Direct Grey D, listed in the Colour Index as C.I. Direct Black 22 and assigned CAS 6473-13-8, is a disazo direct dye supplied as a dark grey-to-black powder or presscake. The product is an anionic, water-soluble colorant for exhaustion dyeing of cellulosic substrates without a separate mordant or reducing agent. Commercial grades are standardised to 100% or 150% relative dye strength against a manufacturer reference lot; moisture content is normally held below 5.0% and water-insoluble matter below 0.2% by mass. Direct Grey D dissolves in demineralised water at 80°C to give usable stock solutions of at least 80 g/L under visual clarity testing after 30 min. The product is incompatible with quaternary ammonium softeners in the same bath because ion-pair precipitation produces dark specks and package filtration.

The product is supplied in powder and presscake forms. Powder grades may be dust-reduced with mineral oil or binders; the choice between 100% and 150% standardised strengths affects the powder weight required to reach a target shade. A 150% grade contains approximately two-thirds of the diluent mass of a 100% grade at equal colour value, but the exact factor must be verified by spectrophotometric dye strength measurement because supplier standardisation is not identical.

Incoming dyehouse quality control should request a REACH Annex XVII entry 43 compliance certificate for restricted aromatic amines; the regulatory limit is 30 mg/kg in the finished dye article. Because direct dye batches are standardised with diluents, the effective colour yield per kilogram varies among suppliers, and a dye strength correction factor should be applied when changing suppliers.

Stock solutions are prepared by pasting the powder with cold water, then diluting with demineralised water at 80°C in a stainless steel mixing tank equipped with a high-shear disperser. Filtration through a 100-mesh stainless steel filter before the dye bath reduces speck formation. Storage of prepared stock beyond 24 h at room temperature can allow sedimentation or biological growth, especially when non-sterile soft water is used; a preservative or fresh preparation is required for extended holding.

What Controls Exhaustion and Levelling in Hard-Water Dye Baths?

The exhaustion of Direct Grey D onto cotton is salt-dependent and temperature-sensitive. Production dye baths are typically set at 40–50°C with 0.5 g/L nonionic wetting agent and 0.2–0.5 g/L polyacrylate sequestrant, then raised to 90–98°C after dye addition. Sodium sulfate or sodium chloride is injected in three equal portions at 10 min intervals after the bath reaches 80°C; pale-to-medium shades require 10–30 g/L salt, while deep black-grey shades require 30–60 g/L. Single-shot salt addition causes aggregation and uneven migration. On package dyeing machines with a 1:8 liquor ratio and flow reversal every 2 min, inner-to-outer shade variation can exceed 0.8 ΔE when salt is added too rapidly; staged addition over 20 min typically reduces the variation to below 0.3 ΔE under D65/10° reflectance measurement.

Hardness above 50 mg/L as CaCO₃ should be corrected before dye addition. Calcium and magnesium ions bind to sulfonate groups, lowering solubility and creating filtration residues on package yarn. A bath pH of 8.0–9.5 with soda ash or sodium phosphate buffers the alkalinity and maintains dyeing reproducibility. pH values above 10.5 are not required and may accelerate hydrolysis of cotton pectins, increasing oil spot re-deposition.

Cellulosic Exhaust Dyeing Without Oxidative Development

Direct Grey D develops its colour on cellulose by substantivity, hydrogen bonding, and van der Waals aggregation; no oxidative development step is required. This distinguishes it from C.I. Sulphur Black 1, which must be reduced with sodium sulfide and re-oxidised inside the fibre. It also differs from C.I. Reactive Black 5, which forms a covalent cellulose ether bond under alkaline fixation. The absence of a covalent bond means unfixed dye remains available for desorption, making afterwashing the main control point for wet fastness. For a 2.0% owf dyeing on bleached cotton, wash fastness under ISO 105-C06 C2S is commonly reported as 3–4 grey scale before fixation, and wet crocking under AATCC TM8 is typically 2–3. Aftertreatment with a cationic fixative shifts these boundaries upward but may reduce light fastness.

Mercerised cotton exhibits higher accessible surface area and can build depth with 10–20% lower dye concentration than unmercerised cotton at equal reflectance, as monitored by reflectance at λmax. Dyeings on regenerated cellulose, such as viscose, absorb Direct Grey D more rapidly; the rate of temperature rise should be limited to 1.0–1.5°C/min between 50°C and 90°C to avoid barré.

In paper mass dyeing, Direct Grey D is applied at 0.02–0.15% on oven-dry pulp. The anionic sulfonate groups give low retention on bleached kraft unless a cationic retention aid or alum is present. With 0.02% cationic polyacrylamide on pulp mass, retention measured on a dynamic drainage jar at 0.05% dye dose is typically 65–85%; without retention aid, retention can fall below 40%, producing visible colour in white water and felt-side shade variation. For leather crust dyeing, published data for this specific Colour Index entry is limited; where Direct Grey D is used at 1.0–2.0% wet-blue weight with anionic syntans, drum trials at pH 4.5–5.5 are required to establish grain penetration and crock resistance on the finished article.

When Direct Grey D Replaces Sulphur Black 1 in Low-Odor Dyehouse Operations

Substitution of Direct Grey D for C.I. Sulphur Black 1 eliminates the sodium sulfide reducing bath and its associated hydrogen sulfide hazard. Sulphur dyeing typically uses sodium sulfide at 80–100 g/L of dye bath, and spent baths require oxidation to reduce sulphide species. Local discharge permits frequently set sulphide limits at 2 mg/L or lower; Direct Grey D avoids this parameter entirely, shifting the effluent burden to colour, salt, and cationic fixative. The substitution is not universally applicable because Direct Grey D without fixation rarely meets ISO 105-C06 wash fastness requirements of 4 or higher on deep black shades. It is therefore used when low-odor handling, simple effluent treatment, or shading without oxidative development outweighs wet fastness demands, or when a cationic aftertreatment is already installed in the dyehouse.

Cationic Fixation Shifts the Wet Fastness Boundary

Aftertreatment with a polyamine or dicyandiamide-formaldehyde condensate at 2–4 g/L and 40°C for 15–20 min forms a low-solubility complex with the sulfonate groups of Direct Grey D inside the fibre. This treatment typically raises wet crocking from 2–3 to 3–4 and wash fastness from 3 to 4 on a 2.0% owf dyeing, but it can reduce lightfastness by 0.5 grey scale step and dull shade by 0.3–0.5 ΔE units. The fixative concentration should be kept within the supplier range because overdosing causes handle stiffening and surface dye migration; a warm rinse at 40°C for 10 min after fixation is required to remove uncomplexed residue. Dyehouse QC should evaluate the fixed shade under ISO 105-A05 for shade change, ISO 105-C06 for laundering, ISO 105-E04 for perspiration, and ISO 105-B02 for lightfastness.

Because Direct Grey D is anionic, contact with cationic wetting agents, cationic fixatives, or cationic softeners outside the aftertreatment step must be controlled. In continuous pad-dry operations, the fabric should be free of residual alkali and salt before entering the dryer; surface deposition above 130°C on direct dyeings can cause migration to fibre surfaces and increased crocking.