The Factors that Affect Proper Adhesion and Cohesion of PSA Tape
There are many factors that can impact the bond strength of pressure sensitive adhesive (PSA) tapes. When adhesion or cohesion are compromised by one or more factors—from surface texture to environmental conditions—it inevitably reduces the effectiveness and longevity of the tape, which is why choosing the right PSA tape based on the substrates and environments you are working with is critical to getting optimal performance. What impacts proper adhesion or cohesion when it comes to PSA tapes? There are certain critical elements to consider.
A Closer Look at Adhesion and Cohesion
Before we go deeper into the actual factors that impact bond strength, it’s important to define and fully understand the two major terms that define the strength of a bond when it comes to adhesive tapes: adhesion and cohesion.
Adhesion is the attraction between the adhesive and a specific substrate. When referring to adhesion in terms of PSA tape, we are referring to how well the pressure sensitive adhesive tape adheres to the substrate is being placed on.
Cohesion is the force of attraction between like substances. It’s how well the molecules of the PSA hold together and resist splitting when placed under stress.
Failed adhesion would be because the PSA tape did not adhere properly to a particular surface. Failed cohesion refers to the adhesive itself splitting due to stress.
With this in mind, we can review the factors that impact bond strength.
Surface Details
When choosing a PSA tape for any application, it’s critical to consider everything there is to know about the surface the tape will adhere to. The smallest changes in the qualities of a surface can have a large impact on the adhesion of the tape.
Surface factors that impact PSA tape adhesion include:
Surface texture: rougher or uneven surfaces require a thicker adhesive that allows it to, given enough dwell time, flow into the small crates and valleys on the surface of the material.
Surface energy: impacts the wetting (how quickly the adhesive spreads on a surface) of the adhesive. Substrates with a higher surface energy are easier to adhere to, while low surface energy materials inhibit most adhesive wet-out.
Surface contour: contoured surfaces cause internal stress on tapes that are thin or too firm; they require thicker PSA tapes that have softer adhesive that helps increase surface contact.
Forces Acting on the PSA Tape
In many applications and environments, PSA tapes are subjected to a number of external forces that attempt to break the bond between the tape and the surface. There are four common types of stress that can cause strain on either the adhesion or cohesion of PSA tape:
Tensile stress: tensile force is the pulling of a material, which creates tension and strain on the material as it tries to resist stretching or pulling away from the surface to which it is adhered. If two objects are bonded together with a piece of tape, any attempt to pull them directly apart from one another will result in tensile stress on the tape. The force is distributed equally over the entire bond area.
Shear stress: shearing refers to lateral forces that are applied to PSA tapes. These forces cause substrates to slide over one another. Shear stress is concentrated over the entire bond area.
Peel: peeling refers to stress concentrated at one edge of the bond of the PSA tape where one substrate is flexible. It is the force per unit width that’s required to peel a piece of tape from the surface.
Cleavage: cleave is the prying force exerted at one end of the bond. It is similar to peel force, except that each of the bonded substrates are rigid.
In most cases, there are two types of stress: dynamic and static. Static stress is a constant force that is applied and does not quickly change over time (e.g., a hanging weight). Dynamic stress is a force that changes over time (e.g., quickly pulling on a piece of tape repeatedly). When choosing a PSA tape for an application, it’s best to design joints that evenly distribute stress over the bond area and it’s important to understand the dynamic and static stresses the bond will experience.
Environmental Factors
Where the PSA tape needs to operate is just as important as the surface it will be adhering to. Certain solvents and chemicals, for example, can weaken adhesive tapes by causing them to swell, soften, and dissolve. Certain plasticizers or leaching agents contained in the substrates can also impact the strength of PSA tapes by causing the adhesive to become stringy and soft.
Exposure to UV light, varying temperatures, and humidity can also weaken tape over time. UV light and low temperatures can cause tapes to become brittle, while high temperatures will soften adhesives, which can make them more susceptible to shearing.
Initial Application Factors
Perhaps the most critical factor which impacts the bond strength of PSA tapes is proper initial application. Key steps include:
Surface contamination: dust, debris, loose fibers, oil, grease, etc. will all reduce the bond strength of PSA tape. So, it is vital that substrates should be thoroughly cleaned before tape is applied. Often a 50:50 mixture of isopropyl alcohol and water and a clean rag will do the trick. For very oily metal, multiple cleaning steps may be required.
Dwell time: dwell time is how long the adhesive needs in order to completely flow into the texture of the surface. The rougher the surface, the longer the dwell time needed. Dwell times can range from 24 hours to 72 hours depending on the firmness of the adhesive. It is best to allow 72 hours before testing the ultimate adhesion strength.
Application pressure: pressure on the bond accelerates the adhesive flow, helps improve surface contact, and helps to eliminate trapped air. While hand pressure can be adequate, more uniform pressure from tools like J-rollers is best practice.