• Temperature rise too rapid so dye has been adsorbed too quickly
• Inadequate salt mixing causing areas of highly positively charged areas on the fabric for which the dye will have high affinity
• Poor dissolving of dye before addition to dye bath, large aggregates of dye on the fabric will cause darker areas or dye 'spots'
• Poor fabric transport, e.g. the fabric is moving too slowly around the machine or that the liquor fabric interchange is too low
• Poor preparation of the fabric before dyeing, if the fabric has not been scoured efficiently completely the dye may not penetrate the 'unscoured' areas.

What is 'right first time' dyeing?

Right first time is an important term in dyeing process. It means to get the right shade of fabric first time without the need for re-shading or washing off to reduce the depth of shade. When using optimised dyeing recipes and practices, less dye, auxiliaries and energy are used.

Why it is necessary to get "right first time"?

Right first time has many advantages including:

Saving money in terms of energy, labour and reprocessing costs Less dye will be in the effluent and thus they can reduce pollution Increased productivity since there is no re-processing so time to dye more production Increased customer confidence

Is the effect of pH in dyeing? What is the optimal pH?

Every chemical reaction works best at a specific pH , each dye class requires a certain pH at which it will be most efficient.

How do fibre reactive dyes work?

The dyeing of cotton does not work in the same way as dyeing protein fibres. Cotton is negatively charged in water and so are the reactive dyes which mean that there is ionic repulsion, the dye will not be attracted to the fibre. To overcome this repulsion salt is added to the dye bath. The positively charged ions from the salt are attracted to the negatively charged cotton so 'mask' the negative charge, this will allow the dye to be adsorbed onto the fibre. Alkali is necessary because it facilitates the reaction between the dyestuff and the fibre. The important point is not the type or amount of alkali but rather the pH of the dye bath since there are various reactive groups on different reactive dye classes each requiring a specific pH to allow reaction of this reactive group with the fibre.

For example vinyl sulphone reactive dyes require a pH of 11.5, whereas monochlorotriazine dyes require a pH of 10.5, therefore the pH must be closely supervised.

The most suitable pH for dyeing varies with the temperature, being approx. 11.5 for common warm dyeing dyeing at 60, 10.5 for hot dyeing 80, there are dyes which can be dyed at low temperatures of 40oC (cold brand) but these dyes require sophisticated equipment and very good control of the dyeing process since these dyes are very susceptible to hydrolysis making them unreactive so will increase effluent loading. It is always advisable to contact the dyestuff supplier for information on the correct application conditions for the dyes one have purchased.

What are the factors that influence the performance of reactive dyes?

There are many factors that influence the performance of specific reactive dyes, some general factors are listed below:-

• Molecular structure of dye, will dictate the applications conditions required
• Material being dyed, including the preparation before dyeing
• Application conditions, e.g. temperature, pH, time, liquor ratio etc