Introduction
Surface treatment is one of the most critical steps in BOPP film production. Untreated BOPP film has a low surface energy of approximately 30-31 mN/m, which is insufficient for effective ink adhesion, coating application, or adhesive bonding. Surface treatment technologies modify the film surface to achieve higher surface energy levels, enabling the film to perform reliably in downstream converting processes including printing, laminating, and coating. Understanding these technologies is essential for anyone specifying, purchasing, or converting BOPP film.
Key Principle: Surface treatment does not change the bulk properties of BOPP film. It only modifies the top 5-50 nanometers of the surface by introducing polar functional groups (primarily -OH, -COOH, and C=O) that increase surface energy and improve wettability.
1. Corona Treatment
Corona treatment is by far the most widely used surface treatment method for BOPP film, applied to over 95% of commercially produced BOPP films. It is valued for its simplicity, speed, cost-effectiveness, and ability to treat film in-line at production speeds exceeding 400 m/min.
1.1 Principle of Operation
Corona treatment works by exposing the film surface to a high-voltage electrical discharge (corona) in the air gap between a dielectric-covered electrode and a grounded roll. The electrical discharge ionizes the surrounding air, generating a plasma of free electrons, ions, and excited molecules. These reactive species bombard the polypropylene surface, breaking C-H and C-C bonds and creating free radicals. These radicals react with atmospheric oxygen to form polar functional groups on the film surface.
1.2 Key Process Parameters
The effectiveness of corona treatment depends on several controllable parameters:
| Parameter | Typical Range | Effect on Treatment |
|---|---|---|
| Treatment Power | 1-8 kW per meter of width | Higher power = higher dyne level, but excessive power causes pinholes and backside treatment |
| Line Speed | 100-500 m/min | Higher speed reduces treatment dose (power/speed ratio) |
| Air Gap | 1-3 mm | Narrower gap = more uniform treatment, but risk of arcing |
| Electrode Type | Ceramic, silicone rubber | Ceramic electrodes provide more uniform and durable treatment |
| Treatment Side | One-side or two-side | Most BOPP films are treated on one side only (printing side) |
| Film Thickness | 12-60 micron | Thinner films require lower power to achieve same dyne level |
1.3 Treatment Level (Dyne Level)
The dyne level, measured in dynes/cm (or mN/m), quantifies the surface energy of the treated film. For BOPP film, the target dyne level depends on the intended application:
- Basic printing (flexo/gravure): β₯ 38 dynes/cm
- High-quality printing with UV inks: β₯ 40 dynes/cm
- Lamination and adhesive bonding: β₯ 42 dynes/cm
- Coating applications: β₯ 44 dynes/cm
1.4 Treatment Decay (Aging)
Corona treatment is not permanent. The surface energy of treated BOPP film gradually decreases over time due to the reorientation of polar groups away from the surface and the migration of low-molecular-weight additives (slip agents, antiblock agents) to the surface. Typical decay rates:
- Immediately after treatment: Peak dyne level (e.g., 44-46 dynes/cm)
- After 1 week: Drops 2-4 dynes/cm
- After 1 month: Drops 4-6 dynes/cm
- After 3 months: Drops 6-10 dynes/cm
- After 6 months: May fall below functional threshold
Production Tip: For films requiring a minimum dyne level of 38 dynes/cm for printing, manufacturers typically target 42-44 dynes/cm at the time of production to ensure adequate surface energy after storage and shipping. Always check the production date of BOPP film and verify dyne level before converting.
2. Plasma Treatment
Plasma treatment is an advanced surface modification technology that offers several advantages over corona treatment, particularly for specialty and high-value BOPP film applications.
2.1 Principle of Operation
Unlike corona treatment (which occurs at atmospheric pressure), plasma treatment typically operates under vacuum or low-pressure conditions. A gas (oxygen, air, nitrogen, or argon) is ionized to create a plasma containing energetic ions, electrons, and neutral species. These species react with the polymer surface more uniformly and controllably than corona discharge.
2.2 Types of Plasma Treatment
| Type | Pressure | Advantages | Limitations |
|---|---|---|---|
| Low-pressure plasma | 0.1-1 mbar | Highly uniform, precise control, multiple gas options | Batch process, slower, higher cost |
| Atmospheric plasma (APGD) | Ambient | In-line capable, no vacuum needed | Less uniform than low-pressure, newer technology |
| Dielectric Barrier Discharge (DBD) | Ambient | Similar to corona but with controlled discharge | Equipment cost higher than corona |
2.3 Advantages Over Corona Treatment
- More uniform treatment: Plasma provides consistent surface modification across the entire film width without edge effects
- Greater depth control: Treatment can be precisely controlled to modify only the surface layer without affecting bulk properties
- Longer-lasting effect: Plasma-treated surfaces typically show slower decay rates than corona-treated surfaces
- Functional group versatility: Different gases can be used to create specific surface chemistries (e.g., fluorine plasma for oleophobic surfaces)
- No backside treatment: Plasma can be directed precisely to one side, eliminating backside treatment issues common with corona
3. Flame Treatment
Flame treatment is one of the oldest surface treatment methods, primarily used for thicker films, sheets, and molded polypropylene products. For thin BOPP film (below 30 micron), flame treatment is rarely used due to the risk of heat distortion.
3.1 Principle
The film surface is passed through an oxidizing flame at controlled speed and distance. The flame's free radicals and excited oxygen species oxidize the surface, introducing polar functional groups similar to corona treatment.
3.2 Process Parameters
- Flame temperature: 1,000-1,800Β°C (at the combustion zone)
- Film-to-flame distance: 5-15 mm
- Line speed: Typically slower than corona (50-200 m/min for film)
- Fuel type: Natural gas, propane, or butane with excess air
4. Chemical Treatment
Chemical treatment involves applying chemical primers or coating solutions to the BOPP film surface to improve adhesion. While less common than corona treatment for standard applications, chemical treatment plays an important role in specialty BOPP films.
4.1 Primer Coatings
Polyurethane-based or acrylic-based primers are applied via gravure coating to create an adhesion-promoting layer between the BOPP film and subsequent printing inks or adhesives. Primers are particularly useful when:
- Extremely high adhesion is required (e.g., for metallic foil lamination)
- Long shelf life of treated film is needed (primed films maintain adhesion properties for 12+ months)
- The film will be exposed to harsh environments (moisture, heat, chemicals)
4.2 Online Coating Treatments
Some advanced BOPP films incorporate surface treatment during the manufacturing process itself, such as:
- Co-extruded tie layers: A thin co-extruded layer of PP copolymer or functional polymer provides inherent printability without post-treatment
- Aqueous acrylic coatings: Applied inline during film production for enhanced printability and sealability
5. Measuring Surface Energy
Accurate measurement of surface energy is essential for quality control and process optimization. The two most common methods are:
5.1 Dyne Pen / Dyne Solution Method (ASTM D2578)
Dyne pens contain solutions of known surface tension applied to the film surface. The dyne level is determined by the highest-tension solution that forms a continuous film (wets the surface) for at least 2 seconds without beading. This is the most common field method due to its simplicity and speed.
5.2 Contact Angle Measurement (ASTM D7334)
A goniometer measures the angle between a liquid droplet and the film surface. Lower contact angles indicate higher surface energy. Water and diiodomethane are used as test liquids to calculate total surface energy and its polar/dispersive components using the Owens-Wendt method. This laboratory method provides more detailed information than dyne pens.
6. Treatment and Printability
The relationship between surface treatment and printability is direct but nuanced:
- Minimum threshold: Below 36 dynes/cm, most printing inks exhibit poor adhesion and may delaminate
- Optimal range: 38-44 dynes/cm provides reliable adhesion for standard flexographic and gravure inks
- Over-treatment risk: Excessive treatment (>46 dynes/cm) can cause the film surface to become too polar, leading to blocking (film layers sticking together), reduced slip properties, and poor heat seal performance
- Ink compatibility: Match the ink system to the treatment level β solvent-based inks generally require lower surface energy than water-based or UV-cured inks
Quality Control Rule: Always measure dyne level at three positions across the film width (left edge, center, right edge) and document the results. A variation of more than 3 dynes/cm across the width indicates uneven treatment and may result in inconsistent print quality.
SHUNZHAN: Precision Surface Treatment on Every Roll
Our state-of-the-art corona treatment systems with ceramic electrodes ensure consistent dyne levels across every roll. We offer custom treatment levels from 38 to 46 dynes/cm to match your specific converting requirements.
Request Custom Treatment Specifications