Continuous polymerization in tubular reactors with prepolymerization: Analysis using two‐dimensional phenomenological model and hybrid model with neural networks

Continuous polymerization processes have advantages when large amounts of product are required; moreover, higher quality can be obtained because of the elimination of variability between batches. Tubular reactors are economically attractive because of their simple geometry and high heat exchange area; however, they are not commonly used for commercial purposes, mainly because of the large radial profiles. This study elucidates the operation of this kind of reactors in three different ways: first a detailed two‐dimensional mathematical model was developed, in which a complete visualization of all axial and radial profiles is possible, allowing a safe analysis at different operating conditions. In a second step a system composed of a continuously stirred tank reactor in series with a tubular reactor was used. A reduction in radial profiles can be clearly observed when prepolymerization is taken into account, improving both the homogeneity and the end properties of the polymer. In a third approach neural networks (NNs) were used in parallel with a one‐dimensional model. The objective of this study was to illustrate how NNs can improve the prediction capability when it is not possible to build a reliable model because of uncertainties in parameters and incomplete knowledge of the system. The NNs generated good results, showing that the hybrid model was able to accurately simulate the reactor, even when uncertainty in kinetic and diffusional parameters was imposed to the model. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 871–882, 2004     

Automatic image analysis and morphology of fibre reinforced concrete

Automatic image analysis is an efficient tool to quantify the morphology of materials. Moreover, it can aid to understand their mechanical behaviour. Several applications of automatic methods are presented to investigate concrete reinforced by ribbon shaped amorphous cast iron fibres. Introducing ribbons into the plain matrix entrapped air voids. This affected the workability and, later on, the compressive strength of the fibre reinforced concrete (FRC). Both were improved by additions of superplasticizer in order to keep the water to cement ratio constant. The influence of the superplasticizer and fibre contents on the compactness of the FRC was characterized by the dimensional and the spatial distributions of the air voids. The orientations of fibres and microcracks were quantified by Fourier image transforms. Due to the casting procedure of the FRC, the fibres were located in “horizontal layers”, perpendicular to the casting axis. Whatever the direction of compression with respect to the layers of fibres, the microcrack network was getting more and more oriented in the direction of compression as stresses increased. The analysis of fibre and microcrack orientations suggests that, under uniaxial compression, the inelastic strain domain should be characterized by an anisotropic biaxial damage model, whose principal axes are the orthogonal and parallel directions to the layers of fibres.     

14C labelling of lignins of normal and bm3 maizes for fermentation studies

[U-¹⁴C] phenylalanine (phe*) and [O¹⁴CH₃] sinapic acid (sin*) were infused into the cut ends of normal and bm3 maizes (anthesis stage) under or above the last node or at mid-internode, with or without the leaf, in light or in darkness. Radioactivity was measured in the organs, and in phenolic constituents of the cell wall and saponified residues of the bases and tops of the apical inter-node. In both maize genotype labelled under the node the radioactivity was distributed more evenly in the organs with sin* than with phe*. Infusion above the node and at mid-internode greatly increased radioactivity in the bases and tops, respectively. Removal of the leaf only slightly increased the radioactivity, mainly in the bases, and no clear-cut effect of darkness was observed. Phe* labelled the phenolic acids and the three lignin units, but the syringyl units of bm3 maize were only slightly labelled. Sin* specifically labelled the syringyl units, which represented the least condensed fraction of lignins. Both the native and labelled lignins were highly alkali soluble. There were differences in lignin biogenesis between the bases and tops, and between normal and bm3 maizes. The newly formed lignins were slightly different from the native lignins but had similar types of heterogeneity, with variations in the internode and between genotypes similar to those in native lignins. Provided due allowance is made for the distinguishing characteristics of newly formed lignins, the [¹⁴C-lignin] cell walls, which are strongly labelled on complementary structures, seem suitable model substrates for fermentation studies.     

Metal catalyst residues in carbon nanotubes decrease the thermal stability of carbon nanotube/silicone composites

An anomalous decrease in the thermal stability of silicone was observed when carbon nanotubes (CNTs) were added as fillers. The decreased thermal stability is found to result from the residues of cobalt nanoparticles in CNTs, whereas CNTs synthesized with other metal catalysts do not show such a phenomenon. The analysis of thermal degradation products indicates that CNT fillers do not change the mechanism of the thermal degradation of silicone but cobalt nanoparticles within CNTs may accelerate the degradation through free radical generation. Radical scavengers such as hindered amines and impurity-free CNTs, or removal of cobalt nanoparticles by acid treatment, can mitigate the accelerated thermal degradation.     

Dynamics near the filler surface in natural rubber-silica nanocomposites

We study the effect of in situ synthesized 10 nm silica nanoparticles on the glass transition and dynamics of natural rubber networks using differential scanning calorimetry, broadband dielectric relaxation spectroscopy and thermally stimulated depolarization currents. Even in the absence of specific polymer-filler interactions, polymer segments within a few nanometers of the filler particles exhibit relaxation times up to 2-3 orders of magnitude slower and reduced heat capacity increment at the glass transition compared to bulk natural rubber. These effects are only observed when the nanoparticles are uniformly distributed in the polymer matrix.     

Butyltin(IV) Benzoates: Inhibition of Thioredoxin Reductase,Tumor Cell Growth Inhibition,and Interactions with Proteins

Thioredoxin reductase (TrxR) is overexpressed in cancer cells and is therefore a putative cancer target. Inhibition of this enzyme is considered an important strategy for the development of new chemotherapeutic agents with a specific mechanism of action. Organotin compounds have been described as experimental antitumor agents, yet their mechanism of action remains largely unknown. Based on the outcome of a virtual screening study, various di‐ and tri‐n‐butyltin(IV) carboxylates were synthesized, and their biological properties were evaluated. All synthesized compounds were able to inhibit TrxR selectively within the micromolar range and showed potent antitumor activity against HT‐29 and MCF‐7 cancer cell lines. Moreover, tin(IV) organometallics were found to strongly induce apoptosis in the BJAB lymphoma cell line. Mass spectrometry and atomic absorption spectroscopy experiments revealed metal binding to proteins, and efficient cellular uptake was observed using a di‐n‐butyltin(IV) complex as an example.     

Fluidized‐bed reactor modeling for polyethylene production

Gas‐phase technology for polyethylene production has been widely used by industries around the world. A good model for the reactor fluid dynamics is essential to properly set the operating conditions of the fluidized‐bed reactor. The fluidized‐bed model developed in this work is based on a steady‐state model, incorporating interactions between separate bubble, emulsion gas phase, and emulsion solid polymer particles. The model is capable not only of computing temperature and concentration gradients for bubble and emulsion phases, calculating polymer particle mean diameter throughout the bed and polyethylene production rate, but also of pinpointing the appearance of hot spots and polymer meltdown. The model differs from conventional well‐mixed fluidized‐bed models by assuming that the particles segregate within the bed according to size and weight differences. The model was validated using literature and patent data, presenting good representation of the behavior of the fluidized‐bed reactor used in ethylene polymerization. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 321–332, 2001     

A Dynamic Risk Assessment Methodology for Maintenance Decision Support

The failure mode and effect analysis (FMEA) is a widely applied technique for prioritizing equipment failures in the maintenance decision‐making domain. Recent improvements on the FMEA have largely focussed on addressing the shortcomings of the conventional FMEA of which the risk priority number is incorporated as a measure for prioritizing failure modes. In this regard, considerable research effort has been directed towards addressing uncertainties associated with the risk priority number metrics, that is occurrence, severity and detection. Despite these improvements, assigning these metrics remains largely subjective and mostly relies on expert elicitations, more so in instances where empirical data are sparse. Moreover, the FMEA results remain static and are seldom updated with the availability of new failure information. In this paper, a dynamic risk assessment methodology is proposed and based on the hierarchical Bayes theory. In the methodology, posterior distribution functions are derived for risk metrics associated with equipment failure of which the posterior function combines both prior functions elicited from experts and observed evidences based on empirical data. Thereafter, the posterior functions are incorporated as input to a Monte Carlo simulation model from which the expected cost of failure is generated and failure modes prioritized on this basis. A decision scheme for selecting appropriate maintenance strategy is proposed, and its applicability is demonstrated in the case study of thermal power plant equipment failures. Copyright © 2016 John Wiley & Sons, Ltd.     

Controlling interfacial instabilities in PP/EVOH coextruded multilayer films through the surface density of interfacial copolymers

Coextruded polypropylene/tie/ethylene vinyl alcohol/tie/polypropylene (PP/tie/EVOH/tie/PP) films often exhibit optical defects which appear as randomly distributed scattering objects, in the submillimeter range. These defects may strongly affect the film transparency and prevent their practical use in packaging. Based on an objective optical test aimed at quantifying the film transparency, and on a systematic analysis, through optical microscopy, of transverse cuts of films obtained in various coextrusion conditions, the nature of the defects could be identified as resulting from a modulation of the thickness of the inner ethylene vinyl alcohol (EVOH) layer, with no variation in the overall thickness of the multilayer films. Thanks to a recently developed method to dose the surface density of interfacial copolymers, a clear correlation between the amplitude of the thickness modulation of the inner EVOH layer and the density of copolymer molecules formed in situ at the EVOH/tie layer interface during the coextrusion process was established. These results open the way to a better design of tie layers composition to avoid these kinds of defects.     

Polymorphic transformation morphology of isotactic poly(1-butene)/poly(propylene-co-1-butene-co-ethylene) blends

The aim of this work was to investigate the polymorphic transformation morphology of isotactic poly(1-butene) (iPB-1) in blends with poly (propylene-co-1-butene-co-ethylene) (terPP). The blends were prepared by solution casting followed by compression-molding and were examined immediately after preparation as well as after aging for over 30 days. Morphological changes were firstly observed by polarized optical microscopy, which showed that the terPP promotes the reduction of the spherulite mean size during the crystallization of iPB-1 from the melt. Digital pulsed force microscopy provided data about the terPP interspersion between the interfibrilar and spherulitic regions of the iPB-1 crystals, causing iPB-1 spherulite deformation. However, a better distribution of the amorphous component on the iPB-1 crystals was observed after phase II to I transition, favored by the rearrangement of iPB-1 lamellae and lamellar stacks during phase transition.