Testing of adhesive properties

Tanknoto, Y Saeki, H. Kimoto, S. Nishiwaki, T. Nishiyama,N. Evaluation of adhesive properties of three resilient denture liners by the modified peel test method. Acta. Biomater., 2009, 5(2), 764-769. 

The thickness of the adhesive layer is of importance in most adhesive joints and shonld be measnred and controlled in Tests of adhesion. Adhesive layer thickness is often determined by a nnmber of factors, some intentional in the design process and some resnlting from a variety of items related to the adhesive properties and the processing steps. Factors affecting the final bond thickness inclnde adhesive viscosity (which may fall dramatically and then rise during elevated temperature cure), clamping pressure and... 

The study of fundamental adhesion has been hampered because standard Tests of adhesion provide a result that is a complicated combination of fundamental adhesion, the physical properties of the adherend and the viscoelastic/plastic character of the adhesive (see Adhesion - fundamental and practical, Peel tests). Our understanding of adhesion has been significantly improved with the advent of mechanical devices that are able to probe the forces of adhesion under conditions that minimize all of the confounding effects of adherend, viscoelasticity, and so on. The Surface Forces Apparatus (SFA) as developed by Israelachvili and Tabor is a mechanical device that has allowed adhesion scientists to directly measure the forces of adhesion under very low rate, light loading, almost equilibrium conditions. Attention is also drawn to Atomic force microscopy. 

Other tests involve nondestructive evaluations of adhesive properties such as density, shelf life, tack, working life, and flow characteristics. These properties are important in the manufacturing and processing of structural adhesive bonds. 

Besides using these tests as a method for comparing various adhesives, they are also applicable for determining the dependence of adhesive properties on different variables. These variables can be test temperature, test rate, bond thickness, or environmental conditions. 

The results of fracture tests of adhesive bonds are almost never independent of the experimental geometry because the presence of the interface with its discontinuity in elastic properties ensures that the stress field at the interface depends on both the external loading and the elastic properties mismatch as discussed in chapters on hard adhesives. However, soft adhesives have the added complication to dissipate energy, not only in a restricted plastic zone near the interface, but over a large volume, often the entire volume of the sample. This means that there is a very strong coupling between the boundary conditions of the test (thickness of the layer, size of the probe and stiffness of the probe) and the observed deformation mechanisms. 

As mentioned before, adhesion is one of the major properties of a protective coating. Testing of adhesion seems dierefore of prime importance for the characterization of organic coatings. In practice it is, however, rather difficult to measures the adhesion of organic coatings. 

Testing of adhesives is necessary in order to determine the level of performance and/or predicted durability. Some of the tests provide information on the working properties of the adhesive, such as viscosity, which affects mixing, application, and spreadability as well as wetting and penetration ofthe substrate. Othertest methods measure the amount of resin present (solid content). This not only influences the viscosity of the adhesive, but also the performance of the bonded assembly.PI... 

There are a number of challenges associated with the testing of adhesive and sealant systems. The performance of these systems is never solely due to material properties. Often, the results of a test are a function of the test rather than the system being tested. This can be due to a number of variables including the... 

A range of sonic vibration techniques may be used for non-destructive testing of adhesive joints [143,147,148]. Most of these depend upon a void causing a local change in stiffness and hence a change in vibrational properties of the structure. They involve applying vibration excitation at a test point and... 

Not only is adhesive testing carried out under dry conditions but also under wet conditions. Exposing the painted part to a humidity chamber (typically 100% relative humidity at 38°C) for 96 to 240 hours can increase the likelihood of paint delamination as moisture can penetrate through the coating layer into the substrate. Increasing the temperature to 140 or 160°C as in the Cleveland Humidity Chamber can further test the adhesive properties of the painted part. With the formulator performing testing under conditions that are much more severe than specified by the OEM, it is likely to increase the chance of success at the customer, even under conditions that are usually less than ideal. 

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