Main Classifications of Inkjet Printing Inks
Main Classifications of Inkjet Printing Inks
Introduction
Inkjet printing is a pressureless dot-matrix technology. It is a low-cost, reliable, fast, and convenient method for printing digital files. Since its initial adoption from the 1950s to the 1970s, inkjet printing has achieved significant commercial success. Over the past two decades, it has also become an important topic in scientific research and technological development.
Compared with other deposition methods—such as pad printing, screen printing, spraying, photography, and lithographic printing—inkjet printing offers major advantages, including single-step processing, low cost, compact equipment, and compatibility with a wide range of substrates.
Beyond traditional graphic applications, inkjet printing has expanded into non-traditional fields, including precision-manufactured devices such as transistors, integrated circuits, conductive polymer devices, structural polymers, ceramic components, biomaterials, and even living tissues for printed growth scaffolds. Inkjet technology is also used in the fabrication of complex three-dimensional objects and micro-electromechanical systems (MEMS).
In the electronics industry, inkjet printing enables the production of flexible displays, radio-frequency identification (RFID) tags, sensors, organic light-emitting diodes (OLEDs), and photovoltaic (PV) devices, including solar cells. Using conductive inks, inkjet printing of printed circuit boards (PCBs) provides a low-cost solution for manufacturing large-area electronic substrates such as paper, polymers, glass, metals, and ceramics.
★ Tips
In principle, inkjet printing is simple: ink droplets are ejected from a small orifice in the printhead and deposited onto designated positions on a substrate in response to electrical signals, thereby forming an image. Most inkjet printers are based on drop-on-demand (DOD) technology, primarily thermal bubble and piezoelectric methods.
However, reliable operation depends on careful design, implementation, and system-level optimization, which are usually application-specific. One of the main challenges is ink formulation. Ink must be tailored to different printing devices and substrates with diverse physicochemical properties, while also fulfilling the functional requirements of the printed application.
Because of the complexity of inkjet inks, their design and preparation are often highly challenging. In addition to traditional requirements such as shelf life and color properties, inks must meet strict physical parameters specific to the printhead. For example, each printhead operates within a defined range of surface tension and viscosity to ensure proper jetting. Piezoelectric printheads typically require ink viscosities in the range of 8–15 cP, whereas thermal inkjet printheads usually require viscosities below 2 cP.
The choice of ink vehicle is equally critical, as it significantly affects print quality, substrate compatibility, and the operating environment. Functional inks often introduce additional challenges beyond graphical performance. For example, conductive inks must provide sufficient electrical conductivity after printing. Conductive inks are commonly composed of metal nanoparticles, which require polymeric stabilizers to maintain dispersion stability. However, these stabilizers act as electrical insulators and inhibit particle-to-particle contact after printing, making it difficult to achieve high conductivity.
A similar contradiction exists in UV-curable inks. To achieve high throughput in industrial printing systems, printed patterns must cure rapidly to maintain high resolution. However, rapid curing limits droplet spreading and leveling over large areas, resulting in low coverage. Improving coverage requires depositing more ink, which increases material consumption and cost.
Therefore, when developing innovative inkjet inks, it is essential to consider the impact of every component on overall ink performance, including cartridge lifetime, jetting behavior, substrate interaction, and potential effects on human health and the environment.
The formulation of inkjet inks follows specific principles and key parameters, including preparation methods, component selection, and ink–substrate interactions, to achieve optimal overall performance.
Ink Formulation
Inkjet inks consist of functional materials and a liquid medium that serves as the carrier for these materials. This definition also applies to hot-melt inks, which are solid before and after printing but become liquid during the printing process. Inks typically include additives such as surfactants, preservatives, and initiators, and often contain polymers that fix functional molecules—such as colorants or conductive particles—onto the substrate after printing.
Ink media components are selected based on the printing technology and the intended functional application.
Based on the properties of the ink vehicle, inks can generally be classified into three main categories:
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Water-based inks
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Non-aqueous (solvent-based) inks
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100% solid inks
For the first two types, once ink droplets reach the substrate, the liquid phase must disappear, usually through evaporation. In the third type, no solvent evaporation occurs. Instead, the ink undergoes a phase change: hot-melt inks are solid at room temperature, melt at jetting temperature, and solidify again on the substrate; UV inks are liquid initially and solidify through polymerization upon UV exposure.
Multiphase inks may include polymer-based systems (e.g., polyester carriers for hydrophobic colorants), micellar systems with dyes dissolved in micelle cores, oil-in-water microemulsions, fine emulsions containing water-insoluble dyes, and lipid-based microcapsules.
Functional materials may be dissolved in the ink vehicle (dye-based inks) or dispersed as micro- or nanoparticles (pigment-based inks). In the latter case, stabilizers are required to prevent particle aggregation and to ensure colloidal stability. When used as colorants, pigment-based inks generally exhibit superior lightfastness and water resistance compared with dye-based inks. In principle, dye-based inks are thermodynamically stable, whereas most pigment-based inks are kinetically stable.
Functional Materials
Inkjet technology is no longer limited to graphical printing and is now widely applied in scientific and industrial fields. A wide range of functional materials can be incorporated into inks, including pigments and dyes for graphic arts; metal nanoparticles and organometallic compounds for conductive patterns; carbon nanotubes; polymers for light-emitting devices; photopolymerizable monomers for 3D printing; ceramics; biomedical materials; and even living cells for bioprinting.
These functional materials may be dissolved in the ink vehicle (e.g., dyes, polymers, organometallic compounds, biomaterials) or dispersed as particles.
The concentration of functional materials in an ink formulation is determined by solubility (for dye-based inks) or by dispersibility and stability (for pigment-based inks). Considering overall ink composition and system performance, typical colorant concentrations range from 0.5% to 10% by weight, while conductive inks may contain up to approximately 60% metal nanoparticles.
Instability in inks without undissolved materials can arise from interactions between components, such as monomer polymerization, phase separation due to temperature changes, or adsorption of ink components onto container walls. For high-solids dispersions, irreversible aggregation and sedimentation are major concerns, particularly when particle density exceeds that of the liquid medium, as in metal nanoparticle inks. Carbon particles also tend to aggregate in aqueous systems.
To prevent aggregation, repulsive mechanisms must counteract attractive forces. Electrostatic stabilization can be achieved by imparting surface charge to pigment particles. For example, anionic surfactants such as sodium dodecyl sulfate (SDS) can adsorb onto hydrophobic particles (e.g., carbon black), generating electrostatic repulsion that inhibits aggregation. This mechanism is particularly effective in aqueous inks due to their high dielectric constant.
Steric stabilization provides an alternative mechanism. Polymer molecules adsorbed onto pigment surfaces create spatial repulsion between particles. For example, carbon black particles can be stabilized by polymers with hydrophobic segments that anchor to the particle surface and hydrophilic segments that extend into the solvent. This approach is effective in both aqueous and non-aqueous inks.
Numerous commercial polymeric dispersants are available, including Efka, Tegsperse, Solsperse, and Disperbyk. Selecting the appropriate dispersant and optimal concentration is critical. Dispersants contain anchoring groups that bind to pigment surfaces, while solvated chains extend into the medium to maintain dispersion.
Dispersant concentration strongly affects ink performance. Excess polymer dispersant in conductive inks, for example, can prevent close contact between nanoparticles even after printing, thereby reducing conductivity. In such cases, the use of minimal dispersant concentrations is recommended.
Dispersant concentration also influences viscosity and is a key parameter in printhead compatibility. To achieve stable dispersions, pigment particle sizes should generally be below 200 nm.
★ Tips: Solvents
Solvents are the primary ink vehicles, enabling functional materials and other components to dissolve or remain suspended. Solvent content in inkjet inks typically ranges from 25% to 80% by weight. Common solvents include organic liquids (for solvent-based inks) and water (for aqueous inks).
Solvent-Based Inks
Typical solvents include oxygen-containing organic compounds such as alcohols, methyl ethyl ketone, ethyl acetate, propylene carbonate, glycol ether acetates, diethylene glycol monobutyl ether, toluene, as well as small amounts of hydrocarbons such as tetradecane. Solvent mixtures are often used to tailor ink properties such as viscosity, evaporation rate, and surface tension.
Solvent-based inkjet inks have been widely used in industry for many years due to their excellent print quality, durability, fast drying, and compatibility with a wide range of substrates, including metals, glass, ceramics, plastics, wood, leather, and food packaging.
Solvent-based conductive inks containing dispersed silver or copper nanoparticles have recently been demonstrated for precision electronics manufacturing and semiconductor applications.
However, solvent-based inks also present drawbacks, including environmental and health concerns related to volatile organic compounds (VOCs), strong odors, and the risk of printhead nozzle clogging due to rapid solvent evaporation.
Water-Based Inks
Water-based inks are widely used in office and home printers. They are relatively inexpensive and environmentally friendly but have limitations. They typically require porous or specially treated substrates to achieve durability and do not adhere well to non-porous surfaces or plastics.
Additionally, some industrial piezoelectric printheads are incompatible with water-based inks, although this is gradually changing due to increasing demand for applications such as biocompatible printing and food-grade inks.
UV-Curable Inks
UV-curable inks and coatings have been used commercially for many years, and inkjet printers are now widely employed as deposition tools for UV-curable materials. These inks remain stable liquids until exposed to light of specific wavelengths and intensities.
UV-curable ink formulations typically include monomers and oligomers (often acrylate derivatives), functional materials (such as pigments or dyes), photoinitiators, and various additives involved in the curing process.
UV curing occurs via two primary mechanisms: free-radical polymerization (commonly initiated by α-hydroxyketones) and cationic polymerization (typically initiated by photoinitiators such as benzophenone).
UV-curable inks are successfully used in many inkjet applications, including product coatings and packaging labels. They offer advantages such as zero VOC emissions, high durability, and excellent abrasion resistance due to cross-linked film formation. Current limitations include food-contact applications, higher material costs, and the need for UV curing hardware.
Hot-Melt (Phase-Change) Inks
Hot-melt inks, also known as phase-change inks, are typically based on natural waxes or synthetic compounds containing long hydrocarbon chains, such as stearic acid, behenic acid, sebacic acid esters, and stearamides. Before jetting, hot-melt inks are maintained in a molten state, typically at operating temperatures of 50–120°C.
Pigmented hot-melt inks can be formulated with excellent opacity, which is particularly important for printing on non-porous or dark substrates such as black plastics.
Compared with conventional pigment inks, phase-change inks offer very fast solidification and improved storage stability due to their solid-state matrix. They are also considered environmentally friendly. Their rapid solidification limits ink spreading, making print quality easier to control.
The main drawbacks of phase-change inks are limited durability and poor abrasion resistance.