Introduction
The flexible packaging industry relies on a diverse range of polymer films to meet the varied requirements of modern packaging applications. Among the most widely used are BOPP (Biaxially Oriented Polypropylene), CPP (Cast Polypropylene), PET (Polyethylene Terephthalate), and PE (Polyethylene). Each of these materials has unique characteristics that make it suitable for specific applications.
Understanding the differences between these four core film types is essential for packaging engineers, brand owners, and converters who need to select the optimal material for their specific requirements. This comprehensive guide provides detailed information on each film type, their properties, advantages, and optimal applications.
BOPP Film (Biaxially Oriented Polypropylene)
What is BOPP Film?
BOPP film is produced by extruding polypropylene resin into a sheet and then biaxially orienting it—both in the machine direction and transverse direction. This orientation process stretches the polymer chains in both directions, significantly improving mechanical strength, optical properties, and barrier characteristics compared to non-oriented polypropylene.
Key Characteristics
- High tensile strength: 100-200 MPa in both directions
- Excellent clarity: Gloss values of 90-95%
- Low density: 0.90-0.91 g/cm³ (lightest of the four)
- Superior moisture barrier: WVTR of 4-6 g/m²·day
- Good printability: After corona treatment
- Cost-effective: Lower cost compared to PET
Limitations
- Moderate oxygen barrier (OTR: 1500-2000 cc/m²·day)
- Limited temperature resistance (up to 100-120°C)
- Requires surface treatment for printing and lamination
Best Applications
- Food packaging (snacks, biscuits, confectionery)
- Overwrapping films
- Labels and tapes
- Textile packaging
- Cigarette packaging
- Flexible laminates (as outer layer)
CPP Film (Cast Polypropylene)
What is CPP Film?
CPP film is produced by cast extrusion, where molten polypropylene is extruded through a flat die and cooled on a chilled roller. Unlike BOPP, CPP is not oriented, resulting in different properties. CPP is available in various grades including homopolymer, random copolymer, and impact copolymer formulations.
Key Characteristics
- Good seal strength: Excellent heat seal properties
- High clarity: Good optical properties
- Moisture barrier: Good, though not as good as BOPP
- Flexibility: Softer and more flexible than BOPP
- Good aroma barrier: Excellent for odor-sensitive products
- Low temperature sealability: Can seal at lower temperatures
Limitations
- Lower tensile strength than BOPP
- Higher haze compared to BOPP
- Less stiff than BOPP
- Moderate temperature resistance
Best Applications
- Flexible packaging inner/heat seal layer
- Fresh produce packaging
- Medical packaging
- Industrial packaging
- Textile packaging
- Lamination substrate
PET Film (Polyethylene Terephthalate)
What is PET Film?
PET film is produced from polyethylene terephthalate resin, typically in biaxially oriented (BOPET) form. The biaxial orientation process provides excellent mechanical strength, thermal stability, and barrier properties. PET is one of the most versatile packaging films available.
Key Characteristics
- Excellent tensile strength: 150-250 MPa
- Superior temperature resistance: Up to 200°C (short term)
- Good oxygen barrier: OTR of 50-100 cc/m²·day
- Excellent dimensional stability: Low shrink
- Good printability: Naturally higher surface energy
- High gloss: 80-90%
Limitations
- Higher density: 1.40 g/cm³
- Higher cost than BOPP and CPP
- Moderate moisture barrier (WVTR: 15-25 g/m²·day)
- Can be brittle in certain conditions
Best Applications
- Hot-fill food packaging
- Microwaveable packaging
- Electronics packaging
- Pharmaceutical packaging
- Premium laminates
- Capacitor dielectrics
- Graphic arts
PE Film (Polyethylene)
What is PE Film?
PE film is produced from polyethylene resin and is available in several grades including LDPE (Low-Density Polyethylene), LLDPE (Linear Low-Density Polyethylene), and HDPE (High-Density Polyethylene). Each grade has distinct properties suited to different applications.
Key Characteristics
- Excellent flexibility: Especially LDPE and LLDPE
- Superior moisture barrier: WVTR of 5-15 g/m²·day
- Excellent sealability: Wide heat seal range
- Good chemical resistance: Resistant to many chemicals
- FDA approved: Excellent for food contact
- Cost-effective: Generally lowest cost option
Limitations
- Poor oxygen barrier compared to PET and PA
- Lower tensile strength
- Limited temperature resistance
- Can be prone to stretching
- Lower clarity (especially HDPE)
Best Applications
- Food packaging (as inner layer)
- Shopping bags
- Agricultural film
- Industrial packaging
- Medical packaging
- Stretch and shrink films
Quick Comparison Summary
The following table provides a side-by-side comparison of all four film types:
| Property | BOPP | CPP | PET | PE |
|---|---|---|---|---|
| Orientation | Biaxial | Non-oriented | Biaxial | Varies |
| Density (g/cm³) | 0.90-0.91 | 0.89-0.91 | 1.40 | 0.91-0.96 |
| Tensile Strength | High | Medium | Very High | Medium |
| WVTR (Moisture) | Excellent | Good | Moderate | Good |
| OTR (Oxygen) | Moderate | Moderate | Good | Poor |
| Heat Seal | Requires coating | Excellent | Poor | Excellent |
| Temp Resistance | 100-120°C | 100-130°C | Up to 200°C | 70-100°C |
| Cost | Medium | Medium-Low | High | Low |
| Printability | Good | Good | Excellent | Moderate |
| Clarity | Excellent | Very Good | Very Good | Varies |
| Recyclability | PP stream | PP stream | PET stream | PE stream |
How to Combine Films in Laminates
Modern flexible packaging often uses multi-layer laminates that combine the best properties of different films. Understanding the role of each layer helps in selecting the optimal structure:
Common Laminate Structures
BOPP/PE Laminate
Structure: BOPP outer layer + PE inner layer
Applications: Snack food packaging, candy wrappers
Benefits: BOPP provides printability and moisture barrier; PE provides sealability and food contact compliance
PET/PE Laminate
Structure: PET outer layer + PE inner layer
Applications: Frozen food, retort packaging
Benefits: PET provides strength and temperature resistance; PE provides sealability
BOPP/CPP Laminate
Structure: BOPP outer layer + CPP inner layer
Applications: Garment packaging, gift boxes
Benefits: Excellent optics, good seal strength
PET/Al/PE Laminate
Structure: PET outer + Aluminum foil middle + PE inner
Applications: High-barrier food packaging, pharmaceutical blisters
Benefits: Maximum barrier properties for sensitive products
Selecting the Right Film
When choosing between these four film types, consider the following factors:
Moisture Sensitivity
For products requiring excellent moisture barrier, BOPP is the top choice, followed by CPP and PE. Avoid PET for moisture-sensitive products due to its higher WVTR.
Oxygen Sensitivity
For products requiring oxygen barrier, PET excels. BOPP and CPP are moderate, while PE provides poor oxygen barrier.
Temperature Requirements
For high-temperature applications (hot-fill, microwave, oven), PET is the clear winner. For standard room temperature applications, BOPP or CPP are more cost-effective choices.
Print Quality
PET offers the best printability without extensive treatment. BOPP and CPP require corona treatment but can achieve excellent results. PE is more challenging to print.
Sealing Requirements
CPP and PE offer excellent inherent sealability. BOPP and PET require functional coatings or additional layers for heat sealing.
Cost Constraints
PE is the most economical choice, followed by CPP and BOPP. PET commands a premium price but offers superior performance in demanding applications.
Conclusion
Each of these four film types—BOPP, CPP, PET, and PE—has distinct advantages and optimal applications. BOPP excels in applications requiring excellent moisture barrier, high clarity, and cost-effective packaging. CPP is ideal for heat seal layers and applications requiring good seal strength. PET provides superior temperature resistance and oxygen barrier for demanding applications. PE remains the go-to choice for inner layers requiring excellent sealability and food contact compliance.
In practice, most flexible packaging uses multi-layer laminates that combine these films to achieve the required performance at optimal cost. Understanding the properties of each material enables packaging designers to create structures that precisely match application requirements.
At Shandong Shunzhan New Materials Co., Ltd., we specialize in manufacturing high-quality BOPP films for diverse packaging applications. Our technical team can help you select the optimal film solution or laminate structure for your specific requirements. Contact us today to discuss your packaging needs.