177,231 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
Connecting existing cemeteries saving good soils (for livings)
No full textBackground: Urban sprawl consumes and degrades productive soils worldwide. Fast and safe decomposition of corpses requires high-quality functional soils, and land use which competes with both agriculture and buildings. On one hand, cremation does not require much land, but it has a high energy footprint, produces atmospheric pollution, and is unacceptable to some religious communities. On the other hand, as exhumations are not practiced, "green burials" require more surface area than current burial practices, so a new paradigm for managing land use is required. Conclusions: In this paper, we propose a concept for 'green belt communalities' (i.e., ecological corridors with multiple, yet flexible, uses and services for future generations). With the expansion of urban centers, ecological corridors gradually disappear. Cemeteries for burial plots preclude alternative uses of the land for a long time. By combining these two aspects (need for connectivity and land take imposed by cemeteries), two positive results can be achieved: protecting memories of the past and connecting ecosystems with multiple-use corridors. This new paradigm works best in flat or hilly terrain where there are already several urban agglomerations that contain traditional cemeteries. Stakeholders who might consider this concept are local administrators, planners, and the communities of individuals who share specific beliefs on burial systems
Letteratura, teoria della
No full textL’espressione "Teoria della letteratura", elaborata nel terzo decennio del 20° secolo entro la corrente dei formalisti russi, s’impose negli anni Quaranta grazie a un fortunato manuale di R. Wellek e A. Warren (Theory of literature). Essa venne a identificare una disciplina in cui la visione s’intreccia continuamente con le tecniche.The expression, elaborated in the third decade of the 20th century within the current of the Russian formalists, prevailed in the forties thanks to a lucky manual by R. Wellek and A. Warren (Theory of literature). It came to identify a discipline in which vision is continually intertwined with techniques
Poly(Lactic Acid): Flow-Induced Crystallization
No full textPoly(lactic acid) is surely one of the most interesting commercially available biodegradable polymers. Being a slowly crystallizing material, it generally does not have the time to crystallize at the cooling rates involved in the common processing techniques. However, the properties induced by crystallinity are extremely interesting for tuning the characteristics of the obtained parts, and this leads the research to find the routes to an enhancement of PLA crystallization kinetics. During processing, polymer melts are subjected to very high deformation rates, and the stretch resulting from this orientation gives rise to shorter crystallization times and also to peculiar crystalline structures. The so-called flow-induced crystallization is therefore a phenomenon that for PLA can be considered strategic toward the realization of parts with enhanced properties. In this work, a state of the art on flow-induced crystallization of PLA is presented
Effect of pressure on viscosity at high shear rates by using an injection molding machine
No full textThermodynamic properties and crystallization kinetics of isotactic polypropylene under pressure
No full textThe effect of pressure on the processing and, in particular, on the crystallization kinetics of a polymeric material, is often ignored because of the difficulties involved in the experimental measurements. As a consequence, crystallization behaviors were rarely described by using a model which takes into account the effect of pressure on the crystallization. In this study, experimental data on the dependence of the specific volume on temperature and pressure of isotactic polypropylene using a high-pressure dilatometer up to 150 MPa are reported and discussed. In the complex polymorphism of isotactic polypropylene, the main way to obtain γ structure for a high molecular weight homopolymer is to perform crystallization under high pressure. From the subsequent heating step after the isothermal crystallization, the melting temperatures at the same pressures have been determined to assess the pressure effect on the equilibrium melting temperature. The crystallization times of isothermal experiments have been analyzed
Transport properties of water vapor through hemp fibers modified with a sustainable process: Effect of surface morphology on the thermodynamic and kinetic phenomena
No full textThe paper is focused on a deep analysis of transport properties of natural hemp fibers (HF) functionalized through a mechano-chemical treatment assisted by high energy ball milling. The effect of milling time on fibers’ morphology was investigated by means of the scanning electron microscopy and FTIR spectroscopy. The results show that the mechanical effect induced a reduction of the mean diameter of hemp fibers and, simultaneously, the achievement of a close packing of cellulose chains. The barrier properties appeared to be dependent either on the packing of cellulose or the water cluster formation. Fick's law solution in cylindrical coordinates was used to simulate experimental sorption kinetics and to extrapolate the diffusion parameters. The change of diffusion with equilibrium moisture content was described through an empirical relationship. Sorption isotherms were explained on the basis of Park model which accounts for a dual mode sorption at low activities as well as the water clustering phenomenon occurring at high activities. The mean cluster size (MCS) of water molecules was estimated through the application of Zimm Lundberg theory. Finally, the spreading pressure calculation allowed to estimate the driving force of diffusion inside the systems
Applicazione di metodi chemiometrici alla ricerca dello standard interno ottimale per la determinazione di metalli pesanti nei sedimenti, mediante ICP-AES
No full textFlow-induced crystallization of a Poly(Lactic acid): Effect of the application of low shear rates on the polymorphous crystallization
No full textThe study of the crystallization process of biodegradable polymers is crucial to predict the final properties of the industrial plastic parts. In this work, the effect of the application of low shear rates on the polymorphous crystallization of the Poly (lactic acid), PLA, has been studied. Results in terms of nucleation and growth rate of crystalline structures in the spherulitic regime revealed a non-monotonous dependence of the growth rate upon the shear rate. DSC and WAXD analyses on samples crystallized at 120 °C allowed to assess that in quiescent conditions the more ordered α phase prevails, but even at low shear rates a predominance of the α′ phase can be found. By increasing the shear rate, the amount of the more ordered α phase increases again. The transition from α′ to α seems to inhibit the spherulitic growth rate, that reaches a relative minimum at γ ̇ = 0.2 s−1. Similar observations hold true also at other temperatures in the range 110–130 °C. A further increase in the shear rate leads to a large prevalence of the α-phase, as in the case of quiescent conditions
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