TOK PAG-1000 is a cationic photoinitiator sold as a crystalline triarylsulfonium hexafluorophosphate. The active compound is identified on the commercial technical data sheet by CAS 68156-13-8, with a molecular weight of 516.50 g/mol. Typical technical literature describes the material as a white to pale yellow crystalline powder with an HPLC assay of ≥99.0% and a melting range of 101–104 °C. The product is soluble in propylene carbonate, acetone, and chloroform, and is practically insoluble in water. A 50% solution in propylene carbonate is a common intermediate for low-dust handling in UV-curable epoxide and vinyl ether formulations. The UV absorption envelope extends from approximately 220 nm to 370 nm, allowing direct excitation at the 365 nm mercury emission line. Upon irradiation, the sulfonium salt generates a Brønsted acid that initiates ring-opening polymerization of cycloaliphatic epoxides, oxetanes, and vinyl ethers. Unlike Norrish Type I free-radical photoinitiators used for acrylate systems, TOK PAG-1000 is not consumed by oxygen inhibition; the cationic propagation mechanism is instead terminated by water, alcohols, amines, and basic fillers.

Formulation work with TOK PAG-1000 on production-scale lines has centered on cationic clearcoats, adhesives, and electronic encapsulants where the concentration window is narrow. At 0.5–2.0 wt% active photoinitiator in a cycloaliphatic epoxide resin, surface cure dose is governed by lamp irradiance and film thickness. On a medium-pressure mercury line operated at 120 W/cm, a 25 µm clear epoxide film can reach tack-free cure between 1500 mJ/cm² and 3000 mJ/cm²; the same film under a 395 nm UV-LED array may remain tacky unless a sensitizer such as 0.25–0.50 phr of 2-isopropylthioxanthone or an anthracene derivative is added. Published data for this specific film configuration is limited, but the absorption cutoff of triarylsulfonium hexafluorophosphate salts makes 365 nm excitation more reliable than 385 nm or 395 nm excitation without a sensitizer.

What Limits Cationic Cure Depth When the Photoinitiator Is Used Below 2 wt%?

At loadings below 2 wt%, through-cure becomes dose-limited because fewer photogenerated acid molecules are available to overcome termination by residual moisture and nucleophilic impurities. The Beer-Lambert relationship controls the light-intensity gradient within the film. In a clear 10 µm film with a photoinitiator loading of 0.5 wt%, absorbance at 365 nm may be low enough to allow nearly uniform acid generation through the film, but the total acid yield is low. At 2 wt%, surface absorbance can exceed 1.0 AU, increasing surface crosslinking while reducing photon flux at the substrate interface. Pigmented formulations containing TiO₂ or carbon black show a more severe limitation because scattering and absorption compete with the photoinitiator. In such systems, a longer-wavelength sensitizer and higher irradiance are normally required. Process engineers evaluate through-cure by attenuated total reflectance FTIR, following attenuation of the epoxide ring vibration at 910 cm⁻¹, and by solvent rub testing according to ASTM D5402-19. An acceptable residual epoxide level is usually specified below 5% of the initial peak area; above this threshold, hardness and chemical resistance decline disproportionately.

Viscosity Control and Solubility Boundaries in Epoxide Formulations

TOK PAG-1000 is supplied as a solid, and ambient-temperature solubility in cycloaliphatic epoxide resins is limited. The preferred dissolution method is to heat the epoxide carrier to 50–60 °C under high-shear mixing in a jacketed vessel, add the photoinitiator in portions, and then cool rapidly to 25 °C after complete dissolution. Prolonged exposure above 70 °C should be avoided because sulfonium salts can undergo thermally induced decomposition and release acid. For low-viscosity inks and coatings, a 50% propylene carbonate solution is often used; this stock solution is added at the letdown stage. Crystallization can occur if the dissolved solid concentration exceeds 5 wt% in a nonpolar epoxide diluent or if the formulation is stored at 4–8 °C. Viscosity drift is monitored with ISO 3219:2021 cone-plate viscometry at 25 °C and 100 s⁻¹. A drift of less than 10% over 72 h in a closed container is generally considered acceptable for cationic adhesives; larger drift indicates premature oligomerization or moisture ingress. When ambient relative humidity exceeds 60%, dry nitrogen blanketing of the mixing vessel is used to reduce water pickup.

Production-scale dispensing of cationic epoxide adhesives containing TOK PAG-1000 requires control of dark cure. In a static mix dispense system, the mixed resin is held at 20–25 °C to limit viscosity increase. Raising the mixing head temperature to 40 °C can shorten the working life from several days to less than 8 h for highly reactive cycloaliphatic epoxy systems. This behavior is consistent with acid-catalyzed homopolymerization after incidental photolysis or thermal acid generation. Equipment should be shielded from sunlight and fluorescent lamps because low-level UV exposure can generate acid during storage and produce viscosity rise or localized gel particles. Needle blocks and dead volumes should be flushed at shift end to prevent cured deposits from accumulating.

When TOK PAG-1000 Replaces Hexafluoroantimonate or Iodonium Salts in UV-Cured Inks

Compared with a triarylsulfonium hexafluoroantimonate salt, TOK PAG-1000 has a less acidic photogenerated anion, which improves hydrolytic stability but reduces cure speed in cycloaliphatic epoxide inks. In a thin-film ink applied at 10 g/m² and cured under a 120 W/cm mercury lamp, a PF₆-based system may require 20–30% additional UV dose to reach equivalent epoxide conversion relative to an SbF₆-based system. The actual offset is formulation-specific; monomer polarity, moisture content, and pigment loading shift the ratio. Where fast cure is critical, formulators may combine TOK PAG-1000 with an anthracene-type photosensitizer rather than selecting the antimonate salt. Compared with diaryliodonium hexafluorophosphate, TOK PAG-1000 provides absorption at longer wavelengths and higher thermal stability, but diaryliodonium salts may produce lower color in very thin clear films because their absorption cutoff is shorter.

TOK PAG-1000 is typically handled as a skin and eye irritant under the supplier safety data sheet. Nitrile gloves, safety goggles, and local exhaust ventilation are required during weighing and dissolution. The powder should be stored in a tightly closed container at 15–25 °C and protected from light and moisture. Under REACH, the product may be subject to registration obligations at annual tonnages above 1 t. Under RoHS Directive 2011/65/EU, cationic photoinitiators are not among the restricted homogeneous materials, but the presence of fluorine and phosphorus in the anion requires review in waste and recycling streams. No statement can be made concerning FDA 21 CFR food-contact status without supplier food-contact documentation and specific end-use migration testing.

The Product Is Supplied as a Crystalline Triarylsulfonium Hexafluorophosphate

Because TOK PAG-1000 is a solid, batch-to-batch variability in particle size can affect dissolution time. Production records show that agglomerates formed during prolonged storage at 30 °C can require extended high-shear dispersion; passing the powder through a 500 µm sieve before charging reduces undissolved residue. The material should not be milled or micronized without inert gas because airborne dust may form explosive mixtures; the supplier’s safety data sheet should be consulted for dust explosion data. In a typical batch addition, the photoinitiator is charged after the resin and before the final letdown, so that high-shear mixing for 15–30 min at 50–60 °C is sufficient for full dissolution. Filtering the final formulation through a 5 µm absolute filter cartridge removes residual particles and prevents downstream screen clogging in inkjet or precision dispensing heads.

In UV powder coating development using a co-rotating twin-screw extruder with L/D 40:1, early barrel zones are kept at 70–80 °C because the sulfonium salt can undergo thermal decomposition above 90 °C; melt residence times below 60 s are preferred. Production batches have shown yellowing and gel particles when the die temperature exceeds 90 °C. Published data for this specific configuration is limited, but the behavior is consistent with the thermal sensitivity of triarylsulfonium hexafluorophosphate salts.

Through-cure of a cationic clearcoat is measured by scraping or cross-sectioning the cured film after UV exposure and examining the substrate-side layer. Under-cured cationic films exhibit soft, tacky substrate-side material and poor solvent resistance. FTIR microscopy maps epoxide conversion across the film depth; the epoxy ring absorbance at 910 cm⁻¹ should decrease by more than 90% for a fully cured clearcoat. Adhesion to aluminum and glass is commonly evaluated with ASTM D4541-17 pull-off adhesion or ISO 2409:2020 cross-cut adhesion. Cured films that show cohesive failure within the coating and no delamination are considered formulation-appropriate for the substrate. These tests provide objective release criteria for process commissioning when TOK PAG-1000 is used in UV-cured industrial coatings.