Shear adhesion failure test

Another type of shear test involves testing under heated conditions. The shear adhesion failure test (SAFT) (FINAT FTM 8, PSTC-7) is a static shear test administered to a PSA sample under increasing temperature at a rate typically of 1 or 2°C per minute. The recorded SAFT value is the temperature at which the adhesive bond fails. 

There is second shear adhesion test that is now being reported with increasing frequency, the so-called SAFT, or shear adhesion failure temperature test. It is particularly popular among block copolymer PSA developers. In this test [15], a shear specimen with an overlap area of 2.54 cm x 2.54 cm is prepared and suspended in a circulating air oven. A 1-kg weight is attached to the tape and the oven temperature is raised continuously 5.5°C per 15 min until failure. An industry wide standard has not yet been written for this test. 

One method used to evaluate the upper working temperature of block copolymer systems is to measure the shear adhesion failure temperature (SAFT), a useful method to discover exactly what has been gained in the upper working temperature limits when resins are added to the styrene domain of polystyrene end-block systems to increase the effective glass transition temperature. Typically the test is set up as a standard shear test, either to the standard stainless steel panel or to polyester, with a 1 in. by 1 in. (25 mm by 25 mm) area and a load of 1 kg. The set-up is placed in an oven that can be accurately controlled so that the temperature is increased by 2.0°C (3.6°F) per minute, the temperature at which failure occurs being recorded as the SAFT. 

As with other pressure-sensitive adhesive tests, the temperature is an important variable and tests are typically carried out at room temperature. A variation on this method is the SAFT test (shear-adhesion-failure temperature) in which the test apparatus is placed in an oven where the temperature is set to rise at 4.5°C/min. The temperature at which the tape fails is recorded as the SAFT value. 

Shear adhesion failure temperature (SAFT) gives information on the resistance of the PSA to stress at elevated temperature. The geometry is identical to the shear adhesion test (Fig. 13). The test is performed in a forced-air oven with temperature rising at constant rate (0.4 °C/min). The SAFT is the temperature at which the bond fails and the load falls down. As this method is not standardized yet, there are some differences in overlap and load between companies [225], [226]. 

Polybutylene polymers slow oystallization rates are helpful in forming hot-melt adhesives with long open times [67]. Besides the hot-melt viscosity, lap shear strength, T-peel, and open time, the shear adhesion failure temperature (SAFT) of the bonded substrate is an inqxntant noperty for evaluating the effectiveness of the adhesive. The polymer MFI plays an inqxntant part in determining the surface tenqterature at whidi shear adhesion fidlure could occur, as can be seen from Fig. 9.75. The top curve represents the SAFT test results with a 0.5-kg load, whereas the bottom curve shows test results with a 1-kg... 

Since creep is considered as a key weaknesses of pressure sensitive adhesives, various specific test methods and standards have been developed to evaluate the creep resistance of pressure sensitive adhesives like, for example, in the European Standard EN 1943 ( Self adhesive tapes-Measurement of static shear adhesion ), FINAT (Federation Internationale des Fabricants et Transformateurs d Adhesives et ThermocoUants sur Papier et autres Supports), test method FTM 8 (Resistance to shear from a standard surface), or the Pressure Sensitive Tape Councils test method PSTC 107 (International Standard for Shear Adhesion of Pressure Sensitive Tape) by either monitoring the time- and load-dependent displacement of an adhesive specimen under shear load or simply recording the time to failure. The result of the so called SAFT-test ( Shear Adhesion Failure Temperature ) indicates the temperature at which a sample that has been subjected to an environment with steadily rising temperature under static shear load has failed. 

Alternative indications for durability of PSAs in specific applications can be obtained from modifications to these methods for determining performance properties, and the two most commonly used tests are the peel adhesion failure time (PAFT) and shear adhesion failure... 

Pressure-sensitive adhesives are used in a great variety of applications, most commonly for adhesive tapes. In that case, they have to be tested by static shear test or dynamic shear test The difference between these two methods is that in static shear test a standard force is being applied to the test specimen and the adhesive failure is reported as the time it takes for failure to occur. The dynamic shear test involves a force being applied to the PSA tape at a specific rate of speed (typically 0.25 mm or 0.1 in. per minute). The value reported is as the peak force per unit area (Ib/in, also abbreviated as psi, and in SI units MPa) required to cause adhesive failure. The standards for adhesion shear tests are ASTM D3654, ISO EN 1943, and PSTC-107.i i ... 

A standard test report usually documents the resulting measurements, such as tensile shear strength and peel strength. It should also indicate all the pertinent conditions that are required to ensure reproducibility in subsequent testing. It is often very useful to describe the failure mode of the tested specimens. An analysis of the type (or mode) of failure is an extremely valuable tool to determine the cause of adhesive failure. The failed joint should be visually examined to determine where and to what extent failure occurred. The percent of the failure that is in the adhesion mode and that in the cohesion mode should be provided. A description of the failure mode itself (location, percent coverage, uniformity, etc.) is often quite useful. The purpose of this exercise is to establish the weak link in the joint to better understand the mechanism of failure. 

The shear and tension tests after submersion of specimens in boiling water for 2 hours are not tests accepted or specified by any regulatory or testing society or agency. Gluing experience over many years, however, has shown that important conclusions with respect to reasons for adhesive failure, as well as important pointers for adhesive development, can be obtained by careful interpretation of the comparison of test results after boil with those from dry and vacuum-pressure testing. 

The international standard method for shear adhesion is now incorporated into the standard for shear modulus, ISO 1827 [12], which has been discu,ssed in Chapter 15. The quadruple clement test piece provides a still construction that will will remain in alignment under the high stresses that can be expected when the test is continued to failure point to measure adhesion. The test piece is strained at 50 mm min. and the ma.ximum force is recorded in line with the other adhesion-to-metal tests. The result is expressed as the maximum force divided by the total bonded area of one of the double sandwiches. 

B. Automated shear adhesion apparatus In the standard industry test for shear adhesion (Pressure Sensitive Consul lPSTC-2) a fixed load is applied to a defined area of adhesive in a vertical plane. Performance is measured as the total time to failure. 

Lap joint tests conducted to yield a rate of shear strain of about 1.0/minute, except for Ref. 87 double lap specimen A adhesive failure C cohesive failure... 

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