The document summarizes research on the influence of the surfactant sorbitan monolaurate on the suspension polymerization of polyacrylic acid superabsorbent polymers. The study found that a weight ratio of acrylic acid to sorbitan monolaurate of 20/2 produced polymers with the highest swelling properties. Analysis using FT-IR showed the presence of carboxyl groups, and SEM images revealed that samples with a 20/3 ratio had a more porous and compact morphology compared to other ratios. The optimum amount of surfactant was found to produce smaller, more uniform polymer particles and therefore better water absorption capabilities.

1. Influence of Surfactant Sorbitan Monolaurate to Suspension Polymerization of Polyacrylic Acid Superabsorbent (Dewi Sondari)
Akreditasi LIPI Nomor : 536/D/2007
Tanggal 26 Juni 2007
INFLUENCE OF SURFACTANT SORBITAN
MONOLAURATE TO SUSPENSION POLYMERIZATION OF
POLYACRYLIC ACID SUPERABSORBENT
Dewi Sondari, Nuri Astrini, Lik Anah and Agus Haryono
Research Center for Chemistry (P2K) - LIPI
Kawasan Puspiptek Serpong 15314, Tangerang
e-mail : sondaridewi@yahoo.com
ABSTRACT
INFLUENCE OF SURFACTANT SORBITAN MONOLAURATE TO SUSPENSION
POLYMERIZATION OF POLYACRYLIC ACID SUPERABSORBENT. A superabsorbent polymer has
been prepared by suspension polymerization reaction using acrylic acid, sodium hydroxide, cyclohexane,
potassium persulfate as initiator and sorbitan monolaurate as surfactant. The effect of different weight ratio of
acrylic acid/sorbitan monolaurate 20/1, 20/2 and 20/3 were investigated based on its swelling properties,
chemical structure by using Fourier Transform-Infra Red (FTIR) Spectroscopy and the surface morphology
by Scanning Electron Microscopy (SEM). The molecular weight of polyacrylic acid superabsorbent was
determined by intrinsic viscosity measurements in 2 M aqueous sodium hydroxide at 25 oC. The study showed
sample with weight ratio of acrylic acid/sorbitan monolaurate = 20/2 have best swelling properties.
Key words : Superabsorbent polymer, Suspension polimerization, SEM, FT-IR, Surfactant
ABSTRAK
PENGARUH SURFAKTAN SORBITAN MONOLAURAT TERHADAP POLIMERISASI
SUSPENSI SUPERABSORBEN ASAM POLIAKRILAT. Polimer superabsorben telah dibuat secara reaksi
polimerisasi suspensi menggunakan asam akrilat, sodium hidroksida, sikloheksan, potassium persulfat sebagai
inisiator, dan sorbitan monolaurat sebagai surfaktan. Pengaruh dari berbagai rasio berat asam akrilat/sorbitan
monolaurate = 20/1, 20/2 and 20/3 telah dipelajari berdasarkan sifat mengembang (swelling properties),
struktur kimia yang dianalisis menggunakan spektroskopi Fourier Transform-Infra Red (FT-IR) dan morfologi
permukaan yang diamati menggunakan Scanning Electron Microscope (SEM). Berat molekul superabsorben
poliasam akrilat dihitung dengan cara mengukur kekentalan intrinsik dalam larutan natrium hidroksida 2M
pada suhu 25 oC. Hasil penelitian menunjukkan sampel dengan perbandingan berat asam akrilat/sorbitan
monolaurat = 20/2 mempunyai nilai sifat mengembang yang terbesar.
Kata kunci : Polimer superabsorben, Polimerisasi suspensi, SEM, FT-IR, Surfaktan
INTRODUCTION
Superabsorbent polymers can be described as For superabsorbent polymers used as medical
polymer having a network structure and a moderate and sanitation materials, they must have high absorption
degree of cross-linking. They can absorb a large amount rates and low residual levels of toxic chemicals. Usually,
of water, considerably more than their dry mass. The the porous superabsorbent polymers are prepared by
amount of the water absorbend can be in a range of adding chemical reagents to produce bubbles in the
hundreds to thousands time to that of the polymer. These polymer [1].
polymers have been extensively used as absorbents in Recent developments were also suggest the
personal care products, such as infant diapers, feminine production of super-absorbent polymers from acrylic
hygiene products, and incontinence product. They have acid and a cross-linker by solution or suspension
also received considerable attention for a variety of more polymerization. The type and quantity of cross-linker
specialized applications, including matrices for enzyme control both the swelling capacity and gel modulus. The
immobilization, bioabsorbents in preparative synthesis and the use of cross-linker polyacrylate super-
chromatography, materials for agricultural mulches, and absorbents have been a popular topic in the polymer
matrices for controlled release devices [1]. technology research and development [2].
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2. Jurnal Sains Materi Indonesia Edisi Khusus Desember 2008, hal : 21 - 24
Indonesian Journal of Materials Science ISSN : 1411-1098
In general, the functions of the surfactant used samples have been averaged over 3 independent
in the polymerization, are that the polymerization measurements.
mixture forms a stable emulsion, the surfactant does The water uptake is expressed in terms of
not retard the polymerization reaction, and uniform swelling ratio (Q, g/g) was calculated by:
polymer particles are formed without coagulation nor
separation [3]. Q = (Wt - Wo - Wn) / Wo
In this study, we investigated influence of
surfactant sorbitan monolaurate to polyacrylic acid Where :
superabsorbent properties into suspension W n = Weight of nylon cloth
polymerization. Wo = Weight of dry hydrogel
W t = Weight of swollen hydrogel
EXPERIMENTAL METHODS
FT-IR Spectrum and SEM Analysis
Materials Infrared absorption spectra of polyacrylic
Acrylic acid purity 99,8% was purchased from superabsorbent was studied by FT-IR using IR
Aldrich (Singapore), sodium hydroxide, cyclohexane Prestige-21 Shimadzu (Shimadzu Co., Japan). The
were reagent grade from E-Merck, Potassium persulfate polymers were mixed with KBr and pressed to a plate for
pure grade was purchased from E-Merck, and Sorbitan measurement. All of the spectra were measured 16 scans
Monolaurate was purchased from Aldrich. at a resolution of 4 cm-1. A background spectrum
containing no sample was substracted from all spectra.
The surface morphology was observed using scanning
Methods
electron microscopy (SEM).
Synthesis of Superabsorbent Polymers Polyacrylic
Acid Measurement of Molecular Weight by
Intrinsic Viscosity
In 200 mL flask, acrylic acid having a purity of
99.8% by weight was placed. While cooling it, 76.5 g of For the determination of viscosity-average
22.6% by weight aqueous sodium hydroxide solution molecular weight (Dalton), the polyacrylic acid was
was dropped thereinto with stirring to neutralize dissolved in 2 M aqueous sodium hydroxide. Five
the acrylic acid. Then, potassium persulfate was added different dilute solutions were used to do the
and stirring was continued at room temperature to experiments on measurement of intrinsic viscosity (η).
dissolve it. The Mark-Houwink equation relating to intrinsic
213 g of cyclohexane and sorbitan monolaurate viscosity with empirical viscometer constant
were charged into a 500 mL flask equipped with a reflux K = 0.0422 (mL/g) and a = 0.64 for polyacrylic acid was
condenser, and the surfactant was dissolved at room used to calculate the molecular weight using this
temperature with stirring. The aqueous solution equation [4].
of partially neutralized acrylic acid salt was dropped to
be suspended therein. Polymerization reaction was RESULTS AND DISCUSSION
carried out for 3 hours while keeping the bath temperature
at 70 oC [3]. Swelling Studies
With the aim of investigating the effect of
Swelling Analysis surfactant on the material’s sorption properties an
equilibrium swelling measurements in distilled water were
The water absorption capability of hydro-gel performed on hydrogel samples, obtained from three
were measured by tea bag method using nylon cloth [5].
The hydrogel (approximately 500 mg ) is weighed 250
and poured into the bag. The open side of the bag 225
200
is closed. The bag was then immersed into the 175
Swelling ratio
20/1 ratio
distilled water to swell. After a suitable period of time, 150
20/2 ratio
125
the bag was taken out from the water. The bag was 100
20/3 ratio
20/2>20/3=20/1
then hanged for 15 minutes to let the excess water to 75
50
dripp off and then weighed to determine the amount of
25
distilled water absorbed by the hydro-gel. Equilibrium 0
swelling measurements in distilled water were carried 0 20 40 60 80 100 120
Tim e (m inute)
out for all samples using an electronic microbalance
(AdamAAA250L) with an accuracy of ±10-4 g. Reported Figure 1. Swelling ratio in distilled water vs time of
values of equilibrium water uptakes for each different various weight ratio of acrylic acid/sorbitan monolaurate
22

3. Influence of Surfactant Sorbitan Monolaurate to Suspension Polymerization of Polyacrylic Acid Superabsorbent (Dewi Sondari)
different weight ratio of acrylic acid/sorbitan (20/1, 20/2
and 20/3). The resulted swelling ratios are shown in
Figure 1.
Figure 1 shows that the water uptake capability
of hydrogel was influenced by sorbitan monolaurate
content. Higher swelling capacity will be produced when
weight ratio of acrylic acid/sorbitan monolaurate = 20/2
was put into reaction. The influence of sorbitan
monolaurate as the surfactant forms a stable suspension.
A stable polymer will be obtained when the amount of
surfactant, which is added, is optimum, in which the Figure 3. Scanning Electron Micrograph of the hydrogel
polymer product will be less harder and water swelling surface with weight ratio of acrylic acid/sorbitan
monolaurate 20/1
capacity will be higher. In the other hand, the increase of
surfactant amount, the formed polymer during the
polymerization becomes a block and has such a low
water absorption.
Optimizing the amount of added surfactant to
polymerization mixture, result in a more stable
suspension. This formed polymer particle sizes are small
and uniform, so that, increases the water absorption
capability. When the particle sizes were smaller, the
surface area is better, and then the better surface area
will be have better water absorption.
Figure 4. Scanning Electron Micrograph of the hydrogel
surface with weight ratio of acrylic acid/sorbitan
FT-IR and SEM Analysis monolaurate 20/2
The infrared spectroscopy was used to determine
the type of bonds present in the hydrogel. The infrared
spectra in Figure 2 indicate that the hydrogel is containing
carboxyl group (-COOH). Spectra peak at 1734 cm-1 and
800 cm-1 arised from symmetric stretching vibration of
C=O, and –C-C respectively.
Another new peak is at 2852 cm-1 and 2924 cm-1
arised from stretching vibration of –C-H. The
polymerization between acrlylic acid using potassium
persulfate as initiator was showed by disappearance
of spectrum peak at 3100-3000 cm-1 from double bond Figure 5. Scanning Electron Micrograph of the hydrogel
of -HC=CH-. surface with weight ratio of acrylic acid/sorbitan
monolaurate 20/3
sample 4
sample 5
SML 2x
%T
Samples with different surfactant amounts
were collected, and their surface morphologies
were investigated by SEM. A series of SEM
micrographs of polyacrylic acid hydrogel with various
surfactant are shown in Figure 3, 4, and 5.
A From SEM analysis, it can be shown
that morphology of the hydrogel surface
B micrograph (weight ratio acrylic acid/sorbitan
monolaurate 20/1 and 20/2) were less porous and
compact compared with SEM image of 20/3 sample
C
in Figure 5. It is obviously illustrated in Figure 5
that 20/3 sample has more porous and compact
morphology.
4000
SML 2x
3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600
1/cm
The surfactant of sorbitan monolaurate
Wave number (cm-1 )
influenced to the formed polymer particle size. An
Figure 2. Infrared spectra of the polyacrylic acid
optimum surfactant amount will provide a more porous
superabsorbent with various weight ratio acrylic acid/
sorbitan monolaurate. (A) 20/1; (B) 20/2 and (C) 20/3 and compact of polymer structure.
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4. Jurnal Sains Materi Indonesia Edisi Khusus Desember 2008, hal : 21 - 24
Indonesian Journal of Materials Science ISSN : 1411-1098
Molecular Weight Determination by Intrinsic [5]. Y. ZHAO, H. SU, L. FANG and TAN, Polymer, 46
Viscosity (2005) 5368-5376
The molecular weight of polyacrylic acid
superabsorbent polymer were reported in Table 1.
Table 1. Molecular weight of the polyacrylic acid superabsorbent
polymer
Weight ratio acrylic acid/ Molecular
No
sorbitan monolaurate weight
1 20/1 135,397
2 20/2 191,594
3 20/3 217,274
Molecular weight depends on polymer chain,
longer polymer chain will be produced when less initiator
was put into reaction and depend on the amount of
monomer. The functions of the sorbitan monolaurate as
surfactant used in the polymerization, are that the
polymerization mixture forms a stable suspension, the
surfactant does not retard the polymerization reaction,
and uniform polymer particles are formed without
coagulation nor separation.
CONCLUSIONS
Superabsorbent polymer has been prepared by
suspension polymerization reaction using acrylic acid
and sorbitan monolaurate as surfactan. Sample with
weight ratio of acrylic acid/sorbitan monolaurate = 20/2
have best swelling properties.
The morphology of polymer made of acrylic acid/
sorbitan monolaurate with weight ratio of 20/3 has porous
and compact structure more than others.
ACKNOWLEDGEMENTS
The authors are thankful for the financial
supported by DIPA thematic from Indonesian Institute
of Sciences (LIPI)
REFERENCES
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of macroporous cellulose-based superabsorbent
polymer through the precipitation method,
BioResources, 3 (1) (2008) 247-254
[2]. Absorbent Polymers: AMCOL International, http:/
/www.amcol.com/fctbook/absor.htm, (1996)
[3]. United States Patent. 4,340,706. July. 20, 1982.
Alkali metal acrylate or ammonium acrylate polymer
axcellent in salt solution-absorbency and process
for producing same, (1982)
[4]. BRANDRUP, J., IMMERGUT, E.H., and GRULKE,
E.A., Polymer Handbook, fourth edition, A Wiley-
Interscience Publication, Canada.
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