Note: Descriptions are shown in the official language in which they were submitted.
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Absorbent Structure
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Background of the rnvention
The present invention relates to an absorbent structure
containing non-delignified wood pulp fibers which struc-
ture is particularly suitable for use in absorbent productsrequiring a su~stantial liquid holding capacity~
Non-delignified wood pulp fi~ers, such as the thermo-
mechanically produced ~ood pulp fibers, refiner produced
wood pulp fi~ers, or the like, have become quite important
in the last few years. These wood pulp fibers also refer-
red to as "high yield" w-ood pulp fibers, have become
increasingly important for several reasons. The processes
used to produce the fibers utilize more of the raw material.
For instance, these types of processes use 90% or more of
the tree as compared to approximately 50% use of the
tree in the instance of typical chemical processing.
Furthermore, the non-delignified wood pulp processes
reduce the environmental problems caused by chemical
processing. Specifically, the "high yield" processes
cause considerably less air pollution and water pollution
than do the counterpart chemical processes. These various
factors and the concomitant economic considerations make
the high yield processes, such as the thermomechanical
pulp process, very attractive.
Non-delignified wood pulp processes have been known for
some time and are usually developed primarily for paper
grade wood pulps, newsprint, and the like. These wood
pulps have not been well accepted in absorbent type
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products, such as sanitary napkins, disposable diapers,
and the like, primarily because of their relatively poor
per~ormance as the absorbent core for such products.
Conventional chemically processed wood pulp fibers have
a degree of cohesive strength when placed in an air-~aid
web structure. Typically chemically processed wood pulp
fibers are somewhat collapsed and appear in ribbon-like
form. This form permits fiber entanglement during the
air-laid web processing and hence results in a web having
a degree of cohesiveness and fibrous web integrity.
In contrast, the non-delignified wood pulp fibers are
non-collapsed, stiffer and more resilient. Webs formed
of these fibers, although possessing a greater potential
liquid holding capacity, have poor integrity and hence
tend to break apart.
Furthermore, absorbent structures made from non-delignified
wood pulp fibers are substantially hydrophobic and not
readily wettable. For any a~sorbent structure to be
satisfactory, it is highly desirable for the structure to
~1) readily accept liquid, ~2~ easily transport the liquid
from one portion of the structure to another and ~3) hold
~5 the liquid accepted.
Various techniques have been developed or suggested for
improving the a~sor~ent characteristics of non-delignified
wood pulp, such as removing the fines from the wood pulp
30 product or providing various solvent or other chemical
treatments to the wood pulp product to both bleach the
pulp and improve its absor~ency However, these techniques
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increase the econo~ics or cost of the wood pulp and, in
some instances, increase the pollution problem and, hence,
do not take full advantage of the non-delignified wood
pulp process.
Other techniques or developing absorbent products util-
izing non-delignified wood pulps have been suggested.
One technique is disclosed in British Patent 1,500,Q53 and
uses fibers of specific measurement; that is, length and
diameter, The surface hydrophilicity of the fibers is
increased by bleac~ing and the hydrophilic fibers are air-
laid in we~ form and compressed to a specific density.
Bleaching followed by compression substantially increases
the wetta~ility of the otherwise hydrophobic structure,
lS but at the same time, reduces the liquid holding capacity
of an absorbent structure made from non-delignified wood
pulp fibers.
As mentioned above, for any absorbent structure to be
satisfactory, it is not only necessary for the structure
to hold liquid but also t,o readily accept liquid and
transport it. The liquid holding capacity of the absorbent
structure relates to the pore size of the fibrous bed and
the wet bending modulus of the fibers. If the pore si~e
(i.e. the spaces surrounding the fibers) is large and the
wet bending modulus ~,i.e. stiffness) of the fibers is high,
then the structure will have a relatively high liqùid
holding capacity but generally does not accept and trans-
port (wick2 liquid readily. On the other hand if the pore
3Q size is smaller and the bending modulus relatively low,
the structure readily accepts and wicks liquid but will
have a lower liquid holding capacity.
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The fibers from the non-delignified wood pulp process can
provide an a~sorbent structure having a large pore size
and a high wet bending modulus of the fibers, however, such
absorbent structures do not readily accept liquid, nor
will the structure be readily densified or embossed to
promote wicking~
Summary_of t`he Present Invention
It has been discovered that an a~sor~ent structure can
be made from non-delignified ~ood pulp fibers which will
readily accept and wick liquid w~ile retaining a desirable
substantial liquid holding capacity Furthermore, the new
absorbent structure of the present invention takes advan-
tage of the improved economics available when using non-
delignified wood pulp fibers~
In its broadest aspect, the present invention provides an
absorbent structure comprising a fibrous bed of non-
2Q delignified wood pulp fibers and at least a~out 1% byweight of a hydroxyl-bearing polymer component commingled
therewith.
The new absorbent structure is made by defibrating non-
delignified wood pulp feed material and air-laying the
fibers. The feed material is in the form of a board,
crumbs (clumps~, or bale material, or the like, with
which the hydroxyl-bearing polymer has been commingled.
Detailed Description of the Invention
The non-delignified wood pulp processes do not remove the
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lignin from the wood fibers as t~e chemical processes
do. The presence of the lignin in or on the fiber provides
the fi~er with resilience and resistance to wet compression.
When providing an a~sor~en~ structure made from a fibrous
batt w~ich is to be used in products such as disposable
diapers, sanitary napkins, and the like, it is desirable
to provide the batt ~ith the ability to wick the liquid
from t~e point of entry of the liquid to other areas o~
the absor~ent structure.
The "wicking" o~ liquid is the transporting of liquid
from one point to another in the absorbent structure.
~Yicking has heretofore ~een accomplished by densifying
a selected area to create a smaller capillary structure
resulting in an increased capillary pressure. T~is concept
is discussed in further detail hereinafter. One method of
densifying an area is disclosed in Burgeni U. S. Patent
3,017,304 wherein a "paper-like" skin is formed on one
surface of an absor~ent batt used in a diaper. Another
method of densifying a selected area of fibers is found
in Rep~e, U.S. Patent 3,938,522 wherein densified regions
are provided.
In these absor~ent structures wherein a portion of the
fibrous batt is densified to decrease the capillary radius
between adjacent fibers resulting in an ability ~o wick
more liquid in a given period of time, the densified area
does not hold as much liquid resulting in a reduction of
the liquid holding capacity of the absorbent structure.
The present invention improves the wickability and inte-
grity, i.e., stability of the absorbent structure without
this loss of liquid holding capacity.
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The absorbent structure of the present invention is a
fibrous batt formed of loosely compacted non-delignified
wood pulp fibers which are primarily held together by
interfiber ~onds requiring no added adhesive. In order
for the absorbent structure to perform, the batt should
be substantially wettable. It has been discovered that
the addition of at least about 1% of a hydroxyl-bearing
polymer component commingled with the fibers of the fibrous
batt permits rapid wetting of the fibrous batt.
The fibers used to form the absorbent structure of the
present invention may be produced from any of the various
soft woods, such as spruce, balsam, western hemlock,
douglas fir, white fir, and the various pines, especially
southern pine, such as loblolly and slash pines. For ex-
ample, thermomechanical pulp is formed when ~he wood is
initially cut into chips and the chips are pre-steamed
in a pressurized steaming vessel. The pre-steaming
softens the wood and allows defibration by a disc refining
technique. The forming of non-delignified wood pulp
fibers is generally well known in the art.
The hydroxyl-bearing polymer is starch, polyvinyl alcohol,
guar gum or a hydroxyl-bearing polymer which is dispersible
in water. The starch may be any starch material whether
it be cereal starch, tuber starch, waxy, or a high amylose
starch. The starch may ~e cross linked or otherwise modi-
fied to form a cationic starch, an anionic starch or non-
ionic starch. Derivatives such as starch phosphates, hydro-
xyethyl starch, starch acetate, and the like, may be used.In addition, polyvinyl alcohol has been found to be suit-
able, as is guar gum The hydroxyl-bearing polymer compon-
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ent is present in an amount at least a~out 1% by weight,
preferably from a~out 1% to a~out 20% and most preferably
from about 1% to about 5~
In a preferred embodiment of the present invention, the
hydroxyl-bearing polymer in the form of starch, ~in an
amount from about 1% to about 2Q% ~y weight of the thermo-
mechanical pulp fibersl is slurried in water to form a
slurry less than about 30% by weig~t solids. The slurry
lQ is then heated to ~5C and the starch component is "cooked"
for about 20 minutes~ rn the meantime, non-delignified
thermomechanical wood pulp fibers from the refinery are
provided in dilute aqueous sl~rry form. The two water
slurries are combined and mixed. The resulting mixture
lS is then formed into board After formation of the board,
the board is ground and the absorbent structure is air-
laid as a we~.
The absorbent structure in the form of a batt may be
2Q laminated with a moisture-impermeable film, such as a
backing on one side and a moisture~permeable facing on
the other side, to form a diaper. On the other hand, the
batt may be wrapped in a suitable wrapper to produce a
sanitary napkin or it may 6e processed in various ways
so as to be incorporated in tampons, underpads, dressings
or other absorbent products.
In the instance wherein the absorbent structure is to be
used in a disposable diaper, it is particularly advanta-
3Q geous to treat the absor~ent batt so as to provide wicking.When an infant voids into the diaper structure, all of
the urine enters the diaper in one limited area. Thus it
is highly desirable to provide a wicking or transporting
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system to the absorbent structure to rapidly wick the
urine away from the point of entry. As heretofore men-
tioned there are two well-kno~n systems for rendering
absorbent structures effective for wicking liquid.
The Burgeni ~or paper-like skin) generally formed on the
side of the batt adjacent the backing of the diaper,
assists in drawing the urine from ~he entry point to the
back side of the diaper a~ay from the infant. Densified
regions generally applied as longitudinal lines on the
absorbent batt assist in wicking the liquid to the ends
of the absorbent batt.
Non-delignified wood pulp fi~ers air-laid as a web are not
readily densified nor can a Burgeni skin be formed effec-
tively. However, surprisingly,an absorbent batt formed
in accordance with the present invention readily accepts
densified regions and, if desired, a Burgeni skin. In
other words, the incorporation of the hydroxyl-bearing
2Q polymer into the absorbent batt structure greatly enhances
the ability of the fibrous structure to receive densified
regions. Furthermore, a paper-like (Burgeni) skin is
readily formed by conventional methods. Either the densi-
fied regions or the Burgeni skin or a combination thereof
greatly enhance the integrity of the absorbent batt.
Heretofore it has not been possible to improve both the
~icking properties and the integrity of an absorbent batt
consisting of non-delignified wood pulp fibers. At this
time it is theorized that in the situation of densified
3a regions and Burgeni skin as applied to an absorbent batt
of chemical pulp, the presence of moisture causes a ~iber
collapse thus creating the densified region. In the
instance of an absorbent batt formed from non-delignified
wood pulp fibers 5 the fibers do not collapse in the presence
of moisture. The addition of ~he hydroxyl-bearing polymer
for some reason permits densifying of the batt fibers in
S the presence of pressure and/or moisture.
It is another embodiment of the present invention to
provide an absorbent structure comprising non-delignified
wood pulp fibers in com~ination with up to about 50% by
10 weight of chemically processed wood pulp fibers, cotton
linters, mixtures thereof or mixtures containing small
amounts of rayon or the like.
These absorbent structures containing fibers o~her than
15 non-delignified wood pulp fibers may be formed by pre-
mixing the fibers, forming a feed material and then
grinding and forming the air-laid web, or the absorbent
structure may be formed of a layer of non-delignified
wood pulp fibers in close association with or laminated
20 to another layer of chemically processed wood pulp fibers
or the like.
The following example illustrates a preferred embodiment
for forming an absorbent structure in accordance with the
25 present invention~
Example
Seventy grams of non-delignified thermomechanical wood
3Q p~llp fibers are soaked in water and made into a twelve
by twelve inch board in a sheet former. This sample is
used as a control sample.
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Another seventy grams of the pulp are soaked in water.
A five hundred milliliter slurry is prepared containing
seven grams of a slightly acid-modified starch. The
starch mi~ture is cooked at 95C for twenty minutes and
added to the thermomechanical pulp slurry and mixed.
The mixture is formed into a board of the same size and
type as the control board. The control board weighs
66.2 grams and the board containing the starch weighs 70.1
grams. The boards are ground in a hammer mill and the
fibers air-laid to provide a fibrous ~at~. The batt is
divided into samples and tested for wicking, tensile
strength, embossed lines, etc. in accordance with the
following test procedure. A test to determine the wicking
properties of a fi~rous web supported in a vertical
position is as follows.
The test apparatus consists of two matching clear-plastic
plates measuring 5" x 11". The gap between the plates
is regulated with adjusting screws so as to adjust the
thickness of the batt to obtain the desired density.
Fibrous batt strips measuring 3" ~ 8" (the batts generally
weighing about 8 o2/yd2~ are placed between the plates
with the bottom edge of the batt strip matching the bottom
edge of the plates. The plate gap is adjusted to give
whatever dry density is desired. The apparatus is then
hung in a vertical position. A large Petri dish, contain-
ing 1.59~ saline solution,is placed on an adjustable stand
and is raised to contact the bottom surface of the batt
strip. The distance wicked by the liquid and the weight
of the solution wicked is determined after a prescribed
period of time.
The results of the tests are in Table 1 below.
TABLE 1
INCHES LIQUID WICKED
DENSITY WICKED GmLiq/
5 SAMPLE GM WEIGHT gm/cc IN 8 MIN. Total Grams GmSample
Control
A 5.48 0.1 4.50 20.1 3.7
B 4.72 0.1 4.25 16.8 3.6
C 5.25 0.1 5.00 18.3 3.5
D 5.16 0.1 4.75 19.6 3.8
With Starch
. . . _
E 3.83 0.1 4.50 16.0 4.2
F 4.70 0,1 4.50 20.7 4.4
G 4.75 0.1 4.75 19.1 4.0
15 H 5.18 0.1 4.50 22.4 4.3
When comparing the control samples with the samples
containing starch, it is noted that the distance the
liquid wicked in 8 minutes is about the same. However,
2Q the amount of liquid wicked by the samples containing
starch is greater by at least 13% in each instance as
shown in the last column of the table above.
Embossed lines were placed on the panels. Each line was
one-eighth inch in width and was applied by an embossing
machine at a pressure of 20 psi. The embossed line was
then cut out of the panel and subjected to a tensile
strength test on an Instron instrument wherein the full
scale load was 100 grams. The embossed lines of the
control sample exhibited a tensile strength of 38.6 grams
whereas the lines of the sample with starch had a tensile
strength of 60.2 grams.
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It is readily seen from the data above that the starch improves
the amount of liquid wicked by at least 10%. Thus more liquid is
wicked just as far in the same time period. Furthermore the
integrity of the fibrous structure is improved by more than 50%.
The foregoing description and example are illustrative but are
not to be taken as limiting. Other variations and modifications
are possible without departing from the spirit and scope of the
present invention~
