Re: Gum hardening: top down experiment

----- Original Message -----
From: "Yves Gauvreau" <gauvreau-yves@sympatico.ca>
To: <alt-photo-process-l@usask.ca>
Sent: Thursday, April 13, 2006 1:49 PM
Subject: Re: Gum hardening: top down experiment

> The point is this, when a photon comes in to meet with this emulsion it
will
> either strike an atom on its path through all these molecules or it will
go
> through, it could even go through the substrate if it's not completely
> opaque (obviously). This is definitely top-down or to be more accurate
> photon always hit atom from the side they come from or if you prefer
> "hardening always happens in the same direction as the path of the
photons".
>
> Now, why in the world would someone be led to believe that it could be the
> other way around?

In certain photopolymer systems you might just find that kind of anomalies.
To a large extent oxygen inhibts radical polymerization. Hence, you might
have full curing in the depth of the layer whereas the surface remains soft
and tacky. One way to get around this, is to "sandwich" the photosensitive
solution between two plates/films that protect from atmospheric oxygen
(glass, PET, PVDC etc.). Incidentally, paper is certainly not a particularly
efficient oxygen barrier...
Having said that, I've no idea whether atmospheric oxygen does play a
significant role in dichromated systems though.

> There is a
> very low probability if any that enough UVC and UVB photons will go
through
> the emultion, hit an atom in the substrate and be emitted as UVB or UVA
such
> that they could hit an atom in the emultion and cause sufficient hardening
> from the back to outweight the combine effect of all UVs coming directly
> from the source.

I guess at the high levels of dichromate used for gum printing it becomes
unlikely that radiation shorter than say, 450nm does penetrate the layer.

Martin
Received on Fri Apr 14 05:48:57 2006