In a message dated 20/04/99  03:53:58PM, you write:

> 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
>  [log in to unmask]

Glass transition temperature (Tg) is the temperature at which the bonds in a
polymer such as epoxy relax becoming longer and less rigid. This means that
above the Tg the material will have different physical properties. eg it will
be less dense, more flexible, have different coefficients of expansion,
conductivity and so on. Various test methods can be utilised to measure these
differences, and this is not too difficult on either side.
Measuring the point at which they change is a little less absolute, depending
on the property selected for investigation  you are looking at  different
effects which come in at slightly different rates, especially for
multicomponent blends, and there is usually a finite time for the transition
rather than a switch. So there maybe room for intepretation even within the
same method of measurement. eg do you measure at the completion of the event,
when it starts or take a mid point and so on. Most original suppliers will
give a number which they feel is representative of what users should expect
to see in their product, but in the small print somewhere will be words like
" when mixed and cured in accordance with the instructions". This
pre-supposes a degree of idealism in preparation and handling in a lab
prepared sample by polymer experts which may not be present in a real world
situation. This and the above comments usually accounts for most differences.
Personally I do not think that the small differences reported are that
significant given the above and would not normally be the subject of a
quality issue between yourself and your vendor for example. As long as the
numbers are consistent, and consistent in any differences I would not be too
concerned.
So far as which method is preferred I have found these vary according to the
chemists concerned - each one having some logical reason for his view which
as a mere exploiter of their wares I can only accept.
Perhaps there is a proper polymer chemist on Technet with a better informed
view.
Finally, I would be interested to know why Tg is significant for you, nothing
perjorative in this remark I am just curious.

Mike Fenner

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