For producing man-made fibres, the polymers must be unfolded and extended uni-directionally to extremely large dimension to get high length to diameter ratio and high orientation. In spinning, a small amount of polymer may be elongated to over thousands of kilometers of length while the other dimension (diameter) is only in microns. Manmade/synthetic fibre manufacturing process i.e. spinning process includes the process of extruding the polymer extrusion through the spinneret. The extruded polymer is then solidified in fiber form.
The different types of spinning are as follows:
1. Melt Spinning
Melt spinning is one of the most convenient and economic method of manufacturing polymeric/synthetic fibres. In melt spinning polymer (fibre-forming substance) is melted and pumped through a spinneret (die) with numerous holes (one to thousands). The molten fibres are cooled, solidified, and wound on a suitable package. Melt spinning uses the heat to melt the polymer to a viscosity which is suitable for the extrusion through the spinneret. Polymer chips are obtained from previously reacted chemical combinations. These polymeric chips are melted and then pumped at specific rate through a spinneret. The extruded filaments are cooled to solidify continuous filaments. Stretching of the fibres in both the molten and solid states can be done to improve orientation of the polymer chains along the fibre axis. Fibres like Polyester, Nylon, Olefin, Polypropylene, Saran, etc. can be spun by melt spinning.
2. Dry Spinning
Dry spinning technique is used for such kind of Polymers which won’t melt but degrade on heating. Dry spinning is required for polymers with a melttemperature equal to or close to their thermal degradation temperature, therefore they require dissolving in a solvent in order to be processed into fibres. In dry spinning the polymer is dissolved in its solvent and then extruded. As fibres emerge through the spinneret the solvent is evaporated off with hot air. This process may be used for the production of fibres such as Acetate, Tri-acetate, Acrylic, Modacrylic, PBI, Spandex and Vinyan.
3. Wet Spinning
Wet spinning is the oldest, most complex and also the most expensive method of man-made yarn manufacture. Wet spinning is applied to polymers which do not melt and dissolve only in non-volatile or thermally unstable solvents. The principle of wet spinning process is shown in the following figure.
It consists of feeding of polymer and solvent and mixing homogeneously in the mixing vessel. The solution (the concentration of the polymer in the solutions is 5-20 %) is transferred to the spin block. The feeding of the solution is controlled by means of the gear pump and it is filtered before extrusion in the coagulating bath. The fibre solution is extruded into a liquid (coagulating bath) that will draw out the solvent, leaving behind only the polymer. The solidification process of the polymer solution consists of extraction of the solvent by the chemicals present in the coagulating bath. The extruded fibre can be stretched in the coagulation bath. The degree of stretching can be as high as 30 times. After stretching fibres are washed to remove any residual solvent or any other chemical present in the fibre. The finish is applied on washed fibres to reduce static charge generation, then drying of the fibre take is done and filament is wound on to the bobbin to get the filament and send to crimping operation to produce staple fibre.
Being extruded into a liquid provides a greater drag force on the filament than those extruded directly into air, therefore the speeds at which this occurs is reduced from that of melt and dry spinning. Once evaporated these fibres then have to be drawn or stretched in order to orient the polymers to give the fibre its strength. Fibres spun using this process include Acrylic, Rayon, Aramid, and Spandex.