Polymer Forming & Fibre Reinforcement

Question 1

State at least 3 reasons for why typical fibre reinforcement (e.g. glass, carbon) has extremely small diameters

Answer 1

1. A lower probability of a large flaw being present (size effect) maximises the fibre strength.
2. Small diameters provide large surface area for bonding the fibre with the matrix.
3. Small diameters provide sufficient flexibility in fibre production

Question 2

How do you calculate the maximum crack length?

Answer 2

a[max] = 2EGlass Surface Energy / (pi*fibre tensile stress)

Question 3

How do you calculate the surface to volume ratio?

Answer 3

Sf/V = 4/d

Sf = pi*d*L
V = (pi*d^2*L) / 4

Question 4

What are the processing methods for thermoplastics?

Answer 4

The most common options for processing thermoplastic polymers:

• Extrusion: Plastics are melted and pushed through a die to form a continuous profile
• Injection moulding: Moulding is achieved by final injection through reciprocating-screw
• Blow moulding: Film thickness and diameter are controlled through roller velocity, cooling rate and pressure inside the bubble
• Compression moulding: Involves heating the polymer in a cavity and consolidating the melt under pressure.
• 3D Printing: A form of additive manufacturing technology, in which 3D objects are created by laying down successive layers of materials.

Question 5

What effect does the molar mass have on processing

Answer 5

Critical/minimum value of molar mass required for the
formation of entanglements, which can form physical cross-linking.

From the point of view of ease of processing, the polymer should have a molar mass which is below the critical value of molar mass; however, many of the advantageous physical properties (e.g. strength, toughness etc) of polymer are only achieved by high molar mass materials.

Blends of different molar mass polymers to achieve the desired balance of processability and physical properties in the final product.

Question 6

What effect does cold drawing have on polymer properties

Answer 6

Cold drawing is the process whereby the neck extends along load direction.

Cold drawing induces anisotropic property in polymers with higher mechanical properties (e.g. strength, modulus) along the drawing direction (ie. chain length axis) and lower mechanical properties in other directions.

Typically found when blow moulding, however extrusion blow moulding polymers are not necessarily subjected to any significant cold drawing in production.

Question 7

What is size effect in fibre reinforcement

Answer 7

Small fibres are stronger than its bulk form because of a lower probability of a large flaw being present – so called size effect.

Stiffness is determined by the bulk material structure and
molecular orientation but strength of a brittle material is
practically determined by its artificial surface flaws/defects.

Question 8

Why do glass and carbon fibres play a critical role in lightweight engineering applications?

Answer 8

Lightweight engineering structure requires a material with high specific stiffness and specific strength.
Carbon fibres offer the highest specific modulus among all the fibres commercially available today, while glass fibres have the best performance-to-price ratio and
make them the best economic reinforcement fibres.