Synthetic fibers production :
Generally, fibers divide into two groups: natural fiber and synthetic fibers. Natural fibers divide into three groups: vegetable fibers, protein fibers (animal) and, mineral fibers. In this writing, we are going to explain the production process of synthetic fibers.
Synthetic fibers have two natural and synthetic sources. The most important natural source is vegetable cellulose, for instance, viscose rayon and acetate are synthetic cellulosic fibers.
The second group is synthetic fibers with a synthetic polymer which are petroleum's derivatives. For example, polyester, nylon, PP and acrylic are the samples of these kinds of fibers. Synthetic fibers are produced by one of these two methods: melt spinning and solution spinning.
Melt spinning is one of the simplest, fastest and most economic spinning systems. Melt spinning is suitable for thermoplastic polymers which can be melted, cellulosic polymers are thermoset; this means when they are exposed to high temperature their structure would ruin.
The first step for melt spinning is polymer synthesis, for the next step the polymer enter to the spinning factory in different forms like granule, chips, powder, and slab, they should be converted to molten liquid for spinning. Today's industries do this by extruder, extruder's design is very important and it would affect on the filament's structure directly. In the extruder melting, mixing and homogenization are done. This is very important that impurities be separated from the polymer in extruder because they could cause breakpoints in the final filament. Molten liquid passes through a spinneret and becomes to filaments, in this stage, the filaments should be cooled and this step is very important because if filaments cool rapidly the amount of crystallinity would be low and the mechanical properties of the fiber would reduce. The cross-section shape follows the spinneret shape and fineness is determined by the amount of draw and spinneret shape. Drawing is done in two stages; one of them is done by the speed difference between feed pomp and take-off roller and another is done in drawing zone by two rollers. Nylon and polyester are produced by melt spinning.
This is obvious that we need solvent in solution spinning, extraction of the solvent divides solution spinning into two methods; wet spinning and dry spinning.
When we have a volatile solvent and thermoset polymer we choose the dry spinning method. The polymer is solved and after purification, the solution passes through spinneret after that filaments need to be converted into solid, for this, the solvent should be extracted and by heating, we can remove solvent from the filament structure, then it should be cooled. Cooling should be done gradually in order to have oriented filaments. In dry spinning, drawing is done by the speed difference between feed pomp and take-off roller. The cross-section shape doesn't follow the spinneret shape because of the evaporation. Acrylic and acetate are produced by dry spinning.
We do same as dry spinning in wet spinning but, the solvent should be removed in coagulation bath in this method, the solvent is removed in the spinning bath by the presence of the non-solvent. Viscose rayon and acrylic are produced by wet spinning.
Melt spinning have advantages to solution spinning. For example, in dry spinning we has solvent and it should be recycled. These actions make the solution spinning expensive and toxic; additionally, solution spinning is more complicated than melt spinning.