Play Putty Experiment

Play Putty Experiment


Play Putty Problem
by Mark J. Anderson, Stat-Ease, Inc.
(e-mail: mark@statease.com)

(This article is greatly expanded from a document contributed by Paul N. Sheldon of Honeywell International as an exercise for learning design of experiments (DOE).)

Description:
Mix ordinary white glue (Elmer's®) and a cross-linking agent: borax (20 MULE TEAM® brand from your local grocery store). Eureka! You've made play putty. To make things more interesting, add laundry starch (STA-FLO® concentrated liquid) to the mixture. See how well you can do with this home-made material in comparison to the real thing sold commercially as a toy: Silly Putty® (see note below).

Procedure:
These instructions leave plenty of room to be creative! Plan on taking at least two hours to complete the project. Have fun!

1. Brainstorm a list of desirable properties and how to quantify them. (Hints: consider bounciness (percent rebound), elongation (percent stretch) and possibly surface sheen and tackiness (ability to pick up printed copy from newspaper).)
2. Select at least two properties and decide exactly how you will measure them. Do your best with materials at hand. At the very least, establish ratings on a 1 to 9 scale, with 1 being worst :(, 9 being best :) and 5 so-so :|.
3. With the aid of Design-Expert® software, plan a mixture experiment. Here's a specific recipe modified from www.elmers.com:
a. Cover your work area with newspaper or paper towels.
b. In one bowl, combine 50 milliliters (ml) of glue and 25 ml cold water. Stir with a plastic spoon.
c. In another bowl combine 2.5 ml (1/2 teaspoon) of borax and 25 ml hot water. Mix with a straw.
d. Using the output from Design-Expert software for the recipe, add the borax/water (and/or the liquid starch) to the glue mixture. Stir with the spoon. Then knead it by hand when it forms a glob that's too stiff for the spoon. Lift the glob from the bowl and allow the moisture to drain off. You've got play putty!
e. Add food coloring if you wish (wear rubber gloves unless you want your hands the same color!). Let it dry for about an hour. It should become smooth and rubber like. Store the putty in an airtight container (such as a Ziploc® bag). It should last about 2 weeks before drying out.
4. Do the experiment and analyze the results.
5. Find the most desirable composition.

Other Things to Try:
· Does cooling improve the "bounceability"? If cooled too much, in a freezer for example, will the putty shatter when bounced?
· How does your putty deform with time? (Suggestion: Take a blob, roll it in a ball and stick it to the side of a metal cabinet or refrigerator. See how far it flows as days go by.) Does temperature affect the deformation?
· What happens if you hit a ball of putty with a heavy object, such as a hard-cover book? (Hint: Putty behaves as a "dilatant," which means it reacts differently to hard, fast pressure than it does to slow, even pressure. As the Zen proverb says: "Only when you can be extremely pliable and soft can you be extremely hard and strong.")"

Consider a second set of mixture experiments that incorporates a gas-producing reaction to reduce density of the putty. This can be done by adding a few spoonfuls of baking soda (Arm & Hammer®) to the borax solution and vinegar to the glue on a 1:1 basis. What will this do to the physical properties of the putty? Will it be putty, or something else altogether?

Scientific Details:
Play putty involves a reaction between polyvinylacetate in the white glue and borax (Na2B4O7-10H2O) to form a highly-flexible, cross-linked polymer. Two glue molecules (the monomer) become cross-linked by a borax molecule via reaction of alcohol end groups. Acetic acid is generated as the byproduct - two molecules per cross-link. Many borax cross-links occur, thus "glomming" together many polymer molecules to form a pliable putty material.

Adding commercially-produced starch in liquid form creates complications in the chemistry. For example, the STA-FLO brand lists not only water and corn starch as ingredients, but also (in order of concentration?) borax, "processing aids," preservative, "ironing aid" and perfume. As noted above, borax acts as a cross-linker, but the starch itself acts as a co-polymer or perhaps in some other way to modify the properties of the putty. (Sorry, I am a chemical engineer, not a chemist! Mark.)

Baking soda is sodium bicarbonate (NaHCO3) - a base. The vinegar, a weak form of acetic acid (CH3COOH), reacts with the baking soda to produce water and carbon dioxide (the gas).

The Story of Silly Putty:
During World War II, while looking for a cheap substitute for rubber, an engineer for General Electric, James Wright, accidentally developed Silly Putty®, now a famous toy.* Silly Putty is an organosiloxane polymer made from silicone oil and boric acid. Unlike home-made "play putty" it will not dry out, because it's not water based. Silly Putty has flexible molecules that, when 'smooshed' by fingers, slide over each other and cause the material to flow. For years, prominent physicists have pondered over the super bouncing properties of Silly Putty, which rebounds 80% or more. (For more fascinating facts on this incredible material, see www.sillyputty.com. They offer a short, but riveting, movie on how it's manufactured at http://www.sillyputty.com/silly_science/making.htm.)

According to the makers of Silly Putty and sources at Alfred University (Alfred, NY), Silly Putty has been put to many experiments in the past. Don't try these at home!

· It's flammable and when lit, the flame is a very bright white. Though it burns slowly, the left over ash from the putty crumbles very easily.
· When Silly Putty is microwaved in a drinking glass for about 3 minutes, it becomes very sticky. However, when cooled the putty returns to the same state it was in before.
· When it's baked at 450 degrees F for 15 minutes, it gives off very bad fumes.
· In 1989 a graduate student at Alfred University dropped a 100-pound ball of Silly Putty from the roof of the Engineering Building. It bounced about 8 feet into the air, returned to Earth, and shattered on the second impact.
· On Apollo 8, the astronauts used it to stabilize their tools in zero gravity.

One thing you should try at home, or better yet - the office, is using Silly Putty (or the like) as a stress reliever. According to the Wall Street Journal (see reference in footnote) "a nice, chunky handful, massaged and stretched and squeezed, is the perfect workplace stress-reliever. Some say ricocheting it off their office walls helps them think. Others spend hours sculpting characters, shapes and animals."

*An alternate story in the September 10, 2002 Wall Street Journal (WSJ) gives credit to a Dow Corning scientist, Earl Warrick, now 91 years old, for accidentally inventing this nontoxic substance while searching for a silicone-based rubber substitute during World War II. Dow Corning, who specializes in silicones, still makes the material under the brand-name "3179 Dilatant Compound." It's available only in 50-pound quantities. (For ideas on how to order the stuff in lesser quantities, but more than what's normally sold in stores, see http://www.hart.com/putty/ or http://www.puttyworld.com/.). According to WSJ, Dow-Corning sells most of its 100,000-pound annual production of dilatant compound to Crayola maker, Binney & Smith Inc., in Easton, PA, which processes it and sells it under its Silly Putty brand. (See the article Wall Street Journal article, entitled "Some Grown-Ups Go Silly for Buying Putty in Bulk," at http://online.wsj.com/article_email/0,,SB1031611692226887235,00.html).