can be considered: the 'classical geotextlles', typical products of the
textile industry, including woven, knitted and nonwoven fabrics; and the
'special geotextiles', products recently developed to be used in combma-
tion with, or in place of, classical geotextiles. (Although they are not
products of the textile industry, special geotextiles are included in the
classification of geotextiles because of similarities in appearance or
applications with classical geotextiles.)
2.2.1. Classical geotextiles
The manufacturing process of a classical geotextile includes two steps.
The first step consists of making linear elements such as filaments, fibers,
slit films (tapes) or yarns.
Filaments. These are produced by extruding melted polymer through
dies or spinnerets. Since this process is continuous, filaments are
sometimes called 'continuous filaments'. After extrusion, a filament is
usually drawn (i.e. pulled along its longitudinal axis) to orient its
molecules in the same direction. As a result of the draw, the modulus of
the filament is increased (i.e. when subjected to a given force, a drawn
filament exhibits a smaller elongation than a nondrawn filament). A
multifilament yarn is made from filaments aligned together.
Short ('staple') fibers. These are obtained by cutting filaments to a
short length, typically 2 to 10 cm (1 to 4 in). Almost always, the filaments
are crimped before cutting. A spun yarn is made from staple fibers
interlaced or twisted together.
Slitfilms. These are small tapes, typically 1 to 3 mm (40 to 120 mils*)
wide, produced by slitting an extruded plastic film with blades. After the
slitting of the film, the tapes are drawn. As a result of the draw, molecules
become oriented in the same direction and the modulus of the tapes
increases.
Fibrillated yarn. This is a film strip which has been nicked and broken
up into fibrous strands (in other words, a bundle of tapelike fibers that
can still be partially attached to each other).
The second step of the manufacturing process of a classical geotextile
consists of combining filaments, fibers, slit films or yarns to make a
planar structure called a fabric. The three types of fabrics are wovens,
knitted and nonwovens.
*One mil = 10- 3 in.
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