Aramid Fibers From Fiber Chemistry 3rd
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Aramid Fibers Vlodek Gabara, Jon D. Hartzler, Kiu-Seung Lee, David J. Rodini, and H.H. Yang
CONTENTS 13.1
13.2 13.3 13.4
13.5
13.6
Introduction ............................................................................................................. 976 13.1.1 Historical Perspective .................................................................................. 976 13.1.2 Aramid—Definition .................................................................................... 977 13.1.3 Examples of Compositions.......................................................................... 977 Aramid Products: Forms and Properties ................................................................. 977 Producers of Aramid Products................................................................................. 978 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 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 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
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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 applications 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 management of the DuPont Technical Division launched very broad, long-range research programs with goals, among others, of developing very high-strength fibers and high-temperatureresistant 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 significant 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 mid1930s. 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 processability. This is very likely the reason why after half-a-century of research only four compositions have reached commercial stage: poly(m-phenylene isophthalamide) (MPDI), PPTA, copoly(p-phenylene=3,4’-diphenyl ether terephthalamide) (ODA=PPTA), poly[5-amino2-(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. a
Kevlar—a registered trademark of E.I. DuPont de Nemours & Co., Inc., Wilmington, Delaware, USA.
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O
O
C
C
N
N
H
H
n
poly (m-phenylene isophthalamide) MPDI O
O C
C N
N H
H
poly (p-phenylene terephthalamide) O
PPTA
O
O C
n
O C
C
C N
O
N
N
H
H
H
N
n
H
copoly(p-phenylene/3,4'-diphenyl ether terephthalamide) ODA/PPTA
H
|
N
HN
NH
OC
N
CO
n
poly [5-amino-2-(p -aminophenyl)benzimidazole terephthalamide]–SVM
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 homopolymer.
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 structure, 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
b
Nomex—a registered trademark of E.I. DuPont de Nemours & Co., Inc., Wilmington, Delaware, USA. Teijinconex, Technora—registered trademarks of Teijin, Ltd., Japan. d Twaron—a registered trademark of Akzo Nobel, The Netherlands. e Mictron—a registered trademark of Toray Co., Japan. f Aramica—a registered trademark of Asahi Co., Japan. c
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TABLE 13.1 Crystallinity of Homopolymers
Crystal system Lattice constant ˚) a (A ˚) b (A ˚) c (A a (degree) b (degree) g (degree) Number of chains in a unit cell Density (g=cm3) Calculated Observed
PPTA
MPDI
Orthorhombic
Triclinic
7.80 5.19 12.9
90 2 1.50 1.43–1.45
5.27 5.25 11.3 111.5 111.4 88.0 1 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 CC and CN 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 homopolymers. 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 5308C. 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
CHEMICAL PROPERTIES
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 resistance. 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 PROPERTIES Typical properties of commercial aramid fibers are given in Table 13.4. While yarns of maramids 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 13.2 Thermal Properties of Aramid Fibers Trade name polymer
Nomex MPDI
Fiber type Property Specific heat (J=kg-K) Thermal conductivity (W=m-K) Coefficient of thermal expansion (cm=cm-8C) Heat of combustion (J=kg) Flammability LOI (%) Decomposition (in N2) Temperature (8C)
Teijinconex MPDI
Twaron PPTA
K-29
HT
Std
430
Kevlar PPTA
K-49
Std
HM
Technora ODA=PPTA
72
60
60
81
81
81
81
96
0.25
0.11
0.11
2.5
2.5
—
—
0.5
1.8105
1.5105
1.5105
4.0106
4.9106
3.5106
3.5106
6106
28106
—
—
35106
35106
—
—
—
28
29—32
29—32
29
29
29
29
—
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.
TABLE 13.3 Chemical Resistance of Aramid Fibers Trade name Polymer Chemical
Time (h)=temp. (8C)
40% H2SO4 10% H2SO4 10% H2SO4 10% HCl 10% HNO3 10% NaOH 10% NaOH 40% NaOH 28% NH4OH 0.01% NaClO 10% NaClO 0.4% H2O2 10% NaCl 100% Acetic acid 90% Formic acid 90% Formic acid 100% Acetone 100% Acetone 100% Benzene 100% Ethyl alcohol 100% Ethyl alcohol 50% Ethylene glycol 100% Gasoline 100% Methyl alcohol 100% Perchloroethylene 100% Tetrahydrofuran
100=95 100=21 1000=21 1000=21 100=21 100=95 1000=21 1000=21 1000=21 1000=21 100=95 1000=21 1000=21 1000=21 100=21 100=99 1000=21 100=56 1000=21 1000=21 100=77 1000=99 1000=21 1000=21 10=99 1000=21
Nomex MPDI
Kevlar PPTA
Technora ODA=PPTA
Percent
Strength
Retention
90–100
90–100 95 35 90–100
90
20–60 60–80 90–100 80–90 90–100 90–100
90 76 90–100
90–100 90–100
56–75
90–100 60–80 90–100 80–90 90–100 90–100 90–100 80–90 90–100 90–100 90–100
35* 10* 20–60 75 46 65* 16 55 100* 90* 90–100 0–20
90–100 90–100 90–100 90–100 90–100 90–100 90–100
100 100 90–100 60–80 90–100 90–100
90–100 90–100 90–100 90–100 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 13.4 Properties of Aramid Fibers Trade name Polymer
Fiber type Density (g=cm3) Strength (Gpa) Elongation (%) Modulus (Gpa)
Nomex MPDI
Teijinconex MPDI
Kevlar PPTA
Twaron PPTA
430
std
HT
K-29
K-49
std
HM
Technora ODA=PPTA
1.38 0.59 31 11.5
1.38 0.61–0.68 35–45 7.9–9.8
1.38 0.73–0.86 20–30 11.6–12.1
1.44 2.9 3.6 71
1.44 3.0 2.4 112
1.44 2.9 3.6 70
1.45 2.9 2.5 110
1.39 3.4 4.6 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 orientation 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 properties. 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 13.5 Properties of Aramid Films Producer
Mictron toray
Aramica asahi
Thickness (mm) Density (g=cm3)
25 1.5
25 1.4
Mechanical properties: Direction Tensile strength Elongation Tensile modulus Initial tear strength Long-term heat resistance Thermal expansion Moisture absorption At 75% RH and At room temp. Electrical properties: Dielectric constant at 1 KHz Dissipation factor at 1 KHz Volume resistivity Surface resistivity Dielectric strength
GPa % GPa Kg 8C (1=8C 105)
machine 0.5 60 13 — 180 0.1
cross
%
1.5
2.8
V=cm V=cm KV=mm
— — 5 1017 — 300
4 0.02 1 1016 1 1016 230
9
machine 0.5 15 19 25 ~200 0.2
cross 0.3 25 10
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 processing 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 significantly 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 13.6 Properties of Aramids Paper Polymer
Nomex MPDI
Nomex MPDI
Nomex PPTA
Producer Type
DuPont 410
DuPont 410
DuPont N710
Thickness Density Mechanical properties: Direction Tensile strength Elongation Tensile modulus Initial tear strength Long-term heat resistance Thermal expansion Moisture absorption At 55% RH and At room temp. Electrical properties: Dielectric constant at 1 KHz Dissipation factor at 1 KHz Volume resistivity Surface resistivity Dielectric strength
mm g=cm3
127 0.87
127 0.87
97 0.64
GPa % GPa Kg 8C (1=8C 105)
machine 0.1 16 — 3.3 ~200 —
cross 0.05 13 — 1.6
machine 0.2 1.5 5.4 — ~200 0.7
%
—
1.6
V=cm V=cm KV=mm
2.4 0.006 5 1016 1 1016 25
3.9a 0.02a — — 82a
a Measurements 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 SYNTHESIS OF INGREDIENTS 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-Phenylene Diamine
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.
ß 2006 by Taylor & Francis Group, LLC.
NO2
NO2
NO2
NO2
H2SO4
2 HNO3
2 H2O
ð13:1Þ
NO2 NO2 Major
Minor
Minor
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
NH2
NO2
Catalyst 6 H2
NH2
ð13:2Þ
4 H2O
(Pt, Pd, Fe) Isomeric mixture of dinitrobenzene
13.5.2.2
MPD crude
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 C
2 N2 O3 þ 6 H2 O
ð13:3Þ
This mixture is then reacted with four moles of aniline to produce diphenyltriazine as follows:
N2O3
4
NH2
N
2
N
N
3 H2O
H
ð13:4Þ Diphenyltriazine is rearranged to form a mixture of p- and o- aminoazobenzene using nitric acid as a catalyst HNO3 N
N N H
Rearrangement
N
N
N
N
NH2
H2N
ð13:5Þ
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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 1 NH2
H2N
2H2
NH2
NH2
1
(13.6a) N
N 1
2H2
H2N
1
NH2
H2N H2N
13.5.2.3
(13.6b)
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 dimethylformamide (DMF) or DMAc produces 3,4’-POP. The resulting 3-nitro-4’-aminodiphenyl ether is then hydrogenated [42]. O2N
O2N NO2
⫹
K2CO3 NH2
HO
O
NH2
DMF/DMAc 3-nitro-4'-aminodiphenyl ether
ð13:7Þ H2N
O2N Pd/C O
NH2
O
NH2
H2,DMF,110C 3-nitro-4'-aminodiphenyl ether
3,4'-diaminodiphenyl ether
ð13:8Þ A mixture of 4-aminophenol, 1-3-dinitrobenzene and K2CO3 in DMF was treated at 1508C for 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].
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HO
O
O
C
C
O
DMF
OH 2Cl C Cl
Cl
O
O
C
C
Cl 2HCl 2CO2
ð13:9Þ The reaction involves formation of a catalyst complex between DMF and phosgene, which then reacts with terephthalic acid. Cl − O
CH3 N
C
O H
+
Cl
CH3
Cl C Cl
N
CH3
C
ð13:10Þ
CO2
+
H
CH3 "Complex" Cl −
O HO
O
C
C
CH3 OH + 2
CH3
N
Cl C
−2HCl H
Cl
O
O
C
C
Cl + 2
O
CH3 N
C
H
CH3
"Complex"
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 highmolecular-weight aromatic polyamides because of the reduced reactivity of aromatic diamines 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 Scho¨tten– Baumann reaction [44–49]. O RC
C
R1 Cl
H N
+
O
NaOH R
R1 +
N
C
R2
NaCl
+
H2O
ð13:12Þ
R2
Condensation polymers are formed if the complementary reagents are difunctional. O nH2N
R
NH2 + nCl
C
O R'
C
2nNaOH Cl
H
H
O
N
N
C
R
O R'
+ 2nNaCl + 2nH2O
C 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
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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). O
O N
nO C
R1 N
C
O + nHO
C R2
C
H-transfer OH
O
H
C
N
R1
H
O
N
C
O O
C
O R2
C
O n
O
H
H
O
O
O
H
Heat C
N
R1
N
C
O
C
R2
C
O
N
R1
H
O
N
C
O R2
+ 2n CO2
C n
n
ð13:14Þ 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
Reaction Mechanism
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. pi-cloud overlap H N
O +
X
Aromatic amidation
C
H
inductive effect H CH2
N
O +
HO
C
Aliphatic amidation CH2
H
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 quaternary 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
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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.
H2N
H
O
O
N
C
C X
H
H2N
X
H
O
O
N
C
C
H
X
X
Transition complex
ð13:15Þ
H 2N
H
O
O
N
C
C
H
X
X
H
H
O
O
H2N
N
C
C
X
X
Transition complex
ð13:16Þ
Amine regeneration H H 2N
H N
O C
O C
X
N
H2N
X
H
O
O
N
C
C
X + X
N H
ð13:17Þ Factors that can limit the extent of the polymerization reaction include deactivation of chainends, stoichiometric imbalance of reagents, monofunctional impurities, and insufficient mobility 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 monomers 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 13.7 Energetics of Aromatic Polycondensation DGr(T) (KJ=mole)
DGr(T) (KJ=mole)
Diamine
T8K
IPA
DMI
DPI
ICl
TPA
DMT
DPT
TCl
MPD
298 400 298 400
8.5 23.0 35.5 50.0
32.5 — 59.5 —
79.5 — 106.5 —
158.0 179.5 186.0 207.5
8.5 4.0 21.5 49.5
17.0 47.5 47.0 92.0
63.5 — 94.5 —
145.0 168.5 175.0 214.0
PPD
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).
OPh OPh PhO
P
O OPh
+
HO
N
N
O
NH2
H P O C PhO OPh
C
5 O
H
C
N
O +
H
P
OPh
+
OH
OPh
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-phosphonium salt 4 reacts with the carboxylate anion to give 5.
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OPh
OPh PhO
P
OPh
+
N
H
H
P
OPh
OPh 2 O OPh N H PhO
P
OPh
HO C N
N OPh OPh
3
H P PhO OPh 4
H PhO
P
O O
C
OPh
5
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 Interface In 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 ‘‘diffusion-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
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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(CH2 O =Csolvent). The values have been used to estimate the relative tendency of diamines to transfer to the organic phase under polymerization conditions. 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 Acylation At 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 chlorideterminated oligomer and some acid chloride. The reaction proceeds by an irreversible coupling 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 Elimination Hydrogen 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 Variables Variables affecting the polymerization include temperature, monomer ratio and concentration, 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 nonequilibrium 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. ß 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 two moles of acid acceptor per mole of diamine are needed to maximize the yield of high polymer [65]. Of water soluble inorganic acid acceptors used in MPDI polymerizations in a water–DMeTMS solvent system, sodium carbonate appeared to be the most promising. 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, potassium 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-molecularweight 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 Factors Both 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 ß 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 chainends, 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
O C
Cl
+ N
R
R
O
R
C
N
Cl
R
R
O
R
C
N
R +R
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).
2
O
R
C
N
Cl R
O R
C
R
O O
C
+
2R
NH
Cl
R
ð13:21Þ
ß 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 Rates Solution polycondensation employs the same reactions as used in interfacial polycondensation 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 impurities. 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
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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 Acceptors Polycondensation 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 preparation 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, is used to quantify the base strength of amines in organic 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 and has shown that E1=2 is parallel to the pKa scale in water. Table 13.8 lists these values for some acid acceptors frequently used for solution polyamidation. 13.5.5.2.5 Solvent The 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 polymer sufficiently to permit the completion of the polymerization [75–77]. The solution polycondensation process requires a stronger polymer–solvent interaction than does the
TABLE 13.8 Basicity of Amine Acid Acceptors E1=2 (mV)a Acid acceptor
pKa
Ethyl acetate
Acetonitrile
tert-Butylamine Diisobutylamine Triethylamine Tri-n-propylamine Tri-n-butylamine N-Ethylpiperidine N-Ethylmorpholine N,N-Diethyl-m-toluidine N,N-Diethylaniline Pyridine
10.45 10.59 10.74 10.70 10.89 10.45 7.70 7.24 6.56 5.26
130 207 197 228 210 190 290 — 467 —
— — 66 — — 84 221 — 425 —
a
E1=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.
ß 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 Aids Occasionally, 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). Li
CH3
C
O
Polarization
O
CH3
CH3 CH3
+ LiCl
N
C
ð13:22Þ
N CH3
CH3
Cl
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). CH3 Cl H3C
CH2CH2CH2CH3 N
N
CH2CH2CH2CH3 +
CH2CH2CH2CH3
Cl O
CH3 N
O H3C
CH2CH2CH2CH3 N
CH2CH2CH2CH3
CH2CH2CH2CH3
ð13:23Þ 13.5.5.2.7 Reactivity of Precipitated Polymer In 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.
ß 2006 by Taylor & Francis Group, LLC.
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. Scrubber
Inert gas
Monomer A vapor
Inert gas
Mixer
Quench
Reactor
Monomer B vapor
Separator
Inert gas recycle
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 internal 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). O N
O
O
C
C
O
O
C
C
O N
NH2
H2N +
ð13:24Þ
O O N
N
H
H
+ 1−x
C
H CH2
4
N x
+
N
H
The melt polycondensation of isophthaloyl-N,N-bis (valerolactam) with m-phenylene diamine yielded the aromatic polyamide MPDI plasticized by liberated valerolactam. A small ß 2006 by Taylor & Francis Group, LLC.
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 solutions. 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 SOLUTIONS 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-Aramid Solutions
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 inorganic 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.
g
Exception Teijinconex mono-filament process.
ß 2006 by Taylor & Francis Group, LLC.
le sib ver Irre mit li
Polymer conc, %
30
20
10
ble
rsi
R
e ev
it
lim
Solution region
0 0
100 Temperature, 8C
FIGURE 13.1 Stability of Teijinconex spin solution. (From Fujie, H., Nikkyo Geppo, 40, 8, 1987. With permission.)
13.6.2 ANISOTROPIC 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 conformation 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
ß 2006 by Taylor & Francis Group, LLC.
Brookfield viscometer reading
50 40 30 20 10
0
5
10 15 20 Concentration, wt %
25
30
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
Rheological Properties
Lyotropic solutions generally exhibit viscoelastic behavior. They are pseudoplastic and exhibit shear thinning with increasing shear rate. For polymers of near-linear chain conformation, 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 investigators [92–97]. Figure 13.3 and Figure 13.4 show the relationship between shear viscosity, h , and shear rate, g, for Kevlar–H2SO4 solutions of various concentrations at 25 and 608C, respectively. Figure 13.5 is a plot of shear viscosity vs. shear stress for PPTA solutions at 258C [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 Spinning
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
ß 2006 by Taylor & Francis Group, LLC.
105
10% 104
12%
h, poise
8% 3
10
6%
102 0.5%
100 ⫺2 10
100 10⫺1 . ⫺ 1 γ, sec
101
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 maramid 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 104
10% 12%
h, poise
103
8%
102
101
6%
10⫺3
10⫺2
10⫺1 100 γ, sec−1
101
102
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.)
ß 2006 by Taylor & Francis Group, LLC.
105 12%
h, poise
104 10% 8%
6%
103
102 1 10
102
103
104
σ12, dynes/cm2
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 evaporation 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 dogbone. 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 (55 y=min (>50 m=min) l0–l500 0.002–0.004 in. (0.05l–0.l02 mm) 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 Time Tension
250–5508C 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 relationship 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.
ß 2006 by Taylor & Francis Group, LLC.
Staple supply
Fibrid supply
Beater
Stock tank
Mixer
Head box
Product reel
Calender
Dryer rolls
Wet press
Fourdiner machine
Broke
Slusher
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 Kajaani P as2 aP 200 instrument and is reported in P terms of a length-weighted average length ( n l = nili) or i i P weight-weighted average length ( nili3= nili2). The length-weighted average length of typical 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.
ß 2006 by Taylor & Francis Group, LLC.
EP6381TR3 Pulp KE IF538 10μm Grand = 1.00 KX
Detecteur = SE1 WD = 12mm
Nom Utilisateur = PIERDOC Date :7 Juil 2003
(Merge 1f538)
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 characteristics 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 paramids 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 respectively) both glass transition temperature and melting temperature of the crystalline phase are
ß 2006 by Taylor & Francis Group, LLC.
high (2758C and 4258C respectively) in flame 100% m-aramid garments exhibit some shrinkage, 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 environments 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 applications 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 maramid 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.
ß 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 2008 C. For equipment operating at lower temperatures (150–1608 C), 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.
h
Teflon1—a registered trademark of E.I. DuPont de Nemours & Co., Inc., Wilmington, Delaware, USA.
ß 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 mechanical 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 densification. 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 products 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
ß 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, flechettes, and fragments from mortars, grenades, and mines. The design of the optimum protective 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].
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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 minimized. 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 paramid 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 applications 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 capability, handling, and comfort. Additional factors that favor increasing aramid usage in automobile 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.
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PT belts can be divided into two categories—v-belts and synchronous belts. Strength, dimensional stability, fatigue resistance, and adhesion are key reinforcement criteria. Polyester 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 reinforcing 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, longerlived 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 elongation that results in improved impact resistance. Composite structures are found in a host of applications including aerospace components, automobile parts, boats, sporting goods, protruded 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 demonstrated satisfactory performance in secondary aircraft structures. Aramid’s high tensile strength lends itself well to the manufacture of canoes where weight can be reduced significantly while providing greater tear strength and puncture resistance than fiberglass composites. 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.
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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 construction 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 environment. 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 considerations 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
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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 transportation 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 Engineering
Use of composite materials for concrete infrastructure repair that was initiated in the mid1980s 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 in weight and can be easily handled without heavy machinery and can be 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 13.8.4.1
P-ARAMID
PAPER
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
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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 fabrication 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 Facings Asbestos 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 formulations. 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 50
9.5 >2 400–450 2 0.5 1.7 530 >50
In epoxy resin—3-point bending test. Source: From Magellan International; Teijin Ltd., Teijinconex Heat Resistant Aramids Fiber 02.05.
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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 functionality 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
N
O
O
PBO OH N
NH
N
N
NH OH
M5 N
N
NH
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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