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Crystallization Modalities in Polymer Melt Processing Fundamental Aspects of Structure Formation von Janeschitz-Kriegl, Hermann (eBook)

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Crystallization Modalities in Polymer Melt Processing

Structure formation in crystallizing polymers, as occurring during processing, has not been treated so far in a coherent form. This fact explains, why this monograph is written as the ?rst book devoted to this subject. A quarter of a century ago the underdevelopment of this subject was obvious. Trial and error dominated. In fact, other apposite subjects as polymer melt rheology or heat transfer, had reached high levels. A great number of books has been devoted to them. Mold ?lling of amorphous polymers and the solidi?cation of these polymers by vitri?cation can nowadays be simulated numerically with a high degree of accuracy. In the solidi?ed sample even residual stresses and corresponding birefringence effects can accurately be 1 calculated . However, semicrystalline polymers, which form the majority of industrial po- mers, have been excluded from these considerations for good reasons. In fact, great uncertainties existed about the formation of quality determining crystalline str- tures. In particular, polyole?ns suffered from this shortcoming. In 1983 this fact instigated the polymer research group at the Johannes Kepler University in Linz to start with pertinent activities. The urgency of this kind of studies becomes evident, if advantages and hitches of these polymers are considered. 1. Versatility of processing: Injection molding into a great variety of shapes and sizes, from thin walled beakers to garden chairs, not to forget pipe and pro?le extrusion, cable coating, ?ber spinning, ?lm blowing. 2. Product qualities: Ductility, low density, good electric insulation, corrosion resistance, surface quality.


    Format: PDF
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    Seitenzahl: 221
    Sprache: Englisch
    ISBN: 9783211876275
    Verlag: Springer-Verlag
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Crystallization Modalities in Polymer Melt Processing

Chapter 2 Kinetics and Structure Formation in Unloaded Quiescent Melts (p. 69-70)

2.1 Introductory Remarks

It goes without saying that structure formation in a permanently quiescent unloaded melt does almost never occur in practical polymer processing. In fact, flow and pressurization can almost never be avoided. Nevertheless, the present chapter will appear of great importance, as flow or pressure induced crystallization cannot be understood without a profound basic knowledge of the processes occurring in a permanently quiescent melt, which has not been put under pressure. Such a melt must be cooled down in its quiescent state from a temperature well above the equilibrium melting point, where the residues of previous crystallization processes are erased.

2.2 Empirical Techniques

2.2.1 Number Density of Nuclei

The number density of nuclei is of particular interest for the structure formation. This quantity determines the number density of spherulites, which is finally obtained and which determines various properties of the product. In fact, a fine grained structure is preferable in many cases because of the good ductility obtained with such a structure. On the other hand, as has been pointed out in Sect. 1.2.1, the growth speed of the spherulites mainly determines the speed of the solidification process and, as a consequence, the heat transfer problems.

From the early times up to now there has been a discussion, whether the assumption of sporadic nucleation with a temperature dependent rate of nucleation or just the assumption of a predetermined number density of nuclei as a unique function of temperature will be most appropriate for a description of the solidification process in quiescent polymer melts (see the alternative use of (1.11) or (1.12)). The use of the latter assumption and equation has been advocated by Van Krevelen in an early paper [1] just because of the easier handling. However, fortunately it has turned out meanwhile that there are strong physical reasons for the latter assumption. One can be quite sure that below the melting temperature of the spherulites (see Fig. 1.3) all nuclei are of the athermal type [2,3], which means that they have their specific temperature of activation. As a consequence they become effective immediately, when this temperature is reached during the cooling process. A theoretical discussion will be given below. In the present section just the experimental methods will be described, which lead to the determination of the said number densities as functions of temperature.

Janeschitz-Kriegl, Ratajski and Wippel [3] described a successful method for the determination of the number density of an industrial PP as a function of temperature. For the purpose a cylindrical sample of a diameter of 4 mm was prepared in the solid state. In the heart of this sample a thin hole was drilled from one end down to half the length of the sample. In this hole a thin thermocouple with a diameter of 0.3 mm was placed, so that its junction was at the end of the hole. This sample was wrapped into a metal foil and suspended in a horizontal position in the coil of a wire.

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