40207_10

ORGANIC FIBERS 10

Linda L. Clements

10.1 INTRODUCTION

Before the first aramid fibers were introduced in the 1960s and 1970s, organic fibers were relatively low performance materials, primarily used in textile applications. Now several different types of high performance organic fibers exist, all competitive with inorganic fibers in some or even most of their properties. The market demand for these fibers exceeds one billion dollars (Adams and Farrow, 1993a).

The main applications for high performance organic fibers today are in asbestos replacement, ballistics, rubber reinforcement, ropes and cables and composites. Most of the usage is of aramid fibers, with over 18000 metric tons used each year. Both usage and existing capacity for other organic fibers are only a fraction of this value (Adams and Farrow, 1993a).

Tlus broad market for organic fibers is a direct outgrowth of applying the basic princi- ples of polymer science to produce a new and exceptional engineering material. In the 1950s it was recognized that if a means could be found to form certain intractable polymers into extended chain fibers, very high stiffnesses, strengths and use temperatures could be achieved. The difficulty of producing such fibers was solved in the 1960s by spinning from liquid crystalline solutions. The first fibers produced by th s process were the aramids, which have since been followed by other such fibers.

Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7

A different type of hgh performance organic fiber, extended chain polyethylene fibers, was added in the 1970s. While inferior to inorganic fibers in some properties, organic fibers provide combinations of properties not available with inorganic fibers and so have made possible new designs and applications.

In this chapter, only high performance organic fibers which are commercially available will be discussed in detail, although fibers which are nearing commercialization will be discussed briefly. For a more complete review of both commercially available and experimental high performance organic fibers, see Yang (1989, 1992).

10.2 ARAMID FIBERS

10.2.1 OVERVIEW

Aramid fiber is the generic term for a specific type of ’aromatic polyamide fiber.’ The US Federal Trade Commission defines an aramid fiber as ‘a manufactured fiber in which the fiber-forming substance is a long-chain synthetic polyamide in which at least 85% of the amide linkages are attached directly to two aromatic rings.’

Thus, in an aramid, most of the amide groups are directly connected to two aromatic rings, with nothing else intervening. It should not be surprising that aramids have quite different properties from nylons and other conventional polyamides since the latter polymers contain few if any aromatic groups in the main chain of the polymer.

Aramid fibers 203

Aramid fibers can be separated into two types: the para- aramids and the meta-aramids. In para-aramids, the chain-extending bonds are in the para-position on the aromatic ring, as in poly-p-phenylene terephthalamide (PPTA) (Fig. 10.1 (a)), co-poly-p-phenylene /3,4’-oxydiphenylene terephthalamide (Fig. lO.l(b)) and poly-p-phenylene-benzimidazole-terephthalamide (Fig. lO.l(c)). In meta-aramids, on the other hand, the chain-extending bonds are in the meta-position on the aromatic ring, as in poly-m-phenylene isophthalamide (MPIA) (Fig. 10.1 (d)). Commercially available para-aramids

-0 0 I I

. C \

H

k-(=&A I \ H

H 1 H I

0

0 I1 C

H

0

H I

0

C I1

include DuPont’s KevlarO, Akzo’s TwarorP, Teijin’s TechnoraO and Kaiser VIAM‘s Amosa and S W @ fibers, while meta-aramids include DuPont’s Nomexs and Teijin’s TeijinconexO fibers. Hoechst AG also markets a para-aramid fiber in Europe. The para-aramids are the fibers used in high performance applications and thus will be emphasized in this chapter.

10.2.2 MANUFACTURE

Historically, meta-aramid fibers were the first to be produced, with DuPont’s Nomex fiber

-0;- 0

0 I1 C \

H N- I d+- 0

H 1 I

N

0 -

-0 \

-0 H I

Fig. 10.1 Structural formulae of (a) the para-aramid poly-p-phenylene terephthalamide (PPTA), (b) the para-aramid co-poly-p-phenylene/3,4’-oxydiphenylene terephthalamide, (c) the para-aramid poly-p- phenylene-benzimidazole-terephthalamide (PBIA), and (d) the meta-aramid poly-m-phenylene isophthalamide (MPIA).

204 Organic fibers

being introduced in the 1960s. The first para- aramids were synthesized in 1965 by S.L. Kwolek of DuPont (Kwolek, 1971; Kwolek, 1972; Kwolek, 1974). Forming these into usable fibers is very difficult because para-aramids show no melting point and are soluble in a limited number of solvents.

The problem of spinning the polymer into fibers was solved for PPTA following the dis- covery that the polymer would dissolve in strong acids to form a liquid crystalline solution. Undiluted sulfuric acid is the solvent usually used. Blades (1973, 1974) devised a special manufacturing process - known as continuous dry jet wet spinning - for forming the liquid crystalline solution into filaments. The polymer solution is extruded through spinnerets at elevated temperature through an air layer into a coagulating water bath. The cold water bath also contains a base to neu- tralize and remove the retained acid.

Continuous dry jet wet spinning is the manufacturing technique used for most para-aramid fibers. Teijin’s Technora fiber, however, is produced by wet spinning followed by drawing (Hongu and Phillips, 1990).

H I

-N

\ c - e ll

I

H

0 II

- C \

10.2.3 STRUCTURE

The excellent properties of para-aramids result from both chemistry and physical microstruc- tures. In both meta- and para-aramids, the aromatic rings in the backbone chain produce high thermal resistance. In addition, in para- aramids the orientation of the chain-extending bonds produces a polymer which is an extended-chain rigid rod. Spinning produces a fiber made up of extended-chain crystallites which are almost completely aligned parallel to the draw direction and to each other. The crystallites have a very high length-to-diameter ratio and extensive interconnection of molecules between crystallites. Thus, an unbroken ’infinite’ filament can be formed.

Within the crystallite the chains are bonded to one another by hydrogen bonds, as shown in Fig. 10.2. Although these bonds are not nearly as strong as the covalent bonds which occur within the molecules, hundreds or even thousands of such bonds form between adjacent para-aramid molecules. Since the molecules are rigid, the only way to separate them in tension is to break all of the hydrogen bonds at once. This requires a large force and

0

0

H

*-(=&A I \ H

H

0

0 -0

H

Fig. 10.2 Schematic showing hydrogen bonding between PPTA molecules in the crystallite.

Aramid fibers 205

is the reason para-aramid fibers are exception- ally strong in axial tension. However, since the bonds can be broken easily one at a time, the fibers are quite susceptible to damage by bending, buckling or transverse loading.

In meta-aramids, on the other hand, a crooked chain results. Since even in pure tension the chain-extending bonds can flex and rotate, meta-aramids are much less rigid than para-aramids and not as strong. However, because the chains are more flexible, meta- aramids are easier to manufacture than para-aramids and are less expensive.

10.2.4 PROPERTIES

Aramid fibers offer some significant advantages over other fibers, but also have their drawbacks and limitations. Both advantages and limitations will be described more fully in the sections on properties and in the sections on design considerations and applications.

Both DuPont’s Kevlar family of fibers and Akzo‘s Twaron fibers are based upon PPTA (Fig. lO.l(a)). Teijin’s Technora fiber and the para-aramid marketed by Hoechst AG in Europe, on the other hand, are a para-aramid copolymer, co-poly-p-phenylene/3,4’-oxydiphenylene terephthalamide (Fig. lO.l(b)). It is likely that Kaiser VIAM’s SVM fibers are poly- p-phenylene-benzimidazole-terephthalamide (PBIA), (Fig. lO.l(c)) rather than PPTA (Gerzeski, 1989). Kaiser VIAM’s Armos fiber may be PBIA or PPTA. Both DuPont’s Nomex and Teijin’s Teijinconex fibers are based upon MPIA (Fig. lO.l(d)). These chemical and structural differences produce different properties for the fibers. In addition, differences in spinning conditions and, most importantly, post-spinning heat treatments are used to alter properties further. For example, by changing processing conditions, Kevlar fibers can be produced with elastic moduli ranging from 63 to 143 GPa (9 to 21 Msi) and elongations at break from 1.5 to 4.4%.

Because of the anisotropy of their microstructure, para-aramid fibers have very anisotropic

mechanical, thermal, physical and other properties. This anisotropy may produce design limitations, but can also be used to advantage.

Physical and thermal properties

Table 10.1 compares the physical and thermal properties of some representative aramid fibers.

Due to their highly aromatic and ordered structure, aramids have very high thermal resistance for organic materials. They do not melt prior to decomposition, in spite of the fact that they are technically classified as thermoplastics. This is because melting of the crystalline phase, like rupturing the fiber in tension, would require that all of the hydrogen bonds between two molecules be severed at once. Nonetheless, because of decomposition, their temperature resistance is not equal to that of inorganic fibers. Thermogravimetric analysis of Kevlar fibers shows that weight loss begins at above 350°C (660°F) in air (Penn and Larsen, 1979; Yang, 1992), with complete decomposition occurring at between 427 and 482°C (800 and 900°F) (DuPont, 1992a).

Exposure to elevated temperature will degrade the properties of aramid fibers. Figure 10.3 shows the strength retention of Kevlar 29 and Technora fibers as a function of time and temperature. This change in properties occurs as a result of slow oxidation. For this reason, the long-term use temperature of para-aramid fibers is typically limited to about 150-175°C

In the transverse direction para-aramids are like most other materials in that they expand with increasing temperature. However, in the longitudinal direction the fibers actually contract somewhat as temperature increases. The negative thermal expansion coefficient of para-aramids can be used to advantage to design composites with tailored or zero thermal expansion coefficient.

Aramids are flame resistant but can be ignited. While pulp or dust of Kevlar may continue to smolder once ignited, fabrics do not

(300-350°F).

206 Organic fibers

Table 10.1 Physical and thermal properties of representative aramid fibers

Fiber Kevlar 49 Twaron HM Technora Nomex Teijinconex

Type para-aramid para-aramid para-aramid copolymer

Teijin 1989,1993

1.39 (0.0502)

-

meta-aramid meta-aramid

Reference for data DuPont

Density 1.44 g cm-3 (lb in-?) (0.0520)

Melting temperature -538°C" (1000°F)

Decomposition 427482°C temperature in air (800-900°F)

temperature in air (300-350°F)

1992a

Long-term use 149-177°C

Longitudinal linear -4.9 thermal expansion (-2.7) coefficientb 10-6/ "C

Transverse linear +66"

coefficientb: /"C ( / O F )

Specific heatb 1.42 kJ/kg K (BTU/lb OF) (0.34)

Longitudinal thermal conductivityb 4.11'

W/m K (2.38) BTU/h ft OF)

O F )

thermal expansion (+37)

Transverse thermal conductivityb 4.82'

W/m K (2.79) BTU/h f t OF)

moisture contentb

Typical filament 12 or 15 diameter (0.48 or 0.59) pm in)

Typical filament round shape

Equilibrium 3.5%

Akzo 1990,1991

1.45 (0.0524)

>5OO0C (>932"F)

500°C (930°F)

DuPont 1981,1993g

1.38 (0.0499)

>371"C (>700"F)

371°C (700°F)

Teijin 1991 1.38

(0.0499)

500°C (930°F)

-

400430°C (750-805°F)

-3.5 (-1.95)

-6.2 (-3.4)

+15 (+8.3)

+20 (+11)

1.42 (0.339)

1.09 (0.26)

1.21 (0.29)

1.05 (0.25)

4.0 (2.3)

0.13 (22)

0.13 (22)

5.0 (2.9)

3.5% 2.0% 4.5% 5.0-5.5%

12 (0.48)

12 (0.48)

max: 15-17 (0.6-0.7)

-10 to 15 X 45 (-0.4 to 0.6 X 1.1)

round round oval to dogbone

oval to dogbone

* Data from Yang, 1992. Varies with temperature; room temperature values are given. Data from Chiao and Chiao, 1982.

Arumidfibers 207

r I I I I

100

be 80 .. C 0

C

w L11

m

- +

2 60

f I= 40 e iz

20

L

-

- Technorag -

0 ' I I I I L

0.1 1 10 100 1000

Time, h

Fig. 10.3 Strength retention of Kevlar 29 and Technora fibers following elevated temperature exposure (DuPont, 1992a; Teijin, 1989).

continue to burn when the flame source is removed (DuPont, 1992a). The lower thermal conductivity of aramids compared to inorganic fibers can improve the fire resistance of their composites, since aramids do not readily conduct heat into the more volatile matrix.

Mechanical properties

Composite materials are most commonly used because of their superior strength and/or stiffness at a given weight as compared to conventional structural materials. Figure 10.4 compares the specific strengths and specific stiffnesses of various reinforcing fibers. (The strengths and stiffness in Fig. 10.4 are expressed in units of grams per denier (gpd). This is a textile term often used for organic fibers which measures specific strength and/or stiffness. This term is further explained in the appendix to this chapter.) As can be seen, aramid fibers perfonn very well. In fact, until the emergence of high strength intermediate modulus carbon fibers and the commercialization of polyethylene

fibers in the mid-l980s, aramid fiber composites had the highest specific strengths of all composite materials. Although composites from newer fibers have taken over that position, aramids still offer outstanding combinations of properties, such as high specific strength, toughness, creep resistance and moderate cost, for specific applications.

Table 10.2 compares the mechanical properties in axial tension of several commercially available aramid fibers.

Aramid fibers have some definite limitations. They are weak in bending and show obvious damage if subjected to kinking or buckling. As a result, they are also weak in compression (where microbuckling is inevitable) and in transverse tension (where bond-by-bond breakage of hydrogen bonds is likely). In addition, even though the para-aramid chain is quite polar in nature, almost all of the polar groups are fully involved in hydrogen bonding to other aramid molecules. As a result, para- aramid fibers do not form strong bonds with other materials such as composite matrices,

208 Organicfibers

Table 10.2 Axial tensile mechanical properties of representative aramid fibers

Fiber Reference Spec$c Initial tensile Tensile strength, Elongation at gravity modulus, GPa (Msi) MPa ( h i ) break, %

Bare" Epoxy- Bare" Epoxy- Bare" impregnatedb impregnatedb

Kevlar Type 956, 1500 denier

Kevlar 29 Type 964, 1500 denier



Kevlar 119 1500 denier

Kevlar 129 denier unspecified

Type 965A, 1140 denier

Kevlar 149

Kevlar HT Type 964C, 1000 denier

Kevlar KM2 850 denier

Twaron

DuPont, 1993h 1.44 71.8 (10.4)

2920 (424)

- 3.6

DuPont, 1992a, DuPont, 19938

1.44 70.5 (10.2)

83.0 (12.0)

2920 (424)

3600 3.6 (525)

DuPont, 1992a, DuPont, 1993g

1.44 112.4 (16.3)

124.0 (18.0)

3000 (435)

DuPont, 1993g 1.44 99.13 (14.4)

3050 (442)

- 2.9

DuPont, 1990

DuPont, 1993i

- 54.6 (7.9)

1.44 96.0 (13.9)

3050 (442)

(490) 3380

- 4.4

- 3.3

DuPont, 19938 1.47 142.7 (20.7)

2340 (339)

- 1.5

DuPont, 19938 1.44 99.1 (14.4)

3370 (489)

- 3.3

DuPont, 1992d

Akzo, 1991

- 63.4 (9.2)

(10.2) 1.44 70

1.44 88 (12.8)

3280 (476)

(406)

(468)

2800

3230

- 4.0

3500 3.6 (508)

- 3.3 Twaron Perkins, 1993 Type 2000, 930 denier 'microfilament'

Twaron HM Akzo, 1991 1.45 103

- 147 (14.9)

(21.4)

1.43 123 (17.8)

3500 2.5

- 3.2 (508)

Armos Kaiser VIAM,

SVM Kaiser VIAM, 58.8(300) 1993g' X17-1000 Gerzeski, 1989

58.8 tex 1993a'

Continued on next page

Aramid fibers 209

50

40

30

2o

Table 10.2 Continued

. . . . I . . I . I . . . I l l l l l l . . . . l l . . I I I I . . I . I . .

- PED HM - Armos

Carbon TlOOOG 0

0 'pectra 'Oo0

Dyneema SKBO Carbon

'spectra 900 - Technora Tekmilon

. Vectran HS

. Twaron O O S ~ 0 Carbon T-300

. .waron HM - S-Glass

. PE (H.C.) Boron

0

0 Kevlar 49

0 Kevlar 149 0 Carbon T-50

E-Glass

Fiber Reference Specific gravity

. Steel

Technora Teijin, 1989 1.39

Nomex DuPont, 1993g 1.38 Type 430, 1200 denier

Teijinconex Teijin, 1991 1.38

Teijinconex Teijin, 1991 1.38 HT

Initial tensile Tensile strength, Elongation at modulus, GPa (Msij MPa (ksij break, %

Bare" Epoxy- Bare" Epoxy- Bare" impregnatedb impregnatedb

~~ ~

73 - (10.6) 11.6 -

(1.68)

7.9-9.7 -

(1.1-1.4) -

11.6-12.2 -

(1.7-1.8)

3440 - 4.6 (498)

596 - 28.0 (86.6)

610-670 - 3545 (88-97)

730-850 - 20-30 (110-120)

a Data for DuPont fibers taken from conditioned yarns tested according to ASTM Standard D885. Modulus data for Akzo fibers from testing according to ASTM Standard D885M. Test technique unspecified for Akzo fiber strengths and elongations and for all data from Kaiser VIAM and Teijin fibers. Data for DuPont fibers taken from epoxy-impregnated strands tested according to ASTM Standard D2343. Data for Akzo fibers from testing according to impregnated strand test method DIN 65356, part 2. Test technique unspecified for Kaiser VIAM fibers. Preliminary data.

1 Carbon P-100

2 10 Organic fibers

further aggravating the poor transverse, bending and compressive properties of the fiber itself.

The basic chemical structure differences between the aramid fibers produce many of the mechanical property differences seen in Table 10.2. The ether (-0-) linkages in the backbone of the Technora copolymer fiber produce a lower modulus than that of Kevlar and Twaron PPTA-based fibers. On the other hand, the additional cyclic ring in the SVM PBIA- based fibers produces a higher basic modulus. However, heat treatment and other fabrication steps can also alter mechanical properties significantly, as is seen in the property differences between the various Kevlar fibers.

The mechanical properties of aramid composites are illustrated in the data of Table 10.3. For this filament-wound composite the longitudinal compressive strength was about one-eighth that in longitudinal tension, the in- plane shear strength was one-seventy-fifth and the transverse tensile strength over two hun- dred times smaller. While the relative values of properties may change for composites made from other aramid fibers and/or other matrices

and/or prepared using other fabrication processes, the general trend is valid: aramid fiber composites have poor off-axis properties.

In axial tension, both aramid fibers and their composites are linear to failure. In spite of this fact, the same microstructural characteristics which lead to the weakness of aramid fibers in buckling also make them very tough. During failure, the widespread bending, buckling and other internal damage to the fibers absorbs a great deal of energy. Similarly, the strength of aramid fibers is not very strain rate sensitive: an increase in strain rate of more than four orders of magnitude only decreases the tensile strength by about 15%. (Abbott et al., 1975) This property alone provides design advantages over all inorganic and many other organic fibers.

The mechanical properties of aramid fibers decrease with increasing temperature. Figure 10.5 shows the fiber elastic modulus as a function of temperature for several organic fibers. At 177°C (350°F) the modulus of para-aramid fibers is about 80% of that at room temperature. Figure 10.6 compares the fiber tensile

Table 10.3 Mechanical properties of a filament-wound composite of 60 vol YO aramid fiber in a room-temperature curable epoxy matrix (Clements and Moore, 1977)

Fiber: DuPonf’s Kevlar 49, Type 968, 1420 denier Matrix: 100 parts Dow Chemical DER 332 (diglycidyl ether of bisphenol-A epoxy) and 45 parts Jefferson

Chemical reffamine T-403 polyether triamine Cure: 1 day at room tnnperuture, postcure 16 h ut 85°C (185°F)

-

Elastic constants:

Longitudinal Young’s modulus E,,, GPa (Msi) Transverse Young’s modulus E,,, GPa (Msi) Shear modulus G,,, GPa (Msi) Major Poisson’s ratio vl, 0.310 k 0.035 Minor Poisson‘s ration u,,

81.8 f 1.5” 5.10 k 0.10 1.82 k 0.09

(11.9 k 0.22) (0.74 & 0.014) (0.26 f 0.013)

0.0193 f 0.0014

Ultimates: Tension Compression

Longitudinal strength, MPa (ksi) Longitudinal ultimate strain, Yo 2.23 f 0.06 0.48 5 0.3 Transverse strength, MPa (ksi) Transverse ultimate strain, Yo 0.161 f 0.023 1.41 f 0.12

1850 f 50 (268 f 7.3)

7.9 k 1.1 (1.15 f 0.15)

235 f 3 (34.1 k 0.4)

53 f 3 (7.7 f 0.4)

Shear stress at 0.2%, offset, MPa (ksi) - -

Shear strain at 0.2% offset, Yo - -

In-plane shear - - - -

24.2 f 2.4 (3.51 k 0.35) 1.55 k 0.16

a Limits are 95% confidence limits. Each value is the result of five or more tests.

Aramidfibers 211

si

25

s - 5

-

3500

2500

8 n = g 2000

0,

1500 t? 3i

1992a; Teijin,

500 -1 a - 1 8 - m q

Techno ra

- 400 1 Kevlar

.- 300 Y"

f

?! m c

3000:; : -

- 200 Polyester

1989).

- Nylon

500

looo;,

0 , ,:

Fig. 10.6 Tensile strength as a function of temperature for two para-aramid fibers and for two polymer fibers and steel (DuPont, 1993h; Teijin, 1989).

100

0

21 2 Organic fibers

strength as a function of temperature for several organic fibers. For Kevlar fiber the strength at 177°C (350°F) is about 80% of that at room temperature, while for Technora the strength is about 70% of the room temperature value. On the other hand, at cryogenic temperatures modulus increases slightly and strength is not degraded.

The presence of moisture also reduces the mechanical properties of aramid fibers and their composites. The effect upon longitudinal tensile properties is relatively small, but the loss is pronounced for off-axis properties. Table 10.4 illustrates this loss for Kevlar 49 fiber in a room-temperature curable epoxy. The longitudinal tensile strength in water at room temperature was 88% of that for composites equilibrated at room temperature and 52% relative humidity (r.h.). The wet longitudinal compressive strength, on the other hand, was only 75% of the 52% r.h. value, while the

wet transverse tensile and in-plane shear strengths were only about half of the 52% r.h. values. The data in boiling water illustrate that the drops in strength due to the presence of moisture alone were almost as severe as those due to the combined presence of moisture and elevated temperature. This relative loss in properties is less for the Technora para-aramid co-polymer fiber. Care must be exercised when using aramid composites in high moisture applications.

Both para-aramid co-polymers and homopolymers exhibit very little creep. In general, creep strain increases with increasing temperature, increasing stress and decreasing fiber modulus. Like all high performance fibers, under long term loading, para- aramids are subject to stress rupture, i.e. failure of the fiber under sustained loading with little or no accompanying creep. Figure 10.7 compares the stress rupture performance of Kevlar 49 to that

Table 10.4 The effect of environments on the mechanical properties of a filament-wound composite of 50 vol Yo of an aramid fiber in a room-temperature curable epoxy matrix (Wu, 1980)

Fiber: DuPont's Kevlar 49, 4560 denier

Matrix: 100 parts Dow Chemical DER 332 (diglycidyl ether of bisphenol-A epoxy) and 45 parts Jefierson Chemical Jeffamine T-403 polyether triamine

Infrared heating, postcure 2 h at 100°C (212°F) Cure:

Strength, MPa (ksi)

23"C, dry 23°C' 52% r.k. 23"C, water 1OO"C, water

Longitudinal 1370 f62" 1340 f 112 1190 f 62 1150 f 1 2 4 tension (199 f 9) (194 f 16) (173 f 9) (167 f 18)

Longitudinal 188 f 1 2 169 f 20 126 f 22 107 f 2 1 compression (27.3 f 1.7) (24.5 f 2.9) (18.3 f 3.2) (15.5 f 3.0)

Transverse 7.6 f 1.6 74 f 1.2 3.9 f 0.7 3.6 f 0.2 tension (1.10 f 0.23) (1.07 k 0.17) (0.57 f 0.10) (0.52 f 0.03)

Transverse 31.3 f 3 . 2 29 f 4.0 22.5 f 3 . 2 22.1 f 23.6 compression (4.54 kO.46) (4.21 f 0.58) (3.26 f 0.46) (3.20 f 3.42)

In-plane shear 27 f3 .0 26.5 f 1.6 13.8 f 2.2 13.6 f 2.5 (3.92 f 0.44) (3.84 f 0.23) (2.00 f 0.32) (1.97 f 0.36)

Hygrothermal Properties

________

Equilibrium moisture - 4.1 7.8 8.9 concentration, Yo Limits are 95% confidence limits. Each strength is the average of five tests.

~

0 +, 100

!! 2 90 .I4

1 W o 80

d . 7 0

u ," 60

a 0 50 .rl

a 2 40

a a

a

rl

Aramid fibers

K e v l a f l 4 9

213

10-2 10-1 1 io io2 1 0 3 104 105

Lifetime, h

Fig. 10.7 Stress-rupture behavior of epoxy-impregnated Kevlar 49 fibers compared to that of epoxy- impregnated S-glass fibers (Chiao, Chiao and Sherry, 1976).

of Sglass. Para-aramids perform well under these conditions, but the phenomenon of stress rupture must be considered in any design where long term loading is anticipated. Strength retention cannot be used to estimate the remaining life of aramid fibers or composites under long term load (Chiao, Sherry and Chiao, 1976), so estimates of long term behavior must be derived from actual data, or acceler- ated testing methods (Chiao and Chiao, 1982).

Para-aramid fibers and their composites perform very well in fatigue. For aramids, tension-tension fatigue generally is not of significant concern in applications where an adequate static safety factor has been used (Yang, 1992). Aramid composites have been found to be superior to glass fiber composites in both tensile-tensile and flexural fatigue loading. For the same lifetime (cycles to failure), Kevlar 49/epoxy composites can operate at a significantly larger percentage of their static strength than can glass-reinforced composites

(DuPont, 1986). Para-aramids also can be expected to perform better than carbon fibers in fatigue (Teijin, 1989; Yang, 1992). Technora para-aramid co-polymer is found to have even better fatigue resistance than the para-aramid homopolymer fibers (Teijin, 1989).

Chemical and environmental properties

PPTA fibers are quite stable chemically; their resistance to neutral chemicals is usually very high. They are, however, subject to attack by acids and bases, especially by strong acids. Because the spin process used for Teijin's Technora para-aramid co-polymer produces a very pure polymer, the chemical and environmental resistance of Technora is superior to that of the PPTA fibers. Table 10.5 reports the resistance of Kevlar and Technora fibers to various chemicals. Technora has better acid and alkali resistance than PPTA and its steam resistance is also superior.

214 Organicfibers

Table 10.5 Stability of para-aramid fibers in various chemicals

Concentration, Temperature, Time, Effect on breaking strength"

Chemical Yo "C ( O F ) hr None

Acids Acetic 40

40 Formic 90

90

10 Nitric 10

10 10

Sulfuric 10 20 40

Hydrochloric 20

Phosphoric 10

Alkalis Ammonium hydroxide 28

10 Portland cement saturated

saturated

Sodium hydroxide 10

Organic solvents Acetone 100 Benzene 100

100 Carbon tetrachloride 100

Ethylene glycol/water 50/50

Gasoline 100 Gasoline-leaded 100

N-Methyl pyrrolidone 100

Sodium chloride 3 10 10

Sea water 100 Sea water (New Jersey) 100 Steam 100

100 100 100

Ethylene chloride 100

Ethylene glycol 100

Methyl alcohol 100

Other

Water, tap 100

21 (70)

21 (70)

20 (68) 71 (160)

95-99 (203-210)

95-99 (203-210)

20-21 (68-70) 20-21 (68-70)

21 (70) 99 (210) 99 (210) 95 (203) 95 (203)

21 (70) 21 (70)

95 (203) 180 (356)

95-99 (203-210)

boil 20 (68) 21 (70)

boil 20 (68)

95 (203) 20 (68) 21 (70) 21 (70) 95 (203)

99 (210)

21 (70)

121 (250) 95 (203)

120 (248) 150 (302) 150 (302) 200 (392)

99 (210)

-

99 (210)

1000 100 Tb 100 K 100 100 T 10 100 T 100 K,T 1000 100 10 100 T 100 T

1000 K 1000 100 100 T 15

100 K 784 T 1000 K 100 1000 T 1000 300 T 784 T 1000 K 1000 K,T 100

1000 K 100 K 100 1000 T 1 yr K 400 T 48 100 T 100 100 K

Slight Moderate Appreciable Degraded

Kb

T

K

K

K

K K

K K

K T K

T

K

K

T

K

K

T

a None, 0-10% strength loss; slight, 11-20% strength loss; moderate, 2140% strength loss; appreciable, 41430% strength loss; degraded, 81-100% strength loss. K is for Kevlar aramid fiber (DuPont 1989,1993h); T is for Technora aramid fiber (Teijin, 1989).

Aramid fibers 215

Para-aramids are strong ultraviolet (W) absorbers. Upon exposure, the yellow or gold fibers turn first orange and then brown, due to degradation. The degradation occurs only in the presence of oxygen and is not enhanced by either moisture or atmospheric contaminants (DuPont, 1992a). Extended exposure may cause a loss of mechanical properties. Bare 1667 dtex (1500 denier) Kevlar 29 was found to have 71% strength retention after 1 month of outdoor exposure in Wilmington, DE and 43% after 4 months (Yang, 1992). In both processing and applications, para-aramids must be protected from W exposure, such as by painting or coating. However, since para-aramids are self-screening, UV protection may also be effected simply by dense packing of the fiber itself, with or without a matrix. Thus, bare 12.7mm (0.5 in) 3-strand Kevlar 49 rope was found to have 90% strength retention after 6 months outdoors in Florida and 69% strength retention after 24 months (DuPont, 1986).

Unlike inorganic fibers, aramid fibers absorb water. For some aramid fibers the equilibrium moisture content (see appendix on page 241 for defirution) is quite high (5% for SVM, 7% for Kevlar, Kevlar 29 and Twaron), moderate for others (3.5% for Kevlar 49 and Twaron HM) and reasonably low for some (2% for Armos and Technora and about 1% for Kevlar 149) (Akzo, 1991; Kaiser VIAM, 1993a; Teijin, 1989; Yang, 1992). The equilibrium moisture content is directly proportional to the relative humidity, rising for Kevlar 49 to 6.2% at 96% r.h. (DuPont, 1992a). Absorbed moisture has only a small effect upon the tensile properties of the fibers, but a significant effect upon the transverse tensile, compressive, shear and flexural properties of the composite. The gain of moisture is completely reversible and once removed produces no permanent property changes.

Electrical and optical properties

Aramid fibers are electrical insulators. The process used to make the Technora fiber, however, leaves it with fewer ionic impurities than

in the para-aramid homopolymers and thus improved electrical properties. Technora fiber has a resistivity of 5 x lo’* Q/cm (Teijin, 1989). The dielectric constant of PPTA is 3.85 (Allied, 1989).

The refractive index of Kevlar 49 fiber is 2.0 parallel to the fiber axis and 1.6 perpendicular (DuPont, 1986). Aramid fibers are opaque and are yellow to gold in color.

10.2.5 TREATMENTS

Unlike inorganic fibers, few surface treatments are used on aramid fibers to promote matrix adhesion. One reason is the futility of increasing the matrix bonding to the surface of a fiber which readily fails by defibrillation. Most dra- matic improvements in fiber/matrix bonding give only modest improvements in off-axis strengths since they simply move the locus of failure from the surface to the interior of the filament. In other cases, longitudinal tensile strengths are adversely affected by otherwise successful surface treatments. Not all attempts at designing surface treatments have been unsuccessful, but for the most part the surface treatment used on commercial fibers is minimal compared to that used for inorganic fibers.

Finishes - lubricants which aid in subse- quent processing steps - are applied to aramid fibers for some applications. Available finishes are designed for such purposes as lubrication during weaving operations, improving abrasion resistance for cable applications or better performance in rubber goods. If the fiber is to be used in a high performance composite, however, the user will usually wish to avoid or remove any finish before impregnating the fiber with a matrix.

Commercial aramid fibers may also be twisted. Twist may be quite useful in some applications and a small amount of twist will increase the strength of bare yarn or cord. [This optimum twist for Kevlar fibers occurs at a twist multiplier of 1.1. At about this value, the strength of bare yarn is the highest and the modulus is only slightly decreased from the

2 16 Organic fibers

untwisted level (DuPont, 1992b).] Twist will make the fiber easier to handle, make subse- quent weaving or braiding operations easier and will improve the abrasion resistance of the fiber. It is also required for rope and cable applications. However, once the fiber is used in composite matrix fiber, twist is not desirable. This is because twist interferes with full impregnation of the fiber with resin and with stress transfer between adjacent fiber bundles. It also increases stress concentrations, particularly at higher twist levels. For this reason, most of the aramid fiber manufacturers supply most or all of their fibers untwisted or with minimal twist.

10.2.6 FORMS AND AVAILABILITY

Table 10.6 lists most of the commercial types of para-aramid fibers. Some of these fibers are readily available in a variety of fiber deniers, package sizes, finishes and so forth, while others are available only in limited quantities for specific applications. Due to constant changes in market conditions and other factors, the user is advised to check with the fiber manufacturer concerning current availabilities. The mechanical properties of fibers with different deniers and/or finishes and other treatments will vary somewhat from each other and from the nominal values given in Table 10.2.

In addition to the yarns, tows and rovings listed in Table 10.6, DuPont’s Kevlar fibers are also available as staple (short fibers), floc (pre- cision cut fibers of very short lengths) pulp (very short and high fibrillated fibers) and in specialty compounded forms (DuPont, 1992a, 1993h). A variety of fabrics are also produced. In addition, DuPont produces a colored fiber, Kevlar 100, in sage green, yellow, black and royal blue (Yang, 1992). Both Teijin’s Technora and Akzo’s Twaron fibers are available as staple and chopped fiber (very short lengths) and in a variety of fabrics (Teijin, 1989; Akzo, 1991). Technora is also marketed in black as well as natural color. Kaiser VIAM‘s Armos and SVM fibers are also expected to be offered as tape,

staple, pulp and in various fabrics. While the meta-aramid fibers are not usu-

ally used as fiber reinforcements in composites, they are used extensively as reinforcements for honeycomb sandwich core materials. The use of such materials along with composite face sheet panels has greatly extended the overall usage of composite materials, particularly in the aerospace industry.

Information about the availability and package sizes of the fibers shown in Table 10.6, about other products and about special formulations can be obtained from Table 10.7. At the time of this publication, Kaiser VIAM’s Armos and SVM fibers are just being imported from Russia. For this reason, information on the fibers and their availability is limited in this chapter, but should be readily available later from the contact given in Table 10.7.

Pricing

Para-aramid fibers are currently priced from about $20 per pound for the larger denier fibers to about $60 per pound for most of the small denier, higher modulus fibers. (However, some of the very fine denier specialty fibers from some manufacturers cost hundreds of dollars per pound.) Prices can vary significantly for similar fibers of different deniers or from different manufacturers and thus price quotes should always be obtained before any decision is made upon use of a specific fiber.

10.2.7 DESIGN CONSIDERATIONS

In the 1970s and early 1980s aramids began to replace carbon and glass fibers in many applications. However, the development of high strength intermediate modulus carbon fibers in the mid-1980s and the commercialization of tough, high strength polyethylene fibers reversed this trend. Today aramid fibers are used mainly in applications where they offer a unique combination of properties, such as high specific strength combined with toughness and creep resistance.

Aramidfibers 217

Table 10.6 Availability of commercial para-aramid fibers"

Product Count Filament Comments/typical applications (reference) number/ diameter

~ _ _ _ _ _ _ _ _

dtex (den) yarn pm ( 1 C 3 i n )

Kevlarb (DuPont, 1993g; Yang, 1992) Type 950 1110 (1000) 666 12 (0.48)

1670 (1500) 1000 12 (0.48) 2500 (2250) 1000 15 (0.59) 3330 (3000) 1333 15 (0.59)

Type956 800 (720) 490 12 (0.48) 1110 (1000) 666 12 (0.48) 1670 (1500) 1000 12 (0.48) 2500 (2250) 1000 15 (0.59) 3330 (3000) 1333 15 (0.59)

Kevlar 29 (DuPont, 1992b,1993a, 1993b, 1993f, 19938) Type 960 1670 (1500) 1000 12 (0.48)

3330 (3000) 1333 15 (0.59) 17 OOO(15 000) 100OOR' 12 (0.48)

Type 961 1110 (1000) 666 12 (0.48) 1670 (1500) 1000 12 (0.48) 3330 (3000) 1333 15 (0.59) 5000 (4500) 3000 12 (0.48)

17 OOO(15 000) lOOOOR 12 (0.48)

Type 962 1670 (1500) 1000 12 (0.48) 3330 (3000) 1333 15 (0.59)

Type 963 3330 (3000) 1333 15 (0.59)

Type964 215 (200) 134 12 (0.48) 430 (400) 267 12 (0.48)

1110 (1000) 666 12 (0.48) 1670 (1500) 1000 12 (0.48)

Type 965 61 (55) 25 15 (0.59) 215 (195) 134 12 (0.48) 420 (380) 267 12 (0.48)

1270 (1140) 768 12 (0.48) 1580 (1420) 1000 12 (0.48) 2400 (2160) 1000 15 (0.59)

Type968 215 (195) 134 12 (0.48) 420 (380) 267 12 (0.48)

1270 (1140) 768 12 (0.48) 1580 (1420) 1000 12 (0.48) 2400 (2160) 1000 15 (0.59) 3160 (2840) 1333 15 (0.59) 4800 (4320) 2000R 15 (0.59) 5070 (4560) 3200R 12 (0.48) 7900 (7100) 5000R 12 (0.48)

Kevlar 49 (DuPont, 1992b, 1993c-g)

Finish: tire reinforcement

Mechanical rubber goods: hoses, belts, etc.

Cordage finish: high lubricity for improved abrasion resistance; ropes and cables

Textile finish; ropes and cables

No finish; ropes and cables

Textile finish; non-apparel ballistic armor

Textile finish; ballistics and apparel, ignition cables

Textile finish; woven reinforcement in aerospace composites, ballistic armor, and printed circuit boards

No finish; marine composites, fiber optic cable reinforcement, ropes, filament-wound composites


218 Organicfibers


Product Count Filament Commenfs/ fypical applications (reference) number/ diameter

dtex (den) yarn pm (1Pin)

2400 (2160) 1000 15 (0.59) abrasion resistance; ropes and cables 5070 (4560) 3200R 12 (0.48)

2400 (2160) 1000 15 (0.59) reinforcement 3160 (2840) 1333 15 (0.59) 4800 (4320) 2000R 15 (0.59) 6300 (5680) 2666R 15 (0.59) 7900 (7100) 5000R 12 (0.48) 9500 (8520) 4000R 15 (0.59)

~ _ _ Type 978 1580 (1420) 1000 12 (0.48) Cordage finish: high lubricity for improved

Type 989 1580 (1420) 1000 12 (0.48) Textile finish; fiber optic cable

Kevlar 68 (DuPont, 1992b, 1992c, 19938) Type9568 215 (195) 90 12 (0.48) High performance mechanical rubber goods

Type9898 420 (380) 267 12 (0.48) Textile finish; fiber optic cable

1580 (1420) 1000 12 (0.48)

1580 (1420) 1000 12 (0.48) reinforcement 2400 (2160) 1000 15 (0.59) 3160 (2840) 1333 15 (0.59) 4800 (4320) 2000 15 (0.59) 7900 (7100) 5000R 12 (0.48)

Kevlar 129 (DuPont, 1990, 1993h) Type 956E 1670 (1500) 1000 12 (0.48) Power transmission belts, high-performance

tires, high fatigue applications Kevlar 129 (DuPont, 1992c, 1993h, 1993i)

Type 956C 1110 (1000) 666 12 (0.48) Mechanical rubber goods

Type964C 830 (750) 500 12 (0.48) Personal body armor 930 (840) 6OOL 12 (0.48)

1110 (1000) 666 12 (0.48) 1580 (1420) 1000 12 (0.48)

Kevlar 249 (DuPont, 1992c, 19938) Type965A 420 (380) 267 12 (0.48) Woven reinforcement in aerospace composites,

1270 (1140) 768 12 (0.48) hard ballistic armor, printed circuit 1580 (1420) 1000 12 (0.48) boards

1580 (1420) 1000 12 (0.48) cable reinforcements, ropes, filament-wound 4730 (4260) 3000R 12 (0.48) composites 7890 (7100) 5000R 12 (0.48)

Type 968A 1270 (1140) 768 12 (0.48) No finish; marine composites, fiber optic

Kevlar HT (DuPont, 19938)

Kevlar K M 2 (DuPont, 1992d)

Type 964C 1110 (1000) 666 12 (0.48) Advanced ballistic protection

945 (850) 560 12 (0.48) Ballistic protection: helmets, composite armor


Ararnidfibers 219


Product Count Filameizf Comments/typical applications (reference)

number/ diameter dtex (den) yarn Fnz ( lC3in)

Twarond (Akzo, 1990,1991; DeCos, 1993) Type 1000 420 (380)

840 (760) 1100 (990) 1260 (1130) 1680 (1510) 2520 (2270) 3360 (3020)

Type1001 420' (380) 840 (760)

1100' (990) 1260 (1130) 1680 (1510) 3360 (3020)

Type 1010 1680 (1510) 3360 (3020)

Type 1020 1680 (1510)

Type 1030 17 OOO(15 300)

Type 1031 14 OOO'(12 600)

Type1040 420 (380) 840 (760)

1100' (990) 1260 (1130) 1680 (1510)

Type 1041 1260' (1130) 1680' (1510)

Type2000 930 (840)

1680 (1510)

Type 1055 1210 (1090) 1610 (1450) 2420 (2180) 3220 (2900) 4830 (4350) 6440 (5800) 8050 (7245)

Type 1056 1210 (1090) 1610 (1450) 2420 (2180) 6440 (5800) 8050 (7245)

1110 (1000)

Twaron HM (Akzo, 1991)

250 12 (0.48) Standard finish; multipurpose 500 12 (0.48) 750 10.5 (0.41) 750 12 (0.48)

1000 12 (0.48) 1500R 12 (0.48) 2000R 12 (0.48) 250 12 (0.48) Adhesive-activated finish; tires, 500 12 (0.48) mechanical rubber goods, composites 750 10.5 (0.41) 750 12 (0.48)

1000 12 (0.48) 2000R 12 (0.48)

1000 12 (0.48) Very low finish level; composites 2000R 12 (0.48) 1000 12 (0.48) Special finish for increased abrasion

5000R 12 (0.48) PTFE + silicone oil impregnated; braided packings

5000R 12 (0.48) PTFE + silicone oil impregnated; braided packings

resistance; cables, ropes, nets

250 12 (0.48) Tangled yarn; multipurpose 500 12 (0.48) 750 10.5 (0.41) 750 12 (0.48)

1000 12 (0.48)

750 12 (0.48) Adhesive-activated finish; fabrics 1000 12 (0.48)

1000 6.6 (0.26) Standard finish; high tenacity for 1000 8 (0.31) ballistic applications. 930 dtex 1000 12 (0.48) fiber is 'microfilament'.

750 11.5 (0.45) Standard finish; multipurpose 1000 11.5 (0.45) 1500R 11.5 (0.45) 2000R 11.5 (0.45) 3000R 11.5 (0.45) 4000R 11.5 (0.45) 5000R 11.5 (0.45)

1000 11.5 (0.45) 1500R 11.5 (0.45) 4000R 11.5 (0.45) 5000R 11.5 (0.45) Continued on next page

750 11.5 (0.45) Very low finish level; composites

220 Organic fibers


Product Count Filament Comments/typical applications (reference) number/ diameter

-

dtex (den) yarn pm (1Win)

Twaron IM (Akzo, 1991) Type 1111 420' (380) 250 12 (0.48) Easily removed finish; fiber optic cable

1260 (1130) 750 12 (0.48) reinforcements, ballistics, composites 1680 (1510) 1000 12 (0.48) 2520 (2270) 1500R 12 (0.48)

Armos (Kaiser VIAM, 1993a)

SVM(Kaiser VIAM, 1993b-j)

588 (530) - - - Twisted 48 t/m; multipurpose

63 (57) - - - Type A1 lubricating finish 143 (130) - - - Lubricating finish on 'acidic' fiber 294 (265) - - - Lubricating finish on 'neutral' fiber 294 (265) - - - Lubricating finish on 'acidic' fiber 294 (265) - - - Type A1 lubricating finish on 'acidic' fiber 588 (530) - - - Two different heat treatments available 588 (530) - - - Type A1 lubricating finish on 'acidic' fiber

Technoru' (Teijin, 1989; Mahn 1993) T-200 1110 (1000)

1670 (1500)

T-202 440 (400) 1670 (1500)

T-220 1110 (1000)

T-221 1110 (1000)

1670 (1500)

1670 (1500)

T-230 1670 (1500)

T-240 60 (55) 110 (100) 220 (200)

1110 (1000) 440 (400)

T-241 1670 (1500) 8330 (7500)

T-360 608 (55)

440 (400)

1670 (1500)

220 (200)

1110 (1000)

T-370 220 (200) 440 (400)

For footnotes see next page

666 1000

1667 1000

666 1000

666 1000

1000

36 67

133 267 666

1000 5000R

36 67

133 267

1000

133 267

12 (0.48) 12 (0.48)

12 (0.48) 12 (0.48)

12 (0.48) 12 (0.48)

12 (0.48) 12 (0.48)

12 (0.48)

12 (0.48) 12 (0.48) 12 (0.48) 12 (0.48) 12 (0.48)

12 (0.48) 12 (0.48)

12 (0.48) 12 (0.48) 12 (0.48) 12 (0.48) 12 (0.48)

12 (0.48) 12 (0.48)

Rubber reinforcement

Rubber reinforcement, pre-activated type

Rope, cable, and cord

Rope, cable, and cord

Fiber-reinforced plastics, rope

Woven and knitted fabrics, fiber-reinforced plastics

Woven and knitted fabrics, fiber-reinforced plastics

'Spunnized' yarn (made up of long but not continuous filaments) for protective clothing and other fabric applications

High tenacity 'spunnized' yarn for reinforcement of rubber, etc.

Aramid fibers 221

Table 10.7 Sources of information on commercial aramid fibers

Product Information source Armos and SVM fibers

Kevlar fibers

Kaiser VIAM; 880 Doolittle Drive, San Leandro, CA 94577, USA

DuPont Fibers; P.O. Box 80705, Wilmington, DE 19880-0705, USA, (800)

Teijin Limited, 11, 1-chome, Minamihonmachi, Chuo-ku, Osaka 541, Japan Teijin America Inc; 10 East 50th Street, New York, NY 10022, USA

Akzo, Aramide Maatschappij v.o.f., P.O. Box 9300,6800 SB A r h e m , Westervoortsedijk 73, The Netherlands Akzo Fibers Inc., 801-F Blacklawn Rd., Conyers, GA 30207, USA

4-KEVLAR

Technora fibers

Twaron fibers

The outstanding toughness of aramids is often the reason they are used over cheaper, stiffer or even stronger fibers. Unlike glass and carbon composites, aramid composites loaded in compression, flexure or shear fail in a non-brittle manner, with significant work being required to fail the composite. Their fatigue resistance is also excellent. If other concerns such as cost or stiffness preclude the use of aramid composites, aramids are often used as a hybrid with another fiber to improve the toughness of the composite.

The poor off-axis and compressive properties of aramid fibers must be considered in any design. However, because of their high strength in axial tension and their toughness, aramid fibers are often outstanding in applications such as pressure vessels where the loading is almost totally in longitudinal tension.

Aramid fibers absorb moisture. Where either the physical swelling of the fiber or the amount of moisture absorbed is of significant concern, one of the lower absorption aramids, such as Kevlar 149, Armos, or Technora should be considered.

Aramids are strong UV absorbers and dete- riorate when exposed to ultraviolet light. Protective coatings or the self-screening ability of the fiber should be used to avoid deterio- ration.

Aramid fibers are opaque and thus the penetration of resin into the fiber bundles cannot be determined visually for a aramid composite as it can for those made with glass fibers.

In fabric applications the weave used is important to the resulting properties. The same is true for sandwich construction. In these cases, the fiber, fabric, or honeycomb supplier can provide design assistance.

The choice of resin system for use with aramid fibers is an important one. Epoxy resins give better translation of fiber properties than do polyesters, producing better shear strength and flexural properties, but lower impact resistance. Vinyl ester resins give both good shear strength and impact resistance. Thermoplastic matrices are also used, particularly in chopped fiber composites, because of their improved impact resistance over thermosets. However,

Footnotes for Table 10.6 a All availabilities are subject to market conditions and should be verified with the manufacturer.

All Kevlar fibers are supplied untwisted. E The ' R indicates that this fiber is a 'roving,' meaning in this case that it is composed of more than one 'end' of yam.

Twaron fibers are normally supplied untwisted. In some circumstances twist may be supplied on special request. e Under development. ' Technora fibers are supplied with twist as requested. Finishes are supplied as requested or as is appropriate to the

application. For special applications, Technora fibers can be supplied in larger than 12 pm filament diameters. These fibers, and others 'spun' yarns (composed of discontinuous filaments) are normally measured by '(English) cotton count' (ECC) rather than dtex or denier, where ECC = 5315/denier.

222 Organic fibers

for thermoplastics the penetration of the resin into the fiber bundle and the quality of the fiber-matrix bond is almost always of concern.

Because aramids are very tough fibers, they are somewhat difficult to cut and their composites can be difficult to machine. Special shears and other tools are available for cutting aramids and many successful machining techniques have been developed. The fiber manufacturers are an excellent source of information in this area.

As with all high performance fibers, aramids should be handled with care before and during processing. Rough handling will damage any high performance fiber. In addition, because of their sensitivity to ultraviolet light, aramids should be protected from such exposure. The fibers also should not be exposed to excessive moisture prior to processing. If the fiber is to be twisted, braided, or woven, it is preferable to condition the fiber for one to two days at room temperature and intermediate moisture content prior to processing (DuPont, 1993h). However, if the fiber is to be resin-impregnated and processed directly into a composite, so long as fiber handling is careful, superior properties may be attained by drying the fiber prior to processing. Tlus is because of improved bonding of resin to the filament surfaces.

Aramid fibers present minimal safety or environmental concerns. In lifelong animal inhalation studies with Kevlar fibers, no health effects were observed at any workplace levels. Nonetheless, as with any textile fiber, inhalation of fibrous particles should be avoided. Extensive animal and human skin patch tests with Kevlar fibers have shown no sensitivity and little irritation, and rat feeding studies have shown oral toxicity to be very low. Combustion by-products are similar to wool. Aramid yarns are also essentially inert in the environment (DuPont, 1993h).

10.2.8 APPLICATIONS

Aramid fibers are used in numerous applications, some of which are listed in Table 10.6.

Many of these are not as structural composites. For example, aramids are used in many rope and cable applications. In mooring ropes to secure oil tankers and to anchor off-shore oil platforms, the lighter weight compared to steel makes the aramid ropes much easier to handle. In addition, they do not corrode, are easier to maintain and have an extension under load which is far superior to both steel and other organic fibers.

Aramids are widely used to reinforce mechanical rubber goods. The largest volume of such usage is in pneumatic tires, where aramids are lighter than steel and offer higher strength and modulus than other organic fibers. Significant usage is also seen in belts and hoses. The excellent fatigue and creep resistance of aramids are important factors in their usage in these applications. Corrosion resistance and electrical resistivity may also be important. Aramids are also used in athletic shoes and in rubberized sheet materials as used in aircraft evacuation slides and life rafts.

In some cases, non-composite applications have led to composite uses. For example, aramids have long been used in soft body armor, where the fibers absorb and disperse bullet impact energy to other fibers in the fabric weave. This application has now seen a derivative usage in rigid composite ballistic armor, composite helmets and composite spa11 liners. In these applications the toughness, RF transparency and fire and corrosion resistance of aramid fibers were significant factors in their selection.

In spite of significantly higher fiber costs than glass, aramids are used in canoes, kayaks, racing shells and small boats where maximiz- ing strength and minimizing weight are important. Aramids offer weight savings for superior speed and better handling and/or improved range and fuel economy. Toughness and overall durability and vibrational damping are also superior with aramids. The superior properties of aramids allow boats to be built at an overall cost only 10-15% higher than with glass fibers and with superior perfor-

Extended h i i i polyefhylcviefibers 223

mance (DuPont, 1983). These same properties have led to the use of aramids in skis.

Their high strength-to-weight ratio combined with outstanding toughness has led to numerous applications of aramids in aerospace. In both civilian and military aircraft, the toughness of aramids - and resulting resistance to damage from impacts ranging from bird strikes to shrapnel - insures their continued usage. Engine nacelles and the tail cone on the McDonnell Douglas DC-9-80 are made from Kevlar composites and approximately 10% of the empty airframe weight of De Havilland Aircraft's DASH-8 turboprop com- muter aircraft is Kevlar composite. Aramid composites are also widely used in rotorcraft and other vertical lift aircraft.

10.2.9 CONCLUSIONS

Although composites of other fibers have now supplanted aramid composites as having the highest specific strengths, aramids still offer combinations of properties not available with any other fiber. For example, aramids offer high specific strength, toughness and creep resistance, combined with moderate cost. However, the applications of aramid composites continue to be limited by their poor compressive and off-axis properties and in some applications, their tendency to absorb water. Nonetheless, aramids will continue to be the fiber of choice where properties such as outstanding impact resistance combined with creep resistance are critical.

10.3 EXTENDED CHAIN POLYETHYLENE FIBERS

10.3.1 OVERVIEW

High performance polyethylene fibers, with outstanding strength-to-weight and stiffness- to-weight performance, show promise in various specialized applications. While such fibers are not as widely known as aramid and carbon fibers, they possess many superior

properties but they also have limitations that must be considered in design.

Commercially available high strength, hgh modulus polyethylene fibers include Spec trakh' fibers from Allied-Signal Corporation, DyneemaO SK60 from Dyneema Vof, Tekmilon" from Mitsui Petrochemicals and a new, as yet unnamed, fiber from Hoechst Celanese.

10.3.2 MANUFACTURE

The traditional method of producing fibers from polyethylene is to spin them from a polymer melt. This technique yields fibers composed of folded-chain crystalline regions with non-crystalline regions interspersed. With extraordinary means, the modulus of the absolute best of such fibers can be brought to about 80 GPa (11.5 Msi). It was long recognized, however, that if polyethylene could somehow be produced with extended chain crystallinity, a very high modulus fiber would result. [The theoretical modulus for polyethylene is 320 GPa (46 Msi) (Adams and Eby, 1987).]

Following earlier work by Pennings, in the late 1970s Smith and Lemstra of DSM (The Netherlands) developed a process with commercial potential which yielded a highly oriented extended-chain polyethylene fiber (Hongu and Phillips, 1990). At the same time, both Toyobo Inc. of Japan and Allied Chemical Company in the USA were working on a similar approach. DSM, however, was the first to patent the process and both Toyobo and Allied judged it impossible to circumvent the basic patent filed by DSM. Thus, both companies entered into technical association with DSM to produce polyethylene fibers. Toyobo Inc. linked with DSM to form the joint venture - Dyneema Vof - to produce and market the new fiber. In the USA, Allied-Signal is licensed from DSM/Stamicarbon to produce and market a similar fiber.

The process which is used to produce most commercial high strength, high modulus polyethylene fibers is called gel spinning, the name derived from the gel-like appearance of the

224 Organicfibers

as-spun and quenched fibers. Ultra-high molecular weight linear polyethylene is dissolved in a volatile solvent to form a dilute isotropic solution that is then spun through a spinneret and quenched in cold water to form a gel precursor fiber. Following solvent extraction, this fiber is then hot-drawn to a very high draw ratio (= 30), yielding a very highly oriented, highly crystalline, lightweight fiber (Dyneema, 1987; Jaffe, 1989; Ward and McIntyre, 1986; Yang, 1992).

Another approach to producing a high strength polyethylene fiber is melt extrusion followed by multiple stage drawing of a much lower molecular weight polyethylene. The modulus of an experimental fiber of this type, 220 GPa (32 Msi), is the highest ever achieved for polyethylene (Adams and Eby, 1987). The new polyethylene fiber from Hoechst Celanese is the only commercial version of such a fiber. This fiber has only about 50% of the strength and 75% of the modulus of gel spun fibers. In this case, the expense of dealing with a volatile and potentially toxic solvent is avoided, low- ering the overall price of the fiber significantly.

10.3.3 STRUCTURE

Figure 10.8 illustrates the difference between conventional polyethylene fibers and gel-spun or melt-extruded and drawn extended-chain fibers. Figure 10.8(a) is a schematic of a conventional melt-drawn polyethylene fiber. The fiber consists of folded chain crystallites, mostly oriented in the draw direction, which are joined to one another by tie molecules and have between them interspersed non-crystalline material. Figure 10.8(b) is a schematic of a gel-spun and hot-drawn extended-chain polyethylene fiber. Such fibers show minimal chain folding, high crystallinity and a very high degree of axial orientation (>95%).

Since these fibers are based on polyethylene, they have a density of only two-thirds that of aramid fibers and about one-half that of carbon fibers. However, the polyethylene crystallites have neither relatively strong

Fig. 10.8 Schematic illustrating the difference between (a) conventional polyethylene fibers and (b) gel-spun extended chain fibers.

hydrogen bonds nor strong covalent bonds between them. They are, in fact, held together by weak dispersion-type van der Waals bonds which have a distinct effect upon properties.

10.3.4 PROPERTIES

Polyethylene fibers offer a unique combination of properties: low specific gravity, high specific modulus, high specific strength, high energy to break, high abrasion resistance, excellent chemical resistance, good ultraviolet resistance and low moisture absorption. They have outstanding anti-ballistic and vibrational damping characteristics, as well as a low dielectric constant. However there are trade- offs involved in the use of polyethylene fibers. They are limited to fairly low use temperatures,

Extended chain polyethylene fibers 225

they produce composites with poor off-axis and compressive properties and have poor creep resistance.

As with aramid fibers, the anisotropy of their microstructure gives polyethylene fibers anisotropic mechanical, thermal and physical properties which can be used to advantage in some applications.

Physical and thermal properties

Polyethylene fibers have a relatively low melting point [147"C (297"F)I and thus a low use temperature. In general, polyethylene fibers are limited to use below 100°C (212°F). They will, however, tolerate brief exposure (30 min or less) at temperatures near the melting point without major property loss (Dyneema, 1987; Weedon and Tam, 1986).

As would be expected from the lower melting temperature, the properties of polyethylene fibers are much more sensitive to temperature than are aramids. Like aramid fibers, polyethylene fibers contract with temperature in the axial direction, while expanding in the transverse direction. The thermal expansion coefficient of a composite of 60 vol% plasma-

treated Spectra 900 fiber in an epoxy matrix was found to be -9 x lO"/OC (-5 x 104/OF) in the axial direction and 100 x lO"/"C (56 x 104/"F) in the transverse direction. The axial thermal expansion coefficient of a similar composite of Spectra 1000 fiber was -10 x lO"/"C (-5.6 x 10"/OF) and the transverse coefficient was 105 x lO"/OC (58 x 10"/"F) (Allied, 1989).

Polyethylene fibers are the only high performance fibers with a specific gravity of less than 1 and thus are the only fibers that float. Their density is about two-thirds that of aramid fibers and about half that of carbon fibers. Polyethylene fibers will burn slowly if ignited, decomposing into carbon dioxide and water.

The filament diameters of commercial polyethylene fibers are relatively large, typically 23-38 pm (0.91-1.50 x in), although the diameter of Mitsui's Tekmilon monofilament fibers can be as large as 121 pm (4.76 x in). The filament cross-section is typically irregu- lar and somewhat elliptical.

Mechanical properties

Gel-spun polyethylene fibers offer some tremendous advantages over other fibers. As

Table 10.8 Axial tensile mechanical properties of representative high performance polyethylene fibers

Fiber

-

Dyneema SK60

Hoechst Celanese fiber

Spectra 900

Spectra 1000

Tekmilon monofilament

Tekmilon multifilament

Reference Fiber type

Dyneema, 1987 gel-spun

Hoechst melt- Celanese, 1993 extruded

Allied, 1993 gel-spun

Allied, 1993 gel-spun

Mitsui, 1989 gel-spun

Mitsui, 1989 gel-spun

Spec@ gravity

0.97

0.96

0.97

0.97

0.96

0.96

Tensile modulus,

GPa (Msi)

87 (12.7)

55 (8.0)

86-103 (12.5-14.9)

128-171 (18.6-24.8)

59-98 (8.6-14.2)

88.3 (12.8)

Tensile strength,

MPa (ksi)

Elongation at break, %

2620 (380)

(189) 1300

2080-2400 (300-350)

2740-3000 (397435)

1470-3430 (213498)

2450 (356)

-

4

3.6-3.7

2.8-3.1

4-6

3

226 Organic fibers

can be seen in Fig. 10.4, these fibers offer very high specific stiffnesses and specific strengths, equivalent or superior to all of the aramid fibers and to most of the carbon fibers. This superior performance is offered at a lower price than that of competitive fibers.

Table 10.8 compares the mechanical properties of representative commercially available polyethylene fibers.

Like aramid fibers and for similar reasons, polyethylene fibers have poor compressive and off-axis properties. Since the fiber is held together internally by only very weak van der Waals bonds, the transverse strength of the fiber is even worse than that for the aramids. In addition, the inertness of the polyethylene fiber means that the untreated fiber bonds

very poorly to a matrix. Although gas plasma surface treatment can improve the interfacial bond strength significantly, polyethylene fiber composites will still have poor off-axis properties.

Table 10.9 gives mechanical properties for Spectra fiber composites, including those made from plasma-treated fibers.

In spite their weak transverse strength, but because of the non-stick nature of polyethylene and thus its low coefficient of friction, polyethylene fibers perform much better than aramids in abrasion resistance and polyethylene fabrics are much less easily damaged than are those of aramid fibers. The abrasion resistance of polyethylene fibers can be up to ten times that of aramids (Dyneema, 1987) and can be improved

Matrix: Bisphenol A based epoxy __ -

Spectra 900 Spectra 900 P T ~ Spectra 1000

Table 10.9 Mechanical properties of Spectra polyethylene fiber composites” (Allied, 1989)

~

- Spectra 100 PT

Axial Tensile Properties: Volume percent fiber Modulus, GPa (Msi)

Strength, MPa (ksi)

Elongation, %

Volume percent fiber Modulus, GPa (Msi)

Axial Compressive Properties:

Strength, MPa (ksi)

Elongation, YO

Flexural Properties: Volume percent fiber Modulus, GPa (Msi)

Strength, MPa (ksi)

Short Beam Shear Properties: Volume percent fiber Strength, MPa (ksi)

58 27 f 1

(4.0 f 0.1) 552 zk 90 (80 f 13) -

70 32 f 5

(4.7 k 0.7) 52 k 2

(7.5 f 0.3) -

58 22 k 1

(3.2 f 0.2) 145 f 7 (21 k 1)

58 8.3 f 0.7

(1.2 f 0.1)

50 24 f 1

676 f 103 (98 * 15) 3.6 f 0.2

(3.5 f 0.1)

70 40 k 5

(5.8 + 0.7) 59 + 1

(8.6 c 0.2) -

54 3 0 f 1

(4.3 f 0.2) 200 f 7 (29 f 1)

54 28.3 f 0.7 (4.1 f 0.1)

53 50 (7.3)

1034 c 228 (150 f 33)

-

55 1 9 c 6

(2.7 f 0.9) 72 f 3

(10.5 f 0.4) 3.8 k 0.5

54 23 zk 1

159 +. 7 (23 f 1)

(3.3 k 0.2)

54 9.0 c 0.7

(1.3 f 0.1)

54 50 f 3

(7.2 f 0.5) 889 f 55

(129 f 8) 2.1 i-0.4

65 54 f 3

(7.8 f 0.4) 69 f 1

3.8 f 0.2 (10.0 f 0.2)

53 38 f 3

(5.5 f 0.5) 214 f 7 (31 k 1)

53 21 e 3

(3.1 f 0.4) a Numbers of specimens tested and criteria for limits not specified.

PT indicates a fiber with gas plasma surface treatment.


100

0 75 a c)

ul 3 3

z

- 50

25

0

even further by the use of lubricants. Because of their high strength, polyethylene

fibers exhibit very high energy to break. On a per-weight basis, the impact energy absorption of polyethylene composites is superior to that of all other fiber composites.

Polyethylene fibers are more affected by temperature than are higher melting point fibers. The loss in modulus as function of temperature is shown in Fig. 10.9 for Tekmilon multifilament fiber and Spectra fibers. Fig. 10.10 shows the loss in strength as a function of temperature for Tekmilon multifilament, Spectra 900 and Spectra 1000 fibers. Because of their very high specific strength at room temperature, however, polyethylene fibers still outperform most other fibers to about 100°C (212°F).

Room temperature strength retention of polyethylene fibers following annealing at temperatures of up to 125°C (260°F) is excellent, while modulus loss following such

r L 1 . 8 8 I , I I I I , , I S *

- 15 -

- .- ul

i - 10 =

- 3 - U 0 I

- 5 -

~ ~ . l l ~ l . ~ ' ~ ~ ~ ~ l ~ ~ ~ ~ I * l l l l ~ . ~ ~ 0

exposure is 20-30%. The loss in both modulus and strength are reduced if annealing is per- formed under tensile loading.

Unlike aramid fibers and their composites, polyethylene fibers and composites show very little or no loss of properties, axial or off-axis, when exposed to moisture.

Creep resistance of extended-chain polyethylene is of concern. Because of its low melting temperature, the resistance of the fiber to creep, even at room temperature, is less than ideal. This is significant, since the creep of carbon, glass and aramid fibers is minimal. Spectra 1000 is a 'stabilized' version of the fiber, which shows better creep resistance than the Spectra 900 fiber. Figure 10.11 shows the creep response of the two Spectra fibers at room, elevated and low temperatures. At low load levels at room temperature and/or at low temperatures the creep encountered is not severe, especially for the Spectra 1000 fiber, but at higher loads or temperatures the creep is much

Fig. 10.9 Modulus as a function of temperature for Spectra 900, Spectra 1000, and Tekmilon multifilament polyethylene fibers (Prevorsek, 1989; Mitsui, 1989).

228 Organic fibers

Temperature, O F

50 100 150 200 250 300 500

3000 -

2500 - 0

% 2000 1 f 5 1500 1 i7l

VI

1000 -

500 -

- 400

.- - 300 -$

f

e - 200 3;

0, C

- 100

0 0 25 50 75 100 125 150

Temperature, O C

Fig. 10.10 Strength as a function of temperature for Spectra 900, Spectra 1000, and Tekmilon multifilament polyethylene fibers (Allied, 1991e; Mitsui, 1989).

- Spectra 900

RT, 10% Load ___-- ------ . 0 ~ ~ ~ ~ ~ ~ * ~ ~ ~ ~ ~ * ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

0 10 20 30 40 50 60 70 80 90 100

time, h

<a)

Fig. 10.11 Creep of Spectra extended chain polyethylene fibers (a) at room temperature and 10% of static ultimate and at room temperature and 30% of static ultimate.


- Spectra 900 .___ Spectra 1000

6: t C .- e G 1

0 25 50 75 100 125 150 175

time, h

(a)

time, h

< C )

Fig. 10.11 (Continued) Creep of Spectra extended chain polyethylene fibers (b) at 5°C (41°F) and 20% of static ultimate, and (c) at 70°C (160°F) and 275 MPa (40 ksi), which is 18% of static ultimate for Spectra 900 and 11% of static ultimate for Spectra 1000 (Allied, 1991a, 1991b, 1991c, 1991d).

230 Organic fibers

more significant. The creep of polyethylene fibers does not preclude their use in applications such as sailcloth or structural reinforcement, but does require that the creep demands of an application be carefully evalu- ated. Because of their relatively poor creep resistance, polyethylene fibers are often hybridized with other, more creep resistant fibers in applications where prolonged loading is anticipated.

The fatigue resistance of polyethylene fibers is excellent. In one test of loading and unload- ing of ropes, polyethylene fiber ropes withstood approximately eight times the cycles before break as aramid fiber ropes (Weedon and Tam, 2986). T ~ E indicates a superiority in tensile fatigue even to aramid fibers, which are known for their excellent fatigue resistance.

Chemical and environmental properties

Polyethylene is inert. It is stable in almost all organic solvents and in a variety of other chemicals. It is also biologically inert. It is the best of all high modulus fibers in an alkaline environ-

ment (Jaffe, 1989). It shows superior chemical resistance to PPTA in hydrochloric, nitric and sulfuric acid (Dyneema, 1987). Table 10.10 compares the chemical resistance of Spectra polyethylene fiber to that of aramid fiber.

Polyethylene fibers also show good resistance to UV exposure. After 100 hours UV exposure in a fadeometer, Dyneema SK60 retained 70% of its original strength and after 1500 hours, retained 25% strength. This latter exposure is equivalent to about 2 years of outdoor exposure (Dyneema, 1987).

Polyethylene fibers are hydrophobic and thus absorb very little moisture. The moisture regain of polyethylene fibers is less than 1%. Their weatherability is excellent: after 600 hours exposure in a Weatherometer, Tekmilon fiber retained 80% of its strength and 90% of its modulus. Following similar exposure, an aramid fiber had only 40% strength retention (Mitsui, 1989). Because of their excellent chemical and moisture resistance, articles made with polyethylene fibers can be cleaned in soap and water.

Table 10.10 Comparison of strength retention after chemical immersion for polyethylene and aramid fibers (Allied, 1989)

Strength retention, YO

Chemical

Spectra Aramid

6 months 2 years 6 months 2 years (4380 h) (1 7 500 h) (4380 h) (1 7 500 h)

Sea water Hydraulic fluid Kerosene Gas o I in e Toluene Glacial acetic acid 1M hydrochloric acid 5M sodium hydroxide Ammonium hydroxide (29%) Perchloroethylene 10"/0 detergent solution Chlorine bleach

100 100 100 100 100 100 100 100 100 100 100 91

100 100 100 100 100 100 100 100 100 100 100 73

100 100 100 93 72 82 40 42 70 75 91 0

98 87 97

a

Too weak to test

Electrical and optical properties

Polyethylene fibers are electrically non-con- ductive. The dielectric constant of Spectra fiber is 2.2, with a loss tangent of 2 x lo4 (Allied, 1993). This compares to dielectric constants for aramid fibers of 3.85, quartz fibers of 3.78 and E-glass fiber of 6.31 (Allied, 1989). They are white in color and are transparent to X-rays, radar and sonar.

10.3.5 TREATMENTS

Because of their chemical inertness, polyethylene fibers bond poorly to matrices, with consequent negative effects upon the mechanical properties of their composites. Surface treatment by acid etch, plasma etch or corona discharge can significantly improve the

Table 10.11 Availability of commercial polyethylene fibers


bonding of the fiber to matrices. Through 1992, Allied-Signal marketed plasma-treated Spectra fibers. However, polyethylene surface treatments are available after fiber purchase from specialty companies. The fiber manufacturers can suggest sources of these services.

The inertness and high abrasion resistance of polyethylene fibers means that little or no finish is required. This is fortuitous since adhesion to polyethylene is so difficult that finishes generally will not stay on the fibers. All Spectra fibers are supplied with ’low percent process finishes’ whose purpose is simply to aid in holding the filaments together in the bundle.

Like aramid fibers, polyethylene fibers may be twisted, particularly for applications such as marine cables. For bare fiber, twist initially increases the strength, although it decreases the modulus of the fiber. For Spectra fiber, the

Product (reference)

Dyneema SK60 (Dyneema, 1987)

Hoechst Celanese fiber (Adams, 1993)

(Allied, 1990, 1993) Spectra 900

Spectra 1000 (Allied, 1990, 1993)

Tekmilon (Mitsui, 1989) Monofilament

Multifilament

Count

dtex (den)

Fihmen t Corn men t s

number/ diameter yarn pm (IO”in)

444 (400) 888 (800)

1780 (1600)

100 (90)b 200 (180)

722 (650) 1333 (1200) 5333 (4800)

239 (215) 417 (375) 722 (650)

1444 (1300)

5.6 (5) 22.2 (20) 111 (100)

1110 (1000) 555 (500)

60 38 (1.50) All Spectra fibers are supplied 120 38 (1.50) untwisted and with only sufficient 480 38 (1.50) finish to hold the fiber bundle

together

60 23 (0.91) 60 30 (1.18)

120 28 (1.10) 240 28 (1.10)

1 27 (1.06) 1 54 (2.13) 1 121 (4.76)

50 =38 (~1.5) 100 =38 (~1.5)

a All availabilities are subject to market conditions and should be verified with the manufacturer. This fiber is newly commercially available. It will be supplied in multiples of 100 dtex.

232 Organicfibers

strength of the bare fiber is optimized at a twist multiplier of 3, but at this value there is also about a 20% loss in modulus. As with aramids, however, twist is not desirable in fibers used in composites. Polyethylene fiber manufacturers provide most of their fibers untwisted.

10.3.6 FORMS AND AVAILABILITY

Table 10.11 lists the availability of most of the commercial high performance polyethylene fibers. In most cases, each denier of fiber is available in only one package size and weight. The mechanical properties of different deniers may vary somewhat from the nominal values given in Table 10.9.

In addition to the fiber forms listed in Table 10.11, Dyneema SK60 is available in various fabrics. Spectra fibers are also available as chopped fiber in lengths from 6 to 20 rnm (0.25 to 0.8 in) and in fabrics of various weaves. Tekmilon is available as monofilament, multifilament and tape.

Information about the availability of the fibers shown in Table 10.11; information about other products and about special formulations can be obtained from the sources given in Table 10.12.

Pricing

Spectra fibers are currently priced from about $15/lb for larger denier fibers to about $45/lb for the small denier fibers. Dyneema fibers are not marketed in the USA. Because of the strong yen, at the time of writing Tekmilon fibers are significantly more expensive than Spectra fibers in the USA. The new melt-extruded polyethylene fiber from Hoechst Celanese is designed to offer good properties at a lower cost than the gel spun fibers. It costs 15-50% less than the comparable Spectra fibers.

10.3.7 DESIGN CONSIDERATIONS

Like aramid fibers, polyethylene fibers are mainly used in applications where they offer a unique combination of properties. This property combination includes outstanding specific strengths and stiffnesses, high toughness, outstanding abrasion resistance and very low density.

In any design, the relatively low melting point and low use temperature of polyethylene fibers as well as the relatively poor creep resistance must be considered. The creep resistance is improved in a fiber such as

Table 10.12 Sources of information on commercial polyethylene fibers

Product Information source

Dyneema fibers Dyneema Vof, Dr. Nolenslaan 119A, PO Box 599,6130 AN Sittard, The Netherlands Dyneema Japan Ltd., 2-8, Dojima Hama 2-chome, Kita-ku, Osaka 530,

Hoechst Celanese Corporation, PO Box 32414, Charlotte, NC 28232-2414, USA

Japan

Hoechst Celanese’s high performance polyethylene fiber

Spectra fibers

Tekmilon fibers

Allied Fibers, Allied-Signal Inc., High Performance Fibers Technical Center, PO Box 31, Petersburg, VA 23804, USA Mitsui Petrochemical Industries Ltd., Advanced Materials and Products Department, Kasumigaseki Bldg., 2-5, Kasumigaseki 3-chome, Chiyoda- ku, Tokyo 100, Japan Mitsui Petrochemicals (America), Ltd., 1000 Louisiana, Suite 5690, Houston, TX 77002, USA


Allied-Signal’s Spectra 1000, which has significantly improved creep resistance compared to Spectra 900. Polyethylene fibers are often hybridized with other more creep-resistant fibers in applications where prolonged loading is anticipated. Without hybridization polyethylene fibers must be limited to applications where long term, high load level, or elevated temperature loading is not anticipated, or where creep is otherwise not of concern.

As with aramids, the poor off-axis and compressive properties of polyethylene fiber composites may also be concern. This is not a problem, of course, in applications where the loading is mainly in axial tension.

The X-ray and radar transparency of the fiber can present significant design advantages. The UV resistance of the fiber is very good and thus does not present a design problem in most applications.

Because of its chemical inertness, polyethylene fibers are almost impossible to dye, although color can be added during the fiber spinning process. As with aramids, polyethylene fibers are optically opaque, so resin penetration within a composite cannot be determined visually.

Polyethylene fibers can be used with a variety of resins, including polyurethanes, epoxies, vinyl esters, polyesters and thermoplastics, so long as the composite can be processed below 120°C (250°F). Polyesters are economical, vinylester resin systems provide outstanding impact properties and epoxies give better translation of structural properties. The preferred thermosetting matrix cure temperature is 93-104°C (200-220°F) (Allied, 1990). The fiber manufacturers can be very helpful in choosing an appropriate resin system for an application.

As with aramids, polyethylene fiber composites are relatively hard to machine and producing a smooth final machined surface requires special techniques. Machining techniques developed for aramid composites can be used successfully, as can hot knife or hot wire cutting (Allied, 1989).

Polyethylene fibers can be damaged by rough handling and should be handled with care before and during processing. They present minimal safety or environmental concerns and most are biocompatible, offering another potential design advantage.

10.3.8 APPLICATIONS

Most of the current applications of polyethylene fibers are not in structural composites. One of the main uses is in ropes and cables, particularly in marine and off-shore applications. The fibers are used because of their high strength, outstanding abrasion resistance (up to ten times that of aramids), low density (since they float), good UV stability, resistance to seawater and high durability. The fibers are also used as marine sewing threads.

In another major application, UV- and water-resistance are again important. Both Spectra and Dyneema fabrics are used, typically with a film coating, in sails. Unlike aramids, polyethylene sail can be folded and repacked numerous times without damage. This latter quality also led to the selection of Spectra fabric for the anchor balloon of the Hilton Earthwinds round-the-world balloon flight project and to their usage in lightweight, durable backpacks.

Polyethylene fabrics are used as filter cloths, where the excellent chemical resistance is a tremendous advantage. Spectra fabric has been used in oil containment and recovery systems following the Persian Gulf war. The fabric is treated so that water passes through it but oils and other floating pollution do not. Because of their excellent biocompatibility, polyethylene fibers are used as sutures and as artificial ligaments. They are also used in surgical gloves, because of their excellent cut resistance, biocompatibility and low absorption of fluids. While the poor temperature resistance rules out the use of polyethylene fibers in thermal protection, they are used in industrial protective clothing and in ballistic protection and impact shields, with or without

234 Organic fibers

a matrix. Allied makes a special non-woven polyethylene fabric called Spectra Shield@, which has alternating unidirectional layers held together by a polymer matrix and gives outstanding performance in such applications.

As composites, polyethylene fibers and fabrics are used in boat hulls, water skis, sailboards, canoes and kayaks. They have been explored for sporting goods ranging from archery bows to ski poles. In all these examples the excellent impact resistance they impart to their composites is another significant advantage. In applications such as skis and tennis rackets, as well as speaker cones, the excellent vibrational damping capability is also an advantage. However, in spite of this superior performance, the much higher cost of polyethylene compared to glass has limited the usage in boat hulls and sporting goods to high end applications such as racing competi- tions where performance and/or safety are more important than cost. In some cases, however, the addition of polyethylene fiber actually lowers the overall cost of the product. One such application is in wrapping ice hockey sticks with Spectra fiber to improve their durability. In this case, the additional cost to wrap the stick is more than offset by the longer life achieved.

Polyethylene fibers are also used to reinforce rubbers and elastomers. In many of these applications, their excellent vibrational damping characteristics are important. However, the temperature sensitivity limits the usage in tires to off-road vehicles (Dyneema, 1987). Because of their X-ray, radar and sonar transparency and low dielectric constant, they have significant potential in applications such as radomes, sonar domes and X-ray tables.

As mentioned before, polyethylene can be hybridized with other fibers to provide significant improvements in impact resistance. Current hybrid applications range from bike frames to impact shields.

CONCLUSIONS

For many applications extended-chain polyethylene fibers are superior to all other fibers, particularly when properties such as toughness, dielectric constant, and/or hydrolytic stability are of concern. However, because of their relatively low melting temperature, polyethylene fibers must be limited to moderate temperatures and to applications where the creep response is acceptable.

For sail-cloth, marine rope, pressure vessels and other applications where the service temperature is not a governing factor, polyethylene fibers can be expected to make serious inroads into or even dominate their respective industries. However, they cannot supplant aramid, carbon, or glass fibers in applications where elevated service temperature or creep resistance are critical.

10.4 OTHER ORGANIC FIBERS

10.4.1 AROMATIC POLYESTER FIBERS

Aromatic polyester fibers are prepared by spinning from a liquid crystalline melt followed by heat treatment to form high strength, high modulus fibers. While many such fibers have been synthesized since the late 1970s, the only fiber commercially available in the USA today is Vectran@ fiber from Hoechst Celanese Inc. The general structural formula of this fiber is shown in Fig. 10.12.

Vectran was developed in the 1970s in response to tire customers who wanted equivalent performance to aramid fibers but at a lower

p: Y

Fig. 10.12 General structural formula of Vectran. aromatic polyester fiber.

Other organic fibers 235

cost. Hoechst Celanese was successful in pro- viding a fiber with the desired performance, but unfortunately the resulting cost was even hgher than the aramids. Shortly thereafter, Hoechst Celanese stopped marketing Vectran fiber and marketed Vectran resin instead. Vectran resin quickly became the material of choice in the electronics and computer industries for small, very close tolerance connectors, plugs and other components. Based upon this success, in 1989 Hoechst Celanese reintroduced Vectran as a fiber product. Since the fiber is more expensive, by 1.5 to 3 times, than aramids, the marketing focus is on areas where aramids do not meet the performance requirements (Adams and Farrow, 1993a).

Vectran is a polyester-polyarylate fiber. Unlike the aramids, Vectran melts at high temperature. It is melt spun on conventional polyester spinning equipment and the as-spun fibers are then heat treated in a sequence of steps (Adams and Farrow, 1993b). It is the only commercially available melt-spun liquid crystalline polymer fiber (Hoechst Celanese, 1990).

Properties

Vectran HS offers a unique combination of properties: high strength, no creep, low moisture absorption, negative coefficient of thermal expansion, good property retention

over a broad temperature range and excellent chemical resistance.

The density of Vectran HS is 1.41 g ~ m - ~ (0.0509 lbs in") (Hoechst Celanese, 1990). It melts at 330°C (636°F). Like aramid fibers, Vectran HS has a negative axial coefficient of thermal expansion. From 20°C (68°F) to 145°C (293°F) its longitudinal linear thermal expansion coefficient is 4 . 8 x lo4 /"C ( -2 .7~ /OF). The coefficient increases to -14.6 x 10" /"C (-8.1 x lo4 / O F ) from 145°C (293°F) to

200°C (392°F) and to -26.7 x lo4 /"C (-14.8 x / O F ) from 200°C (392°F) to 290°C (554°F)

(Beers and Ramirez, 1990). It has good temperature resistance, although not as good as aramid fibers since it melts at high temperature. Its shrinkage in hot air at 177°C (350°F) or in boiling water is less than 0.5%.

Vectran HS fiber is outstanding in its mechanical properties. Its axial mechanical properties are summarized in Table 10.13. Vectran displays no creep when tested for 2760 h at 50% of its ultimate tensile strength. This behavior is significantly better than both aramid and polyethylene fibers. Vectran also has excellent vibrational damping characteristics, better than aramids. Vectran HS has superior abrasion resistance to Kevlar 29, although not as good as polyethylene fibers. In flexural fatigue, Vectran HS braid exhibited a 10% reduction in strength after one million

Table 10.13 Axial tensile mechanical properties of representative non-aramid, non-polyethylene organic fibers

Fiber

Vectran HS

PBO (Dow)

PBO, high modulus (Dow)

Reference Specific gravity

Hoechst Celanese, 1.41 1990

Adams and Farrow, 1.41 1993a

Burk, 1993 1.56

Burk, 1993 1.56

Tensile modulus

GPa (Msi)

64.8 (9.4)

62-86 (9.0-1 2.5)

152 (22)

(40) 276

Tensile strength, MPa (ksi)

2840 (412)

2500-3100 (363450)

5650 (820)

(800) 5520

Elongation at break, %

3.3

2.2-2.5

3.5

1.5

236 Organicfibers

cycles (and maintained this strength level to five million cycles), while a Kevlar 29 braid showed a 30% strength reduction under the same conditions (Beers and Ramirez, 1990).

Vectran absorbs very little water, having a moisture regain of less than 0.1%. It is hydrolytically stable. It has excellent chemical resistance being resistant to organic solvents, to acids at less than 90% concentration and to bases at less than 30% concentration (Hoechst Celanese, 1990).

Vectran's dielectric constant is 3.3 at 1 kHz (Hoechst Celanese, 1990).

Like aramid and polyethylene fibers, Vectran is difficult to cut and its composites are difficult to machine. Typical aramid composite machining techniques can be used successfully.

Forms, availability and treatments

Vectran is produced in the USA by Hoechst Celanese Corporation and in Japan by the Kuraray Corporation under license from Hoechst Celanese. It is available as Vectran HS, a high strength reinforcement fiber, Vectran M, a high performance matrix fiber, and as engineered, commingled combinations of Vectran HS with Vectran M and S-2 Glass fiber with Vectran M.

Vectran HS fiber is available in dtex (deniers) of 222 (200), 833 (750), 1000 (900) and 1667 (1500). These fibers are composed of 40,150,180 and 300 filaments respectively, with the filaments being 23 pm (0.91 x 10" in) in diameter. Vectran HS fibers are offered with or without a standard textile finish to assist in processing and/or to provide (dramatically improved) abrasion resistance. Commingled Vectran HS or M fibers have no finish. (In the commingled S-2

glasslvectran M products, the glass fibers are producer-sized.) (Hoechst Celanese, 1990).

More information on Vectran can be obtained from the source listed in Table 10.14.

Applications

As with aramid and polyethylene fibers, ropes and cables are an important usage. Marine cables, fish nets, towing ropes, cargo tie downs, slings, sails, bicycle brake cables and optical fiber reinforcement all have been made from Vectran HS. Olympic target archers use bow strings from Vectran HS having a propri- etary abrasion resistant finish. The result is increased arrow speed with no creep of the string. At the last America's Cup yacht races, Vectran HS was used in at least six yachts, either in sails or in marine cables. Vectran sails stretch far less than either aramid or polyethylene and they have four to six times the life of aramid sails (Adams and Farrow, 1993a).

Safety materials and protective garments have also been made of Vectran in industries ranging from meat packing to metal working. In these applications, Vectran is superior to aramids, which have poor resistance to bleach, and to polyethylene fibers, which are sensitive to the high temperatures used in drying laun- dered garments.

Vectran composites have been used in aerospace applications and in recreation and leisure applications such as canoes, golf clubs, baseball bats, hockey sticks, tennis rackets, bicycles, skis, ping pong paddles, paragliders and stereo speaker cones. Vectran properties of importance in these applications include the low moisture absorption, the excellent damping characteristics, high stiffness, lack of creep

Table 10.14 Sources of information on non-aramid, non-polyethylene organic fibers

Product Itzj?ormation source

Vectran fibers

PBI fibers

PBO fibers

Hoechst Celanese Corporation, P.O. Box 32414, Charlotte, NC 28232-2414, USA

Hoechst Celanese Corporation, P.O. Box 32414, Charlotte, NC 28232-2414, USA

The Dow Chemical Company, Midland, MI 48674, USA

Other organic fibers 237

and good flexural fatigue properties. Vectran is used where the cost of the fiber is secondary to its performance (Adams and Farrow, 1993a).

Hybrid tennis rackets have been made by Prince Manufacturing Company and Dunlop. Vectran HS is combined with carbon fiber to give greater speed and power with vibration characteristics as good as wood. Jennifer Capriatti played with a Vectran racquet at Wimbledon in 1992.

Other actual or potential uses include antenna guy wires, chemical resistant packings and gaskets, heat and creep resistant belting, medical and surgical equipment, pressure vessels, printed circuit board substrates and aerial tow ropes.

10.4.2 AROMATIC HETEROCYCLIC POLYMER FIBERS

Two aromatic heterocyclic polymer fibers are currently available or in development in the United States. These are PBI fiber from Hoechst Celanese and PBO fiber from Dow Chemical Company.

PBI fiber

PBI fiber is produced from a high performance polybenzimidazole. Chemically it is poly-2,2'- rn-phenylene-5,5'-benzimidazole, with the structural formula shown in Fig. 10.13. The fiber was commercialized by Hoechst Celanese

Fig. 10.13 Structural formula of poly-2,2'-rn-phenylene-5,5'- benzimidazole (PBI) (Yang, 1992).

Fig. 10.14 Structural formula of poly-p-phenylene benzobisoxazole (PBO) (Yang, 1992).

in 1983. It has excellent chemical and solvent resistance and does not burn. It is, however, more expensive than the aramids and has an intrinsically high moisture absorption. It is used mainly in woven form in fireblocking layers, including aircraft seat cushions and fire-fighting overgear. It was also used in chemical warfare suits in Operation Desert Storm. In order to reduce cost, PBI is also used in blends with aramids for thermal protective apparel. PBI has potential applications as a fiber reinforcement in composites, but currently its only composites application is as a matrix resin or as a matrix-precursor for carbon-carbon composites (Yang, 1992; Conrad, 1993). More information on PBI can be obtained from the source given in Table 10.14.

PBO fiber

PBO fiber is a polybenzoxazole, specifically poly-p-phenylene benzobisoxazole, with the structural formula shown in Fig. 10.14. PBO fiber resulted from a US Air Force program aimed at developing high strength fibers for advanced composites. In the late 1980s, Dow Chemical purchased worldwide rights to the polymer. Dow has now constructed pilot plant facilities for monomer, polymer and fiber and the fiber is available for evaluation in pre-production quantities (Burk, 1993).

As with aramids, PBO fibers are spun from a liquid crystalline solution using dry-jet wet spinning. This is, however, a more difficult process than for aramids. The fiber is then heat stretched to improve its orientation and properties (Wolfe, 1990).

238 Organic fibers

PBO is one of the most thermally and thermo-oxidatively stable organic polymers known. No weight loss was observed for PBO held at 316°C (600°F) (Wolfe, 1990) and weight loss of only O.O6%/h was observed at 370°C (700°F) (Burk, 1993). Its decomposition temperature is 600°C (1110°F) (Burk, 1993). Exposed to flame, PBO chars, but does not support combustion. (Wolfe, 1990) Dow's PBO fiber has a longitudinal coefficient of thermal expansion of -6 x 104/OC (-3.3 x lO"/"F) (Burk, 1993).

PBO fiber has a significantly higher tensile strength and modulus than any other known organic fiber. PBO fibers have been produced with tensile moduli of as high as 470 GPa (68 Msi). Dow's current pre-production fibers do not achieve these high levels, but do nonetheless have excellent axial mechanical properties, as shown in Table 10.13. However, like all other high performance organic fibers, PBO fibers are quite weak in compression, with a fiber compressive strength comparable to that of aramids (Burk, 1993). They also bond poorly to epoxy matrices, so their off-axis properties are also poor (Wolfe, 1990). For these reasons, as with other organic fiber composites, PBO composites are limited to applications where structural loading is mainly in axial tension.

Moisture regain for Dow's PBO is 2.0% for the standard fiber and less than 0.570 for the high modulus version. The moisture resistance is significantly better than aramids (Burk, 1993). PBO is highly resistant to hydrol- ysis, acid chemical attack, bases, solvents, electron bombardment and laser radiation. Its UV stability is outstanding (Wolfe, 1990). Dow's PBO fiber has a lower and more stable dielectric constant than that of aramids, 3.0 at 100 kHz (Burk, 1993).

The price for commercial PBO will be volume dependent, but will be higher than that for aramids. PBO fiber will be used where aramids and other fibers do not meet the performance needs, particularly for strength, modulus and flammability. Potential applica-

tions include composites loaded in tension, such as pressure vessels, missile cases and tensile beams. PBO fiber composites may also be used in non-load-bearing applications where high temperature exposure or harsh chemical environments are anticipated, such as rocket insulation systems and brake and transmission systems. The high strength could also lower the weight of composites used in space- craft and in recreation and sporting goods. PBO also has significant potential application to ballistics, where, as a fabric or composite, it performs equally well at half the weight of an aramid. PBO composites could provide outstanding containment systems for high speed rotors and turbines where high temperature exposure is of concern. The fibers also have potential for bomb containment systems, for fire resistant and cut resistant apparel and fire blocks, as well as ropes and cables (Burk, 1993).

More information on PBO can be obtained from the source given in Table 10.14.

10.5 CONCLUSIONS

While high performance organic fibers are not competitive with inorganic fibers in all of their properties, they offer certain properties and combinations of properties that are unavail- able with inorganic fibers. All suffer from certain limitations, such as poor off-axis and compression properties and/or temperature limitations. However, if these limitations are properly considered, high performance organic fibers can make possible designs that can be achieved in no other way.

REFERENCES

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Adams, P.M. 1993. Private communication. 10-28-93. Charlotte, NC: Hoechst Celanese Corporation.

References 239

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DuPont. 199213. Kevlar aramid, properties and uses of Kevlar 29 aramid, Kevlar 49 aramid, Kevlar 68 aramid in fiber optic and electromechanical cables. Information bulletin K- 506C, revised November 1992. H-37390. Wilmington, DE: DuPont Fibers Department.

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DuPont. 1993e. Prices for Kevlar 49 yarns used in ropes and cables. Price list effective 1/4/93. Wilmington, DE: DuPont.

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Appendix 241

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as the weight in grams of 1000 meters of the material. A related term is decitex (dtex), 0.1 tex, which is often used in order to be comparable to the US quantity 'denier.'

Equilibrium moisture content: moisture absorbed by a fiber after it has been dried at 50°C (122°F) for 2 h and then equilibrated at 20°C (68°F) and 55% relative humidity

Strength retention: percent of room temperature strength retained following exposure to the conditions indicated

Tenacity: the ultimate failure strength of a fiber per unit original area per unit weight. The most commonly used units are 'grams per denier' (gpd) and 'newtons per tex' (N/ tex).

10.6 APPENDIX Conversion factors

DEFINITIONS AND CONVERSION FACTORS

Definitions

Denier: Term in common usage in the fiber industry in the USA to describe the fineness of a fiber or fiber bundle. The denier is defined as the weight in grams of 9000 meters of the material. This is also known as the 'count'. Its inverse measure is the 'yield', expressed in yards per pound or meters per kilogram.

Tex: Term in common usage in the fiber industry outside the USA to describe the fineness of a fiber or fiber bundle. The tex is defined

Fiber size: 1 tex = 9 denier = 10" kg/m Twist: 1 tpi (turns per inch) = 39.37 t/m (turns per meter) Twist multiplier = [t/m (dtex)'/*]/3000 = [tpi (denier)'/'/ 731 Modulus, stress, strength, and tenacity: 1 kgf / mm2 = 9.806550 MPa 1 ksi = 6.894757 MPa 1 cN/tex = 0.01 N/tex = 10 pf MPa = 1.45 pf ksi

where pf is the specific gravity of the fiber 1 gpd = 8.826 cN/tex = 88.26 pf MPa = 12.8 pf

ksi

40207_10

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