Polymer Science and Fick's Laws Quiz

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.