Polypropylene is chemically inert. A plain BOPP film straight off the tenter frame has a surface energy of only 28-32 dyne/cm — far too low for printing inks, adhesive laminates, or vacuum metallization to bond reliably. Surface treatment is therefore not optional; it is a fundamental part of BOPP film engineering.
Two methods dominate the industry: corona discharge treatment and chemical primer coating (also called AC coating, or anchor coat). They work on different principles, deliver different performance profiles, and are suited to different end applications. This article explains how each works, compares their key properties, and outlines when one is preferred over the other.
1. How Corona Treatment Works
Corona treatment passes the film over a grounded roll while high-frequency, high-voltage electricity discharges across a narrow air gap between an electrode bar and the film surface. The discharge generates ozone and free radicals that oxidize the PP surface, introducing polar functional groups — primarily carbonyl (C=O) and hydroxyl (C-OH) — within microseconds.
This oxidation raises the surface free energy to 38-44 dyne/cm for standard printing grades, or up to 50+ dyne/cm for films destined for metallization. The treatment is fast (inline, at production speed), adds no weight, and requires no drying oven — making it highly cost-efficient.
The critical limitation is aging: corona-activated surfaces gradually lose energy over days to weeks as the polar groups migrate inward or are contaminated by airborne hydrocarbons. Most BOPP rolls must be used within 3-6 months of production and must be stored away from heat, humidity, and UV light to preserve dyne values.
2. How Chemical Primer Coating (AC) Works
An anchor coat (AC) is a thin functional polymer layer — typically polyurethane (PU), polyethyleneimine (PEI), or an acrylic copolymer — applied to the film either inline (between MDO and TDO, then co-stretched) or offline on a separate coating station.
Unlike corona, which modifies the existing surface, an AC coating physically deposits a new layer (0.05-0.5 um dry thickness) that acts as a chemical bridge between the non-polar PP substrate and the polar materials applied later, such as printing inks, adhesives, PVDC barrier coatings, or vacuum-deposited aluminum.
Because the functional groups are locked into a solid polymer matrix rather than surface radicals, AC-treated films are dramatically more shelf-stable — typically 12-18 months — and perform consistently even after prolonged storage, temperature cycling, or exposure to mild humidity.
3. Corona vs. AC Coating: Side-by-Side Comparison
4. The Science Behind Surface Energy
Surface energy (measured in dyne/cm or mN/m) determines wettability — whether a liquid ink or adhesive will spread and bond to a film surface or bead up and repel. The general rule: the surface energy of the film must be at least 10 dyne/cm higher than the surface tension of the liquid to be applied.
Standard offset and flexographic printing inks have surface tensions of 28-36 dyne/cm, so a corona-treated film at 38-40 dyne/cm is typically sufficient. However, water-based adhesives and vacuum metallization aluminum layers require 42-50+ dyne/cm — at which point AC coating provides a more reliable and lasting anchor.
For PVDC barrier coating — where the PVDC emulsion must bond intimately to the PP substrate to form a continuous, pinhole-free barrier layer — AC primer is almost universally required. Even a brief lapse in corona activity during production can create weak spots; a deposited AC layer provides consistent adhesion across the entire roll.
5. Application Guide: Which Treatment for Which End Use?
6. Inline vs. Offline AC Coating
AC coating can be integrated into the BOPP production line or carried out separately. Each approach has distinct implications for coating uniformity, flexibility, and cost.
Inline coating (between MDO and TDO) subjects the wet coating to the full TDO stretching cycle, which co-stretches the coating along with the film. This produces extremely thin, highly uniform coatings (0.05-0.2 um) with outstanding adhesion — the coating is mechanically interlocked into the film surface during stretching. Inline coating is more capital-intensive and limits formula flexibility, since the coating must tolerate oven temperatures of 155-175 C.
Offline coating uses a dedicated coating line (gravure, reverse gravure, or slot-die) after film production. It allows thicker coatings (0.2-0.5+ um), more complex formulations, and easy grade switching without interrupting film production. This is the preferred route for specialty functional coatings such as anti-fog, anti-fingerprint, or PVDC barrier layers.
7. Quick Reference: Surface Treatment Parameters
Conclusion
Corona treatment and AC primer coating are complementary technologies, not competitors. Corona is the workhorse — fast, cheap, and effective for the majority of printing and standard lamination applications. AC coating is the precision tool — deployed when shelf stability, high-barrier performance, metallization adhesion, or demanding lamination chemistry requires a more durable and chemically tailored surface.
Most high-performance BOPP grades on the market today use both: corona treatment as the baseline surface activation step during production, followed by offline AC coating for specialty functional requirements. Understanding which layer does what — and when each is needed — allows packaging engineers to specify films with confidence and avoid costly adhesion or printability failures downstream.
Shunzhan New Materials (szbopp.com) supplies both corona-treated and AC-coated BOPP grades across 13 product lines, including anti-fingerprint, anti-fog, matte soft-touch, and PVDC-coated films. Contact us to discuss surface treatment specifications for your specific application.