Flashcards in CH 15 - Characteristics, Applications & Processing of Polymers Deck (26):
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
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?
-soften with heating
EX: polyethylene, polypropylene, polycarbonate, polystyrene
What is a thermoset?
-hard & brittle
-don't soften w/heating
EX: vulcanized rubber, epoxies, polyester resin, phenolic resin
What is the influence of temp on thermoplastics?
What is the influence of strain rate on thermoplastics?
increasing strain rate:
What things increase chain stiffness?
-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?
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