Paper making and fiber liberation – Processes and products – Running or indefinite length work forming and/or treating...
Reexamination Certificate
1997-10-21
2001-08-14
Fortuna, Jose (Department: 1731)
Paper making and fiber liberation
Processes and products
Running or indefinite length work forming and/or treating...
C162S205000, C162S135000, C162S207000, C034S443000, C034S445000, C100S038000, C427S361000, C427S363000, C427S364000
Reexamination Certificate
active
06274001
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to hot hard calendering of paper and paperboard to improve smoothness. In particular, the invention relates to hot hard calendaring of paper/paperboard which has been surface sized with starch.
BACKGROUND OF THE INVENTION
One of the methods of improving the smoothness of paper/paperboard during the calendering operation is to pass the paper/paperboard through one or more heated nips which are at a temperature higher than the temperature of the web. The surface of the paper/paperboard that is to be finished is pressed against the heated roll. The applied heat raises the surface temperature of the paper/paperboard to the glass transition temperature, which causes the fibers to soften and conform to the surface of the roll. Moisture (i.e., water or steam) can also be added before the nip to the surface that is treated to further lower the glass transition temperature. Thus, gradients in the temperature and moisture level tend to lower the glass transition temperature preferentially on the external surfaces of the paper/paperboard and allow the sheet to achieve a desired smoothness without significant reduction in caliper.
Temperature gradient calendering is a known method of enhancing the smoothness of paper/paperboard without sacrificing caliper. However, high temperature alone cannot yield the degree of smoothness that is required for some grades. Moisture is essential for the smoothness development. This can be observed in
FIG. 1
where Hagerty smoothness of bleached board calendered in a steel-steel nip at 350° F. is shown as a function of the apparent density of the final product. It should be noted that within the range of applied niploads, calendering without moisture addition can only result in a minimum smoothness of 180 units while the sheet calendered with addition of moisture to raise the total moisture content by 0.5% can attain a smoothness of 130 Hagerty units.
The prevalent method of moisture addition for most board and bristol grades is by using waterboxes on at least one calender stack. This process usually consists of overdrying the sheet to obtain a flat moisture profile of 1-2% and then adding water using waterboxes. The moisture pickup is typically greater than 10-12% of the conditioned weight of the paper/paperboard and can sometimes be as much as 15-18% of the conditioned weight of the paper/paperboard. One purpose of overdrying is to correct for any moisture nonuniformity. Overdrying the sheet can only be achieved by running the machine slower, thereby reducing production. In addition, waterboxes tend to cause several operational problems, including breaks and difficulty during the threading process. The high level of moisture added with the waterboxes in a multiroll calender stack makes it necessary to employ drying means so that the reel moisture can be maintained at 5-8%. This drying is typically accomplished using intercalender dryers which are usually run without any supporting fabric. Following this drying, the paper sheet is passed through a multiroll stack with several nips to be subjected to increasing pressure, whereby the web develops good smoothness and high density. One of the advantages of waterboxes is that the water applied can incorporate other functional additives, such as dyes, lubricants, binders such as starch, and film formers such as polyvinyl alcohol.
Addition of the moisture in such a way that only the surface is moistened to a lower glass transition temperature can result in a paper/paperboard having high smoothness and low density. An application similar to this has been suggested in U.S. Pat. No. 5,378,497, wherein the moisture is applied before the calendering nip using a metering size press. The '497 patent further discloses that the smoothness developed is irreversible and is useful in operations that involve rewetting the sheet, such as coating/tinting. The metering size press method applies a lower amount of moisture than does the waterbox but cannot correct profile nonuniformities. In addition, installation of a metering size press involves significant cost and machine reconfiguration.
In addition to the foregoing, the temperature gradient calendering of moistened paper/paperboard that is surface sized is impeded by starch and associated fibers picking and sticking on the heated roll. This problem tends to be aggravated at the higher niploads and temperatures required for achieving high smoothness levels. The sticking and fiber pulling are caused by the starch or other binders that are added in the size press and lead to runnability problems.
SUMMARY OF THE INVENTION
The present invention is a method and an apparatus for calendering which improves the smoothness of paper/paperboard without sacrificing caliper or stiffness. The concept in accordance with the invention can be applied for improving the smoothness of file folder, bleached liner, liquid packaging board, rawstock for coated bristols, linerboard and paper that have a high smoothness requirement. All of these grades are surface sized with variations of starch (pearl, preoxidized, hydroxyethyl), typically in a size press.
The calendering method in accordance with the present invention enhances the smoothness of surface sized paper/paperboard by a combination of temperature and moisture gradient calendering processes without the fiber sticking/picking problems that affect runnability. The moisture is applied to one or both sides of the web using one or more banks of independently controllable showers which allow the cross direction moisture profile to be corrected. One or more heated calender rolls form one or more hot nips having a temperature greater than the temperature of the moisturized web. Lubricant is applied to both sides of the web to prevent fiber sticking/picking in the hot pressure nips. The lubricant may be applied by the size press, by the moisturizing showers or by separate lubricant showers. The smoothness developed by moisturizing and hot nip calendering is substantially irreversible. Calendering of the moisturized web in the hot pressure nips produces high smoothness levels.
The present invention is further directed toward the retrofitting of a conventional machine calendering line having waterboxes with one or more banks of moisturizing showers. In this case, the waterboxes are not used to apply moisture to the paperboard, i.e., the waterboxes are either removed or left in place but not activated to apply liquid to the paperboard.
REFERENCES:
patent: 4207143 (1980-06-01), Glomb et al.
patent: 4596633 (1986-06-01), Attwood
patent: 4606264 (1986-08-01), Agronin et al.
patent: 4624744 (1986-11-01), Vreeland
patent: 4823688 (1989-04-01), Wedel et al.
patent: 4945654 (1990-08-01), Mason
patent: 5045342 (1991-09-01), Boissevain et al.
patent: 5065673 (1991-11-01), Taylor et al.
patent: 5106655 (1992-04-01), Boissevain et al.
patent: 5316624 (1994-05-01), Racine
patent: 5378497 (1995-01-01), Johnson et al.
patent: 5505820 (1996-04-01), Donigian et al.
patent: 5522312 (1996-06-01), Johnson
patent: 5632101 (1997-05-01), Oechsle
patent: 5771603 (1998-06-01), Kotitschke et al.
patent: 2097724 (1997-12-01), None
Gratton et al., “Temperature-gradient calendering of foodboard,” Jan., 1988, Tappi Journal, pp 81-86.*
Vreeland et al., “Substrata thermal molding, Part 2: putting theory to pratice,” Dec. 1988, Tappi Journal, pp 201-206.*
Vreeland et al., “Substrata Thermal Molding, Part 1”, Nov. 1989 Tappi Journal, pp. 139-145.
Krenkel, “Glätteerzeugung und Glättemessung”, Wochenblatt für Papierfabrikation, 17 (1976), pp. 621-624.
Fortuna Jose
International Paper Company
Ostrager Chong & Flaherty LLP
LandOfFree
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