Acid Dyes

Acid dyes are water-soluble anionic colorants containing one or more sulfonic acid groups that dissociate under acidic to neutral conditions to form negatively charged dye anions. Commercial formulations are supplied as sodium salts with standardized strength, typically declared as 100% or 200% relative to a reference batch. The molecular weight of acid dyes commonly falls between 300 g/mol and 900 g/mol, and sulfonation degree controls solubility, aggregation, and affinity for protonated fiber substrates. Representative product identifiers include the Colour Index generic names and constitution numbers: C.I. Acid Yellow 36 (CI 13065), C.I. Acid Red 52 (CI 45100), C.I. Acid Blue 113 (CI 26360), C.I. Acid Black 1 (CI 20470), and 1:2 metal complex products such as C.I. Acid Brown 349. In exhaust dyeing, the dye anion binds electrostatically to protonated amino groups in wool, silk, and polyamide fibers. On nylon 66, amino end group concentration is generally in the range 30 mmol/kg to 50 mmol/kg, which limits saturation dye uptake and influences shade depth. The anionic character separates acid dyes from cationic basic dyes; their water solubility and ionic bonding mechanism separate them from nonionic disperse dyes and from reactive dyes that form covalent bonds under alkaline processing. Solubility of commercial acid dyes in cold water is frequently above 20 g/L at 80°C; high-molecular-weight milling acid dyes with low sulfonation may require pre-dissolution at 90–95°C before addition to the dyebath to avoid filtration and specking on package yarn.

Physical forms include dust-free granules, powders, and liquid concentrates. Granulated products are preferred in automated dispensing systems because they reduce dusting and improve metering accuracy to ±0.1 g per stroke. Liquid acid dye formulations typically contain 10–30% active dye content and may include glycol or other humectants to maintain stability at low temperatures. The pH of a 1% aqueous solution is commonly in the range 5.0–9.0; highly sulfonated products may give solution pH below 4.0. Strength assignment is performed by spectrophotometric comparison to a reference standard at the wavelength of maximum absorbance, following ISO 105-Z10:1997 for colour strength determination in solution. Particle size distribution for powdered products is controlled by spray drying and grinding; a typical maximum residue on a 200 mesh screen is 0.5%, but this specification varies by manufacturing site and product class.

What governs the selection of leveling versus milling acid dye ranges?

The primary selector is sulfonation degree, molecular size, and the resulting balance between migration and wetfastness. Leveling acid dyes have higher sulfonation and lower molecular weight; they migrate readily in acid media and give shade uniformity on raw wool and on nylon with physical barre. In industrial exhaust dyeing, leveling acid dyes are exhausted at pH 2.0–4.0 with acetic or formic acid, and the bath is raised from 40°C to 98°C at 1.0–1.5°C/min. Milling acid dyes have lower sulfonation and higher molecular weight, and they are applied at pH 4.5–7.0 with ammonium sulfate or ammonium acetate. On wool, the temperature interval between 80°C and 98°C is critical for milling acid dyes; heating rates above 1°C/min in this zone can produce uneven strike and reduced levelness. Wetfastness is assessed with ISO 105-C06:2010; leveling acid dyes on nylon may show shade-change grades below 3 under condition C2S, whereas milling and metal-complex acid dyes are specified when the same procedure requires grade 4 or higher. In beam dyeing of woven nylon, liquor flow of 20–40 L/kg/min is typical to maintain level migration without fiber distortion; the operating window must be adjusted when package density exceeds 0.35 g/cm³ because channeling and differential pressure rise become process risks.

On nylon 66 knit goods, pre-scoured fabric is introduced into a jet dyeing machine at a liquor ratio of 1:10 to 1:15. The dyebath is set at 40°C with 0.5–1.0 g/L of a monosodium phosphate/acetic acid buffer; the dissolved dye is metered into the bath over 20 min to avoid local over-concentration. The bath is raised at 1.0–1.5°C/min to 98°C, held for 30–45 min, and cooled to 70°C before overflow rinsing. Package dyeing of nylon yarns with acid dyes requires a filter gap below 50 µm and a differential pressure across the package below 0.8 bar to prevent channeling; production records show that insufficient filtration of undissolved dye leads to specking and package-to-package shade variation. Staining on adjacent nylon and wool in multifiber fabric is evaluated with ISO 105-C06:2010 condition A2S, and lightfastness is determined using ISO 105-B02:2014 with a xenon-arc lamp. When the specification requires shade change no greater than gray scale grade 4, dye selection shifts from simple monoazo leveling dyes to disazo or metal-complex structures with higher exhaustion and larger molecular size.

Metal-complex acid dye structures and chromium discharge thresholds

1:1 chromium complexes and 1:2 metal-complex acid dyes are produced by coordinating one or two dye ligand molecules to a central chromium or cobalt atom. The 1:2 chromium complexes are important for wool automotive upholstery and nylon floor coverings where lightfastness and wetfastness requirements exceed grade 4 under ISO 105-B02:2014 and ISO 105-C06:2010. These dyes are applied in the pH range 5.5–7.0, and their exhaustion is highly sensitive to pH; a drift of ±0.3 pH units during the hold phase can reduce final depth by 10–15% at saturation. On wool, temperature is held at 90–98°C for 40–60 min to allow diffusion through the cuticle. The main operational limitation is the presence of chromium in the dyestuff and in exhausted dyebath residues. Under EU textile wet-processing BAT conclusions, unfixed chromium in discharge is controlled by segregating chrome-containing liquors and by selecting 1:2 metal-complex dyes with low complex stability in acidic perspiration media. Leather dyed with metal-complex acid dyes may be tested for extractable chromium according to ISO 17075-1:2017; for chromium(VI), a common limit is 3 mg/kg in adult leather goods, but published data for specific acid dye formulations is limited because of differences in leather retainage and post-tanning chemistry.

When acid dyes replace reactive dyes in high-throughput polyamide exhaust dyeing

Reactive dyes can be applied to nylon to produce high wetfastness through covalent bonding, but they require alkaline fixation and prolonged afterwashing to remove hydrolyzed dye. Acid dyes replace reactive dyes when shorter cycle times and more uniform barre coverage are prioritized, but the wetfastness ceiling is lower unless milling or metal-complex structures are selected. In direct comparison on nylon 66 warp knits, a disazo acid dye produced shade-change grade 4–5 under ISO 105-C06:2010 condition C2S, whereas a bifunctional reactive dye produced grade 4–5 with lower adjacent-fiber staining; published data for this specific configuration is limited because fastness depends on dye depth, fixing agent, and finishing chemistry. The acid dye process operates at pH 2.5–7.0 and requires no alkali addition, while the reactive process typically requires sodium carbonate to reach pH 10.5–11.0 and a separate fixation stage at 60–80°C. This eliminates one alkaline reduction-clearing step and reduces total processing time by approximately 30–60 min per batch in overflow jet equipment, but final washfastness of acid dyes on nylon remains sensitive to residual anionic surfactant from pre-scouring; residual surfactant above 0.2 g/L in the dyebath can reduce exhaustion by competing for protonated amino groups.

Compatibility in ternary matching is assessed by comparing adsorption isotherms under a standard temperature ramp from 40°C to 98°C at 1°C/min; dyes with similar affinity and diffusion coefficients maintain shade fidelity across batch loads. Incompatible combinations can produce shade drift between the front and rear sections of a jet machine because the faster-striking dye exhausts preferentially on the first contact with the fabric. This failure mode is observed when a leveling acid dye is combined with a 1:2 metal complex dye without sufficient compatibility screening. The addition of 0.5–2.0 g/L of sodium sulfate can retard uptake of leveling acid dyes, but for milling acid dyes electrolyte addition often has limited effect because the dye is already highly substantive. Published data for specific mixture compatibility is limited because it is proprietary and dye-specific.

In leather drum dyeing, acid dyes are applied to chrome-tanned leather at 55–65°C and pH 4.0–6.0 for 30–60 min, with penetration depth controlled by drum speed and liquor temperature. Direct dyes are also water-soluble anionic dyes, but they are differentiated from acid dyes by their planar geometry and high substantivity for cellulosic fibers through hydrogen bonding and van der Waals forces; they do not require the protonated amino groups that acid dyes require. On wool, direct dyes may offer lower cost but generally poorer washfastness under ISO 105-C06:2010 than milling acid dyes. Basic dyes are cationic and are used mainly for acrylic and modacrylic fibers; they cannot be directly substituted for acid dyes because the charge relationship with the fiber is reversed. For silk fabric, acid dyes are preferred over direct dyes because the smooth surface and fine denier require a dye with controlled strike and good leveling; dyeing is commonly carried out at pH 4.0–5.5 with acetic acid and a temperature hold at 85–90°C for 30–40 min. The anionic nature of acid dyes makes them incompatible with cationic fixing agents in the same bath; when an aftertreatment is required, it is applied in a fresh bath after the exhausted dyebath has been drained and rinsed to avoid precipitation and staining.

For liquid acid dye formulations, viscosity is commonly specified in the range 250–1000 mPa·s at 25°C using a Brookfield viscometer with spindle LV-2 at 60 rpm; specifications vary by supplier and transport requirements. Strength and shade are controlled spectrophotometrically using ISO 105-Z10:1997 for determination of colour strength in solution. Regulatory compliance for acid dyes must be confirmed against the formulation-specific safety data sheet and applicable standards such as OEKO-TEX Standard 100, ZDHC MRSL, and REACH Regulation (EC) No 1907/2006 Annex XVII. Heavy metal and aromatic amine limits are matrix-dependent; exact values should not be assumed from a single class designation because synthesis routes and post-processing can vary across manufacturers. For applications involving skin contact, the finished article is tested according to ISO 105-C06:2010 for wetfastness and ISO 105-B02:2014 for lightfastness, and the relevant limit values depend on the end-use category declared by the retailer or brand.