712 FIBERS
ature of the precipitation bath affects fiber structure and properties (167). 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.
713FIBERS
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
714 FIBERS
regain at 65% rh is about 2rc. A novel highly absorbent acrylic fiber, which
absorbs 30-50% 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 equilibrium-moisture 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, relFIBERS 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 heatset 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 (174-176). 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 energy-driven 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.
716 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
Vol. 6
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 yirradiation 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 fiberforming 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 hotsolvent 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 thermal-bonding 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 high-performance aramid fibers. Ultraoriented HDPE fibers are also
produced by a solid-state 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-l-pentene), poly(4-methyl-l-hexene), and polystyrene, which can be
ob718 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(4-methyl-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 returnspontaneously 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
fiber-forming 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 threestage 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 fiberforming 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 poly- acrylonitrile
(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:
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 300-1500°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
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 six-carbon 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
benzene-ring structures integrated with alkyl chains and possessing molecular
weights from 700-2400. 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 high-performance 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
acetone-benzene. 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 flameretardant 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 dryspinning 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 2.6-3.5 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 fireprotective 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 clas-
Vol. 6
726 FIBERS
sification. There are three major classes: side-by-side, sheath-core, and matrixfibril, 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 bonding-fibers 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 3MgO-2SiOo*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 surface-protective spin finish which for glass is called a
sizing agent. After extrusion and sizing, the filaments are wound in
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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 (157159) (see also POLYAMIDES, AROMATIC). Aramid fibers are characterized
either by flame resistance and thermal stability or by high strength and stiffiiess.
Combinations of these high-performance 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-phenyleneterephthalamide). 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
FIBERS708
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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.
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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.
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
710
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 full-shade 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.
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FIBERS 711
Another approach to dyeing has been to modify the basic structure by incorporatipn 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 wet-spinning process involving solvent removal
and subsequent coagulation and gelation of the filament. The temper-