New challenges in polymer foaming: A review of extrusion processes assisted by supercritical carbon dioxide

It is well known that supercritical carbon dioxide (sc-CO₂) is soluble in molten polymers and acts as a plasticizer. The dissolution of sc-CO₂ leads to a decrease in the viscosity of the liquid polymer, the melting point and the glass transition temperature. These properties have been used in several particle generation processes such as PGSS (particles from gas saturated solutions).It is therefore highly likely that extrusion processes would benefit from the use of sc-CO₂ since the rationale of the extrusion processes is to formulate, texture and shape molten polymers by forcing them through a die. Combining these two technologies, extrusion and supercritical fluids, could open up new applications in extrusion.The main advantage of introducing sc-CO₂ in the barrel of an extruder is its function as a plasticizer, which allows the processing of molecules which would otherwise be too fragile to withstand the mechanical stresses and the operating temperatures of a standard extrusion process. In addition, the dissolved CO₂ acts as a foaming agent during expansion through the die. It is therefore possible to control pore generation and growth by controlling the operating conditions.This review focuses on experimental work carried out using continuous extrusion. A continuous process is more economically favourable than batch foaming processes because it is easier to control, has a higher throughput and is very versatile in the properties and shapes of the products obtained.The coupling of extrusion and supercritical CO₂ technologies has already broadened the range of application of extrusion processes. The first applications were developed for the agro-food industry 20 years ago. However, most thermoplastics could potentially be submitted to sc-CO₂-assisted extrusion, opening new challenging opportunities, particularly in the field of pharmaceutical applications.This coupled technology is however still very new and further developments of both experimental and modelling studies will be necessary to gain better theoretical understanding and technical expertise prior to industrial use, especially in the pharmaceutical field.     

FREEZE DEHYDRATION OF FOAMED MILK BY MICROWAVES

Previously published experiments on freeze-drying of foamed milk by microwaves are explained with an alternative theoretical model. The frozen milk foam is assumed to consist of parallel thin plates. A low-pressure gas stream consisting of air and water vapor mixture is believed to circulate freely in voids between plates. Air stream provides additional energy for the freeze-drying, helps to remove the sublimating water vapor and prevents an excessive icrowave heating of the dry milk foam.     

Phase transformation characterization of olivine subjected to high temperature in air

Olivine is one of the well-suited materials for fluidized bed reactor technology. After calcination at high temperature, olivine undergoes phase transformations resulting in dehydration and oxidation of fayalite to hematite and magnetite. The transformation mechanisms of olivine subjected to a calcination process at 1400 °C for 4 h are studied. Calcined olivine is characterized by X-Ray Diffraction (XRD), in situ XRD at varying temperatures and Raman spectroscopy. This paper explains the contribution of Raman spectroscopy to the study of iron oxide with regard to XRD. The heterogeneous distribution of hematite, magnetite and forsterite in the calcined material is exhibited by Raman mapping.     

A new protein folding algorithm based on hydrophobic compactness: Rigid Unconnected Secondary Structure Iterative Assembly (RUSSIA). I: Methodology

We present an algorithm that is able to propose compact modelsof protein 3D structures, only starting from the predictionof the nature and length of regular secondary structures. Helicesare modeled by cylinders and sheets by helicoid surfaces, allstrands of a sheet being considered as a single block. It meansthat relative topology of the strands inside one sheet is aprerequisite. Loops are only considered as constraints, givenby the maximal distance between their C extremities accordingto their sequence length. Unconnected regular secondary structuresare reduced to a single point, the center of their hydrophobicfaces. These centers are then repeatedly moved in order to obtaina compact hydrophobic core. To prevent secondary structuresfrom interpenetrating, a repulsive term is introduced in thefunction whose minimization leads to the compact structure.This RUSSIA (Rigid Unconnected Secondary Structure Assembly)algorithm has the advantage of relying on a small number ofvariables and therefore many initial conformations can be tested.Flexibility is produced in the following way: helices or sheetsare allowed to rotate around the direction leading to the centerof the model; residues in a sheet can slide along the main directionof the strand where they are embedded. RUSSIA is fast and simpleand it produces on a test set several neighbor good models withan r.m.s. to the native structures in the range 1.4–3.7Å. These models can be further treated by statisticalpotentials used in threading approaches in order to detect thebest candidate. The limits of the present method are the following:small proteins with few secondary structures are excluded; multidomain proteins must be split into several compact globulardomains from their sequences; sheets of more than five strandsand completely buried helices are not treated. In this firstpaper the algorithm is developed and in Part II, which follows,some applications are presented and the program is evaluated. Received July 25, 2003; revised October 25, 2003; accepted October 30, 2003     

Evolution of the water–monomer dynamic interfacial properties during methyl methacrylate radical polymerization in a single monomer droplet: dependence on the chemical structure of the surfactant

This paper deals with the monitoring of the methyl methacrylate (MMA) droplet interface during its polymerization in one single monomer droplet by using a drop tensiometer. The droplet is stabilized by various molecular and polymer surfactants, in particular Pluronic® F68 and pullulan‐block‐Jeffamine®. The polymerization in one single droplet of the tensiometer can be seen as a way to study the interface of polymer nanoparticles during a mini‐emulsion polymerization assuming the concept of a perfect nanoreactor. It was found that the experimental results (surface tension and interfacial rheology) can be connected with MMA radical polymerization. These results demonstrate that the drop tensiometer technique enables interface evolution to be followed during polymer polymerization. © 2013 Society of Chemical Industry     

Pressure drops for purely viscous non-newtonian fluid flow through beds packed with mixed-size spheres

Extensive new measurements on pressure drop for the flow of purely viscous non-Newtonian fluids through packed beds made up of binary-and-quaternary size spheres are reported herein. These results have been interpreted using the previously available Sabiri and Comiti capillary model (1995), which has been quite successful in correlating the pressure drop data for the beds of uniform size spherical and nonspherical particles. The resulting predictions are also very good for mixed beds if some results are available for a Newtonian fluid, thereby enabling the evaluation of the tortuosity factor. However, since such data is always not available, an empirical scheme for estimating the tortuosity that allows the prediction of pressure drop with a mean relative error of about 10% is also presented. This sort of accuracy is quite acceptable for process engineering design calculations.     

Force Curve Measurements with the AFM: Application to the In Situ Determination of Grafted Silicon-Wafer Surface Energies

The atomic force microscope (AFM) can be used to perform surface force measurements in the quasi-static mode (cantilever is not oscillating) to investigate nanoscale surface properties. Nevertheless, there is still a lack of literature proposing a complete systematic and rigorous experimental procedure that enables one to obtain reproducible and significant quantitative data. This article focuses on the fundamental experimental difficulties arising when making force curve measurements with the AFM in air. On the basis of this AFM calibration procedure, quantitative assessment values were used to determine, in situ, SAM (or Self Assembled Monolayer)-tip thermodynamic work of adhesion at a local scale, which have been found to be in good agreement with quoted values. Finally, determination of surface energies of functionalised silicon wafers (as received, CH₃, OH functionalised silicon wafers) with the AFM (at a local scale) is also proposed and compared with the values obtained by wettability (at a macroscopic scale). In particular, the effect of the capillary forces is discussed.     

Multimodal support for social dynamics in co-located meetings

In this paper, we present a system that employs perceptual technologies (i.e. technologies that perceive the context through sensors such as cameras and microphone) to provide feedback about people’s behaviour in small group meetings. The system measures aspects of behaviour that are relevant to the social dynamics of the meeting, speaking time and gaze behaviour, and provides visual feedback about these aspects to the meeting participants through a peripheral display. We describe the system properties and the perceptual components. Also, we present a study aimed at evaluating the effect of such a system on meeting behaviour. Groups of participants, amounting to 82 participants in all, discussed topics of general interest. Analysis of the data of 58 participants showed that feedback influenced the behaviour of the participants in such a way that it made over-participators speak less and under-participators speak more. Analysis of the micro-patterns of six participants indicated that feedback on gaze behaviour had little effect on the interaction dynamics. We conclude that perceptual technologies can be used to build services that may help people to improve their meeting skills and we consider some ways in which such systems may be deployed in meetings.     

Solving a rich vehicle routing and inventory problem using column generation

The livestock collection problem (LCP) is a rich vehicle routing problem (VRP) extended with inventory constraints. The LCP is a complex planning problem taken from the meat industry, and the goal is to construct a set of vehicle routes to collect animals from farms for slaughter at a slaughterhouse. Several constraints dealing with animal welfare are added, some of these lead to a loading problem where the vehicle capacity depends on the loading sequence. In addition, global constraints to handle production and inventory at the slaughterhouse are needed. This paper presents an exact solution method for the LCP, based on column generation, that solves much larger instances to optimality than what has been done before. The algorithm presented here also solves a richer model that is closer to the underlying real-world problem than previously published work on exact methods for this problem is based on.