Degree of Cure 1

[Hexergy]

Does any one have any information relating degree of cure to Elastic Modulus and Shear Modulus for PMCs. How is degree of cure calculated? Or is it observed? I have tensile tested coupons of S-2 Glass(90o) and Carbon-Carbon and was curious if this could have an effect on ultimate strength?

Thanks,
Hex

 

[Compositepro]

Degree of cure is usually measured by DSC (differential scanning calorimetry). The instrument measures the heat flow into or out of a very tiny sample (usually 10 to 50 milligrams) as the temperature is ramped up (usually 10C/minute). The total heat of the curing reaction is measured on uncured material. If any reaction heat
(exotherm, delta H, in joules per gram) can be measured on a an already cured sample then it was not fully cured. The percentage of cure can be calculated by dividing the residual delta H by the total delta H.

Degree of cure can have a very strong effect on strength and modulus. Some materials can be over-cured as well as under-cured. Over-curing causes excessive crosslinking which makes the polymer more brittle.

Degree of cure is not relevant to carbon-carbon because the polymer has been pyrolyzed into carbon and isn't a polymer anymore.

 

[Hexergy]

Compositepro,

Is incomplete pyrolyzation possible? If so would an additional heat treatment rectify the condition?

Thanks

 

[Compositepro]

With carbon-carbon composite heat treatment times and temperatures are important. It will determine how much non-carbon elements like nitrogen and oxygen are left and also the crystal structure of the carbon. Very high temperatures , on the order of 2000C will cause amorphous carbon to convert to graphite. High modulus carbon fibers are graphite fibers. Standard modulus carbon fibers are not graphite fibers although many people still call them graphite.

Fiber properties change with heat treatment as well as matrix properties. So carbon-carbon parts are often made with cheaper oxidized PAN fibers or standard modulus fibers which are much less fragile that the high modulus fibers.