Classification and Discussion of Melting Mechanisms

After the polymer has been shaped into the desired form, we are faced with the solidification problem (i.e., the inverse of the melting problem). We will find that, some of the solution methods developed in this chapter with regard to melting, are also valid for solidification. 

The physical mechanisms that can bring about melting or heating of any substance are included in the terms of the thermal energy balance, Eq. 2.9-14 

Let us now discuss, in physical terms, how important each of the previously discussed mechanisms is to the melting of polymers, and the limitations or advantages of each one due to the physical nature of polymers. 

Example 5.1 Thermal Degradation Characterization Thermal degradation is characterized by two temperature-dependent parameters, the induction time 6 (7) for the onset of degradation, as shown in Fig. E5.1(a) for unplasticized Polyvinyl chloride (PVC), and the rate of degradation. The latter is shown in Fig. E5.1(b) in terms of the rate of change of the consistency index of the Power Law parameter, as a function of time and temperature, and can be expressed by the following equations  

Because of these limitations, and in particular because of the fact that, in such a mechanism, the temperature gradient at the wall that determines the heat flux to the solids drops exponentially with time, this melting mechanism is rather inefficient. However, the latter drawback can be alleviated if some mechanism continuously removes the molten layer. This, as shown in Fig. 5.3, can be accomplished either by applying a force normal to the heated surface, forcing out the melt by pressure flow, or by having the contact surface move parallel to its plane, dragging away the molten layer. These comprise the two 

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Classification and Discussion of Melting Mechanisms Important to Polymers... 

In this entry, the classification, preparation, properties, fabrication, safety considerations, and economics of fluoropolymers are discussed. Monomer synthesis and properties have also been discussed. Increasing the fluorine content of a polymer increases chemical and solvent resistance, flame resistance, and photostability, improves electrical properties, such as dielectric constant, lowers coefficient of friction, raises melting point, increases thermal stability, and weakens mechanical properties. 

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