Non Circular /Profile fibres

Non Circular /Profile fibres

Fibres with non-circular cross-section and hollow fibres

The cross-section or geometry of melt spun fibre essentially depends on the design of spinneret holes through which the initial extrusion takes place. Synthetic fibres such as polyester and nylon are generally produced using spinneret with circular holes and have a circular cross-section.While in the case of solution spinning and coagulation process which involve mass transfer result in non-circular cross-section even when circular holes are used.

Why fibres with non circular cross-section?

The natural fibres do not have a perfect circular shape, cotton has a bean shaped cross-section and silk has a roughly triangular cross-section. Non-circular cross-sections are observed to result in greater lusture, high covering power, improved handle, light-weight and warm preservation.The non circular cross-sections of these fibres result in excellent capillarity and wicking properties for removing moisture, therefore are used for diapers, sanitary towels,filter media,shoe liners and wound dressings. Therefore, filaments with non-circular cross-sections are desirable for achieving these special effects.

Approaches for producing non-circular fibres:

The modified filament cross-sections result either by coalescence or fusion of melt streams below the spinneret or directly from extrusion through profiles capillaries.

The major challenge for achieving non circular fibres is spinneret design and optimization of spinning parameters for different polymers with unique rheological properties. The cross-sections of fibres and spinnerets used for producing these are shown in Figure 1.

Figure 1. The cross-sections of fibres and its spinnerets

The reshaping of filament along the spinline depends primarily on the time of solidification,the longer this time period, the greater the degree of reshaping. The factors that increase the possibility or extent of reshaping are:

Lower melt viscosity Increased spinning temperature Lower throughput rate Increased spinning speed (as it reduces the time available for solidification) and Contour of spinneret

The structural and cross sectional shape can also be changed in high-speed melt spinning.

In general, the spinnerets orifices of diameter 0.18-0.45 mm and length 0.4-0.8 mm are used for producing circular cross-sectional shape. For producing some other cross-sectional shapes typical dimensions and design of spinnerets is given below.

Trilobal

The trilobal cross-section was the first departure from circular cross-section and was introduced by DuPont in USA to produce lustorous yarn and fabric.The fibres can be produced using a typical spinneret orifice shown in Figure 2.

Figure 2. Trilobal cross-section of fibre

Twin trilobal fibres resemble silk fibres and are very lustorous.These can be produced by conjugated bicomponent spinning of trilobal cross-section yarn.The three tips of the trilobal crosssection may consist of polymer that is easily soluble and results in special twin trilobal cross section ( Figure 3).

Figure 3. Twin trilobal cross-section

The rectangular shaped cross-section fibres are produced using rectangular spinnerets. A typical spinneret with dimensions and the dimensions of the fibre produced using this are shown in Figure 4.

Figure 4. Spinneret for fibre with rectangular cross-section

For hollow fibre production, it is important to consider the die swell and the spinneret dimensions are designed accordingly. A typical spinneret with dimensions and the resulting hollow fibre with dimensions is shown in Figure 5.

Figure 5. Spinneret designs for hollow fibre and extruded hollow fibre

Other methods like separation of bicomponent fibre and thin-film splitting are also used. These are discussed in the next chapter.