Polymer Composites

Question 1

What are polymer composites 2 phases?

Answer 1

same as copolymer:
1. matrix
2. dispersed

Question 2

what is the difference between composite and blend?

Answer 2

composites dispersed phase are non-polymer compounds

• composites create designs that can compete with metal and ceramics for mechanical properties

Question 3

Why do we add composites?

Answer 3

To improve properties or to add a desired functionality (ex. electric conductivity)

fibre-reinforced polymer composites are popular choices for rigid, lightweight structural materials (ex. hockey sticks, canoes)

Question 4

How are thermoplastic composites made?

Answer 4

• typically prepared with melt blending using extrusion
• polymer and dispersed particles both added into extruder where the polymer is melted and they are mixed
• has a low degree of particle alignment
• maximum dispersed particle fraction is lower (~40%) because need to be able to flow

Question 5

How are thermoset composites made?

Answer 5

• can use wet layup process where a fibre mat is rolled out and resin is poured around it in a mold
• time intensive
• difficult to make small parts (but good for large materials)
• good alignment of dispersed particles
• up to 80% polymers
• lots of batch processes (can use filament winding for continuous but is a very slow process)
• very difficult to recycle

Question 6

What are some processes to create thermoset composities?

Answer 6

1. wet lay-up process
2. filament winding
3. pultrusion
4. resin transfer moulding

Question 7

What factors affect composite reinforcement?

Answer 7

1. concentration
   - (more dispersed = better properties to a certain point)
2. size
   - interfacial area decreases with increasing size
3. shape, aspect ratio
   - aspect ratio = largest dimension/smallest dimension
   - high aspect ratio aligns more with flow
4. distribution
   - want homogenous mixed
   - want to avoid agglomeration
5. orientation
   - if fibres perpendicular to load, do not get reinforcing effect

Question 8

What is the fiber reinforced composite effect?

Answer 8

• size of polymer aligned with the force
• the interfacial tension is low enough, will get properties of fibre when stress is applied (fibre often 10-100X stronger than polymer)
• there is a critical length the fibre must be to to have a noticeable stiffening effect (balance of the tensile strength of the fibre and the shear yield strength of the interface)

Question 9

What happens if below or above the critical length?

Answer 9

equal or above, stress reaches maximum applied load and remains there

below, it never reaches maximum applied load as polymer breaks from fibre

Question 10

How does continuous aligned fibres impact the strength and stiffness of mixtures?

Answer 10

aligned, follows the typical lower and upper values based on their volume fraction (when L > Lc)

Question 11

How does discontinuous aligned fibres impact strength and stiffness of mixtures?

Answer 11

If L>Lc, then get a matrix and a fibre phase, same as when L<Lc (but here we have the shear strength at the interface directly)

(just 2 different equations)

Question 12

How does discontinuous random fibres impact strengh and stiffness of composites?

Answer 12

random orientation are relatively isotropic

• follows the rule of mixture, modified by K on the fibre property
• K less than 1 (usually between 0.1 and 0.6)

Question 13

How to add conductivity to composite materials?

Answer 13

• need to add conductive particles but also need them to form a pathway (or need percolation)
• There is a threshold where below this volume fraction, conductivity is low but above grows exponentially to a fully formed network of dispersed particles

Question 14

What are ways to improve conductivity of composite materials with the same or less conductive dispersed particle?

Answer 14

• since conductive materials are expensive, use strategies for controlling percolation
• selection of filler volume fraction and aspect ratio
• cocontinuous blends where conductive particles localize at 1 (or at the interphase)
• Use foaming - this creates bubbles that push dispersed particles together where the bubbles touch
  -pathway formation through annealing and crystallite exclusion (same mechanism as foaming but with crystals instead)

Question 15

What is the desired aspect ratio of dispersed particles for packaging?

Answer 15

• want high aspect ratios
• dispersed is often considered impermeable (or less than polymer), so can create longer flow backs with high aspect ratio particles aligned against the flow path

Question 16

What is the issue with adding biocomposites into polymers? And why would we want to do that?

Answer 16

pros:
- biodegradeable, so is a green approach that is compostable and renewable
- natural fibres have excellent mechanical properties
- can be used in thermoplastics and thermosets

cons:
- poor compatibility/interface with matrix
- processing challenges