Structure Development and Mechanical Performance of Polypropylene Structure Development and Mechanical Performance of Polypropylene by Tim B. van Erp. Technische Universiteit Eindhoven, 2012. A catalogue record is available from the Eindhoven University of Technology Library ISBN: 978-90-386-3164-6 Reproduction: University Press Facilities, Eindhoven, The Netherlands. Cover design: Paul Verspaget (Verspaget & Bruinink) and Tim van Erp This research is part of the research programme of the Dutch Technology Foundation STW, ”Predicting Catastrophic Failure of Semi-Crystalline Polymer Products”. Structure Development and Mechanical Performance of Polypropylene PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de rector magnificus, prof.dr.ir. C.J. van Duijn, voor een commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op donderdag 5 juli 2012 om 16.00 uur door Tim Bernardus van Erp geboren te Helmond Dit proefschrift is goedgekeurd door de promotoren: prof.dr.ir. G.W.M. Peters en prof.dr.ir. H.E.H. Meijer Copromotor: dr.ir. L.E. Govaert Contents Summary ix Introduction 1 Background 1 Processing-Structure-Properties Relation 3 Scope of the Thesis 5 References 6 1 Quantification of Non-Isothermal, Multi-Phase Crystallization 7 1.1 Introduction 8 1.2 Theory 9 1.3 Experimental 12 1.3.1 Materials 12 1.3.2 Fast Cooling Experiments 12 1.3.3 Differential Fast Scanning Calorimetry 13 1.3.4 Multipass Rheometer (MPR) 13 1.3.5 Dilatometry 13 1.3.6 X-Ray 14 1.4 Results and Discussion 15 1.4.1 Experimental Approach 15 1.4.2 Fast Cooling Experiments 16 1.4.3 Pressurized Cooling Experiments 21 1.4.4 Dilatometry 23 1.5 Conclusions 27 References 27 v vi Contents 2 Rate, Temperature and Structure Dependent Yield Kinetics 31 2.1 Introduction 32 2.2 Experimental 33 2.2.1 Materials 33 2.2.2 Fast Cooling 34 2.2.3 X-Ray 34 2.2.4 Mechanical Testing 35 2.3 Results 35 2.3.1 Processing - Structure Relation 35 2.3.2 Yield Kinetics 38 2.3.3 Time-to-Failure 42 2.3.4 Structure - Properties Relation 43 2.3.5 Discussion 45 2.4 Conclusions 48 References 49 3 Structure Development during Cooling at Elevated Pressure and Shear Flow 53 3.1 Introduction 54 3.2 Experimental 55 3.2.1 Material 55 3.2.2 Dilatometry 55 3.2.3 X-Ray 57 3.2.4 Transmission Electron Microscopy (TEM) 58 3.3 Methods 58 3.3.1 Normalized Specific Volume 58 3.3.2 Weissenberg Number 59 3.3.3 Dimensionless Numbers 60 3.4 Results and Discussion 60 3.4.1 Dilatometry 60 3.4.2 Morphology 65 3.5 Conclusions 73 References 73 Contents vii 4 The Oriented Gamma Phase 77 4.1 Introduction 78 4.2 Experimental 79 4.3 Results and Discussion 79 4.4 Conclusions 83 References 83 5 Flow-Enhanced Crystallization Kinetics during Cooling at Elevated Pressure 85 5.1 Introduction 86 5.2 Experimental 87 5.2.1 Material 87 5.2.2 Dilatometry 87 5.2.3 X-Ray 88 5.3 Methods 88 5.3.1 Normalized Specific Volume 88 5.3.2 Weissenberg Number 89 5.3.3 Dimensionless Numbers 90 5.4 Modeling 90 5.4.1 Quiescent Crystallization 90 5.4.2 Flow Effects on Crystallization 92 5.5 Results and Discussion 94 5.6 Conclusions 99 References 100 5.7 APPENDIX 102 6 Prediction of Yield and Long-Term Failure of Oriented Polypropylene 103 6.1 Introduction 104 6.2 Experimental 105 6.2.1 Material 105 6.2.2 Mechanical Testing 106 6.3 Experimental Results 106 6.4 Constitutive Modeling 108 6.4.1 Viscoplastic Model 108 6.4.2 Equivalent Stress 109 6.4.3 Flow Function 110 6.4.4 Time-to-Failure 110 viii Contents 6.5 Model Application 112 6.5.1 Characterization. 112 6.5.2 Validation 113 6.6 Conclusions 114 References 116 7 Mechanical Performance of Injection Molded Polypropylene 117 7.1 Introduction 118 7.2 Experimental 119 7.2.1 Material 119 7.2.2 Injection Molding 120 7.2.3 Optical Microscopy 120 7.2.4 Fourier Transform InfraRed (FTIR) Spectrometry 120 7.2.5 X-Ray 121 7.2.6 Mechanical Testing 121 7.3 Results and Discussion 121 7.3.1 Microstructure. 121 7.3.2 Mechanical Properties 124 7.3.3 Model Application 126 7.4 Conclusions 129 References 130 Conclusions and Recommendations 133 Conclusions. 133 Recommendations. 134 References 137 Samenvatting 139 Dankwoord 141 Curriculum Vitae 143 List of Publications 145 Summary Polymers are known for their ease of processability via automated mass production technologies. The most important process is injection molding that, due to its freedom in material choice and product design, allows producing a wide variety of thermoplastic products. Mechanical failure of these products, either upon impact or after prolonged exposure to load, limits their ultimate useful lifetime. To predict and control lifetime, understanding of the route from production to failure, i.e. the processing-structure-property relation, is necessary. This is a complex issue; especially in the case of semi-crystalline polymers. These are heterogeneous systems comprised of amorphous and crystalline fractions, of which the latter can be highly anisotropic with size and orientation that are strongly dependent on the precise processing conditions. As a consequence, these structural features in the microstructure, and the associated mechanical properties, generally exhibit distributions containing different orientations throughout a single processed product. Understanding polymer solidification under realistic processing conditions is a prerequisite to predict final polymer properties, since only a complete characterization of the morphology distri- bution within a product can lead to a meaningful and interpretable mechanical characterization. In this thesis we study the relation between processing conditions, morphology and mechanical performance of a semi-crystalline polymer, isotactic polypropylene. Key issue is the accurate control over all relevant processing parameters. Therefore, different experimental techniques are used to obtain samples at different high cooling rates, at elevated pressures, and high shear rates. A custom designed dilatometer (PVT- ˙ T -˙γ-apparatus) proves to represent the most important and useful technique. First, a predictive, quantitative model is presented for the crystallization kinetics of the multiple crystal structures of polypropylene, under quiescent conditions. The approach is based on the nucleation rate and the individual growth rate of spherulites of each type of polymorphism (α-, β-, γ-and mesomorphic phase), during non-isothermal, isobaric solidification. Using Schneider’s rate equations, the degree of crystallinity and distribution of crystal structures and lamellar thickness is predicted. Next, the effect of flow is introduced. Flow strongly influences the kinetics of the crystallization process, especially that of nucleation. Three regimes are observed in the experiments; quiescent crystallization, flow enhanced point nucleation and flow-induced creation of oriented structures. To assess the structure development under flow, a molecular-based rheology model is used. Combining the models derived for quiescent and for flow-induced crystallization, yields the tool that is capable of predicting the volume distributions of both isotropic and oriented structures, under realistic processing conditions. ix x Summary The kinetics of mechanical deformations strongly depend on the anisotropy in the crystalline morphology, thus the local orientation. To study this, uniaxially oriented tapes with a well defined, and high, degree of anisotropy are used as well as injection molded rectangular plates. Yield and failure are described using an anisotropic viscoplastic model, applying a viscoplastic flow rule. It uses the equivalent stress in Hill’s anisotropic yield criterion, and combines the Eyring flow theory with a critical equivalent strain. Factorization is used and the model is capable to quantitatively predict the rate, the angle and the draw ratio dependence of the yield stress, as well as the time-to- failure in various off-axis tensile loading conditions. To use the model, also for other polymers, characterization of only the isotropic state is sufficient. Therefore, the influence of the cooling rate on the deformation kinetics is studied in-depth on isotropic systems. Different cooling rates induce different crystal phases, both the stable α-phase and the mesomorphic phase, while also the degree of crystallinity and lamellar thickness are influenced. The deformation kinetics prove to be the same for the different microstructures, which means that the activation volume and energy are independent of the thermodynamic state. Differences in thermal history are, consequently, solely captured by two rate constants which are a function of the microstructure. [...]... method is presented to quantify the effect of thermal and pressure history on the isotropic and quiescent crystallization kinetics of four important crystalline structures of isotactic polypropylene, i.e the α-, β-, γ- and mesomorphic phase Subsequently, the mechanical performance of PP-based systems comprised of only α- and mesomorphic phase as a result of systematic variations in thermal history... 1 Abstract The structure of semi-crystalline polymers is strongly influenced by the conditions applied during processing and is of major importance for the final properties of the product A method is presented to quantify the effect of thermal and pressure history on the isotropic and quiescent crystallization kinetics of four important structures of polypropylene, i.e the α-, β-, γ- and mesomorphic... non-isothermal and isobaric crystallization and structure development of isotactic polypropylene (iPP) and β-nucleated isotactic polypropylene (β-iPP) The influence of flow is ongoing work The most established physical picture of quiescent crystallization is nucleation and subsequent growth of spherulites; crystalline lamellae grow in three dimensions starting from point-like nuclei The nucleation density and growth... with high contents of γ-phase (Chapter 4) In Chapter 6 the mechanical performance of uniaxially oriented polypropylene tape is discussed An anisotropic viscoplastic model is presented based on factorization of the rate and draw ratio dependence and is capable of quantitatively predicting the rate, angle and draw ratio dependence of the yield stress as well as time-to-failure in various off-axis tensile... analysis of European plastics production, demand and recovery for 2010 Technical report, www.plasticseurope.org, 2007 [8] B A G Schrauwen Deformation and Failure of Semicrystalline Polymer Systems: Influence of Micro and Molecular Structure Ph.D thesis, Eindhoven University of Technology, 2003 Quantification of Non-Isothermal, Multi-Phase xxxxxxx Crystallization: The Influence of Cooling Rate and Pressure... thesis, we aim to identify basic principles and tools for process-induced structure development, but also provide direct assessment of its influence on the resulting short and longterm mechanical performance of the final product This thesis focusses on two aspects, both related to the processing -structure- property relationship of isotactic polypropylene; isotropic and anisotropic systems In Chapter 1 a method... important class of polymers are the polyolefins; mainly PE and PP The basis of the dynamic development of polyolefins and their still tremendous potential lies in [4]: • Their versatility with respect to physical and mechanical properties and applications • Their nontoxicity and bioacceptability • The energy savings during their production and use, in comparison with other materials • Their low cost and the... real-time WAXD collection By means of deconvolution, the evolution of the different crystal fractions as a function of time and temperature is accessed Figures 1.5 and 1.6 show the evolution of the α- and 17 Results and Discussion mesomorphic phase for six cooling rates for iPP1 and iPP2, respectively Crystallization is a kinetic process and, therefore, the formation of α-phase is suppressed with increasing... (sections A, B and C resp.) From this simple example the complexity of the processing -structure- property relation becomes clear, and it is, therefore, evident that the ability to predict the mechanical properties of polymer products is uniquely linked to the capability to assess the development of the various structures during processing within a product 70 x 70 x 1 mm A B A injection of polymer C B... polymer C B C Figure 6: Variation in microstructure over the thickness in a simple product, and the resulting different mechanical responses of samples cut from different parts of a typical injection molded plaque of high-density polyethylene Scope of the Thesis Catastrophic failure of polymer artifacts, either upon impact (e.g of protective products such as airbags and helmets) or after prolonged exposure . Structure Development and Mechanical Performance of Polypropylene Structure Development and Mechanical Performance of Polypropylene by. Catastrophic Failure of Semi-Crystalline Polymer Products”. Structure Development and Mechanical Performance of Polypropylene PROEFSCHRIFT ter verkrijging van de