Direct-melt process

The direct melt process economically produces thick optical fibers (250-400 pm in diam.), which is advantageous, but their relatively high attenuation (3-20 dB/km) due to impurities is not. As a result, they are limited to short distance multimode applications. 

Figure 7.7 Direct melting process of making optical glass fiber. Qadding and core rods are melted in a double crucible and wound on a drum.

The industrially most important process is the direct melt process in which the glass melt passes through a forehearth before being spun. This zone consists of a brick-lined channel in which the melt is cooled to the ca. 1250°C necessary for satisfactory spinning with spinning jets mounted on its side arms. 

Conversion of glass melts into l ibers is achieved u.sing the direct ttielt, tnarble melt and rod drawing processes with the direct melt process dominating. 

Continuous fibers can be formed in a direct melt process (Figure 1), where the melt is formed from mixed raw materials, or in a remelt process, where the melt is formed from marbles which had previously been made from batch in a separate process step. Continuous fibers... 

Figure 1. Direct melt process for the production of continuous glass fibers. Redrawn from J. F. Dockum, Fiberglass, in Handbook of reinforcements for plastics, J. V. Milewski and H. S. Katz, Editors, 233-286, Van Nostrand Reinhold, New York, NY (1987).

The production of glass hbres is by the direct melt process in which fine filaments diameter 3-24 pm are produced by continuous and rapid drawing from the melt (Hollaway, 2008). The two most important fibres which are used in bridge engineering are ... 

Direct Reduction. Direct reduction processes are distinguished from other ironmaking processes in that iron oxide is converted to metallic iron without melting. Because this product, called direct reduced iron (DRI), is soHd, it is most suitable for melting in an electric arc furnace (EAF) as a substitute for scrap (see Furnaces, electric). The briquetted form of DRI, hot briquetted iron (HBI) is used when the product is to be transported. Briquetting increases density and chemical stabiUty. The predominant direct reduction processes (MIDREX and HyL III) are based on natural gas as a fuel and reductant source. They are economically attractive in regions where natural gas is cheap and abundant, especially if iron ore is available nearby (see Iron BY DIRECT reduction). ... 

Low molecular weight PET and PBT resins are made by melt processes. For higher molecular weight resins, both melt processes or soHd-state polymerization are used. Although terephthaHc acid can be directly esterified, the most common process involves transesterification of dimethyl terephthalate with ethylene glycol or 1,4-butanediol in the presence of trace amounts of metal ion catalysts (67,68). 

The type of manufacturing process, reaction conditions, and catalyst are the controlling factors for the molecular structure of the polymers [4-8]. The molecular features govern the melt processability and microstructure of the solids. The formation of the microstructure is also affected by the melt-processing conditions set for shaping the polymeric resin [9]. The ultimate properties are, thus, directly related to the microstructural features of the polymeric solid. 

Since the possibility of direct melt intercalation was first demonstrated [11], melt intercalation has become a method of preparation of the intercalated polymer/ layered silicate nanocomposites (PLSNCs). This process involves annealing, statically or under shear, a mixture of the polymer and organically modified layered fillers (OMLFs) above the softening point of the polymer. During annealing, the polymer chains diffused from the bulk polymer melt into the nano-galleries between the layered fillers. 

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