The mechanism of drawing of capron fibres

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4. V. K. IRZHAK, L. M. ROMANOV and N. S. YENIKOLOPYAN, Vysokomol. soyed. 5: 1638, 1963 5. N. F. PROSHLYAgOVA, I. F. SANAYA and N. S. YENIKOLOPYAN, Vysokomol. soyed. 5: 1632, 1963 6. N. F. PROSHLYAKOVA, I. F. SANAYA and N. S. YENIKOLOPYAN, Vysokomol. soyed. 5: 1776, 1963 7. I. M. BEL'GOVSKII, N. S. YENIKOLOPYAN and L. S. SAKHONENKO, Vysokomol. soyed. 4: 1197, 1962 8. I. MEIZLIK, I. MENCHTKOVA~and Z. MAKHACHEK, Vysokomol. soyed. 4: 776, 1962 9. A. A. SHAGINYAN and N. S. YENIKOLOPYAN, Vysokomol. soyed. 7: 1866, 1965

THE MECHANISM OF DRAWING OF CAPRON FIBRES*t A. SH. GOIKHMAN, M. P. NOSOV, YU. N. TRET'YAKOV and V. G. OLEINIK Kiev Branch of the Scientific-Research Institute of Synthetic Fibre (Received 1 December 1964)

IN A previous communication [1] the morphological changes occurring in polycaproamide during the drawing of Capron fibres were studied. It was found that there is a definite relationship between the structure of the polymer, its behaviour during drawing and the distribution of the mechanical characteristics of the drawn fibres. It is of interest to study the connection between the behaviour and mechanical properties on drawing on the one hand and the orientation and crystallinity of the polymer on the other. EXPERIMENTAL The material studied was the same Capron fibre as in reference [1]. X-ray analysis was used to determine the orientation of the crystallites of the ~-form $. The mean angle of orientation, v (the angle between the longitudinal axis of the crystallite, coinciding with the 1 axis of the unit cell, and the axis of the fibre) was selected as a characteristic of the orientation of the crystallites [2]. For determination of v for the monoclinic form the intensity along the interference rings was plotted. From these diagrams the azimuthal half-widths of two equatorial interferences of the monoclirfic ~-form, ~ (200) and ~h (002), were determined. The angle ~ was calculated by means of Urbanczyk's formula [2]: ~ = a r e cos

sin 2 ~-- (cos a--cos 65 ° cos ~)~ COS2 25 °

* Vysokomol. soyed. 7: No. 11, 1877-1883, 1965. t Communication X in the series "Study of the process of drawing of synthetic fibres". $ The ~-form is the monoclinie Crystalline modification of polyeaproamide. The hightemperature hexagonal modification is refeq[,red to as the fl-form in this communication.

Mechanism of drawing of Capron fibres

2061

where cos ~ c o s [90°--~h (200)]" cos ~ (200); cos a--~eos [90°--~h (002)]" cos ~ (002); ~ is the Wolff-Bragg angle and p = 6 5 ° - - t h e angle between the a and c axes of the monoclinic cell of Capron. The diehroism of the 930 cm -1 band, corresponding to the skeletal vibrations of the C O - - N H groups, was measured for the purpose of determining the orientation of the molecular chains. The coefficient of dichroism, R, was calculated from the formula

R=DcL/D±, where Dit and D ± are the optical densities with the electrical vector of the illumination parallel and perpendicular to the axis of the fibre. The specimens were in the form of strips made up of parallel fibres and the spectra were recorded in an IKS-12 spectrometer. The polarized spectra were recorded in an IPO-12 instrument. I t is well known t h a t change in molecular structure is clearly detected when swollen polymers are studied [3-5]. Swelling can be expressed quantitatively in various ways, namely by the degree of swelling (by weight or volume), by the index of the anisotropy of swelling or by the increase in length of the specimen (axial swelling). We used the last method, it being the simplest and most convenient. Capron fibre specimens, 200 m m long, were suspended in a glass vessel. The upper end was firmly fixed and the minimal load (less than 0.1% of the breaking load), required to straighten the specimen was suspended on the lower end. The vessel was filled with a 3% aqueous solution of phenol and the change in length of the specimen with time was recorded by means of a cathetometer. The relative change in length, expressed as a percentage, was taken as a measure of the swelling. Variation in the crystallinity of the specimens was measured in terms of the size of the band at 1201 cm -1, corresponding to the crystalline ~-form of polyeaproamide. I n order to avoid errors due to variation in the thickness of the fibres and penetration of the light between the fibres, the band at 1120 cm -1, which gives a measure of the total content of polycaproamide, was used as an internal standard. The index of crystallinity was obtained from the ratio Ax2ox/An~o, where A1201 and An~ 0 are the sizes of the 1201 and 1120 cm -1 bands.

Results on the measurement of the mean angle of orientation, v, are given in Fig. 1, where values obtained at a number of drawing temperatures are combined. The shape of the curve, which to a sufficient degree of approximation is general for the temperatures studied, shows that orientation of the crystallites of the ~-modification practically ceases at a draw ratio of 2----3. Similar results were obtained by Novak and Vettegren' [6], who by measurement of infrared dichroism determined the orientation in Capron fibres of different degrees of crystallinity. These authors found that above a degree of drawing of 2----3 orientation of the macromolecules in the crystals remained practically constant, whereas orientation in the amorphous regions continued to increase. Further drawing proceeds without significant change in the axial orientation of the a-crystallites, and evidently involves nothing but slipping of the crystallites relatively to one another, without axial rotation. The draw ratio of 2----3 represents the threshold of true orientation (in the sense of rotation of the crystallites with respect to the axis) and further change in the linear dimensions of the fibre involves another mechanism.

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A . SH. GOIKHMAN et al.

It is interesting to note that change in strength with ~ is not of the same diminishing nature at high values of ~ [7, 8]. Taking into consideration the fact that data on average molecular orientation show that it increases to some extent even at high values of 4, it must be assumed that the increase in strength when A>3 is associated with ordering of the amorphous regions. This is in accord with a number of studies of the connection between the orientation and strength of fibres [8, 6].

1800 X

R ~8

30

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180° ,~200°

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o

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1"4

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,

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I

I

I

I

2

3

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FIG. 1

2

3

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4

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FIG. 2

FIG. I. Dependence of the m e a n angle of orientation, T, on the draw ratio, 4, at various

temperatures. FIa. 2. Variation in the coefficient of diehroism, R, of Capron fibre with the degree of drawing (the figures on the curves denote the temperature of drawing).

Results on measurement of the coefficient of dichroism are given in Fig. 2. I t is seen from this diagram that at first the orientation of the fi-form increases up to a draw ratio of approximately 3. From 3 to 3.5 the orientation decreases as a result of transition of the fi-form to the ~-form. From 3.5 upwards orientation of the ~-form is predominant. The reduction in orientation at the draw ratio of 2-2-2.5 is explained b y the considerable structural inhomogeneity of the polymer in this region of drawing. The axial swelling is closely connected with the index of molecular orientation. A typical rate curve of axial swelling is shown in Fig. 3. Each point on the curve represents the mean of ten measurements. All the curves have the following characteristic features:-an initial curved section (up to I ram) corresponding to wetting of the specimen and surface absorption of the phenol; a linear section corresponding to diffusion of the phenol into the interior of the fibre with rapid

Mechanism of drawing of Capron fibres

2063

change in the length of the specimen, and a linear section parallel to the abscissa corresponding to the establishment of equilibrium, which is reached within 15-20 min. Also in the curves for specimens showing elongation (positive axial swelling) there is a maximum immediately before the attainment of equilibrium. It is seen from Fig. 3 that specimens drawn to ratios up to )~----2-0-2.2 lengthen on swelling, and specimens with )~>2.5 contract. When ~=2.0-2.5 the specimens undergo little change in length and in some experiments the curves for these intersect the abscissa. /l ~100%

18

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.

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Time, rain 18

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4'5 FIG. 3. Effect of the degree of drawing on the kinetics of swelling of Capron fibre in a 3% aqueous solution of phenol (drawing temperature 180°). It is interesting to note that this region of draw ratios (4----1.8-2.5) corresponds to the maximal orientation of the crystallites of the fl-form, as shown by X-ray analysis. Figure 4 shows the dependence of the equilibrium axial swelling (Aleq/lo) on the draw ratio for various temperatures and solvents. It is seen that change in sign of the elongation occurs within the range of draw ratios of ~=2.0-2.5, independently of the temperature of drawing, or of the medium in which swelling takes place. An important fact is that statistical studies disclose a close connection between the distribution of the mechanical characteristics of the fibre and of the equilibrium axial swelling with respect to the degree of drawing (Fig. 5).

2064

A. SH. GorKH~A~ et al.

lo

c~c~

÷16

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-8 -12

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),

FIG. 4 Fro. 5 FIG. 4. Dependence of the equilibrium axial swelling of Capron fibres on the draw ratio, ~, at different temperatures and in various solvents: 1--180, 3~o p h e n o l solution; 2--20, 3O/o phenol solution; 3-- 180, 2~/o phenol solution; -- 180, distilled water. FIG. 5. Variation in the coefficients of variation with respect to elongation (ce), the work of rupture (Ca) and equilibrium swelling (Csw), with degree of drawing.

A ~12o 1800

~o

I

I

I

I

1

2

3

4

.

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FIG. 6. Variation in the degree of crystallinity of Capron fibre with draw ratio, 2, (the figures on the curves denote the temperature of drawing).

Mechanism of drawing of Capron fibres

2065

I n Figure 5 the ordinate represents the coefficients of variation with respect to elongation (c,), the absolute work of rupture (Ca) and the equilibrium axial swelling (Csw.). The abscissa represents the draw ratio. It is seen from the diagram that the maxima in the distribution of these measurements occur in the range of draw ratios from 1.8 to 2.5. The results of measurement of the degree of crystallinity are shown in Fig. 6. It is seen from Fig. 6 t h a t the degree of crystallinity of Capron fibre increases with increase in drawing and increases slightly with increase in the temperature of drawing. The small decrease in erystallinity at a draw ratio of 2.2 is explained by inhomogeneity of the fibre in this region of drawing. DISCUSSION

The above studies of the process of drawing of polyamide fibres enable some conclusions to be drawn on the mechanism of the structural changes occurring in the polymer during drawing. The undrawn fibre in the structural sense consists of a complex of ordered and unordered regions. The ordered regions consist mainly of crystallites of the fl-modification. It is known that the fl-modification is unstable in the region of the stresses and temperatures normally encountered in practice. In addition the undrawn fibre contains a certain proportion of crystallites of the a-modification. Under the action of stresses and change in temperature the fl-crystallites orientate and break down, changing to a-crystallites. X-ray analysis shows that at low degrees of drawing orientation of the fl-crystallites without breakdown predominates [1]. This occurs in the region of draw ratios up to 4=2.5. In this region orientation of the fl-crystallites is the predominating process. At these "critical" degrees of drawing the polymer is evidently in a special state. The crystalline regions, consisting of fl-crystallites, are oriented to the maximal extent. The a-crystallites present in the fibres are oriented to a considerably smaller extent (see Fig. 1; at 4=2.0 r is about 27°). Orientation of the macromolecules in the amorphous regions is also low (see [6]). Thus the fibres contain regions with highly oriented fl-crystallites and at the same time regions with low axial orientation of a-crystallites and of the molecular chains in the amorphous regions. This must lead to considerable lack of uniformity with respect to the mechanical characteristics, and this is confirmed by results from the tensometric study of the drawing process and on the distribution of the mechanical characteristics and equilibrium swelling (Fig. 5). With increase in the degree of drawing breakdown of the fl-crystallites occurs, and this practically ceases at ~ 3 . 5 . Simultaneously with breakdown of the fl-crystallites formation of the stable a-form occurs. The resulting a-crystallitos are at first oriented to a smaller extent than the fl-crystallites that precede them. However orientation of the crystallites of the a-modification practically ceases at ~--=3.0-3.2. It is a well known fact that the further increase in strength when

2066

A. SH. GOI]g:H~L~N"et a/.

4 > 3 . 2 can be explained by orientation of the chains in the amorphous regions (the increase in t h e average molecular orientation shown by measurement of birefringence and infrared diehroism) or possibly by reconstruction of supermolecular structural elements [8]. In the swelling of fibres one of two effects is seen, either elongation or contraction. Elongation is characteristic of the swelling of isotropic fibres and shrinkage is characteristic of highly oriented fibres. I t is reasonable to suppose t h a t in real fibres the mechanism of the change in linear dimensions is a combination of the two above-mentioned processes. Without going into detail about the mechanism of swelling it m a y be supposed t h a t in fibres t h a t are not uniform in structure, i.e. t h a t contain highly oriented regions and regions with a low degree of ordering, neither contraction nor elongation will predominate and such a fibre will undergo practically no change in linear dimensions in a solvent. I t is this situation t h a t we encounter in fibres extended to a draw ratio of 2.0-2.5. This provides confirmation of the suggestion made earlier t h a t these specimens have the maximal degree of inhomogeneity. CONCLUSIONS

(1) A study has been made of the change in orientation of crystallites of the ~-form, and of the average molecular orientation in polycaproamide, with the degree of drawing. 2. I t was found t h a t orientation of the ~-crystallites, characterized by the mean orientation angle, z, practically ceases at ~=3.0-3-2. The variation in the average molecular orientation (the coefficient of dichroism) displays a more complex relationship. 3. I t is confirmed t h a t the crystallinity of the polymer increases with increase in the degree of drawing. 4. I t is shown t h a t there is a definite relationship between the equilibrium axial swelling and the structure of the fibre. Fibres drawn to the extent of A~--1 to ~ = 2 elongate on swelling. Fibres at draw ratios of 2.0-2.5 remain almost unchanged in linear dimensions. Above A : 2 . 5 only contraction occurs. Translated by E. O. PHILLIPS REFERENCES

1. A. Sh. GOIKHMAN, M. P. NOSOV and Yu. P. TRET'YAKOV, Khimich. volokna, No. 6, 1965 2. G. URBANCZYK, Kolloid. Z. 176: 128, 1961 3. E. K. MANKASH and A. B. PAKSHVER, Zh. prikl, khim. 26: 830, 1953 4. H. KANETSUNA, Bull. Text. Res. Inst. 49: 69, 1959 5. M. JAMBRICH and J. DICEK, Faserforch. und Textilteelm. 12: 11, 1961 6. I. I. NOVAK and V. I. VETTEGREN', Vysokomol. soyed. 6: 706, 1964 7. P. P. KOBENKO, Amorfnye veshehestv~. (Amorphous Substances.) Izd. AN SSSR, Moscow, 1952 8. N. V. MIIKHAILOV,Khimieh, volokna, No. 1, 7, 1964