Film-forming

Tredgold R H, Allen R A and Hodge P 1987 X-ray-diffraction and optical studies of Langmuir-Blodgett films formed from azobenzene derivatives Thin Solid Films 155 343-52... 

Resins vary from soft, elastic, film-forming materials to hard plastics. 

As with polyesters, the amidation reaction of acid chlorides may be carried out in solution because of the enhanced reactivity of acid chlorides compared with carboxylic acids. A technique known as interfacial polymerization has been employed for the formation of polyamides and other step-growth polymers, including polyesters, polyurethanes, and polycarbonates. In this method the polymerization is carried out at the interface between two immiscible solutions, one of which contains one of the dissolved reactants, while the second monomer is dissolved in the other. Figure 5.7 shows a polyamide film forming at the interface between an aqueous solution of a diamine layered on a solution of a diacid chloride in an organic solvent. In this form interfacial polymerization is part of the standard repertoire of chemical demonstrations. It is sometimes called the nylon rope trick because of the filament of nylon produced by withdrawing the collapsed film. 

Regeneration. The regeneration process is the oldest of the film-forming processes and was used exclusively for the manufacture of... 

Nylon. Nylon is the designation for a family of thermoplastic polyamide materials which in film form are moderate-oxygen barriers. The gas-barrier properties are equal to odor and flavor barrier properties important in food appHcations. Nylon films are usually tough and thermoform able, but are only fain moisture barriers (see Polyamides). 

Uses. Alginates are used in a wide range of appHcations, particularly in the food, industrial, and pharmaceutical fields (25—27). As shown in Table 5, these appHcations arise from the properties of gelation, thickening/water holding, emulsification, stabilization/binding, and film forming. 

The excellent corrosion-resistant lead dioxide, Pb02, film formed on anodes and lead—acid battery positive grids in sulfuric acid has enabled lead insoluble anodes and lead—acid batteries to maintain the dominant positions in their respective fields. 

Sometimes the formation of oxide films on the metal surface binders efficient ECM, and leads to poor surface finish. Eor example, the ECM of titanium is rendered difficult in chloride and nitrate electrolytes because the oxide film formed is so passive. Even when higher (eg, ca 50 V) voltage is apphed, to break the oxide film, its dismption is so nonuniform that deep grain boundary attack of the metal surface occurs. 

If conditions aie such that the film does not form, such as in the case of acids, then the reaction proceeds until all the metal is consumed. The reaction of magnesium with hydrofluoric acid [7664-39-3J is an exception to this rule, because a stable fluoride film forms. 

In neutral and alkaline environments, the magnesium hydroxide product can form a surface film which offers considerable protection to the pure metal or its common alloys. Electron diffraction studies of the film formed ia humid air iadicate that it is amorphous, with the oxidation rate reported to be less than 0.01 /rni/yr. If the humidity level is sufficiently high, so that condensation occurs on the surface of the sample, the amorphous film is found to contain at least some crystalline magnesium hydroxide (bmcite). The crystalline magnesium hydroxide is also protective ia deionized water at room temperature. The aeration of the water has Httie or no measurable effect on the corrosion resistance. However, as the water temperature is iacreased to 100°C, the protective capacity of the film begias to erode, particularly ia the presence of certain cathodic contaminants ia either the metal or the water (121,122). 

Spray Drying. Spray-dry encapsulation processes (Fig. 7) consist of spraying an intimate mixture of core and shell material into a heated chamber where rapid desolvation occurs to thereby produce microcapsules (24,25). The first step in such processes is to form a concentrated solution of the carrier or shell material in the solvent from which spray drying is to be done. Any water- or solvent-soluble film-forming shell material can, in principle, be used. Water-soluble polymers such as gum arable, modified starch, and hydrolyzed gelatin are used most often. Solutions of these shell materials at 50 wt % soHds have sufficiently low viscosities that they stiU can be atomized without difficulty. It is not unusual to blend gum arable and modified starch with maltodextrins, sucrose, or sorbitol. 

Chemical modification of the wax can improve smear resistance (5). Sihcones, which do not harm furniture finishes (6), are incorporated as film-forming ingredients in furniture pohshes. The lubricant properties of sihcones improve ease of apphcation of the pohsh and removal of insoluble soil particles. In addition, sihcones make dry films easier to buff and more water-repeUent, and provide depth of gloss, ie, abihty to reflect a coherent image as a result of a high refractive index (7). Wax-free pohshes, which have sihcones as the only film former, can be formulated to dehver smear resistance (8). Another type of film former commonly used in oil-base furniture pohshes is a mineral or vegetable oil, eg, linseed oil. 

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