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Description  |
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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 structures. More
specifically, this invention is concerned with individualized, crosslinked
cellulosic fibers, processes for making such fibers, and absorbent
structures containing cellulosic fibers which are in an individualized,
crosslinked form.
BACKGROUND OF THE INVENTION
Fibers crosslinked in substantially individualized form and various methods
for making such fibers have been described in the art. The term
"individualized, crosslinked fibers", refers to cellulosic fibers that
have primarily intrafiber chemical crosslink bonds. That is, the crosslink
bonds are primarily between cellulose molecules of a single fiber, rather
than between cellulose molecules of separate fibers. Individualized,
crosslinked fibers are generally regarded as being useful in absorbent
product applications. 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, the improvement in absorbency
is reported in terms of absorbent capacity. Additionally, absorbent
structures made from individualized crosslinked fibers generally exhibit
increased wet resilience and increased dry resilience relative to
absorbent structures made from uncrosslinked fibers. The term "resilience"
shall hereinafter refer to the ability of pads made from cellulosic fibers
to return toward an expanded original state upon release of a
compressional force. Dry resilience specifically refers to the ability of
an absorbent structure to expand upon release of compressional force
applied while the fibers are in a substantially dry condition. Wet
resilience specifically refers to the ability of an absorbent structure to
expand upon release of compressional force applied while the fibers are in
a moistened condition. For the purposes of this invention and consistency
of disclosure, wet resilience shall be observed and reported for an
absorbent structure moistened to saturation.
In general, three categories of processes have been reported for making
individualized, crosslinked fibers. These processes, described below, are
herein referred to as dry crosslinking processes, aqueous solution
crosslinking processes, and substantially non-aqueous solution
crosslinking processes.
Processes for making individualized, crosslinked fibers with dry
crosslinking technology are described in U.S. Pat. No. 3,224,926, L. J.
Bernardin, issued Dec. 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. Pat. No. 3,224,926, wherein crosslinking is caused to occur while
the fibers are in an unswollen, collapsed state, are referred to as
processes for making "dry crosslinked" fibers. Dry crosslinked fibers are
generally highly stiffened by crosslink bonds, and absorbent structures
made therefrom exhibit relatively high wet and dry resilience. Dry
crosslinked fibers are further characterized by low fluid retention values
(FRV).
Processes for producing aqueous solution crosslinked fibers are disclosed,
for example, in U.S. Pat. No. 3,241,553, F. H. Steiger, issued Mar. 22,
1966. Individualized, crosslinked fibers are produced by crosslinking the
fibers in an aqueous solution containing a crosslinking agent and a
catalyst. Fibers produced in this manner are hereinafter referred to as
"aqueous solution crosslinked" fibers. Due to the swelling effect of water
n 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. Pat. 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 structures made from dry crosslinked fibers.
In U.S. Pat. No. 4,035,147, Sangenis et al., issued Jul. 12, 1977, a method
is disclosed for producing individualized, crosslinked fibers by
contacting dehydrated, nonswollen fibers with crosslinking agent and
catalyst in a substantially nonaqueous solution which contains an
insufficient amount of water to cause the fibers to swell. Crosslinking
occurs while the fibers are in this substantially nonaqueous solution.
This type of process shall hereinafter be referred to as a nonaqueous
solution crosslinked process; and the fibers thereby produced shall be
referred to as nonaqueous solution crosslinked fibers. The nonaqueous
solution crosslinked fibers disclosed in U.S. Pat. No. 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 crosslinking agents most widely
referred to in the literature are formaldehyde and formaldehyde addition
products known as N-methylol agents or N-methylolamides, which,
unfortunately, cause irritation to human skin and have been associated
with other human safety concerns. Removal of free formaldehyde to
sufficiently low levels in the crosslinked product such that irritation to
skin and other human safety concerns are avoided has been hindered by both
technical and economic barriers.
As mentioned above, the use of formaldehyde and various formaldehyde
addition products to crosslink cellulosic fibers is known in the art. See,
for example, U.S. Pat. No. 3,224,926, Bernardin, issued on Dec. 21, 1965;
U.S. Pat. No. 3,241,553, Steiger, issued on Mar. 22, 1966; U.S. Pat. No.
3,932,209, Chatterjee, issued on Jan. 13, 1976; U.S. Pat. No. 4,035,147,
Sangenis et al, issued on Jul. 12, 1977; and U.S. Pat. No. 3,756,913,
Wodka, issued on Sep. 4, 1973. Unfortunately, the irritating effect of
formaldehyde vapor on the eyes and skin is a marked disadvantage of such
references. A need is evident for cellulosic fiber crosslinking agents
that do not require formaldehyde or its unstable derivatives.
Other references disclose the use of dialdehyde crosslinking agents. See,
for example, U.S. Pat. No. 4,689,118, Makoui et al, issued on Aug. 25,
1987; and U.S. Pat. No. 4,822,453, Dean et al, issued on Apr. 18, 1989.
The Dean et al reference discloses absorbent structures containing
individualized, crosslinked fibers, wherein the crosslinking agent is
selected from the group consisting of C.sub.2 -C.sub.8 dialdehydes, with
glutaraldehyde being preferred. These references appear to overcome many
of the disadvantages associated with formaldehyde and/or formaldehyde
addition products. However, the cost associated with producing fibers
crosslinked with dialdehyde crosslinking agents such as glutaraldehyde may
be too high to result in significant commercial success. Therefore, there
is a need to find cellulosic fiber crosslinking agents which are both safe
for use on the human skin and also commercially feasible.
The use of polycarboxylic acids to impart wrinkle resistance to cotton
fabrics is known in the art. See, for example, U.S. Pat. No. 3,526,048,
Roland et al, issued Sep. 1, 1970; U.S. Pat. No. 2,971,815, Bullock et al,
issued Feb. 14, 1961 and U.S. Pat. No. 4,820,307, Welch et al, issued Apr.
11, 1989. These references all pertain to treating cotton textile fabrics
with polycarboxylic acids and specific curing catalysts to improve the
wrinkle resistance and durability properties of the treated fabrics.
It has now been discovered that ester crosslinks can be imparted onto
individualized cellulosic fibers through the use of specific
polycarboxylic acid crosslinking agents. The ester crosslink bonds formed
by the polycarboxylic acid crosslinking agents are different from the
crosslink bonds that result from the mono- and di-aldehyde crosslinking
agents, which form acetal crosslinked bonds. Applicants have found that
absorbent structures made from these individualized, ester-crosslinked
fibers exhibit increased wet resilience and dry resilience and improved
responsiveness to wetting relative to structures containing uncrosslinked
fibers. Importantly, the polycarboxylic acids disclosed for use in the
present invention, are nontoxic, unlike formaldehyde and formaldehyde
addition products commonly used in the art. Furthermore, the preferred
polycarboxylic crosslinking agent i.e., citric acid, is available in large
quantities at relatively low prices making it commercially competitive
with formaldehyde and formaldehyde addition products, without any of the
related human safety concerns.
It is an object of this invention to provide individualized fibers
crosslinked with a polycarboxylic acid crosslinking agent and absorbent
structures made from such fibers wherein the absorbent structures made
from the crosslinked fibers have higher levels of absorbent capacity
relative to absorbent structures made from uncrosslinked fibers, and
exhibit higher wet resilience and higher dry resilience than structures
made from uncrosslinked fibers.
It is a further object of this invention to provide individualized fibers
crosslinked with a polycarboxylic crosslinking agent and absorbent
structures made from such fibers, as described above, which have a
superior balance of absorbency properties relative to prior known
crosslinked fibers.
It is additionally an object of this invention to provide commercially
viable individualized, crosslinked fibers and absorbent structures made
from such fibers, as described above, which can be safely utilized in the
vicinity of human skin.
SUMMARY OF THE INVENTION
It has been found that the objects identified 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
an effective amount of a polycarboxylic acid crosslinking agent,
preferably between about 0.5 mole % and about 10.0 mole %, more preferably
between about 1.5 mole % and about 6.0 mole % crosslinking agent,
calculated on a cellulose anhydroglucose molar basis, reacted with the
fibers in the form of intrafiber crosslink bonds. The polycarboxylic acid
crosslinking agent is selected from the group consisting of C.sub.2
-C.sub.9 polycarboxylic acids. The crosslinking agent is reacted with the
fibers in an intrafiber crosslinking bond form. Such fibers, which are
characterized by having water retention values (WRV's) of from about 28 to
about 60, have been found to fulfill the identified objects relating to
individualized, crosslinked fibers and provide unexpectedly good absorbent
performance in absorbent structure applications.
DETAILED DESCRIPTION OF THE INVENTION
Cellulosic fibers of diverse natural origin are applicable to the
invention. Digested fibers from softwood, hardwood or cotton linters are
preferably utilized. Fibers from Esparto grass, bagasse, kemp, flax, and
other lignaceous and cellulosic fiber sources may also be utilized as raw
material in the invention. The fibers may be supplied in slurry, unsheeted
or sheeted form. Fibers supplied as wet lap, dry lap or other sheeted form
are preferably rendered into unsheeted form by mechanically disintegrating
the sheet, preferably prior to contacting the fibers with the crosslinking
agent. Also, preferably the fibers are provided in a wet or moistened
condition. Most preferably, the fibers are never-dried fibers. In the case
of dry lap, it is advantageous to moisten the fibers prior to mechanical
disintegration in order to minimize damage to the fibers.
The optimum fiber source utilized in conjunction with this invention will
depend upon the particular end use contemplated. Generally, pulp fibers
made by chemical pulping processes are preferred. Completely bleached,
partially bleached and unbleached fibers are applicable. It may frequently
be desired to utilize bleached pulp for its superior brightness and
consumer appeal. 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.
Crosslinking agents applicable to the present development include aliphatic
and alicyclic C.sub.2 -C.sub.9 polycarboxylic acids. As used herein, the
term "C.sub.2 -C.sub.9 polycarboxylic acid" refers to an organic acid
containing two or more carboxyl (COOH) groups and from 2 to 9 carbon atoms
in the chain or ring to which the carboxyl groups are attached. The
carboxyl groups are not included when determining the number of carbon
atoms in the chain or ring. For example, 1,2,3 propane tricarboxylic acid
would be considered to be a C.sub.3 polycarboxylic acid containing three
carboxyl groups. Similarly, 1,2,3,4 butane tetracarboxylic acid would be
considered to be a C.sub.4 polycarboxylic acid containing four carboxyl
groups.
More specifically, the C.sub.2 -C.sub.9 polycarboxylic acids suitable for
use as cellulose crosslinking agents in the present invention include
aliphatic and alicyclic acids either olefinically saturated or unsaturated
with at least three and preferably more carboxyl groups per molecule or
with two carboxyl groups per molecule if a carbon-carbon double bond is
present alpha, beta to one or both carboxyl groups. An additional
requirement is that to be reactive in esterifying cellulose hydroxyl
groups, a given carboxyl group in an aliphatic or alicyclic polycarboxylic
acid must be separated from a second carboxyl group by no less than 2
carbon atoms and no more than three carbon atoms. Without being bound by
theory, it appears from these requirements that for a carboxyl group to be
reactive, it must be able to form a cyclic 5- or 6-membered anhydride ring
with a neighboring carboxyl group in the polycarboxylic acid molecule.
Where two carboxyl groups are separated by a carbon-carbon double bond or
are both connected to the same ring, the two carboxyl groups must be in
the cis configuration relative to each other if they are to interact in
this manner.
In aliphatic polycarboxylic acids containing three or more carboxyl groups
per molecule, a hydroxyl group attached to a carbon atom alpha to a
carboxyl group does not interfere with the esterification and crosslinking
of the cellulosic fibers by the acid. Thus, polycarboxylic acids such as
citric acid (also known as 2-hydroxy-1,2,3 propane tricarboxylic acid) and
tartrate monosuccinic acids are suitable as crosslinking agents in the
present development.
The aliphatic or alicyclic C.sub.2 -C.sub.9 polycarboxylic acid
crosslinking agents may also contain an oxygen or sulfur atom(s) in the
chain or ring to which the carboxyl groups are attached. Thus,
polycarboxylic acids such as oxydisuccinic acid also known as
2,2'-oxybis(butanedioic acid), thiodisuccinic acid, and the like, are
meant to be included within the scope of the invention. For purposes of
the present invention, oxydisuccinic acid would be considered to be a
C.sub.4 polycarboxylic acid containing four carboxyl groups.
Examples of specific polycarboxylic acids which fall within the scope of
this invention include the following: maleic acid, citraconic acid also
known as methylmaleic acid, citric acid, itaconic acid also known as
methylenesuccinic acid, tricarballylic acid also known as 1,2,3 propane
tricarboxylic acid, transaconitic acid also known as
trans-1-propene-1,2,3-tricarboxylic acid, 1,2,3,4-butanetetracarboxylic
acid, all-cis-1,2,3,4-cyclopentanetetracarboxylic acid, mellitic acid also
known as benzenehexacarboxylic acid, and oxydisuccinic acid also known as
2,2'-oxybis(butanedioic acid). The above list of specific polycarboxylic
acids is for exemplary purposes only, and is not intended to be all
inclusive. Importantly, the crosslinking agent must be capable of reacting
with at least two hydroxyl groups on proximately located cellulose chains
in a single cellulosic fiber.
Preferably, the C.sub.2 -C.sub.9 polycarboxylic acids used herein are
aliphatic, saturated, and contain at least three carboxyl groups per
molecule. One group of preferred polycarboxylic acid crosslinking agents
for use with the present invention include citric acid also known as
2-hydroxy-1,2,3 propane tricarboxylic acid, 1,2,3 propane tricarboxylic
acid, and 1,2,3,4 butane tetracarboxylic acid. Citric acid is especially
preferred, since it has provided fibers with high levels of absorbency and
resiliency, is safe and non-irritating to human skin, an has provided
stable, crosslink bonds. Furthermore, citric acid is available in large
quantities at relatively low prices, thereby making it commercially
feasible for use as a crosslinking agent.
Another group of preferred crosslinking agents for use in the present
invention includes saturated C.sub.2 -C.sub.9 polycarboxylic acids
containing at least one oxygen atom in the chain to which the carboxyl
groups are attached. Examples of such compounds include oxydisuccinic
acid, tartrate monosuccinic acid having the structural formula:
##STR1##
and tartrate disuccinic acid having the structural formula:
##STR2##
A more detailed description of tartrate monosuccinic acid, tartrate
disuccinic acid, and salts thereof, can be found in U.S. Pat. No.
4,663,071, Bush et al., issued May 5, 1987, incorporated herein by
reference.
Those knowledgeable in the area of polycarboxylic acids will recognize that
the aliphatic and alicyclic C.sub.2 -C.sub.9 polycarboxylic acid
crosslinking agents described above may be present in a variety of forms,
such as the free acid form, and salts thereof. Although the free acid form
is preferred, all such forms are meant to be included within the scope of
the invention.
The individualized, crosslinked fibers of the present invention have an
effective amount of the C.sub.2 -C.sub.9 polycarboxylic acid crosslinking
agent reacted with the fibers in the form of intrafiber crosslink bonds.
As used herein, "effective amount of crosslinking agent" refers to an
amount of crosslinking agent sufficient to provide an improvement in at
least one significant absorbency property of the fibers themselves and/or
absorbent structures containing the individualized, crosslinked fibers,
relative to conventional, uncrosslinked fibers. One example of a
significant absorbency property is drip capacity, which is a combined
measured of an absorbent structure's fluid absorbent capacity and fluid
absorbency rate. A detailed description of the procedure for determining
drip capacity is provided hereinafter.
In particular, unexpectedly good results are obtained for absorbent pads
made from individualized, crosslinked fibers having between about 0.5 mole
% and about 10.0 mole %, more preferably between about 1.5 mole % and
about 6.0 mole % crosslinking agent, calculated on a cellulose
anhydroglucose molar basis, reacted with the fibers.
Preferably, the crosslinking agent is contacted with the fibers in a liquid
medium, under such conditions that the crosslinking agent penetrates into
the interior of the individual fiber structures. However, other methods of
crosslinking agent treatment, including spraying of the fibers while in
individualized, fluffed form, are also within the scope of the invention.
Applicants have discovered that the crosslinking reaction can be
accomplished at practical rates without a catalyst, provided the pH is
kept within a particular range (to be discussed in more detail below).
This is contrary to the prior art which teaches that specific catalysts
are needed to provide sufficiently rapid esterification and crosslinking
of fibrous cellulose by polycarboxylic acid crosslinking agents to be
commercially feasible. See, for example, U.S. Pat. No. 4,820,307, Welch et
al., issued Apr. 11, 1989.
However, if desired, the fibers can also be contacted with an appropriate
catalyst prior to crosslinking. Applicants have found that 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 if one is
used), the crosslinking agent is caused to react with the fibers in the
substantial absence of interfiber bonds, i.e., while interfiber contact is
maintained at a low degree of occurrence relative to unfluffed pulp
fibers, or the fibers are submerged in a solution that does not facilitate
the formation of interfiber bonding, especially hydrogen bonding. This
results in the formation of crosslink bonds which are intrafiber in
nature. Under these conditions, the crosslinking agent reacts to form
crosslink bonds between hydroxyl groups of a single cellulose chain or
between hydroxyl groups of proximately located cellulose chains of a
single cellulosic fiber.
Although not presented or intended to limit the scope of the invention, it
is believed that the carboxyl groups on the polycarboxylic acid
crosslinking agent react with the hydroxyl groups of the cellulose to form
ester bonds. The formation of ester bonds, believed to be the desirable
bond type providing stable crosslink bonds, is favored under acidic
reaction conditions. Therefore, acidic crosslinking conditions, i.e. pH
ranges of from about 1.5 to about 5, 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 hereafter 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. Pat. No. 3,987,968, issued to D. R. Moore and O.
A. Shields on Oct. 26, 1976, said patent being hereby expressly
incorporated by reference into this disclosure. The fluffing device
described in U.S. Pat. No. 3,987,968 subjects moist cellulosic pulp fibers
to a combination of mechanical impact, mechanical agitation, air agitation
and a limited amount of air drying to create a substantially knot-free
fluff. The individualized fibers have imparted thereto an enhanced degree
of curl and twist relative to the amount of curl and twist naturally
present in such fibers. It is believed that this additional curl and twist
enhances the resilient character of absorbent structures made from the
finished, crosslinked fibers.
Other applicable methods for defibrating the cellulosic fibers include, but
are not limited to, treatment with a Waring blender and tangentially
contacting the fibers with a rotating disk refiner or wire brush.
Preferably, an air stream is directed toward the fibers during such
defibration to aid in separating the fibers into substantially individual
form.
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 65% moisture.
Mechanical refining of fibers at high consistency or of partially dried
fibers may also be utilized to provide curl or twist to the fibers in
addition to curl or twist imparted as a result of mechanical defibration.
The fibers made according to the present invention have unique combinations
of stiffness and resiliency, which allow absorbent structures made from
the fibers to maintain high levels of absorptivity, and exhibit high
levels of resiliency and an expansionary responsiveness to wetting of a
dry, compressed absorbent structure. In addition to having the levels of
crosslinking within the stated ranges, the crosslinked fibers are
characterized by having water retention values (WRV's) of less than about
60, more preferably between about 28 to about 50, and most preferably
between about 30 and about 45, for conventional, chemically pulped,
papermaking fibers. The WRV of a particular fiber is indicative of the
level of crosslinking. Very highly crosslinked fibers, such as those
produced by many of the prior art known crosslinking processes previously
discussed, have been found to have WRV's of less than 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. Applicable methods of crosslinking include dry
crosslinking processes and nonaqueous solution crosslinking processes as
generally discussed in the Background Of The Invention. Certain preferred
dry crosslinking and nonaqueous solution crosslinking processes for
preparing the individualized, crosslinked fibers 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.
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