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Rubber Band Experiment
Name: Jane M.
Status: student
Age: 17
Location: N/A
Country: N/A
Date: 3/22/2004
Question:
I am doing a physics investigation into different effects
on elasticity of a rubber band. I have carried out 3 different methods
for ageing of rubber bands
1) Boil rubber bands in boiling water for 15/30/45/60 minutes
2) Freezing in ice for 1 day and 10days
3) Soak rubber bands in Motor Oil for 3 - 9 days
So far I can see that after boiling the rubber bands.. it becomes easier
(takes less weight) to stretch the rubber band 4 times its original
length. I have calculated the work done by counting the no of boxes under
the load extension graph and see that work done decreases as the boiling
time increases.. My questions : is the result correct according to the
theory? And what is the theory to explain this result I got?
For the freezing bit.. I could not actually see any effect on the work
done. My questions: What does the effect suppose to be? What is the
theory beyond this? Please suggest me how i should improve my experiment to make
it works.
Replies:
Motor Oil: from the experiment, I could see that the work done decreases
as the number of days soaking increases. (same effect as boiling but a
little bit more dramatic). Also, after soaking, I could see that the
length of the rubber bands have increased.. they became longer than when
first soaked them. My questions: Are my results correct? And i
desperately want to know the theory to explain why motor oil has such
effect on rubber band elasticity. I have searched everywhere, but I still
cannot find out why.
Finally, thank you very much for reading my questions..and thank you very
much for answering them.
I am looking forward to hearing from you
Replies:
1. Boiling can produce two effects. The first is to remove some additive
that may have been added to the rubber in manufacture to control its
elasticity. The second in the breaking of the ---S-/-S-- sulfur to sulfur
bonds that cross link the rubber.
2. Freezing should have no effect I can think of.
3. Motor oil may be absorbed by the rubber band and cause it to swell and be
lubricated. That should increase its elasticity (less weight required).
Vince Calder
Dear Jane,
I do not know if there is a "theory" about aging rubber bands, but I thought
of an additional experiment; try exposing the rubber bands to lots of sun,
or leave them outdoors. I noticed any rubber bands I left out tended to
degrade sooner than rubber bands in the house. It might be worth a trial or
two.
Good Luck
Martha Croll
Jane-
A very cool experiment. Believe your results.
2) why do you think that freezing a rubber band in water should degrade it?
Of course it is an important thing to try.
But maybe you have just discovered that freezing _does_not_ significantly degrade your
rubber-bands?
This would be a nice thing to discover. I think I will freeze my stock so they get
crumbly and useless every few years.
Freezing usually damages things by:
a) water penetrates it, then crystals form and grow, cracking or cutting the substance
around them.
b) the limber substance becomes stiffer when cold, then cracks instead of stretching
when it is pushed around by anything.
I think you have discovered that neither mechanism happens to your rubber-bands.
You might think about why not, and about what it would take to check.
1) Rubber is long, zig-zaggy string-molecule, which is also called a polymer.
(Zig-zaggy-ness helps for being stretchy, but not necessarily for the reasons you
would think.)
Pure molecular stringiness is not enough to be solid, for many such oil-like
substances.
A pure long-string molecule often makes a goopy, sticky, viscous liquid.
To be solid requires a little "cross-linking" between strings, so the strings cannot
ever slip all the way past each other.
(That is what "vulcanization" does, add cross-links.)
I am not sure, but when you boil the rubber, maybe you are breaking a percentage of
the cross-links that the manufacturer made the rubber with.
Then the strings can slip farther before the last molecular strands stretch taught and
stop it.
Then you perceive easier stretching and lower breaking strength.
3) The strength and stretch-resistance you feel is proportional to the density of
cross-links, multiplied by some fudge-factor due to the kinkiness or tangled-ness of the
basic string molecule. There is a class of chemicals called "plasticizers" which
are just liquids which can dissolve into your solid, lubricating the molecules so they
slip past each there easier. Rubber is a hydrocarbon, so any hydrocarbon liquid dissolves
into it well, and will be a either a plasticizer or dissolver. The oils
which cannot finish dissolving rubber will all be plasticizers for your rubber-bands.
Motor oil has a bunch of things in it, some of which will be strong solvents and
plasticizers. For a more limited effect, you might try drug-store mineral oil, or
"pure, non-smoky" candle oil.
Weigh your rubber-bands after you do these things to them. For boiling or freezing,
they will weigh the same or less. For oil-soaking, they will weigh more, because
there is oil stuck in the rubber now. 10% more by weight is a large number, 1% is
small.
The more plasticizer in your rubber, the stretchier and weaker and tackier it will be.
Some rubbers are never soft unless they add plasticizer.
In principle, a cross-linked polymer can never be completely dissolved by a liquid, not
without breaking molecules.
Some densely cross-linked plastics never dissolve in any liquid or melt at any
temperature. When heated, they just get a little softer then burn or char.
Sometimes in practice, some cross-links just break in a strong solvent, perhaps because
they were not too strong to begin with, perhaps because once the solid is a pasty glob,
all the forces pulling the solid apart focus on them. Or perhaps because we
aren't being critical about the difference between a pure, clear liquid and an opaque
pasty glob, which is a mix of solid and liquid.
1), theory #2) maybe the heat and water is helping the rubber molecules permanently
un-tangle themselves to the maximum extent possible without breaking cross-links. Not
sure yet how to measure the difference.
It makes a nice big picture that all fits together-
Jim Swenson
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