Download 712 FIBERS Vol. 6 ature of the precipitation bath affects fiber

712 FIBERS
ature of the precipitation bath affects fiber structure and properties (167). Vol. 6
By decreasing the precipitation temperature from50-0°C, a more perfectly
developed fibrillar structure is obtained after drawing.^ The fibrillar structure of
acrylic fibers is important and determines to what extent of stretch drawing the
filaments can be subjected; therefore, it influences mechanical properties, density,
and sorption characteristics. As in all synthetic fibers, the extent of drawing largely
determines the fiber’s mechanical properties (168). Acrylic fibers used as staple ■
fibers are drawn less than multifilament yarns.
Typical properties of acrylic fibers are shown in Table 17. Mechanical properties are strongly temperature-dependent, particularly in the wet state (Fig. 33).
The stress-strain curve of acrylic staple fibers resembles wool, and these fibers are
frequently blended with wool or processed on the woolen system of yarn
manufacture.
Table 17. Typical Properties of Acrylic Fibers
Continuous
Property
tenacity at break. N/tex* 65%
rh/21°C
wet
extension at break, %
65% rh. 2i°ti
wet
elastic modulus, N/tex°
65% rh, 2l°C
moisture-regain at 65% rh, %
specific gravity
approx. volumetric swelling in
water. %
a 'To convert N/tex to g-f/den, multiply by 11.3.
Extension. %
filament
Staple
0.40-0.44
0.35-0.40
0.22-0.26
0.18-0.26
15-20
20-30
25-35
35-45
5.S-6.2
1.6-2.0
1.17
slight
2.2-3.5
1.8-2.5
1.17
2-5
.Fig. 33. Effect of temperature on the load-extension curves in water of acrylic fibers (168).
713
FIBERS
The dyeing of acrylic fibers is done mostly with cationic or basic dyes at temperatures
above the boiling point of water (under pressure). Suitable dye sites in other vinyl
monomers in the polymer chain make possible the use of cationic dyes at normal dyeing
temperatures. A method for dyeing with acid dyes requires the presence of cuprous ions;
the ions add to the cyanide groups of PAN forming dye sites for the acid-dye anion.
Disperse dyes are relatively slow, but in the presence of certain dye assistants,
satisfactory light shades can be produced.
As a general class, acrylic fibers do not have well-developed crystalline structures,
although there are strong dipolar interactions of the polymer chains through the nitrile
groups. Crystallization tendency is low’even at high temperatures, and as a result
acrylic fibers cannot be heat-set. The fibers undergo considerable length shrinkage upon
exposure to water at elevated temperatures. This longitudinal contraction is due to the
relaxation of internal strains imposed during drawing. This shrinkage tendency allows
manufacture of yarns with desirable bulk and loftiness. Bicomponent acrylic fibers are
permanently and structurally crimped as a result of differential shrinkage of the two
polymer components (169,170). Structurally crimped acrylic fibers were the first
synthetic fibers to adopt the cortical asymmetry of keratin fibers, and the method of
crimping developed for regenerated cellulose fibers (135). The high bulking properties of
these crimped acrylic fibers (shown in Fig. 34) and other physical bulking processes
have made acrylic fibers readily adaptable for wool-like applications, eg, blankets,
sweaters, and carpeting.
Fig. 34. Longitudinal section of a differentially dyed crimped bicomponent acryl fiber (169).
Acrylic fibers have good resistance to chemical and.microbial attack. Th do not have a
characteristic melting point.' but have softening temperatures about 250°C. In general,
thermal stability is high. The equilibrium moist1
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714 FIBERS
regain at 65% rh is about 2rc. A novel highly absorbent acrylic fiber, which absorbs 3050% of water, has been commercialized (171). The high water absorption is achieved not
by chemical modification, but by special extrusion techniques to produce a fiber with a
porous inner core. An important and growing use of acrylic fibers is as precursors to
carbon fibers. Regularly extruded acrylic fibers are subjected to high degrees of
stretching to improve mechanical properties, and then to a two-stage heating and
carbonization process to yield carbon and graphite fibers.
Several commercial modacrylic fibers are produced primarily because of their flame
resistance resulting from their high halogen content. In one mod- ; acrylic fiber, the
fiber-forming substance is a copolymer of acrylonitrile and vinyl chloride in ca a 40:60
ratio by weight: Another modacrylic fiber is a copolymer of acrylonitrile and vinylidene
chloride; other, copolymeric structures with hal- • ogen containing monomers have been
reported. Modacrylic fibers are wet spun from solution into aqueous precipitation baths.
Solvents for modacrylic polymers include acetone, dime thy lformamide,
dimethylacetamide, and dimethyl sulfoxide. After extrusion and coagulation, the
filaments are subjected to hotrdrawing and annealing at elevated temperatures. The
fibers have a tenacity at break of 0.22-0.26 N/tex (2.5-3.0 g-f/den) and extensibilities
between 30 and 45%. Specific gravity varies from 1.25-1.35, and the equilibriummoisture regain, from about 1.5-2.5% under standard conditions, depending on chemical
composition. In general, the modacrylic fibers are more easily dyed with disperse and
basic dyes than the 'acrylic fibers.
Comprehensive reviews of acrylic and modacrylic fibers have been prepared (172,178)
Olefin Fibers
Polypropylene. The discovery of stereospecific polymerization in 1954 opened the way
for polypropylene to join the ever-growing family of fiber-forming polymers. High
molecular weight isotactic polypropylene was found to be a most suitable raw material
for fiber formation.
These long-chain molecules are helical in shape and can be easily crystallized and
oriented. The fiber is manufactured by the melt-spinning process, but because of the
very high molecular weight of fiber-forming polypropylene, ca 200,000, the extrusion
temperature is more than 100°C above the crystalline melting point in order to decrease
the melt viscosity to practical levels for processing. The molten polymer is extruded
through a spinneret under pressure after preliminary filtration and deaeration. The
newly formed filaments are either ai.r- or water- quenched. The rate of cooling largely
controls the crystalline texture that is obtained. Rapid, low temperature quenching
retards crystallization; slower, rel-
FIBERS 715
atively high temperature quenching permits more complete development of crystallites.
The ability to undergo subsequent drawing and consequently the mechanical properties
depend on the quenching process (see also PROPYLENE POLYMERS).
Normally the quenched filaments are heated and drawn to develop molecular
orientation along the fiber axis. To relieve internal strains, the filaments are heat-set or
annealed. This last step also aids in the development of a more perfect crystal structure.
Fibers with degrees of crystallinity of about 70% can be obtained under optimum
quenching and annealing conditions. Several studies relate molecular weight, degree of
crystallinity, degree of orientation, and crystal size and habit, with fiber properties (174176). Orientation (qv) has been found to be the most important structural variable as far
as mechanical properties are concerned. Orientation can be achieved by drawing'or by
high-speed spinning. In the latter case, the spinline stress on the filaments is the
critical factor (Fig. 35). The effect of increasing windup speed from 1000-7000 m/min, is
an increase in elastic modulus and the tenacity at break, and a decrease in extensibility.
Similar effects are produced by drawing. Supertenacity polypropylene fibers have been
made with tenacities at break up to 1.15 N/tex (13 g-f/den) by inducing extremely high
orientations and well-developed crystal structures (175). The elastic recovery of
polypropylene filaments is quite high, although strongly time-dependent.
Among various special polypropylene fibers that have been prepared are those referred
to as "hard” elastic fibers. These are crystalline fibers with relatively high elastic
moduli, and with high elastic recoveries (50-95%) from large deformations (see
ELASTICITY, hard elastic behavior). In polypropylene they can be produced by adjustments of
spinning and annealing conditions to create porous structures that provide an energydriven recovery mechanism (177,178).. This is in contrast to those elastic fibers that are
based on elastomeric polymers (rubbers) and spandex fibers whose recovery mechanism
is largely entropy-driven.
7,16 FIBERS
Polypropylene fibers are also extensively produced from film (179). In those processes,,
the film is formed by normal melt extrusion and high levels of drawing, after which fiber
is obtained by slitting, splitting, or cutting.
Polypropylene fibers are the lightest of the commercial textile fibers, with a specific
gravity of about 0.90-0.92. The fibers absorb virtually no moisture from the atmosphere
because of the high crystallinity and lack of polar-sorption sites. As a result, the
mechanical properties are almost completely insensitive to variations in rh. Strength,
extensibility, and stiffness in the wet state are identical to those under standard
conditions. Polypropylene is essentially inert to aqueous chemical systems, but it may be
swollen and in certain, instances dissolved in ' organic solvents at elevated
temperatures. Decalin and chlorinated hydrocarbons are particularly effective
polypropylene swelling agents. The crystalline melting point is about 165PC and the
fiber softens about 10° below its true melting point.
Polypropylene is subject to oxidative degradation, particularly when initiated in the
presence of light. Adequate^ stabilization (qv) is provided by incorporation of free
radical scavengers in the polymer melt prior to extrusion. More efficient stabilization is
achieved by compounds which also absorb ultraviolet radiation. Polypropylene is subject
to static electrification in normal processing and use because of its negligible moisture
regain. Appropriate surface finishing agents are required to minimize the static problem
(see ANTISTATIC AGENTS). The
properties of polypropylene and other olefin fibers are summarized in Table 18.
■ -> ' \ ■ \\ ■ .* < ,
’■
The fibers are available for textile purposes as continuous filaments and in staple form.
As is generally the case, staple fibers are subjected to less drawing and consequently,
are somewhat weaker than continuous filaments. Normal polypropylene has a circular
cross section; however, interesting- surface effects can be produced with irregular fiber
cross sections resulting from filament extrusion through diversely shaped spinneret
orifices.
The lack of water-sorption sites, the high crystallinity, and the general • chemical
inertness of polypropylene poses a major dyeing problem. None of the
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FIBERS 717
existing classes of dyes provide an adequate range and depth of color with good fastness.
Several approaches can be taken to solve the polypropylene dyeing problem. The
incorporation of pigment in the melt prior to extrusion or dope dyeing provides a
number of basic colors. The drawbacks of this method are the relatively high cost, the
limited number of subtle shades available, and the often deleterious effects of the
pigment on fiber properties. Resin bonding of pigments on the fiber surface (in fabric
form) is another method of imparting color, although it is not used very often. The most
promising approach is that of chemically modifying the polypropylene structure to
provide dye-receptive sites. This can be accomplished by the addition of organic or
inorganic compounds to the melt prior to extrusion, chemical treatment of the
polypropylene in fiber form, and graft polymerization. Depending on the method of
chemical modification and the type of dye site introduced, polypropylene can be dyed
with acid, basic, and disperse dyes to provide a good range of colors with adequate
fastness.
Several excellent reviews of the production, structure, properties, and uses of
polypropylene have.been prepared (180-182).
Polyethylene. High pressure polymerization techniques yield low density polyethylene
(LDPE), a polymer more suitable for plastic than fiber applications. Fibers from LDPE
are quite weak and highly extensible even after extensive drawing, reflecting a poorly
developed crystal structure, as does the low specific gravity. Another drawback is the
low melting point (ca 115°C). Cross-linking by y-irradiation improves thermal stability,
but it is still quite inadequate for textile applications (see also Ethylene polymers).
High density polyethylene (HDPE), obtained by the polymerization of ethylene in the
presence of certain metal catalysts, is a somewhat more promising fiber-forming
polymer. Fibers from HDPE have better molecular packing and higher crystallinity,
with a melting point of about 135°C. The fibers are melt- spun or hot-solvent dry-spun,
and drawn to improve orientation. Although chemically quite inert, the fibers shrink
when exposed to elevated temperatures in the presence of water, indicating poorly
developed crystal structure and lack of intermolecular attractive forces. Polyethylene is
used almost exclusively in in- - dustrial applications in monofilament form, and no
staple is now being produced.
Polyethylene-polypropylene bicomponent fibers in a sheath-core configuration, with the
polyethylene forming the sheath around the polypropylene-core, are used as thermalbonding fibers in nonwovens. This specialty bonding fiber is mixed with other base
fibers, usually polypropylene. The low melting polyethylene sheath provides good
interfiber bonding when the nonwoven structure is subjected to temperatures above
100°C, frequently while under compression.
One of the most important new developments is high strength, ultrahigh modulus
polyethylene fibers intended for special high-performance applications (183). These
fibers are produced by modification of a gel-spinning process of HDPE (184,185). After
fiber formation and quenching, the filaments are subjected to very high degrees of
drawing to achieve modulus and strength values equivalent at least to those of highperformance aramid fibers. Ultraoriented HDPE fibers are also produced by a solidstate extrusion (qv) process (186).
Other Polyolefins. Other olefinic polymers have been considered for fiber purposes
including poly(l-butene), polyf 1-pentene), poly(3-methyl-1-butene), poly(4- methyl-lpentene), poly(4-methyl-l-hexene), and polystyrene, which can be obVol. 6
718 FIBERS
tained with adequate molecular weight for fiber formation. The polymers are readily
crystallizable after melt extrusion and the orientation attained in drawing can be
stabilized by crystallization. The melting points, with the exception of pdly(l-butene)
and poly(l-pentene), are above 200°C. making them quite suitable for fiber applications.
The fibers have adequate mechanical properties and low densities. Crystalline poly(4methyl-l-pentene) was thought to be particularly promising' as a fiber, but its high
temperature strength retention is poor, a characteristic true to some degree for all the
olefin fibers (qv).
Spandex Elastic Fibers
An elastic fiber may be defined as one that recovers completely from long- range
deformations immediately upon removal- of the deforming force. Fibers made from
natural and synthetic latex rubbers were the only fibers from which elastic fabrics could
be manufactured. Although the-elasticity of these fibers is high, the force of recovery
from deformation and ultimate strength are not quite adequate. Their susceptibility to
oxidative-chemical degradation, and their poor dyeability,, are further.serious
drawbacks. The development of synthetic elastic fibers follows the principles established
by the classical theory of rubberlike elasticity (qv) (187,188). According to this theory, it
is necessary that long-chain, liquidlike polymer segments be joined at discrete points to
provide the recovery forces. The polymer chains -must be flexible and easily extendable
to an oriented configuration after a tensile load has been applied to the fiber, but must
also be able to return-spontaneously to a disordered state after the load has been
removed. Thus, a synthetic elastic fiber should contain soft, extendable polymer
segments and hard tie regions that bind the chains together to provide the retractive
forces.
These structural requirements are found in polyurethanes (qv) (189-191). The fiberforming substance in spandex (elastic) fibers is a long-chain, synthetic polymer
composed at least 85% by weight of a segmented polyurethane. The term segmented
refers to alternating soft and hard regions in the polymer structure. The formation of
the segmented polyurethane structure for spandex fibers takes several distinct steps.
The first step involves the formation of flexible linear polyglycol chains, which may be
either polyesters or polyethers and are referred to as macroglycols. They have molecular
weights between 500-4000 and have reactive hydroxyl groups at both ends. The next
step in the formation of the soft segment involves the reaction Of the macroglycol with
an excess of a diisocyanate, usually aromatic. This results in the formation of an
isocyanate--terminated soft- segment prepolymer. In the next step, the hard segments
are formed by reaction of the isocyanate-terminated prepolymer with low molecular
weight glycols or diamines. This results in a polymer with hydrogen bonding sites
through either urethane or urea groups. These bonding sites provide the tie points in
the segmented polyurethane structure that are responsible for long-range elasticity.
FIBERS 719
Spandex fibers can be formed as continuous filaments by traditional dry- and wetspinning processes. Dry spinning with dimethylformamide or dimethyl- acetamide as
solvent is by far the most extensively used method. Some use has been made of reaction
spinning (192,193). In reaction spinning, the diisocyanate- terminated soft segment
prepolymer is extruded into an environment containing the glycol or diamine. The
reaction that creates the urethane or urea groupings, takes place after fiber formation. *
Spandex fibers have high extensibilities and low elastic moduli, and very high elastic
recoveries from large deformations. It is- quite evident from Table 19 that the
mechanical properties of spandex fibers closely approximate those of the natural rubber
fibers, with the added feature of greater strength. Spandex fibers are resistant to
chemical degradation, light, and uv radiation. The fibers also have adequate thermal
stability, with softening temperatures somewhat above 200°C. Spandex fibers may be
dyed with many different classes of dyes; the best results are obtained with acid and
disperse dyes.
.
Spandex fibers are usually processed into fabrics as covered yarns (191,194). Covering
the elastic fiber with either staple or continuous filament hard fibers, eg, polyester,
polyamide, cotton, and wool, protects the elastic fiber and modifies the physical/chemical
properties of the composite. Core spinning, a means of forming a composite yarn, feeds
partially extended spandex continuous filament into a spinning frame together with
staple hard fibers. In a core-spun composite yarn, the spandex filaments form an inner
core with a sheath of staple fibers around it. Due to the partial extension of the spandex
filaments before core spinning, a fabric woven or knitted from these yarns will shrink
during wet- finishing. The final fabric has high stretch and, most importantly, high
recoverability (see FIBERS, ELASTOMERIC).
Vol. 6
720 FIBERS
Carbon Fibers
Because these fibers combine low density with exceptional mechanical properties, they
have found increasing use as reinforcing elements in fiber-reinforced composites (qv)
(195,196). Carbon fibers (qv), along with certain aramid and glass fibers, provide
strength and durability in terms of engineering thermosetting and thermoplastic resins
to make composites one of the most exciting materials of the future. The fibers consist of
small crystallites of "turbostratic” graphite, which is structurally similar to crystalline
graphite. Both have layer planes of hexagonal ly arranged carbon atoms held together
by strong covalent bonds; weaker van der Waal interactions occur between layer planes.
Thus, the elastic modulus parallel to the layer planes is many times larger than that
perpendicular to the planes. As a result, the high-performance characteristics of carbon
fibers depend upon the preferential orientation of the graphite layers parallel to the
fiber axis.
Carbon fibers typically are fabricated, from continuous precursor fibers by a three-stage
procedure. Initially, the precursor fibers are stabilized at low temperatures to prevent
fusion or melting in later stages. Noncarbon elements are then eliminated during a
carbonization heat-treatment step; and finally, a high temperature graphjtization stage
enhances the mechanical properties of the carbon fiber. Depending on the particular
precursor, preferential orientation of the graphite layers parallel to the fiber axis may
occur during any portion of the fabrication process. Orientation may be achieved by
spinning hydrodynamics during the fiber-forming process, by stretching during
stabilization, or by plastic deformation during the graphifcization stage. Although many
materials may be converted to carbon fibers, a successful precursor must have a high
carbon yield relative to its cost, and it must maintain its filamentary morphology during
the conversion process. Carbon fibers have been successfully produced from many
polymers, but large-scale production of carbon fibers is currently limited to polyacrylonitrile (PAN), cellulose, and pitch.
PAN As a Precursor. PAN-based high-performance carbon fibers currently comprise
approximately 80% of the carbon fiber market. The initial stage in the conversion of
PAN precursor fibers to carbon fibers is a low temperature stabilization, involving
temperatures of 200-300°C for several hours in. an inert or air atmosphere. This process
converts thermoplastic PAN to a thermally stable, cyclized structure thought to involve
the formation of a ladder polymer:
— CH, ^CH, /CH, ^ CH '"CH ""CH
Secondary scission reactions involving the polymer backbone are also believed to occur.
To prevent shrinkage of the fiber and to align the ladder polymer chains parallel to the
fiber axis, the stabilization stage is performed under tension. Typically, a fiber is
stretched to over fifteen times its original length. Carbonization of the thermally
stabilized fibers proceeds in an inert gas or under vacuum at temperatures of 3001500°C. During this process heteroatoms in the ladder polymer are removed as volatiles,
and the turbostratic layer planes are developed. The principal volatiles removed are
HCN. NH3, and N2. The fiber loses approxVol. 6
Vol. S FIBERS 721
imately 50% of its mass during carbonization, and the resultant fiber contains more
than 90% carbon.
In the final step, graphitization of the carbonized fiber occurs upon" heating to
temperatures above 2500°C. At these temperatures, the ordering and orientation of the
layer planes parallel to the fiber axis are improved without further loss in mass. The
mechanical properties of the carbon fiber are directly related to the final graphitization
temperature.
Cellulose As a Precursor. Rayon fibers were among the first to be carbonized (197). Low
temperature degradation in a reactive environment, eg, air, chlorine, or hydrogen
chloride, at temperatures up to 400°C, is the first stage in the conversion process. The
result of the degradation is the formation of four- carbon residues, which are believed to
consist of furan derivatives. Subsequent carbonization of the residues occurs in an inert
atmosphere from 400~1500°C, during which the furan derivatives condense into sixcarbon graphitic structures. The graphitic structure developed during carbonization
lacks a preferred direction, and orientation of the layer planes must occur during high
temperature graphitization. This last step is performed under tension for short times at
temperatures exceeding 2800°C, and longitudinal orientation of the graphitic layers
with respect to the fiber axis occurs by plastic deformation. Again, the mechanical
properties of the fiber are directly related to the final graphitization temperatures as
well as the magnitude of applied strain.
Pitch As a Precursor. The development of pitch-based carbon fibers has occurred over
the last two decades. In general, pitch materials are collections of condensed benzenering structures integrated with alkyl chains and possessing molecular weights from 7002400. The suitability of a pitch material for conversion to carbon fibers depends on its
ability to be spun into fibers and then heat-treated to an infusible stage. Commonly
used isotropic pitches are obtained from coal tar and petroleum asphalt. After the
molecular weight and chemical composition of the pitch is adjusted in a pretreatment
stage, the bulk pitch is extruded into fibrous form. Subsequent oxidation (stabilization),
carbonization, and graphitization stages are similar to those of other precursor
materials. During the carbonization stage all preferred orientation is lost and
graphitization must be performed under strain. This costly process, coupled with a
lengthy oxidation stage, makes the fabrication of high-performance carbon fibers from
isotropic pitches industrially unattractive.
The use of mesophase pitches does not require stretch graphitization. A mesophase
pitch is obtained by heating certain isotropic pitches for prolonged periods of time at
temperatures above 350°C. Condensation reactions occur and aggregates of large
molecules form an anisotropic liquid crystalline phase, the mesophase. Since mesophase
pitches are thermodynamically stable, they will not revert to an isotropic liquid unless
heated above the mesophase liquid transition temperature. Since the decomposition
temperature for most pitches is lower than the mesophase liquid transition
temperature, a mesophase pitch-based fiber retains its preferred orientation during
carbonization and graphitization. Preferred orientation is induced during the fiber
forming process. The mesophase pitch is melt-spun through a spinneret to produce a
"green yarn” as the aromatic molecules align parallel to the fiber axis because of the
high strain rates employed. Conversion of the mesophase pitch to a completely infusible
stage is accomplished
722 FIBERS
in an oxidative environment. This process is limited by the rate at which oxygen can
diffuse to the isotropic domains of the fiber. After stabilization, the precursor fiber is
carbonized and graphitized in a manner similar to PAN precursor materials. The end
product is a highly oriented, high-performance carbon fiber obtained without stress
graphi'tization.
Fiber Properties. The physical properties of carbon fibers depend on internal structure,
which in turn depends on the precursor and on processing conditions. A typical highperformance carbon fiber may have a diameter of 8 /xm, a specific gravity of i.95, an
elastic modulus of 390 GN/m2, and a tensile strength of 2.2 GN/m2: The elastic modulus
increases with rising graphitization temperature (Fig. 36), but the tensile strength
passes through a maximum at a graph- itization temperature of about 1300°C (198). In
general, strength is limited by both internal and external flaws, in the fiber structure. It
has also been shown that the elastic modulus increases with increasing orientation of
the graphitic .layers with respect to the fiber axis (199).
.
.
Graphitizing temperature °C
Since carbon fibers are used mostly as reinforcing elements in composites, when
adhesion between fiber and matrix is of critical importance, the surface texture and
properties of these fibers is usually given special attention. Carbon fibers are relatively
smooth with specific surface areas of 0.1-2 m2/g. There are usually some longitudinal
striations, but otherwise no major surface roughness. To improve adhesion to both
thermosetting and thermoplastic resins, carbon fibers are usually subjected to
controlled surface treatments, including chemical vapor deposition, oxidative etching,
and polymer coating. These fibers are still in the developmental stage, and improved
and less expensive carbon fibers can be expected in the future.
Vol. 6
FIBERS 723
VinyS Fibers
Two principal fiber types fall under the general category of vinyl fibers (200): Those
fibers that contain at least 85% by weight of vinyl chloride are generically referred to as
vinyon fibers; those that are composed of at least 50% by weight of vinyl alcohol are
referred to as vinal fibers. Other fibers in this category are based on vinylidene chloride
or contain fluorine.
Vinyon Fibers. Poly(vinyl chloride) (PVC) fibers, generally containing about 10% vinyl
acetate units, are produced by dry spinning using mixed solvents such as acetonebenzene. One particular fiber is wet-spun from cyclohexanone solution and coagulated
in an aqueous bath. Some PVC fiber is melt-spun, but special conditions must be used
because of the polymer’s limited thermal stability and high melt viscosity. The extruded
fiber is drawn in steam or in water at nearly 100°C, and frequently is also subjected to
thermal-setting treatments. Typical values of tensile strength for a drawn fiber are
0.18-0.26 N/tex (2.0-3.0 g-f/den) with extensibilities ranging from 10-20%. Vinyon fibers
are inherently flame- retardant in view of their high halogen content (see also VlNYL
CHLORIDE POLYMERS).
Vinyon fibers have equilibrium-moisture regains less than 0.5% under standard
conditions, and their mechanical properties are relatively unaffected by moisture.
Chemical'and microbial resistance are particularly good for these fibers. Specific gravity
is 1.35 and melting point is about 135-150°C, although the fibers soften and become
tacky above 80°C. In view of the relatively low softening point, these fibers are
frequently used as bonding agents in nonwovens.
Vinal Fibers. Vinal fibers, or poly(vinyl alcohol) fibers, are currentiy not made in the
United States, but the fiber is produced commercially in Japan where the generic name
vinylon is used (201). The poly(vinyl alcohol) (PVA) polymer is made by saponification of
poly (vinyl acetate), which in turn is obtained from free radical polymerization of vinyl
acetate. PVA fiber is produced by wet spinning from an aqueous solution into a
coagulating bath containing sodium sulfate. The fibers are drawn under wet or dry
conditions (or both) to develop orientation and crystallinity, and further heat-treated to
improve hot water resistance. Thermal treatments at temperatures up to 220°C induce
further crystallization. PVA fibers are frequently treated with formaldehyde under
acidic conditions to cross-link the polymer chains. These acetalization reactions improve
the stability of th'ese fibers to high temperature water environments. Another method of
fiber manufacture involves wet spinning from an aqueous solution into NaOH solutions,
and dry-spinning methods have also been explored.
The mechanical and other physical properties of PVA fibers depend on processing
conditions (drawing and heat setting) arid on the degree of acetalization. Typically, PVA
fibers are quite strong, up to 0.79 N/tex (9 g-f/den), under both wet and dry conditions,
with extensibilities in the 10-20% range. They can have reasonably high elastic modulus
values, up to 8.8 N/tex (100 g-f/den), and high resilience. They have a specific gravity of
1.26, an equilibrium moisture regain under standard conditions of 3.5-5.0%, and a
softening temperature of 220-230°C (see also VINYL ALCOHOL POLYMERS).
Vinylidene Fibers. Fibers based on poly(vinylidene chloride), from the addition
polymerization of vinylidene chloride, CH2=CCl2? are generically known
Vol. 6
FIBERS 725
novolac resin (202). These fibers are highly flame-resistant and decompose to form a
protective char. The fibers have reasonable textile properties (strength, extensibility,
etc) and are used primarily in protective materials and as reinforcement or fillers in
certain thermosetting resins. Their thermal properties allow carbonization of novoloid
fibers with maintained configuration of the precursor material.
Poly(phenylene sulfide) Fibers. The PPS polymer is produced by reaction of pdichlorobenzene and sodium sulfide, and is used as an engineering thermoplastic resin.
It has good dimensional stability and inherent flame resistance, as well as thermal
stability and chemical resistance. Improvements in the original polymerization process
allow the production of a fiber-grade linear PPS polymer, and fiber is now produced by
melt spinning followed by drawing at elevated temperatures (203). High crystallinity
can be achieved by further annealing of the drawn fiber. PPS fiber has a tenacity at
break of 0.3 N/tex (3.5 g-f/den), extensibility of 30%, and an elastic modulus of about 2.63.5 N/tex (30-40 g-f/den). Melting point is 285°C, specific gravity is 1.37, and the
equil'ibrium-moisture regain is 0.67c. The fiber retains strength even after long-term
exposure to a temperature of 232°C, comparable to other high-performance fibers. It is
used in the form of both woven and nonwoven fabrics in air filtration applications, and
as conveyor belts in high temperature drying operations (see PoLY(ARYLENE SULFIDES)).
PBI Fibers. Polybenzimidazole (PBI) polymer, which is produced from diphenylisophthalate and 3,3',4,4'-tetraaminobiphenyl, is dry spun from dimethylacetamide solution (204). After thorough water washing and drying, the fiber is drawn
at high temperatures (above 400°C). The fiber is stabilized by a two-stage sulfuric acid
treatment after drawing. The fiber has a tenacity at break of 0.27 N/tex (3.1 g-f/den),
extensibility of 30%, and an elastic modulus of 4.0 N/tex (45 g-f7den), although
considerable variations in mechanical properties can be achieved by modifications in
processing conditions. It has a specific gravity of 1.43 and an equilibrium-moisture
regain of 15%. PBI fiber is nonflammable and has good thermal stability at elevated
temperatures. Its major uses are in fire-protective clothing, in filter media, and as an
asbestos replacement (205) (see POLYBENZ- IMID AZOLES). '
Polymer Blend Fibers. The use of two or more different polymers in the manufacture of
a synthetic fiber to produce a polymer blend fiber is becoming increasingly important
(206.207). These fibers can be either homogeneous, ie, the two polymers are intimately
mixed into a single phase, or heterogeneous, in which a multiphase structure is
developed. In general, homogeneous polymer blend fibers have not been developed
because they have no particular advantages over those produced by the use of a single
specially designed polymer. Furthermore, maintaining an intimate homogeneous blend
of two different polymers is quite difficult because of incompatibilities and the
thermodynamic tendency to phase separation (see COMPATIBILITY). Heterogeneous
polymer blend fibers, on the other hand, have generated a great deal of interest, and
many different types are commercially available. In the generally accepted
nomenclature, "biconstituent” fibers are those in which the two polymers belong to
genericallv different classes, whereas in "bicomponent” fibers the two polymers belong
to the same generic class. The geometric arrangement of the two phases serves as the
basis of clasVol. 6
726 FIBERS
sification. There are three major classes: side-by-side, sheath-core, and matrix- fibril,
together with many variants of these. Heterogeneous polymer blend fibers are formed
by special extrusion techniques involving controlled flow of two liquid polymer streams
and their delivery to specially designed spinneret orifices. The properties of these fibers
depend on the properties of the component polymers and on their spatial arrangement.
The side-by-side polymer blend fibers, with differential shrinkage properties, are the
basis of structurally crimped, fibers (170). Sheath-core structures, containing outer
material with a relatively low softening or melting temperature, are used as bondingfibers in thermally bonded nonwovens. Other special properties, and effects produced
with heterogeneous polymer blend fibers are high absorbency, flame retardance, silklike
luster, ul- trafine fibers, enhanced dyeability, and soil-hiding capability (see also
POLYMER BLENDS).
• Inorganic- Fibers
Asbestos. Asbestos is a generic name for a number of minerals that occur naturally in
fibrous form (208,209). Over 75% of the commercial asbestos is obtained from Canada.
Other major producers of asbestos are the Soviet Union, South Africa, Rhodesia, and the
United States. Although there are several different classes of asbestos, chrysotile is by
far the most important for textiles. Its structure is that of a hydrated basic silicate of
magnesia with varying water content (12-15%) that can be formulated as 3MgO2SiOo*2HoO. Other forms of asbestos, which contain appreciably less water, are
anthophyllite, tremolite, and actinolite. These more rare forms of asbestos contain
various quantities of iron, calcium, and magnesium as part of the hydrated-silicate
structure.
Chrysotile asbestos is obtained in lengths varying from a fraction of a cm up to 5 cm.
Asbestos fibers are extremely fine but have more than adequate strength for textile
purposes, although they have low extensibilities. The fibers are extremely resistant to
heat and will not burn. Chrysotile asbestos fibers lose. less than 15% of their weight
after a 2-h exposure to temperatures up to ca 1000°C, and adequately retain mechanical
properties.
Products manufactured from asbestos fibers have been used primarily in industrial
applications when heat, resistance is required in combination with mechanical stability.
Because asbestos has been identified as a carcinogen, its use is now strictly regulated
and severely restricted.
Glass. Glass fiber is the only inorganic synthetic fiber that is used extensively in the
textile industry, particularly for industrial products and household items such as
drapery material (210). Continuous glass filaments are manufactured by the meltspinning process. The main ingredients of a typical glass are silica (sand), limestone,
aluminum hydroxide, soda ash, and borax. The glass is formed into small beads which
are the starting material for the manufacture of glass fiber. The glass marbles are
melted and the molten glass is extruded through bushings (spinnerets) in the usual
manner. The jets of molten glass solidify almost immediately into filaments, and as in
the case of all synthetic filaments, they are immediately treated with a surfaceprotective spin finish which for glass is called a sizing agent. After extrusion and sizing,
the filaments are wound in
Vol. 6
FIBERS 707
Many aromatic polyamide polymers have been synthesized, but only a few have found
general use as fiber-forming materials. Reviews of the structure and properties of
aromatic polyamide polymers and aramid fibers have appeared (157-159) (see also
POLYAMIDES, AROMATIC). Aramid fibers are characterized either by flame resistance and
thermal stability or by high strength and stiffiiess. Combinations of these highperformance properties are also possible. In general, flame resistance and thermal
stability are characteristic of the m-isomers, whereas high strength and stiffness are
associated with the p-isomers.
The first aramid fiber to be commercialized successfully is based on poly(mphenyleneisophthalamide) under the trade name of Nomex. It is flame-resistant and
retains its physical properties even after long-term exposure to temperatures as high as
300°C. Flame resistance and thermal stability can be enhanced even further by various
chemical additives, ie, by incorporating phosphorus.
Aramids
As a result, after extrusion in a modified (dry-jet or air-gap) wet-spinning proces very
high orientation and crystallinity are achieved. These fibers are extreme strong and
stiff, especially when considered on the basis of strength-to-weig ratio (160). The tensile
strength of Kevlar and nylon-6,6 filaments as a functi temperature are compared in
Figure 30. Two principal forms of Kevlar are produc the regular Kevlar 29, and a more
highly drawn Kevlar 49. These high strenj and high modulus fibers are used as tire and
advanced composite reinforceme ropes and cables, and for ballistic protection. A new
version called Kevlar p is used as an asbestos replacement in friction products such as
brake lini (161) (see FIBERS, ENGINEERING). Physical properties of the principal comme:
aramid fibers are summarized in Table 15.
An aramid fiber based on poly(p-benzamide) was introduced in 1970 as a high strength
fiber for use as tire reinforcement. This fiber has been replaced by the aramid fiber
Kevlar, the nominal structure of which is poly(p-phenyleneter- ephthalamide). The
rigidity of this linear structure gives the chain a rodlike conformation, and the pol}rmer
takes a nematic, liquid-crystalline form in 100% sulfuric acid solution.
FIBERS
708
Vol. 6
Polyester Fibers
Synthetic fibers, formed from a synthetic polymer composed of at least 85% by weight of
an ester of a dihydric alcohol and terephthalic acid, are known as polyesters (qv) (162).
In one polyester fiber type, 1,4-dimethylolcyclohexane is used. The most common
polyester fibers have ethylene glycol as the dihydric alcohol.
FIBERS 727
standard packages for subsequent textile processing. Staple-length glass fibers,
sometimes known as blown fibers, are manufactured by a somewhat different process
from that used for other synthetic fibers. Upon extrusion, the continuous filaments of
glass are subjected to the action of high-pressure steam jets which attenuate the glass
filaments just prior to solidification, and break them up into staple lengths. Some staple
glass fiber is processed into textile spun yams, but a great deal of this material is used
in bat or web form for filtration and insulation.
The outstanding properties of glass fibers are their chemical and thermal resistance,
nonflammability, and inertness to microbial degradation. Glass fibers have extremely
high electrical resistance and are dimensionally stable when exposed to elevated
temperatures. The filaments and staple fibers are strong, although inextensible and
quite brittle. In comparison with the more common textile fibers, they are dense, with a
specific gravity of —2.5. Glass fibers absorb virtually no moisture from the atmosphere,
and their mechanical properties are nearly identical .under wet and dry conditions.
Glass fibers have no affinity for any of the common textile dyes, but they can be colored
by incorporating a suitable pigment into the molten glass before extrusion. Another
method of coloring glass fiber in fabric form is by padding a latex or synthetic resin on
the fabric which is substantive to the glass fiber and which will be capable of accepting
one of the standard classes of dyes. In addition to providing dye sites, the resin and
other surface-coating agents protect the fiber against abrasion. The development of
these glass-fiber surface additives has enabled many successful applications of
continuous and spun yarns in industrial and apparel products.
Both blown-glass fibers (short staple length) and continuous-filament-glass yarns can be
used as reinforcements in thermosetting and thermoplastic matrices of composites. The
chemical and physical structure of the glass fiber surface is critical in the development
of strong and effective bonding between the fiber and matrix. Chemical modification
treatments of glass fiber surfaces improve interfacial adhesion. A common treatment
involves silane coupling agents (qv) which are most effective with epoxy thermosetting
resins (211). Glass yarns are also used as reinforcements in pneumatic tires.
Metallic Fibers. Fibers and yarns can also be produced from metallic substances. For
example, a plastic-coated aluminum fiber is a common metallic yarn. An aluminum
sheet or foil is coated on both sides with a cellulose acetate—butyrate or a polyester
plastic and cut into filaments of desired dimensions. Such metallic yarns are used
primarily for decorative purposes. Metallic fibers are also manufactured in continuous
form by repeated attenuation and drawing through diamond and tungsten dies.' It is
possible to weave traditional textile fabrics from steel filaments. Such filaments are
expensive and are used only in special applications.
Other Inorganic Fibers. A number of fibers have been developed with inorganic lattice
structures. Inorganic fibers (qv) are being made from alumina, silica, silicon carbide,
boron nitride, and boron carbide. These fibers are extremely strong, although
inextensible, and are intended for ultrahigh temperature uses. One method of
manufacture involves the extrusion of a cellulose inorganic filament by the standard
viscose process. These filaments are then ignited and sintered to remove the cellulosic
component.
Vol. 6
728 FIBERS
BIBLIOGRAPHY
"Fibers” in EPST 1st ed., Vol. 6. pp. 505-573, by Ludwig Rebenfeld, Textile Research Institute.
1.
Text. Organon. 56, (1985).
2.
B. C. Goswami, J. G. Martindale, and F. L. Scardino, Textile Yarns; Technology, Structure and
Applications, John Wiley & Sons. Inc., New York, 1977.
31 J. W. S. Hearle, P. Grosberg, and S. Backer, Structural Mechanics of Fibers, Yarns, and Fabrics, John Wiley &
Sons, Inc:, New York, 1969.
4.
J. Luhenschloss and W. Albrecht, Nonwoven Bonded Fabrics, Halsted Press, John Wiley & Sons, Inc.,
New York, 1985.
5.
R. Krcma, Manual of Nonwovens. Textile Trade Press, Manchester, UK; W. R. C. Smith Publishing
Co., Atlanta, Ga, 1971.
6.
J. Lunenschloss and W. Albrecht, Vliesstoffe, Georg Thieme Publishers, Stuttgart, FRG., New York,
1982 (in German).
7.
R.
JefTries,'J. Text. Inst. 51. T339. T399. T441 (I960).
8.
J.
W. Rowen and R. L. Blaine. Ind. Eng. Chem. 39, 1659 (1947).
9.
J.
W. S. Hearle and R. H. Peters. Moisture in Textiles, The Textile Institute,
Butterworths
Scientific Publications, Manchester. UKV 1960.
10.
J.
F. Fuzek, Ind. Eng. Chem. Prod. Res. Dev. 24, 140-144 (1985).
11.
T.
Vickersta ff, The. Physical Chemistry of Dyeing, 2nd ed., Wiley-Interscience Publishers,
Inc.,
New York, 1954.
12.
R. H. Peters, "Textile Chemistry” in The Physical Chemistry of Dyeing, Vol. 3, Elsevier Science
Publishing Co., New York, 1975.
13.
H. D. Smith, Proceedings of the American Society for Testing and Materials, Vol. 44, p. 542, 1944.
14.
"Textiles” in 1985 Annual Book of ASTM Standards, American Society for Testing- and Materials,
Philadelphia, Sect. 7.
15.
D. J. Montgomery and W. T. -Milloway, Text. Res. J. 22, 729. (1952).
16.
J. H. Dillon, Ind. Eng. Chem. 44, 2115 (1952).
17.
W. J. Hamburger j Text. Res. J. 18, 102 (1948).
18.
G. Susich and S.. Backer, Text. Res. J. 21, 482 (1951).
19.
R. Meredith, Mechanical Properties of Textile Fibers, Interscience Publishers, Inc., New York, 1956.
20.
J. D. Ferry, Viscoelastic Properties of Polymers, 2nd ed., John Wiley & Sons, Inc., New York, 1970.
21.
W. J. MacKnight and J. J. Aklonis, Introduction to Polymer Viscoelasticity, 2nd ed., John Wiley & Sons,
Inc., New York, 1983.
22.
D. C. Prevorsek and W. J. Lyons. Rubber Chem. Technol. 44, 271-293 (1971).
23.
W. J. Lyons, Impact Phenomena in Textiles, MIT Press, Cambridge, Mass., 1963.
24.
W. E. Morton and J. W. S. Hearle, Physical Properties of Textile Fibers, 2nd ed., The Textile Institute,
Butterworths Scientific Publications, Manchester, London, 1975.
25.
L. R. G. Treloar, Phys. Today, 23-30, (Dec. 1977).
26.
J. W. S.- Hearle, "Polymers and Their Properties” in Fundamentals of Structure and Mechanics, Vol. 1,
Halsted Press, John Wiley & Sons, Inc., New York, 1982.
27.
A. V. Tobolsky, Properties and Structure of Polymers, John Wiley & Sons, Inc., New York, I960.
28.
R. J. Samuels, Structured Polymer Properties, John Wiley & Sons, Inc., New York, 1974.
29.
L. Mandelkern, Crystallization of Polymers, McGraw-Hill, New York, 1964.
30.
F. Happey, Applied Fiber Science, Vols. 1, 2, and 3, Academic Press, New York, 1978, 1979.
31.
Milton Harris, ed., Handbook of Textile Fibers, Textile Book Publishers, John Wiley & Sons, Inc., New
York, 1954.
32,. E. Pacsu' in L. Zechmeister, ed., Fortschritte der Chemie Organischer Naturstoffe, Vol. 5, Springer- Verlag,
Vienna, 1948, p. 128.
33. E. Ott, H. M. Spurlin, and M. W. Grafflin, eds., Cellulose and Cellulose Derivatives, Wiley- Interscience, New
York, 1954; -N. M. Bikales and L. Segal, eds., Pts. IV and V, 1971.
* 34. G. Jayme and F. Lang, "Methods in Carbohydrate Chemistry,” in R. L. Whistler, ed., Cellulose, Vol. 3,
Academic Press, .New York, London, 1963, p. 75.
Vol. 6
709
The free terephthalic acid, or its methyl ester, is polymerized with ethylene glycol in
vacuum by a condensation mechanism at elevated temperatures. The.polymer may be
isolated and formed into chips for subsequent handling, but the current trend is toward
continuous processes where fiber formation immediately follows polymerization.
Polyester fibers are melt-spun, normally through circular spinneret holes, although a
variety of profiled fibers can be produced by specially designed spinneret orifices, as in
the case of polyamide fibers. The molten polymer jets solidify almost immediately after
extrusion. The filaments are drawn to develop orientation and crystallinity by a factor of
about four, and wound for direct use as continuous multifilament yarn. Alternatively,
the continuous filaments can be crimped after drawing and cut into desired lengths for
use as staple fiber. Since the Tg of polyesters is ca 80°C, the filaments are hot-drawn in
most cases. Some very coarse polyester monofilaments can be cold-drawn to low-draw
ratios. The microfibrillar structure model (154) for a fully drawn polyester fiber is shown
in Figure 31.
Microfibrils i \iSli (jjllili-u •: 1
i
\
.
Fig. 31. Morphology of polyester fibers (154).
As in the case of polyamide fibers, high-speed spinning is beginning to replace the
traditional two-step spinning and drawing process (151,163). Similar, but not fully
equivalent, crystalline structures are developed in polyester by highspeed spinning as
well as by the two-step process. The effects of drawing (draw ratio) and increasing
windup speed on the tensile properties of polyester are shown by the stress-strain
curves in Figure 32.
The properties of polyester fibers are summarized in Table 16. The tensile stiffness or
elastic modulus at low strains is much higher for drawn polyesters than for
corresponding polyamides. Polyesters have high elastic recoveries, particularly from
small deformations. An important characteristic of polyesters is that their mechanical
properties in the wet state are virtually unchanged from those under standard
conditions. Polvester fibers have excellent resistance to
Table 16. Typical Propertied of Polyester Fibers
Property
Continuous
filament
Staple
tenacity at break, N/tex" 65% rh, 0.35-0.53
21°C
0.31-0.44
wet
0.35-0.53
extension at break, % rh, 21°C ■' 15-30
0.31-0.44
25-45
wet
elastic modulus, N/tex“ 65% rh,
21CC
moisture regain at 65% rh, %
specific gravity
approx. volumetric swelling
in water, %
15-30
7.9
25-45
7.9
0.4
1.38
none
0.4 •
1.38
none
a
To convert N/tex to g-f/den, multiply by 11.3.
acids, alkalies, and microbial attack. They have good! resistance to light and actinic
degradation. Moisture regain under standard conditions is about 0.4% which
contributes to the fibers’ high electrical resistivity and makes polyester subject to static
electrification.
Dyeing of polyester fibers is difficult because of the lack of hydrophilic sites and the
inherent stiffness of polyester molecules. Certain disperse dyes have been used by
conventional methods, but new techniques of dyeing (qv) had to he developed for fullshade ranges with adequate fastness. These methods are high temperature dyeing
(under pressure above 100°C), carrier dyeing (use of polyester-swelling agents such as
phenylphenols), and the Thermosol method of dye fixation. The latter process is
continuous and particularly effective for polyester/cotton blend fabrics. It involves
padding of the fabric with a paste of dyestuff, usually a mixture of vat and disperse
dyes, followed by passage of the dried, prepadded fabric through an oven or over heated
rolls at temperatures of about 200°C for short periods of time (ca 1 or 2 min). At these
temperatures the dyes dissolve in the polyester fiber, thereby penetrating into the
internal structure.
710 FIBERS
Fig. 32. Effects of draw ratio and windup speed on the load-extension curves of PET
fibers (164). Winding speeds in m/min.
•
Vol. 6
■’
Strain, % Strain.
.
%
•
■
'•
V'v
;
\
.
'
'
Vol. 6 FIBERS 711
Another approach to dyeing has been to modify the basic structure by incorpo- ratipn of
other dibasic acids or dihydric alcohols in the polyester backbone. Sulfonic acid or other
active groups may also be incorporated as dyeing sites for basic dyes and also to
enhance dyeability with disperse dyes.
Their ability to be set into desired configurations is an outstanding characteristic of
polyester fibers. Such setting operations, conducted in the yam or fabric stage at
temperatures about 160°C, are essentially recrystallizations. The changes in fiber
properties depend on the mechanical constraints on the fiber during heat setting. If free
shrinkage is permitted, the fibers increase in extensibility and decrease in tenacity. The
reverse effects are obtained if heat setting is performed under tension. Polyester yarn
and fabrics are heat-set to stabilize yarn twist, increase wrinkle resistance, obtain
durable creases and pleats, and impart dimensional stability. Yarn texturing to confer
bulk and loftiness to polyester yarns is similar to heat setting in that recrystallization
and setting in a crimped fiber configuration is involved.
Polyester fibers are hydrophobic with water having only minor effects on their physical
properties, but they are affected by a wide range of organic solvent systems. The
interactions between polyester and interactive chemical systems can lead to depression
of the Tg, secondary crystallization, and loss of orientation, which can have an important
effect on mechanical and physical properties. Such interactions are due to irreversible
structural modifications as well as solvent absorption and reversible swelling (165,166).
Liquid crystalline wholly aromatic polyester fibers based on poly(p-hydroxybenzoic acid)
are becoming increasingly important as high performance fibers (see POLYESTERS,
AROMATIC).
Acrylic and Modacrylic Fibers
Acrylic fibers (qv) are long-chain polymers composed of at least 85% by weight of
acrylonitrile units. A modacrylic fiber has less than 85%, but at least 35% by weight, of
acrylonitrile units.. Polyacrylonitrile (PAN) is formed by the addition polymerization of
acrylonitrile (vinyl cyanide). In most commercial acrylic fibers, various quantities of
other vinyl monomers are copolymerized with acrylonitrile. They confer specific
chemical and physical properties, and include such monomers as vinyl acetate, vinyl
chloride, styrene, vinylpyridine, acrylic esters, and acrylamide. In commercial
modacrylic fibers the comonomer is either vinyl chloride or vinylidene chloride.
These fibers are manufactured by either the dry- or wet-spinning processes. In dry
spinning dimethylformamide is a common solvent. The filaments are hot- drawn after
extrusion and usually have dog-bone, cross-sectional shapes. In wet spinning the
polymer is dissolved in solvents such as dimethylacetamide, dimethylformamide,
aqueous sodium-thiocyanate solutions, and aqueous nitric acid. The polymer solution is
extruded into aqueous precipitation baths containing various inorganic and organic
additives. Wet-spun acrylic fibers generally have circular or slightly elliptical cross
sections. Complex diffusional processes take place during precipitation in the wetspinning process involving solvent removal and subsequent coagulation and gelation of
the filament. The temper-