Advancing reaction

Advancement Process. In the advancement process, sometimes referred to as the fusion method, Hquid epoxy resin (cmde diglycidyl ether of bisphenol A) is chain-extended with bisphenol A in the presence of a catalyst to yield higher polymerized products. The advancement reaction is conducted at elevated temperatures (175—200°C) and is monitored for epoxy value and viscosity specifications. The finished product is isolated by cooling and cmshing or flaking the molten resin or by allowing it to soHdify in containers. 

So important are lattice imperfections in the reactions of solids that it is considered appropriate to list here the fundamental types which have been recognized (Table 1). More complex structures are capable of resolution into various combinations of these simpler types. More extensive accounts of crystal defects are to be found elsewhere [1,26,27]. The point which is of greatest significance in the present context is that each and every one of these types of defect (Table 1) has been proposed as an important participant in the mechanism of a reaction of one or more solids. In addition, reactions may involve structures identified as combinations of these simplest types, e.g. colour centres. The mobility of lattice imperfections, which notably includes the advancing reaction interface, provides the means whereby ions or molecules, originally at sites remote from crystal imperfections and surfaces, may eventually react. 

The systematic treatment of interface advance reactions given by Jacobs and Tompkins [28] remains a valuable survey of the kinetics of solid phase decompositions. A later account was given by Young [29]. Greater mathematical emphasis is to be found in the books by Delmon [30] and by Barret [31]. 

KINETIC EXPRESSIONS DERIVED FOR INTERFACE ADVANCE REACTIONS... 

Advanced thin films, 1 723—726 U.S. market trends, l 726t Advanced visual technology, 15 469-470 Advanced waste recycling, 21 454 Advancement process, 10 361-364, 387 Advancement reaction catalysts, 10 362-363... 

An homologous series of advancement reactions was synthesized using standard reaction conditions on a 300 gram scale. The branch concentration was increased from zero to 0.677 branches/molecule. Viscosity and gel permeation (GPC) data were obtained. Table II summarizes the results. 

Advancement reactions were carried out on a 300 gran scale, unless otherwise specified, using commercially supplied Epon 328, BPA and DEN 438. All resins were used as received, with no further purification. Triphenylphosphine was used as received from Polysciences (lot 2-2401). 

Advancement reactions were performed using electric heating mantles. Temperatures of the resin, mantle/beaker interface, and mantle interior were recorded via thermocouples. Reaction mixtures were not protected from atmospheric oxygen. The advancement reaction was initiated at 120 C using O.IZ (weight/weight) triphenylphosphine. 

An exothermic chemical reaction that propagates with such rapidity that the rate of advance of the reaction zone into the unreacted material exceeds the velocity of sound in the unreacted material, that is the advancing reaction zone is preceded by a shock wave. The rate of advance of the reaction zone is termed detonation rate or detonation velocity. When this rate of advance attains such a value that it will continue without diminution thru the unreacted material, it is termed a stable detonation velocity. The exact value of this term is dependent upon a number of factors, principally the chemical and physical properties of the material. When the detonation rate is equal to or greater than the stable detona-... 

In the first step, all the coal bulk which is readily available in the mixture of coal and solvent goes to Ri which is solid, Li which is liquid, and Gi which is gas. The rate constant for this unimolecular reaction is ki. When Reaction 1 is well advanced, Reaction 2 becomes the main route with the extraction of Ri (rate constant fa) and so on. According to Dryden (2), extraction occurs by the removal of units of colloidal size directly from the coal, and he proposes a model for extraction as follows ... 

Periodic reactions of this kind have been mentioned before, for example, the Liese-gang type phenomena during internal oxidation. They take place in a solvent crystal by the interplay between transport in combination with supersaturation and nuclea-tion. The transport of two components, A and B, from different surfaces into the crystal eventually leads to the nucleation of a stable compound in the bulk after sufficient supersaturation. The collapse of this supersaturation subsequent to nucleation and the repeated build-up of a new supersaturation at the advancing reaction front is the characteristic feature of the Liesegang phenomenon. Its formal treatment is quite complicated, even under rather simplifying assumptions [C. Wagner (1950)]. Other non-monotonous reactions occur in driven systems, and some were mentioned in Section 10.4.2, where we discussed interface motion during phase transformations. 

Topley and Hume [36] developed a model based on the rapid initial formation of (on average) a single nucleus on the surface of each particle of reactant, represented as a sphere of radius, a. The advancing reaction interface penetrates the reactant at an equal rate in all inward directions (k - dr/d/) and the volume of material reacted at time t is that volume of a sphere, having its centre at the site of surface nucleation and of radius k t, which falls within the reactant. The fractional reaction, which is the zone of interpenetrating spheres, at time t is ... 

Synchrotron radiation studies [92] have identified systematic changes in the interplanar spacing in the vicinity of the microscopically visible reaction interface, showing that the thickness of the reaction zone is about 150 pm. There is apparently significant loss of water fi-om the zone ahead of the recrystallization plane identified as the advancing reaction interface. 

Water elimination and product recrystallization linked together in the vicinity of a single advancing reaction interface... 

The dissociations described above have mostly been identified as interface advance reactions for which the nucleation step occurs relatively readily. The dominant kinetic feature is the progress of the reaction zone inwards to the particle centres. The Polanyi-Wigner reaction model (Chapter 7) was developed to account for the rates of such processes. Shannon [83] identified 29 different chemical changes of this type and found that only one-third of the reported kinetic parameters were within an order of magnitude of the theoretically expected values. From these, the dissociations of CaCO, and MgCOj were selected for more quantitative application of the absolute reaction rate theory. The known crystal structure and physical properties of the participating bonds were used to represent the transition state as follows ... 

Safety using nitric acid at high temperatures and pressures requires a well-conceived and continually executed safety concept, using advanced reaction technology. 

Pales and Stroeve [31] investigated the effect of the continuous phase mass transfer resistance on solute extraction with double emulsion in a batch reactor. They presented an extension of the perturbation analysis technique to give a solution of the model equations of Ho et al. [29] taking external phase mass transfer resistance into account. Kim et al. [5] also developed an unsteady-state advancing reaction front model considering an additional thin outer liquid membrane layer and neglecting the continuous phase resistance. 

Dutta et al. [32] modified the pseudo-steady-state advancing reaction front model of Stroeve and Varanasi [30] by considering the polydispersity of the emulsion globules and the external phase mass transfer resistance. They also included the outer membrane film resistance in their model [5]. Their results were in good agreement with experimental data for phenol extraction. 

Lorbach and Hatton [56] analyzed the polydispersity and back mixing effects in terms of the advancing reaction front model by assuming pseudosteady-state diffusion within the macrodrop so that the zero order solution to the perturbation expansion could be used. Mok et al. [57] proposed a... 

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