Wet coating methods

PCC is normally used in fatty acid coated form in thermoplastic applications. The means by which the coating is applied is treated as proprietary by most producers but it is probably by the wet coating method, with a solution of the fatty acid salt added to an aqueous slurry of the filler, followed by filtration and drying. As described earlier, the use of fatty acid coatings can improve product filtration and drying characteristics. 

There are other methods of preparation that iavolve estabhshing an active phase on a support phase, such as ion exchange, chemical reactions, vapor deposition, and diffusion coating (26). For example, of the two primary types of propylene polymerization catalysts containing titanium supported on a magnesium haUde, one is manufactured usiag wet-chemical methods (27) and the other is manufactured by ball milling the components (28). 

In the case of a paint coating the film thickness may be determined when the film is dry or when it is still wet both methods are described and discussed in detail in BS 5493, but some discussion is appropriate here. 

In addition to these techniques, there are passive samplers for N02 that have been used for unique situations such as indoor measurements. For example, in the Palmes Tube, N02 diffuses through to a surface coated with triethanolamine and is trapped in the form of NOJ. The nitrite is subsequently measured colori-metrically (e.g., see Boleij et al., 1986 Miller, 1988 and Krochmal and Gorski, 1991). As with most, if not all, such wet chemical methods, interferences can arise, for example, from PAN (Hisham and Grosjean, 1990) and HONO (Spicer et al., 1993b). 

On the other hand, wet process metal coating method was applied to the surface-modified nylon 12 particles (24). Nylon 12 particles modified by alumina or silica fine particles were given metallic coating by an electroless plating method. 

Jan 1969, it is stated that A-5 shall consist of RDX Type A or B, complying with the requirements of Spec MIL-R-398 and Stearic Acid, complying with the requirements of Spec MIL-S-271. The composition shall be made by coating the RDX with stearic acid by the wet slurry method (See Note after procedure 4.4.5. Determination of Bulk Density)... 

B = Volume of compacted sample in ml Note The following wet slurry method of coating has been found satisfactory. Add... 

Precipitation from aqueous solution is able to produce high purity materials of a wide chemical range and with very small particle sizes. Because of the formation of the filler as a suspension in an aqueous medium, precipitation methods lend themselves to the incorporation of wet coating procedures. The particles produced by precipitation are generally very small, and difficult to filter and often form a filter... 

The different application methods can be broadly divided into dry and wet processes. In addition many coatings can be applied during composite production, in effect using the compounding procedure to carry out the coating. This method is widely used but is outside the scope of this article. The advantages and limitations of the various coating methods have been discussed elsewhere [49]. 

As with fatty acids, organo-silanes can be applied by both wet and dry coating methods. In the present context, wet coating is principally restricted to glass fibre coating, where it can be incorporated into the size, which is already applied from water. Dry coating is the preferred method for coating mica, wollastonite clays and metal hydroxides. 

Dry coating is probably the main method used in preparing products for thermoplastic applications. Due to the cost of additional drying operations, it is the method of choice when the filler itself is produced by a dry process. It is also useful where wet coating procedures would give rise to effluent problems. 

A number of coatings are offered with magnesium hydroxide. These include saturated and unsatuxated fatty acids and silanes. The coating methods have not been disclosed, but presumably wet coating is used where practical. A number of pubhcations, especially from Japan, refer to the use of coatings of various types to protect the filler from attack by carbon dioxide [106],but it is not clear whether this is a real problem and whether these are in commercial use. 

ConffMV method Base material Coating composition Usual coaling speed (in min " 1 Viscosity range, OnPas) Wet-coating thickness range (pm)... 

B = Volume of compacted sample in ml Note The following wet slurry method of coating has been found satisfactory. Add molten stearic acid at approx 70°C to a hot water slurry of RDX at approx 70° and then, after thorough mixing, cool, filter and dry the coated product... 

---