Pigment Red 122 (C.I. 73915) is a high-performance quinacridone pigment — and the answer to a critical formulation challenge brought to TRESAI Chemical by a leading packaging and printing company in Southeast Asia. The company operates high-volume flexographic and UV printing facilities, producing pressure-sensitive labels and flexible packaging for major consumer brands in the region.
Challenge: Color Fading During UV Curing
The customer’s production line utilizes narrow-band UV LED curing systems at 395 nm wavelength. Their existing red pigment—a mid-grade azo-based product—exhibited three critical deficiencies:
- Insufficient color saturation on synthetic films: When printed on PET and OPP films, the pigment produced flat, grayish magenta tones that failed to meet brand color guidelines. Measured CIE Lab* values showed the a* value fell below the target specification, resulting in insufficient vibrancy.
- Photochemical degradation under UV LED irradiation: The azo chromophore underwent partial decomposition under intense 395 nm UV LED radiation, causing a measurable shift in hue angle (Delta H) after each curing cycle. This was particularly problematic for multi-color printing jobs requiring precise color registration.
- FDA compliance requirements: The customer’s end-use included food packaging labels, requiring a pigment system compliant with FDA 21 CFR 178.3297 (Colorants for Polymers) and EU 10/2011 regulations, without the need for additional functional barrier layers.
Why PR122: Advantages of Quinacridone
Pigment Red 122 (C.I. 73915)—a high-performance quinacridone pigment capable of addressing each identified challenge—was recommended by the TRESAI technical team after analyzing the customer’s formulation requirements and substrate conditions.
Key characteristics that make PR122 the ideal choice include:
- Excellent lightfastness: The fully aromatic quinacridone ring system provides superior resistance to UV photochemical degradation. Unlike azo pigments that rely on the relatively unstable N=N bond, the fused-ring conjugated structure of the quinacridone chromophore offers inherently greater resistance to the 395 nm wavelength used in modern curing systems. Blue Wool lightfastness ratings reach 7–8 (ISO 105-B02).
- High tinting strength and color saturation: TRESAI’s PR122 grade achieves excellent tinting strength compared to commercial reference standards. This enables formulators to obtain deeper, more saturated magenta tones at reasonable pigment addition levels, reducing overall formulation costs while improving optical density on transparent film substrates.
- Excellent dispersion stability in UV oligomers: The surface of TRESAI PR122 particles is treated with proprietary organic modifiers, improving compatibility with common UV-curable oligomer systems (including epoxy acrylates, polyurethane acrylates, and polyester acrylates). This yields stable dispersions with minimal viscosity increase and no sedimentation in long-term storage tests.
- Regulatory compliance: PR122 is listed on positive lists for multiple food-contact applications, including FDA 21 CFR 178.3297 and EU 10/2011 regulations, with heavy metal contents well below regulatory limits (Pb < 1 ppm, Cd < 0.5 ppm, Hg < 0.1 ppm).
Implementation and Results
TRESAI application engineers collaborated with the customer’s ink laboratory over a six-week development cycle:
- Benchmark comparison (Week 1): Using the customer’s existing UV ink vehicle, laboratory drawdowns were prepared replacing the pigment with PR122 at equivalent tinting strength. Initial spectrophotometer readings showed a significant improvement in a* values compared to the previous pigment.
- Formulation optimization (Weeks 2–3): The TRESAI team adjusted the pigment dispersion process, optimizing bead mill parameters (0.3 mm zirconia media, 4000 RPM, multiple passes) to achieve Hegman fineness of 7.5+. Dispersant dosage was fine-tuned to a reasonable percentage of pigment weight, using polyamine-type dispersants compatible with the customer’s acrylic oligomer system.
- Pilot production (Weeks 4–5): Full-scale batch production was conducted on the customer’s production equipment. Print trials on PET and OPP films confirmed laboratory results, with color saturation controlled within acceptable ranges relative to Pantone targets.
- Durability validation (Week 6): Accelerated weathering tests were conducted per ASTM G155 standard using a xenon-arc weatherometer. The PR122 formulation maintained color integrity after 500 hours of exposure, with total color difference (Delta E) well within the customer’s acceptance criterion of Delta E < 3.0.
The final formulation achieved a substantial improvement in color saturation (measured by CIE a* coordinate) compared to the previous pigment, while maintaining equivalent gloss levels and passing all FDA migration test requirements.
Key Takeaways
- Quinacridone pigments such as PR122 outperform azo pigments in UV LED curing environments because their inherently stable molecular structure resists photodegradation at 395 nm.
- Higher tinting strength enables formulators to optimize pigment content—achieving superior coloration density while reducing raw material costs.
- Regulatory compliance and performance are not mutually exclusive. PR122 meets stringent food-contact regulations (FDA, EU 10/2011) without compromising tinting strength, lightfastness, and gloss retention.
- Proper dispersion processing is essential to unlock the pigment’s full potential. Optimized bead mill parameters and dispersant selection release the complete tinting strength of PR122, achieving Hegman fineness of 7.5+ and stable viscosity.