Analysis Methods for Poor Peel Strength Issues



In the performance evaluation of laminated packaging films, peel strength is one of the most critical indicators. During the processing and application of laminated films, "poor peel strength" is a frequently encountered problem.

Through summarization and induction, feedback regarding "poor peel strength" from customers and downstream users can be categorized into two types:

  1. "Below Design Value" Type: The measured peel strength is lower than the designed or planned value.

  2. "Unsatisfied Application Requirements" Type: The peel strength is inferred to be non-compliant based on other observed phenomena.

I. Essential Knowledge and Skills Before Analysis

Prior to analyzing poor peel strength, one should master the following:

  1. Orientation Identification: Correctly judging the relationship between the length/width of the pouch and the machine direction (MD) / transverse direction (TD) of the film.

  2. Geometric Dimensions: Including design/nominal size, measured size, and the difference (precise to 0.1mm) between the actual width at wide-knife seals versus non-wide-knife seals.

  3. Thermal Shrinkage: Rates in both MD and TD.

  4. "Secondary Tack": The concept and its testing methods.

  5. Exudation Identification: Recognizing the migration of additives and low-molecular-weight substances (oligomers).

  6. Interface Understanding: Correctly identifying the number of potential separation/failure interfaces. For PET/CPE laminates, failure can occur within the substrates, between PET and ink, between ink layers, between PET and adhesive, between ink and adhesive, within the adhesive, or between adhesive and CPE.

  7. Failure Types: Including cohesive failure of the substrate, delamination of the substrate, cohesive failure of the adhesive, and adhesive (interfacial) failure.

  8. Strain Energy Concepts: * Thermal Strain Energy: The retraction energy released by a substrate with a high shrinkage rate after cold/heat treatment.

    • Elastic Strain Energy (Type A): Energy released by a substrate with a high elastic strain rate during lamination when external force is removed.

    • Elastic Strain Energy (Type B): Retraction energy in Al/Plastic laminates released during post-processing by a substrate that underwent significant plastic strain but retained some rebound capacity.

  9. Curling Status: The direction and degree of curling (heat-sealed edges must be trimmed first).

  10. Surface Tension: Actual wetting tension values required for substrates, ink layers, and adhesive layers.

  11. Adhesive Distribution: Determining which side the adhesive adheres to after separation (using eight identification methods).

    Note: High peel strength is typically only achieved during cohesive failure of the adhesive. Other types (adhesive failure, substrate delamination, or substrate/ink cohesive failure) result in relatively lower peel strength.

II. Analysis Methods for Causes of Poor Peel Strength

The core logic is to confirm the failure interface, the failure type, and the distribution of the adhesive on the separated surfaces.

(A) Analyzing "Below Design Value" Issues

The first step is to test the peel strength in both MD and TD.

  • While MD strength is usually lower than TD, and many companies only focus on MD, there are cases where MD exceeds TD.

  • The gap between MD and TD can range from <1N/15mm to 4-5N/15mm.

  • Action: If a customer complains, check if they are measuring MD or TD. Request data for the other direction and the "secondary tack" values for both.

Key Factor: Curling and Strain Energy

Significant differences between MD and TD peel strength are usually caused by Type A elastic strain energy or thermal strain energy, manifested as curling.

  • Curling = Shear Stress: Smaller curling diameters indicate higher shear stress and lower peel strength in that direction.

  • Tension Mismatch (Type A): If curling exists immediately after lamination and doesn't change after curing, it's due to improper tension settings.

  • Thermal Mismatch (Thermal Energy): If curling only appears after curing, the substrates have different thermal shrinkage rates at curing temperatures. Solution: Use low-temperature, long-term curing or specify stricter shrinkage requirements during procurement.

Analyzing Overall Low Values (Both MD & TD)

Identify the failure mode:

  1. Cohesive Failure (Adhesive): Analyze adhesive grade or application conditions.

    • Fully Cured (No Secondary Tack): If strength is historically low, change the adhesive. If historically high but currently low, check coating weight, mixing ratios, or solvent purity (active hydrogen).

    • Incomplete Curing (Secondary Tack present): Heat the sample at 80°C for 0.5–1h. If tack decreases, check the curing room/process. If tack remains, check the hardener ratio or solvent quality.

  2. Adhesive Failure (Interfacial): Analyze substrate surface tension or additive migration. (If adhesive stays on Substrate A, look for issues with Substrate B).

  3. Substrate/Ink Failure: Look for causes in the material properties (e.g., ink transfer, Al-layer transfer, PET oligomer migration).

(B) Analyzing "Unsatisfied Application Requirements" Issues

First, verify if the product meets national standards. If it meets standards but the customer still reports "poor peel strength," investigate the specific "adverse phenomena" reported.

Common issues often mistaken for poor peel strength:

  • Tunnels: Caused by excessive Type A elastic or thermal strain energy.

  • Seal Wrinkles/Delamination: Often caused by excessive thermal strain energy or high modulus of the outer substrate during heat sealing/retorting.

  • Test Failures (Pressure/Drop): Usually due to overly aggressive test conditions or insufficient mechanical properties of the pouch.

  • Al-Laminate "White Spots": Caused by excessive knurling depth or high modulus of the surface layer.

In many of these cases, the measured peel strength in non-affected areas is actually quite high. The root cause is often the shear stress triggered by high shrinkage of the heat-seal layer rather than "wet adhesive" or "poor glue quality."

III. Conclusion

Peel strength is a comprehensive manifestation of the material system, processing technology, and application conditions. Addressing "poor peel strength" requires systemic thinking. By accurately identifying failure modes and interfaces, companies can optimize formulations, adjust process parameters, and ultimately ensure packaging reliability and market competitiveness.

www.eufuho.com

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