Composites- Matrices Flashcards
Functions of the matrix
Bind reinforcements together. Maintain shape. Transfer stress onto reinforcement. Protect reinforcements from chemical and moisture attack and mechanical abrasion. Add mechanical properties
What mechanical properties can matrices add?
Toughness to improve energy absorption. Shear strength to support against bending and torsional loading. Compressive strength to support against buckling
What is manufacture of composites controlled by?
The processing of the matrix phase. This considers viscosity, cure temperature and safety
Examples of thermoset polymer matrices
Most commonly used.
Epoxies, unsaturated polyesters, vinylesters, phenolics, bismalamides, cyanate esters
Examples of thermoplastic polymer matrices
Polyketones (PEEK), polyphenylene sulfide, polysulfones, thermoplastic polyamides
Other possible matrices
Metallic (alloys of Al, Ti, Mg)
Ceramic (Al2O3.SiO2, Al2O3, SiC, concrete, carbon)
General advantages of thermoset matrices
Low viscosity reactants. Good adhesions. Thermal stability. Chemical resistance. Low creep
General disadvantages of thermoset matrices
Limited storage life. Long cure time. Low strain to failure
General advantages of thermoplastic matrices
High impact strength. High fracture toughness. High strain to failure. Good storage life. Low fabrication time
General disadvantages of thermoplastic matrices
High melt viscosity. Low creep resistance. Low thermal stability
Unsaturated polyesters
Thermoset matrix. Linear, unsaturated polyester backbone. Has good mechanical or thermal properties, easy to cure, low cost. But is thermally unstable, there is shrinkage on cure and health concerns due to use of styrene
How to cure unsaturated polyesters
Use reactive monomer like styrene and an initiator such as MEKP (methyl ethyl ketone peroxide). Peroxide group easy to separate into radicals. React with styrene to make styrene radical which reacts with polyester to make polyester radical. Combinations of radicals react with each other to make a cross-linked system and form a solid. This is the curing reaction
Epoxy resins
Thermoset matrix. Contain unstable epoxied rings (2 Cs bonded to same O) which are opened with a curing agent (often amine or anhydride). Increased crosslink density increases Tg, modulus, thermal stability and decreases strain to failure and fracture toughness. Wide variety of properties available.
Advantages of epoxy resins
Most mechanical or thermal properties good or excellent. No volatiles on cure. Low shrinkage on cure. Excellent chemical resistance and adhesion to fibres.
Disadvantages of epoxy resins
Relatively high cost. Health concerns. Elevated cure temperature
Vinylesters
Thermoset matrix. Compromise between epoxies and unsaturated polyesters with properties between the two. Easy to process but enhanced shrinkage on cure and low adhesion to fibres.
Phenolic resins
Thermoset matrix. High temperature applications (char layer forms and ablates heat away). Used for aircraft interiors, automotive engine parts, electrical parts
Bismalamides and cyanate esters
Thermoset matrix. For high temperature applications (200-300C)
Polyketones
Thermoplastic matrix. Most common PEEK (poly ether ether ketone). Has backbone phenyl rings (impart excellent mechanical properties) as part of ether groups and ketone group between some.
Advantages and disadvantages of polyketones
High mechanical properties and temperature stability. Good solvent resistance. High fracture toughness. Low water absorption.
But high cost. Very hard to process (need 400C)
How to improve properties of matrix
Use additives/fillers
What can additives do for matrix properties?
Reduce cost. Alter mechanical or thermal properties. Reduce cure shrinkage. Control viscosity/processability. Improve electrical or environmental resistance. Give better surface finish or change colour
What is CaCO3 used for?
Added to unsaturated polyesters and vinylesters to reduce cost and cure shrinkage
Common composite combinations
Glass fibres (long or short) in unsaturated polyester matrix for low cost and moderate properties. Carbon fibres (long) in epoxy resin matrix for moderate cost and high properties. Glass fibre in epoxy resin matrix (not as common but possible)
