Note: Descriptions are shown in the official language in which they were submitted.
1340278
INDtVlDUALlZED, CROSSLINKED FIBERS
AND PROCESS FOR MAKING SAID FIBERS
FIELD OF INVENTION
This invention is concerned with cellulosic fibers having high
fluid absorption properties, absorbent structures made from such
cellulosic fibers and processes for making such fibers and
S structures. More specifically, this invention is concerned with
absorbent cellulosic fibers, structures made from such fibers and
processes for making such fibers and absorbent structures
utilizing cellulosic fibers which are in an individualized,
crossl in ked form .
1() BACKGROUND OF THE INVENTION
Fibers crosslinked in substantTally individualized form and
various methods for making such fibers have been described in
the art. The term "individualized, crosclinked fibers", refers to
cellulosic fibers that have primarlly intrafiber chemical crosslink
15 bonds. That is, the crosslink bonds are primarily between
cellulose molecules of a single fiber, rather than between cellulose
molecules of separate fibers. Individuallzed, crosslinked fibers
are generally regarded as beTng useful in absorbent product
2 1340278
apphcations. In general, three categories of processes have been
reported for making TndivldualizPd, crosslinked fibers. These
processes, described below, are herein referred to as 1 ) dry
crosslinklng 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 "resilTence"
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 fTbers are in a moistened condTtion. 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.
3,224,926 issued to L. J. 8ernardin on Dece.,.ber 21, 1965.
Individualized, crosslinked fibers are produced by impregnating
swollen fibers in an aqueous solution with crosslTnking agent,
dewaterlng and defiberizing the fibers by mechanical actTon, and
drying the fibers at elevated temperature to effect crosslinking
while the fibers are in a substantially individual state. The
fTbers are inherently crosslTnked 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, whereTn
3 ~340278
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 .5. Patent No. 3,224,926. ThTs time 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. H. Steiger on March 22, 1966. Individualized, crosslinked
tlbers are produced by crosslinklng 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 solutTon 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.
In U.S. Patent No. 4,035,147, issued to S. Sangenis, G.
Guiroy and J. Quere on July 12, 1977, a method is dTsclosed 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 flbers are in this substantially
nonaqueous solution. This type of process shall hereinafter be
4 13~0278
referred to as a nonaqueous solution crosslinked process; and the
flbers 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.
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
2~ 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.
For example, dry crosslinked fibers and nonaqueous solution
crosslinked fibers, have generally resulted in fibers of excessive
stiffness and dry resiliency, thereby making them difficult to
form into densified sheets for transport and subsequently refluff
without fiber damage. Furthermore, when compressed in a dry
state, pads made from these fibers have exhibited a low
3û responsiveness to wetting. That is, once compressed in a dry
state, they have not shown the ability to regain substantial
amounts of their prior absorbent capacity upon wetting.
Another difficulty which has been experienced with respect
to dry and nonaqueous solution crosslinked fibers is that the
fibers rapidly flocculate upon wet-laying on a foraminous forming
1340278
wire. This has hindered formation of absorbent wet laid
structures as well as formation of densified sheets which
would facilitate economic transport of the fibers to a
converting plant.
Aqueous solution crosslinked fibers, while useful for
certain higher density absorbent pad applications such as
surgical dressings, tampons and sanitary napkins wherein
densities ordinarily are about 0.40 g/cc, are excessively
flexible when in a wet state and therefore result in
absorbent structures which have low wet resllience.
Furthermore, upon wetting, aqueous solution crosslinked
fibers become too flexible to structurally support the pad
at lower fiber densities. The wetted pad therefore
collapses and absorbent capacity is reduced.
It is an object of an aspect of this invention to
provide individualized, crosslinked fibers and absorbent
structures made from such fibers wherein the absorbent
structures made from the crosslinked fibers have high
levels of absorbency relative to absorbent structures made
from uncrosslinked fibers, exhibit higher wet resilience
and lower dry resilience than structures made from prior
known dry crosslinked and nonaqueous solution crosslinked
fibers, and exhibit higher wet resilience and structural
integrity than structures made from prior known aqueous
solution crosslinked fibers.
It is an object of an aspect of this invention to
provide individualized, crosslinked fibers and absorbent
structures made from such fibers, as described above, which
have improved responsiveness to wetting relative to prior
known crosslinked fibers and conventional, uncrosslinked
fibers.
It is an object 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.
JA~
~J
6 1?~ 7 8
It is an object of an aspect of this invention to
provide improved processes for forming individualized,
crosslinked fibers into wet-laid sheeted forms.
SUMMARY OF THE INVENTION
It has been found that the objects identifed above may
be met by individualized, crosslinked fibers and
incorporation of these fibers into absorbent structures, as
disclosed herein. In general, these objects and other
benefits are attained by individualized, crosslinked fibers
having between about 0.5 mole % and about 3.5 mole %
crosslinking agent, calculated on a cellulose anhydro-
glucose molar basis, reacted with the fibers in the form of
intrafiber crosslink bonds wherein the crosslinking agent
is selected from the group consisting of C2 - C8
dialdehydes, C2 - Cg dialdehyde acid analogues having at
least one aldehyde functionality, and oligomers of such C2
~ C8 dialdehydes and dialdehyde acid analogues. Such
fibers, which are characterized by having water retention
values (WRV's) of less than about 60, have been found to
fulfill the identilied objects relating to individualized,
crosslinked fibers and provide unexpectedly good
absorbent performance in absorbent structure applications.
Accordingly, such fibers may be obtained by practicing the
following process, which comprises the steps of:
a) providing cellulosic fibers;
b) contacting the fibers with a crosslinking agent
selected from the group consisting of C2 - C8 dialdehydes,
C2 ~ C8 dialdehyde acid analogues, and oligomers of such C2
~ C8 dialdehydes and dialdehyde acid analogues; and
c) causing the crosslinking agent to react with
the fibers while the fibers are maintained in substantially
individual form, whereby intrafiber crosslink bonds are
formed;
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-- 7
wherein the fibers are contacted with a sufficient
amount of crosslinking agent such that between about 0.5
mole ~ and about 3.5 mole ~ of crosslinking agent,
calculated on a cellulose anhydroglucose molar basis,
react with the fibers to form crosslink bonds. The
fibers, subsequent to crosslinking, are characterized by
having WRV's of less than about 60.
Preferably, the fibers are crosslinked while in a
highly twisted condition. In the most preferred
embodiments, the fibers are contacted with crosslinking
agent and a catalyst in an aqueous solution, dewatered,
mechanically separated into substantially individual
form, and then dried and caused to crosslink under
substantially unrestrained conditions. The dewatering,
mechanical separation and drying stages allow the fibers
to become highly twisted prior to crosslinking. The
twisted condition is then at least partially but less
than completely set as a result of crosslinking.
Alternatively, cellulosic fibers in individualized form
may be crosslinked in a substantially nonaqueous
crosslinking solution containing a water-miscible polar
diluent, such as acetic acid, and insufficient amount of
water to cause the fibers to swell to a level greater
than, that corresponding to a 30~ aqueous moisture
content for equivalent fibers.
Various aspects of the invention are as follows:
Individualized, twisted and curled, crosslinked
cellulosic fibers, said fibers comprising cellulosic
fibers in substantially individual form having between
about 0.5 mole ~ and about 3.5 mole ~ crosslinking
agent, calculated on a cellulose anhydroglucose molar
basis, 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 of said dialdehydes replaced by a carboxyl group,
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- 7a - 1340278
and oligomers of said dialdehydes and said acid
analogues, said fibers having been maintained in
substantially individual form during drying and
crosslinking, said crosslinking agent being sufficiently
reacted with said fibers in intrafiber crosslink bond
form to cause said fibers to have a water retention
value of from about 28 to about 45.
Individualized, twisted and curled, crosslinked
fibers, said fibers comprising cellulosic fibers in
substantially individual form having between about 0.5
mole ~ and about 3.5 mole ~ crosslinking agent,
calculated on a cellulose anhydroglucose molar basis,
said crosslinking agent being selected from the group
consisting of glutaraldehyde, glyoxal and glyoxylic
acid, said fibers having been maintained in
substantially individual form during drying and
crosslinking, said crosslinking agent being sufficiently
reacted with said fibers in an intrafiber crosslink bond
form, that the water retention value of said fibers is
from about 28 to about 45.
A process for making individualized, twisted and
curled, crosslinked cellulosic fibers, said process
comprising the steps of:
a. providing cellulosic fibers;
b. contacting said fibers with a solution
containing a crosslinking agent selected from
the group consisting of C2-C8 dialdehydes,
acid analogues of said dialdehydes derived by
having one aldehyde group of each of said
dialdehydes replaced by a carboxyl group, and
oligomers of said dialdehydes and said acid
analogues;
c. mechanically separating said fibers into
substantially individual form; and
B
1 ~ 2 ~ 3
- 7b -
d. drying said fibers and reacting said
crosslinking agent with said fibers to form
crosslink bonds while said fibers are in
substantially individual form, to form
intrafiber crosslink bonds;
said fibers being contacted with a sufficient
amount of crosslinking agent such that between
about 0.5 mole ~ and about 3.5 mole ~
crosslinking agent, calculated on a cellulose
anhydroglucose molar basis, reacts with said
fibers to form said intrafiber crosslink bonds
and causes said fibers, subsequent to
crosslinking, to have water retention values
of from about 28 to about 45.
A process for making individualized, twisted and
curled, crosslinked, cellulosic fibers, said process
comprising the steps of:
a. providing cellulosic fibers;
b. contacting said fibers with a solution containing a
crosslinking agent selected from the group
consisting of glutaraldehyde, glyoxal, and
glyoxylic acid;
c. mechanically separating said fibers into
substantially individual form; and
d. drying said fibers and reacting said crosslinking
agent with said fibers to form crosslink bonds
while said fibers are in substantially individual
form, to form intrafiber crosslink bonds;
said fibers being contacted with a sufficient
amount of crosslinking agent such that between
about 0.5 mole ~ and about 3.5 mole ~ crosslinking
agent, calculated on a cellulose anhydroglucose
molar basis, reacts with said fibers to form said
intrafiber crosslink bonds, and causes said fibers,
subsequent to crosslinking, to have water retention
values of from about 28 to about 45.
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- 7c - 1340278
Other processes, fibers and structures made
according to the present invention in addition to those
specific processes described above, are meant to be
within the scope of this invention, which are defined in
the claims.
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 Exparto 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
B
8 i3~û~8
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 partTcular 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 preferred to utilize fibers from southern softwood
pulp due to their premium absorbency characteristics.
2~)~ Crosslinking agents applicable to the present development
t include C2 - C8 dialdehydes, as well as acid analogues of suchdialdehydes wherein the acid analogue has at least one aldehyde
group, and oligomers of such dialdehydes and acid analogues.
These compounds are capable of reacttng with at least two
hydroxyl groups in a single cellulose chain or on proximately
located cellulose chains in a single flber. 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 acld 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. PartTcular crosslinking agents
contemplated for use with the invention are glutaraldehyde,
9 134027~
glyoxal, and glyoxylTc acTd. Glutaraldehyde is especially
preferred, 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 additlonal 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.
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
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
20 ? .'~ ~, crosslinking agent penetrates into the interior of the individual
fiber structures. However, other methods of crosslTnking agent
treatment, including spraying of the fibers while in
individualized, fluffed form, are also within the scope of the
inventlon .
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 crosslTnking 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
in the substantial absence of interfiber bonds, t.e., while
interfiber contact is maintalned at a low degree of occurrence
relative to unftuffed pulp fibers, or the fibers are submerged in
1~40278
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.
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 condi-
tions 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 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
11 13~0278
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 Waring blender
and tangentially contacting the fibers with a rotating disk refiner
or wire brush. Preferably, an air stream is directed toward the
fibers durlng such defibratlon 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 provlde curl or twist
to the fibers in addition to curl or twist imparted as a result of
mechanical defibratlon.
;~ The fibers made according to the present inventlon have
unique combinations of stiffness and resiliency, which allow
....
absorbent structures made from the fibers to maintain high levels
of absorpt7vity, and exhibit high levels of resiliency and an
expansionary responsiveness to wetting of a dry, compressed
absorbent structure. In additlon to having the levels of
crosslinking within the stated ranges, the crosslinked fibers are
characterized by having water retention values tWRV s) of less
2 5 than about 60, and preferably between about 28 and 45, for
conventional, chemically pulped, papermaking fibers. The WRV of
a particular fiber is indicatlve of the level of crosslinking and the
degree of swelling of the fiber at the time of crosslinking. Those
skil1ed 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
l34û27~
12
about 25, and generally less than about 20. The particular
crosslinkinc~ 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 crosslTnking 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.
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
3~ longitudinal axis of the fiber. "Twist" refers to a rotation of thefiber about the perpendicular cross-section of the longitudinal
axis of the fiber. For exemplary purpose only, and without
intending to specifically limit the scope of the invention,
individualTzed, crosslinked fibers within the scope of the
invention having an average of about 6 (six) twists per millimeter
of fiber have been observed.
'3 ~3~U~78
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
1() 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
embodiment, the fibers are contacted with a solution whTch
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 . I n
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.
In general, any substance which catalyzes the crosslinking
mechanism may be utilized. Applicable catalysts include organic
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
sulfuric acid, hydrochloric acid and other mineral and organic
14 1340278
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
completlon 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. ~.1ineral acids are useful for
adjusting pH of the fibers whlle being contacted with the
crosslinking agent in solution, but are 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 conditlons, especially temperature and pH.
~ - In general, based upon technical and economic considerations,
catalyst Ievels of between about 10 wt. % and about 60 wt. %,
based on the weight of crosslinking agent added to the cellulosic
fibers, are preferred. For exemplary purposes, in the case
~herein 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. Most preferably, between about 5% and about 30%,
based upon the weight of the g lutaraldehyde, 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, more preferably between about pH
2 . 5 and about pH 3. 5, during the period of contact between the
crosslinking agent and the fibers.
15 i~0~78
The cellulosic fibers should generally be dewatered and
optionally dried. The workable and optimal consistencies will
vary depending upon the type of fluffing equipment utili2ed. In
the preferred embodiments, the cellulosic fibers are dewatered
and 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 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 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
~1 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
2 5 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 fTbers, 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 initlal 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
.... .. . ... .
16 13~0278
for the curing of the crossllnking agent. Conventional paper
drylng temperatures, (e.g ., 1 20~F to about 150 F~, for periods of
between about 30 mlnutes and 60 minutes, under static,
atmospheric conditlons will generally provide acceptable curing
efficiencies for fibers hav'ng moisture contents less than about
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 about 1 60~C, since exposure of the fibers to such
lt~ high temperatures in excess of about 1 60~C may lead to yellowing
or other damaging of the fibers.
The maximum level of crossltnking will be achieved when the
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~ relatlve 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
2t) : - centrifuged to remove interfiber fluid. (The FRV is further
defined and the Procedure For Determining FRV, is described
below. ) The amount of fluld 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
swelllng to a maximum level. This, Tn turn, is dependent upon
the level of crosslinking. As the level of intrafiber crosslinklng
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 descrlptive 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 fluld medium for analysis.
Generally, the FRV of a particular tiber crosslinked by
procedures wherein curing Is largely dependent upon drying,
17 13~0278
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 about S0, greater than about 25, and are preferably
between about 28 and about 45. 81eached SSK fibers having
between about 0. 5 mole % and about 2 . S mole % glutaraldehyde
reacted thereon, calculated on a cellulose anhydroglucose molar
~ basis, have been observed to have WRV's respectively ranging
from 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 higher than described herein, and have
WRV's less than about 25. Such fibers, as previously discussed,
have been observed to be exceedingly 5tiff and to exhibit lower
absorbent capabilities than the fibers of the present invention.
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 appticable to dry crosslinking
processes, also described above. The fibers, ~Nhile in sheeted
form, are dried and caused to crosslink preferably by heating the
fibers to a temperature of between about 1 20~C and about 1 60~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 fTbers made according to this
sheet crosslinking process are treated with a sufficient amount of
crosslinking agent such that between about 0. 5 mole % and about
. . . _,
18 i34û~78
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 o- 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 wett~ng of a dry absorbent
structu re.
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
2r~ with a sufficient amount of crosslinking agent such that between
about 0. S mole % and about 3. S 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 additlonal unexpected benefits as
hereinafter more fully dTscussed. 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
~ 19 1~0~78
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 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 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 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.
The relative amounts of curl and twist imparted to the
fibers is in part dependent upon the moisture content of the
fibers.
~k.',
. . _ . . .
20 i~4~J~78
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, mechanical treatment of moist fibers
S initlally 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 pract1ced on high
consistency moist pulp or pulp which has been de~atered to fiber
consistency of about 45% to about 55% (determined prior to
initialization of defibration).
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 defibratlon 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.
.
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 solutTon should be less than about 18%
and is preferably less than about 9%.
A bat of flbers which have not been mechanically defibrated
may also be contacted with a crosslink~ng solution as described
above.
The amounts of crosslinking agent and acid catalyst utilized
will depend upon such reaction conditions as consistency,
temperature, water content in the crosslinklng solution and
21 13~0278
~ibers, type of crosslinking agent and diluent In the crossllnking
solution, and the amount of crosslinking desired. Preferably, the
amount of crosslinking agent utilized ranges from about .2 wt ~ to
about 10 wt ~ (based upon the total, fiber-free weight of the
crosslinking solutionl. Preferred acld catalyst content is
additlonally 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 solut10n weight bas;s)
in crosslinking solutions containlng an acetic acid diluent,
preferred levels of glutaraldehyde, and a limited amount of water.
Slurries of fibers and crossllnking solution having fiber
consistencies of less than about 10 wt % are preferred for
crosstinking in conjunction with the crosslinking solutions
described above .
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
-~ 2 0 contacted with, and crosslinked in, the crosslinking solution. In
~i one embodiment, the fibers are contacted with the solution which
initially contains both the crossltnking agent and the acid
catalyst. The fibers are allowed to soak in the crosslinking
solut~on, during which time crosslTnking occurs. In another
embodiment, the fibers are contacted with the diluent and allowed
to soak prior to addTtion 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 withln the scope of this
3~ invention.
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
reactlon to occur in substantlal part after the crosslinklng agent
22 1~402~8
has had sufficient time to penetrate into the fibers. Reaction
condit;ons are preferably chosen so as to avoid instantaneous
crosslinking unless the crosslinking agent has already penetrated
into the fibers. Periods of reactlon 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, inc1uding substantlally instantaneous
crosslinking, and longer periods are meant to be within the scope
of this invention.
It is also contemplated to only partlally cure while in
solution, and subsequently complete the crosslinking reaction later
in the 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 and dried to completion. Preferably, the fibers are
subjected to a second mechanical defibration step which causes
2 0 - the crosslinked fibers to curl, e. g, ., fiufflng by defibration,
- ~ between the defluidizing and drying steps. Upon drying, the
curleci 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 twlst and curl described in
connection with the first mechanical defibratlon step are applicable
to this second mechanTcal defibration step. As used herein, the
term "defibratlon" shall refer to any of the procedures which may
be used to mechanically separate the fibers into substantially
3() 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 substantlally individual form if they
were not already in such form, and b) imparts curl and twist to
the fibers upon drying.
23 ~34~278
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
conf7guration 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 defibratlon 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.
I t 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 vvhile in a condition which is twisted
but partially 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
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 solution due to the low water content of the solutlon.
As discussed before, a critical aspect of the crosslinking solution
is that it does not cause any substantial swelling of the fibers.
3t) 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 conditlon.
24 1340278
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 ~hile 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
1~ while in an unswollen state. Fibers having this increase 7n 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 partlally swollen
crosslinked fibers shall be water retention values (\~IIRV). 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.
... .
- 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-m7scible crosslinking solution
containing a catalyst and crosslinking agent as previously
described. The fibers are preferably mechanically defibrated into
fiuff 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 voîatilTze the
crosslinking agent thus, possibly leading to atmospheric
contamination by, or high air treatment investments due to, the
crosslinking agent.
25 i3~0278
In a modification of the process described immediately above,
the fibers are defibrated and then presoaked in a high
concentratlon solution of crossl;nking agent and a fiber-swelling
diluent, preferably water. The crosslinking agent concentration
S 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
~ound to be useful solutions for presoaking the fibers. The
presoaked fibers are defluidized and submerged in a crosslinking
1() 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 defibratlon
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
unexpectedly high absorbency propertles for absorbent pads made
from the crosslinked fibers, even relatlve 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,
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 lald 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.
26 i-~40278
Once the individualized, crosslinked fibers are made, they
may be dry laid and directly formed into absorbent structures, or
wet lald 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 flbers into a smooth, wet laid sheet by conventional
wet sheet formation practices. Thls is because individualized,
crosslinked fibers rapidly flocculate when In solution. Such
flocculat70n may occur both in the headbox and upon deposition
1~) 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 sheet7ng individualized, crosslinked
fibers be provided, whereby wet laid absorbent structures and
densified pulp sheets for transit and subsequent defibration may
be formed.
.
Accordingly, a novet 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 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 dTrected 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 f10cculations of
the fibers and to disperse flocculations of fibers which have
already formed. The fiber setting step is preferably performed
27 1~ 78
with a cylindrical screen, such as a dandy roll, or with another
apparatus analogous in functlon 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.
In addition to inhibitlng 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 addTtional 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 formatlon 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.
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 conventlonal pulp sheet densities. Therefore, it may
be desirable to combine crosslinked fibers with uncrosslinked
fibers, such as those conventionally used Tn the manufacture of
absorbent cores. 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.
Lt is especially preferred to Tnclude between about 5% and about
30% of highly refined, uncrosslinked cellulosic fibers, based upon
the total dry weight of the sheet. Such highly refTned fibers are
28 13~027~
refined or beaten to a freeness level less than about 300 ml CSF,
and preferably less than 100 ml CSF. The uncrosslinked fibers
are preferably mixed with an aqueous slurry of the
individuali~ed, 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 subsequentlv 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 ,ormed 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
fibers, or from mixtures also containing uncrosslinked fibers, will
preferably have basis weights of less than about 800 g/m and
densitTes 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 300 g/m2 and about about 600 g/m2
and densities between 0.15 g/cm3 and about 0.30 g/cm3 are
especially contemplated for direct application as absorbent cores
in disposable articles such as diapers, tampons, and other
catamenial products. Structures hav7ng 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 low density tissue
3S sheets having densities which may be less than 0.10 g/cc.
29 13a~0278
For product applications wherein the crossllnked 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
crosslinklng agent is reduced to at least below about 0.03~, based
on the dry weight of the cellulosic fibers. One series of
treatments found to successfully remove excess crosslTnking agent
comprise, in sequence, washing the crosslinked tibers, allowing
the fibers to soak in an aqueous solution for an appreciable time,
screening the fibers, dewatering the f;bers, e.g., by
centrifuging, to a consistency of between about 40~ and about
80%, mechanically defibrating the dewatered fibers as prevlously
described and air drying the fibers. This process has been
found to reduce residual free crosslinking agent content to
between about 0,01% and about 0.15%.
In another method for reducing residual crosslinking agent,
readily extractable crosslinklng agent is removed by alkaline
washes. Alkalinity may be introduced by basic compounds such
as sodium hydroxide, or alternatively in the form of oxidizing
2 ) 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 Schi ff bases . The pH is preferably
maintained at a level of at least about pH 7, and more preferably
at least about 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 whlch 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 about 0. 0023% for
soaking periods of 30 minutes to two (2) hours. Minimal
additional benent is believed to incur for soaking times in excess
of about 30 minutes and for ammonium hydroxide concentrations Tn
excess of about 0. 01 N .
30 13~278
Both single stage oxidatlon and multiple stage oxidation were
found to be effective methods of extracting residual crosslinking
agent. Single stage washing with 0.1~ available chlorine (av.CI)
to about 0. 8~ av. Cl, based upon the dry ~eight of the fibers,
supplied in the form of sodium hypochlorite was observed to
reduce residual crosslinking agent levels to between about . 0015%
and about . 0025%.
In one novel approach to producing crosslinked,
individualized fibers, the source fibers are subjected to a
conventional multlstage 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 remalnder of the bleaching
sequence is completed. It has been found that acceptably low
crosslinking agent residual levels of less than about 0.0û6~ 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 additlonal 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 crossllnking 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
sodTum hypochlorite). The post-crosslinklng bleaching sequence
stages are preferably alkal1ne treatments performed at pH greater
than about pH 7, and more preferably greater than about pH 9.
In addttlon to providing effectlve reduction of residual
crosslinking agent, post-crosslinking, alkaline treatments have
been observed to facilitate the development of higher FRV lfluid
retention value) f7bers for equivalent levels of crosslinklng. The
hTgher FRV fibers have lower dry resillence, i.e., they are easier
to densify while in a dry state, while retaining substantially the
31 l3 1~278
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, tissue sheets, 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.
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
32 13~0278
2 . 5 mole ~ crosslinking agent, based upon a dry cellulose
anhydroglucose molar basis. Absorbent cores made from such
tibers have a desirabte 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.
In another preferred embodiment, the individualized,
crosslinked flbers 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 fluffed
tibers. The fibers of the present invention can be compressed to
a density higher than the equilibrium wet density, to form a thin
pad ~vhich, upon wetting, will expand, thereby increasing
3n absorbent capacity, to a degree significantly greater than
obtained for uncrosslinked fibers.
Especially high absorbency propertles, 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
33 13~0~78
into absorbent cores having dry densities greater than their
equilibrium wet denslties. Preferably, the absorbent cores are
compressed to densities of between about 0.12 g/cc and about
0.60 g/cc, wherein the corresponding equilibrium wet density is
less than the density of the dry compressed pad. 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 between about 0. 08
g/cc and about 0.12 g/cc, and are less than the densities of the
dry, compressed cores. 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 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 lo~Ner levels of crosslinking. Improved
performance relative to prior known individualized, crosslinked
fibers is obtained for all such structures.
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 flbers and prior known crosslinked fibers.
Such embodiments are meant to be included within the scope of
th is invention .
34 13~0278
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 80-mesh wire basket supported about 1~ 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 ~ravities for 19 to 21
minutes. The centrifuged fTbers are then removed from the
basket and weighed. The weighed fibers are dried to a constant
weight at 1 05~C and rewelghed. The water retention value is
calculated as follows:
(1 ) WRV = ~W D) x100
where,
~ - - W = wet weight of the centrifuged fibers:
20 - - D = dry weight of the f7bers: 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
.''5 combined measure of absorbent capacity and absorbency rate of
the cores.
A four inch by four inch absorbent pad weighing about
7. 5 ~ 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
' 35 13~0278
subsequent to introduction of the synthetic urine divided by the
mass of the fibers, bone dry basis.
PROCEDURE FOR DETER~11NING WET COMPRESStBlLlTY
The following procedure was utilTzed 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.
A four inch by four inch square pad weighing ~. 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
lS 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 equilibrate and the thickness is again
7 '' measured. The densitles are calculated for the pad at the
original 0.1 PSI load, the 1.1 PSI load and the second 0.1 PSI
2~ load, referred to as 0.1 PSIR (PSI rebound~ load. The void
volume reported in cc/g, is then determined for each respective
pressure load. The void volume is the reciprocal of th wet pad
density minus the fiber volume (0.95 cc/g). 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 densities indicate greater resistance to
wet compression and greater wet structural 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.
36 13~278
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 COt~PRESSIBILITY
S The following procedure was utilized to determine dry
compressibility of absorbent cores. Dry compressibility was
utilized as a measure of dry resilience of the cores.
A four inch by four inch square air lald 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 Ibst16 in'. 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 DtTERMlNlNG LEVEL OF
~;LUTARALDEHYDE REACTED WITH CELLULOSIC FIBERS
The follo~ing 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.
A sample of individualized, crosslinked fTbers is extracted
with 0.1 N HCI. The extract Ts 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 (DNPH1. The reaction is allowed to
proceed for 15 minutes after which a volume of chloroform is
3() 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
.
37 13~278
is determined by analyzing the chloroform layer by high pressure
liquid chromatography (HPLC~ for DNPH derivative.
The chromatographic conditions for HPLC analysis utilized
were - Cotumn: C-18 reversed phase: Detector: UV at 360 mm
5 Mobile phase 80:20 methanol: water; Flow rate: 1 mllmin.;
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.
Each of the four chloroform phases for each fiber sample Y~as
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
15 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
crosslinking agent, glutaraldehyde, on the absorbency and
resiliency of absorbent pads made from individualized, crosslinked
fibers. The individualized, crosslinked fibers were made by a
25 dry crosslinking process.
For each sample, a quantity of never dried, southern
softwood 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
3t) a selected amount of 50% aqueous solution of glutaraldehyde, 309~
(based upon the weight of the glutaraldehyde) zinc nitrate
, .
38 13~2~8
hexahydrate, demineral;zed water and a su-ficlent amount of l i~
HCI to decrease the slurry pl~i 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
5 centrifuging. Next, the dewatered fibers were air dried to a
fiber consistency of about 55~ 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 3,987,96a. The defibrated fibers were
lO placed in trays and cured at 145~C in an essentlally 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 about 20~C
water, soaked at 1% consistency for one (1) hour in 60~C water,
15 screened, washed with about 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
20 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
25 Table 1.
TABLE 1 13~0278
Sample Glutaraldehyde WRV Drip Cap. Wet Compressibility
# (mole ~ )@ 8 mlls (cc/gJ
Added/Reacted (9/9)0.1PSI1.1PSi 0.1PSIR
1 010 79.2 NIA 10.68 6.04 6.46
21.7310.44 51.0 6.98 11.25 5.72 6.51
3* NIA10.50 48.3 NIA NIA NIA NIA
4 2.09/0.62 46.7 N/A 11.25 6.05 6.09
s 3.16/0.99 36.3 15.72 12.04 6.09 6.86
6 4.15/1.54 35.0 15.46 13.34 6.86 8.22
7 6.46/1.99 32.8 12.87 13.34 6.93 8.31
8 8.42/2.75 33.2 16.95 13.13 7.38 8.67
~ 9 8.8912.32 29.2 13.59 12.56 6.51 7.90
1012.60/3.32 27.7 13.4, 12.04 6.63 7.82
15 * Taken from a separate sample of fibers.
( N / A ) - Not Avai lab le
EXAMPLE 2
The indTvidualized, crosslinked fibers of Example 1 were
formed into dry laid absorbent pads having a dry fiber density of
2~ 0.20g/cc. The pads were allowed to expand under unrestrained
conditions upon wetting with synthetic urine during execution of
the drip capacity procedure. The pads were subsequently tested
for absorbency resllfency and structural integrity according to
the previously outlined wet compressibility procedure. The
25 results are reported in Table 2. Drip capacity and ~vet
compressibility increased significantly at 0.50 mole %
glutaraldehyde .
TABLE 213~0278
SampleDrip Capacity Wet Cornpressibility
~t @ 8 ml/s rcclg)
(9l9) 0.1 PSI 1.1 PSI 0.1 PSIR
1 4.56 8.95 5.38 5.90
2 ~.84 8.31 q.80 5.72
3~ 11.05 11.71 6.63 7.31
4 9.65 8.90 5.11 6.10
12.23 11.87 6.35 ~.52
6 13.37 10.54 6.04 7.25
7 11.09 9.80 5.67 6.92
8 12.04 9.69 5.72 6.86
9 7.99 9.80 5.50 6.74
3.57 9.25 5.50 6.46
~ Taken from a separate sample of fibers.
EXAMPLE 3
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.
2n 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.
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 5~ available chlorine (av. Cl~ at about pH 2.5
and about 38~C for 30 minutes; caustic extraction - 12%
consistency slurry treated ~vith 1.4 g/l NaOH at about 74~C for 60
minutes; and hypochlorite treatment (H) - 12% consistency slurry
41 ~3~0278
treated with sufficient sodium hypochlorite, at 11 -l l . 5 pH between
38~C and 60~C for 60 minutes, to provide a 60-65 Elretho
brightness and a 15.5 - 16.5 cp viscosity. l'he partially bleached
fibers were processed into individualized, crosslinked fibers
5 utilizing glutaraldehyde as the crosslinking agent in accordance
with the process described in Example l. The fibers retained
2.29 mole % glutaraldehyde, calculated on a drv fiber cellulosic
anhydroc~lucose 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~,
extraction (E), and sodium hypochlorite (H) sequence (DEH). In
the chlorine dioxide stage (i~), individualized, crosslinked fibers
were soaked in a 10% consistency aqueous slurry also containing a
15 sufficient amount of sodium chlorite to provide 2% available
chlorine on a dry fiber weight basis. After mixing, the pH of
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
20 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
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.
Finally, for the sodium hypochlorite stage (H~, a 10%
consistency slurry of the fibers containing suffTcient 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,
30 rinsed to pH 5.0 and centrifuged to 62. 4% consistency. The
dewatered fibers were air dried, fluffed and dried to completion
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
35 level of extractable glutaraldehyde believed to be acceptable for
~2 13~0278
applications whereln the fibers are utilized in proximity to human
skin .
Also, it was found that pads made from the fibers which
were partially bteached, crosslinked and then bleached to
5 completion had unexpectedly higher fluid retentlon 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
10 bleaching sequence, were more compressible in a dry state.
Substantially equivalent results were obtained when a
peroxide b1eaching stage 1 P) was substituted for the final
hypochlorite stage (HJ. In the P stage, a 10~ consistency slurry
was treated with 0. 5% hydrogen peroxide, fiber weight basis, at
15 11-11 . 5 pH and 80~C for 90 minutes.
E X AM PLE 4
This example shows the effect of mixing an organic acid with
- an inorganic salt catalyst on the level of crosslinking reaction
completion. The fibers were crosslinked by a dry crosslinking
2 0 process .
A first sample of individualized, crosslinked fibers was
prepared as described in Example 1, wherein 4. 0 mole %
glutaraldehyde was retained subsequent to dewatering. Analytical
measurements of the fibers subsequent to crosslinking indicated
25 that the level of glutaraldehyde reacted on the fibers was 1. 58
mole %, corresponding to a reaction completion percentage of
about 37~.
A second sample of individualized, crosslinked fibers was
prepared the same way as the first sample described in this
30 example, except that in addition to the zinc nitrate catalyst, a
quantity of citric acid equivalent to 10 wt. % of the
43 1340278
glutaraldehyde mixed with the zinc nitrate in the pulp slurry as
an additional catalyst. Analytical measurements of the fibers
subsequent to crosslinking indicated that the 1evel of
glutaraldehyde reacted on the fibers was 2 . 45 mole %,
5 corresponding to a reaction completion percentage of about 61%,
tmolar basis) a 55.1~ increase in reaction completion relative to
the unmixed zinc nitrate catalyst sample.
EXAMPLE 5
This example disclosed the use of low levels of glyoxylic
ln acid, a dialdehyde acid analogue having one aldehyde group, in a
dry crosslinking process as described in Example 1.
A fibrous slurry of never dried SSK containing a sufficient
amount of glyoxylic acid to provide an estimated 1. 2% glyoxylic
acid reacted with cellulosic fibers, on a cellulose anhydroglucose
15 molar basis and a zinc nitrate hexahydrate catalyst was prepared.
The centrifuged fibers had a fiber consistency of about 38~ and
contained about 1. 06 wt % glyoxylic acid, on a dry fiber basis .
The catalyst to crosslinking agent ratio was about 0. 30. The pH
of the slurry at the start of crosslinking was about 2.16. The
20 fibers were individualized and crosslinked according to the
procedures described in Example 1.
In a second sample about 0.53 wt % glyoxylic acid, based on
a dry fiber weight basis, was addèd to the fibers to provide an
estimated level of glyoxylic acid reacted with the fibers of about
25 0.6 mole %, calculated on a cellulose anhydroglucose molar basis.
The individualized, crosslinked fibers were otherwise prepared in
accordance with the sample described immediately above, except
that the slurry pH at the start of crosslinking was about 2. 35 .
Absorbent structures of 0.1 g/cc acid 0. 2 gtcc densit7es
30 were made from the individualTzed, crosslinked fibers as described
in Example 2. The drip capacities, the wet compressibilities at
44 ~3~ 8
0,1 PSI, 1.1 PSI and 0.1 PSIR, and the wicking o- the pads were
signiflcantly greater than for similar density absorbent structures
made from conventional, uncrosslinked fibers.
E XAM PLE 6
This example discloses a method for making individualized,
crosslinked fibers by a nonaqueous solutlon cure crosslinking
process, wherein the fibers are crosslinked in a substantially
nonswollen, collapsed state.
Never dried, SSK bleached fibers are provided and dried to
fiber weight consistency of about 679~. The fibers are
mechanically defibrated utilizing a three-stage fluffing device as
described in U.S. Patent No. 3,987,968. The defibrated fibers
are then dried to completion at 105~C for a period of four (4)
hours. The dried fibers are next placed in a 10% consistency
slurry of fibers and crosslinking solutlon, wherein the
crosslinking solution contains between about 0. 5 wt ~ and about
6 . o wt % of 50~ glutaraldehyde solution, an additional quantity of
water of between about 1 . 5 wt % and about 13 wt %, between
about 0.3 wt ~ and about 3.0 wt ~ acid catalyst (HCI or H2SO4~,
and a balance of acetic acid. The fibers are maintained in the
crosslinking solution for a period ranging between 0 . 5 hours and
6 hours, at a temperature of about 25~C, during which time the
primarily intrafiber crosslink bonds are formed. The fibers are
then washed with cold water and centrifuged to a fiber
consistency of between about 60 wt % and about 65 wt %,
defibrated with a three-stage fluffer and dried at 105~C for a
period of four hours. Such fTbers will generally have between
about 0. 5 mole % and about 3.5 mole ~ crosslinking agent,
calculated on a cellulose anhydroglucose molar basis, reacted
therein. The dried fibers may be air laid to form absorbent
structures and compressed to a density of 0.10 glcc or 0.20 g/cc
with a hydraulic press, similar to the pads formed in Examples 1
and 2, or to another density as desired.
13~0278
EXAMPLE 7
This example discioses a method for making individualized,
crosslinked fibers by a nonaqueous solution cure crosslinking
process, wherein the fibers are crosstinked in a partially, but not
5 completely swollen condit70n.
The process followed is identical to that described in
Example 6, except that the never dried SSK fibers initially are
dried to a 50-SS wt % fiber consistency before defibration, and
the defibrated fibers are dried to a moisture content of between
10 about 18 wt % and about 30 wt % as a result of such defibration
and, if required, an additional drying step. The fibers, which
have a partlally swollen conflguration, are then crosslinked,
washed, centrifuged, defibrated and dried as described in
Example 6. Relative to the crosslinked fibers of Example 6, the
lS partially swollen, crosslinked fibers of this example having
substantially equivalent glutaraldehyde levels have higher WRV
and make absorbent structures having higher drip capacity and
wet compressibility.
EXAMP~E 8
This example discloses a method for making individualized,
crosslinked fibers by a nonaqueous solution crossiinking process
wherein the fibers are presoaked in a high concentration aqueous
solution containing glutaraldehyde prior to crosslinking in a
substantlally nonaqueous crosslinking solution.
Never dried SSK fibers are mechanicatly separated by the
defibration apparatus described in U. S. Patent No. 3, 987, 968 and
presoaked in an aqueous solution containing 50 wt %
glutaraldehyde and S0 wt % water for a period of between about 2
minutes and about 30 minutes. The fibers are then mechanically
pressed to provide partially swollen glutaraldehyde-impregnated
fibers. The fibers are next crosslinked in the presence of a
catalyst, washed, centrifuged, defibrated and dried as described
46 13 ~0~78
in Example 6. Relative to crosslinked fibers of Examples 6 or J
having equivalent levels of crosslinking, the fibers of the present
example made absorbent structures having higher dr;p capacities
and wet compressibilities.
EXAMPLE 9
Individualized, crosslinked fibers were prepared according to
the process described in Example 7. The crosslinking solutions
contained: 2% glutaraldehyde, t.29% H2SO4, 3% water, balance
acetic acid for Samples 1 and 2; and 0 . 5% glutaraldehyde, 0 . 6%
H2SO4, 1. 2% water, balance acetic acid for Samples 3 and 4. The
moisture content of the fibers going into the crosslinking solution
was 30% for samples 1 and 2, and 18% for samples 3 and 4.
Glutaraldehyde reacted to form crosslink bonds with the fiber.
WRV, drip capacity and wet compressibility rebound 10.1 PSIR
were measured and are reported below in Table 3.
TABLE 3
SampleFiber Glutar. Density WRV Drip Wet
#Moist. Cont. Reacted 8 mlls Compress.
(cc/g)
(mole %1(mole %) (glcc) (~)(9/9)0.1 PSIR
1 30 3.2 0.10 55 NIA 8.4
2 30 3.2 0.20 55 14.4 ~.7
3 18 1.6 0.10 46 N/A NIA
4 18 1.6 0.20 46 12.6 J.2
25 (NIA) - Not Available
Example 1 0
The purpose of thls example is to exemplify a process for
making wet-laid sheets containing individualized, crosslinked
fibers .
47 139,~)~78
A 0. 55% consistency slurry of a blend of fibers containing
90% individualized, crosslinked fibers made according to the
process described in Exampte 1 and 10~ conventional,
uncrosslinked fTbers having a freeness of Iess than 100 CSF were
deposited in flocculated, clumped fibers on a conventlonal 84-mesh
Fourdrinier forming wire. The papermaking flow rate out of the
headbox was 430 kglmin. ImmedTately after deposition, a serles
of five streams of water of sequentially decreasing flow rates were
directed upon the fibers. The five streams of water provided a
cumulative flow ratio 85 kg . watertkg. bone dry (b.d. ) fiber.
The showers were all spaced ~vithin an approximately 1 meter long
area parallel to the direction of travel of the forming wire. Each
stream of water was showered onto the fibers through a linear
series of 1/8 (3.2 mm) ID circular aperatures spaced 1/2 (12.7
lS mm) apart and extending across the width of the forming wire.
The approximate percentage of flow, based upon the total flow
rate, and velocity of flow through the aperatures for each of the
showers was as follows: Shower 1-37% of total fiow, 170 m/min.;
Shower 2-36% of total flow, 165 m/min.; Shower 3-13% of total
flow, 61 m/min.; Shower 4-9% of total flow, 41 m/min.; Shower
S-S~ of total flow, 20 m/min. Immediately after the fifth shower,
the fibers were set by treatment wTth a cylindrical, screened roll
known in the art as a Dandy Roll. The Dandy Roll pressed the
fibers, which at the time of setting were in a high consistency
slurry form, against the forming wire to set the fibers to form of
a wet sheet. The sheet was similar in appearance to conventional
fibrous pulp sheets.
The scope of the invention is to be defined according to the
following claims.
