Geometrical parameters of yarn cross-section in plain woven fabric

Indian Journal of Fibre & Textile Research Vol. 38, June 2013, pp. 126-131 Geometrical parameters of yarn cross-section in plain woven fabric Siavash Afrashteh 1,a, Ali Akbar Merati 2 & Ali Asghar Asgharian Jeddi 3 1 Department of Textile Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran 2 Advanced Textile Materials and Technology Research Institute, 3 Department of Textile Engineering, Amirkabir University of Technology, 424 Hafez Ave., Tehran, Iran Received 14 November 2011; revised received and accepted 1 May 2012 To study the effect of yarn twist on the geometrical parameters of yarn in a plain woven fabric such as its cross-sectional shape, the samples with various twist levels of warp and weft yarns have been woven and the yarn cross-section parameters are measured using their images obtained by Projectina microscope. The image processing analyses show that the shape of yarn cross-section in the fabric is affected by the twist level. The statistical analysis shows that the increase in yarn twist factor (α tex ) from 0 to 4600 changes the yarn cross-section shape into a lens shape and then elliptical shape and finally circular-like shape. The major diameter of the elliptical shape of the yarn cross-section decreases as the yarn twist increases, while the minor diameter of the elliptical shape of yarn cross- section increases as the yarn twist increases. The results also show that the twist of one group of yarns (warp) affects the cross-sectional shape of other group of yarns (weft). The ellipticity ratio of the yarns (minor diameter divided by major diameter) increases from 0.35 to 0.86 as the yarn twist factor increases from 0 to 4600. Keywords: Ellipticity ratio, Fabric structure, Plain fabric, Twist level, Weaving, Yarn cross-section 1 Introduction The geometrical parameters of yarn inside a fabric play an important role in the fabric geometry and some physical properties of fabric such as handle and appearance. Fabric geometry is a very complex structural parameter that depends on the component materials and the manufacturing process as well as the weave design 1. It has considerable effect on fabric behaviour and its physical and mechanical properties. The yarn geometry in the fabric determines the fabric thickness, comfort, thermal, and draping properties 1. The geometry of the fabric cannot be described by a simple mathematical forms based on basic geometry, but efforts are needed to idealize the fabric geometry to explain it. To represent the configuration of yarn in woven fabrics, many different forms of geometry have been put forward by textile researchers 1-11. In conventional approaches, the general characters of fabrics were idealized into simple geometrical forms. These studies were often based on the assumption of arbitrary geometrical models for the wave crimp and yarn cross-sectional shapes. The researchers treated the micromechanics of fabrics on the basis of the unite cell. The yarn configuration in a Corresponding author. E-mail: afrashteh57@yahoo.com fabric is mainly determined by the form of crimp waves and the cross-sectional shape of yarns in given position. Numerous models have been developed for yarn and fabric structures and their behaviours. In the case of yarn structure in woven fabric, several approaches have been used to describe the shape of the yarn path along the fabric (sine curves, straight lines, elastica forms, etc.) and the yarn cross-sectional shape (circular, elliptical, lens, etc.) 2-11. The configuration of the spun yarns inside the woven fabric cannot be the same throughout the whole fabric. Hence, the cross-sectional geometry of the spun yarns inside the woven fabric cannot be only circular, elliptical or of any other shape. In other words, the assumption of yarns having invariable cross-sectional shape inside the woven fabric needs more attention and consideration 4. Yarn flattening during the weaving process demands extensive attention in understanding the geometry of fabric structure as the cross-sectional dimensions of yarn and the changes in yarn structure significantly affect the thickness, comfort, thermal and draping properties of the final cloth 5. Hofstee and Keulen 6 stated that 3D fabric geometry is frequently defined by sweeping a constant crosssection along the centre line, which implies parallel fibre paths. To this effect, a variable fibre bundle

AFRASHTEH et al.: GEOMETRICAL PARAMETERS OF YARN CROSS-SECTION IN PLAIN WOVEN FABRIC 127 cross-section was introduced in their study. Jiang and Chen 7 also emphasized the gap in the definition of yarn cross-section shape to create more realistic and flexible geometric fabric models. They introduced irregular cross-sections of yarns created with variability in both shape and size along the yarn path. In all various studies 1-11, researchers assume a constant uniform yarn configuration throughout the whole fabric. The common aspect in prior researches is that the theoretical configuration of one unit cell is extended to the whole fabric, giving the image of a woven fabric that is composed of thousands of uniform cells 8. Gong and Ozgen 11 found that an ellipse model of yarn cross-section in plain woven fabric is a suitable approach to generate realistic 3D yarn images. They found that the attention on variable cross-sectional shape of the yarn along the yarn path gives a more realistic representation of yarn appearance in the woven fabric. Alamdar-yazdi and Heppler 11 studied the cross-sectional shapes of the yarn in cotton gray woven fabric in weaving process and in relaxed fabric. They found that the cross-sectional shapes of the yarns at the early stage of weaving are circular, elliptical, or a combination of two circles or an asymmetric elliptical. The deformation of yarn cross-section at the contact point in the fabric is highly dependent upon the yarn twist and the surface contour of the crossing yarn. In this research, the yarn twist range was limited because the fabric samples were made of staple cotton yarn. The geometry of any textile fabric can be represented in a generic way by specifying yarn path and yarn cross-sections independently. Characterisation of fabric geometry is difficult due to the large variability observed in measurement of fabric parameters. However, by given accurate input measurements, an accurate geometric model of the fabric can be created.. A number of assumptions about the path and shape of the cross-sections were made for 2D woven fabrics in general and validated against four different fabrics. TexEng Software Ltd have developed two products named TechText CAD and Weave Engineer 12. TechText CAD is software aimed at transferring academic work on the structural mechanics of textiles into a CAD package that is easy to use and directed at industrial needs. In TexGen, the algorithms were implemented to create 2D woven fabric geometric models using these assumptions with minimal input data. In this research, filament yarn is used to expand the yarn twist range, and the effect of twist of both group of yarns (weft and warp) on their cross-sectional geometry in the woven fabric has been studied. The interactional effect of twist levels of two groups of yarn on the yarn geometry is also investigated. 2 Materials and Methods 2.1 Yarn and Fabric Production The polyester filament yarns of 300 denier/96 mono filament/flat were used to twist at various twist levels and then to weave into a plain fabric of 25 ends/cm and 23 picks/cm. The polyester filaments were dull with a solid circular cross-section. For a given yarn count, the spinning twist is the most important factor influencing yarn thickness and compressibility under the various extensional and compression forces in the fabric. To study the geometry of the yarns in the fabric and its variation affected by the twist, the polyester multi-filaments were twisted at different twist factors (α tex ) of 1150, 2300, 3450 and 4600 and then used in warp and weft of the fabric samples. The polyester yarns of 1150, 2300, 3450 and 4600 α tex twist factors were used in warp while yarns of 0, 1150, 2300, 3450 and 4600 α tex twist factors were used in the weft of the fabric. These levels of twist were chosen, since they gave considerably different yarn compactness to the produced yarn sample. The yarns were twisted on a two-for-one (RAPPI) twisting system. In order to achieve more suitable packages that contained enough stability of yarn to use in weaving, yarns were heat set on special steaming fixation machine (OBEM). After the yarn winding process, the cones were conditioned at 20 C and 75% humidity for 4 h. They were then left in the conditioning box for 24 h. This was followed by weaving. For this purpose, the CCI sample sizing and weaving machine was used, although the yarns were not sized during this process. The fabrics were then woven on a CCI weaving machine with eight heald frames. Finally, twenty plain fabrics of different yarn twist levels were woven on the CCI sample sizing and weaving machine with a weft insertion speed of 45 picks/min. 2.2 Measuring the Yarn Cross- section Shape In this study, the fabrics woven by the yarns of various twist levels were examined. In sample selection, it should be noted that the areas of fabric near the selvage do not have a good sample qualification condition and hence the samples should not be selected from these areas.

128 INDIAN J. FIBRE TEXT. RES., JUNE 2013 To measure the geometry of the yarn cross-section in the fabric, the fabric samples were selected randomly and then immersed in a liquid resin under a relax condition inside a special designed metallic mold. The resin is a mixture of polyester, methyl-ethyl-ketone-peroxide as hardener and cobalt octoate as catalyser and drier. The hardener in the resin makes it hard while the fabric sample is inside. The metallic mold inner dimensions were 15 cm length, 8 cm wide and 2 cm depth. Therefore, the hardened samples were a solid body of 15 8 2 cm with fabric sample inside. The prepared solid bodies were then cut in pieces of 2 mm thickness in a manner that the cutter is perpendicular to the fabric surface and one group of either warp or weft ends to cut them vertically. Then the cut pieces were polished carefully to smoothen their surface and to achieve clear images. Ten pieces of hardened samples of 4 2 0.2 cm for each direction of warp and weft were prepared and then photographed using a Projectina microscope ( 20 magnification) equipped with a digital camera. The photographs of yarn cross-section were saved on a computer and then processed using image processing toolbox of MATLAB software. Regarding the weave density of the fabrics, each piece of cut and polished samples of 4 2 0.2 cm includes 100 and 92 cross-sections of warp and weft respectively. Twenty images in warp direction and 20 images in weft direction were captured and saved (Fig. 1A). To obtain the shape of the yarn cross-section, we measured the major diameter a in the plane approximately parallel to the fabric surface and minor diameter b in the plane approximately perpendicular to the fabric surface of the elliptical yarn cross section in each image (Fig. 1B). 3 Results and Discussion The results of microscopic examination of the shape of the yarn cross-section are given in Fig. 2. The cross-section of the yarns of high twist remains approximately in circle form in the woven fabric (Fig. 2d) and it changes into an ellipse when the yarn twist decreases (Fig. 2a). To investigate the effect of twist on yarn cross-section, the major and minor diameters of elliptical shape of the each yarn crosssection were measured as shown in Fig. 1. One should note that the highest twisted yarns render the roundest cross-sectional shapes (close to a circle) with low contact area, whereas the lowest twisted yarns result in a symmetrical or asymmetrical ellipse or amygdaloidal with relatively higher contact area. Fig. 1 Measuring method of yarn cross-section shape (A) photograph of yarn cross-section in the fabric, and (B) measuring method Figure 3 shows the effect of yarn twist on major diameter of yarn cross-section. Obviously, the major diameter of warp yarn cross-section decreases as the warp yarn twist increases (Fig. 3a). Similarly, the major diameter of weft yarn cross-section decreases as the weft yarn twist increases (Fig. 3b). Also, it is obvious that the minor diameter of warp yarn cross-section increases as the warp yarn twist increases (Fig. 3a). Similarly, the minor diameter of weft yarn cross-section increases as the weft yarn twist increases (Fig. 3b). These results show that the yarns of lower twist deform easily by compression forces in the contact point of weft and warp yarns in the fabric. The greater deformation of yarns of lower twist is because of their loose structure. Therefore, because of bigger major diameter of low twist yarns in the fabric, these yarns cover more area of fabric surface and make it softer. Figures 3 and 4 also show that the twist of warp or weft yarns affect on the cross-section shapes of each other at the contact point. These results show that the effect of tightness of one group of yarn on the crosssectional parameters such as major and minor diameters of another group of yarn is significant. The major diameter of warp yarn cross-section decreases as the weft yarn twist increases (Fig. 3a).

AFRASHTEH et al.: GEOMETRICAL PARAMETERS OF YARN CROSS-SECTION IN PLAIN WOVEN FABRIC 129 Fig. 3 Effect of yarn twist on major diameter of (a) warp and (b) weft yarns cross-section Fig. 2 Typical photograph of warp yarn cross-section in the fabric with various twist levels (a) warp and weft twist factor 1150, (b) warp twist factor 1150, weft twist factor 2300, (c) warp twist factor 1150, weft twist factor 3450, and (d) warp twist factor 1150, weft twist factor 4600 (magnification 20) Similarly, the major diameter of weft yarn crosssection decreases as the warp yarn twist increases (Fig. 3b). On the contrary, the minor diameter of warp yarn cross-section increases as the weft yarn twist increases (Fig. 4a). Similarly, the minor diameter of weft yarn cross-section increases as the warp yarn twist increases (Fig. 4b). As Figure 3 shows, at the contact point of warp and weft yarns, the major diameter of group of yarns (warp or weft) decreases due to the increase in twist of other group of yarns. This is because of the bigger minor diameter of the higher twist yarns. When the twist of one group of yarns (warp) increases, their minor diameters increase and this causes the decrease in free space for expansion of major diameter of weft yarns. This phenomenon also explains the bigger minor diameter of warp yarns at higher twist of crossed weft yarns. The statistical analyses show the differences in the major diameter of one group of yarns, e.g. warp in the fabrics of various weft twists is significant at the 5% level. The same statistical results are also obtained for minor diameter.

130 INDIAN J. FIBRE TEXT. RES., JUNE 2013 Fig. 4 Effect of yarn twist on minor diameter of (a) warp and (b) weft yarns cross-section To consider the effect of yarn twist on the cross-sectional geometry of yarns in the fabric, the ratio of minor diameter to major diameter (ellipticity ratio) of both warp and weft cross-sections was calculated (Fig. 5). The figure shows the ellipticity (b/a ratio) of warp and weft cross-sections in a plain fabric. The ellipticity ratio changes between 0 and 1. The bigger the ellipticity ratio, the closer is the circular-like cross section. The ellipticity of warp yarn increases as the warp yarn twist increases (Fig. 5a). Similarly, the ellipticity of weft yarn cross-section increases as the weft yarn twist increases (Fig. 5b). Figure 5 also shows that the twist of one group of yarns such as warp does significantly affect the ellipticity of another group. The statistical analyses show that the differences in the ellipticity of the warp yarns of various twists are statistically significant at the 5% level. Similarly, the differences in the ellipticity of the weft yarns of various twists are statistically significant at the 5% level. Also, the differences in the ellipticity of the warp yarns in the fabrics of various weft twist levels are statistically significant at the 5% level. Fig. 5 Effect of yarn twist on ellipticity shape (b/a ratio) of (a) warp and (b) weft yarns cross-section Similarly, the differences in the ellipticity of the weft yarns in the fabrics of various warp twist levels are statistically significant at the 5% level. The results show that the elliptical shape of one group of yarns (warp or weft) in the fabric depends not only on their own twist but also on the twist of other group of yarns (warp or weft). The above finding may help to consider the effect of yarn twist on fabric structural parameters such as its thickness. The thickness of the fabric was measured according to ASTM D 1777 using a digital tester (KARDOTEC Co). The yarn twist affects the yarn cross-sectional dimensions such as yarn minor and major diameter. Obviously, the fabric thickness is the measurable dimensional parameter of fabric and is a function of yarn diameter particularly the yarn minor diameter. Theoretically, the fabric thickness is equal to the sum of the minor diameter of

AFRASHTEH et al.: GEOMETRICAL PARAMETERS OF YARN CROSS-SECTION IN PLAIN WOVEN FABRIC 131 measuring the yarn cross-sectional deformation in the fabric show that the shape of yarn cross-section in the fabric is affected by the twist level. The major and minor diameters of yarns are greatly affected by their own twist levels while the twist level of one group of yarns (warp or weft) shows a significant effect on major and minor diameters of another group of yarns. Increasing the twist factor from 0 to 4600 makes the yarn cross-section shape into a lens shape and then elliptical shape and finally circular-like shape. The major diameter of the elliptical shape of the yarn cross-section decreases as the yarn twist increases, while the minor diameter of the elliptical shape of yarn cross-section increases as the yarn twist increases. The ellipticity ratio of the yarns (minor diameter divided by major diameter) increases from 0.35 to 0.86 as the yarn twist factor increases from 0 to 4600. Acknowledgement The authors wish to thank Yazdbaf Textile Co. for their help and efforts in weaving fabric samples. Fig. 6 Relationship between the (a) weft and (b) warp yarns minor diameter and fabric thickness the crossed yarns. Therefore, the changes in yarn minor diameter in the fabric by the yarn twist may have an obvious effect on fabric thickness. The results of measuring the fabric thickness show that there is a linear relationship between the fabric thickness and the minor diameters of the constituent yarns (Fig. 6). The experimental results also show that the fabric thickness is approximately equal to the sum of the minor diameter of the warp and weft yarns. 4 Conclusion To study the effect of yarn twist on the geometrical shape of yarn cross-section in the fabric, the plain fabric samples were woven using polyester filaments of various twist levels in warp and weft. The results of References 1 Jinlian H, Structure and Mechanics of Woven Fabrics (The Textile Institute, Woodhead Publishing Limited, CRC Press), 2004, 61. 2 Afrashteh S, Merati A A & Jeddi A A A, J Text Sci Technol, 5 (2) (2009) 25. 3 Hamilton J B, A General System of Woven-fabric Geometry (The Textile Institute, UK), 1964. 4 Milasius V, Fibers Text Eastern Eur, 23 (4) (1998) 48. 5 Ozdi N, Marmarali A & Kretzschmar S, Int J Thermal Sci, 46 (12) (2007) 1318. 6 Hofset J & Keulen F, Compos Struct, 54 (2-3) (2001) 179. 7 Jiang Y & Chen X, J Text Inst, 96 (4) (2005) 237. 8 Gong R H & Ozgen B, Text Res J, 81 (7) (2011) 738. 9 Gong R H, Ozgen B & Soleimani M, Text Res J, 79 (11) (2009) 1014. 10 Dolatabadi M K & Kova R, J Text Inst, 98 (1) (2007) 1. 11 Alamdar-Yazdi A & Heppler G R, J Text Inst, 102 (3) (2011) 248. 12 Hearle J W S, Indian J Fibre Text Res, 31 (1) (2006) 142.