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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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