Structural and Functional Analysis of Polypropylene Films for Packaging: Construction Differences, Industrial Applications and Use of Recycled Polymers

by Marco Arezio. 

In the modern packaging landscape, flexible plastics play a central role not only for their ability to protect contents, but also for the efficiency with which they support logistics, visual communication, and product preservation.

Polypropylene-based (PP) films embody this versatility perfectly, offering an attractive balance of mechanical properties, barrier performance, visual appeal, and processability. However, defining a film simply as “PP” is an oversimplification that fails to do justice to the variety and complexity of the materials actually in use.

Among the most commonly used variants, three distinct categories stand out: non-oriented PP, CPP (Cast Polypropylene) and BOPP (Biaxially Oriented Polypropylene). Each exhibits unique characteristics due to different production processes and molecular structures, with critical impacts on performance and application areas—from food preservation to label printing.

Polypropylene (PP): Polymer Structure and Base Versatility

Polypropylene is created through the polymerization of the monomer propylene, a hydrocarbon with three carbon atoms. Its molecular chain can adopt various stereochemical configurations: isotactic, atactic, or syndiotactic. The most widely used form in the packaging sector is isotactic PP, characterized by an ordered, crystalline structure that gives the material good rigidity, transparency, and strength.

- Its fundamental properties make it ideal for use in plastic films:

- Relatively high melting point (160–165 °C), suitable for thermal applications

- Low density (0.90–0.91 g/cm³), translating into greater yield per unit weight

- Excellent heat sealability and resistance to chemical agents, particularly fats and organic solvents

- Electrical inertia, enabling its use in electronics and electrical packaging

PP can be used in its pure form (homopolymer) or modified with ethylene (random or block copolymers) to improve impact resistance, flexibility, and processability, depending on the intended application.

CPP – Cast Polypropylene: Structure, Advantages, and Limitations

CPP is produced through a flat die extrusion process, where molten polymer is cast onto a chilled surface and rapidly solidified. This process does not include molecular orientation, resulting in a relatively amorphous structure. As a result, CPP film is more flexible and easier to seal, though it is mechanically weaker than BOPP.

Key strengths of CPP include:

- Uniform thickness, useful for applications requiring precise dimensional control

- Excellent sealability, even at low temperatures, making it ideal as the inner layer in multilayer laminates

- Good transparency and gloss, useful for visually appealing packaging

- Superior flexibility, advantageous for packaging that needs to conform to irregular product shapes

However, CPP also presents some clear limitations:

- Lower tensile and puncture resistance

- Reduced dimensional stability, particularly in fluctuating temperature conditions

- Insufficient rigidity, making it less suitable for high-speed automatic packaging lines

For these reasons, CPP is often laminated with other films, such as PET or BOPP, to combine their properties and obtain more tailored and performant solutions.

BOPP – Biaxially Oriented Polypropylene: Molecular Orientation for High Performance

BOPP represents a true engineering advancement in the world of plastic films.

Thanks to these properties, BOPP stands out for:

- High tensile strength in both machine (MD) and transverse (TD) directions

- Excellent dimensional stability, essential for accuracy in printing and packaging lines

- Flat and glossy surface, improving the aesthetics and readability of packaging

- Good moisture barrier, though less effective against oxygen and aromas

- Surface treatability, allowing for metallization, acrylic coatings, or corona treatments to enhance printability and barrier properties

Applications range from the food industry (flow packs for snacks and baked goods) to cosmetics, self-adhesive labeling, and technical adhesive tapes. However, BOPP’s high rigidity and welding difficulties without surface treatment can present disadvantages in certain operating environments.

The Sustainable Evolution: Recycled Polymers in PP, CPP and BOPP Films

Within the framework of ecological transition and the circular economy, plastic films are also undergoing significant transformation. Increasingly, recycled polypropylene (rPP) is being integrated into packaging materials. However, this is far from straightforward: PP recycling poses both technical and regulatory challenges, especially for food-contact applications.

There are two main ways to obtain rPP:

- Mechanical recycling, through the sorting, washing, and regranulation of post-consumer or post-industrial waste

- Chemical recycling, through depolymerization into monomers and subsequent repolymerization into virgin-like material

From a manufacturing standpoint, CPP films are more tolerant of rPP usage, due to less stringent molecular homogeneity requirements. BOPP films, on the other hand, require a very precise molecular weight distribution to enable defect-free biaxial orientation: introducing rPP can lead to issues with strength and consistency.

Promising strategies include:

- Monomaterial projects, where only PP-based films (CPP + BOPP) are laminated together to ease recycling within homogeneous streams

- Environmental certifications (such as ISCC+, RecyClass) attesting to the traceability of recycled content

- Recycling-compatible surface treatments, such as easily removable coatings

However, one significant barrier remains: use in regulated sectors (food, pharmaceutical, cosmetic) requires traceability, sanitary certifications, and closed-loop supply chains, where post-industrial material is reused under strictly controlled conditions.

Conclusions: An Engineering and Environmental Choice

Choosing between PP, CPP and BOPP is not merely a matter of mechanical performance or visual clarity—it is a strategic design decision, taking into account multiple factors. From the nature of the product being packaged to the packaging line requirements, from environmental sustainability to regulatory compliance, the selection of the film involves a delicate balance between technical performance, production efficiency, and ecological impact.

Today—and even more so in the future—the real challenge is to combine innovation and circularity: thinner films, easily separable layers, materials with recycled content, and packaging that meets the highest standards for barrier performance, sealability, and industrial compatibility.

Only an integrated approach—combining material science, process engineering, and sustainable design—will allow the packaging industry to evolve in harmony with the planet’s needs.

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