EP0613979A1

EP0613979A1 - Soft layered tissues and method for making same

Info

Publication number: EP0613979A1
Application number: EP94400347A
Authority: EP
Inventors: Steven Lawrence Edwards; Peter John Allen; Oliver Paul Renier
Original assignee: Kimberly Clark Worldwide Inc; Kimberly Clark Corp
Current assignee: Kimberly Clark Worldwide Inc
Priority date: 1993-03-02
Filing date: 1994-02-17
Publication date: 1994-09-07
Also published as: MX9401013A; CA2098327A1; JPH07317000A; KR940021823A; AU5488494A

Abstract

The formation of paper webs useful for facial tissue, bath tissue, paper towels or the like is substantially improved by forming the web in layers in which the second-formed layer has a consistency which is significantly less than the consistency of the first-formed layer.

The web is formed in layers by using a layered headbox (1) wherein first and second stock layers are separated by a headbox divider (2).

The resulting improvement in formation provides a significant improvement in softness, particularly for wet-pressed tissues, while reducing the degree to which fibers protrude from the surface of the tissue.
This also reduces the tendency of the web to lint.

Description

Background of the Invention

The use of layering to make tissue products such as facial and bath tissue is well known in the art. Layering affords an opportunity to more precisely engineer the tissue by placing different fibers in the inner and outer layers to take advantage of the different properties that the different fibers offer. Because improving softness is frequently an objective for many tissue products, it is logical to place the softer fibers in the outer layers while other fibers occupy the center of the tissue. Eucalyptus fibers are well known for their softness properties, in part due to their short fiber length. By placing the relatively short eucalyptus fibers in the outer layer, these fibers can be made to stand on end to protrude from the surface of the tissue to create a velvety feel. As the number of protruding surface fibers increases, the softness also increases. However, increasing the short fiber content of the outer layers of tissues often leads to excessive linting, which is undesirable and is a common complaint among soft tissue users. Hence there is a need for a method of making a softer tissue without increasing the number of protruding surface fibers.

Summary of the Invention

It has now been discovered that the softness of layered tissues is greatly influenced by how well the tissue sheet is formed. While the number of protruding surface fibers can improve perceived softness of tissues at low levels of sheet formation, at higher formation levels the perceived softness has been found to improve with increasing formation, while at the same time the number of protruding surface fibers decreases (as measured by the Surface Fiber Index, hereinafter defined). The net result is a softer tissue with fewer protruding fiber ends and hence a lesser tendency to create dust or lint during use, which addresses a common complaint among current soft tissue users. Hence, contrary to the teachings of the prior art, the softness of a tissue can be improved even though the number of protruding surface fibers is decreased.
More specifically, it has been found that the formation of a layered tissue can be greatly improved by adjusting the relative consistencies of the stock layers as the tissue sheet is formed such that the consistency of one or more of the second, third, fourth, etc. stock layers is (are) less than the consistency of the first layer. (As defined herein, the "first" stock layer is the only stock layer which comes in direct contact with the forming fabric or is the first to come in direct contact with the forming fabric, as the stock jet is deposited onto the forming fabric. Also as used herein, "consistency" is the weight percent fiber in an aqueous fiber suspension or stock layer.) Preferably, the consistency of successive stock layers is progressively less. It has been found that the resulting tissues have substantially better overall formation, as measured by the Formation Index (hereinafter defined), and correspondingly have substantially better softness. At the same time, the number of protruding surface fibers is reduced. Formation improvements, as measured by the Formation Index, can be about 15 percent or greater as compared to the same tissue sheet made with all stock layers having the same consistency.
Hence in one aspect the invention resides in an improved method of forming a tissue web using a layered headbox in which first and second stock layers, separated by a headbox divider, are continuously deposited onto an endless forming fabric to form a wet web such that the second stock layer is superposed on top of the first stock layer, said wet web being thereafter dried and preferably creped to form a tissue web, the improvement comprising providing a ratio of the consistency of the second stock layer to the consistency of the first stock layer of from about 0.95 to about 0.1 or less, more specifically from about 0.7 to about 0.1 or less, and still more specifically from about 0.5 to about 0.1 or less. A suitable range is from about 0.1 to about 0.7 and a particularly suitable range is about 0.3 to about 0.5.
In another aspect, the invention resides in a soft tissue having a Formation Index of about 150 or greater, suitably from about 150 to about 250, and more specifically from about 160 to about 200. Such a tissue can be further characterized by a high Void Volume (hereinafter defined), which for wet-pressed tissues can be raised to levels heretofore associated only with throughdried tissues. More specifically, the Void Volume can be about 9 or greater, preferably about 10 or greater, and suitably from about 9 to about 12.
In another aspect, the invention resides in a soft tissue having a Surface Fiber Index of about 60 or less, preferably of about 50 or less, and most preferably of from about 40 to about 55.
As used herein, a tissue web or sheet is a paper web suitable for use as a facial tissue, bath tissue, kitchen towel, dinner napkin or the like. Such webs can be creped or uncreped. They can be made by wet-pressing or throughdrying tissue making processes well known in the tissue making arts.
Papermaking fibers for making the tissue webs of this invention include any natural or synthetic fibers suitable for the end use products listed above including, but not limited to: nonwoody fibers, such as abaca, sabai grass, milkweed floss fibers, pineapple leaf fibers; softwood fibers, such as northern and southern softwood kraft fibers; hardwood fibers, such as eucalyptus, maple, birch, aspen, or the like. Because of commercial availability, softwood and hardwood fibers are preferred and the papermaking fibers can be a blend of softwood fibers and hardwood fibers.
As used herein, a layered headbox is a headbox having one or more headbox dividers which create separate flow channels or layers of papermaking stock issuing from the headbox. The dividers need not extend beyond the headbox lips, but such extended dividers are preferred in order to preserve layer purity by minimizing intermixing of the layers. As defined above, the stock layer (an aqueous suspension of papermaking fibers) within the divided headbox which directly contacts the forming fabric is referred to herein as the "first" stock layer. This is the stock layer through which most or all of the water in the newly-formed web must pass as the web is dewatered through the forming fabric. Superposed on top of the first stock layer, as the papermaking stock leaves the headbox for deposition onto the forming fabric, are one or more successive stock layers of fiber suspensions, the number of which depends on the number of headbox dividers. Each of these successive superposed stock layers is generally referred to herein as a "second" aqueous suspension of papermaking fibers, unless the individual superposed stock layers are otherwise identified. There can be two, three, four or more distinct stock layers, although two or three are preferred for practical commercial reasons.
For embodiments of this invention where there are three or more stock layers, the consistencies of each stock layer can be adjusted to provide a wide range of consistency ratios relative to the consistency of the first stock layer. It is preferable that the consistencies of successive stock layers decrease when progressively going from the first stock layer to the second stock layer to the third stock layer and so on. However, it is within the scope of this invention that successive stock layers have the same consistency. In a three-layer stock system, the second and third stock layers can have the same consistency provided they are less than the consistency of the first stock layer. Alternatively, the first and second stock layers can have the same consistency while the consistency of the third stock layer is less than that of the first two.
It is important to note that the fiber composition of the stock layers can be the same or different. If the fiber composition of all of the stock layers is the same, regardless of the number of stock layers, a blended tissue product having improved formation will result. However, additional product benefits can be obtained if the fiber compositions of the stock layers are different. In this regard, in a two layer stock system for example, the first stock layer can comprise primarily softwood fibers and the second stock layer can comprise primarily hardwood fibers, although the reverse can also be used. The preferred layer compositions may vary depending on the particular type of former being used and the desired product attributes.
For example, using a crescent former where the web is formed between a fabric and a felt, a preferred manner of operating could be to place the hardwood fibers on the fabric side at relatively higher consistency than the strength-developing softwood fibers, which would be placed on the roll side. This configuration results with the hardwood fibers being placed against the Yankee dryer during creping and subsequently being plied into a two-ply product with the hardwood fibers on the outside surfaces of the product. Alternatively, the softwood fibers could be placed on the forming wire at relatively higher consistency and the hardwood fibers placed on the roll side at relatively lower consistency. In this mode, the softwood, or strength, fibers would be placed against the Yankee during creping. During the plying process, the hardwood fibers could be placed on the outside of the multi-ply product to produce a soft tissue. An advantage of this configuration is an increase in sheet opacity at comparable basis weights.
In a suction breast roll former, the more dilute side of the sheet is the top side or the side against the "roof" of the headbox. The side with the higher consistency, be it hardwood or softwood, is placed onto the forming fabric. Similarly, in the "S" wrap twin wire former, the higher consistency side would be first laid onto the fabric side while the lower consistency would be placed on the roll side. In all of the formers mentioned above, the higher consistency side is ultimately placed in contact with the surface of the Yankee dryer.
By way of contrast, when using a twin wire "C" former, the roll side of the formed sheet, which is the more dilute side, is placed against the surface of the Yankee dryer. A preferred mode of operation might be to place the hardwood layer on the roll side of the former and ply the hardwood fibers on the outside of the product. Alternatively, the hardwood fibers can be placed on the fabric side and the softwood fibers on the roll side, the softwood fibers being the relatively more dilute layer. In this mode the softwood fibers are against the surface of the Yankee during creping, but the plying is carried out such that the hardwood fibers are still on the outside of the product.
The "Formation Index" is measured using a digital image analysis system with a minimum pixel density of 512 (horizontal) by 480 (vertical) and 8 bit resolution (giving 256 gray levels). Several commercial systems are available with these specifications including the Zeiss IBAS image analysis system (available from Carl Zeiss, Inc. in Thornwood, NY) and the Leica/Cambridge 900 Series image analysis system (available from Leica, Inc. in Deerfield, IL). Alternatively, an image analyzer suitable for the measurement of the Formation Index can be constructed from a "386 Class" personal computer containing a video frame grabber card such as the Imaging Technology VP1400-KIT-640-U-AT (manufactured by Imaging Technology Inc. of Bedford, MA) or equivalent frame grabbers from Data Translation (of Boston, MA) or other vendors. Such personal computer-based systems are most effectively operated using specialized image analysis software such as Optimas (available from Optimas Inc., Edmonds, WA). Many other such software packages are available for the different frame grabber cards.
Whatever image analysis system is used, a video camera system is used for image input. Either image tube cameras or solid state cameras such as those utilizing Charge Coupled Devices may be used. The chosen camera must have a gamma value of between 0.9 and 1.0. One such camera is a Dage Model 68 camera containing a Newvicon sensing tube (available from Dage MTI, Michigan City, IN).
A 35 mm. focal length lens is used with the camera. Any high quality lens may be used, such as the Nikon Nikkor 35 mm., f/2 autofocus lens (manufactured by Nikon, Inc., Japan). The lens is attached to the camera through suitable adapters. Typically, the lens is operated with its aperture set to f/5.6.
The camera system views a tissue sample sandwiched between a plate of diffuser plastic and window glass. This sandwich is placed on the center of a lightbox having dimensions of greater than 20.32 cm (8 inches) in each direction. Whatever lightbox is used, it must have a uniform field of Lambertian (diffuse) illumination of adjustable intensity. The method of intensity adjustment must not change the color temperature of the illumination. One appropriate lightbox is the ChromoPro Model 65 illuminator with optional diffuser table (available from Byers Photo Equipment Co. of Portland, Oregon).
Specifically, samples for the Formation Index are single-ply tissue sheets cut to 10.1 cm x 10.1 cm (4-inch by 4-inch) squares, with one side aligned with the machine direction of the test material. Each specimen is placed on a square 10.1 cm x 10.1 cm (4-inch by 4-inch) piece of nominally 0.32 cm (1/8-inch) thick Plexiglas MC acrylic sheet (available from Rohm and Haas, Philadelphia, PA) such that the side of the tissue sheet that contacted the Yankee dryer during manufacture is facing up, away from the acrylic sheet. The tissue sheet is then covered with a 10.1 cm x 10.1 cm (4-inch by 4-inch)by nominally 0.32 cm (1/8-inch) thick Piece of window glass containing non visible scratches or optical imperfections.
The specimen "sandwich" is set, glass side up, on the lightbox so that the center of the sandwich is aligned with the center of the illumination field. All other natural or artificial room light is extinguished. The camera is adjusted so that its optical axis is perpendicular to the plane of the tissue sheet and so that its video field is centered on the center of the specimen sandwich. The machine direction of the specimen is aligned with the vertical direction of the camera field. The camera is then positioned along its optical axis until its entire field of view contains exactly two inches of the specimen in the horizontal direction. The camera is focused so that the resulting picture contrast, measured as the standard deviation of the pixel array formed by digitization of the image, is maximized.
Next, the sample sandwich is replaced with a 10.1 cm x 10.1 cm (4-inch by 4-inch) piece of the acrylic sheet that does not have a specimen mounted. This acrylic sheet also is placed in the center of the lightbox, but it is not covered with a piece of window glass. The lightbox intensity is adjusted so that the mean value of the pixel array formed by digitization of this image averages 160 gray levels, plus or minus 0.4 gray levels. 32 frames of this image are then averaged into the frame grabber memory as a shading correction image.
The specimen sandwich is again placed on the lightbox, in the same position and alignment as it was previously. The lightbox illumination is adjusted so that the mean value of the resulting pixel array representing the tissue picture is again 160 gray levels plus or minus 0.4 gray levels. 32 frames of the tissue image are averaged into another part of the frame grabber memory.
The Formation Index is calculated by correcting the tissue image for lightbox shading, preferably by using an additive shading correction procedure. A precursor of the Formation Index is then calculated from the variance of the shading corrected pixel array as: $Precursor = (16 / [pixel array variance])$

Image analyzer systems have intrinsic response differences due to design differences between various manufacturers and also due to normal component variation. Therefore, an image analysis system must be calibrated against a set of fourteen known tissue standards before the final Formation Index can be calculated. These tissue standards (available from Kimberly-Clark Corporation, Neenah, WI) are tested on a "standard" image analysis system and are individually rated as to the expected value of the Formation Index along with its standard deviation when tested on appropriate equipment. The list of standards used for calibration are listed below:

Standard Identification	Nominal Formation Index
FSTD-1	81
FSTD-2	85
FSTD-3	91
FSTD-4	93
FSTD-5	101
FSTD-6	102
FSTD-7	109
FSTD-8	106
FSTD-9	101
FSTD-10	97
FSTD-11	89
FSTD-12	80
FSTD-13	160
FSTD-14	180

The image analysis system is calibrated against these tissue standards by measuring each standard on the system and obtaining a Precursor value. Each standard is individually measured at least three times and the average Precursor value for each standard is used as the independent variable in a least squares linear regression utilizing the specified standard's Formation Index as the dependent variable. If the equipment is properly set up, the coefficient of determination for this regression should be greater than 0.95.
The linear regression procedure gives a slope value, which is herein referred to as the "m" value, and an intercept value, which is herein referred to as the "b" value. The Formation Index can be calculated for any specimen by measuring its Precursor value and using the following equation. $(Formation Index) = m * Precursor + b$
The image analysis system must have new values of the calibration coefficients, m and b, calculated occasionally. While the frequency of this calibration depends, in general, on the stability of the image analysis system, best measurement of the Formation Index is made when calibration is carried out at each power-up of the formation analyzer system, or on a daily basis, if the image analyzer is left powered-up.
As used herein, "Void Volume" is determined by saturating a sheet with a nonpolar liquid and measuring the volume of liquid absorbed. The volume of liquid absorbed is equivalent to the void volume within the sheet structure. The Void Volume is expressed as grams of liquid absorbed per gram of fiber in the sheet. More specifically, for each single-ply sheet sample to be tested, select 8 sheets and cut out a 2.54 cm x 2.54 cm (1 inch by 1 inch) square (2.54 cm (1 inch) in the machine direction (MD) and 2.54 cm (1 inch) in the cross-machine direction (CMD)). For multi-ply product samples, each ply is measured as a separate entity. Multi-ply samples should be separated into individual single plies and 8 sheets from each ply position used for testing. Weigh and record the dry weight of each test specimen to the nearest 0.001 gram. Place the specimen in a dish containing PORIFIL^TM pore wetting liquid of sufficient depth and quantity to allow the specimen to float freely following absorption of the liquid. (PORIFIL^TM liquid, having a specific gravity of 1.875 grams per cubic centimeter, available from Coulter Electronics Ltd., Northwell Drive, Luton, Beds., England; Part No. 9902458.) After 10 seconds, grasp the specimen at the very edge (1-2 millimeters in) of one corner with tweezers and remove from the liquid. Hold the specimen with that corner uppermost and allow excess liquid to drip for 30 seconds. Lightly dab (less than 1/2 second contact) the lower corner of the specimen on #4 filter paper (Whatman Ltd., Maidstone, England) in order to remove any excess of the last partial drop. Immediately weigh the specimen, within 10 seconds, recording the weight to the nearest 0.001 gram. The Void Volume for each specimen, expressed as grams of PORIFIL per gram of fiber, is calculated as follows: $Void Volume = [ (W₂ - W₁)/W₁ ], wherein$

"W₁" is the dry weight of the specimen, in grams; and
"W₂" is the wet weight of the specimen, in grams.
The Void Volume for all eight individual specimens is determined as described above and the average of the eight specimens is the Void Volume for the sample.
As used herein, the Surface Fiber Index is a measure of a number of surface fibers of a sheet which exhibit an observable starting point on the sheet and a loose unbonded end that measures 0.1 millimeter or greater. In general, it is determined by folding a portion of the sheet over the edge of a glass slide and counting the number of fibers which meet the foregoing criteria. More specifically, a rectangular test sample measuring 8.89 cm (3-1/2 inches) long x 6.03 cm (2-3/8 inches) wide is cut out of the center of the sheet at a 45° angle relative to the machine direction of the sheet as illustrated in Figure 7. The rectangular test sample is inserted into the bottom of a sample sled as shown in Figures 8 and 9 with the side of the sample to be tested facing out. The sled and attached sample are placed onto a brushing fabric (low pile, crush-resistant acetate velvet available from Wimpfheimer American Velvet Company, 22 Bay View Ave., Stonington, Connecticut) which has been secured to a flat planar surface. The sled is pulled across the brushing surface by hand as shown in Figure 9. Brushing of the sample takes place in one direction in one continuous motion at a speed of 5 centimeters per second for a distance of 10 centimeters under an applied load of 5 grams per square centimeter. The applied load includes the weight of the sled and any additional weight necessary to attain 5 grams per square centimeter. After brushing, a scissors is used to cut a piece out of the middle of the brushed sample about 1 inch wide, being careful not to touch the surface of the sample. The sample is then folded over a No. 1-1/2 glass cover slip with the brushed side out and carefully placed between two glass slides (Corning Micro Slit slide, #2947, 75x50 millimeters) as shown in Figure 10. The sample orientation at the coverslip edge represents a 45° angle to the machine direction of the tissue sheet. The slides can be secured using two rubber bands.
The number of fiber ends can be counted by placing the prepared sample under a microscope. An Olympus compound microscope, model BH-2, can be used with transmitted lighting using a 4X DPIAN objective which yields a 40X magnification of the fibers ends at the eye piece. Alternatively the image can be projected via a video camera connected to a video monitor (Sony B/W with 850 lines of resolution). The number of fibers exhibiting an observable starting point and a loose unbonded end measuring 0.1 millimeter or greater per 12.7 millimeters (0.5 inch) of sample is the Surface Fiber Index for the sample. A sufficient number of slides should be prepared to take 20 measurements; the average reading from these twenty measurements is the Surface Fiber Index for the tissue sample.

Brief Description of the Drawing

Figure 1 is a schematic diagram of the forming zone of a typical tissue machine, illustrating the formation of multiple layers in accordance with this invention.
Figure 2 is a schematic diagram of a tissue making process using a crescent former in accordance with this invention.
Figure 3 is a plot of Void Volume as a function of basis weight for wet-pressed and throughdried tissues, illustrating an advantage of the method of this invention as applied to wet-pressed tissue products.
Figure 4 is a plot of Stiffness, as determined by a trained sensory panel, as a function of Void Volume for wet-pressed tissues, illustrating decreasing stiffness (and hence increasing softness) with increasing Void Volume, as well as illustrating the low stiffness of the products of this invention.
Figure 5 is similar to Figure 4 and is a plot of Stiffness, as represented by MD (machine direction) Modulus, as a function of Void Volume for wet-pressed tissues, further illustrating the low stiffness of the products of this invention.
Figure 6 is a plot of Surface Fiber Index as a function of Formation Index for wet-pressed tissues, illustrating the relationship that, for a given operating mode, the number of protruding surface fibers decreases as formation is improved. The absolute values will depend on the particular process, including the particular fibers and the machine being used.
Figure 7 is a plan view of a tissue sheet to be tested for the Surface Fiber Index, illustrating the orientation of the test sample.
Figure 8 is a perspective view of the sample sled used to brush the test sample in measuring the Surface Fiber Index.
Figure 9 is a side view of the test sample brushing operation for determining the Surface Fiber Index, illustrating the sample sled being pulled over the brushing surface.
Figure 10 is a cross-sectional view of the brushed test sample mounted between glass slides for measuring the Surface Fiber Index, illustrating the protruding fiber ends which are exposed as the test sample is folded over the glass cover slip.

Detailed Description of the Drawing

Referring to Figure 1, the invention will be described in greater detail. Figure 1 is a schematic diagram of a layered forming process illustrating the sequence of layer formation. Shown is a two-layered headbox 1 containing a headbox layer divider 2 which separates the first stock layer (the lower or bottom layer) from the second stock layer (the upper or top layer). The two stock layers each consist of a dilute aqueous suspension of papermaking fibers having different consistencies. In general, the consistencies of these stock layers will be from about 0.04 percent to about 1 percent. An endless travelling forming fabric 3, suitably supported and driven by rolls 4 and 5, receives layered papermaking stock issuing from the headbox and retains the fibers thereon while allowing some of the water to pass through as depicted by the arrows 6. In practice, water removal is achieved by combinations of gravity, centrifugal force, and vacuum suction depending on the forming configuration. As shown, the first stock layer is the stock layer which is first to make contact with the forming fabric. The second stock layer (and any successive stock layers if a headbox having more than one divider is utilized) is the second-formed layer and is formed on top of the first layer. As shown, the second stock layer never contacts the forming fabric. As a result, the water in the second and any successive layers must pass through the first layer in order to be removed from the web by passing through the forming fabric. While this situation might be considered to be disruptive of the first layer formation because of all the additional water which is deposited on top of the first stock layer, it has been found that diluting the second and successive stock layers to lower consistencies than that of the first stock layer provides substantial improvements in the formation of the second and successive layers without detriment to the formation of the first layer.
Figure 2 is a schematic flow diagram of the method of this invention placed in context of a conventional tissue making process. The specific formation mode illustrated is commonly referred to as a crescent former. Shown is a layered headbox 21, a forming fabric 22, a forming roll 23, a papermaking felt 24, a press roll 25, a Yankee dryer 26, and a creping blade 27. Also shown, but not numbered, are various idler or tension rolls used for defining the fabric runs in the schematic diagram, which may differ in practice. As shown, a layered headbox 21 continuously deposits a layered stock jet between the forming fabric 22 and the felt 24, which is partially wrapped around the forming roll 23. Water is removed from the aqueous stock suspension through the forming fabric by centrifugal force as the newly-formed web traverses the arc of the forming roll. As the forming fabric and felt separate, the wet web stays with the felt and is transported to the Yankee dryer. At the Yankee dryer, the web is pressed by the pressure roll between the surface of the Yankee and the felt, where additional water is squeezed out of the web. The dewatered web adheres to the surface of the Yankee and is dried before impacting the doctor blade, where it is creped and dislodged from the Yankee surface and wound into a soft roll.
Figure 3 is a plot of Void Volume (expressed as grams of Porofil liquid per gram of fiber) versus basis weight (expressed as grams per square meter) for a number of tissue products, illustrating how the method of this invention can transform a layered wet-pressed product into a throughdried-like product in terms of fiber structure. As will be illustrated hereinafter, increases in Void Volume correlate with improved softness. Shown in the plot of Figure 3 are a number of commercial wet-pressed tissue products, labelled "WP", and several commercial throughdried tissue products, labelled "TD". The wet-pressed tissue products made in accordance with this invention are labelled "INV". As shown, the wet-pressed tissue products of this invention have a Void Volume of about 11, which is equivalent to the Void Volume of the higher Void Volume throughdried products.
Figure 4 is a plot of sheet stiffness, as determined by a trained sensory panel, as a function of the Void Volume for a number of wet-pressed tissue samples. As shown, the stiffness of the products of this invention, designated by the points labelled "INV", is very low relative to most of the other wet-pressed products.
Figure 5 is a plot similar to that of Figure 4, but substituting MD Modulus for the sensory panel measurement of stiffness. The relationship is generally the same, with the sheets of this invention having a significantly lower MD Modulus than all of the conventional wet-pressed samples tested.
Figure 6 is a plot of the Surface Fiber Index as a function of the Formation Index for a number of wet-pressed tissues formed on the same machine at different formation levels, illustrating the discovery that the number of protruding surface fibers decreases as the formation of the tissue improves.
Figures 7-10 have been referred to above in connection with the description of method for measuring the Surface Fiber Index. Illustrated in Figure 7 is the proper orientation of the test sample to be taken from a tissue sheet in order to measure the Surface Fiber Index. Shown is the tissue sheet 30 with the machine direction represented by arrow 31. The test sample 33 is cut from the middle of the tissue sheet at an angle of 45° to the machine direction as indicated by double arrow 34.
Figure 8 is a perspective view of the sample sled 40 used to brush the test sample after it has been cut out of the tissue sheet. Shown is the base plate 41, the sample clamp 42, two spring-loaded screws 43 which keep pressure on the sample clamp to hold the sample firmly in place, and a yoke 44 used to pull the sled during brushing of the sample.
Figure 9 illustrates the test sample brushing process used to increase the visibility of the fiber ends on the surface of the tissue sample. Shown is the brushing sled base plate 41, the yoke 44, the sample 33 firmly positioned underneath the base plate, the velvet brushing fabric 50, and a line 51 pulling the sample sled in the direction of the arrow 52.
Figure 10 illustrates an end view of the test sample prepared for viewing under the microscope to count the number of fiber ends protruding from the surface of the sample. Shown is the test sample 33, the cover slip 61 over which the test sample is folded, and two glass slides 62 and 63 which protect the sample and firmly hold it in place for viewing. Also schematically depicted are numerous fiber ends 64 protruding from the surface of the test sample at the point where the sample is folded over the edge of the cover slip.

Examples

Example 1 (This Invention).

In order to further illustrate the invention, a creped sheet was made using the crescent former illustrated in Figure 2. More specifically, aqueous suspensions of 100% virgin papermaking fibers, one suspension 100% hardwood and one 100% softwood, were prepared containing about 0.1 weight percent fibers. The hardwood portion of this furnish, representing half the total sheet weight, was fed to the forming zone, contacting the wire side of the forming unit, at about 0.15 weight percent fibers. Simultaneously delivered to the roll side of the forming unit was the softwood portion, representing half the total sheet weight, in a suspension containing about 0.075 weight percent fibers. Both these suspensions were delivered from the same headbox but were kept separated by an extended divider sheet until just before contacting the forming zone. The headbox used was of three chamber design, two of which were devoted to delivering the lower consistency softwood fibers while one chamber was devoted to the higher consistency hardwood. The forming fabric used was an Albany 94M, a typical tissue weight forming fabric traveling at a speed of about 914 m (3000 feet) per minute. The felt was an Albany Super Fine DURACOMBE SG, a typical felt used in tissue production. The sheet was delivered to the pressure roll and Yankee dryer at about 10 weight percent consistency. The pressing was done with a relatively wide nip with an applied pressure of about 136 kg (300 pounds) of loading force per 2.54 cm (per inch) of contact length on the Yankee dryer. Following attachment of the sheet to the Yankee dryer the consistency of the web was at about 40 weight percent fibers. The sheet was then creped off the Yankee dryer using a typical metal creping blade set up with a typical 80 to 90 degree creping pocket angle so as to provide efficient sheet breakup without undue loss of sheet strength. The resulting sheet was then wound into a softroll and exhibited the following characteristics: basis weight, 15 grams per square meter (gsm); geometric mean tensile strength, 650 grams per 7.62 cm (3 inches) of width (grams) tested with two plys together to simulate an actual tissue sheet; Formation Index of 180; a Surface Fiber Index of 45; and a caliper of 0.0342 mm (0.0135 inches) tested with two sheets plied together such that creped sides are out.

Example 2 (This Invention).

A creped sheet made as described in Example 1 except that the relative positions of the hardwood and softwood fibers were changed. The same hardwood fibers were delivered to the headbox on the roll side of the former at the relatively lower consistency while the softwood fibers were delivered to the former on the wire side of the former at the relatively higher consistency. All other conditions remained the same except for some adjustments in the creping chemicals applied to the Yankee dryer to account for the different adhesive properties between the hardwood and softwood fibers. The resulting properties of the base sheet were as follows: basis weight, 15 g/m²; geometric mean tensile strength, 600 grams; Formation Index of 160; Surface Fiber Index, 40; and a caliper of 0.0317 mm (0.0125 inches) tested with two sheets plyed together such that the uncreped sides are out.

Example 3.

For comparison, several creped sheets were made in a conventional layered mode in which the same fibers as in Example 1 were delivered to the headbox at 0.1 weight percent consistency. In this case both the hardwood and softwood portions, each representing half the total sheet weight, were delivered to the forming zone at the same 0.1 weight percent consistency. The softwood fibers were formed on the roll side of the sheet while the hardwood fibers were formed on the wire side of the sheet. In this case, two extended dividers separated the three chambers of the headbox. Other conditions were maintained the same as that in Example 1. The resulting properties of the sheets are as follows: basis weight, 15-18 g/m²; geometric mean tensile strength, 650-850 grams; Formation Index, 120-140; Surface Fiber Index, 50-60; and caliper of 0.0190-0.0241 mm (0.0075-0.0095 inches) tested with two sheets plyed together such that the creped sides are out.
It will be appreciated that the foregoing examples, given for purposes of illustration, are not to be construed as limiting the scope of this invention, which is defined by the following claims and all equivalents thereto.

Claims

In a method of forming a tissue web of papermaking fibers with a layered headbox (1) wherein first and second stock layers, separated by a headbox divider (2), are continuously deposited onto an endless forming fabric (3) to form a wet web such that the second stock layer is superposed on top of the first stock layer and the first stock layer directly contacts the forming fabric (3), said wet web being thereafter dried to form a tissue web, the improvement comprising providing a ratio of the consistency of the second stock layer to the consistency of the first stock layer of about 0.95 or less.
The method of Claim 1 wherein the consistency ratio is about 0.7 or less.
The method of Claim 1 wherein the consistency ratio is about 0.5 or less.
The method of Claim 1 wherein the consistency ratio is from about 0.1 to about 0.7.
The method of Claim 1 wherein the consistency ratio is from about 0.3 to about 0.5.
The method of Claim 1 wherein there are only two stock layers.
The method of Claim 1 further comprising a third stock layer superposed on top of the second stock layer, wherein the ratio of the consistency of the third stock layer to the consistency of the first stock layer is about 0.95 or less.
The method of Claim 7 wherein the ratio of the consistency of the third stock layer to the consistency of the first stock layer is about 0.7 or less.
The method of Claim 7 wherein the ratio of the consistency of the third stock layer to the consistency of the first stock layer is from about 0.7 to about 0.1.
The method of Claim 1 wherein the papermaking fibers of the first stock layer are substantially the same as the papermaking fibers of the second stock layer.
The method of Claim 10 wherein the papermaking fibers are a blend of softwood fibers and hardwood fibers.
The method of Claim 1 wherein the papermaking fibers of the first stock layer are different from the papermaking fibers of the second stock layer.
The method of Claim 1 wherein the papermaking fibers of the first stock layer are predominantly softwood fibers.
The method of Claim 1 wherein the papermaking fibers of the first stock layer are predominantly hardwood fibers.
The method of Claim 1 wherein the papermaking fibers of the first stock layer are predominantly softwood fibers and the papermaking fibers of the second stock layer are predominantly hardwood fibers.
The tissue web made by the method of Claim 1.
A soft tissue having a Formation Index of about 150 or greater.
The tissue of Claim 17 having a Formation Index of from about 150 to about 250.
The tissue of Claim 17 having a Formation Index of from about 160 to about 200.
The tissue of Claim 17 having a Surface Fiber Index of about 60 or less.
The tissue of Claim 17 having a Surface Fiber Index of about 50 or less.
The tissue of Claim 17 having a Surface Fiber Index of from about 40 to about 55.
The tissue of Claim 17 having a Void Volume of about 9 or greater.
The tissue of Claim 17 having a Void Volume of about 10 or greater.
The tissue of Claim 17 having a Void Volume of from about 9 to about 12.
The tissue of Claim 17 having a Void Volume of about 11.
A wet-pressed tissue having a Void Volume of about 9 or greater.
The tissue of Claim 27 having a Void Volume of about 10 or greater.
The tissue of Claim 27 having a Void Volume of from about 9 to about 12.