Exhaustion coating process

Unlike the processes described elsewhere in this book, the film-coating process is inherently much more complex, since the list of parameters that contribute to overall success is, potentially, exhaustive. Thus, the complexities of the scale-up process are potentially more challenging. In basic terms, these three components of the film-coating process all contribute, in a very much interactive manner, to the overall success of the process ... 

Tablet coating is done for a variety of reasons. The tablet core may contain an active substance that imparts undesirable organoleptic qualities to the tablet. The active moiety of the tablet may be unstable to specific environmental conditions and may need protection. The core may need to be coated to meet marketing requirements. The coating may be needed to impart enteric properties or sustained-release characteristics to the active release profile. Typical coating processes require a solvent, organic or aqueous. Utilities required include compressed air, steam, and electrical service. Provisions should be made for air handling. The exhaust system used must be capable of handling the solvent used in this process.

The precursor chemicals should be selected with care because potentially hazardous or toxic vapors may result. The exhaust system should be designed to handle any reacted and unreacted vapors that remain after the coating process is complete. Other waste effluents from the process must be managed appropriately. Retrieval, recycle, and disposal methods are dictated by the nature of the chemical. For example, auxiliary chemical reactions must be performed to render toxic or corrosive materials harmless, condensates must be collected, and flammable materials must be either combusted, absorbed, or dissolved. The extent of these efforts is determined by the efficiency of the process. ... 

In a world increasingly conscious of the dangers of contact with chemicals, a process that is conducted within the walls of a vacuum chamber, such as the VDP process for parylene coatings, offers great advantages. Provided the vacuum pump exhaust is appropriately vented and suitable caution is observed in cleaning out the cold trap (trace products of the pyrolysis, which may possibly be dangerous, would collect here), the VDP parylene process has an inherently low potential for operator contact with hazardous chemicals. 

Cordierite [12182-53-5] Mg Al Si O g, is a ceramic made from talc (25%), kaolin (65%), and Al O (10%). It has the lowest thermal expansion coefficient of any commercial ceramic and thus tremendous thermal shock resistance. It has traditionally been used for kiln furniture and mote recently for automotive exhaust catalyst substrates. In the latter, the cordierite taw materials ate mixed as a wet paste, extmded into the honeycomb shape, then dried and fired. The finished part is coated with transition-metal catalysts in a separate process. 

Precious Meta.1 Ca.ta.lysts, Precious metals are deposited throughout the TWC-activated coating layer. Rhodium plays an important role ia the reduction of NO, and is combiaed with platinum and/or palladium for the oxidation of HC and CO. Only a small amount of these expensive materials is used (31) (see Platinum-GROUP metals). The metals are dispersed on the high surface area particles as precious metal solutions, and then reduced to small metal crystals by various techniques. Catalytic reactions occur on the precious metal surfaces. Whereas metal within the crystal caimot directly participate ia the catalytic process, it can play a role when surface metal oxides are influenced through strong metal to support reactions (SMSI) (32,33). Some exhaust gas reactions, for instance the oxidation of alkanes, require larger Pt crystals than other reactions, such as the oxidation of CO (34). 

The ACC is a stainless steel cylinder 3 in. in diameter and 1 ft long through which exhaust gases pass before they enter the muffler and tailpipe. The converter contains an extmded ceramic honeycomb stmcture with square pores 0.5 mm in diameter through which gases pass. The walls of these pores are coated with porous y-Al203 on which Pt, Pd, and Rh are deposited, along with several catalyst promoters such as cerium oxide. The residence time in the ACC is -0.05 sec, because the volume of gas that must be processed... 

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