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Bleaching of Cotton


BLEACHING OF COTTON Cotton is bleached in the raw state (loose cotton) as yarn (cops, hanks, or warps) and in the piece. The aim of the bleacher is to remove as far as possible all the impurities present in the cotton without injuring the fibre, and thus obtaining pure cellu lose in the form of complete fibres. In the raw state, and in the form of yarn, only the natural impurities have to be considered. These include cotton wax, fatty acids, pectic substances, colour ing matters, albuminoid and mineral matter, amounting together to about 5% of the weight of the material. There are also fragments of the cotton seed husk (motes). The bleaching of cloth involves, in addition to these, the removal of the sizing materials with which the manufacturer strengthens the warp before weaving. In all cases three main operations are involved: boiling (bowking), chemicking, and souring. Much depends on the condition of the material and the use to which it is subse quently to be put as to the extent to which these operations are carried out. In the case of piece bleaching they are generally supplemented by additional processes. Loose cotton is rarely bleached before spinning in England, but some American firms who carry all the textile processes through in one works find it convenient to do so. It is certainly never subjected to drastic treatment with boiling alkali, which would tend to cause the fibres to mat together and to remove the cotton wax which seems to be a valuable assistant in spinning. The three processes can be applied when the material is to serve for the manufacture of cotton wool, or of gun cotton, in which instances the boiling operation is very important, as it helps to remove the wax and to render the material easily wetted with water. For such pur poses good wetting out is of greater importance than a particu larly good white. Cotton yarn is first boiled with an alkali such as 3 to 4% soda ash or 2% caustic soda, with or without the addition of soap, in a boiler which is known as a "kier" working at low pressure (up to iolb.) for six to eight hours. The yarn is then washed in the kier and transferred to a stone cistern provided with a false bottom. Bleaching liquor 1 to 2° Tw. is drawn by means of a centrifugal pump, from a well situated below the floor line, to the top of the cistern and is showered over the yarn. The liquor falls through the goods and back again to the well and is caused to circulate in this way for about one and a half to two hours. After rinsing with water, the goods are steeped in hydrochloric or sulphuric acid of 2° Tw. in another circulating cistern, and are then washed thoroughly. If the yarn is intended for the market to be sold as white it is often tinted with a little blueing material such as . a suspension of ultramarine in weak soap solution, or a very little Victoria blue 4R or acid violet. During the boiling process most of the im purities, with the exception of some of the cotton wax and the colouring matters, are removed. The removal of the wax is more thorough if some form of soap is employed in the boiling process, and it has been shown that resin soap is very effective in this respect. In the second operation the calcium hypochlorite of the bleaching powder—to some extent by direct action, but also owing to the fact that it decomposes on coming into contact with the carbon dioxide of the air yielding chlorine (R. L. Taylor) —destroys the colouring matter by oxidation. At the same time the motes, which were swelled up by the alkali, are removed. The souring operation has for its object the removal of lime deposited by the bleaching liquor and the attack and solution of any metallic oxides.

The rinsing which follows souring is of great importance in order to ensure the removal of the acid, which would otherwise cause the yarn to become tender.

The largest bulk of cotton is bleached in the piece, in which condition it is necessary not only to remove the natural impuri ties, but also such materials as starch, paraffin wax, soap, zinc chloride, magnesium chloride, which may be present as constit uents of the size, as well as dust, dirt, and mineral oil, which may have become incorporated with the fibre during the manu facture of the cloth. In bleaching for whites ("market bleach ing") it is essential that the white should be as perfect as possible, and the goods are invariably blued after bleaching; but probably the most thorough process is that which is commonly employed in the case of goods intended for calico printing. The ordinary process is carried out as follows:— The pieces are first examined for faults and marked in gas tar with distinctive letters and numbers. They are then sewn end to end with a machine in chain stitch, which is easily removed after the bleaching is complete. Unless the cloth is intended for flannelette or other raised cloth, it is usual to remove the "nap," that is, the ends of the cotton fibres which project on the surface of the cloth, by "singeing." The operations may be effected in three different ways: (I) plate singeing, (2) revolving roller singeing, (3) gas singeing. The first method (fig. I) consists in running the cloth at full width over a couple of arched copper plates (a and b) heated to a full red heat by being mounted over the flues of a coal fire. As a rule one plate is considerably hotter than the other and the cloth goes over the hotter one last, where the singeing actually takes place. The first plate dries the cloth, and between the two a block (d) carrying two rails is arranged so that the pressure of the cloth on the plates can be regulated or if necessary entirely removed. This process has the disad vantage that owing to irregular heating the singeing may be uneven or "stripey" and show corresponding defects in the finished cloth.

In revolving roller singeing a cast iron cylinder is heated by causing the flame of a fire to be drawn through it. The roller in this process revolves in the reverse direction to the cloth, which passes over it.

Gas singeing (fig. 2) is more convenient, economical, and also more effective, and can be applied to figured as well as to plain goods. It consists in running the goods over a non-luminous gas flame, the breadth of which slightly exceeds that of the piece. After singeing the goods are passed through hot water or through sulphuric acid 2-3° Tw. They are allowed to lie in a pile over night but are never allowed to dry. The operation has the effect which is shown in fig. 4, consists essentially of a wooden vat, over which there is a pair of heavy wooden (sycamore) bowls or squeezers. The pieces enter the machine at each end, as indicated by the arrows, and pass rapidly through the bowls down to the bottom of the vat, over a loose roller, thence between the first pair of guide pegs through the bowls again, and travel thus in a spiral direction until they arrive at the middle of the machine, when they leave at the side opposite to that on which they entered. The same type of machine is used for liming, chemicking, and souring.

The next operation is the "grey sour," in which the goods are run through a washing machine containing hydrochloric acid of 2° Tw. strength, with the object of dissolving out the lime, which of hydrolysing starchy material. Some bleachers submit cloth to a treatment with malt extract, with the same object in view. After rinsing in water the cloth is subjected to the action of boiling alkali. For calico intended for printing it is found best to carry out the lime boil, but in recent methods of bleaching this is omitted. After saturating the goods with milk of lime in a similar machine to that used for washing they pass, in what is known as rope form, into the kier and are carefully plated down. The amount of lime (CaO) with which they are impregnated is about 4% of their weight and the rope formation is given by passing the goods through smooth porcelain rings (pot-eyes) before entering the kier.

It is of the greatest importance that the goods should be evenly packed, for, if channels or loosely-packed places are left, the liquor circulating through the kier, when boiling is subse quently in progress, will follow the line of least resistance, and the result is an uneven treatment. Of the numerous forms of kier in use, the injector kier is the one most generally adopted. This consists of an egg-ended cylindrical vessel constructed of stout boiler plate, and shown in sectional elevation in fig. 3. The kier is from i o to i 2ft. in height and from 6 to 7 f t. in diameter, and stands on three iron legs, riveted to the sides but not shown in the figure. The bottom exit pipe E is covered with a shield-shaped false bottom of boiler plate, or (and this is more usual) the whole bottom of the kier is covered with large rounded stones from the river bed, the object in either case being simply to provide space for the accumulation of liquor and to prevent the pipe E being blocked. The cloth is evenly packed up to within about 3 to Oft. of the manholes M, when lime water is run in through the liquor pipe until the level of the liquid reaches within about 2ft. of the top of the goods. The manholes are then closed, and steam is turned on at the injector J by opening the valve V. The effect of this is to suck the liquor through E and to force it up through pipe P into the top of the kier, where it dashes against the umbrella-shaped shield U and is distributed over the pieces, through which it percolates, until, on arriving at E, it is again carried to the top of the kier, a continuous circulation being thus effected. As the circulation proceeds the steam condensing in the liquor rapidly heats the latter to the boil, and as soon as, in the opinion of the foreman, all air has been expelled the blow-through tap is closed and the boiling is continued for periods varying from six to 12 hours under 20-601b. pressure. Steam is then turned off, and by opening the valve V the liquor, which is of a dark-brown colour, is forced out by the pressure of the steam it contains.

The pieces are then run through a continuous washing machine, which is provided with a plentiful supply of water. The machine, the goods retain in considerable quantity after the lime boil. The goods are then well washed and are boiled again in the ash bowking kier, which is similar in construction to the lime kier, with soda ash (3%) and a solution of resin (I I%) in caustic soda (4%) %) for eight to ten hours. For white bleaching the resin soap is omitted, soda ash alone being employed.

The pieces are then washed free from alkali and the bleaching proper or "chemicking" follows. This operation may be effected in various ways, but the most efficient is to run the goods in a washing machine through bleaching powder solution at -°—i ° Tw., and allow them to lie loosely piled over night, or in some cases for a longer period. They are then washed, run through dilute sulphuric or hydrochloric acid at 2° Tw. ("white sour") and washed again. Should the white not appear satisfactory at this stage (and this is usually the case with very heavy or dense materials), they are boiled again in soda ash, chemicked with bleaching powder at i° Tw., or even weaker, soured, and washed.

sulphuric acid, after which they are washed and run through old kier liquor from a previous operation. They are then packed evenly in the wagons, which are pushed into the kier, and, the door having been closed, they are boiled f or about eight hours at 7-15lb. pressure with a liquor containing soda ash, caustic soda, resin soap, and a small quantity of sulphite of soda. The rest of the operations (chemicking, souring, and washing) are the same as in the old process.

A somewhat different principle is involved in the Thies-Herzig process. In this the kier is vertical, and the circulation of the liquor is effected by means of a centrifugal or other form of pump, while the heating of the liquor is brought about outside the kier in a separate vessel between the pump and the kier by means of indirect steam. The sequence of operations is similar to that adopted in the Mather-Koechlin process, differing chiefly from the latter in the first operation, which consists in running the goods, after singeing, through very dilute boiling sulphuric or hydrochloric acid, containing in either case a small proportion of hydrofluoric acid, and then running them through a steam box, the whole operation lasting from 20 to 6o seconds.

Open Width Bleaching.

For bleaching goods which contain effect threads, especially when these are dyed with vat colours, or are composed of artificial silk, the process of full width bleach ing has been greatly improved, and is to be recommended. After singeing the goods are run through a steaming chamber (to put out any fire) impregnated with caustic soda at 2° Tw. on a hatch ing machine in which the cloth is wound perfectly straight on to one of the two batch rollers of the kier until the batch is 3f t.– 3f t. 6in. in diameter. The two rollers are driven into the kier on a batch carriage (fig. 6), where they are kept at a constant distance apart by means of an oscillating drum, which remains in contact with the two batches although they are continually increasing and decreasing in size as the winding proceeds. The carriage automatically couples with driving and reversing gear It is of the utmost importance that the final washing should be as thorough as possible, in order to remove the acid completely, for if only small quantities of the latter are left in the goods they are liable to become tender in the subsequent drying, through formation of hydrocellulose. The best known, and probably the most widely practised of processes which dispense with the lime boil, is one which was worked out by the late M. Horace Koechlin in conjunction with Sir William Mather, and this differs from the former process not only in the sequence of the operations but also in the construction of the kier. This consists of a horizontal egg-ended cylinder, and is shown in fig. 5.

The goods to be bleached are packed in two wagons W outside the kier, and when filled these are pushed home into the kier, so that the pipes p fit with their flanges on to the fixed pipes at the bottom of the kier. The heating is effected by means of steam pipes at the lowest extremity of the kier, while the circulation of the liquor is brought about by means of the centrifugal pump p which draws the liquor through the pipes p from beneath the false bottoms of the wagons and showers it over distributors D on to the goods. By this mode of working a considerable economy is effected in point of time, as the kier can be worked almost continuously; for as soon as one lot of goods has been boiled the wagons are run out and two freshly packed wagons take their place. The following is the sequence of operations :—The goods are first steeped over-night in dilute at the back of the kier, so that the cloth is continually wound off one roller and on to the other during the boiling process. The caustic soda liquor is entered boiling and is circulated by means of a pump. After two or three hours the cloth is removed from the kier to a special machine for chemicking. This process may also be applied with advantage in the bleaching of gauze and bandage cloths, sateens, reps, poplins, canvas, and gaberdines. Defects.—As shown by the late Charles O'Neill, carefully bleached cotton ought to be stronger than the same cotton in the unbleached condition. Tendering may occur through the presence of air in the kier during boiling, or by the excessive action of bleaching liquor. Cotton which has been affected in this manner is said to contain oxycellulose. Such cotton readily becomes yellow and more tender on boiling with a dilute solution of caustic soda (about io° Tw.). It also dyes lighter with a direct colour, such as chlorazol sky blue, than does ordinary cotton, but after boiling with caustic soda it dyes normally. This test dis tinguishes oxycellulose tendering from the tender product which is formed when dilute sulphuric acid is dried into cotton, for in the latter case light dyeing with chlorazol sky blue is observed even after boiling with caustic soda. Both kinds of tendered cotton dye darker with basic' colour than does ordinary well bleached cotton. Hydrochloric acid tendered cotton does not show such differences in dyeing. Any free acid which is left in cotton can be detected by testing the aqueous extract with a drop of methyl orange, which, in the presence of acid, is turned pink.

kier, liquor, cloth, boiling and soda