MadSci Network: Physics

Re: Why does heated rubber bands stretch the most?

Date: Thu Dec 5 01:06:58 2002
Posted By: James Griepenburg, , Chemical consultant, Chemmet Services
Area of science: Physics
ID: 1022440599.Ph


We tried stretching [by hand] various types of rubber bands and found that 
room T, freezer chilled, and hot water heated bands stretched almost the 
same amount and that the differences were more dependent on the person 
stretching the bands than on the temperature.  This is a very hard 
experiment to quantify and do reproducibly. Also if you stretch a rubber 
band and hold it against your cheek you will notice it got warm. This 
means that it is difficult to do the experiment at a constant temperature
[isothermally is the big word to describe such conditions].  If you 
stretch a rubber band quickly, thereby increasing its energy content and 
causing changes in molecular orientations, any heat produced does not 
equilibrate with the surroundings[this is called an adiabatic process] and 
the temperature changes.  Therefore to study the stretching one would have 
to use a device which measures force as a function of displacement [The 
Instron Corp. makes such machines] The process has to be carried out at a 
slow enough speed to maintain temperature equilibrium with the 
surroundings. I can't answer your question without quantitative 
reproducible data.

Also the chemical handbooks show rubbers to have a positive coeffecient of 
expansion, you heat them they expand.  Most rubber materials contain oil-
like plasticizers which can leach out at higher temperatures causing an 
apparent shrinkage and certain materials will crosslink[vulcanize] on 
heating again causing contraction. Natural rubber softens on heating and 
probably does stretch more and recover less at a higher temperature. I 
don't think many things are made of totally uncrosslinked natural rubber 
tho.  Again this can get complicated and the material has to be studied 

Mechanical properties of rubber materials are notoriously difficult to 
measure accurately.  A given material usually has a range of acceptable 
values rather than a definite value of its various properties.  You picked 
interesting but difficult material to study.   

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