Shear pressure-sensitive tape

We may now make some quantitative estimates, using plausible numbers for the physical and mechanical properties that are involved. For a fibril drawn from the adhesive of a pressure-sensitive tape, we may assume an order of magnitude for rf, of about 0.01 mm, and ryy -5 rf, i.e. about 0.05 mm. We assume the substrate to be hard, strong and smooth, and that "physical" forces act across the interface, so that AG will be of the order of 100 ergs/cm = 0.10 j/m. The shear strength of the polymer, for a practical rate of elongation, may be about 1.0 X 10 N/m ( ) or about 140 psi. Then 0.004. 

Adhesive properties of pressure-sensitive tapes are characterized primarily by three parameters peel adhesion, shear adhesion and tack. 

Shear Adhesion (Creep). The most significant example of creep is a tape gliding down under shearing load. Therefore, most commonly creep is characterized by the time a tape holds under defined shear load (Fig. 13). This often is called holding power or shear adhesion. Further examples of creep are the telescoping of rolls of stretchable pressure-sensitive tapes and the greasy spots occurring on a paper sheet mended by a transparent adhesive tape. 

Nonstructural adhesives are characterized by low shear strength (usually less than 1000 lb/ in ) and poor creep resistance at slightly elevated temperatures. The most common nonstructural adhesives are based on elastomers and thermoplastics. Although these systems have low strength, they usually are easy to use and fast setting. Most nonstructural adhesives are used in assembly-line fastening operations or as sealants and pressure-sensitive tapes. 

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 ... 

Shear holding test with 12 mm x 12 mm overlap bond (Reproduced with permission from the Pressure Sensitive Tape Council)... 

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. 

Excessive shear strain should generally be avoided in adhesive joints because cohesive failure is likely to occur once shear deformation leads to a buildup of tensile stress at the end of the overlap bond area. In practice, it has been found that strain in assemblies with rubbery moisture curing one-component polyurethane adhesives should be limited to values of shear strain tany below 0.5. Specific viscoelastic pressure sensitive tapes may even tolerate shear strains of tany up to 2. The maximum tolerable strain of structural adhesive joints using adhesives with a highly cross-linking density, however, is typically more than one order of magnitude lower than that. 

Pressure-sensitive adhesives (PSAs) are materials used to bond other materials on their surfaces by the bonding processes of adhesion and cohesion. They have the abihty to adhere instantaneously (also known as tack), resist removal by peel, and hold their position when shear forces are applied. Pressure-sensitive adhesives have been utilized in a range of medical applications from simple pressure-sensitive tapes to implant adhesives. Silicone PSAs are commonly used in transdermal drug delivery systems drug-loaded adhesive... 

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 ... 

Pressure-sensitive adhesives (PSAs) are used in various forms for packaging, medical and masking tapes and labels. PSAs are viscoelastic materials that exhibit sohd-like and liquid-like behavior so that their performance can be evaluated by tack (the capability of a PSA to adhere instantly under light pressure), peel strength (the tensile force required to remove a PSA) and shear strength (flow resistance) [1, 2]. 

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. 

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. 

Adh6SiVGS. Acrylic emnlsion polymers are nsed in a wide variety of adhesives. Pressure-sensitive Adhesives, which typically have Tg values less than 20° C, are the main type of acrylic adhesive. Acrylic polymers and copolymers find use as PSAs in tapes, decals, and labels. Along with their aforementioned superior chemical resistance properties, acrylics possess an excellent balance of tack, peel, and shear properties which is crucial in the adhesives market (218,219). Other types of adhesives that employ acrylics include construction formulations and film-to-film laminates. 

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. 

In 1966, Dahlquist(i7) defined the requirement for a good pressure-sensitive adhesive as an adhesive with one-second shear creep compliance greater than J(t) = 1 x 10cm /dyn (Figure 17). Recently, we characterized the viscoelastic properties of many conmiercial tapes and label adhesives and found that the glass transition temperature and modulus (G ) at the application temperature are the most important requirements for good performances (Figures 18, 43, and 52). The requirements for tape and label adhesives are somewhat different for... 

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

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 ... 

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