Tensile Deformation of Fibers Used in Textile Industry

Transcription

1 Tensile Deformation of Fibers Used in Textile Industry Application Note Sandip Basu, Ph.D. Agilent Technologies Introduction Fibers are almost synonymous with textile industry. The word Textile is actually defined as a flexible woven material consisting of a network of natural or artificial fibers [1]. We have been using different type of fibers from plants and animals to make fabric for last several thousands of years, and knowingly or unknowingly selected these fibers based on their different attributes. In recent years, as natural resources started to get expensive, we have also invented many artificial fibers to replace the natural ones in our clothes, carpets and other apparels. Selection of fiber materials for making a fabric depends on many considerations, such as their chemical stability, thermal and electrical conductivity, as well as the mechanical properties. It may seem farfetched at first, but the knowledge of these physical properties of the fibers enables the designers to make new fabric for specific applications, such as for the athletes or for the astronauts. The knowledge of mechanical property of the fibers is also important for designing the automated textile plants where each fiber is introduced to different amounts of tensile loads during the weaving process. In recent years, it has also been an important research direction to understand the structure-property relations in the natural fiber materials, which then can be successfully mimicked in an artificial material. Given the importance of understanding mechanical behavior in textile industry it is reasonable to assume that mechanical behavior of these textile materials has already been reported since long time in literature [2, 3]. Despite these efforts, very little is available in the open literature about the mechanical behavior of single individual fibers comprising the yarns in many textiles. It is important not only from the perspective of a materials engineer, designing new materials for textile industry, but also from the perspective of the person who designs advanced fabrics for specific applications based on existing materials. This lack of information is in part due to unavailability of commercially available instruments which can precisely measure the deformation of a thin single fiber, which are often only a few microns in diameter. The Agilent UTM T150 is specifically designed for the purpose of measuring the tensile deformation behavior of extremely thin fibers [4, 5]. Along with the high force and displacement resolution, the patented continuous dynamic analysis (CDA) module enables the T150 to measure storage and loss modulus of a material continuously

2 Lumen Secondary wall with several layers Winding layer Primary wall (1st layer) Primary wall (2nd layer) Cuticle Figure 1. Schematic of the microstructure of cotton fiber (left), and electron micrograph showing the twisted morphology of processed cotton fibers (right). ( com/2009/06/11/fiber-files-cotton/). during a tensile experiment. The CDA is an important tool to characterize the change in the inherent structure of materials during deformation, and especially important for polymeric materials. The capabilities of the T150 along with the CDA were successfully utilized to characterize tensile deformation of many technical fibers, including polypropylene [6], copper, tungsten [6], basalt [6] and spider silk [7] (a potential material for the textile community). Hence it is evident that the T150 is a powerful tool to characterize the deformation mechanisms in single fibers used in textile industry. The following section briefly describes the natural and artificial fiber materials studied in the current study. Fiber Materials Cotton: Despite all the competition from their artificial counterparts, cotton is still the best-selling fiber material in the world, and mostly in the textile industries [8]. This natural cellulosic fiber does not contain many harmful chemical effects of the artificial fibers. The individual cotton fibers are actually individual plant cells. The structure of a cotton fiber is shown in Figure 1 (schematic). It is composed of concentric layers of closely packed cellulosic crystalline fibrils. The innermost part, known as lumen, contains the cell nucleus and protoplasm before the boll opening. However, when the cell interior dries out it causes the twists observed in individual cotton fibers (Figure 1, image on right). The cotton fibers also go through a number of chemical treatments (as is the case with the fibers used in the present study) such as bleaching and mercerization to improve their luster and sorption properties. After all these treatments, what remains of the cotton fibers is mostly crystalline cellulose, which, in scientific language, consists of a linear chains of hundreds or thousands of glucose units. Wool: The most popular natural fiber material that comes from an animal is wool. It is mostly fibrous protein from specialized skin cells of sheep [9]. Unlike the continuous surface of cotton fiber, wool fiber consists of scales and crimps. These qualities make the wool fibers easy to spin, and the air retained in the 2

3 Cell membrane complex Macrofibril Complex inner matter 4 outer cuticle layers Macrofibril Matrix Left-handed, coiled-coil rope Para cell and ortho cell cortex Right-handed α-helix Figure 2. Schematic of the microstructure of a wool fiber ( Fibre-Modification.htm) crimped space gives rise to the thermal insulation in the fabric. Figure 2 shows the hierarchical structure of a single wool fiber. One wool fiber is also not a single cell like cotton, but consists of multiple cells. There are mainly two different types of cells in a wool fiber the internal cells of the cortex and external cuticle cells. Other than their usual application in warm clothing, wool is safer for fire hazards compared to cotton and synthetic fibers and is used in clothing and carpets in environments where there is a likelihood of fire exposure. Polyester: Polyester, also known as polyethylene terephthalate (PET), fibers are the most extensively used manmade material in the textile industry. Polyester fabrics show improved wrinkle resistance, durability and high color retention. Most characteristic properties of polyester fibers are usually attributed to the benzene rings in the polymer chain, which leads to chain stiffness. PET fibers consist of crystalline, oriented semi-crystalline as well as noncrystalline (amorphous) regions [10]. Rayon: Another widely used fiber in the textile industry is rayon. It is a regenerated cellulose fiber made from naturally occurring materials [11]. Rayon fibers exhibit a lot of similarity with natural fibers like cotton and wool; however, it does not insulate the body heat. So, rayon fabrics are ideal for hot and humid climate. Experimental Details Commercially available yarns of 100% mercerized cotton, wool, polyester and rayon were purchased. Individual fibers of each material were carefully separated from the yarn and mounted on card templates. The gage section of each fiber specimen is measured using a caliper, and the cross-sectional areas of the fibers were measured using scanning electron microscopy (SEM). The template was then mounted on the T150 (Figure 3) using the template-grips and the sides of the template were clipped to release the fiber for testing. The micro-positioner is used to make sure proper alignment of the fibers before testing. The NanoSuite test method named UTM T150 Standard Toecomp CDA was used for tensile testing of the textile fibers, along with continuous dynamic analysis. All the tests were performed with a strain rate of 1x10-3 s -1. The continuous dynamic analysis during each test was performed using a force amplitude of 2mN at a frequency of 20Hz. The fiber morphology and the fracture surface of the fibers were imaged in the SEM. Figure 3. Individual textile fiber mounted on the Agilent UTM T150. Note that the sides of the card template were cut to release the fiber before testing. 3

4 Engineering Stress Storage Modulus Engineering Stress (MPa) Storage Modulus (GPa) Loss Modulus (MPa) Loss Modulus Results and Discussion Cotton: The engineering stress-strain curve for a typical test on a single strand of cotton is shown in Figure 4. The elastic modulus, measured from the slope of the initial linear region, the tensile strength, and the strain at failure for individual cotton fibers are listed in Table 1. As expected of a natural fiber, there were higher variation in sizes of the cotton fibers, which resulted in the variation in elastic modulus and tensile strength (Table 1). Although the cellulose crystals in the mercerized cotton fibers exhibit high modulus and strength, they are also the least ductile compared to the other fibers studied herein. The electron micrograph in Figure 5 clearly shows the anisotropic cross-section of the cotton fiber. Hence, the fiber sizes were measured post-test and a modified fiber dimension was entered in NanoSuite to recalculate the tensile test results. At this point, it is difficult to compare the results with literature as most of the previous tensile studies of cotton have been conducted on yarns [3], rather than individual fibers. Moreover, the mechanical properties of cotton also vary with the length of the fiber and the chemical treatment it undergoes before application. The variation in dynamic storage and loss modulus with increasing strain is shown in Figures 4a and 4b, respectively. The loss factor (ratio of loss modulus and storage modulus) is also plotted in Figure 4b to get a better understanding of the damping behavior. Figure 4. Continuous dynamic analysis of cotton fiber during tensile test. (a) Variation in engineering stress and storage modulus with increasing strain; (b) Variation in loss modulus and loss factor with increasing strain. Figure 5. SEM micrograph of the cotton fiber near the fractured surface. Note the anisotropic shape of the cotton fiber. Young s Tensile Strain at Fiber Diameter Modulus Strength Failure (µm) (GPa) (MPa) (%) Cotton 9 ± 1 30 ± ± 41 5 ± 1 Wool 41 ± ± ± ± 12 PET 17.0 ± ± ± ± 1 Rayon 13.0 ± ± ± ± 2 Table 1. Mechanical properties of individual textile fibers. 4

5 Engineering Stress Storage Modulus Loss Modulus Engineering Stress (MPa) Storage Modulus (GPa) Loss Modulus (MPa) The storage modulus is about 20GPa during the initial linear elastic regime, but it increases significantly up to about 100GPa, before failure. This increase in storage modulus is due to the increased alignment of cellulose crystals along the fiber axis. The macroscopic twists in the mercerized cotton fiber may also cause some changes in the measured storage modulus. However, more work is needed to understand the exact microstructural changes during tensile deformation. The initial increase of the loss factor is also in agreement with the alignment phenomenon. Once most of the crystallites are aligned along the fiber axis, the deformation is mainly due to stretching of bonds in the cellulose crystals, which in turn reduces the loss factor. Wool: The tensile deformation behavior in an individual wool fiber is shown in Figure 6. The tensile strength, the initial elastic modulus and the strain to failure are listed in Table 1. The wool fibers exhibited the lowest strength and modulus among the four different types of fibers characterized during this study. However, these fibers can be stretched about 30% of their original length before failure, much higher strains compared to other fibers. The surface morphology of wool fibers can be seen in Figure 7. Although the fiber diameter is uniform along the length of the fibers, the expected defect distribution in the natural fiber is higher. This causes the variation in tensile strength and elastic modulus of wool fibers (Table 1). Figure 6. Continuous dynamic analysis of wool fiber during tensile test. (a) Variation in engineering stress and storage modulus with increasing strain; (b) variation in loss modulus and loss factor with increasing strain. Figure 7. SEM micrograph of the wool fiber. Note the crimps on the fiber surface. When the variation in dynamic storage modulus with strain is plotted (Figure 6a), there is a slight drop corresponding to the yield in engineering stress-strain curve. This correlates to the molecular movement in the microfibrils (Figure 2) to align themselves along the fiber axis. As this alignment process dissipates energy, it increases the loss factor (Figure 6b). After the molecules in the microfibrils are aligned, the deformation is mostly due to stretching of various hierarchical layers along the fiber axis. The drop in loss factor in this regime suggests that the deformation of the hierarchical structure dissipates less energy compared to the molecular alignment. However, more systematic microstructural characterization is needed to completely understand the deformation process. 5

6 Engineering Stress (MPa) Engineering Stress Storage Modulus Storage Modulus (GPa) Loss Modulus (MPa) Loss Modulus Polyester: A typical engineering stress-strain curve from tensile test of individual polyester (PET) fiber is shown in Figure 8. The initial modulus, tensile strength and strain to failure are listed in Table 1. As shown in Figure 9, the PET fibers, used during this study, had very uniform diameter. According to previous literature [12], one PET fiber consists of microfibrils aligned along the fiber axis. These microfibrils, in turn, consist of crystalline and amorphous regions, and connected to other microfibrils by another kind of amorphous phase, known as mesamorphous phase. The different regions observed in the tensile stress-strain curve can be explained by the deformation of the different microstructural regions mentioned above. During the initial deformation, the amorphous regions within the microfibrils align themselves in the similar orientation as the mesamorphous phase. The stress-strain curve goes through another point of inflexion when the applied load starts to strain the bonds in both amorphous and crystalline phases. The final part of the curve represents slippage between microfibrils. When the storage modulus, calculated from the continuous dynamic analysis, were plotted together with the engineering stress and strain (Figure 8a), it shows almost a monotonous increase in stiffness of the material with strain. The alignment of amorphous regions, as discussed earlier, supports the small Figure 8. Continuous dynamic analysis of cotton fiber during tensile test. (a) Variation in engineering stress and storage modulus with increasing strain; (b) variation in loss modulus and loss factor with increasing strain. Figure 9. SEM micrograph of the cotton fiber near the fractured surface. Note the anisotropic shape of the cotton fiber. drop observed in the storage modulus. The dynamic storage modulus during the initial deformation is in good agreement with the elastic modulus listed in Table 1, as well as the quasistatic elastic modulus reported earlier. Similarly, the dynamic loss modulus also increases with increasing strain (Figure 8b). However, the real damping behavior can be observed form the variation of loss factor with strain, where the loss factor reaches a maximum during the alignment of amorphous regions, and then decreases as the crystalline and amorphous regions within the fiber becomes more aligned with the fi b e ra x i s. 6

7 Loss Modulus Engineering Stress (MPa) Engineering Stress Storage Modulus (GPa) Loss Modulus (MPa) Storage Modulus Rayon: The tensile deformation behavior of individual rayon fibers is similar to wool fibers. The tensile stress-strain curve (Figure 10a) exhibit an initial linear elastic region followed by a hardening behavior. The diameter of the rayon fiber was also needed to be modified post-test because of its irregular cross-section. The electron micrograph of the rayon fiber (Figure 11) clearly revealed the striations on the surface of the fiber. This is most probably due to the arrangement of cellulose microfibrils along the length of the rayon fiber. Similar to wool and PET, rayon fibers also exhibit a drop in dynamic storage modulus during the yielding process (Figure 10a). This corresponds to the alignment of the amorphous regions within the microfibril in a similar orientation with the amorphous chains between the microfibrils. This alignment mechanism dissipates more energy, which results in higher loss factor (Figure 10b). As both the crystalline and amorphous regions get aligned the energy dissipation drops, decreasing the loss factor with increasing strain. By comparing the results on four different individual textile fibers (Figures 4 11, Table 1), it is evident that fiber morphology plays an important role in their deformation process. However, a more detailed study is required to understand the exact nature of the deformation. Figure 10. Continuous dynamic analysis of rayon fiber during tensile test. (a) Variation in engineering stress and storage modulus with increasing strain; (b) variation in loss modulus and loss factor with increasing strain. Figure 11. SEM micrograph of the rayon fiber. Note the striations on the fiber surface. Conclusions The Agilent UTM T150 is successfully employed to test four different individual textile fibers cotton, wool, polyester and rayon under tensile loading. The continuous dynamic analysis (CDA) module enabled us to study the variation in dynamic storage and loss modulus of the textile fibers with increasing strain. The combination of storage modulus and loss factor calculation not only confirms that the stiffness of the individual fibers increases with strain, but also can be used to identify the energy dissipation mechanisms during deformation of natural and artificial polymeric fibers. 7

8 References Eyring, H. and G. Halsey, The Mechanical Properties of Textiles, III. Textile Research Journal, (1): p Orr, R.S., et al., Physical Properties of Mercerized and Decrystallized Cottons. Textile Research Journal, (4): p Basu, S., Tensile Stress-Strain Response of Small-diameter Electrospun Fibers. Agilent Technologies Application Note, Basu, S., Tensile Test of Copper Fibers in Conformance with ASTM C1557 using Agilent UTM T150. Agilent Technologies Application Note, Hay, J., Quasi-static and Dynamic Properties of Technical Fibers. Agilent Technologies Application Note, Blackledge, T.A., J.E. Swindeman, and C.Y. Hayashi, Quasistatic and continuous dynamic characterization of the mechanical properties of silk from the cobweb of the black widow spider Latrodectus hesperus. The Journal of Experimental Biology, : p Lechat, C., et al., Mechanical behaviour of polyethylene terephthalate & polyethylene naphthalate fibres under cyclic loading. Journal of Materials Science, (6): p Nanomeasurement Systems from Agilent Technologies Agilent Technologies, the premier measurement company, offers high precision instruments for nanoscience research in academia and industry. Exceptional worldwide support is provided by experienced application scientists and technical service personnel. Agilent s leading-edge R&D laboratories ensure the continued, timely introduction and optimization of innovative, easy-to-use nanomeasurement system technologies. Americas Canada (877) Latin America United States (800) Asia Pacific Australia China Hong Kong India Japan 0120 (421) 345 Korea Malaysia Singapore Taiwan Thailand Europe & Middle East Austria 43 (0) Belgium 32 (0) Denmark Finland 358 (0) France * *0.125 /minute Germany 49 (0) Ireland Israel /544 Italy Netherlands 31 (0) Spain 34 (91) Sweden Switzerland United Kingdom 44 (0) Other European Countries: Product specifications and descriptions in this document subject to change without notice. Agilent Technologies, Inc Printed in USA, April 2, EN