1,720,982 research outputs found
Surfactant Assisted Polymerization of Tetrafluoroethylene in Supercritical Carbon Dioxide with a Pilot Scale Batch Reactor.
No full textChain-free radical polymerization of tetrafluoroethylene (TFE) was carried out in supercritical carbon dioxide (scCO(2)), at 50 degrees C and 121-133 bar, with tertiary-amyl-per-pivalate as a free radical initiator, using a 5-L pilot scale batch reactor. Experiments were carried out both in the absence and in the presence of perfluoropolyether (PFPE) carboxylate surfactants. Stabilizers were employed under the free acid form and as sodium and calcium salts. An expanded fibrillated poly(TFE) was obtained in all experiments. In the case of surfactant-free polymerizations, the product was mainly constituted by irregular shape macroparticles having size in the range 200-500 pm. Quite interestingly, when the free acid surfactant was used, a clear acceleration of the polymerization rate was observed and smaller polymer particles with more regular quasi-spherical morphology were obtained. Melt fusion crystallinity of as-polymerized poly(TFE) seemed not substantially affected by the presence of the stabilizers and was rather high (80-86%) suggesting that polymerization mainly occurs at polymer particle surface. All these elements suggest that FLUOROLINK(R) 7004H PFPE carboxylic acid decreases coagulation of primary polymer particles leading to an increase in polymer surface area
Estimation of Critical Micellar Concentration Through Ultrasonic Velocity-measurements
No full textA technique based on ultrasound propagation velocity measurements has been developed to investigate surfactant's behavior in aqueous solutions. Applications to the estimation of the critical micellar concentration of two emulsifiers, sodium dodecyl sulfate and dodecylbenzene sulfonic acid sodium salt, and a mixture of them are discussed. A simple model was developed to interpret the experimental ultrasound propagation velocity measurements. The model provides a theoretical foundation to the experimental results as well as a valuable tool for smoothing out the disturbances affecting the experimental data. (C) 1995 John Wiley and Sons, Inc
Propagation velocity of the reaction front in addition polymerization systems
No full textTraveling polymerization fronts in unstirred solutions of methylmethacrylate, methacrylic acid, or acrylamide with some free radicals initiators (through thermal decomposition) have been observed experimentally. A local heating of the initial reactant mixture, under suitable conditions, leads to a reaction front that propagates along the space coordinate with a constant velocity. In this article, a physical interpretation of this phenomenon is provided through a mathematical model that accounts for the depolimerization reaction and is based on the constant pattern approach. Moreover, an approximate explicit analytic expression for the velocity of propagation of the polymerization front is proposed. The theoretical values are compared with those measured experimentally as a function of the initiator concentration for different addition polymerization systems. (C) 1997 John Wiley & Sons, Inc
Characterization of Emulsion Polymerizations Through Ultrasound Propagation Velocity-measurements
No full textThe application of the measurement of ultrasound waves propagation velocity for characterizing emulsion polymerization systems is discussed. The ultrasound velocity sensor provides informations about various properties of interest in emulsion polymerization systems. These include the evaluation of monomer solubility and emulsifier critical micellar concentration in water, the in-situ and on-line monitoring of conversion in homo- and co-polymerizations, the investigation of the polymer glass transition and finally the measure of monomer solubility in submicron polymer particles
Dominanza della marca e successo del cobranding: una verifica sperimentale
No full textL’obiettivo di ricerca perseguito consiste, in primo luogo, nel confermare l’effetto positivo che l’esistenza delle due fondamentali tipologie di consonanza percettiva (product fit e brand fit) può esercitare sull’accettazione di un nuovo prodotto proposto nell’ambito di un accordo di co-branding. In secondo luogo, il paper approfondisce l’impatto della dominanza della marca ospitante, al fine di verificare se e come essa interagisca con tali consonanze percettive.
Il paper è così strutturato: il paragrafo 2 sintetizza le risultanze emergenti dalle ricerche empiriche che hanno approfondito lo studio dei fattori suscettibili di influenzare il successo dei nuovi prodotti lanciati con il co-branding e le ipotesi di ricerca adottate; il paragrafo 3 illustra la metodologia d’indagine utilizzata; il paragrafo 4 analizza i risultati ottenuti. Infine, nelle conclusioni vengono discusse alcune implicazioni di marketing dell’analisi svolta
Online Conversion Monitoring Through Ultrasound Propagation Velocity-measurements In Emulsion Polymerization
No full textAn on-line sensor for estimating conversion from measurements of ultrasound propagation velocity in emulsion polymerizations has been developed. The sensor does not require a sampling circuit, because it can be directly plugged in the reacting mixture. The performance of the sensor has been tested by comparison with off-line gravimetric measurements of conversion under various operating conditions. These include the homopolymerization of methyl methacrylate and vinyl acetate, with different values of the monomer to water ratio and the initial concentrations of emulsifier and initiator. The results obtained using either an empirical calibration curve or a suitable a priori model indicate excellent reliability of the sensor with somewhat lower accuracy at low conversion values, i.e., when monomer droplets are present. (C) 1995 John Wiley & Sons, Inc
Free radical controlled polymerization of fluorinated copolymers produced in microemulsion
No full textControlled free-radical polymerization is an emerging technique which can in principle produce macromolecules with well defined microstructure, possibly tailored to specific applications. This work describes the application of such a process to the production of several hundreds of tons of a fluorinated copolymer per year. The aim is to control the microstructure of the product and, in particular, to produce branched macromolecules with relatively uniform chain lengths. The process operates in microemulsion and is complicated by the presence of branching reactions. A complete kinetic model is developed to identify the most appropriate operating conditions to produce copolymers with desired characteristics
Kinetics of the emulsion polymerization of vinylidene fluoride and hexafluoropropylene
No full textThe kinetics of emulsion polymerization of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) has been investigated both experimentally and through the development of an appropriate model. Despite the practical relevance of this copolymer, information about the polymerization kinetics in the literature are rather scarce. A reasonable polymerization mechanism involves quite a number of kinetic parameters, which all need to be evaluated independently. This was obtained by running a limited number of polymerization reactions, but performing a rather accurate characterization of the polymer produced, including the polymerization rate, molecular weight, and chain end group distribution. These quantities allow for the reliable estimation of the involved kinetic parameters. The reliability of the developed model has been assessed by comparison with a second independent set of experimental data, where different operating conditions, in terms of pressure, initial concentration of the initiator, and chain-transfer agent, have been considered
On-line monitoring of emulsion polymerization reactors through ultrasonic velocity measurements
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