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Ink formulations

The basic problem with inkjet inks is the conflicting requirements for a coloring agent that will stay on the surface vs. rapid dispersement of the carrier fluid.

Desktop inkjet printers, as used in offices or at home, tend to use aqueous inks based on a mixture of water, glycol and dyes or pigments. These inks are inexpensive to manufacture, but are difficult to control on the surface of media, often requiring specially coated media. HP inks contain sulfonated polyazo black dye (commonly used for dyingleather), nitrates and other compounds. Aqueous inks are mainly used in printers with thermal inkjet heads, as these heads require water to perform.

While aqueous inks often provide the broadest color gamut and most vivid color, most are not waterproof without specialized coating or lamination after printing. Most Dye-based inks, while usually the least expensive, are subject to rapid fading when exposed to light. Pigment-based aqueous inks are typically more costly but provide much better long-term durability and ultraviolet resistance. Inks marketed as "Archival Quality" are usually pigment-based.

Some professional wide format printers use aqueous inks, but the majority in professional use today employ a much wider range of inks, most of which require piezo inkjet heads and extensive maintenance:

  • Solvent inks: the main ingredient of these inks are volatile organic compounds (VOCs), organic chemical compounds that have high vapor pressures. Color is achieved with pigments rather than dyes for excellent fade-resistance. The chief advantage of solvent inks is that they are comparatively inexpensive and enable printing on flexible, uncoatedvinyl substrates, which are used to produce vehicle graphics, billboards, banners and adhesive decals. Disadvantages include the vapour produced by the solvent and the need to dispose of used solvent. Unlike most aqueous inks, prints made using solvent-based inks are generally waterproof and ultraviolet-resistant (for outdoor use) without special over-coatings. The high print speed of many solvent printers demands special drying equipment, usually a combination of heaters and blowers. The substrate is usually heated immediately before and after the print heads apply ink. Solvent inks are divided into two sub-categories:
    • Hard solvent ink offers the greatest durability without specialized over-coatings but requires specialized ventilation of the printing area to avoid exposure to hazardous fumes.
    • Mild or "Eco" solvent inks, while still not as safe as aqueous inks, are intended for use in enclosed spaces without specialized ventilation of the printing area. Mild solvent inks have rapidly gained popularity in recent years as their color quality and durability have increased while ink cost has dropped significantly.[6]
  • UV-curable inks: these inks consist mainly of acrylic monomers with an initiator package. After printing, the ink is cured by exposure to strong UV-light. The advantage of UV-curable inks is that they "dry" as soon as they are cured, they can be applied to a wide range of uncoated substrates, and they produce a very robust image. Disadvantages are that they are expensive, require expensive curing modules in the printer, and the cured ink has a significant volume and so gives a slight relief on the surface. Though improvements are being made in the technology, UV-curable inks, because of their volume, are somewhat susceptible to cracking if applied to a flexible substrate. As such, they are often used in large "flatbed" printers, which print directly to rigid substrates such as plastic, wood or aluminum where flexibility is not a concern.

UV Curable Ink Properties and Functions:
• Photoinitiators: Absorb the UV energy from the light source on the print head. Chemical reaction occurs that converts the liquid ink into a solid film.
• Monomers: Used as solvents because of their ability to reduce viscosity (thickness) and combine with other ink components. 100% percent solids and do not release VOCs (volatile organic compounds). Monomers also add improved film hardness and resistance properties.
• Oligomers: Determine the final properties of the cured ink film, including its elasticity, outdoor performance characteristics and chemical resistance.
• Colorants: Can be dye-based or pigment-based. Usually, pigment-based because of the greater light fastness and durability of pigments compared with dyes. Pigments used in outdoor advertising and display applications have similar requirements to those used in automotive paints. Consequently, there is some crossover of use. While a pigment is selected on the basis of the required application, size control and reduction along with dispersion technique are major components of ink formulation.

UV Ink Printing Process:

  1. Ink is exposed to UV radiation where a chemical reaction takes place where the photo-initiators cause the ink components to cross-link into a solid.
  2. Typically a shuttered mercury-vapor lamp is on either side of the print head, and produces a great amount of heat to complete the curing process (this lamp is used for free radical UV ink, which is what the majority of flatbed inkjet systems use).
  3. UV inks do not evaporate, but rather cure or set as a result from this chemical reaction.
  4. No material is evaporated or removed, which means about 100% of the delivered volume is used to provide coloration.
  5. This reaction happens very quickly, which leads to instant drying that results in a completely cured graphic in a matter of seconds. This also allows for a very fast print process.
  6. As a result of this instant chemical reaction no solvents penetrate the substrate once it comes off the printer, which allows for high quality prints.

[7] [8]

  • Dye sublimation inks: these inks contain special sublimation dyes and are used to print directly or indirectly on to fabrics which consist of a high percentage of polyester fibres. A heating step causes the dyes to sublimate into the fibers and create an image with strong color and good durability.

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