US 3041200 A
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United States 3,041,200 Patented June 26, 1962 This invention is concerned with a process for producing a printing paper, particularly such a paper suitable for use as labels, for cans, packages and the like, liners, covers, etc.,'or more broadly to, that class of stock commonly referred to as litho stock, or coat one side paper.
Printing papers of the type referred to are readily available on the market. Because of their use, they are generally characterized by brightness and opacity which make them suitable for lithographic or label printing where, in many cases, multiple colors may be used. Generally, litho papers are provided with some type of coating to render one surface uniformly receptive of inks used in printing and, further, such papers are also subject toacalendering operation to render the surface smooth and to provide a high gloss thereon. As is well known, calendering has an adverse effect on opacity.
Since brightness and opacity are critical, consideration must be given to the problem of securing suitable opacity and brightness either or both of which are affected by such factors as .color of the pulp, coatings, and tints to be applied to the finished stock, and the like. In many cases, because the pulp used is productive of a stock which lacks opacity at a given weight of paper, or because the calende r-ing operation reduces such opacity as the paper may have to an unacceptable level, it has been customary to a d fi e in he s k during eat n Pr r t e y of the pulp into the paper machine. Such fillers increase the scattering of light within the stock rendering the finished sheet more opaque than it would naturally be, if no fillers were added. However, in many cases opacity in creases result in a'loss of brightness which is totally undesirable from the printing and sales viewpoint. On the other hand, it is also known that the addition of certain pigments, principally titanium dioxide, TiO either into the pulp during beating or in the form of coatings, or both, will not only increase opacity but will also add to the brightness of the finished stock. The latter method, however, has a major distadvantage in that the [addition of 'IiO adds to the manufacturing cost of the paper and in some cases Where the pulp is of such 'a nature as to produce a paper severely lacking in. opacity, the loading thereof with TiO produces adverse results. [For example, such loading raises costs to an almost prohibitive figure as well as weakens the stock creating problems in the printing thereof, particluarly in high speed continuous lithographic printing machines such that only a given amount of fillers may be used without adversely afiecting the printing qualities of the paper, or a minimum of filler is used resulting in a paper lacking in opacity and brightness.
production of a bright, opaque, highly acceptable paper for use in lithographic printing processes.
Another object of the invention is to produce a paper of maximum opacity and brightness obtained by the minimum use of fillers and the like.
Still a further object of the invention is to produce a The present invention, therefor, has for an ob ect the coated paper of acceptable brightness and opacity, wherein the manufacturing costs may be substantially reduced.
Another object of the invention is to produce a bright, opaque paper that is economical to manufacture, commercially competitive, yet permits of substantial reductions in the use of fillers, brightening agents, etc., such as TiO An additional object of the invention is to produce a bright, opaque paper which is particularly adapted for use as labe1's',"cdvers, liners, etc. i
\A further object of the invention is to produce an all purpose paper stock wherein acceptable shades may be obtained along with suitable opacity and brightness while permitting a substantial reduction in the use of fillers, brighteners or the like.
These and other objects of the invention may be accomplished by the method of applying to a paper stock, by one means or another, .one of a class of dyes having the particular characteristic of increasing opacity but without substantial reduction in brightness, the dye being applied solely to the backside of the stock within a range of certain concentrations and further, in some'cases, in controlling the tints that may be inclnded in the pplp and/or. coatings if used or both. The front side of the stock is generally the only side coated with a mineral coating that provides a good printing surface. 0
One manner of carrying out the instant method for producing bright, opaque label paperstock may be by the, use of equipmenfsirnilar to that disclosed in Mont gomery patent 2,251,890. That patent discloses a means ,for applying liquid to a moving web as it passes between the rollsof a supercalender stack. The means includes a suitable Wetting roll to apply liquid to a web and a wiper to remove excess water over and above that necessary to assure proper finishing as theweb passes through the calender. It has been found that the liquid applicator and wiper action of the apparatus disclosed in the patent will apply a fairly well controlled and finedarnount bf liquid .such that varying the concentrations of substances that might. be included in the liquid will result inolose con- .trol of the amount of such substances asmay be deposited onaweb.
Anothermethod of carrying out the invention might well be on the paper machine itself. For example, the dye can be applied to the web at the machinecalender stack or somewhere in the dryer section of .a paper machine. Regardless of the manner of application, it mustbeaccor nplished in such a mannerthatjtheHye solution is applied evenly and'at a well controlled rate on what will be the uncoated side of the web, such that control ofthe shade of the dyed web is a function of the amount of dye contained in the liqui-d'a-ppl-ied to thepaper, rather than a function of the amount of liquid applied to the paper. a i i p "In the practice of the present invention one of the variou yp of l qu a l t s s wa as to apply liquid to one side of a moving web. win all cases the pp icato pp i a mete q u c o a su t e ye to one side of a paper. In most instances the dyeisapplied only to that side of thelweb which is ,not coated or intended as a printing surface. The dye, if of the type to be described,'h as the'efle'ct of increasing'opacity of a sheet of the paper with only a minor reduction in brightness as measured on the print receptive surface In practic- Web forming l...
Coating r pplication to one side only Calender to smooth 7 Product Opposite side Dye application one side and calendertng to smooth opposite side Finished product Finished product I Since shade or #hue of the commercial product is so important the effect of a given dye on this final hue or shade, i.e. color of the printing surf-ace of the paper, must be carefully considered. It was found during the trials on at least two shades of raw stock that, within a limited range, a given dye could be applied in suflicient amounts to increase opacity several points yet produce a specific tint or shade on the printing side of the sheet, while having a effect on brightness of the printing side of the paper. In some cases, the tultreated stock was a neutral shade, that is, it contains little if any coloring or tints and in other cases, the stock was so called regul-ar stock having limited amounts of blue and red tints added as is customary in the art. It has been found that in all cases the opacity of the calendered stock as viewed from the front side was increased by the applicationof Pontamine Fast Blue 3RNP or its equivalent,,on the backside of the stock, and that the controlling factor as to concentration of the dying solution is the shade produced on the front, i.e. printing surface of the sheet. The fol: lowing examples indicate the manner in which the invention can be carried out. In each example, the paper stock was 60# per 25 x 38, 500 sheet ream, all tests for brightness and opacity were made in accordance with the standards set up for the industry by The Institute of Paper Chemistry.
The particular dye used as an example hereinafter is Pontamine Fast Blue 3RNP as manufactured by the E. I. du Pont de Nemours Co. This dye has been selected from among others, of which some are listed below, because, in addition to the specific qualities as regards brightness and opacity, it exhibits other qualities such as light fastness, water solubility, and because generally, its excellent properties make it easy to handle and readily useable. In addition since a blue-white hue is most pleasing to the eye, blue having the quality of making white appear even more white, a blue was believed preferable to other colors, although it has been found that certain other colors of dyes-do have similar properties, though not in all cases quite as good as blue.
Among the dyes which can be used to eifect the inven- 4 v 7 tion are the following, rated in terms of their afiect on opacity with minimum affect on brightness:
Pontacyl Violet C4BN Excellent;
While some of the above listed dyes are rated not quite as good as others, it will be realized that such additional factors as have been mentioned in connection with Pontamine Fast Blue SRNP may dictate the preference for one dye over another. In addition the hue, i.e. color of the paper deemed most desirable may also and, in fact, often does dictate the choice of one dye over another. While the word dye is used throughout the specification it will be appreciated that it is not used in a limit ing sense, but is frieant to include such substances as may be used to give a hue or tint or shade to the paper, such as water soluble dyestuffs, dispersions of pigments and the like. The invention is therefore not intended to be limited to a particular dye, or dyestuif, though water soluble dyes are the most convenient to use because of the extensive use of water in paper-making coating and finishing operations. For example a so called pigmented dyest-uif can produce desirable results if properly dispersed in a liquid medium. A specific example is a pigmented dye such as Au-rasperse Phthalocyani ne Blue W4021 manufactured by Zinnser & Co. dispersed in water and applied to the uncoated side of a standard litho sheet. This pigmented dispersion gave results quite similar to the results outlined below. 7
In the following examples of the manner of practicing the invention a regular tinted stock was used and the re sults after supercalendering were as follows:
Stock Percent Dye Opacity 1 in Solution Brightness 1 (G.E.)
X All measurements on coated, i.e., printing surface.
It was found that in all cases with regular stock, i.e., stock containing tints in both pulp and one side coating, there was an increase in blueness on the printing side of the sheet, which ranged from a deep blue at 0.125% to an acceptable blue white upon the application of solutions containing between 0.0625% and 0.03125% blue dye solutions. It will also be noted that this range gave the greatest increase in opacity while brightness of the sheets was affected the least.
Similar results were obtained when the dye was applied to a neutral stock, that is, a paper stock having little or no tints added in either coating or pulp. Again, the amounts of dye in the aqueous solution were varied to obtain the most favorable ranges of color. In this case, however, since the neutral stock was less blue in shade, as viewed from the coated side, it was found that more dye, approximately 0.2% in solution, could be applied to the back before the shade of the printing surface became too blue .to be acceptable. On a neutral sheet having an initial opacity of 89.4 and a brightness of 79.3. after calendering of the printing surface, it was found that a solution containing 0.1% by weight of the blue dye increased opacity to 92.4 while brightness was 78.4 and the final shade of the sample was adjudged to be exceptionally close to the standards for such paper. With the exception of the fact that the neutral shade permitted the use of more dye on the backside of the sheet, the results were substantially similar to those obtained from tinted stock indicating quite clearly that hue, or final shade was the limiting factor in the amount of dye that can be used. Thus it may be seen that the percentages of dye in solution are a function of the intensity of the dye and that a range of from .015625 to 0.2% by Weight are well within the preferable range of concentrations of solutions giving favorable results, although a Wider range of concentrations may be used on occasion.
From the above results it may be seen that the application of one of a selected group of given dyes having the property of increasing opacity of paper, yet producing a minimum reduction of brightness of the coated side of a sheet, enables the paper to be made with a minimum of filler and further, that various combinations of tints in the stock, tints in the coating applied to the printing surface and tints applied directly to the back of the stock can produce an opaque, bright printing paper having excellent printing properties.
As has been stated, beneficial results can be obtained by the sole application of dye of the disclosed character istics to the opposite side of a paper than that which is to receive printed material. In carrying out the invention on a commercial scale, however, it has been found desirable to not only apply dye to the backside of the sheet, but also to include some dye in both the pulp at the heaters and in the coating. This combination of dye in pulp, coating, and on one side of a sheet is highly effective because it has been found that in many cases the pulp will not always be the same shade before it is to run onto the paper machine. Also in some cases the definite blue hue that is present on the back of a one side coated sheet is objectionable. Accordingly, it has been found that favorable results can be obtained by reducing the normal amount of blue used in the coating, maintaining a fairly constant shade of pulp and then applying to the uncoated side of the paper a concentration of dye sufficient to produce the shade required on the coated side thus resulting in high opacity at the selected shade when the paper is viewed from the coated side. Examples of this practice are as follows, all readings after calendering.
Percent Dye Percent Dye Opacity, 1 Brightness, 1 Stock in Solution in Coating TAPPI G.E.
Backside 0 84. 4 79. 3 0. 1 0 92. 4 78. 4 Neutral 0. 03 0. 03 90. 5 78. 5 0.05 0.02 91. 2 78. 1 0.075 0.005 91. 6 77.9
I As measured on coated, i.e., printing surface.
In the examples cited above, the dye used in both coating and on the back was Pontamine Fast Blue 3RNP. It has been found that a combination of about .04% of a suitable tinting solution applied to the back and about 0.03% weight of dye in the coating produces a paper deemed commercially acceptable and, in fact, so it is, as evidenced by the huge quantities thereof used in the printing trade.
Although the favorable results were obtained on 60#/ream paper it will be appreciated that the advantages of the invention can be realized on any weight paper Where an increase in opacity is desired and wherein the product is to be printed on only one side, such as is the case with a one side coated paper which is intended for use in forming labels, etc., as described above.
Having thus described the invention, it will be apparent that variations in the practice thereof are possible, all of which fall within the spirit and scope of the inventive concept which is limited only as defined in the following claims, wherein, what is claimed is:
1. A lithographic printing paper having the properties of high brightness and low opacity and one coated surface formed by the process of applying a mineral coating on one side thereof, applying a dye in the form of an aqueous solution containing from 0.2% to 0.015625% by Weight of the dye to the opposite side thereof, said dye having the property of increasing opacity of the paper yet having a minimum adverse effect on brightness as measured from the coated surface of the paper, and calendering of the coated, finished paper to improve the printing quality of the coated surface thereof and wherein the paper is characterized in that it is substantially free from filler materials.
2. The method of manufacturing printing paper stock having one coated printing surface and high opacity and brightness, comprising the steps of coating one side of a paper stock with a mineral coating, thereafter applying a aqueous finishing solution containing from 0.2 to 0.0156 percent of a blue dye to the opposite side of the stock while calendering the coated dyed stock to smooth the coated surface, the blue dye having the properties of increasing opacity of the stock without materially affecting the brightness thereof as measured by standard measuring procedure on the coated surface of the stock.
3. The method of increasing the opacity of printing papers having at least one surface adapted to receive printed matter and characterized by the property of 10W opacity comprising the steps of forming a pulp into a web coating the web to form a surface adapted for receipt of printed matter drying the web and thereafter treating the opposite surface of said Web by the application of .an aqueous solution containing from 0.01% to 0.02% of a dye, said dye having the property of increasing the opacity of the sheet while not materially affecting the brightness of the printing surface.
4. The method of manufacturing printing paper stock having at least one surface coated with a mineral pigment to define a printing surface and characterized by its qualities of high opacity and brightness, comprising the steps of forming a dye containing pulp into a web, coating the formed web on at least one surface with a mineral pigment coating containing from .005% to 0.03 percent by weight of a dye having the property of rendering the formed web opaque Without materially affecting the brightness of the coated surface drying the coated web, thereafter applying an aqueous solution containing from 0.03% to .1% by weight of said dye to the opposite surface of said formed substantially dry web.
5. The method of manufacture of printing paper stock as defined in claim 4 wherein the formed web is calendered and wherein said aqueous dye containing solution is applied directly to the surface of the web during said calendering operation.
References Cited in the file of this patent UNITED STATES PATENTS 1,721,576 Reynolds July 23, 1929 1,814,344 Snelling July 14, 1931 1,818,982 Antoine Aug. 18, 1931 2,099,341 Kenyon et al. Nov. 16, 1937 2,348,128 Groak May 2, 1944 2,528,324 Thomas Oct. 31, 1950 2,920,991 Fisher et al. Jan. 12, 1960 2,930,106 Wrotnowski Mar. 29, 1960 2,993,803 Sulich et al. July 25, 1961 FOREIGN PATENTS 496,011 Great Britain Nov. 23, 1938 697,307 Great Britain Sept. 16, 1953 OTHER REFERENCES McGraw- Hill: Pulp and Paper Manufacture, vol 2, McGraw-Hlll Book Co., Inc., New York (1951), pages 492, 510, 511.
Casey: Pulp and Paper, vol II, Interscience Publishers, Inc., New York (1952), pages 912, 913 1110.