What is corona discharge?

1. Definition & Intrinsic Physical Essence of Corona Discharge

Corona discharge belongs to incomplete partial gas breakdown discharge, emerging exclusively at localized high-field-gradient regions including sharp electrode tips and charged curved surfaces under non-uniform alternating high voltage, differing thoroughly from full air-gap arc breakdown. Driven by uneven electric field, ambient air is sequentially ionized to form nonequilibrium low-temperature plasma, accompanied by characteristic bluish glow, ozone and nitrogen oxide byproducts from oxygen/nitrogen molecular cleavage and recombination reactions.

From printing engineering perspective, corona treatment is the only mature ambient-atmosphere dry surface activation technique to resolve inherent low surface energy defect of non-absorbent polyolefin substrates (PP, PE, PET, non-woven). These non-polar plastics feature saturated C-C/C-H molecular backbone without inherent polar binding sites; raw surface tension hovers around 30–32 mN/m, far below the minimum 38–42 mN/m threshold required for water-based/UV ink wetting and stable anchoring. Corona precisely modifies merely 5–10 nm substrate surface layer without altering bulk mechanical, optical or thermal properties of base material, becoming an indispensable pre-printing procedure for flexible packaging and non-woven printing.

2. Technical Evolution Logic of Corona Discharge Technology

Corona physical phenomenon was firstly documented in late 19th century, when physicists observed faint luminescence around pointed conductors under kilovolt high voltage, yet its industrial value remained unmined until 1901, when H.W.Geitel pioneered the hypothesis of utilizing plasma oxidation to restructure solid surface molecular configuration, laying theoretical basement for subsequent surface modification engineering.

Primitive corona equipment in early 20th century adopted low-frequency alternating current with crude open electrodes, limited by unstable electric field distribution; early applications on paper printing only removed surface sizing contaminants with trivial surface energy improvement, unable to handle synthetic polymer substrates. After mid-20th century, booming plastic industry drove core equipment iteration: power source shifted from industrial 50Hz low frequency to dedicated 18–32kHz high-frequency resonant generator, insulated ceramic electrode + grounded cooling roller structure standardized discharge gap (0.5–2mm), enabling precise power density regulation against different substrate thickness and production line speed.

Post-1980s, with green printing reform and VOC restriction tightening, solvent-based chemical priming gradually faded out, accelerating corona’s cross-industry expansion into label converting, PCB coating, optical film lamination and medical-grade non-woven processing. Current mainstream closed ozone-exhaust corona units integrate real-time dyne closed-loop feedback, automatically adjusting output power to counter ambient humidity fluctuation and substrate additive migration drift, solving long-standing unstable treatment consistency pain point of traditional open-type machines.

3. Core Modification Mechanism of Corona Treatment: Triple Synergy of Physical Etching + Chemical Grafting + In-situ Surface Cleaning

Conventional simplified interpretation only attributes performance improvement to ozone oxidation, while actual substrate activation originates from three coupled plasma effects under high-frequency high-voltage ionization (5000–15000V alternating potential):

1. High-energy particle chain scission & physical micro-etching: Plasma electrons (3–20eV kinetic energy) surpass bond energy of polymer C-C/C-H bonds (~3–5eV), bombarding substrate top layer to crack macromolecular chains and generate surface free radicals; meanwhile ion spalling forms uniform microscopic concave pits (Ra=0.05~0.1μm), creating mechanical anchor grooves for ink penetration and interlocking, upgrading physical bonding strength between coating and base material.
2. Oxygen radical in-situ polar group grafting: Ionized ambient oxygen decomposes into nascent oxygen atom and ozone (strong oxidizer), combining with surface free radicals to graft high-polar functional groups including hydroxyl (-OH), carbonyl (C=O), carboxyl (-COOH). These polar groups convert original inert non-polar polyolefin surface into high-dipole surface, lifting surface tension from ~32mN/m up to 42–48mN/m and enabling hydrogen bonding & dipole interaction with polar components inside water-based/UV ink, shifting bonding from weak physical adsorption to stable chemical anchorage.
3. Contaminant decomposition cleaning: Slip agent, anti-blocking wax and low-molecular migrating additives enriched on plastic surface are cracked and oxidized into volatile CO₂/H₂O by high-energy plasma, eliminating interfacial isolation layer which would otherwise trigger ink shrinkage and de-lamination during printing.

Critical note: modification strictly confines within nanoscale skin layer, bulk polymer molecular structure stays intact; excessive power density over 2.0W·min/m² triggers deep-chain degradation, leading to substrate yellowing, pinhole and post-print brittleness failure.

4. Multi-stage Physical Process of In-air Corona Discharge

The full discharge progression divides into four sequential physical phases dominated by space charge migration and air dielectric breakdown:

Phase1: Non-uniform high-field buildup: High-frequency boosted voltage is applied between sharp metal electrode and grounded coated roller, establishing steep field gradient near electrode tip; air gap forms uneven electric field where field intensity peaks at tip edge.

Phase2: Initial electron emission & local gas ionization: When localized field exceeds air ionization threshold, field emission releases free electrons from electrode surface; accelerated electrons collide with ambient O₂/N₂ molecules to generate primary electron-ion pairs, forming discrete micro-plasma clusters around electrode tip, the prototype of corona glow.

Phase3: Space charge accumulation & discharge expansion: Positively charged ions drift toward grounded roller while electrons move oppositely, accumulating space-charge layer which distorts original electric field and further expands ionization range; numerous micro-plasma zones interconnect to form continuous visible corona discharge layer covering substrate passing through gap.

Phase4: Plasma-substrate interfacial reaction: Migratory energetic ions, free radicals and ozone molecules impact moving substrate surface simultaneously to implement triple modification mentioned above, completing surface activation within microsecond-level single pass.

5. Graded Industrial Application & Process Constraint in Printing Sector

Corona’s practical effect varies drastically across substrate categories, hence graded parameter setup is required targeting different printing scenarios, alongside two universal industry constraints: post-corona aging decay (polar groups gradually bury into substrate bulk within 72h due to molecular chain relaxation, dyne value declines continuously) and ambient humidity interference (moisture absorbs free radicals and consumes ozone, lowering treatment efficiency by 15%~30% under RH＞70%).

• Flexible plastic film (BOPP/PE packaging gravure printing): Target dyne 39–43mN/m, moderate power density to avoid over-treatment-induced thermal seal failure; core benefit eliminates water-based ink crawling, edge shrinkage and post-storage peeling defects.
• PP/PE non-woven bag printing: Porous fiber structure accelerates radical dissipation, requires 10%~15% higher power than compact film; qualified treatment solves ink bleeding and surface abrasion drop-off of finished non-woven products.
• Metal sheet pre-coating: Corona removes surface passive oxide and oily contamination, improving coating crosslink uniformity and corrosion resistance of finished painted metal.
• High-end outdoor decorative fabrics: Dual-side corona + slightly boosted treatment intensity to guarantee long-term ink weather resistance against UV aging and rain wash.

Additionally, substrates loaded with high-content slip additives need secondary corona or online double-pass treatment, as migrating lubricant continuously rebuilds low-surface-energy barrier on finished surface within short storage period.