Photoinitiator 819 (chemical name: Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide) is a high-performance Type I (Norrish Type I) radical photoinitiator belonging to the bisacylphosphine oxide (BAPO) family. It appears as a pale yellow powder and is widely recognized for its exceptional deep-curing capability and superior photobleaching effect.
As the UV curing industry rapidly transitions toward 395 nm and 405 nm LED light sources, Photoinitiator 819 has become the go-to solution for tackling challenges in high-opacity systems, thick coatings, and heavily pigmented formulations. This article provides a comprehensive overview of its photochemical mechanism, key advantages, typical applications, synergistic combinations, and formulation guidelines.
1. Chemical Properties and Photolysis Mechanism
1.1 BAPO Structure
Photoinitiator 819 features two acylphosphine oxide groups within its molecular structure. This BAPO configuration distinguishes it from conventional α-hydroxyketone photoinitiators (e.g., 1173, 184) in several ways:
Type I Photolysis: Upon UV exposure, it undergoes direct α-cleavage, generating two active free radicals per molecule without requiring co-initiators such as tertiary amines.
Red-Shifted Absorption: It exhibits strong absorption in the UVA region (320–400 nm) with a maximum absorption peak at approximately 370 nm and a tail extending to 440 nm, making it highly compatible with UV-LED sources (365 nm, 385 nm, 395 nm, 405 nm) .
High Molar Extinction Coefficient: Its superior absorbance in the long-wavelength range enables excellent penetration through light-scattering or light-absorbing media.
1.2 Photobleaching Effect
During photolysis, the chromophoric groups of 819 are rapidly destroyed, causing the photoinitiator to lose its color. This photobleaching property results in minimal yellowing after curing-a critical advantage for clear coatings, white finishes, and optical-grade adhesives.
2. Key Advantages of Photoinitiator 819
| Advantage | Technical Benefit |
|---|---|
| Long-Wavelength Absorption & Deep Cure | Absorbs up to 440 nm, enabling thorough curing through titanium dioxide (TiO₂), black pigments, and thick coatings-eliminating under-cure and adhesion failures |
| Dual Radical Generation | Produces two radicals per molecule, boosting initiation efficiency, reducing required loading, and enhancing crosslink density |
| Excellent Photobleaching | Minimal residual yellowing after cure, ideal for color-sensitive and transparent systems |
| UV-LED Compatibility | Highly responsive at 395–405 nm, aligning with modern LED curing equipment |
| Good Thermal Stability | Stable under standard storage and formulation conditions, outperforming some other acylphosphine oxides |
3. Key Applications
3.1 UV Coatings: High-Opacity and Thick-Film Systems
Photoinitiator 819 excels in pigmented coatings and thick-film applications:
High-TiO₂ White Paints: Titanium dioxide strongly scatters short-wavelength UV light; 819's long-wavelength absorption enables through-cure in white formulations.
Solder Masks: Ensures complete bottom-layer cure in dark green, blue, and other opaque solder mask systems used in PCB manufacturing.
FRP Composites: Provides uniform deep curing in glass-fiber-reinforced materials, preventing surface over-cure while leaving the interior under-cured.
3.2 UV Inks: Solving Deep-Color Curing Challenges
In screen, flexographic, and UV digital inkjet printing, Photoinitiator 819 is essential for curing black, deep blue, and other high-opacity inks:
Penetrates through the entire ink film, ensuring bottom-layer polymerization.
Improves adhesion on challenging substrates such as metals, glass, and plastics.
Enhances rub resistance and durability of the final print.
3.3 UV Adhesives: Optical Bonding and Thick-Section Bonding
Optically Clear Adhesives (OCA) : Photobleaching ensures no yellowing after cure, maintaining optical transparency.
Electronic Structural Adhesives & Potting Compounds: Enables one-step curing of layers several millimeters thick, reducing delamination and shrinkage stress.
3.4 3D Printing: High-Speed Printing with Superior Mechanical Strength
In SLA, DLP, and LCD 3D printing resins, Photoinitiator 819 offers exceptional sensitivity to 395–405 nm light sources:
Increases printing speed, particularly in high-speed resin formulations.
Improves interlayer adhesion and final part strength, reducing the need for extended post-curing.
4. Formulation Strategies: Balancing Surface Cure and Deep Cure
Although 819 provides outstanding deep curing, using it alone may result in surface tackiness due to oxygen inhibition or rapid photobleaching at the surface. The industry standard is to use synergistic combinations to achieve a fully cured surface and bulk:
| Component | Function | Typical Loading (by total resin weight) |
|---|---|---|
| 819 | Deep cure, pigmented system penetration | 0.5% – 2% |
| 184 | Surface cure, low yellowing | 1% – 3% |
| 1173 | Surface cure, low viscosity contribution | 1% – 4% |
Formulation Logic: 819 addresses "through-cure," while surface-cure initiators (184, 1173) address "surface cure." Adding amine synergists (e.g., EDAB, EOHA) can further counteract oxygen inhibition for a tack-free finish.
5. Handling Guidelines and Formulation Considerations
5.1 Solubility
Photoinitiator 819 is readily soluble in most acrylate monomers (e.g., HDDA, TPGDA, DPGDA) and common oligomers. In low-polarity systems (such as pure epoxy acrylates), gentle heating to 40–50°C may be required to ensure complete dissolution. Compatibility testing is recommended for new formulations.
5.2 Weathering and Yellowing
While 819 photobleaches rapidly during cure, any residual, unreacted initiator may contribute to slow yellowing under prolonged outdoor exposure. For high-weatherability applications (automotive coatings, outdoor signage, etc.):
Ensure sufficient UV dose for complete decomposition of the initiator.
Incorporate hindered amine light stabilizers (HALS) and UV absorbers (UVA) .
Consider formulations with minimal residual initiator content.
5.3 Regulatory and Safety Information
Photoinitiator 819 has undergone extensive toxicological evaluation, and its decomposition products are considered low in toxicity. However, for sensitive applications such as food contact materials, medical devices, and toys, it is essential to comply with relevant regulations, including GB 9685, EU No 10/2011, and FDA guidelines, with respect to migration limits.