Föreläsning 8 - polymers, composites and ceramics

Question 1

What are polymers, and why are they used so often?

Answer 1

poly = many
merer = building blocks
==> long molecules
carbon and hydrogen

Plastic = polymer + additives

Main reason for use of plastics:
- Good mechanical properties in relation to density
- Freedom in geometry and design
- Cost-effective

Question 2

What difference does different cross-bondings make in plastic?

Answer 2

Weak atomic bonds between polymer molecules = thermoplastics
ex:
- Polyethylene, PE
- Polyvinyl chloride, PVC
…

Some strong covalent bonds = rubber

Dense strong covalent bonds = thermosetting

Question 3

How does the melting process look for plastics?

Answer 3

• No distinct melting temp
• Becomes gradually softer with higher temp

Question 4

How do we shape different kinds of plastics?

Answer 4

Thermoplastics:
*Injection molding
*Blow-forming
*Vacuum forming
*Extrusion

Thermosettings:
*Casting and hardening

Question 5

What kinds of plastics can be recycled?

Answer 5

• Thermoplastics can be recycled.
  Problems: degradation, various polymers, additives, impurities
• Thermosetting and rubber cannot be recycled

Question 6

What is carbon fiber?What is carbon fiber?

Answer 6

• The C-C binding is strong
• Graphite consists of layers with strong C-C binding
• Weak bond between layers
  ==>
• High E- modulus and stretch limit in two directions
• Low E-modulus and stretch limit in cross direction
• Graphite is not a widely used design matter, high operating temp.

High strength, high stiffness

Question 7

What is graphene?What is graphene?

Answer 7

High E-module parallel to atomic plane

• Not a construction material!
  -High price and high CO_2 footprint
• Interesting properties: electrical, chemical, biological
• Can modify other materials

Question 8

What is glass fiber?What is glass fiber?

Answer 8

• Ordinary glass basically
• Same properties as glass
• Cheap and low CO_2 footprint

Question 9

What is kevlar?What is kevlar?

Answer 9

• Polymer fiber with a crystalline structure
• C-C bindings in the fiber direction => high strength

Question 10

What is spectra/dynema?

Answer 10

• Polyethene spun so that the polymer chains are in the fibre irection
• C-C bindings in the fibre direction => high strength

Question 11

What is conclusion fiber?

Answer 11

• It is possible to make fiber with very good properties, better than homogenous materials
• Ceramic and polymeric fibers are mainly used, but also metallic and natural
• In order to be used as engineering material, the fibers must be held together => composites

Question 12

What are composites?

Answer 12

Combined materials.

• Reinforcement phase = high strength material
• Matrix = the material that binds together, usually a plastic (even metal or ceramic)
• Long fibers, short fibers or particles

Question 13

What is laminate?

Answer 13

Composite built up from lamellas with long fibers in different directions.

Characteristics depend on the directions of the lamellas

UD= undirectional

QI = quasi-isotropic, same properties in all directions (requires fiber in at least 3 directions)

Question 14

What is distance material?

Answer 14

Used to increase bending stiffness.

• Polymer foam, honeycomb of Al or polymer

Question 15

What are some pros and cons of composites?

Answer 15

Pros:
- Exploits fiber properties
- Ofter low density
- Can provide good crime toughness
- Can control properties

Cons:
- More difficult and expensive manufacturing
- Harder to predict properties
- Difficult or impossible to recycle

Question 16

What is special about ceramics?

Answer 16

Compound of metal and non-metal, strong covalent/ionic bonds => high melting temp

No (usually) moving electrons => low electrical conductivity

No dislocation movement => no plasticity, hard and brittle

Much less strength in tension than in compressive load (about 15 times)

Sensitive to defects in material

Cannot be cast or shaped plastically

May have special optical, magnetic and electrical properties.
-superconductors
- dye, pigment
- ruby

sinter at ~2/3 of melting temp

Question 17

What are clays?

Answer 17

silicates in layers (SiO_4)

strong bonds in the layers, weak bonds between the layers -> moldable when the clay is wet

sintered = materials have bonded

Question 18

What are technical ceramics?

Answer 18

Starting materials: pure powders of the ceramics.

Mixed with any binder and shaped into a green body.

sinter at 2/3 of the melting temp. => the ceramics are bound together by diffusion.

Applications: Ceramic bearings, cutting tools, thermal coatings

Question 19

What is glass?

Answer 19

• Amorphous 3D-network of SiO_2

Network modifiers lower forming temp.

Glass can be formed using a variety of methods in a range of temps.