Viscous melts

The carbide impurities tend to distill and are reoxidized in the upper, cooler region of the charge. This process forms cmsts near the top of the charge or around the cooler part of the reaction cmcible and causes trouble in furnace operation. Large amounts of dissolved calcium siUcate and alurninate may form a viscous melt and impede the tapping process. FerrosiUcon is commonly removed from the cmshed carbide by electromagnets. 

In a thermoplastic material the very long chain-like molecules are held together by relatively weak Van der Waals forces. A useful image of the structure is a mass of randomly distributed long strands of sticky wool. When the material is heated the intermolecular forces are weakened so that it becomes soft and flexible and eventually, at high temperatures, it is a viscous melt. 

In injection molding, the polymer is fed from a hopper into a heated barrel where it softens and becomes a viscous melt. It is then forced under high pressure into a relatively cold mold cavity where the polymer has sufficient time to solidify. Then the mold is opened and the fabricated part is ejected. The cycle of operation is then repeated. 

Shear rate When, a melt moves in a direction parallel to a fixed surface, such as with a screw barrel, mold runner and cavity, or die wall, it is subject to a shearing force. As the screw speed increases, so does the shear rate, with potential advantages and disadvantages. The advantages of an increased shear rate are a less viscous melt and easier flow. This shear-thinning action is required to move the melt. 

Thermoplastics are processed by reshaping the finished product in the form of a viscous melt at high temperatures. 

Represents the temperature of the conversion of an amorphous glassy or partially crystalline polymer into a rubbery viscous melt. Important for sensors, because polymers with high Tg require plasticizers for fast analyte diffusion and response time (see section 3.1). 

Problems associated with the stirring of the viscous melt are eliminated in the solid state. These problems become increasingly troublesome in the melt phase as the capacity increases and as the molecular weight increases above ca. 19 000. 

Bubble Growth by Decompression 13.4.1. Thin shell of over-pressured viscous melt... 

The growth of a bubble in a thin shell of over-pressured viscous melt (Fig. 13.1) is given by (Barclay et al., 1995)... 

Information on the morphology of polymers is revealed by techniques such as powder X-ray diffraction (PXRD), which is often called wide-angle X-ray scattering (WAXS) by polymer scientists, and small-angle X-ray scattering (SAXS). The crystallites exist in a polymer sample below the melting temperature T, an order-disorder transition, above which a viscous melt is formed. 

The potential for rapid randomizing processes in the copolyesters at elevated temperatures has been demonstrated conclusively by heating a mixture of the two homopolymers of PHBA and PHNA at 450 °C at a pressure of around several hundred psi [40]. Within a few seconds a viscous melt was observed to extrude from the cracks in the mold. Analysis of this material showed a structure consistent with the random 50/50 copolymer of HBA/HNA (see Figs. 18 and 19). We estimate that at this very high temperature the rate of interchain transesterification reactions corresponds to 1000 ester interchanges/chain/10 s. 

The common feature of the internal reactions discussed so far is the participation of electronic defects. In other words, we have been dealing with either oxidation or reduction. We now show that reactions of the type A+B = AB can take place in a solvent crystal matrix as, for example, the formation of double oxides (CaO +Ti02 = CaTi03) in which atomic (ionic) but no electronic point defects are involved. Although many different solvent crystal matrices can be thought of (e.g., metals, semiconductors, glasses, and even viscous melts and surfaces), we will deal here mainly with ionic crystal matrices in order to illustrate the basic features of this type of solid state reaction. 

Rotation of the core (or its reciprocating rotary vibration) can be even more efficient in processing of high-viscous melts, for example, filled polymers, high- and superhigh-molecular polyethylene (with MM > 10s). We may assume that this is dependent upon two major causes. The introduction of a filler results in a changed spectrum of relaxation time H(9) 41-42-45). Thus, for example, introduction of 10% of chalk (by volume) into polyolefins shifts the spectrum along the axis of coordinates towards... 

Phthalimide (500 g) and 20 g of magnesium were melted in a wide glass tube heated by an oil bath. The internal temperature was kept at 240 -250 C and a vigorous stream of dry ammonia passed, the molten mass being stirred mechanically. A considerable quantity of phthalimide sublimed. After 8 hr heating, the viscous melt was cooled, broken up, and treated with a large bulk of 5% NaOH solution, the vessel being streamed out to remove the excess of metal, and... 

Linear amorphous polymers can behave as either Hookian elastic (glassy) materials, or highly elastic (rubbery) substances or as viscous melts according to prevailing temperature and time scale of experiments. The different transitions as shown schematically in Figure 5.1 are manifestations of viscoelastic deformations, which are time dependent [1]. 

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