Poly(lactic acid) crystallization

Poly (lactic acid) foaming

This study has been dedicated to the foaming of modified poly (lactic acid) with supercritical CO 2 . The first part of this work consisted in a rheological modification of neat PLA through chain extension. Improvement of the melt viscosity and elasticity has been achieved by the use of an epoxy additive during a reactive extrusion process. Rheological characterizations confirmed an increase of the melt strength due to this chain extension process. Foaming was then performed on the neat and modified PLAs using a batch process with supercritical CO 2 as blowing agent. The investigation of the foaming temperature revealed an enlarged processing window for modified PLAs compared to neat PLA. Depending on the foaming parameters, foams with a cellular structure ranging from macro scale to micro scale have been obtained. A concomitant effect of the CO 2 -plasticization and the crystallisation on the melt rheology could explain this wide range of cellular morphologies.

Batch foaming of chain extended PLA with supercritical CO2: Influence of the rheological properties and the process parameters on the cellular structure

The crystallization and melting behaviors of linear polylactic acid (PLA) treated by compressed CO2 was investigated. The isothermal crystallization test indicated that while PLA exhibited very low crystallization kinetics under atmospheric pressure, CO2 exposure significantly increased PLA’s crystallization rate; a high crystallinity of 16.5% was achieved after CO2 treatment for only 1 min at 100 °C and 6.89 MPa. One melting peak could be found in the DSC curve, and this exhibited a slight dependency on treatment times, temperatures, and pressures. PLA samples tended to foam during the gas release process, and a foaming window as a function of time and temperature was established. Based on the foaming window, crystallinity, and cell morphology, it was found that foaming clearly reduced the needed time for PLA’s crystallization equilibrium.

https://www.mdpi.com/1422-0067/10/12/5381/pdf

A Study of the Crystallization, Melting, and Foaming Behaviors of Polylactic Acid in Compressed CO2

Facile preparation of open-cellular porous poly (l-lactic acid) scaffold by supercritical carbon dioxide foaming for potential tissue engineering applications

The effect of CO 2 on the isothermal crystallization kinetics of poly(L-lactide). PLLA, was investigated using a high-pressure differential scanning calorimeter (DSC), which can perform calorimetric measurements while keeping the sample polymer in contact with pressurized CO 2 . It was found that the crystallization rate followed the Avrami equation. However, the crystallization kinetic constant was changed depending upon the crystallization temperature and concentration of CO 2 dissolved in the PLLA. The crystallization rate was accelerated by CO 2 at the temperature in the crystal-growth rate controlled region (self-diffusion controlled region), and depressed in the nucleation-controlled region. CO 2 has also decreased the glass transition temperature. T g , and the melting temperature. T m . As a result, the CO 2 -induced change in the crystallization rate can be predicted from the magnitudes of depression of both T g and the equilibrium melting temperature. The crystalline structure and crystallinity of polymers crystallized in contact with pressurized CO 2 were also investigated using a wide angle X-ray diffractometer (WAXD). The resulting crystallinity of the sample was increased with the pressure level of CO 2 , although the presence of CO 2 did not change the crystalline structure.

Crystallization kinetics of poly(L‐lactide) in contact with pressurized CO2


 System durability is crucially important for the successful commercialization of fuel cell electric vehicles (FCEVs). Conventional accelerated durability testing protocols employ relatively high voltage to hasten carbon corrosion and/or platinum catalyst degradation. However, high voltages are strictly avoided in commercialized FCEVs such as the Toyota MIRAI to minimize these degradation modes. As such, conventional durability tests are not representative of real-world FCEV driving conditions. Here, modified start-stop and load cycle durability tests are conducted on prototype fuel cell stacks intended for incorporation into commercial FCEVs. Polarization curves are evaluated at beginning of test and end of test, and the degradation mechanisms are elucidated by separating the overvoltages at both 0.2 and 2.2 A cm-2. Using our modified durability protocols with a maximum cell voltage of 0.9 V, the prototype fuel cell stacks easily meet durability targets for automotive applications, corresponding to 15-year operation and 200,000 km driving range. These findings have been applied successfully in the development of new fuel cell systems for FCEVs, in particular the second-generation Toyota MIRAI.

Accelerated Durability Testing of Fuel Cell Stacks for Commercial Automotive Applications: A Case Study

Cold start cycling durability of fuel cell stacks for commercial automotive applications

1-dimentional (1-D) physical modeling methods of the dynamic behavior of an integrated fuel cell (FC) system are investigated, which consist of the models of the FC stack and subsystems of the air, hydrogen (H2), and cooling systems. To ensure numerical simulation of the entire FC system in life-long system operation (> 10 years) in acceptable calculation time, a proper model resolution is selected. The subsystem models are integrated with the FC stack model and controllers developed in our previous research to build a closed-loop simulator of an integrated FC system. It demonstrated that the impacts of the FC stack material properties on the overall FC system performance can be investigated by numerical simulation.

Shigeki Hasegawa

Papers

Crystallization kinetics of poly(L‐lactide) in contact with pressurized CO2

Accelerated Durability Testing of Fuel Cell Stacks for Commercial Automotive Applications: A Case Study

Cold start cycling durability of fuel cell stacks for commercial automotive applications

Modeling of the dynamic behavior of an integrated fuel cell system including fuel cell stack, air system, hydrogen system, and cooling system