Lecture 5 - Sustainable recyclable polyolefin packaging Flashcards
What are polyolefins made up of?
Polymers made up of olefin monomers which are alkenes
- e.g : polypropylene (PP), polyethylene (PE).
What are some advantages of polyolefins? [6]
- Cheap
- Lightweight
- Wide range of chemical and optical properties
- Chemically resistant (lack of polar groups)
- High processability
- Flexible and heat sealable
What is a significant disadvantage of polyolefins?
Poor barrier properties against moisture and oxygen.
What is the equation for Blow-up Ratio (BUR) in polyethylene processing?
Defined as Bubble Diameter / Die Diameter.
What does a higher Blow-up Ratio (BUR) indicate?
More stretching, which improves film strength (rigidity). It also means the film is thinner, having better transparency. However, it is at risk of brittleness
What are the two main directions of polymer molecule orientation in film processing?
- Machine Direction (MD)
- Transverse Direction (TD)
What is MDO-PE?
Machine direction oriented-PE, which is from blown film extrusion of HDPE.
MDO-PE has high rigidity and excellent properties.
What is BOPP?
Is its structure crystalline or more amorphous?
Biaxially oriented polypropylene, where PP is stretched in both machine and transverse directions.
- Due to the stretching and alignment of polymers, BOPP has a highly crystalline structure
What is a key limitation of BOPP in packaging?
Not good for retort packaging due to shrinkage of polymers at high temperatures. (Not heat resistant)
BOPP is commonly used as an outer layer or heat sealing layer?
Outer layer.
- Through stretching in both directions, BOPP has highly oriented chains, leading to high stiffness and modulus, making it suitable as a rigid outer layer.
What is CPP and how is it processed (extrusion technique)?
Cast polypropylene.
It is processed through cast extrusion, where the melted resin is passed through an extruder without the orientating / stretching of the polymer chains.
What is one method to improve barrier properties for polyolefins?
Coating with high barrier laminates.
What is EVOH?
Poly(ethylene-co-vinyl alcohol) that has higher barrier properties due to hydrogen bonding capabilities.
What is a disadvantage of using EVOH in packaging?
Sensitive to moisture, which can compromise barrier properties.
- Hydroxyl groups in WVOH can form hydrogen bonding with water, causing swelling of packaging ; higher chain mobility results in lower barrier properties.
What is the rationale behind silicate/polymer nanocomposites in improving barrier properties of polyolefins?
To create a tortuous path for gas molecules, improving barrier properties.
What is metallised plastic packaging?
Involves coating plastic packaging with a thin layer of metal (under vacuum), such as aluminium.
What are the pros of laminated aluminum/plastic (EVOH) packaging?
- High barrier to water vapor and gas
- Enhanced barrier properties with aluminum layer
What is the process of mechanical recycling of polyolefins? [4]
- Collection & Sorting
- Washing
- Separation
- Shredding & Reprocessing
What is a challenge associated with mechanical recycling?
Residue of ink and issues with adhesive affecting mechanical properties / leading to safety concerns
What does chemical recycling involve?
Converting used polyolefin film into chemicals (monomers) via chemical reaction in the presence of a catalyst.
- The monomers can then be used to process other plastic products or used as sustainable aviation fuels (SAF).
What are challenges of chemical recycling? [2]
- Energy-intensive
- Expensive, requiring specialized infrastructure.
State the equation for using iron as an oxygen scavenger.
4 Fe + 3 O2 + 6H2O → 4 Fe(OH)3
How are oxygen scavengers directly incoporated into plastic packaging?
Nano-structured oxygen scavenger (e.g. iron) are ancohored onto plate like fillers and are dispersed into the polymer film / coating uniformly.
For nano particle oxygen scangers, why is it necessary to anchor them onto plate-like fillers before dispersing them into the packaging?
Nano structured materials may aggregate together, reducing their scavenging efficiency.
What 3 factors affect the scavenging capacity of oxygen scavengers and how to increase their scavenging capacity?
- Capacity of scavenger : use scavengers that has higher reactivity
- Amount of scavenger : add more scavenger. However, this may cause food safety issues as scavenger migrate into the food matrix
- Dispersion of scavenger : adhere scavenger onto plate-like fillers or carbon particles (iron) to allow uniform dispersion
How can oxygen scavengers be incoporated into packaging? [3]
- Directly incoporating nanoparticles into the polymer matrix
- Incorporation into a separate layer of film / coating, which is adjacent to the polymer layer
- Adding of sachets containing scavengers into food package
What are oriented and cast polymers and what is the main structural difference between them?
Oriented polymers are polymers that have been stretched, whereas cast polymers are polymers that are directly extruded without stretching.
Oriented polymers have more crystalline domains as they have been stretched and polymer chains are more aligned.
What are some physical properties that oriented polymers possess? Thus, what layer are oriented polymers used for?
Oriented polymers : stretched and aligned, leading to stronger interactions between polymer chains. Oriented polymers are:
- more rigid / stiff
- higher strength
- better barrier properties (lower mobility of polymer chains)
Oriented polymers are used as an outer layer due to their rigidity and better barrier properties
What is the drawback of oriented polymers?
Can they be used in retort packaging?
They are not heat resistant, the stretched chains can move back to their original position at high temperatures.
No, retort packaging involves very high temperatures, thus oriented polymers like OPP/BOPP/ MDOPE cannot be used as packaging integrity will be compromised.
What are the 3 material structure factors which can affect barrier properties of plastics? Explain the rationale.
1) Chain to chain packing : When polymer chains are closely packed due to high crystallinity or molecular orientation / better molecular symmetry, there is less free volume for gas or moisture to diffuse through, lowering permeability and increasing barrier properties.
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2) Interfacial interaction between polymer chains : when polymer chains have some degree of polarity, it leads to stronger interactions between polymer chains → reduced mobility and free vol → lower permeability and high barrier
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3) Polymer chain stiffness : High chain stiffness reduces polymer mobility and free volume, leading to lower permeability.
Lower mobility → lower free vol (??)
