Section
| HS Code | 540533 |
| Productname | Direct Black G |
| Colorindexname | C.I. Direct Black 19 |
| Colorindexnumber | 35255 |
| Chemicalclass | Trisazo direct dye |
| Casnumber | 6428-31-5 |
| Molecularformula | C34H27N13Na2O7S2 |
| Molecularweight | 839.77 |
| Appearance | Black powder |
| Color | Black |
| Solubility | Soluble in water |
| Application | Dyeing and printing of cotton, viscose, paper, and leather |
| Dyeingtemperature | 80-95 °C |
| Electrolyterequirement | Sodium chloride or sodium sulfate |
| Phrange | 6-8 |
| Lightfastness | 3-4 |
| Washfastness | 3-4 |
| Storage | Store in a cool, dry, well-ventilated area away from direct sunlight |
| Packaging | 25 kg woven bags or fiber drums |
As an accredited Direct Black G factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | Direct Black G is supplied in 25 kg net multi-wall paper bags with polyethylene liners, sealed and labeled for industrial dye use. |
| Container Loading (20′ FCL) | Direct Black G, in sealed labeled packages, is palletized, evenly distributed, braced, and secured in a clean, dry 20′ FCL container. |
| Shipping | Shipping description: Direct Black G is not classified as dangerous goods for transport; no UN number, hazard class, or packing group is assigned. Ship in sealed, labeled containers, keep dry and away from oxidizers, avoid dust generation, use appropriate PPE, and comply with local transport regulations. |
| Storage | Store Direct Black G in a cool, dry, well-ventilated area in a tightly closed, labeled container. Keep away from moisture, direct sunlight, heat sources, oxidizers, acids, and alkalis. Prevent dust generation and avoid inhalation; use appropriate PPE. Maintain good housekeeping and separate from food, feed, and incompatible materials. Follow local regulations and the supplier’s safety data sheet. |
| Shelf Life | Direct Black G has a shelf life of about 24 months when stored dry, cool, and protected from light and moisture. |
On a production-scale overflow jet range handling 300–800 kg lots of cotton single jersey, Direct Black G is pre-wetted at a 1:10 dye-to-water ratio and then diluted to a 50 g/L stock solution through a 100 mesh filter cloth; powder that has absorbed moisture above 6% in unlined storage tends to form agglomerates that pass through the filter only under high-shear mixing. Because commercial Direct Black G is sold under different Colour Index designations in different supply chains, the mill certificate must state the Colour Index designation and CAS number before bulk qualification. The dye cycle starts with the fabric loaded at 35–40°C and the dissolved dye added over 5–10 min. After 10 min of running, reagent-grade sodium chloride at 10–30 g/L or anhydrous sodium sulphate at 5–20 g/L is added in 2–3 split portions at 10 min intervals; water hardness above 8°dH requires 0.2–0.5 g/L of a polyphosphate sequestrant in the first salt portion to prevent metal-dye complex formation. The bath is then raised to 95–98°C at 1.0–1.5°C/min and held for 30–45 min for medium-depth shades or 45–60 min for a black on 180–220 g/m² cotton jersey. Draining is delayed until the bath has cooled to 70°C; hot draining at 95°C causes rope marks and crease-set faults in jet machines because mechanical deformation in the wet swollen state relaxes unevenly. The exhausted fabric is rinsed cold and aftertreated with 1–2% owf cationic dye-fixing agent at 45–50°C and pH 5.5–6.5, adjusted with dilute acetic acid; below pH 5.0 the fixer can precipitate residual anionic dye in the bath and deposit on the fabric surface as bronzing. On 4.0% owf dyeing, typical fastness before aftertreatment is ISO 105-C06:2010 change 2–3 and wet rubbing ISO 105-X12:2016 1–2; after cationic fixation, wash fastness improves by 0.5–1.0 grade and wet rub to 2–3. Batch-to-batch shade variation on shared dye lines is reduced by reserving one storage tank for direct black stock because residual cationic softener from previous textile finishing cycles precipitates the anionic dye as a filterable complex on jet nozzle screens. Finished black cotton jersey, rib, and French terry are used in apparel, linings, pocketing, and mattress ticking where the fabric is not specified as high-wet-fastness sportswear. Regulatory acceptance for textile lots requires reductive-cleavage testing to EN ISO 14362-1:2017, confirmation that restricted amines listed in EU REACH Annex XVII entry 43 are below the 30 mg/kg reporting threshold, and supplier documentation against Oeko-Tex Standard 100 Annex 4 and ZDHC MRSL 3.1.
When a pad-steam range is configured for 40–60 m/min line speed on 200–250 g/m² cotton poplin, the limiting variable is not exhaustion but migration during the intermediate drying step. Pad liquor is prepared with 25–60 g/L Direct Black G, 2–5 g/L nonionic wetting agent, and 5–15 g/L sodium sulphate; the pad trough is held at 25–40°C, and a horizontal padder with 2–3 bar nip pressure delivers 65–75% wet pick-up. The fabric is then pre-dried on cylinder dryers at 100–120°C to a residual moisture window of 25–30%. If the moisture content falls below 20%, dye migration to fibre surfaces produces dark selvedges and white-centred fibres; above 35%, the subsequent chemical pad is diluted and final colour yield drops. The dried cloth passes through a chemical pad containing 20–40 g/L sodium chloride and 5–10 g/L sodium carbonate, followed by saturated steam at 102–105°C for 60–120 s in a steamer with controlled condensate removal. Cold rinsing at ≤30°C stops further dye diffusion and removes unfixed colour; the first rinse bath typically carries 15–30% of the applied dye, which must be handled through on-site treatment before discharge. The finished black cotton poplin, drill, and twill are used for workwear linings and light-duty woven outer fabric where a cationic aftertreatment is applied to lift wet rub fastness. Effluent from this continuous route is characterized by conductivity values of 20–30 mS/cm in the salt-rich wash stream; local discharge permits usually cap conductivity and sulphate, so the pad-steam route is not always environmentally feasible without brine recovery. Published data for the exact pad-steam fastness of Direct Black G on every construction is limited; the following process comparison is not a substitute for mill-scale qualification.
| Process variable | Exhaust (jet/overflow) | Pad-steam (woven) |
|---|---|---|
| Dye offer | 2.5–4.0% owf | 25–60 g/L pad liquor |
| Electrolyte | 10–30 g/L NaCl or 5–20 g/L Na₂SO₄ in split portions | 20–40 g/L NaCl plus 5–10 g/L Na₂CO₃ in chemical pad |
| Fixation window | 95–98°C for 30–60 min | 102–105°C saturated steam for 60–120 s |
| Critical moisture control | Cool to 70°C before drain | Predry to 25–30% residual moisture |
| Wet rub before/after aftertreatment | ISO 105-X12:2016 1–2/2–3 | ISO 105-X12:2016 1–2/2–3 |
In tanneries processing bovine side leather for shoe uppers, Direct Black G is introduced as a drum-dyeing colorant after the wet-blue has been neutralized to pH 4.5–5.0 with sodium formate and sodium bicarbonate. The float is set at 150–200% based on shaved wet-blue weight, the dye offer is 1.0–2.5% with 0.3–0.5% penetrating auxiliary, and the drum runs at 8–12 rev/min for 30 min. Fatliquoring sequence is the main process conflict: if the full anionic fatliquor is applied before the dye, competitive adsorption reduces surface depth and can produce a weak, uneven crust; if the full fatliquor is applied after dye fixation, depth increases but grain softness suffers. Production tanneries therefore split the fatliquor, adding 50% pre-dye and 50% after fixation. Fixation is carried out with formic acid diluted 1:10 and added in two portions 10 min apart to a final float pH of 3.6–3.8, followed by a further 20 min run. The leather is washed at 35°C, horsed, and vacuum-dried at 60–70°C; higher dryer temperatures migrate dye toward the grain and create bronzing or weak crock, measured by the standard leather rub test ISO 11640:2018 where dry rub typically reaches 3–4 and wet rub 2–3 for medium-grade black lining leather. Direct Black G is not normally specified for automotive upholstery because light fastness of this class under ISO 105-B02:2014 is generally below grade 4, and published data for the specific Direct Black G configuration in high-UV cabin exposure is limited. Finished outputs include black bovine upper leather, lining leather, belt leather, and bag leather, with each lot requiring EN ISO 14362-1:2017 certification for restricted aromatic amines, EU REACH Annex XVII entry 43 compliance, and ZDHC MRSL 3.1 conformance documented in the tannery chemical inventory.
Beater dyeing with Direct Black G on cylinder board machines is positioned after refining and before the thin-stock pump so that the anionic colorant has 15–30 min contact time at 2.0–3.0% stock consistency before dilution to the headbox. Dye charge is 0.5–2.0% on oven-dry fibre, the stock pH is held at 4.5–5.5 with alum or a cationic polyamine retention aid at 0.02–0.05% on oven-dry fibre, and the wet-end temperature is controlled at 35–45°C. Under these conditions the dye exhausts onto the fibre and the white-water absorbance in the red region is used to monitor retention; a rising absorbance trend after the first 10 min indicates overloading or insufficient cationic charge. For black or grey board, the dyed stock is formed at 200–400 g/m² basis weight and couched onto the cylinder mould, after which the surface may be further coloured at the size press. Size-press formulations use a cooked starch base at 8–12% solids with Direct Black G at 2–10 g/L, applied at 3–5% dry pick-up; the bath viscosity is maintained at 20–60 mPa·s at 60°C to avoid film split patterns. Direct Black G is suitable for black poster board, grey/black folding box board, and non-food kraft liner; for food-contact paper and board the grade is not automatically cleared under FDA 21 CFR 176.170 or the EU Framework Regulation (EC) No 1935/2004 positive-list system, so converters must obtain a written opinion for the specific article before use.
Because low-viscosity aqueous inks for stamp pads and markers must remain free of pigment grind residue, Direct Black G is dissolved at 3–8% by weight in deionized water at 50–60°C with a humectant system of 5–15% glycerin or ethylene glycol and a preservative active against Pseudomonas species in the finished ink. The pH is buffered to 7.0–8.5; acidic pH below 6.5 reduces solubility and may precipitate the sulphonated azo colorant, while pH above 9.0 accelerates hydrolytic degradation of the chromophore at elevated warehouse temperatures. The solution is filtered through 1–5 µm depth media before filling; iron or copper in the water supply above 0.1 mg/L shifts hue and forms insoluble metal-dye complexes, so stainless steel or plastic-lined mixing vessels are used. The finished ink viscosity is below 5 mPa·s at 25°C, which allows flow through felt nibs and ceramic piston pump filling lines without the abrasive wear associated with carbon black dispersions. Light fastness under ISO 105-B02:2014 is limited, so the formulation is confined to stamp pads, markers, and water-based paper printing inks where archival permanence is not specified. Regulatory testing for toy markers follows EN 71-3:2019 migration limits for elements, with REACH Annex XVII restrictions applied to the final liquid as a mixture; because the dye is an azo colorant, EN ISO 14362-1:2017 screening is retained in the raw-material acceptance protocol even though the reduction step is more relevant to fibre-bound dye than to soluble ink.
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Direct Black G is supplied as a black granular or powder preparation and is commercially indexed as C.I. Direct Black 38 with constitution number 30235, CAS Registry Number 1937-37-7, molecular formula C34H25N9Na2O7S2, and molar mass 781.7 g mol⁻¹. The product is a benzidine-derived polyazo direct dye with sulphonated aromatic rings; the dyestuff is strongly anionic in aqueous solution and its solubility is temperature-dependent. Representative commercial specification data are as follows: dye strength 100% relative to the supplier’s reference lot by dual-beam UV-visible integration at the product λmax, moisture content ≤ 5.0% by Karl Fischer titration according to ISO 760, water-insoluble residue ≤ 0.5% by membrane filtration through a 2.5 µm polycarbonate filter, and pH 7.5–9.0 in a 1% aqueous dilution at 25 °C. Bulk density commonly falls between 0.45 g cm⁻³ and 0.65 g cm⁻³. Direct Black G is not a pigment; it is dissolved in the aqueous dyebath before contact with cellulosic fibre, and stock solutions are normally prepared at 20–30 g/L by pasting in cold water and diluting at 80 °C under mechanical agitation. Direct steam injection is not recommended because localised boiling can promote reductive damage. Concentrates above 50 g/L are not recommended for extended storage, because aggregation increases apparent viscosity and can produce metering pump cavitation. Storage in unopened bags at ≤ 40 °C and ≤ 60% RH avoids caking in the weigh room.
Direct Black G is used principally for exhaust and continuous dyeing of cotton, viscose, flax, and cellulosic blends. The product builds to a bluish black at 2–4% on-weight-fibre depth; above 4% incremental build-up becomes inefficient and wet rub fastness deteriorates. Exhaust dyeing starts at 40 °C with a temperature ramp of 1.0 °C min⁻¹ to 90–95 °C. Sodium chloride at 10–20 g/L is divided into at least two additions to avoid aggregation-induced filtration and uneven shade.
The controlling constraint is the combination of high substantivity and electrolyte sensitivity. On mercerised cotton, the fibre has higher amorphous cellulose content and greater accessible surface area, so Direct Black G exhausts more rapidly than on bleached cotton and can produce end-to-end and side-to-centre variation if salt and temperature ramps are not controlled. Exhaustion begins near 40 °C and reaches a practical maximum near 90 °C; heating at 1.0 °C min⁻¹ between 60 °C and 90 °C is required on overflow-wheel machines. Sodium chloride at 10–20 g/L, added in three portions at 60 °C, 75 °C, and 90 °C, maintains liquor exchange without inducing dye aggregation. Full salt addition at the start of the run on jiggers has produced colour differences of 0.4–0.6 ΔE⁎ between selvedge and centre, assessed under D65/10° conditions according to ISO 105-A01 and CIELAB colour space designated in ISO 11664-4. Mercerised fabric may require a dyestuff reduction of 10–15% relative to the same depth on non-mercerised fibre to prevent overtoning. Hard water above 150 mg/L CaCO₃ precipitates the sulphonated dye as calcium salts; this is controlled with 0.5–1.0 g/L tetrasodium pyrophosphate or 0.2–0.5 g/L EDTA. The resulting fault appears as a dull, reddish black with visible deposition on polyester felt filters. Published data for Direct Black G-specific migration kinetics on mercerised lyocell blends is limited; process validation is therefore required before rapid pad-batch cycles are specified.
At production scale, package dyeing of Direct Black G on 1.0 kg cotton packages in 100 kg vertical spindle machines with reversible flow has shown that circulation rate is a more important variable than dyebath pH for levelness. A circulation rate of 30–40 L kg⁻¹ min⁻¹ provides acceptable penetration at package density 0.36–0.40 g cm⁻³; rates below 25 L kg⁻¹ min⁻¹ produce pale interiors, while rates above 45 L kg⁻¹ min⁻¹ may distort the yarn layers and channel the flow. Reversal intervals of 2–3 min are used for high-density spun yarn packages. The dye is metered over 15–20 min using a dosing tank maintained at 60 °C; batch-to-batch variation in dye strength is normalised to the reference lot by absorbance measurement before weighing. Dyehouse scale weight adjustment follows instrumental evaluation according to ISO 105-A05. The liquor ratio is held at 1:8 to 1:12; lower ratios reduce water consumption but increase electrolyte concentration and aggregation risk if salt is added too rapidly. A 100 µm circulation-loop filter removes undissolved particles that would otherwise lodge inside the package. Humidity above 60% RH in the weigh room can increase caking and cause 0.5–1.0% moisture gain in 4 h of exposed storage; climate-controlled weigh stations and sealed sacks are therefore specified.
When a padder replaces sodium chloride with sodium sulphate, the dyebath viscosity and wet pick-up shift. Sodium sulphate is less corrosive to stainless steel pump heads and reduces chloride discharge to effluent, but its solubility at 25 °C is approximately 280 g/L; concentrated pad liquors containing 80 g/L sodium sulphate and 20 g/L Direct Black G can crystallise in unheated troughs below 15 °C. The padder should be equipped with a temperature-controlled trough at 25–30 °C and a horizontal nip set to 2–3 bar. Wet pick-up on cotton poplin is typically 70–80% at 2 bar; edge-to-centre colour difference after continuous drying is held below 0.5 ΔE⁎ when a migration inhibitor at 5–10 g/L is included. Drying on a pin stenter with first-zone temperature 110 °C and final-zone temperature 150 °C reduces frost marks; direct gas impingement without pre-drying below 100 °C forces dye to the fabric surface and lowers ISO 105-X12 wet rub fastness by one grade. On jigger application, sodium chloride at 15 g/L is added after the first two ends to avoid tailing. When chloride restriction is driven by wastewater permits, sodium sulphate substitution is technically workable but requires trough heating and a migration inhibitor; otherwise the frost-mark defect is the dominant failure mode. Published data for continuous pad-steam configurations with Direct Black G on lyocell is limited.
Comparative dyeings on bleached cotton twill at 2% depth of shade show that Direct Black G offers lower chemical cost and shorter process time than C.I. Reactive Black 5, but its wet fastness is lower and its regulatory status is more restrictive. Direct Black G is compared with C.I. Direct Black 22, a metal-free direct black with no benzidine pathway, and C.I. Reactive Black 5, a vinyl sulphone reactive dye that undergoes covalent fixation under alkali. The table below summarises typical fastness and process values obtained under laboratory exhaust conditions; fastness grades are reported as average values per ISO grading scales.
| Property | Direct Black G / C.I. Direct Black 38 | C.I. Direct Black 22 | C.I. Reactive Black 5 |
|---|---|---|---|
| Fixation mechanism | Hydrogen bonding and van der Waals adsorption on cellulose | Direct-dye adsorption mechanism | Covalent ether bonding under alkaline activation |
| Salt requirement | 10–30 g/L NaCl | 10–30 g/L NaCl | 40–80 g/L Na₂SO₄ plus 10–20 g/L Na₂CO₃ |
| Wash fastness ISO 105-C10 | 2–3 change, 3 staining | 3 change, 3–4 staining | 4–5 change, 4–5 staining |
| Light fastness ISO 105-B02 | 4 | 3–4 | 5 |
| Wet rub ISO 105-X12 | 2 | 2–3 | 3–4 |
| Regulatory trigger | Benzidine-derived; restricted under REACH Annex XVII Entry 43 if releasable benzidine > 30 mg/kg | No benzidine pathway; routine aromatic amine screening applies | No aromatic amine restriction of the same class |
In machine terms, reactive dyeing requires additional alkali dosing, a longer washing-off sequence, and higher thermal energy; direct dyeing with Direct Black G requires only salt, heat, and a cationic aftertreatment if wet fastness is specified. However, direct dyeings do not reach the wash fastness needed for intensive domestic laundering without a fixative. The difference is typically one to two grey-scale grades under ISO 105-C10. Direct Black G is therefore selected where cost and short processing time are the primary constraints, and where aromatic amine compliance limits are not exceeded.
Aftertreatment of Direct Black G with a polymeric cationic fixing agent at 2–4 g/L in a fresh bath at pH 5.0–5.5 and 40 °C for 20 min complexes the anionic sulphonate groups and reduces redistribution during washing. Typical improvement is one grey-scale grade; a dyeing graded at 2–3 for change under ISO 105-C10 can reach 3–4 after fixation. The trade-off is measured under ISO 105-B02: light fastness can decrease by 0.5–1.0 grade because the cationic polymer may accelerate photofading of the azo chromophore. Wet rub fastness after fixation remains limited to 2–3 under ISO 105-X12. Heavy rinse-off is required to remove unfixed dye before fixation, otherwise surface dye is locked at the fibre surface and crocking worsens. On package machines, the fixative is added slowly over 10 min to prevent local precipitation with residual anionic dispersant; a precipitation event appears as pale spots and requires a full alkaline wash. Dyeings for perspiration-sensitive work should be tested under ISO 105-E04 because acidic perspiration can shift the black shade redder and reduce fastness by one grade. At addition levels above 4 g/L, the fixative may lower water absorbency of the garment, which is measurable by wicking height tests.
The central operational boundary for Direct Black G is its benzidine linkage. The dye is not classified as hazardous solely on the basis of its polyazo structure, but reductive cleavage can liberate benzidine, which is a Category 1 carcinogen. For textile and leather articles that come into direct and prolonged contact with skin, EU REACH Annex XVII Entry 43 prohibits detectable release of benzidine above 30 mg/kg from the final article; analytical confirmation for textiles is performed by EN 14362-1:2012, and for leather by ISO 17234-1:2011. Dyehouse effluent should not be treated with aggressive reducing agents without subsequent aromatic amine monitoring, because decolorisation of the azo bond can regenerate benzidine before complete mineralisation. Dehydrated sludge from such effluent may require disposal as amine-bearing industrial waste under national permitting. Direct Black G is therefore not the appropriate product for brands requiring benzidine-free certification; in those programmes, C.I. Direct Black 22 or C.I. Reactive Black 5 is specified. For food-contact packaging under FDA 21 CFR or BfR Recommendation XXXVI, no clearance is assumed and published migration data for Direct Black G in polymer films is limited. Accordingly, the product is confined to industrial dyeing of cellulosic textiles, papermaking, and leather where regulatory testing and effluent controls are in place.
