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Journal of Basic & Applied Sciences, 2016, 12, 383-387 383
ISSN: 1814-8085 / E-ISSN: 1927-5129/16 © 2016 Lifescience Global
Synthesis of Superabsorbent Polymer (SAP) via Industrially
Preferred Route
R. Ahmed* and K. Ali Syed
Polymer & Petrochemical Engineering Department, NED University of Engineering & Technology, Karachi,
Pakistan
Abstract: The assigned study is dedicated to the synthesis, improvement and characterization of acrylic-based
superabsorbent polymers (SAPs) which can be used in versatile applications notably in disposabl e diapers and
pharmace utics. The indus trially preferred sol ution polymerization route and low cost monomers were used to synthesize
SAPs. Homopolymer and copolymer based SAPs were prepared with varying amount of cross-linker and initiator
concentrations and compared for swelling rate with a commercially available SAP sample. Swelling capacity linearly
decreases with increase in cross-linker content for both the synthesiz ed SAPs samples whereas it first incre ases and
then decreases with initiator content for the synthesized copolymer SAP. Swelling kinetics of the synthesiz ed and
commercial SAPs were modelled using model equation proposed by Omidian et al. Both the synthesized SAPs showed
substantial increase in swelling capacity whereas copolymer SAP exhibited the highest swelling rate (rate parameter
2.78 min) when compared to homopoly mer SAP and t he commercially available SAP samples. Accordingly, the
copolymer SAP may find its application in disposable diapers or pharmaceutics where the higher swelling rate is of prime
importance. Copolymer and commercially available SAPs depicted significant decrease in swelling capacity even at very
low saline solution conc entrat ion (0.01 %).
Keywords: Copolymer, SAP, swelling capacity, swelling rate, solution polymerization.
INTRODUCTION
Hydrophilic polymers having capability of absorbing
and retaining enormous amount of water, saline and
physiological solutions are known as superabsorbent
polymers (SAPs). SAPs are sometimes mentioned as
“xerogels” or “xerogellents” owing to their dryness and
gelling nature [1,2]. SAPs were developed in mid 1970s
and find applications in personal care, hygienic
products, agriculture, controlled delivery of drugs,
artificial snow for skiing areas and many others [1-4].
Monomer concentration, initiator concentration, cross-
linker concentration, polymerization temperature and
particle porosity are some of the major factors that
affect the final properties of the SAPs [5]. On the other
hand, high swelling capacity, high swelling rate and
good swollen gel strength are some of the desired
properties of a good superabsorbent [6]. To prevent
infinite swelling of polymer chains in water three
dimensional structures or networks commonly known
as cross-links are employed between polymer chains of
a SAP. As polymer (SAP) is immersed in water,
random coils of polymer become more aligned on
swelling which reduces the entropy of the chains.
There is a direct relationship between cross-link density
and polymer characteristics, as increase in cross-link
density decrease swelling capacity and increase gel
strength of the polymer [7-9].
*Address corr espondence to this author at the Polymer & Petrochemical
Engineerin g Department, NED U niversity of Engineer ing & Technology ,
Karachi, Pakistan; Tel: (92-21) 99261261-8; Fax: (92-21) 9926125 5;
E-mail: ahmedr@nedue t.edu.pk
SAPs can be synthesized using either solution [10]
or inverse suspension/emulsion polymerization [11, 12]
techniques. Both of the polymerization techniques have
their own advantages and disadvantages. However the
solution polymerization is industrially preferred in
consequence of its economical process [13, 14].
It is well documented that superabsorbent polymers
are ordinarily polyelectrolytes which contain carboxylic
groups and cross-link sites. Dissociation of carboxylic
groups results in the extension of polymer coils
because of osmotic pressure and electrostatic forces
[1,2]. Many kinds of raw materials, synthesis methods
and properties of SAPs have been reviewed by many
researchers [15-18]. However, acrylic acid and
acrylamide are industrially preferred due to their high
water absorbency and low cost [13, 14].
Majority of the reported research work is devoted to
the high swelling capacity whereas the swelling rate is
of prime importance for certain applications such as
disposable diapers which alone accounts for 80-85% of
the market share [19] and pharmaceutics [20]. On the
other hand, currently in Pakistan no industry is involved
in the production of SAPs.
The main purpose of this study is twofold: first, to
explore SAP synthesis using low cost acrylic-based
monomers and second to compare the swelling rate
(water absorption rate) of the synthesized SAPs with a
commercially available imported SAP sample.
384 Journal of Basic & Applied Sciences, 2016, Volume 12 Ahmed and Ali Syed
EXPERIMENT
Materials
Sample Preparation
All the solutions were prepared in distilled water.
Two different SAP samples namely homopolymer SAP
(Poly-SAP) and copolymer SAP (copoly SAP) were
synthesized and compared with a commercially
available imported SAP sample.
The commercial SAP sample was purchased from
the local market in the form of disposable baby diaper.
The SAP resins were separated from the diaper
wrapping materials. The separated SAP resins were
used for comparison purpose in this study and named
Commercial-SAP hereafter.
Synthesis of Homopolymer SAP (Poly-SAP)
In a typical recipe of homopolymer SAP, acrylic acid
solution (50 % V/V) was neutralized up to 75 % using
0.15 M NaOH solution. The resultant exothermic
mixture was left for about 2 to 3 hours at room
temperature. Cross-linker solution ranging from 0.75 %
to 1.75 % (W/V) was added to the above mixture in a
separate beaker. The beaker was gradually heated on
a silicon oil bath under continuous stirring using
magnetic bead. The initiator solution of constant
concentration (3.75 % (W/V) was added to the mixture
when the final temperature of 55 °C was reached. The
beaker was left for reaction until the gelation was
reached. The gelation was practically observed when
the stirring magnetic bead stopped working.
Synthesis of Copolymer SAP (copolySAP)
In a typical recipe of copolymer SAP, acrylic acid
solution (50 % V/V) was neutralized up to 75 % using
0.15 M NaOH solution. The resultant exothermic
mixture was left for about 2 to 3 hours at room
temperature. In a separate beaker cross-linker solution
ranging from 0.75 % to 1.75 % (W/V) and the
acrylamide solution (0.115 % W/V) of constant
concentration were mixed under continuous stirring
using magnetic bead. The mixture of the two beakers
were mixed in a large beaker and left for approx. 30
min under continuous stirring to homogenize the
content of the beaker. The beaker was gradually
heated on a silicon oil bath. The initiator solution in the
range of 1.88 % to 5.75 % (W/V) was added to the
mixture when the final temperature of 55 °C was
reached. The beaker was left for reaction until the
gelation was reached. The gelation was practically
observed when the stirring magnetic bead stopped
working.
RESULTS AND DISCUSSIONS
Effect of Cross-Linker Concentration on Swelling
Capacity
Figure 1 shows the effect of cross-linker
concentration on the swelling capacity of Poly-SAP and
copolySAP samples. It is apparent from the figure that
Poly-SAP shows higher swelling capacity, for all cross-
linker concentrations, than copolySAP. The higher
swelling capacity of the Poly-SAP sample is attributed
to the high absorption capacity of acrylic acid due to
the repulsion of negatively charged carboxylic groups
present in the main polymer chains [1]. The lower
values of the swelling capacity of copolySAP might be
attributed to the increase number of cross-links per
polymer chain which relatively results in a strong
molecular structure. Consequently, when the cross-
linking density increases, the swelling capacity will
decrease [7-9, 21-23]. However it is interesting to note
that the swelling capacity decreases faster in case of
Poly-SAP samples, see regression lines slopes in the
Figure 1.
Effect of Initiator Concentration on Swelling
Capacity
It is evident from the Figure 2 that the swelling
capacity of copolySAP first increases with the initiator
Table 1:
Raw Material*
Type
Manufacturer
Acrylic acid
Monomer
Lab grade (Shafi Reso Chemicals Pvt. Ltd. – Pakistan)
Acrylamide
Monomer
Commercial grade
Methylene-bis-acryl amide
Cross-linker
Merck
Potassium peroxide
Initiator
Commercial grade
Sodium hydroxide
Neutralizer
Analytical grade (Merck)
*All the raw materials were used as received.
Synthesis of Superabsorb ent Polymer (SA P) via Industrially Pref erred Route Jou rnal of Basic & Applied Sciences, 201 6, Volume 12 385
concentration and then decreases. Similar Gaussian
distribution behavior is also reported by other studies
[24-26]. The initial increase in the swelling capacity
might be attributed to the optimum amount of radicals
to produce high molecular weight SAP with sufficient
density of hydrophilic groups (-COOH, -CONH, -COO-)
and gel strength [25]. The decrease in the swelling
capacity after optimum swelling capacity (228.45 (g/g))
could be due to the synthesis of low molecular weight
SAP [27] which caused low gel strength [18,22,23] and
hence, fast dissolution.
Figure 2: Effect of the initiator content on the swelling
capacity of the copolymer SAP (copolySAP) at 30 °C and
constant cross-linker content of 10 mg. The line is the
Gaussian fit to the data.
Swelling Kinetics of the Synthesized and
Commercial SAPs
The rate of distilled water (pH = 7) absorption
(swelling rate), measured on the same range of particle
size, is compared for the synthesized and commercial
SAP samples in Figure 3. All the samples show faster
absorption in the initial stage and levels of at the later
stages. The highest initial absorption rate is observed
for the copolymer sample which might be due to the
presence of amide groups [28]. Kabiri et al. also repor-
ted similar results for their copoly mer samples [29].
Figure 3: Swelling kinetics (rate of distilled water absorption,
pH = 7) of the synthesized (homopolymer SAP (Poly-SAP)
and copolymer SAP (copolySAP)) and Commercial SAP
(Commercial-SAP) at 30 °C. Contents of the initiator and the
cross-linker are 150 mg and 10 mg, respectively, for both the
synthesized SAPs. The lines show the model fitting (see
equation 1).
The swelling kinetics of the synthesized and
commercially available SAPs can be modelled via
following equation [30]:
SC(t)= SCe [1- exp(-t/R)] (1)
Where SC(t) is the instantaneous swelling capacity
(g/g), SCe (equilibrium swelling capacity) is the swelling
capacity (g/g) at infinite time, t is the swelling time
(min), and R (the rate parameter) is the time (min)
required to reach 63 % of the equilibrium swelling
capacity.
A typical curve is shown in Figure 4 to determine
the model parameters.
The model parameters were determined by non-
linear curve fitting using OriginPro 9.1® and are shown
in Figure 5. It is evident that the lowest rate parameter
is obtained for the copolySAP and the highest swelling
capacity for Poly-SAP. The lower value of the rate
parameter [31] suggests the higher swelling rate of the
copolySAP. Accordingly, the copolySAP may find its
application in disposable diapers or pharmaceutics
where the higher swelling rate is of prime importance.
Figure 1: Effect of the cross-linker content on the swelling
capacity of homopolymer SAP (Poly-SAP) and copolymer
SAP (copolySAP) at 30 °C and constant initiator content of
150 mg.
386 Journal of Basic & Applied Sciences, 2016, Volume 12 Ahmed and Ali Syed
Figure 4: A representative curve of the copolymer SAP
(copolySAP) for the determination of model (equation 1)
parameters.
Figure 5: Rate parameter and equilibrium swelling capacity
versus type of SAP. The lines are drawn to guide the eye of
the reader.
Effect of Ionic Strength on Swelling Capacity
The effect of saline solution concentration on the
swelling capacity is compared for the copolySAP and
Commercial-SAP samples in Figure 6. The swelling
capacity of copolySAP and Commercial-SAP in distilled
water is about 3 times and 2.4 times more than 0.01 %
saline solution, respectively. The decrease in the
swelling capacity might be due to the increase of
sodium ions outside the SAP gel which leads to
lowering in osmotic pressure. The observations made
in this research are in accordance with the findings of
the other researchers [32-34].
CONCLUSIONS
An attempt was made to synthesize acrylic-based
superabsorbent polymers, homopolymer SAP and
copolymer SAP, using low cost monomers (acrylic acid
and acrylamide) and industrially preferred solution
polymerization route. The aim was to produce SAPs
with high swelling rate (water absorption rate) and high
swelling capacity especially for disposable diapers and
pharmaceutics applications. The findings of this study
are:
• Higher cross-linker concentration leads to lower
swelling capacity for both the synthesized
samples.
• Swelling capacity of the copolymer SAP first
increases and then decreases with the initiator
content.
• The swelling kinetics was successfully modeled
by Omidian et al. proposed model.
• Both the synthesized SAP samples showed
higher swelling capacity when compared to
commercially available imported SAP sample.
• The highest swelling rate (water absorption rate)
was obtained for copolymer SAP when
compared to homopolymer and commercially
available SAPs samples.
• Substantial decrease in swelling capacity in
saline solution, even at very low concentration
(0.01 %), was observed for copolymer SAP and
commercially available SAP samples.
The findings of this study suggest that the
synthesized copolymer SAP will find its applications in
Figure 6: Effect of the saline solution concentration on the
swelling capacity of copolymer SAP (copolySAP) and
commercially available imported SAP (Commercial-SAP) at
30 °C. Contents of the initiator and the cross-linker are 150
mg and 10 mg, respectively, for the copolySAP.
Synthesis of Superabsorb ent Polymer (SA P) via Industrially Pref erred Route Jou rnal of Basic & Applied Sciences, 201 6, Volume 12 387
disposable diapers or pharmaceutics where higher
swelling rate is the foremost prerequisite of the final
products.
ACKNOWLEDGEMENTS
The authors are very thankful to Dr. S.I. Ali for his
valuable discussions throughout the research.
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Received on 12-05-2016 Accepted on 15-06-2016 Published on 27-09-2016
http://dx.doi.or g/10.6000/1927-5129.2016.12.59
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