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Rubber and Heat

05/02/2017 5:19 AM

Here is a question.

You hang a weight from a rubber string. You heat up the rubber. What happens?

1. nothing

2. the rubber contracts

3. the rubber expands

Why would it be like you say it is?

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#1

Re: Rubber and heat

05/02/2017 6:12 AM

4. the rubber catches fire

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#16
In reply to #1

Re: Rubber and heat

05/03/2017 5:15 AM

This is not considered!

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#20
In reply to #16

Re: Rubber and heat

05/03/2017 6:14 AM

That's exactly why it needed to be added to the list: it wasn't being considered.

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#26
In reply to #20

Re: Rubber and heat

05/03/2017 9:57 AM

I can smell burned tyre!

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#2

Re: Rubber and heat

05/02/2017 6:49 AM

Depending on what type of rubber and how much its heated any one of the three can happen.

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#3

Re: Rubber and heat

05/02/2017 7:54 AM

It contracts. Watch this...

Interestingly enough, the effect is reversible. Stretching a rubber band will heat it up and releasing it will cool it down. This has to do with the entropy of the rubber.

When the rubber band is relaxed, the rubber molecules are tangled up (high entropy). When stretched, the molecules are aligned (lower entropy). Entropy is a measure of the heat flow from the environment.

http://www.physlink.com/education/askexperts/ae478.cfm

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#5
In reply to #3

Re: Rubber and heat

05/02/2017 9:53 AM

I watched the video of the bicycle wheel 'rubber band heat engine'.

I think that guy has deluded himself (I am reminded of the 'Crooke's Radiometer' bulb in which the vanes 'turn the wrong way').

The intense heat coming off those lamps creates a rising air current that exerts an upward force on the rubber bands on that side of the wheel. The effect of the rising heat is going to be overwhelmingly greater than any minimal effect of the rubber bands contracting and moving the center of gravity a minuscule amount.

I'd like to see that test repeated in a vacuum chamber.

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#11
In reply to #5

Re: Rubber and heat

05/02/2017 8:54 PM

Finding a large hard vacuum chamber with a convenient window for displaying the effect might not be easy.

It would be nice to have a counter-example to remove any doubts about the influence of convection/air currents.

If the heat source were directed at a portion of the spokes comprised of material that has a large positive linear coefficient of thermal expansion, and the wheel rotated the other direction (with the longer, lower tension, hot side falling) then the effects of convection/air currents could be discounted.

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#37
In reply to #11

Re: Rubber and heat

05/03/2017 2:52 PM

Could you indicate where on a straight shaft bicycle spoke the area where the mass is greater to take advantage of an improved level of coefficient thermal expansion ?

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#39
In reply to #37

Re: Rubber and heat

05/03/2017 4:00 PM

No, Tony, he meant that there would be a greater effect if the linear expansion coefficient were large (more massive). Although, I think it might roll about like an eccentric egg.

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#45
In reply to #37

Re: Rubber and heat

05/04/2017 10:39 AM

James is correct. I mean that if the effect makes the wheel turn in one direction when the coefficient is negative, demonstrating that it turns in the other direction when the coefficient is positive would help to substantiate that is what is causing the wheel to turn.

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#14
In reply to #5

Re: Rubber and heat

05/03/2017 4:52 AM

What is going to be proved by testing in a vacuum chamber? For all practical applications, it is only people who live in vacuums (bubbles), but all other daily motion is done in a positive atmosphere.

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#23
In reply to #5

Re: Rubber and heat

05/03/2017 8:56 AM

I think the whole idea of a rubber band engine is lost, since they did not simply wind up the rubber (tensioning), then let it unwind through the drive mechanism.

Another heat engine that does work by alternating hot and cold is the Nitinol engine.

I do not know the source material for the above image, but here they claim a 40 watt engine.

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#24
In reply to #23

Re: Rubber and heat

05/03/2017 9:33 AM

And the reference to the original question is?

Nickel titanium alloy versus rubber link, expansion or contraction? Interesting digression, like many threads here.

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#25
In reply to #24

Re: Rubber and heat

05/03/2017 9:44 AM

Something that actually works, rather than a child's toy that does not work. So Mr. Anonymous, why don't you grow some and step on up to the microphone, and say what you have to say in the open, rather than behind the mysterious curtain of doom?

Nitinol works by memory, and since it was formed to remember being a 10 # spring, and it relaxes in the heat, the wheel turns with considerable force. It all depends on the efficiency of quickly heating, then quickly cooling (so that the wheel might rotate a bit faster). Other Nitinol engines have been made (even by NASA engineers way back in the early days) that are not configured like the one I clipped for display.

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#29
In reply to #25

Re: Rubber and heat

05/03/2017 10:14 AM

Original question: 'You hang a weight from a rubber string. You heat up the rubber. What happens?"

I don't see the link to a nitinol engine, or power source. Perhaps further explanation is required from your standpoint to how this string became an 'engine' question.

This 'Merican English is dreadfully confusing. However, Mr. IQ explained the phenomenon rather well in a few posts, that make sense. Indothermic and exothermic process's can be interesting.

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#41
In reply to #5

Re: Rubber and heat

05/03/2017 8:23 PM

I see your point. I'm sure YouTube videos are inadmissible in a court of law. There is too great a chance of self-trickery (or plain old intentional trickery sometimes).

Perhaps a ruler from the hub to the rim on the heated side would show it shift inward when the rubber is heated, thus unbalancing the wheel and causing the rotation.

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#17
In reply to #3

Re: Rubber and heat

05/03/2017 5:20 AM

Correct!

You should get more good answers for this!

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#36
In reply to #3

Re: Rubber and heat

05/03/2017 2:46 PM

An airplane that flies by stretching a rubber band.

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#4

Re: Rubber and heat

05/02/2017 9:21 AM

I saw the thread title and was hoping it would be to do with something totally different.

Warm rubber, mmmmmm……. Someone had better chuck a bucket of cold water over me.

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#6

Re: Rubber and heat

05/02/2017 10:07 AM

RUBBER AND SILICONE LSR THERMAL EXPANSION

Materials commonly named "RUBBER" are HOT CURED or HIGH TEMPERATURE VULCANISATION ELASTOMERS (HCR or HTV); they expand under temperature increase influence.Their coefficients of linear expansion will directly be function of "loads" rates; they vary according to polymers.

The usual values are from 2 to 4 x 10E-4 (unity = ° KE-1).
For information, silicones we transform into very big quantities have values from 2 to 3 and often closer to 2.

Sample of HCR, which would see its temperature increasing 130 °C, and by ex from 20 °C to 150 °C, would see a average of its initial length L 20 ° becoming :
L 150° = L 20° x (1 + 3 x 0,0001 x delta T (130)) = L 20° x (1 + 0,039)
so 3,9 % increase!

http://www.chevalier-cleret.com/event/rubber-and-silicone-lsr-thermal-expansion.html

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#19
In reply to #6

Re: Rubber and heat

05/03/2017 6:00 AM

Kind of odd as Vulcanised cable joints in HV systems always collapse and compress with heating and form a uniform homogenous water tight seal to the cable sheath. I have yet to see a rubber vulcanised cable joint expand when completed, using heat.

Most rubber compounds (everyday use) are good for 120°C as an emergency temp, (short duration's of a few hours), but operate very well at 70-90°C for long duration's.

Simple test: stick an elastic band in the oven or microwave and watch it contract when heated.

If your formula was correct and applicable to rubber, in theory, car tyres would be falling off rims each time you took a long drive and air would be leaking out on the rim seal.

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#35
In reply to #19

Re: Rubber and heat

05/03/2017 12:02 PM

The difficulty is that each compound of "rubber" can have different properties.

I'm pretty certain that heat cured rubbers react differently to high temperatures than room temperature cure rubbers

Since some rubbers do have a coefficient of thermal expansion on their data sheets I would imagine that they do actually expand when heated.

However Natural rubber which is an isoprene and what most rubber bands are made of does shrink under heat

However Silicone Rubber is resistant (to an extent )

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#7

Re: Rubber and heat

05/02/2017 10:27 AM

It could end badly, especially the part about the smell of burning rubber, and the sound of a woman screaming.

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#8
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Re: Rubber and heat

05/02/2017 5:52 PM

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#9
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Re: Rubber and heat

05/02/2017 7:27 PM

Welcome to my typical day online with certain assholes people's posts.

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#10
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Re: Rubber and heat

05/02/2017 7:38 PM

I thought it was pretty funny actually.

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#12

Re: Rubber and Heat

05/03/2017 2:26 AM

I guess it kind of depends a bit on the rubber type as well. Silicone is not what I would call a rubber.

The answer is it contracts.

.

The reason I post maybe tomorrow.

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#13

Re: Rubber and Heat

05/03/2017 4:41 AM

Contracts on heating and expands on cooling.

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#15
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Re: Rubber and Heat

05/03/2017 5:14 AM

Why!

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#18
In reply to #15

Re: Rubber and Heat

05/03/2017 5:26 AM

Because it has been proved to be so and it is the inherent nature of rubber and taken we are talking rubber and not a mixed compound such as EPDM, Nitriles etc.

When atoms are warmer, they vibrate faster. Objects with a simple molecular structure will tend to expand when they’re heated because their vibrating atoms bump into each other harder. What happens to rubber is less straightforward because the atoms in molecules that comprise rubber have a more complex structure: they’re arranged a lot like links in long chains. When an atom in one of these chains gets bumped by another atom, the bumped atom puts a kink in its chain, shortening it. Imagine this process happening to billions of rubber molecules every second. Rubber is constantly contracting because of billions of tiny, vibrating “chains” putting kinks in each other, and it’s all because of heat!

When you added more heat to your system with your blow dryer, you enabled the rubber molecules to pull harder because you caused individual atoms to vibrate faster. When you rubbed the ice cube on your rubber band, you took a lot of thermal energy away, which caused the chains to loosen up and straighten out.

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#27
In reply to #18

Re: Rubber and Heat

05/03/2017 10:01 AM

Yepp, I have seen to late Rixters reply between the Youtube Vids.

Thanks for the explanation, saves me the lengthy effort.

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#38
In reply to #18

Re: Rubber and Heat

05/03/2017 3:21 PM

In your explanation, you say that the bumped atom creates a kink in the chain. This predisposes the idea that of two adjacent atoms, one is moving and one is stationary.

If heat is applied equally to the entire length of the chain then they should all be bumping and this bumping should both create kinks and decreate kinks simultaneously.

What is the controlling factor that allows some to bump whereas others do not ?

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#40
In reply to #38

Re: Rubber and Heat

05/03/2017 4:15 PM

I would not overthink this if I were you. Think more of the volume coefficient of expansion, and not so much of the linear one. The whole mass of rubber in a rubber band is that of entangled polymeric chains of essentially isoprene (bonded to the next isoprene by opening up the double bond).

As the chains approach macro length (no longer really on the molecular level), they are able to have a "random" chain orientation, as if someone threw a chain out on a floor, except the floor is in zero gravity, and the chain essentially goes everywhere. Now add millions more chains. Then billions and trillions more.

At some point we reach the microscopic level, and eventually we have the complete rubber band.

Now suppose we have a band that is cut into a rubber "string". It is laid out flat, but on a cold plate. Its length is X. We put the cold plate on a nearby hot plate (stove burner not at home), and as it warms, the rubber "string" changes length, width, and thickness, but we only measure the length, and it is X + dX (only heated a tiny amount, so that dX is also very tiny). What sign does dX have? It is negative. However, we find on further measurements that dY, and dZ turn out to be positive. Why? This is the polymer thread entanglement issue, playing against order since we turned up the entropy by heating. Oddly enough, even though dX is negative and decidedly so, dY and dZ are sufficiently large that dV >0, even though one of the dimensions apparently shrunk. That is a fun one to wrap the neurons around.

V=XYZ, dX = X(δX/δT), and so on, but dV ≠dXdYdZ, and some genius mathematician here can prove it.

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#43
In reply to #38

Re: Rubber and Heat

05/04/2017 2:27 AM

Only yourelf predisposed. You assumed something without reading further; Imagine this process happening to billions of rubber molecules every second. Rubber is constantly contracting because of billions of tiny, vibrating “chains” putting kinks in each other, and it’s all because of heat!

Where is one atom standing still as heat exists even at ambient temps. Heat is heat and rubber atoms move, none stand still as you have assumed.

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#28
In reply to #15

Re: Rubber and Heat

05/03/2017 10:13 AM

Dangle a rope so that it hangs straight down. Note the height of the free end above the floor. Now wiggle it hard. What happens? Is the free end farther from the floor?

What of the width of the space the rope occupies? Is it wider to accommodate the wiggle amplitude?

The same thing happens to the rubber band. The polymer chains making up the band are the same length as before, but now, at the higher temperature, that length is distributed over the wiggles, drawing the endpoints closer together.

By the same token, the band is wider at the higher temperature because the amplitude of the wiggles occupy more space laterally.

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#30
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Re: Rubber and Heat

05/03/2017 10:31 AM
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#42
In reply to #28

Re: Rubber and Heat

05/03/2017 11:58 PM

So the linear thermal coefficient of expansion is anisotropic? How does the heat know which way to vibrate? Is there a polarizing effect for any heat not absorbed?

If we had the rubber bands tensioned by attachments to the sides (rather than long ends) the effect would be opposite?

What about the orthogonal to those two directions..positive of negative linear thermal coefficient of expansion.

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#21

Re: Rubber and Heat

05/03/2017 6:18 AM

You don't know? Why don't you know?

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#22
In reply to #21

Re: Rubber and Heat

05/03/2017 6:25 AM

The same as people do not know the answer to 'how long is a piece of string?' Do you know the defined acceptable answer?

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#31
In reply to #22

Re: Rubber and Heat

05/03/2017 10:35 AM

Beauty. Twice the distance from the centre to one end.

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#32
In reply to #31

Re: Rubber and Heat

05/03/2017 10:39 AM

A string is never longer than the length required to join the two ends once folded. Never longer than the length measured from one end to the other. If you pull on it hard enough, it multiplies, but the strings won't grow.

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#33
In reply to #32

Re: Rubber and Heat

05/03/2017 10:50 AM

If you do have string longer than the length required between 2 securing points you have string left over. So how long is the string?

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#34
In reply to #33

Re: Rubber and Heat

05/03/2017 11:37 AM

Only as long as is necessary.

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#44

Re: Rubber and Heat

05/04/2017 9:25 AM

https://www.thenakedscientists.com/sites/default/files/media/heatingRubber.mp4

watch the above video for proof. Note that some load on the rubber is needed for the dX<0 effect to be seen (I think), otherwise, you just see normal expansion.

Thermal energy results in expansion in the other degrees of freedom of cross-linked chains. (explained in fewer words this time).

why rubber shrinks when heated along the direction of tensile force

easy to read article with excellent links at the bottom

Obviously, an such heat engine relying upon tension on rubber bands would need some serious mass, and that thermal inertia would slow down action, unless the heat was applied evenly, and throughout the rubber instantaneously (microwaves)? at 0 degree point of rotational axis, and then cooled as quickly as possible at some position past TDC.(90 degrees). On another note, could this apply as well to other materials of natural origin, and be used to lift massive objects using levels, and tensioned strands under heat (with a detent mechanism of course)?

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#46

Re: Rubber and Heat

05/04/2017 1:39 PM

For what it's worth, I was just in the lab and noticed I had everything I needed to try it out: rubber band; weight (pair of mole grips); a place to hang it (vice), and, a heat shrink gun.

Before

After

It actually went up about twice as far, but, had obviously started to cool by the time I could take the picture.

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#47
In reply to #46

Re: Rubber and Heat

05/04/2017 2:50 PM

Do it again, and mike the thickness and width both bold and hot and report back here by Monday!

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#48

Re: Rubber and Heat

05/09/2017 9:06 AM

It all depends on what type of weight is on that rubber. It could either contract or expand. But usually it expands when its not yet spent. Just saying..

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