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Elements > Chapter 15 > Flashcards

Chapter 15 Flashcards

1
Q

What are the three “categories” of the stress-strain behavior of polymers?

A

brittle, plastic, and elastomer

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2
Q

How do the three different types of polymers behave on a stress-strain curve?

A

Brittle polymer: (brittleness) steep slope, experiences significant deformation with a small increase in stress, little ability to deform plastically before fracture.

Plastic polymer: (plasticity) initial steep slope and then a slope of about 0. The material undergoes elastic deformation up to a certain stress and then transitions into plastic deformation with little increase in stress

Elastomer: (elasticity) small slope in a stress-strain curve. High elasticity and deforms easily under low stress.

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3
Q

What is the fracture strength of polymers in comparison to those for metals?

A

10% of those for metals

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4
Q

What is the deformation strain for polymers? metals?

A

polymers > 1000%
metals <10%

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5
Q

Is the elastic modulus greater for those of polymers or metals?

A

greater for metals

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6
Q

What are some structures of brittle polymers? What do they look like initially and near failure?

A

crosslinked and network

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7
Q

What is the structure of plastic polymers?

A

semicrystalline polymers

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8
Q

What are the five steps to the failure of a semicrystalline (plastic) polymer?

A

1) undeformed structure
2) amorphous regions elongate
3) crystalline regions align: ONSET OF NECKING
4) crystalline block segments separate
5) fibrillar structure- near failure

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9
Q

What are the steps of predeformation?

A

drawing and annealing

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10
Q

What is predeformation by drawing? Results? Examples?

A

Drawing
1) stretches the polymer prior to use
2) aligns chains in the stretching direction

Results
1) Increases elastic modulus (E)
2) Increases tensile strength (TS)
3) decreases ductility (%EL)

Annealing
1) decreases chain alignment
2) reverses effects of drawing (lower E, TS, and increases % EL)

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11
Q

What are the three steps in the mechanism of deformation for elastomers?

A

1) Initial: amorphous chains are kinked, cross-linked
2) deformation is reversible (elastic)
3) final: chains are straighter, still and cross-linked

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12
Q

What are the differences between thermoplastics and thermosets?

A

Thermoplastics
1) little crosslinking
2) ductile and soft with heating
3) soften with heating

Thermosets
1) significant crosslinking (10 to 50% of repeat units)
2) hard and brittle
3) do NOT soften with heating

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13
Q

What are 4 examples of thermoplastics? thermosets?

A

Thermoplastics:
polyethylene, polypropylene, polycarbonate, polystyrene

Thermosets:
1. vulcanized rubber
2. epoxies
3. polyester resin
4. phenolic resin

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14
Q

What is the influence of decreasing temperature (T) on thermoplastics?

A
  1. increases E
  2. increases TS
  3. decreases %EL
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15
Q

What are the effects of increasing strain rate on thermoplastics?

A
  1. increases E
  2. increases TS
  3. decreases %EL
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16
Q

How does increasing chain stiffness affect Tm and Tg?

A

Tm and Tg increase with increasing chain stiffness

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17
Q

What is chain stiffness increased by?

A
  1. bulky sidegroups
  2. polar groups or sidegroups
  3. chain double bonds and aromatic chain groups
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18
Q

How does the regularity of repeat unit arrangements affect Tm and Tg?

A

only affects Tm

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19
Q

What test do you use to study time-dependent deformation? Formula?

A

Stress relaxation test

relaxation modulus

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20
Q

What happens in the stress relaxation test?

A
  1. strain a material to a certain ε0 and hold it there
  2. observe decrease in stress with time
21
Q

What is the variable for the relaxation modulus?

A

E sub r

22
Q

What happens to Er in relation to T?

A

Decrease in Er when T>Tg

23
Q

What are the function of polymer additives?

A

1) improve mechanical properties
2) processability
3) durability

24
Q

What are 6 types of polymer additives?

A

1) fillers
2) plasticizers
3) stabilizers
4) lubricants
5) colorants
6) flame retardants

25
Q

Why are fillers added? 2 examples.

A

1) improve tensile strength, abrasion resistance, and toughness
2) decrease cost

ex: carbon black, silica gel

26
Q

Why are fillers added?

A

1) to reduce the glass transition temperature Tg below room temperature
2) transforms a brittle polymer into a ductile one
3) commonly added to PVC- otherwise brittle

27
Q

What are the benefits of adding stabilizers?

A

1) antioxidants
2) UV protectants `

28
Q

What are the benefits of lubricants? Example.

A

1) Added to allow easier processing
2) polymer “slides” through dies easier

sodium stearate

29
Q

What are examples of colorants?

A

Dyes and pigments

30
Q

What elements do flame retardants contain?

A
  1. chlorine
  2. fluorine
  3. boron
31
Q

How are thermoplastics processed?

A
  1. They are reversibly cooled and reheated (recycled)
  2. Heat until soft, shape as desired, then cool
32
Q

How are thermosets processed?

A
  1. Heated to form molecular network
  2. degrades (doesn’t melt) when heated
  3. a prepolymer molded into desired shape, then a chemical reaction occurs
33
Q

What are the four types plastic processing?

A
  1. compression molding
  2. injection molding
  3. extrusion
  4. blown-film extrusion
34
Q

What types of plastics can be compression molded?

A

thermoplastics and thermosets

35
Q

What is the process of compression molding?

A
  1. polymers and additives are placed in mold cavity
  2. mold is heated and pressure applied
  3. fluid polymer assumes shape of mold
36
Q

What types of plastics can be injection molded?

A

Thermoplastics and some thermosets

37
Q

How does injection molding work?

A
  1. The ram retracts and plastic pellets drop from the hopper into the barrel
  2. the ram forces plastic into the heating chamber (around the spreader) where the plastic melts as it moves forward
  3. molten plastic is forced under pressure (injected) into the mold cavity where it assumes the shape of the mold
38
Q

What types of plastics can be extruded?

A

thermoplastics

39
Q

How does extrusion work?

A
  1. plastic pellets drop from hopper onto the turning screw
  2. they melt as the screw pushes them forward by the heaters
  3. molten polymer is forced under pressure through the shaping die to form the final product (extrudate)
40
Q

How does blown-film extrusion work?

A
41
Q

What are the five polymer types?

A
  1. fibers
  2. coatings
  3. adhesives
  4. films
  5. foams
42
Q

What is the typical length/diameter of polymer fibers?

A

> 100

43
Q

What are fiber characteristics?

A
  1. high tensile strengths
  2. high degrees of crystallinity
  3. structures containing polar groups
44
Q

How are fibers formed?

A

formed by spinning:

  1. extrude polymer through a spinneret
  2. the spun fibers are drawn under tension
  3. leads to highly aligned chains-fibrillar structure
45
Q

How are polymer coatings used?

A

Coatings are thin polymer films applied to surfaces (paints, varnishes,…)

  1. protects from corrosion/degradation
  2. decorative- improves appearance
  3. can provide electrical insulation
46
Q

How are polymer adhesives used?

A

they bond two solid materials (adherands)

47
Q

What are the two bonding types of adhesives?

A
  1. secondary- van der Waals forces
  2. mechanical- penetration into pores/crevices
48
Q

What’s an example of an advanced polymer? Its properties? Applications?

A

Ultahigh molecular weight polyethylene

Properties:
1. high impact strength
2. resistance to wear/abrasion
3. low coefficient of friction
4. self-lubricating surfaces

Applications
1. bullet-proof vests
2. golf ball covers
3. hip implants

49
Q

What are the characteristics of thermoplastics? thermosets?

A

thermoplastics:
1. smaller E, yield strength, and T application
2. larger Kc- fracture toughness

thermosets:
1. larger E, yield strength, T application
2. smaller Kc

Elements (15 decks)

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