From melts number distribution

Metal Particles This test is required only for ophthalmic ointments. The presence of metal particles will irritate the corneal or conjunctival surfaces of the eye. It is performed using 10 ointment tubes. The content from each tube is completely removed onto a clean 60-mm-diameter petridish which possesses a flat bottom. The lid is closed and the product is heated at 85 °C for 2 h. Once the product is melted and distributed uniformly, it is cooled to room temperature. The lid is removed after solidification. The bottom surface is then viewed through an optical microscope at 30x magnification. The viewing surface is illuminated using an external light source positioned at 45 ° on the top. The entire bottom surface of the ointment is examined, and the number of particles 50 pm or above are counted using a calibrated eyepiece micrometer. The USP recommends that the number of such particles in 10 tubes should not exceed 50, with not more than 8 particles in any individual tube. If these limits are not met, the test is repeated with an additional 20 tubes. In this case, the total number of particles in 30 tubes should not exceed 150, and not more than 3 tubes are allowed to contain more than 8 particles [15]. 

When an ionic salt such as NaCl melts, the ionic lattice (see Figure 5.15) collapses, but some order is stiU retained. Evidence for this comes from X-ray diffraction patterns, from which radial distribution functions reveal that the average coordination number (with respect to cation-anion interactions) of each ion in liquid NaCl is 4, compared with 6 in the crystalline lattice. For cation-cation or anion-anion interactions, the coordination number is higher, although, as in the solid state, the intemuclear distances are larger than for cation-anion separations. The solid-to-liquid transition is accompanied by an increase in volume of il0-15%. The number of ions in the melt can be determined in a similar way to that described in Section 8.8 for H2SO4 systems in molten NaCl, v = 2. 

Soldering and brazing are used to join two metal parts by means of a filler metal that has a lower melting point than the base metal. The joint may consist of one or more metals. The filler metal is normally distributed in the joint by capillary action, with additional flux dissolving unwanted oxides. The necessary heat can be provided from a number of sources, including furnaces or fuel gas torches. 

As has been demonstrated by Bodor [65], the number distribution of the crystalline particle size in partially crystalline polymers can be determined by a method related to line broadening of X-ray scattering peaks. Figure 6.17 shows the particle size distribution of a linear PE sample that was cooled down from the melt atl60°Ctol25°C. 

Figure6.17 Number distribution of c stalline particle size of a linear polyethylene sample melted for 2 h at 160°C and annealed for two days at 125 °C. Adapted with permission from Ref [63] 1985, Springer.

Usually, crystallization of flexible-chain polymers from undeformed solutions and melts involves chain folding. Spherulite structures without a preferred orientation are generally formed. The structure of the sample as a whole is isotropic it is a system with a large number of folded-chain crystals distributed in an amorphous matrix and connected by a small number of tie chains (and an even smaller number of strained chains called loaded chains). In this case, the mechanical properties of polymer materials are determined by the small number of these ties and, hence, the tensile strength and elastic moduli of these polymers are not high. 

Sulfur is widely distributed as sulfide ores, which include galena, PbS cinnabar, HgS iron pyrite, FeS, and sphalerite, ZnS (Fig. 15.11). Because these ores are so common, sulfur is a by-product of the extraction of a number of metals, especially copper. Sulfur is also found as deposits of the native element (called brimstone), which are formed by bacterial action on H,S. The low melting point of sulfur (115°C) is utilized in the Frasch process, in which superheated water is used to melt solid sulfur underground and compressed air pushes the resulting slurry to the surface. Sulfur is also commonly found in petroleum, and extracting it chemically has been made inexpensive and safe by the use of heterogeneous catalysts, particularly zeolites (see Section 13.14). One method used to remove sulfur in the form of H2S from petroleum and natural gas is the Claus process, in which some of the H2S is first oxidized to sulfur dioxide ... 

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