Senshoku nouhau no rironka received favorable reviews in Japan, the first edition of the monograph, published in 1985, being followed by the second edition, published on 1 June 1986. Since then, however, it has been out of print. Due to persistent demand, however, we have taken this opportunity to reintroduce some of the contents of this monograph in Textile Info to registered readers. The content is as follows.

1. Alkaline Treatments

Mercerization, the treatment of cotton with a strong caustic alkaline solution in order to improve the luster, hand and other properties, was named after its discoverer, John Mercer, and has been in use for some time. It has been seeing an increase in application recently.

Recently, there has been wide use of so-called alkaline reduction processing, which treats polyester with a strong caustic alkaline solution to dissolve and remove the surface film in order to improve the hand.

The methods and effects involved in the processing of cotton and polyester are different, but, both involve treatment with a strong alkaline solution before dyeing to improve the properties of the fiber, and so both can be considered together to be alkaline treatments.

Furthermore, in the handling of blended and union weaves of polyester and cotton, both fibers can be treated effectively with alkalis at the same time, and so it is important that the two treatments be given equal consideration in such a case.

1.1 Mercerization Processing

If cotton is dipped into a strong alkaline solution such as lithium hydroxide, caustic soda, or potassium hydroxide, the fibers will swell and shrink. If the fibers are placed under tension while in this swollen state and then rinsed with water, the alkali will be removed and a permanent silk-like luster will result.

Alternatively, after swelling, if the alkali is rinsed off when the fiber is in its shrunk state, an increase in luster may not be discernable, but the fibers will fix in that shrunk state, thus giving good elasticity to external stress.

The former is known as tension mercerization and is often simply called mercerization, while the latter is referred to as slack mercerization. Due to considerations of cost and efficacy, only caustic soda is used as the alkali in industry.

The effects of mercerization

1. improved luster
2. increased ability to absorb dye
3. improved reactions with a variety of chemicals
4. improved stability of form
5. improved strength/elongation
6. improved smoothness
7. improved hand

In presenting here basic information regarding mercerization, the work of J.I. Marsh (Mercerising, Chapman and Hall Ltd, 1951) and Matsui (Senshoku kogyo, Vol. 21, No. 11, pp.10-27) were used as references. A few points that are considered important for dyeing in the future are discussed here.

1.1.a The Effect of Caustic Alkalis on Cotton

In the dyeing of cotton, it is well known that if too much caustic soda is used in vat dyes and other dyes which use caustic soda, the dye's ability to be absorbed will decline, this tendency being especially strong in weak alkaline vat dyes.

This is thought to be the result of competition for absorption between the dye and the caustic soda. Caustic soda has an affinity for cellulose fibers, and through routine dyeing experience, it is well known that the removal of caustic soda through rinsing is very difficult when compared with the removal of acid.

However, within the range of concentrations of caustic soda generally used in dyeing, the properties and form of cotton does not incur any particular effects, but if the alkaline concentration is gradually increased, they will be affected.

Due to the different effects on different yarns, which are a collection of single fibers, or on different knits and wovens (and, in fact, its effect on yarn or knits and wovens is that which is desired) a variety of factors have complex cumulative effects, and the basic behavior of cotton exposed to certain alkalis is difficult to ascertain accurately, but clarification has come through using cotton hairs (single cotton fibers).

Qualitative observations of the behavior of cotton when exposed to caustic soda solutions of different concentrations were first conducted by Pop and Hubner (J.S.C.I. 23, p.404, 1904).

Subsequently, researchers have repeatedly conducted experiments which included quantitative measurements, but the results have lacked consistency. While the reasons for this may be related to experimental procedure or certain errors, essentially, factors involved in the type and the maturation process of natural macro molecules like those in cotton can result in considerable differences in the resulting properties, structure and configuration.

In addition to the variations in the responses to alkalis which result from these factors, a precise experimental procedure is difficult to determine, and this can also be considered a factor contributing to the difficulties.

In the results observed to date, the behavior of cotton hairs exposed to different concentrations of caustic alkaline solutions can be summarized as follows.

If a cotton hair is dipped in different alkaline solutions, no change in appearance will be visible up to 10°Bé, but above 11°Bé, the hair slowly loses its natural twist (this usually being in the order of 150-300 times per inch).

Above 13°Bé, untwisting and shrinkage in the longitudinal direction now gradually starts to increase, and as the concentration nears 16°Bé, untwisting and shrinkage advance rapidly. Between 18 and 22°Bé, shrinkage of the length reaches its maximium and untwisting for the most part ceases.

However, in the untwisting and shrinkage which have occurred to this point, while both are related to the swelling of the fiber, the untwisting usually occurs first, and is followed subsequently by the swelling. Nearing 24°Bé, swelling and untwisting occur at the same time, and between 33 and 44°Bé, swelling occurs before untwisting, and the rate of shrinkage that occurs with increases in the concentration of the alkaline solution decreases.

After the point at which the greatest rate of shrinkage is reached, the rate of increase of untwisting slows down, but increases more or less linearly with increases in the concentration of the alkali.

In the data reported by different researchers, the greatest discrepancies exist in the data related to the rates of shrinkage and swelling of the hairs. The concentrations which showed the greatest rate of shrinkage straddled the area between 18 and 22°Bé, and the concentrations for the greatest rate of swelling were distributed from 18 to 26°Bé.

In these very widely distributed results, at concentrations displaying the greatest rate of contraction, these being in the range from approximately 18-22°Bé to around 33°Bé, not only did the actual alkaline concentrations displaying the greatest rates of shrinkage and swelling differ, but also the rate of shrinkage itself also varied greatly, sometimes displaying an undulating decline, and sometimes displaying a smooth and gradual decline. In many cases, however, the rate of shrinkage started to decrease rapidly above 33°Bé.

As can be seen, in mercerization, the results observed for the behavior of hairs over a range of alkaline concentrations, while important, show great disparities, and many points are still awaiting clarification.

While many conjectures can be made regarding these problems, the essence of mercerizing cotton is that in the swelling of cellulose fibers due to exposure to alkalis, the natural crystalline structure of the cellulose relaxes and under an appropriate tension, the dimensions can be set by the conditions, and rinsing with water while these conditions are maintained removes the alkali and converts the cellulose to a new crystalline structure, fixing the dimensions. These being the basic principles, the degree of swelling of the cellulose is the most important factor and it is related to the alkaline concentration.

However, because the swelling of the cellulose hair in the alkaline solution accompanies a change in the form of the hair itself, accurate measurements are extremely difficult to attain, and the results to date for the alkaline concentrations which display the greatest degree of swelling are, as previously noted, spread over a wide range of between 18 and 22°Bé.

Other methods of measuring the degree of mercerization of cellulose include the X-ray diffraction method in which the degree of mercerization can be determined by comparing cellulose I, which has the crystalline structure of natural cellulose, with cellulose II, which has the crystalline structure of mercerized cellulose.

The results concerning the degree of mercerization of a cellulose hair in different concentrations of caustic alkaline solutions obtained with the X-ray diffraction method showed that in 17°Bé NaOH 10% mercerized, in 18-19°Bé 40-45%, in 19-20°Bé 70%, in 20-21°Bé 80%, 22°Bé 90-95%, 23-24°Bé 95-100%, and in concentrations above 24-25°Bé 100% mercerized.

According to these results, the concentration of the caustic alkaline solution at normal temperatures needs to be above approximately 24°Bé to ensure the complete mercerization of cellulose fibers (cotton hairs) in their free state. This gives consistency to the effects of practical mercerization, and, at this concentration, the swelling and the untwisting of the cotton hair start more or less simultaneously. At alkaline concentrations in which the swelling occurs subsequently to the untwisting, the crystalline structure of the cellulose fiber shows signs that it has not undergone complete mercerization.