Transcript Training - Plymouth University

Fibre reinforcements
John Summerscales
ACMC University of Plymouth
Glossary of fibre/textile terms
• Fibre/textile terms are defined at:
o
http://www.tech.plym.ac.uk/sme/MATS324/MAT
S324A9%20FibreGlossary.htm
Principal fibres
•
•
•
•
basalt, boron
carbon fibres
glass fibres
rigid-rod polymers (aramid and PBX fibres)
o
e.g. Kevlar, Twaron
• polyethylene fibres
o
e.g. Dyneema, Spectra
• natural fibres
o
flax, hemp, jute, kenaf, sisal
• surface treatments on fibres
Griffith crack theory
• Alan Griffith (1920) studied strengths of glass rods and fibres
• fibre strength becomes markedly higher
as fibre diameter decreases to ~10 micrometres
• critical stress above which
cracks of a given size will spontaneously propagate.
• critical stress level is higher for small cracks.
• AG’s very fine fibres were strong
because cracks in them would be very small.
• AG’s work was the key to
present understanding of brittle fracture in all materials.
• the strength of the modern fibreglass industry is
"a fitting memorial to his pioneering efforts".
Glass fibres
• A: high alkali grade
o
originally made from window glass
• C: chemical resistance or corrosion grade
o
for acid environments
• D: low dielectric
o
good transparency to radar: Quartz glass
• E: electrical insulation grade
o
E = most common reinforcement grade (E ~70 GPa)
• L: high lead content for radiation absorption
• M: high modulus grade
• R: reinforcement grade
o
European equivalent of S-glass
• S: high strength grade (a common variant is S2-glass)
o
o
fibre with higher Young’s modulus and temperature resistance
significantly more expensive than E-glass
Glass-forming oxides
Oxide
% in E-glass
SiO2
54
65
Na2O
trace
trace
high thermal expansion, moisture sensitivity
K 2O
-
-
high thermal expansion, moisture sensitivity
Li2O
-
-
high thermal expansion, moisture sensitivity
CaO
17.5
trace
resistance to water, acids and alkalis
MgO
4.5
10
resistance to water, acids and alkalis
B2O3
8.0
trace
Al2O3
14
25
Fe2O3
trace
trace
green colouration
ZnO
-
-
chemical durability
PbO
-
-
increased density and brilliance (light transmission)
and high thermal expansion
BaO
-
-
high density and improved chemical durability
TiO2
F2
% in S-glass Effect on Fibre Properties
very low thermal expansion
low thermal expansion
improved chemical durability
improved chemical durability especially for alkali
trace
Glass fibres: beware!
• Handling fibres causes damage
salts on the skin can displace bonding ions
from the glass structural network
o oil and grease on the skin
transfer to fibre and act as release agents
o
• Health and safety issues
o
Commercial fibres should NOT be
respirable as diameter is > 5 μm
Surface finish (known as “size”)
•
•
•
•
protect fibre surfaces from damage
lubricate fibres during mechanical handling
impart anti-static properties
bind fibres together for easy processing
• coupling agent promotes interfacial bond
Carbon fibres
• natural graphite has
o
o
Young’s modulus of 910-1000 GPa in-plane
Young’s modulus of 30 GPa through plane
• carbon fibre
o
o
turbostratic layered structure of contiguous benzene rings
a single layer of graphite = graphene
.
• standard (high strain/high strength) fibres
o
E > 210 GPa (E is equivalent to steel)
• high-modulus (HM-) fibres
o
o
E > 350 GPa
when E>400 GPa incorrectly called “graphite fibre” in USA
Carbon fibres
• precursor materials are:
polyacrylonitrile (PAN)
o pitch, and
o rayon (regenerated cellulose) and lignin
o
• manufacturing imposes orientation by:
spinning of polymer to fibre
o stretching polymer precursor
o graphitisation (pyrolysis) under tensile stress
o HM fibres pyrolysed at >1650°C
o
Carbon fibres: beware!
• as fibre modulus rises, strain to failure falls
• carbon fibres conduct electricity
• longitudinal coefficient of thermal expansion
of carbon fibres is slightly negative
o
this effect increases in magnitude
with increasing modulus
Rigid rod polymers: aramid
• aramid is derived from poly aryl amide
• commercial reinforcements fibres are:
o
Kevlar (DuPont) reinforcement,

o
o
molecule is poly(para-phenylene tere-phthalamide) [PPTA]
Twaron (Akzo) reinforcement
Nomex (DuPont) for paper and honeycombs

molecule is poly(meta-phenylene iso-phthalamide)
Aramid fibres
Fibre
Character
Kevlar 29
high-toughness,
high-strength,
intermediate modulus
for tire cord
Kevlar 49
high modulus,
high-strength
for composite
reinforcement
Kevlar 149 ultra-high modulus
recently introduced
E
σ'
ε'
(GPa) (GPa) (%)
83
3.6
4.0
131
3.6
2.8
186
3.4
2.0
Aramid fibres: beware!
• very low resistance to axial compression
o
o
o
typically ~20% of corresponding tensile strength
poor transverse properties
low longitudinal shear modulus
• fibres break into small fibrils (fibres within the fibre)
o
fibrils from rod-like structure of liquid crystal precursor
• fibres are hygroscopic
o
they absorb water
• fibre surfaces degrade in ultraviolet (UV) light.
Rigid-rod polymer fibres
• aramid (PPTA)
C
O
O
N
N
H
C
• aramid chemical structure alternates
o
o
aromatic (aryl) benzene rings, and
the amide (CONH) group.
• PBX: poly benz[x]azole
PBX rigid rod polymers
PBI
PBO
PBT
O
N
S
Polyethylene fibres
• made from UHMWPE
(ultra-high molecular weight polyethylene)
• trade names
Dyneema (DSM), and
o Spectra (Allied Corporation)
o
• excellent modulus and strength-to-weight
properties (similar to aramid)
• lower density than aramid
o
weight specific properties are superior
(almost match those of HM carbon fibres?)
Polyethylene fibres: beware!
• fibres melt at ~150°C
• fibre surface is effective release agent
Natural fibres
• reinforcement mostly uses the structural
fibres from plant stems (bast fibres)
• the fibres most used are
temperate zone: flax, hemp
o Tropical zone: jute, kenaf and sisal
o
• MATS324: topic dealt with in separate lecture
• MATS231: natural fibre less than ideal when wet
Summary
• density
o
aramid (1.44) < carbon (1.6-1.8) < glass (2.56)
• modulus of standard fibre is
o
glass (70 GPa) < aramid (140 GPa) < carbon (210 GPa)
• strength of synthetic reinforcement fibres
o
usually ~ 1 GPa (if not virgin fibre)
• toughness
o
carbon (brittle) < glass < aramid (tough)
• beware!: each fibre has different problems