CBEE 101 First Year Orientation Fall 2008

ENGINEERING DESIGN: Disposable Diapers and Reverse Engineering

Jason Hower and Dr. Skip Rochefort

Chemical Engineering Department

Oregon State University

Background

Over 20 billion disposable diapers are sold and used each year. 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 from 2 years to 3 years old. There is a reason that so many homes use so many disposable diapers; they work so well!

But where do they all go? To your local landfill…which just keeps getting bigger and bigger each day!

Objective

Our goal is to discover why these disposable diapers work so well and what makes them different from the old cloth diapers. This is also a great lesson in the concept of reverse engineering, which is used by virtually every manufacturing company in the world, and especially the “big two” diaper manufacturers - Kimberly-Clark and Proctor & Gamble.

Materials

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

Cotton squares (1’ x 1”) typically used for face cleansing.

One disposable diaper (per group of four people)

One Ziploc bag, gallon size and Scissors

Two small containers, like Petri dish halves

One syringe, pipette or graduated cylinder (10 ml size)

Water (approx. 200 mL per group) and Salt Water (approx. 50 ml per group)

DISPOSABLE DIAPER REVERSE ENGINEERING

(should be done in groups of two people)

1.  Examine your disposable diaper. Think about form and function, why and how does it work? Cut the diaper in half down its length and give one-half of the diaper for each group of two people. As you examine your diaper, create a cross-sectional schematic diagram of the important parts (see Fig.1 as a basic example). Make sure to note ALL the parts. Does the cotton feel like normal cotton?

Figure 1. Schematic cross-sectional diagram of a baby diaper (very basic).

NOTE: This is an example of a common engineering practice called “reverse engineering”. By disassembling a product and making a schematic diagram with careful notes, engineers at company A can determine the design of a product by company B. For example, if you worked for Proctor and Gamble (manufacturers of Pampers diapers) you could cut into a Huggies diaper (manufactured by Kimberly-Clark) and see how engineers at Kimberly-Clark have designed their product, which just happens to be your biggest competitor.

2.  Remove the cotton batting from the diaper and place it in the plastic bag.

3.  Cut (or break-off) an approx. 1” x 1” square piece and put it aside for later use.

4.  Break-up the batting in the bag and separate the grainy crystals from the cotton. After breaking up the cotton some white crystals (they look like salt granules) will collect in the bottom of the bag. These are the “white gold”, Super Absorbant Polymer (SAP).

ABSORBANCY EXPERIMENTS

  1. Obtain a “cotton square” from the TA or Dr. Skip. This is the type of cotton typically used for cleansing your face. Place the cotton square in a petrie dish and add 10ml of DIW. Place the 1” square you recovered from your diaper in a petrie dish and add 10ml of DIW. This is an example of the difference between a cloth diaper (physical absorbancy) and the disposable diaper with SAP (chemical absorbancy). Make some observations, note the differences and arrive at an explanation for the difference between physical and chemical absorbancy.
  1. Weigh out approximately 0.25 grams of your recovered SAP into a petrie dish and record the exact mass. These crystals are called Super Absorbent Polymer (SAP). SAP is a dehydrated particle (powder) that will rapidly pick up water.
  1. In small increments, roughly 5 mL, add deionized water (DIW) to the SAP in the plastic dish. Note how the crystals rapidly absorb the water. Allow 30-60 sec. for the SAP to absorb the water. Continue adding water in small increments until the SAP appears to be “saturated”. The best test for test is what we call the “snail trail test”. Slowly tilt the petrie dish and watch to see if the SAP slides and leaves a slight liquid trail (like a snail moving) on the plastic dish. Once you see the “snail trail” wait about 60 sec. and then repeat the test. If it is gone, add a bit more water. When you see the “snail trail” repeatedly, you are done. This is a subjective determination, so have both partners agree when the experiment is complete!
  1. Record the volume of water that was absorbed by your SAP.

Calculate the Absorbancy Ratio (AR) = ml DIW/g SAP

  1. Repeat steps 2-4 using salt water with 0.5 g SAP.

Calculate AR = ml Salt Water/g SAP.

  1. REPEAT steps 2-4 for the commercial ChemSol SAP supplied by Dr. Skip.

7.  REPORT your DATA for DIW and SALT WATER to the lab TA or Dr. Skip. This data will be later shared with the class for an analysis of the average absorbancy ratio (AR) and experimental error (standard deviation).

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 stay dry and end up with a nice gel pad for softer landings!

SAP is not only used in diapers. Today SAP is used in a wide range of applications, such as: clean-up of all types of spills, including oil spills; as a fire retardant squirted on the roofs of homes to prevent destruction in wildfires; as artificial snow in indoor ski hills, predominantly in Japan; in garments (hats and bandanas) that can be soaked and worn to prevent overheating; and in planting soil to keep overhead plants from dripping or to decrease the frequency of watering; to prevent erosion of topsoil in some of the worlds leading food producing areas.