Polymer fibre

reinforced composites

  Prepared by

Dr. K. PadmanabhanProfessor Asst Director, CENC,Manufacturing Division School of MBSVIT-UniversityVellore 632014August, 2010

Contents

• High strength and modulus polymer fibres• Flexibility and mechanical behaviour• Structure property correlation• Moisture attack• Thermal characteristics• Case studies from publications• Applications

High-modulus high-strength organic fibers

• Theoretical estimates for covalently bonded organics show strength of 20-50 GPa (or more) and modulus of 200 – 300 GPa

• Serious processing problems• New fibers developed since the early 1970s: high axial

molecular orientation, highly planar, highly aromatic molecules

• Major fibers: Kevlar (polyaramid); Spectra (PE); polybenzoxazole (PBO) and polybenzothiazole (PBT).

• The latest entry being Zylon made by Toyobo Company of Japan.

N H CH2 6 NH C

O

CH2 4 C

O

C

O

N H

N H

C

O C

O

H N

N H

N H

C

O

C

O

N H

N H

C

O

Nylon 6,6

Poly(m-phenylene isophthalamide) (Nomex)

Poly(p-phenylene terephtalamide)PPT (Kevlar)

Aramid Fibers

• Aramid (aromatic polyamide) fibers = poly(paraphenylene terephthalamide)

• Kevlar behaves as a nematic liquid crystal in the melt which can be spun

• Prepared by solution polycondensation of p-phenylene diamine and terephthaloyl chloride at low temperatures. The fiber is spun by extrusion of a solution of the polymer in a suitable solvent (for example, sulphuric acid) followed by stretching and thermal annealing treatment

Liquid crystal Conventional (PET)

Solution

Extrusion

Solidstate

Nematic structureLow entropy Random coil

High entropy

Extended chain structureHigh chain continuityHigh mechanical properties

Folded chain structureLow chain continuityLow mechanical properties

Schematic representation of structure formationduring spinning, contrasting PPT and PET behavior

Producing Kevlar fibers

Stephanie Kwolek--Synthesized Kevlar

Phase diagram of the anisotropic solution of PPT in 100% H2SO4

•Various grades of Kevlar fibers: Kevlar-29, 49, 149 (Kevlar-49 is the more commonly used in composite structures) and Kevlar 981.

•X-ray diffraction: the structure of Kevlar-49 consists of rigid linear molecular chains that are highly oriented in the fiber axis direction, with the chains held together in the transverse direction by hydrogen bonds. Thus, the polymer molecules form rigid planar sheets.

•Strong covalent bonds in the fiber axis direction - high longitudinal strength•Weak hydrogen bonds in the transverse direction - low transverse strength. ( Van Der Vaal’s bond)

•Aramid fibers exhibit skin and core structures – Core = layers stacked perpendicular to the fiber axis, composed of rod-shaped crystallites with an average diameter of 50 nm. These crystallites are closely packed and held together with hydrogen bonds nearly in the radial direction of the fiber.

0.51 nm

Schematic diagram of Kevlar® 49 fibershowing the radially arranged

pleated sheets

Microstructure of aramid fiber

Kevlar fibers

Kevlar-49 Structure

Kevlar - High flexibility but poor compressive

performance

Also low shear performance, moisture-sensitive, UV-sensitive

Twaron(Akzo)

Twaron

HM

Kevlar

29

Kevlar

49

Kevlar

149

HM-50(Teijin

) Density, g/cm3 1.44 1.45 1.44 1.44 1.47 1.39Tensile strength, GPa 2.8 2.8 2.8 2.8 2.8 3.0Tensile modulus, GPa 80 125 62 124 186 74Tensile strain, % 3.3 2.0 3.5 2.5 1.9 4.2Coefficient of thermal

expansion,10-6oC

Longitudinal:0 to 100 oC … … -2.0 … -- …

Radial: 0 to 100 oC … … 59 … -- …

The Aramid fiber family

Kevlar/epoxy בא

Note the fibrillar structure of the fiber

•Little creep only•Excellent temperature resistance (does not melt,

decomposes at ~500°C)•Linear stress-strain curve until failure in Tension•Low density : 1.44•Negative CTE along the axis•Fiber diameter = 11.9 micron•Fiber strength variability

Kevlar fiber

Zylon Fibrewww.toyobo.co.jp

ZYLON   consists of rigid-rod chain molecules of poly(p-phenylene-2,6-benzobisoxazole)(PBO).

Tensile Strength : 5.8 GPa

Tensile Modulus : 270 GPa

Ref: K. Padmanabhan , Toyobo Confidentiality Report, 2002.

Polyethylene fibers

The theoretical elastic modulus of the covalent C-Cbond in the fully extended PE molecule is 220 Gpa.

Experimental value in PE fibres - 170 Gpa.

Stretching

Entanglement network Fibrillar crystal

Dyneema or SpectraOrientation > 95%Crystallinity up to 85%

Normal PEOrientation lowCrystallinity < 60%

Extended chain polyethylene

minimum chain folding

UHMWPE fibre structure: (a) macrofibril consists of array of microfibrils;(b) microfibril; (c) orthorhombic unit cell; (d) view along chain axis

•UHMWPE (Spectra or Dyneema) are highly anisotropic fibers•Even higher specific properties than Kevlar because of lower density (0.98 g/cc)•Limited to use below 120°C•Creep problems; weak interfaces•Applications – ballistic impact-resistant structures

UHMWPE

UHMWPE (Spectra) – high flexibility and toughness, poor

interfacial bonding

Poly(p-phenylene benzobisthiazole)

PBT or PBZT

Kevlar

Spectra

Flexibility, compressibility, and limit performance of fibers

FLEXIBILITYIntense bending strains and stresses applied to fibers during manufacturing operations (weaving, knitting, filament winding, etc)

Definition of flexibility:Bending of an elastic beam: M = (EI)/R = (EI) Units: [N/m2][m4]/[m] = [N*m]M = bending momentI = second moment of area of cross-section R = radius of curvature to the neutral surface of cross-section

dAyI 2

E = Young’s modulusEI = flexural rigidity (≈ resistance of beam to bending)= curvature = 1/R

Intuitively: the flexibility of a fiber is the highest when:o The radius of curvature is as small as possible (or the

curvature is as large as possible)o The bending moment necessary to reach a given curvature is as small as possible

o The appropriate parameter to focus on is = /M, which must be maximized for highest flexibility.

b

hM M

12

3bhI

64

4dI MM d

Moment of inertia

Flexibility is thus defined as (for a circular fiber)

where E and d are the fiber bending modulus and diameter, respectively

As seen, the effect of size (diameter) on flexibility is by far the strongest, and thus nanoscale reinforcement promotes high flexibility. Low modulus also promotes high flexibility.

Units of flexibility are [1/Nm2]

464 dE

ASSUMING A CONSTANT DIAMETER:

material d (m) E (Pa) [N-1m-2]

E-glass 1.00E-05 72.0 E+9 28 E+9max

HM carbon 1.00E-05 750 E+9 2.7 E+9 min

HS carbon 1.00E-05 250 E+9 8.2 E+9

Kevlar 49 1.00E-05 130 E+9 16 E+9

Nicalon 1.00E-05 190 E+9 11 E+9

(Steel) 1.00E-05 210 E+9 9.7 E+9

Performing a ‘gedanken’ experiment:

Using real diameters and moduli:

USING REAL DIAMETERS AND MODULI:material d (m) E (GPa) [N-1m-2]

E-glass 1.10E-0572.0 E+9 19 E+9

HM carbon 8.00E-06 750 E+9 6.6 E+9HS carbon 8.00E-06 250 E+9 20 E+9 maxKevlar 49 1.20E-05 130 E+9 7.6 E+9Nicalon 1.50E-05 190 E+9 2.1 E+9 min

SWNT 1.10E-091200 E+9

1.16E+25 !

Glass fibers and HS carbon fibers are more tolerant to bending

Compressibility

• The compressive strength of single fibers is very difficult to measure and is usually inferred from the behavior of composites including the fibers.

• Euler buckling is one possible mode of compressive failure: it occurs when a fiber under compression becomes unstable against lateral movement of its central region.

EILkPcr 2

2

EULER’s WORK ON BUCKLING

0 10 20 30 40 500.01

0.1

1

10

E S

GLASS

K29

SiC

K49 K149

LM CF

BORON

PBTPBO

HM CF

STEEL

SPECTRA 1000

THEORETICALLIMIT FOR GRAPHITE

SPEC

IFIC S

TREN

GTH,

106 m

SPECIFIC MODULUS, 106 m

21

1260

E

Hydrolytic Stability of Kevlar

Moisture Regain of Kevlar

TGA of Kevlar

Multiple Fibre Pull out Test

Kevlar Fibre Surface after Treatment with Acetic Anhydride

Kevlar Fibre Surface

ILSS of Kevlar/Epoxy Composites

Mechanical Testing

Ref: K. Padmanabhan and Kishore , ` Failure behaviour of carbon/epoxy composites in pin ended buckling and bending tests’, Composites, Vol:26,No: 3, 1995, p201.

Asymmetric Hybridization

Kevlar Fibre Fibrillation

Composites in Defense

A Bulletproof Vest A missile material case

Uses• Performance Apparel• Adhesives and Sealants• Belts and Hoses• Composites

Fiber-Optic and ElectroMechanical Cables

• Friction Products and GasketsProtective ApparelTires

Uses• Ropes and Cables

Uses• Ballistics& • Defense

Winners don’t do different things but they do things differently - Shiv Khera