at lowest cost The basic apparatus constituting a part of this invention is depicted in the drawing which further illustrates the preferred type of cast coating operation to which the present invention is especially adaptable. Referring to the drawing, the improved cast coating means of the instant invention comprises the rotatable drum means 18 which means further comprise heating means 35, the latter preferably comprising steam injection and condensate discharge means. A system of roller means, for example the rollers 15 and 17, is provided for conveying the endless metal belt 12 around a portion of the surface of the drum 18, the outer surface of the belt 12 being adapted to serve as a drying and casting surface. The web conveying means 21 is positioned and adapted (a) to bring a wet coated web 10 into contact with the outer surface of the belt 12 at least by the point at which the belt first meets the drum 18, (b) to define at the point of said contact a nip capable of receiving a puddle 11 of the coating composition, and (c) to maintain contact beyond the point at which the belt 12 first meets the drum 18. In combination with the metal belt 12, which as brought out above is preferably a chromium plated extremely smooth surface flexible metal belt, is cooling means 13 which is preferably water maintained at about room temperature within container means 14.
Thus it can now be appreciated that among the advantages and features of the present invention is that the belt 12 keeps the puddle 11 away from the hot drum 18 and thus minimizes the amount of heat picked up by the puddle of coating material 11. This coupled with the fact that the belt 12 is additionally cooled by cooling means 13 makes it possible to operate with a very liquid flooded nip 11 while avoiding spattering of the coating composition. As indicated above, spattering of the coating composition at the entrance is extremely undesirable inasmuch as the distinct particles of coating materials will coalesce and adhere to the casting surface and upon subsequent contact with the wet coated web undergoing drying, will not blend in and form an integral part of the coating but rather will remain a separate entity the configuration of which is assumed and reflected in the cast coating, viz. an imperfect cast coated web having mottled surface. Even if spattering should occur in the practice of this invention, the above effect is minimized since the distinct particles of coating material do not contact the casting surface when it is hot. Consequently, the particles of coating material are not dried upon the casting surface and hence absorbed by the puddle maintained in the entrance nip. Additional advantageous features of the instant invention is that the metal belt 12 serves as a casting surface unlike the conventional smooth surface dryer drum as presently employed in the art. This minimizes the cost of operation inasmuch as in the event of damage to the casting surface, the metal belt need only be replaced in lieu of the rather expensive dryer drum. The amount of down time is also minimized since the metal belt can be more quickly replaced than the dryer drum. Moreover, employing the same production line, it is possible to produce economically coated products having various degrees of surface smoothness or embossing thereon by merely substituting a flexible metal belt having the desired surface finish. Otherwise, it is necessary to change the dryer drum, a practice which is obviously cumbersome, or at least have different production lines which is obviously more expensive.
As brought out above, the drawing depicts the preferred method and means wherein the practice of this invention can be conducted. The wet coated paper stock as fed to the casting unit has thereon a coating composition-which is applied at about 8 to about 25 pounds per 3,000 square feet of base stock. The coating contains a heat-vaporizable liquid, e.g. water as employed in conventional pigmented coatings, e.g. clay, titanium, dioxide, and the like based coatings which further comprise an adhesive, e.g. starch, casein, and the like. It is understood of course that the coating composition can contain other materials, for example waterproofing agents, defoamers, wetting agents, release agents, and the like. In fact, an ancillary feature of the present invention is that the loss of volatile additives by vaporization occurring at the flooded nip is similarly greatly reduced or eliminated by operating in this manner. In terms of our preferred embodiment, the wet coated paper 10 is fed to the casting unit comprising the chromium plated smooth surfaced fiexible metal belt 12 in combination with the dryer drum 18 and heating means 35. The wet coated paper web 10 is fed into the nip defined by the position of the drum 18 and roller 21 wherein the puddle 11 is maintained between the wet coated web 10 and metal belt 12. The metal belt 12 is cooled by conveying it over roller 15 into contact with a source of cooling water 13 within the container means 14. Wiping means 16, such as conventional papermakers felt, is provided to remove excess water from the flexible metal belt which is then conveyed over roller means 17 into contact with the flooded nip 11 and the heated dryer drum 18. The web 10 is pressure'cast by being compressed by the pressure belt 20 over the area defined by the extent or degrees of Wrap of the pressure belt 20 around the casting surface 12 which conforms to the shape of the dryer drum 18. The degrees of wrap of the casting surface area is determined by the position of the rollers 21 and 24 which together with rollers 22 and 23 constitute a conveying system for the pressure belt 20. Operating in combination with the pressure belt 20 is vapor receiving means 19, generally conventional papermakers felt. The vapor receiving means 19 receives moisture emanating from the coating formulation which is driven therefrom during the course of casting it upon the paper web 10. The vapor receiving means 19 can be separately dried if desired by such means as the heated drum 27 which together with rollers 25, 26, and 28-32 constitute the conveying system for means 19. After casting, the cast coated paper then leaves the heated dryer drum over the roller means 33.
By way of example, employing the apparatus arrangement depicted in the drawing a paper web 10 comprising an uncalendered board having a density of 7 lbs./ream/ mil and 18 mils thick as prepared from 50 percent Kraft and 40 percent Groundwood is first given a wet coating composed of parts clay, 18 parts protein, and sufficient water to make a 55 percent solid solution which is applied to the web 10 in sufficient quantity to produce a coating of 20 lbs/ream. The wet coated web 10 is fed into the casting unit comprising the dryer drum 18 which is heated to a temperature of about 350 F. A puddle 11 of the coating material is easily maintained at the entrance nip at all times. The pressure belt 20 by virtue of tensioning means 34 is caused to exert a pressure of about 33 p.s.i. continuously and uniformly upon the paper web 10 during its contact with the metal belt 12. The vapor receiving means 19 comprises A inch thick conventional papermakers felt. The temperature of the cooling water 13 is maintained at essentially room temperature. By means of this procedure employing a dryer drum of 6 foot diameter, production rates exceeding 400 feet per minute are easily attained without entailing excessive attention on the part of the machine operator to maintain a cool flooded nip or puddle 11 of the coating material. Moreover, rapid changes in production rates can be readily accomplished, viz. from 450 f.p.m. to 550 f.p.m. in a matter of minutes without adjusting the consistency of the coating composition but still having the same number of machine operators in attendance. Operating in this manner produces a cast coated paper having an extremely smooth finish and high gloss.
What is claimed is:
1. A process for pressure cast coating a web with a thermosettable mineral coating comprising, in combination,
(a) passing said web through an entrance nip into a pressure area defined by the contact of at least two moving endless belts one of which is metal and one of which is a porous material,
(b) maintaining a puddle of liquid solution of unset said thermosettable mineral coating at said entrance nip between said web and the one moving endless metal belt, said porous belt contacting the uncoated web surface,
(c) heating the moving endless metal belt after contact with said puddle at said entrance nip to a temperature at least equivalent to the boiling point of the liquid of said liquid solution,
(d) maintaining the coated web within said pressure area for a period sufiicient to cause said thermosettable mineral coating to become pressure-cast on said web,
(e) separating said at least two moving endless belts to release the pressure on said web,
(f) recovering the resulting thermoset pressure castcoated web, and
(g) cooling the separated said one moving endless metal belt to a temperature at least below the boiling point of said liquid prior to its contact with said puddle.
2. The process of claim 1 further characterized by the heating of the moving endless metal belt being accomplished by contacting the belt with a rotating heated drum.
3. The process of claim 1 further characterized by said heating being to a temperature of from about 220 F. to about 350 F. and said pressure area between said at least two moving endless belts being sufiicient to compress the moving web there between at a pressure of from about 5 p.s.i. to about 50 psi. in excess of ambient pressure.
4. The process of claim 3 further characterized by said temperature being from about 240 F. to about 280 F. and said pressure being from about 10 psi. to about 35 psi.
5. The process of claim 1 further characterized by said one moving endless metal belt being chromium plated at least on the side in contact with said puddle.
6. The process of claim 1 further characterized by said cooling being accomplished by Water.
7. The process of claim 1 further characterized by said moving endless metal belt being smooth on the side in contact with said puddle.
8. The process of claim 1 further characterized by said moving endless metal belt having an embossing pattern on the side in contact With said puddle.
9. The process of claim 1 further characterized by said liquid comprising water.
References Cited UNITED STATES PATENTS 1,944,600 1/1934 Greene 11764 2,442,443 6/ 1948 Swallow 117-64 2,919,205 12/ 1959 Hart 11764 2,943,954 7/ 1960 Robinson et al 117-64 3,110,612 11/1963 Gottwald et a1. 11764 FOREIGN PATENTS 794,460 5/ 1958 Great Britain.
MURRAY KATZ, Primary Examiner.
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|US1944600 *||Jan 28, 1931||Jan 23, 1934||Photostat Corp||Photographic print drier|
|US2442443 *||Dec 9, 1944||Jun 1, 1948||Bakelite Corp||Apparatus for pressing plastic sheeting|
|US2919205 *||Sep 18, 1956||Dec 29, 1959||Warren S D Co||Process for finishing coated paper|
|US2943954 *||Feb 10, 1958||Jul 5, 1960||Mead Corp||Casting surface for producing castsurfaced mineral coated paper|
|US3110612 *||Dec 20, 1960||Nov 12, 1963||Albemarle Paper Mfg Company||Method and apparatus for cast coating paper|
|GB794460A *||Title not available|
|Citing Patent||Filing date||Publication date||Applicant||Title|
|US3911180 *||Oct 13, 1972||Oct 7, 1975||Fuji Photo Film Co Ltd||Method of making a resin coated paper having a smooth surface|
|US4277524 *||Dec 26, 1979||Jul 7, 1981||Mitsubishi Paper Mills, Ltd.||Method and apparatus for finishing coated papers|
|US4420502 *||Mar 18, 1982||Dec 13, 1983||Conley Kenneth E||Apparatus and method for producing a flexible sheet material having a predetermined surface characteristic|
|US4520048 *||Jan 16, 1984||May 28, 1985||International Octrooi Maatschappij "Octropa" B.V.||Method and apparatus for coating paper and the like|
|EP0117054A1 *||Jan 17, 1984||Aug 29, 1984||Unilever Plc||Method and apparatus for coating paper and the like|
|U.S. Classification||427/362, 118/60, 118/407|
|International Classification||B41F31/00, B05C9/14|
|Cooperative Classification||D21H23/40, D21H5/0015|
|European Classification||D21H23/40, D21H5/00C8B|