Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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STABILIZED ABSORBENT BOARDS
BACKGROUND OF TH~ INVEN~ION
This invention concerns methods and products utilizing
fibrous absorbent bodies for absorbing fluids. In partic-
ular, the invention concerns products such as catamenial
tampons, diapers, sanitary napkins and the like and is
specifically directed toward fibrous absorbent bodies
which are easily handled in processes for manufacturing
such products and which maintain their integrity when wet
with body fluids.
The vast majority of body fluid absorbent products now in
use comprise, at least in their formative stages, pads of
loosely associated fibrous, and generally cel]ulosic,
absorbent materials such as comminuted wood pulp fluff,
rayon staple~ cotton, cotton linters and the like. For
~enerations, these materials have proven to be useful and
effective in dressings, diapers and sanitary protection
devices in that such materials are absorbent, inexpensive,
and, in the case of absorbent products which must be worn
by the user for substantial periods of time, such mater-
ials are flexible and comfortable. Unfortunately,balanced against these highly desirable properties, is the
fact that pads manufactured from the loosely associated
fibrous materials are relatively weak, having little
tensile strength and must be handled gingerly throughout
any manufacturing process.
The manufacturing problems associated with these loosely
associated fibrous materials has been aggravated to some
extent by the desire to incorporate into the body of such
fibrous materials certain cellulosic materials ~herein
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termed hydrocolloidal) which exhibit substantially
increased absorptive properties by virtue of chemical
modificationA Examples of such materials are the grafted
cellulosic copolymers described in U. S. Patent 3,889,-
578 issued to Pronoy Chatterjee, et al. on June 17,1975
and the cross-linked carboxyalkyl cellulosic materials
described in U.S. Patents 3,731,686 and 3,858,585
issued to Pronoy Chatterjee on May 8, 1973 and June
7, 1975; and in U. S. Patent 3,589,364 issued to W.
L. Dean, et al. on June 29, 1971. These hydrocolloidal
- materials are in the form of highly swellable and
highly retentive fibers. It is desirable to combine
; these fibers with the more conventional absorbent
materials such as rayon, woodpulp, cotton or the like
to produce an absorbent body having increased fluid
retentive properties. Unfortunately, when mixing such
fibrous materials, it is not an easy processing task
to get an even distribution and this adds to the burden
of producing an absorbent body for the products of
interest herein.
In my Canadian Patent No. 1,153,502, issued September
13, 1983, I describe a method for avoiding the difficul-
ty of handling such materials and specifically describe
a method whereby the materials are formed into a board
and the board is rendered flexible by virtue of being
; dry compressed after its formation. While this method
has indeed allowed the use of fibrous systems in a
moxe readily processible form and produces products
which are comfortable to the user, there are still
drawbacks associated with this product. Specifically
it has been found that when boards of fibrous systems
incorporatiny such chemically modified fibers as those
described above become wet with body fluids such as
urine or menstrual fluid, the boards deform greatly,
particularly under the influence of pressures common
in the use of produc-ts such as tampons, diapers and
the like. Under the influence of
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pressures exerted by the wearer in normal use of such
products, the boards tend to collapse, flow and deform
thereby greatly reducing their abilities to trap fluids in
the interstices between the ibers and allcw the
penetra~ion of additional fluid. Said in other words, the
deformation tends to block the ability of fluid to
penetrate and hence fully utilize the absorbent capacities
of these absorbent, board-like materials.
Accordingly, there is a need for producing a densified
board-like absorbent material which can be readily handled
during processing, which is comfortable when incorporated
in absorbent products worn by the user, and which will
resist deformation when subjected to pressure in the wet
state.
SUMMARY OF THE INVENTION
In accordance with the teachings of this invention it has
now been discovered that absorbent material comprising
hydrocolloidal fibers may now be provided in board form
having substantial structural integrity both in the dry
state and after being wetted with body fluids. Specifi~
cally, it has been discovered that a board of hydrocol-
loidal fibers may be prepared which will exhibit a sub-
stantial resistance to deformation in the wet state and
hence be capable of absorbing further body fluids without
collapsing and blocking penetration of fluid therein.
In accordance with the teachings herein, a board compris-
ing hydrocolloidal fibers is subjected to a heat treatment
step whereby the board is heated, in the dry state, at a
temperature ranginq from about 150C~ to about 200C., for
a period of about 3 to 30 minutes and preferably 170
180C. for 10-15 minutes.
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The board may be formed by such conventional means as
wet forming wherein a wet web of hydrocolloidal fibers
is fo~ned from a slurry by such means as by depositing
the slurry onto a screen and drawing water away with the
aid of a vacuumO The wet web is then dried to form
relatively hard inflexible board. As described in my
above-mentioned Canadian Patent No. 1,153,502, the board
may now be densified and rendered flexible by application
of pressure. The resulting board is then subjected to
the heat treatment being taught herein where~y the board
is rendered resistant to deformation when wetted.
Brief Description of the Drawinqs
Fig. 1 is a graphical illustration of the time-load
relationship when boards are subjected to pressure in
the wet state;
Fig. 2 is a graphical illustration of`the relationship
between percent relaxation value and heat treatment
time in accordance with the teachings of this invention;
Fig, 3 is a graphical illustration of the same relation-
ship depicted in Fig. 2, further illustrating the effect
of varying temperatures of heat treatment;
- Fig. 4 is a graphical illustration of the same relation~
ship depicted in Fig. 2 utilizing a different hydro-
colloidal material, and
FigO 5 is the same graphical representation utilizing
still another hydrocoll~idal material.
DETAILED DESCRIPTION OF THE INVE~TION
The boards with which this invention is concerned com-
prise fibers herein characterized as hydrocolloidal. The
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base of these fibers may be such commonly used cellulosic
materials as, for example, wood pulp, cotton, grasses or
regenerated cellulose fibers and the like. It is general-
ly preferred that these fibers lie in ~he range of about
lO0 to about 3,000 microns in length. Currently, because
of both C05t and availability considerations, wood pulp is
the cellulosic fiber of choice. The base fibers are
rendered hydrocolloidal by virtue of chemical modification
whereby they become water swellable and capable of absorb-
ing water in an amount which is at least about ten timestheir cwn weight, in the dry form, and preferably about
fifteen to about thirty times their dry weight. The
chemical modif ication consists of chemically bonding to
the polymeric backbone of the base materials, hydrophilic
groups or polymers containing hydrophilic groups.
Included in this class of materials are base materials
which are modified by being carboxyalkylated, phosphono-
alkylated, sulfoalkylated or phosphorylated to render them
highly hydrophilic. S~ch modified polymers may also be
crosslinked to enhance their hydrophilicity and render
them water insoluble~
.
These same base materials may also serve, for example, as
the backbone onto which other polymer moieties may be
bonded by grafted copolymerization techniques. Such
grated polysaccharides and the method of manufacture are
descrlbed in U.S. Patent No. 3,889,678 to Chatterjee,
et al~ and may be described as polysaccharide chains
having grafted thereon a hydrophilic chain of the general
formula:
~ (CH2)q - CRl ~ (CH2)p - CR3 1
l C=O ~ L C-O
m n
~ ,~
i2~
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wherein Rl and R2 are selected from the group consisting
of hydrogen and alkyl having 1 to 4 carbon atoms, X and Y
are selected from the group consisting of -OH, -O(alkali
metal), and -NH2, wherein m i5 an integer having a value
of 0 to about 5000, n is an integer having a value of 0 to
about 5000, the total number of m and n moieties on a
chain is at least 500, p is an integer having a value of
zero or 1, and q is an integer having a value of l to 4.
Preferably the hydrophilic chain is a hydrolyzed copolymer
of acrylonitrile and me~hyl methacrylate or ethyl acrylate
monomers as are set out in U.S. Patent 3,889,678.
The grafted polysaccharides described above and, in
particular, grafted cellulose as well as carboxy
methylcellulose are the hydrocolloidal fibers of choice.
The hydrocolloidal fibers may be combined with other
cellulosic or non-cellulosic absorbent fibers in producing
the board of this invention.
The fibers are com~ined with water in a slurry forming
station to form a slurry which may be conveyed to a web
forming station. Generally, the slurry should comprise of
no more than about 0.1 and preferably no more than about
0.05% solids. The slurry may be formed in several ways
known in the art associated with wet laying of fibrous
webs. For instance it may be prudent to form the slurry
at high solids concentration, e.g., about 1.5% by weight
solids and then further dilute the slurry with the
addition of more water to the desired concentration.
Irrespective of how the slurry is formed, it is next
passed to a web forming station where a wet web is formed,
for example, by depositing the slurry onto a continuous
belt and maintaining a differential pressure across the
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face of the belt to remove a preponderance of the water
and leave a loosely compacted wet web of fibers. A~ this
point in the process it is desirable that the web has a
solids content of no more than about 30% by weight of wet
web and not less than about 6. The wet web is next passed
to a drying station wherein the web is dried to a water
content of less than about 10% by weight and preferably
less than about 5%.
In accordance with this i~vention the web, ncw dried to
what is essentially ambient conditions, is heat treated by
being subjected to a heating step at a temperature of
about 150C. to about 200C. for a time period of about 3
to 30 about minu~es. It shculd be understood that at the
higher temperature range the period may be nearer the
lower limits of the time range and vice versa. The heat
treatment of the sheet can be accomplished by passing the
sheet through an air circulated tunnel dryer operated at
the appropriate temperature and belt speed.
The resulting product is a board that ls highly resistant
to deformation under pressure when wet. This property may
be quantified by use of a ~est to determine the Percent
Relaxation Value, as defined hereinafter. In accordance
with this test, a sample board of a given size is wet out
completely and any excess water is removed by blotting the
sample between paper towels. The wet board is then placed
on a compression cell in an Instron Tester. The head of
the Instron Tester is lowered until it nearly touches the
wet board and the recorder is started. The Instron head
is then slowly lowered until a maximum compression load of
a given arbitrary weight (e.g. 25 kilograms) is indicated
on the chart of the recorder. When a 25 kilogram maximum
compression load is reached, the Instron is stopped and
the sample is allowed to equilibrate under the load.
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As ~he sample is allowed to equilibrate, tha compression
load decreases as the sample deforms under the forces
exerted by the Instron tester head. The deformation
begins rapidly and then slowly decreases until a constant
load, in a deformed state, prevails. The Percent Relaxa-
tion Value is defined as the difference between the 25
kilogram load and the load at equilibrium divided by the
25 kilogram load and multiplied by 100.
Figure 1 illustrates the relationship graphically and
represents a plot of the compression load versus time.
The force of the Instron head is gradually increased to
the value of P over a period of time and P may be selected
arbitrarily e.g., 25 kilograms. After the board is
allowed to equilibrate, it reaches a constant co~pression
load o the value R. Percent relaxation value is then
defined as ((P-R~/P)xlO0. It will be understood that the
lower ~he value for percent relaxation, the higher the
stability of the board under wet conditions and,
conversely, the higher the percenl: relaxation value, the
the lower the wet stability and the more likely the board
will be to deform under pressure when wet.
EXAMPLE 1
A series of sample boards are prepared from hydrocolloidal
fibrous material made in accordance with example III of
the aforementioned U.S. Patent 3,889,678. This material
is a cellulose graft copolymer consisting o a cellulose
backbone having grafted thereto hydrolyzed polymer
moieties of poly(acrylonitrile) and poly(ethyl acrylate)
by combining wood pulp, acrylonitrile and ethyl acrylate
as the starting materials in the following weight proper-
ties, 1 part wood pulp to 1.40 parts ethyl acrylate to 0.8
parts acrylonitrile. This hydrocolloidal material is in
fibrous form, the fibers having an average fiber length of
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approximately 0.8~ millimeters. In accordance with
aforementioned ~.S. Patent 3,889,678, the grafted material
is subjected to a hydrolysis step. Table 2 sets out the
conditions under which these specific samples are
hydrolyzed to produce hydrolyæed product having variable
degrees of hydrolysis and, hence, being variably
hydrophilic.
Table 1
10S~MPLE HYDROLYSIS
Time (HRS ? Temperature(C)
A 1.75 80
B 2.75 80
C 3.7~ 80
lS
The hydrocolloidal fibrous material is formed into a board
b~ first dispersing the fibers in water to yield a slurry
having a consistency of 1.17% by weight solids. One liter
of the slurry is placed in a handsheet mold measuring 7.5
inches by 7.5 inches and manufactured by Williams
Apparatus Company of Watertown, Nlew York. The slurry is
then diluted to a consistency of 0.01% by weight solids in
accordance with the procedure set out in TAPPI Standard
Method T-2050S71.
After mixing thoroughly, the water is allowed to gravity
drain, leaving a we~ hydrocoll~id fibrous web having a
solids content of about 5~ based on the weight of the dry
web. The wet web is then blotted with blotter boards,
squeezed to remove excess water, and then dried in an air
circulated oven to a water content of about 2% water, by
weight of dry material.
The resulting boards have a basis weight of 320 lbs. per
3000 sq. feet.
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In accordance with the teachings of this invention, the
various board samples are subjected to a heat treatment
step wherein the boards are heated at 175C. for various
periods of time. The sample boards are then tested to
determine their Percent Relaxation Value as described in
accordance wi~h the compression test set out above.
Figure 2 illustrates the results of this test with respect
to samples A, B and C of Table 1, which samples have been
subjected to heat ~reatments of from zero to 60 minutes at
175C. As ca`n be noted from Figure 2, at zero minutes of
heat treatment i.e., no heat treatment, the Percent
Relaxation Value is basically a function of the degree of
hydrolysis or said in other words, the hydrophilicity of
the hydrocolloidal material. Sample C, having a high
degree of hydrophilicity, has a Percent Relaxation Value
of 81, that is to say, a low wet stability. Sample A,
having a lcw degree of hydrophilicity, has a Percent
Relaxation Value of 44 indicating a relatively high wet
stability. Sample B, having medium degree of
hydrophilicity exhibits a Percent Relaxation Value of 71,
an intermediate wet stability. As these samples are
subjected to heat treatment for periods of time from zero
to 50 minutes, it will be noted that the Percent
Relaxation Value drops dramatically and tends ~o stabilize
for these samples at values ranging from 15 to 25 percent
after a heat tre~tment of from 10 to 20 minutes.
~hereafter, the Percent Relaxation Value seems to
stabilize and no longer decreases with increasing heat
treatment time. As a control, a sample of wood pulp is
also subjected to the same board-forming process and
subsequent heat treatmentO As shown in Figure 2, the
sample wood pulp has approximately the same Percent
Relaxation Value at zero minutes of heat treatment
(untreated) as does sample A, i.eO, the sample having the
lowest degree of hydrophilicity. However in marked
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contrast to the hydrocolloidal board of this invention,
wood pulp shows essentially no decrease in Percent
Relaxation Value with increasing heat treatment time and
instead remains substantially less wet stable than the
hydrocolloidal boards of this invention.
EXAMPLE II
_._
To illustrate the time-temperature relationship of the
materials treated in accordance with the teachings of this
invention, sample C of Example I abo~e is subjected to
temperatures of 150, 175, and 200C. The results of the
Percent Relaxation Value test for these materials is
illustrated in Figure 3. As can be seen from this figure,
it requires 60 minutes to reach a Percent Relaxation Value
of 20 when the heat treatment occurred at 150C. At a
heat treatment of 175C. it requires 10-15 minutes, and at
a heat treatment temperature of 200C. it requires only 5
minutes to reach this Percent Relaxation Value. As a
control, the pulp sample heated at a heat treatment
temperature of 175C. is also illustrated in Figure 3 and
essentially does not vary in Percent Relaxation Value with
the time of heat treatment.
EXAMPLE III
__
A sample D of hydrocolloidal material o the type
described with respect to Example I, is heat treated at a
temperature of 175C. for varying time periods. This
sample D is made in accordance with the process of Example
I with the exception that the s~arting materials used are
in the weight ratio of 1 part cellulose to 2.75 parts
ethyl acrylate to 1.6 parts acrylonitrile. The sample is
hydrolyzed at 80C. for two hours. Figure 4 illustrates
graphically the change in Percent Relaxation Value with
varying time of heat treatment at 175C. ~s can be seen
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from this figure, the sample boards are stabilized in
about 10-15 minutes ~o a Percent Relaxation Value of
approximately 20. Superimposed on Figure 4 are the
results of Example I which more or less perform
equivalently to the sample of this Example with respect to
Percent Relaxation Value. Also superimposed is wood pulp
as a control.
EXAMPLE IV
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Absorbent fiberous board is provided using the method
described in E~ample I with the exception that the fibrous
mix consists of 50% wood pulp fibers and 50% by weight of
a carboxymethylated cellulose fiber that has been
crosslinked and is sold b~ the Hercules Company of
Wilmington, Delaware, under the trademark "Aqualon'l. The
board produced by the method of Example I is tested for
percent relaxation value after being heated at 175C. for
various ~ime intervals. The results are shown in Figure 5
and as can be seen from this Figure, the heat treated
carboxymethyl cellulose containing board, Sample E reach a
constant Percent Relaxation Value; i.e., stability, in
about 4 minutes of heat treatment at 175C. This is
substantially faster than the grafted cellulose copolymers
of the prior examples.
