Different Degrees Of Conversion Research Articles

Investigating the activity of modified TiO2 photocatalysts used for the photoreforming of biomass derivatives

Different TiO2 modified (with Pt, Cu2O, Nb) photocatalysts have been compared for the photo-reforming of glucose and fructose at ambient conditions with the aim of linking the photoactivity to some structural and surface characteristics. A different degree of conversion, distribution of intermediates and H2 production were observed with the various photocatalysts. Moreover, the results obtained with the same catalysts were slightly different with the two substrates, highlighting the importance of the interaction between the catalyst surface and the organic compound. Bare TiO2 was inactive towards H2 production, Cu2O was effective in replacing Pt for hydrogen generation, and the presence of Pt/Nb was beneficial for both H2 production and selective oxidation. Moreover, Pt not only works as a sink for the photoproduced electrons, as it is well known in the literature, but also modifies the surface acid-base properties of catalysts as revealed by DRIFT and TPD measurements.

Determination of the degree of conversion, the diffuse reflectance, and the color stability after different aging processes of gingiva-colored composite resins.

To determine the degree of conversion (DC) and spectral diffuse reflectance of four different gingiva-colored composites and to evaluate their color stability after various aging processes. The gingiva-colored composites were assigned to four experimental groups (Anaxgum (AG), Crea.lign paste Gum (CB), Gradia Gum (GR), SR Nexco Gum (NC)). A total of 120 disc-shaped specimens (10 × 2mm) (n = 30/group) were polymerized in a Teflon mold. The nature of chemical bonding was studied by Fourier transform infrared spectroscopy (FTIR). Diffuse reflection spectra of the polymerized specimens were gathered using an ultraviolet-visible-near infrared (UV-Vis-NIR) spectrophotometer. Specimens subjected to aging methods were divided into three subgroups (n = 10): ultraviolet aging, hydrothermal aging, and autoclave aging. Color differences (ΔE*ab and ΔE00 ) were determined by colorimetry before and after aging. The statistical analysis was done using a two-way ANOVA along with paired sample t-test and Bonferroni's post hoc test. Conversion degrees varied between 26.9% and 59.7% and all groups showed 3 or 4 maxima at different positions in the visible region of the spectrum. Both ΔE*ab and ΔE00 values were significantly different from the groups of different brands for all types of aging processes. Similarly, there were significantly different ΔE*ab and ΔE00 values according to the aging procedure for all groups of particular brands, except for ΔE00 of SR Nexco Gum (NC). The aging procedures resulted in significant color differences between similar shades of four commercial gingiva-colored composites. The composite resins showed different degrees of conversion and diffuse reflectance spectra. The aging conditions tested affected the color stability. Patients with gingiva-colored indirect restorations should be informed about time-dependent discoloration.

Investigation of physio-mechanical properties of cross-linked Bis-GMA/TEGDMA dental resins: A molecular dynamics study

In this work, molecular dynamics simulation was employed to investigate the cross-linked network structure formation and resulting physio-mechanical properties of typical Bis-GMA/TEGDMA dental resin models. An automated polymerization protocol was initially proposed to mimic the curing process and cross-linked Bis-GMA/TEGDMA models were constructed with different degrees of conversion (DCs). The network structure of the cured chains was then characterized by examining radial distribution function (RDF) and molecular weight fraction profiles. The RDF peaks at ∼1.54 Å indicated the polymerization successfully proceeds, and the molecular weight fraction of the largest cross-linked chain increases with DC, while that of the second largest molecule starts to decrease around a DC of 40 %, signifying the formation of the cross-linked network. In addition, the effects of the 3D network formation on the physio-mechanical properties, i.e., density, volume shrinkage, glass transition temperature (Tg), and elastic modulus, were also investigated. As the DC rises, both the density and volume shrinkage exhibit a significant increase due to the double bond conversion. Furthermore, Tg was evaluated and adjusted at various DCs and their relation (Tg vs. DC) agrees well with the DiBenedetto’s equation. More intriguingly, the elastic modulus was found to increase almost linearly with DC; while discrepancy still exists for models with DC < 40 % as the dental resins present a rubber-like state (T > Tg). The findings in this work are believed to provide a robust automated polymerization protocol as well as the atomic/molecular scale understanding of the cross-linked network structure and physio-mechanical properties of Bis-GMA/TEGDMA dental resins.

Elucidating reaction pathways occurring in CO2 hydrogenation over Fe-based catalysts

A detailed experimental analysis of reaction pathways proceeding in CO2 hydrogenation over Fe-based catalysts was performed. The catalytic activity was measured at different degrees of conversion and used to analyze the production/consumption rates of reagents and products along the catalyst bed. It was demonstrated that except for the reverse water gas shift reaction and the subsequent hydrogenation of CO via Fischer-Tropsch (FT) mechanism, additional routes of hydrocarbon formation from CO2 exist in the CO2 hydrogenation. One of them is the methanation reaction which takes place over all studied catalysts. Another one is a direct pathway of C2+ hydrocarbon formation from CO2 that plays a significant role at least for some catalysts. In this route, iron carbides may be responsible for hydrogenating CO2 into C2+ hydrocarbons. Based on these results, the concept of requiring iron oxides as an essential active component for catalyzing the RWGS reaction in CO2-FT should be reviewed.

Study on Steam Co-Gasification of Waste Tire Char and Sewage Sludge

The large and growing volume of tire waste and sewage sludge requires disposal, for which thermochemical processes such as gasification can be used. Co-gasification of these two waste products allows the tire char to be used as a charge stabilizer and the sewage sludge to improve reactivity and efficiency. The purpose of this study was to evaluate the effect of the composition of a waste tire char and sewage sludge fuel blend on the gasification process, using steam as the gasification agent. Tests were carried out for tire char, municipal sewage sludge, and blends of the two in ratios of 90:10 and 67:33. An analysis of the materials used was carried out (ultimate and proximate analysis as well as ash composition), and isothermal measurements of steam gasification were taken using the thermal volumetric method for temperatures of 800, 850, and 900 °C at an elevated pressure of 1 MPa. On the basis of the results, the formation curves of the main gasification products (H2, CO, CO2, and CH4) were created, the curves for the degree of carbon conversion were plotted, the reactivity indexes were determined for different degrees of conversion (0.25, 0.5, and 0.75), and the quantity and composition of the resulting gas were analyzed. Using the grain model, the kinetic parameters (activation energy and pre-exponential factor) of the gasification reaction were calculated. The addition of municipal sewage sludge had a positive effect on the reactivity of tire char and increased the efficiency of gasification, because it contained components that act as catalysts in the gasification process. There was a favorable effect from the addition and higher amount of sewage sludge on lowering both the activation energy (49.5 kJ/mol and 89.2 kJ/mol for 90:10 and 67:33 blends, respectively) and the pre-exponential factor. A significant improvement in reactivity, with a high degree of conversion and the best gas composition, was obtained for a 90:10 blend at 900 °C.

Do high translucency zirconia shades contribute to the degree of conversion of dual-cure resin cements?

The opacity of the zirconia ceramic restoration may influence the degree of conversion (%DC) of dual-cured resin cements. The aim of this study was to evaluate the degree of conversion of resin luting agents photocured under different high translucency zirconia shades. The opacity of each ceramic shade was evaluated. The opacity percentage of Lava Frame and three Lava Plus High Translucency Zirconia ceramic shades (W1, A2, and B4) was determined. A spectrophotometer MiniScan was used to measure the opacity percentage of each specimen (0.7 mm thick) and then the opacity was calculated. Specimens from three different resin cements (Panavia F2.0, RelyX Unicem 2 Automix Self-Adhesive, and PermaFlo DC) were prepared with 100-µm thickness. The specimens were photocured according to the manufacturers' instructions under a ceramic block (0.7 mm thick). Specimens photocured without the ceramic block were used as control. Fifteen groups (n = 3) were evaluated. Micro-ATR/FTIR (micro-attenuated total reflectance/Fourier-transformed infrared spectroscopy) spectrometry was used to evaluate the extent of polymerization of all specimens after 24 hours. The %DC was determined using experimentally polymerized versus maximally polymerized composite. The opacity percentages (mean ± SD) of W1, Lava Frame, A2, and B4 ceramics were 72.41% ± 0.04%, 74.24% ± 0.09%, 77.63% ± 0.11%, and 78.17% ± 0.12%, respectively. There was a statistically significant difference in %DC among the different cements investigated (P = .003) and a statistical difference in %DC was also found among the different ceramic shades studied (P = .030). For the Lava Frame and A2 ceramic blocks, the resin cements tested presented the lowest %DC except for the RelyX Unicem resin cement. The %DC of RelyX Unicem 2 cement under low zirconia opacities (W1 and Lava Frame) was greater compared to high zirconia opacities (A2 and B4). For the Panavia F2.0 cement, no effect on the %DC under different high translucency shades was observed. The highest %DC of PermaFlo cement was exhibited when the ceramic shade W1 was used. Dual-cured resin cements exhibited different degrees of conversion depending on the opacity of the zirconia restoration. However, different brands demonstrated higher or lower dependency of the light transmittance on their degree of conversion.

Comparison of the Properties of Natural Sorbents for the Calcium Looping Process.

Capturing CO2 from industrial processes may be one of the main ways to control global temperature increases. One of the proposed methods is the calcium looping technology (CaL). The aim of this research was to assess the sequestration capacity of selected carbonate rocks, serpentinite, and basalt using a TGA-DSC analysis, thus simulating the CaL process. The highest degrees of conversion were obtained for limestones, lower degrees were obtained for magnesite and serpentinite, and the lowest were obtained for basalt. The decrease in the conversion rate, along with the subsequent CaL cycles, was most intense for the sorbents with the highest values. Thermally pretreated limestone samples demonstrated different degrees of conversion, which were the highest for the calcium-carbonate-rich limestones. The cumulative carbonation of the pretreated samples was more than twice as low as that of the raw ones. The thermal pretreatment was effective for the examined rocks.

Comparative Analysis of the Thermal Decomposition Kinetics of Polyethylene Glycol Fumarate–Acrylic Acid Copolymers

The thermal decomposition of copolymers based on polyethylene glycol fumarate with acrylic acid in oxygen and nitrogen were studied. The activation energies at different degrees of conversion were evaluated by the Friedman and Flynn–Ozawa–Wall kinetic methods. It was found, using the nonparameteric kinetic methods, that the reaction rate depends on the temperature and degree of conversion in air and inert media.

The activation energies at glass transition temperatures and dynamic fragility of (Se90Te10)100-x Inx glassy alloys

Chalcogenide glasses of (Se90Te10)100-x Inx (x = 0, 3, 6, 9, 11) alloys are prepared by melt quench technique. Differential scanning calorimetry (DSC) technique with different heating rate is used to study the dependence of the glass transition temperature (Tg) and fragility dynamics m on the In content in the Se-Te matrix. It is observed an increase of Tg with increasing In content. The glass transition temperature is differently defined according to the different degree of conversion of the glass. The activation energy of the glass transition (Eg) is evaluated using Kissinger, Moynihan kinetic and the partial area (isoconversional) methods. Analysis of experimental data shows that Eg changes with the degree of conversion from the glassy to the supercooled phase. The variation of the fragility with indium content is attributed to the formation of network structure. The employed theoretical models to describe the kinetics of the glass transition are valid.

Thermomechanical characterization of BFRP and GFRP with different degree of conversion

Thermomechanical behavior of basalt fiber- and glass fiber-reinforced plastics (BFRP and GFRP) was characterized under the same conditions. The effect of an EDI binder formulation on Martens temperature was studied by varying the contents of a curing agent from 75 to 95 parts by weight (pbw) and of an accelerator from 1 to 2 pbw with respect to 100 pbw of an ED-22 resin. The Martens temperature was found to vary from 90 to 113 °С, depending on the component ratio. An optimum binder formulation was chosen, exhibiting stable results on heat resistance after curing at 150 °С for 2 and 10 h. To achieve different degree of conversion, BFRPs and GFRPs were fabricated using the chosen formulation and were cured at 100-110 °С for 30 min and then at 150 °С for 1 and 4 h. The said formulation can be recommended, with the maximum transition point of BFRPs and GFRPs reaching 137 °С.

Dentin Bonding Durability of Two-step Self-etch Adhesives with Improved of Degree of Conversion of Adhesive Resins.

To evaluate (1) the initial and long-term microtensile bond strengths of two-step self-etch adhesives with different degrees of conversion (DC); (2) the elastic modulus of the respective adhesive resins; (3) the water sorption of the respective adhesive resins. Two two-step self-etch adhesives, Clearfil SE Bond (CSE) and Clearfil SE Bond 2 (CSE2) were used in this study. The DC was determined using ATR/FT-IR with a time-based spectrum analysis. Midcoronal flat dentin surfaces of 24 human molars were prepared with 600-grit SiC paper for microtensile bond strength (µTBS) testing. CSE and CSE2 were applied to the dentin surfaces according to the manufacturer's instructions, followed by composite buildups. The µTBS was measured after water storage for 24 h, 6 months, and 1 year. The elastic modulus (before and after 1 month of water immersion) was determined by the three-point flexural bending test and water sorption values by the water sorption test. CSE2 showed significantly higher DC than CSE. The µTBS of CSE2 was significantly higher than that of CSE in all water storage periods. One-year water storage decreased the µTBS of CSE; however, it did not decrease that of CSE2. Regarding the polymerized adhesive resins, the elastic modulus of CSE2 was significantly higher than that of CSE before and after water immersion (p < 0.001), and the water sorption of CSE was higher than that of CSE2. The higher DC of adhesive resins of two-step self-etch adhesives resists water aging and improves the initial bond strengths and durability of the resin-dentin bond.

Kinetics and Mechanism of Calcium Hydroxide Conversion into Calcium Alkoxides: Implications in Heritage Conservation Using Nanolimes.

Nanolimes are alcohol dispersions of Ca(OH)2 nanoparticles used in the conservation of cultural heritage. Although it was believed that Ca(OH)2 particles were inert when dispersed in short-chain alcohols, it has been recently shown that they can undergo transformation into calcium alkoxides. Little is known, however, about the mechanism and kinetics of such a phase transformation as well as its effect on the performance of nanolimes. Here we show that Ca(OH)2 particles formed after lime slaking react with ethanol and isopropanol and partially transform (fractional conversion, α up to 0.08) into calcium ethoxide and isopropoxide, respectively. The transformation shows Arrhenius behavior, with apparent activation energy Ea of 29 ± 4 and 37 ± 6 kJ mol(-1) for Ca-ethoxide and Ca-isopropoxide conversion, respectively. High resolution transmission electron microscopy analyses of reactant and product phases show that the alkoxides replace the crystalline structure of Ca(OH)2 along specific [hkl] directions, preserving the external hexagonal (platelike) morphology of the parent phase. Textural and kinetic results reveal that this pseudomorphic replacement involves a 3D diffusion-controlled deceleratory advancement of the reaction front. The results are consistent with an interface-coupled dissolution-precipitation replacement mechanism. Analysis of the carbonation of Ca(OH)2 particles with different degree of conversion into Ca-ethoxide (α up to 0.08) and Ca-isopropoxide (α up to 0.04) exposed to air (20 °C, 80% relative humidity) reveals that Ca-alkoxides significantly reduce the rate of transformation into cementing CaCO3 and induce the formation of metastable vaterite, as opposed to stable calcite which forms in untransformed Ca(OH)2 samples. Similar effects are obtained when a commercial nanolime partially transformed into Ca-ethoxide is subjected to carbonation. Such effects may hamper/delay the strengthening or consolidation effects of nanolimes, thus having important implications in the conservation of cultural heritage.

An Investigation of Thermal Decomposition Kinetics of Nano Zinc Oxide Catalyzed Composite Propellant

Thermal decomposition and the combustion mechanism of ammonium perchlorate/hydroxyl-terminated polybutadiene propellant has been widely investigated during the past decades. The focus in the present work is to investigate the catalytic effect of zinc oxide (ZnO) nano particles on the decomposition mechanism and kinetic behavior of composite solid propellant. Thermal cum kinetic evaluation of the catalyzed and non-catalyzed version of composite propellant has been carried out by four different kinetic methods, including Kissinger method, Flynn–Wall–Ozawa method, Friedman method, and Kissinger–Akahira–Sunose method. Differential scanning calorimetry and thermogravimetry have been used to identify the changes in the thermal and kinetic behavior of the catalyzed composite propellant. The major effect in the catalyzed propellant was lowering of the decomposition temperature by nearly 52°C. The activation energy also decreased, whereas the rate constant increased by the addition of nano zinc oxide confirming the catalytic activity in the composite solid propellant. The activation energy graphs for both versions of the propellant have shown variations with respect to different degrees of conversion.

Curing kinetics of self-healing epoxy thermosets

The curing kinetics of self-healing epoxy compositions was investigated by non-isothermal differential scanning calorimetric (DSC) studies. Cycloaliphatic epoxy resin was encapsulated in urea–formaldehyde (UF) using emulsion polymerisation technique to prepare epoxy-loaded UF microcapsules. Triethylene tetramine (TETA) hardener was immobilised on a mesoporous siliceous substrate (SBA 15) and both these additives were dispersed into an epoxy resin, which was subsequently cured using TETA. DSC studies revealed the autocatalytic nature of epoxy curing, which remained unaltered due to addition of the above-mentioned fillers, responsible for introducing self-healing functionality. The kinetic parameters of the curing process were determined using both Friedman and Kissinger–Akahira–Sunose (KAS) method. The activation energy at different degrees of conversion (E α ) was found to decrease with increasing degree of cure (α). Although UF resins possess secondary amine functionalities, which have the potential to react with the epoxy groups, no significant differences in the curing kinetics of the base resin were observed. Kinetic parameters were used to predict the curing behaviour of compositions at higher heating rates using KAS method. As expected, the onset curing temperature (T onset) and peak exotherm temperature (T p) of epoxy shifted towards higher temperatures with increased heating rate; however, introduction of fillers does not affect these characteristic temperatures significantly. Also, the overall order of reaction does not vary significantly which supports the autocatalytic nature of curing reaction. The results suggests that although 2° amino groups are available with the UF resin, these do not directly participate in the curing reaction, as the primary amino groups in TETA are more easily accessible.

Effect of the mechanochemical treatment of peat on the composition of aromatic hydrocarbons separated from peat lipids

The studies of the composition of aromatic hydrocarbons in the lipids of peats of different degrees of conversion are reported. The chemical transformations of mono-, bi-, tri-, and tetracyclic aromatic compounds under the mechanochemical action on peat are considered.

VEGETABLE OIL-BASED HYPERBRANCHED POLYOLS IN FLEXIBLE FOAMS

Novel polyols were synthesized by controlled polymerization of hydroxylated fatty acids from methyl soyate. Polymerization of methyl esters of hydroxy fatty acids gives a hyperbranched (HB) product with different content of hydroxyl groups depending on the degree of conversion. Molecular weights, functionality and hydroxyl equivalent of polyols can be controlled by controlling the degree of conversion but also using monofunctional components. A range of hyperbranched polyols with acceptable viscosities and functionalities, suitable for flexible applications, was obtained by stopping the reaction at different degrees of conversion. A polyol with 60% of hydroxylated soybean fatty acid methyl esters and 40% of methyl esters from castor oil was selected for detailed analysis and was used in flexible foams. Monte-Carlo simulation of the polymerization of hydroxylated methyl soyate gave molecular weights and distributions which were compared with experimental values. Flexible foams were obtained with 65% of HB polyol in a mixture with a petrochemical triol. Increasing concentration of the HB polyol improved load bearing properties but decreased mechanical strengths and in particular tear strength.

VEGETABLE OIL-BASED HYPERBRANCHED POLYOLS IN FLEXIBLE FOAMS

Novel polyols were synthesized by controlled polymerization of hydroxylated fatty acids from methyl soyate. Polymerization of methyl esters of hydroxy fatty acids gives a hyperbranched (HB) product with different content of hydroxyl groups depending on the degree of conversion. Molecular weights, functionality and hydroxyl equivalent of polyols can be controlled by controlling the degree of conversion but also using monofunctional components. A range of hyperbranched polyols with acceptable viscosities and functionalities, suitable for flexible applications, was obtained by stopping the reaction at different degrees of conversion. A polyol with 60% of hydroxylated soybean fatty acid methyl esters and 40% of methyl esters from castor oil was selected for detailed analysis and was used in flexible foams. Monte-Carlo simulation of the polymerization of hydroxylated methyl soyate gave molecular weights and distributions which were compared with experimental values. Flexible foams were obtained with 65% of HB polyol in a mixture with a petrochemical triol. Increasing concentration of the HB polyol improved load bearing properties but decreased mechanical strengths and in particular tear strength.

Continuous-flow biodiesel production using slit-channel reactors

Slit-channel reactors are reactors whose active surface areas are orders of magnitude higher than those of micro-reactors but have low fabrication costs relative to micro-reactors. We successfully produced biodiesel with different degrees of conversion using homogeneous catalyst in the slit-channel reactor. The reactor performance shows that percent conversion of soybean oil to biodiesel increases with channel depth, as expected, due to more efficient mixing. Shallow slit-channels require short average residence times for complete product conversion. Present results show that the slit-channel reactor provides an improved performance over traditional batch reactors using homogeneous sodium alkoxide catalyst. It is aimed to couple the reactors with solid catalysts in converting soybean oil to biodiesel and implementation method is suggested. The cost advantages resulting from the ease of fabrication of slit-channel reactors over micro-reactors and how these factors relate to the oil conversion efficiency to biodiesel are briefly noted and discussed.

Intrinsic char reactivity of plastic waste (PET) during CO 2 gasification

Char reactivity has a strong influence on the gasification process, since char gasification is the slowest step in the process. A sample of waste PET was devolatilised in a vertical quartz reactor and the resulting char was partially gasified under a CO 2 atmosphere at 925 °C in order to obtain samples with different degrees of conversion. The reactivity of the char in CO 2 was determined by isothermal thermogravimetric analysis at different temperatures in a kinetically controlled regime and its reactive behaviour was evaluated by means of the random pore model (RPM). The texture of the char was characterised by means of N 2 and CO 2 adsorption isotherms. The results did not reveal any variation in char reactivity during conversion, whereas the micropore surface area was affected during the gasification process. It was found that the intrinsic reaction rate of the char can be satisfactorily calculated by normalizing the reaction rate by the narrow micropore surface area calculated from the CO 2 adsorption isotherms. It can be concluded therefore that the surface area available for the gasification process is the area corresponding to the narrow microporosity.

Molecular beam mass spectrometry and modelling of CH 4–CO 2 plasmas in relation with polycrystalline and nanocrystalline diamond deposition

CH 4–CO 2 microwave plasmas have been studied by optical emission spectroscopy, microwave interferometry, Langmuir probing and molecular beam mass spectrometry. The variations of plasma parameters and the concentration variations of both stable species and radicals in the plasma had been reported previously as a function of the power density; the influence of the total inlet flow rate is reported here. While the power density influences directly the plasma kinetics, the flow rate changes the residence time in the plasma and then the degree of conversion of the chemical system that is the extent to which the gas composition moves toward its steady-state composition. This is studied by modelling of plasma kinetics taking into account the coupled fluid dynamics of the gaseous species and the gas-phase chemistry including electron dissociation and surface recombination at the reactor wall. The experimental and modelling studies are used for correlating: – the relative concentration of important hydrocarbon radicals and etching radicals in the plasma and the gradients of all these species in front of the surface; – to the deposition domain, the structure (polycrystalline or nanocrystalline) and the quality of diamond films, which is the ratio of sp 3 to (sp 3 + sp 2)-hybridized carbon in the film. All results evidence the plasma kinetic effect on the diamond deposition domain and the diamond deposition quality and structure, due to different degrees of conversion of the chemical system. The deposition of diamond coating from CH 4–CO 2 is shown to be a versatile process that permits deposition of a great variety of diamond films. However it requires particular attention because of the variation of the deposition conditions and then diamond quality and structure of the deposits depending on the extent of conversion of the inlet species to various intermediate and finally stable species formed in the plasma chemical system.