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THE
DYEING OF COTTON FABRICS
A PRACTICAL HANDBOOK FOR THE DYER AND STUDENT
BY
FRANKLIN BEECH
PRACTICAL COLOURIST AND CHEMIST
ILLUSTRATED BY FORTY-FOUR ENGRAVINGS
LONDON SCOTT, GREENWOOD & CO. 19 LUDGATE HILL, E.C.
1901
[All rights reserved]
PREFACE.
In writing this little book the author believes he is supplying a want which most Students and Dyers of Cotton Fabrics have felt—that of a small handbook clearly describing the various processes and operations of the great industry of dyeing Cotton.
The aim has not been to produce a very elaborate treatise but rather a book of a convenient size, and in order to do so it has been necessary to be brief and to omit many matters that would rightfully find a place in a larger treatise, but the author hopes that nothing of importance has been omitted. The most modern processes have been described in some detail; care has been taken to select those which experience shows to be thoroughly reliable and to give good results.
FRANKLIN BEECH.
May, 1901.
CONTENTS.
CHAPTER I. PAGE STRUCTURE AND CHEMISTRY OF THE COTTON FIBRE 1 Action of Alkalies 6 Action of Acids on Cellulose 9 Action of Sulphuric Acid on Cotton 10 Action of Hydrochloric Acid 11 Action of Nitric Acid 12 Action of Oxidising Agents on Cellulose or Cotton 16
CHAPTER II.
SCOURING AND BLEACHING OF COTTON 23 Stains and Damages in Bleached Goods 50
CHAPTER III.
DYEING MACHINERY AND DYEING MANIPULATIONS 53 Hand Dyeing 53 Dyeing Machines 57 Dyeing, Slubbing, Sliver or Carded Cotton and Wool 58 Cop Dyeing 64
CHAPTER IV.
THE PRINCIPLES AND PRACTICE OF COTTON DYEING 82 (1) Direct Dyeing 85 (2) Direct Dyeing followed by Fixation with Metallic Salts 112 (3) Direct Dyeing followed by Fixation with Developers 128 (4) Direct Dyeing followed by Fixation with Couplers 139 (5) Dyeing on Tannic Mordant 147 (6) Dyeing on Metallic Mordants 156 (7) Production of Colour Direct upon Cotton Fibres 181 (8) Dyeing Cotton by Impregnation with Dye-stuff Solution 198
CHAPTER V.
DYEING UNION (MIXED COTTON AND WOOL) FABRICS 208
CHAPTER VI.
DYEING HALF SILK (COTTON-SILK, SATIN) FABRICS 225 Method of Dyeing 225
CHAPTER VII.
OPERATIONS FOLLOWING DYEING 239 Washing, Soaping, Drying 239
CHAPTER VIII.
TESTING OF THE COLOUR OF DYED FABRICS 257
CHAPTER IX.
EXPERIMENTAL DYEING AND COMPARATIVE DYE TESTING 262
LIST OF ILLUSTRATIONS
FIG. PAGE 1. Cotton Fibre 5 1A. Cross-section of Cotton Fibre 5 2. Mercerised Cotton Fibre 7 2A. Cross-section of Mercerised Cotton Fibre 7 3. Silkified Cotton Fibre 9 3A. Cross-section of Silkified Cotton Fibre 9 4. Mather & Platt's Low-pressure Bleaching Kier 31 5. Mather & Platt's Yarn-bleaching Kier 49 6. Rectangular Dye-tank 54 7. Round Dye-tub 54 8. Section of Dye-vat 56 9. Delahunty's Dyeing Machine 58 10. Obermaier Dyeing Machine 59 11. Holliday's Yarn-dyeing Machine 60 12. Klauder-Weldon Dyeing Machine 62 13. Graemiger Cop-dyeing Machine 65 14. Graemiger Cop-dyeing Machine 66 15. Beaumont's Cop-dyeing Machine 67 16. Warp-dyeing Machine 70 17. Warp-dyeing Machine 71 18. Dye-jiggers 72 19. Dye-jigger 73 20. Jig Wince 75 21. Cloth-dyeing Machine 76 22. Dye Beck 77 23. Holliday's Machine for Hawking Cloth 78 24. Continuous Dyeing Machine 79 25. Padding Machine 80 26. Padding Machine 81 27. Dye-tub for Paranitroaniline Red 191 28. Padding Machine for Paranitroaniline Red 192 29. Developing Machine for Paranitroaniline Red 194 30. Indigo Dye-vat for Cloth 199 31. Squeezing Rollers 240 32. Yarn-washing Machine 243 33. Dye-house Washing Machine 244 34. Cloth-washing Machine 245 35. Cloth-washing Machine 247 36. Washing and Soaping Vats 248 37. Steaming Cottage 249 38. Steaming and Ageing Chamber 250 39. Hydro-extractor 251 40. Hydro-extractor 252 41. Automatic Yarn-dryer 253 42. Truck Yarn-dryer 254 43. Drying Cylinders 255 44. Experimental Dye-bath 263
CHAPTER I.
STRUCTURE AND CHEMISTRY OF THE COTTON FIBRE.
There is scarcely any subject of so much importance to the bleacher, textile colourist or textile manufacturer as the structure and chemistry of the cotton fibre with which he has to deal. By the term chemistry we mean not only the composition of the fibre substance itself, but also the reactions it is capable of undergoing when brought into contact with various chemical substances—acids, alkalies, salts, etc. These reactions have a very important bearing on the operations of bleaching and dyeing of cotton fabrics.
A few words on vegetable textile fibres in general may be of interest. Fibres are met with in connection with plants in three ways.
First, as cuticle or ciliary fibres or hairs; these are of no practical use, being much too short for preparing textile fabrics from, but they play an important part in the physiology of the plant.
Second, as seed hairs; that is fibres that are attached to the seeds of many plants, such, for instance, as the common thistle and dandelion; the cotton fibre belongs to this group of seed hairs, while there are others, kapok, etc., that have been tried from time to time in spinning and weaving, but without much success. These seed hairs vary much in length, from 1/4 inch to 1-1/2 inches or even 2 inches; each fibre consists of a single unit. Whether it is serviceable as a textile fibre depends upon its structure, which differs in different plants, and also upon the quantity available.
The third class of fibre, which is by far the most numerous, consists of those found lying between the bark or outer cuticle and the true woody tissues of the plant. This portion is known as the bast, and hence these fibres are known as "bast fibres". They are noticeable on account of the great length of the fibres, in some cases upwards of 6 feet, which can be obtained; but it should be pointed out that these long fibres are not the unit fibres, but are really bundles of the ultimate fibres aggregated together to form one long fibre, as found in and obtained from the plant. Thus the ultimate fibres of jute are really very short—from 1/10 to 1/8 of an inch in length; those of flax are somewhat longer. Jute, flax, China grass and hemp are common fibres which are derived from the bast of the plants.
There is an important point of difference between seed fibres and bast fibres, that is in the degree of purity. While the seed fibres are fairly free from impurities—cotton rarely containing more than 5 per cent.—the bast fibres contain a large proportion of impurity, from 25 to 30 per cent. as they are first obtained from the plant, and this large quantity has much influence on the extent and character of the treatments to which they are subjected.
As regards the structure of the fibres, it will be sufficient to say that while seed hairs are cylindrical and tubular and have thin walls, bast fibres are more or less polygonal in form and are not essentially tubular, having thick walls and small central canals.
The Cotton Fibre.—The seed hairs of the cotton plant are separated from the seeds by the process of ginning, and they then pass into commerce as raw cotton. In this condition the fibre is found to consist of the actual fibrous substance itself, containing, however, about 8 per cent. of hygroscopic or natural moisture, and 5 per cent. of impurities of various kinds, which vary in amount and in kind in various descriptions of cotton. In the process of manufacture into cotton cloths, and as the material passes through the operations of bleaching, dyeing or printing, the impurities are eliminated.
Impurities of the Cotton Fibre.—Dr. E. Schunck made an investigation many years ago into the character of the impurities, and found them to consist of the following substances:—
Cotton Wax.—This substance bears a close resemblance to carnauba wax. It is lighter than water, has a waxy lustre, is somewhat translucent, is easily powdered, and melts below the boiling point of water. It is insoluble in water, but dissolves in alcohol and in ether. When boiled with weak caustic soda it melts but is not dissolved by the alkali; it can, however, be dissolved by boiling with alcoholic caustic potash. This wax is found fairly uniformly distributed over the surface of the cotton fibre, and it is due to this fact that raw cotton is wetted by water only with difficulty.
Fatty Acids.—A solid, fatty acid, melting at 55 deg. C. is also present in cotton. Probably stearic acid is the main constituent of this fatty acid.
Colouring Matter.—Two brown colouring matters, both containing nitrogen, can be obtained from raw cotton. One of these is readily soluble in alcohol, the other only sparingly so. The presence in relatively large quantities of these bodies accounts for the brown colour of Egyptian and some other dark-coloured varieties of cotton.
Pectic Acid.—This is the chief impurity found in raw cotton. It can be obtained in the form of an amorphous substance of a light yellow colour, not unlike gum in appearance. It is soluble in boiling water, and the solution has a faint acid reaction. Acids and many metallic salts, such as mercury, chloride and lead acetate, precipitate pectic acid from its solutions. Alkalies combine with it, and these compounds form brown substances, are but sparingly soluble in water, and many of them can be precipitated out by addition of neutral salts, like sodium and ammonium chlorides.
Albumens.—A small quantity of albuminous matter is found among the impurities of cotton.
Structure of the Cotton Fibre.—The cotton fibre varies in length from 1 to 2 inches, not only in fibres of the same class but also in fibres from different localities—Indian fibres varying from 0.8 in the shortest to 1.4 in the longest stapled varieties; Egyptian cotton fibres range from 1.1 to 1.6 inches long; American cotton ranges from 0.8 in the shortest to 2 inches in the longest fibres. The diameter is about 1/1260 of an inch. When seen under the microscope fully ripe cotton presents the appearance of irregularly twisted ribbons, with thick rounded edges. The thickest part is the root end, or point of attachment to the seed. The free end terminates in a point. The diameter is fairly uniform through 3/4 to 7/8 of its length, the rest is taper. In Fig. 1 is given some illustrations of the cotton fibre, showing this twisted and ribbon-like structure, while in Fig. 1A is given some transverse sections of the fibre. These show that it is a collapsed cylinder, the walls being of considerable thickness when compared with the internal bore or canal.
Perfectly developed, well-formed cotton fibres always present this appearance. But all commercial cottons contain more or less of fibres which are not perfectly developed or are unripe. These are known as "dead fibres"; they do not spin well and they do not dye well. On examination under the microscope it is seen that these fibres have not the flattened, twisted appearance of the ripe fibres, but are flatter, and the central canal is almost obliterated and the fibres are but little twisted. Dead fibres are thin, brittle and weak.
Composition of the Cotton Fibres.—Of all the vegetable textile fibres cotton is found to have the simplest chemical composition and to be, as it were, the type substance of all such fibres, the others differing from it in several respects. When stripped of the comparatively small quantities of impurities, cotton is found to consist of a substance to which the name of cellulose has been given.
Cellulose is a compound of the three elements, carbon, hydrogen and oxygen, in the proportions shown in the following analysis:—
Carbon, 44.2 per cent., Hydrogen, 6.3 per cent., Oxygen, 49.5 per cent.,
which corresponds to the empirical formula C{6}H{10}O{5}, which shows it to belong to the group of carbo-hydrates, that is, bodies which contain the hydrogen and oxygen present in them in the proportion in which they are present in water, namely H{2}O.
Cellulose may be obtained in a pure condition from cotton by treatment with alkalies, followed by washing, and by treatment with alkaline hypochlorites, acids, washing and, finally, drying. As thus obtained it is a white substance having the form of the fibre from which it is procured, showing a slight lustre, and is slightly translucent. The specific gravity is 1.5, it being heavier than water. It is characterised by being very inert, a property of considerable value from a technical point of view, as enabling the fibres to stand the various operations of bleaching, dyeing, printing, finishing, etc. Nevertheless, by suitable means, cellulose can be made to undergo various chemical decompositions which will be noted in some detail.
Cellulose on exposure to the air will absorb moisture or water. This is known as hygroscopic moisture, or "water of condition". The amount in cotton is about 8 per cent., and it has a very important bearing on the spinning properties of the fibre, as it makes the fibre soft and elastic, while absolutely dry cotton fibre is stiff, brittle and non-elastic; hence it is easier to spin and weave cotton in moist climates or weather than in dry climates or weather. Cotton cellulose is insoluble in all ordinary solvents, such as water, ether, alcohol, chloroform, benzene, etc., and these agents have no influence in any way on the material, but it is soluble in some special solvents that will be noted later on.
ACTION OF ALKALIES.
The action of alkalies on cellulose or cotton is one of great importance in view of the universal use of alkaline liquors made from soda or caustic soda in the scouring, bleaching and dyeing of cotton, while great interest attaches to the use of caustic soda in the "mercerising" of cotton.
Dilute solutions of the caustic alkalies, caustic soda or caustic potash, of from 2 to 7 per cent. strength, have no action on cellulose or cotton, in the cold, even when a prolonged digestion of the fibre with the alkaline solution takes place. Caustic alkali solutions of from 1 to 2 per cent. strength have little or no action even when used at high temperatures and under considerable pressure—a fact of very great importance from a bleacher's point of view, as it enables him to subject cotton to a boil in kiers, with such alkaline solutions at high pressures, for the purpose of scouring the cotton, without damaging the fibre itself.
Solutions of caustic soda of greater strength than 3 per cent. tend, when boiled under pressure, to convert the cellulose into soluble bodies, and as much as 20 per cent. of the fibre may become dissolved under such treatment. The action of strong solutions of caustic soda or caustic potash upon cellulose or cotton is somewhat different. Mercer found that solutions containing 10 per cent. of alkali had a very considerable effect upon the fibre, causing it to swell up and become gelatinous and transparent in its structure, each individual cotton fibre losing its ribbon-like appearance, and assuming a rod-like form, the central canal being more or less obliterated. This is shown in Fig. 2 and 2A, where the fibre is shown as a rod and the cross section in Fig. 2A has no central canal. The action which takes place is as follows: The cellulose enters into a combination with the alkali and there is formed a sodium cellulose, which has the formula C{6}H{10}O{5}2NaOH. This alkali cellulose, however, is not a stable body; by washing with water the alkali is removed, and hydrated cellulose is obtained, which has the formula C{6}H{10}O{5}H{2}O. Water removes the whole of the alkali, but alcohol only removes one half. It has been observed that during the process of washing with water the fibre shrinks very much. This shrinkage is more particularly to be observed in the case of cotton. As John Mercer was the first to point out the action of the alkaline solutions on cotton, the process has become known as "mercerisation".
Solutions of caustic soda of 1.000 or 20 deg. Tw. in strength have very little mercerising action, and it is only by prolonged treatment that mercerisation can be effected. It is interesting to observe that the addition of zinc oxide to the caustic solution increases its mercerising powers. Solutions of 1.225 to 1.275 (that is from 45 deg. to 55 deg. Tw. in strength) effect the mercerisation almost immediately in the cold, and this is the best strength at which to use caustic soda solutions for this purpose. In addition to the change brought about by the shrinking and thickening of the material, the mercerised fibres are stronger than the untreated fibres, and at the same time they have a stronger affinity for dyes, a piece of cloth mercerised taking up three times as much colouring matter as a piece of unmercerised cloth from the same dye-bath.
The shrinkage of the cotton, which takes place during the operation of washing with water, was for a long time a bar to any practical application of the "mercerising" process, but some years ago Lowe ascertained that by conducting the operation while the cotton was stretched or in a state of tension this shrinkage did not take place; further, Thomas and Prevost found that the cotton so treated gained a silky lustre, and it has since been ascertained that this lustre is most highly developed with the long-stapled Egyptian and Sea Island cottons. This mercerising under tension is now applied on a large scale to produce silkified cotton. When viewed under the microscope, the silkified cotton fibres have the appearance shown in Fig. 3, long rod-like fibres nearly if not quite cylindrical; the cross section of those fibres has the appearance shown in Fig. 3A. This structure fully accounts for the silky lustre possessed by the mercerised fibres. Silky mercerised cotton has very considerable affinity for dye-stuffs, taking them up much more readily from dye-baths, and it is dyed in very brilliant shades.
In the chapter on Scouring and Bleaching of Cotton, some reference will be made to the action of alkalies on cotton.
ACTION OF ACIDS ON CELLULOSE.
The action of acids on cellulose is a very varied one, being dependent upon several factors, such as the particular acid used, the strength of the acid, duration of action, temperature, etc. As a rule, organic acids—for example acetic, oxalic, citric, tartaric—have no action on cellulose or cotton. Solutions of sulphuric acid or hydrochloric acid of 2 per cent. strength have practically no action in the cold, and if after immersion the cotton or cellulose be well washed there is no change of any kind. This is important, as in certain operations of bleaching cotton and other vegetable fibres it is necessary to sour them, which could not be done if acids acted on them, but it is important to thoroughly wash the goods afterwards. When the acid solutions are used at the boil they have a disintegrating effect on the cellulose, the latter being converted into hydrocellulose. When dried, the cellulose is very brittle and powdery, which in the case of cotton yarn being so treated would show itself by the yarn becoming tender and rotten. The degree of action varies with the temperature (the higher this is the stronger the action), and also according to the strength of the acid solution. Thus a 10 per cent. solution of sulphuric acid used at a temperature of 80 deg. C. begins to act on cotton after about five minutes' immersion, in half an hour there is a perceptible amount of disintegration, but the complete conversion of the cotton into hydrocellulose requires one hour's immersion. A dilute acid with 8 volumes of water, used in the cold, takes three hours' immersion before any action on the cotton becomes evident.
ACTION OF SULPHURIC ACID ON COTTON.
When cellulose (cotton) is immersed in strong sulphuric acid the cotton becomes gradually dissolved; as the action progresses cellulose sulphates are formed, and some hydrolytic action takes place, with the formation of sugar. This fact has long been known, but only recently has it been shown that dextrose was the variety of sugar which was formed. On diluting the strong acid solution with water there is precipitated out the hydro or oxycelluloses that have been formed, while the cellulose sulphates are retained in solution.
By suitable means the calcium, barium, or lead salts of these cellulose-sulphuric acids can be prepared. Analysis of them shows that these salts undergo hydrolysis, and lose half their sulphuric acid.
The action of strong sulphuric acid has a practical application in the production of parchment paper; unsized paper is immersed in strong acid of the proper strength for about a minute, and then immediately rinsed in water. The acid acts upon the surface of the paper and forms the cellulose-sulphuric acid which remains attached to the surface. On passing into the water this is decomposed, the acid is washed away, and the cellulose is deposited in an amorphous form on the paper, filling up its pores and rendering it waterproof and grease-proof. Such papers are now largely used for packing purposes.
ACTION OF HYDROCHLORIC ACID.
Dilute hydrochloric acid of from 1 deg. to 2 deg. Tw. in strength, used in the cold, has no action on cellulose. Cotton immersed in acids of the strength named and then well washed in water is not materially affected in any way, which is a feature of some value in connection with the bleaching of cotton, where the material has to be treated at two points in the process with weak acids. Boiling dilute hydrochloric acid of 10 deg. Tw. disintegrates cellulose very rapidly. The product is a white very friable powder, which if viewed under the microscope appears to be fragments of the fibre that has been used to prepare it. The product has the composition C{12}H{22}O{11}, and is therefore a hydrate of cellulose, the latter having undergone hydrolysis by taking up the elements of water according to the equation 2C{6}H{10}O{5} + H{2}O = C{12}H{22}O{11}. By further digestion with the acid, the hydrocellulose, as it is called, undergoes molecular change, and is converted into dextrine. In composition hydrocellulose resembles the product formed by the addition of sulphuric acid which has received the name of amyloid. It differs from cellulose in containing free carboxyl, CO, groups, while its hydroxyl groups, HO, are much more active in their chemical reactions.
Hydrocellulose is soluble in nitric acid, 1.5 specific gravity, without undergoing oxidation. Nitrates are formed varying in composition.
The formation of hydrocellulose has a very important bearing in woollen manufacture. It is practically impossible to obtain wool free from vegetable fibres, and it is often desirable to separate these vegetable fibres. For this purpose the goods are passed into a bath of hydrochloric acid or of weak sulphuric acid. On drying the acid converts the cotton or vegetable fibre into hydrocellulose which, being friable or powdery, can be easily removed, while the wool not having been acted on by the acid remains quite intact. The process is known as "carbonising". It may not only be done by means of the acids named but also by the use of acid salts, such as aluminium chloride, which on being heated are decomposed into free acid and basic oxide. For the same reason it is important to avoid the use of these bodies, aluminium chloride and sulphate, zinc and magnesium chlorides, etc., in the treatment of cotton fabrics; as in finishing processes, where the goods are dried afterwards, there is a great liability to form hydrocellulose with the accompaniment of the tendering of the goods.
ACTION OF NITRIC ACID.
The action of nitric acid on cellulose is a variable one, depending on many factors, strength of acid, duration of action and temperature. Naturally as nitric acid is a strong oxidising agent the action of nitric acid on cellulose is essentially in all cases that of an oxidant, but the character of the product which is obtained varies very much according to the conditions just noted. When cellulose or cotton in any form is immersed in nitric acid of 1.4 to 1.5 specific gravity for a moment, and the fibre be well washed, there is a formation of hydrate of cellulose which has a gelatinous nature. This is deposited on the rest of the material, which is not materially affected so far as regards strength and appearance, but its power of affinity for dyes is materially increased. There is some shrinkage in the size of the cotton or paper acted upon.
Nitric acid changes all kinds of cellulose into nitro products, the composition of which depends upon the strength of the acid, the duration of treatment, and one or two other factors. The nitrocelluloses are all highly inflammable bodies, the more highly nitrated burning with explosive force. They are produced commercially and are known as "gun cotton" or "pyroxyline". The most highly nitrated body forms the basis of the explosive variety; the least highly nitrated forms that of the soluble gun cotton used for making collodion for photographic and other purposes.
The products formed by the action of nitric acid are usually considered to be nitrocelluloses. It would appear that they are more correctly described as cellulose-nitrates, for analysis indicates the presence of the NO{3} group, which is characteristic of nitrates, and not of the NO{2} group, which is the feature of nitro bodies in general. Further, nitro compounds, when subject to the action of reducing agents, are converted into amido compounds, as is the case, for instance, with nitro-benzene, C{6}H{5}NO{2}, into aniline, C{6}H{5}NH{2}, or with nitro-naphthalene, C{10}H{7}NO{2}, which changes into naphthylamine, C{10}H{7}NH{2}.
But the nitric acid derivatives of cellulose are not capable of conversion by reducing agents into similar amido compounds. They have the following properties, which accord more closely with nitrates than with nitric bodies: alkalies remove the nitric acid; cold sulphuric acid expels the nitric acid, cellulose sulphates being formed; boiling with ferrous sulphate and hydrochloric acid causes the elimination of the nitric acid as nitric oxide (on which reaction a method for determining the degree of nitration of gun cotton is based). It is best therefore to consider them as cellulose nitrates. Several well-characterised cellulose nitrates have been prepared, but is an exceedingly difficult matter to obtain any one in a state of purity, the commercial articles being always mixtures of two or three. Those that are best known and of the most importance are the following:—
Cellulose Hexa-nitrate, C{6}H{4}O{5}(NO{3}){6}. This forms the principal portion of the commercial explosive gun cotton, and is made when a mixture of strong nitric acid and strong sulphuric acid is allowed to act on cotton at from 50 to 55 deg. F. for twenty-four hours. The longer the action is prolonged, the more completely is the cotton converted into the nitrate, with a short duration the finished product contains lower nitrates. This hexa-nitrate is insoluble in ether, alcohol, or in a mixture of those solvents, likewise in glacial acetic acid or in methyl alcohol.
Cellulose Penta-nitrate, C{6}H{5}O{5}(NO{3}){5}, is found in explosive gun cotton to a small extent. When gun cotton is dissolved in nitric acid and sulphuric acid is added, the penta-nitrate is thrown down as a precipitate. It is not soluble in alcohol, but is so in a mixture of ether and alcohol, it is also slightly soluble in acetic acid. Solutions of caustic potash convert it into the di-nitrate.
Cellulose Tetra-nitrate, C{6}H{6}O{5}(NO{3}){4}, and Cellulose Tri-nitrate, C{6}H{7}O{5},(NO{3}){3}, form the basis of the pyroxyline or soluble gun cotton of commerce. It has not been found possible to separate them owing to their behaviour to solvents being very similar. These nitrates are obtained by treating cotton with nitric acid for twenty or thirty minutes. They are characterised by being more soluble than the higher nitrates and less inflammable. They are freely soluble in a mixture of ether and alcohol, from which solutions they are precipitated in a gelatinous form on adding chloroform. Acetic ether, methyl alcohol, acetone and glacial acetic acid, will also dissolve these nitrates.
Cellulose Di-nitrate, C{6}H{8}O{5}(NO{3}){2}, is obtained when cellulose is treated with hot dilute nitric acid, or when the high nitrates are boiled with solutions of caustic soda or caustic potash. Like the last-mentioned nitrates it is soluble in a mixture of alcohol and ether, in acetic ether, and in absolute alcohol. The solution of the pyroxyline nitrates in ether and alcohol is known as collodion, and is used in photography and in medical and surgical work.
One of the most interesting applications of the cellulose nitrates is in the production of artificial silk. Several processes, the differences between which are partly chemical and partly mechanical, have been patented for the production of artificial silk, those of Lehner and of Chardonnet being of most importance. They all depend upon the fact that when a solution of cellulose nitrate is forced through a fine aperture or tube, the solvent evaporates almost immediately, leaving a gelatinous thread of the cellulose nitrate which is very tough and elastic, and possesses a brilliant lustre. Chardonnet dissolves the cellulose nitrate in a mixture of alcohol and ether, and the solution is forced through fine capillary tubes into hot water, when the solvents immediately evaporate, leaving the cellulose nitrate in the form of very fine fibre, which by suitable machinery is drawn away as fast as it is formed. Lehner's process is very similar to that of Chardonnet. Lehner uses a solution of cellulose nitrate in ether and alcohol, and adds a small quantity of sulphuric acid; by the adoption of the latter ingredient he is able to use a stronger solution of cellulose nitrate, 10 to 15 per cent., than would otherwise be possible, and thereby obtains a stronger thread which resists the process of drawing much better than is the case when only a weak solution in alcohol and ether is employed. By subsequent treatment the fibre can be denitrated and so rendered less inflammable.
The denitrated fibres thus prepared very closely resemble silk in their lustre; they are not quite so soft and supple, nor are they in any way so strong as ordinary silk fibre of the same diameter.
Artificial silk can be dyed in the same manner as ordinary silk.
ACTION OF OXIDISING AGENTS ON CELLULOSE OR COTTON
Cellulose resists fairly well the action of weak oxidising agents; still too prolonged an action of weak oxidising agents has some influence upon the cotton fibre, and it may be worth while to point out the action of some bodies having an oxidising effect.
Nitric acid of about 1.15 specific gravity has little action in the cold, and only slowly on it when heated. The action is one of oxidation, the cellulose being transformed into a substance known as oxycellulose. This oxycellulose is white and flocculent. It tends to form gelatinous hydrates with water, and has a composition corresponding to the formula C{6}H{10}O{6}. It is soluble in a mixture of nitric and sulphuric acids, and on diluting this solution with water a tri-nitrate precipitates out. A weak solution of soda dissolves this oxycellulose with a yellow colour, while strong sulphuric acid forms a pink colouration. It is important to note that nitric acid of the strength given does not convert all the cellulose into oxycellulose, but there are formed also carbonic and oxalic acids. When cotton is passed into strong solutions of bleaching powder and of alkaline hypochlorites and then dried, it is found to be tendered very considerably. This effect of bleaching powder was first observed some thirteen years ago by George Witz, who ascribed the tendering of the cotton to the formation of an oxycellulose. Although the composition of this particular oxycellulose so formed has not yet been ascertained, there is reason to think that it differs somewhat from the oxycellulose formed by the action of the weak nitric acid. A notable property of the oxycellulose now under consideration is its affinity for the basic coal-tar dyes, which it will absorb directly. The oxycellulose is soluble in alkaline solutions.
In the ordinary bleaching process there is considerable risk of the formation of oxycellulose by the employment of the bleaching solutions of too great a strength, or in allowing the goods to lie too long before the final washing off. The presence of any oxycellulose in bleached cotton may be readily determined by immersing it in a weak solution of Methylene blue, when, if there be any oxycellulose present, the fibre will take up some of the dye-stuff.
Permanganate of potash is a very powerful oxidising agent. On cellulose neutral solutions have but little action, either in the cold or when heated. They may, therefore, be used for the bleaching of cotton or other cellulose fibres.
Alkaline solutions of permanganate convert the cellulose into oxycellulose, which resembles the oxycellulose obtained by the action of the nitric acid.
Chromic acid, when used in the form of a solution, has but little action on cellulose. In the presence of mineral acids, and used warm or boiling, chromic acid oxidises cellulose into oxycellulose and other products.
It is therefore always advisable in carrying out any technical process connected with cotton which involves its treatment with oxidising agents of any kind, and where it is desired not to alter the cotton, to allow these actions to be as short as possible.
Dyes and Cotton Dyeing.—An account of the chemistry of the cotton fibre would not be complete unless something is said about the reactions involved in the processes of dyeing and printing cotton. This is a most interesting subject and opens up quite a number of problems relating to the combination of the fibre with colouring matter of various kinds, but here only a brief outline of the principles that present themselves in considering the behaviour of the cotton fibre as regards colouring matter will be given.
When the question is considered from a broad point of view, and having regard to the various affinities of the dyes for cotton; we notice (1) that there is a large number of dye-stuffs—the Benzo, Congo, Diamine, Titan, Mikado, etc., dyes—that will dye the cotton from a plain bath or from a bath containing salt, sodium sulphate, borax or similar salts; (2) that there are dyes which, like Magenta, Safranine, Auramine and Methyl violet, will not dye the cotton fibre direct, but require it to be mordanted or prepared with tannic acid; (3) that there are some dyes or rather colouring matters which, like Alizarine, Nitroso-resorcine, barwood, logwood, etc., require alumina, chrome and iron mordants; (4) that there are some dyes which, like the azo scarlet and azo colours in general, cannot be used in cotton dyeing; (5) that there are a few dyes, i.e., indigo, which do not come under this grouping.
From the results of recent investigations into the chemistry of dyeing it is now considered that for perfect dyeing to take place there must be formed on the fibre a combination which is called a "colour lake," which consists of at least two constituents; one of these is the dye-stuff or the colouring matter itself, the other being either the fibre or a mordant, if such has to be used. The question of the formation of colour lakes is one connected with the molecular constitution of the colouring matter, but much yet remains to be done before the proper functions and mode of action of the various groups or radicles in the dye-stuffs can be definitely stated. While the constitution of the dye-stuff is of importance, that of the substance being dyed is also a factor in the question of the conditions under which it is applied.
In dealing with the first of the above groups of dyes, the direct dyes, the colourist is somewhat at a loss to explain in what manner the combination with the cotton fibre is brought about. The affinity of cellulose for dyes appears to be so small and its chemical activities so weak, that to assume the existence of a reaction between the dye-stuff and the fibre, tending to the formation of a colour lake, seems to be untenable. Then, again, the chemical composition and constitution of the dyes of this group are so varied that an explanation which would hold good for one might not do so for another. The relative fastness of the dyes against washing and soaping precludes the idea of a merely mechanical absorption of the dye by the fibre; on the other hand the great difference in the fastness to soaping and light between the same dyes on cotton and wool would show that there has not been a true formation of colour lake.
The dyeing of cotton with the second group of dyes is more easily explained. The cotton fibre has some affinity for the tannic acid used in preparing it and absorbs it from the mordanting bath. The tannic acid has the property of combining with the basic constituents of these dyes and forms a true colour lake, which is firmly fixed on the fibre. The colour lake can be formed independently of the fibre by bringing the tannic acid and the dye into contact with one another.
In the case of the dyes of the third group, the formation of a colour lake between the metallic oxide and the colouring matter can be readily demonstrated. In dyeing with these colours the cotton is first of all impregnated with the mordanting oxide, and afterwards placed in the dye-bath, the mordant already fixed on the fibre then reacts with the dye, and absorbs it, thus dyeing the cotton. To some extent the dyeing of cotton with the basic dyes of the second group and the mordant dyes of the third group is almost a mechanical one, the cotton fibre taking no part in it from a chemical point of view, but simply playing the part of a base or foundation on which the colour lake may be formed. In the case of the dyes of the fourth group, there being no chemical affinity of the cotton known for them, these dyes cannot be used in a successful manner; cotton will, if immersed in a bath containing them, more or less mechanically take up some of the colour from the liquor, but such colour can be almost completely washed out again, hence these dyes are not used in cotton dyeing, although many attempts have been made to render them available.
Indigo is a dye-stuff that stands by itself. Its combination with the cotton fibre is chiefly of a physical rather than a chemical nature; it does not form colour lakes in the same way as Alizarine and Magenta do.
Cellulose can be dissolved by certain metallic solutions and preparations:—
(1) Zinc Chloride.—When cotton or other form of cellulose is heated with a strong solution, 40 to 50 per cent., it slowly dissolves to a syrupy liquid. On diluting this liquid with water the cellulose is thrown down in a gelatinous form, but more or less hydrated, and containing some zinc oxide, 18 to 25 per cent., in combination.
(2) Zinc Chloride and Hydrochloric Acid.—When zinc chloride is dissolved in hydrochloric acid a liquid is obtained which dissolves cellulose; on dilution the cellulose is re-precipitated in a hydrated form. It is worth noting that the solution is not a stable one: on keeping, the cellulose changes its character and undergoes hydrolysis to a greater or less extent.
(3) Ammoniacal Copper.—When ammonia is added to a solution of copper sulphate, there is formed at first a pale blue precipitate of copper hydroxide, which on adding excess of ammonia dissolves to a deep blue solution—a reaction highly characteristic of copper. The ammoniacal copper solution thus prepared has, as was first observed by John Mercer, the property of dissolving cellulose fairly rapidly, even in the cold.
If instead of preparing the ammoniacal copper solution in the manner indicated above, which results in its containing a neutral ammonium salt, the copper hydroxide be prepared separately and then dissolved in ammonia a solution is obtained which is stronger in its action.
The cupra-ammonium solutions of cellulose are by no means stable but change on keeping. When freshly prepared, the cellulose may be precipitated from them almost unchanged by the addition of such bodies as alcohol, sugar and solutions of neutral alkaline salts. On keeping the cellulose undergoes more or less hydrolysis or even oxidation, for it has been observed that oxycellulose is formed on prolonged digestion of cellulose with cupra-ammonium solutions, while there is formed a fairly large proportion of a nitrite.
On adding lead acetate to the cupra-ammonium solution of cellulose, a compound of lead oxide and cellulose of somewhat variable composition is precipitated. It is of interest also to note that on adding metallic zinc to the cupra-ammonium solution the copper is thrown out and a solution containing zinc is obtained.
This action of cupra-ammonium solutions on cellulose has been made the basis for the production of the "Willesden" waterproof cloths. Cotton cloths or paper are passed through these solutions of various degrees of strength according to requirements, they are then passed through rollers which causes the surface to become more compact. There is formed on the surface of the goods a deposit of a gelatinous nature which makes the surface more compact, and the fabric becomes waterproof in character while the copper imparts to them a green colour and acts as a preservative. The "Willesden" fabrics have been found very useful for a variety of purposes.
CHAPTER II.
SCOURING AND BLEACHING OF COTTON.
Preparatory to the actual dyeing operations, it is necessary to treat cotton in any condition—loose cotton, yarn, or piece—so that the dyeing shall be properly done. Raw cotton contains many impurities, mechanical and otherwise; cotton yarns accumulate dirt and impurities of various kinds during the various spinning operations, while in weaving a piece of cotton cloth it is practically impossible to keep it clean and free from dirt, etc. Before the cotton can be dyed a perfectly level and uniform shade, free from dark spots or light patches, these impurities must be removed, and therefore the cotton is subjected to various scouring or cleansing operations with the object of effecting this end. Then again cotton naturally, especially Egyptian cotton, contains a small quantity of a brown colouring matter, and this would interfere with the purity of any pale tints of blue, rose, yellow, green, etc., which may be dyed on the cotton, and so it becomes necessary to remove this colour and render the cotton quite bright. This is commonly called "bleaching". It is these preparatory processes that will be dealt with in this chapter.
Scouring Cotton.—When dark shades—blacks, browns, olives, sages, greens, etc., are to be dyed it is not needful to subject the cotton to a bleaching operation, but simply to a scouring by boiling it with soda or caustic soda. This is very often-carried out in the same machine as the goods will be dyed in; thus, for instance, in the case of pieces, they would be charged in a jigger, this would be filled with a liquor made from soda or from caustic soda, and the pieces run from end to end, while the liquor is being heated to the boil—usually half to three-quarters of an hour is sufficient. Then the alkali liquor is run out, clean water run into the jigger and the pieces washed, after which the dyes, etc., are run into the jigger and the dyeing done. There is usually used 2 lb. to 3 lb. of caustic soda, or 3 lb. to 4 lb. of soda for each 100 lb. of goods so treated.
If the ordinary dyeing machines are not used for this purpose, then the ordinary bleachers' kiers may be used. These will be described presently.
Bleaching of Cotton.—Cotton is bleached in the form of yarn, or in the finished pieces. In the latter case the method depends very largely on the nature of the fabric; it is obvious that fine fabrics, like muslins or lace curtains, cannot stand the same rough treatment as a piece of twilled calico will. Then, again, the bleaching process is varied according to what is going to be done with the goods after they are bleached; sometimes they are sent out as they leave the bleach-house; again, they may have to be dyed or printed. In the first case the bleach need not be of such a perfect character as in the last case, which again must be more perfect than the second class of bleach. There may be recognised:—
(1) Market or white bleach. (2) Dyers or printers' bleach. (3) Madder bleach.
As the madder bleach is by far the most perfect of the three, and practically includes the others, this will be described in detail, and differences between it and the others will be then pointed out. A piece is subjected to the madder bleach which has afterwards to be printed with madder or alizarine. Usually in this kind of work the cloths are printed with mordant colours, and then dyed in a bath of the dye-stuff. This stains the whole of the piece, and to rid the cloth of the stain where it has to be left white, it is subjected to a soap bath. Now, unless the bleach has been thorough, the whites will be more or less stained permanently, and to avoid this cloths which are to be printed with alizarine colours are most thoroughly bleached. The madder bleach of the present day generally includes the following series of operations:—
(1) Stitching. (2) Singeing. (3) Singeing wash. (4) Lime boil. (5) Lime sour. (6) Lye boil. (7) Resin boil. (8) Wash. (9) Chemicing. (10) White sour.
(1) Stitching.—The pieces are fastened together by stitching into one long rope, which is passed in a continuous manner through all operations in which such a proceeding is possible. This stitching is done by machines, the simplest of which is the donkey machine, whereby the ends of the pieces, which are to be stitched together, are forced by a pair of cogwheels working together on to the needle carrying a piece of thread, this is then pulled through and forms a running stitch, a considerable length of thread being left on each side so as to prevent as far as possible the pulling asunder of the pieces by an accidental drawing out of the thread.
Birch's sewing machine is very largely used in bleach works. It consists essentially of a Wilcox & Gibb machine fitted on a stand so as to be driven by power. The pieces are carried under the needle by a large wheel, the periphery of which contains a number of projecting pins that, engaging in the cloth, carry it along.
There is also a contrivance by which these pieces to be sewn can be kept stretched, this takes the form of an arm with clips at the end, which hold one end of the cloth while it is running through the machine. The clip arrangement is automatic, and just before the end passes under the needle it is released, and the arm flies back ready for the next piece; it is, however, not necessary to use this arm always. This machine gives a chain stitch sufficiently firm to resist a pull in the direction of the length of the pieces, but giving readily to a pull at the end of the thread.
The Rayer & Lincoln machine is an American invention, and is much more complicated than Birch's. It consists of a sewing machine mounted on the periphery of a large revolving wheel. This carries a number of pins, which, engaging in the cloth to be stitched, carry it under the needle of the machine. Besides sewing the pieces together this machine is fitted with a pair of revolving cutters which trim the ends of the pieces as they pass through in a neat clean manner. There is also an arrangement to mark the pieces as they are being stitched. Like Birch's it produces a chain stitch.
What is important in sewing the ends of pieces together is to get a firm uniform stitch that lies level with the cloths without any knots projecting, which would catch in the bleaching machinery during the processes of bleaching, and this might lead to much damage being done.
Should it be necessary to mark the pieces so that they can be recognised after bleaching, the best thing to use is printers' ink. Gas tar is also much used, and is very good for the purpose. Coloured inks do not resist the bleaching sufficiently well to be used satisfactory. Vermilion and Indian red are used for reds, yellow ochre is the fastest of the yellows, there is no blue which will stand the process, and Guignet's green is the only green that will at all resist the process, umber will serve for brown. All these colours are used in the form of printing ink.
The next operation is a very important one, which cannot be too carefully carried out, that is:—
(2) Singeing.—For printing bleaches the cloths are singed. This has for its object the removal from the surface of the cloth of the fine fibres with which it is covered, and which would, if allowed to remain, prevent the designs printed on from coming out with sufficient clearness, giving them a blurred appearance.
Singeing is done in various ways, by passing the cloth over a red-hot copper plate, or over a red-hot revolving copper cylinder, or through a coke flame, or through gas flames, and more recently over a rod of platinum made red hot by electricity.
Plate singeing is the oldest of these methods and is still largely used. In this method a semi-cylindrical copper plate is heated in a suitable furnace to a bright red heat, the cloths are rapidly passed over it, and the loose fibres thereby burnt off. One great trouble is to keep the plate at one uniform heat over the whole of its surface, some parts will get hotter than others, and it is only by careful attention to the firing of the furnace that this can be obtained. To get over these difficulties Worral introduced a roller singeing machine in which the plate was replaced by a revolving copper roller, heated by a suitable furnace; the roller can be kept at a more uniform temperature than the plate. The singe obtained by the plate and roller is good, the principal fault being that if the cloths happen to get pressed down too much on the hot plate the loose ends are not burnt off as they should be. With both plate and roller the cloths are singed only on one side, and if both sides require to be singed a second passage is necessary. Both systems still retain their hold as the principal methods in use, notwithstanding the introduction of more modern methods.
Singeing by passing the cloths over a row of Bunsen burners has come largely into use. This has the great advantage of being very cleanly, and of doing the work very effectually, much more thoroughly than any other method, which is due to the fact that while in the methods described above only the loose fibres on the surface are burnt off; with gas all the loose fibres are burnt off. This is brought about by the gas flame passing straight through the cloth. It is not necessary to describe the gas singeing machine in detail. Singeing machines should be kept scrupulously clean and free from fluff, which is liable to collect round them, and very liable to fire. Some machines are fitted with a flue having a powerful draught which carries off this fluff, away from any source of danger.
(3) Singeing Wash.—After being singed the cloths are run through a washing machine to remove by water as much of the loose charred fibres as possible. The construction of a washing machine is well known. It consists of a pair of large wooden rollers set above a trough containing water and into which a constant stream of water flows. In the trough is also fixed another wooden roller and the pieces are passed round this bottom roller and between the top rollers. The cloth is passed through and round the rollers several times in a spiral form so that it passes through the water in the trough frequently, which is a great advantage, as the wash is thus much more effectual. The pressure between the two top rollers presses out any surplus water. The operation scarcely needs any further description.
(4) Lime Boil.—After the cloth leaves the singeing or grey wash, as it is often called, it passes through the liming machine, which is made very similar to the washing machine. In this it passes through milk of lime, which should be made from freshly slaked lime. The latter maybe prepared in a pasty form in a stone cistern. The lime used should be of good quality, free from stones, badly burnt pieces or any other insoluble material, so that when slaked it should give a fine smooth pasty mass.
Lime should not be slaked too long before using, as it absorbs carbonic acid from the atmosphere, whereby carbonate of lime is formed, and this is useless for liming cloth. The pasty slaked lime may be mixed with water to form the milk of lime, and this can be run from the cistern in which it is prepared into the liming machine as it is required; the supply pipe should be run into the bottom of the trough of the liming machine and not over the top, in which latter case it may splash on to the cloths and lead to overliming, which is not to be desired on account of its liability to rot the cloth. The amount of lime used varies in different bleachworks, and there is no rule on the subject; about 5 lb. to 7 lb. of dry lime to 100 lb. of cloth may be taken as a fair quantity to use.
The lime boil has for its object the removal or rather the saponification of the resinous and fatty matters present in the grey cloth, either naturally or which have been added in the process of weaving, or have got upon the cloth accidentally during the processes of spinning and weaving. With these bodies the lime forms insoluble lime soaps; these remain in the cloth, but in a form easily decomposable and removable by treatment with acids and washing. Soda or potash is not nearly so good for this first boiling as lime—for what reason is somewhat uncertain, but probably because they form with the grease in the cloths soluble soaps, which might float about the kier and accumulate in places where they are not required and thus lead to stains, whereas the insoluble lime soap remains where it was formed. The lime also seems to attack the natural colouring matter of the cotton, and although the colour of the limed cloth is darker than before boiling, yet the nature of the colour is so altered that it is more easily removed in the after processes. Besides these changes the starchy matters put into the cloth in the sizing are dissolved away. Great care should be taken to see that the goods are evenly laid in the kiers, not too tight, or the liquor will not penetrate properly; and not too slack, or they will float about and get entangled and more or less damaged. Then again care should be taken, especially when using low-pressure kiers, to see that the supply of liquor does not get too low, in which case the goods in the upper part of the kier are liable to get dry and are tendered thereby. So long as the goods in the kiers are not allowed to get dry there is no risk of damage; this trouble rarely arises with the Barlow and injector kiers. The inside of the kiers should be kept well limed, so that the goods shall not come in contact with the bare iron or metal of which the kier is constructed, as this would be very likely to lead to stains being produced which are by no means easy to remove. It is usual, and it is a good plan with almost all kinds of kiers, except the Mather and Edmeston kiers, to put a number of large pebbles or small stones at the bottom of the kier, which serves to make a false bottom on which the goods rest and through which the liquor penetrates and flows away. Before using, the stones should be well washed to free them from dirt and grit.
The lime boil is carried out in what are called "kiers". Many forms of kiers have been devised, but the one in most general use is that known as the "injector kier," of which a drawing is given in Fig. 4, of the form made by Messrs. Mather & Platt of Salford. Injector kiers are made to work either under a pressure of 40 lb. to 50 lb. of steam per square inch, when they are called high-pressure kiers, or at a pressure of 15 lb. to 20 lb., when they are called low-pressure kiers. The one shown in the drawing is intended for low-pressure kiers. The principle of construction is the same in all, the details varying somewhat with different makers. Injector kiers consist of a hollow, upright iron cylinder made of plates riveted together; the top is made to lift off, but can be fastened down tightly by means of bolts and nuts as shown in the drawing. From the bottom, and placed centrally, rises a pipe, known as the puffer pipe; this terminates at the top in a rose arrangement. The lower end of the pipe is perforated. A jet of steam is sent in at the bottom of this pipe, and by its force any liquor at the bottom of the kier is forced up the puffer pipe and distributed in a spray over any goods which may be in the kier. The liquor ultimately finds its way to the bottom of the kier ready to be blown up again. This circulation of the liquor can be maintained for any length of time and through its agency every part of the goods gets thorough and effectual treatment.
The length of the boil depends upon the kiers; with the open kier about ten hours are usually given, with the Barlow and injector kiers, working at a pressure of 40 lb. to 50 lb., six to seven hours are given.
(5) Lime or Grey Sour.—After the lime boil, the next operation is that of the lime sour or grey sour as it is often called to distinguish it from a subsequent souring. The souring is done in a machine constructed in the same way as a washing machine; the trough of the machine is filled with hydrochloric acid at 2 deg. Tw., which is kept ready prepared in a stone cistern and run into the machine as required (it is not advisable to use acid stronger than this). After passing through the sour the goods are piled in a heap on the stillage for a few hours. The acid attacks the lime soap which was formed during the lime boil, decomposes it and dissolves out the lime with the formation of calcium chloride, while the fat of the soap is liberated, the former is washed away in the subsequent washing, while the latter remains to a large extent on the goods, and is removed by the lye boil that follows. Sulphuric acid is not so satisfactory to use for the lime sour as hydrochloric acid, because it forms with the lime the insoluble sulphate of calcium, which is difficult to entirely remove from the goods, whereas the chloride is very soluble and is entirely eliminated from the goods by the washing that follows.
It is advisable to keep the acid at a uniform strength in the machine. The Twaddell is here of no use as an indicator of the actual strength, because the lime which the acid dissolves, while it neutralises and reduces the strength of the acid, actually raises the Twaddell, under which circumstance the only safe method is a chemical test. This can be carried out very simply and with a sufficient degree of accuracy by the workmen, and if it be done at regular intervals during the souring, and the supply of the fresh acid be regulated, the sour will be kept at a more uniform strength and more uniform results will be obtained than if the souring were done in a more empirical fashion. The test is best and most easily done as follows:—
Prepare a solution of 1 oz. of the powdered high strength 98 per cent. caustic soda in 1 pint of water, weighing and measuring these quantities very carefully. Now take a tall, narrow, white bottle of about 5 oz. capacity and make a mark on the neck. Fill this bottle with the test solution.
Now take exactly 5 ozs. of freshly prepared sour of 2 deg. Tw., pour into a jar, and add carefully some of the soda-test solution until a piece of cloth dyed with turmeric is turned brown, when the acid is neutralised. Now make a mark on the bottle of soda to show how much has been used. In all subsequent tests of the sour 5 ozs. should always take the same quantity of soda solution; if it takes less it is too weak, if more it is too strong; the remedy in each case is obvious. It is worth while to graduate the test bottle for 1 deg., 3 deg., 4 deg., 5 deg. Twaddell, as well as for 2 deg. Tw. acid.
After the souring it is often the custom to pile the goods on to a wooden stillage, but the goods should not be left too long so piled up for they may become dry, either entirely or in parts. In any case, as the goods dry the acid becomes concentrated and attacks them and makes them tender, which is not at all desirable. Therefore, if it is not convenient to proceed with them for some time after souring, they should be moistened with water from time to time, but it is best to wash them off at once, whereby they are made ready for the next operation.
(6) Lye Boil.—This is, perhaps, the most important operation in the whole process of bleaching, especially if the cloths are going to be printed in the so-called madder style with alizarine colours, or otherwise stains are liable to occur in the final stage, and it is then sometimes difficult to put the blame for these upon the right shoulders.
In principle the lye boil is simple, consisting in boiling the goods with a solution of soda ash, or caustic soda. The quantity of ash used varies in different works, as might naturally be expected; from 170 lb. to 200 lb. of ash to 10,000 lb. of cloth is a fair proportion to use. The length of boil averages about four hours, certainly not less than three should be given, and it is not necessary to give more than five hours in either ordinary kiers, with central puffer pipe, or in injector kiers.
Care should be taken to see that the goods are well packed into the kiers, not too tightly or the lye will fail to penetrate equally all through, and this is important if a uniform bleach is desired; neither should they be too loose, or they will float about and get torn. It is not necessary to be particular about the quantity of water used, except that it must be sufficient to keep the goods well covered, and still have enough to keep the circulation energetic. When the water is not sufficient in amount the goods get somewhat dry; there is then a liability to tendering, but with plenty of water there is no fear of any damage being done during a boil with alkali. Some works use caustic soda instead of soda ash in which case less is required, from 120 lb. to 150 lb. to 10,000 lb. of cloth, otherwise no alteration is made in the mode of boiling.
This lye boil clears away the fatty and waxy matter left in the goods after the lime sour, and thus prepares the way for the next boil. There is no advantage in using caustic soda in this preliminary boil, soda ash being just as effective and cheaper.
(7) Resin Boil.—Following the lye boil is the resin boil which consists in boiling the goods in a resin soap liquor. This is made as follows: a soda ash liquor of about 15 deg. to 20 deg. Tw. is prepared, and into this is thrown resin, broken up into small pieces.
The whole is boiled up until the resin is dissolved, and then as much more is added in small quantities as the alkali will take up. The soda liquor should not be much weaker than 20 deg. Tw., it will then be heavier than the resin which will float on the top, it is found to dissolve quicker and better than when the liquor is weak, in which case, the resin would sink to the bottom of the boiler and would there melt into a single mass difficult to dissolve. The resin soap liquor when made is ready to be used. The proportions of resin and alkali used in the boil vary in different works, but, as a rule, the quantities for 10,000 lb. of goods are 430 lb. of 58 per cent. soda ash, 180 lb. of resin, and 80 lb. of 70 per cent. caustic soda. Too much resin should be avoided, as it is found that with an excess the whites obtained are not nearly so good as when the right quantity is used; on the other hand, too little acts much in the same way. It may be taken that from 1-1/2 to 1-3/4 per cent. of the weight of the goods is about the right proportion; 1 per cent. being too little, and 2 per cent. too much. The quantity of soda used should be rather more than twice that of the resin, from 3-1/2 to 4 per cent. The length of boil is usually about twelve hours in a low-pressure kier; in a high-pressure kier about seven hours is sufficient.
What the special function of the resin is in this boil is not definitely known; but experience, both on a large and small scale, proves that it is essential to obtaining a good white for alizarine printing; without it, when the goods are dyed with alizarine after the mordants have been printed on, they frequently take a brown stain—with the resin this never or but rarely happens.
(8) Wash.—After the lye boils the goods must be washed, and it is important that this be done in as thorough a manner as possible. With the object of accomplishing this most thoroughly many washing machines have been invented, the main idea in all being to bring every part of the goods into contact with as much water as possible. Bridson's is an old form, and a very good one, the principle of this machine is to cause the cloth to pass to and fro, and to flap upon the surface of the water in the trough of the machine.
Furnival's square beater works on much the same principle, and does its work effectively. More modern washing machines are those of Birch, Farmer, Mather & Platt, and Hawthorne, where by the peculiar construction of the rollers and the use of beaters the cloth is very effectually washed. These machines are much more economical in the use of water than the older forms, and yet they do their work as well, if not better.
(9) Chemicing.—This is the actual bleaching operation, familiarly known as "chemicing," that is, the treatment of the goods with bleaching powder. The previous operations have resulted in obtaining a cloth free from grease, natural or acquired, and from other impurities, but it still has a slight brownish colour. This has to be removed before the goods can be considered a good white, which it is the aim of every bleacher they should be.
To get rid of this colour they are subjected to some final operations, the first of which is now to be considered. The chemicing consists in running the goods through a weak solution of bleaching powder (chloride of lime), piling the goods up into heaps, and allowing them to lie overnight, the next day they are finished. As the cloth has received, or ought to have received, a thorough bottoming, only a weak bath of chemic is required, about 1/2 to 1 deg. Tw. is quite sufficient. The solution is prepared in a stone cistern. There is very little difficulty in making it, the only precaution necessary is to have the solution quite clear and free from undissolved particles, for if these get upon the cloth they will either lead to the production of minute holes, or they may overbleach the fibre, which in such case will have the power of attracting excess of colour in any subsequent dyeing process and thus lead to stains, the origin of which may not be readily grasped at the first sight.
It is best, therefore, either to allow the solution to settle in the cistern till quite clear, which is the simplest way, or to filter through cloth.
The chemicing is best done cold and with weak solution, at 1/2 deg. Tw. rather than 1 deg. Tw. Warming the liquor increases the rapidity of the bleaching action, but there is a risk of over-chloring, which must be avoided as far as possible, because there is then danger of tendering the fibre, moreover, such over-chlored cloth has an affinity for colouring matters that is not at all desirable, as it leads to the production of stains and patches in the dyeing operations. It is much better, when a single chemicing does not bleach the cloth sufficiently and give a white, to run the cloth twice through a weak liquor rather than once through a strong liquor.
Although the chemicing is followed by a sour, which, acting on the bleaching powder, liberates chlorine that bleaches the fabric, yet the greatest proportion of the bleaching effect is brought about while the pieces are being piled up into heaps between the chemicing and the sour. In this state they should be left for some hours, covered over with a damp sheet, care being taken that they are not left piled so long as to become dry, as in this event there is a great risk of tendering the cloth or fabric; it is, therefore, a good plan to moisten them with a little water from time to time. They should not be tightly piled up, but be as loose as possible, so that the air can get to them, as it is the carbonic acid and other acid vapours in the air, that by acting on the chemic causes slow liberation of chlorine, which effects the bleaching of the goods.
(10) White Sour.—After the chemicing the goods are treated to a sour, for which purpose either hydrochloric acid or sulphuric acid may be used.
Hydrochloric acid possesses the advantage of forming a more soluble salt of lime (calcium chloride) than does sulphuric acid (calcium sulphate), and it has a more solvent action upon any traces of iron and other metallic oxide stains which may be present in the goods. On the other hand, on account of its fuming properties, it is unpleasant to work with. The souring is done by passing the goods through an acid liquor at 2 deg. Tw. strong and piling for two or three hours, after which it is washed. This final washing must be thorough, so that all traces of acid and chemic are washed out, otherwise there is a tendency for the goods to acquire a yellowish colouration.
So far the routine has been described of the so-called madder bleach, the most perfect kind of bleach applied to cotton cloths. Besides this two other kinds of bleach are distinguished in the trade. Turkey red and market bleach. The former is used when the cloth or yarn is to be dyed plain or self-coloured with delicate shades with Alizarine; the latter is used for cloth sold in the white. As the operations involved in producing these are identical in their method of manipulation to those already described, it will only be necessary to give an outline of the process for each one.
Turkey Red Bleach—(1) Rinse through water into a kier and boil for two hours. (2) Lime boil for three to four hours. The amount of lime required is rather less than what is used with the madder bleach, from 2-1/2 lb. to 3 lb, lime to each 1 cwt. of goods being quite sufficient. (3) Souring as in the madder bleach. (4) Lye boil, using about 100 lb. caustic soda to a ton of goods, and giving ten hours' boil. (5) Second lye boil using about 50 lb. soda ash to a ton of goods, after which the goods are well washed. (6) Chemicing as with the madder bleach. (7) Souring as with the madder bleach, then washing well.
This represents an average process, but almost every bleacher has his own methods, differing from the above in some of the details and this applies to all bleaching processes. It is obvious that the details may be varied to a great extent without changing the principles on which the process depends.
Market Bleach—Here all that requires to be done is to get the cloth of a sufficient degree of whiteness to please the eye of the customer. Market bleachers have, however, to deal with a wider range of goods than is dealt with in the former kinds of bleaches, from very fine muslins to very heavy sheetings. Now it is obvious from a merely mechanical point of view, that the former could not stand as rough a process as the latter, therefore there must be some differences in the details of muslin bleaching and sheeting bleaching. Then again with goods sold in the white, it is customary to weave coloured headings or markings, and as these have to be preserved, to do so will cause some slight alteration of the details of the bleach with this object. On all these points it is difficult to lay down general rules because of the very varying feature of the conditions which are met with by the market bleacher.
The resin boil may be omitted, only two lye boils being required, and these need not be so long or of such a searching character as the corresponding boils of the madder bleach. Outlines of two or three such processes, which are now in use in bleach works, will serve to show the general routine of a market bleach. The proportions given are calculated for 10,000 lb. of goods:—
(1) Lime boil, using 500 lb. of lime, and giving a twelve-hours' boil. (2) Grey sour, hydrochloric acid of 2 deg. Tw., then wash well. (3) Lye boil, 100 lb. caustic soda, 70 per cent. solid, ten to twelve hours' boil; wash. (4) Second lye boil, 100 lb., 58 per cent. soda ash, twelve-hours' boil. (5) Chemic, bleaching powder liquor at 1 deg. Tw., boil for three hours; wash. (6) White sour, sulphuric acid at 2 deg. Tw.; wash well.
The length of boil with the lime and lyes will depend upon the quality of the goods, heavy goods will require from two to three hours longer than will light goods, such as cambrics, the time given above being that for heavy goods, sheetings, etc.
Another process is the following:—
(1) Lime boil, using 480 lb. lime, and boiling for ten hours. (2) Grey sour, hydrochloric acid at 2 deg. Tw.; wash. (3) Lye boil, 300 lb. soda ash, 58 per cent.; 50 lb. caustic soda, 70 per cent., and 30 lb. soft soap, giving ten hours' boil; wash. (4) Chemic as above. (5) White sour as above; wash well.
A smaller quantity of lime is used here, but on the other hand the lye boil is a stronger one. This process gives good results. Some bleachers do not use lime in their market bleaches, but give two lye boils, in which case the process becomes:—
(1) Lye boil, using 140 lb. caustic soda, of 70 per cent., giving ten hours' boil and washing well. (2) Second lye boil, using 120 lb. soda ash, 58 per cent., and giving ten hours' boil; wash. (3) Chemic as above. (4) White sour as above; wash well.
Light fabrics, such as laces, lace curtains, muslins, etc., cannot stand the strain of the continuous process, and they are therefore subjected to a different bleaching process, which varies much at different bleach works. One method is to lime by steeping for an hour in a weak lime liquor, using about 2 lb. of lime to 100 lb. of goods; they are then boiled in the kier for eight hours, after which they are washed. This washing is done in what are called dash wheels, large hollow wheels, the interior of each being divided into four compartments. Into these the goods are put, and the wheel is caused to revolve, while at the same time a current of water flows with some force into the interior of the wheel and washes the goods.
The wheels do their work well, and the action being gentle the finest fabrics can be washed without fear of any damage. It is necessary that the speed at which they are driven should be such that as the wheel revolves the goods are thrown from side to side of each compartment; if the speed be too slow they will simply slide down, and then they do not get properly washed; on the other hand, if the speed be too great then centrifugal action comes into play and the goods remain in a stationary position in the wheels with the same result. As to the amount of washing, it should be as before. After this washing they are boiled again in the kier with soda ash, using about 8 lb. ash for 100 lb. goods and giving seven hours' boil, which, after washing, is followed by a second boil with 5 lb. ash and 4 lb. soft soap for each 100 lb. of goods, giving eight hours' boil. They are then washed and entered into the chemic. The chemicing is done in stone cisterns, which are fitted with false bottoms, on which the goods can rest, and which allow any insoluble particles of bleaching powder to settle out and prevent them from getting on the goods. The liquor is used at the strength of about 1/2 deg. Tw., and the goods are allowed to steep about two hours; they are then placed in a hydro-extractor and the surplus chemic is whizzed out, after which they are steeped in sour of hydrochloric acid at 1 deg. Tw., kept in a stone cistern, the goods being allowed to steep for two hours. Next they are washed, well whizzed, passed through a blueing water, whizzed again, and dried. The remarks made when describing similar operations of the madder bleach as to the action, testing, etc., of the chemicals, are equally applicable here.
Another plan of bleaching fine fabrics is shown in outline in the following scheme:—
(1) Wash; boil in water for two hours. (2) Boil in soda for five hours, using 80 lb. soda ash of 58 per cent., and 30 lb. soft soap for 1,000 lb. of goods. (3) Second soda boil, using from 40 lb. to 50 lb. soda ash, and 15 lb. to 20 lb. soft soap, giving four hours' boil; after each soda boil the goods should be washed. (4) Chemic, using bleaching powder liquor at 1/2 deg. Tw., allowing them to steep for two hours, then wash and whiz. (5) White sour, using hydrochloric acid at 2 deg. Tw., steeping two hours; wash.
A further extension of the same process is sometimes given for the best goods, which consists, after the above, in giving:—
(6) A third soda boil, using 25 lb. to 30 lb. soda ash and 15 lb. to 20 lb. soft soap, giving one hour's boil; washing. (7) Chemic as before. (8) Sour as before, after which the goods are well washed.
In the bleaching of Nottingham lace curtains for the soda boils there is used what is called the "dolly," which consists of a large round wooden tub about 5 feet to 6 feet in diameter and about 2 feet 6 inches to 3 feet deep; this is made to revolve slowly at about one revolution per minute. Above the tub on a strong frame are arranged four stampers or beaters, which are caused to rise and fall by means of cams. The goods are placed in the tub with the scouring liquors and the dolly is set in motion, the beaters force the liquor into the goods, and the revolution of the tub causes the beaters to work on a fresh portion of the goods at every fall.
This is rather an old-fashioned form of machine, and is being replaced by more modern forms of boiling kiers. In bleaching certain kinds of muslins in which the warp threads are double, and in the case of lace curtains, it is necessary to endeavour to keep the threads as open and prominent as possible. This cannot be done with the continuous process, which puts a strain on the threads and thus effaces their individuality. To avoid this the fabrics have to be dealt with in bundles or lumps, and on these no strain is put, therefore every thread retains its individuality. The process above described is applicable.
Yarn Bleaching.—Yarn is supplied to the bleacher in two forms: (1) warps in which the length of the threads may vary from as little as 50 to as much as 5,000 yards; these can be dealt with in much the same manner as a piece of cloth, that is, a continuous system can be adopted; (2) hanks, which are too well known to require description. Sometimes yarn is bleached in the form of cops, but as the results of cop bleaching are not very satisfactory it is done as little as possible.
Warp Bleaching.—The warp, if very long, is doubled two, three or four times upon itself, so as to reduce its length. Care should be taken that the ends of the warp are tied together to prevent any chance of entangling, which would very likely happen if the ends were left loose to float about. As a rule, warps are not limed, but the adoption of the liming would assist the bleaching. In outline warp bleaching consists of the following operations:—
(1) Lye boil, using 30 lb. caustic soda, 70 per cent., and 50 lb. soda ash, 58 per cent., giving six hours' boil, and washing. (2) Sweeting, boil with 80 lb. soda ash, 58 per cent., for two hours. (3) Washing. (4) Chemicing, bleaching powder liquor at 1 deg. Tw., washing. (5) Sour, sulphuric acid at 2 deg. Tw,. washing well. (6) Hydro extracting and drying.
About 2,000 to 3,000 lb. of warps are usually treated at one time.
The machinery used may be the same as that used in the cloth bleach, and each operation may be conducted in the same manner. In some warp bleachworks, while the kiers are made in the same way, the other machines are made differently. The chemicing and souring is done in strong cisterns provided with a false bottom; in these the warps are allowed to remain for about two hours. A more complicated form of chemicing cistern is also in use. This is made of stone, and is provided with a false bottom. Above is a tank or sieve, as it is called, having a perforated bottom through which the liquor flows on the warp in the cistern below.
Under the chemicing cistern is a tank into which the liquor flows, and from which it is pumped up into the sieve above. A circulation of liquor is thus kept up during the whole of the operation. Owing to the action of the chemic or acid on the metal work of the pump there is great wear and tear of the latter, necessitating frequent repairs. This is a defect in this form of chemicing machine. For drying the warps a hydro-extractor is first used to get the surplus liquor from the goods. This machine is now well known, and is in use in every bleachworks, where it is familiarly known as the "whiz," and the operation is generally called whizzing. Hydro-extractors are described under the head of "Dyeing Machinery".
The actual drying of the warps is done over the "tins" as they are called. These are a number of large cylinders measuring about 20 inches in diameter and about—for warp drying—5 feet long. Usually they are arranged vertically in two tiers, each tier consisting of about five cylinders, not arranged directly one above another but in a zig-zag manner, the centres of the first, third and fifth being in one line, and the centres of the others in another line. The cylinders are made to revolve by suitable driving mechanism, and into them is sent steam at about 5 lb. to 10 lb. pressure, which heats up the cylinders, whereby the warp passing over them is dried. This drying may be partial or complete, being regulated by the speed at which the warps pass over the cylinders and by the quantity of steam passed into the same. The quicker the speed and the smaller the amount of steam, the less the warps are dried; while, on the other hand, the slower the speed and the larger the amount and greater the pressure of the steam, the quicker and more thoroughly are the warps dried. As there is a great deal of water formed in the cylinders by the condensation of the steam, means are always provided for carrying off this water, as its retention in the cylinders often leads to serious results and damage to the machine.
Hank Bleaching.—So far as the chemical part of hank bleaching is concerned it does not differ from that of warp bleaching; the same operations and proportions of chemicals may be used and in the same order, but there is some difference in the machinery which is used. The hanks may be manipulated in two ways: they may be either kept in separate hanks, which is the method mostly in vogue in modern bleach-houses, or they may be linked together in the form of a chain. In the latter case the operations and the machinery may be the same as used in the madder bleach, with a few unimportant minor differences. In the final washing the dumping machine is used, which consists of two wooden bowls set over a wooden trough containing the wash waters. The top bowl is covered with a thick layer of rope and merely rests on the bottom bowl by its own weight, and is driven by friction from the latter. The chain of hanks passing through between the two bowls has the surplus liquor squeezed out of it, and as there is considerable increase in the thickness at the points of linkage between the hanks, when these pass through the bowls they lift up the top bowl, which, when the thick places have passed through, falls down with a sudden bump upon the thin places, and this bumping drives out all the surplus liquor and drives the liquor itself into the very centre of the hanks, which is sometimes an advantage.
In modern bleach-houses the chain form is gradually giving place to the method of bleaching separate hanks, partly because so many improvements have been made in the hank-bleaching machinery of late years, which enables bleachers to handle the yarn in the form of separate hanks better than they could do formerly; and as bleaching in separate hanks means that the cotton is kept in a more open form, and is thus more easily penetrated by the various liquors which are used, it follows that the bleach will be better and more thorough, which is what the bleacher aims at. At the same time weaker liquors or, what is the same thing, less material can be used, which means a saving in the cost of the process. For bleaching yarn in the hank the following process may be followed with good results:—
(1) Lye boil, using 1,000 lb. yarn, 40 lb. caustic soda of 70 per cent., and 50 lb. of soda ash of 58 per cent., giving five to six hours' boil at low pressure. (2) Wash through washing machine. (3) Second lye boil, using 40 lb. soda ash of 58 per cent., and giving two to three hours' boil, wash again through a washing machine. (4) Chemic as in warp bleaching. (5) Sour as in warp bleaching. (6) Wash well. (7) Hydro extract and dry.
Sometimes, if the yarn is to be sold in thread form, before the last operation it passes through another, viz., treating with soap and blue liquors, which will be dealt with presently.
The lye boils are done in the ordinary kiers, and do not call for further notice, except that in filling the goods into the kiers care should be taken that while sufficiently loose to permit of the alkaline liquors penetrating through the hanks properly, yet they should be so packed that they will not float about and thus become entangled and damaged.
The washing is nowadays done in a special form of washing machine, designed to wash the hanks quickly and well with as little expenditure of labour and washing liquor as possible. There are now several makes of these washing machines on the market, most of them do their work well, and it is difficult to say which is the best. Some machines are made to wash only one bundle at once, while others will do several bundles. Generally the principle on which they are constructed is the same in all, a trough containing the ash liquor, over which is suspended a revolving reel or bobbin, usually made of wood or enamelled iron, the bobbin being polygonal in form so that it will overcome readily any resistance the yarn may offer and carry the hank round as it revolves. The hank dips into the wash liquor in the trough, and as it is drawn through by the revolution of the bobbin it is washed very effectually. The moving of the hank opens out the threads, and thus the wash liquor thoroughly penetrates to every part of the hank, so that a few minutes' run through this machine thoroughly washes the yarn. A constant stream of clean water is passed through the trough. This machine may also be used for soaping and sizing the hanks if required. By extending the trough in a horizontal direction, and increasing the number of reels or bobbins, the quantity of material that can be washed at one time can be extended, although not to an indefinite extent. The workman can start at one end of the machine and fill all the bobbins with yarn, by the time he has finished this the first bobbinful will have been washed sufficiently and can be taken off and replaced with another quantity of yarn, and thus one by one the bobbins may be emptied and refilled, which means that a considerable amount of material can be got through in the course of a day. To avoid the labour of walking to and fro to fill and refill the bobbins, washing machines are constructed in which the trough is made in a circular form. The bobbins are placed at the ends of radial arms which are caused to revolve round over the trough, the workman is stationed constantly at one part of the circle, and as the arms pass him in their motion round the trough he takes off the washed hanks and puts on the unwashed hanks. By this machine he is saved a very considerable amount of labour, and is able to do his work in a more convenient manner. The yarn is well washed in such a machine. The trough may be entire or it may be divided into a number of compartments, each of which may contain a different kind of wash liquor if necessary. Of course it goes almost without saying that in all these machines the liquors in them may be heated up by means of steam pipes if required.
The chemicing and souring of the hanks does not call for special mention, beyond the fact that these operations are done in the same manner as warp bleaching. In Fig. 5 is shown Mather & Platt's yarn-bleaching kier, which is designed to bleach cotton yarn, either in hanks or in the warp forms, without removing it from the vessel into which it is first placed. The process is as follows: The hot alkali solution is circulated by means of a distributing pipe through the action of an injector or centrifugal pump to scour the yarn; then water is circulated by means of a centrifugal pump for washing. The chemic and sour liquors are circulated also by means of pumps, so that without the slightest disturbance to the yarn it is quickly and economically bleached.
STAINS AND DAMAGES IN BLEACHED GOODS.
Some of the stains in bleached goods which are met are beyond the control of the bleacher to avoid, while others are due to various defects in the process. Now the subject of stains can only be dealt with in a very general way, because of the varying manner in which they arise. The recognition of the particular way in which the stains have been formed is sometimes difficult to discover. First, there are iron stains, which are the most common kind of stains that a bleacher is troubled with. These generally make their appearance in the form of red spots of greater or less extent. As a rule they are not visible before the pieces are fully bleached. Their origin is varied. Sometimes they arise from the machinery; if the kiers are not kept thoroughly whitewashed out, there is a great liability to produce iron stains. Every other machine which is used in the process is made of iron, and should be kept free from rust, or the chances of stains are considerably increased. The water used in the bleaching must be free from iron. A small trace will not make much difference, but some waters contain a great deal of iron, so much so that they are absolutely unusable for bleaching purposes. Iron stains are often due to a very curious cause: the dropping of the oil used in the spinning or weaving machinery on to the cotton during the process of manufacture. This oil is often charged with iron derived from the wear and tear of the machinery, and which often gets fixed in the form of red spots of oxide on the fibre. Iron stains cannot readily be extracted.
Oil stains are also common. These take the form of bright yellow stains in various shapes, sometimes extending along the piece in streaks, at other times in patches in various places about the piece. Generally these oil stains do not make their appearance as soon as the piece is bleached, and often the bleacher sends out his goods quite white and apparently all right, and yet soon afterwards comes a complaint that the goods are stained yellow. One cause of these yellow oil stains can be traced to the use of paraffin wax in the sizing of the warps. In this case the stains are more or less streaky in form, and extend along the length of the piece. They are due to the fact that paraffin wax is not saponifiable by the action of the alkalies used in the process, and is therefore not extracted. When the goods are chemiced the chlorine acts upon the paraffin and forms chlorine compounds, which are acted upon by light, and turn yellow by exposure to that agent and to the atmosphere. Paraffin, when used for the sizing of warps, may sometimes be completely extracted from the fabric, but this depends upon the proportion of tallow or other fat which is used in the composition of the sizing grease. If the paraffin is only present in small quantities, and the grease well mixed, then it may be possible to extract all the paraffin out of the fabric during the bleaching process, but if the paraffin is in large proportion, or the grease not well mixed, it is scarcely possible to extract it all out, and stains must be the result. These stains can hardly be considered the fault of the bleacher, but are due to the manufacturer of the cloth using cheap sizing compositions on his warps. There are no means which can be adopted before bleaching to ascertain whether paraffin exists in the cloth. If found to be present, the remedy which is the easiest practically is to saturate the cloth with a little olive oil, or better, pale oleic acid. Allow the fatty matter to soak well in, and then boil the goods in a little caustic soda. Another cause of oil stains is the use of mineral oils in the lubrication of cotton machinery. These mineral oils partake of the nature of paraffin in their properties, and therefore they are unsaponifiable by the action of alkalies. Like paraffin wax, they resist the bleaching process, and much in the same manner produce stains. Oil stains show themselves in various forms—sometimes as spots. These may be due to the splashing of oil from the spindles during the process of spinning, or they may be in patches of a comparatively large size over the pieces. |
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