Digital BW, The Print

> ... 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