Chapter 3: Formulation and Rheology of UV LED Inkjet Inks

The formulation and rheology of UV LED inkjet inks lie at the heart of achieving high-quality digital printing performance. The interplay between ink chemistry and fluid dynamics directly impacts print fidelity, jetting behaviour, curing efficiency, and final product durability. This chapter delves into the fundamental principles and practical considerations governing the development of UV LED inkjet inks, with a focus on optimizing formulations for reliable jetting and curing under the unique constraints posed by UV LED curing technology. 

Overview of UV LED Inkjet Ink Formulation 

UV LED inkjet inks are complex formulations comprising multiple components, each serving a specific function. Core ingredients include reactive monomers and oligomers that polymerize under UV exposure, photo initiators that trigger the curing reaction, pigments or dyes that provide colour, and various additives that enhance jetting performance and stability. 

Unlike traditional solvent or water-based inks, UV LED inks are formulated as liquid resins designed to solidify rapidly via photopolymerization. The monomers and oligomers must balance low viscosity for efficient jetting with high reactivity to ensure complete curing. Photo initiators must be carefully selected to absorb the narrow wavelength emissions of UV LED lamps (typically 365–405 nm) to initiate polymerization effectively. 

Pigments used in UV LED inks must not only provide vibrant colours but also maintain dispersion stability and be compatible with the curing chemistry. Additives such as surfactants, flow modifiers, and adhesion promoters are introduced to fine-tune properties like surface tension, wettability, and substrate adhesion. 

Rheological Properties Critical to Inkjet Printing 

Rheology, the study of flow and deformation of materials, is a critical discipline in ink formulation. For inkjet inks, the viscosity and surface tension must fall within precise ranges to ensure the formation of stable droplets that can be ejected accurately from printheads without clogging or satellite formation. 

Viscosity affects the ease with which the ink passes through microscopic nozzles. UV LED inkjet inks typically target viscosities between 10 to 30 millipascal-seconds (mPa·s) at jetting temperatures to balance flow and stability. Too high viscosity leads to nozzle clogging and irregular droplet formation, while too low viscosity can cause splashing or misting, reducing print quality.