6 июля, 2015 
Malyar Consultant, Charlotte, North Carolina, U. S.A.
It is assumed in almost all cases, that any pressure-sensitive adhesive to be tested has already been applied to a flexible carrier, which is either in tape form, or which can be cut into tapes for testing, virtually all test methods making this assumption. If this is not the case, then it would be necessary to coat the adhesive onto a suitable flexible carrier, usually 25 pm polyester film, which may need to be suitably pre-primed, the prime coat used depending on the adhesive type. The coat weight chosen should be that used for the practical application of that adhesive, or if this adhesive is still under development, then a series of coat weights can be run, to determine which provides optimum performance. An exponential relationship will be found between coat weight and resulting adhesion.
The need for the physical testing of a pressure-sensitive adhesive can vary considerably; such reasons include the determination that a given pressure-sensitive adhesive will perform satisfactorily for its intended use, that it meets a specific standard, that uniformity exists within a given population, or between populations, or that it could be to compare one system to other similar systems—all of which demand that any test method must be accurate and reproducible. The thermoplastic nature of pressure-sensitive systems can make this objective very difficult to achieve, without a full understanding of their behavior and without observing a number of precautions.
Recognizing that the behavior of a pressure-sensitive system varies according to temperature and to rate, and that a pressure-sensitive adhesive easily deforms under pressure, which affects the degree of contact, then the conditions under which any pressure-sensitive adhesive is both prepared for testing, then tested, must be rigidly controlled. Otherwise considerably different values will be found for each uncontrolled evaluation of the same adhesive system. This also applies to the geometry of the test, where the bending of a flexible adhesive carrier plays a key part, as in peel adhesion testing, the relative stiffness of the backing altering the intended angle of the test.
The nature of the bond that is formed with a specific adhesive depends both on the adhesive design and on the nature of the surface to which it is adhered. This not only applies to its material of construction, but also whether it is porous or nonporous, the degree of surface roughness, and from this, the contact that can be obtained, 100%
contact during testing being a rarity. So now we must add to our protocol for standardization of Testing, standardized test surfaces both in material and in surface roughness, recognizing that nonporous and porous substrates will behave quite differently, keying into the micro-irregularities of a porous surface being one means of attachment.
The internationally recognized standard material chosen for a nonporous test surface is stainless steel [1-5], both the European and United States Adhesive Tape Councils agreeing on Type 304, although there are differing opinions as to surface roughness. For a porous test surface, in the United States, a standardized cardboard, Standard Reference Material 1810A from the National Institute of Standards and Testing, is used, other testing authorities again having their own choice. Since any result obtained using a standardized test surface will be applicable to that surface only, accurate data for alternative surfaces will require a retest with that new surface.
The application of any stress that a pressure-sensitive adhesive encounters in practical use ranges from a very rapid rate, as in unwinding a pressure-sensitive tape on an automatic application machine, to a continuous slow stress as would be the case with a packaging tape in use, or with a double-sided tape used as a mounting tape. In the very rapid rate case, the viscous component of the adhesive has little or no time to respond and under excessively rapid rates, separation may even be in the form of a brittle fracture. This behavior would be duplicated if the adhesive were stressed at extremely low temperature, due to the time/temperature relationship [6]. In the case of a continuous low stress, there is ample time for viscous response, and resultant molecular disentanglement and failure may well be cohesive, again with a corresponding behavior if tested at high temperature. For the most part, both in use and in testing, there is a simultaneous elastic and plastic response to stress by pressure-sensitive adhesives, each to a greater or lesser extent, depending on the rate and/or temperature.
Regarding test panels, it is common practice to reuse standard test panels over and over again, and the need for a contamination-free surface at each test is essential to obtain reliable data. While it appears that a specific pressure-sensitive test leaves no residue on removal, and the panel may appear perfectly clean, a simple dusting, as is done for fingerprint identification, will show that there is indeed an adhesive residue remaining after each test, and further, this micro-trace can be quite difficult to remove. There are standard procedures as to the correct solvent for test panel cleaning [7], but with the variety of pressure-sensitive systems now available, the correct method is the one that removes the last applied adhesive system most satisfactorily, and leaves the test panel clean and dry. It may well involve the need for two solvents, one to remove the adhesive then another to remove the last traces of the first solvent. Because of this inherent adhesive micro-contamination, even following the standard cleaning procedures, there are those who prefer to restrict the use of a series of test panels to a specific adhesive system, to further standardize their test surfaces and to prevent any cross-contamination, and others who use microscope slides as their standardized surface, the slides being discarded after each test. Other possible sources of contamination of both the panel and the sample under test are perspiration from the fingers, dust, and moisture, all of which can act as a weak boundary layer between the adhesive and the test panel. Although care in handling and panel cleaning can correct the perspiration and dust, the humidity of the air controls the moisture, and so testing must be in a controlled humidity environment, normally 50% relative humidity, and at a temperature of 23°C (73.5°F) [8], to restrict the testing to a known condition.
If all of the foregoing is appreciated, then not only will the observed behavior of a pressure-sensitive adhesive during testing be better understood, but it will also be possible either to adapt existing test methods, or to devise others, which are more appropriate to the practical application of a specific adhesive.
Finally, it should be recognized that, when a stress is applied to a pressure-sensitive adhesive, it is either a tensile stress, a shear stress, or a combination of both. In the specific end use of a given pressure-sensitive adhesive system, a clear understanding should exist as to what type of stresses can be encountered, to ensure that the test methods applied bear a relationship to use. The various standard test methods can now be considered.
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