UV Inkjet Printing Stability Solution

UV Inkjet Printing Stability Solution

Table of content.

UV Inkjet Printing — Balancing Viscosity, Reactivity, Stability, and Performance

UV inkjet printing requires achieving an optimal balance between low viscosity, reactivity, stability, and performance characteristics. The following explains how each factor influences inkjet formulation:

 

 ① Low Viscosity

 

A key objective in UV ink formulation is to achieve low ink viscosity.

High-viscosity inks tend to form thicker films and are more prone to bubbles and pinholes.

This goal is achieved by selecting suitable low-viscosity monomers:

* Monofunctional monomers provide the lowest viscosity.

* However, lower functionality reduces reactivity, slowing cure speed and requiring longer exposure to achieve full polymerization.

* Low crosslinking results in softer cured films, with lower scratch and stain resistance—though this can be desirable when printing on flexible substrates.

* Acrylate monomers typically have lower viscosity than oligomers and therefore, are the main components in UV inkjet inks.

* Hyperbranched acrylate oligomers offer lower viscosity than traditional oligomers at equivalent functionality, enabling higher reactivity at low viscosity.

* DCPD (Dicyclopentadiene) acrylate monomers exhibit viscosity between 7–25 mPa·s at 25°C, and are excellent reactive diluents for improving handling and flow.


 ② Safety


Thiols are commonly used as curing accelerators.

However, they are associated with strong odors and potential health concerns.

  * Odor originates mainly from trace impurities.

  * High-purity thiols significantly reduce odor levels.

Acrylate monomers may irritate skin, raising safety concerns.

Certain DCPD acrylate monomers produced via specialized processes exhibit much lower skin irritation, reducing irritation levels by up to 50% in ink formulations.

 ③ Stability


Impurities in monomers and oligomers—such as peroxides or metal ions—may trigger premature polymerization and gelation.

Ink shelf life is significantly shortened at temperatures above 30°C.

To improve storage stability:

* Use high-purity monomers and oligomers.

* Avoid monomers produced via esterification dehydration, as they are less stable in color and acid value.

* DCPD acrylate monomers produced via transesterification show superior stability and reduce polymerization shrinkage due to their bulky structure.

Adding secondary thiols increases double-bond conversion and reactivity under UV exposure.

Increasing photoinitiator concentration also improves reactivity.

Typical formulations contain 5–15% photoinitiator.


 ④ Improving Ink Performance


Certain substrates—such as ABS, PC, PMMA—are difficult to bond with UV inks.

DCPD monomers significantly improve adhesion on hard-to-bond substrates.

Adding secondary thiols enhances adhesion on glass, stainless steel, PET, etc.

Additional performance considerations:

* DCPD’s alicyclic structure increases the glass transition temperature (Tg).

* For higher image brightness and color gamut, use fine pigments with narrow particle-size distribution (~100 nm).

* Narrow size distribution prevents:

     * High viscosity

     * Nozzle clogging

     * Flocculation and sedimentation

* Use high-performance pigments (azo, polycyclic) for superior light and weather resistance.

* Add high-molecular-weight dispersants for stable dispersion and controlled wetting.


 ⑤ UV-LED Compatibility


Mercury UV lamps are traditionally used for curing, but present environmental and health hazards.


UV-LED is safer, but provides lower light energy.


Thiols enhance curing efficiency under weak UV-LED light by:

* Acting as chain-transfer agents

* Regenerating free radicals during polymerization

* Enabling deep curing even with low light intensity


 ⑥ Overcoming Oxygen Inhibition


Acrylic polymerization suffers from oxygen inhibition.

Peroxy radicals consume active radicals, preventing full curing.


This is especially severe in:

* Low-viscosity inks (oxygen diffuses faster)

* Surface layers of the ink film

Oxygen inhibition causes surface tackiness even when the bulk layer is cured.

Solutions:

1. Add secondary thiols

   – Donate hydrogen to peroxy radicals, regenerating curing radicals.

2. Use amine synergists

   – Tertiary amines serve as hydrogen donors for photoinitiators.

3. Increase UV intensity or irradiance

   – Extra radicals compensate for oxygen loss.

4. Use oxygen-resistant photoinitiators

5. Increase photoinitiator concentration




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