Pressure sensitive adhesive shear tests

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 ... 

Devices are secured to the skin by use of a skin-compatible pressure-sensitive adhesive, usually based on silicones, acrylates or polyisobutylenes. These adhesives are evaluated by shear-testing and assessment of rheological parameters (Musolf 1987). Standard rheological tests include creep compliance (measurement of the ability of the adhesive to flow into... 

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. 

While a static load test is the commonly accepted procedure, in practice many variations in areas and weights are used, to compensate for the various qualities of adhesive evaluated, so that the test results will fall into a similar time frame, so it becomes difficult to compare different adhesive systems from accumulated data. It has the disadvantage of giving variable results for the same adhesive system, and is essentially a pass/ fail test, as many products remain in place at the end of the test period. Experience has shown that the shear properties of pressure-sensitive adhesives to porous and nonporous substrates can be quite different, and each must be judged on its own merits. 

Because of the unique properties of pressure-sensitive adhesives, special tests not applicable to other types have been developed. While standard physical tests such as nonvolatile content, viscosity, and specific gravity are performed to ensure consistency of application, these tests do not predict adhesive performance. For pressure-sensitive adhesives, three critical performance characteristics are usually measured tack, peel, and shear strength. 

This test is a measure of the ability of a pressure-sensitive adhesive to withstand ereep. A standard area of coated film is bonded to a steel plate and a weight suspended from it. The assembly is plaeed in an oven. In some shear tests the time for the assembly to delaminate at a fixed temperature is recorded, while in other tests the temperature at which failure occurs when the oven temperature is increased at a certain rate is the shear value quoted. 

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. 

A primary method used to characterize adhesives is the lap shear test. A diagram of the test is shown in Fig. 27.1 and is described in Standard Test Method ASTM D1002 [4]. The specimen is usually 1 in. (2.54 cm) wide. The lap shear test places the adhesive in normal as weU as shear stress [5]. This type of test is used for many types of adhesives, with the exception of pressure-sensitive adhesives (PSAs, defined below). In the tables presented later, lap shear strength is presented in units of mega Pascal (MPa) and pounds per square inch (psi). The latter is shown in parenthesis. The temperature of the test will always be room temperature. 

Standard tests used to characterize the adhesion properties of tapes are for the assessment of shear strength (see Shear tests) (the ability of a tape joint to resist a load applied in the shear mode), peel strength (see Peel tests) (the resistance of a tape joint to peeling under specified conditions) and Tack (the ability of a pressure-sensitive adhesive to form a bond immediately on contact with another material). There are many standard test specifications laid down by different authorities to assess these properties and many differences in detail between them (e.g. see Appendix). No attempt will be made to describe them comprehensively, but the principles of the tests will be discussed separately. 

Pressure-sensitive adhesives - adhesion properties D W AUBREY Shear and peel strengths tack tests... 

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. 

Figure 13. Standard test methods for peel and shear adhesion of pressure-sensitive adhesive tapes...

The chemistry and structure of the tackifying terpene resins are developed by E. Ruckel al. These resins, produced commercially from pine turpentine, since the mid-Thirties, are formulated with natural rubber to produce pressure sensitive adhesives. More recently, the scope of their use has been broadened by formulation with elastomers and waxes for hot melt applications. Empirical application tests have developed a broad knowledge of utility but little science or predictability. By use of sophisticated high polymer techniques, polymerization mechanisms are used to explain how the minor structural differences between the beta-pinene and dipentene resins suit these resins respectively for pressure sensitive and hot melt adhesive usage. Again for use application the critical aspects of the formulation are its adhesive and cohesive properties as demonstrated by tack, shear and peel properties. 

Most adhesive tapes are composed of a flexible backing (paper, plastic, cloth, metal foil, etc.) to which a pressure-sensitive adhesive has been applied to one side (both sides for double-sided tapes). Pressure-sensitive adhesives typically consist of a rubbery material with a modifying tactifier that may be applied to the tape by a solvent system, hot melt, or by other means. One would expect such materials to be sensitive to the mode of stress (tensile versus shear) in the region where debonding occurs. Furthermore, since tacky rubbers of the type used in pressure-sensitive adhesive are viscoelastic, one would anticipate material properties to be time- and rate-dependent. Are these expectations consistent with the observations from your simple peel test ... 

Properties and Testing. The adhesive requirements for a pressure sensitive adhesive will vary greatly with end use. While each application will have certain specific tests, the three most common adhesive tests are peel adhesion, shear (creep) resistance, and tack. There are standard test procedures developed for these tests by PSTC, " ASTM, and the Tag and Label Manufacturers Institute. 

The Pressure Sensitive Tape Council has developed a series of test procedures for the determination of tack, adhesion, and shear properties of pressure sensitive adhesives. The most commonly used tests are ... 

Abstract This chapter gives a brief description of special mechanical tests for various types of materials and sample geometries, such as blister tests for membranes/adhesives/coatings, tensile tests and shear tests for sealants/foam adhesives, indentation and scratch tests for coatings, tack tests for pressure-sensitive adhesives (PSAs), and bimaterial curvature tests for characterizing residual stress, stress-free temperature (SFT), and coefficient of thermal expansion (CTE) of adhesives bonded to substrates of interest. In addition, some applications of these tests, including the nano-/micrometric scale, are also described in this chapter. 

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. 

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. 

In instrumented creep tests taken to failure, one learns not only how long specimens last but also how deformation increases throughout the creep process. For lap joints, delay times have been seen in creep tests, probably due to the increasing uniformity of the shear stress state, as predicted by the shear lag model as the creep compliance of the adhesive increases with time. In other situations, no such delay time is seen. A schematic illustration of a creep curve for an adhesive bond consisting of a butt joint bonded with a pressure sensitive foam tape is shown in Fig. 2, exhibiting classical primary, secondary and tertiary regions of creep behaviour. 

Test Method for Measuring Strength and Shear Modulus of Nonrigid Adhesives hy the Thick-Adherend Tensile-Lap Specimen-, and ASTM D2979, Standard Test Method for Pressure-Sensitive Tack of Adhesives Using an Inverted Probe Machine. 

Influence of type and amount of clay added on the shear strength of adhesives. The FTM 8 test method was used to determine the ability of an adhesive to withstand static forces applied in the same plane as the coated adhesive. Resistance to shear surface is defined as the time required for a standard area of pressure-sensitive coated material to slide from a standard flat surface in a direction parallel to the surface. 

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