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Materialteknik 2.0 > Materialteknik 2017 > Flashcards

Materialteknik 2017 Flashcards

1
Q

Vad är molekylvikten ett mått på hos polymerer?

A

Polymerkedjans längd

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2
Q

Vad är det för skillnad mellan en sampolymer och en polymerblandning?

A

Sampolymerer är polymerer med två eller fler monomerer i samma kedja (polymeriserade) medan polymerblandningar innehåller två eller fler olika typer av polymerer som är blandade (ej polymeriserade)

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3
Q

Hur påverkar temperaturen viskositeten hos polymerer?

A

En ökning i temperatur orsakar en minskning av viskositeten.

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4
Q

What are polymeric materials?

A

Large molecules, repetitive units

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5
Q

What molecular forces hold a polymer component?

A
  1. Strong forces C-C (covalent bonds; backbone).

2. Weak forces (van-der-Waals; intermolecular). These dominate the strength of a polymer component

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6
Q

How do we quantify the size of polymer molecules?

A
  1. The molecular weight
  2. The molecular number
  3. Weight averaged
  4. The polydispersity index
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7
Q

How are polymers defined in terms of structure/conformation/configuration/arrangement?

A

rotation around the carbon bonds ->

  1. Flexibility
  2. Tacticity
  3. Branching/Topology
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8
Q

What are the main types of polymers based on their molecular arrangement and their characteristics?

A

Thermoplastics, can be reshaped and remelted:

  1. Amorphous, random orientation of chains
  2. Semi-crystalline, defined by degree of crystallinity which dictates how organized the polymer i

Cross-linked polymers, obtained via crosslinking/curing and cannot be reshaped or remelted:

  1. Thermosets
  2. Elastomers
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9
Q

What is polymerization?

A

The process by which monomers are added to each other through a chemical reaction to become a
polymer

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10
Q

What can polymers contain?

A
  1. Copolymers
  2. Blend
  3. Thermal stabilizers
  4. Pigments
  5. Additives
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11
Q

What are the thermal characteristics of polymers and what do thermal properties tell us about the microstructure of polymers?

A

As a polymer is heated the distance between the molecules increases as the vibration amplitude increases. At certain temperatures, the material flows at macroscopic level.

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12
Q

What transition temperatures are there?

A
  1. Tg: Glass transition temperature.
  2. Melting temperature.

For amorphous: usefulness below Tg.
For semi-crystalline: usefulness between Tg & Tm.
For thermosets: usefulness below Tg

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13
Q

What can you say about the thermal conductivity of polymers?

A

Polymers are excellent thermal insulators, best: foams.

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14
Q

What are the fundamental concepts of polymer melt flow?

A
  • Both amorphous and semi-crystalline polymers have similar structure in the melt state
  • polymers are viscoelastic when deformed in the melt state (viscous part: molecular friction between molecules)

> small/slow deformations -> no change in orientation; large/fast deformations: molecular orientation in the flow direction

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15
Q

What are the flow characteristics of polymers in the melt state?

A
  1. Thermoplastic polymers: viscosity versus shear rate: polymer materials exhibit shear-thinning behavior; the viscosity function has two regions.
  2. Thermosets: generally start from a viscous liquid (viscosity is independent of the shear rate), during curing they become viscoelastic (with shear thinning behavior) until they become solid.
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16
Q

How do we measure rheological properties?

A

Melt flow indexer and the melt flow index

17
Q

What are the main processing methods for polymers and how do they work?

A
  1. Extrusion, parts: https://i.imgur.com/uRvEW3e.png
2. Injection modling, parts:
injection unit (think extruder), mold, clamp
18
Q

Explain the solidification of thermoplastics & p-V-T diagrams

A

As temperature is lowered, polymers gradually loose their ability of move freely. Influencing parameters: Pressure & Temperature.

Higher cooling rate results in ‘freeze-in’
free volume between molecules

p-V-T: https://imgur.com/a/EN8H2

19
Q

What post-solidification problems can occur in polymers?

A
  1. Thermoplastics: residual stresses, warpage, shrinkage, anyisotropy
  2. Thermosets: non-uniform
    curing, voids, cracks, imperfections inside the part.
20
Q

What is the Boltzmann superposition principle?

A

The deformation of a polymer component is the sum/ superposition of all strains that result from various loads acting on the part at different times.

21
Q

Explain creep & stress relaxation

A

Stress relaxation: A material held at a constant strain will require less stress to hold it there with time.

Creep: A material held at a constant stress will strain over time.

https://i.imgur.com/FjUwuCa.png

22
Q

What is tacticity in polymers? Name two tactic configurations that can be achieved and represent them.

A

Tacticity - side groups can be arranged in different directions.

Ex of configurations:

Isotactic - All arranged on one side
Atactic - Arranged randomly
Syndiotactic - Alternatingly arranged

23
Q

Briefly explain the working principle of film casting as secondary shaping operation for extrusion. Use a sketch and brief description
thereof.

A

https://i.imgur.com/g0xPXPv.png

24
Q

Briefly explain the working principle of film casting as secondary shaping operation for extrusion. Use a sketch and brief description
thereof.

A

https://i.imgur.com/g0xPXPv.png

25
Q

What is ‘necking’ in polymers and in what type(s) of polymer(s) does it occur?

A

‘necking’ - localized reduction in
cross-sectional area.

Occurs in semi-cristalline polymers.

26
Q

Consider the morphology of semi-crystalline thermoplastic polymers. Name the possible state of the crystals
and how can they be obtained.

A
  1. Single crystals - formed in solution
  2. Spherulites - form at the solidification of semi-crystalline materials (no deformation)
  3. Deformed crystals (ordered lamellae) - if a semi-crystalline material is deformed during crystallization
    .4 Shish-kebab (plate and ribbon structures) - shear deformation during solidification
27
Q

Describe 4 different types of copolymers

A
  1. Alternating
  2. Block
  3. Random
  4. Graft
28
Q

What is the effect of strain rate on the occurrence of crazes in thermoplastic amorphous polymers at room temperature?

A
  1. At high def speeds: c. are small, appear shortly before failure, or do not appear at all!
  2. At low def speeds: c. are large and appear earlier during loading
29
Q

Förklara lite om isokront och isometrisk krypning.

A

Isokront: x-axel= töjning, y-axel= spänning. linje= tid
Isometrisk: x-axel= tid, y-axel= spänning. linje= töjning

30
Q

Vad är den eutektiska & eutektoida punkten?

A

Eutektisk punkt: Den lägre gränsen för det likvida enfasområdet, utgörs av en punkt där likviduslinjerna möts. Detta är den eutektiska punkten.

Eutektoid punkt: Den lägre gränsen för ett fasta enfasområde, utgörs av en punkt där fasgränslinjerna sammanfaller i ett V. Detta är den eutektoidiska punkten.

Materialteknik 2.0 (1 decks)

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