Mercerisation

Under the strongly alkaline conditions of mercerising, addition of either gluconate or triethanolamine with a little HEDP is useful. The presence of a polycarboxylate helps to prevent precipitation on machine components. 

Mercerisation, scouring and alkali boiling of cellulosic fibres... 

In chain mercerising, the weft threads of the fabric are kept under tension on a clip stenter. In a chainless merceriser the fabric dimensions are controlled by a series of rollers. The fabric is usually woven slightly wider to allow for some weft shrinkage. Yarn is mercerised in hanks between two movable rollers which create the required tension whilst knitted constructions may be mercerised in either slit or tubular forms. Chain and chainless mercerising have been compared [276]. 

Table 10.31 Comparison of hot mercerising with conventional cold mercerising of cotton [274,277]...

Table 10.32 Comparison of cold and hot mercerising processes for bleached and unbleached cotton and for modal fibres [275,277] ...

There is a so-called dry mercerisation process [275] in which the fabric is padded with caustic soda liquor at 20-25 °C and then dried in a stenter at about 130 °C. An immersion time in the pad trough of 7-10 seconds is sufficient but the goods need a total saturation time in the alkaline liquor of 30-40 seconds, i.e. from the nip to entry into the drying zone. 

Ideally, the maximum possible degree of mercerisation would be obtained if the goods were repeatedly mercerised (for example, twice at around 70 °C and then for a third time at 10-15 °C) but such a procedure is economically impractical [274]. In practice it is essential to aim for an optimum rather than the maximum degree of mercerisation, a compromise between what is desirable or ultimately possible and what is economically feasible on available machinery. 

The properties of a wetting agent for mercerising can be summarised as follows [235,280] ... 

Figure 10.42 Mercerising shrinkage rate using 5 g/l of a commercial wetting agent [280]...

An electrochemical system combining scouring, mercerising and bleaching has been proposed. It is a non-polluting method based on an electrochemical cell, the cathode of which produces the base to mercerise and bleach, whilst the anode produces an acid to neutralise the base remaining after mercerisation [321]. 

One of the earliest fibre pretreatments for improving the dyeability of cotton is of course mercerisation (section 10.5.4). However, more recent research interest in this area has been generated by environmental concerns about reactive dyeing, aiming to enhance substantivity for the modified fibre so that higher absorption and fixation are obtained. This results in less dye (hydrolysed or still active) in the effluent. A further objective is to minimise the usage of electrolyte in the application process. This area has been thoroughly reviewed [392,393]. 

The paramount importance of efficient preparation in producing goods of thorough and uniform absorbency for continuous dyeing cannot be over-emphasised [157,158] all continuous dyeing systems are heavily dependent on this prerequisite. Typical build-up curves on mercerised and unmercerised cotton fabrics in Figure 12.20 illustrate the... 

Figure 12.20 Typical dye build-up curves for pad-steam process variants A and B on mercerised and unmercerised cotton [158]...

Recipe pH Pad liquor stability (min) Bleached cotton Mercerised cotton... 

Washing Exhaust dyeing of Cl Reactive Red 123 on bleached cotton Continuous dyeing of Cl Reactive Blue 104 on mercerised cotton ... 

Higher substantivity for cellulose. Dye at 45-50 °C with more caustic soda and less salt (none for mercerised cotton or regenerated cellulosic fibres). 

Further, somewhat different characters may be observed according to the process used and especially according to the degree to which the merceri-sation has been carried, intermediate stages existing between the typical forms of mercerised cotton and those of ordinary cotton. 

Unstretched mercerised cotton is less characteristic and resembles more closely the original fibres. The surface is rough, with moderately frequent... 

Stripping test for acid colours. Some direct or " salt dyestuffs are partly decolorised by dilute ammonia and hence might be regarded as add colouring matters. To avoid this error it is useful to add, when this test is made, a piece of mercerised white cotton. With an add dye, the cotton remains white, or is scarcely coloured, and then becomes white again when boiled a second time with dilute ammonia. 

Test for salt colouring matters. A piece of the fabric is boiled in the soap or sodium carbonate solution in presence of white mercerised cotton to see if the cotton remains coloured. 

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