Abstract
In the early 1980’s Johnson & Johnson developed a revolu-
tionary process for the production of webs containing superab-
sorbent polymers (SAP) produced by in-situ polymerization of
partially neutralized acrylic monomers directly on a synthetic
nonwoven substrate [1, 2, 3, 4]. A fresh look at this forgotten
technology will be presented from both a manufacturing and
application perspective. In particular, In-situ, SAP-containing
nonwovens offer many unique properties for application in the
personal hygiene industry, such as, improved fluid acquisition,
permeability, compressibility and pH control. These materials
also provide a more homogenous SAP distribution, eliminate the
need for SAP powder handling and have superior wet integrity
as compared to conventional fluff pulp/SAP air laid structures.
This technology also offers some unique opportunities for
designing and manufacturing profiled absorbent articles with
specific zones tailored to perform specific functions.
Introduction
Typically, conventional superabsorbent powder polymer pro-
duction begins with solution polymerization of partially neutral-
ized acrylic acid along with a small amount of a crosslinking
agent in water. The polymerization results in a water insoluble,
water swellable gel containing approximately 25 to 40 % solids,
which must then be cut, dried, milled and sifted to produce a
powdered SAP product with a typical particle size ranging
between 100 to 800
µ
m. The sifting operation typically generates
a fines stream that must be recycled back into the production
process creating a production bottleneck. The finished SAP
product is then shipped to a hygiene-industry converter where it
is blended with fibrillated wood fluff to form the absorbent core
structure of a personal hygiene article such as a diaper.
In an in-situ SAP process, the partially neutralized acrylic acid
monomer solution is applied directly to a nonwoven substrate
and polymerized. The web may be fed to the process either as a
pre-manufactured roll good or, preferably, made in-line from
bulk staple fiber using a carding operation. The monomer solu-
tion may be applied to the web using a variety of application
techniques such as brush coating, pressurized liquid spray, air-
assisted spray or airless spray. Initiation of the polymerization
may be carried out by using a redox package, thermal initiation,
UV, electron beam radiation or a combination of methods.
Partial drying of the web may be achieved using the heat of poly-
merization if the acrylic acid concentration of the monomer solu-
tion is above 30%. The moisture content of the final product may
be adjusted to the desired level using either a through-air, air
flotation, or forced-air infrared dryer. The dried product is then
mechanically softened, slit to the desired use width and wound.
A schematic comparison of a conventional SAP production
process and an in-situ process is shown in
Figure 1
.
The in-situ SAP process offers a number of potential advan-
tages for hygiene converters over conventional technology. Since
the process produces stable, immobilized SAP structures, it elim-
inates the need for SAP powder handling and the associated
dust exposure issues. It also offers the potential to provide more
uniform SAP distribution in absorbent core structures and will
remain stable during transport. The immobilized SAP particles
also remain stable even in the hydrated state. Other possibilities
include zoned SAP gradients and strategic placement of SAP’s
with different performance properties within the core structure.
The technology further offers the possibility of reducing produc-
tion costs through the minimization of SAP processing steps.
Nonwoven Substrate
One of the major objectives of the in-situ process is the forma-
tion and bonding of discrete SAP micro-droplets to individual
web fibers as shown in
Figure 2
. An optimized nonwoven struc-
ture is critical to the achievement of this state. Typically, a high-
loft nonwoven structure is required, depending on the desired
SAP loading and application. Key substrate properties include
the basis weight, bulk density, loft and porosity. A delicate and
intricate balance of these properties must be maintained in order
Nonwovens Containing
Immobilized Superabsorbent
Polymer Particles
By Darryl L. Whitmore, BASF Corp., Portsmouth, Virginia
ORIGINAL PAPER/
PEER-REVIEWED
35
INJ Fall 2003
•
to avoid saturation of the web structure during the monomer
coating process. Saturation, where droplets of monomer solution
coalesce on the web, leads to agglomeration and film formation.
Figure 3
shows the functional relationship between SAP basis
weight and weight percent loading for 55, 80 and 100 gsm sub-
strates at a constant substrate density of 0.018 g/cc. Once the sat-
uration zone is reached for a given substrate, film and agglomer-
ate formation begins to occur and negatively impacts the flexibil-
ity of the structure. In addition, this phenomenon also
adversely affects the absorbency rate of the applied SAP.
The monomer coating process is also critical to SAP
droplet formation. For spray application, the degree of
atomization is controlled by: (1) the viscosity of the
coating (the higher the solution viscosity with the high
shear rate encountered going through the nozzle ori-
fice, the larger the particle size), (2) atomization gas
pressure (higher gas pressure giving smaller particle
size), (3) diameter of the nozzle orifice (smaller orifice
giving smaller particles), (4) pressure forcing the coat-
ing through the orifice (higher pressure leading to
smaller particle size), and (5) surface tension (lower
surface tension yielding smaller particle size) [5]. By
maintaining a critical balance among all of these para-
meters, the SAP particle size on the web can be con-
trolled. Both fiber type and finish also play a crucial
roll in droplet formation. As shown in
Figure 4
,
hydrophobic fibers and finishes promote more spheri-
cal droplet formation, while hydrophilic types tend to
promote film formation of the applied hydrophilic
monomer mixture.
Other factors to consider when choosing a nonwoven for in-
situ SAP polymerization include flexibility, softness, compress-
ibility, resilience and elasticity. For hygiene applications, the final
product must, of course, remain soft and flexible even though it
is generally compressed to produce a thin structure. Resiliency
and elasticity are important aspects of the nonwoven in that the
substrate must be able to expand as the SAP particles begin to
swell. Any impedance to the swelling of the SAP particles in the
36
INJ Fall 2003
Figure 1
PAD FORMATION — CONVENTIONAL VS. IN SITU
Figure 2
IN-SITU SAP POLYMERIZATION ON
A FIBROUS SUBSTRATE
SAP Particle (<400 mm)
SAP Bond Point
Web Fiber
Pore
Volume
Highly permeable structures with good absorption properties that
simplify the manufacturing process for superabsorbents and diapers
xyz directions may prohibit the SAP from achieving its full
absorptive capacity.
Applications
One unique property of in-situ SAP structures is their ability to
rapidly generate a large, mechanically stabilized pore volume,
which makes the structure become highly permeable [6]. In-situ
materials provide such a mechanism through the swelling of the
immobilized SAP particles following exposure to fluid. Both the
degree and rate of expansion may be optimized by adjusting
such parameters as: the degree of loading of SAP particles on the
web, the SAP particle size, the degree of neutralization of the SAP
particles and their respective crosslink density. The degree of
expansion is an indication of the generated pore volume avail-
able for fluid uptake (i.e. larger volume correlates with better per-
formance) and by increasing the speed at which the pore volume
is generated, the likelihood of leakage during rapid fluid appli-
cation is diminished.
The degree and rate of expansion of in-situ materials may be
determined by measuring the free swell expansion volume
(FSEV) [6]. The FSEV is determined by measuring the height
(thickness) change, in millimeters, of a compressed, 6.0 cm diam-
eter circle of the test material during hydration using a single 20
ml dose of 0.9% saline under an applied load of 0.6 KPa. The
thickness of the sample as a func-
tion of time is measured with the
aid of a linear variable differential
transducer (LVDT) interfaced to a
computer (additional details about
this method may be found in ref-
erence 6). The results obtained for
an in-situ material, containing 100
gsm of SAP applied to a 55 gsm,
thermally bonded high-loft non-
woven, is shown in
Figure 5a
. A
similar test, expansion volume
under load (EVUL), was also con-
ducted on the same sample using
an applied load of 3.0 KPa and is
shown in
Figure 5b
. As shown in
Figures 5a and 5b,
the in-situ mate-
rial expands quite rapidly once
hydrated, even under an applied
load of 3.0 KPa.
Following both the FSEV and EVUL tests, the hydrated in-situ
sample was removed from the sample cell and the free fluid con-
tained within the void spaces of the material was removed by
blotting the sample between two stacks of filter paper under an
applied load of 3.0 KPa for two minutes. The resulting weight
loss allowed an estimation of the void volume of the sample. The
corresponding gel volume, measured by the weight change of
the dewatered sample, gives the amount of the fluid contained
within the in-situ SAP particles on the web. The results of this
determination are shown in
Table I
. As can be seen from the data,
the in-situ material generates a substantial pore volume com-
pared to the uncoated high loft nonwoven substrate and the SAP
particles provide fluid storage.
Since in-situ materials swell rapidly and generate a large open
pore volume, they lack sufficient capillary pressure to effectively
wick and distribute fluids. This limitation is effectively overcome
through the formation of laminate structures with cellulosic
materials. Suitable laminates may be prepared by either air lay-
ing fluff pulp directly on the in-situ web or by simply laminating
the webs with an air-laid, fluff-pulp roll good as shown in
Figure
6
. The resulting laminate may then be either layered or folded as
shown in
Figure 7
to produce a structure with the desired basis
weight and SAP loading.
A four layer, in-situ and fluff-pulp laminate structure is shown
in
Figure 8
. A comparison of the inclined wicking performance
against two commercial adult diapers is shown in
Figure 9
.
Inclined wicking is determined by measuring the saline uptake
of the sample up a 30 degree incline. One end of the sample is
immersed in a constant-level reservoir containing 0.9% saline.
Fluid uptake is measured by an electronic balance interfaced to a
computer. As shown in Figure 9, the wicking performance of the
in-situ laminate structure is comparable to that of the two com-
mercial adult diapers. Experimental diapers were prepared by
removing the cores from both the low SAP (10 wt%) and high
SAP (40 wt%) diapers and replacing them with the four layer, in-
situ laminate. The results obtained from rewet testing are shown
in Figures 10a and 10b. As shown in Figure 10a, a significant
37
INJ Fall 2003
Figure 3
FUNCTIONAL RELATIONSHIP BETWEEN SAP BASIS WTS. AND WT.
% LOADING
Figure 4
EFFECT OF FIBER FINISH ON SAP
DROPLET FORMATION
600
500
700
SA
P
(gsm)
improvement in rewet performance was observed for the low-
SAP diaper containing the in-situ laminated core, while compa-
rable performance was obtained for the modified high-SAP dia-
per. A dramatic improvement in strikethrough performance was
observed (Figure 10b) for both diapers containing the in-situ
core, demonstrating the superior fluid acquisition properties of
in-situ materials.
Considering the acquisition and rewet data, we can
address a common, undesirable trend seen among many
commercially available diapers on the market today: the
fact that acquisition times tend to increase with succes-
sive doses of fluid during rewet testing. With conven-
tional fluff-based absorbent structures, the cellulosic
fibers can lose resiliency and collapse when wetted. As a
result, the liquid uptake rate of the wetted structures
may become too low to adequately accommodate subse-
quent fluid insults. In these conventional structures,
where absorbent gelling particles are incorporated
between the fibers to hold them apart, the gelling parti-
cles swell and do not release the fluid. Swelling of the
particles can then diminish the void volume of the
absorbent structure and reduce the ability of the struc-
ture to rapidly uptake fluid. The degree to which the
swelling of the absorbent gelling particle negatively
impacts the rate of fluid uptake is dependent upon a
number of factors, such as the concentration of superab-
sorbent used in the absorbent core, the degree of cross-
linking in the polymer, the uniformity of the distribution
of SAP within the structure, the particle size distribution
of the powder and the hydrophobicity of the particles.
While many of these factors are difficult to control in
conventional cores, each of these factors is easily con-
trolled with an in-situ core structure and may be opti-
mized to achieve the desired performance properties for
a given absorbent article.
In many current, commercially manufactured
absorbent products, a considerable amount of pressure
is applied during manufacture to produce an "ultrathin"
product. This compression decreases the interstitial pore
volume of the structure, resulting in slower fluid uptake.
Currently, a section of a high-loft nonwoven is inserted
between the topsheet and the absorbent core to compen-
sate for this effect and aid in the uptake of fluid. Key
properties of this type of acquisition layer are the wet
and/or dry modulus of the constituent fibers, the hydrophilicity
of the fibers and the resiliency of the fabric structure [7]. Such
properties contribute to the acquisition layer’s ability to stay
open under load, maintain void volume, resist wet collapse,
enhance the desorption properties of the fabric, and preserve
void volume capacity after successive applications of fluid.
Acquisition materials used in current products have no real
38
INJ Fall 2003
Figure 5a
FREE SWELL EXPANSION VOLUME (0.6 KPa)
Figure 5b
EXPANSION VOLUME UNDER LOAD (3.0 KPa)
T
ABLE
1
SUMMARY OF RESULTS OF FSEV AND EVUL TESTS FOR IN-SITU SAP NONWOVEN
Initial
Final
Expansion
Gel
Capillary
Test
height (mm)
Height (mm)
Volume (ml)
Volume (ml)
Volume (ml)
FSEV
0.72
3.5
9.8
3.1
6.2
EVUL
0.61
2.5
7.1
3.4
3.1
FSEV Uncoated Web
0.45
0.45
1.3
0.03
1.1
mechanism to reopen after being compressed other than the
memory effects preserved in the fibers themselves. In-situ core
structures possess their own built-in acquisition capability pro-
vided by their rapid generation of a large, mechanically stabi-
lized pore volume.
In-situ SAP polymerization also offers the potential to produce
profiled absorbent structures in which each individual web sur-
face has been spray coated with superabsorbent particles of dif-
ferent performance characteristics that have been optimized to
perform a specific function, such as acquisition, fluid distribution
or fluid storage. An example of such an optimized structure
would be a web structure in
which one coated web surface has
been optimized for fluid acquisi-
tion and the other for fluid stor-
age. Webs prepared in this man-
ner may be used either alone in an
absorbent article to provide the
sole means of fluid acquisition
and storage or in combination
with conventional fluff and SAP
cores to provide improved fluid
uptake and increased capacity in
selected zones within the article.
The performance characteristics of
spray-coated webs are dependent
on a number of different factors,
such as the crosslink density and
degree of loading of the formed
superabsorbent polymer as well
as the density and basis weight of
the substrate itself.
Typically, acquisition zones are
highly crosslinked to both mini-
mize fluid retention and increase
swelling speed, while storage
zones are optimized for capacity.
It is also possible to produce SAP
gradients in both the xy plane and
z direction of the nonwoven.
Such possibilities do not easily
exist with conventional absorbent
core forming technologies.
It is well known that an absorbent
hygiene product capable of lower-
ing skin pH within the range of
4.0 to 5.5 is beneficial in prevent-
ing, or at least reducing, the inci-
dence of diaper rash [8, 9, 10].
Attempts to lower the pH of dia-
pered skin through the addition of
various acidic pH control agents
suffer from a number of draw-
backs including: decreasing the
absorptive capacity of the
absorbent core, safety and comfort
factors associated with leaching of the pH-control materials from
the article and processing problems associated with the place-
ment and distribution of the acidic material within the absorbent
core. Through simple control of the degree of neutralization of
the applied acrylic monomer solution, in-situ SAP technology
offers a convenient means to lower the pH both within the inte-
rior of the diaper as well as the surface without the difficulties
mentioned above [6].
Conclusions
In-situ, SAP-containing nonwovens have been shown to pos-
39
INJ Fall 2003
Figure 6
IN-SITU LAMINATED CORE STRUCTURES
Figure 7
IN-SITU LAMINATED CORE STRUCTURES
sess a number of unique properties for application in the per-
sonal hygiene industry, such as improved fluid acquisition, per-
meability, compressibility and pH control. These materials also
provide a more homogeneous SAP distribution, eliminate the
need for SAP powder handling and have superior wet integrity
compared to conventional fluff pulp and SAP air-laid structures.
This technology also offers some unique opportunities for
designing and manufacturing profiled absorbent articles with
specific zones tailored to perform specific functions.
References
1. Pieniak, et al., “Superthin absorbent product,†U.S. Patent
4,573,988, Mar. 4, 1986
2. Erdman, et al., “Stable disposable absorbent structure,†U.S.
Patent 4,676,784, Jun. 30, 1987
3. Pieniak, et al., “Superthin absorbent product,†U.S. Patent
4,537,590, Aug. 27, 1985
4. Pieniak, et al., “Method of forming a superthin absorbent
product,†U.S. Patent 4,540,454, Sep. 10, 1985
5. Z. Wicks, Jr., F. Jones, S. Pappas, Organic Coatings (2nd ed.),
pg. 420, John Wiley & Sons, New York, NY (1999)
6. Whitmore, D., Engelhardt, F., “Absorbent article and process
for preparing an absorbent article,†U.S. Patent 6,417,425 B1, Jul.
9, 2002
7. Latimer et al., “Absorbent structure having improved fluid
surge management and product incorporating same,†U.S.
Patent 5,364,382, Nov. 15, 1994
8. Zimmerer, “Disposable absorbent articles,†U.S. Patent
4,657,537, Apr. 14, 1987
9. Blaney, “Disposable diaper containing ammonia inhibitor,â€
U.S. Patent 3,964,486, Jun. 22, 1976
10. Alonso, et al., “Composition for treatment and prevention
of malodorous generating skin conditions,†U.S. Patent
4,382,919, May 10, 1983
— INJ
40
INJ Fall 2003
Figure 8
IN-SITU WEB LAMINATE FOR DIAPER
CORE CONSTRUCTION
Figure 9
INCLINED WICKING COMPARISON OF IN-
SITU COMPOSITE AND ADULT BRIEF CORES
Figure 10a
REWET COMPARISON OF ADULT MEDIUM
BRIEFS AND IN-SITU SAP WEB CORE
Figure 10b
STRIKETHROUGH COMPARISON OF ADULT
MEDIUM BRIEFS AND IN-SITU SAP WEB CORE
S
TRIKETHR
O
UGH (SEC)
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