Resistivity epoxy coatings

The secondary hydroxyl groups react further to form a tight, crosslinked network which imparts the optimum solvent and chemical resistance. Flexibility of the film is reduced, however, compared to other epoxies and discoloration can be a problem. If a lighter color is needed, urea-formaldehyde resins are reacted with the epoxy with little loss in chemical and solvent resistance. Epoxy coatings are used to line the interior of cans, drums, tanks and piping. 

Epoxies provide excellent chemical, moisture, and abrasion resistance. However, because a solvent can sometimes dissolve the epoxy present in the laminate, rework on an epoxy coating requires heat or abrasion to remove the coating. 

Galvanised steel provides increased corrosion resistance in carbonated concrete. In concrete with more than 0.4% chloride ion with respect to the cement content, there is an increased risk of corrosion and at high chloride contents the rate of corrosion approaches that of plain carbon steel. In test conditions the rate of corrosion is greater in the presence of sodium chloride than calcium chloride. Fusion-bonded epoxy-coated steel performs well in chloride-contaminated concrete up to about 3.9% chloride ion in content. 

Austenitic stainless steels resist corrosion at levels of chloride contamination greater than that which can be resisted by epoxy-coated bar. 

In the case of some tanks used to carry wine or chlorinated solvents the final coat applied over an epoxy coating is sometimes an oil-free polyurethane enamel because this paint resists chlorinated solvents better than do epoxies, does not taint wines and is not stained by red wines. 

The thickness and water resistance of the normal air-cured film can now be much improved by the incorporation of suitable coal-tar pitch material. A typical coal tar/epoxy coating material would be constituted as follows ... 

The paints were spray applied to a number of substrates. Two coatings gave a final dry film thickness between 85 to 130 microns. For ac impedance studies the coatings were applied to shot (steel) blasted cold rolled steel plates. For interface metal-loss studies by electrical resistance the coatings were applied to 10 micron Fe foils (99.85 purity), adhesively attached by Araldite 2003 epoxy paste on one side to grit blasted glass plates. 

A micro computer system allowed voltage and current measurements to be synchronised plus data logging and averaging of measurements. Alongside each "measurement" foil was a "control" foil, coated with paint plus a protective epoxy coating, which did not corrode and allowed resistance measurements to be normalised. 

Oligophenylethoxysiloxanes are used as modifiers for various polymers to improve their weather resistance and other technical characteristics, as well as to increase the heat resistance of coatings. E.g., PES-50 is used to modify polyethers, aciylic and epoxy polymers PES-80 is used to modify alkyd and urea-formaldehyde resins. Besides, PES-80 is used as an additive in paints and enamels (to improve their flow properties, gloss and colour), as well as in concrete mixes (to improve the water resistance and durability of concrete works). 

Catalyzed epoxy coatings lacked sufficient high-temperature resistance to soaked insulation for the long-term exposure. 

Many of the attributes of solvent-borne epoxy coatings could be carried over to the waterborne epoxy coatings. These same attributes are useful in the application of waterborne epoxies as adhesive systems. They include good adhesion to a variety of substrates such as metals, wood, concrete, glass, ceramics, and many plastics chemical resistance low shrinkage toughness and flexibility and abrasion resistance. 

A solid plastic may be ground to a solid powder and then used to apply a plastic coating to a metal product, either for decorative reasons (color) or for functional performance such as insulation or corrosion resistance. Plastics most often used in this way include nylon, vinyl, acrylic, polyethylene, polypropylene, and epoxy. Coating without solvents is beneficial both economically and environmentally, and 100 percent utilization of material eliminates waste. After the powder is bonded to the metal surface, it is often reheated to flow into a more uniform coating and, in the case of thermosetting resins, to complete the cure reaction. 

The vinyl copolymers can be used most efficiently in special applications such as hospital and dental equipment where durability is more important than initial cost. For laboratory equipment, epoxy resins may be preferred because the vinyls are sensitive to some solvents. The vinyl coating systems consisting of corrosion-inhibiting primer and chemical-resistant finish coats are used on new equipment for chemical plants. The metal conditioner based on zinc chromate and polyvinylbutyral are widely used over sandblasted steel as a use for vinyl systems on both industrial and marine equipment. 

Solid adducts Isolated amine products, in particular, offer advantages such as low colour, low free amine content and low irritation potential they also provide non-yellowing, bloom-free films with good chemical and solvent resistance Amine adducts solvent-based, two-pack coatings, e.g. primers, finishes and coal tar epoxy coatings... 

They are used mainly as curing agents for solvent-based coatings, epoxy coal tar coatings, putties, adhesives and solvents. Polyamide-cured epoxy coatings have the drawback of being somewhat softer and less chemically resistant than other coatings. 

Epoxy coatings are considered a versatile choice for the construction industry because of their excellent adhesion to concrete, steel and metal surfaces and their chemical resistance, toughness and corrosion resistance. A variety of brands are marketed by manufacturers for different end-uses. In general, the protective purpose of these coatings is resistance to corrosion and chemical attack. 

It is also possible to formulate epoxy coatings by using glass flakes. This improves the abrasion resistance. Such coatings can be applied at a dft of between 1500 xm and 3000 xm (International Paints, 1996). 

Apart from good mechanical properties and chemical and corrosion resistance, another advantage of solventless epoxy coatings is that there is no emission of solvents and less of a possibility of contamination, making them more environmentally friendly than other options. Ecological concerns have led to increasing use of these materials. 

Coal tar-epoxy coatings are resistant to a wide range of chemicals and chemical solutions, including freshwater, seawater, distilled water, sewage, detergents, dilute mineral adds and alkalis, barnacle growth, exhaust... 

Figure 103 The weathering resistance of epoxy-polysulphide coatings after one month of outdoor exposure [A] LP-epoxy modified coating [B] unmodified epoxy coating. Substrate = oily, rusty and wet mild steel. Reproduced with permission from Morton International...

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