The Rheology of Hot Melt Adhesive I
Hot melt adhesives are generally thermoplastic polymers of relatively high molecular weight, giving them both strength and high viscosity. But high molecular weight polymers alone generally do not have sufficient adhesion, so these polymers are blended with a variety of additives, which can include plasticizers, tackifiers and stabilizers, to increase adhesive performance. Since hot melts are classified as thermoplastics, exposure to high temperatures can cause the adhesive to re-melt. Therefore, most hot melt applications are limited to room temperature and near room temperature applications.
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 adhesives are applied in a molten state, and most must flow smoothly onto surfaces to ensure both wetting and adhesion. Thus, viscosity as a function of temperature is a key to propel hot melt performance. In addition, factors such as bond strength, flexibility, tack and set time are intimately related to the adhesive’s rheology. By knowing the rheological 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 of 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 adhesive.
In a typical rheological test, the material is placed between two fixtures. 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 can perform either method. The choice of method depends upon the nature of the material and the specific behavior being studied.
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