SODIUM POLYACRYLATE (C3H3NAO2) N: STRUCTURE, PROPERTIES, USES, RISKS - CHEMISTRY - 2025

The sodium polyacrylate is an organic compound of high molecular weight formed by the union of many units or pieces of a smaller compound, sodium acrylate. The molecular formula of sodium polyacrylate is (C ₃ H ₃ NaO ₂) _n, and the extended formula of its monomer is –CH ₂ –CH (COONa) -.

Sodium polyacrylate has many repeating pieces all the same. It is an anionic polyelectrolyte because it has many anions carboxylate -COO ^-, which are negatively charged. This is what gives it its main physicochemical properties and favors its many uses.

Formula of the monomer or repeating piece of the sodium polyacrylate polymer, where n represents the number of times it is repeated. Muso. Source: Wikimedia Commons.

It has a great affinity for water. It absorbs it very easily, forming a gel. Absorbs all types of fluids or aqueous solutions. This property makes it used for example in disposable diapers and sanitary napkins.

It is also used in packaging for the fresh food industry to absorb excess water that they may produce.

Due to its large amount of negative charges, –COO ^- is used in detergents to help suspend dirt particles. It also works in industrial processes where it is required to trap positive metal ions such as calcium, aluminum and magnesium.

Sodium polyacrylate is not toxic, but it is very important not to ingest it as it can absorb water from saliva, increase its volume and cause suffocation.

Chemical structure

Sodium polyacrylate is a polymer, so it is made up of many individual pieces all the same and joined together. Such chunks, pieces or units are called monomers.

Sodium polyacrylate is generated by the joining of many molecules of sodium acrylate. When joined, the pieces of sodium acrylate lose the double bond.

For this reason, sodium polyacrylate has a structure without double bonds, with many carboxylate anions –COO ^- and many sodium Na ⁺ ions attached to them.

Because it has so many anionic groups –COO ^{- it} is said to be a polyelectrolyte.

The monomer or individual piece of sodium polyacrylate has a chain of two carbon atoms and a group –COO ^- Na ⁺ attached to it, as indicated below: –CH ₂ –CH (COO ^- Na ⁺) -.

Structure of the sodium polyacrylate monomer. The dotted lines indicate the bonds with other monomers identical to this one. Edgar181. Source: Wikimedia Commons.

The amount of monomers can vary from a few tens to several hundred.

For this reason an n is placed in the polymer formula, since n symbolizes the number of monomers and this number can change according to what the polymer manufacturer wants.

Nomenclature

-Sodium polyacrylate

-Sodium salt of polyacrylic acid

-Acrylic acid polymer sodium salt

-Sodium salt of 2-propenoic acid homopolymer (homopolymer means that it is a homogeneous polymer or that it is made up of many units of a single type of molecule)

Properties

Physical state

White solid powder or granules.

Molecular weight

It depends on the number of monomers that make up the polymer. Sodium polyacrylate with molecular weights of, for example, 1200, 2100, 8000 and 15000 is produced commercially.

Density

1.32 g / mL at 25 ° C.

pH

6-9

Solubility

Very soluble in water.

Chemical properties

Sodium polyacrylate has a high affinity for water. Aqueous sodium polyacrylate solutions are highly viscous and exhibit high stringiness.

It is an extremely absorbent polymer, especially of water or aqueous fluids. Its water absorption speed is very high.

Water is easily attracted to this polymer, which when absorbed turns into a gel. And it has a very high capacity to retain absorbed water.

All this is explained because the hydrogen in the water forms bonding bridges with the carboxylate anion –COO ^- of the polymer. The polarity of water and the carboxylate group favors these hydrogen bonds: –C – O- - -H – O – H.

Sodium polyacrylate also stabilizes oil-in-water emulsions. The parts of the polyacrylate molecule that are related to the oil –CH ₂ –CH– bind to it and the ionic or water related parts –COO ^- Na ⁺ remain in it. Thus, the oil droplets remain stable in the water.

Another of its characteristics is that due to its large quantity of carboxylate anions –COO ^- sodium polyacrylate can easily attract positive ions or metal cations such as calcium Ca ²⁺, magnesium Mg ²⁺ or aluminum Al ³⁺. It joins them easily and does not let go.

It is not toxic or polluting and is biodegradable.

Obtaining

To obtain sodium polyacrylate, acrylic acid CH ₂ = CH-COOH and sodium hydroxide NaOH are reacted first, in order to obtain sodium acrylate CH ₂ = CH-COO ^- Na ⁺.

The latter is subjected to heating in the presence of ammonium persulfate (NH ₄) ₂ S ₂ O ₈ to accelerate the polymerization reaction, where each molecule of sodium acrylate is attached to the next using the double bond.

The solution obtained contains sodium polyacrylate. Then by evaporation under certain conditions the dry polymer is obtained.

Applications

As an absorbent in various applications

Sodium polyacrylate is used as a superabsorbent in several widely used products. One gram of this polymer can absorb 300 to 1000 grams of pure water.

For example, it is used in disposable diapers, sanitary napkins, breastfeeding pads, and other household items. In these cases, it efficiently absorbs aqueous body fluids, such as urine or blood.

It is also used to prevent liquid spills by solidifying them. It allows you to deal with body fluids by trapping them in a dry, semi-solid gel, making their disposal easier.

In hospitals and clinics it is used for the management of spills and in bottles for suction of fluids. This enables savings on laundry, less embarrassment for patients, and reduced slips and hustle and bustle for their healthcare professionals.

In cleaning products and in laundry

Sodium polyacrylate serves as a thickener in hypochlorite solutions that are used in bleaching mixtures.

It is also part of detergent compositions because during laundry it acts as a dispersant for dirt particles, keeping them in suspension, increasing whiteness and improving the general cleaning performance of detergents.

This is due to the multiple negative or anionic charges that help suspend dirt particles, making it most effective in granular or powder detergents and washing at alkaline (i.e. basic) pH.

Detergents contain sodium polyacrylate which allows them to better wash clothes. Author: Frank Habel. Source: Pixabay.

In the food industry

It has been widely used as a thickener and preparation agent utilizing its physicochemical properties.

It is used, for example, as a dispersant for mineral pigments or fillers such as calcium carbonate or kaolin during the manufacture of paper or cardboard that comes into contact with all kinds of food, except for use in contact with infant formulas or breast milk.

It also serves as a fluid absorbent in materials that come into contact with food. It absorbs excess water from all kinds of fresh foods, such as chicken, fish, meat, vegetables, and fruits.

Fresh food packages may contain sodium polyacrylate to absorb the water that they can release. Author: Shutterbug75. Source: Pixabay.

In sugar-producing industries, its use during the evaporation of water from beet or sugar cane juices allows the trapping of metal ions and thus control the formation of mineral incrustations in equipment.

In experiences to improve the recovery of residual oil

In the oil industry, it is currently used in conjunction with other acrylic polymers to thicken the water injected into some wells and favor the effective removal of residual oil for recovery.

In addition, sodium polyacrylate has been used in tests to prevent calcium and magnesium precipitation and thus prevent fouling of equipment during oil recovery from polymer flooding wells.

In the plastics industry

Sodium polyacrylate is often added to polyvinyl chloride, or PVC (PolyVinyl Chloride) plastic as an impact strength modifier and processing aid.

PVC pipes used in the construction of houses and buildings may contain sodium polyacrylate to increase their resistance. Author: Pisauikan. Source: Pixabay.

Among the characteristics that it must possess to fulfill these functions is that of having some non-miscibility (not mixing) with the other polymer, to form small independent areas.

At the same time, it must generate some strong interfacial bonds with the base or matrix polymer to allow good stress transfer through these bonds.

If the impact modifier is too compatible or miscible it does not improve impact resistance, but if it is completely incompatible the base compound loses its strength or toughness.

For this reason, sodium polyacrylate is useful in this application.

In various applications

Sodium polyacrylate is used in the pulp and paper industry as a dispersant.

This is due to its ability to sequester or trap (and not release) harmful multivalent (ie, multi-charged) cations, such as aluminum Al ³⁺ and calcium Ca ²⁺, complexing with these.

It is resistant to high pH, ​​high temperatures, and high shear or shear forces. However, if the multivalent cations are in high concentration they can precipitate the polyacrylate causing it to lose its effectiveness.

It is also used in the cosmetic industry as a thickener and emulsifier.

Due to its ability to absorb water quickly, it is used as a water blocker in fiber optic cables. These are the cables used for the transmission of the Internet and telephone signal.

Inside the fiber optic cables there is sodium polyacrylate to absorb any moisture that can penetrate them and prevent damage. Author: Planet fox. Source: Pixabay.

Sodium polyacrylate is the one that fills the cold gel packs that are used to treat pain caused by blows or injuries, or to transport medicines or flowers.

Cold gel bag used to relieve pain from sports injuries. yamada kazuyuki from Higashi-betsuin, Japan. Source: Wikimedia Commons.

Small dolls are made with sodium polyacrylate that expand or increase in volume when left soaking in water.

Risks

Although sodium polyacrylate is not toxic, ingesting its granules orally causes them to expand in contact with saliva.

Consequently, ingesting it is a significant risk because it can cause airway obstruction. This means that it can impede breathing. On the other hand, if it is inhaled in small amounts, not enough to cause obstruction, it is irritating to breathing due to its drying properties.

Sodium polyacrylate is increasingly being used in medical and geriatric wards, placed in urine bottles next to patients, making it a risk that confused elderly patients may ingest it.

For this reason, these types of patients must be continuously monitored.

It is not irritating to the skin. It is not flammable.

References

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