Influence of Dyeing parameters

Influence of Dyeing parameters


1.Effect of electrolyte:

Electrolytes act mainly as retarding or levelling agent for strong acid dyes

CH3COO-H3N+–W–COOH+R–SO3Na ↔ RSO3-H3N+–W–COOH+CH3COONa

Dyeing proceeds with elimination of CH3COONa. At the initial stages of dyeing – due to higher attraction between cation of fibre and anion of dye – more and more salt will be liberated due to faster dye–fibre attachment developing chances of uneven dyeing. A levelled shade can only be produced through reduction in the formation of salt by adding excess salt in bath at the start of dyeing to ensure backward process – stripping – higher the concentration of salt added, higher is the rate of stripping. In other words, excess of salt
enhances stripping and acts as a retarding or levelling agent. For super-milling dyes, which are often applied from a neutral bath, salt plays opposite effect (similar effect as dyeing of cellulose with direct dyes), i.e., salt promotes dye uptake on protein fibres by reducing zeta potential. Wool probably acquires a negative electrical potential in neutral solutions,
which normally repels the negatively charged acid dye anions present in dye bath. Presence of an electrolyte reduces this surface negative charge difference between dye and fibre through absorption of positive sodium ions –released from salt.

2.Effect of acid:

Levelling and milling acid dyes are not taken up by protein fibre unless acid
is added to bath. As stated earlier, immersion of protein fibre in water transfers the terminal H atom from –COOH and at another end the –NH2 group is protonated. During this process, carboxylic ion is formed at one end of fibre (COO-) while amine ion (NH3+) at the other making the fibre oppositely charged (zwitter ion). Addition of acid makes the fibre cationic in nature through attachment of H atom of acid to the COO- ion making it COOH
again. Absence of acid in bath, marked by pH at or above 7.0 will cause repulsion of more dye anions with little dye uptake, as shown in Fig. 15.1 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. When different acids are used in equivalent quantities, these produce similar extent of exhaustion on wool only if dyeing time is short; if dyeing is done
below boil then weaker acid is preferred in larger quantities than the stronger
ones at lower concentration. 

3.Effect of temperature: 

Protein fibres are ionised when dipped in bath.

H2O
H2N–W–COOH → H3N+–W–COO-

Due to this ionisation, fibres possess inherent negative tendency to absorb acid when the acid is added to bath; anionic COO- group at one end attracts H+ released by acid and is neutralised thereby making the fibre highly cationic in nature. Acid dyes added to bath can attach themselves with cationic site of fibre through replacement of anion released by acid (CH3COO-) which remains attached with the cationic site of fibre. 
This replacement is not possible at room temperature but it is possible only
when the bath is heated up causing acceleration of dye molecules to generate the required momentum. Efficient 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; slow rise in temperature causes replacement of CH3COO- ions at a specific rate avoiding unlevelled dyeing. Acid dyes are not transferred from bath to fibre below 39°C, beyond this temperature rate of adsorption increases and the trend varies from one type of dye to other.
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.

4.Effect of levelling agents: 

Levelling agents are occasionally used in dyeing with acid dyes. Pyridine can break and stop aggregation of dye molecules promoting levelling through formation of pyridine–dye complex
Influence of Dyeing parameters Influence of Dyeing parameters Reviewed by Suraj Gupta on April 25, 2020 Rating: 5

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