Mesoporous carbon as Pt support for PEM fuel cell

A mesoporous carbon (MP) supported Pt nanocatalyst was evaluated as anode and cathode catalyst for PEM fuel cell. Kinetics study of the oxygen reduction reaction were characterized by using the rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) techniques in acid media. Membrane electrode assemblies (MEAs) were prepared using Pt supported on MP as anodic and cathodic catalysts and the fuel cell performance evaluated. Polarization and power curves show a similar performance as cathode catalyst when compared to commercial catalyst while there is an 8% improvement when used as anode catalyst.     

Vinylogous Urethane Vitrimers

Vitrimers are a new class of polymeric materials with very attractive properties, since they can be reworked to any shape while being at the same time permanently cross‐linked. As an alternative to the use of transesterification chemistry, we explore catalyst‐free transamination of vinylogous urethanes as an exchange reaction for vitrimers. First, a kinetic study on model compounds reveals the occurrence of transamination of vinylogous urethanes in a good temperature window without side reactions. Next, poly(vinylogous urethane) networks with a storage modulus of ≈2.4 GPa and a glass transition temperature above 80 °C are prepared by bulk polymerization of cyclohexane dimethanol bisacetoacetate, m‐xylylene diamine, and tris(2‐aminoethyl)amine. The vitrimer nature of these networks is examined by solubility, stress‐relaxation, and creep experiments. Relaxation times as short as 85 s at 170 °C are observed without making use of any catalyst. In addition, the networks are recyclable up to four times by consecutive grinding/compression molding cycles without significant mechanical or chemical degradation.     

Physicochemical properties of novel pectin/<Emphasis Type="Italic">Aloe</Emphasis> gel membranes

Nowadays, an increased interest on the development of novel biomaterials for different applications has been observed. The biopolymers such as proteins, lipids, polysaccharides or their combinations have been used for the manufacturing of novel membranes. The pectin is a polysaccharide that previously has been used only or combined with polymeric matrices, but the formulation of PEC/AG has not been evaluated yet. The aim of this work was to evaluate the physicochemical properties of a novel PEC/AG membrane. A mixture of pectin citric (PEC)/Aloe gel (AG) was prepared to manufacture membranes at 100/0 (control), 90/10, 80/20, 70/30, 60/40 and 50/50 proportions (% v/v) by casting method. Water vapor permeability (WVP), solubility, ATR-FTIR spectroscopy, microstructure, mechanical and optical properties were assessed for the produced membranes. The PEC/AG membranes showed a yellowish color, low UV light transmission at 200 nm and no significant changes in the opacity values. In addition, the microstructure by scanning electron microscopy (SEM) showed changes on the surface appeared as differently sized structures. An analysis of the total area of the 3284 cm^?1 showed rearrangement of hydrogen bonds of the polysaccharide macromolecules, suggesting an enhanced interaction between the PEC and AG chains. In addition, all the membranes of PEC/AG showed high solubility (100%), low WVP and better toughness, extensibility and plasticity as compared with the control. The assessed physicochemical properties of the produced membranes suggested that they may be used as biomaterial for multiple applications in the medical, pharmaceutical, cosmetic or food industries.     

Kinetics of ascorbic acid degradation in fruit-based infant foods during storage

The kinetics of ascorbic acid (AA) degradation in a fruit-based beikost product added with AA were determined after storage at 4, 25, 37 and 50 °C during 4, 8, 12, 16 and 32 weeks in plastic polypropylene/ethylene–vinyl alcohol vacuum packaging. It was confirmed that AA degradation followed an Arrhenius first-order kinetics, with an activation energy of 20.11 ± 0.33 kcal mol⁻¹. No AA losses at 4 °C were recorded during the entire storage period. In contrast, a time- and temperature-dependent decrease (p < 0.05) in AA was observed at the other tested temperatures – the degradation rate decreasing from 50 °C to 25 °C, as expected. AA percentage retention at the end of storage ranged between 6.4% (50 °C/16 weeks) and 100.9% (4 °C/32 weeks).     

Evolution of the Population of Bubbles in 2D Fluidized Beds

The size of the bubbles in a solid‐gas 2D fluidized bed was estimated by image capture and digitalization using a CCD camera. It was confirmed that the size distribution for the bubbles in a slice of the bed was skewed, varying with the location of the slice. The results were analyzed using the Agarwal (1985) model, which is based on a population balance. For this model, four adjustable parameters are required, as well as the knowledge of bubble size just above the distributor. Experimental data were compared with values predicted by the model. The parameters that the model needs were obtained by three procedures: i) the constants supplied by Agarwal (1985) and Rowe and Everett (1972); ii) the parameters obtained from experimental data for a 1 cm wide slice bed; and iii) the parameters obtained by fitting the bubble diameter to all the bubbles in the bed. The second and third methods provided the best results.     

Influence of porosity on cavitation instability predictions for elastic–plastic solids

Cavitation instabilities have been found for a single void in a ductile metal stressed under high triaxiality conditions. Here, the possibility of unstable cavity growth is studied for a metal containing many voids. The central cavity is discretely represented, while the surrounding voids are represented by a porous ductile material model in terms of a field quantity that specifies the variation of the void volume fraction in the surrounding metal. As the central void grows, the surrounding void volume fractions increase in nonuniform fields, where the strains grow very large near the void surface, while the high stress levels are reached at some distance from the void, and the interaction of these stress and strain fields determines the porosity evolution. In some cases analysed, the porosity is present initially in the metal matrix, while in other cases voids nucleate gradually during the deformation process. It is found that interaction with the neighbouring voids reduces the critical stress for unstable cavity growth.     

Limitations of the Log-Logistic Model for the Analysis of Sigmoidal Microbial Inactivation Data for High-Pressure Processing (HPP)

This study identified limitations of the log-logistic model to evaluate microbial inactivation kinetics by high-pressure processing (HPP) including the need to assign a numerical value to “approximate” the undefined expression log₁₀ t?=?0 and the misinterpretation of its parameters due to a derivation flaw. Peer-reviewed HPP microbial inactivation data were adjusted to a sigmoidal equation (SIG), the original “vitalistic” log-logistic models (VIT-1, VIT-6), and two functions that did not follow the original derivation procedure (LOG-1, LOG-6). Their goodness of fit was determined utilizing the coefficient of determination (R ² ) and Akaike information criteria (AIC). The shape of the survival curve greatly influenced the performance of log-logistic models. VIT and LOG models performed equally when the kinetic curve showed a sigmoidal shape, and the numerical values of their parameter estimates were identical regardless of the log₁₀ (t?=?0) approximation. Conversely, most concave curves yielded inaccurate parameter estimates for all models. LOG-1 and VIT-1 performed best when log₁₀ t?=?0 was ?1 or ?2, whereas LOG-6 and VIT-6 yielded best results for values of ?3 to ?9. SIG ranked last for most datasets but occasionally performed best (Akaike weight factor w_AICi ?=?0.40–1.00) when microbial survival counts showed clear sigmoidal shapes. VIT models consistently displayed R ² ?≥?0.98, and their parameters can be interpreted within a “biological” context using the corrected derivation shown for LOG models. However, concave curves are more frequently observed for HPP microbial inactivation, and fitting the experimental data to log-logistic models deems unnecessary.