US2759847A

US2759847A - Method of producing high gloss mineral-coated paper and resultant product

Info

Publication number: US2759847A
Application number: US343584A
Authority: US
Inventors: Frederick H Frost; Philip S Leighton
Original assignee: Warren SD Co
Current assignee: Warren SD Co
Priority date: 1953-03-20
Filing date: 1953-03-20
Publication date: 1956-08-21
Anticipated expiration: 1973-08-21
Also published as: USRE25039E

Description

F. H. FROST ET ODUCING HIGH GLOSS PAPER AND RESULTANT PRODUCT Filed March 20, 1955 United States Patent O NIETHOD OF PRODUCING HIGH GLOSS MIN- ERAL-COATED PAPER AND RESULTANT PRODUCT Frederick H. Frost, Portland, and Philip S. Leighton,

Westbrook, Maine, assignors to S. D. Warren Company, Boston, Mass., a corporation of Massachusetts Application March 20, 1953, Serial No. 343,584

Claims. (Cl. 117-64) This invention relates to the production of mineralcoated sheet material coated on one or both sides and having a very high gloss and a very level surface, and to the glossy mineral-coated sheet material so produced.

In the production of mineral coated paper a paper base which has been surface-sized, surface-filled, or which may be coated or uncoated, is coated with an aqueous composition comprising mineral pigment such as clay, blanc xe, calcium carbonate, talc, titanium dioxide, various colored pigments or the like and adhesive material such as casein, starch, glue, polyvinyl alcohol, synthetic elastomer, or the like. The coated surface is then dried and smoothed. Customarily from a minimum of about one pound to a maximum of about seven pounds of dry weight of coating is applied to each 1000 sq. ft. of surface coated. The lighter Weights of coating are used if the base paper has been previously coated whereas the heavier weights are used on base paper which has not been previously coated or surface lled. Thus the coating may be applied in one llayer or in two or more layers as desired; either one or both sides of the web may be coated. Most coated papers are first dried and then smoothed by calendering and such papers constitute the standard glossy coated papers of commerce. Such papers possess an attractive sheen but do not have a high specular or l mirror-like gloss approximating, for instance, that of polished metal surfaces.

For producing a high specular gloss on coated paper surfaces which are normally used for commercial printing purposes there are presently in use only three cornmercial methods, namely: dint-glazing, bufng as disclosed by Clark Patent No. 2,349,704, and cast coating as disclosed by Bradner Patent No. 1,719,166 and Warner Patent No. 2,316,202. A fourth method of producing glossy-coated paper is by friction-glazing, by which the coated surface is burnished by a rapidly turning metal roll, usually heated. The result so produced is somewhat sirnilar, but generally quite inferior, to that obtained by flint-glazing. Generally speaking, the friction-glazed product is not suliiciently high in specular gloss to be in the same class as the products previously mentioned.

The present invention has for its object the production of mineral-coated paper having high specular gloss at least comparable to that of paper produced by the before mentioned known methods.

The present invention is based on the discovery that the application of aqueous liquid to the surface of a hard, dry, `densifed supercalendered coating comprising hydrophilic material on a sheet of paper just as it passes into the nip between a highly polished roll and a backing roll with the coating facing the polished roll will cause the hydrophilic material at the surface of the coating to swell, become moldable and by expansion to be pressed against and molded by the polished roll, thereby receiving a surface which is complementary to that of the polished roll and that the novel paper so produced may have a gloss Iat least equal to that made by any other process of which we are aware.

2,759,847 Patented Aug. 21, 1956 According to the invention a paper web or other flexible sheet material is coated by any desired means, e. g., roll coater, brush coater, etc., with one or more applications of an aqueous hydrophilic coating composition comprising a finely -divided coating pigment such as clay, a hydrophilic material and sufficient adhesive material to bind the pigment to the paper base. The hydrophilic material and the adhesive material may be identical or different. Suitable hydrophilic materials include casein, starch, soy protein, glue and polyvinyl alcohol, which may be used singly, mixed with each other, or mixed with a substantial proportion of an elastomer such as synthetic rubber latex. The paper web so coated is then dried and densied by calendering, under considerable pressure, as by a supercalender. It is not necessary to calender enough to develop high gloss, but it is necessary to use enough pressure to densify the coating and it is advantageous to render the surface fairly level; it is permissible, but not essential, to calender until a high supercalendered gloss is obtained. The weight of coating of the finished paper may be the same as in the prior methods referred to.

In contrast to this procedure in the method of cast coating described in Patents 1,719,166 and 2,316,202, the cast coated paper is neither air-dried nor calendered. Instead, the wet freshly coated paper is rolled tightly against the surface of a heated drum and is there dried. Thus cast-surfaced coated paper is made Without the use of a supercalender with its high pressures and the coating on its surface has a higher bulk and greater ink absorbency than paper which is made according to the present invention.

In the method of this invention, the densitied coated surface is wet with an aqueous liquid as the paper passes into the nip of a pair of rolls one of which is heated and has a polished surface complementary to that of the glossy surface of the paper to be produced. The other roll is preferably a rubber-covered backing roll. The aqueous liquid is applied to the densitled surface as closely as possible to the nip of the two rolls, so that the swelling of the hydrophilic component of the coating takes place as nearly coincidentally as possible with the passage of the wetted portion of the coating into and through the nip of the rolls, and before the aqueous liquid has had time to penetrate the coating and pass into the backing sheet. The coating remains in contact with the heated polished surface until it has dried. When dry, the paper cornes away easily from the polished support and has a surface which is substantially the mirror image of the polished roll surface. If they roll surface is highly polished, the coated paper dried against it will exhibit an extremely high gloss.

The most satisfactory way of applying an aqueous liquid to the coating without penetration to the backing sheet is by keeping the nip between the paper and the polished roll surface lled or flooded with aqueous liquid. Liquid may be injected directly into the nip, or it may be applied to the polished roll surface before the latter comes into contact with the surface of the coating or both in quantity sufficient to keep the nip filled and Hooded.

With some coatings water alone is an entirely satisfactory wetting liquid; with other coatings which are more water-resistant, such as coatings containing a considerable proportion of a synthetic elastomer or partially waterproofed casein, it is advantageous to use an aqueous solution of wetting and/or swelling agent or the like. A dilute solution of ammonia is effective in some such instances.

It is advantageous in some instances to include in the coating composition applied to the paper a small quantity of an anti-sticking or parting agent to facilitate release of the dry coated'surface from the polished surface after it has dried in contact therewith. Soaps and solid polyethylene glycol are useful anti-sticking agents. lf the dried coating parts from the polished surface with difhculty, parting may be facilitated by adding a small quantity of anti-sticking agent to the aqueous liquid in the nip; a trace of glycol monolaurate or ammonium stearate dissolved in the aqueous liquid is found very effective.

The best results appear to be obtained when the following conditions are observed:

l. That the sheet material used bear on at least one side a surface layer of mineral-coating containing some hydrophilic material, preferably a hydrophilic adhesive material, which is swellable by absorption of aqueous liquid.

2. That the coating be densified and the gross irregularities in the levelness thereof removed.

3. That the coated sheet be dry, e. g. substantially airdry, before aqueous liquid is applied to the surface thereof according to the invention.

4. That the application of aqueous liquid to the surface of the coating take place as nearly simultaneously as practicable with the entrance of the wetted portion into the nip of the rolls so that the wetted surface will be in the nip of the rolls as the hydrophilic component of the coating swells.

5. That the quantity of aqueous liquid applied be limited so that the swelling of the hydrophilic material will take place before substantially any of the aqueous liquid can penetrate into the paper base.

6. That the wetted surface of the coating remain in contact with the polished drying surface until substantially dry.

Normally, when mineral-coated paper is densified by calenderng the surface produced looks reasonably level to the unaided eye. Under a microscope, however, the surface is seen to be made up of juxtaposed high, glossy areas, and lower, less glossy areas.

Commonly, there are also visible under the microscope fairly numerous and relatively deep depressions or frothpits in the coated surface. The supercalendering operation considerably decreases the thickness of the coating originally applied and densities it, accomplishing that result by pushing nearer together the individual particles of mineral pigment and thus decreasing the size, but not the number, of the interstices therebetween. At the same time the elevation of the high spots above the low spots is considerably decreased; froth-pits, however, are so comparatively large that little difference in their appearance is noticed after the supercalendering operation.

When such a supercalendered mineral-coated paper is passed through a nip which is kept filled or ooded with aqueous liquid, the quantity of said aqueous liquid passing the nip apparently will be the sum of the following quantities; (l) the quantity which forms a liquid film between the surfaceof the coating and the surface of the drum, (2) the quantity which penetrates into the interstices between particles, and (3) the quantity which is actually absorbed by the hydrophilic material present in the coating. Of these three quantities the second and third are by far the most important in relation to the quality of the finished product of the invention. In the case of properly supercalendered mineral-coated paper the total quantity of aqueous liquid taken up in practicing the invention is so comparatively little that it can be readily retained at the surface of the coating or without penetrating the underlying paper base. On the other hand, in the case of a coating which has not been densified the roughness of the coating permits the trapping of a considerable quantity of aqueous liquid between the surface thereof and the surface against which it may be passed in a nip, e. g. the finishing drum surface-a quantity which may be greater than the coating layer itself may be able to absorb completely. Moreover, when the interstices between pigment particles have not been partly closed, as by a calendering operation, they alord effective passages to the underlying paper base which are easily penetrable by aqueous liquid; and when the paper base once becomes wetted thereby, the wet fibers may then act as wicks to drain liquid from the coating. This may be the explanation of the noted fact that uncalendered mineral-coated papers do not yield as satisfactory results when treated by the polishing process of the invention.

Ideally in the practice of the invention the surface of the coating acquires a mirror image of the polished surface against which it is pressed and dried. Variables which affect the degree to which the ideal is approached are (l) the degree in which the hydrophilic adhesive or other hydrophilic material quickly swells, and (2) the external pressure applied in the nip which forces the paper surface against the polished drying surface. Both variables aid in obtaining intimate contact between the surface of densifled coating and the surface of the drum.

The most feasible polished surface against which the mineral-coated paper may be pressed in the practice of the invention is a metal drum of good size, say 6 feet or more in diameter, which is internally heated, and suitably is plated with chromium, polished and maintained by conventional methods. For use to press the coated paper undergoing treatment against the drum the most suitable means appears to be a rubber-covered roll soft enough to ensure substantially even pressure across the entire sheet or web of the paper. In such a set-up, a nip of appreciable width, i. e., the distance from front to back, is produced and while the nip pressure readily may amount to at least one hundred pounds per linear inch across the width of the sheet, such pressure is far less than the high pressures customarily used in supercalenders. If a supercalendered mineral-coated paper containing no hydrophilic material which is in the least swelled by aqueous liquid applied thereto, is treated according to the invention, no amount of pressure expedient to apply will appreciably affect that sheet.

If, however, the hydrophilic matter present in the surface of the coating is swelled, even to a very slight extent, by aqueous liquid applied thereto, the coating is to some extent rendered moldable and when pressed against the polished surface it is molded to conform to the surface of the polished roll. In such a case the degree of deformation of the surface of the coating and consequent increase in polish or gloss varies with the pressure which is applied to it in the nip. The gloss obtainable by such treatment is very high indeed. rl`he operation itself differs lfrom conventional calendering, in that in part at least, the pressure which molds the surface of the sheet is supplied by the swelling of the hydrophilic component of the densified coating and that the swollen or moldable mineral-coating adheres firmly to the surface of the polished drum until the coating has become dry again. The effect of the treatment on the paper is that the gloss imparted thereby is much higher than that obtainable by the severest supercalendering, but dinginess or blackening is not increased and may be decreased.

Since only a very small quantity of aqueous liquid passes through the nip, the drying problem is much less diflicult and relatively high speeds can be attained even on a drum of small diameter and the expense for equipment is reduced since polished drums even of small diameter cost thousands of dollars. Moreover, and more irnportant, because only a very small amount of aqueous liquid is involved and has to escape as water vapor through the reverse side of the sheet, it is .possible to turn the sheet over and subject the reverse side to the same process without appreciably affecting the finish previously developed on the first side. Thus the process of the invention is capable of producing mineral-coated paper having both sides with substantially identical appearance and high gloss free from visible imperfections while by the prior processes it has been ditiicult or perhaps impossible to produce 'commercially a coated paper having satisfactory high gloss on both sides, except by flint-glazing or buiiing.

As the swelling increases, the surface is reshaped and molded against the polished drum. If the coating swells enough small froth-pits and other small surface defects originally present may even disappear. The coating after being dried may be found to have expanded slightly and to have become slightly less dense; though the change is exceedingly small. The small decrease in density, however, is effective in decreasing or removing dinginess or mottle that may have been present as a result of supercalendering. Concomitantly the brightness of the coating is improved and the opacity may be slightly increased by the treatment. The coated surface embodying the present invention is free from the cross-grain microscopic scratches observable in the surface of glossy dint-glazed paper and from the longitudinal microscopic scratches observable on the surface of the glossy buffed paper made according to aforementioned Clark Patent No. 2,349,704. Moreover, the product does not show under a microscope the outlines of fibers which are often observable in commercial so-called cast-coated papers, doubtless because under the invention the paper base itself is not wetted by the aqueous liquid.

Because the coating of the product of the invention has been so compressed or densified, the product can be printed by regular printing inks commonly used for printing ordinary supercalendered, glossy, mineral-coated printing paper, and does not necessitate the use of the special transparent inks required for use on prior art cast-coated paper nor such careful handling by the printer. The densified coating of the paper embodying our invention does not absorb so much of the vehicle of ordinary printing ink into its interstices that the ink pigment left on the surface is chalky and dull-looking. The fact that the product of the present invention, while retaining all the advantages of appearance possessed by cast-coated paper, can at the same time be printed readily and satisfactorily by use of regular printing inks, both letter-press and lithographie inks, is a very valuable consideration from the viewpoint of the printer.

Another important advantage of the densified product of the invention is that it is not much more subject to damage from breaking of the coating during handling than are conventional supercalendered mineral-coated printing papers. The papers embodying our invention do not have to be handled with special care while being manipulated in the press room.

The swelling of the hydrophilic coating material in making our coated product may be controlled by variation of either one or both of two separate factors, namely; the composition of the coating itself and the composition of the aqueous liquid used. For example, variations can be made in the particular kind and in the quantity of hydrophilic material used in the coating. Commonly the hydrophilic material will be the adhesive, such as casein, glue, soy protein, polyvinyl alcohol, starch, etc., or mixtures of the same. The degree of swelling of the hydrophilic material will likewise depend upon whether or not any water-proofing or insolubilizing treatment has been given the coated product. For instance, it is common practice to subject the common hydrophilic adhesives used in coatings to the action of formaldehyde or other aldehyde to render them more or less insoluble. Such treatment definitely decreases the swellability of the material.

If the coating is found to be slow to swell upon application of water alone, it is frequently found that an aqueous ammoniacal solution will be much more eflicacious in swelling the material. On the other hand, if an unusually susceptible material, such for example as a highly alkaline sodium caseinate, swells too much or takes up too much water, a little formaldehyde dissolved in the aqueous liquid used to Wet the coating will decrease the swelling to the desired degree.

As has been stated, water or aqueous liquid must not be drained away into the paper base before'the hydrophilic matter has had time to swell against the polished surface of the drying drum. By proper selection of the aqueous liquid applied, and control of the quantity thereof as Well as by proper densiiication or calendering of the coating, it is possible to practice the invention when the paper base itself has little or no resistance to being wet by water. It may be advantageous to have the paper base surface-sized or surface filled, possibly because such treatment tends to lay down fibers which otherwise might project into the coating layer and act as wicks to lead away water therefrom at a later step in the process.

In practicing this invention the maximum temperature to which the drum is heated becomes an important consideration. If the drum is heated to a temperature so that the aqueous liquid which is fed to the nip trough boils, a large part of the coating on the paper entering the nip will be removed and become incorporated in the aqueous uid in the nip. If this situation occurs the previously dried and compacted coating loses its identity and the resulting product is worthless. The resultant surface tends to resemble stippled plaster and has no finish which would be capable of receiving a printed impression.

It is desirable to maintain the temperature of the drum as high as possible in order to facilitate the rate of drying of the small amount of aqueous liquid which passes through the nip. However, care must be exercised so that this liquid does not evaporate at a rate of sucient magnitude to form steam bubbles between the surface of the coating and the surface of the drum which would break the contact between these two surfaces. The generally preferred drum temperature is from about to about F.

It is believe that the most nearly perfect surfaces resulting from practice of the invention occur in those cases in which the hydrophilic coating swells the most quickly and to the greatest degree in the pressure nip. In such cases, however, it is desirable to avoid substantial swelling before the coating is pressed against the drying surface. This is accomplished by maintaining a pool of aqueous liquid in the nip trough formed by the press roll and the polished roll. A pool in the nip trough of an inch or two in depth is ample. The permissible depth, however, depends in part upon the speed at which the paper passes through the nip; in most cases the speed will be at least 100 feet per minute. At this speed and at liquid depths of one or two inches the surface of the coating is in contact with the aqueous liquid for only from 1/0 to 1/10 of a second before it passes into the nip and into contact with the polished roll. By keeping short the period of contact between the surface of the coating and the aqueous liquid the amount of material removed from the surface of the coating and washed away with the nip efuent is reduced. This holds the loss of coating weight to a minimum in this process.

Variations in glossy appearance of the product can be obtained by varying the exact conditions of treatment, the optimum conditions in general being best determined by experiment in each separate case. Nevertheless, very excellent appearance generally results in all cases even when no attempt is made to reach absolutely optimum conditions. The specular gloss values of papers embodying the present invention may be measured by a Scotty Aminco goniophotometer made by the American Instrument Company of Silver Spring, Maryland. In making the tests, the sample was illuminated by a beam of light making an angle of 70 degrees from normal to the sample surface, and the reilected light measured was at an angle also 70 degrees to the normal to the paper surface but on the opposite side of the normal, so that if the illumination was at plus 70 degrees the reflected light was at minus 70 degrees to the normal. The reflected light was passed through a lens and then through a slit aperture placed at the focal distance from the lens. The slit subtended an angle of 1 38 in the plane of the light beam and an angle of 5 2 across the plane ofthe beam, the

angles beingy from the optical center o'f the lens.v The numerical figures so obtained for specular' gloss'- are' percentages of the light reflected" fr'orna'higl'il'y polished fiat black plate glass" standard.

Measurements made as described upon'usual supercalen-l dered glossy-coated printing papersof the' quality at present sold as No. 1 quality glossy-coated" printing papers'v usually yield values of specular gloss between "a`nd 15l Especially severe supercalenderin'g may yield papersiliaving described, of at le'ast 30, and preferably o'fabout 3'5` or' Most samples fall between and 60' specular' above. gloss, but frequently specular gloss values over" 60 are reached. It is found, however; that increasing values of specular gloss beyond a certain' point' are not worthwhile because any sheet havingal specular gloss' as high a's 40 is so excellent in appearance that little furtherV improvement either in appearance or in printing qualities is observable by the users'.

The improvement in printing" qualities maybe determined quantitatively by the tests for' ainity for ink and ink-setting time described in patent No. 2,395,992 of John W. Clark. Briefly, it'm'ay be said that the inksetting time is time in seconds between the time the sheet is printed with a standard printingink and the time the ink applied ceases to look wet; it is a reliableY indica`- tion of the receptivity of the sheet for printing ink. Generally speaking, papers having an' ink-setting time of not over seconds are considered to have excellent ink receptivity. The products of the invention usually show ink-setting times Well under 40' seconds. Glossy-coatcd papers which are' finished by the Hint-glazing' process or by the bui-"ting process' ofthe Clark' Patent No. 2,439,704' invariably have ink-setting times considerably in` excess of 40 seconds, and are considerably more difficult to print satisfactorily than are' the products of this invention. The product of the invention is also free from the characteristic scratches found in fiint-g'lazed and buffed paper.

Conventional mineral coatings can generally be treated according to the process of the invention provided they contain a hydrophilic water-swellabte component which is not too water-resistant. Conventional pigment ma'- terials or mineral fillers suitable for' use' in coatings to be treated according to the invention include: clay, calcium carbonate, blanc, talc, titanium dioxide, ochr'e, iron oxide, ultramarine, colored lakes and toners and the like. Some embodiments of the invention are' shown by the following specific examples:

Example 1 y A coating composition was prepared of the followingingredients:

Parts by weight,` dry Fine paper coating clay 7 Fine paper coating calcium carbonate 30 Casein (solvated by ammonia) f 15 Water to make solids content This composition was applied by means of an air-knife coater to one side of a paper body stock of pounds ream weight which had been surface-sized with a starch solution. Enough coating wasapplied to amount to 15 pounds, dry weight, per ream of 500 sheets 25 x 38 inches in size. The coated web was dried and supercalendered to an average supercalendered finish. The supercalendered surface was then run into a nipagainst a heated chromiumplated drum while water was fed into the nip. The paper was removed from the drumhwhen` dry and Was foundV to have a specular gloss value of 47.

Cil

ExamplelZV A coating was prepared of the following ingredients: Par-ts by weight, dry

Fine particleY English clay 30 Fine particlefGeorgiaclay 35 Fine'particle calcium'carbonate 35 f-Casein (solv'ated by Na-iPzO'z) 15 Water tomake solidsrcontent 44%.

This composition was applied to one side of a surfacesized paper body stock oft` 13'5'pounds weight per ream in quantity amounting" to 16 pounds, dry weight, per ream and'supe'rcalendered to a normal supercalendered finish. The supercalendered surface was then run at feet per minute into a nip against a steam heated polished chromuuu-plated drum four feet in diameter while an aqueous solution of 0.4% formaldehyde was fed into the nip and maintained at a depth of about 1/z inch. The drum surface adjacent the nip was about F. The paper was removed from the drum when dry and was found to have a specular gloss value of 50.4.

Example 3 A coating composition was prepared of the following ingredients:

Parts by weight, dry Fine coating clay 60 Fine calcium carbonate 40 Starch, enzyme converted' 16 Casein, solvated by ammonia 4 styrene-butadiene copolymer (latex form) 4 Water to make solids content 46 A coating composition was prepared of the following ingredients.

Parts by weight, dry Fine paper coating clay 100 Caesin (solvated by ammonia) 17.4 Ammonium stearate 0.65

Water to makey solids of 46.5%

This coating was applied to one side of a surface filled paper of about 5l pounds ream weight, about 15 pounds dry weight of coating being applied per ream. The paper was dried and supercalendered. The supercalendered surface was run into a nip with the coated side against a heated chromium-plated drum while water was fed into the nip. The surface of the coating was dried in contact with the drum and released cleanly when dry. The specular gloss of the finished sheet was 59.5.

Example 5 A coating composition was made of the following ingredients:

Parts by weight, dry

Fine paper-coating clay 65 Fine paper-coating calcium carbonate 35 Casein (solvated by ammonia) 16 Water" to make solids 42% This coating composition was applied to both sides of a surface-sized paper base of about 68 pounds ream weight, about 16 pounds, dry weight, of coating being applied to each side of the sheet. The paper was dried,

and supercalendered on both sides. The supercalendered web was then run into a nip with one side against a heated polished chromium-plated drum while the nip was kept ooded with an aqueous solution containing 1.0 per cent of dissolved ammonia and 0.25 percent of dissolved ammonium stearate. The sheet was dried in contact With the polished drum and then removed. The sheet was then reversed and run into a nip with the other coated side in contact with a heated polished chromium-plated drum while the nip was kept flooded With an aqueous solution containing 0.8 per cent ammonia, 0.4 per cent hexamethylenetetramine and 0.25 per cent of ammonium stearate. 'Ihe sheet was dried in contact with the drum and was then removed. The specular gloss of the first treated side Was 40 and of the second treated side was 41.

Accordingly we believe ourselves to be the first to discover that a superior high gloss can be produced on a supercalendered coated sheet by wetting the surface of the densiied coating suiciently to cause the hydrophilic component of the coating to swell and bringing the wetted surface into contact with the highly polished surface of a heated roll under pressure part of which may be supplied by the swelling of at least part of the hydrophilic component as it passes between the nip of the polishing roll and a suitable backing roll, and that by so doing a paper sheet may be produced having on one or both sides a densiiied mineral-coating of a density comparable to a usual supercalendered mineral-coating, but having a specular gloss of at least 30, and preferably at least 35, as measured as described on a Scott-Aminco goniophotometer; and the coated surface being easily printable by the same techniques and same printing inks customarily used in printing supercalendered mineral-coated paper.

The invention is illustrated in the accompanying drawings in which:

Fig. 1 represents a greatly magnified cross-section of coated paper prior to supercalendering,

Fig. 2 represents a cross-section of the same paper after supercalendering,

Fig. 3 represents a cross-section of the same paper after the gloss treatment, and

Fig. 4 is a ow sheet of the process.

In Fig. 1, 1 is the paper body stock and 2 the coating comprising the pigment particles 3 and the adhesive 4. The porosity of the body stock and of the coating cannot be illustrated but as appears in Fig. 2 both the body stock and the coating have been densified by reduction of the degree of porosity. The number and size of the pigment particles has not been reduced but they have been brought into closer proximity to each other. It will be seen further that the surface of the coating of the supercalendered sheet in Fig. 2 is more nearly level than in Fig. l. As shown in Fig. 3 the coating has been slightly swelled or expanded by the gloss treatment and its surface has been further smoothed and levelled.

It will be appreciated that the illustrations in Figs. l to 3 are not intended accurately to represent the structure of the paper but are given merely to show applicants conception of the effects of the supercalendering and gloss treatments upon the body stock and coating of a sheet of coated paper.

In Fig. 4, 5 is a feed roller f-or the paper body stock, 6 is a conventional air knife coating unit, 7 a dryer, 8 a supercalender, 9 the molding drum having a highly polished chromium surface, y10 a pressure roller, =111 a windup roller and 12 the paper web. The movement of the paper through the apparatus from the feed roller 5 to the Windup roller 1\1 is indicated by arrows. The pool of wetting liquid in the nip between the molding drum land the paper web is shown at 1B.

It is believed that in carrying out the preferred method of this invention a thin iilm or layer of aqueous liquid is formed on the surface of drum 9 as the coated paper passes through the nip of the press roll surface 10 and the drum surface. Hence it vis believed that the wetted surface of the coating after passage through the nip p'ool is pressed against a thin lm or layer of aqueous liquid on the drum-surface and as the paper revolves with the hot drum surface, Water from both said aqueous liquid in the lm and the wetted coating surface is evaporated and .passes through the paper as vapor. The terms static adhesive contact with, static contact with, immovable contact with and forcible contact with a polished surface, polished finishing surface or the surface of -the iinishing roll as used in the specification and claims include contact with such a surface which carries a thin lilm or layer of the aqueous liquid which is supplied t-o the nip of the press and the finishing rolls.

This application is filed as a continuation in pant of our application Serial No. 210,214, filed February 9, 1951, which was a substitute for our application Serial No. 207,112 filed January 22, i1, both now abandoned.

Although it has been attempted to explain the theory of the present invention, it is not intended that the present invention be limited to such theory.'

It should be understood that the present disclosure is for the purpose of illustration only and that the present invention includes al-l m'odiiicat-ions and equivalents which fall within the scope of the lappended claims.

We claim:

1. A method comprising applying aqueous liquid to the dry surface of the coating of a supercalendered coated paper, the coating :of which comprises a water s'wellable hydrophilic material, subsequently pressing the resul-ting moist surface into static adhesive contact with a highly polished finishing surfa-ce and drying 4the paper while in contact with said finishing sunface.

2. A method comprising applying aqueous liquid to the surface of the coating of a dry, supercalendered coated paper, the coating of which contains a water-swellable hydrophilic material, as nearly simultaneously as possi- -ble with the application of said liquid, pressing said surface into static contact with a highly polished finishing surface and drying the paper while in contact with said finishing surface.

3. The process of claim 2 wherein said nishing surface is wet with aqueous liquid when said surface of the coating is pressed thereagainst.

4. A method comprising applying a coating composition comprising a water swellable hydrophilic material to a web of paper, drying the resulting coated paper, supercalendering the dried coated paper, passing the surface of the coating of said dried, supercalendered coated paper through a pool of aqueous liquid, directly thereafter without removing the same from said pool pressing said surface into static adhesive contact with a heated, polished finish-ing surface and drying the paper while in contact with said finishing surface.

5. The process of claim 4 wherein said finishing surface is maintained wet with aqueous liquid when said surface of the coating is pressed thereagainst by passage through said pool.

6. In a process comprising applying aqueous liquid to the dry surface of the coating of a coated paper, the coating of which comprises a swell-'able hydrophilic material, subsequently passing said coated paper through a nip between a highly polished roll and a backing roll with the coating facing said polished roll and drying the paper While in static adhesive contact with said polished roll, the improvement comprising drying and subsequently supercalendering 'said coated paper before applying aqueous liquid t-o the surface of the coating and applying said aqueous liquid to said surface of the coating as it passes into said nip.

7. The process of claim 6 wherein said aqueous liquid is applied to said surface of the coating as it passes into said nip by maintaining a pool of aqueous liquid in said nlp;

8. In a process comprising applying aqueous liquid to the surface of the coating of a coated paper, the coating of which comprises a swellab'le I hydrophilic mate-rial' and subsequently passing said' coated paper through ai nipbetween` a highly polishedA roll andg av backing roll with` thecoating facingsaid polishing roll'and drying-the paperr while in static adhesive Contact withv said polished roll, the improvement' comprising drying and subsequently supercalcndering said'coated paper before applying aqueous liquid to the'same.

9. That: improvement in the art of7 makingl polished paper which comprises compressing' dry coated paper byy super-calend'ering,4 passing the compressed paper between a press roll? and a: highly polished-nishingl-roll, wetting the surface of the coating-with aqueous liquid as theing, and while theswelling is taking place and'beforethel body stock has been-swollen bringing thewetted". surface of the coating into forcible contact with a polishcdl surface and drying: the moistened surface while still in static adhesive contactcwith the polished surface.

11. That step in the` process of making highly polished' mineral-coated paper which consists, in swelling the surface of a hard*` dry supercalend'ered'sheet by the application of moisture to the coating as nearly as possible at the moment when the sheetpasses` between a pair of rolls one of which has a highly polished' surface and drying it while instatic adhesive Contact therewith, said coating comprising1 awater swellable hydrophilic material.

12. A method comprising applying aqueous liquid` to' theAdry surface ofthe coating of coated paper which has beencalendered withl enough pressure to-densify thecoating and to render the surface fairly level, said coatingcomprisingv a water swellable hydrophilic material, subsequentlypressing the resulting moist surface into static adhesive contact with a highly polished nishing surface and drying the paper while in` contact with said finishing surface.

173. A method as defined inA claim 12 in which the aqueous liquidi's applied to the coating by passing the paper througha pool thereofv which is in contact with said finishing surface.

1'4. A method as defined inclaim 12 in which the aqueous liquid is applied to the coating by passing the paperv through a pool thereof which is supported in thenipbetween a highly polished rolll and a backing roll.

1'5. A high glossmineral-coated paper substantially identicalA with the product obtainable by the method deiined4 in claim` 12.

References Cited in the file of this patentr UNITED STATES PATENTS 2,003,065 Boyce May 28, 1935 2,069,786. Van der Meulen Feb. 9, 193.7 2,293,278 Cates Aug. 18, 1942 2,304,818 Grupe Dec. 15, 19,42 2,304,819 Grupe Dec. 15, 1942 2,611,717 Sody Sept. 23, 1952

Claims

1. A METHOD COMPRISING APPLYING AQUEOUS LIQUID TO THE DRY SURFACE OF THE COATING OF A SUPERCALENDERED COATED PAPER, THE COATING OF WHICH COMPRISES A WATER SWELLABLE HYDROPHILIC MATERIAL, SUBSEQUENTLY PRESSING THE RESULTING MOIST SURFACE INTO STATIC ADHESIVE CONTACT WITH A HIGHLY POLISHED FINISHING SURFACE AND DRYING THE PAPER WHILE IN CONTACT WITH SAID FINISHING SURFACE.