Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
1999-04-30
2002-07-23
Silverman, Stanley S. (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S009000, C162S158000, C162S162000, C162S181200, C162S189000, C162S190000, C162S181100, C162S123000, C162S126000, C162S127000, C162S132000, C162S134000
Reexamination Certificate
active
06423183
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to paper products. More particularly, the invention concerns methods for applying chemical additives to cellulosic fibers and the paper products that can be obtained by the methods.
In the manufacture of paper products, it is often desirable to enhance physical and/or optical properties by the addition of chemical additives. Examples of properties that are developed or enhanced through the addition of chemical additives include but are not limited to dry strength, wet strength, softness, absorbency, opacity, brightness and color. During the papermaking process, chemical additives are commonly added to fiber slurries in the wet end, before the fibers are formed into a web, dewatered and dried. Traditionally, wet end additives are added to a fiber slurry that is between 0.5 and 5 percent consistency. The slurry may then be further diluted in the papermaking process before a final dilution at the fan pump to the ultimate forming consistency.
Wet end chemical addition has several advantages over topical spray, printing or size press chemical addition methods. For instance, wet end chemical addition provides a uniform distribution of chemical additives on the fiber surfaces. Additionally, wet end chemical addition allows a selected fiber fraction to be treated with a specific chemical additive in order to enhance the performance of the paper and/or the effectiveness of the chemical additive. Further, wet end chemical addition enables multiple chemistries to be added to a fiber slurry, either simultaneously or sequentially, prior to formation of the paper web.
One difficulty associated with wet end chemical addition is that the water soluble or water dispersible chemical additives are suspended in water and are not completely adsorbed onto the cellulosic fibers. To improve adsorption of wet end additives, chemical additives are often modified with functional groups to impart an electrical charge when in water. The electrokinetic attraction between charged additives and the anionically charged fiber surfaces aids in the deposition and retention of chemical additives onto the fibers. Nevertheless, the amount of chemical additive that can be retained in the wet end generally follows an adsorption curve exhibiting diminishing effectiveness, similar to that described by Langmuir. As a result, the adsorption of water soluble or water dispersible chemical additives may be significantly less than 100 percent, particularly when trying to achieve high chemical additive loading levels.
Consequently, at any chemical addition level, and particularly at high addition levels, only a fraction of the chemical additive is retained on the fiber surface. The remaining fraction of the chemical additive remains dissolved or dispersed in the suspending water phase. These unadsorbed chemical additives can cause a number of problems in the papermaking process. The exact nature of the chemical additive will determine the specific problems that may arise, but a partial list of problems that may result from unadsorbed chemical additives includes: foam, deposits, contamination of other fiber streams, poor fiber retention on the machine, compromised chemical layer purity in multilayer products, dissolved solids build-up in the water system, interactions with other process chemicals, felt or fabric plugging, excessive adhesion or release on dryer surfaces, physical property variability in the finished product, and the like.
Therefore, what is lacking and needed in the art is a method for applying adsorbable chemical additives, particularly a dye, onto cellulosic fiber surfaces in the wet end of the papermaking process such that the amount of unadsorbed chemical additives in the process water is reduced or eliminated. The method minimizes the associated manufacturing and finished product quality problems that would otherwise occur.
SUMMARY OF THE INVENTION
It has now been discovered that chemical additives can be adsorbed onto cellulosic papermaking fibers at high levels with a minimal amount of unadsorbed chemical additives present in the papermaking process water. This is accomplished by treating a fiber slurry with an excess of the chemical additive, allowing sufficient residence time for adsorption to occur, filtering the slurry to remove unadsorbed chemical additives, and redispersing the filtered pulp with fresh water. Because the filtrate from the thickening process contains unadsorbed chemical additive, it is not sent forward in the process with the chemically treated fibers. Rather, the filtrate may be sent to the sewer or reused in a processing step prior to the filtration step.
Hence in one aspect, the invention resides in a method for applying chemical additives to cellulosic fibers. The method comprises the steps of: creating a fiber slurry comprising water, cellulosic fibers, and an adsorbable chemical additive; dewatering the fiber slurry to remove unadsorbed chemical additive; and redispersing the fibers with fresh water. This method for processing cellulosic papermaking fibers enables chemical additives to be adsorbed by fibers while at the same time maintaining significantly lower levels of unadsorbed chemical additive in the water phase compared to traditional wet end chemical addition. Thus, higher concentrations of the chemical additive on the fiber relative to the process water can be achieved as compared to what has been possible with prior methods.
For purposes of the present invention, the term “cellulosic” refers to papermaking fibers comprising an amorphous carbohydrate polymer, in contrast to synthetic fibers. The term “adsorbable” is used herein to refer to a chemical additive that can be assimilated by the surface of a cellulosic fiber, in the absence of any chemical reaction involving the chemical additive and the cellulosic fiber. The term “unadsorbed” refers to any portion of the chemical additive that is not adsorbed by the fiber and thus remains suspended in the process water. The term “fresh water” is used herein to refer to water that is substantially free of the unadsorbed chemical additive. Most desirably, the fresh water is completely free of the chemical additive.
The fiber slurry is desirably dewatered to increase the consistency of the fiber slurry to about 20 percent or greater, and particularly to about 30 percent or greater, in order to remove the majority of the water containing the unadsorbed chemical additive. The fibers are thereafter redispersed, desirably to decrease the consistency of the fiber slurry to a level suitable for papermaking, to about 20 percent or less, and more particularly to about 5 percent or less, such as about 3 to about 5 percent.
The present method allows for the production of fiber furnishes that are useful for making paper products, and particularly layered paper products. Thus, another aspect of the invention resides in a fiber furnish that has a higher chemical additive loading than could otherwise be achieved in combination with the relatively low level of unadsorbed chemical additive in the water. This is because chemical additive loading via traditional wet end addition is often limited by the level of unadsorbed chemical and its associated processing difficulties such as foam, deposits, chemical interactions, felt plugging, excessive dryer adhesion or release or a variety of paper physical property control issues caused by the presence of unadsorbed chemical in the water.
In one embodiment, a fiber furnish of the present invention comprises water, cellulosic fibers, and an adsorbable chemical additive. The amount of chemical additive adsorbed onto the fibers is about 2 kilograms per metric ton or greater, and the amount of unadsorbed chemical additive in the water is between 0 and about 20 percent of the amount of chemical additive adsorbed onto the fibers. In particularly desirable embodiments, the amount of adsorbed chemical additive is about 3 kg/metric ton or greater, particularly about 4 kg/metric ton or greater, and more particularly about 5 kg/metric ton or g
Gentile Victor Michael
Georger Jill A.
Goulet Mike Thomas
Polderman Denise Alice
Wyatt Maurice Alan
Charlier Patricia A.
Connelly Thomas J.
Kimberly--Clark Worldwide, Inc.
Silverman Stanley S.
Walls Dionne A.
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