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Which of two superabsorbent polymers is the most absorbent? (page 2)

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Author: Mike Calhoun

Sodium polyacrylate is a superabsorbent polymer primarily used as an absorbent for water and aqueous solutions in baby diapers, adult incontinence products, feminine hygiene products, and similar applications.

A  polymer is a chainlike, high-molecular-weight natural or synthetic organic compound, whose structure is made up of repeating smaller units called monomers. Polyethylene, rubber, and cellulose, are examples of polymers.

Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration.

crystal or crystalline solid is a solid material whose atoms, molecules, or ions are arranged in an orderly, repeating geometric pattern.

This superabsorbent polymer compound has very long carbon chains bonded with sodium atoms in the center of the molecule in repeating units of -CH2-CH(COONa). When sodium polyacrylate is exposed to water, the higher concentration of water outside the polymer draws the water into the center of the molecule via osmosis. Sodium polyacrylate will continue to absorb water until there is an equal concentration of water inside and outside the polymer. This forms a polymeric gel many times larger in size than the original powdered compound.

Superabsorbent polymers are being considered for use to solidify liquid waste because the polymer can absorb many times it weight in water with less than 1% increase in waste volume. Avoiding an increase in the waste volume also means fewer loads of waste being shipped to landfills. In this project, figure out which polymer would be most absorbent.

Safety:

  • Sodium polycrylate is non-toxic! However, if it gets in your eyes, they will become dry and irritated. Inhaling the powder will also be irritating to nasal membranes. Dispose of excess powder and/or gelled material in the trash. Do not put it down the sink!
  • Do not ingest the hydrogel crystals or drink the water used with these crystals. These crystals are extremely slippery, so be careful around spillage. Never flush or pour these crystals down the drain, as crystal swelling could clog drainpipes.

Materials

  • Hydrogel crystals
  • Superabsorbent baby diapers
  • Wax paper
  • Scissors
  • 8 oz plastic glasses
  • Measuring cup
  • Plastic spoons
  • Distilled water
  • Colander or funnel
  • Digital camera
  • Box of table salt

Research Questions

  • What is a polymer?
  • What is a superabsorbent polymer?
  • How are hydrogel crystals different from sodium polyacrylate? How are they the similar?
  • How does the absorbency of the hydrogel compare with sodium polyacrylate?
  • In addition to its use in disposable diapers, what are some other uses for sodium polyacrylate?
  • Based on the results of the investigation which superabsorbent polymer would be best to use to solidify liquid waste?

Procedure

  1. Place a new super-absorbent diaper on a piece of wax paper, cut the diaper in half and pull out some of the fibers.
  2. Shake the fibers and pull them apart, allowing the powdery granules to fall onto the paper.
  3. Discard the fibers. The white granules that remain are sodium polyacrylate.
  4. Repeat steps 1-4 with another diaper until there is 1 tablespoon of the powder.
  5. On another sheet of wax paper, place 10 hydrogel crystals in a pile.
  6. Next to them pour an equal amount of sodium polyacrylate granules removed from the diapers.
  7. Carefully pour the two polymers into separate 8 oz plastic glasses or cups and label them.
  8. Using a measuring cup add 50 ml (or 6 oz) of distilled water to each glass.Stir the water in both glasses with plastic spoons.
  9. The sodium polyacrylate will start absorbing water and swelling, and the hydrogel crystals will begin to grow immediately.
  10. Observe and record what happens to the two substances after 30 minutes, 1 hour, and 1 ½ hours. 
  11. Allow the hydrogel crystals to grow to the point where they cannot be seen clearly inside the glass and allow the sodium polyacrylate to gel until it reaches its maximum expansion.
  12. Measure the unabsorbed water in each glass by poring the liquids into a measuring cup, using a strainer, colander, or funnel to catch the crystals and gel. 
  13. Record how much water is left after crystal and gel absorption by subtracting the amount left from 50 ml (6 ounces) and record in a table similar to the one shown below.

Name of Super-absorbent

Initial Amount    

Amount after absorbent addition   

Amount Absorbed

Sodium Polyacrylate

50 ml

 
 

Hydrogel Crystals

50 ml

 
 
 
  1. Using the data in the table plot a line or bar graph of amount of liquid absorbed by the two absorbents along the Y-axis verses their names along the X-axis.

Digging Deeper

As an extension to this project, take the unused sodium polyacrylate granules removed from the diaper and place them in a dish. Add water, and once again the polymer will start absorbing the water and swelling. Keep adding water until the granules stop swelling.

Next, add table salt to the gel. This addition will cause it to turn back to a liquid because salt has a higher concentration of sodium ions on the outside of the polyacrylate particles. The water is attracted to the salt, which causes the water to leave the polymer particles. The particles then collapse and become a liquid again. This process is called osmosis.

Follow the same procedure using the hydrogel crystals to see if the same thing happens when salt is added to these crystals. 

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