What does thermal lag mean wrt DSC? 2

[mta02hm]

Hi,

I've ran a polymer same on the DSC at 5 different temperature and the intensity and position of the peak has changed as a function of heating rate. I.E The melting point peak is at a higher temperature and more intense at a heating rate of 50C than at 30C or 20C. I've read this is due to "thermal lag" but what is thermal lag?

Any help greatly appreciated!

 

[MikeHalloran]

"Thermal lag" is a fancy way of saying that polymers don't conduct heat well. So, for example, if you apply a step function in temperature, or a fast ramp, to one side of a polymer wall, there will be a time delay before you can measure a change on the other side of the wall, the step function will have become a ramp, and the fast ramp will have become a slower ramp.

It's just like a lag in a control system. Does that help?

p.s. "50C" is a difference, not a heating _rate_. Perhaps you meant "50C/min" or "50C/sec".



Mike Halloran
Pembroke Pines, FL, USA

 

[Demon3]

As Mike explained it refers to the poor thermal conductivity of polymers. The consequence is that the temperature of the DSC pan is not the same as the temperature of your sample. The difference between the pan and sample temperature will increase as heating rate increases. This can shift your peak slightly.

One way to avoid / minimise it is to make a very thin sample and clamp the pan so the polymer is squeezed into contact with the pan. Another way is to add an inert oil (like silicone oil) to help conduct the heat.

There are other things that may cause your peak to shift other than thermal lag.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[mta02hm]

Yes, I did mean 50C per min!

Thank you very much for your responses - having no good text book on the subject and no one in the lab to ask I find it difficult to find information!

What other things could cause the peaks to shift?

I'm also finding it difficult to find information on what is meant by "thermal history" I've been told to run a number of cycles to eliminate this but no one has given me a satisfactory explanation what thermal history is.

I've read Thermal/Heat history is the accumulated total thermal energy input into the polymer system but how is it "accumulated"?

Oh, and if anyone could suggest a good textbook on DSC that would be great!

Thank you again for your responses

 

[patprimmer]

Thermal history is exactly what it says, the amount of exposure of the plastic to heat.

A long term or high temperature thermal history suggests thermal degradation will have occurred. This is mostly in the form of oxidation, chain scission and cross linking.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.