Unstable adhesion after corona treatment is a common and tricky issue involving equipment, processes, materials, and the environment.
Core Concept: Unstable Adhesion = Loss of Process Parameter Control
To resolve this instability, corona treatment must be converted from a "black box" operation to a measurable, monitorable, and repeatable precision process.
Phase 1: Precise Control of Electrode and Discharge System (Source)
Electrode and discharge system control is the most common yet easily overlooked source of unstable adhesion. For electrode cleaning and maintenance, electrode surfaces (especially on silicone rollers) tend to accumulate ozone, dust, precipitated additives like slip agents, and may even be ablated by electric sparks; this contaminant layer forms an insulating barrier, leading to uneven discharge energy, "weak corona" or arcing, and compromised treatment effectiveness. To address this, strict cleaning procedures must be formulated-electrodes and ground rollers should be thoroughly cleaned with anhydrous ethanol or isopropanol before each shift or daily production, avoiding silicone or grease-containing cloths. Additionally, regular checks on electrode wear are necessary to inspect for pits, rust, or deformation, as damaged electrodes cause electric field concentration, arcing, and film surface damage, and ensuring electrode parallelism is critical to maintain a consistent gap across the entire electrode-ground roller width, as misalignment results in uneven treatment intensity. Regarding the corona treatment machine itself, equipment aging, unstable power output, and degraded high-frequency generator performance are common issues; solutions include regularly calibrating the machine's power meter to ensure displayed power matches actual output (focusing on power in watts/min/cm² rather than just voltage or current) and inspecting silicone or other dielectric rollers to ensure they are intact without cracks or local aging to prevent breakdown.
Phase 2: Standardization and Optimization of Process Parameters (Core)
Simply "turning on the corona" is insufficient, as precise control of all process parameters is mandatory. For critical process parameters (CPP), treatment power is not a case of "higher is better"-insufficient power leads to under-treatment, while over-treatment degrades surface molecular chains and reduces adhesion, so gradient experiments should be conducted to find the optimal power window for each material. Treatment speed and power are closely linked, with faster speed meaning shorter discharge time and thus higher required power; the two must be controlled in tandem to achieve a stable surface energy (dyne value) slightly above ink or adhesive requirements, such as 42-44 mN/m instead of just 38 mN/m. The electrode gap should be kept at a fixed value (e.g., 1.5-2.0mm) and not changed arbitrarily, as the gap directly affects electric field strength. In terms of environmental control, ambient temperature and humidity impact discharge characteristics-moist air conducts better, so workshop conditions should be stabilized, and power should be fine-tuned during humid seasons like the plum rain season. Meanwhile, effective cooling of electrodes and rollers must be ensured to prevent heat accumulation, which impairs treatment uniformity and equipment life.
Phase 3: In-depth Understanding and Adaptation of Materials Themselves (Fundamental)
Materials are the internal cause of adhesion issues, with corona treatment acting as the external driver, so external factors can only take effect through internal ones. Substrate surface contamination is the primary culprit of unstable adhesion: low molecular weight migrants (LLM) such as slip agents (erucamide, oleamide) in PE and antistatic agents in PP migrate to the surface during production and storage, forming an invisible "isolation film"; residues of processing aids like lubricants and mold release agents, as well as storage contamination such as dust and oil stains, also contribute. Solutions include communicating with suppliers to request low-migration or non-migration raw material grades, implementing "first-in, first-out" inventory management to minimize slip agent migration, adding online cleaning processes (e.g., plasma air knife, electrostatic brush) for films with existing migration, and performing dyne pen testing on each film roll or batch during incoming inspection to record initial values. Different substrate types and polarities respond differently to corona treatment-non-polar materials like PE and PP have a significant corona effect but fast attenuation, while polar materials like PET, PA, and PVC have inherent polarity that ensures durable treatment but may require higher energy for activation. Corresponding solutions involve targeted parameter setting (using lower power for PE/PP while monitoring the "time window" to subsequent processes, and potentially higher power for PET activation) and understanding attenuation laws by mastering dyne value attenuation curves via experiments to ensure surface energy remains above safe thresholds during lamination or printing. Additionally, mismatched ink or adhesive can undermine excellent corona treatment, so collaborative testing (conducting adhesion tests like cross-cut and tape tests with corona-treated substrates when selecting inks and adhesives) is necessary, and for difficult adhesion combinations (e.g., water-based ink on BOPP) or fast corona attenuation, applying a thin primer post-corona as a "bridge layer" can enhance adhesion stability.
Phase 4: Establish a Full-Process Quality Monitoring System (Guarantee)
The principle "no measurement, no management" underscores the importance of establishing a full-process quality monitoring system. To implement online monitoring, installing an online dyne value detector is ideal, as it enables real-time full-width monitoring of the film and alarms when values drop, which is critical for closed-loop control. Strengthening offline testing requires standard operation of performing dyne pen or solution testing at the start of each roll, during production, and when changing rolls, as well as rigorous recording and traceability-documenting supplier, batch number, treatment parameters (power/speed), pre and post-treatment dyne values, and test results for each roll, which is critical for troubleshooting. Final effect verification involves conducting regular destructive tests such as cross-cut and peel strength tests to ensure the final adhesion meets requirements.