Naphthol AS-BS

Naphthol AS-BS (IUPAC: 3-hydroxy-N-(3-nitrophenyl)naphthalene-2-carboxamide; CAS 135-65-9; C.I. Azoic Coupling Component 17; molecular formula C₁₇H₁₂N₂O₄; molar mass 308.29 g mol⁻¹) is supplied as a yellow-brown to brown fine powder for azoic dyeing of cellulosic substrates. The product is not a finished dye but a coupling component that reacts within the fibre after application in alkaline solution and subsequent development with a diazotized primary aromatic amine. Commercial technical grades are specified with an HPLC area-purity of not less than 98.5%, moisture by Karl Fischer titration not more than 0.5%, methanol-insoluble matter not more than 0.1%, and residue on a 100 µm sieve not more than 0.5%. The powder is practically insoluble in water and soluble in dilute sodium hydroxide solution; a 2% aqueous suspension usually exhibits a pH between 6.0 and 8.0.

In continuous processing, dissolution is normally carried out by pasting one part of Naphthol AS-BS with 2–3 parts of denatured ethanol, followed by the addition of softened water heated to 80 °C and 1.0–1.5 molar equivalents of sodium hydroxide relative to the naphthol. The resulting sodium naphtholate bath is then diluted to a working concentration of 10–20 g/L for medium shades on cotton. Baths prepared below 70 °C frequently show cloudiness and require in-line filtration through a 50 µm screen to prevent speck deposition during padding. Water hardness above 150 mg/L CaCO₃ should be sequestered with a polyphosphate or chelating agent compatible with the subsequent diazonium salt.

Two physical grades are commonly available. The technical powder is used for exhaust dyeing and continuous pad-batch processing; the micronized grade, with a median particle size below 5 µm, is intended for printing pastes and ink formulations where coarse particles can cause screen blocking. Both grades are supplied with the same chemical specification.

What structural and colorimetric consequences arise from the meta-nitro substituent?

The anilide ring in Naphthol AS-BS carries an electron-withdrawing nitro group in the meta position. Relative to Naphthol AS (C.I. Azoic Coupling Component 2) and Naphthol AS-D (C.I. Azoic Coupling Component 4, which carries an ortho-methyl group), this substitution increases molecular mass and alters the affinity of the naphtholate for cellulose. The substantive effect is observable as lower initial exhaustion under identical salt and temperature conditions; published comparative reflectance data for this specific configuration is limited, but industrial pad-bath monitoring indicates that the sodium sulfate concentration may require an increase of 10–15 g/L when replacing Naphthol AS-D with Naphthol AS-BS to maintain comparable apparent depth. The hue direction with the same diazo component shifts toward dull red and Bordeaux, and the electron-poor anilide ring reduces the brightness of the final azoic pigment.

Azoic dyeings produced from Naphthol AS-BS on mercerized cotton poplin after a boiling soap aftertreatment are commonly assessed at light fastness ratings of 4–5 under ISO 105-B02:2014, washing fastness ratings of 4 under ISO 105-C06:2010 A2S, and dry rubbing ratings of 4 with wet rubbing ratings of 3–4 under ISO 105-X12:2016. The wet rubbing value is highly dependent on the diazo component and on afterwash shear; residual loose pigment from unreacted naphthol lowers the rating by one grey-scale step when soaping temperature falls below 90 °C.

On a continuous pad-batch line for cotton woven fabric of 180–220 g/m², the naphtholating bath is applied at 25 °C with a 2-dip/2-nip padder set to a nip pressure of 3–5 bar and a wet pickup of 68–72%. The padded fabric is dried at 100–110 °C; drying at or above 120 °C creates visible migration of the sodium naphtholate and side-to-side shade variation after development. The dried fabric should enter the developing bath with moisture regain below 5% to limit hydrolysis of the diazonium salt. Development is performed with a diazonium salt buffered to pH 4.5–5.5 using sodium acetate or acetic acid, followed by air passage, cold rinse, and boiling soap at 95 °C for 20 min. Production-scale colour measurement using a Datacolor 850 spectrophotometer under D65 illuminant and 10° observer geometry provides K/S values used for batch release against an internal standard.

In package dyeing of cellulosic yarns on a high-temperature yarn carrier at a flow rate of 4–6 L/kg/min, the naphtholating bath is circulated at 30 °C for 20 min before salt addition. The diazonium salt is prepared separately and added after cooling to 10–15 °C; coupling is considered complete within 30 min, and circulation is continued for a further 15 min to release trapped gas. A boiling soap at 95 °C for 20 min with 1–2 g/L of a suitable dispersing agent is necessary to remove unfixed naphthol and develop full fastness. Omitting the soaping step lowers washing fastness by more than one grey-scale step under ISO 105-C06:2010 A2S in most combinations.

Naphthol AS-BS is most commonly paired with diazo components of moderate to strong electrophilic character. Diazo components based on para-nitroaniline and substituted para-nitroanilines give Bordeaux and maroon shades on cotton. Because the coupling rate is influenced by the electron density at the naphthol 1-position, process conditions should maintain the developing bath at low temperature and controlled pH.

When Naphthol AS-BS Is Discharged in Rotary Screen Printing with Sulfite-Based Reducing Agents

In rotary screen printing, azoic ground shades produced from Naphthol AS-BS can be locally discharged with sodium sulfoxylate formaldehyde or zinc sulfoxylate formaldehyde. The reduction window is narrower than for Naphthol AS-D because the electron-withdrawing meta-nitro substituent alters the reactivity of the azo bond toward reducing agents. Discharge paste pH below 4.0 produces brown halo formation at the edge of the discharge area; paste pH above 6.5 reduces white clarity. On a 12-colour rotary screen line with 80 mesh screens and a hot-air dryer set to 140 °C, a residence time exceeding 90 s is associated with pink shift in discharge whites. A two-stage cure at 120 °C followed by 150 °C reduces halo formation, but published data for this specific configuration is limited. Where a bright discharge white is required, formulations based on Naphthol AS-D or Naphthol AS-PH may be preferred because the nitro-containing coupling component tends to leave a residual tint.

Naphthol AS-BS is structurally differentiated from other common Naphthol AS products by the substituent on the anilide ring. The table below lists the molecular mass and C.I. designation for the unsubstituted parent, the ortho-methyl derivative, and the meta-nitro derivative; the typical hue direction assumes coupling with the same diazonium component and is based on laboratory dyeings on bleached cotton sheeting.

The presence of the meta-nitro group also changes storage and solution behaviour. Naphthol AS-BS should be stored in closed containers at or below 30 °C and relative humidity below 60%. Alkaline naphtholate solutions are susceptible to aerial oxidation and should be used within 24 h of preparation; prolonged standing causes yellow-brown discoloration and a measurable loss in coupling efficiency. The product is incompatible with concentrated mineral acids and with nitrous acid under poorly controlled diazotization. Accidental acidification of the alkaline naphtholate bath below pH 9 precipitates the free naphthol and causes uneven dyeing. Premature coupling in the bath occurs if the product is combined with aromatic amines that have not been fully diazotized, consuming both the naphthol and the diazo component.

When the objective is a bright blue-red, the ortho-methoxy derivative Naphthol AS-OL or the ortho-anisidide Naphthol AS-BO is normally selected instead of Naphthol AS-BS. When the objective is a dull Bordeaux with good lightfastness, Naphthol AS-BS is retained because the nitro substituent depresses brightness and shifts the shade toward the brown-red region. A 1% depth produced with Naphthol AS-BS often appears darker and less saturated than the same nominal depth produced with Naphthol AS-D; the visual assessment is made under D65 illumination and should be confirmed with reflectance data from a spectrophotometer with specular excluded.

Regulatory and Quality-Control Documentation for Technical Grade Shipments

Each shipment should be accompanied by a certificate of analysis reporting batch assay, moisture, pH, and sieve residue. Safety data sheets prepared in accordance with REACH Regulation (EC) 1907/2006 classify the product as a skin and eye irritant under GHS; dust formation must be controlled with local exhaust ventilation and dust masks conforming to regional standards. No specific occupational exposure limit for Naphthol AS-BS is established; when no substance-specific limit exists, control of total inhalable dust below 10 mg/m³ is commonly applied as a default for organic dye intermediates. The product does not contain intentionally added substances of very high concern under REACH Annex XIV. Quality-control laboratories typically use reverse-phase HPLC with a C18 column, acetonitrile/water mobile phase, and UV detection at 254 nm for assay. Batch-to-batch variance in assay for technical material is commonly less than 0.5 area%, and a moisture value above 0.5% requires recalculation of the naphtholating bath concentration to compensate for the effective decrease in active coupling component.

Transport classification follows the GHS classification in the safety data sheet. Fines dispersed in air can form an explosive dust cloud; pneumatic transfer lines and dust collection systems should be grounded and bonded according to NFPA 652, and the micronized grade should not be allowed to accumulate in electrostatic discharge zones. Retention of quality-control records for the commercial lifetime of the finished textile is recommended; certification schemes for brand manufacturing may require retention for 10 years or more.