Time Required
About 5 class periods

Advance Preparation
You will need to collect or purchase several samples each of three different brands of diapers. (The brands will be determined by the class survey results.) Each team of students will need one diaper of each brand. In addition, each team will need a "super-absorbent" diaper of any brand from which it will remove the powdered white polymer in the first session.

To extract the polymer, students will remove the plastic layer from one side of their diapers, place them in a plastic bag, and shake the bag until at least one teaspoon of the powdered polymer collects at the bottom of the bag. They can extract additional powder by teasing the fibers in the diapers with their fingers.

Student teams will need two large plastic cups—one empty and one full of lukewarm distilled water. They will also need a stirring stick, a spoonful of table salt, a plastic bag, a paper towel, and a pair of scissors. Students should cover their workstations with newspaper before beginning to facilitate cleanup.

Materials
disposable diapers (various brands)
distilled water
large plastic cups
newspaper
paper towels
plastic bags
plastic spoons
scissors
stirring sticks
table salt

Safety Information
The polymer inside the diapers (sodium polyacrylate) is not toxic. However, it may cause irritation if it gets into students’ eyes, nose, or mouth as it will absorb water and dry out those tissues. Students should wipe their hands off with paper towels after touching the polymer. They should wash them thoroughly with soap and water at the end of the activity.

Teaching Strategies
Disposable diapers have largely replaced traditional cloth diapers as they are both more convenient and more reliable. While traditional diapers employ only cotton fibers to hold in moisture, the fibers of disposable diapers contain super-absorptive polymers—large, often artificially engineered molecules made up of smaller repeating units called monomers.

When these polymers combine with water, they undergo a dramatic chemical reaction, forming a gel which traps the water in place. Polymers can absorb several hundred times their own weight instantaneously. The distribution of these polymers within the fibers allows the disposable diaper to hold more water than a cloth diaper.

While disposable diapers enjoy a longer wearing time than cloth diapers, they cannot hold moisture forever. This is because urine salts that accumulate after several wettings begin to break down the gelled polymer, allowing bound water to be released. Students can feel this breakdown by adding salt to their polymer gel and then rubbing the gel between their fingers.

In the final portion of the activity, students are asked to design a test to determine which of three diaper brands holds the most water. One way to do this is to cut a small hole in the plastic layer on the back side of the diaper and pour water into the inside. Water not absorbed by the fibers/polymers will drip through the hole. The diaper that drips last is the most absorbent. As students will discover, more expensive brand-name diapers are not always more absorptive than cheaper bargain-brand diapers, although they may offer other advantages such as a more comfortable design or better fit.

Extensions
Have students research the role of osmosis in the absorption of water by the polymers.

Have students research other industries which make use of super-absorptive polymers.

Have you ever wondered how disposable diapers can hold so much baby pee? What’s in them anyway? A giant sponge? A billion cotton balls? Let’s open one up and look inside. Don’t worry, we’ll give you a clean one.

Problem
Why do disposable diapers hold more liquid than cloth diapers?

___<There are intertwined fibers and some white powder.>___
________________________________________________
________________________________________________

3.  Place the diaper in a plastic bag and shake it until you see white powder collecting at the bottom of the bag. Use your fingers to pull the fibers apart and loosen the powder. Stop when you have about a teaspoon of powder in your bag.

4.  Carefully pour the powder into a large plastic cup. Examine it closely, but do not allow it to get in your eyes, nose, or mouth. What do you think its function is?

________________<Answers will vary.>_________________
_________________________________________________

5.  To find out what the powder does, pour a small amount of water into the cup and stir it with your stirring stick. Now try to pour the water out onto the newspaper. What happens?

_________<The water does not pour out. The powder plus the water
have formed a gel.>___________________________________
__________________________________________________

6.  Add more water to the cup. Keep adding small amounts of water at a time until you can no longer invert your cup without spilling some of the liquid inside. How much water could one teaspoon of the powder hold? Are you surprised by what you saw?

_________<Answers will vary, but the powder can hold a
startling amount of water.>____________________________

7.  Rub a little of the material in your cup between your fingers. How would you describe it? (Wipe your hands off on a paper towel after you have finished touching the material.)

9.  So, the white powder in disposable diapers allows the diaper to hold more water. But wait a minute…. Baby pee is not pure water! It actually contains small amounts of dissolved salts. After several hours of accumulation, a good quantity of salts can build up inside the diaper.

10.  Your teacher will give you a small amount of salt in a spoon. Pour the salt a little bit at a time into the cup, stirring after each addition. Stop when you notice a change. What do you observe?

________<Water seems to be coming out of the gel.>_______

11.  Rub a bit of the material between your fingers as you did before. Do you notice a difference? (Don’t forget to wipe your hands off on a paper towel again after handling the material.)

13.  Do you think all diapers hold the same amount of liquid? Conduct a survey of some parents you know. Ask them which brand of diaper they prefer and which variety of that brand (for example, Overnight, Super-absorbency, and so on). Collect your data in the chart below.

14.  Pool your data with the rest of the class. Your teacher will come up with a list of the three most popular diaper brands/varieties.

15.  Your teacher will now give you one of each of these three kinds of diapers. Design a test to determine which of the three holds the most water. Describe how your test will work in the space below.

_______<Answers will vary.>___________________________
__________________________________________________

16.  Perform your test. Record the results in the chart below.

 17.  Share your results with the people you surveyed. If they chose a more expensive brand that turned out not to be the most absorbent, can they give you other reasons for buying those diapers? What are they?

______<The diapers may fit their baby better. Or maybe they
just associate the brand name with quality.>_______________
________________________________________________