Re: Tanning theory of dichromated colloids (was gelatin

MARTINM wrote:

>
> 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.

Here you have lost me totally. Pre-hardening step? Formaldehyde? None of
this is part of the gum process. You just put dichromate with gum and
go, that's all there is to it. So I don't understand what you're talking
about here.

 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 7.52¡Ñ10-2
> mol/dL, ceric ammonium sulfate at 15.81¡Ñ 10-4 mol/dL, H2SO4 at 0.037
> mol/dL, temperature at 50°C 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.
>

I'm afraid that here you have lost me again. I don't understand what the
point of adding this monomer would be. The dichromated gum process works
fast and the hardened gum is quite rigid. It's impervious to dilute
acid, to ammonia, and to boiling water either soaking or by pouring on
with force. The only thing that I tried that disturbed the hardened gum
was full strength household bleach. So I guess I don't understand why
one would need to complicate the process by adding another step to speed
up the process and add rigidity, as we really don't need more of either.
Katharine
Received on Mon May 17 13:32:23 2004