Föreläsning 8 - polymers, composites and ceramics Flashcards
What are polymers, and why are they used so often?
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
What difference does different cross-bondings make in plastic?
Weak atomic bonds between polymer molecules = thermoplastics
ex:
- Polyethylene, PE
- Polyvinyl chloride, PVC
…
Some strong covalent bonds = rubber
Dense strong covalent bonds = thermosetting
How does the melting process look for plastics?
- No distinct melting temp
- Becomes gradually softer with higher temp
How do we shape different kinds of plastics?
Thermoplastics:
*Injection molding
*Blow-forming
*Vacuum forming
*Extrusion
Thermosettings:
*Casting and hardening
What kinds of plastics can be recycled?
- Thermoplastics can be recycled.
Problems: degradation, various polymers, additives, impurities - Thermosetting and rubber cannot be recycled
What is carbon fiber?What is carbon fiber?
- 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
What is graphene?What is graphene?
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
What is glass fiber?What is glass fiber?
- Ordinary glass basically
- Same properties as glass
- Cheap and low CO_2 footprint
What is kevlar?What is kevlar?
- Polymer fiber with a crystalline structure
- C-C bindings in the fiber direction => high strength
What is spectra/dynema?
- Polyethene spun so that the polymer chains are in the fibre irection
- C-C bindings in the fibre direction => high strength
What is conclusion fiber?
- 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
What are composites?
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
What is laminate?
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)
What is distance material?
Used to increase bending stiffness.
- Polymer foam, honeycomb of Al or polymer
What are some pros and cons of composites?
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
What is special about ceramics?
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
What are clays?
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
What are technical ceramics?
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
What is glass?
- 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.
