Major Coating Processes

The major coating processes for refractory carbides and nitrides are listed in Table 15.2.I W ] 

CVD and PVD belong to the class of vapor-transfer processes which are atomistic in nature, that is the deposition species are atoms or molecules or a combination of these. The coatings are also commonly known as thin-fllms when their thickness is less than 10 pm. This is an arbitrary limitation and perhaps a better definition would be a coating that adds essentially nothing to the mass of the substrate. CVD and PVD coatings of Tie, TiN, Si3N4, and AljOj, have major industrial applications (see Ch. 16). 

Thermal-spray coatings consist mostly of WC, Cr2C3, and to a lesser degree TiC and AIN. They are relatively inexpensive and widely used in corrosion and wear applications particularly in the gas-turbine industry. 

These processes, CVD, PVD, and thermal spray have reached the stage of large industrial production with a constant R D effort, particularly in the development of new source materials with greater purity, the refinement of processii parameters, and the improvement of the equipment. In the next sections, the processes are reviewed as they pertained to the refractory carbides and nitrides. This review can only touch on the major aspects of these extended and complex technologies and the references should be consulted for further information. 

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Handbook of Refractory Carbides and Nitrides 1.2 Major Coating Processes... 

Table 15.2 Major Coating Processes for Refractory Carbides and Nitrides...

Coating application processes have changed significantly over the years. Three major coating processes are used today roll coaters, air knife coaters, and blade coaters. 

Another major type of coating process is wire covering. The tremendous demand for insulated cables in the electrical industry means that large tonnages of plastic are used in this application. Basically a bare wire, which may be heated or have its surface primed, is drawn through a special die attached... 

The two major plastics processing techniques of extrusion and injection moulding are used for coating metals. 

The major optical coating process remains evaporation. However, sputtering, solgel, and CVD are making inroads because of the better properties they provide.fi l At this time, it is not clear which coating process will prove best but CVD, because of the excellent bond and hardness of the deposited materials and the relatively moderate cost of the equipment, is a good candidate. 

Hydroformylation is an important industrial process carried out using rhodium phosphine or cobalt carbonyl catalysts. The major industrial process using the rhodium catalyst is hydroformylation of propene with synthesis gas (potentially obtainable from a renewable resource, see Chapter 6). The product, butyraldehyde, is formed as a mixture of n- and iso- isomers the n-isomer is the most desired product, being used for conversion to butanol via hydrogenation) and 2-ethylhexanol via aldol condensation and hydrogenation). Butanol is a valuable solvent in many surface coating formulations whilst 2-ethylhexanol is widely used in the production of phthalate plasticizers. 

Since the majority of film-coating operations around the world utilize aqueous coating processes, it is often useful to apply thermodynamic models to the process. In this way, the development-scale process can be fundamentally characterized, based on application of the first law of thermodynamics, as suggested by Ebey (6), allowing more accurate predictions for operating... 

From the foregoing discussion, there are clearly many issues to be confronted when scaling up the pan-coating process. Evidently, the more definitive data that are developed initially, the less likely that major problems will occur later on, especially problems that could inevitably delay a product launch and cost much in the way of lost revenues, particularly when dealing with a potentially blockbuster drug product. 

Surface tension and contact angle phenomena play a major role in many practical things in life. Whether a liquid will spread on a surface or will break up into small droplets depends on the above properties of interfaces and determines well-known operations such as detergency and coating processes and others that are, perhaps, not so well known, for example, preparation of thin films for resist lithography in microelectronic applications. The challenge for the colloid scientist is to relate the macroscopic effects to the interfacial properties of the materials involved and to learn how to manipulate the latter to achieve the desired effects. Vignette VI provides an example. 

A summary of the response variables obtained in this preliminary study are shown in Table 14, and the order ranking for the influence of process variables on the critical responses associated with coating process efficiency and drug release are provided in Table 15. As can be seen from the summary provided in Table 14, there is clearly an influence of the processing conditions used on ultimate drug release characteristics. On further examination, it was concluded that the major causes of the magnitude of differences in drug release characteristics were primarily due to ... 

To date, use of such continuous processes has been restricted primarily to the manufacture of large-volume products, an application for which continuous processes potentially have a major advantage. Nonetheless, continuous coating processes provide a potentially viable alternative for the scale-up of any film-coating process, where many of the tasks potentially become much simpler, since they would always be the same irrespective of whether the production batch size is 250 kg or 5(K)() kg. Simplification arises from reducing or eliminating tasks that would... 

One approach to increasing CO2 solvent strength relies on the fact that most volatile materials (such as alcohols, ketones, and hydrocarbons) are soluble in supercritical and near-critical CO2. This allows the employment of a wide range of cosolvents to enhance the C02 s solvation properties. The use of such cosolvent-modified CO2 as a solvent medium is most recognized in the Unicarb spray-coating process commercialized by Union Carbide (now with Dow Chemical, Midland, MI) in the early 1990s. In this process, the majority of traditional solvents used in the spray coatings are replaced by supercritical CO2. This process has been implemented in automotive and furniture industries. 

The modification of the optical fiber involves two major steps, for chemical sensor application [19, 20] (1) removal of the passive cladding (fiber etching) and (2) application of active cladding (fiber coating). The etching and coating processes are explained next. An all-silica MM fiber with core/cladding/jacket dimension of 105/125/250 pm was selected for such application. 

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