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Characteristics of Marine paints that protect ships hull against corrosion at sea

Paint characteristics onboard

Paint consists of pigment dispersed in a liquid referred to as the ‘vehicle’. When spread out thinly the vehicle changes in time to an adherent dry film. The drying may take place through one of the following processes.

(i) When the vehicle consists of solid resinous material dissolved in a volatile solvent, the latter evaporates after application of the paint, leaving a dry film.

(ii) A liquid like linseed oil as a constituent of the vehicle may produce a dry paint film by reacting chemically with the surrounding air.

(iii) A chemical reaction may occur between the constituents of the vehicle after application, to produce a dry paint film. The reactive ingredients may be separated in two containers (‘two-pack paints’) and mixed before application. Alternatively ingredients which only react at higher temperatures may be selected, or the reactants may be diluted with a solvent so that the reaction occurs only slowly in the can.

Corrosion-inhibiting paints for application to steel have the following vehicle types:

(1) Bitumen or pitch: Simple solutions of bitumen or pitch are available in solvent naphtha or white spirit. The bitumen or pitch may also be blended by heat with other materials to form a vehicle.

(2) Oil based :These consist mainly of vegetable drying oils, such as linseed oil and tung oil. To accelerate the drying by the natural reaction with oxygen, driers are added.

(3) Oleo-resinous :The vehicle incorporates natural or artificial resins into drying oils. Some of these resins may react with the oil to give a faster drying vehicle. Other resins do not react with the oil but heat is applied to dissolve the resin and cause the oil to body.

(4) Alkyd resin: These vehicles provide a further improvement in the drying time and film forming properties of drying oils. The name alkyd arises from the ingredients, alcohols and acids. Alkyds need not be made from oil, as an oil-fatty acid or an oil-free acid may be used.

(5) Chemical-resistant :Vehicles of this type show extremely good resistance to severe conditions of exposure. As any number of important vehicle types come under this general heading these are dealt with individually.

(a) Epoxy resins :Chemicals which may be produced from petroleum and natural gas are the source of epoxy resins. These paints have very good adhesion, apart from their excellent chemical resistance. They may also have good flexibility and toughness where co-reacting resins are introduced. Epoxy resins are expensive owing to the removal of unwanted side products during their manufacture, and the gloss finish may tend to ‘chalk’ making it unsuitable for many external decorative finishes. These paints often consist of a ‘two-pack’ formulation, a solution of epoxy resin together with a solution of cold curing agent, such as an amine or a polyamide resin, being mixed prior to application. The mixed paint has a relatively slow curing rate at temperatures below 10 °C. Epoxy resin paints should not be confused with epoxy-ester paints which are unsuitable for underwater use. Epoxy-ester paints can be considered as alkyd equivalents, as they are usually made with epoxy resins and oilfatty acids.

(b) Coal tar/epoxy resin: This vehicle type is similar to the epoxy resin vehicle except that, as a two-pack product, a grade of coal tar pitch is blended with the resin. A formulation of this type combines to some extent the chemical resistance of the epoxy resin with the impermeability of coal tar.

(c) Chlorinated rubber and isomerized rubber: The vehicle in this case consists of a solution of plasticized chlorinated rubber, or isomerized rubber. Isomerized rubber is produced chemically from natural rubber, and it has the same chemical composition but a different molecular structure. Both these derivatives of natural rubber have a wide range of solubility in organic solvents, and so allow a vehicle of higher solid content. On drying, the film thickness is greater than would be obtained if natural rubber were used. High build coatings of this type are available, thickening or thixotropic agents being added to produce a paint which can be applied in much thicker coats. Coats of this type are particularly resistant to attack from acids and alkalis.

(d) Polyurethane resins: A reaction between isocyanates and hydroxylcontaining compounds produces ‘urethane’ and this reaction has been adapted to produce polymeric compounds from which paint film, fibres, and adhesives may be obtained. Paint films so produced have received considerable attention in recent years, and since there is a variety of isocyanate reactions, both one-pack and twopack polyurethane paints are available. These paints have many good properties; toughness, hardness, gloss, abrasion resistance, as well as chemical and weather resistance. Polyurethanes are not used under water on steel ships, only on superstructures, etc., but they are very popular on yachts where their good gloss is appreciated.

(e) Vinyl resins :Vinyl resins are obtained by the polymerization of organic compounds containing the vinyl group. The solids content of these paints is low; therefore the dry film is thin, and more coats are required than for most paints. As vinyl resin paints have poor adhesion to bare steel surfaces they are generally applied over a pretreatment primer. Vinyl paint systems are among the most effective for the underwater protection of steel.

(6) Zinc-rich paints: containing metallic zinc as a pigment in sufficient quantity to ensure electrical conductivity through the dry paint film to the steel are capable of protecting the steel cathodically. The pigment content of the dry paint film should be greater than 90 per cent, the vehicle being an epoxy resin, chlorinated rubber, or similar medium.

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