Lecture 2 - Categories of food packaging Flashcards
Describe the 3 steps involved in addition polymerisation in detail.
1. Initiation : The polymerisation process is initated by a free radical attacking C atom within C=C bond. A new free radical is formed
2. Growth : The new free radical attacks another C=C bond of another monomer.Thus, resulting free valences are joined together, forming polymers.
3. Termination : reaction terminates when 2 radicals react to form a bond
What aspects of polymer does a catalyst affect in addition polymerisation?
1) Conformation : catalyst may cause chain transfer during polymerisation, forming graft polymers.
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2) Molecular weight : catalyst regulate how many monomer units are added to the chain before termination, affecting the MW of the polymer.
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3) MW distribution : Catalysts influence how evenly the polymer chains grow, determining whether the polymer has a narrow or broad distribution.
narrower MWD → most polymer chains within polymer are similar in size, leading to consistent properties
What is copolymerisation?
It is when 2 or more different monomers are combined to form a polymer chain
What does the monomer reactivity ratio tell us (in copolymerisation)?
It tells us the mole fraction of each monomer in the final polymer and tells us about the composition / structure of the polymer
Monomer A has a higher monomer reactivity ratio than monomer B. What does this mean?
it means that monomer A is more reactive, and the polymer chain is likely to add more of monomer A to it than monomer B during chain growth
In condensation reactions, what are the reaction conditions employed? [2]
- High temperature → for high MW with narrow distribution
- High vacuum → remove water to favour forward reaction, as condensation reactions produce water as a product
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In the plastic identification code (PIC), what is plastic is number 1?
Polyethylene Terephthalate (PETE)
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In the plastic identification code (PIC), what is plastic is number 2?
High density polyethylene (HDPE)
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In the plastic identification code (PIC), what is plastic is number 3?
Polyvinyl chloride (V)
aka pvc
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In the plastic identification code (PIC), what is plastic is number 4?
Low density polyethylene (LDPE)
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In the plastic identification code (PIC), what is plastic is number 5?
Polyproylene (PP)
PP comes before PS, P comes before S
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In the plastic identification code (PIC), what is plastic is number 6?
Polystyrene (PS)
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In the plastic identification code (PIC), what is plastic is number 7?
Other
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What are some limitations of plastics as packagings? [3]
- Non-degradable
- Safety issue when used inappropriately : mass transfer of monomers / oligomers or catalysts into foods
- Permeable to gas and vapour, low barrier (which could have minimal impact on shelf life extension, thus generating food waste)
What is the difference in polymer structure for LDPE and HDPE?
How does this affect their physical properties? [3]
HDPE (high density) has more linear chains that pack more closely together and thus has a more crystalline structure, while LDPE has more branched chains that have less tight packing.
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Thus, HDPE has:
- higher melting and boiling point (stronger interaction)
- harder and more rigid (while LDPE is more flexible)
- less transparent
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What are some advantages to plastic packaging? [4]
- Easily processed to produce packaging with many shapes and types with various functions
- Low cost
- Light weight
- Low energy consumption during production
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The principal ingredient of glass is….?
Silicate (derived from sand, flint or quartz)
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What are some advantages of glass packaging? [5]
1. Highly inert : does not react with food
2. Diverse forms available : colour, transparency and shape
3. Sustainable : recyclable and re-usable
4. Sterilisable
5. Perfect at barrier, high barrier against oxygen, moisture etc
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What are some disadvantages of glass packaging? [3]
- Fragile
2. Heavy - Cause harm to people if broken
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What are some advantages of metal packaging (steel, aluminium)? [4]
1. High mechanical strength
2. Application to a wide range of temperature
3. Impermeable to gas, vapour, light
4. Recyclable
What are bio-based materials and what are their advantages?
Materials derived from annually renewable sources.
- some are biodegradable, thus environmentally friendly
- Increased sustainability
What are some limitations for biobased materials for food packaging? [4]
- Limited mechanical and other properties
- Poor ductility (flexibility, how much material can stretch before breaking)
- Highly permeable to gas and vapour / low barrier
- Costly (compared to petroleum based plastic packaging)
What are some sources that bio-based packaging can be made from?
Renewable raw materials such as :
- plants (sugarcane, corn)
- microbes (yeast)
What is a biobased and biodegradable packaging material?
PLA, poly lactic acid / polylactide
Mechanical properties: - high glass transition temperature - lower melting point - higher modulus : more rigid and stiff
