Method and apparatus for crosslinking individualized cellulose fibers

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BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of making intrafiber crosslinked cellulose and the product resulting from the process. The invention is especially directed to a crosslinked cellulose having a high wet and dry resilience, high bulk, and superior absorbency.

2. General Discussion of the Background

It is known in the art that resilient bulking fibers are useful for the preparation of bulky and more absorbent paper structures. Such paper structures are useful for the manufacture of products such as handsheets, towels, tissues, filters, paperboard, diapers, sanitary napkins, hospital dressings and the like. Crosslinked cellulose materials may be generated by reacting cellulose fibers with crosslinking agents which are capable of combining with at least two hydroxyl groups within a single cellulose molecule, or between adjacent cellulose molecules. The crosslinking agent must be difunctional such that it will react with at least two of the hydroxyl groups in the cellulose molecule to form the crosslink.

One method for obtaining resilient bulking fibers is by crosslinking cellulose fibers by treatment with a chemical compound. U.S. Pat. No. 3,819,470 discloses cellulosic fibers having a substantive polymeric compound reacted with and attached to the fibers. The modified fibers are characterized by reduced swellability and a reduced capability of natural fiber-to-fiber bonding when compared to unmodified cellulosic fibers. U.S. Pat. No. 4,431,481 discloses modified cellulosic fibers produced by treating the fibers with copolymers of maleamic acid. Other known techniques include treatment of fibers with cationic urea formaldehyde resins, (U.S. Pat. No. 3,756,913), methylol ureas and melamines (U.S. Pat. No. 3,440,135), formaldehyde (U.S. Pat. No. 3,224,926), with the condensation product of acrolein and formaldehyde, (U.S. Pat. No. 3,183,054), bis-acrylamides (Eur. Patent No. 213,415), and treatment with glyoxal or glutaric dialdehyde (WO 88104704, U.S. Pat. Nos. 4,822,453, and 4,853,086). Copending U.S. patent application Ser. No. 07/607,268 discloses a crosslinking process in which the crosslinking agent is dimethyldihydroxy-ethylene urea (DMDEU).

A drawback of many of these prior crosslinking agents is that they are inefficient crosslinkers or are toxic. The problem of toxicity is a particular concern with formaldehyde crosslinkers. Formaldehyde is toxic when inhaled, and can be strongly irritating to the skin and mucus membranes. Concerns have also been expressed that formaldehyde is teratogenic and carcinogenic. Public anxieties about environmental safety and occupational hazards have provided a special impetus to find new, non-formaldehyde crosslinkers.

Three techniques have generally been used to produce intrafiber crosslinked material. They are dry crosslinking, aqueous crosslinking, and crosslinking in a non-aqueous solution. In the dry crosslinking process, the cellulose fibers are crosslinked while in an unswollen, collapsed state. Dry crosslinked fibers are stiffened by crosslink bonds, such that absorbent structures made from the fibers have high wet and dry resilience, and low fluid retention. Aqueous solution crosslinked fibers are produced by crosslinking fibers in an aqueous solution, such that the swelling effect of water causing the fibers to be crosslinked in a swollen condition. Compared to dry crosslinked fibers, aqueous crosslinked fibers have increased flexibility, reduced stiffness, higher fluid retention, and lower wet and dry resilience. Nonaqueous crosslinking occurs when individualized, dehydrated, nonswollen fibers are contacted with a crosslinking agent in a substantially nonaqueous solution. The resulting fibers are stiff and exhibit high wet and dry resilience.

An example of using dry crosslinking technology is U.S. Pat. No. 3,440,135 to Chung. This patent discloses a technique of pre-soaking cellulose fibers in an aqueous solution of a crosslinking agent to reduce interfiber bonding. The treated fibers are then aged prior to carrying out a drying stage, in which the fibers are heated to effect crosslinking. The Chung patent suffers from the drawback that the wet fiber mat must be stored between 16 and 48 hours, in order to minimize nit formation resulting from incomplete difiberization.

Another example of dry crosslinking technology is U.S. Pat. No. 3,224,926 to Bernardin. That patent describes treating cellulosic material with a crosslinking agent such as formaldehyde or dimethylolurea. Individualized, crosslinked fibers are produced by impregnating swollen fibers in an aqueous solution with a crosslinking agent, dewatering and then mechanically defiberizing the fibers, and then drying the fibers at an elevated temperature to crosslink the fibers while they are substantially individualized. The fibers are crosslinked in an unswollen, collapsed state as a result of being dehydrated prior to crosslinking. The products made by this dry crosslinking process exhibit high absorbency and high wet and dry resilience.

An example of an aqueous crosslinking process is U.S. Pat. No. 3,241,533 to Steiger, in which the cellulose fibers are crosslinked in an aqueous solution with a crosslinking agent and a catalyst. The product made from this process was said to exhibit high fluid retention and great flexibility compared to a product made from a dry crosslinking process. Finally, an example of a nonaqueous crosslinking process is U.S. Pat. No. 4,035,147 to Sangenis et al. In this process, the lack of water present in the solution keeps the cellulose fibers in a state similar to that in the dry crosslinking process. While in the nonaqueous solution, the cellulose fibers are crosslinked with a crosslinking agent and a catalyst. Like dry crosslinked fibers, the nonaqueous crosslinked fibers are very stiffened by crosslink bonds, and absorbent materials made from these fibers have high wet and dry resilience.

Various devices are known in the art for treating fibers with crosslinking agents in mat form and thereafter breaking the mats into individual fibers. For example, U.S. Pat. No. 3,440,135 to Chung discloses a mechanism for applying a crosslinking agent to a cellulosic fiber mat. The mat is then aged and passed (while still wet) through a fiberizer, such as a hammermill to fiberize the mat. The resulting loose fibers are then dried in a two stage dryer. The first dryer stage is at a temperature sufficient to flash water vapor from the fibers and the second dryer stage is at a temperature that cures the crosslinking agent. A cyclone separator then separates the fibers from the gas for subsequent collection. The Chung apparatus suffers from the drawback of requiring the inconvenient and costly storage of wet fiber mats (e.g. in roll form) for a substantial period of time in order to minimize nit formation.

Unfortunately, fiberization processes known in the art which employ currently available fiberizing or comminution machinery yield crosslinked fibers that have too many nits and knots to be acceptable for many uses. A probable reason is that such machinery has excess dead space where fibers are excessively pressed together and/or has localized regions of elevated temperature hot enough to cause premature curing of the crosslinking agent while fibers are in intimate contact with each other. Since fiberization is performed on a mat that is still wet with the uncured crosslinking agent, dead spaces and hot spots in the fiberizer would encourage the formation of interfiber bonds, which form nits, that virtually cannot be broken by downstream equipment.

Interfiber bonding in a conventional fiberizer apparatus can also lead to production of excessive amounts of "fines," which are undesirably short fibers due principally to fiber breakage. Crosslinking imparts substantial brittleness to cellulose fibers, which thereby exhibit limited compliance to mechanical stresses. Nits are especially susceptible to mechanical stresses because of their density which is much greater than the density of individual fibers. Excess fiber breakage and fines not only degrade absorbency but can substantially reduce the loft and resiliency of a product made from crosslinked fibers.

Hence there is a need for a process of producing a product made of individualized crosslinked cellulose fibers that have minimal nits and knots. It is therefore an object of the invention to produce treated fibers, such as intrafiber crosslinked cellulose, having a nit level lower than levels obtainable with existing equipment. There is also a need for such an apparatus that will produce fibers from a mat comprised of crosslinked cellulose while not causing significant breakage of individual fibers of the mat.

It is yet another object to provide crosslinking agents that are less toxic and provide a product having high wet and dry resilience, high bulk, and superior absorbance.

Finally, it is an object to provide a crosslinking process that operates at a pH that is compatible with standard unmodified papermaking equipment.

These and other objects of the invention will be understood more clearly by reference to the following detailed description and drawings.

SUMMARY OF THE INVENTION

The foregoing objects are achieved by providing a method of forming an intrafiber crosslinked cellulose product in which cellulose fibers or individualized cellulose fibers are exposed to an aqueous solution comprising a catalyst and a crosslinking agent selected from the group consisting of cyclic N-sulfatoimide; a mixture of glyoxal and imidazolidone; a periodate or salt thereof; dimethoxyethanal and OH--R.sub.1 --R.sub.2 --R.sub.1 --OH wherein R.sub.1 is ethyl and R.sub.2 is sulfonyl or ##STR1## The catalyst and crosslinking agent are exposed to the cellulose fiber in a sufficient amount for a sufficient period of time at a sufficient temperature to crosslink molecules of cellulose in the fibers. The crosslinking agent can be used to crosslink various cellulose products, such as liner board, wood, and individualized cellulose fibers for absorbent products such as paper towels, diapers, and sanitary products.

In some specific embodiments, the method further comprises the step of individualizing the cellulose fibers before crosslinking the molecules of cellulose in the fibers. The fibers are exposed to the crosslinking agent and catalyst by spraying them on a mat of cellulose fibers at a fiber treatment zone, then conveying the mat through the fiber treatment zone directly into a fiberizer without stopping to cure the crosslinking substance. The fibers are then separated in a fiberizer by hammering them into substantially unbroken individual cellulose fibers, and then drying and curing the individual cellulose fibers. The fiberizer of the present invention individualizes fibers such that they have a nit level of no more than about 3 after individualization in the fiberizer.

The cyclic N-sulfatoimide crosslinker preferably comprises: ##STR2## or N-sulfatophthalimide, but is most preferably an N-sulfatosuccinimide salt. ##STR3## A basic catalyst is used with the N-sulfatoimide, preferably sodium hydroxide in an amount of 1-25% by weight of the treated cellulose, preferably 10% by weight. The sulfatoimide crosslinking agent is preferably present in an amount of 1-20% by weight, more preferably 5-10%, most preferably 10% by weight.

In yet other embodiments, the crosslinking agent is sulfonyldiethanol and the catalyst is a basic catalyst. The sulfonyldiethanol crosslinking agent is preferably present in an amount of 10% by weight of the treated product. The ratio by weight of crosslinker to catalyst is preferably about 5:1. In especially preferred embodiments the catalyst is NaOH, and is preferably present in an amount of 1-2% by weight.

In another embodiment, the crosslinker is glyoxal and 2-imidazolidone in the presence of an acidic catalyst. The glyoxal and imidazolidone are preferably present in a molar ratio of 1:1 to 3:1, more preferably 2:1. The glyoxal is preferably present in an amount of 1-5% by weight, while 1-4% of the imidazolidone is used.

When the crosslinking agent is sodium periodate, the catalyst should be an acidic catalyst, preferably one that lowers the pH to less than 5, and most preferably to the range of 2-5. An especially preferred catalyst is alum or Al.sub.2 (SO.sub.4).sub.3.

In those embodiments wherein the crosslinking agent is dimethoxyethanal, the dimethoxyethanal may optionally be combined with an imidazolidone, for example 2-imidazolidone, wherein the dimethoxyethanal and imidazolidone are present in a molar ratio of 1:1 to 3:1, preferably 2:1. The dimethoxyethanal can also be combined with a urea compound, such as N-N' dimethylurea, preferably in a 2:1 molar ratio. An acid catalyst is used to catalyze the dimethoxyethanal crosslinking reaction. The dimethoxyethanol is preferably present in an amount of 2-14% by weight of the treated cellulose product, preferably 9% by weight.

The present invention also includes crosslinking compounds selected from the group consisting of Compounds I-IV above.

The invention also includes compositions comprising a mixture of glyoxal and imidazolidone, preferably in a molar ratio of 1:1 to 3:1, more preferably 2:1. An acid catalyst may be present in the composition. In other embodiments, the invention further includes cellulose products produced by the crosslinking method of the present invention.

The present method is preferably used to prepare a quantity of individual crosslinked cellulose fibers from one or more mats comprising non-crosslinked cellulose fibers. The method is preferably performed with an apparatus that includes an applicator which applies a crosslinking substance to a mat of cellulose fibers at a fiber treatment zone; a fiberizer having a fiberizer inlet; and a conveyor that conveys the mat through the fiber treatment zone and directly to the fiberizer inlet without stopping for curing. The fiberizer provides sufficient hammering force to separate the cellulose fibers of the mat into a fiber output of substantially unbroken individual cellulose fibers. A dryer coupled to the fiberizer receives the fiber output, dries the fiber output, and cures the crosslinking substance, thereby forming dried and cured fibers. The fiberizer preferably fiberizes the treated mat to form a fiber output having a low nit level, such as a nit level of no more than about 3.

Representative conveyors include, but are not limited to, conveyor belts and roller mechanisms. In the fiber treatment zone, the crosslinking substance can be applied to the mat via any suitable means including, but not limited to, spraying, roller coating, and a combination of spraying and roller coating. The applicator that applies the crosslinking agent is preferably a shower spray and a subsequent impregnation roller that presses the crosslinking substance into the

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of the components of one embodiment of the apparatus of the present invention that is used to individualize and crosslink cellulose fibers.

FIG. 2 is an isometric external view of a preferred embodiment of an attrition device, where certain details of the mat feeder assemblies have been omitted for clarity.

FIG. 3 is a transverse sectional view of a mat feeder assembly of the preferred embodiment of the attrition device.

FIG. 4 is an isometric view of the rotor of the attrition device of FIG. 2.

FIG. 5 is a plan view of a hammer plate used in the rotor of FIG. 4.

FIG. 6 is an isometric view of a stack of hammer plates used in the rotor of FIG. 4.

FIG. 7 is an isometric view of the exterior of a preferred embodiment of a fluff generator included as an option in the apparatus of the present invention.

FIG. 8 is a transverse sectional view through a housing portion and rotor of the fluff generator of FIG. 7.

FIG. 9 is a plan sectional view of the fluff generator of FIG. 7.

FIG. 10 is an enlarged view of the crosslinking applicator portion of the diagram of FIG. 1.

FIG. 11 is a graph showing properties of an absorbent material crosslinked with a variety of crosslinking agents.

DETAILED DESCRIPTION

The use of the crosslinking agents of the present invention are illustrated in the following examples. mat. In especially preferred embodiments, the shower spray includes a pair of opposing shower spray applicators that direct droplets of the crosslinking agent toward each face of the mat.

The dryer of the apparatus preferably includes a drying zone for forming dried fibers, and a curing zone for curing the crosslinking substances on the dried fibers. The drying zone preferably includes the expansion chamber, which has an inlet for receiving the individual cellulose fibers from the restricted diameter conduit. The dryer inlet has a temperature within the range of about 200.degree.-315.degree. C. so as to flash evaporate moisture from and expand the cellulose fibers. The subsequent curing zone has an outlet through which the dried and cured fibers are delivered from the dryer. The outlet of the curing zone preferably has a temperature within a range of about 140.degree.-180.degree. C.

The fiberizer apparatus comprises at least an attrition device which produces a low nit level fiber output. The fiberizer may also optionally include a disk refiner of conventional design coupled to the attrition device and a fluff generator of novel design coupled to the disk refiner.

The present invention also includes a method of producing crosslinked cellulose fibers by applying one of the crosslinking substances of the present invention to a mat of cellulose fibers at a fiber treatment zone, then conveying the mat from the fiber treatment zone directly into a fiberizer without stopping to cure the crosslinking substance. The fiberizer separates the fibers by hammering them into substantially unbroken individual cellulose fibers, preferably having a nit level of no more than about 3. The separated fibers are then dried at a temperature of about 200.degree.-315.degree. C. so as to flash evaporate water from the fiber output, and then cured at a temperature of about 140.degree.-180.degree. C.

EXAMPLE I

N-sulfatosuccinimide Crosslinker

This example shows the effect of using N-sulfatosuccinimide as a crosslinking agent. The N-sulfatosuccinimide was prepared by combining 11.5 g of N-hydroxysuccinimide and 15.8 g of pyridine in 150 ml of dichloromethane. To this solution, 11.7 g of chlorosulfonic acid was added over a 20-minute period with cooling to maintain the temperature at 5.degree.-15.degree. C. The solution was stirred at room temperature for about 16 hours, then suction filtered, and the solid precipitate washed with three 100 ml portions of dichloromethane to remove pyridine hydrochloride. The resulting colorless solid was then dried to yield approximately 22.5 g of pyridinium N-sulfatosuccinimide.

The crosslinking of the cellulose with pyridinium N-sulfatosuccinimide was carried out by dissolving 1.6 g of pyridinium N-sulfatosuccinimide and 1.0 g of sodium bicarbonate catalyst in 10 ml of water. The solution was washed twice with dichloromethane in a separatory funnel to remove pyridine, and the water phase was diluted to 15.5 ml with water and immediately distributed dropwise over the surface of a 15.5 g piece of pulpsheet made from NF105 pulp available from the Weyerhauser Company of Federal Way, Wash. The pulpsheet was made from 2:1 of NF105:Buckeye pulp. The wet pulp was placed in a plastic bag and pressure rolled to evenly distribute the liquid. The pulpsheet was then converted to fluff by passing it twice through a pen mill fluffer, and the fluff was cured in an oven at 150.degree. C. for 20 minutes.

The dry bulk, wet bulk and fluff absorbance quality (FAQ) of the product was next determined. A measured quantity of pulp was air laid into a mat in a plexiglas cylinder tube. A pressure of 0.6 kPa was exerted on the mat and the volume (dry bulk) of the mat was measured to give a measure of its dry compressibility. Next, a pressure of 2.5 kPa was placed on the mat, and the mat volume (dry bulk) was again measured. Water was then introduced into the bottom of the cylinder to determine the amount of water absorbed by the fluff under a pressure of 2.5 kPa, and this amount of water (wet bulk) was measured. The FAQ capacity is expressed as the grams of water absorbed per gram of pulp, and this value is an indication of the absorbency of the crosslinked fibers. Table I illustrates that the N-sulfatosuccinimide crosslinking agent of this example significantly enhanced the FAQ capacity and bulk of a handsheet.

TABLE I ______________________________________ FAQ CAPACITY AND BULK OF UNTREATED AND TREATED MAT Hand Sheet Bulk FAQ (2:1 Treated pulp: Capacity Buckeye pulp) ______________________________________ Untreated pulp 11.6 g/g 5.4 cc/g Pulp treated 16.5 g/g 12.0 cc/g with 10% Compound I ______________________________________

The N-sulfatosuccinimide crosslinking agent was similarly tested at a variety of reactant and catalyst concentrations. The results of those runs are shown in the following Tables IIA and IIB, wherein all concentrations are on a wt/wt % of solids basis. Handsheets are made from a 2:1 mixture of treated crosslinked NF105 pulp:Buckeye Pulp (Buckeye Pulp may be obtained from Procter and Gamble of Cincinnati, Ohio).

TABLE IIA __________________________________________________________________________ PROPERTIES OF N-SULFATOSUCCINIMIDE TREATED MATS Reactant Catalyst Cure Cure Dry Bulk Run Reactants Catalyst Conc. Conc. Temp Time Color 0.6 kPa __________________________________________________________________________ 1 Pyridinium NaHCO.sub.3 10% 6.7% 150.degree. C. 20 min Lt. cream 47.4 N-Sulfatosuccinimide 2 Pyridinium NaHCO.sub.3 10% 3.1% 150.degree. C. 20 min white N-Sulfatosuccinimide 3 Pyridinium NaHCO.sub.3 10% 6.1% 150.degree. C. 20 min Lt. cream N-Sulfatosuccinimide 4 Pyridinium NaHCO.sub.3 10% (12.1%) 150.degree. C. 20 min cream N-Sulfatosuccinimide 5 Pyridinium NaHCO.sub.3 10% (24.3%) 150.degree. C. 20 min cream N-Sulfatosuccinimide 6 Pyridinium NaHCO.sub.3 5% 3.0% 150.degree. C. 20 min Lt. cream N-Sulfatosuccinimide 7 Pyridinium NaHCO.sub.3 20% 12.1% 150.degree. C. 20 min Lt. cream N-Sulfatosuccinimide 8 Pyridinium none 10% none 150.degree. C. 20 min Lt. tan 46.5 N-Sulfatosuccinimide 9 Pyridinium Na.sub.2 CO.sub.3 10% 6.7% 150.degree. C. 20 min cream 51.1 N-Sulfatosuccinimide 10 Pyridinium NaHCO.sub.3 10.0% 6.1% max. pp 51.9 N-Sulfatosuccinimide 11 Pyridinium NaHCO.sub.3 10.0% 6.1% 150.degree. C. 15 min 52.8 N-Sulfatosuccinimide __________________________________________________________________________

TABLE IIB __________________________________________________________________________ PROPERTIES OF N-SULFATOSUCCINIMIDE TREATED MATS (Continued) Dry Bulk Absorb Wet Bulk Wet Bulk FAQ Bulk Permeability Run Reactants 2.5 kPa Time 2.5 kPa 0.6 kPa Capacity (cc/g) (cu ft/sq ft) __________________________________________________________________________ 1 Pyridinium 26.5 6.3 12.8 15.7 16.5 12.0 95 N-Sulfatosuccinimide 2 Pyridinium 4.6 23 N-Sulfatosuccinimide 3 Pyridinium 10.1 78 N-Sulfatosuccinimide 4 Pyridinium 8.9 68 N-Sulfatosuccinimide 5 Pyridinium 7.7 53 N-Sulfatosuccinimide 6 Pyridinium 9.8 68 N-Sulfatosuccinimide 7 Pyridinium 11.2 91 N-Sulfatosuccinimide 8 Pyridinium 5.9 33 N-Sulfatosuccinimide 9 Pyridinium 28.4 6.2 9.4 11.2 12.1 N-Sulfatosuccinimide 10 Pyridinium 27.4 4.9 12.4 15.0 15.5 8.5 73 N-Sulfatosuccinimide 11 Pyridinium 28.1 5.0 13.4 16.1 16.5 10.3 87 N-Sulfatosuccinimide __________________________________________________________________________

All concentrations are on a wt/wt % solids basis, except those in parentheses, which are on a wt/wt % solution "as is" basis. Better bulk results were noted when the ratio of crosslinking agent to catalyst was about 2:1 or more of catalyst. Crosslinking concentrations of 5-20%, preferably 5-10%, most preferably 10%, were used.

EXAMPLE II

Crosslinking with 2,2'dimethoxyethanal

In this example, 2,2'-dimethoxyethanal (DME) was used as the crosslinking agent. DME was obtained from Hoechst Celanese Corporation, Specialty Chemicals Division in Charlotte, N.C. DME has the structural formula shown below: ##STR4## DME is a colorless liquid with a fruity odor, a molecular weight of 101.4, a boiling point of 65.degree. C. at 140 mm Hg, and a pH at 20.degree. C. of 7. The DME was completely miscible in water at 20.degree. C., and had a viscosity of 0.28 and a density of 1.173 at 20.degree. C.

The DME and catalyst were dissolved in 10 ml of water. The water phase was then diluted to 15.5 ml with water and immediately distributed dropwise over the surface of a 16.5 g piece of pulpsheet from NF105 pulp. The NF105 pulpsheet was treated with the aqueous solution of DME, catalyst and urea derivative (if any) to a 50% consistency level, then fiberized and cured at 150.degree. C. for 20 minutes. The results are shown in the following Table III, where all concentrations are on a wt/wt % solids basis, except those in parentheses which are in a wt/wt % solution "as is" (molar) basis. Handsheets are from a 2:1 mixture of crosslinked NF105 pulp:Buckeye pulp. Numerical color ratings in Table II are: 0=no color, bright; 1=trace of color, but possibly acceptable; 2=very noticeable color, probably not acceptable; 3=intense color.

TABLE IIIA __________________________________________________________________________ PROPERTIES OF DME TREATED MATS Reactant Catalyst Cure Cure Dry Bulk Run Reactants Catalyst Conc. Conc. Temp Time Color 0.6 kPa __________________________________________________________________________ 1 DME Al.sub.2 (SO.sub.4).sub.3 9.0% 2.0% 150.degree. C. 20 mi 1 49.7 2 DME + Urea (2:1) Zn(NO.sub.3).sub.2 9% DME 2.0% 150.degree. C. 20 mi 2 52.1 3 DME + 2-Imida- Zn(NO.sub.3).sub.2 9% DME 2.0% 150.degree. C. 20 mi 1 50.8 zolidone (2:1) 4 DME + 2-Imida- Zn(NO.sub.3).sub.2 13.5% 3.0% 150.degree. C. 20 mi 1 51.1 zolidone (2:1) 5 DME + 2-Imida- Zn(NO.sub.3).sub.2 9% DME 2.0% 150.degree. C. 20 mi 1 51.3 zolidone (2:1) 6 DME + 2-Imida- Zn(NO.sub.3).sub.2 4.5% DME 1.0% 150.degree. C. 20 mi 1 50.5 zolidone (2:1) 7 DME + 2-Imida- Zn(NO.sub.3).sub.2 2.3% DME .5% 150.degree. C. 20 mi 1 48.9 zolidone (2:1) __________________________________________________________________________

TABLE IIIB __________________________________________________________________________ PROPERTIES OF DME TREATED MATS - (Continued) Dry Bulk Absorb Wet Bulk Wet Bulk FAQ Bulk Permeability Run Reactants 2.5 kPa Time 2.5 kPa 0.6 kPa Capacity (cc/g) (cu ft/sq ft) __________________________________________________________________________ 1 DME 29.2 7.7 15.6 18.5 18.8 12.9 103 2 DME + Urea (2:1) 30.2 7.9 15.3 18.0 18.2 12.1 88 3 DME + 2-Imida- 28.5 4.9 15.1 18.6 18.8 16.2 108 zolidone (2:1) 4 DME + 2-Imida- 29.3 4.8 15.0 18.2 18.8 11.9 99 zolidone (2:1) 5 DME + 2-Imida- 29.1 5.6 14.9 18.1 18.6 11.5 103 zolidone (2:1) 6 DME + 2-Imida- 28.6 6.5 13.9 16.7 17.2 10.0 75 zolidone (2:1) 7 DME + 2-Imida- 27.6 5.7 12.2 14.5 15.2 7.2 88 zolidone (2:1) __________________________________________________________________________

EXAMPLE III

Periodate Crosslinking

An NB316 pulp from the Weyerhaeuser Company was treated with the sodium periodate in an aqueous slurry for 2-4 days at 22.degree. C. The slurry was filtered and the pulp mat was washed several times with water and air dried. The catalyst was then added and the crosslinking reaction carried out at high temperatures, as in Example II above. This fiber had high absorbency and was easily densified by pressure application into an air-laid pad form to 0.2-0.3 g/cc and the pad exhibited higher total capacity and better wicking properties than uncrosslinked NB316 at a similar density. Total capacity and wicking were measured as in Example VI below. It was also found that when the sodium periodate oxycellulose was treated with 2% (wt/wt) alum and treated to 100.degree.-150.degree. C. for 20 minutes, the resulting fiber exhibited high bulk properties in a wet-laid handsheet when combined with Buckeye pulp in a 2:1 ratio. Densification of the crosslinked cellulose is more fully described in copending U.S. patent application Ser. No. 07/665,761 filed Mar. 7, 1991, which is incorporated by reference. The results with undensified material are reported in Table IV below, while results with a densified pad are shown in Table V below.

TABLE IV ______________________________________ PROPERTIES OF PERIODATE CROSSLINKED NON-DENSIFIED CELLULOSE FAQ Absorption Handsheet Reactants FAQ Capacity Time Bulk ______________________________________ NB316 Periodate 16.8 g/g 5.7 sec 6.0 cc/g Oxycellulose NB316 Periodate 19.8 g/g 5.8 sec 10.8 cc/g Oxycellulose + alum NF105 11.6 g/g 5.0 sec 4.7 cc/g ______________________________________

TABLE V ______________________________________ PROPERTIES OF PERIODATE CROSSLINKED DENSIFIED CELLULOSE Wicking Reactants Total Capacity Wicking Capacity Time ______________________________________ NB316 Periodate 16.1 g/g 9.9 g/g 19 sec Oxycellulose NB316 9.6 g/g 7.5 g/g 40 sec ______________________________________

Crosslinking in non-densified pads was studied using a variety of catalysts and reaction conditions shown in Table VI below, where all concentrations are on a wt/wt % solids basis (expressing the concentration of reactants as a weight percentage of the final treated product). Handsheets are again from a 2:1 mixture of additive pulp:Buckeye pulp. Numerical color ratings are the same as the Example II above. The pulp used in these runs was NB316.

TABLE VIA __________________________________________________________________________ PERIODATE CROSSLINKING WITH VARIOUS CATALYSTS AND CONDITIONS Reactant Catalyst Cure Cure Dry Bulk Run Reactant Catalyst Conc. Conc. Temp Time Color 0.6 kPa __________________________________________________________________________ 1 Sodium Periodate none 6.4% 0.0% 150.degree. C. 20 min 0 30.4 2 Sodium Periodate Al.sub.2 (SO.sub.4).sub.3 6.4% 2.0% 150.degree. C. 20 min 2 38.2 3 Sodium Periodate Zn(NO.sub.3).sub.2 6.4% 2.0% 150.degree. C. 20 min 1 43.5 4 Sodium Periodate DEG + Zn(NO.sub.3).sub.2 6.4% 3% + 2% 150.degree. C. 20 min 1 43.0 5 Sodium Periodate ethylenediamine 6.4% 2.0% 150.degree. C. 20 min 2 6 Sodium Periodate ethylenediamine 6.4% 2% + 1% 150.degree. C. 20 min 3 Al(SO.sub.4).sub.3 7 Sodium Periodate none 3.3% 0.0% 155.degree. C. 20 min 1 43.2 8 Sodium Periodate Al.sub.2 (SO.sub.4).sub.3 3.3% 2.0% 155.degree. C. 20 min 2 38.9 9 Sodium Periodate Zn(N(.sub.3).sub.2 3.3% 1.0% 155.degree. C. 20 min 1 10 Sodium Periodate DEG + Zn(NO.sub.3).sub.2 3.3% 3% + 2% 155.degree. C. 20 min 2 41.2 11 Sodium Periodate Urea + Zn(NO.sub.3).sub.2 3.3% 10% + 2% 155.degree. C. 20 min 2 12 Sodium Periodate Urea 3.3% 10.0% 150.degree. C. 20 min 2 13 Sodium Periodate none 12.8% 0.0% 155.degree. C. 20 min 0 42.3 14 Sodium Periodate Al.sub.2 (SO.sub.4).sub.3 12.8% 2.0% 155.degree. C. 20 min 3 39.6 15 Sodium Periodate Al.sub.2 (SO.sub.4).sub.3 12.8% 2.0% 150.degree. C. 20 min 2 43.7 16 Sodium Periodate Al.sub.2 (SO.sub.4).sub.3 12.8% 2.0% 125.degree. C. 20 min 1 47.1 17 Sodium Periodate Al.sub.2 (SO.sub.4).sub.3 12.8% 2.0% 100.degree. C. 20 min 0 47.4 18 Sodium Periodate Zn(NO.sub.3).sub.2 12.8% 1.0% 155.degree. C. 20 min 2 42.9 19 Sodium Periodate DEG + Zn(NO.sub.3).sub.2 12.8% 3% + 2% 155.degree. C. 20 min 3 39.3 20 Sodium Periodate Urea + Zn(NO.sub.3).sub.2 12.8% 10% + 2% 155.degree. C. 20 min 3 21 Sodium Periodate Urea 12.8% 10.0% 155.degree. C. 20 min 2 22 Sodium Periodate NaHSO.sub.3 12.8% 5.0% 150.degree. C. 20 min 1 40.0 23 Sodium Periodate NH.sub.4 Cl 12.8% 5.0% 150.degree. C. 20 min 3 24 Sodium Periodate HCl 12.8% 1.0% 150.degree. C. 20 min 3 33.6 25 Sodium Periodate MgCl.sub.2 12.8% 2.0% 150.degree. C. 20 min 2 44.3 26 Sodium Periodate (NH.sub.4)SO.sub.4 12.8% 2.0% 150.degree. C. 20 min 2 27 Sodium Periodate NH.sub.4 OH 12.8% 10.0% 150.degree. C. 20 min 3 28 Sodium Periodate Borax 12.8% 2.0% 150.degree. 20 min 1 29 Sodium Periodate (NH.sub.4).sub.2 HPO 12.8% 2.0% 150.degree. 20 min 3 (dibasic) 30 Sodium Periodate NH.sub.4 H.sub.2 PO.sub.4 12.8% 2.0% 150.degree. 20 min 3 (monobasic) 31 Sodium Periodate none 12.8% 0.0% none none 0 48.8 __________________________________________________________________________

TABLE VIB __________________________________________________________________________ PERIODATE CROSSLINKING WITH VARIOUS CATALYSTS AND CONDITIONS (Continued) Dry Bulk Absorb Wet Bulk Wet Bulk FAQ Bulk Permeability Run Reactants