Patent Publication Number: US-2007107862-A1

Title: Sodium sulfate treated pulp

Description:
FIELD OF THE INVENTION  
      The present invention relates to cellulose pulp that has been treated with sodium sulfate and to methods for applying sodium sulfate to cellulose pulp.  
     BACKGROUND OF THE INVENTION  
      Cellulose fibers have found widespread application in absorbent articles, such as diapers and feminine hygiene products. The cellulose fibers are generally used as an absorbent medium to acquire, transport, and hold fluids. While cellulose fibers are effective at acquiring, transporting, and holding fluids, many improvements to cellulose fibers have been made over the past decades to improve the performance properties of cellulose fibers in absorbent products. For example, U.S. Pat. Nos. 6,340,411 and 5,547,541 describe that webs of cellulose fibers treated with certain polymeric and nonpolymeric materials require less pressure to densify a web of the fibers to a given density as compared to the pressure needed to densify a similar web of fibers without the polymeric or nonpolymeric material present.  
      The cellulose fibers treated with the compositions described in U.S. Pat. No. 5,547,541 are manufactured by applying the desired compositions to a wet laid web of cellulose fibers which has been produced, for example, using a Fourdrinier machine. The treated wet laid web of cellulose fibers is generally formed into a roll for bulk delivery to an absorbent product manufacturer. The absorbent product manufacturer typically unrolls the roll and processes the web in a fiberization unit that individualizes the fibers and prepares them for further processing.  
      Absorbent products including an absorbent core of superabsorbent material and cellulose fibers are typically manufactured by a process that combines cellulose fibers and superabsorbent material. In such a process, rolls or bales of cellulose fibers without superabsorbent material are fiberized by a fiberizing apparatus such as a hammermill. These fiberized cellulose fibers are entrained in air and superabsorbent material is introduced to the air entrained fibers. The air entrained combination of cellulose fibers and superabsorbent material is delivered to an air lay device such as a pad former, which draws the fibers and superabsorbent material onto a screen and forms the fibers and superabsorbent material into a particular shape. These formed pads are then removed from the pad former for further processing, including subjecting the formed pads to compression in order to densify the pad by decreasing its thickness.  
      Reducing the thickness of the formed pads which are used in diapers is important to diaper manufacturers so that they can reduce the size of packaging which allows them to ship more diapers per volume and to display a larger number of diapers in a limited amount of shelf space. In addition, consumers find thinner diapers more desirable.  
      With this background, the present inventors have worked to address the challenges above and have developed compositions that can be compressed to achieve articles of desirable densities and methods of providing and utilizing such compositions.  
     SUMMARY OF THE INVENTION  
      The present invention provides cellulose pulp sheets and cellulose fibers treated with sodium sulfate that are useful in absorbent cores formed from the treated pulp or fibers. The compositions of the present invention can be formed into absorbent articles for absorbing fluids such as aqueous fluids like urine or blood. The compositions are useful in absorbent articles such as diapers, incontinent devices and feminine hygiene products. The compositions of the present invention can be compressed to densities that manufacturers of absorbent articles should find desirable.  
      In one aspect, the present invention relates to a cellulose pulp sheet that includes cellulose fibers, water and sodium sulfate applied to the cellulose fibers in an amount ranging from about 0.1 to 15 weight percent based on dry fiber weight. The cellulose pulp sheet can be fiberized into individualized fibers, laid into a pad, and then compressed. The invention also relates to a method for producing a cellulose pulp sheet which includes the steps of providing cellulose pulp, forming a cellulose pulp sheet from the cellulose pulp, and applying sodium sulfate to the cellulose pulp sheet.  
      In another aspect, the present invention relates to a method for producing a densified web of cellulose fibers that includes the step of providing cellulose fibers treated with sodium sulfate. The treated cellulose fibers are fiberized and formed into a web. The web is compressed to form a densified web.  
      In yet another aspect, the present invention relates to fibers that have been treated with sodium sulfate. The cellulose fiber before treatment with sodium sulfate exhibit a first density after application and release of a compression load. The treated fibers include water and sodium sulfate. The cellulose fibers after treatment with sodium sulfate densify to a second density after application and release of the compression load. The sodium sulfate is present in an amount so that the second density is at least 5 percent greater than the first density.  
      The sulfate treated fibers of the present invention may be further treated with an oil in order to provide fibers that retain materials such as superabsorbent materials within a web of the fibers.  
      Manufacturers of absorbent articles will find the sodium sulfate treated pulp and fibers of the present invention useful in their absorbent products due to the densification properties of cellulose pulp fibers treated in accordance with the present invention. The methods of the present invention provide suitable means for producing the treated cellulose pulp fibers that exhibit densification properties that absorbent article manufacturers should find desirable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
       FIG. 1  is a graph illustrating the results of compression testing to determine the densification properties of absorbent structures containing cellulose fibers treated in accordance with the present invention;  
       FIG. 2  is a graph illustrating the results of compression testing to determine the densification properties of structures containing cellulose fibers treated in accordance with the present invention; and  
       FIG. 3  is a schematic illustration of a wet laid web manufacturing line illustrating the application of sodium sulfate to a wet laid web of cellulose fibers in accordance with the present invention.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      As used herein, the term “fiber” refers to natural or synthetic fibers. Such fibers may be physically pretreated, e.g., by subjecting the fibers to steam, or chemically treated, e.g., by crosslinking the fibers. The fibers may also be twisted or crimped as desired.  
      A particular type of fiber are cellulose fibers. A particular example of a cellulose fiber is wood pulp fiber. Wood pulp fibers can be hardwood pulp fibers or softwood pulp fibers. The cellulose pulp fibers may be chemical, thermomechanical, chemithermomechanical or combinations thereof. Such wood pulp fibers can be obtained from well known chemical processes such as the kraft or sulfite processes. Other cellulose fibers include lyocell, linen, chopped silk fibers, bagasse, hemp, jute, rice, wheat, bamboo, corn, sisal, cotton, flax, kenaf, peat moss, and mixtures thereof. When the fibers are cellulose fibers, they may be pretreated with chemicals to result in lignin or cellulose-rich fiber surfaces. In addition, the fibers may be bleached.  
      Examples of synthetic fibers include acrylic, polyester, carboxylated polyolefin, and polyamine fibers.  
      Sodium sulfate (Na 2 SO 4 ) is available in the form of white crystals or powder from numerous commercial sources.  
      As used herein, the term “superabsorbent material” refers to polymers that swell on exposure to water and form a hydrated gel (hydrogel) by absorbing large amounts of water. Superabsorbent materials exhibit the ability to absorb large quantities of liquid, i.e., in excess of 10 to 15 parts of liquid per part thereof. These superabsorbent materials generally fall into three classes, namely starch graft copolymers, crosslinked carboxymethylcellulose derivatives and modified hydrophilic polyacrylates. Examples of such absorbent polymers are hydrolyzed starch-acrylonitrile graft copolymer, a neutralized starch-acrylic acid graft copolymer, a saponified acrylic acid ester-vinyl acetate copolymer, a hydrolyzed acrylonitrile copolymer or acrylamide copolymer, a modified crosslinked polyvinyl alcohol, a neutralized self-crosslinking polyacrylic acid, a crosslinked polyacrylate salt, carboxylated cellulose, and a neutralized crosslinked isobutylene-maleic anhydride copolymer.  
      Superabsorbent particles are available commercially, for example starch graft polyacrylate hydrogel fines (IM 1000F) from Hoechst-Celanese of Portsmouth, Va., or larger particles such as granules. Other superabsorbent particles are marketed under the trademarks SANWET (supplied by Sanyo Kasei Kogyo Kabushiki Kaisha), SUMIKA GEL (supplied by Sumitomo Kagaku Kabushiki Kaisha and which is emulsion polymerized and spherical as opposed to solution polymerized ground particles), FAVOR (supplied by Stockhausen of Greensboro, N.C.), and NORSOCRYL (supplied by Atochem).  
      The term oil as used generally applies to a wide range of substances. Oils may be derived from animals or from plant seeds or nuts, and these types of oils tend to be chemically identical with fats, with the only difference being one of consistency at room temperature. Animal and plant oils are composed largely of triglycerides of the fatty acids, oleic, palmitic, stearic, and linolenic acid. Oils may also be derived from petroleum sources. Petroleum-based oils generally include a mixture of hydrocarbons. As used herein, the term “oil” refers to oils that have melting points below the temperature at which the oil is applied to the fibers as described below in more detail. Such temperature will generally be below 25° C., but could be higher. If the melting point of the oil is greater than the ambient temperature at which the oil is applied to the fibers, the oil can be heated to liquefy it. This ensures that the oils remain liquid during their application to the fibers. Oils useful in the present invention should also have a vapor pressure sufficiently low to prevent evaporation either during their application or during use.  
      The oil should not penetrate the walls of the fibers so rapidly that it becomes unavailable to retain the superabsorbent material when superabsorbent material is contacted with the oil treated fibers. The oil preferably resides on the surface of the fibers during the useful life of the absorbent article made from the fibers. To that end, oils of higher molecular weight penetrate the fiber wall more slowly than oils of a lower molecular weight.  
      Examples of “oils” as that term is used herein include fats and their component fatty acids. As described above, fats are naturally occurring esters of long chain carboxylic acids and the triol glycerol. These esters are also referred to as triglycerides. The hydrolysis of fats yields glycerol and three component carboxylic acids. These straight chain carboxylic acids which may be obtained from the hydrolysis of fats are called fatty acids and include one carboxylic acid group. Fatty acids may be saturated or unsaturated. The most common saturated fatty acids are lauric acid, myristic acid, palmitic acid, and stearic acid. Other fatty acids include oleic acid, linoleic acid, and linolenic acid. Generally, the melting point of a fat depends on the amount of unsaturation in the fatty acids. Fats with a preponderance of unsaturated fatty acids generally have melting points below about 25° C. Specific examples of oils as that term is used herein include soybean oil, cottonseed oil, linseed oil, tung oil, castor oil, coconut oil, olive oil, canola oil, safflower oil, corn oil or jojoba oil. Jojoba oil is a light yellow liquid at room temperature that is not technically an oil or fat, but rather is a wax. A wax is an ester of fatty acids with long chain monohydric alcohols. The term oil as used herein is intended to include jojoba oil and other waxes that are liquid at temperatures that they are applied to fibers. It should be understood that the foregoing is a list of exemplary oils and that oils useful with the sodium sulfate treated fibers of the present invention are not necessarily limited to the foregoing oils. It should be understood that use of the term “oil” in this application refers not only to the oil itself comprising a mixture of various fat and fatty acid components, but also includes the individual isolated fats, and the isolated fatty acids that result when the fats are hydrolyzed. For example, the term “oil” as used herein also refers to the fatty acids oleic, palmitic, stearic, and linolenic, that form the most common triglycerides in many oils derived from animals and plants and would be useful to retain superabsorbent material in an absorbent structure comprising oil-treated fibers and superabsorbent material.  
      The term “oil” as used herein also refers to unsubstituted alkanes, alkenes, alkynes, cycloalkanes, cycloalkenes, cycloalkynes, aromatics, and mixtures thereof derived from petroleum or animal sources that have melting points below the temperature at which the oil is applied to the fibers, e.g., about 25° C. Such oils are generally derived from petroleum sources, but may also be derived from animal sources. Oils of this type should have vapor pressure sufficiently low to prevent evaporation of the oil during application or use. Specific examples of these types of oils include mineral oil, paraffin oil, hexadecane, squalane, and squalene.  
      As used herein, mineral oil is an example of a highly refined liquid petroleum derivative. Mineral oil is light, clear, colorless, and odorless and is also referred to as medicinal oil. Mineral oil is used medicinally as an internal lubricant and for the manufacture of salves and ointments.  
      Paraffin oil is an example of an oil that is either pressed or dry distilled from paraffin distillate obtained from the distillation of petroleum.  
      Squalane is an example of an alkane derived from animal sources, such as the sebum. Squalene is an example of an alkene; more specifically, a terpene derived from animal sources, such as the human sebum or shark liver oil. Squalene may also be isolated from oils derived from plants, such as olive oil, wheat germ oil, rice bran oil, and yeast.  
      In accordance with the present invention, the amount of sodium sulfate added to the cellulose pulp sheet can vary over a wide range. Amounts of sodium sulfate solids in the treated cellulose sheets can range from about 0.1 wt. % to 15 wt. % based on the dry fiber. weight. A narrower range of amounts is about 1.0 to 10.0 wt. % based on dry fiber weight, and an even narrower range is about 1 to 7 wt %. These amounts of sodium sulfate can be provided in the treated cellulose sheets by applying an aqueous solution of sodium sulfate. The amount of sodium sulfate in the aqueous solution can vary over a wide range. Preferably, the amount of sodium sulfate in the aqueous solution that is applied to the cellulose pulp sheet is chosen such that a desired level of loading of sodium sulfate solids onto the cellulose pulp sheet is achieved without the water content of the cellulose fiber sheet rising above about 15 to 20 wt. % based on the total product weight. If the water content of the pulp sheet is excessive, the pulp sheet is difficult to fiberize and may be susceptible to premature degradation, such as from mold growth or rotting. In certain embodiments, when the aqueous solution of sodium sulfate is applied to the pulp sheet before the pulp sheet is dried, larger amounts of water can be introduced into the pulp sheet provided that the subsequent drying steps reduce the water content of the pulp sheet to a level below about 20 wt. % based on the total product weight. Sufficient amounts of sodium sulfate solids should be added to the cellulose pulp sheet so that when the cellulose pulp sheet is fiberized the resulting fibers exhibit densification properties that are superior to the densification properties of fibers that have not been treated with sodium sulfate.  
      Aqueous solutions containing from 5 to 33 wt. % sodium sulfate are useful in accordance with the present invention. Aqueous solutions containing from about 5 to 30 wt. % or about 5 to 27 wt. % sodium sulfate are preferred because it has been observed that the percent increase in the density of the fibers when the aqueous solution contains about 25 wt. % sodium sulfate is greater than when the aqueous solution contains about 33 wt. % sodium sulfate.  
      The sodium sulfate solution can be applied to the cellulose pulp sheet in a number of different ways. The present invention is not limited to any particular application technique. Examples of suitable application techniques include spraying, rolling, dipping, and the like. The solution can be applied to one or both sides of the cellulose pulp sheet. Alternatively, the solution can be applied to fibers that are not in sheet form, e.g., individualized fibers. The solution can be heated prior to its application, although this is not required. Alternatively, the cellulose pulp sheet can be at a temperature above room temperature when the sodium sulfate solution is applied. In view of the decreasing solubility of sodium sulfate in water as the temperature of the solution decreases, in certain embodiments, particularly those where the concentration of the sodium sulfate in the solution is near its solubility limit, it is advantageous to preheat the solution or the pulp sheet in order to reduce crystallization of sodium sulfate from the solution during or right after its application. Heating the aqueous solution of sodium sulfate or heating the cellulose pulp sheet prior to application of the sodium sulfate solution is one means for introducing more sodium sulfate into the web.  
      As described above, when the sodium sulfate treated fibers are subjected to and released from a compression load, they densify to a density that is higher than the density that is achieved when fibers that have not been treated with sodium sulfate are subjected to the same compression loading and releasing. In some instances, the density is increased  10  % or more.  
      As described above, oil can be applied to the sodium sulfate treated fiber. The particular way that oil is applied to the fibers is not critical. Examples of techniques for applying oil to the fibers include the use of a gravure-type roll coater to coat a web of the fibers. Alternatively, oil can be sprayed onto a web of the fibers or the fibers can be immersed in a bath of oil. The oil may also be added to the fibers as a web of the fibers is being broken up, such as in a hammermill. The amount of oil applied to the fibers should be sufficient to achieve retention of superabsorbent material, but not so much as to have a significant adverse affect on the fluid absorption properties of the fibers, such as the fluid acquisition rate or the amount of fluid absorbed by a web of the fibers. Manufacturers of absorbent articles that include absorbent structures containing oil-treated fibers desire that the fluid absorption properties of such structures be similar to or superior to the fluid absorption properties of the absorbent structures that the manufacturer is considering replacing. Ideally, the absorbent structures would exhibit fluid acquisition properties that are at least as desirable as the fluid acquisition properties of similar absorbent structures manufactured from fibers that have not been treated with oil. The amount of oil applied to the fibers should also not be so great that it adversely impacts the fiberization of the web of oil-treated fibers. Suitable amounts of oil applied to the fibers include about 0.5 wt. % to about 20 wt. % oil based on the weight of oven dried fibers. A narrower range is 1.0 wt. % to about 15 wt. % oil based on the weight of oven dried fibers and an even narrower range is 1.0 wt. % to about 10 wt. % oil based on the weight of oven dried fibers.  
      The form of the fibers to which the oil is applied can vary. If a roll coater is used, the fibers can be in the form of a sheet of fibers. For example, the oil can be applied to a wet laid sheet of fibers having a basis weight of at least 350 grams per meter 2  and a density of at least about 400 kg/meter 3 .  
      The oil may be added neat, or it may be diluted with solvent that evaporates after application of the oil to the fibers. The solvent should not adversely affect the attachment of superabsorbent material to the fibers or the fluid acquisition and fluid retention properties of an absorbent article that contains the oil treated fibers.  
       FIG. 3  illustrates a wet laid sheet manufacturing line such as a wood cellulose pulp sheet manufacturing line  10 . In this manufacturing line, a pulp slurry  12  is delivered from a headbox  14  through a slice  16  and onto a Fourdrinier wire  18 . The pulp slurry  12  typically includes wood pulp fibers and may also include synthetic or other non-cellulose fibers as part of the slurry. Water is drawn from the pulp deposited on wire  18  by a conventional vacuum system, not shown, leaving a deposited pulp sheet  20  which is carried through a dewatering station  22 , illustrated in this case as two sets of calendar rolls  24 ,  26  each defining a respective nip through which the pulp sheet or mat  20  passes. From the dewatering station, the pulp sheet  20  enters a drying section  30 . In a conventional pulp sheet manufacturing line, drying section  30  may include multiple canister dryers with the pulp mat  20  following a serpentine path around the respective canister dryers and emerging as a dried sheet or mat  32  from the outlet of the drying section  30 . Other alternate drying mechanisms, alone or in addition to canister dryers, may be included in the drying stage  30 . The dried pulp sheet  32  has a maximum moisture content pursuant to the manufacturer&#39;s specifications. Typically, the maximum moisture content is no more than 10% by weight of the fibers and most preferably no more than about 6% to 8% by weight. Unless overly damp fibers are immediately used these fibers are subject to degradation by, for example, mold or the like. The dried sheet  32  is taken up on a roll  40  for transportation to a remote location, that is, one separate from the pulp sheet manufacturing line, such as at a user&#39;s plant for use in manufacturing products. The dried pulp sheets have a basis weight of about 200 g/m 2  to about 1000 g/m 2  or more and a density on the order of at least about 0.5 g/cm 3  to about 1.2 g/cm 3 . Dried pulp sheets having the foregoing basis weights are structurally distinct form lighter basis weight sheets of wet laid or airlaid wood pulp fibers such as tissue paper, paper towels, or other types of paper-like wet laid or airlaid webs of cellulose fibers. Alternatively, the dried sheet  32  is collected in a baling apparatus  42  from which bales of the pulp  44  are obtained for transport to a remote location.  
      The sodium sulfate solution can be applied to the pulp sheet from one or more applying devices, one of which is indicated at  50  in  FIG. 3 . Any applying device may be used, such as streamers, sprayers, roll coaters, curtain coaters, immersion applicators, or the like. Sprayers are typically easier to utilize and incorporate into a pulp-sheet manufacturing line. As indicated by the arrows  52 ,  54 , and  56 , the sodium sulfate may be applied at various locations or at multiple locations on the pulp sheet manufacturing line, such as ahead of the drying stage  30  (indicated by line  52 ), intermediate the drying stage  30  (as indicated by line  54 ), or downstream from the drying stage  30  (as indicated by the line  56 ). At location  52 , the water remaining in the sheet or mat  20  at this stage tends to interfere with the penetration of the materials into the sheet. Consequently, application of the sodium sulfate solution after some drying has taken place, for example at location  54 , is preferable. If the sodium sulfate solution is applied at location  56  in an amount which would cause the moisture content of the sheet to exceed the desired maximum level, an additional drying stage (not shown) may be included in the pulp manufacturing line to bring the moisture content down to the desired level.  
      The oil can be applied to the pulp sheet from the same types of devices and locations as described above with respect to the sodium sulfate solution.  
      The rolls  40  or bales  44  of the treated wet laid web of fibers may be transported to a remote location for use by a user. These rolls or bales are then refiberized by a fiberizing device, such as a hammermill which may be used alone or in conjunction with other devices such as picker rolls or the like for breaking up the sheet  32  or bales  42  into individual fibers. Depending on the end use, the individualized fibers may be combined with particulate material, such as superabsorbent particles, and/or airlaid into a web and densified.  
      With this approach, the end user of the treated fibers may readily select particles to be combined with the fibers. The user has flexibility in air laying or otherwise processing the treated fibers of the present invention into a finished product.  
      The treated fibers and superabsorbent material can be combined and then formed into an absorbent structure in the following manner. Rolls or bales of treated fibers, without particles, are fiberized by a fiberizing device such as a hammermill. The individualized fibers are air entrained during which time the superabsorbent material can be added thereto. The air entrained fibers and superabsorbent material are then delivered to an air laying device, such as a pocket former, and formed into a desired shape. The formed pad is removed from the air laying device for further processing, including subjecting the pad to a compression load to reduce the thickness of the pad and increase its density. The formed pads are in the form of a web or mass of fibers used as absorbent structures in absorbent articles such as the ones discussed above.  
      It should be understood that in an alternative embodiment, the sodium sulfate solution and oil can be applied to the fibers while they are air entrained.  
      As illustrated in the examples that follow, fibers treated with a sodium sulfate solution in accordance with the present invention exhibit desirable densification properties.  
      The following examples are intended to illustrate certain embodiments of the present invention and are not intended to limit the scope of the present invention.  
     EXAMPLE 1  
     Preparation of Sodium Sulfate Treated Cellulose Pulp  
      Southern Pine wood cellulose pulp sheet available from Weyerhaeuser Company under the designation NB 416 from New Bern, N.C. with a starting moisture content of 6% by weight (based on total sheet weight) was brought to a temperature of 120-140° F. by storing in a zippered plastic bag in an oven. The pulp sheet was then quickly removed from the bag and coated in a Black Brothers gravure-type roll coater with a solution of sodium sulfate. The gravure coater results in the application of a uniform coating of the sodium sulfate solution over one entire surface of the pulp sheet from where it is rapidly soaked up by the sheet. The sodium sulfate was obtained from Sigma-Aldrich (CAS number 7757-82-6 anhydrous sodium sulfate, 99% reagent grade). The sodium sulfate solution had a solids content of 24.8% with the balance being water. The sodium sulfate is dissolved in water at 33° C. This 24.8 wt. % solution was applied to the wood pulp sheet at a rate of 10.5 parts by weight solution to 100 parts by weight of pulp sheet, resulting in a loading of active (dry basis) sodium sulfate solids of ˜2.8 wt. % based on the dry fiber content of the pulp sheet. The final total moisture content of the wood pulp cellulose sheet treated with sodium sulfate is ˜12.6 wt. % based on the total final product weight.  
      The treated sheet was stored in a plastic zippered bag for 24 hours at room temperature to allow the added moisture to migrate and reach equilibrium within the whole sheet. The sheet was then fiberized in a laboratory hammermill and the resultant fluff was fed to a rotary pocket former and resulted in fluff pads measuring ˜12″×5″ with a basis weight of ˜300 grams per square meter (gsm). The pads were placed in zippered plastic bags to preserve moisture until used in the densification test below.  
      After removal from the bags, the rectangular 12″×5″ pads were cut into smaller square pads measuring 10 cm×10 cm using a die. The 10×10 cm pads were densified in a hydraulic flat press under loads of either 50 psi, 100 psi, and 150 psi. The pressure was only held momentarily and then released. Different pads were used for each of the successively higher loads. Caliper (thickness) of the pads was determined using a caliper gauge with a wide “foot” designed to apply only moderate pressure to the pad of ˜0.2 psi (i.e., it does not materially densify the pad in the act of determining caliper). The densities of the pads were calculated from the caliper and basis weight measurements.  
      Results of the density measurements versus applied pressure are shown in  FIG. 1  and show that the sodium sulfate treated pulp (2.8% loading from 24.8 wt. % solution) attains a higher density (up to about 14%) for a given pressure compared to the untreated NB416 pulp having a moisture content of 6 wt. % which was incorporated as a control.  
     EXAMPLE 2  
     Preparation of Sodium Sulfate Treated Cellulose Pulp  
      Example 2 was identical to Example 1 in every respect except that the 24.8 wt. % solution of sodium sulfate was applied to the wood pulp sheet at a rate of 21.6 parts by weight solution to 100 parts by weight of pulp sheet, resulting in a loading of active on a (dry basis) sodium sulfate solids of ˜5.7 wt. % based on the dry fiber content of the pulp sheet. The final total moisture content of the wood pulp cellulose sheet treated with sodium sulfate is ˜18.3 wt. % based on the total final product weight.  
      Again, results of the density measurements versus applied pressure are shown in  FIG. 1  and show that the sodium sulfate treated pulp (5.7% loading from 24.8 wt. % solution) attains a higher density (up to about 25.7%) for a given pressure compared to the untreated NB416 pulp having a moisture content of 6 wt. % which was incorporated as a control. The data also shows that the higher loading of sodium sulfate at higher final moisture content is advantageous.  
     EXAMPLE 3  
     Preparation of Sodium Sulfate Treated Cellulose Pulp  
      Several air dry (˜6% moisture content)  4 ″ width strips of NB416 pulp sheet weighing about 40 g were brought to a temperature of 120-140° F. by placing in zippered plastic bags in an oven. The strips were quickly removed from the oven and, whilst hot, were treated with 5.7 g of a 33 wt. % solution of sodium sulfate solution that had been preheated to 33° C. Application of the solution was made to one side of the wood pulp sheet using a syringe. Liquid was applied in lines along the full length of the machine (long) direction of the paper and were spaced apart by approx. ¼″. This resulted in a loading of active (on a dry basis) sodium sulfate solids of ˜5.0 wt. % based on the dry fiber content of the pulp sheet. The final total moisture content of the wood pulp cellulose sheet treated with sodium sulfate is ˜13.61 wt. % based on the total final product weight. Control strips of NB416 having a moisture content of 6 wt. % (no sodium sulfate addition) were also included in the fiberization and densification procedures that follow below.  
      The pulp strips thus treated are placed in zippered plastic bags for 24 hours and then removed and fiberized using a laboratory Fitz hammermill. Resultant fluff was stored for about 16 hours in a room at 50% RH (in bags with the tops open) and then is formed (using a laboratory pad former) into 6″ diameter round pads of ˜300 gsm basis weight from which 10 cm×10 cm square pads are cut and subject to the densification procedure as described in Example 1. Density of the resultant pads is shown in  FIG. 2 .  
      Results illustrated in  FIG. 2  show that the sodium sulfate treated pulp (5.0% loading from 33 wt % solution) attains a higher density for a given pressure compared to the untreated NB416 pulp which was incorporated as a control. However, in comparison with  FIG. 1  it is clear that the level of density increase over the control is reduced when using a 33% sodium sulfate solution versus the 24.8% solution. In the case of the 33% solution (at 5% chemical add-on) the increase at 150 psi is ˜4% higher than the control (0.17 g/cc vs. 0.163 g/cc) whereas in the case of the 24.8% solution of Example 2 (at a similar 5.7% chemical add-on) the density at 150 psi is 25.7% higher than the control (0.176 g/cc vs. 0.14 g/cc). It should be pointed out that in these types of experiments the density that the control pulp attains varies considerably with the type of fiberizer, pad former and ambient humidity conditions. Given this, each experiment had its own internally consistent control as the basis for judging the performance of treated pulps.