Polymer Science and Fick's Laws Quiz

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Questions and Answers

What does Fick's first law describe in the context of permeation?

The thickness of the polymer affecting permeation

The direct proportionality between the rate of permeation and the concentration gradient (correct)

The relationship between concentration and pressure

The area through which diffusion occurs

In the equation for calculating the quantity of permeant Q, which variable represents the concentration difference?

p1 - p2

c2 - c1 (correct)

What does the solubility coefficient S reflect in Henry's law?

The vapor pressure of the permeant

The concentration of the gas in the polymer

The amount of permeant in the polymer (correct)

The permeability of the polymer to gases

Which variable is NOT assumed to be constant when using Fick’s first law for calculating diffusion in polymers?

c (concentration) Signup and view all the answers

When measuring permeant in gases, what is more convenient to use instead of actual concentration?

Vapor pressure Signup and view all the answers

What happens to tensile strength as polymer chain length increases?

Tensile strength increases Signup and view all the answers

At what temperature does polystyrene become strong and stiff?

100°C Signup and view all the answers

Which of the following statements is true for polymers with a Tg below service temperature?

They are weaker and more ductile Signup and view all the answers

What does ASTM D882 primarily test?

Tensile properties of thin plastic films Signup and view all the answers

What are some advantages of using aluminum for packaging materials?

Good recycling properties and corrosion resistance Signup and view all the answers

What type of cans are aluminum primarily used for?

Beverage cans (most) Signup and view all the answers

Why is glass considered optically isotropic when homogeneous and stress-free?

It allows uniform light transmission Signup and view all the answers

What role do metallic oxides play in the optical properties of glass?

They create absorption bands Signup and view all the answers

Which of the following is NOT a disadvantage of metal containers?

High resistance to physical and thermal shock Signup and view all the answers

What are some common additives used to enhance paper packaging materials?

Waxes and resins Signup and view all the answers

Which of the following is NOT a main color of glass used for food containers?

Red Signup and view all the answers

What effect do color additives have on glass?

They change the optical transmission Signup and view all the answers

What is a key characteristic of aluminum foils thinner than 20 µm?

They are prone to pinholes. Signup and view all the answers

Which of the following metals is NOT mentioned as an alloying agent for aluminum?

Nickel Signup and view all the answers

What is a feature of metal containers that provides added security to the products inside?

Tamper evident seals Signup and view all the answers

Which of these packaging materials requires modifications to enhance barrier properties for food products?

Plain paper Signup and view all the answers

What factor primarily controls the magnitude of permeability (P) in polymers?

Chemical structure of the polymer Signup and view all the answers

What does the transmission rate (TR) formula represent?

The amount of permeant passing through the polymer over time Signup and view all the answers

For which of the following gases is the independence of diffusion coefficient (D) and solubility coefficient (S) from concentration generally applicable?

Oxygen (O2) Signup and view all the answers

What are the typical units used for water vapor transmission rate (WVTR)?

(g m-2 day-1) Signup and view all the answers

What is the effect of polymer thickness on the transmission rate of gases?

Thicker films generally reduce TR Signup and view all the answers

Which of the following conditions is NOT commonly used for the performance parameters of oxygen transmission rate (OTR)?

30 °C/50% RH Signup and view all the answers

Why might steady-state diffusion take longer for larger molecules in glassy polymers?

Physical barriers formed by polymer structure Signup and view all the answers

What does it imply when deviations from steady-state assumptions occur in composite materials?

There is significant interaction between polymer and permeant Signup and view all the answers

What factor primarily influences the tensile strength of a polymer?

Length of the polymer chains Signup and view all the answers

Which type of polymer is characterized as strong, stiff, and sometimes brittle due to having a Tg above the service temperature?

Polystyrene Signup and view all the answers

Which material is considered optically isotropic when it is homogeneous and free from stresses?

Glass Signup and view all the answers

What is the effect of coloring additives, such as metallic oxides or sulfides, on glass?

Controlling light transmission Signup and view all the answers

Which color of glass is typically used to provide light protection for food containers?

Amber or brown Signup and view all the answers

What happens to polymers with a Tg below the service temperature?

They become weaker and more ductile. Signup and view all the answers

Which of the following properties is NOT associated with the mechanical behavior of polymers?

Thermal conductivity Signup and view all the answers

What primarily contributes to the barrier properties of plastic packaging?

Molecular structure of the polymer Signup and view all the answers

What does the permeability coefficient P represent in the context of gas transmission?

Ratio of gas quantity to thickness and pressure difference Signup and view all the answers

In the provided example, what is the Oxygen Transmission Rate (OTR) of the OPP film?

1550 mL m−2 day−1 Signup and view all the answers

What is the permeability coefficient for the OPP film calculated in the example?

2.8 × 10−10 barrer Signup and view all the answers

Which factor does NOT directly affect the permeability of moisture-sensitive materials?

Type of gas through the material Signup and view all the answers

How is Water Vapor Transfer Rate (WVTR) expressed based on relative humidity difference?

kg m−2 s−1 Signup and view all the answers

For the permeability coefficient in the example for WVTR, which conversion factor is used from kg to barrer?

1 kg = 3.12 × 10^6 barrer Signup and view all the answers

What is the relationship between the thickness of the film and the permeability coefficient?

Inversely proportional to the permeability coefficient Signup and view all the answers

What units are used to express the Oxygen Transmission Rate (OTR)?

mL m−2 day−1 Signup and view all the answers

At 37.8 oC, the saturated vapor pressure of water is given as approximately what value?

4.9167 cm Hg Signup and view all the answers

What calculation method is mentioned to determine the permeability for multilayer packaging materials?

Modifying equation based on average partial pressure Signup and view all the answers

Study Notes

Polymers Used in Food Packaging

Polyethylene Terephthalate (PET or PETE) (#1) is a good barrier to moisture, oxygen, and aroma, and has good heat resistance.
High Density Polyethylene (HDPE) (#2) is also a good barrier to moisture
Low Density Polyethylene (LDPE) (#4) is a excellent moisture barrier.
Polypropylene (PP) (#5) is an excellent moisture barrier, but a poor barrier to oxygen and aroma.
Polyvinyl chloride (PVC or V) (#3) is a good moisture barrier, but a poor barrier to oxygen and aroma.
Polystyrene (PS) (#6) is a poor moisture, oxygen and aroma barrier.
Nylon or EVOH copolymer inner layer (Other - #7) is an excellent oxygen and aroma barrier, but a poor moisture barrier
PVDC copolymer coating (Other - #7) is an excellent oxygen, aroma and moisture barrier.

Additives in Plastics

Additives are auxiliary ingredients that enhance the properties of the parent polymer without altering its chemical structure.
In food packaging, all additives should have clearance from the appropriate food regulatory authority.

Processing Additives

Polymer degradation frequently involves oxidation reactions due to free radical mechanism and interaction of O₂ with C-H bonds forming hydroperoxide groups, which decompose into OH radicals and molecular scissions.
Primary stabilizers or antioxidants (hindered phenols or aromatic amines) interrupt the chain reaction by combining with free radicals.
Secondary stabilizers or peroxide decomposers (organic thioesters, phosphites and metal thiocarbamates) react with hydroperoxides.
Chelating agents or metal deactivators (organic phosphites and hydrazides) protect the polymer by immobilizing metal ions.
With PVC, heat stabilizers or acid absorbers are used to retard the decomposition of PVC into HCl and dark, degraded polymers.
The tendency for polymers such as PVC and polyolefins to stick to metal parts during processing can be reduced by adding lubricants such as PE waxes, fatty acid esters and amides, metallic stearates and paraffin.

Plasticizers

Brittle polymers such as PVC must be plasticized to obtain flexible films and containers.
Plasticizers also give the material the limp and tacky qualities found in "cling" films. About 80% of all plasticizers are used in PVC.
Phthalic esters, epoxidized oils, and low-MW polyesters are commonly used plasticizers.
Internal plasticization can be achieved by copolymerization, such as in the case of PVC, which can be copolymerized with vinyl acetate, ethylene or methylacrylate.

Antiaging Additives

Aging is the process of deterioration of materials resulting from the effects of atmospheric radiation, temperature, O₂, water, microorganisms.
Antioxidants (BHT) are used as free radical scavengers.
Organophosphites act as hydroperoxide decomposers. For a synergistic effect, different antioxidants may be used together.
UV stabilizers absorb high-energy UV radiation and release it as lower-energy radiation. They prevent the deterioration of polymeric films by photooxidation.

Antifogging Films

Moisture tends to condense as small droplets on the internal surface of some food packaging applications.
Antifogging films let consumers clearly see food in packages.
Antifogging polymeric films prevent the formation of fog inside food packages (e.g., fresh produce or raw meat packages).
The antifogging property is essential for packaging film used in frozen products.

Antifogging Agents

Antifogging materials are typically surface-active agents that consist of two parts: a hydrophilic head and a lipophilic tail.
Examples include glycerol fatty acid ester, polyglycerol fatty acid ester, fatty acid ester of polyethylene glycol, alkyl ether of polyethylene glycol, ethoxylated alkyl phenol, sorbitan ester, and ethoxylated sorbitan ester.
When incorporated in a polymer matrix, antifogging agents migrate to the film surface, reducing the interfacial tension between the polymer and the water drops, thus spreading water drops across the surface and forming a continuous thin layer.
These antifogging agents can be applied on the surface or compounded internally in the packaging material at levels ranging from 0.5% to 4%.

Antistatic Agents

Static electricity is generated on polymer surfaces by friction or rubbing against other surfaces or during converting operations, or filling lines.
Antistatic agents are used to prevent electrical charge accumulation in polymers, which are nonconductors of electricity.
Electrification results from charge segregation (electrons and ions) when two surfaces are parted after close initial contact.
Ethoxylated fatty amines, polyhydric alcohols, derivatives, nonionic and quaternary ammonium compounds migrate to the surface, forming a conducting layer via moisture absorption, enabling electron discharge.

Antiblocking Agents

Many packaging films or sheets tend to stick together due to their nonconducting nature (blocking).
Antiblock agents reduce friction between packages allowing for easy separation.
These agents help prevent blocking (that could develop under pressure during storage or use) and include colloidal silica, clays, starches, and silicones.
Antiblock agents are typically used at levels ranging from 0.1% to 0.5%.

Antiblocking Agents (Mechanism)

Chemically inert, inorganic antiblock additives migrate to the film surface, partially sticking out to create a microroughness on the film surface.
A barrier layer is thereby formed on the plastic film preventing the two adjacent plastic film layers from adhering to each other.

Slip Agents

Slip is when polymeric films slide parallel over each other.
It is quantified via the coefficient of friction (COF). High COF leads to film sticking together rather than sliding.
Fatty acid amides are commonly used slip agents.

Optical Properties Modifiers

Optical properties in packaging materials are described as the ability of the material to transmit light, exhibit color, and reflect light from the surface (gloss).
Food packaging films are commonly not pigmented; however, some are colored by dyes (soluble in the plastic and tending to migrate) or pigments (insoluble in plastic matrix).
The principal pigments used in packaging are carbon black, white titanium dioxide, red iron oxide, yellow cadmium sulfide, molybdate orange, ultramarine blue, blue ferric ammonium ferrocyanide, and chrome green.
The FDA has raised concerns about the use of some colorants in packaging materials that may migrate into the food.

Foaming Agents

Foaming or blowing agents are used for producing cellular products, classified into physical or chemical types.
Physical types use inert gases, whereas chemical types employ thermal decomposition reactions that result in gas evolution.
Today, expanded and extruded polystyrene (PS) foams are typically produced using CO₂ or light aliphatic hydrocarbons like pentane or butane as blowing agents.
Expanded polyethylene terephthalate (PET), polypropylene (PP), and polyvinyl chloride (PVC) foams are produced using chemical blowing agents such as CO₂ and N₂.

Plastic Food Packaging Material Advantages

Low cost in manufacturing and storage.
Often practical for inline package formation.
Moldable, heats sealable, light weight, transparent, and microwaveable for some.
Dual-ovenable for some, non-corrodible, physical and thermal shock-resistant, and stronger barrier than paper.
Recyclable for some plastics.

Plastic Food Packaging Material Disadvantages

Permeable to moisture, O2, CO2, ethanol, & aromas.
Lower compressive strength than glass & metal, less heat resistant than glass & metal.
Potential for interaction with food.
Some plastics are not recyclable.
Multi-layer plastic packages are not recyclable.

Polymers Used in Food Packaging (alternate list)

Polyethylene (PE)
High Density Polyethylene (HDPE)
Low Density Polyethylene (LDPE)
Polypropylene (PP)
Polyvinyl chloride (PVC)
Polyvinylidene Chloride (PVDC)
Polystyrene (PS)
Polyethylene Terephthalate (PET or PETE)
Ethylene Vinyl Alcohol Copolymer (EVOH)
Polyamide (Nylon)

Packaging Materials: Glass

Glass is an inert packaging material with an absolute barrier to gases and moisture.
It is versatile for preserving flavor and freshness of delicate food products (e.g., beer and wine).
It can withstand high thermal processing, provides good insulation, is formable into various shapes, and commonly comes in clear, amber, or green colors with varying levels of transparency.
Additional oxide coatings enhance mechanical properties and provide a barrier against chemical attack.
Disadvantages: heavy weight, and fragility to internal pressure, impact and thermal shock.

Packaging Materials: Glass (Properties)

Glass is an amorphous, inorganic product of fusion that has cooled to a rigid state without crystallizing.
It appears as a highly viscous liquid in a vitreous or glassy state.
A typical formula for soda-lime glass includes silica (68-73%), calcia (10-13%), soda (12-15%), alumina (1.5–2%), and iron oxides (0.05-0.25%).

Packaging Materials: Metals

Common metals used for food packaging include aluminum (Al), Chromium (Cr), Steel, and Tin (Sn).
Steel is plated with tin (tinplate) or chromium (ECCS/TFS) to prevent corrosion.
Tinplate and electrolytically coated steel are typically used for retort processing of various food items: fruits, vegetables, meats, etc.
Aluminum is used in the form of alloys (containing magnesium and manganese) for flexible packaging (foil, laminated), and metallized films.

Packaging Materials: Metals (Advantages)

Metal packaging is impervious to water, oxygen, carbon dioxide, and aromas.
Metals are lighter compared to glass.
Double-seam closures on metal cans and hermetical sealed containers are impervious to light.
Higher heat resistance, good resistance to physical and thermal shock, faster heating than glass, and can have tamper-evident seals.
Metal is easily recyclable.

Packaging Materials: Metals (Disadvantages)

Multi-step manufacturing is commonly used for shaped steel containers.
Cylindrical shape is not ideal for storage space optimization.
Viewing the contents of the package is not readily possible, which results in non-hygienic drinking opportunities.
Not microwaveable.
Perceived as outdated or old fashion style.
Metal containers are not easily reclosable or refillable.

Packaging Materials: Paper

Paper is commonly used for a wide variety of food products whether plain or coated with additives like (lacquers, waxes, and resins).
Kraft paper is often used in bags, and wrappings.
Sulfite paper is widely used for higher print quality, and is used in laminates.
Densified and greaseproof papers (glassine, parchment) are resistant to fats while being porous to moisture.
Paperboard (whiteboard, solidboard, chipboard, fiberboard) is a secondary package for the handling and distribution of food products.

Packaging Materials: Paper (Treatment/Disadvantages)

Paper is typically treated by embossing, coating, laminating, to form the desired shapes and sizes (boxes, bags).
Paper and paperboard have poor barriers to gases and water vapor.
Uncoated paper only offers limited light and minor mechanical protection.

Packaging Materials: Paper (Barrier Properties)

A barrier against water vapor and gases, like oxygen, can be created using sizing agents or by coating the paper with plastic polymers.
Beverage cartons typically comprise layers of coated paperboard to become water/liquid impermeable.
Aluminum foil, used as a gas/odor/light barrier, is often included in multilayer constructions.

Packaging Materials: Paper (Advantages/Disadvantages)

Advantages (Paper/Paperboard): Offers light barrier, various shapes like bags and boxes, coating or lamination gives better barrier properties, and recyclability for uncoated versions.
Disadvantages (Paper/Paperboard): Poor barrier to gases and moisture when not coated or laminated, and not heat-sealable when not treated appropriately.

Packaging Material Properties

Mechanical properties of glass and plastic packaging
Optical properties of glass and plastic packaging
Barrier properties of plastic packaging
Factors affecting permeability

Barrier Properties of Plastic Packaging

Plastic packaging is permeable unlike metals and glass
A good barrier plastic has a low permeability
Food protection depends on the integrity of packages and the permeability of the packaging materials.

Barrier Properties of Plastic Packaging (Continued)

Polymer permeability is related to the concentration gradient via Fick's first law :
J = -Dc/δx
Where J is the flux (permeant rate of passage), D is the diffusion coefficient, and c is concentration (function of geometry).

Barrier Properties of Plastic Packaging (Additional Details)

The quantity of permeant can be calculated by substitution : Q= D(c2-c1) At/X
Rather than actual concentration, the vapor pressure (P) is more convenient and is measured at equilibrium with the polymer.
Henry's law applies to low concentrations: c=Sp (where S is the solubility coefficient of the permeant in the polymer)
The product DS is the permeability coefficient (P).

Permeability Coefficient

The permeability coefficient (P) is the product of the kinetic term (D) which reflects the penetrant-polymer system dynamics, and the thermodynamic term (S) which depends on penetrant-polymer interactions.
P quantifies how easily a gas permeates a polymer under a pressure gradient.
Permeability can be described as a function of time, area, thickness, pressure drop, or concentration gradient.

Factors Affecting Permeability

Polymer, chemical composition, structure/orientation/plasticizers/pigments
Permeant, compatibility with polymer, size/shape
Temperature (and sometimes relative humidity)

Aseptic Packaging, Modified Atmosphere Packaging (MAP)

Aseptic packaging involves using a special packaging process to preserve the food with no additives or preservatives
MAP involves modifying the atmosphere around food by altering gas compositions (O2, CO2, N2, C2H4)
This technique aims to prevent bacterial growth, extend shelf life, and enhance quality attributes.

Active Packaging

Active packaging is a system where the package, product, and environment interact positively to extend food shelf life, maintaining quality and sensory properties.
Active packaging incorporates components that release or absorb substances into/from the food or the environment.

Oxygen Scavengers, Purge Absorption

Oxygen scavengers are used to remove oxygen, retarding oxidative reactions, and slowing microbial growth.
Purge absorption removes liquids from food items without affecting the microbial spoilage profile.
In addition to active additives like antimicrobial compounds, carbon dioxide generators, and pH reducers, polymeric purge absorbers can enhance the preservation process.

Others (Miscellaneous)

Other packaging systems may include anti-fogging, anti-sticking, light absorbing/regulating, microwave susceptors & insect repellant.

Future Trends in Food Packaging

Consumer preferences drive innovation in packaging, now extending to evolving needs in sea-food and meat industries.
Upcoming demands include self-heating and self-chilling functions in packaging.

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Description

Test your understanding of Fick's first law and its applications in permeation and diffusion within polymers. This quiz covers key concepts such as concentration differences, solubility coefficients, and the mechanical properties of polymers. Ideal for students studying materials science and polymer engineering.