Supplied as a dry powder, Lithol Rubine 6B298 is a commercial pigment prepared as the calcium salt of C.I. Pigment Red 57, Constitution Number 15850:1, CAS registry 5281-04-9, EINECS 226-109-5. The suffix 6B298 is a manufacturer-specific grade code; it does not alter the Colour Index classification but may designate a defined particle-size distribution, surface treatment, or wetting-agent package. The dry powder exhibits a blue-shade red masstone and a bluish red undertone in letdown. Colouristic evaluation in a standard offset ink prepared and printed according to ISO 2834-1:2020 is used to release each lot.

Table 1 consolidates class-typical specification ranges for calcium lithol rubine powder. The actual certificate of analysis for lot release governs the product.

The dry powder is hydrophobic but can retain moisture when stored in unlined kraft bags. Water content is measured by ISO 787-2:2006; if volatile matter exceeds 0.5 %, the pigment should be re-dried at 80 °C for 2 h before incorporation into moisture-sensitive resin systems. This is especially important when dosing into polyamide or thermoplastic polyester at the lower end of the melt-processing range.

What Limits Thermal Stability of Lithol Rubine 6B298 in Polyolefin Processing?

The primary process conflict occurs in polypropylene injection moulding. Polypropylene barrel zones are commonly set between 220 °C and 240 °C, while the calcium lithol rubine class shows visible shade drift and pigment decomposition above roughly 200 °C. On single-screw extruders with 30:1 L/D ratio, die-lip plateout and loss of chroma have been reported when the pigment was run in neat PP at 220 °C; therefore the 6B298 grade is not a direct substitution into unfilled PP without validation.

The decomposition mechanism in the calcium lithol rubine class is not a single sharp event but a progressive loss of sulfonic acid and carboxylic acid fragments, which can generate gaseous products and shift the visible absorption maximum toward shorter wavelengths. In extrusion, this appears as a move from blue-shade red to orange-red and as a visible colour difference measured by ΔE* values greater than 1.5 on white letdown plaques. Batch-to-batch control therefore includes injection-moulded chips at 160 °C, 180 °C, and 200 °C and comparison of ΔE* under D65/10° illumination per ISO 11664-4:2019.

For low-density polyethylene and ethylene-vinyl acetate masterbatch carriers, the melt temperature is held at 170–190 °C. Pre-drying at 80 °C for 2 h is required if storage relative humidity exceeds 60 %. Thermogravimetric analysis under nitrogen at 10 K/min may be used to establish the safe upper limit for each lot, but published data for the specific 6B298 surface treatment is limited; class data for calcium lithol rubine indicate an onset of mass loss near 200–220 °C.

Letdown into polypropylene can be performed at the injection unit only when the pigment is part of a pre-dispersed masterbatch in a carrier that melts below 180 °C. The thermal history in the barrel should be kept below 200 °C, and residence time at temperature should not exceed 5 min in screw designs with low compression ratios. Batch-to-batch shade drift in white letdowns is evaluated by absorbance spectroscopy after injection-moulded plaques are conditioned according to ISO 291:2008.

In lithographic ink manufacture, the pigment is predispersed in an alkyd-moistened cake or flush base on a three-roll mill or horizontal bead mill. A water-cooled bead mill charged with 0.3–0.4 mm yttria-stabilized zirconia media is operated at tip speeds of 8–10 m/s; stock temperature is maintained below 45 °C because calcium lithol rubine dispersions can thicken and lose transparency when shear heating exceeds this threshold. Fineness of grind is checked according to ISO 1524:2020, and ink tack is recorded on an inkometer per ASTM D4361.

In sheetfed offset inks, the pigment is typically used at 12–18 % by weight of the finished ink. The dispersion must reach ≤5 µm on a grind gauge per ISO 1524:2020 to avoid plate wear and poor transfer. Ink tack is measured at 30 °C and 400 rpm on an inkometer according to ASTM D4361; the target is set by the press configuration and substrate class, not by pigment identity alone. The pigment's metal salt structure can interact with alkyd resin driers, so cobalt/manganese drier systems are pre-tested for viscosity stability over 24 h at 23 °C after ink preparation.

The pigment's solubility in common packaging ink solvents is selective. In ethanol-rich flexographic ink systems, the calcium salt exhibits pronounced bleed, while toluene-containing gravure formulations show lower migration but still require caution. Therefore the product is usually confined to non-food-contact packaging or graphic inks where bleed resistance is specified by a solvent-rub panel test such as ASTM D5402-19 rather than by total extraction testing. Published data for the exact 6B298 configuration is limited, so a comparative bleed test in the intended solvent mixture is required before substitution.

When a Solvent-Based Packaging Gravure Ink Requires Toluene Compatibility and Low Plateout

In toluene-ethyl acetate-nitrocellulose gravure inks, the 6B298 grade is dispersed on a horizontal media mill at 10–12 m/s tip speed using 0.3–0.4 mm zirconia beads. The milling vessel is jacketed and maintained at 35–40 °C; plateout on the gravure cylinder chrome surface is reduced when the grind reaches ≤5 µm by ISO 1524:2020. Residual solvent retention in printed film is measured by headspace gas chromatography after printing at 60–80 m/min on a pilot gravure press.

Formulations containing the calcium salt may require 2–3 % lower pigment loading than a barium-precipitated C.I. Pigment Red 57:2 grade to reach equivalent optical density on high-opacity white polyethylene. This shift is assessed by black-and-white drawdowns according to ISO 2834-1:2020, not by visual inspection alone. At equal pigment loading, the 6B298 grade may exhibit more bluish undertone but lower solvent resistance; published data for the exact 6B298 configuration is limited, so a comparative bleed test in the intended solvent mixture is required before substitution.

In water-based inks, the calcium salt is difficult to stabilize without surface treatment. High-pH amine-stabilized acrylic lattices can ionize the pigment surface and cause bleed into the aqueous phase; anionic dispersants at 0.5–1.0 % on pigment weight and a final pH of 7.5–8.5 are typical starting points. The dispersion is milled in a vertical bead mill at 6–8 m/s tip speed and checked for particle-size stability after 24 h. A water-based ink trial must include freeze-thaw and shear stability testing because published data for the specific 6B298 surface treatment is limited.

Powder Handling, Dust Explosion Potential, and Spill Control

As a finely divided organic pigment, Lithol Rubine 6B298 must be handled with local exhaust ventilation and grounding to limit combustible dust clouds. Dry-powder transfers into mixer hoppers should be performed in an inerted or ventilated zone; organic pigment dusts are classified as combustible dust and are subject to NFPA 652. Spills are wetted before collection to prevent resuspension, and collected waste is classified according to national waste regulations.

Operators should verify sieve residue on a 45 µm sieve according to ISO 787-7:2003; a high sieve residue indicates agglomeration from moisture contact and requires re-drying before use. Bulk density and flowability should be recorded on each lot with a 1 L powder funnel apparatus to monitor batch-to-batch variance. Dust generation is highest during bag transfer and mixer charging; a ventilated weigh booth with a face velocity of 0.5–1.0 m/s and filter cartridges rated for fine organic powders is used to reduce operator exposure.

Static discharge from high-speed screw feeders is controlled by bonding and grounding; conductive hoses are used for vacuum transfer. Dust clouds must be kept below the lower explosive limit; in the absence of product-specific explosivity data for 6B298, the process is designed according to NFPA 652 as if the dust were combustible.

Regulatory status and resin compatibility boundaries to confirm before use

Lithol Rubine 6B298 is not automatically approved for food-contact packaging. Compliance with Regulation (EC) No 10/2011 requires a positive listing or migration testing according to EN 1186-1, and no approval can be assumed from the C.I. classification alone. For toy and childcare articles, EN 71-3:2019 + A1:2021 migration testing is required for barium, lead, cadmium, and chromium; the calcium salt reduces but does not eliminate barium carryover from incomplete precipitation.

The product is unsuitable for exterior coatings, automotive coatings, and high-temperature powder coatings because full-shade lightfastness of the calcium lithol rubine class is rated no better than 3–4 on the blue wool scale under ISO 105-B02:2014. Avoid combination with strong alkaline pigment wetting agents and with reactive amine-based additives that can promote shade drift; the pigment is most stable in neutral-to-slightly-acidic, low-moisture systems.

When replacing a barium-precipitated C.I. Pigment Red 57:2 with 6B298, the formulator should not assume equal density or oil absorption. The calcium salt has a lower density than the barium analogue; consequently, the same mass loading may occupy a greater volume in the dry mix and alter the volumetric colour strength. Oil absorption according to ISO 787-5:1980 is also salt-dependent, and the calcium salt may require a different dispersant demand in alkyd and acrylic vehicles. A drop-in replacement is therefore validated on a laboratory three-roll mill at 0.5 bar roll pressure and 30 rpm apron speed before production scaling.

For applications requiring exterior durability and heat resistance above 250 °C, a quinacridone red such as C.I. Pigment Red 122 is specified instead; full-shade lightfastness under ISO 105-B02:2014 is typically 7–8 for quinacridone, while the calcium lithol rubine class rarely exceeds 4. The 6B298 grade is therefore positioned for graphic inks, indoor PVC, and low-cost plastic masterbatches where the service life is short and the colour cost per unit area is critical.

In solventborne architectural and industrial coatings, the pigment is used only in interior tinting pastes because it lacks exterior durability. Exposure testing under ISO 16474-2:2013 or ASTM G154 is not generally run for this product class; if specified, the expected gloss loss and mass tone shift limit use to non-structural interior applications. The pigment can be stabilized in long-oil alkyds and nitrocellulose lacquers, but the addition of zinc oxide or other basic pigments should be avoided to prevent hue shift.

Flexible PVC plastisol preparation begins with a high-shear mixer at 1500–3000 rpm for 10–15 min to wet the pigment into plasticizer; the premix is milled on a three-roll mill or bead mill to 20 µm or finer before addition to the main plastisol. Calendering roll temperatures are held at 150–170 °C; because the processing window may be as narrow as ±5 °C, temperature deviations across the roll width can produce visible shade variation in white letdowns. The gelation profile is monitored with a Haake torque rheometer at 160 °C and 40 rpm; a rapid torque increase indicates premature fusion and is corrected with additional plasticizer rather than higher shear. The pigment remains suitable only for indoor graphic films and short-lifetime articles due to the class-limited lightfastness.