Wednesday, March 12, 2008

Concept of pH in Dyeing

A variety of factors can influence the quality of dyeing and batch-to-batch reproducibility, but pH ranks as one of the most important factors. The pH of dye bath solution is most critical because of its effect on the dyeing cycle with respect to level and reproducible dyeing5. Even if the pH of the dye bath has been adjusted prior to the dyeing process, it may then be affected by various factors such as the absorption of acid by the fiber itself, increased alkalinity by boiling temporarily hard water and reduce alkalinity by loss of ammonia where ammonium salts are present in the bath at elevated temperatures in open-dyeing systems.

The control of pH in dyeing is ensured by three fundamentally different techniques.

A) Maintenance of a relatively high degree of acidity or alkalinity.
B) The control of pH within narrow tolerance mainly near the neutral region.
C) The gradual shifting of pH as dyeing proceeds.

Approach A is normally easiest to control and is used in the application of leveling type of acid dyes and 1:1 metal complex dyes to wool and nylon, and of the reactive, sulphur, vat dyes to cellulose. The agents traditionally used are the acids such as sulphuric, hydrochloric, formic acid, and alkalies like sodium bicarbonate and caustic soda.

Approach B needs a greater awareness of the factors that not only determine pH, but also stabilize it against interferences. Most of the dye-fibre systems requiring this approach are operated in the near neutral region (pH 4-9) and are much more sensitive to minor changes in pH. The pH of water supply may vary, or drift, during heating.

Approach C is particularly useful for non-migrating acid dye on wool and nylon. More recently, similar systems have been proposed for reactive dyes on cellulosics. In this case, pH control involves a deliberate shift of pH during processing in a consistent direction, rather than randomly.

pH control has received considerable attention in dyeing processes because of its critical role in quality assurance8,9. In dyeing, pH exhibits strong nonlinearity and time varying behavior. Some dyeing processes such as acid dyes on nylon, etc., are very dependent on pH. In dyeing process, pH not only responds to the addition of acids or base, but also varies as the temperature increases.

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