Specialized Melting Methods

Heavy metal halide glasses also require melting in an oxygen-free atmosphere to preserve their optical properties. These glasses are often melted under a reactive atmosphere, such as CCI4 or SFg, which, due to decomposition of the atmospheric gas, contains free halide. The atmosphere acts as both a getter for oxygen, and as a source of halide to replace volatilization losses and maintain the stoichiometry of the 

Define the role of each of the following in a glass batch. Give at least 2 examples of each  

Express the following compositions in stoichiometry formulae. (example-Li2Si205 for lithium disilicate)  

Write the general oxide formula for a series of glasses where  

Why is there an optimum size for the sand used in glass melting, i.e., what problems occur if the sand is (a) too coarse, or (b) too fine  

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Processability will be important insofar as the need for complex equipment will add to cost, and it may ultimately render the material useless for this type of application. An example is melting point. Even if all other factors were attractively addressed, a high melting point would require special melting equipment and may in fact be dan-gerons to the patient. There are, of course, alternative methods of delivery to melting, snch as solvent evaporation, but the risks associated with those methods must also be evaluated. 

Polycondensation at room temperature between two or more fast-reacting intermediates is becoming widely used because of its convenience and speed. The interfacial polycondensation system, in particular, which employs two immiscible liquids, is applicable to a wide voriety of chemical structures amides, urethanes, esters, sulfonates, sulfonamides, and ureas. Many products can be made at low temperature which could not be formed by melt methods because of their infusibility or thermal instability. The low temperature procedures are subject to the effect of many variables, but these are readily controlled and acceptable conditions for use with new polymers or intermediates can usually be found. The processes are readily scaled up in simple batch equipment or continuous reactors. Special areas of application are the direct formation of fibers from the reactants and polycondensation on fiber substrates. 

The basic methods for forming film or sheeting materials may be classified as follows melt extmsion, calendering, solution casting, and chemical regeneration. Of special note is the use of biaxial orientation as part of the critical manufacturing steps for many film and sheet products. 

Ionomer resins are produced in multiple grades to meet market needs, and prospective customers are provided with information on key processing parameters such as melt-flow index. Nominal values for many other properties are Hsted in product brochures. The ASTM test methods developed for general-purpose thermoplastic resins are appHcable to ionomers. No special methods have been introduced specifically for the ionomers. 

Molecular Weight. PE mol wt (melt index) is usually controlled by reaction temperature or chain-transfer agents. Reaction temperature is the principal control method in polymerization processes with Phillips catalysts. On the other hand, special chemical agents for chain transfer are requited for... 

The melting points, optical rotations, and uv spectral data for selected prostanoids are provided in Table 1. Additional physical properties for the primary PGs have been summarized in the Hterature and the physical methods have been reviewed (47). The molecular conformations of PGE2 and PGA have been determined in the soHd state by x-ray diffraction, and special H and nuclear magnetic resonance (nmr) spectral studies of several PGs have been reported (11,48—53). Mass spectral data have also been compiled (54) (see Mass spectrometry Spectroscopy). 

The second method, based on measurements of IR reflection spectra, is simpler and enables working with larger volumes of molten salt. No special problems involving temperature and atmosphere control exist. The method was used successfully by Fordyce and Baum [336-338] in the investigation of fluoride melts containing tantalum and niobium. 

The separation of the excess unreacted sodium is also subject to investigation and development. Miyazaki and Kuroki [588] proposed a reactor system for the reduction of K2TaF7 or K2NbF7 with Na or K, respectively. According to this method, the reduced melt is tapped into a special chamber for solidification, while unreacted Na or K is trapped with a condenser in the gas phase for recycling. 

Data on high-temperature melts are still limited. Conventional methods are difficult to apply because of the high values of thermal conductivity. Other difficulties in measuring molten salts are their corrosiveness, high electrical conductivities, and the necessity of careful preparation. Special care should be taken to exclude convection errors, which are usually the most serious source of errors, even at room temperature. 

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