Why Hot Melt Polyurethane Rheology is Important
Rheology can be defined as the study of the deformation and flow of materials. Typically, materials exhibit both elastic (the ability to store energy) and viscous (the ability to dissipate energy in the form of heat) behavior. These behaviors change as a function of temperature, time and rate or degree of deformation.
Hot melt polyurethane are applied in a molten state, and most must flow smoothly characteristics of a given hot melt, users can determine its suitability for a given task, or modify the formulation to customize it for a specific application.
One of the best ways to study the rheological behavior ofonto surfaces to ensure both wetting and adhesion. Thus, viscosity as a function of temperature is a key to proper hot melt performance. In addition, factors such as bond strength, flexibility, tack and set time are intimately related to the polyurethane’s rheology. By knowing the rheological hot melts is through dynamic oscillatory measurements. This technique involves oscillating, or twisting, the material at different frequencies and amplitudes and studying the resultant behavior. Use of this technique enables the user to observe changes in both viscosity and elasticity as a function of temperature or rate — without changing the structure of the polyurethane.
In a typical rheological test, the material is placed between two fixtures (parallel plates in the case of melts, torsion clamps in the case of solids). The test can be performed in one of two ways. The material can be deformed by rotating one of the fixtures by a known amount, and the resulting force is measured. This is known as a controlled strain test. Controlled stress tests can also be performed. These are facilitated by holding the sample in the same manner, but generating data by applying a known force and measuring the resultant deformation. Both methods work well, and instruments such as the MCR can perform either method. The choice of method depends upon the nature of the material and the specific behavior being studied.
Elasticity can be defined as a material’s ability to store deformational energy, and is represented by G’, or storage modulus. In simple terms, the elastic component of a material can be thought of as a spring; when the deformation is removed, the material uses this energy to return to its original shape.
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