CHAPTER XX

ARTIFICIAL SILK--ITS PRODUCTION AND USE IN THE MANTLE INDUSTRY

The history of the artificial silk industry, since its foundation about the year 1890, illustrates curiously the rapidity with which isolated facts, of apparently merely academic interest, are seized upon and adapted to the needs of modern civilisation. It is during this period, especially, that the bonds between science and industry, in a dozen different directions, have been drawn so close that to-day it is in many cases impossible to differentiate the two. The pure science of to-day is the technology of to-morrow--and not always even of to-morrow, but of to-day. But we have moved even beyond this; the industrial needs of the day are creating and extending our science at a rate which shows how relatively poor a stimulus has been the mere desire for knowledge. Such has been the history of the artificial silk industry. No sooner had Chardonnet shown that the preparation of a new fabric was not only possible but profitable, than a thousand aspects of the problem were taken up. Patents were taken out on all sides--the majority, as usual, valueless, one or two of great importance. Companies were formed, factories built, machines invented; numberless applications were proposed, mostly again worthless, whilst patient research and innumerable experiments have carried one or two suggestions to a successful place in practice. Among these has been the adaptation of artificial silk to the manufacture of mantles, which will be outlined in the present chapter. Before taking up this question, however, a short account of the manufacture of the fabric itself must be given.

~Chardonnet Process.~--In the Chardonnet process, an account of which was published about 1890, continuous fibres are obtained by forcing through tiny jets a viscous solution of collodion, or nitrocellulose, as it has been misnamed, in a mixture of ethyl alcohol and ether. In the original form of the process, the solution was forced into water, which, by removing the alcohol and ether, caused an instantaneous coagulation of the surface, so that a filament was obtained which could be wound directly on to a spool. More generally, however, the jets deliver the solution into a chamber through which warm air is passed; this is equally effective in removing the solvents and causing surface coagulation, and the filaments are woven directly into threads of ten to forty strands, according to the purpose for which the fabric is required, fifteen to twenty being used for silk from which mantles are to be made. On account of its inflammability, the thread is denitrated by means of a solution of ammonium sulphide.

The raw material for the process is cellulose, usually in the form of cotton. Treatment of this with a suitable mixture of concentrated sulphuric and nitric acids replaces some of the hydroxyl groups by the ‘nitrate radicle,’ NO₃, a mixture of various nitrates of cellulose being formed, in which the so-called tetra-, penta-, and hexa-nitrates predominate.[525] The product, cellulose nitrate or collodion, very closely resembles the original cellulose in appearance and structure. It is washed thoroughly to free it from traces of acid--which render it liable to explode spontaneously--and after drying, dissolved in the minimum quantity of the mixed solvents;[526] the solution is filtered from insoluble impurities through wads of cotton, pressures of thirty to sixty atmospheres being required. This filtration purifies and thoroughly mixes the solution, so that perfect uniformity is obtained in the product. The glass jets through which the solution is now forced, under a pressure of forty to fifty atmospheres, have a diameter of 0·08 mm., but the threads obtained contract on the removal of the solvents, so that fibres of 0·01-0·02 mm. are formed.

[525] The cellulose esters are usually named as if they were derived from a compound C₁₂H₂₀O₁₀, the formula for cellulose being (C₆H₁₀O₅)_{_n_}. Thus the formation of the ‘hexa-nitrate’ would be represented--

C₁₂H₂₀O₁₀ + 6HNO₃ = C₁₂H₁₄O₄(NO₃)₆ + 6H₂O.

[526] In the Lehner process, in which collodion is also used, larger quantities of solvent are employed, so that much more dilute solutions are obtained; these require low pressures to form the thread, which is then hardened chemically.

Chardonnet probably began his work about 1885. It is interesting to observe that an Englishman, Swan, had proposed in 1883 to use a solution of collodion in acetic acid, fabrics prepared by his process being shown at the London Exhibition of 1884.[527]

[527] _Vide_ Böhm, _Zeitsch. angew. Chem._ 1912, ~25~, 657. There is no account of this process in the English patent literature.

~The Pauly or Cuprammonium Process.~[528]--It has long been known that a solution of copper hydroxide in ammonia solution--Schweitzer’s reagent--will dissolve cellulose. The use of this solvent for the production of artificial silk was proposed about 1900, and the method has become a serious rival of the older Chardonnet process. The solvent is prepared on a large scale by passing air through an ammonia solution to which copper turnings have been added. After addition of the cellulose, and filtration, the solution is forced through tiny jets into a bath of dilute acid, which removes the copper and precipitates the cellulose again.

[528] A full account of this and of the other processes employed in the manufacture of artificial silk will be found in the work of Piest, _Die Zellulose_, Stuttgart, 1910.

The solution of cellulose by Schweitzer’s reagent is undoubtedly a chemical action. Cellulose is to be regarded as a polyhydric alcohol, with one or several atoms of hydrogen of the hydroxyl groups replaceable by metals. According to Piest (_loc. cit._) a ‘Cupramine base’ is formed by the replacement of this hydrogen by copper and the amino-group, NH₂. The action of sodium hydroxide on cellulose, however, is generally regarded rather as an additive reaction, the product, ‘alkali cellulose,’ being usually written C₆H₁₀O₅,NaOH. A careful chemical investigation alone can reveal the actual nature of the compound formed; such an investigation, apart from its scientific interest, might yield results of considerable technical importance.

~The Viscose Process.~--Shortly after the introduction of the Chardonnet process, patents were taken out which protected a very cheap and simple method of dissolving cellulose,[529] which had been discovered by two well-known English authorities. Cross and Bevan. They found that mercerisation, _i.e._ the action of the sodium hydroxide on cellulose, produces a swollen, transparent mass, which very readily takes up carbon disulphide. When exposed to the action of this liquid for three or four hours, at ordinary temperatures, the mass swells further, gelatinising and becoming soluble in water. On treatment with water, a yellowish, extremely slimy solution is obtained, from which cellulose is precipitated on prolonged standing, by heating, or by oxidation. The substance is apparently a cellulose xanthate, and may be written NaS·CS·O·C₆H₉O₄,NaOH.[530] On account of the extremely viscous nature of the aqueous solution, Cross and Bevan gave it the name Viscoid.

[529] _Vide_, _e.g._ Cross, Bevan, and Beadle, _D. R. P._ 70999, granted September, 1893.

[530] _Vide_ Beltzer, _Zeitsch. angew. Chem._ 1908, ~21~, 1731.

During the last few years this method of dissolving cellulose has been employed in the manufacture of artificial silk, under the name ‘Viscose Process.’ The product obtained is very suitable for the manufacture of incandescent mantles, and is considerably cheaper than either the Chardonnet or Pauly silk.

~The Acetate Process.~--Quite recently numerous experiments have been carried out with the object of finding methods for employing the cellulose esters of organic acids in the preparation of fabrics. The acetate, which is generally used, gives solutions from which fibres can be obtained which are comparable to natural silk in strength, and which have the further advantage of being non-inflammable, and far less readily affected by water than artificial fabrics obtained by the above methods. It is prepared by treating cellulose with dilute acid, by which the so-called ‘hydrocellulose’ is obtained; this is treated with a mixture of glacial acetic acid and acetyl chloride, and the whole, after addition of a little concentrated sulphuric acid, warmed to 65°-70°C. As early as 1894, Cross and Bevan[531] had patented a process for this preparation by the action of acetyl chloride in the cold on an intimate mixture of cellulose and zinc chloride.

[531] _E._ 9676, 1894.

From the solution obtained, the acetates are precipitated by water, washed and dried. The mixture of esters dissolves in chloroform, nitromethane, acetic acid, phenol, pyridine, etc., and is re-precipitated by addition of alcohol, benzene, or ligroin (petroleum ether). On account of its non-inflammable character, cellulose acetate, as the product is called, is being used instead of the nitrate in the manufacture of celluloid; it is also used for non-inflammable cinematograph films. Fibres can be obtained by forcing the solutions through jets, and removing the solvent, as in the above processes; these are spun into threads which are coming into increasing use, on account of their extremely low conducting power, for the insulation of very fine electric leads. The product is at present too expensive, however, for use in the textile industries, or for the manufacture of mantles.

A solvent which had at one time some technical importance is zinc chloride.[532] The concentrated aqueous solution of the salt will take up cellulose in considerable quantity; and the solution has been used in the preparation of carbon filaments for glow lamps.

[532] Gulbrandsen, _Prog. Age_, 1912, ~30~, 77; Wynne and Powell, _E._ 16805, December, 1884.

The fabrics prepared by the processes which have been mentioned above are of great technical value. In lustre they far surpass natural silk, and they take dyes very well, but owing to the ease with which they tear, they cannot be woven alone for textiles, but are always used in ‘mixed’ materials. The acetate silk, which approaches the natural fibre in strength, is not much less expensive. Whilst the price of natural silk is roughly 35 francs per kilo. (approx. 13_s._ 3_d._ per lb.), the costs of production of the artificial fabrics are--Chardonnet 15 frs., Pauly 12 frs., Viscose 7 frs. per kilo. (respectively 5_s._ 8_d._, 4_s._ 6_d._, and 2_s._ 8_d._ per lb.). Artificial silk, however, has uses distinct from the natural fibre, and is at present a competitor with it in one or two small fields only. Thus the production of natural silk is ten times that of artificial silk (50,000,000 kilos. per annum to 5,000,000 kilos.) in spite of the difference in price.

Artificial silk is very susceptible to the action of water, which weakens it very considerably. Its resistance is said to be greatly increased by the action of formaldehyde; the fabric is plunged into a bath containing an aqueous solution of the aldehyde, to which a little lactic acid has been added. The chemistry of the change is discussed at length by Beltzer (_loc. cit._).

The threads of artificial silk far surpass in lightness those spun from vegetable fibres. A thread of twenty strands weighing one pound avoirdupois would be more than twenty miles long. At the same time the filaments have not the irregular tubular structure of vegetable fibres, but are solid cylinders. The fact that the filaments are continuous, so that there is relatively little torsion in threads spun from them, gives artificial silk its great advantage over the natural vegetable fibres for the manufacture of mantles. For this purpose the Pauly or Cuprammonium silk is most suitable, though Viscose silk is almost as good; the fibre obtained by the Chardonnet process is not quite so useful in this direction.

~The Manufacture of Mantles from Artificial Silk.~--Whilst the fabrics made by the various processes outlined above are more expensive than the cotton and ramie formerly exclusively used in the mantle industry, they have the advantage, in addition to the fact that they produce better and more lasting mantles, that they do not need the laborious and troublesome process of washing which is so essential in the case of the vegetable fibres. From the nature of the methods used in its manufacture, artificial silk can contain no mineral residue; hence the fabric is immediately ready for impregnation.

As early as the year 1892 Schlumberger and Sinibaldi proposed the use of Chardonnet silk for the manufacture of mantles; but their patent, a Belgian one,[533] attracted little attention, although they stated clearly that the denitrated silk will readily take up the lighting fluid. Ignorance of this fact deferred the successful application of this fibre for ten years. In 1894 De Mare suggested the preparation of mantles by addition of the necessary salts to the collodion solution before squirting; in the following year Knöfler used the same process, recommending in addition the use of ammonium sulphide to denitrate the impregnated threads. These two attempts, which were found to be unworkable, owing to the difficulty of obtaining a homogeneous product before squirting, were merely efforts to compete against the Auer monopoly, resting on Welsbach’s patents, which covered impregnation of any natural fibre. In Knöfler’s process,[534] the salts were dissolved in alcohol and added to the collodion solution, which was then forced through jets into water, to which ammonia was added to prevent removal of the nitrates in solution; the threads were then denitrated with ammonium sulphide. The ammonia treatment of course converts the nitrates into the insoluble hydroxides, a departure which was followed in most of the numerous patents inspired by Knöfler’s process.

[533] _Vide_ Böhm, _Zeitsch. angew. Chem._ 1912, ~25~, 657. Apparently this patent was not taken up; no account of it has been found in the published patents of the Belgian Government.

[534] _E._ 11038, 1895, granted July, 1895.

The first indications of the method which ultimately led to success are to be found in a patent taken out by Plaisetty, in 1901.[535] The specification protects the addition of thorium and cerium hydroxides to the cuprammonium solution of cellulose, but apparently without any inkling of the results that were to follow, and more or less incidentally, he includes in this patent the impregnation of the finished fabric and the subsequent treatment with ammonia. In the following year he applied for a German patent,[536] which was granted in May, 1903, in which he definitely protects the impregnation of the finished fabric, and the ammonia treatment, the fabric being then washed and dried, and burnt off as usual.

[535] _E._ 20747, 1901.

[536] _D. R. P._ 141244.

~Impregnation.~--Since the filaments from which artificial silk is obtained are solid and rod-like in form, as opposed to the tubular structure of cotton and ramie filaments, it is rather surprising that the fabric should take up the lighting fluid in the necessary quantity (_vide_ p. 295). It is found that a 50 per cent. solution of nitrates gives the best results, the impregnation requiring half an hour; a warm bath is usually employed. It is usual to add to the bath a quantity of thorium hydroxide, since the thorium nitrate of commerce generally contains nitric acid, which has a bad effect on the fabric.[537] The excess of solution is removed by means of a glass or porcelain centrifuge, not, as with cotton or ramie mantles, by use of a wringer; drying must be carried out very slowly. The fabric is not cut into lengths before impregnation, as in the case of cotton or ramie, but is immersed in the lighting fluid in long strips.

[537] _Vide_ Buhlmann, _D. R. P._ 188427, 1907; also _E._ 6828, 1907.

~‘Fixing.’~--If the dried fabric, impregnated with the necessary salts, be finished and burnt off in the usual way, the oxide skeleton is extremely fragile, and soon falls to powder. The reason for this lies probably in the explosive decomposition of the nitrates, the weight of organic matter relative to the salts being very much less than in the case of ramie or cotton fabrics (_vide_ p. 295). The additional ammonia bath advocated by Knöfler (_vide supra_) was therefore adopted by Plaisetty, and the nitrates in the dried impregnated fabric are converted into hydroxides by this treatment. For this process, to which the name ‘Fixing’ has been given by Böhm, numberless alternative proposals have been made. Plaisetty’s ammonia fixing gives a mantle which, after burning off, is exceedingly elastic and strong, but it is nevertheless open to serious objections. Thus the nitrates may be to some extent dissolved out by the fixing bath before precipitation of the hydroxides has occurred; to remedy this, the impregnated fabric must be very thoroughly dried before fixing, and in this case, apart from the trouble involved, the acid of the commercial nitrates will attack the fabric unless addition of thorium hydroxide has been made to the impregnating fluid. Again, the conversion into oxides is not complete, the outer layer first formed preventing free diffusion of the alkaline fluid. Finally, since ammonium nitrate is formed in the reaction, a very thorough washing is necessary to remove this salt.

It would be impossible to mention all of the numberless proposals which have been put forward for fixing; nor are the great majority worthy of mention.[538] One of the most important was that of Albrecht,[539] by which hydrogen peroxide is substituted for ammonia. This reagent, as is well known, precipitates from solutions of thorium salts ‘peroxide’ compounds (_vide_ p. 255); from the fabric impregnated with the nitrate, free nitric acid is liberated in the reaction. Since the peroxide is soluble in nitric acid, two baths are used, the fabric being allowed to remain for a short time only in the first, which becomes strongly acid, and being then transferred to the second, in which the precipitation is completed. The burning off of the product so treated proceeds quite quietly, and leaves a very hard and elastic skeleton. The method, however, has the grave disadvantage that cerium salts are not precipitated under these conditions, but escape into the solution. To remedy this, arbitrary additions of cerium nitrate are made to the fixing bath, but the percentage of cerium, and therefore the lighting power (_vide_ p. 293) of mantles fixed by this method is liable to fluctuate. A modification has been introduced[540] in which various salts are added to the hydrogen peroxide fixing baths to prevent the withdrawal of the cerium salt; these are chiefly acetates of the alkali metals and allied compounds. The fabric requires washing after this treatment.

[538] A large number of patents are mentioned by Böhm, _J. Gasbel._ 1909, ~52~, 855.

[539] _D. R. P._ 188427, September, 1907; _E._ 15295, 1907.

[540] _Vide_ _E._ 2240, 1908. Cerofirm Gesellschaft, by Brit. Cerofirm Co.

A rather similar proposal[541] substitutes for ammonia an alkaline solution of hydrogen peroxide, obtained by dissolving sodium peroxide in water. After saturation for a minute or so, the fabric is wrung out and dried, there being no necessity, according to the patent, for any subsequent washing. The same company in an earlier patent[542] suggest a fixing bath of ‘an alkali or amine with an acid which can form insoluble basic double salts with the earth metals,’ the said salt to be precipitated on the fibre, whilst an alkali or amine nitrate goes into solution; acetic and benzoic acids and phenol are mentioned. Apparently this process did not give satisfactory results. The same may be said of the hypochlorite method of Visseaux.[543] Equally interesting, and doubtless equally practical is the proposal[544] to treat the dried impregnated mantle with ozone in a closed chamber, which possibly inspired the even more original suggestion[545] that the fabric be first washed in ozonised water, dried, then impregnated and dried, and finally treated with ozone. According to yet another French patent,[546] the impregnated and dried fabric is to be treated with an alcoholic solution of hydrofluoric acid, which will give a viscous insoluble mixture of thorium and cerium fluorides in the fabric, and at the same time will remove any impurities. Another patent[547] proposes the use of hydrazine and similar bases, cerium nitrate being added to the fixing solution.

[541] _D._ 247940, June, 1912; _F._ 430417, August, 1911.

[542] _F._ 403433, September, 1909, Bruno Co.

[543] _F._ 408807, February, 1910.

[544] _F._ 414700, June, 1910.

[545] _F._ 422643, January, 1911.

[546] _F._ 426156, April, 1911.

[547] _E._ 11904, 1909.

Innumerable suggestions have been made for fixing by means of ammonia gas, or vapours of organic bases.[548] An English patent granted in February 1910[549] protects ‘various improvements,’ which consist in carrying out a preliminary treatment with steam, impregnation with the solution of nitrates, conversion of nitrates into oxides either by steam carrying ammonia, pyridine, etc., or by the action of these vapours without steam in a vacuum, all in one chamber, which can be exhausted or filled with various solutions or vapours as required.

[548] _Vide_, _e.g._ _D. R. P._ 199615 of June, 1908.

[549] _E._ 25549, 1908.

More recently, the use of organic salts of thorium and cerium for impregnation has been proposed by Dr. F. W. Wirth;[550] in fixing the impregnated fabric with hydrogen peroxide, the cerium is not removed in solution, since the weak organic acid formed will not dissolve cerium peroxide. The same author has also suggested[551] the addition to the fixing bath of substances which absorb oxygen from the air--_e.g._ sodium hydrogen sulphite, resorcinol, tannin--to prevent removal of cerium. He has also advocated impregnation with amorphous salts,[552] which will obviate the necessity for any subsequent fixing treatment, the hypophosphites and double compounds with ammonium oxalate being specified. Attempts have been made to achieve the same end by other methods. Thus Silbermann[553] has proposed a preliminary treatment with alkalies (mercerisation); the fabric is treated with concentrated sodium hydroxide solution in absence of air for half an hour, pressed through rollers, and plunged into the impregnating solution. Two years previously a patent was taken out by Drossbach[554] to protect the use of colloidal solutions of the hydroxides. To a boiling suspension of well-washed, freshly-precipitated thorium hydroxide, a solution of a small quantity of the nitrate is gradually added; after half an hour a colloidal solution is obtained, which, after the addition of the required quantity of cerium nitrate, and dilution to a suitable extent, is employed directly for impregnation. The patent states that this solution is more readily absorbed than the ordinary nitrate solution, but the statement has been questioned.

[550] _Chem. Zeitg._ 1911, ~35~, 752.

[551] _Zeitsch. angew. Chem._ 1912, ~25~, 922.

[552] _Chem. Zeitg._ 1911, ~35~, 752.

[553] _Chem. Zeitg._ 1911, ~35~, 1037.

[554] _D. R. P._ 212842, August, 1909; _vide_ also Kreidl and Heller, _E._ 17862, 1909, and _D. R. P._ 228203, 1910.

Artificial silk itself is of a colloidal nature, the solidification of the filaments of cellulose during the manufacture being rather in the nature of a coagulation than of a precipitation; it is to this fact that the fibres owe their solid, rod-like structure, and it is probably this circumstance also which determines the very intimate absorption of the hydroxides or peroxides produced by fixing. It is well known that colloidal substances under some circumstances possess the property of clinging tenaciously to foreign bodies, exhibiting the so-called phenomenon of adsorption. The strength and elasticity of the oxide skeleton, obtained when the fixed and dried fabric is subjected to the operation of burning off, are presumably to be referred to such a relation between the cellulose of the fibres and the insoluble thorium and cerium compounds, precipitated by one of the methods of fixing described.

~The Final Stages.~--The treatment of the fabric after impregnation and fixing differs only slightly from that used for the impregnated ramie and cotton products. The dried strips are cut into suitable lengths, and the head is drawn together with asbestos and threaded across. No tulle or gauze is required, the end being simply turned down before threading. After the ordinary strengthening process for the head (_vide_ p. 296) the process of manufacture was, until recently, finished, the goods being sent out in the unburnt condition, on account of the difficulties of collodinisation. These have now been overcome, so that the mantles are burnt off and collodinised as usual. Burning off and shaping are now frequently effected in one operation by machine; the nature of the methods by which the fibre is made produces a uniform fabric, and if the earlier processes are carefully carried out, a uniform product is obtained, which is therefore suitable for machine treatment.