Aramid Fibers From Fiber Chemistry 3rd
Transcript of Aramid Fibers From Fiber Chemistry 3rd
13 Aramid Fibers
� 2006 by Taylor & Francis Group
Vlodek Gabara, Jon D. Hartzler, Kiu-Seung Lee,David J. Rodini, and H.H. Yang
CONTENTS
13.1 Introduction .............................................................................................................976
13.1.1 Historical Perspective .................................................................................. 976
13.1.2 Aramid—Definition .................................................................................... 977
13.1.3 Examples of Compositions.......................................................................... 977
13.2 Aramid Products: Forms and Properties ................................................................. 977
13.3 Producers of Aramid Products.................................................................................978
13.4 Structure–Property Relationship..............................................................................979
13.4.1 Fine Structure..............................................................................................979
13.4.2 Thermal Properties ......................................................................................980
13.4.3 Solubility and Chemical Properties .............................................................981
13.4.4 Fiber Mechanical Properties .......................................................................981
13.4.5 Films and Papers......................................................................................... 984
13.5 Polymerization of Aromatic Polyamides..................................................................985
13.5.1 Introduction ................................................................................................985
13.5.2 Synthesis of Ingredients............................................................................... 986
13.5.2.1 m-Phenylene Diamine ..................................................................986
13.5.2.2 p-Phenylene Diamine ................................................................... 987
13.5.2.3 3,4’-Diaminodiphenyl Ether ......................................................... 988
13.5.2.4 Diacid Chlorides .......................................................................... 988
13.5.3 Polymerization Fundamentals..................................................................... 989
13.5.3.1 Reaction Mechanism ................................................................... 990
13.5.3.2 Reaction Energetics ..................................................................... 991
13.5.4 Direct Polymerization by Catalysis. ............................................................ 991
13.5.5 Polymerization Methods ............................................................................. 993
13.5.5.1 Interfacial Polymerization............................................................ 993
13.5.5.2 Solution Polymerization...............................................................995
13.5.5.3 Vapor-Phase Polymerization........................................................999
13.5.5.4 Plasticized Melt Polymerization................................................. 1000
13.6 Aramid Solutions ................................................................................................... 1001
13.6.1 Isotropic Solutions. ................................................................................... 1001
13.6.1.1 m-Aramid Solutions................................................................... 1001
13.6.1.2 p-Aramid Solutions. ................................................................... 1001
13.6.2 Anisotropic Solutions................................................................................ 1002
13.6.2.1 Phase Behavior........................................................................... 1002
13.6.2.2 Rheological Properties ............................................................... 1003
, LLC.
13.7 Preparation of Aramid Products............................................................................ 1003
13.7.1 Fibers......................................................................................................... 1003
13.7.1.1 Dry Spinning.............................................................................. 1003
13.7.1.2 Wet Spinning ............................................................................. 1005
13.7.1.3 Dry-Jet Wet-Spinning ................................................................ 1006
13.7.2 Film ........................................................................................................... 1009
13.7.3 Fibrids ....................................................................................................... 1010
13.7.4 Pulp ........................................................................................................... 1011
13.8 Applications ........................................................................................................... 1012
13.8.1 m-Aramid Fiber......................................................................................... 1013
13.8.1.1 Protective Apparel ..................................................................... 1013
13.8.1.2 Thermal and Flame-Resistant Barriers ...................................... 1014
13.8.1.3 Elastomer Reinforcement........................................................... 1015
13.8.1.4 Filtration and Felts .................................................................... 1015
13.8.2 m-Aramid Paper ........................................................................................ 1015
13.8.2.1 Electrical .................................................................................... 1015
13.8.2.2 Core Structures .......................................................................... 1016
13.8.2.3 Miscellaneous............................................................................. 1017
13.8.3 p-Aramid Fiber.......................................................................................... 1017
13.8.3.1 Armor ........................................................................................ 1017
13.8.3.2 Protective Apparel ..................................................................... 1018
13.8.3.3 Tires and Mechanical Rubber Goods ........................................ 1018
13.8.3.4 Composites................................................................................. 1019
13.8.3.5 Optical and Electromechanical Cables....................................... 1019
13.8.3.6 Ropes and Cables ...................................................................... 1020
13.8.3.7 Reinforced Thermoplastic Pipe.................................................. 1020
13.8.3.8 Civil Engineering........................................................................ 1021
13.8.4 p-Aramid Paper ......................................................................................... 1021
13.8.4.1 Core Structures .......................................................................... 1021
13.8.4.2 Printed Wiring Boards ............................................................... 1022
13.8.4.3 p-Aramid Pulp ........................................................................... 1022
13.9 Conclusions and Direction..................................................................................... 1024
References ........................................................................................................................ 1025
13.1 INTRODUCTION
13.1.1 HISTORICAL PERSPECTIVE
Development of aromatic polyamides had a very unique beginning. Its origin in an industrial
corporation (DuPont) led to a combination of fundamental science, engineering, and ap-
plications research from the very early stages of the development. In 1948, with the
commercialization of nylon fiber and the near-development of a polyester fiber, the manage-
ment of the DuPont Technical Division launched very broad, long-range research programs
with goals, among others, of developing very high-strength fibers and high-temperature-
resistant fibers.
The first phase covered a decade from the early 1950s to the early 1960s. Clearly, materials
with unusual properties are not easy to process and they would not have been possible without
the development of low-temperature solution polymerization techniques by Paul Morgan’s
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group at DuPont [1]. The next critical step was to understand factors governing solubility of
these difficult to dissolve polymers. Beste and Stephens [2] elucidated the role of certain salts
that help in obtaining good solutions of these polymers. This work culminated in the
commercialization of Nomex, the first high-temperature-resistant, m-aramid fiber [3,4].
Starting in the early 1960s the work focused on new fibers with a performance superior
to Nomex, and p-aramids became a logical choice. Stephanie Kwolek focused her initial
work on the more tractable poly(1,4-benzamide) polymer and produced, in the mid-1960s, a
fiber with a spectacular modulus of 400 gpd. After additional work, a yarn with 7.0 gpd
tenacity and an unheard of modulus of 900 gpd was prepared. This fiber was known as fiber
B. Subsequent work shifted to poly( p-phenylene terephthalamide) (PPTA). After signifi-
cant effort by many in both polymerization and spinning areas, Herb Blades made a
processing breakthrough by focusing on the air-gap spinning of concentrated solutions of
high-molecular-weight PPTA polymer. The first PPTA fibers were produced by this process
in early 1970, and by 1972 Kevlara was introduced to the market place. This was clearly a
significant achievement considering the novelty and complexity of the technology involved
and the speed at which it was accomplished. In addition to the impressive blend of science and
engineering required to commercialize Kevlar, this was also the first demonstration of fiber
mechanical properties predicted by theoretical considerations developed as early as the mid-
1930s. This provided a fundamental basis as well as an impetus to study and commercialize
other materials with comparable properties.
13.1.2 ARAMID—DEFINITION
As alluded to in the introduction, properties of aromatic polyamides differ significantly from
those of their aliphatic counterparts. This led the U.S. Federal Trade Commission to adopt
the term ‘‘aramid’’ to designate fibers of the aromatic polyamide type in which at least 85% of
the amide linkages are attached directly to two aromatic rings.
13.1.3 EXAMPLES OF COMPOSITIONS
The superior properties of these materials were the reason why significant research effort
has been devoted to this chemistry. Yang [5] showed at least 100 different compositions
in this area and that number has doubled during the past 15 years since Yang’s book was
published.
The early work by Sweeny, Kwolek, and others demonstrated that progress in this
area of technology was the result of a constant trade-off between properties and processa-
bility. This is very likely the reason why after half-a-century of research only four compo-
sitions have reached commercial stage: poly(m-phenylene isophthalamide) (MPDI), PPTA,
copoly(p-phenylene=3,4’-diphenyl ether terephthalamide) (ODA=PPTA), poly[5-amino-
2-(p-aminophenyl)benzimidazole terephthalamide] (SVM), and its copolymers.
13.2 ARAMID PRODUCTS: FORMS AND PROPERTIES
The outstanding thermal and mechanical properties that can be derived from these
compositions led to the exploration, as well as commercial realization, of various product
forms. Currently these product forms include fibers, fibrids and pulps, films, papers,
and particles.
aKevlar—a registered trademark of E.I. DuPont de Nemours & Co., Inc., Wilmington, Delaware, USA.
� 2006 by Taylor & Francis Group, LLC.
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The largest commercial volume of these materials is in the form of fibers. Continuous filament
yarns are preferred where very high mechanical properties are required and staple fiber is used
for textile applications. The significant volumes involved in these applications led to the
development of special spinning processes designed to produce these forms.
The excellent thermal properties of these materials led to high volume applications where
these materials were used as binders or as short fiber reinforcing agents. This required the
development of both fibrids and pulps. This chapter discusses both the processes of formation
as well as the principles of applications of these forms.
Various nonwoven structures have been developed as well. The least important among
sheet structures are films. There are two film products (see Section 13.3) based on p-aramids
and none on m-aramids. The significant cost differential is the most likely reason for this
situation. On the other hand a very large market has been developed for papers based on both
p-aramids and m-aramids. In general, these papers are based on short fibers (floc) and a
binder (fibrids), but other components have been explored as well. A very small market exists
for particles other than fibrids and pulps.
13.3 PRODUCERS OF ARAMID PRODUCTS
The basic development and the first commercial introduction of these materials were done by
DuPont, which continues to be the largest producer. m-Aramid fiber products (staple,
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continuous filament yarn, and floc) with the trademark Nomexb are produced by DuPont in
the United States as well as Spain. The paper products come from the U.S. plant as well as
from a facility in Japan. The only other major m-aramid producer is Teijin, with its fiber
product Teijinconexc produced in Japan.
The situation is very similar on the para side of chemistry. The first and the largest
producer—DuPont—has three facilities throughout the world. The largest one in the United
States produces essentially all product forms except films. Fiber is also produced in Ireland
and Japan. The other producer of p-aramids is Teijin Co., which produces two basic fibers:
Twarond based on PPTA and Technorac based on a copolymer. Twaron is produced in the
Netherlands while Technora is manufactured in Japan.
A small amount of p-aramid fiber (Armos and Rusar) is produced in Russia. Both are
copolymers based on diaminophenylbenzimidazole—a unique but expensive monomer.
There are two producers of p-aramid film. The first one was Toray with its Mictrone
film based on a copolymer and Asahi with a product (Aramicaf) based on PPTA homo-
polymer.
13.4 STRUCTURE–PROPERTY RELATIONSHIP
13.4.1 FINE STRUCTURE
In general aramid homopolymers crystallize with relative ease. PPTA is a highly crystalline
material. Two structures have been identified for this polymer: the first was proposed by
Northolt [6] and the second by Haraguchi [7]. Haraguchi [7] and Roche [8] proposed
mechanisms for their formation. In both cases they proposed an interaction between the
solution and the coagulation process. Roche proposed that to form the Haraguchi struc-
ture, PPTA solution has to crystallize into a crystal solvate [9] prior to the removal of
sulfuric acid. After acid removal and drying the Haraguchi polymorph is formed. This is
the less stable form and at an elevated temperature rearranges into the Northolt form.
Coagulation of PPTA solution leads to the Northolt structure, according to Roche, and
that is why all commercial fibers exhibit essentially the Northolt structure. Northolt [6] and
later Tashiro [10] reported their estimates of the size of the orthorhombic unit cell. The
values are listed in Table 13.1. Commercial fibers based on PPTA are highly crystalline.
Estimates of the degree of crystallinity of Kevlar 29 are 68 to 85% and as high as 95% for
Kevlar 49 [11,12].
In addition to crystallinity, PPTA fibers exhibit a larger scale organization. It has been
proposed that PPTA fibers have an unusual radial orientation of pleated hydrogen-bonded
sheets [13]. This unique morphology has a significant impact on the mechanical properties of
the fibers.
MPDI has a triclinic unit cell and is significantly less crystalline than PPTA (Table 13.1).
Savinov [14] proposed that crystallinity depends on the conditions of polymer precipitation
from solution. Precipitation of polymer in water leads to a noncrystalline material while
precipitation in water containing some solvent leads to a crystalline form. Krasnov [15]
showed that increased fiber orientation leads to higher crystallinity. SVM, the Russian
bNomex—a registered trademark of E.I. DuPont de Nemours & Co., Inc., Wilmington, Delaware, USA.cTeijinconex, Technora—registered trademarks of Teijin, Ltd., Japan.dTwaron—a registered trademark of Akzo Nobel, The Netherlands.eMictron—a registered trademark of Toray Co., Japan.fAramica—a registered trademark of Asahi Co., Japan.
� 2006 by Taylor & Francis Group, LLC.
TABLE 13.1Crystallinity of Homopolymers
PPTA MPDI
Crystal system Orthorhombic Triclinic
Lattice constant
a (A) 7.80 5.27
b (A) 5.19 5.25
c (A) 12.9 11.3
a (degree) 111.5
b (degree) 111.4
g (degree) 90 88.0
Number of chains in a unit cell 2 1
Density (g=cm3)
Calculated 1.50 1.45
Observed 1.43–1.45 1.38
Source: From Northolt, M.G.; Eur. Polym. J., 10, 799, 1974; Haraguchi, K.,
Kajiyama, T., and Takayanagi, M.J., J. Appl. Polym. Sci., 23, 915, 1979;
Roche, E.J., Allen, S.R., Gabara, V., and Cox, B., Polymer, 30, 1776, 1989;
Gardner, K.H., Matheson, R.R., Avakin, P., Chia, Y.T., and Gierke, T.D.,
Polym. Prepr. (Am. Chem. Soc. Div. Polym. Chem.), 24(2), 469, 1983;Tashiro,
K., Kobayashi, M., and Tadokoro, H., Macromolecules, 10(2), 413, 1977.
product based on poly[5-amino-2-(p-aminophenyl) benzimidazoleterephthalamide] is the only
other commercial product based on a homopolymer. This material is noncrystalline, as might
be expected, based on the structural irregularities that can arise from the orientation of repeat
units in the polymer chain (cis–trans, head–tail).
Copolymers are noncrystalline materials. Blackwell has studied the fine structure of
Technora fiber [16].
13.4.2 THERMAL PROPERTIES
The search for materials with very good thermal properties was the original reason for
research into aromatic polyamides. Bond dissociation energies of C��C and C��N bonds
in aromatic polyamides are ~20% higher than those in aliphatic polyamides. This is the reason
why the decomposition temperature of MPDI exceeds 4508C [17]. Conjugation between
the amide group and the aromatic ring in PPTA increases chain rigidity as well as the
decomposition temperature, which exceeds 5508C [17,18].
Obviously, hydrogen bonding and chain rigidity of these polymers translates to very high
glass transition temperatures. Using low-molecular-weight polymers, Aharoni [19] measured
glass transition temperatures of 2728C for MPDI and over 2958C for PPTA (which in this
case had low crystallinity). Others have reported values as high as 4928C [20]. In most cases
the measurement of Tg is difficult because PPTA is essentially 100% crystalline. As one would
expect, these values are not strongly dependent on the molecular weight of the polymer above
a DP of ~10 [21].
We have discussed above the crystalline nature of most of the fibers based on homo-
polymers. While information on melting of the crystalline phase of these polymers differs, all
quoted melt temperatures are very high. For MPDI most values are similar to 4358C as
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determined by Takatsuka [17]. On the other hand, most authors report the decomposition
temperature of PPTA to be lower than its melting point [17]. Chaudhuri [18] reported a value
of 530 8C. Table 13.2 summarizes some of the thermal properties of commercial aramid fibers
[22,140–146].
The almost perfect orientation of p-aramid fibers is reflected in the anisotropic behavior of
its thermal expansion coefficient. The linear expansion coefficient for these materials is
negative (Table 13.2). Because the volumetric thermal expansion coefficient is not affected
by orientation, the radial coefficient must increase as fiber orientation increases. The negative
expansion coefficient of these materials has opened a whole field of applications in electronics
(see section 13.8.4.2).
13.4.3 SOLUBILITY AND C HEMICAL P ROPERTIES
The same structural characteristics that are responsible for the excellent thermal properties of
these materials are responsible for their limited solubility as well as good chemical resistance.
PPTA is soluble only in strong acids like H2SO4, HF, and methanesulfonic acid. Preparation
of this polymer via solution polymerization in amide solvents is accompanied by polymer
precipitation. As expected, based on its structure, MPDI is easier to solubilize then PPTA. It
is soluble in neat amide solvents like N-methyl-2-pyrrolidone (NMP) and dimethylacetamide
(DMAc), but adding salts like CaCl2 or LiCl significantly enhances its solubility.
The significant rigidity of the PPTA chain (as discussed above) leads to the formation of
anisotropic solutions when the solvent is good enough to reach a critical minimum solids
concentration. The implications of this are discussed in greater detail later in this chapter.
It is well known that chemical properties differ significantly between crystalline
and noncrystalline materials of the same composition. In general, aramids have very good
chemical resistance as shown in Table 13.3. Obviously, the amide bond is subject to a
hydrolytic attack by acids and bases. Exposure to very strong oxidizing agents results in
a significant strength loss of these fibers. In addition to crystallinity, structure consolidation
affects the rate of degradation of these materials.
The hydrophilicity of the amide group leads to a significant absorption of water by all
aramids. While the chemistry is the driving factor, fiber structure also plays a very important
role; for example, Kevlar 29 absorbs ~7% water, Kevlar 49 ~4%, and Kevlar 149 only 1%.
Fukuda explored the relationship between fiber crystallinity and equilibrium moisture in
great detail [23].
Because of their aromatic character, aramids absorb UV light, which results in an
oxidative color change. Substantial exposure can lead to the loss of yarn tensile properties
[24]. UV absorption by p-aramids is more pronounced than with m-aramids. In this case a
self-screening phenomenon is observed, which makes thin structures more susceptible to
degradation than thick ones. Very frequently p-aramids are covered with another material
in the final application to protect them.
The high degree of aromaticity of these materials also provides significant flame resist-
ance. All commercial aramids have a limited oxygen index in the range of 28–32%, which
compares with ~20% for aliphatic polyamides (Table 13.2). The utilization of these properties
is discussed in greater detail in the Applications section of this chapter.
13.4.4 FIBER MECHANICAL P ROPERTIES
Typical properties of commercial aramid fibers are given in Table 13.4. While yarns of m-
aramids have tensile properties that are no greater than those of aliphatic polyamides, they do
retain useful mechanical properties at significantly higher temperatures. The high glass
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TABLE 13.2Thermal Properties of Aramid Fibers
Trade name Nomex Teijinconex Kevlar Twaron
polymer MPDI MPDI PPTA PPTA
Fiber type 430 Std HT K-29 K-49 Std HM
Technora
ODA=PPTA
Property
Specific heat (J=kg-K) 72 60 60 81 81 81 81 96
Thermal
conductivity (W=m-K) 0.25 0.11 0.11 2.5 2.5 — — 0.5
Coefficient of thermal
expansion (cm=cm-8C) 1.8�10�5 1.5�10�5 1.5�10�5 �4.0�10�6 �4.9�10�6 �3.5�10�6 �3.5�10�6 �6�10�6
Heat of
combustion (J=kg) 28�106 — — 35�106 35�106 — — —
Flammability
LOI (%) 28 29—32 29—32 29 29 29 29 —
Decomposition (in N2)
Temperature (8C) 400–420 400–430 400–430 520–540 520–540 520–540 520–540 500
Source: From DuPont Technical Guide for Kevlar Aramid Fiber, H-77848, 4=00; DuPont Technical Guide for Nomex Brand Aramid Fiber, H-52720, 7=01; Teijin Ltd.,
Teijinconex Heat Resistant Aramids Fiber 02.05; Teijin Ltd., High Tenacity Aramids Fibre: Technora TIE-05=87.5; Akzo Nobel, Twaron—Product Information:Yarns, Fibers
and Pulp.
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TABLE 13.3Chemical Resistance of Aramid Fibers
Trade name Nomex Kevlar Technora
Polymer MPDI PPTA ODA=PPTA
Chemical Time (h)=temp. (8C) Percent Strength Retention
40% H2SO4 100=95 90
10% H2SO4 100=21 90–100 90–100
10% H2SO4 1000=21 95 35*
10% HCl 1000=21 20–60 35 10*
10% HNO3 100=21 60–80 90–100 20–60
10% NaOH 100=95 75
10% NaOH 1000=21 90–100 90 46
40% NaOH 1000=21 80–90 76
28% NH4OH 1000=21 90–100 90–100 65*
0.01% NaClO 1000=21 90–100 16
10% NaClO 100=95 55
0.4% H2O2 1000=21 90–100 56–75
10% NaCl 1000=21 90–100 100*
100% Acetic acid 1000=21 90–100 90*
90% Formic acid 100=21 90–100 90–100 90–100
90% Formic acid 100=99 60–80 90–100 0–20
100% Acetone 1000=21 90–100 90–100
100% Acetone 100=56 80–90 90–100
100% Benzene 1000=21 90–100 90–100 100
100% Ethyl alcohol 1000=21 90–100 90–100 100
100% Ethyl alcohol 100=77 90–100 90–100
50% Ethylene glycol 1000=99 80–90 90–100 60–80
100% Gasoline 1000=21 90–100 90–100 90–100
100% Methyl alcohol 1000=21 90–100 90–100 90–100
100% Perchloroethylene 10=99 90–100 90–100
100% Tetrahydrofuran 1000=21 90–100
*Measurements made after 3 months (2200 h) exposure at room temperature.
Source: From DuPont Technical Guide for Kevlar Aramid Fiber, H-77848, 4=00; DuPont Technical Guide for
Nomex Brand Aramid Fiber, H-52720, 7=01; Teijin Ltd., High Tenacity Aramids Fibre: Technora TIE-05=87.5.
TABLE 13.4Properties of Aramid Fibers
Trade name Nomex Teijinconex Kevlar Twaron
Polymer MPDI MPDI PPTA PPTA
Fiber type 430 std HT K-29 K-49 std HM
Technora
ODA=PPTA
Density (g=cm3) 1.38 1.38 1.38 1.44 1.44 1.44 1.45 1.39
Strength (Gpa) 0.59 0.61–0.68 0.73–0.86 2.9 3.0 2.9 2.9 3.4
Elongation (%) 31 35–45 20–30 3.6 2.4 3.6 2.5 4.6
Modulus (Gpa) 11.5 7.9–9.8 11.6–12.1 71 112 70 110 72
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transition temperature leads to low (less than 1%) shrinkage at temperatures below 2508C. In
general, mechanical properties of m-aramid fibers are developed on drawing (see below). This
process produces fibers with a high degree of morphological homogeneity, which leads to very
good fatigue properties.
The mechanical properties of p-aramid fibers have been the subject of much study. This is
because these fibers were the first examples of organic materials with a very high level of both
strength and stiffness. These materials are practical confirmation that nearly perfect orienta-
tion and full chain extension are required to achieve mechanical properties approaching those
predicted for chemical bonds. In general, the mechanical properties reflect a significant
anisotropy of these fibers—covalent bonds in the direction of the fiber axis with hydrogen
bonding and van der Waals forces in the lateral direction.
Termonia has proposed a kinetic model for fiber strength [25–27]. His calculations suggest
that molecular mass, its distribution, and intermolecular forces control fiber strength. Allen’s
work linked the failure mode of these fibers with their morphology very closely [16, 28–30].
He was able to show that fiber pleating is responsible for the fact that one needs to consider
the asymptotic modulus (modulus close to the fiber breaking point) of these fibers rather than
the initial modulus to explain mechanical properties. This interpretation confirmed a clear
dependence of fiber strength on both local orientation (as measured by the asymptotic
modulus) and secondary interactions (as measured by shear properties).
The use of p-aramids in composites has focused much research effort on the compressive
properties of these fibers. Excellent tensile properties, approaching 80% of the theoretical
modulus, and 30% of the theoretical strength are not matched by their compressive proper-
ties. PPTA fiber yields under compression at ~0.5% of strain. This is caused by a buckling
phenomenon that is attributed to the relatively weak lateral properties of these highly
anisotropic fibers. However, aramids with their hydrogen bonding have significantly better
compressive strength than UHMWPE, which has extremely weak lateral properties. Allen
[31] measured compressive strength by a recoil test and obtained 258N=tex for Kevlar 49
compared to 7.5 N=tex for UHMWPE. Aramids also compare well with PBO, which has a
compressive strength of 0.133 N=tex. All high strength organic fibers yield under compressive
stress with formation of kink bands. However this, significant dislocation does not lead to
major tensile strength loss. At a strain of 3% the loss is only ~10%.
This high degree of anisotropy of the p-aramids is reflected in fatigue properties. Tension–
tension fatigue is very good. Wilfong [32] reported no failure after 107 cycles with loads at
60% of breaking strength. Compressive fatigue is not as good—especially at higher strains. At
a strain of 0.5% no strength loss is observed even after 106 cycles but at a strain of 1% the
strength loss begins at about 103 cycles [33].
Creep (long-term failure of fibers at loads below their breaking strength) is the final
mechanical property for review. The kinetic model of fiber failure was applied by Termonia
[25] to estimate creep behavior. His calculations suggest that the activation energy of covalent
bond breaking controls the lifetime of materials. That is why UHMWPE fails after 2.5 min
when strained to 50% of its breaking strain (measured at 1 sec). PPTA under the same
conditions fails after 100 years. Lafitte [34] measured creep strain for Kevlar 29 at a load of
50% of its breaking strength and found a strain of 0.3% after 107 sec.
13.4.5 FILMS AND PAPERS
Although the primary focus of this chapter is on fibers, we have included some illustrations of
sheet products based on this chemistry.
There are two examples of commercial p-aramid films. Toray produces a terpolymer film
under the trade name Mictron, while Asahi introduced a PPTA homopolymer film called
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TABLE 13.5Properties of Aramid Films
Mictron Aramica
Producer toray asahi
Thickness (mm) 25 25
Density (g=cm3) 1.5 1.4
Mechanical properties:
Direction machine cross machine cross
Tensile strength GPa 0.5 0.5 0.3
Elongation % 60 15 25
Tensile modulus GPa 13 9 19 10
Initial tear strength Kg — 25
Long-term heat resistance 8C 180 ~200
Thermal expansion (1=8C � 10�5) 0.1 0.2
Moisture absorption
At 75% RH and
At room temp. % 1.5 2.8
Electrical properties:
Dielectric constant at 1KHz — 4
Dissipation factor at 1 KHz — 0.02
Volume resistivity V=cm 5 � 1017 1 � 1016
Surface resistivity V=cm — 1 � 1016
Dielectric strength KV=mm 300 230
Source: From Yasufuku, S., IEEE Elec. Insu. Mag., 11(6), 27, 1995; Teijin Ltd., High Tenacity Aramids Fibre:
Technora TIE-05=87.5; Asahi Chemical Industry America, Inc., Technical Brochure, Aramica Film, 1991; Akzo
Nobel, Twron Product Information: Yarns, Fibers and Pulp.
Aramica. In both cases the product goal was a high strength, thin film for mass storage
devices. Film properties are shown in Table 13.5.
Aramids papers are found in a much broader range of applications than films (see
Applications section). Most papers are comprised of a composite structure of short fibers
and a binder. Paper properties can be tailored by changing the composition and the process-
ing conditions. Selected properties are illustrated in Table 13.6.
13.5 POLYMERIZATION OF AROMATIC POLYAMIDES
13.5.1 INTRODUCTION
We began this discussion with a description of the high melting point and difficult solubility
of aromatic polyamides. Very clearly these properties present a significant challenge in their
synthesis and fabrication.
First, the infusible nature of many of these polymers precludes the use of conventional
bulk polymerization and melt processing techniques. Second, aromatic diamines are signifi-
cantly less reactive than aliphatic diamines toward polyamidation. This requires the use of
more reactive dicarboxylic acid intermediates or some activation mechanism to complete the
polycondensation in a reasonable period of time. Some technological breakthroughs were
necessary to make progress in the synthesis of aromatic polyamides. These came in the late
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TABLE 13.6Properties of Aramids Paper
Nomex Nomex Nomex
Polymer MPDI MPDI PPTA
Producer DuPont DuPont DuPont
Type 410 410 N710
Thickness mm 127 127 97
Density g=cm3 0.87 0.87 0.64
Mechanical properties:
Direction machine cross machine
Tensile strength GPa 0.1 0.05 0.2
Elongation % 16 13 1.5
Tensile modulus GPa — — 5.4
Initial tear strength Kg 3.3 1.6 —
Long-term heat resistance 8C ~200 ~200
Thermal expansion (1=8C � 10�5) — 0.7
Moisture absorption
At 55% RH and
At room temp. % — 1.6
Electrical properties:
Dielectric constant at 1 KHz 2.4 3.9a
Dissipation factor at 1 KHz 0.006 0.02a
Volume resistivity V=cm 5 � 1016 —
Surface resistivity V=cm 1 � 1016 —
Dielectric strength KV=mm 25 82a
aMeasurements made after three months (2200 hrs) exposure at room temperature.
Source: From E.I. DuPont de Nemours & Co., Inc, NOMEX Aramid Paper Type 410—Typical Properties, H-22368,
8=98; Magellan International; Hendren, G.L., Kirayoglu, B., Powell, D.J., and Weinhold, M., Adv. Mater., 10(15),
1233, 1998; Yasufuku, S., IEEE Elec. Insn. Mag., 11(6), 27, 1995.
1950s and the early 1960s when it was demonstrated that high molecular weight wholly
aromatic polyamides could be prepared by low-temperature interfacial [35] and solution
[36,37] polycondensation processes.
13.5.2 S YNTHESIS OF I NGREDIENTS
It was also imperative to develop synthetic routes to high purity ingredients for these
polymerizations to be successful. The syntheses of commercially important ingredients will
be described here. Only one of several alternative routes will be illustrated. It must also be
noted that the chemistry is constantly being modified to achieve less costly, more efficient and
environmentally friendly processes.
13.5. 2.1 m -Phenylen e Dia mine
The first step in m-phenylene diamine (MPD) synthesis is the nitration of benzene in 20%
oleum (Equation 13.1). The nitration is a two-stage continuous process [38] replacing two
protons on the benzene ring with two nitro groups by the catalytic action of sulfuric acid. The
m-isomer is the dominant product.
� 2006 by Taylor & Francis Group, LLC.
�
NO2
NO2
NO2
NO2
NO2
NO2
� �
MinorMinorMajor
H2SO4
�2 HNO3 2 H2O ð13:1Þ
The resulting isomer mixture is washed with water and ammonia, centrifuged to remove acid
and phenolic by-products and then catalytically hydrogenated [39]. MPD is isolated from the
crude diamine mixture and purified by selective distillation (Equation 13.2)
NO2
NO2 � 6 H2 NH2
NH2
Catalyst
(Pt, Pd, Fe)� 4 H2O
MPD crudeIsomeric mixtureof dinitrobenzene
ð13:2Þ
13.5.2.2 p-Phenylene Diamine
The synthesis of p-phenylene diamine (PPD) starts with air oxidation of ammonia to form
N2O3 (in equilibrium with NO and NO2) (Equation 13.3)
4 NH3 þ 6O2 ��������!Pt=Ru Catalyst
1000�C2N2O3 þ 6H2O ð13:3Þ
This mixture is then reacted with four moles of aniline to produce diphenyltriazine as follows:
N2O3 � 4 NH2N N
H
� 3 H2O2 N
ð13:4Þ
Diphenyltriazine is rearranged to form a mixture of p- and o- aminoazobenzene using nitric
acid as a catalyst
N NN
H
HNO3
Rearrangement
N N
NH2
NN
H2N
�
ð13:5Þ
� 2006 by Taylor & Francis Group, LLC.
2
Finally, the aminoazobenzenes are hydrogenated to the corresponding phenylene diamines
[40–42]. A mole of aniline is regenerated for every mole of phenylene diamine and is recycled.
The phenylene diamine isomers are then separated, and the o-isomer is sold as an ingredient
for the production of various fungicides.
N N
NH2
N N
H2N
2H2
1 2H2
NH2 H2N
H2N
H2N
1
NH
(13.6a)
(13.6b)
NH2
1 1
13.5.2.3 3,4’-Diaminodiphenyl Ether
The synthesis of 3,4’-diaminodiphenyl ether (3,4’-POP) is more complex than that of simple
aromatic diamines such as MPD and PPD and hence this monomer is more expensive.
Condensing 1,3-dinitrobenzene with 4-aminophenol using potassium carbonate in dimethyl-
formamide (DMF) or DMAc produces 3,4’-POP. The resulting 3-nitro-4’-aminodiphenyl
ether is then hydrogenated [42].
3-nitro-4'-aminodiphenyl ether
DMF/DMAc
K2CO3
O2N
O NH2� NH2HONO2
O2N
ð13:7Þ
3,4'-diaminodiphenyl ether
H2N
O NH2H2,DMF,110�C
Pd/C
O2N
O NH2
3-nitro-4'-aminodiphenyl ether
ð13:8Þ
A mixture of 4-aminophenol, 1-3-dinitrobenzene and K2CO3 in DMF was treated at 1508Cfor 4 h to give 96.3% 3-nitro-4’-aminodiphenyl ether. This was treated with Pd on C in DMF
at 1108C and H2(3 atm) for 5 h to give 98.0% 3,4’-diaminodiphenyl ether.
13.5.2.4 Diacid Chlorides
Terephthaloyl chloride (TCl) and isophthaloyl chloride (ICl) are produced by reacting the
corresponding dicarboxylic acid with phosgene [43].
� 2006 by Taylor & Francis Group, LLC.
OH � 2ClDMF
Cl � 2HCl � 2CO2CCl
O
C
O
C
O
ClCHO
O
C
O
ð13:9Þ
The reaction involves formation of a catalyst complex between DMF and phosgene, which
then reacts with terephthalic acid.
"Complex"
CO2+
Cl−
NCH3
CH3
C
Cl
HCl C
O
Cl+NCH3
CH3
C
O
H ð13:10Þ
2 NCH3
CH3
C
Cl
H
"Complex"
−2HCl
2 NCH3
CH3
C
O
H+CCl
O
C
O
Cl+CHO
O
C
O
OH
Cl−
ð13:11Þ
The reaction is carried out in a slurry of TPA, DMF, and TCl with countercurrent injection of
phosgene. The product, TCl, is degassed, heated to destroy the catalyst complex, and then
distilled to remove impurities.
13.5.3 POLYMERIZATION FUNDAMENTALS
The usual methods for preparing aliphatic polyamides are not suitable for preparing high-
molecular-weight aromatic polyamides because of the reduced reactivity of aromatic dia-
mines and the high melting point of the resulting polymers. Polymerization of wholly
aromatic polyamides is usually carried out in solution, instead of in bulk, using highly reactive
diacid chlorides vs. diacids. The reaction is fast and takes place at a much lower temperature
than conventional melt polymerizations. The synthesis is based on the familiar Schotten–
Baumann reaction [44–49].
H2ONaCl ++R
O
N
R1
R2
NaOHH
R1
R2
+RC
O
C Cl N C ð13:12Þ
Condensation polymers are formed if the complementary reagents are difunctional.
N
H
R
H
C
O
R' C
O
+ 2nH2O2nNaOH
nnClnH2N + + 2nNaClC
O
R' C
O
ClNH2R N
ð13:13Þ
A large amount of salt is generated in this reaction following neutralization of the by-product
hydrochloric acid (HCl). The high salt concentration in the process stream requires the
� 2006 by Taylor & Francis Group, LLC.
use of expensive corrosion resistant materials—one of the key contributors to the high cost of
aramid fibers.
An alternative route to aromatic polyamides is referred to as a hydrogen transfer reaction
[50]. This reaction between a diacid and diisocyanate is run at a low temperature to form an
intermediate polymer that loses carbon dioxide on subsequent heating to form the aromatic
polyamide (Equation 13.14).
H-transfer
HeatCO22n+
nN
H
R1 N
H
C
O
R2 C
O
C
O
N
H
R1 N
OH
O
O
R2 C
O
O
n
nHOnO
n
C
O
N
H
R1 N
OH
O
O
R2 C
O
OC
O
R2 C
O
OHNC R1 N O +C C C
C C
ð13 :14 Þ
This reaction is not widely used because of the higher cost of diisocyanates and the difficulty
in eliminating all the carbon dioxide.
13.5. 3.1 Reactio n Mech anism
The first step in the condensation reaction is the attack of the amine nitrogen at the carbonyl
carbon of the dicarboxylic acid. The local electron density at the aromatic amine nitrogen is
greatly reduced by participation of the lone pair electrons with the aromatic p-cloud, whereas
the local electron density of the aliphatic counterpart is enhanced by the inductive effect of
aliphatic hydrocarbon. This leads to a significant difference in the polycondensation reaction
rate between aromatic polyamides and aliphatic polyamides.
+ Aromatic amidation
HO C
O
CH2+
Aliphatic amidation
N
H
H
CH2
N
H
H
CX
O
pi-cloud overlap
inductive effect
To compensate for reduced electron density at the amine nitrogen, the dicarboxylic acid is
activated by increasing the partial positive charge at the carbonyl carbon. Halogen atoms (X)
have proven to be effective because of their high electronegativity. An amide linkage is
formed from the transition complex (Equation 13.15) by eliminating HX (Equation 13.16).
Because the eliminated acid, HX, will react with the opposing amine to form a quater-
nary ammonium salt, it must be removed for the polymerization to continue. An organic
amine, such as pyridine, is often used as an acid acceptor to regenerate the amine end
� 2006 by Taylor & Francis Group, LLC.
from the quaternary salt (Equation 13.17). Polymerization solvents such as N, N-dimethyl
acetamide (DMAc) and N-methylpyrrolidone (NMP) are sufficiently basic to function as acid
acceptors as well.
Transition complex
C
O
X
C X
O
NH2N
H
H
N
O
X
H
H
H2N
O
XC C
ð13 :15 Þ
N C
OH
H2N C
O
X
X
H
N
O
X
H
H
H2N
O
X
Transition complex
C C
ð13 :16 Þ
+
X N
H
Amine regeneration
N C
OH
H2N C
O
XN
N
OH
H2N
O
X
X
H
C C
ð13 :17 Þ
Factors that can limit the extent of the polymerization reaction include deactivation of chain-
ends, stoichiometric imbalance of reagents, monofunctional impurities, and insufficient mo-
bility of growing chain-ends. Some of these factors are used to control polymer molecular
weight.
13.5.3.2 Reaction Energetics
As shown in Table 13.7, the free energy of reaction of aramid polymerizations is reported to
be negative even with aromatic acid, ester, and diamine monomers. In spite of this driving
force, the rate of reaction is extremely slow because of the high activation energy of the
polymerization reaction [51].
13.5.4 DIRECT POLYMERIZATION BY CATALYSIS
Several different classes of catalysts, so-called condensing agents, have been reported in the
literature [52–55] for the polycondensation reaction of aromatic diamines with aromatic
diacids. This polycondensation is called ‘‘direct polymerization’’ because unmodified mono-
mers can be used in the reaction. The condensing agents, which are generally derived
from phosphorus or sulfur compounds, activate the dicarboxylic acid in situ during the
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TABLE 13.7Energetics of Aromatic Polycondensation
DGr(T) (KJ=mole) DGr(T) (KJ=mole)
Diamine T8K IPA DMI DPI ICl TPA DMT DPT TCl
MPD 298 �8.5 32.5 �79.5 �158.0 8.5 �17.0 �63.5 �145.0
400 �23.0 — — �179.5 �4.0 �47.5 — �168.5
PPD 298 �35.5 �59.5 �106.5 �186.0 �21.5 �47.0 �94.5 �175.0
400 �50.0 — — �207.5 �49.5 �92.0 — �214.0
MPD: m-Phenylenediamine
DPI: Diphenylisophthalate
DMI: Dimethyl isophthalate
TPA: Terephthalic acid
DPT: Diphenylterephthalate
DGr(T): Free energy of reaction
PPD: p-phenylenediamine
IPA: Isophthalic acid
ICl: Isophthaloyl chloride
DMT: Dimethylterephthalate
TCl: Terephthaloyl chloride
Source: From Hand, D.R., Hartert, R., and Bottger, C., Stab resistant and Anti-ballistic material. Method of making
the same, U.S. Patent Application Publication U.S. 2004=0023580 A1, February 5, 2004; Karyakin, N.V. and
Rabinovich, I.B., Dokl. Akad. Nank. SSSR, 271(6), 1429, 1983.
polymerization. The best-known route involves an N-P type intermediate as the activated
complex. As an example, triphenyl phosphite is reacted with a carboxylic acid in the presence
of a tertiary amine (e.g., pyridine) to form the N-phosphonium salt 5, which gives the
corresponding amide on aminolysis (Equation 13.18).
5
OPh
+ + OHP OPhH
O
OPh
C
O
N
H
NH2NH
N
O C
O
PhO OPh+ CHO
O
P
OPh
OPhPhO P
ð13:18Þ
The reaction mechanism involves protonation of the triphenyl phosphite by a carboxylic acid
to form 2, which is transformed by pyridine into transition states 3 and 4. The N-phospho-
nium salt 4 reacts with the carboxylate anion to give 5.
� 2006 by Taylor & Francis Group, LLC.
543
2
H
N O
O
PhO
OPhCHO
O
OPh
N
POPhPhO
H
N
POPhPhO
H
NP
OPh
OPh
OPh
HH+P
OPh
OPhPhO
OPhOPh
CP
ð13:19Þ
In other words, the aromatic carboxylic acid is activated by the pyridinyl triphosphonate
cation so that the weakly basic aromatic amine can effectively attack the carbonyl center. The
reaction has not been utilized commercially because the costs of recovering and regenerating
triphenylphosphite far outweigh the cost advantage of using unmodified diacids.
Similar activation mechanisms of the P-O-P type [56], C-O-P type [57], and N-S=C-O-S
type [58], and reactions activated by silicon tetrachloride [59] and aromatic halo compounds
such as picryl chloride have also been reported in the literature [60].
13.5.5 POLYMERIZATION METHODS
The two principal methods used for the synthesis of aromatic polyamides are interfacial
polymerization and solution polymerization. Vapor-phase polymerization and plasticized
melt techniques have also been demonstrated but have not been adopted for practical use.
13.5.5.1 Interfacial Polymerization
In the interfacial method, the two fast-reacting intermediates are dissolved in a pair of
immiscible liquids, one of which is preferably water. The water phase contains the diamine
and any added alkali. The second phase consists of the diacid halide in an organic liquid such
as carbon tetrachloride, dichloromethane, xylene, or hexane, etc. The two solutions are
brought together with vigorous agitation and the reaction takes place at or near the interface
of the two phases; hence, the name interfacial polymerization.
13.5.5.1.1 Reaction at the InterfaceIn interfacial polycondensation, the polymerization reaction occurs very close to the interface
between the aqueous and organic layers generally just within the organic solvent layer that
contains the diacid chloride [60,61]. The adjacent aqueous phase generally contains, in
addition to the diamine, a basic reagent capable of neutralizing hydrogen chloride liberated
in the reaction. The reaction rate is so fast that the polymerization reaction becomes ‘‘diffu-
sion-controlled.’’ As the polymerization proceeds, the diffusion of additional monomers
through the formed polymer layer becomes increasingly difficult. As a result, the number of
growing chains is limited. For this reason, polymers with much higher molecular weights are
� 2006 by Taylor & Francis Group, LLC.
formed than are obtained in a normal step-growth polymerization reaction and these high
molecular weights are achieved at less than quantitative conversion. Furthermore, because
the polymerization reaction is diffusion controlled, it is not mandatory to start with an exact
balance of the two monomers in the respective phases.
There is no evidence that the interface has any special orienting or aligning effect on the
reactants, but it does provide, through solubility differences, a controlled introduction of the
diamine in the aqueous phase into an excess of diacid halide in the adjacent organic phase.
When the two phases are brought into contact, both reactants and solvents tend to
become partitioned with the opposing phase. The diamine nearly always has an appreciable
partition toward the organic phase, whereas the acid chloride has very little solubility in
water. Measured equilibrium partition coefficients for diamines in useful solvent systems have
varied from 400 to less than 1(CH2O=Csolvent). The values have been used to estimate the
relative tendency of diamines to transfer to the organic phase under polymerization condi-
tions. Partition equilibria are never achieved during polymerization because mass transfer of
diamine is the rate-controlling step at all concentrations and acylation takes place in the
organic phase as rapidly as diamine is transferred.
13.5.5.1.2 Amine AcylationAt the onset of the polycondensation reaction, diamine monomer sees excess acid chloride
and is presumably acylated at both ends. Ensuing diamine encounters a layer of acid chloride-
terminated oligomer and some acid chloride. The reaction proceeds by an irreversible coup-
ling of the oligomers by the diamine. The concentration and size of oligomers increase until a
layer of high polymer is obtained. Thus, high polymer forms because of the high reaction rate
and the increasing probability that the diamine will react with an acid chloride-terminated
oligomer rather than with a free acid chloride monomer.
13.5.5.1.3 Acid EliminationHydrogen chloride, the product of the fast reaction between amine and acid chloride, diffuses
to the aqueous phase. Any amine hydrochloride that might be formed is usually very
insoluble in the organic phase but is soluble in the aqueous phase. Both hydrogen chloride
and amine hydrochloride have to be neutralized in the aqueous phase with inorganic bases.
13.5.5.1.4 Major VariablesVariables affecting the polymerization include temperature, monomer ratio and concentra-
tion, impurities, additives, acid acceptor, and mode of addition. The polymerization of MPDI
is used as a model for interfacial polymerization in the following discussion.
13.5.5.1.4.1 Temperature
Most interfacial polycondensation are initiated at ambient temperature. Because the reactions
are rapid there is no need for heating and, in fact, cooling is frequently employed to
control the temperature rise, especially on a larger scale [62–64]. Raising the temperature
will change the solubility of both polymer and intermediates and will accelerate side reactions
as well as the desired polymerization reaction.
13.5.5.1.4.2 Reactant Equivalence
The molecular weight of polymers made by interfacial polycondensation is far less sensitive to
nonequivalence of reactants than that of polymers prepared by melt or solution methods for
reasons already discussed—high reaction rate, diffusion control of monomers, and the none-
quilibrium nature of the polymerization. The molecular weight of polymers precipitating as a
coherent film from an unstirred interface is completely insensitive to the contents of the
system as a whole, whereas the molecular weight of polymers from a stirred interface is
generally more sensitive to reactant nonequivalence.
� 2006 by Taylor & Francis Group, LLC.
13.5.5.1.4.3 Impurities and Additives
Interfacial polymerization will tolerate the presence of impurities in the reactants that simply
dilute the material and thereby produce nonequivalence of reactants. These diluents might be
water or inert contaminants in the acid chloride. Reactive monofunctional species are harmful
in either phase. To maximize molecular weight, it is essential to use high purity monomers.
Molecular weight control can be achieved, if desired, with appropriate use of monofunctional
reagents. Examples of impurities interfering with the interfacial polyamidation of MPDI
are half hydrolyzed acid chloride, monoamide, partially oxidized amines, and reactive
surfactants.
13.5.5.1.4.4 Acid Acceptors
Salts of basic diamines and strong acids are not sufficiently dissociated to permit the amine to
react further.At least twomoles of acid acceptor permole of diamineare needed tomaximize the
yield of high polymer [65]. Of water soluble inorganic acid acceptors used in MPDI polymeriza-
tions in a water–DMeTMS solvent system, sodium carbonate appeared to be the most prom-
ising. Use of two equivalents of sodium carbonate gave white polymer with inherent viscosity of
2.48 in 100% yield. With 1.1 equivalents of sodium carbonate, white polymer with an inherent
viscosity of 2.70 was obtained in 100% yield, while further reduction to one equivalent gave a
polymer with an inherent viscosity of 1.97. Polymer with an inherent viscosity of 1.83 (98.5%
yield) was obtained using two equivalents of sodium bicarbonate. Calcium hydroxide, potas-
sium carbonate, and sodium hydroxide all gave polymers with lower inherent viscosity.
13.5.5.1.4.5 Reactant Addition
The mode of addition of reactants will also influence the reaction. Perhaps the best procedure
would be to use a high-speed, low-volume mixer into which both solutions are charged
simultaneously. In a typical batch polymerization process, rapid addition of the diacid
chloride solution to a vigorously stirred diamine solution has given the best results. Rapid
initial stirring appears to be an essential requirement for obtaining high-molecular-
weight MPDI in water–DMeTMS. In two experiments employing rapid and slow stirring,
respectively, in a Waring blender, polymers with inherent viscosity of 2.48 and 0.66
were obtained. In another experiment polymer obtained with initial low speed stirring for
one minute followed by high speed stirring for an additional four minutes had a viscosity of
only 0.41 [66].
These and other factors affecting the interfacial polycondensation reaction are discussed
in more detail in P.W. Morgan’s book entitled, ‘‘Condensation Polymers,’’ published by
Interscience Publishers, John Wiley & Sons, 1965 [66].
13.5.5.2 Solution Polymerization
Solution polycondensation is carried out in an inert organic solvent.
Tertiary amines typically serve as the acid acceptor. The procedure generally starts with all
the ingredients in solution but this is not always an essential requirement. The polymer may
remain in solution or precipitate at any time.
13.5.5.2.1 Interfering FactorsBoth physical and chemical factors can limit the polymerization reaction. Several effects that
are classified as physical, even though they are physicochemical interactions, are the quality
of stirring, precipitation of diamine salts, and precipitation of the polymer. Chemical factors
include reactions with impurities and acid acceptors.
13.5.5.2.1.1 Impurities
The fast, low-temperature solution polymerization reactions are surprisingly tolerant of
impurities but this tolerance varies considerably. The purity of the reactants and solvents
� 2006 by Taylor & Francis Group, LLC.
must exceed the level required by the interfacial method. This is because all of the materials
are in intimate proximity in a single-phase system.
Nonreactive impurities in the solvent are of minor significance except as they might
depress the solubility of the polymer. Nonreactive impurities in the intermediates lead to an
imbalance in the reactants thereby limiting molecular weight.
Reactive impurities are substances that can react with the monomers, the growing chain-
ends, or the acid acceptor to terminate the polymerization prematurely. They can be
introduced with the solvent or with the intermediates. The acid chloride may contain
impurities originating in its synthesis or storage such as hydrogen chloride, thionyl chloride,
phosphorus halides, or monoacid halides. The diamine may contain monoamines, water, or
carbonates. It may degrade oxidatively in air or absorb moisture and carbon dioxide. The
degree of interference caused by these impurities depends on both the quantity of
the impurities as well the relative reaction rates of the desired polymerization vs. those
of the impurities.
13.5.5.2.1.2 Solvent Reactivity
The solvent should not react with either the amine or the acid halide during the course of
the polymerization. Solvent interference can be limited by minimizing the contact time
between the monomer and the solvent; for example, the intermediates can be dissolved and
allowed to react simultaneously. Alternatively, a small amount of nonreactive solvent can be
used to dissolve one or both intermediates prior to polymerizing them in a more reactive
medium.
13.5.5.2.1.3 Side Reactions with Acid Acceptors
Secondary amine acid acceptors can terminate chain growth by reacting with the diacid halide
unless amine reactivity is minimized by steric effects. Reactions between a tertiary amine acid
acceptor and the acid halide or certain solvents must also be avoided. An acid chloride and a
tertiary amine can react to form a monoamide and an alkyl halide (Equation 13.20). This
reaction is known to occur in fair yield at high temperatures and probably takes place to some
extent at room temperature [67–69]. In the usual preparative method wherein diacid halide is
added to a solution of diamine and a strongly basic acid acceptor, no difficulty is experienced
if the polycondensation reaction is rapid. As the polycondensation reaction rate decreases, the
potential for interference by side reactions increases. In a polymerization system, this would
be a chain terminating reaction.
R Cl+C
O
N
R
RC
O
N
R
R
RCl
N
R
R
R
+C
O
Cl
ð13:20Þ
A reaction that can occur between an acid chloride and a tertiary amine in the presence of
moisture is the formation of an acid anhydride (Equation 13.21).
NH
R
R
Cl+ 2RC
O
N
R
R
RCl
C
O
O
O
2 C
ð13:21Þ
� 2006 by Taylor & Francis Group, LLC.
An anhydride group in the polymer chain is a hydrolytically weak link and would likely be
subject to cleavage on isolation of the polymer in water.
13.5.5.2.1.4 Diacylation
Diacylation of an amine by the acid halide leads to branched and network polymers. This side
reaction has also been observed in interfacial polycondensation reactions [70].
13.5.5.2.2 Reaction RatesSolution polycondensation employs the same reactions as used in interfacial polycondensa-
tion and similar reaction rates are involved. This means that the fastest reactions have rates
on the order of 102–106 l=mole-sec. Polycondensations involving such reactions may be
completed in a few minutes at room temperature.
13.5.5.2.3 Physical and Mechanical Effects
13.5.5.2.3.1 Temperature
Solution polycondensation reactions between diamines and diacid halides produce polymers
with maximum molecular weight when carried out at room temperature or below.
While reaction rates and polymer solubility would be expected to increase with increasing
temperature, the rates of competitive side reactions will also increase.
13.5.5.2.3.2 Concentration
Solution polycondensation reactions have not shown any marked sensitivity to reactant
concentration except as the concentration affects stirrability or temperature control. Lower
concentrations are uneconomical and introduce relatively larger amounts of solvent impur-
ities. Higher concentrations may yield unstirrable masses when the polymer or by-product
salt precipitates, and the heat of the reaction is more difficult to control when reactants are
mixed rapidly at high concentration.
13.5.5.2.3.3 Equivalence of Reactants and Mixing
Although both interfacial and solution polycondensation reactions show unusual insensitivity
to nonequivalence of reactants, solution polycondensations are appreciably more sensitive to
reactant balance.
Features common to both polymerization methods include: (1) use of fast reacting
intermediates; (2) reaction irreversibility; (3) the reaction takes place essentially as fast as
the contact of complementary reactants occurs; and (4) the growing polymer is in solution or
highly swollen during the polymerization process. Unlike the interfacial process, the solution
process has no interface to provide for the flow of one reactant into a higher concentration of
the complementary reactant. It is this liquid–liquid interface that plays a significant role in
attaining reactant balance in the interfacial process. The success of the solution process shows
that an interfacial boundary, while helpful as a regulating device, is not essential for the
formation of a high-molecular-weight polymer.
A key rationale for the insensitivity to nonequivalence of reactants in a single-phase
system is that the rate of polymerization is often faster than the rate of mixing even in
the absence of an interfacial boundary. It is presumed that in a solution polymerization
system there are temporary interfaces or zones within which polymerization is proceeding
independently of any potential effect of the ratio of the two reactants in the system as a whole.
Thus, even a single drop of acid chloride solution in a large volume of diamine solution reacts
rapidly with the local, or immediately surrounding diamine, before the droplet is dispersed.
This leads to oligomers and polymer with higher molecular weight than would be obtained
from a random reaction at the known reactant ratio. Further dropwise addition of one
reactant continues this effect because each successive drop goes into a large system that
� 2006 by Taylor & Francis Group, LLC.
consists in part of an active polymer with a higher than random degree of polymerization.
Eventually as the system approaches equivalence and the concentration of reactive groups is
reduced, there is a greater chance of a wider distribution of the increment of added reactant
and the occurrence of random reaction [68]. Theoretical treatments of the effects of monomer
ratio as well as side reactions have been described by Flory [71]. Kilkson has analyzed the
problem of irreversible polymerization in both batch and steady-state reactors [72].
13.5.5.2.4 Acid AcceptorsPolycondensation reactions between diamines and diacid chlorides require the removal of the
by-product hydrogen chloride. The acid acceptor need not be a basic substance but must retain
the by-product acid in some way while the reaction proceeds. A variety of amines and some
sterically hindered secondary amines have been used as acid acceptors in the solution prepar-
ation of polyamides. From an empirical point of view, the base strength of the acid acceptor
should be about equal to or greater than the base strength of the terminal amine group at the end
of an oligomer or polymer chain. The pKa scale in water is used for base strength. A different
measure,E1=2, isused toquantify thebase strengthofamines inorganic solvents.Hall [73,74]has
defined the E1=2 of an amine as a potential (in millivolts) of solution at the half-titration point
with perchloric acid andhas shown thatE1=2 is parallel to the pKa scale inwater. Table 13.8 lists
these values for some acid acceptors frequently used for solution polyamidation.
13.5.5.2.5 SolventThe solvent has many roles. It dissolves the monomers and provides for their mixing and
contact; it dissolves or swells the growing polymer so that the reaction is maintained; it carries
the acid acceptor and facilitates the disposition of by-product salts; it influences the reaction
rate by polarity or solvation effects; and it absorbs the heat of reaction.
The solvent should be inert and should ideally be able to dissolve the intermediates before
the polymerization is started. A primary requisite for high polymer formation in all solution
polycondensation reactions is that the solvent must be able to dissolve or swell the poly-
mer sufficiently to permit the completion of the polymerization [75–77]. The solution
polycondensation process requires a stronger polymer–solvent interaction than does the
TABLE 13.8Basicity of Amine Acid Acceptors
E1=2 (mV)a
Acid acceptor pKa Ethyl acetate Acetonitrile
tert-Butylamine 10.45 130 —
Diisobutylamine 10.59 207 —
Triethylamine 10.74 197 66
Tri-n-propylamine 10.70 228 —
Tri-n-butylamine 10.89 210 —
N-Ethylpiperidine 10.45 190 84
N-Ethylmorpholine 7.70 290 221
N,N-Diethyl-m-toluidine 7.24 — —
N,N-Diethylaniline 6.56 467 425
Pyridine 5.26 — —
aE1=2 is the millivolt reading at the half-titration point at 258C with perchloric acid as the titrant
from the work of Hall.
Source: From Hall, H.K., J. Am. Chem. Soc., 79, 5439, 1957; Hall, H.K., J. Phys. Chem., 60, 63,
1956.
� 2006 by Taylor & Francis Group, LLC.
interfacial polycondensation method. The combination of solvent, diamine, and acid acceptor
must be such that the diamine does not precipitate as a salt with limited solubility.
Although little is known about the effects of solvent polarity, viscosity, and specific gravity on
these reactions, the reaction rate tends to increase with an increase in solvent polarity [78,79].
13.5.5.2.6 Solubilizing AidsOccasionally, solubilizing aids or auxiliary solvents are added to boost the solvating power of
the primary solvent. The polymerization of PPTA requires the presence of a solubilizing aid
to obtain a high-molecular-weight polymer. Alkaline or alkaline earth metal halides such as
CaCl2 and LiCl are known to be effective solubilizing aids in substituted amide solvents such
as NMP and DMAc. Solubilizing aids apparently increase the polarity of the solvent by
complexing with the carbonyl group (Equation 13.22).
CH3 C
O
N
CH3
CH3
+ LiCl
Li
CH3 C
O
N
CH3
CH3
Cl
Polarization ð13:22Þ
More recently, quaternary ammonium halides such as methyl tri-n-butyl ammonium chloride
were used in the polymerization of PPTA in NMP [80]. Effective shielding of the ammonium
cation by bulky alkyl groups stabilizes the ionized species in an organic medium so that it can
facilitate the polarization of NMP (Equation 13.23).
Cl
+
N
CH3
ON
CH3
O
NH3C
CH2CH2CH2CH3
CH2CH2CH2CH3
CH2CH2CH2CH3
Cl
NH3C
CH2CH2CH2CH3
CH2CH2CH2CH3
CH2CH2CH2CH3
ð13:23Þ
13.5.5.2.7 Reactivity of Precipitated PolymerIn the solution polymerization of PPTA in NMP–CaCl2 solvent, significant chain growth takes
place after the polymer precipitates. At the beginning of the reaction, the polymerization
proceeds in solution. As the molecular weight of the polymer increases, the viscosity of the
solution increases rapidly to a gel point and eventually the polymer precipitates. At this stage, the
molecular weight of the polymer is still very low (inherent viscosity ~2), but the polymerization
continues in the precipitated state to an inherent viscosity of >6, in the absence of interfering
contaminants such as water. This is a clear evidence that the chain-ends of the polymer are not
deactivated on precipitation but retain enough mobility to react with the neighboring active
groups. However, the rate of reaction becomes very slow after the polymer precipitates.
13.5.5.3 Vapor-Phase Polymerization
Vapor-phase polymerization has been described in the patent literature as an alternative route
to aromatic polyamides from aromatic diamines and aromatic diacid chlorides [81]. The
reaction is carried out in the gas phase by mixing vapors of the two monomers in the presence
of an inert gas. The temperature at the reaction zone has to be higher than the glass transition
temperature of the polymer to achieve segmental mobility of the growing polymer chain.
� 2006 by Taylor & Francis Group, LLC.
� 2006 by Taylor & Francis Group, LLC.
l
Polymer decomposition is minimal because the reaction time is very short. The polymer is
deposited on removable inorganic or organic substrates maintained in the reaction zone.
Monomer Avapor
Monomer Bvapor
Inert gas Mixer
Rea
ctor
Que
nch Sep
arat
or
Inert gas
Scrubber
Inert gasrecycle
Schematic of the Vapor-Phase Polymerization Process
Vapors of two different monomers (A and B) together with a hot inert gas are fed to a
mixer (such as a jet mixer, a simple short tube, or a combination of both) and then to the
reactor inlet. Additional inert gas can be introduced as needed. The reactor effluent stream
consisting of some polymer, possible oligomers, and by-product acid, is conducted through a
quench chamber where the stream is cooled by a flow of relatively cold inert gas. The cooled
stream is then led through a separator such as combination of a cyclone separator and filters
to remove solid material. The filtered stream is then passed through a water scrubber to
remove hydrogen halide and vented to the atmosphere or recycled.
Vapor-phase polycondensation has the distinct advantage of not having to use solvent and
it makes possible the elimination of by-product HCl in the gas phase. However, the resulting
polymers are usually highly branched due to the high reaction temperature required to
maintain chain mobility. In addition, the stoichiometric balance of reagents is much more
difficult to maintain than in the case of a condensed phase reaction.
13.5.5.4 Plasticized Melt Polymerization
Most aromatic polyamides cannot be made by a melt polymerization process because the
polymer melt temperature exceeds the decomposition temperature. Singh developed a unique
procedure for preparing certain aromatic polyamides by a melt process using an interna
plasticizer generated in-situ during the polymerization [82]. The following reaction scheme
was used to prepare aromatic polyamides in the absence of a solvent (Equation 13.24).
N
O
H
+
N
OC
O
C
O
N
O
x4C
O
CH2 N
H
+1−x
C
O
NC
H
O
N
H
+
NH2H2N
ð13:24Þ
The melt polycondensation of isophthaloyl-N,N-bis (valerolactam) with m-phenylene dia-
mine yielded the aromatic polyamide MPDI plasticized by liberated valerolactam. A small
amount of valerolactam is polymerized to poly(valerolactam) during the polymerization,
which the author claims can be minimized by adjusting the reaction parameters. It is proposed
that the plasticizer can be removed by water extraction after the shaping process thereby
recovering the infusible aromatic polyamide.
13.6 ARAMID SOLUTIONS
Aramid polymers have high melting points or melt with decomposition that makes fiber
processing by melt spinning impractical [1]g. Fibers are therefore spun from polymer solu-
tions. These polymers not only do not melt but also are not easy to dissolve. Highly polar
solvents, with or without the aid of inorganic salts such as lithium chloride or calcium
chloride, or acids like concentrated sulfuric acid have to be used [88].
13.6.1 ISOTROPIC S OLUTIONS
Some aramids are processed from isotropic solutions. Flexible chain homo-polymers like MPDI
can be dissolved in solvents like NMP and DMAc [88] to form such solutions but the degree of
solubility can be further enhanced by copolymerization [83]. Isotropic solutions can be also
obtained with p-aramids but in this case copolymerization is required to enhance solubility.
13.6. 1.1 m-Aram id So lutions
As previously mentioned, DuPont and Teijin are the two major manufacturers of m-aramid
fibers. Russian scientists also developed a commercial process for the manufacture of MPDI
polymer and fiber under the trade name of Fenilon [84]. However, at this point Fenilon
production has been suspended.
DuPont’s m-aramid polymer, MPDI, is polymerized using essentially a 1:1 molar ratio of
m-phenylenediamine and isophthaloyl chloride [85]. Patent literature indicates that the fiber,
Nomex, is spun directly from the polymerization solution in DMAc, which contains calcium
chloride. MPDI polymer solutions containing >3% by weight calcium chloride are quite
stable [2].
Teijin’s product, trademarked Teijinconex, is a 100 =97 =3 copolymer of MPD =ICl =TCl
[83]. The polymer is prepared by interfacial polymerization, isolated and dissolved in NMP to
form spin dopes of approximately 20% solids concentration [86]. The resulting isotropic
solutions are stable at 1008C and are suitable for wet spinning. The solution has two solubility
limits that include reversible and irreversible regions, as shown in Figure 13.1 [87]. If the
irreversible limit is exceeded, the polymer becomes soluble only in sulfuric acid.
The Russian Fenilon process utilizes low-salt content MPDI solutions [89]. Most of the
hydrochloric acid generated during the polymerization process is removed by treatment with
ammonia. The resulting insoluble ammonium chloride is filtered from the polymerization
solution. Residual HCl is likely neutralized with an organic base. The neutralized solution is
suitable for wet spinning of fibers.
13.6.1.2 p-Aramid Solutions
p-Aramids are soluble in strong acids and in highly polar solvents in the presence of in-
organic salts. They form isotropic solutions only at low polymer concentrations. Among
commercial products, copolyamides from the SVM family as well as copoly(p-phenylene=3,4’-diaminodiphenylether terephthalamide) (Teijin’s Technora base polymer) remain soluble in
their polymerization mixture [90] and can be spun directly from that solution.
gException Teijinconex mono-filament process.
� 2006 by Taylor & Francis Group, LLC.
10
20
30
00 100
Pol
ymer
con
c, %
Reversible
limit
Solution
region
Irreversible
limit
Temperature, 8C
FIGURE 13.1 Stability of Teijinconex spin solution. (From Fujie, H., Nikkyo Geppo, 40, 8, 1987. With
permission.)
13.6.2 A NISOTROPIC SOLUTIONS
13.6. 2.1 Phase Behavior
A distinctive feature of semirigid polymers such as p-aramids is that their solutions develop
molecular orientation under shear or extension with great ease. This results in a unique
difference in properties in the direction of shear or extension vs. those perpendicular to the
shear direction. There are two classes of materials that have this characteristic: lyotropic,
which form anisotropic solutions; and thermotropic, which form anisotropic melts. As
aramids do not melt we will focus here on lyotropic systems. Anisotropic solutions differ
from isotropic solutions in many physical characteristics including light depolarization,
rheological properties, phase behavior, and molecular orientation.
Observed structures of a lyotropic material are classified into three categories: nematic,
smectic, and cholesteric. Nematic and cholesteric mesophases can be readily identified by
microscopic examination. The existence of a smectic mesophase is not well defined and is only
suggested in some cases. Solvent, solution concentration, polymer molecular weight, and
temperature all affect the phase behavior of lyotropic polymer solutions. In general, the phase
transition temperature of a lyotropic solution increases with increasing polymer molecular
weight and concentration. It is often difficult to determine the critical concentration or
transition temperature of a lyotropic polymer solution precisely. Some polymers even degrade
below the nematic–isotropic transition temperature so that it is impossible to determine the
transition temperatures. Phase behavior is also affected by the polymer molecular conform-
ation and intermolecular interactions.
A good example of a lyotropic solution is that of PPTA in sulfuric acid. Figure 13.2
shows the viscosity–concentration relationship of a solution of PPTA of moderate molecular
weight [91]. At low polymer concentrations, the solution viscosity increases with increasing
concentration just like an isotropic solution of a flexible chain polymer. However, above a
critical concentration of ~12%, the solution viscosity decreases abruptly with increasing
concentration. This behavior is caused by the close packing of the rigid chain polymer
molecules to form ordered domains. The solution viscosity reaches a minimum point at
about 20% solids and then abruptly increases with additional solids. A solid phase will
eventually appear when the solution becomes supersaturated. The anisotropic PPTA–
H2SO4 solution exhibits liquid crystal behavior. It has the flow properties of a liquid and is
crystal-like with the ability to depolarize cross-polarized light. When the solution is subjected
� 2006 by Taylor & Francis Group, LLC.
0
10
20
30
40
50
5 10 15 20 25 30
Bro
okfie
ld v
isco
met
er r
eadi
ng
Concentration, wt %
FIGURE 13.2 Bulk viscosity vs. concentration of PPTA–H2SO4 solution. (From Bair, T.I. and Morgan,
P.W., U.S. Patent 3,673,143, 1972; U.S. Patent 3,817,941, 1974. With permission.)
to shear or elongational flow, the liquid crystal domains become aligned in the direction of
flow to achieve a high degree of molecular orientation.
For fiber preparation, a lyotropic solution is best processed at a solids concentration near
the minimum solution viscosity and at a temperature close to its anisotropic transition
temperature (Figure 13.2). These conditions maximize solution ordering prior to spinning.
13.6. 2.2 Rheologi cal Propert ies
Lyotropic solutions generally exhibit viscoelastic behavior. They are pseudoplastic and
exhibit shear thinning with increasing shear rate. For polymers of near-linear chain conform-
ation, their lyotropic solutions are known to give less die swell and are less tractable than
isotropic solutions. The PPTA–H2S04 solution was the first to be used commercially and has
been studied most extensively.
The rheological properties of PPTA–H2S04 solutions have been studied by several inves-
tigators [92–97]. Figure 13.3 and Figure 13.4 show the relationship between shear viscosity, �hh,
and shear rate, g, for Kevlar–H2SO4 solutions of various concentrations at 25 and 60 8C,
respectively. Figure 13.5 is a plot of shear viscosity vs. shear stress for PPTA solutions at 25 8C[97]. The change in the slope of these curves between 8 and 10% solutions shows the effect of
the isotropic–anisotropic phase transition. The viscosity–shear stress curves for 10 and 12%
solutions tend to infinity, indicating the presence of a yield stress [94].
13.7 PREPARATION OF ARAMID PRODUCTS
13.7.1 FIBERS
13.7. 1.1 Dry Spinnin g
Solutions of m-aramid polymers are currently produced using dry-or-wet spinning processes.
Processing steps after spinning can include drawing, drying, and heat treatment.
In the dry-spinning process, a solution of polymer is extruded through a spinneret that is
mounted at the top of a heated column. As the solution is extruded in the presence of hot inert
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10�2 10�1
0.5%
6%
8%
12%
10%
100100
102
103
104
105
101
γ, sec�1
h, p
oise
.
FIGURE 13.3 Shear viscosity vs. shear rate for re-dissolved Kevlar–H2SO4 solution at 258C. (From
Aoki, H., White, J.L., and Fellers, J.F., J. Appl. Polym. Sci., 23, 2293, 1979. With permission.)
gas (or air), solvent evaporates from the incipient fiber. The temperature of the heated gases
in the column is above the boiling point of the solvent. The solidified fiber is collected at the
bottom of the column. The polymer solvent must be inert, stable at its boiling point, and a
good solvent for the polymer. The heat of vaporization of the solvent must not be too high, it
must have sufficient thermal resistance, low toxicity, a very low tendency to produce static
charges, low risk of explosion, and be relatively easy to recover [98]. The dry-spinning
process was initially developed for spinning acrylic fibers and was modified for spinning m-
aramid polymer. DuPont developed processes for dry spinning Nomex from DMF and
DMAc solutions [99]. The m-aramid polymer solution is disordered in the solution state.
Some orientation is imparted during the extrusion of the solution through the spinneret
capillary. The extent of fiber orientation tends to increase as the shear rate through the
spinneret capillary is increased. Radial structural inhomogeneities are generally introduced
during the solvent diffusion and evaporation stages of the dry-spinning process [10]. A skin
10�3 10�2 10�1
γ, sec−1
100 101101
102
10310%
12%
8%
6%
104
102
h, p
oise
FIGURE 13.4 Shear viscosity vs. shear rate for re-dissolved Kevlar–H2SO4 solution at 608C. (From
Aoki, H., White, J.L., and Fellers, J.F., J. Appl. Polym. Sci., 23, 2293, 1979. With permission.)
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101 102
8%6%
10%
12%
102
103
104
105
σ12, dynes/cm2
h, p
oise
103 104
FIGURE 13.5 Shear viscosity vs. shear stress for re-dissolved Kevlar–H2SO4 solution at 258C. (From
Aoki, H., White, J.L., and Fellers, J.F., J. Appl. Polym. Sci., 23, 2293, 1979. With permission.)
core structure forms because the outer skin of the fiber loses solvent faster than the inner core.
As diffusion progresses, the loss of the solvent from the core through the solidified sheath
reduces the mass of the core. This results in the sheath collapsing inward. Since the evapor-
ation rate of the solvent in the sheath of the fiber is faster than the diffusion rate of solvent
from the core of the fiber the cross section shape of the fiber can change from round to dog-
bone. m-Aramid fibers are spun at a spin stretch ratio of 1–20x, which is far lower than
fibers processed from the melt, but this has little impact on fiber properties since there is
very little orientation produced during this part of the process. The resulting m-aramid fibers
at the bottom of the spin cell retain considerably more solvent ( >20%) than dry spun acrylic
fibers (<5%).
The as-spun fiber is then drawn to develop physical properties. Fiber drawing is generally
done in a dilute water solution of solvent. The solvent partially plasticizes the fiber and
facilitates drawing (3–5x). After the drawing step, the fibers are washed with water, dried, and
crystallized by heating at a temperature above the polymer Tg (~275 8C) [100,101]. Typical
fiber properties are in the order of 0.6 GPa with an elongation to break of 30%. A schematic
of the Nomex dry-spinning process is shown in Figure 13.6.
13.7. 1.2 Wet Spinni ng
In the wet-spinning process polymer solution is extruded through a spinneret that is sub-
merged in a coagulating medium consisting of solvent and nonsolvent. On coagulation, the
spinning solution undergoes spinodal decomposition into polymer-rich and polymer-poor
regions and ultimately into a solid phase. It is this polymer solvent–nonsolvent interaction
that has the greatest impact on the structure of the fiber and the ultimate properties that can
be achieved. The relative rates of solvent to nonsolvent diffusion control the process of phase
separation [102]. Important variables controlling this process are polymer solids, solution
composition and temperature, coagulating solution composition and temperature, the extru-
sion rate, and the residence time in the coagulating bath. Control of the size and character of
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Spin solution frompolymerization
Dry spinning
Drawing
Washing
Drying and heattreatment
FIGURE 13.6 m-Aramid dry-spinning process.
the voids formed in such a process is key to achieving fibers with excellent mechanical
properties [103].
A schematic of the Teijinconex wet-spinning process is shown in Figure 13.7 [87]. The
process schematic for producing Fenilon is similar to that shown in Figure 13.7 with the
exception that the polymer solution is spun directly from the polymerization process [104].
While the above processes require little or no inorganic salt content in the spinning solution,
the process described by Tai et al. allows the use of salt-containing solutions [105].
13.7. 1.3 Dry-Je t Wet- Spinning
Kwolek [106] demonstrated in her early work at DuPont that p-aramid fibers could be spun
from amide and salt solutions using a conventional wet-spinning process. These solutions
were typically of low concentration. The resulting fibers had low strength but high modulus
after heat treatment. In later development, p-aramid fibers were spun from more concentrated
solutions using dry-jet wet-spinning processes [107]. These solutions contained aramid poly-
mer above a critical solids concentration and were anisotropic.
In 1970, Blades [108] discovered that high-strength, high-modulus fibers could be
spun from anisotropic solutions of aramid polymers by dry-jet wet spinning (Figure 13.8).
His process is shown schematically in Figure 13.8. The key feature of this process is that an
anisotropic solution is extruded through an air gap between the spinneret and the coagula-
tion bath. The coagulated filaments are washed, neutralized, and dried. This process
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Dissolver
Wet spinning
Quenching
Washing
Wet draw
Dry polymer from interfacialpolymerization
Amide solvent
Drying, hot draw andheat treatment
FIGURE 13.7 Teijinconex wet-spinning process. (From Fujie, H., Nikkyo Geppo, 40, 8, 1987.)
produces a fiber with tenacity and initial modulus 2–4 times that of a fiber prepared by a
conventional wet-spinning process.
The mechanistic model of polymer molecular orientation in a dry-jet wet-spinning process
is shown in Figure 13.9 [109]. Shear at the capillary wall causes the liquid crystalline domains
to orient along the direction of flow when an anisotropic solution is extruded through a
spinneret capillary. At the capillary exit, some deorientation of liquid crystalline domains
occurs because of solution viscoelasticity. However, this deorientation is quickly overcome by
threadline tension on the attenuating filament in the air gap. The attenuated filaments retain
this highly oriented molecular structure on coagulation giving rise to highly crystalline, highly
oriented fibers.
� 2006 by Taylor & Francis Group, LLC.
=
Spin dope
Spinneret
Transfer line
Air gap
Coagulating liquid
Spin tube
GuidePump
Tube
p
Filaments
Container
Rotating bobbing
Container
Spinning block
p
FIGURE 13.8 Dry-jet wet-spinning process. (From Blades, H., U.S. Patent 3,767,756, 1973.)
Orientation
Partial deorientation
Reorientation
Air gap
Spinneret
Quenchwaterbath
FIGURE 13.9 Molecular orientation during dry-jet wet spinning. (From Yang, H.H., Aramid fibers, in
Fibre Reinforcement for Composite Materials, Bunsell, A.R., Ed., Elsevier, Amsterdam, 1988. With
permission.)
� 2006 by Taylor & Francis Group, LLC.
The operating conditions for dry-jet wet spinning are proprietary for fiber producers and
are therefore not revealed in detail. A review of the literature shows that the general
conditions are as follows [110]:
Polymer molecular weight 5,000–35,000
Polymer inherent viscosity 3–20 dL=g
Spinning speed >55 y=min (>50 m=min)
Number of filaments l0–l500
Spinneret hole diameter 0.002–0.004 in. (0.05l–0.l02 mm)
Filament size 1–6 denier=filament
The as-spun fiber from dry-jet wet spinning can be heat treated at high temperatures and
high tension to increase its crystallinity and degree of crystalline orientation [111]. The heat
treatment conditions are generally in the following ranges:
Temperature 250–5508CTime <10 min
Tension 5–50% of breaking strength
As discussed above, isotropic solutions are typically converted to fibers by a wet-spinning
process. Ozawa [90] disclosed that the polymerization mixture of copoly(p-phenylene=3,4’-diaminodiphenylether terephthalamide) remained isotropic. He deviated from traditional
spinning techniques and spun fiber from this solution using dry-jet wet spinning. Although
as-spun fiber tensile properties were modest, high strength fiber was achieved with subsequent
drawing. This fiber product was later commercialized as Technora aramid fiber by Teijin Ltd.
The use of dry-jet wet spinning to prepare fibers from isotropic solutions has since been
widely practiced.
The dry-jet wet-spinning process is unique in that the temperature of the spinning nozzle is
different than that of the spin bath. In comparison, the spinning nozzle in a conventional wet-
spinning process is immersed in the coagulation liquid and is therefore at the same temp-
erature. This gives rise to several inherent limitations with the wet-spinning process. First,
the coagulant temperature must exceed the freeze point of the spinning solution. Second, the
spinning solution is exposed to the coagulant as soon as it exits the spinneret holes. This can
limit attenuation of the incipient filament. The dry-jet wet-spinning method allows the use of
a low temperature coagulant without concern for freezing the spin solution. The air gap
permits the extruded solution to be more fully attenuated and to develop a higher degree of
molecular orientation.
Dry-jet wet spinning is, however, a much more mechanically complicated process and
requires careful control of both the air gap and the flow dynamics of the coagulant fluid.
13.7.2 FILM
Aramid films have been in development since the late 1990s by several Japanese com-
panies including Toray, Teijin, and Asahi. As with fibers, aramid solutions can be extruded
through flat dies to form films. The conventional wet process can be employed to produce
unidirectional and bi-oriented films from isotropic aramid solutions. Production of films
from anisotropic solutions requires unique processes as shown by the example of PPTA film.
Forming films from anisotropic solution is extremely difficult because of the ease with
which these solutions orient. Obviously once the films orient in the machine direction they are
� 2006 by Taylor & Francis Group, LLC.
Dry polymer H2SO4
Dissolver
Drum/Beltcasting
Steamtreatment
Coagulation
Acid andwater
Washing
Wetstretching
Drying
FIGURE 13.10 PPTA film process. (From Imanishi, T. and Muraoka S., U.S. Patent 4,752,643,
June 21, 1988.)
very weak in the cross direction and, as a result, tend to fibrillate. Asahi has developed a
process leading to a coherent film [112]. A schematic of this process is shown in Figure 13.10.
An anisotropic PPTA solution in sulfuric acid is extruded through a die onto a drum or a
belt where it is initially exposed to warm, humid air. Under these conditions, the solution
reverts to an isotropic state as moisture is absorbed to reduce the effective acid concentra-
tion and raise the temperature. This is the critical step as it leads to the formation of an
isotropic film. The structure is fixed on coagulation after which the film is washed with
water to remove the remaining sulfuric acid. The wet film is biaxially stretched to develop
mechanical properties in both directions and then dried. Finally, the film can be heat-treated
to further improve properties.
13.7.3 F IBRIDS
Fibrids are film-like particles that are formed when—aramid solutions are precipitated in a
nonsolvent under high shear [113,116]. The dimensions of as-formed fibrids are around 100
mm � 700 mm � 0.01 mm [113,114]. Fibrids have a high surface area, around 200–300 m2=g,
and can function as a thixotrope or a reinforcing agent in composite, sealing, coating, and
elastomer applications [114,115]. Fibrids are used primarily in aramid papers. Aramid papers
are composed of a mixture of fibrids and short Nomex fibers referred to as floc (Figure 13.11).
Fibrids serve as a binder for the short fibers and also improve the dielectric properties of high
temperature, heat-resistant aramid papers (Figure 13.12) [115,116]. A process for making m-
aramid papers is shown in Figure 13.13 [113].
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Fibrid Floc
FIGURE 13.11 Photomicrographs of aramid fibrid and floc.
13.7.4 PULP
p-Aramid pulp is a highly fibrillated material that retains the key chemical and physical
properties, low creep performance, and high temperature and wear resistance of the precursor
p-aramid fiber. These characteristics make p-aramid pulp an excellent candidate to replace
asbestos in friction products such as brake linings and clutch facings, gaskets, and industrial
papers. The highly fibrillated structure of Kevlar pulp is characterized by a combination of
high fibril aspect ratio (>100) and high specific surface area [116,117]. The fibrils can be
attached to, or detached from, the core fiber.
Pulp is produced by passing a dilute slurry of short cut length, p-aramid fiber through one
or more high shear refiners. The highly oriented, crystalline fiber is cut and readily split into
fibrils of smaller diameter because of the relatively low compressive strength of the fiber. The
refining process is controlled to produce a certain balance between the final fiber length and
the degree of fibrillation or the degree of new surface generation. The optimum relation-
ship between these two parameters is dictated by the process or product performance
requirements of the specific end-use application. Water is removed from the resulting pulp
slurry to produce a wet product or, with additional drying, a dry product. Wet pulp contains
50–70% moisture depending on the producer. Dry pulp contains 4–8% moisture. Handling of
the pulp becomes difficult at lower moisture levels because of static problems.
FIGURE 13.12 Photomicrograph of a cross section of Nomex Type 411 paper.
� 2006 by Taylor & Francis Group, LLC.
Staple supply
Fibrid supply Stock tankBeater Head box
Product reel Dryer rolls Wet press Fourdinermachine
Broke
Slusher
Mixer
Calender
FIGURE 13.13 Process for making m-aramid papers.
Pulp is characterized in terms of fiber length, length distribution, and the degree of
fibrillation. Absolute fiber (or fibril) length typically ranges from less than a millimeter to
about 6 millimeters. The fiber length distribution is measured using a device such as a Kajaani
200 instrument and is reported in terms of a length-weighted average length (P
ni li2=P
ni li) or
weight-weighted average length ( P
ni li3=P
ni li2). The length-weighted average length of typ-
ical commercial pulps is in the range of 0.6–1.1 mm. The degree of fibrillation is related to the
specific surface area of the pulp or to the drainage rate of an aqueous pulp slurry determined
by the Canadian Standard Freeness or Schopper–Riegler methods. There is a fiber–fibril
diameter or width distribution in pulp just as there is a length distribution. The diameter will
range from 12 to 15 mm, the diameter of the precursor fiber, to less than 1 mm for the smallest
fibrils. Pulp specific surface area ranges from about 7 to 15 m2=g reflecting the breakdown of
the initial fiber, with a surface area of about 0.2 m2=g, into a broad distribution of smaller
diameter fibrils. Canadian Standard Freeness values range from about 100 ml for ‘‘high’’
surface area pulps to about 600 ml for less highly refined pulp merges. The highly fibrillated
morphology characteristic of p-aramid pulp is shown in Figure 13.14.
13.8 APPLICATIONS
The broad range of properties of aramids is the main reason for their utility in diverse
applications. Here we will attempt to illustrate how previously described properties of these
fibers are exploited in their applications.
� 2006 by Taylor & Francis Group, LLC.
EP6381TR3 Pulp KE IF538 Detecteur = SE1 Nom Utilisateur = PIERDOC
Date :7 Juil 2003WD = 12mmGrand = 1.00 KX
(Merge 1f538)
10μm
FIGURE 13.14 Scanning electron micrograph of Kevlar brand pulp.
13.8.1 m-ARAMID FIBER
Many of the applications of m-aramid fibers are due to their unique combination of flame
resistance with thermal and textile properties. Some applications also benefit from the fact
that m-aramid fibers are available in colored form. In general, these fibers are very difficult to
dye and thus most producers offer producer colored (pigmented) fiber. While pigments offer
in general better UV stability, this approach limits the number of colors available. At this time
only DuPont offers piece dyeable products. In general, dyeing of Nomex fibers requires the
use of carriers, and dyeing technology is kept as proprietary information by dye houses.
In general, flammability as well as thermal properties are bulk properties of the material.
When these properties are critical, compositions comprising 100% aramid fibers are used.
Blends with nonaramid materials do come into play when other fiber properties or charac-
teristics are desired.
13.8.1.1 Protective Apparel
Fabrics of m-aramids are widely used in thermal protective apparel because of their unique
combination of thermal and textile properties. The fibers from which these fabrics are made
are inherently flame resistant and do not melt or drip. A measure of the fiber’s flammability is
its limiting oxygen index (LOI), which is the concentration of oxygen in air that is required to
support combustion once the material is ignited. Materials with an LOI> 21 are considered
nonflammable. The inherent flame resistance of m- and p-aramids is essentially the same with
LOI values of ~28–29. For apparel applications, m-aramids are generally preferred over p-
aramids because the fabrics have a more comfortable, textile-like hand as a consequence of
lower fiber modulus and higher elongation. Even though m-aramids fibers exhibit high glass
transition temperature and high crystalline phase melting points (2758C and 4258C respect-
ively) both glass transition temperature and melting temperature of the crystalline phase are
� 2006 by Taylor & Francis Group, LLC.
high (2758C and 4258C respectively) in flame 100% m-aramid garments exhibit some shrink-
age, which in turn can lead to fabric ‘‘break opening’’ and loss of protective barrier. Blends
with p-aramids are often utilized to stabilize the protective garment against shrinkage and to
reduce fabric ‘‘break-open’’ during flame exposure. At higher exposure to flame MPDI
carbonizes and forms a tough char at a temperature of ~8008F (4278C). The intumescent
nature of the char provides additional protection. Decomposition products on combustion
will vary depending on the heating rate and the amount of oxygen present. In general,
combustion by-products are similar to those obtained on burning wood, wool, cotton,
polyester, and acrylic [118,121].
Both continuous filament and staple yarns are used in protective apparel fabrics. Typical
filament deniers range from 0.85 to 2. Staple fiber length is 1.5–2 in. for processing on the
cotton system. Yarns are available in dyeable and producer colored forms. Fabric forms
include woven, knit, and nonwoven. The mechanical toughness of the fiber results in higher
fabric strength than FRT cotton fabrics of even greater weight. Higher resistance to tear and
abrasion also provides greater durability and longer useful garment life. Ultimately fabric
selection will depend on the application and the end-use performance requirements such as
the degree of protection required, flammability, durability, comfort, cost, style, etc.
m-Aramid fabrics are widely used in industrial, military, fire fighting, and auto racing
applications. Chemical, petrochemical, and utility workers wear flame-resistant protective
clothing where flash fire or electrical arc hazards exist. Military applications include flight
suits and coveralls for combat vehicle and shipboard engineering crews. In firefighting
apparel, m-aramids and blends with p-aramids find use in turnout gear, station uniforms,
hood, gloves, and boots. The turnout is a three-component system (an outer shell, a moisture
barrier, and a thermal barrier) designed to provide basic thermal protection in hot environ-
ments and in flashover conditions in addition to maximizing comfort and minimizing the
potential for heat stress. Race car drivers and their crews wear clothing to protect themselves
from flash fires resulting from crashes and pit accidents. The protective gear includes suits,
underwear, socks, and gloves.
13.8.1.2 Thermal and Flame-Resistant Barriers
The same fiber properties that make m-aramids suitable for protective apparel applications
find utility in thermal and flame-resistant barrier fabrics found in transportation (aircraft,
train, and automobile) end-uses and in contract furnishings for hotels, offices, auditoriums,
hospitals, and day care centers. Fabrics involved in aircraft and railroad car interior appli-
cations include upholstery, floor coverings, bulkheads, wall coverings, and blankets.
Fire-blocking materials increase the probability of safe egress of passengers from the cabin
in a fire emergency. A fire-blocking fabric or thermal liner in aircraft seating provides a
barrier between the flame source and, for example, a high fuel content polyurethane seat
cushion. A typical construction would be a layer of a spunlaced fabric quilted to a woven m-
aramid fabric to provide both durability and lightweight. The fire-block is designed to retard
or delay ignition of the cushion once the flame has penetrated the outer upholstery fabric.
Because the fibers are inherently flame-resistant, there are no topical treatments that can
wear off or be removed during routine laundering. The abrasion resistance and toughness of
the fiber allows for easy maintenance of fabrics without concern for fading, cracking, or
degradation.
Yarns can be dyed or are producer colored. This allows for the design of attractive
interiors and at the same time, provides the safety of a flame-resistant material. The filament
denier for these applications is higher than that of yarns for apparel fabrics and is generally in
the range of 3–10.
� 2006 by Taylor & Francis Group, LLC.
13.8.1.3 Elastomer Reinforcement
There are a few elastomer reinforcement applications where m-aramid yarns are superior to
p-aramid yarns. Continuous filament m-aramid yarn is used in a loose knit construction
to reinforce automotive heater hose. Yarn on yarn abrasion resistance, and not strength, is
key to performance in this application where the hose is exposed to significant thermal,
impulse, and vibrational stresses. A second growing use is in the reinforcement of silicone
elastomer hose for automobile turbochargers where m-aramid provides high thermal stability.
13.8.1.4 Filtration and Felts
Filter bags of m-aramid fiber felts are the material of choice in the bag houses of the hot mix
asphalt (HMA) industry as well as in a variety of other applications. Bag houses are the
preferred air cleaning system because they provide compliance with pollution codes and
provide economic advantages over scrubbers. Bags can be manufactured from a variety of
materials including Teflon1h, fiberglass, polyester, and polyphenylene sulfide, but m-aramids
are the most suitable for HMA plants. Key factors determining this include filtration
performance, chemical resistance, tensile strength, durability, cost, temperature resistance,
and combustibility [119,122].
Bags of Nomex fiber can withstand a continuous operating temperature of 4008 F
(2048C). Additionally the fiber remains dimensionally stable at this temperature—neither
growing nor shrinking more than 1%. The common felt in the industry is a 14 oz=yd2 felt
made of 2 dpf fibers.
m-Aramid felts and fabrics are ideal for heavy-duty laundry textile covers used on
calendars and ironing presses. These materials can meet the thermal stability requirements
of calendars and presses operating at temperatures of up to 2008C. For equipment operating
at lower temperatures (150–1608C), m-aramid fabrics provide greater reliability than lower
cost polyester press covers whose use is still permissible at this temperature range. While heat
resistance is the key criterion for covers, m-aramids also have the advantages of abrasion
resistance, dimensional stability, and very good resistance to hydrolysis.
13.8.2 m-ARAMID PAPER
As we have mentioned earlier, m-aramid papers are produced exclusively by DuPont and thus
most of the application data are based on Nomex papers.
13.8.2.1 Electrical
In the form of paper or pressboards, m-aramids provide an optimum balance of properties for
use as electrical insulation in transformers, motor, generators, and other electrical equipment.
Properly used, these materials can extend the life of an electrical equipment, reduce the
frequency of premature failures, and protect against random electrical stress situations.
Papers and pressboards are made from two m-aramid forms—floc and fibrids. Floc is
yarn cut to a short length. Floc retains the intrinsic properties of the yarn and gives the paper
mechanical strength. Fibrids are microscopic film-like particles that provide dielectric
strength and bind the floc particles together to give the sheet integrity.
Key properties are inherent dielectric strength, mechanical toughness, thermal stability,
chemical compatibility, cryogenic capability, moisture insensitivity, and radiation resistance.
hTeflon1—a registered trademark of E.I. DuPont de Nemours & Co., Inc., Wilmington, Delaware, USA.
� 2006 by Taylor & Francis Group, LLC.
Depending on product type and thickness, densified products can withstand high short-term
electrical stresses without further treatment with varnishes or resins. Densified products have
good resistance to tear and abrasion and, in thin grades, are flexible. Electrical and mechan-
ical properties are unaffected at temperatures up to 2008C. Useful properties are maintained
for at least 10 years of continuous exposure at 2208C. Like m-aramid yarns, papers do not
melt and do not support combustion. Products are compatible with all classes of varnishes
and adhesives, transformer fluids, lubricating oils, and refrigerants. At the boiling point of
nitrogen (778K), selected types of Nomex paper and pressboards have tensile strengths
exceeding values at room temperature. In equilibrium at 95% relative humidity, densified
products retain 90% of their bone-dry dielectric strength. Products are unaffected by 800
megarads of ionizing radiation and retain useful electrical and mechanical properties after
eight times this exposure [120,123].
Papers are available in many forms varying in thickness, degree of densification, and
composition (additive type or floc to fibrid ratio). Pressboards, which differ from paper in
thickness and rigidity, are likewise available in several thicknesses and degrees of densifica-
tion. The product of choice will depend on many factors including end-use thermal and
mechanical performance requirements, formability or ease of fabrication, and the desired
degree of saturability.
Applications in transformers include conductor wrap, layer and barrier insulation, coil end
filler, core tubes, section or phase insulation, lead and tap insulation, case insulation,
and spacers. In motors and generators, the superior thermal properties of m-aramid pro-
ducts can enhance both performance and reliability. Their strength and resilience can also
help extend the life of rotating equipment in severe operation conditions. Insulating parts
where m-aramids are used in rotating equipment include conductor wrap, coil wrap, slot
liners, wedges, phase insulation, end-laminations, pole pieces and coil supports, commutator
V-rings, bushings, and lead insulation.
13.8.2.2 Core Structures
Core structures are more commonly referred to as honeycomb structures or cores. Cores of
m-aramid honeycombs with carbon-fiber skins were first used in flooring panels of the British
Aerospace VC-10 BOAC in the late 1960s. In 1970, Boeing’s new generation aircraft, the 747,
flew with a number of interior and exterior components fabricated with aramid core. Since
then, aramid honeycomb cores have become a standard design material for flooring panels,
fairings, radomes, rudders, elevators, cowlings, and thrust reversers. The primary purpose of
core structures is to minimize weight while [121] maximizing stiffness. Lower weight translates
to increased payloads and reduced fuel costs [124].
Aramid cores are made from paper (typically 1.5–4 mil in thickness) comprising m-aramid
floc and fibrids, similar to the papers used in electrical applications discussed in the previous
section. Adhesive node lines are printed on paper sheets that are then stacked, pressed, and
heated to cure the adhesive. The resulting block is expanded. The adhesive-free areas form the
hexagonal cells of the honeycomb configuration. The core is dipped several times in an epoxy
or phenolic resin solution until the desired density and mechanical property levels are
reached. The core is then cut into slices of the desired thickness. Face sheets are glued to
each side of the core. The most common face sheet today is a composite of carbon fiber and
epoxy resin.
Aramid cores have many attributes. m-Aramids have high thermal tolerance and are
compatible with resins with cure temperatures to 4008F. Cores can be fabricated in a wide
range of densities from 1.5 to 10 lb=ft3. They have higher specific shear strength than foam
cores and higher toughness, at equal density, than aluminum, glass, or foam cores. They have
� 2006 by Taylor & Francis Group, LLC.
high wet strength and exhibit excellent creep and fatigue performance. Aramid cores do not
corrode and do not promote galvanic action in contact with metals. They are easy to fabricate
and the self-extinguishing character of m-aramids allows the structures to meet stringent
flammability, smoke generation, and toxicity standards.
13.8.2.3 Miscellaneous
Tags and labels of m-aramid paper for in-process bar coding are used where high temperature
stability and chemical resistance are required. In loudspeakers, m-aramid sheets are used
for voice coil insulation and for the speaker cone itself. Bus bars in lithium ion batteries for
portable telephones and computers are insulated with m-aramid paper. Photocopiers and laser
printers that operate at high temperatures use cleaning rollers and webs made from m-aramid
paper.
13.8.3 P-ARAMID FIBER
As m-aramid fibers are best known for their flame resistance, p-aramid fibers are universally
recognized as the material of choice for ballistic protection. While p-aramids do play a critical
role in this application we will attempt to show that their unusual properties are also suitable
for a wide variety of other end-uses.
13.8.3.1 Armor
Aramid-based armor systems are designed to protect individuals and equipment against a
variety of threats in both civilian and military environments. Handgun bullets and knives are
the primary threats encountered in civilian law enforcement work. Military threats are more
wide ranging and generally deal with higher velocity projectiles including rifle bullets, flech-
ettes, and fragments from mortars, grenades, and mines. The design of the optimum protect-
ive system must take into consideration the nature of the threat and therefore civilian and
military systems will necessarily differ. Armor systems can be roughly divided into soft and
hard categories. Soft armor systems are assemblies of woven fabrics that are used to make
bullet-resistant vests, flak jackets, and soft structures such as blankets, curtains, and liners.
Hard or composite armor systems are used in helmets and in structures designed to protect
vehicles, vessels, or shelters. These systems are made of multiple fabric layers impregnated
with a vinyl ester or phenolic–polyvinylbutyral resin binder. Spall liners that are fitted inside
armored military vehicles and protect against fragments resulting from hits by high velocity
shells are a classical example of hard armor.
Beginning in the 1970s high strength fibers—particularly p-aramids—generally displaced
glass and nylon as the preferred fibers for ballistic protection in soft armor. The evolution of
vest design continues today with ever-increasing demands for greater ballistic protection,
less weight, and greater comfort. Initial aramid-based vests of the 1970s had a weight of
1.26 lb=ft2 compared to 1.3 for the incumbent nylon reinforced vests of the 1950s. Today’s
vest weighs even less, about 0.95 lb=ft2, while providing greater ballistic protection. These
advances have been made possible through the use of higher strength yarns with a broader
range of deniers, achieved through spinning process modifications, and by optimizing the
weave pattern of the reinforcing fabrics.
Vests providing ballistic protection do not necessarily provide adequate protection
against threats from sharp implements such as knives. For civilian use, particularly in penal
institutions, vests incorporating p-aramids have been designed that provide protection against
penetration by knife, ice pick, and awl [122,123,125,126,127,128]. Designs that offer both
ballistic and stab protection have also been claimed [124–130].
� 2006 by Taylor & Francis Group, LLC.
13.8.3.2 Protective Apparel
p-Aramid yarns are used in protective apparel where cut resistance, thermal resistance, or
abrasion resistance is critical. Applications include gloves and sleeves for automotive, glass,
steel and metal workers, chainsaw chaps and trousers for lumberjacks, and other apparel such
as aprons and jackets. p-Aramid yarn does not support combustion and does not melt
in contrast to competitive products made from nylon, polyester, and polyethylene. Gloves
of p-aramids offer exceptional cut resistance and can substantially reduce the risk of hand and
finger injuries in glass and metal handling operations.
Gloves are made primarily from spun yarns, although some are made from textured
continuous filament yarns for applications where the tendency to form lint must be minim-
ized. Yarn denier per filament can vary from 0.85 to 4.2 dpf with 2.25 dpf the predominant
product. Generally, cut resistance increases as the denier is increased but dexterity is
sacrificed. Gloves are made from 100% aramid yarns or from blends with other fibers,
such as nylon or polyester, to reduce cost or to improve comfort or abrasion resistance.
Yarns can also be spun with steel fibers to provide superior cut resistance. Most gloves are
made of a knit construction although some are cut and sewn from woven fabric. Some p-
aramid gloves are coated or ‘‘dotted’’ with elastomers to enhance grip; others have leather
sewn over the palms and fingers to provide puncture resistance or to increase abrasion
resistance.
p-Aramid gloves can be cleaned using conventional laundering or dry-cleaning processes
with minimal impact on cut resistance. Unlike cotton, these gloves do not shrink when
exposed to hot water or hot air. Overall cost per use can be reduced with cleaning and
reuse, rather than disposal, of soiled items.
13.8.3.3 Tires and Mechanical Rubber Goods
p-Aramids are particularly well suited as reinforcing agents for belts of radial tires and for a
variety of mechanical rubber goods because of their high strength and modulus, excellent
dimensional stability, high temperature durability, and favorable strength to weight ratio. In
spite of these attributes, lower cost steel wire continues to be the reinforcement of choice for
passenger car tires. Nevertheless, aramid cords have slowly made inroads into tire applica-
tions since their introduction in the mid-1970s, particularly in the high performance arena
where the performance to weight ratio is critical. Key performance criteria are speed capabil-
ity, handling, and comfort. Additional factors that favor increasing aramid usage in automo-
bile and truck tires are the ongoing efforts to reduce vehicle weight and to reduce rolling
resistance to reduce energy consumption. Aramids also find use in aircraft, motorcycle, and
bicycle tires where the performance attributes often outweigh cost. Typical yarn deniers for
tire applications are 1000–3000 with a 1.5–2.25 dpf fiber. Product variants include so-called
‘‘adhesion activated’’ yarns that have a surface treatment that facilitates adhesion to the
elastomer and can simplify subsequent tire cord and fabric processing steps by eliminating a
dip-coating step [128,131].
Mechanical rubber goods include hoses, power transmission (PT) belts, and conveyor
belts. Aramids compete with nylon, polyester, glass, and steel in these applications. Steel
dominates the rubber hydraulic hose market and polyester is the reinforcement of choice
in lower pressure thermoplastic hoses. Advantages of aramid vs. other textiles in hose
applications include higher strength, which can lead to constructions with fewer plies and
less weight, and better thermal stability, dimensional stability, and chemical resistance. When
compared with steel, aramid will not corrode and can be fabricated into lower weight, more
flexible hoses.
� 2006 by Taylor & Francis Group, LLC.
PT belts can be divided into two categories—v-belts and synchronous belts. Strength,
dimensional stability, fatigue resistance, and adhesion are key reinforcement criteria. Poly-
ester is the primary reinforcing fiber in v-belts where cost considerations are most important.
Aramids can replace polyester in those applications where strength, shock loading, and
dimensional stability requirements outweigh cost. Glass has been the primary reinforc-
ing fiber in timing belts. However, aramid yarn is beginning to replace glass where higher
fatigue performance is required to meet increasing demands for more durable, longer-
lived belts.
In conveyor belts, as in hoses and PT belts, the superior performance potential of
aramid reinforcement must be weighed against the higher material cost. Compared to steel,
equivalent belt strength is achieved at one fifth the weight resulting in ease of handling, lower
energy costs, and lower installation costs. Maintenance and repair costs are reduced because
the fiber does not corrode. Personnel safety is enhanced by the absence of sparking potential.
Aramid reinforced belts have higher strength and modulus than nylon or polyester belts and
can be made thinner or constructed with fewer plies to lower belt weight, simplify handling, or
increase section length by reducing the number of splices.
Yarns are available in high tenacity, high modulus, or high elongation versions to meet the
performance requirements of specific end-uses.
13.8.3.4 Composites
p-Aramids are widely used in composite materials as the sole matrix-reinforcing agent or as a
hybrid in combination with carbon or glass. Composite property balance will differ from
application to application but the key requirement is cost-effective performance at reduced
weight. Glass has lower strength and modulus and higher density than aramid or carbon but
is the most widely used reinforcing fiber because of its low cost. Carbon fibers have the
highest strength and modulus but the lowest elongation. Aramid fibers have a combination of
high strength and modulus (although lower than carbon) with low density and high elonga-
tion that results in improved impact resistance. Composite structures are found in a host of
applications including aerospace components, automobile parts, boats, sporting goods, pro-
truded articles, and pressure vessels. In aircrafts, aramids are used in storage bins, air ducts,
and a variety of core (honeycomb) structures. In general, aramid composites have demon-
strated satisfactory performance in secondary aircraft structures. Aramid’s high tensile
strength lends itself well to the manufacture of canoes where weight can be reduced signifi-
cantly while providing greater tear strength and puncture resistance than fiberglass compos-
ites. Hockey shafts, golf club shafts, fishing rods, skis, and tennis rackets have incorporated
aramid composites. Fishing rods with unidirectional carbon fibers to provide longitudinal
stiffness and aramid fibers woven to provide lateral stiffness yield a high performance rod that
is both light weight and stable. In skis, aramid fibers dampen vibration for smoother, more
comfortable skiing.
13.8.3.5 Optical and Electromechanical Cables
The primary function of p-aramid yarns in fiber optic and electromechanical cables is to
protect the optic glass fiber and ductile power conductors from excessive loading or axial
strain. p-Aramids are well suited to this task because of their high strength and modulus, low
density, and resistance to creep. Yarn is used in two forms. Untwisted yarn is laid along the
length of the cable to provide maximum modulus to resist stretching. Twisted yarn is inserted
as a ripcord to provide maximum strength for tearing the protective sheathing when installing
or repairing cable.
� 2006 by Taylor & Francis Group, LLC.
Initial usage as a reinforcing agent in ground cables has largely been replaced by less costly
glass fiber that can provide the necessary strength and modulus where cable weight is not a
critical factor. Aramid yarn is widely used in ADSS (all dielectric self-supporting) aerial
cables where glass is unsuited because of its weight. Higher modulus aramid merges are
used in this application to minimize cable sag and to prevent the cable from coming into close
contact with neighboring electrical lines. Typical yarn deniers are 2840 and multiples thereof.
More recent applications are in so-called premise cables that are used to connect devices
within buildings. These cables provide more bandwidth, have lower power requirements, and
are less costly to maintain than copper lines. Cable diameter is important in this application
and therefore lower yarn deniers are used. These range from 380 to 1420. In addition to the
attributes cited above, the aramid yarn is nonflammable, which allows the cable to pass
mandated burn tests.
For electromechanical cables that are subject to fluctuating loads in use, tension–tension
fatigue performance is key. For this application, aramids are superior to galvanized improved
plow steel wire in fatigue resistance [129,132]. The high strength-per-unit weight of aramids
also allows the cable designer to maximize payload or working length while retaining the ease
of handling of a smaller and lighter system.
13.8.3.6 Ropes and Cables
Like fiber optic and electromechanical cables, p-aramids provide high strength and modulus
and permit the design of cordage with high load carrying capability with smaller, lighter
systems. Yarns are used in a variety of rope and cordage designs such as eight-strand plaited,
single and double braids, parallel strands, and wire-lay construction. The choice of construc-
tion will depend on the balance of properties required for a specific application. Applications
include mooring cables for ship, towlines, elevator cables, and deep-sea cables. Compared to
heavy cables of steel wire, p-aramid cables provide equivalent strength at one fifth the weight
and have a creep rate that approaches that of steel. Lower cable weight can be a significant
factor in enhancing worker safety by reducing the potential for back injuries related to
handling mooring lines. Unlike steel, aramid ropes will not corrode in an aqueous environ-
ment. Aramid ropes must be designed and handled in a way that minimizes the potential for
severe internal or external abrasion and subsequent strength loss. This includes consider-
ations of both rope construction and the appropriate sheave size for a given rope diameter.
A recent innovative machine-room-less traction elevator (ISIS) from ThyssenKrupp takes
full advantage of the properties of p-aramid in the design of the hoist cable and associated
traction sheaves [130–133]. The cable has three times the life of a steel rope, is smaller in size,
and weighs 90% less than a steel rope at a comparable strength rating. The smaller size
permits the use of smaller sheaves thereby decreasing torque requirements and operating
costs. No lubrication is required because the inner strands are Teflon coated. Finally, the
cable transmits less noise and provides a smoother, quieter ride.
Yarns are available in a variety of deniers and merge types that vary in the balance of
tensile properties. Special finishes can be applied to increase lubricity, improve fatigue in wet
applications, or provide better UV resistance. Ropes using Kevlar or Twaron are particularly
useful for static applications or where maximum modulus is required. Technora-based ropes
are suited for dynamic applications where resistance to fatigue is important.
13.8.3.7 Reinforced Thermoplastic Pipe
Reinforced thermoplastic pipe (RTP) is a relatively new composite product. At present there
are four suppliers with products ranging in diameter from 4 to 10 in. and with pressure ratings
� 2006 by Taylor & Francis Group, LLC.
up to 100 bars. The pipes are made in continuous lengths of polyethylene with p-aramid
reinforcement [131,134]. Like the ISIS elevator example above, RTP takes full advantage of
the intrinsic attributes of p-aramid fibers in the design of this new fluid transport system.
The oil industry is a major user of pipelines to transport oil and gas. In the oil field, flow
lines connect individual wells to trunk lines that carry the crude to loading docks or to
processing plants. Steel piping has traditionally been used for this application but the pipe
is subject to corrosion from within or without over its lifetime. Leakage caused by corrosion is
inevitable. Prior to the development of RTP, no suitable alternative to steel piping had
been found. The pipeline operator has value for a system that can reduce installation and
lifetime maintenance costs per unit length of pipe while meeting temperature and pressure
requirements. RTP designs incorporating aramid reinforcement appear to have the necessary
characteristics to replace steel piping in the flow line application.
Pipes are constructed with twisted cords to ensure the flexibility required to reel long
lengths of pipe of relatively small diameter. The pipes are lightweight for ease of transporta-
tion and installation. Long lengths simplify installation and maintenance by reducing the
number of couplings. Pipes are corrosion resistant, damage tolerant, and able to withstand
high temperatures and pressures. Advantages of aramids over other reinforcement materials
such as carbon or glass fiber include flexibility, ease of assembly, and damage tolerance
during assembly.
13.8. 3.8 Civil En gineering
Use of composite materials for concrete infrastructure repair that was initiated in the mid-
1980s finally began to proliferate in the mid-1990s. Carbon and glass fiber reinforced epoxy
resin composites have received the most interest. Aramid-based reinforcement has been
viewed as a more specialty product for applications requiring high modulus and where the
potential for electrical conductivity would preclude the use of carbon; for example, in Japan,
aramid sheet is used for all tunnel repair. Product forms include dry fabrics or unidirectional
sheets as well as pre-cured strips or bars. Fabrics or sheets are applied to a concrete surface
that has been smoothed (by grinding or blasting) and wetted with a resin (usually epoxy).
After air pockets are removed using rollers or flat, flexible squeegees, a second resin coat
might be applied. The process is repeated for additional plies [132,135].
Reinforcement of concrete structures is important in earthquake prone areas such as Japan,
Turkey, and Taiwan. Although steel plate is the primary material used to reinforce and repair
concrete structures, higher priced fiber-based sheet structures offer advantages for small sites
where ease of handling and corrosion resistance are important. The high strength, modulus, and
damage tolerance of aramid-reinforced sheets makes the fiber especially suitable for protecting
structures prone to seismic activity. The use of aramid sheet also simplifies the application
process. Sheets are light inweight and canbe easilyhandledwithout heavymachinery and canbe
applied in confined working spaces. Sheets are also flexible, so surface smoothing and corner
rounding of columns are less critical than for carbon fiber sheets [133,136].
13.8.4 P-ARAMID PAPER
13.8.4.1 Core Structures
p-Aramid core structures are analogous to core structures based on m-aramids (Section
13.8.2.2) but the base paper uses stronger and stiffer p-aramid floc instead of m-aramid
floc. In addition the component ratio of floc to fibrid is increased. This results in a more
porous sheet structure that allows better penetration of the matrix resin in the dipping step. In
addition to retaining all the attributes of m-aramid based cores, p-aramid cores have higher
� 2006 by Taylor & Francis Group, LLC.
shear strength, higher modulus, and greater fatigue resistance at similar cell size and density.
They also have higher hot–wet shear and compression properties than the m-analogues.
p-Aramid cores also bring process advantages because of the lower thermal expansion
coefficient and lower moisture-regain of the component fibers. This translates to improved
dimensional stability and the ability to retain shape and dimensions throughout the fabrica-
tion and part consolidation process.
Because of their superior compression, shear, and fatigue properties, structures based on
p-aramid cores allow even greater weight reduction than incumbent m-aramid cores. Recent
commercial adoptions include flooring panels in weight critical programs such as the
extended Airbus A-340 and the double deck Airbus A-380. p-Aramid cores have also replaced
m-aramid cores in the elevators and rudders of these aircrafts [121,124], because of their
superior hot–wet characteristics.
13.8.4.2 Printed Wiring Boards
Printed wiring boards (PWDs) made of p-aramid papers take advantage of the low axial
coefficient of thermal expansion (CTE) of the fiber to restrain in-plane expansion of the
impregnated resin when heat is applied to the composite laminate. Low CTE boards reduce
the strain on solder joints of leadless ceramic chip carriers used in traditional avionics and
military applications. In addition, low CTE laminates provide a reliable base for mounting
new high-density chip packages where solder joint failure due to thermal cycling is a concern.
These include the thin small outline package (TSOP) used for memory chips, the solder grid
array (SGA) microprocessor package, and the high lead count ball grid array (BGA).
Nonwoven aramid reinforcement is prepegged with epoxy resin on the same vertical path
treaters that are used to process fine weave E-glass. At a resin loading of 45–55% by weight,
the finished PWB has an in-plane CTE of 9–11 ppm=8C. [134,139].
13.8.4.3 p-Aramid Pulp
13.8.4.3.1 Brake Linings or Pads and Clutch FacingsAsbestos was the primary reinforcing agent used in friction materials before it was banned by
Congressional legislation in 1978 for health reasons. Two classes of formulations were
developed to replace asbestos: semimetallic and nonasbestos organic. Each has its own
specific limitations and attributes. p-Aramid in the form of pulp is one of the few organic
materials suited to the thermal demands of friction applications. Acrylic fiber in the form of
pulp has also been used where temperature requirements are less severe. Pulp retains the
strength, stiffness, and thermal properties of the precursor fiber and, in addition, provides
surface area in the order of 7–15 m2=g. This high surface area serves as a processing aid in
certain manufacturing steps and also as a retention aid for multicomponent brake formula-
tions. High fiber strength can lead to higher pad shear strength and increased resistance to
cracking. Fiber thermal stability can influence the nature of the critical transfer layer that
forms between the pad and the rotor. Brake formulations are optimized for a variety of
performance characteristics such as wear, frictional behavior, and noise. Aramid pulp, at
volume percentage levels of <1 to ~10, will influence each of these properties but overall
performance is highly dependent on the combined performance of all of the components in
the formulation.
Clutch facings are made from wet pulp and staple yarn. Friction papers for automatic
transmissions are made from wet pulp that is formed into a sheet on a paper making machine
and then impregnated with phenolic resin. Pulp provides strength in the initial paper making
process and tensile strength in the final composite structure. The fibrillar pulp also influences
sheet porosity. Sheet porosity is essential in this application to ensure adequate permeation of
� 2006 by Taylor & Francis Group, LLC.
the transmission fluid to dissipate heat generated in service. The combined attributes of
strength, heat and wear resistance, and durability that pulp brings to friction papers have
become increasingly important as designers continue to reduce the number and size of plates
in the transmission and, at the same time, auto manufacturers extend the warranty period for
the transmission.
Manual transmissions use a clutch facing made from a resin impregnated wound structure
composed of staple yarns. p-Aramid in yarn form provides more strength and durability than
a pulp-based paper sheet in this more demanding application. Although aramid reinforced
facings have sufficient thermal stability for this application, compositions based on glass and
metal fibers dominate this market.
13.8.4.3.2 GasketsLike friction materials, asbestos was widely used in high temperature, high performance
gaskets prior to the legislation in 1978. Asbestos was highly effective, very cheap, and
comprised 80–85% of the weight of the gasket. Aramid pulp brought high strength and
thermal stability to this end-use but the fiber cost was an order of magnitude higher than
that of asbestos. To reduce this cost penalty, formulations with only 5–20% aramid and 60–
80% inert fillers have been developed that provide goal performance in both compressed and
beater-add type gaskets.
Compressed gaskets are made on a two-roll calendar from a mix of pulp, elastomer, fillers,
curing agents, and toluene. Final gasket properties are very dependent on both the processing
conditions and the specific gasket formulation. Tensile strength depends primarily on the
amount and type (length and surface area) of pulp selected. Stress retention, compression and
recovery, and sealability depend on a combination of factors including the relative amounts
of fiber and elastomer, the type and particle size of the filler materials, as well as the mixing
and calendaring conditions.
Beater-add gaskets are made in an aqueous paper making type process. Ingredients such
as pulp, elastomer, fillers, curing agent, precipitation regulator, and precipitant are com-
bined in water. The resultant slurry is laid down on a screen to drain the water and form a
sheet that is then calendared and press cured. As with compressed gaskets, properties
will depend on both process conditions and the relative amount and type of ingredients.
Beater-add gaskets have been used primarily in cylinder head and other engine gaskets.
Today, many auto manufacturers are replacing these beater-add gaskets with gaskets of
multilayered steel.
13.8.4.3.3 Elastomer and Resin Reinforcementp-Aramid in pulp and short fiber (1.5–6 mm length) forms is an effective reinforcing agent in
both elastomer and thermoplastic resin matrices. Compared to traditional particulate reinfor-
cing agents in elastomers such as carbon black and silica, aramid pulp provides superior
reinforcement at much lower loadings. Advantages of pulp-reinforced elastomers include
high low-strain modulus, property anisotropy, greater cut and abrasion resistance, improved
wear performance and, in tire stocks, lower rolling resistance. These attributes are achieved,
however, only when the pulp is fully dispersed in the rubber stock. Because high surface area
pulp is rather difficult to open and wet out using standard rubber compounding processes,
concentrated pulp masterbatches (Kevlar engineered elastomer and Rhenogran) have been
formulated that allow compounders to more easily achieve adequate dispersion using stand-
ard mixing techniques. These masterbatches are available in a variety of elastomers including
SBR, NBR, natural rubber, polychloroprene rubber, and EPDM [135,136,138,139].
Applications utilizing pulp and short fiber reinforcement include PT belts, tires, roll
covers, hoses, and footwear. In v-belts, both wear resistance and durability increase. In
� 2006 by Taylor & Francis Group, LLC.
synchronous timing belts, pulp placed in the tooth area increases modulus and reduces the
propensity of tooth chunking or chipping. Pulp is used in high performance bicycle and
motorcycle tires to improve handling characteristics and to increase puncture resistance. Use
in several components of automobile tires continues to be investigated. In roll covers,
improvements in tear and abrasion resistance are achieved without affecting compound
hardness or processibility.
Use of p-aramids in molded or extruded thermoplastic parts offers performance advan-
tages over neat resins or glass-reinforced resins. Aramid reinforced parts exhibit improved
mechanical and thermal properties and superior wear resistance with no abrasion to the
counter surface. Because the fiber is not abrasive, there is less damage to processing equipment
and machining of parts is simplified.
13.8.4.3.4 Sealants and AdhesivesDry pulp is used as a thixotrope in sealants and adhesives to provide viscosity control at low
cost. Viscosity is presumably built through the formation of physical networks of entangled
fibrils of the high surface area pulp. Sag or run of applied sealants, adhesives, or coatings is
thereby minimized. With shear, the viscosity of these fluids decreases as the networks break
down, which facilitates application by spraying, brushing, or other means.
Compared to a common thixotrope such as fumed silica, pulp provides equivalent
viscosity at less than one tenth the weight in a typical epoxy resin. In addition, fluid viscosity
is unaffected by further processing (agitation) or aging—in contrast to fumed silica modified
resin where viscosity drops and is not fully recovered under similar conditions. Pulp can also
provide reinforcement in an adhesive matrix as shown by the significant increase in tensile
strength, modulus, and tear strength of both a PVC plastisol adhesive and a silicone sealant
on the addition of pulp [137,140].
13.9 CONCLUSIONS AND DIRECTION
This brief review of aramid fibers has summarized the very broad range of unusual function-
alities that these products bring. While the chemistry plays an important role in defining the
scope of applications for which these materials are suited, it is equally important that the final
parts are designed to maximize the value of the inherent properties of these materials.
TABLE 13.9Properties of High Performance Fibers
Fiber Twaron HM Carbon HS PBO M5 experimental M5 target
Tenacity GPa 3.2 3.5 5.5 5.3 9.5
Elongation % 2.9 1.4 2.5 1.4 >2
E modulus GPa 115 230 280 350 400–450
Compressivea strength GPa 0.48 2.1 0.42 1.6 2
Compressivea strain % 0.42 0.9 0.15 0.5 0.5
Density g=cm3 1.45 1.8 1.56 1.70 1.7
Onset of thermal degradation 8C 450 800 550 530 530
LOI % 29 N=A 68 >50 >50
aIn epoxy resin—3-point bending test.
Source: From Magellan International; Teijin Ltd., Teijinconex Heat Resistant Aramids Fiber 02.05.
� 2006 by Taylor & Francis Group, LLC.
It is very clear that these unusual properties are derived from structures that are quite
different from those of incumbent materials; for example, to obtain very high strength and
stiffness the polymer molecules must be perfectly oriented and fully extended, which leads to
the highly anisotropic nature of the fibers. That is one of the major reasons why associated
applications research efforts have gained such importance. The ultimate products have to be
designed to take this anisotropy into account.
We hope that we were able to clearly exemplify the constant trade-off between function-
ality and processability that is an ongoing challenge with these advanced materials. The
functionality that allows these materials to perform under extreme conditions has to be
balanced against processability that allows them to be economically shaped into useful
forms. This requirement is responsible for the fact that from hundreds of compositions
evaluated in the laboratory only a handful are commercially viable.
The fundamental science of structure–property relationship developed as a result of work
on aramids is being extended to other chemistries and offers the potential to develop materials
with even more impressive properties (Table 13.9).
N
O
N
O
PBO
OHN
OH
N
NH
N
NH
M5
N
NH
N
NH
PBI
The structures shown above illustrate the movement to a higher level of aromatic ‘‘content’’
to obtain even better thermal and flame performance. In the case of PBO and M5, the
structures are even more rigid than those of p-aramids and offer the potential for even greater
properties. This is achieved at the expense of ease of processability and at a significantly
higher cost. It is very clear that these compositions will not replace p-aramids but will likely be
an important supplement to our ‘‘tool box’’ of solutions to problems that we face.
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