Re: Energy, Power, time...

From: Guy Allen Brady (
Date: Thu Jan 22 1998 - 19:52:38 EET

On Thu, 22 Jan 1998, Marshall Burns wrote:

> Werdinius Christian wrote:
> > If I have calculated a certain value for energy per areaunit, then I can
> > reach this amount either by low power and long exposure time or the opposite.
> > [Energy=Power*Time]
> > Does it make any difference how I "get" the energyamount?
> > Is there a "powerbarrier" under which no polymerisation occur at all - a
> > kind of activation energy?
> That's an interesting question Christian. As I say in the book, the
> phenomenon of critical exposure is an assumption of the Evans-Jacobs theory. If
> the threshold of gelation depends in a more complicated way on the photopower,
> that would change the theory. This is something that can and should be tested
> experimentally.

Well, I should have caught on to this before. But this is more clear now.
The critical exposure is not something that is indirectly measured (its
very impractical to physically measure a polymer at the gel point with
calipers). But, it's not the same as activation energy. The activation
energy for polymerization won't change with power- that's a fixed

You have a very different situation than most with your tunable laser. By
varying power, you effectively vary radiation intensity. That's an
important distinction. It is known for photopolymerization of acrylates
that the rate of polymerization depends upon Intensity to about 0.5 power.
This is proably true for the most parts for the SL resins. So, changing
the intensity (or power), you're monkeying with the kinetics of
polymerization. It could very well be your resin parameters will change
with laser power. (It will definitely change with wavelength). So, yes, it
does matter how you "get" the energy. There should be a threshold, or
range of low powers where things may develop too slowly, however, I
suspect this will be very low powers.

          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 48109-2136 work: 313/936-0177 fax:
313/747-4807 email:

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