| HS Code | 173339 |
| Chemical Class | Small organic molecules or organometallic compounds |
| Mechanism | Generate free radicals upon absorption of UV/visible light |
| Photoinitiation Type | Type I (cleavage) or Type II (hydrogen abstraction/electron transfer) |
| Absorption Spectral Range | Typically 250–450 nm depending on chemical structure |
| Typical Forms | Liquid or solid crystalline powders |
| Solubility | Soluble in acrylate monomers, oligomers, and organic solvents |
| Curing Mechanism | Initiate radical polymerization of acrylates, methacrylates, and unsaturated polyesters |
| Application Areas | UV-curable inks, coatings, adhesives, dental materials, and 3D printing resins |
| Reactivity Characteristics | High initiation speed and low migration tendency after cure |
| Storage Conditions | Store in cool, dark, dry conditions away from direct light and heat |
As an accredited Radical Photoinitiator factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | 25 kg light-resistant fiber drum with inner polyethylene liner, sealed moisture-proof lid, hazard labeling. |
| Container Loading (20′ FCL) | Load 20′ FCL with sealed, light-protected drums; secure firmly, avoid heat and ignition sources, ensure ventilation, and follow chemical safety protocols. |
| Shipping | Radical Photoinitiators ship in light-protective, sealed containers away from UV, heat, and ignition sources. Depending on formulation, they may be classified as non-hazardous or as environmentally hazardous substances under ADR/IMDG. Documentation includes a Safety Data Sheet; temperature-controlled, ventilated transport is recommended to maintain stability and shelf life. |
| Storage | Store Radical Photoinitiators in a cool, dry, well-ventilated area away from heat, sparks, and direct sunlight. Keep containers tightly sealed to prevent oxygen or moisture exposure, which can reduce activity. Avoid incompatible materials like strong oxidizers. Use proper grounding and label clearly, ensuring segregation from peroxide-forming agents. |
| Shelf Life | Shelf life typically 1–2 years when stored in a cool, dark, dry place, away from UV light and moisture. |
In prefinished wood and plastic flooring coatings, Norrish Type I radical photoinitiator packages containing 2,4,6-trimethylbenzoyldiphenylphosphine oxide (TPO) and bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (BAPO) are adjusted to mercury-gallium and LED emission lines. For 395 nm LED-cured clear sealers, the addition ratio is typically 0.8–2.2 wt% BAPO or 1.0–3.0 wt% ethyl (2,4,6-trimethylbenzoyl) phenylphosphinate based on total reactive solids; for medium-pressure mercury systems, 2.0–4.0 wt% 1-hydroxycyclohexyl phenyl ketone is paired with 2.0–3.0 wt% amine synergist. Loading above 3.5 wt% acylphosphine oxide creates an absorbance barrier in the first 10–20 µm of the wet film, reducing bottom cure on pigmented vinyl wear layers and increasing residual photoinitiator migration into the lacquer. Adhesion and film hardness are benchmarked against ASTM D3359-17 and ASTM D4366-14; when the clear coat is specified for cosmetic closures or food-contact packaging surfaces, the cured film falls under FDA 21 CFR 175.300 for resinous and polymeric coatings and must be confirmed non-migrating under the relevant food type and temperature condition. EU formulators additionally require REACH registration of all photoinitiators and reactive diluents in the formulation.
Downstream application on a production roller coater deposits 10–50 g/m² wet film before inline UV modules delivering 300–800 mJ/cm² UV-A at line speeds of 8–25 m/min. Ga-doped mercury lamps with peak irradiance in the 365–420 nm band or 385/395 nm LED arrays are matched to the photoinitiator absorbance window. Pigmented vinyl wear layers containing 2–5 wt% titanium dioxide reduce UV penetration; production lines therefore increase surface dose to 600–900 mJ/cm² or shift to acylphosphine oxide grades with absorption tails extending past 420 nm. Production-scale failure modes include oxygen-inhibited surface haze at coat weights below 8 g/m² when line speed exceeds 20 m/min, and yellowing after a 60°C stack test when tertiary amine synergist exceeds 2 wt% in the formulation. Finished article types include prefinished engineered wood flooring, rigid vinyl plank wear layers, and UV-sealed polypropylene cosmetic closures.
Photoinitiator molecular weight and photofragment volatility determine whether a flexographic UV ink can pass European food-contact regulation. Conventional isopropylthioxanthone (ITX) and benzophenone packages at 4–6 wt% of total ink formula provide adequate radical generation but create low-molecular-weight extractables; the 2005 ITX milk packaging episode demonstrated transfer through aluminum-polyethylene laminate under ambient storage. Low-migration grades replace them with difunctional or polymeric thioxanthone and high-molecular-weight aminobenzoate synergists at 2–5 wt% total photoinitiator package, while maintaining surface cure at 300–600 mJ/cm² on narrow-web flexographic presses running 50–150 m/min. For LED-cured low-migration inks at 395 nm, published data for some polymeric thioxanthone grades is limited, and formulation work must verify both cure speed and extractable photofragment concentration before production approval.
| Framework | Scope | Boundary |
|---|---|---|
| Regulation (EU) No 10/2011 | Plastic food-contact materials and multilayer laminates | Overall migration ≤ 10 mg/dm²; unlisted photoproducts must undergo Article 3 risk assessment |
| Swiss Ordinance SR 817.023.21 | Printing inks for food packaging | Positive list only; ITX and benzophenone excluded from low-migration suites |
| FDA 21 CFR 175.300 | Resinous and polymeric coatings | Compliance conditioned by food type, temperature, and coating thickness |
| EUPIA Good Manufacturing Practices | Food-safe ink manufacture | GMP risk assessment, raw material traceability, and contamination control |
The flexographic process applies 5–10 µm wet ink film through anilox rollers engraved at 400–1200 lpi, followed by medium-pressure mercury or LED UV modules. End products include primary food-contact labels, flexible snack packaging, and paper-based laminate pouches. For low-migration systems, the operational boundary is narrow: addition of free amine synergist above 3 wt% increases extractable amine odor and migration risk, while omitting it in a thioxanthone-based system slows through-cure and produces set-off on the chill drum. Batch-to-batch variance in pigment absorbance at 365–385 nm shifts cure energy demand; production-scale ink rooms monitor real-time UV power density with radiometers and quarantine batches below 250 mJ/cm² surface dose. Unlisted substances are assessed under Article 3 of Regulation (EC) No 1935/2004 rather than assumed compliant, and target non-detection limits for photoinitiator photoproducts are confirmed by LC-MS/MS before food-contact print runs are released.
At 405 nm exposure, depth of polymerization follows the Jacobs equation Cd = Dp ln(E/Ec). In LCD and DLP resin systems, radical photoinitiator concentration is set between 0.3–1.2 wt% of total liquid resin; ethyl (2,4,6-trimethylbenzoyl) phenylphosphinate is selected over TPO because its absorption tail to 410 nm reduces loss of penetration depth Dp at 405 nm. Increasing photoinitiator above 1.5 wt% raises the critical curing energy Ec and lowers Dp, producing delamination at 100 µm layer thickness when exposure drops below 15 mJ/cm² on low-power LCD printers. The processing window is therefore bounded by surface oxygen inhibition at low dose and by viscous uncured residue in build trays at high photoinitiator loading. Resin viscosity for standard LCD formulations is typically maintained at 500–1500 mPa·s at 25°C to ensure uniform recoating between layers without excessive resin carry-over on the build platform.
Production-scale LCD and DLP equipment operate with 405 nm LED arrays emitting 2–10 mW/cm², layer thicknesses of 50–100 µm, and burn-in layers exposed at 200–600 mJ/cm². Post-cure in a 40–60°C UV chamber for 30–60 min completes conversion of residual acrylate groups on part surfaces. Mechanical properties of printed articles are measured under ASTM D638-14 or ISO 527-2:2012 for tensile and ISO 178:2019 for flexural; dental model and surgical guide resins require ISO 10993-5:2009 cytotoxicity data. End-product types include dental diagnostic casts, thermoforming templates, investment casting patterns, and hearing aid shells. Operational incompatibilities include BAPO yellowing in clear parts when loaded above 1 wt% and exposed to 60°C post-cure, and recrystallization of TPO in resin stored below 15°C. Addition of amine-based accelerators in this process is avoided because it increases surface tack and photoyellowing without resolving the oxygen inhibition threshold at the resin-air interface.
Pressure-sensitive adhesive coating lines running at 80–120 m/min on siliconized polyethylene terephthalate carriers use radical photoinitiator levels below 1.5 wt% based on acrylic solids to prevent post-cure yellowing in label face stocks. BAPO at 0.5–1.5 wt% or ethyl (2,4,6-trimethylbenzoyl) phenylphosphinate at 0.8–2.0 wt% is dispersed into a 100% solids acrylic syrup before slot-die or reverse-roll application at 50–80 µm wet film, with slot-die temperature maintained at 40–65°C to lower coating viscosity. The adhesive is cured under nitrogen-inerted UV chambers maintaining residual oxygen below 500 ppm because radical polymerization at the adhesive surface is otherwise arrested by oxygen addition to propagating acrylic radicals, producing a tack-free underlayer beneath a liquid surface. Adhesion performance is measured under ASTM D3330/D3330M for peel, and medical-grade tapes are screened for cytotoxicity under ISO 10993-5:2009. Food-packaging label adhesives fall under FDA 21 CFR 175.125 for pressure-sensitive adhesives; residual acrylic monomer is quantified by gas chromatography against the migration limits calculated for the final packaging application rather than a single fixed residue limit. Production-scale failure modes include edge lifting during high-speed slitting when through-cure conversion at the 910 cm⁻¹ acrylate absorption band remains below 80%, and surface contamination from silicone release liner transfer that inhibits radical cure in spots. End products include transparent label stock, dermal wear medical tape, and window film mounting adhesives.
Acrylated epoxy oligomer solder mask formulations employ a type II radical photoinitiator pair at 2–4 wt% isopropylthioxanthone plus 2–3 wt% amine synergist based on ink solids, or a type I acylphosphine oxide at 1.5–3 wt% when 385 nm LED exposure replaces medium-pressure mercury. The ink is screen-printed at 20–40 µm dry film thickness onto FR-4 panels and conveyed through a UV chamber delivering 400–800 mJ/cm² in the 365–420 nm band. Thermal post-bake at 150°C for 60 min completes epoxy crosslinking in shadow areas and increases chemical resistance to molten lead-free solder at 260–288°C for 10 s double-side reflow. Qualification follows IPC-SM-840E for permanent solder mask and UL 746E for long-term thermal aging of printed wiring boards; RoHS Directive 2011/65/EU restricts lead, cadmium, mercury, and hexavalent chromium in the formulated ink. End products include rigid multilayer FR-4 circuit boards, flexible polyimide circuits, and aluminum-core LED boards. Operational boundaries include storage at 20–25°C in red-light areas because ambient UV exposure initiates polymerization in the screen mesh, and incompatibility with high-pH aqueous developers above pH 11.5, which attack the uncured acrylate network before post-bake. A type II amine synergist also reduces one-package storage stability at temperatures above 30°C; production-scale dispensing rooms therefore control ink inventory and monitor viscosity drift against ISO 2884-1 rotational viscometry.
Competitive Radical Photoinitiator prices that fit your budget—flexible terms and customized quotes for every order.
For samples, pricing, or more information, please contact us at +8615365186327 or mail to admin@ascent-chem.com.
We will respond to you as soon as possible.
Tel: +8615365186327
Email: admin@ascent-chem.com
Flexible payment, competitive price, premium service - Inquire now!
Ultraviolet cure of acrylate and methacrylate resins proceeds when a radical photoinitiator absorbs at the emission wavelength of a mercury arc or UV-LED source and undergoes α-cleavage or hydrogen abstraction. The Radical Photoinitiator product line comprises three grades identified as RPI-1173, RPI-819, and RPI-TPO-L. RPI-1173 is 2-hydroxy-2-methyl-1-phenylpropan-1-one, supplied as a clear liquid with an absorption maximum at 331 nm. RPI-819 is phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, a yellow powder with absorption maxima at 370 nm and 405 nm. RPI-TPO-L is ethyl (2,4,6-trimethylbenzoyl) phenylphosphinate, a pale-yellow liquid with an absorption maximum at 380 nm. The product line is specified for acrylate-terminated oligomers, unsaturated polyesters, and thiol-ene systems; it is not formulated for epoxide or vinyl ether resins that require cationic initiation. Radical cure terminates when irradiation stops, whereas cationic systems continue dark cure, which changes line-speed control and post-cure packing windows.
Type I radical photoinitiators generate radical pairs through intramolecular α-cleavage after photon absorption. RPI-1173 cleaves at the carbon–carbon bond adjacent to the carbonyl chromophore to produce a benzoyl radical and an alkyl radical. RPI-819 and RPI-TPO-L are acylphosphine oxide and phenylphosphinate systems that undergo photobleaching; their absorption spectra decay as photolysis proceeds. This permits deeper light penetration in pigmented coatings and filled systems because the initiator does not continually filter incident radiation at the cure wavelength. Type II radical photoinitiators such as benzophenone require a hydrogen-donating co-initiator, usually a tertiary amine, and proceed through bimolecular hydrogen abstraction. The type II route is more viscosity-dependent and is not represented in the RPI series.
Oxygen inhibition is the dominant surface-cure limitation in radical systems. Dissolved oxygen in low-viscosity acrylate formulations at 25 °C is typically near 2×10⁻³ mol/L, and oxygen diffusion coefficients are on the order of 1×10⁻⁵ cm²/s. Triplet oxygen quenches the initiator triplet state and scavenges carbon-centered radicals to form low-reactivity hydroperoxyl radicals. In clearcoats below 5 µm dry film thickness, RPI-1173 alone can leave a tacky surface because radical consumption at the air interface exceeds radical generation. The conventional correction is addition of an amine synergist at 2–5 wt%, use of a thiol-functional dilution agent, lamination under inert gas, or reformulation with an acylphosphine oxide such as RPI-819 or RPI-TPO-L. Surface conversion after cure is quantified by attenuated total reflectance infrared spectroscopy at the acrylate double-bond absorption near 810 cm⁻¹, while tack-free state is assessed according to ISO 9117-1.
Under steady-state radical concentration, polymerization rate scales approximately with the square root of incident irradiance. Doubling lamp output raises cure speed by a factor of about 1.4, not 2.0. Radiometric dose at the substrate, measured with a calibrated UV radiometer in the 320–420 nm band, commonly falls between 150 mJ/cm² and 300 mJ/cm² for clearcoat cure. This square-root dependence explains why line-speed increases require photoinitiator reformulation rather than simple irradiance scaling.
On flexographic printing lines equipped with a chambered doctor blade and an anilox roll delivering 4.0 BCM, a UV-LED-curable cyan ink formulated with only RPI-1173 exhibits incomplete bottom cure when press speed exceeds 150 m/min under 395 nm LED arrays. Replacing part of the initiator package with RPI-TPO-L at 1.5 wt% restores through-cure at 200 m/min at a film weight of 1.2 g/m². The liquid grade disperses directly under high-shear mixing, whereas RPI-819 powder requires a separate pre-dispersion step on a three-roll mill at 40 °C to avoid visible yellow particles in low-viscosity ink. Ink viscosity is maintained at 180 mPa·s at 25 °C using a cone-and-plate viscometer per ISO 3219, and cure is checked by cross-hatch adhesion per ISO 2409 and methyl ethyl ketone double rubs according to ASTM D5402-19. This process difference is the main reason UV-LED ink lines often select RPI-TPO-L over RPI-819 despite the powder grade’s higher absorption at 405 nm.
Mercury arc sources emit strongly at 254 nm, 313 nm, 365 nm, and 405 nm; UV-LED arrays concentrate peak irradiance at 365 nm, 385 nm, 395 nm, or 405 nm depending on diode binning. Grade selection is therefore based on molar absorption at the source wavelength and on physical form. Table 1 lists supplier technical data sheet values.
| Designation | CAS registry number | Molecular weight | Physical form | Absorption maxima | Typical dosage | Preferred source |
|---|---|---|---|---|---|---|
| RPI-1173 | 7473-98-5 | 164.2 g/mol | Clear liquid | 245 nm, 280 nm, 331 nm | 1.0–4.0 wt% | Mercury arc |
| RPI-819 | 162881-26-7 | 418.46 g/mol | Yellow powder | 370 nm, 405 nm | 0.3–2.0 wt% | UV-LED 365–405 nm |
| RPI-TPO-L | 84434-11-7 | 316.32 g/mol | Pale-yellow liquid | 380 nm | 0.5–3.0 wt% | UV-LED 385–405 nm |
Typical batch variation for RPI-819 melting range is ±3 °C, and for liquid-grade viscosity is ±5% of the technical data sheet nominal. Storage below 25 °C in amber glass or opaque HDPE containers is required to prevent premature photolysis and thermal decomposition.
For optical-grade UV adhesives used in display lamination, haze is measured according to ASTM D1003. RPI-TPO-L at 0.5 wt% in an aliphatic urethane acrylate avoids the scattering losses associated with poorly dispersed RPI-819 particles. Cured films below 100 µm must maintain luminous transmission above 95%; filtration through a 5 µm absolute filter is required before dispensing to remove particulate defects. This formulation approach is selected when the adhesive bond line is inspected by automated vision systems that reject visible bright spots larger than 10 µm.
Low-migration clearcoats for food-contact packaging differ from general industrial coatings because unreacted photoinitiator and photolysis fragments must be controlled at migration-test limits. RPI-TPO-L is selected in liquid formulations because it is directly miscible in low-viscosity acrylate oligomers and does not require the high-shear dispersion step that RPI-819 powder demands. The molecular weight of RPI-TPO-L is 316.32 g/mol, lower than RPI-819 at 418.46 g/mol; however, molecular weight alone does not determine migration. Crosslink density, free volume, coating thickness, and final conversion have greater influence on the diffusion coefficient of a residual initiator in the cured matrix.
Low-migration clearcoats frequently use high-functionality acrylate monomers or urethane acrylate oligomers with acrylate functionality between 6 and 9 to increase crosslink density and reduce free volume. The photoinitiator loading is then minimized to 0.5–1.0 wt% and inert-gas lamination is used to suppress surface tack without amine synergists, because many amine synergists are themselves migration-relevant small molecules.
For food-contact materials, Regulation (EU) No 10/2011 requires that substances not positively listed in the Union List may be used only behind a functional barrier that prevents migration above 0.01 mg/kg food or food simulant. FDA 21 CFR 175.300 sets conditions for resinous and polymeric coatings on food-contact surfaces. Migration testing is performed with food simulants specified in EN 13130-1; for fatty and alcoholic contact, 95% ethanol at 40 °C for 10 days is common. Residual photoinitiator in the cured clearcoat is determined by LC-MS/MS with a reporting limit of 0.01 mg/kg. Published migration data for this specific photoinitiator in all multilayer packaging constructions is limited; converter validation on the final packaging geometry and coating weight is required before compliance can be claimed.
Pre-drying of RPI-819 powder is required when ambient relative humidity exceeds 60%; absorbed moisture increases agglomeration and reduces gravimetric dosing accuracy on single-screw volumetric feeders. The RPI series should not be stored in one-pot formulations with primary or secondary amine-functional oligomers, because Michael addition of amines to acrylate double bonds raises viscosity above 20,000 mPa·s within 24 h at 40 °C. Tin-based condensation catalysts used in hybrid urethane-acrylate systems should be screened separately because accelerated dark reactivity in the presence of residual acrylate groups can alter pot life; published data for every catalyst grade is limited.
Table 2 compares radical, cationic, and hybrid initiation mechanisms. The comparison is limited to operational differences that affect line speed, surface cure, and monomer selection.
| Process variable | Radical photoinitiator series | Cationic initiation | Hybrid radical/cationic |
|---|---|---|---|
| Dark cure after irradiation stops | None; radical recombination terminates growth | Continues; living cationic centers react after UV exposure | Partial; radical component stops, cationic component continues |
| Oxygen sensitivity | High in films below 5 µm; surface tack unless synergist or inert gas | Low; oxygen does not terminate cationic active centers | Moderate; radical portion remains oxygen-sensitive |
| Typical resin scope | Acrylates, methacrylates, unsaturated polyesters, thiol-ene | Epoxides, oxetanes, vinyl ethers | Mixed acrylate-epoxide formulations |
| Typical addition rate | 0.3–4.0 wt% depending on grade and pigmentation | 1.0–5.0 wt% for sulfonium or iodonium salts | Sum of both packages; formulation-dependent |
| Compliance screening | Heavy metals under RoHS; registration status under REACH via supplier safety data sheet | Same screening | Same screening; potential acid byproducts require pH control in finished article |
For cured-film mechanical properties after formulation adjustment, tensile testing per ASTM D638-14 with type IV specimens and cross-cut adhesion per ISO 2409 are used. Impact resistance of UV-cured acrylate films is assessed by ASTM D2794 reverse impact; published data for the RPI series in every formulation matrix is limited.
The operational difference between radical and cationic systems is most evident on humid production lines. Cationic cure slows when moisture terminates oxonium intermediates; radical cure is less moisture-sensitive but requires strategies for oxygen inhibition. Hybrid systems offer reduced oxygen sensitivity while retaining acrylate monomer scope, but storage stability and acid byproduct formation require acid scavengers and tight water control below 0.05%.
Three-dimensional vat photopolymerization using 405 nm laser-galvo systems requires sufficient absorption to cure the liquid resin without excessive attenuation that degrades depth resolution below 50 µm. RPI-819 at 0.2 wt% in a methacrylated oligomer is specified for such systems because its photobleaching window allows dose-dependent layer formation. Working curves are generated according to ISO 17296-3 or equivalent additive manufacturing characterization protocols; cure depth is determined by film thickness after solvent wash and optical profiling. RPI-TPO-L at 0.5–1.0 wt% may reduce yellowing in visible-light-sensitive clear builds, but published data for this specific resin configuration is limited and requires calibration on the target vat polymerization machine.
