Peel adhesion force is one of the most important adhesion performance reported on the technical data sheet (TDS) of pressure sensitive adhesives (PSAs). Several standard test methods are available from the following associations.
Based on the above test methods, we can always observe various fracture modes upon peeling. Very often, two HMPSAs generating the same peel forces may present completely different fracture modes. This fact reveals that two adhesives possess different rheological properties upon peeling. Below are descriptions of several possible fracture modes observed upon peeling a tape or label off a substrate.
Face stock Tear mode: This is a common fracture mode for paper labels. The tear strength of paper is normally lower than the adhesion force and cohesion strength of used adhesive.
Transfer mode or interfacial failure between face stock and adhesive. For a difficult adhered face stock, such as a low surface energy plastic, the coated adhesive may be detached from face stock and leave on the top of bonded substrate upon peeling.
Cohesive Failure mode: When the cohesive strength of an adhesive is weaker than the tear strength of face stock and adhesion force, adhesive may be split upon peeling.
Adhesive Failure mode or interfacial failure between adhesive and substrate: This is a typical or desired fracture mode for most PSA tapes. No adhesive residues are left on the bonded substrate.
Substrate Tear mode: When the cohesive strength of a bonded substrate is the weakest spot among all, substrate tear can be occurred. This fracture mode is often observed on weaker substrates, such as papers or cardboards.
Stick-Slip mode: For certain adhesive formulations, although the peeling rate is the same for the entire testing, the actual strain rate upon peeling may not be consistent because a very high elongation or deformation of the adhesive mass appears at the vicinity of its Tg area. The force at stick (peak) point is a result of the greatest elongation. On the other hand, immediately after the peak value, the test specimen is fully relaxed without any strain rate. As a result, a very low or almost no force is detected. Both peak and valley forces and the distance (wavelength, ë) between peaks should be reported for this type of fracture mode.
The above fracture modes may be observed from different adhesive formulations based on a standard test condition at room temperature. They may also be observed at different testing temperature, peeling rate, and peeling angle for the same adhesive formulation.
For more information call or email Pierce Covert,
Glue Machinery Corporation