Enamels types

The preceding chapter (7 ) described synthesis of model LC oligomeric diols and their incorporation into coatings binders of the baking enamel type. The enamels had far better hardness and impact resistance than control enamels made from amorphous or crystalline oligomeric diols. The model LC oligomers were synthesized by a Schotten-Bauman method that would be costly for large-scale production. 

Amaechi, B. T., Higham, S. M., and Edgar, W. M. (1999). Factors influencing the development of dental erosion in vitro Enamel type, temperature and exposure time. J. Oral. Rehab. 26, 624-630. 

This value is higher than those cited earlier, possibly because of our use of relatively newly erupted enamel, which is less crystalline, and hence more soluble than enamel which has matured [63], However, it is very close to one value quoted for dissolution of bulk human enamel [64] and very close indeed to the value calculated by Zhang et al. [46], who suggested that demineralisation kinetics were controlled in part by a mineral phase with an apparent Ken of 2 X 10-58 mol9l 9. Given the divergence of reported values for both human and bovine enamel, it seems unlikely that comparison of Ken values alone is useful when comparing the behaviour of enamel types, except in cases where experimental conditions are very similar. 

In aqueous systems which are initially infinitely undersaturated with respect to enamel, differences between human and bovine enamel can be very small. During enamel demineralisation studies, initial enamel dissolution rate was used to match samples prior to pH-cycling [Lynch, unpubl. data]. In this case, there was no significant difference between the two enamel types. Mean calcium demineralisation rates (SD, n > 50) for human and bovine enamel were 0.74 (0.26) and 0.66 (0.14) xg mm 2 h 1 respectively. 

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. 

Stripes may be appHed to wire coated with ETEE fluoropolymer over DuLite 817-5002 fluoropolymer clear enamel or other bases. Thermally stable pigments are required. Stripes may be appHed by gravure-wheel-type appHcators and oven-cured in-line. 

Types of internal enamel for food containers include oleoresins, vinyl, acryflc, phenoHc, and epoxy—phenoHc. Historically can lacquers were based on oleoresinous products. PhenoHc resins have limited flexibiHty and high bake requirements, but are used on three-piece cans where flexibiHty is not required. Vinyl coatings are based on copolymers of vinyl chloride and vinyl acetate dissolved in ketonic solvents. These can be blended with alkyd, epoxy, and phenoHc resins to enhance performance. FlexibiHty allows them to be used for caps and closures as weU as drawn cans. Their principal disadvantage is high sensitivity to heat and retorting processes this restricts their appHcation to cans which are hot filled, and to beer and beverage products. 

Activators. Activators are often added to removers to make them more efficient. Acids such as phenol [108-95-2] phosphoric acid [7664-38-2] acetic acid [64-19-7] formic acid [64-18-6]., and citric acid [5949-29-1] are used to increase the cutting abiHty on epoxide-type paints and other modem finishes. Strongly alkaline activators are effective on enamel andlatex paints. Other activators include ammonia [7664-41-7] monoethyl amine [75-04-7], and /V-phenyIdiethan ol amines. Acid and base activators shorten the shelf life of some removers. 

The rate of stripping or the stripabiUty on cataly2ed urethane and epoxy resin finishes can be increased by adding formic acid, acetic acid, and phenol. Sodium hydroxide, potassium hydroxide, and trisodium phosphate [10101-89-0] may be added to the formula to increase the stripabiUty on enamel and latex paints. Other activators include oleic acid [112-80-17, trichloroacetic acid [76-85-9], ammonia, triethanolamine [102-71-6], and monoethyl amine. Methylene chloride-type removers are unique in their abiUty to accept cosolvents and activators that allow the solution to be neutral, alkaline, or acidic. This abihty gready expands the number of coatings that can be removed with methylene chloride removers. 

Polymers based on trimellitic anhydride are widely used in premium electromagnetic wire enamels requiring high temperature performance. Several types of trimellitic anhydride-derived polymers are used as wire enamels poly(amide—imide)s (133), poly(ester—imide)s (134), and poly(amide—imide— ester)s (135). Excellent performance characteristics are imparted by trimellitic anhydride-based polymers for wire enamel requirements of flexibiUty, snap, burnout, scrap resistance, heat shock, and dielectric strength. 

Another large appHcation is as an ingredient in the production of charcoal briquettes. The amount of sodium nitrate used in charcoal briquette manufacture depends on the type and amount of wood and coal used. Typically charcoal briquettes contain up to almost 3% sodium nitrate. Sodium nitrate is also used in the manufacture of enamels and porcelain as an oxidizing and fluxing agent. In porcelain—enamel frits used for metal coating, the amount of sodium nitrate in a batch varies with the various metal bases to be coated, typically from about 3.8 to 7.8 wt %. 

The uses of boric oxide relate to its behavior as a flux, an acid catalyst, or a chemical iatermediate. The fluxing action of B2O2 is important ia preparing many types of glass, gla2es, frits, ceramic coatings, and porcelain enamels (qv). 

Table 11 Hsts the types and appHcations of special pigment-grade carbon blacks. Included in this Hst are thermal black and lampblack. Over 40 special black grades have been developed based on the furnace process having a broad range of surface areas, from 20 m /g to over 1500 m /g. The lower surface area products are used in printing inks and tinting. The high area, more expensive products find use in high color enamels and lacquers.

Other Metals. Metals such as the austenitic series. Types 301—347, and the ferritic series. Types 409—446, of stainless steels may be enameled, as well as a number of other alloys (17). The metal preparation usually consists of degreasiag and grit blasting. Copper, gold, and silver are also enameled. These metals are usually prepared for appHcation by degreasiag. Copper is pickled usiag either a nitric acid [7697-37-2] or a sulfuric acid [7664-93-9] solution, followed by a dilute nitric acid dip. Silver may be pickled in hot dilute sulfuric acid followed by a dip in a nitric acid solution (18). 

Enamel firing temperature is dictated by the coating composition, metal thickness, and the type of metal used. Enamels for aluminum are fired at... 

These paints are resin based and the paints required for sheet-metal surfaces are genertilly alkyd-based resins. For general industrial applications, tiny of the following types of enamel paints may be used,... 

Magnesium anodes are widely used in conjunction with enamel coatings. This type of corrosion protection is particularly economical and convenient in small-and medium-sized boilers. The anode only has to ensure protection of small de-... 

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