Lecture 5 - Sustainable recyclable polyolefin packaging

Question 1

What are polyolefins made up of?

Answer 1

Polymers made up of olefin monomers which are alkenes
- e.g : polypropylene (PP), polyethylene (PE).

Question 2

What are some advantages of polyolefins? [6]

Answer 2

• Cheap
• Lightweight
• Wide range of chemical and optical properties
• Chemically resistant (lack of polar groups)
• High processability
• Flexible and heat sealable

Question 3

What is a significant disadvantage of polyolefins?

Answer 3

Poor barrier properties against moisture and oxygen.

Question 4

What is the equation for Blow-up Ratio (BUR) in polyethylene processing?

Answer 4

Defined as Bubble Diameter / Die Diameter.

Question 5

What does a higher Blow-up Ratio (BUR) indicate?

Answer 5

More stretching, which improves film strength (rigidity). It also means the film is thinner, having better transparency. However, it is at risk of brittleness

Question 6

What are the two main directions of polymer molecule orientation in film processing?

Answer 6

• Machine Direction (MD)
• Transverse Direction (TD)

Question 7

What is MDO-PE?

Answer 7

Machine direction oriented-PE, which is from blown film extrusion of HDPE.

MDO-PE has high rigidity and excellent properties.

Question 9

What is BOPP?
Is its structure crystalline or more amorphous?

Answer 9

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

Question 10

What is a key limitation of BOPP in packaging?

Answer 10

Not good for retort packaging due to shrinkage of polymers at high temperatures. (Not heat resistant)

Question 11

BOPP is commonly used as an outer layer or heat sealing layer?

Answer 11

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.

Question 12

What is CPP and how is it processed (extrusion technique)?

Answer 12

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.

Question 13

What is one method to improve barrier properties for polyolefins?

Answer 13

Coating with high barrier laminates.

Question 14

What is EVOH?

Answer 14

Poly(ethylene-co-vinyl alcohol) that has higher barrier properties due to hydrogen bonding capabilities.

Question 15

What is a disadvantage of using EVOH in packaging?

Answer 15

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.

Question 16

What is the rationale behind silicate/polymer nanocomposites in improving barrier properties of polyolefins?

Answer 16

To create a tortuous path for gas molecules, improving barrier properties.

Question 17

What is metallised plastic packaging?

Answer 17

Involves coating plastic packaging with a thin layer of metal (under vacuum), such as aluminium.

Question 18

What are the pros of laminated aluminum/plastic (EVOH) packaging?

Answer 18

• High barrier to water vapor and gas
• Enhanced barrier properties with aluminum layer

Question 19

What is the process of mechanical recycling of polyolefins? [4]

Answer 19

• Collection & Sorting
• Washing
• Separation
• Shredding & Reprocessing

Question 20

What is a challenge associated with mechanical recycling?

Answer 20

Residue of ink and issues with adhesive affecting mechanical properties / leading to safety concerns

Question 21

What does chemical recycling involve?

Answer 21

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).

Question 22

What are challenges of chemical recycling? [2]

Answer 22

1. Energy-intensive
2. Expensive, requiring specialized infrastructure.

Question 23

State the equation for using iron as an oxygen scavenger.

Answer 23

4 Fe + 3 O₂ + 6H₂O → 4 Fe(OH)₃

Question 24

How are oxygen scavengers directly incoporated into plastic packaging?

Answer 24

Nano-structured oxygen scavenger (e.g. iron) are ancohored onto plate like fillers and are dispersed into the polymer film / coating uniformly.

Question 25

For nano particle oxygen scangers, why is it necessary to anchor them onto plate-like fillers before dispersing them into the packaging?

Answer 25

Nano structured materials may aggregate together, reducing their scavenging efficiency.

Question 26

What 3 factors affect the scavenging capacity of oxygen scavengers and how to increase their scavenging capacity?

Answer 26

1. Capacity of scavenger : use scavengers that has higher reactivity 2. Amount of scavenger : add more scavenger. However, this may cause food safety issues as scavenger migrate into the food matrix 3. Dispersion of scavenger : adhere scavenger onto plate-like fillers or carbon particles (iron) to allow uniform dispersion

Question 27

How can oxygen scavengers be incoporated into packaging? [3]

Answer 27

1. Directly incoporating nanoparticles into the polymer matrix 2. Incorporation into a **separate layer of film / coating, which is adjacent to the polymer layer** 3. Adding of sachets containing scavengers into food package

Question 28

What are oriented and cast polymers and what is the main structural difference between them?

Answer 28

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.

Question 29

What are some physical properties that oriented polymers possess? Thus, what layer are oriented polymers used for?

Answer 29

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

Question 30

What is the drawback of oriented polymers? Can they be used in retort packaging?

Answer 30

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.

Question 32

What are the 3 material structure factors which can affect barrier properties of plastics? Explain the rationale.

Answer 32

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.
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
3) **Polymer chain stiffness** : High chain stiffness reduces polymer mobility and free volume, leading to lower permeability. ## Footnote **Lower mobility → lower free vol (??)**