Plastics Flashcards
First synthetic plastic? What was it made from?
Bakelite (1907)
phenol + formaldehyde -> cross-linked resin
Define ‘plastics’
synthetic polymers + additives
What is a polymer?
large molecule made of repeating units (monomer)
What is a monomer?
molecule that combines with others (same or different) w/ covalent bonds -> forms polymer
Current synthetic polymers are made from _____. What elements are they made of?
petrochemicals
C, H, O (sometimes Cl, F, N, Si, P, S)
2 sources of petrochemicals?
raw natural gas
crude oil
How are the materials for plastic synthesis obtained from raw natural gas?
gas -> processing (release methane) -> produce ethane, propane, butane
STEAM CRACKER -> ethylene, propylene, benzene, butadiene, byproducts
How are the materials for plastic synthesis obtained from crude oil?
oil -> refinery -> Naphtha, gas oil -> STEAM CRACKER -> ethylene, propylene, benzene, butadiene, byproducts
What does the Steam Cracker do?
uses catalyst to process raw hydrocarbon materials -> monomers with doub bond
What is “BTX?”
Benzene, toluene, xylenes (produced from crude oil -> refining)
What is BTX used for?
processed into monomers for polystyrenes, nylons, polyesters
The chemical process that joins monomers is called ______. What rxn can be used? (2)
polymerization
addition
condensation
2 types of polymers (based on monomer content)?
Homopolymer (1 monomer type)
Copolymer (2+ monomers types)
3 descriptive characteristics of polymers (and monomers)
molecular weight
chemical composition
molecular structure
How does molecular weight/size affect melting point?
longer chain length (larger size) -> increased melting point
Why does polyethylene have a higher melt point than wax?
much longer chain length (molecule size) - 70 vs 1500
The higher the molecular weight, the greater the ________ (5)
melt point tensile strength hardness stiffness barrier properties
True/False: increased molecular weight of a polymer will increase solubility
false: will decrease solubility
What monomers is polyethylene made of? what properties does it have?
ethylene
poor O2 barrier, good moisture barrier
What monomers is polyvinyl alcohol made of? What properties does it have?
vinyl alcohol
good O2 barrier, poor moisture barrier (O2 soluble)
Copolymer made from PE and PVA:
What is the advantage?
ethylene-vinyl alcohol
excellent O2 barrier, reduced water solubility (properties from each type)
Why is PVA a poor moisture barrier?
water soluble (OH groups)
What is the proportion of PE vs PVA in ethylene vinyl alcohol? why is this important?
27-48% PE (52-73% PVA)
properties will be shifted towards the polymer with higher %
what is polarity?
how electrons are shared:
equal sharing = no dipole moment = nonpolar
unequal sharing = dipole (‘charged’) = polar
polar molecules are attracted to ____ and not attracted to _____
polar molecules
nonpolar molecules
water is (polar/nonpolar). CO2 is (polar/nonpolar)
polar
nonpolar
What is ‘like dissolves like?’
molecules with same polarity -> soluble
What is a complete barrier? what materials qualify?
prevents substances from passing through (moisture, gas)
glass, metal
Are plastics complete barriers?
No; selective permeability
Why are most plastics generally better as moisture barriers than gas barriers?
water = polar molecule (repelled by nonpolar plastic)
O2 and CO2 = nonpolar, small -> pass through easier
What are the nonpolar plastics? (4)
PE, PP, PS, PTFE
What are the polar plastics? (3)
Polyester, Polyamide, PVC
Why is molecular shape important?
determines packing of molecules (amorphous or crystalline)
affects clarity, melt point, barrier properties, stiffness…
Regular shapes can be packed into ___ structures. Irregular shapes become _____ structures.
crystalline
amorphous
True/False: melted polymers can be amorphous or crystalline
False; all are amorphous when melted
How does the cooling process affect structure formation?
rapid cooling -> amorphous
slow cooling -> crystalline
Compare clarity in amorphous vs crystalline:
amorphous: clear
crystalline: hazy (unless very small crystals)
T/F: crystalline structures are stiffer then amorphous structures
true
Which is a better barrier, crystalline or amorphous structures?
crystalline
T/F: amorphous structures have a higher melt point than crystalline
False; lower melt point
____ _____ will align molecules, making amorphous structures more crystalline (organized)
molecular orientation
Types of molecular orientation: (2)
Uniaxial (1 direction)
Biaxial (2 directions)
What are ‘memory’ plastics? How are they made?
Shrinkable plastics (when heated)
heat to soften -> stretch -> cool quickly (will retain ‘memory’)
When reheated to same temperature, will shrink
What happens if a plastic is heat stretched, and cooled slowly?
becomes heat stable (will not shrink with re-heating)
Most plastics used for food are _____
thermoplastics
What are the properties of thermoplastics?
soften when heated (no set melt point)
solidify when cooled
can be re-melted
viscoelastic
Examples of thermoplastics (4)
polyethylene
polypropylene
polystyrene
polyvinyl chloride
What is a thermoset plastic and its properties?
plastic that cannot be melted/re-formed once it is set (polymer chains are crosslinked into 1 giant molecule)
Examples of thermoset polymers: (3)
urethane
bakelite
melmac
Can Urethane be used to make shrink-wrap?
No; it is a thermoset plastic (cannot be melted/re-formed)
Polyethylene was heat-stretched and cooled quickly. What will happen when it is reheated to the same temperature?
shrinkage
What are some plastic additive types? (8)
plasticizer stabilizer flame retardant antistatics slip agents foaming agents pigments fillers
What is the purpose of plasticizers?
increase flexibility & plasticity
reduce flow temperature & hardness
How are plasticizers incorporated into the polymer?
expose polymer to plasticizer -> diffuses into polymer network
examples of plasticizers: (5)
phthalate (P4) di-n-octylphthalate (P5) tri-n-butyl citrate (P1) dioctyl adipate (P3) dioctyl sebacate (P2)
What are stabilizers for?
protect from degradation from UV, heat, oxygen
examples of plastic stabilizers: (5)
Carbon black, TiO, hydroxybenzophenone, organometallic compounds, antioxidants
examples of flame retardants: (2)
PBDE (poly brominated diphenyl ethers)
Al(OH)3
What compounds are used as antistatics?
long chain aliphatic amines
What is a pelletizer used for?
making materials (plastic powder/pellets) for further processing
recycling
What is the process of plastic pelleting?
raw materials (resins, hardeners, pigment, flow additive) -> premix extruder -> cooling belt -> cutter -> grinder (reduce to powder) -> sieve (remove coarse powder)
powder -> packaging manufacture
What is plastics extrusion?
raw plastic melted -> forced through extruder -> single continuous form
Types of extruders:
single screw twin screw (conical, parallel)
main zones of an extruder screw:
- solids conveying (feeds resin into extruder)
- melting (most material is melted, channel gets smaller)
- metering (melts last particles, mixes for even temp/composition)
What melts the plastic in an extruder? What gives it the shape?
heated barrel, intense pressure/friction from screw
forced through die at end
The different rotation and alignment patterns for twin screws:
intermeshing, non-intermeshing
co-rotating, counter-rotating
Plastic can be extruded into what forms?
tubing, pipes, sheets, films, specific shapes (structural parts)
The resin (raw material) is added to the extruder through the _____
feed hopper
The product exiting the extruder is called the _____
extrudate
types of molds connected to extruders?
blown mold
injection mold
compression mold (hot press)
how does a blown mold work?
split mold with parison inside
mold closed -> air blown in -> expands plastic (press against mold)
mold opened -> bottle
How does a injection mold work?
inject liquid plastic into closed mold
open mold to eject (automatic ejector pins)
What is compression molding for plastics?
place plastic material (charge) in mold
Use hot press (heat + pressure) -> compress into desired shape
open press to remove
What is blown film extrusion?
extrude molten plastic through die -> inflate with air -> thin film bubble -> collapse to make film
What are the parts of a blown film extrusion process?
- extruder (molten plastic pumped out)
- air forced through tubing die
- tenter frame (stabilizes inflated tube)
- Collapser (tube -> film)
- Chill rollers (annealing/cool slowly under tension)
- wind-up roll
Why is anealing under tension important in blown film extrusion?
reduces residual stresses
prevents shrinkage
What is the orientation formed in the blown film structure?
bi-axial:
machine direction + transverse direction
(formed during blowing)
What is the ‘double bubble’ process?
Similar to normal blown film process, but after the first inflation the bubble/tube is preheated and re-inflated (secondary bubble)
-> collapsed, annealed, collected
What are the advantages of the double bubble process?
- cooling extrudate limits crystallization/spherulite growth -> better optical properties
- reheating/stretching while maintaining orientation -> better tensile properties
- further biaxial orientation
- rapid air cooling will freeze orientation formed in stretching
- annealing under tension reduces residual stress, prevent shrinkage
printing is a ____ treatment
surface
What affects printability?
polarity
additives
Printing is improved by: (3)
electric discharge (corona)
flame treatment
plasma treatment
The density range of plastics:
0.9-1.4g/mL
List the common 6 plastics from least to greatest density
PP (0.89-0.91) LDPE (0.91-0.93) HDPE (0.94-0.97) PS (1.04-1.08) PVC (1.35) PET (1.35-1.4)
important properties of plastic to consider for packaging: (7)
density breakage/sharp edges sealability flexibility strength/durability permeability (gas, odor, light) printability
The 7 plastic identification codes:
- PETE
- HDPE
- V
- LDPE
- PP
- PS
- Other
What is PETE? How is it produced?
polyethylene terephthalate
acid (terephthalic acid) + alcohol (ethylene glycol) => condensation rxn
Which plastic has the highest tensile strength?
polyethylene terephthalate
T/F: PET is a good moisture barrier, but a poor gas barrier
false: good moisture barrier AND good gas barrier
Disadvantages of PET:
No heat sealability lack printability (chemical inertness)
Advantages of PET:
Stable (low/high temp, humidity) clarity chemical resistance tear resistance good barrier properties high tensile strength
____ printing can be done for poor-printability materials
flexographic
How can PET be improved? (2)
2 way stretching (bi-orientation)
high-temp crystallization
What does 2 way stretching achieve for PET?
Improves tensile strength, flexibility, tear strength
What does high temp crystallization achieve in PET?
improve thermal stability + barrier properties
Types of PET:
Amorphous (APET)
Crystalline (CPET)
Shrinkable (30%)
____ PET is better in heat applications
crystalline
Applications of PET in food packaging:
films, oven trays/pouches, heat shrink film, bottles, thermoformed containers
PE is made of ____ monomers. What is the difference between HDPE and LDPE production?
Ethylene
HDPE: low pressure + catalyst => linear
LDPE: high pressure + catalyst => Long branched chain
How does branching affect properties in PE?
increase clarity, elongation
decrease crystallinity, moisture/grease barrier, density, tensile strength, melt point
structure/strength properties of HDPE:
crystalline, low branching
high stiffness/tensile strength, low impact/tear strength, translucent
Barrier properties of HDPE:
POOR oxygen barrier, good moisture barrier
T/F: HDPE is not stable at low temp
False; retains properties at low temp (good for freezer use)
Applications of HDPE:
Grocery bags, box liners, laminates, containers, tubs, crates/cases, pails/drums
T/F: LDPE softens at a lower temp than HDPE
True (softens at very low temp)
a plastic with a low melt point is good for ______
heat sealability
advantages of LDPE:
heat sealable
non reactive
clarity
elongation (shrink wrap)
disadvantages of LDPE:
High tear strength
low stiffness
very poor O2 barrier, fair moisture barrier
Applications of LDPE:
Stretch wrap, heat seal film/coating, bags, liners, shrink film, squeeze bottles, caps/closures
What is plastic #3? How is it made?
PVC (polyvinyl chloride)
polymerize vinyl chloride monomers (ethylene Cl -> ethylene dichloride -> heat -> vinyl chloride + HCl -> vinyl chloride -> polyvinyl chloride)
Advantages of PVC:
excellent clarity good thermoforming properties heat-shrinkability cling, tear resistance sealability chemical resistance high moisture barrier moderate gas perm (breathable) high tensile strength elongation printable
Applications of PVC:
cling film, shrink film, blister packs, portion control packs, chilled packaging, clear/strong bottles and jars
What is plastic #5? What is the monomer unit?
polypropylene
made from propylene monomers
What has a higher softening point, PE or PP?
PP
advantages of PP over PE:
higher softening point (can hot-fill)
better clarity
better barrier (O2, moisture)
better grease resistance
describe the structural strength of PP:
high tensile strength
low density
resists scratching/cracking
brittle at low temp
How can low temperature affect PP?
brittleness
Is PP printable?
yes, after treatment with corona discharge
applications of PP:
pouches/bags, clear wraps, clear labels, shrink packages, laminates, dairy tubs (no freezer), hot-fill bottles
What is plastic #6?
polystyrene (styrene monomers)
styrene monomers are linked to produce PS through _______
free radical vinyl polymerization
advantages of PS:
Low cost
hard & stiff
crystal clear (transmit all light)
disadvantages of PS:
Brittle
poor resistance to solvents (reactive, acid damage)
poor barrier (moisture/gas)
Of the common plastics, which is the most clear?
Polystyrene
What are some modified PS types? what qualities do they have?
HIPS (high impact PS): with rubber -> better impact strength
EPS (expanded PS): foamed with hexene/N2 -> better insulation
OPS (oriented): orientation increases strength (but not WVTR)
How is styrofoam made?
expanded PS: foam with hexene and N2 initiator -> better insulation (but doesn’t improve gas barrier properties)
PS applications:
Labels, window panels for bag/boxes, soak pads (meat), boxes, jars, liners, tray/tub/protective packaging, cups
What plastics are classified under #7? (6)
polyamide (nylon) polyvinylidene chloride ionomers polyvinyl alcohol ethylene-vinyl acetate ethylene vinyl alcohol
What is nylon made from?
(acid + amide)
adipic acid + hexamethylene diamide => nylon 6, 6
or from caprolactam => nylon 6
good nylon properties:
good barrier (gas, oil) clear chemical/grease resistant tough & stiff temp resistant (high and low)
disadvantage of nylon:
poor moisture barrier
Applications of nylon:
films for process meats/cheese, strapping, boil-in-bag, seafood film, multilayer bottles, laminates (with EVA, PVDC, LDPE)
What #7 polymer is very difficult to make? How is it usually made?
PVDC (polyvinylidene chloride)
made of vinylidene Cl units; made as a copolymer with PVC
properties of PVDC:
excellent moisture/gas barrier,
high cling
clarity
strong
What is PVDC usually used for?
thin layers to coat other films (improve them)
What are ionomers?
made from ethylene methacrylic acid copolymer (EMAA)
Some H on carboxyl groups replaced by Na or Zn (charged ionic areas)
ionomer polymers are used when?
when a strong seal is needed (greasy, crumbly products)
in laminates, skin packaging
properties of ionomers:
strong (puncture/impact resistant) excellent adhesive excellent clarity excellent moisture barrier oil resistant
PVA is made from:
polyvinyl acetate -> NaOH, methanol -> PVA
