Constituent Materials Flashcards
To appreciate the range of constituent materials used in modern composite materials
different fibre types
thermoset and thermoplastic matrices
fibre/matrix interface
additives
What are the requirements for fibres, and what are the different fibre types?
Fibres can be considered as synthetic or natural, and we only consider synthetic in this module. They must have:
* High strength, high modulus
* Low density
* Low cost
* Readily available
* Compatibility with common matrices
* Compatiblity with composite manufacturing processes
The most important fibres for polymer composites are glass, carbon and aramid. Also boron and HDPE.
What are the glass fibres types, and how are they produced?
Glass fibres are the most common fibre type. They have generally good specific properties at low cost.
E = 70 GPa, ρ = 2540 kg/m^3
They are manufactured by melting and extruding molten glass. They are also non-crystalline with no particular microstructure. They are isotropic which is unique for fibres.
* A-glass - Soda-lime-silica compositions, lower mech. properties than E-glass
* C-glass - Formulated for chemical resistance
* E-glass - Originally for electrical resistance but also used in reinforcing polymers. Common in marine applications
* S-glass - Developed for high strength applications, but high production cost rules it out for general use. Used for blast applications
What are the carbon fibre types, and how are they produced?
Starts as graphitised precursor, which is stretched in hot water and steam at 300 degrees C. It is carbonised at 1000C and graphitised at 2000C, and finally surface treatments are added. Sizing is a chemicall treatment to provide protection and promote adhesion with the matrix.
The final carbon content is dependent on graphitisation temperature.
* PAN-based CF - Polyacrylonitrile (PAN) fibres make up 96% of the market and have higher tensile strength.
* Pitch-based CF - Much more brittle but extremely low negative coefficient of thermal expansion and used in space applications such as satellites, very expensive (up to £1000/kg).
What are the aramid fibre types, and how are they produced?
The term “aramid” refers to aromatic polyamide fibres. They are anisotropic due to their microstructure. They are produced by combining polymer powder to sulphuric acid which forms an aramid solvent solution. This is extruded at high temperatures to remove the solvent and then cold drawn to further align the fibres.
* Kevlar - Made by Du Pont, available as Kevlar 29 and 49. 29 has a higher modulus and strain at failure.
* Twaron - Made by AKZO
* Technora - Made by Teifin
Compare glass, carbon and aramid fibre properties.
Density (g/cm3)
glass > carbon > aramid
ρ_glass=2.54, ρ_carbon=1.80, ρ_aramid=1.45
Specific Tensile Properties
Modulus: carbon > aramid > glass
Strength: carbon > aramid > glass
Axial compression
* glass: σ_comp ~ σ_tens (glass fibres are isotropic)
* carbon: σ_comp < σ_tens
* aramid: σ_comp «_space;σ_tens
Fracture
* Carbon : brittle
* Glass : brittle
* Aramid : ductile
Tmax
* Carbon <2500C
* Glass < 250C
* Aramid < 250C
What makes an ideal matrix, and what are the matrix types?
An ideal matrix is:
* Cheap
* Readilly available
* Non-toxic/allergenic
* Good mechanical properties
* Good thermal properties/ temperature resistance
* Compatible with fibres
* Easy to process
For polymer matrices, there are two main types: thermosets and thermoplastics
What are polyesters?
Unsaturated polyesters are most commonly used in industry, and typically used with glass fibres. They cross-link (cures) from a liquid to a solid.
Catallysts a mixed in with the base resin to start the reaction, and accelerators to speed it up. Additives can include, pigments, fillers and fire retardants.
They have low viscosity, and a short pot-life.
Used in boat hulls, bath and shower trays, fibre glass repair kits, cladding, body panels for cars, storage tanks etc.
What are epoxy resins?
Epoxy resins are high performing thermosets and widely used in aerospace for structural applications (fuselage, wing skins). Typically used with carbon fibres and sometimes glass.
The resin cross-links (cures) from a liquid to a solid by adding a hardener (typically amine). No catalyst is required. There is low volumetric shrinkage which reduces residual stresses and improves component appearance, making demoulding more difficult.
Used in aerospace, motorsports, paints and coatings, adhesives, composite tooling, insulation for electronics, and large wind turbine blades.
What are vinyl-esters?
Vinylesters are thermosets that are similar in structure to polyesters. Simply put, they have epoxy-like performance with the processing ease of polyesters. Reactive sites are at the end of the molecular chains so have a higher toughness vs polyesters, and fewer ester groups thus less susceptible to water degradation, used as barrier coat.
Costs are intermediate between polyester and epoxy.
What are the advantages and disadvantages of thermoset matrices?
Advantages:
* Dimensionally stable
* Cost effective
* Low viscosity (easy to wet out fibres)
Disadvantages:
* Long cycle times due to cross-linking
* Generallly brittle
* Cannot be remoulded or reshaped
* Difficult to recycle
* Poor fire, smoke and toxicity (FST) rating
What are thermoplastics?
Thermoplastics are polymers that do not form cross links, so they have unlimited shelf-life. They are commonly used without reinforcement or with very short fibres. They can be reformed and are more easily recycled than thermosets, with potentially lower cost materials but high processing costs. This is because processing is challenging, their viscosity is 1000 times higher.
They are amorphous (polyetherimide (PEI) and acrylonitrile butadiene styrene (ABS)) and semi-crystalline (polypropylene (PP), nylon, PEEK).
Compare amorphous to semi-crystalline thermoplastics
Amorphous thermoplastics
* Have a randomly ordered molecular structure
* Lack a sharp melting point
* Soften gradually with increasing temperature
* Presence of hydrocarbons makes them sensitive to stress cracking
* Isotropic flow gives good dimensional stability
* Translucent
Semi-crystalline thermoplastics
* Highly ordered molecular structure
* Sharp melting point causes them to rapidly change into a low viscosity fluid
* Good toughness
* Anisotropic flow gives high shrinkage transverse to flow direction
* Opaque
What is meant by interface in a composite?
- The interface region is an infinitesimally thin layer between the fibre and matrix (might not actually exist).
- It is acknowledged to have a unique set of mechanical properties that can differ to those of the matrix.
- A coupling agent is used to assist with fibre/matrix bonding
- A sizing agent is added to protect the fibres and provide cohesion, and coupling agent added to increase chemical adhesion
How does the performance of the interface affect the performance of the composite?
The mechanical performance of the interface at the microscale directly influences the mechanical performance of the moulded composite at the macroscale. Load is transferred fibre to fibre via the interface. The interface is particularly important for discontinuous fibre composites, since the fibres are not directly loaded.
Shear lag is a model used to describe composite micromechanical behaviour, which assumes an ideal interface with no slippage. It assumes stress is transferred from the matrix to the fibres via shear stress at interface.
Aspect ratio also affects fibre stress.
