By: D. Gopalakrishnan
Sardar Vallabhbhai Patel Institute of Textile Management,
Coimbatore - 641004

dgk_psgtech@yahoo.co.in

In current scenario, textiles not meant only for satisfying the basic needs of human, but also have to aid and enhance the human performance and comfort. This has lead to intense growth of functional textiles known as technical textiles. The most commonly emerging advanced applications include medical textiles, geotextiles, package textiles, industrial textiles, automotive textiles, protective textiles etc. With the development of new fibres and new manufacturing processes for yarn and fabrics, new area of applications for various sectors of technical textiles are in rapid growth. Over the years, the applications of manmade fibres have increased tremendously and that too in diverse fields. Yet there is no ideal single fibre which can satisfy all these requirements. This has lead to the development of Bicomponent fibres, which can be designed to suit the requirements of end user.

Bicomponent Fiber Capabilities

Bicomponent can provide:

� Thermal bonding
� Self bulking
� Very fine fibers
� Unique cross sections
� The functionality of special polymers or additives at reduced cost

Advantages of Bicomponent Thermal Binder Fibers

� Uniform distribution of adhesive
� Fiber remains a part of structure and adds integrity
� Customized sheath materials to bond various materials
� Wide range of bonding temperatures
� Cleaner, environmentally friendly (no effluent)
� Recyclable/ Lamination / molding / densification of composites

Production

Bicomponent and multi-component fibers are fibers which are generated during the spinning process from two or more polymers which have different chemical or physical characteristics. Two extruders are used for melting the chips in the simplest two-component-spinning process. The polymer melts are separately led to the spin packs or holes and thereafter spun to filaments.


CLASSIFICATION OF BICOMPONENT FIBRES

Bicomponent fibres are sometimes referred as 'composite', 'conjugate' or 'hetro' fibers. These can be divided into several groups according to the component distribution within the fibre cross section area [chart 1], as given below:

� Side-by-side (s/s) fibers
� Sheath-core (s/c) fibers
� Matrix-fibril Bicomponent fibers
� Segmented pie structure
� Polymer blends

SIDE-BY-SIDE (S/S)

These fibers contain two components lying side-by-side (Fig.1.). Generally, these fibers consist of two components divided along the length into two or more distinct regions. In most cases, the components must show very good adhesion to each other; otherwise, the process will result in obtaining of two fibers of different compositions. The way to connect the two components mechanically is described in patent literature and is shown in (Fig.1). Generally, there are several approaches for producing side-by-side Bicomponent fibers. Two components, either in the form of solution or melt, are fed directly to the spinneret orifices or are combined into Bicomponent fibers near the orifices. Two components are first formed into multi-layered structure and slowly fed (without turbulence) in the orifices. The orifices are positioned so that they intersect the interfaces of various layers of the polymer.

Two components are also formed into layered structure but the orifices do not follow exactly the interfaces, which leads to production of fibers of a wide range of compositions, varying from 100% of one component to 100% of the other through all intermediate possibilities. Two polymer components are slit-extruded into a layered film, which is then cut into stripes, drawn, cut into staple and fibrillated by a carding machine and then crimped by heat relaxation.

Use of Side-by-side Bicomponent Fibers

Side-by-side fibers are generally used as self-crimping fibers. There are several systems used to obtain a self-crimping fiber. One of them is based on different shrinkage characteristics of each component. All commercially available fibers are of this type. There have been attempts to produce self-crimping fibers based on different electromeric properties of the components; however, this type of self-crimping fiber is not commercially used. Some types of side-by-side fibers crimp spontaneously as the drawing tension is removed and others have "latent "crimp, appearing when certain ambient conditions are obtained. Some literature mentions "reversible "and "non-reversible" crimp, when reversible crimp can be eliminated as the fiber is immersed in water and reappears when the fiber is dried. This phenomenon is based on swelling characteristics of the components. Several factors are crucial to the fiber curvature development:

� The difference in the shrinkage between the components,
� The difference between modulus of the components,
� The overall cross-sectional fiber shape and individual cross-sectional shapes of each component, and the thickness of the fiber.

Different melting points on the sides of the fiber are taken advantage of when fibers are used as bonding fibers in thermally bonded non-woven webs. The example of such bonding fibers is EA & ES of Chisso, Japan, with polyethylene as the low melting component (Tm = 110oC), along with polypropylene. Side-by-side fibers have also been reported to be a base fiber for producing so called "Splittable" fibers, which split in a certain processing stage, yielding fine filaments of a sharp-edged cross section. One of the components could be removed by dissolving or the fiber could split by just heating and the fiber would split by a flexion action.

SHEATH-CORE (S/C) FIBERS

Sheath-core Bicomponent fibers are those fibers where one of the components (core) is fully surrounded by the second component (sheath) (Fig.2). Adhesion is not always essential for fiber integrity. This structure is employed when it is desirable for the surface to have the property of one of the polymers such as luster, dyeability or stability, while the core may contribute to strength, reduced cost and the like. A highly contoured interface between sheath and core can lead to mechanical interlocking that may be desirable in the absence of good adhesion.

Sheath-core Fiber Production

The most common way of production of sheath-core fibers is a technique where two polymer liquids are separately led to a position very close to the spinneret orifices and then extruded in sheath-core form. In the case of concentric fibers, the orifice supplying the "core" polymer is in the center of the spinning orifice outlet and flow conditions of core polymer fluid are strictly controlled to maintain the concentricity of both components when spinning. Eccentric fiber production is based on several approaches: eccentric positioning of the inner polymer channel and controlling of the supply rates of the two component polymers; introducing a varying element near the supply of the sheath component melt; introducing a stream of single component merging with concentric sheath-core component just before emerging from the orifice; and deformation of spun concentric fiber by passing it over a hot edge.

Other, rather different techniques to produce sheath-core fibers are coating of spun fiber by passing through another polymer solution and spinning of core polymer into a coagulation bath containing aqueous latex of another polymer. Modifications in spinneret orifices enable one to obtain different shapes of core or/and sheath within a fiber cross section. There is considerable emphasis on surface tensions, viscosities and flow rates of component melts during spinning of these fibers.

Use of Sheath-core Bicomponent Fibers

Besides the sheath-core bicomponent fiber used as a crimping fiber, these fibers are widely used as bonding fibers in nonwovens industry. The sheath of the fiber is of a lower melting point than the core and so in an elevated temperature, the sheath melts, creating bonding pints with adjacent fibers - either bicomponent or mono component. The first commercial application of sheath-core binding fiber (I.C.I. Heterofil) has been in carpets and upholstery fabrics. The newest trend in bicomponent fiber production is to focus on tailoring a fiber according to the customer's needs.

A considerable emphasize was put on the processing optimization (depending strictly on machinery used) and on the desired look of the final product. It appears that concentricity/eccentricity of the core plays an important role. If the product strength is the major concern, concentric bicomponent fibers are used; if bulkiness is required at the expense of strength, the eccentric type of the fiber is used.

Other uses of sheath-core fibers derive from characteristics of the sheath helping to improve the overall fiber properties. A sheath-core fiber has been reported whose sheath is made of a polymer having high absorptive power for water, thereby having obvious advantages for use in clothing. Other sheath-core fibers showed better dye ability, soil resistance, heat insulating properties, adhesion etc. Production of ceramic sheath-core bicomponent fibers is another application utilizing the difference of sheath and core. The fiber precursors are first spun in a sheath-core arrangement and then cured by oxidation, UV and electron beam, heating or by chemical means. These fibers are used as a composite reinforcement.

Matrix-Fibril (Biconstituent) Bicomponent Fibers

These are also called islands-in-the-sea fibers. Technically these are complicated structures to make and use. In cross section they are basically areas of one polymer in a matrix of a second polymer. These types of Bicomponent structure facilitate the generation of micro denier fibers. The 'islands' are usually a melt spin able polymer such as nylon, polyester or polypropylene. The sea or matrix can be formed by polystyrene water soluble polyesters and plasticized or saponified polyvinyl alcohol. The finer deniers that can be obtained are normally below 0.1 denier.

Production of Matrix-fibril Bicomponent Fibers

Basically, these fibers are spun from the mixture of two polymers in the required proportion, where one polymer is suspended in droplet form in the second melt. An important feature in production of matrix-fibril fibers is the necessity for artificial cooling of the fiber immediately below the spinneret orifices. Different spin abilities of the two components would almost disable the spin ability of the mixture, except for low concentration mixtures (less than 20%).

Use of Matrix-fibril Bicomponent Fibers

A matrix-fibril fiber called "Source" is produced by Allied Chemicals Ltd. The fiber is based on PET fibrils embedded in a matrix of Nylon 6. The presence of PET fibrils is supposed to increase the modulus of the fiber, to reduce moisture regain, to reduce the dye ability, improve the texturing ability and give the fiber a unique lustrous appearance.

About the author:

I am doing PG Diploma in Home Textile Management. I did my Diploma in Textile Technology & B. Tech in Textile Technology from PSG College of Technology & Polytechnic College. After my diploma I worked as a Production & maintenance Supervisor in Cambodia Mills (NTC) Coimbatore, after three years of experience I came back to my B. Tech. I did 17 paper presented in various technical symposiums, national & international conferences in all over India and i participated in various technical workshops & innovative project works. I published several articles in journals, magazines.

Area of Interest: innovative textiles, Technical textiles
Coimbatore - 641 004, Email: dgk_psgtech@yahoo.co.in


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