1.0 Friction Spinning

1.1 Introduction:

The need to rotate the package at the twist insertion rate coupled with rapid increase in spinning tension with spinning speed sets a limit to the spindle speeds achievable in ring spinning. Open end spinning methods where open-end of the yarn alone needs to be rotated for imparting twist was therefore, developed to achieve high delivery rates. Rotor spinning which is one of first methods developed on this principle has well established itself as an alternate to ring spinning in course count range for certain end uses. Friction spinning represents an alternate open-end spinning method to rotor spinning which holds promise of still higher delivery rates

1.1.1 The development and potential: There has been a tremendous revolution in short staple spinning technologies in the form of rotor, air-jet and friction spinning systems. Rotor spinning is successful in in the courser to medium count ranges up to 40s. Air-jet though established but it’s restricted to synthetic and their blends only in the count range of 15-60s. Friction spinning is slowly taking its market share in course count ranges up to 20s. In the year of 1977, commercial Dref-2 friction spinning machines were put into production. Presently 1,400 machines are operative in about 50 countries producing about 3,00,000 tones of course count yarns annually (Gastu.M, Textile Horizon, 1997). Later in 1981 Dref-3 machines were introduced to the world market. Besides alternative to conventional yarns, friction- spinning technology is particularly suitable for the production of multi-component and multi-layered yarns, core yarns and some specialty yarns. The principal advantage of this technology is its very high production speeds (Brockmanns, K.J,Textile Month,1984) (Fig. 1.1 &1.2.) along with this ability of forming big packages . Yarn production cost is also low (Fig.1.3). The excellent possibilities of automation, low-end breakage rates, high efficiency and flexibility in the use of raw material are few essential merits of this spinning process.

1 Fig. 1.1 Fig.1.2

(Delivery speeds in different spinning (Production rates in different spinning technology) technologies) A-Ring spinning

B-Friction spinning (B1: Masterspinner, B2: Dref-2 & B3: Dref-3) C-Rotor spinning, D-Air-jet spinning (Murata) E-Wrap spinning

A-Ring Spinning B-Friction Spinning C-Rotor Spinning D-Air-jet Spinning Fig. 1.3 Percentage difference in production costs in comparison with ring spinning (assuming 100%)

2 2.0 Working Principle: Frictions spinning technologies works on the principle of open-end or wrap (Fasciated)/core spinning. The general principle of working of friction spinning in its simplest form can be described as below.

Fig. 2.1 The principle of friction spinning As in the diagram (Fig.2.1) separating them from slivers generates a stream of fibres, which is transported through a duct. The fibres are then directed towards the nip of two rotating drums called friction drums .The fibres are collected close to the nip of these drums. The friction drums have perforations on it and suction from inside holds the fibres on the surface. There is a long slot inside the friction drums located close to the nip point along the length of friction drums and the rest of the area inside the perforated drum has a shield. So, the yarn at the nip of the friction drum is subjected to a radial force generated as a result of airflow over yarn.

Fig.2.2 Forces acting on the yarn tail and the vector diagram. This is shown in figure 2.2 , This radial force serves as a normal load and frictional force is generated between the yarn and the surface of friction drum. It can be observed from the same figure that at the closest proximity of the friction drums they rotate in opposite direction and the frictional force thus developed, produces a torque on the yarn tail. Continuous rotation of the drum produces twist in the yarn leading to gradual integration

3 of fibres in the yarn tail. The yarn is withdrawn in the direction of the axis of the friction drums and is wound on a bobbin. 3.0 Different types of friction Spinning Machines: The principle of various types of commercially available friction spinning machines is described below. 3.1 Dref-2 System:

Fig.3.1 Schematic diagram of Dref-2 system A schematic diagram of Dref-2 system is shown in figure 3.1. There is a drafting system through which one or more slivers are directed to an opening roller clothed with saw teeth. The opening roller individualizes the fibres in the strand and they are stripped from the roller surface by an air -current from blower. The stream or cloud of fibres thus generated is guided through a duct and finally gets collected at the nip of two perforated friction drums. There is a suction from inside the two perforated drums that causes the fibres to be adhered just above the nip point. The fibres are thus gradually added to the open –end and get twisted thereafter because of the rotation of the drums. The yarn thus formed is wound on a bobbin. Dref-2 is mainly useful for course counts and requires medium to long staple fibres as input materials.

4 3.2 Masterspinner (PLS) System:

Fig.3.2 Masterspinner (PLS) System: As figure 3.2 shows, the machine also works on the principle of open-end spinning like Dref-2 machine but there is a difference particularly with respect to fibre feed and flow direction and construction of friction drums A single draw frame sliver is fed to opening roller which opens the fibre in the same way as it does in rotor spinning. The stream of fibres is guided through the suction duct making an acute angle to the yarn withdrawal direction. It is claimed that feeding the fibre in this particular style improves the fibre extent and orientation of the fibres in the final yarn. Similarly, the fibres are collected on friction drums. Hence one friction drum is perforated and it acts as suction drum and the other one is blind solid drum with a special surface to ensure effective friction transfer. The yarn is twisted by the drums and then packaged in a bobbin. This machine is able to produce yarns in the range of 10s-30s using cotton and synthetic fibres up to 40mm length and their blends

5 3.3Dref-3 system:

Fig.3.3 Dref-3 system:

This is an extension of Dref-2 spinning system that adds a drafting system at one side of the friction drums to attenuate the sliver that finally forms the core of the yarn. It is not an open –end spinning method like Dref-2 or PLS Master spinner. It works on friction fibre wrapping i.e. the parallelly delivered fibres or filaments are false twisted and simultaneously wrapped with sheath fibres in the spinning zone. As a result it produces a core sheath structure of yarn. The Dref-3 spinning system is shown in figure 3.3. This machine runs at a maximum speed of 300 m/min and the finest count that can be spun is around 20s.Each spinning head in this system is composed of following five sections

1) Drafting unit 1 The drafting unit 1 is meant for supplying the staple fibre core component. There are two models of this unit. One is suited for short staple fibres up to 40 mm and processing cotton with filament core. 2) Drafting Unit 2 This unit consists of a 1-over –1 and a 1-over –2 roller combinations as guide rollers for slivers, two oppositely rotating opening rollers and a transport duct for feeding the fibres onto a pair of perforated friction drums .The shape of transport duct is such that

6 it is conducive for acceleration of the air-current passing through it and thus intended orientation of fibres in air stream is maintained. 3) Spinning zone It consists of a pair of perforated drums known as friction drums .The friction drums have air suction from inside. The stream of fibres after being transported through the duct are collected above the nip point of drums .The surface of the friction drums have a special coating to attain a better twisting effect.

4) Winding – up mechanism After passing through the spinning zone, the yarn is drawn off via three outlet rollers and finally delivered to the winding device. Between the outlet rollers and the winding device, there are two deflection rollers that evens out yarn tension and actuates stop motion. 4.0 Operations involved in friction spinning: The basic operation involved in friction spinning process are as follows 1) Opening of slivers 2) Transporting the stream of slivers through a duct to the yarn formation zone 3) Collecting and reassembling the fibres 4) Consolidation and twisting of fibres into a yarn 5) Withdrawal of the resulted yarn for package formation

4.1 Opening of fibres: A high degree of opening of the slivers almost to the extent of individual fobres is required and this is performed with the help of opening rollers. High drafting of slivers is also an essential operation where staple fibres are used .The degree of longitudinal orientation and the straightness of the fibres extent a considerable influence on yarn properties. 4.2 Fibre transportation through duct: The fibres from the opening rollers are directed through the transport duct into the nip of the friction drums .The fibres are in free flight under the influence of air –current

7 through the duct. During this free flight the fibres lose most of its orientation that not only affects the yarn characteristics but also the spinning limit. Lord et al.(Lord.P.R., J.Text .Inst, 1987 ) have reported that there is a practical problem of assembling fibres in the yarn with good degree of fibre orientation because of varying amount of turbulence in the fibre transport duct .The fibres in the duct move at a very high speed but on arrival at the rotating mass of fibres close to the nip of friction drum, they suffer a huge deceleration . This not only disrupts the fibre orientation but also the mass density of the fibre stream. As a result, fibres are buckled, hooked and looped. As for example, it is shown in (Lord.P.R., J.Text .Inst, 1987 ) that the fibre in the air -stream are carried at a speed of 1000 m/ min. Thus the fibres are decelerated from 1000 m/ min to only 200 m/min over a distance of 2 mm or so. Though it is intended by the machine designer to collect the fibres in straight condition, in practice it does not happen. Moreover, the buckled fibres land from different conditions on the collecting surface. The design of the duct can exercise a great influence on this phenomenon

4.3 Fibre collections or Accumulation:

Fig. 4.1 Fibre collection systems a) on the ingoing perforated friction drum b)on the rotating mass of fibres

The fibers from the transportation duct are deposited on the surface of ingoing perforated drum and subsequently carried to the nip or directly on the rotating mass at the nip. Here the fibres encounter a surface that moves at a much slower speed at which they

8 were moving through the duct. As a result fibre buckling is inevitable and fibres in the yarn body are looped.

Fig. 4.2 Effect of counter air flow and surface velocity on the lying of fibres on the perforated friction drum

There are basically two types fibre collection systems presently available in friction spinning machine (Lord.P.R., and Rust .J.P., J.Text.Inst. 1991). 1. Fibres are made to land on the surface of the ingoing perforated friction drum and subsequently transported to the nip of the friction drums. 2. Fibres are collected directly on the rotating mass The two methods have been shown in figure ( 4.1 ) In the first case where the fibres are deposited on the ingoing drum, suction is used to hold them on the surface and finally carried to the rotating mass of fibre. It has been stated that the fibres will land end first onto the drum surface and this end of fibre being firmly held, the remainder of the fibre sweeps past because of the effect of inertia and drag from counter flow of air. This is shown in fig (4.2 ) Others have the opinion that “ Fibres may be trapped at any point along its length because it has a three dimensional shape as it travels through the feed duct and only portions of it are

9 straightened . It is not certain that the leading end will land first on the perforated surface, this tends to produce hooks “ 4.4 Twisting of fibres: The torque generated by the friction drum imparts twist in the yarn tail. The twist integrates the loose fibres at the tail of yarn. The amount of torque generated is dependent on fibre to meal friction and the radial reaction pressure at the contact point of the yarn tail and friction drums. Both of these quantities appear to be uncontrolled in nature leading to variable slippage of yarn tail. As a result there is an irregularity in twist in friction yarns.

Figure. 4.3 Forces acting on the yarn tail and the vector diagram

In figure ( 4.3 ) it has been shown that R1 and R2 are the normal radial reaction forces acting on the growing yarn . The corresponding tangential frictional forces are μR1 and

μR2 trying to rotate the yarn in opposite direction and thereby producing a torque.

Torque = (μR1 + μR2) (d/2) Where, d = diameter of the yarn

4.5 Yarn withdrawal: The yarn delivery speed in friction spinning can be as high as 300m/ min .The spinning tension is very low in this process and hence the end breakage rate is also less. Therefore, tension has practically no influence on the spinning limit .The yarn is wound on a big package directly and so rewinding is eliminated

10 4.6 Twist insertion in open-end friction spinning: Figure (4.4) represents a fibre being applied to the open- end of a yarn in the friction spinning process. Fibres such as those shown by positions a, b, c, d and e rotate about the conical end of the yarn x, y, z. The fibre may be anchored at d while position b rotates relative to the cone xy .The resultant structure will consist of fibre abcde may or may not end up as a perfect spiral. Also, the geometry of the cone xy may changes as a function of time and spinning conditions

Fig. 4.4 Fibre rotation about the conical yarn tail The fibres arriving in the spinning zone are carried in to the mesh with the yarn tail by one of the spinning drums. These envelopes of fibres at the open end rotate and twist the arriving fibres. The fibres near the tip are loosely wrapped around the yarn tail, where as those away from the tip are progressively more tightly wrapped. Lord et al (Lord.P.R., Joo.C.W., and Ashizaki.T., J.Text .Inst, 1987) explained how the swirling mass of fibres tightened up on to the tapered non –rotating and departing yarn (Fig 4.5) .There was a sort of balloon with a continuous surface shaped like an artist’s paintbrush. Simulation studies have shown that the envelop of this brush was indented where it came in into contact with the friction drums. As fibres tighten on to the tapered yarn tail, their rotational speed is diminished. When they are completely tightened on the structure, there us little or no further rotation with respect to the yarn axis unless there us considerable instability in the system.

11 Fig.4.5 Twisting of fibres as they assemble on the yarn The yarn does not emerge in conical form but it is surrounded by rotating sleeve of fibres of approximately cylindrical form .The rotating sleeve is formed by the fibres arriving from the feed unit. The fibre sleeve rotates on the perforated drum surface, particularly slippage free without any movement in the direction of sleeve axis and the actual yarn tail is formed within this sleeve, during this period of tail formation, the fibre sleeve does not rotate. Subsequently the fibres are transferred from the rotating sleeve on to the non-rotating core, which is axially withdrawn. The twist is imparted to the yarn by the rotation of the fibre sleeve while fibres are being peeled from it. The fibres from within the sleeve are peeled off through the rotation of the fibre sleeve and wound up in the Z direction of the yarn tail. The fibre sleeve and the mode of yarn formation are shown in Fig (4.6)

Fig. 4.6 Fibre sleeve and mode of yarn formation

12 5.0 Formation of friction –spun yarn:

a)

b) Fig.5.1 Dref-2 yarn formation structure As the figure (5.1) depicts, the friction drums are longer than the width of the fibre feed .The yarn formation therefore occurs along two parts of the friction drums, first in zone1, the fibre supply zone and second in zone 2, where the forming yarn receives no more fibres .Lawrence et al (Lawrence.C.A, Fundamentals of Spun Yarn Technology, CRC Press, 2002 ) has observed that the fibres landing on to the friction drums in zone 1 form a conical yarn end or tail between the drums.

13 a)

b) Fig. 5.2 Transportation, deposition and twisting of fibre in Dref-2 spinning Figure ( 5.2 a ) shows that the fibres traveling from the opening roller to the friction drums and figure ( 5.2 b) shows the fibres landing and being twisted to form the yarn. The fibres are individually twisted onto the conical yarn tail during their deposition. The formation of Dref-2 yarn structure is therefore a buildup of fibre layers from the sliver feed. As the forming yarn length moves in the direction of take-up, the separated fibres from each consecutive sliver are deposited onto the previous layer. Thereby, the fibres present in particular sliver become integrated into the corresponding concentric layer of the yarn; the fibres from the first sliver farthest from the delivery rollers forming the center of the yarn. It is therefore possible to produce the yarns with each concentric layer being composed of different fibre type. It is evident that any migration between layers is very small and that the yarn is much more compact in the region of the core. Since spinning

14 tension is low, it is unlikely that this compaction is caused by any applied axial tension on the fibres (as is the case of ring spinning) and is therefore more likely to be the result of a higher twist level at the yarn core. In zone1, the twist level is low, and centrifugal forces cause the yarn tail to swell. The twist in zone2 is much greater, and the yarn diameter decreases .As the yarn leaves the friction drums the amount of twist in each radial position will depend on the length of time fibres in that position stayed within the two zones. This is because the twist gained is cumulative between the point where a fibre lands and the end of friction drums. This means that fibres forming the yarn core are the most highly twisted. In Dre-2 spinning, the individual fibres are blown off the opening roller and during transport to the friction drums or rollers, they become buckled. On landing and during twisting (fig 5.2) , fibres have hooked,folded,entangled and looped configurations.

Fig. 5.3 (a) Yarn formation in fine-count friction spinning

Stalder, Lord and others (Stalder.H.,and Soliman.H., Melliand Textilber, 1989 ) have established that ,when fibres are delivered (usually from a single sliver feed ) in an air-flow down a channel inclined acutely to the yarn length between the friction drums, the fibres are deposited to form a sleeve ,lofty in structure ,around the conical end of yarn (Fig.5.3). The sleeve is usually squashed within the nip of friction drums. Importantly it is the fibre sleeve that rotates by frictional contact with the drums. The yarn tail is found within the sleeve, but it does not rotate, the forming yarn length is only pulled away with the velocity of the delivery rollers.

15 The fibres are individually twisted on to the sleeve during their deposition. The fibers may be deposited, preferably, at the interface of the sleeve and drum surface (fig.5.3 b.) or directly on to the fibre sleeve (fig.5.3 c).

b) C)

) d) Fig .5.3) Yarn formation in fine-count friction spinning The hypothesis is that the leading end of a fibre makes first contact with the friction drum surface and the momentum of the trailing end causes the fibre length to fillip over its leading end as the fibre is being twisted into the rotating sleeve. This fillip- over action tends to give some degree of fiber straitening and results in the sleeve fibers having an S –twist helix. The preferred state of fibre deposition at the sleeve drum interface is obtained by employing only one perforated drum with applied suction, the

16 other drum (friction drum) being a solid surface and by poisoning the exit of fibre of the fibre transport channel close to the interface (fig.5.4).

Fig. 5.4 Fibre deposition at the sleeve-friction drum interface Fibre ends projecting from the rotating sleeve provides the means of capturing the leading ends of the depositing fibres for the later to be twisted on to the sleeve. Fibres may also be captured as illustrated in fig.(5.5)

. Fig.5.5 Fibre capture by rotating sleeve At (a), the fibre approaches and is pulled into the interface of the sleeve and the first drum surface. The sleeve rotates at the surface velocity VY < VR1. It is, however, lightly that the torsions resistance of the sleeve is sufficiently small that slippage between itself and drum is negligible in comparison with the Dref-2 system. At (b), the fibre contacts the second drum surface, or is entangled in the sleeve, and moves towards the second interface. In positions (c) and (d), the fibre becomes twisted on to the sleeve.

17 Fig.(5.6) depicts how fibres forming the sleeve are subsequently twisted on to the conical tail of the yarn. The leading ends of the fibres in the rotating sleeve become attached to the yarn tail (figure a) and, as the yarn length is pulled away by the delivery rollers, the attached fibres are Z-twisted on to the tail (fig b). The fig. Shows a fibre with S-helix angle γ being twisted with Z-helix angle β on to the yarn tail at a point where, locally, the diameter is indicated as “d” (fig.c ) from the geometrical parameters given in the diagram, (fig c and d)the fallowing equation can be derived for the yarn twist (Stalder.H.,and Soliman.H., Melliand Textilber, 1989)

Fig.5.6 Twisting of fibres onto yarn tail

18 1 ( DTY / ) TwistR AV cos  (sin  / Y ) Yarn 2 [1 (2 dG T AV / 3) ]

Where, DR = Fiber Sleeve Diameter

TAV = Average yarn twist Y =Ratio of Drum Surface Speed to Yarn Speed

dG = Average Yarn Diameter 5.1 Mechanism of Yarn Formation:

Fig.5.7 Sketches of yarn tails during OE-friction spinning

The process of yarn formation can be visualized as follows: The fibre mass in the vicinity of the nip of the friction drums is a rotating cloud of fibres around a core. There are two views on the nature and the structure of “rotating mass” of fibres the first one is that “rotating mass” is nothing but an agglomeration of discrete fibres around a core. The other is the “rotating mass” is an annular ring of fibre sleeve onto which the fibres are gradually added externally and from which fibres are transferred to the core internally (Krause, H.W., Soliman, H.A., and Stalder.H., The Textile Institute 1998 Annual World Conference, The Textile Institute, Manchester,1998.)The former view is supported by Johnson and Lord (Johnson, N.A.G., and Lord. P.R., , J. Text. Inst. 1988) Lord and Rust (Lord.P.R.,and Radhakrishnianh.P., J.Text.Inst, 1987 ) have supported the first view and have shown with help of photography that the fibres are being loosely wrapped around the yarn tail. They have

19 prepared few sketches from the original photograph of the yarn tail and those sketches have been depicted for better understanding in fig (5.7). It can be observed that the fibres away from the yarn tip are more tightly wrapped. But according to Stalder and Soliman (Stalder.H.,and Soliman.H., Melliand Textilber, 1989 ) the separated fibres after landing on friction drums surface proceeds towards the nip where it experiences a torque generated by the friction drums. This causes the fibres to form a rotating sleeve continuously pass onto the non- rotating yarn end that rests co-axially within the sleeve. a seed yarn has to be fed initially around the stationery yarn end once one of its end gets entangled by the yarn. The yarn is withdrawn and the process continuous.

5.2 Formation of sleeves:

Fig. 5.8 Fig. 5.9

(Formation of cylindrical sleeve of fibres (Fibres transfer from feed channel to yarn) tail via fibrous sleeve)

Stalder and Soliman (Stalder.H.,and Soliman.H., Melliand Textilber,1989) have established with ultra high speed photography that there is a stationery conical yarn tail and it is surrounded by a more or less cylindrical sleeve of fibres as shown in (fig 5.8 and 5.9) . It us reported that fibre sleeve rotates on the perforated drum particularly slippage free, without any movement in the direction of the axis. The actual yarn tail is

20 formed within this sleeve. It is difficult to perceive the idea that the sleeve, which is not positively gripped between the drums, can rotate in slippage free state. Lord and Rust (Lord.P.R, and Rust. J.P., J.Text.Inst, 1990) has suggested the existence of torpedo shaped fibre sleeve that contains the tapered yarn tail.

6.0 Fibre consolidation and twisting:

Fig.6.0 Theoretical torque distribution along rthe length of fibre assembly

The two friction drums rotate in the same direction but at the close vicinity they move in opposite directions. The vector diagram of forces has been shown in fig (6.0) .The amount of torque can be calculated as under

RR1 2  Torque( y / 2)    F FF  Where, y = local yarn diameter and other notations are as stated earlier and F= the force produced due to air flow over the yarn

The geometry of the system has a great influence in deciding the ratios R1/F and R2/F and so the separation between the torque rollers (g) and the local yarn diameter (y). In open –end friction spinning the yarn tail is tapered and fibres are added continuously to it. Thus yarn formation takes place. There is a mass equilibrium between the arriving fibres and the outgoing yarn, but the linear density of the forming yarn is not constant.

21 Depending upon the local force gradient, the yarn dimension changes along the length of the formation zone .The theoretical torque distribution is shown in figure (6.0). But in practice, the effective yarn diameter in the formation zone, the coefficient of friction and the reaction forces along the length of the friction drums are not constant and sometimes unpredictable. This causes a deviation in the torque distribution from the ideal theoretical one. Consequently, the twist distribution along the length of yarn varies .The twist distribution also gets affected by the changing stiffness of the fibrous material and various annular layers of fibres in the yarn are likely to have different twist level.” The behavior of the tip of the tail was interesting. Torque built in the tip of the tail but was released by intermittent slippage, which caused the tip to rotate relative to the rest of the tail for short period of time. There was a sort of slip/stick phenomenon and when the slip part was operative, the tail was quite unstable “ It has been reported by Lord & Rust (Lord.P.R.,and Rust .J.P., J.Text.Inst. ,1990). 6.1 Fibre integration at yarn tail:

22 Fig. 6.1 Fibre transfer process from rotating sheath to non-rotating core

It is important to know how the fibres from sleeve get transferred to the yarn tail continuously. Wrapping of fibres around the open end of the yarn would need a relative speed to be generated between the sleeve and the yarn tail and establishment of a connection between them by fibre to be wrapped. There could be three options:

1) A rotating sleeve holding stationery tapered tail within it. 2) A non-rotating sleeve holding a rotating tapered tail. 3) A rotation of both sleeve and tail in the same direction but at different speeds.

Though the possibilities of first two have been stated, the third alternative has not been suggested. Krause et al.( Krause, H.W., Soliman, H.A., and Stalder.H., J.Text . Inst.

23 1991) suggested that fibres are added to the yarn tail through a transient location in the form of a rotating fibre sheath or sleeve .It is termed as epicycle theory and describes how fibre is withdrawn from a rotating sheath and transferred to a non rotating yarn end. It is said that the fibres are transferred from the perforated drum to the rotating sheath in the same way as ink in a rotary printing machine is passed to the papers. But how the non-rotating tail catches a fibre has not been suggested. This process of fibre transfer from rotating sheath to non-rotating yarn core, has been suggested by Lord and Rust (Lord.P.R.,and Rust .J.P., J.Text.Inst, 1990) and showed in fig (6.1) .It is reported that the rotating sheath has a number of fibres ends projected from the surface of the sheath .these hair strand out quite rigidly when the sheath rotates .An approaching fibre easily gets trapped as the hair is folded back and new fibre continues to be trapped until the centrifugal force straightens out the hair again .Thus fibres are added to the rotating sheath .The fibres are peeled of the inner surface of the rotating sheath and are laid on the non-rotating core attached to the outgoing yarn .The centrifugal force forms a barrier to this transfer .Once the fibre is anchored in the core ,there is probably no difficulty but when a new fibre attempts to make the transfer ,it can do so only if it is entangled with one already in transit .If the fibres are randomly placed on the inside surface of the rotating sheath some of the entanglement is likely to act so as to prevent a clear peeling action .If there were an equal number of fibre connections in both the forward and backward directions .,there would not be any pilling action .There are some disputes as pointed out by Lord and rust (Lord.P.R.,and Rust .J.P., J.Text.Inst. 1991) .They are confident that there is always a slippage between the rotating sheath or sleeve and perforated drums the claim of Krause et al(Krause, H.W., Soliman, H.A., and Stalder.H., The Textile Institute 1998 Annual World Conference, The Textile Institute, Manchester,1998.) that the sheath surface speed is particularly same as that of friction drum.

The alternative way (Lord.P.R.,and Radhakrishnianh.P., J.Text.Inst, 198) by which fibres can be captured at the yarn tail , does not require other fibres for gripping .This mechanism requires that the suction draws the fibre into the nip between the yarn tail and ingoing roller .

24 Fig. 6.2 The process of loop formation.

The different stages have been depicted in fig (6.2). As the process of self –entrapment progresses, the fibre do curl back towards the yarn in the vicinity of nip between the outgoing drum and yarn tail .It is possible to spin a loosely constructed yarn with just one roller. But in two-roll case the fibre end may then become entangled with the yarn or it may pass between the yarn tail and the second roll. A loop is thus formed below the yarn tail. “ A loop may be formed by one of the two mechanisms. First, if the yarn tail is assumed to be cylindrical, a loop can be formed when the fibre tip is caused to curl back towards the tail and comes into contact with it. The magnitude of that loop is then determined by the local shear. The relative fibre velocities f1 and f2 (the fibre velocities at the point of contact with the ingoing and outgoing rollers respectively) are related to the shear between the perforated roller and the yarn tail. If f2 > f1 the loop diminishes, if f2 < f1, the loop grows and, if f2 = f1, the loop remains constant in size. The second mechanism is related to the first .In this case, we take into account the shape of the tail, which is usually tapered at the tip and again upstream of the fibre accumulation zone .If a cone is placed between two cylinders rotating in the friction spinning process and a fibre is allowed to wrap around the cone by the action of rotating cylinders, a loose wrapping occurs at positions at which the cone diameter is less than that of the drive zone. There will be loops above and below the point of contact of the cone and the cylinders. This occurs

25 owing to the required slippage between the cone and cylinder in the regions not in the drive zone” 6.2 Structure of friction spun yarns: The structure of friction yarn is different from that of other yarns, is characterized by poor fibre orientation, buckled and folded fibre configurations and loose packing of fibres in yarns. The structure is determined by the way the fibres are assembled into yarn .The structure has a significant effect on properties .The appearance of OE-friction spun yarn is closer to ring yarn though there is a varying degree of twist present in different layers of the yarn. But Dref-3 yarns have a distinct core sheath structure where the core is false twisted (Lord.P.R., Joo.C.W., and Ashizaki.T. J.Text .Inst, 1987) and the sheath has wrapped fibres at various helix angles around the core .Alagha et al.( Alagha, M.J.Oxenhan, Text. Res. J, 1994) have reported that percentages of fibres near the yarn center is greater for yarns produced at higher speeds (i.e., high core packing density) whereas higher packing density towards the outer sections is observed for the yarns produced at lower speeds .The packing density at middle layers remains unaffected by production speeds. They also added that in general the fibre angle increases from center to surface of yarn. According to them the yarn consists of highly twisted core, but core twist exhibits a sharp reduction as production speed is increased beyond 200-m/ min. Sett et al (Sett.S.K.,Mukherjee.A., and Sur.D., Melliand Textilber, 1995) have reported that Dref-2 yarn consists of entangled fibres with a very irregular helical structure .The mean fibre helix is stated to be more widely distributed as compared to ring and rotor The fibre extent in friction yarns is found to be of the order of 50% (Lawrence.C.A, Foster.W., Wilding.M.,Howard.A and Kudo.R, J.Text.Inst, 1988.) whereas the same for ring and rotor yarns are 90- 95% and 70-80% respectively .The low average fibre extent can be ascribed to folded and buckled fibre configuration in the yarn which also account for its poor strength . 6.3 Stacked cone model of OE- friction yarns: It has been described by different researchers (Lawrence.C.A, Foster.W., Wilding.M.,Howard.A and Kudo.R.,6thInternational IZMIR TEXTILE SYMPOSIUM,1992.) that the friction yarns resemble a series of cones inside the other

26 as shown in fig (6.3) . A fibre follows a helical path originating somewhere at the surface and gradually moving towards the apex of the conical surface as in fig (6.3) . The same fibre is confined to a particular conical surface only. Rust and Lord (Realff.M.L., Seo. M., Boyce.M.C., Schwartz.P., and Backer.S., Text .Res.J. ….. ) have shown that the amount of position deviations from the conical surface is very small. In absence of such positional deviations, there will be no intermingling; crossing or bonding of fibres to give a well interlocked structure because of the shear strength between layers the structure will collapse even at lower stresses.

Fig. 6.3 A simplified model of friction yarn

6.4 Migration in Friction Yarns:

In ideal migration for conventional ring yarns, a fibre would have been a confined in particular cylindrical layer. In such a case, discrete cylindrical layers of the yarn could have been easily separated. But Morton and Yen (Morton.W.E., and Yen.K.C., J.Text.Inst, 1952) have found that in reality the fibre in reality the fibres in ring yarns actually migrate among the different cylindrical zones .This causes a good bonding or locking among the different cylindrical zones and the yarn exhibits a high load bearing capacity. But this is just not true for friction yarns because of the prevalent concept conical deposition of fibres in these yarns. This can be attributed to the fundamental

27 difference in the twisting process. The tension variation in the fibres results in migration .In friction spinning the level of spinning tension is relatively low .So, migration, as expected in ring yarns can not be achieved in friction yarns. Hence, the parameters described by Hearle et al (Gupta.B.S., Text Hearle.J.W.S., and. Res. J., 1995) to measure migration in ring yarns are not suitable to estimate migration in friction yarns.

But Alagha et al.( Alagha, M.J.Oxenhan, Text. Res. J, 1994) have a different opinion .They have reported that mean fibre position of friction yarns are quite similar to yarns produced on ring and rotor system except when friction yarns are produced at speeds higher than 200 m / min .However amplitude of migration (rms) was found to highest for friction yarns .

Stalder and Soliman (Stalder.H.,and Soliman.H., Melliand Textilber, 1989) have established with photography that the sleeve rotates on the perforated friction drum without any slippage .The twist per meter is higher inside a friction yarn than at its surface ,though it is dependent on the spinning system .But Lawrence et al.( Lawrence.C.A, Foster.W., Wilding.M.,Howard.A and Kudo.R, 6th International IZMIR TEXTILE SYMPOSIUM, 28th Oct-1st Nov. Proceedings, S43-S60,1992.) have concluded that there is a high level of twist on the surface of yarn and it gradually reduces towards the core . It is just reverse to what is claimed by Stalder and Soliman (Stalder.H.,and Soliman.H., Melliand Textilber, 1989).

6.5 Tensile Strength of friction Yarns:

The strength of friction yarns is 50-60% of ring and 70-80% of rotor yarns () The low strength of these yarns can be ascribed to a) low spinning tension b) folded or buckled configuration of the fibres c) ineffective twisting or wrapping of sheath fibres .The parameters that can influence the yarn strength are

1) Machine parameters

28 2) Process parameters and 3) Material parameters 7.0 Influence of Machine parameters: 7.1 Design of Opening Roller: The opening roller in friction spinning machine have a strong influence on yarn properties .The design of opening roller particularly clothing characteristics such as type of teeth, wire point density and surface finish of teeth is of paramount importance. Simpson and Murrey (Simpson.J., and Murray.M.F., Text.Res.J, 1979) have found that for rotor spinning a combing roller with forward rake angle 15º yielded better results in respect of fibre parallelization and yarn strength .Dyson (Dyson.E., J.Text.Inst, 1974) have reported that fibre breakage can be minimized with low wire point density of the clothing and running the opening roller at low speed .He pointed out that that optimum wire design should have a front angle of 89˚.He also added that pinned type roller has a gentle effect on fibre which reduces fibre breakage.

There may be one or two opening roller running at different speeds depending on the type of machine. In Dref-3 friction spinning machine there are two opening rollers running at a speed of 12,000 rpm (Dr. Ernst Fehrer-AG). Ulku-et-al. (Ulku.S., Ozipek.B., Acar.M., Text. Res.J., 1995)has reported that the fibre breakage increases with opening roller speed on a open-end friction spinning machine of PSL (Masterspinner). They have found a clear correlation between fibre length characteristics and opening roller speed for cotton, polyester and viscose fibres but for acrylic fibres the degree of association between these two factors is small. This means that the extent of fibre breakage or damage is dependent on the type of fibrous material. So, the selection of opening roller types and speed significantly influences the yarn properties such as strength and elongation Rust and Lord (Rust .J.P., and Lord .P.R., Text.Res.J, 1991) have reported an increase in yarn tenacity when the opening roller assembly in Dref –3 machine is replaced with suction roll and an opening roller from a Platt 881 rotor spinning system. The change in opening roller also led to improvement in fibre

29 orientation before being assembled onto the yarn tail and finally increased the fibre extent in the yarn.

7.2 Design of transportation duct: The design of transport channel and its angle of inclination with respect to the yarn withdrawal direction also influence the yarn quality .In case of rotor spinning Lawrence and Chen (Lawrence.C.A, and Chen.K.Z., J.Text.Inst. 1988) have shown that a narrow rectangular cross-section of the transport channel will improve the straightness of fibres in duct as well as in yarn and thus the yarn strength .This behavior is also expected in case of friction spinning but the extent of fibre straightness is far more poor because of the much less speed of the friction drums .This matter is further complicated because of the fact that the fibres from the feed duct can land on the ingoing drum or directly on the rotating mass of fibre called “sleeve” .By feeding fibres directly on sleeve helps to obtain higher yarn strength . The strength of the yarn is also affected by the angle of the feed duct to the yarn withdrawal direction. Rust and Lord (Rust .J.P., and Lord .P.R., Text.Res.J, 1991) have observed that with 30˚ feed angle the yarn was strongest. With 45˚, 60˚ and 75˚ there were a gradual drop in yarn strength but they were close to each other. But all them have a significant different in yarn strength compared to that the twisting or wrapping of fibres are more effective and tight though the fibre extent is somewhat less.

7.3 Design of friction drum: The twisting mechanisms of friction spinning system are of various types, which exercise a strong influence on yarn properties especially on strength. These devices (Lunenschloss.J., and Brockmanns.K.J., , Int.Text.Bul. 1989) can consist of a) two perforated suction drums b) one perforated suction drum and one solid blind drum cc) one perforated suction drum one perforated but without suction and d) one perforated suction drum with a supporting disk etc. The system with one perforated friction drum and one solid roller exhibit lowest yarn tension (Stalder.H.,and Soliman.H., Textile Asia, 1987).So , the kind of twisting device has a strong influence on yarn tension and consequently on yarn strength.

30 7.4 Position of the suction slot and duct exit point:

The increasing air force on the yarn ((Stalder.H.,and Soliman.H., Textile Asia, 1987) tail causes the yarn tension to increase . Higher setting of the feed channel relative to the blind friction drum also increases yarn tension as the air thrust on the yarn tail increases yarn tension as the air thrust on the yarn tail increases. This is shown in fig (7.0 a). The position of suction slots inside the perforated drum affects the air force and consequently the yarn tension. This is shown in figure (7.0 b) .If the lower edge of the suction slot is above the normal position (e> 1.5), the air stream has a tendency to bypass the yarn tail and thus reducing the yarn tension. On the other hand, if the suction slot is positioned below the level of yarn end, the air pressure increases and thus the tension on yarn tail. The increase on yarn tension leads to higher strength of the yarn produced.

8.0 Influence of process parameters: 8.1 Friction ratio: Stalder and Soliman (Stalder.H.,and Soliman.H., Melliand Textilber, 1989) have reported that friction ratio ( f) is an important parameter in friction spinning .It is defined as the ratio of friction drum surface speed to tarn withdrawal speed .The twist in yarn increases with increasing (f) values i.e. when friction drum speed is increased for a constant yarn delivery speed. They have claimed that f values cannot be reduced beyond 1.5 for all practical purposes. Brokmmans and Lunescholoss (Brockmanns, K.J, and Lunenschloss,J., Int. Text Bul, 1984) have also mentioned that yarn strength its very much dependent on friction ratio. They added that for coarser cont yarns twist could be influenced significantly by friction ratio due to better friction transfer. 8.2 Yarn delivery speed: Lawrence et al (Lawrence.C.A, Foster.W., Wilding.M.,Howard.A and Kudo.R., 6th International IZMIR TEXTILE SYMPOSIUM, 28th Oct-1st Nov. Proceedings, S43-

31 S60,1992. ) have reported that for a constant optimum twist, the yarn tenacity for viscose fibre gradually decreased with higher production i,e delivery speed . Alagha et al (Alagha, M.J.Oxenhan, Text. Res. J, 1994) have found reasonable co relation between change in yarn tenacity with production speed and emigrational characteristics. They have pointed out that increase in production speed are related to change in yarn structure and so to strength. Fibre helix angle reduces whereas yarn diameter increases as the production speed is increased. From the same study it has been pointed out that a higher production speed leads to higher core packing density. Also, at low delivery speed, the packing density at the outer surface is relatively high. The yarn consists of high twisted core but this abruptly reduces as the production speed is beyond 200 m/min. 8.3 Friction drum speed: The friction drum(s) are the elements in the machine that imparts twist in the yarn. It has been reported by Padmanabhan and Ramakrishna (Padmanabhan.A.R., and Ramakrishnan.N., Ind.J.F.Text.Res. 1993) that strength of Dref-3 yarns increases with increase in friction drum speed from 3000 to 5000 rpm. It is understood that the yarn twist is directly influenced by the speed of friction drum for a constant delivery speed. As drum speed is increased, the twist in the yarn becomes more.

Fig. 7.0 Setting of a) Transportation duct b) Suction slot

32 8.4 Suction pressure:

Konda et al (Konda.F., Okamura.M, Merati.A.A., Text. Res. J. 1996 ) have reported that the effect of suction pressure (in the yarn formation zone ) on yarn structure and mechanical properties .An increase in air pressure leads to a linear rise in yarn tension and so yarn tenacity. When the air suction pressure increases, the yarn is held more firmly against the surface of friction roller and the amount of frictional forces increases and thus the yarn tension. The mean fibre speed along the feed channel increases as the suction pressure increases. Thus the fibre orientation inside the feed channel and finally the fibre length utilization in the yarn improves .As a result, increased fibre packing density and fibre extent produce stronger yarn structure. They have also reported the breakage behaviour of yarn produce at lower suction pressure is dominated by fibre slippage. Whereas the yarn produced at high suction pressure breaks sharply indicating dominance of fibre breakage over slippage.

8.5 Spinning tension: Spinning tension is one of the most critical parameters and it is highly variable in friction spinning. The spinning tension in friction spinning is in the range of 5-15 cN, which is considered too low, compared to ring and rotor spinning. Due to low spinning tension, yarn strength as well as yarn breakage during spinning is less. Stalder and Soliman (Stalder.H.,and Soliman.H., Melliand Textilber, 1989 ) have shown mathematically as well as verified experimentally that the spinning tension is influenced by the factors viz, air force on the yarn tail, co efficient of friction between metal and fibre and yarn tail diameter . They have also commented that these variables are not easy to measure and difficult to vary during spinning operation. As the spinning tension is low, sufficient friction among the fibres is not developed and consequently yarn strength is low. Konda et al (Konda.F., Okamura.M, Merati.A.A., and Youki.T., Text.Res .J,1996) have pointed out that spinning tension is dependent upon suction pressure yarn diameter ,width of the suction slit and yarn to friction roller drag coefficient . They have shown that yarn increases with increase in air suction pressure because at higher suction

33 pressure yarn is held more firmly to rotating friction surface and as a result friction between yarn surface and the surface of friction roller increases.

8.6 Yarn diameter: Spun yarn diameter in general depends on fibre properties, method of twisting, amount of twist, packing density and processing parameters. In friction spinning, the yarn tail in the yarn-forming zone is rotated by friction drum. Merati et al ( ) have reported that as the yarn diameter decreases, the length of yarn subjected to frictional forces is reduced which reduces torque accumulation and yarn tension resulting in loss of spinning tension. Thus yarn diameter plays important role in yarn tension during friction spinning. When the yarn diameter is less then the gap between friction drums, lack of contact between the yarn and both friction surfaces simultaneously results in lesser magnitude of frictional force generation. Though the yarn tail is voluminous, the tip of yarn in this zone is unstable (Lord.P.R.,and Radhakrishnianh.P., J.Text.Inst, 1987) . Moreover, fiber in this zone do not have sufficient cohesion and the forces experience by the yarn tail is negligible (Merati.A.A., Konda.F., Okamura.M., and Maarui.E., Text.Res. J, 1997).

Yarn tension increases with yarn diameter but the relationship is not linear. At smaller diameters, yarn tension increases slowly till to the. Level of the gap between friction drums and then increases sharply. Lord et al (Lord.P.R., Joo.C.W., and Ashizaki.T., J.Text .Inst, 1987) have also pointed out that ( y-g) is an important factor so far torque is concerned, where g is separation or gap between friction drums and y= the local yarn diameter at yarn formation zone.

9.0 Influence of Material parameters: 9.1 Coefficient of friction: In case of OE –friction spinning system consisting of one perforated drum and one blind drum, it has shown that by increasing the coefficient of friction between fibre and perforated drum on relation to that of fibre and the blind drum (Stalder.H.,and Soliman.H., Textile Asia, 1987) causes the torque available for twisting to increase .Thus spinning tension and finally the yarn strength increases.

34 9.2 Fibre fineness: The study taken by Lawrence et al. (Lawrence.C.A, Foster.W., Wilding.M.,Howard.A and Kudo.R., 6th International IZMIR TEXTILE SYMPOSIUM, 28th Oct-1st Nov. Proceedings, S43-S60,1992 ) in which he concluded that the OE –friction yarns made from finer fibres are stronger and more extensible than that of from coarser fibres .This high strength and extensibility can be ascribed to the increased number of fibre cohesion of the yarn .Fibre fineness also influences the twist transmission through friction drums .For air-jet yarns , finer fibres produce stronger yarns as reported by Bhortakke et al (Bhortakke, M.K.,Nishimura,T.,Matsuo, T., Inoue, Y., and Morihashi, Text. Res. J1997). 9.3 Fibre length: The friction yarns have poor fibre length utilization in the yarn. This causes the yarn structure to be weak as far as load-bearing capacity is concerned .The folded, hooked and deformed fibre configurations are due to some unavoidable design problems. The extent of fibre folding can be influenced to some extent by manipulating few machine and process parameters such as suction pressure, design of transport channel, design and speed of opening roller etc. Lawrence et al (Lawrence.C.A, Foster.W., Wilding.M.,Howard.A and Kudo.R., 6th International IZMIR TEXTILE SYMPOSIUM, 28th Oct-1st Nov. Proceedings, S43-S60,1992) have reported that for OE-friction spun yarns maximum tenacity is achieved around 40 mm length of viscose fibres . Fibre length is an important parameter for fascinated yarns. Santjer (Santjer.H.J., Textile Asia, 1991) has reported that longer fibres in core increases yarn strength for air-jet yarns 9.4 Type of fibre material: The actual material properties can also influence the process of friction spinning or the properties of final yarn to a certain extent .The intrinsic fibrous material properties viz. frictional coefficient, bending and tensional rigidity, resistance to mechanical process etc can play an important role in friction spinning.

Materials and methods:

35 This chapter deals with material specifications, sample preparations and test methods. Fibres: Cotton fibres of fibre length of 35mm length (mean length after 16% noil extraction) and fineness 3.97 micronaire and Polyester fibres of 44mm and fineness 1.2 denier were used for making the yarns. Preparation of yarn sample: Dref-2 friction spun yarns were produced from Dref-3 machine on Dref-2 mode. The cotton and polyester slivers of 3.38Ktex and 2.87Ktex were used for the above purpose. Three sets of 7 yarn samples of 98.4, 59 and 42.2 tex yarn were produced keeping drum speed 3100rpm and delivery speed of 110 mpm. Yarn samples were categorized as A, B, C, D, E, F, G. & H.

A Structure B Structure C Structure D Structure

E Structure F Structure G Structure H Structure

36 Structure A: Composed of 100% polyester Structure B: Three inside layers of polyester and two outside layers with cotton Structure C: Three outside layers with cotton and two inside layers of polyester Structure D: Three outside layers with polyester and two inside layers with cotton Structure E: Three inside layers of cotton and two outside layers with polyester Structure F: Three cotton layers sandwiched between two layers of cotton Structure G: Three polyester layers sandwiched between two layers of polyester Structure H: Composed of 100% cotton The delivery speed of drafting unit 2 was calculated as V V  A Nm n  m

VA = Delivery speed

Nm=Metric count to be spun n=Number of slivers fed m=gm/mtr of fed sliver V=Drafting speed

Sample Arrangement of V2 V2 m/min V2 m/min sliver in drafting (m/min) unit II A PPPPP 0.726 0.442 0.318 B PPPCC 0.679 0.413 0.297 C PPCCC 0.657 0.4 0.288 D CCPPP 0.679 0.413 0.297 E CCCPP 0.657 0.4 0.288 F PCCCP 0.657 0.4 0.288 G CPPPC 0.679 0.413 0.297

Yarn Testing

37 The following tests were carried out to assess the effect of sheath composition on twist and tensile behaviour of Dref-2 friction spun yarn. All the tests were performed on sample conditioned in a standard atmosphere of 65  2% RH and 27  2° C Twist A mechanical twist tester was used to calculate the twist. The yarn samples 0f 10- inch length were twisted in the direction of original twist . Twisting was continued till the break and the number of turns required to break the yarn (N1) was noted . The test was repeated but this time twisting in a direction opposite to the direction of original twist and again the number of turns required to break the yarn (N2) was noted. The test was repeated 40 times for each sample. The twist was then calculated by using the following relationship ()NN TPM 2 1 39.37 2

Tensile properties Zwick universal tensile tester was used to evaluate the tensile properties . Tenacity and breaking extension of Dref-2 yarns were measured at 150 mm/min cross head speed with 500 mm gauge length. The average of 40 test results were taken for each sample.

Structural integrity The decay (a measure of structural integrity of the fibre assembly in the yarn) was evaluated by carrying out the cyclic loading test after 25 cycles of loading and unloading

38 under constant load condition (20% breaking load of the weakest yarn in the group) on a Zwick universal tensile tester . The decay was calculated using the following relation:

AA25 1 Decay%  100 A1

Where A1- Area of hysteresis curve at first cycle

A25 -Area of hysteresis curve at twenty fifth cycle

Results and discussions (FOR TWIST) The value of twist in different structures is given in table1and figure( ) is the graphical representation of the results . From the table it is seen that the

39 Twist of Yarns

300

250

) 200 M

P 98.4 tex T ( t 150 59 tex s i w 42 tex T 100

50

0 A B C D E F G H structure

Fig Variation of Twist for different structures

Table :1

40 Structure Twist (TPM) 98.4 tex 59 tex 42 tex A 174.5 231.7 267.4 (5.4) (5.9) (8.9) B 160.2 220.8 259.2 (12.4) (4.9) (4.4) C 156.9 208.3 236.4 (7.1) (5.0) (8.9) D 170.4 209.2 241.1 (5.9) (8.9) (8.0) E 162.7 204.5 241.5 (5.3) (5.5) (8.5) F 156.2 212.6 248.5 (10.9) (6.4) (6.5) G 166.8 229.7 266.7 (5.4) (6.7) (7.6) *Values inside the bracket indicate CV %

From the table it is seen that the twist in the structures range from 156.2 tpm to 267.4. It is clear from the Table 1 and Fig 1, that the twist in structure A is the highest (174.5) at all count level. The level of twist reduces as the outer polyester layers are replaced by cotton as in the case of structures B & C. Similar decreasing trend in twist is also observed in structures D and E, where the inner polyester layers are replaced by cotton .When the cotton layers are placed in between two innermost and outermost polyester layers as in the structure F, the twist reduces and when the innermost and outermost polyester layers are replaced by cotton (Structure G) the twist reduces as compared to the structure A, though the reduction is not very significant. Similar observation is valid for other counts as well. The twist is seen to increase from 174.5 to 267 for structure A, from 160 .2 to 259.2 for structure B,156.9 to 236.4 for structure C,! 70 .4 to 241.5 for structure D,162.7 to 241.5 for structure E,156.2 to 248.5 for structure F and 166.8 to 266.7 for structure G. As the count of yarn become finer, the amount of twist friction spun yarn also increases.

41

The amount of twist generated in each layer and hence the overall twist in an open-end friction spun yarn will depend on the following factors: a) The length of time, the fibres originating from a fed sliver stay in the twisting zone. Fibres originating from the first sliver will have more residence time than the fibres originating from the fifth sliver. So, the initial layer of fibres should possess more twist. Thus it can be postulated that the twist retention by a layer would reduce as one moves towards the surface of the yarn, where the fibres receive minimum twist owing to least time of residence at the twisting zone. b) Friction condition of the incoming fibre with metallic drum surface and with the fibres surrounding it i.e., at the interface of the layers. The friction condition with the drum will decide the extent of torque transfer while the friction condition with surrounding fibres will decide the degree of torque retention by the layer deposited previously. c) Amount of suction pressure inside the drum. The suction pressure decides the pressure with which the fibre assembly will be pressed on the drum surface during the torque application. The normal pressure acting on fibres as a result of pressure difference due to suction acting through the perforations of the friction drums .The relationship of pressure difference  p  an d air flow  F  across a fibre mass packed into a constant volume is given by (Sinha S.K., and Chattopadhyay.R., Ind.J.Fib and Text. Res, June 2006,.]

p k s2 L 2   F  AL m where k is the factor of form ;, the viscosity of air; s , the mean specific surface of fibres ; m, the total mass of fibres;  , the fibre density; L ,the length of the section of the chamber in which fibres are packed ; and A , the area of the section .

42 When all other parameters are constant, the pressure difference  p  would be proportional to total specific surface area of fibres. The specific surface area , in turn is inversely related to fibre diameter. Finer the fibre , the lesser is the diameter and more would be the specific surface area. In the present cotton-polyester blended yarn, the polyester is finer than cotton. Hence, more is the proportion of polyester, more will be the specific surface area of fibre agglomerates on the friction drum and hence more would be the pressure difference d) The diameter of the rotating sleeve in the nip of the two rotating drums. In case of structure A (100 % polyester), the coefficient of friction (fibre to metal ) for polyester is more than the coefficient of friction (fibre to metal ) of cotton, the fibre to fibre friction at the interface of each layer will also be more as the fibres are of same generic nature, so the resistance to untwisting of previously laid layer will be more. Hence the overall twist in the yarn is more. In case of structure B the last two layers are of cotton. While the last three layers are of cotton for structure C. Here the late deposition of cotton layers causes less twist generation in them. The frictional coefficient between cotton and polyester is also less and the fibre to fibre friction and fibre to metal friction for cotton is also less. These cotton layers will not be able to trap the generated twist in the previously laid polyester layer and the twist in cotton layer will also be less because the normal force which will be acting on them is less as compared to that on polyester, as they are coarser. The fibre length of cotton being less may hinder the wrapping of the structure. So , the twist in structure B reduces as compared to structure A and it reduces further with additional numbers of cotton layers in structure C for similar reasons. It may also be mentioned here that last cotton layers generate less twist and there retention is also less the resultant effect is less realization of twist in the yarn. In case of structure D and E , the initial layers are of cotton. As these cotton layers arrive the friction drum first, the generation of twist on them will be more and their untwisting

43 will be resisted by the subsequent polyester layer. So, the structure D posseses comparable twist as in structure A. However, in case of structure E the amount of twist in last two polyester layer might be less due to there short residence time. Hence, the overall twist in structure reduces with further increases in cotton component. The twist in structure F, were three cotton layers are sandwiched in between two polyester layer is less than structure A. Though the initial polyester layer can generate more twist in it, but the last layer of polyester may not be able to generate much twist due to less residence time which may be insufficient in restricting the untwisting action of the previously laid cotton layers due to the following reasons: 1) The inner cotton fibre layers will not be able to generate much twist as compared to polyester layer being placed at that position. 2) The frictional condition of the fibres at the interface of the last two layers may be insufficient in restricting their untwisting. 3) The last polyester layer itself will posses less twist. In case of structure G the twist increases in comparison to structure F. Although the initial and final layers are of cotton but the presence of intermediate three layers of polyester is probably the reason for the structure showing higher twist as compared to structure F. One interesting point to note here that, the twist in structure C and F are same when the count is coarse. The presence of two polyester layers at the 1st and 2nd layers are likely to receive more twist owing to their feed position, while three cotton layers at 3rd, 4th and 5th position in the case of structure C might have caused to yield moderate twist value. In case of structure F contribution from the outermost polyester layer in restricting the untwisting of the inner cotton layers might have contributed in generating moderate twist as in structure C. Twist has been found to increase as the yarn becomes finer. Table 2 Tenacity of Yarns

44 Structure Tenacity (g/tex)

98.4 tex 59 tex 42.2 tex 7.3 10.1 9.8 A (12.2) (15) (13.8) 9.2 10.2 9.9 B (9.5) (12) (10.6) 9.1 9.2 8.9 C (8.4) (9.8) (9.2) 8.9 9.4 8.9 D (7.4) (10.4) (10.5) 9.0 9.4 9.8 E (6.1) (8.8) (16.9) 9.6 9.5 9.2 F (7.6) (7.1) (9.8) 7.6 9.1 8.2 G (11.3) (10.4) (11.2) 8.5 9.5 9.4 H (5.8) (7.7) (9)

Tenacity of Yarn

12

10

) 8 x e t /

g 98.4 tex ( y t i 6 59 tex c a

n 42.2 tex e T 4

2

0 A B C D E F G H Structure

Figure-

45 References 1. Alagha, M.J.Oxenhan, Influence of Production Speed on the Tenacity and Structure of Friction Spun Yarns, Text. Res. J 64(4), 185-189 (1994) 2. Alga, M.J.Oxenhan, The use of an Image Analysis Technique for Assessing the Structural Parameters of Friction Spun Yarns, J. Text. Inst.85 (3), 383-388(1994) 3. Artzt,P., Muller, H., Joas, W.,and Wittmann M., Influence of the Fibre Length and Fibre Fineness of Polyester and Model Fibres on Spinning Stability and Yarn Properties with 32mm Rotor Diameter , Milliand Textilber (English part) 3/92, E78-E80(1992) 4. Barella, A., and Manich, A.M., Friction Spun Yarns Versus Ring and Rotor Spun Yarrns: Resistance to Abrasion and Repeated Extensions, Text. Res.J 59(12),767- 769(1989) 5. Bhortakke, M.K.,Nishimura,T.,Matsuo, T., Inoue, Y., and Morihashi, T., High Speed Yarn Production with Air-Jet Spinning: Effects of Some Fibres Parametrs , Text. Res. J .67 (2), 101-108(1997) 6. Booth, J. E, : Principles of Textile Testing, Newness-Butterwoths & Co., 1974 7. Brockmanns, K.J, and Lunenschloss,J., Friction Spinning Analyzed, Int. Text. Bul. Yarn Forming 2/84, 5-23 (1984) 8. Brockmanns, K.J, and Lunenschloss,J., Friction Spinning Analyzed, Int. Text. Bul. Yarn Forming 3/84,19-32 (1984) 9. Brockmanns, K.J, Posiibilities of OE –Friction Spinning, Textile Month , Dec,10- 16(1984) 10. Chattopadhyay, R., Influence of Plying on the Tenacity,Breaking Extension and Flexural Rigidity of Air- jet Spun Yarns., J. Text. Inst.88 (1) , 76-78(1997) 11. Chattopadhyay. R, Salhotra. K. R, Dhamija.S., and Kaushik.R.C.D., Influence of Friction Ratio on Quality of Friction Spun Yarns, Ind. J. F. Text. Res, 23,131- 135(1998)

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