Sheetfed Lithographic Inks Demand UV-A Photoinitiator Loading Across Roller and Blanket Dynamics

UV lithographic inks formulated for sheetfed offset presses typically contain high pigment volume concentrations and require photoinitiator packages that maintain through-cure at press speeds above 12,000 sheets/h. In this environment Irgacure 369 is used at 3.0–6.0 wt% of total ink mass, commonly with 1.0–3.0 wt% of 2-isopropylthioxanthone to extend the absorption envelope at 365 nm and 385 nm. The α-amino ketone is dissolved in acrylate diluents before incorporation because its melting range of 110–115 °C can generate crystalline residues if dry powder is added directly during high-speed dispersion. Compliance for commercial offset inks is assessed under REACH Regulation (EC) No 1907/2006, while print process control follows ISO 12647-2:2013 and ink colorimetry follows ISO 2846-1:2017. For printed matter that may contact food through a functional barrier, formulation components are screened under the EuPIA Good Manufacturing Practice and the Swiss Ordinance SR 817.023.21; Irgacure 369 is not recommended for direct food-contact ink without migration testing.

Ink manufacturing for this segment proceeds through a high-speed disperser followed by a three-roll mill, with rolls maintained at 25–35 °C to keep the mill base below radical generation temperature. The mill gap is adjusted to produce a final grind below 5 µm, and vacuum deaeration is applied to prevent microfoam from reducing optical density. On a sheetfed press, the UV curing unit is positioned after the delivery or between printing units; lamp irradiance is typically 120–180 W/cm in the 300–400 nm band, and the reflector geometry is selected to maintain peak irradiance without overheating the substrate. The printed and cured film is subjected to tape adhesion and rub resistance tests according to ASTM D3359-17 and a modified Sutherland rub test, because matt and dark pantone inks using Irgacure 369 at the upper addition window can exhibit residual amine at the film surface if under-cured. Terminal article types include folding cartons for cosmetics and pharmaceuticals, book covers, direct mail covers, and printed metal decorating blanks for beverage and confectionery tins.

What Limits Through-Cure in Narrow-Web Flexo Inks at Low Film Weights?

In narrow-web flexo, the anilox roller deposits inks with cell volumes between 2.5 BCM and 4.0 BCM and line counts of 800–1,200 LPI, yielding wet films of 6–12 µm; this low film weight creates a high surface-to-volume ratio in which oxygen inhibition consumes radicals at the surface. Irgacure 369 is employed at 3.5–7.0 wt% of total formulation in TiO₂-containing whites and carbon-black blacks, while lighter monochromatic inks are formulated at 2.0–4.0 wt% to limit yellowing and residual amine odour. In heavily pigmented grades, a thioxanthone derivative such as 2-isopropylthioxanthone is added at 0.5–2.0 wt% to improve through-cure under the 365 nm emission line of doped mercury lamps. Print process control is based on ISO 12647-6:2020; raw material and finished ink compliance is managed under REACH 1907/2006, and for low-odour shrink sleeve and label applications the Swiss Ordinance SR 817.023.21 migratables framework is applied.

Production of UV flexo ink concentrates is carried out in bead mills or high-shear batch mixers, with pigment grinds below 5 µm and viscosity adjusted to 50–250 mPa·s for chambered doctor-blade application. The press configuration often includes UV lamp modules rated 8–16 kW and inerting is not usually employed; therefore the photoinitiator package must compensate for oxygen diffusion. Irgacure 369 is pre-dissolved in acrylate monomer under yellow lighting before the final letdown, and the millbase is maintained below 35 °C to prevent premature polymerization at hot spots. Cure adequacy is monitored by methyl ethyl ketone double rubs or by ATR-FTIR acrylate conversion at the print surface. Terminal converted articles include pressure-sensitive labels for beverage and durables, shrink sleeves for PET containers, flexible packaging pouches for non-food and food-contact applications requiring a functional barrier, and thermal transfer label overprint webs.

For membrane switch overlays and appliance panels, screen-ink formulations are applied as thixotropic films that are deposited through mesh openings and then subjected to UV exposure in conveyorized tunnels. Irgacure 369 is added at 2.5–5.0 wt% of total ink mass; the exact amount is set by mesh count, film thickness and pigment type. At the lower end, clear conductive dielectric overlays are used, while black and white graphic layers with heavy pigmentation require the upper end. The ink is manufactured on a triple-roll mill with temperature-controlled rolls, and the grind gauge reading is held below 15 µm to avoid mesh clogging. Compliance testing for cured prints on polyester and polycarbonate includes cross-hatch adhesion according to ASTM D3359-17 Method B and pencil hardness according to ASTM D3363-20; the finished membrane switch assemblies are evaluated under ISO 1518-1:2019 scratch resistance or equivalent OEM specifications.

The downstream process uses screens with 90–150 threads/cm and squeegee durometer 60–85 Shore A, with an off-contact distance of 1.0–2.5 mm for graphics and 0.5–1.0 mm for conductive silver traces when combined with UV dielectric layers. The wet film thickness after screen printing is commonly 15–35 µm, significantly higher than flexo or offset, which reduces surface-to-volume cure stress but increases sensitivity to bulk absorption; Irgacure 369’s UV-A activity is used to maintain cure through pigmented graphic blacks. Conveyorized UV tunnels are set to 80–120 W/cm with belt speed 5–15 m/min, and curing dose is referenced to a calibrated radiometer operating in the 320–390 nm band. Production handling requires exclusion of ambient white light because the photoinitiator has long-wavelength sensitivity; stock is stored at ≤30 °C and <40% RH in sealed containers. Terminal product types include membrane switch overlays, industrial nameplates, in-mold decoration inserts for appliance fascia, and UV-cured screen-printed polycarbonate panels for medical display housings.

When Opaque Wood Fillers and White Cabinetry Coatings Are Cured by Medium-Pressure Mercury Lamps

For white cabinetry coatings, the critical variable is the UV competition between TiO₂ pigment and photoinitiator at 365 nm; titanium dioxide absorbs strongly below 380 nm, while Irgacure 369 has sufficient UV-A absorption to initiate polymerization beneath the pigmented film surface. Addition ratios range from 2.0–4.5 wt% in transparent grain fillers and sanding sealers to 3.0–6.0 wt% in white pigmented topcoats and opaque primers. In high-build white formulations, a bisacylphosphine oxide photoinitiator is co-added at 0.5–2.0 wt% to maintain surface cure and reduce oxygen inhibition; the combination is dissolved in acrylate oligomers under low shear. Cured coating durability is tested under ASTM D3359-17 for adhesion, ASTM D523-14(2021) for specular gloss, and DIN 68861-1 for furniture surface resistance to chemical agents.

Application on flat panels is performed with roller coaters or curtain coaters at coating weights of 15–40 g/m², followed by UV curing units equipped with medium-pressure mercury lamps at 100–160 W/cm; the lamps are arranged in a dual-bank configuration to deliver a total UV dose of 300–800 mJ/cm² in the UV-A band. The high film weight and white pigmentation require that line speed be derated relative to clear coatings, otherwise the lower layers remain under-cured and show post-embrittlement or delayed oil exudation. In sanding sealer operations, the first UV pass is partially cured and sanded with 320–400 grit abrasive before topcoat; incomplete through-cure at this stage causes clogging of sanding belts and exaggerated grain raising. Irgacure 369 is pre-dissolved in monomer at 40–50 °C under light-safe conditions before addition to the coating batch. Terminal finished products include kitchen cabinet doors, office furniture panels, flat-pack furniture components, and UV-primed wood flooring substrates.

Pigmented Plastic Coatings for Automotive Interior Trim and Consumer Electronics Enclosures

After surface activation of PC/ABS substrates, robotic spray lines apply UV-curable coatings at dry film thicknesses of 10–20 µm, where the photoinitiator package must deliver both surface cure and film-through cure without generating residual odour or extractable amine. Irgacure 369 is used at 2.0–5.0 wt% of total coating formulation, with the lower end reserved for transparent clearcoats and the upper end for black or metallic pigmented layers that absorb strongly across the UV-A band. Adhesion is qualified according to ISO 2409:2020 cross-cut classification 0–1, and pencil hardness is measured under ISO 15184:2020; chemical resistance is assessed with ASTM D1308-20 spot tests using artificial sebum, sunscreen lotion, and automotive interior cleaners.

Surface activation of the plastic substrate is performed by atmospheric plasma or low-pressure UV and is controlled to a surface energy of 56–64 mN/m before coating application. The coating is applied in water-wash spray booths, flashed at 40–50 °C for 5–10 min, and cured under focused elliptical reflector lamps rated 80–120 W/cm with a peak irradiance of 200–600 mW/cm² in the UV-A band. The use of Irgacure 369 in pigmented systems is typically paired with acylphosphine oxide photoinitiators at 0.5–2.0 wt% to support surface cure, while the α-amino ketone maintains deep conversion in shaded areas near part edges. Production control includes periodic ATR-FTIR acrylate unsaturation checks and methyl ethyl ketone rub testing after ageing at 60 °C for 72 h. Terminal product types comprise automotive centre console trim, steering wheel bezels, consumer electronics housings, cosmetic compacts, and scratch-resistant covers for medical diagnostic devices.

In adhesive lamination, the competing requirement is that UV radiation must pass through a transparent substrate or penetrate pigmented adhesive bulk to generate sufficient radical flux at the opposite interface. Irgacure 369 is employed at 0.5–2.5 wt% of total adhesive formulation in clear laminating adhesives for PET, BOPP and PVC films, and at 1.5–3.0 wt% in pigmented or UV-absorbing assembly adhesives for rigid polymer components. The addition ratio is kept below the solubility limit in the chosen acrylic or urethane acrylate oligomer mixture, and the photoinitiator is dissolved at 40–50 °C before compounding to prevent particulate contamination in nip coaters. Compliance for structural and laminating adhesives is tested under ISO 4587:2019 for lap-shear strength of bonded metal or plastic specimens and ASTM D1876-19 for T-peel resistance of flexible laminates; for indirect food-contact packaging, the adhesive film is assessed under FDA 21 CFR 175.105 conditions or EU Regulation 10/2011 with a demonstrated functional barrier.

Processing is carried out on a laminating line with a gravure or slot-die application station, a nip pressure of 2–4 bar, and UV curing through the transparent web using an LED lamp at 365 nm or a doped mercury lamp with a UV-A reflector. When the adhesive contains pigments or fillers, a 385 nm LED source is preferred because longer-wavelength radiation penetrates deeper into the bondline and reduces the surface-to-depth cure gradient. The irradiance at the adhesive plane is monitored with a calibrated radiometer, and the line speed is adjusted to maintain a UV-A dose of 400–1,200 mJ/cm². For UV-LED curing of clear adhesives, Irgacure 369 may be supplemented with thioxanthone at 0.3–1.0 wt% because the α-amino ketone alone has moderate absorbance at 385 nm. Fully cured laminates are subjected to heat-ageing at 50 °C for 72 h followed by re-testing of peel adhesion to detect post-cure drift. Terminal finished products include pressure-sensitive label stocks, film-to-film lamination pouches, optical display bonding gaskets, and rigid plastic assembly adhesives for medical device housings where published data on long-term skin contact for this specific configuration is limited and must be generated under ISO 10993-1.