Dr. Skip's Corner: K-12 Teaching Adventures @ OSU Engineering
March 2004

Disposable Diapers and SAP Activity

Jason Hower and Skip Rochefort, Chemical Engineering Department
Ellen Momsen, COE Women and Minorities Program

Background
Over 20 billion disposable diapers are sent to landfills each year. (1) This is enough diapers to cover a football field 3 miles deep. Within the last 10 years the average age to potty train children has increased to 3 years old. (2) There is a reason that so many homes use so many disposable diapers; they work so well!

Objective
Our goal is to discover why these disposable diapers work so well and what makes them different from old cloth diapers.

Materials
In order to complete this investigation you will need the following supplies for each group:

  • One old cloth diaper or piece of cotton cloth
  • One disposable diaper
  • One Ziploc bag, quart size
  • Two small containers, like Petri dish halves
  • One syringe or pipette or graduated cylinder
  • Water, roughly 500 mL per group

Procedure

  1. Begin by adding water to the cloth diaper. Note how the diaper feels after the water has been added. Note also what happens when the diaper is squeezed. List any design improvements you would make on the cloth diaper.
  2. Examine your disposable diaper. Think about form and function, why and how does it work? Tear into the diaper and investigate each part. Make sure to note ALL the parts. Does the cotton feel like normal cotton? As you examine your diaper, create a cross section schematic of the important parts. Figure 1 is an example of a cross section schematic.
Figure 1. Disposable Diaper Schematic
  1. Remove the cotton batting from the diaper and place it in the plastic bag. Break up the batting in the bag and separate the grainy crystals from the fluffy cotton. After breaking up the cotton some white crystals, they look like salt, will collect in the bottom of the bag. Remove the cotton and set aside for later use.
  2. Pour a small amount of the crystals into the plastic dish and record the mass. The crystals are called Super Absorbent Polymer, SAP for short. They are the gold in the disposable diaper. SAP is like small gel beads that have all the water removed. When water is reintroduced, the crystals suck the water in and begin to grow into a gel.
  3. In small increments, roughly 10 mL, add water to the SAP in the plastic dish. Note how the crystals change into a gel. What does the SAP resemble now? How does it feel? If you squeeze the SAP, does the water come out? Flip the dish over, does the gel fall?
  4. Continue adding water in small increments until the gel swells to large and falls from the plastic container. Record how much water you added. Using the original mass of SAP and the volume of water added, determine how many times it own weight, SAP can absorb in water. Your answer should be at least 100 times.
  5. After experimenting with the SAP gel, dispose of it in a trash can. The experiment can be repeated using the SAP plus cotton from the diaper.

Discussion
After completing this experiment, and seeing the amazing properties of the SAP, it is clear why kids want to be in diapers longer. They feel dry right away thanks to the wicking polypropylene layer. The diaper can hold over 10 pees thanks to the SAP. Their clothes stay dry due to the polyethylene shell. Not only that, but when they pee, they create a nice gel pad to sit on or fall on with less pain. So, the result is, people are not going back to cloth diapers.

SAP is not only used in diapers. Today SAP is used in planting soil to keep overhead plants from dripping or to decrease the frequency of watering. SAP is used to prevent homes near wildfires from going up in smoke. SAP is even used as artificial snow in indoor ski hills in Japan.

If that 3 mile high pile concerns you, you should pursue a career in biodegradable disposable diapers. Chemical Engineering is the place for you! If the amazing properties of SAP interest you and you want to find other ways to use it, chemical engineering is for you!

References

  1. http://www.knowaste.com/diaper_facts.htm
  2. http://www.parentsplace.com/features/pottytrain/articles/0,,258706_115583,00.html