Hi everyone!
I need to compare the thermal conductivity of two plastics. The only equipment available is DSC, but since I don't need the absolute values, only comparison, I was thinking I could do something like this:
Make a DSC program fom maybe 70ºC to 150ºC at a very high heating rate (as fast as the equipment can go). Then make it hold at 150ºC until the sample temperature doesn't change. Then I would plot the time each sample took to get to that temperature. The one with a higher heating speed would have a larger thermal conductivity. I would repeat this with at least 3 samples.
Do you think this could work? Any advice on how I can improve this method?
Thank you very much!
Probably your DSC is able to register only the temperature of the reference side (have to check this).
If your materials have the same heat capacity, try to run DSC and use the first material as the sample and the second as the reference. You should be able to detect increasing or decreasing DSC signal. The result is the difference in heat flow through the samples, but if everything else is the same - you could assume the change is the result of differences in thermal conductivity of the samples. LFA is recommended but if only want to know more/less you can try this. The sample shape and mass, surface contact with the crucible, should be the same.
Hello Graça Brotas ,
Thermal conductivity is usually only measured with a steady state apparatus, see Method Thermal Conductivity Comparator
. The reason for using a steady state apparatus is that a transient apparatus, such as a DSC and LFA, is influenced by the sample's thermal conductivity, k, heat capacity at constant pressure, Cp, and its mass density, ρ. In other words, using the time it takes for a sample to reach a certain temperature depends on three parameters, and not just the the sample's thermal conductivity. It would be unlikely that your two different plastics only differ in their respective thermal conductivities. An example of a transient apparatus is the LFA, seeMethod Laser Flash Diffusivity Measurement
, which directly measures the thermal diffusivity, k/(Cp*ρ), of a sample; in order to calculate the thermal conductivity of the sample from the measured thermal diffusivity, you need to have an independent measure of the sample's heat capacity at constant pressure and its mass density. Note, because of the way thermal diffusivity is defined, you could compare two different plastics with a LFA even if their heat capacity at constant pressure, Cp, and mass density, ρ, were different if the products of these two parameters were the same in the two different plastics.Regards,
Thomas Cuff
Thermal conductivity is defined under stationary heat conduction for a constant temperature gradient. In transient heat conduction situations the temperatur evolution at one specific place dependes on density, heat capacity and thermal conductivity, or better on Thermal diffusivity. Additionally, for thermal conductivity you have to measure the gradient. If you take the the temperatur at one place you do not have information enough.
Hello Graça Brotas ,
There is one caveat, which I forgot mention. In the statement of your question, you mentioned using a DSC apparatus. This type of instrumentation is usually used to detect phase transitions in materials over some temperature range, i.e., an inert reference material and a sample material have their temperature swept over a fixed temperature range, and any heat evolved or absorbed by the sample, due to phase transition, is measured with reference to the inert reference material; the inert reference material is chosen not to exhibit any phase transitions in the swept temperature range. If you employ a steady state apparatus (thermal conductivity comparator or an absolute thermal conductivity setup) to measure the thermal conductivity, make sure that the material(s) subjected to the spatial temperature gradient do not undergo a phase transition anywhere in the temperature range of the gradient, as this will usually cause the thermal parameters of the material to change, at that point in the material(s), and invalidate the measurement.
Regards,
Thomas Cuff
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