1. 8. High Performance Fibers
8.1 Aramid fiber
In today’s fast growing market, apart from general apparel clothing, some special applications are
expected from textile fibers. Such fibers are used specifically for protective clothing and thus
require certain high performance properties. This generation of fibers has been recently developed
in the 20 th century and are called high performance fibers. A class of these fibers have very high
tenacity and high modulus which are used in applications such as bullet proof jackets, whereas
another class of fibers would have high thermal or chemical resistance which can be used as flame
resistant fabrics etc.
High performance fibers include polymeric fibers such as aramids, aromatic copolyesters,
extended chain flexible polyolefin etc.; carbon fiber; glass fibers; ceramic fibers and metallic
fibers. High performance polymeric fibers being used for high mechanical properties should be
highly oriented, linear aliphatic or aromatic molecules since flexible chains would give low melting
polymers and thus low thermal resistance. On the other hand, carbon fibers are planar graphite
structures with outstanding mechanical properties. Inorganic fibers have a three dimensional
structure compared to a one dimensional and two dimensional structure of polymeric materials
and carbon fibers respectively. They have very good mechanical properties but are brittle and
have the highest thermal resistance. However, carbon fibers as well as inorganic fibers (except
glass fibers) are very expensive. In the following sections, particularly, polymeric fibers would be
discussed in detail.
I. Polymeric fibers
1. Aramid fiber
Introduction
"Aramid" is a generic name for synthetic polyamide fibers that are derived from aromatic
molecules. Defining aramid fibers, it can be said that; Aramid fibers are synthetic fibers
manufactured of long chain synthetic polyamide in which at least 85% of the amide, (-NHCO-),
linkages are attached directly to two aromatic rings".
The aromatic structure of the aramid fibers makes them very strong and highly resistant to high
temperature. The differences between aramid fibers are due to the relative positions of the amine
and carboxyl groups in the benzene derivatives which are taken as the monomers for polymer
manufacture.
There are two very important commercial aramid fibers, called Nomex and Kevlar, both
invented by Du Pont Ltd., USA.
Nomex
Introduction
Nomex are meta-aramid fibers synthesized by polymerization of m-phenylene diamine and
dichloride of m-isopthalic chloride. This is perhaps the best known and most widely used of the
aramid fibers and was first produced by Dupont in 1961. They are known majorly for their
combination of heat resistance and strength. Additionally, meta-aramid fibers do not ignite, melt
or drip; a significant reason for their huge success in Flame repellent apparel market. In
comparison to commodity fibers, meta-aramids offer better long-term retention of mechanical
properties at elevated temperatures. Except for the positions of –NH- and –CO- groups, the
meta-phenylene isophthalamide molecule is identical to para-phenylene terephtalamide (or
Kevlar) molecules. This different positioning of –NH- and –CO- groups creates a zig-zag
molecular structure that prevents it from as high crystallization as Kevlar thus resulting in
relatively poorer tensile strength property. Nomex fibers have a relatively soft hand and can be
processed similarly to conventional fibers, giving them a wide range of converted products. Meta-
aramids are available in a variety of forms, anti-stat, conductive, in blends (with other high
performance fibers), etc.
2. Properties
Under microscopic view, the fiber is cylindrical in longitudinal view and dumb-bell or peanut shell
shaped in cross section. Nomex fiber is a strong fiber with a tenacity of 3.8-7.2g/denier. It has
high elastic property and its breaking elongation is 25-40%. It has a density similar to polyester
of 1.38g/cc. This fiber has average moisture regain of 4.5% in standard atmosphere.
Due to the aromatic rings present in the structure, the fiber is inherently flame retardant. It does
not melt, but decomposes at about 4000
C.
The chemical properties of the fiber are fair. It is unaffected by most acids under normal
conditions and has good resistance to alkalis, bleaches and solvents. Also, the fiber has excellent
resistance to sunlight, mildew and aging.
Uses of Nomex
Nomex finds applications in variety of products such as Space suits, Children's fire-proof dresses,
Fire fighters' coats, Racing drivers' overalls, Foundry workers' clothing, Protective gloves and
footwear, Covering cloth over an ironing board, High temperature filtration, Refinery Operators
uniform, Automotive cushion material, etc.
Kevlar
Introduction
Kevlar fibers are para aramid fibers rather than meta aramid structure of Nomex. The monomers
normally used for the production of the polymer for Kevlar fibers are p-phenylene diamine and
terephthaloyl chloride. In Kevlar, the para-substitution of the monomers allows the benzene rings
to lie centrally along the molecular axis. Due to this arrangement, greater number of
intermolecular bonds and a stronger and a more thermally and chemically resistant fiber is
formed than that for Nomex. These fibers have high tensile strength, high tensile modulus and
high heat resistance because of the highly oriented rigid molecular structure. Kevlar is about five
times lighter than steel in terms of the same tensile strength. This high strength Kevlar is
produced by a special spinning process called the Liquid crystal spinning. Now there exists a
series of first, second and third generations of para-aramids. For example, Kevlar HT which has
20% higher tenacity and Kevlar HM which has 40% higher modulus than the original Kevlar 29
are largely used in the composite and the aerospace industries.
Properties of Kevlar
Kevlar is a very strong fiber. In fact, it is the strongest textile fiber available today. It has
tenacity in the range 22-26 gpd with a breaking elongation of 2.5-4.4%. The fiber is cylindrical in
the longitudinal view of microscopic view and circular in cross-sectional view. It is a dense fiber
with density of 1.44-1.47g/cc. Similar to nomex, it has average moisture regain of about 4.3% in
standard atmosphere. Due to the aromatic structure of the polymer, Kevlar is difficult to ignite.
Para aramids generally have high glass transition temperatures nearing 370°C and do not melt or
burn easily, but carbonise at and above 425°C.
Regarding the chemical properties of the fiber, the fiber is unaffected by most acids under normal
conditions and has good resistance to alkalis and solvents, but not resistant to bleaches. It also
has excellent resistance to mildew and aging. All aramid fibers are however prone to
photodegradation and need protection against the sun when used out of doors.
Uses of Kevlar
Kevlar is used where its high strength and modulus are required. It is therefore used in Radial
tyres (as tyre cord), Conveyor belts and high-power transmission belts (as the reinforcing
component), Composites (as the reinforcing component), Aircraft parts and other high-
performance products (as the reinforcing fiber/yarn), Ropes and cables and mainly used in
Ballistics (bulletproof fabric/vests) and friction products.
Other aramid fibers
Some more aramid fibers containing paraphenylene rings have been produced using normal melt
5. on electroless Cu plating was studied. Favorable Ni weight gain rate of approximate 10% was
obtained. With this modification, the conductivity of the fiber due to deposits was increased and
the breaking strength of Ni-Cu coated Kevlar fibers obtained was 45N.
II. Inorganic Fibers
Inorganic fibers such as glass, asbestos and more recently carbon are well known and have been
extensively used for their unique characteristics. Glass fibers have been used since ancient Syrian
and Egyptian civilizations. However the stiffness or very high modulus of these fibers means that
they can be easily damaged by surface marks and defects and are quite brittle. Thus, they are
best utilised in composites wherein they are embedded inside a matrix and are fully protected.
Common resins used for making composites of glass fibers are epoxy resins, cement, polyester
and other polymers. By using glass fibers in composites their strength can be well used while
protecting the fiber. This fiber has good resistance to heat and very high melting point and so can
be used to make effective insulating materials.
Asbestos is a general name given for a variety of silicates occurring naturally in rocks. Anciently
fibers of asbestos were used as a reinforcing material in clay rich cooking pots in Finland. The
fibers extracted from these silicates have all textile-like properties of fineness, strength, flexibility
and good resistance to heat with high decomposition temperature of around 5500
C. These
materials are now used in reinforcing brittle matrices such as cement, pipes, plastics and as heat
insulators. However, knowing their carcinogenic hazards, their use has slowly declined.
Carbon Fibers
Carbon fiber is a fiber that contains at least 90% carbon in it and is obtained by controlled
pyrolysis of appropriate precursor fiber. The precursor fiber generally used for production of
carbon fibers is polyacrylonitrile fibers. It was first developed in 1963 in the laboratories of the
Royal Aircraft Establishment, UK. Carbon fibers are of great importance in the field of composites
as reinforcement fibers.
Properties of Carbon Fiber
Carbon fibers are black in colour and have a smooth surface and a silky lustre. They are commonly
of round cross-section, sometimes with flattened sides. The fine structure of carbon fiber is similar
to that of graphite which is pure carbon i.e. the carbon atoms are arranged in layers of hexagonal
arrays of the atoms, producing a crystalline structure. Due to close packing of the carbon atoms in
orderly manner, the carbon fiber is very strong and has a high modulus.
6. Carbon fibers are high strength, high-modulus fibers and ultramodulus fibers. The tenacity of the
fiber is around 11 – 24 gpd and the elongation-at-break ranges from 0.5 to 1.5%. The s pecific
gravity is in the range 1.77 to 1.96. Thus, carbon fiber is thus the second heaviest textile fiber.
Carbon fibers have zero percent moisture regain. The fibers have excellent resistance to acids and
alkalis even at high temperature. Strong oxidising agents will degrade the fiber. They are inert to
all common solvents but sensitive to hypochlorite. The fibers have excellent resistance to
microbiological organisms, aging and sunlight. Carbon fibers do not melt. It oxidises slowly at
temperatures above 3000
C.
Uses of carbon fiber
The applications of carbon fiber are only for industrial purposes. The high strength and modulus of
the fiber combined with relatively low extensibility means that they are best utilised in association
with epoxy or melt-spinnable aromatic resins as composites. Thus, t he biggest application of
carbon fibers is in composites, i.e. fiber-reinforced plastics. Carbon fiber composites are used in
aircraft structural components and in brakes and engines. They are also used in the construction
of space vehicles. These fibers are used to a great extent in sports and other areas where strength
is important. Thus in rackets of tennis, badminton, fishing rods carbon fibers are used. A large
application comes in the chemical and allied industry where pressure vessels and protective
clothing which requires inertness of chemical and heat is required are made of carbon fibers.
Recently work has led to development of carbon nanotubes which are spun into yarn and then
held together using a polymer resulting into a tough and strong but flexible material. It is a new
high performance fiber that can be used in replacement of Kevlar for bullet proof vests and
parachutes since it is better at absorbing energy without breaking. Further applications of this
material can be in composite materials for vehicles, airplanes and satellites in the future. The fiber
has been engineered by U.S. Department of Defence.
Recent Developments
In a recent application of carbon fiber, Carbon-fiber microelectrodes (CFMEs) were developed
which are typically constructed from glass capillaries pulled to a fine taper or from a polyimide-
coated capillary that is 90 μm in outer diameter. In a research work carried out by Zestos et al., a
new fabrication method is developed to insulate carbon-fiber microelectrodes with a thin epoxy
