Re: gloy for tricolor on yupo?

From: Katharine Thayer <kthayer@pacifier.com>
Subject: Re: gloy for tricolor on yupo?
Date: Mon, 03 Apr 2006 10:47:52 -0700

> Now there's a categorical statement. I would tend to agree that it
> seems odd to classify gum arabic as a protein; I would be more likely
> to classify it as a complex polysaccharide, since 98-99% of it is
> comprised of monosaccharides, (the bulk of these galactose,
> arabinose, rhamnose, and glucuronic acid) and only 1-2% is amino
> acids.

I agree that it is very odd to classify gum arabic as a protein unless
your focus is on the protein part. The reference you gave, Williams et
al is focusing on the protein nature of gum arabic and it's not too
odd that they call it protein.

Generally protein is defined as molecules comprising mostly linear
chains of amino acids connected by amide bonds. The maximum of 2% in
gum arabic is amino acids, and how much of it is protein?

> Sorry, I'm having trouble keeping up; I'm not sure I'm seeing all
> your posts.

I don't know if you are seeing but you didn't respond to my reply
about your argument of DCPVA work of Bolte. I'm warning that applying
her recent results to other cases is risky.

> Okay, so new information about PVA crosslinking makes
> it unlikely that PVA crosslinking and gum arabic crosslinking are
> similar, so you would no longer cite literature on PVA crosslinking
> to answer questions about gum crosslinking? In which case, we don't
> even have a clue about how crosslinking occurs in gum.

Why are you disregarding the wealth of relevant chemical information
in the literature and jumping the gun like that? You just need to use
good analytical skills to extract relevant pieces and think about the
most likely mechanism, and then test it.

Below is my post to which you didn't respond. To add a mini summary,
Bolte's and others' work indicate that the reactions involved in DCPVA
depends on the purity of PVA and the moisture content. You are
referring to Bolte's paper on very pure PVA and completely dry film
and trying to generalize to dirtier PVA or gum, which contains some
moisture, and in case of gum printing, receiving postexposure
rinsing. This is the problem.

From: Ryuji Suzuki <rs@silvergrain.org>
Subject: Re: A PVA for printing "gum" from Mike Ware
Date: Fri, 31 Mar 2006 11:50:58 -0500 (EST)

> From: Katharine Thayer <kthayer@pacifier.com>
> Subject: Re: A PVA for printing "gum" from Mike Ware
> Date: Fri, 31 Mar 2006 07:41:49 -0800
>
> > As to the latter, I'll quote the Bolte paper again (the
> > introduction): "For a long time chromium (III) the final chromium
> > species was thought to be responsible for the crosslinking process
> > in dichromated systems. This assumption is quite logical with
> > dichromated gelatin, chromium (III) being the only chromium species
> > remaining after development. On the contrary, it is more surprising
> > when dealing with dichromated PVA, a material in which chromium (V)
> > has been proved to be stable..." I suppose it's statements like
> > that which make me wonder if maybe the mechanism for gelatin might
> > be different than that for PVA/gum.
>
> I didn't read Bolte's 2005 paper, but in my understanding of her work
> from a couple of years ago, she is recently interested in the behavior
> of completely *dried,* dichromated *pure* PVA *film* with no
> postexposure rinsing. If you read her papers carefully, you'll know
> that moisture content of dichromated PVA or introduction of a few
> percents of carboxyl groups (for example, by oxidation) can change the
> results dramatically.
>
> There is no good way to make a usable hologram from dichromated
> gelatin without rinsing. Gelatin also contains significant fraction of
> carboxyl groups. So in dichromated gelatin, Cr(V) is not found after
> the exposure is turned off and the material is rinsed.
>
> In the case of dichromated PVA *solution,* Cr(V) is rapidly dropped
> after the exposure is turned off. This indicates that the presence of
> moisture changes the reaction and Cr(V) becomes unstable; it's
> converted to Cr(III). However, the measurement methods used by Bolte's
> lab is not very efficient in detecting Cr(III) because it gives very
> weak UV-vis spectroscopic signal at the amount sufficient to crosslink
> the polymer, so absence of Cr(III) discussion/data in their paper does
> not mean absence of Cr(III). Cr(III) can be measured more effectively
> by low frequency dielectric loss, for example.
>
> > [...] the fact that when I asked a question about gum crosslinking,
> > Ryuji cited a paper about PVA crosslinking, suggests that he's
> > probably in agreement with the former.
>
> I think the reaction with gum is similar to not overly pure PVA (which
> is what Bolte and others are using lately), or a blend of PVA and a
> few percents of poly(acrylic acid).
>
Received on Mon Apr 3 16:01:35 2006