Zinc-rich epoxy coatings

Alternatively, some coating systems are characterized by the application method used. For example, for coil-coated metal sheet (commonly steel or galvanized steel), very specialized high-speed roller application equipment is used to coat the sheet steel as it is unwound from a coil. The paint used in the coil-coating process can be of virtually any generic type, although alkyds, polyesters, epoxies, and zinc-rich epoxy coatings are the most prevalent. 

Figure 19.60 illustrates a steel surface which should have been blast cleaned to a high standard before being coated at works with a zinc-rich epoxy primer and on site with two-pack intermediate and finishing coats. After exposure for 18 months in a marine environment, flaking millscale from beneath the paint was observed, and a survey showed that the paint... 

Prime coating is either a two-pack, zinc-rich epoxy primer with a DFT of ca. 30-40 pm, or a chromate-free, two-pack epoxy primer. Topcoats based on PVC copolymerisates, chlorinated rubber, silicone-reinforced epoxy esters, or alkyds were used for many years. Now, the standard European topcoat is based on acrylic resins with terminal epoxy groups that are cross-linked with amine adducts or aminoamide resins. 

Coil coatings are normally applied by roller coating machines, but are sometimes sprayed. The topside of the metal band is normally painted with a primer (DFT ca. 5 pm) and a topcoat (DFT 20-22 pm). Zinc-rich coil coatings are applied with a DFT of 5-20 pm, plastisol films have a DFT of 80-400 pm. The reverse side of the coils is coated with backing coats based on binders, such as alkyd, polyester, and epoxy resins (DFT 8-10 pm). 

Chem. Descrip. Polyamidoamine (70%) in xylene Uses Crosslinking agent for epoxy resins in soiv.-based primers and coatings for min. and metallic substrates, anticorrosion primers, zinc-rich primers, coatings for lock gates, concrete, and underground conduits (in combination with tar)... 

FIGURE 1.24 Bode plots of coated steel panels for different times of immersion in 5% NaCl solution, (a) 2-C epoxy topcoat without primer and (b) zinc-rich epoxy primer and 2-C epoxy topcoat. (Reprinted from Wessling, B. and Posdorfer, J., Electrochim. Acta., 44, 2139, 1999. With permission. Copyright 1999 Elsevier Science)... 

The most significant marine use of resins is actually in the form of paint corrosion protection systems for hulls. These include polyurethane and epoxide systems, the latter giving good alkali and solvent resistance in addition to providing superior adhesion to most substrates. Such systems take the form of zinc-rich epoxy and epoxy coal-tar combination hull paints. Epoxy powder coatings are also commonly used for the protection of steel pipelines, both on land and offshore. 

Taken from Ref. 3 Change of potential with immersion time for steel coated with various zinc-rich epoxy primers... 

Most pigments can be used in any type of binder therefore, paints cannot be identified by pigment type alone. For example, micaceous iron oxide pigment is traditionally in an oil-based binder but is being increasingly used in epoxies, etc. In the paint coating film, the pigment content may vary from 15 to 60 per cent. In the special case of zinc rich primers, it is over 90 per cent. 

Use Anticorrosive pigment for solvent-based epoxies and as auxiliary pigment for 1-package zinc-rich coatings. Also used in water-based coatings. 

The epoxy-polyamide system may also provide a base for paper and board coatings applied by rotogravure or roller-coating techniques. It is useful for coating plastics and particularly valuable as a resin system for zinc-rich paints. It may be formulated with exempt solvents specified under Rule 66 (37). Higher solid formulations, particularly those formulated with fatty amido amines, are available. These formulations reduce solvent pollution and allow for higher film builds (37, 38). 

Anti-corrosion coating based on epoxy rein and zinc-rich primer ... 

As indicated in Table 10.18, a zinc-rich primer is often recommended. It can be an organic zinc-epoxy or an inorganic zinc-ethyl-silicate primer. Zinc-rich primers are also used as so-called shop primers, or prefabrication primers, for temporary protection of semi-manufactured steel goods. After fabrication, e.g. of welded steel structures, the shop primer surface must be cleaned (degreased), and possible shop primer defects and weld joints have to be blast cleaned and coated with a primer before the whole structure is painted. Iron oxide is also used as a pigment in some shop primers. These must not be overpainted with a zinc-rich paint. 

C3 Interiors with high relative humidity, some air pollution. Exteriors in urban and industrial Sa2 /2 Zinc-rich primer. Finishing coats vinyl, chlorinated rubber or epoxy 3-4 175-240... 

Chem. Descrip. Barium metaborate monohydrate-based Uses Corrosion inhibitor, pigment for use in trade saies primers, indus-triai coatings, heavy-duty maintenance coatings, aq. systems (emuisions, water-reducibie aikyds), and soiv.-based systems (aikyds, urethanes, epoxies) stabiiizer for cerfain mefais and metai saits, esp. zinc in zinc-rich coatings... 

Uses Epoxy curing agent for two-pkg. air-dry coatings, e.g., industrial maintenance primers, midcoats, and topcoats, anticorrosive marine primers, zinc-rich primers, methanol-resist, tank coatings, concrete floor coatings... 

Uses Epoxy for high-performance, two-package systems tor coatings (structural steel primers, concrete sealers and surfaces, tank linings, topcoats for zinc rich primers)... 

Uses Thixotrope for high polarity and oxygenated solv. systems such as nail lacquers, paints/coatings (nitrocellulose lacquers, sol n. vinyls, epoxies, acrylics, wash primers, inorg. zinc-rich primers, chlorinated mb-ber, unsat. polyesters)... 

Araldte GY 281 Araldte GY 9580 epoxy, zinc-rich coatings... 

Two types of paint systems were used for experiments. First system used was zinc phosphate primer (50 % Zn as compound) with micaceous iron oxide (MIO) as intermediate coat followed by polyurethane (PU) as top coat denoted as ZP. Other system used was zinc rich primer (80 % Zn as dust) with MIO as intermediate coat and PU as top coat denoted as ZR. This system is costlier with respect to ZP and considered for laboratory evaluations purpose only. ZP is designed with respect to places where test panels are deployed for exposure based on manufacturers catalogues and standards [4]. These systems possess good resistance from water permeability, weathering, abrasion and good adhesion to maintain a proper barrier to the environment. The drying mechanism of the systems is the reaction between epoxy resin and polyamide. The details of each paint system used comprising primer, intermediate and top coats with the respective layer of film thickness are shown in Table 2.2. 

Combinations of zinc dust (particle diameter 12-20 xm) and extrafine zinc dust (particle diameter 3-6 p,m) yield optimal corrosion resistance. Small amounts of aluminum paste or powder, added to obtain a more silvery appearance, will however decrease corrosion resistance—as found in comparative weathering tests by Van Eijnsbergen (personal communication). Generally, nonsaponifiable binders, such as vinyl copolymers, chlorinated mbber, methacrylic resins, and epoxy resins, are used. For welding zinc-rich primers, epoxy-polyamide binders are widely used. They allow a wider range of sealers and top coatings. 

The standard does not relate to the coating systems based on zinc-rich, silicone-epoxy, polysiloxane and polyurethane materials. 

Organic (epoxy) zinc-rich primers are used in place or to repair imperfections of the inorganic zinc-rich primer. Their advantages over inorganic zincs include improved adhesion to the epoxy primer coating and better tolerance to poor surface cleaning. 

Zinc-rich coatings can be classified as organic or inorganic. The organic zinc-riches have organic binders, with polyamide epoxies and chlorinated rubber binders being the most common. Other types such as urethane zinc-rich are also available. These latter coatings are more easily applied than the other zinc-rich... 

Heavy paint coating systems are used in severe atmospheres such as marine and industrial, and consist of a spray coating of zinc or aluminum, or a zinc-rich paint primer, followed by several layers of synthetic resin paints such as epoxy, tar epoxy, or polyurethane resins. The total film thickness is more than 250 jxm. 

Many of the binder systems previously discussed, particularly the epoxies, are formulated as zinc-rich coatings resulting in changes in the characteristics of the formulated coating. Urethane zinc-rich coatings are more easily applied than other zinc-rich coatings and still provide galvanic protection. 

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