Formulation and Performance of Water-Based Inkjet Inks
Formulation and Performance of Water-Based Inkjet Inks
Typical Water-Based Ink Formulation
Ink Droplet Ejection Cycle
In thermal inkjet (TIJ) printheads, droplet ejection occurs when an electrical pulse is applied to a resistor. Within 2–5 microseconds, the resistor surface temperature rises to 200–300 °C, causing the ink adjacent to the resistor to boil. The resulting high-pressure vapor bubble forces ink out through the nozzle. Subsequently, ink is replenished into the chamber from the ink reservoir by capillary action.
In contrast, in piezoelectric inkjet (PIJ) printheads, ink is expelled by membrane deflection driven by a piezoelectric actuator at each nozzle. The timescale of droplet ejection in PIJ heads is similar to that of TIJ heads, allowing both systems to eject approximately 10,000–30,000 droplets per second per nozzle.
Typical nozzle diameters (d = 10–50 µm), ink viscosity (η = 1–5 cP), surface tension (σ = 20–50 dyn/cm), and ink density (ρ = 0.9–1.1 g/mL) are comparable for TIJ and PIJ printers used in office and home applications.
Influence of Ink Viscosity and Surface Tension
The effects of ink viscosity and surface tension on droplet mass and velocity have been extensively studied both experimentally and computationally. These properties are also influenced by factors such as the internal fluidic geometry of the printhead. Generally, increasing ink viscosity results in reduced droplet volume and lower droplet velocity.
In TIJ printheads, thermal energy transfer to the ink ceases immediately after vapor bubble formation, effectively limiting droplet energy. Consequently, TIJ heads are primarily suitable for low-viscosity inks (1–5 cP). In contrast, the mechanical displacement of PIJ membranes can be more easily increased, enabling the ejection of inks with viscosities up to ~50 cP. However, at such high viscosities, the maximum firing frequency must be reduced to accommodate slower ink refill rates.
Droplet Breakup Analysis
After ejection, an ink jet rapidly (10–50 µs) breaks up into a series of elongated, spherical droplets, typically consisting of a primary droplet and several smaller satellite droplets, which may cause printing issues. Due to higher deceleration from viscous air drag, satellite droplets may deviate from the intended landing position within the typical flight time (~100 µs). As a result, satellites may land on unintended areas of the substrate or deposit onto internal printer components.
Although satellite and aerosol droplet formation is complex, general atomization studies indicate that higher Weber and Reynolds numbers promote the formation of smaller, finer droplets, which correspond to higher flow velocities and inks with lower viscosity and surface tension.
Because many ink surfactants require more than 100 µs to migrate to the liquid–air interface, dynamic surface tension plays an important role in both droplet breakup and spreading. One formulation strategy to suppress aerosol formation is the addition of small amounts of high-molecular-weight polymers, which significantly increase extensional viscosity and resist the formation of long liquid filaments.
Some PIJ printheads also employ subtle vibrations of the drive membrane to control the meniscus position at the ink–air interface, promoting clean droplet detachment.
Droplet Breakup Dynamics in Inkjet Printheads
Inevitably, some ejected ink accumulates on the orifice plate (the exterior surface of the printhead near the nozzle). Large ink pools near the nozzle can compete with newly formed droplets for momentum, causing droplet misdirection or complete suppression of ejection. Therefore, minimizing ink accumulation on the orifice plate is critical.
Printhead design modifications—such as using non-wetting orifice plate materials or recessing the nozzle—can help keep the plate clean. Ink composition also plays a significant role. In particular, fluorinated surfactants have been shown to inhibit ink puddle formation. Although the exact mechanism is not fully understood, it is believed that fluorinated hydrophobic groups preferentially migrate to the ink–air interface rather than the ink–solid interface, reducing surface tension without promoting wetting of the orifice plate.
The strong surface-tension-lowering effect of fluorinated surfactants may also suppress surface tension gradients, thereby preventing Marangoni flow that would otherwise draw ink away from the nozzle onto the orifice plate.
Resistor Fouling and Droplet Deceleration
TIJ printheads exhibit inherent complexity due to the gradual formation of deposits on the resistor surface, which reduces heat transfer efficiency to the ink. This leads to decreased droplet mass and velocity over time, a phenomenon known as kogation.
Although the chemistry of deposit formation is complex, a key factor is the solubility of ink solutes. During each firing event, approximately 0.02 ng of liquid evaporates to generate a 20 ng droplet. Nonvolatile ink components (e.g., dyes and pigments) remain behind on the resistor. If these solutes do not readily re-dissolve or re-disperse, they accumulate on the resistor surface.
The rate of solute re-dissolution is critical. Slowly re-dissolving components can produce a transient kogation-like phenomenon known as “decel” (deceleration), in which droplet velocity decreases during continuous firing. After a rest period of seconds to minutes, droplet velocity may temporarily recover before declining again.
Effect of Kogation and Decel on Droplet Velocity
Certain ink components may also exhibit specific affinity for the resistor surface. For example, trace inorganic compounds are suspected of depositing via a chemical-plating-like mechanism on the tantalum oxide layer covering the resistor, which is why chelating agents are commonly used.
Ink formulation strategies to prevent resistor fouling focus on incorporating components that either passivate the resistor surface or slowly etch deposits. In the latter case, a careful balance must be maintained between removing unwanted deposits and avoiding corrosion of the resistor material.
Nozzle Clogging
Water rapidly evaporates from ink in uncapped, inactive nozzles, leading to significant changes in ink physical properties and phase behavior. This can cause poor firing performance or complete nozzle blockage.
To mitigate these effects, commercial inkjet inks incorporate low-volatility humectant co-solvents, typically at 5–50% by weight of the total ink formulation.
Selecting the correct combination of co-solvents is essential to achieve a long “open time” or “idle time”, during which the printhead can remain inactive yet still reliably eject droplets upon restart.
Water Loss and Diffusion Effects
The resistance to water diffusion within the ink channel
(R_ink ≈ L / D_water,ink)
is approximately 100 times greater than the resistance to water transfer from the nozzle opening into the vapor phase
(R_air ≈ δ / D_water,air),
because the mass-transfer boundary layer thickness (δ) at the liquid–air interface is typically comparable to the diffusion path length (L) in the ink channel, while the diffusion coefficient of water in air is about 100× higher than in ink.
As a result, printhead water loss is diffusion-limited, leading to steep water concentration gradients within the ink channel. Under typical ambient relative humidity conditions (30–80%), the equilibrium water content in exposed nozzle ink is estimated to range from 5–30 wt%.
The characteristic timescale for gradient formation and evaporation is
t ≈ L / D.
For a typical ink channel length of L = 50 µm, assuming D ≈ 5 × 10⁻⁶ cm²/s, the ink in the nozzle region can equilibrate with ambient air in as little as 5 seconds.
Cross-Section of Nozzle Region and Ink Channel
This diagram illustrates the steep water concentration gradient that develops at an inactive, exposed nozzle.
Implications for Ink Formulation
Managing water loss is critical when formulating water-based inkjet inks. Understanding how ink properties (e.g., viscosity) and solute solubility vary with water concentration is essential for both TIJ and PIJ printheads.
Because PIJ printheads can eject more viscous inks, PIJ formulations often contain higher solute loadings (e.g., pigments and polymers). As water evaporates, these inks may exhibit increased risks of precipitation and deposition.
Furthermore, water concentration gradients in the ink channel have been shown to induce rapid migration of charged pigment particles toward the nozzle, altering ink composition in complex ways. Overall, while these guidelines help develop high-performance inks, empirical experience remains indispensable.
Nozzle Clogging Caused by Other Substances
The narrow ink channels (10–100 µm) in inkjet printheads are highly susceptible to blockage. For example, air bubbles can become trapped in the channels, preventing ink flow. Surfactants used in inks can stabilize bubbles, so defoamers are sometimes added to suppress excessive bubble formation.
Microbial growth can also block ink channels; therefore, biocides are commonly included in ink formulations. Pigment-based inks introduce additional complexity, as large pigment particles (>500 nm) may settle in the nozzle, especially during prolonged idle periods. Consequently, pigment dispersions must be sufficiently fine to ensure sedimentation is not a major concern.