Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2001-09-20
2003-04-22
Fortuna, Jose A. (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S168300, C162S164100, C162S175000, C162S178000, C162S181200, C162S181400, C162S181800, C162S206000
Reexamination Certificate
active
06551457
ABSTRACT:
The present invention relates to paper making and more specifically to a process for the production of paper wherein a web of paper is formed, dewatered and then dried by means of impulse pressing (drying) in the press section at temperatures above the boiling point of water. In the process, a chemical system comprising at least one polymer component in combination with micro- or nanoparticles are added to the furnish or paper web before passing an impulse unit. By the use of the process according to the invention delamination of the paper web can be avoided and the tendency of adhesion to the press roll and formation of deposits on the roll is removed or decreased. By means of the process according to this invention paper with improved physical properties, such as densification of the outer layer, high smoothness and increased strength can be produced.
BACKGROUND
In the paper making art, an aqueous suspension containing cellulosic fibers, fillers and additives, referred to as the stock, is fed into a headbox which ejects the stock onto a forming wire. Water is drained from the stock through the forming wire so that a wet web of paper is formed on the wire and the web is further dewatered in the press section and dried in the drying section of the paper machine. Water obtained by dewatering the stock, referred to as the white water, which usually contains fine particles, i.e. fine fibers, fillers and additives, is usually recirculated in the paper making process. Drainage and retention aids are conventionally introduced into the stock in order to facilitate drainage and increase adsorption of fine particles onto the cellulosic fibers so that they are retained with the fibers on the wire.
In order to increase the productivity in paper production a proposed solution has been to increase the speed of the web through the paper machine. However, increasing the speed at which the paper web is produced creates problems in the dry section of the paper making process. Thus, as the web speed increases, heat transfer to the dry paper web from each drying cylinder decreases. To solve the heat transfer problem the dry section of paper making machines must be made longer. Another solution of said problem is to use an impulse press. An impulse press employs a high temperature roll which is heated above 100° C. In impulse pressing, or impulse drying, the paper web after being formed is passed through a number of roll pairs, the rolls usually unheated, to remove water by mechanical pressing and is then contacted by the heated roll to remove water by evaporation in the heated press nip. The heated roll can be of a temperature of, for example, from 100 to 400° C. An endless porous felt is usually located in the nip and passes around the unheated roll. The combination of heat and pressure exerted on the web by the nips of the rolls substantially increases the dry solids contents. However, it has been noted that impulse pressing usually has the undesirable effect of delaminating the web.
The potential of the impulse pressing technology has been very limited owing to this delamination problem and this has reduced or prevented the industrial use of this technology.
Different solutions have been proposed in order to solve the problem with web delamination after the web leaves the nip. Several solutions deal with the design and construction of the pair of rolls used in impulse drying. Thus, European Patent Application No. 0 723 612 relates to an impulse dryer roll with a shell of high thermal diffusivity in order to improve the heat transfer to the paper web being dried. The U.S. Pat. No. 5,404,654 relates to a paper web impulse drying apparatus wherein web delamination is prevented by both (a) a steam chamber on the exit side of the nip through which the web passes, and (b) heating the web prior to its entrance into the nip. European Patent Application No. 0 742 312 relates to a method and apparatus for drying a wet fiber web by impulse drying and then introducing the web into a gas pressurized zone followed by reducing the pressure in the zone wherein the reduction preferably is effected with cooling of the fiber web.
International Patent Application Publication No. WO 99/36620 relates to an impulse dried paper having a three-dimensional pattern of alternating raised and recessed portions which is conveyed to the paper in connection with impulse drying. The object of the invention described in said publication is to provide a method of producing an impulse dried paper having a three-dimensional pattern where the paper has a high bulk and a high absorption capacity and where the three-dimensional structure should be maintained in dry as well as in wet condition. Said object is stated to be achieved by the fact that the paper contains at least 0.05% by weight, based on the dry fiber weight, preferably at least 0.25% by weight, of one or more additives which in connection with impulse drying undergoes a chemical reaction, so that they contribute in stabilizing the pattern structure that has been conveyed to the paper at the impulse drying. The additives proposed are reactive polymers, such as wet strength agents, fixing agents, polysaccharides, polyvinyl alcohol or a polyacid such as polyacrylic acid and copolymers thereof. This publication does not at all deal with or even mention the delamination problem in connection with impulse drying.
In addition to delamination of the paper web, other undesirable effects observed in impulse drying include adhesion of the sheet to the press roll and occurrance of deposits on the roll.
THE INVENTION
According to the present invention it has unexpectedly been found that the problems with delamination of the paper web and the tendency of adhesion to the press roll and forming of deposits on the roll can be removed or substantially decreased by addition of a chemical additive system containing micro- or nanoparticles. More specifically, the present invention relates to a process for the production of paper from an aqueous suspension containing cellulosic fibres, and optional fillers, which comprises draining the suspension to obtain a paper web and subjecting the paper web to impulse pressing, or impulse drying, by passage through at least one press nip having at least one heated roll which is in contact with the web and heated to a temperature above 100° C., wherein a polymer component and micro- or nanoparticles are added to the suspension or the paper web before the paper web passes the press nip of the impulse unit. The polymer component and micro- or nanoparticles are also referred to herein as chemical system, or micro- or nanoparticle system. The invention thus relates to a process as further defined in the appended claims.
The micro- or nanoparticle system according to the present invention can be used alone or in combination with wet strength agents as well as sizing agents. The chemicals are added to the suspension, furnish or paper web before the web passes the impulse unit. The chemicals can be added at any position in the wet end before draining the suspension, such as, for example, the pulp chest, machine chest, constant level box, fan pumps, screen, etc., and the chemicals can be added before or after these steps as well as during them. They can also be added to the dilution flow of a dilution headbox or in one or several layers of a multilayering headbox. It is also possible to apply them wet-in-wet within a headbox by using a method and a device similar to that described in the European Patent Application No. EP 0 824 157. These differentiated additions in the headbox can be used for z-layered additions.
A micro- or nanoparticle system refers to a chemical system comprising a polymer component and micro- or nanoparticles, preferably an anionic microparticulate material. The polymer component can be selected from anionic, amphoteric, non-ionic and cationic organic polymers and mixtures thereof. The use of such polymers is known in the art. The polymers can be derived from natural or synthetic sources, and they can be linear, branched or cross-linked.
Laine Janne
Lindstrom Tom
Norell Maria
Westman Caroline
Akzo Nobel N.V.
Fortuna Jos'e A.
Parker Lainie E.
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