US3702779A - Coating of webs by freeze-drying and products therefrom - Google Patents

Abstract

COATED PAPER IS PRODUCED BY APPLYING A COMPOSITION COMPRISING A POLYMERIC MATERIAL WITH OR WITHOUT ADDED PIGMENTS DISPERSED IN A LIQUID VEHICLE, FREEZING THE COATING, SUBJECTING THE FROZEN COATING TO A VACUUM TO REMOVE THE MAJORITY OF THE LIQUID VEHICLE, AND RETURNING THE DRIED OR PARTIALLY DRIED COATING ON THE WEB TO AMBIENT TEMPERATURE.

Description

Nov. 14, 1972 T. A. FADNER ETAL 3,702,779

COATING OF WEBS BY FREEZE-DRYING AND PRODUCTS THEREFROM Filed April 21, 1970 Li I OOOOOOOOO INVENTQRS THOMAS A. FADNER KARL V- KRASKE flhad 7 f I TORNEYS United States Patent 3,702,779 COATING OF WEBS BY FREEZE-DRYING AND PRODUCTS THEREFROM Thomas A. Fadner, Oxford County, Maine, and Karl V.

Kraske, South Hadley, Mass, assignors to Ethyl Corporatiou, Richmond, Va. Continuation-impart of application Ser. No. 749,008,

July 31, 1968. This application Apr. 21,1970, Ser.

Int. Cl. B05c 11/10 US. Cl. 117102 R 31 Claims ABSTRACT OF THE DISCLOSURE Coated paper is produced by applying a composition comprising a polymeric material with or without added plgments dispersed in a liquid vehicle, freezing the coatmg, subjecting the frozen coating to a vacuum to remove the majority of the liquid vehicle, and returning the dried or partially dried coating on the web to ambient temperature.

BACKGROUND OF THE INVENTION Conventional coated printing papers and other coated papers are made by applying a coating consisting of inorganic pigments, binders and a vehicle such as water to a paper web by any one of several well-known methods. In all of these, the vehicle is evaporated from the liquid state to leave a solid coating on the surface of the paper. This drying step has usually been accomplished through the use of such techniques as high velocity air impingement, by contact with heated drums or by subjecting the wet coated sheet to infrared radiation, microwave radiation and the like. These conventional processes are similar in that the vehicle, for instance, water, is removed from the coating by transforming the liquid vehicle into vapor. When drying a coating from the substantially liquid state, the forces arising due to the surface tension of the residual liquid cause the coating film to shrink as the vehicle is removed such that upon more or less complete removal of the vehicle, the coating more or less conforms to the original surface of the web which was coated. This shrinking effect and the tendency of the coating to conform to the web surface during drying has been a major reason for continual development efforts towards high solids coatings applied for instance by trailing blade techniques, or towards drying the wet coating when in position against a polished drum, in attempts to obtain better printing surfaces. Despite these developments, this shrinkage and conforming property produces a surface rough enough that the additional step of supercalendering is generally required to smooth the surface of the coating so that it is useful in high quality printing applications. Typically, with the prior art procedures described above, the conversion of a paper base to a high quality printing paper is attended by an increase in apparent density from an original to 12 pounds per 3300 sq. ft. per mil of thickness to values ranging from about 18 to 24 Ice pounds per 3300 sq. ft. per mil. As a consequence, it has become common in the art of printing papers to associate heavyweight or high density with high quality.

Attempts have been made to improve the quality of presently available coated papers and to produce a high quality coated product without using heavyweight paper and without increasing the thickness of the coated sheet. Generally, such attempts, which have involved the formation of porous type coatings prepared from two-phase emulsified oil in water systems, have not been commercially successful to any great extent. With such two-phase systems, the porous nature of the coating is the result of forming globules of oil as a discontinuous phase in the liquid carrier for the coating material. The production of pores in the final product requires removal of this discontinuous oil phase generally subsequent to substantial removal of the continuous phase portion of the liquid carrier. This removal procedure, however, increases the overall cost of manufacture of the coated product and does not eliminate further processing, such as supercalen-' dering or other smoothing operations, as is usually required with normal coating procedures to produce an acceptable high quality product. Additionally, the ability to form uniform pore sizes in the dried coating from these prior art two-phase coating formulations is severely limited by the practical difiiculty of forming a uniform particle size emulsion coating formulation. Such emulsions will generally have a broad distribution in particle size, which broadens further with the age of the formulation, temperature changes, etc. Consequently, in practice, a number of the resulting pores or voids will have dimensions either below or above the most useful size range for optimum scattering of visible light. Maximum opacifying and brightening efiiciency is therefore difficult to obtain and difficult to reproduce. Further, this aging coalescence can readily occur during drying of the emulsion coating. It is not uncommon to have a void size variation as great as 50 fold.

In addition to a more or less random and broad distribution of void dimensions, this prior art two-phase coating technology results in a more or less random distribution of the material surrounding these voids. Variations as great as fold are common. A physical configuration of this type will have little structural strength. The thin membranes will break readily under compressive, shearing or printing forces. Thus, with the two-phase coating systems, smooth, printable coatings are diflicult to obtain without either adding pigments, using a subsequent surface treatment, or overcoating. All of these treatments, however, serve to reduce the appearance, optical properties, lightweight and/or economical advantages which might otherwise be characteristic of these coatings.

In addition to the two-phase coating systems, plastic or resinous compositions have been formed into porous films for application onto a backing material. The porous film is formed by first freezing a dispersion or emulsion of the composition. The freezing produces expanded ice structures in the mass; whereupon, subsequent thawing produces a resinous mass which is generally sponge like. The thawed mass is then broken down into a moldable or coatable composition. This composition can be applied to a suitable backing material; and with the application of heat, the dispersing medium is removed and the composition fused together into a porous film.

SUMMARY OF THE INVENTION The present invention provides a method which permits the direct production of a lightweight coating with high smoothness and opacity without necessity for subsequent smoothing, coating or surface treating operations and without the necessity of using a cumbersome twophase liquid system. Briefly, the coated product is one which is formed by coating a web or sheet of paper with a mixture consisting of a polymeric material dispersed in a liquid vehicle with or without pigments and adhesives, freezing the applied coating before the vehicle has evaporated or been substantially absorbed into the paper, introducing the frozen coated sheet into a vacuum chamber where liquid vehicle is substantially sublimed away from the frozen coating, and returning the substantially dried web to normal temperature.

We have found that the thickness of the dried coatings prepared according to this disclosure remain substantially the same as its wet thickness when freshly applied to the substrate. The surface of the coating thus prepared has substantially the same high degree of smoothness and gloss as the wet coating at the time it was frozen. The opacity of the resulting lightweight coating is markedly improved over an equivalent weight of the best known conventionally dried pigmented coatings. Further advantages of the coatings of this invention include low density and high gloss. The dried coating thickness is also substantially the same as the wet thickness.

The invention is particularly advantageous for the continuous manufacture of lightweight opaque printing paper. Coated paper products of this invention are readily made water insensitive, of high brightness and of strength commensurate with that encountered in known conventional printing processes.

Freeze-drying as a process is well known and has been used in the food industry for a number of years. However, it has generally been considered merely as a means for removing water from, for instance, a food product without damaging the material, that is, without destroying its natural structure as would occur for instance by normal high temperature drying processes. In fact, in the area of food technology, one of the desirable properties which has been obtained is ease of subsequent water recovery into the solid when the product is later to be used, that is, water could be restored to the solid rapidly because the resulting product was extremely porous. Of course, this water-regain property is highly undesirable in coated papers and we have therefore, developed coatings which attain a high degree of water resistance, even though dried by the freeze-drying process. These coatings have in addition other desirable properties as described herein. It has, for instance, been found that a high degree of opacity can be obtained in thin films from virtually any normally solid polymeric material dispersed in a liquid vehicle provided that the vehicle can be conveniently frozen and subsequently sublimed away from the polymeric material. The reasons why greater opacity is obtained according to this invention are not completely understood but it is believed that the opacity results from the relatively uniform void size and intervoid spaces left by removal of the solid vehicular crystals from the frozen coating, and that the degree of opacity is a function of the void volume, the relatively narrow void size distribution and the index of refraction of the polymeric material employed.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a schematic view showing the components of the processing equipment for carrying out the process of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS There are literally hundreds of polymeric materials and vehicles that can be used in this invention and all of these cannot conveniently be listed here. The primary limitation to the operation of our process is that the vehicle in the coating formulation can be frozen and removed by sublimation from the solid coating formulation. Two practical limitations can be stated as advantageous but not as in- 4 herently limiting to the practice of this invention. Thus, the polymer liquid vehicle combination should have rheological properties suitable for coating application processes known in the art and sufficiently high solids should be obtainable within appropriate viscosities to avoid undue expense for removal of the liquid vehicle. It is particularly advantageous to work with aqueous solutions or aqueous suspensions.

The polymeric materials that can be used according to this invention either alone or in admixture include all polymeric materials capable of being coated on a substrate such as paper and particularly those used in the paper industry and well known to those skilled in the art.

Examples of polymeric materials that appear particularly advantageous include starch, dextrine, casein, soya protein, polyvinyl alcohol, regenerated cellulose, poly (styrene/maleic anhydride), poly (styrene/butadiene), acrylic polymers and copolymeric mixtures thereof, and the like.

In the practice of this invention, it is preferred to utilize formulations at a solids level between 5% and 60% by weight and even more preferably between 15% and 40% by weight in a single phase liquid carrier.

Organic and inorganic pigments which are well known in the paper coating art may be added to the formulation in essentially any proportion for special effect such as color, but some special opacifying pigments such as rutile titanium dioxide actually decrease the opacifying power of the freeze-dried coating when compared on a pound for pound basis with a similar coating not containing the pigment. Except as indicated herein, the coating formulation need not be treated physically or chemically in any unusual way prior to its application to the web.

Pigments are included in conventional paper coatings not only to increase opacity and whiteness of the coating, but also to provide microporosity necessary to trap printing ink, thereby preventing setolf from one sheet to another during printing, and to allow adequate ink pickup during printing to obtain desired image density. Examples of pigments which can be used include coating clays, zinc oxide, titanium dioxide and satin white. The freeze-dried coatings according to this invention contributes similarly to these aspects of the printing process and produce printing results superior to conventional coatings.

It might be anticipated that the dried porous or cellular coatings of this invention would have little or no measurable strength and indeed it has been found that unless proper steps are taken, this effect is observed. Various procedures can be employed, however to impart additional or the necessary strength to the coatings prepared by the freeze-drying technique. These methods should be construed as instructive to the practice of this invention rather than as limiting to it.

One method for assuring adequate strength in the final coating involves preparing formulations consisting of two or more dissimilar polymeric materials. The freeze-dried coatings prepared from such systems exhibit unexpected strength. Although the exact reasons for this unexpected strength are not known, it is believed that these systems obtain their strength by virtue of the fact that freezing does not actually occur instantaneously regardless of how fast the coating appears to have been frozen. We have postulated that certain molecular regions within the coating are probably occupied by dissimilar functional groups from polymeric molecules and that some water molecules do not become frozen even at apparent ambient temperatures as low as minus C. Hence, when these water molecules are removed, sufiicient residual surface tension exists which forces the group of polymeric molecules involved in that region into suificiently close contact that bonding can take place between them even though the greater part of any one molecule may be substantially free of this intermolecular bonding.

Various dissimilar polymers, i.e. those having dissimilar functional groups such as in Example VII, can be used according to this invention to obtain sufiicient strength in the coating for printing purposes.

Another method for achieving strength in freeze-dried coatings involves freezing the coating slowly and limiting the final temperature to which the frozen coating is subjected, that is maintaining the temperature as high as possible and yet retain a frozen coating. Whether or not dissimilar polymer materials are used, this approach accomplishes substantially the same effect as the immediately preceding method. Thus, it is believed that all of the polymeric molecules would be associated with sufiicient liquid vehicle molecules to permit good intermolecular bonding while the crystals of vehicle which have been allowed to grow to the desired size provide the void spaces which contribute to the exceptionally high opacity characteristics of the freeze-dried coatings made accord ing to this invention.

Another technique involves the addition of an emulsified thermoplastic or heat fusible material to the coating formulation, for instance, in the case of an aqueous system, an acrylic emulsion polymer or a wax emulsion respectively. In this case, after the coating has been freeze dried, it is subject to a source of heat suflicient to cause fiow of the thermoplastic or fusible material, thereby effecting substantial intermolecular bonding. Such coated products when subsequently returned to normal temperatures have the strength required of a printing coating as illustrated in Example XVI. It should be apparent that the major polymeric ingredient of the coating must not have thermoplastic or fusion properties similar to the ad ditive just described. Different functional groups are not required using this technique.

Another technique of imparting strength to a freezedried coating according to this invention, involves subjecting the coating to a bond forming chemical reaction after the coating has been dried wherein at least one part of the reacting system is added to the coating after it has been dried. In this case, it is desirable to limit the quantity of reactive material taken up by the dried coating so as not to impair the inherent opacity of the coating. Typical reaction system might include vapor phase isocyanate reactions, epoxy reactions, carboxyl, amine formaldehyde,

and hydroxy reactions, polymerization of ethylenimine,

or other well known chemically reactive polymeric or adhesive systems such as that set forth in Example V.

The processing equipment required for practice of this invention is unique in that the various components necessary to the invention have to our knowledge, never been assembled into one apparatus, despite the fact that each of the major components exist and are well known to those schooled in the art of coating, vacuum treatment of webs, and freeze drying. That is to say, the process equipment necessary to this invention is made up of a series of already known operational units. The overall process is described in the following paragraphs.

Although this invention may be carried out on a batch basis in which a roll of paper would be loaded into a vacuum chamber wherein the entire process is carried out and then the finished product unloaded, our invention is advantageously directed toward the use of a continuous or air-to-air process. The operational units required for the continuous process are an unwind stand, a coating application unit, a freezing unit, a vacuum chamber unit, a heating and condensing system connected to the vacuum unit, and a subsequent windup stand. These process units are indicated schematically in the drawing,

It should be obvious that iteins such as unwind stands, windup stands, and various continuous web conveying and splicing devices which are well known can be employed and these need no further comment. The coating unit may be selected from any of the types which can apply fluids of suitable rheology so as to obtain desired coat weight within desired tolerances when applied to a paper web.

For example, trailing blade, inverted blade, air knife, roll coating, reverse roll coating, wire wound rod doctor, extrusion or any of the other well-known techniques for applying coatings to a web may be employed. This list is intended to express the breadth and scope of our invention and not to limit the applicable techniques to those named.

Likewise the freezing step may be carried out by any one of several techniques. A blast. of cold air or other gas may be used, the sheet may be brought into contact with the cold surface, such as for instance, the chilled backing roll for a trailing blade, or a large chilled drum roll, or the sheet may be dipped into a cold immiscible non-wetting fluid, each of which is known in various segments of industry which utilize freezing as a process operation.

Equipment appropriate to leading a web of material from atmospheric conditions into and subsequently out of a reduced pressure chamber in a continuous or semi-com tinuous manner has been developed for vacuum metallizing of strip metals, glass, plastics, and paper. Similar units can be advantageously employed in the practice of our invention. However, in the case of freeze-drying, the mechanical problems are less severe since the pressure used in the vacuum chamber can be orders of magnitude higher than that required for vacuum metalizing. Thus, a pressure of 50 to 500 microns of mercury is adequate for freeze-drying processes whereas vacuum metallizing frequently requires pressures as low as 0.5 micron of mercury.

This portion of the process involves passing the web through for instance, rotary seals or narrow slits into succeeding chambers at lower pressure. Each chamber can be individually pumped and can be of very low volume. The number of such entry and exit chambers required will depend upon the efiiciency of the rotary seals and the capacity of the pumps. Once the coated sheet reaches the main chamber in which the ambient pressure is maintained below the sublimation pressure of ice at the temperature of the sheet, the drying process begins. The rate of sublimation can be accelerated by supplying heat to the sheet at a rate equal to the rate of heat usage in sublimation. This heat may be supplied in a number of ways, for instance, by use of microwave or dielectric heaters, infrared radiation, electrical resistance through a conductive material and many others. It is important, however, during the sublimation process that the coating not be grossly melted thereby destroying the porous structure. To improve the rate at which water can be removed from the chamber, it is desirable to condense the water vapor out of the vacuum chamber before the vapor gets to the vacuum pumps. This is ordinarily accomplished by interposing heat exchanging condensers in the vacuum line prior to the vacuum pumps. The heat exchangers are maintained at a temperature well below the ice point characteristic of the operating pressure, which temperature is consequently also below the temperature of the coated web. A second or alternate heat exchanger can also be provided so that the ice buildup on the primary heat exchanger may be removed without interrupting the continuous operation.

After the web has been dried in the vacuum chamber, it is led out of the chamber through a similar set of seals, given a further heat or chemical treatment if desired, then rewound into a roll. The same process can serve for a sheet which has been coated on either one side or two, with only minor modifications to the geometry required for a two-side coated sheet as will be recognized by those skilled in the art.

With proper mechanical design of the coating application system and entry seals to the vacuum chamber, the wet coated sheet could be lead directly into the vacuum chamber without necessity for first freezing the coating. The overall process remains similar in this case, since the coating will freeze rapidly in the vacuum due to the cooling effect of the vaporization. Once frozen in this man- 7 ner, the process continues substantially as we have already described. This is illustrated by the Example XVII disclosed herein.

The coatings that can be used according to this invention can comprise conventional paper coating compositions having solids contents in the usual and conventional range of coat weights or wet thicknesses as previously practiced in the paper industry. With respect to wet thickness of the coating, it is significant to point out that the coatings according to this invention have the same thickness both wet and dry, while conventional air drying of these same coatings reduces the thickness of the wet coatings. The invention is of course not limited to paper but can be practiced on various substrates, such as plastic films, particularly where the applied coating is to be printed.

This invention can be more fully understood by reference to the following examples.

EXAMPLE I A sheet of paper rawstock having a basis weight 44 pounds per 3300 sq. ft. was placed on a metal plate cooled by Dry Ice. A coating formulation consisting of aqueous ammonium caseinate, at 15% by weight solids was applied to the sheet by means of a Bird Film Applicator at a rate of 2 pounds of solids per 3300 sq. ft. and frozen immediately. The paper was placed in a vacuum chamber and evacuated to 500 microns and held there until the frozen coating was dried by sublimation and then returned to ambient conditions. The drying only took about 2 minutes. Casein, freeze dried in the above manner forms an opaque, white coating with an exceptionally smooth, glossy surface. The properties of the paper were considerably improved by the coating as noted below:

2 lbs. C 1 S sheet stock 78 82. 5 85 90 65 12 Hunter gloss 8 66 The freeze-dried, casein coated paper was printed on a Diamond Gardner rotogravure press and produced sharp images superior to those produced on a commercial grade of paper designed for rotogravure printing.

EXAMPLE II EXAMPLE III A coating formulation containing 80% casein and 20% of paraffin wax emulsion (Paracol 404 g.) at 15% total solids was coated on paper and freeze dried as in Example I. The film was white and opaque and after a heat treatment at 100 C. for 1 minute was substantially water resistant.

EXAMPLE IV A coating formulation consisting of 68% casein and 32% of a 55% solids styrene/butadiene copolymer (60/ 40) latex, marketed by Koppers under the name Dylex K-55, was coated at 15% total solids on a paper web as in Example I. This coating was white and opaque and more resistant to water than the coating of Example I.

8 EXAMPLE v A coating formualtion consisting of 78% casein, 19.5% of methylated methylol melamine-formaldehyde resin, marketed by Monsanto under the trade name Scriptite 31, and 2.5% diammonium phosphate was coated on paper at 15% total solids and frozen as in Example I. The freeze-dried coating was then cured for 20 seconds at 180 C. The film produced was white, opaque and water resistant.

EXAMPLE VI Soya protein was substituted for casein in Examples I-V and very little difference was noted. The dark color of the protein however decreased the brightness by several points relative to Examples I-V.

EXAMPLE VII A coating formulation consisting of 68% dextrine and 32% of a partial ester of a maleic anhydride-styrene copolymer, marketed by Monsanto Chemical Company under the trademark Scripset 550, was applied and freeze dried at 23% total solids as in Example I. The coating was white, opaque, and water resistant. The Shefiield smoothness was 10 and the Hunter gloss measured which indicated a smoother surface than was attained using casein or protein.

EXAMPLE VIII A coating formulation containing HT clay and 20% oxidized starch at 40% total solids was coated on paper and freeze dried as in Example I, and compared to a conventionally air-dried sheet with the same coating and coat weight. Freeze-drying improved the brightness from 80 to 86, the opacity from 90 to 94, and tripled the Bekk smoothness 80 to 2140.

EXAMPLE IX A coating formulation containing 80% HT clay and 20% polyvinyl alcohol at 20% total solids was coated on paper and freeze dried as in Example I and compared to an air-dried control with the same coat weight. The brightness on the control was 77 and the opacity 87. The freeze-dried sheet had a brightness of 85.5 and an opacity of 92.5.

EXAMPLE X The coating formulation described in Example VII was applied as in Example I at 23% total solids and put in the vacuum chamber and kept frozen while the chamber was evacuated. At 1000 microns total pressure the sheet was removed from the cold metal support and suspended parallel to an infrared heater in vacuum which was turned on at a wire temperature of 1200 C. for 10 seconds. The chamber was then returned to atmospheric pressure and the coating was found to be dry. The total time that the paper was in the vacuum was 2 minutes. The coat weight was measured at 5.5 lbs, per ream and the gloss and smoothness corresponded to those measured in Example VII.

EXAMPLE XI An aqueous casein coating formulation at 15% solids was applied to paper rawstock and allowed to penetrate for 1 second before freezing on a cold metal plate and freeze-drying as in Example I. The coating produced had a fiber tearing bond to the paper and similar properties described in Example I.

EXAMPLE XII A coating formulation consisting of oxidized starch (Stayco C) at 15 solids was applied as in Example I and freeze dried. A 5.4 lb./ream coating raised the papers brightness from 79 to 88 and its opacity from to 94. The sheet was smooth and glossy.

9 EXAMPLE XIII An aqueous formulation containing 100 parts of ammonia cut Polish casein and 20 parts of Azite (Azite 900 Liquefier, American Cyanamid Co.) at 24% solids was applied to a 42-pound per 3300 square foot paper base, immediately frozen by a 1-2 second contact of the sheet with a glass plate previously cooled by contact with Dry Ice, placed in an air tight chamber connected with a vacuum pump and the frozen water removed from the coating by vacuum sublimation. The characteristics of the resulting dried, coated paper are given below.

Density of base stock 1 13 Weight of base stock 42 Weight of coating 2 6-7 Density of product 1 12-13 Caliper of product (mils) 4.0-4.5 Weight of product 2 48-49 Opacity 95-97 Brightness 88-90 Smoothness 13-23 Olfset printability Good 1 Pounds/3300 sq. ft. per mil of thickness. 9 Pounds/3300 sq. ft.

The product of Example XIII is smoother than conventionally coated, air-dried, supercalendered products. The apparent density of Example XIII coating is substantially the same as that for its base stock, which compares favorably with conventional products where the product density is typically doubled to achieve product properties comparable to those of Example XIII.

EXAMPLE XIV An aqueous formulation containing 75 parts of ammonia cut Polish casein, 7.5 parts of Azite and 200 parts of HT clay (kaolinite, :Edgar Clay Company), at 41.4% solids was applied to a 34 /2 pounds per 3300 square feet paper base, immediately frozen and vacuum dried as described in Example XIII. The characteristics of the resulting dried, coated sheet are given below:

Density of base stock 14 Weight of base stock 34-35 Weight of coating 6-8 Density of product 13-15 Caliper of product 2.8-3.0 Weight of product 40-43 Opacity 93-96 Brightness 87-88 Smoothness 12-24 Offset printability (1) Fair-good.

A coating formulation containing 100 parts of enzymeconverted starch, 600 parts of No. 2 coating clay and 100 parts of calcium carbonate (Purecal 0, Wyandotte Chemical Co.), dispersed in 600 parts of water to 58% solids was applied in a manner similar to that described in Example XIII. The dried product characteristics are listed below:

To apply the coating composition of Example XV to a paper in the conventional manner with supercalendering would require twice the coating weight to obtain opacity and brightness values approaching those of Example XV. Also, higher densities; would be required.

EXAMPLE XVI An aqueous coating formulation containing by weight parts of a 15% ammoniacal Polish casein dispersion, 32.6 parts of an acrylic emulsion polymer (Rhoplex AC-73, Rohm and Haas Company), 2 parts of a condensation product of melamine and formaldehyde, marketed by American Cyanamid Company as 'Parez'613 and 0.64 part of a 25% ammonium chloride solution was freeze dry coated in a manner similar to Example I. The dried coated paper was then heated for 30 seconds at C. Strength improvement was noted by a marked increase in resistance of the coating to marking by a metal stylus.

EXAMPLE XVII An aqueous coating formulation containing 100 parts by weight of casein, 25 parts of a styrene-maleic anhydride copolymer marketed by Monsanto Chemical Company as Scripset 54 and 25 parts of a synthetic rubber emulsion polymer marketed by E. I. du Pont, Inc. as a Neoprene emulsion, was applied to a paper base, placed in a vacuum chamber and evacuated to about 250 microns and held there until the coating was dried, re- Sliltd in an opaque coating having a brightness value 0 84.

The coated products produced according to the above examples have various degrees of strength depending upon the particular process used as discussed above, but all are useful and the particular method chosen will depend upon the particular end use intended for the coated substrate. Dilferent printing processes, for example, require different coating strengths or tack strength as is well known in the art. The coated substrates can of course be used for purposes other than printing where higher opacity, brightness, and smoothness are desired.

The coatings of this invention allow greater latitude in the design of printing paper products than heretofore possible. Thus, rawstocks having base weights in the range from about 15 to 80 pounds per 3300 sq. ft. can be used, with or without added pigments or fillers to manufacture products of essentially nonvarying, low apparent density but ranging from about 3.0 to 6.0 mils in caliper. Thus, coatings of this invention having substantially greater thicknesses than the prior art coatings can be used to produce coated products having total thicknesses comparable to prior art products.

With the coated product of the present invention, fiber showthrough of the undnerlying base material is virtually eliminated. This is so because the coating lies essentially completely on the paper surface and the surface of the coating is well above the uppermost fibers of the paper base. With the prior art coatings, fiber showthrough often occurs due to shrinkage during conventional high temperature drying. This non-uniformity, smoothness defect in the coating surface can cause subsequent printing problems which are not exhibited with any of the coatings described above.

From the above examples, it is seen that coatings produced in accordance with the teachings of the present invention are uniform in thickness and may range from about 0.1 to 10 mils. Also, coatings of the present in vention are chaarcterized as having anhywhere from 20 to 90% void volume where the void volume is made up of numerous cells which may range from 0.1 to microns in size. However, we have found that in any particular coating, the size of the voids advantageously varies no more than about four fold. Furthermore, the intervoid thickness of the coatings of the present invention Will be substantially uniform to form a continuous matrix surrounding the voids. The intervoid distances may vary from 0.1 to 5 microns; with these distances in any particular application varying only from about two to four fold.

When the coatings of the present invention are applied to paper, the density of the coated product is substantially the same as that of the base material to which the coating is applied. More particularly, the density of the coating may range from 0.06 to 1.2 grams per cubic centimeter with the density of the coated product being no greater than 20% more than the density of the original base material. These characteristics of the coated product of the present invention compare favorably with products produced according to the prior art techniques where thicker and denser coatings are required to produce acceptable high quality paper.

Paper products coated in accordance with the teachings of the present invention generally have TAPPI opacity values ranging from 90 to 100, standard brightness values ranging from 80 to 100, Shefiield smoothness values ranging from about to 50, apparent density values from about 10 to 18 pounds per 3300 sq. ft. per mil of product thickness, good to excellent printability and good marring and crush resistance.

From the foregoing examples, it is apparent that the present invention offers a technique of producing a high quality lightweight opaque printing paper surface having gloss, smoothness and inherent printability qualities so that the usual subsequent operation of supercalendering may be entirely eliminated. Elimination of the supercalendering step reduces the need for capital and labor in the manufacture of coated papers and eliminates a high wastage of product and time normally associated with the supercalendering step.

It will also be apparent from the foregoing disclosure that the polymeric materials for use in our coating compositions can be selected from a wide variety of natural polymers such as casein and starches and their chemical modifications, as well as addition-type polymers and copolymers of vinyl chloride, acrylonitrite, styrene, acrylic and methacrylic acid and their polymerizable derivatives, ethylene, butadiene, vinyl acetate, fiuorinated monomers and the like, and also from condensation polymers such as polyesters, polyamides, phenol-formaldehyde resins and the like, provided that the dried residue of these polymeric compositions is a solid, more or less permanent mass substantially as illustrated by the examples in this disclosure and that the composition can be dissolved, dispersed or otherwise suspended in a liquid medium which can be frozen and sublimed under reduce pressure in accordance with our disclosure.

We claim:

1. The process which comprises:

(a) coating a paper web with a coating comprising a polymeric composition contained in a liquid carrier;

(b) freezing the applied coating by reducing its temperature to below the freezing point of the applied coating to solidify the coating;

(c) removing a sufficient amoun of the frozen liquid carrier substantially as a vapor directly from the solidified coating under reduced pressure that the coating will remain solid and substantially dry when the web is returned to atmospheric pressure and 12 ambient temperature and will possess a thickness substantially the same as that of the coating when first applied; and

(d) returning the frozen web to ambient pressure and temperature conditions.

2. The process according to claim 1 in which:

(a) thermal energy is applied to the frozen web under reduced pressure.

3. The process according to claim 2 in which:

(a) thermal energy is applied to the frozen web under reduced pressure in the form of infrared radiation.

4. The process according to claim 2 in which:

(a) thermal energy is applied by microwave or dielectric heating.

5. The process according to claim 2 in which:

(a) thermal energy is applied by conduction from a heated contacting surface.

6. The process according to claim 1 wherein:

(a) the liquid vehicle is water; and

(b) the polymeric composition is selected from at least one member of the group consisting of starch, oxidized starch, dextrine, casein, soya protein, polyvinyl alcohol, styrene-butadiene copolymers, emulsified wax, and partial esters of styrene-maleic anhydride copolymers.

7. The process according to claim 1 wherein:

(a) the liquid vehicle is water; and

(b) a mixture of polymeric composition is used in which:

(1) one is selected from at least one member of the group consisting of starch, oxidized starch, casein, soya protein, dextrine or polyvinyl alcohol, and

(2) the second is selected from at least one member of the group consisting of polymers of melamine-formaldehyde or urea-formaldehyde, partial esters of styrene-maleic anhydride copolymers, styrene-butadiene copolymers and emulsified waxes.

8. The process of claim 1 wherein:

(a) the Web is subjected to the further heat treatment after coming out of the vacuum chamber to attain added strength or added strength and water resistance.

9. The process according to claim 8 in which:

(a) the liquid vehicle is water; and

(b) the polymeric composition is selected from at least one member of the group consisting of starch, oxidized starch, dextrine, casein, soya protein, polyvinyl alcohol, styrene-butadiene copolymers, emulsified wax, or partial esters of styrene-maleic anhydride copolymers.

10. The process according to claim 8 in which:

(a) the liquid vehicle is Water; and

(b) a mixture of polymeric materials is used in which:

(1) one is selected from at least one member of the group consisting of starch, oxidized starch, casein, soya protein, dextrine and polyvinyl alcohol, and

(2) the second is selected from at least one member of the group consisting of pre-polymers of melamine-formaldehyde prepolymers of ureaformaldehyde, partial esters of styrene-maleic anhydride copolymers, styrene-butadiene copolymers, and emulsified wax.

11. The process according to claim 1 wherein:

(a) the solvent is benzene; and

(b) the adhesive is at least one member of the group consisting of polystyrene and a cross-linkable styrene copolymer.

12. The process of claim 1 in which:

(a) the liquified composition coated on the web contains an inorganic pigment.

13. The process of claim 1 in which:

(a) up to about 60 percent by weight of the polymeric 13 composition is replaced with inorganic pigments selected from at least one member of the group consisting of coating clays, zinc oxide, titanium dioxide, and satin white.

14. The process of claim 1 in which:

(a) the coating of the Web contains a polymeric composition chemically reactive with at least one member of the group consisting of isocyanate, epoxy, carboxyl, amine, hydroxy containing compounds and formaldehyde; and

(b) contacting the coated web containing the chemically reactive polymeric composition after it has been removed from the vacuum chamber and dried with at least one member of the group consisting of isocyanate, epoxy, carboxyl, amine, hydroxy containing compounds and formaldehyde in the vapor phase to impart added strength and water resistance to the coated web.

15. The process which comprises:

(a) coating a base material a paper web with a coating comprising a polymeric composition contained in a single phase liquid carrier;

(b) solidifying said liquid carrier at discrete locations throughout said coating; and

(c) removing the liquid carrier in its vapor state from said coating.

16. The process according to claim 15 in which:

(a) the coating is frozen to elfect the solidification of said liquid carrier.

17. The process according to claim 16 wherein:

(a) the liquid carrier is removed by sublimation.

18. A coated product comprising:

(a) a base material comprising a paper web, and

(b) a thin coating of a polymeric composition adhered to said base material and comprised of (l) the dried residue of a frozen dispersion of said polymeric composition in a liquid carrier.

19. A coated product comprising:

(a) a base material comprising a paper web; and

(h) a coating of polymeric composition adhered to said base material and comprised of:

(1) the dried residue of a wet coating consisting of a dispersion of said polymeric composition in a single phase liquid carrier, said dried residue having a thickness substantially the same as that of said wet coating.

20. A coated product comprising:

(a) a base material comprising a paper web; and

(b) a coating of a polymeric composition adhered to said base material and comprised of:

(1) the dried residue of a dispersion of said polymeric composition in a single phase liquid carrier, said residue having voids spaced therethrough at discrete locations previously occupied by said liquid carrier prior to drying of said coating.

21. A coated product according to claim 20 wherein:

(a) said coating contains an inorganic pigment.

22. A coated product according to claim 21 wherein:

(a) said inorganic pigment is selected from at least one member of the group consisting of coating clays, zinc oxide, titanium dioxide, and satin white.

23. A coated product according to claim 20 wherein:

(a) the polymeric composition is selected from at least one member of the group consisting of starch, oxidized starch, dextrine, casein, soya protein, polyvinyl alcohol, styrene butadiene copolymers, emulsified wax, and partial esters of styrene-maleic anhydried copolymers.

24. A coated product according to claim 20 wherein:

(a) the polymeric composition consists of dissimilar polymeric materials.

25. A coated product according to claim 24 wherein:

(a) one of the polymeric materials is one selected from at least one member of the group consisting of starch, oxidized starch, casein, soya protein, dextrine or polyvinyl alcohol; and

(b) another of the polymeric materials is one selected from at least one member of the group consisting of polymers of melamine-formaldehyde or urea-formaldehyde, partial esters of styrene-maleic anhydride copolymers, styrene-butadiene copolymers and emulsified waxes.

26. A coated product according to claim 20 wherein:

(a) the apparent density of said product is no more than about 20% greater than that of said base material.

27. A coated product according to claim 26 wherein:

(a) the apparent density of said product is substantially the same as that of said base material.

28. The product according to claim 26 wherein:

(a) the apparent density of said coating is from between about .06 to 1.2 grams per cubic centimeter.

29. A coated product according to claim 20 wherein:

(a) said voids are uniformly spaced throughout said coating.

30. A coated product according to claim 29 wherein:

(a) said voids vary up to about four fold in size.

31. A coated product according to claim 30 wherein:

(a) the intervoid distances of said coating vary from about two to four fold.

References Cited UNITED STATES PATENTS 3,245,151 4/1966 Eichmanns 34-5 3,403,046 9/1968 Schlacke et al. 117-1 19.2 X 3,376,158 4/1968 Buser 117-1192 3,228,786 1/ 1966 Fitzgerald et a1. 117-62 3,428,584 2/ 1969 Riley 260-15 3,349,749 10/ 1967 Utschig 117-119.2 X 2,668,364 2/1954 Colton 34-5 X 2,897,094 7/1959 Hayes et a1. 117-62.1

WILLIAM D. MARTIN, Primary Examiner M. R. LUSIGNAN, Assistant Examiner US. Cl. X.R.

34-5; 117-119, 119.2,, 119.6, UA, 155 L, 156, 158

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