“Run another data set — reduce the TPO-L photoinitiator dosage to 2.1%, immerse the printed sample in 50% ethanol simulant for 72 hours, and focus on the migration values.”

The instruction came clearly from Li, head of Starcolor’s R&D center. Photoinitiator migration in UV flexographic inks has long been one of the most persistent challenges in food packaging. If these substances migrate from the ink layer into food, they may pose safety risks and regulatory non-compliance.

Over 18 months of continuous optimization, Starcolor’s R&D team refined dozens of micro-level formulation and process details. The latest validated results show photoinitiator migration controlled below 0.005 mg/kg under laboratory testing conditions — far below the commonly referenced benchmark limit of 0.6 mg/kg aligned with EU Regulation (EU) No 10/2011.

1: Screening from the Source

“Migration risks originate at the raw material level. Selecting the right photoinitiator solves half of the problem.”

At the early stage, the team evaluated 16 commonly used photoinitiators. Market-standard benzophenone (BP), while offering fast curing, showed migration levels as high as 0.8 mg/kg in fatty food simulants — exceeding acceptable safety benchmarks.

The team established a three-layer screening strategy:

• Chemical stability: Preference for higher molecular weight and higher polarity structures.
• Preliminary migration testing: 72-hour immersion in water, 4% acetic acid, 20% ethanol, and n-hexane, analyzed by HPLC.
• Cost feasibility: Exclusion of excessively high-cost photoinitiators.

Reactive photoinitiator TPO-L demonstrated excellent migration control. However, its curing speed was approximately 20% slower than BP, limiting suitability for high-speed flexographic lines.

To balance safety and performance, the team developed a low-dosage hybrid system using TPO-L combined with a minimal amount of fast-reacting ITX. After testing 12 ratio combinations, an optimized formulation of 2.2% TPO-L + 0.3% ITX achieved both acceptable curing speed and controlled migration below 0.05 mg/kg under test conditions.

2: Building a Resin “Framework”

If photoinitiators represent the potential migration source, the resin matrix acts as the containment structure.

Traditional acrylic-based UV flexo resins exhibited relatively low crosslink density, allowing molecular pathways for migration. The team introduced polyurethane segments into the acrylic backbone to create a denser three-dimensional network.

After extensive testing, a formulation containing 12% polyurethane and 88% acrylic resin, combined with increased acrylic molecular weight, achieved optimal balance: viscosity around 290 mPa·s, improved crosslink density, and photoinitiator migration reduced to approximately 0.02 mg/kg.

An additional 0.5% silane coupling agent was introduced to further bind photoinitiator molecules within the resin network, reducing the amount of free, migratable species.

3: Precision Process Optimization

Even the best formulation can fail without precise process control.

Two key risks were identified on the production line: pigment agglomeration and insufficient UV curing energy.

By upgrading dispersion from conventional high-speed mixing to pre-dispersion followed by triple-roll milling, pigment particle size was controlled within 1–2 μm, eliminating micro-voids that could facilitate migration.

Curing parameters were optimized by installing real-time UV energy monitoring. Optimal curing energy was determined to be 90 mJ/cm² for PE films and 100 mJ/cm² for paper substrates. Additionally, the curing wavelength was adjusted from 365 nm to 395 nm, closer to the absorption peak of TPO-L, improving reaction efficiency and reducing residual photoinitiators by approximately 40%.

4: Full-Scenario Extreme Testing

Laboratory success must be validated under real-world conditions.

The team conducted comprehensive extreme testing, including freezing at -20°C, high-temperature storage at 70°C, thermal cycling, and six-month shelf-life simulation.

Following formulation refinements for low-temperature flexibility, all scenarios demonstrated stable migration levels below 0.005 mg/kg under validated test methods.

Additional controls ensured heavy metals ≤ 0.001 mg/kg and VOC content ≤ 10 g/L through solvent-free formulation design.

Conclusion

From pilot trials to mass production, Starcolor’s R&D team completed nearly 200 experimental data sets, 32 formulation revisions, and 16 process optimizations.

There were no dramatic technological breakthroughs — only relentless attention to detail: adjusting a ratio by 0.1%, testing one more scenario, validating one more data point.

Today, the team is advancing toward near-zero migration formulations for infant food packaging. In the highly regulated field of direct food contact inks, Starcolor’s engineers continue to build trust by eliminating migration risks at their source — one detail at a time.