Tack tests

Perhaps the most significant complication in the interpretation of nanoscale adhesion and mechanical properties measurements is the fact that the contact sizes are below the optical limit ( 1 t,im). Macroscopic adhesion studies and mechanical property measurements often rely on optical observations of the contact, and many of the contact mechanics models are formulated around direct measurement of the contact area or radius as a function of experimentally controlled parameters, such as load or displacement. In studies of colloids, scanning electron microscopy (SEM) has been used to view particle/surface contact sizes from the side to measure contact radius [3]. However, such a configuration is not easily employed in AFM and nanoindentation studies, and undesirable surface interactions from charging or contamination may arise. For adhesion studies (e.g. Johnson-Kendall-Roberts (JKR) [4] and probe-tack tests [5,6]), the probe/sample contact area is monitored as a function of load or displacement. This allows evaluation of load/area or even stress/strain response [7] as well as comparison to and development of contact mechanics theories. Area measurements are also important in traditional indentation experiments, where hardness is determined by measuring the residual contact area of the deformation optically [8J. For micro- and nanoscale studies, the dimensions of both the contact and residual deformation (if any) are below the optical limit. 

A loop tack (Fig. 2c) test consists of allowing a tear-shaped loop of conditioned tape to drape into contact with a test surface of specified area (usually 25.4 x 25.4 mm), with the force of contact limited to the weight of the tape itself (ASTM Ref. D-6195). The ends of the loop are held in a tensile tester. After a momentary contact time the tester is engaged and the tape is removed at a specified speed. The maximum in the removal force is ordinarily observed just at the point where the two peel fronts Join. The value is reported in a force per area of tape width, or lb in. -. While this tack test has some popularity, it is perhaps more of a very short dwell time peel test, and it has variables more associated with that test, especially backing effects, since heavier backings lead to higher tack values. 

In another tack test, a steel ball of specified diameter is rolled down a grooved incline onto a conditioned surface area of pressure sensitive adhesive (ASTM D 3121, PSTC-6). The length of travel before it stops is the rolling ball tack (Fig. 2d) reported in millimeters. It is relatively inexpensive and simple to set up. Similar test variables to the probe tack test apply. 

Fig. 2. Tack tests and results, (a) Probe tack, (b) Probe tack vs. temperature for a natural rubber PSA. (c) Loop tack, (d) Rolling ball.

The methods to evaluate PSAs include the rolling ball test (ASTM D3121, PSTC-6, BS EN 1721), loop tack test (ASTM D6195, FINAT Test Method 9, BS EN 1719), and quick stick test (PSTC-S). ... 

When the shutter closes and the stage retracts, a second timer is activated which controls the interval between exposure and administration of the tack test. This interval can also be varied from one second to 15 minutes in increments of one second. 

The tack test is administered by a small air-driven piston, to one end of which is attached a probe consisting of a ball of absorbent cotton. Pressure on the probe is controlled by air pressure driving the piston. A tacky coating is readily recognized by the tendency of cotton 1 inters to adhere to it. The number of seconds following exposure for a coating to become hard enough that it does not pull cotton 1 inters from the probe is defined as the "tack-free time". 

The tack test was performed in fixed experimental conditions the probe (a glass cylinder of 2 mm diameter) was pressed with a force of 6 N onto the PSA film surface. After 10 s of contact time the probe was withdrawn with a constant velocity of 1050 pm s . In Fig. 26.4, horizontal cuts for both sample systems are displayed. The curves correspond to different distances between the... 

Fig. 26.6 Representative images as detected by an optical microscope during tack tests through a transparent substrate. Highly inhomogeneous structures develop on a macroscopic scale. Polymeric partition walls keep neighboring cavities separate. A polygonal, foam-like structure develops. The diameter of the punch is 2 mm.

Figure 4 Rolling ball tack testing (a) douglas (b) PSTC (c) dow. 

Figure 5 Quick-stick tack testing (a) compressive effect of 90° peel (b) loop tack testing.

Variations in test results for tack from test to test for a given sample are common, possibly because such a small area of the adhesive surface is xmder examination at each test, and for this reason, whatever test method is used for tack testing, a number of checks are necessary to obtain a statistically meaningful result. 

Test Procedures and Conversion Tables, V, Loop Tack Test, Publication 6512, Gelva, Monsanto, p. 20. 

The classic test for tack of a pressure-sensitive adhesive film is the rolling ball tack test. Here a ball is rolled down an inclined plane onto a film of the adhesive. The length the ball travels across the film before stopping is a measure of the tack of the film. This test gives a good indication of tack with elastomer adhesives but is unreliable with water-based systems. 

A more universal test is the probe test, in which the end of a cylinder of standard diameter is brought lightly into contact with the film for a very short time and the force required to separate it from the surface is measured. Similar in principle is the loop tack test, in which a loop of coated film is lowered onto a steel plate, making contact under its own weight, and the force required to withdraw the plate is then measured. All of these tests are markedly affected by the cleanliness of the ball, probe, or plate. Figure 4 illustrates the loop tack test. 

Most of the testing we did early on was pretty simple. There are three basic properties of all pressure-sensitive adhesives peel, tack and shear. Most of the testing we did was pretty limited to peel adhesion tests, tack tests and shear tests. We weren t doing a lot of application-related testing. Testing was done in lab conditions on stainless steel test panels, and customers were trying to translate that information into whether the adhesive would actually perform on their specific substrate. 

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