Paint systems

Wa.ter-Ba.sedPaint. Fluorescent pigments also have some use in water-based paint systems, with many being sensitive to aqueous media, especially at pH above 7.5. For extended shelf stabiHty, Radiant P-1600 or P-1700 Series, Day-Glo T and GT pigments, and Sinloihi FZ-3000 Series are recommended. 

D. Hawke and K. Gaw, "Effects of Chemical Surface Treatments on the Performance of an Automotive Paint System on Die Cast Magnesium," Paper 920074, Society of Automotive Engineers International Congress and Exposition, Detroit, Mich., 1992. 

A variety of thermosetting resins are used in SMC. Polyesters represent the most volume and are available in systems that provide low shrinkage and low surface profile by means of special additives. Class A automotive surface requirements have resulted in the development of sophisticated systems that commercially produce auto body panels that can be taken direcdy from the mold and processed through standard automotive painting systems, without additional surface finishing. Vinyl ester and epoxy resins (qv) are also used in SMC for more stmcturaHy demanding appHcations. 

High stmcture blacks ia unvulcanized mbber give higher Mooney viscosities, lower die swell, faster extmsion rates, and better and more rapid dispersion after iacorporation. In vulcanized mbber higher modulus is obtained. High stmcture blacks give lower bulk densities and high vehicle demand ia paint systems. 

Figure A13.1 A liquid painting system illustrating typical layout of a medium-size paint shop showing a seven-lank pre-treatment process...

These materials are now widely used for coating both steel and concrete surfaces that are subject to a particularly aggressive environment (e.g. North Sea oil platforms). There is less validity for their use under normal atmospheric conditions since they are relatively expensive and tend to chalk on exposure to sunlight. However, their use as zinc phosphate, pretreatment or blast primers for blast-cleaned steel which is subsequently overcoated by any other paint system is an extremely valuable contribution to the painting of new steel work. 

Steel, whether in structural form or as a sheet, can be protected by many different coating systems, such as paint, plastic materials, concrete and other metals, either singly or in combination (such as a metal coating followed by a paint system, or a plastic coating). Examples of this compound type of... 

Paint is the most widely used protective coating for steelwork and normally acts as a barrier between the metal and environment. The choice of type of paint and the final thickness required depends on the conditions of service, and the more severe the conditions the thicker and more resistant the paint film needs to be. Also the more sophisticated the paint system the more demanding is the surface preparation required. 

Dietl, B. and Gleason, J. D., Compatibility of Impressed Current Cathodic Protection with Paint Systems , Mat. Prot. and Perf., 10 No. 11, 44, Nov. (1971)... 

Organic and inorganic paint systems are widely used to protect and enhance appearance because they are often economic and always flexible in use. In many industries paint is chosen when other coating processes are technically impossible or uneconomic. Because it is so often used upon subjects which are difficult to coat, sensible design detailing becomes even more important. 

The processes are dealt with fully in Chapters 11, 14 and 15. Because many paint systems include an initial surface pretreatment, e.g. chromated aluminium or phosphated steel, BS4479 1990, Part 3 deals with conversion coatings and should be consulted by designers. Whatever the method of treatment, liquids must be able to drain quickly and freely from the surfaces. Crevices where liquids can become entrapped are best avoided. The surface configuration needs to be such that active solutions can be washed away, leaving the surface to be painted completely free from unreacted pretreatment solution. Failure to achieve the requisite level of freedom from the surplus chemicals causes paint failure, e.g. osmotic blistering. 

Tubular structures are easier to coat than those produced from profiled metal, e.g. a flare stack constructed of welded tubes can increase the life of a brush- or roller-applied paint system by as much as four times over the same process applied to a similar structure built of welded angled steel. 

A wide variety of paint systems may, however, be used over the pretreatment primers. Applying a coat of inhibitive primer such as zinc chromate after the pretreatment primer is less advantageous than applying an additional coat of finishing paint indeed, unless the inhibitive primer is carefully formulated, it may contribute problems of intercoat adhesion and blistering. 

Exposure tests of 5-years duration in the atmosphere and during immersion in fresh water or sea-water by Welding Institute and by Stanners and Watkins have indicated the best paint systems for aluminium-coated steel. For immersed conditions two systems giving good performance are ... 

If protection by paints or varnish films is due to their ability to restrict the penetration of corrosive ions, then it follows that resistance measurements should form the basis of the prediction of their behaviour. In 1948 Bacon eta/. measured the resistance of over 300 paint systems immersed in seawater using a d.c. technique, and concluded that for good performance coatings should have a resistance in excess of 10 0cm Coatings having resistances in the range 10 -10 0cm were found to be unreliable, and those of lower resistance behaved poorly. 

Paint failure is related to climatic conditions, and the weather prevailing during application of the paint and during subsequent exposure will determine the life of the paint system. This applies, of course, particularly to outdoor work. In unfavourable weather conditions, cracking and blistering can be promoted as a consequence of the expansion of the products of corrosion, and in the case of iron and steel this can lead to under-rusting. 

Stripping of top coats soaked by rain or sea-water has occurred with alkyd-resin-based paint systems, mainly on ships. The risk of such intercoat failure is reduced if the time interval between application of coats is reduced, but is best controlled by modification of the alkyd resin with a proportion of a different material. 

Painting systems are selected by the manufacturers of industrial articles, advised by their paint and their equipment suppliers, taking into account the following factors ... 

It is not possible, in a section of this size, to deal adequately with the painting systems used by all industrial finishers. Instead a selection will be covered. 

This is the last of the end usages in this section for which exterior durability is required from the painting system. The substrate here is mainly aluminium... 

The paint system needs to be chosen carefully for demanding environments, particularly marine situations. 

Thi.s is the most important factor deitermining the life of a protective paint system on steel.. The best surface is one free from mri, scale, grease, dirt... 

Details of typical marine painting systems are set out in Table 14.5. 

The cargo tanks of oil tankers present a special case, because of the high cost of steel renewals in unprotected tanks. For a 30 0001 tanker, costs in the region of 500000 for the initial painting of the tanks have been quoted if the life of the paint system is 6-8 years, the total cost over the normal 20-year life of a tanker is expected to be appreciably less than the sum otherwise spent on steel renewals, which may amount to several hundred thousand pounds. 

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