Paper making and fiber liberation – Processes and products – Running or indefinite length work forming and/or treating...
Reexamination Certificate
2001-02-23
2002-11-26
Fortuna, Jose (Department: 1731)
Paper making and fiber liberation
Processes and products
Running or indefinite length work forming and/or treating...
C162S361000, C100S038000, C427S366000, C427S361000
Reexamination Certificate
active
06485611
ABSTRACT:
TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION
The invention relates to a method for smoothening a web in a papermaking operation before it is coated. The steps in the smoothening operation include calendering a moistened web in a heated hard nip. This process improves the receptivity of the web surface to coating applications, and to subsequent printing operations.
BACKGROUND OF INVENTION
It is known that an otherwise untreated web of paper or paperboard rawstock will react adversely to the application of a liquid by becoming rougher, since the surface fibers of the web absorb moisture leading to fiber swelling, breaking of hydrogen bonds and fiber reorientation. If the liquid application is in the form of a coating or the like, the web may actually become smoother because of the presence of smoothing ingredients in the coating, but the improvement in smoothness is not as great as would be expected because the surface of the rawstock beneath the coating becomes roughened in the presence of the liquid phase of the coating. Because of this phenomenon, and because one of the objectives in the production of paper and paperboard is to produce a smooth finish, it is desirable to smoothen the surface of the rawstock web before coating.
One of the known techniques for smoothing the surface of a web of paper or paperboard rawstock before coating is to pre-calender the web. This may be accomplished on a paper machine after the web is dried. In the pre-calendering step, the web is passed through one or more nips between the rolls of a calender, the nips providing a degree of compression to the paper surface to flatten the surface fibers, thereby creating a smoothened surface. The degree of finish obtained by this method depends upon the nip pressure reached in the calender. However, the smoothness obtained by pre-calendering a dried web is short lived when the web is subsequently brought into contact with water or a composition containing water, since dry cellulose fibers will not bond under pressure. On wetting, there will be substantial reorientation of the fibers at the web surface due to fiber swelling and release of stress. An improvement that alleviates this unwanted result is to moisten the dry web before pre-calendering. The compressing of moistened cellulose fibers will consolidate the web without undue stress, and the smoothened surface that is achieved will retain a substantial amount of its integrity for a longer time after wetting than is common for a web that is compressed in an essentially dry state.
Conventional processes for wet calendering employ either a water box on the calender, water sprays, or steam showers. In most cases, the quantity of moisture transferred to the web by these means is uneven, too great or too small, resulting in unsatisfactory results. Nevertheless, recent innovations in the paper industry have provided equipment that is capable of applying controlled amounts of water to a web in a uniform manner, particularly as disclosed in pending U.S. patent application Ser. No. 09/296,394, owned by the present assignee herein. The ability to apply a controlled amount of moisture to the web at the pre-calender yields better results. Nevertheless, there remains a need in the art to further enhance the process, and, accordingly, the present invention is designed to advance the process efficiency and product quality that can be achieved in a pre-calendering operation.
The importance of having a smooth rawstock surface and its effect on the printability of coated paper is well known. Pigmented coatings tend to hide or cover up some of the surface imperfections in a paper rawstock. However, the effectiveness of a coating in smoothing the surface of a paper web depends upon the condition of the surface on which the coating is applied. The effect on paper due to the application of a coating is discussed in the article entitled “Water-Paper Interaction During Paper Coating”, by J. Skowronski and P. Lepoutre, published in the November 1985 issue of TAPPI Journal, at pages 98-102. Meanwhile, the effect of pretreatment or pre-calendering of a paper product before coating is discussed in the articles “Effect of Pretreatment of LWC Basestock on Coated Paper Properties”, by P. Lepoutre, W. Bichard and J. Skowronski, published in the December 1986issue of TAPPI Journal, at pages 66-70, and in the article entitled “Precalendering and its Effect on Paper-Coating Interaction”, by G. Engstrom and J. F. Lafaye, published in the August 1992 issue of TAPPI Journal, at pages 117-122.
Likewise, the effect of applying moisture to a paper web before the web enters a calender nip, and various methods for making water application are also known, and are shown, for example, in U.S. Pat. Nos. 5,378,497; 5,505,820; 5,522,312; and 5,607,553, all owned by the present assignee herein. However, none of these prior art references deal with the concept of smoothening the web while reducing loss of bulk by drying the web to a very low moisture content before the application of moisture, passing it through a hard calender nip, and heating at least one roll of the calender nip as disclosed herein. Nor do the prior art references teach the use of the heated roll to dry the applied moisture as the web exits the calender nip as disclosed herein, especially in connection with a pre-calendering treatment on paper machine.
SUMMARY OF THE INVENTION
When a web of paper rawstock is exposed to water during a coating process, the surface beneath the coating layer becomes roughened as described above. In order to offset this increase in roughness, the present invention is directed to a process whereby the paper rawstock is pre-calendered before coating on the paper machine. More particularly, the present invention is directed to a process whereby a thin film of liquid is uniformly applied to the surface of the rawstock web using an apparatus particularly as disclosed in U.S. patent application Ser. No. 09/296,394, which is herein incorporated by reference. The application of a thin film of liquid to the paper surface allows only the surface fibers of the web to be plasticized, so that, upon pre-calendering in a heated, hard nip formed, for example, by a pair of steel rolls, the surface of the web is fixed. At the same time, loss of bulk of the web is reduced because it does not become wetted throughout with the liquid. According to the present invention, the paper surface may be characterized as being “fixed,” in that it is substantially irreversibly smoothened by practicing the process described herein. Because of the fixed surface, subsequent coating application will have a minimal impact on re-roughening the surface of the web. In the preferred method of the invention, the web enters the calender nip prior to the time that the liquid application has a chance to completely penetrate the full thickness of the web. The liquid used to moisten the paper surface can be plain water, or a solution or dispersion of a binder, thickener, or one or more other functional additives, such as lubricants or surface-active agents, which may be heated or at ambient temperature.
Another benefit of using the device of the present invention is that it eliminates the presence of a standing pond of water or finishing liquid on or adjacent to the heated roll, and, as a result, the inherent cooling effect that would be associated with such an arrangement is eliminated. Because the pond is removed, maintaining a roll temperature above the boiling point of water then becomes practical, and operating temperatures in excess of 212° F. (100° C.) are possible. These higher temperatures in turn are capable of producing even greater smoothening results. In this respect, the temperature of the heated roll can range up to about 400° F. or higher, for example, from about 300° F. to about 500° F., and is limited only by the potential for causing adverse heat effects, such as charring, on the substrate being processed.
The present invention may be practiced on any grade of paper or paperboard that contains moisture sensitive fi
Bowman Donald L.
Fortuna Jose
Schmolz Richard L.
Westvaco Corporation
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