1,720,961 research outputs found
Curing Kinetics of Bioderived Furan-Based Epoxy Resins: Study on the Effect of the Epoxy Monomer/Hardener Ratio
No full textThe potential of furan-based epoxy thermosets as a greener alternative to diglycidyl ether of Bisphenol A (DGEBA)-based resins has been demonstrated in recent literature. Therefore, a deep investigation of the curing behaviour of these systems may allow their use for industrial applications. In this work, the curing mechanism of 2,5-bis[(oxiran-2-ylmethoxy)methyl]furan (BOMF) with methyl nadic anhydride (MNA) in the presence of 2-methylimidazole as a catalyst is analyzed. In particular, three systems characterized by different epoxy/anhydride molar ratios are investigated. The curing kinetics are studied through differential scanning calorimetry, both in isothermal and non-isothermal modes. The total heat of reaction of the epoxy resin as well as its activation energy are estimated by the non-isothermal measurements, while the fitting of isothermal data with Kamal’s autocatalytic model provides the kinetic parameters. The results are discussed as a function of the resin composition. The global activation energy for the curing process of BOMF/MNA resins is in the range 72–79 kJ/mol, depending on both the model used and the sample composition; higher values are experienced by the system with balanced stoichiometry. By the fitting of the isothermal analysis, it emerged that the order of reaction is not only dependent on the temperature, but also on the composition, even though the values range between 0.31 and 1.24
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Polyurethane foams: novel processing and novel additives for improved thermal insulation properties
Full text linkGlobal energy consumption is expected to increase by more than 50% within the next ten years, as a result of the energy efficiency technologies that still do not keep up with the rapid growth of new buildings and the refrigeration sectors. As a consequence of the rising energy demand, environmental issues are becoming more apparent. Moreover, regulations on thermal insulation are becoming ever stricter. For these reasons, there is high industrial interest to design innovative and efficient materials that are capable of lowering the thermal conductivity and then reducing the heat loss in buildings. In this field, polyurethane foams already proved to be one of the most efficient materials in reducing the thermal conductivity. In this dissertation, focus was given to develop new knowledge and gain fundamental understanding about the PU foaming process and the various influencing factors. New strategies were developed in order to obtain rigid polyurethane as well as polyisocyanurate foams with low foam density and reduced cell size that could lead to improved thermal insulation properties. In particular, two main strategies have been followed and defined that can be resumed as follows. The first approach consisted in the investigation of foaming process of rigid polyurethanes obtained by high-pressure CO2. High-pressure CO2 foaming technology proved very effective with thermoplastics. In fact, with the pressure quench method, utmost performances in terms of cell number densities have been reached with numerous thermoplastic polymers: microcellular (cell size in the range 1–10 µm) and, more recently, nanocellular (cell size in the range 1–100 nm) foams have been produced, characterized by improved thermal insulating and mechanical properties as compared to standard cell-sized foams. With regard to thermosetting foams, the same approach used in thermoplastics has been implemented here in this work. To this aim, firstly, (i) the governing factors that affect the curing process of the rigid PU under high-pressure CO2 were monitored and studied by FT-NIR spectroscopy. (ii) Once that the effect of the CO2 on the curing process have been clarified, it is necessary to develop a new strategy to cope with the very different timescales of the processes under consideration, namely the rapid depressurization of the sorbed CO2 and the slow curing reaction. In this way, the polymer does not undergo to excessive stresses due to the depressurization and suitable polyurethane foams can be obtained. In this approach, by pushing on CO2 bubble nucleation, (iii) microcellular foams by high-pressure CO2 have been produced. The second approach consisted in the investigation and monitoring of the foaming process of rigid polyurethane and polyisocyanurate foams obtained by liquid-type organofluorine additives. Liquid-type additives have been extensively used in the last years to reduce the average cell size. Several works have shown the improvement of polyurethane and polyisocyanurate foam morphologies (and, correspondingly with improved mechanical and thermal properties) as a consequence of the introduction of these additives. However, besides the achievements based on characterization results only, the mechanisms induced by addition of these liquids have not been deeply studied. In our study, it has been found that organofluorine additives are suitable to this purpose, by inhibiting the Ostwald Ripening that affect the late foaming process. With the aim of investigating the effect of these additives, firstly, (i) the competing bubble formation mechanisms and how these are affected by the cell degenerations mechanisms were studied and monitored by in-situ optical observation. (ii) Once that the mechanisms affecting the final morphology are elucidated, it is necessary to develop a new strategy that allows to regulate the aforementioned governing factors. In this approach, by pushing on the depletion of the cell degeneration mechanisms, (iii) polyurethane as well as polyisocyanurate foams by liquid organofluorine additives have been produced. In conclusion, the developed methodologies allow to control the processing conditions in order to obtain the desired foam morphology
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Role of Air Bubble Inclusion on Polyurethane Reaction Kinetics
Full text linkIn this study, we investigated the influence of mixing conditions on the foaming process of water blown polyurethane (PU) foams obtained at different mixing speeds (50, 500, 1000 and 2000 rpm). In particular, the morphological evolution during the foaming process, in terms of the bubble size and bubble density, was studied via optical observations, while the effects on the reaction kinetics were monitored using in situ FTIR spectroscopy. At the slow mixing speed (50 rpm), no air bubbles were included and the early foaming process was characterized by the formation of new bubbles (CO(2) nucleation), provided by the blowing reaction. Later on, it was observed that the coalescence affected the overall foaming process, caused by the gelling reaction, which was inhibited by the indigent mixing conditions and could not withstand the bubbles expansion. As a result, a PU foam with a coarse cellular structure and an average bubble size of 173 µm was obtained. In this case, the bubbles degeneration rate, dN/dt, was −3095 bubble·cm(−3)·s(−1). On the contrary, at 500 rpm, air bubbles were included into the PU reaction system (aeration) and no formation of new bubbles was observed during the foaming process. After this, the air bubbles underwent growth caused by diffusion of the CO(2) provided by the blowing reaction. As the gelling reaction was not strongly depleted as in the case at 50 rpm, the coalescence less affected the bubble growth (dN/dt = −2654 bubble·cm(−3)·s(−1)), leading to a PU foam with an average bubble size of 94 µm. For the foams obtained at 1000 and 2000 rpm, the bubble degeneration was first affected by coalescence and then by Ostwald ripening, and a finer cellular structure was observed (with average bubble sizes of 62 µm and 63 µm for 1000 rpm and 2000 rpm, respectively). During the first foaming stage, the coalescence was less predominant in the bubble growth (with dN/dt values of −1838 bubble·cm(−3)·s(−1) and −1601 bubble·cm(−3)·s(−1), respectively) compared to 50 rpm and 500 rpm. This occurrence was ascribed to the more balanced process between the bubble expansion and the PU polymerization caused by the more suitable mixing conditions. During the late foaming stage, the Ostwald ripening was only responsible for the further bubble degeneration (with dN/dt values of −89 bubble·cm(−3)·s(−1) and −69 bubble·cm(−3)·s(−1), respectively)
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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