Polymer fibre composites like Kevlar fibre composites are described here. Their processing and properties are discussed.

1. Polymer fibre
reinforced composites
Prepared by
Dr. K. Padmanabhan
Professor
Asst Director, CENC,
Manufacturing
Division
School of MBS
VIT-University
Vellore 632014August, 2010

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

3. 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.

4. 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)

5. 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

6. Liquid crystal Conventional (PET)
Solution
Extrusion
Solid
state
Nematic structure
Low entropy
Random coil
High entropy
Extended chain structure
High chain continuity
High mechanical properties
Folded chain structure
Low chain continuity
Low mechanical properties
Schematic representation of structure formation
during spinning, contrasting PPT and PET behavior

7. Producing Kevlar fibers

8. Stephanie Kwolek--Synthesized Kevlar

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

10. •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.

11. 0.51 nm
Schematic diagram of Kevlar® 49 fiber
showing the radially arranged
pleated sheets
Microstructure of aramid fiber
Kevlar fibers

12. Kevlar-49 Structure

13. Kevlar - High flexibility but poor
compressive performance
Also low shear performance,
moisture-sensitive, UV-
sensitive

14. 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 1.39
Tensile strength, GPa 2.8 2.8 2.8 2.8 2.8 3.0
Tensile modulus, GPa 80 125 62 124 186 74
Tensile strain, % 3.3 2.0 3.5 2.5 1.9 4.2
Coefficient of thermal
expansion,
10-6o
C
Longitudinal:
0 to 100 o
C … … -2.0 … -- …
Radial:
0 to 100 o
C … … 59 … -- …
The Aramid fiber family

15. Kevlar/epoxy ‫א‬‫ב‬
Note the fibrillar structure of the fiber

16. •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

17. Zylon Fibre
www.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.

18. Polyethylene fibers
The theoretical elastic modulus of the covalent C-C
bond in the fully extended PE molecule is 220 Gpa.
Experimental value in PE fibres - 170 Gpa.
Stretching
Entanglement network Fibrillar crystal
Dyneema or Spectra
Orientation > 95%
Crystallinity up to 85%
Normal PE
Orientation low
Crystallinity < 60%

19. 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

20. •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

21. UHMWPE (Spectra) – high
flexibility and toughness, poor
interfacial bonding

22. Poly(p-phenylene benzobisthiazole)
PBT or PBZT

25. Kevlar
Spectra
Flexibility, compressibility, and
limit performance of fibers

26. FLEXIBILITY
Intense 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 = κ(EI)
Units: [N/m2
][m4
]/[m] = [N*m]
M = bending moment
I = second moment of area of cross-section
R = radius of curvature to the neutral surface of cross-section
∫= dAyI 2

27. E = Young’s modulus
EI = flexural rigidity (≈ resistance of beam to bending)
κ= 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 ϕ = κ/M, which
must be maximized for highest flexibility.

28. b
h
M M
12
3
bh
I =
64
4
d
I
π
= MM d
Moment of inertia

29. 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
]
4
64 dEπϕ =

30. ASSUMING A CONSTANT DIAMETER:
material d (m) E (Pa) ϕ [N-1
m-2
]
E-glass 1.00E-05 72.0 E+9 28 E+9 max
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:

31. Using real diameters and moduli:
USING REAL DIAMETERS AND MODULI:
material d (m) E (GPa) ϕ [N-1
m-2
]
E-glass 1.10E-05 72.0 E+9 19 E+9
HM carbon 8.00E-06 750 E+9 6.6 E+9
HS carbon 8.00E-06 250 E+9 20 E+9 max
Kevlar 49 1.20E-05 130 E+9 7.6 E+9
Nicalon 1.50E-05 190 E+9 2.1 E+9 min
SWNT 1.10E-09 1200 E+9 1.16E+25 !
Glass fibers and HS carbon fibers are more tolerant to bending

32. 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.

34. EI
L
k
Pcr 2
2
π
=
EULER’s
WORK ON
BUCKLING

36. 0 10 20 30 40 50
0.01
0.1
1
10
E S
GLASS
K29
SiC
K49 K149
LM CF
BORON
PBT
PBO
HM CF
STEEL
SPECTRA 1000
THEORETICAL
LIMIT FOR GRAPHITE
SPECIFICSTRENGTH,10
6
m
SPECIFIC MODULUS, 10
6
m
2
1
1260 





=
ρρ
σ E

37. Hydrolytic Stability of Kevlar

38. Moisture Regain of Kevlar

39. TGA of Kevlar

41. Multiple Fibre Pull out Test

42. Kevlar Fibre Surface after
Treatment with Acetic Anhydride

43. Kevlar Fibre Surface

45. ILSS of Kevlar/Epoxy Composites

47. 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.

50. Asymmetric Hybridization

52. Kevlar Fibre Fibrillation

55. Composites in Defense
A Bulletproof Vest A missile material case

56. Uses
• Performance Apparel
• Adhesives and Sealants
• Belts and Hoses
• Composites
Fiber-Optic and
ElectroMechanical Cables
• Friction Products and
Gaskets
Protective Apparel
Tires

57. Uses
• Ropes and Cables

58. Uses
• Ballistics&
• Defense

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