Characterizing fabrics from generated yarns

Tensile drawing of rotor spun yarn: Part II: Characterizing fabrics from generated yarns

Hu, Jinlian

ABSTRACT

Rotor yarns subjected to a tensile drawing process, as reported in Part I, not only differ from their original form in terms of yarn structure, but also in bulk, mechanical, and surface properties. The fabrics produced from drawn rotor spun yarns are no doubt affected by these changes. This paper presents a report on changes in fabric characteristics. Experimental observation reveals an appreciable improvement in most fabric mechanical properties. What is more important, dimensional stability and pilling performance are not seriously influenced. Both subjective evaluations and rational experimental results particularly indicate that comfort significantly improves as well. Other features of modified yam fabrics include a whiter appearance, a slight increase in fabric weight-perunit-area, and a rise in fabric thickness.

In Part I of this series [4], we reported the results of using tensile drawing to modify rotor spun yarn. We focused on the changes in yarn properties as well as the influence of processing conditions on the drawing effect. As presented in Part I, the treated rotor yarns not only differed from their original form in terms of yarn structure, but also in bulk, mechanical, and surface properties. In more detail, after tensile drawing, yarn diameter decreased noticeably, yarn evenness improved significantly, and yam packing density was markedly augmented. In particular, the yarns’ resistance to different exterior loads, ie., Young’s modulus and bending modulus, improved appreciably. Moreover, yarn tenacity was also potentially enhanced. These changes will no doubt be introduced into downstream fabrics in subsequent production. In an effort to supply additional information about this aspect, we have undertaken further study of fabric characteristics, the focus of our discussion in this paper. We present a comparison of the characteristics fabrics produced from original and drawn yarns and analyze them with the help of changes in yarn characteristics.

Experimental

All the yarn samples (cotton only, with variables listed in Table I of Part I), whether before or after drawing, were made into 1 X 1 rib knitted fabrics on a circular knitting machine, adopting the same fabric variables with common processing parameters. Gray fabric samples were then scoured to release stress. The Kawabata Evaluation System was the main testing apparatus used here due to its unique ability to examine fabric physical and surface properties as well as automatically integrate these properties into data to express the total hand value of the fabrics. All sixteen parameters describing fabric mechanical properties were determined according to the prescribed procedure and required conditions. We also examined fabric skewness, dimensional stability, whose marking scheme is shown in Figure 1, abrasion/pilling performance, air permeability, and draping (Figure 2).

The scouring details were as follows: scouring was done in a Winch dyeing machine in Sandopan DTC (1.5 g/l) anionic detergent at a 40:1 liquor ratio at 80 deg C for 30 minutes, then extracted in a hydro extractor and dried in an oven at 80 deg C.

Results and Analysis

Although all fabric samples were tested, the results reported here for each facet will be restricted to a few samples. We will emphasize those samples most representative of a certain fabric property. Moreover, only the results of finished fabric samples are presented here, since fabric properties in the gray state are of only minor interest.

STRUCTURAL CHARACTERISTICS

Thickness

The thickness of the fabrics produced from the treated yams increased remarkably. This phenomenon seems to be a discrepancy from the notable decrease in yarn diameter. In fact, this increased thickness reflects the significant development of yarn bending rigidity, since yarn curvature in a fabric is bending-rigidity dependent. Accordingly, the resultant higher bending rigidity will no doubt bring about a stronger three-dimensional structure of yarn loops. Moreover, the drawn yarn’s more compact structure also provides some explanation, since this change makes these yarns more difficult to squash.

Dimensional Stability

The effect of tensile drawing on fabric shrinkage deserves special attention, since dimensional stability is particularly important for apparel products. Table I presents the fabric dimensions before and after scouring. In order to determine the significance of shrinkage property differences between fabrics produced from original yarns and those from drawn yarns, we conducted a t-test at a 5% level of probability based on the null hypothesis that differences between pairs of values are not significantly different. We found no statistically significant difference between the shrinkage of fabrics produced from drawn rotor spun yarns and those from original yarns. This is quite encouraging, because what researchers, and also potential users, worry most is whether tensile drawing will result in deteriorated shrinkage. Apparently, the plastic strain accumulated during the process of tensile drawing contributes most in this aspect. In addition, there was no skewness in the scoured specimens.

MECHANICAL CHARACTERISTICS

The mechanical properties of fabric are very important in determining performance during use. The sixteen parameters describing fabric mechanical properties measured on the Kawabata instruments are shown in Table 11, and a detailed analysis follows.

Tensile Property

EM is a measurement of extensibility: the largerer the value of EM, the more extensible the fabric. From Table II, it is easy to see that fabric produced from tensile treated yarns is more extensible compared with fabrics from untreated yarns. Extensibility is closely related to the ease of crimp removal, which, in turn, is determined by the mobility of the yarns within the fabric. Therefore, the increased extensibility implies that the treated yarns within the fabric have more easily removed crimps. Considering that all samples have the same structure, crimp removal is principally determined by yarn bulk and surface properties. Admittedly, lower bulk results in lower yarn surface-to-surface friction. Therefore, the increased EM values agree well with the reduced yarn packing density. On the other hand, this change also implies that the increase in yarn hairiness is not large enough to cut extensibility. Besides, we believe that the stronger three-dimensional yarn structure stemming from higher bending rigidity also provides yarns with more mobile freedom, thus contributing to fabric extensibility.

Moreover, the linearity of the stress-strain curve LT, the tensile energy per unit area WT, and the tensile resiliency RT are all enhanced in fabrics knitted from treated yarns. Since all these values depend, once again, on the ease of crimp removal, the elasticity of the yarn, and the fabric tightness, no further explanation needs to be given here. Remember that a higher value of the tensile parameters is supposed to be better.

Bending Property

A fabric’s bending is apparently a function of the bending property of its constituent yarns. As mentioned above, the bending rigidity B and bending hysteresis 2HB of treated yarn increased markedly. Therefore, it is not strange that both bending rigidity and bending hysteresis have higher values for the fabrics produced from the treated yarns. This implies that these fabrics are more difficult to bend and recover from bending deformation. In addition, the higher fabric thickness without question also contributes to higher fabric bending rigidity.

Shear Property

Whenever bending occurs in more than one direction, so that the fabric is subject to double curvature, shear deformations of the fabric are involved. Fabric behavior in which shear plays a part includes bias properties, handle and creasing, tear strength, and the general mechanics of fabric deformation [3]. Therefore, shear is very important in terms of a fabric’s wearing performance. As revealed by its definition, this property is highly related to bending, so it is not unusual to find the increased G and 2HG, and the reason is too plain to reiterate.

Compression

Fabric compression, a characteristic integrated by the fabric structure as well as the constituent fibers and/or yarn surface properties and lateral compression [1], is one of the most important factors when assessing a fabric’s mechanical properties. It is also a property highly related to fabric handle, i.e., softness and fullness, and to a fabric’s surface smoothness.

All fabric compression parameters depend on the compression behavior of the yarns and the fabric’s thickness. In this respect, fabrics produced from drawn yarns mostly present greater compression properties, except for the slight decrease in LC. As stated above, drawn yarns are more difficult to compress. Hence, this phenomenon shows us that yarns should not be judged merely by their behavior in the free state, but should be assessed by their behavior in fabric structures. Nevertheless, we also found that the increase in the 18s fabric’s compression properties was much less than that in the 16s fabric. We believe that a combined effect of the different extent of drawing, the generated decrease in yarn diameter, and the different extent of increased yarn rigidity can provide a good explanation.

FABRIC PERFORMANCE

These aforementioned facts reveal that just as predicted, the notable change in yarn structural and mechanical properties gives the downstream fabric an amended appearance and modified properties. However, among all the changed fabric characteristics, the largest success is the important improvement in fabric comfort.

Fullness

Fukurami, a measure of fabric softness and fullness, depicting a bulky, rich, well-formed feeling, has acquired the most positive improvement according to whichever standard [1]. Table III, presenting three selected primary hands and the total hand value, supports this statement well. This fact, as the term describes, reflects pleasant changes in fabric bulk, softness, and fullness. And once again, it is a necessary result of the modified yarn mechanical properties, especially bending.

Handle

Many researchers have reported that rotor spun yams are under very low tension, with shallow migration during formation. It follows that rotor spun yarns possess less internal stress, so the bulk of rotor spun yarns is higher, usually between 5 to 8% greater than for ring spun yarns [5]. In this connection, however, we encounter a strange phenomenon. Normally, a bulky yarn is a fluffy, soft, woolly material, which at equal yarn counts occupies more space than a lean yarn. Although rotor spun yarns do exhibit volumn and bulkiness, they lack this soft hand, which is considered the most significant demerit of rotor spun yarn [2].

Fortunately, this disadvantage has been dramatically amended by our research. Except for the objective experimental results in Table III, subjective evaluations suggest that fabrics produced from drawn rotor spun yarns, compared with those from original rotor spun yarns, show prominently improved hand, as shown in Table IV. This fact can be regarded as a major breakthrough in our research. We believe that this pleasing change comes from the combined effects of better evenness in yarn diameter, reduced/better distributed yarn twist, improved fiber alignment, and changes in yarn/ fabric physical/construction properties. Furthermore, the increased yarn hairiness also contributes a great deal.

Air Permeability

Table V lists the results of air permeability for all five fabric samples produced from 12s yarn. Fabrics produced from drawn yarns have a slightly higher air permeability than those from original yarns. This result seems to dispute the fact that drawn yarns possess higher hairiness, and can be attributed to the higher crimp levels and the resulting higher yarn bending/torsional/tensile modulus after drawing. It is also due to the fact that a drawn yarn opens up less than an untreated yarn, as revealed by a cross-sectional examination.

Pilling Performance

Pilling is very important in textile products, for it not only detracts from the appearance and hand, but also affects the service life of the product. Therefore, in this research we also investigated the propensity of our fabrics to pill, even though this is normally not a problem for cotton fabrics. We attempted to determine the sensitivity of this parameter to tensile drawing and to determine whether drawn yarns suffer defects in this respect. Table VI presents the fabric weight changes after a certain number of rubs on a fabric abrasion tester. In addition, we also used scanning electron microscopy to study the various stages of pilling development.

According to the averaged results for all evaluation periods, in any fabric constructed of different yarn counts ranging from 12s to 20s, the initial pilling development in drawn yarn fabric is slightly higher than that of the original yarn fabric. This might be regarded as evidence that tensile drawing has a slight detrimental effect on end-use fabrics. Apparently, it is the hairier surface of the drawn yarn that leads to this defect, because more fiber ends are available to be raised and hence favor fiber fuzz and roll-up. Fortunately, in our experiments, the drawn yarn fabric performed nearly equally as well, if not better, as the original yarn fabric, which may be due to the more densely packed yarn structure.

Draping Profile

Fabric draping is closely related to the bending rigidity of the constituent yarns and of the fabric itself, as well as the fabric’s thickness. In this research, we have found that this characteristic decreases significantly after tensile drawing (see Figure 3). This may be caused by increased yarn bending rigidity. Furthermore, the growth in thickness would definitely help to reduce this property.

The decreased draping characteristic is of great interest, since a soft hand is always a concomitant of higher draping characteristics, whereas our tensile drawing process makes these two extremely incompatible properties coexist within one fabric. This would no doubt lead to new thinking by fashion designers. Especially in the case of knitted fabrics, egregious drape limits their application mainly to underwear and skirting.

Appearance

It is well known that rotor spun yarn fabrics generally have a duller and more matte appearance in comparison with ring spun yarn fabrics, even when bright fibers are used. This could be associated with a combination of the peculiar nature of the rotor spun yarn surface and the resulting turbid light refraction. Besides, rotor fabrics are darker [3]. All these disadvantages are amended to some extent by tensile drawing, due to improved fiber alignment and fiber structural evenness.

Conclusions

Rotor spun yarns, despite their growing popularity, suffer from the disadvantages of lower tenacity, limited count range, and harsh hand in their end fabrics. The objective of this series is to probe the possibility of improving rotor spun yarns and fabrics by tensile drawing.

In Part I of this series, we reported that rotor yarns subjected to tensile drawing not only differ from their original form in terms of yarn structure but also in bulk, mechanical and surface properties. These changes, when the yams are made into fabrics, will surely be introduced into downstream articles. In this part of the series, we present a detailed depiction of characteristic changes generated in the fabric as well as the reasons for these changes, demonstrated by a whiter appearance with better luster, improved performance, and a fuller, thicker, smoother hand. These improvements no doubt indicate the dramatic potential of commercializing this tensile drawing process for rotor spun yarns.

Literature Cited

1. Behera, B. K., Ishtiaque, S. M., and Chand, S., Comfort Properties of Fabrics Woven from Ring-, Rotor-, and Friction-spun Yarns, J. Textile Inst. 88, 255-264, (1997).

2. Dyson, Eric, “Rotor Spinning: Technical and Economics Aspects,” Textile Trade Press, Stockport, U.K., 1975.

3. Hearle, J. W. S., et al., “Structural Mechanics of Fibers, Yams, and Fabrics,” Wiley-Interscience, NY, 1969.

4. Hu, J., and Jiang, X., Tensile Drawing of Rotor Spun Yarn, Part I: Consideration of Processing Variables and Property Changes in Generated Yams, Textile Res. J. 72, 741-748 (2002).

5. Mohamed, M. H., and Lord, P. R., Comparison of Physical Properties of Fabrics Woven from Open-End and Ring Spun Yarns, Textile Res. J. 43, 154-166 (1973).

Manuscript received February 27, 2001; accepted September 11, 2001.

JINLAN HU AND XIUYING JIANG

Institute of Textile and Clothing, The Hong Kong Polytechnic University, Hung Horn, Kowloon, Hong Kong

RON POSTLE

Department of Textile Technology, University of New South Wales, Sydney 2052, Australia

Copyright Textile Research Institute Sep 2002

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