US3300364A - Process for smoothly laminating a thin film on an unsmoothed paper base - Google Patents

Description

' Jan. 24, 19 7 Filed May 5, 1964 B. C. GOTTWALD ETAL PROCESS FOR SMOOTHLY LAMINATING A THIN FILM ON AN- UNSMOOTHED PAPER BASE 2 Sheets-Sheet l Unsmbolked Paperfioard Wkly/211g Less 771cm A-boui j2lbs./K?eccm.

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661/1611? T/zz'zz Decoralivc F0 zl Over Goad/i139? ApplyfieleaseLaycr 0w!" Coeziirqgfi 24,, 1967 B. c. GOTTWALD ETAL 3, PROCESS FOR SMOOTHLY LAMINATING A THIN FILM ON AN UNSMOOTHED PAPER BASE Filed May 5, 1964 2 Sheets-Sheet 2 United States Patent 3,300,364 PROCESS FOR SMOOTHLY LAMINATING A THIN FILM ON AN UNSMOOTHED PAPER BASE Bruce C Gottwald and John M. Haigh, Richmond, Va.,

assignors to Albemarle Paper Company, Richmond, Va.,

a corporation of Virginia Filed May 5, 1964, Ser. No. 365,102 4 Claims. (Cl. 156-281) This application is a continuation-in-part of prior copending application Serial No. 77,174, filed December 20, 1960, and now abandoned.

This invention relates generally to an economical cast coated paper which provides for a paper product having superior gloss and smoothness. More particularly, the present invention is concerned with an unpigmented cast coated paper which has greater stiffness, tearing strength and printability which can be used as either a smooth coating base material which permits the attaining of greater brightness with lower coat weight or as a superior laminating base.

The manufacturer today, as is well known, utilizes a colorful and attractive packaging or wrapping for his product, which is appealing to the prospective purchaser for the reason that such eye catching packagings are a significant factor in the sale of any individual product. This is particularly true when competitive products are often side by side on the retail shelf, each vying for the attention of the consumer. Thus, more attractive wrappings or packages have been sought by manufacturers to sell their product without adding appreciably to the cost of the product.

In the past, almost all grades of paper and paperboard, except for bulky saturating grades, have a finish imparted to the surface by means of calenders located on the end of the paper machine. The paper or board is passed between a series of heavy rolls, stacked one upon the other. By a combination of the weight of the rolls and a slight slippage effect between the rolls, a relatively smooth surface is imparted to the paper. Due to the high pressure exerted by these rolls on the paper, the relatively high areas in the paper caused by fiber flocculation during the formation of the paper receive a higher finish than do the lower areas, resulting in an imperfect surface. In addition to the smoothing effect of the calendering, the sheet is also compacted during the calendering operation.

It has been found that the resulting calendered paper is not satisfactory in many instances for at least two reasons: (a) The paper board is compressed permanently by the high pressure calendering rollers, usually resulting in a high density paper of at least 12 pounds per mil per ream, as compared to the uncalendered paper density of less than pounds per mil per ream, thus reducing the stiffness and tearing strength of the paper, since each characteristic is normally related to density; and (b) the smoothness resulting from the calendering is still not desirable for laminating thin foils for example, and not smooth enough to permit the use of a desirably low coating weight. e

A second method sometimes used for finishing paper and paperboard is referred to as machine glazing. In

this method the wet paper web is pressed against a Yankee finishing dryer, usually 10-12 feet in diameter, by means of a pressure roll. The paper or board will partially dry while against the drum and the fibeTs will conform to the surface of the drum, resulting in a high finish. While a superior finish can be obtained by this method as opposed to calendering, a true mirror image 3,300,364 Patented Jan. 24, 1967 of the Yankee drum cannot be obtained as the fibers are not sufficiently flexible so as to conform completely to the drum and hence the voids between the fibers which do not touch the drum remain as voids in the finished product. While this method does largely overcome the strength problems encountered in compacting by calendering, it does not provide a completely smooth finish, and thus is not a satisfactory product.

Additionally, it has been consistently experienced that the printability of a paper or paperboard is related to its smoothness; thus, the smoother the paper, the more easily it is printed. The reason for this is simply that the ink will only contact the paper and produce a continuous print where the lowest points on the paper surface are not lower than the thickness of the ink film on the printing plate. Due to the usual roughness of conventional paper, it is not unexpected that it is extremely difficult to obtain a solid colored print that is satisfacory.

Also, when such conventional papers are coated with a mineral or pigmented coating by any of the usual methods, such as an air knife, roll, blade, cast, etc., the coating must of necessity be of a heavy coat weight in order that all the paper fibers are covered and all the small undulations and surface variations normally present in the board are filled. All the fibers and hills in the paper must be substantially covered with the mineral coating if the paper is to attain any desirable brightness, smoothness and uniformity of appearance.

Regardless of what method of application of the coating is used, the heavier coat Weight that is necessary with conventional paper poses problems when the paper is to be scored or folded. Such problems, however, are not apparent in a paper with a lighter coat weight. Addi tionally, a high coat weight often prevents proper gluing of the coated paper, whereas, the low coat weight paper generally has superior gluing properties. However, one of the most significant and important facts concerning low coat weight paper is that it has greater strength per pound of product. For this factor alone, the low coat weight paper is desirable if it also has the desired qualities of brightness, smoothness and uniformity of appearance.

In regard to the use of paper as a base for laminations, such as aluminum foil or other similarly thin material, as distinguished from a liquid coating, the usual paper is again too rough to have a very thin layer of foil placed directly thereon since this paper roughness will emboss the thin layer superposed on it, thus producing an unsightly product.

Consequently, it is an important object of this invention to provide a paper having a smooth surface.

An additional object of the present invention is the provision of an unpigmented coating on a rough paper base to smooth the surface sufficiently so that the base may be laminated with a thin foil without the foil being substantially embossed by the paper.

Another object of the present invention is the production of a paper stock for coating which permits the attaining of superior brightness at relatively low coating weights as compared to conventional coated paper stock.

It is also an object of this invention to provide a paper with a smooth coating of low weight, while maintaining the paper at a substantially low density and therefore high bulk. A further and more limited object of this invention is to provide a rough paper with a smooth unpigmented coating which permits the use of the resulting product as a smooth base material requiring only a low coating weight to produce superior brightness.

Other objects and advantages of the present invention will become apparent to those skilled in the art from the following description when read in conjunction with the accompanying drawings, wherein:

FIG. 1 shows the steps of a method of preparing coated paper boards pursuant to the present invention; and

FIG. 2 shows the brightness obtainable with one particular conventional pigmented coating weight on various treated base stocks.

Briefly, this invention recognizes that a rough paper stock can be cast coated with an unpigmented hardenable and thermo-setting adhesive coating to produce a smooth surface that requires only a very light pigmented coat weight applied by conventional means to attain a superior brightness, or can be laminated with a very thin preformed sheet of material, such as aluminum foil, without being undesirably embossed by the surface of the under layer. Specifically, the present process comprises: (1) coating a rough paper with a coating formulation consisting essentially of a liquid and an unpigmented hardenable and thermo-setting adhesive selected from the group consisting of protein, starch, and synthetic polymers; (2) casting the coating upon a rough paper under heat and pressure so as to fix said coating and render it smooth; and ('3) superposing on said coated rough paper an integral, essentially continuous, and non-penetrating second coating. The coating effected in step (1) above is preferably a protein or starch coating (more so the latter) because these materials are readily available and hence offer a cost advantage. The superposed coating is preferably a pigmented or a preformed metal foil coating.

The base paper with which this invention is concerned is a paper of relatively low density, preferably less than about 12 pounds per mil thickness per ream. Such paper is conventional and is well known to have surface variations with considerable high spots and low spots caused not only from grouping of the fibers into hills and valleys, but also from the fact that the fibers position themselves erratically and often protrude above other portions of the paper surface. A paper in this state has a relatively low density usually of less than about 12 pounds per mil per ream and is stated to be a bulky paper. Such paper has an inherent stiffness due to its high bulk and thickness which is desirable in packaging since bowing or bending of the paper is lessened. Moreover, the tear strength will be greater with a paper that has a greater bulk. Again, this is a desirable characteristic to retain in a package.

In accordance with the present invention, a rough paper as above can be highly smoothed, without resort to the normal degree of calendering which reduces both the tear strength and stiffness of a paper, by cast coating thereon a low weight clear coating of approximately /2 to approximately 9 lbs/ream of hardenable and thermosetting adhesive. For most applications it is preferred to employ at least about 2 lbs/ream of the adhesive, hence a preferred range is from about 2 to about 9 lbs./ ream of hardenable and thermo-setting adhesive. An even more preferred range is from about 4 to about 7 lbs/ream. Exemplary of the hardenable and thermosetting adhesives suitable for use in the instant invention are: starches, e.g., a starch derivative of the oxidized, chlorinated, enzyme converted, corn or potato type, and the like; proteins, e.g., casein, and the like; and synthetic polymers, e.g., a styrene-butadiene water based emulsion, carboxy methylcellulose, methylcellulose, alkydmelamines, and the like. Pursuant to the first step in the present process, by casting is meant the application of both heat and pressure simultaneously to the web while it is in contact with a smooth surface. In this manner, the very smooth surface imparted to the coating is fixed by drying the coating at the point of application of heat and pressure, unlike for example, in inking or similar operations where the coating (which is neither integral nor continuous) is subsequently dried. When operating in this manner, speeds within the range of from about 50 to about 200 f.p.m. are readily realized utilizing conventional cast coating equipment. In fact, at low coating weights and utilizing the preferred casting apparatus described hereinafter, speeds in excess of 200 f.p.m. are readily achieved.

The apparatus for cast coating such an adhesive can be of conventional design, such as in Bradner 1,719,166, and hence need not be shown herein. However, a more preferred cast coating apparatus is that as shown and described in applicants Patent 3,110,612 titled, Coating Apparatus and Method, based upon an application filed concurrently with applicants application Serial No. 77,174 of which the present application is a continuationin-part.

When casting the initial adhesive unpigmented coating, it is preferred to do so at a temperature above 212 F., exerting suflicient pressure upon the coating to keep it in contact with the heated smooth casting surface, viz. to prevent blowing. In accordance with the preferred apparatus of U.S. Patent 3,110,612, it is especially preferred to press the coating upon the smooth casting surface heated to a temperature sufficient to boil the liquid in the coating, the pressure being applied by vapor receiving means caused to urge the coating against the heated casting surface.

A particularly preferred embodiment comprises casting the adhesive coated paper at a temperature within the range of from about 220 F. to about 500 F., especially within about 260 F. to about 360 F., while compressing it within the range of from about 20 to about 150 p.s.i.a., especially within about 40 to about p.s.i.a. As moisture is picked up by the vapor receiving means during the casting operation, it can be removed therefrom by pressing and/or drying such means as desired.

In the conventional cast coating apparatus, and in applicants Patent 3,110,612, the pressure exerted by the usual rollers at the nip or around the dryer drum are slight compared to the high pressures exerted against the paper during calendering. Thus, in casting the initial adhesive coating upon the surface of the rough paper the important high bulk characteristics of the paper are maintained, and at the same time the paper is given a very smooth coating of light weight.

The initial coating thus applied fills in the low spots on the paper and is suflicient to cover the high spots with a very thin coating such that there will be a continuous smooth and integral coating or film of the clear adhesive on the paper which coating is cast to the smoothness of the chromium plated drum. Since relatively little pressure is used in the casting operation, the paper maintains its original high bulk with its inherent advantages.

The following table shows a comparison of the important characteristics between a conventionally calendered paper board and a protein cast board prepared pursuant to the instant invention, both having a similar natural kraft board base:

The smoothness test was made on an air leak smoothness tester (Gurley R.D. Smoothness Tester) in which the time required for a given quantity of air to flow between a flat surface and the paper under test was measuredthe greater the time, the smoother the paper.

Conventional Unpigmented Conventional This Inven- Calendered Protein Cast Calendercd tin-Protein Board Board of This and Coated Cast Board Invention Board Taber Stiffness; Basis Weight, lbs/ream 126 126 Machine Direction 220 336 rotcin 0 4 Cross Direction 102 191 Gloss: (Gardner Portable Gloss Total Basis Weight 126 130 Meter) 28 Clay Coating, Wt. lbs/ream Applied" 18 10 10 Total Coated. Basis, Wt. lbs./ ream 144 140 By this comparison, the cast board of this invention was Hunter Brightness, percent 7a 75 shown to be not only much smoother, but also consider- Gurley smoothness Secs 12 28 ably stiffer. Such a board, therefore, has the ideal qualities to serve as a base sheet for subsequent coating or laminating operations.

It has been determined that an unpigmented cast coated paper board prepared pursuant to the teachings of this invention can be coated by means of the usual air knife with a conventional pigment coating formulation such as:

Parts Clay 100 Rubber latex 18 together with enough water to have 55 percent solids in the aqueous medium, with the result that less pigmented coating weight is required on the unpigmented cast coated base board of this invention to achieve not only the same brightness, but a superior brightness when compared to a base board treated in any one of the conventional manner, such as: machine glazing, calendering, or applying the pigmented coating directly to an uncalendered board.

The plot of pigmented coating weight (pounds per.

ream) vs. Hunter brightness in FIG. 2 graphically shows the significant advantage of a cast starch coating according to this invention on a base of kraft paper board before the conventional pigmented coating is applied with an air knife. Within a coating range of 2 pounds/ream and 20 pounds/ream the advantage of the present smooth cast starch base board is easily recognizable since the brightness of the resulting board is considerably higher with lower coat weights, than with higher coat weights on conventional paper bases.

The following table from which the above plot was derived compares the brightness obtained on various coated bases at different coat weights. In every test the original semi-bleached base board was identical and weighed 158 pounds/ream and had an initial Hunter brightness of 50'.

From the above, it is further evident that starting with a conventional kraft paper base board and applying thereon a cast starch coating in a light coat weight pursuant to the present process, (1) much less clay coating is necessary with the cast starch undercoat than without it to produce a product having superior surface properties and without subtracting or reducing the inherent physical properties of the base board, (2) a smoother surface is realized, viz. more than twice as good, and (3) superior brightness is realized.

Another distinction between the cast board of the present invention and conventional calendered board can be made with respect to conventional metal foil lamination. In the case of the present cast board, the foil surface retains the smooth glossy and mirror-like appearance of the original foil, whereas with the conventional calendered board, the foil surface is embossed, diffused, and irregular in appearance.

The term functional layer as applied to the outer or superposed coating is intended to include only (1) pigmented or mineral containing coatings applied by conventional means, e.g. brush coater, air knife, or the like, and conventionally dried (the super-posed coating could be cast if desired), (2) foils or film layers, such as: aluminum foil (which is especially preferred); paper foil; plastic foil; or the like, either of which foils may include a layer of glue or adhesive preparatory to laminating same to the coated base web, and (3) release coatings. The functional layer can be substantially continuous, allowing for pinholes and the like, or substantially discontinuous such as when employing an outer coating or film having special out out portions, e.g. for decorative purposes. In no instance is the term functional layer intended to read on an ink coating or system which is neither integral nor continuous and is penetrating.

HUNTER BRIGHTNESS VS. COgFQggEgGHT-VARIOUS COATED BASE Starch Cast (4 lbs/ream) Machine Glazed Calender-ed Pigment (This Invention) Coated Pigment Brightness Pigment Brightness Pigment Brightness Coat Wt. Coat Wt. Coat Wt momma c363 fiss w s z s OQDFQQQOWQQ During the compilation of the above data, it was determined that the base stock ceased to be a major factor in the ultimate brightness after the base stock was covered sufficiently. At this point, further coating only added to the coating weight without any substantial increase in brightness.

A further comparison of a paper base produced accord ing to this invention and a conventional calendered kraft paper base is shown below.

7 the province of those skilled in the art. However, it is intended that all such variations not departing from the spirit of the invention be considered as within the scope thereof as limited solely by the appended claims.

We claim:

1. A process for preparing a strong paper-foil laminate in which the foil forms a very smooth surface layer, which process comprises the steps of providing an unsmoothed paper board web having a density less than about 12 pounds per mil per ream, cast coating on one face of the unsmoothed web an unpigmented layer of adhesive selected from the group starch, protein and synthetic polymers, said layer weighing about 4 to about 7 pounds per ream, and cementing a thin decorative foil over the cast coating.

2. A process for preparing a strong smooth-surfaced paper board with a release coating, which process comprises the steps of providing an unsmoothed paper board web having a density less than about 12 pounds per mil per ream, cast coating on one face of the unsmoothed 8 web an unpigmented layer of adhesive selected from the group starch, protein and synthetic polymers, said layer weighing about 4 to about 7 pounds per ream and forming a smooth surface on the board, and applying a layer of release coating over the smooth surface of the cast coating. 3. The process of claim 1 in which the foil has cut-outs. 4. The process of claim 1 in which the foil is aluminum.

References Cited by the Examiner UNITED STATES PATENTS 1,719,166 7/1929 Bradner 117156 X 2,214,565 9/1940 Montgomery et a1. 117156 X 2,286,259 6/1942 Cagle 11783 2,754,240 7/1956 Kinney 156336 X 2,955,970 10/1960 Rice et a1. 117-64 X 2,999,787 9/1961 Down et a1 11760 X EARL M. BERGERT, Primary Examiner.

J. F. MATHEWS, I. P. MELOCHE, Assistant Examiners.

Claims (1)

1. A PROCESS FOR PREPARING A STRON PAPER-FOIL LAMINATE IN WHICH THE FOIL FORMS A VERY SMOOTH SURFACE LAYER, WHICH PROCESS COMPRISES THE STEPS OF PROVIDING AN UNSMOOTHED PAPER BOARD WEB HAVING A DENSITY LESS THAN ABOUT 12 POUNDS PER MIL PER REAM, CAST COATING ON ONE FACE OF THE UNSMOOTHED WEB AN UNPIGMENTED LAYER OF ADHESIVE SELECTED FROM THE GROUP STARCH, PROTEIN AND SYNTHETIC POLYMERS, SAID LAYER WEIGHING ABOUT 4 TO ABOUT 7 POUNDS PER REAM, AND CEMENTING A THIN DECORATIVE FOIL OVER THE CAST COATING.

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