High-performance ballistic fibers
and tapes
1
T. Tam, A. Bhatnagar
Honeywell International Inc., United States
1.1
Introduction to high-performance fibers and tapes
High-performance ballistic fibers and ballistic tapes are engineered for lightweight ballistic fabrics, composites, and other industrial applications. These are generally used
for niche life-saving products such as flexible body armor, molded breastplates, and
molded ballistic helmets and panels for armoring helicopters, military cargo planes,
the hulls of navy ships, high-speed coast guard boats, and military ground vehicles.
Some of the industrial applications of high-strength fibers and tapes include
cut-resistant gloves, premium fishing lines, large fishing nets, ropes, sail cloth, and a
host of other applications.
1.1.1
Requirements for high-performance fibers and tapes
To achieve high-performance fibers and tapes with exceptional tenacity and modulus
properties, there are at least three necessary requirements:
1. The molecule must be highly oriented in the fiber axis direction.
2. The molecular weight or the molecular chain length must be very high.
3. The fiber must be highly crystalline with few defects.
There are generally two approaches in manufacturing high-performance fibers to
meet the above criteria. One can start with a highly oriented chemical rigid-chain,
rod-like polymer (Fig. 1.1) such as aramid (lyotropic) or liquid crystal (thermal tropic).
The relatively low-molecular-weight liquid crystal rigid-chain polymer is spun into
fiber, and the resulting fiber is “solid-state polymerized” to a high molecular weight
with drawing and annealing processes. The spinning of aramid fibers is an example
of this approach.
On the other hand, one can start with an ultrahigh-molecular-weight, flexible,
long-chain, randomly coiled polymer like ultrahigh-molecular-weight polyethylene
(UHMWPE) (Fig. 1.2).
Since the ultrahigh-molecular-weight polymer cannot be melt-spun (the polymer
will decompose before it will flow at the melting temperature), the polymer is dissolved in a solvent to form a dilute solution that is then spun into filaments. In this
dilute solution, the ultrahigh-molecular-weight polymeric chain is “uncoiled” and
the spun filaments are subsequently formed into a network called a gel. By this
“gel-spinning” method, a long-chain molecule with a loosely connected network
Lightweight Ballistic Composites. http://dx.doi.org/10.1016/B978-0-08-100406-7.00001-5
Copyright © 2016 Elsevier Ltd. All rights reserved.
2
Lightweight Ballistic Composites
Figure 1.1 Random rods of polymers (Bhatnagar, 2006).
Figure 1.2 Random coils of polymers (Bhatnagar, 2006).
xerogel fiber can be made. The xerogel fiber can be drawn into a highly oriented, highly crystalline, high-performance fiber via specially developed drawing techniques.
High-performance UHMWPE fibers like Spectra® or Dyneema® fibers are examples
of these processes.
1.1.2
Manufacturing of high-performance fibers
In general, high-performance fiber manufacturing requires unique, relatively high-cost
processes such as the gel-spinning process for UHMWPE fibers. The gel-spinning process involves dissolving the polymer in a first solvent to “disentangle” the UHMWPE
polymer into a dilute solution (eg, 10% solid). The dilute polymer solution is spun with
a melt-spinning-type process, forming a solvent-containing gel fiber upon quenching
and optionally extracting the first solvent with a second solvent, followed by drying or
evaporating the second solvent from the solvent-rich fiber to form a solid fiber.
The solid fiber is then drawn at least once or in several steps to develop its
high-strength, high-modulus, and highly oriented structure.
Fabricating aramid fibers also require a solvent-based process to dissolve the
“rigid” aromatic polyamide polymer chain followed by a “dry-jet” wet spinning process. The high-temperature melt-spinning of the liquid crystal polymer requires an
annealing and drawing process to develop its molecular weight for strength, which
increases the manufacturing cost compared with the conventional melt-spinning
processes of nylon, polyester, polypropylene, etc.
High-performance ballistic fibers and tapes
3
Owing to the high cost and solvent-recovery steps of the gel-spinning process, an
alternative process to make high-performance UHMWPE tape (fiber) was developed
using a compression and sintering process followed by slitting and drawing to
develop its strength. However, this compressed/sintered UHMWPE tape/fiber has
significantly lower tenacity (about 50%) than its gel-spun counterpart but a reasonable modulus.
1.2
High-performance ballistic fibers and tapes
The high-performance ballistic fibers and tapes are different from high-performance
structural fibers, such as glass and carbon fibers, in many aspects. For applications
in which both ballistic performance and structural performance are required, a compromise is usually achieved.
This chapter will focus on high-performance fibers and tapes which are used for ballistic applications only.
1.2.1
UHMWPE fibers
The UHMWPE fiber is a type of polyolefin fiber. The fibers are made up of extremely
long chains of polyethylene, which are aligned in the same direction. Each chain is
bonded to the other with many van der Waals bonds. This provides the superior physical properties attractive for a number of military and industrial applications.
The UHMWPE fiber polymer chains can attain an orientation greater than 95% and
a level of crystallinity of up to 85%.
The weak bonding between olefin molecules allows local thermal excitations to
disrupt the crystalline structure and therefore UHMWPE fibers have lower heat resistance than other high-strength fibers. The melting point of UHMWPE fibers is around
144e152 C and, generally, UHMWPE fibers are not used at temperatures exceeding
80e100 C for long periods of time. However, the UHMWPE fibers maintain performance at below 50 C.
Owing to the molecular structure of UHMWPE fibers, they exhibit surface and
chemical properties that are rare in high-performance polymers and do not absorb
water readily. For the same reason, skin does not interact with it strongly, making
the UHMWPE fiber surface feel slippery. The UHMWPE fibers are resistant to water,
moisture, most chemicals, ultraviolet (UV) radiation, and microorganisms.
The density of gel-spun UHMWPE fibers is 0.97 g/cm3.
1.2.2
Aramid fibers
Aramid fibers are human-made fibers having molecules that are characterized by relatively rigid polymer chains. These molecules are linked by strong hydrogen bonds that
transfer mechanical load very efficiently, making it possible to use chains of relatively
low molecular weight with much higher tenacity and elastic modulus.
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Lightweight Ballistic Composites
The aramid fibers have a high degree of orientation, similar to the UHMWPE fibers.
The fibers are known for high strength, good impact and ballistic properties, low flammability, no melting point, and good resistance to chemicals and abrasion.
The density of aramid fibers varies from 1.44 to 1.46 g/cm3.
1.2.3
UHMWPE tapes/ribbons
UHMWPE thin tapes and ribbons are made by solid-state extrusion of special-grade
low-entangled UHMWPE polymers. The molecular structure after solid-state extrusion and drawing is not perfectly aligned as achieved by the gel-spinning process.
This results in lower performance compared to UHMWPE tapes.
UHMWPE tapes and ribbons exhibit low shrinkage, high abrasion, high strength
and modulus, and excellent chemical resistance.
The main features of UHMWPE tapes and ribbons are high dimensional stability,
low creep resistance, translation efficiency (polymer molecular weight vs tape molecular weight), and ease of surface modification for higher adhesion, and increased UV
stability.
The density of UHMWPE tapes and ribbon is 0.97 g/cm3.
1.2.4
Ballistic fiberglass
Glass fibers (commonly referred to as fiberglass) are made of various types of crushed
glass depending upon the fiberglass use. The crushed glass contains silica with varying amounts of oxides of calcium, magnesium, and sometimes boron. For fiberglass
applications, care is taken during manufacturing to achieve a very low level of
defects.
Fiberglass filaments are manufactured by a pultrusion process. In the manufacturing
process, large furnaces gradually melt the sand, limestone, kaolin clay, fluorspar, colemanite, dolomite, and other minerals into liquid form. The liquid is then extruded
through platinum bushings, which are bundles of very small orifices (typically
5e25 mm in diameter for E-glass and 9 mm for S-glass).
Just after the pultrusion process, when the filaments become solid, a sizing (coating)
with a chemical solution is applied through a spray process. The coated and solid fibers
are then combined into bundles to provide a roving.
The two most common types of glass fiber used in ballistic applications are E-glass,
which is aluminoborosilicate glass with less than 1% w/w alkali oxides, mainly used
for glass-reinforced plastics, and S-glass (aluminosilicate glass without CaO but with
high MgO content), with high tensile strength.
The density of E-glass is 2.58 g/m3 and S-glass is 2.46 g/m3.
1.2.5
Carbon fibers
The raw material for manufacturing carbon fiber, also referred as graphite fiber or CF,
is called the precursor. About 90% of carbon fibers manufactured are made from polyacrylonitrile. The process involves melt extrusion followed by pyrolysis. The balance
High-performance ballistic fibers and tapes
5
is made from rayon or petroleum pitch. During the fiber manufacturing process, a
variety of gases and liquids are used. Some of these materials react with the fiber
and other materials are designed not to react or to prevent certain reactions with
the fiber.
The carbon fibers are about 5e10 mm in diameter and composed mostly of atoms.
Carbon fibers are not ballistic fibers because carbon fibers and composites reinforced with carbon fibers are brittle in nature. However, in certain applications, single
or multiple layers of carbon fabric are used to provide structural integrity, repeated
compression improvements, and other benefits.
The density of carbon fibers is 1.88 g/m3.
1.2.6
Other fibers
There are a number of other fibers which can be combined with the high-performance
ballistic fibers to meet specific performance needs or higher specific values.
1.2.6.1
High-modulus polypropylene fibers
High-modulus polypropylene (HMPP) fibers are manufactured using a unique hot
melt-spinning process designed to crystallize filaments while the polymer is in a highly
relaxed, highly disoriented state. This permits high draw ratios and efficient chain
orientation to be achieved in the subsequent drawing operation.
The drawn HMPP fibers have high levels of crystallinity and orientation, but the
density of the HMPP fibers is about 0.67 g/cm3, which is well below the density of
industrial polypropylene in the amorphous state (0.85 g/cm3).
1.2.6.2
Ceramic fibers for ballistics
Ceramic fibers were designed and developed for applications in which the composite
matrix/resin temperature can go, for example, as high as 1000 C in a corrosive and
oxidizing environment.
The ceramic fibers are made from precursor fibers or a very thin tungsten-core wire.
Materials like boron and silicon carbide vapors are deposited onto a red-hot precursor
moving very slowly. Some of the ceramic fibers are large-diameter monofilaments.
The ceramic fibers show high-strength and high-modulus properties in both tension
and compression applications. In compression, unidirectional boron composite stresse
strain curves are linear to failure (400,000 psi failing stress) and exhibit a modulus of
30 million psi.
Because ceramic fibers have large diameters, prepreg tapes formed from the fibers
are usually unidirectional only.
The ceramic fibers are uniquely suited to handle the high-temperature consolidation
conditions of titanium and ceramic matrix composites. Only limited quantities of
ceramic fibers are manufactured annually but production can be rapidly expanded to
meet new demands.
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Lightweight Ballistic Composites
For ballistic applications, including reinforcing ceramic tiles, prepregs are crossplied and cured using an autoclave.
There are a number of other fibers which can be combined with the highperformance ballistic fibers to meet specific performance needs or higher values.
1.3
UHMWPE fibers (Prevorsek, 1996)
1.3.1
Chemical structure and morphology of UHMWPE fibers
The chemical structure of polyethylene is the simplest repeating molecular unit of CH2
as shown in the schematic below:
ðCH2 CH2 Þ
UHMWPE generally refers to molecular weights higher than 1 million (8 intrinsic
viscosity (IV)) to 5e6 million (30 IV). Depending on the polymerization technique,
the structure or the morphology of the UHMWPE polymer may have different features.
The polymer morphology and structure have a great impact on the gel-spinning
processes, the ultimate fiber morphology, and the final physical properties of the
UHMWPE fibers.
For example, some UHMWPE polymers have different particle sizes and shapes
(Fig. 1.3, cauliflower) as viewed by scanning electron microscopy (Figs. 1.3 and 1.4).
When magnified, the individual UHMWPE polymer powder particles show that there
is a fibril structure between the “gaps” within the particles themselves. These fibrils
are speculated to be highly oriented structures within the polymer particle which
may have an elevated melting point in comparison with the rest of the bulk particles.
#2a 5.0 kV x100 100 µm
Figure 1.3 Particle sizes and features of individual UHMWPE polymer particles.
High-performance ballistic fibers and tapes
7
#1b 5.0 kV x2000 5 µm
Figure 1.4 Fibril morphology between the UHMWPE polymer particles.
The particle size, particle size distribution, and morphology have great impact on both
fiber processing and the properties of the fibers.
Some UHMWPE polymers show different features compared to others. For
example, Figs. 1.5 and 1.6 show more uniform particle size and particle size distribution. There is no fibril observed in the gaps within the particles.
Naturally, different processing techniques will be required to maximize the potential fiber properties from other polymer types.
#3a 5.0 kV x100 100 µm
Figure 1.5 Uniform UHMWPE particle size and particle distribution.
8
Lightweight Ballistic Composites
#2d 5.0 kV x2000 5 µm
Figure 1.6 No fibril structure within UHMWPE polymer particles.
1.3.2
Gel-spinning process
With the extremely high molecular weight of UHMWPE polymers, the UHMWPE
polymers cannot be melt-spun like convention nylon or Polyethylene terephthalate
(PET). The UHMWPE polymer will degrade before it can flow. As a result, the
gel-spinning process has been developed to handle the UHMWPE polymer (Fig. 1.7).
There are two general types of gel-spinning processes: one-solvent systems and
two-solvent systems. In a one-solvent gel-spinning process, a solvent such as decalin is
used to disentangle the UHMWPE polymer to form a solution having a polymer concentration up to about 15%. The polymer solution will behave like nylon or PET melts at
Suspension
UHMWPE
Continuous extrusion/solutions
Metering pump
Spinneret
Figure 1.7 UHMWPE polymer fiber gel-spinning process (Bhatnagar, 2006; Van
Dingenen, 2001).
High-performance ballistic fibers and tapes
9
elevated temperatures and can be spun through a spinneret using conventional
melt-spinning equipment. After the fiber solution exits the spinneret, it is passed through
an evaporation chamber in which the solvent is flashed off to form a gel fiber with a limited
amount of solvent remaining in the “solid precursor fiber.” This solid precursor fiber
(tenacity about 20 g/denier) can then be drawn in a drawing apparatus wherein residual
solvent may be evaporated. During the drawing process, the polymer molecules are
aligned, enhancing the tensile strength of the fiber.
In an alternate version of this process, the solution fiber is extruded through the
spinneret, passed through an air gap, and then quenched in liquid bath to form the
gel fiber. The gel fiber then will be drawn in a heated oven in which the solvent is evaporated and the polymer molecules are oriented to develop the high-strength
high-modulus (stiffness) fibers.
In a two-solvent system, a low-molecular-weight solvent, such as mineral oil, wax, or
paraffin wax, is used as the first solvent to disentangle the UHMWPE molecules to make
a solution having a polymer concentration up to about 15%. At elevated temperatures,
the solution can be melt-spun with conventional melt-spinning equipment. After extrusion, the solution fiber is quenched in a liquid bath to form a gel fiber, optionally
stretched, and then the spinning solvent is extracted with a second low-flash-point solvent. During the solvent extraction step, the low-molecular-weight solvent (eg, mineral
oil) is replaced with a second solvent. The yarn with the second solvent is then dried and
optionally stretched to form a “solid fiber.” The solid fiber is then drawn in different
stages with a different drawing apparatus to maximize the fiber tensile properties.
The high cost of the gel-spun processes (Fig. 1.8) can be attributed to:
11
12
10
A
13
38
15
14
22
D
16
16
25
20
28
19
18
24
23
27
E
B
45
C
33
32
47
37
31
30
41
40
52
56
F
58
63
61
54
55
52
68
65
62
Figure 1.8 Schematic of gel-spinning process (Bhatnagar, 2006).
66
72
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Lightweight Ballistic Composites
1. The concentration of the polymer in solution is low. For example, assuming a 10%
concentration, one has to process 100 lb of solution material to get 10 lb of polymer
(fiber).
2. The solvent used during the gel-spinning process must be recovered. Assuming a 10%
concentration, one must process 90 lb of solvent to get 10 lb of fiber.
As a result, a less expensive way of making UHMWPE fibers using a nonsolvent
process has been developed. This disruptive technology will be discussed later in
the UHMWPE tape session.
1.3.3
Morphology of UHMWPE fibers
There are several steps during the gel-spinning process leading to the final UHMWPE
fiber morphology.
In the solution, the UHMWPE molecules become disentangled. The solution is
then spun through a spinneret just like in a conventional melt-spinning process.
The spun solution is then quenched, forming a loosely connected network called
a gel. After quenching or cooling of the solution into gel fibers, the loosely
entangled molecules of the gel fibers can be drawn at a very high draw ratio.
Fig. 1.9 shows various stages from spinning of the solution into gel fibers to drawing into high-performance fibers. During the extraction or evaporation step of the
solvent, the gel fiber could be drawn further.
Like most high-performance fibers, the UHMWPE fiber contains microscopic and
macroscopic fiber morphology. A scanning electron micrograph (SEM) of Spectra®
fiber is shown in Photo 1.1. The SEM shows regular micro- and macrostructures
(see Fig. 1.10).
The longitudinal structure of the fibrils consists of microfibrils having a proposed
structure as shown in Fig. 1.11, in which nearly perfect crystals are covalently linked
through a relatively small amorphous domain.
This microfibrillated structure is far from the perfect uniaxial fiber structure
in Fig. 1.12 and thus the strength of the UHMWPE fiber, while 15 times
stronger than steel, is still far from the theoretical strength of the covalent CeC
bonds.
It is speculated that an increase in the number of “extended-chain” molecules
that span the amorphous domain would increase both strength and modulus.
The potential is certainly there to further advance the properties of the UHMWPE
fibers.
Fig. 1.13 represents a proposed model for the macrofibrils. Because
amorphous matter also exists between the microfibrils, the structure appears to be a
composite of near perfectly oriented crystalline microfibrils embedded in an amorphous matrix. However, there are extended-chain molecules that can bridge
through several layers of the “amorphous” region. It is speculated that the more of
this type of “bridging” molecule or, as called by a new term, extended-chain tie
molecule, the stronger and more dimensionally stable the UHMWPE fiber will be.
High-performance ballistic fibers and tapes
11
(a)
(b)
Breaking of
clusters
Discontinuity
Newly formed
fibril
Constriction
(c)
T = 100–133ºC
Ea = 50 kJ/mol
T = 133–143ºC
Ea = 150 kJ/mol
T = 143–150ºC
Ea = 300–600 kJ/mol
Figure 1.9 Morphology of UHMWPE during various stages of production (Bhatnagar, 2006).
12
Lightweight Ballistic Composites
Photo 1.1 Microfibration of UHMWPE fibers.
Spectra
PET
Fiber
Kevlar
Kevlar
Macrofibrils
100–150 nm
Microfibril
6–10 nm
Spectra
Extended
molecules
0.5–1.0 nm
Figure 1.10 Micro- and macrofibrillar structure of PET, aramid, and UHMWPE fibers
(Bhatnagar, 2006).
High-performance ballistic fibers and tapes
13
To scale
Figure 1.11 Proposed longitudinal structure of Spectra® microfibrils (Bhatnagar, 2006).
Reduced scale
Figure 1.12 Perfect uniaxial fiber structure assumed in the calculations of theoretical strength
(Bhatnagar, 2006).
Interfibrillar
amorphous
phase
Crystallites
Intrafibrillar
amorphous
phase
Figure 1.13 Model showing crystallites and amorphous phase (Bhatnagar, 2006).
1.3.4
Physical properties of UHMWPE fibers
The UHMWPE fiber properties are listed in Table 1.1. As the gel-spinning and drawing technology evolves with time, fiber properties improve to dovetail different end
uses. As a result, there are different grades of commercial UHMWPE fibers. In short,
the new-generation product tends to be in lower denier per filament, with higher
tenacity and higher modulus (Fig. 1.14).
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Lightweight Ballistic Composites
Table 1.1 Properties of UHMWPE fibers (United States
Patent US 8,361,366, 2013)
Highly drawn,
high-performance fibers
Yarn property
a
Tensile strength, g/denier (GPa)
37.5e70.0a
(3.21e5.99)a
Initial modulus, g/denier (GPa)
1320e2000a
(113e171)a
Density, g/cm3
0.97
Estimated. All values are exemplary.
New spectra
40
Spectra 2000
Tenacity (g/d)
Spectra 1000
30
Spectra 900
‘S’ glass
20
K - 129 aramid
K - 29 aramid
HT graphite
‘E’ glass
HM graphite
10
Steel
0
0
50
100
150
Tensile modulus (g/d)
200
250
Figure 1.14 Tensile strength and tensile modulus of high-performance fibers.
1.3.5
Ballistic application of UHMWPE fibers
UHMWPE fiber-based woven and unidirectional (UD) crossplied materials have been
developed for soft, hard, and vehicle armor and a host of other lightweight composite
applications.
Soft ballistic vest materials are designed particularly for use in flexible vests for law
enforcement and military personnel. The range of materials provides the highest ballistic protection against handgun bullets and fragments.
Hard ballistic UHMWPE materials are available for molded ballistic inserts and
helmets to protect against both handgun and rifle bullets and fragments.
High-performance ballistic fibers and tapes
15
UHMWPE has also been used in numerous vehicles and body armor products,
including vests, helmets, and inserts, by a rapidly growing number of end users since
the early 1990s.
1.4
Aramid fibers
Aramid fibers, like nylon fibers, are polyamides derived from aromatic acids and
amines. Figs. 1.15 and 1.16 illustrate nylon 6 and nylon 6,6 polymers, which have flexible chains between the amide groups. Figs. 1.17 and 1.18 illustrate meta-aramid
(Nomex®) and para-aramid (Kelvar®) polymers, which have aromatic chains between
the amide groups that give these fibers their unique properties.
Because of the stability of the aromatic rings and the added strength of the amide
linkages, due to conjugation with the aromatic structures, aramids exhibit higher tensile
strength and thermal resistance than the aliphatic polyamides (nylons). The para-aramids
(trade name Kevlar® and Twaron®) based on terephthalic acid and p-phenylene diamine
(PPD-T), or p-aminobenzoic acid, exhibit higher strength and thermal-resistance properties than those with the linkages in the meta positions on the benzene rings (trade name
O
O
NH
H 2O
*
N
n
*
Figure 1.15 Structure of nylon 6.
O
O
HO
O–
OH
O
O–
O
H3 N +
H 2N
NH3+
NH2
Heat &
vacuum
O
*
H
N
N
H
O
Figure 1.16 Structure of nylon 6,6.
NH
Figure 1.17 Nomex® structure.
NH
CO
CO
n
*
16
Lightweight Ballistic Composites
NH
NH
CO
CO
Figure 1.18 Structure of aramid fiber.
Nomex®). The greater degree of conjugation and more linear geometry of the para linkages, combined with the greater chain orientation derived from this linearity, are primarily responsible for the increased strength. The high impact resistance of the para-aramids
makes them popular for first-generation “bullet-resistant” body armor. Aramid fibers
can be chopped into staple form to make felt for applications such as chain
saw-protective garments, or they may be blended with other fibers for other end uses.
Aramid fiber is lyotropic. It is solution-spun and it melts at a lower temperature than
a thermotropic liquid crystal fiber.
1.4.1
Dry-jet wet aramid fiber spinning
The aramid solution is spun by a process called dry-jet wet spinning (Fig. 1.19). In this
process, an anisotropic solution of PPD-T is extruded through an air gap into a coagulation bath as shown in Fig. 1.19. The resulting yarn after coagulation is washed and dried.
Spin dope
Spinneret
Transfer line
Spinning block
Air gap
Container
Filaments
Coagulating liquid
Spin tube
Tube
P
Pump
Rotating bobbin
Guide
O
Container
Figure 1.19 Schematic diagram of the dry-jet wet spinning process for aramid fibers
(Bhatnagar, 2006).
High-performance ballistic fibers and tapes
17
Spinneret
Orientation
Partial deorientation
Air gap
Reorientation
Quench
water
bath
Figure 1.20 Orientation through the capillary die: elongation and shear flow
(Bhatnagar, 2006).
The keys to the dry-jet wet spinning method to orient the anisotropic molecule are
shear orientation and elongation flow through the spinneret capillaries, as is represented graphically in Fig. 1.20. In addition, the “relaxation” of the molecule after
exiting the capillary is kept at a minimum by filament tension or attenuation in the
air gap and through the coagulation bath as the filament is precipitated into the highly
oriented crystalline fiber. This fiber is also heat treated under tension to increase its
modulus.
1.4.2
Aramid fiber structure and morphology
Aramid fibers contain several levels of microscopic and macroscopic morphology.
A brief discussion of each is given below using individual fibers as a starting point.
1.4.3
Skin core fibril structure
When aramid fiber is subjected to tensile testing, its typical fracture mode is generally a
fibrillated-type failure represented in the following figures. This fracture mode represents a highly ordered lateral fiber structure. The proposed failure mode is shown in
Fig. 1.21 with a skin core structure as in Figs. 1.22 and 1.23.
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Lightweight Ballistic Composites
Figure 1.21 Failure mode of aramid fiber (Bhatnagar, 2006).
1.4.4
Fiber fibrillar structure
Aramid fiber fibrillates easily upon abrasion, especially in the direction perpendicular
to the fiber axis. In fact, almost all highly oriented fibers like UHMWPE (such as
Spectra® fibers) are easily fibrillated. Aramid fibers are easily fibrillated because the
macromolecules are held together only by weak van der Waals forces and/or weak
hydrogen bonding. Fig. 1.24 is a proposed model of the fibrillar structures for most
of the highly oriented performance fibers. The individual fibrils are the load-bearing
elements for the fiber, whereas the tie molecule is the load-bearing element for the conventional fibers. The width of the fibrils is about 600 nm and they are up to several
centimeters long.
Drilling a layer down on the fibril structure, each “column” of Fig. 1.24 is called a
fibril. On each of the fibrils, the straight line represents a PPD-T molecular chain. In
most of the chain ends, bends are contained in an alternating “defect” or amorphous
layer. These defects or amorphous layers are the weak links in the fiber structure.
High-performance ballistic fibers and tapes
19
Crack propagation path
Core
Fiber axis
Skin
Surface
Core
Skin
Figures 1.22 and 1.23 Aramid fracture morphology showing long tails fracture mode
(Bhatnagar, 2006).
Fibril
Ordered
lamella
Detect zone
Fiber axis
Tie point
°
6000 A
Figure 1.24 Fibrillar structure model of aramid fiber (Bhatnagar, 2006).
20
Lightweight Ballistic Composites
However, some of the PPD-T chain can be oriented and extended to bridge several
amorphous or defect layers. This unique “extended-chain tie molecule” should give
satisfactory fiber strength.
1.4.5
Pleat structure
Aramid fiber has a unique feature when observed under a cross-polarized microscope
light field, featuring transverse bands (Fig. 1.25). However, these transverse bands are
diminished when the filament is under tension (Fig. 1.26). This leads to the hypothesis
that aramid fiber has a pleated structure (Fig. 1.27). The occurrence of a pleat sheet
structure in aramids is not well understood.
To explain the formation of the pleated structure, it has been hypothesized that during the coagulation of the aramid fiber the skin is first formed and is subjected to attenuation stress on a spun filament. This allows the “core” of the fiber to relax and form
pleats at a uniform periodicity. The formation of the pleat structure gives the fiber an
inherent elongation or elasticity. That may be the reason that, when aramid fiber is under stress, the transverse bands diminish as observed under the microscope.
Figure 1.25 Cross-polarized microscope light field featuring transverse bands.
High-performance ballistic fibers and tapes
Figure 1.26 Diminishing transverse bands under stress.
Figure 1.27 Pleat structure model of aramid fiber (Bhatnagar, 2006).
21
22
1.4.6
Lightweight Ballistic Composites
Crystalline structure
Aramid fiber has a highly crystalline, highly ordered molecular structure. Wide-angle
X-ray diffraction (Fig. 1.28) shows no amorphous halo indicating a highly crystalline
fiber. There is a pair of sharp rings in the equatorial scan indicating that the fiber may
contain a few percent unoriented crystals.
Northolt and Van Aartsen assumed a centered monoclinic (pseudo-orthorhombic)
unit cell and proposed a crystal lattice model of PPD-T.
The top view of Fig. 1.29 is a projection of top-view parallels of the c-axis. There
are two repeat units of PPD-T per crystal lattice, one at each corner of the crystal lattice
and one at the center. The lower view is a projection parallel to the a-axis. It reveals the
phenylene rings of the PPD-T repeat unit in the bc plane of the unit cell and its corresponding bonds. The crystal lattice dimensions are a ¼ 7.80 Å, b ¼ 5.19 Å, and the
fiber axis c ¼ 12.9 Å. The a angle ¼ 90 degrees.
1.4.7
Ballistic application of aramid fibers
Threats to military and law enforcement have multiplied in recent years, creating the
need for protection against armor-piercing bullets and improvised explosive devices.
Today, both woven aramid fabrics and UD crossplied materials provide greater protection, more comfort, and advantageous performance/weight ratios for military, police, and other law enforcement people than older aramid materials. A number of
civilians who face ballistic threats such as prison guards, cash carriers, and private people benefit from aramid fiber-based composites. Table 1.2 provides some typical properties of aramid yarns.
Aramid-coated fabrics are extensively used for manufacturing military helmets and
providing spall liners inside military vehicles.
Aramid fibers are used in the armoring of police and civilian vehicles while keeping
in mind their maneuverability. Even tanks and other military vehicles can be made
lighter and safer with aramid fiber composites. The aramid fiber composites can reduce
the weight of armored vehicles by 30e60% compared to steel. Aramid fiber ballistic
solutions exist for a number of threat levels, ranging from direct fire and shell fragments to high explosives.
Figure 1.28 X-ray photograph of aramid fiber (Handbook of Textile Fiber Structure, 2009).
High-performance ballistic fibers and tapes
23
a
b
1/4
1/4
1/4
1/4
1/4
c
b
Figure 1.29 PPD-T crystal lattice by Northolt (Handbook of Textile Fiber Structure, 2009).
Table 1.2 Typical properties of aramid yarns (Bhatnagar, 2006)
Yarn property
Standard fibers
High-modulus fibers
Tensile strength, g/denier (GPa)
23.0e26.5 (1.97e2.27)
18.0e26.5 (1.54e2.26)
Initial modulus, g/denier (GPa)
550e750 (47e64)
950e1100 (81e94)
Elongation, %
3.6e4.4
1.5e2.8
Density (g/cm )
1.44
1.44
Moisture regain, % 25 C, 65% RH
6
1.5e4.3
3
24
1.5
Lightweight Ballistic Composites
UHMWPE tape/ribbon
Typical gel-spun UHMWPE fiber requires a solvent system to dilute and disentangle
the extremely long chain molecules, thus enabling a drawing process to highly orient
the molecular chain for increased tenacity and tensile modulus. The gel-spinning process is expensive but is capable of producing extremely high-tenacity and
high-modulus fibers. On the other hand, a lower cost, nonsolvent-based, compression
or sintering process may be used to make UHMWPE tapes/ribbons/fibers, which was
developed by Nippon Chemical in the 1990s (called the Milite process). This nonfibrous tape/fiber has high modulus (about 1400 g/denier) but a comparatively lower
tenacity (20 g/denier). Currently, there are several companies investing into research
and development resources to further enhance the physical properties of this nonfibrous, nonsolvent UHMWPE process.
1.5.1
UHMWPE polymer for tape/ribbon
Just like other methods of forming UHMWPE fibers, the compression, sintering nonsolvent process needs specially tailored UHMWPE morphology to reach its highest
potential strength. In general, the less entangled the polymer chain in the polymer,
the better it is for the compression/sintering nonsolvent process.
1.5.2
Extrusion and pressing process
Fig. 1.30 is a schematic representation of the compression and sintering process. By
selecting the right polymer morphology, the polymer powder is first compacted into
a thick sheet at below or near the melting point of UHMWPE, followed by calendaring
it into a thinner sheet. The sintered thin sheet is then subject to further calendaring and
drawing in one or more steps. The sheet at this stage could be as wide as 12 in. or more
and can be wound up on a package for further drawing.
Since it is difficult to draw a sheet of sintered UHMWPE, the sheet is generally
silted into a ribbon, as narrower ribbons can be effectively drawn/stretched.
There are several published patents detailing the equipment and processing steps of
making nonsolvent-based tape/ribbon. As shown in Figs. 1.31 and 1.32, the polymer
powder is fed and dropped down to an “endless” moving belt (24). The powder is then
compressed and compacted under a weight (26) into a cohesive sheet. The sheet is subjected to one or more calendaring or roll extrusion (“rolltrusion”) steps and/or drawing
under heat to further reduce its thickness and at the same time develop partial orientation in the machine direction. The tape can then be wound up into a package as an
interim product, which can be slit into narrow ribbons for further drawing. While
this nonsolvent process is of lower cost, the tenacity of the tape is about 20 g/denier,
but with a respectable modulus of about 1400 g/denier.
High-performance ballistic fibers and tapes
25
Polymer
powder
Draw
Slit
UHMWPE tape
Figure 1.30 UHMWPE polymer compression and sintering process for tape/ribbon
(Game Changing Technology, 2008).
1.5.3
Drawing of the slit tape/ribbon
The drawing of the tape is accomplished by a multiple-stage drawing process schematically shown in Fig. 1.33. In fact, multiple stages of drawing are used in most methods
of forming UHMWPE tape/fiber to develop its high strength properties. In this case,
the sheet from the package is first slit into a narrow ribbon about 3/8 to 2 in. wide.
The tape/ribbon is then drawn over a long heater plate by passing the tape back and
forth over the heater plate surface. Resistance time, strain rate, drawing temperature,
and tension are all important variables during this process which are typically proprietary to each individual fiber manufacturer.
26
Lightweight Ballistic Composites
Raw
material
22
26
28
30
32
36
34
37
43
43
24
33
38
31
42
Figure 1.31 Compaction, sintering, and rolltrusion steps of the UHMWPE tape process (United
States Patent 8,236,119, 2012).
32
30
28
34
36
37
40
33
38
Figure 1.32 Additional rolltrusion and drawing steps before winding (United States Patent
8,236,119, 2012).
44
46
48
50
52
54
56
58
60
62
Figure 1.33 Tape drawing stages (United States Patent 8,236,119, 2012).
1.5.4
High-tenacity and high-modulus fibrous tape/ribbon
To have the best, a tape with tenacity and modulus, Honeywell International, Inc., has
developed a one-step drawing and tape converting process as outlined in US Patent
8,236,119. This process starts with a gel-spun UHMWPE precursor fiber that is drawn
High-performance ballistic fibers and tapes
27
50
100
10
20
51
101
102
80
30
31
60
70
32
20
60
Figure 1.34 High-tenacity and high-modulus fibrous tape/ribbon process (United States Patent
6,277,773, 2001).
in a heated oven to enhance its tensile properties. The UHMWPE fiber is then compressed into a tape/ribbon.
The schematic in Fig. 1.34 shows the multiple-filament yarn (10) being fed by a set
of rolls (20) (feed step). The fiber is drawn under tension to develop its yarn strength
between rolls 30, 31, and 32. Roll 32 also compresses the fiber to convert it into a flat
tape. The tape is then transported out by a set of rolls (60) (take-up rolls) with the speed
determined by the desired draw ratio. This particular process allows the tape to retain
most of the fiber strength during the compression step. The tape/ribbon strength is
about double, or more than that of tapes obtained through the nonsolvent, nonfibrous
sintering process.
1.5.5
Morphology of UHMWPE tape/ribbon
There is an obvious visual difference between a nonfibrous sintering process tape and a
fibrous tape converted from a gel-spun fiber. Figs. 1.35e1.38 show comparisons of the
Figure 1.35 Drawn nonfibrous UHMWPE tape/ribbon.
28
Lightweight Ballistic Composites
Figure 1.36 Drawn/fused/pressed fibrous tape/ribbon.
Figure 1.37 Nonfibrous tape/ribbon showing tape/ribbon nonuniformity.
nonsolvent UHMWPE tape made by a sintering process to the fibrous tape. It appears
that the surface is not smooth, not uniform, and not homogeneous, as if the polymer
particles are still intact. On the other hand, the tape made by gel-spun fiber via the
drawn/fused/compressed process is homogeneous and smooth.
High-performance ballistic fibers and tapes
29
Figure 1.38 Drawn/fused/pressed showing uniformity.
1.5.6
Differential scanning calorimetry characteristics
of nonfibrous tape vs fibrous tape
Owing to the differences in the processing of the nonsolvent, nonfibrous tape and the
fibrous tape, the nonfibrous tape has a lower melting point component at 138.5 C. It is
speculated that the sintering process melts part of the UHMWPE surface causing
“adhesion” of the particles, resulting in this lower melting point component. On the
other hand, the drawn/fused/pressed process fibrous tape does not have the lower
melting point component, allowing it to retain most of its original tenacity.
1.5.7
Ballistic application of UHMWPE tape/ribbon
The biggest difference between a UHMWPE tape/ribbon and a UHMWPE fiber is the
aspect ratio. In general, the aspect ratio of the tape/ribbon is at least 3:1 instead of a
round fiber. It is speculated that this high aspect ratio may be the reason a
lower-tenacity fibrous tape could have a higher ballistic performance than the fibers
from which it is fabricated with a comparative strength.
1.6
Ballistic fiberglass
1.6.1
Raw materials
The primary component of glass fiber is silica, but it also includes varying quantities of
feldspar, sodium sulfate, anhydrous borax, boric acid, and many other materials. The
30
Lightweight Ballistic Composites
Raw materials
Limestone Silica sand Boric acid
Fluorspar
Clay
Coal
Hopper
Binder
formulation
Platinum
bushings
Tank
Screw feeder
Automatic controls
Binder
applicator
Mixer
Hopper
High-speed
winder
Figure 1.39 Glass fiber manufacturing (Fiberglass).
raw materials are weighed according to the desired product recipe and then blended
well before their introduction into the melting unit. The weighing, mixing, and
charging operations may be conducted in either batch or continuous mode (Fig. 1.39).
1.6.2
Glass melting and refining
In the glass-melting furnace, the raw materials are heated to temperatures ranging from
1500 to 1700 C (2700e3100 F) and are transformed through a sequence of chemical
reactions to molten glass. The furnaces are generally large, shallow, and well-insulated
vessels that are heated from above. In operation, raw materials are introduced continuously on top of a bed of molten glass, where they slowly mix and dissolve. Mixing is
effected by natural convection, by gases rising from chemical reactions, and, in some
operations, by air injection into the bottom of the bed.
Glass-melting furnaces can be electric, gas-fired, or oil-fired. Electric furnaces are
currently used only for wool glass fiber production because of the electrical properties
of the glass formulation.
1.6.3
Textile glass fiber spinning
Molten glass from either the direct melting furnace or an indirect marble-melting
furnace is temperature regulated to a precise viscosity and delivered to forming
stations.
At the forming stations, the molten glass is forced through heated platinum bushings containing numerous very small openings to form fibers. The continuous fibers
emerging from the openings are drawn over a roller applicator, which applies a coating
of a water-soluble sizing and/or a coupling agent. The coated fibers are then gathered
High-performance ballistic fibers and tapes
31
and wound into a spindle. The spindles of glass fibers are next conveyed to a drying
oven where moisture is removed from the sizing and/or coupling agents.
1.6.4
Fiberglass structure and morphology
Fiberglass is an amorphous material that is neither solid nor liquid. Fiberglass does not
possess either the crystalline structure of solids or the flow characteristics of liquids.
Chemically, fiberglass comprises primarily a silica (SiO)2 backbone in the form of
(eSiO4e)n groups.
Since silica by itself requires an extremely high temperature for liquefaction and fiber spinning, modifiers are utilized to reduce glass temperatures to workable levels as
well as obtaining molten-glass viscosities suitable for fiber spinning. Table 1.3 lists
typical properties of fiberglass.
1.6.5
Applications of fiberglass
Fiberglass, either in woven form or in UD crossplied form, is not used for flexible body
armor applications because of its relatively low ballistic resistance against handgun
bullets.
Fiberglass for molded armor applications typically is provided with a starch finish,
which provides tailed bonding between the fiberglass and later applied resins for
achieving high ballistic performance without shattering or too much delamination.
For example, fiberglass yarns (generally called rovings) are often used to weave
2 2 basket-weave fabrics wherein the fabric is coated with a phenolic/polyvinyl
butyral (PVB) resin system.
Both autoclave and hydraulic press molding can be used for converting fiberglass/
phenolic/PVB prepregs into molded ballistic panels.
Depending upon the type of metal and metal thickness in a vehicle, molded fiberglass spall liners may be designed for military vehicles. For certain armor-piercing bullets, a ceramic is typically added to the front of the panel facing the armor-piercing
bullet. On impact, the ceramic blunts and in some cases tumbles the bullet, and the
molded fiberglass backing behind the ceramic absorbs the leftover kinetic energy of
the bullet and fragments of the ceramics and bullets.
Table 1.3 Typical properties of fiberglass (Fiberglass)
Yarn property
E-glass
S-glass
Tensile strength (GPa)
3.4
4.5
Initial modulus (GPa)
72e80
87e90
Elongation (%)
3e4
5.4
3
2.55
2.49
Density (g/cm )
32
1.7
Lightweight Ballistic Composites
High-modulus polypropylene fiber
(Elizabeth Cates, 2015)
HMPP fiber is a melt-spun fiber based on highly oriented polypropylene. These fibers
are characterized by high toughness, excellent chemical resistance, and low density.
Innegra™ S from Innegra Technologies is the only commercially available HMPP
fiber at the time of writing.
1.7.1
Manufacturing process
HMPP fiber is spun from molten polymer in an extrusion process (Fig. 1.40). The
rheological limits of the fiber melt-spinning process place certain practical limits on
the molecular weight of the polymers used, in contrast with the gel-spinning process
used to produce UHMWPE fibers. The molten polymer is quenched to a solid shortly
after exiting the spinneret. As with most of the high-performance fibers, the characteristic structure of the fiber is developed by drawing the filaments to increase the crystallinity and alignment of the polymer crystals within the fiber.
1.7.2
Structure of fiber
HMPP fibers crystallize under tension, producing a microfibrillar structure similar to
UHMWPE and para-aramid fibers. During the drawing process, the transition of the
polymer from the lower density amorphous structure to the higher density crystalline
structure results in the formation of voids in the fiber as the polymer chains reorient.
The resulting void content creates a fiber with a bulk density lower than the polymer
density (Fig. 1.41).
Wide-angle X-ray (WAXS) of HMPP fibers clearly shows the high degree of crystallinity and orientation of the polymer chains, with crystallinity levels over 70% and
Herman’s orientation function over 0.7. The crystalline phase is the thermodynamically favored a-monoclinic form. Crystallite size is estimated to be around 100 Å
based on WAXS measurements (Fig. 1.42).
Metering pump
Spinneret
Extruder
Quench
Figure 1.40 Manufacturing process of HMPP.
Drawing process
High-performance ballistic fibers and tapes
S4800 5.0 kV 7.8 mm × 4.00 k SE(M) 3/18/2008
33
10.0 mu
Figure 1.41 Scanning electron micrograph of HMPP fiber cross section showing microfibrillar
structure with voids. Fiber axis is horizontal in image.
Figure 1.42 Wide-angle X-ray of HMPP fiber.
34
Lightweight Ballistic Composites
1.7.3
Properties
1.7.3.1
Tensile properties
The tensile properties of HMPP fall between those of high-performance fibers and
commodity fibers. The predicted ultimate tensile strength of polypropylene is substantially lower than that of polyethylene, so this difference in tensile properties is expected. The higher elongation at break, relative to high-performance fibers, gives
the HMPP fibers a higher degree of toughness. This is especially evident upon cryogenic exposure of the fibers, in which they have proven to be resistant to cryofracturing
for structural examination. Properties of HMPP fiber are given in Table 1.4.
1.7.3.2
Thermal properties
In examination of the thermal properties of HMPP by differential scanning calorimetry
(DSC), multiple endotherms may be observed, with the initial peak melting range of
160e164 C and a higher melting endotherm range around 171e175 C. This higher
melting endotherm has been attributed variously in the literature to a more perfect crystal structure, which is dependent on the isotacticity of the base polymer, or to crystalline transformation from the a1-monoclinic C2/c space group to the higher order
a2-monoclinic P21/c space group, where the polymer chain helices pack more
compactly. It is unclear if the higher endotherms measured by DSC are truly an attribute of the fiber or if they are a result of recrystallization of the polymer on the time
scale of the DSC scans. Regardless, the DSC scans of HMPP fiber tend to yield
sharper, more intense peaks than those of conventional polypropylene or even
high-tenacity polypropylene fibers.
1.7.3.3
Chemical and moisture properties
HMPP is a hydrophobic material with very low moisture regain of <0.1%. HMPP
fibers have been demonstrated to have excellent chemical resistance to most classes
Table 1.4 Properties of HMPP (United States Patent
Application US2011/0268951 A1, 2011)
Property
HMPP fiber
Density(g/cm3)
0.91 (polymer)
0.85e0.85 (bulk)
Tenacity (g/denier)
9e10
Tensile elastic modulus (g/denier), GPa
200
16
Strain to tensile failure (%)
8e10
Creep at 20% ultimate tensile strength (%)
3.2
Peak melting range ( C)
160e164
High-performance ballistic fibers and tapes
35
of chemical threats. They are virtually unaffected by aqueous solutions of salts, acids,
or bases and most polar organic solvents such as alcohols. The resistance to salt-water
degradation and low density of HMPP make it especially well suited to marine applications, in which flotation is a desirable attribute.
1.7.3.4
Comments about use in ballistics
While the relatively low tensile strength of HMPP, in comparison to high-performance
fibers such as para-aramid or UHMWPE, would seem to preclude its use in ballistic
applications, it has been found to have some interesting attributes that warrant consideration. The toughness of the HMPP fiber, in particular, may provide some benefits in
antiballistic structures.
The sonic velocity of fibers is frequently used as a quick gauge of their suitability
for ballistic applications. HMPP has been shown to have an unexpectedly high sonic
velocity of approximately 5500 m/s (Fig. 1.43).
Fabrics made from HMPP have been found to have good performance against fragmentation impacts. The inclusion of HMPP into a ballistic package can improve fragment trapping and can be used to reduce back-face deformation in some
configurations. The higher strain to failure and resulting toughness of HMPP fiber
make it an especially good partner for brittle fibers in impact-resistant laminates.
1.8
Recycling of ballistic fibers and converted products
The current procurement of high-performance ballistic products has been on a long
road toward the responsible recycling of all ballistic vests and military and police helmets after their service life.
During the manufacturing of flexible vests for military and law enforcement and after
the replacement of old vests, a significant amount of ballistic material is available
7000
Sonic velocity (m/s)
6000
5000
4000
3000
2000
1000
0
Para-aramid
HMPP
Std PP
Figure 1.43 Sonic velocity of para-aramid, HMPP, and standard polypropylene (Std. PP) fibers.
36
Lightweight Ballistic Composites
for recycling. Recycling of ballistic materials is technically achievable, but it costs
money and a firm commitment from fiber manufacturers and demand from the end users.
Three processes are used for recycling: grinding, incineration, and pyrolysis.
Grinding reduces material to small pieces or powders to be reused in other products.
Potentially all material that can be reground can be used as a recyclate; there is little or
no unused waste.
Incineration, or “thermal oxidation,” is burning the material to create heat for other
purposes, such as making steam to power turbines that generate electricity.
Pyrolysis is the process of chemically decomposing or transforming a material into
one or more recoverable substances by heating it to very high temperatures in an
oxygen-free environment. This is different from incineration, which takes place in an
open atmosphere. Pyrolyzed fiberglass ballistic panels decompose into three recoverable substances: pyro-gas, pyro-oil, and solid by-productdall of which can be recycled.
To pyrolyze fiberglass/phenolic panels, they are shredded into 2-inch squares that
are fed directly into the pyrolysis reactor by vacuum assist, which also draws off most
of the oxygen in the atmosphere. The reactor is then heated to around 14,000 F
(7760 C). At about 5000 F (2760 C), the hydrocarbons in the resin decompose into
gas. The gas is drawn off and sent through a scrubber, which separates it into
pyro-gas and pyro-oil. The pyro-gas is very clean and has energy content similar to
that of natural gas. It can be sold as a natural gas replacement, and it fuels the burners
of the pyrolysis reactor so that the reaction is self-sustaining. Pyro-oil is similar to
heavy crude oil and, as such, it has less value than normal crude oil, but it can be
blended with other fuel oils or incorporated into asphalt. Pyro-gas and pyro-oil
comprise about 25% of the pyrolysis reaction output in roughly equal amounts.
They are free of sulfur, halogens, phosphorus, heavy metals, or other elements that
can cause environmental problems.
1.8.1
UHMWPE fibers and tapes
One of the advantages of disposal of UHMWPE fibers is the chemistry of the
UHMWPE fibers. No toxic fumes or residue powder is left after incineration, or thermal oxidation, of the fibers. The process creates heat for other purposes, such as making steam to power turbines that generate electricity.
1.8.1.1
Waste from woven and uncoated fabric
The fibers from woven and uncoated fabrics can be chopped to a certain length and
used for continuous staple fibers for industrial applications. Both UHMWPE fibers
and woven fabrics can also be incinerated to generate energy for local power supply
companies.
1.8.1.2
Waste from coated fabric and crossply unidirectional
materials
A number of companies have attempted to separate out fibers. However, because of the
different chemistries of coated resins on different crossplied UD materials, only some
have succeeded in removing the majority of the coatings. Once fabric coatings are
removed, the fibers can be chopped and converted into staple fibers.
High-performance ballistic fibers and tapes
37
Also, both UHMWPE coated fabric and crossply UD materials can be incinerated
and generate energy for local power supply companies.
1.8.2
Aramid fibers
The suppliers of aramid fibers and a few other converter companies offer recycling of
aramid fibers, aramid fabrics, coated aramid prepregs, and UD crossplied aramid fiber
materials.
The recycled fibers are converted into pulp, which can be used as an asbestos
replacement in numerous high-end applications. Such recycled aramid fibers may
also be converted into spun yarns for other applications. This variety of recycling options enables a broad range of aramid scrap to be accepted.
Another method of recycling aramid fibers and fabrics requires chopping and mixing them with phenolic resin for molding into brake pads for passenger cars.
1.8.2.1
Waste from weaving and uncoated fabric
The waste of aramid woven fabrics and uncoated fabrics can be chopped and mixed
with virgin fibers and phenolic resin and molded into brake linings and brake pads.
1.8.2.2
Waste from coated fabric and crossply UD materials
Both coated and crossplied aramid waste is ground and mixed with phenolic resin for
brake-lining applications.
1.8.3
1.8.3.1
Glass fibers
Waste from weaving and uncoated fabric
Fiberglass and woven fabrics may be ground into small pieces or powders that can be
reused in other products as filler, especially for sheet molding compound and bulk
molding compound.
1.8.3.2
Waste from coated fabric and crossply UD materials
A by-product of incinerated coated fiberglass and incinerated crossply UD fiberglass is
ash, which usually goes straight to a landfill. The heat content of fiberglass molded ballistic composites comes from the organic materials in the resin coating. Since fiberglass ballistic composites are typically resin starved, having resin content from 10%
to 20%, the ash content is high. The ash is primarily calcium oxide, which comes
from the calcium carbonate, boron, and other oxides in the glass.
Acknowledgment
Thanks to Elizabeth Cates, Ph.D., Vice President, Research & Development of Innegra Technologies, LLC, for providing information about high-modulus polypropylene fibers.
38
Lightweight Ballistic Composites
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