1,720,994 research outputs found
Opacification Kinetics of PLA during Liquid Water Sorption
No full textWhen in contact with water, poly(lactic acid), PLA, undergoes several physical changes. A very evident one is opacification, namely the change from the typical transparent appearance to a white opaque color. This phenomenon is particularly significant for many applications, including packaging, since opacity hinders the possibility of a clear look of the packed goods and also worsens the consumers’ perceptions. In this work, we report an analysis of the time evolution of the phenomenon in different conditions of temperature and water concentration. The results allow us to define a time-scale of the phenomenon and to put it in relationship with the temperature and water content inside the material. In particular, opacification proceeds from the outer surface of the specimens toward the center. Both craze formation due to hydrolysis and crystallization contribute to the opacification phenomenon. Opacification becomes faster as temperature increases, whereas the increase in the solution density has the opposite effect. A model for describing the evolution of opacification was proposed and found to be consistent with the experimental data
A method to obtain the quantitative orientation of semicrystalline structures in polymers by atomic force microscopy
No full textMolecular orientation can determine the final properties in polymer parts during processing: In optoelectronic devices, the emission efficiency is strongly dependent on the orientation of the emitter materials; mechanical performances in polymer parts depend on the orientation and dimension of crystalline structures. A simpler and faster method to obtain the quantitative orientation of crystalline structures, based on atomic force microscopy, is introduced as a powerful alternative to the techniques mentioned above. This method is based on the acquisition of topographical maps along with the sample thickness and applying the directionality analysis to each map to obtain the distribution of orientation on the map. Such a distribution was analyzed following two approaches: The first one is based on Herman’s analysis; it is quite similar to the one adopted for calculating the Herman’s factor from the wide-angle X-ray scattering. The second one is simpler; it is based on the standard deviation of the distribution. Both approaches allowed the determination of an orientation parameter: The orientation parameter was close to 1 in the regions where a high number of oriented fibrils were found, vice versa, the orientation parameter was close to zero where spherulites were found. The orientation parameter was found highly consistent with Herman’s factor for injection molded samples obtained with different mold temperatures, thus with different distributions of orientation and morphology
Prediction of morphology distribution within injection molded parts: Description of fibrils formation
No full textShort processing times and high dimensional accuracy made injection molding a widespread process for polymeric part production. Recently introduced fast cavity heating cycles (FHC) enhance part accuracy at the micrometrical level, duration, and the mechanical properties of the parts. Despite several works devoted to the experimental observations of the effect of FHC on part properties, only a few works focused on the prediction of morphology development during the process. Particularly concerning the formation of fibrils in semicrystalline parts, which mainly determines mechanical properties. The proposed two-steps approach allows predicting fibril formation in the case of a well-characterized polypropylene. The first step describes the process; the second step adopts the outputs of the first one for describing fibril formation. The simulation outputs for temperature, pressure evolutions, and morphology distributions successfully compare with the experimental findings
Modeling spherulitic and fibrillar crystallization kinetics in injection molding: Analysis of process variables and morphological evolution
No full textInjection molding of polypropylene was conducted using a heating device to control mold surface temperature over time. By varying the temperature and heating times, a significant effect on cavity pressures, temperatures, and morphology distribution in the resulting samples was obtained. The University of Salerno's UNISA code, a simulation model for injection molding, was used to simulate the experimental conditions. This code incorporates a crystallization kinetics model that accounts for the competing formation of spherulites and fibrils from the same amorphous material, considering the effects of temperature, pressure, and flow evolution during different stages of the process. The simulation results showed good agreement with experimental data for temperature evolutions within the mold and pressure variations at critical points during the filling, packing, and cooling stages. Additionally, the simulation consistently predicted the formation of a fibrillar layer near the mold wall. Overall, a comprehensive framework for understanding and simulating the injection molding process is provided, with relevant implications for the optimization of manufacturing conditions and improving part quality
Modeling of the injection molding process coupled with the fast mold temperature evolution
No full textThe modulation of mold temperature during injection molding is a strategic issue since it allows modulating/calibrating interesting properties of the moldings. In this work, thin heating devices were layered on the cavity surface allowing its fast temperature evolution between injection and cooling channels temperatures. The heating devices were made by a conductive layer between two insulating layers with thicknesses selected in order to realize a heating/cooling cycle as fast as possible. Several tests were performed, injecting polypropylene (iPP), using different heating powers and heating times to analyze the effect of the fast cavity surface temperature evolution on the molding morphology and properties. The heat transfer through the mold was modeled, accounting for the Joule effect in the conductive layer of the heating devices. To validate the proposed modeling of the heat exchange during the process, the simulated temperature evolutions at the polymer–cavity and the heating device–mold interfaces were satisfactorily compared with the experimental ones recorded during the tests conducted adopting different mold temperature evolutions. Furthermore, pressure evolutions during the process recorded in different position along the flow path were satisfactorily compared with the simulated ones to validate the predictions of the thermo–mechanical histories experienced by the polymer
Structure/properties relationship within injection molding samples obtained by fast modulation of the cavity temperature
No full textThe tailoring of the properties developed in the plastic objects only by the process is mandatory in order to improve the sustainability of the plastic objects. The possibility to tailor the properties developed within the molded object is essentially related to the understanding of the relationship between the properties and the process. One of the main process parameters that allows tailoring properties of molded objects is the mold temperature. In this work, a thin electrical heater was developed and located below the cavity surface in order to obtain rapid heating/cooling cycles during the process. An isotactic polypropylene was adopted. The modulation of the cavity temperature was found to control the distribution of the properties along the sample thickness. In particular, elastic modulus and lamellar thickness distributions were strongly influenced by the temperature field experienced by the polymer chains. Both distributions show the highest level in the shear layer and the intermediate values in the sample core. The correlations between these two properties is due to the structuring phenomenon occurring during the process
Modeling and Analysis of Morphology of Injection Molding Polypropylene Parts Induced by In-Mold Annealing
No full textIt is generally recognized that high-temperature treatments, namely annealing, influence the microstructure and the morphology, which, in turn, determine the mechanical properties of polymeric parts. Therefore, annealing can be adopted to control the mechanical performance of the molded parts. This work aims to assess the effect of annealing on the morphology developed in isotactic polypropylene (iPP) injection-molded parts. In particular, a two-step annealing is adopted: the polymer is injected in a mold at a high temperature (413 or 433 K), which is kept for 5 min (first annealing step); afterward, the mold temperature is cooled down at 403 K and held at that temperature for a time compatible with the crystallization half-time at that temperature (second annealing step). The characterization of morphology is carried out by optical and electronic scanning microscopy. The temperature of the first annealing step does not influence the thickness of the fibrillar skin layer; however, such a layer is thinner than that found in the molded parts obtained without any annealing steps. The second annealing step does not influence the thickness of the fibrillar skin layer. The dimension of spherulites found in the core is strongly influenced by both annealing steps: the spherulite dimensions enlarge by the effect of annealing steps. A model that considers spherulite and fibril evolutions is adopted to describe the effect of molding conditions on the final morphology distribution along the part thickness. The model, which adopts as input the thermo-mechanical histories calculated by commercial software for injection molding simulation, consistently predicts the main effects of the molding conditions on the morphology distributions
Morphology predictions in molded parts: a multiphysics approach
No full textInjection molding of polymer parts at a micro-scale is successfully applied in the fabrication of electronics and biomedical devices where high geometrical accuracy is required. Microinjection molding is more challenging than conventional injection molding due to the necessity to account for the presence of air in the cavity, which slows down the process, and a strong flow field that may induce premature solidification of semi -crystalline polymers. The process modeling and simulation are crucial steps toward predicting all the final parts' properties. For this reason, a multiphysics approach was used to model microinjection molding under different mold cycle temperatures. A well-characterized polypropylene was selected for this purpose. A model for tracking the polymer-air interface during the filling was adopted to account for air's effect in the micro-cavity. Additionally, a model accounting for the effect of crystallization on viscosity was implemented. Models describing the evolution of morphology into fibrils were previously proposed in steady-state conditions. In this work, a model for describing the crystallization into fibrils was proposed and adapted for the first time to the transient conditions of microinjection molding. The aim was the prediction of the final morphology developed during the process. The morphology evolution toward fibrillar structure is consistent with those observed experimentally; in particular, the fibrillar layer thickness decreased with the increase of the mold temperature.(c) 2022 Institution of Chemical Engineers. Published by Elsevier Ltd. All rights reserved
A layer-by-layer approach based on APTES/Cloisite to produce novel and sustainable high performances materials based on hemp fiberboards
No full textThe paper presents a hybrid and smart methodology, using the layer-by-layer approach, to produce high performant multi-layered composites based on hemp fiberboards with improved physical properties. The effect of an inorganic component, such as a natural clay, and the surface covering with an organic compound, such as stearic acid, was investigated through the analysis of morphological, spectroscopic, thermal, mechanical and barrier properties of the composites. Vertical burning test confirmed the improvement in flame retardancy of layered composites since the presence of clay. The oil adsorption capacity was evaluated and correlated to the number of clay layers. Finally, a possible mechanism of combustion was proposed
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