Acid Ink Red A, commercial model designation for C.I. Acid Red 88 and C.I. 15620 (CAS 1658-56-6), is a water-soluble monoazo acid dye supplied as a dark red to brown-red powder. The chromophore is formed by diazotisation of 1-naphthylamine-4-sulfonic acid and coupling to β-naphthol, yielding the sodium salt of 4-[(2-hydroxy-1-naphthalenyl)azo]naphthalene-1-sulfonic acid with molecular formula C₂₀H₁₃N₂NaO₄S and molecular weight 400.39 g/mol. The dye content is standardised to not less than 85% by visible spectrophotometry at λ_max 506 nm in distilled water buffered to pH 7.0, measured in a 1 cm quartz cell. The product is intended for aqueous ink concentrates, writing inks, water-based flexographic inks, leather dyeing, and paper coatings where an anionic sulfonate colorant is compatible with the formulation vehicle. A typical saturated solution at 20°C contains between 50 g/L and 80 g/L dye in deionised water; the pH of a 1% aqueous dispersion is 6.0 to 8.0. The ink-grade lot is controlled for conductivity below 500 µS/cm in a 5% solution to limit inorganic electrolyte carryover from the isolation step. The product is packaged in 25 kg fibre drums with polyethylene liners and carries a batch-specific certificate of analysis that includes dye strength, moisture, sieve residue, and heavy metals.

What Distinguishes Acid Ink Red A from Other Acid Red Colorants?

Acid Ink Red A belongs to the monoazo class, whereas many commercial acid reds used in graphic arts are disazo or xanthene derivatives. The monoazo structure produces a relatively narrow absorption band in the visible region; the half-band width measured in pH 7.0 phosphate buffer is typically 85–95 nm. This is narrower than the 105–120 nm half-band width observed for C.I. Acid Red 73 under the same conditions. The result is a cleaner blue-leaning red hue with less yellow tailing in magenta process sets. Unlike C.I. Acid Red 87, a xanthene colorant of substantially higher molecular weight and lower photostability, Acid Ink Red A exhibits better resistance to photochemical fading in aqueous ink films; lightfastness on paper measured according to ISO 12040 with blue wool reference ISO 105-B02 is generally 3–4 at a 1.0 optical density drawdown, whereas typical xanthene acid reds rate 2 or below. The lower molecular weight of Acid Ink Red A also supports faster diffusion into porous substrates, which can reduce surface bronzing but increases the need for a suitable fixing resin. The table below summarises the key comparative properties.

Across the pH range 4.0–9.0, Acid Ink Red A exhibits a bathochromic shift of less than 4 nm, which is sufficiently low for process-color reproduction when the ink is buffered. The dye is not recommended for use with strong reducing agents such as sodium sulfite above 0.2 wt% because the azo bond is susceptible to reductive cleavage; if oxygen scavenging is required in the ink package, isoascorbic acid should be evaluated by accelerated storage at 60°C for 7 days before production approval. This stability profile differs from C.I. Acid Red 52, a sulfonated xanthene with broader pH tolerance but weaker thermal stability in thermal inkjet printheads.

In thermal inkjet formulations based on water, 2-pyrrolidone, and glycerol humectants, Acid Ink Red A is typically introduced as a 10–15 wt% stock solution and then let down to 3–8 wt% dye in the finished ink. The finished ink is processed through sequential 5 µm and 1 µm nylon membrane filters at 0.2–0.4 bar differential pressure. On a production line using a 316L stainless steel jacketed vessel and a 4-blade pitched-turbine agitator at 300 rpm, dissolution of a 500 kg batch completes within 35–45 min at 40°C. Filtration throughput decline is observed when the dye stock solution is cooled below 15°C or when the conductivity of the process water exceeds 10 µS/cm; the resulting aggregate phase plugs the 1 µm membrane and increases differential pressure above 0.6 bar. The use of hard water with calcium carbonate concentration above 150 mg/L is not recommended because calcium salts of the sulfonate group reduce solubility and form insoluble residues. The pH of the finished ink is maintained between 6.5 and 8.0 with sodium acetate/acetic acid buffer; outside this window, the chromophore remains soluble but shifts its λ_max by up to 4 nm, which can alter the hue in process-color builds. High-shear dissolution at 2000 rpm for 15 min is used only when lumping is observed after prolonged storage at relative humidity above 60%.

For dye-based inkjet inks, the final filtered ink is evaluated for nozzle open time and sustained jetting stability. A sustained jetting test on a printhead with 9 µm orifice diameter and 12 kHz firing frequency at 25°C and 45% relative humidity is run for 30 min; a recoverable nozzle drop count above 5% indicates insufficient salt removal or incomplete filtration. The addition of 0.2–0.5 wt% of a nonionic acetylenic diol surfactant improves re-wetting of the nozzle plate but does not compensate for high divalent cation content. Foam generation in the recirculating ink path is controlled by vacuum degassing to 20–30 mbar absolute pressure before the ink enters the printhead manifold. Ink viscosity measured by ASTM D2196 is adjusted to 8–12 mPa·s at 25°C with glycerol or polyethylene glycol 200; viscosity values above 15 mPa·s reduce drop velocity and degrade print resolution at 600 dpi on coated paper.

When Acid Ink Red A Replaces Direct Red 81 in Bleed-Sensitive Ink Systems

Direct dyes such as C.I. Direct Red 81 exhibit high substantivity to cellulose and are often selected for low-bleed paper inks. Their strong fibre affinity can also produce uneven strike-through and mottling on lightweight coated grades. Acid Ink Red A has lower substantivity, which typically reduces paper pulp dye adsorption and provides a more uniform print edge when evaluated under ISO 2836 for water bleed and intercolor staining. The trade-off is a measurable loss of wet-rub resistance unless a cationic fixing polymer or blocked polyisocyanate crosslinker is added at 0.5–1.5 wt% of the finished ink. For a given uncoated 80 g/m² offset paper, the bleed distance is a function of substrate sizing, humectant type, and fixing polymer dose; no universal bleed reduction factor is assigned. The use of a nitrile-edge blade coater at 10 m/min is recommended for reproducing the drawdown; wire-wound rods give wider variation in film thickness at low dye loadings. Compatibility with polyurethane dispersion resins should be checked by measuring zeta potential because strongly cationic dispersions will precipitate the anionic dye at addition levels above 2 wt%.

In water-based flexographic inks for surface printing on porous paper, Acid Ink Red A is incorporated as a 20 wt% aqueous solution into a styrene-acrylic or acrylic solution varnish at 15–25% of the finished ink. Unlike pigment-based flexo inks, no bead milling or pigment wetting cycle is required; the dye dissolves directly in the letdown phase under low-shear agitation at 100–200 rpm. This reduces manufacturing time but also lowers the maximum optical density available on film and recycled board. On clay-coated board, a 1.0 optical density drawdown requires 6–9 g/m² wet film deposition; the resulting print has a gloss level of 30–40 measured at 60° geometry under ASTM D523. The dried ink film is sensitive to water spotting until 24 h cure; a crosslinked acrylic overprint varnish is recommended for wet-rub resistance.

For chrome-tanned leather drum dyeing, Acid Ink Red A is added to the wet-end float at 2–6% on shaved weight and exhausted at pH 3.5–4.5 with formic acid and 10% sodium sulfate at 60°C. Exhaustion on chrome-tanned stock exceeds 95% after 45 min when the drum is operated at 12 rpm; residual dye in the spent float is measured by visible spectrophotometry at 506 nm. The product gives a dark cherry red on full chrome grain, with crockfastness values under ISO 11640 of 3 dry and 2–3 wet on a standard chrome-tanned upper leather. The dye penetrates the grain layer more rapidly than higher-molecular-weight disazo acid reds, which can reduce buffing losses but may also require a low-pH fixation step to avoid uneven surface concentration in through-dyed crust stock. Cationic retanning agents and aluminium tanning salts should be separated from the dye float because coacervation occurs at pH above 5.0 and results in grain specking.

Specification Data and Test Methods for Incoming Inspection

Incoming inspection of Acid Ink Red A is performed on each batch. The lot is released only when the spectrophotometric strength is 95–105% relative to the retained standard at 506 nm in 1 cm cells. The specification below applies to the standard ink-grade material; custom desalted and low-dusting grades may differ.

Under the EU REACH regulation, the registration and classification status of C.I. Acid Red 88 must be confirmed against the current ECHA C&L inventory before shipment to European Economic Area markets. The product is not intended for food-contact inks unless specific migration testing under Regulation (EU) 10/2011 demonstrates compliance for the finished printed article. Published migration data for this specific colorant in indirect food-contact inks is limited; each finished article must be tested against its intended use. Acid Ink Red A should not be formulated with strongly cationic wetting agents, polyethyleneimine retention aids, or benzothiazole-based optical brighteners at acidic pH because precipitation and filter blockage occur. When the powder is stored in unopened 25 kg fibre drums at 10–30°C and relative humidity below 60%, the retest interval is 24 months. Carbon steel process vessels are unsuitable because ferric ion contamination dulls the hue and can form insoluble iron-dye complexes; 316L or glass-lined equipment is required.