U.S. patent application number 16/549903 was filed with the patent office on 2020-03-19 for grafted crosslinked cellulose.
This patent application is currently assigned to INTERNATIONAL PAPER COMPANY. The applicant listed for this patent is INTERNATIONAL PAPER COMPANY. Invention is credited to Karen Brogan, Torsten Lindner, Charles E. Miller, Maike Siemons, Angel Stoyanov.
Application Number | 20200087435 16/549903 |
Document ID | / |
Family ID | 56800129 |
Filed Date | 2020-03-19 |
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United States Patent
Application |
20200087435 |
Kind Code |
A1 |
Brogan; Karen ; et
al. |
March 19, 2020 |
GRAFTED CROSSLINKED CELLULOSE
Abstract
Grafted, crosslinked cellulosic materials include cellulose
fibers and polymer chains composed of at least one
monoethylenically unsaturated acid group-containing monomer (such
as acrylic acid) grafted thereto, in which one or more of said
cellulose fibers and said polymer chains are crosslinked (such as
by intra-fiber chain-to-chain crosslinks). Some of such materials
are characterized by a wet bulk of about 10.0-17.0 cm.sup.3/g, an
IPRP value of about 1000 to 7700 cm.sup.2/MPasec, and/or a MAP
value of about 7.0 to 38 cm H.sub.2O. Methods for producing such
materials may include grafting polymer chains from a cellulosic
substrate, followed by treating the grafted material with a
crosslinking agent adapted to effect crosslinking of one or more of
the cellulosic substrate or the polymer chains. Example
crosslinking mechanisms include esterfication reactions, ionic
reactions, and radical reactions, and example crosslinking agents
include pentaerythritol, homopolymers of the graft species monomer,
and hyperbranched polymers.
Inventors: |
Brogan; Karen; (Seattle,
WA) ; Miller; Charles E.; (Federal Way, WA) ;
Stoyanov; Angel; (Federal Way, WA) ; Lindner;
Torsten; (Schwalbach am Taunus, DE) ; Siemons;
Maike; (Schwalbach am Taunus, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
INTERNATIONAL PAPER COMPANY |
Memphis |
TN |
US |
|
|
Assignee: |
INTERNATIONAL PAPER COMPANY
Memphis
TN
|
Family ID: |
56800129 |
Appl. No.: |
16/549903 |
Filed: |
August 23, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14808010 |
Jul 24, 2015 |
|
|
|
16549903 |
|
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|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
D21C 9/005 20130101;
B01J 20/265 20130101; D21H 11/20 20130101; D21H 27/002 20130101;
B01J 20/28023 20130101; D01F 2/00 20130101; B01J 20/267 20130101;
D21H 11/16 20130101; D01F 11/02 20130101; C08B 15/005 20130101;
D21H 17/37 20130101; D21H 21/22 20130101; C08B 3/08 20130101; C08F
251/02 20130101; B01J 20/24 20130101 |
International
Class: |
C08F 251/02 20060101
C08F251/02; B01J 20/28 20060101 B01J020/28; B01J 20/26 20060101
B01J020/26; B01J 20/24 20060101 B01J020/24; D01F 11/02 20060101
D01F011/02; D21H 17/37 20060101 D21H017/37; D21H 11/20 20060101
D21H011/20; D21H 11/16 20060101 D21H011/16; D21H 27/00 20060101
D21H027/00; C08B 15/00 20060101 C08B015/00; C08B 3/08 20060101
C08B003/08; D21H 21/22 20060101 D21H021/22; D21C 9/00 20060101
D21C009/00 |
Claims
1. A cellulosic material comprising a cellulose fiber and polymer
chains composed of at least one monoethylenically unsaturated acid
group-containing monomer grafted thereto, wherein one or more of
said cellulose fiber and said polymer chains are intra-fiber
crosslinked, and wherein at a given IPRP value (in cm.sup.2/MPasec)
from 800 to 5400, the material has an MAP value (in cm H.sub.2O)
higher than the corresponding MAP value possessed by non-grafted,
crosslinked cellulose fiber.
2. The material of claim 1, wherein the at least one
monoethylenically unsaturated acid group-containing monomer
includes acrylic acid.
3. The material of claim 1, characterized by a graft yield of 5-35
weight %.
4. The material of claim 3, characterized by a graft yield of 10-20
weight %.
5. The material of claim 1, wherein the material has a wet bulk at
least 6% greater than untreated cellulose fiber.
6. The material of claim 1, wherein the material has a wet bulk of
about 10.0-17.0 cm.sup.3/g.
7. The material of claim 1, wherein the material has an IPRP value
of about 1000 to 5400 cm.sup.2/MPasec and a MAP of about 8.0 to 20
cm H.sub.2O.
8. The material of claim 1, characterized in that at a given MAP
value (in cm H.sub.2O) from 7.0 to 20, the material has an IPRP
value that is equal to or higher than the corresponding IPRP value
possessed by non-grafted, crosslinked cellulose fiber.
9. A cellulosic material comprising a cellulose fiber and polymer
chains composed of at least one monoethylenically unsaturated acid
group-containing monomer grafted thereto, wherein one or more of
said cellulose fiber and said polymer chains are intra-fiber
crosslinked, and wherein at a given MAP value (in cm H.sub.2O) from
7.0 to 20, the material has an IPRP value (in cm.sup.2/MPasec)
higher than the corresponding IPRP value possessed by non-grafted,
crosslinked cellulose fiber.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. patent
application Ser. No. 14/808,010, filed Jul. 24, 2015, the entire
disclosure of which is incorporated herein by reference.
TECHNICAL FIELD
[0002] This disclosure relates to cellulose fibers, and in
particular to grafted, crosslinked cellulose.
BACKGROUND
[0003] Cellulosic fibers find utility in many applications,
including absorbents. Indeed, cellulosic fibers are a basic
component of many absorbent products such as diapers. The fibers
form a liquid absorbent structure, a key element in an absorbent
product.
[0004] Cellulosic fluff pulp, a form of cellulosic fibers, has been
used for absorbent applications because the fluff pulp form
provides a high void volume, or high bulk, liquid absorbent fiber
structure. However, this structure tends to collapse upon wetting,
which reduces the volume of liquid that can be retained in the
wetted structure. Further, such collapse may inhibit transfer of
liquid into unwetted portions of the cellulose fiber structure,
leading to local saturation.
[0005] Whereas the ability of an absorbent product containing
cellulosic fibers to initially acquire and distribute liquid (such
as from an initial liquid insult) relates to the product's dry bulk
and capillary structure, the ability of a wetted structure to
acquire additional liquid (such as from subsequent and/or extended
liquid insults) relates to the structure's wet bulk. Due to
diminished acquisition and capacity properties related to loss of
fiber bulk associated with liquid absorption, the potential
capacity of a dry high bulk fiber structure such as cellulosic
fluff pulp may not be fully realized, with the liquid holding
capacity instead determined by the structure's wet bulk.
[0006] Intra-fiber crosslinked cellulose fibers and structures
formed therefrom generally have enhanced wet bulk as compared to
non-crosslinked fibers. The enhanced bulk is a consequence of the
stiffness, twist, and curl imparted to fibers as a result of
crosslinking. Accordingly, crosslinked fibers are incorporated into
absorbent products to enhance their wet bulk and liquid acquisition
rate.
[0007] In addition to wet bulk and liquid acquisition rate, a
material's suitability for use in absorbent products may be
characterized in terms of other performance properties, such as
liquid permeability. As noted above, performance properties tend to
result from different fiber characteristics such as fiber length,
fiber stiffness, and so forth. However, relationships between some
performance properties indicate the existence of trade-off trends
for many cellulose fiber (and other) materials. For example, liquid
permeability tends to decrease as capillary pressure, expressed in
terms of medium absorption pressure, increases. As explained in
greater detail below, this particular relationship manifests in a
manner that can be mathematically approximated as a power curve
function of the two properties, which is characteristic for many if
not all materials used in absorbent applications, including
cellulose fiber materials, synthetic fiber materials, blends, and
so forth. Of these materials, the "trade-off" curve for cellulose
fiber products is the highest, but successful efforts to raise this
curve higher--that is, to produce materials that exhibit better
liquid permeability value at a given capillary pressure value (and
vice versa) than as predicted by the power curve function described
by cellulose fibers--have not yet been observed.
[0008] There are a number of methods for preparing crosslinked
cellulose fibers; several are summarized in U.S. Pat. No. 5,998,511
to Westland, et al. Much effort has been spent improving
crosslinking processes, such as to lower production and/or material
costs, to modify absorbent and/or other fiber properties of the
products, and so forth. In one example, polycarboxylic acids have
been used to crosslink cellulosic fibers (such as in U.S. Pat. Nos.
5,137,537, 5,183,707, and 5,190,563, all to Herron, et al., and so
forth), to produce absorbent structures containing cellulosic
fibers crosslinked with a C2-C9 polycarboxylic acid. Despite
advantages that polycarboxylic acid crosslinking agents provide,
cellulosic fibers crosslinked with low molecular weight (monomeric)
polycarboxylic acids, such as citric acid, have been found to
undergo reversion to a non-crosslinked condition and thus have a
useful shelf-life that is relatively short. Polymeric
polycarboxylic acid crosslinked fibers, however, such as disclosed
in U.S. Pat. Nos. 5,998,511, 6,184,271, and 6,620,865, all to
Westland, et al., amongst others, resist such aging or reversion,
due in part to the participation of the polymeric polycarboxylic
acid molecule in the crosslinking reaction with an increased number
of reactive carboxyl groups than is the case with monomeric
polycarboxylic acids such as citric acid. In another example, U.S.
Pat. No. 8,722,797 to Stoyanov, et al., discloses the use of a
comparatively low molecular weight polyacrylic acid having
phosphorous (in the form of a phosphinate) incorporated into the
polymer chain as a crosslinking agent to achieve crosslinked
cellulose fibers having improved brightness and whiteness (as well
as other properties) as compared to those prepared with higher
molecular weight phosphinated agents or polyacrylic acid agents
without phosphinates.
[0009] Thus, there is a continuing need to produce crosslinked
cellulose fibers and compositions and materials including such
fibers suitable for use in absorbent and other applications.
SUMMARY
[0010] This summary is provided to introduce a selection of
concepts in a simplified form that are further described below in
the Detailed Description. This summary is not intended to identify
key features of the claimed subject matter, nor is it intended to
be used as an aid in determining the scope of the claimed subject
matter.
[0011] Various embodiments of crosslinked cellulosic materials
composed of a cellulosic substrate, such as cellulose fibers,
having grafted polymer chains, with such cellulose fibers and/or
grafted polymer chains being crosslinked, and various methods of
producing such compositions, are disclosed herein. The fibrous
materials produced according to the present disclosure are also
referred to herein as "grafted crosslinked cellulose" and may be
described as "compositions" as well as "materials." In embodiments
of the grafted crosslinked cellulose of the present disclosure, the
polymer chains are composed of monoethylenically unsaturated acid
group-containing monomers (non-limiting examples include acrylic
acid, maleic acid, methacrylic acid, etc., and combinations
thereof), which may be crosslinked in a variety of manners
(non-limiting examples include ester intra-fiber crosslinks via
crosslinking agents such as homopolymers, hyperbranched polymers,
pentaerythritol, and so forth).
[0012] In some embodiments, acrylic acid is used as the
monoethylenically unsaturated acid group-containing monomer. Some
embodiments are characterized by a graft yield of 5-35 weight %,
and more particularly 10-20 weight %. Some embodiments are
characterized by a wet bulk at least 6% greater, and up to at least
40% greater, than untreated cellulose (referring to the cellulose
substrate in an untreated, i.e., non-grafted and non-crosslinked,
state). Some embodiments are characterized by a wet bulk of about
10.0-17.0 cm.sup.3/g, and more particularly of about 15.0-17.0
cm.sup.3/g. Some embodiments include mainly intra-fiber
chain-to-chain crosslinks composed of a crosslinking agent such as
pentaerythritol or a hyperbranched polymer. Some embodiments
include intra-fiber chain-to-cellulose crosslinks.
[0013] In some of the aforementioned embodiments, the material is
characterized by an IPRP value of about 1000 to 7700
cm.sup.2/MPasec and a medium absorption pressure (MAP) of about 7.0
to 20 cm H.sub.2O. Some embodiments are further characterized by
power curve function wherein for a given IPRP value y (in
cm.sup.2/MPasec) from 1000 to 7700, the MAP value of the material
(in cm H.sub.2O) is within +/-30% of the value of x in the formula
y=mx.sup.z; wherein m is from 600 to 1200, and wherein z is from
-0.590 to -0.515. Some embodiments are more particularly
characterized in that z is from -0.560 to -0.520 and/or m is from
800 to 1100. In some embodiments, at a given IPRP value (in
cm.sup.2/MPasec) from 800 to 5400, the material has a MAP value
that is equal to or higher (e.g., 0-20% higher) than the
corresponding MAP value possessed by non-grafted, crosslinked
cellulose fiber, and/or at a given MAP value (in cm H.sub.2O) from
7.0 to 20, the material has an IPRP value that is equal to or
higher (e.g., 0-15% higher) than the corresponding IPRP value
possessed by non-grafted, crosslinked cellulose fiber. Some
embodiments have an IPRP value of 5400 cm.sup.2/MPasec or
above.
[0014] Example methods of producing grafted crosslinked cellulose
in accordance with the present disclosure include grafting polymer
chains of at least one monoethylenically unsaturated acid
group-containing monomer from a cellulosic substrate to produce a
grafted cellulosic material, followed by crosslinking the grafted
cellulosic material by treating the material with a crosslinking
agent adapted to effect crosslinking of one or more of the
cellulosic substrate or the polymer chains. In some methods, the
grafting is performed in situ and may include reacting the monomer
with the cellulosic substrate in the presence of a grafting
initiator such as cerium(IV) sulfate. In some methods, acrylic acid
is used as the monomer. Some methods include varying the amounts of
the initiator and/or the ratio of cellulose to monomer to achieve a
desired graft level.
[0015] Such methods may include any of a variety of crosslinking
procedures. Some methods include establishing intra-fiber
crosslinks via an esterification reaction via one or more
crosslinking agents such as pentaerythritol, a polymeric
crosslinking agent (for example, a homopolymer formed of the at
least one monoethylenically unsaturated acid group-containing
monomer), a hyperbranched polymer, and so forth. Some methods
include establishing intra-fiber crosslinks via an ionic reaction
via a multivalent inorganic compound (such as aluminum sulfate) as
a crosslinking agent. Some methods include establishing intra-fiber
crosslinks via a radical reaction via a suitable inorganic salt
(such as ammonium persulfate) as a cross-linking agent. Some
methods include at least partially neutralizing the grafted polymer
side chains by treating the grafted cellulosic material with an
alkaline solution.
[0016] The materials, concepts, features, and methods briefly
described above are clarified with reference to the detailed
description below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The foregoing aspects and many of the attendant advantages
of this invention will become more readily appreciated as the same
become better understood by reference to the following detailed
description, when taken in conjunction with the accompanying
drawings, wherein:
[0018] FIGS. 1 and 2 show partially schematic views of an equipment
assembly used in the In-Plane Radial Permeability (IPRP) Test
described herein.
[0019] FIG. 3 shows a partially schematic view of an equipment
assembly used in the Medium Absorption Pressure (MAP) Test
described herein.
[0020] FIG. 4 is a graph comparing best-fit curves showing the
relationship between MAP and IPRP values exhibited by example
embodiments of grafted cellulose materials produced in accordance
with the present disclosure (including crosslinked as well as
non-crosslinked cellulose materials, collectively represented as
the solid line) and example non-grafted cellulose materials
(including crosslinked as well as non-crosslinked cellulose
materials, collectively represented by the dashed line).
DETAILED DESCRIPTION
[0021] The complete disclosures of the aforementioned references,
and those of all of the other references cited herein, are
incorporated in their entireties for all purposes.
[0022] There has been much research on grafting copolymers,
including grafting copolymers from cellulosic materials, such as
with grafting polymer chains or "arms" consisting of acrylic acid
monomers, from holocellulose (see, e.g., Okieimen, E. F., and
Ebhoaye, J. E., Grafting Acrylic Acid Monomer on Cellulosic
Materials, J. Macromol. Sci.-Chem., pp 349-353 (1986)). However,
there has been no investigation of subjecting grafted cellulose
structures to conditions suitable to effect crosslinking in
non-grafted cellulose materials.
[0023] The inventors have discovered, however, that various
absorbent and other properties of certain grafted cellulose
structures are changed upon undergoing crosslinking by various
reactions and mechanisms. In particular, absorbent properties of
some grafted, crosslinked cellulose structures, such as wet bulk,
absorbent capacity, permeability (e.g., in-plane radial
permeability or IPRP), capillary pressure (e.g., as measured by
medium absorption pressure or MAP), as described in greater detail
below, are consistent with or improved relative to those achieved
by crosslinked cellulose products produced by other methods, and
are favorable when compared to cellulose fibers that are not
crosslinked and/or not grafted.
[0024] As an example of a non-crosslinked, non-grafted cellulose
fiber, a bleached kraft pulp product available from Weyerhaeuser NR
Company under the designation CF416 has a wet bulk of 11.71
cm.sup.3/g and an absorbent capacity of 11.75 g/g. When grafted
with polymer chains composed of monoethylenically unsaturated acid
group-containing monomers (with a graft yield in the range of about
5-35 weight %) the resulting grafted CF416 exhibited lower wet bulk
values (of about 9.8-11.2 cm.sup.3/g, with acrylic acid used as the
graft species) and absorbent capacity values (of 10.0-11.3 g/g,
with acrylic acid used as the graft species), but when subjected to
subsequent crosslink treatment, the grafted, crosslinked cellulose
structures produced in accordance with the present disclosure
exhibited improved wet bulk values of about 15.0-17.0 cm.sup.3/g,
and/or absorbent capacity values of about 15.0-17.0 g/g. A
crosslinked (and non-grafted) fiber product available from
Weyerhaeuser NR Company under the designation CMC530, useful as a
control, has a wet bulk of approximately 16.4 cm.sup.3/g and an
absorbent capacity of 16.5 g/g. Accordingly, the grafted,
crosslinked cellulose fibers of the present disclosure may have
suitability, for example, in absorbent applications similar to
those for which non-grafted crosslinked cellulose fibers are
used.
[0025] As another example of this suitability, the grafted
cellulose structures produced in accordance with the present
disclosure exhibit IPRP and MAP values consistent with or improved
relative to non-grafted cellulose fibers. As explained in greater
detail below, IPRP and MAP values indicate a trade-off relationship
that approximates a power law function of the two properties. IPRP
and MAP values relating to CMC530 and other non-grafted controls as
compared to that of example embodiments of grafted cellulose
structures indicate that the grafted, crosslinked structures
exhibit (or are predicted to exhibit, according to formulae
expressing best-fit curves generated by measured IPRP and MAP data)
comparable or increased values, up to 15 or 20%, or even
greater.
[0026] Cellulosic fibers useful for making the grafted crosslinked
cellulose of the present disclosure are derived primarily from wood
pulp. Although suitable wood pulp fibers may be obtained from
chemical processes such as the kraft and sulfite processes, with or
without subsequent and/or prior mercerization and/or bleaching, the
pulp fibers may also be processed by thermomechanical or
chemithermomechanical methods, or various combinations thereof.
Ground wood fibers, recycled or secondary wood pulp fibers, and
bleached and unbleached wood pulp fibers may be used. One example
starting material is prepared from long-fiber coniferous wood
species, such as southern pine, Douglas fir, spruce, hemlock, and
so forth. Details of the production of wood pulp fibers are known
to those skilled in the art. Suitable fibers are commercially
available from a number of sources, including the Weyerhaeuser NR
Company. For example, suitable cellulose fibers produced from
southern pine that may be used as the cellulose substrate in the
materials of the present disclosure are available from the
Weyerhaeuser NR Company under the designations CF416, CF405, NF405,
NB416, FR416, FR516, PW416, and PW405, amongst others.
[0027] The graft species suitable for grafting to the cellulosic
fiber "backbone" to produce the grafted crosslinked cellulose
materials of the present disclosure include those that may be
described as monoethylenically unsaturated acid group-containing
monomers, which include, for example, acrylic acid, methacrylic
acid, crotonic acid, isocrotonic acid, maleic acid, fumaric acid,
itaconic acid, vinylsulfonic acid, 2-acrylamido-2-methyl-1-propane
sulfonic acid, vinyl acetic acid, methallyl sulfonic acid, and so
forth, as well as their alkali and/or ammonium salts, and various
combinations of the aforementioned examples. The choice of suitable
graft species is guided in part by the nature of the backbone from
which the grafted arms are grown, achieving a suitable polymer
architecture, the desired end result of the crosslink treatment,
and so forth.
[0028] For example, grafting, in the context of polymer chemistry,
refers in general to the synthesis of polymer chains attached to a
substrate, and thus encompasses mechanisms such as "grafting to,"
which refers to a polymer chain adsorbing onto a substrate out of
solution, as well as "grafting from," which refers to initiating
and propagating a polymer chain (such as by step-growth addition of
monomer units) at a grafting site on the substrate. The latter
mechanism is generally considered to offer greater control over the
resulting polymer architecture, density of grafting sites, polymer
chain lengths and linearity, and so forth. Considering these
factors, graft species that include one or more acid groups are
chemically appropriate when considered against a goal of
establishing intra-fiber crosslinks, such as chain-to-chain
crosslinks between grafted arms of an individual cellulose fiber.
Further, monoethylenenically unsaturated graft species are suitable
for the grafted crosslinked cellulose materials herein because of
their ability to graft to a cellulose substrate without creating
additional branches or side chains (as opposed, for example, to
species with more than one unsaturated group, the use of which is
more difficult to control). Monomeric graft species are considered
to be easier to control, in terms of reactivity, density of polymer
chains, establishing desired chain lengths and polymer chain
architectures (e.g., linear, unbranched polymers grafted at one end
to the cellulose backbone), suppressing crosslink reactions from
occurring in the grafting stage, and so forth.
[0029] "Grafting," as the term is used herein, refers collectively
to the processes of initiation, growth, and termination of growth
polymerization of the (monomeric) graft species from one or more
grafting sites on the cellulosic substrate. Typically, grafting
according to methods discussed herein is performed in situ, in
which an initiator is used, such as to create active centers on the
substrate and, usually to a lesser extent, initiate
homopolymerization in the aqueous phase. As such, although the
grafting processes described herein proceed mainly by way of the
"grafting from" mechanism described above, the term does not
exclusively refer to this mechanism. Rather, "grafting" also
encompasses the "grafting to" mechanism, other mechanisms, and/or
combinations thereof. Moreover, the terms "to" and "from," when
used when referring to grafting, do not exclusively refer to the
corresponding grafting mechanism, but instead may each encompass
some degree of the other grafting mechanism (or mechanisms).
[0030] It was found that by controlling the amount of graft species
used, various levels of grafting were obtained, characterized by
graft yield %, defined below as the additional weight of a grafted
sample attributable to the polymerized graft species. For example,
acrylic acid readily grafted from cellulose up to approximately 30%
graft yield. Additionally, by varying the levels (e.g., weight %)
of initiator, the number of graft sites available on the cellulosic
substrate was altered. Variations in graft yield were also obtained
by varying the ratio of cellulose to graft species.
[0031] In general, grafted cellulose fibers were produced in situ
by dissolving a measured amount of initiator, such as cerium(IV)
sulfate, in deionized water, and then dissipating a designated
amount of graft species, such as 4-60 weight % acrylic acid (based
on the oven-dry weight of the cellulose), in the solution. The
levels of cerium(IV) sulfate initiator were varied between about
4.5-7.0 weight % (based on the oven-dry weight of the cellulose).
The grafting solution was added to the cellulosic substrate, the
treated cellulose was allowed to react, then washed and filtered to
remove unreacted grafting solution and excess homopolymers of the
graft species, dried, and weighed to determine graft yield %. In
some examples, cellulose-graft-poly(acrylic) acid was then treated
with dilute solutions of sodium hydroxide to at least partially
neutralize the grafted arms on the cellulosic substrate.
[0032] A variety of cross-linking agents and reaction mechanisms
were applied to various grafted cellulose materials, including
ionic crosslinking reactions using multivalent inorganic compounds
(such as aluminum sulfate, a trivalent salt, and titanium-based
crosslinking agents), covalent ester crosslinking reactions using
polymeric crosslinking agents (for example, a homopolymer of the
graft species, such as poly(acrylic) acid or "PAA" when acrylic
acid was used as the graft species), pentaerythritol, and various
hyperbranched polymers ("HPB"), and a radical-based cross-linking
mechanism using ammonium persulfate. Each was performed, at various
levels, on various graft yield levels of fiberized, grafted
cellulose and/or partially neutralized grafted cellulose, with
various factors determining the conditions and amounts of reagents
selected (e.g. solubility in water, ability to distribute evenly
across the grafted cellulose, etc.).
[0033] In general, the grafted structures assumed a polymer
architecture consisting mainly of linear, unbranched polymer chain
arms attached to the cellulose backbone at one end. When subjected
to the various cross-linking reactions, the resulting crosslinked,
grafted structures generally exhibited intra-fiber crosslinks
between grafted arms (such as via pentaerythritol, or a
hyperbranched polymer), also referred to as chain-to-chain
crosslinks. Additionally, in some cases, for example when treated
with polymeric crosslinking agents, some of the chain arms attached
to the cellulose backbone at more than one point, also referred to
as chain-to-cellulose crosslinks, and some polymer bonded along the
cellulose backbone. While not being bound by theory, it is believed
that the stiffness and resiliency imparted by establishing
intra-fiber chain-to-chain and/or chain-to-cellulose crosslinks
strengthen the high void volume structure and provide the observed,
improved absorbent properties as compared to non-crosslinked
grafted cellulose fibers and non-crosslinked, non-grafted cellulose
fibers.
[0034] Although the methods disclosed herein primarily establish
intra-fiber crosslinks (such as chain-to-chain crosslinks between
grafted arms of an individual cellulose fiber, chain-to-cellulose
crosslinks between a grafted arm of a cellulose fiber to elsewhere
on the cellulose fiber, and so forth), some materials also
exhibited a minor degree of inter-fiber crosslinking, such as
between separate cellulose fibers and/or grafted chains thereon. In
general, inter-fiber crosslinking is thought to increase the number
of "knots" in the resulting fibers. Although a higher knot content
is generally not considered to be a desirable characteristic in
cellulosic material used in absorbent applications (both for the
sake of overall product appearance, as well as for the comparative
ease of processing lower knot content fibers), a greater degree of
inter-fiber crosslinking may be achieved by using the processes
described herein, such as, for example, by applying and curing the
crosslinking agent to cellulose in sheet form, rather than to
fiberized cellulose. Other variations may include selecting graft
species and/or crosslinking agent(s) appropriate for establishing
such inter-fiber crosslinking, suitably varying process conditions,
and so forth, to achieve a desired amount of inter-fiber
crosslinking. All of such variations are considered to be within
the scope of this disclosure.
[0035] AFAQ Analysis
[0036] Some performance properties of the grafted, crosslinked
cellulosic compositions according to the present
disclosure--specifically, absorbent properties of wet bulk, wick
time, wick rate, and absorbent capacity--were determined using the
Automatic Fiber Absorption Quality (AFAQ) Analyzer (Weyerhaeuser
Co., Federal Way, Wash.), according to the following procedure:
[0037] A dry 4-gram sample of the pulp composition is placed
through a pinmill to open the pulp, and then airlaid into a tube.
The tube is then placed in the AFAQ Analyzer. A plunger then
descends on the airlaid fluff pad at a pressure of 0.6 kPa. The pad
height is measured, and the pad bulk (or volume occupied by the
sample) is determined from the pad height.
[0038] The weight is increased to achieve a pressure of 2.5 kPa and
the bulk recalculated. The result is two bulk measurements on the
dry fluff pulp at two different pressures.
[0039] While the dry fluff pulp is still compressed at the higher
pressure, water is introduced into the bottom of the tube (to the
bottom of the pad), and the time required for water to wick upward
through the pad and reach the plunger (defined as wick time) is
measured. The bulk of the wet pad at 2.5 kPa is also calculated.
From distance measurements used to calculate the bulk, the wick
rate is determined by dividing the wick time by the distance
traveled by the water (e.g. the height of the wetted fluff pad).
The plunger is then withdrawn from the tube and the wet pad is
allowed to expand for 60 seconds. In general, the more resilient
the sample, the more it will expand to reach its wet rest state.
Once expanded, this resiliency is measured by reapplying the
plunger to the wet pad at 0.6 kPa and determining the bulk. The
final bulk of the wet pad at 0.6 kPa is considered to be the "wet
bulk at 0.6 kPa" (in cm.sup.3/g, indicating volume occupied by the
wet pad, per weight of the wet pad, under the 0.6 kPa plunger load)
of the pulp composition. When the term "wet bulk" is used herein,
it refers to "wet bulk at 0.6 kPa" as determined according to this
procedure.
[0040] Absorbent capacity is calculated by weighing the wet pad
after water is drained from the equipment and reported as grams
water per gram dry pulp.
[0041] In-Plane Radial Permeability (IPRP) Analysis
[0042] Permeability generally refers to the quality of a porous
material that causes it to allow liquids or gases to pass through
it and, as such, is generally determined from the mass flow rate of
a given fluid through it. The permeability of an absorbent
structure is related to the material's ability to quickly acquire
and transport a liquid within the structure, both of which are key
features of an absorbent article. Accordingly, measuring
permeability is one metric by which a material's suitability for
use in absorbent articles may be assessed.
[0043] The following test is suitable for measurement of the
In-Plane Radial Permeability (IPRP) of a porous material. The
quantity of a saline solution (0.9% NaCl) flowing radially through
an annular sample of the material under constant pressure is
measured as a function of time.
[0044] Testing is performed at 23.degree. C..+-.2 C.degree. and a
relative humidity of 50%.+-.5%. All samples are conditioned in this
environment for twenty four (24) hours before testing.
[0045] The IPRP sample holder 400 is shown in FIG. 1 and comprises
a cylindrical bottom plate 405, top plate 410, and cylindrical
stainless steel weight 415.
[0046] Top plate 410 comprises an annular base plate 420 9 mm thick
with an outer diameter of 70 mm and a tube 425 of 150 mm length
fixed at the center thereof. The tube 425 has an outer diameter of
15.8 mm and an inner diameter of 12 mm. The tube is adhesively
fixed into a circular 16 mm hole in the center of the base plate
420 such that the lower edge of the tube is flush with the lower
surface of the base plate, as depicted in FIG. 1. The bottom plate
405 and top plate 410 are fabricated from Lexan.RTM. or equivalent.
The stainless steel weight 415 has an outer diameter of 70 mm and
an inner diameter of 15.9 mm so that the weight is a close sliding
fit on tube 425. The thickness of the stainless steel weight 415 is
approximately 22 mm and is adjusted so that the total weight of the
top plate 410 and the stainless steel weight 415 is 687 g.+-.1 g to
provide 2.0 kPa of confining pressure during the measurement.
[0047] Bottom plate 405 is approximately 25 mm thick and has two
registration grooves 430 cut into the lower surface of the plate
such that each groove spans the diameter of the bottom plate and
the grooves are perpendicular to each other. Each groove is 1.5 mm
wide and 2 mm deep. Bottom plate 405 has a horizontal hole 435
which spans the diameter of the plate. The horizontal hole 435 has
a diameter of 8 mm and its central axis is 15 mm below the upper
surface of bottom plate 405. Bottom plate 405 also has a central
vertical hole 440 which has a diameter of 8 mm and is 10 mm deep.
The central hole 440 connects to the horizontal hole 435 to form a
T-shaped cavity in the bottom plate 405. The outer portions of the
horizontal hole 435 are threaded to accommodate pipe elbows 445
which are attached to the bottom plate 405 in a watertight fashion.
One elbow is connected to a vertical transparent tube 460 with a
total height of 175 mm measured from the bottom of bottom plate 405
(including elbow 445) and an internal diameter of 6 mm. The tube
460 is scribed with a suitable mark 470 at a height of 100 mm above
the upper surface of the bottom plate 420. This is the reference
for the fluid level to be maintained during the measurement. The
other elbow 445 is connected to the fluid delivery reservoir 700
(described below) via a flexible tube.
[0048] A suitable fluid delivery reservoir 700 is shown in FIG. 2.
Reservoir 700 is situated on a suitable laboratory jack 705 and has
an air-tight stoppered opening 710 to facilitate filling of the
reservoir with fluid. An open-ended glass tube 715 having an inner
diameter of 10 mm extends through a port 720 in the top of the
reservoir such that there is an airtight seal between the outside
of the tube and the reservoir. Reservoir 700 is provided with an
L-shaped delivery tube 725 having an inlet 730 that is below the
surface of the fluid in the reservoir, a stopcock 735, and an
outlet 740. The outlet 740 is connected to elbow 445 via flexible
plastic tubing 450 (e.g. Tygon.RTM.). The internal diameter of the
delivery tube 725, stopcock 735, and flexible plastic tubing 450
enable fluid delivery to the IPRP sample holder 400 at a high
enough flow rate to maintain the level of fluid in tube 460 at the
scribed mark 470 at all times during the measurement. The reservoir
700 has a capacity of approximately 6 liters, although larger
reservoirs may be required depending on the sample thickness and
permeability. Other fluid delivery systems may be employed provided
that they are able to deliver the fluid to the sample holder 400
and maintain the level of fluid in tube 460 at the scribed mark 470
for the duration of the measurement.
[0049] The IPRP catchment funnel 500 is shown in FIG. 2 and
comprises an outer housing 505 with an internal diameter at the
upper edge of the funnel of approximately 125 mm. Funnel 500 is
constructed such that liquid falling into the funnel drains rapidly
and freely from spout 515. A stand with horizontal flange 520
around the funnel 500 facilitates mounting the funnel in a
horizontal position. Two integral vertical internal ribs 510 span
the internal diameter of the funnel and are perpendicular to each
other. Each rib 510 is 1.5 mm wide and the top surfaces of the ribs
lie in a horizontal plane. The funnel housing 500 and ribs 510 are
fabricated from a suitably rigid material such as Lexan.RTM. or
equivalent in order to support sample holder 400. To facilitate
loading of the sample it is advantageous for the height of the ribs
to be sufficient to allow the upper surface of the bottom plate 405
to lie above the funnel flange 520 when the bottom plate 405 is
located on ribs 510. A bridge 530 is attached to flange 520 in
order to mount two digital calipers 535 to measure the relative
height of the stainless steel weight 415. The digital calipers 535
have a resolution of .+-.0.01 mm over a range of 25 mm. A suitable
digital caliper is a Mitutoyo model 543-4926 or equivalent. Each
caliper is interfaced with a computer to allow height readings to
be recorded periodically and stored electronically on the computer.
Bridge 530 has a circular hole 22 mm in diameter to accommodate
tube 425 without the tube touching the bridge.
[0050] Funnel 500 is mounted over an electronic balance 600, as
shown in FIG. 2. The balance has a resolution of .+-.0.01 g and a
capacity of at least 1000 g. The balance 600 is also interfaced
with a computer to allow the balance reading to be recorded
periodically and stored electronically on the computer. A suitable
balance is Mettler-Toledo model MS6002S or equivalent. A collection
container 610 is situated on the balance pan so that liquid
draining from the funnel spout 515 falls directly into the
container 610.
[0051] The funnel 500 is mounted so that the upper surfaces of ribs
510 lie in a horizontal plane. Balance 600 and container 610 are
positioned under the funnel 500 so that liquid draining from the
funnel spout 515 falls directly into the container 610. The IPRP
sample holder 400 is situated centrally in the funnel 500 with the
ribs 510 located in grooves 430. The upper surface of the bottom
plate 405 must be perfectly flat and level. The top plate 410 is
aligned with and rests on the bottom plate 405. The stainless steel
weight 415 surrounds the tube 425 and rests on the top plate 410.
Tube 425 extends vertically through the central hole in the bridge
530. Both calipers 535 are mounted firmly to the bridge 530 with
the foot resting on a point on the upper surface of the stainless
steel weight 415. The calipers are set to zero in this state. The
reservoir 700 is filled with 0.9% saline solution and re-sealed.
The outlet 740 is connected to elbow 445 via flexible plastic
tubing 450.
[0052] An annular sample 475 of the material to be tested is cut by
suitable means. The sample has an outer diameter of 70 mm and an
inner hole diameter of 12 mm. One suitable means of cutting the
sample is to use a die cutter with sharp concentric blades.
[0053] The top plate 410 is lifted enough to insert the sample 475
between the top plate and the bottom plate 405 with the sample
centered on the bottom plate and the plates aligned. The stopcock
735 is opened and the level of fluid in tube 460 is set to the
scribed mark 470 by adjusting the height of the reservoir 700 using
the jack 705 and by adjusting the position of the tube 715 in the
reservoir. When the fluid level in the tube 460 is stable at the
scribed mark 470 initiate recording data from the balance and
calipers by the computer. Balance readings and time elapsed are
recorded every 10 seconds for five minutes. The average sample
thickness B is calculated from all caliper reading between 60
seconds and 300 seconds and expressed in cm. The flow rate in grams
per second is the slope calculated by linear least squares
regression fit of the balance reading (dependent variable) at
different times (independent variable) considering only the
readings between 60 seconds and 300 seconds.
[0054] Permeability k is then calculated by the following
equation:
k = ( Q / .rho. i ) .mu. ln ( R 0 / R i ) 2 .pi. B .DELTA. p ( 1 )
##EQU00001##
[0055] Where:
[0056] k is the permeability (cm.sup.2);
[0057] Q is the flow rate (g/s);
[0058] .rho..sub.1 is the liquid density at 20.degree. C.
(g/cm.sup.3);
[0059] .mu. is the liquid viscosity at 20.degree. C. (Pas);
[0060] R.sub.0 is the outer sample radius (cm);
[0061] R.sub.i is the inner sample radius (cm);
[0062] B is the average sample thickness (cm); and
[0063] .DELTA.p is the pressure drop (Pa) calculated according to
the following equation:
.DELTA. p = ( .DELTA. h - B 2 ) g .rho. i 10 ( 2 ) ##EQU00002##
[0064] Where:
[0065] .DELTA.h is the measured liquid hydrostatic pressure
(cm);
[0066] g is the acceleration constant (m/sec.sup.2); and
[0067] .rho..sub.1 is the liquid density (g/cm.sup.3).
[0068] In-plane radial permeability is dependent on the fluid being
used, so the IPRP value (in cm.sup.2/MPasec) may be defined and
calculated as follows:
IPRP value=(k/.mu.) (3)
[0069] Where:
[0070] k is the permeability (cm.sup.2); and
[0071] .mu. is the liquid viscosity at 20.degree. C. (MPas).
[0072] MAP Analysis
[0073] Capillary pressure can be considered representative of a
material's ability to wick fluid by capillary action and is
expressed in the context of the present disclosure in terms of
Medium Absorption Pressure (MAP), as explained below.
[0074] Capillary pressure measurements are made on a
TRI/Autoporosimeter (TRI/Princeton Inc. of Princeton, N.J.). The
TRI/Autoporosimeter is an automated computer-controlled instrument
for measuring capillary pressure in porous materials, which can be
schematically represented in FIG. 3. Complimentary Automated
Instrument Software, Release Version 2007.6WD, is used to capture
the data. More information on the TRI/Autoporosimeter, its
operation and data treatments can be found in The Journal of
Colloid and Interface Science 162 (1994), pp. 163-170, incorporated
here by reference.
[0075] As used herein, determining capillary pressure hysteresis
curve of a material as function of saturation, involves recording
the increment of liquid that enters a porous material as the
surrounding air pressure changes. A sample in the test chamber is
exposed to precisely controlled changes in air pressure which at
equilibrium (no more liquid uptake/release) corresponds to the
capillary pressure.
[0076] The equipment operates by changing the test chamber air
pressure in user-specified increments, either by decreasing
pressure (increasing pore size) to absorb liquid, or increasing
pressure (decreasing pore size) to drain liquid. The liquid volume
absorbed (or drained) is measured with a balance at each pressure
increment. The saturation is automatically calculated from the
cumulative volume.
[0077] All testing is performed at 23.degree. C..+-.2 C.degree. and
a relative humidity of 50%.+-.5%. A saline solution of 0.9% weight
to volume in deionized water is used. The surface tension (mN/m),
contact angle (.degree.), and density (g/cc) for all solutions are
determined by any method known in the art. Alternatively (as done
for measuring the Examples below), reference values for these
parameters may be provided to the TRI/Autoporosimeter's
software.
[0078] Surface tension (mN/m), contact angle (.degree.), and
density (g/cm.sup.3) is provided to the instrument's software.
Reference values used for the tests described herein were as
follows: surface tension of 72 mN/m; contact angle of 0.degree.;
and liquid density of 1 g/cm.sup.3. The balance is leveled at 156.7
g and equilibration rate set to 90 mg/min. The pore radius protocol
(corresponding to capillary pressure steps) scans capillary
pressures according to the following equation:
R=2.gamma. cos /.DELTA.p (4)
[0079] Where:
[0080] R is the pore radius;
[0081] .gamma. is the surface tension;
[0082] is the contact angle; and
[0083] .DELTA.p is the capillary pressure.
[0084] Tests are performed with the sample compressed with an
applied load of approximately 0.3 psi. The weight applied to the
sample is 428 g and is 50 mm in diameter.
[0085] The pressure sequence in Table 1, below, is applied to the
measurement cell in the standard test protocol which corresponds to
an individual pore radius as indicated.
TABLE-US-00001 TABLE 1 Height (mm) Radius (.mu.m) 1 600 24.5 2 450
32.7 3 350 42.0 4 300 49.0 5 250 58.8 6 200 73.5 7 150 98.0 8 100
147 9 80 184 10 60 245 11 40 368 12 20 735 13 10 1470 14 20 735 15
40 368 16 60 245 17 80 184 18 100 147 19 150 98.0 20 200 73.5 21
250 58.8 22 300 49.0 23 350 42.0 24 450 32.7 25 600 24.5
[0086] The sample is cut into a circle with 5 cm diameter and then
conditioned at 23.degree. C..+-.2 C.degree. and a relative humidity
50%.+-.5% for at least 24 hours before testing. The sample weight
(to .+-.0.001 g) is measured. The empty sample chamber is closed.
After the instrument has applied the appropriate air pressure to
the cell, the liquid valve is closed and the chamber is opened. The
specimen and confining weight are placed into the chamber and the
chamber is closed. After the instrument has applied the appropriate
air pressure to the cell, the liquid valve is opened to allow free
movement of liquid to the balance and the test under the radius
protocol is started. The instrument proceeds through one
absorption/desorption cycle (also called a hysteresis loop). A
blank (without specimen) is run in like fashion.
[0087] For calculations and reporting, the mass uptake from a blank
run is directly subtracted from the uptake of the sample. Medium
Absorption Pressure (MAP) is the pressure at which 50% of the
liquid uptake has been achieved--or, in other words, the pressure
that corresponds to 50% of the total liquid absorbed on the
absorption branch of the hysteresis loop generated by the
autoporosimeter.
[0088] The following examples summarize representative methods of
treating cellulose fibers in accordance with the methods and
concepts discussed above, and are illustrative in nature. The
reagent amounts, times, conditions, and other process conditions
may be varied from those disclosed in the specific representative
procedures disclosed in the following examples without departing
from the scope of the present disclosure.
Example 1: Grafting
[0089] Representative procedure: cellulose pulpsheets (CF416, 93%
solids, Columbus Mill, from Weyerhaeuser NR Company, Federal Way,
Wash.) were cut into rectangles (of 13.25''.times.4''), weighed,
and placed into re-sealable plastic bags in pairs. A Ce.sup.4+
catalyst solution was produced by stirring and dissolving a
measured quantity of ammonium cerium(IV) sulfate (94%, from Sigma
Aldrich) in 150.0 mL deionized water. Acrylic acid (99%, with
180-200 ppm MEHQ inhibitor, from Sigma Aldrich) in a measured
volume was then added to the Ce.sup.4+ solution and stirred for 5
minutes. The resulting solution was slowly poured over the
cellulose pulpsheets, on both sides, in the bag, which was then
sealed and allowed to equilibrate at room temperature
overnight.
[0090] The sealed bag was then cured in a ventilated oven at
50.degree. C. for 2 hours, followed by cooling to room temperature.
The treated cellulose (cellulose-graft-poly(acrylic) acid) was then
washed with 2.5 L deionized water in a Waring Blendor at low speed.
Unreacted grafting solution, excess homopolymers of poly(acrylic)
acid, and other impurities, were removed via vacuum filtration with
Buchner funnel and filter paper, washed and vacuum filtered again,
then oven-dried overnight at 50.degree. C.
[0091] Graft yield was calculated using the following formula:
% Graft Yield=[(W.sub.2-W.sub.1)/W.sub.1].times.100 (5)
Where W.sub.1=weight of starting cellulose material, and
W.sub.2=grafted product weight.
[0092] Representative data indicating weights and volumes used in a
number of runs performed according to Example 1, and graft yields
achieved, are shown in Table 2.
TABLE-US-00002 TABLE 2 Wt Starting AA Catalyst Wt Product Graft
Yield Run # Material (g) (mL) (g) (g) (%) 1 48.79 2.2 3.44 51.06
4.66 2 48.62 15.5 2.92 57.60 18.47 3 48.55 9.0 2.92 54.40 12.06 4
48.57 2.2 3.44 50.74 4.46 5 48.26 9.0 2.92 53.82 11.53 6 48.64 2.7
2.40 50.62 4.07 7 48.37 9.0 2.92 53.73 11.08 8 48.65 9.0 2.92 54.44
11.91 9 48.87 16.0 2.40 58.04 18.76 10 48.74 10.0 2.40 54.33 11.47
11 48.72 9.0 2.92 54.33 11.51 12 48.18 2.7 2.40 50.35 4.50 13 48.44
8.5 3.44 54.24 11.97 14 48.75 2.5 2.92 50.83 4.27 15 48.21 14.0
3.44 56.49 17.17 16 (control) 48.92 0 0 49.04 0.25
[0093] A variety of cross-linking agents and reaction mechanisms
were then applied to cellulose-graft-poly(acrylic) acid materials
prepared in accordance with the procedure in Example 1, as
described in Examples 2-7 infra.
Example 2: PAA Ester Cross-Linking
[0094] Representative procedure: using
cellulose-graft-poly(acrylic) acid prepared according to the
procedure in Example 1, 12''.times.12'' British handsheets were
prepared using all the material from one run (there was some loss
during process). The handsheets were equilibrated to 93% solids in
a humidity-controlled room. Each handsheet was cut into strips (of
12''.times.4'').
[0095] Polyacrylic acid ("PAA") crosslinking agent (Aquaset.TM.
1676 available from The Dow Chemical Company; other suitable
examples of suitable crosslinking agents are listed in U.S. Pat.
App. Pub. No. 20110077354 of Stoyanov, et al.) was applied, in some
cases in the presence of a sodium hypophosphite ("SHP") catalyst,
to the handsheet strips. The treated strips were allowed to
equilibrate, then air-dried, fiberized with a Kamas hammermill, and
cured.
[0096] Representative data indicating weights and volumes used are
shown in Table 3.
TABLE-US-00003 TABLE 3 Wt Handsheet @ PAA PAA Run 93% (g) % (g) 1
53.05 0 0 2 59.16 1.5 1.49 3 56.40 0 0 4 52.78 3 2.96 5 55.70 1.5
1.47 6 52.68 0 0 7 55.58 1.5 1.48 8 56.38 3 2.97 9 60.01 0 0 10
56.31 1.5 1.49 11 56.34 1.5 1.49 12 52.37 3 2.94 13 56.19 1.5 1.48
14 52.86 1.5 1.49 15 58.35 0 0 16 (control) 51.31 6.5 6.44
Example 3: Pentaerythritol Ester Cross-Linking
[0097] Representative procedure: using
cellulose-graft-poly(acrylic) acid prepared according to the
procedure in Example 1, an aqueous slurry was prepared using all
the material from one run, which was then vacuum-filtered using a
Buchner funnel to produce a pad. The pad was then oven-dried at
50.degree. C. to constant weight.
[0098] A solution of 5.00 g pentaerythritol (from Sigma Aldrich)
and 0.15 g sodium hypophosphite in 60 mL deionized water was
prepared at 40.degree. C., which was then cooled to room
temperature and evenly applied to both sides of the pad via
transfer pipette, and the treated pad was placed in a sealed
plastic bag and allowed to equilibrate at room temperature
overnight.
[0099] The pad was then fiberized in a Waring Blendor, and cured in
an oven at 193.degree. C. for 5 minutes.
Example 4: Trivalent Salt Ionic Cross-Linking
[0100] Representative procedure: using
cellulose-graft-poly(acrylic) acid prepared according to the
procedure in Example 1, an aqueous slurry was prepared using all
the material from one run, which was then vacuum-filtered using a
Buchner funnel to produce a pad. The pad was then air-dried to
43.5% solids content.
[0101] An aluminum sulfate solution was prepared by dissolving
10.00 g of aluminum sulfate octodecahydrate (from Sigma Aldrich) in
250 mL deionized water. The solution was evenly applied to both
sides of the air-dried pad via transfer pipette, and the treated
pad was placed in a sealed plastic bag and allowed to equilibrate
at room temperature overnight.
[0102] The treated pad was then vacuum filtered in a Buchner funnel
and gently rinsed, once, with 500 mL deionized water, then
air-dried at room temperature until constant weight was
achieved.
Example 5: Titanium-Based Ionic Cross-Linking
[0103] Representative procedure: using
cellulose-graft-poly(acrylic) acid prepared according to the
procedure in Example 1, an aqueous slurry was prepared using all
the material from one run, which was then vacuum-filtered using a
Buchner funnel to produce a pad. The pad was then oven-dried to
constant weight.
[0104] A solution was prepared by dissolving 6.61 g of Tyzor.RTM.
LA (lactic acid titanate chelate, from DuPont) in 26.4 mL deionized
water at room temperature. The solution was evenly applied to both
sides of the air-dried pad via transfer pipette, and the treated
pad was placed in a sealed plastic bag and allowed to equilibrate
at room temperature overnight.
[0105] The pad was then fiberized in a Waring Blendor, and cured in
an oven at 175.degree. C. for 15 minutes.
Example 6: Radical Cross-Linking
[0106] Representative procedure: using
cellulose-graft-poly(acrylic) acid prepared according to the
procedure in Example 1, 12''.times.12'' British handsheets were
prepared using 50 g of the material. The handsheets were
equilibrated to 93% solids in a humidity-controlled room. Each
handsheet was cut into strips (of 12''.times.4'').
[0107] A solution of 2.22 g of ammonium persulfate (from Sigma
Aldrich) in 45 mL deionized water was prepared and evenly applied
across the handsheet strips via transfer pipette, and the treated
strips were placed in a sealed plastic bag and allowed to
equilibrate at room temperature overnight.
[0108] The treated strips were then air-dried to about 70% solids
and then fiberized with a Kamas hammermill. The material was then
gently and evenly sprayed with 110 mL deionized water, placed in a
foil pouch that was perforated to allow evaporation, and oven-cured
at 390.degree. F. (199.degree. C.) for 15 minutes. After removal
from the oven, the pouch was cooled to room temperature, and then
the material was removed and allowed to air-dry at room temperature
until constant weight was achieved.
Example 7: Ester Cross-Linking with Hyperbranched Polymers
[0109] Representative procedure: using
cellulose-graft-poly(acrylic) acid prepared according to the
procedure in Example 1, 12''.times.12'' British handsheets were
prepared using 50 g of the material. The handsheets were
equilibrated to 93% solids in a humidity-controlled room. Each
handsheet was cut into strips (of 12''.times.4'').
[0110] A solution of a measured quantity of a hyperbranched polymer
(e.g., 0.22 g Lutensit.RTM. Z96 or 0.95 Lutensol.RTM. FP620, both
from BASF) along with 0.14 g sodium hypophosphite in 45 mL
deionized water was prepared and evenly applied across the
handsheet strips via transfer pipette, and the treated strips were
placed in a sealed plastic bag and allowed to equilibrate at room
temperature overnight.
[0111] The treated strips were then air-dried to about 70% solids,
fiberized with a Kamas hammermill, air-dried overnight, and cured
at 370.degree. F. (187.8.degree. C.) for 5 minutes in a large
dispatch oven.
[0112] AFAQ analysis was performed on the
cellulose-graft-poly(acrylic) acid materials prepared in accordance
with the representative procedure of Example 1, as well as on
various crosslinked cellulose-graft-poly(acrylic) acid materials
prepared in accordance with the representative procedures of
Examples 2-7. The representative values presented in Table 4
(below) are averages from multiple runs of the indicated crosslink
method on the indicated graft yield % level of
cellulose-graft-poly(acrylic) acid.
TABLE-US-00004 TABLE 4 AFAQ Analysis Wick Wick Wet Bulk Absorbent
Time Rate 0.6 kPa Capacity Sample Description Sample # sec mm/s
cm.sup.3/g g/g % graft crosslink method 1 2.1 9.42 9.73 9.93 11
ionic (Al.sup.3+) 2 2.6 8.03 12.07 12.49 11 ester (pentaerythritol)
3 2.0 10.58 10.55 10.62 11 titanium (Tyzor LA) 4 4.6 6.63 16.53
16.74 12 ester (PAA) 5 4.0 6.89 11.19 11.32 6.6 none 6 3.8 8.43
15.92 15.83 6.6 ester (HPB) 7 3.8 7.12 10.93 11.01 11 none 8 3.7
6.32 11.26 11.56 11 radical 9 3.3 9.11 16.73 16.43 11 ester (HPB)
10 3.4 7.22 10.12 10.17 19.8 none 11 2.9 9.51 16.15 15.75 19.8
ester (HPB) 12 3.2 6.82 9.84 9.97 25.2 none 13 2.85 8.72 15.53
15.09 25.2 ester (HPB) Control A (CMC530) 2.3 11.87 16.39 16.45 n/a
n/a Control B (CMC530) 2.6 10.71 16.45 16.65 n/a n/a Control C
(CF416) -- -- 11.71 11.75 n/a n/a
[0113] Table 4 shows, for example, that
cellulose-graft-poly(acrylic) acid produced from CF416 cellulose
pulp fiber tends to exhibit lower wet bulk and absorbent capacity
values as compared with untreated (i.e. non-grafted,
non-crosslinked) CF416, across a range of graft yield levels
tested. However, when subjected to subsequent crosslink treatment,
the grafted, crosslinked cellulose structures produced thereby
generally exhibited improved wet bulk values and/or absorbent
capacity values. For example, ester cross-linking using PAA or a
hyperbranched polymer as the cross-linking agent yielded
improvements in wet bulk and absorbent capacity values, from about
6% to over 40%, as compared to untreated cellulose. Indeed, the
improved values of these properties were found to be comparable
with those exhibited by non-grafted, crosslinked cellulose samples
(e.g., CMC530). Some crosslinking treatments, however, such as
ionic trivalent salt and titanium-based reactions, did not
increase, and in some cases further decreased, wet bulk and
absorbent capacity values as compared to untreated cellulose. Also,
in general, cellulose-graft-poly(acrylic) acid materials produced
from CF416 exhibited lower wick rates than non-grafted, crosslinked
cellulose samples, but when subjected to subsequent crosslink
treatment, wick rates were seen to further decrease with some
PAA/SHP and radical treatment processes, but increase with others
(such as with ester cross-linking with HPB and pentaerythritol, and
ionic processes). Thus, various absorbent properties may be
modified through a selection of graft species, cross-linking
reaction, and other process conditions.
[0114] Another performance metric by which the grafted, crosslinked
cellulose materials produced in accordance with the present
disclosure may be characterized and/or compared is by means of
liquid permeability and capillary pressure, two properties
important for absorbent products. Liquid permeability may be
measured by in-plane radial permeability (IPRP) and capillary
pressure may be measured by medium absorption pressure (MAP),
according to the tests described above.
[0115] As noted above, there is a trade-off between IPRP and MAP
with known absorbent materials, including cellulose materials,
synthetic fibers, blends, and so forth, in which IPRP tends to
decrease as MAP increases. This trade-off is illustrated, for
example, in FIG. 4, in the form of a dashed line following the
least-squares best-fit curve that corresponds to IPRP and MAP
values exhibited by example cellulose fiber controls, including
crosslinked cellulose products such as CMC530 (used as a control in
the Examples and subsequent AFAQ analysis described above) as well
as non-crosslinked cellulose products such as NB416, CF416, and so
forth. The IPRP and MAP values are shown in Table 5, below. The
curve can be described mathematically as a power law function
y=mx.sup.z, with IPRP value as the abscissa and MAP as the
ordinate. For the Table 5 data, the best-fit curve for the
non-grafted controls can be expressed by the formula
y=896.38x.sup.-0.549, with R.sup.2=0.9479.
TABLE-US-00005 TABLE 5 IPRP Value MAP (cm.sup.2/MPa sec) (cm
H.sub.2O) Material 594 21.3 non-crosslinked 651 23.9 control 736
23.7 737 25.6 759 31.6 2144 12.9 crosslinked 2360 12.3 control 2674
11.8 2723 11.5 3061 11.6 3219 10.3 3453 9.7 3659 10.6 3708 9.8 4350
9.0 5189 8.1 5296 8.0
[0116] As exemplified, for example, in FIG. 4 (and Table 5),
cellulose fibers have been observed to be bounded by a maximum IPRP
value of about 5400 cm.sup.2/MPasec and a maximum MAP value of
about 32 cm H.sub.2O. Higher IPRP values have been achieved, but
only with blends of cellulose with synthetic fibers (e.g.
polyethylene, polypropylene and/or polyester fibers) or synthetic
nonwovens produced from, for example, polyethylene, polypropylene
and/or polyester fibers or filaments.
[0117] Focusing in particular on crosslinked cellulose fibers, such
products have been observed to be bounded by a maximum MAP value of
about 13 cm H.sub.2O.
[0118] The grafted, crosslinked cellulose materials prepared in
accordance with the present disclosure, however, exhibit IPRP
values as high as 7700 cm.sup.2/MPasec and MAP values up to 20 cm
H.sub.2O.
[0119] In some examples, IPRP and MAP values for grafted,
crosslinked cellulose materials approximate a trade-off curve that
is slightly shifted (i.e. raised) and also elongated (i.e. spans a
broader IPRP range), with respect to that exhibited by the
non-grafted cellulose controls, as shown in FIG. 4. The trade-off
for the grafted materials is shown as a solid line following the
best-fit curve for the example data presented in Table 6,
below.
TABLE-US-00006 TABLE 6 IPRP Value MAP (cm.sup.2/MPa sec) (cm
H.sub.2O) Material 517 27.67 grafted, non- 601 37.55 crosslinked
1113 19.13 grafted, 1139 20.02 crosslinked 1143 18.06 1267 18.31
1465 17.56 1479 17.01 1555 16.62 1774 16.45 1868 14.79 2122 13.44
2129 13.09 2139 13.58 2152 13.34 2270 13.17 2270 13.20 2308 13.31
2384 13.13 2386 13.16 2470 12.95 2502 12.95 2532 12.94 grafted,
2564 12.69 crosslinked 2649 12.55 2651 12.45 2710 11.78 2714 12.33
2722 12.69 2801 12.18 2832 11.80 2851 12.36 2875 12.15 2879 11.98
2940 12.20 2971 11.16 3816 9.39 4143 9.34 4434 10.07 5038 8.00 5483
9.24 5713 8.49 5999 9.61 7672 7.55
[0120] The best-fit curve generated by the example data set in
Table 6, corresponding to grafted cellulose materials, can be
expressed by the formula y=869.93x.sup.-0.538 (with
R.sup.2=0.9471). Best-fit curves for example materials prepared in
accordance with the present disclosure can be characterized by the
same general power law function represented by the formula
y=mx.sup.z, with m values ranging from about 600 to about 1200 (and
more particularly from about 800 to about 1100), and z values
ranging from about -0.590 to -0.515 (and more particularly from
about -0.560 to about -0.520). These best-fit curve models for the
grafted materials of the present disclosure correspond to or
predict the IPRP value for a given MAP value (and vice versa)
within about +/-30% of the value of x (or y) in the respective
formula, particularly at IPRP values y (in cm.sup.2/MPasec) ranging
from about 1000 to about 7700.
[0121] Comparing the best-fit curves for the example non-grafted
controls (dashed line) and the example grafted materials (solid
line), the "shift" visible in FIG. 4 (and shown by the data in
Tables 5 and 6) illustrates that, in the range of IPRP values
exhibited by non-grafted cellulose fiber control materials (that
is, a range of from about 600 to about 5400 cm.sup.2/MPasec), the
example grafted materials exhibit (or are predicted to exhibit) MAP
values equal to or higher than the corresponding MAP values
possessed by non-grafted cellulose materials. Also, for a given MAP
value (in cm H.sub.2O) in a range of from about 7.0 to about 25,
the grafted materials exhibit (or are predicted to exhibit) IPRP
values equal to or higher than the corresponding IPRP values
possessed by non-grafted cellulose fiber. Focusing specifically on
the data corresponding to crosslinked materials, the difference in
IPRP values is generally up to about 20% higher for the example
grafted materials as compared to the example non-grafted controls
over an MAP range of about 7.0 to 20 (although the differences are
even greater in some instances, for example with IPRP values
exhibited at MAP values between about 7.0 and about 10), and the
difference in MAP values is generally up to about 15% higher for
grafted materials over a range of IPRP values of about 800 to 5400
cm.sup.2/MPasec.
[0122] Comparing the best-fit curves for the example non-grafted
controls and the example grafted materials, the "elongation"
visible in FIG. 4 (and shown by the data in Tables 5 and 6)
illustrates that IPRP values greater than those achieved with
non-grafted cellulose products (e.g., IPRP values greater than
about 5400 cm.sup.2/MPasec) are exhibited by the grafted,
crosslinked cellulose materials of the present disclosure.
[0123] Accordingly, the grafted, crosslinked cellulose of the
present disclosure may have suitability, for example, in absorbent
applications similar to those for which non-grafted, crosslinked
cellulose fibers are used, as well as other applications.
[0124] Although the inventive subject matter for which protection
is sought is defined in the appended claims, other illustrative,
non-exclusive examples of inventive subject matter according to the
present disclosure are described in the following enumerated
paragraphs:
[0125] A. A cellulosic material comprising a cellulose fiber and
polymer chains composed of at least one monoethylenically
unsaturated acid group-containing monomer grafted thereto, wherein
one or more of said cellulose fiber and said polymer chains are
crosslinked.
[0126] A.1. The material of A, wherein the cellulose fiber is wood
fiber.
[0127] A.2. The material of A or A.1, wherein the material includes
mainly intra-fiber crosslinks.
[0128] A.3. The material of any of A through A.2, wherein the at
least one monoethylenically unsaturated acid group-containing
monomer includes one or more of acrylic acid, maleic acid, and
methacrylic acid.
[0129] A.4. The material of any of A through A.3, wherein the at
least one monoethylenically unsaturated acid group-containing
monomer is acrylic acid.
[0130] A.5. The material of any of A through A.4, characterized by
a graft yield of 5-35 weight %.
[0131] A.6. The material of any of A through A.5, characterized by
a graft yield of 10-20 weight %.
[0132] A.7. The material of any of A through A.6, wherein the
material has a wet bulk at least 6% greater than untreated
cellulose fiber.
[0133] A.8. The material of any of A through A.7, wherein the
material has a wet bulk at least 40% greater than untreated
cellulose fiber.
[0134] A.9. The material of any of A through A.8, wherein the
material has a wet bulk of about 10.0-17.0 cm.sup.3/g.
[0135] A.10. The material of any of A through A.9, wherein the
material has a wet bulk of about 15.0-17.0 cm.sup.3/g.
[0136] A.11. The material of any of A through A.10, wherein the
material has an absorbent capacity of about 10.0-17.0 g/g.
[0137] A.12. The material of any of A through A.11, wherein the
material has an absorbent capacity of about 15.0-17.0 g/g.
[0138] A.13. The material of any of A through A.12, wherein the
material has an IPRP value of about 1000 to 7700 cm.sup.2/MPasec
and a MAP of about 7.0 to 20 cm H.sub.2O.
[0139] A.14. The material of any of A through A.13, wherein for a
given IPRP value y (in cm.sup.2/MPasec) from 1000 to 7700, the MAP
value of the material (in cm H.sub.2O) is within +/-30% of the
value of x in the formula y=mx.sup.z; wherein m is from 600 to
1200, and wherein z is from -0.590 to -0.515.
[0140] A.15 The material of A.14, wherein z is from -0.560 to
-0.520.
[0141] A.16. The material of A.14 or A.15, wherein m is from 800 to
1100.
[0142] A.17. The material of any of A through A.16, characterized
in that at a given IPRP value (in cm.sup.2/MPasec) from 800 to
5400, the material has a MAP value that is equal to or higher than
the corresponding MAP value possessed by non-grafted, crosslinked
cellulose fiber.
[0143] A.18. The material of A.17, wherein the material at the
given IPRP value has a MAP value that is between 0 and 20% higher
than the corresponding MAP value possessed by non-grafted,
crosslinked cellulose fiber.
[0144] A.19. The material of any of A through A.17, characterized
in that at a given MAP value (in cm H.sub.2O) from 7.0 to 20, the
material has an IPRP value that is equal to or higher than the
corresponding IPRP value possessed by non-grafted, crosslinked
cellulose fiber.
[0145] A.20. The material of A.19, wherein the material at the
given MAP value has an IPRP value that is between 0 and 15% higher
than the corresponding IPRP value possessed by non-grafted,
crosslinked cellulose fiber.
[0146] A.21. The material of any of A through A.12, wherein the
material has an IPRP value of 5400 cm.sup.2/MPasec or above.
[0147] B. A fibrous cellulosic material comprising
cellulose-graft-poly(acrylic) acid having a graft yield of 10-20
weight % and wherein two or more grafted polymer chains of
poly(acrylic) acid are intra-fiber crosslinked.
[0148] B.1. The material of B, wherein the material has a wet bulk
of about 15.0-17.0 cm.sup.3/g.
[0149] B.2. The material of B or B.1, wherein said two or more
polymer chains of poly(acrylic) acid are intra-fiber crosslinked by
a hyperbranched polymer.
[0150] B.3. The material of any of B through B.2, wherein said two
or more polymer chains of poly(acrylic) acid are intra-fiber
crosslinked by pentaerythritol.
[0151] C. A method of producing a grafted, crosslinked cellulosic
material, the method comprising (a) grafting polymer chains of at
least one monoethylenically unsaturated acid group-containing
monomer from a cellulosic substrate to produce a grafted cellulosic
material, and (b) subsequently crosslinking the grafted cellulosic
material by treating the material with a crosslinking agent adapted
to effect crosslinking of one or more of the cellulosic substrate
or the polymer chains.
[0152] C.1. The method of C, wherein the cellulosic material is
cellulose fiber.
[0153] C.2. The method of C of C.1, wherein the grafting is
performed in situ.
[0154] C.3. The method of any of C through C.2, wherein the
grafting includes reacting the at least one monomer with the
cellulosic substrate in the presence of a grafting initiator.
[0155] C.3.1. The method of C.3., wherein the grafting initiator
includes ammonium cerium(IV) sulfate.
[0156] C.3.2. The method of C or C.3.1., wherein the grafting
includes varying one or more of the weight percent of the
initiator, and the ratio of the cellulosic substrate to the
monomer, to achieve a desired graft yield level.
[0157] C.4. The method of any of C through C.3.2., including using
acrylic acid as the at least one monomer.
[0158] C.5. The method of any of C through C.4., wherein the
crosslinking includes establishing intra-fiber crosslinks via an
esterification reaction, and wherein the crosslinking agent
includes one or more of pentaerythritol, a homopolymer formed of
the at least one monoethylenically unsaturated acid
group-containing monomer, and a hyperbranched polymer.
[0159] C.5.1. The method of C.5., wherein the crosslinking also
includes establishing intra-fiber chain-to-cellulose
crosslinks.
[0160] C.6. The method of any of C through C.5.1, wherein the
crosslinking includes establishing intra-fiber chain-to-chain
crosslinks via an ionic reaction, and wherein the crosslinking
agent includes a multivalent inorganic compound.
[0161] C.6.1. The method of C.6, wherein the crosslinking agent
includes aluminum sulfate.
[0162] C.7. The method of any of C through C.5.1., wherein the
crosslinking includes establishing intra-fiber chain-to-chain
crosslinks via a radical reaction, and wherein the crosslinking
agent includes an inorganic salt of a strong acid and a weak
base.
[0163] C.7.1. The method of C.7, wherein the crosslinking agent
includes ammonium persulfate.
[0164] C.8. The method of any of C through C.7.1., further
including, prior to the crosslinking, at least partially
neutralizing the grafted polymer side chains by treating the
grafted cellulosic material with an alkaline solution.
[0165] C.9. The material produced by the method of any of C through
C.8.
[0166] Although the present invention has been shown and described
with reference to the foregoing principles and illustrated examples
and embodiments, it will be apparent to those skilled in the art
that various changes in form, detail, conditions, and so forth, may
be made without departing from the spirit and scope of the
invention. The present invention is intended to embrace all such
alternatives, modifications and variances that fall within the
scope of the appended claims.
* * * * *