Cross-linked enamels

A thermosetting appliance enamel consists of a terpolymer comprising about 72 parts of vinyl toluene (70/40 meta/para) with about 20 parts of ethyl acrylate (to reduce brittleness of the copolymer) and 8 parts of an acidic vinyl comonomer. The acid is incorporated in the copolymer to provide sites for subsequent cross-linking with a diepoxide. It seems reasonable to expect that grease and slain resistance of the cross-linked enamel will be enhanced if the cross-links are not clustered and almost all initial polymer molecules contain at least one or a few cross-linking sites. To achieve this in a batch copolymerization, what are the best reactivity ratios (approximately) of the major component (vinyl toluene) and the vinyl acid comonomer Show you reasoning. 

The other broad class of refinish coatings is cross-linking coatings, caHed enamels in the trade. In the United States, most shops cure the enamels at room temperature in Europe, cure is commonly carried out at temperatures of 60—75°C. TraditionaHy, the vehicle was an oxidi2ing medium oH alkyd. 

The cured resins are stable at 150-200°C depending on cross-linking agents used. They are resistant to chemical attack and are flexible and strongly adhesive. They are used as surface coatings and yield an excellent enamel after esterification which is used for floors, walls, tanks, domestic equipment, etc. 

Embrittlement. This can occur where the curing process continues throughout the life of the coating—for instance, alkyd enamel drying by oxidative cross-linking. 

The photolnltlated degradation of a typical melamine cross-linked acrylic enamel studied by using this technique will be discussed. 

Polyurethane Enamels. Small volumes of this enamel type are used in applications for which the temperature does not rise above approximately 105 C. These enamels are made from hydroxyl-bearing polyesters and blocked isocyanates. During the heating cycle in the enameling process the isocyanates become unblocked and react with hydroxyl groups of the polyesters to form a cross-linked polyurethane film. Wires so insulated can be directly soldered, without prior removal of the polyurethane insulation. 

Polyester Enamels. The various commercially available enamel formulations contain a polymer made from ethylene glycol and phthalic acids as the main resin ingredients. Cross-linking... 

Miscellaneous Extrusion-Applied Polymers. As mentioned earlier, there is a tendency to develop solventless magnet wire enamel formulations, and extrudable polymer systems would fulfill that requirement. There have been reports about extrusion of thin coatings of polyesters over copper wire. At this point, the state of the art allows extrusion of thin insulating films only with thermoplastic materials. The reliable extrusion of uniform and concentric insulating films of approximately 0.001-0.002 in. wall thickness is already an improvement over the more traditional extrusions of polyethylene, poly(vinyl chloride), and several fluoropolymers in much greater wall thicknesses. Because cross-linked insulation is ultimately required for most magnet wire applications, further materials development needs to be done to provide polymer compositions that are both extrudable as thin films and can be cross-linked in an economical process suitable for large-scale industrial application. 

Degradation Chemistry of Primary Cross-links in High-Solids Enamel Finishes Solar-Assisted Hydrolysis... 

A notable apphcation of polyacrylic acid is for cements in dentistry. These are made by mixing an aqueous solution of the polymer with zinc oxide when the zinc salt precipitates as a highly cross-linked gel that rapidly sets to a hard mass under oral conditions. In a variation of this reaction, the zinc oxide is replaced with a tooth-colored glass powder that releases Al and Ca ions. These cements, called ASPA (aluminosihcate polyacryhc acid) or glass ionomer, set very rapidly, bond well to tooth enamel, and are compatible with hving tissue. 

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