Читать книгу The Dyeing of Woollen Fabrics - Franklin Beech - Страница 7

Table of Contents

Wool scouring takes place at two stages in the process of manufacture into cloth. First, in the raw state, to free the wool from the large amount of grease and dirt it naturally contains; second, after being manufactured into cloth, it is again scoured to free it from the oil that has been added to the scoured raw wool to enable it to spin easily. This oiling is generally known as wool batching, and before the spun yarns or woven fabrics can be dyed it is necessary to remove it.

Raw wool is a very impure substance, containing comparatively little wool fibre, rarely more than 50 to 60 per cent. in the cleanest fleeces, while it may be as low as 25 per cent. in the dirtiest.

First there is a small quantity of dirt; there is what is called the suint, a kind of soapy matter, which can be removed by washing in hot water. This soap has for its base potash, while its acids are numerous and complex. The wool contains a fatty-like substance of the nature of wax, called cholesterine, and this imparts to the fatty matter, which be extracted from the wool fibre, very peculiar properties. Besides these there are several other bodies of minor importance, all of which have to be removed from the wool before it can be manufactured into cloth.

Marker and Schulz give the following analysis of a good sample of raw wool:--

Moisture 23·48 per cent.

Wool fat 7·17 "

Wool soap (suint), soluble in water 21·13 "

Soluble in alcohol 0·35 "

Soluble in ether 0·29 "

Soluble in dilute hydrochloric acid 1·45 "

Wool fibre 43·20 "

Dirt 2·93 "

-------

100·00

Two principles underlie the methods which are in use for this purpose. The first principle and the one on which the oldest method is based is the abstraction of the whole of the grease, etc., from the wool by means of an alkaline or soapy liquor at one operation. This cannot nowadays be considered a scientific method. Although it extracts the grease, etc., from the wool, and leaves the latter in a good condition for after processes, yet with it one might almost say that the whole of the soap or alkali used, as well as the wool grease itself, is lost as a waste product; whereas any good process should aim at obtaining the wool grease for use in some form or another. The second principle which underlies all the most recent methods for extracting the grease from the wool, consists in treating the fibre with some solvent like benzol, carbon bisulphide, petroleum spirit, carbon tetrachloride, etc., which dissolves out the cholesterine and any other free fatty matter which is in the wool fibre, leaving the latter in such a condition that by washing with water the rest of the impurities in the wool can be extracted. By distilling off and recondensing the solvent can be recovered for future use, while the wool fat can also be obtained in a condition to use for various purposes. This is rather a more scientific method than the old one, but it has not as yet come into extensive use.

Wool Scouring. Old Methods.--In the early days of wool scouring this operation was done in a very primitive fashion, generally in a few tubs, which could be heated by steam or otherwise, and in which wool was worked by means of hand forks. These primitive processes are still in use in some small works, especially where the wool is dyed in the loose condition, but in all the large works machinery has been adopted, which machinery has been brought to a high state of perfection, and does its work very well, and without much attendant manual labour.

The alkaline substances used in this process of scouring demand some notice. These comprise soda ash, soda crystals, caustic soda, silicate of soda, potash, caustic potash, soaps of various kinds, stale urine, ammoniacal compounds. Which of these may be used in any particular case depends upon a variety of reasons. Potash is the best alkaline agent to use. It agrees better with the fibre than any other, leaving it soft and elastic. Ammonia is the next best, but it does not take out the grease as well as the potash. Soda does not suit as well as potash, as it has a tendency to leave the fibre harsh in feel and somewhat brittle, yet on account of its being so much cheaper it is the most largely used. The use of silicate of soda cannot be recommended, as it has a great tendency to leave the fibre hard, which may be ascribed to the deposition of silica on the fibre.

The caustic alkalies cannot be used as they have too solvent an action on the fibre. The carbonates, therefore, in the form of soda ash or potash, or pearl ash, are used, or better still, soap is used as it has a greater solvent action on the fatty matter of the wool than have the alkalies, and in this respect a potash soap is better than a soda soap.

The character of the wool determines the alkali to be used; fine, long-stapled wools, which are usually very free from grease in excess, should always be treated with potash, or a potash soap, which will remove the whole of the grease from the wool, leaving the latter in a fine, soft, silky condition.

Short-stapled wools can be treated with soap and a little soda ash, but too much of the latter is to be avoided. Coarse and greasy wools may be scoured with soap and soda ash, or other alkali which is almost necessary to remove the large amount of grease these wools contain.

Practically the only alkaline products now in use are the various hard and soft soaps, and the carbonates of soda and potash in their various forms of soda ash, soda crystals, potashes, pearl ash, etc. Ammonia and its compounds are rarely used, while stale urine, which acts in virtue of the ammonia it contains has practically gone out of use.

Hand-Scouring.--Wool scouring by hand is easily done and requires few appliances, simple tubs or vats of sufficient capacity in which steam pipes are placed, so that the scouring liquors can be heated up. The best temperatures to use are about 130° to 140° F., and it is not advisable to exceed the latter, as there is then some risk that the alkali may act on the fibre too strongly.

The strength of the scouring liquor necessarily varies with the kind of wool being treated, and with the kind of alkaline product used; if soft, fine wools are being treated, then the liquor may be made with 1 to 2 lb. of soap to 10 gallons of water (if a mixture of soap and alkali is used, then it may contain from ¼ to ½ lb. soda ash, and ½ lb. to 1 lb. of soap). For coarse, greasy wools these quantities may be increased by about one-half. The best plan of scouring by hand is to treat the wool in a tub with a scouring liquor for about half an hour, then to squeeze out the surplus liquor and to treat again in a new liquor for half an hour; this liquor may be used for a new batch of wool. The wool is often put into nets, and these are lifted up and down in the liquor so as to cause it to penetrate to every part of the wool.

It is not advisable to work the wool about too much, otherwise felting might ensue and this must be avoided. The felting of the wool is one of the troubles of the wool-scourer and is often difficult to avoid, it is mostly brought about by excessive working of the wool during the process, and by the use of too high a temperature in the scouring bath. The remedies are obvious to the practical man, as little handling of the wool as possible, and at as low a temperature as possible. Still it is necessary to see that the scouring liquor penetrates to every part of the wool which is being treated.

To ensure this, care must be taken not to scour too much at one time, so that the wool is loosely placed in the scouring tub, if placed loose in the latter, the workmen can by means of forks work it to and fro while in process of treatment. After the wool has been through these scouring liquors it is thrown on a scray to drain, and is next placed in cisterns which have perforated false bottoms. In these cisterns it is washed with cold water two or three times, the water being run off from the wool between each washing; it is then spread out in a room to dry. As a rule, a man can wash from 500 lb. to 600 lb. of wool in a day by this method. Another plan which is sometimes adopted so as to avoid handling the wool as much as possible, and thus prevent felting, is to place the wool in cages having perforated sides which will hold about 1 cwt. of wool. They are lowered by means of cranes into the washing liquors, and the wool in them is then worked for a quarter of an hour, when the cages and their contents are lifted out and the surplus liquor allowed to drain off. They are then lowered into the next bath, treated or worked in this, again lifted out and dropped into the wash waters.

There is by this plan a saving of handling, and more wool can be got through in the same time, but it requires two men to work it. These hand processes are only in use in small works, having been replaced in all large works by mechanical methods described below.

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.