Chapter 15 Flashcards
What are the three “categories” of the stress-strain behavior of polymers?
brittle, plastic, and elastomer
How do the three different types of polymers behave on a stress-strain curve?
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.
What is the fracture strength of polymers in comparison to those for metals?
10% of those for metals
What is the deformation strain for polymers? metals?
polymers > 1000%
metals <10%
Is the elastic modulus greater for those of polymers or metals?
greater for metals
What are some structures of brittle polymers? What do they look like initially and near failure?
crosslinked and network
What is the structure of plastic polymers?
semicrystalline polymers
What are the five steps to the failure of a semicrystalline (plastic) polymer?
1) undeformed structure
2) amorphous regions elongate
3) crystalline regions align: ONSET OF NECKING
4) crystalline block segments separate
5) fibrillar structure- near failure
What are the steps of predeformation?
drawing and annealing
What is predeformation by drawing? Results? Examples?
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)
What are the three steps in the mechanism of deformation for elastomers?
1) Initial: amorphous chains are kinked, cross-linked
2) deformation is reversible (elastic)
3) final: chains are straighter, still and cross-linked
What are the differences between thermoplastics and thermosets?
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
What are 4 examples of thermoplastics? thermosets?
Thermoplastics:
polyethylene, polypropylene, polycarbonate, polystyrene
Thermosets:
1. vulcanized rubber
2. epoxies
3. polyester resin
4. phenolic resin
What is the influence of decreasing temperature (T) on thermoplastics?
- increases E
- increases TS
- decreases %EL
What are the effects of increasing strain rate on thermoplastics?
- increases E
- increases TS
- decreases %EL
How does increasing chain stiffness affect Tm and Tg?
Tm and Tg increase with increasing chain stiffness
What is chain stiffness increased by?
- bulky sidegroups
- polar groups or sidegroups
- chain double bonds and aromatic chain groups
How does the regularity of repeat unit arrangements affect Tm and Tg?
only affects Tm
What test do you use to study time-dependent deformation? Formula?
Stress relaxation test
relaxation modulus
What happens in the stress relaxation test?
- strain a material to a certain ε0 and hold it there
- observe decrease in stress with time
What is the variable for the relaxation modulus?
E sub r
What happens to Er in relation to T?
Decrease in Er when T>Tg
What are the function of polymer additives?
1) improve mechanical properties
2) processability
3) durability
What are 6 types of polymer additives?
1) fillers
2) plasticizers
3) stabilizers
4) lubricants
5) colorants
6) flame retardants
Why are fillers added? 2 examples.
1) improve tensile strength, abrasion resistance, and toughness
2) decrease cost
ex: carbon black, silica gel
Why are fillers added?
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
What are the benefits of adding stabilizers?
1) antioxidants
2) UV protectants `
What are the benefits of lubricants? Example.
1) Added to allow easier processing
2) polymer “slides” through dies easier
sodium stearate
What are examples of colorants?
Dyes and pigments
What elements do flame retardants contain?
- chlorine
- fluorine
- boron
How are thermoplastics processed?
- They are reversibly cooled and reheated (recycled)
- Heat until soft, shape as desired, then cool
How are thermosets processed?
- Heated to form molecular network
- degrades (doesn’t melt) when heated
- a prepolymer molded into desired shape, then a chemical reaction occurs
What are the four types plastic processing?
- compression molding
- injection molding
- extrusion
- blown-film extrusion
What types of plastics can be compression molded?
thermoplastics and thermosets
What is the process of compression molding?
- polymers and additives are placed in mold cavity
- mold is heated and pressure applied
- fluid polymer assumes shape of mold
What types of plastics can be injection molded?
Thermoplastics and some thermosets
How does injection molding work?
- The ram retracts and plastic pellets drop from the hopper into the barrel
- the ram forces plastic into the heating chamber (around the spreader) where the plastic melts as it moves forward
- molten plastic is forced under pressure (injected) into the mold cavity where it assumes the shape of the mold
What types of plastics can be extruded?
thermoplastics
How does extrusion work?
- plastic pellets drop from hopper onto the turning screw
- they melt as the screw pushes them forward by the heaters
- molten polymer is forced under pressure through the shaping die to form the final product (extrudate)
How does blown-film extrusion work?
What are the five polymer types?
- fibers
- coatings
- adhesives
- films
- foams
What is the typical length/diameter of polymer fibers?
> 100
What are fiber characteristics?
- high tensile strengths
- high degrees of crystallinity
- structures containing polar groups
How are fibers formed?
formed by spinning:
- extrude polymer through a spinneret
- the spun fibers are drawn under tension
- leads to highly aligned chains-fibrillar structure
How are polymer coatings used?
Coatings are thin polymer films applied to surfaces (paints, varnishes,…)
- protects from corrosion/degradation
- decorative- improves appearance
- can provide electrical insulation
How are polymer adhesives used?
they bond two solid materials (adherands)
What are the two bonding types of adhesives?
- secondary- van der Waals forces
- mechanical- penetration into pores/crevices
What’s an example of an advanced polymer? Its properties? Applications?
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
What are the characteristics of thermoplastics? thermosets?
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
