Re: Gelatin-polymer blend

From: Loris Medici <mail@loris.medici.name>
Subject: RE: Gelatin-polymer blend (was Re: Gesso sizing)
Date: Sat, 21 Jan 2006 23:59:02 +0200

> Why the addition of gelatine to acrylic is necessary? Can't one use
> direcly diluted acrylic medium?

That's because adhesion will be very poor. Even when gelatin is
blended, if the fraction of acrylic polymer is too high, the adhesion
suffers, even with gelatin emulsions, which generally has excellent
adhesion compared to other binder systems.

The basic strategy in multilayer coating is that, you want to make
small stepwise change in hydrophilicity and swelling from one layer to
the next. For example, in the case of sheet films, the base is
polyester, which is hydrophobic and nonswelling. The first layer of
subbing is weakly hydrophilic and weakly swelling, like latex
polymers. The next layer will be more hydrophillic and more swelling,
and so on. You don't want an abrupt change between adjacent layers;
damage due to mechanical distortion is a lot more likely when the
material is wet and swollen. This can be emulsion flaking off,
reticulation, or other forms of mechanical damage.

Gum is very hydrophilic. So the layer under gum shouldn't be very
hydrophobic. Straight acrylic medium is a poor choice for this reason.

> BTW, IIRC Getty Museum is running acrylic longevity tests... I guess
> we'll have much more info about this issue soon.

Archival property of acrylic medium with pigments is a different story
from using it as a part of photographic material, though. I'm less
worried about acrylic medium itself, but I am very leery of accepting
titanium white as a part of sizing layer without doing extensive
tests. Exactly what AGFA, Ilford, Kodak, Fuji, Mitsubishi and Konica
did to remedy this problem is not widely known. For this matter, I
don't even trust RC papers made in eastern European countries. (Their
baryta papers are generally fine, though.)
Received on Sat Jan 21 16:26:23 2006