Composites and polymers

Question 1

Why are thermoplastics usually recyclable while thermosets are not recyclable?

Answer 1

1. Thermoplastics: manufactured by addition of polymers
   Thermoplastics can be torn apart into individual polymers and recycled usually by applying heat and reshaping them
2. Thermoset are manufactured by condensation polymerization. Normally, the by-product is lost in the reaction. Recycling is not possible because there is no efficient way of getting the by-product like water molecules back into the individual components

Question 2

Why do fibres gain strength when the fibre diameter decreases?

Answer 2

1. Size effect: increased volume for a same area will increase the chances of a critical flaw and thus reduce strength
2. Reducing the diameter reduces the volume which then reduces the probability of a critical flaw meaning that strength is increased
3. In the perpendicular direction to the fibres, the diameter is reduced which reduces the size of the flaws effectively eliminating them and increasing the overall strength

Question 3

Why do FRPs have good impact resistance?

Answer 3

1. When an impact occurs, energy has to be absorbed by the material
2. In the case of an FRP, the energy absorbed leads to fibers breaking
3. When a lot of fibers break, a lot of energy is absorbed which massively reduces the kinetic energy of the object impacting the FRP
4. The polymer matrix is capable of absorbing massive amounts of energy although the FRP might need to be replaced to be effective again

Question 4

How is it possible to increase the surface bond of pultruded FRP rebar with concrete? Why is it difficult to take the same approach as done in steel rebar?

Answer 4

1. To increase the surface bond, you need to increase the surface area between the rebar and the concrete
2. This can be done by using a surface treatment on the rebar such as a sand coating increasing surface area and mechanical interlock between the rebar and the concrete.
3. Harder approach for FRP than for steel because steel can be deformed during manufacturing to create ridges and increase surface area
4. FRP has a constant cross-section which makes it harder to create ridges like steel

Question 5

What is a polymer and what are they made of?

Answer 5

1. Plastic
2. Synthetic materials made from repeating molecules to create a long flexible chain
3. These repeating molecules which make up a polymer is called a monomer (monomer + monomer = polymer)

Question 6

What is the difference between addition polymerization and condensation polymerization?

Answer 6

1. Addition polymerization adds 2 monomers without the creation of a by-product like water
2. Condensation polymerization adds 2 monomers with the creation of a by-product

Question 7

What are the 3 groups of polymers?

Answer 7

1. Thermoplastics
2. Thermosets
3. Elastomers

Question 8

What is a thermoplastic?

Answer 8

Polymers obtained by addition polymerization

Question 9

What is a thermoset?

Answer 9

Polymers obtained by condensation polymerization

Question 10

What is an elastomer?

Answer 10

Polymer obtained by condensation and addition. They have the ability to elastically deform under low loads without permanently changing shape

Question 11

What are the characteristics of a high performance/high quality fiber and what do they do?

Answer 11

1. Small diameter (increase tensile strength)
2. High flexibility (allows for easy fabrication)
3. High aspect ratio (increases surface area/volume ratio which increases strength)