Note: Descriptions are shown in the official language in which they were submitted.
~3(~ 4
PROCESS FOR MAKING WT-LAID STRUCTlJRES
CONTAINING INDIVIDUALIZED, STIFFENED FIBERS
FIELD OF INVENTION
This invention is concerned with individualized, stiffened
fibers and processes for forming such fibers into wet-laid
structures .
BACKGROUND OF THE INVENTION
Fibers stiffened in substantially individualized form and
various methods for making such fibers have been described in
the art. The term "individualized, stiffened, crosslinked fibers",
refers to cellulosic fibers that have primarily intrafiber chemical
lt) crosslink bonds. That is, the crosslink bonds are primarily
bet~een cellulose molecules of a single fiber, rather than between
cellulose molecules of separate fibers. Individualized, crosslinked
fibers and other individualized, stiffened fibers are generally
regarded as being useful in absorbent product applications. In
15 general, three categories of processes have been reported for
making individualized, stiffened fibers by forming intrafiber
crosslink bonds. These processes, described belo~, are herein
referred to as 1 ) dry crosslinking processes, 2) aqueous solution
crosslinking processes, and 3) substantially non-aqueous solution
crosslinking processes. The fibers themselves and absorbent
structures containing individualized, stiffened 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
S capacity. Additionally, absorbent structures made from individ-
ualized crosslinked fibers generally exhibit increased ~et
resilience and increased dry resilience relative to absorbent
structures made from uncrosslinked fibers. The term "resilience"
shall hereinafter refer to the ability of pads made from cellulosic
10 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 vvhiie the fibers are in a substantially
dry condition. Wet resilience specifically refers to the ability of
15 an absorbent structure to expand upon release of compressional
force applied while the fibers are in a moistened condition. For
the purposes of this invention and consistency of disclosure, wet
resilience shall be observed and reported for an absorbent
structure moistened to saturation.
2Q Processes for making individualized, crosslinked fibers with
dry crosslinking technology are described in U . S . Patent No.
3,224,926 issued to L. J. Bernardin on December 21, 1965.
Individualized, crosslinked fibers are produced by impregnating
swollen fibers in an aqueous solution with crosslinking agent,
dewatering and defiberizing the fibers by mechanical action, and
drying the fibers at elevated temperature to effect crosslinking
while the fibers are in a substantially individual state. The
fibers are inherently crosslinked in an unswollen, collapsed state
as a result of being dehydrated prior to crosslinking. Processes
as exemplified in U.S. Patent Nos. 3,224,926, wherein
crosslinking is caused to occur while the fibers are in an
uns~ollen, 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
13~
April 22, 1969, to soak the fibers in an aqueous solution of a
crosslinking agent to reduce interfiber bonding capacity prior to
carrying out a dry crosslinking operation similar to that described
in U.S. Patent No. 3,224,926. This time consuming pretreatment,
5 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
fibers are produced by crosstinking 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
15 on cellulosic fibers, aqueous solution crosslinked fibers are
crosslinked while in an uncollapsed, swollen state. Relative to
dry crosslinked fibers, aqueous solution crosslinked fibers as
disclosed in U.S. Patent No. 3,241,553 have greater flexibility
and less stiffness, and are characterized by higher fluid retention
20 value ( FRY) . 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 disclosed for
25 producing individualized, crosslinked fibers by contacting
dehydrated, nonswollen fibers with crosslinking agent and
catalyst in a substantially nonaqueous solution which contains an
insufficient amount of water to cause the fibers to swell.
Crosslinking occurs while the fibers are in this substantially
3n nonaqueous solution. This type of process shall hereinafter be
referred to as a nonaqueous solution crosslinked process: and the
fibers thereby produced, shall be referred to as nonaqueous
solution crosslinked fibers. Like dry crosslinked fibers,
nonaqueous solution crosslinked fibers are highly stifféned by
13(~
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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.
One difficulty which has been experienced with
respect to individualized, crosslinked fibers,
especially dry crosslinked and nonaqueous solution
crosslinked fibers, is that the fibers rapidly
flocculate upon wet-laying on a foraminous forming 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.
It is an object of an aspect of this invention to
provide improved processing for forming individualized
crosslinked fibers into wet-laid structures.
SUMMARY OF THE INVENTION
Various aspects of the invention are as follows:
A process for continuously making a fibrous sheet
comprising crosslinked cellulosic fibers which
characteristically flocculate in an aqueous slurry and
which have a high propensity for flocculating when an
aqueous slurry of said fibers is deposited on a forming
wire in a papermaking-type apparatus, said process
comprising the steps of:
a. providing an aqueous fibrous slurry comprising
said fibers and water, said fibers having been
air dried and crosslinked while individualized
and unrestrained with a crosslinking agent
selected from the group consisting of C2-C8
dialdehydes, C2-C8 dialdehyde acid analogues
6~
~ 4a
having at least one aldehyde group, and
oligomers of said dialdehydes and dialdehyde
acid analogues, said fibers having been
contacted with a sufficient amount of said
crosslinking agent that between about 0.5 mole
~ and about 3.5 mole % of crosslinking agent,
calculated on a cellulose anhydroglucose molar
basis, have been reacted with said fibers to
form intrafiber crosslink bonds, and so that
said fibers have a water retention value of
from about 25 to about 60;
b. depositing said slurry on a traveling
foraminous forming wire in a papermaking-type
apparatus whereupon the free water in said
slurry drains through said traveling
foraminous forming wire;
c. downwardly directing a plurality of showers of
water directly onto the slurry as draining
progresses, said showers being oriented in the
cross machine direction and being spaced from
each other in the machine direction, said
showers also being of progressively less flow
rates and velocities but having sufficient
flow rates and velocities to substantially
disperse flocculations of said fibers and
inhibit further formation of flocculations of
said fibers to provide said fibers in
substantially unflocculated form; and then
d. setting said fibers in sheeted form while said
fibers are substantially unflocculated by
pressing them against said formation wire with
a screen covered cylindrical roll.
A process for continuously making a densified-form
fibrous sheet comprising crosslinked cellulosic fibers
which characteristically flocculate in an aqueous slurry
and which has a high propensity for flocculating when an
aqueous slurry of said fibers is deposited on a forming
~.
~4b
wire in a papermaking-type apparatus, said process
comprising the steps of:
a. providing an aqueous fibrous slurry comprising
from about 70% to about 95%, by weight, of
said crosslinked fibers, said fibers having
been air dried and crosslinked while
individualized and unrestrained with a
crosslinking agent selected from the group
consisting of C2-C8 dialdehydes, C2-C8
dialdehyde acid analogues having at least one
aldehyde group, and oligomers of said
dialdehydes and dialdehyde acid analogues,
said fibers having been contacted with a
sufficient amount of said crosslinking agent
that between about 0.5 mole % and about 3.5
mole % of crosslinking agent, calculated on a
cellulose anhydroglucose molar basis, have
been reacted with said fibers to form
intrafiber crosslink bonds, and so that said
fibers have a water retention value of from
about 25 to about 60, from about 30% to about
5%, by weight, of highly refined,
uncrosslinked cellulosic fibers having a
freeness level not greater than 300 ml CSF and
water;
b. depositing said slurry on a traveling
foraminous forming wire in a papermaking-type
apparatus whereupon the free water in said
slurry drains through said traveling
foraminous forming wire;
c. downwardly directing a plurality of showers of
water directly onto the slurry as draining
progresses, said showers being oriented in the
cross machine direction and being spaced from
each other in the machine direction, said
showers also being of progressively lesser
flow rates and velocities but having
~'
94
4c
sufficient flow rates and velocities to
substantially disperse flocculations of said
fibers and inhibit further formation of
flocculations of said fibers to provide fibers
in substantially unflocculated form; and then
d. setting said fibers in a densified sheeted
form while said fibers are substantially
unflocculated by pressing them against said
formation wire with a screen covered
cylindrical roll.
~3~6~4
The step of setting the ibers into sheeted form may be
performed by pressing the fibers against the forming wire
with a screened roll, such as a cylindrical Dandy Roll.
Preferably, a plurality of streams fo fluid having
sequentially decreasing volumetric flow rates are directed
upon the fibers. The individualized, stiffened fibers may
also be mixed with conventional, unstiffened fibers or highly
refined, unstiffened fibers while in slurry form.
DETAILED DESCRIPTION OF THE INVENTIO~I
Individualized, stiffened fibers made from cellulosic fibers of
diverse natural origin are applicable to the invention. Digested
fibers from softwood, hardwood or cotton linters are preferably
utilized. Fibers from Esparto grass, bagasse, kemp, flax, and
other lignaceous and cellulosic fiber sources may also be utilized
as raw material in the invention. The fibers may be supplied in
slurry, unsheeted or sheeted form. Fibers supplied as wet lap,
dry lap or other sheeted form are preferably rendered into
unsheeted form by mechanically disintegrating the sheet,
preferably prlor to contacting the fibers with the crosslinking
agent. Also, preferably the fibers are provided in a wet or
moTstened 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. Also applicable to the present 7nvention
are individual fibers of synthetic origin which tend to flocculate
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in solution due to fiber chemisty, geometry or a combination of
these or other factors.
The optimum cellulose fiber source utilized in conjunction
with this invention will depend upon the particular end use
5 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,
10 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
15 softwood pulp due to its premium absorbency characteristics.
Any individualized, stiffened fibers which flocculate in solution
are intended to be within the scope of this invention. These
include the fibers made according to the dry crosslinking
processes and nonaqueous solution crosslinking processes
20 disclosed in the Backround Of The Invention. Also contemplated
are individualized, stiffened fibers treated with resins or other
polymeric compounds as exemplified in U.S. Patent No. 3,819,470,
issued to Shaw, D. L., et al, on June 25, 1974 and U . S. Patent
No. 3,756,913, issued to Wodka, E. A., on September 4, 1973.
25 This invention is believed to be most useful for individualized
stiffened fibers which have twisted, curled configurations.
Processes for making such fibers with monomeric crosslinking
agents are discussed below.
CrosslinkTng agents applicable to the present development
30 which are preferred include C2 - C8 dialdehydes, as well as acid
analogues of such dialdehydes wherein the acid analogue has at
least one aldehyde group, and oligomers of such dialdehydes and
acid analogues. These compounds are capable of reacting with at
least two hydroxyl groups in a single cellulose chain or on
35 proximately located cellulose chains in a single fiber. Those
13~P6894
knowledgeable in the area of crosslinking agents will recognize
that the dialdehyde crosslinking agents described above will be
present, or may react in a variety of forms, including the acid
analogue and oligomer forms identified above. All such forms are
5 meant to be included within the scope of the preferred
embodiments. Reference to a particular crosslinking agent shall
therefore hereinafter refer to that particular crosslinking agent as
well as other forms as may be present in an aqueous solution.
Particular crosslinking agents contemplated for use with the
) invention are glutaraldehyde, glyoxal, and glyoxylic acid.
Glutaraldehyde is especially 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,
15 crosslink bonds.
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
20 pads made from individua!ized, crosslinked fibers having between
about 0. S mole ~ and about 3 . S mole ~ crosslinking agent,
calculated on a cellulose anhydroglucose molar basis, reacted with
the fibers.
Preferably, the crosslinking agent is contacted with the
25 fibers in a liquid medTum, under such conditions that the
crosslinking agent penetrates into the ;nterior of the indb, idual
fiber structures. However, other methods of crosslinking agent
treatment, including spraying of the fibers while in
individualized, fluffed form, are also within the scope of the
30 invention.
Generally, the fibers will also be contacted with an
appropriate catalyst prior to crosslinking. The type, amount,
and method of contact of catalyst to the fibers will be dependent
upon the particular crosslinking process practiced. These
variables will be discussed in more detail below.
Once the fibers are treated with crosslinking agent and
catalyst, the crosslinking agent is caused to react with the fibers
5 in the substantial absence of interfiber bonds, i.e., while
interfiber contact is maintained at a low degree of occurrence
relative to unf1uffed pulp fibers, or the fibers are submerged in
a solution that does not facilitate the formation of interfiber
bonding, especially hydrogen bonding. This results in the
10 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
20 under acidic reaction conditions. Therefore, acid catalyzed
crosslinking conditions are highly preferred for the purposes of
this invention.
The fibers are preferably mechanically defibrated into a low
density, individualized, fibrous form known as fluff prior to
25 reaction of the crosslinking agent with the fibers. Mechanical
defTbration may be performed by a variety of methods whTch 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.
30 One type of devlce which has been found to be particularly useful
for defibrating the cellulosic fibers is the three stage fluffing
device described in U . S . Patent No. 3, 987, 968, issued to D . R .
Moore and O. A. Shields on October 26, 1976.
~`:
13U~
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
5 individualized fibers have imparted thereto an enhanced degree of
curl and twist relative to the amount of curl and twist naturally
present in such fibers. It is believed that this additional curl
and twist enhances the resilient character of absorbent structures
made from the finished, crosslinked fibers. It is also believed
10 that this additional curl and twist enhances the degree of
flocculation of the individualized, crosslinked fibers.
Other applicable methods for defibrating the cellulosic fibers
include, but are not limited to, treatment with a ~laring blender
and tangentially contacting the fibers with a rotating disk refiner
15 or wire brush. Preferably, an air stream is directed toward the
fibers during such defibration to aid in separating the fibers into
substantially individual form.
Regardless of the particular mechanical device used to form
the fluff, the fibers are preferably mechanically treated while
20 initially containing at least about 209~ moisture, and preferably
containing between about 409~ and about 50~ moisture.
Mechanical refining of fibers at high consistency or of
partially dried fibers may also be utilized to provide curl or twist
to the fibers in addition to curl or twist imparted as a result of
25 mechanical def?bration.
The fibers made according to the present invention have
unique combinations of stiffness and resiliency, which allow
absorbent structures made from the fibers to maintain high levels
of absorptivity, and exhibit high levels of resiliency and an
30 expansionary responsiveness to wetting of a dry, compressed
absorbent structure. In addition to having the levels of
crosslinking within the stated ranges, the crosslinked fibers are
characterized by having water retention valuec (WRV's) of less
than about 60, and preferably between about 28 and 4S, for
conventional, chemically pulped, papermaking fibers. The WRV of
a particular fiber is indicative of the level of crosslinking and the
degree of swelling of the fiber at the time of crosslinking. Those
5 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
10 about 25, and generally less than about 20. The particular
crosslinking process utilized will, of course, affect the WRV of
the crosslinked fiber. However, any process which will result in
crosslinking levels and WRV's within the stated limits is believed
to be, and is intended to be, within the scope of this invention.
15 Applicable methods of crosslinking include dry crosslinking
processes and nonaqueous solution crosslinking processes as
generally discussed in the 8ackground Of The Invention. Certain
preferred dry crosslinking and nonaqueous solution crosslinking
processes, within the scope of the present invention, will be
20 discussed in more detail below. Aqueous solution crosslinking
processes wherein the solution causes the fibers to become highly
swollen will result in fibers having WRV's which are in excess of
about 60. These fibers will provide insufficient stiffness and
resiliency for the purposes of the present invention.
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,
30 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 ~hile the
fibers are maintained in substantially individual form. The
defibration step, apart from the drying step, is believed to
35 impart additional curl. Subsequent drying is accompanied by
twisting of the fibers, with the degree of twist being enhanced
1306~
11
by the cur!ed geometry of the fiber. As used herein, fiber
"curl" refers to the geometric curvature of the fiber about the
longitudinal axis of the fiber. "Twist" refers to a rotation of the
fiber about the perpendicular cross-section of the longitudinal
5 axis of the fiber. For exemplary purposes, and without intending
to specifically limit the scope of the invention, individualized,
crosslinked fibers within the scope of the invention having an
average of about 6 (six) twists per millimeter of fiber have been
observed .
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
15 conditions. On the other hand, drying fibers in sheeted form
results in dried fibers which are not twisted and curled as fibers
dried in substantially individualized form. It is believed that
interfiber hydrogen bonding "restrains" the relative occurrence of
twisting and curling of the fiber.
There are various methods by which the fibers may be
contacted with the crosslinking agent and catalyst. In one
embodiment, the fibers are contacted with a solution which
initially contains both the crosslinking agent and the catalyst. In
another embodiment, the fibers are contacted with an aqueous
solution of crosslinking agent and allowed to soak prior to
addition of the catalyst. The catalyst is subsequently added. In
a third embodiment, the crosslinking agent and catalyst are added
to an aqueous slurry of the cellulosic fibers. Other methods in
addition to those described herein will be apparent to those
skilled in the art, and are intended to be included within the
scope of this invention. Regardless of the particular method by
which the fibers are contacted with crosslinking agent and
catalyst, the cellulosic fibers, crosslinking agent and catalyst are
preferably mixed andlor allowed to soak sufficiently with the
13(~
12
fibers to assure thorough contact with and impregnatior ,f the
individual fibers.
In general, any substance which catalyzes the crosslinking
mechanism may be utilized. Applicable catalysts include organic
5 acids and acid salts. Especially preferred catalysts are salts
such as aluminum, magnesium, zinc and calcium salts of chlorides,
nitrates or sulfates. One specific example of a preferred salt is
zinc nitrate hexahydrate. Other catalysts include acids such as
sulfuric acid, hydrochloric acid and other mineral and organic
) acids. The selected catalyst may be utilized as the sole
catalyzing agent, or in combination with one or more other
catalysts. It is believed that combinations of acid salts and
organic acids as catalyzing agents provide superior crosslinking
reaction efficiency. Unexpectedly high levels of reaction
15 completion have been observed for catalyst combinations of zinc
nitrate salts and organic acids, such as citric acid, and the use
of such combinations is preferred. Mineral acids are useful for
adjusting pH of the fibers while being contacted with the
crosslinking agent in solution, but are preferably not utilized as
20 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 uti1ized is, of course,
dependent upon the particular type and amount of crosslinking
agent and the reaction conditions, especially temperature and pH.
In general, based upon technical and economic considerations.
catalyst levels of between about 10 wt. ~ and about 60 wt. %,
based on the weight of crosslinking agent added to the cellulosic
fibers, are preferred. For exemplary purposes, in the case
wherein the catalyst u.ilized 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
13~'6~t~
13
preferred. Most preferably, between about 5% and about 30g~,
based upon the weight of the glutaraldehyde, of an organic acid,
such as citric acid, is also added as a catalyst. It is additionally
desirable to adjust the aqueous portion of the cellulosic fiber
5 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.
The cellulosic fibers should generally be dewatered and
10 optionally dried. The workable and optimal consistencies will
vary depending upon the type of fluffing equipment utilized. In
the preferred embodiments, the cellulosic fibers are dewatered
and optimally dried to a consistency of between about 30% and
about 8096. More preferably, the fibers are dewatered and dried
to a consistency level of between about 40% and about S096.
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
20 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,
2 5 under conditions such that the utilization of high temperature for
an extended period of time is not required. Excessively high
temperature at this stage of the process may result in the
premature initiation of crosslinking. Preferably, temperatures in
excess of about 1 60~C are not maintained for periods of time in
30 excess of 2 to 3 seconds. Mechanical defibration is performed as
previously described.
The defibrated fibers are then heated to a suitable
temperature for an effective period of time to cause the
crosslinking agent to cure, i . e., to react with the cellulosic
14
fibers. The rate and degree of crosslinking depends upon
dryness of the fibers, temperature, amount and type of catalyst
and crosslinking agent and the method utilized for heating and/or
drying the fibers while crosslinking is performed. Crosslinking
5 at a particular temperature will occur at a higher rate for fibers
of a certain initial moisture content when accompanied by a
continuous air through drying than when sub~ected to
drying/heating in a static oven. Those skilled in the art will
recognize that a number of temperature-time relationships exist
lO for the curing of the crosslinking agent. Conventional paper
drying temperatures, (e.g., 120F to about 150CF), for periods of
between about 30 minutes and 60 minutes, under static,
atmospheric conditions will generally provide acceptable curing
efficiencies for fibers having moisture contents less than about
15 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 60C, since exposure of the fibers to such
high temperatures in excess of about l 60C may lead to yellowing
20 or other damaging of the fibers.
The maximum level of crosslinking will be achieved when the
fibers are essentially dry (having less than about 596 moisture).
Due to this absence of water, the fibers are crosslinked while in
a substantially unswollen, collapsed state. Consequently, they
25 characteristically have low fluid retention values lFRV) relative to
the range applicable to this invention. The FRV refers to the
amount of fluid calculated on a dry fiber basis, that remains
absorbed by a sample of fihers that have been soaked and then
centrifuged to remove interfiber fluid. (The FRV is further
30 defined and the Procedure For Determining FRV, is described
below. ) The amount of fluid that the crosslinked fibers can
absorb is dependent upon their ability to swell upon saturation
or, in other words, upon their interior diameter or volume upon
swelling to a maximum level. This, in turn, is dependent upon
35 the level of crosslinking. As the level of intrafiber crosslinking
increases for a given fiber and process, the FRV of the fiber will
~3C~6~3~34
decrease until the fiber does not s-vetl at all upon wetting.
Thus, the FRV value of a fiber is structurally descriptive of the
physical condition of the fiber at saturation. Unless otherwise
expressly indicated, FRV data described herein shall be reported
5 in terms of the ~ater retention value (WRV) of the fibers. Other
fluids, such as salt water and synthetic urine, may also be
advantageously utilized as a fluid medium for analysis.
Generally, the FRV of a particular fiber crosslinked by
procedures wherein curing is largely dependent upon drying,
10 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 50, 9 reater than about 25, and are preferably
15 between about 28 and about 45. Bleached 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
20 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
25 WRV's less than about 25. Such fibers, as previously discussed,
have been observed to be exceedingly stiff and to exhibit lower
absorbent capabilities than the fibers of the present invention.
.
In another process for making individualized, crosslinked
3() fibers by a dry crosslinking process, cellulosTc 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
35 contains one of the catalysts applicable to dry crosslinking
processes, also described above. The flbers, while in sheeted
form, are dried and caused to crosslink preferably by heating the
i3~
fibers to a temperature of between about 1 20C and about 1 60C .
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. 3,9a7,968 or may be performed with
other methods for defibrating fibers as may be known in the art.
The individualized, crosslinked fibers made according to this
sheet crosslinking process are treated with a sufficient amount of
crosslinking agent such that between about 0. 5 mole % and about
3.5 mole % crosslinking agent, calculated on a cellulose
anhydroglucose molar basis and measured subsequent to
defibration are reacted with the fibers in the form of intrafiber
crosslink bonds. Another effect of drying and crosslinking the
fibers while in sheet form is that fiber to fiber bonding restrains
lS 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
20 relatively untwisted fibers made the sheet curing process
described above would be expected to exhibit lo~er wet resiliency
and lower responsiveness to wetting of a dry absorbent
structure .
Another category of crosslinking processes applicable to the
25 present invention is nonaqueous solution cure crosslinking
processes. The same types of fibers applicable to dry
crosslinking processes may be used in the production of
nonaqueous solution crosslinked fibers. The fibers are treated
with a sufficient amount of crosslinking agent such that between
:~0 about 0. 5 mole ~ and about 3. 5 mole % crosslinking agent
subsequently react with the fibers, wherein the level of
crosslinking agent reacted is calculated subsequent to said
crosslinking reaction, and with an appropriate catalyst. The
crosslinking agent is caused to react while the fibers are
35 submerged in a solution which does not induce any substantial
levels of swelling of the fibers. The fibers, however, may
13(3t~ 4
17
contain up to about 30~ water, or be otherwise swollen in the
crosslinking solution to a degree equivalent to fibers having about
a 30~ moisture content. Such partially swollen fiber geometry has
been found to provide additional unexpected benefits as
5 hereinafter more fully discussed. The crosslinking solution
contains a nonaqueous, water-miscible, polar diluent such as, but
not limited to, acetic acid, propanoic acid, or acetone. Preferred
catalysts include mineral acids, such as sulfuric acid, and halogen
acids, such as hydrochloric acid. Other applicable catalysts
l0 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 .
(2uere, on July 12, 1977. The crosslinking solution may
15 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
2 0 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 solutTon itself or in a prior
25 treatment stage including, but not limited to, saturation of the
flbers 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
30 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
-
13(?6~3~-~
18
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 sùch as those previously
5 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.
Without limiting the scope of the invention, it is believed that the
10 fibers naturally twist upon drying under conditions wherein fiber
to fiber contact is low, i . e ., when the fibers are in an
individualized ~orm. Also, mechanical treatment of moist fibers
initially causes the fibers to become curled. When the fibers are
then dried or partially dried under substantially unrestrained
15 conditions, they become twisted with the degree of twist being
enhanced by the additional amount of curl mechanically imparted.
The defibration fluffing steps are preferably practiced on high
consistency moist pulp or pulp which has been dewatered to fiber
consistency of about 45% to about 55% (determined prior to
20 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 defibration step
has not already provided fibers having moisture contents within
25 that range. The drying step should be performed ~vhile 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
13~
19
content of the crosslinking solution should be less than about 18
and is preferably less than about 9~.
A bat of fibers which have not been mechanically defibrated
may also be contacted with a crosslinking solution as described
5 above.
The amounts of crosslinking agent and acid catalyst utilized
will depend upon such reaction conditions as consistency,
temperature, water content in the crosslinking solution and
fibers, type of crosslinking agent and diluent in the crosslinking
10 solution, and the amount of crosslinking desired. Preferably, the
amount of crosslinking agent utilked ranges from about .2 wt % to
about 10 wt ~ (based upon the total, fiber-free weight of the
crosslinking solution~. Preferred acid catalyst content is
additionally dependent upon the acidity of the catalyst in the
15 crosslinking solution. Good results may generally be obtained for
catalyst content, including hydrochloric acid, between about . 3 wt
% and about 5 wt % (fiber-free crosslinking solution weight basis)
in crosslinking solutions containing an acetic acid diluent,
preferred levels of glutaraldehyde, and a limited amount of water.
;~0 Slurries of fibers and crosslinking solution having fiber
consistencies of less than about 10 wt ~ are preferred for
crosslinking in conjunction with the crosslinking solutions
described above .
The crosslinking reaction may be carried out at ambient
2~ temperatures or, for accelerated reaction rates, at elevated
temperatures preferably less than about 40C.
There are a variety of methods by which the fibers may be
contacted with, and cross;inked in, the crosslinking solution. In
one embodiment, the fibers are contacted with the solution which
30 initially contains both the crosslinking agent and the acid
catalyst. The fibers are allowed to soak in the crosslinking
solution, during which time crosslinking occurs. In another
embodiment, the fibers are contacted with the diluent and allowed
6t~
to soak prior to addition of the acid catalyst. The acid catalyst
subsequently is added, at ~hich time crosslinking begins. Other
methods in addition to those described will be apparent to those
skilled in the art, and are intended to be ~ithin the scope of this
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
reaction to occur in substantial part after the crosslinking agent
10 has had sufficient time to penetrate into the fibers. Reaction
conditions are preferably chosen so as to avoid instantaneous
crosslinking unless the crosslinking agent has already penetrated
into the fibers. Periods of reaction during which time
crosslinking is substantially completed over a period of about 30
15 minutes are preferred. Longer reaction periods are believed to
provide minimal marginal benefit in fiber performance. However,
both shorter periods, including substantially instantaneous
crosslinking, and longer periods are meant to be within the scope
of th is invention .
It is also contemplated to only partially 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
the crosslinked fibers to curl, e.g.., fluffing by defibration,
) between the defluidizing and drying steps. Upon drying. the
curled condition of the fibers imparts additional t-vist as
prevTously described in connection with the curling treatment
prior to contact with the crosslinking solution. The same
apparatuses and methods for inducing twist and curl described in
l~U ~ L~
tl
connection w;th the first mechanical defibration step are applicable
to this second mechanical defibration step. As used herein, the
term "defibrat;on" shall refer to any of the procedures which may
be used to mechanically separate the fibers into substantially
5 individual form, even though the fibers may already be provided
in such form. "Defibration" therefore refers to the step of
mechanically treating the fibers, in either individual form or in a
more compacted form, to a mechanical treatment step which a)
would separate the fibers into substantially individual form if they
10 were not already in such form, and b) imparts curl and twist to
the fibers upon drying.
This second defibration treatment, after the fibers have been
crosslinked, has been found to increase the twisted, curled
character of the pulp. This increase in the twisted, curled
1~ configuration of the fibers leads to enhanced absorbent structure
resiliency and responsiveness to wetting. A second defibration
treatment may be practiced upon any of the crosslinked fibers
described herein which are in a moist condition. However, it is a
particular advantage of the nonaqueous solution crosslinking
20 method that a second defibration step is possible without
necessitating an additional drying step. This is due to the fact
that the solution in which the fibers are crosslinked keep the
fibers flexible subsequent to crosslinking even though not causing
the fibers to assume an undesirable, highly swollen state.
;~5 It has been further unexpectedly found that increased
degrees of absorbent structure expansion upon wetting
compressed pads can be obtained for structures made from fibers
which have been crosslinked while in a condition which is twisted
but partially swollen relative to fibers which have been
30 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
3~
22
wherein a fiber is dried to completion prior to being contacted
with the crosslinking solution, it is in a nonswollen, collapsed
state. The fiber does not become swollen upon contact with the
crosslinking solution due to the low water content of the solution.
5 As discussed before, a critical aspect of the crosslinking solution
is that it does not cause any substantial swelling of the fibers.
However, when the diluent of the crosslinking solution is
absorbed by an already swollen fiber, the fiber is in effect
"dried" of water, but the fiber retains its preexisting partially
1~) swollen condition.
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
15 swollen state relative to fibers crosslinked ~vhile in a less swollen
state, all other factors being equal. Without limiting the scope of
the invention, it is believed that partially swollen, crosslinked
fibers with increased FRV's have greater wet resilience and
responsiveness to wetting than fibers which have been crosslinked
;~ while in an unswollen state. Fibers having this increase in wet
resilience and responsiveness to wetting are more readily able to
expand or untwist when wetted in an attempt to return to their
natural state. Yet, due to the stiffness imparted by
crosslinking, the fibers are still able to provide the structural
2~ support to a saturated pad made from the fibers. Numerical FRV
data described herein in connection with partially swollen
crosslinked fibers shall be water retention values (\YRV). As the
WRV increases beyond approximately 60, the stiffness of the
fibers is believed to become insufficient to provide the wet
30 resilience and responsiveness to wetting desired to support a
saturated absorbent structure.
I n 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
~5 subsequently submersed in a water-miscible crosslinking solution
13~
23
containing a catalyst and crossllnking agent as previously
described. The fibers are preferably mechanically defibrated into
fluff form subsequent to defluidization and prior to additional
drying, in order to obtain the benefits of enhanced twist and curl
5 as previously described. Mechanical defibration practiced
subsequent to contacting the fibers with the crosslinking agent is
less desirable, since such defibration ~vould volatilize the
crosslinking agent thus, possibly leading to atmospheric
contamination by, or high air treatment investments due to, the
10 crosslinking agent.
I n a modification of the process described immediately above,
the fibers are defibrated and then presoaked in a high
concentration solution of crosslinking agent and a fiber-swelling
diluent, preferably water. The crosslinking agent concentration
15 is sufficiently high to inhibit water-induced swelling of fibers.
Fifty percent, by weight, aqueous solutions of the crosslinking
agents of this invention, preferably, glutaraldehyde, have been
found to be useful solutions for presoaking the fibers. The
presoaked fibers are defluidized and submerged in a crosslinking
20 solution containing a water-miscible, polar diluent, a catalyst, and
a limited amount of water, and then crosslinked as previously
described. Also as described above, the crosslinked fibers may
be defluidized and subjected to a second mechanical defibration
step prior to further processing into a sheet or absorbent
;~5 structure.
Presoaking the fibers with crosslinking agent in an aqueous
solution prior to causing the crosslinking agent to react provides
unexpectedly high absorbency properties for absorbent pads made
from the crosslinked fibers, even relative to pads made from
30 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
~3(~
24
are the product of the present invention. The crosslinked ibers
of the present invention may be utili~ed directiy in the
manufacture of air laid absorbent cores. Additionally, due to
their stiffened and resilient character, the crosslinked fibers may
5 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.
lU Once the individualized, crosslinked fibers are made, they
may be dry laid and directly formed into absorbent structures, or
wet laid and formed into absorbent structures or densified pulp
sheets. However, it is difficult to form such fibers into a
smooth, wet laid sheet by conventional wet sheet formation
15 practices. This is because individua1ized, crosslinked fibers
rapidly flocculate when in solution. Such flocculation may occur
both in the headbox and upon deposition into the foraminous
forming wire. Attempts to sheet individualized, crossiinked fibers
by conventional pulp sheeting methods have been found to result
;~ in the formation of a plurality of clumps of flocculated fibers.
Without limiting the invention, it is believed that 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
25 commercial concern that a practicable process for sheeting
individualized, crosslinked fibers be provided, whereby wet laid
absorbent structures and densified pulp sheets for transit and
subsequent defibration may be formed.
Accordingly, a novel process for sheeting individualized,
30 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
35 individualized, cross1inked fibers, these fibers are deposited on
3 ~ 3 4
the forming wire in a plurality of clumps of fibers. At least one
stream of fluid, preferably water, 1s directed at the deposited,
clumped fibers. Preferably, a series of showers are directed at
the fibers deposited on the forming wire, wherein successivè
5 showers have decreasing volumetric flow rates, The showers
should be of sufficient velocity such that the impact of the fluid
against the fibers acts to inhibit the formation of flocculations of
the fibers and to disperse flocculations of fibers which have
already formed. The fiber setting step is preferably performed
10 with a cylindrical screen, such as a dandy roll, or with another
apparatus analogous in function which is or rnay 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
15 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 inhibiting the formation of and dispersing
20 flocculations of fibers, the fluid showered onto thé fibers also
compensates for the extremely fast drainage of individualized,
crosslinked fibers, by providing additional liquid medium in which
the fibers may be dispersed for subsequent sheet formation. The
plurality of showers of decreasing volumetric flow rates facilitates
~5 a systematic net increase in slurry consistency while providing a
repetitive dispersive and inhibiting effect upon flocculations of
the fibers. This results in the formation of a relatively smooth
and even deposition of fibers which are then promptly, i.e.,
before reflocculation, set into sheeted form by allowing the fluid
30 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
individualized, crosslinked fibers are more difficult to compress to
conventional pulp sheet dens;ties. Therefore, it may be desirable
~3C~
26
to combine crosslinked fibers with uncrosslinked fibers, such as
those conventionally used in the manufacture of absorbent cores,
Pulp sheets containing stiffened, crosslinked fibers preferably
contain between about 5~ and about 90% uncrosslinked, cellulosic
5 fibers, based upon the total dry weight of the sheet, mixed with
the individuali2ed, crosslinked fibers. It is especially preferred
to include between about 5% and about 30% of highly refined,
uncrosslinked cellulosic fibers, based upon the total dry weight of
the sheet. Such highly refined fibers are refined or beaten to a
10 freeness level less than about 300 ml CSF, and more preferably
less than about 100 ml CSF. The uncrosslinked fibers are
preferably mixed with an aqueous slurry of the individualized,
crosslinked fibers. This mixture may then be formed into a
densified pulp sheet for subsequent defibration and formation into
lS absorbent pads. The incorporation of the uncrosslinked fibers
eases compression of the pulp sheet into a densified form, while
imparting a surprisingly small loss in absorbency to the
subsequently formed absorbent pads. The uncrosslinked fibers
additionally increase the tensile strength of the pulp sheet and to
;~0 absorbent pads made either from the pulp sheet or directly from
the mixture of crosslinked and uncrosslinked fibers. The blend
of crosslinked and uncrosslinked fibers may be first made into a
pulp sheet and then comminuted to form an absorbent pad or
formed dTrectly utilized as an absorbent pad. The fibers, if
25 comminuted, may be air-laid or wet-laid as previously described.
Wet-lais~ 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/m2 and densities of less than about 0.60 g/cm3. Although it is
30 not intended to limit the scope of the invention, sheets having
basis weights between about 300g/m2 and about 600 g/m2 and
densities bet~Neen about 0.15 g/cc and about 0.3 glcc are
especially contemplated for direct application as absorbent cores
in disposable articles such as diapers, tampons, and other
35 catamenial products. Structures having basis weights and
densities higher than these levels are believed to be most useful
131J61~94
27
for subsequent comminution and air-laying or wet-laying to form a
lower density and basis weight structure which is more use-ul for
absorbent applicaeions. Other applications contemplated for the
fibers of the present invention include low density tissue sheets
5 having densities which may be less than 0.10 glcc
The absorbent structures made by the process 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
10 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
15 is particularly contemplated. Such articles are described
generally in U.S. Patent 3,860,003, issued to Kenneth B. Buell
on January 14, 1975.
PROCEDURE FOR DETERMINING FLUID RETENTION VALUE
.
The following procedure was utilized to determine the water
20 retention value of cellulosic flbers.
A sample of about 0 . 3 9 to about 0 . ~ 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
25 transferred to an 80-mesh wire basket supported about 1 t inches
above a 60-mesh screened bottom of a centrifuge tube. The tube
is covered with a plastic cover and the sample is centrifuged at a
relative centrifuge force of 1500 to 1700 gravities for 19 to 21
minutes. The centrifuged fibers are then removed from the
30 basket and weighed. The weighed fibers are dried to a constant
weight at 1 05C and reweighed . The water retention value is
calculated as follows:
.,~
~`
~3~
28
(l) WRV = (W D) xl00
where,
W = wet weight of the centrifuged fibers;
D = dry ~eight of the fibers; and
W-D = weight of absorbed water.
PROCEDURE FOR DETERMINING LEVEL OF
GLUTARALDEHYDE REACTED WITH CELLULOSIC FIBERS
The following procedure was utilized to determine the level
1(~) of glutaraldehyde which reacted to form intrafiber crosslink bonds
with the cellulosic component of the individualized,
glutaraldehyde- crosslinked fibers.
A sample of individualized, crosslinked fibers is extracted
with 0.1 N HCI. The extract is separated from the fibers, and
the same extraction/separation procedure is then repeated for
each sample an additional three times. The extract from each
extraction is separately mixed with an aqueous solution of
2,4-dinitrophenylhydrazone (DNPH). The reaction is allowed to
proceed for lS minutes after which a volume of chloroform is
2U added to the mixture. The reaction mixture is mixed for an
additional 45 minutes. The chloroform and aqueous layers are
separated with a separatory funnel. The level of glutaraldehyde
is determined by analyzing the chloroform layer by high pressure
liquid chromatography ( HPLC) for DNPH derivative.
The chromatographic conditions for HPLC analysis utilized
were - Column: C-18 reversed phase: Detector: UV at 360 mm:
Mobile phase 80:20 methanol: water: Flow rate: 1 ml/min.:
measurement made: peak height. A calibration curve of peak
height and glutaraldehyde content was developed by measuring
l3a96~
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 was
analyzed by HPLC, the peak height measureJ, and the
5 corresponding level of glutaraldehyde determined from the
cal ibration curve . The glutaraldehyde concentrations for each
extraction were then summed and divided by the fiber sample
weight (dry fiber basis! to provide glutaraldehyde content on a
fibers weight basis.
Two glutaraldehyde peaks were present for each of the HPLC
chromatograms. Either peak may be used, so long as that same
peak is used throughout the procedure.
EXAMPLE 1
This example discloses a preferred process for making
15 individualized, crosslinked fibers. The individualized,
crosslinked fibers were made by a 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% tequivalent to 37. 6% consistency) .
'0 A slurry was formed by adding the fibers to a solution containing
a selected amount of 50% aqueous solution of glutaraldehyde, 30%
(based upon the weight of the glutaraldehyde) zinc nitrate
hexahydrate, demineralized water and a sufficient amount of 1 N
HCI to decrease the slurry pH to about 3. 7. The fibers were
25 soaked in the slurry for a period of 20 minutes and then
dewatered to a fiber consistency of about 3~% to about 35% by
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
30 were defibrated utilizing a three-stage fluffing device as
described in U.S. Patent 3,987,968. The defibrated fibers were
placed in trays and cured at 1 45C in an essentially statTc drying
13~ S
oven for a period of 45 minutes. Crosslinki~g was completed
during the period in the oven. The crosslinked, individualized
fibers were placed on a mesh screen and washed with about 20C
water, soaked at 1% consistency for one ( 1 ) hour in 60C water,
screened, washed with about 20C water for a second time,
centri fuged to 60% fiber consistency, defibrated in a three stage
fluffer as previously described, and dried to completion in a
static drying oven at 105C for four (4) hours. The fibers had
between 0 mole % and 3 . 3 mole % glutaraldehyde reacted in the
l O form of crosslink bonds . The corresponding WRV's varied
between 51% and about 28%.
Example 2
The purpose of this example is to exemplify a process for
making wet-laid sheets containing individualized, crosslinked
15 fibers.
A 0. SS~ consistency slurry of a blend of fibers containing
90% individualized, crosslinked fibers made according to the
crosslinking process described in Example 1 and 10% conventional,
uncrosslinked fibers having a freeness of less than 100 CSF were
2() deposited in flocculated, clumped fibers on a conventional 84-mesh
Fourdinier forming wire. The papermaking flow rate out of the
headbox was 430 kg/min. Immediately after deposition, a series
of five streams of water of sequentially decreasing flow rates were
directed upon the fibers. The five streams of water provided a
25 cumulative flow ratio 85 kg water/kg bone dry (b.d. ) fiber. The
showers were all spaced within 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
30 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 flow , 1 70 m/min .;
Shower 2-369~ of total flow, 165 m/min. Shower 3-13~ of total
9~
fiow, 61 mlmin. Shower 4-9~ of total f1OW, 41 mlmin.; Shower
5-5% of total flow, 20 mlmin. Immediately after the fifth shower,
the fibers were set by treatment with a cylindrical, screened roll
known in the art as a Dandy Roll. The Dandy Rolt pressed the
5 fibers, which at the time of setting were in a high consistency
slurry form, against the forming wire to set the fibers to form 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
1(~ follo~ving claims.
