1.

2.

3.

LITERATURE CITED

Inventor's Certificate 825696 (1979) (USSR).

V. P. Kim and M. M. Arefeva, Khim. Volokna, No. 2, 51 (1988).

A. T. Serkov, Viscose Fibres [in Russian], Khimiya, Moscow (1981).

F O R M A T I O N O F S E C O N D A R Y G E L P A R T I C L E S I N V I S C O S E

L. A. Rykova, V. A. Dmitriev, and A. A. Serkov UDC 677.463.021.123.014.2

In the manufacture of viscose fibres the purity of the spinning solution -- the viscose -- has great importance. Especially

important is the presence of heterogeneous (suspended) particles, which adversely affect the processes of filtration and spinning,

and also affect the quality of the fibre obtained. They can fog the spinneret holes, break down the stability of the spinning

process, and cause the formation of defects on the fibres, which leads to a reduction in their strength [1]. A content of the so-

called gel particles is especially undesirable. They are easily deformed, penetrating through the filter medium, and have a

tendency to subsequent agglomeration.

There are two types of gel particles in viscose -- primary and secondary. The primary gel particles are residues of the

starting cellulose which have not dissolved for some reason or other [2-5]. The origin of secondary gel particles in viscose has

been connected with processes of association or local crystallization [5-8]. The content of primary gel particles ~ has been reduced

by increasing the reactivity of the cellulose, optimization of technical parameters in mercerization, xanthation, and dissolution.

After careful filtration, as a rule, their negative effect can be eliminated to a considerable extent. As concerns the secondary gel

particles, here there are still significant difficulties, which in our opinion, are connected in a definite degree with the absence of

adequate concepts about the mechanism of their generation. The present communication is an attempt to explain this phenome-

non.

The suggestion has previously been made [9] that the secondary gel particles arise because of the formation of a polymer

residue on solid surfaces of the equipment and pipelines because of dexanthation of the immobile layer of viscose next to the

wall, which is always formed in laminar flow past a solid surface. The higher the temperature, the more intensively should

decomposition of the xanthate in the immobile layer take place, and correspondingly, the more rapidly gel particles will accumu-

late in the solution. To check out this mechanism, we have studied the build-up of secondary gel particles in a closed circulating

volume of viscose at various temperatures.

Our studies were carried out on a laboratory set-up (Fig. 1). By gear-pump 4, the viscose is delivered into a filter

instrument 6 (without a charge), then via pipeline 7 into cylinder 2, which is located in a thermostat 3. From above the cylinder

is closed with a cap having valve 1. From the cylinder by the reverse pipeline 5, the viscose is delivered again to pump 4. The

viscose in the cylinder is thermostated at a definite temperature for an assigned period of time with circulation through a closed

loop.

Samples were taken from the cylinder after every 4 h and the gel content of these was determined. To determine the gel-

particle content of the viscose, we used the microscopic method employing the contrast indicator Congo red [10].

Because of the intensive coloration of the gel particles, this method gives more accurate results. Out of each sample

withdrawn, we examined ten drops under the microscope; in a drop - - ten fields; counting of the colored gel particles was carried

out and we found the mean number (mean squared relative error in the coefficient of variation did not exceed 5%).

From the data given in the table, it is evident that the gel-particle content increases with elevation in heating tempera-

ture and with increase in circulation time. There were five particles in the starting viscose. After circulation for four hours at

20°C, the number of these rose to 14; and at temperatures of 30 and 40°C, it rose respectively to 18 and 32 particles. At 50°C

Translated from Khimicheskie Volokna, No. 4, pp. 35-36, July-August, 1990.

0015-0541/90/2204-0263512.50 ©1991 Plenum Publishing Corporation 263

TABLE 1. Increase in Content of Secondary Gel Particles in

Viscose at Various Temperatures

I

Circula- I Mean gel-particle cont. at indicated temp., in °C

tion time, !

| 20 30 35 40 I 50 h I I

I

18 19 22 24 38

22 23 32 40 47

23 32 41 44 45

32 Coagulation 39 65. 70 Coagulation

- I - -

0 4 8 24 28 32 48.

2 J

J / /

Fig. 1 Fig. 2

Fig. 1. Scheme of set-up for studying the process of secondary gel-particle

build-up in viscose: 1) valve; 2) cylinder containing viscose; 3) thermostat; 4)

gear pump; 5, 7) viscose pipeline; 6) filter.

Fig. 2. Formation of wall (diffusion) layer in flow of viscose: 1) Pipeline wall;

2) immobile layer of viscose at the wall; 3) profile of velocities in stream.

after circulation for four hours, the viscose coagulated. Increasing the duration of circulation to 24 h led to an increase in gel-

particle content at 20°C -- up to 19, at 30°C, to 22; at 40°C, to 65 particles.

The results obtained support the suggestion made above about the mechanism of formation of secondary gel particles.

Let us examine a case of flow of viscose in a pipeline (Fig. 2). Because of the high viscosity of the viscose and the low rate of

flow, a laminar character of distribution of velocities takes place which is close to parabolic. On the surface of the pipeline there

is an immobile, so-called diffusion, layer of viscose.

We shall denote its thickness by 60. It is usually equal to 0.1-0.5 mm and decreases with increase in stream velocity. The

dwell time of dissolved molecules of cellulose xanthate in the zone of the diffusion layer depends on the diffusion coefficient D

and the thickness of the layer. In conformity with the data of [11], let us take a value of D equal to 10 --9 cm2/sec and the

minimum possible value of 60 as 0.1 ram. Bearing in mind the quadratic dependence between the dwell time of polymer

molecules in the diffusion zone and their linear dimensions in diffusion processes, the mean dwell time of viscose (r, h) in the

zone of the wall diffusion layer may be expressed as follows:

6s 0,01 ~ 'v ~ -D 1 0 _ 9 - - 28 .

Consequently, in the wall layer the viscose is not renewed, at least over the course of days. In view of the catalytic effect

of the metallic surface, one may consider that during this time local coagulation of the viscose takes place, with formation of

secondary gel particles.

A temperature increase leads to an acceleration both of diffusion and also of the decomposition process of cellulose

xanthate. The acceleration of diffusion shortens the dwell time of the xanthate macromolecules in the zone of the diffusion layer.

However, the temperature coefficient of diffusion exceeds the initial coefficient by a factor of 1.3, while the chemical reaction of

264

the xanthate decomposition increases 2.3-fold on elevation of temperature by every 10 degrees. From this comparison the sharp

increase in gel-particle content in the circulating viscose upon temperature elevation becomes understandable (see Table 1).

Thus, the formation of secondary gel particles in viscose is connected with the duration of the dwell time of polymer

molecules in the immobile wall layer, where decomposition and coagulation take place. Increasing the stream velocity reduces

the dwell time of the polymer in the wall layer and the intensity of formation of secondary gel particles. Increasing the tempera-

ture leads to an acceleration of polymer decomposition and, in spite of the shortening of the reaction time, it is accompanied by

a sharp increase in the rate of formation of secondary gel particles. The assumption may be expressed that the proposed

mechanism has a general character for solutions and melts of other polymers. The difference consists in the rate of the polymer

degradation and hydrodynamic situation.

CONCLUSIONS

- - It has been shown that the content of secondary gel particles in viscose rises significantly with temperature increase.

-- The formation of secondary gel particles takes place because of decomposition of the cellulose xanthate in an

immobile wall layer of the viscose.

LITERATURE CITED

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

E. Treiber, J. Polymer Sci., 51,297-315 (1961).

L. H. Sperling, J. Appl. Polymer Sci., 7, 1411-1423 (1963).

M. M. Iovleva, et al., Khim. Volokna, No. 6, 41 (1964).

M. V. Golubev, Khim. Volokna, No. 1, 47 (1966).

M. M. Iovleva and S. P. Papkov, Khim. Volokna, No. 2, 3 (1968).

S. A. Glikman, et al., Vysokomol. Soed., No. 5, 598 (1963).

A. I. Virezub, V. G. Kulichikhin, and D. N. Arkhangel'skii, Khim. Volokna, No. 3, 76 (1972).

L. H. Sperling and M. Easterwood, J. Appl. Polymer Sci., No. 4, 25-32 (1960).

A. T. Serkov, Viscose Fibres [in Russian], Khimiya, Moscow (1981), pp. 143-149.

I. I. Krasova, et al., Sv. Nauchn. Tr. Pod Red. A. T. Serkova, VNIIVProekt, Mytishchi (1986), p. 87.

A. T. Serkov, Viscose Fibres [in Russian], Khimiya, Moscow (1981).

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