Poor adhesion is a common technical challenge when printing on PVC and PET substrates. These materials have low surface energy, strong chemical inertness, and plasticizers in PVC are prone to migration, all of which make it difficult for inks to adhere firmly. Systematically solving this problem requires addressing substrate characteristics, surface treatment, ink selection, printing process, and environmental control:

I. Root Cause Analysis

1. Low Substrate Surface EnergyPVC/PET surface energy is typically below 38-40 dynes/cm, making it difficult for inks to wet and spread.

2. Surface Contamination or Additive MigrationPlasticizers, lubricants in PVC, or mold release agents in PET remaining on the surface form a weak boundary layer.

a. Absorption of oils or dust during storage or production.

3. Poor Ink CompatibilityMismatch between the polarity of the ink resin and the substrate (e.g., polar PET with non-polar ink).

a. Ink solvents cannot slightly dissolve/etch the substrate surface to form anchorage.

4. Improper Printing ProcessInsufficient drying/curing (temperature, time too low).

a. Excessive ink film thickness leading to high internal stress.

5. Environmental FactorsHigh humidity affecting solvent evaporation or UV curing efficiency.

II. Systematic Solutions

✅ 1. Substrate Surface Pretreatment (Key Step)

● Corona TreatmentApplies to: PVC/PET film rolls (gravure, flexo printing).

○ Function: Increases surface dyne level via high-voltage discharge (needs to reach >42 dynes/cm).

○ Key Point: Printing should be done within 48 hours after treatment (avoiding surface energy decay).

● Flame TreatmentApplies to: Thick PVC sheets (e.g., signs).

○ Function: Oxidizes the surface generating polar groups (-OH, -COOH).

● Chemical PrimerApplies to: Irregularly shaped or small-batch products (e.g., PET bottles).

○ Recommendations:PVC: Vinyl chloride / Polyurethane primers (enhance resistance to plasticizer migration).

■ PET: Polyester polyol or modified acrylic primers.

○ Operation: Thin coat (2-3μm) → Dry at 70°C for 1-2 minutes.

● Physical AbrasionApplies to: Rigid PVC sheets (e.g., credit cards).

○ Method: Slight roughening with 800-1200 grit sandpaper (avoid excessive damage).

✅ 2. Ink Selection and Adjustment

● Solvent-based Inks: Add 5-8% Cyclohexanone/MEK (slightly etches PET surface).

● UV Inks: Select formulas containing adhesion-promoting monomers for PET (e.g., Caprolactone acrylate).

● Water-based Inks: Require matching water-based primers (e.g., cationic corona treatment agents).

✅ 3. Process Optimization

● Drying/Curing Control:PVC: 60-70°C hot air drying → Avoids rapid plasticizer migration.

○ PET: 80-90°C pre-drying + sufficient UV curing (energy ≥800 mJ/cm²).

● Ink Film Thickness:Control screen printing to 15-25μm, gravure printing to 8-15μm (too thick easily peels).

● Post-Printing Curing:Place PET prints in a 50°C oven for 24 hours → Promotes molecular chain penetration/interdiffusion.

✅ 4. Environment and Equipment Management

● Control workshop humidity at 40-60% → Reduces moisture’s effect on the interface.

● Clean guide rollers/printing plates → Avoids cross-contamination from release agents.

● Regularly check corona treatment intensity (dyne pen/test fluid).

III. Adhesion Quick Test Methods

1. Cross-cut Test (ASTM D3359):Score 11x11 grids (1mm spacing) → Apply and peel 3M tape → ≤5% removal is qualified.

2. Tape Peel Test:Quickly peel tape at a 180° angle → Observe ink transfer.

3. Rub Test:Fold sample and rub 50 times → No cracking means pass (essential for flexible PVC).

IV. Emergency Troubleshooting

If poor adhesion has already occurred, handle according to severity:

1. Localized Ink Lifting:Clean surface → Apply primer locally → Re-print.

2. Large-Area Failure:Remove ink with ethanol/isopropanol → Re-surface treatment → Print.

3. Late Failure due to Plasticizer Migration:Completely remove old ink layer → Use anti-migration primer (e.g., epoxy-modified) → Switch to low-migration ink.

V. Preventive Maintenance Suggestions

● Test surface dyne level of incoming materials for each batch (PVC≥42, PET≥48).

● Regularly clean corona rollers/flame treater nozzles.

● Conduct 48-hour aging tests for inks after adding adhesion promoters (simulates plasticizer migration).

Key Point Summary: PVC/PET adhesion is fundamentally an interface engineering problem.

● Rigid PET → Relies on chemical bonding (coupling agents + UV monomers).

● Flexible PVC → Resists plasticizer migration (chlorinated polyether resins + barrier primers).

● Through the triple assurance of Pretreatment + Ink Matching + Process Control, the risk of peeling can be completely resolved.