Vapor-phase fibers

Vapor-phase fibers are produced by the catalytic decomposition of a hydrocarbon such as methane or benzene. The seed catalysts are iron particles or iron metallo-organics such as ferrocene, (C5H5)2Fe. Growth occurs in the temperature range of 1000 -1150°C. 

Vapor-phase fibers are only produced in short lengths at the present time. Maximum reported length is 50 mm with diameters from 0.5 - 2 pm. Such short-length fibers would be suitable for the random reinforcement of composites and in the production of carbon-carbon (see Ch. 9). 

Chloro-2,4,6-trifluoropyrimidine [697-83-6] has gained commercial importance for the production of fiber-reactive dyes (465,466). It can be manufactured by partial fluoriaation of 2,3,5,6-tetrachloropyrimidine [1780-40-1] with anhydrous hydrogen fluoride (autoclave or vapor phase) (467) or sodium fluoride (autoclave, 300°C) (468). 5-Chloro-2,4,6-trifluoropyrimidine is condensed with amine chromophores to provide the... 

Fig. 21. Schematic illustration of the four primary vapor-phase deposition processes used in optical-fiber fabrication outside vapor deposition (OVD), modified chemical vapor deposition (MCVD), plasma vapor deposition (PVD), and vapor axial deposition (VAD) (115).

Xylenes. The main appHcation of xylene isomers, primarily p- and 0-xylenes, is in the manufacture of plasticizers and polyester fibers and resins. Demands for xylene isomers and other aromatics such as benzene have steadily been increasing over the last two decades. The major source of xylenes is the catalytic reforming of naphtha and the pyrolysis of naphtha and gas oils. A significant amount of toluene and Cg aromatics, which have lower petrochemical value, is also produced by these processes. More valuable p- or 0-xylene isomers can be manufactured from these low value aromatics in a process complex consisting of transalkylation, eg, the Tatoray process and Mobil s toluene disproportionation (M lDP) and selective toluene disproportionation (MSTDP) processes isomerization, eg, the UOP Isomar process (88) and Mobil s high temperature isomerization (MHTI), low pressure isomerization (MLPI), and vapor-phase isomerization (MVPI) processes (89) and xylene isomer separation, eg, the UOP Parex process (90). 

Vapor-Phase Processing. Optical fiber preforms are prepared by vapor-phase techniques because of the superior clarity of the products. 

This technology also allows the control of refractive index profiles by doping. AH vapor-phase techniques use a vapor stream of volatile haUdes such as SiCl, GeCl, BCl, or POCl, and gases such as CI2 or O2. The reactants are oxidized and deposited onto a substrate to produce a soHd glass preform which is then drawn into a fiber. The variations of the technique differ in the way the reactants are oxidized (16). 

Tibbetts, G.G., Gorkiewicz, D.W., and Alig, R.A. A new reactor for growing carbon fibers from liquid- and vapor-phase hydrocarbons, Carbon, 993, 31(5), 809 814. Tibbetts, G.G., Bernardo, C.A., Gorkiewicz, D.W. and Alig R.L. Role of sulfur in the production of carbon fibers in the vapor phase. Carbon, 1994, 32(4), 569 576. 

Tibbetts, G.G., Endo, M. and Beetz, Jr. C.P., Carbon fibers grown from the vapor phase A novel material, SAMPE, Sept., Oct., 1986, 22(5). 

There are four principal processes that may be used to manufacture the glass body that is drawn into today s optical fiber. "Outside" processes—outside vapor-phase oxidation and vertical axial deposition— produce layered deposits, of doped silica by varying the concentration of SiCl4 and dopants passing through a torch. The resulting "soot" of doped silica is deposited and partially sintered to form a porous silica boule. Next, the boule is sintered to a pore-free glass rod of exquisite purity and transparency. 

Figure 5. ESCA spectra of polyethylene film (PE) and polyethylene fiber (split film PE-F) after grafting for 2 min. with the vapor phase method.

Two methods for modification of polymer surfaces by photoinitiated graft copolymerization have been developed a discontinuous method (1) with vapor phase transfer of initiator and monomer and a continuous method (2) with presoaking of a film strip or a fiber bundle in a solution of initiator and monomer. Both methods have been applied to polyelefins and linear polyester. 

Tibbetts, G.G., Gorkiewicz, D.W., and Alig, R.A. A new reactor for growing carbon fibers from liquid- and vapor-phase hydrocarbons, Carbon, 1993, 31 (5), 809 814. 

A gaseous sample is passed through a solid material, such as silica gel or polyurethane foam (PUF), in a tube. A glass fiber filter is often put in front of the solid support to capture particle-phase constituents, while the vapor-phase compounds are captured on the solid support. This is used for semivolatile analytes, such as polycyclic aromatic hydrocarbons and pesticides. The solid support is then usually extracted in the lab with a solvent (see techniques described later in this chapter), and then the techniques used for liquid samples are followed. 

Isotope fractionation between the vapor phase and the dissolved aqueous phase has been studied only for toluene and trichloroethylene (carbon only [545, 690]). Fractionation associated with adsorption has been quantified only for toluene in regard to sample extraction using a poly(dimethylsilo-xane)-coated solid-phase microextraction fiber [373] and qualified for benzene, toluene, and ethylbenzene based on high-pressure liquid chromatography analyses of isotopically labeled and unlabeled compounds (carbon and hydrogen [692]). Isotope fractionation associated with the reductive dechlorination of chlorinated ethylenes by zero-valent iron and zinc has been... 

Baird, B.R. (1969). Dimensional stabilisation of wood by vapor phase chemical treatments. Wood and Fiber, 1(1), 54—63. 

To collect a sample, the probe with a SPME fiber installed is inserted into the soil. Air is pumped through the probe, drawing subsurface soil vapors into the probe tip and across the SPME fiber. Pumping air across the fiber increases uptake of target analytes by the SPME fiber relative to what is collected by molecular diffusion alone. Once a sample is collected, the SPME fiber is removed from the probe for analysis. To analyze the sample, the SPME fiber is inserted into a modified inlet system attached to the Fido sensor. The modified inlet serves to heat the SPME fiber, causing rapid and quantitative desorption of trapped molecules of analyte. The vapor-phase analyte is then swept into the sensor for analysis by a flow of carrier gas. 

A.1 Fundamentals of Vapor Phase Synthesis. In this section we will concentrate on the vapor phase synthesis of some structural ceramics, such as carbides and nitrides. The principles described here apply equally well to the production of oxide ceramics, but we reserve some of this description for later sections, particularly with respect to the formation of optical fibers. 

Polybrominated Diphenyl Ethers. Like PCBs, air samples containing PBDEs are usually collected by pumping air through a sampler containing a glass fiber filter and adsorbent trap to separate the particle bound and vapor phase fractions, respectively (Dobber et al 2000a Hillery et al 1997). The filters and adsorbants are then Soxhlet extracted with acetone/hexane, and the extracts are cleaned-up and analyzed by high resolution GC techniques. 

Notwithstanding the physical presentation of the polymeric backbone—fabric, fiber or film—the immersion media should contain the monomer, with or without addition of a solvent, either in liquid or in vapor phase. The initiation proceeds by direct attack to the polyamide or by the preliminary introduction of labile groups that are then removed by monomer attack, usually by exposure to gamma-rays. 

On the other hand, chemicals might be chosen as agents for generation of active centers in the polymeric backbone and grafting of monomer in the vapor phase. Ammonium persulfate and styrene (189), acrylic acid (190), butyl acrylate (191), potassium persulfate and n-butyl maleate or 2-ethyl hexyl acrylate (55,134) and ammonium ceric nitrate and methacrylic add (192) are examples of this method. Benzoyl peroxide deposited on polycaprolactam fibers initiates the grafting of styrene in vapor phase (193). 

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