U.S. patent application number 10/010711 was filed with the patent office on 2002-07-11 for cellulose fibers having low water retention value and low capillary desorption pressure.
This patent application is currently assigned to BKI HOLDING CORPORATION. Invention is credited to Bell, Robert Irvin, Gross, James R., Schoggen, Howard Leon, Smith, David Jay.
Application Number | 20020090511 10/010711 |
Document ID | / |
Family ID | 26938439 |
Filed Date | 2002-07-11 |
United States Patent
Application |
20020090511 |
Kind Code |
A1 |
Smith, David Jay ; et
al. |
July 11, 2002 |
Cellulose fibers having low water retention value and low capillary
desorption pressure
Abstract
The present invention provides cellulose fibers having low
median desorption pressures and low water retention values (WRV),
which exhibit improved drainage and fluid flow properties. These
fibers are particularly well suited for use in acquisition,
distribution, and acquisition-distribution layers, or in absorbent
core structures. One embodiment of the invention is a method for
preparing cellulose fibers by refining cellulose fibers to a
freeness ranging from about 300 to about 700 ml CSF and
crosslinking the refined fibers. Another embodiment of the
invention is fibers crosslinked with at least one saturated
dicarboxylic acid, aromatic dicarboxylic acid, cycloalkyl
discarboxylic acid, bifunctional monocarboxylic acid, or amine
carboxylic acid. A crosslinking facilitator, such as oxalic acid,
may be present during the crosslinking reaction to improve the
efficacy of the crosslinking agent. Yet another embodiment of the
invention is an absorbent core comprising SAP particles and
reversible crosslinked fibers.
Inventors: |
Smith, David Jay;
(Germantown, TN) ; Schoggen, Howard Leon;
(Southhaven, MS) ; Bell, Robert Irvin;
(Collierville, TN) ; Gross, James R.; (Cordova,
TN) |
Correspondence
Address: |
DARBY & DARBY P.C.
805 Third Avenue
New York
NY
10022
US
|
Assignee: |
BKI HOLDING CORPORATION
Wilmington
DE
|
Family ID: |
26938439 |
Appl. No.: |
10/010711 |
Filed: |
November 8, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60247078 |
Nov 10, 2000 |
|
|
|
60286298 |
Apr 25, 2001 |
|
|
|
Current U.S.
Class: |
428/392 ;
428/311.71 |
Current CPC
Class: |
A61F 13/537 20130101;
Y10T 428/249965 20150401; Y10T 428/2964 20150115; A61L 15/28
20130101; A61F 13/15203 20130101; A61L 15/28 20130101; C08L 1/02
20130101; D21H 11/20 20130101 |
Class at
Publication: |
428/392 ;
428/311.71 |
International
Class: |
D02G 003/00 |
Claims
What is claimed is:
1. Cellulose fibers having a median desorption pressure, as
determined in a capillary absorption-desorption cycle, of 15 cm or
less.
2. The cellulose fibers of claim 1, wherein the cellulose fibers
have a median desorption pressure of 14 cm or less.
3. The cellulose fibers of claim 1, wherein wherein the cellulose
fibers have a median desorption pressure of 13 cm or less.
4. The cellulose fibers of claim 1, wherein the cellulose fibers
have a median desorption pressure of 12 cm or less.
5. The cellulose fibers of claim 1, wherein the cellulose fibers
have a water retention value of 45 percent or less.
6. The cellulose fibers of claim 5, wherein the cellulose fibers
have a water retention value of 38 percent or less.
7. The cellulose fibers of claim 6, wherein the cellulose fibers
have a water retention value of 30 percent or less.
8. The cellulose fibers of claim 1, wherein the cellulose fibers
are crosslinked.
9. An acquisition and distribution layer comprising the cellulose
fibers of claim 1.
10. An acquisition layer comprising the cellulose fibers of claim
1.
11. A distribution layer comprising the cellulose fibers of claim
1.
12. An absorbent structure comprising: (a) a top layer comprising
cellulose fibers having a median desorption pressure, as determined
in a capillary absorption-desorption cycle, of 15 cm or less; and
(b) a bottom layer comprising SAP particles, the second layer being
in fluid communication with the first layer.
13. The absorbent structure of claim 12, wherein the cellulose
fibers have a median desorption pressure of 14 cm or less.
14. The absorbent structure of claim 13, wherein the cellulose
fibers have a median desorption pressure of 13 cm or less.
15. The absorbent structure of claim 14, wherein the cellulose
fibers have a median desorption pressure of 12 cm or less.
16. The absorbent structure of claim 12, wherein the cellulose
fibers have a water retention value of 45 percent or less.
17. The absorbent structure of claim 16, wherein the cellulose
fibers have a water retention value of 38 percent or less.
18. The absorbent structure of claim 17, wherein the cellulose
fibers have a water retention value of 30 percent or less.
19. An absorbent structure comprising the cellulose fibers of claim
1.
20. An absorbent structure comprising the acquisition and
distribution layer of claim 9.
21. An absorbent structure comprising the acquisition layer of
claim 10.
22. An absorbent structure comprising the distribution layer of
claim 11.
23. A method for preparing cellulose fibers comprising the steps
of: (a) refining cellulose fibers to a freeness of from about 300
to about 700 ml CSF; and (b) crosslinking the refined cellulose
fibers.
24. The method of claim 23, wherein the cellulose fibers to be
refined in step (a) are wet lap.
25. The method of claim 23, wherein step (a) comprises refining the
cellulose fibers to a freeness of from about 500 to about 700 ml
CSF.
26. The method of claim 25, wherein step (a) comprises refining the
cellulose fibers to a freeness of from about 650 to about 700 ml
CSF.
27. The method of claim 23, wherein step (b) comprises: (i) mixing
the refined cellulose fibers with a crosslinking agent; and (ii)
curing the cellulose fibers in the mixture.
28. The method of claim 23, wherein step (b) comprises: (i) mixing
the refined cellulose fibers with a crosslinking agent; (ii)
fluffing the cellulose fibers in the mixture; and (iii) curing the
cellulose fibers in the mixture.
29. The method of claim 28, wherein step (b)(iii) comprises drying
the cellulose fibers and curing the dried cellulose fibers.
30. The method of claim 28, wherein curing is performed at a
temperature ranging from about 150 to about 175.degree. C.
31. Cellulose fibers prepared by the method of claim 23.
32. A method of preparing an absorbent structure comprising (a)
preparing cellulose fibers by the method of claim 23; and (b)
incorporating the cellulose fibers into an absorbent structure.
33. Cellulose fibers crosslinked with at least one crosslinking
agent selected from saturated dicarboxylic acids, aromatic
dicarboxylic acids, cycloalkyl dicarboxylic acids, bifunctional
monocarboxylic acids, and amine carboxylic acids and having a
median desorption pressure as measured in a capillary
absorption-desorption cycle of 25 cm or less.
34. The cellulose fibers of claim 33, wherein the saturated
dicarboxylic acid has 2 to 8 carbon atoms.
35. The cellulose fibers of claim 34, wherein the saturated
dicarboxylic acid has 2 to 6 carbon atoms.
36. The cellulose fibers of claim 35, wherein the saturated
dicarboxylic acid has 2 to 4 carbon atoms.
37. The cellulose fibers of claim 34, wherein the saturated
dicarboxylic acid is selected from oxalic acid, malonic acid,
succinic acid, glutaric acid, adipic acid, pimelic acid, suberic
acid, and any combination of any of the foregoing.
38. The cellulose fibers of claim 33, wherein the saturated
dicarboxylic acid is a saturated hydroxy carboxylic acid.
39. The cellulose fibers of claim 38, wherein the saturated hydroxy
carboxylic acid has 2 to 8 carbon atoms.
40. The cellulose fibers of claim 39, wherein the hydroxy saturated
dicarboxylic acid is selected from glycolic acid, tartaric acid,
malic acid, saccharic acid, mucic acid, and any combination of any
of the foregoing.
41. The cellulose fibers of claim 33, wherein the aromatic
dicarboxylic acid has the formula 4wherein R.sup.1, R.sup.2,
R.sup.3, and R.sup.4 independently are hydrogen, hydroxy,
C.sub.1-C.sub.4 alkoxy, C.sub.1-C.sub.4 amino, halogen, or
nitro.
42. The cellulose fibers of claim 41, wherein the aromatic
dicarboxylic acid is phthalic acid.
43. The cellulose fibers of claim 33, wherein the cycloalkyl
dicarboxylic acid has the formula 5wherein R.sup.6, R.sup.7,
R.sup.10, and R.sup.11 are independently hydrogen, hydroxy,
halogen, C.sub.1-C.sub.4 alkoxy, C.sub.1-C.sub.4 alkyl, amino, or
nitro; and R.sup.8 and R.sup.9 are independently hydrogen, halogen,
C.sub.1-C.sub.4 alkoxy, or C.sub.1-C.sub.4 alkyl.
44. The cellulose fibers of claim 43, wherein the cycloalkyl
dicarboxylic acid is 1,2,5,6-tetrahydrophthalic acid.
45. The cellulose fibers of claim 33, wherein the bifunctional
monocarboxylic acid is selected from salts of a haloacetate,
hydroxy monocarboxylic acids, acid derivatives of hydroxy
monocarboxylic acids, and any combination of any of the
foregoing.
46. The cellulose fibers of claim 45, wherein the salt of a
haloacetate is sodium chloroacetate.
47. The cellulose fibers of claim 33, wherein the amine carboxylic
acid is an amino acid.
48. The cellulose fibers of claim 47, wherein the amino acid has
the formulaH.sub.2N--CH.sub.2--R.sup.12--C(O)OHwherein R.sup.12 is
a bond, C.sub.1-C.sub.12 alkyl, or C.sub.1-C.sub.12 alkyl
substituted with one or more of carboxyl, hydroxy, C.sub.1-C.sub.4
alkoxy, C.sub.1-C.sub.4 alkyl, amino, and nitro.
49. The cellulose fibers of claim 47, wherein the amino acid has
the formula 6where R.sup.5 is a linear or branched C.sub.1-C.sub.8
alkyl.
50. The cellulose fibers of claim 49, wherein R.sup.5 is a
C.sub.2-C.sub.4 alkyl.
51. The cellulose fibers of claim 47, wherein the amino acid is
selected from aspartic acid, glutamic acid, and any combination of
any of the foregoing.
52. The cellulose fibers of claim 33, wherein the amine carboxylic
acid is ethylenedinitrilotetraacetic acid.
53. The cellulose fibers of claim 33, wherein the cellulose fibers
have been crosslinked with from about 5 to about 21 mole percent of
crosslinking agent, calculated on a cellulose anhydroglucose molar
basis.
54. The cellulose fibers of claim 33, wherein the cellulose fibers
have been crosslinked in the presence of a crosslinking
facilitator.
55. The cellulose fibers of claim 54, wherein the crosslinking
facilitator and the crosslinking agent are different.
56. The cellulose fibers of claim 54, wherein the crosslinking
facilitator is oxalic acid.
57. The cellulose fibers of claim 54, wherein the cellulose fibers
have been crosslinked in the presence of from about 1.8 to about 9
mole percent of crosslinking facilitator, calculated on a cellulose
anhydroglucose molar basis.
58. The cellulose fibers of claim 54, wherein the cellulose fibers
have been crosslinked with from about 0.5 to about 40 mole percent
of crosslinking agent and crosslinking facilitator, calculated on a
cellulose anhydroglucose molar basis.
59. The cellulose fibers of claim 58, wherein the cellulose fibers
have been crosslinked with from about 1 to about 30 mole percent of
crosslinking agent and crosslinking facilitator, calculated on a
cellulose anhydroglucose molar basis.
60. The cellulose fibers of claim 33, wherein the cellulose fibers
are derived from wood pulp.
61. The cellulose fibers of claim 33, wherein the cellulose fibers
have been refined prior to crosslinking.
62. The cellulose fibers of claim 61, wherein the cellulose fibers
have been refined to a freeness of from about 300 to about 700 ml
CSF prior to crosslinking.
63. The cellulose fibers of claim 62, wherein the cellulose fibers
have been refined to a freeness of from about 500 to about 700 ml
CSF prior to crosslinking.
64. The cellulose fibers of claim 63, wherein the cellulose fibers
have been refined to a freeness of from about 650 to about 700 ml
CSF prior to crosslinking.
65. The cellulose fibers of claim 33, wherein the cellulose fibers
have been cured at a temperature of from about 105 to about
225.degree. C.
66. The cellulose fibers of claim 65, wherein the cellulose fibers
have been cured at a temperature of from about 150 to about
190.degree. C.
67. The cellulose fibers of claim 66, wherein the cellulose fibers
have been cured at a temperature of from about 160 to about
175.degree. C.
68. The cellulose fibers of claim 33, wherein the cellulose fibers
have been cured in the presence of a reducing agent.
69. The cellulose fibers of claim 68, wherein the reducing agent is
a hypophosphite.
70. The cellulose fibers of claim 69, wherein the reducing agent is
sodium hypophosphite.
71. The cellulose fibers of claim 33, wherein the water retention
value of the cellulose fibers is 50 percent or less.
72. The cellulose fibers of claim 71, wherein the water retention
value of the cellulose fibers is 45 percent or less.
73. The cellulose fibers of claim 72, wherein the water retention
value of the cellulose fibers is 38 percent or less.
74. The cellulose fibers of claim 73, wherein the water retention
value of the cellulose fibers is 30 percent or less.
75. The cellulose fibers of claim 33, wherein the median desorption
pressure of the cellulose fibers as measured in a capillary
absorption-desorption cycle is 20 cm or less.
76. The cellulose fibers of claim 75, wherein the median desorption
pressure of the cellulose fibers as measured in a capillary
absorption-desorption cycle is 18 cm or less.
77. The cellulose fibers of claim 76, wherein the median desorption
pressure of the cellulose fibers as measured in a capillary
absorption-desorption cycle is 15 cm or less.
78. The cellulose fibers of claim 33, wherein the crosslinking is
substantially reversible.
79. The cellulose fibers of claim 33, wherein the crosslinking
agent is oxalic acid and the crosslinking is substantially
reversible.
80. Uncrosslinked cellulose fibers prepared by uncrosslinking the
cellulose fibers of claim 33.
81. The uncrosslinked cellulose fibers of claim 80, wherein the
crosslinking agent contains 4 carbon atoms or less.
82. The uncrosslinked cellulose fibers of claim 81, wherein the
crosslinking agent is oxalic acid.
83. The uncrosslinked cellulose fibers of claim 81, wherein the
crosslinking agent is sodium chloroacetate.
84. The uncrosslinked cellulose fibers of claim 80, wherein the
uncrosslinking step comprises soaking the cellulose fibers in
water.
85. The uncrosslinked cellulose fibers of claim 84, wherein the
uncrosslinking step comprises soaking the cellulose fibers in water
for from about 0.5 to about 4 hours.
86. A sheet comprising the uncrosslinked cellulose fibers of claim
80.
87. An absorbent structure comprising the fibers of claim 33.
88. A method of preparing crosslinked cellulose fibers comprising
intrafiber crosslinking the cellulose fibers with at least one
saturated dicarboxylic acid, aromatic dicarboxylic acid, cycloalkyl
dicarboxylic acid, bifunctional monocarboxylic acid, or amine
carboxylic acid.
89. The method of claim 88, wherein the saturated dicarboxylic acid
has 2 to 8 carbon atoms.
90. The method of claim 89, wherein the saturated dicarboxylic acid
has 2 to 6 atoms.
91. The method of claim 90, wherein the saturated dicarboxylic acid
has 2 to 4 carbon atoms.
92. The method of claim 89, wherein the saturated dicarboxylic acid
is selected from oxalic acid, malonic acid, succinic acid, glutaric
acid, adipic acid, pimelic acid, suberic acid, and any combination
of any of the foregoing.
93. The method of claim 88, wherein the saturated dicarboxylic acid
is a saturated hydroxy carboxylic acid.
94. The method of claim 93, wherein the saturated hydroxy
carboxylic acid has 2 to 8 carbon atoms.
95. The method of claim 94, wherein the C.sub.2-C.sub.8 hydroxy
saturated dicarboxylic acid is selected from glycolic acid,
tartaric acid, malic acid, saccharic acid, mucic acid, and any
combination of any of the foregoing.
96. The method of claim 88, wherein the aromatic dicarboxylic acid
has the formula 7wherein R.sup.1, R.sup.2, R.sup.3, and R.sup.4
independently are hydrogen, hydroxy, C.sub.1-C.sub.4 alkoxy,
C.sub.1-C.sub.4 alkyl, amino, halogen, or nitro.
97. The method of claim 96, wherein the aromatic dicarboxylic acid
is phthalic acid.
98. The method of claim 88, wherein the cycloalkyl dicarboxylic
acid has the formula 8wherein R.sup.6, R.sup.7, R.sup.10, and
R.sup.11 are independently hydrogen, hydroxy, halogen,
C.sub.1-C.sub.4 alkoxy, C.sub.1-C.sub.4 alkyl, amino, or nitro; and
R.sup.8 and R.sup.9 are independently hydrogen, halogen,
C.sub.1-C.sub.4 alkoxy, or C.sub.1-C.sub.4 alkyl.
99. The method of claim 98, wherein the cycloalkyl dicarboxylic
acid is 1,2,5,6-tetrahydrophthalic acid.
100. The method of claim 88, wherein the bifunctional
monocarboxylic acid is selected from salts of a haloacetate,
hydroxy monocarboxylic acids, acid derivatives of hydroxy
monocarboxylic acids, and any combination of any of the
foregoing.
101. The method of claim 100, wherein the salt of a haloacetate is
sodium chloroacetate.
102. The method of claim 88, wherein the amine carboxylic acid is
an amino acid.
103. The method of claim 102, wherein the amino acid has the
formulaH.sub.2N--CH.sub.2--R.sup.12--C(O)OHwherein R.sup.12 is a
bond, C.sub.1-C.sub.12 alkyl, or C.sub.1-C.sub.12 alkyl substituted
with one or more of carboxyl, hydroxy, C.sub.1-C.sub.4 alkoxy,
C.sub.1-C.sub.4 alkyl, amino, and nitro.
104. The method of claim 102, wherein the amino acid has the
formula 9where R.sup.5 is a linear or branched C.sub.1-C.sub.8
alkyl.
105. The method of claim 104, wherein R.sup.5 is a C.sub.2-C.sub.4
alkyl.
106. The method of claim 102, wherein the amino acid is selected
from aspartic acid, glutamic acid, and any combination of any of
the foregoing.
107. The method of claim 88, wherein the amine carboxylic acid is
ethylenedinitrilotetraacetic acid.
108. The method of claim 88, wherein the mole percent of
crosslinking agent ranges from about 5 to about 21 mole percent,
calculated on a cellulose anhydroglucose molar basis.
109. The method of claim 88, wherein the crosslinking step is
performed in the presence of a crosslinking facilitator.
110. The method of claim 109, wherein the crosslinking agent is
different than the crosslinking facilitator.
111. The method of claim 109, wherein the crosslinking facilitator
is oxalic acid.
112. The method of claim 109, wherein the mole percent of
crosslinking facilitator ranges from about 1.8 to about 9 mole
percent, calculated on a cellulose anhydroglucose molar basis.
113. The method of claim 109, wherein the mole percent of
crosslinking agent and crosslinking facilitator ranges from about
0.05 to about 40, calculated on a cellulose anhydroglucose molar
basis.
114. The method of claim 113, wherein the mole percent of
crosslinking agent and crosslinking facilitator ranges from about 1
to about 30, calculated on a cellulose anhydroglucose molar
basis.
115. The method of claim 88, wherein the crosslinking step
comprises: (i) mixing the cellulose fibers with the crosslinking
agent; and (ii) curing the cellulose fibers in the mixture.
116. The method of claim 115, wherein the crosslinking step
comprises: (i) mixing the cellulose fibers with the crosslinking
agent; (ii) fluffing the cellulose fibers in the mixture; and (iii)
curing the cellulose fibers in the mixture.
117. The method of claim 116, wherein step (iii) comprises drying
the cellulose fibers and curing the dried cellulose fibers.
118. The method of claim 115, wherein curing is performed at a
temperature ranging from about 150 to about 175.degree. C.
119. The cellulose fibers of claim 88, wherein the fibers are
crosslinked in the presence of a reducing agent.
120. The cellulose fibers of claim 119, wherein the reducing agent
is a hypophosphite.
121. The cellulose fibers of claim 120, wherein the reducing agent
is sodium hypophosphite.
122. The method of claim 88, wherein the cellulose fibers are
refined prior to the crosslinking step.
123. The method of claim 122, wherein the cellulose fibers are
refined to a freeness of from about 500 to about 700 ml CSF.
124. The method of claim 123, wherein the cellulose fibers are
refined to a freeness of from about 650 to about 700 ml CSF.
125. Cellulose fibers prepared by the method of claim 88.
126. A method of preparing uncrosslinked fibers comprising the
steps of intrafiber crosslinking cellulose fibers with at least one
saturated dicarboxylic acid, aromatic dicarboxylic acid, cycloalkyl
dicarboxylic acid, bifunctional monocarboxylic acid, or amine
carboxylic acid; and uncrosslinking the crosslinked cellulose
fibers.
127. The method of claim 126, wherein the crosslinking agent
contains 4 carbon atoms or less.
128. The method of claim 127, wherein the crosslinking agent is
oxalic acid.
129. The method of claim 127, wherein the crosslinking agent is
sodium chloroacetate.
130. The method of claim 126, wherein the uncrosslinking step
comprises soaking the crosslinked cellulose fibers in water.
131. The method of claim 130, wherein the uncrosslinking step
comprises soaking the crosslinked cellulose fibers in water for
from about 0.5 to about 4 hours.
132. A method of preparing a sheet of uncrosslinked cellulose
fibers comprising the steps of preparing uncrosslinked cellulose
fibers by the method of claim 126 and forming the uncrosslinked
cellulose fibers into a sheet.
133. A method of preparing crosslinked cellulose fibers comprising
the steps of: (a) preparing uncrosslinked cellulose fibers by the
method of claim 126; and (b) crosslinking the cellulose fibers.
134. A method of preparing an absorbent structure comprising (a)
preparing cellulose fibers by the method of claim 88; and (b)
incorporating the cellulose fibers into an absorbent structure.
135. An absorbent core comprising superabsorbent polymer particles
and reversible crosslinked cellulose fibers.
136. The absorbent core of claim 135, wherein the reversible
crosslinked cellulose fibers are crosslinked with oxalic acid,
sodium chloroacetate, or a mixture thereof.
137. The absorbent core of claim 136, wherein the reversible
crosslinked cellulose fibers are crosslinked with oxalic acid.
138. The absorbent core of claim 135, wherein the absorbent core
comprises from about 30 to about 70% by weight of superabsorbent
particles and from about 70 to about 30% by weight of reversible
crosslinked fibers, based on 100% total weight of the absorbent
core.
Description
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/247,078, filed Nov. 10, 2000, and U.S.
Provisional Application No. 60/286,298, filed Apr. 25, 2001, both
of which are hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] This invention relates to cellulose fibers having low water
retention values and low median desorption pressures, as measured
in a capillary absorption-desorption cycle, methods for preparing
these fibers, and-absorbent structures containing these fibers.
BACKGROUND OF THE INVENTION
[0003] Absorbent structures are important in a wide range of
disposable absorbent articles including infant diapers, adult
incontinence products, sanitary napkins and other feminine hygiene
products and the like. These and other absorbent articles are
generally provided with an absorbent core to receive and retain
body liquids. In a conventional absorbent structure, the absorbent
core is placed between a liquid pervious topsheet, whose function
is to allow the passage of fluid to the core, and a liquid
impervious backsheet whose function is to contain the fluid and to
prevent it from passing through the absorbent article to the
garment of the wearer of the absorbent article.
[0004] An absorbent core for diapers, adult incontinence pads and
feminine hygiene articles frequently includes fibrous batts or webs
constructed of defiberized, loose, fluffed, hydrophilic, cellulosic
fibers. Such fibrous batts form a matrix capable of absorbing and
retaining some liquid. However, their ability to do so is limited.
Thus, superabsorbent polymer (SAP) particles, granules, flakes or
fibers (collectively particles), capable of absorbing many times
their weight of liquid, are often included in the absorbent core to
increase the absorbent capacity of the core, without having to
substantially increase the bulkiness of the core. In an absorbent
core containing matrix fibers and SAP particles, the fibers
physically separate the SAP particles, provide structural integrity
for the absorbent core, and provide avenues for the passage of
fluid through the core.
[0005] Absorbent cores containing SAP particles have been
successful, and in recent years, market demand has increased for
thinner, more absorbent and more comfortable absorbent articles. As
core absorbency has improved, the ability of the core to rapidly
drain fluid from the absorbent article topsheet has become critical
to maintaining a dry environment between the skin of the wearer of
the absorbent article and the topsheet of the article.
[0006] The ability of the absorbent core to drain fluid from the
layer immediately above it in the absorbent structure is controlled
by gravity, by the number, size and spatial orientation of
unoccupied volumes (voids or pores) in the absorbent core, and by
the characteristics of the core components that impact fluid flow
such as the wettability of the components by the acquired fluid, as
indicated by contact angle, the surface tension of the acquired
fluid and the viscosity of the acquired fluid. An acquisition
layer, distribution layer, or acquisition and distribution layer
can be included in the absorbent structure between the top sheet
and absorbent core to facilitate draining of fluid into the
absorbent core.
[0007] For optimum performance of an absorbent structure in terms
of fluid capacity and core utilization, it is critical for fluids
acquired by the absorbent core to move quickly from the moistened
regions of the core to the dry regions of the core. The ability of
the absorbent core to move fluids rapidly from a moistened region
of the core to a dry region of the core may be described in terms
of its permeability performance. The permeability of an absorbent
core is defined as the ability of a liquid to flow through the
absorbent core.
[0008] The ability of a first substrate, such as an absorbent core,
to drain fluid primarily by capillary forces from a second
substrate, such as an acquisition and distribution layer, is known
as the partition property of the substrates.
[0009] It is known to those skilled in the art that absorbent
structures containing absorbent cores with good fluid partition
properties also exhibit poor fluid permeability. Similarly,
absorbent structures having good fluid permeability exhibit poor
fluid partition properties. Consequently, it is important that
fibers used in an acquisition layer have a higher degree of
stiffness or resiliency (measured as dry compressibility) under the
weight of the diaper wearer than conventional fibers used in an
absorbent core. This resiliency enables the interfiber spaces or
voids in the acquisition layer to be maintained while the diaper is
worn so that fluids can be quickly absorbed through the liquid
permeable topsheet into the absorbent structure of the diaper.
[0010] It is also important for core fibers not to densify when
wet, to the point that fluid flow into and through the absorbent
core is restricted. In addition, core fibers must have sufficient
physical integrity to maintain separation of wet SAP particles in
an absorbent core so that as the particles swell, gel blocking is
minimized or eliminated.
[0011] One method for increasing the stiffness and resiliency of
fibers is by crosslinking them. Cellulose fibers can be stiffened
by intrafiber crosslinks, i.e., crosslinks between two different
portions of the same fiber, and to a lesser degree by interfiber
crosslinks, i.e., crosslinks between two different fibers.
[0012] Intrafiber crosslinking with certain aliphatic and alicyclic
C.sub.2-C.sub.9 polycarboxylic acids is disclosed in U.S. Pat. No.
5,190,563 issued to Herron et al. A "C.sub.2-C.sub.9 polycarboxylic
acid" as defined by Herron et al. is an organic acid containing two
or more carboxyl groups and from 2 to 9 carbon atoms in the chain
or ring to which the carboxyl groups are attached. Suitable
C.sub.2-C.sub.9 polycarboxylic acids contain at least three
carboxyl groups or two carboxyl groups with a carbon-carbon double
bond present at the alpha, beta position relative to one or both
carboxyl groups. When two carboxyl groups are separated by a
carbon-carbon double bond or are both connected to the same ring,
the two carboxyl groups must be in the cis configuration. Examples
of such polycarboxylic acids include citric acid,
1,2,3-propanetricarboxylic acid, 1,2,3,4-butanetetracarboxylic acid
(BTCA), and oxydisuccinic acid. Herron et al. also found that
cellulosic fibers crosslinked with aliphatic alkanes containing 4
carboxyl groups, namely, BTCA, had lower water retention values
than those containing 3 carboxyl groups, namely, citric acid and
1,2,3-propane tricarboxylic acid. Typically, fibers with lower
water retention values are stiffer than those having higher water
retention values.
[0013] In contrast to cellulosic fibers having intrafiber
crosslinks, cellulosic fibers having interfiber crosslinks, such as
those found in most papers, are stiff when dry but do not
necessarily maintain their stiffness when wet. Interfiber
crosslinking of paper with citric acid and
1,2,3,4-butanetetracarboxylic acid and fabrics with maleic acid,
citric acid, and 1,2,3,4-butanetetracarboxylic acid is disclosed in
D. F. Caulfield, TAPPIJ., 77(3): 205-212 (1994); D. Horie & C.
J. Biermann, TAPPIJ., 77(8):135-140 (1994); Y. J. Zhou, P. Luner
& P. Caluwe, J. Appl. Polymer Sci., 58:1523-1534 (1995); and D.
D. Gagliardi and F. B. Shippee, Am. Dyestuff Reptr., 52:300
(1963).
[0014] Zhou et al., supra, studied the wet strength of paper
crosslinked (interfiber) with certain polycarboxylic acids.
Generally, interfiber crosslinking increases the wet strength of
paper fibers. Zhou et al. found that the wet strength of the paper
increased as the functionality of the polycarboxylic acid (i.e. the
number of carboxyl groups in the polycarboxylic acid) increased.
For example, 1,2,3,4-butanetetracarboxyli- c acid (BTCA) (4
carboxyl groups) was found to be more effective than tricarballylic
acid (TCA) (3 carboxyl groups), which in turn was found to be
significantly more effective than succinic acid (2 carboxyl
groups). Paper treated with succinic acid exhibited very little wet
strength.
[0015] H. J. Campbell and T. Francis, Textile Res. J., 35:260
(1965), crosslinked cotton cellulose with specific polycarboxylic
acids. The reaction was catalyzed with trifluoroacetic anhydride
(TFAA), necessitating the use of a non-aqueous solvent, in this
case benzene, to prevent hydrolysis of the TFAA. Campbell and
Francis reported that succinic acid and glutaric acid showed only
slight reactivity with cotton cellulose. Furthermore, they reported
that esterification (or crosslinking) did not take place with
oxalic acid. Malonic acid was found to react readily with cotton
cellulose producing fabrics which were yellowed to an extent
depending upon the degree of reaction.
[0016] Frequently, crosslinked cellulosic fibers are manufactured
at a location remote from where they are incorporated into
absorbent structures. Since the crosslinked fibers are bulky and
have little fiber to fiber contact, they do not bond well to one
another. Hence, sheets formed from crosslinked fibers fall apart
easily. As a result, crosslinked cellulosic fibers are generally
shipped in bales. This increases the cost of shipping the
crosslinked fibers and the cost of manufacturing the absorbent
structure. It would, therefore, be desirable to prepare sheets of
cellulosic fibers containing a crosslinking agent.
[0017] A "crosslinkable" cellulosic fibrous product formed into a
web or sheet is disclosed in International Publication No. WO
00/65146. The crosslinkable product is formed by applying a
crosslinking agent to a mat of cellulosic fibers and then drying
the treated mat (without heating to a temperature sufficient to
cure the crosslinking agent) such that substantially no
crosslinking occurs and the product is substantially free from
crosslinks.
[0018] U.S. Pat. No. 6,059,924 disclose a process for enhancing the
dry compression characteristics and the wicking property of fluff
pulp. The process includes mildly refining a chemical pulp slurry
prior to formation of a fluff pulp sheet.
[0019] There is a continuing need for improved cellulose fibers
that have low water retention values and low median desorption
pressures for incorporation into acquisition, distribution, and
acquisition-distributio- n layers. There is also a need for core or
matrix fibers that facilitate fluid flow into and through the
absorbent core and maintain sufficient physical integrity to
minimize or eliminate the gel blocking of swollen SAP particles.
Finally, there is a need for methods of preparing sheets of
crosslinkable cellulosic fibers.
SUMMARY OF THE INVENTION
[0020] This invention provides cellulose fibers having low median
desorption pressures, as measured in a capillary
absorption-desorption cycle, and low water retention values (WRV),
which exhibit improved drainage and fluid flow properties. These
fibers are particularly well suited for use in acquisition,
distribution, and acquisition-distribution layers, and in absorbent
core structures.
[0021] According to one embodiment, the fibers of the present
invention are crosslinked and have a median desorption pressure, as
determined in a capillary absorption-desorption cycle, of 15 cm or
less. Preferably, the cellulose fibers also have a WRV of 45% or
less. These fibers may be prepared by refining cellulose fibers to
a freeness ranging from about 300 to about 700 ml Canadian Standard
Freeness (CSF) and crosslinking the refined fibers. According to a
preferred embodiment, the fibers are crosslinked with citric acid
after refining.
[0022] Another embodiment of the invention is fibers crosslinked
with at least one saturated dicarboxylic acid, aromatic
dicarboxylic acid, cycloalkyl dicarboxylic acid, bifunctional
monocarboxylic acid, or amine carboxylic acid. A crosslinking
facilitator, such as oxalic acid, may be present during the
crosslinking reaction to improve the efficacy of the crosslinking
agent. According to one preferred embodiment, the cellulose fibers
are refined prior to crosslinking in order to further stiffen
them.
[0023] Another embodiment is a method of preparing crosslinkable
cellulose fibers comprising the steps of (a) crosslinking cellulose
fibers with at least one crosslinking agent selected from saturated
dicarboxylic acids, aromatic dicarboxylic acids, cycloalkyl
dicarboxylic acid, bifunctional monocarboxylic acids, and amine
carboxylic acids and (b) uncrosslinking the crosslinked cellulose
fibers. Preferably, the crosslinking agent in this embodiment
contains 4 carbon atoms or less. Two preferred crosslinking agents
are oxalic acid and sodium chloroacetate. The crosslinkable fibers
can be formed into sheets to ease their transport. Furthermore, the
crosslinkable fibers can be re-crosslinked by curing the
uncrosslinked cellulose fibers.
[0024] Yet another embodiment of the present invention is an
acquisition, distribution, or acquisition and distribution layer
comprising the cellulose fibers of the present invention.
[0025] Yet another embodiment is an absorbent core comprising
cellulose fibers of the present invention. The absorbent core
exhibits improved fluid flow properties into and through the core.
According to one preferred embodiment, the absorbent core comprises
SAP particles and reversible crosslinked fibers. The reversible
crosslinked fibers separate the SAP particles and provide channels
for fluid flow around the SAP particles from the wet to the dry
areas of the absorbent core. Additionally, the reversible
crosslinked fibers facilitate absorption of large volumes of urine
(or other fluid) over a short period of time (e.g., a gush). Once
urine or other fluid enters the absorbent core, the crosslinked
fibers begin to uncrosslink. The uncrosslinked fibers hold and
retain the urine or other fluid to a greater extent than fibers
that are permanently crosslinked. As a result, the absorbent core
has improved initial gush capacity compared to absorbent cores
containing conventional fluff fibers and improved rewet performance
compared to absorbent cores containing permanently crosslinked
fibers.
[0026] Yet another embodiment is an absorbent structure comprising
the acquisition, distribution, or acquisition and distribution
layer of the present invention and/or the absorbent core of the
present invention. Preferably, the absorbent structure contains a
top (acquisition, distribution, or acquisition and distribution)
layer and a bottom (storage) layer in fluid communication with the
top layer. The absorbent structure exhibits superior partitioning
from the acquisition and/or distribution layer to the storage layer
compared to conventional absorbent structures.
[0027] Yet another embodiment is an absorbent article containing
the absorbent structure of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0028] Definitions
[0029] The term "capillary absorption-desorption cycle" (also known
as the capillary sorption cycle or CSC) refers to the process of
determining the relationship between the pore volume of an
absorbent structure and capillary pressure during absorption of a
liquid into the absorbent structure and subsequent drainage of the
liquid from the absorbent structure. The capillary
absorption-desorption cycle is indicative of the ability of an
absorbent structure to attract, retain, and distribute fluid in the
pores between the fibers of an absorbent structure. An absorbent
structure may be subjected to this cycle by systematically lowering
or raising the capillary pressure in narrow intervals, such as by
the method described in the examples of this application;
"Capillary Sorption Equilibria in Fiber Masses", A. A. Burgeni and
C. Kapur, Textile Research Journal, 37:356-366 (1967); and P. K.
Chatterjee, Absorbency, Textile Science and Technology 7, Chapter
II, pp. 63-65, Elsevier Science Publishers (1985), which are hereby
incorporated by reference.
[0030] The "median desorption pressure" as determined in a
capillary-desorption cycle refers to the water swollen cellulose
fibers ability to release water. For example, a sample of cellulose
fibers that strongly retains water exhibits a much higher median
desorption pressure than a sample of swollen cellulose fibers that
readily releases water. The median desorption pressure as discussed
herein is determined by the method described in the examples of
this application; "Capillary Sorption Equilibria in Fiber Masses",
A. A. Burgeni and C. Kapur, Textile Research Journal, 37:356-366
(1967); and P. K. Chatterjee, Absorbency, Textile Science and
Technology 7, Chapter II, pp. 63-65, Elsevier Science Publishers
(1985), which are hereby incorporated by reference. This test
method measures the ability of water swollen cellulose fibers to
retain water against hydrostatic pressure.
[0031] The "water retention value" (WRV) of a cellulose fiber may
be determined by the methods described in TAPPI Useful Methods, UM
256, and P. K. Chatterjee, Absorbency, Textile Science and
Technology Z, Chapter II, pp. 62-63, Elsevier Science Publishers
(1985), which are both hereby incorporated by reference. The test
measures the weight of water remaining in a sample of water
saturated cellulose fibers after centrifugation and expresses that
quantity as a weight percent based on the dry weight of the fibers.
The WRV of a cellulose fiber is related to its drainage
ability.
[0032] Any "cellulose fibers" known in the art, including cellulose
fibers of any natural origin, such as those derived from wood pulp,
may be used as starting materials in the methods of the present
invention. Preferred cellulose fibers include, but are not limited
to, digested fibers, such as kraft, prehydrolyzed kraft, soda,
sulfite, chemi-thermal mechanical, and thermo-mechanical treated
fibers, derived from softwood, hardwood or cotton linters. More
preferred cellulose fibers include, but are not limited to, kraft
digested fibers, including prehydrolyzed kraft digested fibers.
[0033] Generally, cellulose fibers having thicker walls are
preferable since they are coarser and stiffer than similar fibers
having thinner walls. The fiber walls of a fiber are defined by the
lumen of a fiber (i.e. the hollow interior of the fiber) and the
outer surface of the fiber. For example, since the fiber walls of
southern softwoods are on average thicker than those of northern
softwoods, fibers derived from southern softwoods are preferable.
More preferably, the cellulose fibers are derived from softwoods,
such as pines, firs, and spruces.
[0034] Other suitable cellulose fibers include those derived from
Esparto grass, bagasse, kemp, flax and other lignaceous and
cellulosic fiber sources. The cellulose fibers may be supplied in
slurry, unsheeted or sheeted form.
[0035] The optimum fiber source utilized in conjunction with this
invention will depend upon the particular end use contemplated.
Generally, pulp fibers made by chemical pulping processes are
preferred. Completely bleached, partially bleached and unbleached
fibers are applicable. It may frequently be desirable to utilize
bleached pulp for its superior brightness and consumer appeal. For
products, such as paper towels, and absorbent pads for diapers,
sanitary napkins, catamenials, and other similar absorbent paper
products, it is especially preferred to use cellulose fibers
derived from southern softwood pulp due to their premium absorbency
characteristics.
[0036] More preferred cellulose fibers include, but are not limited
to, bleached Kraft southern pine fibers sold under the trademark
Foley Fluff.TM. which are available from Buckeye Technologies Inc.
of Memphis, Tenn.
[0037] The cellulose fibers may have any fiber length. Typically,
longer fibers produce crosslinked cellulose fibers having lower
desorption pressures and water retention values than those produced
from shorter fibers.
[0038] Refined and Crosslinked Fibers
[0039] It has been surprisingly and unexpectedly discovered that
when cellulose fibers are refined and crosslinked, the resulting
fibers have low median desorption pressures as measured in a
capillary absorption-desorption cycle and low water retention
values (WRVs). Furthermore, these fibers exhibit improved fluid
drainage in acquisition and/or distribution layers compared to
similar unrefined fibers.
[0040] The cellulose fibers are crosslinked and have a median
desorption pressure, as determined in a capillary
absorption-desorption cycle, of 15 cm or less. Without being bound
by any theory, the inventors believe this property is the result of
intrafiber crosslinking within the cellulose fibers. More
desirably, the cellulose fibers of this invention have a median
desorption pressure, as determined in a capillary
absorption-desorption cycle, of 14 cm or less; still more
desirably, the fibers of this invention have a median desorption
pressure, as determined in a capillary absorption-desorption cycle
of 13 cm or less; and still more desirably, the fibers of the
invention here a median desorption pressure, as determined in a
capillary absorption-desorption cycle, of 12 cm or less.
[0041] The refined and crosslinked cellulose fibers typically have
a WRV of 45 percent or less; more desirably, 38 percent or less;
and still more desirably 30 percent or less.
[0042] The cellulose fibers may be prepared by refining cellulose
fibers to a freeness ranging from about 300 to about 700 ml CSF and
crosslinking the refined fibers. According to one preferred
embodiment, the starting cellulose fibers to be refined are wet
lap. According to another preferred embodiment, the cellulose
fibers are bleached and/or fluffed prior to being refined. The
refined fibers may be crosslinked by any method known in the art,
e.g., by reacting the fibers with a crosslinking agent.
[0043] Fibers having improved median desorption pressures and water
retention values can be produced by refining the fibers first and
then crosslinking the fibers with any of a wide variety of
crosslinking agents. Suitable crosslinking agents include, but are
not limited to, those described below as well as other
polycarboxylic acids, such as aliphatic and alicyclic
C.sub.2-C.sub.9 polycarboxylic acids. As used herein, the term
"C.sub.2-C.sub.9 polycarboxylic acid" refers to an organic acid
containing two or more carboxyl (COOH) groups and from 2 to 9
carbon atoms in the chain or ring to which the carboxyl groups are
attached. The carboxyl groups are not included in determining the
number of carbon atoms in the chain or ring. For example,
1,2,3-propanetricarboxylic acid would be considered to be a C.sub.3
polycarboxylic acid containing three carboxyl groups. Similarly,
1,2,3,4-butanetetracarboxylic acid would be considered to be a
C.sub.4 polycarboxylic acid containing four carboxyl groups.
[0044] The C.sub.2-C.sub.9 polycarboxylic acids suitable for use as
cellulose crosslinking agents in the present invention preferably
include aliphatic and alicyclic acids either olefinically saturated
or unsaturated with at least three and preferably more carboxyl
groups per molecule if a carbon-carbon double bond is present
alpha, beta to one or both carboxyl groups. Additionally, in order
to be reactive in esterifying cellulose hydroxyl groups, a given
carboxyl group in an aliphatic or alicyclic polycarboxylic acid is
preferably separated from a second carboxyl group by no less than
two carbon atoms and no more than three carbon atoms. Without being
bound by theory, it appears that for a carboxyl group to be
reactive, it must be able to form a cyclic 5 or 6-member anhydride
ring with a neighboring carboxyl group in the polycarboxylic acid
molecule. Where two carboxyl groups are separated by a
carbon-carbon double bond or are both connected to the same ring,
the two carboxyl groups must be in the cis configuration relative
to each other if they are to interact in this manner.
[0045] Novel Crosslinked Fibers
[0046] Another embodiment of the present invention is cellulose
fibers crosslinked with at least one saturated dicarboxylic acid,
aromatic dicarboxylic acid, cycloalkyl dicarboxylic acid,
bifunctional monocarboxylic acid, or amine carboxylic acid which
exhibit low median desorption pressures as measured in a capillary
absorption-desorption cycle and low water retention values. These
crosslinked fibers exhibit improved fluid drainage in acquisition
and/or distribution layers and improved permeability in absorbent
cores.
[0047] Generally, these crosslinked cellulose fibers have a median
desorption pressure, as determined in a capillary
absorption-desorption cycle, of 25 cm or less. Without being bound
by any theory, it is believed that this property is the result of
intrafiber crosslinking within the cellulose fibers.
[0048] More desirably, the crosslinked cellulose fibers have a
median desorption pressure, as determined in a capillary
absorption-desorption cycle, of 20 cm or less; still more
desirably, the fibers have a median desorption pressure, as
determined in a capillary absorption-desorption cycle, of 18 cm or
less; still more desirably, the fibers have a median desorption
pressure, as determined in a capillary absorption-desorption cycle,
of 15 cm or less; still more desirably, the fibers have a median
desorption pressure, as determined in a capillary
absorption-desorption cycle, of 14 cm or less; still more
desirably, the fibers have a median desorption pressure, as
determined in a capillary absorption-desorption cycle, of 13 cm or
less; and still more desirably, the fibers have a median desorption
pressure, as determined in a capillary absorption-desorption cycle,
of 12 cm or less.
[0049] The crosslinked cellulose fibers typically have a WRV of 50
percent or less; more desirably, 45 percent or less; still more
desirably, 38 percent or less; and still more desirably 30 percent
or less.
[0050] The crosslinked cellulose fibers generally have a saturated
capacity as measured by the procedure described in the examples of
this application; Burgeni et al., supra; and Chatterjee et al.,
supra, of at least 10 grams of saline per gram of sample (g/g).
According to a preferred embodiment, the crosslinked cellulose
fibers have a saturated capacity of at least 11, 12, 13, 14, or 15
g/g.
[0051] The crosslinked cellulose fibers of the present invention
are prepared by crosslinking cellulose fibers with one or more of
the crosslinking agents of the present invention. The median
desorption pressure and water retention value of the crosslinked
fibers may be reduced by refining the fibers prior to crosslinking
them. Furthermore, the crosslinking reaction may be performed in
the presence of one or more of the crosslinking facilitators of the
present invention to improve the efficacy of the crosslinking
agents.
[0052] Refining
[0053] Refining may be performed by any method known in the art,
including mechanical refining. Pulp refining involves application
of work onto fibers, generally but not exclusively carried out in
an aqueous slurry. For example, the fibers may be refined by
cutting them, thereby reducing the average fiber length.
Alternatively, the fibers may be refined by rubbing the fibers
against each other and irregular surfaces under force or pressure.
This causes the exterior fiber surfaces to increase in area, due to
scoring and abrasion of the surfaces. In addition, the work put
into the fibers during refining causes delamination of the interior
fiber walls and surfaces. The result is weakened fiber walls that
allow the fibers to absorb more water and swell to a greater extent
than unrefined fibers. Never-dried refined fibers are also more
flexible than similar unrefined never-dried fibers. Furthermore,
when consolidated into a sheet, refined fibers, after drying,
produce greater strength and stiffness in the sheet than is
produced in a sheet of dried unrefined fibers.
[0054] Methods of refining cellulose fibers, including, but not
limited to, beating and fibrillation, are described in J. d'A.
Clark, Pulp Technology and Treatment for Paper, 2.sup.nd Ed.,
Chapter 8, pp. 160-183, Chapter 12, pp. 277-305, Chapter 13, pp.
306-355, Chapter 14, pp. 356-407, Miller Freeman Pub., San
Francisco (1985). A preferred method of refining cellulose fibers
is fibrillation. The cellulose fibers may be refined with, for
example, a disc refiner or a Valley beater available from Valley
Mill Corporation of Lee, MA. Refining is typically performed at
ambient temperature and pressure. For example, cellulose fibers may
be refined by running an aqueous slurry of cellulose fibers through
a Valley beater for 15 minute intervals until the desired freeness
is obtained.
[0055] Generally, the fibers are wetted or moistened prior to being
refined. According to one preferred embodiment, the cellulose
fibers are bleached prior to being refined.
[0056] The cellulose fibers are broadly refined to a freeness of
from about 300 to about 700 ml CSF and preferably to a freeness of
from about 500 to about 700 ml CSF. According to a preferred
embodiment, the cellulose fibers are refined to a freeness of from
about 650 to about 700 CSF. The freeness of cellulose fibers as
discussed herein is determined by TAPPI Method T-227.
[0057] Crosslinking
[0058] The refined or unrefined cellulose fibers are stiffened by
intrafiber covalent crosslinking. Preferably, the cellulose fibers
are wet or moist, prior to being reacted with the crosslinking
agent and crosslinking facilitator. Desirably in some embodiments,
the cellulose fibers are never-dried cellulose fibers.
[0059] The fibers are crosslinked by reacting them with a
crosslinking agent and, optionally, a crosslinking facilitator of
the present invention, such as those described below. Preferably,
the fibers are crosslinked while in a highly twisted condition.
Typically, the reaction step is performed under substantially
unrestrained conditions, i.e., individual fibers are free to move
without interacting with neighboring fibers and are not under any
substantial tension or pressure. The fibers may be reacted with the
crosslinking agent and, optionally, a crosslinking facilitator by
curing the fibers in the presence of the crosslinking agent and,
optionally, the crosslinking facilitator.
[0060] Generally, the fibers are crosslinked by (i) mixing them
with a crosslinking agent and, optionally, a crosslinking
facilitator of the present invention and (ii) curing the fibers
under conditions sufficient to cause intrafiber crosslinking. An
effective amount of crosslinking agent and optionally crosslinking
facilitator to cause formation of intrafiber crosslink bonds is
typically mixed with the cellulose fibers. Preferably, an effective
amount of crosslinking facilitator to increase the number or rate
of intrafiber crosslink bonds formed by the reaction of the fibers
with the crosslinking agent is mixed with the cellulose fibers.
Generally, from about 0.5 to about 40 mole percent and preferably
from about 1 to about 30 mole percent of crosslinking agent and
crosslinking facilitator, calculated on a cellulose anhydroglucose
molar basis, is mixed with the fibers. When the crosslinking agent
is a dicarboxylic crosslinking agent, generally from about 5 to
about 21 mole percent of crosslinking agent, calculated on a
cellulose anhydroglucose molar basis, is mixed with the fibers.
Generally, from about 1.8 to about 9 mole percent of crosslinking
facilitator, calculated on a cellulose anhydroglucose molar basis,
is mixed with the fibers. The mixture containing the fibers and
crosslinking agent preferably contains from about 5 to about 10% by
weight of crosslinking agent based upon the dry weight of the
fibers.
[0061] After the crosslinking agent is mixed with the fibers, the
fibers are preferably separated and individualized, by, for
example, fluffing the fibers or disintegrating and fluffing the
fibers. By separating the fibers, intrafiber crosslinking is
maximized while interfiber crosslinking is minimized. Preferably,
the fibers are crosslinked by formation of intrafiber covalent
bonds.
[0062] Cellulose fibers supplied as wet lap, dry lap or other
sheeted form may be separated by mechanically disintegrating them
to unsheeted form. In the case of dry lap, it is advantageous to
moisten the fibers, for example to 40% moisture (60% solids
content, based on the total weight of fiber and water), prior to
mechanical disintegration in order to plasticize the fibers and
minimize damage to the fibers.
[0063] When the crosslinking agent is applied to the fibers in an
aqueous solution, the fibers are dried prior to being cured. The
fibers are preferably dried to remove all the water in the fibers
and cured to establish intrafiber crosslinking. Drying may be
performed by any method known in the art. Typically, drying is
performed by heating the fibers at a temperature of from about 50
to about 225.degree. C. Preferably, drying is performed at from
about 105 to about 175.degree. C. The fibers are typically dried to
constant weight. The temperature of the fibers during the drying
process generally does not exceed 100.degree. C., the boiling point
of water, irrespective of the temperature at which the fibers are
dried, until all of the water has been evaporated from the fibers.
As discussed in T. Lindstrom, Paper Structure and Properties,
International Fiber and Technology Series 8, Chapter 5, pp 104-105,
Marcel Dekker Inc., New York (1986), drying of cellulose fibers
typically leads to an irreversible reduction in the swelling
ability of the fibers on rewetting. This phenomenon is commonly
referred to as hornification. Without being bound to any theory, it
is believed that the microfibrils in the fiber walls bond together
during the drying process thereby reducing the size of pores in the
fiber walls. This results in stiffened fibers, compared to the
fibers before drying. The subsequent curing stage facilitates
formation of the intrafiber covalent bonds that lock in the dried
fiber stiffness and geometry.
[0064] Curing is generally performed at a temperature sufficient to
cause intrafiber covalent bonds to form. Curing is broadly
performed at a temperature of from about 105 to about 225.degree.
C. Preferably, the cellulose fibers are cured at a temperature of
from about 150 to about 190.degree. C. More preferably, they are
cured at a temperature of from about 160 to about 175.degree. C.
Curing may be performed for 15, 30, 45, or 60 minutes or
longer.
[0065] According to one preferred embodiment, the fibers are
crosslinked by (i) contacting an aqueous solution of the
crosslinking agent and, optionally, crosslinking facilitator with
an aqueous mixture containing the cellulose fibers, (ii) removing
water from the aqueous mixture, (iii) mechanically separating the
fibers into substantially individual form, (iv) drying the fibers,
and (v) reacting the fibers with the crosslinking agent to cause
crosslinking in the fibers. Generally, step (ii) involves removing
the majority of water from the aqueous mixture. Preferably, enough
water is removed from the aqueous mixture to obtain a mixture
having from about 40 to about 80% by weight of solids, based upon
100% total weight of fibers and water. According to a more
preferred embodiment, step (ii) involves removing water from the
aqueous mixture to obtain a mixture having about 60% by weight of
solids, based upon 100% total weight of fibers and water. The water
removal, separating, and drying steps cause the fibers to become
highly twisted. The twisted condition generally is at least
partially, but less than completely, permanently set by the
crosslinking reaction.
[0066] For example, the fibers may be crosslinked by the method
described in U.S. Pat. No. 5,190,563, which is hereby incorporated
by reference, substituting the crosslinking agents and crosslinking
facilitators of the present invention for the crosslinking agent in
U.S. Pat. No. 5,190,563. In U.S. Pat. No. 5,190,563, cellulosic
fibers are contacted with a solution containing a C.sub.2-C.sub.9
polycarboxylic acid crosslinking agent. The fibers are then
mechanically separated into substantially individual form, dried,
and reacted with the crosslinking agent while remaining in
substantially individual form so that intrafiber crosslink bonds
form. The individualized cellulosic fibers are contacted with an
amount of crosslinking agent effective to cause the fibers to form
intrafiber crosslink bonds. Preferably, from about 0.5 mole percent
to about 6.0 mole percent crosslinking agent, calculated on a
cellulose anhydroglucose molar basis, is contacted with the
fibers.
[0067] When the crosslinking agent contains an amino or amine group
to be reacted, the crosslinking agent is preferably activated prior
to or simultaneously with the crosslinking reaction. The term
"activated" as used herein refers to modifying the crosslinking
agent so that the nitrogen atom of the amino or amine group is in a
more reactive condition, i.e., more prone to reaction. The
crosslinking agent may be activated by any method known in the art.
For example, the amine or amino containing crosslinking agent can
be reacted with nitrous acid to activate the nitrogen atom of the
amine or amino group.
[0068] The fibers may be crosslinked in the presence a reducing
agent (antioxidant) to prevent yellowing of the fibers during the
crosslinking reaction. Suitable reducing agents include, but are
not limited to, hypophosphites, such as sodium hypophosphite;
sodium bisulfite; sodium phosphite; and any combination of any of
the foregoing. A preferred reducing agent is sodium
hypophosphite.
[0069] The fibers may be bleached during or after the crosslinking
reaction to improve their appearance. For example, the fibers may
be bleached by reacting them with a bleaching agent. Any bleaching
agent known in the art may be used. Suitable bleaching agents
include, but are not limited to, hydrogen peroxide.
[0070] For example, the bleaching agent may be included in an
aqueous solution containing the crosslinking agent that is applied
to the fibers. Preferably, the aqueous solution contains a
sufficient amount of bleaching agent so that the mixture obtained
from adding the aqueous solution to the fibers contains from about
2.5 to about 5% by weight of bleaching agent, based on the dry
weight of the fibers.
[0071] Saturated Dicarboxylic Acid Crosslinking Agents
[0072] The term "saturated dicarboxylic acid" refers to
dicarboxylic acids that do not contain any carbon-carbon double or
triple bonds. The saturated dicarboxylic acids may contain linear
or branched aliphatic chains, i.e., they are acyclic. Preferred
saturated dicarboxylic acids include, but are not limited to,
C.sub.2-C.sub.8 saturated dicarboxylic acids. The term
"C.sub.2-C.sub.8 saturated dicarboxylic acid" refers to a
dicarboxylic acid in which the total number of carbon atoms
(including those in the carboxyl groups) ranges from 2 to 8.
Non-limiting examples of C.sub.2-C.sub.8 saturated dicarboxylic
acids are oxalic acid, malonic acid, succinic acid, glutaric acid,
adipic acid, pimelic acid, and suberic acid. Special mention is
made of C.sub.2-C.sub.6 saturated dicarboxylic acids and
C.sub.2-C.sub.4 saturated dicarboxylic acids.
[0073] According to one preferred embodiment, C.sub.3 and higher
saturated dicarboxylic acids, such as C.sub.3-C.sub.8 saturated
dicarboxylic acids, are applied to the cellulose fibers in
conjunction with a crosslinking facilitator, such as oxalic
acid.
[0074] Another class of saturated dicarboxylic acids is saturated
hydroxy dicarboxylic acids. The term "saturated hydroxy
dicarboxylic acid" refers to saturated dicarboxylic acids that
contain at least one hydroxy substituent. Suitable saturated
hydroxy dicarboxylic acids include, but are not limited to,
C.sub.2-C.sub.8 hydroxy saturated dicarboxylic acids (i.e. those
containing from 2 to 8 carbon atoms). Special mention is made of
C.sub.2-C.sub.8 polyhydroxy saturated dicarboxylic acids.
Non-limiting examples of C.sub.2-C.sub.8 hydroxy saturated
dicarboxylic acids are tartaric acid, malic acid, saccharic acid,
and mucic acid.
[0075] Aromatic Dicarboxylic Acid Crosslinking Agents
[0076] The term "aromatic dicarboxylic acid" refers to aromatic
compounds having the formula HOOC--R--COOH wherein R is a
substituted or unsubstituted phenyl group. The term "substituted"
as used herein includes, but is not limited to, at least one of the
following substituents: hydroxy, C.sub.1-C.sub.4 alkoxy,
C.sub.1-C.sub.4 alkyl, amino, halogen, and nitro.
[0077] A preferred aromatic dicarboxylic acid has the formula 1
[0078] where R.sup.1, R.sup.2, R.sup.3, and R.sup.4 independently
are hydrogen, hydroxy, C.sub.1-C.sub.4 alkoxy, C.sub.1-C.sub.4
alkyl, amino, halogen, or nitro. A preferred aromatic dicarboxylic
acid is phthalic acid.
[0079] Cycloalkyl Dicarboxylic Acid Crosslinking Agents
[0080] The term "cycloalkyl dicarboxylic acid" refers to cycloalkyl
dicarboxylic acids that do not contain carbon-carbon double bonds
in the .alpha. or .beta. positions relative to the carboxyl groups.
According to one embodiment, the cycloalkyl dicarboxylic acid has
the formula 2
[0081] wherein
[0082] R.sup.6, R.sup.7, R.sup.10, and R.sup.11 are independently
hydrogen, hydroxy, halogen, C.sub.1-C.sub.4 alkoxy, C.sub.1-C.sub.4
alkyl, amino, or nitro; and
[0083] R.sup.8 and R.sup.9 are independently hydrogen, halogen,
C.sub.1-C.sub.4 alkoxy, or C.sub.1-C.sub.4 alkyl. A preferred
cycloalkyl dicarboxylic acid is 1,2,5,6-tetrahydrophthalic
acid.
[0084] Bifunctional Monocarboxylic Acid Crosslinking Agents
[0085] A "bifunctional monocarboxylic acid" refers to an organic
acid having (a) only one carboxyl group and (b) a functional group,
which is not a carboxyl group, capable of reacting with the
carboxyl, carboxylic acid, amino, or hydroxyl groups of a polymer.
Preferably, the bifunctional monocarboxylic acid includes only two
functional groups, i.e., a carboxyl group and a second functional
group.
[0086] Suitable bifunctional monocarboxylic acids include, but are
not limited to, amino acids, salts of haloacetates, hydroxy
monocarboxylic acids, and acid derivatives of hydroxy
monocarboxylic acids, such as acid esters of hydroxy monocarboxylic
acids.
[0087] A preferred salt of a haloacetate is sodium chloroacetate.
Without being bound by any theory, it is believed that when an
aqueous mixture of sodium chloroacetate and cellulose fibers is
dried and cured, an ether is formed by reaction of a cellulose
hydroxyl group with the chlorine containing carbon of the sodium
chloroacetate molecule. This etherification reaction releases a
molecule of hydrochloric acid, which is immediately neutralized by
the sodium salt of the newly formed cellulose based acid. At
elevated temperature, this acid is available for esterification to
proximate hydroxyl groups in the fibers, with concomitant release
of water. After the ether and ester formation, sodium chloride
remains as a byproduct.
[0088] Suitable hydroxy monocarboxylic acids and acid derivatives
thereof include, but are not limited to, glycolic acid, methane
sulfonic acid ester of glycolic acid, and para-toluene sulfonic
acid ester of glycolic acid.
[0089] Amine Carboxylic Acid Crosslinking Agents
[0090] Suitable amine carboxylic acids include, but are not limited
to, primary, secondary, and tertiary amines and aromatic amines.
Preferred primary amines include, but are not limited to, amino
acids. Special mention is made of amino acids having the
formula
H.sub.2N--CH.sub.2--R--C(O)OH
[0091] where R.sup.12 is a bond, C.sub.1-C.sub.12 alkyl, or a
C.sub.1-C.sub.12 alkyl substituted with one or more of carboxyl,
hydroxy, C.sub.1-C.sub.4 alkoxy, C.sub.1-C.sub.4 alkyl, amino, and
nitro.
[0092] Preferred amino acids include, but are not limited to, those
having the formula 3
[0093] where R.sup.5 is a linear or branched C.sub.1-C.sub.8 alkyl.
According to a preferred embodiment, R.sup.5 is a C.sub.2-C.sub.4
alkyl. Non-limiting examples of suitable amino acids include
aspartic acid and glutamic acid.
[0094] Other suitable amine carboxylic acid crosslinking agents
include, but are not limited to, ethylenedinitrilotetraacetic acid
(EDTA).
[0095] Crosslinking Facilitators
[0096] The crosslinking facilitators of the present invention
increase the efficacy of the crosslinking agents. A preferred
crosslinking facilitator is oxalic acid. Without being bound by any
theory, it is believed that oxalic acid (pK.sub.a=1.23) may serve
as an acid catalyst for esterification of the crosslinking agent.
Alternatively, oxalic acid may form a mixed anhydride with the
crosslinking agent which then facilitates esterification of the
cellulose fibers.
[0097] Crosslinking Reversibility
[0098] Fibers crosslinked with the short crosslinking agents of the
present invention, i.e., those containing 4 carbon atoms or less
(e.g. 3 carbon atoms or less), such as oxalic acid and sodium
chloroacetate, can be uncrosslinked and crosslinked thereafter. The
crosslinking of such fibers is generally substantially reversible,
i.e., at least about 50% by weight of the crosslinked fibers can be
uncrosslinked. According to one embodiment, at least about 60, 70,
80, 90, or 95% by weight of the crosslinked fibers can be
uncrosslinked.
[0099] The crosslinked fibers can be uncrosslinked by soaking them
in water for a time sufficient to uncrosslink them. Typically, the
crosslinked fibers are soaked for from about 0.5 to about 4 hours.
According to a preferred embodiment, the fibers are soaked for
about 2 hours. The crosslinked fibers can also be uncrosslinked by
subjecting them to a capillary absorption-desorption cycle as
described in the examples of this application; Burgeni et al.,
supra; and Chatterjee et al., supra.
[0100] The fibers can be re-crosslinked by drying or drying and
curing them. This phenomenon was not observed with the covalently
crosslinked fibers disclosed in U.S. Pat. Nos. 5,137,537; 5,183,707
and 5,190,563.
[0101] Without being bound by any theory, it is believed that as
the crosslinked fibers absorb water and swell, the crosslinks are
strained as the cellulose polymer chains move apart to accommodate
the absorbed water. When the crosslink molecules are very short in
length, as with fibers treated with oxalic acid or sodium
chloroacetate (two carbon atoms separate the hydroxyl groups on the
neighboring cellulose polymer chains), the strain of the fibers
swelling is sufficient to facilitate hydrolysis and cleavage of one
of the two covalent bonds that crosslink the fibers. In contrast, a
citric acid crosslinked fiber as disclosed in U.S. Pat. Nos.
5,137,537; 5,183,707 and 5,190,563 has a much longer molecule
bridging the space between cellulose polymer chains (four or five
carbon atoms separate the hydroxyl groups on the neighboring
cellulose chains). Consequently the strain on the longer
crosslinking molecule as the fibers absorb water and swell, is not
sufficient to facilitate cleavage of one of the crosslinking
covalent bonds.
[0102] Since the fibers of the present invention can be
uncrosslinked, they can be dried and transported or stored in
sheeted form instead of in bulk or baled form. This reduces
shipping and storage costs. The fibers can be re-crosslinked at the
destination or whenever desired by, for example, separating and
curing them. Once the fibers are re-crosslinked, they can, for
example, be incorporated into an absorbent structure.
[0103] The terms "reversible crosslinked fibers" and "reversible
crosslinked cellulose fibers" as used herein refer to crosslinked
fibers or crosslinked cellulose fibers in which at least about 50,
60, 70, 80, 90, or 95% by weight of the crosslinked fibers become
uncrosslinked after being soaked in water for up to 4 hours and in
which at least 50, 60, 70, 80, 90, or 95% of the uncrosslinked
fibers can re-crosslinked by drying the fibers at a temperature of
105.degree. C. or higher.
[0104] The reversible crosslinked fibers of the present invention
are particularly suitable for use in an absorbent core containing
superabsorbent polymer (SAP) particles. The crosslinked fibers
separate the SAP particles and provide channels for fluid flow
around the SAP particles from the wet to the dry areas of the
absorbent core. Additionally, the reversible crosslinked fibers
facilitate absorption of large volumes of urine (or other fluid)
over a short period of time (e.g., a gush). Once urine or other
fluid enters the absorbent core, the crosslinked fibers begin to
uncrosslink. According to one embodiment, after 0.5 to 4.0 hours of
exposure to the urine or other fluid, the majority of wet fibers
are uncrosslinked. The uncrosslinked fibers hold and retain the
urine or other fluid to a greater extent than fibers that are
permanently crosslinked. As a result, the absorbent core has
improved initial gush capacity compared to absorbent cores
containing conventional fluff fibers and improved rewet performance
compared to absorbent cores containing permanently crosslinked
fibers.
[0105] Absorbent Structures
[0106] The cellulose fibers of the present invention may be
incorporated into any disposable or non-disposable absorbent
structure intended to absorb and contain body exudates, and which
are generally placed or retained in proximity with the body of the
wearer. Such absorbent structures are commonly employed in
disposable and non-disposable absorbent articles. Examples of
disposable absorbent articles include, but are not limited to,
infant diapers, adult incontinence products, training pants,
sanitary napkins and other feminine hygiene products. Examples of
absorbent structures in which the cellulose fibers of the present
invention may be incorporated include, but are not limited to,
those described in International Publication Nos. WO 98/47456, WO
99/63906, WO 99/63922, WO 99/63923, WO 99/63925, WO 00/20095, WO
00/38607, WO 00/41882, WO 00/71790, and WO 00/74620 and U.S. Pat.
No. 5,695,486, all of which are hereby incorporated by
reference.
[0107] Acquisition and Distribution Layers
[0108] The cellulose fibers of the present invention may be
incorporated into an acquisition, distribution, or
acquisition-distribution layer. Such layers are commonly employed
in absorbent structures contained in disposable absorbent articles.
The acquisition and/or distribution layer may be prepared and
incorporated into an absorbent structure by any method known in the
art. According to one embodiment, the absorbent structure comprises
a top layer, which includes the acquisition and/or distribution
layer of the present invention, and a bottom storage layer (also
known as an absorbent core). The acquisition and distribution layer
may be a single layer or two separate layers, i.e., a top
acquisition layer and a lower distribution layer. The lower
distribution layer rapidly drains fluid from the acquisition layer
and distributes the fluid into the storage layer.
[0109] The acquisition layer of the present invention typically
includes from about 90 to about 100% by weight of the cellulose
fibers of the present invention, based upon 100% total weight of
acquisition layer. The density of the acquisition layer broadly
ranges from about 0.04 to about 0.07 g/cm.sup.3.
[0110] Absorbent Core
[0111] The cellulose fibers of the present invention may be
incorporated into an absorbent core (also known as a storage
layer). The absorbent core may include any material known in the
art that absorbs liquid. Suitable materials include, but are not
limited to, fibrous batts or webs constructed of defiberized,
loose, fluffed, and/or hydrophilic cellulosic fibers or the fibers
of the present invention; superabsorbent polymer (SAP) particles,
granules, flakes or fibers (collectively particles); and any
combination of the foregoing. Generally, SAP particles are capable
of absorbing many times their weight of liquid and significantly
increase the absorbent capacity of the absorbent core without
substantially increasing the bulkiness of the layer.
[0112] The term superabsorbent polymer particle or SAP particle is
intended to include any particulate form of superabsorbent polymer,
including irregular granules, spherical particles (beads), powder,
flakes, staple fibers and other elongated particles. SAP refers to
a normally water-soluble polymer which has been cross-linked to
render it substantially water insoluble, but generally capable of
absorbing at least ten, and preferably at least fifteen, times its
weight of a physiological saline solution. The SAP particles may be
of any size or shape. Numerous examples of superabsorbers and their
methods of preparation may be found, for example, in U.S. Pat. Nos.
4,102,340; 4,467,012; 4,950,264; 5,147,343; 5,328,935; 5,338,766;
5,372,766; 5,849,816; 5,859,077; and Re. 32,649. Examples of
suitable SAP particles include, but are not limited to, starch
graft copolymers, such as hydrolyzed starch-acrylate graft
co-polymer; cross-linked carboxymethylcellulose and derivatives
thereof; and modified hydrophilic polyacrylates, such as saponified
acrylic acid ester-vinyl co-polymer, neutralized crosslinked
polyacrylic acid, and cross-linked polyacrylate salts.
[0113] Preferably, the SAP particles form hydrogels upon absorbing
fluid. More preferably, the SAP particles have high gel volumes or
high gel strength as measured by the shear modulus of the hydrogel.
Such SAP particles typically contain relatively low levels of
polymeric materials that can be extracted by contact with synthetic
urine (so-called extractables). An example of such SAP particles is
starch graft polyacrylate hydrogel, available as IM1000.RTM. from
Hoechst-Celanese of Portsmouth, Va. Other examples of hydrogels
containing SAP particles include, but are not limited to, those
sold under the trademarks SANWET.TM., available from Sanyo Kasei
Kogyo Kabushiki of Japan; SUMIKA GEL.TM. available from Sumitomo
Kagaku Kabushiki Haishi of Japan; and FAVOR.TM. available from
Stockhausen of Garyville, La.; and ASAP.TM. available from BASF of
Aberdeen, Miss.
[0114] According to one preferred embodiment, the absorbent core
contains (a) SAP particles and (b) fluff fibers, matrix fibers, the
fibers of the present invention, or any combination of the
foregoing. The fibers provide structural integrity and avenues for
the passage of fluid through the absorbent core.
[0115] According to another embodiment, the absorbent core contains
from about 30 to about 70% by weight of SAP particles and from
about 70 to about 30% by weight of the cellulose fibers of the
present invention, based on 100% total weight of the absorbent
core. Generally, the weight density of the absorbent core ranges
from about 0.15 to about 0.25 g/cm.sup.3.
[0116] According to yet another embodiment, the absorbent core
comprises SAP particles and reversible crosslinked fibers of the
present invention. According to one preferred embodiment, the
reversible crosslinked fibers are crosslinked with oxalic acid,
sodium chloroacetate, or a mixture thereof. Generally, the
absorbent core contains from about 30 to about 70% by weight of SAP
particles and from about 70 to about 30% by weight of reversible
crosslinked fibers of the present invention, based on 100% total
weight of the absorbent core. According to another preferred
embodiment, the acquisition and/or distribution layers and the
absorbent core contain fibers crosslinked with oxalic acid.
[0117] The absorbent structure of the present invention may be
incorporated into disposable and non-disposable absorbent articles,
such as diapers, adult incontinence pads and feminine hygiene
articles. The absorbent article can include a liquid pervious
top-sheet above the acquisition and/or distribution layer, whose
function is to allow the passage of fluid to the acquisition and/or
distribution layer, and a liquid impervious back-sheet, whose
function is to contain the fluid and to prevent it from passing
through the absorbent article to the garment of the wearer of the
absorbent article.
[0118] According to one embodiment, the absorbent structure of the
present invention is incorporated into a disposable infant diaper
which generally includes a front waistband area, a rear waistband
area, and a crotch region there between. The structure of the
diaper generally includes a liquid pervious top-sheet, a liquid
impervious back-sheet, the absorbent structure, elastic members,
and securing tabs. Representative disposable diaper designs may be
found, for example, in U.S. Pat. Nos. 4,935,022 and 5,149,335.
[0119] According to another embodiment, the absorbent structure of
the present invention is incorporated into a feminine hygiene pad,
such as that disclosed in U.S. Pat. No. 5,961,505.
[0120] The following examples illustrate the invention without
limitation. All parts and percentages are given by weight unless
otherwise indicated. All the crosslinking agents, crosslinking
facilitators, and other chemical reagents used in the examples are
available from Aldrich Chemical Company of Milwaukee, Wis.
[0121] Measurement of Capillary and Desorption Pressures and
Saturated Capacity
[0122] The capillary absorption and desorption pressures were
determined according to the procedure described in "Capillary
Sorption Equilibria in Fiber Masses", A. A. Burgeni and C. Kapur,
Textile Research Journal, 37:356-366 (1967). This procedure is
described in detail below.
[0123] A 0.75 g sample of individualized fibers is formed into a
disc approximately 55-60 mm in diameter. The sample is placed on
the frit of a 150 ml coarse frit Pyrex.RTM. glass funnel, Corning
No. 36060 (available from VWR of Suwanee, Ga.). A weight sufficient
to apply 0.22 psi pressure, of diameter comparable to that of the
sample is placed on the sample. The bottom of the funnel is
attached to an adapter with decreasing diameter such that
Tygon.RTM. tubing No. R-3603, approximately two feet in length, is
attached to the adapter on one end, and on the other end to a fluid
reservoir resting on an electronic scale capable of measuring 0.01
g. The tubing is attached to the side of the reservoir at the
bottom. The fluid reservoir contains 0.9% saline solution. The
height of the saline solution in the fluid reservoir is
approximately 1 inch above the tubing attachment. The tubing is
filled with saline, as is the funnel below the frit, such that the
frit is damp with the saline, but no saline is above the frit. The
saline column from the reservoir to the frit is continuous without
any air in the column.
[0124] The absorption cycle is as follows. Starting at a height of,
for example, 20, 30, or 80 cm above the level of saline in the
reservoir, the sample is allowed to absorb saline to equilibrium or
steady state. Steady state is determined as no change in the weight
of saline on the electronic scale beneath the reservoir greater
than 0.04 g over a period of one minute. When steady state is
achieved, the sample is lowered 5 cm closer to the level of saline
in the reservoir and held there until equilibrium is achieved. The
sample is lowered another 5 cm and the procedure repeated. When the
sample is at equilibrium at the same height as the saline level in
the reservoir, the sample is subjected to a desorption cycle by
reversing the procedure, ie., by moving the sample upward in 5 cm
increments.
[0125] The weight of saline in the sample at the same level as the
saline in the reservoir is the saturated capacity of the sample.
The height (reported in cm) of the sample above the saline level in
the reservoir at 50% of saturated capacity on the downward
(absorption) cycle (median absorption pressure) and on the upward
(desorption) curve (median desorption pressure) are determined by
interpolation. The value of saturated capacity is reported in table
as grams of saline per gram of sample.
EXAMPLES 1-8
Examples 1-8 in Table 1 were Prepared as Follows
[0126] Unrefined cellulose fibers having a freeness of 740 ml CSF,
available as Foley Fluff.TM. fibers from Buckeye Technologies Inc.
of Memphis, Tenn., were slurried in water and refined in a valley
beater at ambient temperature and pressure to the appropriate
degree of freeness. The fibers were centrifuged, separated by hand,
and air dried to 60% solids. The fibers were crosslinked with the
appropriate concentration of citric acid (dry fiber basis) by
spraying the fibers with an aqueous solution of citric acid having
sufficient dilution to bring the sheet to 40% solids content. The
fibers were then air dried to 60% solids content, fluffed, and
dried to constant weight and heated for an additional 30 minutes at
the temperature indicated in Table 1.
[0127] The water retention value, saturation capacity, capillary
absorption pressure, and capillary desorption pressure of the cured
cellulose fibers were determined. The water retention value of the
cured cellulose fibers was determined according to the procedure
described in TAPPI Useful Methods, UM 256. The capillary absorption
and desorption pressures were determined according to the procedure
described above.
[0128] The results are shown in Table 1.
COMPARATIVE EXAMPLE 9
[0129] Unrefined cellulose fibers having a freeness of 740 ml CSF,
available as Foley Fluff.TM. fibers from Buckeye Technologies Inc.
of Memphis, Tenn., were dried at 100.degree. C. The water retention
value, saturation capacity, capillary absorption pressure, and
capillary desorption pressure of the cured cellulose fibers were
determined as described in Example 1.
[0130] The results are shown in Table 1 below.
COMPARATIVE EXAMPLE 10
[0131] Unrefined cellulose fibers having a freeness of 740 ml CSF,
available as Foley Fluff.TM. fibers from Buckeye Technologies Inc.,
were sprayed with sufficient water to produce a 40% solids content.
The fibers were air dried to 60% solids content, mechanically
fluffed, and dried to constant weight at 150.degree. C. The fibers
were then heated for an additional 30 minutes at the same
temperature. The water retention value, saturation capacity,
capillary absorption pressure, and capillary desorption pressure of
the cured cellulose fibers were determined as described in Example
1.
[0132] The results are shown in Table 1.
COMPARATIVE EXAMPLE 11
[0133] Unrefined cellulose fibers having a freeness of 740 ml CSF,
available as Foley Fluff.TM. fibers from Buckeye Technologies Inc.,
were slurried in water and refined in a valley beater to a freeness
of approximately 500 ml CSF. The fibers were centrifuged, separated
by hand, air dried to 60% solids content, fluffed, and dried to
constant weight at 150.degree. C. The fibers were then heated for
an additional 30 minutes at 150.degree. C. The water retention
value, saturation capacity, capillary absorption pressure, and
capillary desorption pressure of the cured cellulose fibers were
determined as described in Example 1.
[0134] The results are shown in Table 1.
COMPARATIVE EXAMPLE 12
[0135] Unrefined cellulose fibers having a freeness of 740 ml CSF,
available as Foley Fluff.TM. fibers from Buckeye Technologies Inc.,
were slurried in water and refined in a valley beater to a freeness
of approximately 500 ml CSF. The fibers were centrifuged, separated
by hand, and air dried to 60% solids. The cellulose fibers were
sprayed with an aqueous sulfuric acid solution at pH 3 to 40%
solids content in order to adjust the pH of the fiber water mixture
to the same pH as that observed in Example 1, when the fibers were
treated with aqueous citric acid. The fibers were air dried to 60%
solids content, fluffed, and dried to constant weight at
150.degree. C. The fibers were then heated for an additional 30
minutes at 150.degree. C. The water retention value, saturation
capacity, capillary absorption pressure, and capillary desorption
pressure of the cured cellulose fibers were determined as described
in Example 1.
[0136] The results are shown in Table 1.
COMPARATIVE EXAMPLE 13
[0137] Unrefined cellulose fibers having a freeness of 740 ml CSF,
available as Foley Fluff.TM. fibers from Buckeye Technologies Inc.,
were crosslinked with 5% citric acid (dry fiber basis) by spraying
the fibers with an aqueous solution of citric acid having
sufficient dilution to bring the sheet to 40% solids content. The
sheet was air dried to 60% solids content, mechanically fluffed,
dried to constant weight at 150.degree. C. The sheet was then
heated for an additional 30 minutes at 150.degree. C. The water
retention value, saturation capacity, capillary absorption
pressure, and capillary desorption pressure of the cured cellulose
fibers were determined as described in Example 1.
[0138] The results are shown in Table 1.
COMPARATIVE EXAMPLE 14
[0139] The procedure described in Comparative Example 13 was
repeated, except the fibers were crosslinked with 10% citric acid
instead of 5% citric acid. The results are shown in Table 1.
1TABLE 1 Crosslinked Absorption Desorption (% Citric Acid) Cure
Saturation Pressure at 50% Pressure at 50% Freeness (Dry Fiber
Temperature WRV Capacity of Saturated of Saturated Example (ml CSF)
Basis) (.degree. C.) (%) (g/g) Capacity (cm) Capacity (cm) Example
1 500 5 150 44.8 8.2 3.9 13.4 Example 2 500 10 150 38.1 8.7 4.4
13.5 Example 3 300 5 150 42.7 8.2 5.3 14.7 Example 4 300 10 150
41.5 8.6 5.1 14.9 Example 5 500 5 175 36.7 8.5 4.4 14.0 Example 6
500 10 175 29.5 8.3 3.1 11.9 Example 7 300 5 175 33.4 7.9 4.2 15.0
Example 8 300 10 175 29.9 7.5 3.0 13.0 Comparative Example 9 740 --
100 83.0 9.3 12.3 30.5 Comparative Example 10 740 -- 150 73.6 11.0
12.0 27.4 Comparative Example 11 500 -- 150 92.1 8.2 8.1 20.7
Comparative Example 12 500 -- 150 78.6 8.3 5.3 16.3 Comparative
Example 13 740 5 150 43.3 12.3 -- 17.9 Comparative Example 14 740
10 150 38.4 12.1 6.7 18.4
[0140] As shown by the results in Table 1, the refined and
crosslinked fibers exhibited lower WRVs and desorption pressures
than similar unrefined and crosslinked fibers.
EXAMPLE 15
[0141] Refining
[0142] An aqueous slurry containing 2.75-3.25% by weight of never
dried Foley Fluff.TM. fibers, available from Buckeye Technologies
Inc., was prepared. The aqueous slurry was passed through a Bauer
Model No. 444, 24" pump through refiner, at ambient temperature and
pressure. The Bauer refiner plates were No. A24313.
[0143] The refiner was operated at a current of 178 amps and a
slurry flow rate of 255 gallons per minute. These conditions
produced loading of 30-60 horsepower hours per bone dry short ton
of fibers. The fibers produced had a freeness of 680 ml CSF.
[0144] Demineralizing
[0145] The refined pulp slurry was pumped to a false bottom tank at
a consistency of 2.75-3.25%. While stirring the pulp slurry,
sulfuric acid was added until a nominal pH of 2.0 was obtained.
After at least 10 minutes of stirring, the aqueous slurry was
allowed to de-water through the false bottom screen for a minimum
of 3 hours. The slurry was then diluted with sodium softened water
to 2.0% consistency and the pH was adjusted to 4.5-5.0.
[0146] Sheeting
[0147] Pulp sheeting was performed on a paper machine available
from Sandy Hill Corporation of Hudson Falls, N.Y. The deckle (sheet
width) was a maximum of 36 inches. The 2% pulp slurry was pumped
through a stuff box and a basis weight valve and into a white water
silo at a controlled flow rate. The temperature in the silo was
increased to 130-150.degree. F. with direct steam and the slurry in
the silo was diluted with white water to a consistency of
1.0-1.25%.
[0148] The slurry was then fed into a paper machine headbox and
moved onto a moving wire at the Fordrinier section of the paper
machine. Natural drainage and vacuum assisted drainage were
provided until the formed sheet exited the couch press at about 32%
consistency. After formation of the sheet, but before the couch
press, the wet sheet was trimmed with two jets of water to deckle
of 24 inches. The sheet of fibers having a consistency of about 32%
then passed through two wet presses where further water removal and
sheet densification occurred. After exiting the second wet press,
the sheet of fibers entered a first dryer section at approximately
48% consistency. In the first dryer section, the pulp sheet passed
over thirteen rotating steam cans at approximately 300-325.degree.
F. The pulp sheet was then passed over eight rotating steam cans in
a second dryer section and exited the dryer at a moisture content
of 4-8%. The sheet was wound into a roll at a deckle of about 23
inches. The basis weight of this rolled sheet was approximately
0.126 pound per square foot and the density was approximately 0.60
grams per cubic centimeter.
[0149] Slitting
[0150] The rolls of pulp were rewound onto a new core and slit into
smaller rolls, each 10 inches wide.
[0151] Chemical Application
[0152] A 10 inch wide roll of the pulp sheet was unwound and slowly
passed through a puddle press. At the nip of the puddle press was
an aqueous solution of citric acid and sodium hypophosphite. The
sodium hypophosphite moderates pulp darkening at high temperature.
The weight concentrations of citric acid and sodium hypophosphite
in the flooded nip were approximately 14% and 7%, respectively.
Through the puddle press, the sheet absorbed enough of the aqueous
solution to reach a moisture content of about 40%.
[0153] Sheet Disintegration and Fluffing
[0154] Following the puddle press, the sheet was picked apart into
smaller pieces through a shredder, a pre-breaker and a picker. The
disintegrated pulp was then blown into the inlet of a Sunds
Defibrator Model 3784 RO Fluffer, available from Sunds Defibrator,
AB of Sundsvall, Sweden, with a gap setting of 5.5 mm. The
defibrator fluffed the pulp into masses of separated fibers. The
fluffed pulp was swept out of the RO Fluffer with a high velocity
stream of hot air at approximately 380.degree. F.
[0155] Drying and Curing
[0156] The hot air flow that conveyed the fluffed fiber out of the
RO Fluffer was boosted with a fan through a flash dryer where all
or almost all of the water in the fibers was evaporated. The dried
pulp fell onto a mechanical inlet conveyor forming a low density
high bulk bed on the conveyor. The fibers were then transported
into a Proctor & Schwartz K16476 tunnel dryer, available from
Proctor & Schwartz, Inc. of Horsham, Pa. Through a series of
hot circulating air flows, the fluffed fiber bed was heated and
then allowed to cool through three chambers in the dryer. In
chamber 1, the bed temperature reached 325-330.degree. F. In
chamber 2, the bed temperature increased to 385-390.degree. F. In
chamber 3, the bed temperature decreased to 355-360.degree. F. The
total time in the tunnel dryer was approximately 11.5 minutes.
[0157] Baling
[0158] The crosslinked fibers fell off the conveyer from the exit
side of the tunnel dryer into a baler model no. 3445, available
from American Baler Company of Bellevue, Ohio, where the material
was compressed into bales weighing approximately 70-80 pounds.
EXAMPLE 16
[0159] The samples in Table 2 were prepared as follows.
[0160] Unrefined cellulose fibers having a freeness of 740 ml CSF,
available as Foley Fluff.TM. fibers from Buckeye Technologies Inc.
of Memphis, Tenn., were slurried in water and refined in a Bauer
Model No. 444, 24" pump through refiner, at ambient temperature and
pressure to the appropriate degree of freeness. Optionally, the
fibers were washed with dilute sulfuric acid (acid wash) to remove
minerals. The refined fibers were sheeted and dried.
[0161] A piece of the sheet was submerged in a tray containing a
solution of the crosslinking agent and oxalic acid. The piece was
then flipped over and submerged in a second solution of the
crosslinking agent and oxalic acid. The combined solutions
contained 10% of the crosslinking agent (dry fiber basis) and 5%
oxalic acid (dry fiber basis). The solutions had sufficient
dilution to bring the piece to 40% solids content. The piece was
placed in a sealed polyethylene bag for 1 hour. The fibers were air
dried to 60% solids content, fluffed, and dried to constant weight
and heated for an additional 30 minutes at 175.degree. C.
[0162] The water retention value, saturation capacity, capillary
absorption pressure, and capillary desorption pressure of the cured
cellulose fibers were determined. The water retention value of the
cured cellulose fibers was determined according to the procedure
described in TAPPI Useful Methods, UM 256. The capillary absorption
and desorption pressures were determined according to the procedure
described above.
[0163] The results are shown in Table 2.
2TABLE 2 Absorption Desorption Saturated Pressure (cm) @ Pressure
(cm) @ Crosslinking Freeness Acid WRV Capacity 50% of Saturated 50%
of Saturated Carboxyl Sample Agent (ml CSF) Washed (%) (g/g)
Capacity Capacity (meq/kgm) a Succinic Acid 740 No 31.5 13.9 6.8
14.7 343.1 b Succinic Acid 570 No 36.3 13.5 7.2 17.3 294.1 c
Succinic Acid 570 Yes 36.0 13.7 7.2 16.1 303.9 d Adipic Acid 740 No
36.0 13.5 7.4 16.4 228.6 e Adipic Acid 570 No 39.1 12.6 7.0 16.4
190.6 f Adipic Acid 570 Yes 40.0 13.1 7.8 18.3 179.3 g Pimelic Acid
740 No 35.4 14.7 6.4 17.5 231.4 h Pimelic Acid 570 No 39.4 13.9 6.9
17.7 209.0 i Pimelic Acid 570 Yes 37.9 12.2 7.3 17.3 200.0 j
Malonic Acid 740 No 32.8 16.1 7.2 18.9 75.6 k Malonic Acid 570 No
36.5 11.6 6.1 16.8 78.2 l Malonic Acid 570 Yes 37.7 12.7 7.1 15.8
85.5
EXAMPLE 17
[0164] The samples in Table 3 were prepared as follows.
[0165] Unrefined cellulose fibers having a freeness of 740 ml CSF,
available as Foley Fluff.TM. fibers from Buckeye Technologies Inc.
of Memphis, Tenn., were slurried in water and refined in a Bauer
Model No. 444, 24" pump through refiner, at ambient temperature and
pressure to the appropriate degree of freeness. The refined fibers
were sheeted and dried. If the fibers were wet lap, they were then
centrifuged.
[0166] A piece of the sheet was submerged in a tray containing a
solution of the crosslinking agent, sodium hypophosphite, and,
optionally, oxalic acid. The piece was then flipped over and
submerged in a second solution of the crosslinking agent, sodium
hypophosphite, and, optionally, oxalic acid. The combined solutions
contained 10% of the crosslinking agent (dry fiber basis), 5%
sodium hypophosphite (dry fiber basis) and, optionally, 1% oxalic
acid (dry fiber basis). The solutions had sufficient dilution to
bring the piece to 40% solids content. The piece was placed in a
sealed polyethylene bag for 1 hour. The fibers were air dried to
60% solids content, fluffed, and dried to constant weight and
heated for an additional 30 minutes at 175.degree. C.
[0167] The water retention value, saturation capacity, capillary
absorption pressure, and capillary desorption pressure of the cured
cellulose fibers were determined. The water retention value of the
cured cellulose fibers was determined according to the procedure
described in TAPPI Useful Methods, UM 256. The capillary absorption
and desorption pressures were determined according to the procedure
described above.
[0168] The results are shown in Table 3.
3TABLE 3 Absorption Desorption Pressure (cm) Pressure (cm) Cross-
Concentration Saturated @ 50% of @ 50% of linking of Oxalic Acid
Freeness WRV Capactiy Saturated Saturated Carboxyl Sample Agent (%
w/w) (ml CSF) (%) (g/g) Capacity Capacity (meq/kgm) a Oxalic Acid
-- 740 48.3 12.0 6.2 18.9 292.5 b Maleic Acid -- 740 33.8 15.3 4.7
16.4 529.5 c Succinic Acid -- 740 38.2 14.0 5.3 16.7 414.8 d Adipic
Acid -- 740 45.7 13.4 5.7 17.2 343.9 e Succinic Acid 1 740 37.8
14.5 4.6 14.9 432.2 f Adipic Acid 1 740 45.3 13.0 6.1 17.8 356.3 g
Citric Acid -- 740 33.1 15.6 4.4 15.2 442.0 h Oxalic Acid -- 680
47.3 10.5 6.5 18.4 287.7 i Maleic Acid -- 680 34.2 14.4 4.2 15.0
530.2 j Succinic Acid -- 680 38.0 12.8 5.8 16.6 436.8 k Adipic Acid
-- 680 49.6 11.8 5.4 15.6 351.3 l Succinic Acid 1 680 37.0 14.0 4.4
14.3 424.6 m Adipic Acid 1 680 45.8 12.3 6.3 17.7 358.5 n Citric
Acid -- 680 31.3 15.8 3.7 13.5 450.3 o Oxalic Acid -- 570 59.9 3.2
5.2 12.7 432.0 (Wet Lap) p Maleic Acid -- 570 41.6 5.1 3.8 7.7
474.0 (Wet Lap) q Succinic Acid -- 570 34.8 8.8 3.2 8.3 466.1 (Wet
Lap) r Adipic Acid -- 570 50.1 9.0 3.4 8.7 487.5 (Wet Lap) s
Succinic Acid 1 570 36.1 8.8 3.3 8.7 415.4 (Wet Lap) t Adipic Acid
1 570 44.2 8.6 3.3 8.9 422.1 (Wet Lap) u Citric Acid -- 570 35.0
9.7 3.3 9.2 468.6 (Wet Lap) v Oxalic Acid -- 570 49.6 11.7 5.3 15.8
405.8 w Maleic Acid -- 570 41.5 12.4 5.9 16.7 450.7 x Succinic Acid
-- 570 37.2 13.4 4.8 15.1 510.1 y Adipic Acid -- 570 44.9 11.3 5.3
16.0 328.0 z Succinic Acid 1 570 40.7 12.0 5.4 16.2 455.5 aa Adipic
Acid 1 570 48.6 11.4 5.9 16.7 371.0 bb Citric Acid -- 570 39.9 13.7
3.8 12.9 481.1
EXAMPLE 18
[0169] The samples described in Table 4 below were prepared as
follows.
[0170] Stock solutions of the crosslinking agents and crosslinking
facilitators identified in Table 4 were prepared by dissolving the
indicated amounts of crosslinking agent and crosslinking
facilitator in 22.5 g of distilled water.
[0171] Samples were treated with the appropriate solution as
follows. A 15 g (dry basis) sample of sheeted, unrefined cellulose
fibers having a freeness of 740 ml CSF, available as Foley
Fluff.TM. fibers from Buckeye Technologies Inc. of Memphis, Tenn.,
was treated with the appropriate stock solution containing the
crosslinking agent and crosslinking facilitator. This reduced the
fiber solids content of the mixture to 40%. The mixture was placed
in a sealed container for 60 minutes at ambient temperature and
then air dried to 60% solids content. The fibers were mechanically
separated, individualized, and fluffed in a laboratory fluffer and
dried to constant weight in a forced air oven at 175.degree. C. The
fibers were cured for 30 minutes at the same temperature.
[0172] A control was prepared as follows. A dry unrefined sheet of
cellulose fibers having a freeness of 740 ml CSF, available as
Foley Fluff.TM. fibers from Buckeye Technologies Inc. of Memphis,
Tenn., was diluted to a 40% solids content with water. The pH of
the mixture was adjusted to 3 with sulfuric acid. The stock
solutions applied to the fibers in the procedure above typically
have a pH of 3. The mixture was then placed in a sealed container
for 60 minutes at ambient temperature and then air dried to a 60%
solids content. The fibers were mechanically separated,
individualized and fluffed in a laboratory fluffer and dried to
constant weight in a forced air oven at 175 .degree. C. The fibers
were heated for an additional 30 minutes at the same
temperature.
[0173] Fibers from an acquisition-distribution layer of a
Pampers.RTM. disposable diaper, available from Proctor and Gamble
of Cincinnati, Ohio, were obtained and used as a second
control.
[0174] The water retention value, saturation capacity, capillary
absorption pressure, and capillary desorption pressure of the
fibers were determined. The water retention value of the cellulose
fibers was determined according to the procedure described in TAPPI
Useful Methods, UM 256. The capillary absorption and desorption
pressures and saturated capacity were determined by the procedure
described above.
[0175] The results are shown in Table 4.
4TABLE 4 Amount Amount of Cross- Absorption Desorption of Cross-
linking Pressure (cm) Pressure (cm) Cross- linking Agent Cross-
Facilitator Saturated @ 50% of @ 50% of linking in Stock linking in
Stock WRV Capactiy Saturated Saturated Sample Agent Solution (g)
Facilitator Solution (g) (%) (g/g) Capacity Capacity a None
(Control) -- None -- 59.4 13.6 10.7 24.6 b Pampers .RTM. -- -- --
44.9 6.7 4.4 18.2 (Control) c Sodium 0.75 None -- 49.8 12.7 9.5
22.2 Chloroacetate d Sodium 1.5 None -- 44.6 11.1 8.2 18.5
Chloroacetate e Oxalic Acid 0.15 None -- 47.2 12.9 9.1 23.6 f
Oxalic Acid 0.75 None -- 42.9 13.6 8.6 21.5 g Oxalic Acid 1.5 None
-- 38.5 15.3 7.6 17.8 h Succinic Acid 1.5 None -- 39.8 14.2 7.5
20.4 i Succinic Acid 1.5 Oxalic Acid 0.15 33.8 14.3 7.7 16.6 j
Succinic Acid 1.5 Oxalic Acid 0.75 31.5 13.9 6.8 14.7 k Adipic Acid
1.5 None -- 50.5 14.1 8.0 22.3 l Adipic Acid 1.5 Oxalic Acid 0.15
38.9 13.7 8.6 19.8 m Adipic Acid 1.5 Oxalic Acid 0.75 36.0 13.5 7.4
16.4 n Malonic Acid 1.5 None -- 39.5 13.5 7.8 22.8 o Malonic Acid
1.5 Oxalic Acid 0.75 32.8 16.1 7.2 18.9 p Glutaric Acid 1.5 None --
37.1 15.2 7.5 23.4 q Glutaric Acid 1.5 Oxalic Acid 0.75 33.0 15.9
7.3 21.9 r Pimelic Acid 1.5 None -- 42.7 14.8 7.4 23.1 s Pimelic
Acid 1.5 Oxalic Acid 0.75 35.4 14.7 6.4 17.5 t Suberic Acid 1.5
None -- 56.0 13.7 8.3 25.7 u Suberic Acid 1.5 Oxalic Acid 0.75 41.0
14.2 7.2 21.4 v Phthalic Acid 1.5 None -- 55.4 13.9 8.4 26.3 w
Phthalic Acid 1.5 Oxalic Acid 0.75 42.5 13.0 9.0 24.6 x Tetrahydro-
1.5 None -- 53.4 12.6 8.4 22.5 phthalic Acid y Tetrahydro- 1.5
Oxalic Acid 0.75 39.4 13.3 7.4 19.9 phthalic Acid z Fumaric Acid
1.5 None -- 44.5 13.9 9.1 24.1 aa Fumaric Acid 1.5 Oxalic Acid 0.75
41.5 12.7 8.4 19.2 bb Glycolic Acid 1.5 None -- 47.8 13.3 8.7 22.0
cc Glycolic Acid 1.5 Oxalic Acid 0.75 39.6 14.0 7.3 17.4 dd
Tartaric Acid 1.5 None -- 34.3 13.7 7.3 18.5 ee Tartaric Acid 1.5
Oxalic Acid 0.75 32.2 13.1 7.3 18.3 ff Malic Acid 1.5 None -- 31.6
14.5 6.9 19.0 gg Malic Acid 1.5 Oxalic Acid 0.75 30.1 14.0 6.6 18.7
hh Saccharic Acid 1.5 None -- 49.6 13.4 9.4 22.8 ii Saccharic Acid
1.5 Oxalic Acid 0.75 41.1 11.4 8.2 19.4 jj Mucic Acid 1.5 None --
55.9 12.4 9.9 19.0 kk Mucic Acid 1.5 Oxalic Acid 0.75 40.3 11.9 8.4
17.1 ll Aspartic Acid 1.5 None -- 55.2 13.9 9.5 26.6 mm Aspartic
Acid 1.5 Oxalic Acid 0.75 37.5 14.6 6.5 16.5 nn Glutamic Acid 1.5
None -- 52.8 13.8 7.8 25.1 oo Glutamic Acid 1.5 Oxalic Acid 0.75
37.4 14.2 7.0 17.3 pp EDTA 1.5 None -- 50.7 12.1 8.6 21.9 qq EDTA
1.5 Oxalic Acid 0.75 39.7 12.1 8.0 18.4
EXAMPLE 19
[0176] A sample of crosslinked fibers was prepared by the method
described in Example 18 with 1.5 g sodium chloroacetate. A second
sample was prepared with 1.5 g oxalic acid instead of sodium
chloroacetate.
[0177] For comparison purposes, a sample of crosslinked fibers was
prepared by the procedure in Example 18 using a stock solution
containing 10% by weight of citric acid.
[0178] The samples were subjected to a first capillary
absorption-desorption cycle by the procedure described above. The
samples were then subjected to a second capillary
absorption-desorption cycle by the same procedure. The observed
absorption and desorption pressures for both cycles are shown in
Table 5 below.
[0179] This test was repeated with uncrosslinked Foley Fluff.TM.
fibers.
5 TABLE 5 Crosslinking Agent Oxalic Sodium Citric None Acid
Chloroacetate Acid First Cycle Saturated Capacity (g/g) 13.6 15.3
12.2 18.5 Absorption Pressure (cm) @ 10.7 7.6 10.5 7.0 50% of
Saturated Capacity Desorption Pressure (cm) @ 24.6 17.8 18.2 13.5
50% of Saturated Capacity Second Cycle Saturated Capacity (g/g)
11.8 13.1 9.6 16.5 Absorption Pressure (cm) @ >30 >30 >30
15.3 50% of Saturated Capacity Desorption Pressure (cm) @ 25.8 29.3
24.7 18.4 50% of Saturated Capacity
EXAMPLE 20
[0180] Samples as described in Example 19 were prepared and
subjected to two capillary absorption-desorption cycles. The
samples were dried overnight in a forced air oven at 105.degree. C.
Alternatively, the samples were dried to constant weight at
105.degree. C. and heated for an additional 30 minutes (cured) at
175.degree. C. The saturated capacity and absorption and desorption
pressures of the samples was determined.
[0181] The results are shown in Table 6.
6 TABLE 6 Crosslinking Agent Oxalic Sodium Citric None Acid
Chloroacetate Acid Dried at 105.degree. C. Saturated Capacity (g/g)
12.8 12.3 10.3 17.1 Absorption Pressure (cm) @ 12.2 8.6 11.5 7.1
50% of Saturated Capacity Desorption Pressure (cm) @ 24.7 18.1 19.8
14.3 50% of Saturated Capacity Cured at 175.degree. C. Saturated
Capacity (g/g) 11.3 11.2 9.5 14.4 Absorption Pressure (cm) @ 13.0
6.8 12.6 7.2 50% of Saturated Capacity Desorption Pressure (cm) @
24.6 18.1 19.6 13.7 50% of Saturated Capacity
EXAMPLE 21
[0182] Chemical Application
[0183] An aqueous solution of oxalic acid and sodium hypophosphite
was prepared by mixing 151 pounds of a 10% by weight oxalic acid
solution, 15 pounds of a 50% by weight sodium hypophosphite
solution, and 1 pound of water.
[0184] A 10 inch wide roll of Foley Fluff.TM. fibers, available
from Buckeye Technologies Inc., was unwound and slowly passed
through a puddle press. Flooded at the nip of the puddle press was
the aqueous solution of oxalic acid and sodium hypophosphite. The
sodium hypophosphite moderates pulp darkening at high temperature.
Through the puddle press, the sheet absorbed enough of the aqueous
solution to reach a moisture content of about 47%, based upon 100%
total weight of dry fibers. The treated sheet also contained about
10% by weight of oxalic acid -and 5% sodium hypophosphite, based
upon 100% total weight of dry fibers.
[0185] Sheet Disintegration and Fluffing
[0186] Following the puddle press, the sheet was picked apart into
smaller pieces through a shredder, a pre-breaker and a picker. The
disintegrated pulp was then blown into the inlet of a Sunds
Defibrator Model 3784 RO Fluffer, available from Sunds Defibrator,
AB of Sundsvall, Sweden, with a gap setting of 5.5 mm. The
defibrator fluffed the pulp into masses of separated fibers. The
fluffed pulp was swept out of the RO Fluffer with a high velocity
stream of hot air at approximately 380.degree. F.
[0187] Drying and Curing
[0188] The hot air flow that conveyed the fluffed fiber out of the
RO Fluffer was boosted with a fan through a flash dryer where all
of the water in the fibers was evaporated. The dried pulp fell onto
a mechanical inlet conveyor forming a low density high bulk "bed"
on the conveyor. The fibers were then transported into a Proctor
& Schwartz K16476 tunnel dryer, available from Proctor &
Schwartz, Inc. of Horsham, Pa. Through a series of hot circulating
air flows, the fluffed fiber bed was heated through three chambers
in the dryer. In chamber 1, the bed temperature reached
330-340.degree. F. In chamber 2, the bed temperature increased to
375-385.degree. F. In chamber 3, the bed temperature decreased to
355-360.degree. F. After the three heating zones, the fiber bed
passed through one last insulated chamber with no additional heat
being added. The total time in the tunnel dryer was approximately
11.5 minutes.
[0189] Baling
[0190] The crosslinked fibers fell off the conveyer from the exit
side of the tunnel dryer into a baler model no. 3445, available
from American Baler Company of Bellevue, Ohio, where the material
was compressed into bales weighing approximately 85-100 pounds.
[0191] All references cited herein are incorporated by reference.
To the extent that a conflict may exist between the specification
and the reference the language of the disclosure made herein
controls.
* * * * *