MERCERIZING. The term applied to a process of treating cotton and other vegetable fibres with certain chemical reagents, especially caustic soda, whereby considerable improvement in some properties of the material is effected. Caustic soda solution of over 13.5% strength (3o°Tw) causes cotton to shrink consider ably and to become stiff and translucent. The two latter properties are lost on washing with water, but the threads become coarser and stronger so that a piece of treated calico appears closer and thicker than the original cloth. These observations were first recorded by John Mercer (1843) who suggested the use of 45°Tw caustic soda for obtaining cotton of improved strength and dye ing properties. The treated cotton has a greater affinity for direct, sulphur and vat colours than untreated cotton. The production of crimp effects—by printing caustic soda thickened with British gum on to calico—first suggested by Mercer was revived in 189o-91 and is frequently used, but it was about the only applica tion of mercerizing without tension or as Mercer termed it sodaiz ing or fulling to attain a large measure of commercial success. In 1844 Depoully and Gamier produced crimps on union fabrics by submitting the material to the action of cold caustic soda. The cot ton contracts and the wool and silk remain unaltered. The caustic soda is removed by washing and treating with dilute acid and again washing. The production of a permanent lustre as the result of treating cotton in a stretched condition (under tension) with caustic soda and washing until tension slackens was discovered and patented by H. Lowe in 1889 with a supplementary patent in 189o. The importance of Lowe's discovery was not recognized until 1895 when Thomas and Prevost repatented his invention. Their patent was subsequently annulled on the ground that it had been anticipated. The natural lustre of cotton can be seen when the fibre is viewed by reflected light under the microscope, but cotton mercerized under tension shows a spun silk-like lustre to the naked eye. Considerable change in the appearance of the
cross section of the cotton fibre as seen in the microscope is effected by mercerizing. It is especially noticeable that the fibres appear more rounded. This has probably much to do with the appearance of lustre in the threads of yarn or cloth mercerized. No doubt many of the researches which have been carried out with the object of determining the cause of the lustre (a subject which has given rise to much controversy), have been undertaken with the object of ascertaining whether it would be possible to obtain lustre without applying tension to the cotton and thereby avoiding the use of somewhat elaborate machinery for large scale work. It has been proposed to add substances such as silicate of soda and glycerine to the alkali but these are only applied in con junction with the device of applying tension.
Machines for mercerizing are designed to secure uniform im pregnation and tension with the minimum volume and waste of caustic soda solution. Efficient washing is a matter of great impor tance. An efficient type of yarn mercerizing machine is con structed with a pair of strong steel rollers placed horizontally, arranged to rotate with reversing motion and fitted with hydraulic pistons for moving them apart when a comparatively shallow trough containing caustic soda 6o°Tw is drawn into position under them. The yarn which is at first loosely passed over the rollers is stretched out, then allowed to shrink and again stretched. After a few minutes it is submitted to the action of a squeezing roller, which is caused to press against one of the stretching rollers, and is washed in the stretched condition with hot water from spurt pipes above. The caustic washings are collected by means of a separate trough from which they are conveyed to the recovery plant. In warp mercerizing the tension is supplied by double nip squeezing rollers placed at each end of the first of a set of four tanks or boxes each provided inside with guide rollers.