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EUROPEAN
European Polymer Journal 41 (2005) 1927–1933
www.elsevier.com/locate/europolj
POLYMERJOURNAL
Synthesis and characterization of glass-containingsuperabsorbent polymers
A. Ruttscheid, W. Borchard *
Fakultat fur Naturwissenschaften der Universitat Duisburg—Essen (Campus Duisburg), Lotharstr. 1/65, FAK4,
47048 Duisburg, Germany
Received 31 March 2004; accepted 8 September 2004
Available online 29 April 2005
Abstract
By embedding hollow glass spheres of different sizes and densities into a matrix of crosslinked sodium polyacrylate,
superabsorbent polymers were synthesized using a water-in-oil suspension polymerization. These glass-containing
superabsorbents were capable of taking up water not only in form of a swollen polyacrylate gel, but also in form of
additional free water between the glass spheres and the surrounding polyacrylate matrix. According to swelling mea-
surements the maximum volume swelling degree of the glass-containing superabsorbents is in certain cases nearly dou-
ble as large as the value of a superabsorbent without embedded glass spheres.
� 2004 Elsevier Ltd. All rights reserved.
Keywords: Glass spheres; Polyacrylates; Superabsorbents; Suspension polymerization; Swelling
1. Introduction
Superabsorbent polymers are crosslinked hydrophilic
polymers capable of absorbing large amounts of water
or aqueous salt solutions [1–3]. The most important
application of superabsorbents is their use in personal
hygiene products (such as incontinence products like dis-
posable diapers for infants and adults or feminine
hygiene products). Other applications include conser-
vation of water in agriculture and horticulture [4],
insulation of underwater cables [3], and applications
concerning recreational activities like artificial snow in
indoor skiing arenas [3].
Most of nowadays manufactured superabsorbents
are made of lightly crosslinked poly(acrylic acid) which
0014-3057/$ - see front matter � 2004 Elsevier Ltd. All rights reserv
doi:10.1016/j.eurpolymj.2004.09.005
* Corresponding author. Tel.: +49 0203 379 3316; fax: +49
0203 379 2110.
E-mail address: [email protected] (W. Borchard).
is partially neutralized with sodium hydroxide to im-
prove absorbent properties [3]. For simplification such
partially neutralized poly(acrylic acid)s are called poly-
acrylates (PA) in this text. The theoretical aspects con-
cerning the swelling behaviour of superabsorbents as
well as polyelectrolyte gels in general have been treated
extensively in the literature (see e.g. Refs. [5], [6] and [7]).
Since superabsorbents were introduced to the hygiene
market in the seventies of the last century, their absor-
bent capabilities have been continuously improved.
One of these improvements comprises the development
of so-called surface-crosslinked superabsorbents which
consist of a lightly crosslinked polyacrylate core and a
shell with a higher degree of crosslinking [2,3]. The
advantage of these systems is that the rate of water
absorption is increased by preventing gel blocking dur-
ing swelling.
In general gel blocking is observed especially in par-
ticles of relatively lightly crosslinked polyacrylate [1,3],
ed.
1928 A. Ruttscheid, W. Borchard / European Polymer Journal 41 (2005) 1927–1933
because such systems contain a significant amount of
uncrosslinked polymer which can be extracted by the
swelling agent. Because this soluble fraction increases
the viscosity of the swelling agent in the interstitial vol-
ume of a particle packing, in this case the swelling pro-
cess can only progress by diffusion rather than by
convection of the swelling agent between the particles.
In the course of a thesis [8] another new type of
superabsorbents with core/shell structure was developed.
By embedding hollow glass spheres into a matrix of a
crosslinked polyacrylate, the absorbent capacity of the
crosslinked polyacrylate itself could be improved, be-
cause during swelling in water the core/shell contact
was interrupted and the system was able to imbibe addi-
tional water between the embedded glass spheres and the
surrounding polyacrylate shell (see Fig. 1).
The swelling behaviour of these glass-containing
superabsorbent polymers can be explained as follows.
As soon as water is added to the system, the polyacrylate
shell in which the glass spheres are embedded swells, so
that the added water is absorbed by forming a gel. The
volume of this polyacrylate gel (Vgel) depends on the vol-
ume of the polyacrylate in dry state (VPA, dry) and on the
volume swelling degree of the polyacrylate (QV,PA):
glass
polyacrylate(VPA, dry)
+ water
di
de
polyacrylate + water
(gel, Vgel )
Fig. 1. Schematic representation of the swelling of a glass-conta
polyacrylate; Vgel = volume of the swollen polyacrylate gel; V
de resp. di = external resp. internal diameter of the polyacrylate s
diameter of the polyacrylate shell after swelling).
V gel ¼ V PA;dry � QV;PA ð1Þ
Since the swelling of superabsorbent polymers is isotro-
pic, the external diameter (de) and the internal diameter
(di) of the polyacrylate shell increase during the swelling
process. Because the volume of a sphere depends cubically
on its diameter, the external and the internal diameter of
the polyacrylate shell after swelling (d 0e resp. d 0
i) can
be calculated as follows:
d 0e ¼ de �
ffiffiffiffiffiffiffiffiffiffiffiffiQV;PA
3
qresp: d 0
i ¼ d i �ffiffiffiffiffiffiffiffiffiffiffiffiQV;PA
3
qð2Þ
Thus during swelling of such a system an additional shell
(called ‘‘intershell’’) filled with additional water between
the glass sphere and the polyacrylate shell is formed (see
Fig. 1). The volume of this intershell (Vis) depends on
the volume of the glass sphere (Vg) and on the volume
swelling degree of the polyacrylate (QV,PA):
V is ¼ V g � QV;PA � 1� �
ð3Þ
The increase of de during the swelling process is indepen-
dent of the size of the glass sphere. Eq. (3) is also valid,
if several glass spheres with a total volume of Vg are
embedded into a matrix of crosslinked polyacrylate
(see Fig. 2). Because the synthesis of the samples (see
glass
water(intershell, Vis )
d i'
d e'
di
ining superabsorbent in water (VPA,dry = volume of the dry
is = volume of the intershell filled with additional water;
hell before swelling; de0resp. di
0= external resp. internal
A. Ruttscheid, W. Borchard / European Polymer Journal 41 (2005) 1927–1933 1929
Section 2.1) can not be conducted in such a way that
each particle contains only one glass sphere, the model
depicted in Fig. 2 describes the structure of the glass-
containing superabsorbents better than the model de-
picted in Fig. 1. The comparison of both figures shows
that a mean glass content per particle can always be cal-
culated, which leads to a similar description of the swell-
ing of the particles.
For particles of glass-containing superabsorbents
volume swelling degrees which are referring to the poly-
acrylate fraction of the samples (instead of referring to
the volume of the whole particle) can be calculated.
These volume swelling degrees are larger than the swell-
ing degree of the polyacrylate itself (because of the addi-
tional water in the intershell, Vis), and can be considered
as being ‘‘effective volume swelling degrees’’ (Q�V):
Q�V ¼ V gel þ V is
V PA;dry
¼ QV;PA þ V is
VPA;dryð4Þ
If the volume fraction of the glass spheres (/g) in the dry
sample is known, and if it is assumed that the swelling
degree of the polyacrylate fraction of the samples re-
mains the same with increasing glass content, it is possi-
ble to calculate (starting from Eq. (3) and Eq. (4)) the
expected effective volume swelling degrees (Q�V):
Q�V ¼ QV;PA þ
/g
1� /g
� QV;PA � 1� �
ð5Þ
According to Eq. (5) the effective volume swelling degree
of a particle increases with increasing volume fraction of
the glass spheres. If the glass content approaches a value
of /g 1 (nearly pure glass spheres, only a small
amount of polyacrylate), the effective volume swelling
polyacrylate + water
(gel)
+ water
glass
polyacrylate
glass
glass
glass
glass
glass
Fig. 2. Schematic representation of the swelling of a glass-containing
degree approaches very high values. In superabsorbents
without glass spheres (/g = 0) Q�V is equivalent to the
volume swelling degree of the polyacrylate (QV,PA).
2. Experimental
2.1. Synthesis of a glass-free superabsorbent
For comparision of the swelling behaviour of the
glass-containing superabsorbents a glass-free superab-
sorbent was synthesized. This was accomplished by per-
forming a water-in-oil suspension polymerization of
partially neutralized acrylic acid in the presence of
N,N 0-methylene bisacrylamide as a tetrafunctional
crosslinker as follows.
To a stirred solution of 0.875g sorbitan monooleate
(suspension aid) in 700ml cyclohexane (organic phase
solvent) a solution of 0.33g hydrogen peroxide (30%
by wt) in 60g distilled water was added drop by drop.
To this suspension an aqueous solution of partially neu-
tralized acrylic acid (36g acrylic acid and 15.04g sodium
hydroxide in 84.12g distilled water), which additionally
contained 0.77g N,N 0-methylene bisacrylamide and
0.05g ascorbic acid, was dripped. The combination of
ascorbic acid and hydrogen peroxide forms a redox sys-
tem which releases hydroxyl radicals to initiate the poly-
merization of the acrylic acid. The suspension was
heated to 40 �C. After 1h the water was azeotropically
distilled out of the reaction mixture using a water trap.
The received particles of crosslinked polyacylate were fil-
tered off, dried in an oven at 90 �C, and sieved after dry-
ing. The fraction containing particles sized between 250
glass
glass
glass
glass
glass
glass
water
water
water
water
water
water
superabsorbent with several glass spheres per particle in water.
1930 A. Ruttscheid, W. Borchard / European Polymer Journal 41 (2005) 1927–1933
and 500 lm were used for the additional characteriza-
tions of the sample.
2.2. Synthesis of glass-containing superabsorbents
For the synthesis of glass-containing superabsorbents
hollow glass spheres were embedded into a matrix of
crosslinked polyacrylate. The synthesis of the samples
was performed analogously to the synthesis of the
glass-free sample (see Section 2.1), except that addition-
ally varying amounts of hollow glass spheres of two dif-
ferent diameters and densities were suspended in the
organic phase before the aqueous phases were added
to the reaction vessel. The first kind of hollow glass
spheres had an average diameter of about 10 lm and a
density of 1.100g/cm3 (Aldrich). The second kind had
an average diameter of about 100lm and a density of
0.125g/cm3 (‘‘Scotchlite Microbubbles K1’’ by 3M).
With increasing glass content it was also necessary to in-
crease the amount of the suspension aid (sorbitan
monooleate) to prevent aggregation of the particles dur-
ing the reaction.
2.3. Determination of the glass contents of the glass-
containing superabsorbents
To determine the glass contents of the glass-con-
taining superabsorbents 1g of each sample was
weighed into a porcelain crucible and tempered in an
oven for two days at 800 �C. At this temperature the
glass did not loose its weight wheareas 76% by wt of
the polyacrylate decomposed. Because an aqueous
solution of the residue was alkaline, it is likely that
the residue consisted mainly of sodium oxide, which
has been formed out of the counterions (Na+) of the
polyacrylate. With taking into account the amount of
this residue the glass content of the samples could be
calculated.
2.4. Swelling measurements
The swelling measurements of the samples were per-
formed in distilled water and in aqueous sodium chlo-
ride solution (0.9% by wt) by using a swelling
apparatus developed by Selic and Borchard [9,10]. With
the help of this apparatus the volume swelling degree QV
(defined as the relation of the volume of the swollen
sample Vsw.s. to the volume of the dry sample Vdry s.)
in dependence of time could be measured:
QV ¼ V sw.s.
V dry s.ð6Þ
For the glass-containing samples (in addition to the vol-
ume swelling degrees referring to the volume of the
whole sample) volume swelling degrees referring only
to the volume of the polyacrylate fraction (VPA) of the
samples Q�V, see Eq. (4) and Eq. (5) were calculated as
follows (Vg = volume of the glass spheres):
Q�V ¼ V sw.s.
V PA
¼ V sw.s.
V dry s. � V g
ð7Þ
The values of QV (see Eq. (6)) of the glass-containing
samples were expected to be the same as the volume
swelling degrees of the glass-free sample. This seems to
be plausible if it is assumed that the swelling degree of
the polyacrylate is independent of the increasing glass
content of the samples, because the external diameters
of all samples should increase by the same factor during
swelling (see Eq. (2)). Only if the thickness of the outer
shell is extremely small the crosslinking reaction is no
longer a chemical reaction in bulk. But this can be
excluded.
The maximum volume swelling degrees QmaxV and
Q�;maxV of the glass-containing superabsorbents were
additionally converted to the appropriate maximum
mass swelling degrees (Qmaxm resp. Q�;max
m ), because these
samples (especially the samples containing the embed-
ded glass spheres of a larger diameter and a lower den-
sity) had a relatively low density in the dry state and
therefore a relatively high mass swelling degree. Besides,
if industrial use of superabsorbents is considered (such
as applications in incontinence products), the mass
swelling degree is more important than the volume swell-
ing degree of the samples.
3. Results and discussion
3.1. Swelling behaviour of glass-containing superab-
sorbents with 10lm glass spheres
Fig. 3 shows the dependence of the experimental and
the expected volume swelling degrees QmaxV and Q�;max
V of
the glass-containing superabsorbents with 10lm glass
spheres in water and in aqueous NaCl solution (0.9%
by wt) on the glass content (mass fraction, wg) of the
samples.
Up to a glass content of about 7% by wt the values of
Q�;maxV and Qmax
V correspond approximately to the ex-
pected values, whereas at glass contents between about
7% and 22% by wt the values are higher and at glass con-
tents above about 22% they are lower than the expected
values (see Fig. 3). Thus the initial increase of the max-
imum volume swelling degrees (up to a glass content of
about 7% by wt) of the glass-containing superabsorbents
can be attributed to the uptake of additional free water
in the intershells between the glass spheres and the poly-
acrylate matrix, whereas at higher glass contents addi-
tional effects dominate which can be explained as
follows.
A possible reason for the maximum volume swelling
degrees exceeding the expected values at glass contents
0 5 10 15 20 25 300
20
40
60
80
100
120
140
dotted lines represent expected values
QV
*, max in water QV
*, max in NaCl solution (0.9% by wt)
QV
max in water QV
max in NaCl solution (0.9% by wt)
QV
*, max
resp.
QV
max
wg in % by wt
Fig. 3. Dependence of the maximum volume swelling degrees (QmaxV and Q�;max
V ) of the glass-containing superabsorbents with 10 lmglass spheres in water and in aqueous NaCl solution (0.9% by wt) on the glass content (wg) of the samples (see text).
A. Ruttscheid, W. Borchard / European Polymer Journal 41 (2005) 1927–1933 1931
between 7 and 22% by wt might be that the crosslinked
polyacrylate in these glass-containing samples exhibits a
higher maximum swelling degree than the glass-free ref-
erence sample, although (besides adding glass spheres)
the same conditions during the synthesis of these sam-
ples were applied. This can be attributed to a lower de-
gree of crosslinking (e.g. as a consequence of a lower
extent of self-crosslinking) and a lower homogeneity in
the distribution of crosslinks in the network. The extent
of self-crosslinking [3] as well as the homogeneity of the
network [11] decrease, if the polymerization is per-
formed in a more diluted solution.
In the actual case a dilution of the reaction mixture
during the synthesis of the samples might be attributed
to the glass spheres suspended in the aqueous phase of
the water-in-oil suspension, because the glass spheres
cause a decrease in the probability of collisions between
the reactants. Besides, by the presence of the glass
spheres embedded into the matrix of crosslinked polyac-
rylate the network also contains inhomogeneities which
can cause an increase of the swelling degree. Moreover,
dissociation of Na+ ions at the surface of the glass
spheres before and during the polymerization process
can lead to an increase of the degree of neutralization
and therefore to an increase of the maximum swelling
degree of the crosslinked polyacrylate matrix.
The decrease of the maximum volume swelling de-
grees at higher glass contents might be attributed to
the effect of gel blocking caused by higher soluble frac-
tions (as a consequence of a lower degree of crosslinking
and a lower homogeneity of the network). Also a partial
aggregation of glass spheres at higher glass content
might be the reason.
The maximum mass swelling degrees of the glass-
containing samples with 10lm glass spheres exhibit a
similar dependence on the glass content as the maximum
volume swelling degrees. Therefore a plot of these values
has been omitted.
3.2. Swelling behaviour of glass-containing superab-
sorbents with 100lm glass spheres
Fig. 4 shows the dependence of the experimental and
the expected volume swelling degrees of the glass-con-
taining superabsorbents with 100lm glass spheres in
water and in aqueous NaCl solution (0.9% by wt) on
the glass content (wg) of the samples.
The maximum volume swelling degrees (referring to
the polyacrylate fraction of the samples, Q�;maxV ) of the
glass-containing superabsorbents with 100lm glass
spheres are significantly lower than the expected values
and exhibit a similar dependence on the glass content
as the samples with smaller glass spheres (see Section
3.1).
The Q�;maxV values of the samples containing larger
glass spheres in water initially increase with increasing
glass content up to a glass content of about 20% by
wt. This maximum value is 76% larger than the maxi-
mum volume swelling degree of the glass-free sample.
If the glass content exceeds 20% by wt, the Q�;maxV values
decrease again. In contrast the QmaxV values decrease con-
tinuously with increasing glass content (see Fig. 4).
0 5 10 15 20 25 30 35 400
20
40
60
80
100
120
140
QV
*, max in water QV
*, max in NaCl solution (0.9% by wt)
QV
max in water QV
max in NaCl solution (0.9% by wt)
dotted lines represent expected valuesQV
*, max
resp.
QV
max
wg in % by wt
Fig. 4. Dependence of the maximum volume swelling degrees (QmaxV and Q�;max
V ) of the glass-containing superabsorbents with 100 lmglass spheres in water and in aqueous NaCl solution (0.9% by wt) on the glass content (wg) of the samples (see text).
1932 A. Ruttscheid, W. Borchard / European Polymer Journal 41 (2005) 1927–1933
That the samples with larger glass spheres fail to
reach the expected maximum swelling degrees, might
be attributed to gel blocking, as well as the increase of
the QmaxV values with decreasing glass content and the de-
crease of the Q�;maxV values at higher glass content.
0 5 10 150
10
20
30
40
50
60
70
80
Qm
*, max
Qm
max
Qm
*, max
Qm
max
Qm
*, max
resp.
Qm
max
wg in
Fig. 5. Dependence of the maximum mass swelling degrees (Qmaxm an
glass spheres in water and in aqueous NaCl solution (0.9% by wt) on
The dependence of the maximum mass swelling de-
grees of the samples containing 100lm glass spheres
on the glass content is similar to the dependence of the
maximum volume swelling degrees of the samples con-
taining 10lm glass spheres on the glass content (see sec-
20 25 30 35 40
in water
in water
in NaCl solution (0.9% by wt)
in NaCl solution (0.9% by wt)
% by wt
d Q�;maxm ) of the glass-containing superabsorbents with 100 lm
the glass content (wg) of the samples (see text).
A. Ruttscheid, W. Borchard / European Polymer Journal 41 (2005) 1927–1933 1933
tion 3.1). The difference in the plots of Qmaxm (see Fig. 5)
in comparision to QmaxV (see Fig. 4) is attributed to the
significantly lower density of the embedded glass spheres
(0.125g/cm3) and the appropriate lower density of the
whole particles. Thus the Qmaxm values of the samples con-
taining larger glass spheres increase with increasing glass
content (at least up to a certain glass content), although
the QmaxV values decrease in the same region.
This is particularly relevant if industrial applications
of the superabsorbents (e.g. for use in incontinence
products) are considered, because in this case the mass
swelling degree of the particles is more important than
the volume swelling degree. By applying the presented
method of embedding hollow glass spheres into a matrix
of crosslinked polyacrylate, an improvement of the max-
imum mass swelling degree can be achieved by simply
substituting a fraction of the relatively heavy crosslinked
polyacrylate (density 1.667g/cm3 [3]) by relatively light
hollow glass spheres (density 0.125g/cm3).
4. Conclusion
Except the QmaxV values of the samples containing
100lm glass spheres, an increase of the maximum swell-
ing degrees (the values referring to the polyacrylate frac-
tion, Q�;maxV and Q�;max
m ), as well as the values referring to
the whole particles, QmaxV and Qmax
m was achieved by
embedding hollow glass spheres into a matrix of cross-
linked polyacrylate. The highest increase of the Q�;maxV
values was found for the samples containing 10lm glass
spheres. At a glass content of about 9% by wt these sam-
ples exhibited maximum volume swelling degrees being
nearly double as large as the value of the glass-free ref-
erence sample.
Although the maximum volume swelling degrees
QmaxV of the samples containing 100lm glass spheres de-
crease with increasing glass content, the respective max-
imum mass swelling degrees (Qmaxm ) increase (at least up
to a certain glass content).
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