This is the sort of question that is easily answered across a desk
with a pad, but difficult across keyboards, I'll have a go.....
Yes epoxies are thermosets and the process of curing is permanent.
That is once all the bonds - cross links - are set in place they
remain in place until the epoxy is destroyed. The Tg is not the
temperature at which this happens however, what happens at the Tg is
the qualitative nature of the bonds changes. They are still there but
they relax and expand slightly, become a little more flexible. So the
epoxy is a still a solid lump of stuff, but overall its mechanical
properties are measurably changed. A reasonable analogy is to imagine
a large scaffold structure. This is made of a lot of vertical uprights
held in place by horizontal and diagonal links with rigid joints.
Thermosets are similar structure [but a good deal less regular]. It is
in the nature of the epoxy that when heated up, at a certain temp
these links expand and become softer/pliant. In the scaffold it would
be the same as if all the joints were loosened slightly. The scaffold
would still be there, but not as rigid or strong as before. The
temperature at which the epoxy resin transits from its natural hard
glassy state to the softer more compliant one and back is the glass
transition temperature(Tg).
IF you continue to heat the epoxy beyond the Tg eventually the bonds
will start to fall apart and the material will begin to char (the char
point) etc
If you want a complete set of terms and to know more suggest you go to
your friendly glue supplier for their literature. Loctite for example
has published an excellently pragmatic book on adhesives for non
chemist users. (Illustrated by examples of their products, but the
practical information is pretty well separated from the product info).
TWI also has some good articles in this area, which have the advantage
of being of easily readable length compared to a book.
Mike
----- Original Message -----
From: "michael" <[log in to unmask]>
To: "Mike Fenner" <[log in to unmask]>; <[log in to unmask]>
Sent: Thursday, July 13, 2000 3:15 AM
Subject: Re: Re: [TN] Glass Transition Temperature (Tg) Questions
> Epoxies are thermosetting. Surely this process is not indefinitely
two-way? Or
> should I say: beyond what temperature is the process no longer
two-way (and what
> is the technical term for this temp.)?
>
> Michael Fennema
>
> -----原始郵件-----
> 寄件者: Mike Fenner <[log in to unmask]>
> 收件者: [log in to unmask] <[log in to unmask]>
> 日期: Thursday, July 13, 2000 05:58
> 主旨: Re: [TN] Glass Transition Temperature (Tg) Questions
>
>
> |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
> |>
> |> ##############################################################
> |> TechNet Mail List provided as a free service by IPC using
LISTSERV
> |1.8c
> |> ##############################################################
> |> To subscribe/unsubscribe, send a message to [log in to unmask] with
> |following text in
> |> the body:
> |> To subscribe: SUBSCRIBE TECHNET <your full name>
> |> To unsubscribe: SIGNOFF TECHNET
> |> ##############################################################
> |> Please visit IPC web site (http://www.ipc.org/html/forum.htm) for
> |additional
> |> information.
> |> If you need assistance - contact Keach Sasamori at [log in to unmask]
or
> |> 847-509-9700 ext.5315
> |> ##############################################################
> |>
> |
> |##############################################################
> |TechNet Mail List provided as a free service by IPC using LISTSERV
1.8c
> |##############################################################
> |To subscribe/unsubscribe, send a message to [log in to unmask] with
following
> text in
> |the body:
> |To subscribe: SUBSCRIBE TECHNET <your full name>
> |To unsubscribe: SIGNOFF TECHNET
> |##############################################################
> |Please visit IPC web site (http://www.ipc.org/html/forum.htm) for
additional
> |information.
> |If you need assistance - contact Keach Sasamori at [log in to unmask]
or
> |847-509-9700 ext.5315
> |##############################################################
>
>
##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in
the body:
To subscribe: SUBSCRIBE TECHNET <your full name>
To unsubscribe: SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information.
If you need assistance - contact Keach Sasamori at [log in to unmask] or
847-509-9700 ext.5315
##############################################################
|
