1. Introduction

UV-curable pigment dispersions, commonly known as UV color pastes, are concentrated pigment preparations designed specifically for radiation-curable coating and ink systems. Unlike solvent-based or water-based color pastes, UV color pastes use reactive diluents—such as tripropylene glycol diacrylate (TPGDA), 1,6-hexanediol diacrylate (HDDA), and trimethylolpropane triacrylate (TMPTA)—as the carrier medium. These reactive diluents participate directly in the free-radical polymerization during UV curing, eliminating solvent emissions and ensuring complete film formation. Dongguan DENSON Functional Materials Co., Ltd. (DENSON) offers the FLUV series of UV-curable color pastes, covering organic pigments, inorganic pigments, and fluorescent pigments with a comprehensive color spectrum.

This article provides a technical overview of FLUV UV-curable color pastes, focusing on dispersion mechanism, curing compatibility, application parameters, and quality control considerations for industrial coating and ink formulators.

Figure 1. Automated UV-curable coating production line with conveyor-fed curing oven

2. Technical Features and Mechanism

2.1 Dispersion Stabilization in Reactive Diluent Media

The stabilization of pigment particles in UV-curable systems follows the combined mechanisms of electrostatic repulsion and steric hindrance. Hyperdispersants with tailored anchoring groups (acidic, basic, or aromatic moieties) adsorb onto pigment surfaces through hydrogen bonding, ion-pair interactions, or π-π stacking, while solvated chains (polyacrylate or polyester segments) extend into the reactive diluent medium, creating a steric barrier that prevents particle flocculation. Journal of Applied Polymer Science (2025, DOI: 10.1002/app.56144) reported that PEGylated partially hydrolyzed poly(N-vinylamide) dispersants for carbon black rely primarily on hydrogen bonding and π-π interactions for adsorption in non-aqueous polar media, with steric hindrance from solvated chains as the dominant stabilization force—a mechanism directly applicable to UV reactive diluent systems.

A critical requirement for UV color paste dispersants is the absence of polymerization-inhibiting groups (such as phenolic or quinonic structures). DENSON FLUV series uses specifically selected dispersants that do not interfere with free-radical polymerization, ensuring that the addition of color paste does not extend cure time or reduce crosslink density.

2.2 Pigment-Photoinitiator Competition During UV Curing

Pigments in UV coatings absorb and scatter UV radiation, competing with photoinitiators for photon energy. This competition reduces the photon flux reaching deeper layers, potentially causing under-cure at the coating-substrate interface—particularly in dark colors (carbon black, phthalocyanine blue) with high UV absorption. ACS Applied Polymer Materials (2026, 8(6), 4573–4586, DOI: 10.1021/acsapm.6c00349) proposed a soft-core/hard-shell gradient copolymer encapsulation strategy for pigments, which builds a gradient-refractive-index shell around pigment particles to reduce scattering and improve light transmittance—an approach that addresses the root cause of pigment-induced cure inhibition.

DENSON addresses this challenge through three strategies: (1) nano-scale dispersion (fineness ≤5 μm for organic pigments per GB/T 1724-1979) to minimize scattering; (2) surface treatment of inorganic pigments to passivate catalytic sites that may quench free radicals; (3) formulation guidance recommending dual photoinitiator systems (e.g., Irgacure 184 for surface cure + Irgacure TPO for through-cure) for dark color systems.

2.3 Weatherability and Light Fastness

The FLUV series employs high-performance weather-resistant pigments—quinacridones, phthalocyanines, DPP reds (organic), and rutile titanium dioxide, iron oxides (inorganic)—whose molecular structures or crystal lattices resist photodegradation under UV exposure. In QUV accelerated weathering tests (GB/T 1865-2009, equivalent to ISO 11341), coatings formulated with FLUV color pastes exhibit color change ΔE ≤ 2.0 after 500 hours of exposure.

Figure 2. DENSON FLUV-6500 cyan blue UV-curable color paste product

3. Application Case 1: UV Wood Coating — Transparent Staining

Application: UV-curable transparent topcoat for laminate flooring, requiring high clarity to preserve wood grain visibility.

Process: Three-roll coating line: UV transparent putty (30 μm) → sanding → color coat (FLUV transparent color paste + UV finish, 15 μm) → mercury lamp cure (800 mJ/cm²) → wear-resistant UV clear coat (20 μm) → mercury lamp cure (1000 mJ/cm²).

Formulation (walnut color target, ΔE ≤ 1.5):

Results: Coating transparency excellent with clear wood grain; cure speed equivalent to unpigmented control (surface dry ≤3 s); workshop VOC concentration reduced from 180 mg/m³ to below 30 mg/m³ (compliant with GB 30981-2020); abrasion resistance (GB/T 1768-2006, 500 g/500 cycles) gloss loss ≤5%.

4. Application Case 2: UV 3C Electronic Coating — PC/ABS Housing

Application: UV primer-topcoat integrated coating for laptop computer housings (PC/ABS alloy substrate), requiring high adhesion, solvent resistance, and low halogen content.

Process: Plasma surface treatment (surface energy ≥45 dyn/cm) → UV primer (rotary bell electrostatic spray, 15 μm, LED-UV 395 nm, 500 mJ/cm²) → UV topcoat (rotary bell electrostatic spray, 20 μm, LED-UV 395 nm, 600 mJ/cm²).

Formulation (space gray target, ΔE ≤ 1.0):

Results: Adhesion 0 grade (GB/T 9286-1998 cross-cut); pencil hardness 2H (GB/T 6739-2006); impact resistance 40 kg·cm (GB/T 1732-1993); no color migration under 3M 610 tape test (500 g load, 24 h); ethanol rub test (50 cycles) no visible change; halogen content ≤50 ppm (IEC 61249-2-21), meeting RoHS 2.0 requirements.

Figure 3. UV LED conveyor curing line for electronic component coating

5. Application Case 3: UV Inkjet Printing — Flatbed and Cylindrical

Application: UV-curable inkjet inks for flatbed printers (acrylic, PVC, metal substrates) and cylindrical printers (plastic cups, bottles), requiring fine particle size, low viscosity, and reliable jetting performance.

Key parameters: FLUV organic pigment color pastes achieve fineness ≤5 μm (GB/T 1724-1979), which after filtration through 1 μm filters satisfies the requirements of industrial inkjet printheads (nozzle diameter 20-50 μm) without clogging. The low viscosity (500-3000 mPa·s at 25°C, GB/T 9751.1-2008) and high pigment loading (30-60%) minimize the dilution effect on the ink formulation.

Curing: LED-UV 395 nm, 300-500 mJ/cm², achieving instant cure at print speeds up to 30 m/min. Adhesion on non-porous substrates (acrylic, glass, metal) is achieved through substrate-specific primer treatment or UV-curable adhesion promoters in the ink formulation.

Figure 4. UV LED curing of cylindrical plastic substrate printing

6. Key Selection Parameters and Usage Recommendations

Usage recommendations: (1) Stir thoroughly before use to ensure uniform pigment dispersion; (2) Conduct small-scale trials to determine optimal dosage and cure parameters, especially for dark colors (black, deep blue) which may require 20-50% higher cure energy; (3) Avoid mixing with additives containing phenolic inhibitors; (4) Store in cool, dark conditions at 5-35°C; (5) For cationic UV systems (epoxy/vinyl ether), consult DENSON technical service for compatibility assessment, as the FLUV series is primarily designed for free-radical acrylate systems. Additional technical resources are available in the DENSON literature section.

7. Conclusion

The DENSON FLUV series of UV-curable color pastes provides a reliable coloring solution for UV-curable coatings, inks, and 3D printing systems. Through hyperdispersant anchoring-solvation dual stabilization, inhibitor-free dispersant selection, high-performance weather-resistant pigments, and low-halogen formulations, FLUV color pastes achieve nano-scale stable dispersion in reactive diluent media without interfering with free-radical polymerization, while meeting EU regulatory requirements. With verified performance in wood coatings, 3C electronic coatings, and UV inkjet applications, FLUV series offers formulators a technically sound and cost-effective alternative to imported UV color pastes.

8. FAQ

Q1: Can FLUV color pastes be used in cationic UV-curable systems?

A: The FLUV series is primarily designed for free-radical UV-curable systems (acrylate-based). Cationic UV systems (epoxy/vinyl ether) have different dispersant and pigment requirements due to the cationic polymerization mechanism. Before use in cationic systems, consult DENSON technical service for compatibility assessment; custom cationic-system color pastes can be developed upon request.

Q2: Why do dark UV coatings sometimes exhibit under-cure at the bottom layer?

A: Dark pigments (carbon black, phthalocyanine blue) strongly absorb UV radiation, reducing the photon flux reaching deeper layers and causing insufficient photoinitiator excitation. Solutions include: increasing cure energy (additional lamps or slower line speed); using long-wavelength-sensitive photoinitiators (TPO, BAPO) for through-cure; reducing color paste dosage; applying double-sided cure; or increasing bottom-layer photoinitiator concentration.

Q3: What are the characteristics of FLUV fluorescent pigment color pastes?

A: FLUV fluorescent color pastes use daylight fluorescent pigments that exhibit vivid fluorescent colors under visible light, with enhanced effect under UV lamp illumination. They are suitable for safety signage, decorative coatings, and advertising inks. Note that fluorescent pigments generally have lower light fastness than conventional organic pigments; for outdoor applications, addition of UV absorbers is recommended.

Q4: Will FLUV color pastes affect coating adhesion and hardness?

A: Within the recommended dosage range (≤8%), FLUV color pastes do not significantly affect coating adhesion (GB/T 9286) or pencil hardness (GB/T 6739). The reactive diluent carrier participates directly in curing, leaving no residual solvent as in solvent-based color pastes. Excessive dosage may reduce film density due to high pigment volume concentration, so dosage should be controlled within the recommended range.

Q5: How can I determine if FLUV color paste has flocculated or expired?

A: Check for visible separation, sedimentation, or caking in the container; measure fineness with a Hegman gauge (a significant increase beyond twice the出厂 value indicates flocculation); and draw down a small amount on a glass plate to check for color specks, particles, or uneven color distribution. If any of these occur, re-disperse at high speed and retest. If the issue persists after re-dispersion, the paste should not be used.