Section
| HS Code | 697448 |
| Product Name | Direct Sky Blue 6B |
| Colour Index Name | C.I. Direct Blue 6 |
| Colour Index Number | 22610 |
| Cas Registry Number | 2429-74-5 |
| Molecular Formula | C32H20N6Na4O14S4 |
| Molecular Weight | 992.85 g/mol |
| Chemical Class | Disazo direct dye |
| Appearance | Dark blue powder |
| Colour Shade | Blue |
| Water Solubility | Soluble in water |
| Ionic Character | Anionic |
| Substrate Affinity | Cellulose fibres, paper, and leather |
| Typical Dyeing Temperature | 90-95 °C |
| Light Fastness | Moderate, approximately 3-4 |
| Washing Fastness | Moderate, approximately 3 |
| Storage Conditions | Cool, dry, well-ventilated area away from light and moisture |
As an accredited Direct Sky Blue 6B factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | Direct Sky Blue 6B is supplied in 25 kg moisture-proof paper sacks with inner plastic liners, suitable for industrial dye storage. |
| Container Loading (20′ FCL) | Direct Sky Blue 6B chemical is packed in drums/bags and loaded into a 20′ FCL container, securely braced for export transport. |
| Shipping | Direct Sky Blue 6B is generally shipped as a non-hazardous industrial dye in sealed, labeled, moisture-proof containers. Store and transport in a cool, dry, ventilated area away from oxidizers and direct sunlight. Follow the SDS and local transport regulations; avoid dust generation and container damage during transit. |
| Storage | Store Direct Sky Blue 6B in a tightly closed, labeled original container in a cool, dry, well-ventilated area away from direct sunlight, moisture, acids, and oxidizing agents. Protect from dust, avoid inhaling dust, and prevent spills. Keep separate from food, feed, and drinking water. Use appropriate personal protective equipment when handling and follow local regulations. |
| Shelf Life | Stable when stored cool, dry, and away from light; typical shelf life is two years in sealed containers. |
Direct Sky Blue 6B, listed as C.I. Direct Blue 1, is applied to cotton, viscose, modal, and lyocell knit goods by exhaust dyeing without a mordant. The dye is dissolved separately at 80–90°C under mechanical agitation; undissolved aggregates block screen filters and cause specking in jet overflow machines. A stock concentration above 20 g/L is avoided because the tetrasulfonated bisazo dye tends to aggregate in cold pipework. The dyebath is set at 40–50°C with a liquor-to-goods ratio of 1:10–1:20 for jet overflow and 1:6–1:10 for air-jet machines. Sodium sulfate or sodium chloride is added in two portions to control initial substantivity. The first 5–10 g/L is added after 10 min, and the remaining 5–15 g/L is added after 25 min. The bath is raised to 90–95°C over 30–45 min and held for 45–60 min. At 2% on weight of fabric, light fastness under ISO 105-B02 normally falls in the 2–3 grade range. Washing fastness under ISO 105-C06 C2S is typically 2 before fixation. The dye is derived from o-dianisidine. For EU-bound textile articles, finished goods are screened under EN ISO 14362-1; free o-dianisidine must not exceed 30 mg/kg in the final article under REACH Annex XVII Entry 43. End products include lining fabrics, decorative warp knits, craft textiles, and casual garments where the bright azure tone is prioritized over high wet-fastness.
| Parameter | Range / value | Test method / equipment |
|---|---|---|
| Dye dissolution temperature | 80–90°C | High-shear mixing tank |
| Dyebath start temperature | 40–50°C | Jet overflow or air-jet machine |
| Electrolyte total addition | 10–25 g/L | Sodium sulfate or sodium chloride |
| Final dyeing temperature | 90–95°C | Exhaust dyeing machine |
| Light fastness at 2% owf | 2–3 blue scale grade | ISO 105-B02 |
| Washing fastness before fixation | 2 grade | ISO 105-C06 C2S |
| Free o-dianisidine in finished textile | <30 mg/kg | EN ISO 14362-1 |
On package dyeing machines, levelness is controlled less by liquor ratio than by pump flow direction and package density. Cotton yarn cones are wound to 0.36–0.42 kg/L. Higher density forces flow through the core and produces a darker inside layer. The pump is operated at 30–50 L/kg/min, and flow direction is reversed every 180–300 s. Unidirectional flow beyond 5 min creates a radial shade gradient that cannot be corrected by extended migration time. Because Direct Sky Blue 6B has strong substantivity at 60°C, dye addition begins at 40°C and electrolyte is withheld until the dye has circulated for 10–15 min. Full electrolyte is then metered over 20–30 min to avoid tailing. Hard water above 10 °dH should be corrected with a polyphosphate or polycarboxylate sequestrant. Calcium ions bridge sulfonate groups and form dye aggregates that deposit on the winding core. After the 90°C hold, the bath is cooled to 60°C before the first rinse to reduce thermal shock on package columns. For EU export, dyed yarns are tested under EN ISO 14362-1 after stripping only when the final article falls within the skin-contact scope of REACH Annex XVII Entry 43. End uses in yarn-dyed form include woven shirting stripes, canvas footwear trim, and crochet yarns for articles washed infrequently.
In papermaking, Direct Sky Blue 6B is added at the wet end after refining and before headbox screening. Dose rates range from 0.02% to 0.5% on dry fiber mass depending on target shade. Pastel tissue requires 50–200 mg/kg, while deep cover paper requires 0.3–0.5%. The anionic dye binds to cellulose by hydrogen bonding and van der Waals forces, but retention in an alkaline fine-paper furnish is poor without a cationic mordant. Poly-DADMAC at 0.05–0.2% on dry fiber improves retention to 85–95% measured by filtrate absorbance at 620 nm. Alum at 0.5–1.5% on dry fiber works at pH 4.5–5.5. Above pH 6.0, aluminum hydrolysis products lose charge density and dye bleed increases. Process water hardness above 15 °dH reduces anionic charge density and shifts retention downward. The dyed stock is passed through a headbox at 0.3–1.0% consistency. Excessive turbulence at the slice causes shade variation because fine fiber fractions absorb dye more rapidly. End products include blue-tinted specialty papers, non-food-contact packaging board interlayers, and decorative tissue where direct dye migration is controlled by a closed wet-end loop.
When Direct Sky Blue 6B is applied at the size press or in a coating kitchen, the operating window narrows because the dye is an anionic aromatic sulfonate and can interfere with cationic starch viscosity. A solution of 0.1–0.5% dye is metered into oxidized starch at 55–65°C. Starch solids are held at 6–10%. Cationic starch above 0.3% on dry solids can flocculate the dye before the nip, producing visible blue specks on the sheet. Size press pick-up is controlled at 1.5–3.0% dry pick-up. Below 1.0%, the shade is too light for visual match. Above 3.5%, the sheet becomes embrittled. The dyed surface is dried with cylinders at 90–110°C. Residence time longer than 30 s at 110°C can shift the shade greener due to partial reduction of the azo chromophore in reductive papermaking broke. End products include technical cover stock, folder board, and non-food-contact packaging where surface brightness and uniform blue tone are specified.
In histochemical laboratories, the same dye is used as a collagen contrast stain under the synonym Chicago Sky Blue 6B. Stock solutions are prepared at 0.1–1.0% w/v in distilled water and acidified with acetic acid to 0.5–2.0%. Tissue sections fixed in neutral buffered formalin are stained for 10–30 min. Frozen sections require shorter times of 2–5 min because direct dye binding to collagen is rapid. Differentiation in 0.5% acetic acid removes background staining from elastin and cytoplasm. The stain is compatible with alum hematoxylin counterstaining, but picrate-containing fixatives reduce uptake by competing for cationic amine groups on tissue proteins. Published data for quantitative collagen morphometry using this dye are limited. Fluorescence microscopy is not reliable because the molecule has low quantum yield. Standard product specifications are not used in this segment; internal staining protocols are validated against archived control sections. End products are stained slides, frozen-section archive material, and teaching specimens.
Direct Sky Blue 6B can be applied to vegetable-tanned or chrome-tanned leather as a surface tone, but its use is constrained by low wet-fastness and reductive cleavage risk under REACH. In drum dyeing, the dye is added at 0.5–2.0% of wet-blue or vegetable-tanned weight after neutralization to pH 5.5–6.5. The drum runs at 20–30 rpm with a float of 150–300% at 45–55°C. Penetration depth is generally shallow. The dye is therefore used as a topping agent rather than a through-dye. Fatliquoring before dyeing reduces surface adsorption and should be reversed when a heavy surface blue is specified. Wet fastness under ISO 11640 is typically no better than 2–3 on the grey scale without cationic fixing. For EU-bound leather goods, free o-dianisidine is determined under EN ISO 17234-1; the limit remains 30 mg/kg under REACH Annex XVII Entry 43. End products include bookbinding covers, lining leather, and low-flex decorative panels that are not subjected to repeated washing.
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Direct Sky Blue 6B is supplied under the Colour Index Generic Name C.I. Direct Blue 1, C.I. constitution number 24410, and Chemical Abstracts Service Registry Number 2610-05-1. The commercial product is the tetrasodium salt of a disazo dye; its empirical formula is C34H24N6Na4O16S4 and its relative molecular mass is 992.79 g/mol. The molecule carries four sulfonate groups, which provide anionic character and substantial water solubility under neutral and alkaline dyebath conditions. The visible absorption maximum in neutral aqueous solution is reported between 610 nm and 620 nm, corresponding to a reddish-blue to sky-blue hue on cellulosic substrates.
No ISO or EN model code exists for this colourant; procurement is therefore keyed to the Colour Index designation and the CAS number rather than to a proprietary model number. Where supplier-specific grade codes are used, they separate technical powder, low-salt powder, and purified stain-grade material marketed under the synonym Chicago Sky Blue 6B. The grade differences are not reflected in colour index nomenclature and must be verified from the certificate of analysis. Commercial powders are commonly standardised with sodium sulfate or sodium chloride to a batch-specific colour strength. The diluent type and residual free amine content should be declared because both influence dyeing behaviour and regulatory status.
Exhaustion uniformity is controlled primarily by temperature ramp, electrolyte addition profile, and liquor ratio. On vertical package dyeing machines operating at a liquor ratio of 1:8 to 1:12, a dyebath is started at 40 °C and raised to 95 °C at 1.0–1.5 °C/min. Sodium sulfate is introduced in divided portions—commonly at 40 °C, 60 °C, and 90 °C—to a final concentration of 10–30 g/L for medium-depth shades. A single-shot salt addition at the start of the cycle reduces the negative surface charge of the cellulose too rapidly and produces unlevelness on high-density packages with differential flow resistance. For shades above 2.0% on weight of fibre, a dispersing agent is added before the final salt increment; without it, dark-blue agglomerates can form and block beam or package-machine filters.
In the absence of electrolyte, the sulfonated dye and the negatively charged cellulose repel one another. Addition of sodium sulfate compresses the electrical double layer and permits van der Waals and hydrophobic contact between dye and fibre; final exhaustion values above 85% are typical for well-optimised exhaust dyeing, although published data for specific equipment configurations is limited. Continuous pad-steam operation uses a pad bath containing 10–40 g/L dye and 5–20 g/L sodium sulfate at 60–80% pick-up. Fixation is performed in saturated steam at 102 °C for 60–90 s. Intermediate drying must remain below 120 °C; higher temperature promotes dye migration toward water-exposed surfaces and creates edge-to-centre shade variation. On a woven cotton range running at 40 m/min, infrared pre-drying before drum drying reduces migration defects. Hard water above 100 mg/L as CaCO3 should be sequestered with polyphosphate or EDTA-based auxiliary; calcium and magnesium salts of the sulfonated dye show reduced solubility and may deposit as dark specks.
Quality-control release of Direct Sky Blue 6B is performed by dissolving the powder in deionised water at 80 °C and measuring absorbance at the visible maximum against a reference lot. A typical calibration range uses 0.01–0.05 g/L dye in a 10 mm quartz cell, with maximum absorbance held between 0.2 and 0.8 AU to remain within the linear response of the spectrophotometer. Residual insolubles are assessed by filtration through a 10 µm polypropylene membrane; a rising insolubles value across batches often indicates hard-water contamination or incomplete clearing during manufacture. Moisture content is determined by loss on drying at 105 °C. Caked powder from humid storage should not be used until dye strength is rechecked. Stock solutions at 10–20 g/L are practical for machine dosing, but held solution should be used within 8 h unless clarity and pH are verified. Lot-to-lot variance in dye strength is usually caused by diluent standardisation rather than by a change in the chromophore. A certificate of analysis including dye content, moisture, insolubles, and residual 3,3'-dimethylbenzidine is required for lot acceptance. For stain-grade material, supplier-specific limits on heavy metals and residual salts apply, and solubility in histology protocols is sensitive to these residuals.
In comparison with C.I. Direct Blue 15, Direct Sky Blue 6B carries methyl substituents on the central biphenyl unit, whereas Direct Blue 15 carries methoxy groups. This structural difference alters solubility, aggregation tendency, and shade position; Direct Blue 1 is generally perceived as a reddish sky blue, while Direct Blue 15 is a greener blue. Instrumental ΔE values under ISO 105-J03:2009 can exceed 1.5 CIELAB units between the two dyes at equal visual depth. Direct Blue 1 does not form covalent bonds with cellulose; wet fastness is therefore controlled by molecular aggregation inside the fibre and by post-fixation with cationic fixatives. Under ISO 105-C06:2010, unfixed Direct Blue 1 on cotton shows moderate staining; a cationic fixative applied at 2–5% on weight of fibre in a fresh bath after rinsing can improve the wash-fastness rating by one to two grey-scale steps. The trade-off is that some fixatives reduce light fastness under ISO 105-B02:2014 and shift the shade duller.
Against C.I. Direct Blue 199, a phthalocyanine direct dye, Direct Sky Blue 6B provides a brighter reddish-blue shade and easier stripping with reducing agents, but generally lower light fastness. Direct Blue 199 also requires more aggressive vessel cleaning because phthalocyanine residues can contaminate subsequent pastel batches. Compared with C.I. Reactive Blue 19, Direct Sky Blue 6B avoids alkaline fixation and the associated hydrolysed dye loss, but lacks the covalent cellulose-dye bond that reactive blue chemistry provides. Selection therefore turns on the end-use specification: reactive blue is used where high wash fastness is required; Direct Blue 1 is used where simple salt-only fixation and lower process cost are primary constraints.
| Property | C.I. Direct Blue 1 | C.I. Direct Blue 15 | C.I. Direct Blue 199 | C.I. Reactive Blue 19 |
|---|---|---|---|---|
| Chromophore class | Disazo | Disazo | Phthalocyanine | Anthraquinone/vinyl sulfone |
| Key substitution | 3,3'-dimethylbiphenyl | 3,3'-dimethoxybiphenyl | Copper phthalocyanine | Reactive vinyl sulfone anchor |
| Fixation to cellulose | Non-covalent substantivity | Non-covalent substantivity | Non-covalent substantivity | Covalent bonding under alkali |
| Shade position | Reddish sky blue | Greener blue | Turquoise | Royal to navy blue |
| Wet-fastness control | Cationic fixative required | Cationic fixative required | Fixative or aftertreatment depending on depth | Alkali fixation followed by soaping-off |
Regulatory compliance for finished textile articles containing Direct Sky Blue 6B centres on the potential reductive cleavage of the azo bond to release 3,3'-dimethylbenzidine, a restricted aromatic amine. Under REACH Regulation (EC) No 1907/2006, Annex XVII Entry 43, textile and leather articles that come into direct and prolonged contact with human skin or the oral cavity must not release this amine above 30 mg/kg of finished article when tested according to EN ISO 14362-1:2017. Because Direct Blue 1 is manufactured from 3,3'-dimethylbenzidine, lot-specific certification is required; a certificate that only reports total dye content is insufficient. The dye is not intended for food-contact use and is not listed in 21 CFR colour additive regulations. Discharge printing and reductive stripping operations require contained processing, because reducing conditions such as sodium hydrosulfite at 80 °C and pH 11 intentionally cleave the azo bonds and may liberate regulated amine fragments.
Operational boundaries include avoiding addition of cationic fixatives to the main dyebath; the anionic sulfonate groups of the dye form insoluble complexes with quaternary ammonium compounds and produce precipitation. Fixation with a cationic agent should be performed in fresh liquor after rinsing. Powder storage in bulk-bag unloading or silo feeding should be maintained below 60% relative humidity. Caked product can be screened through a 500 µm sieve, but dye strength must be revalidated after de-agglomeration. Avoid reductive stripping of Direct Sky Blue 6B on the same equipment without an alkali wash, because residual amine contamination can cross-contaminate goods destined for skin-contact markets.
