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Different methods are employed for converting the solid polymers into liquid form. Accordingly the extrusion also differs. Based on these, three methods can be evolved for production of man-made and synthetic fibres. These are termed as ‘chemical spinning’ or ‘extrusion’.
Melt spinning: Melt spinning is applicable to the polymers which are capable of melting at elevated temperatures. This is the simplest and most inexpensive process, as it involves only melting, extrusion and cooling. The polymer is melted, extruded through the holes of a spinnerette which is a perforated metal plate of small diameter. Each hole may be as small as 0.5 mm diameter or even smaller. Each spinnerette may contain 1 to 100 holes depending on the type of yarn or tow produced. A tow is a rope of filaments collected from different spinnerettes that are used for making staple fibres for blending.
In the manufacture of melt spun filaments, the polymers in chip form are fed through a hopper and fall on a hot grid where it melts. The molten polymer is collected and filtered to remove suspended impurities that block the polymer flow and also vacuum treated to remove the air bubbles, so as to ensure continuous flow of polymer liquid that produces continuous filament.
The polymer stream collected in a tank is forced through the holes of the spinnerette in to a cooled air chamber which aids in solidification of the filament. The fibres that are produced by melt extrusion are polyamide, polyester and polypropylene.
The temperature employed for melt extrusion is generally 30°C above the melting point of the polymer, so that the viscosity of the fibre is low enough for extrusion. The viscosity of a molten polymer is also a measure of the length of the molecular chains. The higher the viscosity, the longer the polymer chain.
The filaments thus produced have the polymer chains arranged at random. To improve its orientation, the filaments are cold drawn. To facilitate cold drawing, filaments are drawn between two sets of rollers which operate at progressively higher speeds. The second set revolves at least 4 times the speed of the first set, so that drawing of the filament takes place. The randomly arranged molecular chains are oriented towards fibre axis and the filament becomes finer and stronger. This also facilitates the crystal formation when the polymer chains are drawn closer.
Dry/Solvent spinning: When a polymer cannot transform into liquid through melting, an alternative method is employed, wherein the polymer is dissolved in a suitable solvent. If the solvent is volatile, it leaves the polymer in a desired shape when the solvent dries. Therefore this method is termed as dry spinning.
The polymer is dissolved in a solvent up to 25% and transformed into polymer solution. Sometimes the solution is heated to ensure sufficient flow of polymer through the tiny holes of the spinnerete. At this stage, certain additives like coloured pigments (spun dyed/dope dyed) can be introduced. Delustering agents such as titanium dioxide can also be added to the solution to produce dulled filaments. The polymer solution is filtered, vacuum treated to remove suspended particles that block the spinnerette holes and air bubbles that interrupt the formation of continuous filament.
The solution from the storage tank in forced through the tiny holes of the spinnerette into a heated air chamber where hot air or nitrogen is in circulation. The hot air evaporates the solvent from the polymer and carries it to another chamber wherein the solvent recovery takes place. After spinning, the filaments require washing.
This process is considered to be a costly one as it needs huge investment for a solvent recovery plant. About 10 % of the solvent is lost during this process, part of which may be present in these filaments. The solvents used may be expensive, toxic and flammable. Generally 3 to 6 kgs of solvent is used per kg of polymer. However, this process is considered to be fast (500-1000 metres per minute).
Unlike melt spinning, the cross sectional shape of the filament does not resemble the hole of the spinnerette as the drying up of solvent leaves the polymer in a different shape like a lobular shape in case of acetate fibre. Fibres such as cellulose diacetate, cellulose triacetate, polyacrylonitrile and modacrylics can be modified to become cellulose acetate and dissolved in a cheaper solvent such as acetone and on evaporation, acetate fibres are produced.
The cellulose acetate filaments generally are not drawn after spinning as the orientation of the molecular chains seems to be sufficient where as acrylic fibres are cold drawn like polyamide and polyester fibres to improve orientation. The cross sectional shapes of the filaments produced through melt spinning resembles the shapes of the holes of the spinnerette. Thus, the shapes of the fibres can be altered by changing the shapes of the holes to alter the lustre and bulking properties.
Wet spinning: Wet spinning is applicable to those polymers which cannot be either melt or dissolved in a volatile solvent to change it from a solid to liquid state. A non-volatile solvent is used to change it to a liquid state. As the solvent can’t be evaporated, it is made to pass through a chemical bath, wherein the filament coagulates leaving the solvent, hence the name wet spinning.
Wet spinning is a slow process (30-80 metres/ minute) as the filament is regenerated through coagulation. As the coagulation takes place in a chemical bath, the spinnerettes should be made of corrosion resistant precious metal alloys, which again needs huge investment. The holes used are much smaller when compared to other two methods. Around 20,000 holes are present if the filament is turned into a tow.
The polymer solution is filtered to remove suspended particles and vacuum treated to free from air bubbles and then collected in tanks. Additives such as pigments and delustering agents can be added. Then it is forced through spinnerette into a chemical bath containing a dilute acid and gets coagulated in filament form. The filaments are then washed and drawn into desired diameter. Certain additives are introduced into the coagulating bath to alter the handle characteristics of the filaments.
Fibres that are manufactured using this method include viscose rayon and polyacrylonitrile. The non-volatile solvents are generally cheaper and therefore, recovery is not attempted. The cross sectional shapes of the filaments are often serrated (viscose rayon) due to chemical reaction that leads to coagulation of the filament.
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