Paints are the chief material used on board ships for corrosion prevention. There are different types of paints available as per requirements and place of application on ship. Paints when applied on a structure create barrier between the structural surface and corrosion causing phenomenon.
Now, before we proceed to types of Paints, let us first understand the constituents of paint
Constituents of paints
Paints constitute of following
- Binder (Sometime the binders are termed as vehicle, medium, resin, film former or polymer)
- Pigment and Extender
Binder and the pigment in the paint form the dry paint film. Solvents in the paint help in the application of the paint.
Binders determine the physical and chemical characteristics of the paint coating. Binder can be of liquid type, solid type or solid type with curing agents.
Paints are generally named after their Binder component. For Example: Epoxy Paints, Chlorinated Rubber Paints, Alkyd Paints etc.
Binders are generally classified into two classes
- Convertible Binders
- Non-Convertible Binders
Convertible binders are associated with chemical reaction and work in two stages
Solvent is lost from the film by evaporation and the film becomes dry to the touch.
The film progressively becomes more chemically complex by:
- Reaction with atmospheric oxygen
- Reaction with an added chemical curing agent
- Reaction with moisture in the atmosphere.
- Artificial heating
- Solar Heating.
Non-Convertible binders do not involve any chemical reaction, but loss of solvent by evaporation.
Simple solutions of various resins or polymers are dissolved in suitable solvent(s). Drying of paint is by solvent evaporation and there is no chemical change.
Binders which are in this category are:
- Chlorinated Rubber
Pigments and Extenders
Pigments and extenders are used in the form of fine powders which are dispersed into the binders of various particle sizes. Pigments can be divided into the following types:
- Anticorrosive Pigments
- Barrier Pigments
- Colouring Pigments
Solvents are used in paints principally to facilitate their application. The function of the solvent in paint is to:
- Dissolve the binder and reduce the viscosity of the paint to a level which is suitable for the various methods of application. E.g., brush, roller, airless spray etc.
- After application the solvent evaporates
Types of Solvents
- White Spirit
- Iso Propyl Alochol
- Methyl Ethyl Ketone (MEK)
Types of Paints
The following are various types of paints used on ships, These are classified as per their binders / vehicle as explained earlier.
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.
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 used.
The vehicle incorporates natural or artificial resins into drying oil. Some of these resins may react with drying oil to give a faster drying vehicle.
These vehicles provide a further improvement in the drying time and film forming properties of drying oils. The name alkyd arises from the ingredients alcohol and acids. Alkyd need not be made from oil, as an oil-fatty acid or an oil free acid may be used.
Vehicles of this type show extremely good resistance to severe weather conditions
1. Epoxy Resins
Chemicals which may be produced from petroleum and natural gas are the source of epoxy resins. These paint have a good adhesion, apart from there excellent chemical resistance. They may also have good flexibility and toughness. They are expensive owing to removal of unwanted side products and also have a gloss finish. These normally come in two pack consisting of epoxy resin and a curing agent (generally an amine or a polyamide resin) which is mixed prior to application. Epoxy resins should not be confused with Epoxy-Ester Paints. Epoxy-Esters can be considered as alkyd paints, as they are usually made of epoxy resin and oil fatty acids.
2. Coal Tar / Epoxy Resin
The vehicle type is similar to epoxy resin, except that instead of the curing agent, a grade of coal tar pitch is blended with the resin. A formulation of this type combines to some extent the chemical resistance of epoxy resin with the impermeability of coal tar.
3. Chlorinated Rubber and Isomerised 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. It has same chemical composition but a different molecular structure. Both these derivatives of rubber have a wide range of solubility in organic solvents, and so allow a vehicle of higher solid content. On drying the film thickness would be greater than it would have been obtained from natural rubber. Coats of this type are particularly resistant to attack by acids and alkalis.
4. Polyurethane Resins
A reaction between isocyanates and hydroxyl containing compounds produces urethane and this reaction has been adopted to produce polymeric compounds from which paint film, fibres and adhesives may be obtained. These paint have good properties such as toughness, hardness, gloss, abrasion resistance as well as chemical and weather resistance. These are not applied underwater but only on superstructures. They are very popular on yachts due to their high gloss.
5. Vinyl Resins
Vinyl Resins are obtained by the polymerisation of organic compounds containing the vinyl group. The solid contents of this paint is low thus it has a very thin film and more costs are required as compared to other paints. Since these have a poor adhesion to bare steel, they are used over a pre-treatment primer. These are among the most effective for underwater protection of steel.
Zinc Rich Paints
These paints contain metallic zinc as a pigment in sufficient quantity to ensure electrical conductivity through the dry film capable of protecting the steel cathodically. The pigment content of the dry film should be more than 90%, the vehicle being epoxy resin, chlorinated rubber or similar medium.