Section
| HS Code | 146580 |
| Productname | Direct Sky Blue 5B |
| Colorindexname | C.I. Direct Blue 15 |
| Colorindexnumber | C.I. 24400 |
| Casnumber | 2429-74-5 |
| Chemicalclass | Disazo direct dye |
| Molecularformula | C34H24N6Na4O16S4 |
| Molecularweight | 992.8 g/mol |
| Appearance | Dark blue powder |
| Colorshade | Bright blue |
| Watersolubility | Soluble in water |
| Applicationsubstrate | Cellulose fibers such as cotton and viscose |
| Dyeingmethod | Exhaustion direct dyeing |
| Dyeingtemperature | 80-100 °C |
| Electrolyterequirement | Sodium sulfate or sodium chloride is commonly used |
| Phrange | Neutral to slightly alkaline, approximately pH 7-9 |
| Lightfastness | Moderate |
| Washingfastness | Moderate |
| Storageconditions | Store in a cool, dry, well-ventilated place away from moisture and direct sunlight |
As an accredited Direct Sky Blue 5B factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | Direct Sky Blue 5B is packed in 25 kg net fiber drums with inner moisture-resistant plastic liners for secure storage. |
| Container Loading (20′ FCL) | Direct Sky Blue 5B in 25 kg bags on pallets, loaded into a 20′ FCL container for secure export shipment. |
| Shipping | Direct Sky Blue 5B is shipped as a non-hazardous chemical dye; it is not regulated for transport. Keep containers closed, dry, and protected from moisture, light, and contamination. Avoid dust generation and use standard industrial hygiene. No UN number or hazard class required. |
| Storage | Store Direct Sky Blue 5B in a cool, dry, well-ventilated area away from direct sunlight, heat, and ignition sources. Keep containers tightly closed and clearly labeled, preventing dust generation and moisture ingress. Separate from strong oxidizers, acids, and incompatible materials. Use appropriate secondary containment and follow local regulations. Wear suitable PPE when handling. Maintain good housekeeping to avoid spills and contamination. |
| Shelf Life | Direct Sky Blue 5B has a shelf life of about 24 months when kept dry, sealed, and away from light and moisture. |
In exhaust dyeing of cellulosic knit constructions intended for industrial and technical textile uses, Direct Sky Blue 5B (C.I. Direct Blue 15, CAS 2429-74-5) is metered at 0.5–2.5% on weight of fibre (o.w.f.) into a dyebath buffered to pH 6.5–7.5 with sodium acetate/acetic acid. The dye's tetrasulfonate structure produces only moderate strike-up below 60 °C; accelerated exhaustion begins after staged addition of sodium chloride or sodium sulfate in 5 g/L increments every 10 min up to a total electrolyte concentration of 15–40 g/L. On a soft-flow overflow machine with a liquor ratio of 1:8–1:12, a temperature ramp of 1.0–1.5 °C/min to 85–95 °C and a hold time of 30–45 min are required to reach adequate penetration on single-jersey cotton and cotton/viscose blends; nozzle pressure is maintained at 0.4–0.8 bar and fabric speed at 200–300 m/min to avoid rope marks. After dyeing, the substrate is rinsed and may be aftertreated with a cationic fixative, because direct dye wet fastness remains the primary operational liability measured by ISO 105-C10:2013. Compliance for EU-bound skin-contact articles is constrained by Regulation (EC) No 1907/2006 Annex XVII Entry 43: the chromophore contains the bisazo component 3,3'-dimethylbenzidine, which must be quantified by EN ISO 14362-1:2017; a released amine concentration above 30 mg/kg in the finished textile makes the article non-compliant for skin-contact applications. For this reason, the recommended downstream products are industrial wiping cloths, filtration media, technical interlinings, and other non-apparel textile articles that fall outside the skin-contact scope of Entry 43 or are destined for jurisdictions with equivalent technical testing rather than automatic exclusion.
Wet-end dosing on a fourdrinier or twin-wire paper machine running 600–1200 m/min introduces Direct Sky Blue 5B as a 5–10 g/L stock solution into thick stock at 2–4% consistency after refining; the addition ratio on dry fibre is 0.01–0.20% for pale to deep blue shades in beater dyeing. Because the dye is anionic, first-pass retention depends on cationic polyacrylamide or polyaluminium chloride addition at 0.05–0.10% on dry fibre; without retention aid, dye loss to white water increases and shade depth per unit of dye added falls. For surface size-press coloration, the dye is metered into an oxidised starch bath at 8–12% solids and 55–65 °C, with dye concentration controlled at 0.2–1.5 g/L; rod metering or puddle size press application is followed by after-dryer temperatures of 110–130 °C. Compliance for food-contact paper is not automatically granted under EU Regulation 1935/2004 or FDA 21 CFR 176.170, and no blanket approval is presumed; the dye is therefore recommended only for non-food decorative and industrial paper grades. Terminal products include decorative laminating base paper, file folders, industrial wipes, non-food gift wrap, and coloured kraft for technical wrapping.
After desalination of Direct Sky Blue 5B prior to ink formulation, piezoelectric nozzle stability is determined principally by residual electrolyte content and particle count. The dye is dissolved in deionized water with conductivity below 10 µS/cm, then processed by nanofiltration or ultrafiltration until the retentate conductivity does not exceed 100 µS/cm after dilution to the target dye loading of 1.0–3.5 wt%. The aqueous ink matrix includes 10–20 wt% humectant, 5–15 wt% water-soluble co-solvent, 0.5–1.5 wt% nonionic surfactant, and 0.1–0.2 wt% biocide; final viscosity is adjusted to 2.0–4.5 mPa·s, surface tension to 28–38 mN/m, and pH to 7.5–9.0 before filtration through a 0.45 µm membrane followed by a 0.22 µm absolute-rated cartridge. Compliance for writing inks and inkjet refills sold into the EU requires documentation under REACH; for children's stationery, chemical migration assessment under the EN 71 series is required, although EN 71-3:2019 alone addresses only inorganic element migration and does not substitute for dye-specific toxicological screening. The principal technical limit is light fastness: direct dye-based inks exhibit lower resistance to fade than pigment-based systems, so the recommended terminal types are plain-paper desktop inkjet cartridges, refill bottles, highlighter inks, and absorbent-pad writing inks for indoor documents rather than outdoor or archival prints.
For split suede and industrial leather stock, a neutralised wet-blue surface pH is required before Direct Sky Blue 5B is added at 0.2–1.5% on shaved weight in a rotating drum at 6–10 rpm. Float volume is set at 200–300% and bath temperature at 40–50 °C; the dyebath pH is held between 5.0 and 6.5, and the drum is run for 20–40 min after dye addition. Direct dye fixation on leather is weaker than acid dye fixation; therefore a cationic aftertreatment or formic acid-assisted exhaustion is applied before rinsing to reduce crocking. Compliance for leather articles placed on the EU market is governed by EN ISO 17234-1 when testing for reductive cleavage of restricted amines under REACH Annex XVII Entry 43; because the dye can release 3,3'-dimethylbenzidine, finished leather components intended for prolonged skin contact must not exceed 30 mg/kg of the free amine. Recommended terminal products are consequently non-skin-contact technical leathers such as book covers, industrial gaskets, strap reinforcement, and luggage components sold in non-EU markets or with batch test results verifying the limit.
Because the critical defect in continuous pad dyeing of woven cotton and cotton/viscose fabrics is not bath exhaustion but differential migration of dye solids toward the fabric surface during pre-drying, pad liquor is prepared at 5–20 g/L Direct Sky Blue 5B with 5–10 g/L electrolyte, and the trough temperature is held at 40–60 °C to maintain solubility while limiting viscosity drift. Nip pressure at the padder is set to achieve 70–80% wet pick-up; infrared pre-drying at 110 °C reduces moisture content to below 30% before cylinder drying at 120–130 °C, because the highest migration risk occurs between 30% and 8% residual moisture. A moisture sensor across the fabric width should record a deviation of no more than ±2% entering the cylinder section; larger gradients produce frosty, surface-heavy dye distribution and lower crock fastness. Process validation uses ISO 105-C10:2013 for wash fastness and ISO 105-X12:2016 for rubbing fastness. Compliance mirrors textile requirements: EN ISO 14362-1:2017 must be applied to woven articles that may contact skin under REACH Annex XVII Entry 43; because of the 3,3'-dimethylbenzidine cleavage pathway, the 30 mg/kg free amine limit controls EU market access. Terminal products are woven twill, canvas, and plain-weave technical fabrics for industrial workwear only where batch testing supports compliance, or for non-EU applications in cargo covers, shade cloth, and binding tape.
Published data for this specific dye in viscose dope are limited, and pilot-scale validation is required before production adoption. Direct Sky Blue 5B is dissolved in softened water and metered into the viscose solution at 0.05–0.8 wt% on α-cellulose content before final ripening; the dye's sulfonated structure is compatible with alkaline viscose, but its anionic charge can alter spinning solution filtration characteristics and spinnability, so candle filters of 15–20 µm are installed before the spinneret. The coagulation bath typically contains 80–130 g/L sulfuric acid, 180–260 g/L sodium sulfate, and 10–20 g/L zinc sulfate at 50–55 °C; dye loss to the acid bath occurs during cellulose regeneration, and effluent colour load must be managed before discharge. Terminal products include coloured viscose staple fibre for technical nonwovens, industrial felts, embroidery threads, and specialty paper reinforcement fibre. Compliance for final textile or nonwoven articles sold in the EU follows EN ISO 14362-1:2017 and REACH Annex XVII Entry 43; the same 30 mg/kg restricted amine limit applies to skin-contact viscose articles, which restricts this route to non-skin-contact or non-EU technical products unless reduced amine formation is demonstrated.
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Direct Sky Blue 5B is the commercial designation for the water-soluble disazo direct dyestuff indexed as C.I. Direct Blue 15, CAS 2429-74-5, C.I. 24400. The product is supplied as a dark blue powder or granular solid, standardised with sodium sulfate or sodium chloride to 100% strength against a manufacturer’s reference lot. The chromophore is based on a 3,3′-dimethoxybiphenyl-4,4′-diyl bis(azo) framework coupled to two 5-amino-4-hydroxy-2,7-naphthalenedisulfonic acid residues; the tetrasodium salt form has an approximate relative molecular mass of 992.8 g/mol. Aqueous solutions show a visible absorption maximum in the 580–620 nm range at pH 6–8, producing a bright blue shade with weak green undertones. Commercial specifications typically control moisture by ISO 787-2 at ≤8.0%, water-insoluble matter by hot-water filtration at ≤0.5%, and pH of a 1% aqueous solution at 7.0–9.0.
Optical strength is quantified by UV/VIS spectrophotometry at the absorption maximum against a certified reference lot; bulk dyehouse contracts usually adopt a tolerance of ±3%. Product differences between suppliers arise mainly from the standardising filler, residual electrolyte content, granulation method, and dusting tendency. The colouristic core is identical, but the dyeing rate, solubility, and suspending characteristics in high-concentration stock solutions are not interchangeable without re-qualification. Compared with fibre-reactive blue chromophores such as C.I. Reactive Blue 19, Direct Sky Blue 5B does not contain a vinyl sulfone or monochlorotriazine leaving group and therefore does not form a covalent cellulose bond. Its substantivity for cotton, viscose, modal, and lyocell results from the coplanar disazo structure, sulfonate groups, and hydrogen bonding with cellulosic hydroxyl groups. This distinction is operationally significant: exhaust dyeing with Direct Sky Blue 5B does not require sodium carbonate addition, but laundering, perspiration, and water-contact fastness are lower unless a cationic fixative is applied.
The product is classified as a direct dye for medium-fastness applications rather than a fibre-reactive dye for high wet-fastness end-uses. Compared with pigment blue 15:3 colouration on cotton, Direct Sky Blue 5B does not need a film-forming binder, but its crock fastness under ISO 105-X12 is generally lower in deep shades. Against acid blue dyes for nylon, it exhibits less build-up and inferior wet fastness, though it may stain polyamide components in mixed loads. Direct Sky Blue 5B has no substantivity for polyester; on polyester-cotton blends, only the cotton component is coloured, and the polyester remains white unless a disperse dye is applied in a separate step.
| Parameter | Typical acceptance interval | Test procedure |
|---|---|---|
| Strength vs reference | 100±3% | UV/VIS spectrophotometry at λmax |
| Moisture content | ≤8.0% | ISO 787-2 |
| Water-insoluble matter | ≤0.5% | Hot-water extraction and gravimetric filtration |
| pH of 1% aqueous solution | 7.0–9.0 | Electrometric pH measurement |
| Visible λmax in distilled water | 580–620 nm | UV/VIS spectrophotometer |
In exhaust dyeing on cotton knitgoods, the dyestuff is added to a bath set at 40–60 °C, and then sodium sulfate is charged in divided portions. Typical production recipes for 1.0–3.0% owf use 10–20 g/L anhydrous sodium sulfate at a liquor ratio of 1:10–1:20. The electrolyte suppresses electrostatic repulsion between the sulfonated dye and the cellulosic surface. If salt is added too rapidly, dye strike at liquor entry points becomes uneven and results in listing or shade variation across the fabric width. Excess salt above 25 g/L can aggregate the dyestuff and reduce penetration on tightly wound packages.
Package dyeing machines require controlled heating between 60 °C and 95 °C; a temperature rise of 1–2 °C/min is commonly specified to limit radial shade deviation from outer to inner layers. The bath is held at 90–95 °C for 30–60 min, depending on depth and liquor flow reversal. Below 80 °C the diffusion rate into secondary cell walls decreases; above 100 °C the risk of dye aggregation and shade dulling increases, particularly in hard water. Process water with calcium ion concentrations above 150 mg/L should be sequestered with tetrasodium EDTA or a polyacrylate dispersant; otherwise insoluble calcium sulfonate precipitates may deposit as dark blue specks.
After the exhaust phase, the bath is cooled to 70 °C before sampling. Sampling at the boil can produce a misleading shade assessment because thermochromic shifts and residual electrolyte affect the apparent depth. Production-scale checks are normally carried out under D65 illumination at 10° observer geometry, with ΔE tolerances of ≤0.5 CIELAB units against a laboratory reference.
Pad-batch and continuous operations require different handling because the dyestuff remains mobile until drying is complete. A pad bath containing Direct Sky Blue 5B at 10–30 g/L is applied at 60–80% wet pickup on cotton broadcloth. The fabric is batched for 4–12 h at 20–35 °C under polyethylene film to prevent edge drying. Without alkali fixation, the main process failure is migration to the fabric surface during slow drying. Migration is controlled by adding 5–10 g/L urea and by maintaining residual moisture before drying. In continuous ranges, the dye is applied by slop-padding followed by infrared pre-drying at 80–110 °C and cylinder drying. Cylinder temperatures above 130 °C can yellow residual reducing sugars or finishing auxiliaries and shift the shade toward green.
For paper wet-end addition, Direct Sky Blue 5B is introduced at the fan pump or size press depending on retention configuration. The strong anionic charge gives high retention on bleached kraft pulp in the presence of cationic retention aids, but the product is sensitive to alum coagulation at low pH. A stock pH below 5.0 can reduce colour strength and increase white water tinting. In leather processing, the dyestuff is added after retanning in a drum at 60–70 °C and 1.0–3.0% of wet-blue weight. Penetration in vegetable-tanned leather is slower; extended drumming is required to avoid dark grain and pale suede sides.
Without aftertreatment, Direct Sky Blue 5B shows moderate wet fastness on cotton. Commercial fastness maps typically quote light fastness in the 3–4 range at deep depth under ISO 105-B02 and lower values at pale depths, but published data for this specific configuration is limited and varies with depth, substrate, and test method. Washing fastness under ISO 105-C10 is generally insufficient for machine-washable garments unless a cationic fixative is applied. Aftertreatment with 0.5–1.5% owf of a cationic fixing agent raises wash fastness by approximately 0.5–1.0 grade and reduces staining on multifibre, but it can reduce light fastness by up to 0.5 grade and stiffen handle above 2.0% owf.
Fixation is performed after rinsing at 40–50 °C for 15–20 min at pH 6.0–7.0. Acidic pH below 5.5 can precipitate the cationic complex and produce backstaining. The effect of fixation is not permanent under all laundering conditions; severe alkaline detergents at 60 °C may hydrolyse the electrostatic complex and release stain onto adjacent fibres. For articles labelled as machine washable, the dyehouse should verify fastness after 5 repeated ISO 6330 washing cycles rather than relying on single-cycle data.
For stock solution preparation, Direct Sky Blue 5B is pasted with cold water before dilution to 1–5% concentration. Direct dyes are generally soluble at 80–100 g/L at 80 °C, but stock solutions above 50 g/L may require heating and continuous agitation. Undissolved particles are removed by a 30–50 µm inline filter before the dyeing machine. Storage of concentrated stock for more than 8 h is not recommended because hydrolytic degradation and bacterial growth can reduce tinctorial strength. Direct dye solutions should not be mixed with cationic fixing agents, cationic softeners, or alum in the same bath; coacervation occurs at specific concentration ratios and deposits as blue specks on fabric.
The powder is hygroscopic; storage should be in sealed containers at 10–30 °C and relative humidity below 60%. Pre-drying is not normally required unless the powder has been exposed to humid air, in which case lump formation and dusting tendency change. Dust exposure in the dye kitchen should be controlled below 2 mg/m³ as inhalable dust. Spilled dye should be collected dry before wet cleaning to avoid high-colour effluent load. Wastewater from Direct Sky Blue 5B dyeing carries anionic colour and residual salt. Conventional activated sludge treatment removes only a fraction of direct dye; colour in discharge is controlled by measuring absorbance at 590 nm after membrane filtration. Production-scale effluent treatment should use adsorption onto activated carbon or coagulation with ferric sulfate at pH 5.5–6.5.
Because Direct Sky Blue 5B is synthesised from 3,3′-dimethoxybenzidine, dyed textile and leather articles intended for the EU market fall within REACH Annex XVII entry 43. Test methods EN 14362-1 for textiles and EN 14362-2 for leather determine reductive cleavage of restricted aromatic amines; the detection limit for 3,3′-dimethoxybenzidine is typically 30 mg/kg. A dyestuff certificate alone does not replace article-level testing, because perspiration and laundering can generate the free amine if the azo bond is not fully reduced. Production-scale dyehouses should retain reductive-cleavage quality-control data for each dyed article lot and avoid strong reducing agents in subsequent wet processing.
In addition, the product may be subject to REACH registration when manufactured or imported in quantities above 1 tonne/year per legal entity. Users must verify that the safety data sheet includes exposure scenarios for dyehouse manual weighing, dust extraction, and effluent treatment. Colouration of paper and food-contact board is not permitted unless the specific grade is cleared under national food-contact legislation and migration testing indicates no transfer of free parent amine above the legal detection limit.
