Acid Green 28 is supplied as a water-soluble anionic colourant listed under the Colour Index as C.I. Acid Green 28. The commercial product is standardised as a dark green powder or granule with batch-to-batch strength controlled by visible spectrophotometric assay against a reference lot. Because the Colour Index designation defines the chemical identity rather than a fixed formulation, supplier-specific grades differ in diluent content, particle size distribution, residual moisture, and permitted trace-metal levels. A representative industrial grade is controlled for water-insoluble matter below 0.2% by mass and moisture below 5% by Karl Fischer titration; these values are derived from standard acid-dye powder specifications and are not a universal certificate of analysis.

The chromophore is anthraquinone-based, which places Acid Green 28 in the same general class as Acid Green 25 but with narrower regulatory recognition in cosmetic and food-contact jurisdictions. Its solubility in deionized water is strongly temperature-dependent; cold additions to a working bath can form dispersed aggregates rather than true solutions. Stock solutions are typically prepared at 70–80°C under mechanical agitation and then transferred to the process bath through a 50–100 µm filter to remove undissolved fines. The product is pH-sensitive: outside the working range of approximately pH 4.5–6.5, shade drift and precipitation can occur, particularly in hard water where calcium and magnesium form sparingly soluble colourant salts.

The dry product is hygroscopic, and repeated opening of the container under humid conditions increases caking and reduces free-flow properties. In automated dispensing systems, bridging in hoppers can occur when the powder has a cohesive particle size distribution; vibratory feeders or screw conveyors with 20–50 mm auger diameters are preferred to maintain consistent dye additions. Industrial users typically specify a residue on a 250 µm screen below 1% to avoid slow incorporation in stirred tanks.

In aluminium anodizing, Acid Green 28 is used as a pore-filling dye after electrolytic formation of an anodic oxide film. The typical operating envelope comprises a dye concentration of 1–5 g/L, bath temperature of 55–65°C, and immersion time of 5–15 min. These values are standard starting points for anthraquinone acid dyes on sealed Type II anodic coatings, not absolute rules for every alloy. Film thickness exerts a first-order effect on colour depth: coatings below 10 µm produce weak, non-uniform dye uptake on extruded AA 6063, while thicknesses above 25 µm can increase dye uptake but may require extended sealing to prevent bleed. The pH is maintained with acetic acid or an acetate buffer; uncontrolled pH drift above 6.5 accelerates pore sealing during dyeing and produces a patchy surface, whereas drift below 4.0 risks precipitation of the colourant and increased aluminium dissolution products in the bath.

Production-scale dyeing tanks require high-volume recirculation without air entrainment. Eductor or recirculating pump systems generating enough turnover to prevent stagnant zones are employed, because dye circulation failure produces streaks along anodized extrusions that are not correctable after hot-water or nickel acetate sealing. In batch operations on long architectural profiles, adjacent profiles can differ by more than 1.0 CIELAB ΔE if rack density or immersion time varies by 2 min across a 20 µm film. Verification by colourimetric drawdown panels under ASTM D2244-23 is therefore used at shift start and after every bath top-up.

Sulfuric acid anodizing at 15–20% by volume and 18–22°C produces pore diameters that accept Acid Green 28; hard anodizing in sulfuric acid near 0°C creates smaller, denser pores that may retard dye uptake and require longer immersion. Phosphoric acid anodizing produces larger pores but is less common for decorative colouring. The processor must match dyeing time to the formation conditions; using a dyeing cycle developed for conventional Type II sulfuric acid films on hard-anodized surfaces can result in insufficient colour saturation.

What Limits the Dyeing Window in Sealed Anodic Films?

Sealing chemistry imposes a narrow processing conflict. Acid Green 28 is adsorbed in the unsealed anodic pores; if the dyeing bath temperature is too high or the pH too alkaline, partial sealing initiates before the desired colour depth is reached. This action closes the outer pore mouths and blocks further dye penetration. Conversely, if the anodic coating is immersed too long or at excessive concentration, dye molecules saturate the outer oxide region and increase the risk of bleed during subsequent sealing. Hot-water sealing at 96–100°C or nickel acetate sealing at 80–90°C hydrates the oxide and chemically fixes the dye by pore closure; incomplete sealing is detectable by the ASTM B680-19 acid dissolution or dye spot test.

The operational boundary for Acid Green 28 in architectural anodizing is therefore a balance between the dye uptake rate, the oxide pore volume, and the sealing response. For sealed Type II coatings of 15–20 µm, a dyeing time of 10 min at 60°C is a reproducible midpoint on a conventional batch line; however, this is a starting condition rather than a fixed production rule. Alloys high in copper or silicon, such as AA 2024 or AA 6061 with poor anodizing response, may produce a darker or uneven green because the oxide pore structure is less uniform than on AA 6063. Published data for this specific configuration is limited, so alloy-specific panels must be colour-matched before full production.

Rinsing after dyeing must remove unbound colourant from the surface without leaching dye from the pores. A post-dye immersion rinse of 30–60 s in deionized water at ambient temperature is typical, followed by sealing without air drying. If the dyed film is allowed to dry before sealing, residual colourant crystallises on the pore walls and can create a bronzing or non-uniform appearance after seal.

Batch-to-Batch Viscosity and Solubility Controls

Powder particle size distribution controls the dissolution rate more than the final bath performance. Coarse granules can remain undissolved in high-demand anodizing lines where the dye is added directly to the working tank. Processors therefore pre-paste the powder with deionized water at 1:5 to 1:10 mass ratio and use a high-shear disperser to eliminate lumps before dilution. The dissolved colourant does not significantly alter bath viscosity at normal working concentrations; viscosity remains within 1–2 mPa·s at 60°C, close to water, so pumping and filtration are not limiting factors. However, undissolved fines can accumulate on filter media and cause premature blinding of 50 µm bag filters, reducing recirculation rate.

Storage stability of the dry powder is limited by moisture adsorption. In ambient relative humidity above 60%, caking and strength loss can occur if the container is left open; the powder should be stored in sealed, moisture-resistant packaging at 5–35°C. Pre-drying of the powder is not recommended because the dye degrades at elevated temperatures above approximately 150°C; instead, caked material is rejected or dispersed with high-shear mixing after confirming strength by spectrophotometric assay.

Batch-to-batch variance in dye strength is managed by adjusting the mass addition relative to the standard reference, not by visual inspection. A strength deviation of ±3% relative to the reference lot can produce an observable shade difference on adjacent profiles, particularly in light-toned pastel blends. Dyehouse control samples are measured in solution at the absorbance maximum of the standard lot under ISO 9001-documented procedures; the absorbance maximum is supplied in the certificate of analysis and is not assumed from other green acid dyes.

When Acid Green 28 Replaces Acid Green 25 in Dyehouse Operations

For polyamide and wool dyehouses, replacement of Acid Green 25 with Acid Green 28 requires a documented shade comparison rather than a direct substitution. Acid Green 25 is listed as D&C Green No. 5 under 21 CFR 74.1205 for certain cosmetic and drug applications, while published regulatory listings for Acid Green 28 in FDA-governed cosmetic or food-contact uses are limited. Therefore, replacement in any application requiring FDA colourant certification is not permissible unless the specific Acid Green 28 grade is explicitly cleared.

In textile dyeing of wool and polyamide, the two green dyes differ in migration behaviour, build-up, and pH sensitivity. Acid Green 28 generally follows the behaviour of an acid-leveling dye with slower strike under weakly acidic conditions, while Acid Green 25 may offer better leveling in some nylon formulations. Published comparative fastness data for Acid Green 28 versus Acid Green 25 in polyamide is limited; mills evaluate washing fastness per ISO 105-C10 and light fastness per ISO 105-B02 on the specific substrate before commercial substitution. The user should not infer equivalence from shade similarity alone.

Differences in trace-metal content and solubility can also affect bath stability in hard water. Acid Green 28 is sensitive to calcium and magnesium at levels above roughly 150 ppm as CaCO₃, producing a shade shift toward duller, more yellow green. Acid Green 25 may exhibit a different hardness tolerance, so replacement in locations with hard process water requires either softened or reverse-osmosis make-up water. Sequestrant use should be validated because anionic chelates can compete with the dye for positively charged adsorption sites in the anodic film, lowering apparent depth.

In anodized aluminium, replacement must include new colourimetric reference panels. The hue angle and chroma of Acid Green 28 can differ from Acid Green 25 even at matched absorbance strength. Operators should establish a colour difference specification, for example ΔE*ab ≤ 1.0 under ASTM D2244-23, and verify through shade sorting of production racks against a master standard. Without such a specification, adjacent profiles in a building façade may exhibit visually objectionable variation under daylight.

In wool dyeing, the dye is applied in an acid bath using acetic or formic acid; the strike rate is controlled by temperature ramp rather than by electrolyte addition. A typical cycle includes a start pH of 4.0–5.0, a ramp of 1–2°C per min to 95–98°C, and a hold time of 30–45 min. For pale shades, anionic leveling agents may be needed to reduce uneven adsorption. The fastness of Acid Green 28 on wool should be confirmed by ISO 105-C09 for severe washing and ISO 105-E04 for perspiration; published data for this specific colourant is limited, so trials are necessary. Nylon dyeing requires pH control closer to 5.5–6.0 to slow the initial strike, because polyamide exhibits rapid acid-dye uptake at lower pH. The use of amphoteric leveling agents and a delayed acid addition improves uniformity. The colourant’s high affinity for polyamide can lead to tippy dyeing if the fibre has uneven molecular orientation; fabrics are heat-set before dyeing to minimise this defect.

Compliance Checklist Matrix for Industrial Colourant Specifications

Regulatory and quality requirements for Acid Green 28 depend on the final article and jurisdiction. The table below lists the standard designations most often referenced on certificates of analysis and quality agreements. The absence of a substance-specific restriction under REACH or RoHS does not remove the obligation to verify each supplied lot.

The cadmium and lead values shown are RoHS maximum concentration values in homogeneous materials and are not inherent guarantees for every Acid Green 28 lot. Finished-article compliance must be established by analytical testing of the final coloured component, not by inference from raw colourant documentation.

Wastewater containing Acid Green 28 should not be discharged without treatment because the anthraquinone chromophore resists simple neutralization. Treatment commonly includes pH adjustment to 6–8, followed by adsorption onto activated carbon or coagulation with ferric salts. The specific removal efficiency depends on the dye bath concentration and the presence of sealing salts. Operators must verify discharge parameters against local permits; no universal dilution factor should be assumed.