The Dyeing of Woollen Fabrics Part 2

#Machine Scouring.#--Wool-scouring machinery has been brought to a high state of perfection by the successive efforts of many inventors, and by their means wool washing has been much simplified and improved.

Wool-washing machinery is made by several firms, among whom may be mentioned Messrs. J. & W. McNaught, and John Petrie, Junior, Limited, both of Rochdale.

[Ill.u.s.tration: Fig. 6.--Wool-washing Machine.]

Fig. 6 shows one form of wool-washing machine. It consists of a long trough which contains the scouring liquor. In this machine the wool enters at the left-hand end, and is seized by a fork or rake and carried forward by it a short distance, then another rake seizes it and carries it further forward to another rake, and this to the last rake of the machine, which draws it out of the machine to a pair of squeezing rollers which press out the surplus liquor, and from these rollers the scoured wool pa.s.ses to a travelling band for delivery from the machine. Sometimes the wool is not entered into the trough direct, but is put on a travelling ap.r.o.n which opens it and delivers it in a more open form into the trough. The movement of the forks causes some degree of agitation in the scouring liquor which facilitates the penetration of the liquor through the wool, and thus brings about a better scouring.

After the wool has pa.s.sed through the machine it is taken and run once more through the machine. Some scourers use the same liquor, but it is better to use fresh liquors, after which it is washed in the same machine with water two or three times. With a single machine there is some time and labour lost in transferring the wool from one end to the other between the separate treatments, and in large works where a great deal of wool is scoured it is usual to place three or four of these machines end to end.

The first is filled with strong scouring liquor, the second with (p. 022) a weaker liquor, while the third and fourth contains wash waters, and the wool is gradually pa.s.sed by the action of the machine through the series without requiring any manual aid. Between each machine it is pa.s.sed through squeezing rollers as before, and finally emerges thoroughly scoured. A good plan of working in connection with such a series of machines is to have four as above, two washing machines and two soaping machines, the soap liquor is run through these in a continuous stream, entering in at the delivery end of the second soaper and pa.s.sing out at the entering end of the first soaper. The wool as it first enters the machine comes into contact with rather dirty soap liquor, but this suffices to rid it of a good deal of loose dirt; as it pa.s.ses along the machine it comes in contact with cleaner and fresher soap liquor, which gradually takes all grease and dirt out of it, and, finally, when it pa.s.ses out it comes in contact with fresh liquor, which removes out the last traces of dirt and grease. In the same way it pa.s.ses through the washers, being treated at the last with clean water. By this plan the scouring is better done, while there is some saving of soap liquor and wash water, for of these rather less is required than by the usual system. These are matters of consideration for wool scourers. The wool-washing liquors after using should be stored in tanks to be treated for recovery of the grease which they contain.

The temperature of the scouring liquors should be about 100 F., certainly not more than 120 F., high temperatures are very liable to bring about felting, while tending to increase the harshness of the wool, particularly when soda is the agent used. By this method all the wool fat, suint, etc., of the wool find their way into the soap liquors. These were formerly thrown away, but they are generally treated with acid and the fat of the soap and wool recovered, under the name of wool grease or Yorkshire grease. (_Vide_ G. H. Hurst, (p. 023) "Yorkshire Grease," _Jour. Soc. Chem. Ind._, February, 1889.)

The wool fat consists largely of a peculiar fat-like body known as cholesterine. This, however, is unsaponifiable, and cannot be made into soap; at the same time when it gets into, as it does, the recovered wool grease it spoils the latter for soap-making purposes.

Cholesterine has some properties which make it valuable for other purposes; it is a stable body not p.r.o.ne to decomposition, it is capable of absorbing a large quant.i.ty of water, and it is on these accounts useful for medicinal purposes in the production of ointments, and it might be useful in candle-making. When it gets into recovered grease it cannot be extracted from it in an economical manner. The wool suint consists largely of the potash soaps of oleic and stearic acids. These two fatty acids find their way into the recovered wool grease but the potash salts are lost, while they would be valuable for various purposes if they could be recovered.

Another form of wool-washing machine has a frame carrying a number of forks arranged transversely to the machine. The forks are by suitable gearing given a motion which consists of the following cycle of movements. The forks are driven forwards in the trough of the machine, carrying the wool along with them, they are then lifted out, carried back, and again allowed to drop into the machine, when they are ready to go forward again. Thus the forks continually push the wool from one end of the machine to the other.

It is a common plan to have three machines placed end to end, so that the wool pa.s.ses from one to the other; in a set of this kind the first machine should have a capacity of 1,500 gallons or thereabouts, the second 1,000 gallons, and the third 750 gallons.

#Wool Scouring by Solvents.#--Of late years processes have been (p. 024) invented for the scouring of wool, either raw or spun by means of solvents, like carbon bisulphide, benzol, petroleum spirit, etc. Such processes are in a sense rather more scientific than the alkali processes, for whereas in the latter the grease, etc., of the wool and the oil used in batching it are practically lost for further use, and therefore wasted, being thrown away very often, although they may be partially recovered from the used scouring liquors, in the solvent processes the grease and oil may be recovered for future use for some purpose or other.

The great objection to these processes is the danger that attends their use, owing to the inflammable character of the solvents. Several other objections may be raised, some of which are mechanical, and due to the want of proper machinery for carrying out the processes. There are many ways in which solvents may be applied, some are the subject of patents. It is not possible to describe the details of all these, but two of the most recent will be mentioned.

In Singer's process, which was described in detail by Mr. Watson Smith some time ago before the Society of Dyers and Colourists, carbon bisulphide is used. The raw wool is placed between two endless bands of wire, and it is carried through a series of troughs containing bisulphide of carbon; during its pa.s.sage through the troughs the solvent takes out the grease, and loosens the other const.i.tuents of the wool. After going through the bisulphide the wool is dried and pa.s.sed through water which completes the process. The carbon bisulphide that has been used is placed in steam-heated stills, distilled off from the grease, condensed in suitable condensers, and used over again. In this process, with care, there is very little loss of solvent. The grease which is recovered can be used for various purposes, one of which is the manufacture of ointments, pomades, etc.

The disadvantages of bisulphide are: (1) It tends after some time (p. 025) to cause the wool to acquire a yellow cast, due to the free sulphur which it contains, and which being left in the wool gradually causes it to turn yellow. By using redistilled bisulphide this defect may be avoided. (2) Another defect is the evil odour of the solvent.

This, however, is less with redistilled bisulphide than with the ordinary quality, and with suitable apparatus is not insuperable. (3) Another defect is the volatility and inflammability of carbon bisulphide. On the other hand, bisulphide possesses the very great advantage of being at once heavier than, and insoluble in, water, and it can be, therefore, stored under water very much more safely than can any of the other solvents which are used.

Burnell's machine has two troughs filled with benzoline. In these are arranged a large central roller round which are some smaller rollers.

The wool pa.s.ses round the large roller and is subjected to a number of squeezings in pa.s.sing the smaller rollers. A current of the benzoline is continually pa.s.sing through the machine. The whole is enclosed in a hood to avoid loss of solvent as far as possible. After pa.s.sing through the benzoline trough the wool pa.s.ses through a similar trough filled with water. Benzoline is better than carbon bisulphide in that there is no tendency on the part of the wool to turn yellow after its use, on the other hand it is more inflammable, and when it does take fire is more dangerous, and being lighter than water is not so readily and safely stored. Another feature is that it is not so completely volatile at steam temperatures, so that a little may be left in the grease and thus tend to deteriorate it. Coal-tar benzol, the quality known as 90's, would be better to use.

The solvent processes are well worth the attention of wool scourers, all that is required for their proper development being the production and use of suitable machinery.

After the raw wool has been scoured it is batched, _i.e._, it is (p. 026) mixed with a quant.i.ty of oil for the purpose of lubricating the wool to enable it more easily to stand the friction to which it is subjected in the subsequent processes of spinning and weaving by giving it greater pliability.

For this purpose various kinds of oil are used. Olive oil is the princ.i.p.al favourite, the variety mostly used being Gallipoli oil.

Ground-nut oil is also extensively employed, and is cheaper than olive. Oleic acid a by-product of the candle industry, is extensively used under the name of cloth oil, there is also used oleine, or wool oil, obtained by the distillation of Yorkshire grease.

So far as merely oiling the wool is concerned there is not much to choose between these different oils, olive perhaps works the best and agrees best with the wool. Mineral oils have been and can be used either alone or mixed with the oils above mentioned, and so far as lubricating the wool is concerned do very well and are much cheaper than the fatty oils named above.

The following are some a.n.a.lyses of various oils used as cloth oils which the author has had occasion to a.n.a.lyse.

1. 2. 3. 4.

Specific gravity at 60 F. 09031 09091 06909 08904 Free fatty acid 5502 6442 5152 6805 Unsaponifiable oil 3456 995 3280 952 Saponifiable oil 1032 2532 1568 1243 ------ ------ ------ ------ 10000 10000 10000 10000

Nos. 1 and 2 are prepared from Yorkshire grease. The unsaponifiable matter in these is purely natural, it will be seen it varies within wide limits. Nos. 3 and 4 are made from the oleic acid of the candle factories, and the unsaponifiable matter is due to their containing mineral oil which has been added to them.

So far as regards the object for which the wool is oiled, the mineral oils will answer almost as well as the fatty oils and with most (p. 027) satisfactory results from an economical point of view, for they are much cheaper. But this is not the only point to be considered. The oil has to be got out of the wool before the latter can be dyed. Now while the fatty oils can be easily removed, by treatment with soap, and they can be recovered along with the fat of the soap, mineral oils cannot be entirely removed from the wool, what remains in will interfere very much with the satisfactory dyeing of the wool, and what is got out finding its way into the covered wool grease, spoils this for soap making and other uses, so that on the whole what is gained in lessened cost of oiling is lost by the increased liability to defects in dyeing and consequently depreciation in value of the wool, and to decrease in value of the recovered grease.

The amount of oil used varies from 7 per cent. with the best wools to 15 per cent. with shoddy wools. The scouring agents generally used are the same as those used in loose wool scouring, namely, carbonate of soda for coa.r.s.e woollen yarns, soap and soda for medium yarns, and soap and ammonia for fine yarns. Prior to treating the yarns it is best to allow them to steep in hot water at about 170 F. for twenty minutes, then to allow them to cool. The actual scouring is often done in large wooden tubs, across which rods can be put on which to hang the hanks of yarn, and in which are placed steam pipes for heating up the bath. The best temperature to treat the yarn at is about 150 F.; too high a temperature must be avoided, as with increased heat the tendency to felt is materially augmented, and felting must be avoided.

The hanks are treated for about twenty minutes in the liquor, and are then wrung out, drained, and again treated in new scouring liquor for the same length of time. After rinsing in cold water they are dried and finished.

When the oiled wool has been spun into yarns, whether worsted or (p. 028) woollen, and pa.s.ses into the hands of the dyer, it is necessary to remove from it all the oil before any dyeing operations can be satisfactorily carried out. This oil is removed by the use of soap and weak alkaline liquors, using these at about 110 to 120 F. The most common way is to have the liquor in a rectangular wooden tank, and hang the hank of yarn in by sticks resting on the edges of the tank; from time to time the hanks are turned over until all the oil has been washed out, then they are wrung out and pa.s.sed into a tank of clean water to wash out the soap, after which the yarn is ready for dyeing.

When the yarn is of such a character that it is liable to curl up, shrink and become entangled, it is necessary that it be stretched while it is being treated with the soap liquor; this is effected by a stretching apparatus consisting of two sets of rollers connected together by a screw attachment, so that the distance between the two sets of rollers can be varied. The hanks are hung between each pair of rollers, and can be stretched tightly as may be required.

For pressing out the surplus liquor from the hanks of yarn a pair of squeezing rollers is used.

#Scouring Woollen Piece Goods.#--Very often before weaving the yarns are not scoured to remove the oil they contain, as the weaving is more conveniently done with oily yarn than with a scoured yarn. Before dyeing the oil must be taken out of the pieces, and this can be conveniently done by scouring in a washing machine such as is shown in figures 7 and 8, using soap and soda liquors as before, and following up with a good rinse with water.

[Ill.u.s.tration: Fig. 7.--Cloth-washing Machine.]

The soap liquors used in scouring yarns and pieces become charged with oil, and they should be kept, and the oil recovered from them together with the fatty matter of the soap, by treatment with sulphuric acid.

By subjecting the grease or fatty matter so obtained to a boil with caustic soda soap is obtained which may be again used in scouring (p. 029) wool.

[Ill.u.s.tration: Fig. 8.--Cloth-washing Machine.]

#Bleaching Wool.#--The wool fibre has to be treated very differently from cotton fibre. It will not stand the action of as powerful bleaching agents, and, consequently, weaker ones must be used. This is a decided disadvantage, for whereas with cotton the colouring matter is effectually destroyed, so that the bleached cotton never regains its original colour, the same is not the case with wool, especially with sulphur-bleached wool, here the colouring matter of the fibre is, as it were, only hidden, and will under certain circ.u.mstances return.

The two materials chiefly used for bleaching wool are sulphur and peroxide of hydrogen.

[Ill.u.s.tration: Fig. 9.--Sulphur Bleach House.]

#Sulphur Bleaching.#--Bleaching wool by sulphur is a comparatively (p. 030) simple process. A sulphur house is built, the usual size being 12 feet high by 12 feet broad, and about 17 feet long. Brick is the most suitable material. The house should have well-fitting windows on two sides, and good tight doors at the ends (see fig. 9). Some houses have a (p. 031) small furnace at each corner for burning the sulphur, two of these furnaces are fitted with hoods, so that the sulphur gases can be conveyed to the upper part of the chamber, but a better plan, and one mostly adopted where the chamber is used for bleaching pieces, is to construct a false perforated bottom above the real bottom of the chamber, the sulphur being burnt in the s.p.a.ce between the two floors. If yarn is being bleached the hanks are hung on wooden rods or poles in (p. 032) the chamber, while with pieces an arrangement is constructed so that the pieces which are st.i.tched together are pa.s.sed in a continuous manner through the chamber.

When all is ready the chamber doors are closed, and the furnaces are heated, some sulphur thrown upon them, which burning evolves sulphur dioxide gas, sulphurous acid, and this acting upon the wool bleaches it. The great thing is to cause a thorough circulation of the gas through every part of the chamber, so that the yarn or pieces are entirely exposed in every part to the bleaching action of the gas.

This is effected by causing the gas to pa.s.s into the chamber at several points, and, seeing that it pa.s.ses upwards, to the ventilator in the roof of the chamber. Generally speaking, a certain quant.i.ty of sulphur depending upon the quant.i.ty of goods being treated is placed in the chamber and allowed to burn itself out; the quant.i.ty used being about 6 to 8 per cent. of the weight of the goods. After the sulphuring the goods are simply rinsed in water and dried.

Sulphur bleaching is not an effective process, the colouring matter is not actually destroyed, having only entered into a chemical combination with the sulphur dioxide to form a colourless compound, and it only requires that the wool be treated with some material which will destroy this combination to bring the colour back again in all its original strength; washing in weak alkalies or in soap and water will do this. Another defect of the process lies in sulphur being volatilised in the free form, and settling upon the wool causes it to turn yellow, and this yellow colour cannot be got rid of.

The goods must be thoroughly rinsed with water after the bleaching, the object being to rid the wool of traces of sulphuric acid, which it often contains, and which if left in would in time cause the disintegration of the wool.

Sometimes the wool is washed in a little weak ammonia or soda (p. 033) liquor, but this is not advisable, as there is too much tendency for the colour of the wool to come back again, owing to the neutralising of the sulphur dioxide by the alkali.

Instead of using the gas, the sulphur dioxide may be applied in the form of a solution in water. The goods are then simply steeped for some hours in a solution of the gas in water until they are bleached, then they are rinsed in water and dried. In this method it is important that the solution of the gas be freshly made, otherwise it is liable to contain but little sulphurous acid, and plenty of sulphuric acid which has no bleaching properties, but, on the other hand, is liable to lead to damage of the goods if it be not washed out afterwards.

A better method of utilising the bleaching action of sulphur in a liquid form is to prepare a bath of bisulphite of soda, and acidify it with hydrochloric acid, then to enter the wool, stirring well for some time, and allowing it to steep for some hours, next to expose to the air for a while, and rinse as before.

It is better to allow the wool to steep for about an hour in a simple bath of bisulphite, then enter into a weak hydrochloric acid bath for a few hours. The acid liberates sulphur dioxide in a nascent condition, which then exerts a more powerful bleaching action than if it were already free.

Even with liquid bleaching the bleach is not any more perfect than it is with the gas bleaching; the colour is liable to come back again on being washed with soap or alkali, although there is a freedom from the defect of yellow stains being produced.

Goods properly bleached will stand exposure to air for some considerable time, but those imperfectly bleached exhibit a tendency to regain their yellow colour on exposure to air. One fault which is sometimes met with in sulphur bleaching is a want of softness in (p. 034) the wool, the process seeming to render the fibre harsh.

Washing in a little weak soft soap or in weak soda will remedy this and restore the suppleness of the wool; at the same time care must be taken that the alkaline treatment is not too strong, or otherwise the bleaching effect of the sulphur will be neutralised as pointed out above.

#Bleaching Wool by Peroxide of Hydrogen.#--During recent years there has come into use for bleaching the animal fibres peroxide of hydrogen, or, as the French call it, oxygenated water. This body is a near relation to water, being composed of the same two elements, oxygen and hydrogen; in different proportions in water these elements are combined in the proportion of 1 part of hydrogen to 8 parts of oxygen, while in the peroxide the proportions are 1 of hydrogen to 16 of oxygen. These proportions are by weight, and are expressed by the chemical formulae for water H_{2}O, and for hydrogen peroxide H_{2}O_{2}. Water, as is well known, is a very stable body, and although it can be decomposed, yet it requires some considerable power to effect it. Now the extra quant.i.ty of oxygen which may be considered to have been introduced into water to convert it into peroxide has also introduced an element of instability, the extra quant.i.ty of oxygen being ever ready to combine with some other body for which it has a greater affinity than for the water. This property can be utilised in the bleaching industry with great advantage, true bleaching being essentially a process of oxidation. The colouring matter of the fibre, which has to be destroyed so that the fibre shall appear white, is best destroyed by oxidation, but the process must not be carried out too strongly, otherwise the oxidation will not be confined to the colouring matter, but will extend to the fibre itself and disintegrate it, with the result that the fibre will become tendered and be rendered useless.

Peroxide of hydrogen is a weak oxidiser, and therefore, although (p. 035) strong enough to destroy the colouring matter of the fibre is not strong enough to decompose the fibre itself. Hydrogen peroxide is sold as a water-white liquid, without any odour or taste. Its strength is measured by the quant.i.ty of oxygen which is evolved when one volume of the liquid is treated with pota.s.sium permanganate; the most common strength is 10 volume peroxide, but 30 and 40 volume peroxide is made.

On keeping it loses its oxygen, so that it is always advisable to use a supply up as quickly as possible.