The journey of plastic, from the recycling bin to becoming a newly printed film, is a complex process involving collection, sorting, and reprocessing stages.
The use of post-consumer recycled (PCR) plastics in packaging marks a significant step toward achieving a circular economy. However, this choice introduces considerable technical challenges for the printing industry, requiring targeted engineering solutions to ensure high performance and quality.
The journey of post-consumer plastic: a multi-stage process
The plastic waste cycle begins with separate collection, an essential preliminary step for efficient recycling. Once collected, materials are sent to sorting facilities (MRF – Material Recovery Facility), where they undergo key operations:
- Sorting: This is a critical and complex phase. Manual and automated processes separate plastics from other recyclable materials (glass, paper, metals). Advanced technologies such as near-infrared (NIR) spectroscopy enable polymer classification (PET, HDPE, LDPE), and in some cases, even color sorting. In certain collection models, consumers actively contribute to pre-sorting at home.
- Shredding and Washing: Selected plastics are shredded into flakes to increase surface area and facilitate the removal of contaminants (organic residues, labels, adhesives) through intensive washing.
- Melting and Pelletizing: Clean flakes are melted, extruded into filaments, and then cut into pellets. These pellets become the new raw material, ready for transformation into plastic products, including blown films for packaging.
Challenges of printing on PCR plastic films
Printing on films made from PCR materials is significantly more complex than on virgin substrates due to chemical and physical alterations caused by the recycling process. Key issues include:
- Surface inconsistencies and residual contaminants: Residual contaminants (inks, adhesives, organic substances) compromise surface regularity and ink adhesion. Additionally, gelled particles from ink degradation during extrusion can cause surface defects.
- Color shifts and hazing: PCR materials often exhibit a slight yellowish or grayish tint due to the mix of colored plastics and thermal degradation during recycling. This discoloration can affect the appearance of printed colors, making it difficult to achieve brand-specific tones. Films may also show increased hazing, reducing transparency and vibrancy of printed graphics.
- Non-uniform material quality: The quality and consistency of PCR films can vary significantly depending on source and recycling process, affecting critical parameters such as thickness, surface tension, and uniformity, with direct impacts on printability and final product performance.
- Ink adhesion issues: The surface energy of PCR films is often lower and less uniform than virgin materials, hindering proper ink wetting and adhesion. This can lead to pinholes, uneven coverage, and poor abrasion resistance.
Engineering solutions for printing on PCR Films
To address these challenges, the industry has developed targeted technical solutions and established best practices:
- Advanced surface treatments: Corona or plasma treatments, applied with greater intensity and precision than on virgin films, increase surface energy and improve ink adhesion.
- Specialized ink formulations: Inks specifically designed for PCR substrates feature high-concentration pigments to compensate for color variations and additives to optimize adhesion and durability. PVC-free formulations are preferred to avoid issues in subsequent recycling.
- Advanced printing and control technologies: Modern flexographic printing machines offer the control needed for printing on PCR films.
- Automatic register and pressure control: Real-time micro-adjustments compensate for variations in film thickness and tension, ensuring consistent, high-quality printing.
- High-definition plates: Using high-resolution plates allows for finer, more controlled ink application, helping to mitigate surface imperfections in PCR films.
- Advanced drying and curing systems: Efficient drying and curing systems, such as UV-LED, rapidly and effectively fix inks on challenging substrates, enhancing adhesion and preventing smudging.
- Enhanced quality control: Inline spectrophotometers and vision systems enable continuous monitoring of color output and timely defect detection, ensuring high quality standards.
Recycling post-consumer plastics presents a significant technical challenge, but also a strategic opportunity for the packaging sector.
Thanks to innovations in materials, sophisticated printing technologies, and rigorous process controls, it is now possible to transform plastic waste into functional, high-quality printed films, contributing concretely to closing the plastic loop in a circular economy perspective.
Written by Pino M. | Team Giugni®
