Physical And Chemical Properties of Each of the Generic Types of Coating

What Are Some of the Physical And Chemical Properties of Each of the Generic Types of Coating That Are Most Commonly Used.

An inspector is not expected to be a chemist; however, some basic knowledge of the different types of coating materials and their chemical and physical properties can be advantageous. The most commonly encountered generic types of coating materials are:

• Acrylic • Epoxy Phenolic

• Alkydes • Epoxy Ester

• Bituminous • Inorganic And Organic Zincs

• Chlorinated Rubber • Silicones

• Epoxy Amine • Urethanes

• Epoxy Polyamide • Vinyls

• Epoxy Coal Tar

The more significant features of the different generic types of coating materials are as follows:

Acrylics

Acrylics have excellent colour and gloss retention for outdoor applications and are often combined with other resins because of these properties. They are ideal for use in areas of mild chemical fumes but are not recommended for tank linings. They are somewhat inferior to vinyls or chlorinated rubbers in chemical resistance. They cure by solvent evaporation. They are often used as a topcoat over other chemical resistance coatings because of their colour and gloss retention.

Alkyds

Alkyds are usually natural oils that have been chemically modified to improve the rate of cure, chemical resistance and hardness. They are general-purpose coatings designed for applications to a wide range of substrates. They are easily applied and can be used as primers or topcoats. They provide good colour retention and gloss but exhibit poor chemical resistance. Since they are subject to saponification, which is chemical interaction of fat with an alkali that forms a soap, they are not suitable for applications to alkaline surfaces. They cure by air oxidation.

Bituminous

Bituminous coatings are low-cost, heavy-bodied materials applied either hot or as a cut back with solvent. They provide good moisture barriers, have good-to-fair resistance to chemical fumes and spillage and exhibit good acid resistance and poor solvent resistance. They form very heavy films but have no corrosion inhibiting qualities. When they are damaged, undercutting can be serious problem. They are available in black only. Materials such as clay, slate, mica, asbestos and other powders are sometimes added to increase film thickness and toughness. They cure by solvent evaporation.

Chlorinated Rubber

Chlorinated Rubber coatings are similar to vinyls in that they form fairly rough, thin films that have good abrasion resistance. They have excellent weathering properties and provide excellent resistance to most mineral acids and alkalis, salt and fresh water, and fungus growth. They are frequently modified with alkyd resins to lower their cost and improve their application characteristics. Curing is by solvent evaporation.

Epoxy Amine

Epoxy amine coatings are catalysed or hardened by an amine curing agent. These materials form a hard, abrasion resistant coating with good-to-excellent alkali, acid, and solvent resistance. They must be applied to a thoroughly cleaned surface and are moisture-sensitive during application. They have a tendency to fade and chalk I direct sunlight and to embrittle on ageing.

Epoxy Polyamide

Epoxy Polyamide coatings are not as resistant to acids, alkalis, or solvents as the amines. Polyamides have greater flexibility and provide water and salt solution resistance. They have a tendency to fade and chalk indirect sunlight and do not embrittle as much as the amine epoxies do on ageing.

Epoxy Coal Tar

Amine or polyamide epoxy resins are often modified with coal tar pitch to produce relatively high film build-up for good chemical and moisture resistance. Resistance to hydrogen sulphide and acids in general is dramatically improved over a straight epoxy. They have a tendency to embrittle on ageing, and delimitation between coats or beneath touch-up patches is common unless special pre-cautions or taken during applications. They are normally black in colour.

Epoxy Phenolic

Epoxy Phenolic coatings combine a portion of phenolic resin with epoxy resin. They are usually employed as tank linings but have used extensively in nuclear plants. As interior coatings for containment vessels. They have good resistance to the effects of radiation.

Epoxy Ester

Epoxy ester coatings are a combination of an epoxy resin and a drying oil. These coatings cure by a combination of solvent evaporation and oxidation. They do not require the use of catalyst or hardener. Chemical resistance is better than alkyds but still rather poor. However, they can be applied by relatively inexperienced painter on surface having only marginal surface preparation. They do not lift most old, sound paints. Their whether resistance is fairly good, but they will chalk and yellow after extended exposure to sun. They do no saponify over alkaline surfaces to the same degree as an alkyd.

Inorganic and Organic Zincs

Metallic Zinc is used as the pigmentation with the variety of vehicle for inorganic and organic zinc coatings. They may be one- two- or three- package materials, post cured, moisture cured, solvent based or water based. The end product in all cases is metallic zinc suspended in extremely hard, tough silicate matrix. Acid or alkali resistance is poor but solvent, moisture and salt resistance is excellent. They weather extremely well and are often used as a single cost system for structural steel or tanks. Abrasive blasting is required, and the dry film thickness of the applied film thickness of the applied film is critical.

Silicones

Silicone coatings are semi-inorganic polymers which, when formulated into coatings, have outstanding heat resistance and excellent water repellency. Properties depend on the amount of silicone resin and the type of modifying agents used (i.e. alkyd, acrylics, etc). In high heat applications, such as stakes, they are used to temperatures as high as 12000 F.

Urethanes

Sometimes called polyurethanes or isocyanets, these coatings are a fairly new development but may be obtained in a wide range of formulations. Normally, two-package, catalysed materials, the aliphatic urethanes are hard, tough and abrasion resistant, with excellent weathering properties and gloss retention. They exhibit excellent solvent resistance, only slightly inferior to that of an epoxy, and withstand mild acids and alkalis. Their adhesion properties are excellent; however, they are usually applied over an epoxy primer. Urethanes can be applied at lower than most other chemically cured coatings. During applications, urethanes are moisture sensitive, but after they have cured, they exhibit outstanding resistance to high humidity.

Vinyls

Vinyls are thermostatic materials and cure solely by solvent evaporation. As such, they can be applied at much lower temperatures than any catalysed coating. Acid and alkali resistance is excellent, but solvent resistance is poor. Because they are soluble I their own solvents, repairs are easily made. Water and weather resistance is outstanding, and they have been used for years on the steelwork of many of the country’s largest dams. They are the basis of the Bureau of Reclamation VR-3 and VR-6 systems. The designation VR-6 means “vinyl resin, six (6) coats.” A clean, preferable abrasive blasted surface is required for proper adhesion. They usually employ special primers.