Comparative study of homogeneous and heterogeneous styrene polymerizations with Ni(acac)2/MAO catalytic system

Styrene polymerization was carried out with Ni(acac)₂/MAO and Ni(acac)₂/SiO₂/MAO. The influence of reaction parameters (Al/Ni mole ratio, catalyst concentration, temperature and time polymerization) on styrene polymerization was evaluated. It was observed that both catalytic systems were affected by reaction parameters and that the heterogeneous catalyst presented higher activity than the homogeneous one. Polystyrenes with different molecular weight, stereoregularity and polydispersity were obtained. These results suggest that different active catalyst species could have been present. In addition, two types of methylaluminoxane (MAO) with different molecular weights were also evaluated as cocatalyst. As a result, the catalyst activity and stereospecificity were strongly affected by the MAO type.     

Influence of the Metal Particle Size on the Crack Growth Resistance in Mullite–Molybdenum Composites

The R -curve for mullite–molybdenum (32 vol%) composites, which were obtained at 1650°C under reducing conditions with three different Mo average grain sizes (1.5, 3, and 9 μm), was estimated by the indentation-strength method and compared with that monolithic mullite obtained under similar conditions. The composites material exhibited rising R -curve behavior. The composite with larger grain size, however, displayed better damage tolerance and higher resistance to crack growth. Microscopic observation of the crack path revealed, in the composites, the systematic presence of dispersoids acting as bridging sites in the crack wake. Therefore, the increased fracture toughness of these ceramic-matrix composites with adherent ductile phase can be attributed to clamping forces applied by metal ligaments that bridge the crack faces behind the crack front. These clamping forces retard the crack from opening as an external stress is applied. It was inferred that this superior performance of the larger Mo particle size composite can be attributed mainly to more effective bridging of the metal grains. Because of this, a higher applied stress intensity will be required to propagate the crack tip. These results suggest that the rising R -curve should be proportional to the metal grain size, since the grain bridging area is proportional to the metal grain size.     

The influence of some synthesis parameters on Ziegler-Natta catalyst performance

The role of di-n-butyl ether (DBE) in the synthesis of highly active and stereospecific catalyst for propylene polymerization has been investigated. The ether was used as internal base (IB) or complexed with TiCl₄ and diethylaluminium chloride (DEAC) in iso-octane solution (System A) or complexed with triethylaluminium (TEA) in toluene solution (System B). Many differences were observed in the catalyst performance. The activity, the catalyst stereospecificity and the polymer bulk density were evaluated.     

Controlling the phase stability of polymer blends through the introduction of impenetrable interfaces

In this work, the effect of the introduction of modified solid surfaces into polymer blends on the phase‐separation process was investigated. Glass fibers with surfaces having different chemistries were introduced into polystyrene–poly(methyl methacrylate) blends. The glass fibers used either had fully hydrated surfaces or had surfaces covered with a random copolymer, poly(styrene‐co‐methyl methacrylate). The copolymer was synthesized by free‐radical polymerization of styrene and methyl methacrylate in the presence of previously vinyl silane‐treated glass fibers. The copolymerization and grafting procedures were investigated by FTIR and thermal analysis. Blends containing the fibers were studied using FTIR microscopy and optical microscopy. FTIR microscopy results showed that the composition of the phases in the blends was shifted by using fibers with different surface chemistries. Fibers with grafted copolymers were capable of narrowing the immiscibility region in the phase diagram, while fully hydrated fibers were able to expand the gap. It was proposed that interfacial interactions regulated by a hydrophilic–hydrophobic type of forces were responsible for guiding the described phase‐separation process. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1619–1627, 2003     

The use of magnesium silicate (talc) in a potentiometric sensor for hydrogen ions

The construction and analytical evaluation of a pH sensor based in a matrix containing 20% of [(Mg₆Si₈O₂₀)](OH)₄, a magnesium silicate, talc and 30% graphite dispersed in an epoxy resin (50%) is described. The data obtained from various acid–base titrations were compared with those using a glass electrode in the same conditions. The proposed electrode presented a linear response in the 1–12 pH range with a slope of −39.9±0.3 mV/pH (at 25 °C) and ca. 15 s of response time. The lifetime of this electrode was higher than 8 months (ca. 1500 determinations) and it showed a good performance for pH determination and end-point indication in the potentiometric titrations of acids and bases.     

Accelerated Aging in 3-mol%-Yttria-Stabilized Tetragonal Zirconia Ceramics Sintered in Reducing Conditions

The aging behavior of 3-mol%-yttria-stabilized tetragonal zirconia (3Y-TZP) ceramics sintered in air and in reducing conditions was investigated at 140°C in water vapor. It was observed by X-ray diffraction (XRD) that 3Y-TZP samples sintered in reducing conditions exhibited significantly higher tetragonal-to-monoclinic transformation than samples with similar density and average grain size values but obtained by sintering in air. This fact is explained by the increase of the oxygen vacancy concentration and by the presence at the grain boundary region of a new aggregate phase formed because of the exolution of Fe²⁺ ions observed by X-ray photoelectron spectroscopy.     

Catalyst deactivation in the gas phase destruction of nitrogen-containing organic compounds using TiO2/UV–VIS

Possible application of the TiO₂/UV–VIS photocatalytic process in the destruction of nitrogen-containing malodorous compounds was evaluated. Pyridine (C₅H₅N), propylamine (C₃H₇NH₂) and diethylamine (C₄H₁₀NH) were photodegraded in the presence and in the absence of oxygen. Degradation of nitrogen-containing organic compounds was confirmed by mass balance taking into consideration NH₄⁺ and NO₃⁻ ions trapped at the TiO₂ surface. Photocatalytic deactivation was observed in all cases. On-line mass spectrometry was used to identify byproducts in the gas phase formed during the degradation process. GC–MS analysis of the dichloromethane-extract of aqueous species leached from the surface of deactivated catalyst, as well as pre-concentration in a Tenax column were used to identify intermediates in the gas phase. These byproducts are considered to be the major ones responsible for deactivation of TiO₂.     

Experimental procedure for kinetic studies on egg-shell catalysts: The case of liquid-phase hydrogenation of 1,3-butadiene and n-butenes on commercial Pd catalysts

A study of the liquid-phase hydrogenation of 1,3-butadiene on commercial Pd/Al₂O₃ catalysts of the “egg-shell” type has been performed. Experimental conditions (40°C, 4 atm and high conversion of the di-olefin) were selected in accordance to industrial operating conditions employed for selective hydrogenation of 1,3-butadiene. Three experimental schemes were tested: a slurry reactor, a rotating-basket reactor, and a recirculation system with an external fixed-bed reactor. Significant drawbacks shown by the two former devices were mainly derived from the very high activity and the egg-shell structure of the catalysts. Instead, the recirculation system was found to be an excellent alternative.

Although Pd is present only within a very thin external layer (around 50–250 μm), strong diffusion effects impairing selectivity were observed. Plausible kinetic expressions corresponding to nine series–parallel overall reactions are derived from a mechanistic model. To deal with this network of fast reactions, a rather complex set of computational and predictive tools were employed. A worked out example from several replicates demonstrates the capability of both, experimental and data analysis procedures, for inferring kinetic parameters of the proposed model.     

Zero‐Modified Geometric INAR(1) Process for Modelling Count Time Series with Deflation or Inflation of Zeros

In this article, we propose a first‐order integer‐valued autoregressive [INAR(1)] process for dealing with count time series with deflation or inflation of zeros. The proposed process has zero‐modified geometric marginals and contains the geometric INAR(1) process as a particular case. The proposed model is also capable of capturing underdispersion and overdispersion, which sometimes are caused by deflation or inflation of zeros. We explore several statistical and mathematical properties of the process, discuss point estimation of the parameters and find the asymptotic distribution of the proposed estimators. We also propose a test based on our model for checking if the count time series considered is deflated or inflated of zeros. Two empirical illustrations are presented in order to show the potential for practice of our zero‐modified geometric INAR(1) process. This article contains a Supporting Information.