Low-temperature plasma — specifically atmospheric-pressure uniform glow discharge — has emerged as one of the most effective technologies for modifying polymer film surfaces before printing, coating, or vacuum metallization. Unlike corona discharge, which is the industry standard but has well-known limitations, atmospheric glow discharge delivers higher surface energy, more uniform treatment, and no reverse-side effect. Here is how the technology works and why it is gaining traction in packaging converting.
What Is Atmospheric Glow Discharge Plasma?
Plasma is the fourth state of matter — an ionized gas containing free electrons, ions, neutral atoms, and excited molecular species. In low-temperature plasma, the electron temperature can reach thousands of degrees Celsius, but the gas temperature remains near ambient — typically 30–70°C at atmospheric pressure. This means the plasma can activate polymer surfaces through energetic electron bombardment and chemical reactions without melting, distorting, or degrading the bulk material.
Electrons in the plasma gain energy from the applied electric field. These energetic electrons collide with surrounding gas atoms and molecules, generating a cascade of excited species — free radicals, metastable atoms, ultraviolet photons — that are chemically reactive. When these species contact a polymer surface, they initiate oxidation, crosslinking, grafting, and functional group formation reactions that fundamentally change the surface chemistry without affecting the bulk properties.
The bonding energies of most polymer molecules are in the range of several to over ten electron volts. The reactive species in glow-discharge plasma carry energies well above these bond strengths, enabling them to break existing bonds and form new functional groups — primarily carbonyl (C=O), carboxyl (COOH), and hydroxyl (OH) groups — on the film surface.
Why This Matters for Packaging Film Converting
Polyolefin films — PP, BOPP, PE, CPP — and polyester films like PET are inherently low-surface-energy materials. Their untreated surface tension is typically 28–34 dynes/cm. For water-based inks, water-based coatings, and adhesive lamination to bond reliably, the surface tension must be above 38 dynes/cm — preferably above 42 dynes/cm for demanding applications.
Corona discharge, the incumbent surface treatment technology, raises PET film surface tension to approximately 45 dynes/cm. It has three structural disadvantages:
- Limited peak performance: 45 dynes/cm is often insufficient for water-based ink adhesion on demanding print jobs and for ultra-thin coating layers (a few microns) that require maximum wet-out.
- Non-uniform treatment: Corona discharge forms filamentary micro-discharges that are distributed unevenly across the film width, producing spotty treatment at high line speeds.
- Reverse-side effect: Corona penetrates thin films and treats the reverse side, which can reduce the coefficient of friction and cause winding or converting problems. Treated film also ages — surface energy decays over days to weeks after corona treatment, requiring just-in-time processing.
Atmospheric uniform glow discharge plasma addresses these limitations. The discharge is truly uniform across the electrode width — no filamentation, no micro-arcing. Surface tension on PET can reach 72 dynes/cm or higher — a level that allows water-based coatings and inks to wet out completely, producing defect-free thin coatings in the micron range. Treatment is confined to the exposed surface only, with no reverse-side effect. Treated film retains its surface energy longer than corona-treated film, extending the shelf life of treated rolls.
Application: Vacuum Metallization Pretreatment
Vacuum metallization — the deposition of a thin aluminum layer onto polymer film to create metallized film for barrier packaging — places extreme demands on surface quality. Before entering the vacuum chamber, the film must be pretreated to maximize adhesion between the aluminum layer and the polymer substrate. The conventional pretreatment is corona discharge.
Replacing corona with atmospheric glow discharge plasma produces measurably better metallized film. Adhesion between the aluminum layer and the substrate improves — peel strength values are higher and more consistent across the web. The uniformity of the plasma treatment translates into uniform aluminum deposition and consistent optical density. CPP, BOPP, and PET films pretreated with glow discharge plasma produce metallized films with higher bond levels, better barrier properties, and fewer pinholes than corona-pretreated equivalents.
The plasma treatment unit operates at atmospheric pressure (no vacuum chamber required), can be installed inline on the metallizer or the coater, and consumes no more energy than a corona treater. Shanghai Zhijing’s One Atmosphere Uniform Glow Discharge Plasma system achieves international-standard performance and fills a gap that previously existed in Chinese atmospheric-pressure glow discharge technology.
Summary of Advantages
Atmospheric uniform glow discharge plasma offers six advantages over corona for polymer film surface treatment in packaging converting:
- Dry process — no chemicals, no waste, no drying energy
- Short treatment time at high line speed
- No substrate geometry restrictions — works on films, nonwovens, and 3D shapes
- Excellent treatment uniformity regardless of film width
- Low gas temperature — safe for heat-sensitive materials
- Surface-only modification — bulk mechanical, optical, and barrier properties are unaffected
As the packaging industry shifts toward water-based inks and coatings to reduce VOC emissions, the surface energy and wettability limitations of conventional corona treatment become increasingly binding on production capability. Atmospheric glow discharge plasma is the enabling technology that removes this constraint.
References
- Wikipedia: Plasma: Fourth state of matter, plasma classification by temperature and pressure, and industrial plasma applications.
- Wikipedia: Glow Discharge: Principles of glow discharge physics, including atmospheric pressure operation and comparison with corona and dielectric barrier discharge.
- Wikipedia: Corona Discharge: Technical description of corona treatment for polymer films — mechanism, limitations, and comparison with plasma treatment.
- Wikipedia: Surface Energy: The physical chemistry of surface energy, contact angle, and wettability as applied to polymer film printing and coating.
- Wikipedia: Vacuum Metallization: Process description of aluminum deposition on polymer films for barrier packaging, including pretreatment requirements.