Page 617 - Carrahers_Polymer_Chemistry,_Eighth_Edition

580 Carraher’s Polymer Chemistry

relatively long, the linear velocity of wet spinning is less than for either melt or dry spinning. Wet
spinning allows the placement of holes in the spinneret face to be closer together allowing produc-
tivity to be increased. Even so, it remains the slowest of the traditional spinning processes. The
equipment used for wet spinning is similar to that used in dry spinning thought it is not necessary to
heat the polymer solution to a high temperature. The spinnerets are immersed in tanks containing
the nonsolvent. Wet spinning is the most complex of the three spinning processes, typically includ-
ing washing, stretching, drying, crimping, finish application, and controlled relaxation to form tow

material. Spinning of natural-derived materials generally include additional steps including ageing
or ripening to achieve the desired viscosity and chain length.
Fibers made from wet spinning generally have high-void contents in comparison to all of the
other processes giving them increased dyeability. The surface is rougher with longitudinal serra-
tions and from a round die hole it has an approximately circular to bean-shaped diameter.
Hollow fibers for gas and liquid separation are prepared through passing air through the material

just prior to entrance into the nonsolvent bath.
18.3.5 OTHER SPINNING PROCESSES

There are a number of lesser used, but still important, spinning processes. Following is a summary
of some of these. In reaction spinning, a prepolymer is generally used that is further reacted upon
by a material that is may be in solution in a bath. Further treatments may included cross-linking

of the fiber. The most important example is the production of selected segmented polyurethane
(PU) elastomeric fi bers. Here the prepolymer is the soft segment generally a low molecular weight
polyether or polyester. Reaction with an aromatic diisocyanate converts the end groups, generally
hydroxyls, into isocyanate groups. The bath contains a diamine such as 1,2-diaminoethane. The
reaction between the amine and isocyanate forms the hard urea linkages. Some segmented PUs,
such as Lycra (TM), are formed using conventional dry spinning.
Fibers can be formed from intractable materials such as ceramics and polytetrafl uoroethylene

through extrusion of a suspension of fine particles in a solution of a matrix polymer. The matrix
polymer-intractable material is coagulated embedding and aligning the intractable material in the

matrix polymer. The filament is then heated decomposing the matrix polymer. During this process,
the material is sintered and drawn giving small, often with little fl exibility, fi brils.
Gel spinning is used to produce high strength and modulus fi bers. High molecular weight (such
6
as 10 Da for polyethylene) polymer is dissolved in a high-temperature solvent at low concentration
(1%). The hot solution is extruded into a cooling zone such as a liquid nonsolvent. The resulting

gel-like filament contains polymer with lots of entrapped liquid. This gel-like filament can be easily

highly drawn. The drawing can be carried out even though liquid is removed before drawing. What
occurs is that the low density of polymer chains in the gel allows a decreased chain entanglement

allowing greater linear chain conformations to occur as the fiber is drawn.
Some low orientation polymers exhibit what is referred to as necking. In necking, a fi lament
extends preferentially at only selected sites known as necks or necking sites. This behavior occurs
with many thermoplastic materials near their T . At lower temperatures brittle fracture may occur
g

at high tensions instead of necking. At higher temperatures filament extension occurs uniformly
without preferred necking sites. Commercially, filament extension is carried out at suffi cient tem-

peratures to avoid necking. For multifilament yarns, filament elongation is generally carried out by

first heating the filament with subsequent application of the stress necessary to stretch the fi lament.
For monofilaments and tows, the heating and application of the stress occur together.

Along with centrifugal spinning, there are several additional fiber forming techniques that are

employed in fiber formation that do not employ a spinneret. In electrostatic spinning, a high voltage,

generally >5,000 V, is applied to a viscous solution of the polymer dissolved in a volatile solvent
with a high dielectric constant but low conductivity contained in a fine capillary tube. A stream of

filaments emerge from the capillary. These filaments are collected on a suitable surface.
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