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There are two main printhead technologies on the market, one that produces
drops continuously and one that produces drops on demand. The
drop-on-demand printhead design can be categorized into four methods:
piezoelectric, thermal, acoustic and electrostatic ink jet. Currently, the
thermal ink jet technology dominates the low-end color printer market [5,
6]. Both the electrostatic ink jet and acoustic ink jet methods are still
in the development stage, and there are only a few products commercially
available.
In
principle,
Continuous InkJet
printing means that the
ink supply is pressurized sufficiently to create a jet. The jet will break
up into varying drop sizes based on surface waves produced by a
piezoelectric vibrator. Thus, there is a continuous flow of droplets, and
the drops have to be deflected, either to the material or to the gutter,
to create an image. Usually the deflection force is electrostatic and the
ink jet drops are charged as they brake away from the jet stream.
In all drop-on-demand methods, the ink supply is not sufficiently
pressurized to form a jet. The ink is held in a small chamber and forms a
meniscus at the orifice. The ink droplet is only produced when it is
required to form a dot on the medium. There is no deflection needed and
the drops do not need to be charged.
In piezoelectric inkjet printing a
piezoelectric element is used to squeeze individual drops out of a small
chamber by changing its shape. When an electric field is applied to a
piezo activated wall of the chamber, the wall's dimension changes a minute
amount, proportional to the applied voltage. Depending on the polarity of
the applied voltage, it is either a minute contraction or a minute
expansion. In the later case an ink drop is pushed out of the nozzle.
Thermal inkjet printing is based on the
concept that when a liquid is vaporized its volume expands tremendously.
In a thermal ink jet printhead the ink is heated up by a resistor, and a
vapor bubble is formed. The pressure inside the chamber increases due to
the growth of the bubble, and a drop is forced through the nozzle. When
the heat is suddenly cut off, the drop breaks free and the bubble
collapses back onto the heater. At the same moment the pressure decreases
and the chamber refills with ink from an ink reservoir and the cycle
starts over again.
The various ink jet applications and printhead designs require different
ink formulations. The ink chemistry and formulation not only determine the
drop ejection characteristics and the reliability of the printing system
but also dictate the quality of the printed image. In addition to matching
the color specifications, the ink should not penetrate so deeply into the
medium that it can be seen from the back. Such bleed-through of ink also
reduces the image resolution. On the other hand, this property is needed
to reduce drying time, smearing and inter-color bleed. Further, the print
should be light- and water-fast to meet the users requirements.
To ensure print reliability the ink has to be formulated to allow stable
drop formation under either continuous or drop-on-demand operation. The
two main properties to control the drop formation are viscosity and
surface tension. They have to be addressed differently, depending on the
printhead design. A thermal ink jet printhead, for example, requires an
ink that is vaporizable, like an aqueous- or water-based ink. Meanwhile, a
piezoelectric printhead needs a viscous ink to achieve ink flow through
the structure. Further, the ink must be compatible with the various
components of the fluid system. This means that the ink should not show
any chemical reaction, like corrosion, swelling or adverse interactions
with the printhead components, and that can easily be washed off orifices
and charge plates. Finally, the ink cannot pose any health or safety
problem, nor should it support microbial growth.
The above listed performance requirements [5, 9] give only a brief
overview of the desirable physical and chemical properties of ink jet
inks. Indeed, the chemical structure of the ink and its interaction with
the print media do have a great influence on print quality and image
The traditional physical and chemical treatments applied to paper were not
adequate to assure ink jet print quality. To enhance the quality it was
necessary to investigate the interactions that occur when ink jet ink is
printed on paper. The key interactions take place when the ink hits the
surface of the substrate. By modifying the structure of the colorant as
well as the surface of the medium, prints with better image quality and a
higher durability result.
There are a number of factors to consider when a colorant interacts with a
substrate. Depending on the relatively complex chemical structure of both,
different modes of interactions (i.e., covalent bonding, electrostatic or
ionic interactions, ππ
interactions, hydrogen bonding, hydrophobic interactions, dipole-dipole
interactions and Van der Waals force) take place. Further, the energy for
binding colorant to the medium has a significant influence on the fastness
properties of the print. To get the highest quality images for specific
ink jet applications, the choice of the medium and its matching colorant
must be made very carefully.
The typical components of an ink formulation are listed below:
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Colorant
Normally a dye or pigment
Usually 2-8% of the total weight by ink.
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Solvent
Primary ink vehicle that dissolves or suspends the colorant Typical
solvents are: water, alcohols and methyl ethyl ketone Usually 35-80 %.
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Surfactant, Penetrant
Added to lower the surface tension of the ink and to promote penetration
(wetting) into the substrate. Tergitol 15-S-5, a secondary alcohol
ethoxylate, is used as surfactant and isopropyl alcohol is used as
penetrant, for example. Usually 0.1-2.0 % surfactant, and 1-5 % penetrant.
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Solubilizing Agent
Added to promote dye solubility in the primary solvent. This is also
called co-solvent and is used to increase the loading of the dye, which
enhances the ink's optical density. Further it should hold the dye in
solution in case of increasing concentration due to nozzle evaporation,
for example. N-methyl pyrrolidone is used as agent, for example. Usually
2-5 %.
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Dispersant
Added to assists the colloidal suspension of a pigment. Derussol carbon
black, is used as dispersant, for example. Usually 3-8 %.
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Humectant
Added to inhibit evaporation Glycols are typical for aqueous ink. Usually
10-30 %.
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Viscosity Modifier
Added to raise the ink viscosity, often a humectant like glycols. Usually
1-3 %.
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pH
Buffer
A pH adjustment toward the basic side is typically used. This improves
ink-metal compatibility (i.e., less corrosion of the printer's metal
parts). Further pH changes influence color shifts. Triethylamine are used
as buffer, for example. Usually 0.1-1.0 %.
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Chelating agent
Added to complex metal ions to prevent scale buildup where ink may
evaporate. A typical material is EDTA (Ethyldiaminetetra-acetic acid).
Usually 0.1-0.5 %
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Biocide
Added to kill bacterial and other organisms. 1,2 Benzisothiazolin-3-one,
for example. Usually 0.1-0.3 %.
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UV-Blocker, Antioxidant, Free Radical Inhibitor
Added to promote light-fastness, or to prevent degradation of long-chain
dye molecules. Usually 1-5 %.
Not all
of these components are used in an ink formulation, and also some inks
have other ingredients that are not listed above.
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Colorants.
Chemical Structure of Colorants |
In the early days
of dye chemistry the correlation between chemical constitution and color
of organic compounds was investigated. Graebe and Liebermann recognized in
1868 that all dyes contain a system of conjugated C = C double bonds. Witt
postulated in 1876 that a compound is colored due to the presence of
particular groups, the chromophores and auxochromes, which must be linked
to a system of conjugated double bonds. In 1933 Dilthey and Winzinger
divided chromophores into chromophores and antiauxochromes. Later, as
physical and organic chemistry developed, it became apparent that
auxochromes are electron donors, antiauxochromes are electron acceptors,
chromophores are linear or cyclic systems of conjugated double bonds, and
the assembly is sometimes called a chromogen.
In the 1920s chemists started to investigate the chemical structures of
colorants in regard to their spectra, in particular to the wavelength of
the absorption maxima in the visible range. Organic compounds become
colored by absorbing electromagnetic radiation in the visible wavelength
range (400-700 nm). All molecules have electron-filled and empty orbitals,
and the conjugation allows the electrons to be delocalized over the
chain/ring system. The energy (hν) of visible light, and also ultraviolet
light (10-400 nm), is absorbed by the colorant molecule and used to
promote one of the electrons from its ground state into an orbital of
higher energy. Thus, it is the energy gap (ΔE) between the HOMO (highest
occupied molecule orbital) and the LUMO (lowest unoccupied molecule
orbital) that is critical in determining the color of a pigment or a dye.
The Einstein-Bohr frequency condition states that the energy difference (ΔE)
between the ground state and a particular excited state is directly
proportional to the observed frequency (ν), and, hence, inversely
proportional to the wavelength (λ) of the absorbed light:
ΔE =
hν = hc/λ
where h =
Planck's constant and c = speed of light
Shifts from the absorption maxima to longer wavelengths (towards red) are
called bathochromic and shorter wavelengths (towards blue) are called
hypsochromic and are directly related to the degree of conjugation.
Further shifts are produced by the presence of electron donor groups (auxochromes),
such as NH2, NMe2, OH and OR, which release electrons into the
conjugated system, and electron withdrawing groups (antiauxochromes), such
as NO2 and C = O, which take electrons out of the system.
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Classification of Colorants |
In
terms of ink jet, the differentiation of colorants in either dyes or
pigments is very important. Dyes are non-planar molecules and they may
contain solubilizing groups (e.g., sulfonic acid or carboxylic acid). The
dye crystals are less stable due to the fact that their intermolecular
forces are weaker than in pigments. Thus, they are easily broken up
by a solvent to give solution. Based on the solvent used they can be
further classified as water-based or solvent-based dyes.
A pigment, on the other hand, is an aggregation of hundreds or thousands
of molecules, depending on the size of the pigment (0.1-1.0 micrometers).
Pigments are essentially planar molecules, which usually contain strong
hydrogen bonding groups (e.g., amide CONHR and carbonyl C = O).
Further, these molecules' features promote strong intermolecular
attractive forces, which lead to a stable crystal with a high lattice
energy. Thus, pigments are solid particles and therefore practically
insoluble in the applied media. They have to be solubilized by using a
dispersant to act as a bridge between the solvent of the ink and the
pigment's surface molecules.
Chemists in the 19th century discovered synthetic dyes and pigments that
were of organic nature and so opened up the development of a variety of
colorants that could be chemically modified. Most organic pigments are
closely related to dyes (with respect to their chemical structure).
Furthermore, dyes can be formed into pigments by aggregation and binding
the dye molecules into particles
The first colorants
used for ink jet printing were
water-soluble dyes.
Pigments were not used, because they could not reach the color gamut of
dyes, and they did not perform reliable due to the fact that they are not
soluble. However, pigmented inks are now available that perform reliably
and have a color gamut approaching that of dyes. Now, the ink
manufacturers are trying to improve the permanence requirements of prints,
such as light-fastness and water-fastness, mainly in two different ways.
Some are continuing to try to control dye aggregation, while others are
more focused on producing stable pigment dispersions of smaller particle
sizes.
Pigments
have the advantage of better light-fastness primarily because there are
more chromophores in the pigmented particles than in the dye molecules.
Light may break apart the chromophores in both the dye and the pigment
image, but the pigment image lasts longer because the decomposition of
chromophores is less per time. This basically means that all dye molecules
(due to their large surface area) are reached by photofading agents, while
only the pigment molecules at the surface of the particle (10% of the
total) absorb photons. The disadvantage, on the other hand, is that larger
particles lead to light scattering on the surface, which reduces color
saturation and gives a duller or more matte surface. Because the color
gamut depends on chroma and chroma depends on the purity of the reflected
light, colorants with a narrow, symmetrical absorption band display the
highest chroma. Due to their monomolecular state dyes have the advantage
of a narrow absorption band in the visible light spectrum. Aggregation of
molecules leads to a broader absorption curve, which results in dullness
[10]. Further, the individual dye molecules are so much smaller than the
wavelength of light that no light scattering is possible. Pigment
particles in the size of 0.2-1.0 microns on the other hand are able to
scatter light (0.4-0.7 microns wavelength). Pigment particles can also
cause nozzle clogging and crusting problems as well.
Thus, the chemistry of forming a stable, uniform dispersion of solid
particles in a liquid is more difficult than in dye chemistry. The
development of pigment-based inks is a greater technical challenge than
that of dye-based inks. The achievement of a particle dispersion stable
enough to compete with dye solutions was the key to pigmented ink, in the
beginning used mainly in outdoor applications, i.e., advertising, due to
its better durability.
Because of the possibility of modifying the chemistry of a dye to match
that of the medium coating, dyes have greater versatility, and they are
still mostly applied in indoor use, i.e., desktop printers in home or
office environments.
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Light-Fastness
In general, dyes resist fading in a vacuum. In contact with the
atmosphere, the media and other compounds in the ink, they will fade to
varying degrees. The understanding of the controlling mechanisms is
somewhat limited due to the fact that there is no single, well-defined
mechanism explaining the photodegration. More often there is a whole group
of mechanisms that have to be understood in order to improve
light-fastness.
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Photo-oxidation
and Photo-reduction
On absorbing a photon, the dye is excited to a higher singlet state
(S1). Then it can return either to the ground state
(S0) directly, resulting in fluorescence or, by "inter
system crossing" (ISC), to the triplet state
(T1), resulting in phosphorescence.
The energy levels and possible quantum processes for typical dye molecule.
The mechanism of energy transfer between molecules is a resonance
dipole-dipole interaction.
Because UV radiation has more energy than the light of the visible
spectrum it forces more excited singlet states. Visible light gets
absorbed by the dye as well, this is why we can see the color of the dye.
When the dye is photo-exited (D*) it can undergo a photochemical reaction
leading to degradation.
Most colorants undergo oxidative fading in the presence of light,
moisture and oxygen. Either the photo-excited dye can react with water and
the hydrogen peroxide, formed in a second step from oxygen, and the
hydrogen radical destroys the dye, or the dye can react with oxygen
leading to singlet oxygen which also destroys the dye direct or indirect:
Another possible photochemical reaction which destroys the dye's molecule
into a colorless product is the reductive mechanism. Either the dye
picks up a hydrogen or an electron transfer takes place :
The photostability can be increased by reducing the lifetime of the dye in
its excited single state, i.e by adding a substituent (antioxidant) or
changing the dye's molecular structure (auxiliary groups). The mechanism
of decomposition of azo dyes (i.e., magenta dyes) is presented in the
figure below. The oxidative fading of an azo dye has been attributed to
the attack of singlet oxygen on its hydrazone tautomer. This initial
reaction produces an unstable peroxide which rapidly decomposes into a
colorless molecule. While this reaction is promoted by singlet oxygen
sensitizers (e.g., other dyes), singlet oxygen quenchers such as
1,4-diaszabicyclo[2,2,2]-octane (DABCO) and nickel-dibutyldithiocarbamate
(NBC) suppress the fading.
Oxidation mechanism for azo dyes
The reductive fading mechanism of an azo dye under anaerobic conditions is
based on the addition of a hydrogen donor, like alcohols, amines, ketones,
carboxylic acids, ethers and esters. This reaction is greatly accelerated
when either the hydrogen donor or the dye is photo- excited:
Reduction
mechanism for azo dyes (magenta).
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Photocatalysis
As mentioned above, excited dyes can produce singlet oxygen resulting in
oxidative fading. Catalytic fading happens when one dye can transfer its
absorbed energy to another dye at a lower energy level and increases the
other dye's radiative exposure and its fading. The dyes have to be mixed
on the image for catalytic fading to take place. In ink jet, for example,
it is possible to observe catalytic fading of magenta dyes by the presence
of cyan dyes, i.e., in a blue hue where the cyan and magenta dots are
overlapping
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Dye
Aggregation and Pigments
By investigating the light stability of ink jet images it has been
found that dye aggregates (formation of micelles) are more resistant to
fading than their monomolecular state.
The positive effect that aggregation can have on light-fastness has been
attributed to several factors. For example, these larger aggregates
diminish the attack by radicals due to the surface area per unit mass of
the available dye. Since light is absorbed within the surface layers of
the larger aggregates, as the outer layer is degraded, reactants diffuse
more slowly through it to reach the reservoir of unreacted dye in the
interior. Another factor is that the lifetime of the dye's excited state
is possibly shorter in the aggregated state, which allows it less time to
react. These arguments explain the reduction of fading rate over time in
aggregated dyes. They are the same arguments used to explain the better
light-fastness of pigments over dyes. Organic pigments are known to
achieve their light-fastness due to their particle-forming properties.
However, if they are fine enough to meet the requirements of modern
printers, such as passing through the nozzle and matching the color gamut
of dyes, they begin to lose their inherently better light stability due to
the reduced stability of smaller particle sizes.
Dye aggregation can be induced in several ways; for example, it can
be induced by reducing the dye's solubility via a co-solvent. A less basic
pH and the addition of salts also lead to aggregation. Overall, the two
most important variables to control aggregation are relative dye
concentration and solvent concentration in the drying dot in the image
layer. The decrease of dye concentration and/or increase of solvent
concentration leads to fewer dye molecules in a dot, where they are no
longer able to build aggregates of larger particle size. The use of
diluted ink, i.e., in a six-ink system to improve the highlights of an
image, reduces the light-fastness of low and medium densities up to a
factor of two.
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Effect of
Additives
The additives used in an ink formulation can influence the light
stability. For example, optical brighteners, mainly used in paper based
substrates to make the paper appear whiter, can have a great influence on
the photo-fading mechanism. Basically these brighteners are designed to
absorb photons of one energy (usually UV light) and emit a photon of a
lower energy in the visible spectrum. Now, as the dye comes into contact
with the brightener it has the opportunity to absorb energy, not from a
photon but from an optical brightener molecule excited by a photon. These
energy transfer mechanisms are well known and can function as an
additional center where a photochemical reaction begins and cascades.
Other substances are added to protect the chromophores and enhance light
stability. Depending on the photo-fading mechanism, reducing agents or
antioxidants could be added. Further, it should be kept in mind, that the
components chosen to stabilize one colorant may well destabilize others.
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Water-Fastness
Due to the widespread use of water-soluble ink, water-fastness can
be a major issue for water-based dyes. These dyes are needed to achieve
maximum freedom in formulating the ink to perform reliably in the printer,
but once on paper they should not re-dissolve or disperse on contact with
water. There are two successful approaches. One is based on pH and the
other on a zwitterionic-type mechanism.
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Humidity-Fastness
The term " humidity-fastness" describes the durability of an image
under conditions of high humidity at sometimes higher than normal
temperature. These conditions can lead to dye de-aggregation and to dye
diffusion (bleeding) through the medium. These effects should be kept in
mind when looking at accelerated light-fading tests. The high irradiance
tends to dry out the test samples, which often helps to preserve them. But
dry environmental conditions cannot be assumed for all locations where
prints will be displayed; often they are much cooler and more humid. Test
results by Ilford show that density changes are typically 1.5-3 times
higher under humid conditions, than under dry conditions. However, the
separation of these two effects (humidity and light) in accelerated fading
experiments is very difficult. Another issue where humidity has a great
influence is dark storage print life. ISO Standard 10977 recommends
primarily testing at a humidity of 50% ± 3% at different temperatures for
color photographic materials.
For ink jet prints, humidity levels that are higher than 50% are of
greater concern for dark stability. Test results from the measurement of
humidity effects on ink jet prints kept in the dark were given by Kodak at
the NIP 16 Conference in Vancouver. The observed changes, like lateral ink
diffusion (dye smear or blur), density changes (increase or decrease) and
color balance changes (hue shifts) were measured. It was found that a
relative humidity level of 60% is enough to cause significant changes in
the image quality of the test target. At an increase of humidity above 60%
the amount of time eliciting these changes decreased.
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Ozone-Fastness and Other Pollutants
Ozone has been suggested as a reason for fading of inkjet prints,
but it has not yet been proven. There is a hypothesis that exposure to an
ambient level of ozone over time can cause significant fading of inkjet
prints. The impact of high levels of ozone in accelerated testing is easy
to see, but the fact is that these high concentrations of ozone regularly
force chemical reactions that can result in dye fading. However, no one
has yet demonstrated that the level of ozone present in normal air at
ground level is unambiguously an agent that causes significant
degradation. On the other hand, it is well known that airflow (e.g., air
conditioning systems) causes fading in inkjet images. As other pollutants,
such as
NOX and
SO2, are obvious possibilities as well, it has not yet
been proved which one really is the culprit.
The effect of environmental pollution on the aging stability of papers as
archival materials has been addressed in several investigations. The
studies have been focused mainly on the interaction between paper and SO2
or NO2, since the impact of ozone (as a bleaching agent) on
paper degradation has attracted less interest in the field of paper
conservation. It has been found that
NO2 and
SO2 or a mixture of
NO2/SO2 causes yellowness to varying degrees
on tested papers.
The dye-based samples on glossy photo paper show the highest fading rate
overall. The dyes turned out to be fairly unstable, especially under the
50 klx light condition. After only one week of exposure a density loss of
up to 25% (magenta dye) in the low densities was noticeable. The influence
of the media on the stability of the dye could be seen best with the cyan
dye. While the cyan dye was fairly stable against VIS light on both papers
(glass-filtered samples), the dye was very unstable against airflow on
glossy photo paper. There does not necessarily have to be a pollutant like
ozone present to decompose the cyan dye. This was also true for the
magenta dye although it had a lower fading rate. The yellow dye was fairly
stable against both effects.
The cyan samples exposed to airflow (150 lx) in the light fading room
showed significantly more fading after 35 days (real time) than the
samples exposed for one day to 1 ppm
O3, which equals forty days at an average of 0.025 ppm
O3. The cyan dye printed on glossy photo paper reached
nearly the same rate of fading after fourteen days of airflow and two days
in 1 ppm O3. The humidity in both cases was nearly the same
because the samples exposed to airflow were not heated up and so not dried
out by a high irradiance. The light fading room was set to 55% RH and the
pollution chambers to 50% RH. However, it is not possible to determine the
exact rate of fading caused by ambient air or by ozone due to the unknown
concentration of ozone in the ambient air and the possibility of
reciprocity failure in the accelerated pollution test. However, it was
definitely shown that a high airflow (e.g., from air conditioning systems)
without measurable concentration of pollutants can cause a significant
rate of fading in an ink jet image.
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Dye |
UV |
Airflow |
Ozone |
NO2 |
|
Cyan
Magenta
Yellow |
Very unstable
Very unstable
Very unstable |
Unstable
Rel. stable
Stable |
Very unstable
Unstable
Rel. stable |
Unstable
Rel. stable
Unstable |
|
Pigment |
UV |
Airflow |
Ozone |
NO2 |
|
Cyan
Magenta
Yellow |
Unstable
Rel. stable
Rel. stable |
Rel. stable
Stable
Very stable |
Rel. Stable
Rel. Stable
Stable |
Stable
Stable
Stable |
where:
Very stable:
no fading, fading rate below 4%,
Stable:
nearly no fading, fading rate below 10 %,
Rel. stable:
slow fading rate, max. 20% after eight weeks,
Unstable:
approx. 10% fading rate after the first week of exposure, max. up to
40-50%,
Very unstable:
over 15% fading rate after the first week of exposure, max. up to 60-70%.
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