Just for completeness, thet Tg is two way:
When ever the polymer is at a higher temperature than the Tg, the
chemical bonds are relaxed and the material is relatively floppy. When
the polymer goes back below the Tg the chemical bonds tighten back up
and the material reverts to its hard state. The transition from hard
to soft (or soft to hard) is a repeatable process in either direction.

Mike

----- Original Message -----
From: "Alain Savard" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, July 12, 2000 5:35 PM
Subject: Re: [TN] Glass Transition Temperature (Tg) Questions


> My answers marked with 2 asterisks (**) below.
>
> Alain Savard, B.Sc.
> Chemical Process Analyst
> CAE Electronics Ltd.
> e-mail: [log in to unmask]
>
> -----Original Message-----
> From: Hiteshew, Michael [mailto:[log in to unmask]]
> Sent: July 12, 2000 12:06 PM
> To: [log in to unmask]
> Subject: [TN] Glass Transition Temperature (Tg) Questions
>
> Hi,
>         I've been reading up on laminate materials and have some
questions
> that I hope some-one can help me with:
>
> 1. What is the "glass transition" ? What is the glass transitioning
from and
> to? I'm guessing it's the plastic stage that occurs in a material
just
> before it melts.
>
> ** Temperature at which the laminate turns from a relatively hard
and
> potentially brittle state, to a vicious, semi-liquid stage. Epoxy
used in
> the boards is more likely to decompose or burn than it is of ever
melting.
>
> 2. What is gained by a higher Tg in tetrafunctional and
multifunctional
> epoxy-glass?
>
> ** Higher Tg means that you have a better resistance to heat from
any
> source. Your board is more likely to keep it's shape during heat
related
> fabrication process or during use if it has a higher Tg.
>
> 3. What is meant by the prefixes "tetra" and "multi" as they're
applied
> here? Is it a reference to the number on bonds in the chemical
structure of
> the epoxy or glass?
>
> ** It refers to the number of linkage sites of the epoxy prepolymer.
>
> Thanks in advance,
>
> Michael Hiteshew
> Lockheed Martin NE&SS-Baltimore
> [log in to unmask]
> (410) 682-1259
>
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