Q1: What is the difference between acid dyes and reactive dyes?

A: Acid dyes and reactive dyes differ in both their target fibres and their bonding mechanism. Acid dyes are designed for protein-based fibres — wool, silk, leather — and synthetic polyamide (nylon). They bond to the fibre through ionic (electrostatic) attraction under acidic dyebath conditions. Reactive dyes, by contrast, are designed for cellulosic fibres — cotton, viscose, linen — and form a covalent (chemical) bond directly with the cellulose molecule. Acid dyes cannot be used on cotton (zero affinity) and reactive dyes cannot be used on leather or wool with reliable results. Choosing the correct dye class for your substrate is the most fundamental decision in dyehouse technical management.

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Q2: Can acid dyes be used on cotton or polyester?

A: Acid dyes have no affinity for cotton, polyester, acrylic, or other cellulosic or synthetic fibres. Applying acid dyes to cotton will produce minimal or no colour uptake regardless of process conditions. For cotton and cellulosic fibres, reactive dyes or direct dyes are the appropriate choice. For polyester, disperse dyes are required. Acid dyes are specifically formulated for protein fibres (wool, silk, angora, cashmere, feathers) and synthetic polyamide (nylon, PA6, PA6.6).

 

Q3: What pH is required for acid dye application on leather?

A: The dyebath pH for acid dye application on leather typically ranges from 3.5 to 5.5 depending on the specific dye type and leather substrate. Formic acid or acetic acid is commonly used to achieve and maintain the required pH level. Lower pH (3.5–4.0) promotes faster dye uptake and deeper penetration but may affect the surface finish of the leather. Higher pH (4.5–5.5) produces more even, levelled results. The exact pH profile — and the rate of pH reduction during dyeing — has a significant influence on shade penetration depth, uniformity, and final fastness values. Always refer to the technical datasheet for the specific dye being used.

 

Q4: What is the wash fastness of acid dyes on leather?

A: Wash fastness of acid dyes on leather is generally rated at 3–4, which is appropriate for leather goods that are not regularly laundered. For fashion leather goods (handbags, shoes, garments), the more relevant fastness tests are wet and dry crocking (rubbing fastness), perspiration fastness, and light fastness — all of which are rated 4–5 for quality acid dyes. European buyers (Italy, Spain) commonly require compliance with ISO 11640, EN ISO 15701, and EN ISO 15702 test standards for leather colourfastness. Avi Chemicals can provide acid dye recommendations with compliance data for European-standard leather production.

 

Q5: Are your acid dyes chrome-free and REACH compliant for the European market?

A: Avi Chemicals offers chrome-free and heavy-metal-free acid dye options for leather tanneries supplying European brands and retailers with REACH, ZDHC, or chemical compliance requirements. We can provide full Safety Data Sheets (SDS), technical datasheets, and REACH compliance information for all acid dye products. Buyers sourcing for Italian or Spanish leather production — or for brands requiring ZDHC MRSL compliance — should request our compliance documentation pack when ordering samples. Contact exports@avichemicals.com with your specific compliance requirements.

 

Q6: What is the difference between levelling acid dyes and milling acid dyes?

A: Levelling acid dyes have high migration — they move freely in the dyebath before fixation, producing very even, level shades. They offer moderate wash fastness (Rating 3–4) and are best for fashion applications requiring brilliant, even colour. Milling acid dyes have lower migration and higher wash fastness (Rating 4–5). They are preferred for applications where the dyed article will be subjected to washing — wool knitwear, hosiery, nylon swimwear. Super-milling acid dyes have the highest fastness of all acid dye types and are used for technical textiles, carpets, and upholstery where durability under wet conditions is critical.

 

Q7: What temperature is required for acid dye application on nylon?

A: Nylon (polyamide) is typically dyed with acid dyes at 80–90°C, which is lower than the 95–100°C recommended for wool. Higher temperatures may cause nylon to lose lustre or develop texture issues. A controlled temperature ramp — starting at 40°C and raising gradually at 1–2°C per minute to the maximum temperature — is important for achieving level results on nylon. Acid levelling dyes are preferred for nylon due to their high migration, which compensates for nylon's tendency to show barré (streakiness) when dyed. Dyebath pH for nylon should be maintained at 4.0–5.0 using acetic acid.

 

Q8: Can acid dyes be used on silk?

A: Yes. Silk is one of the most beautiful substrates for acid dyes, producing luminous, jewel-bright shades that no other dye class can match on protein fibres. Silk is dyed with levelling acid dyes at 85–90°C at pH 4.0–5.0. The lower fibre density of silk compared to wool means dye uptake is faster — careful temperature control and the use of levelling agents are important to prevent unlevel results. Acid dyes on silk deliver wash fastness of 3–4 and light fastness of 4–5, which is appropriate for silk fashion garments, scarves, and home textiles.

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