Tack, or quick stick, is one of the most important properties of pressure sensitive adhesives (PSAs). The definition of tack is the property of a pressure sensitive adhesive that allows it to adhere to a surface under very slight pressure. It is determined by the ability of the adhesive to quickly wet the surface it contacts.
There are four standard test methods that are used globally to evaluate a pressure sensitive hot melt’s tack. They are Loop Tack, Probe Tack, Rolling Ball Tack, and Quick Stick. Although these test methods do not result in the identical values for a fixed PSA, they can distinguish relative adhesion performance for different PSAs. The descriptions of various test methods are listed below.
Loop tack has recently become the most reliable test method because it can generate reproducible and consistent data. Most tape and label manufacturers now prefer reporting loop tack data only on their published brochures. Microprocessor controlled loop tack testers are now commercially available.
Probe tack was very popular years ago because the action of the probe tack test is very similar to the finger tack test. The deviation of test data results from this method is too great due to its small contact area. The diameter of the probe tip is only 5.0 mm. Tack performance determined by probe tack tester is too sensitive to the consistency of coating weight, smoothness of coating surface, and sample preparation.
The rolling ball tack test is very simple and inexpensive and the test result cannot be used to compare with other tack tests. Very tacky adhesives do not impart good rolling ball tack. In practice, rolling ball tack is a very good test method for online QC testing at the production site. The consistency of a coating on a web is easy to determine immediately after coating by means of this simple test.
Quick stick can be performed at most tensile tests accompanied with a moving sled to generate a 900 peeling angle. The difference of quick stick test and 900 peel test is that no pressure (e.g. 2 Kilograms) is applied on the quick stick test specimen prior to testing. Although this test method is a little bit tedious and requires an additional accessory, the test results generated from this test method are somewhat consistent and comparable to those determined by a 900 peel test.
Finger tack is also commonly used in the real world evaluation which requires no equipment. Many people believe finger tack is more realistic than other tests described above. In fact, finger tack test is not only too subjective, but also very unscientific. No PSAs, except skin contact PSAs, are bonded on human skin. Many variables, such as skin roughness, temperature, grease, and thread perspiration, etc. will significantly affect the feeling of finger tack. Finger tack may mislead adhesive users when they try to select an appropriate PSA.