Composites- Matrices

Question 1

Functions of the matrix

Answer 1

Bind reinforcements together. Maintain shape. Transfer stress onto reinforcement. Protect reinforcements from chemical and moisture attack and mechanical abrasion. Add mechanical properties

Question 2

What mechanical properties can matrices add?

Answer 2

Toughness to improve energy absorption. Shear strength to support against bending and torsional loading. Compressive strength to support against buckling

Question 3

What is manufacture of composites controlled by?

Answer 3

The processing of the matrix phase. This considers viscosity, cure temperature and safety

Question 4

Examples of thermoset polymer matrices

Answer 4

Most commonly used.

Epoxies, unsaturated polyesters, vinylesters, phenolics, bismalamides, cyanate esters

Question 5

Examples of thermoplastic polymer matrices

Answer 5

Polyketones (PEEK), polyphenylene sulfide, polysulfones, thermoplastic polyamides

Question 6

Other possible matrices

Answer 6

Metallic (alloys of Al, Ti, Mg)

Ceramic (Al2O3.SiO2, Al2O3, SiC, concrete, carbon)

Question 7

General advantages of thermoset matrices

Answer 7

Low viscosity reactants. Good adhesions. Thermal stability. Chemical resistance. Low creep

Question 8

General disadvantages of thermoset matrices

Answer 8

Limited storage life. Long cure time. Low strain to failure

Question 9

General advantages of thermoplastic matrices

Answer 9

High impact strength. High fracture toughness. High strain to failure. Good storage life. Low fabrication time

Question 10

General disadvantages of thermoplastic matrices

Answer 10

High melt viscosity. Low creep resistance. Low thermal stability

Question 11

Unsaturated polyesters

Answer 11

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

Question 12

How to cure unsaturated polyesters

Answer 12

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

Question 13

Epoxy resins

Answer 13

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.

Question 14

Advantages of epoxy resins

Answer 14

Most mechanical or thermal properties good or excellent. No volatiles on cure. Low shrinkage on cure. Excellent chemical resistance and adhesion to fibres.

Question 15

Disadvantages of epoxy resins

Answer 15

Relatively high cost. Health concerns. Elevated cure temperature

Question 16

Vinylesters

Answer 16

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.

Question 17

Phenolic resins

Answer 17

Thermoset matrix. High temperature applications (char layer forms and ablates heat away). Used for aircraft interiors, automotive engine parts, electrical parts

Question 18

Bismalamides and cyanate esters

Answer 18

Thermoset matrix. For high temperature applications (200-300C)

Question 19

Polyketones

Answer 19

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.

Question 20

Advantages and disadvantages of polyketones

Answer 20

High mechanical properties and temperature stability. Good solvent resistance. High fracture toughness. Low water absorption.
But high cost. Very hard to process (need 400C)

Question 21

How to improve properties of matrix

Answer 21

Use additives/fillers

Question 22

What can additives do for matrix properties?

Answer 22

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

Question 23

What is CaCO3 used for?

Answer 23

Added to unsaturated polyesters and vinylesters to reduce cost and cure shrinkage

Question 24

Common composite combinations

Answer 24

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)