Optimum Drafting Conditions of Non-circular Polyester and Cotton Blend Yarns

Abstract

In this study, experiments were conducted to examine the drafting behavior of non-circular fiber (cross-shaped profiled polyester) to obtain the optimum drafting conditions for spinning polyester/cotton (P/C) blended yarn. The drafting force in the break draft zone was measured and the effects of roller gage, draft ratio and P/C blend ratio on the drafting force were examined. The 20Tex spun yarn was spun with different P/C blend ratios and its quality was evaluated to shed light on the relationship between drafting force and yarn evenness.

Experimental results showed that the optimum drafting conditions involved using a roller gage of 62 mm and spinning at a draft ratio of 1.20. In addition, blend yam of P80/C20 ratio had the best yarn quality. In today's world, clothes should not only be a good fit, beautiful and fashionable, but also serve other functions, such as absorption and release of moisture. Previous research on clothing materials focused mainly on improving the finishing process and the development of profiled fibers. While repeated washing will diminish the effect of the finishing process on common fibers, non-circular fibers possess a permanent function of moisture absorption and release, making it a widely popular clothing material.

Although 100% cross-shaped polyester fiber had superior characteristics of good diffusion and fast drying, its performance in water absorption was the worst. However, such a defect can be greatly improved by adding an appropriate amount of cotton fibers. Furthermore, the addition of cotton to non-circular polyester fiber can also soften the blended yarn and improve its wicking property. Yarn quality depends heavily on both the fineness and length of the fibers. That is, the finer the fiber used, the better the yarn quality will be. Fibers of longer length and higher uniformity ratio can also improve and enhance the spinning limit. In the same way, the amount of cotton it contains influences the quality of the P/C blended yarn. In order to produce at low cost the best quality P/C blended yarn with ideal function of moisture absorption and release, the optimum blending ratio of non-circular polyester and cotton merits further investigation.

The drafting force during roller drafting and the drafting behavior have been widely studied [1-6]. In addition, earlier experimental work [7-9] had also investigated the correlation between drafting force and yarn quality to determine the optimum spinning-drafting conditions for improving yarn quality. The force required to draft a roving depends upon the frictional properties of its constituent fibers that may be influenced by factors such as fineness, cross-section, and spin finishing. The magnitude of the inter-fiber frictional force will be proportional to the overall geometric area of contact. For the polyester fiber used in this work, its cross-shaped surface is rougher than the circular and smooth surface of regular polyester fibers, and thus the non-circular polyester fiber has smaller inter-fiber frictional force.

In this study, experiments were conducted to examine the drafting behavior of non-circular fiber (cross-shaped profiled polyester) to obtain the optimum drafting conditions of spinning the P/C blended yarn. Cotton was blended with cross-shaped profiled polyester to investigate the influence of back roller gage, break draft ratio, and blend ratio on drafting force. The quality of 20Tex yarns spun on a laboratory spinning frame was evaluated to elucidate the relationship between drafting force and yarn properties. Furthermore, we assessed the yarn evenness to determine the optimum drafting ratio for achieving the best yarn quality. Our results would serve as useful references for practical spinning.

Theory

Drafting Behavior and Drafting Wave

Martindale and others have shown [3-6] that drafting behavior could be divided into three zones, namely the initial draft zone (I), peak zone (III), and fully draft zone (II), as shown in Figure 1. In the (a) region of the initial draft zone (I), drafting straightens fiber crimps and hooks, and no slipping occurs. In the (b) region of zone (I), fibers slip partially, but the static friction has not yet been fully overcome. Beyond the peak zone (III) is an unsteady region where the curve declines quickly because of fiber slippage. In the fully draft zone (II), fibers slip, and the drafting force is caused by fiber friction.

Drafting fibers in the roller-drafting zone can be classified into three types as shown in Figure 2 [10]. Type A fibers are held by the back rollers and move slowly; type B fibers are held by the front rollers and are withdrawn relatively quickly; and type C fibers are not held by either set of rollers but are supported by other fibers, and are thus called floating fibers. Floating fibers tend to result in a thick place and/or a thin place in the drafted strand. Irregularities of this sort, called drafting wave, occur due to improper setting of drafting conditions in the drafting system.

To obtain high-quality spun yarn, the ring spinning conditions, such as the back roller gage and the break draft, must be changed according to the specifications of the roving used. When the break draft is too low, the twisted roving cannot be sufficiently drafted in the back roller zone, resulting in thick places in the spun yarn. However, when the increase in break draft is too great, the back roller gage will frequently cause a drafting wave in the drafted roving, thus undermining the quality of the spun yarn. As the roller gage increases, the decrease in drafting force is due to a decrease in inter-fiber cohesion. However, roller gages that are too large will cause the floating fibers to be out of control. This will influence the drafting wave, and reduce the yarn quality [11].

Grishin's Formula

In 1945, Grishin developed a theoretical formula for dividing the total draft into sectional drafts for drafting with three pairs of rollers [12].

Generally, the division of the total draft into sectional drafts using Grishin's formula involves regular fibers. However, non-circular polyester and cotton fibers were the raw materials used in this study and therefore, the optimum draft conditions derived theoretically were compared with those obtained in the experiments.

Experiment

Figure 3 shows the non-circular polyester fiber (cross-shaped, 1.56 dtex 38 mm) used in this study. Cotton fiber (1.67 dtex 28 mm) was added to prepare P/C roving through the processes of opening, carding, drawing and roving. The drafting force of P/C roving was measured by the cohesion tester. The experiments were carried out at seven draft ratios, namely 1.04, 1.10, 1.20, 1.30, 1.40, 1.60, and 2.0, and roller gages having dimensions of 54, 58, 62 and 66 mm were used between the two sets of rolls. The yarn spinning experiments aimed to examine the relationship between quality of yarn and drafting force. The 20Tex P/C yarns were spun on a laboratory ring spinning frame under the following conditions: 0.543 g/m roving weight; blended ratios of 80P/20C, 60P/40C, 40P/60C and 20P/80C; break draft ratios of 1.10, 1.20, 1.30, 1.40 and 1.50; roller gages of 45/54, 45/58, 45/62 and 45/66 mm; a ZS#04 traveler; a 40.0 twist factor (in the Tex system); 12 000 rpm spindle speed; and 20Tex yarn count. The quality of the yarn thus spun, including its U%, IPI index, and hairiness, was assessed by Uster Tester IV. The IPI index was defined as follows: thick places (degree of sensitivity: 50%), thin places (degree of sensitivity: -50%), and neps (degree of sensitivity: 200%) per 1000 m yarn length. All the experiments were carried out at 20 2C and 65 5% relative humidity

Results and Discussion

Drafting Force

Figure 4 shows the relationship between drafting force and draft ratio corresponding to the four gages used in conjunction with cotton roving. As can be seen, the drafting force of cotton reached the peak (maximum value) at the draft ratio of 1.40. Thereafter, the fibers began sliding and drifted apart from each other, resulting in reduced drafting force. For the polyester roving the maximum drafting force occurred around the draft ratio of 1.30 and when a roller gage of 62 mm was used, as shown in Figure 5. Beyond the peak, the curve declined. Comparing Figures 4 and 5 reveals a left shifting of the peak attributed to the lower inter-fiber cohesive force of the profiled polyester fibers than that of the cotton fibers. This can readily be explained by the fact that the inter-fiber frictional force of the non-circular polyester fiber is smaller than that of the regular polyester fiber and the cotton fiber. Therefore, the experimental results confirmed that a lower draft ratio is needed to achieve the peak force for non-circular polyester fibers.

In roller drafting, the degree of disconnection between the front and back bearings is proportional to the dimension of the roller gage. The drafting force decreases as the size of gage increases, implying an inverse relationship between the two [1]. For the blended roving of the cross-shaped polyester 80% and cotton 20% (P80/C20), namely, in the case of polyester-rich blended roving, P80/C20 and P60/C40, the drafting force reached the peak when the roller gages used were 54 and 58 mm, respectively, corresponding to the draft ratio of 1.40. On the other hand, when roller gages of 62 and 66 mm were used, respectively; the draft force peaked at the draft ratio of 1.30, as shown in Figure 6. The drafting force curve of P60/C40 was similar to that of P80/C20. However, in the case of cotton-rich blended roving, P40/C60 and P20/C80, the cotton fibers dominated the drafting behavior and the two rovings showed similar drafting behavior as seen in the comparison between Figures 4 and 7.

CV of Drafting Force

In order to further examine the relationship between drafting force and draft ratio, the coefficient of variation (CV%) of drafting force versus the draft ratio is plotted. As shown in Figures 8 and 9, the variation in drafting force decreased with increasing draft ratio. Thus, a stabilized and lower value was found for smaller draft ratios of 1.20-1.30. Thereafter, the variation became more prominent, and the drafting force increased with increasing draft ratio. At the draft ratio of 1.40, the drafting force reached a maximum, while the CV% began to increase. Beyond the peak, the fibers began sliding and drifted apart from each other. In this completely drafted zone, worse fiber control caused the fibers to float, resulting in an increase in drafting force as revealed in the rapidly rising CV curve. As shown in the curve with CV% of drafting force versus draft ratio, the draft ratio at which CV% begins to decrease and stabilize can be chosen as the optimum break draft in the spinning process to obtain better yarn quality [2].

Yarn Quality

It was found that before the peak was reached, the variation in drafting force at smaller draft ratios of 1.20-1.30 was smaller and steadier. Beginning with the draft ratio of 1.40, the drafting force CV increased. The draft ratio of 1.20-1.30 was chosen as the optimum break draft in actual spinning. This is a very important parameter, which correlates more with properties of the spun yarn than with the mean drafting force [2, 7-9]. The 20Tex P/C yarn was spun using break draft ratios of 1.10-1.60 and different blend ratios of the P/C blended roving to assess the influence of these variables on yarn quality.

For blended fiber, a greater variation in fiber length will lead to an increase in the number of floating fibers and greater product irregularity. The experimental materials used were non-circular polyester of 38 mm in cut-length and cotton fiber of 28 mm in variable-length. Consequently, the yarn quality of spun polyester was less uneven than that of cotton and P/C blended yarns, which were spun under optimum spinning conditions.

When evaluating the quality of 20Tex P/C yarn spun using different blended ratios of P/C roving, the authors found that the optimum drafting conditions for achieving the best evenness of yarn were draft ratio of 1.20-1.30. Moreover, the fundamental relationships between yarn qualities and drafting conditions show the same trend as that between drafting force CV and drafting conditions. As seen in Table 1, the unevenness of the spun P/C yarn increased with increasing proportion of cotton. Therefore, too much blending of cotton fiber should be avoided in order to achieve a better quality of blended yarn. Experimental results showed that the P80/C20 blended yarn spun using a roller gage of 62 mm and draft ratio of 1.20 could obtain the best yarn evenness, and its IPI value was also lower than that of other yarns.

Correlation between Variation in Drafting Force and Yarn Unevenness

In practical spinning, 20Tex yarns were spun with six kinds of roving using roller gages of 54-66 mm. Results in Table 1 reveal that the best yarn quality would be obtained with lower draft ratios of about 1.20-1.30. Figure 10 displays the relationship between drafting force CV and unevenness of yarn spun using roller gage of 62 mm and P80/C20 roving. As can be seen, when the break draft reached 1.20, the drafting force CV was lower, and the yarn became more even. When the draft ratio exceeded 1.40, the drafting force CV increased and it began to rise rapidly at the draft ratio of 1.60, so the variation in yarn evenness increased markedly. Obviously, examining the relationship between drafting force CV and yarn unevenness can reveal the optimum drafting conditions for spun spinning to obtain the best yarn quality.

Comparisons of Yarn Properties with the World Level

Table 2 shows the comparison between 20Tex P/C blend yarn and world-level yarn quality. From the statistics of yarn properties provided by Zellweger Uster Ltd., the CV% of 11.3 is between 5 and 25% of the world level. In terms of IPI values, thick-places of 15 per 1000 m is superior to 5% of the world level, thin-places of 2.5 per 1000 m is between 5 and 25% of the world level; and neps of 15 per 1000 m is better than 5% of the world level. The hairiness index of 4.0 is equal to 25% of the world level. In sum, the 20Tex yarn spun in this study is of comparable quality between 5 and 25% of the world level.

Comparison between Break Drafts obtained using Grishin's Formula and Experimental Values

As mentioned above, Grishin's formula can be employed to determine the sectional drafts in a roller drafting system [12]. Take the example of the P80/C20 yarn in this study. The average staple length of the blended fiber l is 34.1 mm and the other drafting conditions are front roller gage setting, L^sub 2^, of 45 mm and back roller gage setting, L^sub 1^, of 54, 58, 62 and 66 mm. Substituting these values into formulas (3) and (4), one can obtain the coefficient of variation in staple fiber length (C^sup 2^^sub f^) of 0.162. With this, the coefficients of "floating" β^sub 1^ and β^sub 2^ can also be calculated. Then, substituting β^sub 1^ and β^sub 2^ into formulas (1) and (2) yields the theoretical values of break drafts (Z^sub 1^) as shown in Table 3.

Comparing the theoretical and experimental values of break drafts reveals a consistent trend. The break draft decreased as the roller gage setting increased. However, it was found that the optimum break draft ratio in practical spinning conjugated with 45/62 mm roller gage setting was 1.20 for the P80/C20 blended yarn, which is lower than 1.303 calculated using Grishin's formula. In the experiment, the authors used two materials for the blended yarn, one is the non-circular polyester fiber of 38 mm in cut-length, and the other is cotton fiber of 28 mm in variable-length. The coefficient of variation in staple fiber length of /C blended yarn (Cf2) increased as the two materials blended, resulting in increase in the coefficients of "floating", β^sub 1^ and β^sub 2^. Thus, the theoretical value also became bigger.

In addition, Grishin's formula was developed for dividing the total draft into sectional drafts for drafting with three pairs of rollers, and the laboratory spinning frame equipped with the SKF apron drafting system which controls well the floating fibers. Therefore, the authors adopt the smaller draft ratio to minimize the certainty caused by the floating fibers in the break draft zone, and the main draft zone takes charge of the main draft behavior by apron draft. Therefore, the results confirmed that a lower suitable break draft ratio is sufficient for the P/C blended yarn in this experiment. However, Grishin's theoretical prediction is also accurate indeed and worth consulting when determining the optimum break draft in actual spinning.

Conclusion

For the cotton-rich P/C blended roving, the cotton fiber dominated the drafting behavior, while the polyester-rich P/C blended roving reveals a left shifting of the peak attributed to the lower interfiber cohesive force of the non-circular polyester fibers than that of the cotton fibers. This can be explained by the fact that the inter-fiber frictional force of the cross-shaped polyester fiber is smaller than that of the regular polyester fiber and the cotton fiber.

The cross-shaped polyester yarn has better quality because of its fiber properties. Less cotton fiber should be blended so as to achieve better P/C yarn quality and improve its wicking property. These results showed that the 20Tex blended yarn of P80/C20 resulted in best yarn quality under the optimum drafting conditions of using a roller gage of 62 mm at the draft ratio of 1.20.

Acknowledgements

This research was supported by the National Science Council (NSC92-2216-E-011-025). The authors are grateful to Professor Tsang-Jou Yang of the Taiwan Cotton Spinners Association for his assistance with this experiment.

Literature Cited

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About the author:

Ching-Iuan Su1 and Jun-Xian Fang
Department of Polymer Engineering, National Taiwan
University of Science and Technology, Taiwan
1 Corresponding author: e-mail: cysu@tx.ntust.edu.tw

Copyright Textile Research Institute Jun 2006
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