Soft absorbent tissue

[0002] In the field of soft tissues, such as facial tissue and bath tissue, it is well known that the application of polysiloxanes to the surface of the tissue can impart an improved surface feel to the tissue. However, polysiloxanes are also known to impart hydrophobicity to the treated tissue. Hence it is difficult to find a proper balance between softness and absorbency, both of which are desirable attributes for tissue, particularly bath tissue.

[0003] It has now been discovered that the softness of a tissue can be improved with minimal negative impact on the absorbency or wettability of the tissue by treating one or both outer surfaces of the tissue with a particular group of hydrophilically-modified amino-functional polydiorganosiloxanes. More specifically, suitable polysiloxane structures have one or more pendant groups which contain a terminal amine and at least one ethylene oxide moiety. The terminal amine group and the ethylene oxide moieties can be parts of the same pendant group or different pendant groups. A general structure is as follows:

[0004] wherein:

[0005] X is hydrogen, hydroxy, amino, C₁-C₈straight chain, branched, cyclic, unsubstituted or hydrophilically substituted alkyl or alkoxyl radical;

[0006] m=20-100,000;

[0007] p=1-5000;

[0008] q=0-5000;

[0009] R₁=a C₁-C₆, straight chain, branched or cyclic alkyl radical;

[0010] R₂=a C₁-C₁₀straight chain or branched, substituted or unsubstituted alkylene diradical;

[0011] R₃=—R₅—(CH₂-CH₂—O)_r—(CH₂-CH—O)_s—Z

[0012] wherein

[0013] R₅is an unsubstituted or a hydrophilically substituted C₁-C₁₀alkylene diradical;

[0014] r=1-10,000;

[0015] s=0-10,000; and

[0016] Z=hydrogen, C₁-C₂₄alkyl group, or a G-group, where G is selected from the following: —R₆COOR₇; —CONR₈R₉; —SO₃R₈; and PO R₈R₉, where R₆is a substituted or unsubstituted C₁-C₆alkylene diradical; R₇, R₈, and R₉are independently a hydrogen radical or a substituted or unsubstituted C₁-C₈alkyl radical; and

[0017] wherein

[0018] R₁₀, R₁₁, and R₁₂are independently an unsubstituted or a hydrophilically substituted C₁-C₈alkylene diradical;

[0019] t=0-10,000;

[0020] u=0-10,000;

[0021] w=0-10,000; and

[0022] R₁₃, R₁₄and R₁₅are independently a hydrogen radical, an unsubstituted or a hydroxyl, carboxyl or other functionally substituted C₁-C₁₀straight chain, branched, or cyclic alkyl radical.

[0023] Representative species within the foregoing general structure include the following (the values of “m”, “p” and “q” are as defined above; the terms “EO” and “PO” are shorthanded representations of “ethylene oxide” and “propylene oxide” moieties, respectively):

[0024] The hydrophilically-modified amino-functional polydiorganosiloxanes described above can be applied to the tissue web alone or in conjunction with other chemicals, such as bonders or debonders. They can be applied to the tissue web, particularly an uncreped throughdried web, by spraying or printing. Rotogravure printing of an aqueous emulsion is particularly effective. Add-on amounts can be from about 0.5 to about 15 dry weight percent, based on the weight of the tissue, more specifically from about 1 to about 10 dry weight percent, still more specifically from about 1 to about 5 weight percent, still more specifically from about 2 to about 5 weight percent. The distribution of the deposits of the hydrophilically-modified amino-functional polydiorganosiloxanes is substantially uniform over the printed surface of the tissue, even though the surface of the tissue, such as in the case of uncreped throughdried tissues, may be highly textured and three-dimensional. The printing does limit the deposits to the high points of the textured tissue sheets, thereby ensuring a soft hand feel.

[0025] The Wet Out Time (hereinafter defined) for tissues of this invention can be about seconds or less, more specifically about 8 seconds or less, still more specifically about seconds or less, still more specifically about 5 seconds or less, still more specifically from about 4 to about 6 seconds. As used herein, “Wet Out Time” is related to absorbency and is the time it takes for a given sample to completely wet out when placed in water. More specifically, the Wet Out Time is determined by cutting 20 sheets of the tissue sample into 2.5 inch squares. The number of sheets used in the test is independent of the number of plies per sheet of product. The 20 square sheets are stacked together and stapled at each corner to form a pad. The pad is held close to the surface of a constant temperature distilled water bath (23+/−2° C.), which is the appropriate size and depth to ensure the saturated specimen does not contact the bottom of the container and the top surface of the water at the same time, and dropped flat onto the water surface, staple points down. The time taken for the pad to become completely saturated, measured in seconds, is the Wet Out Time for the sample and represents the absorbent rate of the tissue. Increases in the Wet Out Time represent a decrease in absorbent rate.

[0026] The “Differential Wet Out Time” is the difference between the Wet Out Times of a tissue sample treated with a hydrophilically-modified amino-functional polydiorganosiloxane and a control tissue sample which has not been treated. The Differential Wet Out Time, for purposes of this invention, can be about 5 seconds or less, more specifically about 4 seconds or less, still more specifically about 3 seconds or less, still more specifically about 2 seconds or less, and still more specifically about 1 second or less.

[0027] The ratio of the Wet Out Time, expressed in seconds, to the add-on amount of the hydrophilically-modified amino-functional polydiorganosiloxane in the tissue, expressed as dry weight percent of the weight of the tissue, can be about 3 seconds per weight percent or less, more specifically about 2 seconds per weight percent or less, still more specifically from about 1 to about 3 seconds per weight percent.

[0028] The ratio of the Differential Wet Out Time to the add-on amount of the hydrophilically-modified amino-functional polydiorganosiloxane can be about 2 seconds per weight percent or less, more specifically about 1 second per weight percent or less, still more specifically about 0.5 second per weight percent or less.

[0029] Tissue sheets useful for purposes of this invention can be creped or uncreped. Such tissue sheets can be used for facial tissues or bath tissues. They can have one, two, three or more plies. The basis weight of the tissue product can be from about 25 to about 50 grams per square meter. If used for bath tissue, a single ply tissue having a basis weight of from about 30-40 grams per square meter is particularly suitable.

[0030] FIG. 1 is a schematic diagram of an uncreped throughdried process for making bath tissue in accordance with this invention.

[0031] FIG. 2 is a schematic diagram of the post-manufacturing method of handling the uncreped throughdried web and the rotogravure coating process used to apply the hydrophilically-modified amino-functional polydiorganosiloxane emulsion in accordance with this invention.

[0032] Referring to FIG. 1, shown is a schematic flow diagram of a throughdrying process for making uncreped throughdried tissue sheets. Shown is the headbox 1 which deposits an aqueous suspension of papermaking fibers onto an inner forming fabric 3 as it traverses the forming roll 4. Outer forming fabric 5 serves to contain the web while it passes over the forming roll and sheds some of the water. The wet web 6 is then transferred from the inner forming fabric to a wet end transfer fabric 8 with the aid of a vacuum transfer shoe 9. This transfer is preferably carried out with the transfer fabric traveling at a slower speed than the forming fabric (rush transfer) to impart stretch into the final tissue sheet. The wet web is then transferred to the throughdrying fabric 11 with the assistance of a vacuum transfer roll 12. The throughdrying fabric carries the web over the throughdryer 13, which blows hot air through the web to dry it while preserving bulk. There can be more than one throughdryer in series (not shown), depending on the speed and the dryer capacity. The dried tissue sheet 15 is then transferred to a first dry end transfer fabric 16 with the aid of vacuum transfer roll 17. The tissue sheet shortly after transfer is sandwiched between the first dry end transfer fabric and the transfer belt 18 to positively control the sheet path. The air permeability of the transfer belt is lower than that of the first dry end transfer fabric, causing the sheet to naturally adhere to the transfer belt. At the point of separation, the sheet follows the transfer belt due to vacuum action. Suitable low air permeability fabrics for use as transfer belts include, without limitation, COFPA Mononap NP 50 dryer felt (air permeability of about 50 cubic feet per minute per square foot) and Asten 960C (impermeable to air). The transfer belt passes over two winding drums 21 and 22 before returning to pick up the dried tissue sheet again. The sheet is transferred to the parent roll 25 at a point between the two winding drums. The parent roll is wound onto a reel spool 26, which is driven by a center drive motor.

[0033] Particularly suitable methods of producing uncreped throughdried basesheets for purposes of this invention are described in U.S. Pat. No. 6,017,417 issued Jan. 25, 2000 to Wendt et al. and U.S. Pat. 5,944,273 issued Aug. 31, 1999 to Lin et al., both of which are herein incorporated by reference.

[0034] FIG. 2 illustrates a suitable method for applying the hydrophilically-modified amino-functional polydiorganosiloxane to the tissue basesheet. Shown is the parent roll 25 being unwound and passed through two calender nips between calender rolls 30aand 31aand 30band 31b. The calendered web is then passed to the rotogravure coating station comprising a first closed doctor chamber 33 containing the hydrophilically-modified amino-functional polydiorganosiloxane emulsion to be applied to a first side of the web, a first engraved steel gravure roll 34, a first rubber backing roll 35, a second rubber backing roll 36, a second engraved steel gravure roll 37 and a second closed doctor chamber 38 containing the hydrophilically-modified amino-functional polydiorganosiloxane emulsion to be applied to the second side of the web. If both sides of the web are to be treated, the two emulsions can be the same or different. The calendered web passes through a fixed-gap nip between the two rubber backing rolls where the hydrophilically-modified amino-functional polydiorganosiloxane emulsion is applied to the web. The treated web is then passed to the rewinder where the web is wound onto logs 40 and slit into rolls of bath tissue.

[0035] In order to further illustrate this invention, an uncreped throughdried tissue was produced using the methods described in FIGS. 1 and 2 and treated with a hydrophilically-modified amino-functional polydiorganosiloxane as set forth in structure (14) described herein above.

[0036] More specifically, a single-ply, three-layered uncreped throughdried bath tissue was made using eucalyptus fibers fOr the outer layers and softwood fibers for the inner layer. Prior to pulping, a quaternary ammonium softening agent (C6027 from Goldschmidt Corp.) was added at a dosage of 4.1 kg/Mton of active chemical per metric ton of fiber to the eucalyptus furnish. After allowing 20 minutes of mixing time, the slurry was dewatered using a belt press to approximately 32% consistency. The filtrate from the dewatering process was either sewered or used as pulper make-up water for subsequent fiber batches but not sent forward in the stock preparation or tissuemaking process. The thickened pulp containing the debonder was subsequently redispersed in water and used as the outer layer furnishes in the tissuemaking process.

[0037] The softwood fibers were pulped for 30 minutes at 4 percent consistency and diluted to 3.2 percent consistency after pulping, while the debonded eucalyptus fibers were diluted to 2 percent consistency. The overall layered sheet weight was split 20%/60%/20% among the eucalyptus/refined softwood/ eucalyptus layers. The center layer was refined to levels required to achieve target strength values, while the outer layers provided the surface softness and bulk.

[0038] A three layer headbox was used to form the wet web with the refined northern softwood kraft stock in the two center layers of the head box to produce a single center layer for the three-layered product described. Turbulence-generating inserts recessed about 3 inches (75 millimeters) from the slice and layer dividers extending about 1 inch (25.4 millimeters) beyond the slice were employed. The net slice opening was about 0.9 inch (23 millimeters) and water flows in all four headbox layers were comparable. The consistency of the stock fed to the headbox was about 0.09 weight percent

[0039] The resulting three-layered sheet was formed on a twin-wire, suction form roll, former with forming fabrics (12 and 13 in FIG. 1) being Lindsay 2164 and Asten 867a fabrics, respectively. The speed of the forming fabrics was 11.9 meters per second. The newly-formed web was then dewatered to a consistency of about 20-27 percent using vacuum suction from below the forming fabric before being transferred to the transfer fabric, which was travelling at 9.1 meters per second (30% rush transfer). The transfer fabric was an Appleton Wire T807-1. A vacuum shoe pulling about 6-15 inches (150-380 millimeters) of mercury vacuum was used to transfer the web to the transfer fabric.

[0040] The web was then transferred to a throughdrying fabric (Lindsay Wire T1205-1) previously described in connection with FIG. 2 and as illustrated In FIG. 9). The throughdrying fabric was travelling at a speed of about 9.1 meters per second. The web was carried over a Honeycomb throughdryer operating at a temperature of about 350° F. (175° C.) and dried to final dryness of about 94-98 percent consistency. The resulting uncreped tissue sheet was then wound into a parent roll.

[0041] The parent roll was then unwound and the web was calendered twice. At the first station the web was calendered between a steel roll and a rubber covered roll having a 4 P&J hardness. The calender loading was about 90 pounds per lineal inch (pli). At the second calendering station, the web was calendered between a steel roll and a rubber covered roll having a 40 P&J hardness. The calender loading was about 140 pli. The thickness of the rubber covers was about 0.725 inch (1.84 centimeters).

[0042] The calendered single-ply web was then fed into the rubber-rubber nip of the rotogravure coater to apply the hydrophilically-modified amino-functional polydiorganosiloxane emulsion to both sides of the web. The aqueous emulsion contained 40% amino-functional polydimethlysiloxane, 8.3% surfactant, 0.25% antifoaming agent, 0.2% acetic acid, 0.1% aloe, 0.1% Vitamin E, 0.05% preservative, and the balance water. The gravure rolls were electronically engraved, chrome over copper rolls supplied by Specialty Systems, Inc., Louisville, Ky. The rolls had a line screen of 200 cells per lineal inch and a volume of 6.0 Billion Cubic Microns (BCM) per square inch of roll surface. Typical cell dimensions for this roll were 140 microns in width and 33 microns in depth using a 130 degree engraving stylus. The rubber backing offset applicator rolls were a 75 Shore A durometer cast polyurethane supplied by American Roller Company. Union Grove, Wis. The process was set up to a condition having 0.375 inch interference between the gravure rolls and the rubber backing rolls and 0.003 inch clearance between the facing rubber backing rolls. The simultaneous offset/offset gravure printer was run at a speed of 2000 feet per minute using gravure roll speed adjustment (diferential) to meter the polysiloxane emulsion to obtain the desired addition rate. The gravure roll speed differential used for this example was 1000 feet per minute. This process yielded an add-on level of 3.0 weight percent total add-on based on the weight of the tissue. The tissue was then converted into bath tissue rolls. Sheets from the bath tissue rolls had a silky, lotiony hand feel and a Wet Out Time of 7.0 seconds. (Similarly made tissues without the treatment of this invention had a Wet Out Time of about 4.0 seconds.)

[0043] An uncreped throughdried tissue was produced similarly as described in Example 1 with the following exceptions: (1) prior to pulping, an amino functionalized polydiorganosiloxane (AF2340 from Kelmar Industries) was added to the eucalyptus fibers at a dosage of 2 kg/Mton of active chemical per metric ton of fiber; (2) the overall layered weight was split 30%/40%/30% among the eucalyptus/refined softwood/eucalyptus layers; (3) Parez 631 NC was added to the center layer at 24 kilograms per tonne of pulp based on the center layer; (4) the add-on level of the hydrophilically-modified amino-functional polydimethylsiloxane was 1.5 weight percent (5) the structure of the hydrophilically-modified amino-functional polydimethylsiloxane printed onto the tissue was as set forth in structure (10) herein above; and (6) the hydrophilically-modified amino-functional polydimethylsiloxane constituted 20 weight percent of the aqueous emulsion used to deliver the hydrophilically-modified amino-functional polydimethylsiloxane to the tissue. The resulting bath tissue product obtained had a silky, lotiony hand feel and a Wet Out Time of 4.8 seconds.

[0044] It will be appreciated that the foregoing example and discussion is for purposes of illustration only and is not to be construed as limiting the scope of this invention, which is defined by the following claims and all equivalents thereto.