Types of failure during peel testing of pressure-sensitive adhesives

Types of failure during peel testing of pressure-sensitive adhesives

 

　　When conducting peel tests on pressure-sensitive adhesives, the type of failure observed is just as important as the peel-test results themselves. This is because, even if the test results are identical, the performance they represent can differ significantly depending on the type of failure that occurs. During a peel-strength test on a pressure-sensitive adhesive product that has been properly applied, six different types of failure may occur, depending on where the failure takes place: interfacial adhesion failure, cohesive failure within the adhesive layer, failure at the substrate-adhesive interface, cohesive failure within the adherend, cohesive failure within the substrate, and mixed failure. Different pressure-sensitive adhesive products require different types of peel failure to be observed.

 

　　Interfacial adhesive failure occurs at the bond interface between the pressure-sensitive adhesive and the adherend; strictly speaking, it happens within the weak interfacial layer located near the bond interface, either in the adhesive itself or on the surface of the adherend. An obvious indicator of complete interfacial adhesive failure is that, after peel testing, the pressure-sensitive adhesive product separates cleanly from the adherend surface without leaving any residual adhesive visible to the naked eye on the adherend surface, nor any adherend material remaining on the adhesive layer surface. Therefore, only when complete interfacial adhesive failure occurs can the measured peel strength truly reflect the magnitude of the interfacial adhesion force. If the cohesive strength of the adhesive layer is weaker than the interfacial adhesion force, cohesive failure within the adhesive layer will occur during peel testing. After the specimen fails in peel, both the adherend surface and the substrate will visibly retain a layer of adhesive. In such cases, the test results obtained reflect solely the cohesive strength of the adhesive layer. On the other hand, if, after peel failure, the entire adhesive layer remains firmly attached to the adherend surface while no adhesive residue is visible on the substrate, this constitutes complete substrate-adhesive-layer interfacial failure. Under these circumstances, the peel test result obtained can only serve as an indication of the substrate-adhesive adhesion strength, whereas both the interfacial adhesion force and the adhesive's cohesive strength are actually greater than this value. When using packaging tapes to affix cardboard boxes or other paper products, peel failure often results in cohesive failure within the adherend—that is, the paper product is torn apart along its internal layers. In such cases, the test results can only characterize the interlayer peel strength of the paper product itself, and the peel strength of the pressure-sensitive tape used will certainly exceed this value. Similarly, when applying pressure-sensitive paper labels to objects and conducting peel tests, cohesive failure within the substrate—meaning the label paper itself is torn apart along its internal layers—is a common occurrence. In these instances, the test results can only reflect the interlayer peel strength of the label paper. If two or more of the five typical failure modes described above occur simultaneously during a single peel test, this is referred to as mixed failure.

 

　　With the exception of the few pressure-sensitive adhesive products mentioned above, which may exhibit cohesive failure of the adherend or substrate during peel testing, virtually all other pressure-sensitive adhesive products are required to show complete interfacial adhesion failure upon peeling—particularly those used for surface protection and masking, reusable pressure-sensitive labels, pressure-sensitive advertising stickers, office-use pressure-sensitive adhesive tapes, and medical pressure-sensitive adhesive products. Certain permanently adhesive pressure-sensitive adhesive products—such as pressure-sensitive tapes for pipeline protection and permanent pressure-sensitive labels—as well as high-strength double-sided pressure-sensitive adhesive products—are expected to have the highest possible peel strength and durability. During peel testing, these products often exhibit a mixed failure mode, typically involving both interfacial adhesion failure and cohesive failure within the adhesive layer. Failure at the substrate-adhesive interface is absolutely unacceptable.