Twice-coated cellulosic-fiber structure and its manufacture

United States Patent

[J 3,607,348

<td> [72]</td><td> Inventors</td><td> Joseph A. Wray Columbus;</td>

<td></td><td></td><td> Herbert N. Johnston, Columbus, Ohio; Robert L. Leaf, Shawano, Wis.</td>

<td> [21]</td><td> Appl. No.</td><td> 735,486</td>

<td> [22]</td><td> Filed</td><td> May 14,1968</td>

<td> [45]</td><td> Patented</td><td> Sept. 21,1971</td>

<td> [73]</td><td> Assignee</td><td> Little Rapids Pulp Company Shawano, Wis.</td>

[ 541 TWICE-COATED CELLULOSIC-FIBER STRUCTURE AND ITS MANUFACTURE 14 Claims, 2 Drawing Figs.

[52] U.S. Cl........................................................ 117/38,

117/29, 117/45, 117/64 R, 117/76 P, 117/8(1’

117/83, 117/86, 117/91, 117/92, 117/98, 117/111’

R, 117/135.5

[51] Int.CI......................................................... B44d 1/14,

D21h 1/21

[50] Field of Search............................................ 117/76 P

F, 76 T, 45,98, 135.5, 38, 92,86,111,64

[56] References Cited

UNITED STATES PATENTS

1,966,74.7 7/1934 Zechmanek.................. 117/98

2,407,549 9/1946 Gurwick....................... 117/98 X

2,470,493 5/1949 Karfioletal.................. 117/76 PX

2,955,970 10/1960 Rice etal...................... 117/86 X

2,962,385 11/1960 Rees etal..................... 117/45

2,970,932 2/1961 Edlund.......................... 117/92 X

3,196,031 7/1965 Unmuth....................... 117/76 PX

3,257,226 6/1966 Thwaites...................... 117/111 X

Primary Examiner— William D. Martin Assistant Examiner—Ralph Husack Attorney—Olson, Trexler, Wolters &amp; Bushnell

ABSTRACT: A sheet product comprised of a substrate of nonwoven cellulosic fibers (which substrate either has a substantial porosity or is water-sensitive), a discontinuous prime coating of hydrophobic material adhering to the cellulosic fibers, and a second material filling surface interstices, and process for making same.

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PATENTED SEP211971

3,607,348

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3,607,348

I

TWICE-COATED CELLUI.OSIC-FIBER STRUCTURE AND ITS MANUFACTURE

This invention relates generally to the art of coated sheet products and in one particular embodiment to coated papers.

Lightweight, rather porous tissue can be produced in an inexpensive manner from comparatively long cellulosic fibers. However, the uses of such economical tissue are restricted by its poor wet strength and inability to resist permeation by such substances as carbon inks and printing inks. In the past, various efforts have been made to overcome these deficiencies, typical of such efforts being the use of various additives at either the pulping or papermaking stage. A lengthy pulp beating has also been employed in an effort to provide more dense paper made up of relatively short fibers. Both of these general approaches have been found to add considerable expense to the ultimate product and therefore tend to defeat one of the principal advantages of the tissue, namely its low cost. Coating of the tissue has also been proposed to remedy its high porosity and weakness when wet. The latter approach has been characterized by relatively high coating weights of a single material in order to achieve the desired result.

Accordingly it is an important object of the present invention to provide a paper product which is based on porous tissue or the like but which is itself of comparatively low porosity and which has appreciable resistance to permeation by such substances as carbon inks.

A more general object of the invention is to provide a novel coated sheet product which is economical to produce.

Another object of the invention is to provide an inexpensive, lightly coated tissue paper product.

Still another object of the invention is to provide a novel method of producing a coated sheet product.

These and other objects and features of the invention will become apparent from a consideration of the following descriptions and examples.

The term “air permeability” is used herein to mean the ability of a sheet of paper or other cellulosic product to permit air flow therethrough under a pressure difference across the sheet and, in particular, as determined by using the Bendtsen Smoothness and Porosity Tester, Model 5, as manufactured by Andersson andS0rensonof Copenhagen, Denmark.

Unless otherwise specified, the term “ream” is used herein to refer to a quantity of paper or other sheet material comprised of 500 rectangular sheets each measuring 20 inches by 30 inches.

In the drawing:

FIG. 1 is a schematic representation in magnified plan view of an product made in accordance with the invention; and dicated generally in the drawing by the reference numeral 10. The paper product 10 is shown to comprise a substrate sheet 12 which is composed of a multitude of randomly oriented interlaced and interlocked cellulosic fibers 14 which define a network of surface and internal interstices 16. It is important to realize that certain of the interstices 16 are interconnected forming, variously, direct and devious routes from the surface to the interior of sheet 12 and, in some circumstances, passageways extending completely through the sheet. In compliance with certain features of the invention, the porous sheet 12 is of comparatively lightweight stock, weighting approximately 8 pounds per ream. In such circumstances, the sheet 12 may have a substantial porosity, indicated by an air permeability of at least about 300 ml./min. and frequently in excess of 500 ml./min. and thus would be susceptible to invasion by carbon inks for example.

On at least one surface, the sheet 12 carries a hydrophobic coating 18 that comprises a thermoplastic material, such as a mineral wax, or some other hydrophobic material as described more fully hereinafter. The coating 18 adheres to the fibers 14, and the discontinuous nature of this coating is attested to by the fact that it does not substantially alter the porosity of sheet 12. The hydrophobic coating 18 is intentionally arranged to be discontinuous, and this character of the coating 18 is achieved, for example, by employing low application levels, on the order of about 0.4 to about 2.0 pounds per ream. Cooperating in the establishment of the hydrophobic material as a discontinuous coating is the application of the material in a molten state, where convenient, and in a discontinuous pattern.

In further compliance with the features of the present invention, a second material 20, such as starch, is selectively deposited in the discontinuities of the hydrophobic coating 18. Selective deposition of the latter material is achieved by employing low coating weights, on the order of about 0.1 to about 1.5 pounds per ream, and by applying the material from an aqueous medium onto the hydrophobically coated substrate. Other methods of selective deposition may, of course, be employed. While substantially all of the discontinuities in the hydrophobic coating are filled with the top coat material, not every interstice is filled; and such incomplete deposits are most likely to occur at openings of passageways penetrating through the sheet, such as for example the pinhole 22. As a result of such scattered imperfections, the paper product 12 has a low porosity. In specific embodiment, this low porosity has been measured by an air permeability of less than about 50 ml./min.

TABLE I

<td rowspan="2"> Property</td><td rowspan="2"> Porous tissue</td><td rowspan="2"> Prime coated (A)</td><td colspan="2"> Dual coated</td><td rowspan="2"> Conventional carbonizing tissue (uncoated)</td>

<td> (B)</td><td> (C)</td>

<td> Caliper (mils)_______________________________</td><td> __________ 1.1</td><td> 1. 25</td><td> 1.35</td><td> 1. 35</td><td> 0.9</td>

<td> Total weight (lbs./2.083 it.²)..................</td><td> __________ 8.1</td><td> 9.3</td><td> 9.6</td><td> 9.9</td><td> 8.5</td>

<td> Total coating weight (Ibs./ream)_____________</td><td></td><td> 1.2</td><td> 1.5</td><td> 1.8 .</td><td></td>

<td> Tensile » strength (lbs./linear inch)........... Air permeability (ml./min.): ^b</td><td> ......... 6.3±0.4</td><td> 6. 6±0. 6</td><td> 7. 2±0. 6</td><td> 7. 3±0.8</td><td> 8. 5±0.4</td>

<td> 4 sheets..................................</td><td> __________ 340-400</td><td> 410-490</td><td> 10-15</td><td> 7-10</td><td> 25-50</td>

<td> Single sheet______________________________</td><td> __________ &gt;500</td><td> &gt;500</td><td> 15-35</td><td> 7-20</td><td> 100-200</td>

^a Tensile strength determined on bench Model Instron Tester using initial gap of 3.75 inches and head speed of 1 inch per minute. Deviation is approximate standard deviation.

b Bendsten Porosity Tester No. 5 using the 75 MM-water column Monostate-weight.

(A) Wax base coated (see Example II).

(B) Starch coated with zero rod on top of wax.

(C) Starch coated with No. 6 rod on top of wax.

FIG. 2 is an enlarged, sectional view taken along the line 2-2 of FIG, 1.

in compliance with the invention, a sheet of nonwoven cellulosic fibers is treated with hydrophobic material so as to form a discontinuous prime coating. Subsequently, a hydrophilic material is applied to the discontinuously coated sheet; and the hydrophilic coating material includes a substance to fill at least surface interstices in the sheet. The dual coatings of the invention work a substantial reduction in the permeability or water-sensitivity of the substrate sheet.

A typical paper product forming one embodiment of the invention and produced by the procedures of the invention is in70

In Table I, certain physical measurements are set forth for: a porous tissue of the type contemplated for use as a substrate sheet of the instant invention, prime coated tissue, dual coated tissue, and a dense tissue of short fibers such as is conventionally used in making carbon paper. The substantial reduction in permeability which is achieved by the instant invention will be apparent from the data presented; and additional, subjective, visual examination has indicated that the dual coated products described in Table 1 exhibited no penetration by a commercial carbon ink applied hot (200° F.) using a smooth (zero) metering rod under heavy pressure. By comparison, the untreated porous tissue exhibited a heavy penetration.

3,607,348

As described hereinabove, the substrate sheet of the present invention comprises a nonwoven network of interlaced cellulosic fibers characterized either by substantial porosity, as indicated for example by a corresponding degree of air permeability, or by being adversely effected by water. In the practice of the invention, various water-sensitive and/or porous open sheets are contemplated, such as unsized paper, tissue paper, paperboard, cardboard, kraftboard, paper felts, boxboard, newsprint, and the like; and included in the useful substrates are those consisting essentially of or composed primarily of cellulosic fibers. The cellulosic fibers of chief interest are those of natural origin, primarily those derived from various woods but also those of cotton flax, hemp, ramie and jute. Regardless of the type of fiber employed, the substrate comprises a major content of the cellulosic fibers but may also include minor amounts of other natural fibers or synthetic fibers, as well as dyes, resins, adhesives, gums and other additives conventionally used in pulp and paper manufacture.

The hydrophobic material which is applied as a prime or base coating is intended to impart to the substrate a substantial degree of either water-repellency or structural integrity or both; and application of this material from a molten state or in solution or as either an organisol or a plastisol is contemplated. Useful hydrophobic substances for use in the practice of the invention include those that are thermoplastic and are substantially solid at about 20° C.; and such classes of materials as natural and synthetic polymers, resins and waxes may be employed. Specific illustrative substances include paraffin and microcrystalline waxes; carnauba, candelilla and like natural waxes; polyethylene, polypropylene, polyisobutylene and other polyolefins; ethyl cellulose and the various esters and ethers of cellulose; ester gums; hydrogenated and otherwise modified resins; phenolic, alkyd and maleic esters; polyamides; vinyl resins and copolymers; zein and similar vegetable products; chlorinated and cyclized rubber; cumar and terpene resins; and similar materials known in the art of papermaking. When formulated into coating compositions, the foregoing substances are commonly combined with plasticizers, pigments, dyes, fillers, organic solvents and diluents, and like constituents.

The hydrophobic coating is applied so as to be discontinuous and, advantageously, in a generally predetermined or fixed pattern so as to establish uniformity in the coating weight. An engraved applying surface, such as for example an intaglio rotogravure roll, has proved eminently useful in this regard, the hydrophobic coating composition being selected, in such instance, to be thermoplastic in order that it may be deposited in a molten state from recesses in the engraved surface onto the porous substrate. Other means of applying the hydrophobic coating are useful in the invention. For example, an organisol, a plastisol or an organic solvent solution of the hydrophobic coating substance may be applied by silk screen printing or by the spray spattering of droplets and globules. Roll, rod and knife coating may also be employed to produce a discontinuous coating when the substrate has a rough or grained surface.

In accord with important features of the present invention, the hydrophobic coating is discontinuously applied so as to expose at least about 10% of the gross surface area of the substrate and nor more than about 70% of such area. Coverages by the hydrophobic discontinuous coating of less than about 30 percent of the substrate gross surface area are suitable only if some degree of detrimental effect from the subsequent hydrophilic coating can be tolerated or if the integrity of the substrate would be otherwise substantially retained.

In further accord with features of the present invention, at least about 90 percent of the individual discontinuities are arranged to take a configuration having a maximum dimension not exceeding about 1 mm. With regard to the corresponding minimum dimension of the discontinuities in the hydrophobic coating, economic considerations favor individual discontinuities approaching the 1 mm. maximum dimension in combination with the larger total exposed surface area of the substrate because the hydrophobic coating substances are generally more costly than the subsequently applied hydrophilic coating substances. However, where minimum permeability in the final product is desired, the discontinuities are preferably of extremely small size and, where the oblong configuration, desirably have an average width not exceeding about 0.2 to about 0.3 mm.

In accordance with the invention, a secondary coating is applied to the prime coated sheet. One material for the secondary coating comprises an aqueous medium containing at least about 30 percent or more by weight of water. Advantageously, the secondary coating substance itself is readily wet by water and possesses a substantial solubility in water, although water dispersions of less soluble substances or hydrophobic substances may also be employed. In the latter instance, the included water serves to distribute the dispersed substance as a discontinuous coating. Where the substance of the top coating is itself hydrophilic, it may be applied either with or without admixture with water or another hydrophilic carrier; and the secondary coating may even comprise exclusively hydrophobic material. In any event, after the second coating is applied to the hydrophobic coated substrate, any included water or other vehicle is evaporated or otherwise driven off before further processing or utilization of the product.

Illustrative substances for use as the second coating include casein, vegetable protein, glue, starch, water soluble cellulose products and synthetic polymers, carbonxymethyl-cellulose, methyl cellulose and other cellulose derivatives, polyvinyl alcohol, hydrophilic clays, acetylated starches, sodium alginates, vinyl pyrrolidone polymers and copolymers, and similar substances. Preferably, the hydrophilic material contains at least about 50 percent of either water, a hydrophilic coating substance or a combination of the two.

In order to describe the invention more fully, the following specific examples are given without, however, intending to limit the invention to the precise details and conditions set forth except as required in the appended claims.

Example 1

A hydrophobic coating composition of

Microcrystalline wax of 163° F. congealing point (ASTM D-93 8) (Aristowax 165) 9 parts

Microcrystalline wax of 143° F. congealing point (ASTM D-938) (Aristowax 143) 9 parts

Ethylcnc/vinyl acetate copolymer (about 27-29% vinyl acetate content) (Elvax 220) 2 parts

Aluminum silicate particles (ASP-900 Clay) 16.2 parts was prepared by melting and stirring together the first three constituents at about 250° F. until uniformly mixed and then adding and stirring therein the clay.

This hydrophobic coating composition in a molten condition was applied by a 200-line gravure metering roll to a porous tissue paper, specifically 8-pound wrapping tissue, and followed by a hot smoothing rod, the coating being of discontinuous nature both before and after application of the smoothing rod.

To the surface of the resulting tissue paper bearing a discontinuous coating of the hydrophobic coating composition there was then applied an aqueous-based hydrophilic coating composition of 2 percent by weight of sodium alginate and 0.4 percent of wetting agent in water. The sodium alginate employed in the hydrophilic coating composition was a high viscosity sodium derivative of algin, specifically Kelgin HV, a watersoluble derivative of alginic acid, a natural gum product extracted from giant kelp grown in the coastal waters of Southern California. Other sodium alginates, such as Kelgin LV, Kelgin XL, and Kelgin S, have also been used. The employed wetting agent was a commercially available alkyl aryl polyether alcohol, specifically Advawet No. 10. This hydrophilic coating composition was applied by wire wound rod, in

3,607,348 metered amounts, so as after drying and volatilization of the water therefrom, to provide an applied dry coating weight of about 1.0 Ib./ream. Typical and illustrative porosity measurements (The Bendsten Smoothness and Porosity Tester) for the untreated tissue paper and the prime coated sheet was greater than 500 ml. air/min. and for the twice-coated tissue paper product from 20 to 0 ml. air/min.

Visual microscopic examination of the discontinuous hydrophobic coating of this Example revealed the individual discontinuities to be irregular in shape without any apparent tendency toward elongular configuration. Approximately 90 percent or more of these discontinuities, as determined by visual examination, possessed a maximum dimension not exceeding about 0.3 mm. In addition, the discontinuous hydrophobic coating covered, as determined by visual examination, about 50 percent of the total surface area of the substrate sheet.

A conventional hot-melt carbon ink was applied by reverse roll coating to the employed untreated tissue paper, and an extensive heavy bleed through a penetration of the applied ink to the reverse side of the tissue paper was observed. A conventional hot-melt carbon ink also was applied by conventional means to tissue paper bearing only the discontinuous coating of the hydrophobic coating composition, and here too a heavy bleed through and penetration of the applied ink to the reverse side of the tissue was observed. In contrast, upon application by conventional means of a conventional hot-melt carbon ink to the twice-coated tissue paper product of this example, slight to no penetration and bleed through the applied ink to the reverse side of the product was detected.

Example 2

The tissue paper and hydrophobic coating composition of Example 1 was employed as a base for an aqueous based coating composition of

7% corn starch acetate ester in waler (KOFILM 10)

4% methyl cellulose aqueous solution MC 1500 Methyl cellulose)

Calcium chloride parts parts

0.117 parts

Example 3

A hydrophobic coating composition was prepared by uniformly mixing 45 parts of ASP 900 clay with 55 parts of a ₅ hot-melt blend, 9:1 ratio, respectively, of a microcrystalline wax of 143° F. congealing point (Aristowax 143) and an ethylene/vinyl acetate copolymer of about 27-29 percent vinyl acetate content (Elvax 220). This composition was mixed with heating to about 145° C. for application by a IQ gravure metering roll to porous tissue paper. The amount of deposited coating weight being deposited was varied from 0.5 to 2.0 Ibs./ream.

To the resulting tissue paper bearing a discontinuous coating of the hydrophobic coating composition there was applied 15 a commercial particulate aluminum silicate (ASP-900 clay) in a dry state with buffing so as to deposit and to fill substantially the discontinuities in the hydrophobic coating composition. About 0.5 Ib./ream of clay sufficed to fill the discontinuities and yield a useful, twice-coated tissue paper product.

_r , _Λ

Example 4

The same hydrophobic coating composition as employed in Example 1, in a molten condition at about 200° F., was applied by a 150-line gravure metering roll at a web speed of 100 ft./min. to the uncoated side of a one-side-printability-coated white paper litho label stock of a weight of 60±2 lbs. per 3300 sq. ft. ream so as to deposit thereon a discontinuous hydrophobic coating weight of one-half to 1 lb. per such ream. Visual microscopic examination of the deposited discontinuous hydrophobic coating showed it to contain numerous discontinuities of an irregular slitlike configuration with the discontinuous coating covering approximately 60 percent of the total surface area to which the coating had been applied. Over 90 percent of the numerous discontinuities in the resulting discontinuous coating were slitlike and did not exceed an average length of 1.0 mm. and a average width of 0.2 mm. with the area of the litho label stock exposed by the sum total of the numerous individual discontinuities approximating 40 percent of the total surface area to which the coating had been applied.

Tow hydrophilic coating formulations of the following compositions then were prepared:

This top coating was prepared by blending all of its con- 45 stituents together.

This aqueous-based hydrophilic coating composition was applied by use of a smooth metering rod on a pilot Dixon paper coater at a feed rate of 75 feet per minute on top of the tissue paper bearing on discontinuous coating of the 5Q hydrophobic coating composition. The aqueous-based coating composition was metered to provide a dry coating weight application of about 0.7 Ib./ream. The resulting twice-coated tissue paper product had a measured porosity of 15-35 ml. air/min. in comparison to a porosity of greater than 500 ml. 55 air/min. for the untreated tissue paper.

Microscopic examination of this twice-coated tissue paper product revealed it to be a porous substrate composed of cellulosic fibers and having thereon a firstly deposited discontinuous patterned solid coating of the hydrophobic coating 60 composition and a secondly deposited discontinuous coating of the starch composition, substantially filling the discontinuities of the firstly deposited discontinuous coating. Because of the contrasting hydrophobic and hydrophilic properties of the applied coatings and the meager amounts thereof applied, ami 65 secondly applied coating concentrated selectively ami primarily in the discontinuities of the firstly applied coating.

A conventional two-coat barrier system consisting of a flexible undercoating and a high barrier topcoating was applied to the twice-coated tissue paper product of this example to pro- 70 vide a useful article for packaging applications. A flexible undercoating of a water-dilutable styrene-butadiene latex was applied by a No. 12 wire-wound rod and dried; and thereupon, a topcoating of a poly vinylidene chloride copolymer latex was applied by a No. 12 wire-wound rod. ⁷⁵

Composition A

Aqueous styrene-butadiene latex of 48% solids content, such as Dow Latex 636

Sodium alkylaryl sulfonate surfactant, such as Santomerse 3 paste

Aqueous 2% solution of a watersoluble derivative of alginic acid, such as Kclgin HV

Composition B

Aqueous polyvinylidenc chloride latex, about 50% solids content, such as Daran 220

Weight

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Hydrophilic coating Composition A was applied by a No. 12 wire-wound rod onto the discontinuous-hydrophobic-coated

3,607,348 litho label stock. Coating was followed by a drying at about 100° C. for 3 minutes. After air cooling at room temperature, hydrophilic coating Composition B was applied by a No. 12 wire-wound rod onto the now twice-coated surface of the litho label stock. The resulting sum total dry weight of coating Compositions A and B approximated 13.8 lbs. per 3300 sq. ft. ream.

Example 5

In this example, the substrate for coating was a clay-sized, white, bleached paperboard. There was applied to its back side, i.e. the side appearing to have the poorest clay sizing thereon, the same hydrophobic coating composition, as employed in Example 4, by a hot-melt technique and a gravure metering roll. The resulting deposited coating was one-fourth to 1 lb. per 1000 sq. ft. Visual microscopic examination indicated that the discontinuities were predominantly irregular slitlike configurations with at least 90 percent thereof having an average length less than 1 mm. and an average width less than 0.1 mm. The discontinuous hydrophobic coating covered about 80-85 percent of the total surface area with the sum of the numerous discontinuities exposing about 15-20 percent of the total surface area of the paperboard to which the hydrophobic coating composition had been applied.

On top of this discontinuous-hydrophobic-coated paperboard there were applied in sequence two hydrophilic coating compositions, the same latices as employed in Composition A and B of Example IV, by a wire-wound rod technique so as, after drying and cooling, to provide a resulting sum total dry weight of the two coatings of about 8 to 9 lbs. per 1000 sq. ft. ream.

Example 6

In this example the substrate for coating was a 60 lb. per 3000 sq. ft. ream natural kraft paper. It was coated by a hotmelt technique and a gravure metering roll with the same hydrophobic coating composition as employed in Example 4 so as to deposit about a one-half to 2 lbs. per 3000 sq. ft. ream of the continuous hydrophobic coating. On top of this discontinuous-hydrophobic-coated kraft paper there were applied in sequence the two hydrophilic coating compositions of Example 4 in a manner alike that of Example 4 to provide a sum total dry weight of the two hydrophilic coatings of about 10 to 12 lbs. per 3000 sq. ft. ream.

From the foregoing, it will be apparent that preferred embodiments of the invention have been described with specificity. Equivalents, variations, and modifications of the principles of our invention falling within the scope of the description herein will be apparent in light of the specific disclosures herein and it is intended that the true spirit and scope of the invention be limited only as defined in the appended claims.