Digital BW, The Print

TiO2 is the common white pigment used in commerical white paints and in most
artist's white paint. Compared to the lead pigments that it replaced, it
doesn't tend to darken with exposure to sulphides and the like. I have used
tens of pounds of the stuff in making rabbit glue gesso panels for my wife's
egg tempera paintings and if it fluoresces, it sure ain't much. It is
inherently extremely stable. I have a fifty pound sack of the stuff that is
unchanged over at least ten years. I no longer have access to analytical
intruments but if someone does, a quick scan with a recording
spectrofluorimeter would answer the question quickly. I certainly would not
consider it a typical  OBA. I have no doubt that it reflects UV as well as
the rest of the visible spectrum since it is, afterall, a very white white.
 
Since it reflects UV that could certainly add to the damage UV causes to
plastics with time since it would intensify the UV exposure and hence the
degradation of the plastic.
 
Roger
 
Roger L Sopher 
rlsopher@... 
http://deCorrales.com <http://decorrales.com/>  

 

  _____  

From: DigitalBlackandWhiteThePrint@yahoogroups.com
[mailto:DigitalBlackandWhiteThePrint@yahoogroups.com] On Behalf Of Paul
Roark
Sent: Friday, April 06, 2007 1:05 PM
To: DigitalBlackandWhiteThePrint@yahoogroups.com
Subject: RE: [Digital BW] Kayenta yellowing too!





> ... There are no white "Dyes" for inclusion in paper for
> Chemical and process reasons,

I think the OBAs are usually in the coating, not the paper itself. Many, if
not most, of these OBAs are considered "dyes." See
http://en.wikipedia <http://en.wikipedia.org/wiki/Optical_brightener:>
.org/wiki/Optical_brightener: "Optical brighteners,
optical brightening agents, fluorescent brightening agents or fluorescent
whitening agents (FWAs) are dyes that absorb light in the ultraviolet and
violet region of the electromagnetic spectrum, and re-emit light in the blue
region..."

My questions with respect to the TiO2 claim are more with respect to whether
it is a true OBA and whether it has other properties that make it
undesirable in papers. 

"Titanium dioxide (TiO2) reflects ultraviolet light, ...
[I]n the presence of sunlight and water, TiO2 catalyzes the formation of
hydrogen peroxide, ..."
http://www.ingentac
<http://www.ingentaconnect.com/content/klu/bico/2003/00000048/00000005/05122
4> onnect.com/content/klu/bico/2003/00000048/00000005/051224
62 

If TiO2 reflects UV rather than absorbs it and re-emits it as blue light
like the OBAs we usually talk about, it might still be considered a
"brightener" in that it is very white, but it is not the same as the dyes
that convert UV to visible light. This might explain why with respect to at
least one Premier Art paper claimed to be "Bright White" I did not detect
fluorescence under my UV light.

TiO2's catalytic properties were apparently what caused the polyethylene
cracking in the original RC papers (according to an old Kodak publication).
I wonder if that same property has any effects on fading, which, after all,
is an oxidation process.

On the other hand the above paper, which is not talking of paper, mentions
that TiO2 can be coated to eliminate the catalytic properties. So, it may
be that the coating companies are using an advanced form of the very white
pigment to make their papers very white, even if not fluorescent. 

At any rate, I'm not an expert in this, but it is interesting. Having a
very white paper that did not yellow quickly would clearly appeal to many
people, and it may be that some paper companies have found a way to do this
with a form of TiO2 that is much longer lived than the old OBA dyes that
"faded" (lost activity) rather quickly. I have not had the time to do a
comparison test of the various papers, but the Premier Art paper that is
"Bright White" yet does not fluoresce under my black light would definitely
be one I'd want to have in a comparative test.

Paul
www.PaulRoark.com 



 


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