Why Acid Dye Is So Called?
The term acid dye derives from the use of acidic dye baths that were originally employed for the application of the dyes to wool and silk. The acid dyes are large dyes containing one or more sulfonic or carboxylic acid salt functional groups. However, although some members of this dye class now require acidic conditions for their application to these two protein fibers and also nylon, many acid dyes exhibit considerable substantivity towards these substrates from neutral dyebaths. In this article we will discuss on dyeing mechanism of acid dyes, dyeing parameters, chemical structure and fastness properties of acid dyes.
Acid dyes are highly water soluble, and have better light fastness than basic dyes. The textile acid dyes are effective for protein fibers such as silk, wool, nylon and modified acrylics. They contain sulphonic acid groups, which are usually present as sodium sulphonate salts. These increase solubility in water, and give the dye molecules a negative charge. In an acidic solution, the -NH2 functionalities of the fibers are protonated to give a positive charge: -NH3+. This charge interacts with the negative dye charge, allowing the formation of ionic interactions. As well as this, Van-der-Waals bonds, dipolar bonds and hydrogen bonds are formed between dye and fiber. As a group, acid dyes can be divided into two sub-groups: acid-leveling or acid-milling.
Chemical Structure of Acid Dyes:
These dyes are normally very complex in structure but have large aromatic molecules, having a sulphonyl or amino group which makes them soluble in water. Most of the acid dyes belongs to following three main structural molecules,
- Anthraquinon type
- Azo dye type
- Triphenylmethane type.
Although other classes of acid dyes namely nitro, indigoid, quinoline, azine, phthalocynanine, xanthane, carbolan dyes, etc., provide individually important classes of dye. Azo dyes represent the largest and most important group and are followed by anthraquinone and triarylmethane dyes.
Dyeing Parameters of Acid Dyes:
There are different types of dyeing parameters for different types of dyes. These parameters have to be maintained at an optimal level to attain a satisfactory dyeing result. For acid dyes, there are 3 parameters that are of utmost importance. Let’s take a look.
a) Effect of Electrolyte:
Electrolytes in acid dyeing act as levelling or retarding agent. Due to opposite electrical nature of dye and fiber, strike rate and uptake of acid dye is faster.
Also, the electrolytes slow down the initial rate of dyeing by creating a temporary bond with the dye or fiber which disassociates when the temperature is raised. Thus, electrolyte prevent unlevel dyeing.
b) Effect of Acid:
Not only the total amount of the dye adsorbed is influenced by amount of acid, but the rate of exhaustion is also dependent upon acidity or pH of bath.
c) Effect of Temperature:
Acid dyes added to bath can attach themselves with cationic site of fiber through the replacement of anion released by acid. And it is not possible at room temperature, rather only when the bath is heated up, causing acceleration of dye molecules to generate the required momentum.
Excellent dyeing results are obtained if dyeing is started at about 40°C, raised slowly to boil and dyeing is further carried out at boil for desired time. Acid dyes are not transferred from bath to fiber below 39°C.
Milling acid dyes have a minimum temperature of exhaustion at 60°C, but at 70°C, transfer of dye is fast. Super-milling dyes cause level dyeing only at boil.
Mechanism of Dyeing with Acid Dyes:
Dissolution of dyes in aqueous solvent, produces a colored anion,
The protein and polyamide fibers produce cationic sites in water under acidic conditions, as the acidity of the solution is increased more cationic sites are produced under these strongly acidic conditions. These cationic sites are thus available for the acid dye anions to combine with through hydrogen bonding, vander waals forces or ionic bonding. These linkages are strong enough to break, and thus dyeing produced are fast .
Reaction between an acid dye and wool can be represented by following equation,
Dyeing Mechanism of Acid Dyes on Wool:
Based on the chemical structure of wool, it may be simply represented as H2N –W–COOH, where ‘W’ denotes the main non-reacting body of the wool structure. When wool is immersed in water, the following reaction takes place –
H2N-W-COOH + H2O →H3N+-W-COO−
Now, when acid is added –
H3N+-W-COO−+ CH3COOH → H3N+−CH3COO-W-COOH
After the addition of dyes –
H3N+−CH3COO-W-COOH + R– SO3−+Na → R– SO3−+H3N-W-COOH + CH3COONa
Electrolyte in the acid dye bath acts as a retarding agent because of chlorides ions attracted by the positive sites at the fiber and in the competition between. Addition of acid acts as an exhausting agent, because strongly acidic conditions makes more cationic sites available and thus available dye anions got combined with these.
The dyeing is generally carried out at boiling temperature for 30- 60 minutes depending upon the depth of the shade and dyestuffs used.
Dyeing leveling agents:
In the case dyeing with acid dyes, mainly cationic agents such as ethoxylated fatty amines are used as leveling agents.
Heating rate is generally kept 1-3oC/min
Washing off process:
A typical dyeing cycle of nylon filament dyeing with acid dyes is shown in the above chart,
Wool dyeing method with acid dyes:
At A set bath at 50° with:
- 4% Sulphuric Acid (96%)
- 5% Glaubers Salt anhydrous,
- pH 2.5 to 3.5
At B add required amount to dyestuff.
At A set bath at 50° with:
- 2% Formic Acid (85%)
- 5% Glaubers Salt anhydrous,
- pH 3.5 to 4.5
At B add required amount of dye.
At C add 2% Sulphuric Acid (96%) or 2% Formic Acid (85%).
Thoroughly rinse after dyeing to remove loose color.
A dyeing cycle for nylon filament dyeing:
Fastness Properties of Acid Dyes:
The wet and light fastness properties of the acid dyes varies from poor to excellent, depending upon the molecular structure of the dyes.
The fastness properties as per the category are as follows:
Neutral acid dyes: Since these dyes have very good leveling and migration properties, and have a low affinity for the fiber, therefore the wet fastness properties of this class are generally poor.
Weak acid dyes or half milling dyes: These dyes have a medium to good affinity for the fiber and are generally applied in a weakly acidic bath, shows medium to good wet fastness properties.
Strong acid dyes or super milling dyes: These dyes have poor exhaustion properties, therefore applied under very strong acidic condition, exhibit good fastness properties.
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The wet fastness of acid dyes for wool, as with metal-complex dyes, is inversely proportional to their migration properties. Below Figure illustrates this fact and indicates the limitations on potential applications for acid dyes. The relative position of chrome mordant dyes has been included in this chart to illustrate the effect of mordanting on dyes that are in many respects similar to acid levelling types. As previously indicated, mordant dyes exhibit very good wet fastness, while also possessing good migration properties.
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.