Paper making and fiber liberation – Processes and products – Non-uniform – irregular or configured web or sheet
Reexamination Certificate
1996-10-11
2002-07-16
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-uniform, irregular or configured web or sheet
C162S111000, C162S113000, C162S164100, C162S164300, C162S164600, C162S166000, C162S168300, C162S175000, C162S177000, C162S178000, C162S183000, C162S198000, C162SDIG004
Reexamination Certificate
active
06419789
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of making a paper web having superior strength, absorbency and softness. The invention further relates to a non-compacted paper web produced with a headbox furnish composition maintained at an average anionic charge level in a specific range. More particularly, the invention relates to a non-compacted paper web made from a refined long fiber furnish containing high levels of wet strength additives at an average anionic charge level in the headbox in a specific range. Still more particularly, the present invention relates to a single ply towel product having improved strength, softness and absorbency.
BACKGROUND OF THE INVENTION
Folded and roll paper toweling, such as that used in commercial, “away-from-home” dispensers, is a relatively modest product normally sold almost exclusively on the basis of cost since the purchaser is rarely the user. Because improved performance rarely justifies even a minimal increase in cost, techniques for improving the quality of this product have previously centered around those satisfying the most stringent of economic criteria. Recent market trends have seen a shift toward improved product characteristics; however, economics are still closely monitored.
Traditionally, the production of away-from-home toweling occurs by one of three basic technologies: (i) conventional wet press technology with wet creping and embossing; (ii) conventional wet press technology with dry creping and embossing; and most recently (iii) through-air-drying without creping. Each of these technologies has its own advantages and disadvantages.
Conventional wet press technology with wet creping and embossing results in a product having good strength when saturated with aqueous liquids. This technology suffers from the disadvantage that the product lacks sufficient absorbent capacity and softness. As described in U.S. Pat. No. 5,048,589 to Cook et al., herein incorporated by reference in its entirety, towels made from a conventionally wet pressed, wet crepe process “are normally strong even when saturated with liquid, but often lack desirable levels of absorbent capacity, absorbent rate, and softness.”
Conventional wet press technology with dry creping and embossing results in a product having good absorbent capacity and softness; but the product lacks strength when saturated with aqueous liquids. U.S. Pat. No. 5,048,589 to Cook et al. describes products made by this method as “. . . soft towels [that] possess high levels of absorbent capacity and absorbent rate, however, these soft towels are also very weak and tend to break apart when saturated with liquid.”
Through-air-drying without creping is also disclosed, for example, in U.S. Pat. No. 5,048,589. The '589 patent discloses towels with good absorbent capacity and strenght when saturated with an aqueous liquid. Uncrepe technology as described in the '589 patent was developed to overcome some of the difficulties in making soft, strong, and absorbent wiper towels.
Although through-air-drying with both creping and embossing can result in a product that is relatively soft and absorbent, this product is generally regarded as a retail in-home towel because of its marginal strength. For example, a particularly successful through air dried towel marketed as a retail in-home product is two-ply Bounty®. Two successful high quality away-from-home folded towels are single-ply KC Surpass® 50000 and Scott Select® 189. The geometric mean wet tensile strength of Bounty® is approximately 895 g/3″, while the geometric mean wet tensile strengths of KC Surpass® 50000 and Scott Select® 189 are generally 1297 g/3″ and 970 g/3″, respectively. Clearly, conventional retail in-home through-air dried towel products are lower in strength. So, for applications where strength is an important consideration, e.g., in the area of away-from-home toweling, traditionally through-air-drying is not coupled with operations that lead to a decrease in strength, for example, dry creping or embossing.
The present invention provides a method of overcoming the disadvantages associated with each of the prior art technologies. The method according to the present invention produces a single-ply towel using through-air-drying, creping, and embossing that does not suffer from the marginal strength of prior art towel products while maintaining both high softness and good absorbency. This is accomplished through the use of an anionic/cationic thermally cross-linking strength additive system at a headbox charge controlled to a specific anionic range; preferably in conjunction with a furnish having as its major component, refined long fibers; and high levels of wet strength/dry strength resins.
Prior art through-air-drying processes do not provide a method for making a strong, soft, and absorbent away-from-home hand drying towel using high levels of refined softwood, adding high levels of wet strength resin, and adding wet/dry strength resins to appropriately control headbox charge to a specified anionic range.
U.S. Pat. No. 3,998,690 to Lyness et al., incorporated herein by reference in its entirety, discloses a chemical flocculation technique for using short fiber to make bulky webs. Flocculation of the furnish tends to produce aggregates that apparently cause a short fiber furnish to act like a long fiber furnish. Lyness et al. discloses the use of wet strength resins or other cationic agents and anionic agents for inclusion in a bifurcated furnish which requires the use of a complex stock system. Although Lyness et al. discloses that a stoichiometric charge density balance of the anionic/cationic pairs can be used, they do not include the furnish as part of the charge balance. Furthermore, measuring and controlling headbox charge to a specific anionic range for improved wet strength is not considered by Lyness et al.
There are numerous schemes for measuring the charge state of a wet end system. Two of the most common methods are described below: zeta potential via micro-electrophoresis and titratable charge.
When a negatively charged particle, such as a wood pulp fiber, is suspended in an aqueous solution, the negative surface attracts a considerable number of positive counterions next to the electrified interface. The counterions next to the electrified interface are strongly attracted into a thin layer referred to in the literature as the Stem layer. When a particle moves in solution, liquid immediately adjacent to the particle surface moves with the same velocity. This unknown boundary layer is referred to as the shear surface and contains the Stem layer. Therefore, in a fiber furnish, solution and counterions are bound to the moving electrified fiber particle in the shear/Stem layer.
Counterions tend to diffuse away from an electrified surface because of thermal motion, but they are also attracted by coulombic forces. These opposing effects cause charge concentration variations which effect the double layer potential in solution. Zeta potential is the double layer electrical potential at the shear surface. Salts added to a solution suppress the electrical potential or double layer potential in solution, and thus, reduce the zeta potential without changing the charge on the particle.
The most common technique for measuring zeta potential is by microelectrophoresis. Microelectrophoresis techniques require a particle dispersion to be placed in a cell and an electric field applied. The velocity of the particles is determined, e.g., microscopically. The mobility is calculated as the particle velocity per unit electric field. The zeta potential is then calculated from the Helmholtz-Smoluchowski equation as the mobility times the viscosity of medium divided by the dielectric constant of medium.
The electrostatic charge associated with papermaking particles and polyelectrolyte additives defines the cationic or anionic demand of a papermaking system. The most popular technique for measuring the state of charge of a wet end system is to titrate a papermaking sample, like a
Kershaw Thomas N.
Ostrowski Henry S.
Worry Gary L.
Yeh Kang Chang
Chin Peter
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Fort James Corporation
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