Note: Descriptions are shown in the official language in which they were submitted.
CA 02315748 2000-06-19
w0 99/33420 PCT/SE98/02454
1
Absorbent article containing superabsorbent material
Technical field
The present invention refers to an absorbent structure in an absorbent
article, such as a
diaper, sanitary napkin, incontinence guard, wound dressing, bed protection
etc., said
structure containing a superabsorbent material possibly in combination with
other
absorbent and/or carrier materials and where the superabsorbent material has a
content
of renewable and/or biodegradeable raw material of at least 20 % by weight,
calculated
on the dry weight of superabsorbent material.
The superabsorbent material can be in particulate form, a g grains, granules,
flakes or
fibers, or as a film, a foam or a coating on fibers and has a high capability
to absorb
liquids, such as waer and body liquids, a g urine and blood while swelling and
forming
a non water-soluble gel.
Background of the invention
So called superabsorbents are crosslinked polymers having the capability to
absorb
liquid several times, 10 times or more, their own weight. They further have
capability
to retain liquid even when they are exerted to an external pressure. They are
extensively
2 o used in hygiene article such as diapers, incontinence guards, sanitary
napkins and the
like, at which they are usually present in particulate form such as a g
grains, granules,
flakes or fibers and are mixed or layered with other absorbent material,
usually
cellulosic fibers.
Superabsorbents usually are crosslinked poiymers of different chemical
composition.
Commonly occurring superabsorbents are crosslinked polyacrylates. Acrylic acid
and
polyacrylates are synthetic products based on oil raw material, i a a non
renewable raw
material. The increasing environmental conciousness however involves a desire
to
change over to materials based on renewable and/or biodegradeable raw
materials. This
3 0 applies especially to disposable products, which after use are thrown
away. Poly-
saccharides such as cellulose and starch are examples of materials which are
both
CA 02315748 2000-06-19
WO 99/33420 PCT/SE98/02454
2
biodegradeable and renewable and can be used as starting materials for
superabsor-
bents. Other examples of useful polysaccharides are xantane, alginate,
chitosane, pectin,
guar gum. Proteins and peptides are also examples of polymers from a renewable
source.
The performance of a certain superabsorbent in an absorbent article depends on
many
factors, such as where and how the superabsorbent is mixed into the absorbent
structure, particle shape, particle size and physical and chemical properties
such as
absorption capacity, gel strength and liquid retaining capacity.
to
A phenomena called gelblocking can also negaitvely effect the absorption
capacity in
for example a fibrous structure containing superabsorbents. Gelblocking means
that
superabsorbent particles when wetted forms a gel which blocks the pores in the
fibrous
structure or between the particles and by that obstructs liquid transport from
the wetting
area to the rest of the absorbent article and liquid transport to all
particles. This involves
a poor utilization of the total absorption capacity of the absorbent body and
involves a
risk for leakage.
A high gel strength - cross linking of the superabsorbent involves a decreased
risk that
2 0 gelblocking will occur. A too high gel strength however results in a too
low absorption
capacity. With gel strength is meant the tendency to gel deformation, which
can be
defined as the shear modulus of the gel. This is measured in a rheometer and
is
calculated as the relationship between (a) the shear stress/deformation that
is applied to
the sample and (b) the relative deformation/shear stress showed by the sample.
The
2 5 shear modulus, G', is measured in the unit Pa. This is described in a g EP-
B-254 476.
Another common method to characterize a superabsorbent and its capacity to
perform
in an absorbent article such as a diaper, incontinence guard, sanitary napkin
and the like
is to measure its capacity to absorb liquid under pressure, so called AUL
(absorbency
3 0 under load). This is disclosed i a g EP-B- 0,339,461. One measures here in
a special
apparatus the ability of the superabsorbent to absorb a saline solution
according to the
CA 02315748 2000-06-19
WO 99/33420 PGT/SE98/02454
3
so called demand wettability principle under a certain load, usually 2kPa,
which is an
established standard load within this technical field,
Other common measuring parameters is the fi~ee swell capacity {FSC) which is
measured as g absorbed liquid per g superabsorbent when this is allowed to
swell
freely, and the capacity to retain liquid after centrifugation, centrifuge
retention
capacaity (CRC). According to the later method the superabsorbent is allowed
to
absorb liquid freely until saturation, after which the sample is centrifugated
and it is
measured how much of the absorbed liquid that is retained after
centrifugation.
None of these methods of characterizing the pmperties of a superabsorbent
however
tells the whole truth and does not always correlate to the performance of the
super-
absorbent in a product concept. This especially applies to superabsorbents
wholly or
partly based on renewable and/or biodegradeable raw materials, which usually
have a
low AUL value and a low gel strength and where neither AUL or any of the other
above
stated measuring methods is a good measure of the performance of the
superabsorbent
in an absorbent article.
The object and most important features of the invention
2 0 The object of the present invention is to provide an absorbent structure
in an absorbent
article of the above mentioned kind where the absorbent structure contains a
super-
absorbent material wholly or partly based on renewable and/or biodegradeable
raw
materials and which shows a very good performance in the absorbent structure
measured as absorption capacity and acquisition rate. This has according to
the
2 5 invention been achieved by the fact that the superabsorbent material has a
phase angle
8 s 8 °defined as the phase shift between deformation and strain
measured at rheological
measurements.
The superabsorbent material preferably has a phase angle 8 s 6 °,
preferably s 4 ° and
3 o most preferably s 3 °.
CA 02315748 2000-06-19
WO 99/33420 PCT/SE98/02454
4
The superabsorbent material should preferably have a content of renewable
and/or
biodegradeable raw material of at least 40, preferably at least 60 and most
preferably at
least 80% by weight, calculated on the dry weight of the superabsorbent
material.
According to an alternative the superabsorbent material comprises a
crosslinked
polymer, where the polymer is based on renewable and/or biodegradeable raw
material
and the crosslinking agent either is renewable and/or biodegradeable or non-
renewable/
non-biodegradeable.
The absorbent structure can according to an embodiment contain a combination
of
hydrophilic fibers and superabsorbent material, at which the hydrophilic
fibers
preferably are cellulosic fluff pulp, tissue or hydrophilic synthetic fibers,
and the
absorbent structure contains between 5 and 80 % by weight superabsorbent
material
calculated on the total weight of the structure.
According to another embodiment the absorbent structure contains a combination
of an
absorbent foam material and a superabsorbent material.
According to a further embodiment the superabsorbent materialet is in the form
of a
2 0 foam and according to another embodiment in the form of a coating on a
fibrous
rmaterial
The superabsorbent material can have a relatively low absorption capacity
under load
(AUL), more precisely no more than 25, preferably no more than 20 and most
2 5 preferably no more than 18 ml of an acqueous solution containing 0,9 % by
weigh of
sodium chloride, per g superabsorbent material under a load of 2kPa.
Description of the invention
The absorbent structure according to the invenrion is intended to be used in
an
3 o absorbent article of the kind comprising a liquid pervious cover sheet, a
liquid
impervious bottom sheet and an absorbent structure applied therebetween. The
CA 02315748 2000-06-19
WO 99/33420 PCT/SE98/02454
absorbent article can be a diaper, a sanitary napkin, an incontinence guard, a
wound
dressing, a bed protection etc. The shape and construction of the absorbent
article is of
course completely optional and the invention can be applied to all types of
such articles.
5 The increasing environmental conciousness has however involved that there is
an aim
towards replacing the materials in absorbent articles, a g diapers, that are
based on non-
renewable/non-biodegradeable raw materials with materials that are completely
or
partly based on renewable and/or biodegradeable raw materials. The materials
concerned are the liquid impervious bottom sheet in the form of a plastic
film, the
liquid pervious cover sheet in the form of a nonwoven and the superabsorbent.
Superabsorbents based on renewable and/or biodegradeable raw materials, such
as
starch, cellulose, alginate etc often have a low gel strength and a low AUL
value. In
tests that have been performed it has proved that those superabsorbents having
essentially equal FSC (free swell capacity), CRC (centrifugation retention
capacity) and
AUL can have very different absorption capacity if they are combined with
hydrophilic
fibers, usually cellulosic fluff pulp, in an absorbent structure. The
inventors of the
present invention have therfore searched for another parameter showing a clear
correlation with the absorption capacity of the superabsorbenten when
incorporated in
2 0 an absorbent structure, as it is intended to be in an absorbent article, a
g a diaper.
Rheology is the science of the deformation and and flow properties of
materials. The
term rheology includes all deformation of a material that occurs when it is
exerted to
forces, both the usual gravitation or if higher forces are needed. Rheology
deals with
2 5 the relationship between FORCE, DEFORMATION and TIME.
In the cases where all deformation energy is spread as heat in the material
this is
denoted as viscous and one says that the material flows. In the cases where
all
deformation energy is stored in the material.the material is denoted as
ELASTIC and no
3 0 flowing ocurs. A material in which a certain part of the deformation
energy is stored
and a certain part in spread as heat is denoted as VISCOELASTIC.
CA 02315748 2000-06-19
w0 99/33420 PGT/SE98/02454
6
At measurements of viscoelastic material the material is exerted to sinus-
shaped strain
(or deformation, depending on which type of instrument that is used) and the
sinus-
shaped deformationen (or strain) in the material is measured: The phase shift
between
deformation and strain is called the phase angle 8. This is 0° for an
ideally elastic
material and 90 ° for an ideally viscous material.
By making a vector analysis of the strain/deformation realtionship there is
obtained a
component, G', in phase with the deformationen and a component, G", 90°
offset in
phase with the deformation.
The higher values the modules G' and G" have the more elastic and viscous
respectively the material is. According to the above mentioned EP-B-254 476 it
is the
module G' that is measured, which according to the invention has proved to be
insufficient in order to foresee the function of the superabsorbent in an
absorbent
structure.
It has now according to the invention shown that it is the phase angle 8
defined as
tan b = G"/G' that is such a parameter that shows a clear correlation with the
properties
of the superabsorbent in a concept in an absorbent structure. Those
superabsorbents
2 0 having a value of 8 s 8° show an especially good product function
in the form of
absorption capacity and acquisition rate. It is especially surprising that
this applies also
to superabsorbents having such a low AUL value as lOg/g at 2 kPa, which is
evident
form table 1 below.
2 5 The Theological measurements are made on a Cam Med CSLZ 100 from TA
Instruments. The rheometer is a "controlled stress"- instrument. The
measurment are
performed according to established Theological methods, so that the person
skilled in
the art should easily be able to repeat the measurments.
3 o In all measurements the materials are prepared as samples according to the
principle: 1 g
absorbent material to 20 g synthetic urine. In order to test the materials at
an
CA 02315748 2000-06-19
WO 99/33420 PCT/SE98/0Z454
7
equilibrium position in the swelling process the mixture sample + synthetic
urine is
allowed to stand for 2 hours before tested, however within 6 hours from the
mixture
was prepared
The measurement is divided into two steps, where step one inciudes a so called
"stress-
sweep" with which the linear area of the material is def ned. The linear
viscoelastic area
area occurs when the deformation is so small that the material only
neglectably changes
from its equilibrium position. From this linear position the shear stress is
chosen, at
which the material is to be tested in the coming frequency sweep. From the
frequency
sweep 0,1-lOHz the quantities G', G" and 8 are obtained All G', G" and 8-
values
shown in the table below are taken at I,13 Hz.
The superabsorbent based on renewable raw material that were tested were a
number of
test substance based on crosslinked CMC {carboxy methyl cellulose): A1, A2,
A3, A4,
A5, A6, A7, A8, A9, A10, Al l, A12, A13 och A14. Preparation of
superabsorbents
based on crosslinked CMC is descrubed in a g EP-B-201 895. Some test
substances
based on crossiinked carboxy methyl starch: B I, B2 och B3, were also tested,
and
which can be produced analogously with crosslinked carboxy methyl cellulose
(CMC).
Further some test substance based on crosslinked polyaspartate: C1, C2 och C3,
were
2 0 tested and which a g may be produced according to W096/08523,
As reference material there was also tested a commercially available
superabsorbent
from Hoechst called Sanwet IM 7100 which is a crosslinked sodium polyacrylate
and
pure cellulosic fluff pulp without admixture of superabsorbent.
The absorption concept tested consisted of 20 % by weight of a superabsorbent
mixed
with cellulosic fluff pulp of chemical type. The basis weight of the pulp
structure was
500 g/mz.
3 o Besides the b-value there was also measured FSC (g/g), CRC (g/g) and AUL
(g/g) at
CA 02315748 2000-06-19
WO 99/33420 PCT/SE98/02454
8
2 kPa of the superabsoerbents. The test liquid was in all cases synthetic
urine according
to the following recipe : 60 mmol KCl (4,5 g), 130 mmol NaCI (7.6 g), 3.5 mmol
MgS04 (0.42 g) , 2.0 mmol CaS04*2H20 (0.34 g), 300 mmol urea ( 18 g) and 1 g
0.1
triton were dissloved in 1 liter destilled water.
The measuring methods used for testing the function of the superabsorbent in
an
absorbent structure according to above were the following:
The absorption capacity in a sloping plane:
The sample body was placed in a 30° inclination in order to imitate the
position of the
diaper during use and the lower portion was, in contact with a liquid bath
(synthetic
urine) and was absorbing liquid by suction. The sample body was weighed before
and
after liquid absorption and the absorption was noted as g absorbed liquid per
g
absorbent body.
Acquisition time:
Three additions of each 60 ml liquid (synthetic urine) was made with a time
interval of
10 minutes. The time it took before all liquid had been absorbed was measured
(visual
observation).
The result of the performed measurements are shown in Table 1 below.
CA 02315748 2000-06-19
WO 99/33420 PCT/SE98/02454
Table 1
MaterialFSC CRC AUL 8 () Abs.capacityAcquisition
(g/g)(g/g) (g/g) in time (s)
a sloping 1, 2, 3
plane
(gig)
Puip - 6.5 31, 87, 127
IM 46 28 28 1.9 13.5 20, 15, 22
7100
A1 27 13 13 13.0 9.2 19, 34, 58
A2 29 22 11 16.0 8.8 21, 73,134
A3 39 23 9 13.0 9.5 21, 45, 98
A4 40 20 13 15.0 9.0 19, 38, 81
A5 45 27 7 16.0 8.9 24, 75, 145
A6 43 25 13 15.0 9.1 ~ 22, 53, 123
A7 27 13.6 14 14.0 9.0 20, 44, 83
A8 25 13 14 15.0 9.2 20, 41, 97
A9 30 18 14 15.0 9.8 18, 50, 77
A10 54 32 11 20 9.6 21, 67, 210
AI 1 32 13 16 8.8 10:0 13, 23, 36
A12 58 18 12 21 9.0 18, 90, 960
A13 37 21 14 14 10.7 16, 60,131
2 0 A14 29 16 12 8.8 9.6 24, 24, 40
B 1 26 14 15 6.0 10.7 22, 28, 42
B2 27 17 6 9.9 9.4 25, 60, 79
B3 20 12 9 10.2 23, 40, 63
C 1 31 17 8 2.9 12.1 22, 30, 49
2 5 C2 28 13 14 2.4 11.3 21, 30, 49
C3 23 10 18 2.0 9.8 21, 33, 54
CA 02315748 2000-06-19
WO 99/33420 PCT/SE98/02454
Correlation studies on these show that the 8-value correlates positively with
the
acquisition times 2 and 3, i a low acquisition times at low 8-values, and
negatively with
the absorption capacity, i a high absorption capacity at low 8-values.
5 Multiple correlation studies show that CRC in combination with a-values
explain the
result of absorption capacity and of acquisition time 3.
In the absove stated tests the superabsorbent was in the form of particles
which were
mixed with cellulosic fluff pulp for forming an absorbent body. The invention
is
1 o however not limited to such absorbent bodies, but it would also be
possible to apply
superabsorbent particles in the form of a layer between layers of cellulosic
fluff pulp or
other optional hydrophilic fibers, natural or synthetic. The superabsorbent
can also be
applied between layers of soft paper (tissue), nonwoven or other material. The
superabsorbent can also be present in the form of fibers, as a film or as a
coating on
another material or as a foam. Instead of or besides cellulosic fluff pulp the
absorbent
body can contain other types of hydrophilic fibers, absorbent foam material or
the like.
