How to Prevent Yellowing in UV Flexo Varnish for Label Printing?

Date: Jul 08 2025 From: Star Color Views:

In label printing, UV flexographic varnish is widely used to enhance surface appearance, improve durability, and increase chemical or abrasion resistance. However, for labels that frequently appear outdoors, they may inevitably turn yellow due to exposure to ultraviolet radiation. This discoloration can damage visual quality and brand perception, especially in some cosmetic labels.

At Starcolor, we address this challenge by developing non yellowing UV varnish optimized for long-term transparency and color stability. Here is how yellowing occurs and what measures can be taken in actual production environments to prevent it.

Why Does Yellowing Occur in UV Flexo Varnishes?

Yellowing typically results from one or more of the following:

1. Photoinitiator Degradation
Many photoinitiators used in UV-curable formulations tend to yellow when exposed to light, especially over time under UV or sunlight. Low-quality initiators can oxidize, resulting in a visible yellow tint.

2. Overexposure to UV Light
Excessive UV energy or multiple passes through curing lamps can degrade both the ink and the varnish layers. This breakdown leads to yellowing, especially with thin film layers.

3. Thermal Aging
If the label is exposed to elevated temperatures during storage or transportation, thermal oxidation of the cured varnish can occur, leading to discoloration.

4. Formulation Imbalance
Incorrect ratios of oligomers, monomers, or stabilizers can contribute to unstable film properties, especially under environmental stress.

How to Prevent Yellowing in UV Flexo Varnish for Label Printing?

How to Prevent Yellowing in UV Flexo Varnish for Label Printing?

How Starcolor Prevents Yellowing in UV Varnishes

Our R&D team has developed UV flexo varnishes that remain clear, non-yellowing, and stable even after long-term exposure. Here's how:

1. Non-Yellowing Photoinitiators
We use advanced photoinitiators that offer low yellowing index and are thermally and photochemically stable, reducing color shift over time.

2. Optimized Resin Systems
Our varnishes are based on aliphatic urethane acrylates, which are less prone to yellowing compared to aromatic systems. They provide both clarity and flexibility.

3. Precision UV Curing
We recommend precise UV energy calibration to avoid over-curing. Starcolor offers technical curing guidelines for different press speeds and lamp types to maintain film integrity.

4. Thermal and Light Stabilizers
Formulations include anti-oxidants and UV absorbers that help block degradation due to light and heat.

Recommended Use Cases

Starcolor’s non-yellowing UV flexo varnish is ideal for:

Clear-on-clear labels

White or metallic substrates

High-end health and beauty products

Pharmaceutical labels requiring long shelf life

Beverage and wine labels with strong brand color fidelity

Preventing yellowing in UV flexo varnish starts with the right formulation, controlled curing conditions, and careful application. At Starcolor, we provide UV varnishes engineered for clarity, longevity, and performance, backed by lab testing and global customer success.

Say goodbye to yellowing—and hello to long-lasting clarity.