The document describes the application of super absorbent polymer. It finds use in diapers, female sanitary napkins, agricultural application as water retainer, etc...

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Superabsorbent Polymers
chandran.udumbasseri@gmail.com
Technical Consultant
Introduction - Market
The global market for superabsorbent is approximately $8 billion in next two years that
is by 2020. Annual growth rate accounts to 7-6% in the coming years. The market is
heading for thinner personal care diapers. The market share of agricultural applications
is also growing fast. The water conservation is an very important field of application in
agriculture. The demand for food increases arithmetically with the population growth
which makes it more imperative to concentrate water conservation and utilization. Asia
Pacific region especially India and China is going to be largest market for SAP.
Introduction - SAP Applications and Chemistry
Superabsorbent polymers (SAPs) are chemicals that can absorb large quantities of
water and retain most of the fluid in their polymeric net work even when pressure is
applied. Absorbent fibers like cotton absorb large amount of water, but cannot retain
when pressure is applied.
A superabsorbent retains
Large quantities of fluid, more than 15times
It retains the fluid under pressure
Superabsorbents are used in disposable baby diapers, feminine hygiene products,
surgical appliances. It finds use in retaining water in agriculture and industrial use
Starch-acrylonitrile graft co polymer was the first developed superabsorbent for
agricultural application. Superabsorbent was used
These molecules are cross linked to achieve good absorption capacity. Usually only a
small amount of cross linking (0.1 to 1.0 mole %) is required. Molecules with two or
more polymerizable olefinic linkage and reactive functional groups can produce polymer
back bone with cross linking. Multivinyl compounds such as divinyl benzene, methylene
bis acrylamide, tri methylol propane triacrylate, diallyl amine, triallylamine, etc are cross
linkers that are added to monomer prior to polymerization to form superabsorbent
polymer.
Post polymerization cross linking agents is classified into four categories.
1. Diglycedyl ethers

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2. Halo epoxy compounds
3. Aldehydes
4. Coordinating metal cations.
Examples
1. Polyethylene glycol diglycedyl ether
2. Glyoxal
3. Epichlorohydrin
4. Glutaraldehyde
5. Formaldehyde
6. Borates
7. Metal ions (Zn, Mg, Ca, Al, and their oxides
Some cross linking is possible by using free radicals with preformed polymers. Sodium
persulfate the polymerization initiator can promote cross linking.
Mechanism
The absorption capacity and speed of absorption depends on
1. Hydrofilicity of polymer
2. Cross linking density and uniformity
3. Particle size distribution
4. Particle porosity
5. Particle surface treatment
The hydrophilic sites hold water molecules by hydrogen bonding
As the polymer uncoils more and more water molecules are absorbed
The gelled –polymer make a barrier and prevent further water absorption. This gel
blocking occurs for polymers with less cross linking.
The capillary action creates channels which allow fluid distribution throughout the
polymer body. As cross linking increases pore volume decreases. This leads to less
absorption. When pore size approaches to that of fluid molecules then further
absorption gets impeded. Microscopic capillary action can speed up absorption.
The absorbed water is three types
1. Bound water – strongly bound to the hydrophilic moieties and accounts for a
small fraction
2. Interfacial water – are weekly bound
3. Free water – accounts for 80% of absorbed water and are mobile
Increasing the hydrophilic sites increases its absorption capacity. Neutralizing a portion
of poly acrylic acid can increase its absorbing ability. Neutralization increases ionic
charge which cause repulsion in polymer backbone by which uncoiling occurs.
Electrolytes cause salt intolerance. Increasing ionic strength causes collapse of the
polymer network and dramatically reduces internal volume. Increased electrolyte

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reduces osmotic pressure driving the swelling mechanism. Presence of Ca and Mg
even reduces the absorbing ability.
Superabsorbents are available as irregular granules, spherical beads, fibers, and
dispersants in non aqueous media. Fine granules absorb fast but suffer from gel
blocking and wetting problems. Large granules reduce gel blocking but absorption is
slow
Capacity is a measure of absorption of a fluid by the polymer and expressed as g/g
units.. a certain amount of polymer is allowed to absorb the fluid until equilibrium is
reached. The time needed to reach equilibrium is the absorption speed.
For personal care applications the superabsorbent capacity is checked by 0.9% NaCl
solution, synthetic urine (synthetic urine contains Na, K, Ca, Mg and PO4 ions) and not
plane water.
Gel strength is the stiffness and elasticity of swollen gel. Gel with low gel strength
easily deforms and absorbed fluid loses.
Speed of absorption is the rate at which a certain amount of fluid is absorbed or the
time required to reach its capacity. For personal care applications the fluid should be
absorbed fast.
Gel blocking occurs when surface of particles absorb liquid and swell tp prevent
passage of the liquid into the centre of the particle. Gel formed at the surface of the
particle blocks the entry of further fluid. Gel blocking occurs most often with very small
sized particles.
Blocking can be avoided by controlling the particle size distribution. Fines and small
particles can be eliminated. Particle size distribution to obtain optimum performance of
the superabsorbent must be determined for each application. For personal care
applications fines and avoided and particle size below 90 microns (170 mesh) are
eliminated. Very large particles (over 840 microns) absorb fluid slowly. For diapers the
particle size should between 90 and 840 microns.
Process
Acrylic acid is partially neutralized with caustic soda solution. This is suspended in oil
phase of low boiling hydrocarbon like heptane or cyclohexane and a surfactant. The
polymerization is initiated with ammonium persulfate at 45-60o
C. The reaction mass is
allowed to boil at the boiling point of the solvent. The polymer mass is partially dried by
removing water by azeotropic distillation until the polymer content is 70-80%. The
solvent is removed by centrifuging. The dried polymer is then powdered.

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Molecular weights
1. Acrylic acid, 72.06
2. NaOH, 40
Batch Quantity
Reactor Capacity = 3000 liters
AA = 255 Kgs
NaOH (50%) = 184 Kgs
H2O = 213 Kgs
-------------------------------
Total 652 Kgs
====================
Total water = 213 + 92 = 305 Kgs
652 - 305 = 347Kgs
Solid content = 347 Kgs
Duration of processing
1. Monomer preparation and charging = 1hr
2. Adding initiator and reaction temperature= 1hr
3. Reaction time= 30 min
4. Temperature maintenance = 1 ½ hrs
5. Water removal = 2hrs
6. Unloading the slurry ½ hr
7. Centrifuging and crushing and packing 1 hr
1+ 1 + ½ +1 ½ +2 + ½ +1 = 7 ½ hrs
Daily batches: 3;
cycle time: 8hrs
Reactor: 3000 Liters
Output per day = 347x3 = 1041 Kgs
Yearly output = 1041x300 =312300 Kgs = 312 MT/year

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