Fibrous structures comprising acidic cellulosic fibers and methods of manufacturing the same
The invention relates to fibrous structures having desirable physical properties, such as good tensile strength, low stiffness and high bulk, manufactured using a fiber furnish comprising cellulosic fibers having a pH of 5.0 or less and at least one strength resin. Not only do structures prepared with acidic fibers have desirable physical properties, they may also be manufactured in an energy efficient manner. To achieve the greatest energy savings it is generally desirable that acidic fibers not be subjected to mechanical treatment, such as by refining, prior to forming the fiber into a fibrous structure. Further, it may be desirable to subject the remainder of the fiber furnish to a minimal degree of mechanical treatment, such as by refining, so as to produce a furnish having a freeness greater than about 550 mL.
1. A method of manufacturing a fibrous structure comprising the steps of:
a. providing a first fibrous furnish comprising acidic cellulosic fibers having a pH less than 4.5;
b. providing a second fibrous furnish comprising cellulosic fibers having a pH greater than about 6.0;
c. adding from about 1.0 to about 2.5 kilograms (kg) strength resin per metric ton (MT) of dry fibrous furnish to the first or the second fiber furnish;
d. depositing the first and second fibrous furnishes on a forming fabric to form a wet fibrous web;
e. partially dewatering the wet fibrous web; and
f. drying the fibrous web to a consistency greater than about 95 percent.
2. The method of claim 1 wherein the wet fibrous web has a Water Retention Value (WRV) from about 0.90 to about 1.40 g/g.
3. The method of claim 1 wherein the second fibrous furnish comprises cellulosic fibers selected from the group consisting of softwood fibers, hardwood fibers, secondary fibers and combinations thereof.
4. The method of claim 1 wherein the second fibrous furnish comprises softwood fibers having a freeness from about 500 to about 700 mL.
5. The method of claim 1 further comprising the step of refining the second fibrous furnish and wherein the refined second fibrous furnish has a freeness from about 500 to about 700 mL.
6. The method of claim 1 wherein the acidic cellulosic fibers comprise hardwood fibers selected from the group consisting of Acacia, Eucalyptus, Maple, Oak, Aspen, Birch, Cottonwood, Alder, Ash, Cherry, Elm, Hickory, Poplar, Gum, Walnut, Locust, Sycamore and Beech.
7. The method of claim 1 wherein the acidic cellulosic fibers have a Water Retention Value (WRV) less than about 1.20 g/g.
8. The method of claim 1 wherein the acidic cellulosic fibers have a Water Retention Value (WRV) from about 0.90 to about 1.10 g/g.
9. The method of claim 1 wherein the first fibrous furnish consists essentially of hardwood kraft pulp fibers having a pH from about 3.0 to 5.0 and a Water Retention Value (WRV) from about 0.90 to about 1.10 g/g.
10. The method of claim 1 wherein the first fibrous furnish is not subject to mechanical treatment and has a freeness from about 550 to about 750 mL.
11. The method of claim 1 wherein the strength resin is added to the second fibrous furnish.
12. The method of claim 1 wherein the wet fibrous web comprises from about 20 to about 80 percent, by dry weight of the wet fibrous web, acidic cellulosic fibers-having a pH less than 4.5 and from about 20 to about 80 percent, by dry weight of the wet fibrous web, cellulosic fibers having a pH greater than about 6.0.
13. The method of claim 1 wherein the strength resin is a modified starch or carboxymethyl cellulose resins and the amount of strength resin added to the first or the second fiber furnish is from about 1.0 to about 2.5 kg per metric ton of dry fiber.
14. The method of claim 1 wherein the strength resin is selected from the group consisting of polyamide-polyamine epichlorohydrin resins, glyoxalated polyacrylamide resins, carboxymethyl celluloses, starch, starch derivatives, and combinations thereof, and the amount of strength resin added to the first or the second fiber furnish is from about 1.0 to about 2.5 kg per metric ton of dry fiber.