Re: Tanning theory of dichromated colloids (was gelatin

From: MARTINM ^lt;martinm@SoftHome.net>
Date: 05/17/04-11:42:08 AM Z
Message-id: <002a01c43c37$ef18bc20$5e0adb50@MUMBOSATO>

"So in this case they knew they already had Cr(III) but the EDTA inhibited
the crosslinking?"

I am not sure if this applies to Cr(III) specifically but certainly to
"dichromate". If the quantity of EDTA (actually the dichromate/EDTA ratio)
reaches a certain threshold, it will impair any crosslinking to take place.
In former dye sensitized DCG systems EDTA used to be added as an electron
donor for the dye.

"I didn't completely understand what he was saying so I haven't incorporated
it into my thinking. But is this what you're talking about?"

I was thinking of adding monomers to the colloid. I expect this to enhance
both speed and rigidity of the colloid. E.g. it actually might "replace" the
pre-hardening step - hence the handling of formaldehyde and the like. In
some areas grafting colloids seems to be a common thing. To give you an
example of a
graftet gum, see:

"Synthesis and Characterization of Poly(methyl methacrylate) Grafted from
Acacia Gum

P. Chowdhury*, S. Samui, T. Kundu, B. Saha
Department of Chemistry, Siksha Bhavana, Visva-Bharati, Santiniketan-731235,
India

Abstract. Graft polymerization of methyl methacrylate onto acacia gum
has been studied in detail. The grafting was found to be optimal under the
following reaction conditions: gum at 0.4 g/dL, monomer at 7.5210-2
mol/dL, ceric ammonium sulfate at 15.81 10-4 mol/dL, H2SO4 at 0.037
mol/dL, temperature at 50C and time at 3.0 h. Fourier transform infrared
spectroscopy was sed for the confirmation of grafting. Thermal and physical
properties of the copolymer were studied. A probable mechanism of
polymerization has been suggested based on reaction kinetics."

I suppose methyl methacrylate would not be particularly convenient in the
context of a dichromate system. No doubt there are more promising (more
reactive and hydrophilic) monomers around.
With regards to the grafting catalyst ceric ammonium sulfate I am fairly
sure it would be compatible with dichromate.

"This is a book? Is it in English?"

I don't know. I only have a (bad) copy of that report. If I managed to make
my scanner work, I could send you a copy.

" Thanks for the dialogue,"

Thanks for trying to get back to the very roots of dichromated systems.

Martin

----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Sunday, May 16, 2004 5:07 PM
Subject: Re: Tanning theory of dichromated colloids (was gelatin

> MARTINM wrote:
> >
> >
> > I guess the EDTA thing goes back to Oster & Oster who pointed out that
EDTA
> > interfered with the crosslinking action of Cr(III). That was in the
context
> > of
> > dye sensitized DCG.
>
>
> So in this case they knew they already had Cr(III) but the EDTA
> inhibited the crosslinking?
>
> >
> > "Duncan & Dunn also suggest that their observations rule out the
> > possibility of free radical (polymer) intermediates, but Mannivannan et
> > al show clearly the presence of such an intermediate."
> >
> > That's interesting. I wonder whether its (the free radical's) action
would
> > be sufficient for certain monomers to initiate polymerization.
>
> My chemist friend, after reading Mannivannan et al last weekend, said
> that the formation of a polymer radical suggests to him that once the
> radical is formed (by transferring electron to chromium) then it could
> start a series of reactions among the gum molecules to effect the
> hardening without further chromium being required. I didn't completely
> understand what he was saying so I haven't incorporated it into my
> thinking. But is this what you're talking about?
>
> > Incidentally, Sasaki, Honda, Kichuchi (Studies on photosensitive
dichromated
> > materials, Tokyo 1979)
>
> This is a book? Is it in English?
>
> suggest possible intermediate Cr(V) or Cr(IV)
> > formation upon photoreduction of HCrO4 to Cr(III).
>
> Mannivannan et al (1993) say Cr(V) but not Cr(IV), in their
> investigation of dichromated PVA.
>
> Thanks for the dialogue,
> Katharine
>
>
>
> > ----- Original Message -----
> > From: "Katharine Thayer" <kthayer@pacifier.com>
> > To: <alt-photo-process-l@sask.usask.ca>
> > Sent: Friday, May 14, 2004 9:05 AM
> > Subject: Re: Tanning theory of dichromated colloids (was gelatin
> >
> > > Martin M wrote:
> > >
> > > Katharine Thayer wrote:
> > >
> > > Duncalf and Dunn, which I am waiting to read myself
> > > >> before I draw any conclusions about its utility, but which
according to
> > > >> Manivannan et al doesn't establish whether the chromium is bonded
to
> > the
> > > >> gum. It's hard to see how this study could address that question
> > anyway,
> > > >> since it's pretty obvious even from the brief summaries that it's a
> > > >> qualitative study. There's always going to be some trivalent
chromium
> > > >> there; the question is whether it's bonded to the colloid or not.
> > >
> > > >I assume that's the Canadian group (Lessard, Couture, Changkakoti,
Bolte,
> > > >Solano, Capollo etc.)...
> > >
> > > Yes, that's right. Perhaps Ryuji answered this already, but I haven't
> > > read his posts in this thread past the one where he accused me of
having
> > > an "anti-scientific mind" because after going through another crisis
> > > about my father, I don't have any emotional energy left to deal with
the
> > > sneering and jeering, and besides 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 do have Duncalf and Dunn in my hands now. The qualitative analysis
> > > that I'd seen summarized elsewhere is only a part of the research;
most
> > > of the work is UV spectrometry. The qualitative analysis is just as I
> > > thought:
> > >
> > > "Insoluble film from which residual dichromate had been extracted was
> > > dissolved in 0.1N hydrochloric acid and reprecipitated by running the
> > > solution into acetone. The precipitate was insoluble in water and
> > > contained chromium."
> > >
> > > 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.
> > >
> > > 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." Unfortunately, just as Mannivannan et
al
> > > commented, there is no direct proof to back up the attribution of the
> > > crosslinking to this purported coordination. The attribution is based
> > > on these two observations: (1) "insolubilization of the film on
exposure
> > > to light could be prevented by incorporating at least enough
> > > ethylenediaminetraacetic acid to combine with 75% of the chromium
> > > present." The assumption of the authors seems to be that the reason
> > > crosslinking is prevented by doing this is that the chromium becomes
> > > unavailable to coordinate with the polymer. But it seems more
reasonable
> > > to me, if I were speculating something, to speculate that the reason
the
> > > crosslinking doesn't occur is because the chromium, by bonding with
the
> > > reagent, has been rendered incapable of participating in the electron
> > > transfer that makes the crosslinking possible. (2) insoluble films
could
> > > be dissolved with the same reagent. This observation does suggest
> > > chromium coordination to me, but it doesn't constitute anything like
> > > proof.
> > >
> > > 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)."
> > >
> > > Duncan & Dunn also suggest that their observations rule out the
> > > possibility of free radical (polymer) intermediates, but Mannivannan
et
> > > al show clearly the presence of such an intermediate.
> > >
> > > P.S. One of my guests last weekend was a chemist; he saw the pile of
> > > books about chromium chemistry etc and was curious. I told him I was
> > > trying to figure out the chemistry of the gum process in the absence
of
> > > any research on the topic, and that I was especially interested in the
> > > question of what happens to the chromium, whether it attaches to the
> > > matrix or not. He was intrigued and read all the material I have, and
> > > when he was done I asked, "So what happens to the chromium?" He said,
> > > "It looks to me like no one knows what happens to the chromium." Which
> > > is what I've been saying for five years. He says that the chromium
could
> > > just as easily stand by to participate in the electron transfer
without
> > > actually coordinating with the colloid.
> > >
> > > Those of you who are quick to jeer for little reason should understand
> > > that I've never said that the chromium DOESN"T attach to the matrix.
> > > All I'm saying is that no one knows whether it does or not. The
argument
> > > that it "must" do so because that's the kind of thing chromium likes
to
> > > do, is theoretical and no matter how much sense it makes logically, it
> > > has to be demonstrated empirically; so far such empirical
demonstration
> > > seems hard to come by.
> > > Katharine Thayer.
Received on Mon May 17 11:55:00 2004

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