YARN STRUCTURE AND PROPERTY RELATIONSHIP FOR FRICTION SPUN YARN

Introduction Textile materials are non homogeneous materials. If we consider over the fabric, it consists of two sets of threads embedded together to frame a structure termed as fabric. If we consider the yarns, they are composed of large number of fibers in its section in its own order. The yarn has to satisfy the customer or user to the maximum extend. The life of the yarn is determined by its strength value and the performance of the yarn is predicted with respect to its structure and other properties. Quest over the structure property relationship starts from the below said factors,

• Structure of yarns.
• Surface characteristics and Geometry

Structure of yarn mainly deals with the twist and number of fibers in its cross section and its arrangement. Surface and geometry means idea about the individual fiber and its role on structure, surface as well as sectional related behaviors.

In general yarn properties are evaluated under the following factors,

• Performance of yarn during weaving / subsequent processes.
• Surface appearance, mechanical properties, visual and aesthetic of end product. The process like weaving and knitting require tensile properties, the variation of breaking strength give a detail over the weak places (2). The yarn friction, hairiness, and compression are also playing an important role over the end product's properties. The fabric appearance can be giving an impact by the irregularities in the yarn.
  Analyzing the structure and property needs idea over the formation of its structure, hence for this we need to answer for the following,
  How the friction spun yarn is produced? (concept of formation of yarn)
• Components of friction spun yarn?

Role of these components over the properties.

• Principle of operation:

Friction spinning is an open end or a core type form of spinning, in which the yarn formation takes place with the aid of frictional forces in the spinning zone.

• Opening and individualization of fibers from a sliver or slivers.
• Reassembling of individualized fibers.
• Twisting of the fiber assembly.
• With-drawl of final spun yarn. As shown in fig,
  
  

  The feed materials are fed to the machine as two components, the main process involved here are, the opening, stretching, supplying fibers to drum, formation of yarn and winding.
  The structure of friction spun yarn is shown in fig,
  

It comprised of concentric layers of fibers having core form at center. The yarn formation is represented in fig. ( 2, 5 ).

The opened fibers from the end are fed to the friction drum, where the fibers are formed as yarn in the form of concentric circles. This concentric layer formed by the system of feeding of slivers. The yarn formation takes movement as the take up takes place. Hence the adjacent opened layers from the slivers are layered over the formed yarn. Hence this leads to the formation of concentric layers. This we can study by using the different colours of slivers as feed (1).

The schematic view of concentric layers is shown below which represents the flow of twist in that zone, Due to this structure it's very difficult for the fibers to migrate from their position. The fibers in the core are packed compactly. The twist level in the structure is depending on the time or position of the fibers in the friction drum from its feeding to withdrawal. Due to this the core fibers are having more twist than the others. Hence this is the reason behind the compactness of core (3).

The fibers are fed to the drum by the air blown technique. Due to this technique the fibers may get disturbed while feeding to the drum and passage. When the fed to drum the fibers may become buckled, entangled, looped structure. Hence the suction technique is better than the blown technique (3).
YARN STRUCTURE:
A friction spun yarn is differing from the ring and rotor by its internal structure. The structure comprises of the lesser orientation and loose packing of the fiber in its cross section. The structure of the yarn is differing with respect to the method of yarn formation i.e. DREF – 2 or DREF – 3. Hence the structure of core sheath type and friction type yarns will discuss here in detail.
Open end friction spun yarns: (DREF – 2)
The internal structure of an open end spun yarn comprises less oriented, buckled, folded fibers in its cross section. The fibers are packed loosely in its cross section due to its low tension during the yarn formation (3, 9).
Fiber migration:
In ring spun yarns the fiber migrates from the core towards the surface and back towards the core many times along their length due to the variation in the tension and path length in the yarn formation zone. In friction spun yarn the mechanism which causes migration in ring spun cannot be present and the fibers are fed with out difference in tension. Tension while assembling the yarn is too low. The fibers are more or less individualized by the opening roller hence reduction over the friction also gives less tension over the fibers (3, 2)
Model for the structure of open end spun yarn:
The structure of friction spun yarn is similar to that of stacked series of cones one inside the other. It is assumed that every fiber originates somewhere near or the surface and follows helical path with respect to one of the conical surfaces. So the structure is weak enough due to the lack of cohesiveness and shear strength between the successive layers (3, 2).
Core sheath type yarn:
DREF 3 friction spun yarns have two distinct components one is core and other is sheath. The core consists of filaments or bundle of fibers which is false twisted. The sheath is wrapped helically over the core. Thus exhibits effect over the properties. The strength of this structure is depending on the magnitude of radial pressure exerted by the sheath to the core and on the frictional characteristics of both core and sheath (15).
Structure of the core and Structure of the sheath:
The core in the friction spun yarn was virtually twist less and considered like a parallel bundle of fibers. The research shows that the core fibers are having entrapped false twist. The helix angle is less for the core fibers. Some times it exhibits the straight and twisted configuration. In case of continuous filament as core, and the sheath becomes important factor which determining the yarn properties. In the DREF 3 yarns the sheath fibers are wrapped helically over the core and exhibits Z twist. Due to the different feed positions, the sheath fibers are expected to have varying structural parameters (3).
Yarn characteristics:
Yarn characteristics can be discussed with respect to the following variables (3),

• Influence of yarn delivery speed.
• Influence of spinning drum speed.
• The friction ratio.
• Suction air pressure.
• Core sheath ratio.
• The yarn linear density.
• The spinning tension.

Every process parameters has its own effect over the product. The speed of the spinning drum influences the amount of twist inserted. But in practice twist insertion has its limitation over the speed of the drum. Increasing the drum speed the diameter of the yarn reduces and tenacity increases. Decrease in diameter occurs due to compactness of the yarn (10).
Fig: Relation between delivery speed and yarn count (3)

Tenacity and breaking strength of friction spun yarn decreases with increasing the delivery speed. The yarn twist becomes lower at higher delivery speeds due to increased slippage (14). The yarn twist also depends on the factor said to be 'friction ratio'. It means the ratio of spinning drum surface speed to the yarn delivery rate (14).

Suction air pressure also influences over the twist. Higher air pressure causes the fibers to get firmly attached with the drum. This causes high friction between them. So to over come this it needs increased torque, as a result the amount of twist, yarn tension and twisting efficiency are considerably increased. The latest research shows that decrease in air pressure increase the strength variability as well as mass variability which Results a weaker and more irregular yarn. Found by using Uster Evenness Tester.

Core sheath ratio plays a role over the strength of the yarn. Researches are carried out by varying the ratio of sheath – core and type of material. PES-CO core – sheath yarn shows a drop in strength when the sheath proportion was reduced. Some work shows that at 60-40 ratio we can achieve maximum tenacity (15).

Frictional contact between drums and its diameter also influences the strength and elongation due to the less torque generation. DREF 3 yarn exhibits that decrease in yarn tenacity as the yarn becomes finer. Finer yarns make less effective surface contact with the drum which reduces the intensity of wrapping the core by the sheath and thus results in lower yarn tenacity (3). The drum and the yarn diameter are shown in fig.

The influences of fiber parameters:

• Fiber friction


• Strength


• Fineness


• Length


• Cross section shape

The level of fiber friction influences more on the yarn properties. Unlike ring spinning we can't control the fibers while twisting the fibers. The twisting rate is largely depends on the friction between the fibers and the drum surface. Fiber assembly is subjected to two kinds of friction, namely, sliding and rolling friction. So there should be both inter fiber and fiber to fiber friction necessary to achieve the well twisted structure. The decrease in the friction causes the following,

• Decrease in tenacity.
• Reduction in yarn twist.
• Loss in yarn integrity.

The minimum number of fiber required in friction spinning is higher. Finer fibers are more prone to damage during opening and may get buckle. Longer fibers are also more prone to get damage, may lap around the opening roller and also exhibit a higher buckling tendency in the spinning zone. Fibers of ribbon like section results a higher yarn twist than those with circular section.
YARN PROPERTIES:
The various literatures gives details over the comparison of yarns in terms of raw materials, spinning condition, machine variables etc.
Open end yarns are possessing only 60% of the tenacity of ring spun yarns; Due the migration behavior of the yarn. The inward to outward and outward to inward migration does not exist in this structure. (9, 3)
Fig: The distribution of tenacity

Fig: comparison of strength

The strength of friction spun yarns is 10% to 15% lower than that of rotor yarns. The breaking elongation was observed to be equal for all three. The mass irregularity is higher in the friction than the other. Fiber orientation and arrangement are poorer than ring and rotor spun yarns. Friction spun yarns are more hairy all other. Evenness is satisfactory. Number of imperfection is high. Tendency towards snarl. Stiffness as compared with rotor spun yarn. Huddle in structure gives better cover (3, 11, 8).
References:

1. Klein series – short stable spinning.
2. Fundamentals of spun yarn technology- by carl.c.lawrence.
3. Friction spinning – The Textile institute.
4. http://www.textileinfoonline.com/
5. http://www.textileindustries.com/
6. http://www.fischerwolle.com/.
7. w.oxenham, zhu, leaf, "observation on the tensile properties of friction spun yarns" 1992, J.T.I.
8. P.R.Lord, C.W. Joo, T.Ashizaki, "The mechanics of friction spinning", 1987, vol4, J.Text.Inst.
9. A.R.Padmanabhan, "A comparative study of the properties of cotton yarns spun on DREF3, Ring and Rotor spinning systems", 1989, vol4, J.Text.Inst.
10. M.J.Alagha et al, "Influence of production speed on the tenacity and structure of friction spun yarns" 1994, vol64 (4), Textile Res J.
11. A.K.Sengupta et al "Influence of spin finish and core sheath ratio on the mechanical properties of friction spun yarns", 1992, vol(73), Melliand Textilberichte.
12. Kalyanaraman-AR, Frictional behaviour of DREF yarn, air-jet yarn and ring-spun yarn with reference to textile contact surfaces", Indian-Journal-of-Textile-Research. 1988; 13: No.2, June, 92-94.
13. Chaudhuri-A; Basu-G, "Studies on the properties of dref-spun acrylic yarns", Indian-Journal-of-Fibre-and-Textile-Research. 1998; 23(1): 8-12.
14. Chattopadhyay-R; Chakraborty A, "Influence of additional twist on tensile behaviour of friction-spun yarns", Indian-Journal-of-Fiber-and-Textile-Research. 1998; 23(4): 223-228.
15. Ishtiaque-SM; Salhotra-KR; Gowda-RVM,"Influence of filament core surface structure on tensile properties of DREF-3 yarns", Indian-Journal-of-Fiber-and-Textile-Research. 2002; 27(1): 18-24.