Shear strength, pressure sensitive

Materials that are already pressure-sensitive adhesives but can be modified by radiation (UV or EB) to introduce or increase cross-linking (which increases their adhesive properties, shear value, peel strength, service temperature range and solvent resistance). 

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. 

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]. 

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. 

At the other end of the spectrum, pressure-sensitive adhesives, contact adhesives, aqueous varnishes, clear sealants and caulks, overprint varnishes for metal decorating and printing are essentially pigment free. Minor amounts of silica are used for gloss reduction in the case of varnishes. Carpet, construction and ceramic/brick adhesives may contain substantial amounts of filler, not only to reduce cost but also to provide shear strength. Gloss and semi-gloss latex enamels... 

It has been shown that the plateau modulus of rubber-resin systems is a function of the volume fraction of the polymer, while the tan <5 peak temperature is related to both the concentration and glass transition temperature of the resin additive. Pressure sensitive adhesive performance of rubber-resin systems is also related to the concentration of the resin. The optimum balance of tack peel strength, and shear strength for typical systems can be found at a specific resin concentration. Therefore, performance zones can be identified on plots of tan 6 peak temperature and plateau modulus. This is significant because... 

The presence of a cross-linked network again led to the development of good pressure-sensitive properties. A plot of peel strength vs. composition for the cured PVEE-MCEA system is shown in Pig. 4. In subsequent experiments, the pressure-sensitive properties were maximized at 3.8 pli/48 hour shear for the 55/45 ratio. 

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... 

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. 

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