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INKJET INKS for PRINTERS and WIDE FORMAT PLOTTERS INTERNET SHOP

     INKJET PRINTOUT STABILITY

   Inkjet Printing

   Ink Design
   Chemical Structure of Colorants
   Classification of Colorants
   Inkjet Image Stability
   Concluding remarks

 

     Inkjet printing can be classified as a masterless digital process. In other words, compared to an conventional process, which uses a physical image carrier or plate (master) to transfer ink to paper, the digital data can be repeatedly converted directly into a printed product. All processes called inkjet share one common factor. To create an image, ink is ejected through very small orifices to form droplets that are directed to a medium. Inkjet printing is a non impact printing process. Because of the simplicity of this process there is a great variety of applications and technologies on the market today. Scientists have developed a wide range of methods to eject and form droplets. The only restrictions in applying this technology to new or higher-quality digital image solutions are imagination and physical limitations. Due to the fact that no one inkjet process or print head fabrication has served every sort of requirement, there are many technical differences among the various inkjet methods that are in use.

   On the basis of performed tests we recommend, however, to overcoat the media due to the fact that no one pigmented ink is so scratch resistant like dye ink.
The relatively scratch resistance problem is coming because the pigment ink is not going inside the substrate (printout base) but dry on while the dye is going inside one.
Overcoating also protects from torsional deflection destruction when design is printed on a flexible media and is used as a flag or a banner.

Inkjet Printing

   There are two main print head technologies on the market, one that produces drops continuously and one that produces drops on demand. The drop-on-demand print head design can be categorized into four methods: piezoelectric, thermal, acoustic and electrostatic inkjet. Currently, the thermal inkjet technology dominates the low-end color printer market [5, 6]. Both the electrostatic inkjet and acoustic inkjet methods are still in the development stage, and there are only a few products commercially available.

  • Continuous Inkjet Printing

  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 inkjet drops are charged as they brake away from the jet stream.

  • Drop-on-Demand Inkjet Printing

   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 inkjet print head 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 inkjet applications and print head 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 print head design. A thermal inkjet print head, for example, requires an ink that is vaporizable, like an aqueous- or water-based ink. Meanwhile, a piezoelectric print head 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 print head 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 inkjet 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

  • Ink and Media Interaction

   The traditional physical and chemical treatments applied to paper were not adequate to assure inkjet print quality. To enhance the quality it was necessary to investigate the interactions that occur when inkjet 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,
π—π 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 inkjet applications, the choice of the medium and its matching colorant must be made very carefully.

Ink Design

    The typical components of an ink formulation are listed below:

  • Colorant
    Normally a dye or pigment
    Usually 2-8% of the total weight by ink.

  • Solvent
    Primary ink vehicle that dissolves or suspends the colorant Typical solvents are: water, alcohols and methyl ethyl ketone Usually 35-80 %.

  • 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.

  • 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 %.

  • Dispersant
    Added to assists the colloidal suspension of a pigment. Derussol carbon black, is used as dispersant, for example. Usually 3-8 %.

  • Humectant
    Added to inhibit evaporation Glycols are typical for aqueous ink. Usually 10-30 %.

  • Viscosity Modifier
    Added to raise the ink viscosity, often a humectant like glycols. Usually 1-3 %.

  • 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 %.

  • 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 %

  • Biocide
    Added to kill bacterial and other organisms. 1,2 Benzisothiazolin-3-one, for example. Usually 0.1-0.3 %.

  • 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.

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.

 

Classification of Colorants

 

   In terms of inkjet, 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 inkjet 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 photo fading 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.

 

Inkjet Image Stability
  • 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.

  • 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).

 

  • 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 inkjet, 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
     

  • Dye Aggregation and Pigments

       By investigating the light stability of inkjet 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.
     

  •  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.
     

  • 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.
     

  • 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 inkjet prints, humidity levels that are higher than 50% are of greater concern for dark stability. Test results from the measurement of humidity effects on inkjet 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.
     

  • 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.
     

Concluding remarks

 

     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 inkjet image.
 

  • Dye ink

      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

 

  • Pigmented ink

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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