Polymer Fibers

Polymer Processing

Shaping Polymers

Extrusion

Molding

Fibers

Coatings

Product Shaping / Secondary Operations

EXTRUSION

Shaping through die

Final Product (pipe, profile)

Preform for other molding processesBlow molding (bottles),Thermoforming (appliance liners)Compression molding (seals)

Secondary operation Fiber spinning (fibers) Cast film (overhead transparencies, Blown film (grocery bags)

Fibers• A Fiber is a long, thin thing!

– Aspect ratio >100– At diameters > 75 µ, the fiber is a rod

• Long means:– > 1 kilometer

• At a density of 1.4 and a denier of 5, 1 kilometer weighs less than 5 grams

– > 1 kilogram• 1.5 kilograms at 5 dpf is 20,000 miles

• Few commercial fibers are produced at a scale of less than 500 tons– The length at 5 dpf is ~ .01 lightyear

• Typical melt spinning speeds are in excess of 100 miles/hour– To be viable, polymer to fiber conversions must be ~ 90%

• Minimum property CVs are < 10%• Real fibers are hard to make!!

MACROSCALE vs MICROSCALE

Griffith’s experiments with glass fibers (1921)

FIBER DIAMETER (micron)

Strength of bulk glass: 170 MPa

Extrapolates to

11 GPa

1

2

3

TENSI

LE S

TRE

NGT

H (GP

a)

020 40 60 80 100 1200

Griffith’s equation for the strength of materials

21

2

=a

E

πγσ a = length of defect

γ = surface energy

• Thus, going from the macroscale to the atomic scale (via the nanoscale), defects progressively become smaller and/or are eliminated, which is why the strength increases (see equation).

• Note that the Griffith model predicts that defects have no effect on the modulus, only on strength

• But note: the model also predicts that defects of zero length lead to infinitely strong materials, an obvious impossibility!

Fibers1000 X longer than diameterOften uniaxial strengthKevlar-strongest organic fiber • Melt spinning technology can be applied to polyamide (Nylon),

polyesters, polyurethanes and polyolefins such as PP and HDPE.

• The drawing and cooling processes determine the morphology and

mechanical properties of the final fiber. For example ultra high

molecular weight HDPE fibers with high degrees of orientation in the

axial direction have extremely high stiffness !!

• Of major concern during fiber spinning are the instabilities that arise

during drawing, such as brittle fracture and draw resonance. Draw

resonance manifests itself as periodic fluctuations that result in

diameter oscillation.

TABLE 4.2. Fiber Propertiesa

Fiber Type

Natural Cotton WoolSynthetic Polyester Nylon Aromatic polyamide (aramid)c

Polybenzimidazole Polypropylene Polyethylene (high strength)Inorganicc

Glass Steel

Tenacityb

(N/tex)

0.26-0.440.09-0.15

0.35-0.530.40-0.711.80-2.0

0.270.44-0.792.65d

0.53-0.660.31

SpecificGravity

1.501.30

1.381.141.44

1.430.900.95

2.567.7

aUnless otherwise noted, data taken form L. Rebenfeld, in Encyclopedia of Polymer Science and Engineering (H. f. Mark,N. M. Bikales, C. G. Overberger, G. Menges, and J. I. Kroschwitz, Eds.), Vol. 6, Wiley-Interscience, New York, 1986,pp. 647-733.bTo convert newtons per tex to grams per denier, multiply by 11.3.cKevlar (see Chap. 3, structure 58.)dFrom Chem. Eng. New, 63(8), 7 (1985).eFrom V. L. Erlich, in Encyclopedia of Polymer Science and Technology (H.F. Mark, N. G. Gaylord, and N. M. Bikales, Eds.), Vol. 9, Wiley-Interscience, New Uork, 1968, p. 422.

Polymer fibers

Organicpolymers

Flexiblemolecules

Stiffmolecules

Meltspinning

Wetspinning

Meltspinning

Wetspinning

Normalspinning

Superstretching

NylonPP, PE

UHMWPE

HMWPE

Aromaticpolyesters

Aramides

Dyspinning

CelluloseAcetate

Fibers

Dry Spinning: From solution

Melt Spinning: From Melt

Nylon 6,6 & PETECellulose Acetate

Wet Spinning: From solution into solution

Kevlar, rayon, acrylics, Aramids, spandex

Fiber Spinning: MeltFiber spinning is used to manufacture synthetic fibers. A filament is continuously extruded through an orifice and stretched to diameters of 100 µm and smaller. The molten polymer is first extruded through a filter or “screen pack”, to eliminate small contaminants. It is then extruded through a “spinneret”, a die composed of multiple orifices (it can have 1-10,000 holes). The fibers are then drawn to their final diameter, solidified (in a water bath or by forced convection) and wound-up.

Bobbin

Extruded Fiber Cools

and Solidifies Here

Metered Extrusion(controlled flow)

Melting Zone

Polymer Chips/Beads

PumpFilter and SpinneretAir Diffuser

Heating Grid

Pool

Lubrication by oil disk and trough

Packaging

Bobbin drive

Yarn driver

Feed rolls

Moisture Conditioning Steam Chamber

Nylon 6,6 & PETE

Feed

Filtered polymer solution

Metered extrusion Pump

Filter and spinneret

Solidif ication by solvent evaporation

Heated chamber

Lubrication

Air inlet

Feed roll and guide

Yarn driving

Balloon guide

Packaging

Ring and traveler

Bobbin transverse

Spindle

Dry Spinning

Dry

Spi

nnin

g of

Fib

ers

from

a S

olut

ion

Cellulose Acetate

Wet Spinning (e.g. Kevlar)

Kevlar, rayon, acrylics

Aramids, spandex

feedline

take-upgodet

filaments

spinneretdrawingelements

coagulation bath plastisizing bath

Melt spinning

Acrylic Fibers

• 85% acrylonitrile• Wet spun• Acrylic's benefits are:

– ･ Superior moisture management or wickability ･– Quick drying time (75% faster than cotton) ･– Easy care, shape retention ･– Excellent light fastness, sun light resistance ･– Takes color easily, bright vibrant colors ･– Odor and mildew resistant

• Nanotube effecting crystallization of PP• Sandler et al, J MacroMol Science B, B42(3&4), pp 479-

488,2003

Why are strong fibers strong?The source of strength: van der Waals forces

Flexible molecules,normally spun

Flexible moleculesultra stretched

Rigid moleculesliquid crystallinity

N

N

O

O

H

H

N

N

O

O

H

H

N

N

O

O

H

H

Fib

er o

rient

atio

n

•High Tensile Strength at Low Weight •Low Elongation to Break High Modulus (Structural Rigidity) •Low Electrical Conductivity •High Chemical Resistance •Low Thermal Shrinkage •High Toughness (Work-To-Break) •Excellent Dimensional Stability •High Cut Resistance •Flame Resistant, Self-Extinguishing

Kevlar

•High Tensile Strength at Low Weight •Low Elongation to Break High Modulus (Structural Rigidity) •Low Electrical Conductivity •High Chemical Resistance •Low Thermal Shrinkage •High Toughness (Work-To-Break) •Excellent Dimensional Stability •High Cut Resistance •Flame Resistant, Self-Extinguishing

Kevlar or Twaron

Polypropylene elastomers

99n Rn

H e-beam

99n Rn

99n Rn

R

Aramide fibersthe complete spinning lineH2SO4

80 wt%

H2O

PPD-T20 wt%

icemachine

H2SO4 ice

mixer

extruder

spinneret

Washingcsulf.ac. < 0.5 %

neutralisingdrying2000C

winding

H2SO4 + H2O

air gap

Long washing traject(initially difficult to control)Sometimes post-strech of 1%to enhance orientation

Strong fibers from flexible chains

Super-stretched polyethylene:

Mw = 105 (just spinnable)conventional melt spinningadditional stretching of 30 to 50 times

below the melting point

Wet (gel) spinning of polyethylene

Mw = 106 (to high elasticity for melt spinning)decalin or parafin as solventformation of thick (weak) fibers without stretchingremoval of the solventstretching of 50 to 100 times close to melting point

POLYETHYLENE (LDPE)

H2C CH2

RH2C CH2

x20-40,000 psi150-325° C

Molecular Weights: 20,000-100,000; MWD = 3-20 density = 0.91-0.93 g/cm3

Highly branched structure—both long and short chain branches

15-30 Methyl groups/1000 C atoms

Tm ~ 105 C, X’linity ~ 40%

Applications: Packaging Film, wire and cable coating, toys, flexible bottles, housewares, coatings

CH2

H3CCH3

CH3

H3C

CH3

H3C

H3C

H3C

H3C

Polyethylene (HDPE)

CH3

Essentially linear structure

Few long chain branches, 0.5-3 methyl groups/ 1000 C atoms

Molecular Weights: 50,000-250,000 for molding compounds250,000-1,500,000 for pipe compounds >1,500,000 super abrasion resistance—medical implants MWD = 3-20 density = 0.94-0.96 g/cm3Tm ~ 133-138 C, X’linity ~ 80%Applications: Bottles, drums, pipe, conduit, sheet, film

Generally opaque

UHMWPE fibers: Dyneema or Spectra

http://www.dyneema.com

Gel spinning process

Structure of UHMWPE, with n = 100,000-250,000

Comparison of mechanical properties

Strength Modulus stretch (Gpa) (Gpa) (%)

Classical fibres• nylon 1.0 5.6 18• glass 2.7 69 2.5• steel 2.8 200 2Strong fibres• superstretched PE 0.7 4.7• wet spun PE (Dyneema) 2.2 80 3.4• melt spun PE (Vectran) 3.2 90 3.5• wet spun aramide 2.7 72 3.3• idem with post-stretch 3.6 130 2.3

Aramide fibersthe spinning mechanism

removal ofsulfuric acid

platinumcapillary 65µ

polymer inpure sulfuric acid

at 850C

air gap 10 mm withelongational stretch (6x)

coagulationbath at 100C

Specific points:

solvent: pure H2SO4

polymer concentration 20%

general orientation in the capillary

extra orientation inthe air gap

coagulation in cooled diluted sulfuric acid

O

O

O

O

m

n

Vectran

Vectran fiber is thermotropic, it is melt-spun, and it flows at a high temperature under pressure

O

HNHN NH

O

n

Aramid

n

Ultra High Molecular Weight Polyethylene

O

O

O

O

m

n

Vectran

O

N

poly(p-phenylene benzobisoxazole)

O

N

n

Zylon

Carbon Fibers: Pyrolyzing Polyacrylonitrile Fibers

C C C C C C C

N N N N N N N

N N N N N N N N

N N N N N N N N

Young’s Modulus 325 GpaTensile Strength 3-6 GPa

Electrospinning of Fibers

–Driving force is charge dissipation, opposed by surface tension–Forces are low–Level of charge density is limited by breakdown voltage – Taylor cone formationFiber diameter α [Voltage]-1

–“Inexpensive” and easy to form nanofibers from a solution of practically any polymer (Formhals 1934)–Only small amount of material required

5-30 kV

Human hair (.06mm)

Electrospun polymers

Fibers

1000 X longer than diameterOften uniaxial strengthKevlar-strongest organic fiber

tensile strength 60GPaYoung’s modulus 1TPa)

Making Carbon Nanotubes

Carbon Nanotube Fibers

Nature 423, 703 (12 June 2003); doi:10.1038/423703a

1cm

Fig. 4. Scanning electron micrograph of a dry ribbon deposited on a glass substrate. The black arrow indicates the main axis of the ribbons, which corresponds to the direction of the initial fluid velocity. Despite the presence of a significant amount of carbon spherical impurities, SWNTs bundles are preferentially oriented along the main axis. Scale BAR=667 nm

SWNT Fiber after drawing

25 µm

Fibers• Large aspect ratio (length/diameter) & strong (fewer defects)• Common fibers: cellulose acetate, viscous cellulose,

polyethylene, polypropylene, acrylics (acrylonitrile copolymers), nylon’s, polyester (PETE), PMMA (optics), urethane (Spandex).

• High performance fibers: polyaramides (Kevlar), Uniaxially oriented gels (UHMWPE), Liquid crystals (Vectran)

• Carbon fibers (Black Orlon or pitch based), carbon nanotubes• Methods for preparing:

-Dry spinning-Wet spinning-Melt spinning-Gel spinning-electrospinning-growing (self-assembly)

Polymides (PI) - Vespel®, Aurum®, P84®, and more. Polybenzimidazole (PBI) - Celazole®Polyamide-imide (PAI) - Torlon® Polyetheretherketone (PEEK) - Victrex®, Kadel®, and more. Polytetrafluoroethylene (PTFE) - Teflon®, Hostaflon®Polyphenylene Sulfide (PPS) - Ryton®, Fortron®, Thermocomp®, Supec® and more. Polyetherimide (PEI) - Ultem®Polypthalamide (PPA) - Amodel®, BGU®, and more. Aromatic Polyamides - Reny®, Zytel HTN®, Stanyl®Liquid Crystal Polymer (LCP) - Xydar®, Vectra®, Zenite®, and more. Other Polymers - Nylon, Polyacetal, Polycarbonate, Polypropylene, Ultra High Molecular Weight Polyethylene, ABS, PBT, and mor