Re: adhesion

From: Katharine Thayer <kthayer@pacifier.com>
Subject: Re: adhesion
Date: Fri, 04 Feb 2005 18:12:34 +0000

> 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.

I doubt it. I have an emulsion which makes the image area black if
there is a defect in the sizing layer (such as small area where sizing
is not completely applied) and chemically contact the paper base. You
know what, plain 3% gelatin size hardened with glut is good enough to
completely prevent this problem, indicating that glut hardened gelatin
size acts as a chemical barrier between the paper and the
emulsion. (Note: not all emulsions do this. Some are perfectly fine
when I coat them directly on unsized paper.)

> [...] 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.

That's a problem with the use of unhardened gelatin and unrelated to
your point. Unhardened gelatin swells and expands all directions and
is mechanically weak. You can't expect good adhesion in such a
situation.

But anyway you acknowledged that gum can adhere on smooth, gelatin
sized surface, thereby recognized that the adhesion based on
intermolecular force (and probably heterogeneous crosslinking between
gelatin and gum due to exposed dichromate) was sufficient for them to
adhere.

Anyway, I suggest you read a decent intro book on industrial chemistry
of adhesives, cements, paints, and these kinds of stuff. The role of
intermolecular forces and effects of surface roughness are some of the
basic backgrounds treated in literatures of that area. That should
give you a framework in which you can better interpret your
observation.

> 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.

More available carboxyl groups in the gelatin should work the same way
in contact with both glass substrate and gum coating because they are
both polar substrates that engage with hydrogen bonding, which is an
intermolecular force. This is very analogous to how paper glues, wood
glues, PVA adhesive, acrylic paints, etc. work. Cellolose is rich of
-OH groups available for hydrogen bonding and these glues and paints
also contain a lot of -OH and -COOH that involve in hydrogen bonding.
Gum arabic, of course, is rich in -OH. Then those carboxyl groups of
the gelatin can also engage in crosslinking with gum molecules through
chromium (III).

--
Ryuji Suzuki
"People seldom do what they believe in.  They do what is convenient,
then repent." (Bob Dylan, Brownsville Girl, 1986)