Note: Descriptions are shown in the official language in which they were submitted.
13~043~
PROCESS FOR MAKING INDIVIDUALIZED,
CROSSLINKED FIBERS HAVING REDUCED
RESIDUALS AND FIBERS THEREOF
FIELD OF INVENTION
This invention is concerned with processes for making
individualized, crosslinked fibers. More specifically, this invention
is concerned with processes for making such fibers wherein reduced
S levels of residual crosslinking agent are obtained.
BACKGROUND OF THE INVENTION
Fibers crosslinked in substantially individualized form and
various methods for making such fibers have been described in the
art. The term "individualized, crosslinked fibers", refers to
10 cellulosic fibers that have primarily intrafiber chemical crosslink
bonds. That is, the crosslink bonds are primarily between cellulose
molecules of a single fiber, rather than between cellulose molecules
of separate fibers. Individualized, crosslinked fibers are generally
regarded as being useful in absorbent product applications. In
15 general, three categories of processes have been reported for
making individualized, crosslinked fibers. These processes,
~'
.. ., . ~ .. ..
2 1~043~
described below, are herein referred to as 1 ) dry crosslinking
processes, 2) aqueous solution crosslinking processes, and 3)
substantially non-aqueous solution crosslinking processes. The
fibers themselves and absorbent structures containing
S individualized, crosslinked fibers generally exhibit an improvement
in at least one significant absorbency propertv relative to
conventional, uncrosslinked fibers. Often, this improvement in
absorbency is reported in terms of absorbent capacity.
Additionally, absorbent structures made from individualized
10 crosslinked fibers generally exhibit increased wet resilience and
increased dry resilience relative to absorbent structures made from
uncrosslinked fibers. The term "resilience" shall hereinafter refer
to the ability of pads made from cellulosic fibers to return toward
an expanded original state upon release of a compressional force.
15 Dry resilience specifically refers to the ability of an absorbent
structure to expand upon release of compressional force applied
while the fibers are in a substantially dry condition. Wet resilience
specifically refers to the ability of an absorbent structure to
expand upon release of compressional force applied while the fibers
20 are in a moistened condition. For the purposes of this invention
and consistency of disclosure, wet resilience shall be observed and
reported for an absorbent structure moistened to saturation.
Processes for making individualized, crosslinked fibers with
dry crosslinking technology are described in U.S. Patent No.
~5 3,224,926 issued to L. J. Bernardin on December 21, 1965.
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
30 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. Processes as
exemplified in U.S. Patent Nos. 3,224,926, wherein crosslinking is
caused to occur while the fibers are in an unswollen, collapsed
35 state, are referred to as processes for making "dry crosslinked"
fibers. Dry crosslinked fibers are characterized by low fluid
retention values ( FRV) . It is suggested in U . S. Patent No.
134043~
3,440,135, issued to R. Chung on April 22, 1969, to soak the fibers
in an aqueous solution of a crosslinking agent to reduce interfiber
bonding capacity prior to carrying out a dry crosslinking operation
similar to that described in U.S. Patent No. 3,224,926. This time
5 consuming pretreatment, preferably between about 16 and 48 hours,
is alleged to improve product quality by reducing nit content
resulting from incomplete defibration.
Processes for producing aqueous solution crosslinked fibers are
disclosed, for example, in U . S . Patent No. 3,241,553, issued to F .
10 H. Steiger on March 22, 1966. Individualized, crosslinked fibers
are produced by crosslinking the fibers in an aqueous solution
containing a crosslinking agent and a catalyst. Fibers produced in
this manner are hereinafter referred to as "aqueous solution
crosslinked" fibers. Due to the swelling effect of water on
15 cellulosic fibers, aqueous solution crosslinked fibers are crosslinked
while in an uncollapsed, swollen state. Relative to dry crosslinked
fibers, aqueous solution crosslinked fibers as disclosed in U. S.
Patent No. 3,241,553 have greater flexibility and less stiffness, and
are characterized by higher fluid retention value ( FRV) .
20 Absorbent structures made from aqueous solution crosslinked fibers
exhibit lower wet and dry resilience than pads made from dry
crosslinked fibers.
In U.S. Patent No. 4,035,14', issued to S. Sangenis, G.
Guiroy and J . Quere on July 12, 1977, a method is disclosed for
25 producing individualized, crosslinked fibers 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. Crosslinking occurs
while the fibers are in this substantially nonaqueous solution. This
30 type of process shall hereinafter be referred to as a nonaqueous
solution crosslinked process; and the fibers thereby produced, shall
be referred to as nonaqueous solution crosslinked fibers. The
nonaqueous solution crosslinked fibers disclosed in U.S. Patent
4,035,147 do not swell even upon extended contact with solutions
35 known to those skillec' in the art as swelling reagents. Like dry
crosslinked fibers, they are highlv stiffened by crosslink bonds,
.. . ... . . ..
4 1~0~34
and absorbent structures made therefrom exhibit relatively
high wet and dry resilience.
Crosslinked fibers as described above are believed to be
useful for lower density absorbent product applications
such as diapers and also higher density absorbent product
applications such as catamenials. However, such fibers
have not provided sufficient absorbency benefits, in view
of their detriments and costs, over conventional fibers to
result in significant commercial success. Commercial
appeal of crosslinked fibers has also suffered due to
safety concerns. The most widely referred to crosslinking
agent in the literature, formaldehyde, unfortunately causes
irritation to human skin and has been associated with other
human safety concerns. Removal of free formaldehyde to
lS sufficiently low levels in the crosslinked product such
that irritation to skin and other human safety concerns are
avoided has been hindered by both technical and economic
barriers.
Crosslinked fibers made from crosslinking agents other
than formaldehyde may also have residual levels of
crosslinking agent which are higher than preferred for
applications in the vicinity of human skin. Therefore, it
may be desirable to treat the fibers subsequent to
crosslinking in such a way to reduce the level of unstable,
residual crosslinking agent on the fibers. One method for
doing this is to wash the fibers with water subsequent to
crosslinking. This method is effective, but does not
~ f
13~0~3~
4a
reduce the level of residual crosslinking agent as low as
may be desirable. Additionally, such post-crosslinking
washes necessitate additional capital and operating costs
associated with the washing and drying of the fibers.
It is an oblect of an aspect of this invention to
provide commercially viable individualized crosslinked
fibers and absorbent structures made from such fibers, as
described above, which can be safely utilized in the
vicinity of human skin.
It is an object of an aspect of this invention to
provide a process for making individualized, crosslinked
fibers having low
1~043~
levels of residual crosslinking agent wherein capital investrr ent for
equipment and related operating costs are minimized.
SUMMARY OF THE INVENTION
It has been found that the objects identified above may be met
5 and individuali2ed, crosslinked fibers having reduced levels of
unstable, residual crosslinking agent may be obtained by practicing
the following process, which comprises the steps of:
~ a) providing cellolosic fibers and contacting the fibers with a
crosslinking agent
b) reacting the crosslinking agent with the fibers in the
substantial absence of interfiber bonds, to form intrafiber
crosslink bonds; and
c) washing the fibers with an alkaline solution.
The crosslinking agent is preferably an aldehyde crosslinking
15 agent selected from the group consisting of C2 - C8 dialdehydes,
C2 ~ C8 dialdehyde acid analogues containing at least one aldehyde
group, and oligomers of said dialdehydes and dialdehyde acid
analogues .
Preferably, the fibers are washed with an alkaline solution
;~0 having a pH greater than about 9. Also, preferably, the fibers are
contacted with a sufficient amount of one of the preferred
crosslinking agents such that between about 0. 5 mole % and about
3.5 mole % crosslinking agent, calculated on a cellulose
anhydroglucose molar basis, are reacted with said fibers and said
25 fibers have a water retention value of less than about 60.
Surprisingly, fibers made according to the present invention
have higher water retention values than fibers having the same
level of crosslinking --.ade by otherwise equivalent crosslinking
.. . . . . ., . ,, .. ~ , . .
6 13404~
processes performed on fully bleached fibers.
Corresponding absorbent structures made from the fibers
of the present invention have higher absorbency
properties, including wet resiliency and responsiveness
to wetting.
Another aspect of this invention is as follows:
A process for making individualized, crosslinked,
cellulosic fibers, said process comprising the steps of:
a. providing cellulosic fibers and contacting
said fibers with a crosslinking agent in an
aqueous solution, said crosslinking agent
being selected from the group consisting of
C2-C8 dialdehydes, acid analogues of said
dialdehydes derived by having one aldehyde
group of each said dialdehyde replaced by a
carboxyl group, and oligomers of said
dialdehydes and said acid analogues, said
fibers being contacted with a sufficient
amount of said crosslinking agent that, when
said crosslinking agent has been reacted,
between about 0.5 mole % and about 3.5 mole %
crosslinking agent, calculated on a cellulose
anhydroglucose molar basis, reacts with said
fibers so that said fibers have a water
retention value of from about 28 to about 45;
b. dewatering said fibers to a consistency of
from about 30% to about 80% of fibers by
weight, and defibrating said fibers into
substantially individual form;
c. air drying said fibers while individualized
under conditions that obviate fiber to fiber
contact and reacting said crosslinking agent
with said fibers to form intrafiber crosslink
bonds in the substantial absence of interfiber
bonds; and
B
6a 13~0~3~
d. reducing the level of unstable and unreacted
crosslinking agent on the individualized
fibers by washing such fibers with an alkaline
solution, said alkaline solution having a pH
greater than about 7 and comprising a
constituent for decomposing hemiacetal bonds
while being neutral towards acetal bonds.
DETAILED DESCRIPTION OF THE INVENTION
Cellulosic fibers of diverse natural origin are
applicable to the invention. Digested fibers from
softwood, hardwood or cotton linters are preferably
utilized. Fibers from Esparto grass, bagasse, kemp,
flax, and other lignaceous and cellulosic fiber sources
may also be utilized as raw material in the invention.
The fibers may be supplied in slurry, unsheeted or
sheeted form. Fibers supplied as wet lap, dry lap or
other sheeted form are preferably rendered into
unsheeted form by mechanically disintegrating the sheet,
preferably prior to contacting the fibers with the
crosslinking agent. Also, preferably the fibers are
provided in a wet or moistened condition. Most
preferably, the fibers are never-dried fibers. In the
case of dry lap, it is advantageous to moisten the
fibers prior to mechanical disintegration in order to
minimize damage to the fibers.
The optimum fiber source utilized in conjunction
with this invention will depend upon the particular end
use contemplated. Generally, pulp fibers made by
chemical pulping processes are preferred. Completely
bleached, partially bleached and unbleached fibers are
applicable. It may frequently be desired to utilize
bleached pulp for its superior brightness and consumer
appeal. In one novel embodiment of the invention,
hereinafter more fully described, the fibers are
partially bleached, crosslinked, and then bleached to
B
6b 1340434
completion. For products such as paper towels and
absorbent pads for diapers, sanitary napkins,
catamenials, and other similar absorbent paper products,
it is especially preferred to utilize fibers from
southern softwood pulp due to their premium absorbency
characteristics.
B ~
1~0~3~
Crosslinking agents applicable to the present development
inctude C2 - C8 dialdehydes, as well as acid analogues of such
dialdehydes wherein the acid analogue has at least one aldehyde
group, and oligomers of such dialdehydes and acid analogues.
5 These compounds are capable of reacting with at least two hydroxyl
groups in a single cellulose chain or on proximately located cellulose
chains in a single fiber. Those knowledgeable in the area of
crosslinking agents will recognize that the dialdehyde crosslinking
agents described above will be present, or may react in a variety
~ 10 of forms, including the acid analogue and oligomer forms identified
above. All such forms are meant to be included within the scope of
the invention. Reference to a particular crosslinking agent shall
therefore hereinafter refer to that particular crosslinking agent as
well as other forms as mav be present in an aqueous solution.
15 Particular crosslinking agents contemplated for use with the
invention are glutaraldehyde, glyoxal, and glyoxylic acid.
Glutaraldehyde is especially preferred, since it has provided fibers
with the highest levels of at-sorbency and resiliency, is believed to
be safe and non-irritating to human skin when in a reacted,
20 crosslinked condition, and has provided the most stable, crosslink
bonds. Monoaldehydic compounds not having an additional
carboxylic group, such as acetaldehyde and furfural, have not been
found to provide absorbent structures with the desired levels of
absorbent capacity, resilience, and responsiveness to wetting.
;~5 It has been unexpectedly discovered that superior absorbent
pad performance may be obtained at crosslinking levels which are
substantially lower than crosslinking levels previously practiced.
In general, unexpectedly good results are obtained for absorbent
pads made from individualized, crosslinked fibers having between
30 about 0.5 mole % and about 3.5 mole % crosslinking agent, calculated
on a cellulose anhydroglucose molar basis, reacted with the fibers.
Preferably, the crosslinking agent is contacted with the fibers
in a liquid medium, under such conditions that the crosslinking
agent penetrates into the interior of the individual fiber structures.
35 However, other methoc', of crosslinking agent treatment, including
8 13~0~34
spraying of the fibers while in individualized, fluffed form, are also
within the scope of the invention.
Generally, the fibers will also be contacted with an appropriate
catalyst prior to crosslinking. The type, amount, and method of
5 contact of catalyst to the fibers will be dependent upon the
particular crosslinking process practiced. These variables will be
discussed in more detail below.
Once the fibers are treated with crosslinking agent and
catalyst, the crosslinking agent is caused to react with the fibers
10 in the substantial absence of interfiber bonds, i.e., while
interfiber contact is maintained at a low degree of occurrence
relative to unfluffed pulp fibers, or the fibers are submerged in a
solution that does not facilitate the formation of interfiber bonding,
especially hydrogen bonding. This results in the formation of
15 crosslink bonds which are intrafiber in nature. Under these
conditions, the crosslinking agent reacts to form crosslink bonds
between hydroxyl groups of a single cellulose chain or between
hydroxyl groups of proximately located cellulose chains of a single
cellulosic fiber.
Although not presented or intended to limit the scope of the
invention, it is believed that the crosslinking agent reacts with the
hydroxyl groups of the cellulose to form hemiacetal and acetal
bonds. The formation of acetal bonds, believed to be the desirable
bond types providing stable crosslink bonds, is favored under
25 acidic reaction conditions. Therefore, acid catalyzed crosslinking
conditions are highly preferred for the purposes of this invention.
The fibers are preferably mechanically defibrated into a low
density, individualized, fibrous form known as "fluff" prior to
reaction of the crosslinking agent with the fibers. Mechanical
~() defibration may be performed by a variety of methods which are
presently known in the art or which may hereinafter become known.
Mechanical defibration is preferably performed by a method wherein
knot formation and fiber damage are minimized. One type of device
which has been found to be particularly useful for defibrating the
., . .. , .. .... .. , . .. .. ... ~ .. ..
13~0~3~
cellulosic fibers is the three stage fluffing device
described in U. S. Patent No. 3,987,968, issued to D. R.
Moore and 0. A. Shields on October 26, 1976. The fluffing
device described in U. S. Patent No. 3,987,968 subjects
moist cellulosic pulp fibers to a combination of
mechanical impact, mechanical agitation, air agitation and
a limited amount of air drying to create a substantially
knot-free fluff. The individualized fibers have imparted
thereto an enhanced degree of curl and twist relative to
the amount of curl and twist naturally present in such
fibers. It is believed that this additional curl and twist
enhances the resilient character of absorbent structures
made from the finished, crosslinked fibers.
Other applicable methods for defibrating the cellulosic
fibers include, but are not limited to, treatment with a
WaringT~blender and tangentially contacting the fibers with
a rotating disk refiner or wire brush. Preferably, an air
stream is directed toward the fibers during such
defibration to aid in separating the fibers into
substantially individual form.
Regardless of the particular mechanical device used to
form the fluff, the fibers are preferably mechanically
treated while initially containing at least about 20%
moisture, and preferably containing between about 40% and
about 60% moisture.
Mechanical refining of fibers at high consistency or of
partially dried fibers may also be utilized to provide curl
or twist to the fibers in addition to curl or twist
imparted as a result of mechanical defibration.
~. ~ .
1~4043~
9a
The fibers made according to the present invention have
unique combinations of stiffness and resiliency, which
allow absorbent structures made from the fibers to maintain
high levels of absorptivity, and exhibit high levels of
resiliency and an expansionary responsiveness to wetting of
a dry, compressed absorbent structure. In addition to
having the levels of crosslinking within the stated ranges,
the crosslinked fibers are characterized by having water
retention values (WRV's) of less than
13~0~
about 60, and preferably between about 28 and 45, for
conventional, chemically pulped, papermaking fibers. The WRV of a
particular fiber is indicative of the level of crosslinking and the
degree of swelling of the fiber at the time of crosslinking. Those
5 skilled in the art will recognize that the more swollen a fiber is at
the time of crosslinking, the higher the WRV will be for a given
level of crosslinking. Very highly crosslinked fibers, such as
those produced by the prior known dry crosslinking processes
previously discussed, have been found to have WRV's of less than
10 about 25, and generally less than about '0. The particular
crosslinking process utilized will, of course, affect the WRV of the
crosslinked fiber. However, any process which will result in
crosslinking levels and WRV's within the stated limits is believed to
be, and is intended to be, within the scope of this invention.
lS Applicable methods of crosslinking include drv crosslinking
processes and nonaqueous solution crosslinking processes as
generally discussed in the Background Of The Invention. Certain
preferred dry crosslinking and nonaqueous solution crosslinking
processes, within the scope of the present invention, will be
20 discussed in more detail below. Aqueous solution crosslinking
processes wherein the solution causes the fibers to become highly
swollen will result in fibers having WRV's which are in excess of
about 60. These fibers will provide insufficient stiffness and
resiliency for the purposes of the present invention.
2~ Specifically referring to dry crosslinking processes,
individualized, crosslinked fibers may be produced from such a
process by providing a quantity of cellulosic fibers, contacting a
slurry of the fibers with a type and amount of crosslinking agent
as described above, mechanically separating, e.g., defibrating, the
30 fibers into substantially individual form, and drying the fibers and
causing the crosslinking agent to react with the fibers in the
presence of a catalyst to form crosslink bonds while the fibers are
maintained in substantially individual form. The defibration step,
apart from the drying step, is believed to impart additional curl.
:~5 Subsequent drying is accompanied by twisting of the fibers, ~ith
the degree of twist be ng enhanced by the curled geometry of the
fiber. As used herein, fiber "curl" refers to a geometric curvature
ll 13~0~3~
of the fiber about the longitudinal axis of the fiber. "Twist" refers
to a rotation of the fiher about the perpendicular cross-section of
the longitudinal axis of the fiber. For exemplary purposes onlv,
and without intending to specifically limit the scope of the
5 invention, individualized, crosslinked fibers within the scope of the
invention having an average of about six (6) twists per millimeter
have been observed.
Alaintaining the fibers in substantially individual form during
drying and crosslinking allows the fibers to twist during drying
10 and thereby be crosslinked in such twisted, curled state. Drying
fibers under such conditions that the fibers may twist and curl is
referred to as drying the fibers under substantially unrestrained
conditions. On the other hand, drying fibers in sheeted form
results in dried fibers which are not twisted and curled as fibers
1~ dried in substantially individualized form. It is believed that
interfiber hydrogen bonding "restrains" the relative occurrence of
twisting and curling of the fiber.
There are various methods by which the fibers may be
contacted with the crosslinking agent and catalyst. In one
20 embodiment, the fibers are contacted with a solution which initially
contains both the crosslinking agent and the catalyst. In another
embodiment, the fibers are contacted with an aqueous solution of
crosslinking agent and allowed to soak prior to addition of the
catalyst. The catalyst is subsequently added. In a third
~5 embodiment, the crosslinking agent and catalyst are added to an
aqueous slurry of the cellulosic fibers. Other methods in addition
to those described herein will be apparent to those skilled in the
art, and are intended to be included within the scope of this
invention. Regardless of the particular method by which the fibers
30 are contacted with crosslinking agent and catalyst, the cellulosic
fibers, crosslinking agent and catalyst are preferably mixed and/or
allowed to soak sufficiently with the fibers to assure thorough
contact with and impregnation of the individual fibers.
In general, any substance which catalyzes the crosslinking
:~5 mechanism may be utilized. Applicable catalysts include organic
12 13~0~3~
acids and acid salts. Especially preferred catalysts are salts such
as aluminum, magnesium, zinc and calcium salts of chlorides,
nitrates or sulfates. One specific example of a preferred salt is
zinc nitrate hexahydrate. Other catalysts include acids such as
5 sulfuric acid, hydrochloric acid and other mineral and organic
acids. The selected catalyst may be utilized as the sole catalyzing
agent, or in combination with one or more other catalysts. It is
believed that combinations of acid salts and organic acids as
catalyzing agents provide superior crosslinking reaction efficiency.
10 Unexpectedly high levels of reaction completion have been observed
for catalyst combinations of zinc nitrate salts and organic acids,
such as citric acid, and the use of such combinations is preferred.
Mineral acids are useful for adjusting pH of the fibers while being
contacted with the crosslinking agent in solution, but are
15 preferably not utilized as the primary catalyst.
The optimum amount of crosslinking agent and catalyst utilized
will depend upon the particular crosslinking agent utilized, the
reaction conditions and the particular product application
contemplated .
The amount of catalyst preferably utilized is, of course,
dependent upon the particular type and amount of crosslinking
agent and the reaction conditions, especially temperature and pH.
In general, based upon technical and economic considerations,
catalyst levels of between about 10 wt. % and about 60 wt. %, based
25 on the weight of crosslinking agent added to the cellulosic fibers,
are preferred. For exemplary purposes, in the case wherein the
catalyst utilized is zinc nitrate hexahydrate and the crosslinking
agent is glutaraldehyde, a catalyst level of about 30 wt. %, based
upon the amount of glutaraldehyde added, is preferred. Alost
30 preferably, between about 5~ and about 30%, based upon the weight
of the glutaraldehyde, of an organic acid, such as citric acid, is
also added as a catalyst. It is additionally desirable to adjust the
aqueous portion of the cellulosic fiber slurry or crosslinking agent
solution to a target pH of between about pH 2 and about pH 5,
35 more preferably between about pH 2 . 5 and about pH 3 . 5, during
the period of contact between the crosslinking agent and the fibers.
.... . . .
13~0~
13
The cellulosic fibers should generally be dewatered and
optionally dried. The workable and optimal consistencies will vary
depending upon the type of fluffing equipment utilized. In the
preferred embodiments, the cellulosic fibers are dewatered and
5 optimally dried to a consistency of between about 30% and about
80%. More preferably, the fibers are dewatered and dried to a
consistency level of between about 40% and about 60%. Drying the
fibers to within these preferred ranges generally will facilitate
defibration of the fibers into individualized form without excessive
10 formation of knots associated with higher moisture levels and
without high levels of fiber damage associated with lower moisture
levels .
For exemplary purposes, dewatering may be accomplished by
such methods as mechanically pressing, centrifuging, or air drying
15 the pulp. Additional drying is preferably performed by such
methods, known in the art as air drying or flash drying, under
conditions such that the utilization of high temperature for an
extended period of time is not required. Excessively high
temperature at this stage of the process may result in the
2Q premature initiation of crosslinking. Preferably, temperatures in
excess of about 1 60"C are not maintained for periods of time in
excess of 2 to 3 seconds. Mechanical defibration is performed as
previously described.
The defibrated fibers are then heated to a suitable temperature
25 for an effective period of time to cause the crosslinking agent to
cure, i.e., to react with the cellulosic fibers. The rate and degree
of crosslinking depends upon dryness of the fibers, temperature,
amount and type of catalyst and crosslinking aaent and the method
utilized for heating and/or drying the fibers while crosslinking is
30 performed. Crosslinking at a particular temperature will occur at a
higher rate for fibers of a certain initial moisture content when
accompanied by a continuous air through drying than when
subjected to drying/heating in a static oven, Those skilled in the
art will recognize that a number of temperature-time relationships
35 exist for the curing of the crosslinking agent. Conventional paper
drying temperatures, (e.g., 120~F to about 150''F), for periods of
13~0~
14
between about 30 minutes and 60 minutes, under static, atmospheric
conditions will generally provide acceptable curing efficiencies for
fibers having moisture contents less than about 5%. Those skilled
in the art will also appreciate that higher temperatures and air
S convection decrease the time required for curing. However, curing
temperatures are preferably maintained at less than about 1 60~C,
since exposure of the fibers to such high temperatures in excess of
about l 60~C may lead to yellowing or other damaging of the fibers.
~ The maximum level of crosslinking will be achieved when the
l0 fibers are essentially dry (having less than about 5% moisture).
Due to this absence of water, the fibers are crosslinked while in a
substantially unswollen, collapsed state. Consequently, they
characteristically have low fluid retention values (FRV) relative to
the range applicable to this invention. The FRV refers to the
15 amount of fluid calculated on a dry fiber basis, that remains
absorbed by a sample of fibers that have been soaked and then
centrifuged to remove interfiber fluid. (The FRV is further
defined and the Procedure For Determining FRV, is described
below. ) The amount of fluid that the crosslinked fibers can absorb
20 is dependent upon their ability to swell upon saturation or, in other
words, upon their interior diameter or volume upon swelling to a
maximum level. This, in turn, is dependent upon the level of
crosslinking. As the level of intrafiber crosslinking increases for a
given fiber and process, the FRV of the fiber will decrease until
25 the fiber does not swell at all upon wetting. Thus, the FRV value
of a fiber is structurally descriptive of the physical condition of the
fiber at saturation. Unless otherwise expressly indicated, FRV data
described herein shall be reported in terms of the water retention
value (WRV! of the fibers. Other fluids, such as salt water and
30 synthetic urine, may also be advantageously utilized as a fluid
medium for analysis. Generally, the FRV of a particular fiber
crosslinked by procedures wherein curing is largely dependent upon
drying, such as the present process, will be primarily dependent
upon the crosslinking agent and the level of crosslinking. The
35 WRV's of fibers crosslinked by this dry crosslinking process at
crosslinking agent levels applicable to this invention are generally
less than about 50, greater than about 25, and are preferably
13 ~0 ~ 3 ~
between about 28 and about 45. Bleached SSK fibers having
between about 0. 5 mole % and about 2. 5 mole % glutaraldehyde
reacted thereon, calculated on a cellulose anhydroglucose molar
basis, have been observed to have WRV's respectively ranging from
5 about 40 to about 28. The degree of bleaching and the practice of
post-crosslinking bleaching steps have been found to affect WRV.
This effect will be explored in more detail below. Southern
softwood Kraft (SSK) fibers prepared by dry crosslinking processes
known prior to the present invention, have levels of crosslinking
10 higher than described herein, and have WRV's less than about 25.
Such fibers, as previously discussed, have been observed to be
exceedingly stiff and to exhibit lower absorbent capabilities than
the fibers of the present invention.
In another process for making individualized, crosslinked
15 fibers by a dry crosslinking process, cellulosic fibers are contacted
with a solution containing a crosslinking agent as described above.
Either before or after being contacted with the crosslinking agent,
the fibers are provided in a sheet form. Preferably, the solution
containing the crosslinking agent also contains one of the catalysts
20 applicable to dry crosslinking processes, also described above.
The fibers, while in sheeted form, are dried and caused to
crosslink preferably by heating the fibers to a temperature of
between abcut 120~C and about 160~C. Subsequent to crosslinking,
the fibers are mechanically separated into substantially individual
;~5 form. This is preferably performed by treatment with a fiber
fluffing apparatus such as the one described in U.S. Patent No.
3,987,968 or may be performed with other methods for defibrating
fibers as may be known in the art. The individualized, crosslinked
fibers made according to this sheet crosslinking process are treated
30 with a sufficient amount of crosslinking agent such that between
about 0. S mole % and about 3 . 5 mole % crosslinking agent, calculated
on a cellulose anhydroglucose molar basis and measured subsequent
to defibration are reacted with the fibers in the form of intrafiber
crosslink bonds. Another effect of drying and crosslinking the
35 fibers while in sheet form is that fiber to fiber bonding restrains
the fibers from twisting and curling with increased drying.
Compared to individualized, crosslinked fibers made according to a
13~043~
16
process wherein the fibers are dried under substantially
unrestrained conditions and subsequently crosslinked in a
twisted, curled configuration, absorbent structures made
the relatively untwisted fibers made the sheet curing
process described above would be expected to exhibit lower
wet resiliency and lower responsiveness to wetting of a dry
absorbent structure.
Another category of crosslinking processes applicable to
the present invention is nonaqueous solution cure
crosslinking processes. The same types of fibers
applicable to dry crosslinking processes may be used in the
production of nonaqueous solution crosslinked fibers. The
fibers are treated with a sufficient amount of crosslinking
agent such that between about 0.5 mole % and about 3.5 mole
% crosslinking agent subsequently react with the fibers,
wherein the level of crosslinking agent reacted is
calculated subsequent to said crosslinking reaction, and
with an appropriate catalyst. The crosslinking agent is
caused to react while the fibers are submerged in a
solution which does not induce any substantial levels of
swelling of the fibers. The fibers, however, may contain
up to about 30% water, or be otherwise swollen in the
crosslinking solution to a degree equivalent to fibers
having about a 30% moisture content. Such partially
swollen fiber geometry has been found to provide additional
unexpected benefits as hereinafter more fully discussed.
The crosslinking solution contains a nonaqueous,
' ~ '
~ 13~0~3~
16a
water-miscible, polar diluent such as, but not limited to,
acetic acid, propanoic acid, or acetone. Preferred
catalysts include mineral acids, such as sulfuric acid,
halogen acids, and hydrochloric acid. Other applicable
catalysts include salts of mineral acids and halogen acids,
organic acids and salts thereof. Crosslinking solution
systems applicable for use as a crosslinking medium also
include those disclosed in U. S. Patent No. 4,035,147,
issued to S. Sangenis, G. Guiroy, and J. Quere, on July 12,
1977. The crosslinking solution may include some water or
other fiber swelling liquid, however, the amount of water
is preferably insufficient to cause a level of swelling
corresponding to that incurred by 70% consistency pulp
fibers (30% aqueous moisture content). Additionally,
crosslinking solution water contents less
13~043~
than about 10% of the total volume of the solution, exclusive of the
fibers are preferred. Levels of water in the crosslinking solution
in excess of this amount decrease the efficiency and rate of
crosslinking .
Absorption of crosslinking agent by the fibers may be
accomplished in the crosslinking solution itself or in a prior
treatment stage including, but not limited to, saturation of the
fibers with either an aqueous or nonaqueous solution containing the
crosslinking agent. Preferably, the fibers are mechanically
10 defibrated into individual form. This mechanical treatment may be
performed by methods previously described for fluffing fibers in
connection with the previously described dry crosslinking process.
It is especially preferred to include in the production of fluff
a mechanical treatment which causes the moist cellulosic fibers to
15 assume a curled or twisted condition to a degree in excess of the
amount of curl or twist, if any, of the natural state of the fibers.
This can be accomplished by initially providing fibers for fluffing
which are in a moist state, subjecting the fibers to a mechanical
treatment such as those previously described methods for
;~0 defibrating the fibers into substantially individual form, and at
least partially drying the fibers.
The relative amounts of curl and twist imparted to the fibers
is in part dependent upon the moisture content of the fibers.
Without limiting the scope of the invention, it is believed that the
2~ fibers naturally twist upon drying under conditions wherein fiber to
fiber contact is low, i.e., when the fibers are in an individualized
form. Also, mechanical treatment of moist fibers initially causes the
fibers to become curled. When the fibers are then dried or
partially dried under substantially unrestrained conditions, they
3() become twisted with the degree of twist being enhanced by the
additional amount of curl mechanically imparted. The defibration
fluffing steps are preferably practiced on high consistency moist
pulp or pulp which has been dewatered to fiber consistency of
about 45% to about i;% (determined prior to initialization of
~5 defibràtion).
18 13~
Subsequent to defibration, the fibers should be dried to
between 0% and about 30% moisture content prior to being contacted
with the crosslinking solution, if the defibration step has not
already provided fibers having moisture contents within that range.
5 The drying step should be performed while the fibers are under
substantially unrestrained conditions. That is, fiber to fiber
contact should be minimized so that the twisting of the fibers
inherent during drying is not inhibited. Both air drying and flash
drying methods are suitable for this purpose.
The individualized fibers are next contacted with a
crosslinking solution which contains a water-miscible, nonaqueous
diluent, a crosslinking agent and a catalyst. The crosslinking
solution may contain a limited amount of water. The water content
of the crosslinking solution should be less than about l 8% and is
15 preferably less than about 9%.
A bat of fibers which have not been mechanically defibrated
may also be contacted with a crosslinking solution as described
above .
The amounts of crosslinking agent and acid catalyst utilized
2() will depend upon such reaction conditions as consistency,
temperature, water content in the crosslinking solution and fibers,
type of crosslinking agent and diluent in the crosslinking solution,
and the amount of crosslinking desired. Preferably, the amount of
crosslinking agent utilized ranges from about .2 wt % to about 10 wt
25 % (based upon the total, fiber-free weight of the crosslinking
solution). Preferred acid catalyst content is additionally dependent
upon the acidity of the catalyst in the crosslinking solution. Good
results may generally be obtained for catalyst content, including
hydrochloric acid, between about . 3 wt % and about 5 wt %
(fiber-free crosslinking solution weight basis) in crosslinking
solutions containing an acetic acid diluent, preferred levels of
glutaraldehyde, and a limited amount of water. Slurries of fibers
and crosslinking solution having fiber consistencies of less than
about 10 wt % are preferred for crosslinking in con junction with the
35 crosslinking solutions described above.
,9 1340~3~
The crosslinking reaction may be carried out at ambient
temperatures or, for accelerated reaction rates, at elevated
temperatures preferably less than about 40"C.
There are a variety of methods by which the fibers may be
S contacted with, and crosslinked in, the crosslinking solution. In
one embodiment, the fibers are contacted with the solution which
initially contains both the crosslinking agent and the acid catalyst.
The fibers are allowed to soak in the crosslinking solution, during
which time crosslinking occurs. In another embodiment, the fibers
10 are contacted with the diluent and allowed to soak prior to addition
of the acid catalyst. The acid catalyst subsequentlv is added, at
which time crosslinking begins. Other methods in addition to those
described will be apparent to those skilled in the art, and are
intended to be within the scope of this invention.
Preferably, the crosslinking aaent and the conditions at which
crosslinking is performed are chosen to facilitate intrafiber
crosslinking. Thus, it is advantageous for the crosslinking
reaction to occur in substantial part after the crosslinking agent
has had sufficient time to penetrate into the fibers, Reaction
20 conditions are preferably chosen so as to avoid instantaneous
crosslinking unless the crosslinking agent has already penetrated
into the fibers. Periods of reaction during which time crosslinking
is substantially completed over a period of about 30 minutes are
preferred. Longer reaction periods are believed to provide minimal
25 marginal benefit in fiber performance. However, both shorter
periods, including substantially instantaneous crosslinking, and
longer periods are meant to be within the scope of this invention.
It is also contemplated to only partially cure while in solution,
and subsequently complete the crosslinking reaction later in the
30 process by drying or heating treatments.
Following the crosslinking step, the fibers are drained and
washed. Preferably, a sufficient amount of a basic substance such
as caustic is added in the washing step to neutralize any acid
remaining in the pulp. After washing, the fibers are defluidized
13~0~3~
and dried to completion. Preferably, the fibers are subjected to a
second mechanical defibration step which causes the crosslinked
fibers to curl, e.g.., fluffing by defibration, between the
defluidizing and drying steps. Upon drying, the curled condition
5 of the fibers imparts additional twist as previously described in
connection with the curling treatment prior to contact with the
crosslinking solution. The same apparatuses and methods for
inducing twist and curl described in connection with the first
mechanical defibration step are applicable to this second mechanical
10 defibration step. As used herein, the term "defibration" shall refer
to any of the procedures which may be used to mechanically
separate the fibers into substantially individual form, even though
the fibers may already be provided in such form. "Defibration"
therefore refers to the step of mechanically treating the fibers, in
15 either individual form or in a more compacted form, to a mechanical
treatment step which a) would separate the fibers into substantially
individual form if they were not already in such form, and b)
imparts curl and twist to the fibers upon drying.
This second defibration treatment, after the fibers have been
20 crosslinked, has been found to increase the twisted, curled
character of the pulp. This increase in the twisted, curled
configuration of the fibers leads to enhanced absorbent structure
resiliency and responsiveness to wetting. A second defibration
treatment may be practiced upon any of the crosslinked fibers
25 described herein which are in a moist condition. However, it is a
particular advantage of the nonaqueous solution crosslinking method
that a second defibration step is possible without necessitating an
additional drying step. This is due to the fact that the solution in
which the fibers are crosslinked keep the fibers flexible subsequent
~~ to crosslinking even though not causing the fibers to assume an
undesirable, highly swollen state.
It has been further unexpectedly found that increased degrees
of absorbent structure expansion upon wetting compressed pads can
be obtained for structures made from fibers which have been
35 crosslinked while in a condition which is twisted but partially
, . . . .. . .. .. . . . . .. . .
13~0~34
swollen relative to fibers which have been thoroughly dried of water
prior to crosslinking.
Improved results are obtained for individualized, crosslinked
fibers which have been crosslinked under conditions wherein the
5 fibers are dried to between about 18% and about 30% water content
prior to contact with the crosslinking solution. In the case wherein
a fiber is dried to completion prior to being contacted with the
crosslinking solution, it is in a nonswollen, collapsed state. The
- fiber does not become swollen upon contact with the crosslinking
ln solution due to the low water content of the solution. As discussed
before, a critical aspect of the crosslinking solution is that it does
not cause any substantial swelling of the fibers. However, when
the diluent of the crosslinking solution is absorbed by an already
swollen fiber, the fiber is in effect "dried" of water, but the fiber
lS retains its preexisting partially swollen condition.
For describing the degree to which the fiber is swollen, it is
useful to aaain refer to the fluid retention value ( FRV~ of the fiber
subsequent to crosslinking. Fibers having higher FRV's correspond
to fibers which have been crosslinked while in a more swollen state
20 relative to fibers crosslinked while in a less swollen state, all other
factors being equal. Without limiting the scope of the invention, it
is believed that partially swollen, crosslinked fibers with increased
FRV's have greater wet resilience and responsiveness to wetting
than fibers which have been crosslinked while in an unswollen
25 state- Fibers having this increase in wet resilience and
responsiveness to wetting are more readily able to expand or
untwist when wetted in an attempt to return to their natural state.
Yet, due to the stiffness imparted by crosslinking, the fibers are
still able to provide the structural support to a saturated pad made
30 from the fibers. Numerical FRV data described herein in connection
with pzrtially swollen crosslinked fibers shall be water retention
values (WRV). As the WRV increases beyond approximately 60, the
stiffness of the fibers is believed to become insufficient to provide
the wet resilience and responsiveness to wettina desired to support
35 a saturated absorbent structure.
22 13~0~34
In an alternative method of crosslinking the fibers in solution,
the fibers are first soaked in an aqueous or other fiber swelling
solution, defluidized, dried to a desired level and subsequently
submersed in a water-miscible crosslinking solution containing a
5 catalyst and crosslinking agent as previously described. The fibers
are preferably mechanically defibrated into fluff form subsequent to
defluidization and prior to additional drying, in order to obtain the
benefits of enhanced twist and curl as previously described.
Mechanical defibration practiced subsequent to contacting the fibers
- 10 with the crosslinking agent is less desirable, since such defibration
would volatilizc the crosslinking agent thus, possibly leading to
atmospheric contamination by, or high air treatment investments due
to, the crosslinking agent.
In a modification of the process described immediately above,
15 the fibers are defibrated and then presoaked in a high
concentration solution of crosslinking agent and a fiber-swelling
diluent, preferably water. The crosslinking agent concentration is
sufficiently high to inhibit water-induced swelling of fibers. Fifty
percent, by weight, aqueous solutions of the crosslinking agents of
20 this invention, preferably, glutaraldehyde, have been found to be
useful solutions for presoaking the fibers. The presoaked fibers
are defluidized and submerged in a crosslinking solution containing
a water-miscible, polar diluent, a catalyst, and a limited amount of
water, and then crosslinked as previously described. Also as
;~5 described above, the crosslinked fibers may be defluidized and
subjected to a second mechanical defibration step prior to further
processing into a sheet or absorbent structure.
Presoaking the fibers with crosslinking agent in an aqueous
solution prior to causing the crosslinking agent to react provides
30 unexpectedly high absorbency properties for absorbent pads made
from the crosslinked fibers, even relative to pads made from
crosslinked fibers of the prior described nonaqueous solution cure
processes wherein the fibers were not presoaked with a solution
containing crosslinking agent.
,3 1 ~ ~0 ~3~
The crosslinked fibers formed as a result of the preceding dry
crosslinking and nonaqueous solution crosslinking processes are the
product of the present invention. The crosslinked fibers of the
present invention may be utilized directly in the manufacture of air
5 laid absorbent cores. Additionally, due to their stiffened and
resilient character, the crosslinked fibers may be wet laid into an
uncompacted, low density sheet which, when subsequently dried, is
directly useful without further mechanical processing as an
absorbent core. The crosslinked fibers may also be wet laid as
10 compacted pulp sheets for sale or transport to distant locations.
Once the individualized, crosslinked fibers are made, they may
be dry laid and directly ~ormed into absorbent structures, or wet
laid and formed into absorbent structures or densified pulp sheets.
The fibers of the present invention provide a variety of substantial
15 performance advantages. Itowever, it is difficult to form such
fibers into a smooth, wet laid sheet by conventional wet sheet
formation practices. This is because individualized, crosslinked
fibers rapidly flocculate when in solution. Such flocculation may
occur both in the headbox and upon deposition into the foraminous
;~() forming wire. Attempts to sheet individualized, crosslinked fibers
by conventional pulp sheeting methods have been found to result in
the formation of a plurality of clumps of flocced fibers. This
results from the stiff, twisted character of the fibers, a low level
of fiber to fiber bonding, and the high drainabilitv of the fibers
2~ once deposited on a sheet forming wire. It is therefore a
significant commercial concern that a practicable process for
sheeting individualized, crosslinked fibers be provided, whereby
wet laid absorbent structures and densified pulp sheets for transit
and subsequent defibration may be formed.
Accordingly, a novel process for sheeting individualized,
crosslinked fibers which tend to flocculate in solution has been
developed, wherein a slurry containing individualized, crosslinked
fibers are initially deposited on a foraminous forming wire, such as
a ~ourdrinier wire in a manner similar to conventional pulp sheeting
;~5 processes. However, due to the nature of individualized,
crosslinked fibers, these fibers are deposited on the forming wire
.. . ..
1340~3~
24
in a plurality of clumps of fibers. At least one stream of fluid,
preferably water, is directed at the deposited, clumped fibers.
Preferably, a series of showers are directed at the fibers deposited
on the forming wire, wherein successive showers have decreasing
5 volumetric flow rates. The showers shou!d be of sufficient velocity
such that the impact of the fluid aaainst the fibers acts to inhibit
the formation of flocculations of the fibers and to disperse
flocculations of fibers which have already formed. The fiber
setting step is preferably performed with a cylindrical screen, such
10 as a dandy roll, or with another apparatus analogous in function
which is or may become known in the art. Once set, the fibrous
sheet may then be dried and optionally compacted as desired. The
spacing of the showers will varv depending upon the particular rate
of fiber floccing, line speed of the forming wire, drainage throuqh
1~ the forming wire, number of showers, and velocity and flow rate
through the showers. Preferably, the showers are close enough
together so that substantial levels of floccing are not incurred,
In addition to inhibiting the formation of and dispersing
flocculations of fibers, the fluicl showered onto the fibers also
20 compensates for the extremely fast drainage of individualized,
crosslinked fibers, by providing additional liquid medium in which
the fibers may be dispersed for subsequent sheet formation. The
plurality of showers of decreasing volumetric flow rates facilitates a
systematic net increase in slurry consistency while providing a
~5 repetitive dispersive and inhibiting effect upon flocculations of the
fibers. This results in the formation of a relatively smooth and
even deposition of fibers which are then promptly, i.e., before
reflocculation, set into sheeted form by allowing the fluid to drain
and pressing the fibers against the foraminous wire.
3(J Relative to pulp sheets made from conventional, uncrosslinked
cellulosic fibers, the pulp sheets made from the crosslinked fibers
of the present invention are more difficult to compress to
conventional pulp sheet densities. Therefore, it may be desirable
to combine crosslinked fibers with uncrosslinked fibers, such as
35 those conventionally used in the manufacture of absorbent cores.
1340~3~
Pulp sheets containing stiffened, crosslinked fibers preferably
contain between about 5% and about 90% uncrosslinked, cellulosic
fibers, based upon the total dry weight of the sheet, mixed with
the individualized, crosslinked fibers. It is especially preferred to
5 include between about 5~ and about 30~ of highly refined,
uncrosslinked cellulosic fibers, based upon the total dry weight of
the sheet. Such highly refined fibers are refined or beaten to a
freeness level less than about 300 ml CSF, and preferably less than
about 100 ml CSF. The uncrosslinked fibers are preferably mixed
lû with an aqueous slurry of the individualized, crosslinked fibers.
This mixture may then be formed into a densified pulp sheet for
subsequent defibration and formation into absorbent pads. The
incorporation of the uncrosslinked fibers eases compression of the
pulp sheet into a densified form, while imparting a surprisingly
15 small loss in absorbency to the subsequently formed absorbent
pads. The uncrosslinked fibers additionally increase the tensile
strength of the pulp sheet and to absorbent pads made either from
the pulp sheet or directly from the mixture of crosslinked and
uncrosslinked fibers. Regardless of whether the blend of
20 crosslinked and uncrosslinked fibers are first made into a pulp
sheet and then formed into an absorbent pad or formed directly into
an absorbent pad, the absorbent pad may be air-laid or wet-laid as
previously described.
Sheets or webs made from the individualized, crosslinked
25 fibers, or from mixtures also containing uncrosslinked fibers, will
preferably have basis weights of less than about 800 g/m2 and
densities of less than about 0.60 g/cm3. Although it is not
intended to limit the scope of the invention, wet-laid sheets having
basis weights between about 300g/m2 and about 600 gtm2 and
~~ densities between about O.lSg/cm3 and about 0.30g/cm3 are
especially contemplated for direct application as absorbent cores in
disposable articles such as diapers, tampons, and other catamenial
products. Structures having basis weights and densities higher
than these levels are believed to be most useful for subsequent
35 comminution and air-laying or wet-laying to form a lower density
and basis weight structure which is more useful for absorbent
applications. Although, such higher basis weight and density
26 I 340~3L
structures also exhibit surprisingly high absorptivity and
responsiveness to wetting. Other applications contemplated for the
fibers of the present invention include tissue sheets, wherein the
density of such sheets may be less than 0.10 g/cc.
For product applications wherein the crosslinked fibers are
disposed next to or in the vicinity of a person's skin, it is
desirable to further process the fibers to remove excess, unreacted
crosslinking agent. Preferably, the level of unreacted crosslinking
agent is reduced to at least below about 0 . 03%, based on the dry
lU weight of the cellulosic fibers. One series of treatments found to
successfully remove excess crosslinking agent comprise, in
sequence, washing the crosslinked fibers, allowing the fibers to
soak in an aqueous solution for an appreciable time, screening the
fibers, dewatering the fibers, e.g., by centrifuging, to a
15 consistency of between about 40% and about 8096, mechanically
defibrating the dewatered fibers as previously described and air
drying the fibers. This process has been found to reduce residual
free crosslinking agent content to between about 0. 01% and ahout
0.15%.
;~0 In another method for reducing residual crosslinking agent,
readily extractable crosslinking agent is removed by alkaline
washes. Alkalinity may be introduced by basic compounds such as
sodium hvdroxide, or alternatively in the form of oxidizing agents
such as those chemicals commonly utilized as bleaching agents,
25 e.g., sodium hypochlorite, and amino-containing compounds, e.g.,
ammonium hydroxide, which hydrolyze hemiacetal bonds to form
Schiff bases. The pH is preferably maintained at a level of at least
pH 7, and more preferably at least about pH 9, to inhibit reversion
of the acetal crosslink bond. It is preferred to induce
30 decomposition of hemiacetal bonds, while being neutral towards
acetal bonds. Therefore, those extracting agerts which operate at
highly alkaline conditions are preferred. Single wash treatments
with 0 . 01 N and 0 .1 N ammonium hydroxide concentrations were
observed to reduce residuals content to between about 0 . 0008% and
:~5 about 0.0023% for soaking periods of 30 minutes to two (2) hours.
~.1inimal additional benent is believed to incur for soaking times in
.. . .. . . ..
27 13~3~
excess of about 30 minutes and for ammonium hydroxide
concentrations in excess of about 0 . 01~ .
Both single stage oxidation and multiple stage oxidation were
found to be effective methods of extracting residual crnsslinking
5 agent. Single stage washing with 0.1~ available chlorine (av.CI) to
about 0. 8% av . Cl, based upon the dry weight of the fibers,
supplied in the form of sodium hypochlorite was observed to reduce
residual crosslinking agent levels to between about . 0015% and about
. 00259~.
In one novel approach to prnducing crosslinked, individualized
fibers, the source fibers are subjected to a conventional multistage
bleaching sequence, but at a midpoint during the sequence the
bleaching process is interrupted, and the fiberc are crosslinked in
accordance with the present invention. Subseauent to curing, the
remainder of the bleaching sequence is completed. It has been
found that acceptably low crosslinking agent residual levels of less
than about 0.006~ can be obtained in this manner. This method is
believed to embody the preferred manner of producing crosslinked
fibers, since the capital expense and processing inconvenience of
20 additional washing and extraction equipment and additional process
steps are avoided due to mer~er of the bleaching and residual
reduction steps. The bleaching sequences practiced and the point
of interruption in the sequences for crosslinking may vary widely,
as will be evident to one of ordinary skill in the art, However,
;~5 multi-stage bleaching sequences, wherein DEP* or DEH* stages
follow crosslinking, have been found to provide desirable results.
(*D - chlorine dioxide, E - caustic extraction, P - peroxide, H -
sodium hypochlorite). The post-crosslinking bleaching sequence
stages are preferably alkaline treatments performed at pH greater
3() than about pH 7 and more preferably greater than about pH 9.
In addition to providing effective reduction of residual
crosslinking agent, post-crosslinking alkaline treatments have been
observed to facilitate the development of higher FRV (fluid
retention value) fibers for equivalent levels of crosslinking. The
35 higher FRV fibers have lower dry resilience, i.e., they are easier
... . . . .. ..
13~0~3 1
28
to densify while in a dry state, while retaining
substantially the same wet resilience and moisture
responsiveness as the otherwise equivalent fibers
crosslinked subsequent to completion of bleaching. This
was especially surprising considering that higher FRV
heretofore resulted in reduced absorbency properties.
The crosslinked fibers herein described are useful for a
variety of absorbent articles including, but not limited
to, disposable diapers, catamenials, sanitary napkins,
tampons, and bandages wherein each of said articles has an
absorbent structure containing the individualized,
crosslinked fibers described herein. For example, a
disposable diaper or similar article having a liquid
permeable topsheet, a liquid impermeable backsheet
connected to the topsheet, and an absorbent structure
containing individualized, crosslinked fibers is
particularly contemplated. Such articles are described
generally in U. S. Patent 3,860,003, issued to Kenneth B.
Buell on January 14, 1975.
Conventionally, absorbent cores for diapers and
catamenials are made from unstiffened, uncrosslinked
cellulosic fibers, wherein the absorbent cores have dry
densities of about 0.06 g/cc and about 0.12 g/cc. Upon
wetting, the absorbent core normally displays a reduction
in volume.
~, ,",,;
13~043~
28a
It has been found that the crosslinked fibers of the
present invention can be used to make absorbent cores
having substantially higher fluid absorbing properties
including, but not limited to, absorbent capacity and
wicking rate relative to equivalent density absorbent cores
made from conventional, uncrosslinked fibers or prior known
crosslinked fibers. Furthermore, these improved absorbency
results may be obtained in conjunction with increased
levels of wet resiliency. For absorbent cores having
densities of between about 0.06 g/cc and about 0.15 g/cc
which maintain substantially constant volume upon wetting,
it is especially preferred to utilize crosslinked fibers
having crosslinking levels of between about 2.0 mole % and
about 2.5 mole ~ crosslinking agent, based upon a dry
cellulose anhydroglucose molar basis. Absorbent
, . . ,, ,, .. , ~ , . . .... .. . . . . . . ...
13~0~3~
cores made from such fibers have a desirable combination of
structural integrity, i.e., resistance to compression, and wet
resilience. The term wet resilience. in the present context, refers
to the ability of a moistened pad to spring back towards its original
5 shape and volume upon exposure to and release from compressional
forces. Compared to cores made from untreated fibers, and prior
known crosslinked fibers, the absorbent cores made from the fibers
of the present invention will regain a substantially higher
proportion of their original volumes upon release of wet
10 compressional forces.
In another preferred embodiment, the individualized,
crosslinked fibers are formed into either an air laid or wet laid
(and subsequently dried) absorbent core which is compressed to a
dry density less than the equilibrium wet density of the pad. The
15 equilibrium wet density is the density of the pad, calculated on a
dry fiber basis when the pad is fully saturated with fluid. When
fibers are formed into an absorbent core having a dry density less
than the equilibrium wet density, upon wetting to saturation, the
core will collapse to the equilibrium wet density. Alternatively,
:~0 when fibers are formed into an absorbent core having a dry density
greater than the equilibrium wet density, upon wetting to
saturation, the core will expand to the equilibrium wet density.
Pads made from the fibers of the present invention have equilibrium
wet densities which are substantially lower than pads made from
25 conventional, uncrosslinked fibers. The fibers of the present
invention can be compressed to a density higher than the
equilibrium to form a thin pad which, upon wetting, will expand,
thereby increasing absorbent capacity to a degree significantly
greater than obtained from uncrosslinked fibers.
Especially high absorbency properties, wet resilience, and
responsiveness to wetting may be obtained for crosslinking levels of
between about 0. 75 mole % and about 1 . 25 mole ~, calculated on a
dry cellulose molar basis. Preferably, such fibers are formed into
absorbent cores having dry densities greater than their equilibrium
~5 wet densities. Preferably, the absorbent cores are compressed to
densities of between about 0.12 glcc and about 0.60 g/cc, wherein
. .
13~043~
the corresponding equilibrium wet density is less than the density
of the dry, compressed core. Also preferably, the absorbent cores
are compressed to a density of between about 0.12 g/cc and about
0.40 g/cc, wherein the corresponding equilibrium wet densities are
5 between about 0 . 08 9 tcc and about 0 .12 9 /cc . Relative to
crosslinked fibers having crosslinking levels of between 2 . 0 mole %
and about 2 . 5 mole %, the former fibers are less stiff, thereby
making them more suitable for compression to the higher density
range. The former fibers also have higher responsiveness to
L0 wetting in that upon wetting they spring open at a faster rate and
to a greater degree than do fibers having crosslinking levels within
the 2 . 0 mole % to 2 . 5 mole % range, have higher wet resiliency, and
retain almost as much absorbent capacity. It should be recognized,
however, that absorbent structures within the higher density range
15 can be made from crosslinked fibers within the higher crosslinking
level range, as can lower density absorbent structures be made
from crosslinked fibers having lower levels of crosslinking.
Improved performance relative to prior known individualized,
crosslinked fibers is obtained for all such structures.
~Vhile the foregoing discussion involves preferred embodiments
for high and low density absorbent structures, it should be
recognized that a variety of combinations of absorbent structure
densities and crosslinking agent levels between the ranges disclosed
herein will provide superior absorbency characteristics and
25 absorbent structure integrity relative to conventional cellulosic
fibers and prior known crosslinked fibers. Such embodiments are
meant to be included within the scope of this invention.
PROCEDURE FOR DETERMINING FLUID RETENTION VALUE
The following procedure was utilized to determine the water
:~~ retention value of cellulosic fibers.
A sample of about 0. 3 9 to about 0. 4 9 of fibers is soaked in
a covered container with about 100 ml distilled or deionized water at
room temperature for between about 15 and about 20 hours. The
soaked fibers are collected on a filter and transferred to an
13~0~3~
80-mesh wire basket supported about 1t inches above a 60-mesh
screened bottom of a centrifuge tube. The tube is covered with a
plastic cover and the sample is centrifuged at a relative centrifuge
force of 1500 to 1700 gravities for 19 to 21 minutes. The
5 centrifuged fibers are then removed from the basket and weighed.
The weighed fibers are dried to a constant weight at 1 05~C and
reweighed. The water retention value is calculated as follows:
(1 ) WRV = (W D) x100
~0 where,
W = wet weight of the centrifuged fibers;
D = dry weight of the fibers: and
W-D = weight of absorbed water.
PROCEDURE FOR DETERMINING DRIP CAPACITY
The following procedure was utilized to determine drip capacity
of absorbent cores. Drip capacity was utilized as a combined
measure of absorbent capacity and absorbency rate of the cores.
A four inch by four inch absorbent pad weighing about 7 . 5 9
is placed on a screen mesh. Synthetic urine is applied to the
20 center of the pad at a rate of 8 ml/s. The flow of synthetic urine
is halted when the first drop of synthetic urine escapes from the
bottom or sides of the pad. The drip capacity is calculated by the
difference in mass of the pad prior to and subsequent to
introduction of the synthetic urine divided by the mass of the
25 fibers, bone dry basis.
PF~OCEDURE FOR DETERMINING WET CO~PRESSIBILITY
The following procedure was utilized to determine wet
compressibility of absorbent structures. Wet compressibility was
utilized as a measure of resistance to wet compression, wet
30 structural integrity and wet resilience of the absorbent cores.
32 1~4043~
A four inch by four inch square pad weighing 7 . 5 9 is
prepared, its thickness measured and density calculated. The pad
is loaded with synthetic urine to ten times its dry weight or to its
saturation point, whichever is less. A 0.1 PSI compressional load
is applied to the pad. After about 60 seconds, during which time
the pad equilibrates, the thickness of the pad is measured. The
compressional load is then increased to 1.1 PSI, the pad is allowed
to equilibrate, and the thickness is measured. The compressional
load is then reduced to 0.1 PSI, the pad allowed to equilihrate and
10 the thickness is again measured. The densities are calculated for
the pad at the original 0.1 PSI load, the 1.1 PSI load and the
second 0.1 PSI load, referred to as 0.1 PSIR (PSI rebound! load.
The void volume, reported in cc/g, is then determined for the
respective pressure loads. The void volume is calculated as the
15 reciprocal of the wet pad density minus the fiber volume (0. 75
cc/g1. The 0.1 PSI and 1.1 PSI void volumes are useful indicators
of resistance to wet compression and wet structural integrity.
Higher void volumes for a common initial pad density indicate
greater resistance to wet compression and greater wet structural
20 integrity The difference between 0.1 PSI and 0.1 PSIR void
volumes is useful for comparing wet resilience of absorbent pads.
A smaller difference between 0.1 PSI void volume and 0.1 PSIR void
volume, indicates higher wet resilience.
Also, the difference in caliper between the dry pad and the
2 5 saturated pad prior to compression was found to be a useful
indicator of the responsiveness to wetting of the pads.
PROCEDURE FOR DETER~INING DRY CO~1PRESSIBILITY
The following procedure was utilized to determine dry
compressibility of absorbent cores. Dry compressibility was utilized
30 to determine dry resilience of the cores as a measure of dry
resilience of the cores.
A four inch by four inch square air laid pad having a mass of
about 7 . 5 is prepared and compressed, in a dry state, by a
hydraulic press to a pressure of 5500 Ibs/16 in~. The pad is
... ..
1 3 0 4~4
inverted and the pressing is repeated. The thickness of the pad is
measured before and after pressing with a no-load caliper. Density
before and after pressing ;5 then calculated as
massl (area X thickness) . Larger differences between density
5 before and after pressing indicate lower dry resilience.
PROCEDURE FOR DETERMINING LEVEL OF
GLUTARALDEHYDE REACTED WITH CELLULOSIC FIBERS
The following procedure was utilized to determine the level of
glutaraldehyde which reacted to form intrafiber crosslink bonds with
l0 the cellulosic component of the individualized, glutaraldehyde-
crosslinked fibers.
A sample of individualized, crosslinked fibers is extracted with
0.1 N HCI. The extract is separated from the fibers, and the same
extraction/separation procedure is then repeated for each sample an
15 additional three times. The extract from each extraction is
separately mixed with an aqueous solution of
2,4-dinitrophenylhydrazone (DNPH). The reaction is allowed to
proceed for 15 minutes after which a volume of chloroform is added
to the mixture. The reaction mixture is mixed for an additional 45
20 minutes. The chloroform and aqueous layers are separated with a
separatory funnel. The level of glutaraldehyde is determined by
analyzing the chloroform layer bv high pressure liquid
chromatography (HPLC) for DNPH derivative.
The chromatographic conditions for HPLC analysis utilized were
~5 - Column: C-18 reversed phase; Detector: UV at 360 mm; Mobile
phase 80:20 methanol: water; Flow rate: 1 ml/min.; measurement
made: peak height. A calibration curve of peak height and
glutaraldehyde content was developed by measuring the HPLC peak
heights of five standard solutions having known levels of
30 glutaraldehyde between 0 and 25 ppm.
Each of the four chloroform phases for each fiber sample was
analyzed by H PLC, the peak height measured, and the
corresponding level of glutaraldehyde determined from the
~34~
34
calibration curve. The glutaraldehyde concentrations for each
extraction were then summed and divided by the fiber sample
weight (dry fiber basis) to provide glutaraldehyde content on a
fibers weight basis.
Two glutaraldehyde peaks were present for each of the HPLC
chromatograms. Either peak may be used, so long as that same
peak is used throughout the procedure.
EXAMPLE 1
This example shows the effect of varying levels of a
10 crosslinking agent, glutaraldehyde, on the absorbency and
resiliency of absorbent pads made from individualized, crosslinked
fibers. The individualized, crosslinked fibers were made by a dry
- crosslinking process.
For each sample, a quantity of never dried, southern softwood
15 kraft (SSK) pulp were provided. The fibers had a moisture
content of about 62.4% (equivalent to 37.6~ consistency). A slurry
was formed by adding the fibers to a solution containing a selected
amount of 50% aqueous solution of glutaraldehyde, 30% (based upon
the weight of the glutaraldehyde) zinc nitrate hexahydrate,
20 demineralized water and a sufficient amount of 1 N HCI to decrease
the slurry pH to about 3.7. The fibers were soaked in the slurry
for a period of 20 minutes and then dewatered to a fiber
consistency of about 34% to about 35% by centrifuging. Next, the
dewatered fibers were air dried to a fiber consistency of about 55%
25 to about ' 56% with a blow through dryer utilizing ambient
temperature air. The air dried fibers were defibrated utilizing a
three-stage fluffing device as described in U.S. Patent 3,987,968.
The defibrated fibers were placed in trays and cured at 145~C in
an essentially static drying oven for a period of 45 minutes.
30 Crosslinking was completed during the period in the oven. The
crosslinked, individualized fibers were placed on a mesh screen and
washed with about 20~C water, soaked at 1% consistency for one (1)
hour in 60~C water, screened, washed with about 20~C water for a
second time, centrifuged to 60% fiber consistency, defibrated in a
.. , . . . , , . , ., ~, . ..
134043~
three stage fluffer as previously described, and dried to completion
in a static drying oven at 105~C for four (4) hours. The dried
fibers were air laid to form absorbent pads. The pads were
compressed with a hydraulic press to a density of 0.10 g/cc. The
5 pads were tested for absorbency, resiliency, and amount of
glutaraldehyde reacted according to the procedures herein defined.
Glutaraldehyde reacted is reported in mole % calculated on a dry
fiber cellulose anhydroglucose basis. The results are reported in
Table 1.
TABLE 1
SampleGlutaraldehyde WRVDrip Cap. Wet Compressibility
# (mole %) (%)~ 8 ml/s (cc/g)
Added/Reacted (9/9)0.1PSI l.lPSI O.lPSIR
1 0/0 79.2 NIA 10.68 6.04 6.46
2 1.73/0.44 51.06.98 11.25 5.72 6.57
3* N/A/0.50 48.3 N/A N/A N/A NtA
4 2.09/0.62 46.7 N/A 11.25 6.05 6.09
5 3.16/0.99 36.315.721~.04 6.09 6.86
6 4.15/1.54 35.015.4613.34 6.86 8.22
7 6.46/1.99 32.812.8713.34 ~ 6.93 8.31
8 8.42/2.75 33.216.9513.13 7.38 8.67
9 8.8912.32 29.213.5912.56 6.51 7.90
1012.6013.32 27.713.4712.04 6.63 7.82
25 * Taken from a separate sample of fibers.
(NIA) - Not Available
EXAMPLE 2
The purpose of this example is to show that low levels of
extractable crosslinking agent may be obtained by subjecting the
30 fibers to bleaching sequence steps subseauent to crosslinking.
The level of extractable crosslinking agent was determined by
36 1340~3~
soaking a sample of the fibers in 40~C deionized water at 2 . 5%
consistency for one ( 1 ) hour . The glutaraldehyde extracted by
the water was measured by HPLC, and reported as extractable
glutaraldehyde on a dry fiber weight basis. The fibers were
5 crosslinked by a dry crosslinking process.
Southern softwood kraft pulp (SSK) was provided. The
pulp fibers were partially bleached by the following bleaching
sequence stages: chlorination (C) - 3-4% consistency slurry
treated with about 59~ available chlorine (av. Cl) at about pH 2 . 5
10 and about 38~C for 30 minutes caustic extraction - 12%
consistency slurry treated with 1.4 9/l NaOH at about 74~C for 60
minutes and hypochlorite treatment (H) - 12% consistency slurry
treated with sufficient sodium hypochlorite, at 11-11 . 5 pH between
38~C and 60~C for 60 minutes, to provide a 60-65 Elretho
15 brightness and a 15 . 5-16 . 5 cp viscosity. The partially bleached
fibers were processed into individualized, crosslinked fibers
utilizing glutaraldehyde as the crosslinking agent in accordance
with the process described in Example 1. The fibers retained
2 . 29 mole % glutaraldehyde, calculated on a dry fiber cellulosic
2(~ anhydroglucose molar basis. Typically, such fibers have
extractable glutaraldehyde levels of about 1000 ppm (0.1%).
Bleaching of the partially bleached, individualized fibers was
then continued and completed with a chlorine dioxide (D~,
25 extraction (E), and sodium hypochlorite (H) sequence (DEH). In
the chlorine dioxide stage (D), individualized, crosslinked fibers
were soaked in a 10% consistency aqueous slurry also containing a
sufficient amount of sodium hypochlorite to provide 2% available
chlorine on a dry fiber weight basis. After mixing, the pH of
30 the slurry was reduced to about pH 2 . 5 by addition of HCI and
then increased to pH 4. 4 by addition of NaOH. The pulp slurry
was next placed in a 70~C oven for 2 . 5 hours, screened, rinsed
with water to neutral pH and centrifuged to 61. 4~ consistency.
In the extraction stage, a 10~ consistency aqueous slurry of
the dewatered fibers were treated with 0.33 9 NaOH/liter water
3, 13~0434
for 1 . S hours in a 40~C. The fibers were then screened, rinsed
with water to neutral pH and centrifuged to 62. 4% consistency.
Finally, for the sodium hypochlorite stage (H), a 10%
consistency slurry of the fibers containing sufficient sodium
5 hypochlorite to provide 1.5% available chlorine on a dry fiber
weight basis was prepared. The slurry was mixed and heated in
50~C oven for one ( 1 ) hour. The fibers were then screened,
rinsed to pH 5.0 and centrifuged to 62.4% consistency. The
dewatered fibers were air dried, fluffed and dried to completion
10 in a 1 05~C oven for one ( 1~ hour . The level of extractable
glutaraldehyde of the fully bleached, individualized, crosslinked
fibers was 25 ppm (0.0025%). This is well below the maximum
level of extractable glutaraldehyde believed to be acceptable for
applications wherein the fibers are utilized in proximity to human
15 skin.
Also, it was found that pads made from the fibers which
were partially bleached, crosslinked and then bleached to
completion had unexpectedly higher fluid retention value and
wicking rate and at least equivalent drip capacity and wet
;~0 resilience as individualized fibers which were crosslinked
subsequent to being fully bleached. However, as a result of the
higher WRV, the fibers crosslinked at an intermediate point of the
bleaching sequence were more compressible in a dry state.
Substantially equivalent results were obtained when a
25 peroxide bleaching stage (P) was substituted for the final
hypochlorite stage (H). In the P stage, a 10% consistency slurry
was treated with 0. 5% hydrogen peroxide, fiber weight basis, at
11-11. S pH and 80~C for 90 minutes.
The scope of the invention is to be defined according to the
30 following claims.
. , ,
