What Controls Salt Addition Rate in Exhaust Dyeing of Bleached Cotton?

Exhaust dyeing of bleached cotton knitgoods with Direct Scarlet 4BS (C.I. Direct Red 23, CAS 3441-14-3) is governed by electrolyte partition, ramp rate, and hold time at maximum temperature. The commercial dye is predissolved at 1:10 in demineralized water heated to 70–75°C; hard water above 150 mg/L CaCO₃ must be conditioned with a sequestering agent before dye addition to prevent insoluble sulfonate salt formation. A medium-depth shade is applied at 0.8–1.2% on weight of fabric. Sodium sulfate anhydrous is preferred over sodium chloride in closed stainless steel machines because chloride reduces the pitting resistance of 316L heat-exchanger plates at 95°C. The electrolyte is split into three aliquots of 10 g/L each: the first at 40°C, the second after 15 min, and the third before the ramp to 95°C. A ramp rate of 1.0°C/min and a hold of 45 min are used on overflow machines at a liquor ratio of 1:10. Spent liquor retains anionic dye; it is treated with cationic polymeric flocculant before discharge to reduce colour load. Untreated Direct Scarlet 4BS has limited wet fastness; under ISO 105-C06:2010 A1S at 40°C, wash fastness for 1/1 standard depth is typically graded 2–3. Cationic aftertreatment with a polyquaternary ammonium fixer raises wash fastness to 3–4, but light fastness under ISO 105-B02:2014 may decrease by half a grade. Because the chromogen contains a benzidine core, all skin-contact textile goods must be tested under EN ISO 14362-1:2017; the benzidine release limit under EU REACH Annex XVII entry 43 is 30 mg/kg. Reductive stripping agents, including sodium dithionite or sulfur-dye reduction systems, cleave the azo system and are incompatible.

On pad-batch ranges processing cotton/viscose shirting, liquor pickup is fixed before diffusion begins, and the drying phase decides whether the shade remains level. The trough liquor contains 10–30 g/L Direct Scarlet 4BS, 1–2 g/L nonionic wetting agent, and 20–50 g/L urea. Horizontal mangle bowls are specified at 70–75 Shore A to maintain wet pickup at 70–85%; harder bowls above 85 Shore A cause edge squeezing and lower pickup. Low-viscosity sodium alginate at 5–10 g/L is added when predrying cylinders exceed 100°C, because uncontrolled water removal drives dye to the surface and creates centre-to-edge shade variability. The batched fabric is wrapped in polyethylene film and rotated slowly for 8–12 h at 25–30°C. On a continuous pad-steam range, saturated steam at 100–102°C for 30–60 s accelerates dye diffusion without chemical fixation. Viscose-containing lots require storage below 60% RH after washing; higher humidity activates dye migration at selvedges. Residual peroxide from upstream bleaching must be destroyed with catalase before padding; oxidative species at 2 mg/L or above decrease tinctorial yield and shift the red a* coordinate. Rubbing fastness is verified with ISO 105-X12:2016 on the dried, washed fabric.

Because direct dyes of the C.I. Direct Red 23 class show high affinity for hydrated cellulose, stock dyeing of bleached sulfite pulp is carried out after refining in the thin-stock loop. Direct Scarlet 4BS is prepared as a 1–2% aqueous solution and metered at the machine chest or fan pump at 0.05–0.5% on dry fiber. The stock pH is held at 4.5–6.5 with alum or dilute sulfuric acid; alum at 1.0–3.0 kg/t dry fiber promotes retention of the anionic sulfonate dye, but overdosing above 5.0 kg/t produces dye-lake agglomerates that appear as red specks in the sheet. In furnishes containing deinked pulp, residual sodium hydrosulfite must be oxidized before dye injection; otherwise reductive cleavage of the azo system releases the benzidine core and consumes colourant. Cationic poly-DADMAC at 0.05–0.1% on dry fiber improves retention in high-anionic-trash systems, but it is added after the dye to avoid local precipitation in approach piping. Two-sidedness on the fourdrinier is controlled by limiting white-water dissolved-solids accumulation; high sulfate content in the top layer produces a redder wire side. Formed handsheets are measured under ISO 2469:2014 with a benchtop spectrophotometer; colour strength is expressed as Delta E against the approved base white.

Chrome-Tanned Leather Drum Dyeing at Neutralised pH

Chrome-tanned crust leather after neutralisation to pH 5.5–6.5 is dyed in a stainless steel drum at 60°C using 1.0–2.5% Direct Scarlet 4BS on shaved weight. The float ratio is 1:2–1:4, and drum speed is 12–16 rev/min. The dye is predissolved at 1:10 in hot water and injected through the hollow axle; adding powder directly through the door produces dark specks on the grain. Uptake is rapid during the first 10 min because the anionic sulfonate groups ionically interact with cationic chromium-collagen sites, yielding high surface colour with limited penetration. For suede splits, a weak anionic retarding agent is required to improve depth of penetration. After 40–60 min of running, 0.5–1.0% formic acid is split into two additions at 10 min intervals, bringing final float pH to 3.8–4.2. Fatliquoring after dyeing shifts the hue slightly and changes wet rub behaviour; finished leather is assessed under ISO 11640:2018 for dry and wet rubbing, and wet rub is at least one grade lower than dry rub. Because leather is a skin-contact article, benzidine release is measured by EN ISO 17234-1:2015; the EU REACH Annex XVII entry 43 limit is 30 mg/kg. Reductive bleaching with sodium dithionite before dyeing is incompatible with this dye class.

When Direct Scarlet 4BS Is Applied Through Textile Printing Paste

Textile printing with Direct Scarlet 4BS on cotton poplin uses a sodium alginate thickener to immobilize the dye before steaming. The paste is prepared by dissolving 20–40 g/kg dye in water at 80°C, cooling to 25°C, and mixing into a 4–6% alginate paste containing 100–150 g/kg urea. Final viscosity is adjusted to 3,000–5,000 mPa·s at 25°C on a Brookfield RVT viscometer with spindle No. 5 at 20 rpm. Printed cotton poplin is dried at 100°C to 8–10% moisture and steamed at 100–102°C for 10–20 min in a star steamer. Wash-off is a two-stage sequence: cold overflow rinse to remove thickener, followed by 60°C detergent washing. Backstaining of unprinted white areas is reduced by 0.5–1.0 g/L polyvinylpyrrolidone in the wash bath. The process is unsuitable for discharge printing because the benzidine-based azo structure is cleaved by sodium sulfoxylate formaldehyde or tin chloride; discharge quantities above 20 g/kg produce complete colour loss and free amine release. Long print runs above 10,000 m require viscosity checks every 2 h because alginate depolymerizes in hot paste rooms, shifting print definition and colour strength.

In radial-flow loose-stock dyeing of viscose staple for melange yarn, Direct Scarlet 4BS is charged at 0.3–1.5% owf in a packed cage at a liquor ratio of 1:8–1:12. Pump pressure is held below 0.8 bar differential to avoid fibre compression and channeling; viscose staple loses wet modulus above 90°C, so the maximum temperature is limited to 80–85°C. Sodium sulfate is dosed at 10–30 g/L in three increments, and the hold time is 30–45 min. After rinsing, a formaldehyde-free cationic fixer is exhausted at 40–50°C to reduce colour transfer during carding. Drying in a loop dryer is set at 80–100°C to a final moisture of 8–10%; overdrying below 6% generates static charge and increases fly generation in carding. The melange yarn is tested for washing fastness under ISO 105-C06:2010 and for benzidine release under EN ISO 14362-1:2017. Because viscose staple is often used in next-to-skin garments, the benzidine-based direct dye is generally excluded from European hygiene and infant product programs; this is an operational boundary rather than a formulation correction.