UV Ink Components and Common Formulation Ratios (Highly Practical Reference)

UV Ink Components and Common Formulation Ratios (Highly Practical Reference)

Table of content.

UV Ink Components and Typical Formulation Tables

 

 

I. Concept of UV Ink

UV is the abbreviation for Ultraviolet (Ultraviolet Radiation, UV), which was discovered by the German scientist Ritter. Ultraviolet radiation refers to electromagnetic waves with wavelengths ranging from 100 to 400 nm.

UV-curable inkjet inks designed for piezoelectric ceramic DOD printheads are subject to specific physical limitations. The viscosity must be relatively low—typically 8–13 cP at jetting temperature. Most printheads are equipped with onboard heaters and, in many cases, can operate at temperatures up to 60 °C.

 

 

II. Main Components of UV Ink

UV inks are primarily composed of the following components:

  • Reactive prepolymers (oligomers)

  • Photoinitiators

  • Reactive diluents / photosensitive monomers

  • Colorants

  • Additives

 

01. Reactive Prepolymers (Oligomers)

Reactive prepolymers are the most critical components determining the performance of UV ink coatings. They serve as the film-forming material and play a key role in both the curing process and the final properties of the cured ink film.

★ Tips

Reactive monomers and oligomers form the foundation of inkjet formulations and provide most of the fluid’s physical properties. These materials must contain functional groups that allow free-radical addition polymerization.

The four steps of free-radical polymerization are:

  1. Formation of free radicals

  2. Initiation

  3. Propagation

  4. Termination

 

 

02. Photoinitiators

All UV-curable formulations must contain photoinitiators to initiate polymerization.

Photoinitiators are substances that absorb UV radiation and undergo chemical changes to generate reactive intermediates capable of initiating polymerization. They are indispensable in any UV-curing system.

Photoinitiators are classified into:

  • Hydrogen-abstraction type (Type II) – require a co-initiator containing active hydrogen to form free radicals

  • Cleavage type (Type I) – generate free radicals directly through molecular cleavage upon UV excitation

UV inks with excellent curing performance typically use photoinitiators that exhibit:

  • High UV absorption efficiency

  • Good thermal and storage stability

★ Tips

  • Pigments are often surface-treated with acidic or alkaline materials to prevent flocculation. If these interact with photoinitiators, ink shelf life may be compromised.

  • Ensure that the maximum absorption wavelength of the photoinitiator matches the emission wavelength of the UV lamp and does not overlap with other components (especially pigments).

  • Most UV inks contain multiple photoinitiators.

Common UV curing light sources:

  1. Mercury vapor lamps (H lamps) – broadband output, strong surface curing

  2. Iron-doped mercury lamps (D lamps) – enhanced UVA output for deeper curing

  3. Gallium-doped lamps – strong output at 405–420 nm

  4. Indium-doped lamps – UV output up to 450 nm

  5. UV-LED lamps – single wavelength (typically ≥395 nm), low energy density

 

 

03. Reactive Diluents / Photosensitive Monomers

Reactive diluents (also called crosslinking monomers) are functional monomers used to adjust viscosity, curing speed, and film properties.

They contain unsaturated C=C bonds (e.g., acrylate, methacrylate, vinyl, allyl groups). Acrylates are most widely used due to their fast curing speed.

Based on functionality, reactive diluents are classified as:

  • Mono-functional

  • Di-functional

  • Multi-functional

Higher functionality leads to faster curing but weaker viscosity reduction.

Traditional reactive diluents such as styrene and early-generation acrylates exhibit strong toxicity and skin irritation. To reduce irritation:

  • Increase molecular weight via ethylene oxide or propylene oxide ring-opening

  • Modify ester structures

  • Replace esterification with alcohol addition

For example, neopentyl glycol diacrylate synthesized via esterification has a skin irritation index (PH value) of 4.96, whereas the addition method reduces it to 0.3.

Eco-friendly UV inks favor monomers with:

  • Low viscosity

  • Fast curing

  • Good adhesion

  • Low skin irritation and toxicity

  • No residual odor

★ Tips

  • Higher acrylate functionality increases crosslink density, improving solvent, abrasion, and scratch resistance—but may reduce adhesion due to curing shrinkage.

  • Key adhesion-promoting monomers include:

    • 2-Phenoxyethyl acrylate

    • Alkoxylated phenol acrylates

    • Ethoxylated tetrahydrofurfuryl acrylate

  • Diacrylates effectively reduce ink viscosity.

 

 

04. Colorants

Colorants are pigments or dyes dispersed or dissolved in the reactive carrier.

Pigments are insoluble colored powders and are classified as:

  1. Inorganic pigments – metal oxides or insoluble salts (e.g., natural or synthetic mineral pigments)

  2. Organic pigments – synthetic organic compounds with superior color strength and performance

★ Tips

  • Smaller pigment particle sizes can negatively affect weather resistance, color gamut, gloss, and opacity.

  • Typical printhead nozzle diameters are 30–60 µm; pigments must be milled to <1 µm, preferably much smaller, to ensure jetting reliability and long printhead life.

  • Low-viscosity reactive diluents and dedicated dispersants are recommended to fully wet and stabilize organic pigments.

 

 

05. Additives

Additives are used to improve overall ink performance and typically include:

  • Dispersants

  • Defoamers

  • In-can stabilizers

  • Surfactants

  • Waxes

Functions:

  • Dispersants: Improve pigment wetting, reduce grinding time, lower oil absorption, prevent agglomeration and sedimentation

  • Defoamers & surfactants: Improve leveling, prevent pinholes, enhance surface gloss

★ Tips (Surface Tension)

Inkjet inks are often adjusted via surfactants to control surface tension. Typical printheads require static surface tension of 20–30 dyn/cm.

  • Too low: excessive wetting of the nozzle plate, unstable jetting

  • Too high: poor internal wetting, inconsistent droplet formation

In-can stabilizers prevent thermal polymerization during storage, extending shelf life. Free-radical scavengers are commonly used. Adequate headspace in containers is important since oxygen inhibits radical polymerization.

Stabilizers, HALS, and UV absorbers must be added sparingly, as they also inhibit curing.

Waxes:

  • Modify rheology

  • Improve water resistance and printability

  • Reduce blocking, scuffing, and paper picking

  • Form a smooth surface wax layer after curing to enhance abrasion resistance

  • Reduce oxygen inhibition and improve surface curing

 

 

III. Typical UV Ink Formulation Tables (For Reference Only)

  • Formulation A: Contains four raw materials (or five if photoinitiators are counted separately). It exhibits all essential properties of UV inkjet ink: low viscosity from monomer selection and polymerization capability from photoinitiators.

  • Formulation B: Uses multifunctional acrylates to ensure a hard, durable cured film. Additional additives and monomers may be incorporated to meet end-user performance requirements.

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