CELLULOSE CAUSTIC

XANTHATE

DISSOLUTION

IN AQUEOUS

SODA SOLUTIONS UDC 661.728.84 UDC 677.463.021.123.1

V. G. Kulichikin, D. N. Arkhangelskii, E. B. Kostikova, and A. P. Luvishis

The alkali concentration used by the various rayon plants for dissolving the cellulose xanthate (CX) varies 4 - 12%, the optimum being determined either experimentally or being fixed once and adhered to without change. A thorough analysis of the dissolution process can help to disclose means and methods of improving viscose quality and accelerating the viscose preparation processes. The aim of the work reported here was to evolve a reliable method of determining the polymer proportion dissolved at a given instant of time and to use this method for investigating the influence of the alkali concentration on the dissolution rate of the CX. A weighed portion of CX is placed into a cylindrical tube fabricated from metal gauze of 0.3 x 0.3 mm mesh. The tube containing the CZ is immersed in an alkali solution of known concentration contained in a glass vessel which is then placed on a laboratory type shaker. The undissolved part of the CX is thereby effectively separated from the solution. The dissolved proportion of the CX is determined spectrophotometrically. The method consists in essence of determining the absorption of UV radiation at 303 nm, the wavelength of the oscillations of CS 2 groups /1/. Preliminary experiments showed that the degree of esterification remains near-constant over a period of 3 h so

$0

4

3

x

'

~

o

g

~2~r

o

,FO

,'~

15o

200

a,¢o

t, min

NaOH, wt. %

Fig. 1

Fig.2

Fig.1. The variation of the CX concentration in the liquid phase with the dissolution time at alkali concentrations of 3% (@)~ 4% (A), 10% (0), 12% (X), and 14% (K]). Fig.2. The CX dissolution concentration.

rate as a function

of the NaOH

All-Union Scientific R e s e a r c h Institute for Man-Made Fibre. Translated from Khimicheskie Volokna, No.l, pp. 25 - 26, January - February, 1973. Original article submitted December 31, 1971.

1973 Consultants Bureau, a division of Plenum Publishing Company Limited, Davis House (4th Floor), 8, Scrubs Lane, Harlesden, London NW10 65E, England. AH rights reserved. This article cannot be reproduced for any purpose whatsnever without permission of the publisher, a copy of this article is availzble from the publisher for $15.00.

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X O

0

i

I

,

~oo

300

J00

' #OO

~E

I

'

0

VNaOH/mCX

.50

NaOH/C~-cell

Fig.3 Fig.3. The dissolution N o t a t i o n as in Fig.].

I

I 36

Fig.4 rate as a function of the ratio VNaOH/mCX.

Fig.4. The variation of the dissolving capacity of the system with the ratio N a O H / a - c e l l at various NaOH concentrations (wt.%). Notation as in Fig.]. that the absorption intensity is proportional to the polymer tion C (%) which is calculated from the known equation

concentration

in the solu-

A o C = K -Y where A o is the molar absorption, p e r i m e n t a l constant.

y is the degree of CX esterification

and K is an ex-

A comparison of the results of determinations of the concentration dure and by the oxidiometric method showed that they agreed.

by this proce-

The method proposed here gives the kinetic curve of the CX dissolution rapidly and with adequate precision. Samples for analysis (about 5 - 7 ml) are e x t r a c t e d at intervals of 15 min during the first hour of the dissolution process and subsequently every half hour. The decrease in the total volume of the solution of known concentration was taken into account in the analysis of the experimental results. The experiments were conducted with CX containing about 29% a-cellulose and 15% N a O H with a y of about 50 - 55 and a mean degree of polymerization of 350 - 4 0 0 . The concentration of the dissolution alkali was varied 0 - 15%. The temperature of the experiments was 20 ~ 1°C. The kinetic curves of the solution process are shown in Fig. 1. The initial dissolution modulus in this series of experiments with varied alkali concentration was I/9, i.e. 50 g CX was dissolved in 450 ml NaOH solution. After the completion of the dissolution process the a - c e l l u l o s e concentration of the solution was about 3%. The results showed that a v a r i a t i o n in the alkali concentration produced sharp changes in the rate of the dissolution process which was determined in terms of the tangent of the angle of inclination of the straight sections of the AC/At curves. The non-monotonic variation of the dissolution rate with the NaOH concentration is conspicuous, i.e. the dissolution rate increases with an increase in the NaOH concentration to 10% and then begins to fall (Fig.2). The influence of the dissolution modulus of CX in alkali of various concentrations on the process rate Was studied at various VNaoH/mcx values obtained by varying either the weight of the CX portion (mcx) or the proportion of solvent (VNaOH). The sharp variation of the dissolution m o d u l u s when extracting samples in experiments in which the ratio VNaOH/mCX was small, the process rate in these experiments was determined, from the relation between the proportion of CX dissolved in 30 min and the initial weight of the polymer portion. The diagram in Fig.1 shows that the points relating to t = 30 min lie on the n e a r - s t r a i g h t initial sections of the kinetic curves so that the corresponding polymer c o n c e n t r a t i o n s in the solution are proportional to the &C/At values. The experimental data is p l o t t e d in Fig.3. It was found that for every Na0H concentration there is an optimum ratio (VNaoH/mCX) ~ corresponding to the peak dissolution rate. The quantity

40

(VNaoH/mcx)* decreases with an increase in the alkali concentration and its peak is less extended along the modulus axis. No characteristic peak in the dissolution rate occurs with water alone; in this case an increase in the modulus results in a slight increase in the rate of dissolution - evidently as a result of the diffusion processes (i.e. the increase in the difference between the polymer concentrations in the liquid and solid phases). The experimental data plotted in Fig.3 is characterized by another singularity. When argument VNaOH/mCX is replaced by NaOH/a-cl (where N a O H is the proportion of dry caustic soda in the solvent and a-cl the a-cellulose content of the CX), this quantity remains constant for the peak dissolution rate whatever the NaOH concentration (Fig.4). Moreover, the experimental points relating to NaOH concentrations of 2.4 and 10% on the rising branches of the curves almost coincide but the tendency for the peak rate to increase above a 10% alkali concentration remains. These results lead to the conclusion that the nature of the processes taking place in the dissolution of the CX remains the same at all alkali concentrations. The decisive factor seems to be the blocking of the reactive functional macromolecules of the CX by the solvated N a 0 H molecules and their subsequent transition into the solution. The evidence is provided by the fact that for a peak dissolution rate in alkali of a given concentration each xanthate group must be associated with a strictly defined number of NaOH molecules. A quantitative assessment of this relation is difficult in the present case owing to the variation of the molecules in the dissolution process. Suffice it to state that a near-constant rate of dissolution (CNaOH 2 - 10%) can be achieved when using NaOH/a-cl = 2 - 15 as the criterion. The peak process rate remains the same as for 10% NaOH when the alkali concentration is increased and NaOH/a-cl > 15. The part which water plays in the solvent probably consists of forming a hydration film round the N a O H molecules the size of which governs their initial penetration into the CX /4/ and predetermines the subsequent process of transition. Thus, while for an alkali concentration CNaOH > 10% there is not enough water for producing a hydration film of optimum thickness, for Na0H/a-cl > 15 the water shortage precludes the solvated CX macromolecules being diffused at the peak rate. It follows that the e x p l a n a t i o n / 2 / for the dissolution rate being highest for a 10% alkali concentration lies in the fact that the hydrate formula (NaOH-2OH20) is the most suitable one for the structure of a CX of the above composition. Under production condition, therefore, CX dissolution should be carried out in 10% alkali. Technological studies at the Krasnoyarsk viscose rayon plant have shown that viscose quality is in fact improved when cellulose xanthate is dissolved in alkali of that concentration.

The authors are indebted to E.Z.Fainberg of the All-Union Scientific Research Institute for his interest in the work and his helpful contribution to the discussion of the results. LITERATURE 1. 2. 3. 4.

CITED

S.Dunbrant and O.Samuelson, J.Appl.Polymer Sci., ~, 136 (1957). R.Bartunek, Koll.-Z., 146, 35 (1956); Khim.i Tekhnol.Polimerov, No.6 (1967). L.A.Molochnaya and M.E.Shor, Fibre Chemistry, No.2 (1969). G.N.Musatova, D.N.Arkhangelskii, M.A.Ginzberg, and E.M.Mogilevskii, Khim.Volokna, N o . 4 (1962).

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