Indian 10urnal of Fibre & Textile Research Vol. 27, March 2002, pp. 48-5 1

Tensile properties of polyester/cotton blended yams

V K Kothari", S M Ishtiaque & V G Ogale

Department of Textile Technology, Indian Institute of Technology, New Delhi 1 10 0 1 6, India

Received 4 January 2oo};accepted 2 February 2oo}

Effect of blend ratio, strain rate and cone winding from ring bobbins on the tensile properties of polyester/cotton blended yarns under static and dynamic conditions has been studied using USTER Tensorapid and Constant Tension Transport testers. It is observed that the static tensile test overestimates tensile strength and breaking elongation. The static tensile strength and breaking elongation decrease after winding but the dynamic elongation of cotton, polyester and 67:33 polyester/cotton yarn increases after winding.

Keywords: Blended yarns, Constant tension transport tester, Cone winding, Dynamic tensile test, Static tensile test

1 Introduction An important aspect of testing the yarns is the

possibility of predicting the performance of yarn in subsequent processing. Among the various yarn properties, the tensile properties of yams significantly affect the behaviour of yarns during their conversion into fabrics and the properties of the final textile structure. A number of methods have been developed over the years for evaluating tensile properties of yarns and these are used for a variety of reasons.

Lea strength, although useful for routine process control, has poor correlation with yarn performance in subsequent processes and the test is not sensitive to small changes in yarn properitesl . Single thread tensile testing of yarns is based on the small sample size and only statistical inferences about the yarn tensile properties can be drawn based on the limited number of samples normally tested. The single thread tensile test method gives the value of tensile strength which is sometimes referred to as static yarn strength. The static yarn strength cannot accurately predict the running behaviour of yarn on the subsequent machines.

One of the important purposes of single thread testing is to assess how the yarn will perform in subsequent processes and the interest is in the weakest sections of yarn. A weak place in one process may not be a weak place in another process as the type and the speed of operation of the fabric-forming machine as well as fabric structure affect the stress applied on the

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yarn2• Continuous tensile testing of yarn involves transporting the yarn under constant tension at constant output speed. Thus, in continuous testing every inch or millimeter of yarn is tested to generate true elongation of yarn at specific dynamic tensionl; speed condition and tensile characteristics are continuously assessed3.4. The continuous testing simulates actual manufacturing conditions more closely than static tensile testing. These tests can be selected to hunt the weak spots in the yarns at different tension levels and to accurately measure elongation at these tension levels to provide tension­elongation or the stress-strain behaviour of yarns. The effect of extension rate on tensile behaviour has also been reported5• In the present paper, the study on dynamic and static tensile properties of polyester/cotton blended yams before and after winding has been reported.

2 Materials and Methods

30s Ne ( 19.68 tex) ring-spun yam samples were prepared from polyester, Shankar 4 cotton and their blends. The blending was carried out at draw frame stage to get different blends composition. The samples were made at a mill which has the regular production of blended yarns. Yam samples on 1 5 ring bobbins and 5 baby cones prepared frortl. another 1 5 ring bobbins were evaluated for related yarn properties.

2.1 Fibre Properties

Polyester and cotton fibres were tested for fibre denier, tensile strength and breaking elongation using

KOTHARI et al. : TENSILE PROPERTIES OF POLYESTER/COTTON BLENDED YARNS 49

Lenzing Vibroscope and Tensile Tester. Twenty-five tests on each fibre were carried out. The tensile properties were measured at the following conditions: gauge length, 10 mm; test speed, 10 mm1min; and pretension, 100 mgf. Cotton fibre length, uniformity ratio and micronaire value were determined on High Volume Instrument (HVI). Polyester fibre length was measured using 25 fibres on a scale. The properties of cotton and polyester fibres used for the study are shown in Table 1 .

2.2 Yarn Tensile ,Properties Static tensile properties of blended yams were

evaluated using USTER Tensorapid which works on Constant Rate of Extension (CRE) principle and gives force-elongation characteristics, their variation and time required to break the specimen. For dynamic test, Constant Transport Tension (CTT) tester was used which transports yam under constant tension through the test zone at a constant output speed. A controller is used to keep the tension constant by changing the input speed of the yam and it has an ability to maintain constant tension between 0.5 gf and 700 gf at any selected speed between 20 m1min and 360 m1min by means of a servo system. The maximum tension which allows transport of a certain length of the yam without a break gives the dynamic tensile strength and the difference between input and output yam speeds at this stage gives the breaking elongation under dynamic test conditions. Tensile test parameters used during both static and dynamic testings are given in Table 2 .

3 Results and Discussion

Fig. 1 shows the effect of blend proportion on the tenacity of polyester/cotton blended yams using dynamic tensile tester, static tensile tester at a fixed strain rate of 5000 mm1min and static tensile tester with constant time-to-break of 20±3s. It is observed that the dynamic strength of all the yams is lower than the static strength by almost 34-60%. The difference in yarn strength under static condition with fixed strain rate and constant time-to-break is not significant. It is also observed that the percentage difference between static and dynamic strength increases as the polyester fibre percentage increases. The blending of higher strength and higher elongation polyester component with cotton initially leads to slight decrease in yam strength but the addition of polyester beyond 33% in blended yams leads to significant increase in yam strength. Fig. 2 shows that

Property

Table I - Fibre properties

Cotton

2.5% span length, mm 50% span length, mm Cut length, mm Unifortnity ratio, % Micronaire value Fineness, denier Breaking load of single fibre, gf C. V of breaking load, % Breaking strain, %

27.79 1 4.22

48.00 4.06 1 .36 2. 1 5 1 2.5 8 . 1

Table 2 - Tensile testing parameters

Pol)4ester

38

1 .42 6.50 9.5

24.3

Parameter Measuring principle , Static Static Dynamic

Test speed, mlmin 5 Variable 1 00 Time-to-break, s 20±3 Gauge length, mm 500 500 Pretension, cN/tex 0.5 0.5 0.5

3 S,�----------------------------'

5

o 10 '20 30 40 SO 60 70 80 90 1 00 POLYESTER (".)

Fig. I - Effect of blend ratio on tensile strength ( I -dynamic test, 2-static test at 5000 mmlmin strain rate and 3-static test at 20±3 s time-to-break)

14 r--------------------------, 12

� 10 Z o 8 � (!) 6 Z o irl 4 "'-==�'"

2

o ����--�--��--��--� o 20 40 60 80

POLYESTER (%) 100

Fig. 2 - Effect of blend ratio on breaking elongation ( I -<iynamic test, 2-static test at 5000 mmlmin strain rate and 3-static test at 20±3 s time-to-break)

50 INDIAN J. FIBRE TEXT. RES., MARCH 2002

1 6 Cotlon

1 4

1 2

1 0 .64 , .

8

6

4

2

2 3 4 5 6

1 8

1 6 Pol yester : Collon

( SO : 50) 1 4

x � 1 2

� 10 V)

8 V) w ex: 6 l-V) 4

2

0

0 2 4 6 8 1 0

3 5 Pol yes l e r

30

2 5

2 0 .. 1 5

10 5

0

0 2 4 6 8 1 0 1 2 STRAIN(" , )

the elongation of yarn at break is also lower under dynamic test conditions as compared to those under both the static test conditions and ,the elongation-at­break increases as the polyester content in the blended yarn increases.

The average stress-strain curves of yarns with different blend proportions were obtained before and after winding operations using Tensorapid at a fixed strain rate of 5000 mmlmin and CTT dynamic tensile tester. Fig. 3 shows comparison of stress-strain curves of yarns before and after winding in both static and dynamic conditions for 0: 1 00, 33 :67, 50:50, 67 :33

and 1 00:0 polyester/cotton blended yarns. It is observed that the static tensile strength of ring-spun

7

1 2

1 4

1 2 polyes t u : Col t o n ( 33 : 6 7 )

1 0 0

6

4

2

2 3 4 5 6

2 5 P o lyes ter: Collon

( &7 : 33) l U

1 5

1 0

5

2 4 6 8 1 0 1 2 1 4 STRAIN ( " , )

Fig. 3 - Static and dynamic stress-strain diagrams of cotton, polyester and their blends ( I-static test before wind-ing, 2--dynamic test before winding, 3-static test after winding and 4-dynamic test after winding)

yarn decreases after winding for all the yarns as shown by comparing the curves 1 and 3 in Fig. 3 . But the dynamic strength increases after winding for cotton and 3 3 : 67 & 67 : 3 3 polyester/cotton blends as shown by comparing the curves 2 and 4 in Fig. 3 , possibly due to the reduction in weak places after winding. It is also observed that the elongation of yarn after winding increases in case of dynamic test whereas it decreases generally in case of static test.

4 Conclusions

Static tensile testing overestimates tensile strength and breaking elongation of cotton, polyester and cotton/polyester blended yarns. However, the

KOTHARI et al. : TENSILE PROPERTIES OF POLYESTER/COTION BLENDED YARNS 5 1

dynamic testing of these yarns indicates 34-60% decrease in tensile strength than that obtained on static tensile tester. Static tensile strength and breaking elongation decrease after winding but reverse trend is observed in case of dynamic testing, possibly due to the reduction in weak places after winding.

References 1 Parker A, Text Month, March ( 1993) 25.

2 Furter R, Strength and elongation testing of single and ply yams, Manual of Textile Technology (The Textile Institute, UK), 1 985.

3 Nevel A, New concept in dynamic testing of cotton yams, paper presented at the Yam Group Conference, Bradford, 22-24 Feb­ruary 1 994.

4 Lee E K & Oxenham W, Techniques to assess yam tensile properties, paper presented at the 1 998 Beltwide Cotton Con­ference, San Diego CA, USA, 5-9 January 1998.

5 Punj S K; Mukhopadhyay A & Chakraborty A, Effect of exten­sion rate and gauge length on the tensile behaviour of ring and air-jet spun yams, Indian J Fibre Tex Res, 23( 1 ) ( 1 998) 1 9-24.