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Food Packaging > Lecture 6: Plastics > Flashcards

Lecture 6: Plastics Flashcards

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1
Q

plastics consist of what?

A

synthetic polymers and additives

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2
Q

define polymer

A

large molecules made up of many repeating units

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3
Q

define monomer

A

molecule that combines with others by covalent bonds to form a polymer

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4
Q

how are synthetic polymers developed? manufactured?

A

developed by chemist

manufactured in factories

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5
Q

current polymers are made from what?

what do they contain? what do they sometimes contain?

A

petrochemicals

contains C, H, O
sometimes contains Cl, F, N, Si, P, S

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6
Q

what is polymerization?

A

the chemical process that joins monomers to make a polymer

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7
Q

what two main additions are involved in polymerization?

A

addition and condensation

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8
Q

differentiate homo polymer and copolmer

A

homo: made from one kind of monomer
copolymer: made from 2 or more different monomers

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9
Q

in synthetic polymers, as the chain length increases, how does size change? how does this affect melting point of the substance?

A

increases the size which increases melting point

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10
Q

increasing molecular wt means an increase in what properties?

A

tensile strength
hardness
stiffness
barrier

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11
Q

increasing molecular wt means a decrease in what properties?

A

solubility

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12
Q

describe properties of polyethylene

A

poor O2 barrier

excellent moisture barrier

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13
Q

describe properties of poly vinyl alcohol

A

superior O2 barrier

poor moisture barrier (soluble in H2O)

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14
Q

describe properties of ethylene vinyl alcohol

A

excellent O2 barrier

reduced water solubility

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15
Q

how can you change the properties of copolymers?

A

by changing the amount of which monomers are used

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16
Q

what is the polarity of CO2?

A

non-polar

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17
Q

describe permeability of barriers

A

barriers do not allow any substance

if a substance can pass through, it is permeable

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18
Q

what are complete barriers?

A

glass and metal

they don’t allow any substance to pass through

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19
Q

do plastics have barrier or permeability properties?

A

has barrier properties

selective permeability

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20
Q

what molecules can pass through non-polar PE?

A

PE has poor O2 barriers and good moisture barriers

thus, polar water molecules can’t pass

non-polar O2 molecules can pass

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21
Q

what are some non-polar synthetic polymers?

A

poly ethylene
polypropylene
polystyrene
polytetrafluoroethylene

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22
Q

what are some polar polymers?

A

polyester
polyamide
polyvinyl chloride

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23
Q

what are the effects of molecular shape on crystallinity?

A

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

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24
Q

what kind of packing results in crystallinity in polymers?

A

orderly molecular packing

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25
Q

what kind of packing results in amorphous polymers?

A

random molecular packing

26
Q

all polymers are ___ when they are melted

a) amorphous
b) crystalline

A

amorphous

27
Q

rapid cooling encourages ____ regions

a) amorphous
b) crystalline

A

amorphous

28
Q

slow cooling encourages ____ regions

a) amorphous
b) crystalline

A

crystalline

29
Q

what are properties that are affected by the amount of crystalline regions?

A 
clarity
melting point
barrier properties
stiffness
etc..
30
Q

what are the following properties in amorphous polymers?

a) clarity
b) melting point
c) barrier
d) stiffness

A

a) clarity = clear
b) melting point = lower MP
c) barrier = poor
d) stiffness = less stiff

31
Q

what are the following properties in crystalline polymers?

a) clarity
b) melting point
c) barrier
d) stiffness

A

a) clarity = hazy
b) melting point = higher MP
c) barrier = better
d) stiffness = more stiff

32
Q

describe the effects of orientation of molecules in polymers

A

polymers are orientated to improve properties

orientation aligns molecules (makes them more crystalline and less amorphous)

33
Q

how does alignment of molecules in polymers affect crystallinity?

A

increases crystallinity

34
Q

what are two words used to describe orientation?

A

uniaxial (1 direction)

biaxial (2 directions)

35
Q

how do you give “memory” to plastics?

what happens when you cool fast or slow?

A

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

36
Q

what are most commonly used plastics for food packaging?

A

thermoplastics

37
Q

what are properties of thermoplastics?

A
  • soften when heated (don’t have a set MP)
  • solidify when cooled
  • can be re-melted
  • viscoelastic
38
Q

what are examples of thermoplastics?

A

polyethylene
polypropylene
polystyrene
poly vinyl chloride

39
Q

describe thermoset plastics

what are examples?

A

once set, can’t be re-melted or reformed

has crosslinked polymer chains

examples: urethane, bakelite, melmac

40
Q

define plasticizer

A

increases flexibility and plasticity

decreases flow temp and hardness

41
Q

what are additives that can be added?

A
  • plasticizers
  • stabilizers
  • flame retardants
  • antistatics
  • slip agents
  • foaming agents
  • pigments
  • fillers
42
Q

what are techniques used for plastic processing?

A
  1. pelleting
  2. single screw extruder
  3. twin screw extruder
  4. hot press
  5. blown film
  6. double bubble process
43
Q

describe blown film plastic processing

A

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
44
Q

what are important components in the double bubble process

A
  • cooling extrudates
  • reheating
  • biaxial orientation
  • rapid air cooling
  • annealing under tension
45
Q

describe what cooling extrudates does in the double bubble process?

A

limits crystallisation and spherulite growth and yields superior optical properties

46
Q

describe what reheating does in the double bubble process

A

gives mobility to stretch but regain orientation yields for improved tensile properties

47
Q

describe what rapid air cooling does in the double bubble process

A

freezes the orientation produced in stretching

48
Q

describe what annealing under tension does in the double bubble process

A

reduces residual stresses formed during cooling and prevents potential shrinkage of film

49
Q

what kind of treatment is printing?

A

surface treatment

50
Q

printability is affected by what?

A

polarity of polymer

additives

51
Q

printability is improved by what? (3)

A

electric discharge
flame treatment
plasma treatment

52
Q

what are properties of plastic packaging materials?

A
  • density s 0.9-1.4 g/mL
  • easily broken
  • sealability
  • flexibility and strength
  • environmental durability
  • selective permeability (gas, odour, light)
  • printability
53
Q

what do the resin identification codes correspond to?

A
  1. PETE: polyethylene terephthalate
  2. HDPE: high density PE
  3. V: vinyl
  4. LDPE: low density polyethylene
    5: PP: polypropylene
  5. PS: polystyrene
  6. other
54
Q

what is PETE made from?

what reaction is this?

A

acid (TPA) and alcohol (EG)

via condensation rxn

55
Q

what are properties of ETE?

A
  • 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
56
Q

how does two-way stretching improve PET?

A

improves

  • tensile strength
  • flexibility
  • tear strength
57
Q

how does high temp crystallization improve PET?

A

improves

  • thermal stability
  • barrier properties
58
Q

what are common types of PET?

A

amorphous (APET)

crystalline (CPET)

59
Q

what are applications of PET?

A
  1. film:
    - where high strength and improved gas barrier is needed
    - coated to make sealable
    - ovenable trays and pouches
    - heat shrink film
  2. dimensional: clear bottles, soda pop, thermoformed containers
60
Q

how is HDPE made?

how is LDPE made?

A

low pressure + catalyst –> HDPE

high pressure + catalyst –> LDPE

Food Packaging (11 decks)

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