Basic Properties of Pressure-Sensitive Adhesives
The Three Elements of Adhesion
The physical properties of pressure-sensitive adhesives are often evaluated in terms of the three elements of adhesion—adhesive strength, holding power, and tack (stickiness)—when the adhesives are processed into the form of adhesive tape.
Adhesive Strength
Adhesive strength is the force required to peel an adhesive material away from an adherend (the surface to which it is attached). For example, when peeling tape off a wall, you might ask, “How much force do I need to peel this off?” The answer is the adhesive strength of the adhesive material. The higher the force required to peel off the adhesive material, the higher its adhesive strength, and the more firmly it sticks to the adherend. Adhesive strength depends largely on factors such as the temperature and humidity of the surroundings and the material of the adherend.
Test for measuring adhesive strength
Adhesive tape is applied to a test plate made of stainless steel or another material and pressed with a crimping roller. After a defined amount of time has passed, a testing machine is used to peel off the tape at different angles (e.g. 90°, 180°) at a defined speed. Adhesive strength is often expressed in newton-millimeters (N/mm) or newton-meters (N/m). The mm or m correspond to the width of the tape, and newtons are a unit of force. The higher the value, the higher the adhesive strength.
Holding Power
Holding power is the resistance of adhesive tape to moving against an adherend (the surface to which it is attached). Holding power is evaluated by measuring how far adhesive tape moves when a defined load (weight) is applied for a defined amount of time. For example, when the top flaps of a cardboard box are taped shut, the holding power of the tape resists the repulsive force that would cause the flaps to open.
Test for measuring holding power
Adhesive tape is applied to a test plate made of stainless steel or another material and pressed with a crimping roller. After a defined amount of time has passed, a load (weight) is applied to the tape such that it hangs downward. Holding power is expressed as both the distance the adhesive tape moves after a defined amount of time has passed, and, if it falls down before that time has passed, the amount of time that passed before it fell. Therefore, holding power is expressed in millimeters (mm) and minutes (min). The closer the value is to 0, the higher the holding power or resistance to moving.
Tack
Tack is the short-term bonding force under light contact between an adhesive material and an adherend (the surface to which it is attached). For example, tack is the stickiness you feel when you touch a piece of cellophane tape. When tack is strong, the adhesive material bonds with the adherend as soon as it is applied. There are many different ways to measure tack. The most recognizable is the rolling ball tack method.
Test for measuring tack
Adhesive tape is placed adhesive side up and set at an angle, and stainless steel balls are rolled down from the top. The tack of the adhesive material is determined by the largest ball that stops on it for at least five seconds. The larger the ball, the stickier the material. However, tack only covers the initial bond; it does not necessarily indicate the adhesive strength of the material.
When evaluating physical properties, results may vary substantially depending on the conditions of the test, namely temperature, humidity, and adherend pretreatment. Therefore, test results should be evaluated relative to predefined evaluation condition, and by skilled practitioners.
The performance required of adhesive tape depends on the application of the pressure-sensitive adhesive and the purpose of use. Higher adhesive strength, holding strength, and tack are not necessarily better in all situations. For example, protective films for smartphones should be easy to remove, so the adhesive strength cannot be too strong. A broad range of physical properties other than these three are also evaluated. Therefore, when selecting pressure-sensitive adhesives, the key is to conduct tests that evaluate the desired performance and select the one that meets the requirements.
Our lineup of pressure-sensitive adhesives has various features that meet our customers’ needs. Please feel free to contact us to learn more about pressure-sensitive adhesives that meet yours.
- Note: The data listed are for reference only and are not standard values.
In addition, there is no guarantee that the listed uses will not infringe on any patents.