How do PE/PP co-extruded films perform in terms of oxygen and carbon dioxide barrier properties?

PE/PP co-extruded films can offer varying levels of oxygen and carbon dioxide barrier properties, depending on the specific formulation of the materials and the co-extrusion process. Here's a breakdown of how these films perform:

1. Oxygen Barrier Properties
Polyethylene (PE): Standard PE films have relatively low oxygen barrier properties, meaning they are not very effective at preventing oxygen from passing through. This is one reason why PE alone is not ideal for packaging applications that require long shelf life, especially for oxygen-sensitive products such as food.

Polypropylene (PP): Like PE, PP also has limited oxygen barrier capabilities on its own, though it performs slightly better than PE in some cases. PP films are generally used in packaging that requires moderate protection against oxygen.

Combination of PE and PP: When PE and PP are co-extruded, the resulting film can benefit from a combination of properties. The outer layer of the film, typically made of PP, may offer better stiffness and heat resistance, while the inner PE layer can contribute to better sealability. However, the co-extruded structure still generally doesn't provide an optimal barrier for oxygen unless additional layers or coatings are added to enhance this property.

Enhancement Options: To improve the oxygen barrier properties, manufacturers may incorporate additional barrier layers made from materials such as ethylene vinyl alcohol (EVOH), nylon (PA), or PVDC (polyvinylidene chloride), which have significantly better oxygen barrier properties. These barrier layers are often placed between the PE and PP layers during the co-extrusion process.

2. Carbon Dioxide Barrier Properties
PE and PP both have similar limitations when it comes to blocking carbon dioxide (CO₂). Neither material is highly effective at providing a barrier to gases like CO₂, which is essential for maintaining the freshness and quality of certain perishable foods, such as fresh produce or carbonated beverages.
Effectiveness: As with oxygen, the ability of PE/PP co-extruded films to block CO₂ is generally insufficient for applications that require strong barrier properties for long-term freshness. If barrier performance against CO₂ is critical, other specialty films or additional barrier materials such as EVOH or nylon are often used to complement the PE/PP film structure.

3. Layered Structure for Enhanced Barrier Performance
Multi-Layer Co-Extrusion: In PE/PP co-extruded films, the performance of the oxygen and CO₂ barrier properties can be significantly improved by incorporating multi-layer co-extrusion. By using multi-layered films where different materials are combined (e.g., PE/PP with EVOH, PA, or other high-barrier resins), the film can achieve the desired barrier properties.
Typical Structure: A common structure for PE/PP co-extruded films designed to improve gas barrier properties might include:
Outer PP layer: Provides strength, heat resistance, and surface properties for printing.
Inner PE layer: Ensures good sealing and flexibility.
Barrier layer (e.g., EVOH or Nylon): Positioned between PE and PP or as an inner layer to provide a strong oxygen and CO₂ barrier.

4. Applications and Use Cases
Food Packaging: PE/PP co-extruded films are used in food packaging where moderate barrier properties are needed, such as for dry snacks, bakery items, and non-perishable goods. However, for products like fresh meat, dairy, or items requiring extended shelf life, additional barrier layers (such as EVOH) are added to PE/PP films to enhance the oxygen and CO₂ barrier properties.
Modified Atmosphere Packaging (MAP): For MAP, where a specific mixture of gases (including CO₂) is used to extend the shelf life of fresh food, films with high barrier properties are required. In these cases, PE/PP co-extruded films are often combined with other materials that provide the necessary gas impermeability.

5. Performance Trade-offs
Strength vs. Barrier: One of the challenges with PE/PP co-extruded films is that while they provide a balance of good mechanical properties (such as strength, puncture resistance, and flexibility), they often fall short in providing the high-level oxygen and CO₂ barrier required for some critical applications. To balance these factors, manufacturers often rely on multi-layer film structures that combine different polymers for specific properties, or they incorporate specialized additives.