Digital BW, The Print

Paul, you quoted and wrote:

> The majority of pigments used in ink jet ink today are organic.  Organic
> pigments are produced by chemically "stacking" organic dyes to form larger
> particles.  Formerly water soluble dyes are stacked like wet glass plates
on
> top of each other, forming a crystal-like structure that precipitates out
of
> the aqueous solution. (31)
>
> [This could explain the seeming confusion about whether the pigments we
use
> contain dyes.  They may be dyes that are simply "stacked."  Nowhere did I
> see a description of a coated carbon particle.  I'm begining to wonder if
> the color pigments have any carbon in them at all.]

The last was something I have written you a long time ago: there are no
carbon particles in inkjet inks but in the black/grey inks. First appearence
of that carbon black ink must have been in HP inkjets. Carbon is mentioned
further on in your quotes.

> Smaller pigment particles have better color, gloss and transparency but
> worse lightfastness. (32)  [UC inks come to mind.]

The surface to volume increases in smaller particles and so more surface to
interact with fading agents/light. I bet that there is something else going
on in UC inks. First the encapsulation itself contains a colorant, whether
that is dye or the result of a lower stacking of dye molecules. More the
encapsulation of a ball of wooll so to speak. If it is a dye in the coating
then that will fade first, otherwise the more open structure of a less dense
particle will fade faster on light again because more surface is exposed to
light
(but less to chemical reactivity as a result of the acrylic coating).

> The harder a pigment particle is, the more abrasive it will be in the
print
> head and the more it might endanger the integrity of the fragile nozzle
> opening.  Organic pigments are generally softer than inorganic ones. (32)
> [Metal added to black to make it darker might not be the best solution.]

Metal sounds hard but this is a kind of dye component.

> Organic Colorant Classes (35)
>
> Water soluble azo dyes will have a sulfonic acid group (-S03H).  Azo dyes
> are the most common dye molecule types.  [Imagine what that acid does when
> it hits an alkaline buffer.]

Give a good bond ? ;-)

> Copper phtalocyanine cyan dyes are counted among the most brilliant and
> stable of dyes.  [The cyan pigment might be a "stack" of these.  It
appears
> to be the most stable in my fade testing.  The key to a good midtone, in
my
> view, is a pure carbon particle toned with cyan pigment.]
>
> Chemically inert pigments employed in ink jet printing include carbon
black,
> diarylide yellows, metal salt reds, pthalocyamine blues, and mono azos.
> [I'm not sure "chemically inert" implies resistance to being broken by UV
> and visible light.]

Chemically inert: less chance to bind themselves chemically / or be broken
up to / by other substances, gasses etc. Light only plays a role then when
it is part of such a chemical process.

> Pigment particles are large enough to scatter white light on their
surface,
> whereas dye molecules are much smaller than the wavelengths of visible
light
> and thus do not scatter any.

Main problem to get that fade resistance black with a high density.
Also part of the gamut problem with pigment inks.

> Dyes will not only fade due to UV, but also due to the effects of visible
> light. (52)

For the kind of work we do, the last is the main part of light fading.

> UV radiation in the 200 - 400 nm range is primarily what causes
> photochemical reactions and bond ruptures in organic compounds.

Normal Window glass cuts at 350Nm so there is only 350 to 400 left of UV
light. You better keep that part I guess as it will influence the gamut as
well. In the case that there is no fluorescent at all in ink and paper a
more UV cutting glass can be used.

> Reactions between the colorants and the substances in the coatings are
> typically of reductive and oxidative nature.
>
> Colorant Fading
>
> The photolytic deterioration of organic colorants is the main factor.
>
> If a double bond is broken, the conjugation length of the molecule is
> reduced, and the color absorption capabilities are disturbed, either
leading
> to a loss of its absorptive and reflective capabilities which is perceived
> as fading, or leading to a shift in wavelength absorption which results in
a
> color shift.  The complete rupture of a bond (cleavage), such as  is
> possible with azo groups (-N=N-) leads to the division of the molecule
into
> two smaller ones, which will each have shorter conjugation lengths. (53)
>
> [This color shift may be part of the warming we see with most dyes and
> pirments used in B&W pritning.]

I thought that it would fade the other way then, getting colder. Isn't it
that the dye or other added colorant fades and the carbon brown becomes
prominent that gives the warming ?

> The size of the pigment stacks assist in dissipating the energy.   Also,
> already faded surface molecules can act as light barriers for the internal
> molecules of the pigment crystal. (54)  [Perhaps this is part of the
reason
> there seems to be faster fading of pigments at first.]

Correct, often seen in silkscreen inks as well when exposed outdoors, clean
the surface and part of the colour returns.The thicker the layer the longer
it will withstand fading but the top layer will diminish the effect, less so
with transparant inks.

Nice report, will read a hardcopy this evening. There's that older report of
Barbara Vogt that has been mentioned on some lists before. All RIT related
research AFAIK.

http://www.geocities.com/mortenryhl/index.html

Ernst