How Tack Impacts Bonding Performance in Pressure Sensitive Adhesive Tapes
Pressure sensitive adhesive tapes (PSA tapes) are often known for their fast-bonding capabilities and long-term bonding performance, but not all PSA tapes have the same level of quick stick, also known as tack. For those looking for a quick rate of strength build upon their application, understanding tack is crucial, and choosing the right PSA tape with the right tack can be the difference maker in how well your tape holds immediately after it is bonded.
What is tack, and how will it impact the performance of a PSA tape in your application?
What is Tack When it Comes to PSA Tapes?
A PSA tape’s tack is defined by how aggressively a PSA tape’s initially grabs a corresponding substrate when pressure is applied. Also known as “thumb appeal,” tack is the ability of the adhesive to quickly wet the surface (flow across and create a strong bond) it comes into contact with. The higher the tack, the faster the adhesive flows into the microscopic pores of the substrate and the faster bond reaches ultimate adhesion.
High tack tapes are ideal for many applications. Manufacturers may want to use them in assembly processes where quick bonding is necessary or where they may only need a temporary strong bond. If you have the time to allow the PSA tapes to wait and form a strong bond, you may not need to use a high tack tape. Whether or not tack is important in your choice of PSA tape depends on your assembly process and the other application limitations. It may not always be possible to use a higher tack tape depending on your applications (i.e., temperature and exposure to solvents, UV, etc.)
Which PSA Tapes Have the Highest Tack?
There are three major types of PSA tapes that will cover most use cases and applications: rubber, acrylic, and silicone. For a much more fleshed out explanation of these three types of tapes and how to choose the right one for your application, see this detailed guide.
When it comes to tack in these types of PSA tapes, you’ll find that each of these options has their own level of tack:
Rubber PSA tapes: these tapes typically have the highest tack and can bond well to both high and low surface energy substrates almost immediately. They are ideally used for temporary and indoor applications in modest environments, or when you need a strong bond with plastics like UHMW or polypropylene. 3M™ Double Coated Paper Tape 410M, for example, is a high tack rubber-based tape that has been used successfully in industry for decades. While the tack of these tapes is high, they also have the lowest resistance to temperature extremes, solvents, UV light, and plasticizers.
Acrylic PSA tapes: depending on the formulation of these tapes, their tack can range from high to low. They are generally regarded as the “mid-level” range in terms of tack. Higher tack acrylic tapes do sacrifice some strength and temperature performance, but they are still robust in tough environments and can bond to many plastics. 3M™ Adhesive Transfer Tape 950 is a long-standing high tack acrylic PSA that combines high tack with excellent performance. If you are ever in doubt, use an acrylic PSA tape as they are the most versatile option for most bonding applications.
Silicone PSA tapes: these tapes have the lowest tack of all options and are generally not recommended for high tack bonding applications. However, these types of PSA tapes have the widest range of resistances to UV light, solvents, grease, etc., and will perform the best in extreme temperatures. 3M™ Adhesive Transfer Tape 91022 is a great example of a low tack silicone transfer tape that withstands harsh environments. If your application is exposed to harsh environments, extreme temperatures, or need to bond to silicone rubber, silicone tapes are likely the best option despite their lot tack and higher cost profile..
The Environmental Factors that Can Impact Tack
Temperature: High temperatures can soften adhesives, while low temperatures can make them brittle and less effective. So, adhesive tapes are known to become less tacky in cold temperatures and excessively tacky in high temperatures.
Contaminated surfaces: the presence of dust, dirt, or contaminants on a surface can hinder the adhesive's ability to bond effectively. Clean surfaces allow for better initial and ultimate adhesion. Exposure to chemicals, solvents, or oils can also degrade adhesives and reduce tack. Some surfaces may require pre-treatment, such as cleaning, priming, or scuffing, to enhance tape adhesion and tack. These same pre-treatments can also accelerate adhesion build up.
Surface porosity and unevenness: surfaces that are smooth and non-porous generally provide better adhesion when compared to rough or porous surfaces. A good bond requires full wet-out, but it’s also important to consider the dwell time and how long it will take the PSA tape to form a strong bond.
Surface energy: the surface energy of the substrate can also impact initial and long-term adhesion, depending on if the PSA tape is particularly suited to adhere well to that type of substrate. A material's surface energy is a critical component to the bonding process. Find out more about bonding to low surface energy substrates in this guide.
How Tack is Measured in PSA Tapes
When measuring tack in PSA tapes, there are four types of tests that are generally used:
Loop tack: measured through ASTM D6195. The loop tack test involves attaching a piece of tape to a surface with one end hanging off the substrate. A weight is then attached to the hanging piece of tape to find the force required to peel the tape from the substrate at a specified angle and speed.
Rolling ball tack: meets the standards set by the Pressure Sensitive Tape Council (PSTC-6) and ASTM D3121. This test involves placing a small steel ball on the surface of the adhesive tape that is set at a specified angle. The ball adheres to the tape and the angle is increased until the ball begins to roll down the tape. Lower angles indicate higher tack.
Probe tack: measured through ASTM D2979. The probe tack involves placing a small object on the tape (generally a ball or metal cylinder) and measuring how much force is required to peel it away from the adhesive.
Peel Adhesion: the tape is peeled off at a specified angle and speed using a mechanical testing machine. The force required to initiate peeling is measured, providing a quantitative value for adhesion. Running these tests within minutes of making the bond, and after 72 hours, provides a view to the initial tack versus the ultimate peel adhesion.
Other Considerations for PSA Tape Adhesion
Outside of a pressure sensitive adhesive tape’s tack, it’s also important to consider the cohesion and adhesion of the tape. Cohesion measures how well the adhesive holds together to itself (it’s inner bond), while adhesion measures how well the adhesive bonds with the substrate it is attached to. There’s a lot to consider, but we can help you make the right choice. If you have any concerns with the tack or adhesive properties of your current PSA tape, or are looking for better options, get in touch with our bonding specialists at Engineered Materials today.