U.S. patent application number 10/387485 was filed with the patent office on 2004-09-16 for method for making chemically cross-linked cellulosic fiber in the sheet form.
Invention is credited to Chmielewski, Harry J., Hamed, Othman A., Murguia, Tina R., Sears, Karl.
Application Number | 20040177935 10/387485 |
Document ID | / |
Family ID | 32961900 |
Filed Date | 2004-09-16 |
United States Patent
Application |
20040177935 |
Kind Code |
A1 |
Hamed, Othman A. ; et
al. |
September 16, 2004 |
Method for making chemically cross-linked cellulosic fiber in the
sheet form
Abstract
The present invention is directed to cross-linked cellulosic
fiber in the sheet from, obtainable by cross-linking a blend of
mercerized pulp and conventional pulp. The method includes heating
treated cellulosic fibers to promote intra-fiber cross-linking. The
cross-linked fibers are characterized by an improved acquisition
rate, resiliency, absorbency, and absorbency under load. Moreover,
the inventive cross-linked fibers exhibit a reduction in centrifuge
retention capacity, and have low knots, nits and fines contents.
The cross-linked cellulosic fibers of the invention are useful in
the acquisition layer and/or absorbent core of absorbent
articles.
Inventors: |
Hamed, Othman A.; (Jesup,
GA) ; Chmielewski, Harry J.; (Brunswick, GA) ;
Murguia, Tina R.; (Surrency, GA) ; Sears, Karl;
(Jesup, GA) |
Correspondence
Address: |
HUNTON & WILLIAMS LLP
INTELLECTUAL PROPERTY DEPARTMENT
1900 K STREET, N.W.
SUITE 1200
WASHINGTON
DC
20006-1109
US
|
Family ID: |
32961900 |
Appl. No.: |
10/387485 |
Filed: |
March 14, 2003 |
Current U.S.
Class: |
162/9 ;
162/157.6; 604/375; 604/378; 8/116.1 |
Current CPC
Class: |
D21C 9/002 20130101;
A61L 15/28 20130101; A61L 15/28 20130101; A61F 13/537 20130101;
A61F 13/53 20130101; C08L 1/02 20130101 |
Class at
Publication: |
162/009 ;
162/157.6; 008/116.1; 604/375; 604/378 |
International
Class: |
D06M 013/192; D06M
013/00; D21C 009/00 |
Claims
What is claimed is:
1. Cross-linked cellulosic fibers comprising a blend of mercerized
cellulosic fibers and conventional fibers having an absorbent
capacity of at least about 8.0 grams saline/gram of fiber.
2. The cross-linked fibers of claim 1 wherein the fibers have a
centrifuge retention capacity of less than about 0.6 grams of a
0.9% by weight saline solution per gram of fiber.
3. The cross-linked fibers of claim 1, wherein the fibers have an
absorption capacity of at least about 9.0 g saline/g fiber.
4. The cross-linked fibers of claim 1, wherein the fibers have a
centrifuge retention of not more than 0.5 g saline/g fiber.
5. The cross-linked fibers of claim 1, wherein the fibers have a
free swell of at least about 10.0 g saline/g fiber.
6. The cross-linked fiber of claim 1, wherein the fibers have knots
and nits contents of less than about 10%.
7. The cross-linked fiber of claim 1, wherein the fibers have fines
contents of less than about 6.5%.
8. A method of making a cross-linked blend of cellulosic fibers
comprising: forming a wet laid sheet of a blend of mercerized and
conventional cellulosic fibers; supplying a cross-linking agent to
the sheet of fibers to form a sheet impregnated with the
cross-linking agent; and drying and curing the cross-linking agent
on the impregnated sheet of cellulosic fibers to form intra-fiber
cross-links.
9. The method of claim 8, wherein the mercerized cellulose fiber is
a conventional cellulose fiber that has been contacted with an
alkali metal, washed, neutralized, and optionally dried.
10. The method of claim 8, wherein the conventional cellulose fiber
is a wood pulp fiber selected from the group consisting of hardwood
pulp, softwood cellulose pulp obtained from a Kraft or sulfite
chemical process, and combinations or mixtures thereof.
11. The method of claim 10, wherein the hardwood cellulose pulp is
selected from the group consisting of gum, maple, oak, eucalyptus,
poplar, beech, aspen, and combinations and mixtures thereof.
12. The method of claim 10, wherein the soft cellulose pulp is
selected from the group consisting of Southern pine, White pine,
Caribbean pine, Western hemlock, spruce, Douglas fir, and mixtures
and combinations thereof.
13. The method of claim 10, wherein the conventional cellulose
fiber is derived from one or more components selected from the
group consisting of cotton linters, bagasse, kemp, flax, grass, and
combinations and mixtures thereof.
14. The method of claim 9, wherein the mercerized fibers are
prepared by treating conventional cellulosic fibers with an aqueous
solution of sodium hydroxide, washing the fibers, and neutralizing
the treated fibers.
15. The method of claim 14, wherein treating the conventional
cellulosic fibers comprises contacting the fibers with an aqueous
solution containing about 4% to about 40% by weight sodium
hydroxide, based on the total weight of the solution.
16. The method of claim 14, wherein treating the cellulosic fibers
with caustic is carried out in an aqueous solution of sodium
hydroxide at 2 to 10% consistency.
17. The method of claim 14, wherein washing and neutralizing is
carried out until the residual water has a pH of between 3 and
8.
18. The method of claim 9, wherein the cellulosic fibers are
caustic treated in sheet or roll form.
19. The method of claim 9, wherein the caustic treated cellulosic
fiber is in the dry or wet state.
20. The method of claim 9, wherein the .alpha.-cellulose content of
the caustic treated cellulosic fibers is greater than 65%.
21. The method of claim 20, wherein the .alpha.-cellulose content
is at least 90%.
22. The method of claim 8, wherein the cross-linking agent is
selected from one or more acid aldehyde organic molecules
containing aldehyde and carboxylic acid functional groups.
23. The method of claim 23, wherein the acid aldehyde cross-linking
agent is selected from the group consisting of glyoxylic acid,
succinic semialdehyde, and mixtures and combinations thereof.
24. The method of claim 8, further comprising contacting the fibers
with a catalyst.
25. The method of claim 24, wherein the catalyst is Lewis acid
selected from the group consisting of iron(III) Chloride, boron
trifluoride, tin (IV) chloride, aluminum potassium sulfate,
magnesium chloride, zinc chloride, aluminum sulfate, ammonium
chloride, zirconium oxychloride, magnesium nitrate, zinc nitrate,
aluminum chloride sodium, potassium bisulfate, and a like.
26. The method of claim 8, wherein the cross-linking agent is
selected from one or more alkane polycarboxylic acid organic
molecules containing at least two acid functional groups.
27. The method of claim 26, wherein the alkane polycarboxylic acid
cross-linking agent is selected from the group consisting of
1,2,3,4-butantetracarboxylic acid, 1,2,3-propanetricarboxylic acid,
oxydisuccinic acid, citric acid, itaconic acid, maleic acid,
tartaric acid, glutaric acid and mixtures and combinations
thereof.
28. The method of claim 8, wherein the cross-linking agent is
selected from one or more polymeric polycarboxylic acid organic
molecules.
29. The method of claim 28, wherein the polymeric polycarboxylic
acid cross-linking agent is one or more polymer(s) and copolymer(s)
prepared from monomers selected from the group consisting of
acrylic acid, vinyl acetate, maleic acid, maleic anhydride, carboxy
ethyl acrylate, itanoic acid, fumaric acid, methacrylic acid,
crotonic acid, aconitic acid, acrylic acid ester, methacrylic acid
ester, acrylic amide, methacrylic amide, butadiene, styrene, or
combinations and mixtures thereof.
30. The method of claim 8, wherein the cross-linking agent is
supplied as an aqueous solution additionally comprising a
catalyst.
31. The method of claim 30, wherein the catalyst is selected form
the group consisting of alkali metal salts of phosphorous
containing acids such as alkali metal hypophosphites, alkali metal
phosphites, alkali metal polyphosphonates, alkali metal phosphates,
and alkali metal sulfonates
32. The method of claim 8, wherein the cross-linking agent is
selected from one or more polyepoxides having a substituent
selected from the group consisting of hydrogen; hydrophobic
saturated, unsaturated, cyclic saturated, cyclic unsaturated,
branched, and unbranched alkyl groups; and mixtures and
combinations thereof.
33. The method of claim 26, wherein the alkane polycarboxylic acid
cross-linking agent is selected from the group consisting of 1,4
cyclohexanedimethanol diglycidyl ether, diglycidyl
1,2-cyclohexanedicrboxylate, N,N-diglycidylaniline,
N,N-diglcidyl-4-glycidyloxyaniline, diglycidyl
1,2,3,4-tetrahydrophthalat- e, glycerol propoxylate triglycidyl
ether, and mixtures and combinations thereof.
34. The method of claim 32, wherein the one or more polyepoxide
cross-linking agents are supplied as an aqueous solution
additionally comprising a surfactant.
35. The method of claim 34, Wherein the surfactant is selected from
the group consisting of nonionic, anionic, cationic surfactant, or
combinations and mixtures thereof.
36. The method of claim 8, wherein the cross-linking agent is
supplied to the cellulosic fibers in an amount from about 0.5 to
10% by weight based on the total weight of the fiber.
37. The method of claim 8, wherein the cross-linking agent is
supplied to the cellulosic fibers in an amount from about 2 to 5%
by weight based on the total weight of the fiber.
38. The method of claim 8, wherein the cross-linking agent
comprises an aqueous solution of acid aldehyde having a pH from
about 1.5 to about 4.0.
39. The method of claim 8, wherein the cross-linking agent
comprises an aqueous solution of alkane polycarboxylic acid having
a pH from about 1.5 to about 4.0.
40. The method of claim 8, wherein the cross-linking agent
comprises an aqueous solution of polymeric polycarboxylic acid
having a pH from about 1.5 to about 4.0.
41. The method of claim 34, wherein the surfactant is added in an
amount of from about 0.01 to 5% by weight, based on the total
weight of the cross-linking agent.
42. The method of claim 8, wherein drying and curing is conducted
at a temperature within the range of from about 280.degree. F. to
about 435.degree. F.
43. The method of claim 8, wherein drying and curing is conducted
for a period of time of from about 3 minutes to about 15 minutes at
temperatures within the range of from about 320.degree. F. to about
435.degree. F.
44. The method of claim 8, wherein the treated fibers are first
dried then cured.
45. The method of claim 44, wherein the fiber is dried at a
temperature below the curing temperature, and curing is conducted
for about 1 to about 10 min at a temperature within the range of
from about 300.degree. F. to about 435.degree. F.
46. The method of claim 45, wherein drying is conducted at
temperatures within the range of from about 150 to about
300.degree. F., and curing is conducted for about 0.5 to about 5
minutes at temperatures within the range of from about 320.degree.
F. to about 435.degree. F.
47. An absorbent article comprising the cross-linked fiber of claim
1.
48. The absorbent article of claim 47, wherein the absorbent
article is at least one article selected from the group consisting
of infant diapers, feminine care products, training pants, and
adult incontinence briefs.
49. The absorbent article of claim 47, further comprising a liquid
penetrable top sheet, a liquid impenetrable back sheet, an
acquisition layer, and an absorbent structure, wherein the
acquisition layer is disposed beneath the top sheet, and the
absorbent structure is disposed between the acquisition layer and
the back sheet.
50. The absorbent article of claim 49, wherein the acquisition
layer comprises the cross-linked fibers.
51. The absorbent article of claim 49, wherein the absorbent
structure comprises a composite of superabsorbent polymer and
cellulosic fibers.
52. The absorbent article of claim 51, wherein the superabsorbent
polymer is selected from the group consisting of polyacrylate
polymers, starch graft copolymers, cellulose graft copolymers,
cross-linked carboxymethylcellulose derivatives, and mixtures and
combinations thereof.
53 The absorbent article of claim 51, wherein the superabsorbent
polymer is in the form of fiber, flakes, or granules.
54. The absorbent article of claim 51, wherein the superabsorbent
polymer is present in an amount of from about 20 to about 60% by
weight, based on the total weight of the absorbent structure.
55. The absorbent article of claim 51, wherein the cellulosic
fibers comprise the cross-linked cellulosic fibers.
56. The absorbent article of claim 51, wherein the cellulosic
fibers comprises a mixture of the cross-linked cellulosic fibers
and cellulosic fibers.
57. The absorbent article of claim 56, wherein the cellulosic fiber
is a wood pulp fiber selected from the group consisting of hardwood
pulp, softwood cellulose pulp obtained from a Kraft or sulfite
chemical process, mercerized, rayon, cotton linters, and
combinations or mixtures thereof.
58. The absorbent article of claim 57, wherein the cross-linked
cellulosic fiber is present in the mixture of fibers in an amount
of from about 1 to 70% by weight, based on the total weight of the
mixture of fibers.
59. The absorbent article of claim 58, wherein the cross-linked
cellulosic fiber is present in an amount of from about 10 to 40% by
weight, based on the total weight of the mixture of fibers.
60. The absorbent article of claim 56, wherein the mixture of
cross-linked cellulosic fibers and cellulosic fibers is present in
an amount of from about 10 to about 80% by weight, based on the
total weight of the absorbent structure.
61. The absorbent article claim 60, wherein the mixture is present
in an amount of from about 20 to about 60% by weight, based on the
total weight of the absorbent structure.
62. The method of claim 8, wherein the cross-linking agent is a
mixture of cross-linking agents selected from the group consisting
of: a mixture of glyoxylic acid and citric acid; a mixture of
glyoxylic acid and polymaleic acid; and a mixture of glyoxylic
acid, citric acid, and polymaleic acid.
63. The method of claim 8, wherein the cross-linking agents is a
mixture cross-linking agents selected from the group consisting of:
a mixture of glyoxylic acid and citric acid; a mixture of glyoxylic
acid and a terpolymer of maleic acid, vinyl acetate, and ethyl
acrylate; and a mixture of glyoxylic acid, citric acid, and a
terpolymer of maleic acid, vinyl acetate, and ethyl acrylate.
Description
FIELD OF THE INVENTION
[0001] The present invention is directed to a method of making
chemically cross-linked cellulosic fiber in the sheet form and to
the product resulting from the process.
DESCRIPTION OF RELATED ART
[0002] Absorbent articles intended for personal care, such as adult
incontinent pads, feminine care products, and infant diapers
typically are comprised of at least a top sheet, a back sheet, an
absorbent core disposed between the top sheet and back sheet, and
an optional acquisition layer disposed between the top sheet and
the absorbent core. The acquisition layer comprised of, for
example, acquisition fibers, usually is incorporated in the
absorbent articles to provide better distribution of liquid,
increase the rate of liquid absorption, and reduce gel blocking. A
wide variety of acquisition fibers are known in the art. Included
among these are synthetic fibers, a composite of cellulosic fibers
and synthetic fibers, and cross-linked cellulosic fibers.
Cross-linked cellulose fiber is preferred because it is abundant,
it is biodegradable, and it is relatively inexpensive.
[0003] Cross-linked cellulose fibers and processes for making them
have been described in the literature for many years (see, for
example G. C. Tesoro, Cross-Linking of Cellulose, in Handbook of
Fiber Science and technology, Vol. II, M. Lewis and S. B. Sello
eds. pp 1-46, Mercel Decker, New York (1993)). The cross-linked
cellulose fibers are typically made by reacting cellulose with
polyfunctional agents that are capable of reacting with the
hydroxyl groups of the anhydroglucose repeating units of the
cellulose either in the same chain, or in neighboring chains
simultaneously. Cross-linked cellulose fibers generally are
characterized by their high absorbent capacity, and their high
resiliency in the wet and dry states.
[0004] Cellulosic fibers typically are cross-linked in fluff form.
Processes for making cross-linked fiber in the fluff form comprise
dipping swollen or non-swollen fiber in an aqueous solution of
cross-linking agent, catalyst, and softener. The fiber so treated,
usually is then cross-linked by heating it at elevated temperatures
in the swollen state as described in U.S. Pat. No. 3,241,553, or in
the collapsed state after defiberizing it as described in U.S. Pat.
No. 3,224,926, and European Patent No. 0,427,361 B1, the
disclosures of each of which are incorporated by reference herein
in their entirety.
[0005] The art has proposed many solutions to overcome some of the
problems of cross-linking fiber in sheet form. One alleged solution
to this problem is to minimize the contact between fibers in the
dry state. For example, Graef et al. in U.S. Pat. No. 5,399,240,
the disclosure of which is incorporated herein by reference in its
entirety, describe a method of treating fiber in the sheet form
with a cross-linking agent and a de-bonder. Fiber while in the
sheet form is then cured at elevated temperatures. The de-bonder
tends to interfere with the hydrogen bonding between fibers and
thus minimizes the contact between fibers. As a result, fiber is
produced with a relatively low content of knots and nits. In
addition, the long aliphatic chains tend to reduce the fibers'
absorbency and acquisition rate, thus rendering the fibers
unsuitable for applications where high rate of absorbency and fast
acquisition are important, such as in absorbent articles.
[0006] Bernardin et al. in U.S. Pat. No. 3,434,918 disclose a
method of treating fiber in sheet form with a cross-linking agent
and a catalyst. The treated sheet then is wet-aged to render the
cross-linking agent insoluble. The wet-aged fibers are re-dispersed
before curing and mixed with untreated fiber, and then sheeted and
cured. Other documents describing methods of treating fiber in
sheet form include, for example, U.S. Pat. Nos. 4,204,054;
3,844,880; and 3,700,549 (the disclosures of which are incorporated
by reference herein in their entirety).
[0007] The above-described approaches complicate the process of
cross-linking fiber in sheet form, and they render the process time
consuming, and costly.
[0008] In previous work (U.S. patent application entitled:
"Chemically Cross-Linked Cellulosic Fiber and Method of Making the
Same, filed on Jun. 11, 2002, attorney docket number 60892.000002,
and Ser. No. 09/832,634, entitled "Cross-Linked Pulp and Method of
Making Same, filed Apr. 10, 2001 it was shown that mercerized fiber
can be successfully cross-linked in sheet form. The produced
cross-linked fiber showed similar or better performance
characteristics than conventional individualized cross-linked
cellulose fibers. Also, the fiber showed less discoloration and
reduced amounts of knots and nits compared to conventional
individualized cross-linked fiber.
[0009] Fiber mercerization, which is a treatment of fiber with an
aqueous solution of sodium hydroxide (caustic), is one of the
earliest known modifications of fiber. It was invented 150 years
ago by John Mercer (see British Patent 1369, 1850). The process
generally is used in the textile industry to improve cotton
fabric's tensile strength, dyeability, and luster (see, for
example, R. Freytag, J.-J. Donze, Chemical Processing of Fibers and
Fabrics, Fundamental and Applications, Part A, in Handbook of Fiber
Science and Technology Vol. I M. Lewis and S. B. Sello eds. pp.
1-46, Mercell Decker, New York (1983)).
[0010] In addition to the above advantages, mercerization adds to
fibers several other properties. Among these are: (1) mercerized
fibers have high .alpha.-cellulose content, since caustic removes
residuals such as lignin and hemicellulose from fiber leftover from
pulping and bleaching processes; (2) mercerized fibers have a
round, circular shape (rather than the flat, ribbon-like shape of
conventional fibers) that reduces the contact and weakens the
hydrogen-bonding between fibers in the sheet form; and (3)
mercerization converts cellulose chains from their native structure
form, cellulose I, to a more thermodynamically-stable and less
crystalline form, cellulose II. The cellulosic chains in cellulose
II are found to have an anti-parallel orientation rather than
parallel orientation as in cellulose I (see, for example, R. H.
Atalla, Comprehensive Natural products Chemistry, Carbohydrates And
Their Derivatives Including Tannins, Cellulose, and Related Lignins
Vol. III, D. Barton and K. Nakanishi eds. pp 529-598, Elsevier
Science, Ltd., Oxford, U.K. (1999)).
[0011] The description herein of certain advantages and
disadvantages of known cross-linked cellulosic fibers, and methods
of their preparation, is not intended to limit the scope of the
present invention. Indeed, the present invention may include some
or all of the methods and chemical reagents described above without
suffering from the same disadvantages.
SUMMARY OF THE INVENTION
[0012] One feature of an embodiment of the present invention
provides cross-linked fibers with enhanced bulking characteristics,
porosity and rate of acquisition. An additional aspect of the
present invention is to provide cross-linked cellulosic fiber
having long shelf-life and high stability. Further, another aspect
of an embodiment of the present invention provides fibers useful in
an acquisition layer and/or in an absorbent core of absorbent
products. Various aspects of the present invention also provide
absorbent articles comprising the cross-linked fiber of the present
invention.
[0013] In accordance with these and other aspects and features of
embodiments of the invention, there is provided a cross-linked
sheet of a blend of fibers, and a method for cross-linking
cellulose fibers in sheet form. In one aspect of the invention, the
cellulose fibers are a blend of mercerized fibers and conventional
fibers that are cross-linked. In another aspect of the invention,
the cross-linked fibers formed in accordance with the present
invention can be easily defiberized without serious fiber breakage
and with low knot-content and low nit-content. It will be
appreciated, however, that knots and nits are advantageous for some
applications, and accordingly, the present invention is not in any
way limited to producing cross-linked cellulosic fibers
substantially free of knots.
[0014] In accordance with the method, a wet laid sheet of a blend
of mercerized fibers and cellulose fibers are formed, and then
treated a cross-linking agent to form a sheet impregnated with the
cross-linking agent. The cross-linking agent then is dried and
cured to form intra-fiber cross-links.
[0015] These and other objects, features, and advantages of the
present invention will appear more fully from the following
detailed description of the preferred embodiments of the invention,
and the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The drawings show electron microscope photographs of
representative cross-linked fibers of the present invention.
[0017] FIGS. 1a and 1b are photographs at 100.times. and 200.times.
magnifications, respectively, of blends of fibers obtained as shown
in Example 2 from Rayfloc.RTM.-J-LD (southern pine Kraft pulp
commercially available from Rayonier Performance Fibers Division,
Jesup, Ga.) and mercerized fibers in about 1:1 weight ratio using
glyoxylic acid (2%) cross-linking agent.
[0018] FIGS. 2a and 2b are photographs at 100.times. and 200.times.
magnification, respectively, of a blend of fibers of the present
invention prepared from Rayfloc.RTM.-J-LD and mercerized fibers in
1:1 ratio and cross-linked in accordance with Example 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0019] As used herein, the terms "absorbent garment," "absorbent
article" or simply "article" or "garment" refer to mechanisms that
absorb and contain body fluids and other body exudates. More
specifically, these terms refer to garments that are placed against
or in proximity to the body of a wearer to absorb and contain the
various exudates discharged from the body. A non-exhaustive list of
examples of absorbent garments includes diapers, diaper covers,
disposable diapers, training pants, feminine hygiene products and
adult incontinence products. Such garments may be intended to be
discarded or partially discarded after a single use ("disposable"
garments). Such garments may comprise essentially a single
inseparable structure ("unitary" garments), or they may comprise
replaceable inserts or other interchangeable parts.
[0020] The present invention may be used with all of the foregoing
classes of absorbent garments, without limitation, whether
disposable or otherwise. Some of the embodiments described herein
provide, as an exemplary structure, a diaper for an infant, however
this is not intended to limit the claimed invention. The invention
will be understood to encompass, without limitation, all classes
and types of absorbent garments, including those described
herein.
[0021] The term "component" can refer, but is not limited, to
designated selected regions, such as edges, corners, sides or the
like; structural members, such as elastic strips, absorbent pads,
stretchable layers or panels, layers of material, or the like.
[0022] Throughout this description, the term "disposed" and the
expressions "disposed on," "disposed above," "disposed below,"
"disposing on," "disposed in," "disposed between" and variations
thereof are intended to mean that one element can be integral with
another element, or that one element can be a separate structure
bonded to or placed with or placed near another element. Thus, a
component that is "disposed on" an element of the absorbent garment
can be formed or applied directly or indirectly to a surface of the
element, formed or applied between layers of a multiple layer
element, formed or applied to a substrate that is placed with or
near the element, formed or applied within a layer of the element
or another substrate, or other variations or combinations
thereof.
[0023] Throughout this description, the terms "top sheet" and "back
sheet" denote the relationship of these materials or layers with
respect to the absorbent core. It is understood that additional
layers may be present between the absorbent core and the top sheet
and back sheet, and that additional layers and other materials may
be present on the side opposite the absorbent core from either the
top sheet or the back sheet.
[0024] Throughout this description, the expressions "upper layer,"
"lower layer," "above" and "below," which refer to the various
components included in the absorbent material are used to describe
the spatial relationship between the respective components. The
upper layer or component "above" the other component need not
always remain vertically above the core or component, and the lower
layer or component "below" the other component need not always
remain vertically below the core or component. Other configurations
are contemplated within the context of the present invention.
[0025] Throughout this description, the term "impregnated" insofar
as it relates to a cross-linking agent impregnated in a fiber,
denotes an intimate mixture of cross-linking agents and cellulosic
fiber, whereby the cross-linking agent may be adhered to the
fibers, adsorbed on the surface of the fibers, or linked via
chemical, hydrogen or other bonding (e.g., Van der Waals forces) to
the fibers. Impregnated in the context of the present invention
does not necessarily mean that the cross-linking agent is
physically disposed beneath the surface of the fibers.
[0026] The present invention concerns chemically cross-linked
blends of fibers that are useful in absorbent articles, and in
particular, that are useful in forming acquisition layers or
absorbent cores in the absorbent article. The particular
construction of the absorbent article is not critical to the
present invention, and any absorbent article can benefit from this
invention. Suitable absorbent garments are described, for example,
in U.S. Pat. Nos. 5,281,207, and 6,068,620, the disclosures of each
of which are incorporated by reference herein in their entirety
including their respective drawings. Those skilled in the art will
be capable of utilizing the chemically cross-linked cellulosic
fibers of the present invention in absorbent garments, cores,
acquisition layers, and the like, using the guidelines provided
herein.
[0027] Cross-linking of fibers in fluff form is believed to improve
the physical and chemical properties of the fibers in many ways,
such as improving the stiffness, increasing resiliency (in the dry
and wet state), increasing the absorbency, reducing wrinkling, and
improving shrinkage resistance. Unfortunately, it has been found
that such cross-linking, if carried out on a fiber in sheet form,
may create problems in the fiber which render it unsuitable for
many applications. These problems include severe fiber breakage and
increased amounts of knots and nits (hard fiber clumps). These
problems are attributed to the inter-fiber (fiber-to-fiber)
cross-linking that occurs between fibers in close contact during
the curing process. Usually, fibers get into close contact in the
dry state due to (a) mechanical entanglement; (b) hydrogen bonding
between fibers; and (c) pulping and bleaching residuals such as
lignin and hemicellulose. As a result, when fibers treated with a
cross-linking agent are heated for curing, fibers in close contact
tend to form inter-fiber cross-links rather than intra-fiber
cross-links (chain-to-chain within the single fiber).
[0028] Thus, there is a need for a simple, relatively inexpensive
process for cross-linking fiber in sheet form that will provide
cross-linked fibers with low liquid retention, enhanced rates of
acquisition, and reduced amounts of knots and nits. In another
embodiment, the present invention is directed to a method of making
cross-linked fibers in sheet form. The method preferably comprises
treating cellulose fibers in sheet or roll form with an aqueous
solution of a polyfunctional cross-linking agent, followed by
drying and curing at sufficient temperature for adequate time to
accelerate the formation of covalent bonding between hydroxyl
groups of cellulose fibers and functional groups of the
cross-linking agent.
[0029] The method of an embodiment of the invention preferably
comprises reacting a blend of fibers in sheet form with one or more
reagents selected from organic molecules having carboxylic acid, an
aldehyde and carboxylic acid (e.g., an acid aldehyde), or epoxy
functional groups. In one embodiment the method of the present
invention provides cross-linked fibers in sheet form that can be
readily defiberized with low knot-content and without significant
fiber breakage. In another embodiment the method of the present
invention provides cross-linked fibers that are characterized by an
enhanced acquisition rate, resiliency, and absorbency under load.
Moreover, cross-linked fibers of the present invention display a
reduction in centrifuge retention capacity which makes the fiber
especially suited for use in acquisition, distribution and
acquisition-distribution layers in absorbent articles intended for
fluid management.
[0030] In one aspect of the present invention, the fiber in sheet
form comprises a blend of mercerized fibers and conventional
fibers. Throughout this description, the expression "conventional
fibers" denotes cellulose fibers of diverse origins, especially
those primarily derived from wood pulp. Suitable wood pulp can be
obtained from any of the chemical processes known by those of
ordinary skill in the art such as Kraft, and sulfite processes.
Preferred fibers are those obtained from various soft wood pulp
such as Southern pine, White pine, Caribbean pine, Western hemlock,
various spruces, (e.g. Sitka Spruce), Douglas fir or mixtures and
combinations thereof. Fibers obtained from hardwood pulp sources,
such as gum, maple, oak, eucalyptus, poplar, beech, and aspen, or
mixtures and combinations thereof can also be used in the present
invention. Other cellulose fibers derived form cotton linter,
bagasse, kemp, flax, and grass may also be used in the present
invention. The fiber can be comprised of a mixture of two or more
of the foregoing cellulose pulp products. Particularly preferred
"conventional" fibers for use in forming the cross-linked fibers of
the present invention are those derived from wood pulp prepared by
Kraft and sulfite-pulping processes.
[0031] The fibers of the present invention preferably have a high
surface purity of cellulose, but it is not necessarily required
that the cellulosic fibers have a high cellulose bulk purity. It is
preferred that the cellulosic fiber be cross-linked in the sheet
form, and more preferably, be fiber with "high cellulose purity."
The high cellulose purity refers to the surface purity of the
cellulosic fibers. Throughout this description, the expression
"high cellulose purity" refers to pulp comprising at least about
65%, preferably at least 75%, and most preferably, at least about
90% .alpha.-cellulose.
[0032] The preferred fibers in sheet form that are cross-linked in
accordance with the present method are blends of conventional
cellulose and "mercerized fibers." Throughout this description, the
expression "mercerized fibers" denotes any of wood pulp fibers or
fibers from any source described, previously treated with an
aqueous solution of alkali metal. Fiber mercerization can be
carried out by any method known in the art such as those described
in, for example, Cellulose and Cellulose Derivatives, Vol. V, Part
1, Ott, Spurlin, and Grafllin, eds., Interscience Publisher (1954).
Fiber mercerization can be performed by mixing pulp in an aqueous
solution of alkali metal (i.e. sodium hydroxide), washing,
neutralizing, or washing and neutralizing, and optionally drying
the pulp.
[0033] Reagents suitable for mercerization include but are not
limited to, alkali metal hydroxides, such as sodium hydroxide,
potassium hydroxide, calcium hydroxide, and rubidium hydroxide,
lithium hydroxide, and benzyltrimethylammonium hydroxide. Sodium
hydroxide is a particularly preferred reagent for use in fiber
mercerization in accordance with the present invention. The pulp of
the invention preferably is treated with an aqueous solution
containing from about 8 to about 30% by weight sodium hydroxide,
more preferably from about 12 to about 20%, and most preferably 13%
to about 16%. Mercerization may be performed during or after
bleaching, purification, and drying. Preferably mercerization is
carried out during bleaching and or drying processes. After
mercerization, the cellulose fibers preferably contain at least
about 80% by weight .alpha.-cellulose, preferably at least about
90% by weight, more preferably at least about 95% by weight, and
even more preferably at least about 97% by weight
.alpha.-cellulose.
[0034] Commercially available caustic extractive pulp (i.e.,
mercerized pulp") suitable for use in the present invention
includes, for example, Porosanier-J-HP, available from Rayonier
Performance Fibers Division (Jesup, Ga.), and Buckeye's HPZ
products, available from Buckeye Technologies (Perry, Fla.). In one
aspect of the present invention, it is preferred that the pulp
fibers be in sheet or roll form.
[0035] In accordance with the invention, the sheet or roll form of
cellulose preferably is a blend of mercerized fiber and
conventional fiber containing about 10% to about 60% by weight
conventional fiber, more preferably from about 20% to about 60% by
weight conventional fiber, and most preferably from about 30% to
about 50% by weight conventional fiber, based on the total weight
of the mixture of fibers.
[0036] In another aspect of the invention, the fiber can be used in
wet or dry state. It is preferred in the present invention that the
cellulosic fibers be employed in the dry state.
[0037] Cross-linking agents suitable for use in the present
invention are polyfunctional molecules. As used herein, the
expression "polyfunctional molecule" refers to a polymeric or
monomeric molecule able to form a bridge between adjacent cellulose
chains. Accordingly, any material capable of reacting with more
than one hydroxyl group of cellulose chains can be suitable for use
in the present invention. Particularly suitable cross-linking
agents for use in the present invention are those carrying
functional groups such as, for example, carboxyl, aldehyde and
epoxy. Preferably, the cross-linking agents include dialdehydes,
acid aldehydes, polycarboxylic acids, and polyepoxides.
[0038] Cross-linking agents preferred for use in the present
invention include acid aldehydes. As used herein, "acid aldehyde"
refers to organic molecules having carboxylic acid and aldehyde
functional groups, such as, for example, glyoxylic acid and
succinic semialdehyde. A particularly preferred acid aldehyde
cross-linking agent is glyoxylic acid.
[0039] Other suitable cross-linking agents include polycarboxylic
acids. Especially suitable polycarboxylic acids are those having at
least two carboxyl groups, such as, 1,2,3,4-butanetetracarbocylic
acid, 1,2,3-propanetricarboxylic acid, oxydisuccinic acid, citric
acid, itaconic acid, maleic acid, tartaric acid, glutaric acid.
Particularly preferred polycarboxylic acids are
1,2,3,4-butanetetracarbocylic acid and citric acid.
[0040] Other suitable polycarboxylic acids include polymeric
polycarboxylic acids such as those specially prepared from monomers
such as, for example, acrylic acid, vinyl acetate, maleic acid,
maleic anhydride, carboxy ethyl acrylate, itanoic acid, fumaric
acid, methacrylic acid, crotonic acid, aconitic acid, acrylic acid
ester, methacrylic acid ester, acrylic amide, and methacrylic
amide, butadiene, styrene, or combinations thereof.
[0041] Commercially available examples of these polymers and
co-polymers include polyacrylic acid, polymaleic acid, polyitaconic
acid, polyaspartic acid, polymethacrylic acid, poly(acrylic
acid-co-maleic acid), poly(acrylamide-co-acrylic acid),
poly(etheylene-co-acrylic acid), and poly(styrene-co-maleic acid).
Particularly preferred polycarboxylic acids are polymaleic acid,
polyacrylic acid, and a co-polymer of acrylic acid and maleic
acid.
[0042] Other cross-linking agents suitable for use in the present
invention include polyepoxides, particularly those containing
hydrophobic saturated, unsaturated, branched and un-branched
alkyls. Examples of these include 1,4-cyclohexanedimethanol
diglycidyl ether, diglycidyl 1,2-cyclohexanedicrboxylate,
N,N-diglycidylaniline, N,N-diglcidyl-4-glycidyloxyaniline, and
diglycidyl 1,2,3,4-tetrahydrophthalate and glycerol propoxylate
triglycidyl ether. A particularly preferred polyepoxide is
1,4-cyclohexanedimethanol diglycidyl ether.
[0043] In another aspect of the invention, a mixture or combination
of cross-linking agents may be used. In another aspect, the present
invention provides chemically cross-linked fibers in sheet form,
that are cross-linked with a blend of cross-linking agents
including those described above.
[0044] In one embodiment of the invention, the cross-linking agent
may be applied to the cellulose fiber in an aqueous solution.
Preferably the aqueous solution has a pH of from about 1 to about
5, more preferably from about 2 to about 3. The present inventors
have discovered that an aqueous solution of acid aldehyde
cross-linking agent can be used as is without any adjacent or
additional pH control agent.
[0045] In another embodiment of the invention, a water insoluble
cross-linking agent, e.g. polyexpoxide, may be used. When such a
water insoluble cross-linking agent is used, it is preferred to add
a minor amount of surfactant (e.g., a few drops--less than 1% by
weight) to emulsify the cross-linking agent prior to fiber
application. The cross-linking agent may then be applied to the
fiber as a dispersion, instead of an aqueous solution.
[0046] In general, any type of surfactant capable of forming a
dispersion with the water insoluble cross-linking agent can be
used. Suitable surfactants include nonionic, anionic, or cationic
surfactant or mixtures and combinations of surfactants that are
compatible with each other. Preferably the surfactant is selected
from Triton X-100 (Rohm and Haas), Triton X-405 (Rohm and Haas),
sodium lauryl sulfate, and lauryl bromoethyl ammonium chloride,
ethoxylated nonylphenols, polyoxyethylene alkyl ethers,
polyoxyethylene alkyl ethers, and polyoxyethylene fatty acid
esters.
[0047] The cellulosic fiber preferably is treated with an effective
amount of cross-linking agent to achieve the absorbent properties
and physical characteristics described herein. Generally, the
concentration of the cross-linking agent in aqueous solution is
sufficient to provide from about 0.5 to 10.0 weight percent
cross-linking agent on fiber, more preferably from about 1 to 6
weight percent, and most preferably from about 2 to 5 weight
percent.
[0048] Optionally, the method of forming the cellulosic fiber in
accordance with the invention includes a catalyst to accelerate the
formation of an ester linkage between the hydroxyl groups of the
cellulose and the carboxyl groups of the polycarboxylic acid and
acid aldehyde cross-linking agents. A catalyst also may be used to
accelerate the formation of acetal cross-links between hydroxyl
groups of cellulose and aldehyde functional groups of acid aldehyde
cross-linking agents. When an acid aldehyde is used as the
cross-linking agent, however, a catalyst is not required. To the
extent that a catalyst is used, any catalyst known in the art that
is capable of accelerating the formation of an ester cross-link
between a hydroxyl group and an acid group, or capable of
accelerating the formation of an acetal cross-link between a
hydroxyl group of cellulose and an aldehyde group could be used in
the present invention. Suitable catalysts for use in the present
invention to accelerate the formation of ester cross-links include
alkali metal salts of phosphorous containing acids such as alkali
metal hypophosphites, alkali metal phosphites, alkali metal
polyphosphonates, alkali metal phosphates, and alkali metal
sulfonates. A particularly preferred catalyst of this type is
sodium hypophosphite.
[0049] Suitable catalysts for use in the present invention to
accelerate the formation of acetal cross-links are Lewis acids
consisting of a metal and a halogen, such as for example
FeCl.sub.3, AlCl.sub.3, and MgCl.sub.2.
[0050] A catalyst may also be used to promote the reaction between
polyepoxides and cellulose hydroxyl groups, to the extent a
cross-linking agent containing polyepoxide groups is used as a
cross-linking agent. Any catalyst known in the art to accelerate
the formation of an ether bond or linkage between the hydroxyl
groups of cellulose and an epoxide group can be used in the present
invention. Preferably, the catalyst is a Lewis acid selected from
aluminum sulfate, magnesium sulfate, and any Lewis acid consisting
of a metal and a halogen, including, for example FeCl.sub.3,
AlCl.sub.3, and MgCl.sub.2. The catalyst can be applied to the
fiber as a mixture with the cross-linking agent, before the
addition of the cross-linking agent, or after the addition of
cross-linking agent to cellulosic fiber. Preferably, the ratio of
catalyst to cross-linking agent is, for example, from about 1:2 to
1:10, more preferably from about 1:4 to 1:8.
[0051] Optionally, in addition to the cross-linking agent, other
finishing agents such as water repellent, softening, and wetting
agents also can be used to treat the cellulosic fiber. Examples of
softening agents include fatty alcohols, fatty acids amides,
polyglycol ethers, fatty alcohols sulfonates, and N-stearyl-urea
compounds. Examples of wetting agents include fatty amines, salts
of alkylnaphthalenesulfonic acids, alkali metal salts of dioctyl
sulfosuccinate, and the like.
[0052] Any method of applying the cross-linking agent(s) to the
fiber can be used in carrying out the cross-linking method of the
invention. Acceptable methods include, for example, spraying,
dipping, impregnation, and the like. Preferably, the fiber is
impregnated with an aqueous solution of cross-linking agent.
Impregnation usually creates a uniform distribution of
cross-linking agent on the sheet and provides a better penetration
of cross-linking agent into the interior part of the fiber.
[0053] In one embodiment of the invention, a sheet including a
blend of mercerized and conventional fibers in roll form is
conveyed through a treatment zone where a cross-linking agent(s) is
applied on both surfaces by conventional methods such as spraying,
rolling, dipping, knife-coating or other manners of impregnation. A
preferred method of applying the aqueous solution of the
cross-linking agent(s) to fiber in roll form is by puddle press,
size press, and blade coater.
[0054] In one embodiment of the present invention, a blend of
fibers in sheet or roll form after having been treated with a
solution of cross-linking agent then preferably is transported by a
conveying device such as a belt or a series of driven rollers
through a two-zone oven for drying and curing.
[0055] Fibers in roll or sheet form after treatment with the
cross-linking agent preferably are dried and cured in a two stage
process, and even more preferably dried and cured in a one stage
process. Such drying and curing removes water from the fiber,
thereupon believed to induce the formation of an ester and an ether
linkage between hydroxyl groups of fiber and cross-linking
agent(s). Curing usually is carried out in a forced draft oven
preferably from about 300.degree. F. to about 450.degree. F., and
more preferably from about 320.degree. F. to about 430.degree. F.,
and most preferably from about 350.degree. F. to about 420.degree.
F. Curing preferably is carried out for a certain period of time
that permits complete fiber drying and efficient cross-linking. It
is preferred that the cellulosic fiber is cured for a period of
time ranging from about 5 min to about 25 min, and more preferably
from about 7 min to about 20 min, most preferably from about 10 min
to about 15 min.
[0056] The blend of cellulosic fibers cross-linked in accordance
with the present invention can be characterized as having an
absorbent capacity within the range of about 7.0 g/g to about 12.0
g/g. Preferably, the fibers have an absorbent capacity of at least
8.0 g/g, more preferably at least 9.0 g/g, even more preferably at
least 10.0 g/g or higher. The cross-linked cellulosic fibers can
have a centrifuge retention as determined by the Hanging Cell Test
method within the range of about 0.4 g/g to about 0.6 g/g.
Preferably, the fibers have a centrifuge retention of less than
about 0.6 g/g, more preferably less than about 0.55 g/g, and most
preferably less than about 0.5 g/g.
[0057] The cellulosic fibers cross-linked in accordance with the
present invention preferably possess characteristics that are
desirable in absorbent articles. For example, the cross-linked
cellulosic fibers preferably have a centrifuge retention capacity
of less than about 0.6 grams of synthetic saline per gram of fiber
(hereinafter "g/g"). The chemically cross-linked cellulose fibers
also have desirable properties, such as a free swell of greater
than about 10 g/g, an absorbent capacity of greater than about 8.0
g/g, an absorbency under load of greater than about 8.0 g/g, less
than about 10% of knots, less than about 6.5% of fines, and an
acquisition rate upon the third insult (or third insult
strike-through) of less than about 13.0 seconds. The particular
characteristics of the cross-linked cellulosic fibers of the
invention are determined in accordance with the procedures
described in more detail in the examples.
[0058] The centrifuge retention capacity measures the ability of
the fiber to retain fluid against a centrifugal force. It is
preferred that the blend of fibers of the invention have a
centrifuge retention capacity of less than about 0.6 g/g, when
cross-linked with any cross-linking agent, more preferably, less
than about 0.55 g/g, even more preferably less than 0.5 g/g. The
cross-linked cellulosic fibers of the present invention can have a
centrifuge retention capacity as low as about 0.40 g/g. It also is
preferred that the fibers of the invention have a centrifuge
retention capacity of less than about 0.60 g/g when cross-linked
with an acid aldehyde cross-linking agent, more preferably, less
than about 0.55 g/g, even more preferably less than 0.50 g/g, and
most preferably less than about 0.45 g/g.
[0059] It is preferred that the fibers of the invention have an
absorbent capacity of more than about 8.0 g/g, more preferably,
greater than about 8.5 g/g, even more preferably greater than about
9.0 g/g, and most preferably greater than about 10.0 g/g.
[0060] The absorbency under load measures the ability of the fiber
to absorb fluid against a restraining or confining force of 0.3 psi
over a given period of time. It is preferred that the blend of
fibers of the invention have an absorbency under load of greater
than about 7.0 g/g, more preferably, greater than about 7.5 g/g,
and most preferably, greater than about 8.0 g/g.
[0061] The third insult strikethrough measures the ability of the
fiber to acquire fluid, and is measured in terms of seconds. It is
preferred that the fibers of the invention have a third insult
strike-through of less than about 15.0 seconds, more preferably,
less than about 14 seconds, even more preferably less than 13
seconds, and most preferably less than about 12 seconds.
[0062] The cross-linked fibers of the present invention preferably
have less than about 10% of knots, more preferably less than about
8% knots, and most preferably, less than about 5% knots. The
cross-linked fibers of the present invention also preferably have
less than about 8.0% of fines, preferably less than about 7.0%
fines, and most preferably, less than about 6.0% fines.
[0063] In addition to being more economical, there are several
other advantages for cross-linking fibers in sheet form in
accordance with the present invention. Fibers cross-linked in sheet
form have typically been expected to have an increased potential
for inter-fiber cross-linking which leads to "knots" and "nits"
resulting in poor performance in some applications. For instance,
when a standard purity fluff pulp, Rayfloc-J, is cross-linked in
sheet form, the "knot" content increases substantially, indicating
increased deleterious inter-fiber bonding. Examination of these
"knots" recovered by classification showed they contained true
"nits" (hard fiber bundles). Surprisingly, it was found that a
blend of mercerized pulp and conventional pulp cross-linked in
sheet or roll form actually yields fewer knots and nits than
control pulps having conventional cellulose purity cross-linked
under the same conditions. Significantly, a blend of fibers in
sheet or roll form that were cross-linked in accordance with the
present invention were found to contain fewer knots than commercial
individualized cross-linked cellulose fibers, like those produced
by the Weyerhaeuser Company, commonly referred to as HBA (for
high-bulk additive), and by The Proctor & Gamble Company
("P&G").
[0064] In another aspect of the invention, it also has been
discovered that the presence of knots to a certain level in the
cross-linked fiber of the present invention enhances the
performance of the cross-linked fiber when used as an acquisition
layer in hygiene products. In this instance, the knots present in
the cross-linked fiber are within the range of 2% to 15%, more
preferably from 3% to 12%, and most preferably from 5% to 10%.
[0065] Another benefit of the present invention is that cellulose
fibers cross-linked in sheet form in accordance with the present
invention enjoy equal performance characteristics to conventional
individualized cross-linked cellulose fibers, but avoid the
processing problems associated with dusty individualized
cross-linked fibers.
[0066] Scanning electron microscope (S360 Leica Cambridge Ltd.,
Cambridge, England) photographs illustrated in FIGS. 1A and 1B
represent cross-linked fibers of the present invention obtained by
cross-linking a blend of conventional fiber (Rayfloc.RTM.-J-LD) and
Porosanier, in 1:1 ratio by weight, using glyoxylic acid (2%) as
the cross-linking agent. The photographs were taken at 100.times.
magnification for FIG. 1A, and 200.times. magnification for FIG.
1B.
[0067] Scanning Electron Microscope (SEM) photographs illustrated
in FIGS. 2A and 2B represent cross-linked fibers of the present
invention obtained by cross-linking a blend of conventional fiber
(Rayfloc.RTM.-J-LD) and Porosanier, in 1:1 ratio by weight, using
DP-60 (5%) as the cross-linking agent. The photographs were taken
at 100.times. magnification for FIG. 2A, and 200.times.
magnification for FIG. 2B.
[0068] As shown in these figures, the cross-linked fibers of the
present invention are twisted and curled. The photographs show a
mixture of round, circular-shaped fibers and flat, ribbon-like
fibers. The round, circular-shaped fibers represent the mercerized
fibers while the flat, ribbon-like fibers represent the
conventional fiber Rayfloc.RTM.-J-LD.
[0069] Cross-linked cellulosic fibers prepared in accordance with
the present invention can be utilized, for example, as a bulking
material in the manufacture of high bulk specialty fiber
applications that require good absorbency and porosity. The
cross-linked fibers can be used, for example, in non-woven, fluff
absorbent applications. The fibers can be used independently, or
preferably can be incorporated with other cellulosic fibers to form
blends using conventional techniques, such as air laying. In an
airlaid process, the cross-linked fibers of the present invention,
either alone or blended with other fibers, are blown onto a forming
screen. A wet laid process may also be used, combining the
cross-linked fibers of the invention with other cellulosic fibers
to form sheets or webs of blends.
[0070] The cross-linked fibers of the present invention can be
incorporated into various absorbent articles intended for body
waste management such as adult incontinent pads, feminine care
products, and infant diapers. The cross-linked fibers can be used
in an acquisition layer in the absorbent articles. The cross-linked
fibers also can be utilized in the absorbent core of the absorbent
articles. Towels, wipes and other absorbent products such as
filters also may be made with the cross-linked fibers of the
present invention. Accordingly, an additional feature of the
present invention is to provide an absorbent core and an absorbent
article that includes the chemically cross-linked fibers of the
present invention.
[0071] As known in the art, absorbent cores typically are prepared
using fluff pulp to wick the liquid, and an absorbent polymer
(oftentimes a superabsorbent polymer ("SAP")) to store liquid. As
noted previously, the cross-linked blend of fibers of the present
invention have high resiliency and high free swell capacity.
Furthermore, the blend of cross-linked fibers are highly porous.
Accordingly, the cross-linked fibers of the present invention can
be used in combination with the SAP to make an absorbent composite
(or core) having improved porosity, bulk, resiliency, wicking,
softness, absorbent capacity, absorbency under load, low third
insult strikethrough, low centrifuge retention capacity, and the
like. The absorbent composite could be used as an absorbent core in
absorbent articles intended for body waste management.
[0072] It is preferred in the present invention that the
cross-linked fibers be present in the absorbent composite in an
amount ranging from about 10% to about 80% by weight, based on the
total weight of the composite. More preferably, the cross-linked
fibers are present in an amount ranging from about 20% to about 60%
by weight. A mixture of cellulosic fibers and cross-linked fibers
of the present invention along with the SAP may also be used to
make the absorbent composite. Preferably, the cross-linked fibers
of the present invention are present in the mixture in an amount
ranging from about 1% to about 70% by weight, based on the total
weight of the fiber mixture, and more preferably present in an
amount from about 10% to about 40% by weight. Suitable additional
conventional cellulosic fibers include any of the wood fibers
mentioned previously, cold caustic treated fibers, conventional
fibers, mercerized fibers and mixtures and combinations
thereof.
[0073] Any suitable superabsorbent polymer, or other absorbent
material, can be used, to form the absorbent composite. The SAP can
be in the form of, for example, fiber, flakes, or granules capable
of absorbing several times its weight of saline (0.9% solution of
NaCl in water) and/or blood. The SAP also preferably is capable of
retaining the liquid when it is subjected to load. Non-limiting
examples of superabsorbent polymers applicable for use in the
present invention include any SAP presently available on the
market, including, but not limited to, polyacrylate polymers,
starch graft copolymers, cellulose graft copolymers, and
cross-linked carboxymethylcellulose derivatives, and mixtures and
combinations thereof.
[0074] An absorbent composite made in accordance with the present
invention preferably contains superabsorbent polymer in an amount
from about 20% to about 60% by weight, based on the total weight of
the composite, and more preferably from about 30% to about 60% by
weight. The SAP may be distributed throughout an absorbent
composite within the voids in the cross-linked fiber or the mixture
of cross-linked fibers and cellulosic fibers. In another
embodiment, the SAP is attached to the fiber via a binding agent
that includes, for example, a material capable of cross-linking the
SAP to the fiber via hydrogen bonding (see, for example, U.S. Pat.
No. 5,614,570, the disclosure of which is incorporated by reference
herein in its entirety).
[0075] A method of making an absorbent composite of the present
invention may include forming a pad comprising cross-linked fibers
or a mixture of cross-linked fibers and cellulosic fibers and
incorporating particles of superabsorbent polymer in the pad. The
pad can be wet laid or airlaid, preferably the pad is airlaid. It
also is preferred that the superabsorbent polymer and cross-linked
fibers, or mixture of cross-linked fibers and cellulosic fibers,
are air laid together.
[0076] Absorbent cores containing cross-linked fibers and
superabsorbent polymer preferably have dry densities ranging from
about 0.1 g/cm.sup.3 to 0.50 g/cm.sup.3 and more preferably from
about 0.2 g/cm.sup.3 to 0.4 g/cm.sup.3. The absorbent core can be
incorporated into a variety of absorbent articles intended for body
waste management, such as diapers, training pants, adult
incontinence, feminine care products, and toweling (e.g. wet and
dry wipes).
[0077] To evaluate the various attributes of the present invention,
several tests were used to characterize the cross-linked fibers'
performance improvements resulting from the presently described
method.
[0078] The invention will be illustrated but not limited by the
following examples.
[0079] In the examples, all percentages are by weight and all
temperatures in degrees Celsius, unless otherwise noted. Also, when
referring to pulp weight, the measurement includes equilibrium
moisture content. When subjected to testing, all fiber contains
about 5% to 7% moisture.
EXAMPLES
[0080] The following test methods were used to measure and
determine various physical characteristics of the inventive
cross-linked cellulosic fibers.
Test Methods
[0081] The Absorbency Test Method
[0082] The absorbency test method was used to determine the
absorbency under load, absorbent capacity, and centrifuge retention
capacity of the cross-linked fibers of the present invention. The
absorbency test was carried out in a one inch inside diameter
plastic cylinder having a 100-mesh metal screen adhering to the
cylinder bottom "cell," containing a plastic spacer disk having a
0.995 inch diameter and a weight of about 4.4 g. In this test, the
weight of the cell containing the spacer disk was determined to the
nearest 0.0001 g, and then the spacer was removed from the cylinder
and about 0.35 g of cross-linked fibers having a moisture content
within the range of from about 4% to about 8% by weight were
air-laid into the cylinder. The spacer disk then was inserted back
into the cylinder on the fiber, and the cylinder group was weighed
to the nearest 0.0001 g. The fiber in the cell was compressed with
a load of 4 psi for 60 seconds, the load then was removed and the
fiber pad was allowed to equilibrate for 60 seconds. The pad
thickness was measured, and the result was used to calculate the
dry bulk of the cross-linked fiber.
[0083] A load of 0.3 psi then was applied to the fiber pad by
placing a 100 g weight on the top of the spacer disk, and the pad
was allowed to equilibrate for 60 seconds, after which the pad
thickness was measured. The cell and its contents then were hanged
in a Petri dish containing a sufficient amount of saline solution
(0.9% by weight saline) to touch the bottom of the cell. The cell
was allowed to stand in the Petri dish for 10 minutes, and then it
was removed and hung in another empty Petri dish and allowed to
drip for 30 seconds. While the pad still was under the load, its
thickness was measured. The 100 g weight then was removed and the
weight of the cell and contents was determined. The weight of the
saline solution absorbed per gram fiber then was determined and
expressed as the absorbency under load (g/g).
[0084] The absorbent capacity of the cross-linked fiber was
determined in the same manner as the test used to determine
absorbency under load above, except that this experiment was
carried out under a load of 0.01 psi. The results are used to
determine the weight of the saline solution absorbed per gram fiber
and expressed as the absorbent capacity (g/g).
[0085] The cell from the absorbent capacity experiment then was
centrifuged for 3 min at 1400 rpm (Centrifuge Model HN,
International Equipment Co., Needham HTS, USA), and weighed. The
results obtained were used to calculate the weight of saline
solution retained per gram fiber, and expressed as the centrifuge
retention capacity (g/g).
[0086] Fiber Quality
[0087] Fiber quality evaluations were carried out on an Op Test
Fiber Quality Analyzer (Op Test Equipment Inc., Waterloo, Ontario,
Canada) and Fluff Fiberization Measuring Instruments (Model 9010,
Johnson Manufacturing, Inc., Appleton, Wis., USA).
[0088] Op Test Fiber Quality Analyzer is an optical instrument that
has the capability of measuring the average fiber length, kink,
curl, and fines content.
[0089] Fluff Fiberization Measuring Instrument is used to measure
nits and fine contents of fiber. In this instrument, a sample of
fiber in fluff form was continuously dispersed in an air stream.
During dispersion, loose fibers passed through a 16 mesh screen
(1.18 mm) and then through a 42 mesh (0.36 mm) screen. Pulp bundles
(knots) which remained in the dispersion chamber and those that
were trapped on the 42-mesh screen were removed and weighed. The
former are called "knots" and the latter "accepts." The combined
weight of these two was subtracted from the original weight to
determine the weight of fibers that passed through the 0.36 mm
screen. These fibers were referred to as "fines."
Example 1
[0090] This example illustrates a method for making mercerized
fibers and a method for forming handsheets from a blend of
mercerized and Rayfloc.RTM.-J-LD fibers.
[0091] Fiber mercerization was carried out as follows: A sample of
Rayfloc.RTM.-J-LD (never dried) was obtained as a 33.7% solid wet
lap from the Rayonier mill at Jesup, Ga. Rayfloc.RTM.-J-LD is an
untreated southern pine Kraft pulp commercially available from
Rayonier Performance Fibers Division, Jesup, Ga. A 70.0 g (dry
weight base) sample of Rayfloc.RTM.-J-LD was treated with an
aqueous solution of 16% (w/w) sodium hydroxide at room temperature
at a consistency of about 3.5%. The mixture was agitated for about
10 min, then excess NaOH was removed by suction filtration or
centrifuge. The resulting mercerized pulp then was washed with
excess water, neutralized to a pH of 6.4 with acetic acid solution
(0.01 M) at a consistency of about 3.5%, and optionally sheeted and
dried.
[0092] Handsheets of blends of fibers then were formed by
thoroughly agitating a slurry of Rayfloc.RTM.-J-LD and mercerized
fiber in water at consistency of about 3% for about 10 min. The
blend of fibers then was formed into sheets on a standard
12.times.12 inch laboratory sheet mold. The wet sheets were
optionally dried. The formed sheets had approximately the same
basis weight (762 gsm) and had a density in the range of 3.2 to 4.6
g/cm.sup.3.
[0093] The sheets prepared in accordance with this example 1 then
were cross-linked as described in the following examples.
Example 2
[0094] This example illustrates a method for cross-linking a blend
of fibers in the sheet form prepared in the manner described above
in example 1. In order to determine the effect of increasing the
amount of conventional fiber on absorbent properties of fibers
cross-linked in the sheet form, sheets with various proportions of
Rayfloc.RTM.-J-LD and mercerized fibers were prepared as described
in example 1 and used in this example. Sheets were soaked in a bath
of 2% solution of glyoxylic acid (obtained as 50% solution in water
commercially available from Clarinet Corporation, Charlotte, N.C.)
for about 1 to 2 min and then pressed to a pick-up that affords
about 2% of glyoxylic acid on fiber. Sheets then were dried and
cured in one step process at 190.degree. C. for 15 min.
[0095] The absorbent capacity, absorbency under load, centrifuge
retention, knots and fine contents of the cross-linked sheets were
determined. The results are summarized below in Table 1.
1TABLE 1 Ab- sorben- Ab- Cen- cy Under sorbent trifuge Knots Fiber
Blend Load Ca- Re- and Rayfloc .RTM.- Mercerized (0.3 psi) pacity
tention Fines nits J-LD (%) Fiber (%) (g/g) (g/g) (g/g) (%) (%) 0
100 9.8 8.3 0.46 30 70 10.3 8.6 0.43 7.2 0.47 50 50 10.0 8.7
0.46
Example 3
[0096] This example illustrates a method for cross-linking a blend
of fibers in sheet form using DP-60 as a cross-linking agent
(Belclene.RTM. DP-60 is a mixture of polymaleic acid terpolymer
with the maleic acid monomeric unit predominating (molecular weight
of about 1000) and citric acid commercially available from BioLab
Industrial Water Additives Division). Sheets with various
proportions of Rayfloc.RTM.-J-LD and mercerized fibers were
prepared as described above in Example 1, and then were soaked in a
bath of DP-60 solution (5%) for about 1 to 2 min and then pressed
to a pick-up that affords about 5% of DP-60 on fibers. Sheets were
then dried and cured in a one step process at 190.degree. C. for
about 15 min.
[0097] The absorbent capacity, absorbency under load, centrifuge
retention, knots and fine contents of cross-linked sheets were
determined. The results are summarized in Table 2.
2TABLE 2 Ab- sorben- Ab- Cen- cy Under sorbent trifuge Knots Fiber
Blend Load Ca- Re- and Rayfloc .RTM.- Mercerized (0.3 psi) pacity
tention nits Fines J-LD (%) (%) (g/g) (g/g) (g/g) (%) (%) 0 100 9.1
10.8 0.52 1.7 5.7 30 70 9.0 11.0 0.52 0.3 4.7 40 60 9.4 11.0 0.52
2.6 4.7 50 50 9.6 11.0 0.51 4.4 4.7 60 40 8.4 10.0 0.50 15.5
4.8
[0098] The results of examples 2 and 3 demonstrate that
conventional fibers (for example Rayfloc.RTM.-J-LD) can be used in
an amount of up to about 50% in a blend with mercerized fibers to
produce cross-linked fiber in the sheet form with properties
similar to those obtained using mercerized fiber alone. As shown in
Table 2, the presence of Rayfloc.RTM.-J-LD in amounts up to amount
50% showed little or no effect on the absorbent properties of
cross-linked fibers and revealed a slight increase in the amount of
knots and nits.
Example 4
[0099] This example illustrates the effect of curing temperature on
the absorbent properties of representative cross-linked fibers
formed in accordance with the present invention.
[0100] Sheet were formed as described above in example 1 using a
blend of Porosanier-J-HP 70% by weight (mercerized fiber available
from Rayonier Performance Fibers Division (Jesup, Ga.)) and
Rayfloc.RTM.-J-LD 30% by weight. Cross-linking was carried out
using glyoxylic acid in the presence and in the absence of a
catalyst. Sheets were soaked in a bath of an aqueous solution of
glyoxylic acid (2%) and magnesium chloride hexahydrate (0.25%) for
about 1 to 2 min and then pressed to a pick-up that affords about
2% of glyoxylic acid and 0.25% of magnesium chloride hexahydrate on
fiber.
[0101] Sheets after treatment were dried in a 50.degree. C. oven to
about 5 to 7% moisture contents then cured at various temperatures
for about 5 min. The above experiment was repeated without a
catalyst.
[0102] The absorbent capacity, absorbency under load, centrifuge
retention, knots and fine contents of the cross-linked fibers, and
sheet density were determined. The results are summarized below in
Table 3.
3TABLE 3 Absorbency Under Load Absorbent Centrifuge Cure (0.3 psi)
(g/g) Capacity (g/g) Retention (g/g) Temperature No With No With No
With .degree. C. Catalyst Catalyst catalyst Catalyst catalyst
catalyst 130 8.6 7.5 10.0 8.7 0.63 0.52 150 8.7 6.8 10.0 8.4 0.57
0.45 .sup. 190.sup.1 8.7 6.7 9.9 8.0 0.54 0.46 190 8.5 7.4 10.2 8.6
0.43 0.40 .sup.1Curing time in this experiment was about 2 min.
[0103] The results summarized in Table 3 demonstrate that cure
temperature has little or no effect on absorbency under load and
absorbent capacity of cross-linked fibers, whereas centrifuge
retention capacity decreases with increasing cure temperature.
These results indicate that higher temperatures are preferred to
attain higher degrees of cross-linking. In addition, the results in
Table 3 illustrate the effect of using a catalyst on cross-linking
efficiency. As shown in Table 3, fiber cross-linked in the presence
of a catalyst showed lower centrifuge retention compared to those
cross-linked in the absence of a catalyst, especially when
cross-linking is carried out at temperatures below 190.degree. C.
However, at cure temperatures of about 190.degree. C., the presence
or absence of catalyst had little or no effect on cross-linking
efficiency.
Example 5
[0104] This example illustrates the effect of curing time on
absorbent properties of representative cross-linked fibers formed
in accordance with the present invention.
[0105] Cross-linking was carried out as described above in Example
2, except that, in this example a catalyst, magnesium chloride
hexahydrate (0.25%) was used in addition to the cross-linking
agent. The fiber was cured at 150.degree. C. for various curing
times. Sheets used in this experiment were formed as described in
Example 1 using Rayfloc.RTM.-J-LD and Porosanier-J-HP in a ratio of
1:1. The results are summarized below in Table 5.
4TABLE 5 Absorbency Cure Time Under Load (0.3 psi) Absorbent
Centrifuge (min) (g/g) Capacity (g/g) Retention (g/g) 5.sup.1 7.3
8.6 0.50 5 7.3 8.6 0.45 7 6.9 8.3 0.45 10 7.3 8.6 0.44 .sup.1In
this experiment the curing was carried out at 130.degree. C.
[0106] As shown in Table 5, cure times ranging from about 5 to
about 10 minutes had little or no effect on absorbency under load
and absorbent capacity. However, the centrifuge retention
decreasing by increasing curing time indicates that cross-linking
efficiency can be increased by increasing the curing time.
Example 6
[0107] This example illustrates the effect of using various amounts
of cross-linking agent on absorbent properties of produced
fibers.
[0108] Sheets used in this experiment were formed as described
above in example 1 using 70% by weight Porosanier-J-HP and 30% by
weight Rayfloc.RTM.-J-LD. Fiber treatment was carried out as shown
in Example 2 except that in this example various amounts of
cross-linking agents were used. Glyoxylic acid was used as a
cross-linking agent in the presence of a constant amount of the
catalyst magnesium chloride hexahydrate (0.25%). Treated sheets
were dried at 50.degree. C. then cured at 150.degree. C. for 5 min.
The results are summarized below in Table 6.
5TABLE 6 Cross-linking Centrifuge agent Absorbency Under Absorbent
Retention (%) Load (0.3 psi) (g/g) Capacity (g/g) (g/g) 1.0 7.5 7.8
0.48 2.0 6.8 8.4 0.45 3.0 7.4 8.4 0.44 4.0 7.8 8.5 0.42
Example 7
[0109] This example illustrates the effect of using varying amounts
of catalyst on the absorbent properties of cross-linked fiber. The
sheets used in this example were prepared as described in example 1
using 70% Porosanier-J-HP by weight and 30% Rayfloc.RTM.-J-LD by
weight.
[0110] Fiber treatment was carried out as described above in
Example 2 except that in this example, glyoxylic acid (2%) was used
as a cross-linking agent in the presence of various amounts of
magnesium chloride hexahydrate. Treated sheets were dried at
50.degree. C., and then cured at 150.degree. C. for 5 min.
[0111] The results are summarized below in Table 7.
6TABLE 7 Catalyst Centrifuge MgCl.sub.2.6H.sub.2O Absorbency Under
Absorbent Retention (%) Load (0.3 psi) (g/g) Capacity (g/g) (g/g)
0.00 8.7 10.0 0.57 0.05 9.4 8.8 0.46 0.10 8.2 8.9 0.46 0.25 6.8 8.4
0.45 0.40 7.5 8.4 0.45
[0112] The results summarized in Table 7 indicate that increasing
the amount of catalyst showed little or only a slight effect on
cross-linking efficiency, since as can be seen in Table 7,
increasing the amount of catalyst from 0.05% to about 0.4% showed
minimal changes in fiber absorbent properties.
Example 8
[0113] This example illustrates an airlaid method for forming a
representative absorbent structure of the present invention.
[0114] An airlaid absorbent core formed in accordance with the
present invention was prepared using an airlaid apparatus known to
experts in the art. Fiber (Rayfloc.RTM.-J-LD) and superabsorbent
particles (P-02-055-01 obtained from BASF), were loaded into the
airlaying apparatus. Vacuum then was applied, fibers and
superabsorbent particles traveled through plastic tubing and were
combined through an air vortex into a plastic cylinder having a
100-mesh metal screen adhering to the cylinder bottom. After fibers
and superabsorbent particles were completely combined, the vacuum
was discontinued and the resulting pad was removed from the
cylinder. The pad was then compressed by a hydraulic press to a
pressure of 700 PSI for 3 min. The pressure was then released, and
pad was allowed to equilibrate for 60 seconds. The pad thickness
was measured before and after pressing and the density was
calculated. The total weight of the pad is about 3.0 g composed of
55% by weight superabsorbent particles and 45% by weight fiber.
Example 9
[0115] This example illustrate the acquisition times of
cross-linked fibers made in accordance with the present
invention.
[0116] The acquisition time was determined by the SART test method.
The test evaluates the performance of cross-linked fibers as an
acquisition layer in absorbent article. The test measures the time
required for a dose of saline to be absorbed completely into the
absorbent article. The test is conducted on a sample of absorbent
core made in accordance with the method described in example 8. The
core included about 45% by weight fiber (Rayfloc.RTM.-J-LD) and 55%
by weight superabsorbent particles (P-02-055-01 obtained from BASF)
based on the total weight of the core. The core had a circular
shape with a diameter of 60.0 mm, a density of 0.2, and weighed
about 3.0 g (+0.1 g).
[0117] In this test a sample of cross-linked fibers made in
accordance with the present invention was airlaid into a 60.0 mm
pad. The pad weighed about 0.7 g and was compressed with a load of
a 7.6 PSI for 60 seconds before being tested. The core was placed
into the testing cell, which consisted of a plastic base and a
funnel cup. The base used to hold the sample was a plastic cylinder
with an inside diameter of 60.0 mm. The funnel cup was a plastic
cylinder with a star shape hole, the outside diameter of the funnel
cup was 58 mm. The funnel cup was placed inside the plastic base on
top of the sample and a load of 0.6 PSI having a donut shape was
placed on top of the funnel cup.
[0118] The cell and contents were placed on a level surface and
dosed with three successive 9.0 mL insults of saline solution; the
time interval between doses was 20 min. The doses were added with a
Master Flex Pump (Cole Parmer Instrument, Barrington, Ill., USA) to
the funnel cup, and the time in seconds required for each dose of
saline solution to disappear from the funnel cup was recorded and
expressed as an acquisition time.
[0119] The results are summarized in Table 8.
7TABLE 8 % of Cross- 3.sup.rd Insult Fiber Cross-linking Agent
linking agent (sec) Porosanier 21.0 Porosanier Fiber (sheet DP-60
5.0 14.0 form) Blend (Rayfloc 50% and DP-60 5.0 12.0 Porosanier
50%) Blend (Rayfloc 50% and Glyoxylic acid 2.0 14.2 Porosanier 50%)
Blend (Rayfloc 40% and Glyoxlic acid 3.0 10.3 Porosanier 60%)
Porosanier Glyoxylic acid 2.0 10.5 Blend (Rayfloc 30% and Glyoxylic
acid 2.0 10.2 Porosanier 70%)
[0120] The above examples reveal that crosslinked blends of
mercerized fibers and conventional cellulose fibers have improved
absorbency, absorbency under load, retention capacity, and
acquisition times. The blends of the invention therefore preferably
are useful in absorbent articles as an acquisition layer and/or an
absorbent core.
[0121] While the invention has been described with reference to
particularly preferred embodiments and examples, those skilled in
the art recognize that various modifications may be made to the
invention without departing from the spirit and scope thereof.
* * * * *