Precursor Handling

Provision must be made for an area with ready access, permitting clean and dry storage of the precursor. 

Creel Oven 1 Oven 2 Plaiter LT furnace HT HM Surface Drying and sizing Collection 

If a creel is used to hold precursor fiber that is to be twisted (e.g. rayon), then running speeds are significantly faster and it is best if all ends are fitted with tow tension devices, which will also help detect breakages and indicate when all the fiber has been unwound from the package. 

A DC drive system with a tachometer feedback is preferred for fiber drives, although AC drives with an inverter have acceptable accuracy and are preferred for controlling fans with 

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The apparatus can be thought of as being made up of three main constituent parts—precursor handling, the reactor, and the exhaust of by-products. 

It has been found that the flavor of fmit can be increased by a process called precursor atmosphere (PA) (77). When apples were stored in a controlled atmosphere containing butyl alcohol [71-36-3] the butyl alcohol levels increase by a factor of two, and the polar products, butyl ester, and some sesquiterpene products increase significantly. The process offers the possibiUty of compensating for loss of flavor in fmit handling and processing due to improper transportation conditions or excessive heat. 

The product of a successful suspension polymerization is small, uniform polymer spheres. For certain appHcations, they are used directly, eg, as the precursors for ion-exchange resins or bead foams. For others, they may be extmded and chopped to form larger, more easily handled mol ding pellets. 

The radioactive isotopes available for use as precursors for radioactive tracer manufacturing include barium [ C]-carbonate [1882-53-7], tritium gas, p2p] phosphoric acid or pP]-phosphoric acid [15364-02-0], p S]-sulfuric acid [13770-01 -9], and sodium [ I]-iodide [24359-64-6]. It is from these chemical forms that the corresponding radioactive tracer chemicals are synthesized. [ C]-Carbon dioxide, [ C]-benzene, and [ C]-methyl iodide require vacuum-line handling in weU-ventilated fume hoods. Tritium gas, pH]-methyl iodide, sodium borotritide, and [ I]-iodine, which are the most difficult forms of these isotopes to contain, must be handled in specialized closed systems. Sodium p S]-sulfate and sodium [ I]-iodide must be handled similarly in closed systems to avoid the Uberation of volatile p S]-sulfur oxides and [ I]-iodine. Adequate shielding must be provided when handling P P]-phosphoric acid to minimize exposure to external radiation. 

Vapors from Hquids can be put into the gas stream by bubbling the hydrogen or a carrier gas through the Hquid or by using a hot surface to vaporize the Hquid into the gas stream. Liquid precursors are generally metered onto a hot surface using a peristaltic pump and the gas handling system is kept hot to... 

Neoprene Type TW was shown to have low oral toxicity in rats. The LD q was found to be in excess of 20,000 mg/kg. Human patch tests with Types GN, W, WRT, and WHV showed no skin reactions (169). The FDA status of Du Pont Neoprene polymers is described (172). Although polychloroprene itself has not been shown to have potential health problems, it should be understood that many mbber chemicals that may be used with CR can be dangerous if not handled properly. This is particularly tme of ethylenethiourea curatives and, perhaps, secondary amine precursors often contained in sulfur modified polychloroprene types. Material safety data sheets should be consulted for specific information on products to be handled. 

The electrophilic carbene carbon atom of Fischer carbene complexes is usually stabilised through 7i-donation of an alkoxy or amino substituent. This type of electronic stabilisation renders carbene complexes thermostable nevertheless, they have to be stored and handled under inert gas in order to avoid oxidative decomposition. In a typical benzannulation protocol, the carbene complex is reacted with a 10% excess of the alkyne at a temperature between 45 and 60 °C in an ethereal solvent. On the other hand, the non-stabilised and highly electrophilic diphenylcarbene pentacarbonylchromium complex needs to be stored and handled at temperatures below -20 °C, which allows one to carry out benzannulation reactions at room temperature [34]. Recently, the first syntheses of tricyclic carbene complexes derived from diazo precursors have been performed and applied to benzannulation [35a,b]. The reaction of the non-planar dibenzocycloheptenylidene complex 28 with 1-hexyne afforded the Cr(CO)3-coordinated tetracyclic benzannulation product 29 in a completely regio- and diastereoselective way [35c] (Scheme 18). 

A 1,3-diol sometimes represents a more convenient precursor to cyanohydrin acetonides. For these instances, an alternate procedure has been developed. Selective oxidation of a 1,3-diol with TEMPO/NaOCl generates a sensitive -hydroxy aldehyde (see also Sect. 3.2). The neat -hydroxy aldehyde is prone to dimerization, but can be handled in solution without significant dimerization. Conversion to the cyanohydrin acetonide is accomplished in a manner similar... 

More recently, Carreira reported a [Fe(TPP)Cl]-catalyzed diastereoselective synthesis of trifluoromethyl-substituted cyclopropane in aqueous media [56]. The carbene precursor trifluoromethyl diazomethane is difficult to be handled, generated in situ from trifluoroethyl amine hydrochloride, and reacts with styrene in the presence of [Fe(TPP)Cl] to give the corresponding cyclopropanes in high yields and with excellent diastereoselectivities (Scheme 12). 

Historically, the rhodium catalyzed carbonylation of methanol to acetic acid required large quantities of methyl iodide co-catalyst (1) and the related hydrocarboxylation of olefins required the presence of an alkyl iodide or hydrogen iodide (2). Unfortunately, the alkyl halides pose several significant difficulties since they are highly toxic, lead to iodine contamination of the final product, are highly corrosive, and are expensive to purchase and handle. Attempts to eliminate alkyl halides or their precursors have proven futile to date (1). 

The diamine and diacid monomers used to make type AABB nylons are typically rather difficult to handle in their pure form. Diamines are liquids or semisolids at room temperature, while the diacids are crystalline solids. These monomers become much more manageable when they are combined to form nylon salts, as shown in Fig. 23.7 a). Nylon salts are solids that can be easily handled and ensure a stoichiometric balance between the diacid and diamine, which is necessary to produce high molecular weight polymers. In the case of nylon 66, the precursor salt is made by boiling adipic acid and hexamethylene diamine in methanol, from which the nylon salt precipitates. 

Adam, W. et al., J. Amer. Chem. Soc., 1994,116(17), 7581 This molecule sometimes detonated spontaneously even below 0°C. The trimethyl and tetramethyl homologues should not be handled above 0°C and the precursor bromohydroperoxides are also hazardous. 

The title product and several precursors are highly pyrophoric, and all leaks must be eliminated from vacuum handling equipment. 

Features common to all CVD reactors include source evaporators with an associated gas handling system to control input gases and gas-phase precursor concentrations, a reactor cell with a susceptor heated by either radio frequency or infrared radiation, and an exhaust system to remove waste products (which may include a vacuum pump for low-pressure operations). Substrate temperatures can vary from less than 200 °C to temperatures in excess of 1000 °C, depending on the nature of the material layer and precursor used. Schematic diagrams of some simple CVD reactors are shown in Figure 4. 

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