Pressure Sensitive Adhesives: To Stick or Not to Stick

Written By: Greg Zyck

As most of the world sees pressure sensitive adhesives, the assumption is that if you need a tape to stick something together, go to the store or your supplier and find the first available product. While I will admit there are some good general-purpose tapes out there that will work for a period of time, not every tape or adhesive is the best alternative for every situation. Especially if you need it to work for a long period of time, say in a permanent display fixture or securing an assembly in an aircraft.  

Whether you need the tape for yourself or you are in a position to recommend a product, there are many factors that go into the successful use of adhesives. For right now, though, we are going to focus on Pressure Sensitive Adhesives (PSA) and the basic concepts of four factors:

1. Substrate being bonded to
2. Time
3. Pressure
4. Cleanliness

The one factor that is right out in front of most people when selecting pressure sensitive adhesives, but they miss, is the type of substrate they are bonding to. Any adhesive will stick to any surface, right?  As stated earlier, not if you really need it to work. 

One important factor in bonding to any substrate comes down to its surface energy. Most surfaces, especially when you are referring to plastics, are classified as either a High Surface Energy (HSE) or a Low Surface Energy (LSE) substrate. They are classified based on their dyne level. Examples of an HSE are stainless steel or polyester.  An example of an LSE is polypropylene. For the most part if a substrate is rated as an HSE, just about any adhesive will be able to bond to it.  The tricky part is with LSE products.

Because of the low dyne levels of LSE substrates pure acrylics typically have a hard time bonding to them. The adhesives either have to be a modified acrylic, with added tackifiers, or a rubber based adhesive. These types of adhesives wet out better on LSE substrates.

Time is another factor to take into consideration. Does the PSA need to be up to its optimal holding strength immediately or is there time for it to dwell? If you need for the adhesive to bond immediately then a rubber based adhesive or a modified acrylic is your best bet.  If there is some dwell time available, typically at least 24 hours, then a pure acrylic may be of use. If you need to put any type of weight or stress onto an acrylic adhesive after the initial application, it may hold for a short period of time, but it will eventually pull off causing a failure.

Since this post is about PSA’s we need to include the topic of pressure. All adhesives need to be activated to allow them to wet out properly.  For PSA’s that activator is pressure. A common mistake, as an example, is to apply a 1” wide tape to a surface and then to rub a finger down the center of it to get the adhesive to bond. What you will frequently see happen in this type of scenario is that the inner ½” will bond correctly. While the outer edges, which didn’t receive the benefit of pressure from your finger, are not sticking at all. If the application is one that the entire 1” width is necessary then, in this example, you will most likely have application failure. 

The pressure does not need to be extreme but it does need to be complete. A typical nip roller on a laminator, or a hand roller, will usually do the trick. If the carrier of the adhesive has a thickness, such as foam, then more pressure needs to be applied then you would with a film carrier. 

We all know the saying about cleanliness. It is definitely something that is necessary or a positive virtue when it comes to adhesives. If there is dirt, dust, water, oil, or any type of contaminant on a surface adhesives will not bond properly. 

Say you are trying to bond to glass that has condensation on it. Sure, there will be areas where there will be dry glass. There will also be areas with a water droplet on it, which is what the adhesive is actually bonding to. That water is floating on the glass and will eventually evaporate.  Similar situation with dust. If full adhesive coverage is vital to the application then you have a high potential for application failure, in this example. 

I have to warn, though, be careful with what you are cleaning with. Cleaners such as rubbing alcohol or solvents such as toluene actually clean and then evaporate. Other cleaners, such as window cleaners, can leave a residue. That residue is what the adhesive is bonding to and will also be a problem. 

Adhesives can really help bring the world together and there are a few more factors that need to be considered. This quick overview of these four basic factors, though, are ones that I have seen come up the most often and are easily diagnosed and controllable. While the tape specifications and test values are very important, keep these factors in mind when you are trying to determine for yourself which tape to use or attempting to recommend any PSA.

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