How to Solve Insufficient Adhesion on PET Film?

Solving the problem of insufficient adhesion on PET (Polyethylene Terephthalate) film is a systematic engineering task. PET film has low surface energy, strong chemical inertness, and is smooth, making it inherently difficult to adhere to. Its surface properties must be altered through physical or chemical methods to achieve strong coating, ink, or adhesive bonding. Below is a comprehensive and effective set of solutions, listed from the most common to more specialized methods.

Core Problem-Solving Strategy Diagram

The strategies for solving PET film adhesion problems can be summarized into the following three core methods, with the choice depending on production processes and performance requirements:(Note: The original text mentions a diagram/image illustrating the three core methods:

1. Surface Treatment, 2. Material Selection, 3. Process Control. The translation reflects this concept.)

I. Surface Treatment (The Most Critical and Effective Method)

1. This is the most important step to improve adhesion, aiming to increase the surface energy of the PET film (to above 38 dyne/cm or even higher) and enhance surface micro-roughness.

Corona Treatment

a. Principle: Uses high-frequency, high-voltage discharge to ionize the air around electrodes into plasma. These active particles bombard the PET surface, introducing polar groups such as carbonyl, carboxyl, and hydroxyl groups onto its molecular chains, significantly increasing surface energy.

b. Advantages: The most economical and widely applied method, suitable for high-speed continuous web production (e.g., printing, lamination lines).

c. Key Points:Temporal Effectiveness: The treatment effect decays over time (days to weeks). Subsequent processing should be performed immediately after treatment.

i. Measurement: Use dyne pens or dyne solutions to test the treatment level, ensuring the surface energy meets the required standard (typically 38-42 dyne/cm for printing, 44-48 dyne/cm for lamination).

2. Plasma Treatment

a. Principle: Under vacuum or atmospheric pressure, process gases (e.g., oxygen, nitrogen, argon) are introduced and energized to generate highly active plasma, which performs more precise and thorough cleaning and activation of the surface.

b. Advantages: More durable and uniform effect than corona treatment, no ozone pollution, capable of treating three-dimensional structures or thinner materials.

c. Disadvantages: Higher equipment cost; vacuum plasma batch processing can be slower.

3. Flame Treatment

a. Principle: Scorches the surface with a specific gas flame. Reactive free radicals in the flame react with the PET surface, introducing oxygen-containing polar groups.

b. Application: More suitable for thicker PET sheets, preforms, or certain specific films.

4. Applying a Primer/Coating

a. Principle: Apply a dedicated primer coating onto the PET film. This primer has extremely strong bonding to the PET substrate while also providing a new, active surface that is easy to adhere to.

b. Advantages: Most reliable and durable effect, the “ultimate weapon” for solving high-difficulty adhesion problems.

c. Common Primers: Modified vinyl resins (e.g., VAGH), polyester-based primers, polyurethane-based primers, etc.

If you have technical questions or need to request samples, please contact our online engineers.

II. Material Selection (Choosing Coatings, Inks, or Adhesives Compatible with PET)

1. Even after surface treatment, choosing the wrong material can lead to failure.Use Adhesion Promoters

a. Add adhesion promoters specifically designed for PET to the coating or ink formulation. They typically contain groups that can form strong hydrogen bonds or chemical bonds with PET’s ester linkages.

b. Common Types: Organic phosphates, maleic anhydride-modified polymers, specialty polyester additives.

2. Select Dedicated Resins

a. Vinyl Resins (e.g., VAGH): One of the most classic and effective resins for improving PET adhesion, often used as a primer or blended into formulations.

b. Polyester Resins: Due to their chemical similarity to PET (both contain ester bonds), they follow the “like dissolves like” principle, offering good compatibility.

c. Specialty Polyurethane Resins: Choose grades with polar groups in the molecular chain specifically designed for PET.

3. Use Dedicated Inks and Adhesives

a. Procure inks and adhesives clearly labeled “Suitable for PET.” These products typically already contain the aforementioned promoters and resins.

III. Process Control (Ensuring Treatment Effects are Maintained)

1. Thorough Cleaning: Ensure the PET surface is free from contaminants like oils, dust, and mold release agents before treatment. Wiping with solvents like isopropyl alcohol (IPA) is effective.

2. Control Production Environment: Maintain appropriate workshop temperature and humidity. Excessive humidity can affect treatment results and adhesive wetting.

3. Ensure Complete Curing: Whether UV or thermal curing, ensure adequate energy, temperature, and time are provided for the coating or adhesive to fully cross-link and achieve final strength.

4. Testing and Monitoring:Cross-Hatch Test (ASTM D3359): The most direct adhesion test method. Score a grid on the coating, apply and pull adhesive tape, and rate adhesion based on the area removed (0 is best, 5 is worst).

a. Regular Checks with Dyne Pens: Ensure surface treatment equipment is functioning correctly.

If you have technical questions or need to request samples, please contact our online engineers.

Summary and Practical Troubleshooting Flow

1. When encountering insufficient adhesion on PET film, follow this flow for investigation and resolution:

2. Step 1: Check Surface Energy

a. Test with a dyne pen. If below 36 dyne/cm, poor adhesion is inevitable.

b. Solution: Optimize corona/plasma treatment parameters (power, speed, gap) and ensure immediate use after treatment.

3. Step 2: Check Cleanliness

a. Thoroughly clean a small area with solvent and retest adhesion. If adhesion improves, it indicates contamination in the production environment or on the film roll itself.

b. Solution: Enhance cleaning procedures and workshop management.

4. Step 3: Evaluate Material Compatibility

a. If surface energy is sufficient (>38 dyne/cm) and the surface is clean, but adhesion is still poor, the issue likely lies with the material.

b. Solution: Add an “adhesion promoter” to the coating/ink, switch to dedicated resins like vinyl resins, or directly purchase PET-dedicated inks/adhesives.

5. Step 4 (Final Solution): Use a Primer

a. If all above methods fail, or if adhesion requirements are extremely high (e.g., resistance to boiling water, retort sterilization), using a primer is essential.

b. Solution: Select a dedicated primer compatible with both PET and the topcoat material. Apply, dry, and then proceed with subsequent operations. This is the most reliable guarantee.

Through the above systematic approach, the vast majority of PET film adhesion problems can be perfectly resolved.