Re: adhesion

I do agree that "tooth" is a rather vague designation, but when I speak
of "tooth" I am speaking purely of physical properties of a material,
not chemical characteristics. Tooth in paper, even very smooth paper,
consists of fibers that extend from the surface of the paper; it is
these fibers that the gum clings to, to hold itself to the paper. If
there's "tooth" for the gum to hang onto, it will stick, if there's not,
it will slide right off in the water bath. In fiberless material such
as glass or metal or plastic, "tooth" can be created by etching or
sanding or applying paint or other ground vertically and horizontally
to form a crosshatch pattern; in other words by abrading the surface or
adding some kind of ground to it, one can create a semblance of tooth
which will help the gum to stick.

On plastic surfaces like mylar or yupo, gum will not stick, but if the
mylar or the yupo are abraded with sandpaper, the gum sticks quite well.
The only difference between the surface that sticks to gum and the
surface that doesn't stick to gum, in this case, is the surface
roughness of the material.

Actually, I didn't think of it at the time, but I think a case of sorts
for crosslinking providing some measure of tooth could be drawn loosely
from my own observation that unhardened gelatin coated on a slick
surface like glass or plastic does not improve the adhesion of gum to
the surface, coupled with the observations of people who have perfected
printing on glass, who say that hardened gelatin used as a size does
improve the adhesion of the gum. So it seems like there must be
something about the structure of the crosslinked gelatin that gives the
gum something to hang onto in a way it can't hang onto unhardened
gelatin.

Your pthalated gelatin sounds great for certain applications, but it's
hard to picture how it would apply to gum printing on glass, for
example. Sure, the gelatin would stick well to glass, but so does
regular gelatin. The more important question is: would the gum stick to
the pthalated gelatin better than to glass or unhardened regular
gelatin? It's an interesting question, but I don't see any reason to
assume on the face of it that it would. But I'll be interested to hear
the results if someone decides to try it. And if you mixed the
pthalated gelatin with the gum emulsion to improve its adhesion to the
glass, then you'd have some hybrid process but it wouldn't be gum. This
is not to say it might not be great, but it wouldn't be gum.
Katharine

Ryuji Suzuki wrote:
>
> Adhesion of coated materials is determined by intermolecular force of
> substrate and the material in the coating. The adhesion of multiple
> layer coating is basically the same. Such a vague characterization as
> "tooth" is a poor predictor of adhesion. Surface roughness may help
> through increased area of contact, but roughness itself is not
> sufficient for good adhesion.
>
> Gelatin has good adhesion on many substrates including smooth glass
> because of many available carboxyl groups. There is a kind of gelatin
> where available carboxyl groups are increased through chemical
> modification. This type of gelatin has particularly excellent
> adhesion. My emulsions and sizing solutions contain this type of
> gelatin and the coated layers give me excellent adhesion. When I coat
> this type of emulsion directly on unsubbed glass, emulsion sticks to
> the glass very well and I don't experience frilling. People who use
> glass as the substrate may find it very useful to use this type of
> gelatin as a part of the subbing layer and/or the emulsion itself.
>
> --
> Ryuji Suzuki
> "People seldom do what they believe in. They do what is convenient,
> then repent." (Bob Dylan, Brownsville Girl, 1986)
Received on Fri Feb 4 22:57:14 2005