Cotton Convolutions.pdf

158

CHAPTER 8 FIBRE MORPHOLOGICAL FEATURES The effects of aqueous swelling and stretching on the morphological features of the cotton fibre such as convolutions and reversals are discussed in this chapter. 8.1

CONVOLUTIONS AND CONVOLUTION ANGLE Convolutions appear on cotton fibres when it dries for the first time in

order to relieve internal stresses developed during desiccation process. Table 8.1 gives the data on number of convolutions and convolution angle of raw and aqueous stretched samples of six varieties of cotton from Gossypium hirsutum species. The number of convolutions counted in one centimetre of cotton fibre has ranged from 47 to 66 in raw cottons. The frequency of convolutions is drastically reduced on tension - dried samples. Even slack-dried fibres have shown less number of convolutions compared to the control. Very few convolutions are seen on tension-dried samples of 10% stretch. Correspondingly the convolution angle is also decreased in all water stretched cottons. It is found that there is a higher decrease in convolution angle of tension dried samples than slack-dried samples. High coefficient of variation is found in all cases as individual fibres within a variety showed high variation. It is evident from the above data that water swelling followed by stretch has removed the convolutions partly. In tension-dried samples, the deconvolution effect produced is permanent, as the fibre, due to annealing action of the water sets its structure permanently. In slack-dried samples, the drying takes place in slackcondition and thus the fibre tends to pick up convolutions again due to stress relaxation during the process of drying. The changes between drying conditions

159 TABLE 8.1 : DATA ON MORPHOLOGICAL FEATURES OF RAW AND AQUEOUS SWOLLEN AND STRETCHED COTTON FIBRES

No. of Convolu­ Cotton variet^state convolutions CV% tion angle C.V% 6 parent

Distance No. of between reversals CV% reversals per cm cm

SRT-l-Raw

62

28 (0.98)

13.5

44 (0.007)

27

22 (036)

0.037

8% S.D

43

20 (1.08)

6.11

19 (0.002)

26

17 (064)

0.038

8% T.D

23

30 (0.97)

2.99

36 (0.002)

25

36 (1.28)

0.040

10% S.D

37

20 (1.04)

5.95

27 (0.003)

24

13 (0.46)

0.042

10% T.D

11

30 (0.48)

1.8

30 (0.001)

22

19 (0.59)

0.045

Deviraj-Raw

66

25 (2.27)

11.82

46 (0.008)

25

21 (0.72)

0.040

8% S.D

40

25 (1.2)

538

28 (0.002)

23

19 (1.28)

0.043

8% T.D

25

10 (0.33)

3.24

36 (0.002)

22

27 (0.86)

0.045

10% S.D

28

23 (0.91)

3.6

30 (0.002)

20

17 (0.59)

0.050

10% T.D

15

50 (0.74)

152

20 (0.001)

18

25 (0.58)

0.056

Deviraj-mercerized 10% T.D

-

21

14 (0.49)

0.048

-

Jayalakshmi-Raw 47

41 (2.72)

9.53

40 (0.004)

26

31 (1.14)

0.038

8% S.D

34

16 (0.78)

4.49

22 (0.001)

24

22 (0.85)

0.042

8% T.D

18

25 (1.6)

106

38 (0.002)

22

24 (0.91)

0.045

10% S.D

22

33 (1.03)

194

40 (0.002)

23

22 (0.73)

0.043

10% T.D

10

40 (0.57)

1.0

6 (0.00)

20

14 (0.41)

0.05

160

No. of Cotton varietj/state convolutions per cm

Convolu­ GV% tion angle CV%

e

No. of reversals CV% per cm

Distance between reversals cm

B-1007-Raw

57

36 (2.28)

12.98

40 (0.003)

27

29 (1.13)

0.037

10% S.D

31

28 (1.22)

4.5

24 (0.0(G)

24

23 (0.78)

0.042

10% T.D

11

45 (0.86)

1.42

46 (0.001)

21

26 (0.72)

0.048

Sarada-Raw

60

9 (0.78)

11.8

32 (0.01)

25

35 (1.09)

0.045

10% S.D

33

22 (1.01)

4.94

33 (0.003)

21

27 (0.8)

0.048

10% T.D

8

47 (0.84)

1.17

48 (0.002)

20

23 (0.64)

0.05

Bicanaire Narnia-Raw

66

19 (1.8)

12.14

43 (0.01)

29

30 (1.2)

0.03

10% S.D

21

39 (1.2)

3.24

42 (0.002)

25

26 (0.91)

0.04

10% T.D

9

44 (0.8)

1.26

47 (0.001)

22

17 (0.48)

0.045

Note: Values in parentheses indicate the standard error.

S.D. - Slack-dried fibres. T.D. - Tension-dried fibres.

161 and cotton types and in between cotton types are found to be significant at 1% level. Several scientists have related convolution angle with x-ray and spiral angles. They also found relation between convolution angle and strength of cotton fibres. In this investigation too, comparision is made between the convolution angle and various angles of orientation, (Figures 8.1 and 8.2) and between convolution angle and strength (Figure 8.3) as the correlation coefficients on pooled data (paired replicas) are found to be highest The variation in the x-ray angle of different varieties of cotton has been attributed to the variation in the convolution angle among the varieties. A subtraction of mean convolution angle from the corresponding x-ray angle yields a constant value of spiral angle of around 21° for most cottons. The aqueous swelling and stretching treatment that has been given to few varieties of cotton has led to a significant reduction in convolutions and it will be interesting to see the effect of reduced convolution angle on x-ray angles and optical spiral angle. Table 8.2 shows the true 40%, 50%, 75% and mean orientation X-ray angles and optical spiral angle with convolutions angle subtracted from the original angles obtained by X-ray and Becke-line method. It may be observed that though the effect of convolutions is eliminated, the X-ray angles are still found to be less than the optical spiral angles. The first conclusion that emerges from this observation is that the data supports the general assumption that the Becke-line technique leads to a spiral angle at or near the fibre surface and that X-ray method'fields a statistical distribution of the orientation through out the cell wall Le., the mean orientation angle for the whole wall. Another conclusion is that the structure of cotton fibres confirms to a constant pitch arrangement If cotton possesses a constant angle structure, the X-ray angle and the optical spiral angle would be equal or nearly equal if experimental errors do not come into play. Moreover it may be observed from the data that the successive increases in the amounts of stretch casue a gradual narrowing of the difference between X-ray

MEAN

ORIENTATION

ANGLE (oC)

162

CONVOLUTION

ANGLE (S')

FIGURE 8-1. RELATIONSHIP BETWEEN CONVOLUTION ANGLE AND

MEAN

ORIENTATION ANGLE

BIREFRINGENCE

163

CONVOLUTION ANGLE (6*)

FIGURE8-2-RELATIONSHIP AND

BETWEEN CONVOLUTION ANGLE

BIREFRINGENCE

TENACITY (g m s /te x )

164

CONVOLUTION

FIGURE 8-3-RELATIONSHIP AND

SINGLE

ANGLE (9)

BETWEEN CONVOLUTION ANGLE FIBRE

TENACITY

165 TABLE 8.2 : TRUE ORIENTATION ANGLES OF RAW AND AQUEOUS SWOLLEN AND STRETCHED FIBRES FREE FROM THE EFFECT OF CONVOLUTION ANGLE (0°) Cotton Variety/State

40% X-ray angle - 0°

50% X-ray angle - 0°

75% X-ray angle - 0°

Mean orientation angle - 0°

Optical Spiral angle-0°

Raw

15.83

13.4

4.5

4.65

25.26

8% S.D.

20.36

17.74

8.51

10.74

2739

8% T.D.

18.55

15.23

8.76

12.13

2837

10% S.D.

20.2

17.11

8.37

10.9

29.83

10% T.D.

1532

12.56

7.2

10.61

2739

Raw

27.68

24.08

15.18

10.48

2737

8% S.D.

26.62

23.47

15.62

13.81

31.27

8% T.D.

25.61

22.14

14.45

14.44

31.22

10% S.D.

27.4

23.85

15.4

14.93

32.64

10% T.D.

20.17

17.48

10.17

13.16

26.27

Jayalakshmi • Raw

25.47

22.47

13.93

10.37

26.72

8% S.D.

23.97

20.51

13.21

12.77

27.07

8% T.D.

16.79

14.09

7.94

11.12

26.59

10% S.D.

20.06

18.06

11.56

12.07

25.71

10% T.D.

14.0

11.69

5.92

10.93

2136

Raw

20.02

16.02

7.02

6.75

24.17

10% S.D.

21.64

18.21

9.86

12.08

29.96

10% T.D.

19.45

17.08

10.32

11.5

27.77

Raw

22.87

19.2

9.2

8.52

26.28

10% S.D.

2139

18.39

10.06

1232

313

10% T.D.

18.83

15.83

8.83

13.01

26.41

Bicanaire Namftaw

18.86

14.86

6.36

6.28

2633

10% S.D.

22.09

18.76

11.76

12.98

31.22

10% T.D.

18.07

15.74

8.74

12.55

2733

SRT-1 -

Deviraj -

B-1007 -

Sarada -

S. D. - Slack-dried Fibres T. D. - Tension-dried Fibres

166

angles and optical spiral angle. This effect is explainable on the score that as the helices are opened out due to the imposed stretch, the range of spiral angles covering the fibre cross-section, is gradually narrowed down and this brings the value of the optical spiral angle of the outer layer in close proximity with that representing the average spiral angle for the whole wall. The concept of a constant pitch structure for cotton throws a new light on the theory of X-ray diffraction of fibres with a spiral structure. Stephens has made a reference to the bearing that a constant pitch structure would have on the X-ray diffraction pattern. It would be worthwhile, to discuss in brief, the contentions of one school of thought which holds that the spiral angles of all cottons is constant around 21°. It has been shown quite conclusively that cotton has a constant pitch structure. It has also been shown that the estimated spiral angle is largely dependent on this method by which it is determined. The X-ray method leads to a mean spiral angle of the crystallites over the whole cell wall, whereas the Becke-line method leads to a higher estimate without doubt, at least to the outer wall layers. Corresponding compensator and refractometer methods lead to a mean spiral angle for the whole fibre, that is comparable with that given by X-ray methods. It is Meredith (1946) who stated for the first time that the spiral angle deduced from refractive indices is nearly the same for all varieties if the effect of convolutions is corrected for. Later Betrabet et al (1963), Hebert (1967), Duckett and Tripp (1967), Morosoff and Ingram (1970) and many others have supported the view. More recently Iyer et al (1985) have shown a range of 7.7° that exists between true spiral angles in air-dried cottons. Similar relation may not be possible considering the data on aqueous stretched fibres, because aqueous stretching will straighten the spirals to a certain extent which is reflected in decrease in x-ray angles and optical spiral angle. The decrease in all orientation angles may be attributed partially to the decrease in convolution angles and partly to the straightening of spirals.

167

The tensile properties of cotton fibres are affected by the presence of convolutions [Hearle and Sparrow (1979)]. Cotton fibres having a high convolution angle will have a lower initial modulus but will be more extensible, while those with a low convolution angle will have a high initial modulus and be less extensible. High negative correlation was found between the convolution angle and initial modulus of water stretched and mercerized cottons to support the view expressed by Hearle and Sparrow (1979). High negative correlation was found with breaking load and tenacity. It should be noted that in the untreated fibres, the straightening of the convolutions will lead to additional stresses which contribute to the cause of breakage. But water treatment removes convolutions to a certain extent and alleviates the internal stresses and thus brings down the effect of convolutions and results in higher modulus. As the initial extension in any fibre is due to deconvolution, the extensibility of these water stretched samples shows a decrease due to non-availability of initial extension and thus highly correlated with the convolution angle. These aspects are already discussed under Chapter 6. Convolution angle is shown to correlate highly with ribbon width, wall thickness, perimeter, degree of thickening, circularity, linear density, number of reversals and breaking twist angle. A highly convoluted fibre increases the linear density as convolutions contract the length of the fibre and there by result in higher weight But the effect may not be high. The relations between convolutions and circularity may be explained as follows: The never dried fibre has a cylindrical shape without any convolutions. When it dries up, it takes up a flattened structure and convolutions appear. When such a flattened tube is wetted in water and stretched, the convolutions are removed and the fibre tends to take up a cylindrical form and come closer to its own shape before boll opening.

168 8.2.

STRUCTURAL REVERSALS Reversals are the most complicated and much debated morphological

features of cotton fibres. The coefficient of variation is high for counting the reversal frequency. The data on reversal frequency and the distance between the reversals of aqueous stretched cottons are given in Table 8.1, from which it is clear that the frequency of reversals is decreased with increase in stretch. As a result the distance between reversals is increased. This may be attributed to the straightening of reversals, which is shown by decreased spiral angle. The scanning electron micrographs also confirm this finding. The effect of convolutions does not come into picture here, as measurements are made on unconvoluted fibres. The decrease in number of reversals is found to be significant at 1% level between drying conditions and cotton types. Reversal frequency is correlated positively with all orientation angles and negatively with Hermans and optical orientation factors. High negative correlations are found with all tensile parameters namely breaking load, tenacity, elongation and secant modulus. Reversal frequency is also correlated with ribbon width, perimeter, degree of thickening, circularity, number of convolutions and convolution angle. The distance between reversals which is deduced from the reversal frequency is correlated with all the parameters mentioned above but with lower level of correlation. The contribution of reversals for fibre strength is a much debated aspect and a number of studies are reported in the literature review. It may be worthwhile to find the cause for decrease in reversal frequency as it helps to interpret its relation with orientation and strength parameters. From the literature review, it is clear that convolutions reverse their direction at the reversal point When a convoluted fibre is allowed to swell in water, convolutions are removed and due to swelling, there is lateral expansion of the fibre, and as a result, the extra length due to deconvolution is not shown. But when the swollen

TE

NACITY ( gms

/

t«x)

169

FIGURE 84-RELATIONSHIP BETWEEN REVERSAL FREQUENCY AND SINGLE

FIBRE TENACITY

170

fibre is stretched, lateral contraction occurs and the fibre elongates to the level of stretch. Convolutions are not completely removed during this process and in slackdried samples they are picked up again during drying. Then, the extra length due to stretch may be the result of partial deconvolution and straightening of fibrils. The x-ray orientation and birefringence values clearly show that straightening of fibrils has taken place in aqueous stretched samples. As a result, the frequency of reversals on these stretched samples decreased, increasing the distance between the reversals. The straightening of fibrils between reversals reduces the helix angle and relieve the internal stresses between helices by wetting to a certain extent As reversals are already the sources of high orientation, the collective force of the fibrils increases thereby showing higher strength. This clearly shows the association of reversals and its frequency with orientation and strength parameters and convolutions (Figure 8.4). 83

CONCLUSION Interesting changes are observed in the morphological features of cotton

fibres due to swelling and stretching in water. Frequency of convolutions is found to decrease with a corresponding decrease in convolution angle. The convolutions are drastically reduced in tension-dried samples especially in 10% stretch. The relation of convolutions with initial modulus, tenacity and extension are discussed. The constancy of the spiral angle without the effect of convolutions, as observed by previous workers, is discussed and found that such a relation does not exist in water stretched samples. Convolutions are correlated with other fibre properties. Reversals are found to decrease on water stretched samples and as a result the distance between the reversals is increased. This is attributed to the straightening of spirals between the reversal points. Corrections are found with other fibre properties.