CH 15 - Characteristics, Applications & Processing of Polymers

Question 1

How does fracture strength & deformation strains of polymers compare to that of metals?

Answer 1

• fracture strength of polymers roughly 10% of those for metals
• deformation strains for polymers > 1000%
• deformation strains for most metals

Question 2

What is drawing?

Answer 2

• stretches the polymer prior to use

- aligns chains in the stretching direction

Question 3

What are the results of drawing?

Answer 3

• increases the elastic modulus in the stretching direction
• increases the tensile strength in the stretching direction
• decreases ductility

Question 4

What is the effect of annealing after drawing?

Answer 4

• decreases chain alignment

- reverses effects of drawing (reduces E and TS, enhances %EL)

Question 5

What is a thermoplastic?

Answer 5

-little crosslinking
-ductile
-soften with heating
EX: polyethylene, polypropylene, polycarbonate, polystyrene

Question 6

What is a thermoset?

Answer 6

-significant crosslinking
-hard & brittle
-don’t soften w/heating
EX: vulcanized rubber, epoxies, polyester resin, phenolic resin

Question 7

What is the influence of temp on thermoplastics?

Answer 7

decreasing temp:

• increases E
• increases TS
• decreases %EL

Question 8

What is the influence of strain rate on thermoplastics?

Answer 8

increasing strain rate:

• increases E
• increases TS
• decreases %EL

Question 9

What things increase chain stiffness?

Answer 9

presence of:

• bulky sidegroups
• polar groups or sidegroups
• chain double bonds and aromatic chain groups

Question 10

What is the stress relaxation test?

Answer 10

• strain in tension to e-knot and hold

- observe decrease in stress with time

Question 11

What is craze?

Answer 11

formation prior to cracking

Question 12

What happens during craze?

Answer 12

• plastic deformation of spherulites

- formation of microvoids and fibrillar bridges

Question 13

What are the two types of polymer formation?

Answer 13

• addition (chain) polymerization

- condensation (step) polymerization

Question 14

What are the three steps of addition/chain polymerization?

Answer 14

(1) initiation
(2) propagation
(3) termination

Question 15

What are types of polymer additives?

Answer 15

fillers, plasticizers, stabilizers, lubricants, colorants, flame retardants

Question 16

What is the benefit of adding fillers to a polymer?

Answer 16

• improve tensile strength & abrasion resistance, toughness, decrease cost
• EX: carbon black, silica gel, wood flour, glass, limestone, talc

Question 17

What is the benefit of adding plasticizers to polymers?

Answer 17

• added to reduce the glass transition temp below room temp

- presence of plasticizer transforms brittle polymer to a ductile one

Question 18

What are examples of stabilizers?

Answer 18

antioxidants, UV protectants

Question 19

How are thermoplastics processed?

Answer 19

• can be reversibly cooled and reheated (recycled)

- heat until soft, shape as desired, then cool

Question 20

How are thermosets processed?

Answer 20

• when heated forms a molecular network
• degrades (doesn’t melt) when heated
• prepolymer molded into desired shape, then chemical reaction occurs

Question 21

What are the characteristics of fibers?

Answer 21

• high tensile strengths
• high degrees of crystallinity
• structures containing polar groups
• formed by spinning

Question 22

What is spinning?

Answer 22

• extrude polymer through a spinneret
• spun fibers are drawn under tension
• leads to highly aligned chains

Question 23

What are the types of polymers?

Answer 23

fibers, coatings, adhesives, films, foams

Question 24

What is a polymer coating?

Answer 24

• thin polymer films applied to surfaces
• protects from corrosion/degradation
• decorative (improves appearance)
• can provide electrical insulation

Question 25

How are films produced?

Answer 25

produced by blown film extrusion

Question 26

How are foams produced?

Answer 26

gas bubbles incorporated into plastics