Table of Contents

INTRODUCTION

I-History and Development of Kevlar……………………………….….…………..1

II-DuPont's Kevlar Production Facility………..……………………..…………….2

III-Composition of Kevlar……………………………………………..…………….4

A) Grades of Kevlar……………………………………..…………….…………4

B) Mechanical Properties of Kevlar……………………………….……………4

C) Technical Properties……………...……………………………..……………6

D) Advantages and Disadvantages of Kevlar………….……….….…...………6

E) Applications of the Kevlar Fibers………………………………….………..7

F) A comprehensive list of Kevlar application………………..………………8

IV-PROCESS DESCRIPTION

A) The Spinnerets………………………………………………………………….9

B) Types of Spinning Methods…………………...………..…………………….10

1- Wet Spinning…………………...……………..…………………………10

2- Dry Spinning…………………...……...…………………………………11

3- Melt Spinning……………………...…………………………………….12

4- Gel Spinning……………...….…….…………………………………….13

V-DETAIL MANUFACTURING OF HIGH STRENGTH KEVLAR FIBERS

A) Affects of the Crystallinity of Kevlar……………...………….……………..14

B) Spinning Solvent………………………………….……..……………………..14

C) Dry Jet Wet Spinning Method………………………….…………………….15

CONCLUSION

Reference ………………………………………………….….…………………….16

0

Manufacturing Of High Strength Kevlar Fibers

INTRODUCTION

All fibers used in polymer engineering composites can be divided into two

categories, namely synthetic fibers and natural fibers. Synthetic fibers are the most

common. Although there are many types of synthetic fibers, glass, carbon and aramid

fibers represent the most important.

Kevlar is an aromatic polyamide or aramid fiber introduced in early 1970s by

DuPont. It was the first organic fiber with sufficient tensile strength and modulus to

be used in advanced composites. It has approximately five times the tensile strength

of steel with a corresponding tensile modulus. Originally developed as a replacement

for steel in radial tires, Kevlar is now used in a wide range of applications. It is a trade

name of aramid fiber. [13]

The U.S. Federal Trade Commission gives a good definition of 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" [14].

History and Development of Kevlar

Kevlar's history goes back to 1948, when DuPont, with its invention of nylon

behind it, made a decision to pursue work in a broad area of fibers with unusually

high thermal, elasticity, and strength properties. [13]

The possibility of making polyaramid plastic was hypothesized in 1939. It was

synthesized and identified at DuPont in 1960, but polyaramid fiber could not be

produced until 1965, when Stephanie Kwolek, a chemist at DuPont, discovered a

practical solvent. At about the same time, a team at Akzo, Inc., a multinational firm

headquartered in Holland, independently discovered a practical solvent and applied

for a patent for the manufacture of polyaramid fiber, which DuPont named Kevlar®

and Akzo later (1984) named Twaron®. DuPont contested the patent. A consent

decree of the International Trade Commission settled the dispute; terms of the

settlement included cross-licensing but barred Akzo from marketing Twaron® in the

United States until late 1990.

1

Kevlar 29, introduced in the early 1970s, was the first generation of bullet

resistant fibers developed by DuPont and helped to make the production of flexible,

concealable body armor practical for the first time. In 1988, DuPont introduced the

second generation of Kevlar fiber, known as Kevlar 129. According to DuPont, this

fabric offered increased ballistic protection capabilities against high energy rounds

such as the 9mm FMJ. In 1995, Kevlar Correctional was introduced, which provides

puncture resistant technology to both law enforcement and correctional officers

against puncture type threats.

The newest addition to the Kevlar line is Kevlar Protera, which became

available in 1996 by DuPont. DuPont contends that the Kevlar Protera is a high-

performance fabric that allows lighter weight, more flexibility, and greater ballistic

protection in a vest design due to the molecular structure of the fiber. Its tensile

strength and energy-absorbing capabilities have been increased by the development of

a new spinning process. [17]

Before Kevlar® was used for body armor, it was used as a substitute for steel

in the manufacture of radial tires, including those designed for police cars. “Kevlar” is

a registered trademark of DuPont de Nemours and Co., Inc. “Twaron” is a registered

trademark of Akzo, Inc.

DuPont's Kevlar Production Facility

DuPont will expand its Kevlar para-aramid fiber production facility in

Richmond, Va., by adding a new production line at the site, increasing its capacity by

the end of 2002. Total investment for this expansion is expected to be roughly $50

million. [5]

The company has already completed the first phase of an expansion begun

early in 2001 and has increased global production capacity for Kevlar fiber by 15

percent. The second expansion will address a two year trend of growing demand for

high-performance, high strength para-aramid fibers, which has exceeded global

manufacturing capabilities. The capacity expansion is based on technology developed

and patented by DuPont and used in the company's European operations for the past

four years [5]. For example, Toray Industries Inc. of Japan and E.I. DuPont de

2

Nemours of the U.S. said they have agreed to build a joint plant in Japan to

manufacture Du- Pont's kevlar fiber [7].

Composition of Kevlar:

The chemical composition of Kevlar is poly para-phenyleneterephthalamide

(PPD-T) and it is more properly known as a para-aramid. It is oriented para-

substituted aromatic units. Aramids belong to the family of nylons. Common nylons,

such as nylon 6,6 do not have very good structural properties, so the para-aramid

distinction is important. Aramid fibers like Nomex or Kevlar, however, are ring

compounds based on the structure of benzene as opposed to linear compounds used to

make nylon. The aramid ring gives Kevlar thermal stability, while the para structure

gives it high strength and modulus. Like nylons, Kevlar filaments are made by

extruding the precursor through a spinneret. The rod form of the para-aramid

molecules and the extrusion process make Kevlar fibers anisotropic--they are stronger

and stiffer in the axial direction than in the transverse direction. In comparison,

graphite fibers are also anisotropic, but glass fibers are isotropic. [14]

Figure1: Chemical composition of Kevlar [17]

It is made from a condensation reaction of para-phenylene diamine and

terephthaloyl (PPD-T) chloride. The resultant aromatic polyamide contains aromatic

and amide groups which makes them rigid rod like polymers. The rigid rod like

structure results in a high glass transition temperature and poor solubility, which

makes fabrication of these polymers, by conventional drawing techniques, difficult

Instead, they are melt spun from liquid crystalline polymer solutions as described

later. The Kevlar fiber is an array of molecules oriented parallel to each other like a

package of uncooked spaghetti. This orderly, untangled arrangement of molecules is

described as a crystalline structure. Crystallinity is obtained by a manufacturing

3

process known as spinning, which involves extruding the molten polymer solution

through small holes. [14]

When PPD-T solutions are extruded through a spinneret and drawn through an

air gap during fiber manufacture, the liquid crystalline domains can orient and align in

the flow direction. Kevlar can acquire a high degree of alignment of long, straight

polymer chains parallel to the fiber axis. The structure exhibits anisotropic properties,

with higher strength and modulus in the fiber longitudinal direction than in the axial

direction. The extruded material also possesses a febrile structure. This structure

results in poor shear and compression properties for aramid composites. Hydrogen

bonds form between the polar amide groups on adjacent chains and they hold the

individual Kevlar polymer chains together [8]. It is shown as in the following figure:

Figure 2: Hydrogen bonds form between the polar amide groups

Grades of Kevlar

There are three grades of Kevlar available: Kevlar 29, Kevlar 49, and Kevlar

149. Tensile modulus is a function of molecular orientation. As a spun fiber, Kevlar

29 (a high toughness variant) has a modulus of 62 GPa (9 Mpsi). Heat treatment under

tension increases crystalline orientation. The resulting fiber, Kevlar 49, has a modulus

of 131 GPa. [14]

Mechanical Properties of Kevlar:

The tensile strength of Kevlar ranges from about 2.6 to 4.1 GPa. This is more

than twice that for conventional fibers like Nylon 66. Tensile failure initiates at the

fibril ends and propagates via shear failure between the fibrils. The table below shows

4

the differences in material properties among the different grades. Kevlar cloth is most

likely to be Kevlar 49.

Grade Densityg/cm^3

TensileModulus

GPa

TensileStrength

GPa

TensileElongation

%

29 1.44 83 3.6 4.0

49 1.44 131 3.6--4.1 2.8

149 1.47 186 3.4 2.0

Table 1: Differences in material properties among the different grades of Kevlar. [14]

The tensile modulus and strength of Kevlar 29 is roughly comparable to that

of glass (S or E), yet its density is almost half that of glass. Thus, to a first

approximation, Kevlar can be substituted for glass where lighter weight is desired.

Kevlar 49 or 149 can cut the weight even further if the higher strength is accounted

for. Of course, Kevlar's weight savings does come at a price: Kevlar is significantly

more expensive than glass [13]. DuPont sees kevlar as a replacement for steel cable

on offshore oil rigs [12].

Kevlar behaves elastically in tension. In compression, it shows nonlinear,

ductile behavior. It exhibits yield at compression strains of 0.3 to 0.5%. This

corresponds to formation of structural defects known as kink bands. These bands are

related to compressive buckling of the aramid molecules. Aramid fibers are noted for

toughness and general damage tolerance. Kevlar 29 has the lowest modulus and

highest toughness and the tensile elongation of Kevlar 29 is about 4%. The fibrillar

structure and compression behavior contribute to composites that are less notch-

sensitive and which fail in a ductile, non-catastrophic manner, as opposed to glass and

carbon. [8]

The aromatic structure gives the fibers a high degree of thermal stability. They

decompose in air at about 425°C and are inherently flame resistant. Aramids have a

slight negative longitudinal coefficient of thermal expansion of about -2 x 10 -6/K and

a positive transverse expansion of 60 x 10-6/K. They also have a low thermal

conductivity that varies by about an order of magnitude in the longitudinal versus

transverse direction. [8]

5

Technical Properties:

Technical properties claimed of kevlar can be summarized as follows:

* High strength to weight ratio.

* Low ductility.

* High modulus of rigidity (structural rigidity).

* Low electrical conductivity.

* High chemical resistance.

* Low coefficient of thermal expansion.

* High toughness (work-to-break).

* Excellent dimensional stability.

* Low machinability.

* Flame retardant, self-extinguishing. [3]

Advantages and Disadvantages of Kevlar:

Advantages are:

It has the lowest specific gravity.

It has the highest tensile strength-weight ratio.

Only fiber for structural application.

Disadvantages are:

Low compressive strength.

Difficult to machine (Low machinability). [9]

6

Applications of the Kevlar Fibers:

Because of its chemical nature and linearly oriented polymer structure, kevlar

claims an excellent combination of physical properties. High tensile strength, high

stiffness and damage tolerance, and high thermal stability with self-extinguishing

properties (kevlar fiber does not melt), make kevlar fibers ideal for a huge range of

demanding applications. These exceptional properties, particularly its high strength to

weight ratio, temperature resistance and versatility, are thought responsible for kevlar

being used in a huge range of demanding applications ranging from deep sea

umbilical lines and premium sports goods to high performance structural composites

in boat hulls, aircraft components and high-performance cars. [3]

The kevlar fiber is suggested to be used in flywheel of a commuter car because

the kevlar fiber glass flywheel has a greater strength-to-weight ratio so that it can spin

much faster than flywheels made of other materials which would shatter. [1]

Kevlar is now being used to produce lightweight bulletproof body armor, and

kevlar vests are currently being worn by many of the country's policemen. However,

kevlar's main chance seems to lie in markets created by the energy problem.

Replacing heavier materials with Kevlar in airplanes, for instance, saves on fuel. Du

Pont also sees kevlar as a replacement for steel cable on offshore oil rigs. [12]

Kevlar fiber and butacite laminated glasses that make buildings safer, more

durable and more efficient. Kevlar is a lightweight, synthetic fiber, five times stronger

than steel on an equal weight basis, used to protect buildings from blasts. [6]

Wheels made with a single carbon fiber and Kevlar spoke can run

continuously from rim to rim, wrapping around, but not ending at the hub. The Kevlar

and carbon fibers are manipulated into an airfoil shape and coated with a thin layer of

clear thermoplastic resin. The spokes are then shaped to pass through the carbon-

composite hub shell and span across the wheel. The spokes are joined to a standard

rim with stainless steel fasteners and a standard alloy nipple.

Carbon fiber components are used in manufacturing of motorcycles. Baxter, a

textile engineering graduate of Clemson University, went on to invent Draggin' Jeans,

7

which use 100% Kevlar in denim jeans. About 11 inches of the protective fabric is

used in the knees, seam to seam, and the entire rear is covered with it. [2]

A comprehensive list of Kevlar applications

A comprehensive list of Kevlar applications includes:

•Adhesives and sealants — Thixotropes;

•Ballistics and defense — Anti-mine boots, cut-resistant gloves, composite helmets,

and bullet- and fragmentation-resistant vests;

•Belts and hoses — Automotive heating/cooling systems, industrial hoses, and

automotive and industrial synchronous and power transmission belts;

•Composites — Aircraft structural body parts and cabin panels, boats, and sporting

goods;

•Fiber optic and electromechanical cables — Communications and data transfer

cables; ignition wires; and submarine, aerostat and robotic tethers;

•Friction products and gaskets —Asbestos replacement, automotive and industrial

gaskets for high-pressure/high-temperature environments; brake pads; and clutch

linings;

•Protective apparel — Boots; chain-saw chaps; cut-resistant industrial gloves;

helmets (both for firefighters and consumer bicyclists); and thermal- and cut-

protective aprons, sleeves, etc.;

•Tires — Aircraft, automobiles, off-road, race vehicles and trucks; and

•Ropes and cables — Antennae guy wires, fishing line, industrial and marine utility

ropes, lifting slings, mooring and emergency tow lines, netting and webbing, and

pull tapes. [19]

8

PROCESS DESCRIPTION

Most synthetic and cellulosic manufactured fibers are manufactured by

“extrusion” — forcing a thick, viscous liquid (about the consistency of cold honey)

through the tiny holes of a device called a spinneret to form continuous filaments of

semi-solid polymer.

In their initial state, the fiber-forming polymers are solids and therefore must

be first converted into a fluid state for extrusion. This is usually achieved by melting,

if the polymers are thermoplastic synthetics (i.e., they soften and melt when heated),

or by dissolving them in a suitable solvent if they are non-thermoplastic cellulosics. If

they cannot be dissolved or melted directly, they must be chemically treated to form

soluble or thermoplastic derivatives. Recent technologies have been developed for

some specialty fibers made of polymers that do not melt, dissolve, or form appropriate

derivatives. For these materials, the small fluid molecules are mixed and reacted to

form the otherwise intractable polymers during the extrusion process. [16]

The spinnerets:

The spinnerets used in the production of most manufactured fibers are similar,

in principle, to a bathroom shower head (see figure 3). A spinneret may have from

one to several hundred holes. The tiny openings are very sensitive to impurities and

corrosion. The liquid feeding them must be carefully filtered (not an easy task with

very viscous materials) and, in some cases, the spinneret must be made from very

expensive, corrosion-resistant metals. Maintenance is also critical, and spinnerets

must be removed and cleaned on a regular basis to prevent clogging.

Figure 3: Spinnerets

9

As the filaments emerge from the holes in the spinneret, the liquid polymer is

converted first to a rubbery state and then solidified. This process of extrusion and

solidification of endless filaments is called spinning, not to be confused with the

textile operation of the same name, where short pieces of staple fiber are twisted into

yarn. There are four methods of spinning filaments of manufactured fibers: wet, dry,

melt, and gel spinning. Table 1 lists the different types of spinning methods with the

fiber types produced by each method. [16]

Table 2: Types of spinning methods and fiber types produced.

Types of Spinning Methods

(1) Wet Spinning:

Wet spinning is the oldest process. It is used for fiber-forming substances that

have been dissolved in a solvent. The spinnerets are submerged in a chemical bath

and as the filaments emerge they precipitate from solution and solidify. [16]

The process begins by dissolving polymer chips in a suitable organic solvent,

such as dimethylformamide (DMF), dimethylacetamide (DMAc), or acetone, or in a

weak inorganic acid, such as zinc chloride or aqueous sodium thiocyanate. In wet

10

spinning, the spinning solution is extruded through spinnerettes into a precipitation

bath that contains a coagulant (or precipitant) such as aqueous.

Because the solution is extruded directly into the precipitating liquid, this process for

making fibers is called wet spinning. Acrylic, rayon, aramid, modacrylic and spandex

can be produced by this process. [10]

Figure 4: Wet spinning

(2) Dry Spinning:

Dry spinning is also used for fiber-forming substances in solution. However,

instead of precipitating the polymer by dilution or chemical reaction, solidification is

achieved by evaporating the solvent in a stream of air or inert gas. The filaments do

not come in contact with a precipitating liquid, eliminating the need for drying and

easing solvent recovery. [10]

The dry spinning process begins by dissolving the polymer in an organic

solvent. This solution is blended with additives and is filtered to produce a viscous

polymer solution, referred to as "dope", for spinning. The polymer solution is then

extruded through the spinnerets as filaments into a zone of heated gas or vapor. The

solvent evaporates into the gas stream and leaves solidified filaments, which are

further treated using one or more processes (See Figure 5.) [10]

11

Figure 5: Dry spinning

This process may be used for the production of acetate, triacetate, acrylic,

modacrylic, PBI, spandex, and vinyon. [16]

(3) Melt Spinning

In melt spinning, the fiber-forming substance is melted for extrusion through

the spinneret and then directly solidified by cooling. Nylon, olefin, polyester, saran

and sulfur are produced in this manner. [10]

Melt spinning uses heat to melt the polymer to a viscosity suitable for

extrusion. This type of spinning is used for polymers that are not decomposed or

degraded by the temperatures necessary for extrusion. Polymer chips may be melted

by a number of methods. The trend is toward melting and immediate extrusion of the

polymer chips in an electrically heated screw extruder. Alternatively, the molten

polymer is processed in an inert gas atmosphere, usually nitrogen, and is metered

through a precisely machined gear pump to a filter assembly consisting of a series of

metal gauges interspersed in layers of graded sand. The molten polymer is extruded at

high pressure and constant rate through a spinneret into a relatively cooler air stream

that solidifies the filaments. Lubricants and finishing oils are applied to the fibers in

the spin cell. At the base of the spin cell, a thread guide converges the individual

filaments to produce a continuous filament yarn, or a spun yarn, that typically is

composed of between 15 and 100 filaments. Once formed, the filament yarn either is

12

immediately wound onto bobbins or is further treated for certain desired

characteristics or end use. [10]

Melt spun fibers can be extruded from the spinneret in different cross-

sectional shapes (round, trilobal, pentagonal, octagonal, and others). Trilobal-shaped

fibers reflect more light and give an attractive sparkle to textiles. Pentagonal-shaped

and hollow fibers, when used in carpet, show less soil and dirt. Octagonal-shaped

fibers offer glitter-free effects. Hollow fibers trap air, creating insulation and provide

loft characteristics equal to, or better than, down. [16]

(4) Gel Spinning

Gel spinning is a special process used to obtain high strength or other special

fiber properties. The polymer is not in a true liquid state during extrusion. Not

completely separated, as they would be in a true solution, the polymer chains are

bound together at various points in liquid crystal form. This produces strong inter-

chain forces in the resulting filaments that can significantly increase the tensile

strength of the fibers. In addition, the liquid crystals are aligned along the fiber axis

by the shear forces during extrusion. The filaments emerge with an unusually high

degree of orientation relative to each other, further enhancing strength. The process

can also be described as dry-wet spinning, since the filaments first pass through air

and then are cooled further in a liquid bath. Some high-strength polyethylene and

aramid fibers are produced by gel spinning. [16]

13

DETAIL MANUFACTURING OF HIGH STRENGTH KEVLAR FIBERS

Although the specific details of the manufacturing of aramid fibers remain

proprietary secrets, it is believed that the processing route involves solution

polycondensation of diamines and diacid halides at low temperatures. [15]

Affects of the Crystallinity of Kevlar

The most important point is that the starting spinnable solutions that give high

strength and high modulus fibers have liquid crystalline order. Various states of

polymer in solution depend on the type of polymer chain. Two-dimensional, liner,

flexible chain polymer in solution is called random coils. If the polymer chain can be

made of rigid units, that is, rod like, they can be represented like a random array of

rods. Any associated solvent may contribute to the rigidity and to the volume

occupied by each polymer molecule. With increasing concentration of rod like

molecules, one can dissolve more polymers by forming regions of partial order, that

is, regions in which the chains form a parallel array. This partially ordered state is

called a liquid crystalline state. When the rod like chains become approximately

arranged parallel to their long axes but their centers remain unorganized or randomly

distributed, we have what is called a nematic liquid crystal. It is this kind of order that

is found in the extended chain polyamides. [15]

Liquid crystal solutions, because of the presence of the ordered domains, are

optically anisotropic, that is birefringent. The parallel array of polymer chains in the

liquid crystalline state becomes even more ordered when these solutions are subjected

to shear. It is this inherent property of liquid crystal solutions which is exploited in the

manufacture of aramid fibers (trade name of kevlar). The characteristic fibrillar

structure of aramid fibers is due to the alignment of polymer crystallites along the

fiber axis. [15]

Spinning Solvent

Organic Fibers Researchers at DuPont discovered a spinning solvent for poly

p-benzamide (PBA) and were able to dry spin quite strong fibers from

tetramethylurea-LiCI solutions. This was the real breakthrough. The modulus of these

as spun organic fibers was greater than that of glass fibers. P-Oriented rigid diamines

14

and dibasic acids give polyamides that yield, under appropriate conditions of solvent,

concentration, and polymer molecular weight, the desired nomadic liquid crystal

structure. One would like to have, for any solution spinning process a high molecular

weight to obtain improved mechanical properties, a low viscosity to ease processing

conditions, and a high polymer concentration to achieve a high yield. For para aramid,

poly p-phenyleneterephthalamide (PPD-T), trade name Kevlar, the nematic liquid

crystalline state is obtained in 100% sulfuric acid at polymer concentration of about

20%. The polymer solution is often referred to as the dope. The various spinning

processes available are classified as dry, wet and dry jet-wet spinning process

(mentioned earlier). [15]

Dry Jet Wet Spinning Method

For aramid fibers, the dry jet wet spinning method is employed. It is believed

that solution-polycondensation of diamines and diacid halides at low temperatures

(near 00C) gives the aramid forming polyamides. Low temperatures inhibit by product

generation and promote linear polyamide formation. The resulting polymer is

pulverized, washed, and dried. This is mixed with a strong acid (e.g., concentrated

sulphuric acid) and extruded through spinnerets at 100 0C through about 1-cm air

layer is to cold water (0-4 0 C). The fiber precipitates in the air gap and the acid is

removed in the coagulation bath. The spinneret capillary and air gap cause rotation

and alignment of the domains resulting in highly crystalline and oriented as-spun

fibers. At the end of this process, the Kevlar is produced. [15]

CONCLUSION

In this report, it is shown that fibers are formed by forcing a viscous fluid or

solution of the polymer through the small orifices of spinnerets and immediately

solidifying or precipitating the resulting filaments. This prepared polymer may also be

used in the manufacture of other nonfiber products such as the enormous number of

extruded plastic and synthetic rubber products.

15

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14- www.carb.com/. “Kevlar (Aramid) Comparisons”, 30/9/2003.

15- www.eng.uab.edu/compositesLab/b_fiber.htm. “Fiber Types”, 13/10/2003.

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Cellulosic Fiber Formation Technology”, 30/92003.

16

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17