Malaysian Knitting Manufacturers Association

Newsletter: May 2004

CHAPTER 1 CHEMISTRY AND TECHNOLOGY OF TEXTILE DYEING

1.1 Yarn Types and Structure
Textile yarn can consist of a single fibre of monofilament or many fibres of multi- filament. The textile manufacturing process begins with the production of filaments or yarns. Yarn is a generic term for continuous strand of textile fibres, filament, or material in a form suitable for knitting, weaving, or otherwise intertwining to form a textile material. Yarn is a product of substantial length and relatively small cross-section consisting of fibres and/or filament(s) with or without twist. Yarn occurs in the following forms: a number of fibres twisted together – spun yarn; a number of filaments laid together without twist – a zero twist yarn; a number of filaments laid together with a degree of twist multiple filament yarn; a single filament with or without twist – monofilament yarn; or a narrow strip of material, such as paper, plastic film, or metal foil, with or without twist, intended for use in textile construction. Assembles of fibres or filaments are usually process during the stages that lead to the production of yarn, e. g. tow, slubbing, sliver, or roving. Except in the case of continuous-filament or tape yarn, any tensile strength possessed by assemblies at these stages is generally the minimum that can hold them together during process.

1.1.1 Yarn from Staple Fibre:
The formation of a yarn by a combination of drawing or drafting and twisting prepared strand of fibres, such as roving is term as staple yarn. Manufactured staple fibres are produced over a similar range of length. They are normally prepared from extruded filaments by cutting or breaking into length suitable for the subsequent processing system or end-use; they may also be crimped. Staple fibres from natural and manufactured can be spun into yarns by draw out and twisting into staple yarns and threads. Staple yarns have more surface texture and softness. A yarn can be made by blending several different types of fibres in the form of hybrid or blended yarns for unique properties. Thread is the result of twisting together in one or more operations two or more single or folded yarns.

1.1.1.1 Spinning:
The present participle of the verb ‘to spin’ used verbally, adjectivally, or as a noun, meaning the process, or the processes, used in the production of yarns or filaments. The term may applied to:

The drafting and, where appropriate, the insertion of twist in natural or staple manufactured fibres to form a yarn, such as cap spinning, double roving spinning, friction spinning, jet spinning, open-end spinning, ring spinning, self-twist spinning, twistless spinning, and warp spinning. etc.
The extrusion of filaments by spiders or silkworms; or
The production of filaments from glass, metals, fibre-forming polymers or ceramics.

In the spinning of manufactured filaments, fibre-forming substances in the plastic or molten state, or in solution, are forced through the holes of a spinneret or die at a controlled rate of extrusion. There are five general methods of spinning manufactured filaments, but combinations of these methods may be used. These filaments spinning method are: dispersion spinning, dry spinning, melt spinning, reaction spinning, and wet spinning.

In the bast and leaf –fibre industries, the terms ‘wet spinning’ and ‘dry spinning’ refer to the spinning of fibres in the wet state and in the dry state respectively.

1.1.1.2 Spinning Stages:
In the tradition textile process, natural or manufactured staple fibres are converted into yarn through a series of processes, in principle may be divided into two stages:

Spinning preparation: In the first stage a tightly packed mass of staple fibres is gradually opened, clean, mired, and finally attenuated to form fibre strand that is ready to spun.
Spinning: In the second stage: the fibre strand is further attenuated to obtain a desirable yarn thickness, and some form of fibre-to-fibre cohesion is introduced to provide the necessary strength to the yarn.

1.1.1.3 Spinning Preparation
In order to convert the bulk of fibres represented by a fibre bale about 50 billion fibres into a long thin yarn typically, 100 fibres/cross section, a number of preparatory processes are used. These processes are listed in table 1.1.
Table 1.2 Spinning Preparatory Processes

Process	Function	Application
Blending, opening and cleaning	Opening and cleaning of fibres Mixing and blending of different fibres	Mandatory
Carding	Remove of trash Additional opening Removal of neps Sliver formation	Mandatory for some fibre types
Drawing	Straightening and orientation of fibres Reduction of the thickness of fibres strand Blending by doubling	Mandatory
Combing	Further straightening and orientation of fibres Removal of trash and short fibre content Removal of neps	Essentially used for high quality fine ring spun yarns. It may be used for medium to fine opened-end spun yarns. It may be used for air-jet spun yarns
Roving	Further reduction in fibre strand thickness	Used primarily for ring spinning

Some of these processes are mandatory in all yarn making operations and others may be need depending on the type of fibres to be spun, yarn count, and the spinning system utilized.

1.1.1.4 The Principle of Spinning
The spinning process is the final stage of staple yarn manufacturing. It is the final opportunity for a fibre to interact with the machine. In the spinning process, the goodness of fibre preparation through the different preceding processes can be easily be evaluated. A failure in spinning is often a result of a default in the preparatory process.

There are different spinning techniques available in to-day’s technology. Each technique is unique in its principle and in its requirements of fibre quality. New spinning techniques involve high drafting where the input fibre strand（the drawn sliver is separated, partially or fully, into approximately single fibres, flown in an air stream, and reconsolidated to form the yarn. These techniques are therefore, highly sensitive to fibre quality, and to the presence of fine trash and rust.

In any spinning method, three main mechanisms should be used to convert fibres into a yarn. These mechanisms are:

Drafting mechanism: The process of drawing out laps, slivers, slubbings, and rovings to decrease the linear density.

Fibre coherence mechanism: The resistance to separation of fibres in contact with one another in a fibre assembly in a sliver.

Winding mechanism: The transferring of yarn from one package to another.

Figure 1.1 Basic Mechanisms of Spinning

The drafting mechanism works on the same principle as the drawing process; sliding fibres over one another without elongation or stretching them. The objective of drafting in the spinning process is to reduce the size of the fibre strand to the desirable size of yarn.

The fibre coherence mechanism produces cohesive force to hold the fibres together in the yarn by introducing inter-fibre three-dimensional cross-linking. In any spinning system, the coherence mechanism is responsible for providing yarn strength.

The winding mechanism involves winding on yarn on a package bobbin or a cone, and building the yarn along the length of the package. Proper yarn winding is extremely important particularly in weaving preparation. Yarn tension should be uniform and the appearance of the package is a critical factor.

1.1.1.5 Classification of Spinning Techniques:
Different type of staple yarn process with varies manufacturing techniques, there are four major spinning technologies:

Ring Spinning : A continuous system of spinning in which twist is inserted into a yarn by using a circulating traveller. The yarn is wound on to the package since the rotational speed of the package is greater than that of the traveller. Ring spinning is a system of spinning using a ring-and traveller take-up wherein the drafting of the roving and the twisting and winding of the yarn on to the bobbin proceed simuitaneously and continuously. Ring frames are suitable for spinning all counts up to 150s, and they usually give a stronger yarn and are more productive than mule spinning frames. The later innovation in ring spinning involves the use of a revolving ring to increase productivity. Ring spinning equipment is also widely used to take-up manufactured filament yarns and insert producer twist at extrusion.

Rotor Spinning（Open-end spinning）: A method of open spinning which uses a rotor, a high speed centrifuge to collect and twist individual fibres into a yarn. The fibres on entering the rapidly rotating rotor are deposited around its circumference and temporarily held there by centrifugal force. The yarn is then withdrawn from the rotor wall and, because of the rotation of the rotor, twisted is inserted.

Friction spinning : A method of open-end which uses two surfaces moving in opposing directions to insert twist into an assembly of fibres positioned between them. Usually the external surfaces of the two rollers are used, at least one of which is perforated so that air can be drawn through it to facilitate fibre collection. Twisting occurs near the line of closest proximity of the rollers where the fibre assembly is rotated by frictional contact with the roller surfaces.

Air-jet spinning : A system of staple-fibre spinning which utilises an air vortex to apply the twisting couple to the yarn during formation.

Provide different yarn structures and hence properties. The twist level of a yarn is another variable that can affect texture and strength. These spinning techniques may be classified in may different ways. In principle, spinning techniques may be divided into two main categories:

Continuous spinning
Interrupted spinning

Figure 1.2 Classification of Spinning Techniques

In the continuous spinning, the fibres flow is continuous from the feeding point to the delivery point; and fibres are under a full mechanical control. The conventional ring spinning is a continuous spinning process. In interrupted spinning, fibres undergo a complete or partial separation（rotor, and air-jet spinning, respectively before they are reconsolidated into a yarn. The primary reason for the interruption in fibre flow is to allow separation of the fibre coherence mechanism and the winding mechanism. This separation results in producing large yarn packages, increasing production, and introducing strength at minimum energy consumption. Table 1.3 shows comparison of the various spinning techniques:

Table 1.3 Comparison of different spinning Techniques

type	Input strand	Rotating element		Yarn delivery Speed. M/min	Yarn Count, Ne	Yarn structure
		Type	Speed , rpm
Ring Rotor Air-jet Friction	Roving Sliver Sliver Sliver	Bobbin Rotor Air Yarn	Up to 25,000 Up to 130,000 3 million 250,000	Up to 40 Up to 179 Up to 250 Up to 300	3-200 5-30 15-80 10-30	True twist True/partial twists Parallel core/ wrapping fibres True twist/ loops

There have been many attempts to establish a ranking of fibre characteristics according to their contributions to the quality of yarns produced using a particular spinning system. These attempts have mainly been based on long experience and understanding of the principle involved in converting fibres into yarns in a particular spinning system. Table 1.4 provides a suggested general ranking of fibre properties according to their contributions in different spinning systems.

Table 1.4 Ranking of Fibres Properties According to Their Contribution to the yarn Quality for Different Spinning Systems

Ring Spinning	Rotor Spinning	Air-jet Spinning	Friction Spinning
Fibre strength Fibre length Fibre fineness	Fibre strength Fibre fineness Fibre length Small foreign particles	Fibre length Fibre fineness Fibre strength Small foreign particles Fibre friction	Fibre strength Fibre fineness Fibre stiffness Fibre friction

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