Photoinitiator 819 (Bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide) is a high-performance Type I (cleavage) radical photoinitiator belonging to the bisacylphosphine oxide (BAPO) family. Its pale yellow powder form belies an exceptional capability: delivering superior deep‑cure performance in high‑opacity formulations, pigmented coatings, and thick-layer applications. With strong absorption extending into the visible range (up to 440 nm), Photoinitiator 819 is the go‑to choice for UV-LED curing at 395 nm and 405 nm, making it indispensable for modern industrial UV systems.
Key Chemical & Physical Properties
| Property | Description |
|---|---|
| Chemical Name | Bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide |
| Appearance | Pale yellow powder |
| Absorption Range | 350–440 nm (peak in UVA/visible region) |
| Type | Type I (cleavage) radical initiator |
| Mechanism | Generates two radicals upon photolysis → high initiation efficiency |
Unlike conventional photoinitiators that absorb mainly in the short UV region (250–300 nm), Photoinitiator 819's red‑shifted absorption enables efficient curing through titanium dioxide (TiO₂), carbon black, and other high‑pigment systems where shorter wavelengths are scattered or blocked.
Why Photoinitiator 819 Stands Out
1. Broad & Intense Long‑Wavelength Absorption
The absorption tail of 819 extends to 440 nm, perfectly matching the emission spectra of modern UV‑LED lamps (especially 365 nm, 385 nm, 395 nm, and 405 nm). This ensures high energy utilization and rapid polymerization even in challenging formulations.
2. Dual Radical Generation
Upon exposure, each molecule of 819 cleaves into two active radicals. This dual‑radical mechanism dramatically increases the initiation rate, leading to faster cure speeds and higher crosslink density compared to monoacylphosphine oxide (MAPO) initiators like TPO.
3. Superior Photobleaching
A unique advantage of 819 is its photobleaching property: the chromophore that gives the pale yellow color is destroyed during the curing process. The final coating, adhesive, or ink exhibits minimal yellowing, making it suitable for clear coats, white finishes, and optically clear applications.
4. Deep‑Cure & Through‑Cure
Long‑wavelength light penetrates deeper into coatings and substrates. Photoinitiator 819 excels in:
Thick‑film applications (>500 µm)
Highly pigmented systems (e.g., white, black, blue, red)
Fiber‑reinforced composites (FRP, prepregs)
Primary Applications
UV Coatings
High‑pigment industrial coatings: White paints containing TiO₂, opaque color paints, and阻焊油墨 (solder masks).
Thick‑film coatings: Clearcoats and fillers requiring full cure from top to bottom.
Composite materials: Glass fiber reinforced plastics (FRP) where deep penetration is essential for structural integrity.
UV Inks
Screen inks: Especially for dark shades (black, deep blue) that block short UV wavelengths.
Flexographic & gravure inks: High‑speed printing lines benefit from fast, thorough cure.
Digital inkjet: Compatible with UV‑LED inkjet systems, providing reliable jetting stability and pinning performance.
UV Adhesives
Optically clear adhesives (OCA) : Photobleaching ensures negligible color shift after cure.
Electronic structural adhesives: Deep cure through opaque substrates and gap‑filling layers.
Potting & encapsulation: Thick‑layer adhesives where full cure is mandatory.
3D Printing
Photoinitiator 819 is highly sensitive to 395–405 nm light sources used in DLP (digital light processing) and LCD‑based 3D printers. It delivers:
High depth of penetration (Dp)
Reduced printing time
Excellent green strength and final mechanical properties
Formulation Strategies: Combining for Surface & Deep Cure
While Photoinitiator 819 provides outstanding through‑cure, it is often combined with surface‑active photoinitiators to overcome oxygen inhibition and achieve a tack‑free surface. Typical synergistic combinations include:
| Combination | Purpose |
|---|---|
| 819 + 184 (1‑Hydroxycyclohexyl phenyl ketone) | Ideal for UV coatings: 184 drives surface cure (low yellowing), while 819 handles deep cure and pigmented layers. |
| 819 + 1173 (2‑Hydroxy‑2‑methylpropiophenone) | Common in inks and adhesives where fast surface drying and deep‑cure balance are required. |
| 819 + ITX / 369 | For deep‑color ink systems (black, dark blue) requiring extended visible‑light absorption. |
Typical addition levels range from 0.5 % to 3 % of total formulation weight, depending on film thickness, pigment loading, and light source intensity.
Handling & Storage Recommendations
Solubility: Photoinitiator 819 is readily soluble in most acrylate monomers (e.g., HDDA, TPGDA, DPGDA) and oligomers. Ensure complete dissolution to avoid crystallization.
Stability: Store in a cool, dry place away from direct light and heat. While 819 offers better thermal stability than TPO, prolonged exposure to elevated temperatures or humidity may lead to premature decomposition.
Regulatory compliance: For applications involving food contact, toys, or medical devices, always verify regional regulatory status (e.g., EU REACH, US TSCA, FDA 21CFR) and migration limits.
Conclusion
Photoinitiator 819 is the benchmark BAPO‑type photoinitiator for demanding UV curing applications requiring deep penetration, high‑pigment tolerance, and minimal yellowing. Its excellent compatibility with UV‑LED light sources, combined with dual‑radical efficiency and photobleaching properties, makes it indispensable for:
High‑opacity coatings and white paints
Thick‑layer adhesives and encapsulants
Dark‑color screen and digital inks
High‑speed 3D printing resins
When formulating, pairing 819 with a surface‑cure initiator like 184 or 1173 unlocks the full potential of your UV system-delivering fast, thorough, and visually flawless results.