Re: Increase in GFM in time

From: Guy Allen Brady (
Date: Fri Jul 25 1997 - 20:43:19 EEST

On Fri, 25 Jul 1997, Christian Antoniutti wrote:
> I read on Jacobs' "Stereolithography and other RP&M Tecnologies" that an
> epoxy resin like SL 5170 and SL 5180 increase their GFM (green flexural
> modulus) for more than 25 hours after building.
> Can anyone tell me when this increase stops (I think it should stop,
> because this is due to completion of photopolimerization)?

Well, if you're an engineer, it stops for all practical purposes after 25
hours, but if you're a scientist, it does not stop. During (photo)
polymerization, there is a transition from liquid monomer to solid
polymer and the polymerization reaction slows down. The growing polymer
chain branches become "immobilized" due to entanglement and crosslinking
so the unreacted monomer must diffuse around to find these chain ends to
keep the reaction going. The increase in elastic modulus is just
reflecting the increase in monomer-->polymer conversion (and
crosslinking). This process will technically not stop until all reactive
sites are quenched (either with oxygen or monomer or more polymer).

If you just wait for the reaction to stop rather than slow down to
immeasureable speeds, then you'll wait many weeks, but you can help things
along by heating to ~80C for a few hours to get things moving along
(diffusion is a thermal thing).

Sorry for the length, but I've recently read many many many papers on this
and some just spilled out.

          G. Allen Brady -- Graduate Research Assistant
Materials Science and Engineering - The University of Michigan
2219 H.H. Dow Bldg. 2300 Hayward Street Ann Arbor, MI 48105
work: 313/936-0177 fax: 313/747-4807 email:

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