In winch dyeing of loose wool stock destined for woven apparel interlining, Acid Brilliant Scarlet 3R (C.I. Acid Red 18, CAS 2611-82-7, λmax 508 nm in deionized water) is introduced only after the dyebath has reached 40°C in a conical pan dyeing machine operating at a liquor-to-goods ratio of 1:20. The prewetted stock is circulated with 5–10 g/L anhydrous sodium sulfate and 2–4% o.w.f. ammonium sulfate; the pH falls from approximately 6.5 to 2.8–3.2 as ammonia is liberated during the heating phase, generating the acid conditions needed to protonate amino side chains in wool type 1. The temperature is raised at 1.0–1.5°C/min to 98°C and held for 60–90 min. Because the dye carries three sulfonate groups, its strike becomes rapid below pH 4.0; dosing the dissolved dye over 15–20 min through the circulation loop reduces tip-to-root unevenness in unlevel stock, a failure mode documented on loose wool dyeing ranges processing mixed fleece lots.

At 2% o.w.f. depth, exhaustion commonly exceeds 95% after 45 min under the above buffer system, but the full holding time is retained to allow penetration into damaged cuticle regions and to minimize surface dye that would otherwise transfer during carding. The bath is cooled to 70°C before 1–2% o.w.f. formic acid is added to maximize fixation; residual hydrolyzed dye is monitored by absorbance at 508 nm in the spent dyebath. Unfixed colorant can lead to red bleed during alkaline shampooing, so wet fastness must be assessed according to ISO 105-C06 procedure A2S, alkaline perspiration according to ISO 105-E04, and light fastness according to ISO 105-B02 method 3. For machine-washable wool specifications requiring a grey scale result above 4, a cationic aftertreatment with 2–4% o.w.f. of a dicyandiamide polycondensate is used, but the hue shift and slight reduction in light fastness must be evaluated per ISO 105-B02 before shade sign-off.

What Shift in pH and Retarding Agent Ratio Prevents Nylon 6 Barre Marking?

Because anionic sulfonate groups on Acid Brilliant Scarlet 3R compete for terminal amino sites, uneven uptake on filament nylon 6 knit plated with variable amine end-group content produces barre unless the dyebath is buffered in the pH range 4.5–5.5. Nylon 6.6 apparel grades are commonly supplied with amine end-group values between 20–70 mmol/kg, while nylon 6 can range upward to 90 mmol/kg; a difference of 10 mmol/kg between cones is sufficient to produce visually detectable depth variation at a 1% o.w.f. dyeing. The dyebath is prepared with 1–2 g/L sodium acetate/acetic acid buffer, 0.5–1.5 g/L anionic leveling agent, and the dye is added at 40°C before the temperature is ramped to 105°C in a high-temperature jet dyeing machine.

Above pH 6.0 the equilibrium uptake is too low for adequate build-up on deep red shades, but below pH 4.0 the rate of dye exhaustion increases to the point that leveling agents cannot mask differences in fiber morphology and texturing history. The hold time at 105°C is limited to 30–45 min; longer exposure yields negligible additional depth and increases risk of dye migration to crease marks in rope form. After cooling to 60°C, a quaternary ammonium fixative at 2–4% o.w.f. is applied for 20 min to improve wash fastness, followed by rinse cycles. Wash fastness is tested on multifibre adjacent fabric per ISO 105-C06 and AATCC 61 test 2A. The terminal textile forms include tricot lingerie, warp-knit swimwear, and raschel lace; each construction requires adjustment of jet nozzle pressure and fabric speed to avoid ribboning.

Low-swelling acid donor systems have replaced free acetic acid in silk hank processing because Acid Brilliant Scarlet 3R exhibits rapid strike at pH below 4.0, and localized free acid in the hank can produce deep bands where the liquor flow is restricted by stick spacing. Degummed silk hanks are processed in an overflow hank dyeing machine at a liquor-to-goods ratio of 1:20–1:40 with 1–3% o.w.f. tartaric acid and 2% o.w.f. ammonium sulfate. The dyebath starts at 40°C and is raised at 1°C/min to 85–90°C, held 45–60 min, then cooled to 50°C before rinsing with softened water. Boiling above 95°C is avoided because the associated swelling increases loss of silk sericin and reduces surface smoothness in woven linings.

The liquor flow is kept at 8–12 L/s per stick to produce uniform contact across the skein thickness; axial flow below this range creates pale interiors with deeply colored surfaces, a batch defect observed in hand-loomed silk yarn dyeing. End uses include silk twill, habotai lining, and necktie fabric. Fastness verification is carried out according to ISO 105-C06 procedure A2S for washing and ISO 105-B02 for light. Cationic aftertreatment is limited to shades where the resulting light fastness change is acceptable, evaluated on the same dyeing before release.

Chrome-Tanned Crust Drum Dyeing with Sodium Formate Buffered Float

Before Acid Brilliant Scarlet 3R is added to a 3000–4000 L stainless steel drum, wet-blue chrome-tanned bovine side leather shaved to 1.2–1.4 mm is neutralized and rechromed with 1–2% sodium formate and 0.3–0.5% sodium bicarbonate over 40 min to a float pH of 4.5–5.0. The dye is dissolved separately at 60°C and dosed in three aliquots over 30 min at 50–55°C, at 1.5–3.0% by shaved weight. The drum liquor-to-goods ratio is 1:4. After 60–90 min, 0.5–1.0% formic acid diluted 1:10 is added over 40 min to drive exhaustion and fix the dye at the grain surface. Where fatliquoring is performed before dyeing, anionic sulfonated oils compete with the dye for cationic chromium complexes and reduce color yield; the fatliquor must be applied after dye exhaustion or selected as a nonionic emulsifier system.

Fastness evaluation on finished crust follows ISO 11640 for to-and-fro rubbing, ISO 11641 for perspiration, and ISO 11642 for water. The main operational boundary is pH drift: if the float drifts below 3.5 due to over-acidification, the dye precipitates on the grain and produces blotchy full-grain aniline leather. Reverse aftertreatment with a cationic leather dyeing auxiliary can improve wet rub fastness on deep tones but will flatten the nap of the grain, making it unsuitable for aniline pull-up articles. Terminal products include full-grain aniline footwear upper leather and vegetable-retannish-free upholstery leather.

In continuous dyeing of tufted nylon 6 carpet, the short steaming window makes Acid Brilliant Scarlet 3R suitability dependent on wet-pickup control rather than on prolonged exhaust equilibrium. The dye liquor is applied on a continuous carpet dye range at 300–400% wet pickup through a rotary screen or fluid applicator, with 0.5–5.0 g/L dye, 0.5–1.5 g/L citric acid, and 0.2–0.5% locust bean gum thickener. The thickened liquor viscosity is held at 1500–3000 mPa·s at 20°C to prevent dye migration after application and to minimize steam-condensation bleeding on carpet edges. The carpet is steamed at 98–100°C for 5–10 min, then washed cold to remove thickener residues and unfixed dye. Running speeds of 10–30 m/min are typical on production lines equipped with single-pass steamers.

Batch-to-batch variation arises primarily from nylon 6 tufting yarns with different heat-set histories; deep red shades on high-twist insert yarns require prewetting with 0.5 g/L nonionic wetting agent to prevent frosty appearance. Fastness to rubbing is evaluated according to ISO 105-X12, and color retention under wet cleaning according to ISO 105-C06. Automotive and commercial carpet specifications requiring light fastness above blue wool 6 should not use this dye as the sole red component, because its light fastness is adequate for residential indoor carpet but insufficient for continuous daylight exposure.

At pH values above 7.5, brightness gain in bleached softwood furnish is irrelevant because Acid Brilliant Scarlet 3R is anionic and will not fix on cellulosic fibre without a cationic bridge. In beater or pulper dyeing, the dye is added at 0.05–0.5% on oven-dry fibre at 3–4% consistency after alum addition of 2–5% on dry fibre. The resulting pH is 4.5–5.0, and contact time is 15–30 min before sheet formation. For two-sided tinting control, 0.05–0.1% cationic polyamine retention aid on dry fibre is introduced after the dye has been uniformly distributed. Surface coloration at the size press uses a starch solution of 5–8% solids with 0.2–1.0% dye based on size solids and a pickup of 8–12% on paper substrate.

The process is used for colored envelope stock, kraft paper, tissue, and cover paper. Color coordinates are verified by ISO 5631. The principal incompatibility is with alum-free alkaline papermaking containing calcium carbonate filler: at a furnish pH above 7.5, dye retention drops sharply and the red shade shifts to bluish pink in the white water. Closed white-water systems increase color carryback, so the dyeing sequence must be isolated from bright paper grades to prevent tinted broke contamination.

When Sulfuric Acid Anodized Profiles Enter the Dye Bath Before Sealing

The adsorption mechanism in anodic pores differs from textile dyeing because the aluminium oxide surface is positively charged at pH 5.0–6.0, allowing the sulfonated Acid Brilliant Scarlet 3R anion to adsorb into the pore wall before hydrothermal sealing. Aluminium profiles are anodized in 15–18% H₂SO₄ at 18–21°C with a direct current density of 1.2–1.8 A/dm² for 30–45 min, generating an oxide thickness of 18–25 µm and pore diameters in the 10–30 nm range. After triple rinsing to remove acid carryover, the profiles are immersed in a dye bath containing 1–2 g/L Acid Brilliant Scarlet 3R at 50–60°C for 20–30 min. The pH is maintained with acetic acid/sodium acetate buffer, and the bath is filtered continuously through 10–25 µm polypropylene cartridges to prevent particulate build-up on the profile bottom edge.

Sealing follows dyeing in boiling deionized water at 95–100°C for 30–45 min or in nickel acetate 5 g/L at pH 5.5–6.0 for 30–60 min. Sealing completeness is verified by the dye spot test defined in ISO 2143; incomplete sealing leaves visible red bloom after exposure to cleaning agents. End products include red anodized electronic front panels, nameplates, and indoor architectural profiles. For exterior applications, the anodized dyeing requires a UV-stable topcoat or substitute dye because Acid Brilliant Scarlet 3R is not designed for prolonged outdoor sunlight exposure.

Aqueous Ink Concentrates for Flexographic Paperboard

In water-based flexographic inks for uncoated kraft paperboard, Acid Brilliant Scarlet 3R functions as a low-viscosity dye colorant rather than a pigment dispersion. The ink concentrate is formulated with 1–5% dye, 5–10% diethylene glycol monobutyl ether, 0.2–0.5% nonionic surfactant, and 0.1–0.3% defoamer, adjusted to pH 8.5–9.5 with ammonium hydroxide. Viscosity at press is held at 30–60 s Zahn cup 2, and printing speeds on narrow web flexo presses range from 100–200 m/min. Drying is by hot air at 120–140°C, with residual moisture below 2% required before rewinding to prevent blocking on glossy packaging surfaces.

The anionic dye remains soluble in the alkaline ink vehicle; contact with low-pH coatings or cationic primers causes immediate precipitation and loss of color strength. Light fastness of printed paperboard is evaluated according to ISO 2836. The typical terminal products are gift wrap, paperboard folder stock, and envelope overprinting where water immersion resistance is not specified. For wet packaging or direct food contact, this dye-based system is not suitable unless the printed surface is laminated or overvarnished to prevent migration and bleed.