Superabsorbent polymer (SAP)

2021-04-15

*This page is about superabsorbent polymer, slush powder, potassium polyacrylate, sodium polyacrylate, SAP.

Brief introduction

The origins of superabsorbent polymer chemistry trace back to the early 1960s when the U.S. Department of Agriculture developed the first superabsorbent polymer materials.

As a function polymer material, superabsorbent polymer (SAP) has a strong water-absorbing ability.

  • It can absorb hundreds of times its weight and lock water firmly.
  • After absorbing, SAP powder (or granule) will turn into water gel.

According to different usage, there are many types of superabsorbent polymers.


What is SAP

There are two main types of SAPs:

  1. sodium polyacrylate,
  2. potassium polyacrylate.

SAP (Superabsorbent Polymer) is a functional polymer with higher water absorption and superior water retention capability.

It :

  • Absorb plenty of water (or other inorganic solution),
  • Lock water and
  • Become water gel form after absorbing.

The advantageous aspect of using SAP is that it is:

  • Non-Toxic,
  • Non-pollutant, and
  • Harmless in every aspect.

Tips: Its most beneficial property lies in the fact that it absorbs liquids equivalent to several hundred times of its body mass and transforms it into a Gel; that is why it is able to retain or stop the water.


How does SAP work?

It consists of a set of polymeric chains that are parallel to each other and regularly linked to each other by cross-linking agents, thus forming a network.

When water comes into contact with one of these chains, it is drawn into the molecule by osmosis. Water rapidly migrates into the interior of the polymer network where it is stored.


Manufacturing Process

Aqueous solution polymerization

Since most of the monomers of SAP are water-soluble, SAP can be produced by solution polymerization using water as the solvent. More than 90% of the world's production capacity uses this method.

The reason why gamma rays and microwaves are not commonly used is that they require a special container, and the radiation generating device has many inconveniences in operation. Although gamma rays are reported to have a high rate of initiation, they do not have the advantage of being popular in the field of civilian production due to expensive equipment and dangerous radiation leakage. Thermally induced products generally turn yellow, have a high residual monomer rate, and are not uniformly polymerized, so they are not suitable for manufacturing high-performance products.

Advantage

The aqueous solution polymerization method has the advantages of low cost, low equipment requirements, low investment, simple process, high production efficiency, safe operation, pure system, uniform cross-linking structure, etc.

Disadvantage

Increase drying, pulverizing and screening processes, long product production lines, uneven particle size distribution after product crushing, etc.

Production process

Initiator initiation can easily control the initiation rate and adjust the water absorption performance, so it is widely used. Using water as a solvent, acrylic acid and mixed monomers of acrylic acid and sodium acrylate partially neutralized by alkali are prepared by cross-linking polymerization, drying and crushing in the presence of a cross-linking agent and an initiator.

The production method does not produce toxic and harmful substances, and the whole process can be carried out in an environment-friendly atmosphere.


Inverse Suspension Polymerization

The synthesis of SAP by reversed-phase suspension polymerization is different from the synthesis of SAP from aqueous solution.

Advantage

Inverse suspension polymerization SAP products have fast water absorption rate, high water absorption capacity and uniform particle size distribution.

The processing is simple, the product can be obtained only by drying, and the beaded product can be directly obtained without crushing and screening, which solves the problems of heat transfer and difficulty in stirring in the aqueous solution polymerization method.

Disadvantage

The disadvantage of this method is that the material requirements of the main equipment are high, the equipment investment is large, and since the organic solvent is used in the production process, a solvent recovery device is required, which is prone to pollution.

In addition, the reversed-phase suspension polymerization method can only carry out intermittent production, the equipment utilization rate is low, and the production efficiency is low.

Production process

The synthesis process is that organic solvent is used as dispersion medium as oil phase, and the mixed aqueous solution of sodium acrylate and acrylic acid partially neutralized by alkali is used as water phase, which is dispersed in oil phase medium in the form of droplets, and formed under the action of suspending dispersant and stirring. A stable dispersion of water-in-oil droplets, i.e., a water-in-oil suspension, where the initiator and crosslinking agent are dissolved in the water-phase droplets to carry out the polymerization process.

After the polymerization reaction, the organic solvent in the oil phase needs to be removed by distillation under reduced pressure, and then the gel is dried to obtain the SAP product.


List of SAP's Manufacturers

The global consumption of superabsorbent polymer is composed of hygiene products accounting for 92% (including baby diapers accounting for 78%, adult incontinence pads accounting for 10%, women's sanitary napkins accounting for 4%), agriculture accounting for 4%, construction industry accounting for 1%, and other fields accounting for 3%.

CompanyMain product
SAPGELPotassium basic SAP and Sodium basic SAP)
M2 PolymerSAP for sewage treatment
EvonikSodium basic SAP
SantosSodium basic SAP
sanyo-chemicalSodium basic SAP
sumitomoSodium basic SAP
BASF SESodium basic SAP
shokubaiSodium basic SAP
LG ChemSodium basic SAP
Formosa Plastics GroupSodium basic SAP

Quality grade

SAP's main compositions are sodium polyacrylate or potassium polyacrylate.

The grades of SAP as follows,

  • Improved SAP
    • Starch-based SAP - The biodegradable type SAP;
    • Minerals based SAP (for plants) - Available in soil for years;
    • SAP with improved catalyst - High expansion ability (for water blocking of cable);
    • Cation type SAP - concrete additive.
  • Common SAP
    • Potassium polyacrylate - mainly used in agriculture as a water retaining agent;
    • Sodium polyacrylate - widely used in industrial production such as Paper diapers, Sanitary napkins, Ice bags, Air fresher, Expanding rubber, etc.
  • Defective SAP
    • Recycled SAP: the SAP was used and recycled;
    • Burnt SAP: the SAP was produced in over-temperature.

Uses of SAP

For plants

Super Absorbent Polymer, also known as SAP, hydrogel, absorbent polymers, absorbent gels, water-retaining agent, super slurpers, water gel, is a new type of macromolecular synthetic water-absorbing polymer material. It has a water uptake potential as high as 100,000% of its weight in a short period by osmosis and form granules in the soil to enhance soil properties. SAPs are generally white sugar-like hygroscopic materials that swell in water to form a clear gel made of separate individual particles and can retain moisture even under pressure without risk of conflagration or rupturing/blasting.

Super Absorbent Polymers used in agriculture are mostly prepared from acrylic acids and a cross-linking agent like potassium by solution or suspension polymerization. The polymer so formed is called a polyacrylate whose swelling capacity and gel modulus depends greatly on the quantity and type of cross-linker used. Polyacrylates are non-toxic, non-irritating, and non-corrosive and tested to be biodegradable with a degradation rate of 10%-15% per year. They demonstrate high water absorbance potential and can freely release 95% of the same under suction pressure by plant roots.

Important indicators

  • Absorbing capacity in soil;
  • Granular size;
  • The photolysis resistance;
  • Period of validity.

Benefits

The three most common soil conditions that hinder plant growth and crop yield are low water retention capability, high evapotranspiration rate, and soil moisture leaching. Apart from these, factors like unforeseen drought conditions, degradation and salination, overuse of synthetic fertilizers and pesticides, and improper irrigation practices severely affect soil and plants, often rendering permanent damage to soil biota as well.

The three most common soil conditions that hinder plant growth and crop yield are low water retention capability, high evapotranspiration rate, and soil moisture leaching. Apart from these, factors like unforeseen drought conditions, degradation and salination, overuse of synthetic fertilizers and pesticides, and improper irrigation practices severely affect soil and plants, often rendering permanent damage to soil biota as well.

Desirable characteristics

  • High absorption capacity in saline and hard water conditions.
  • Optimized absorbency under load (AUL).
  • Lowest soluble content and residual monomer.
  • Low price.
  • High durability and stability in the swelling environment and during storage.
  • Gradual biodegradability without the formation of toxic species.
  • pH‐neutrality after swelling in water.
  • Photostability.
  • Re‐wetting capability.
  • Hydrogel agriculture technology, as it is popularly called, has the following advantages.
  • Improves soil quality, preserves water, and resists drought stress.
  • Increases seed sprouting and seedling development leading to better farm success.
  • From the environmental aspects, it is nonpolluting and biodegradable, helps in reducing irrigation frequency and water consumption, and creates a simple cyclic process to provide water directly to roots and prevent soil compaction.
  • In agriculture and agroforestry, SAPs act as micro water reservoirs at plant roots. They absorb natural and supplied water 400500 times their own weight and release it slowly on account of root capillary suction mechanism thus preventing water loss in soil by leaching and evaporation.
  • SAPs form a consistent cyclic process of absorption and release of water; the water so released can provide optimum moisture for quick germination and seedling maturation. Thus it reduces seedling mortality by several folds in nurseries.
  • In cold regions, death during germination and maturation is common due to moisture freezing in & around plant root tissue. Absorbed moisture in hydrogels does not freeze and makes easy accessibility to plants. It also regulates seedling growth temperature preventing death by freezing.
  • SAPs can help save water and labor by reducing irrigation frequency, help overcome drought conditions and act as soil conditioners, prevent leaching in sandy soils, runoffs in mountainous and sloping fields, improve virescence efficiency, and restore soil biota.
  • SAPs can reduce the overuse of fertilizers and pesticides in fields. The chemicals so absorbed with water are slowly released thus extending the operational life and uptake efficacy by root systems.
  • SAPs act as soil matter flocculants. They closely bind loose soil thus forming loams that can help better root latching. Simultaneously, the repeated absorbrelease mechanism prevents overcompaction of soil minerals and provides space for aeration and development of soil edaphon.
  • It has a wide area of application ranging from agriculture, forestry, industrial planting, municipal gardening, drought management, water conservation, It helps reduce soil erosion by surface runoffs, fertilizer and pesticide leaching to groundwater, reducing the cost of water and irrigation, and success rate at growth and high yields of crops.

For hygiene products

With its awesome properties of liquid absorption and locking it has found very useful application in products prone to moisture like:

  • Diapers,
  • Sanitary napkins,
  • Nursing pads and
  • Pet pads.

Important indicator as follows,

IndicatorsNote
absorbent rateIt affects the urine-absorbent rate of diapers.
liquid permeabilityIt helps diapers absorb urine fast & evenly.
absorption in 0.9% NaClIt affects the urine absorption ability of diapers.
absorption under pressureIt affects the urine absorption ability of diapers (under pressure ).
retention capacity after centrifugationIt affects the urine absorption ability of diapers (while were switched).

Baby diapers

Baby diapers consist of three basic designs: a porous top layer, an absorbent core layer, and a leak-proof bottom layer. The water-absorbing core layer contains superabsorbent resin.

Baby diapers have the following requirements for SAP:

  • good water absorption,
  • Absorbs the baby's ability to absorb urine after several urinations,
  • Under the action of the shearing force generated by the movement of the baby's body, it remains free from spillage and has a continuous ability to absorb urine.

Adult diapers

Most of the users of adult incontinence materials are adults or the elderly who are paralyzed, incontinent, or incapacitated due to various injuries and diseases, and a small number are consumers who have to use it due to inconvenience in working for a long time, mainly in the form of diapers.

The structure of adult diapers is mainly divided into 4 layers, from the top layer to the bottom layer are the surface layer covering material, the liquid collecting diffusion transfer layer, the liquid absorbing core, and the back layer material, in addition to the leg guard, elastic material and hot melt adhesive, etc. member.

Among them, the absorbent core is the key functional layer of the adult incontinence material, which is mainly composed of fluff pulp and granular superabsorbent resin.


Sanitary napkins

The liquid that sanitary napkins have to absorb is more complex, a more viscous mixture of water, salt, and cells.

Since the cells are very large, it is difficult for the superabsorbent polymer to absorb them into the network structure. Therefore, surfactants are often applied to the surface of the superabsorbent polymer particles to improve the dispersibility to blood.


Improve SAP in hygiene products

At present, the research on SAP mainly focuses on: reducing cost, improving gel strength after water absorption, improving Polymer salt resistance.

improving gel strength after water absorption

To improve the strength of hydrogel after water absorption, surface cross-linking is mainly carried out post-treatment to reduce the content of water-soluble parts. Scientists Lind and Smith treated acrylic resins with a mixture of surfactants and olefin additives to reduce the residual monomer content to 50 mg/kg.

improving polymer salt resistance

In terms of improving the salt resistance of the polymer, the mixing method of SAP and ion exchange resin is adopted, and the ion exchange characteristics of the ion exchange resin are used to reduce the ion concentration of the aqueous solution, thereby improving the water absorption of SAP to the saline solution. The complex of SAP and the inorganic hydrogel is based on the good salt tolerance of inorganic hydrogel, and the combination of anionic SAP and inorganic hydrogel to form a gel complex is beneficial to improving the salt tolerance of SAP.


For miscellaneous industrial products

With each passing day, SAP is being employed in more and more industries and applications due to its increasing popularity and efficiency in water deterrence.

Some applications as follows,

For waterproofing cables

SAP is a highly coveted product as far as the manufacture of water-proof tapes and ointments for all sorts of cables are concerned; even, optical fiber cables use SAP. The extent and the speed of absorption are quite high and so are the overall inflation rate and its quantum of inflation upon encountering water.

Important indicator as follows,

  • Expansion height;
  • Granular size.

In Ice Bags

Freshwater is directed at the individual sheets of ice packs to activate them; Superabsorbent Polymer instantly starts absorbing the water and converting it to a Gel by placing them under fresh tap water; in about 20 minutes time it is seen to soak up enough water to form small cushion-shaped Ice Cubes. They are now ready for use by placing them in the freezer.

Important indicator as follows,

  • Hydration time;
  • Price;
  • Granular size.

Inflatable Anti-Flood Sandbags

SAP has this unique application in Disaster Management where it is used in preparing sandbags which, when coming in contact with water, quickly absorb it and expand to form a barrier to the advancing water. Though we call them Sandbags, they actually do not contain any sand; they are so-called because that is how they have been known traditionally.

These Sand-less Sandbags are quite small in size and light in weight initially but they soak up the water to assume a solid shape. They are much easier to store and transport and take up lesser space when unused.

Important indicator as follows,

  • Absorbent rate;
  • The photolysis resistance.

Concrete Protection

  • Its permeability happens to be quite low; it totally shuts out the water content,
  • Due to its fibrous texture, it can be stored to form a vertical barrier,
  • Since it is an effective sealant, it finds application in Cement-based items.

Other Applications

It finds extensive usage in a number of species of products due to its amazing properties; some of them may be listed here:

  • As an additive for functional coating in many specific applications,
  • As an additive in Air fresheners,
  • For expanding rubber requirements,
  • To dispose of tattoo liquid waste
  • Absorption of wastewaters from industries,
  • In the mining industry for disaster management and normal seepages,
  • In certain cosmetics.

Due to its awesome capacity to act on moisture, SAP (Superabsorbent Polymer) is finding newer purposes for which it is being employed.


Frequently asked questions

Can

Can potassium polyacrylate be reused?

Yes. Potassium polyacrylate becomes gel form after absorbing water. After gel releases the water to the roots of the plant, it will return to the granular state. There potassium polyacrylate granular can absorb rainwater or irrigation water again.

The life of potassium polyacrylate in the soil is related to the degree of soil salinity and local temperature. It can usually reach several years.


Can sodium polyacrylate go down the drain?

Yes for sodium polyacrylate gel. It'll slowly be decomposed into the water, carbon dioxide, and some sodium ions, which will not cause environmental pollution.

tip: Sodium polyacrylate powder has a strong water swelling property. This will cause the toilet to block.


How

How is sodium polyacrylate manufactured?

Many people ask me "is sodium polyacrylate natural?". We need to know the sodium polyacrylate's manufacturered process first.

There are four ways to produce sodium polyacrylate: Copolymer chemistry, Gel polymerization, Solution polymerization, and Suspension polymerization.

Here's the common process picture for your conference:

As you see, sodium polyacrylate is not a natural product.

tip: there is a sodium polyacyrlate produced by starch grafting. The manufacturer claims that it is a "natural product" as a gimmick.

In theory, sodium polyacrylate can be applied to all applications that need to absorb water or lock water. But as mentioned before, it cannot be used for agricultural water retention. This will be explained in detail later.


How to use potassium polyacrylate for plants?

You can make the potassium polyacrylate gel first then mix it with soil.


Is

Is potassium polyacrylate safe for the environment (biodegradable)?

Yes. Unlike sodium polyacrylate, potassium polyacrylate can be decomposed into water, carbon dioxide, and potassium that can improve soil and increase fertilizer efficiency.


Is sodium polyacrylate OK in food?

Although the claim of "food grade sodium polyacrylate" has existed for many years, no manufacturer has claimed to be able to produce it. This may be related to the production process.


Is sodium polyacrylate safe/good for skin?

Although sodium polyacrylate is not toxic, there're extremely strict requirements on technical parameters especially used in hygiene products like diapers, sanitary napkins, nursing pads.

As a manufacturer of hygiene products, it's necessary to check out the MSDS (Material Safety Data Sheet) of sodium polyacrylate.

As a customer of hygiene products, please try to choose well-known brand products.

note: acrylic acid content is a very important indicator. It may cause skin allergies and rashes when the content exceeds the standard.


Is sodium polyacrylate safe for plants (as fertilizer)?

Although the price of sodium polyacrylate is much lower than potassium polyacrylate, it's whimsical to use it as a water-retaining agent.

List a few reasons why you should NOT use sodium polyacrylate for plants:

  • As a water retaining agent, sodium polyacrylate can hardly absorb water underground cause the high ion concentration soil leachate.
  • The water gel from sodium polyacrylate is easily melted and photodegraded at a lower temperature (such as 40°C).
  • Sodium polyacrylate is decomposed into a large amount of sodium ions that cannot be absorbed by plants, which leads to increased soil salinization.

Note: different from A, B, which is specially designed for the water-retaining agents, effectively solves the above three problems.


Is sodium polyacrylate toxic to dogs/animal/insect?

Strictly speaking, the answer can be yes only if sodium polyacrylate meets the food-grade standard.


What

What is potassium polyacrylate used for?

Potassium polyacrylate is a kind of super absorbent polymer used for agriculture. It can improve soil

Mixing it in the soil can greatly increase the water holding capacity of the soil.


What's the melting point?

It's 150℃.

In addition, after sodium polyacrylate absorbs water and becomes a hydrogel, it will be easily melted and photodegraded at a lower temperature (such as 40°C). This is one of the reasons why sodium polyacrylate cannot be used as a water-retaining agent for agriculture and forestry.


What's the Ph value?

Generally speaking, sodium polyacrylate is weakly acidic because it has very little acrylic residue. (Sodium polyacrylate is formed by the polymerization reaction of acrylic acid and sodium hydroxide. )


What's the water absorption?

As the strongest water-absorbing material at present, sodium polyacrylate can absorb up to several thousand times its own weight of pure water.

In addition, whether the water is pure or not directly affects the absorption of sodium polyacrylate. If salt is added to the water, the water absorption will be greatly reduced. The concentration of water ions in the soil is extremely high, and the water absorption of sodium polyacrylate is greatly reduced -- This is one reason why it is not suitable for use as a water-retaining agent.


Where

Where can we buy sodium polyacrylate/potassium polyacrylate?

Purchase sodium polyacrylate bulk.

For bulk purchasing, usually, the minimum purchase quantity of the supplier is 500KG.

A simple Google search will find that the professional suppliers of this product like Www.Sapgel.com.

Purchase sodium polyacrylate online

You may only need 500g or 1kg sodium polyacrylate for some scientific experiments.

Then you can consider purchasing it from Amazon.


Where to find/get sodium polyacrylate at home?

As I said, you can easily find sodium polyacrylate in sanitary napkins or paper diapers.


Why

Why does sodium polyacrylate absorb water?/how does sodium polyacrylate work?

You may have been wondering how does sodium polyacrylate work. In fact, the entire water absorption process has undergone complex physical and chemical reactions.

Simply speaking, there are network molecular chains in the molecular structure of A. It electrolysis occurs immediately after encountering water and dissociates into positively and negatively charged ions. The positively and negatively charged ions have a strong affinity with water. Therefore, it has strong water absorption and water retention capabilities.


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