Solid-state processing

In sintering, the green compact is placed on a wide-mesh belt and slowly moves through a controlled atmosphere furnace (Fig. 3). The parts are heated to below the melting point of the base metal, held at the sintering temperature, and cooled. Basically a solid-state process, sintering transforms mechanical bonds, ie, contact points, between the powder particles in the compact into metallurgical bonds which provide the primary functional properties of the part. 

Radical induced grafting may be carried out in solution, in the melt phase,292 29 or as a solid state process.296 This section will focus on melt phase grafting to polyolefin substrates but many of the considerations are generic. The direct grafting of monomers onto polymers, in particular polyolefins, in the melt phase by reactive extrusion has been widely studied. Most recently, the subject has been reviewed by Moad1 9 and by Russell.292 More details on reactive extrusion as a technique can be found in volumes edited by Xanthos," A1 Malaika and Baker et a 21 7 The process most often involves combining a frcc-radical initiator (most commonly a peroxide) and a monomer or macromonomer with the polyolefin as they are conveyed through the extruder. Monomers commonly used in this context include MAII (Section 7.6.4.1), maleimidc derivatives and malcate esters (Section 7.6.4.2), (meth)acrylic acid and (meth)acrylate esters (Section 7.6.43), S, AMS and derivatives (Section 7.6.4.4), vinylsilancs (Section 7.6.4.5) and vinyl oxazolines (Section 7.6.4.6). 

Measurements of product gas evolution, mass loss or evolved gas analysis may all be used to study the kinetics of a solid—solid interaction provided that there is strict adherence to the condition that gas evolution occurs concurrently with the solid state process. Clearly this approach is only applicable if there is direct experimental support for a single step process. For example, carbon dioxide release is identified [410] as being... 

The reactions of some aromatic metal carboxylates are on the borderline of classification as solid-state processes. While there is no evidence of liquefaction, rates of decomposition in the poorly crystallized or vitreous reactant obey kinetic expressions more characteristic of reactions proceeding in a homogeneous phase. 

In the case of selective oxidation catalysis, the use of spectroscopy has provided critical Information about surface and solid state mechanisms. As Is well known( ), some of the most effective catalysts for selective oxidation of olefins are those based on bismuth molybdates. The Industrial significance of these catalysts stems from their unique ability to oxidize propylene and ammonia to acrylonitrile at high selectivity. Several key features of the surface mechanism of this catalytic process have recently been descrlbed(3-A). However, an understanding of the solid state transformations which occur on the catalyst surface or within the catalyst bulk under reaction conditions can only be deduced Indirectly by traditional probe molecule approaches. Direct Insights Into catalyst dynamics require the use of techniques which can probe the solid directly, preferably under reaction conditions. We have, therefore, examined several catalytlcally Important surface and solid state processes of bismuth molybdate based catalysts using multiple spectroscopic techniques Including Raman and Infrared spectroscopies, x-ray and neutron diffraction, and photoelectron spectroscopy. 

Solid-state phase transitions usually take place at a slower rate than solution-mediated phase transitions because the motion of the molecules is restricted in the solid state. Processing stresses such as grinding [28], heating [28], humidity... 

Picture of solid-state process hydrogen sensor implemented in a process stream. (PCB—printed circuit board.)... 

Hannay, N. B. (1967). Solid-State Chemistry. Prentice-Hall, Englewood Cliffs, NJ. An older book that gives a good introduction to solid-state processes. 

The dual treatment in the Buhler botde-to-bottle process is an important aspect in food safety considerations. The bulk of the contaminants are removed in the extruder. However, the SSP process provides a back-up to remove any residual contaminants, which are now homogeneously distributed in the PET pellets. The cleaning becomes a well-defined and predictable diffusion controlled process, which is defined by pellet diameter, treatment temperature and time. The same parameters also regulate the SSP process. For products with similar reactivity, a known increase in molecular weight during the solid-state process will also provide a known cleaning efficiency. 

The CEG content decreases during the solid-state process for producing bottle-grade PET, under the standard conditions, by ca. 20-40 %. For the production... 

The observed reduction in the content of oligomers during the solid-state process is shown in Figure 5.26. These contents drop during SSP upon increasing temperatures and time. However, these low levels are increased by remelting in subsequent processing. 

Photodimerization of cinnamic acids and its derivatives generally proceeds with high efficiency in the crystal (176), but very inefficiently in fluid phases (177). This low efficiency in the latter phases is apparently due to the rapid deactivation of excited monomers in such phases. However, in systems in which pairs of molecules are constrained so that potentially reactive double bonds are close to one another, the reaction may proceed in reasonable yield even in fluid and disordered states. The major practical application has been for production of photoresists, that is, insoluble photoformed polymers used for image-transfer systems (printed circuits, lithography, etc.) (178). Another application, of more interest here, is the use that has been made of mono- and dicinnamates for asymmetric synthesis (179), in studies of molecular association (180), and in the mapping of the geometry of complex molecules in fluid phases (181). In all of these it is tacitly assumed that there is quasi-topochemical control in other words, that the stereochemistry of the cyclobutane dimer is related to the prereaction geometry of the monomers in the same way as for the solid-state processes. 

In this and the other solid-state processes described in this section, we see that the factors determining the products are as follows ... 

McBride and co-workers have studied extensively the reactions of such free-radical precursors as azoalkanes and diacyl peroxides (246). By employing a variety of techniques, including X-ray structure analysis, electron paramagnetic resonance (EPR), and product studies, and comparing reactions in the crystal and in fluid and rigid solvents, they have been able to obtain extremely detailed pictures of the solid-state processes. We will describe here some of the types of lattice control they have elucidated, and the mechanisms that they suggest limit the efficacy of topochemical control. 

Both solid-solid and solid-gas types of reactions lead from solid reactants to a solid product without the use of solvents. Solvent-less processes, however, are not necessarily solid-state processes. Indeed, it has been argued [8d,e] that many solid-state syntheses cannot be regarded as bona fide solid-solid reactions because they occur with the intermediary of a liquid phase, such as a eutectic phase or a melt, or may require destruction of the crystals prior to reaction. This latter situation is often observed, for instance, in the case of reactions activated by co-grinding, since the heat generated in the course of the mechanochemical process can induce local melting at the interface between the different crystals, or when kneading, i.e. grinding in the presence of small amounts of solvent, takes place (vide infra). 

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