Direct Black EX (C.I. 30235, CAS 1937-37-7) is treated in this application section as a benzidine-derived direct azo colorant whose reductive cleavage behaviour defines the compliance and processing boundaries in each downstream segment.

In mill-scale exhaust dyeing of scoured and bleached cotton interlock, Direct Black EX is introduced into the dye bath only after metered pre-dissolution at 80–90 °C and edge filtration through 50–60 μm mesh to prevent agglomerated dyestuff particles from depositing on fabric contact points. The dyestuff is handled under the assumption that reductive cleavage will liberate benzidine; production dyers therefore isolate dyestuff weighing stations, use local exhaust ventilation, and run weekly aromatic amine swab checks on process piping and mixing decks.

The dyeing cycle starts at 40 °C with a liquor ratio of 1:10–1:20. Direct Black EX is dosed at 0.5–3.0% o.w.f. for medium-through-heavy shades, with black depth above 3.0% o.w.f. rarely producing a linear build-up because the high substantivity of the colorant causes surface saturation on mercerized cotton before the diffusion front reaches the fibre core. Sodium sulfate is partitioned in two stages: 1.0–2.5 g/L after the dye bath reaches 40 °C and a further 3.0–5.0 g/L at 95 °C. Hold time at 95–98 °C is 45–60 min, followed by cooling to 70 °C before a cationic fixing agent based on dicyandiamide-formaldehyde resin is applied at 2.0–4.0% o.w.f. Without this aftertreatment, wet fastness on cellulosic substrates under ISO 105-C10 can degrade by more than one grey scale grade when stained against cotton and viscose multifibre adjacent fabric.

Compliance boundaries for cotton articles are set by Regulation (EC) No 1907/2006 Annex XVII entry 43, which restricts azocolourants that release benzidine or other listed aromatic amines at a threshold of 30 mg/kg in textile articles intended for skin or mouth contact. Direct Black EX is outside the permissible range for skin-contact cotton apparel, underwear, bed linen, and towels sold in the EU/EEA. Non-EU industrial cotton end uses may still require customer RSL screening against ZDHC MRSL 3.1; terminal finished products in permitted sectors include cotton duck for machinery covers, non-apparel technical filter cloth, transportation webbing where the construction prevents skin-contact, and black cotton upholstery backing.

What Limits Package Density in Direct Black EX Dyeing of Viscose Spun Yarn?

Viscose spun yarn package dyeing imposes a narrower liquor-circulation window than cotton because regenerated cellulose swells more rapidly in hot aqueous electrolyte. Direct Black EX is charged at 1.0–2.5% o.w.f. on bone-dry yarn weight, and package density is held at 0.32–0.36 g/cm³ on stainless steel dye springs. Package densities above 0.38 g/cm³ routinely produce a centre-to-shoulder shade difference that cannot be corrected after fixation because the inner package layers receive reduced flow once the yarn bundle compresses under axial pressure.

The production-scale package carrier should be equipped with reversible-flow circulation at 30–50 L/min/kg; the pressure drop across the package stack may exceed 1.5–2.5 bar if the full electrolyte charge is introduced before the fibre has reached thermal equilibrium. Sodium chloride is split into three additions at 40 °C, 70 °C, and 90 °C to limit the hydronium screening layer that accelerates pressure drop through the yarn bundle. The dye bath is held at 95 °C for 40–50 min, after which the bath is drained and the packages are washed at 60 °C until conductivity in the rinse drop. Foaming in the expansion tank should not exceed 0.15 kg/m³ at any point; entrained air at the pump inlet can collapse circulation and create an unlevel inner core that is only detected after package re-winding.

Compliance testing for viscose spun yarn shipped to European textile converters follows the same REACH Annex XVII entry 43 boundary as cotton; if the yarn is intended for lining fabrics or furniture covers that contact skin, the final article must meet EN 14362-1:2012 for aromatic amine release. Aftertreatment with a cationic fixing agent at 2.0–3.0% o.w.f. is standard, and the dyed yarn is evaluated for wash fastness by ISO 105-C06 and light fastness by ISO 105-B02. Terminal product types in non-EU industrial sectors include black viscose embroidery thread, non-apparel lining substrates, and technical upholstery yarn where article-level skin-contact restrictions do not apply.

Drum dyeing of chrome-free wet-white leather with Direct Black EX requires a staged pH reduction sequence rather than single-shot formic acid addition because the amphoteric charge reversal of the wet-white substrate slows penetration at pH values below 4.0. Direct Black EX is charged at 0.5–2.0% on shaved wet-white weight after neutralization with 1.5–2.0% ammonium bicarbonate or sodium formate at 35–40 °C. The drum is run at 8–12 rpm for 30–45 min, after which formic acid is added incrementally in three equal additions to bring the float pH from 5.8–6.2 down to 3.8–4.2.

A drop in float pH below 3.5 during fixation precipitates unexhausted dye onto the grain as bronzing spots; this is a common production failure mode on conventional stainless steel mixing drums with no automatic pH metering. Because Direct Black EX has high affinity for the leather protein when fully acidified, adding the full acid charge at once leads to rapid surface strike and prevents the dye from penetrating into the reticular layer. The same effect is observed when the float temperature is raised above 50 °C too early; production schedules therefore hold 40–45 °C until the first two acid splits have been absorbed.

Compliance verification for leather uses EN ISO 17234-1:2015 for aromatic amine release under reducing conditions, with the 30 mg/kg threshold applied to skin-contact leather articles in the EU/EEA. Direct Black EX is not suitable for EU shoe upper leather, garment nappa, watch straps, or leather gloves because the colorant is outside the REACH Annex XVII entry 43 permitted range for skin-contact articles. For non-EU industrial leather where such restrictions do not apply, terminal product types include heavyweight rigging leather, leather gaskets, bookbinding covers, and non-contact furniture trims; wet and dry rub fastness are assessed by ISO 11640, and perspiration fastness by ISO 11641. Without a polymeric or lacquer topcoat, direct-dyed leather of this class typically falls below the fastness level achieved by chrome-complex acid dyes.

Papermaking Wet-End Retention Dynamics for Direct Black EX on Bleached Kraft Pulp

Direct Black EX is applied in the thin stock line after the machine chest and before the fan pump at 0.2–1.0% on oven-dry pulp for deep black packaging and decorative paper grades. Because the colour field of the dye is anionic and the bleached kraft fibre carries a negative surface charge at neutral pH, first-pass retention cannot be assumed from beaker-scale dyeing tests; it must be measured from white water samples at the forming section and validated against coated sheet drawdowns.

The stock pH is held at 5.5–6.5 with alum at 0.5–1.5 kg/t oven-dry pulp, and the retention system is metered separately to avoid competitive adsorption between the dye and high-cationicity polyacrylamide. A typical sequence uses a short-chain cationic coagulant at 0.3–0.8 kg/t, followed by a high molecular weight retention aid at 200–500 g/t dry fibre. Headbox consistency is maintained at 0.5–1.0%. On machines running above 800 m/min, closed-loop in-line colour sensors provide more useful shade control than laboratory drawdowns because sheet ash content and wire retention shift the effective dyestuff demand across the web.

Compliance for paper and board is concentrated on end-use exclusions rather than textile article restrictions. Food-contact paper and board under EU Regulation EC 1935/2004 and US 21 CFR 176.170 cannot be assumed for this dyestuff because the benzidine-liberating profile is incompatible with migration limits for aromatic amines. The allowable industrial scope covers non-food-contact black packaging board, abrasive paper backing, partition board, poster stock, and decorative base paper where the final article is not mouthed by children and not governed by EU Directive 2009/48/EC Toy Safety.

When Direct Black EX Is Metered into Jute Hank Dyeing Baths for Carpet Backing and Sack Fabric Use

Jute hank dyeing operates at a lower temperature than cotton to avoid lignin redeposition on the fibre surface; Direct Black EX is therefore applied at 60–70 °C rather than full boil. The addition ratio is 1.5–3.0% on conditioned jute weight, with sodium chloride added as a single charge of 10–15 g/L after the dyebath has circulated for 10 min. The lower application temperature extends the dyeing cycle to 60–90 min, but it reduces the brown background shift that would otherwise appear when jute lignins solubilize at high pH and temperature.

Hank carriers are stacked on rectangular dyeing machines with variable circulation; loading above 0.25 kg/L hank mass per liquor volume creates a characteristic dark outer layer and pale inner layer because the jute fibre swells anisotropically and restricts flow through the hank bundle. After rinsing, the hanks are dried at 70–80 °C on a steam-heated forced-air dryer, then fixed with a non-formaldehyde cationic auxiliary at 2.0–4.0% o.w.f. to reduce crocking onto adjacent packaging materials. Dye fixation on jute is lower than on cotton because a portion of the direct dye is adsorbed onto lignin-rich surface components rather than cellulosic hydroxyl groups; this loss is accounted for by dyeing against a pre-established calibration curve rather than by assuming the same exhaustion behaviour as cotton.

Compliance for jute sack fabric and carpet backing in EU industrial packaging does not fall under REACH Annex XVII entry 43 if the final article is non-skin-contact; however, export consignments to EU retailers are routinely screened for aromatic amines using EN 14362-1:2012 or equivalent in-house LC-MS/MS methods. Terminal product types include black carpet backing, jute twine, hessian packaging, erosion-control matting, and agricultural tie cord where skin contact is incidental rather than prolonged.