Latest posts by Dr Beth Cope (see all)
- Teaching Resources Website—explore the resources and join the conversation - 15th July 2014
- Lightweight aluminum beryllium alloys - 30th October 2012
- The Not-So-Silly Science of ‘Silly Putty’ - 14th September 2012
A Silly Mistake?
No one set out to develop Silly Putty: it was a novelty by-product during research aiming at new silicone elastomers to replace scarce rubber. In 1943, an engineer at General Electric, James Wright, was working in the New Haven laboratory when he accidentally dropped boric acid into silicone oil: the result was a bouncing silicone putty. The new material stretched more than rubber, even at high temperatures, but it also had some more interesting and unusual properties. Over long timescales or at high temperatures it flowed like a fluid. But at shorter timescales it bounced and behaved like an elastic solid. GE started marketing it, but it only really took off when the novelty value of this new material caught the attention of Peter Hodgson, a marketing consultant. He bought the rights from GE, and started marketing his ‘solid liquid’ as ‘Silly Putty®’.
The Not-So-Silly Science
Silly Putty’s main ingredient is polydimethylsiloxane, PDMS, a silicone polymer containing a -Si-O-Si-O- backbone that terminates in Si-OH groups. The chain ends are crosslinked b
y hydrogen bonds formed through the addition of boric acid, B(OH)3. These crosslinks hold together the flexible polymer chains (made up of stronger covalent bonds), which can coil and uncoil. However, the weak hydrogen bonds in the crosslinks make Silly Putty a viscoelastic material: it can flow as well as bounce. When the putty is pulled slowly apart, e.g., under gravity, some of these weak bonds break and reform, allowing viscous flow. But under application of very rapid stress, likebouncing it against a wall, the hydrogen bonds in the polymer don’t have time to break and re-form; therefore the deformation is elastic. Beyond the elastic limit, quickly applying avery high stress causes the backbone covalent bonds to break: then the putty snaps cleanly apart.
Playing With Silly Putty: Not-So-Silly A Proposition In Materials Education
Speaking at the 3rd North American Materials Education Symposium earlier this year, Dr John Nychka, from the Department of Chemical and Materials Engineering, University of Alberta, Canada, said: “Silly Putty is my favorite material—of all time. I routinely take 5lbs of Silly Putty to my lectures. Many materials science and engineering concepts can be visualized with Silly Putty—many more than you may even think: grains, grain boundaries, grain shape changes during plastic deformation, stress states, creep, ductile to brittle transitions, stress concentrations and fracture mechanics, impact failure, pearlite formation, crystal structures, atomic bonding, viscoelasticity, composites, strain rate dependent properties, hardness, thermal shock, and yes, even more!”
Perhaps it is no surprise that Dr Nychka has recently been awarded the 2012 Engineers Canada Medal for distinction in engineering education.
Some Silly and Not-So-Silly Science Experiments
Melting Models: mold it into shapes and put it on display: eventually it will start to ooze (the hydrogen bonds have time to break and reform, so it flows over long timescales). Try leaving it on the back of your hand, and watch it flow over your fingers.
Bouncing Balls: roll it up and bounce it; it can have a rebound of 80%, depending on temperature (the hydrogen bonds do not have time to break and re-form, so deformation is elastic). In fact, it still bounces even it isn’t in rolled into a ball.
Bobbing Boats & Balls: Silly Putty normally sinks (the specific gravity is 1.14, denser than water), but when shaped like a boat it will float. To make balls of Silly Putty float, roll it into tiny spheres and drop them into a glass of water containing little vinegar and baking soda. (The reaction produces bubbles of carbon dioxide gas, which stick to the spheres of putty and cause them to float. As the gas bubbles fall off, the putty will sink.)
No-more Newsprint Copying: go back a generation, and kids delighted in using Silly Putty to copy newsprint. However, the switch from petroleum-based dyes (whose pigment-oil suspension was readily adsorbed by Silly Putty) to cheaper soy-based dyes means this no longer works so well.
Stretchy String:pull the Putty gently, and you’ll get long strips of string-like putty (as you uncoil those polymer chains, breaking and reforming the hydrogen bonds. Keep pulling and you’ll break apart the cross-links and split the material between the polymer chains).
Silly Putty Races: we were excited to hear from Eurocopter about their Silly Putty Races. See if you can spot the weekends on this snapshot… (When the temperature in the building dropped, it slowed the flow…)
The 2012 Materials Education Symposia were some of the world’s largest events focused on undergraduate materials related education. Next year’s events will again include presentations, workshops, discussion sessions, and extended poster sessions. www.materials-education.com