How to Solve the Challenge of Corona Breakdown in EVOH Barrier Films
In the world of premium food and pharmaceutical packaging, Evoh (ethylene vinyl alcohol copolymer) films are prized for their exceptional barrier performance. However, the corona treatment process—necessary to improve ink adhesion—introduces a serious risk: corona breakdown. Invisible pinholes caused by excessive discharge act like micro-cracks in a vault, instantly compromising the film’s oxygen barrier and rendering the entire roll unusable, with potentially severe economic consequences.
1.Why is EVOH Film Especially Vulnerable to Corona Breakdown?
Corona breakdown occurs when localized electric fields exceed the dielectric strength of the material. EVOH polymer's molecular characteristics make it more sensitive to this risk than many common packaging plastics.
1.1 High Molecular Polarity
The hydroxyl groups in EVOH give it inherently high surface polarity and hydrophilicity. During corona treatment, these features contribute to a complex dielectric response. Unlike non-polar polyolefins (like PE or PP), EVOH plastic is more prone to polarization under high voltage. If standard polyolefin treatment settings are applied to EVOH, the critical energy threshold can easily be surpassed, triggering localized breakdown.
1.2 Multilayer Structures with Thin Barrier Layers
In co-extruded multilayer films (e.g., PE/Tie/EVOH/Tie/PE), the dielectric constants differ across layers, resulting in uneven electric field distribution. If the EVOH layer is particularly thin (3–5 microns), it can become the electrical weak point. Even when overall power settings appear within safe limits, localized concentration of energy may still occur in the EVOH layer, leading to failure.
1.3 Resin Grade Sensitivity
Different EVOH resin grades vary significantly in dielectric behavior and processing tolerances. For example, ultra-high barrier grades with ethylene content below 32 mol% tend to be more rigid and crystalline, and are thus more susceptible to energy shocks from corona treatment. These grades require tighter control over treatment conditions.
2.Building a Stable and Safe Corona Treatment System for EVOH
2.1 Equipment Maintenance and Precision
The condition of electrodes, including their cleanliness and flatness, and the quality of grounding rollers—roundness, hardness, surface finish—are essential to ensure uniform discharge. Minor defects in these components can distort the electric field, creating local hotspots that trigger breakdown.
2.2 Optimizing Process Parameters with Data Control
Excessive power application is a leading cause of breakdown. Surface energy should meet adhesion requirements—more is not necessarily better. Use corona systems equipped with Automatic Compensation Power (ACP) and real-time arc suppression. ACP dynamically adjusts power in line with web speed to maintain constant energy per square meter. Arc suppression systems can detect and cut off abnormal discharges within microseconds, effectively preventing film damage.
2.3 Incoming Film Quality Control
Uniform film thickness acts as a natural safeguard. Persistent thin spots are weak points susceptible to breakdown. Real-time thickness monitoring across the film web is essential. Surface cleanliness is also critical—contaminants like additives or dust can significantly lower local dielectric strength.
3.Frequently Asked Questions (FAQ)
Q: Is there a reliable way to detect corona-induced damage during production?
A: Yes. Install inline visual inspection systems or full-width pinhole detectors. These tools continuously scan the film and immediately flag any micro-defects, offering 100% inspection coverage. This is especially important in pharmaceutical packaging lines with zero-defect requirements.
Q: What precautions should be taken when treating bio-based EVOH films?
A: Bio-based content may alter the crystallinity and dielectric behavior of the material. It’s essential to start with low-power test runs and validate the process window through small-batch trials.
Q: How can we balance surface energy needs and corona risks for ultra-thin EVOH layers?
A: In addition to adopting pulse-type corona systems, maintaining precise thickness control of the base film is key. Slower line speeds can also ensure more even energy distribution during treatment.
Corona breakdown is a serious challenge for EVOH film, but it is one that can be effectively managed. By understanding the material’s intrinsic properties and implementing data-driven, system-level quality control—from resin selection to real-time process monitoring—manufacturers can minimize risk and ensure reliable barrier performance in even the most demanding applications.