Understanding the Forces That Affect PSA Tapes
The very nature of adhesive bonding is to resist the forces and stressors induced by the use in the application. While heat, UV light, and many other environmental factors can impact the performance of a pressure sensitive adhesive (PSA) tape, some of the most common contributors to adhesive failure are the forces placed on the tape bond-line. When choosing a PSA tape for your application, consider the loads, friction, vibrations, and other sources of stress that can cause the tape to fail.
These are some of the most common forces placed on a tape.
Dynamic vs Static Loads
Simply put, loads are linear forces placed on adhesive tapes. Static loads are non-changing forces that act on a piece of tape, creating a consistent force that adds stress to the tape. Think of attaching a weight to the ceiling with a piece of tape; the force pulling on the tape is a static load.
Dynamic loads are time variant; they change in intensity over time. The load can increase or decrease in magnitude or direction, similar to a person repeatedly pulling straight downward on a metal I-beam that is attached to the ceiling using a piece of tape.
Adhesion vs. Cohesion
Adhesion and cohesion refer to the strength of the bond and the adhesive itself. Adhesion is the ability of the tape to stick to the substrate. Cohesion is the internal strength of the adhesive itself. If the bond breaks because of adhesive failure, the result would be the tape pulling away cleanly from the substrate. If the bond breaks because of the result of a failure of cohesion, the adhesive itself would pull itself apart (you would normally see some adhesive still left on both substrates, as if the adhesive split in two).
Both adhesive and cohesive failure can be the result of various joint stresses, namely cleavage, shear, peel, and tensile (more discussed on these below). Bonds with excellent adhesion, that are designed for cohesive failure with tapes that have strong cohesive strength, provide the strongest bonds.
Tensile Stress
Tensile Stress
Tensile stress is the force that is exerted and distributed equally over the entire joint, pulling in a straight, in-plane direction away from the adhesive bond. This force can be seen in either static, dynamic, or both.
Tensile strength is tested by pulling on the ends of the tape, measuring the force it takes to break the backing material, or pulling linearly on the two bonded materials to see if the tape or adhesive fails from the substrate. Tensile strength is measured in pounds per inch of tape.
Shear Stress
Shear Stress
Shear is a force directed across the adhesive, forcing the substrates to slide over one another. The force is in-plane and is distributed across the entire area of the bond line. The loads that can be placed on the tape and cause shearing can be static or dynamic (as noted above), resulting in both static and dynamic shear strengths measured within a specific adhesive film.
Since shear is the pull that is directed across the adhesive, it’s static holding power is measured by placing a piece of adhesive tape on a surface and then hanging a weight on the tape so that it pulls the tape downward, a force that is parallel to the surface to which the tape is adhered. To test the dynamic shear of a double-sided piece of tape, the tape is attached to two separate plates with a force applied in opposite directions. The force that is placed on the tape is once again distributed over the entire bond line.
Cleavage Stress
Stiffer tapes with higher shear values are also generally better at withstanding higher temperatures and chemical exposure.
Cleavage Stress
Cleavage is the stress placed on the edge of the tape’s joint. It is a prying force that is placed on the edge of the bond, creating stress only on one side of the adhesive joint and forcing it to open. The other side of the bond remains under no stress. Cleavage occurs when two rigid substrates are being bonded.
Peel Stress
Peel is like cleavage stress in that the force takes place at one edge of the bond. Peel is a pull that is also concentrated at one edge of the joint. The difference from cleavage stress is that one or both substrates must be flexible. This results in a highly concentrated load at the point of stress.
Peel Stress
180-degree and 90-degree peel tests are very common for pressure sensitive adhesives. They provide a key measurement of both cohesive tape strength and adhesive strength. Results are provided as the pound force per inch of width. This value highlights the pinpointed nature of stress.
Designing the Right Adhesive Joint for Any Application
Certainly, selecting the right tape for the application is a critical part of joint design. A great rundown of tape selection is available in our article about choosing a double-sided tape. Ensuring that the tape provides the optimum adhesion to the substates and has enough cohesive strength for the application is critical. In most permanent tape applications, the goal of adhesion should be that adhesion to the substrates out-performs the cohesive strength of the tape. In other words, if the bond fails, the goal should be that the tape fails cohesively.
The best joint design looks to ensure that bond lines only see shear and/or tensile stresses because these forces distribute loads across the entire adhesive area. Meanwhile, it is important to limit cleavage and peel stresses that the bond joint may experience since these are more likely to lead to either adhesive or cohesive failure. By limiting the stress factors and ensuring substrate adhesion, tapes may be selected with enough cohesive strength to withstand the specific static and dynamic stress of the bond.
Obviously, in addition to stress types, it’s also important to consider the dimensions of the bond. Since adhesive strength improves as bond area increases, the bond dimensions must be enough to withstand the loads placed on them.
Lastly, time is an important variable to consider. Will the bond immediately see stresses requiring a secure bond need to be formed immediately? Can the bond be built up and strengthened over time, since bond stress will not be seen for days or weeks? Bonds that need to be formed immediately require adhesive tapes that are high in tack or require additional surface treatment to ensure faster wet out. For bonds that require high overall strength for long periods of time it is probable that firm acrylic adhesive tapes are needed, like 3MTM VHB (very high bonding) tapes or high performance double-sided and adhesive transfer tapes.