Life of coatings

A Imosphere Corrosion rate Life of coating with average thickness (years) ... 

Prediction of the useful life, or the remaining life, of coatings from physical or analytical measurements presents many problems in data analysis and interpretation. Two important considerations are that data must be taken over a long period of time, and the scatter from typical paint tests is large. These considerations require innovative application of statistical techniques to provide adequate prediction of the response variables of interest. 

The mechanism of action of antimony trioxide has been discussed in some detail in Chapter 7. It has been widely used in bonded film formulations since the mid-nineteen-sixties to improve the wear life of coatings. The greatest improvement is obtained at higher temperatures, and Kinner found a small deterioration in performance at low temperatures when antimony trioxide was used. He also found that the use of lead oxide, PbO, together with antimony trioxide gave markedly improved performance at both high and low temperatures. 

Since residual coating stresses influence the quality and the service life of coatings, in particular their adhesion and wear performance, it is the goal of any coating development to minimise such stresses. The origin of residual stresses is twofold. First, rapid quenching of the molten particles at the substrate interface results in frozen-in particle contraction according to... 

W.J. van Ooij, Improved service life of coated metals by engineering the polymer-metal interface. In Serviee Life Prediction of Organie Coatings - A Systems Approach, ed. D.R. Bauer and J.W. Martin, ACS Symposium Series No. 722, American Chemical Society, Oxford University Press, 1999, 354-377. 

Catalytic cathodes in membrane cell operations exhibit a voltage savings of 100—200 mV and a life of about 2 + yr using ultrapure brine. However, trace impurities such as iron from the caustic recirculation loop can deposit on the cathode and poison the coating, thereby reducing its economic life. 

Bonded Solid-Film Lubricants. Although a thin film of soHd lubricant that is burnished onto a wearing surface often is useful for break-in operations, over 95% are resin bonded for improved life and performance (62). Use of adhesive binders permits apphcations of coatings 5—20 p.m thick by spraying, dipping, or bmshing as dispersions in a volatile solvent. Some commonly used bonded lubricant films are Hsted in Table 12 (62) with a more extensive listing in Reference 61. 

Substrate Properties. It is clear from equation 5 that higher hardness of the substrate lowers friction. Wear rate of the film also is generally lower. Phosphate undercoats on steel considerably improve wear life of bonded coatings by providing a porous surface which holds reserve lubricant. The same is tme for surfaces that are vapor- or sandblasted prior to appHcation of the soHd-film lubricant. A number of typical surface pretreatments are given in Table 13 to prepare a surface for solid-film bonding (61). 

Other Uses. Photochromic glass contains silver chloride (80) and silver molybdate [13765-74-7] (81) (see Chromogenic materials). An apparatus coated with silver nitrate has been described for the detection of rain or snow (82). Treatment with silver-thiosulfate complex has been reported as dramatically increasing the post-harvest life of cut carnations (83). Silver sulfate has been used in the electrolytic coloring of aluminum (84). Silver sulfate also imparts a yellowish red color to glass bulbs (85). 

Sucrose monoesters (SMEs) are used as nonionic surfactants, in detergents and as emulsifiers in foods. Some SMEs have bacteriostatic activity and are used to prevent spoilage in beverages. Semperfresh, an SME produced in the U.K., is used as a coating to extend the shelf life of fmits and vegetables (21,29). 

Water-borne adhesives are preferred because of restrictions on the use of solvents. Low viscosity prepolymers are emulsified in water, followed by chain extension with water-soluble glycols or diamines. As cross-linker PMDI can be used, which has a shelf life of 5 to 6 h in water. Water-borne polyurethane coatings are used for vacuum forming of PVC sheeting to ABS shells in automotive interior door panels, for the lamination of ABS/PVC film to treated polypropylene foam for use in automotive instmment panels, as metal primers for steering wheels, in flexible packaging lamination, as shoe sole adhesive, and as tie coats for polyurethane-coated fabrics. PMDI is also used as a binder for reconstituted wood products and as a foundry core binder. 

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