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Design with Ultra Strong Polyethylene Fibers

26 Aug.,2024

Design with Ultra Strong Polyethylene Fibers

Materials Sciences and Applications

, , 2, 319-330

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doi:10./msa.. Published Online May (http://www.SciRP.org/journal/msa)

MSA

319

Design with Ultra Strong Polyethylene Fibers

Roelof Marissen

1,2

DSM Dyneema, Urmond, The Netherlands;

Delft University of Technology, Faculty of

Aerospace Engineering, Delft, The Nether-

lands.

: roelof.marissen@dsm.

com,

Received January 4

, ; revised March 11

, , accepted March 23

, .

ABSTRACT

Ultra strong polyethylene fibers can be made by gel-spinning of Ultra High Molecular Weight Polyethylene

(

UHMWPE

)

Such fibers exhibit extraordinary properties. They show very high tensile strength and stiffness and low density. On the

other hand

the axial and transverse compression strength is low.

This is a large difference with other advanced fibers

like glass and carbon fibers. Additionally

the fibers are chemically inert and the bonding strength to other materials

like resins is weak. Moreover, the coefficient of friction is very low

so the fiber is extremely slippery. Another property

is viscoelasticity

;

the fiber elongates due to creep

at higher loads or temperatures

. This exceptional combination of

properties explains why gel-spun UHMWPE fibers are not always applied in straight forward ways

e.g. like glass and

carbon fibers in composites. On the other hand

weaknesses like the limited compression strength are related to very

damage tolerant behavior on a micro scale. This opened application areas like providing of cut resistance. This paper

describes some established applications and shows the relationship between the properties and the applications. Fur-

thermore

some emerging applications are discussed and it is demonstrated how weaknesses can be turned into advan-

tages.

Keywords

Tensile

Compression

Friction

Creep

Density

Impact

1. Introduction

Very strong fibers have found various applications in

technology. Glass fibers were about the first non-metallic

fibers with strength levels exceeding 2 GPa. Structural

application of glass fibers in composites is well estab-

lished. Such composites are lightweight and high-strength

materials. Carbon fibers were developed later and are

stronger, stiffer and lighter th

an glass fibers. Carbon fiber

reinforced plastics are superior construction materials

with unsurpassed specific strength. Polymer fibers could

initially not reach strength levels that are comparable to

glass and carbon fiber s. However, so lvent based sp inning

technologies enabled the development of ultra strong

polymer fibers. Two classes of such fibers can be distin-

guished. One class is based on rigid rod molecules. Well-

known products are Kevlar®, Twaron® or Zylon®. The

molecular chains of these fibers exhibit some bending

stiffness. The other class is made of the very flexible

polyethylene molecules. A well known trade name is

Dyneema® from DSM. Spectr

a® is a similar fiber from

Honeywell. The interaction between the polymer mo-

lecular chains is low for polyethylene. Therefore, very

long chains are necessary to provide sufficient load

transfer between the macrom

olecules. Such a polyethy-

lene with long chains is called Ultra High Molecular

Weight Polyethylene (UHMWPE). The gel spinning

process starts from a high temperature solution of UHM-

WPE. Cooling causes crystallization from the solution,

thus a gel is obtained, containing polymer crystals and

solution. These crystals contain disentangled molecular

chains. The disentanglement is conserved during the re-

moval of the solvent. These disentangled chains can be

unravelled during a subsequent drawing process. The

drawing ratio is very high (about 100) and thus causes

extreme orientation of the UHMWPE chains. The paral-

lel oriented chains are again arranged in a crystalline

configuration. The crystallinity is high. Demco

et al.

[1]

analyzed various phases w

ith NMR. Roughly summa-

rized about 90% of the fiber material is crystalline. The

longitudinal chain orientation &#;translates&#; external ten-

sion loads to loads on the st

rong covalent bond s between

the carbon atoms of the chains, and this explains the high

tensile strength of the fibers. More detailed information

on the production and properties of such fibers is des-

cribed in earlier publications, e.g. by Smith and Lemstra

[2], and Jacobs [3]. An early application field that was

anticipated during the development of these very strong

Design with Ultra Strong Polyethylene Fibers

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polymer fibers was the use as reinforcement in compo-

sites. However, this application remains limited due to

low axial compression strength of the polymer fibers.

Carbon and glass fibers can sustain large axial compres-

sion stress if supported by a sufficiently strong and stiff

well adhering resin, thus all structural loads can be car-

ried. However, polymer fibers tend to respond with plas-

tic deformation under axial compression. The compres-

sion yield strength of rigid rod polymer fibers is typically

around 10% of their tensile strength. The compression

yield strength of high strength UHMWPE is even lower

and is around 1% of the tensile strength. The explanation

is a kind of molecular buckling. This buckling stress is

influenced by the low bending stiffness of the molecular

chains and by the low interaction between the chains.

Consequently, fibers made from rigid rod molecules will

show higher compression strength than polyethylene

fibers. Rigid rid fibers show more chain-chain interactions,

because they contain hydrogen bridges. The Vander Waals

bonds and crystalline interactions between UHMWPE

chains are much weaker than hydrogen bonds.

Figure 1

shows a Scanning Electron Microscope (SEM) picture of

gel-spun UHMWPE fibers with kink bands due to com-

pression loading. The kink bands are the microscopic

manifestation of the molecular buckling process. The

limited compression strength explains why such fibers

are hardly chosen as a reinforcement for structural com-

posites. On the other hand, several investigations e.g. by

Marissen

et al.

[4,5] indicate that hybridizing glass or

carbon fiber composites with gel-spun UHMWPE fibers

can improve the impact resistance, with a small penalty

on flexural strength only.

One of the rare applications of composites with

gel-spun UHMWPE fibers, without a significant amount

of glass or carbon fibers, is for the walls of air cargo pa-

nels. The walls are connected to an aluminum frame at

the edges of the container and only initial indentation

causes real bending stresses and the associated compres-

sive stresses. Larger indentations will cause membrane

stresses that are tension by nature. The membrane

stresses are transferred to the aluminum corner frame.

Consequently, compression strength of the panels is

hardly needed. Stiffness and impact strength are required.

Air cargo containers are subject

ed to severe impact loads

during their handling on airports. Gel-spun UHMWPE

fibers provide the stiffness and excellent impact resis-

tance at low weight. The low weight is desired in view of

fuel cost savings and saving of carbon dioxide emissions

during flight. Lightness in aviation is of extreme impor-

tance. Every kilogram mass saved on flying equipment

saves a multitude of kilogram&#;s fuel consumption per

year, and accordingly saves carbon dioxide emission.

Figure 2

shows examples of such air cargo containers.

Figure 1. SEM picture of kink bands in compression loaded

gel-spun UHMWPE fibers.

Figure 2. Two air cargo containers with panels made from

gel-spun UHMWPE fibers and a Turane resin (Courtesy

DoKaSch Aircargo equipment GmbH Staudt, Germany).

The colored panels in these containers replace aluminum

sheets at about half the panel weight, yet offering about

triple impact resistance. Thus a considerable redu ction of

repair costs is obtained.

Kink bands are common for high strength polymer fi-

bers. However, they are reversible in UHMWPE fibers.

They disappear under subsequent tensile loading, without

causing noticeable damage. The tensile strength hardly

decreases if kink bands were present in these fibers. On

the other hand, the compression yielding may be related

to the fiber&#;s damage tolerance on a microscopic scale.

Glass and carbon fibers behave like elastic rods and

bending fracture of those rods occurs if the elastic

strength limit of the material is exceeded. High strength

polymer fibers will rather show compressive yielding

than fracture. The highest toughness may be expected for

the fibers with the lowest compression yield strength.

Indeed, such effects can be observed.

Figure 3

shows a

SEM picture of a knot in a single Dyneema® filament.

Extreme curvature and transverse deformation is visible,

yet signs of tensile fracture are absent.

Figure 4

shows a

Design with Ultra Strong Polyethylene Fibers

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Figure 3. SEM picture of a knot in a Dynee m a® filame nt.

Figure 4. SEM picture of a Dyneema® filament stretched

over the cutting edge of a razor blade.

Dyneema® filament that is stretched over the cutting

edge of a razor blade. Again extreme curvature occurs.

The filaments deform in the transverse direction and ra-

ther spread out over the blade than being cut.

Another special feature of gel-spun UHMWPE fibers

is that the melting point is much lower than that of other

high strength fibers. Polyethylene is just a low melting

polymer. A typical melting point of normal non oriented

UHMWPE is about 135

C. However, the rather perfect

longitudinal orientation of the molecular chains in the

crystals causes some increase of the melting point. Typi-

cal melting point is about 150

C, it is slightly depending

on specific crystal morphologies. Detailed discussions on

morphology are presented by Demco

et al.

[1]. Still

150

C is low compared to other high performance fibers.

This limits the application area.

However, it also offers

new unique processing options. Gel-spun UHMWPE

fibers can be fused under pressure at a temperature some-

what below the melting point. Such a sintering process

enables the production of &#;matrix free composites&#; with

a slight loss of fiber properties only. Ward

et al.

[6,7]

published on the fusion of highly oriented polyethylene

fibers. Ward stated that local partial melting of the fibers

is important for fusion. His patent mentions a preferred

pressure range of about 5 - 20 Bar [7]. However, gel-

spun UHMWPE fibers can also be fused without notice-

able melting. This can be done at much higher pressures.

Pressures of more than 100 Bar should than be chosen.

The long crystals of the different fibers can be fused un-

der sufficient pressure. Indeed, the lo w transverse strength

of the fibers allows deformation and creation of intense

contact between the fibers. A slight rearrangement of

molecular chains under pressure may cause fusion of

crystals with parallel orientation.

Figure 5

presents a

SEM micrograph of a cross section showing partly fused

fibers being deformed from circular to about hexagon

shapes. Some boundaries are still visible; others have

disappeared indicating complete fusion.

Polyethylene is &#;the simplest polymer&#;, it contains

only covalent carbon bonds in the chains, and hydrogen

atoms as side groups. The absence of other bonds, like

ester- or amide bonds implies the absence of properties

related to those bonds. So polyethylen e (and UHMWPE)

fibers are non-polar and insensitive to hydrolysis. This

means a good chemical resist

ance, but also poor bond ing

to resins and dyes. Some activation of the surface is pos-

sible, e.g. with corona or plasma treatments. However,

providing very strong bonding to resins, or providing

saturated colors to the fibers were as yet unsuccessful.

The above discussion shows that properties and useful

technology application of high strength polyethylene

fibers is very different from application of other fibers.

This paper describes some applications of high strength

polyethylene fibers. Firstly, some established applica-

tions are briefly presented and discussed in Chapter 2. A

Figure 5. SEM picture of fused Dyneema® filaments.

Design with Ultra Strong Polyethylene Fibers

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322

more extensive presentation of established applications

has been presented previously by Vlasblom and Van

Dingenen [8]. Secondly, more

recent applications are

discussed in Chapter 3. This chapter also presents some

&#;laboratory principles&#; that did not yet find wide spread

commercial application, but may inspire engineers to

utilize the peculiar properties to the advantage of tech-

nology innovation. In all cases the chosen fiber is the

Dyneem a® fiber prod uc ed by DSM.

2. Well Established Applications of Ultra

High Strength Polyethylene Fibers

2.1. Armor

Stopping fast projectiles with high strength fibers is well

accepted technology. Flexible bullet resistant vests are

made from fibrous sheets. Hard armor plates can also be

made from fibers. Various slightly different views are

possible regarding the physics on the projectile stopping

mechanism. The view by Cu

nniff [9] is chosen here,

mainly because it is elementary and straight forward.

Cunniff argues that the quality of fibrous armor will be

related to the amount of armor that contributes to the

projectile catching effect. This contibuting amount in-

creases with increasing soni

c speed in the longitudinal

direction of the fibers, so with an increasing (

)

1/2

val-

ue, where

is Young&#;s modulus and

is density. An-

other factor is the amount of deformation energy that can

be &#;consumed&#; by that contributing amount of material.

That energy will be proportional to (

), where

the fracture stress and

is the fracture strain. The final

equation by Cunniff is obtained by multiplying the

amount of contributing material and the energy con-

sumed by that material:

&#;&#;

fr fr

&#;

&#;&#; &#;

&#;

(1)

where

is an armor performance parameter. Very high

strength fibers show approx

imately linear stress strain

behavior, especially for the high loading rates which are

typical for ballistics, so

and

. Substitu-

tion of this last relationship in Equation (1) yields:

&#;&#;

&#;

(2)

Equation (2) contains elementary fiber properties only.

It shows that a cho ice of high strength f ibers is of highest

importance, because strength occurs in the Equation (2)

with the highest exponent. Low density is the next im-

portant property and low Young&#;s modulus is of some

importance for stopping fast projectiles.

Table 1

shows

some elementary fiber properties. Indeed Dyneema®

fibers exhibit very high strength and low density, thus

they are a typical armor material. Of course projectiles

Table 1. Basic fiber properties of a Dyneema® SK75 yarn

with 176 tex.

Tensile strength [GPa]Modulus [GPa] Density [kg/m

]

3.4 110 975

should not be able to travel

between the fibers. Therefore

fibrous armor contains two perpendicular fiber directions.

This reduces the splitting between fibers that would al-

low easy passing through of projectiles. Hence fibrous

armor is made from cross plies of fibers.

Figure 6

presents an impression of such armor. Indeed many suc-

cessful applications of gel-spun UHMWPE fibers armor

do exist at present. More details on projectile stopping

mechanisms of such armor can be found e.g. in a paper

by Jacobs and Van Dingenen [10], and by Van der Werff

et al.

[11]. Van der Werff also provided perspectives on

future performance of fiber-based armor. Laboratory

investigations on fiber production by Van der Werff [12]

and by Wang

et al.

[13] with very low polymer concen-

trations in the initial solution indicate that strength and

modulus values could exceed the comercial values with

almost a factor two. Equation (2) indicates that the armor

performance can be approximately doubled in terms of

energy absorption. Note that Van der Werff mentioned a

lower number. This is because he argued in terms of

projectile speed. The projectile kinetic energy is propor-

tional to the second order of speed. Making such ulti-

mately strong fibers on an industrial scale is quite a chal-

lenge. Yet, the argumentation elucidates a future poten-

tial.

Gel-spun UHMWPE fibers are viscoelastic materials.

At room temperature, elasticity is dominant under short

term loadings (up to days or weeks). However, time-

dependent behavior is dominant under long-term loading

(years). Creep may be considerable under long-term

loading, depending on stress and temperature. The mo-

lecular origin of the creep deformation is mainly the slip

of the individual molecular

chains through the crystals.

Some chain scission may not be excluded. However, the

large creep strains that can be observed (> 50%) can only

be explained by chain slip. Indeed, the low interactions

(atomic bonds) between the chains and the rather perfect

longitudinal crystals with few entanglements can allow

such chain slip. Jacobs [3] elaborates extensively on the

various aspects of chain slip

and the corresponding creep

behavior.

The creep of gel-spun UHMWPE fibers is a disadvan-

tage for long-term loaded applications. However, it can

advantageously be used for processing. The creep rate

increases at high temperatures and it is sufficiently high

at temperatures above about 130

C for processing in suf-

ficiently short time.

Figure 7

shows two helmet shells

Design with Ultra Strong Polyethylene Fibers

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323

(a)

(b)

Figure 6. Illustration of fi

ber cross plies for armor, (a)

Schematic picture of build up, (b) SEM micrograph of a

cross section.

Figure 7. Two helmet shells made by creep-forming of gel-

spun UHMWPE fibers [14].

made by creep-forming, from a type of cross-ply material

as shown in

Figure 6

. Typically deep drawn helmets of

such material show wrinkles due to the 3-D deformation,

or they have to be made from a kind of &#;flower-cut&#;

stack of plies. This cutting provides fiber ends in the

helmet and thus may reduce protection. The creep elon-

gation of the fibers allows 3-D draping without wrinkles.

The helmets in

Figure 7

were made at about 130

C and

they are free of wrinkles and

all fibers are intact. More

details on creep forming can be found in [14].

Another way of making helmets also utilizes the typi-

cal features of gel-spun UHMWPE fibers. The po ssibility

to fuse the fibers under pres

sure at high temperature al-

lows omission of a matrix in &#;composites&#; produc ts. This

allows dry filament winding without leading the fibers

through a resin impreg na tion bath . It simplifies th e set-up

considerably and allows fast winding with many yarns

simultaneously.

Figure 8

shows dry winding around an

ellipsoid mandrel with about 30 yarns simultaneously.

After completion of the winding process, a hot knife is

used to separate the wound ellipsoid shape in two pre-

forms. The fibers melt at the cutting location and thus are

welded together, yield ing two shaped pr eforms.

Figure 9

shows the &#;welded&#; cut line. The resulting halves are

stable preforms that can be shaped and consolidated to

helmet shells in a hot press. The disadvantage of the low

melting temperature is turned here into an advantage, as

it allows the simultaneous cutting and fusion of the edges.

Thus a practical production of stable preforms is possi-

ble.

2.2. Cut Resistance

Gel-spun UHMWPE fibers are difficult to cut.

Figure 4

gives an impression on a part of the physics of cut resis-

tance. However, cut resistance in actual prod ucts is much

more complex. The angle between the blade and the fi-

bers is important as well. Protective textiles are often

hybrids with other fibers. Gel-spun UHMWPE fibers are

rather stiff as compared to other polymer fibers. Blending

with very elastic fibers helps to make products like

gloves even more comfortable. Blending may also im-

prove the cut resistance. If the very stiff gel-spun UHM-

WPE fibers are blended under tension with a stretched

elastic fiber, the gel-spun UHMWPE fibers will form

loops after relaxation of the tension. Loops remain al-

most free of stress until the cutting blade has stretched

the loop completely.

A striking property of cut resistant gloves made from

gel-spun UHMWPE fibers is their comfortable cool

feeling. Indeed such fibers have a very high thermal

conductivity along the fiber

axis. Thus, excessive heat

from the hands is easily transferred to the usually colder

environment. The combination of cut resistance and cool-

Design with Ultra Strong Polyethylene Fibers

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324

Figure 8. Filament winding w ith many yarns.

Figure 9. Preform with &#;fiber

weld&#; at the cutting location.

ing is especially attractive in suits for sh ort track skating.

The suits offer protection against the sharp skates in case

of falling and collision accident

s. The suits also provide

good tran sfer of ex cessiv e heat th at is typically gen erated

in high level exercise. An article by S. Shen

et al.

[15]

highlights this behavior on very small scale. Of course,

the conductivity is even larger fo r the perfect crystals that

can be made on nano-scale, as

compared to industrial

scale. However, the less perfect large scale industrial

grade of their fibers is already on the market for years,

e.g. Dyneema®. It is not un

common that ultimate pro-

perties are approached on small laboratory scale, where

properties associated with large scale industrial produc-

tion are somewhat lower.

2.3. Ropes, Cables, Wires and Nets

The combination of high tensile strength, stiffness, low

density and damage tolerance (as illustrated in the

Fig-

ures 3

and

), make gel-spun UHMWPE fibers ideal

If you are looking for more details, kindly visit UHMWPE Fiber.

material for tensile structures

that are subjected to fre-

quent handling. Ropes and cables are used in various

designs and dimensions, varying from thin lines for kites

and fishing lines to very thick cables for offshore moor-

ing. The density of about 0.975 g/cm

makes cables al-

most weightless under water, thus they do not have to

carry their own weight in deep water mooring.

Parallel fusion of the fibers at increased temperature,

but below the melti ng point allows the prod uct i on o f wire

like monolines that are attractive for fishing.

Figure 10

shows a SEM picture of such a wire. The individual fi-

laments can still be recognized to some extent. Yet it

behaves as a single line. Nets for fishing and fish farming

can be made from gel-spun UHMWPE fibers. Again,

lightness and small size at high strength are attractive.

The bite resistance makes them especially suitable for fish

farming. Nets for air cargo pallets show high strength

and lightness and thus allow for a reduction of fuel con-

sumption.

Gel-spun UHMWPE fibers show low friction coeffi-

cients. Measurement results of friction coefficients (

)

yield somewhat varying values. A coefficient of friction

= 0.05 is not uncommon. Karuppiah

et al.

[16] investi-

gated the effect of crystallinity on the friction co efficient

of UHMWPE. They found reduced friction for increasing

crystallinity. Consistently, the very high crystallinity of

gel-spun U H MWPE fib ers explai

ns the low coefficient of

friction. It can be speculated that this is related to the

lower mobility of the molecular chains in the crystal.

Lower mobility implies lower interaction between the

sliding parts. The low friction causes some difficulties

regarding connections of ropes and connections to ropes.

On the other hand, the relative sliding of filaments allo ws

bending of ropes with little damage. Thus bending fati-

gue resistance of such cables is large. Smeets

et al.

present some quantitative results on bending fatigue of

gel-spun UHMWPE cables [17].

Figure 10. Monofilament made by fusing gel-spun UHMWPE

fibers.

Design with Ultra Strong Polyethylene Fibers

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325

3. Recent and Emerging Technology with

Ultra High Strength Polyethylene Fibers

3.1. Medical Applications

Dyneema Purity® is a very clean grade of gel-spun

UHMWPE fiber. It is a fairly recent development, based

on a proprietary spinning process. Dyneema Purity® is

an ideal material for interactions with the human body,

both during demanding surgeries, and over the longer life-

span required for implantable devices. The high strength,

softness and abrasion resistance are very valuable assets

in demanding applications like sports medicine. The ma-

terial&#;s low elongation and fatigue resistance offer a su-

perior alternative to traditional materials for surgeons and

device manufacturers. Some typical uses include: high-

strength sutures, ligament repair, arthro scopic procedures,

motion-preserving spinal applications, trauma and surgery

of the spine. A striking example is rotator cuff repair.

Thousands of rotator cuff repair shoulder surgeries with

Dyneema Purity® have been performed already with good

results and this surgery is becoming standard practice.

The wear resistance and the possibility for thermal fu-

sion suggest the use of this pu

re variant as a liner in arti-

ficial implantable joints. Such joints may not only pro-

vide good wear-resistance, but the fused fibers will also

provide high strength and resi

stance to cracking that is

still observed sometimes after long term in vivo use of

conventional joint materials. A young company named

Cartificial explored this field. The results were very en-

couraging. Unfortunately, testing of medical devices is

very expensive and this start-up company did run out of

cash before the commercial potential was established

sufficiently and the company was discontinued. Yet, the

potential might be materialized later.

3.2. Kite Based Wind Energy

Wind turbines provide an increasing part of electric en-

ergy production. A much lighter alternative for wind tur-

bines are kites. Although many practical problems have

to be solved, kites are potentially more powerful and

more effective energy gener ators than convention al wind

turbines. Ockels is one of the pioneers of this concept,

see e.g. Podgaets and Ockels [18]. His kite based ladder

mill design should generate approximately 100 MW.

Many concepts for kite based energy generation have

already been proposed. All kite based energy generation

systems will require lightweight cables, allowing much

&#;handling&#;. The high tenac

ity (specific strength) and

damage tolerance of g el-spun UHMWPE fiber will make

it a first choice material for the required cable systems.

3.3. Connections

The low friction coefficients of gel-spun UHMWPE fiber

were discussed before. The low coefficient of friction

makes gel-spun UHMWPE fibers notorious for knot

slippage. The friction coefficien

t of fibers against steel is

about 0.1. The fiber-fiber friction coefficient is about

0.05 only. The behavior of knots suggests that these

numbers may even decrease under high normal forces.

Strong knots in gel-spun UHMWPE fibers require addi-

tional loops. A square knot that provides good hold in

conventional fibers behaves as a &#;large force sliding

knot&#; in gel-spun UHMWPE fibers. This may be imprac-

tical for standard connections. However, it may be turned

into a great advantage. A square knot can be used to

connect parts with a loop. S

ubsequently, the loop can be

tensioned by a large mechani

cal force at the end of the

fibers. The strong fibers will allow that large force.

About 10% of this force will be transferred through the

square knot to the loop, thus tensioning the connected

parts considerably. Most of this tens ion will remain after

removing the force at the ends. An additio nal square kno t

(or optionally more knots) will cause a durable fixation.

Variations of this procedure are under development for

surgery, allowing for a safer procedure involving tempo-

rary fixation, optionally adjustment, and final fixation.

A variation of this slip-block technology is the use of

auxiliary features. A system that is easily tensioned and

provides excellent holding power is based on the use of

small metal rings. An assembly of two or three rings can

be used.

Figure 11

shows a schematic picture of a clamping

method with a three ring clamping system. The dimen-

sions are optimized for a Dyneema® cable of tex

( gram/kilometer). The u

pper ends in the figure are

the tension ing ends. The lo wer ends are co nnecting ends,

e.g. parts of a loop to be tensioned. The high strength of

the fibers, together with the low friction, allows firm ten-

sioning. On the other hand, th

e loading force at the con-

necting end presses the rings together and thus prevents

reversed sliding. The extra loop configuration has the

best holding force, about N for the above mentioned

cable; the standard winding pattern allows easier and

Figure 11. Schematic presen

tation of ring clamping.

Design with Ultra Strong Polyethylene Fibers

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326

Figure 12. Pictures of the installed three-fold ring clamp with extra loop.

faster tensioning. Many variations are possible. All have

in common that the tensioning ends are between the rings

and the loaded ends are around the rings.

Figure 12

shows photographs of the assembled rings with the fibers.

Figure 13

shows a variation with two rings.

Connections may be desired to be permanent, but

sometimes it is important that quick release and connec-

tion is possible. Metal shackles, or carabiners are used in

such cases. However, they are heavy and hard. A metal

shackle at the end of a swinging rope can be a hazard. A

recent alternative is a soft shackle. Soft shackles are typ-

ically made of gel-spun UHMWPE fibers. Reasons for

this choice are the high tensile strength and damage to-

lerance. Another reason is that the slippery character of

such fibers allows easy use. Opening can even be done

easily after high loading . The special shape still provide s

a good locking behavior if closed and loaded.

Figure 14

shows a LIROS XTR soft shackle in opened and closed

condition. Colligo Marine offers a similar &#;softie&#; with

an additional feature for keeping it closed. Such soft

shackles are light, strong

and practical in use.

3.4. Hinges

Connecting rigid structures with cables may allow for

some flexibility. The conn ection can be designed in such

a way that the flexibility is optimized. This will create a

kind of cable hinge, or in general a fiber hinge. The fi-

bers may be present as yarns, cables, or fabrics, depend-

ing on the specific design. The use of gel-spun UHM-

WPE fibers in such hinges is especially advantageous. A

rough but effective way of making a line-hinge is to

make a composite plate of gel-spun UHMWPE fibers,

e.g. by impregnating a fabric with a resin and curing the

resin, followed by folding the plate. Indeed, it will not

break completely, only the resin breaks! This is unlike

other composites and it is attributed to the typical fiber

properties. Folding it a few times in both directions and

pressing the fold line creates

a strong and flexible line-

hinge in the plate. Another way of making a hinge is

shown in

Figure 15

. If made from Dyneema Purity® and

a surgical steel quality, it may potentially be used in

arthroplasty as a strong

artificial finger joint.

Figure 16

shows a possible joint with the kinematics resembling

that of a ball bearing. This could b e an artificial hip joint

that does not generate wear pa

rticles. Wear particles are

the cause for loosening of the stem-femur connection on

long term for conventional implants. The four cables

prevent relative translation of both parts, but allow rota-

tion. The body weight will be carried mainly by the low-

er cable. This cable may be designed thicker than the

other ones. The extreme tensile strength of Dyneema

Purity® and the possibility to apply rather thick cables

allow overdesign of the critical cable up to load carrying

capacity of a few tons, whereby creep will be virtually

eliminated.

3.5. Fiber Modifications

Some &#;weaknesses&#; of UHMWPE fibers may even be

enhanced. Ropes made of UHMWPE fibers allow fre-

quent bending on winches and sheaves. A lubricating

coating on the fibers may even enhance this effect. A so

called bending optimized fiber was developed this way.

Details are presented by Smeets [17]. Cables made from

this modified fiber show even further improved bending

fatigue properties.

Also the production of gel-spun UHMWPE fibers al-

lows some flexibility. Pigments and other functional

constituents can be incorporated into the fibers during

Figure 13. Ring clamp variation with two rings.

Design with Ultra Strong Polyethylene Fibers

MSA

327

(a) (b)

Figure 14. Closing a Liros soft shackle; a) Initial condition, b)

Opening the loop, c) Feeding the knot through the loop, d)

Closing the loop.

Figure 15. Set of fiber-hinges

with one degree of freedom;

rotation around one axis. The size as compared to a 2 Euro

coin illustrates the possibility of

design as a potential finger

joint implant.

fiber production. A new implantable blue fiber &#;Dyneema

Purity® BLUE&#; was recently developed and presented at

the Medical Design and Manufacturing East Conference

& Exposition (MD&M East) [19]. Blue fibers pro-

vide improved visibility for surgeons, due to improved

contrast to body tissue.

Another modification of gel-spun UHMWPE fibers is

the incorporation of short thin mineral fibers in the

gel-spun filaments.

Figure 17

shows a SEM micrograph

of gel-spun UHMWPE fibers containing a mineral fiber.

The mineral fiber is made visi

ble (lighter grey scale) by

using back scattered electrons. So far, this technique is

only published in patents [20]. The commercial launch

was at Expoprotection in Paris (France) at 4 November

. These fibers with incorporated mineral fibers exhi-

bit about twice the cut resist

ance of the already cut resis-

tant non modified fibers, without reducing the wearing

comfort. The constitution of the mineral fibers is such

that they will dissolve in the human body, in case they

would be released from the polyethylene filaments

Design with Ultra Strong Polyethylene Fibers

MSA

328

(a) (b)

Figure 16. Fiber-hinge with rotational freedom in various directions. Designed as a potential hip joint implant, (a) View

showing the head rotated in two different positions, (b)-(d) Various views on the inside showing three white cable pairs to

the &#;equator&#; and one black cable to the &#;pole&#; of the c up.

and would enter the human body.

The above innovations demonstrate that new future

variants of gel-spun UHMWPE fibers are possible and

may become available, allowing even more applications.

4. Discussions

Gel-spun UHMWPE fibers already have numerous ap-

plications in a wide variety of technology fields. How-

ever, also many application trials failed because of

structural limitations of these fibers. This is in contrast to

the situation with g lass or carbon fibers which are exten-

sively applied in composite materials, but application

does not extend much b eyond this technology field. This

creates an almost paradoxical situation: Fibers with most

&#;complete&#; properties (tension, compression and trans-

verse strength) are applied in a rather narrow field (com-

Design with Ultra Strong Polyethylene Fibers

MSA

329

Figure 17. Gel-spun UHMWPE fiber filaments with incur-

porated short mineral fibers.

posites), whereas gel-spun UHMWPE fibers with &#;in-

complete&#; properties (tension strength only, weak in

compression and in transverse direction) have applica-

tions in many different fields. This paradox is discussed

in some detail below an d resolved to some extent.

Johan Cruijff once stated (in Dutch): &#;Every disad-

vantage comprises an advantage&#;. This wisdom was re-

lated to soccer, but it also applies to gel-spun UHMWPE

fibers. Such fibers could roughly be qualified as &#;having

all properties but one being negligible&#;. The not-negligi-

ble property is that it allows application of extreme ten-

sion loading. This character is due to the uniaxial mole-

cular structure of such fibers.

However, the lack of other properties can indeed be an

advantage for some applications. The extreme &#;micro

toughness&#; as illustrated in the

Figures 3

and

is related

to the limited transverse stress transfer in the filaments,

thus the tensile stresses are equalized over the filament

cross section. High local peak stresses in the fibers are

thus annihilated. Of course, it is important that the re-

sponse in the weak directions is deformation and not

material separation, so in fact not all non-tension proper-

ties are low. For example, the fracture strain in compres-

sive and transverse direction is high. So weaknesses in

those directions are only appa

rent in terms of force.

It was demonstrated above that the high tenacity, to-

gether with disadvantages like creep deformation, a low

friction coefficient, and low melting point can all be

turned into an advantage,

for specific applications,

enabling the design of products or processes that show

properties with some unique aspects. Finding such possi-

bilities may require creativity but when found, they often

lead to unique advantages. Some opportunities for such

special products or processes are presently known.

However, still a challenge remains. There is no reason to

assume that possibilities for new attractive designs with

such exceptional fibers are exhausted. Moreover, the

gel-spinning process is versat

ile, and fibers optimized for

specific applications, like ballistic protection, surgery, or

cut resistance can be ( further ) d ev e loped.

REFERENCES

[1]

D. E. Demco, C. Melian, J. Simmelink, V. M. Litvinov

and M. Möller, &#;Structure and Dynamics of Drawn

Gel-Spun Ultrahigh-Molecular-

Weight Polyethylene Fi-

bers by

C and

129

XE NMR,&#;

Macromolecular Che-

mistry and Physics

, Vol. 211, No. 24, , pp. -

doi:10./macp.

[2]

P. Smith and P. J. Lemstra, &#;Ultrahigh Strength Polye-

thylene Filaments by Solution Spinning/Drawing,&#;

Jour-

nal of Materials Science

, Vol. 15, No. 2, , pp. 505-

514.doi:10./BF

[3]

M. J. M. Jacobs, &#;Creep of Gel-Spun Polyethylene Fi-

bres,&#; Ph.D. Thesis, Eindhoven University of Technology,

Eindhoven, December .

[4]

R. Marissen, L. Smit and C.

Snijder, &#;Dyneema Fibers in

Composites, the Addition of

Special Mechanical Func-

tionalities,&#;

Conference Proceedings

Advancing with

Composites

, Naples, October .

[5]

J. G. H. Bouwmeester, R. Marissen and O. K. Bergsma,

&#;Carbon/Dyneema® Intralaminar Hybrids: New Strategy

to Increase Impact Resistance or Decrease Mass of Car-

bon Fiber Composites,&#;

ICA

Conference Anchor-

age

, Alaska, September .

[6]

I. M. Ward and P. J. Hine, &#;The Science and Technology

of Hot Compaction,&#;

Polymer

, Vol. 45, No. 5, , pp.

-. doi:10./j.polymer..11.050

[7]

I. M. Ward, P. J. Hine and

K. Norris, &#;Polymeric Mate-

rials,&#; Patent US, August .

[8]

M. P. Vlasblom and J. L. J. van Dingenen, &#;The Manu-

facture, Properties and Applications of High Strength,

High Modulus Polyethylene Fibers,&#; In: A. R. Bunsell,

Ed.,

Handbook of Tensile Properties of Textile and Tech-

nical Fibres

, Woodhead Publishing Ltd., Cambridge,

doi:10./.2.437

[9]

P. M. Cunniff, &#;Dimensionle

ss Parameters for Optimiza-

tion of Textile-Based Body Armor Systems,&#;

Proceed-

ings of

th International Symposium on Ballistics, San

Antonio

, November .

[10]

M. J. N. Jacobs and J. L. J.

van Dingenen, &#;Ballistic Pro-

tection Mechanisms in Personal Armour,&#;

Journal of

Materials Science

, Vol. 36, No. 13, , pp. -.

doi:10./A:

[11]

H. van der We rff, U. Heisse

rer and S. L. Phoenix, &#;Mod-

elling of Ballistic Impact on Fiber Composites,&#; Personal

Armour Systems Symposium , Quebec City, Sep-

tember .

[12]

H. van der Werff and A. J.

Pennings, &#;Tensile Deforma-

tion of High Strength and High Modulus Fibers,&#;

Colloid

& Polymer Science

, Vol. 269, No. 8, , pp. 747-763.

doi:10./BF

[13]

J. Wang and K. J. Smith, &#;The Breaking Strength of Ul-

Design with Ultra Strong Polyethylene Fibers

MSA

330

tra-High Molecular Weight Polyethylene Fibers,&#;

Polymer

Vol. 40, No. 26, , pp. -.

doi:10./S-(99)-8

[14]

R. Marissen, D. Duurkoop, H. Hoefnagels and O. K.

Bergsma, &#;Creep-Forming of High Strength Polyethylene

Fiber Prepregs for the Production of Ballistic Protection

Helmets,&#;

Composites Science and Technology

, Vol. 70,

No. 7, , pp. -.

doi:10./j.compscitech..03.003

[15]

S. Shen, A. Henry, J. Tong, R. T. Zheng and G. Chen,

&#;Polyethylene Nanofibres with Very High Thermal Con-

ductivities,&#;

Nature Nanotechnology

, Vol. 5, No. 4,

March , pp. 251-255.

doi:10./nnano..27

[16]

K. S. K. Karuppiah, A. L.

Bruck, S. Sundararajan, J.

Wang, Z. Q. Lin, Z. H. Xu and X. D. Li, &#;Friction and

Wear Behavior of Ultra-High Molecular Weight Polye-

thylene as a Function of Polymer Crystallinity,&#;

Acta

Biomaterialia

, Vol. 4, No. 5, , -.

doi:10./j.actbio..02.022

[17]

P. J. H. M. Smeets, M. P. Vlasblom and

J. C. Weis, &#;Lat-

est Improvements on HMPE Rope Design for Steel Wire

Rope Applications,&#;

Proceedings

OIPEEC

, 3

In-

ternational Ropedays

, Stuttgart, March .

[18]

A. R. Podgaets and W. J. Ockels, &#;Laddermill Sail: A

New Concept in Sailing,&#;

International Conference on

engineering Technology

ICET

, Kuala Lumpur, 11-

14 December .

[19]

&#;DSM Dyneema Launches Dyneema Purity® BLUE,&#;

http://www.dyneema.com/en_US/public/dyneema/page/n

ewsitems/PurityBLUE.htm

[20]

R. Marissen, E. F. F. de Danschuttter and E. Müller, &#;Cut

Resistant Yarn, a Process

for Producing the Yarn and

Products Containing the Yarn,&#; Patent WO,

April .

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