Analysis of Poor Adhesion of UV Ink on PE Film

When issues such as poor adhesion, tape peeling and ink rub-off occur during UV printing on PE film, corona treatment is not the main cause in 90% of cases. Inconsistent curing between the surface and inner layer of UV ink is also a major trigger for the above problems.

PE is a non-polar substrate with low surface energy. If the UV ink is cured unevenly — fully dried on the surface but under-cured inside, or over-cured on the top while lacking cross-linking at the bottom — the printed layer will look dry on the outside but remain soft at the bottom. This results in weak adhesion and large-area delamination after storage. This article explains the root causes of ink peeling on PE film from the perspective of UV ink curing performance.

1. Core Cause: Inconsistent Curing Between Surface and Inner Layer

For UV ink curing, ultraviolet rays first reach the ink surface and then penetrate down to the inner layer which contacts the PE film. PE film hardly absorbs or conducts light, and reflects most UV rays. Such characteristics easily lead to two typical problems:

• The ink surface is over-cured and becomes brittle
• The inner layer remains under-cured, seemingly dry but actually soft

The final result: The print looks bright and dry, yet a weakly cross-linked layer exists between the ink bottom and PE film. The ink will peel off under external force or delamination automatically after several days of storage.

2. Abnormal Curing on UV Ink Surface

Surface curing problems rarely cause immediate peeling, but they make the whole ink film brittle and prone to shrinkage, which will crack and pull off the inner layer. These are the main reasons for delayed peeling, chipping and flaking.

2.1 Over-curing and excessive cross-linking on the surface

Excessively high UV lamp power, long exposure time or low printing speed will cause instant high-density cross-linking on the ink surface:

• The surface film hardens, shrinks and turns brittle rapidly
• Shrinkage stress on the surface exceeds the adhesion of the inner layer, tearing the incompletely cured ink off the PE film
• No obvious defect right after printing, but large-area peeling occurs after rolling up or 24 hours of storage

Symptom: The surface feels hard and glossy, while the whole ink layer peels off easily with slight tearing.

2.2 Surface skin formation blocks UV penetration

High-power UV lamps instantly form a solid crust on the ink surface, which blocks ultraviolet rays from penetrating deeper:

Fully cured surface → UV ray obstruction → Completely under-cured inner layer

This is the most typical pseudo-drying phenomenon in UV printing on PE film.

2.3 Surface oxygen inhibition leading to tackiness and poor rub resistance

The ink surface is exposed directly to air. Oxygen will restrain UV cross-linking reaction and leave the surface slightly sticky and slippery. In fact, incomplete cross-linking loosens the ink structure, so the ink powder rubs off easily during friction.

3. Abnormal Curing in UV Ink Inner Layer

The inner layer refers to the ink bottom attached to PE film. Overall adhesion entirely depends on sufficient cross-linking of the inner layer. Under-cured or pseudo-dried inner layer is the primary cause of ink peeling on PE film.

3.1 Insufficient UV penetration and incomplete curing (Most common)

PE film is opaque and reflective, and cannot transmit ultraviolet light. UV rays only irradiate from the top without supplementary light from the bottom. If the ink film is thick, or the pigment concentration is high especially for dark colors such as black, blue and red, pigments will block UV rays and prevent them from reaching the bottom layer:

• The ink surface becomes hard and dry
• The inner layer stays semi-liquid, gelatinous and soft
• No effective chemical bonding forms between ink bottom and PE film

Symptom: The surface looks dry, while the whole ink layer peels off completely when pulled by adhesive tape, and the bottom remains sticky and soft.

3.2 Low cross-link density of inner resin and inactive adhesion system

Special UV ink for PE film relies on adhesion monomers and wetting resin at the bottom to stick firmly to the substrate. If curing energy is insufficient:

• Hard resin on the surface completes cross-linking first
• Adhesion resin at the bottom fails to finish curing in time
• The resin cannot anchor to the corona-treated layer of PE film

Result: The ink forms a complete film but loses adhesion to the substrate, like a suspended layer that peels off under slight friction.

3.3 Residual active monomers and unreacted photoinitiators in inner layer

Incomplete curing leaves a large number of unreacted monomers and additives inside the ink. These substances migrate gradually over time:

• Form an isolation layer between ink and PE film
• Destroy interfacial adhesion
• Cause automatic delamination after days of storage

4. Influence of Ink Formulation

4.1 Strong light-shielding pigments hinder deep curing

Dark-colored and high-opacity UV ink contains pigments with high light absorption. UV rays can hardly penetrate deep into the ink layer, resulting in fully dried surface but severely under-cured bottom.

4.2 High ink viscosity and excessive ink film thickness

Thicker ink film raises the difficulty of UV penetration and worsens uneven curing. Thick ink layer inevitably leads to pseudo-drying at the bottom and weak adhesion.

4.3 Mismatched ink system

Ordinary UV ink for paper or general plastic lacks dedicated adhesion monomers for bottom layer. After curing, such ink features hard surface and fragile bottom, and cannot adhere to non-polar PE surface effectively.

5. Equipment and Process Factors Causing Uneven Curing

• Fast printing speed: The surface barely gets cured while the inner layer has no time for cross-linking.
• Aging UV lamps: Reduced luminous intensity leads to insufficient energy for deep curing, only drying the ink surface.
• Single-side curing mode: Only top UV irradiation without bottom reflection results in poor curing of the ink inner layer on PE film.
• Excessive heat accumulation: Severe thermal shrinkage on the surface pulls the inner ink layer and causes delamination.

6. Summary: Distinction between Surface and Inner Layer Curing Problems