Spray surface structure

Glycerol, used in spray mounting techniques may decorate the surface of the specimen on the mica surface. The protein surface structure of the TMV particle, which shows clearly a helical pattern in low-dose negative-stained and freeze-dried shadowcast images (106, 131), is not visible if the particles are applied to mica via glycerol spray. 

The surface structure of spray-dried particles can be regarded as a key for fine particle adhesion in customer appUcations. This effect is needed for example, for inhalation therapies when fine active particles with d < 5 im are added to spray-dried carrier particles within the size range 60-100 im in order to obtain a dose unit with reasonable fiowabihty and with proper detachment properties of the fines. Experiments with mannitol (Maas et ol., 2009) gave rise to quite different surface morphologies when sprayed into air at different temperatures. In the case of low air exhaust temperatures, T= 80 °C, the carrier particles exhibit a fairly smooth surface. At an exhaust air temperature of T= 130 °C, the particles show rough surfaces formed by crystals covering the particle surface, see Fig. 6.5. In this case, the air temperature is a vehicle to adjust the desired adhesion properties. 

Figure 14.18 shows two SEM pictures of the SBS qualities used for these studies. Micronised SBS (Fig. 14.18, left) with a X5o,3 = 1.69 0.01 pm revealed needlelike structures, whereas spray dried SBS (Fig. 14.18, right) with a 50.3 = 2.35 0.02 pm showed spheres with a golf ball-like surface structure. The particle size measurements of both batches ensured that the drug is small enough to be inhaled [11, 33]. 

Fig. 14.27 SEM images (5000 fold magnification) of interactive mixtures after impaction analysis caught in the pre-separator of the NGI. Rough surface structure (a) and smooth surface structure (b) of blends with the spray dried model drug Bud...

The more automatic the method of coating application, the more economical and efficient it is, since automation lends itself more readily to more even coatings than do manual methods, e.g. large surface areas lend themselves more readily to spraying techniques, whereas open work structures are more suitable for dipping methods. The coating should also be applied to a specified minimum thickness which is adequate for the service conditions and life envisaged. 

The coatings produced by metal spraying have an unusual structure which is characteristic of the method of formation. They are composed of small particles usually not more than 0-01 mm in diameter which, having reached the surface in the molten condition, have splashed outwards and then solidified. Figure 12.27 (left) shows in section the irregular form of the flattened particles. In transverse section the surface profile is undulating (Fig. 12.27 (right)). 

Sprayed coatings These are of the greatest importance, particularly for the protection of structural steel or certain aluminium alloys. The metal to be coated must be grit blasted shortly before spraying to provide a clean rough surface. Chilled iron grit is used for most steelwork, while alumina or silicon... 

In the Forth, Severn and many other suspension bridges, zinc coatings have an important function. The whole main structure is of steel and has been zinc-sprayed on the external surfaces, while the main cable and hanger ropes have been coated by continuous hot-dip galvanising. Case histories of galvanised multi-truss bridges cover more than 30 years. 

Sprayed coatings have structures in which fine pores thread tortuous paths through the deposit, and it is necessary to apply a coating thickness of about 350 ftm if all these paths are to be closed. Scratch-brushing of the deposit, however, makes it possible to consolidate the surface and to achieve adequate continuity in thinner deposits, e.g. 200 m. 

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