Lecture 6: Plastics Flashcards
plastics consist of what?
synthetic polymers and additives
define polymer
large molecules made up of many repeating units
define monomer
molecule that combines with others by covalent bonds to form a polymer
how are synthetic polymers developed? manufactured?
developed by chemist
manufactured in factories
current polymers are made from what?
what do they contain? what do they sometimes contain?
petrochemicals
contains C, H, O
sometimes contains Cl, F, N, Si, P, S
what is polymerization?
the chemical process that joins monomers to make a polymer
what two main additions are involved in polymerization?
addition and condensation
differentiate homo polymer and copolmer
homo: made from one kind of monomer
copolymer: made from 2 or more different monomers
in synthetic polymers, as the chain length increases, how does size change? how does this affect melting point of the substance?
increases the size which increases melting point
increasing molecular wt means an increase in what properties?
tensile strength
hardness
stiffness
barrier
increasing molecular wt means a decrease in what properties?
solubility
describe properties of polyethylene
poor O2 barrier
excellent moisture barrier
describe properties of poly vinyl alcohol
superior O2 barrier
poor moisture barrier (soluble in H2O)
describe properties of ethylene vinyl alcohol
excellent O2 barrier
reduced water solubility
how can you change the properties of copolymers?
by changing the amount of which monomers are used
what is the polarity of CO2?
non-polar
describe permeability of barriers
barriers do not allow any substance
if a substance can pass through, it is permeable
what are complete barriers?
glass and metal
they don’t allow any substance to pass through
do plastics have barrier or permeability properties?
has barrier properties
selective permeability
what molecules can pass through non-polar PE?
PE has poor O2 barriers and good moisture barriers
thus, polar water molecules can’t pass
non-polar O2 molecules can pass
what are some non-polar synthetic polymers?
poly ethylene
polypropylene
polystyrene
polytetrafluoroethylene
what are some polar polymers?
polyester
polyamide
polyvinyl chloride
what are the effects of molecular shape on crystallinity?
the shape of the molecule helps determine how they fit together
regular shapes can back together in regular and repeating patterns (results in crystalline)
irregular shapes can pack together in irregular and random patterns (results in amorphous
what kind of packing results in crystallinity in polymers?
orderly molecular packing
what kind of packing results in amorphous polymers?
random molecular packing
all polymers are ___ when they are melted
a) amorphous
b) crystalline
amorphous
rapid cooling encourages ____ regions
a) amorphous
b) crystalline
amorphous
slow cooling encourages ____ regions
a) amorphous
b) crystalline
crystalline
what are properties that are affected by the amount of crystalline regions?
clarity melting point barrier properties stiffness etc..
what are the following properties in amorphous polymers?
a) clarity
b) melting point
c) barrier
d) stiffness
a) clarity = clear
b) melting point = lower MP
c) barrier = poor
d) stiffness = less stiff
what are the following properties in crystalline polymers?
a) clarity
b) melting point
c) barrier
d) stiffness
a) clarity = hazy
b) melting point = higher MP
c) barrier = better
d) stiffness = more stiff
describe the effects of orientation of molecules in polymers
polymers are orientated to improve properties
orientation aligns molecules (makes them more crystalline and less amorphous)
how does alignment of molecules in polymers affect crystallinity?
increases crystallinity
what are two words used to describe orientation?
uniaxial (1 direction)
biaxial (2 directions)
how do you give “memory” to plastics?
what happens when you cool fast or slow?
heat them to soften, then stretch
if cooled slowly, will be heat stable
if cooled fast, will have a “memory”. When heated to the same temp again, it will shrink
what are most commonly used plastics for food packaging?
thermoplastics
what are properties of thermoplastics?
- soften when heated (don’t have a set MP)
- solidify when cooled
- can be re-melted
- viscoelastic
what are examples of thermoplastics?
polyethylene
polypropylene
polystyrene
poly vinyl chloride
describe thermoset plastics
what are examples?
once set, can’t be re-melted or reformed
has crosslinked polymer chains
examples: urethane, bakelite, melmac
define plasticizer
increases flexibility and plasticity
decreases flow temp and hardness
what are additives that can be added?
- plasticizers
- stabilizers
- flame retardants
- antistatics
- slip agents
- foaming agents
- pigments
- fillers
what are techniques used for plastic processing?
- pelleting
- single screw extruder
- twin screw extruder
- hot press
- blown film
- double bubble process
describe blown film plastic processing
5 main components: chill rollers, wind up roll, collapser, tenter frame, extruders
- annealing under tension reduces residual stresses formed during cooling and prevents potential shrinkage of film
- the process is controlled by the extruder feed, air volume, roller speed
- orientation occurs in the machine and transverse direction during blowing
what are important components in the double bubble process
- cooling extrudates
- reheating
- biaxial orientation
- rapid air cooling
- annealing under tension
describe what cooling extrudates does in the double bubble process?
limits crystallisation and spherulite growth and yields superior optical properties
describe what reheating does in the double bubble process
gives mobility to stretch but regain orientation yields for improved tensile properties
describe what rapid air cooling does in the double bubble process
freezes the orientation produced in stretching
describe what annealing under tension does in the double bubble process
reduces residual stresses formed during cooling and prevents potential shrinkage of film
what kind of treatment is printing?
surface treatment
printability is affected by what?
polarity of polymer
additives
printability is improved by what? (3)
electric discharge
flame treatment
plasma treatment
what are properties of plastic packaging materials?
- density s 0.9-1.4 g/mL
- easily broken
- sealability
- flexibility and strength
- environmental durability
- selective permeability (gas, odour, light)
- printability
what do the resin identification codes correspond to?
- PETE: polyethylene terephthalate
- HDPE: high density PE
- V: vinyl
- LDPE: low density polyethylene
5: PP: polypropylene - PS: polystyrene
- other
what is PETE made from?
what reaction is this?
acid (TPA) and alcohol (EG)
via condensation rxn
what are properties of ETE?
- highest tensile strength
- elongation 150%
- good moisture barrier
- good gas barrier
- no heat sealability
- tear resistant
- stable at low and high temp and humidity
- clarify
- chemical resistance
how does two-way stretching improve PET?
improves
- tensile strength
- flexibility
- tear strength
how does high temp crystallization improve PET?
improves
- thermal stability
- barrier properties
what are common types of PET?
amorphous (APET)
crystalline (CPET)
what are applications of PET?
- film:
- where high strength and improved gas barrier is needed
- coated to make sealable
- ovenable trays and pouches
- heat shrink film - dimensional: clear bottles, soda pop, thermoformed containers
how is HDPE made?
how is LDPE made?
low pressure + catalyst –> HDPE
high pressure + catalyst –> LDPE
