Re: Could someone summarize that gum up or down discussion?

On Fri, 14 Apr 2006 22:22:07 +0000, "Marek Matusz"
<marekmatusz@hotmail.com> said:
> Perhaps we could test some of the hypotheis put forward. For example: the
> light penetration of the gum layer. With some assumption one could
> calculate
> if the light penetrates to the paper interface or not. Anybody still
> remember their freshman chemistry or physiscs. If I get bored enough with
> gum transfer experiments I might do the calculations as well. For the
> very
> thick, heavily pigmented gum layers that I am using for back printing
> there
> is no visible ligt passing through and since carbon absorbs everything
> indiscriminately no UV passes through as well. I will test that to make
> sure.

I proposed such a model yesterday, describing exponential decay. It is a
simple differential equation that appears in many aspects of science and
engineering.

Assuming a uniform layer, the UV light intensity falls as the depth
increases in the form of exp(-x/c) where x is the depth (surface is at
x=0) and c is depth constant. In other words, c is the thickness of the
layer which gives UV density of 0.43, which is log exp -1. In this
sense, the rate at which the UV is attenuated is described by one number
and it can be compared across different solutions, etc. This is very
analogous to power loss that occues when RF signal is transmitted over a
long coaxial cable (or any constant-impedance transmission line for that
matter), to help understanding of radioheads.

Since Mike Ware mentioned a possibility that dichromate concentration is
not necessarily homogeneous in the coated layer, I suggest to make two
measurements. One is UV-optical density of gum plus pigment at any known
thickness, and another is that of gum, pigment and dichromate at any
known thickness. This way, we can calculate two depth constants. Those
numbers should be measured from films made on nonabsorbent support, and
may be measured with the support or measured as a film peeled off from
it. By using these two numbers, we can easily conclude for any given
coating thickness of actual dichromated gum material, how much UV would
reach the "bottom" if the dichromate is uniformly distributed, or if all
dichromate is absorbed by the paper substrace and available only at the
gum-paper interface.

There are three factors that people didn't mention, though.

1. The UV-optical density of dichromated gum will change during the
course of UV exposure. (This is a real problem.)

2. There _may_ be significant scattering of light at the gum-paper
surface.

3. The number of crosslinking necessary to render the macromolecule
insoluble depends on the MW and other factors. When the gum can
crosslink with the sizing layer (which would be the case with
glut+gelatin sizing).

This is an interesting question, but I am not set up to do UV
densitometry so I'll have to leave it to someone else.

For one case where much of the UV-density is built by the binder and
pigment, I think there is little question for top-down theory. If, in
such situation, one wants to make a uniform "all-together" hardening, it
would require that the dichromate concentration would be in the form of
exp(+x/c) and, and it is unlikely that actual dichromate concentration
is anything like this. If pigment and the binder are both
UV-transparent, then the situation is a bit better for bottom hardening.

Some of you might have been bothered by the term UV here, but if it
helps, read "monochromatic 370nm light" or something like that.
Received on Sat Apr 15 20:09:18 2006