What is tack, peel, & shear?
Lift the corner on a removable film and it should pull away cleanly and easily. Try the same on a shipping label and it should adhere stubbornly to the package. Both are adhesive applications yet the needs are very different. The way an adhesive performs is strongly influenced by the latex polymer emulsion included in the formulation. An apparently minor change or difference in the latex can have a big impact on the nature of the bond.
Three properties of an adhesive
Adhesive performance can be characterized in terms of tack, peel and shear. These properties define the type of bond created between two surfaces so it’s important to understand what they mean.
Tack is a term used with pressure sensitive adhesives and is a measure of how quickly a bond is formed. Two surfaces are brought together briefly under light pressure, then pulled apart. The more force needed to separate them the higher the tack. On the contrary, low tack allows an adhesive film or tape be repositioned.
Peel is a measure of the force needed to break the bond between an adhesive tape and the surface it’s been applied to. This is what tells us the strength of the bond. In peel testing an adhesive tape is applied to a surface, allowed to sit, and then pulled away. For consistency of results, the parameters like peel angle or direction, application pressure and the time for which the surfaces stay bonded are standardized.
Shear can be thought of as one surface sliding over another. In a shear test the sample is mounted vertically and has a weight attached. The time it takes for the sample to slip off the substrate shows the durability of the bond.
The latex emulsion contribution
When formulating an adhesive it’s important to consider how the latex emulsion used will affect final properties. The glass transition temperature (Tg), gel content and degree of carboxylation all have a bearing on tack, peel and shear performance. A carboxylated emulsion tends to build bond strength over time. It’s something you might look for in a shipping or package label. Non-carboxylated emulsions tend not to build adhesion over time, an important attribute in removables. Tg influences pliability, especially at low temperatures. (Think labels applied in the freezer section). Gel content is a measure of cross-linking within a styrene-butadiene emulsion polymer. Less cross-linking results in higher tack, but more cross-linking provides higher shear. If necessary, a tackifier can be added to the emulsion to raise tack even more.
It’s difficult to maximize all three properties simultaneously, and few applications call for that anyway. In particular, tack and peel are usually in opposition to shear. Instead, the approach to formulating a latex emulsion for adhesive applications revolves around understanding the combination of tack, peel and shear needed.
Work with your supplier
How an adhesive performs is strongly influenced by the latex polymer emulsion included in the formulation. Different emulsions provide varying levels of tack, peel and shear. When standard product offerings don't provide quite the combination needed, a specialty manufacturer can customize the properties of their emulsion to meet the application requirements.
If you're in the business of formulating adhesives, a conversation with Mallard Creek Polymers will be time well spent. They'll take the time to understand what combination of tack, peel and shear you need, helping you to develop superior products.