Naphthol AS-LC

Commercially supplied as a pale beige to off-white powder, Naphthol AS-LC is the 4-chloro-2,5-dimethoxyanilide of 3-hydroxy-2-naphthoic acid. The substance is indexed as C.I. Azoic Coupling Component 23, C.I. 37555, CAS 4273-92-1. Its molecular formula is C19H16ClNO4, and the molecular weight is 357.79 g/mol. Naphthol AS-LC belongs to the naphthol AS class of coupling components used in azoic dyeing and printing of cellulosic textiles. In the alkaline sodium naphtholate state, it is applied to cotton or regenerated cellulose and subsequently coupled with a diazotized base or stabilised fast colour salt to form an insoluble azo chromophore inside the fibre. The anilide ring carries a chloro substituent at the 4-position and methoxy groups at the 2- and 5-positions, which differentiates this coupler from unsubstituted Naphthol AS and from Naphthol AS-D or AS-OL. This substitution pattern changes both the solubility behaviour in caustic soda and the final hue obtained with a given diazonium salt.

In continuous pad-dry-chemical pad processing, the powder is prepared as a concentrated naphtholate stock. A typical stock formulation disperses 10–20 g/L Naphthol AS-LC with an anionic wetting agent such as sulphated castor oil at 1:1 to 1:2 powder-to-wetting-agent mass ratio. Sodium hydroxide 32.5% w/w is added at 6–10 mL/L, and the solution is heated to 60–80 °C under high-turbulence stirring. The naphtholate is then diluted into the pad trough. On cotton poplin, a wet pickup of 65–75% is common. The padded cloth is dried at 90–120 °C before the diazonium coupling step. Drying above 130 °C can increase naphtholate migration and impair levelness; published product-specific data for higher drying temperatures is limited.

What Limits the Stable Dissolution Window in Hard Water and Low-Temperature Baths?

Hard water above 150 mg/L CaCO3 can precipitate the naphtholate as a calcium salt or cause sludge at bath temperatures below 45 °C. A sequestrant based on sodium gluconate or polyphosphate is added at 1–2 g/L when the supply water exceeds this hardness. The dissolution endpoint is checked by visual clarity and absence of sediment after 15 minutes of circulation. In low-temperature wash boxes used for subsequent rinsing, residual alkali from the naphthol padding step can redissolve unfixed coupler and create cross-staining if the first rinse is below 40 °C. The recommended first rinse is therefore maintained at 40–50 °C with 1–2 g/L sodium carbonate. These boundaries are mill parameters rather than molecular properties, and they are derived from standard azoic dyeing practice described in dyehouse manuals.

When Coupling Bath pH and Temperature Drift Outside the Acetate Buffer Range

Coupling of Naphthol AS-LC with diazotized bases is performed in a sodium acetate-acetic acid buffer. The operational pH range is typically 4.5–6.0. Below 4.0, the free naphthol form can precipitate and the coupling rate falls, while above 7.0 the diazonium salt becomes less stable and can form diazoamino compounds that dull the shade. Temperature is held at 10–20 °C because diazonium decomposition follows first-order kinetics in aqueous solution and is accelerated by heat. A production-scale coupling bath can be assembled by dissolving the fast colour salt at 8–12 g/L and adding buffer salts to maintain the pH after the padded cloth enters the bath. Continuous coupling units with inline pH probes are preferred over manual dip-nip lines because pH drift of more than 0.3 pH units across a batch produces visible side-to-side depth variation.

Physical and Chemical Specification Matrix

Representative certificate-of-analysis parameters for commercial Naphthol AS-LC are listed below. Actual limits vary by manufacturer and should be confirmed against the supplier SDS and technical data sheet.

The powder should be inspected for caking; lumps that remain after gentle agitation indicate moisture ingress and should trigger re-testing. A mill receiving a container with visible condensation on the liner should quarantine the lot until moisture by Karl Fischer is confirmed below the agreed limit.

Relative to Naphthol AS, the molecular mass is higher by 94.50 g/mol due to the chloro and dimethoxy substitution. This changes the mass of coupler required to give equal molar concentration in a pad bath. Naphthol AS-LC has a calculated molecular mass of 357.79 g/mol, compared with 263.29 g/mol for Naphthol AS and 277.32 g/mol for Naphthol AS-D. When a colour kitchen changes from Naphthol AS to Naphthol AS-LC, the weight per litre must be recalculated on a molar basis; a simple one-to-one weight replacement can produce a hue shift and lower depth. Published shade cards from dye manufacturers provide base-to-coupler combinations, but the final hue is not an intrinsic property of the coupler alone.

Compared with Naphthol AS-BO, Naphthol AS-LC shows lower substantivity in exhaust processes, which makes it less suitable for jigger or circulating liquor machines at liquor ratios above 1:15. This is inferred from standard naphthol AS chemistry; product-specific substantivity curves are not widely published. Mills that attempt direct substitution in exhaust dyeing rather than pad-dry application should run a laboratory dip series at 1:10, 1:15, and 1:20 liquor ratios to determine the tailing and final depth before bulk production.

Confirming Fastness After Soaping and Oxidative Clearing

After coupling, the fabric is rinsed cold, then soaped at 95–98 °C with 1–2 g/L nonionic detergent and 1 g/L sodium carbonate. The soaping step removes loosely bound surface pigment and increases crock fastness. A post-clearing step with hydrogen peroxide 1–2 mL/L at 60–70 °C is used only when the dyehouse requires brighter yellows and the specific diazonium salt is stable. Fastness is verified according to ISO 105-C06:2010 single-cycle A2S and ISO 105-X12:2016 for rubbing. Under these test conditions, azoic dyeings from Naphthol AS-LC can show acceptable wet fastness on cotton, but light fastness depends on the diazonium salt. Published data for light fastness of this coupler with every fast colour base is limited; any claimed ISO 105-B02 rating must be tied to a specific diazonium salt and exposure condition.

Regulatory Testing Under EN ISO 14362-1:2017 and REACH Annex XVII

Because the final dyed textile contains an insoluble azo chromophore, the finished article may be subject to the EU REACH Annex XVII entry 43 restriction on azo colorants that release hazardous aromatic amines. Naphthol AS-LC is not itself a finished azo dye; it is an azoic coupling component that forms the chromophore only after reaction with a diazonium salt. However, the textile processor remains responsible for demonstrating that the final dyeing does not release restricted amines above 30 mg/kg when tested according to EN ISO 14362-1:2017. The anilide fragment of Naphthol AS-LC is 4-chloro-2,5-dimethoxyaniline; the free amine equivalence under reductive cleavage conditions should be evaluated on the final coloured article rather than assumed from the coupler structure. The product SDS should be checked for CLP classification and occupational exposure limits. In production, dust exposure during weighing should be controlled with local exhaust ventilation and a dust mask with a P2 filter.

In rotary screen printing, Naphthol AS-LC can be incorporated through the pre-print naphtholate route or through the pad-dry route followed by printing with a diazonium salt paste. The thickener is typically a sodium alginate with medium viscosity, at 40–60 g/kg in the print paste. Screen mesh counts of 80–100 threads/cm are selected to avoid blocking from precipitated pigment particles. After printing, the cloth is dried at 100–110 °C and then subjected to the standard coupling and soaping sequence. Because the pigment is formed inside the fibre rather than as a surface film, penetration is higher than pigment printing on heavyweight fabrics, but the process has narrower pH control. Low-solids thickener systems with synthetic rheology modifiers can alter the pH buffering capacity and should not be used without pilot trials.

If the Dry Powder Is Stored Above 60% Relative Humidity

Store in closed containers at 15–30 °C and ≤60% relative humidity. Avoid direct contact with strong acids, nitrosating agents, and oxidising agents; the dry powder is combustible as an organic chemical but is not classified as a flammable solid according to standard transport classifications. In standard commercial practice, many suppliers set a retest interval of 24 months for the dry powder when stored under these conditions. After opening, partial bags should be resealed and used within 6 months. Batch-to-batch variance in residual aniline and moisture is the main cause of shade variation in continuous dyehouses, so each incoming lot should be pre-tested in a laboratory pad-dye trial before bulk stocking. If the dry powder is stored above 60% relative humidity, re-drying is not recommended because the product may agglomerate and the melting range can be affected. Such material should be quarantined and re-certified against the original specification.