CH 15 - Characteristics, Applications & Processing of Polymers Flashcards
How does fracture strength & deformation strains of polymers compare to that of metals?
- fracture strength of polymers roughly 10% of those for metals
- deformation strains for polymers > 1000%
- deformation strains for most metals
What is drawing?
- stretches the polymer prior to use
- aligns chains in the stretching direction
What are the results of drawing?
- increases the elastic modulus in the stretching direction
- increases the tensile strength in the stretching direction
- decreases ductility
What is the effect of annealing after drawing?
- decreases chain alignment
- reverses effects of drawing (reduces E and TS, enhances %EL)
What is a thermoplastic?
-little crosslinking
-ductile
-soften with heating
EX: polyethylene, polypropylene, polycarbonate, polystyrene
What is a thermoset?
-significant crosslinking
-hard & brittle
-don’t soften w/heating
EX: vulcanized rubber, epoxies, polyester resin, phenolic resin
What is the influence of temp on thermoplastics?
decreasing temp:
- increases E
- increases TS
- decreases %EL
What is the influence of strain rate on thermoplastics?
increasing strain rate:
- increases E
- increases TS
- decreases %EL
What things increase chain stiffness?
presence of:
- bulky sidegroups
- polar groups or sidegroups
- chain double bonds and aromatic chain groups
What is the stress relaxation test?
- strain in tension to e-knot and hold
- observe decrease in stress with time
What is craze?
formation prior to cracking
What happens during craze?
- plastic deformation of spherulites
- formation of microvoids and fibrillar bridges
What are the two types of polymer formation?
- addition (chain) polymerization
- condensation (step) polymerization
What are the three steps of addition/chain polymerization?
(1) initiation
(2) propagation
(3) termination
What are types of polymer additives?
fillers, plasticizers, stabilizers, lubricants, colorants, flame retardants
What is the benefit of adding fillers to a polymer?
- improve tensile strength & abrasion resistance, toughness, decrease cost
- EX: carbon black, silica gel, wood flour, glass, limestone, talc
What is the benefit of adding plasticizers to polymers?
- added to reduce the glass transition temp below room temp
- presence of plasticizer transforms brittle polymer to a ductile one
What are examples of stabilizers?
antioxidants, UV protectants
How are thermoplastics processed?
- can be reversibly cooled and reheated (recycled)
- heat until soft, shape as desired, then cool
How are thermosets processed?
- when heated forms a molecular network
- degrades (doesn’t melt) when heated
- prepolymer molded into desired shape, then chemical reaction occurs
What are the characteristics of fibers?
- high tensile strengths
- high degrees of crystallinity
- structures containing polar groups
- formed by spinning
What is spinning?
- extrude polymer through a spinneret
- spun fibers are drawn under tension
- leads to highly aligned chains
What are the types of polymers?
fibers, coatings, adhesives, films, foams
What is a polymer coating?
- thin polymer films applied to surfaces
- protects from corrosion/degradation
- decorative (improves appearance)
- can provide electrical insulation
How are films produced?
produced by blown film extrusion
How are foams produced?
gas bubbles incorporated into plastics
