Composites- Fibre Phase 2

Question 1

Chemical structure in carbon fibres

Answer 1

Made of graphite. Hexagonal arrangement of C atoms into flat nasal plane or layer. Strong covalent bonds between C atoms (350kJ/mol). Delocalised π-electron density above and below plane adds to chemical stability. Weaker physical interactions between planes (3kJ/mol). These are van der Waals interactions resulting from motion of electron density causing momentary dipoles which overall creat a weak attractive force

Question 2

Where do carbon fibres differ from other graphitic types of carbon?

Answer 2

Their physical structure. In carbon fibres, the outer skin tends to be more ordered graphitic carbon. The inner core tends to be less ordered turbostratic carbon. Turbostratic carbon mostly oriented along fibres axis but can suffer from folds or crumpling

Question 3

What does folding or crumpling of turbostratic carbon lead to?

Answer 3

Creation of voids and other flaws and misorientation

Question 4

Anisotropy of carbon fibres

Answer 4

Overall they are very anisotopic. Much stronger forces in the fibre direction than transverse to it. Nasal planes aligned with fibre direction but a mixture of voids and other flaws mean the properties are reduced from ideal values. For graphite, YM is 1TPa parallel to basal planes and 35GPa perpendicular to them

Question 5

How can imperfections in the structure of carbon fibres help in composite applications?

Answer 5

Can help with adhesion to matrix

Question 6

Other good properties of carbon fibres

Answer 6

High specific strength and modulus retained at elevated temperatures. Unaffected by moisture and wide range of solvents, acids and bases

Question 7

Types of carbon fibres and their properties

Answer 7

High modulus (HM): 3.5GPa tensile strength, 441GPa tensile modulus.
Intermediate modulus (IM): 5.5GPa tensile strength, 303GPa tensile modulus.
High strength (HS): 7.1GPa tensile strength, 294GPa tensile modulus

Question 8

Composite applications for carbon fibres

Answer 8

High performance and low weight
Aerospace: civil aircraft, military aircraft, spacecraft
Automotive: luxury cars, sports cars
Sports: bicycles, rackets, clubs

Question 9

Precursors for carbon fibre manufacture

Answer 9

Most use polyacrylonitrile (PAN)
Others can use pitch or cellulose

Question 10

Overview of steps in PAN or pitch process

Answer 10

Textile fibre for PAN (melt-spun fibre for pitch)
Stretch
Thermoset
Carbonise
Graphitise
Surface treatment
Sizing
Wound up

Question 11

Spinning of PAN

Answer 11

PAN is spun into fibres. Primarily wet-spun from 15-20% DMF (dimethyl formamide) solution (could be dry-spun or melt-spun). All of spinning processes align polymers along fibre direction. Stretching during processing improves this effect.

Question 12

Co-monomer used in PAN process

Answer 12

Can use a vinyl ester (co-monomer) like methacrylate. Plasticises the PAN making it easier to spin. Controls heat evolved during initial fibre oxidation stage. Using pure PAN homopolymer leads to exotherm and degradation

Question 13

Oxidation step of PAN process

Answer 13

Aka stabilisation or thermosetting. PAN fibres heated at 200-300C in air. Cyclises the PAN into a ladder structure (by cyclisation and dehydrogenation) see diagram page 12. Fibres now won’t melt or fuse together in later steps. Also densifies the fibres from 1.18 to 1.38 g/cm3

Question 14

Carbonisation step of PAN process

Answer 14

Oxidised PAN fibres heated up to 1000C in inert atmosphere. Cyclised PAN chains now combine. Non-carbon content removed as gases (primarily HCN and N2, also H2O and CO2). Increases carbon content. Further densifies to 1.8g/cm3

Question 15

Graphitisation step of PAN process

Answer 15

Carbonised PAN fibres given a very high temperature treatment in an inert atmosphere. Temperature varies but between 2000-3000C is common. Removes any remaining non-carbon content. Tensile modulus significantly increased by this process. Tensile strength decreases above 1500C. Fibres are now carbon fibres (often 8μm diameter)

Question 16

Which step of PAN processes influences properties of fibres?

Answer 16

Graphitisation. The time and temperature gives a range of properties.

Question 17

Strength vs T and TM vs T graphs for carbon fibres

Answer 17

This is the T in graphitisation. Strength increases to a peak at about 1500C then falls a bit like decay curve. YM increases in shallow curve up with temperature

Question 18

Other features of PAN process

Answer 18

Tension applied throughout process to counteract shrinkage and maintain properties. Wash in aqueous electrolytic bath to remove misaligned weaker material (beneficial side effect of leaving OH groups at carbon plane edges which promote adhesion). Size. Process

Question 19

Length types of carbon fibres

Answer 19

Continuous (over 100mm long)
Chopped (6-100mm long)
Milled (30μm-6mm long)

Question 20

Carbon fibre manufacture using pitch

Answer 20

Pitch is by-product of petrochemical refining industry (complex mixture of hydrocarbons, aromatic and heterocyclic). Heated above 350C pyrolyses pitch into pseudo-polymeric form containing many flat, aligned hydrocarbons. This liquid crystal phase is mesophase pitch. Melt-spin polymer into fibres and oxidise to thermoset/stabilise them (otherwise very weak and fusible). Remaining steps very similar to PAN. No applied tension required.

Question 21

Comparing pitch-based to PAN-based carbon fibres

Answer 21

Pitch-based often weaker but can be significantly stiffer than PAN-based

Question 22

Carbon fibre manufacture by pyrolysis deposition

Answer 22

Very expensive alternative to PAN, pitch or cellulose routes. Aka chemical vapour deposition or vapour grown carbon fibre. Carbon fibres will grow in oxygen-free gaseous carbon-containing environment in presence of small metal catalyst particle. Methane, benzene, naphthalene and other precursors have been successful. High T required, 1100C. Thin carbon tube forms and grows into concentric layers of turbostratic carbon forming the fibre. Only shorter lengths (60mm) possible and little diameter control. No commercial use

Question 23

Why do carbon fibres need surface treatments?

Answer 23

To promote adhesion to matrix phase. Interface can’t be too strong or composite can be too brittle.

Question 24

How do you measure adhesion?

Answer 24

Via ILSS (interlaminar shear strength)

Question 25

Oxidative processes for carbon fibre surface treatment

Answer 25

Oxygenate the fibre surface.
Gas phase oxidation: oxygen gas with an inert carrier at high T over time.
Liquid phase oxidation: strong liquid phase oxidising agent HNO3 or KMnO4.
Electrolytic oxidation: similar liquid phase oxidising agent used in electrolyte solution with fibres as one electrode.
Plasma: plasma created from air or O2 gas

Question 26

Nom-oxidative processes for carbon fibre surface treatment

Answer 26

Whiskerisation: grow small whiskers of SiC, S3N4 etc perpendicular to fibre surface.
Sizing: many polymer chemistries used to add functional groups to fibre surface, tailored to specific fibre-matrix combination, solution deposition (cost fibre surface using polymer emulsion), electro-deposition or electro-polymerisation (deposit ionised polymer group suing fibre as electrode, initiate polymerisation using fibres as electrode within cell of monomers), work in this area has led to definition of an interphase