Patent Grant
7056572
Commercially available bath tissues generally fall into either of two categories. One of the categories is the premium segment, where softness is a major characteristic. The softness is at least partially due to high stretch and sheet caliper, which tend to impart a “cushiony” feel. A disadvantage of these premium products is that the number of sheets that can be wound on a roll of bath tissue is limited by the caliper of the sheets. This is because the average bath tissue dispenser can only accommodate rolls of bath tissue having a roll diameter of about 5 inches or less. Simply increasing the sheet count on a roll of premium tissue would result in a roll that is too large in diameter to fit within the average bath tissue dispenser. Alternatively, more sheets could be added to the roll by increasing the winding tension as the roll is wound, but this would remove stretch and reduce the sheet caliper and consequently reduce the softness of the product. As a result, products in the premium category typically have sheet counts of about 600 sheets per roll or less.
The other category is the value segment, which provides a large number of sheets per roll. The number of sheets per roll for bath products in this category is typically about 1000, but the sheets are characterized by low stretch and low bulk and hence exhibit lower softness than the premium products.
Hence there is a need for a bath tissue product that provides more sheets per roll than the premium segment products, but provides a comparably soft product.
It has now been discovered that a relatively high sheet count roll of bath tissue can be made with a tissue sheet having a low caliper (low bulk) and low stretch, yet at the same time has a soft, bulky feel.
Hence, in one aspect, the invention resides in a roll of bath tissue comprising a wound continuous tissue basesheet having spaced-apart transverse lines of perforations which define individual tissue sheets for detachment in use, said tissue basesheet having a geometric mean stretch of 11% or less, a single sheet caliper of about 0.01 inch or less, and a Void Volume of about 8.0 grams or greater per gram of tissue.
More specifically, the roll of bath tissue can have from about 600 to about 800 individual tissue sheets, more particularly from about 650 to about 750 sheets, alternatively from about 600 to about 750 sheets, and alternatively from about 650 to about 800 sheets. Stated differently, the total length of the wound continuous basesheet can be from about 2460 to about 3280 inches.
As used herein, “geometric mean stretch” is defined as the square root of the product of the machine direction stretch and the cross direction stretch and is measured using a constant rate of extension tensile tester. A three-inch wide strip is cut using a standard specimen cutter. The specimen is placed in both the upper and lower clamps of an Instron using as 2 inch jaw span. With the crosshead speed at 10 inches per minute, break sensitivity at 65% and slack compensation at 25 grams, the specimen is elongated until failure. The point of failure is detected and the elongation, measured as a percent of the initial length is recorded as the stretch. A total of ten specimens were run in each direction under standard laboratory conditions (23° C., 50% relative humidity) to obtain average values.
The geometric mean stretch of the tissue sheets of this invention can be about 11% or less, more specifically about 10% or less, and still more specifically from about 7 to about 10%.
The “caliper” of a single sheet is measured using an Emveco Model 200-A Micrometer from Emveco, Inc., Newberg, Oreg. With the clearance set at 0.50 inches and loading pressure at 2.00 kPa (132 g/in2) a specimen is placed between the pressure foot (2500 mm2 in diameter) and anvil. The pressure foot is lowered, contacts the tissue for approximately 3 seconds and then rises back to its original position. The test instrument automatically records each reading and averages them upon completion of a sample. In this work, readings from a total of 5 specimens were averaged together and recorded as one value.
The caliper of the tissue sheets of this invention can be about 0.01 inch or less, more specifically about 0.0095 inch or less, and still more specifically from about 0.007 to about 0.01 inch.
The Void Volume is determined by saturating a tissue sheet with a non-polar liquid and measuring the amount of liquid absorbed by the sheet. The volume of liquid absorbed is equivalent to the Void Volume within the sheet structure. For convenience, however, the Void Volume is expressed as grams of liquid absorbed per gram of fiber in the sheet, hereinafter referred to as “grams per gram of tissue”. The procedure is more specifically described in U.S. Pat. No. 5,494,554 issued Feb. 27, 1996 to Edwards et al., which patent is hereby incorporated by reference.
The Void Volume of the tissue sheets of this invention can be about 8.0 grams or greater per gram of tissue, more specifically about 10.0 grams or greater per gram of tissue, and still more specifically from about 8.0 to about 11 grams or greater per gram of tissue.
Basesheets suitable for purposes of this invention can be made using any process that produces a low density, resilient tissue structure. Such processes include uncreped throughdried, creped throughdried and modified wet press processes. Exemplary patents include U.S. Pat. No. 5,656,132 issued Aug. 12, 1997 to Farrington et al. and U.S. Pat. No. 6,083,346 issued Jul. 4, 2000 to Hermans et al., both of which are hereby incorporated by reference. The softness of the products of this invention is at least in part derived from the low-density, resilient nature of the web produced by the foregoing tissue processes, which can produce unusually high levels of stretch and caliper. By applying a high calendering load to these webs, the stretch and caliper, which are properties that normally enhance softness, are reduced. However, it has been discovered that calendering these webs under a high load still produces a soft absorbent tissue despite the resulting relatively low caliper and stretch. Suitable calendering loads can be about 200 pounds per linear inch (pli) or greater, more specifically about 300 pli or greater, still more specifically from about 250 to about 450 pli, and still more specifically from about 300 to about 400 pli. It is believed that the softness is largely due to the substantial Void Volume and surface smoothness that remains in the tissue after such high load calendering. And because of low caliper, relatively high sheet counts (600–800) of soft tissue can be wound onto a roll.
Referring to
A three-layered tissue in accordance with this invention was made as described in
To form the tissue, a three-layered headbox was employed, through which the two outer layers contained the same treated eucalyptus fibers described above and the center layer contained 100% refined softwood fiber. The resulting three-layer sheet structure was formed on a twin-wire, suction form roll. The speed of the forming fabric was 1920 feet per minute (fpm). The newly-formed web was then de-watered to a consistency of about 20–27% using vacuum suction from below the forming fabric before being transferred to the transfer fabric, which was traveling at 1600 fpm (20% rush transfer). A vacuum shoe pulling about 9–10 inches of mercury vacuum was used to transfer the web to the transfer fabric. The web was carried over a pair of Honeycomb throughdryers operating at temperatures of about 375° F. and dried to a final dryness of about 97–99% consistency. The dried cellulosic web was rolled onto a core to form a parent roll of tissue.
The parent roll tissue was then converted into soft, absorbent rolls of toilet tissue of this invention using high-intensity calendering, wherein the tissue was passed through a calendering nip, at approximately 300 feet per minute (fpm), consisting of a rubber roll on top having a hardness of 4 P&J and a steel roll on the bottom with a 350 pounds per lineal inch (pli) sustainable nip pressure. Once calendered, the tissue was sent through a second calender nip at approximately 300 feet per minute (fpm), consisting of a rubber roll on top having a hardness of 75 Shore A and a steel roll on the bottom with a 100 pounds per lineal inch (pli) sustainable nip pressure. The tissue was then wound into individual rolls of toilet tissue having a sheet count in the range of from 600–700 sheets per roll. The resulting tissue product had a geometric mean stretch of 8.0%, a one-sheet caliper of 0.0095 inch and a Void Volume of 8.6 grams per gram. In an alternate embodiment, the calendered cellulosic web can also be embossed using techniques known in the art.
A tissue was made as described in Example 1, except the tissue was sent through a single nip calender stage with 30 pounds per lineal inch sustainable nip pressure. The resulting tissue product had a geometric mean stretch of 10.0%, a one-sheet caliper of 0.0091 inch and a Void Volume of 8.2 grams per gram.
A three-layered tissue in accordance with this invention was prepared as described in
A three-layered headbox was used to inject the slurry between two Lindsay Wire 2164B forming fabrics, in a twin-wire forming section. While disposed between the two forming fabrics and travelling at 1000 fpm, the embryonic web was transported over four vacuum boxes operating with respective vacuum pressures of approximately 11, 13, 14, and 19 inches of mercury vacuum. The embryonic web was passed through an air press including an air plenum and a collection box that were operatively associated and integrally sealed with one another. The air plenum was pressurized with air to 15 pounds per square inch gauge at approximately 150° F. and the collection box was operated at approximately 11 inches of mercury vacuum. The sheet was exposed to the resulting pressure differential of approximately 41.5 inches of mercury and air flow of 68 standard cubic feet per minute (SCFM) per square inch for a dwell time of 7.5 milliseconds over four slots, each ⅜ inch in length. The consistency of the web was approximately 30% just prior to the air press and 39% upon exiting the air press.
The de-watered web was transferred using a vacuum pick-up shoe operating at approximately 10 inches of mercury vacuum onto an Albany Wire 44X-30 GST throughdrying fabric. A silicone emulsion in water was sprayed onto the sheet side of the 44X-30 GST fabric just prior to transfer from the forming fabric to facilitate eventual transfer to the Yankee. The silicone was applied at a flow rate of 400 milliliters per minute at 1.0% solids. The throughdrying fabric was thereafter pressed against the surface of the Yankee dryer with a conventional pressure roll operating at a maximum pressing pressure of 350 pli. The fabric was wrapped over about 39 inches of the Yankee dryer surface by a transfer roll which was unloaded and slightly removed from the Yankee dryer. The web was adhered to the Yankee surface using an adhesive mixture of polyvinyl alcohol and AIRVOL 532 made by Air Products and Chemical Inc. and sorbitol in water applied by four #6501 spray nozzles by Spraying Systems Company operating at approximately 40 psig with a flow rate of about 0.4 gallons per minute (gpm). The spray had a solids concentration of about 0.5 weight percent. The sheet was creped from the Yankee at a final dryness of approximately 95% consistency and wound into a parent roll.
The parent roll was converted into soft, absorbent rolls of toilet tissue using high-intensity calendering as described in Example 1, except the calendering load was 250 pounds per lineal inch sustainable nip pressure. The resulting tissue product had a geometric mean stretch of 8.4%, a one-sheet caliper of 0.0082 inch and a Void Volume of 10.4 grams per gram.
A tissue was made as described in Example 3, except that the tissue was stratified. The inner two layers consisted of NSWK which made up 50% of the weight of the tissue. The outer layer consisted of eucalyptus which constituted the other 50% of the weight of the tissue. When plied together the resulting tissue product had a geometric mean stretch of 7.9%, a one-sheet caliper of 0.0081 inch and a Void Volume of 10.3 grams per gram.
It will be appreciated that the foregoing examples, given for purposes of illustration, shall not be construed as limiting the scope of this invention, which is defined by the following claims and all equivalents thereto.
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