| HS Code | 894877 |
| Chemical Name | 2-Hydroxy-2-methyl-1-phenylpropan-1-one |
| Cas Number | 7473-98-5 |
| Molecular Formula | C10H12O2 |
| Appearance | Clear, colorless to slightly yellow liquid |
| Assay | ≥ 99% |
| Density | 1.077 g/cm³ at 20°C |
| Viscosity | 15-20 mPa·s at 20°C |
| Melting Point | 4°C |
| Boiling Point | 80-82°C at 0.1 mmHg |
| Flash Point | 114°C (closed cup) |
| Absorption Maxima | Approximately 245 nm and 280 nm |
| Solubility | Soluble in common organic solvents and acrylate monomers |
As an accredited Photoinitiator IGM Omnirad 1173 factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | Supplied in 25 kg opaque HDPE drums, sealed with tamper-evident closures; stored tightly closed to maintain stability. |
| Container Loading (20′ FCL) | 20′ FCL: palletized drums of IGM Omnirad 1173 stowed securely, ventilated, dry, away from heat sources, ensuring safe transport. |
| Shipping | Ship in tightly sealed, UN-approved containers, upright and ventilated. Label per GHS (skin/eye irritation). Avoid sunlight, heat, and incompatible materials. Use secondary containment to prevent spills. Comply with IMDG, ADR, or IATA regulations as applicable. Attach Safety Data Sheet and mark as photoinitiator. Keep away from foodstuffs and ignition sources. |
| Storage | Store Photoinitiator IGM Omnirad 1173 in a tightly sealed container in a cool, dry, well-ventilated area. Keep away from heat, sparks, open flames, direct sunlight, and incompatible materials such as strong oxidizers. Avoid moisture contamination and maintain temperatures below 40°C. Ensure proper labeling and segregate from food items to preserve stability and safety. |
| Shelf Life | Typically, shelf life is two years from manufacture if stored properly in original sealed container away from heat and light. |
Flat-line UV wood coating lines configured with a 1200 mm wide roller coater applying 80–120 g/m² wet film to sanded beech substrates operate at 10–25 m/min. The formulation is a 100% solids aromatic urethane acrylate blend containing 30–40 wt% tripropylene glycol diacrylate and 10–20 wt% 1,6-hexanediol diacrylate. 2-Hydroxy-2-methyl-1-phenyl-1-propanone (CAS 7473-98-5), supplied as Omnirad 1173, is introduced at 2.0–3.5 wt% relative to total formulation. Viscosity of the compounded liquid at 25 °C is typically 500–800 mPa·s measured under ISO 3219. Cure is effected by two 120 W/cm medium-pressure mercury lamps tuned to a UVA dose of 300–500 mJ/cm². Through-cure of a 35 µm dry film is evaluated by pendulum damping using ISO 1522:2022; undercured films exhibit hardness losses greater than 25% after methyl ethyl ketone wiping. Adhesion to sanded beech, sealed with an acrylic primer, is assessed by cross-cut testing according to ISO 2409:2013; cohesive failure within the coating is distinguished from substrate failure by microscopy. Yellowing under accelerated weathering is quantified as Δb* using ASTM D2244-23 after 168 h QUV-B exposure per ISO 16474-3. Omnirad 1173 has lower yellowing potential than benzophenone/tertiary amine systems in this clear-coat application, although the numerical Δb* comparison is resin-dependent and published data for the exact oligomer configuration are limited. Surface oxygen inhibition is the principal process conflict; above 25 m/min or when lamp output decays below 60% of nominal irradiance, residual tack persists and ASTM D5402 acetone double rub values fall below 50 cycles. In such cases, increasing Omnirad 1173 concentration beyond 4.0 wt% does not compensate linearly because internal filler absorption and radical recombination become pronounced; the practical correction is higher irradiance, inert-gas blanketing, or a second lamp station. Sanding between coats with 320–400 grit media removes the oxygen-inhibited surface and improves intercoat adhesion.
Pigmented UV inkjet inks for single-pass label presses require a viscosity of 18–25 mPa·s at 25 °C and surface tension of 24–32 mN/m to function in piezoelectric printheads with 7–12 pL drop volume. Omnirad 1173 loading in cyan, magenta, and yellow formulations generally falls between 3.0 wt% and 5.0 wt%; in high-opacity white inks the working range reaches 5.0–7.0 wt% because titanium dioxide scatters most of the incident UV radiation. Dispersion processing is carried out in a bead mill charged with 0.3 mm yttrium-stabilized zirconia media; the photoinitiator is added post-dispersion to avoid localized overheating in the mill chamber and to maintain consistent initiator distribution. Viscosity measurements follow ISO 3219 using a cone-and-plate rheometer at a shear rate of 50 s⁻¹, and printhead operability is confirmed by a 15-minute idle jetting test under 40 °C printhead temperature. The main constraint in pigmented systems is competitive light absorption: carbon black and yellow pigments attenuate the 330–370 nm region where Omnirad 1173 has its highest extinction. Consequently, black and dense yellow inks frequently combine Omnirad 1173 with an acylphosphine oxide photoinitiator at 0.5–1.0 wt% to extend response into the near-visible range. On 395 nm LED exposure, Omnirad 1173 alone shows limited initiating activity because the absorption maximum is situated near 331 nm in polar solvents; a gallium-doped mercury lamp with significant emission in the 350–380 nm band is the more robust source. Above 6.0 wt% total photoinitiator in a cyan ink, production-scale inkjet lines have recorded nozzle plate fouling after idle periods exceeding 15 minutes, requiring purge cycles that waste 0.5–1.0 L of ink per shift. For food-packaging printing, Omnirad 1173 is not listed in EU Regulation 10/2011 Annex I; if used in a low-migration ink located behind a functional barrier, migration must be assessed under EN 1186-3 and analytical detection at low-µg/kg limits. Cure conversion of the acrylate double bond is monitored at 810 cm⁻¹ by FTIR-ATR; conversion values of 65–80% are typical under 500 mJ/cm² for pigmented films, but exact plateaus depend on pigment loading and oligomer unsaturation. Published data for this specific inkjet formulation configuration are limited; therefore process windows are verified by print trials on the target press rather than by interpolation from clear-film data.
Laminating lines converting 12 µm PET film to 50 µm LDPE at 150–250 m/min using 100% solids UV adhesives rely on extremely thin adhesive layers of 2–5 µm. Omnirad 1173 is incorporated into aliphatic urethane acrylate laminating adhesives at 1.0–2.5 wt% of the total adhesive. Wetting and transfer on a three-roll offset gravure applicator require viscosity of 300–800 mPa·s at 40 °C, measured under ISO 3219. The adhesive is cured through the PET web with 200–400 mJ/cm² UVA from a 160 W/cm mercury lamp; through-cure is sensitive to the UV transmission of the PET film, which drops significantly below 320 nm. Because Omnirad 1173 requires higher-energy UV, unmodified PET that contains UV blocking additives can suppress through-film cure, leaving a weak interlayer boundary. T-peel adhesion is tested according to ASTM D1876-08; undercured laminates fail cohesively within the adhesive rather than at the film-adhesive interface, an indication that monomer conversion was insufficient. Migration compliance is the dominant technical constraint in this application. Omnirad 1173 is not among the authorised additives in EU Regulation 10/2011 Annex I; its use on the non-food-contact side of a functional barrier requires migration screening using food simulants according to EN 1186-3 and analytical methods capable of detection in the low-µg/kg range. For adhesives marketed into the United States, 21 CFR 175.105 provides a framework for indirect food additives, but resin and photoinitiator residues remain subject to good manufacturing practice and must be below the threshold of analytical detection unless conventional testing demonstrates otherwise. At high line speeds, open time on the transfer roll is short; operators record that viscosity drift above 800 mPa·s causes transfer defects at speeds above 200 m/min. The photoinitiator is dissolved into the monomer phase before blending to avoid refractive-index heterogeneity that can create visible striations in clear laminates.
Systematic process-limit comparison across UV-curing platforms
| Application platform | Omnirad 1173 loading range | Principal process constraint | Reference method |
|---|---|---|---|
| Clear wood coatings | 2.0–3.5 wt% | Oxygen inhibition above 25 m/min | ISO 1522:2022 |
| Pigmented UV inkjet inks | 3.0–5.0 wt% | 395 nm LED absorption tail | ISO 3219 |
| Flexible packaging laminating adhesives | 1.0–2.5 wt% | Migration through functional barrier | EN 1186-3 |
| Vat photopolymerization for investment casting | 0.5–1.5 wt% | Cure depth at 405 nm | Jacobs working curve |
| Sheetfed offset overprint varnish | 2.0–4.0 wt% | Gloss loss from surface undercure | ISO 2813:2014 |
| Conformal coating for printed circuit assemblies | 1.5–3.0 wt% | Shadow-area incomplete cure | IPC-TM-650 2.6.3.7 |
In vat photopolymerization for investment casting and rapid tooling, a 405 nm DLP light engine typically delivers 10–25 mW/cm² at the build surface. Omnirad 1173 has limited absorption at 405 nm because its primary absorption region lies below 370 nm; therefore it is normally employed at 0.5–1.5 wt% and is supplemented by a longer-wavelength acylphosphine oxide photoinitiator when the projection source is exclusively 405 nm. The working curve of a clear urethane acrylate resin follows the relationship Cd = Dp ln(E/Ec), where Cd is the cured depth, Dp is the depth of penetration, E is the incident energy dose, and Ec is the critical energy dose. For systems based on 355 nm lasers, Dp values in the range 150–300 µm and Ec values near 10–20 mJ/cm² have been reported for similar resins; at 405 nm the weak absorption of Omnirad 1173 produces larger Dp values and poorer z-axis resolution. This shifts the operational limit onto the imaging step: a 100 µm layer thickness may be over-cured into previous layers if the photoinitiator concentration is too low, while excessive concentration reduces Dp and can leave uncured liquid in deep recesses. In investment casting pattern formulations, residual photoinitiator fragments can volatilize during autoclave dewaxing and burnout; burnout cycles typically reach 650–750 °C at 5–8 °C/min to remove crosslinked polymer without shell cracking. The use of Omnirad 1173 at 1.0 wt% in a filled investment casting resin has been associated with acceptable shell integrity only when a two-stage burnout holds at 300 °C for 1 h; published data for this specific ceramic shell configuration are limited. The resin viscosity under 35 °C vat temperature is maintained below 400 mPa·s for adequate recoating between layers; viscosity is measured using ISO 3219. Process validation is not governed by a single ISO standard; layer thickness and lateral resolution are verified by measuring green parts with optical profilometry before thermal post-cure.
Sheetfed offset presses running at 10,000–18,000 sheets/h apply UV overprint varnish at 2–4 g/m² wet film over freshly dried ink. Omnirad 1173 is dosified at 2.0–4.0 wt% in a low-volatile clear varnish based on epoxy acrylate or polyester acrylate. Curing is provided by one or two 120 W/cm mercury lamps positioned after the delivery; the dwell time under irradiance is typically 0.02–0.10 s, so the initiator must generate radicals with an extremely short induction time. Specular gloss at 60° is measured under ISO 2813:2014; low initiator levels or lamp degradation reduce surface conversion and produce a visible matte band across the sheet. Blocking resistance is evaluated by stacking printed sheets under 5 kPa pressure at 40 °C for 24 h; residual surface tack raises dynamic coefficient of friction measured by ASTM D1894. Overdosing above 5.0 wt% does not improve gloss but increases viscosity and may interfere with wetting over ink; some production lines record gloss loss from surface wrinkling if viscosity exceeds 400 mPa·s at 25 °C. Interaction with offset inks is a formulation-specific risk: benzophenone/tertiary amine systems can generate amine radicals that contribute to yellowing, whereas Omnirad 1173 avoids this route in clear varnishes. Because sheetfed OPV applies a very thin film, the photoinitiator concentration must be verified by weight before each campaign; a 0.5 wt% error at low film weight causes larger relative conversion differences than the same error in wood coating. Published data for this specific sheetfed configuration are limited, so inline gloss measurement is used to maintain process capability rather than relying on offline hardness alone.
Selective spray application of 100% solids acrylated polyurethane conformal coating onto printed circuit assemblies at 50–75 µm dry film thickness uses Omnirad 1173 at 1.5–3.0 wt%. Viscosity at 25 °C is typically 100–250 mPa·s for airless or air-assisted dispensing, measured under ISO 3219. The coating is cured with a 365 nm UV source delivering 500–1000 mJ/cm² to exposed surfaces. Because conformal coating is a line-of-sight process, areas under leadless packages, connector overhangs, and tall capacitors receive no direct UV energy and remain liquid unless a secondary moisture-cure isocyanate or thermal initiator is included. Qualification testing under IPC-CC-830B includes thermal shock, hydrolytic aging at 65 °C/95% RH, and surface insulation resistance measured according to IPC-TM-650 method 2.6.3.7. The low-viscosity nature of Omnirad 1173 supports uniform film formation over low-profile components; however, it does not address shadow cure and must not be considered a substitute for a dual-cure resin network in partial-exposure geometries. In selective spray equipment with a 0.5 mm needle nozzle, clogging and viscosity drift above 250 mPa·s at 25 °C reduce transfer efficiency. UV-A illumination can reveal surface wetness differences in uncured shadow zones, but visual inspection alone is insufficient for ionic contamination risk. Adhesion to FR-4 substrates is verified by cross-cut according to ISO 2409:2013 after 24 h ambient cure; the specific electrical performance after multiple thermal cycles is formulation-dependent and published data for this exact conformal coating layout are limited.
Optical fiber draw towers running at 1,500–3,000 m/min coat 125 µm glass fiber with a 60–80 µm dual-layer UV-curable acrylate system. Omnirad 1173 is used in the inner primary coating at 1.0–2.0 wt% of the primary formulation; viscosity is held below 5,000 mPa·s at 25 °C. Cure is accomplished by a 365 nm source in an inert atmosphere below 50 ppm oxygen to prevent surface inhibition. The fast surface-cure requirement at high draw speed makes Omnirad 1173 suitable in combination with acylphosphine oxide; high-speed drawing exceeds 2,500 m/min, and line speeds of this order require cure dose delivery of 100–300 mJ/cm². Mechanical properties are tested via strip tensile per IEC 60793-1-30; fiber attenuation is measured per IEC 60793-1-40. The primary coating must maintain low microbending loss across -40 °C to 85 °C; photoinitiator residues can affect oxidative stability under aging. Published data for this specific fiber-coating configuration are limited, so process windows are validated on the draw tower with real-time concentricity and attenuation monitoring.
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IGM Omnirad 1173, CAS 7473-98-5, EINECS 231-272-0, chemical name 2-hydroxy-2-methyl-1-phenylpropan-1-one, is a Type I photoinitiator supplied as a clear liquid. The product code Omnirad 1173 is the IGM Resins designation for a photoinitiator formerly known under the Irgacure 1173 trade name. The technical specification includes assay ≥99.0%, colour ≤50 APHA, molecular weight 164.20 g/mol, density 1.07–1.09 g/cm³ at 20 °C, viscosity 25–30 mPa·s at 20 °C, and refractive index 1.527–1.535 at 20 °C. The product is miscible with common acrylate monomers and reactive diluents, including trimethylolpropane triacrylate, tripropylene glycol diacrylate, and bisphenol A epoxy acrylate. As a photofragmenting initiator, it does not require a tertiary amine co-synergist; this distinguishes it from Type II systems such as benzophenone. Compared with 1-hydroxycyclohexyl phenyl ketone, the liquid physical state of Omnirad 1173 removes pre-dissolution operations but carries a higher migration potential because of the lower molecular weight.
Typical application fields are clear overprint varnishes, UV flexographic inks, screen inks, wood coatings, and clear or lightly pigmented coatings on plastics and metal. The usual dosage is 1–5 wt% of total wet formulation; pigmented white systems are often charged at 4–6 wt% because rutile TiO₂ attenuates the activating UV. Omnirad 1173 is not used to initiate cationic epoxy polymerisation; it generates radical species rather than photogenerated Brønsted acid, so it is not interchangeable with triarylsulfonium or diaryliodonium salts.
The strongest absorption band appears at approximately 245 nm in methanol, and the compound shows useful activity at the 313 nm mercury emission line. A medium-pressure mercury lamp provides the necessary short-wave UV output, whereas UV-LED systems emitting at 365 nm, 385 nm, or 405 nm fall outside the efficient absorption range. In LED-only cure, Omnirad 1173 is generally not suitable as the sole photoinitiator. Published data for such configurations is limited, and the low molar absorptivity beyond 365 nm is the physical reason for slow surface cure. Acylphosphine oxide photoinitiators such as TPO-L are preferred for LED curing; hybrid systems combine Omnirad 1173 for short-wave surface cure with TPO-L for long-wave through-cure when the line uses both mercury and LED sources or a broadband additive lamp.
Photolysis follows Norrish Type I α-cleavage: the excited triplet state fragments into a benzoyl radical and a 2-hydroxy-2-propyl radical. Both radicals initiate acrylate unsaturation, but atmospheric oxygen competes for excited states and radical centres. The oxygen inhibition effect is most evident at wet film thickness below 15 µm; residual tack can remain despite bulk double-bond conversion above 80%, as monitored by FTIR disappearance of the acrylic C=C bending vibration near 810 cm⁻¹. On roll-coating lines where film weight is fixed, the standard process response is nitrogen inerting at residual oxygen below 0.5% and a UVA dose above 400 mJ/cm². Raising Omnirad 1173 above 8 wt% under oxygen-rich conditions is ineffective because the initiator itself filters the short-wave photons near the surface and increases residual extractables.
Operational differentiation from common photoinitiators is summarised in the following matrix.
| Parameter | Omnirad 1173 | Omnirad 184 | Benzophenone | TPO-L |
| CAS number | 7473-98-5 | 947-19-3 | 119-61-9 | 84434-11-7 |
| Physical state at 20 °C | Clear liquid | White crystalline solid | White solid | Clear liquid |
| Molecular weight g/mol | 164.20 | 204.26 | 182.22 | 316.32 |
| Initiation mechanism | Type I α-cleavage | Type I α-cleavage | Type II hydrogen abstraction | Type I α-cleavage |
| Amine co-synergist | Not required | Not required | Required | Not required |
| Primary UV source match | Hg arc UV-C/UV-B | Hg arc UV-B/UV-A | Hg arc UV-B | Hg arc UV-A/LED 365–405 nm |
| Characteristic limitation | Weak LED response | Solid dissolution required | Amine yellowing and odour | Risk of yellowing if overdosed in white coatings |
In UV flexographic varnishes, Omnirad 1173 is typically charged at 1–5 wt% of total wet formulation. In white-pigmented inks containing 20–30 wt% rutile titanium dioxide, the initiator level is often raised to 4–6 wt% because pigment absorption and scattering reduce photon availability. The compound is miscible with trimethylolpropane triacrylate, tripropylene glycol diacrylate, and bisphenol A epoxy acrylate; press-side adjustment is performed with low-shear mixing rather than a separate millbase. Viscosity is measured by cone-and-plate at 25 °C under ISO 2884-1:2017, with target press viscosity below 300 mPa·s. On narrow-web flexo lines with 120 W/cm medium-pressure mercury lamps and line speeds of 60–120 m/min, surface cure becomes the limiting defect at the fast end. Tack-free surfaces are restored by increasing lamp power or reducing coating weight; further increase in Omnirad 1173 does not reliably improve surface cure and may raise extractables. Compared with benzophenone/tertiary amine combinations, the amine-free varnish has lower post-cure odour and reduced amine blush, but the absence of an amine hydrogen donor demands tighter lamp maintenance, radiometer verification, and inerting discipline.
With waterborne UV clearcoats, the photoinitiator is added after neutralisation and after high-shear dispersing because it has limited water solubility and partitions into the acrylic phase. Insufficient agitation can produce soft or low-gloss domains after cure. A more critical boundary is volatile loss during forced-air drying before UV exposure. Drying tunnels operating above 60 °C with air velocities above 2 m/s can deplete the surface concentration of Omnirad 1173, producing a cured skin over a tacky underlayer. Where these drying conditions are fixed, the formulation is shifted to a lower-volatility initiator such as Omnirad 184 or to a polymeric photoinitiator. This constraint is more severe for Omnirad 1173 than for higher-molecular-weight photoinitiators and is a recurring source of batch-to-batch variation in waterborne coating lines.
Omnirad 1173 is used in overprint varnishes where yellowing is to be minimised relative to benzophenone/amine systems, but it is not intrinsically non-yellowing in all matrices. Colour change is measured by CIELAB ΔE under ISO 11664-4, and accelerated weathering under ISO 4892-2 provides comparative yellowing data. In low-crosslink-density acrylate films, residual unreacted initiator can diffuse because the molecular weight is only 164.20 g/mol. Food-contact packaging requires article-specific migration testing under Regulation (EU) No 10/2011; raw-material purity and assay ≥99.0% do not establish compliance. Polymeric photoinitiators reduce migratable species but may require higher dose or slow cure. The trade-off is evaluated with pendulum hardness under ISO 1522:2022 and crosscut adhesion under ASTM D3359-17. If pendulum hardness reaches the specification at minimum dose, migration risk is reduced but not eliminated; additional extraction analysis is necessary for regulated end uses.
The operational penalty with benzophenone is not only the amine co-synergist but also the formation of cyclohexadienyl-type intermediates that can lead to coloured by-products in overprint varnishes after ageing. Omnirad 1173 has no abstractable hydrogen donor requirement; its initiation is unimolecular and begins as soon as the short-wave photons are supplied. This makes cure more predictable in thin clear coats. However, the unimolecular route is sensitive to dissolved oxygen; whereas tertiary amines can act as sacrificial oxygen scavengers, Omnirad 1173 consumes oxygen only indirectly through carbon-centred radicals. Therefore, formulations containing Omnirad 1173 often use a small amount of a benzophenone derivative or an amine acrylate only when oxygen inhibition is severe; this reintroduces some yellowing and migration risk, so the addition is kept below 1.5 wt% and validated by ΔE under ISO 11664-4.
At the equipment level, direct dosing of a liquid photoinitiator eliminates the need for solvent pre-dissolution and reduces the number of cleaning cycles on metering pumps and feed lines. In contrast, crystalline photoinitiators can form blockages in narrow feed lines when ambient temperature fluctuates below the solvent dew point. Omnirad 1173 remains liquid down to at least 5 °C, although viscosity increases; pumping at 10 °C may require jacketed lines or reduced flow rates. These observations are based on standard coil-coating and flexo press operation; specific pump accuracy versus viscosity should be confirmed with the equipment manufacturer's pressure-drop data.
In UV screen inks formulated with acrylated epoxy oligomers and monomer blends, the product is introduced during letdown after the pigment dispersion step. High solvent content is not required; the liquid photoinitiator can be added directly to the letdown tank under low-shear agitation at 300–500 min⁻¹. The absence of crystals minimises foam generation, but vacuum deaeration at 50–100 mbar for 10–20 minutes is still required before screen application to remove dissolved air that would otherwise add to oxygen inhibition. The selection of Omnirad 1173 in screen inks is therefore primarily an operational convenience, not a cure-speed advantage compared with solid Type I photoinitiators.
Warehouse handling should follow the current safety data sheet under Regulation (EC) No 1272/2008. Storage in original sealed containers at temperatures between 5 °C and 35 °C, protected from direct sunlight, preserves the specified assay and colour. Shelf life is generally 24 months from date of manufacture when stored under these conditions. Oxidising agents and strong acids should be kept out of the storage area; contamination with metal salts can accelerate discoloration in unconverted aromatic ketone stocks. Before bulk transfer, moisture-exclusion measures are applied because the product may be used in moisture-sensitive resin systems even though the product itself is a liquid at ambient temperature.
When clear acrylic topcoats are applied at 25–50 µm wet film thickness, the cure depth obtained with Omnirad 1173 depends on the overlap between its absorption spectrum and the lamp's spectral irradiance. At equal mass loading, 1-hydroxycyclohexyl phenyl ketone often produces lower surface tack but requires a dissolution step; Omnirad 1173 matches that surface cure in short-wave UV and gives the coater easier handling. The difference appears at the deepest layer: a topcoat containing only Omnirad 1173 may show a slightly higher underlayer conversion than benzophenone/amine because Type I radicals are not dependent on hydrogen abstraction from the amine, but the cure depth is still limited by pigment-free resin absorbance. This is especially true when longwave UV is removed by aged lamps; aging reduces the 365 nm output before the 254 nm output fails, so through-cure is lost before surface tack becomes apparent. Routine lamp maintenance and monitoring of the 365 nm band with a radiometer is required.
In confined matrices such as UV screen inks at high pigment volume concentration, the initial radical pair may recombine within the solvent cage before monomers can react; increased viscosity above 1,500 mPa·s reduces radical escape efficiency. Formulators therefore lower viscosity with reactive diluents rather than relying solely on elevated temperature. At higher processing temperatures above 40 °C, thermal motion increases radical escape but may also accelerate volatile loss of the photoinitiator in open pans. Screen-printing trials use water-jacketed screens to control ink temperature below 35 °C; this compresses the working window and increases the importance of dose control.
When titanium dioxide or carbon black is present at 10–30 wt%, the activating UV is attenuated within the first 5–10 µm of wet film. With Omnirad 1173 as the sole initiator, the cure gradient can produce a hard surface layer over an uncured base; crosscut adhesion classification can fall from 5B to 2B under ASTM D3359-17. Increasing the photoinitiator level beyond 6 wt% does not correct the gradient and may raise residual odour and extractables. The effective response is to pair the product with a long-wave photoinitiator or to reduce pigment loading. This behaviour is observed on flat-bed screen lines; published data for this specific configuration is limited, but the internal-filter mechanism is well established. The product is therefore positioned for clear, lightly pigmented, or thin-film applications rather than heavily pigmented thick layers intended for one-pass cure.
