Glenn:
The DSC would normally given slightly higher readings. In my experience,
especially working with either Perkin-Elmers or Shimadzu systems, the
DSC determines the midpoint by integration techniques, and will usually
report that as the Tg; the TMA does not specifically integrate to the
midpoint but has a user definable (at least in some systems) level after
onset to which it itegrates. Given that both systems integarte flow vs
heat, then one must ensure the consistency of the heating profile,
including instantaneous temperature, heat rate (thermal gradient or
slope), etc. Between the two systems if there is variation in these
factors, you will see offsets in Tg.
The best solution is to perform Tg on a combined system which has both
TMA (or DTMA) and DSC capability. While these systems generally are not
as accurate as a dedicated parametric system, they are very consistent
in their determination of parameters from technique to technique. I
suspect, unless you are in R&D developing polymers, that an
approximation within a degree or two will suffice, since almost no
polymeric compounds have Tg, Mp, and carbonization points that close.
I have always used a N2 air purge (argon works as well) when doing
thermodynamics work, running the systems at 20C/min and always trying to
use 8-12mg in platinum cups (my systems max out at 20mg; pre-baked
aluminum cups will also work as well, except at temperatures above
400C).
Hope this helps...
> Bill Davis, Ph.D.
> Diamond Multimedia Systems
> Senior Scientist
> Tel. 408.325.7868
> Cell. 408.888.5650
> e-mail: [log in to unmask]
>
>
-----Original Message-----
From: glenn pelkey [mailto:[log in to unmask]]
Sent: Tuesday, April 20, 1999 8:42 AM
To: [log in to unmask]
Subject: [TN] DSC vs. TMA
Hello all,
Could somebody tell me how the measurement results differ? I vaguely
recall that when measuring the same board by these two different
methods, one method would show a slightly higher value than the other.
But, I can't recall which one. For example, is 135 C Tg by DSC
equivalent to 130 C Tg by TMA?
I know there's sample and test variation, but I'm looking at results
in general.
And, while we're on it, which test method is your preference,
especially when it comes to specification issues.
Thanks
Glenn Pelkey
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