MadSci Network: Chemistry

Re: How do 'Shrinky Dinks' work?

Date: Sun Feb 28 18:13:23 1999
Posted By: Edward Peterson, Staff, Chemical Engineering, S&B Engineers and Constructors
Area of science: Chemistry
ID: 919973516.Ch

Dear AmeriCorp Student,

I am a chemist and a chemical engineer and have spent over ten years in 
research, and have never asked myself the question posed to you by your 
students.  I do not KNOW for certain how those toys work, but I have a 
good idea.  It is not so obvious as a chemical reaction or magic, but you 
can demonstrate the same principle to your student with a few easy to get 

Get a few good, sturdy normal rubber bands that do not stretch too far.  
Find a drinking glass with smooth outer walls and stretch a few rubber 
bands around the glass.  Two or three should do.  Get some dry ice at a 
grocery or convenience store.  Put the glass with the rubber bands 
stretched around the outside of the glass in an inexpensive drink cooler 
or ice chest.  Place the dry ice around the rubber bands.  Wait long 
enough for the glass to get as cool as the dry ice.  Quickly remove the 
glass from the ice chest (using goggles and insulated rubber gloves, of 
course)and remove the rubber bands from the glass (If they don't come off 
easily, you may have to sprey the glass with a spray cooking oil first).  
You will see that the rubber bands do not behave like rubber bands.  They 
do not shrink back to size when they are very cold.  They retain the 
circumference they had when wrapped around the glass.  When the rubber 
bands warm up again, they do act normally.  You can repeat this experiment 
again and again until the rubber bands break.

The reason that the rubber bands and the shrinky dinks behave this way is 
caused by the molecular structure forced on them when above the 
rubberization temperature.  At room temperature, rubber bands are above 
their rubberization temperature.  They stretch and return to their 
original shape right away.  However, when you stretch a rubber band then 
cool it to below the rubberization temperature, its molecules are locked 
in their "stretched" or forced structure.  The more complex and branched a 
polymer molecule is, the higher the rubberization temperature (on 
average.  Rubber bands have very linear structure so they have a low 
rubberization temperature.  As for the shrinky dinks, their structure is 
different enough from rubber bands that room temperature is below their 
rubberization temperature.  When they were made by a machine known as an 
extruder in sheet form, the sheet of rubbery polymer, while still hot, was 
pulled in the forward and cross direction.  While still in tension, the 
sheet of stretched polymer was cooled, retaining the stretched shape.  
When put into a warm oven, the stretched rubbery material returns to its 
unstretched size. Sometimes the polymer also contains trapped gas because 
it is foamed to add tension in another direction (Thickness).

I hope that helps and that the experiment works.  If dry ice is not cold 
enough for rubber bands, you may have to resort to liquid nitrogen if you 
can find some.  Sometimes research labs and hospitals will have some and 
let you have a little.

I hope this helps.

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