Updated: April, 2004 - Raghavendra R. Hegde, Atul Dahiya, M. G. Kamath
(Praveen Kumar Jangala & Ramaiah Kotra)
Bicomponent fibers can be defined as
"extruding two polymers from the same spinneret with both polymers contained
within the same filament. " A close relative is "co-spun fiber", which is a
group of filaments of different polymers, but a single component per filament,
spun from the same spinneret. The term "conjugate fibers" is often used,
Dupont introduced the first commercial bicomponent
application in the mid 1960s. This was a side-by-side hosiery yarn called
"cantrese" and was made from two nylon polymers, which, on retraction, formed a
highly coiled elastic fiber. In the 1970s, various bicomponent fibers began to
be made in Asia, notably in
The polymers given below can be used as either of the components in the cross sections .
Soluble co polyester
5. PRODUCTION AND CLASSIFICATION
The main objective of producing Bicomponent fibers is to exploit capabilities not existing in either polymer alone. By this technique, it is possible to produce fibers of any cross sectional shape or geometry that can be imagined. Bicomponent fibers are commonly classified by their fiber cross-section structures as side-by-side, sheath-core, islands-in-the-sea and citrus fibers or segmented-pie cross-section types.
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. (h) And (i). 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 .
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 electrometric 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
5.2 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.
5.2.1.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 copolymer 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.
Besides the sheath-core bicomponent fiber used as a crimping fiber, these fibers are widely used as bonding fibers in Nonwoven 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 monocomponent. 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 emphasis 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 dyeability , 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.
These are also called islands-in-the-sea fibers. Technically these are complicated structures to make and use. In cross section, they are 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 spinnable polymer such as nylon, polyester or polypropylene. Polystyrene water-soluble polyesters and plasticized or saponified polyvinyl alcohol can form the sea or matrix. The finer deniers that can be obtained are normally below 0.1 denier .
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 spinnability of the two components would almost disable the spinnability of the mixture, except for low concentration mixtures (less than 20%).
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 dyeability, improve the texturing ability and give the fiber a unique lustrous appearance. The fine fibers produced by this method are used in synthetic leather, specialty wipes, ultra-high filtration media, artificial arteries and many other specialized applications.
This structure as shown in Fig-4 is commonly referred to as "segmented pie structure" or "citrus," Alternate pie or wedges are made of nylon and polyester. The fiber contains around 16 segments. The fibers are made in to web, carded, and fiber web is passed through high-pressure jet of air or water as to split the fibers. This splitting and entanglement makes the resultant fabric more strong.
Fig. 4: Segmented pie structure
Sometimes it is difficult to split individual fibers, and in that case the hollow wedge structure is used as shown in Fig-5 and Fig 6.
Fig. 5: Hollow Pie Wedge Structure
Fig. 6: Conjugate Structure
Sometimes, it becomes very difficult to card this fiber because of its different modulus properties. In order to overcome this problem their structure is a ltered as in Fig-7 and Fig-8 .
Fig-7 Segmented Cross structure
Fig-8 Tipped Trilobal Structure
Polyblends, of polymer alloys, are defined as homogenous or heterogeneous mixtures of structurally different homopolymers or copolymers. The purpose of blending is either to improve processability or to obtain materials suitable for specific needs by tailoring one or more properties with minimum sacrifice in other properties. The behavior of polyblends may be expected to depend on the individual properties of the components in the blend, their relative proportions, degree of heterogeneity and the properties of the interface between the components. Several criteria are used to define the nature of polyblends:
Two polymers are thermodynamically compatible when their free energy of mixing is negative. Because mixing of two materials is generally endothermic and the entropy of mixing long polymer chains is small, the free energy of mixing is rarely negative. This is the reason why blending two polymers usually leads to heterogeneous blend. If the blend shows homogenity, then the behavior of the blend behaves as a single polymer.
In this more common category, two polymers are segregated into spatial regions composed essentially of one or the other pure component. Usually, the two polymers are immiscible but they can be compatible. Considerable emphasis is put on the adhesion between the phases of the blend because it is crucial factor for mechanical properties of the blend.
The theory of modulus tailoring is mainly used in matrix-fibril type of bicomponent fibers. Classification based on the relative moduli of the two components depends to a great extent on the properties and use of the blends. For example, adding of a disperse phase of higher modulus generally increases the overall modulus and is frequently used to reduce the creep of elastomers. In contrast, adding of a low modulus polymer in the blend is generally used to improve the impact resistance and elongation-to-break of rigid plastics.
It is essential that the viscosities of both polymer fluids are of comparable value; otherwise, the higher viscosity component will not tend to rearrange during spinning causing the distortion of the distribution of the components in the cross section of the fiber.
Considerable attention should be also paid to the rate of solidification of each component. It has been shown that during high speed spinning of PP/PET sheath-core fibers  that the PET component achieved higher orientation than would be obtained if the fiber was just monocomponent, while PP component orientation was decreased. This phenomenon is explained in terms of difference in activation energy of the longitudinal viscosity and solidification temperature of both polymers.
Bicomponent fibers made of PP/PE are important material in the nonwoven market. The main applications include:
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