Lecture 3 - Physical Properties of packaging materials

Question 1

What are the 7 key physical properties of packaging materials?

Answer 1

1. Mechanical properties
2. Barrier properties
3. Mass transfer and other properties
4. Thermal properties
   5. Flow properties
   6. Surface properties
5. . Optical properties

Question 2

What are the most 4 important mechanical properties of packaging materials? Give definitions

Answer 2

TTIC
1.Tensile properties - ability of a material to be stretched and resist tension
2.Impact strength - the ability of a material to withstand shock loading (toughness of material + ability to absorb energy applied to it)
3.Tear strength - energy needed for tear initiation and tear propogaion
4.Crease/flex resistance - ability of a material to resist formation of creases or recover from creases

Question 3

What is tensile strength?

Answer 3

The maximum tensile stress in which a material can sustain (before breaking)

Question 4

What is yield strength?

Answer 4

The tensile stress at which the first sign of a non-elastic deformation occurs

Question 4

What is Young’s modulus?
Which region of the stress-strain curve does it fall under?

Answer 4

Young’s modulus is the force required to deform a material by a given amount;; a measure of the stiffness of the material

It falls under the linear region (flexible region) of the stress strain curve (gradient of the graph)

Question 5

What is the equation for Young’s modulus?
(Also state the symbols)

Answer 5

Young’s modulus, E = stress (σ) / strain (ε), where
stress = force per cross sectional unit area N.m²
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strain = ΔL/L₀, the fractional change in length (dimensionless number, no units)
- ΔL : change in length (m)
- L₀ : initial length (m)

Question 5

What is elongation? It is expressed as?

Answer 5

It is the point where the film breaks. It is expressed as % change in length of original material

Question 6

In plastic processing, knowing ultimate tensile stength is more important than yield strength. True or False?

Answer 6

False.
Yield strength > ultimate tensile strength, since once there is a non-elastic deformation, the material cannot go back to its original conformation, thus compromising the integrity of the packaging

Question 7

Why is a polymer more prone to breaking when stretched?

Hint : structure of polymer, coils

Answer 7

• Polymers are arranged as random coils.
• When the material is stretched, the polymer chains align along the direction of the force applied
• This alignment reduces the number of entanglements , thus polymers cannot absorb stress as well and are more brittle (entanglements help absorb stress by allowing chains to slide past each other during deformation.)

Amorphous regions contribute to flexibility but may experience strain hardening before breaking.

Crystalline regions resist deformation but can cause brittleness when excessive stress is applied.

Question 8

What is meant by barrier properties?

Answer 8

Resistance of a material to sorption and diffusion of substances such as gases, vapour, aromatic compounds

Question 9

What are 2 factors that affect how well the food is protected from vapour / gas exchange?

Answer 9

1. Integrity of the package : microscopic pores / cracks can increase gas exchange, reduce barrier properties
2. Permeability of packaging materials

Question 10

The permeability model of O₂ can be used to describe materials such as glass,plastic and metal. True or False?

Answer 10

False, permeability model is only used to describe plastics, as glass and metals are perfect at barrier

Question 11

What is the general equation for gas transmission rate?

Answer 11

Transmission rate = Q/At
where Q : amount of permeant passing through polymer ;
A : area of film ;
t : time (usually day)

Question 12

What is the unit for oxygen transmission rate (OTR), and what conditions is it usually measured under (temp, RH)?

Answer 12

units : cc/(m².day)
- 25°C
- 0/50/90% RH

1 cc = 1 mL

Question 13

What is the unit for water vapour transmission rate (WVTR), and what conditions is it usually measured under (temp, RH)?

Answer 13

Units : g / m².day
- 38°C
- 90% RH

Question 14

What are the 3 main steps in the permeability model of plastics?

Answer 14

1. Adsorption / sorption
2. Diffusion
3. Desorption

Question 15

What are 5 key factors affecting permeability of plastics?

Answer 15

1. Close chain-to-chain packing
2. Stiffness of polymer chains
3. Degree of polarity of polymer chains
4. Inertness of polymer chains to penetrants
5. Glass transition temperature

Question 16

One factor affecting permeability of plastics is close chain-to-chain packing. What 2 factors can affect how close polymer chains are packed to each other?

Answer 16

1. Crystallinity : polymer chains in crystalline form are arranged orderly and thus can pack closer together (compared to amorphous state)
2. Molecular symmetry : higher symmetry of molecules enables closer packing of polymer materials

Question 17

How does chain-to-chain packaging affect permeability?

Answer 17

Closer packing → stronger interaction between polymer chains → lower mobility of polymer chain + lower free volume → lower permeability, higher barrier

Question 18

How does inertness to penetrants (e.g.water) affect permeability?

Answer 18

When there are no polar groups (e.g. -OH) in the polymer chain, polymers will not absorb water and swell up → lower chain mobility → lower permeability. higher barrier

Question 19

How does chain stiffness affect permeability?

Answer 19

When polymer chains are stiff → low mobility and free volume = lower permeability = higher barrier

Question 20

What is glass transition temperature and how does it affect permeability?

Answer 20

• Defintion : temperature range at which a material transitions from a glassy state (hard, brittle, and rigid) to a rubbery state (soft, flexible, and can be easily deformed).
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  At room temperature,
• Materials with higher Tg are more rigid (glassy state) = lower mobility and free volume = more tortuous path for penetrants to pass through = reduced permeability, increased barrier.
• Materials with lower Tg are more flexible (rubbery state) = higher mobility and free volume = increased permeability, decreased barrier.

Question 21

Thermal properties

What is the importance of knowing the glass transition temperature of a material? (explain impact of T_g when selecting materials for frozen food packaging)

Answer 21

Important to know what material to use to package food products, especially during frozen storage (if glass transition temperature is high, such as room temp, the material is rigid at low temperatures and is not suitable to be used as a material to pack frozen meats → more desirable to have more flexible and less rigid packaging for frozen foods, rigid packaging is very brittle

Question 22

Thermal properties

What is the equation of coefficient of thermal expansion? What applications is knowing this value important for?

Answer 22

Equation : degree of expansion / change in temperature (amount a material expands per unit temp)

• Important applications : retort / hot sealing packaging where high temperatures and subsequent cooling of products are involved ;; temp change cannot be too drastic as it will affect integrity of packaging

Question 23

# **Thermal properties** What types of applications is knowing the thermal conductivity (k) important for? [3]

Answer 23

1. Secondary / tertiary packaging used to contain food products : k should be low for cooler boxes to contain chilled or frozen food 2. Frozen and heat sensitive foods : k should not be too high to prevent heat transfer to food 3. Microwave / oven-safe packaging : k should be high enough to cook food in packaging

Question 24

# **Flow properties** What is the important flow property of material? Define it and state its importance (applications)

Answer 24

Apparent viscosity : resisstance to flow under a particular applied force. - important to know apparent viscosity during plastic processing, where it affects how plastic melts and spreads during extrusion, moulding and coating

Question 25

# **Surface properties** What are the 2 surface properties of materials? Define.

Answer 25

1. Wettability : how well liquids spread on a surface 2. Coefficient of friction : how slippery a surface is

Question 26

# **Surface properties** What is the importance of knowing the wettability of a material? [2]

Answer 26

Important for - Coating : wet coating / deposition coating - Printing of colours etc on food packaging Affects printing and coating adhesion

Question 27

What is the importance of knowing the coefficient of friction of a material>

Answer 27

Important for handling and transport of the product.
- At the end of the conveyor belt, packaged food products slide into boxes and are arranged before transport. If COF is too high : Packages don’t slide smoothly into the box, causing jams or misalignment. The packaging line slows down, reducing efficiency.

Question 28

What are the 2 most important optical properties of materials? And what is their importance in food packaging [2]?

Answer 28

Transparency and opacity.
**Importance:** 1. Shelf life extension / preservation of light-sensitive nutrients Fats are sensitive to light, exposure to UV light can facilitate oxidation of fats → oxidative rancidity ;; loss of light sensitive vitamins / phenols 2. Consumer appeal → consumers like transparent packaging to see contents E.g. cooking oil

Question 29

# **Tutorial : barrier properties** Why is it important to study barrier technologies?

Answer 29

Materials, such as polyolefines (PE,PP) have low cost, flexibility and recycability However, they have poor oxygen and moisture barrier properties, which can affect shelf-life of foods and increase food wastage.

Question 30

# **Tutorial** What are the 5 different types of barrier technologies that can be used for plastics and explain how they work.

Answer 30

1) Metallised plastic packaging : Involves coating plastic packaging with a **thin layer** of metal, such as aluminium
2) Laminated alunium / plastic (EVOH) packaging : Involves laminating a thicker layer of aluminium on top of EVOH (ethylene vinyl alcohol) plastic packaging (which has excellent barrier properties)
3) SiO_x coated plastic packaging : A transparent glass-like layer (silicon oxide) is coated on plastic
4) Multi-layered plastic film : sealing multiple layers of plastic together
5) Nano-fillers in polymer matrix : Tiny plate-like fillers are dispersed in polymer film, creating a tortuous path for gas molecules, reducing permeability.

Question 31

What are the pros [2] and cons [4] of metallised plastic packaging?

Answer 31

**Pros** : 1. Low WVTR - impermeable to water vapour 2. Light weight
**Cons** 1. High OTR (thus, high OTR + low WVTR → medium barrier) 2. Non-transparent 3. Expensive vacuum process ;; high equipment cost and energy consumption 4. Cracks easily

Question 32

What are the pros [1] and cons [2] of aluminium-**laminated** EVOH plastic

Answer 32

**Pros** : high barrier to water vapour and gas **Cons** 1. High cost 2. Difficulty in recycling (hard to separate due to thick aluminium layer)

Question 33

What are the pros [3] and cons [3] of silicon oxide coated plastic packaging?

Answer 33

**Pros**: 1. Transparent 2. Light weight 3. Low WVTR - **Cons** 1. High OTR 2. Expensive vacuum process + equipment needed 3. Cracks easily

Question 34

What are the pros [1] and cons [2] of multi-layered plastic packaging?

Answer 34

**Pros** : low OTR **Cons** 1. Complicated processing to achieve multi-layered packaging 2. Difficulty in separating layers for recycling

Question 35

What is the benefit of nano-fillers in food packaging? What materials are most commonly used as fillers

Answer 35

It is low in cost and has high barrier properties - Most common material : silicate fillers

Question 35

What are the 3 methods in which nano-fillers can be incorporated into the polymer matrix? (& explain how they work, and what kinds of polymers they produce)

Answer 35

**1. Layer by layer deposition** : Thin alternating layers of materials (polymer & nanoparticles) are deposited to build a composite, with high control over thickness and structure **2. Vacuum assissted filtration** : Nano particle fillers are pulled into the polymer film matrix with the help of a vacuum → compact, stacked layers with high density **3. In-situ suspension polymerisation** : before conducting co-polymerisation, fillers are mixed together with both monomers and fillers are being incorporated while co-polymerisation occurs → ensure that polymer chains form and grow in direct interaction with the filler ;; strong interfacial bonding between polymer and filler

Question 36

What 3 factors affect the efficiency of the tortrous path by plate-like fillers?

Answer 36

1. **Filler orientation** → allignment with polymer matrix determines the efficiency. Well oriented fillers (vertical) force gases such as O₂ to **travel a longer path**, reducing permeability 2. **Filler content **→ higher filler content leads to **increased number of obstacles**, reduced permeability 3. I**nterfacial bonding betwen filler and polymer matrix** : stronger bonding → **prevents formation of cracks / empty space**, reducing permeability and increasing efficiency

Question 37

What are 3 material structure factors that can affect barrier properties? Explain their rationale

Answer 37

1) **Chain to chain packing** : When polymer chains are closely packed due to high crystallinity or molecular orientation, 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 chain mobility**, leading to lower permeability.