U.S. patent application number 10/200090 was filed with the patent office on 2003-06-12 for superabsorbent cellulosic fiber.
Invention is credited to Neogi, Amar N., Petersen, Brent A., Young, Richard H. SR..
Application Number | 20030106163 10/200090 |
Document ID | / |
Family ID | 22740287 |
Filed Date | 2003-06-12 |
United States Patent
Application |
20030106163 |
Kind Code |
A1 |
Neogi, Amar N. ; et
al. |
June 12, 2003 |
Superabsorbent cellulosic fiber
Abstract
A modified cellulosic fiber having superabsorbent properties is
described. The modified fiber of the invention has a fibrous
structure substantially identical to the cellulosic fiber from
which it is derived. The modified fiber is a water-swellable,
water-insoluble fiber that substantially retains its fibrous
structure in its expanded, water-swelled state. The modified fiber
is a sulfated and crosslinked cellulosic fiber having a liquid
absorption capacity of at least about 4 g/g. In one embodiment, the
modified fiber is an individual, crosslinked, sulfated cellulosic
fiber. In another aspects, the invention provides a rollgood that
includes the modified fiber, absorbent composites and articles that
include the modified fiber, and methods for making the modified
cellulosic fiber.
Inventors: |
Neogi, Amar N.; (Seattle,
WA) ; Young, Richard H. SR.; (Maple Valley, WA)
; Petersen, Brent A.; (Seattle, WA) |
Correspondence
Address: |
WEYERHAEUSER COMPANY
INTELLECTUAL PROPERTY DEPT., CH 1J27
P.O. BOX 9777
FEDERAL WAY
WA
98063
US
|
Family ID: |
22740287 |
Appl. No.: |
10/200090 |
Filed: |
July 19, 2002 |
PCT Filed: |
January 17, 2001 |
PCT NO: |
PCT/US01/01883 |
Current U.S.
Class: |
8/116.1 ; 8/120;
8/195 |
Current CPC
Class: |
D06M 13/192 20130101;
D06M 15/423 20130101; D06M 15/263 20130101; D06M 11/52 20130101;
D21H 11/20 20130101; D06M 13/207 20130101; D06M 2200/00 20130101;
D06M 11/55 20130101; D06M 13/12 20130101; D06M 2101/06 20130101;
D06M 13/432 20130101 |
Class at
Publication: |
8/116.1 ; 8/195;
8/120 |
International
Class: |
D06M 011/00; D06M
023/00; D06M 013/322 |
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A modified cellulosic fiber, comprising a sulfated cellulosic
fiber crosslinked to an extent to render the fiber substantially
insoluble in water.
2. The fiber of claim 1 having a liquid absorption capacity of at
least about 4 g/g.
3. The fiber of claim 1, wherein the average degree of sulfate
substitution is from about 0.1 to about 2.0.
4. The fiber of claim 1, wherein the average degree of sulfate
substitution is from about 0.2 to about 1.0.
5. The fiber of claim 1, wherein the average degree of sulfate
substitution is from about 0.3 to about 0.5.
6. The fiber of claim 1, wherein the cellulosic fiber is a wood
pulp fiber.
7. Individual, water-swellable, water-insoluble, intrafiber
crosslinked, sulfated cellulosic fibers.
8. The fibers of claim 7 having a liquid absorption capacity of at
least about 4 g/g.
9. The fibers of claim 7, wherein the average degree of sulfate
substitution is from about 0.1 to about 2.0.
10. The fibers of claim 7, wherein the average degree of sulfate
substitution is from about 0.2 to about 1.0.
11. The fibers of claim 7, wherein the average degree of sulfate
substitution is from about 0.3 to about 0.5.
12. The fibers of claim 7, wherein the cellulosic fibers are wood
pulp fibers.
13. The fibers of claim 7, wherein the fibers are crosslinked with
a crosslinking agent selected from the group consisting of a
urea-based crosslinking agent, a polycarboxylic acid crosslinking
agent, an aldehyde crosslinking agent, a dialdehyde crosslinking
agent, and mixtures thereof.
14. The fibers of claim 13, wherein the crosslinking agent is
applied to the fibers in an amount from about 0.01 to about 8.0
percent by weight based on the total weight of fibers.
15. The fibers of claim 13, wherein the crosslinking agent is
applied to the fibers in an amount from about 0.02 to about 5.0
percent by weight based on the total weight of fibers.
16. A rollgood comprising the fibers of claims 1 or 7.
17. The rollgood of claim 16 further comprising another fiber.
18. The rollgood of claim 17, wherein the other fiber is at least
one of fluff pulp fibers, crosslinked cellulosic fibers, cotton
fibers, CTMP fibers, and synthetic fibers.
19. The rollgood of claim 16 further comprising an absorbent
material.
20. The rollgood of claim 16 further comprising a binder
material.
21. An absorbent article comprising the rollgood of claim 16.
22. An absorbent composite comprising the fibers of claims 1 or
7.
23. The composite of claim 22 further comprising another fiber.
24. The composite of claim 23, wherein the other fiber is at least
one of fluff pulp fibers, crosslinked cellulosic fibers, cotton
fibers, CTMP fibers, and synthetic fibers.
25. An absorbent article comprising the fibers of claims 1 or
7.
26. The article of claim 25, wherein the article is at least one of
an infant diaper, an adult incontinence product, and a feminine
care product.
27. An absorbent article comprising a liquid pervious topsheet, a
liquid impervious backsheet attached to the topsheet, and an
absorbent member intermediate the topsheet and backsheet, wherein
the absorbent member comprises the fibers of claims 1 or 7.
28. An absorbent article comprising a liquid pervious topsheet, a
liquid impervious backsheet attached to the topsheet, and an
absorbent member intermediate the topsheet and backsheet, wherein
the absorbent member comprises the rollgood of claim 16.
29. A method for making cellulosic fibers, comprising crosslinking
sulfated cellulosic fibers to render the fibers substantially
water-insoluble.
30. The method of claim 29, wherein crosslinking sulfated
cellulosic fibers comprises treating sulfated cellulosic fibers
with an amount of a crosslinking agent sufficient to render the
crosslinked fibers substantially water insoluble.
31. The method of claim 30, wherein the amount of crosslinking
agent ranges from about 0.01 to about 8.0 percent by weight
crosslinking agent based on the total weight of fibers.
32. The method of claim 30, wherein the crosslinking agent is
selected from the group consisting of a urea-based crosslinking
agent, a polycarboxylic acid crosslinking agent, an aldehyde
crosslinking agent, a dialdehyde crosslinking agent, and mixtures
thereof.
33. The method of claim 30, wherein the crosslinking agent is
applied to the fibers as an aqueous alcoholic solution.
34. The method of claim 29, wherein the sulfated cellulosic fibers
have an average degree of sulfate substitution of from about 0.1 to
about 2.0.
35. The method of claim 29 further comprising baling the
crosslinked, sulfated fibers.
36. The method of claim 29 further comprising forming the
crosslinked, sulfated fibers into a rollgood.
37. A method for making cellulosic fibers, comprising the steps of:
reacting cellulosic fibers with a sulfating agent to provide
sulfated fibers; applying a crosslinking agent to the sulfated
fibers; and curing the crosslinking agent to provide crosslinked,
sulfated cellulosic fibers.
38. The method of claim 37, wherein the sulfating agent comprises
sulfuric acid.
39. The method of claim 37, wherein the sulfating agent comprises a
solution of sulfuric acid in an organic solvent.
40. The method of claim 37, wherein the organic solvent is an
alcohol is selected from the group consisting of isopropanol,
propanol, and butanol.
41. The method of claim 40, wherein the ratio of sulfuric acid to
alcohol is about 2.4:1 by mole.
42. The method of claim 37 further comprising treating the sulfated
fibers with a neutralizing agent prior to crosslinking.
43. The method of claim 42, wherein the neutralizing agent
comprises a base.
44. The method of claim 42, wherein the neutralizing agent
comprises a multivalent metal salt.
45. The method of claim 44, wherein the metal salt is selected from
the group consisting of cerium nitrate, magnesium sulfate, and
aluminum sulfate.
46. The method of claim 37, wherein the crosslinking agent is
selected from the group consisting of a urea-based crosslinking
agent, a polycarboxylic acid crosslinking agent, an aldehyde
crosslinking agent, a dialdehyde crosslinking agent, and mixtures
thereof.
47. The method of claim 37, wherein the crosslinking agent is
applied to the fibers as an aqueous alcoholic solution.
48. The method of claim 37, wherein the cellulosic fibers are
reacted with the sulfating agent at a temperature of about
4.degree. C.
49. The method of claim 37 further comprising swelling the fibers
prior to reacting the fibers with the sulfating agent.
50. The method of claim 49, wherein swelling the fibers comprises
treating the fibers with a swelling agent selected from the group
consisting of acetic acid, acetic anhydride, and mixtures
thereof.
51. The method of claim 50 further comprising removing excess
swelling agent prior to reacting the fibers with the sulfating
agent.
52. The method of claim 37, wherein reacting the fibers with a
sulfating agent comprises adding the fibers to an alcoholic
solution of the sulfating agent.
53. The method of claim 52, wherein the fibers are cooled to about
4.degree. C. prior to adding the fibers to the alcoholic solution
of the sulfating agent.
54. The method of claim 52, wherein the alcoholic solution of the
sulfating agent is cooled to about 4.degree. C. prior to the
addition.
55. The method of claim 37, wherein the sulfating agent is reacted
with the fibers at a temperature of about 4.degree. C.
56. The method of claim 42 further comprising separating the
sulfated fibers from excess sulfating agent prior to treating the
sulfated fibers with the neutralizing agent.
57. The method of claim 42 further comprising washing the sulfated
fibers with alcohol solution prior to treating the sulfated fibers
with the neutralizing agent.
58. The method of claim 37, wherein the cellulosic fibers further
comprise magnesium sulfate.
59. The method of claim 37, wherein the sulfating agent further
comprises magnesium sulfate.
60. The method of claim 40, wherein the alcoholic solution of the
sulfating agent further comprises magnesium sulfate.
61. A method for making cellulosic fibers, comprising the steps of:
swelling dry cellulosic fibers with a swelling agent to provide
swelled fibers; separating excess swelling agent from the swelled
fibers; reacting swelled cellulosic fibers with a sulfating agent
to provide sulfated fibers; separating excess sulfating agent from
the sulfated fibers; treating the sulfated fibers with a
neutralizing agent to provide fibers suitable for crosslinking;
applying a crosslinking agent to the sulfated fibers; and curing
the crosslinking agent to provide crosslinked, sulfated cellulosic
fibers.
62. The method of claim 61, wherein the dry cellulosic fibers
comprise never-dried cellulosic fibers solvent exchanged with an
alcohol.
63. The method of claim 61, wherein the swelling agent is selected
from the group consisting of acetic acid, acetic anhydride, and
mixtures thereof.
64. The method of claim 61, wherein the sulfating agent comprises
sulfuric acid.
65. The method of claim 61, wherein the sulfating agent comprises a
solution of sulfuric acid in an alcohol.
66. The method of claim 65, wherein the ratio of sulfuric acid to
alcohol is about 2.4:1.
67. The method of claim 65, wherein the alcohol comprises
isopropanol.
68. The method of claim 61, wherein the neutralizing agent
comprises sodium hydroxide.
69. The method of claim 61, wherein the crosslinking agent is
applied to the fibers as an aqueous alcoholic solution.
70. The method of claim 61, wherein the cellulosic fibers are
reacted with the sulfating agent at a temperature of about
4.degree. C.
71. The method of claim 61, wherein the cellulosic fibers are
reacted with the sulfating agent for a period of time from about 10
to about 60 minutes.
72. The method of claim 61 further comprising washing the sulfated
fibers with an alcohol solution prior to treating the sulfated
fibers with the neutralizing agent.
73. The method of claim 61, wherein the sulfating agent further
comprises magnesium sulfate.
74. The product obtainable by the process of claim 29.
75. The product obtainable by the process of claim 37.
76. The product obtainable by the process of claim 61.
77. An absorbent core for an absorbent article, comprising a
sulfated cellulosic fiber crosslinked to an extent to render the
fiber substantially insoluble in water, wherein the core has a
liquid absorption capacity of at least about 22 g/g.
78. An absorbent core for an absorbent article, comprising
individual, water-swellable, water-insoluble, intrafiber
crosslinked, sulfated cellulosic fibers, wherein the core has a
liquid absorption capacity of at least about 22 g/g.
Description
SUPERABSORBENT CELLULOSIC FIBER
[0001] 1. Field of the Invention
[0002] The present invention relates to a modified cellulosic fiber
having superabsorbent properties and, more particularly, to a
crosslinked and sulfated cellulosic fiber having a structure
substantially identical to the fiber from which it is derived.
[0003] 2. Background of the Invention
[0004] Personal care absorbent products, such as infant diapers,
adult incontinent pads, and feminine care products, typically
contain an absorbent core that includes superabsorbent in a fibrous
matrix. Superabsorbents are water-swellable, generally
water-insoluble absorbent materials having a liquid absorbent
capacity of at least about 10, preferably of about 20, and often up
to about 100 times their weight in water. While the core's liquid
retention or storage capacity is due in large part to the
superabsorbent, the core's fibrous matrix provides the essential
functions of liquid wicking, pad strength and integrity, and some
amount of absorbency under load. These desirable properties are
attributable to the fact that the matrix includes cellulosic
fibers, typically wood pulp fluff in fiber form.
[0005] For personal care absorbent products, U.S. southern pine
fluff pulp is used almost exclusively and is recognized worldwide
as the preferred fiber for absorbent products. The preference is
based on the fluff pulp's advantageous high fiber length (about 2.8
mm) and its relative ease of processing from a wetlaid pulp sheet
to an airlaid web. However, these fluff pulp fibers can absorb only
about 2-3 g/g of liquid (e.g., water or bodily fluids) within the
fibers' cell walls. Most of the fibers' liquid holding capacity
resides in the interstices between fibers. For this reason, a
fibrous matrix readily releases acquired liquid on application of
pressure. The tendency to release acquired liquid can result in
significant skin wetness during use of an absorbent product that
includes a core formed exclusively from cellulosic fibers. Such
products also tend to leak acquired liquid because liquid is not
effectively retained in such a fibrous absorbent core.
[0006] The inclusion of absorbent materials in a fibrous matrix and
their incorporation into personal care products is known. The
incorporation of superabsorbent materials into these products has
had the effect of reducing the products' overall bulk while at the
same time increasing its liquid absorbent capacity and enhancing
skin dryness for the products' wearers.
[0007] A variety of materials have been described for use as
absorbent materials in personal care products. Included among these
materials are natural-based materials such as agar, pectin, gums,
carboxyalkyl starch and carboxyalkyl cellulosic fiber, such as
carboxymethyl cellulose, as well as synthetic materials such as
polyacrylates, polyacrylamides, and hydrolyzed polyacrylonitriles.
Although natural-based absorbing materials are well known, these
materials have not gained wide usage in personal care products
because of their relatively inferior absorbent properties compared
to synthetic absorbent materials such as polyacrylates. The
relatively high cost of these materials has also precluded their
use in consumer absorbent products. Furthermore, many natural-based
materials tend to form soft, gelatinous masses when swollen with a
liquid. The presence of such gelatinous masses in a product's core
tends to limit liquid transport and distribution within the core
and prevents subsequent liquid insults from being efficiently and
effectively absorbed by the product.
[0008] In contrast to the natural-based absorbents, synthetic
absorbent materials are generally capable of absorbing large
quantities of liquid while maintaining a relatively non-gelatinous
form. Synthetic absorbent materials, often referred to as
superabsorbent polymers (SAP), have been incorporated into
absorbent articles to provide higher absorbency under pressure and
higher absorbency per gram of absorbent material. Superabsorbent
polymers are generally supplied as particles having a diameter in
the range from about 20-800 microns. Due to their high absorbent
capacity under load, absorbent products that include superabsorbent
polymer particles provide the benefit of skin dryness. Because
superabsorbent polymer particles absorb about 30 times their weight
in liquid under load, these particles provide the further
significant advantages of thinness and wearer comfort. In addition,
superabsorbent polymer particles are about half the cost per gram
of liquid absorbed under load compared to fluff pulp fibers. For
these reasons it is not surprising that there is a growing trend
toward higher superabsorbent particle levels and reduced levels of
fluff pulp in consumer absorbent products. In fact, some infant
diapers include 60 to 70 percent by weight superabsorbent polymer
in their liquid storage core. From a cost perspective, a storage
core made from 100 percent superabsorbent particles is desirable.
However, as noted above, such a core would fail to function
satisfactorily due to the absence of any significant liquid wicking
and distribution of acquired liquid throughout the core.
Furthermore, such a core would also lack strength to retain its wet
and/or dry structure, shape, and integrity.
[0009] Cellulosic fibers provide absorbent products with critical
functionality that has, to date, not been duplicated by particulate
superabsorbent polymers. Superabsorbent materials have been
introduced in synthetic fiber form seeking to provide a material
having the functionality of both fiber and superabsorbent polymer
particle. However, these superabsorbent fibers are difficult to
process compared to fluff pulp fibers and do not blend well with
fluff pulp fibers. Furthermore, synthetic superabsorbent fibers are
significantly more expensive than superabsorbent polymer particles
and, as a result, have not competed effectively for high volume use
in personal care absorbent products.
[0010] Cellulosic fibers have also been rendered highly absorptive
by chemical modification to include ionic groups such as carboxylic
acid, sulfonic acid, and quaternary ammonium groups that impart
water swellability to the fiber. Although some of these modified
cellulosic materials are soluble in water, some are
water-insoluble. However, none of these highly absorptive modified
cellulosic materials possess the structure of a pulp fiber, rather,
these modified cellulosic materials are typically granular or have
a regenerated fibril form.
[0011] A need exists for a highly absorbent material suitable for
use in personal care absorbent products, the absorbent material
having absorptive properties similar to synthetic, highly
absorptive materials and at the same time offering the advantages
of liquid wicking and distribution associated with fluff pulp
fibers. Accordingly, there is a need for a fibrous superabsorbent
that combines the advantageous liquid storage capacity of
superabsorbent polymers and the advantageous liquid wicking of
fluff pulp fibers. Ideally, the fibrous superabsorbent is
economically viable for use in personal care absorbent products.
The present invention seeks to fulfill these needs and provides
further related advantages.
SUMMARY OF THE INVENTION
[0012] In one aspect, the present invention provides a modified
cellulosic fiber having superabsorbent properties. The modified
fiber formed in accordance with the present invention has a fibrous
structure substantially identical to the cellulosic fiber from
which it is derived. More importantly, the modified fiber is a
water-swellable, water-insoluble fiber that substantially retains
its fibrous structure in its expanded, water-swelled state. The
modified fiber is a sulfated and crosslinked cellulosic fiber
having a liquid absorption capacity of at least about 4 g/g. In one
embodiment, the modified fiber is an individual, crosslinked,
sulfated cellulosic fiber. In another embodiment, the invention
provides a rollgood that includes the modified fiber. In one
embodiment, the rollgood includes other materials such as fibrous,
binder, and absorbent materials. In another embodiment, the
rollgood can be directly inserted as an absorbent core into an
absorbent article.
[0013] In another aspect of the invention, methods for forming the
modified cellulosic fiber are provided. In one embodiment of the
method, a sulfated cellulosic fiber is crosslinked to an extent
sufficient to render the fiber substantially insoluble in water. In
another embodiment, a crosslinked cellulosic fiber is sulfated to
provide the modified fiber. The sulfated cellulosic fiber can be
prepared by reacting the fiber with sulfuric acid in an organic
solvent.
[0014] In others aspects, the invention provides methods for using
the modified fiber and absorbent composites and articles
incorporating the modified fiber are also provided. In one
embodiment, the invention provides an absorbent core having a
liquid capacity of at least about 22 g/g. The absorbent core can be
advantageously incorporated into an absorbent article.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] 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:
[0016] FIGS. 1A-C are scanning electron microscope (SEM)
photographs of representative fluff pulp fibers (bleached kraft
southern pine fibers commercially available from Weyerhaeuser
Company under the designation NB416) at 100.times. magnification
(FIG. 1A), at 300.times. magnification (FIG. 1B ), and at
1000.times. magnification (FIG. 1C);
[0017] FIGS. 2A-C are SEM photographs of representative modified
fibers formed in accordance with the present invention from
bleached kraft southern pine fibers (NB416) at 100.times.
magnification (FIG. 2A), at 300.times. magnification (FIG. 2B), and
at 1000.times. magnification (FIG. 2C);
[0018] FIGS. 3A and 3B are optical microscope photographs of
representative modified fibers formed in accordance with the
present invention, FIG. 3A illustrates modified fibers before
contact with water and FIG. 3B illustrates modified fibers after
contact with water; and
[0019] FIG. 4 is a graph illustrating the absorbent capacity for
representative modified fibers formed in accordance with the
present invention as a function of weight percent crosslinking
applied to the fibers and sulfation reaction time (25 minutes, +;
35 minutes, .box-solid.; 45 minutes, .DELTA.).
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0020] In one aspect, the present invention provides a modified
cellulosic fiber having superabsorbent properties. The modified
fiber formed in accordance with the present invention has a fibrous
structure substantially identical to the cellulosic fiber from
which it is derived. More importantly, the modified fiber is a
water-swellable, water-insoluble fiber that substantially retains
its fibrous structure in its expanded, water-swelled state. The
cellulosic fiber formed in accordance with the invention is
modified cellulosic fiber that has been sulfated and crosslinked.
Water swellability is imparted to the cellulosic fiber through
sulfation and intrafiber crosslinking renders the cellulosic fiber
substantially insoluble in water. The modified cellulosic fiber has
a degree of sulfate group substitution effective to provide
advantageous water swellability. The modified cellulosic fiber is
crosslinked to an extent sufficient to render the fiber water
insoluble. The modified cellulosic fiber has a liquid absorption
capacity that is increased compared to unmodified fluff pulp
fibers. The modified fibers have a liquid absorption capacity of at
least about 4 g/g.
[0021] Cellulosic fibers suitable for use in forming the modified
fiber of the present invention are substantially water-insoluble
and not highly water-swellable. After sulfation and crosslinking in
accordance with the present invention, the resulting modified fiber
has the desired absorbency characteristics, is water-swellable and
water-insoluble, and substantially retains the fibrous structure of
the cellulosic fiber from which it is derived.
[0022] The modified fiber of the invention has the structure of a
pulp fiber including a cell wall structure. In one embodiment, the
modified fiber has the structure of a wood pulp fiber. The modified
fiber includes a lumen (i.e., central cavity) surrounded by a wall
surface having four concentric layers. In addition to an outermost
primary wall (commonly denoted P), the cell wall includes secondary
walls (commonly denoted S1-S3). The secondary walls include an
outer layer (S1) adjacent the primary wall, an inner layer (S3)
adjacent the lumen, and a middle layer (S2) intermediate the outer
and inner secondary layers. The modified fiber's structure also
includes long bundles of cellulosic fibrillar structures, referred
to as macrofibrils, fibrils, microfibrils, and elementary fibrils,
having varying diameters. The diameter of fibrillar material
depends on the extent of fiber processing.
[0023] Cellulose is a principal component of delignified cell
walls. For example, the secondary cell wall can include unbranched
cellulose chains having a degree of polymerization up to about
17,000. Accordingly, the modified fiber of the invention is
primarily cellulosic in nature having cellulose as its principal
chemical component. Cellulose can be considered to be a polymer
containing repeating anhydroglucose units. The term
"anhydroglucose" refers to the repeating unit in cellulose that is
formed by loss of water from glucose on condensation to form the
polymer. The degree of polymerization (DP) for a given cellulose
molecule is the number of anhydroglucose repeating units in the
molecule. The DP for a particular cellulose will depend on its
source and the extent of polymer degradation on processing.
[0024] In addition to cellulose, the modified fiber can include
hemicellulose and lignin. While cellulose is a linear
polysaccharide formed from glucose, hemicellulose can be either an
unbranched or branched polysaccharide that includes sugars other
than glucose. Unlike cellulose and hemicellulose, which are
carbohydrate polymers having repeating saccharide units, lignin is
a highly branched, three-dimensional polymer composed of aromatic
units. Lignin is amorphous in structure and not an integral part of
the fiber's fibrillar system of carbohydrate polymers.
[0025] For native wood fibers, lignin content is greatest in the
outer layers of the cell wall and decreases rapidly to the layer
adjacent the lumen. In contrast, cellulose content is lowest in the
primary wall and increases significantly toward the inner fiber
regions. Hemicellulose content tends to increase gradually from the
outer to the inner regions of the fiber. A description of the
chemical composition and structure of wood fibers is provided in
Pulp and Paper Manufacture, Volume I. The Pulping of Wood, Second
Edition, R. G. MacDonald, Ed., MacGraw-Hill, 1969, pages 39-45.
[0026] The chemical composition of the modified fiber of the
invention depends, in part, on the extent of processing of the
cellulosic fiber from which the modified fiber is derived. In
general, the modified fiber of the invention is derived from a
fiber that has been subjected to a pulping process (i.e., a pulp
fiber). Pulp fibers are produced by pulping processes that seek to
separate cellulose from lignin and hemicellulose leaving the
cellulose in fiber form. The amount of lignin and hemicellulose
remaining in a pulp fiber after pulping will depend on the nature
and extent of the pulping process.
[0027] Thus, the fiber of the invention is a modified pulp fiber
that retains the basic chemical and structural characteristics of a
pulp fiber. The modified fiber has a multiwalled macrostructure as
described above and is composed of primarily of cellulose and can
include some hemicellulose and lignin.
[0028] The modified fiber is substantially insoluble in water. As
used herein, a material will be considered to be water-soluble when
it substantially dissolves in excess water to form a solution,
losing its fiber form and becoming essentially evenly disbursed
throughout a water solution. A sufficiently sulfated cellulosic
fiber that is free from a substantial degree of crosslinking will
be water-soluble, whereas the modified cellulosic fiber of the
invention, a sulfated and crosslinked fiber, is
water-insoluble.
[0029] The modified fiber is a water-swellable, water-insoluble
fiber. As used herein, the term "water-swellable, water-insoluble"
refers to a material that, when exposed to an excess of an aqueous
medium (e.g., bodily fluids such as urine or blood, water,
synthetic urine, or 0.9 weight percent solution of sodium chloride
in water), swells to an equilibrium volume but does not dissolve
into solution. The water-swellable, water-insoluble modified
cellulosic fibers of the invention retain their original fibrous
structure, but in a highly expanded state, during liquid absorption
and have sufficient structural integrity to resist flow and fusion
with neighboring materials. A modified fiber of the invention is
effectively crosslinked to be substantially insoluble in water
while being capable of absorbing at least about 4 times its weight
of a 0.9 weight percent solution of sodium chloride in water under
an applied load of about 0.3 pound per square inch.
[0030] Cellulosic fibers are a starting material for preparing the
superabsorbent cellulosic fiber product of the invention. Although
available from other sources, suitable cellulosic fibers are
derived primarily from wood pulp. Suitable wood pulp fibers for use
with the invention can be obtained from well-known chemical
processes such as the kraft and sulfite processes, with or without
subsequent bleaching. Pulp fibers can also be processed by
thermomechanical, chemithermomechanical methods, or combinations
thereof. Caustic extractive pulp such as TRUCELL, commercially
available from Weyerhaeuser Company, is also a suitable wood pulp
fiber. The preferred pulp fiber is produced by chemical methods.
Ground wood fibers, recycled or secondary wood pulp fibers, and
bleached and unbleached wood pulp fibers can be used. Softwoods and
hardwoods can be used. Details of the selection of wood pulp fibers
are well-known to those skilled in the art. These fibers are
commercially available from a number of companies, including
Weyerhaeuser Company, the assignee of the present invention. For
example, suitable cellulosic fibers produced from southern pine
that are usable with the present invention are available from
Weyerhaeuser Company under the designations CF416, NF405, PL416,
FR516, and NB416. In one embodiment, the cellulosic fiber useful in
making the modified fiber of the invention is a southern pine fiber
commercially available from Weyerhaeuser Company under the
designation NB416. In other embodiments, the cellulosic fiber can
be selected from among a northern softwood fiber, a eucalyptus
fiber, a rye grass fiber, and a cotton fiber.
[0031] Cellulosic fibers having a wide range of degree of
polymerization are suitable for forming the modified cellulosic
fiber of the invention. In one embodiment, the cellulosic fiber has
a relatively high degree of polymerization, greater than about
1000, and in another embodiment, about 1500.
[0032] In one embodiment, the modified fiber has an average length
greater than about 1.0 mm. Consequently, the modified fiber is
suitably prepared from fibers having lengths greater than about 1.0
mm. Fibers having lengths suitable for preparing the modified fiber
include southern pine, northern softwood, and eucalyptus fibers,
the average length of which is about 2.8 mm, about 2.0 mm, and
about 1.5 mm, respectively. Fibers with average lengths less than
about 1.0 mm have relatively poorer wicking properties and provide
composites having diminished pad integrity.
[0033] The modified cellulosic fiber of the invention is a sulfated
cellulosic fiber. As used herein, "sulfated cellulosic fiber"
refers to a cellulosic fiber that has been sulfated by reaction of
a cellulosic fiber with a sulfating agent. It will be appreciated
that the term "sulfated cellulosic fiber" includes free acid and
salt forms of the sulfated fiber. Suitable metal salts include
sodium, potassium, and lithium salt, among others. A sulfated
cellulosic fiber can be produced by reacting a sulfating agent with
a hydroxyl group of the cellulosic fiber to provide a cellulose
sulfate ester (i.e., a carbon-to-oxygen-to-sulfur ester). The
sulfated cellulosic fiber formed in accordance with the present
invention differs from other sulfur-containing cellulosic compounds
in which the sulfate sulfur atom is attached directly to a carbon
atom on the cellulose chain as, for example, in the case of
sulfonated cellulose; or cellulosic compounds in which the sulfate
sulfur atom is attached indirectly to a carbon atom on the
cellulose chain as, for example, in the case of cellulose alkyl
sulfonates.
[0034] The modified cellulosic fiber of the invention can be
characterized as having an average degree of sulfate group
substitution of from about 0.1 to about 2.0. In one embodiment, the
modified cellulosic fiber has an average degree of sulfate group
substitution of from about 0.2 to about 1.0. In another embodiment,
the modified cellulosic fiber has an average degree of sulfate
group substitution of from about 0.3 to about 0.5. As used herein,
the "average degree of sulfate group substitution" refers to the
average number of moles of sulfate groups per mole of glucose unit
in the modified fiber. It will be appreciated that the fibers
formed in accordance with the present invention include a
distribution of sulfate modified fibers having an average degree of
sulfate substitution as noted above.
[0035] A representative method for preparing sulfated fibers is
described in Example 1.
[0036] The modified cellulosic fiber of the invention is an
intrafiber crosslinked cellulosic fiber. Crosslinked cellulosic
fibers and methods for their preparation are disclosed in U.S. Pat.
Nos. 5,437,418 and 5,225,047 issued to Graef et al., expressly
incorporated herein by reference.
[0037] Crosslinked fibers can be prepared by treating fibers with a
crosslinking agent. Suitable crosslinking agents useful in
producing the modified cellulosic fiber are generally soluble in
water and/or alcohol. Suitable cellulosic fiber crosslinking agents
include aldehyde, dialdehyde, and related derivatives (e.g.,
formaldehyde, glyoxal, glutaraldehyde, glyceraldehyde), and
urea-based formaldehyde addition products (e.g., N-methylol
compounds). See, for example, U.S. Pat. Nos. 3,224,926; 3,241,533;
3,932,209; 4,035,147; 3,756,913; 4,689,118; 4,822,453; U.S. Pat.
No. 3,440,135, issued to Chung; U.S. Pat. No. 4,935,022, issued to
Lash et al.; U.S. Pat. No. 4,889,595, issued to Herron et al.; U.S.
Pat. No. 3,819,470, issued to Shaw et al.: U.S. Pat. No. 3,658,613,
issued to Steiger et al.; and U.S. Pat. No. 4,853,086, issued to
Graef et al., all of which are expressly incorporated herein by
reference in their entirety. Cellulosic fibers can also be
crosslinked by carboxylic acid crosslinking agents including
polycarboxylic acids. U.S. Pat. Nos. 5,137,537; 5,183,707; and
5,190,563, describe the use of C2-C9 polycarboxylic acids that
contain at least three carboxyl groups (e.g., citric acid and
oxydisuccinic acid) as crosslinking agents.
[0038] Suitable urea-based crosslinking agents include methylolated
ureas, methylolated cyclic ureas, methylolated lower alkyl
substituted cyclic ureas, methylolated dihydroxy cyclic ureas,
dihydroxy cyclic ureas, and lower alkyl substituted cyclic ureas.
Specific preferred urea-based crosslinking agents include
dimethylol urea (DMU, bis[N-hydroxymethyl]ure- a),
dimrethylolethylene urea (DMEU,
1,3-dihydroxymethyl-2-imidazolidinone)- ,
dimethyloldihydroxyethylene urea (DMDHEU,
1,3-dihydroxymethyl-4,5-dihydr- oxy-2-imidazolidinone),
di-methylolpropylene urea (DMPU), dimethyloihydantoin (DMH),
dimethyldihydroxy urea (DMDHU), dihydroxyethylene urea (DHEU,
4,5-dihydroxy-2-imidazolidinone), and dimethyldihydroxyetbylene
urea (DMeDHEU, 4,5-dihydroxy-1,3-dimethyl-2-imi- dazolidinone).
[0039] Suitable polycarboxylic acid crosslinking agents include
citric acid, tartaric acid, malic acid, succinic acid, glutaric
acid, citraconic acid, itaconic acid, tartrate monosuccinic acid,
maleic acid, 1,2,3-propane tricarboxylic acid,
1,2,3,4-butanetetracarboxylic acid, all-cis-cyclopentane
tetracarboxylic acid, tetrahydrofuran tetracarboxylic acid,
1,2,4,5-benzenetetracarboxylic acid, and benzenehexacarboxylic
acid. Other polycarboxylic acids crosslinking agents include
polymeric poly-carboxylic acids such as poly(acrylic acid),
poly(methacrylic acid), poly(maleic acid), poly(methylvinylether-c-
o-maleate) copolymer, poly(methylvinylether-co-itaconate)
copolymer, copolymers of acrylic acid, and copolymers of maleic
acid. The use of polymeric polycarboxylic acid crosslinking agents
such as polyacrylic acid polymers, polymaleic acid polymers,
copolymers of acrylic acid, and copolymers of maleic acid is
described in U.S. Pat. No. 5,998,511, assigned to Weyerhaeuser
Company and expressly incorporated herein by reference in its
entirety.
[0040] Other suitable crosslinking agents include diepoxides such
as, for example, vinylcyclohexene dioxide, butadiene dioxide, and
diglycidyl ether; sulfones such as, for example, divinyl sulfone,
bis(2-hydroxyethyl)sulfone, bis(2-chloroethyl)sulfone, and disodium
tris(.beta.-sulfatoethyl)sulfonium inner salt; and
diisocyanates.
[0041] Mixtures and/or blends of crosslinking agents can also be
used.
[0042] The crosslinking agent can include a catalyst to accelerate
the bonding reaction between the crosslinking agent and cellulosic
fiber. Suitable catalysts include acidic salts, such as ammonium
chloride, ammonium sulfate, aluminum chloride, magnesium chloride,
and alkali metal salts of phosphorous-containing acids.
[0043] The modified cellulosic fiber of the invention is a
crosslinked cellulosic fiber. The amount of crosslinking agent
applied to the fiber is suitably the amount necessary to render the
modified fiber substantially insoluble in water. The amount of
crosslinking agent applied to the cellulosic fiber will depend on
the particular crosslinking agent and is suitably in the range of
from about 0.01 to about 8.0 percent by weight based on the total
weight of cellulosic fiber. In one embodiment, the amount of
crosslinking agent applied to the fibers is in the range from about
0.20 to about 5.0 percent by weight based on the total weight of
fibers.
[0044] In one embodiment, the crosslinking agent can be applied to
the cellulosic fibers as an aqueous alcoholic solution. Water is
present in the solution in an amount sufficient swell the fiber to
an extent to allow for crosslinking within the fiber's cell wall.
However, the solution does not include enough water to dissolve the
fiber. Suitable alcohols include those alcohols in which the
crosslinking agent is soluble and the fiber to be crosslinked
(i.e., unmodified or sulfated cellulosic fiber) is not.
Representative alcohols include alcohols that include from 1 to 5
carbon atoms, for example, methanol, ethanol, n-propanol,
isopropanol, n-butanol, isobutanol, s-butanol, and pentanols. In
another embodiment, the crosslinking agent can be applied to the
fibers as an ether solution (e.g., diethyl ether).
[0045] It will be appreciated that due to its fiber structure, the
modified fiber of the invention can have a distribution of sulfate
and/or crosslinking groups along the fiber's length and through the
fiber's cell wall. Generally, there can be greater sulfation and/or
crosslinking on or near the fiber surface than at or near the fiber
core. Surface crosslinking may be advantageous to improve modified
fiber dryness and provide a better balance of total absorbent
capacity and surface dryness. Fiber swelling and soak time can also
effect the sulfation and crosslinking gradients. Such gradients may
be due to the fiber structure and can be adjusted and optimized
through control of sulfation and/or crosslinking reaction
conditions.
[0046] A representative method for crosslinking sulfated fibers is
described in Example 2.
[0047] Scanning electron microscope (SEM) photographs of bleached
kraft southern pine fibers (NB416) at 100.times., 300.times., and
1000.times. magnification are illustrated in FIGS. 1A-C,
respectively. SEM photographs of representative modified fibers
formed from NB416 fibers in accordance with the invention at
100.times., 300.times., and 1000.times. magnification are
illustrated in FIGS. 2A-C, respectively. Referring to FIGS. 1A-C
and 2A-C, the modified fibers are ribbon-like and are twisted and
curled, and have a structure substantially identical to the fiber
from which they are derived.
[0048] The modified fiber of the invention has a liquid absorbent
capacity of at least about 4 g/g as measured by the centrifuge
capacity test described in Example 3. In one embodiment, the
modified fiber has a capacity of at least about 10 g/g. In another
embodiment, the fiber has a capacity of at least about 15 g/g, and
in a further embodiment, the fiber has a capacity of at least about
20 g/g. The absorbent capacity of representative modified fibers
formed in accordance with the present invention is described in
Example 3.
[0049] As noted above, the modified fiber retains the structure of
a fiber. FIGS. 3A and 3B are optical microscope photographs of
representative modified fibers formed in accordance with the
invention before and after contact with water. FIG. 3A shows
representative modified fibers that have not been contacted with
water. Referring to FIG. 3A, these fibers are ribbon-like and are
twisted and curled. FIG. 3B shows representative modified fibers
that have been contacted with water. Referring to FIG. 3B, these
swelled fibers have retained their fiber structure and have
expanded diameters that are from about 3 to about 6 times their
original diameter.
[0050] In another aspect of the invention, methods for making a
cellulosic fiber having superabsorbent properties are provided. In
the methods, cellulosic fibers are sulfated and crosslinked to
provide superabsorbent fibers. In one embodiment, cellulosic fibers
are sulfated and then crosslinked. In this method, sulfated
cellulosic fibers are treated with an amount of crosslinking agent
sufficient to render the resulting modified cellulosic fibers
substantially insoluble in water. In another embodiment, cellulosic
fibers are crosslinked then sulfated. In this method, crosslinked
cellulosic fibers are sulfated to render the resulting modified
cellulosic fibers highly water absorptive. The modified cellulosic
fiber formed by either method is highly water absorptive,
water-swellable, water-insoluble, and retains the fibrous structure
of the fibers from which it is derived.
[0051] The modified fiber of the invention is a sulfated cellulosic
fiber. Sulfated cellulosic fibers can be made by reacting
cellulosic fibers (e.g., cellulosic fibers that are crosslinked or
noncrosslinked) with a sulfating agent. Suitable sulfating agents
include concentrated sulfuric acid (95-98%), fuming sulfuric acid
(oleum), sulfur trioxide and related complexes including sulfur
trioxide/dimethylformnamide and sulfur trioxide/pyridine complexes,
and chlorosulfonic acid, among others. In one embodiment, the
sulfating agent is concentrated sulfuric acid.
[0052] The sulfating agent is preferably applied to the fibers as a
solution in an organic solvent. Suitable organic solvents include
alcohols, pyridine, dimethylformamide, acetic acid including
glacial acetic acid, and dioxane. In one embodiment, the organic
solvent is an alcohol having up to about 6 carbon atoms. Suitable
alcohols include methanol, ethanol, n-propanol, isopropanol,
n-butanol, isobutanol, s-butanol, pentanols, and hexanols. In one
embodiment, the alcohol is selected from among isopropanol and
isobutanol.
[0053] The molar ratio of sulfuric acid to alcohol in the solution
can be varied from about 1:1 to about 4:1. In one embodiment, the
molar ratio of sulfuric acid to alcohol is about 2.4:1, for
example, an 80:20 (weight/weight) solution of sulfuric acid in
isopropanol. The weight ratio of sulfuric acid to cellulosic fibers
in the sulfation reaction can be varied from about 5:1 to about
30:1. At low sulfuric acid ratios the reaction is slow and
incomplete and at high sulfuric acid ratios significant cellulose
polymer degradation can occur. In one embodiment, the weight ratio
of sulfuric acid to pulp fiber is from about 10:1 to about 25:1. In
another embodiment, the weight ratio of sulfuric acid to pulp fiber
is about 24:1.
[0054] Highly acidic aqueous environments readily degrade cellulose
fibers. It has been reported that concentrated sulfuric acid cannot
be used to prepare sulfated cellulose because treating cellulose
with sulfuric acid results in a soluble product formed from acid
hydrolysis of the cellulose backbone by the sulfuric acid. See, WO
96/15137. However, a water-soluble cellulose sulfate has been
reportedly prepared from an activated cellulose (20 to 30% water)
by direct action of aqueous sulfuric acid or sulfuric acid
dissolved in a volatile organic solvent such as toluene, carbon
tetrachloride, or a lower alkanol. "Cellulose Chemistry and Its
Applications", Ed. T. P. Nevell and S. H. Zeronian, Halstead Press,
John Wiley and Sons, 1985, page 350.
[0055] Despite the well-known degradation of cellulose in aqueous
acidic solutions, the present invention provides methods for making
sulfated cellulose fibers without significant cellulose hydrolysis.
In the methods of the invention, cellulose fiber degradation (i.e.,
degree of polymerization reduction) is substantially avoided by
treating cellulose fibers with a sulfating agent in a nonaqueous
environment and/or at low temperature (e.g., at or below about
4.degree. C.). To further protect against fiber degradation (e.g.,
hydrolysis), a dehydrating agent to absorb water, including water
formed during the sulfation reaction, can be added to the sulfating
reaction mixture. Suitable dehydrating agents include, for example,
sulfur trioxide, magnesium sulfate, acetic anhydride, and molecular
sieves. In one embodiment, cellulosic fibers are reacted with the
sulfating agent at a temperature of about 4.degree. C. and both the
cellulosic fibers and the sulfating agent are cooled to about
4.degree. C. prior to reaction. In another embodiment, cellulosic
fibers, including cooled fibers, are reacted with the sulfating
agent in the presence of a dehydrating agent.
[0056] Depending upon the extent of sulfation desired, the fibers
and sulfating agent are reacted for a period of time of from about
10 to about 60 minutes. Following this reaction period and prior to
neutralizing the resulting sulfated fibers, the sulfated fibers are
separated from excess sulfating agent. In one embodiment, the
sulfated fibers are washed with an alcohol prior to
neutralization.
[0057] Prior to crosslinking the sulfated cellulosic fibers to
provide the modified fibers of the invention, the fibers can be at
least partially neutralized with a neutralizing agent. The
neutralizing agent is suitably soluble in the sulfation solvent. In
one embodiment, the neutralizing agent is a base such as, for
example, an alkaline base (e.g., lithium, potassium, sodium or
calcium hydroxide; lithium, potassium, or sodium acetate).
Alternatively, the neutralizing agent can include a multivalent
metal salt. Suitable metal salts include cerium, magnesium,
calcium, zirconium, and aluminum salts such as ammonium cerium
nitrate, magnesium sulfate, magnesium chloride, calcium chloride,
zirconium chloride, aluminum chloride, and aluminum sulfate, among
others. The use of multivalent metal salts as neutralizing agents
also offers the advantage of intrafiber crosslinking. Thus, through
the use of a multivalent metal salt, the sulfated cellulosic fiber
can be partially neutralized and partially crosslinked. Fibers so
treated can be further crosslinked with other crosslinking agents
including those described above.
[0058] The extent of fiber sulfation is dependent on a number of
reaction conditions including reaction time. For example, in a
series of representative sulfation reactions, a 25 minute reaction
time provided a fiber that included about 3.8 percent by weight
sulfur; a 35 minute reaction time provided a fiber that included
about 4.9 percent by weight sulfur; and a 45 minute reaction time
provided a fiber that included about 6.4 percent by weight sulfur.
However, in these experiments, the extended sulfation reaction time
had an adverse effect on fiber length (i.e., cellulose hydrolysis
occurred under the prolonged reaction conditions). In viscosity
experiments, the sulfated fibers produced by the 25 and 35 minute
reaction conditions provide cellulose solutions classified as
having a Gardner-Holt bubble tube H viscosity (i.e., about 200
Centistokes), while the sulfated fibers produced by the 45 minute
reaction provided cellulose solutions classified as having C
viscosity (i.e., about 85 Centistokes). The results indicate that
at extended reaction times, significant fiber degradation can
occur. The absorbent capacity of modified fibers prepared from
these sulfated fibers is described in Example 3.
[0059] A representative method for preparing sulfated fibers is
described in Example 1.
[0060] The at least partially neutralized sulfated cellulosic
fibers can then be crosslinked by applying a crosslinking agent to
the fibers. In one embodiment, the crosslinking agent is applied to
the fibers as an aqueous alcoholic solution. In general, the
crosslinking agent solution includes water sufficient to swell but
not dissolve the fibers. Above about 95 percent by weight alcohol,
the crosslinking agent does not penetrate the fiber cell wall
sufficiently and the result is a crosslinked fiber having
nonuniform crosslinking and low absorbent capacity. Suitably, the
aqueous alcoholic solution includes from about 10 to about 50
percent by weight water and from about 50 to about 90 percent by
weight alcohol. In one embodiment, the crosslinking agent solution
is an aqueous ethanol solution (88 percent by weight ethanol).
[0061] After the fibers have been treated with the crosslinking
agent, the crosslinking agent is cured by, for example, heating the
treated fibers, to provide intrafiber crosslinked fibers.
[0062] A representative method for crosslinking sulfated fibers is
described in Example 2. The method of Example 2 describes
crosslinking sulfated fibers that have been isolated and dried.
Alternatively, sulfated fibers formed as described above and in
Example 1 may be directly crosslinked, after neutralization,
without drying the fibers.
[0063] Thus, in one embodiment, the present invention provides a
method for making cellulosic fibers having superabsorbent
properties that includes the step of reacting cellulosic fibers
with a sulfating agent, at least partially neutralizing the
sulfated fibers to provide fibers suitable for crosslinking,
applying a crosslinking agent to the sulfated fibers, and then
curing the crosslinking agent to provide the modified fibers.
[0064] It has been discovered that the nature of the modified fiber
of the present invention can be varied and controlled by the amount
of water present in the crosslinking reaction. For example, when it
is desirable to produce the modified fiber in individual fiber
form, relatively less water is used in the crosslinking reaction.
Conversely, when it is desired that the modified fiber be produced
as a sheet or web (e.g., rollgood), the crosslinking reaction
includes a relatively greater amount of water. It has been found
that water present during the crosslinking reaction effects bonding
between the individual, modified fibers. When the water content is
sufficiently high in the crosslinking reaction, interfiber bonding
can occur to provide a structure having sufficient strength and
integrity to provide a fibrous web or sheet of the modified fiber
suitable for the formation of a rollgood. Where it is desirable to
form the modified fiber in individual form, the modified fiber can
be baled for shipping and subsequent processing.
[0065] Some interfiber bonding and loss of individual fiber
structure occurs when more than about 50 percent by weight water is
present in the crosslinking reaction. Between from about 50 and
about 90 percent by weight alcohol, interfiber bonding occurs
without the loss of individual fiber structure.
[0066] The method described above can further include other steps
to optimize the production of the modified fibers of the invention.
To further assist in preventing fiber hydrolysis during sulfation,
the cellulosic fibers can be dried prior to the sulfation reaction.
The fibers can be dried by any one of a number of drying methods
including heating and chemical methods. For example, the fibers can
be dried by heating in a drying oven; solvent exchange with a
suitable solvent; solvent exchange with a suitable solvent followed
by heating; or treatment with a dehydrating agent such sulfur
trioxide or acetic anhydride. Alternatively, a never-dried fiber
can be dried by solvent exchange using a suitable solvent.
[0067] For effective sulfation, cellulosic fibers, including dried
fibers, can be swelled prior to sulfation using a swelling agent.
Suitable swelling agents include, for example, water, glacial
acetic acid, acetic anhydride, zinc chloride, sulfuric acid, sulfur
trioxide, and ammonia. The fibers can be swelled by mixing the
fibers with the swelling agent followed by removing excess swelling
agent prior to reacting the fibers with the sulfating agent.
[0068] Thus, in another embodiment, the present invention provides
a method for making cellulosic fibers having superabsorbent
properties that includes the steps of swelling cellulosic fibers,
including dry fibers, with a swelling agent; separating excess
swelling agent from the swelled fibers; reacting the swelled fibers
with a sulfating agent; separating excess sulfating agent from the
fibers; at least partially neutralizing the sulfated fibers to
provide fibers suitable for crosslinking; applying a crosslinking
agent to the sulfated fibers; and then curing the crosslinking
agent to provide intrafiber crosslinked, sulfated cellulosic
fibers.
[0069] In another embodiment, the modified cellulosic fibers of the
invention can be formed by crosslinking then sulfating the
cellulosic fibers. In the method, the modified fibers can be
prepared by applying a crosslinking agent to cellulosic fibers;
curing the crosslinking agent to provide crosslinked fibers;
reacting the crosslinked cellulosic fibers with a sulfating agent;
at least partially neutralizing the sulfated, crosslinked fibers;
and then drying the sulfated, crosslinked cellulosic fibers.
[0070] The modified fiber of the invention is formed by methods
that do not include dissolving the fiber in solution. In this way,
the modified fiber retains the structure of the fiber from which it
is derived. The structure of the modified fiber of the invention is
in contrast to other fibrous materials that lack fiber structure
and that are prepared by regeneration from solutions (i.e., formed,
for example, by precipitation, from solutions containing dissolved
cellulosic materials).
[0071] The modified fiber formed in accordance with the present
invention has superabsorbent properties while, at the same time,
has the structure of the cellulosic pulp fiber from which it is
derived. As noted above, the modified fiber of the invention can be
produced as an individual fiber or as sheet or web (e.g., rollgood)
of fibers. The nature of the modified fiber produced depends on the
use for which the fiber is ultimately intended.
[0072] The modified fibers can be incorporated into a personal care
absorbent product. The modified fibers can be formed into a
composite for incorporation into a personal care absorbent product.
Composites can be formed from the modified fibers alone or by
combining the modified fibers with other materials, including
fibrous materials, binder materials, other absorbent materials, and
other materials commonly employed in personal care absorbent
products. Suitable fibrous materials include synthetic fibers, such
as polyester, polypropylene, and bicomponent binding fibers; and
cellulosic fibers, such as fluff pulp fibers, crosslinked
cellulosic fibers, cotton fibers, and CTMP fibers. Suitable
absorbent materials include natural absorbents, such as sphagnum
moss, and synthetic superabsorbents, such as polyacrylates (e.g.,
SAPs).
[0073] In one embodiment, the modified fiber is further treated
with a compatible material to provide a coated modified fiber. The
modified fiber can be coated with a variety of materials including
those noted above as well as binders, pH control agents, and odor
reducing agents, among others.
[0074] Webs that include the modified fibers can be prepared in any
one of a variety of methods known in the web-forming art. The
methods include airlaid and wet forming methods. As noted above,
wet-formed webs that include the modified fibers can be formed by,
for example, adding water in an amount sufficient to bond the
crosslinked sulfated fibers to an extent sufficient to provide a
web with structural integrity. Other materials, such as fibrous and
absorbent materials, can also be included in these webs.
[0075] In some instances, when intended for use in a personal care
absorbent product, the rollgood form of the modified fiber is
desired. One advantage of the modified fiber in rollgood form is
that it can be directly incorporated as received by a diaper
manufacturer by cutting the rollgood into the desired shape and
size, and inserting the shaped and sized web into an absorbent
article. In this way, the modified fiber in rollgood form can be
directly utilized in a diaper manufacturing line. The rollgood
containing the modified fiber can also include any one or more of a
variety of other useful materials such as those identified
above.
[0076] Absorbent composites derived from or that include the
modified fibers of the invention can be advantageously incorporated
into a variety of absorbent articles such as diapers including
disposable diapers and training pants; feminine care products
including sanitary napkins, and pant liners, adult incontinence
products; toweling; surgical and dental sponges; bandages; food
tray pads; and the like. Thus, in another aspect, the present
invention provides absorbent composites and absorbent articles that
include the modified fiber.
[0077] As noted above, the modified fiber of the invention has a
fiber structure that, like other pulp fibers, provides for liquid
wicking. Like superabsorbent materials, the modified fiber has a
high liquid absorbent capacity. Accordingly, the modified fiber can
be useful in absorbent products such as, for example, an infant
diaper, where liquid wicking and liquid storage are required.
Because of its unique, liquid wicking and capacity properties, the
modified fiber can be formed into a composite and utilized as a
storage core in a diaper. Such a core may only include the modified
fiber. For a modified fiber having an absorbent capacity of at
least about 22 g/g, the resulting core has an absorbent capacity of
at least about 22 g/g. Conventional, commercial diaper storage
cores typically include two components: (1) fluff pulp fibers to
wick liquid, and (2) superabsorbent material to store acquired
liquid. The core typically consists of minimally about 25 percent
by weight fluff pulp fibers and maximally about 75 percent by
weight superabsorbent material. Superabsorbent materials generally
have an absorbent capacity of about 28 g/g and fluff pulp fibers
generally have an absorbent capacity of about 2 g/g. Therefore,
such a core has a capacity of about 22 g/g. Cores prepared from a
modified fiber having a capacity of at least about 22 g/g can
exceed the performance characteristics of conventional absorbent
composites. Thus, the modified fibers of the invention provide
advantages related to the manufacture of absorbent cores.
[0078] The following examples are provided for the purposes of
illustrating, not limiting, the present invention.
EXAMPLES
Example 1
The Preparation of Sulfated Cellulosic Fibers
[0079] In this example, a representative method for forming
sulfated cellulosic fibers is described.
[0080] Prior to sulfation, the pulp was activated with acetic acid.
Ten grams of fiberized bleached kraft southern yellow pine fluff
pulp (NB416. Weyerhaeuser Company, Federal Way, Wash.) that had
been oven dried at 105.degree. C. was disbursed in 600 mL of
glacial acetic acid. The pulp/acid slurry was then placed in a
vacuum chamber and the air was evacuated. The slurry was allowed to
stand under vacuum for 30 minutes after which time the chamber was
repressurized to atmospheric pressure. The slurry was then allowed
to stand at ambient conditions for 45 minutes before being
resubjected to a vacuum for an additional 30 minutes. After the
second application of a vacuum the slurry was again allowed to
stand for 45 minutes at atmospheric pressure. The slurry was then
poured into a Buchner funnel where the pulp was collected and
pressed until the weight of the residual acetic acid was equal to
twice the weight of the oven dry pulp (i.e., total weight of the
collected pulp was 30 g.) The collected pulp was placed inside a
plastic bag and cooled to -10.degree. C. in a freezer.
[0081] The sulfation liquor was prepared by mixing 240 g
concentrated sulfuric acid with 60 g isopropanol and 0.226 g
magnesium sulfate. The liquor was prepared by pouring isopropanol
into a beaker that was maintained at 4.degree. C. in an ice bath.
Magnesium sulfate was then added to the isopropanol and the mixture
chilled to 4.degree. C. Sulfuric acid was weighed into a beaker and
separately chilled to 9.degree. C. before being slowly mixed into
the isopropanol and magnesium sulfate mixture. The resulting
sulfating liquor was then allowed to cool to 4.degree. C.
[0082] The cooled acetic acid activated pulp (-10.degree. C.) was
stirred into the cooled sulfation liquor (4.degree. C.). The
resulting slurry of pulp and sulfation liquor was allowed to react
for 35 minutes with constant stirring. After 35 minutes the
pulp/sulfation liquor slurry was poured into a Buchner funnel and
the sulfated pulp was collected and washed over a vacuum with
cooled isopropanol (-10.degree. C.). The collected pulp was then
slurried with cooled isopropanol (-10.degree. C.) in a Waring
blender and poured back into the Buchner funnel where the pulp was
again washed with cooled isopropanol (-10 .degree. C.).
[0083] The nature and quality of the modified fiber formed in
accordance with the invention can depend on the washing step.
First, the acid is preferably washed from the pulp as quickly as
possible to prevent continued and/or accelerated cellulose
degradation. Second, the cool temperature of the pulp is preferably
maintained to prevent cellulose degradation. Third, the acid is
preferably washed from the pulp as thoroughly as possible before
neutralization to prevent the formation of difficult to remove
inorganic salts during the neutralization step. These salts can
adversely impact modified fiber absorbency.)
[0084] The washed sulfated pulp was next slurried in cooled
isopropanol (-10.degree. C.) and an ethanolic sodium hydroxide
solution was added dropwise until the slurry was neutralized. The
slurry was then poured into a Buchner funnel where the neutralized
sulfated pulp was washed with room temperature isopropanol. The
neutralized sulfated pulp was then agitated to remove any inorganic
salts that may have been crusted on the fiber surfaces after which
the neutralized sulfated pulp was again washed with isopropanol in
a Buchner funnel. Finally the collected sulfated pulp was allowed
to air dry.
Example 2
The Preparation of Representative Crosslinked, Sulfated Cellulosic
Fibers
[0085] In this example, a representative method for forming
crosslinked, sulfated cellulosic fibers is described. Sulfated
cellulosic fibers prepared as described in Example 1 were
crosslinked with a representative crosslinking agent.
[0086] A catalyzed urea-formaldehyde system was used to crosslink
the sulfated cellulosic fibers. The catalyst included magnesium
chloride and the sodium salt of dodecylbenzenesulfonic acid
dissolved in 88% ethanol/water. In addition to its primary
function, the catalyst solution served as a diluent for the
crosslinking agent. The crosslinking agent was obtained by
dissolving urea in 37 percent (w/w) aqueous formaldehyde. The
crosslinking agent was combined with the catalyst solution and
applied to the sulfated fibers. The treated fibers were then cured
by placing in a 105.degree. C. oven for 60 minutes.
[0087] In the experiment, varying amounts of crosslinking agents
were applied to the fibers. The amount of crosslinking agent used
ranged from 1-11 percent of the weight of the sulfated fibers and
the amount of catalytic diluent used was 250 percent of the weight
of the sulfated fibers. The materials and their amounts used in
preparing the catalytic diluent and crosslinking agent solutions
are shown in Table 1 below.
1TABLE 1 Composition of Catalytic Diluent and Crosslinking Agent
Solution. Parts Catalytic Diluent Denatured ethanol 44 Deionized
water 6 Magnesium chloride heptahydrate 0.214
Dodecylbenzenesulfonic acid. sodium salt 0.4 Crosslinking Agent
Solution Urea 15 37% (w/w) Formaldehyde 41
Example 3
The Performance Characteristics of Representative Crosslinked,
Sulfated Cellulosic Fibers
[0088] In this example, the performance characteristics of
representative crosslinked, sulfated cellulosic fibers formed in
accordance with the present invention is described. Representative
modified fibers, prepared as described in Examples 1 and 2 above,
with varying levels of crosslinking agent applied to the fibers
were evaluated for absorbent capacity by the total absorptive
capacity/tea bag gel volume test described below. Modified fiber
absorbent capacity as a function of crosslinking agent applied to
the fiber is summarized in Table 2 below.
[0089] The preparation of materials, test procedure, and
calculations to determine absorbent capacity were as follows.
[0090] Preparation of Materials:
[0091] 1) Tea bag preparation: unroll tea bag material (Dexter
#1234T heat-sealable tea bag material) and cut cross ways into 6 cm
pieces. Fold lengthwise, outside-to-outside. Heatseal edges
{fraction (1/8)} inch with an iron (high setting), leave top end
open. Trim excess from top edge to form a 6 cm.times. 6 cm bag.
Prepare 3 tea bags.
[0092] 2) Label edge with sample identification.
[0093] 3) Preweigh tea bag and record weight (to nearest 0.001
g).
[0094] 4) Weigh 0.200 g sample (nearest 0.001 g) on tared glassine
and record weight.
[0095] 5) Fill tea bags with modified fiber sample.
[0096] 6) Seal top edge of tea bag {fraction (1/8)} inch with the
iron.
[0097] 7) Weigh and record total weight of tea bag filled with
modified fiber sample. Store in sealed plastic bag until ready to
test.
[0098] Test Procedure:
[0099] 1) Fill container to a depth of at least 2 inch with 1
percent by weight saline solution.
[0100] 2) Hold tea bag horizontally and distribute modified fiber
sample evenly throughout tea bag.
[0101] 3) Lay tea bag on the liquid surface of the saline solution
(begin timing) and allow tea bag to wet-out before submerging the
tea bag (about 10 sec.).
[0102] 4) Soak tea bag for 30 minutes.
[0103] 5) Remove tea bag from the saline solution with tweezers and
clip to a drip rack.
[0104] 6) Allow tea bag to hang for 3 minutes.
[0105] 7) Carefully remove tea bag from clip and lightly touch
saturated corner of tea bag on blotter to remove excess fluid.
Weigh tea bag and record weight (i.e., drip weight).
[0106] 8) Place tea bag on wall of centrifuge by pressing top edge
against the wall. Balance centrifuge by placing the tea bags around
the centrifuge's circumference.
[0107] 9) Centrifuge at 2800 rpm for 75 seconds.
[0108] 10) Remove tea bag from centrifuge, weigh and record tea bag
centrifuged weight.
[0109] Absorbent Centrifuge Capacity Calculation:
[0110] (Net wet weight sample-Net dry weight sample)/Net dry weight
sample=g/g capacity.
[0111] Net wet weight is the centrifuge weight less the dry weight
of the tea bag and fiber sample. Net dry weight is the dry weight
of the fiber sample.
[0112] The absorbent capacity (g/g), determined as described above,
as a function of sulfation reaction time and crosslinking agent
applied to the fiber for representative modified fibers is
summarized in Table 2 below and illustrated graphically in FIG.
4.
2TABLE 2 Modified Fiber Absorbent Capacity: Crosslinking Level and
Sulfation Reaction Time Effect. Centrifuge Capacity (g/g)
Crosslinking level 25 minute 35 minute 45 minute (percent by
weight) sulfation sulfation sulfation 1.08 13.0 12.1 7.0 1.62 15.3
14.6 10.1 1.94 17.2 2.27 15.1 2.48 17.3 2.27 14.7 18.0 2.97 11.3
3.24 11.9 3.78 8.1 7.9 8.6 4.00 6.6
[0113] As shown in Table 2 and FIG. 4, to a point, absorbent
capacity increases with increasing sulfation. However, at the point
where sulfation results in fiber degradation, absorbent capacity
decreases. The results also demonstrate that absorbent capacity
also increases with increasing crosslinking to a point. At higher
levels of crosslinking, absorbent capacity decreases.
[0114] While the preferred embodiment of the invention has been
illustrated and described, it will be appreciated that various
changes can be made therein without departing from the spirit and
scope of the invention.
* * * * *