WO2002103110A2

WO2002103110A2 - Sodium silicate treatment for printing papers

Info

Publication number: WO2002103110A2
Application number: PCT/US2002/019317
Authority: WO
Inventors: Joseph H. Torras, Sr.; Clifford W. Parker
Original assignee: Eastern Pulp And Paper Corporation
Priority date: 2001-06-19
Filing date: 2002-06-19
Publication date: 2002-12-27
Also published as: WO2002103110A3; AU2002310455A1; CA2451011A1; US20030094253A1

Abstract

Integrating sodium silicate during the paper-making process, or after paper manufacture at any time prior to final image formation. Intimately associating or bonding the fiber and the silicate provides significant benefits, for example, adding additional stiffness to papers that otherwise would not be stiff enough for use in high-speed digital laser printers, and improving the quality of colored image reproduction on high speed digital laser printers. Two specific examples are transfers of financial printing on electronic digital printers to lighter weight papers, and colored image formation in advertising material where the reproductive quality of the image must be of high quality.

Description

SODIUM SILICATE TREATMENT FOR PRINTING

PAPERS

TECHNICAL FIELD

This invention is in the general field of printing papers used for image production such as papers used for wet and dry toner printers (e.g., digital high-speed laser printers and laser-type printers).

BACKGROUND

Fidelity and reproductive quality, brilliance, and gloss are important attributes of a great many types of printed material, such as those used for black, single-color and multicolor printing by wet- or dry-toner digital laser and laser-type processes. Xerox DocuTech® and DocuColor®, Indigo, Heidelberg, Oce, Xeikon, Canon, and IBM and others sell papers used for such process.

Products in which these features are important include magazine covers and text, advertising inserts, greeting cards, postcards, calendars, direct mail advertising, financial printing, office correspondence using word processor and office personal laser printers, etc. Photographs provide a standard against which the final printed materials may be evaluated. In some cases, laser printed base stock is coated to improve its appearance.

Another consideration for print jobs with the above-described printers is the weight of the paper. For some applications, a lighter weight paper is desirable. Publications that are shipped or mailed-e.g., HMO directories, health plan benefit booklets, financial prospectuses and the like-are subject to charges based on the weight of the material mailed. Lighter weight publication mailings reduce postal charges substantially. Lighter weight may be desirable for making reprints or custom prints of textbooks, training books and manuals, directories and similar types of publications, and for other applications. Lightweight papers, in which the thickness normally varies with the weight, are also desirable for ease of handling and compactness of storage. On the other hand, laser printers typically require a minimum stiffness in order to run smoothly. The paper must be stiff enough and have sufficient body to pass through several points in the paper path that can be problematic for light-weight papers. Papers having a basis weight of 50 pounds (25"x38"/500 sheets) or more are commonly used for Xerox's DocuTech® printer and similar high-speed digital laser printers to avoid jams in the paper path. The inability of such printers to use lighter weight papers reduces the economics and market opportunities of such printers for customized "on demand" printing for the reasons discussed above.

Official correspondence may be produced by digital laser printing by a process in which the initial letter is written on a word processor and transmitted to a digital laser printer for final printing. The final printing for such correspondence is done on "letterhead" papers containing cotton fibers, the most expensive of all office papers, which are used for the impression they convey. The market for this product is estimated at 100,000 tons. When these papers are laser printed, the following phenomenon, known as ghosting, may occur. A final letter with the desired appearance is signed, folded and placed in an envelope. When the envelope is processed during mailing, particularly in postal equipment, the image printed on the letter is transferred (in a reverse image) to the portion of the paper that contacts it as a result of folding. Thus ghosting disfigures the letter and undermines the sender's desired impression.

For calling cards such as those printed with a thermographic process, stiffness is additionally important.

SUMMARY

We have discovered that integrating sodium silicate into printing paper provides significant benefits, for example, substantially improving image quality

(particularly for color images) and adding additional stiffness to papers that otherwise would not be stiff enough for use in high-speed dry-toner printers.

The use of sodium silicate in printing as described in this invention introduces to image-producing processes and equipment, such as those mentioned above, the ability to produce images of far greater fidelity and reproductive quality, brilliance, and gloss than can be obtained when the same process and printer uses uncoated printing paper not treated with the invention. The printed result approaches photographic quality.

The invention thus makes possible reproductive results with uncoated paper that are generally only possible using higher cost coated papers. Essentially all applications involving black, single color and multicolor printing with the wet and dry toner digital laser and laser-type processes are improved by the invention's heightened color imaging capability and print fidelity. For example, magazine covers and text, advertising inserts, greeting cards, postcards, calendars, direct mail advertising, financial printing, office correspondence using word processor and office personal laser printers, etc., and as coating base stock to be coated for photographic quality laser printing.

While the paper typically is a bleached kraft printing paper, the invention also includes other papers that comprise sodium silicate applied as a saturation or at or near at least one paper surface, such as bleached or unbleached sulfite paper, with or without groundwood fiber. Thus the invention has application with newsprint and for marketing inserts.

Due to the sodium silicate treatment, the printed areas exhibit higher image fidelity and a greater degree of gloss in comparison to a reference paper that has been printed in the same way, where the reference paper lacks sodium silicate and is otherwise identical in composition and physical properties to the paper containing sodium silicate. The gloss in the solid and half-tone areas of the image derives from the ink/toner and not from the surface reflective characteristics of the unprinted paper. In addition, there is much greater fidelity of image reproduction than with an identical image produced in the same manner by a reference paper lacking sodium silicate and being otherwise identical in composition and physical properties to the paper containing sodium silicate.

The resulting image more closely approaches photographic quality than does an identical image produced in the same manner on a reference paper lacking sodium silicate and being otherwise identical in composition and physical properties to the paper containing sodium silicate.

The invention reduces ghosting, e.g. on high quality executive correspondence such as correspondence laser printed on cotton- fiber containing "letterhead" paper. The invention is also applicable to business reply cards, a form of direct mail advertising primarily in magazines for obtaining and renewing subscriptions and for direct advertising of product in magazines. Papers used for this purpose must meet U.S. Postal Service specifications. This market is about 225,000 tons/year. Color printing and personalizing of these cards increases the return rate (order rate) over the return rate for standard black and white cards. In particular, digital laser color printers can color-print personalized business reply cards. The invention includes papers that satisfy both the postal service demands and the demands of digital laser color printers, an unusual combination of requirements. Yet other applications for the invention are direct mail advertising, magazine covers, and the like. Direct mail advertising is a printed advertisement, usually for a specific product, targeted to a specific individual, company or address. The use of color is particularly effective for such advertising. As with the business reply cards, personalization of direct mail advertising to increase the effectiveness, and digital laser printing enables both color and personalization. The invention is designed for use on these printers, both because it enhances the quality of color printing and because it has the requisite stiffness at lower cost.

Magazine covers, which have traditionally been printed on coated paper, can be printed on paper according to the invention, yielding high quality results comparable to coated papers, on lower cost uncoated paper.

The above applications are examples and are not intended to limit the invention, which can be more generally said to be directed to printing paper (usually bleached) comprising sodium silicate applied at least at or near one paper surface or applied throughout the paper. Another benefit of the invention important in numerous applications is enhanced stiffness of the paper, enabling its use of lighter weight paper on image-making machinery. Preferably, the paper is characterized by gurly stifmess at least 5% greater than the gurly stiffness of a reference paper that lacks sodium silicate and is otherwise similar in composition and physical properties to the paper containing sodium silicate. (Alternatively, the reference paper should be manufactured similarly to the silicate- containing paper, except that the silicate is left out). Gurly stiffness may be measured according to standard TAPPI specification T-543 bending resistance of paper. The paper may be saturated with the sodium silicate solution. Preferably, however, the sodium silicate is present in a higher concentration at or near both of the paper surfaces, to provide improved stiffness with less weight. When applied to both sides or through the paper, it comprises sodium silicate in an amount > about 0.03 lbs/ 1000 square feet of paper. In another embodiment, the sodium silicate is concentrated only at one paper surface, in which case the loading of sodium silicate will be > about 0.015 lbs/ 1000 square feet. The location(s) within the paper at which the NaSi may be incorporated can be determined by controlling variables such as, for examble, the viscosity of the solution from which the NaSi is applied to the paper and the wettability of the paper to the liquid vehicles, typically water. A film- forming compound may also be present in the paper to assist in concentrating the sodium silicate at or near one or both surfaces of the paper.

Typically the amount of NaSi required to obtain increased paper stiffness in accordance with the invention is greater than the amount typically required to achieve enhanced image quality. Paper of enhanced image quality can be obtained according to the invention by disposing the NaSi at or near one or both surfaces of the paper. Even with paper having a relatively low basis weight (less than 50 pounds (25" x 38"/500 sheets), a substantial stiffness is achieved, e.g., a gurly stiffness of > 80 grams. We do not wish to be bound by any specific theory, but, based on magnification of the paper product, it appears that the sodium silicate is intimately entwined with, and even bound to, cellulose fibers in the paper. Sodium silicate is produced by dissolving silicate in a strong application of sodium resulting in a clear solution. A strong molecular surface charge occurs and remains with the silicate when it is finally applied to the paper in an aqueous solution and dried. While we do not wish to be bound to any specific theory of the invention, it appears that the sodium silicate when added as a solution and then dried forms an intimate association with the cellulose fibers. This material then retains its electrical surface charge and this is highly desirable for the attraction of the ink-toner image forming mechanism of the laser printer. This is distinct from the addition of fillers that do not form such an association. It also appears that concentration of the sodium silicate at one or both paper surfaces proves an "I-beam" effect described below. The above described paper (as individual sheets or as a roll) can be run through an image producing machine to produce an image on the paper by any image producing technique including wet or dry toner laser printing, or any other suitable image producing technique. The paper can be produced by any conventional process that converts fiber slurry into a bleached printing paper. The sodium silicate may be added at any point during the paper-production process or it can be added at any point after the paper production process and prior to final image production on the paper. For example, the sodium silicate may be added by wet end addition, by size press, by spraying or by coating on or off of a papermaking machine.

The solution of sodium silicate is added in an aqueous mixture, for example a mixture with a pH high enough to maintain sodium silicate in solution, and low enough to substantially avoid detrimental yellowing of the bleached paper (e.g., pH >6.0 and < 11.0; more preferably pH 7 - pHl 0). It will be recognized by those skilled in this art that the solution can be adjusted to deliver the NaSi at a pH outside that range. Also preferably, a film-forming substance is included in the aqueous mixture. Generally any number of film forming substances may be used. Those in the art will recognize that there are a large number of such substances available, including polyvinyl alcohol, polyvinyl acetate, hydroxyethyl cellulose, carboxymethyl cellulose, methyl cellulose, sodium alginate are film forming agents. Polyurethanes, styrene maleic anhydrides, styrene acrylic acids and styrene acrylic esters are additional examples of film forming agents. Those in the art will understand that the agents are selected to be compatible with the sodium silicate solution, and not all of the above agents will be compatible with a given system. The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

DETAILED DESCRIPTION In general, any paper-making process is suitable for use with the invention. A standard paper-making slurry containing pulp and paper-making chemicals is contained in a headbox of the paper machine. The slurry is processed into paper. Water is removed from the slurry as it passes over a wire mesh, the paper is then pressed and dried and the resulting sheet is then treated at a size press, redried, calendared and wound into a roll. The roll may be sheeted in a separate operation. According to the invention, sodium silicate may be introduced anywhere during or after the paper manufacture process. By way of example only, the silicate may be introduced at the size press. Preferably, but not necessarily, a film-forming additive is included with the silicate to keep the majority of the silicate at the surface of the paper being formed. By way of example and not limitation, two such film forming substances are hydroxyethlcellulose or carboxymethylcellusose. They are included with the silicate solution being applied. A commercial hydroxyethylcellulose, Natrasol® from Hercules Inc., Wilmington Delaware is a satisfactory film forming additive at a Natrasol®: sodium silicate weight ratio of about 1 to 3. However, when we refer to increased concentration of sodium silicate at or near the paper surfaces, we do not mean to exclude the possibility that some sodium silicate penetrates throughout the paper. It may be desirable to saturate the paper with sodium silicate in some cases.

The silicate used can be any of a number of commercial silicate products, such as sodium silicate 9 or "N" grade sodium silicate.

A. Structural Considerations With Regard to Papers Under 50 Pounds Silicate-bonded paper can be compared to a wide flanged I-beam. The silicate-bonded surfaces are the upper and lower surfaces of the beam. The body of the sheet between the surfaces is comparable to the I-beam. The stiffness of the beam depends on the distance that separates the two plates and the ability to hold them apart without buckling. The top plate is in compression and the bottom plate is in tension when force is applied from the top. In use, force is applied to alternate sides of the paper, so both surfaces must have compression and tension strength. The body of paper between the surfaces should have the strength to keep the plates apart. One way to help enhance this I-beam structure is to use a film-forming agent that keeps more of the silicate near the paper's surfaces, enhancing the plate strength thereby increasing stiffness. By increasing the relative amount of silicate that remains near the paper surface, as opposed to being absorbed by the body of the sheet, the film former increases stiffness.

B. Print Quality The invention improves print quality, including improved fidelity of image, gloss, and contrast. There are many aspects to the improvement, including improved gloss, sharper edges to half-tone dots, fewer missed dots, improved reflectance, etc. These improvements manifest as a perception of so-called photographic quality.

C. Rate of Addition of Silicate

Based on a size-press addition rate such that the resulting amount incorporated in the paper is at or above .09 pounds per MSF of paper, a measurable, statistically significant increase in stiffness is observed consistently. When the rate of addition approaches or exceeds about 0.52 pounds per MSF, a yellowing or browning may be observed as the paper is dried. Thus, the preferred rate of addition is between .09 pounds per MSF and .52 pounds per MSF.

A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. Printing paper comprising sodium silicate applied at or near at least one paper surface, said sodium silicate optionally saturating the paper.

2. The printing paper of claim 1 in which the paper is characterized by a gurly stiffness, TAPPI specification T-543 bending resistance of paper, at least 5% greater than the gurly stiffness of a reference paper that lacks sodium silicate and is otherwise similar in composition and physical properties to the paper containing sodium silicate.

3. The printing paper of claim 1 in which the sodium silicate is present in a higher concentration at or near one of the paper surfaces.

4. The printing paper of claim 1 in which the sodium silicate is present in a higher concentration at or near both surfaces of the paper.

5. The printing paper of claim 1 in which the printing paper comprises a film- forming compound that assists in concentrating the sodium silicate at or near one or both surfaces of the paper.

6. The printing paper of claim 1 and 4 in which the paper comprises sodium silicate in an amount > 0.03 lbs/1000 square feet of paper.

7. The printing paper of claim 3 in which the paper comprises sodium silicate in an amount > 0.015 lbs/1000 square feet of paper.

8. The printing paper of claim 1 in which the paper has a basis weight less than 50 lbs. for 500 25" x 38" sheets.

9. The printing paper of claim 1 in which the paper has a basis weight greater than 50 lbs. for 500 25" x 38" sheets.

10. The printing paper of claim 1 in which the paper has a gurly stiffness of > 80 grams.

11. The paper of claim 1 in which the paper has a grade stiffness less than 80 grams

12. The printing paper of claim 1 in which the sodium silicate is intimately entwined with or bound to cellulose fibers in the paper.

13. The printing paper of claim 1 in which the paper contains cotton fibers.

14. The printing paper of claim 1 in which the paper meets both the U.S. Postal Service requirements for business reply postal cards and the requirements of digital laser color printers.

15. The printed paper of claim 1 in which the paper meets specifications for direct mail advertising or magazine covers.

16. A method of forming an image on the paper comprising, providing a paper according to any of claims 1-15; and producing an image on the paper.

17. The method of claim 16 in which the image is produced by a laser or laser- like printing process.

18. A method of producing a bleached printing paper comprising producing a bleached printing paper in a process that converts a fiber slurry into a bleached printing paper; adding sodium silicate at any point during the paper-production process or at any point after the paper production process and prior to final image production on the paper.

19. The method of claim 18 in which sodium silicate is added in an aqueous mixture.

20. The method of claim 19 in which the aqueous mixture further comprises a film- forming compound.

21. The method of claim 19 or claim 20 in which the aqueous mixture has a pH high enough to maintain sodium silicate in solution, and low enough to substantially avoid detrimental yellowing of the bleached paper.

22. The method of claim 19 and 20 in which the aqueous mixture has a pH > 7.0 and < 11.0.

23. The method of claim 18 in which the sodium silicate is added by wet end addition, by size press, by spraying or by coating on or off of a paper-making machine.

24. Printing paper comprising sodium silicate applied at least at or near one paper surface or saturated with sodium silicate said paper being a bleached printing paper or unbleached sulfite printing paper, with or without groundwood fiber.

25. A method of laser printing on a bleached paper comprising, providing a paper comprising sodium silicate; and using a laser to produce an image on the paper, whereby the image exhibits a superior image quality.

26. A method of laser printing an image according to claim 25, in which the only image on the paper is black.

27. A method of laser printing an image according to claim 25, in which the only image on the paper is single-colored or multicolored.

28. A method of laser printing an image according to claims 25-27 in which solid printed areas exhibit a greater degree of gloss than do identical solid areas on a reference paper which has been printed in the same way, said reference paper lacking sodium silicate and being otherwise similar in composition and physical properties to the paper containing sodium silicate.

29. A method of laser printing an image according to claims 25-27 in which the appearance of gloss in the solid and half-tone areas of the image derives from the ink/toner in combination with the sodium silicate and not from the surface reflective characteristics of the unprinted paper.

30. A method of laser printing an image according to claims 25-27 in which there is greater fidelity of reproduction than does an identical image produced in the same manner of a reference paper lacking sodium silicate and being otherwise similar in composition and physical properties to the paper containing sodium silicate.

31. A method of laser printing an image according to claims 25-27 which more closely approaches photographic quality than does an identical image produced in the same manner of a reference paper lacking sodium silicate and being otherwise identical in composition and physical properties to the paper containing sodium silicate.

2. The invention when produced as described herein has clear application as a coating base for coatings developed for digital laser printers, as the sodium silicate will further enhance the coating's surface characteristics to react with the ink toners thereby enhancing the image quality as well as improving stiffness.