Indian Journal of Fibre & Texti le Research Vol. 28, June 2003, pp. 1 34- 146

Effects of swelling and stretching in water on the properties of cotton fibres and yarns

G F S Hussain" & K R Krishna Iyerb

Central Institute for Research on Cotton Technology , Matunga, Mumbai 400 0 19, India

Received 12 November 2001; revised received 5 March 2002; accepted 23 April 2002

The effect of aqueous swelling followed by stretching and drying in taut condition of cotton fibre and yarn on their tensile and rheological features has been studied. Single fibre characteristics such as tenaci ty, init ial modulus, secant modulus and specific work of rupture get enhanced while the extension drops on treatment. Treated fibre is more lustrous and does not undergo any change due to the wetting in water. The treatment .of yarn enhances its tenaci ty, ini tial modulus, secant modulus and immediate elastic recovery quite significantly. Wetting in water augments the tenacity and decreases the immediate elastic recovery of raw yarn. However, no such changes occur on wetting of treated yarn.

Keywards : Aqueous swelling, Cotton, Initial modulus, Rheological features, Secant modulus, Specific work of rupture , Strength-Icngth gradient , Tensile characteristics

1 Introduction Water, as an inter-crystal l ine swell ing agent,

plasticizer and lubricant, is known to bring about many changes in the rheological and morphological features of cotton 1 -'1 . But these changes are reversible and temporary in the sense that once the fibre dries, the structure reverts back to the original state to retain its original characteristics. However, researchers l O- 1 3

have shown that if the aqueous swel ling is accompanied by stretch and the fibres are dried in the stretched state, the structural transformation is permanent and irreversible and the durable changes in tensi le and morphological features would fol low.

The influence of aqueous swel ling accompanied by stretch drying on yarn characteristics was investigated by Doke and Iyerl4. They observed that the aqueous swelling and stretch treatment enhances the tenacity of yarn (kiered) by 8-28%. The increase in tenacity is stretch and count dependant. Coarser yarns show higher improvement in tenacity and decreased extension than finer yarns. Subramaniam and Thambidurai 15 observed that the aqueous swell ing and stretching treatment enhances tenacity of the rotor­spun yarn (scoured) whereas they found no such trend in the case of ring-spun yarn (scoured). They

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attributed better absorption of water due to the lower packing fraction of rotor-spull yarn for its differential behaviour from the ring-spun yarn .

However, there is no data available on the strength­length gradient of treated fibres (i .e . drop in bundle tenacity with the increase in specimen length) which characterizes the uniformity of fibre along its length in respect of the tensile characteristics. Another important gain of this treatment is the improvement in lustre. This improvement in lustre needs to be quantified, literature being totally silent about this aspect. In the case of normal cotton fibres, it is well established that the wet strength is more than dry strength. Doke and lyerlO, in their study dealing with the aqueous swollen stretch dried cotton fibres, indicated that there is no increase in tenacity when the treated fibres are wetted in water but there is a l i ttle decrease in tenacity. They explained the reasons for this decrease but unfortunately the study fai led in respect of statistical test of significance. Also, no data pertaining to static and dynamic moduli and recovery characteristics of the yarn subjected to aqueous swel ling fol lowed by stretch drying is avai lable in the l iterature.

The present paper reports the results obtained from the study of influence of aqueous swel l ing and stretching of fibres and yarns (hereafter referred to as treated fibres and yarns) on their tensile characteristics. The effects of wetting of fibres and yarns, both raw and treated, on the above-mentioned

HUSSAIN & IYER: EFFECT OF SWELLING & STRETCHING IN WATER ON COTION

Table I - Strength-length gradient

Sample Tenacity, g/tex Value of 0" 3.2" 6" 8" l Oa b

1 70C02 Raw 2S.6 IS.7 14.S 1 1 .9 1 1 .6 - 0.328 Treated 34.7 22.0 2 1 .2 19.0 1 8.8 - 0.2S8

LRA.S I66 Raw 27.9 17.4 1 6.2 IS.2 14.4 - 0.274 Treated 32.3 22.9 20.7 20.6 20.6 - 0.197

V.797 Raw 28.0 I S .0 1 2.6 1 1 .7 1 1 .0 - 0.39 1 Treated 34.7 24.4 2 1 .2 20.7 19.7 - 0.239

AKA.840 1 Raw 28. 1 IS.O 13.6 12 . 1 1 1 .4 - 0.376 Treated 34.0 2S.7 22.3 22.0 22.8 - 0.206

AKA.S Raw 30.S 1 8.0 IS.2 IS.O 1 4.1 - 0.324 Treated 33.5 24.0 22.4 2 1 .8 2 1 .S - 0.190

G COL l i Raw 27.6 IS.O 1 2.S 1 2.4 1 1 .3 - 0.37S Treated 32.8 22.S 20.4 19.9 1 8.8 - 0.23S

Mech.1 1 Raw 26.6 19.3 17. 1 IS.3 14.0 - 0.2S8 Treated 3 1 .8 2S . 1 23.4 22. 1 2 1 .3 - 0.1 6S

DCH.32 Raw 29. 1 20.1 19.4 17.8 1 6.8 - 0.221 Treated 38.9 30.1 27.8 26.1 26.0 - 0.1 73

Pima Raw 30.4 20.8 19.0 17.8 1 6.4 - 0.249 Treated 37.S 30.2 28.6 28.1 28.0 - 0.127

Suvin Raw 36.4 27.0 24.9 22.4 20.2 - 0.230 Treated 47.3 4 1 .3 39.8 36.3 34.3 - 0.137

a Gauge length in mm b = Rate of change of tenacity with gauge length; minus sign indicates drop i n tenacity

properties are also analyzed. The influence of treatment on the l ustre and bundle strength of fibres at various gauge lengths is also dealt with.

2 Materials and Methods

1 35

Ten samples of different cotton varieties (Table 1 ) were taken for the investigation. Using a stretching device

' (Fig. 1 ) , the fibres were wetted thoroughly and

then stretched to 1 5% of their ini tial length and dried in an oven in taut state I 2. 1 3 . The dried fibres were removed from the device and stored in a desiccator at 65% RH.

Fig. I-Device used for stretching fibres and drying in taut state

Ten ring-spun yarns of count ranging from 20s to 90s were selected for the study. A gadget (Fig.2) was used to stretch the yarns in the form of leas after swel ling them in any reagent ( in the present case water). The stretch level for each yarn sample was 85% of the stretch at which the yarn segments in the lea started breaking. After stretching the lea to this level, i t was left for drying in the stretched state in stretching gadget. Once the lea dried, i t was removed from the machine and conditioned at 65% RH.

2.1 Tensile Properties

Instron tensile tester was used to determine the bundle tenacity at 0, 3.2, 6, 8 and 10 mm gauge lengths. Cross-head speed for each gauge length was so Fig.2-Gadjet used for stretching yarns and drying in taut state

1 36 INDIAN J. FIBRE TEXT. RES., JUNE 2003

chosen that the time for break was in the range of 1 2-20s. The strength of textile material depends upon the time-to-break and hence to compare the data of raw and treated fibres, it is necessary to maintain time-to­break as far as possible within a prescribed range.

For single fibre dry/wet strength tests, special mounting tabs and mounting bench were used. One end of the fibre was stuck to the centre of tab (called upper tab) with stick fast adhesive. The tab had a hole on the upper middle portion through which a chain could be inserted. This chain was l i nked to the load cell , which was housed in the moving cross-head of the Instron. These tabs were then inserted in the slits in the mounting bench. The mounting bench consists of 6 pairs of rectangular blocks with slits in the middle to insert the tabs. One of the blocks remains fixed while the other · is adjustable. After allowing sufficient time for the adhesive to dry, the free ends of the fibres were fixed with stick fast to other tabs placed in the slits of movable block of the mounting bench. The distance between the two blocks was adjusted such that the length of fibre was approximately 1 cm. These fibres were allowed to dry overnight.

I nstron tensile tester was used to determine the single fibre properties (at a gauge length of 1 cm) such as breaking load, strain-at-break, initial modulus, specific work of rupture, stiffness and secant modulus. To conduct the tests, one of the tabs was attached to the chain l inked to the upper jaw and the other one to the lower jaw. I nitially both the tabs, with the fibre in between, were made to touch each other and the reading on the extension measurement unit was adjusted to show zero mm. Then the upper jaw was allowed to move slowly which straightened the fibre til l the load on the fibre registered (fibre tex/2)g on the chart which was calibrated to show 1 0 g for the ful l scale. At this point, the movement of upper cross­head was stopped. The reading on the extension measurement unit indicated the initial length of the fibre. Later, the fibre was strained til l it ruptured. From the resulting load-elongation curve, all the desired parameters were determined. Tests were carried out in the standard atmosphere of 65 ± 2% RH and 27 ± 2°C. To investigate the effect of wetting, the tabs with the fibre were placed inside a beaker containing water to which wetting agent Auxipan (PN), manufactured by Auxichem Industries, Mumbai, was added. Auxipan is a non-ionic type wetting agent; the concentration is 0.0 1 % of the total l iquor. After 1 0 min, the tabs were l ifted out of water.

One of the tabs was held in hand and the other one was allowed to rotate freely . Once no more rotation took place, which ensured that almost all the convolutions in the fibre were removed, the tabs were mounted on the Instron fol lowing the procedure mentioned earlier. After measuring the initial length, the fibre was again wetted with water by means of wet brush and the wetted fibre was extended til l it ruptured. From the load-elongation curve, the breaking load, strain-at-break, specific work of rupture and moduli values were estimated.

For measuring the tensile properties of yarn, a test specimen of 25 cm length was mounted between the upper (moving) and lower (fixed) jaws under a load of (yarn tex/2)g. The specimen was strained by moving the cross-head (under computer control) with a speed such that rupture occurred in and around 1 2 - 20 s for the reason already mentioned. The broken yarn bits between the grips were removed and weighed accurately to obtain the tex value. From the load­e longation curve, the programme calculated the tenacity, strain-at-break, initial modulus (static modulus) between 1 09 and 50 g load, specific work of rupture and secant modulus. For each sample, 50 tests were conducted and the mean values of al l the parameters were calculated. The experiments were carried out at ambient condition of 65 ± 2% RH and 27 ± 2°C. To determine the tensile properties after wetting, the specimen was thoroughly wetted for 1 5 min in water to which wetting agent was added and the tests were carried out on wetted yarns exactly as of dry yarns.

2.2 Measurement of Lustre

Lustre was measured by an indirect method. Jay Pak 4804 computer colour matching system was used to determine the per cent reflectance at the wavelength of 560 nm and Hunter's whiteness index. Jay Pak 4804 colour matching system consists of spectrophotometer, computer for the conversion of digital data to perform various calcltlations, printer and software. Spectrophotometer measures the amount of l ight reflected from the surface of an opaque sample throughout the visible range of spectrum, i .e. 400-700 nm, at intervals of 1 0 or 20 nm. The computer with an appropriate programme control s the operation of spectrophotometer and the per cent reflectance values at various wavelengths as well as Hunter's whiteness index 1 6 are obtained. There is provision to get the above parameters in two different modes: (i) specular included, and (ii)

HUSSAIN & IYER: EFFECT OF SWELLING & STRETCHING IN WATER ON COTTON 1 37

specular excluded. When the light is incident on the surface of the specimen, the part that is reflected at an equal to the angle of incidence is called the specular reflectance and the part that is reflected at all other angles is called diffuse reflectance. When the spectrophotometer is operated under specular included mode, the measurement is made in such a way that the specular component of the reflectance is included. Under the specular excluded mode, the specular reflectance is trapped and excluded from measurements.

The sample was in the form of bundle of parallel fibres of exactly 50 mg wt mounted on a black paper with a hole such that the width of the tuft was sufficient to cover the aperture of the sample holder. The ends of the tufts were stuck with cellophane tapes so that the fibres remained parallel and taut. The black paper with the sample was placed in the sample holder of spectrophotometer and the experiments were carried out.

2.3 Dynamic Modulus Estimation

The dynamic modulus (sonic modulus) of yarn was measured by means of pulse propagation meter PPM-5. By adopting the procedure standardized in CIRCOTI7, the velocity of sound (C) in krnls in yarn was determined. The dynamic modulus (E) was calculated using the following equation:

E (g/den) = 1 1 .3 x c? For each sample, 75 tests were conducted. The index of elasticity is the ratio of dynamic

modulus to initial (static) modulus. The indices of elasticity of raw and treated yams were calculated from the values of dynamic modulus and initial modulus; later one was obtained from the stress-strain curve.

2.4 Determination of Recovery Parameters

Recovery parameters such as immediate elastic recovery (IER), delayed recovery (DR) and permanent set (PS) were measured with the aid of Instron tensile tester. The procedure was the same as that used by Hussain et a/17 • In the present study, two types of tests were conducted, viz. (i) both raw and treated samples were strained to the same level with respect to their actual breaking extensions, and (ii) raw and treated samples were strained to the same level with reference to the gauge length. In second case, each specimen was strained to 1 .4% of specimen gauge length (which was 25 cm), i .e. 3 .5 mm. For

each sample, 1 0 tests were carried out. The average values of all recovery parameters were obtained.

3 Results 3.1 Strength-Length Gradient (Weak-link Effect)

Bundle tenacity was determined for raw as well as treated cottons at 0, 3 .2, 6, 8 and 1 0 rnm gauge lengths. Table 1 shows that as the specimen length increases, the tenacity decreases for both raw and treated cottons.

It is observed that the tenacity vs gauge length relationships follow Pillai's empirical equation 1 8 [TL =

To (L+ 1 )b] , where TL is the tenacity at gauge length L ; To, the strength at nominal zero gauge length; and b,

the degree of imperfection or strength - length gradient (a measure of the rate of drop of tenacity with test length). The value b determines the uniformity of specimen along its length in respect of tensile characteristics; the lower the value of b, the higher is the uniformity.

It is clear from Table 1 that for treated fibres, the tenacity at zero gauge length (To) is always higher than that of raw fibres. But the strength-length gradient values are always less for treated fibres compared to those for raw fibres. This indicates that the treated sample is more uniform along its length than the raw one. This observation is similar to the one made by Pillai 1 8 for slack mercerized fibres. The main reason for this drop, that signifies improvement in the uniformity, is that the treatments, such as aqueous swelling followed by stretch, slack and stretch mercerization, strengthen the weak points along the length of fibre which leads to enhanced tenacity at every gauge length compared to raw fibres.

Another interesting observation is that the ratio of tenacity of treated sample to that of raw fibre goes on increasing as the gauge length increases. This is shown in Table 2 in which the tenacity ratios averaged for all the ten cottons are shown against gauge lengths. Similar observation was made by

Table 2 - Gauge length vs tenaci ty ratioa

Gauge length , mm

o 3.2

6

8

1 0

Tenacity ratio

1 .23

1 .46

1 .5 1

1 .57

1 .63

Tenacity of treated fi bre a Ratio = __ -'--__ ---::-__

Tenacity of raw fibre

138 INDIAN 1 . FIBRE TEXT. RES. , JUNE 2003

Table 3 - Effect of treatment on single fibre characteristics

Sample Tex Breaking Tenacity Breaking Initial Specific work Secant load g/tex elongation modulus of rupture modulus

g %

Pima Raw 1 65 5.6 33.8 1 1 .6

Treated 1 45 8.0 55.0 5.9

Suvin Raw 1 24 5 .7 45 .8 8.3

Treated 1 I 0 7.9 7 1 .6 5 .3

DCH.32 Raw 1 29 4.8 36.8 10.5

Treated 1 I 3 6.5 58.0 6.8

H.6 Raw 1 65 4.3 23. 1 7.6

Treated 1 40 5.6 4 1 .3 5 .4

AKA.5 Raw 22 1 6.0 26.9 6.5

Treated 20 1 8 .2 40.5 4.2

CJ73 Raw 1 75 4.2 23.8 6.0

Treated 1 57 6.5 43.2 4.7

G CoL l i Raw 1 80 4.5 25. 1 5 .5

Treated 1 65 5.9 35 .9 4.6

V.797 Raw 223 4.9 2 1 .9 6.5

Treated 202 8.4 4 1 .0 4.0

LRA 5 1 66 Raw 1 88 4.6 24.4 1 0.2

Treated 1 63 7.2 43.9 4.8

GA Control 1 99 4.4 22.2 . 4.6

Treated 1 77 7.3 4 1 . 1 3.9

Pillai 18 for mercerized samples. Because of the nature of empirical equation, this ratio cannot go on increasing indefinitely but it wil l get stabil ized at higher gauge lengths.

3.2 Single Fibre Characteristics

Table 3 shows the single fibre characteristics (at 65% RH) of raw and treated fibres. Aqueous swel ling fol lowed by stretching enhances the breaking load of fibres. The per cent increase over the control varies from 3 1 to 7 1 . On account of stretch imparted, the l inear density (tex value) gets reduced by 8- 15%. Combined influence of these two changes results in considerable increase in tenacity of treated fibres. The increase over control ranges from 43% to 87%. Elongation suffers reduction after the treatment; the decrease, as percentage of the control value, varies from 1 5% to 53%. There is a large increase in the initial modulus as well as secant modulus of cotton fibres after the treatment. The extent of increase over the control value ranges from 1 07% to 176% for initial modulus and from 55% to 222% for secant modulus. The specific work of rupture increases after the treatment; the increase ranges from 9% to 56%. In

g/tex g.mm/lex g/tex

462.3 1 7 .8 307.8

1 223.4 1 6.6 992.2

837.3 1 7.7 559.4

228 1 .8 20.8 1 337.5

669.5 1 8 .2 380.8

1 845 . 1 1 9.8 1 1 06.2

699.3 1 2.7 42 1 .4

1 447.9 1 2 . 1 1 000.0

495.0 9 . 1 4 1 0.4

1 294.5 1 2.2 956.7

509.6 7.3 4 1 3 . 1

1 363.7 1 l .4 855.4

680.6 6.6 459.4

1 4 1 8.6 9.3 7 1 3 .9

523 .9 7.7 375.7

1 400.0 1 0.2 936.6

35 1 . 8 1 3 .2 259.7

800. 1 1 l .6 725 .8

602.0 6.7 438. 1

1 370.3 8 .3 96 1 .6

some case, there is a drop in the value which is not statistical ly significant.

3.3 Effect of Wetting

The effect of wetting on tensile characteristics of raw and treated fibres is shown in Tables 4 and 5 re­spectively. It is c lear from Table 4 that for raw fibre on wetting, the tenacity increases on an average by about 24%, breaking extension decreases by about l 7%, and in itial modulus increases by about 2 1 %. The specific work of rupture increases marginally by 8% and secant modulus by 36%. All these changes are statistical ly significant. Table 5 shows the tensile characteristics of treated fibres after wetting. Though there seems to be a reduction in the tensile properties of treated fibres, the decrease is not statistically sig­nificant at al l . This clearly indicates that the wetting does not affect the tensile properties of treated fibres.

3.4 Lustre

Table 6 shows the values of per cent reflectance and Hunter's whiteness index. It is clear that on account of treatment, the per cent reflectance as well as Hunter's whiteness index increase, which i ndicates

HUSSAIN & I YER: EFFECT OF SWELLING & STRETCHING IN WATER ON COTTON 1 39

Table 4 -Influence of wetting on raw fibre characteristics

Sample Tenacity Breaking Initial Specific work Secant g/tex extension modulus of rupture modulus

% g/tex g. mrnltex g/tex

Pima at 65% RH 33.8 1 1 .6 462.3 1 7.8 307.8

Wet 40.0 9 .8 548.5 1 9.9 4 1 7.3

Suvin at 65% RH 45.8 8.3 837.3 1 7 .7 559.4

Wet 56.5 6.9 1 099.2 2 1 .0 834.7

DCH.32 at 65% RH 36.8 10.5 669.5 1 8.2 380.8

Wet 43.0 8.2 824.0 1 8.6 556.0

H.6 at 65% RH 23. 1 7.6 699.3 1 2 .7 42 1 .4

Wet 28.6 5.8 843.3 1 3 .3 634.4

A KA.5 at 65% RH 26.9 6.5 495.0 9 . 1 4 1 0.4

Wet 32.3 5.7 575.0 9.3 500.0

CJ.73 at 65% RH 23.8 6.0 509.6 7.3 4 1 3 . 1

Wet 3 1 .2 5 .3 589.7 7.8 503.9

G COL l i at 65% RH 25. 1 5 .5 680.6 6.6 459.4

Wet 28.8 4.8 748.5 6.6 548.3

V.797 at 65% RH 2 1 .9 6.5 523.9 7.7 375.7

Wet 28.2 5.8 6 14.9 8.5 445.0

LRA.5 1 66 at 65% RH 24.4 1 0.2 35 1 . 8 1 3 .2 259.7

Wet 3 1 .5 8.0 454.8 1 4. 3 400.4

GA at 65% RH 22.2 4.6 602.0 6.7 438 . 1

Wet 28.8 3.6 748.7 7 .8 639.2

Table 5 - Influence of wetting on treated fibre characteristics

Sample Tenacity Breaking Initial Spec i fic work Secant g/tex extension modulus of rupture modulus

% g/tex g.mrnltex g/tex

Pima at 65% RH 55.0 5.9 1 223.4 1 6.6 992.2

Wet 53.8 5 .7 1 1 76.8 1 5 .0 96 1 .4

Suvin at 65% RH 7 1 .6 5 .3 228 1 .8 20.8 1 337.5

Wet 69.3 5 . 1 2 199. 1 1 9.0 1 3 1 1 .4

DCH.32 at 65% RH 58.0 6.8 1 845. 1 1 9.8 1 106.2

Wet 55.9 6.4 1 765.3 1 8 .0 1 033.6

H.6 at 65% RH 4 1 .3 5.4 1447.9 1 2. 1 1 000.0

Wet 38.0 5.0 1 407.9 1 0.7 9 1 8.0

AKA.5 at 65% RH 40.5 4.2 1 294.5 1 2 .2 956.7

Wet 40.5 4.0 1 1 64.8 1 1 .7 888.0

CJ.73 at 65% RH 43.2 4.7 1 363.7 1 1 .4 855.4

Wet 42. 1 4.4 1 257.0 1 0.7 999.9

G CoL l I at 65% RH 35.9 4.6 1 4 1 8.6 9.3 7 1 3 .9

Wet 34.3 4.3 1 398.7 8.9 694.9

V.797 at 65% RH 4 1 .0 4.0 1 400.0 10.2 936.6

Wet 39.3 4.0 1 367.4 8.2 899.9

LRA.5 1 66 at 65% RH 43.9 4.8 800. 1 1 1 .6 725.8

Wet 42.:; 4.8 79 1 .2 1 0.4 758.2

GA at 65% RH 4 1 . 1 3.9 1 370.3 8.3 96 1 .0

Wet 40.4 3.8 1 370.0 7.9 900.0

140 INDIAN J. FIBRE TEXT. RES. , JUNE 2003

Table 6 - Per cent reflectance values at 560 nm and Hunter's whiteness index before and after treatment Sample Per cent reflectance

Raw fibres Treated fibres % increase over raw fibres

H unter' s whiteness index Per cent --R-a-w-fi""l b:-r-es---=T-re-a-te""'d--=fi:-lb-re-s--9!-o-i-nc-r-ea-s-e-o-ve-r- increase

raw fibres in circu-

Specular Specular Specular Specular Specular Specular Specular Specular Specular Specular Specular Specular included excluded included excluded included excluded included excluded included excluded included excluded

larity

Pima 59.93 Suvin 60. 1 6 OCH.32 63.23 H.6 62. 1 6 AKA 5 68. 1 6 C1.73 60.22 V.797 64.6 1 G COL l i 65.33 LRA. 5 1 66 62.30 GA 62.2 1

59. 1 4 60.06 62. 1 8 6 1 .42 68. 1 0 59.65 64.5 1 65. 1 7 62. 1 8 62.00

64.77 65 .8 1 67.4 1 68.44 72.96 68.00 70.64 70.82 69.90 70.04

63.89 65.40 66.38 67.47 72.90 67.60 70.54 70.42 69.60 69.78

8.08 9.39 6.61 1 0. 1 0 7.04 1 2 .92 9.33 8.40 1 2.20 1 2.59

8.03 8.89 6.64 9.85 7.05 1 3.33 9.35 8.06 1 1 .93 1 2.55

56. 1 8 50. 1 6 57.04 56.83 59.80 44.90 59.76 57.33 57.96 58.45

56.00 50.44 57.00 56.52 59.40 44.86 59.56 57. 1 2 56.52 5 8.04

64.90 6 1 .57 65.66 67.23 67.80 60. 6 1 69.60 67.82 66.20 69. 9 1

64.80 6 1 .56 65.63 66.37 67.40 60.39 69.50 67.40 65.98 69.60

1 5 . 1 2 1 .8 1 5 . 1 1 8 .3 1 3 .4 35.0 1 6.5 1 8.3 1 4.2 1 9.6

1 5 .7 22.0 1 5 .0 1 7.4 1 3.5 34.6 1 6.8 1 8.0 1 6.7 1 9.9

8. 1 0 9.60 8 . 1 0 1 1 .70 6.30 1 3 .50 1 1 .00 7.90 7 .80 1 2.80

Table 7 - Si ngle thread properties before and after treatment

Specific work Ini tial modulus Sample (Count)

Tenacity g/tex

Strain % of rupture, g.mm/tex gltex

Secant modulus g/tex

Raw ASTO" % increase

over raw

Raw ASTO" % decrease

over raw

Raw ASTO" % decrease Raw ASTO" % Raw ASTO" % i ncrease

over raw

Suvin (90s) 1 9.7 23.9 Hybrid.6 1 4. 1 1 7.5

(70s) OCH.32

(70s) 1 70 C02

(60s) GA

(50s) Oigvijay

(40s) SRT (40s) CJ .73 (30s)

AKA.5 (20s)

Oeviraj (20s)

1 6.3 19.3

1 1 .0 1 3 .6

1 1 .6 1 4.7

1 0.9 1 3 .8

1 0.4 12 .2

1 1 .2 14.2

10. 1 1 2.6

1 2.5 1 5 .7

2 1 .3 24. 1

1 8.4

23.6

26.7

26.6

1 7 .3

26.8

24.8

25.6

4.80 2.40 5 .50 2.50

7.0 3.70

6.0 2. 1

5 .3 2 .3

4.2 1 .9

4.3 2.3

5 .4 2. 1

7 .5 3 . 1

6.7 3.9

" ASTD--Aqueous swollen and stretch dried

50.0 54.5

47. 1

65.0

56.6

54.8

46.5

6 1 . 1

58.7

4 1 .8

1 22.6 1 00.0

1 49.9

87.7

76. 1

62.3

60.3

86.7

1 07.0

1 04.8

that the fibres have become more lustrous after the treatment.

3.5 Tensile Characteristics of Yarns

The tensi le characteristics of raw and treated yarns are shown in Table 7. It is observed that the breaking load of yarn gets enhanced after the treatment. The l inear density (tex) of yarn gets decreased on account

67.7 49.3

96.4

39.4

40. 1

35. 1

39.7

39.5

57.0

88.6

over raw

44.8 50.7

35.7

55. 1

47.3

43.7

34.2

54.4

46.7

1 5.5

595.8 1 260 3 1 0.2 833.8

202.0 700.5

292. 1 766.8

280.4 766.5

3 1 8.6 792.0

290.6 666.2

23 1 .5 729.5

1 8 1 .0 5 1 4.4

234.0 508.7

increase over raw

1 1 1 .5 1 68.8

4 1 3 .2 984.6 253.9 68 1 .0

1 38.3 1 68.2

246.8 244.5 5 1 5.2 1 1 0.8

1 62.5 688.5 2328.5 238.2

1 73.4 2 1 7 .5 627 . 1 1 88.3

1 48.6 256.9 749.3 1 9 1 .7

1 29.2 243.0 528.0 1 1 7 .3

2 1 5 . 1 233.2 663.4 1 84.4

1 84.2 1 50.7 4 1 3 .8 1 74.6

1 1 7 .4 1 68.6 4 1 1 .8 1 44.2

of stretch imparted. The combined effect of these two factors resul ts in 1 7 .3 - 26.8% increase in tenacity. Strain-at-break suffers 4 1 .8 - 65% reduction after treatment. Treatment lowers the value of specific work of rupture by 1 5 .5 - 55 . 1 % and leads to a l arge increase in moduli values. The range of increase is 1 1 1 .5 - 246.8% for initial modulus and I I I - 238% for secant modulus.

HUSSAIN & I YER: EFFECT OF SWELLING & STRETCHING IN WATER ON COTTON 14 1

Table 8--Influence of wetting o n yarn characteristics

Sample Tenacity Breaking g/tex strain

%

Suvin( 90s)

At 65% rh 1 9.7 4.8

Wet 22.3 6.5

Suvin ASTD

At 65% RH 23.9 2.4

Wet 22.6 3.9

Digvijay (40s)

At 65% RH 1 0.9 4.2

Wet 1 3 . 1 6.5

Digvijay ASTD

At 65% RH 1 3.8 1 .9

Wet 14.0 2 .8

SRT (40s)

At 65% RH 1 0.4 4.3

Wet 1 1 .5 6.8

SRT ASTD

At 65% RH 1 2 .2 2.3

Wet 12 . 1 3.9

A KA .5 (20s)

At 65% RH 1 0. 1 7.5

Wet 1 1 .9 1 0.3

A KA.5 ASTD

At 65% RH 12.6 3. 1

Wet 1 2.3 4.9

Deviraj (20s)

At 65% RH 1 2.5 6.7

Wet 1 4.7 10 . 1

Deviraj STD

At 65% RH 1 5.7 3.9

Wet 14.9 5.2

The characteristics of raw and treated yarns after wetting in water are shown in Table 8 . The effects of wetting on raw yarns characteristics are : ( i) tenacity increases, the extent of i ncrease being 10 .8 - 20.6%, (ii) breaking strain gets enhanced by 36 - 57.5%, (i i i ) specific work of rupture gets increased by 32.3 -68.6%, and (iv) both the moduli ( initial and secant) suffer reduction; the drop in initial modulus ranges from 44.7 - 56.5% and that in secant modulus from 8.7- 19 .0%. Wetting does not bring about any significant change in tenacity of treated yarns. Breaking strain increases in the range of 27.8 - 47.6%. Wetting enhances the specific work of rupture of

Specific work Initial Secant of rupture modulus modulus g.mm/tex g/tex g/tex

1 22.6 595.8 4 1 3.2

1 62.0 285.5 334.6

67.7 1 260 984.6

98. 1 605 568.9

62.3 3 1 8 .6 256.9

1 05.0 1 76.3 230.9

35 . 1 792.0 749.3

56.3 358 307.6

60.3 290.6 243.0

96.4 1 5 1 .0 198.2

39.7 666.2 528.0

65.8 3 1 6.0 298.9

1 07.0 1 8 1 .0 1 50.7

1 47.7 78.8 1 37.6

57.0 5 1 4.4 4 1 3.8

98.7 1 9 1 .0 25 1 . 8

1 04.8 234.0 1 68.6

158.6 1 1 7.9 1 39.2

88.6 508.7 4 1 1 .8

1 10.0 2 1 9.0 272 . 1

treated yam. However, the specific work of rupture of treated yarn after wetting is considerably less than that of raw yarn after wetting. Wetting leads to reduction in moduli values of treated yams; the initial modulus decreases by 52 - 63.6%, while the secant modulus by 33 .8 - 46.9%.

The dynamic and initial (static) moduli of yarns before and after the treatment are given in Table 9. I t is observed that there exists a very high positive correlation between moduli and yarn count. The coefficients of correlation (r) between count and dynamic modulus are 0.93 and 0.93 and those between count and static modulus are 0.86 and 0.88

1 42 INDIAN 1. FIBRE TEXT. RES., JUNE 2003

Table 9 - Modul i values and index of elasticity before and after treatment

Sample Dynamic modulus, g/den Static modulus, g/den Index of elasticity

Raw ASTD % increase Raw over raw

Suvin (90s) 1 20.0 1 78.0 48.3 66.2

Hybrid.6 (70s) 9 1 . 1 1 46.5 60.8 34.5

DCH.32 (70s) 88.6 1 42.9 6 1 .3 22.4

1 70 C02 (60s) 84.8 1 44.9 70.9 32.5

GA (50s) 83.0' 142.4 7 1 .6 3 1 .2

Digvijay (405) 82.4 1 37 .5 66.9 35.4

SRT (40s) 74. 1 1 38.4 86.8 32.3

Cl.73 ( 30s) 74.0 1 32.9 79.6 25.7

AKA.5 (205) 60.8 1 29.9 1 1 3 .7 20. 1

Deviraj (205) 72.9 1 20.2 64.9 26.0

for raw and treated fibres respectively. The values are significant at 1 % level . Owing to the treatment, the yarns experience considerable increase in the values of static and dynamic modul i . However, the extent of increase is much higher in the case of initial (static) modulus compared to that in dynamic modulus. The increase over the control ranges from 48% to 1 1 4% for dynamic modulus and from 1 1 2% to 2 16% for static modulus.

The ratio of dynamic modulus to initial modulus, termed as index of elasticity, reflects the visco-elastic nature of the material . For perfect elastic material, the ratio is one (for example, the ratio is 1 .02 for copper). Higher value of th is ratio suggests that the material is more viscous and less elastic in nature. In the present case, after the treatment, the index of elasticity shows, on the average, 33% reduction. This indicates that the yarn has become more elastic and less plastic (viscous) after the treatment. Thus, the present treatment shifts the yarn towards the elastic side of viscoelastic scale. Hussain et all7 obtained simi lar results for NaOH-swolien and stretched yarns and resin-treated yarns.

Table 10 shows the tensile recovery parameters of control and treated yarns after straining them to 1 .4% of specimen length. Immediate elastic recovery (IER) and total recovery ( IER + DR) of both control and treated yarns show a very high positive relationship with count. In the case of raw sample, the correlation coefficient between IER and count is 0.99 and that between' total te,covery and count, 0.99. For treated samples, the corresponding values are 0.99 and 1 .00

ASTD % increase Raw ASTD % decrease over over raw control

1 40.0 1 1 1 .5 1 . 8 1 1 .27 23.9

92.6 1 68 .8 2.64 1 .58 40. 1

77.8 247.3 3.96 1 .84 53.5

85.2 1 62.5 2 .61 1 .70 34.9

85.2 1 73 . 1 2.66 1 .67 37.2

88.0 1 48.6 2.33 1 .56 33.0

74.0 1 29. 1 2.29 1 .87 1 8 .3

8 1 . 1 2 1 5 .6 2.88 1 .64 43. 1

57.2 1 84.6 3 .02 2.27 24.8

56.5 1 1 7 .3 2.80 2. 1 3 23.9

respectively. All these values are significant at 1 % level. Consequent on the treatment, the IER value increases. The quantum of increase over the control depends upon the yarn count. Delayed recovery (DR) of raw samples is comparatively higher than that of the treated yarns. The total recovery as a result is almost the same for both raw and treated samples. On wetting, for the raw cottons, the IER value decreases by 23% and on an average the total recovery decreases by 8 .3%. The drop in IER value Jor treated yarns is considerably lower (7%) and also there is a proportional increase in DR component. As a result, the total recovery of treated yarns after wetting is almost the same as that of the treated yarn before wetting.

The recovery values after straining the yarns to the same % of breaking strain are given in Table 1 1 . The IER as well as the total recovery values of treated yarns are considerably higher than those of the control yarn. The per cent increase in total recovery over the control varies from 14% for 90s to 40% for 20s count. The permanent set of raw yarn cotton is considerably higher than that of treated yarn.

There exists an excellent relationship between JER and moduli values. This is true for both raw and treated yarns. In the case of the raw yarn, the correlation coefficient (r ) between IER and dynamic modulus is 0.93 and that between IER and static modulus, 0.87. Corresponding figures for treated yarns are 0.92 and 0 .95 respectively. All these valuesare significant at 1 % level . Similar results were also obtained by Hussain et al l7 . They reported a very

HUSSAIN & I YER: EFFECT OF SWELLING & STRETCHING IN WATER ON COTTON 143

Table 10 - Tensi l e recovery values after extending to 1 .4% of specimen length (3.5 mill)

Sample Raw ASTD

lER% DR% Total % PS% IER% DR% Total % PSlJv

Suvin (90s)

At 65% RH 75.6 14.6 90.2 9.8 8 1 .7 1 0.6 92.3 7.7

Wet 62.6 20.2 82.8 1 7 .2 75.7 1 6.6 92.3 7.7

Hybrid (70s)

At 65% RH 73.0 1 6.3 89.3 1 0.7 79.0 1 1 .8 90.8 9.2

Wet 60.3 22.4 82.7 1 7 .3 75. 1 1 6.2 9 1 .3 8.7

GA (50s)

At 65% RH 69.0 1 8.0 87.0 1 3 .0 77.4 1 1 .2 88 :6 1 1 .4

Wet 50.4 29.0 79.4 20.6 7 1 .7 1 6.6 88.3 1 1 .7

CJ 73 (30s)

At 65% RH 62.9 2 1 .4 84.3 1 5 .7 75.4 1 0.7 86. 1 1 3 .9

Wet 47.4 30.0 77.4 22.6 70.4 1 6.7 87 . 1 1 2 .9

AKA.5 (20s)

At 65% RH 60.7 23.3 84.0 16.0 73.0 1 2.0 85.0 1 5 .0

Wet 42.4 34.0 76.4 23.6 66.3 1 8.6 84.9 1 5 . 1

IER- l mmediate elastic recovery PS - Permanent set

DR - Delayed recovery ASTD - Aqueous swollen and stretched dried

Total - (lER + DR)

Table I I - Tensile recovery values from the same percentage of breaking strain

Sample Raw

IER % DR % Total % PS %

Suvin (90s) 6 1 .7 1 9 .3 8 1 .0 1 9 .0

Hybrid (70s) 49.4 23.2 72.6 27.4

GA (50s) 44.6 22.0 66.6 33.4

Cn3 (30s) 42.2 24.0 66.2 33.8

AKA.5 (20s) 4 1 .0 1 9.6 60.6 39.4

high positive relationship between IER and dynamic modulus of yarns both in raw state as well as after stretch mercerization.

4 Discussion The increase in tenacity of single fibres results

from (i) lubricating action of water, ( i i ) removal of built-in strain, ( i i i ) reduction in the morphological features such as density of convolution, reversals and convolution angle which influences the tenacity adversely, and (iv) improvement in orientation and the compact nature of cellulose after the treatment.

Raw fibres on wetting experience increase in tenacity due to the reasons ( i) and ( i i ) enumerated above. On the other hand, the treated fibres behave l ike synthetic fibres and do not show any change in strength on wetting. The reason for this behaviour is

ASTD Per cent increase in ASTD over control

IER % DR % Total % PS % l ER Total

8 1 .2 I l . l 92.3 7.7 3 1 .6 1 4.0

79.0 1 1 .8 90.8 9.2 59.9 25. 1

77.4 1 1 .2 88.6 1 1 .4 73 .5 33.0

75.4 1 0.7 86. 1 1 3.9 78.7 30. 1

73.0 1 2.0 85.0 1 5 .0 78.0 40.3

the absence of built-in strains in the treated fibres. The built-in strains that act as weak l inks in raw fibre are strengthened by water, which act l ike a plasticizer such that the molecules are able to undergo slippage and this permits better load sharing in a tensile test. Since there is no scope for further increase, the tenacity of treated fibres remains unaffected or shows l ittle decrease which is statistically insignificant.

The extension of fibres comprises two components. The first and the major component comes from the fibre morphology and the second from the fine structure. From the morphological front, the convolution plays a major role in the extension of the fibre. Table 1 2 shows that the number of convolutions and the extension-at-break are highly related (r=0.74). Hearle and Sparrow l9 observed that the considerable variation in the number of convolutions is the main

1 44 INDIAN J. FIBRE TEXT. RES., JUNE 2003

Table 1 2 - Effect of treatment on convolution frequency and single fibre extensibi l i ty

Variety Convolution frequency per cm Single fibre extensibi l i ty, %

Raw Treated % decrease Raw Treated % decrease over raw over raw

CJ.73 49.2 26.0 47.2

AKA.5 49.4 24.0 5 1 .4

V.797 49.5 23.6 52.3

G Cot. l l 42.4 22.6 46.6

Suvin 53.5 28.0 47.7

DCH.32 67.0 30 0 55.2

P;ma 64.9 26. 1 59.8

GA 60.0 33.4 44.3

LRA.5 1 66 72.0 33.6 53.5

H.6 60.0 24.4 59.3

reason for the wide spectrum of elongation in cotton fibre and that if the convolutions are eliminated, the spectrum of elongation will narrow down quite considerably. The second component is the fibre fine structure, i .e. its rigidity or flexibil ity and the way the micro-units are inclined with respect to the fibre axis. Higher flexibility and higher inclination of fibrils result in higher extension. Both the components can act simultaneously as well as independently and ultimately decide the breaking extension . In the case of treated fibres, wetting brings down the number of convolutions and the stretching further de-convolutes the fibre. Hence, after the treatment there are very few convolutions left behind. In addition, after the treatment the fibrils become highly oriented and the structure becomes less flexible, leaving less scope for extension of internal elements. . Hence, the contributions from both the components are considerably reduced after the treatment. The elongation registered by the treated fibre is due to de­convolution of a few residual convolutions plus very little alignment of fibrils towards the fibre axis. Hence, the total extension of fibre after the treatment is considerably decreased.

Literature reveals that on wetting, the extensibility of fibre increases. But in the present investigation, the data (Table 4) show that the extension gets decreased after wetting and this decrease is statistically significant. The reason for this is the methodology adopted in measuring the extension. If the fibre is wetted in such a way that it removes most of the convolutions, if not all, present in the fibre, the extension will decrease due to the simple reason that the convolution, which is the major component of

5.97 4.69 2 1 .4

6.47 4.22 34.8

6.52 4.04 38.0

5.53 4.59 1 7 .0

8.3 1 5 .34 35.7

1 0.50 6.82 35.0

1 1 .56 5.94 48.6

4.6 1 3.93 14 .8

1 0.22 4.82 52.8

7 .62 5.40 29. 1

fibre extension, is absent. In the earlier investigations, the extension was measured in such a way that the length of fibre resulting from de-convolution is also taken as extension and hence, the total extension is higher than the dry state extension. Due to the experimental procedure adopted in the present study, the extension was measured on the fibre from which most of the convolutions were removed and the length due to de-convolution did not figure in the extension calculation. Hence, the total extension of wetted raw fibre is significantly less than that of raw fibre.

It is evident from Table 5 that on wetting there is a slight drop in the extension of treated fibre but it is not significant. The treated fibres contain very few convolutions which contribute to the extension in dry state. But on wetting and removing the residual convolutions, the fibre extension decreases because of the absence of the component from de-convolution. Since the number of convolutions are very few indeed, the difference (reduction) in dry and wet state extensions is statistically non-significant.

Lustre is generally understood to reflect the surface characteristics of fibre. If that is true, more uniform fibre with less surface irregularities and less number of convolutions will give strong specular reflection of light fall ing on it and the fibre will be more lustrous. More circular the fibre is, the more lustrous would be its appearance20. In the said treatment, there is . significant improvement in circularity . This improvement in circularity enhances the lustre of the fibre. Actually, there exists a good correlation between per cent increase in circularity and per cent increase in reflectance (r = 0.72 for specular included and 0.74 for specular excluded) as well as between

HUSSAI N & IYER: EFFECT OF SWELLING & STRETCHING IN WATER ON COTION 1 45

per cent increase in circularity and per cent increase in Hunter's whiteness index (r = 0.72 for specular included and 0.67 for specular excludl�d).

From the above correlations, i t cannot be concluded that the circularity alone decides the fibre lustre. Normal rayon has serrated cross-section but is lustrous. Lustre depends primarily on the · optical homogeneIty of the entire fibre volume, from the surface to the lumen. In raw fibre, the microvoids and pores having dimensions close to the wavelength of light must be present. Incoherent scattering must be taking place not only from the surface but also from the inner layers of cellulose when the incident l ight penetrates into the fibre volume. When cotton is swollen in an inter-crystall ine (water) or intra­crystal line (NaOH, EDA and ZnCb) swelling agents in slack condition, no improvement is observed in lustre after the treatment. It is logical to expect that these treatments leave the inherent optical in­homogeneity of fibre practically unchanged. Stretch applied during the swel ling treatments brings about transformation of fibre structure. Fibri ls are better oriented and stabil ized, and microvoids between fibrils and lamella might get eliminated. Even the lumen almost disappears. The enhanced homogeneity must be responsible for the reduced scattering and improved lustre after such treatments.

It has been reportedlO- 13 that the aqueous swelling accompanied by stretching and fol lowed by drying in taut state enhances the tenacity and decreases the extensibility of cotton fibres (single as well as bundle). These changes in fibre characteristics bring about similar changes in tenacity and extensibil ity of yarn subjected to the said treatment, although the quantum of change is different. It is found that a good correlation (r =0.80) exists between per cent increase in bundle tenacity and per cent increase in tenacity of corresponding yarn (Table 1 3) .

It is worth to note that the increase in fibre bundle tenacity is about three times the increase in yarn tenacity. The reasons for this could be attributed to the following factors : ( i ) fibres which constitute the bundle are stretched to the same extent uniformly during the treatment. However, since the yarn is stretched in the lea form, the stretch is not uniform throughout the entire length of the yarn. There are possibilities wherein some portions of yarn are stretched to a higher extent. This uneven stretching can create artificial weak spots and yarn can break up at these weak points. This lowers the extent of increase in tenacity and brings about more reduction

Table 1 3 - Relationship between change in bundle tenacity and yarn tenacity aftcr treatment

Samplc % increase in tenacity Bundle Yarn

Suvin 76.2 2 1 .3

Hybrid.6 9 1 .5 24. 1

DCH 32 63.0 1 8.4

1 70 C02 86.5 23.6

GA 1 05 .9 26.7

Digvijay 78.3 26.6

SRT 68.8 17 .3

CJ 73 106.7 26.2

AKA.5 10 1 .7 24.8

Deviraj 88.7 25.6

in yarn elongation, (ii) in the case of fibres, they are stretched to the maximum possible level. The fibres broken during this process are combed out any way. But in the case of yarn, stretch is restricted to 80% of the breaking extension of the weakest strand constituting the lea, and ( i i i ) had the stretching been done on single strands instead of lea, there is possibility of stretching the yarn more uniformly and the higher levels of stretch would be expected in which case higher increase in tenacity might have resulted. It is pertinent to add here that our data cannot be compared with those of Doke and Iyerl4

and Subramaniam and Thambidurai l5 si nce we had carried out aqueous swel ling and then stretching and drying in taut state on raw yarns while other workers '4. ' 5 carried out the same treatment on scoured yarns. It is well known that the raw yarn differs from scoured yarn in respect of certain mechanical properties.

The specific work of rupture of single fibres is reported 1 3 to increase by - 20% after the said treatment whereas the specific work of rupture of corresponding yams show - 40% reduction after the treatment. After the treatment, the extent of increase in tenacity of yam is extremely lower compared to that of fibre and the yarn elongation suffers enormous drop compared to fibre elongation. This accounts for severe drop in the specific work of rupture of yarns.

As far as tenacity is concerned, the wetting of treated yarn shows similar trend to that of treated fibres on wetting. Treated fibres show no change in tenacity on wetting. However, on wetting the fibres exhibit a sl ight drop in extensibility but the drop is not significant. In the case of treated yarn, there is increase in extensibility after wetting unl ike in the

146 I NDIAN J. FIBRE TEXT. RES .. JUNE 2003

case of fibres. This differential behaviour between the treated yarn and the treated fibre is simply due to the fact that the component of the extension coming from de-convolution is absent in the case of treated fibre whereas the extension due to de-convolution gets added to the extensibility of treated yarn. Since recovery values depend on moduli values, the enhancement in moduli values leads to higher recovery values after treatment.

5 Conclusions

Aqueous swel ling fol lowed by stretch and drying in taut condition brings about fol lowing changes in the tensi le characteri stics of cotton fibres and yarns: (i) fibres become more uniform along their length in respect of tensile characteristics, ( i i ) the rate of drop in bundle tenacity of treated fibres with specimen (gauge) l ength is less than that of raw fibres, (ii i) single fibre tenacity, initial modulus , secant modulus and work of rupture get enhanced while the breaking strain gets reduced, (iv) on wetting, the treated fibres do not exhibit any change unlike the raw fibres which register increase in tenacity after wetting, (v) fibres become lustrous subsequent to treatment, (vi) the said treatment enhances the tenacity, moduli (static, dynamic and secant) and total recovery values, (vii) yarns become more elastic after the treatment, and (vii i) wetting does not affect the tenacity and recovery characteristics of treated yarns .

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