Injection molding of semicrystalline polymers. I. Material characterization

Polymer crystallization under external flow

The general aspects of polymer crystallization under external flow, i.e., flow-induced crystallization (FIC) from fundamental theoretical background to multi-scale characterization and modeling results are presented. FIC is crucial for modern polymer processing, such as blowing, casting, and injection modeling, as two-third of daily-used polymers is crystalline, and nearly all of them need to be processed before final applications. For academics, the FIC is intrinsically far from equilibrium, where the polymer crystallization behavior is different from that in quiescent conditions. The continuous investigation of crystallization contributes to a better understanding on the general non-equilibrium ordering in condensed physics. In the current review, the general theories related to polymer nucleation under flow (FIN) were summarized first as a preliminary knowledge. Various theories and models, i.e., coil-stretch transition and entropy reduction model, are briefly presented together with the modified versions. Subsequently, the multi-step ordering process of FIC is discussed in detail, including chain extension, conformational ordering, density fluctuation, and final perfection of the polymer crystalline. These achievements for a thorough understanding of the fundamental basis of FIC benefit from the development of various hyphenated rheometer, i.e., rheo-optical spectroscopy, rheo-IR, and rheo-x-ray scattering. The selected experimental results are introduced to present efforts on elucidating the multi-step and hierarchical structure transition during FIC. Then, the multi-scale modeling methods are summarized, including micro/meso scale simulation and macroscopic continuum modeling. At last, we briefly describe our personal opinions related to the future directions of this field, aiming to ultimately establish the unified theory of FIC and promote building of the more applicable models in the polymer processing.

Non-Isothermal Crystallisation Kinetics of Polypropylene at High Cooling Rates and Comparison to the Continuous Two-Domain pvT Model

The modelling of the correlation between pressure, specific volume and temperature (pvT) of polymers is highly important for applications in the polymer processing of semi-crystalline thermoplastics, especially in injection moulding. In injection moulding, the polymer experiences a wide range of cooling rates, for example, 60 °C/min near the centre of the part and up to 3000 °C/min near the mould walls. The cooling rate has a high influence on the pvT behaviour, as was shown in the continuous two-domain pvT model (CTD). This work examined the Hoffman–Lauritzen nucleation and growth theory used in the modified Hammami model for extremely high cooling rates (up to 300,000 °C/min) by means of Flash differential scanning calorimeter (DSC) measurements. The results were compared to those of the empirical continuous two-domain pvT model. It is shown that the Hammami model is not suitable to predict the crystallisation kinetics of polypropylene at cooling rates above 600 °C/min, but that the continuous two-domain pvT model is well able to predict crystallisation temperatures at high cooling rates.

High-density polyethylene crystals with double melting peaks induced by ultra-high-molecular-weight polyethylene fibre

High-density polyethylene (HDPE)/ultra-high-molecular-weight polyethylene (UHMWPE) fibre composites were prepared via solution crystallization to investigate the components of epitaxial crystal growth on a highly oriented substrate. Scanning electron microscopy morphologies of HDPE crystals on UHMWPE fibres revealed that the edge-on ribbon pattern crystals that were formed initially on UHMWPE fibres converted afterwards to a sheet shape as crystallization progressed. Wide-angle X-ray diffraction confirmed that the polymer chain oriented along the fibre axis and the orthorhombic crystal form of HDPE remained unchanged in HDPE/UHMWPE fibre composite systems. The thermal behaviour of the fibre composites measured by differential scanning calorimetry showed double melting peaks, the nature of which, as disclosed by partial melting experiments, is ascribed to bilayer components existing in the induced crystals: the inner layer is composed of more regularly folded chain crystals induced by UHMWPE fibres, and the outer layer formed on the inner one with a thinner and lower ordered crystal structure.

Large Amplitude Oscillatory Shear Induces Crystal Chain Orientation in Velocity Gradient Direction

Imposition of large amplitude oscillatory shear (LAOS) on crystallizing polymer melts results in lamellar orientation in the shear gradient direction, in contrast to the flow-orientation observed for steady shear. LAOS enhances the formation of plate-like nuclei and orients their normals in the gradient direction. An Arrhenius temperature dependence (with activation energy ≈ 226 kJ/mol) characterizes the relaxation of crystal orientation with annealing.

Structure Development in Injection Molding: A 3D Simulation with a Differential Formulation of the Kinetic Equations

The purpose of the present work is to introduce a crystallization law into Rem3D, a 3D code written in C++ and dedicated to the injection molding of polymers. We kept the basic hypotheses of Avrami's model and cast the kinetic equations into a differential system that is solved numerically. The variation of the density of potential nuclei with temperature is taken into account. Furthermore, the distribution of mean spherulite sizes can be deduced from the calculations. The second part of the paper is an experimental study of crystallization in well-controlled conditions (2D, isothermal or constant cooling-rate). It establishes a procedure for the determination of the nucleation and growth parameters used in the theoretical model, and gives a first validation of this model. Finally, the crystallization equations are introduced into Rem3D, in order to assess the feasibility of our new approach. Some typical results concerning the evolution of the transformed volume fraction in injection-molded parts are presented.

Simulação do processo de injeção de polipropileno isotático (iPP) utilizando um modelo de cinética de cristalização quiescente

Este trabalho concentrou-se na simulação das fases de preenchimento e resfriamento do processo de injeção do polipropileno isotático. Foi utilizado um modelo matemático baseado nas equações de conservação onde foi considerada a cinética de cristalização quiescente como termo fonte na equação da energia. Os parâmetros do modelo de cinética de cristalização, assim como os do modelo de viscosidade, foram obtidos experimentalmente. Para a solução das equações governantes foi utilizado o método dos volumes finitos. Como resultados foram obtidos os campos de temperatura, pressão, velocidade, taxa de cisalhamento e cristalinidade em diferentes condições de processamento.