U.S. patent application number 17/143051 was filed with the patent office on 2021-05-06 for modified fiber, methods, and systems.
This patent application is currently assigned to International Paper Company. The applicant listed for this patent is International Paper Company. Invention is credited to Charles E. Miller.
Application Number | 20210131038 17/143051 |
Document ID | / |
Family ID | 1000005345562 |
Filed Date | 2021-05-06 |
![](/patent/app/20210131038/US20210131038A1-20210506\US20210131038A1-2021050)
United States Patent
Application |
20210131038 |
Kind Code |
A1 |
Miller; Charles E. |
May 6, 2021 |
MODIFIED FIBER, METHODS, AND SYSTEMS
Abstract
Methods of forming crosslinked cellulose include mixing a
crosslinking agent with an aqueous mixture of cellulose fibers
containing little to no excess water (e.g., solids content of
25-55%), drying the resulting mixture to 85-100% solids, then
curing the dried mixture to crosslink the cellulose fibers. Systems
include a mixing unit to form, from an aqueous mixture of unbonded
cellulose fibers having a solids content of about 25-55% and a
crosslinking agent, a substantially homogenous mixture of
non-crosslinked, unbonded cellulose fibers and crosslinking agent;
a drying unit to dry the substantially homogenous mixture to a
consistency of 85-100%; and a curing unit and to cure the
crosslinking agent to form dried and cured crosslinked cellulose
fibers. Intrafiber crosslinked cellulose pulp fibers produced by
such methods and/or systems have a chemical on pulp level of about
2-14% and an AFAQ capacity of at least 12.0 g/g.
Inventors: |
Miller; Charles E.; (Federal
Way, WA) |
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Applicant: |
Name |
City |
State |
Country |
Type |
International Paper Company |
Memphis |
TN |
US |
|
|
Assignee: |
International Paper Company
Memphis
TN
|
Family ID: |
1000005345562 |
Appl. No.: |
17/143051 |
Filed: |
January 6, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15976654 |
May 10, 2018 |
10900174 |
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17143051 |
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15253450 |
Aug 31, 2016 |
9995000 |
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15976654 |
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14320279 |
Jun 30, 2014 |
9458297 |
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15253450 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C08J 2433/02 20130101;
D21C 9/005 20130101; D21C 9/001 20130101; C08J 2301/02 20130101;
C08J 2333/02 20130101; D21C 9/00 20130101; D21H 11/20 20130101;
C08J 3/24 20130101; B01J 19/2405 20130101; B01J 2219/24 20130101;
D21C 9/002 20130101; C08J 3/246 20130101 |
International
Class: |
D21H 11/20 20060101
D21H011/20; C08J 3/24 20060101 C08J003/24; B01J 19/24 20060101
B01J019/24; D21C 9/00 20060101 D21C009/00 |
Claims
1. A system, comprising: a mixing unit being configured to form,
from an aqueous mixture of unbonded cellulose fibers having a
solids content of about 25-55% and a crosslinking agent, a
substantially homogenous mixture of non-crosslinked, unbonded
cellulose fibers and crosslinking agent, at ambient conditions; a
drying unit downstream of the mixing unit and configured to dry the
substantially homogenous mixture to a consistency of 85-100%
without curing the crosslinking agent; and a curing unit downstream
of the drying unit and configured to cure the crosslinking agent,
thereby forming dried and cured crosslinked cellulose fibers.
2. The system of claim 1, wherein the mixing unit includes a first
zone configured to form the aqueous mixture of unbonded cellulose
fibers, and a second zone configured to receive both the aqueous
mixture and the crosslinking agent and form the substantially
homogenous mixture.
3. The system of claim 2, wherein the first and second zones are
two subsequent regions of an extruder.
4. The system of claim 1, wherein the mixing unit includes a high
consistency mixer.
5. A method of forming a crosslinked cellulose product, comprising:
mixing a crosslinking agent with an aqueous mixture of unbonded
cellulose fibers having a solids content and containing little to
no excess water, the crosslinking agent being added in an amount
suitable to achieve a desired level of crosslinking of the unbonded
cellulose fibers based on the solids content; drying the resulting
mixture to 85-100% solids; and curing the dried mixture under
conditions effective to crosslink the unbonded cellulose fibers,
wherein the amount of crosslinking agent added corresponds to a
chemical on pulp range of about 2-14%, and wherein the crosslinking
agent comprises one or more of a polyacrylic acid and a
polycarboxylic acid.
6. The method of claim 5, wherein the aqueous mixture has a solids
content of about 25-55%.
7. The method of claim 6, wherein the aqueous mixture has a solids
content of about 35-55%.
8. The method of claim 6, wherein the aqueous mixture has a solids
content of about 40-50%.
9. The method of claim 5, wherein the crosslinking agent is mixed
with the aqueous mixture of unbonded cellulose fibers at ambient
conditions.
10. The method of claim 5, wherein the crosslinking agent is added
in an amount no more than that required to achieve a desired level
of crosslinking of the unbonded cellulose fibers.
11. The method of claim 5, further comprising, prior to mixing,
processing the aqueous mixture to reduce fiber clumps.
12. The method of claim 5, wherein mixing is performed in one or
more of an extruder, a refiner, and a high-consistency mixer.
13. The method of claim 5, wherein mixing includes adding the cros
slinking agent at a solids content of 10-50% to the aqueous
mixture.
14. The method of claim 5, further comprising, prior to mixing,
forming the aqueous mixture.
15. The method of claim 14, wherein forming the aqueous mixture
includes mixing one or more of wet lap, previously-dried cellulosic
fibers, and never-dried cellulosic fibers, with water.
16. The method of claim 15, wherein forming the aqueous mixture is
performed in one or more of an extruder, a hydrapulper, and a
high-consistency mixer.
17. The method of claim 15, wherein forming the aqueous mixture
includes adding the cellulose fibers in bale or roll form to a
hydrapulper in the presence of water.
18. The method of claim 14, wherein forming the aqueous mixture
includes mixing cellulose fibers with water at a solids content of
about 25-55%.
19. The method of claim 14, wherein forming the aqueous mixture
includes mixing cellulose fibers with water at a solids content of
lower than about 25%, followed by at least partially dewatering the
mixture in order to achieve a solids content of about 25-55%.
20. A method of forming a crosslinked cellulose product,
comprising: forming an aqueous mixture of unbonded cellulose fibers
having a solids content of about 40-50%; mixing a polyacrylic acid
crosslinking agent with the aqueous mixture in an amount to achieve
a chemical on pulp level of about 2-14%, wherein said crosslinking
agent is mixed at ambient conditions; drying the resulting mixture
to 85-100% solids; and curing the dried mixture under conditions
effective to crosslink the unbonded cellulose fibers.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 15/976,654, filed May 10, 2018, which is a continuation of U.S.
application Ser. No. 15/253,450, filed Aug. 31, 2016 (now U.S. Pat.
No. 9,995,000), which is a division of U.S. application Ser. No.
14/320,279, filed Jun. 30, 2014 (now U.S. Pat. No. 9,458,297), the
entire disclosure of each of which is hereby incorporated by
reference herein.
TECHNICAL FIELD
[0002] This disclosure relates to methods of and systems for
forming modified fiber, in particular intrafiber crosslinked
cellulose fiber.
BACKGROUND
[0003] Traditionally, cellulose fibers from southern pine and other
softwood species are used in absorbent products in large part
because the morphology of these fibers provides good absorbent
performance. Compared to hardwood fibers, southern pine and other
softwood fibers tend to be longer (e.g., having a length weighted
fiber length of about 2.5 mm) and more coarse (e.g., having a
coarseness greater than about 20 mg/100 m), and form low density
pads with sufficient void volume to hold several times their weight
in liquid. Hardwood fibers, on the other hand, are known for their
performance in paper applications where shorter fiber length (e.g.,
about 1 mm) and lower coarseness (e.g., less than about 20 mg/100
m) provide a dense structure and smooth paper surface.
[0004] Crosslinked cellulose fibers are usually produced by
applying a crosslinking agent to a dried sheet or roll of
conventional softwood pulp fibers, generally at a dilute
concentration to ensure chemical impregnation of the sheet,
followed by wet fiberization in a hammermill to generate treated,
individualized cellulose fibers. These fibers are then dried, such
as in a flash drier, and cured, such as in an oven. The resulting
fibers exhibit intrafiber crosslinking in which the cellulose
molecules within a cellulose fiber are crosslinked. Intrafiber
crosslinking generally imparts twist and curl to the cellulose
fiber, and also imparts bulk to the fiber, properties that are
advantageous in some absorbent products.
[0005] One drawback of this method is the high capital cost of the
production process, as well as high energy costs due to drying the
fiber prior to curing. Another drawback is that wet hammermilling
can generate fiber and chemical buildup under usual mill conditions
of heat and high airflow. Additionally, wet hammermilling produces
undesirable features such as knots, which are unfiberized fiber
clumps or pieces of the original pulp sheet. Generally, as
production speeds increase, the level of knots also increases as
the hammermilling efficiency is reduced.
SUMMARY
[0006] 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.
[0007] Various embodiments of methods of forming crosslinked
cellulose products, as well as crosslinked cellulose products
formed therefrom, are disclosed herein. The products may include,
for example, individual crosslinked cellulose fibers, as well as
mats, pads, sheets, webs, and the like generally made from
individual crosslinked cellulose fibers.
[0008] In one aspect, the present disclosure provides methods of
forming crosslinked cellulose products that include mixing a
crosslinking agent with an aqueous mixture of unbonded cellulose
fibers having a solids content and containing little to no excess
water. The crosslinking agent is added in an amount suitable to
effect a desired level of crosslinking in the cellulose fibers
based on the solids content. In some methods, the mixing is
performed at ambient conditions. In some methods, the crosslinking
agent is added in an amount no more than that required to effect
the desired level of cros slinking. The methods further include
drying the resulting mixture to 85-100% solids, then curing the
dried mixture to crosslink the cellulose fibers. In some methods,
the solids content of the aqueous mixture is about 25-55%. Some
methods include forming the aqueous mixture, such as by
hydrapulping cellulose fibers from, for example, fibers provided in
bale or roll form.
[0009] In one particular, non-limiting example of such a method, an
aqueous mixture of unbonded cellulose fibers having a solids
content of about 40-50% is formed, followed by mixing a polyacrylic
acid crosslinking agent with the aqueous mixture in an amount to
achieve a chemical on pulp level of about 2-14%, wherein said
crosslinking agent is mixed at ambient conditions. The resulting
mixture is then dried and cured as above.
[0010] In another aspect, the present disclosure provides
embodiments of a system for forming crosslinked cellulose products,
which include a mixer configured to form, from an aqueous mixture
of unbonded cellulose fibers having a solids content of about
25-55% and a crosslinking agent, a substantially homogenous mixture
of non-crosslinked, unbonded cellulose fibers and crosslinking
agent, at ambient conditions. The system further includes,
downstream of the mixer, a dryer configured to dry the
substantially homogenous mixture to a consistency of 85-100%
without curing the crosslinking agent; and a curing unit coupled to
the dryer that is configured to cure the cros slinking agent,
thereby forming dried and cured crosslinked cellulose fibers.
[0011] In another aspect, the present disclosure provides
intrafiber crosslinked cellulose pulp fibers having a having a
chemical on pulp level of about 2-14% and an AFAQ capacity of at
least 12.0 g/g. In some embodiments, the cellulose fibers are, or
include, hardwood cellulose pulp fibers, such as eucalyptus
cellulose pulp fibers.
[0012] The concepts, features, methods, and component
configurations briefly described above are clarified with reference
to the accompanying drawings and detailed description below.
DESCRIPTION OF THE DRAWINGS
[0013] 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:
[0014] FIG. 1 is a schematic representation of an illustrative,
non-limiting embodiment of a system suitable for producing
crosslinked cellulose fibers in accordance with one aspect of the
present disclosure.
[0015] FIG. 2 is a graph representing a relationship between AFAQ
capacity and MUP capacity at 0.3 psi (2.07 kPa) load of several
samples of crosslinked cellulose fibers prepared by methods in
accordance with the present disclosure.
[0016] FIG. 3 is a graph representing a relationship between AFAQ
capacity and COP of crosslinking agent for representative samples
prepared with a 20% polyacrylic acid crosslinking agent according
to one aspect of the present disclosure, compared with samples
prepared according to the conventional approach.
[0017] FIG. 4 is a graph representing a relationship between AFAQ
capacity and COP of crosslinking agent for representative samples
prepared with a variety of crosslinking agents according to one
aspect of the present disclosure.
[0018] FIG. 5 is a graph representing a relationship between AFAQ
capacity and COP of crosslinking agent for representative
eucalyptus samples prepared according to one aspect of the present
disclosure, compared with southern pine samples prepared according
to the conventional approach.
DETAILED DESCRIPTION
[0019] According to one reference, U.S. Pat. No. 5,183,707 to
Herron et al., there are three basic crosslinking processes. The
first may be characterized as dry crosslinking, which is described,
for example, in U.S. Pat. No. 3,224,926 to Bernardin. In a "dry
crosslinking" process, individualized, crosslinked fibers are
produced by impregnating swollen fibers in an aqueous solution with
crosslinking agent, dewatering and defiberizing the fibers by
mechanical action, and drying the fibers at elevated temperature to
effect crosslinking while the fibers are in a substantially
individual state. The fibers are inherently crosslinked in an
unswollen, collapsed state as a result of being dehydrated prior to
crosslinking. These processes produce what are referred to as "dry
crosslinked" fibers. Dry crosslinked fibers are generally highly
stiffened by crosslink bonds, and absorbent structures made
therefrom exhibit relatively high wet and dry resilience. Dry
crosslinked fibers are further characterized by low fluid retention
values (FRV).
[0020] The second type, which is exemplified in U.S. Pat. No.
3,241,553 to Steiger, involves crosslinking the fibers in an
aqueous solution that contains a crosslinking agent and a catalyst.
Fibers produced in this manner are referred to as "aqueous solution
crosslinked" fibers. Due to the swelling effect of water in
cellulosic fibers, aqueous solution crosslinked fibers are
crosslinked while in an uncollapsed, swollen state. Relative to dry
crosslinked fibers, aqueous solution crosslinked fibers, for
example as disclosed in the '553 patent, have greater flexibility
and less stiffness, and are characterized by higher fluid retention
value (FRV). Absorbent structures made from aqueous solution
crosslinked fibers exhibit lower wet and dry resilience than
structures made from dry crosslinked fibers.
[0021] In the third type, which is exemplified in U.S. Pat. No.
4,035,147 to Sangenis et al., individualized, crosslinked fibers
are produced by contacting dehydrated, nonswollen fibers with
crosslinking agent and catalyst in a substantially nonaqueous
solution which contains an insufficient amount of water to cause
the fibers to swell.
[0022] Crosslinking occurs while the fibers are in this
substantially nonaqueous solution. This process produces fibers
referred to herein as "nonaqueous solution crosslinked" fibers.
Such fibers do not swell even upon extended contact with solutions
known to those skilled in the art as swelling reagents. Like dry
crosslinked fibers, nonaqueous solution crosslinked fibers are
highly stiffened by crosslink bonds, and absorbent structures made
therefrom exhibit relatively high wet and dry resilience.
[0023] As explained in more detail herein, the present disclosure
describes an additional, more viable and flexible approach, as
compared to the three described by Herron.
[0024] In general, crosslinked cellulosic fibers can be prepared by
applying a crosslinking agent(s) to cellulosic fibers in an amount
sufficient to achieve intrafiber crosslinking under suitable
conditions (e.g., temperature, pressure, etc.). Several examples of
polyacrylic acid crosslinked cellulosic fibers and examples of
methods for making polyacrylic acid crosslinked cellulosic fibers
are described in U.S. Pat. Nos. 5,549,791, 5,998,511, and
6,306,251. A system and method that may be considered illustrative
of the conventional approach to forming polyacrylic acid
crosslinked cellulosic fibers is disclosed, for example, in U.S.
Pat. Nos. 5,447,977 and 6,620,865. Accordingly, references to the
"conventional approach" refer to the production of crosslinked
cellulose fibers generally in accordance with that in the
aforementioned patents, which follow the "dry crosslinking process"
as described by Herron. Briefly, the system in these patents
includes a conveying device for transporting a mat or web of
cellulose fibers through a fiber treatment zone, an applicator for
applying a crosslinking agent to the fibers at the fiber treatment
zone, a fiberizer for separating the individual cellulose fibers
comprising the mat to form a fiber output comprised of
substantially unbroken and essentially singulated cellulose fibers,
a dryer coupled to the fiberizer for flash evaporating residual
moisture, and a controlled temperature zone for additional heating
of fibers and an oven for curing the crosslinking agent, to form
dried and cured individualized crosslinked fibers.
[0025] Although current commercial processes for producing
crosslinked cellulose fiber products may use different reagents,
reagent quantities, reaction and other process conditions, and so
forth, than those disclosed in the aforementioned '977 and '865
patents, for the purposes of the present disclosure, references
herein to the current commercial process generally refer to the
conventional approach outlined in these patents.
[0026] Various aspects of the conventional approach are described
in more detail in the following paragraphs. The term "mat" refers
to a nonwoven sheet structure comprising cellulose fibers or other
fibers that are not covalently bound together, but are mechanically
entangled and/or bonded by hydrogen bonds. The fibers include
fibers obtained from wood pulp or other sources including cotton
rag, hemp, grasses, cane, cornstalks, cornhusks, or other suitable
sources of cellulose fibers that may be laid into a sheet. The mat
of cellulose fibers is generally in sheet form, and may be one of a
number of baled sheets of discrete size, or may be a continuous
roll.
[0027] Each mat of cellulose fibers is transported by a conveying
device, which carries the mats through the fiber treatment zone,
where a crosslinking agent solution is applied to the mat. The
crosslinking agent solution is applied to one or both surfaces of
the mat using methods including spraying, rolling, dipping, etc.
After the crosslinking agent solution has been applied, the
solution may be uniformly distributed through the mat, for example,
by passing the mat through a pair of rollers.
[0028] The impregnated mat is then wet fiberized by feeding the mat
through a hammermill. The hammermill disintegrates the mat into its
component individual cellulose fibers, which are then air conveyed
through a drying unit to remove the residual moisture.
[0029] The resulting treated pulp is then air conveyed through an
additional heating zone (e.g. a dryer) to bring the temperature of
the pulp to the cure temperature. In one variant, the dryer
includes a first drying zone for receiving the fibers and removing
residual moisture from the fibers via a flash-drying method, and a
second heating zone for curing the crosslinking agent, to allow the
chemical reaction (e.g., esterification, in some embodiments), to
be completed. Alternatively, in another variant, the treated fibers
are blown through a flash dryer to remove residual moisture, heated
to a curing temperature, and then transferred to an oven where the
treated fibers are subsequently cured. Overall, the treated fibers
are dried and then cured for a sufficient time and at a sufficient
temperature to achieve crosslinking.
[0030] As noted above, the conventional and historical approaches
have some disadvantages. For example, in the conventional ("dry
crosslinking") approach the crosslinking solution is generally very
dilute (and correspondingly very low viscosity, generally lower
than 5 cP) in order to better assure complete impregnation of the
chemical into the pulp sheet. The conventional method involves
adding excess chemical, also to better assure complete
impregnation, which presents additional chemical handling concerns.
In addition, wet fiberization, such as by a hammermill, leads to
fiber and chemical buildup under usual mill conditions (sometimes
referred to as contamination), which must be periodically removed,
requiring production downtime. In addition, wet hammermilling tends
to leave knots, with knot count generally increasing as production
speeds increase, correspondingly decreasing hammermilling
efficiency. Moreover, the conventional approach involves high
energy costs due to wet hammermilling and water removal processes
prior to curing the fiber. A downside to aqueous solution
crosslinking is that a recycle/reclaim loop for excess water and
chemical is needed and must be controlled.
[0031] Also, it has been found that the conventional approach is
limited in terms of the types of cellulose fibers suitable for
effective use with the dry crosslinking process, in which fiber
mats are wetted with the aqueous crosslinking solution and then
passed through rollers before being fed to a hammermill and
fiberized. Accordingly, fibers that do not form mats of sufficient
integrity to withstand mechanical manipulation when impregnated
with a liquid tend to be much more difficult, if not impractical,
to process efficiently on standard crosslinking equipment. As noted
above, hardwood fibers are generally not used for absorbent
products or in crosslinked cellulose fiber applications, because of
their fiber morphology. In addition, however, some hardwood fibers,
such as eucalyptus, form mats that fall apart easily when wet, and
thus are not suitable fibers for use in the conventional
approach.
[0032] As discussed in greater detail herein, systems and/or
methods in accordance with the present disclosure may circumvent
the aforementioned disadvantages, as well as providing an approach
that can be used with a comparatively broader range of cellulose
fibers. For example, mixing a crosslinking agent with an aqueous
mixture of unbonded cellulose fibers that contains little to no
excess water can reduce dryer load and avoid the contamination and
knot content issues associated with wet hammermilling It also may
reduce or eliminate the need for a chemical recycle loop. Further,
it has been found that high solids content aqueous fiber mixtures
can be mixed with crosslinking agents of higher concentrations than
are used in the conventional approach, for example in a high
consistency mixer, and still achieve effective chemical
distribution. This is unexpected, considering that materials having
high solids contents have comparatively higher viscosities (e.g.,
10-50 cP or higher), and it has traditionally been found to be
difficult to achieve substantially homogenous mixtures with fiber
when combining high viscosity materials, especially within
practical processing times. In addition, mechanical manipulation of
an impregnated mat is not a requirement, which may further reduce
capital cost as well as present an option to crosslink cellulose
fibers that have low wet tensile strength or structural integrity,
such as those from hardwood species such as eucalyptus, or
cellulose fibers that are not available in sheet or mat form. In
addition, the methods of the present disclosure may be suitable for
cellulose fibers from plant species other than hardwood or softwood
trees, as well as cellulose that has been treated (such as
mercerized fiber, and the like).
[0033] Mixtures of cellulose fibers and water, also referred to
herein as "aqueous mixtures of cellulose fibers" (or, when it is
clear that water is present, simply "fiber") exhibit different
physical characteristics at different concentration ranges. One way
to characterize the concentration ranges is by the manner in which
water is retained by cellulose fibers. For example, cellulose
fibers can bind a certain amount of water in pores within and on
the surface of fibers, and in spaces between fibers. In general,
water is more tightly bound in smaller pores (sometimes called
micropores) than in larger pores (sometimes called macropores).
Concentrations of such mixtures are conventionally expressed in
terms of solids content, which refers, in this context, to the
weight of the cellulose divided by the weight of the mixture of
cellulose and water.
[0034] In general, mixtures that have solids content of up to about
15-25% exhibit fluid characteristics. A mixture in this solids
content range is sometimes referred to as a slurry. Slurries can be
drained of excess water--that is, water that is not held between
fibers by surface tension forces or in fibrous pores--via
gravitational forces and/or applied vacuum.
[0035] At or above a solids content of about 25%, little to no
excess water remains, and the mixture is generally no longer
flowable. Instead, mixtures at or above 25% solids content (and up
to about 40-50%) often take the form of a moist, lumpy aggregate
sometimes referred to as crumb. Although no more water will drain
from such a mixture, free water--that is, water held between fibers
and in large pores--may yet be removed by mechanical pressure, such
as by standard dewatering equipment including screw presses,
extruders, belt presses, roll presses, and so forth.
[0036] The cellulose fibers in the aforementioned slurry and crumb
aqueous mixtures may be characterized as "unbonded"--that is, the
fibers are not chemically bonded together, for example by covalent,
hydrogen, or other types of chemical bonds, although there may be
some mechanical entanglement.
[0037] At a solids content of about 55%, the remaining water is
bound within the fibers, generally within micropores, and must be
removed by evaporation in order to achieve higher solids content
levels. This is generally done by standard drying equipment such as
ovens, float dryers, drum dryers, flash dryers, and so forth.
Evaporation of bound water is generally accompanied by collapse of
the fibers and formation of hydrogen bonds, internal to, and/or
between, fibers.
[0038] The threshold solids content levels that separate these
three concentration ranges will vary to some extent among different
types of cellulose fiber, due to fiber species, the manner in which
the wood was pulped to generate the fibers, whether and to what
extent the pulp is refined, and so forth.
[0039] Aqueous mixtures of cellulose fibers suitable for use in the
present disclosure may be produced by any suitable method, such as
by mixing cellulose, for example that is in roll or bale form, with
water in a hydrapulper or a similar device, to a desired solids
content. This process is sometimes referred to as slushing.
Optionally, pulp in wet lap or other water-containing form (e.g.,
never-dried cellulose fibers) may be used, with water added or
removed if necessary to achieve a desired solids content. In some
methods, an aqueous mixture of a lower solids content is made and
then dewatered to a desired solids content, such as by means of an
extruder. In some methods, the aqueous mixture may be processed to
remove or reduce fiber clumps, such as by feeding the mixture, once
a desired solids content has been achieved, through one or more
lump breakers, pin mills, or using other suitable means.
[0040] In methods in accordance with the present disclosure, the
crosslinking agent is added to the cellulose mixture at a
concentration at which there is little to no excess water--that is,
at a solids content range of about 25-55%. In a manner somewhat
similar to that in the conventional approach to crosslinking
discussed above, the presence of excess water in the aqueous
cellulose mixture when the crosslinking agent is added requires
additional drying time, representing increased energy costs, and
also may result in chemical buildup in the drying equipment,
increasing the possibility of contamination and/or requiring
downtime for removal. In addition, adding crosslinking agent to a
mixture that contains excess water (such as a slurry) may result in
inadvertent loss of some of the crosslinking agent in solution as
it drains from the cellulose fibers, which can be difficult to
monitor and may reduce process efficiency.
[0041] Also, it is theorized that excess water allows the fibers to
swell and allows some crosslinking agents to fully penetrate the
cell wall. This may interfere with fiber stiffness, a desired
quality in crosslinked fibers. The concept of stiffening cellulose
fiber is explained by the I-beam effect. Stiffer fibers are
obtained when crosslinking is limited to the surface of the fibers.
Chemical that penetrates into the cell wall is less efficient at
generating stiffness. Providing an excess of chemical, due to low
chemical solids (excess water), thus reduces chemical efficiency,
as well as process efficiency by requiring a chemical recovery or
recycle loop, and so forth.
[0042] Accordingly, in the methods disclosed herein, the
crosslinking agent is added to the mixture of unbonded cellulose
fibers and water at a solids content range of about 25-55%, which
addresses many of the efficiency drawbacks noted above. In
accordance with the desire to increase process efficiency by
reducing dryer load, a more preferred range is about 35-55%. The
capacity of current mixing equipment able to handle high-solids
materials, such as high consistency mixers, tends to introduce some
practical limitations, and accordingly a most preferred range is
about 40-50%.
[0043] As used herein, the term "crosslinking agent" includes, but
is not limited to, any one of a number of crosslinking agents and
crosslinking catalysts. The following is a representative list of
useful crosslinking agents and catalysts. Each of the patents noted
below is expressly incorporated herein by reference in its
entirety.
[0044] Suitable urea-based crosslinking agents include substituted
ureas such as methylolated ureas, methylolated cyclic ureas,
methylolated lower alkyl cyclic ureas, methylolated dihydroxy
cyclic ureas, dihydroxy cyclic ureas, and lower alkyl substituted
cyclic ureas. Specific urea-based crosslinking agents include
dimethyldihydroxy urea (DMDHU,
1,3-dimethyl-4,5-dihydroxy-2-imidazolidinone),
dimethyloldihydroxyethylene urea (DMDHEU,
1,3-dihydroxymethyl-4,5-dihydroxy-2-imidazolidinone), dimethylol
urea (DMU, bis[N-hydroxymethyl]urea), dihydroxyethylene urea (DHEU,
4,5-dihydroxy-2-imidazolidinone), dimethylolethylene urea (DMEU,
1,3-dihydroxymethyl-2-imidazolidinone), and
dimethyldihydroxyethylene urea (DDI,
4,5-dihydroxy-1,3-dimethyl-2-imidazolidinone).
[0045] Suitable crosslinking agents include dialdehydes such as
C2-C8 dialdehydes (e.g., glyoxal), C2-C8 dialdehyde acid analogs
having at least one aldehyde group, and oligomers of these aldehyde
and dialdehyde acid analogs, as described in U.S. Pat. Nos.
4,822,453, 4,888,093, 4,889,595, 4,889,596, 4,889,597, and
4,898,642. Other suitable dialdehyde crosslinking agents include
those described in U.S. Pat. Nos. 4,853,086, 4,900,324, and
5,843,061.
[0046] Other suitable crosslinking agents include aldehyde and
urea-based formaldehyde addition products. See, for example, U.S.
Pat. Nos. 3,224,926, 3,241,533, 3,932,209, 4,035,147, 3,756,913,
4,689,118, 4,822,453, 3,440,135, 4,935,022, 3,819,470, and
3,658,613.
[0047] Suitable crosslinking agents include glyoxal adducts of
ureas, for example, U.S. Pat. No. 4,968,774, and glyoxal/cyclic
urea adducts as described in U.S. Pat. Nos. 4,285,690, 4,332,586,
4,396,391, 4,455,416, and 4,505,712.
[0048] Other suitable crosslinking agents include carboxylic acid
crosslinking agents such as polycarboxylic acids. Polycarboxylic
acid crosslinking agents (e.g., citric acid, propane tricarboxylic
acid, and butane tetracarboxylic acid) and catalysts are described
in U.S. Pat. No. 3,526,048, 4,820,307, 4,936,865, 4,975,209, and
5,221,285. The use of C2-C9 polycarboxylic acids that contain at
least three carboxyl groups (e.g., citric acid and oxydisuccinic
acid) as crosslinking agents is described in U.S. Pat. Nos.
5,137,537, 5,183,707, 5,190,563, 5,562,740, and 5,873,979.
[0049] Polymeric polycarboxylic acids are also suitable
crosslinking agents. Suitable polymeric polycarboxylic acid
crosslinking agents are described in U.S. Pat. Nos. 4,391,878,
4,420,368, 4,431,481, 5,049,235, 5,160,789, 5,442,899, 5,698,074,
5,496,476, 5,496,477, 5,728,771, 5,705,475, and 5,981,739.
Polyacrylic acid and related copolymers as crosslinking agents are
described in U.S. Pat. Nos. 5,447,977, 5,549,791, 5,998,511, and
6,306,251. Polymaleic acid crosslinking agents are also described
in U.S. Pat. No. 5,998,511.
[0050] Specific suitable polycarboxylic acid crosslinking agents
include citric acid, tartaric acid, malic acid, succinic acid,
glutaric acid, citraconic acid, itaconic acid, tartrate
monosuccinic acid, maleic acid, polyacrylic acid, polymethacrylic
acid, polymaleic acid, polymethylvinylether-co-maleate copolymer,
polymethylvinylether-co-itaconate copolymer, copolymers of acrylic
acid, and copolymers of maleic acid.
[0051] Other suitable crosslinking agents are described in U.S.
Pat. Nos. 5,225,047, 5,366,591, 5,556,976, 5,536,369, 6,300,259,
and 6,436,231.
[0052] Suitable catalysts can include acidic salts, such as
ammonium chloride, ammonium sulfate, aluminum chloride, magnesium
chloride, magnesium nitrate, and alkali metal salts of
phosphorous-containing acids. In one embodiment, the crosslinking
catalyst is sodium hypophosphite. Mixtures or blends of
crosslinking agents and catalysts can also be used.
[0053] The crosslinking agent is added in an amount suitable to
effect a desired level of crosslinking of the unbonded cellulose
fibers based on the solids content. The determination of a desired
level of crosslinking is often based on several considerations,
such as a trade-off between increased fiber stiffness due to
crosslinking and diminished capillary pressure, as well as material
and energy costs, handling concerns, production rates, and so
forth. The amount of crosslinking agent may be characterized as
"chemical on pulp" (or "COP"), which refers to a mass percent. Some
methods in accordance with this disclosure include adding the
crosslinking agent at a COP of about 2-14%, although other COP
levels and/or ranges are within the scope of this disclosure. In
accordance with principles of process efficiency, in some methods,
the amount of crosslinking agent is no more than is required to
achieve the desired level of crosslinking.
[0054] The concentration of the crosslinking agent is generally
selected such that the addition of the agent to the aqueous mixture
does not increase the water content of the resulting mixture beyond
the desired range. For example, a typical concentration range for
polymeric crosslinking agents is about 5-50%.
[0055] On the other hand, a premature decrease in the water content
(that is, prior to drying) of the resulting mixture below the
desired range may also have undesirable effects. With some
crosslinking agents, water removal may result in the mixture
becoming sticky and/or otherwise difficult to handle, resulting in
slower processing. One example of this may be seen with polymeric
crosslinking agents, in which a lack of water may initiate
polymerization, causing the solids content of the mixture to
increase and become sticky. Accordingly, in methods in accordance
with the present disclosure, the crosslinking agent is added to the
aqueous mixture at ambient conditions, defined herein as a set of
conditions (e.g., temperature, pressure, air flow, time, etc.)
under which water loss from the solution is minimized
[0056] The crosslinking agent may be mixed with the aqueous mixture
of unbonded cellulose fibers in any suitable manner, such as in a
high consistency mixer, an extruder (or a region of an extruder,
such as a section of an extruder downstream of a dewatering
section), a refiner, and so forth. One advantage to the use of a
high consistency mixer, in some embodiments, is that a high
consistency mixer not only allows direct injection of the
crosslinking chemistry into the mixture at solids contents of up to
about 50%, but the mixer also may also fluff the fiber to prepare
it for drying. Once mixed, the methods of the present disclosure
include drying the mixture to about 85-100% solids, such as with
standard drying apparatus (e.g., flash dryers, jet dryers, ring
dryers, and so forth, or combinations thereof).
[0057] Curing refers to the initiation and ensuing chemical
reaction that creates chemical bonds between the crosslinking agent
and the cellulose. Crosslinking occurs by different chemical
reactions, depending on the crosslinking agent. For example,
polyacrylic and polycarboxylic acid crosslinking agents typically
establish chemical crosslinks by means of an esterification
reaction when reacted with cellulose. The present disclosure
encompasses methods that proceed not only by esterification
crosslinking reactions, but also by other crosslinking reactions,
such as etherification and so forth, as well as the reaction
conditions suitable for such reactions. Methods in accordance with
the present disclosure proceed by curing the dried mixture under
conditions effective to crosslink the unbonded cellulose fiber in
the mixture. Curing may be accomplished by any suitable manner,
such as those used in the conventional approach, etc.
[0058] With the illustrative methods discussed above in mind,
including the various steps, concepts, and variants therein, FIG. 1
can be seen to be a schematic representation of an illustrative,
non-limiting embodiment of a system, generally indicated at 10,
that is suitable for producing crosslinked cellulosic compositions
in accordance with aspects of the present disclosure.
[0059] System 10 is shown in FIG. 1 to include a series of boxes
connected by arrows. As will be described, the boxes represent
different functional regions, or units, of system 10. The boxes, as
well as the term "unit," are used for convenience, as each
functional unit may be a single component (such as a machine, piece
of equipment, apparatus, and so forth), or part of a larger
component that also incorporates one or more other functional
units, or may represent multiple components that cooperate to
perform the function(s) of the unit, and so forth. Various
functional units and components of system 10 may be co-located,
such as within a single facility (such as a mill), or located
remotely from each other. The system 10 may be any suitable scale,
from lab scale to industrial/commercial. The arrows generally
represent the direction of the material or product produced or
processed by the various functional units, and, accordingly, may
also represent any suitable means of conveying the material from
one unit to another (such as conduits, conveyors, etc.), and/or
other pieces of processing or handling equipment.
[0060] System 10 is shown to include a mixing unit 20 that is
configured to form, from an aqueous mixture of unbonded cellulose
fibers having a solids content of about 25-55% and a crosslinking
agent, a substantially homogenous mixture of non-crosslinked,
unbonded cellulose fibers and crosslinking agent, at ambient
conditions. As noted above, the mixing unit 20 may thus include,
for example, a high consistency mixer or refiner to which an
aqueous fiber mixture and crosslinking agent are added, as well as
any necessary metering and/or delivery equipment for the mixture
components. Accordingly, the fiber 22 and water 24 may be provided
as a mixture, such as an aqueous mixture having the desired solids
content, for example in embodiments in which the aqueous mixture is
formed upstream of the mixing unit 20, and then mixed in mixing
unit 20 with a crosslinking agent 26. As noted above, equipment
(not shown) such as a hydrapulper, extruder, or other suitable
equipment may produce the mixture. Prior to introduction to the
mixing unit 20, such a mixture may be passed, for example, through
one or more pieces of dewatering, processing, and/or handling
equipment (not shown), such as one or more pin mills, screw
presses, refiners, lump breakers, surge bins or hoppers, conveyors,
and so forth.
[0061] Optionally, in some embodiments, the mixing unit 20 may
incorporate such equipment, and be configured to accept the fiber
22 and water 24 as separate materials, such as to produce an
aqueous mixture that is then mixed with crosslinking agent 26. In
such embodiments, the mixing unit may be characterized as including
a first zone (not separately shown) that is configured to form the
aqueous mixture as described above, and a second zone (not
separately shown) that is configured to receive both the aqueous
mixture and the crosslinking agent and form the substantially
homogenous mixture. As an example of such an embodiment, the first
and second zones may be two subsequent regions of an extruder. In
embodiments in which the aqueous mixture is produced, for example,
at low solids content and then dewatered to the desired solids
content for mixing with the crosslinking chemical, the mixing unit
20 may include a water recycle/reclaim loop, such as from a
dewatering device to a hydrapulper.
[0062] The mixing unit is configured to mix the aqueous fiber
mixture with the crosslinking agent, which as noted above may
include one or more crosslinking chemicals and/or catalysts, as
desired, under ambient conditions, that is, process conditions such
as temperature, pressure, air flow, time, etc., under which water
loss from the solution is minimized The term "substantially
homogenous," when used to describe the mixture of cellulose fibers,
water, and crosslinking agent, indicates that the crosslinking
agent is sufficiently well distributed among the fiber so as to
form consistent and uniform crosslinks throughout each fiber when
dried and cured. As noted above, the mixing unit, such as in
embodiments in which the mixing unit includes a high consistency
mixer, may also fluff the fiber (that is, impart an increase in
bulk density) in the mixture. Optionally, the mixing unit may
include other equipment to fluff the mixture prior to drying.
[0063] Downstream of mixing unit 20 is a drying unit 30 configured
to receive the mixture from the mixing unit and dry the mixture to
85-100% solids. Accordingly, drying unit 30 may include one or more
drying devices, such as one or more ovens, float dryers, drum
dryers, flash dryers, jet dryers, and so forth. In some
embodiments, the drying unit 30 may also bring the fibers up to or
near to curing temperature.
[0064] Finally, the dried fibers are received by a curing unit 40
configured to cure the crosslinking agent, thereby forming dried
and crosslinked cellulose fibers. The curing unit thus may
incorporate additional drying devices, ovens, and so forth. In some
embodiments, the drying unit and/or curing unit may incorporate a
holding area, such as to allow the fibers to equilibrate at a set
temperature and/or time, or such equilibration may occur as the
fibers are conveyed from one functional unit to the next. Some
embodiments may include a recycle/reclaim loop for air/heat from
curing equipment to drying equipment.
[0065] Once formed, the crosslinked fibers exit the curing unit 40
and may be subjected to various post treatment processes, indicated
generally at 50, such as to prepare the fibers for shipment or
storage, for example by being baled according to standard methods,
which may include remoisturizing followed by baling, and so
forth.
[0066] Absorbent properties of crosslinked cellulosic compositions
(such as wet bulk, wick time, wick rate, absorbent capacity, and so
forth), may be determined using the Automatic Fiber Absorption
Quality (AFAQ) Analyzer (Weyerhaeuser Co., Federal Way, Wash.). A
standard testing procedure is described in the following
paragraphs.
[0067] A 4-gram sample (conditioned at 50% RH and 73.degree. F.
(23.degree. C.) for at least 4 hours) of the pulp composition is
placed through a pinmill to open the pulp, and then airlaid into a
tube. The tube is 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. The weight is increased
to achieve a pressure of 2.5 kPa and the bulk is recalculated. The
result is two bulk measurements on the dry fluff pulp at two
different pressures.
[0068] While under the plunger 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 to
reach the plunger is measured. From this, wick time and wick rate
may be determined. The bulk of the wet pad at 2.5 kPa may also be
calculated. 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. Absorbent capacity (or "AFAQ capacity")
may be calculated by weighing the wet pad after water is drained
from the equipment, and is reported as grams water per gram dry
pulp.
[0069] Maximum uptake ("MUP") of a fiber sample is also a capacity
type value, but as measured under a different load. Capillary
pressure measurements of a sample porous material are made on a
TRI/Autoporosimeter (TRI/Princeton Inc. of Princeton, N.J.) to
determine pore volumes and pore size distributions (see, e.g.,
EP2407133A1; see also The Journal of Colloid and Interface Science,
Vol. 162, Issue 1 (January 1994), pp 163-170; the disclosures of
both are incorporated herein by reference). As used in this
application, determining the 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) correspond to the
capillary pressure. 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 (drained) is measured with a balance at each
pressure increment.
[0070] A standard testing procedure is performed at 23.degree.
C..+-.2.degree. C. (73.degree. F.) and a relative humidity of
50%.+-.5%. The test is run using a 0.9% saline solution. The
surface tension (mN/m), contact angle (.degree.), and density
(g/cc) is determined by any method known in the art and provided to
into the instrument's software (in this case the values used are
72, 0, and 1, respectively). The balance leveled at 156.7 g and
equilibration rate set to 90 mg/min The pore radius protocol
(corresponding to capillary pressure steps) to scan capillary
pressures, according to equation R=2y cos .theta./.DELTA.p, is
assigned, where: R is the pore radius, y is the surface tension,
.theta. is the contact angle, and p is the capillary pressure. For
example, a set of pore radius (R) steps for first absorption
(pressure decreasing) are 25, 74, 98, 108, 120, 136, 156, 184, 245,
368, 735, 1470, 2940; and for desorption (pressure increasing) are
1470, 735, 490, 368, 147, 82, 65, 54, 47, 42, 25. A 0.5 g sample is
cut into a 52 mm diameter circular specimen, then conditioned at
23.degree. C..+-.2.degree. C. (73.degree. F.) and a relative
humidity 50%.+-.5% for minimum four (4) hours before testing. The
weight is measured, to .+-.0.0001, and the specimen is placed at
the center of the membrane (MF.TM. cellulose nitrate membrane
filter type 8.0 micron SCWP available from Merck Millipore Ltd.,
Cork, Ireland). The desired load (0.2 psi or 1.38 kPa) is added
onto the sample and the chamber is closed. After the instrument has
applied the appropriate air pressure to the cell, the liquid valve
to allow free movement of liquid to the balance is opened and the
test under the radius protocol begins. The instrument proceeds
through one absorption/desorption cycle. A blank (without sample
specimen) is run in like fashion.
[0071] The mass uptake from a blank run is directly subtracted from
the uptake of the sample at each target pore radius (pressure). The
maximum uptake is the maximum value of liquid absorbed by the
sample that corresponds to the lowest pressure. Saturation at each
capillary pressure step is automatically calculated from liquid
uptake as follows: S=m.sup.1/m.sup.1.sub.max where: S=saturation,
m.sup.1=liquid uptake at the pressure step (mL), and
m.sup.1.sub.max=maximum liquid uptake (mL). Pressure is reported in
cm of water and saturation in %. Data from the first absorption
curve and the desorption curve are used. The liquid absorbed by the
sample at 100% saturation is the maximum uptake. The maximum liquid
uptake in mL provided by the instrument is divided by the liquid
density to provide the liquid weight in grams. The maximum uptake
in grams is divided by the dry sample weight in grams to obtain the
reported value in g/g.
[0072] The aforementioned example embodiments are illustrative of
any number of suitable application methods, as well as combinations
thereof, all of which are understood to be encompassed by the
present disclosure.
[0073] The following examples summarize representative,
non-limiting embodiments and methods of forming crosslinked
cellulose products 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
Lab Scale with PAA and Variable Fiber Solids Content
[0074] Southern pine fiber (CF416, Weyerhaeuser NR Company) was
slushed in a laboratory pulper in 1000 g (OD) batches and then
dewatered in a laboratory centrifuge. The dewatered fiber was
broken down into smaller fiber bundles using a laboratory pin mill.
The solids content of the fiber was measured, and then the desired
amount of fiber for the test was fed via conveyor into a hopper. A
screw at the bottom of the hopper fed the fiber into a laboratory
Sprout refiner fitted with traditional refining plates (C2976) in a
vertical configuration, with the gap set to minimize any fiber
cutting (generally 0.050''-0.300''). Crosslinking chemical
(polyacrylic acid ("PAA") polymer) at 20% solids content was
applied via a chemical port located at the end of the screw
immediately before the fiber enters the refiner. Fiber was
delivered at a fixed rate. The chemical pump speed was changed to
achieve the required chemical addition level. The treated fiber
exited the refiner into a plastic beaker. The fiber was dried in a
Fluid Energy 4'' ThermaJet.TM. jet dryer with a target inlet
temperature of 356.degree. F. (180.degree. C.). Dried fiber was
equilibrated at room temperature before curing at 370.degree. F.
(187.8.degree. C.) for 5 minutes.
[0075] Samples were compared to a commercial control (CMC530,
available from Weyerhaeuser NR Company) prepared under the same
chemical loading and curing conditions, but according to the
conventional method. Representative samples and their corresponding
AFAQ capacity results at constant COP are shown in Table 1 (Sample
UC represents the untreated control, and Sample CC-PAA represents
the commercial control using PAA). Table 1 indicates not only that
effective crosslinking was achieved at high solids, but also that
the AFAQ capacity of samples prepared according to the high solids
methods of the present disclosure is unexpectedly greater as
compared to samples prepared according to the conventional
method.
TABLE-US-00001 TABLE 1 Incoming Fiber AFAQ Sample ID Solids, %
capacity (g/g) Crosslinked? Sample UC n/a 12.6 No Sample CC-PAA n/a
16.7 Yes Sample 1A 35.0 18.7 Yes Sample 1B 39.1 18.9 Yes Sample 1C
41.6 18.9 Yes Sample 1D 43.8 18.4 Yes Sample 1E 43.9 18.6 Yes
EXAMPLE 2
Lab Scale with Citric Acid
[0076] The process of Example 1 was followed using a 18%
polycarboxylic acid (citric acid) solution as the crosslinking
agent. Average solids content of the fiber was 43.3%. Curing
conditions were 340.degree. F. (171.1.degree. C.) for 5 minutes.
AFAQ capacity results for a representative sample are compared with
a commercial control (CMC520, available from Weyerhaeuser NR
Company) using the same citric acid crosslinking agent and with an
untreated control in Table 2 (Sample CC-citric represents the
commercial control using citric acid crosslinker). Again, the AFAQ
capacity of Sample 2, prepared according to the high solids methods
of the present disclosure, is unexpectedly greater as compared to
the commercial control.
TABLE-US-00002 TABLE 2 Sample ID AFAQ capacity (g/g) Crosslinked?
Sample UC 12.6 No Sample CC-citric 14.7 Yes Sample 2 15.7 Yes
EXAMPLE 3
Lab Scale with Variable PAA Solids Content
[0077] The process of Example 1 was conducted over an incoming
solids content range of the polyacrylic acid crosslinking agent of
16.8-40% while keeping the incoming solids content of the fiber
constant. For each sample, several levels of crosslinking agent
were applied. To assess performance, the relationship between two
capacity methods (maximum uptake, or "MUP," and AFAQ capacity) was
compared. For all samples, the same relationship was observed to
apply regardless of the beginning solution strength or crosslinking
agent.
[0078] FIG. 2 is a graph generally showing the correlation between
AFAQ capacity (which increases from left to right on the x-axis)
and MUP capacity at 0.3 psi (2.07 kPa) load (which increases from
bottom to top on the y-axis) of several samples prepared as
described in Example 3 (i.e., with PAA crosslinking agent, over a
range of COP of about 2-14%), and also includes samples prepared as
described in Example 2 (i.e., with 18% citric acid crosslinking
agent, similar COP range), as well as lab controls.
EXAMPLE 4
Lab Scale with Alternative Process Configuration)
[0079] Southern pine fiber (CF416, Weyerhaeuser NR Company) was
slushed in a laboratory pulper in 1000 g (OD) batches, then
dewatered in a laboratory centrifuge. The dewatered fiber was
broken down into smaller fiber bundles using a laboratory pin mill.
The solids content of the fiber was measured to be 43.8%. This
fiber was fed via conveyor into a hopper. A screw at the bottom of
the hopper fed the fiber into a laboratory Sprout refiner fitted
with "devil tooth" mixing plates (C2975A), with the gap was set to
minimize any fiber cutting. PAA crosslinking chemical at 20% solids
content was applied via a chemical port located at the end of the
screw immediately before the fiber enters the refiner. Chemical was
delivered at a fixed rate. The conveyor speed was changed to
achieve the target fiber feed rate to provide the required chemical
dosage. The treated fiber exited the refiner into a plastic bucket.
The fiber was dried in a Fluid Energy Processing & Equipment
Company 4'' ThermaJet.TM. dryer with a target inlet temperature of
356.degree. F. (180.degree. C.). Dried fiber was equilibrated at
room temperature before curing at 370.degree. F. (187.8.degree. C.)
for 5 minutes.
[0080] The product was compared to a commercial control (CMC530)
prepared under the same chemical loading and curing conditions. The
results are shown in Table 3, in which Sample 4 represents the
product and Sample 1D data is also included for comparison.
TABLE-US-00003 TABLE 3 Incoming Fiber AFAQ capacity Sample ID
Solids, % (g/g) Crosslinked? Sample UC n/a 12.6 No Sample CC-PAA
n/a 16.7 Yes Sample 4 43.8 16.8 Yes Sample 1D 43.8 18.4 Yes
EXAMPLE 5
Pilot Scale
[0081] A pilot scale trial using a process in accordance with that
of Example 1 was conducted. Southern pine fiber (CF416,
Weyerhaeuser NR Company) was slushed in a pilot pulper (Black
Clawson 300 gallon capacity) and then fed to a commercial screw
press (Press Technology and Manufacturing Inc, Model 08L200) for
dewatering. Fiber chunks from the screw press were broken apart by
a rotary pin mill. The solids content of the fiber was measured to
be 41.6%. The fiber was placed into a volumetric feeder with a 6''
screw. Metered fiber was dropped onto a conveyer that deposited the
fiber into the inlet of an high consistency ("HC") mixer
(manufactured by Andritz) using C2975A mixer plates in a horizontal
configuration with the gap was set to minimize any fiber cutting.
Crosslinking chemical (22.3% PAA solution) was pumped into the
inlet of the mixer via an injection port. The chemical feed rate
was adjusted to provide the required chemical addition level. The
treated fiber exited the commercial mixer into a drum, and then
dried in a Fluid Energy Processing & Equipment Company 4''
ThermaJet.TM. dryer. Dried fiber was equilibrated at room
temperature before curing at 380.degree. F. (193.3.degree. C.) for
8 minutes. Performance of the pilot scale trial matched the lab
results at the same target chemical loading and curing conditions,
as shown in Table 4.
TABLE-US-00004 TABLE 4 Incoming Fiber AFAQ capacity Sample ID
Solids (%) (g/g) Crosslinked? Sample 5 (pilot) 41.6 17.7 Yes Sample
5 (lab) 41.6 17.4 Yes
EXAMPLE 6
Lab Scale with Variable COP
[0082] The process of Example 1 was conducted over a range of
crosslinking chemical addition levels, using 20% PAA. As shown in
FIG. 3, AFAQ capacity for such fibers over a range of about 2-14%
was found to be improved over crosslinked cellulose fibers produced
according to the commercial method under similar conditions (of
reagent amounts, times, and so forth). Samples prepared according
to Example 1 are indicated as "new process" data points, whereas
the commercial method prepared samples are indicated as "current
process" data points.
[0083] FIG. 4 shows the general trend of AFAQ capacity (which
increases from bottom to top on the y-axis) to increase as COP
increases over the range tested, with both PAA crosslinking agents
(over a range of solids content of the crosslinking agent) and
citric acid crosslinking agent.
EXAMPLE 7a
Alternative Pulps
[0084] Never-dried Douglas fir wet lap, obtained from Weyerhaeuser,
was slushed, dewatered, processed, mixed with PAA at 20%, and dried
and cured in accordance with the process of Example 6. Because the
conventional process uses once-dried pulp sheets, a sample
following the conventional process was not prepared with wet lap
(which is never-dried pulp). Results (representative sample shown
in Table 5) showed favorable AFAQ capacity of crosslinked fibers
formed from wet lap.
TABLE-US-00005 TABLE 5 Incoming Fiber AFAQ capacity Sample ID
Solids (%) (g/g) Crosslinked? Sample 7-FIR 37.7 17.0 Yes
EXAMPLE 7b
Alternative Pulps
[0085] Eucalyptus pulp (Bleached Eucalyptus Kraft Pulp available
from Fibria Veracel mill, Brazil) was processed in accordance with
the process of Example 4. Representative samples (Sample 7-EUC
represents eucalyptus, and Sample UC-EUC represents untreated
eucalyptus) are shown in Table 6. For comparison, Table 6 also
includes data for a commercial control (CMC530, indicated as Sample
CC-PAA), and a lab control prepared from CF416 and crosslinked
according to the same method at the same COP as Sample 7-EUC
(indicated as Sample LC-PAA).
TABLE-US-00006 TABLE 6 Incoming Fiber Solids AFAQ capacity Sample
ID (%) (g/g) Crosslinked? Sample CC-PAA n/a 16.7 Yes Sample LC-PAA
41.8 16.8 Yes Sample UC-EUC n/a 12.0 Yes Sample 7-EUC 41.2 17.4
Yes
[0086] Table 6 indicates not only that effective crosslinking was
achieved with eucalyptus pulp, but also that the AFAQ capacity
thereof exceeds that of conventionally produced southern pine
crosslink.
[0087] Several samples were prepared, varying the COP levels for
the eucalyptus fibers. FIG. 5 is a graph showing that the AFAQ
capacity of eucalyptus fibers crosslinked according to methods of
the present disclosure (indicated as "Eucalyptus" in FIG. 5) were
superior to that of southern pine kraft pulp produced according to
the conventional crosslink method (indicated as "Southern Pine" in
FIG. 5) over a sample COP range.
[0088] Although the present invention has been shown and described
with reference to the foregoing operational principles and
illustrated examples and embodiments, it will be apparent to those
skilled in the art that various changes in form and detail 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.
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