Flexible packaging's performance is fundamentally defined by its component films. Each material brings unique properties to the laminate structure. Here is a detailed guide to the primary films used, categorized by their core function in the packaging structure.

Material Overview by Functional Layer

A typical high-performance flexible laminate consists of three functional layers:

1. Outer Layer: Provides structure, printability, and abrasion resistance.

2. Middle Layer (Barrier): Imparts critical gas, aroma, or moisture barrier.

3. Inner Layer (Sealant): Enables heat sealing and offers product compatibility.

1. Outer/Structural Layer Materials

These films form the package's backbone, offering durability and a printable surface.

A. Polyethylene Terephthalate (PET / BOPET)

• Full Name: Biaxially Oriented Polyethylene Terephthalate.

• Key Properties:

  • High tensile strength and rigidity, excellent dimensional stability.

  • Brilliant clarity and gloss for superior graphics.

  • Good barrier to gases and aromas, fair moisture barrier.

  • Excellent printability and lamination adhesion.

  • Temperature resistant (up to ~150°C).

• Primary Role: Outer layer for pouches, lidding, and as a base for metallization.

• Common Gauges: 12µm, 12µm, 23µm.

B. Biaxially Oriented Polypropylene (BOPP)

• Key Properties:

  • Excellent moisture barrier (best among common plastics).

  • Good clarity and gloss, lower cost than PET.

  • Good stiffness and puncture resistance.

  • Lower heat resistance than PET.

• Primary Role: Outer layer for snack bags, confectionery overwrap, label films.

• Variants: Pearlescent OPP (for aesthetic effect), Metallized OPP (enhanced barrier).

• Common Gauges: 20µm, 25µm, 30µm.

C. Polyamide (PA / Nylon)

• Key Properties:

  • Exceptional toughness, puncture, and abrasion resistance.

  • Good oxygen barrier (when dry), excellent aroma barrier.

  • Poor moisture barrier – properties degrade with humidity.

• Primary Role: Middle or outer layer in packaging for sharp, heavy, or fatty products (e.g., pet food, cheese, frozen meat).

• Common Form: Biaxially oriented (BOPA).

• Common Gauges: 15µm, 20µm.

2. Barrier Layer Materials

These specialized films protect the product from oxygen, moisture, light, or aromas.

A. Aluminum Foil (AL)

• Key Properties:

  • Absolute barrier to gases, moisture, light, and microorganisms.

  • Provides a distinctive metallic appearance.

  • Pinholes can occur at thin gauges, compromising barrier.

• Primary Role: The ultimate barrier layer in retort pouches, aseptic packaging, and high-value dry foods.

• Limitation: Not transparent, non-microwaveable, and makes the package non-recyclable in standard streams.

• Common Gauges: 6µm, 7µm, 9µm.

B. Metallized Films (VMPET, VMCPP, VMBOPP)

• Process: A thin layer of aluminum (< 1 µm) is vacuum-deposited onto a film substrate (PET, CPP, BOPP).

• Key Properties:

  • Very high barrier to gases and moisture (though not absolute like foil).

  • Excellent light barrier, conductive for static dissipation.

  • Retains some flexibility and is more cost-effective than foil lamination.

• Primary Role: High-barrier packaging for coffee, snacks, and dried foods where an absolute barrier isn't required.

• Common Structure: VMPET (e.g., 12µm PET with metallization) is the industry standard for high-barrier laminates.

C. Ethylene Vinyl Alcohol (EVOH)

• Key Properties:

  • Exceptional oxygen barrier (10-100x better than typical films) – but only when dry.

  • Barrier properties drop significantly at high humidity.

  • Must be co-extruded between protective layers (like PE or PP) to function.

• Primary Role: The primary oxygen barrier in "transparent high-barrier" packaging (e.g., liquid pouches, vacuum bags). It is sandwiched as a middle layer.

• Sustainability Note: Enables all-PE or all-PP recyclable structures with high oxygen barrier.

D. Aluminum Oxide / Silicon Oxide Coatings (AlOx, SiOx)

• Process: Ultra-thin ceramic layers applied via vacuum deposition.

• Key Properties:

  • Transparent high barrier to oxygen and moisture.

  • Excellent retort and microwave suitability.

  • More brittle than metallized films, can suffer from micro-cracking.

• Primary Role: Transparent high-barrier packaging for premium products (e.g., baby food, medical devices) and microwaveable retort meals.

3. Sealant/Inner Layer Materials

These polyolefin-based films form the innermost layer, creating the hermetic seal.

A. Polyethylene (PE)

• Types:

  • LDPE (Low-Density PE): Excellent clarity, good sealing range, low heat resistance.

  • LLDPE (Linear-Low-Density PE): Superior seal strength, puncture resistance, and flexibility.

  • MDPE/HDPE (Medium/High-Density PE): Stiffer, better moisture barrier, harder to seal.

• Key Properties:

  • Low melting point enables easy heat sealing.

  • Excellent moisture barrier, poor oxygen barrier.

  • Chemically inert and safe for direct food contact.

• Primary Role: The most common sealant film. PE is often the carrier for recycled content (PCR-PE) in sustainable structures.

B. Cast Polypropylene (CPP) & Retort CPP (RCPP)

• CPP Key Properties:

  • Higher temperature resistance than PE (hot fill applications).

  • Better clarity and stiffness than PE.

  • Good moisture barrier.

• RCPP Key Properties:

  • Specially formulated to withstand retort sterilization (121°C+ for 30+ mins).

  • Prevents delamination and maintains seal integrity under extreme conditions.

• Primary Role: CPP for general sealant; RCPP is mandatory for the inner layer of retort pouches containing ready-to-eat meals, pet food, etc.

C. Chlorinated Polyethylene (CPE)

• Key Properties:

  • Excellent adhesion to aluminum foil and other substrates.

  • Good chemical resistance.

• Primary Role: Often used as a tie layer or adhesive layer in foil laminates (e.g., PET/AL/CPE) or as a sealant where strong foil bonding is required. Less common in modern all-plastic structures.

D. Ionomer (e.g., Surlyn®)

• Key Properties:

  • Exceptional seal-through-contamination (grease, powders).

  • Excellent hot tack strength (seal strength while still hot), crucial for high-speed VFFS machines.

  • High clarity and toughness.

• Primary Role: Premium sealant for challenging applications like oily snacks, cheese, and liquid packaging.

Material Selection Matrix

*Aluminum foil is fragile and lacks tensile strength on its own; it relies on lamination for durability.

Sustainability Considerations

• Mono-Material Focus: The push for recyclability favors structures using only PE or only PP families.

• Recycled Content: Post-Consumer Recycled (PCR) PE is the most commercially available option for incorporation into sealant layers.

• Barrier Dilemma: Traditional high barriers (AL, multi-material EVOH) hinder recycling. Solutions include mono-material PE with EVOH (where EVOH is considered a "minor component") or transparent barrier coatings (AlOx) on recyclable substrates.

Conclusion

Selecting the right combination of films is a balancing act between performance requirements (barrier, strength), processing needs (sealing temperature, machinability), marketing goals (clarity, appearance), and sustainability targets. A high-barrier snack pack might use PET for strength/printability, VMPET for barrier, and LDPE for sealing. A recyclable liquid pouch might use BOPP as the outer, a thin EVOH core, and PE as the sealant layer. Understanding these core materials is the first step in designing effective, efficient, and responsible flexible packaging.