Plastics : Protecting goods from spoilage - Barrier property limitations in the plastic industry -

Barrier property limitations refer to the inherent inability of plastic materials to completely prevent the passage of gases (like oxygen, carbon dioxide), moisture, or other substances through their structure. 

These limitations challenge the effectiveness of plastic packaging in protecting contents from environmental factors that can cause spoilage, degradation, or loss of quality.

Fundamental mechanisms of transmission:

• Permeation: three-step process where molecules dissolve into one surface of the plastic, diffuse through it, and evaporate from the other side
• Diffusion: movement of gas or vapor molecules through the microscopic spaces between polymer chains
• Solubility: ability of permeating molecules to dissolve in the plastic material
• Fick's Law: rate at which gases or liquids pass through a material depends on the concentration difference and thickness—higher difference and thinner material means faster transmission

Key barrier challenges by permeant type:

• Oxygen transmission: critical for food preservation, preventing oxidation and extending shelf life
• Carbon dioxide retention: essential for carbonated beverages and modified atmosphere packaging
• Water vapor transmission: crucial for maintaining moisture content in dry foods or protecting moisture-sensitive products
• Aroma/flavor compounds: important for preserving product quality and preventing off-flavors
• Organic vapors and solvents: necessary for containing chemicals and preventing contamination
• Light transmission: UV and visible light can start degradation reactions

Factors affecting barrier performance:

• Polymer crystallinity: higher crystallinity (degree to which molecules in a solid are arranged in an ordered, repeating pattern rather than positioned randomly) typically improves barrier properties
• Free volume: space between polymer chains that allows molecular passage
• Chain rigidity: how stiff and inflexible the molecular chains in a polymer are, affecting how easily gas or vapor molecules can pass through, stiffer chains typically provide better barriers
• Polarity: property of a molecule having positive and negative electrical charges separated within its structure, determining how it interacts with other substances
• Temperature: higher temperatures increase permeation rates
• Humidity: water acts as a plasticizer (additive that increases flexibility and workability of plastics) in many polymers, reducing barrier properties
• Mechanical stress: stretching or flexing can compromise barrier integrity
• Material defects: pinholes, tears, or inconsistent thickness

Relative barrier performance of common plastics:

• Low barrier: polyethylene (PE), polypropylene (PP), polystyrene (PS)
• Medium barrier: PET, rigid PVC
• High barrier: PVDC, EVOH, nylon (PA), PAN
• Very high barrier: metallized films, AlOx or SiOx coatings, certain nanocomposites

PET (Polyethylene Terephthalate): clear, strong plastic with moderate barrier properties, commonly used for beverage bottles, food containers, and textile fibers.
Rigid PVC (Polyvinyl Chloride): stiff plastic with good clarity and moderate barrier properties, used in packaging, medical devices, and construction.
PVDC (Polyvinylidene Chloride): plastic with excellent barrier properties against oxygen, moisture, and aromas, often used as a coating or in multi-layer food packaging.
EVOH (Ethylene Vinyl Alcohol): resin with exceptional oxygen barrier properties when dry, commonly used as a middle layer in multi-layer food packaging.
Nylon (PA - Polyamide): family of strong, temperature-resistant plastics with good oxygen barrier properties, used in food packaging and technical applications.
PAN (Polyacrylonitrile): polymer with excellent gas barrier properties, commonly used in packaging and as a precursor for carbon fiber.
Metallized films: plastic films coated with a thin layer of metal (usually aluminum) to significantly improve barrier properties against light, moisture, and gases.
AlOx or SiOx coatings: ultra-thin, transparent layers of aluminum oxide or silicon oxide deposited on plastic films to enhance barrier properties without affecting transparency.
Nanocomposites: plastics containing nanoscale particles that create a maze-like path for gases and moisture, improving barrier properties without thick layers.

Industry impact:

• Limited shelf life for perishable products
• Need for refrigeration when barrier properties are insufficient
• Product quality degradation over time
• Package size and thickness constraints
• Processing challenges with high-barrier materials
• Cost premiums for enhanced barrier solutions
• Environmental trade-offs (multi-layer materials may be less recyclable)

Solutions and technologies:

Material strategies:

• Multi-layer structures: combining complementary barrier properties of different materials
• Barrier coatings: thin layers of high-barrier materials like PVDC, metalized films, or oxide coatings
• Nanocomposites: incorporating nanoclays or other nanoparticles to create tortuous paths for permeants
• Oxygen scavengers: active packaging components that absorb oxygen molecules
• Moisture regulators: materials that absorb excess moisture or release it as needed

Design approaches:

• Increased wall thickness: simple but material-intensive approach
• Surface area minimization: reducing the area available for transmission
• Secondary packaging: using additional protective layers
• Modified atmosphere packaging: replacing headspace air with protective gas mixtures
• Cold chain management: reducing transmission rates through temperature control

Testing and qualification:

• Oxygen transmission rate (OTR) testing: ASTM D3985, ASTM F1927
• Water vapor transmission rate (WVTR) testing: ASTM F1249, ASTM E96
• Carbon dioxide transmission rate: ASTM F2476
• Permeation testing: For specific gases or vapors of concern
• Shelf-life studies: Real-world validation of barrier performance

ASTM D3985: standard test method for measuring oxygen gas transmission rate through plastic film and sheeting using a coulometric sensor.
ASTM F1927: standard test method for determining oxygen gas transmission rate through barrier materials using a coulometric sensor.
ASTM F1249: standard test method for measuring water vapor transmission rate through plastic film and sheeting using a modulated infrared sensor.
ASTM E96: standard test methods for determining water vapor transmission of materials using the desiccant method or water method.
ASTM F2476: standard test method for measuring carbon dioxide gas transmission rate through barrier materials using an infrared detector.

Balancing barrier performance with other requirements like cost, processability, transparency, flexibility, and sustainability remains one of the central challenges in plastic packaging development.