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Sodium Polyacrylate Deep Research on Safety & Uses

2024-06-14

Sodium polyacrylate belongs to superabsorbent polymers with the chemical formula [−CH2−CH(CO2K)−]n.

In functionality, it can absorb lots of water and then turns to water gel form.

In the safety aspect, it's non-toxic, non-pollutant, and harmless. It's easy to find the MSDS (Material Safety Data Sheet).


Basic info

NameSodium polyacrylate
Chemical formula(C3H3NaO2)n
CAS Number9003-04-7
Molar massVariable
Appearancewhite granule / transparent gel (before/after absorbing water)
Physical propertyNon-toxic, harmless, non-polluting, natural degradation
Odorimperceptible
FunctionAbsorbing, retaining, or blocking liquid
Main applicationsabsorbing, blocking liquid with expanding
Common test itemsabsorption, absorbent rate, liquid permeability, residual monomer (as AA), particle size distribution.



Types

There're many types of sodium polyacrylate for different uses, such as:

  • starch-based type (which is biodegradable),
  • catalyst improved type (for water blocking of cable),
  • cation type (for concrete additive),
  • *defective type & recycled type (for waste liquid absorption).

How does it 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.

Tip: Metal ions can deeply reduce or destroy the water-absorbing capacity of sodium polyacrylate.

About water absorption

Sodium polyacrylate can

  • quickly absorb hundreds of times to more than a thousand times its own mass in water,
  • and it can also absorb tens to one hundred times of saltwater, blood, urine, and other liquids.

Based on the different compositions of SAP, the water absorption mechanism is also different.

  • For polyacrylate type water-absorbent polymer, mainly relies on osmotic pressure to complete the water absorption process.
  • For non-ionic SAP, it relies on the hydrophilic effect of hydrophilic groups to complete water absorption.

The swelling properties of SAP directly affect its product quality and application.

At present, there are many research reports on the swelling properties of SAP, among which the research theories on the water absorption mechanism of superabsorbent resins can be summarized into three aspects:

  1. thermodynamic mechanism of water absorption;
  2. water absorption mechanism of flexible molecular chains;
  3. swelling kinetic mechanism.

Thermodynamic mechanism of water absorption

The adsorption of SAP on the water can be divided into physical adsorption and chemical adsorption.

Physical adsorption

Physical adsorption refers to the adsorption of water through the capillary, so the water absorption capacity is limited, and it will quickly overflow under a certain pressure.

The SAP molecule contains strong hydrophilic polar groups and has a three-dimensional cross-linked structure. Unlike traditional water-absorbing materials, SAP first absorbs water through capillary adsorption and dispersion, and then the hydrophilic groups of the resin interact with water molecules through hydrogen bonds. The ionic hydrophilic groups begin to dissociate when they meet water, and the anions are fixed at high On the molecular chain, cations are mobile ions.

With the dissociation of the hydrophilic group, the number of anions increases, the electrostatic repulsion increases, and the three-dimensional cross-linked network of the polymer expands. At the same time, in order to maintain electrical neutrality, the cations cannot diffuse to the external solvent, so the concentration increases, resulting in an increase in the osmotic pressure inside and outside the resin cross-linked network, and further infiltration of water molecules.

With the increase in water absorption, the osmotic pressure difference inside and outside the network tends to zero. As the network expands, its elastic contraction force also increases, gradually offsetting the electrostatic repulsion of anions, and finally reaching a water absorption balance. Water molecules penetrate and diffuse into the resin under the capillary action caused by the osmotic pressure difference and the expansion of the three-dimensional cross-linked structure of the resin, so as to achieve the purpose of water absorption.

Chemical adsorption

Chemical adsorption means that the hydrophilic groups in the resin firmly adsorb water molecules through chemical bonds, the adsorption capacity is very strong, and it is difficult to overflow under high pressure.

Due to the cross-network structure of SAP itself and the combination with hydrogen bonds, the adsorption of the resin is limited. The molecular network cannot expand indefinitely when water is present, which ensures that the resin will not dissolve in water after absorbing water.

In this way, there are two forces inside the SAP, one is the osmotic pressure generated by the repulsion between the internal ions, which makes the water enter the resin and causes the expansion of the space network; the other is the elastic force generated by the cross-linking effect, which makes the polymer after water absorption has a certain strength. These two forces restrict each other, and finally reach a balance, the resin is saturated with water, and the water absorption at this time is the water absorption rate.

Water absorption mechanism of flexible molecular chains

The thermodynamic mechanism of water absorption of SAP can well explain the water absorption mechanism of ionic SAP, but it is difficult to explain the water absorption mechanism of non-ionic SAP. Therefore, it is necessary to explain the water absorption mechanism of SAP from the aspect of the molecular chain.

According to the second law of thermodynamics, the system always spontaneously balances in the direction of increasing entropy. In the absence of external energy, the SAP in a completely dry state moves the macromolecular chain randomly, and the conformation of each carbon-carbon σ bond tends to be inconsistent. At this time, the macromolecular chain of SAP always tends to the coiled molecular conformation spontaneously.

For an ideal flexible macromolecular chain, its C-C bond can rotate freely, and its rotation is only limited by Pendant groups and hydrogen bonding effects, with ideal flexibility. However, for SAP, the rotation of the macromolecular chain near the cross-linking point is hindered. In the case of uniform cross-linking density, each cross-linked lattice has the same size. It can be considered that the macromolecular chains that constitute the cross-linked lattice have ideal flexibility. , that is, each water-absorbing network is ideal, and the number of carbon atoms in the molecular chain between each cross-linking point is the same.

Therefore, the lower the cross-linking density of SAP, the stronger the flexibility of the macromolecular chain, the longer the effective chain length, the easier its conformational change, the stronger the water absorption capacity, and the smaller the external energy required to overcome the conformational change of the macromolecular chain. That is, the gel strength of SAP is lower.

From the perspective of macromolecular chain conformational change, the monomer with the greater affinity between the side groups of the main carbon chain and water molecules helps to enhance the flexibility of the SAP macromolecular chain.

Swelling kinetic mechanism

The swelling kinetics of the resin was explained using the Berens-Hopfenberg diffusion relaxation model equation.

The diffusion-relaxation model equation proposes that the diffusion of water molecules and the relaxation of resin macromolecular segments satisfy a linear relationship.

Compared to traditional absorbent materials

The water absorption of traditional water-absorbing materials such as cotton, sponge, paper, etc. is carried out by the principle of capillary, which belongs to physical adsorption.

The super absorbent resin has a certain degree of cross-linking due to its molecular structure, and the internal water cannot be easily extruded by simple mechanical methods, so it has strong water retention.

Comparison of water absorption capacity of several traditional water-absorbing materials and superabsorbent polymers:

Absorbent materialWater absorbing capacity (Weight fraction) / %
Waterman No. 3 filter paper180
Tissue paper400
Polyurethane sponge1050
Wood pulp pile1200
Cotton ball1890
Sodium polyacrylate20200

Applications

Sodium polyacrylate can

  • be used directly;
    • for production: water blocking, inflatable rubber, thickening agent, fire-retardant gel, desiccant agent, thermal storage agent.
    • for building: concrete protection, anti-foggy agent.
    • for environment: waste liquid control, excreta collection.
    • other: drilling fluid, collection of tattoo liquid waste, oil-water separation.
  • be used as the raw material of products.
    • product for animals: pet pads, a drown-free water source for feeder insects.
    • products for personal care: paper diapers, sanitary napkins, nursing mats, and contact lenses.
    • products in home: the ice pack, fragrance carrier, water beads, growing toys, cosmetics, solid waterbed.
    • other: cable, urine bag, anti-flood bag, instant snow, waterproof paint.

List of uses

For 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.

For Cables and Electrical

Sodium polyacrylate is an absorbent material that can absorb & block water up to hundreds of times its own weight.

Protect electrical and optical cables

Sodium polyacrylate 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 it.

Cables and optical cables are the transmission carriers of electricity, communication, etc., and are very sensitive to water. There is a danger of electric leakage when the cable enters the water, and the optical cable cannot transmit information when it enters the water.

SAP is used in cables and optical cables erected or laid underground (especially for submarine laying).

The principle is to make the super absorbent resin layer into waterproof tape and wrap it on the cable core as a waterproof layer.

Once the outer casing is cracked or damaged, water enters the waterproof layer, and the superabsorbent resin contained in it absorbs water and swells to seal the damaged part, which can prevent water from invading the cable and prevent the cable from being damp and reducing reliability. Thereby improving the safety of the cable, and prolonging its service life.

According to the introduction of the Stockhausen company, the cost of the cable with SAP waterproof treatment is low, and the performance is equal to or even better than the traditional treatment method.

At present, the application of SAP in this area is expanding, especially in the water-stop and water-proof materials of optical cables material. The SAPs used as water-stop and water-repellent materials for optical cables mainly include saponified hydrolyzates of starch acrylonitrile graft copolymers, and starch-acrylate graft copolymers, and the like.

In addition, in the electronics industry, SAP can also be used for temperature sensors, moisture measurement sensors, leakage Water detector, photochromic element, photo-responsive element, body electrode, ultrasonic probe, etc.

Features of sodium polyacrylate for cables

  1. Strong expanding capabilityOnce the outer layer of the cable was broken, sodium polyacrylate would quickly absorb water and be expanding (to block water). This process will protect the conductor in cables from moisture.So the expanding capability is the most important point of sodium polyacrylate for cables.
  2. Smaller particle sizeIn general, the smaller particles of sodium polyacrylate bring a faster absorbing rate.Sodium polyacrylate's particle size can be 10-200 mesh. And cable type should be more than 50 mesh.

For 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.

For Construction Industry

Causes of concrete cracking

The cracking of concrete and mortar is a quality accident that often occurs in engineering. This is caused by the large self-shrinking deformation of high-performance concrete with a low water-to-binder ratio and active mineral admixture.

The root cause of self-shrinkage deformation is the self-drying inside the concrete. The structure of high-performance concrete is dense, and it is difficult for external curing water to enter the interior of the concrete, and the water required for cement hydration in the later stage cannot be replenished, which intensifies the self-drying phenomenon inside the cement stone.

Therefore, in order to alleviate the internal self-drying of concrete, the method of internal curing is often used, and pre-water-absorbing ceramsite or ceramic sand, or other water-absorbing artificial lightweight aggregates are used instead of ordinary aggregates. However, the water absorption rate of artificial lightweight aggregate is limited, and the effect of relieving drying and reducing shrinkage is not obvious.

Sodium Polyacrylate for Concrete Protection

Internal curing of concrete after absorbing water with acrylic polymer superabsorbent resin is a cost-effective method.

Studies have shown that the particles of superabsorbent polymer are smaller, disperse evenly in concrete, and absorb large water, which can effectively supplement the digestion water in cement stone.

The self-shrinkage deformation of concrete is significantly reduced after adding pre-absorbent superabsorbent polymer to the mortar.

However, after the concrete is hardened, the superabsorbent polymer leaves closed pores with relatively regular shapes inside the concrete, which reduces the strength of the concrete to a certain extent.

Jensen M and Hansen P F of the Technical University of Denmark jointly proposed that adding ultra-high Strongly water-absorbing polymers can effectively improve self-drying, and this method has been widely recognized by the international academic community. When the hydration of concrete is affected by the decrease of the internal humidity of concrete due to concrete hydration, the polymer material can release the stored water, provide water for further hydration of concrete, and delay the self-drying process of concrete.

Sealing Material

In the construction industry, SAP can also be mixed with natural rubber or synthetic rubber, and then add surface active Additives to improve their compatibility and make sealing materials.

SAP expands sharply when it encounters water or other aqueous fluids, quickly fills the gap, forms a tight seal, and maintains good mechanical strength.

This sealing method has strong resistance to strong acid and strong alkali, especially in the sealing of oil and gas pipelines.

This has been applied in the construction of subsea tunnels in Britain and France, with satisfactory results. In addition, in the emergency rescue of rivers, reservoirs, dams, mines, and other waterproof plugging projects, the application of SAP-containing water plugging agents can greatly improve safety.

For Fire Fighting and Prevention

The German Degussa company has reported that the Firesorb fire extinguishing agent developed by the company is a new type of fire extinguishing agent.of fire extinguishing agents containing SAP.

The fire extinguishing agent has high fire and extinguishing performance, and its heat absorption capacity is 5 times that of water. Using this fire extinguishing agent can not only reduce the water consumption by 50%, but also shorten the fire extinguishing time.

For Hot/cold therapy gel pack

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.

Important indicators for the Gel pack

Hydration time

As we know, some gel pack is disposable and some gel pack can be used multiple times.

Hydration time means how long the gel in the pack would melt. It determines the quality of the gel pack.

Gel strength

When gel strength is too hard, the hand feel of the pack would be terrible.

When gel strength is too soft, the hydration time would be short.

Granular size

The common granular size of SAP is 10-100 mesh. We can purchase the right size according to the gel pack's manufacturing technique.

Another side, the granular size would influence the hydration time and gel strength.

Price

If the purchasing order is big (like more than 16 tons), we can get a good price from SAP factories;

If the purchasing order is not so big, a trading company specialize in SAP would be a better choice.

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,

  • Absorbent rate;
  • Absorption under pressure and retention capacity after centrifugation;
  • Liquid Permeability;
  • Residual Monomer.

For Ice bag/pack

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.

For Waterblocking tape

The 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,

  • Hydration time;
  • Expansion height;
  • Price;
  • Granular size.

For Inflatable sandbag

Different from a traditional sandbag, an inflatable sandbag (sodium polyacrylate inside) has so many advantages:

  1. Transport. Lightweight before they’re used, thousands of inflatable bags can be quickly transported in a van.
  1. Storage. Inflatable bags can be vacuumed-packed. So they can be stored in small depots for a long time.
  2. Use. Compare with a traditional sandbag, inflatable bags are easier to carry & pile up.

important indicators:

  1. Absorbent rate;
  2. The photolysis resistance.

For Leaking stoppage / Spilling treatment

Sodium polyacrylate can absorb hazardous, non-hazardous, contaminated, and other regulated wastes.

Industrial grade sodium polyacrylate is cheap and can absorb plenty of liquid in few minutes. And solid waste is much easier to treat than a liquid one.

important indicators:

  1. Price;
  2. Coagulation time.

Benefits of Sodium polyacrylate

  • Easy to use;
  • High water absorption & absorbent rate;
  • Will not release liquid under pressure;
  • Odor controlling;
  • Customizable property (for the specific waste liquid);
  • Non-biodegradable and has a high absorbency thereby preventing leakage;
  • The waste liquid includes:
    • Hazardous waste;
    • Non-hazardous waste;
    • Contaminated waste;
    • Other regulated wastes.

Get Rid of Horse Manure/Urine Smell

It's really a pain to clean up their horse stable if you have some horses. Especially the urine gets between the cracks of rubber mats is hard to clean and smells like ammonia.

When it meets horse urine, sodium polyacrylate turns from powder to a scentless fluffy substance - which is 3 times the powder form volume and feels dry.

Another side, fluffy property sodium polyacrylate allows air throughout. The oxygen would kill the germ which brings the urine an unpleasant odor.

This is disruptive change and you will have so much fun cleaning them.

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,
  • 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.


Safety

For Human

As a polymer, it sticks together in long chains and neutral. That means sodium polyacrylate is not irritating to the skin.

So far, sodium polyacrylate doesn’t have any side effects. And according to various material safety data sheets (documents created by the U.S. Occupational Safety and Health Administration that list potential hazards of chemicals in great detail), sodium polyacrylate is safe.

How about the “acrylic acid”?

Yes, there’s some kind of inferior sodium polyacrylate that mixed up with small amounts of acrylic acid. And in theory, acrylic acid in large doses could be harmful to a baby’s skin. But as a leftover from the manufacturing process, the acrylic acid value is less than 300 PPM which is totally safe for humans.


For the environment

Sodium polyacrylate can be decomposed into carbon dioxide, water, sodium, etc. This process won’t bring any obvious influence.

But this doesn’t mean sodium polyacrylate is the good choice as a water-retaining agent. Actually, massive amounts of sodium polyacrylate may exacerbate soil salinization.


Tips for safe use

Sodium polyacrylate isn’t toxic. But as a powerful water-absorbent material, it can also cause certain dangers if not handled properly.

-A mask may be a good choice when you directly face the sodium polyacrylate powder. (It can irritate the lungs to inhale the powder.)

-It can absorb water, expand and turn to water gel. So, it’s unwise to eat.

-When it comes in contact with an amount of water in an area, it can cause the area to be very slippery.

-You should not flush down the powder or water gel – it will cause serious clogging.


Choose the good Sodium polyacrylate


The defective sodium polyacrylates

There are three defective sodium polyacrylate types. We can name them: yellow type defective, tiny particles type defective, and recycled type defective.

Where's defective sodium polyacrylate from?


Defects

Yellow type defective

Yellow-type defective sodium polyacrylate would be produced when the temperature is over in the drying process.

Defects:

-The water absorption capacity is weaker than the common product;

-The color usually could be yellow or dark.

Tip: Some improved potassium polyacrylate (agriculture purpose) would also be yellow, dark, or black. There's a big difference between them.

Tiny particle type defective

Tiny particle type defective sodium polyacrylate would be produced by smashing & sifting process.

Defect

The tiny particle will weaken the liquid permeability which is important for paper diapers.

Recycle type

In normal conditions, recycled sodium polyacrylate is most used in wastewater control.

Tip: To earn more money, some profiteers would sell recycled sodium polyacrylate as common sodium polyacrylate.


FAQs

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 the skin?

Although sodium polyacrylate is not toxic, there’re extremely strict requirements on technical parameters especially used in hygiene products like diapers, sanitary napkins, and 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?

Safe but not recommended. Sodium polyacrylate would be decomposed into Sodium ions which cause soil salinization. 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 causing the high ion concentration of 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 number 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/animals/insects?

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


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 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 is the use of sodium polyacrylate?

It can be used as: a. water-retaining agent (for plants); b. water-absorbent material (for diapers); c. expanding material (for inflatable toys); d. thickener (for cosmetics); e. water blocking material (inflatable sandbag); f. cooling storage (ice pad).


What’s 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 to find/get sodium polyacrylate at home?

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


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

You may have been wondering how sodium polyacrylate works. 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. Its 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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