Lecture 3 - Physical Properties of packaging materials Flashcards
What are the 7 key physical properties of packaging materials?
- Mechanical properties
- Barrier properties
- Mass transfer and other properties
- Thermal properties
5. Flow properties
6. Surface properties - . Optical properties
What are the most 4 important mechanical properties of packaging materials? Give definitions
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
What is tensile strength?
The maximum tensile stress in which a material can sustain (before breaking)
What is yield strength?
The tensile stress at which the first sign of a non-elastic deformation occurs
What is Young’s modulus?
Which region of the stress-strain curve does it fall under?
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)
What is the equation for Young’s modulus?
(Also state the symbols)
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)
What is elongation? It is expressed as?
It is the point where the film breaks. It is expressed as % change in length of original material
In plastic processing, knowing ultimate tensile stength is more important than yield strength. True or False?
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
Why is a polymer more prone to breaking when stretched?
Hint : structure of polymer, coils
- 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.
What is meant by barrier properties?
Resistance of a material to sorption and diffusion of substances such as gases, vapour, aromatic compounds
What are 2 factors that affect how well the food is protected from vapour / gas exchange?
- Integrity of the package : microscopic pores / cracks can increase gas exchange, reduce barrier properties
- Permeability of packaging materials
The permeability model of O2 can be used to describe materials such as glass,plastic and metal. True or False?
False, permeability model is only used to describe plastics, as glass and metals are perfect at barrier
What is the general equation for gas transmission rate?
Transmission rate = Q/At
where Q : amount of permeant passing through polymer ;
A : area of film ;
t : time (usually day)
What is the unit for oxygen transmission rate (OTR), and what conditions is it usually measured under (temp, RH)?
units : cc/(m2.day)
- 25°C
- 0/50/90% RH
1 cc = 1 mL
What is the unit for water vapour transmission rate (WVTR), and what conditions is it usually measured under (temp, RH)?
Units : g / m2.day
- 38°C
- 90% RH
What are the 3 main steps in the permeability model of plastics?
- Adsorption / sorption
- Diffusion
- Desorption
What are 5 key factors affecting permeability of plastics?
- Close chain-to-chain packing
- Stiffness of polymer chains
- Degree of polarity of polymer chains
- Inertness of polymer chains to penetrants
- Glass transition temperature
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?
- Crystallinity : polymer chains in crystalline form are arranged orderly and thus can pack closer together (compared to amorphous state)
- Molecular symmetry : higher symmetry of molecules enables closer packing of polymer materials
How does chain-to-chain packaging affect permeability?
Closer packing → stronger interaction between polymer chains → lower mobility of polymer chain + lower free volume → lower permeability, higher barrier
How does inertness to penetrants (e.g.water) affect permeability?
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
How does chain stiffness affect permeability?
When polymer chains are stiff → low mobility and free volume = lower permeability = higher barrier
What is glass transition temperature and how does it affect permeability?
- 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.
Thermal properties
What is the importance of knowing the glass transition temperature of a material? (explain impact of Tg when selecting materials for frozen food packaging)
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
Thermal properties
What is the equation of coefficient of thermal expansion? What applications is knowing this value important for?
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
Thermal properties
What types of applications is knowing the thermal conductivity (k) important for? [3]
- Secondary / tertiary packaging used to contain food products : k should be low for cooler boxes to contain chilled or frozen food
- Frozen and heat sensitive foods : k should not be too high to prevent heat transfer to food
- Microwave / oven-safe packaging : k should be high enough to cook food in packaging
Flow properties
What is the important flow property of material? Define it and state its importance (applications)
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
Surface properties
What are the 2 surface properties of materials? Define.
- Wettability : how well liquids spread on a surface
- Coefficient of friction : how slippery a surface is
Surface properties
What is the importance of knowing the wettability of a material? [2]
Important for
- Coating : wet coating / deposition coating
- Printing of colours etc on food packaging
Affects printing and coating adhesion
What is the importance of knowing the coefficient of friction of a material>
Important for handling and transport of the product.
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- 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.
What are the 2 most important optical properties of materials? And what is their importance in food packaging [2]?
Transparency and opacity.
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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
Tutorial : barrier properties
Why is it important to study barrier technologies?
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.
Tutorial
What are the 5 different types of barrier technologies that can be used for plastics and explain how they work.
1) Metallised plastic packaging : Involves coating plastic packaging with a thin layer of metal, such as aluminium
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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)
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3) SiOx coated plastic packaging : A transparent glass-like layer (silicon oxide) is coated on plastic
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4) Multi-layered plastic film : sealing multiple layers of plastic together
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5) Nano-fillers in polymer matrix : Tiny plate-like fillers are dispersed in polymer film, creating a tortuous path for gas molecules, reducing permeability.
What are the pros [2] and cons [4] of metallised plastic packaging?
Pros :
1. Low WVTR - impermeable to water vapour
2. Light weight
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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
What are the pros [1] and cons [2] of aluminium-laminated EVOH plastic
Pros : high barrier to water vapour and gas
Cons
1. High cost
2. Difficulty in recycling (hard to separate due to thick aluminium layer)
What are the pros [3] and cons [3] of silicon oxide coated plastic packaging?
Pros:
1. Transparent
2. Light weight
3. Low WVTR
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Cons
1. High OTR
2. Expensive vacuum process + equipment needed
3. Cracks easily
What are the pros [1] and cons [2] of multi-layered plastic packaging?
Pros : low OTR
Cons
1. Complicated processing to achieve multi-layered packaging
2. Difficulty in separating layers for recycling
What is the benefit of nano-fillers in food packaging? What materials are most commonly used as fillers
It is low in cost and has high barrier properties
- Most common material : silicate fillers
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)
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
What 3 factors affect the efficiency of the tortrous path by plate-like fillers?
- 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
- **Filler content **→ higher filler content leads to increased number of obstacles, reduced permeability
- Interfacial bonding betwen filler and polymer matrix : stronger bonding → prevents formation of cracks / empty space, reducing permeability and increasing efficiency
What are 3 material structure factors that can affect barrier properties? Explain their rationale
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.
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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 chain mobility**, leading to lower permeability.
