Casting heat, exothermic

Methacrylate polymerizations are accompanied by the Hberation of a considerable amount of heat and a substantial decrease in volume. Both of these factors strongly influence most manufacturing processes. Excess heat must be dissipated to avoid uncontrolled exothermic polymerizations. Volume changes are particularly important in sheet-casting processes where the mold must compensate for the decreased volume. In general, the percent shrinkage decreases as the size of the alcohol substituent increases on a molar basis, the shrinkage is relatively constant (35). 

The reaction vessel (nitrator) is constructed of cast iron, mild carbon steel, stainless steel, or glass-lined steel depending on the reaction environment. It is designed to maintain the required operating temperature with heat-removal capabiUty to cope with this strongly exothermic and potentially ha2ardous reaction. Secondary problems are the containment of nitric oxide fumes and disposal or reuse of the dilute spent acid. Examples of important intermediates resulting from nitration are summarized in Table 3. 

When Plaster of Paris, CaSC>4 5H2O (s), is mixed with water, solid gypsum, CaS04- 2 H2O (s), is formed in an exothermic reaction, that is, a heat-releasing reaction. Thus, as the cast hardens, the plaster cast becomes warm ... 

Gadolinium is produced from both its ores, monazite and bastnasite. After the initial steps of crushing and beneficiation, rare earths in the form of oxides are attacked by sulfuric or hydrochloric acid. Insoluble rare earth oxides are converted into soluble sulfates or chlorides. When produced from monazite sand, the mixture of sand and sulfuric acid is initially heated at 150°C in cast iron vessels. Exothermic reaction sustains the temperature at about 200 to 250°C. The reaction mixture is cooled and treated with cold water to dissolve rare earth sulfates. The solution is then treated with sodium pyrophosphate to precipitate thorium. Cerium is removed next. Treatment with caustic soda solution fohowed by air drying converts the metal to cerium(lV) hydroxide. Treatment with hydrochloric or nitric acid sol-... 

PVC plastisol propellant may be formed into the required size and shape to make the desired grains either by screw extrusion or by casting into molds. No exotherm or volume change is observed as a result of curing (16). Molds are normally bottom-filled, the propellant being forced into them from a pressurized casting pot. Heat to raise the propellant to curing temperature may be supplied by many means. The filled... 

The methyl, ethyl, and butyl esters of acrylic and methacrylic acids are polymerized under the influence of heat, light, and peroxides. The polymerization reaction is exothermic and may be carried out in bulk for castings, or by emulsion, or in solution. The molecular weight decreases as the temperature and catalyst concentration are increased. The polymers are noncrystalline and thus very clear. Such resins are widely used because of their clarity, brilliance, ease of forming, and light weight. They have excellent optical properties and are used for camera, instrument, and spectacle lenses. 

The sulfonation reaction is exothermic, but not highly corrosive, so sulfonation can be conducted in steel, stainless-steel, or cast-iron sulfona-tors. A jacket heated with hot oil or steam can serve to heat the contents sufficiently to get the reaction started, then carry away the heat of reaction. A good agitator, a condenser, and a fume control system are usually also provided. 

Depending on the plastic to be cast, solidification takes place at either room temperature or elevated temperatures. With room temperature systems chemical reaction occurs with the liberation of heat. The rate of heat dissipation can influence the performance and aesthetic characteristics of the hardened product. In thin sections, where a large area in relation to the total volume of the plastic is exposed, the heat of the exothermic reaction is dissipated rapidly and the temperature of casting is not very high. Thin sections can be cast at room temperature with no danger of cracking. When the rate of heat is excessive, application of heat may be necessary to properly control cure rate. 

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