Simulation of the film blowing process for semicrystalline polymers
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We simulate the film blowing process using a fully thermodynamic model developed to study crystallization in polymers. The model stems from a general thermodynamic framework that is particularly well suited to describing dissipative processes during which the symmetry of the material can change; thus, it provides a good basis for studying the crystallization process in polymers. The polymer melt is modeled as a rate-type viscoelastic fluid and the crystalline solid polymer is modeled as an anisotropic elastic solid. The initiation criterian, marking the onset of crystallization and equations governing the crystallization kinetics, arise naturally in this setting in terms of the appropriate thermodynamic functions. The mixture region, wherein the material transitions from a melt to a semicrystalline solid, is modeled as a mixture of a viscoelastic fluid and an elastic solid. The anisotropy of the crystalline phase and consequently that of the final solid depends on the deformation in the melt during crystallization. The polymer melt is simulated using a rate-type model that is a generalization of the Maxwell model, which allows for the relaxation time to depend on the deformation. The results of the simulation agree qualitatively with experimental observations and the methodology described provides a framework in which the film blowing problem can be analyzed. Most previous attempts at describing film blowing terminate at the point at which the bubble reaches its maximum diameter. In fact, a continuation of the simulation in the previous studies, instead of terminating it abruptly on an ad hoc basis as is commonly done, would predict a collapsing bubble because the transition of the material from a fluid model to a solid model is not accounted for. This inadequacy of previous approaches to studying the problem cannot be overemphasized. Rather than just increasing the viscosity of the fluid, which is the usual practice, a true transition to a solid model is adopted in this study and this leads to a model that captures the smooth transition to stable bubble development. Copyright Taylor & Francis Inc.
