Page 329 - Ship Construction.DJ Eyres 6Ed

Ch27-H8070.fm Page 318 Wednesday, October 18, 2006 7:01 AM
318 Ship Construction
(e) Chemical-resistant Vehicles of this type show extremely good resis-
tance 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 petro-
leum and natural gas are the source of epoxy resins. These
--- ใช้เพื่อการศึกษาเท่านั้น---
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
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
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 isomer-
ized 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 since