Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2001-05-23
2002-10-08
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S109000, C162S164300, C162S164600, C162S175000, C162S177000
Reexamination Certificate
active
06461476
ABSTRACT:
BACKGROUND OF THE INVENTION
For tissue products such as facial and bath tissue and paper towels, strength and softness are important properties to many consumers. The strength properties of a product can be expressed in terms of wet strength and dry strength. The dry strength is important from the standpoint of manufacturing, since the product must have sufficient strength to pass through various stages in the manufacturing where the sheet is unsupported and under tension. In the case of paper towels, for example, the dry strength must also be sufficient to enable a towel sheet to be detached from a roll of perforated sheets without tearing and to perform tasks in the dry state without shredding. The wet strength is particularly important because towels are routinely used to wipe up spills. As such, it is necessary that the towel hold up in use after it has been wetted. The amount of wet tensile strength developed using conventional alkaline curing wet strength resins, such as polyamide-epichlorohydrin (PAE) resins (i.e. Kymene® resins from Hercules, Inc.) has been found in practice to be a function of the dry tensile strength of the sheet. Depending upon the furnish, the resin addition level and the water chemistry conditions, the wet tensile strength is generally limited to about 30-40 percent of the dry tensile strength of the sheet. Thus, in order to make tissue or paper products with a high level of wet tensile strength, one has to also develop a high level of dry tensile strength.
Unfortunately, tissues and towels with high dry tensile strengths also exhibit high stiffness and therefore poor hand feel properties since the properties of softness (as characterized by low stiffness) and strength are inversely related. As strength is increased (both wet and dry strength), softness is decreased. Conversely, as softness is increased, the strength is decreased. A high wet/dry strength ratio is desired to provide superior durability when wet, while at the same time exhibiting low stiffness and desirable handfeel properties when dry.
Hence there is a need for a means to increase the wet strength/dry strength ratio while maintaining or decreasing the stiffness of the sheet.
SUMMARY OF THE INVENTION
It has now been discovered that the ratio of the wet tensile strength to the dry tensile strength of a paper sheet, such as an uncreped tissue or towel sheet, can be substantially increased by properly treating the furnish, including adding appropriate amounts of a debonder, a wet strength agent and a dry strength agent. This discovery provides the flexibility to produce a tissue or towel product with increased wet strength while maintaining the current level of stiffness or, alternatively, maintaining the current level of wet strength while reducing the stiffness.
Hence, in one aspect, the invention resides in a method of treating a papermaking pulp useful for making a paper sheet, the method comprising: (a) adding a quaternary ammonium debonder to the pulp in an amount sufficient to significantly reduce the dry cross-machine direction (CD) tensile strength of the sheet; (b) thereafter adding a wet strength agent to the pulp in an amount sufficient to provide the sheet with a ratio of the wet CD tensile strength to the dry CD tensile strength (hereinafter the “Wet/Dry Ratio”) of 0.50 or greater; and (c) thereafter adding a dry strength agent to the pulp in an amount sufficient to increase the dry CD tensile strength of the sheet.
In another aspect, the invention resides in a method of treating an aqueous dispersion of papermaking pulp useful for producing an uncreped throughdried paper sheet comprising: (a) adding to the aqueous dispersion of papermaking pulp from about 5 to about 30 pounds of a quaternary debonder per metric ton of dry fiber; (b) thereafter adding to the pulp from about 5 to about 30 pounds of a wet strength agent per metric ton of dry fiber; and (c) thereafter adding to the pulp from about 5 to about 20 pounds of a dry strength agent per metric ton.
In another aspect, the invention resides in an uncreped paper sheet, such as a tissue or towel sheet, comprising from about 5 to about 30 pounds of a quaternary amine debonder per metric ton of dry fiber, from about 5 to about 30 pounds of a polyamide-epichlorohydrin wet strength resin per metric ton of dry fiber and from about 5 to about 30 pounds of a dry strength agent per metric ton of dry fiber, said paper sheet having Wet/Dry Ratio of 0.50 or greater and a machine direction stiffness of about 30 kilograms or less per 3 inches of width.
In another aspect, the invention resides in an uncreped paper sheet, such as a tissue or towel sheet, comprising from about 5 to about 30 pounds of a quaternary ammonium debonder per metric ton of dry fiber, from about 5 to about 30 pounds of a polyamide-epichlorohydrin wet strength resin per metric ton of dry fiber and from about 5 to about 30 pounds of a dry strength agent per metric ton of dry fiber, wherein the ratio of the Wet/Dry Ratio to the machine direction stiffness is about 1.5 or greater.
The amount of the quaternary ammonium debonder can be about 5 pounds or greater per metric ton of dry fiber, more specifically from about 5 to about 30 pounds per metric ton of dry fiber, still more specifically from about 10 to about 25 pounds per metric ton of dry fiber. Suitable quaternary ammonium debonders include those chemistries containing one or more aliphatic hydrocarbon groups designed to disrupt hydrogen bonding in a paper, tissue or towel product made from wood fibers. Particularly suitable quaternary ammonium debonders include imidazoline quaternary ammonium debonders, such as oleyl-imidazoline quaternaries, dialkyl dimethyl quaternary debonders, ester quaternary debonders, diamidoamine quaternary debonders, and the like. A specific suitable imidazoline quaternary is 1-methyl-2-noroleyl-3-oleyl amidoethyl imidazolinium methylsulfate available from Goldschmidt Corp. under the designation C-6027.
The amount of the wet strength agent can be about 5 pounds or greater per metric ton of dry fiber, more specifically from about 5 to about 30 pounds per metric ton of dry fiber, still more specifically from about 10 to about 25 pounds per metric ton of dry fiber. Suitable wet strength agents include all chemistries capable of forming covalent bonds with cellulose fibers. Alkaline-curing polymeric amine-epichlorohydrin resins, such as polyamide epichlorohydrin resins, poly(diallylamine) epichlorohydrin resins and quaternary ammonium epoxide resins are particularly advantageous. A particularly suitable wet strength agent is a polyamide-epichlorohydrin resin sold by Hercules, Inc. under the trademark Kymene® 6500.
The amount of dry strength agent can be about 5 pounds or greater per metric ton of dry fiber, more specifically from about 5 to about 20 pounds per metric ton of dry fiber. Suitable dry strength agents include all chemistries capable of forming hydrogen bonds with cellulose. These strength resins may include modified starches and gums, modified cellulose polymers and synthetic polymers, including modified polyacrylamide polymers. A particularly suitable dry strength agent is carboxymethylcellulose (CMC), such as one available from Hercules Inc. as Aqualon® CMC 7MCT.
As used herein, dry CD tensile strengths represent the peak load per sample width when a sample is pulled to rupture in the cross-machine direction. The sample must be dry and have been conditioned at 73° F., 50% relative humidity for at least 4 hours prior to testing. Samples are prepared by cutting a 3 inch wide×5 inch long strip in the cross-machine direction (CD) orientation. The instrument used for measuring tensile strengths is an MTS Systems Synergie 100. The data acquisition software was MTS TestWorks® 3.10 (MTS Systems Corp., Research Triangle Park, N.C.). The load cell is selected from either a 50 Newton or 100 Newton maximum, depending on the strength of the sample being tested, such that the majority of peak load values fall between 10-90% of the load cell's full scale value. The gauge lengt
Goulet Mike Thomas
Van Wychen Heath David
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