The Dyeing of Woollen Fabrics Part 1

The Dyeing of Woollen Fabrics.

by Franklin Beech.

PREFACE.

In this little book the author has endeavoured to supply the dyer of woollen fabrics with a conveniently arranged handbook dealing with the various branches of the wool dyeing industry, and trusts that it will be found to meet the want which undoubtedly exists for such a book.

The text on which the book is based is expressed in the t.i.tle "The Dyeing of Woollen Fabrics," and in enlarging upon it the author has endeavoured to describe clearly and in some detail the various processes and operations generally, pointing out the principles involved and ill.u.s.trating these by numerous recipes, showing the applications of a great variety of dyes in the production of the one thousand and one tints and shades the wool dyer is called upon to produce on the fabrics with which he is working. In pursuance of this plan nothing is said of the composition and properties of the various dyes, mordants, chemicals, etc., which are used. This is information every wool dyer should possess, but the author believes it is better dealt with in books devoted to Chemistry proper.

_May, 1902._

CHAPTER I.

THE WOOL FIBRE.

Wool is one of the most important textile fibres used in the manufacture of woven fabrics of all kinds. It belongs to the group of animal fibres of which three kinds are met with in nature, and used in the manufacture of textile fibres; two of these are derived from quadruped animals, such as the sheep, goat, etc., while the third cla.s.s comprises the products of certain insects, _e.g._, silk.

The skin of all animals is covered with more or less of a fibrous coat, which serves as a sort of protecting coat from the weather to the skin underneath. Two different kinds of fibres are found on animals; one is a stiff kind of fibre varying in length very much and called hairy fibres, these sometimes grow to a great length. The other cla.s.s of animal fibres are the woolly fibres, short, elastic and soft; they are the most esteemed for the manufacture of textile fabrics, it is only when the hairy fibres are long that they are serviceable for this particular purpose. There is a slight difference in the structure of the two kinds of fibre, woolly fibres having a more scaly structure than hairy fibres; the latter also differ in being more cylindrical in form.

#Wool.#--By far the most important of the animal fibres is wool, the fibre of the domestic sheep. Other animals, the llama or alpaca, the Angora and Cashmere goats also yield fibres of a similar character, which are imported under the name of wools. There are many varieties of wools Which are yielded by the various breeds of sheep, but they may be roughly divided into two kinds, according to the length of "staple," as it is called. In the long-stapled wools the fibres average from 7-1/2 to 9-1/2 inches in length, while the short-stapled wools vary from 1 to 2 inches long. The diameter varies very considerably from 0.00033 to 0.0018 of an inch.

[Ill.u.s.tration: Fig. 1.--Wool Fibre under the Microscope.]

Two varieties of thread are spun from wool, one is known as "worsted,"

the other as "woollen" yarns; from these yarns two kinds of cloths are woven, distinguished as worsted and woollen cloths; the former are in general not subjected to any milling or felting process, while the latter invariably are.

#Physical Properties.#--When seen under the microscope the wool fibres show a rod-like structure covered with broad scales, the edges of which project from the body of the fibre, and all point in one direction.

Fig. 1 shows typical wool fibres as viewed under the microscope; the sketch shows very well the scales.

The shape of the scales varies in different breeds of wool. The outer scales enclose inner medullary cells, which often contain pigment matter. A transversed section of the wool fibre shows the presence of a large number of cells. Sometimes wool fibres are occasionally met with which have a peculiar white h.o.r.n.y appearance; these do not felt or dye well. They are known as "kempy" fibres. See figure 2. The microscope shows that they are largely devoid of structure, and are formed of very h.o.r.n.y, impenetrable tissue, which is difficult to treat in the milling or dyeing process.

[Ill.u.s.tration: Fig. 2.--Kempy Wool Fibres.]

The curly or twisted character of the fibre is caused by the unequal contraction of the outer scales, and depends in a great measure upon the hygroscopic nature of the wool. It may be entirely removed for the time by wetting the wool in hot water, then drying it in a stretched condition, or the curl may be artificially induced by unequal drying, a fact which is turned to practical account in the curling of feathers and of hair.

The amount of curl in different varieties of wool is very variable, being as a rule greatest in the finer qualities, and diminis.h.i.+ng as the fibre becomes coa.r.s.er. The diameter of the wool fibre varies from 1/2000 to 1/5000 of an inch, and the number of curls from about 30 per cent. In fine wool as little as 1 or 2 per cent. in the thicker fibres.

Elasticity and strength are properties which, in common with silk, wool possesses in a greater degree than the vegetable fibres. When submitted to strain the wool fibre exhibits a remarkable strength, and when the breaking point is reached the fracture always takes place at the juncture of two rings of the outer scales, the embedded edges of the lower layer being pulled out of their seat. The scales themselves are never broken.

[Ill.u.s.tration: Fig. 3.--Wool Fibre showing Medullary Centre.]

When first formed the cells are more or less of a spherical shape, and contain a nucleus surrounded by the ultimate photoplasmic substance.

Those cells which const.i.tute the core or central portion of the fibre retain to some extent this original globular form and pulpy condition.

Surrounding this central portion or medulla, as it has been called (see fig. 3), and forming the main bulk of the fibre, there is a comparatively thick layer of partially flattened cells, which are also elongated in the direction of the length of the fibre, and outside this again there is a thinner stratum which may be compared to the bark of a tree. This outer covering differs materially from the rest of the fibre in its physical structure, but is, probably, nearly identical with it, though possibly not entirely so, in chemical composition. It consists of a series of flattened h.o.r.n.y scales, each being probably an aggregation of many cells. The scales, which have been compared to the scales of a fish or to slates on a housetop, overlap each other, the free edges protruding more or less from the fibre, while the lower or covered edges are embedded and held in the inner layer of cells. The free edges always point away from the root of the fibre, just as do the bracts of a fir cone.

When viewing a section of a wool fibre there is, of course, no sharp line of division between the three portions above described, but the change from the central spherical cells to the elongated cellular portion, and from these again to the flattened h.o.r.n.y scales, is quite gradual, so that the separation into zones, though well marked, is very indefinite in respect of boundaries.

The scaly structure of wool is of great importance in regard to what is known as felting property. When woollen fabrics are worked in boiling water, especially in the presence of soap, they shrink in length and breadth, but become thicker in substance, while there is a greater amalgamation of the fibres of the fabric together to form a more compact and dense cloth; this is due to the scaly structure of the wool fibres enabling them to become entangled and closely united together. In the manufacture of felt hats this is a property of very great value.

#Variations in Physical Structure.#--Wool fibres vary somewhat amongst themselves; fibres from different breeds of sheep, or even from different parts of the same animal, vary greatly, not only in thickness, length, etc., but also in actual structure. A typical wool fibre, such as may be obtained a good merino or Southdown fleece, will possess the typical structure described above, but frequently the type is departed from to such an extent that the central core of globular cells is entirely absent. Also the serrated character of the outermost layer of cells reaches a much higher state of development in some samples of wool than in others.

Wool is a much more hygroscopic fibre than cotton or any of the other vegetable fibres, usually it contains about 18 per cent. of water, but much depends upon the atmospheric conditions that prevail. This water is contained in the wool in two forms: (1) as water of hydration amounting to about 81 per cent., and (2) as hygroscopic water.

Experiments have shown that when a piece of dried wool is exposed to an atmosphere saturated with water vapour it will absorb 50 per cent.

of its weight; cotton under the same conditions will take up 23 per cent.; flax, 275 per cent.; jute, 285 per cent., and silk, 365 per cent.

Heated to about 100 C. it parts with nearly the whole of its water and becomes hard, h.o.r.n.y and brittle, exposed to the air, the dry wool again absorbs water and is restored to its former condition. When heated to 100 C. wool becomes somewhat plastic, so that whatever form is then imparted to it it will retain when it becomes cold, this property is very useful in certain processes of finis.h.i.+ng wool fabrics, making hats, etc.

#Chemical Composition.#--In the natural or raw state each wool fibre is surrounded by a considerable amount of foreign matter, so that in treating of its chemical const.i.tution it is necessary to distinguish between pure wool and the raw fibre. The incrusting substance is technically known as "Yolk," or "Suint," and is princ.i.p.ally composed of a kind of natural soap, consisting of the potash salts of certain fatty acids, together with some fats which are incapable of saponification.

The amount of yolk present upon different samples of wool varies greatly, the finer varieties containing, as a rule, a larger proportion than the coa.r.s.er, and less valuable sorts.

The variation in the relative amount of pure fibres and yolk is well shown in the following a.n.a.lyses which, however, do not by any means represent extreme cases.

a.n.a.lYSES OF RAW MERINO WOOL. DRIED AT 100 C.

No. 1. No. 2.

Moisture 626 104 Yolk 4730 270 Pure Wool 3031 595 Dirt 1113 31 ------ ------ 10000 10000

Yolk consists very largely of two complex substances which have been termed wool perspiration and wool fat. The former is composed of the potash salts of fatty acids, princ.i.p.ally oleic and stearic acids; the latter of the neutral carbohydrate, cholesterine, with other similar bodies. The wool perspiration may be removed by a simple was.h.i.+ng with water, and on the Continent forms a valuable source of potash salts, since the ash after ignition contains 70 to 90 per cent. of pota.s.sium carbonate. The wool fat is insoluble in water, but dissolves readily in ether, benzene, carbon disulphide, etc.

It is also removed from the wool by a treatment with alkali, and it is not easy to explain the action in the case, since the wool fat is not a glyceride, and will not form a soap, but is probably emulsified by the wool perspiration.

#Chemical Composition of the Pure Fibre.#--The following a.n.a.lyses of purified and dried wool fibre indicate its percentage composition:--

Mulder. Bowman.

Carbon 505 per cent. 508 per cent.

Hydrogen 68 " 72 "

Nitrogen 168 " 185 "

Oxygen 205 " 212 "

Sulphur 54 " 23 "

----- ----- 1000 1000

It is sometimes stated that wool fibre consists of a definite (p. 008) substance, keratine, but this view cannot now be admitted, since wool appears to be composed of a mixture or combination of several very complex substances. It is possible and even probable that the outer epidermal scales have a somewhat different composition to the bulk of the fibre, but whether that is the case or not is not known with any degree of certainty, this much can be a.s.serted, that wool is not a simple definite chemical compound.

Sulphur is by far the most variable const.i.tuent of wool, sometimes as little as 15 and occasionally as much as 5 per cent. being found. It appears to be always present in two different forms, one portion being in very feeble combination and easily removed by alkalies, the remainder, which, according to Knecht, amounts to about 30 per cent.

of the total sulphur, cannot be removed without complete disintegration of the fibre. This latter portion does not give a black coloration with plumbite of soda.

The amount of ash left on incinerating dry wool varies from 1 to 2 per cent., and some have considered this inorganic matter as an essential const.i.tuent. It consists princ.i.p.ally of salts of pota.s.sium, calcium and aluminum, with, of course, sulphur.

The chemical composition of the wool fibre is evidently of a most complicated nature; judging from its behaviour in dyeing it is evident that it may contain two bodies, one of a basic character which enables it to combine with the azo and acid series of dyes, the other possessing acid characters enabling it to combine with the basic dyes of the magenta and auramine type. Dr. Knecht has isolated from the wool fibre by extraction with alkalies and precipitation with acids a substance to which the name of lanuginic acid has been given. It is soluble in hot water, precipitates both acid and basic colouring matters in the form of coloured lakes. It yields precipitates with alum, stannous (p. 009) chloride, chrome alum, silver nitrate, iron salts, copper sulphate. It appears to be an alb.u.minoid body. From its behaviour with the dyes, and with tannic acid and metallic salts, it would appear that lanuginic acid contains both acidic and basic groups. It contains all the elements, carbon, hydrogen, oxygen, nitrogen and sulphur, found in wool.