Most Common Processing Methods

Several problems have been recognized even with this simple procedure, however. Firstly substantial retention of high boiling solvents such as NMP does occur, as documented in several studies [45]. Secondly, it also appears that the highest molecular weight (MWt) fractions of even the reduced CP do not dissolve, and are left behind [242]. The CP so obtained thus may not be truly representative of intrinsic behavior of the parent CP. 

With regard to solvation of the de-doped forms of CPs, it may be perceived that this should be applicable to virtually all CPs as a means of processing. It is true that nearly all CPs show substantial solubility in organic solvents in their de-doped states. However, the highest MWt fractions frequently do not dissolve. Additionally, dissolution does not appear to be reproducible in all cases. For instance, if one dissolves a reduced P(Py) film in CHCI3, recasts the film, and attempts to redissolve the recast film, one loses a bit of the polymer in each such cycle, and one observes substantial degradation in electroactivity and other relevant CP properties. Only a few CPs, such as long-chain-alkyl substituted poly (thiophenes) (see below), are reproducibly soluble in this manner. 

Another very common processing method applicable to CPs with moderate thermal stability, such as P(ANi) in its de-doped (emeraldine base) and sulfonate-doped forms, is thermal processing through dispersion in thermoplastics above their melt temperature. This is discussed in more detail further below. 

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Although extrusion is by far the most common processing method for WPC, the processors use a variety of extruder types and processing strategies [89]. Some processors run compounded pellets through single-screw extruders to form the final shape, others compound and extrude final shapes in one step using twin-screw extruders. 

One of the most common processing methods for plastics is injection moulding. Nowadays every home, every vehicle, every office, every factory contains a multitude of different types of articles which have been injection moulded. These include such things as electric drill casings, yoghurt cartons, television... 

Distillation is by far the most commonly used method for the separation of homogeneous fluid mixtures. No attempt should be made to optimize pressure, reflux ratio, or feed condition of distillation in the early stages of design. The optimal values will almost certainly change later once heat integration with the overall process is considered. 

In the most common production method, the semibatch process, about 10% of the preemulsified monomer is added to the deionised water in the reactor. A shot of initiator is added to the reactor to create the seed. Some manufacturers use master batches of seed to avoid variation in this step. Having set the number of particles in the pot, the remaining monomer and, in some cases, additional initiator are added over time. Typical feed times ate 1—4 h. Lengthening the feeds tempers heat generation and provides for uniform comonomer sequence distributions (67). Sometimes skewed monomer feeds are used to offset differences in monomer reactivity ratios. In some cases a second monomer charge is made to produce core—shell latices. At the end of the process pH adjustments are often made. The product is then pumped to a prefilter tank, filtered, and pumped to a post-filter tank where additional processing can occur. When the feed rate of monomer during semibatch production is very low, the reactor is said to be monomer starved. Under these... 

Some alkylphenols in commercial production have low vapor pressures and/or low thermal decomposition temperatures. Eor these products, the economics of distillation are poor and other recovery processes are used. Crystallisation from a solvent is the most common nondistUlation method for the purification of these alkylphenols. 

Chemical recovery ia sodium-based sulfite pulpiag is more complicated, and a large number of processes have been proposed. The most common process iavolves liquor iaciaeration under reduciag conditions to give a smelt, which is dissolved to produce a kraft-type green liquor. Sulfide is stripped from the liquor as H2S after the pH is lowered by CO2. The H2S is oxidized to sulfur ia a separate stream by reaction with SO2, and the sulfur is subsequendy burned to reform SO2. Alternatively, ia a pyrolysis process such as SCA-Bidemd, the H2S gas is burned direcdy to SO2. A rather novel approach is the Sonoco process, ia which alumina is added to the spent liquors which are then burned ia a kiln to form sodium aluminate. In anther method, used particulady ia neutral sulfite semichemical processes, fluidized-bed combustion is employed to give a mixture of sodium carbonate and sodium sulfate, which can be sold to kraft mills as makeup chemical. 

Vanadium metal can be prepared either by the reduction of vanadium chloride with hydrogen or magnesium or by the reduction of vanadium oxide with calcium, aluminum, or carbon. The oldest and most commonly used method for producing vanadium metal on a commercial scale is the reduction of V20 with calcium. Recently, a two-step process involving the alurninotherniic reduction of vanadium oxide combined with electron-beam melting has been developed. This method makes possible the production of a purer grade of vanadium metal, ie, of the quaUty required for nuclear reactors (qv). 

Nylon resins are made by numerous methods (53) ranging from ester amidation (54) to the Schotten-Baumann synthesis (55). The most commonly used method for making nylon-6,6 and related resins is the heat-induced condensation of monomeric salt complexes (56). In this process, stoichiometric amounts of diacid and diamine react in water to form salts. Water is removed and further heating converts the carboxylate functions to amide linkages. Chain lengths are controlled by small amounts of monofunctional reagents. The molten finished nylon resin can be dkectly extmded to pellets. 

A prerequisite for a precise and accurate titration is the reproducible identification of an end point which either coincides with the stoichiometric point of the reaction or bears a fixed and measurable relation to it. An end point may be located either by monitoring a property of the titrand which is removed when the stoichiometric point is passed, or a property which can be readily observed when a small excess of the titrant has been added. The most common processes observed in end-point detection are change of colour change of electrical cell potential change of electrical conductivity precipitation or flocculation. (Electrochemical methods are discussed in Chapter 6 precipitation indicators find only limited use.)... 

The chemistry of nitro compounds forms the basis of a number of well-known processes, such as the Henry or the Nef reactions . Transformations such as the latter permit the interconversion between nitro and other functional groups and are therefore of prime importance. The most commonly employed methods for the reduction of primary nitroalkanes to oximes involve the use of BusSnH, Se/NaBH4, CS2 or SnCla (often in combination with thiophenol) . 

Four-point probe is the most commonly used method to measure sheet resistance for metal films in silicon processing. Given known resistivities of each material, the film thickness can be obtained. Table VI lists the resistivities for the films often used in the back-end interconnect processes. The theory in four-point prohe originally comes from Ohm s law ... 

The most common current method of phenol production is from the cumene hydroperoxide rearrangement process. In this process, benzene reacts with propylene to produce cumene. Cumene is oxidized to cumene hydroperoxide. When cumene hydroperoxide is treated with dilute sulfuric acid, it rearranges and splits into phenol and acetone. Because the reactants are inexpensive and the process is simple, the acidic oxidation of cumene is used to produce more than 95% of the worlds supply of phenol. 

While validating a production process, several steps were listed as they pertained to each of the components of manufacturing equipment, process conditions, personnel, and so forth. These key elements multiply rapidly when it comes to analytical methods validation. Take, for example, HPLC — the most commonly used method of analysis. A typical analytical method would involve use of columns, pumps, heaters, detectors, controllers, samplers, sensors, recorders, computers, reagents, standards, and operators — put together as a system. Each of these components and systems needs independent validation, followed by a validation of the system. Note that when this equipment is used to manufacture a product such a therapeutic proteins wherein HPLC techniques are used for the purification purpose, then all additional requirements of a manufacturing system also apply, including, but not limited to, the requirement that the equipment be of a sanitary kind. This limits the choice for manufacturers, and these considerations should be taken into account in the first selection of equipment. 

Chlorination of Alkanes. Free-radical chlorination is the most commonly used method for the chlorination of a saturated hydrocarbon.31 106-108 111 112 Both thermal and photochemical processes may be carried out in the liquid or vapor phase. The liquid-phase photochemical procedure is preferred for polychlorination gas-phase photochemical reactions can yield either mono- or polychlorinated product. 

In the fractionation of the milk proteins, usually the first step in the process is to separate the so-called whole casein from the whey in a skim milk. A number of procedures are available (McKenzie 1971C), but the most commonly used method is based upon classical acid precipitation at the pH of minimum solubility. Several different temperatures have been employed 2, 20, and 30°C. Except for precipitation at 2°C, where minimum solubility occurs at pH 4.3, the skim milk is adjusted to pH 4.5-4.6 with hydrochloric acid (1 M). A more recent investigation of the relationship of temperature and pH to the completeness of casein precipitation indicated that optimum yield was obtained at pH 4.3 and 35°C (Helesicova and Podrazky 1980). 

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