Digital BW, The Print

But arguably one could make a "dye" ink of very small carbon particles.  I
think the point of an earlier post was that pigment was a generic term
equally applicable to dye inks.  My understanding is the critical issues are
the size of the pigment particle and whether or how it is encapsulated.
Smaller particles sit better on or in paper but their mass to surface ratio
leads to faster oxidation, as you say.  I would like to hear from John
Edmunds on this.  The question is what might the optimum particle size
be...if one found a way to make markedly smaller encapsulated pigment
particles you would gain from their ability to soak into paper better and
from an overall greater saturation of pigment per ounce of ink and hence
better colour gamut.  But what about fade?


> From: Paul Roark <paul.roark@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Fri, 11 Mar 2005 09:31:30 -0800
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: RE: [Digital BW] What is actually in Ultrachrome inks?
> 
> 
>> ... what actually is in UltraChrome ink?
>> Are they a hybred inkset composed of some dye and mostly pigment,
>> or what? ...
> 
> With the caveat that I'm making educated guesses here, since neither Epson
> nor any other pigment company is about to disclose anything to me, let me
> outline some of what I've been able to discern from various materials.
> 
> To start, the main distinction between a dye and a pigment is that pigments
> are solid particles that are suspended in a fluid -- mostly water in our
> case.  Dyes, on the other hand, are dissolved colorants -- like sugar in
> water.  This leads to the main performance differences between them.  The
> dyes go on as a liquid and then dry to become a thin crystalline coating on
> the surface.  This makes for a nice even, high gamut coating, but the
> crystals are very small.  Lightfastness increases with the size of the solid
> crystal particle, the pigments are huge rocks compared to the dye crystals.
> 
> (Note that with an average pigment particle size of about 0.1 micron, our 25
> micron nozzles do not get plugged by these "rocks" in and of themselves.)
> 
> The reason lightfastness is related to particle size is that most of fading
> is oxidation.  The larger the particle, the less surface area it will have
> relative to its mass.  Thus, with less surface exposed to the oxygen, the
> colorant simply fades slower.
> 
> So, for lightfastness, we'd actually like the largest particles possible.
> Unfortunately, the large particles would sink to the bottom of our carts.
> While the pigments are selected, in part, based on their relatively low
> specific gravities, they are all still heavier than the fluid carrier.  To
> keep them in suspension the particles must be very small, the reasons being
> the opposite of what we'd like for lightfastness -- the smaller particles
> have more surface area to mass, thus increasing the effectiveness of
> viscosity and Brownian motion to keep them from sinking immediately.
> 
> Chemically, the dye and pigment colors may be close to the same.  Some of
> the pigments are "dye stacks" -- non-soluble, relatively large crystals of
> dye.
> 
> The pigments that are probably used by Epson and the others are organic
> compounds.  That is, they are based, in part, on carbon.
> 
> (Epson's Material Safety Data Sheets are of little value here because they
> just call almost everything, "proprietary dyes and pigments.")
> 
> The cyan pigment is probably a "phthalocyanine."  This elegant molecule
> looks like a symmetrical snowflake, with 4 carbon rings linked into a flat
> disk by carbon and nitrogen.  There is a copper atom in the middle.  This is
> a very important and stable class of pigments for industry.  The RIT tests
> that MIS has published also show the cyan to be extremely lightfast.
> 
> The magenta is probably a "quinacridone."  This is composed of a linear
> array of 5 carbon rings -- thus the "quin" -- with 2 pairs of oxygen and
> hydrogen hanging off it.
> 
> The yellow is probably a "monoazo (arylide)."  This is a pair of carbon
> rings joined by nitrogen to a central cluster of 4 carbons.  Colors are
> controlled  by various atoms hanging asymmetrically off the sides.  While
> this is only moderately lightfast, it also has the lowest specific gravity,
> thus allowing the particles to be made larger.
> 
> What probably distinguishes the pigments from Epson and the others the most
> is the surface treatment.  Epson uses an acrylic coating, whereas the others
> use different surface attachments to help keep the materials in suspension
> and dispersed, and help them stick to the paper.  There are arguments for
> and against Epson's choice of acrylic.  Since it is not a good oxygen
> barrier, it probably has little effect on lightfastness.  In fact, if it
> makes the primary pigment particle smaller, its effect could be negative.
> 
> I would guess that UltraChrome inks are pure pigment, with no liquid dyes in
> the fluid.  (A more interesting question is whether the acrylic coatings
> have dyes impregnated in them, and I have no clue on this.)
> 
> Anyway, that is my long-winded, best guess as to the pigments that are in
> both the Epson and third-party products.  There are many minor variations of
> these basic pigment classes that alter the exact chemical structures and
> colors.
> 
> Paul
> www.PaulRoark.com
> 
> 
> 
> 
> 
> 
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