Reactive spray coating

Polyurethanes differ from most other polymers in that polymerization frequently takes place at the same time that we are molding or forming them into a usable shape. The three most common processes of this type are reactive foaming, reactive injection molding (RIM), and reactive spray coating. 

In true CVD processes, as compared with the spray coating discussed above, the reactants are transported to the place of reaction, in our case the glass surface, in the form of vapours and gases and not in the form of liquid droplets. Although there are also exceptions, the vapour of the reactive compound, an easily volatalized liquid, is generally prepared by injection of the liquid, into water or oil-bath heated evaporators. From there, the vapour is transported to the reaction zone by a carrier gas. 

Shell materials can be solvent-based, water-based, molten, reactive, or molecnlar. Variations of atomization, spray coating, and coextrusion are available to deposit shell or matrix materials from solvent, water, or as a molten material. For example, spray drying is snitable for encapsnlating with solvent-based or water-based matrix materials, while spray congealing nses molten fats or waxes. Fewer shell material selections are available with the emulsion-based processes. For example, complex coacervation is most often associated with the use of gelatin as the shell, and the generation of polyurea or polymelamine formaldehyde shells is associated with in situ polymerization. Further limited examples include the use of cyclodextrins for molecular complexation or phospholipids for the formation of liposomes. 

Process selection around a core material is dependent upon core material properties such as thermal stability, viscosity if it is a liquid, particle size and shape if it is a solid, density, reactivity, and solubility. Processes such as spray chilling or spray coating can expose core materials to elevated temperatures for longer periods of time, compared to spray drying, as the mixture remains heated as it awaits pumping to an atomization nozzle. Other processes, such as coextrusion or some emulsion processes, can be carried out at or below room temperature. For example, ionic gelation coupled with coextrusion can be used to encapsulate oils or biological materials at or below room temperature." Processes like solvent evaporation can be operated under vacuum to remove the matrix solvent rather than the use of temperature. ... 

H12MDI is less volatile than HDI and is sometimes used as afree diisocyanate in coatings to be applied by roll coating, but not by spray coating. It is a mixture of stereoisomers since both isocyanate groups are secondary, reactivity is lower than HDI or IPDI. Combined allophanate-isocyanurate derivatives are available with low free monomer content. Reference 7 covers the chemistry and uses of H12MDI. 

Spray. In spray-on appHcations the reactive iagredients are impingement mixed at the spray head. Thickness of the foam is controlled by the amount appHed per unit area and additional coats are used if greater than 2.5 cm (1.0 ia.) thickness is required. This method is commonly used for coating iadustrial roofs or iasulatiag tanks and pipes. 

Each type of metallic coating process has some sort of hazard, whether it is thermal energy, the reactivity of molten salt or metal baths, particulates in the air from spray processes, poisonous gases from pack cementation and diffusion, or electrical hazards associated with arc spray or ion implantation. 

The diversity of release products and the wide range of release problems make classification difficult. One approach is by product form, with subdivisions such as emulsions, films, powders, reactive or iaert sprays, reactive coatings, and so on. Another approach is by appHcation, eg, metal casting, mbber processiag, thermoplastic iajection mol ding, and food preparation and packagiag. 

The bed of parallel plates coated with Raney nickel catalyst was much more reactive than the bed of precipitated nickel. This was revealed by the generally lower CO concentration in the product gas during operation with the parallel plate bed for example, after 450 hrs stream time, it was 0.01% with the bed of sprayed Raney nickel (experiment HGR-14) and 0.05% with the bed of precipitated nickel catalyst (experiment HGR-13). 

Coatings of refractory compounds can be deposited reactively in a manner similar to reactive evaporation and sputtering by spraying the pure metal in an atmosphere of either a hydrocarbon or nitrogen. 

---