Polyurethane foams with a hybrid structure between closed cell and open cell were fabricated and fully characterized. Sound absorption measurements were carried out in order to assess their acoustic ...performance at different frequency ranges. The cellular structure of these systems was studied in detail by defining some novel structural parameters that characterize the cell wall openings such as the average surface of holes (
), the number of holes (
), and the area percentage thereof (%HCW). Therefore, these parameters allow to analyze quantitatively the effect of different structural factors on the acoustic absorption performance. It has been found that the parameters under study have a remarkable influence on the normalized acoustic absorption coefficient at different frequency ranges. In particular, it has been demonstrated that increasing the surface of the holes and the percentage of holes in the cell walls allows increasing the acoustic absorption of these types of foams, a promising statement for developing highly efficient acoustic insulators. Additionally, we could determine that a suitable minimum value of hole surface to reach the highest sound dissipation for these samples exists.
Edible coatings for carrots Villafañe, Fernando
Food Reviews International,
01/2017, Letnik:
33, Številka:
1
Journal Article, Book Review
Recenzirano
The use of ready-to-use fresh vegetables has recently increased as a result of the extended use of prepared salads, due to the novel changes in consumer trends. Fresh-cut carrots (Daucus carota L.) ...are one of the most widely used products in ready-to-eat salads. But the carrot market is restricted by the produce deterioration during storage, since several physiological changes reduce their shelflife. Different treatments have been described in order to prevent or slow down these processes. Edible coating benefits include stabilizing the product and thus extending its shelflife. The use of edible coatings for carrots is being largely explored in order to better keep their fresh-like characteristics. This review summarizes the advances on this topic, in an attempt to improve their design and development, which hopefully will lead to an increase in safety and quality of this widely used product.
•This topic has not been so far previously reviewed.•Some processes imply changes in the coordination geometry of the metal.•Considering the positions of the ligands, they migrate and/or they ...scramble.•Establishing rules about the dynamic behavior is so far not possible.•This review could encourage and help future researchers to get into this field.
This work reviews the dynamic behavior in solution of seven-coordinated complexes of different transition metals containing mono and polydentate monohapto ligands. The complexes are classified according to their geometry in the solid state, established by X-ray diffraction: pentagonal bipyramidal (PBP), capped octahedral (cO), capped trigonal prismatic (cTP), and complexes with 4:3 tetragonal base-trigonal base geometry (4:3). For each geometry, the processes are ordered from lower degrees of freedom to higher, thus going from complexes containing more polydentate ligands to those with more monodentate ligands. The processes detected in solution may be classified into two main groups: those implying changes in the coordination geometry of the metals, and those affecting the positions of the ligands. No changes in the coordination geometry of the starting or final products of the dynamic process are usually observed, but the proposal of intermediates of different geometries to that of the starting or final products is common. Two main different types of processes may be differentiated considering the positions of the ligands involved in the dynamic processes: migration and scrambling, and both occur for most of the dynamic processes herein reviewed.
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•Bidentate diimine ligands coordinated to ReI(CO)3 are obtained in situ.•These methods are usually easier than the traditional procedures.•Properties of the final complex may be tuned ...by choosing the adequate substituents.•Similar reactions leading to tridentate ligands are also included.•ReI(CO)3 complexes are interesting due to their photophysical properties.
This work covers the coupling reactions of monodentate ligands coordinated to the “ReI(CO)3” moiety to afford in situ new bidentate diimine ligands. Similar reactions leading to tridentate ligands are also included. The reactions reviewed present evident advantages respect to the usual synthetic methods of these ligands. Besides, choosing the appropriate substituents allows an easy tuning of the properties of the final complexes. Following an approximate chronological order, the formation of bidentate ligands by C-N coupling from a coordinated carbonyl group, from diiminoisoindoline, and the formation of amidino ligands from nitriles are reviewed first. This is followed by the formation of bidentate ligands by C-C coupling, and finally by the formation of tridentate ligands, either by C-C coupling, by B-N coupling, or by using “click” reactions.
Highly transparent polyisocyanurate-polyurethane (PUR-PIR) aerogels were synthesized, and their optical properties were studied in detail. After determining the density and structural parameters of ...the manufactured materials, we analyzed their optical transmittance. It was demonstrated that the catalyst content used to produce the aerogels can be employed to tune the internal structure and optical properties. The results show that the employment of lower catalyst amounts leads to smaller particles forming the aerogel and concomitantly to higher transmittances, which reach values of 85% (650 nm) due to aerogel particles acting as scattering centers. Thus, it was found that the lower this size, the higher the transmittance. The effect of the sample thickness on the transmittance was studied through the Beer-Lambert law. Finally, the scattering mechanisms involved in the light attenuation were systematically evaluated by measuring a wide range of light wavelengths and determining the transition between Rayleigh and Mie scattering when the particles were larger. Therefore, the optical properties of polyurethane aerogels were studied for the first time, opening a wide range of applications in building and energy sectors such as glazing windows.
This research evaluates the effect of temperature and time austempering on microstructural characteristics and hardness of ductile iron, validating the results by means of a statistical method for ...hardness prediction. Ductile iron was subjected to austenitization at 950 °C for 120 min and then to austempering heat treatment in a salt bath at temperatures of 290, 320, 350 and 380 °C for 30, 60, 90 and 120 min. By increasing austempering temperature, a higher content of carbon-rich austenite was obtained, and the morphology of the thin acicular ferrite needles produced at 290 °C turned completely feathery at 350 and 380 °C. A thickening of acicular ferrite needles was also observed as austempering time increased. An inversely proportional behavior of hardness values was thus obtained, which was validated through data analysis, statistical tools and a regression model taking temperature and time austempering as input variables and hardness as the output variable, which achieved a correlation among variables of about 97%. The proposal of a mathematical model for the prediction of hardness in austempered ductile iron represents a numerical approximation which validates the experimental results at 95.20%.
To determine the effect of nanoclays and trapped air on the formation of rigid polyurethane foams, three different production procedures were used. To study the influence of mixing at atmospheric ...pressure, two approaches were carried out employing either an electric or a magnetic stirrer. The third approach was executed by mixing under vacuum conditions with magnetic stirring. The samples thus obtained were characterized, and the effect of trapped air into the reactive mixtures was evaluated by analyzing the cellular structures. Different levels of trapped air were achieved when employing each manufacturing method. A correlation between the trapped air and the increase in the nucleation density when nanoclays were added was found: the cell nucleation density increased by 1.54 and 1.25 times under atmospheric conditions with electric and magnetic stirring, respectively. Nevertheless, samples fabricated without the presence of air did not show any nucleating effect despite the nanoclay addition (ratio of 1.09). This result suggests that the inclusion of air into the components is key for improving nucleation and that this effect is more pronounced when the polyol viscosity increases due to nanoclay addition. This is the most important feature determining the nucleating effect and, therefore, the corresponding cell size decreases.
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