The Rheology of Hot Melt Adhesive II
The successful application and use of hot melt adhesives involves many physical properties that can be measured through rheological testing. Flow properties, optimum application temperature and pot life are a few of the areas where rheological testing can be used to analyze and improve a product or process.
Because resins are often held for several hours in open tanks in dispersing equipment, the viscosity of hot melt adhesives must be stable over time at processing temperatures. Notice how the viscosity drops initially because of molecular weight reduction due to the absorption of water, but over time the viscosity rises slowly. This is caused by slow oxidative crosslinking. A re-formulated resin is also shown that is stable for more than two hours at process temperatures and is better suited to this application.
Generally, the hot melt with pigment has a much steeper slope than the transparent adhesive alone. The addition of pigment has given the adhesive more structure, radically changing its shear behavior. In this case, the process where this adhesive is used will have to be changed to allow for the increased structure of the product with pigment.
The temperature at which an adhesive solidifies can be changed by varying the formulation. Usually, the application temperature for these adhesives is 195 degrees C. At this temperature, the modulus and flow properties of the adhesives are the same. But as the adhesive cools, it is evident that the slow-set adhesive solidifies at a lower temperature.
Hot-melt adhesives must have good flow during application, be stable over time, and form a tough bond. The rheology of the adhesives, as well as the structural features of the material, influences these properties. By using rheological measurements to study material response and predict performance, adhesive chemists can improve existing products, develop new products and customize product formulations for specific applications.
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