Part 5 (1/2)

GROUP 1. The early history of the Chardonnet process is discussed and some incidental causes of the earlier failures are dealt with. The process having been described in detail in so many publications the reader is referred to these for details. [See Bibliography, (1) and (2), (3) and (4).] The denitrating treatment was introduced in the period 1888-90 and of course altogether changed the prospects of the industry; not only does it remove the high inflammability, but adds considerably to softness, l.u.s.tre, and general textile quality. In Table I will be found some important constants for the nitrocellulose fibre; also the fibre after denitration and the comparative constants for natural silk.

TABLE 1.

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Tenacity

Elasticity

(grammes)

(% elongation)

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Nitrocellulose according to Chardonnet

German Patent No. 81,599

150

23

The same after denitration

110

8

Denitrated fibre moistened with water

25

--

Nitrocellulose: Bronnert's German Patent

No. 93,009

125

28

The same after denitration (dry)

115

13

The same after denitration (wetted)

32

--

Natural silk

300

18

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1. Tenacity is the weight in grammes required to break the thread.

2. Elasticity is the elongation per cent. at breaking.

The numbers are taken for thread of 100 deniers (450 metres of 0.05 grammes = 1 denier). It must be noted that according to the concentration of the solution and variations in the process of denitration the constants for the yarn are subject to very considerable variation.

In regard to the manufacture a number of very serious difficulties have been surmounted. First, instead of drying the nitrated cellulose, which often led to fires, &c., it was found better to take it moist from the centrifugal machine, in which condition it is dissolved (5). It was next found that with the concentrated collodion the thread could be spun direct into the air, and the use of water as a precipitant was thus avoided.

With regard to denitration which is both a delicate and disagreeable operation: none of the agents recommended to subst.i.tute the sulphydrates have proved available. Of these the author mentions ferrous chloride (6), ferrous chloride in alcohol (7), formaldehyde (8), sulphocarbonates. The different sulphydrates (9) have very different effects. The calcium compound tends to harden and weaken the thread. The ammonia compound requires great care and is costly. The magnesium compound works rapidly and gives the strongest thread. Investigations have established the following point. In practice it is not necessary to combine the saponification of cellulose ester with complete reduction of the nitric acid split off. The latter requires eight molecules of hydrogen sulphide per one molecule tetranitrocellulose, but with precautions four molecules suffice. It is well known that the denitration is nearly complete, traces only of nitric groups surviving.

Their reactions with diphenylamine allow a certain identification of artificial silks of this cla.s.s. Various other inventors, e.g. Du Vivier (10), Cadoret (11), Lehner (12), have attempted the addition of other substances to modify the thread. These have all failed. Lehner, who persisted in his investigations, and with success, only attained this success, however, by leaving out all such extraneous matters. Lehner works with 10 p.ct. solutions; Chardonnet has continually aimed at higher concentration up to 20 p.ct. Lehner has been able very much to reduce his pressures of ejection in consequence; Chardonnet has had to increase up to pressures of 60 k. per cm. and higher. The latter involves very costly distributing apparatus. Lehner made next considerable advance by the discovery of the fact that the addition of sulphuric acid to the collodion caused increase of fluidity (13), which Lehner attributes to molecular change. Chardonnet found similar results from the addition of aldehyde and other reagents (14), but not such as to be employed for the more concentrated collodions. The author next refers to his discoveries (15) that alcoholic solutions of a number of substances, organic and inorganic, freely dissolve the lower cellulose nitrates. The most satisfactory of these substances is chloride of calcium (16). It is noted that acetate of ammonia causes rapid changes in the solution, which appear to be due to a species of hydrolysis. The result is sufficiently remarkable to call for further investigation. The chloride of calcium, it is thought possible, produces a direct combination of the alcohol with a reactive group of the nitrocellulose.

The fluidity of this solution using one mol. CaCl_{2} per 1 mol.

tetranitrate (17) reaches a maximum in half an hour's heating at 60-70C. The fluidity is increased by starting from a cotton which has been previously mercerised. After nitration there is no objection to a chlorine bleach. Chardonnet has found on the other hand that in bleaching before nitration there is a loss of spinning quality in the collodion. The author considers that the new collodion can be used entirely in place of the ordinary ether-alcohol collodion. With regard to the properties of the denitrated products they fix all basic colours without mordant and may be regarded as oxycellulose therefore. The density of the thread is from 1.5 to 1.55. The thread of 100 deniers shows a mean breaking strain of 120 grammes with an elasticity of 8-12 p.ct. The cardinal defect of these fibres is their property of combination with water. Many attempts have been made to confer water-resistance (18), but without success. Strehlenert has proposed the addition of formaldehyde (19), but this is without result (20). In reference to these effects of hydration, the author has made observations on cotton thread, of which the following table represents the numerical results: