FIELD OF INVENTION
[0001] 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 levels of residual crosslinking agent are obtained.
BACKGROUND OF THE INVENTION
[0002] 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 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 general, three categories of processes have been reported for making individualized,
crosslinked fibers. These processes, 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 individualized, crosslinked fibers generally exhibit an improvement in
at least one significant absorbency property relative to conventional, uncrosslinked
fibers. Often, this improvement in absorbency is reported in terms of absorbent capacity.
Additionally, absorbent structures made from individualized 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. 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 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.
[0003] Processes for making individualized, crosslinked fibers with dry crosslinking technology
are described in U.S. Patent No. 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
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 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. 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 consuming pretreatment, preferably between about 16 and 48 hours, is alleged
to improve product quality by reducing nit content resulting from imcomplete defibration.
[0004] Processes for producing aqueous solution crosslinked fibers are disclosed, for example,
in U.S. Patent No. 3,241,553, issued to F. 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 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). Absorbent structures made from aqueous solution crosslinked fibers exhibit
lower wet and dry resilience than pads made from dry crosslinked fibers.
[0005] In U.S. Patent No. 4,035,147, issued to S. Sangenis, G. Guiroy and J. Quere on July
12, 1977, a method is disclosed for 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 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
known to those skilled in the art as swelling reagents. Like dry crosslinked fibers,
they are highly stiffened by crosslink bonds, and absorbent structures made therefrom
exhibit relatively high wet and dry resilience.
[0006] 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 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.
[0007] Crosslinked fibers made from crosslinking agents other than formaldehyde may also
have residual levels of crosslining 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 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.
[0008] It is an object 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.
[0009] It is additionally an object of this invention to provide a process for making individualized,
crosslinked fibers having low levels of residual crosslinking agent wherein capital
investment for equipment and related operating costs are minimized.
SUMMARY OF THE INVENTION
[0010] It has been found that the objects identified above may be met and individualized,
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.
[0011] The crosslinking agent is preferably an aldehyde crosslinking agent selected from
of C₂ - C₈ dialdehydes, C₂ - C₈ dialdehyde acid analogues containing at least one
aldehyde group, and oligomers of said dialdehydes and dialdehyde acid analogues.
[0012] Preferably, the fibers are washed with an alkaline solution having a pH greater than
9. Also, preferably, the fibers are contacted with a sufficient amount of one of the
preferred crosslinking agents such that between 0.5 mole % and 3.5 mole % crosslinking
agent, calculated on a cellulose anhydroglucose molar basis, are reacted with said
fibers and said fibers have a water retention value of less than 60.
[0013] Surprisingly, fibers made according to the present invention have higher water retention
values than fibers having the same level of crosslinking made by otherwise equivalent
crosslinking 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.
DETAILED DESCRIPTION OF THE INVENTION
[0014] 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.
[0015] 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 completion. For products such as paper towels and absorbent pads
for diapers, sanitary napkins, catamenials, and other similar absorbent paper products,
it is especially perferred to utilize fibers from southern softwood pulp due to their
premium absorbency characteristics.
[0016] Crosslinking agents applicable to the present development include 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. 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 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 may be present in an aqueous solution. Particular crosslinking agents
contemplated for use with the invention are glutaraldehyde, glyoxal, and glyoxylic
acid. Glutaraldehyde is especially perferred, since it has provided fibers with the
highest levels of absorbency and resiliency, is believed to be safe and non-irritating
to human skin when in a reacted, 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.
[0017] 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 0.5 mole % and 3.5
mole % crosslinking agent, calculated on a cellulose anhydroglucose molar basis, reacted
with the fibers.
[0018] 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. However, other methods of crosslinking agent treatment,
including spraying of the fibers while in individualized, fluffed form, are also within
the scope of the invention.
[0019] Generally, the fibers will also be contacted with an appropriate catalyst prior to
crosslinking. The type, amount, and method of contact of catalyst to the fibers will
be dependent upon the particular crosslinking process practiced. These variables will
be discussed in more detail below.
[0020] Once the fibers are treated with crosslinking agent and catalyst, the crosslinking
agent is caused to react with the fibers 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 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.
[0021] 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 acidic reaction conditions.
Therefore, acid catalyzed crosslinking conditions are highly preferred for the purposes
of this invention.
[0022] 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 cellulosic fibers is the three stage fluffing device described in U.S. Patent
No. 3,987,968, issued to D. R. Moore and O. A. Shields on October 26, 1976.
[0023] 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.
[0024] Other applicable methods for defibrating the cellulosic fibers include, but are not
limited to, treatment with a Waring 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.
[0025] Regardless of the particular mechanical device used to form the fluff, the fibers
are preferably mechanically treated while initially containing at least 20% moisture,
and preferably containing between 40% and 60% moisture.
[0026] 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.
[0027] 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 60, and preferably
between 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 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 25, and generally less than 20. 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.
Applicable methods of crosslinking include dry 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 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.
[0028] 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 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. Subsequent drying is accompanied
by twisting of the fibers, with the degree of twist being enhanced by the curled geometry
of the fiber. As used herein, fiber "curl" refers to a geometric curvature of the
fiber about the longitudinal axis of the fiber. "Twist" refers to a rotation of the
fiber about the perpendicular cross-section of the longitudinal axis of the fiber.
For exemplary purposes only, and without intending to specifically limit the scope
of the invention, individualized, crosslinked fibers within the scope of the invention
having an average of six (6) twists per millimeter have been observed.
[0029] Maintaining the fibers in substantially individual form during drying and crosslinking
allows the fibers to twist during drying 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 dried in substantially individualized form. It is believed
that interfiber hydrogen bonding "restrains" the relative occurrence of twisting and
curling of the fiber.
[0030] There are various methods by which the fibers may be contacted with the crosslinking
agent and catalyst. In one 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 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 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.
[0031] In general, any substance which catalyzes the crosslinking mechanism may be utilized.
Applicable catalysts include organic 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 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. 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 preferably not utilized as the primary catalyst.
[0032] 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.
[0033] 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 10 wt. % and 60 wt. %, based 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 30 wt. %, based upon the amount of glutaraldehyde added, is preferred.
Most preferably, between 5% and 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 pH 2 and pH 5, more preferably between pH
2.5 and pH 3.5, during the period of contact between the crosslinking agent and the
fibers.
[0034] 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 optimally
dried to a consistency of between 30% and 80%. More preferably, the fibers are dewatered
and dried to a consistency level of between 40% and 60%. Drying the fibers to within
these preferred ranges generally will facilitate defibration of the fibers into individualized
form without excessive formation of knots associated with higher moisture levels and
without high levels of fiber damage associated with lower moisture levels.
[0035] For exemplary purposes, dewatering may be accomplished by such methods as mechanically
pressing, centrifuging, or air drying 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 premature initiation of crosslinking. Preferably, temperatures in excess of 160°C
are not maintained for periods of time in excess of 2 to 3 seconds. Mechanical defibration
is performed as previously described.
[0036] The defibrated fibers are then heated to a suitable temperature 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 agent and the method utilized for heating
and/or drying the fibers while crosslinking is 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 exist for the curing of the crosslinking agent. Conventional paper drying
temperatures, (e.g., 120° C to about 150°C), for periods of between 30 minutes and
60 minutes, under static, atmospheric conditions will generally provide acceptable
curing efficiencies for fibers having moisture contents less than 5%. Those skilled
in the art will also appreciate that higher temperatures and air convection decrease
the time required for curing. However, curing temperatures are preferably maintained
at less than 160°C, since exposure of the fibers to such high temperatures in excess
of 160°C may lead to yellowing or other damaging of the fibers.
[0037] The maximum level of crosslinking will be achieved when the fibers are essentially
dry (having less than 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 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 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 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 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 WRV's of fibers crosslinked
by this dry crosslinking process at crosslinking agent levels applicable to this invention
are generally less than 50, greater than 25, and are preferably between 28 and 45.
Bleached SSK fibers having between 0.5 mole % and 2.5 mole % glutaraldehyde reacted
thereon, calculated on a cellulose anhydroglucose molar basis, have been observed
to have WRV's respectively ranging from 40 to 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 higher than described herein, and have WRV's less than 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.
[0038] In another process for making individualized, crosslinked 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 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 120°C
and 160°C. Subsequent to crosslinking, the fibers are mechanically separated into
substantially individual 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 with a sufficient amount of crosslinking agent such that between 0.5 mole
% and 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 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 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.
[0039] 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 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 30% water, or be otherwise swollen in the crosslinking solution to a
degree equivalent to fibers having 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, 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, and halogen acids,
such as 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.
[0040] 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 than 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.
[0041] 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 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.
[0042] It is especially preferred to include in the production of fluff a mechanical treatment
which causes the moist cellulosic fibers to 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 defibrating the fibers into substantially individual
form, and at least partially drying the fibers.
[0043] 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 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, machanical 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 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 45% to 55% (determined prior to initialization of defibration).
[0044] Subsequent to defibration, the fibers should be dried to between 0% and 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. 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.
[0045] 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 18% and is preferably less than 9%.
[0046] A bat of fibers which have not been mechanically defibrated may also be contacted
with a crosslinking solution as described above.
[0047] The amounts of crosslinking agent and acid catalyst utilized 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 .2 wt % to 10 wt % (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 .3 wt % and
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 10 wt % are preferred for crosslinking in conjunction with the crosslinking
solutions described above.
[0048] The crosslinking reaction may be carried out at ambient temperatures or, for accelerated
reaction rates, at elevated temperatures preferably less than 40°C.
[0049] There are a variety of methods by which the fibers may be 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 are contacted with the diluent and allowed
to soak prior to addition of the acid catalyst. The acid catalyst subsequently 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.
[0050] Preferably, the crosslinking agent 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 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 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.
[0051] It is also contemplated to ony partially cure while in solution, and subsequently
complete the crosslinking reaction later in the process by drying or heating treatments.
[0052] 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
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 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 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 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.
[0053] This second defibration treatment, after the fibers have been 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 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.
[0054] 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 crosslinked while in a condition which is twisted but partially swollen
relative to fibers which have been thoroughly dried of water prior to crosslinking.
[0055] Improved results are obtained for individualized, crosslinked fibers which have been
crosslinked under conditions wherein the fibers are dried to between 18% and 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 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 retains its preexisting partially swollen condition.
[0056] For describing the degree to which the fiber is swollen, it is useful to again 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 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 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
from the fibers. Numerical FRV data described herein in connection with partially
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 wetting desired to support a saturated
absorbent structure.
[0057] 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 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 with
the crosslinking agent is less desirable, since such defibration would volatilize
the crosslinking agent thus, possibly leading to atmospheric contamination by, or
high air treatment investments due to, the crosslinking agent.
[0058] In a modification of the process described immediately above, 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 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 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.
[0059] Presoaking the fibers with crosslinking agent in an aqueous solution prior to causing
the crosslinking agent to react provides 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.
[0060] 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
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 compacted pulp
sheets for sale or transport to distant locations.
[0061] Once the individualized, crosslinked fibers are made, they may be dry laid and directly
formed 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 performance advantages. However, 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 drainability of the fibers 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.
[0062] Accordingly, a novel process for sheeting individualized, crosslinked fibers which
ten 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
Fourdrinier wire in a manner similar to conventional pulp sheeting processes. However,
due to the nature of individualized, crosslinked fibers, these fibers are deposited
on the forming wire 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 volumetric flow rates. The showers should be of sufficient
velocity such that the impact of the fluid against 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 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 vary depending upon
the particular rate of fiber floccing, line speed of the forming wire, drainage through
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.
[0063] In addition to inhibiting the formation of and dispersing flocculations of fibers,
the fluid showered onto the fibers also 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 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.
[0064] 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 those conventionally
used in the manufacture of absorbent cores. Pulp sheets containing stiffened, crosslinked
fibers preferably contain between 5% and 90% uncrosslinked, cellulosic fibers, based
upon the total dry weight of the sheet, mixed with the individualized, crosslinked
fibers. It is especially preferred to include between 5% and 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 300 ml CSF,
and preferably less than 100 ml CSF. The uncrosslinked fibers are preferably mixed
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 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 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.
[0065] Sheets or webs made from the individualized, crosslinked fibers, or from mixtures
also containing uncrosslinked fibers, will preferably have basis weights of less than
800 g/m² and densities of less than 0.60 g/cm³. Although it is not intended to limit
the scope of the invention, wet-laid sheets having basis weights between 300g/m² and
600 g/m² and densities between 0.15g/cm³ and 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 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 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.
[0066] 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 0.03%, based on the dry 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 consistency of between 40% and 80%, 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 0.01% and 0.15%.
[0067] 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 hydroxide, or alternatively in the form of oxidizing agents such as
those chemicals commonly utilized as bleaching agents, 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 pH 9, to inhibit reversion of the acetal crosslink
bond. It is preferred to induce decomposition of hemiacetal bonds, while being neutral
towards acetal bonds. Therefore, those extracting agents which operate at highly alkaline
conditions are preferred. Single wash treatments with 0.01N and 0.1N ammonium hydroxide
concentrations were observed to reduce residuals content to between 0.0008% and 0.0023%
for soaking periods of 30 minutes to two (2) hours. Minimal additional benefit is
believed to incur for soaking times in excess of 30 minutes and for ammonium hydroxide
concentrations in excess of 0.01N.
[0068] Both single stage oxidation and multiple stage oxidation were found to be effective
methods of extracting residual crosslinking agent. Single stage washing with 0.1%
available chlorine (av.Cl) to 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 .0015% and .0025%.
[0069] In one novel approach to producing 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 fibers are crosslinked
in accordance with the present invention. Subsequent 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 additional washing and extraction
equipment and additional process steps are avoided due to merger 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, 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
than pH 7 and more preferably greater than pH 9.
[0070] 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 higher FRV
fibers have lower dry resilience, i.e., they are easier 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.
[0071] 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.
[0072] Conventionally, absorbent cores for diapers and catamenials are made from unstiffened,
uncrosslinked cellulosic fibers, wherein the absorbent cores have dry densities of
0.06 g/cc and 0.12 g/cc. Upon wetting, the absorbent core normally displays a reduction
in volume.
[0073] 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
0.06 g/cc and 0.15 g/cc which maintain substantially constant volume upon wetting,
it is especially preferred to utilize crosslinked fibers having crosslinking levels
of between 2.0 mole % and 2.5 mole % crosslinking agent, based upon a dry cellulose
anhydroglucose molar basis. Absorbent 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 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 compressional forces.
[0074] 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
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, 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 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.
[0075] Especially high absorbency properties, wet resilience, and responsiveness to wetting
may be obtained for crosslinking levels of between 0.75 mole % and 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 wet densities. Preferably,
the absorbent cores are compressed to densities of between 0.12 g/cc and 0.60 g/cc,
wherein 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 0.12 g/cc and 0.40 g/cc, wherein the corresponding equilibrium wet densities
are between 0.08 g/cc and 0.12 g/cc. Relative to crosslinked fibers having crosslinking
levels of between 2.0 mole % and 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 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 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.
[0076] While 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 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
[0077] The following procedure was utilized to determine the water retention value of cellulosic
fibers.
[0078] A sample of 0.3 g to 0.4 g of fibers is soaked in a covered container with 100 ml
distilled or deionized water at room temperature for between 15 and 20 hours. The
soaked fibers are collected on a filter and transferred to an 80-mesh wire basket
supported 38mm above a 60-mesh (0.25mm opening) 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 centrifuged fibers
are then removed from the basket and weighed. The weighed fibers are dried to a constant
weight at 105°C and reweighed. The water retention value is calculated as follows:

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
[0079] 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.
[0080] A four inch by four inch absorbent pad weighing 7.5 g is placed on a screen mesh.
Synthetic urine is applied to the 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 fibers, bone dry basis.
PROCEDURE FOR DETERMINING WET COMPRESSIBILITY
[0081] 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 structural integrity and wet resilience of the absorbent cores.
[0082] A four inch by four inch square pad weighing 7.5 g is prepared, its thickness measured
and density calculated. The pad is loaded with synthetic urine to ten times it dry
weight or to its saturation point, whichever is less. A 70.31kg/m² 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
773.38kg/m², the pad is allowed to equilibrate, and the thickness is measured. The
compressional laod is then reduced to 70.31kg/m², the pad allowed to equilibrate and
the thickness is again measured. The densities are calculated for the pad at the original
70.31 kg/m² load, the 773.38kg/m² load and the second 70.31kg/m² load, referred to
as 70.31 kg/m² rebound load. The void volume, reported in cc/g, is then determined
for the respective pressure loads. The void volume is calculated as the reciprocal
of the wet pad density minus the fiber volume (0.75 cc/g). The 70.31kg/m² and 773.38kg/m²
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 integrity The difference between 70.31
kg/m² and 70.31kg/m² rebound volumes is useful for comparing wet resilience of absorbent
pads. A smaller difference between 70.31kg/m² void volume and 70.31kg/m² rebound volume,
indicates higher wet resilience.
[0083] Also, the difference in caliper between the dry pad and the saturated pad prior to
compression was found to be a useful indicator of the responsiveness to wetting of
the pads.
PROCEDURE FOR DETERMINING DRY COMPRESSIBILITY
[0084] The following procedure was utilized to determine dry compressibility of absorbent
cores. Dry compressibility was utilized to determine dry resilience of the cores as
a measure of dry resilience of the cores.
[0085] A 100 mm by 100 mm square air laid pad having a mass of 7.5 g is prepared and compressed,
in a dry state, by a hydraulic press to a pressure of 241680 kg/m². The pad is 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 is then calculated
as mass/(area X thickness). Larger differences between density before and after pressing
indicate lower dry resilience.
PROCEDURE FOR DETERMINING LEVEL OF GLUTARALDEHYDE REACTED WITH CELLULOSIC FIBERS
[0086] The following procedure was utilized to determine the level of glutaraldehyde which
reacted to form intrafiber crosslink bonds with the cellulosic component of the individualized,
glutaraldehyde-crosslinked fibers.
[0087] A sample of individualized, crosslinked fibers is extracted with 0.lN HCI at 60°C
for one hour. The extract is separated from the fibers, and the same extraction/separation
procedure is then repeated for each sample an 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 minutes. The chloroform and aqueous layers are separated with a separatory funnel.
The level of glutaraldehyde is determined by analyzing the chloroform layer by high
pressure liquid chromatography (HPLC) for DNPH derivative.
[0088] The chromatographic conditions for HPLC analysis utilized were - 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 glutaraldehyde between 0 and 25 ppm.
[0089] Each of the four chloroform phases for each fiber sample was analyzed by HPLC, the
peak height measured, and the corresponding level of glutaraldehyde determined from
the 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.
[0090] 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
[0091] This example shows the effect of varying levels of a 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.
[0092] For each sample, a quantity of never dried, southern softwood kraft (SSK) pulp were
provided. The fibers had a moisture content of 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, demineralized water and a sufficient amount of 1N HCl to
decrease the slurry pH to 3.7. The fibers were soaked in the slurry for a period of
20 minutes and then dewatered to a fiber consistency of 34% to 35% by centrifuging.
Next, the dewatered fibers were air dried to a fiber consistency of 55% to 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. Crosslinking was completed during the period
in the oven. The crosslinked, individualized fibers were placed on a mesh screen and
washed with 20°C water, soaked at 1% consistency for one (1) hour in 60°C water, screened,
washed with 20°C water for a second time, centrifuged to 60% fiber consistency, defibrated
in a 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
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.

EXAMPLE 2
[0093] The purpose of this example is to show that low levels of extractable crosslinking
agent may be obtained by subjecting the fibers to bleaching sequence steps subsequent
to crosslinking. The level of extractable crosslinking agent was determined by 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 crosslinked by a dry crosslinking
process.
[0094] 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 5% available chlorine (av. Cl) at pH 2.5 and about 38°C for 30 minutes;
caustic extraction - 12% consistency slurry treated with 1.4 g/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 brightness and a 1.55-1.65 x 10⁻² Pa.s. 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 anhydroglucose molar basis. Typically, such fibers have extractable
glutaraldehyde levels of 1000 ppm (0.1%).
[0095] Bleaching of the partially bleached, individualized fibers was then continued and
completed with a chlorine dioxide (D), 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 the slurry was reduced to pH 2.5 by addition of HCl 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.
[0096] In the extraction stage, a 10% consistency aqueous slurry of the dewatered fibers
were treated with 0.33 g NaOH/liter water for 1.5 hours in a 40°C. The fibers were
then screened, rinsed with water to neutral pH and centrifuged to 62.4% consistency.
[0097] Finally, for the sodium hypochloite stage (H), a 10% consistency slurry of the fibers
containing sufficient sodium 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
in a 105°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 skin.
[0098] Also, it was found that pads made from the fibers which were partially bleached,
crosslinked and then bleached to comletion had unexpectedly higher fluid retention
value and wicking rate and at least equivalent drip capacity and wet 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.
[0099] Substantially equivalent results were obtained when a 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 11-11.5 pH
and 80°C for 90 minutes.