Unit 2 - Polymers Flashcards
(242 cards)
1
Q
Define a plastic
A
A plastic is a group of synthetic or natural organic materials that may be shaped when soft and then hardened
2
Q
Define a polymer
A
A polymer is numerous natural and synthetic compounds of high molecular weight consisting of repeat linked units, each is a light and simple molecule
3
Q
What are the 5 key properties that the structure of a polymer affects
A
- Dimensional instability
- Moisture absorption
- Flammability
- Time to degrade
- Chemical instability
4
Q
Give 8 reasons to use plastic in packaging
A
- Versatile - many types and shapes
- Inexpensive - in both materials and processing
- Easily automated and easy to process
- High strength to weight ratio and good insulators
- Corrosion resistance and can be foamed
5
Q
What is the Youngs moduluc of a plastic and a rubber?
A
1-10 and 0.003
6
Q
Describe a thermoplastic
A
- Also called thermosoftening polymers which include PE, PP, and PET
- They soften when heated and harden when cooled
- Consist of strongly bonded molecules held together by weak intermolecular forces
7
Q
Describe thermosetting polymers and name 2
A
- Include urea formaldehyde and cross-linked polyurethanes
- When they cool following the initial melt, the structure is locked into place via crosslinking
- They do not melt when heated - they degrade; this makes then difficult to recycle
8
Q
Describe a thermoplastic elastomer and name 3
A
- Include butyl rubber, butadiene, and PS
- They combine processability of thermoplastics with the performance of rubber
- Highly stretchy and resist deformation
9
Q
How are vulcanised rubbers made?
A
Made by cross-linking polymers with sulphur
10
Q
Describe the structure of an atom, and state what gives atoms their mass
A
- Atoms are made up of a central nucleus containing protons and neutrons
- These give atoms their mass
- This is surrounded by a cloud of negatively-charged electrons - the interaction of these between neighbouring atoms define the chemical properties
11
Q
Name the 6 types of chemical bonding
A
- Ionic
- Covalent
- Metallic
- Van der Waals
- Hydrogen
- Polar forces
12
Q
Describe ionic bonding and give an example of a molecule that uses it
A
- Characterised by electrostatic attraction between oppositely-charged ions in order to achieve a full outer shell
- Tend to conduct electricity in the liquid state
- Usually have high melting points
- An example is sodium-chloride (table salt)
13
Q
Describe metallic bonding
A
- Characterised by outer electrons becoming dissociated from their nuclei resulting in a ‘sea of electrons’ which are free to move around a lattice of positively-charged nuclei
- This is a non-directional form of bonding
- Usually good electrical and thermal conductors as the electrons can move easily
14
Q
Describe covalent bonding
A
- Where atoms share electrons to achieve a full outer electron shell
- Tend to be poor electrical and thermal conductors as electrons are all tied up in bonding
- As electrons repel due to their negative charges, the bonds always form at specific angles
15
Q
What angles do covalent bonds form at if there are 2, 3, and 4 bonding sites
A
180°, 120°, and 109.5°
16
Q
Name the three types of weak forces
A
- Van der Waals
- Hydrogen bonds
- Polar forces
17
Q
Describe Van der Waals forces
A
Van der Waals forces - these are intermolecular forces that are the sum of the attractive and repulsive forces between molecules
18
Q
Describe hydrogen bonding
A
Hydrogen bonds - specifically the interaction between hydrogen atoms and oxygen, nitrogen, or fluorine atoms from the same of an adjacent molecule
19
Q
Describe polar forces
A
Polar forces - occur when residual charges on adjacent molecules attract each other
20
Q
Describe the structure of carbon
A
- Comprised of 6 protons, 6 neutrons, and 6 electrons (2 in the inner shell and 4 in the outer shell)
- These four electrons make it more likely to form covalent bonds
21
Q
Describe atomic weight
A
The atomic weight is the number of protons and neutrons
22
Q
What is Avogadro’s number?
A
- Avogadro’s number is 6.022x10^23
- It is the number of units that make up one mole of any substance
23
Q
What is degree of polymerisation?
A
The degree of polymerisation (DP) represents the average length of the polymer molecules, or the number of -mers that have joined together
24
Q
What 5 effects does increasing the degree of polymerisation have?
A
- The higher the DP, the better the mechanical properties of the material
- This is due to the higher entanglement forces
- It increases chemical resistance as it requires more damage to the main chains to affect the strength
- It increases the strength of the material
- It increases the impact resistance as more bonds need to be broken
- It increases the weather resistance and the viscosity
25
Describe a homopolymer
All the -mers are the same
26
Describe a copolymer, and name the two types
Two types of -mers are polymerised together: random and alternating
27
Describe a grafted copolymer
In a grafted copolymer, sections of one polymer are grafted onto another
28
Describe a block copolymer
A block copolymer is where groups of monomers are attached in alternating fashion
29
Describe tacticity and name the three types
This is the stereochemical arrangement of -mers in a polymer: atatic, isotatic, and syndiotactic
30
Describe atactic polypropylene
In atactic polypropylene, the CH3 side groups are arranged randomly along the main polymer chain
31
Describe isotactic polypropylene
In isotactic polypropylene, the CH3 side groups are all on the same side of the main polymer chain
32
Describe syndiotactic polypropylene
In syndiotactic polypropylene, the CH3 side groups are all in an alternating pattern along the main polymer chain
33
What is the effects of polymer chains having a regular arrangement?
When the side groups have a regular arrangement, the main chains can get closer and can interlock like a jigsaw - this makes the material harder and stiffer
34
Describe branching of polymers
- Side chains, known as branches, help to increase the entanglement
- If the branches are large, or there are lots of them, the main chains stay further apart which leads to a softer and ductile material as the extra space makes it easier for the molecules to flow past each other
- If materials are made using condensation reactions, there is often very little branching - LDPE has more branches than HDPE
35
What will a polymer be if the branches are atactic or the side groups are bulky?
If molecules have bulky side groups or atactic branches, the polymer will be amorphous with a completely random form - these tend to be stiff and brittle
36
Describe a crystalline polymer
Some polymers form neat, regular, and repeating structures as they cool - these are called crystalline materials
37
Give three properties and an example of a semi-crystalline polymer
In between there are semi-crystalline structures which tend to be opaque, tough, and ductile such as PP
38
Why are polymer chains artifically aligned during processing?
To control or impart certain properties
39
How does degree of polymer chain alignment affect strength, stiffness, barrier properties, and thermal stability
Increases strength, stiffness, and barries
Decreases thermal stability
40
How are polymer chains oriented in PET bottles and why?
PET bottles are more oriented along the length to keep the fizz in the drink
41
What is the Tm of a polymer?
Melting point
42
What is the Tg of a polymer?
- Tg is the glass transition temperature
- The temperature at which the polymer can no longer flow and it changes from soft and tough to hard and glassy
43
What types of materials does Tg apply to?
Amorphous material
44
If a polymer is below its Tm, how are the polymers arranged in an amorphous polymer?
Randomly
45
What happens to the polymer chain aligment of an amorphous polymer as it is cooled?
As cooling continues, the molecules continue to rotate and move under stress until Tg is reached
46
What happens to a polymer below its Tg and why?
Below Tg, the polymer becomes glassy, hard, and brittle as only local chain segments can move
47
What happens to a semi-crystalline polymer below its Tm?
Below Tm, structured areas begin to form which makes chain movement difficult, but amorphous areas are still rubbery
48
Describe how double carbon bonds affect a polymer
- Makes molecules very chemically reactive, especially if it is at the end of the molecule
- Also makes molecules more susceptible to chemical attack
49
What effect does a benzene ring have on a polymer chain?
Adds heat stability
50
Name the two polymers formed of only carbon and hydrogen, and give a property of them due to this
PE and PP are formed of only carbon and hydrogen and are very flammable
51
What effect to nitrogen and oxygen bonded to hydrogen have on a polymer?
- When these are bonded to hydrogen, the polymer reactivity increases
- Also becomes hygroscopic and hold moisture so must be dried before processing otherwise the moisture causes the chains to break
52
Why is Nylon so moisture sensitive?
It has oxygen and nitrogen bonded to hydrogen which make it hygroscopic
53
Name 5 things that affect polymer chain flexibility
1. Temperature
2. Types of bonds
3. Side groups
4. Branching
5. Additives
54
Describe how temperature affects polymer chain flexibility
- Polymer chains are constantly vibrating and rotating
- The higher the thermal energy (heat), the more chain movement and the more space between them and so the more flexible it becomes
55
Describe how the types of bonds affect polymer chain flexibility
- Single C-C and C-H bonds move easily
- Double C=C bonds do not rotate and are very rigid
- Oxygen and nitrogen in the main chain provide additional room for movement as they can only form 2 and 3 bonds, respectively
- Benzene rings can add stiffness
56
Describe how side groups affect polymer chain flexibility
These restrict chain movement - the larger the side group, the more rigid the molecule
57
Describe how branching affects polymer chain flexibility
This increases chain flexibility as the larger branches hold the molecules further apart, giving them more room to move
58
Describe how additives affect polymer chain flexibility
They sit between the molecules and can either decrease or increase the flexibility
59
What causes viscoelasticity and why is it important?
This is a result of long covalently-bonded molecules that are held together by weaker intermolecular forces
- Material stretches more over time and stays deformed
- The load overcomes the weak intermolecular forces and allows the molecules to slide past each other
Describes how solid polymers behave and is important as it means thermoplastics have short and long term properties - they flow due to stress over time
60
Describe elastic behaviour of a polymer
You apply a load to a material within its yield strength and it returns to its original shape
61
Describe plastic behaviour of a polymer
When the load exceeds the yield strength and the material stays deformed
62
Name and describe two things that affect viscoelasticity
- Temperature - as this increases, the molecule move further apart giving more free volume
- Strain rate - this is the speed at which the load is applied - at higher rates, the material snaps before the molecules have time to flow past each other
63
Describe the crystallinity of APET, CPET, and PETG
- APET is amorphous
- CPET is semi-crystalline
- PETG is glycol-modified to prevent crystallisation
64
What polymerisation process is used to make PET
Condensation
65
Give 11 properties of PET
1. Transparent
2. Good gloss
3. Strong
4. Tough
5. Puncture resistant
6. High melting point
7. Weak melt strength
8. Good chemical resistance
9. Good gas barrier
10. Average moisture barrier
66
Name 4 types of PET and their uses
- Flexible PET is used for biaxially-oriented films
- Rigid PET is used for blow-moulded bottles
- CPET is used for ovenable trays
- PETG is used for blisters and medical packaging
67
Give 6 properties of the polyethylene family
1. Low cost
2. Good temperature resistance
3. Low Tg
4. Heat-sealable
5. Gas permeable
6. Low surface energy and so must be treated to accept ink
68
Name 3 polyethylene homopolymers
High, medium, and low density polyethylene
69
Name 3 polyethylene copolymers
Very low, linear low density, and ionomer polyethylene
70
Describe the structure of HDPE
Is a linear polymer chain giving it high tensile strength and stiffness
71
Give 7 properties of HDPE
1. Resistant to boiling water
2. Good moisture barrier
3. Poor gas barrier
4. Good chemical resistance
5. Prone to stress cracking
6. Milky-white appearance
7. Stiff / tough
72
Name 3 uses for HDPE
1. Milk bottles
2. Boil-in-the-bag foods
3. Cereal bags
73
Describe the structure of LDPE
Has a branched structure making it harder for the polymer chain to form an organised structure
74
Give 7 properties of LDPE
1. Soft
2. Flexible
3. Easily deformed
4. Slightly translucent finish
5. Good impact strength
6. Poor gas barrier
7. Low melt temperature making it easy to process and handle
75
Give 4 uses for LDPE
1. Shrink / stretch film
2. Coatings for heat-seal
3. Small bottles and tubes
4. Push-on lids
76
Describe the structure of LLDPE
- Is a co-polymer of ethylene with butene, hexene, or octene
- This produces controlled branching where the comonomer choice impacts the strength and price of the film
- Octene produces the highest strength (and therefore highest price)
77
What is LLDPE often blended with and why?
Often blended with LDPE to optimise performance (and cost)
78
Give 10 properties of LLDPE
1. Soft
2. Flexible
3. Very high elongation to failure
4. Higher stiffness than LDPE
5. High tensile strength
6. High puncture resistance
7. Translucent appearance
8. Poor gas barrier
9. Okay moisture barrier
10. Narrow heat-seal range
79
Give 4 uses for LLDPE
1. Shrink film
2. Bags for produce
3. Coatings for heat-seal
4. Snap-on lids / caps
80
Why does polypropylene require surface treatment?
- Also a polyolefin and requires surface treatment for printing due to the low surface energy
- This can cause poor ink and label adhesion
81
Give 4 properties of polypropylene
1. Good stiffness
2. Good resistance to deformation under load
3. High melting point
4. Transluscent
82
What form of polypropylene is used for packaging plastic?
Isotactic
83
How can you improve the toughness, clarity, and lower the melting temperature of polypropylene?
Copolymerising with ethylene improves clarity, toughness, and a lower melting temperature
84
How can you improve the barrier of polypropylene?
Barrier can be improved via orientation or SiOx
85
Give 5 uses of polypropylene
1. Bottles
2. Jars
3. Closures
4. Film for confectionery, biscuits, and snacks
5. Carton overwrap (e.g. teabags)
86
What type of polymer is PVC?
Amorphous
87
Give 7 properties of PVC
1. Transparent
2. Low melting temperature (makes it easy to heat seal)
3. Close melt and decomposition temperatures (makes it difficult to process without stabilizers)
4. Gives off HCL during processing
5. Very hard and brittle
6. Poor moisture barrier
7. Environmental issues such as plasticiser migration and dioxins produced upon burning
88
How can the moisture barrier of PVC be improved?
Addition of a PVdC coating
89
Give 4 uses of PVC
1. Bottle for toiletries / cooking oil
2. Thermoformed blister packs for pharmaceuticals
3. Thermoformed trays and film lids (not cook-in)
4. Collation shrink film
90
What polymerisation process is used to make PVdC?
Additional
91
Give 5 properties of PVdC
1. Heat-sealable due to low melt temperature
2. Very good moisture barrier
3. Good gas barrier
4. Grease resistance
5. Very expensive
92
Give 2 uses of PVdC
- As coatings on other plastics
- Inner layer in coextrusion to give good barriers and grease resistance
93
Describe the structure of polystyrene
An amorphous polymer with benzene rings on the main polymer chain
94
Give 5 properties of polystyrene
1. Transparent
2. Rigid but brittle
3. Low softening and melt temperature so easy to form
4. Poor moisture barrier
5. Poor gas barrier
95
Name 3 types of polystyrene and their uses
- Flexible PS (GPPS) is used for breathable wrap
- Rigid PS (HIPS) is used for pots, trays, and cups
- Expanded PS is used for cushioning and insulating
96
Describe styrene acrylonitrile
Copolymerisation of styrene monomer with acrylonitrile
97
Give 5 properties of styrene acrylonitrile
1. Scratch resistant
2. Chemical resistance
3. Gas barrier
4. 90% transparent
5. Easy to process
98
Describe acrylonitrile butadiene styrene
Made by copolymerisation of styrene with butadiene and acrylonitrile
99
Give 6 properties of acrylonitrile butadiene styrene
1. Good toughness
2. Lower clarity
3. Final properties dependent on polymer ratio
4. Good temperature resistance
5. Good impact resistance
6. Cannot be blow-moulded
100
Descibe polycarbonate
An amorphous polymer
101
Give 6 properties of polycarbonate
1. Tough
2. Rigid
3. Good clarity
4. Poor barriers
5. Heat-resistant
6. Sterilisable
102
Give a use for polycarbonate
Used in large containers to replace glass
103
Describe ethylene vinyl alcohol (EVOH)
A copolymer of ethylene and vinyl acetate, which is hydrolysed to improve toughness and clarity
104
Give 5 properties of EVOH
1. Tough
2. Transparent
3. Excellent gas barrier
4. Poor moisture barrier
5. Water soluble so must be used as a laminate
105
Give 2 uses for ethylene vinyl alcohol
1. Bottles
2. Laminates
106
Describe polyamide
Also known as nylon, it is a condensation polymer of diamines and diacids
107
How are polyamides name?
Name according to the number of carbon atoms in the starting materials e.g. nylon 6,6
108
Give 6 properties of polyamide
1. Good strength
2. Good toughness
3. Good abrasion resistance
4. Good gas barrier
5. Poor moisture barrier
6. Good temperature resistance
109
Give a use for nylon
Part of laminates for meat and cheese
110
Describe ionomers
Copolymers of ethylene and acrylic or methacrylic acids to add metallic ions into the polymer chain
These give some cross-linking and therefore tough film with high impact and puncture resistance
111
Give 8 properties of ionomers
1. Wide heat-seal range
2. Good hot tack
3. Adheres to aluminium foil
4. Good grease resistance
5. Seals through contamination
6. Very tough
7. High puncture resistance
8. Ultra-clear appearance
112
Give 2 uses of ionomers
Skin packaging and sealant layers
113
Describe cellulose film
A clear and sparkling film made from cellulose (rather than petrochemicals)
114
Give 3 properties of cellulose film
1. Excellent clarity
2. Can be coloured
3. Excellent dead-fold properties
115
Give 2 uses for cellulose film
Sweet wraps and carton windows
116
Describe polylactic acid
- A commercial biodegradable thermoplastic based on lactic acid
- Can be produced by 100% renewable sources such as corn
117
Give 2 properties of polylactic acid
1. Not accepted in recycling streams
2. Good thermal processability
118
Give 2 uses for polylactic acid
Used in healthcare and automotive industries
119
Describe ethylene vinyl acetate
A copolymer of ethylene and vinyl acetate
120
Give 2 properties of ethylene vinyl acetate
1. Good clairty
2. Good toughness
121
Give 2 uses of ethylene vinyl acetate
Used as a heat-seal layer in coextrusions and is used as a hot melt adhesive
122
Name two examples of fluoropolymers and describe them
- Poly-tetra-fluoro-ethylene (PTFE - Teflon)
- Highly crystalline and inert material with very low CoF
- Has a very high melt temperature
- Poly-chloro-trifluororo-ethylene (Aclar)
- Very good moisture and gas barrier
- Inert but very expensive
- Usually used in a laminate for pharmaceuticals
123
Describe thermoplastic elastomers
Highly elastic materials which resist deformation and are used in cosmetics and toiletry bottles
124
Give three types of bioplastic
1. Bio-polymers
2. Bio-based polymers
3. Biodegradable polymers
125
Describe bio-polymers
- Generally starch or cellulose-based
- Effective in closed loop recycling systems such as festivals
126
Describe bio-based polymers
- Conventional polymers that are derived from plant-based sources such as sugarcane and seaweed
- Broken down into hydrocarbons
127
Describe bio-degradable polymers
Break down into water, CO2, methane, and biomass
128
Describe fractional distillation
- The starting point is fossil fuel - either crude oil or gas
- Crude oil is formed of hydrocarbons which must be separated in fractional distillation
- Crude oil is heated until it evaporates and the vapour is passed into the bottom of a distillation tower
- Organic compounds rise to different heights depending on their density (the smallest molecules rise to the top)
- As vapours cool, they condense and are collected
129
Describe catalytic cracking
- The main fraction used in the production of polymers is naphtha
- This is broken down into even smaller molecules in a process called catalytic cracking - this is aided by a catalyst of zeolite or alumina
130
Describe addition polymerisation
- The C=C bond is broken - this frees up one bonding site on each carbon atom, which then becomes known as a free radical
- When multiple unsaturated hydrocarbons react, they form covalent bonds to form a polymers
- Involves three stages:
1. Initiation - free radicals are produced
2. Propagation - a free radical compound reacts with another unsaturated molecule and the polymer chain grows
3. Termination - 2 free radicals bond together to form the end of the polymer molecule
131
Name the two thing required for addition polymerisation and give reasons
- An initiating chemical to make the first free radical
- A catalyst to aid the reaction
132
Describe condensation polymerisation
- A portion of the primary -mers react with a portion of the secondary -mers to form a new bond with a by-product such as water or carbon dioxide
- The portion of the -mer that reacts is called a functional group
133
Give a disadvantage of condensation polymerisation and explain why
Usually slower than addition reactions as the reaction rate slows as the molecules are consumed
134
Name 12 plastic additives
1. Colourants
2. Heat stabilizers
3. Light stablizers
4. Lubricants
5. Impact modifiers
6. Plasticisers
7. Flame retardants
8. Coupling agents
9. Foaming agents
10. Anti-fogging agents
11. Antimicrobial agents
12. Antistatic agents
135
What colour are thermoplastics naturally?
Yellow
136
Name 6 things that can affect the final colour of a thermoplastic
1. Material type
2. Thickness
3. Particle size
4. Heat history
5. Shear heating
6. Degree of crystallinity
137
Name and describe the two types of pigments
1. Organic pigments
1. Partially soluble in the polymer
2. Not suitable for high temperature applications as the colourant can degrade
3. They give good opacity
2. Inorganic pigments
1. Naturally occurring minerals from rocks
2. High thermal stability but low acid resistance
138
Describe plastic dyes
Dyes are typically soluble organic compounds that tint clear polymers (such as PS and PVC)
139
Name 3 types of plastic colourants other than pigment and dye
1. Metal flake
2. Pearlescent additives
3. Phosphorescent additives
140
Describe masterbatch colourants
- Masterbatch
- Colourants must be compounded into a concentrate known as masterbatch before it is added to the polymer
- Colourant is diluted to a specific LDR (let-down ratio)
- Carrier resins with a low molecular weight and lubricants help to create a homogenous mix
141
Name and describe 3 ways in which colourants can affect material behaviour
- Lubricants affect flow properties
- Resin in masterbatch can change the viscosity
- Size and shape of colourant can affect toughness due to affecting ability to form crystals
142
Name 3 things that can be done if colourant is appearing too dark
1. Lowering processing temperatures
2. Lowering speed
3. Lowering LDR
143
Name 3 things that can be done if colourant is appearing too light or streaky
1. Raising processing temperature
2. Better mixing
3. Increasing LDR
144
Describe metamerism
Two colours appear the same despite being different
145
Describe dichroism
A colour appears different with different viewing angles
146
How does thermal stability affect colourants?
A high heat history can change the colour by changing the colour of the base resin
147
Describe migration of colourants
Where the colourant moves as a result of partial solubility
148
Name and describe 3 types of colourant migration
- Solvent-bleeding is when colour is released into an organic liquid
- Contact-bleeding is when colour is transferred from a solid to a solid
- Blooming is when pigment particles migrate to the surface
149
Define 'plate-out' of colourants
Pigment-containing plastics are deposited onto a die surface
150
Why are heat stabilizers needed in plastic?
- Heat speeds up oxidation (the aging process) which can result in loss of visual and mechanical properties
- Such as discolouration, yellowing, cracking, loss of strength, and loss of toughness
151
What type of polymers are most sensitive to oxidation and why?
Unsaturated polymers (which have electrons ready to share) are more sensitive to oxidation
152
How do heat stabiliers work in plastics?
The most important antioxidants (heat-stabilizers) disrupt the initiation and propagation reaction steps
153
Name 4 heat stabilizers used in plastic
Include tin, lead, barium, and calcium
154
Why is it important to add in enough heat stabilizer in plastic production?
They are consumes during oxidation prevention and so it is vital to put enough in during production
155
Name 2 light stabilizers
Include 2-hydroxyphenyl benzophones and nickel compounds
156
What concentrations are light stabilizers added in?
Usually added in concentrations of 0.05 to 2%
157
What causes light degradation of plastics?
Light degradation is caused by the production of free radicals when absorbed light excites chromophoric groups on the molecule
158
What 3 things heighten the effects of light degraration?
Effects are heightened by heat, oxygen, and humidity
159
What types of materials are most sensitive to light degradation and why?
Semi-crystalline materials are more sensitive to photo-oxidation as light diffracts off the crystals and spends more time within the polymer
160
Describe the 3 mechanisms of action of light stabilizers
1. Prevention / limitation of the amount of UV absorbed - stabilising compound must be very stable or it will be consumes quickly - not good for films or coatings
2. Deactivation of the excited chromophoric group - quenchers are able to take over the energy of it
3. Transform hydroperoxides into more stable compounds with no generation of free radicals - these are known as free radical scavengers (HALS)
161
Describe how colourants can affect light-stabilising compounds
- Pigments can act as sensitisers and reduce polymer stability - most common with organic reds and yellows
- Some pigments can increase stability, especially in thick samples
162
Describe the 7 ways in which lubricants act as a processing aid
1. Reduce heat shear
2. Reduce viscosity to aid flow
3. Give a glossy finish
4. Help pigment dispersion
5. Reduce cure times
6. Reduce CoF
7. Repel water
163
Describe internal lubricants and how they work
- Internal lubricants are soluble in the polymer and reduce viscosity by lubricating the molecules
- They also reduce the internal heat generation
164
Describe how external lubricants work
- External lubricants prevent the polymer from sticking to the mould
- They also reduce CoF and prevent melt fracture
165
What concentrations are lubricants affected in?
Effective in concentrations of 1% or less
166
Name 3 lubricants and how they are used
- Metal stearates - internal lubricant for PVC
- Paraffin wax - external
- Fatty alcohols - internal and external
167
What do impact modifiers do?
They induce reversible plastic deformation before crack deformation
168
Name 3 things that impact the effectiveness of impact modifiers
- The type and amount of IM additive
- The conditions under which it is incorporated
- Processing conditions of the polymer
169
How do impact modifiers work?
- Works by creating a combination of hard and soft phases within the material
- Mechanical energy is absorbed by the hard phase and transferred to the soft phase so cracks do not form
- Also reduce the degree of crystallinity
170
Name 2 impact modifiers
Common IMs include bulk rubber (EPDM) and grated bulk rubber (SAN)
171
What are plasticisers?
- Substances added to a material to improve its processability, flexibility, and stretchability
- Most are used with PVC
172
Describe external plasticisers and give a disadvantage
Physically bound to the polymer chains, however, they can migrate to the surface
173
Describe internal plasticisers and give a disadvantage
- Involve copolymerising 2 monomers - one with a high Tg and the other with a low Tg
- Not dimensionally stable at high temperatures
174
Describe primary plasticisers
- Primary plasticisers must be used alone
- Typically include phthalates such as DOP and DIOP
175
Describe secondary plasticisers
Secondary plasticisers have limited polymer compatibility and usually require a primary plasticiser to work
176
Describe what anti-plasticisers do and how they work
- Anti-plasticisers make the material harder and more brittle
- They increase the degree of crystallinity by 4-15%
177
Give 6 disadvantages of using plasticisers
1. Cost
2. Compatability
3. Thermal properties
4. Electrical properties
5. Water resistance
6. Toxicity
178
Give two applications that require flame retardants to be added to plastics and why
Plastics are hydrocarbons and inherently flammable, but some applications such as mining and electrics require flame resistance
179
Name and describe three flame redardant additives
- Aluminium hydroxide
- Undergoes an endothermic reaction as it breaks down thus cooling the polymer
- It also releases water vapour and forms a protective barrier layer
- Halogen compounds
- Interfere in gas-phase reactions
- Phosphorous
- Forms a gas shield layer and is good for polymers with oxygen content
180
Name and describe three types of flame retardant additives
1. Reactive flame - used in thermosetting polymers and are built into the molecules
2. Synergistics / secondary - multiple additives react together
3. Smoke - physically coat / dilute by chemically exuding carbon dioxide and carbon monoxide
181
What do coupling agents do?
Chemicals which promote adhesion between polymers and fillers
182
Name two types of coupling agents
- Silanes are used with -OH group fillers
- Titanates are also used with -OH group fillers but are more thermally stable than silanes
183
How do foaming agents work?
- Produce gas through a chemical reaction to produce foam in a polymeric matrix
- Reaction occurs in a narrow, elevated temperature range
- The foam can be open or closed cell and is sometimes mixed with pellets before processing
184
How do anti-fogging agents work?
- Achieved with nucleating agents - these cause a high degree of crystallisation to occur
- Usually much smaller and so increase the transparency
185
Name 4 disadvantages of using antimicrobial agents
1. Staining
2. Odour
3. Increased solvent permeability
4. Enhanced dirt uptake
186
What are antistatic additives used?
- Polymers are susceptible to static charge so they attract dust and can cause processing issues (especially in films)
- It can damage microchips and cause fires
187
List the 3 mechanisms of action of antistatic agents
1. Surface application of external antistatic agent
2. Incorporation of an internal antistatic agent
3. Incorporation of conductive additives to dissipate charge
188
What is the most accurate forming process for plastics?
Injection moulding
189
Describe injection moulding
1. Plastic granules fed via hopper into barrel and heated to a molten state
2. Heated plastic fills barrel - motorised scfewn pushes molten plastic to mould
3. Screw injects molten plastic at pressure into mould cavity via gates and hot runners
4. The plastic product is allowed to cool and solidify before being ejected from the opening mould
190
How can you identify a product that has been formed via injection moulding?
Easily identified by the witness mark that is formed during injection
191
Describe the mould used for injection moulding
- Moulds are usually made from steel and consist of a female and male part
- Many also have water cooling and release pins
192
What must happen to products after they have been formed via injection moulding?
- Components must be trimmed to remove flash
- These are bits of plastic that escaped the mould; they are ground up and fed back in as recyclate
193
Describe multi-shot injection moulding
- Multi-shot injection moulding allows two different materials to be injected that have different properties (such as a cup with different colours inside and out)
- Two-shot over-moulding is a 4-stage process in which components are transferred between stages using robots
- Co-injection moulding gives the appearance of polished stone using depth in colour
194
Name 6 things that affect cost of injection moulding
1. Upfront costs are high
2. Material weight
3. Material thickness
4. Decoration
5. Type of mould
6. Quantity
195
Describe injection (stretch) blow moulding (ISBM)
1. Injection moulded preform heated
2. Preform secured into blow mould
3. Preform stretched with core rod
4. Compressed air simultaneously blown in - preform inflates to mould
5. Product is removed from mould
6. Finished product ready for next production stage
196
Why does ISBM give improved strength, barrier properties, and clarity?
The process aligns the polymer chains
197
What type of products is ISBM normally used for and why?
This process aligns the polymer chains to give improved strength, barrier properties, and clarity
198
How many moulds are required for ISBM?
2
199
What materials are used in ISBM?
PET, PE, PP, PVC, and PS
200
What types of products is extrusion blow moulding (EBM) used to make?
Jars, bottles, drums, and fuel tanks
201
Name the materials that can be used with extrsusion blow moulding
PE, PP, PVC, and PS
202
What makes EMB suitable for making products with handles?
Mould has 2 halves which are both female and so it is possible to make handles
203
What is the 'pinch-off' region in extrusion blow moulding?
Pinch-off region at the bottom of the mould is where the polymer ‘tube’ is squeezed together to form the join
204
Describe moulds used for extrusion blow moulding
- Moulds have a cooling system to ensure it cools quickly to become structurally stable
- Moulds are made from steel or aluminium alloy
205
Give a disadvantage of extrusion blow moulding and the implications of this
Cannot achieve the same accuracy as IM and so cannot be used for carbonated drinks
206
Give three considerations of extrusion blow moulding
- Distribution of material in the mould
- Neck accuracy for sealing
- Waste from top and bottom to recycling
207
Describe extrusion blow moulding
1. Heated plastic is extruded into hollow tube (parison)
2. Mould closes - parison is gripped in place
3. Compressed air blown into parison which inflates
4. Parison fills mould
5. Product is trimmed and removed from mould
6. Finished product ready for next production stage
208
What products can be used for compression moulding?
Used with both thermosoftening and thermosetting plastic
209
Give an advantage of compression moulding
Parts made have no witness marks
210
Name 4 things that a compression mould requires
- Good wear resistance
- Ejection system to push the finished parts out
- No undercuts so product can be removed
- Made from tool steels
211
Describe the process of compression moulding
- Blank placed in mould and pressure applied
- Mould opened and the part is released
212
What is another name for thermoforming?
Vacuum forming
213
What products is thermoforming used to make?
Also called vacuum forming, this process is used to make trays and blister packs
214
Describe the process of thermoforming
- Uses a cast sheet which is fed directly from the casting machine to the thermoformer:
- Sheet is placed over a mould and heated until it becomes soft (but not molten) - therefore, thermoforming is known as a solid-state process
- A vacuum is applied to the underside so the sheet is sucked onto the mould - a plug may be applied to the top of the sheet to ensure that it fully conforms to the correct shape (alternatively, compressed air can be used to push the film onto the mould - this is known as pressure thermoforming)
215
Give 2 advantages of thermoforming
Lower cost than injection moulding
Does not leave witness marks
216
What in particular needs to be taken into consideration during thermoforming?
Thinning of the sheet, especially in the corners, needs to be taken into consideration
217
Name the 5 things that thermoforming moulds can be made from
1. Aluminium alloy
2. Machined steel
3. Plastic resins
4. Wood
5. Plaster of Paris
218
Name 4 requirements of thermoforming moulds
- Good thermal conductivity to quickly cool the polymer
- Vent holes
- No under-cuts as this would prevent the component from being removed
- Generous radii in corners to reduce tinning and aid removal from the mould
219
What is extrusion used to make?
Used to make continuous lengths of material with a constant cross section - can also be used to make feedstock for tube manufacture
220
What are extrusion moulds made from?
Dies are made from hardened tool steels - they are designed to produce a very accurate profile and are highly polished
221
Describe the process of forming an extruded tube
- Polymers such as LDPE, LLDPE, and HDPE are extruded into a continuous tube and cut into sleeves of required length
- Sleeves are transferred to a header unit where an injection moulded shoulder (comprised of nozzle, screw thread, and orifice) is applied
- Tube is transferred to an offset printer for decoration and lacquering
- A tamper-evident membrane is applied is required, followed by a cap
- Other decorations include foil blocking and labels
222
Describe the process of forming a laminated tube
Process begins as flat web stock which is printed using letterpress or gravure, which is then formed into a tube shape and welded before being cut to the correct length - this then has the mould shoulder applied
- Constructed from multiple layers to combine the benefits of an aluminium tube with the look and feel of plastic
- Layers are designed to give barrier properties (aroma and moisture mainly)
223
What type of product is rotational moulding used to make?
Large hollow containers such as drums and tanks
224
List the 9 stages of rotational moulding
1. Material weighed into mould
2. Mould closed and rotated
3. Rotating mould into oven
4. Plastic sticks and is shaped in mould
5. Rotating mould transferred to cooling chamber
6. Plastic solidifies
7. Rotation stopped
8. Mould opened
9. Part removed
225
Name the 3 things that rotational moulding moulds can be made from
1. Cast aluminium alloy
2. Fabricated steel
3. Electroformed nickel
226
Give 4 properties that rotational moulding moulds must have
- Must have good thermal conductivity
- Vents to allow gas to escape
- No under-cuts
- Generous radii
227
Describe the process of cast film / sheet extrusion
- Molten material is forced through a slot die - the die gap determines the gauge of the final film
- Material is cooled on a chilled drum
- Process can be used to orient the polymer molecules in the finished sheet
228
Describe the process of blown film extrusion
- Molten material is forced into an annular (ring) die and air is blown through to form a bubble - this is when cooled and collapsed to form lay-flat tubing
- Can either be left as in or slit into two reels
- Polymer molecules can be oriented for added strength
229
Name and describe the two most important measurements in blown film extrusion
- BUR - blow-up ratio: this is the ratio of the final bubble diameter to the die diameter
- DDR - draw down ratio: the speed at which the bubble is drawn off the top of the machine and the extrusion rate
230
Name the three types of lamination
1. Wet bond lamination
2. Dry bond lamination
3. Extrusion lamination
231
Describe wet bond lamination
- Two layers of film are brought together while the adhesive is still wet, before curing
- An adhesive is applied to one layer
- Second layer of material is applied
- Adhesive is cured in a drying oven
- One side must be porous to allow the products of curing to dissipate
- Laminate is cooled and rewound
232
Describe dry bond lamination
- Adhesive is applied to one layer, which is cured, before the materials are bought together
- Adhesive is applied to one layer
- Adhesive is cured in a drying oven
- Second layer is applied
- Laminate is cooled and rewound
233
Describe extrusion lamination
- No adhesive is used, as a polymer is melt extruded between two outer layers to become to central ply
- Laminate is cooled and rewound
234
What is EPS (expanded polystyrene) used for?
- EPS (expanded polystyrene) is a durable, lightweight material which is an excellent thermal insulator and a good shock absorber
- It is used to protect consumer electronics from mechanical hazards through the supply chain and to maintain the temperature of climate sensitive products
235
Describe the process of expanded polystyrene moulding
- EPS must be conditioned before it can be moulded - this involves expanding spherical PS beads using a small quantity of pentane as a blowing agent
- Beads are heated in a flow of steam which causes the pentane to boil, become a gas, and expand the beads to 40x the original size
- The resulting beads have a closed cell structure (like Aero chocolate)
- As it cools, it returns to the liquid state causing a partial vacuum so they are held for around 12 hours to allow the pressure to stabilize
- Beads are then places in a mould and reheated with steam - this final expansion causes the beads to coalesce to give the final shaped moulding
236
Name the 6 stages of mechanical recycling
1. Sort
2. Shred
3. Wash and dry
4. Melt
5. Granulate
6. Process
237
Name the 5 things that mechanical recyclate is sorted by
- Density - using the principle that some plastics float on water and other sink
- Near infra-red sorting - black plastics have mostly been phased out as this method does not work for them; certain types of plastic are removed from the conveyor belt by jets of compressed air
- X-ray fluorescence
- Laser-aided identification
- Marker systems
238
Give 2 advantages and 2 disadvantages of mechanical recycling
A
1. Very cost effective
2. Does not significantly alter the chemical structure of the polymer
D
1. Quality depends on what is being put in (usually grey coloured)
2. Properties degrade after multiple cycles, usually up to 10 times
239
Describe chemical recycling
Process involves breaking down or purifying the polymer molecules
240
Describe the process of mechanical recycling
- Type 1 - purification: contaminated plastics are dissolved in a solvent when is then purified and the polymer is recrystallised
- This can remove additives and contamination from the polymer and produce a pure material for reprocessing
- Type 2 - depolymerisation: breaking down long polymer molecules back into the monomers to enable new plastics to be made
- Achieved by thermal or catalytic processes
- Only works for condensation polymers such as PET and polyamides
241
Describe feedstock recycling
Feedstock recycling is a thermal process that involves converting the waste polymer back into hydrocarbon molecules which can then undergo petrochemical processing
242
Name and describe the 3 processes of feedstock recycling
- Pyrolysis involves heating in an oxygen depleted atmosphere; also known as cracking. The resultant hydrocarbon vapour can be distilled into various fractions. Useful for contaminated and mixed waste streams
- Gasification involves heating the waste to over 1000°C in an atmosphere containing a small amount of oxygen which breaks it down into a mixture of hydrogen, carbon monoxide, and carbon dioxide (a mixture known as Syngas) that can be used for chemical production
- Hydrothermal treatment uses water heated to 160°C to 240°C under high pressure to dissolve organic compounds and it can be used for recycling plastics (including those in composites)
