Effect of Lowering Twist Levels on Quality Parameters of Rotor Spun Cotton Yarn

Investigations were made to explore the influence of lowering twist level on quality characteristics of rotor spun yarn. Three levels of yarn linear density (i.e. 40, 35 and 30 tex) and five levels of twist (i.e. 700, 600, 550, 500, and 450) were employed during yarn spinning trials. Each twist multiple was investigated at all linear densities for tensile strength, elongation, total CVm (Coefficient of Mass Variation) imperfection index and hairiness. 100% cotton yarn samples were prepared on Reiter R-40 at rotor speed of 90,000 rpm. Determination of elongation, yarn strength, hairiness, mass variation, and total imperfections index was carried out on Uster Tensorapid-4 and Uster Tester-4 as per set standards of ISO standard test methods. Based on investigations it is established that yarn strength and elongation declined minutely (Insignificant) with lowering twist levels but still can be confidently used for knitting yarns. However, significant improvement in total imperfection index and marginal enhancement in CVm were experienced.


INTRODUCTION
I n order to maintain the spinning process; coherence mechanism in rotor spinning is achieved by twist insertion, which as a rule must be higher than the yarn twist required for yarn tenacity. Consequently higher twist levels are used for rotor yarns in comparison to ring spinning to attain the strength equivalent to ring yarn [1][2][3].
Mechanical properties of yarns are a function of their physical structure and morphology [4]. Owing to the yarn surface. Therefore, it is not the overall length of the fiber that contributes to the yarn strength, but only the spun-in part of it as a result optimum yarn tenacity obtained at higher twist multiple than normal [4,9].
Among other factors, residual torque or twist liveliness of a twisted yarn is the most prominent and fundamental factor contributing to the spirality of single jersey fabrics.
If a twist-lively yarn is used for knitting, the resultant loop will no longer be symmetric because of the varying induced torsional strain in the yarn [10]. Moreover, high twist multiple makes the rotor yarn harsher to hand feel.
In order to btain a fabric without spirality, balanced twist factors for both ply yarns and single yarns are essential [11].
A high twist multiple is responsible for producing many unfavorable aspects of the yarn such as poor handling to the fabric, reduced production, complications to snarling, poor cover factor, etc. [12][13][14]. These problems can be addressed by reducing the twist multiple up to certain limits. Lowering of twist levels will improve hand feel, count variation, evenness (mass variation), as well as imperfections [15][16][17]. Though, strength of yarn will decrease further by reducing twist level. However, the low strength yarn can be confidently used for knitting as yarn strength is not a critical factor in knitting as compared to weaving. This lowering of twist multiple will not only help to trim down the unit price of finished product and relief the manufacturer to compete in international market, but also facilitates the knitter by enhancing the smooth running of yarn on knitting machines, as kinkiness of yarn minimizes with low twists [13,[18][19].
Keeping in view the benefits of low twist multiples this study was undertaken to explore the effects of lowering twist level on quality parameters of rotor spun knitting yarn.
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RESULTS AND DISCUSSION
To

Effect of Lowering Twist Levels on Strength and Elongation
Influence of lowering twist levels on tensile strength and elongation are presented in Figs. 1-2. It is evident from Fig. 1 that tensile strength for all linear densities decrease with lowering twist level from 700-450 tpm. Similar trend is observed with respect to elongation. However, coarser counts have higher yarn strength and elongation than finer counts. Reason behind this effect can be discussed as; the function of twist is to hold the constituent fibers in yarn assembly thus providing coherence to yarn [5].
With decreasing twist levels binding effect of yarn or source of coherence reduces. Furthermore with decreasing twist levels delivery of yarn increases which also affects feeding rate. This increase in feeding and delivery rate reduces the interval of fibers' stay in rotor groove [6], which results in reduction of number of core fibers than number of fibers in intermediate zone.
In addition with reducing twist, helix angle of core fibers also decreases due to which lateral force holding the fibers together gets weakened and fibers in yarn are held less tightly. This increases slippage of fibers under tensile loading and the number of fibers contributing to overall strength of the yarn gets reduced. Consequently yarn could not withstand against higher load as well as could not be elongated as much as at higher twist. It is also evident by Fig. 1 that this effect is more pronounced with finer yarn counts.

Effect of Lowering Twist Levels on Quality Parameters of Rotor Spun Cotton Yarn
increases which covers up several imperfections, as their sizes becomes diminutive with respect to yarn diameter.
It isevident from Fig. 5 that hairiness of yarn at all linear densities increases with lowering twist multiples. As a matter of fact hairiness decreaseswith increasing compactness of yarn but lowering twist levels makes yarn soft which results in additional protruding ends from yarn surface [6]. Further, this slight increase in hairiness can also be attributed to wobbly wrapper fibers at low twist [17]. The high rotor speed of 90,000 rpm would alsoplay a negative role of increase in hairiness.

CONCLUSION
Investigations made to explore the effect of lowering twist levels on quality parameters of rotor yarn reveals that at all linear densities, with lowering twist level, tensile strength and elongation declined to some extent.
However, this low strength yarn can be confidently used for knitting as yarn strength is not a critical factor in knitting. On the other hand, fabrication of soft yarn improved evenness and total imperfection index of yarn which will improve hand feel effect and esthetic properties of yarn. An insignificant negative influence on hairiness of yarn was observed at low twist levels.