Guide to Sodium Polyacrylate 2022 (Uses, Safety Research)

December 13, 2022

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What’s Sodium Polyacrylate?

Sodium polyacrylate is a kind of super absorbent polymer (also called SAP).

It is a functional polymer used in a variety of common products such as paper diapers, pets pads, water-retaining material (to help the soil retain water), instant snow, and so on. It is known for its superior absorbency:

Sodium polyacrylate can absorb hundreds of times its own weight in water. It starts out as a powder and as it comes into contact with moisture, it swells into its gel form.

Unlike other absorbent materials, it’s not easy to squeeze the moisture out of this gel. This is what makes it perfect for use in paper diapers — your baby can sit on it, roll around, and sleep for hours in a wet diaper without leaks.


Basis Info

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.


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. )


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.

Why is the water absorption of sodium polyacrylate related to the brine concentration of the liquid?

After a careful study of the entire saline absorption process of sodium polyacrylate, we will find:

Sodium polyacrylate polymer itself is a semi-permeable membrane. The semi-permeable membrane only allows water molecules to pass through, and the semi-permeable membrane wraps sodium chloride to form small spheres. Outside the membrane is a low concentration. When the sodium polyacrylate polymer comes into contact with water, because the concentration of the solution inside and outside the small ball is very different, a large amount of water penetrates from the low concentration solution outside the small ball into the high concentration solution inside the small ball, until the inner and outer solutions are The concentrations are equal. This is called isotonicity.

Based on the principle of osmosis, the amount of water absorbed by a water-absorbing polymer depends on the type of external solution. If it is distilled water, it can absorb 800 times the weight of the polymer itself. If it is tap water, because it contains more ions, it can only absorb 300 times its own weight.


Production Process

In copolymer chemistry, there’re 3 ways to produce sodium polyacrylate: 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.

ps: there is one kind of sodium polyacrylate produced by starch grafting. The manufacturer claims that it is a “natural product” as a gimmick.


How does it work?

You may have been wondering how sodium polyacrylate work. In fact, the entire water absorption process has undergone complex physical and chemical reactions. (It is generally believed that physical reactions account for most of the effects.)

Simply speaking, there are network molecular chains in the molecular structure of sodium polyacrylate. 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.

Details

Sodium polyacrylate is white granular, is a water-absorbing polymer, and the main chain contains -COONa. When water intervenes, it dissociates into -COO- and Na+. Due to the mutual repulsion between the -COO- anions, the originally curved chain expands into a straight chain.

How does sodium polyacrylate work

Since the hydrogen atom in the H2O (H-O-H) molecule is slightly positively charged, and the oxygen atom is slightly negatively charged, the attraction between the slightly positively charged hydrogen atom and the negatively charged -COO- produces hydrogen bonds, so the polymer Sodium acrylate can absorb water.

But sodium polyacrylate is not dissolved in water but forms a colloidal substance. The reason is that when the degree of cross-linking is high, water cannot enter the polymer, and its water absorption capacity is reduced. On the other hand, in the case of a low degree of crosslinking, water molecules easily penetrate into the polymer, and the resin swells, further hydrophilic and gelatinized, and becomes a state of high water absorption.

Tip: When the crosslink density is too low, it may dissolve in water.


Safety Research

Is sodium polyacrylate safe?

Yes. 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 totally safe.

We can also know more through the MSDS (Material Safety Data Sheet) documents like this:

Sodium Polyacrylate Material Safety Data Sheet


How about touching the skin directly?

Sodium polyacrylate itself is not irritating to the skin. As a polymer, it sticks together in long chains that are way too large to be absorbed through the skin.

But some kind of sodium polyacrylate is mixed up with small amounts of acrylic acid, a leftover from the manufacturing process.

In theory, acrylic acid in large doses could be harmful to a baby’s skin. But according to a 2009 report in the Journal of Toxicology and Environmental Health, there isn’t nearly enough acrylic acid in disposable diapers to raise concern. (The study was funded by Procter & Gamble, a major manufacturer of diapers.) Another side, sodium polyacrylate suppliers should test the acrylic acid value and make sure it is less than 300 ppm (part per million).


Food-grade sodium polyacrylate

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.

*ps: this content is updated in 2021.

Is sodium polyacrylate toxic to dogs/animals/insects?

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


Conclusion

Sodium polyacrylate is safe — non-toxic and free from any major safety risks.

But, it can pose certain dangers if not handled properly. But please be sure to take notes of the following hazards and take precautions to avoid any injury or mishaps when handling sodium polyacrylate.

  • If the powder is inhaled, it can irritate the lungs–but that’s not generally a concern.
  • When it comes in contact with a large amount of spilled water in an area, it can cause the area to become very slippery.
  • If it enters sewer or drainage systems in large quantities, it can cause serious clogging and should be dealt with immediately.

FAQs

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.

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


Can sodium polyacrylate be reused?

At this point, the properties of sodium polyacrylate are different.

Ordinary sodium polyacrylate cannot be reused. For example, sodium polyacrylate used in diapers will be discarded after absorbing urine.

For the sodium polyacrylate used in some special purposes such as water-absorbing expansion flood control bags, the manufacturer will carry out special post-processing. The sodium polyacrylate after these special treatments can become solid particles after a certain period of time after absorbing water. State, can repeatedly absorb moisture.


Is sodium polyacrylate and water a chemical reaction?

It is generally believed that physical reactions account for most of the effects.


Is sodium polyacrylate biodegradable?

Sodium polyacrylate is not a natural substance, but a macromolecular structure of organic synthesis, so it is difficult to be biodegraded in natural soil.

But it is worth mentioning that sodium polyacrylate can be decomposed naturally in the soil.

We can confirm this conclusion through a very simple experiment. For example, the gel after absorbing water with sodium polyacrylate will melt under the sunlight for a week.

In addition, in some countries, diaper manufacturers have begun to research and produce sodium polyacrylate types that can be naturally degraded by biotechnology. The principle is that the process of sodium polyacrylate production is grafted with starch or other substances so that it can be decomposed by living microorganisms.


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 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?

Sodium polyacrylate is not toxic. In the process of synthesizing sodium polyacrylate, the content of acrylic acid can be strictly controlled and is not harmful to the human body. 

In fact, sodium polyacrylate has been widely used as an important water-absorbing component of baby diapers.

Although sodium polyacrylate is harmless to the skin, you should see a doctor as soon as possible after your baby or your dog ingests it. This is because it swells so fast that it can cause gastrointestinal discomfort after accidental ingestion.


Where to find/get sodium polyacrylate at home?

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


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