Page 36 - Curriculum Visions Dynamic Book

called wet spinning. In other cases the solvent can be made to evaporate without the use of any other chemical. This is called dry spinning.
Wet spinning is the oldest process and is still commonly used (see the viscose process on page 40). In this case fibres emerge from the spinneret into a chemical bath where the filaments are made to solidify by chemical reaction. Acrylic, rayon, aramid, and modacrylic can be produced this way.
Dry spinning is now used for making specialty fibres such as acetate, triacetate, acrylic, and spandex.
In some cases no solvent is needed at all. This is the case with melt and gel spinning.
The most economical method of spinning is melt spinning. In this case the polymer is heated until it melts and then extruded through the spinneret. It is solidified by cooling. Nylon, olefin, and polyester are often produced this way.
One important property of melt spinning is
that the melt-spun fibres can be extruded from
the spinneret with different cross-sectional shapes (for example, round, square, trilobal, pentagonal, octagonal, and hollow). These shapes can be used
to give important extra characteristics to the fibre. For example, trilobal-shaped fibres reflect more light and give an attractive sparkle to textiles; pentagonal- shaped fibres reflect the light less and do not show dirt as much as other fibres. Octagonal-shaped fibres reflect the light very little. Hollow fibres trap air and so have insulating properties. They make hollow-fill materials used in outdoor garments.
Gel spinning uses a gel for extrusion. A gel is neither a solid nor a liquid, but consists of liquid crystals. Here the polymer chains are bound together at various points even while they are extruded. This can add strength to the fibre.
Stretching (drawing)
In a newly spun synthetic fibre not all of the molecules are in long, straight threads parallel to the fibre. If
(Above) Polyester can be manufactured in a wide variety of forms, including this one, which has a metallic look.
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