High Performance Fibers.pdf

HIGH PERFORMANCE

FIBERS

Introduction

High performance ﬁbers are generally characterized by remarkably high unit

tensile strength and modulus as well as resistance to heat, ﬂame, and chemi-

cal agents that normally degrade conventional ﬁbers. Applications include uses

in the aerospace, biomedical, civil engineering, construction, protective apparel,

geotextiles, and electronic areas.

For many years, plastics reinforced with polymer ﬁbers have been utilized in

the manufacture of boats and sports cars. More recently, ultrahigh strength, high

modulus ﬁbers have been invented and combined into composites whose strength

and stiffness on a speciﬁc basis are unmatched by conventional construction mate-

rials. Composites are now replacing metals in such crucial applications as aircraft

and the space shuttle. The polymeric composites contain carbon or aramid ﬁbers

several times stiffer, weight for weight, than steel. In composite materials, the

ﬁbers support the load which is distributed by the plastic which also prevents

fatigue and failure (1–4) (see C OMPOSITE M ATERIALS ).

In addition to their role in composites, high performance ﬁbers are also found

in coated and laminated textile products, three-dimensional fabric structures,

multifunctional property improvement, and intelligent or self-adaptive materials.

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HIGH PERFORMANCE FIBERS 199

In this article, the preparation and properties of typical high performance

ﬁbers are discussed, then their applications are classiﬁed and detailed. The prin-

cipal classes of high performance ﬁbers are derived from rigid-rod polymers (qv),

gel-spun ﬁbers, modiﬁed carbon ﬁbers (qv), carbon–nanotube composite ﬁbers,

ceramic ﬁbers, and synthetic vitreous ﬁbers.

Rigid-Rod Polymers

Rigid-rod polymers are often liquid crystalline polymers classiﬁed as lyotropic,

such as the aramid Kevlar (DuPont), or thermotropic liquid crystalline polymers,

such as Vectran (Celanese) (see P OLYAMIDES ,A ROMATIC ;L IQUID C RYSTALLINE P OLY -

MERS ,M AIN -C HAIN ;L IQUID C RYSTALLINE T HERMOSETS ).

Liquid Crystallinity. The liquid-crystalline state is characterized by ori-

entationally ordered molecules. The molecules are characteristically rod- or

lathe-shaped and can exist in three principal structural arrangements: nematic,

cholesteric, smectic, and discotic (5,6).

In the nematic phase, within volume elements of the macroscopic sample,

the axes of the molecules are oriented on average in a speciﬁc direction in vari-

ous domains. The centers of gravity of the molecules are arranged in a random

fashion, and consequently no positional long-range order exists. The molecules

are arranged in essentially parallel arrays. Without the presence of an orienting

magnetic or physical force, the molecules exist in random parallel arrays. When

an orienting force is applied, these domains orient easily. The nematic phase is

amenable to translational mobility of constituent molecules.

The cholesteric phase may be considered a modiﬁcation of the nematic phase

since its molecular structure is similar. The cholesteric phase is characterized by

a continuous change in the direction of the long axes of the molecules in adjacent

layers within the sample. This leads to a twist about an axis perpendicular to

the long axes of the molecules. If the pitch of the helical structure is the same as

a wavelength of visible light, selective reﬂection of monochromatic light can be

observed in the form of iridescent colors.

In the smectic phase, the centers of gravity of the rod-like molecules are

arranged in equidistant planes, ie, the ends of the molecules are correlated. The

planes may move perpendicular to the layer normal, and within layers different

arrangements of the molecules are possible. The long axes of the molecules may be

parallel to the layer normal or tilted with respect to it. A two-dimensional short-

or long-range order may exist within the smectic layers. The smectic modiﬁcations

are labeled according to the arrangement of the molecules within the layers using

the symbols A–K.

In the smectic A phase, the director is perpendicular to the planes, while in

the smectic C phase, the director is tilted at an angle less than 90 ◦ to the planes.

In the smectic A and C phases, the molecules diffuse randomly and as a result,

no positional order exists within the planes (positional order exists only in one di-

mension). However, other smectic liquid crystal phases exist in which the molcules

have some degree of order within each plane that results in three-dimensional po-

sitional order (or quasi-three-dimensional order). In this case, molecules diffusing

through the plane spend more time at certain locations than at other locations.

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The smectic B phase is a more ordered analog of the smectic A phase in

which the molecules adopt hexagonal order over distances of ca 150–600 A (6).

The hexagonal S B phase has two tilted analogs called the smectic I and smectic

F phases, in which the hexagonal lattices tilt toward the apex and the side, re-

spectively. In the crystal B phase, the molecules adopt hexagonal order similar

to that of the smectic B phase; however, the hexagonal lattices show long-range

(three-dimensional) positional order. Crystal J and G phases represent hexagonal

lattices with long-range positional order which are analogs of S I and S F , respec-

tively. The crystal E phase results from contraction of a hexagonal lattice which

leads to a herringbone-like structure with restricted rotation. Crystal K and H

phases are the respective tilted analogs of the crystal E phase.

In the discotic phase, disclike molecules form liquid crystal phases in which

the axis perpendicular to the planes of the molecules, orients along a speciﬁc direc-

tion. The nematic discotic phase has orientational order but no positional order.

In the columnar discotic phase, the disclike molecules form columns and therefore

exhibit orientational and positional order. In a chiral discotic liquid crystal, the

director rotates in a helical path throughout the system.

Poly(1,4-benzamide) (PBA) (7) was the ﬁrst nonpeptide synthetic polymer

reported to form a liquid crystalline solution. In order to obtain liquid-crystalline

solutions of poly(1,4-benzamide), it was ﬁrst necessary to prepare the polymer in

the proper solvent. Preparation of the polymer in N,N -dialkylamide solvents at

low temperatures from p -aminobenzoyl chloride hydrochloride produces tractable

PBA polymers with inherent viscosities of as much as 5 dL/g. In solvents such

as N,N -dimethylacetamide [127-19-5] and N,N,N ,N -tetramethylurea [632-22-4]

a coupled polymerization–spinning process in liquid-crystalline solution has been

developed. If polymerization is initiated at temperatures greater than 25 ◦ C, lower

molecular weight polymer is formed. Above 25 ◦ C, chain termination by reaction

of acid chloride chain ends with N,N -dialkylamide is signiﬁcant. To obtain high

molecular weights, a lithium base such as lithium hydride, lithium carbonate, or

lithium hydroxide is added to the polymerization solution after the ﬁrst 1–2 h

of reaction time to neutralize the hydrogen chloride generated. As the reaction

proceeds, the polymerization rate decreases because the increasing amounts of

hydrogen chloride consequently produce fewer free-terminal amine groups.

When pure needle-like crystals of p -aminobenzoyl chloride are polymerized

in a high temperature, nonsolvent process, or a low temperature, slurry process,

polymer is obtained which maintains the needle-like appearance of monomer. PBA

of inherent viscosity, 4.1 dL/g, has been obtained in a hexane slurry with pyridine

as the acid acceptor. Therefore, PBA of ﬁber-forming molecular weight can be

prepared in the solid state.

In 1975, the synthesis of the ﬁrst main-chain thermotropic polymers, three

polyesters of 4,4 -dihydroxy-

α -dimethylbenzalazine with 6, 8, and 10 methy-

lene groups in the aliphatic chain, was reported (8). Shortly thereafter, at the Ten-

nessee Eastman Co. thermotropic polyesters were synthesized by the acidolysis

of poly(ethylene terephthalate) by p -acetoxybenzoic acid (9). Copolymer composi-

tions that contained 40–70 mol% of the oxybenzoyl unit formed anisotropic, turbid

melts which were easily oriented.

Polyesters such as poly( p -phenylene terephthalate), which would be expected

to form liquid crystalline phases, decompose at temperatures below the melt-

ing point. Three principal methods have been used for lowering the melting

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temperatures of thermotropic copolyesters: ( 1 ) the use of ﬂexible groups as spacers

to decouple the mesogenic units and reduce the axial ratio; ( 2 ) the use of unsym-

metrical groups on mesogenic units; and ( 3 ) the copolymerization of rigid units

with nonlinear, bent units which add a “kink” to the rod-like system.

According to patents obtained by Carborundum (10–12), Celanese (13),

DuPont (14–17), and Eastman (9,18) most industrial main-chain thermotrop-

ics are prepared by condensation polymerization involving transesteriﬁcation.

Hydroxy-substituted monomers are acetylated before polymerization by acetic

anhydride in the presence of a suitable catalyst. The transesteriﬁcation reactions

involve acetylated diol, or monosubstituted hydroxybenzoic or hydroxynaphthoic

acids, and diacids. The polymerizations are carried out in an inert atmosphere to

prevent oxidation. A stainless steel stirrer is utilized to improve mixing and to ac-

celerate the release of the reaction by-products. The polymerizations are carried

out at 50–80 ◦ C above the melting point of the highest melting monomer. After

a low melt viscosity prepolymer is obtained, a vacuum is applied to remove the

additional acetic acid and increase the molecular weight of the polymer. Finally,

solid-state polymerization under reduced pressure or in nitrogen at a tempera-

ture of 10–30 ◦ C below the melting point may be utilized to increase the molecular

weight. The heat treatment of spun ﬁbers under these conditions leads to spec-

tacular increases in tensile strength and modulus.

Researchers at DuPont used hydroquinone asymmetrically substituted

with chloro, methyl, or phenyl substituents and swivel or nonlinear bent sub-

stituted phenyl molecules such as 3,4- or 4,4 -disubstituted diphenyl ether,

sulﬁde, or ketone monomers. For example, poly(chloro-1,4-phenylene- trans -

hexahydroterephthalate) and related copolymers were prepared in a melt-

polymerization process involving the reaction of molar equivalents of the diacetoxy

derivatives of diphenols and hexahydroterephthalic acid (19). During polymeriza-

tion, a phase transition from isotropic to anisotropic occurred soon after the rapid

melting of the intermediates to form a clear, colorless liquid.

Also in 1972 (20), Carborundum researchers described a family of aromatic

copolyesters that were recognized later to form liquid-crystalline melts. The poly-

mers are based on a bisphenol monomer. In 1976, in a patent assigned to Car-

borundum, a hydroxybenzoic acid–terephthalic acid–bisphenol system, modiﬁed

and softened with isophthalic acid, was reported to be melt spinnable to produce

ﬁber (21).

Industrial Lyotropic Liquid-Crystalline Polymers (Aramid Fibers).

The ﬁrst polyaramid ﬁber (MPD-1) was based on poly( m -phenylene isophthala-

mide) [24938-60-1]. The ﬁber was not liquid crystalline but was the ﬁrst aramid

ﬁber to be commercialized by DuPont under the trade name Nomex nylon in 1963

and changed to Nomex aramid in 1972 (22). The principal market niche for Nomex

(DuPont) was as a heat-resistant material. Teijin also introduced a ﬁber (trade-

mark Conex) based on MPD-1 in the early 1970s. Fenilon, also based on MPD-1,

was produced in the former USSR for civilian, military, and space exploration ap-

plications. In 1970, DuPont introduced an aramid ﬁber, Fiber B, for use in tires,

which was probably based on polybenzamide PBA spun from an organic solvent.

Fiber B had high strength and exceptionally high modulus. Another version of

Fiber B, based on poly( p -phenylene terephthalamide) [24938-64-5] (PPT) was in-

troduced in the 1970s. This version of Fiber B was spun from sulfuric acid and

had a tensile strength approximately twice that of the Fiber B based on MPD-1.

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An even higher modulus ﬁber based on PPT, in which the modulus was increased

by the drawing of the as-spun ﬁber, was introduced under the name PRD-49 for

use in rigid composites. The undrawn and drawn ﬁbers were later announced as

Kevlar-29 and Kevlar-49, respectively. In 1975, Akzo of the Netherlands reported

the commercialization of an aramid ﬁber, Twaron (Akzo), based on PPT.

Nomex. This ﬁber was commercialized for applications requiring unusu-

ally high thermal and ﬂame resistance. Nomex (DuPont) ﬁber retains useful prop-

erties at temperatures as high as 370 ◦ C. Nomex has low ﬂammability and has

been found to be self-extinguishing when removed from the ﬂame. On exposure

to a ﬂame, a Nomex fabric hardens, starts to melt, discolors, and chars thereby

forming a protective coating (23). Therefore an outstanding characteristic is low

smoke generation on burning. The limiting oxygen index (LOI) value (top down)

for Nomex fabrics is 26.0 (24). Nomex has a tga weight loss of 10% at 450 ◦ C and

a use temperature of 370 ◦ C. Nomex has good to excellent strength, a tenacity of

0.42–0.51N/tex (4.8–5.8 gf/den) (25), good extendability, and a modulus greater

than that of nylon-6,6. The density is 1.38 g/cm 3 (26). Nomex is more difﬁcult to

dye than nylon, but the use of dye carriers allows dyeing to proceed at high temper-

atures with temperature-resistant basic dyes (27). The structure of Nomex may

be represented as follows:

40 ◦ C are desirable to avoid such side reactions

as transamidation by the amide solvent and acylation of m -phenylenediamine by

the amide solvent. Both reactions would lead to an imbalance in the stoichiome-

try and result in forming low molecular weight polymer. Fibers may be either dry

spun or wet spun directly from solution.

Kevlar. In the 1970s, researchers at DuPont reported that the processing of

extended chain all para-aromatic polyamides from liquid crystalline solutions pro-

duced ultrahigh strength, ultrahigh modulus ﬁbers. The greatly increased order

and the long relaxation times in the liquid crystalline state compared to conven-

tional systems led to ﬁbers with highly oriented domains of polymer molecules. The

most common lyotropic aramid ﬁber is poly( p -phenyleneterephthalamide) (PPT)

which is marketed as Kevlar by DuPont. Aramid ﬁber is available from Akzo

under the trade name Twaron. These ﬁbers are used in body armor, cables, and

composites for sports and space applications. Kevlar has the following structure:

MPD-1 ﬁbers may be obtained by the polymerization of isophthaloyl chlo-

ride [99-63-8] and m -phenylenediamine [108-45-2] in dimethylacetamide with 5%

lithium chloride (26). The reactants must be very carefully dried since the presence

of water would upset the stoichiometry and lead to low molecular weight products.

Temperatures in the range of 0 to

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