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  • Indian Journal of Fibre & Textile Research Vol. 27, March 2002, pp. 18-24

    Influence of filament core surface structure on tensile properties of DREF-3 yarns

    S M Ishtiaque" & K R Salhotra

    Department of Text ile Technology, Indian Institute of Technology, New Delhi 11 00 16, India

    and

    R V M Gowda

    Department of Textil e Technology, Bannariamman Institu te of Technology, Sathyamangalam 638 401 , India

    Received 16 October 2000; revised received and accepted 8 January 2001

    The inOuence of core filament surface structure on tensile properties of friction-sp un yarns has been studied. The yarn spun from Z pre-twist filament core has superior properties while that spun from S pre-twist filament core shows inferior quality as against the yarn spun with Oat fi lament core. Yarns spun with air-jet textured fil ament core exhibit significantly lower tenacity, breaking extension , modulus and energy-lO-break as compared to those made from Oat andlwisted forms. However, in respect of sheath con tribution, the yarns spun with air-je t textured fila ment core perform beller. The sheath cOlllribution and sheath slipping force increase wi th the increase in core filament overfeed during texturing. The sheath cOlllribution is highest fo r the yarn spun from filament with 30% overfeed.

    Keywords: DREF-3 yarn, Fi lament- to-fibre frict ion, Flat filament core, Sheath slipping force, Tensile propert ies, Textured filament core, Twisted filament core

    1 Introduction Friction spinning has made a substantial progress

    during the last two decades and has established a strong positi on in the coarse count range. It has got several distinguishing merits, like high production speed, very high twisting rate and low yarn tension during spi nning. It stands unique in the production of multi-component yarns. However, like other spinning systems, it has some drawbacks, especially in regard to the method of fibre feeding and nature of twisting.

    As regards the method of fibre feeding, friction spinning (DREF-2 and DREF-3) employs the standard vertical fibre feeding method where individuali zed fibres carried in the transport duct by aero-dynamic forces are progress ively decelerated as they assemble at the nip of the spinning rollers . This phenomenon, referred as compress ing effed, leads to fibre buckling which results in fibre disorientation, low fibre ex tent and low sheath fibre contribution to the strength of the resultant yarn. Further, the system lacks control over the fibre flow in the transport tube, which results in an uncontrolled fibre assembly in the nip of spinning rollers. The on ly mechan ism of fibre

    "To whom all the correspondence should be addressed. Phone: 6591940; Fax: 0091-01 1-6581 103; E- mail : isilt iaquc54@hotmail.com

    control in both the transport tube and the spinning nip is the fibre-to-fibre, fibre-to-metal and fibre-to-air friction2.

    The twisting in friction spi nning is negative in nature and, therefore, direct twist manipulation is not possible. This causes considerable amount of slippage between the fibre assembly and the fri ction rollers' surface, resulting in the inadequate tightness and frequency of wrapping of sheath fibres over the core component and this leads to differential twist structure.

    The above drawbacks have led to low yarn strength and poor surface integrity, which, in turn, limit the end uses of friction-spun yarn as a general purpose yarn. The friction spinning technology has reached the limit in respect of machine design to overcome the aforementioned drawbacks. Nevertheless , it provides the researchers an ample scope to accomplish it through optimization of process parameters and selection of suitable raw mate rial characteri stics. This would enable them to engineer the yarns for specific end uses and hence exploit the system in a better and more useful manner.

    It is expected that the fibre characteristics in order of importance for opti mum yarn quality in friction spin ning are fric tion, strength, fineness, length and cleanliness3,4. Fibre friction plays a vital role during

  • ISHTIAQUE et al. : TENSILE PROPERTIES OF DREF-3 YARNS 19

    the yarn formation and significantly influences the yam properties. There are basically two forms of friction encountered during yarn formation, namely fibre-to-drum surface and fibre-to-fibre friction. The fibre-to-drum surface friction is of both sliding and rolling types5, while the fibre-to-fibre friction is more of sliding type. These two forms of friction determine the extent of fibre slippage between the drums' surface and yarn surface and the intensity of wrapping of core by the sheath. These two aspects decide the level of radial pressure exerted by the sheath on the core and the cohesive interaction between the two components, which largely govern the mechanical properties of the yarn.

    The frictional cohesion between the core and the sheath can be varied in several ways, more commonly by changing the surface structure of either one or both the components. This can be accomplished through mechanical means by twisting and texturing in case of filaments and chemical means like application of fibre finish in case of staple fibres.

    In a filament core friction-spun yarn, the core offers good yarn strength and uniformity while the sheath provides spun yarn characteristics, besides being a substrate for the application of special finishes, when the yarn is used for industrial textiles. Nevertheless, these yarns suffer from the disadvantage that the staple fibre sheath slips over the filament core due to the abrasive actions during the subsequent processes which therefore restrict their end uses6.

    The present study was aimed at improving the frictional cohesion between the core and sheath by using different filament cores of varying surface structures and hence the tensile properties of the resultant yarns.

    2 Materials and Methods 2.1 Materials

    Acrylic fibres of 38 mm and 1.65 dtex were used for sheath. A 110/48 dtex polyester multifilament yarn was used for core in three different forms, namely flat, twisted and textured. The polyester multifilament yarn with zero twist is considered as flat filament. The Z and S twist filaments with 315 tpm (8 tpi) were made from flat filament. This level of twist was more or less optimum so as to bring about a change in its surface structure without causing much deterioration in its mechanical properties. The flat filament was also air-jet textured with three different overfeed rates (10, 20 and 30%) at a temperature of

    190°C and air pressure of 9 kg/cm2 to produce three types of filament cores with different loop sizes and surface structures. In total, six types of filament core (FI-F6) were used to spin yarns.

    2.2 Fibre!Filament Testing 2.2.1 Tensile Properties

    Acrylic fibres were tested on Lenzing Technik 's Vibrodyn for tensile properties at a gauge length of IOmm and a test speed of 20 mm/min. Polyester filament cores were tested for tensile properties on Instron tensile tester at a gauge length of 500 mm and a test speed of 300 mm/min. The tensile properties of various core filaments and acrylic sheath fibres are given in Table 1.

    2.2.2 Frictional Properties

    Acrylic fibres and core filaments were tested for frictional properties, namely filament-to-fibre and fibre-to-fibre friction using a special device (Fig. I) attached on Instron tensile tester. The fibre fringes of areal density 5 mg/cm2 are prepared by doubling, drawing and combing a sample of fibres taken from sliver. The short and any stray long fibres are removed. The remainder, forming the modal part of the samples with co-terminus ends, is cemented to a piece of card. In case of fibre-to-fibre friction, one fibre fringe is fixed on the metallic platform covered with a sheet of Teflon and the other is hold by the holder slides over the former at the rate of 10 mm/min under an applied normal force of 40 cN. The computer, online with the Instron, plots the friction profile and calculates the frictional force. The test details have already been described by Salhotra et aC. For filament-to-fibre friction, no standard test method is available. However, the following procedure was used to approximately estimate the friction between core-filament surface and the sheath fibres as it happens during the yarn formation. The filament was wrapped onto a thick paper strip with 98 wraps linch. The length and width of the strip were 50 mm and 30 mm respectively. The filaments were firmly attached to the strip at both the ends to avoid any distortion or loss of twist. The paper strip was cut except at the edges, which hold the sheet of filaments fixed to it. This sheet of filaments was fixed at both of its edges on the platform of friction measuring device. The fibre fringe attached to the fibre holder slides over it. The frictional force is computed in the similar way as in case of fibre-to-fibre friction. The frictional characteristics are given in Table 2.

  • 20 INDIAN J. FIBRE TEXT. RES., MARCH 2002

    Table I-Tensile properties of core filaments and sheath fibre

    Core Filament Linear Breaking load Tenacity Breaking Initial modulus Energy-to-break fil ament code density cN cNltex extension cNltex J

    tex %

    Fl at F I 11.11 317.7 28 .6 21.1 724.1 25.4x I0·2

    (3 .9) ( 10.1) (2.8)

    Ztwist F2 11.33 3 18.8 28 .1 20.4 668.6 23 .4x 10-2

    (5 .5) (14.2) (2.1 )

    S twi st F3 11.33 316.6 27 .9 19.9 672.0 23.0X I0-2

    (4.8) (13 .0) (2 .0)

    AJT IO F4 11.4