Crystallization Modalities in Polymer Melt Processing (eBook)

Acknowledgments 6
Preface 7
Contents 10
Required Basic Achievements 12
1.1 Interaction of Three Transport Phenomena: Heat Transfer, Flow and Crystallization Kinetics 12
1.2 Available Theories Describing the Crystallization Process 16
1.2.1 The Kolmogoroff–Avrami–Evans Theory 16
1.2.2 The Rate Equations of Schneider, Köppl and Berger 23
1.3 Examples for Special Cases 26
1.3.1 Minimum Cooling Speed for by-Passing Crystallization, Eder 26
1.3.2 A Dimensionless Process Classification Number 29
1.3.3 Scanning Calorimetry 33
1.3.4 Phenomena of Propagation and Spreading 46
1.4 Crystallization in Confined Spaces 59
1.4.1 A Derivation of Kolmogoroff’s Equation According to G. Eder, Enabling a Generalization 59
1.4.2 Behavior of Confined Samples 62
1.5 Influence of Strong Temperature Gradients 71
1.5.1 Growth of Spherulites in Temperature Gradient 72
1.5.2 Counter-Balance by a Gradient in the Number Density of Nuclei 75
References 76
Kinetics and Structure Formation in Unloaded Quiescent Melts 79
2.1 Introductory Remarks 79
2.2 Empirical Techniques 79
2.2.1 Number Density of Nuclei 79
2.2.2 Growth Speeds of Spherulites 87
2.3 Theoretical Considerations 100
2.3.1 Theory of the Growth Speed of Spherulites 100
2.3.2 On the Nature of Primary Nuclei in Polymer Melts 104
2.3.3 Winter’s Gel Point 112
References 113
Flow Induced Processes Causing Oriented Crystallization 116
3.1 Preamble 116
3.2 Some Comments of Considerable Reach 117
3.3 Survey of Activities in the Field of Flow Induced Crystallization 123
3.3.1 Duct Flow Experiments 124
3.3.2 Flow Induced Small-Sized (“point-like”) Nuclei 143
3.3.3 Relaxation Phenomena 155
3.3.4 Adherence to the Growth Mechanism 172
3.3.5 Uninterrupted Flow Treatments 182
References 198
Closing Remarks 203
4.1 General Aspects 203
4.2 Views on Flow Induced Processes 207
References 219
Subject Index 222
Author Index 225

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.