CIVIL ENGINEERING 365 ALL ABOUT CIVIL ENGINEERING



What Makes Up A Coating System Coating systems, may consists of any number of coats and combinations of materials. In most cases, coating systems will consist of 2 to 4 coats. The coating system you may use will in general be split into three main parts : 1. Primers (may include shop primer) 2. Intermediate coats (mid-coats) 3. Topcoats Each part of the paint system has a definite function, although some paints can play a dual role in a paint system. (See the paragraph later in this article “Variations”). Primers Primer Paints are universal for most anticorrosive coating systems and are considered the most important component of the system. The primer paint is the first coat of an industrial paint system and the base on which the rest of the coating system is applied. As a base, it must have strong adhesion to the substrate surface and must form a good basis for the next layer of the coating system. The most important properties of primers are listed as follows: • Adhesion (strong bonding to substrate). • Cohesion (high internal strength in the film). • Inertness (strong resistance to corrosion & chemicals). • Inter-coat bond (high bonding to intermediate coat). • Appropriate flexibility. Most primers for steel contain anti-corrosive pigments such as red lead, zinc powder (zinc dust), zinc chromate, zinc phosphate, calcium phosphate etc. Due to their toxicity, red lead and zinc chromate are used less frequently nowadays and certainly not in prefabrication (shop-priming) as they will release noxious or toxic fumes during welding and flame-cutting. If the coating system is an inhibitive one, it must contain the inhibitive pigments and be capable of using these pigments in a way which will passivate the metal surface and reduce its tendency to corrode. In cathodically protective primers, zinc silicates to a certain extent react with the steel surface in order to obtain an even greater and stronger adhesion. The primer must also provide a proper and compatible base for the intermediate coat. It must provide a surface that can be thoroughly wetted by the subsequent coat. The generally flat, non-glossy surface must also provide some physical adhesion to the next coat. Where primers are used for immersed conditions or tank linings they must have a chemical resistance equivalent to the remainder of the coating system in order to satisfactorily protect against the chemical cargo / substance in which it is immersed. Intermediate Coats In most paint systems, intermediate coats are applied between primers and topcoat(s). Intermediate coats, often also called undercoats or body coats, must adhere well to the primer; they are essential for obtaining a sufficient “build” to the paint system. The principal purposes of an intermediate is to provide : • Thickness for total coating. • Strong chemical resistance. • Resistance to moisture vapour transfer. • Increase electrical resistance for the coating system. • Strong cohesion. • Strong bonding to primer and topcoat. Midcoats should have high mechanical strength, good covering power and their colour should preferably be nearly the same as that of the top coat, but sufficiently different to allow the contrast to be obvious when applying the final coat. The colour should be lighter than the topcoat to allow good hiding by the topcoat. Often a white, or nearly white, intermediate coat is beneficial for the hiding power of the topcoat. The formulation of the intermediate coat is important. Industrial coatings that build up high thickness of the paint per coat are often termed “High Build coatings”. The body coat (or intermediate coat) must also provide strong adhesion to the primer, as well as a good base for the topcoats. The intermediate coat must have a surface ideal for good adhesion of the topcoat. Without the ability of this material to properly adhere to the primer and to provide proper adherence to the topcoats, the problem of inter-coat adhesion would cause early coating breakdown. Tie-coat A tie coat is designed to improve inter-coat adhesion and/or avoid certain defects like pinholes and popping during application. The main purpose of applying a tie coat is therefore to displace air from narrow gaps and “valleys” on the steel substrate. In order to let the air out from the substrate it is important to apply a thin coat, usually in the range of 20 – 30 microns. Tie coats are mainly applied on inorganic zinc primers or before applying anti-fouling coatings. Topcoats The top coat is the last coat(s) of a paint system, designed to protect the coats beneath from the environment, to contribute to the overall corrosion protection offered by the system and to give the required colour and gloss. Top coats perform several important functions as they : • Provide a resistant seal for the coating system. • Form the initial barrier towards the environment. • Provide resistance towards chemicals, water, and weather. • Provide a tough and wear-resistant surface. • Provide a pleasing appearance. As listed above, the finishing paint or top coat has many functions. Besides being decorative, it should be weather-resistant. The topcoat is the first line of defence against different environments such as: Wind and weather, highly polluted atmospheres, water and aggressive chemicals. It is the initial barrier of the coating system. This means that the finishing paint also must have several properties: Good colour- and gloss retention, non-chalking in bright sunlight and good adhesion under humid conditions. Moreover, the finishing paint may also be required to have good mechanical properties, i.e. resistance against scratching, physical impact and abrasion and resistance to growth. Glossy topcoats are formulated with a low pigment to vehicle ratio. In order to satisfactorily apply a topcoat over a reactive base coat containing zinc, the topcoats must be highly alkali-resistant. Such topcoats like Jotun paint would include vinyls, chlorinated rubbers, epoxies and coal tar epoxies. Variations A coating system need not to be composed of the three different parts; even a single coat can provide a coating system, depending upon the requirements of the coating. Inorganic zinc coatings, for example, provide an excellent one-coat system for the storage of refined oil products and many solvents. A single coating formulation applied in two or more coats may provide the best answer to a specific problem. In most barrier coating systems special intermediate coats are omitted. The surface tolerant mastics can be applied directly on to steel and therefore serves both as a primer and an intermediate coat. As for all paint systems the properties of these products must satisfy a number of requirements, like: • Expected lifetime (design life) • Future maintenance • Classification society requirements • International standard requirements • Health, safety and environmental requirements • Cost for the total design life Hope this helped you design a coating system for your project, but feel free to add a comment below if you need further technical information. Author Bio – Author – Andy Potts – A specialist in business, finance, Paint and spray shop consumables. View Industrial Paint Systems
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