Microcrystalline coatings

Fig. 31. Electron micrographs that compare crystal size of (top) a grain-refined microcrystalline coating and (bottom) a conventional zinc phosphate conversion coating [54].

Xu Q., He Y, Qi H., Wang D., Li Z. et at, (2002b), Microcrystalline coatings deposited by series double-pole electro-pulse discharge and its high-temperature oxidation behavior , Science in China E, 45,477-84. 

Chiral stationary phases in tic have been primarily limited to phases based on normal or microcrystalline cellulose (44,45), triacetylceUulose sorbents or siHca-based sorbents that have been chemically modified (46) or physically coated to incorporate chiral selectors such as amino acids (47,48) or macrocyclic antibiotics (49) into the stationary phase. 

Two pigment production routes ate in commercial use. In the sulfate process, the ore is dissolved in sulfuric acid, the solution is hydrolyzed to precipitate a microcrystalline titanium dioxide, which in turn is grown by a process of calcination at temperatures of ca 900—1000°C. In the chloride process, titanium tetrachloride, formed by chlorinating the ore, is purified by distillation and is then oxidized at ca 1400—1600°C to form crystals of the required size. In both cases, the taw products are finished by coating with a layer of hydrous oxides, typically a mixture of siUca, alumina, etc. 

Microcrystalline ceUuloses ate marketed under the trade name Avicel. The physical characteristics of microcrystalline ceUuloses differ markedly from those of the original ceUulose. The ftee-flowiag powders have particle sizes as smaU as 0.2—10 p.m. Avicel ceUuloses coated with xanthan gum, guar gum, or carboxy-methylceUulose to modify and stabilize their properties are also available. The Avicel products are promoted for use ia low calorie whipped toppiags andiciags andia fat-reduced salad dressiags and frozen desserts (see Fat substitutes). 

Some tablets combine sustained-release and rapid disintegration characteristics. Products such as K-Dur (Key Pharmaceuticals) combine coated potassium chloride crystals in a rapidly releasing tablet. In this particular instance, the crystals are coated with ethylcellulose, a water-insoluble polymer, and are then incorporated into a rapidly disintegrating microcrystalline cellulose (MCC) matrix. The purpose of this tablet is to minimize GI ulceration, commonly encountered by patients treated with potassium chloride. This simple but elegant formulation is an example of a solid dosage form strategy used to achieve clinical goals. 

Wall painting in the Messer Filippo cell, Tower of Spilamberto (Modena, Italy). The painted surface is in an advanced state of deterioration the detachment of the pictorial matter from the plaster is evident, efflorescence is present, and the pictorial coating is almost completely covered by a grey-whitish film formed by microcrystalline gypsum, as proved by micro-Raman investigations. The FA profile, characterized by an A/P ratio of... 

Figure 3.12 Enantioseparation of TrOger s base on (a) microcrystalline cellulose triacetate (CTA I) and (b) 21 coated on silica gel. Column, 25 x 0.46 (i.d.) cm eluent ethanol-HoO (7/3) flow rate, 0.5 ml/min).

We also include in this class of quasi-2D nanostructured materials Titania deposited inside ordered mesoporous silica (because an inner coating of mesoporous silica may be realized), or nano-dot type Titania particles well dispersed in the ordered porous matrix. We do not consider here solids which contain linear or zig-zag type TiOTiO-nanowires in a microcrystalline porous framework, such as ETS-4 and ETS-10, notwithstanding the interest of these materials also as photocatalysts,146-151 because these nanowires are located inside the host matrix, and not fully accessible from the gas reactants (the reactivity is essentially at pore mouth). 

Spherical pellets containing 5% triamcinolone acetonide were prepared by Villar-Lopez and co-workers [59] by extrusion/spheronization following formulation with microcrystalline cellulose and/or a hydrophilic excipient like lactose, sodium earbox-ymethylcellulose, or P-cyclodextrin. Their suitability for coating, with a view toward colonic drug deliveiy, was assessed in terms of their size, sphericity, and dissolution test response. The best results were afforded by a 5 90 5 composition of microcrystalline cellulose, P-cyclodextrin, and triamcinolone acetonide, prepared by complex-ation of triamcinolone acetonide with P-cyclodextrin prior to the addition of microcrystalline cellulose. 

Chromatographic resolution is also dependent on column efficiency (i). Column efficiency is directly dependent on the nature of the support matrix and how well that support is packed in its column. Available chromatographic supports are based on dextran, agarose, polystyrene, acrylic, cellulose, silica gel and a variety of other polymers. Althou cellulosic supports are manufactured in both microcrystalline and leaded forms, most supports are beaded. Newer supports may use hybrid bead construction where the base support is coated with a second materid (e.g., dextran or silica coated with agarose). 

Polarization modulation reflection absorption infrared spectroscopy (PM-RAIRS) was employed to follow the reaction of CO, C2H4 and CO/C2H4 with microcrystalline ]Pd(Me)(OTf)(dppp)] deposited onto a gold coated wafer. Single insertion steps were observed by alternately exposing the catalyst precursor to low CO (500-333 mbar) and ethene (333 mbar) pressures (Figure 7.12). 

The capture threads also comprise similar components but here the microcrystals compose less than 5% of the thread, with both the amorphous and microcrystalline portions arranged in a more random fashion within the thread. Hydrated glue, which coats the thread, acts as a plasticizer imparting greater mobility and flexibility to the chains. It stretches several times its length when pulled and is able to withstand numerous shocks and pulls appropriate to contain the prey as it attempts to escape. Further, most threads are spun as two lines so that the resulting thread has a kind of built-in redundancy. 

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