How does rubber cure physically?
How does rubber cure physically?
(OP)
Hello,
I am trying to simulate the curing of rubber but I am confused with how rubber actually cures in reality. From looking at rheometer results I can see that initially the viscosity of the rubber drops as it is heated, the rubber flows then the viscosity/stiffness of the rubber starts increasing.
With this understanding I am planning on doing the following; run a heat transfer simulation in which I can see how temperature changes within the material when heat is applied to the surface and then apply conditions so that if any part of the material reaches a certain temperature its viscosity starts increasing at a certain rate in time.
Is this how rubber is cured or is my understanding of this incorrect? I just ask because with this theory for example if a small block of rubber takes 10 minutes to cure and a large block takes 60 minutes this would mean that the heat took 50 minutes to get to the center portion of the large rubber block to fully cure the entire material (from surface to center)which from my understanding is unlikely, there are also a few other things that just don't add up with this theory.
I am really getting confused with this and would appreciate any help at all with this problem, even if you can provide anything on this topic please just let me know.
Thank you.
I am trying to simulate the curing of rubber but I am confused with how rubber actually cures in reality. From looking at rheometer results I can see that initially the viscosity of the rubber drops as it is heated, the rubber flows then the viscosity/stiffness of the rubber starts increasing.
With this understanding I am planning on doing the following; run a heat transfer simulation in which I can see how temperature changes within the material when heat is applied to the surface and then apply conditions so that if any part of the material reaches a certain temperature its viscosity starts increasing at a certain rate in time.
Is this how rubber is cured or is my understanding of this incorrect? I just ask because with this theory for example if a small block of rubber takes 10 minutes to cure and a large block takes 60 minutes this would mean that the heat took 50 minutes to get to the center portion of the large rubber block to fully cure the entire material (from surface to center)which from my understanding is unlikely, there are also a few other things that just don't add up with this theory.
I am really getting confused with this and would appreciate any help at all with this problem, even if you can provide anything on this topic please just let me know.
Thank you.





RE: How does rubber cure physically?
"I just ask because with this theory for example if a small block of rubber takes 10 minutes to cure and a large block takes 60 minutes this would mean that the heat took 50 minutes to get to the center portion of the large rubber block to fully cure the entire material (from surface to center)which from my understanding is unlikely, there are also a few other things that just don't add up with this theory."
Rubber is an insulator, i.e. a poor conductor of heat, so yes, it takes more time to cure a thicker/larger section of material when the heat is applied at the outside surface. Uncured rubber, without all of the cross-links, is a poorer conductor than cured rubber, so this affects the rate equation as well. Then, factor in the uncertainties (did the cure agent get properly dispersed in the milling/mixing steps? Are the temperature indicators on the press heater controls reading accurately?).... and most places probably over-cure by a factor of 1.5 to 2x...but better that than an undercured part.
Interested what you think doesn't add up with the theory. It's understandable to me that you are confused, the "science" of rubber is a black art, and a lot of information that is helpful seems to be tied up in trade secrets.
RE: How does rubber cure physically?
". From looking at rheometer results I can see that initially the viscosity of the rubber drops as it is heated, the rubber flows then the viscosity/stiffness of the rubber starts increasing"
It is not intuitive, well wasn't to me, but a rheometer curve is a somewhat ridiculous standard if your goal is to determine the cure time of a molded part. In rheometer testing, the temperature to drive the curing is generated by the internal self-heating of the material as it is continously mechanically mixed/stirred by the rotating disk in the mechanism. This, plus the thorough, continuous mixing of the materials, mean that a rheometer test is going to give a much faster cure rate than the actual molding process (with conduction heating only, and no continuous mixing) would ever be able to achieve.
RE: How does rubber cure physically?
The thing I mentioned that did not add up with the theory was that when I have seen people working with rubber, to see if the part is cured they simply remove the die top and indent the outer surface of the component, if the rubber has its elastic properties in which it rises again back to its original position it would be an indication that the component has cured. This made me think that rubber cures in a homogeneous was in which everything is cured together which goes against the theory that I learnt however, after reading your post I am now thinking that this may have been an inefficient way of testing if the rubber component had cured.
One last thing, the rheometer being used is a moving die rheometer (MDR) and from my understanding (which is very little) this does not mix the material therefore it does not generate heat within the test piece and the majority of it is transferred through heat conduction. If this is not true could you please suggest an alternative method of obtaining results on how a component cures which are fairly accurate.
Again, thank you for your help I really appreciate it, as you said now days everything is a trade secret within the rubber industry and finding information is very difficult.
RE: How does rubber cure physically?
This is true. The die only moves by ±0.5° so there no shear heating is generated.
RE: How does rubber cure physically?
http://digital.ipcprintservices.com/publication/?m...
RE: How does rubber cure physically?
I now have a new question which is also cure related and will help with my simulation. I was thinking about how components are molded, o-rings for example. If you have two o-rings both the same cross section one a much larger diameter than the other, theoretically from my understanding of how rubber cures they should both cure exactly at the same rate as the cross section of the material cures the same and the same temperature is applied to both however, from seeing o-rings get made in reality the larger o-ring requires about 15 minutes to cure whereas the smaller diameter o-ring only requires 5 minutes. This makes no sense, and has really confused me because it goes against my current understanding of how rubber cures.
I would really appreciate if anyone could answer why this happens. Thank you.
RE: How does rubber cure physically?
RE: How does rubber cure physically?
Tom Jablonowski, TSE Industries, Inc.
www.tse-industries.com
RE: How does rubber cure physically?
If the press plate is the same, let's sayd 20x20inches, you probably can fit just one o-ring of 18 inches... all the inner portion will be empty.
If you have 002's o-ring, you will have all the plate full of o-rings, so much more mass to heat, so you need more time to the press stabilize again during the molding cycle.
Also, the preparion method (heat history) will impact the curing... do the o-rings have the same heat history? Was one done by calandering and the other by preforming?
And last, but not least, without trying to discourage you, I have seems people trying to simulate the curing, but it is always using more information than the rheometer can give you.
It has always the thermal coeficients added (thermal conductibility, activation energy, etc). The rheometer probably can approach only of the shear that a compression press do (and if molded slow).
By experience I have seem same compound, pure injection being molded in 2minutes. If injection/compression time = 1min, due the excessive shear that the molten compound suffers)...
RubberDog