Phenol AS-D, also listed as Naphthol AS-D and indexed as C.I. Azoic Coupling Component 18, is chemically 3-hydroxy-N-(2-methylphenyl)naphthalene-2-carboxamide. The molecular formula is C18H15NO2, the relative molecular mass is 277.32 g/mol, the CAS Registry Number is 135-62-6, and the EC number is 205-106-4. The substance is supplied as a free-flowing powder with a pale beige to light brown cast. In azoic dyeing, Phenol AS-D is not applied as a finished dye; it functions as a coupling component that must be converted to the water-soluble sodium naphtholate before it can be exhausted onto cellulosic fibre. The commercial model identifier used in dyehouse recipes is the Colour Index classification C.I. Azoic Coupling Component 18, which distinguishes the o-toluidide derivative from other naphtholate-forming couplers.

What Are the Commercial Specifications and Analytical Limits for Dyehouse Release?

The following values are representative of supplier release data for textile-grade Phenol AS-D and are not a harmonised ISO specification. Incoming lot checks for shade-critical operations should include assay, moisture, residue on ignition, alkali solubility, and coupling strength against a retained working reference. Published data for a single universal specification is limited because manufacturers apply different purification and milling routes; the table below reflects typical release limits rather than a regulatory maximum or minimum.

The HPLC assay should not be interpreted as a direct measure of coupling activity. Alkali-insoluble fractions, over-dried material, or isomeric impurities can pass a simple area-normalisation check while reducing the effective naphtholate concentration in the pad bath. A coupling strength test using a fixed diazonium salt concentration and a standardised cotton substrate is therefore the more discriminating release criterion for colourhouses. In practice, batch-to-batch variance is more visible in the pad bath than in the powder: poor dissolution or residual free phenol manifests as filter-blocking on the padder and as a loss of colour yield at the selvedges.

The product is not water-soluble in its free phenolic form. Aqueous stock solutions must be prepared by pasting the powder with a dispersing agent and adding sodium hydroxide to form the sodium naphtholate. A representative stock concentration is 15–30 g/L Phenol AS-D with 10–15 g/L free sodium hydroxide. The solution is heated to 70–80 °C under gentle agitation until clear or slightly opalescent. In hard water, sequestrant addition at 1–2 g/L is required to prevent calcium naphtholate precipitation. The prepared stock should be checked for filter blockage with a 50 µm screen before transfer to the pad trough.

Continuous azoic dyeing with Phenol AS-D is commonly carried out on a vertical two-bowl padder with 75–85% wet pick-up. The fabric is impregnated with a pad bath containing 8–12 g/L Phenol AS-D, 5–8 g/L free sodium hydroxide, 0.5–1.0 g/L wetting agent, and 1–2 g/L sequestrant. After padding, the substrate is dried uniformly. The critical process window is in the drying step: a two-stage infrared predryer followed by cylinder contact drying at 60–70 °C reduces migration relative to single-pass hot-air drying at 110 °C. Production experience on medium-weight cotton poplin indicates that migration during drying produces more uneven final depth than moderate variation in naphthol concentration.

The predried fabric is coupled in a separate bath containing the selected diazonium salt. For Fast Red TR or Fast Scarlet G, the coupling bath is typically maintained at 10–15 °C and pH 4.5–5.5 with sodium acetate. Coupling is rapid: the diazonium ion attacks the activated naphtholate ortho to the hydroxyl group, and the azo chromophore is formed directly on the fibre. Continuous machines use a dwell time of 30–60 s in the coupling bath, followed by cold rinsing, soaping at 95 °C with 1–2 g/L anionic surfactant, and final rinsing. The soaping step is not optional: it removes loosely bound surface pigment and improves wet rubbing fastness.

When Phenol AS-D Is Substituted for Naphthol AS in Red-Shade Recipes

Phenol AS-D differs from Naphthol AS by the presence of an ortho-methyl group on the anilide ring. This substitution has three consequences for formulation behaviour. First, the molecular planarity of the amide linkage is reduced, which lowers substantivity for cotton. Second, the electron-donating methyl group alters the electron density at the coupling site, shifting red shades toward a yellower angle. Third, the free phenol is less soluble in aqueous sodium hydroxide than the unsubstituted anilide, requiring a higher dispersing agent level and more controlled stock heating.

When replacing Naphthol AS with Phenol AS-D in an existing red recipe, the naphthol concentration should be increased by 10–15% to compensate for the lower substantivity. Pad bath temperature should be held at 20–25 °C; higher temperatures further reduce the effective pick-up of the naphtholate and increase the risk of selvedge-to-centre shade variation. The hue shift is not large enough to require a change in the diazonium salt for most scarlet and red combinations, but it is sufficient to alter laboratory colour matches. If a bluer red is required, Naphthol AS-OL or Naphthol AS-BO should be evaluated rather than increasing the diazonium salt concentration, which changes depth more than hue.

The p-chloroanilide analogue Naphthol AS-E (CAS 92-78-4) shifts the hue bluer and is often selected for pale depths where light fastness is more demanding. In contrast, Phenol AS-D is chosen when a clean yellowish scarlet is required without moving to a different chromophore family. The lower substantivity of Phenol AS-D can be an advantage in continuous pad applications where rapid and uniform diffusion into the fibre is more important than high exhaustion; however, it also means that prolonged dwell times in the pad trough do not compensate for poor predrying control.

Fastness performance of azoic dyeings from Phenol AS-D depends primarily on the diazonium salt and aftertreatment. On cotton, combinations with Fast Red TR and Fast Scarlet G typically give washing fastness according to ISO 105-C10:2006 of grade 4–5, dry rubbing according to ISO 105-X12:2016 of grade 4, and wet rubbing of grade 2–3. Light fastness according to ISO 105-B02:2014 is generally grade 5 or higher for pale-to-medium depths, but deep shades can drop to grade 4. Published data for this specific configuration is limited; dyehouses should confirm fastness on the actual substrate and finish because post-finishing with crosslinking agents can reduce wet rubbing performance.

In batch exhaust applications on cotton knitwear, the naphtholation bath is typically run at 25–30 °C for 30–45 min with a liquor ratio of 10:1 to 15:1. Sodium chloride is not required for the naphtholation step because substantivity is driven by the ionised naphtholate, but a mild alkali buffer such as sodium carbonate at 2–3 g/L stabilises the bath. After hydroextraction or controlled draining, the material is transferred to the coupling bath. Transfer time is critical: excessive delay or uneven draining leads to localised oxidation of the naphtholate and patchy coupling. Production experience on side-paddle and overhead beam machines shows that the most frequent failure mode is not weak depth but tailing caused by diazonium salt decomposition at temperatures above 15 °C.

Fastness Data and Comparative Coupling Behaviour for o-Toluidide-Based Naphtholates

The coupling position of Phenol AS-D is the activated carbon atom ortho to the hydroxyl group of the naphthalene ring. Substituent effects from the o-toluidide group influence the reaction rate without changing the principal coupling site. The ortho-methyl group increases steric hindrance near the amide carbonyl, which weakens intermolecular hydrogen bonding and reduces the tendency of the naphtholate to aggregate. This is a key difference from unsubstituted Naphthol AS, which can form hydrogen-bonded assemblies that increase apparent viscosity in stock solutions. For Phenol AS-D, the lower aggregation tendency generally gives a cleaner pad bath but also a slightly lower colour build per unit of naphtholate.

The following operational boundaries are recommended for routine use. Avoid contact between the naphtholate bath and acidic liquors before coupling; acidification precipitates the free phenol as a fine solid that cannot be re-dissolved uniformly on the fibre. Avoid combining Phenol AS-D with nitrous acid or strong oxidisers. Diazonium salts must be prepared separately and must never be mixed with the naphtholate bath before application, because premature azo coupling in the bath consumes both reactants and produces insoluble pigment that deposits on machine surfaces. The pH of the padding bath should be maintained above 11.0; below this value, free phenol can precipitate. The pH of the coupling bath should be kept between 4.0 and 5.5; below 4.0, diazonium salt decomposition accelerates, and above 5.5, coupling becomes sluggish.

Storage conditions affect assay stability. The product should be kept in closed containers below 40 °C and protected from moisture. Prolonged exposure to relative humidity above 60% may cause caking, assay drift, and slower dissolution in sodium hydroxide. Dried caked material should not be milled aggressively in the dyehouse because shear-generated heat can discolour the product and reduce coupling strength. If pre-drying is required, a vacuum dryer at 50–60 °C is preferable to an atmospheric tray dryer, which can produce local hot spots.

For shade matching, the laboratory should use the same coupling component batch as production and the same aftertreatment sequence. Differences between Phenol AS-D and Naphthol AS are amplified by optical brighteners in the substrate: the yellower hue of Phenol AS-D can appear dull when applied over heavily brightened cotton. The recommended laboratory application method is pad-dry-couple on 100% cotton poplin at 80% expression, with coupling at 12 °C and soaping at 95 °C for 10 min. This protocol reduces the batch-to-batch shade variation that occurs when exhaust methods are used for laboratory screening.