Components of marine paints?
AS A PROTECTIVE COATING marine paints consists of :
Paints intended to protect against corrosion consist 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.
(a) 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.
(b) A liquid like linseed oil as a constituent of the vehicle may produce a dry paint
film by reacting chemically with the surrounding air.
(c)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:
(a)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.
(b)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.
(c)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.
(d)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. (Note. Vehicle types (b) and (d) are not suitable for underwater
service, and only certain kinds of (c) are suitable for such service.)
(e)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.
(i)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 oil- fatty acids.
(ii)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.
(iii)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.
(iv)Polyurethane resins A reaction between isocyanates and hydroxyl- containing 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 sincethere is a variety of isocyanate
reactions, both one-pack and two- pack 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
(v)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
(f)Zinc-rich paints 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