Paper web with pre-flocculated filler incorporated therein

Details Paper web with pre-flocculated filler incorporated therein Paper web with pre-flocculated filler incorporated therein

2002-01-04

2004-12-28

Walls, Dionne A. (Department: 1731)

Paper making and fiber liberation

Processes and products

Plural fiber containing

C162S189000, C162S191000, C162S181800

Reexamination Certificate

active

06835282

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention is in the field of paper manufacturing. More specifically, the invention in one embodiment is directed towards a process for preparing a paper web, such as a sheet of newsprint, from low-grade paper furnishes. In another embodiment, the invention is directed toward a process for preparing a paper web from pulp that contains coated broke. The invention further is directed towards a paper web prepared in accordance with the inventive processes.
BACKGROUND OF THE INVENTION
Conventionally, paper is made by extracting a paper web from a slurry of wood pulp. In accordance with conventional papermaking processes, a furnish composed of wood pulp is provided and is introduced into a paper making machine. To the furnish are added various additives, the additives selected to affect the properties of the paper and the paper machine runnability. At the headbox of the paper making machine, pulp slurry from the headbox is deposited on a wire, and water is removed, thus forming a paper web from the slurry. The web is dried, optionally is collected on a reel, and subsequently is cut into sheets or otherwise converted for its intended application.
Generally speaking, paper may be classified into various grades, with higher grades of paper being known as “fine” papers, and with many lower grades also being known. The grade of paper is largely determined by the quality of the pulp in the paper furnish. One form of lower grade paper is newsprint, the paper stock that is used to print newspapers. Enormous quantities of newsprint are consumed daily by newspaper printers. Because of these high volumes and the short life expectancy of printed newspapers, printers and manufacturers of newsprint find it especially important to minimize the costs of newsprint stock, with cost generally being of somewhat more concern than paper quality. For this reason, newsprint furnishes typically contain lower grades of wood pulp than are found in furnishes that are used for the production of fine papers. For example, newsprint stock typically contains large amounts of groundwood stock and/or recycled paper stock. Typically, at least 30% of the pulp found in newsprint furnishes is composed of groundwood stock and/or recycled stock, and in some instances, as much as 100% of the pulp in the newsprint furnish is so composed.
Both groundwood stock and recycled paper stock typically contain large amounts of impurities. Such impurities can adversely affect the quality of paper produced from such furnishes. For example, groundwood stock is prepared via the high-yield mechanical processing of raw wood. Such stock typically contains relatively large amounts of components that are undesirable in the manufacture of paper, including such components as lignin, residue, pitch, resins, carbohydrates, fatty acids, and fiber fines. All of these components are believed to adversely affect various properties of the paper produced from groundwoods, such as strength, brightness, color, opacity, smoothness, and printability.
Retention aids also are used in connection with the manufacture of fine papers. Recycled stock and groundwood stock are sometimes used in the manufacture of fine papers in relatively smaller amounts than in newsprint. In the manufacture of such papers, the prior art teaches that the brightness, opacity, and other properties of the paper sheet may be improved by incorporating a filler into the paper furnish prior to forming a paper web therefrom. In conjunction with the preparation of such fine papers, fillers are believed to enhance opacity, brightness, color, smoothness, and printability properties. In addition, fillers may be used to provide improved machine runnability and a reduction in sheet cost. Conventional fillers are composed of fine particles, such as clays, which are not easily retained as the paper web is being formed from the pulp slurry. Accordingly, the prior art teaches to add a retention aid to the slurry to assist in retaining the filler material within the paper web. The use of such fillers in connection with retention aids has been satisfactory in connection with the manufacture of fine papers.
One source of recycled paper stock is “broke” stock, which typically is stock that failed to meet the particular specification for a given application and which therefore cannot be sold. Often, broke paper is coated with a variety of coatings, in which case it is termed “coated broke.” Coatings typical of coated broke include pigments and adhesives which are intended to improve the properties of the product. Pigments, which are used in coating color formulations, usually are the main coatings, and generally provide between 80 to 95 percent of the total coating mass. Pigments include various forms of clay, calcium carbonate—both precipitated and ground, and titanium dioxide, among others. Adhesive materials generally are found in lower quantities, typically, between 8 to 20 percent of the total coating mass, and can include various modifications of starch and latex polymers such as styrene-butadiene, polyacrylics and polyacrylates. These components can adversely affect the properties of paper produced from coated broke. Recycled paper stock can contain other undesired components, including fine fibers, other chemical residues from prior paper processing, and possibly other undesired components. Moreover, because the quality and composition of recycled paper stock may vary from one source to another, the composition of recycled paper stock may be largely unknown. Accordingly, when recycled stock is provided in the furnish, a number of difficulties in maintaining the quality of paper prepared from such furnishes are encountered.
One of the problems associated with the production of paper from recycled paper stock, especially stock containing coated broke, is the accumulation of adhesive material in the wet end of a papermaking system. The accumulations known as “white pitch” form blemishes in the finished sheet, reducing its quality and often causing it to fail to meet desired specifications. White pitch also causes frequent web breaks in production runs, causes felt and wire clogging, and limits water drainage, all of which may result in decreased machine speed and increased machine downtime.
The prior art teaches that these difficulties with impurities can be addressed by adding retention aids, such as polyDADMAC, acrylamides, and epichlorohydrin/dimethylamine, to the pulp material. Retention aids cause the problematic substances to bind to the pulp fiber and be removed from the system as the fibers are retained in the newly formed paper web. However, the usefulness of this approach can be limited by the tendency of the treated pulp material to over-flocculate. Over-flocculation reduces machine performance by reducing the efficiency of sheet dewatering, with the result that physical sheet properties including formation, strength, and optical properties decrease. Use of retention aids also can thus be limited in maximizing the retention of broke material, thereby limiting the usefulness of the retention aid in mitigating against white pitch formation.
In recognition of these problems, the prior art has presented a number of proposed solutions. For example, U.S. Pat. No. 4,997,523 (Pease et al.) purports to disclose a method for treating coated broke using cationic and anionic coagulants. Another patent, U.S. Pat. No. 5,131,982 (St. John) purports to disclose treatment of coated broke using polyDADMAC. A third patent, U.S. Pat. No. 5,466,338, purports to disclose the use of a “water-soluble dispersive polymer” to treat repulped broke. These proposed solutions are not always satisfactory. These polymers can have a tendency to cause overflocculation of the sheet as it is being formed, leading to reduced properties including formation, strength, and optical properties.
Another limitation with the use of retention aids arises in the manufacture of newsprint and similar paper webs from lower-grade furnishes. In the manufacture of such webs, it is more difficult,

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