Super absorbant polymers

Assignment # 01 (Super Absorbent Polymers)
1 DepartmentOf PolymerEngineering| National Textile University,Faisalabad.
National Textile University, Faisalabad.
Faculty of Engineering and Technology,
Department of Polymer Engineering.
Title:-
Course Name: Engineering Polymers
Course Code: PE-310.10
Class: PE-6
Name: Adil Naeem
Registration #: 13-NTU-1002
Submission Date: 10-03-2016
Submitted to:-

Introduction:-
Superabsorbent polymers are very high molecular mass, cross-linked polyelectrolytes that can absorb
more than 10 times their mass of water or aqueous solutions. The commercially available
superabsorbent polymers are alkali metal salts of poly(acrylic acid) cross-linked with
multifunctional monomers. The polymers are usually made by means of free-radical-initiated
polymerization of an aqueous solution of the monomers, followed by drying the resulting hydrogel,
and grinding the dry polymers to a granular powder. The principal use of superabsorbent polymers is
as a liquid absorbent in baby diapers, feminine hygiene products, and adult incontinence
products. Smaller volumes are used in food packaging and in water-blocking tapes for electrical
and telecommunication cables. Total global usage of superabsorbent polymers in 2001 amounted to
about 1093 kilo tons.
History of Superabsorbents:-
Superabsorbent polymers were first introduced by Union Carbide in the mid 60's.
They were first developed in the 70's to grow plants in the desert. However, they were used very
little by commercial growers due to their relatively high cost in comparison to their swell capacity.
In the early 80's, super absorbents began to be widely developed for the baby diaper market (hundred
of millions of pounds). This led to newer, higher swell polymers some with long lasting life that
were suitable for agriculture. Polymers like Water-Keep have been recommended by various
Cooperative Extension Services as a beneficial soil amendment. The fine powders have also been
used by nurseries for bare root dipping. Now, a growing use has been seen in nursery container mix,
landscaping (especially golf courses), vegetable and flower gardens and orchards.
Super absorbents like Water-Keep are compatible with any type of living plant, annuals, perennials,
ornamental trees and shrubs, bulbs, vegetable and turf. 1 million pounds will be used in 2003 for
these applications. Water-keep can fulfill the needs of farmers, nursery growers and golf course
landscaping professionals who require a soil amendment which would outperform conventional
potting soils and organic amendments relative to their water holding, releasing and water saving
capacity.

SAPs Types and Classification:-
Classification:-
Resembling the hydrogel family, the SAPs can also be classified based upon different aspects. SAPs
may be categorized to four groups on the basis of presence or absence of electrical charge located in
the cross-linked chains:
1- Non-ionic
2- Ionic (including anionic and cationic)
3- Amphoteric electrolyte (Ampholytic) containing both acidic and basic groups
4- Zwitterionic (polybetaines) containing both anionic and cationic groups in each structural
repeating unit.
Types:-
Synthetic SAPs:-
The greatest volume of SAPs comprises full synthetic or of petrochemical origin.
They are produced from the acrylic monomers, most frequently acrylic acid (AA), its salts and
acrylamide (AM).
(a) Cross-linking polymerization by a polyvinylic cross-linker.
(b) Cross-linking of a water-soluble prepolymer by a polyfunctional cross-linker. R is often CH2 or
another aliphatic group.

Polysaccharide-basedSAPs:-
Although the majority of the super absorbents are nowadays
manufactured from synthetic polymers (essentially acrylics) due to their superior price-to efficiency
balance. The world’s firm decision for environmental protection potentially supports the ideas of
partially/totally replacing the synthetics by "greener" alternatives. Carbohydrate polymers
(polysaccharides) are the cheapest and most abundant, available, and renewable organic materials.
Chitin, cellulose, starch, and natural gums (such as xanthan, guar and alginates) are some of the most
important polysaccharides. Generally, the reported reactions for preparing the polysaccharide-based
SAPs are held in two main groups;
(a) Graft copolymerization of suitable vinyl monomer on polysaccharide in the presence of a cross-
linker.
(b) Direct cross-linking of polysaccharide. In graft copolymerization, generally a polysaccharide
enters reaction with initiator by either of two separate ways. First, the neighboring OHs on the
saccharide units and the initiator (commonly Ce4+) interact to form redox pair-based complexes.
These complexes are subsequently dissociated to produce carbon radicals on the polysaccharide
substrate via homogeneous cleavage of the saccharide C-C bonds. These free radicals initiate the
graft polymerization of the vinyl monomers and cross-linker on the substrate. In the second way of
initiation, an initiator such as per sulphate may abstract hydrogen radicals from the OHs of the
polysaccharide to produce the initiating radicals on the polysaccharide backbone. Due to employing
a thermal initiator, this reaction is more affected by temperature compared to previous method.

Poly(amino acid)-basedSAPs:-
Dissimilar to polysaccharide-based hydrogels, relatively
fewer works have been reported on the natural based SAP hydrogels comprising polypeptides as the
main or part of their structure. Proteins from soybean, fish, and collagen-based proteins are the most
frequently used hetero-polypeptides for preparation of proteinaceous super-swelling hydrogels.
Mechanisms ofswelling in superabsorbentpolymers:-
Before discussing the synthesis of superabsorbent polymers it is
first necessary to understand the reasons why they swell. There are several mechanisms to the
process of swelling, all of which contribute to the final swelling capacity (or centrifuge retention
capacity CRC – which is the amount of 0.9 wt% saline solution that a SAP can retain under free
swelling conditions when surface water has been removed in a centrifuge).
The polymer backbone in SAP is hydrophilic i.e. ‘water loving’ because it contains water loving
carboxylic acid groups (–COOH). When water is added to SAP there is a polymer/solvent
interaction; hydration and the formation of hydrogen bonds are two of these interactions.

Waterabsorbencyof some common absorbent materials:-

How can diapers absorbto much water?
Super absorbent polymers (SAPs) are large chain-like molecules that
can absorb water up to thousands of time their own weight. Their ability to absorb water is strongly
depends on the particle size of the polymer. This is because surface area depends upon particle size.
Because of their ability to absorb the large amount of water SAPs such as poly acrylate are widely
used in diapers.

Two Examples of SAPs Used In Diapers:-
How Do SAPs Retainand Absorb Water:-

Wateris absorbedinto polymer due to:-
 Cross-linked, which prevents the polymer strands to dissolve into water and
provide “pores” for water.
 Hydrogen bond which form between the polymer and water.
Reasonforthat :-

Diaper’s principle:-
Cross-linking in superabsorbent polymers:-
There are two main types of cross-linking in most superabsorbent polymers.
• Bulk or core cross-linking – Which normally takes place during the polymerisation stage of
superabsorbent production.
• Surface cross-linking – Which is a newer process that improves the absorption against pressure
profile of the polymer.
Core cross-linking:-
Cross-linking is the joining of molecules – generally joining two or more
macromolecules with a smaller molecule. The most important type in the case of super absorbents,
and the most common, is the covalent cross-link. In SAP manufacture the most common types of
cross-linker are organic molecule that contain two or more polymerisable double bonds. These
molecules are incorporated into the backbone of the polymer chains as they grow during the
polymerisation reaction.

Some common types of cross-linker:-
Surface cross-linking of superabsorbent polymers :-
Historically it is known that SAPs with no surface treatment
and low internal cross-linking tend to show high swelling capacities but poor absorption against
pressure. Improving the swelling capacity of SAP by decreasing the core cross-linking, i.e.
decreasing the cross-linking density, is limited by the accompanying increase in extractable polymer
content of the gel. Likewise the increase in capacity would lead to a decrease in the uptake of liquid
when the SAP was under load, for example a baby sitting on its diaper. This means that when a
pressure is applied on the SAP, liquid is not absorbed effectively in these areas. Moreover the
pressure on the semi-swollen gel can cause a blockage in the area preventing further liquid entering
the gel bed and being absorbed by dry SAP underneath. This phenomenon is called gel blocking and
causes diapers to leak. Hence increasing the swelling capacity using conventional means (lowering
core cross-linker) leads to an overall decrease in capacity under load.

SAPs properties determination factor:-
SAP TechnicalFeatures:
The functional features of an ideal SAP material can
be listed as follows:
- The highest absorption capacity (maximum equilibrium swelling) in saline
- Desired rate of absorption (preferred particle size and porosity) depending on the application
requirement.
- The highest absorbency under load (AUL)
- The lowest soluble content and residual monomer
- The lowest price
- The highest durability and stability in the swelling environment and during the storage
- The highest biodegradability without formation of toxic species following the degradation
- pH- neutrality after swelling in water
- Colorlessness, odourlessness, and absolute nontoxicity
- Photo stability
- Re-wetting capability (if required).
The following table shows SAP production for these industry leaders:
Stockhausen GMBH 162,000 (metric tons) Dow Chemical 90,000(metric tons)
Nippon Shokubai 137,000 (metric tons) Sanyo 47,000 (metric tons)
Chemdal Corporation 120,000 (metric tons) Hoechst Casella 94,000 (metric tons)

Global Production of SAPs:-

Applications of SAPs:-
Hygienic and Bio-relatedAreas:-
The most volume of SAP produced all over the world is used in disposable
diapers. Therefore, most research works have been focused on hygienic grades which are usually
used with fluff in diapers. Because of the market requests for a thinner diaper, more SAP and less
fluff is being incorporated into the diapers. This approach limits the maximum amount of SAP in a
diaper to about 10 g/piece, and this is required for the AUL to be enhanced. A target for AUL of 35-
40 is achievable using current technology, but it is desirable to have AUL as high as 45-50 g/g to
obtain a much thinner diaper. In addition to the absorbency parameters, the level of residual acrylic
acid (RM, ppm) has dropped over 1000 to less than 30 ppm in 2000s. The extractable fraction (sol
content) of the SAP has also decreased from ~13 to around 4% over time.
The efforts of manufacturers have been stressed on improving the production
and engineering SAPs with higher performance, i.e., higher AUL, lower levels of RM, sol fraction
and fine particles (<50 μm). Some enzymes and additives may be incorporated to prevent infection
and unpleasant smell. Other hygienic applications comprise more or less similar requirements of the
diaper uses. Recently, a new generation of hygienic superabsorbent named Safe and Natural
Absorbent Polymer (SNAP) has been introduced to the market. SNAPs are totally natural with no
residual monomer therefore they are rapidly biodegraded in the environment. However, they possess
lower absorbency and higher price than the full-synthetic counterparts. Most recently, using
superabsorbent fiber and viscous fiber, a method of preparing absorbent core for ultra-thin high-
absorbent sanitary napkins has been presented. SAPs are one of the members of the family of smart
hydrogels, hence they can be potentially employed in separation science and technology, particularly
bio separation. Due to large changes in the swelling ratio, the hydrogels have been used widely in the
separation of various molecules including proteins. In medicine, SAPs may be used for elimination
of body water during surgery, e.g., treatment of edema In the field of pharmaceutics, some super
absorbents called super-porous hydrogels (SPHs) invented by Kinam Park et al. have also been
developed for gastric retention applications. They are different from SAPs since SPHs swell fast,
within minutes, to the equilibrium swollen state regardless of their size. The very fast swelling
property is based on water absorption through open porous structure by capillary force. SPHs have
been designed for controlled delivery of drugs to stomach or intestine. The poor mechanical strength
of SPHs was overcome by developing the second-generation SPH composites and the third-
generation SPH hybrids
Agricultural Areas:-
The presence of water in soil is essential to vegetation. Liquid water ensures
the feeding of plants with nutritive elements, which makes it possible for the plants to obtain a better
growth rate. It seems to be interesting to exploit the existing water potential by reducing the losses of
water and also ensuring better living conditions for vegetation. Taking into account the water
imbibing characteristics of SAP materials, the possibilities of its application in the agricultural field
has increasingly been investigated to alleviate certain agricultural problems. SAPs have been

successfully used as soil amendments in the horticulture industry to improve the physical properties
of soil in view of increasing their water-holding capacity and/or nutrient retention of sandy soils to
be comparable to salty clay or loam. SAP hydrogels potentially influence soil permeability, density,
structure, texture, evaporation, and infiltration rates of water through the soils. Particularly, the
hydrogels reduce irrigation frequency and compaction tendency, stop erosion and water runoff, and
Increase the soil aeration and microbial activity. In arid areas, the use of SAP in the sandy soil
(macro porous medium), to increase its water-holding capacity seems to be one of the most
significant means to improve the quality of plants. The SAP particles may be taken as "miniature
water reservoirs" in soil. Water will be removed from these reservoirs upon the root demand through
osmotic pressure difference. The hydrogels also act as a controlled release system by favoring the
uptake of some nutrient elements, holding them tightly, and delaying their dissolution. Consequently,
the plant can still access some of the fertilizers, resulting in improved growth and performance rates.
On the other hand, SAPs in agriculture can be used as retaining materials in the form of seed
additives (to aid in germination and seedling establishment), seed coatings, root dips, and for
immobilizing plant growth regulator or protecting agents for controlled release. The SAPs used in
the agriculture are polyelectrolyte gels often composed of acrylamide (AM), AA, and potassium
acrylate. Therefore, they swell much less in the presence of monovalent salt and can collapse in the
presence of multivalent ions
Other Applications of super absorbent polymers:-
 Artificial snow
 Ornamental (coloured) products
 Entertaining/educational toys
 Building internal decoration
 Retarding gels
 Fire extinguish
 Cryogenic gels
 Food/meat packaging
 Concrete strengthening
 Yogurt dewatering
 Child diapers
 Biomedical or optical switching applications
Summary :-
SAP is now a mature product that has quickly progressed from specialty chemical
to special commodity. It has revolutionized the baby diaper and adult incontinence industries
over the last 30 years and is now a material that is used by millions of people every day.

References:-
 Buchholz , FL (Editor) & Graham, AT - Modern Superabsorbent Polymer Technology
 Iranian Polymer Journal / Volume 17 Number 6 (2008)
 School of material science and technology, Beijing institute of technology
 www.nclt.com
 Google images
 Ask.com

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