How to Achieve High Adhesion on Polyethylene and Polypropylene

Polyethylene (PE) and Polypropylene (PP) are widely used in flexible films, bottles, caps, pouches, and food packaging. However, these materials have extremely low surface energy, which makes ink adhesion a challenge for inkjet printing. Poor adhesion can result in codes rubbing off, fading, or disappearing during shipping. This article provides proven strategies to improve ink bonding and achieve reliable, long-lasting marks on PE and PP surfaces.

Why Are PE and PP Difficult to Print On?

• Low surface energy (LSE): PE/PP typically have 30–34 dynes/cm, making it hard for ink to “wet” the surface.
• Slick, non-absorbent texture: Ink tends to bead or smear.
• Resistance to solvents: Many inks fail to penetrate or adhere chemically.
• Used in flexible or curved packaging: Increases risk of cracking or rubbing off the print.

Top Methods to Improve Ink Adhesion

1. Surface Treatment

• Corona Treatment: Adds polarity to plastic surface using high-voltage discharge. Widely used inline before printing or lamination.
• Plasma Treatment: Similar to corona but more precise and effective on complex shapes.
• Flame Treatment: Applies high-temperature flame briefly to oxidize the surface and increase wettability.

2. Use Specialized Inks

• Choose MEK-based solvent inks designed specifically for low surface energy plastics
• UV-curable pigmented inks offer excellent adhesion and resistance on treated PE/PP
• Some manufacturers offer PE/PP-formulated thermal inkjet inks—always test before deployment

3. Optimize Printer Settings

• Increase throw distance slightly to reduce ink “splashing” or blotting
• Use bold fonts or high-contrast dot matrix for durability
• Adjust DPI to balance ink deposit vs. drying speed

4. Reduce Environmental Risks

• Minimize exposure to oil, dust, static, or moisture before printing
• Ensure packaging is cool and dry—hot plastic can reduce ink bonding effectiveness

Recommended Equipment for PE/PP Coding

• CIJ (Continuous Inkjet): MEK-based ink + corona-treated surface = best combination
• UV Inkjet (DOD): For high-resolution barcodes or graphics; requires UV curing unit
• TIJ: Only use if packaging has been corona-treated and ink is certified for PE/PP
• TTO: Effective on flat PE films (e.g., snack packs) using thermal ribbon

Adhesion Test Methods

• Tape Test: Apply adhesive tape to the print, pull back, and observe if ink lifts
• Rub Test: Rub printed area with cloth or alcohol swab to test durability
• Water Resistance Test: Spray or soak in water to simulate refrigeration or condensation exposure

FAQs

• Can I print directly on untreated PE or PP? Possible, but not recommended—adhesion is weak and easily rubbed off. Use treatment or specialty ink.
• How do I know if corona treatment worked? Use a dyne pen test—treated film should exceed 38 dynes/cm for good ink bonding.

Conclusion

Printing on PE and PP requires a precise combination of surface treatment, ink chemistry, and printer setup. With the right strategy, it’s possible to achieve high-quality, durable codes even on challenging plastic substrates. For recommendations on PE/PP-compatible ink and printers, contact Cheef Inkjet at sales@cheef.cn.