Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2000-12-20
2002-09-03
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S164100, C162S164500, C162S164600, C162S168100, C162S168200, C162S168300, C162S181600, C162S181800, C162S183000
Reexamination Certificate
active
06444091
ABSTRACT:
TECHNICAL FIELD
This invention is directed to a method for increasing retention and drainage in a papermaking furnish using structurally rigid nonionic and anionic polymers. The structurally rigid polymer may be used alone or in combination with one or more conventional coagulants, flocculants and/or microparticles.
BACKGROUND OF THE INVENTION
In the manufacture of paper, a papermaking furnish is formed into a paper sheet. The papermaking furnish is an aqueous slurry of cellulosic fiber having a fiber content of about 4 weight percent (percent dry weight of solids in the furnish) or less, and generally around 1.5% or less, and often below 1.0% ahead of the paper machine, while the finished sheet typically has less than 6 weight percent water. Hence the dewatering and retention aspects of papermaking are extremely important to the efficiency and cost of the manufacture.
Gravity dewatering is the preferred method of drainage because of its relatively low cost. After gravity drainage more expensive methods are used for dewatering, for instance vacuum, pressing, felt blanket blotting and pressing, evaporation and the like. In actual practice a combination of such methods is employed to dewater, or dry, the sheet to the desired water content. Since gravity drainage is both the first dewatering method employed and the least expensive, an improvement in the efficiency of this drainage process will decrease the amount of water required to be removed by other methods and hence improve the overall efficiency of dewatering and reduce the cost thereof.
Another aspect of papermaking that is extremely important to the efficiency and cost is retention of furnish components on and within the fiber mat. The papermaking furnish represents a system containing significant amounts of small particles stabilized by colloidal forces. The papermaking furnish generally contains, in addition to cellulosic fibers, particles ranging in size from about 5 to about 1000 nm consisting of, for example, cellulosic fines, mineral fillers (employed to increase opacity, brightness and other paper characteristics) and other small particles that generally, without the inclusion of one or more retention aids, would in significant portion pass through the spaces (pores) between the mat formed by the cellulosic fibers on the papermachine.
Greater retention of fines, fillers, and other components of the furnish permits, for a given grade of paper, a reduction in the cellulosic fiber content of such paper. As pulps of lower quality are employed to reduce papermaking costs, the retention aspect of papermaking becomes more important because the fines content of such lower quality pulps is generally greater. Greater retention also decreases the amount of such substances lost to the whitewater and hence reduces the amount of material costs, the cost of waste disposal and the adverse environmental effects therefrom. It is generally desirable to reduce the amount of material employed in a papermaking process for a given purpose, without diminishing the result sought. Such add-on reductions may realize both a material cost savings and handling and processing benefits.
Another important characteristic of a given papermaking process is the formation of the paper sheet produced. Formation may be determined by the variance in light transmission within a paper sheet, and a high variance is indicative of poor formation. As retention increases to a high level, for instance a retention level of 80 or 90%, the formation parameter generally declines.
Various chemical additives have been utilized in an attempt to increase the rate at which water drains from the formed sheet, and to increase the amount of fines and filler retained in the sheet. The use of high molecular weight water-soluble polymers is a significant improvement in the manufacture of paper. These high molecular weight polymers act as flocculants, forming large flocs which deposit on the sheet. They also aid in the dewatering of the sheet.
The high molecular weight water-soluble polymer is typically added after a high shear point in the stock flow system leading up to the headbox of the paper machine. This is necessary since flocs are formed primarily by a bridging mechanism and their breakdown is a largely irreversible process. For this reason, most of the retention and drainage performance of a flocculant is lost by feeding it before a high shear point. To their detriment, feeding high molecular weight polymers after the high shear point often leads to formation problems. The feed requirements of the high molecular weight polymers and copolymers which provide improved retention often lead to a compromise between retention and formation.
While successful, high molecular weight flocculent programs may be improved by the addition of so called inorganic “microparticles” such as copolymers of acrylic acid and acrylamide; bentonite and other clays; dispersed silica based materials; colloidal borosilicate; and naphthalene sulfonate/formaldehyde condensate polymers. The microparticle may be used along with a flocculant as part of a single polymer/microparticle retention and drainage program or along with a coagulant and a flocculant as part of a dual polymer/microparticle retention and drainage program.
In a single polymer/microparticle retention and drainage aid program, a flocculant, typically a cationic polymer, is the only polymer material added along with the microparticle. In such a program, a high molecular weight linear cationic polymer is added to the aqueous cellulosic papermaking suspension before shear is applied to the suspension, followed by the addition of a microparticle such as copolymers of acrylic acid and acrylamide; bentonite and other clays; dispersed silica based materials; colloidal silica; or naphthalene sulfonate/formaldehyde condensate polymers after the shear application. Shearing is generally provided by one or more of the cleaning, mixing and pumping stages of the papermaking process, and the shear breaks down the large flocs formed by the high molecular weight polymer into microflocs. Further agglomeration then ensues with the addition of the microparticle.
Although, as described above, the microparticle is typically added to the furnish after the flocculant and after at least one shear zone, the microparticle effect can also be observed if the microparticle is added before the flocculant and the shear zone.
Another method of improving the flocculation of cellulosic fines, mineral fillers and other furnish components on the fiber mat using a microparticle is in combination with a dual polymer program which uses, in addition to the microparticle, a coagulant and flocculant system. In such a dual polymer/microparticle system one or more coagulants are first added, for instance a low molecular weight synthetic cationic polymer and/or cationic starch. The coagulant may also be an inorganic coagulant such as alum or polyaluminum chlorides. This addition can take place at one or several points within the furnish make up system, including but not limited to the thick stock, white water system, or thin stock of a machine. This coagulant generally reduces the negative surface charges present on the particles in the furnish, such as cellulosic fines and mineral fillers, and thereby promotes a degree of agglomeration of such particles. However, in the presence of other detrimental anionic species, the coagulant serves to neutralize the interfering species enabling aggregation with the subsequent addition of a flocculent. Such a flocculant generally is a high molecular weight synthetic polymer which bridges the particles and/or agglomerates, from one surface to another, binding the particles into larger agglomerates. The presence of such large agglomerates in the furnish, as the fiber mat of the paper sheet is being formed, increases retention. The agglomerates are filtered out of the water onto the fiber web, whereas unagglomerated particles would, to a great extent, pass through such a paper web. In such a program the order of addition of the microparticle an
Carter Phillip W.
Dunham Andrew J.
Ward William J.
Zelenev Andrei S.
Breininger Thomas M.
Chin Peter
Martin Michael B.
Nalco Chemical Company
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