Re: Tanning theory of dichromated colloids (was gelatin

From: Katharine Thayer <kthayer@pacifier.com>
Subject: Re: Tanning theory of dichromated colloids (was gelatin
Date: Fri, 14 May 2004 07:05:50 +0000

> it seems to me that if Ryuji had
> something more persuasive to offer on the subject than Mannivannan et
> al, Duncalf and Dunn, the hardening of gelatin with chrome alum, and
> invisible chromium, he's had ample time to suggest it by now.

I have read more papers than those. I gave you Mannivannan et al first
just because it was relatively recent and I had it in PDF. Other
papers (like Duncalf and Dunn) I didn't give you because I don't have
PDF. You couldn't read PDF so I eventually had to mail them but I just
didn't bother to dig old file and xerox.

Anyway, you have been negating complexing of chromium with gum (and
PVA) for a while. I read over old emails just now and it's very clear
that your argument was towards that direction. And recently you said
that "tanning" reaction cannot happen because there is not enough
carboxyl groups in gum. And you didn't respond to some of what I said
when what I said didn't fit into your belief. One is that carboxyl
groups can be made by dichromate in gum and PVA.

> Since, if we can trust the authors, residual dichromate but not residual
> reduced chromium was removed from the hardened PVA, it goes without
> saying that chromium would be found, just as I said above. This would be
> true whether the chromium was coordinated to the PVA or not coordinated
> to the PVA; there would have to be some reduced chromium present for the
> crosslinking to take place, whether the chromium is part of the matrix
> or not.

Can you explain the reason why you think Cr(III), which is soluble,
cannot be washed out in the way Duncalf and Dunn did, even if it is
not complexed with insolubilized gum?

> The abstract of the paper states that "ultraviolet spectrometry shows
> that secondary hydroxyl groups are oxidized to ketone groups, but
> insolubilization is attributed to crosslinking of polymer chains by
> coordination of alcohold groups to "nascent" chromic ions formed by
> reduction of the dichromate."

That was the point of van Nice and Farlee as well.

> Unfortunately, just as Mannivannan et al commented, there is no
> direct proof to back up the attribution of the crosslinking to this
> purported coordination.

At least one more paper cited by Mannivannan et al (besides Duncalf)
studied this mechanism and they said that up to 3 terminal carboxy
groups (they showd the cases of alpha-ketonic acid and beta-hydroxy
acid) are complexed around hexacoordination of Cr(III). It's Grimm,
L., Hilke, K.-J. and Scharrer, E. 1983. The mechanism of the cross
linking of poly(vinyl alchohol) by ammonium dichromate with
U.V.-light. J. Electrochem. Soc., 130, 1767--71. Also get the other
papers from the same group for more details, because this paper only
gives a roundup of their findings. I don't know why Mannivannan et al
did not discuss their results, especially because the results
presented there are compelling.

> I'm inclined to agree with Mannivannan et al in their assessment of this
> research "...photoreaction of dichromate in PVA films was studied by
> Duncalf and Dunn, and they suggested that the insolubilization of PVA
> was caused by the complex formation between PVA and Cr(III) even though
> no clear evidence was shown for the involvement of Cr(III)."

Grimm et al they cited did say that Cr(III) is complexed and they even
give 3D structures. (which I was going to send you by fax but you said
your fax didn't work or something. Now you figured out how to get
papers from your library so you can get them.)

Grimm et al also showed one more step from there. If the dichromated
PVA does not contain certain amount of moisture, this PVA-Cr(III)
complex undergoes further photosensitive reaction, which loses water
and chromium drops out of the PVA complex and renders the PVA soluble
again. This, they say agrees with the experience from phosphor coating
of color CRT's, but also seems to agree with some of the careful gum
printers in this list talking about moisture level in the coated layer
during exposure.

--
Ryuji Suzuki
"You have to realize that junk is not the problem in and of itself.
Junk is the symptom, not the problem."
(Bob Dylan 1971; source: No Direction Home by Robert Shelton)