Conrad
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Quinacridone-(Quinone) and Oxidation
Hi,
I am interested in quinacridone chemistry. It´s a very common organic pigment(s) and it has a ton of weird crystal modifications and behaviours. One
is the ability to form solid mixed-crystal (phases) with similar molecules such as quinacridonequinone.
One such case of solid solution is pigment orange 49 which is 94% quinacridonequinone and 6% quinacridone. Since both are insoluble pigments, they
need to be dissolved in usually H2SO4, mixed and then dumped in an ice bath to resolidify to obtain the mixed crystal pigment.
A couple of years ago there was sort of an outcry in the artist scene because the beloved pigments orange 48 & 49 went out of production since
BASF aquired Ciba and they deemed them unrentable since the automotive industry turned to better ways of making metallic paints and thus neglected
them.
It goes so far that remaining tubes of paint with PO49 are sold for triple figure prices  .
Oddly quinacridonequinone seems to have its only use for those out-of production pigments and went unobtainium (besides 1g or 5g for waaay too much at
aldrich, etc.) even none of the chinese suppliers is able to aquire it.
So, I figured why not make it myself and have something to paint with.
Total synthesis is surely possible, but I selected to just oxidize the easily obtainable quinacridone (pigment violet 19). I even found a reference
(patent, meh  ) that seemed really easy:
US6972333B2
I even had some persulfate since it is commonly used as an etchant for PCBs.
Ok, so I fetched all the stuff and went to work:
EXAMPLE 1
Quinacridone (100 grams) was dissolved in concentrated sulfuric acid (500 grams), reacted and an iron salt (0.1 grams; FeSO4) was added. The mixture
was slowly heated to 50° C. Sodium peroxydisulfate (100 grams) was added over a 4 hour period, then the mixture was stirred for an additional 2 hours
and added to water (2000 grams). After three hours of stirring, the slurry was filtered and washed with water to a pH 6.5–7.5 and dried. 109 grams
of a bright yellow highly pure quinacridonequinone was produced.
Result was what hot sulfuric acid does to organic substances after 4 hours: mainly charcoal.
But I see the benefit... the pigment gets dissolved thus making it way more accessible to reaction.
Next round:
EXAMPLE 2
Quinacridone (50 grams) was dispersed in water (1000 grams) and potassium hydroxide (50 grams) was added. The resulting slurry was then heated to 65°
C. and sodium peroxydisulfate (200 grams) was added over a 6 hour period. The resulting slurry was stirred continuously for 8 hours at 65° C. then
filtered. The resulting presscake was washed with water to neutral and dried. 55 grams of bright yellow highly pure quinacridonequinone was produced.
OK, i had no KOH at hand, only NaOH but It sort-of worked.
The pigment went more and more brown, but a bright kind of brown, not charcoal brown. After many hours of nothing more happening I washed and filtered
and got a
orange-brown substance, that really looks like pigment orange 48, which is a 40/60 solid solution of quinacridone to -quinone. Sadly no bright yellow
pure quinacridonequinone.
I suspect that due to the particle size? the pigment particles only got oxidized on the outside?
I´d love to hear your comments and ideas since It´s only a partial success and I really hate those.
[Edited on 25-9-2021 by Conrad]
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Tsjerk
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Sounds like a job for a ballmill, maybe first mill it and then oxidize,or maybe do the reaction while milling?
You could use a record player to rotate a PVC tube with caps on the ends with marbles as grinding medium.
I don't know how well this reaction runs at room temperature, but maybe it does and just takes a long time. At least you have a nice visual way to
determine how far the reaction is.
[Edited on 25-9-2021 by Tsjerk]
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Conrad
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Actally.. theres (only almost) nothing I´dont have ;-) .
I bought a cheap chinese ball mill also for other pigment related stuff. and its made of clear acrylic.. but its relatively big.
At first glance the particles of the quinacridone looked very fine, so I didn´t bother with further dispersion but aquaeous dispersions are a bitch
and you usually have conglomerates of particles that need to be smashed. Also It´s only a guess that the particles oxidized on the outside. My WAXS
syncrotron detector is at the car garage for its 100.000 mile check-up so I had to use my eyes 
But, better dispersion using the ball mill would be my next try. I even got some dispersion additives that attach to the pigment surface and keep them
from recombining (if that won´t hinder the reaction)
What I find odd is the use of conc. H2SO4 for dissolving the stuff which totally works, but only for some time... even less when hot.
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Tsjerk
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I have zero experience with reactions like this, but wouldn't it be cool if you could run the ballmill reaction dry? I have read about solventless
reactions done in a ballmill. So just peroxidisulfate and compound to oxidize in a ballmill.
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Boffis
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Hi Conrad, where did you get the starting quinacridone from? or what I am really asking is was it pure?
Have you tried preparing quinacridonequinone directly? I have a patent somewhere that describes its preparation from hydroquinone and anthranilic acid
and it doesn't sound too difficult though I must admit that I have never tried it. I dug out the patent because I have a lot of benzoquinone and was
looking for things to do with it.
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Conrad
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Hi,
ok first the solventless reactions...WOW! I haven´t checked this for a while, actually the last time at university and back then this was a
relatively new thing.. and it seems to have cone quite a way since then. I haven´t found much but the encyclopedia of organic chemistry has a chapter
about solid state reactions and on the topic of oxidations they talk about anthracene autooxidizing to anthraquinone, but only on certain faces of the
crystals due to exposure of the inner ring to oxygen species. I hope it does not relate to quinacridone, but I have a feeling it does. I was wondering
too about how the oxygen can reach the quinacridone in a solid state reaction, so theres that. But heck, what can I loose trying it besides some cheap
stuff.
And regarding the purity.. to be precise I don´t really know, but I suspect fairly pure. It´s quinacridone Pigment Violet 19 straight from BASF
being called Cinquasia as a trade name. Usually their pigments are quite pure, but I don´t know what process steps there have been done to it.
Sonetimes pigments are treated on the surface to allow for better dispersability or they are normalized in their color value by addition of very low
amounts of inert substances to allow for batch to batch standartisation.
Can you send me the patent number?
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S.C. Wack
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It sounds like the reaction is hoped to proceed through an intermediate (di)sulfonic acid...perhaps this can be made similarly to tosic acid; without
iron, and likewise crystallizable as a sodium (or perhaps zinc or aluminum) salt, or the free acid, which can then be oxidized in whichever way?
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