Improving Scratch Resistance of UV Flexo Inks with Additives

Date: Aug 27 2025 From: Star Color Views:

Scratch resistance is a key indicator of the user experience of printed products. Friction during transportation and stacking, or scratches caused by consumers when opening packages, can lead to ink peeling, blurred graphics, and even trigger the safety risk of “ink migration into food.”

According to industry statistics, about 15% of food packaging quality complaints are directly related to insufficient ink scratch resistance. Unlike ordinary inks, UV flexographic inks for food packaging must simultaneously meet dual requirements: improved scratch resistance and compliance with food safety regulations, making additive selection a core consideration.

I. Core Requirements for Scratch-Resistant Additives

Due to the special requirements of food packaging, scratch-resistant additives must go beyond “single performance enhancement” and meet three core criteria, which distinguish them from additives used in industrial packaging inks:

Regulatory Compliance: No Migration in Food Contact
Additives must comply with global food contact material regulations, such as EU 10/2011, U.S. FDA 21 CFR 175.300, and China GB 4806.10-2022, ensuring that the additive itself and its degradation products do not migrate into food.
Performance Compatibility: Adapted to UV Curing
UV flexographic inks rely on rapid ultraviolet curing (typically ≤1 second). Additives must not affect the curing speed or interfere with free radical polymerization, and after curing, they must distribute evenly in the ink layer to avoid local variations in scratch resistance caused by additive aggregation.
Process Compatibility: No Adverse Impact on Print Quality
Additives must be compatible with the UV ink’s resin, monomer, and pigment system. They should not cause defects like pinholes, floating color, or sudden loss of gloss, and they must be suitable for high-speed flexo printing and anilox roller ink transfer without clogging cells.

II. Key Types of Scratch-Resistant Additives

Based on their mechanisms of action, scratch-resistant additives for UV flexographic inks in food packaging fall into three categories: surface lubricants, structural enhancers, and resin modifiers. Each has distinct properties and application scenarios, and must be selected precisely based on packaging needs.

1. Surface Lubricant Additives

These additives form a “lubricating protective layer” on the ink surface, reducing the coefficient of friction between external objects and the ink layer (from 0.6–0.8 down to 0.2–0.4). This minimizes the destructive force of scratches and is the most widely used approach for food packaging UV flexo inks.

Micronized Polyethylene Wax (PE Wax)
- Mechanism: 3–5 μm PE wax microparticles migrate to the ink surface during UV curing to form a uniform lubricating film, without affecting transparency (light transmittance ≥90%).
- Selection Criteria:
  - Regulatory compliance: FDA-approved, heavy metal-free.
  - Melting point: 120–130 °C.
  - Dispersion: surface-modified for uniform dispersion in UV resins, stable without sedimentation for 72 hours.
Polytetrafluoroethylene Wax (PTFE Wax)
- Mechanism: PTFE has extremely low surface energy (18–20 mN/m), forming a “super-lubricating interface” on the ink surface; friction coefficient can drop to 0.15–0.25, with excellent chemical resistance (including acidic food simulants).
- Selection Criteria:
  - Particle size: 1–3 μm (avoiding clogging of 800-line anilox rolls).
  - Purity: PTFE ≥99%, free of PFOA, compliant with EU REACH.
  - Curing compatibility: no significant reduction in UV curing speed.
Natural Plant Waxes
- Mechanism: Long-chain fatty acid esters in natural waxes form weak hydrogen bonds with UV resins, creating a flexible lubricating film on the ink surface, offering both scratch resistance and biodegradability (aligned with sustainable packaging trends).
- Selection Criteria:
  - Source compliance: food-grade natural waxes (e.g., FDA 21 CFR 172.886-certified carnauba wax).
  - Acid value: ≤10 mg KOH/g (avoiding reactions with amine photoinitiators).
  - Thermal stability: melting point 80–90 °C to prevent loss during UV curing.

2. Structural Enhancer Additives

These additives fill voids in the ink layer and increase resin crosslink density, improving the intrinsic hardness and deformation resistance of the ink film. They are particularly suitable for food packaging that requires high hardness (e.g., beverage can labels, frozen food boxes).

Nano Silica (SiO₂)
- Mechanism: 10–50 nm nano SiO₂ modified with silane coupling agents (e.g., γ-methacryloxypropyltrimethoxysilane) reacts with UV resin double bonds, forming an “inorganic–organic hybrid network,” filling microvoids and enhancing hardness and abrasion resistance.
- Selection Criteria:
  - Surface modification: food-grade.
  - Dispersion: transparent dispersion without agglomeration in UV ink.
  - Safety: meets EU 10/2011 food contact inorganic additive requirements; heavy metals ≤0.0005 mg/kg.
Nano Alumina (Al₂O₃)
- Mechanism: Nano Al₂O₃ has high hardness (Mohs 9), forming “rigid support points” within the ink layer to resist localized pressure during scratching; its platelet structure delays crack propagation.
- Selection Criteria:
  - Particle size: 20–40 nm.
  - Surface charge: neutral (pH 6.5–7.5), non-reactive with acidic photoinitiators.
  - Migration: ≤0.005 mg/kg in 10% ethanol simulant.
Food-Grade UV Crosslinkers (e.g., isocyanurate trimers)
- Mechanism: During UV curing, crosslinkers react with hydroxyl and carboxyl groups in the resin to form a 3D network, increasing cohesive strength (from 15 MPa to 25 MPa), reducing ink layer delamination when scratched.
- Selection Criteria:
  - Type: blocked isocyanates (deblocking 80–90 °C, suitable for UV curing plus heat setting).
  - Safety: deblocking products are food-grade alcohols (e.g., butanol), migration ≤0.05 mg/kg.
  - Dosage: ≤3% (avoiding embrittlement; no cracks upon 180° bend).

3. Resin Modifier Additives: Enhancing Flexibility and Adhesion

UV flexographic inks often become brittle after curing; simply increasing hardness may cause “hard but brittle” films prone to cracking under scratches. Resin modifier additives adjust ink film flexibility and substrate adhesion, shifting from “scratch-proof” to “scratch-resistant,” preventing whole-film delamination.

Polyurethane Acrylate Oligomers (PUA)
- Mechanism: PUA contains flexible ether bonds, forming alternating “soft–hard segments” in UV resins, increasing ink film elongation at break (from 5% to 15%), and enhancing adhesion to non-absorbent substrates (e.g., PE film; peel strength from 1.5 N/15 mm to 3 N/15 mm).
- Selection Criteria:
  - Molecular weight: 1,000–3,000 (balance flexibility and curing speed).
  - Certification: FDA 21 CFR 175.300; VOC ≤1 g/L.
  - Compatibility: blends with epoxy acrylate resins (EA) at 1:3 without stratification.
Epoxy-Modified Silicone Additives
- Mechanism: Silicone segments reduce ink surface tension (from 35 mN/m to 30 mN/m), lowering scratch adhesion; epoxy groups crosslink with UV resins to prevent additive migration, ensuring both lubrication and safety.
- Selection Criteria:
  - Silicone content: 10–15% (balance lubrication and compatibility).
  - Migration: not detectable in fatty food simulants.
  - Gloss impact: ≤5 GU loss at 60° measurement angle.

StarColor UV Flexographic Ink

III. Case Study

A snack packaging company in China faced issues with UV flexo inks showing “scratch resistance only 200 cycles (500 g load)” and poor PE substrate adhesion (Grade 2B). After joint development with Huizhou StarColor, the following optimizations were implemented:

Additive Adjustments:
- Replaced standard PE wax with FDA-certified micronized PE wax (3 μm, 4% dosage).
- Added nano SiO₂ (2%, surface-modified) and PUA oligomer (12%).
- Reduced photoinitiator dosage (from 5% to 3%) to prevent side reactions.
Process Adjustments:
- Increased UV curing energy from 80 mJ/cm² to 100 mJ/cm² for complete additive curing.
- Adjusted printing pressure from 1.8 bar to 2.2 bar to enhance adhesion with PE substrate.
Safety Validation:
- Migration testing: additive migration in 4% acetic acid simulant at 0.02 mg/kg, within limits.

After optimization, ink scratch resistance improved to 600 cycles, adhesion reached Grade 4B, and it successfully passed EU infant food packaging certification. The defect rate dropped from 12% to 1.5%, saving approximately $5,000 per month.

Conclusion

Choosing scratch-resistant additives for UV flexographic inks in food packaging requires abandoning the “single hardness or lubrication focus” mindset. Instead, selection must be based on a “regulatory compliance – performance compatibility – process compatibility” framework.

As food packaging safety standards tighten, scratch-resistant additives are evolving toward low-migration, multifunctional, and biodegradable solutions. Emerging additives such as bio-based waxes and nano-composite crosslinkers are already entering industrial-scale applications.