Carbon Fiber Composites

• Composite Materials and Structure-Property Relationship covers Carbon Fibers

Introduction to Carbon Fibers

• Early carbon fibers were initially not used as reinforcement material
• Thomas Edison used cellulosic materials like bamboo, natural cellulose and cotton to make first incandescent electric lamps.
• The first oxidized PAN fiber "Orlon" came from DuPont in 1950.
• The Royal Aircraft Establishment of Farnborough (RAE) patented carbon fibers in 1968.
• Union Carbide (US) made "Thornel 25" from viscose rayon in 1964.
• US production utilized the RAE patent by Hercules and Morganite in 1971.
• JV SGL Group and BMW Group (SGL ACF) formed in 2009.
• Traditional carbon research until 1800 focused on electrodes for Fe, Al & Si production, and graphite parts in solar and semiconductor industries.
• Modern carbon research after 1960 promotes lightweight through CFRP.
• Novel carbon research after 1985 researched fullerenes, nanotubes, and graphene

Structure of Carbon Fibers

• Carbon fibers are fibers from carbon-based precursors converted by pyrolysis into a high tensile strength carbon structure.
• Since 1970 carbon fibers are used as reinforcing materials.
• High strength of carbon fibers is attributed to strong covalent bonds with a binding energy of 350 kJ/mol and a highly oriented graphite structure.

General Carbon Fiber Applications

• Carbon composites are used in: aerospace, automotive, sports & leisure, etc.
• The Boeing 787 Dreamliner is 50% composites by weight.

Carbon Fiber Market

• Global annual production in 2015: Crude Steel at 1.62 billion tons, Aluminum at 57.7 million tons, and CFRP at 58,000 tons.
• Yearly carbon fiber capacity by region (2016) with a total of 130,900 tons: USA & Mexico (35%), Japan (19%), China (10%), Taiwan (7%), South Korea (7%), Hungary (5%), Germany (4%), France (4%), Great Britain (3%), and Rest of World (6%).
• Carbon fiber capacities in September 2019 by manufacturer: Toray + Zoltek, SGL Carbon, MCCFC, Teijin (Toho Tenax), Hexcel, Formosa Plastics, Solvay (Cytec), Zhongfu-Shenying, Jiangsu Hengshen, DowAksa, Hyosung, Kangde and others.
• Global carbon fiber demand (estimate) from 2010-2026.
• Global carbon fiber demand in 2013 totalled 46,500 tons.
• Market segments for carbon fiber demand and revenue in 2013: Aerospace & Defense (30%), Wind Turbines and Sport/Leisure (14% each), Automotive (12%), Molding & Compound and Civil Engineering (11% each), Marine (5%), and Other (2%).
• Global carbon fiber revenue in 2013 totalled US$ 1.7 billion.

Characteristics of Carbon Fibers

• Carbon fibers have a low density of 1.74 – 1.90 [g/cm³]
• The negative thermal expansion coefficient is -0.5 to -1.1 [10-6/°C]
• There are no significant problems at inhalation of filaments < 5 µm
• They are anisotropic in axial and transverse directions.
• Carbon Fibers exhibit high modulus (especially pitch based)..
• They have good thermal stability (in absence of O₂).
• Good thermal conductivity
• Offer high strength (especially PAN based).
• Possess excellent creep resistance.
• General negatives: high cost, low strain to failure, and oxidation at temperatures > 450°C.

Manufacturing of Carbon Fibers

• Strong bonds of Carbon Atoms create hexagonal layers
• The resulting material is stiff and tight
• Carbon layers within the fiber arrange along the fiber direction
• Single, dense thin fibers are 6-7 µm in diameter
• Carbon Fiber tow of 1,000-50,000 single fibres cluster allow cost-efficient handling

Manufacturing Carbon Fibers - Classical Methods vs. Carbon Suitability

• Melt Spinning (Polyester, Glass, Basalt) is unsuitable because carbon doesn't melt.
• Solution spinning (Aramide, viscose rayon) unsuitable because carbon is insoluble.
• Ceramic-like fiber making (binder + nanoparticles extruded & sintered) is unsuitable because it does not achieve the desired internal fiber structure.
• Classical methods are complex and costly.

Process chain of manufacturing

• Crude Oil Acrylonitrile -> PAN C Fiber Precursor -> Carbon Materials Materials
• Polymerization, Spin Dope Preparation, and Spinning are used in material creation.
• Polymerization involves solution, dispersion, or precipitation polymerization.
• Polymerization uses highly polar solvents like Organic: DMAC, DMF, DMSO and Inorganic: ZnCl₂, NaSCN.
• Solvent quality demands extreme purity
• Spinning involves fiber formation in a spinning bath.
• Stretching orients the molecules
• Fiber structure develops during spinning All defects
• Fiber structure develops during spinning
• All defects reduce tenacity of carbon fiber

Stabilisation (Oxidation)

• PAN Fiber (white and flammable) is converted to Oxidised PAN Fiber (black and non-flammable) done via:
• Temperature of 220-280°C is used
• Gases such as (HCN, …) are used

Carbonisation (Graphitisation)

• Temperature above 1000°C with gases e.g. (HCN, CO, …) needed
• Optimum Tenacity happens in: 1300-1500°C which creates HT, IM Fibers Continuous increase of stiffness with temperature.
• Continuous increase of stiffness with temperature happens in: > 2000°C creates HM Fibers

Surface Treatment & Sizing

• Electrolysis is used in the presence of Gases (H₂)

Raw Materials requirements

• Raw materials a.k.a precursors must exhibit sufficient carbon yield (> 50 wt.-%).

• Carbonized structure must show regions with graphitic layers.

• Emerging structure must orient along covalent bonds for high C-C binding energy.

• Every carbonaceous matter exhibits a theoretical carbon yield

• Carbon yield & structure preservation

• There are synthetic or natural, man-made fibers

Precursor Materials

• Polyacrylonitrile (PAN) boasts a 50% carbon yield and is high in terms of orientation of molecular chains and modulus/strength. It commands 95% of the market share.
• Pitch offers 80% carbon yield, capable of being graphitized, but only attains < 5% market share.
• Lignin has a carbon yield of > 40% is considered minimal for research purposes.
• Cellulose has a carbon yield of 25% and is considered minimal

Precursor details

• Polyacrylonitrile (PAN): Molecular formula (C3H3N)ₙ, Molecular weight M = n x 53, Share of carbon atomic mass 36,Theoretic carbon yield 68 wt.-%, Practical carbon yield 50 wt.-%, Practice/Theory 0.74.
• Carbon fibers forms from isotropic pitch, mesophase pitch (MPP), polymerized and condensed from isotropic pitch, formation of spheres of aromatic hydrocarbons, collision and coalescence of spheres, development of solid, infusible and anisotropic coke. The practical carbon yield is 80 wt.-%.

Fiber Production routes

• Using a PAN Based Precursor goes through these phases: PAN Solution -> PAN Precursor Fiber -> Stabilisation -> Carbonisation -> Graphitisation.
• PAN powder dissolves in a spinning solvent.
• The PAN solution is transferred into a fibrous form.
• Washing involves removal of the solvent
• Stretching involves longitudinal alignment of water molecules
• Spinning aligns covalent bonds in longitudinal direction of future carbon fiber
• The chemical reaction occurs under oxygen (oxidation).
• Irreversible thermal stabilization occurs.
• Highly exothermic reaction 2-3x stronger than oxyhydrogen reaction is achieved.
• The process is done under tension to prevent shrinkage and tearing apart, these fibers can then become PANOX.
• PANOX cannot burn under air or melt and is a pre-requisite for carbonization.
• Carbonization is the final step influencing factors involve target density from 1.36-1.42 g/cm³, oxygen content between 10 – 12 wt.-% for maximum carbon yield, good alignment of C-C bonds along longitudinal fiber direction as well as defect minimization.

PAN Applications

• PAN is used in: Reinforcing carbon/carbon aircraft brakes/brake pads in automotive applications (replacement of asbestos)

• Stabilization improves preferred orientation and Young's Modulus.

• Graphization is a higher heat treatment under argon and should use a defect free process.

• Defects can prohibit formation of ideal graphitic structure.

Fibers/Applications

• Carbon fibers are used as: Two-phase emulsion or low molecular weight single phase meso-pitch
• Thermoplastic fiber fiber has high degree of orientation along fiber axis
• Cross-linking of Oxidation and molecules is required while retaining orientation.
• The finalization process is Carbonisation in inert gas.
• Alternative is Graphite which is a Graphitisation technique with High Young's modulus of pitch.

PAN VS Pitch based fibers

• Increased Young's carbon fiber modulus and graphitic layer orientation.
• Properties of carbon fibers in longitudinal fiber direction, based on graphite single crystal.
• Pitch based fibers are used in combination with pan based fibers to create single crystal structures.

Vapor Grown Carbon Fibers

• Vapor Grown Carbon Fibers a.k.a catalytic chemical vapor deposition (CCVD), it grows by a submicron activated catalyst creating diameters of 1-100 μm lengths millimeter to few hundred millimeters eliminating stabilization. This makes for effective fiberglass alternative.

Surface treatment

• The goal for fiber-matrix adhesion is an effective load transfer by the matrix between filaments
• Surface treatment increases the amount of active/reactive surface groups, mainly oxides).
• Roughened fiber surface to increase surface area.
• Polar reactive surface groups are required on carbon fiber surfaces determine adhesion
• Size or surfacing coats the surface
• Applications improves adhesion, fiber wetting and acts lubricant to during subsquent tasks while minimizing damage.
• Reactive groups can be removed to promote the treatment and or matrix material

Surface Finish

• Application of dispersions on polymers
• Fiber treatments can be done via chemical or electrical.
• Good surface bonding is crucial for materials and choosing size dictates what will happen to future materials (like graphite). This alters 3d matrix with interphases and phases

Types and designations

• Monofilament: individual filament d ≈ 0.005 - 0.1 mm, typical 6-8 µm.
• Multi-filaments: several monofilaments (1k = 1000 filaments, 6k = 6000 filaments, 50k = 50,000 filaments)
• Low tow 1-12k for aerospace and 12-24k sports or leisure
• Heavy tow is between 50-320k for mechanical engineering.

Types and application

• There are 4 Fiber types: HT, IM, HM or UHM which dictates aviation, space, industrial applications

Fiber Properties

• HT and IM are result of of burning them at temperatures between 1200-1400°C
• HTRM is a additional treatment that reaches 2000-3000°C

HT|IM|HM|UHM

• --|---|---|--- Tensile strength [MPa]|4000|6000|2500|2150 Young's modulus E₁ [GPa]|240|295|400|450-950 Young's modulus E₂ [GPa]|28 n.a.| 15| n.a.| Density [g/cm³]|1.74| 1.74| 1.81| 1.90

Types of materials

• By testing their density and tensile strength they are found to be: superior

Aerospace carbon fibers

Carbon Fiber Applications

• Used for Automotives, Sports and Leisure